US3141330A - Precipitation sensing system - Google Patents

Description

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PRECIPITATION SENSING SYSTEM 8 Sheets-Sheet 8 Filed Dec. 19, 1960 7 360 figg 364 366 f f3 0 372 E l5 g PULJE MV PUJE i AMPLIFIER AMPLIFIER Pulsa" EAMPLIFIER [Elfen/UFE Jmes E. Murray' United States Patent O PRECIPITATION SENSING SYSTEM James E. Murray, Euclid, Fred H. Gutlr,` Warrensville,

James David Powell, Euclid, and Wendell Victor Peterson, South Euclid, Ghio, assignors to Thompson Ramo Wooldridge Inc., Cleveland, Ohio, a corporation of Ohio Filed Dec. I9, 1960, Ser. No. 76,884 36 Claims. (Cl. 73-170) This invention relates to a weather instrument and more particularly to a precipitation sensing system which can be placed at any desired point to respond to falling precipitation and to automatically produce electrical signals corresponding to the type of precipitation, whether it be hail, sleet, rain, drizzle, freezing rain, freezing drizzle, heavy rain or snow.

Various weather instruments are now available for measuring quantities such as temperature, barometric pressure, relative humidity, wind direction, wind velocity, cloud height, etc. However, no satisfactory instruments have been available for signalling the type of precipitation which may be falling. It has been necessary to rely on human observation which in some respects is eX- tremely sensitive and selective but has the disadvantage in that human judgment as to the type of precipitation is not entirely reliable and reproduceable. This invention was evolved with the general object of eliminating the need for human observation and of providing a reliable system which would respond to falling precipitation and automatically produce output electrical signals accurately corresponding to the type of precipitation. With such a system, it is possible to provide a completely automated weather station.

According to this invention, a system is provided which to a degree simulates the operation of a human in sensing precipitation. The system senses various characteristics of falling precipitation while simultaneously sensing atmospheric conditions to produce a number of control signals. The control signals are fed to logic circuitry which develops an output signal corresponding to the type of precipitation. Important features orf the invention reside in the general arrangement of the sensing devices and in the construction of the logic circuitry to provide such output signals.

Another important feature of the invention is in the recognition of the need for and the provision of a system for automatically producing control signals corresponding to the kinetic energy of falling particles. 'Ihis system has many specific features which enable it to produce such signals with a high degree of accuracy and reliability. Such signals are highly important in distinguishing between drizzle, rain, heavy rain and sleet.

A further important feature of the invention is in the recognition of the need for and lthe provision of a rebound sensing system which automatically produces control signals in response to the rebound of particles from a surface exposed thereto. The rebound sensing system is highly important in distinguishing between precipitation in the form of rain and solid precipitations such as sleet or hail.

Still another important feature of the invention is in the recognition of the need for and the provision of a photo-optical sensing system which develops output signals corresponding to the optical characteristics of falling precipitation. The system of this invention operates to measure the reflectivity of falling particles. The control signals obtained are highly important as au aid in distinguishing between rain and snow.

A still further feature :of the invention is in the recognition of the need for and the provision of a system for measuring the accumulation of precipitation on a sur- ICS face exposed thereto, todevelop output signals which are used in determining the presence of freezing rain or freezing drizzle. The system of this invention is eX- tremely sensitive and accurate in operation.

This invention contemplates other and more specific objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate a preferred embodiment and in which:

FIGURE 1 is a schematic diagram of a precipitation sensing system constructed according to the principles of this invention;

FIGURE 2 illustrates the construction of a mass accumulation sensing apparatus of the system` of FIGURE 1, and a mass accumulation sensor circuit for developing output signals;

FIGURE 3 is a sectional view ott' a sensing plate o-f the apparatus of FIGURE 2, taken substantially along line III-III of FIGURE 2;

FIGURES 4 and 5 are diagrams illustrating the form of signals obtained in the apparatus of FIGURE 2, under different conditions of operation;

FIGURE 6 illustrates the construction of an impact sensor device of the system of FIGURE 1, together with the schematic diagram of an impact sensor circuit shown in block form in FIGURE 1;

FIGURE 7 is a cross-sectional view of the impact sensor device of FIGURE 6, taken substantially along line VII-VII of FIGURE 6;

FIGURE 8 shows the construction of a rebound sensing apparatus of the system of FIGURE l and also shows a block diagram of the rebound sensor circuit of FIGURE l;

FIGURE 9 shows the construction of a photo-optical sensing apparatus of FIGURE l and also shows a block diagram of a photo-optical sensor circuit of FIGURE 1;

FIGURE 10 is a schematic diagram of an AND circuit used at various points in the system;

FIGURE 11 is a schematic diagram of an OR circuit used at various points in the system;

FIGURE l2 is a schematic diagram of an inverter ampliiier circuit used at various points in the system;

FIGURE 13 is a schematic diagram of a preferred form of hip-flop circuit used at certain points of the system;

FIGURE 14 is a schematic diagram of a monostable multivibrator circuit used at various points in the system;

FIGURE 15 is a schematic diagram of a reset timer circuit used at various points in the system;

FIGURE 16 is a schematic diagram of a preferred form of amplitude discriminator or squaring circuit which may be used at various points in the system; and

FIGURE 17 is a schematic diagram of a pulse amplifier circuit which may be used in the reset timer circuit of FIGURE 15.

Referring to FIGURE 1, the precipitation sensing system 10 of this invention generally comprises eight output circuits 11-18 selectively energized through sensing and logic circuitry to indicate hail, sleet, drizzle, freezing drizzle, rain, freezing rain, heavy rain or snow. The inputs of the output circuits 11-18 are connected to a logic circuitry section 19 which has inputs connected to a plurality of sensor circuits, including a mass accumulation sensor circuit 20, an impact sensor circuit 21, a rebound sensor circuit 22., a photo-optical sensor circuit 23, a thunderstorm detector 24 and an air temperature sensor 25.

The mass accumulation sensor circuit 20 is connected to a sensing apparatus 26 and functions to develop output signals in response to the accumulation or non-accumulation of ice on a surface, to distinguish between rain and freezing rain, or drizzle and freezing drizzle. In brief,

the apparatus 26 comprises a pair of plates 27 and 2S exposed to precipitation and mounted on the legs of a tuning fork disposed within a covered tuning fork assembly 29. As ice accumulates on the plates 27 and 28, the frequency of vibration of the tuning fork is lowered and by measuring the change in the frequency, an indication of mass accumulation is obtained. To remove loose snow and water from the plates 27 and 23, a blower section 30 is provided, which also aids in periodic removal of ice and in cooling the plates, after they are heated periodically by suitable means to remove ice. The circuit 29, the apparatus 26 and their operation are described in detail hereinafter in connection with FIGURES 2-5.

The impact sensor circuit 21 is connected to a sensing device 31 and is used to develop signals which aid in clistinguishing between drizzle, rain, heavy rain and sleet. In brief, the illustrated device 31 comprises a hemispherical plate 32 on which the precipitation falls, mechanically connected to a transducer within a housing 33. The circuit 21 and device 3l are described in detail hereinafter in connection With FIGURES 6 and 7.

The rebound sensor circuit 22 is connected to a sensing apparatus 34 and is used to develop output signals which are helpful in distinguishing between precipitation in the form of rain and solid precipitations such as sleet or hail. In brief, the apparatus 34 comprises inclined plates 35, 36, 37, and 38 on which the precipitation falls and from which it may rebound against lower surface portions of an impact sensing structure 39.

The photo-optical sensor circuit 23 is connected to a sensing apparatus 4d and is used to develop output signals which aid in distinguishing between rain or drizzle and snow. In brief, the precipitation falls through a central portion 41 of the apparatus 40, between portions 42 and 43 thereof. Portion 42 houses a light source which projects light through the portion 41 toward the portion 43 and a photoelectric detector for measuring reflection of light from particles of precipitation falling tln'ough the portion 41. Portion 43 simply provides a dark background for contrast. It is found that snow has a much higher degree of reflectivity than -rain and that a very reliable indication can be obtained in this fashion.

The thunderstorm detector 24 is of a type known in the art and its construction and operation are therefore not described and illustrated in detail herein. It is used in providing an output indication when thunderstorm conditions are present, which is helpful in determining the presence of hail. It is also helpful with respect to providing an indication or non-indication of snow, since snow rarely occurs under thunderstorm conditions.

The air temperature sensor 25 is also of a type known in the art and hence is not illustrated and described in detail. Its output is connected to a pair of amplitude discriminators 44 and 45. Amplitude discriminator 44 provides an output signal when the temperature is in excess of a certain value, for example 40 F., while amplitude discriminator 45 provides an output indication when the temperature is in excess of another certain value, considerably lower, for example 15 F.

LOGIC CIRCUITRY 19 (FIGURE l) The logic circuitry 19 operates to selectively energize one of the output circuits 11-18 in response to input signals obtained from the sensor circuits Ztl-25. The controlling input signals for the circuit 19 are produced on various conductors, as follows:

A conductor 46, connected to the output of the mass accumulation sensor circuit Ztl, has an output signal developed thereon, designated as M, when the accumulation of ice on the plate 27 exceeds a certain value, in a certain time interval.

A conductor 47, connected through an inverter 48 to the output of the mass accumulation sensor circuit 20, has an output signal developed thereon, designated as when the accumulation of ice in a certain time interval on the plate 2'7 does not exceed a certain value.

A conductor 49, connected to one of four outputs of the impact sensor circuit 21, has an output signal IY developed thereon under heavy precipitation conditions, i.e. when the amount of precipitation in a certain time interval is in excess of a certain value.

Conductors 50, 51 and 52, connected to the other three outputs of the impact sensor circuit 21, have output signals developed thereon in accordance with the energy of impact of precipitation particles on the sensing plate 32. Conductor 5t) has an output signal IL developed thereon when the amplitudes of the impacts fall within a relatively low range of values. Conductor 51 has an output signal IM developed thereon when the amplitudes of the impacts fall within an intermediate range of values, and conductor 52 has a signal IH developed thereon in response to impacts having amplitudes exceeding a certain relatively high value.

Conductor 53, connected to the output of the rebound sensor circuit 22, has an output signal IR developed thereon in response to rebounds, above a certain amplitude, on the apparatus 34.

Conductor 54 is connected through an inverter 55 to the output of the rebound sensor circuit 22 and has a signal IR developed thereon in response to the absence of a rebound signal on the conductor 53.

Conductors 56, 57 and 58 are connected to outputs of the photo-optical sensor circuit 23. Conductor 56 has a signal PL developed thereon when the reflection of light from the precipitation is in a relatively low range. Conductor 57 has an output signal PM developed thereon when the reflection of light is in a certain intermediate range, and conductor 58 has an output signal PH developed thereon when the reflection of light is in a certain intermediate range, and conductor 58 has an output signal PH developed thereon when the refiection exceeds a certain relatively high value.

Conductor 59 is connected to the output of the thunderstorm detector and has an output signal TS developed thereon under conditions indicative of a thunderstorm.

Conductor 60 is connected through an inverter circuit 6l to the output of the thunderstorm detector 25 and has a signal TS developed thereon in the absence of conditions indicative of a thunderstorm.

Conductor 62 is connected to the output of the amplitude discriminator 44 which, in turn, is connected to the output of the air temperature sensor 25. A signal T40 is developed on the conductor 62 when the temperature exceeds a certain value, for example 40 F.

Conductor 63 is connected through an inverter circuit 64 to the output of the amplitude discriminator 44 and has a signal T40 developed thereon when the temperature is less than the aforesaid predetermined value which may be 40 F.

Finally, conductor 65 is connected to the output of the amplitude discriminator 45, connected to the air temperature sensor 25. A signal T15 is developed on the conductor 65 when the air temperature exceeds a certain relatively low value, for example 15 F.

The logic circuitry 1% will now be described with reference to the various conditions which cause selective operation of the output circuits lll-18.

(l) Hail Indication The input of the hail output circuit 11 is connected to the output of an AND circuit 66 which develops an output signal in response to the concurrent application of input signals to all three of its three inputs. These are connected to the conductors 62, 59 and 53. Accordingly, when there is concurrently a temperature in excess of a certain relatively high value, for example 40 F., thunderstorm conditions and rebounds in the apparatus 34, a signal is applied through the AND circuit 66 to the hail output circuit 1l, energizing the same.

(2) Sleet Indication The input of the sleet output circuit 11 is connected to the output of an AND circuit 67 which develops an output signal in response to the concurrent application of three input signals to all inputs thereof. These inputs are connected to the conductors 52, 63 and 53. Accordingly, the sleet output signal 12 is energized when concurrently there are high amplitude impact signals, rebound signals and a temperature below a certain relatively high value, which may be on the order of 40 F.

(3) Drizzle Indication The input of the drizzle output circuit 13 is connected to the output of an AND circuit 68 which develops an output signal in response to the concurrent application of input signals to both of its two inputs. One of the inputs is connected to the conductor 47. The other input of the AND circuit 68 is connected to the output of another AND circuit 69, having one input connected to the conductor 50, and having another input connected to the output of still another AND circuit 70. Circuit 70 has three inputs connected to conductors 56, 65 and 54. Accordingly, the drizzle output circuit is energized in response to the concurent prence of signals on five conductors 47, 50, 54, 56 and 65. Thus drizzle is indicated by the abensce of mass accumulation, by the presence of lower amplitude impacts, by the absence of rebound, by the presence of low intensity reflections and by the presence of a temperature in excess of a certain relatively low value, for example F.

(4) Freezing Drizzle Indication The input of the freezing drizzle output circuit 14 is connected to the output of an AND circuit 71 having two inputs. The irst input is connected to the output of the AND circuit 69. The second input is connected to the conductor 46 which is connected to the output of the mass accumulation sensor circuit. With this arrangement, the freezing drizzle output circuit is energized in response to the same conditions as those which energized the drizzle output circuit 13, except that the freezing drizzle output circuit 14 is energized in response to mass accumulation, whereas the drizzle output circuit 13 is energized in response to the absence of mass accumulation.

(5) Rain Indication The input of the rain output circuit 15 is connected to the output of an AND circuit 72 having a rst input connected to the conductor 47 and a second input connected to the output of an AND circuit 73 having a first input connected to the output of the AND circuit 70 and a second input connected to the conductor 51. With this arrangement, the rain output circuit is energized in response to the concurrent development of signals on conductors 47, 51, 54, 56 and 65. Thus the rain output circuit is energized When there is an absence of mass accumulation, when there are impacts having amplitudes greater than a certain intermediate value and less than a certain high value, when there is an absence of rebound, when the reflection from the precipitation is within a certain relatively low range, and when the temperature exceeds a certain relatively low value which may be 15 F. v

(6) Freezing Rain Indication The input of the freezing rain output circuit 16 is connected to the output of an AND circuit 74 having a iirst input connected to the output of the AND circuit 73 and a second input connected to the conductor 46. Thus the freezing rain output circuit is energized in response to the same conditions which cause energization of the rain output circuit 15, except that the freezing rain output circuit is energized in response to the presence of mass accumulation, as indicated by a signal on the conductor 46, whereas the rain output circuit 15 is energized in response to the absence of mass accumulation, as indicated by a signal on the conductor 47.

(7) Heavy Rain Indication The input of the heavy rain output circuit 17 is connected to the output of an AND circuit 75 having a first input connected to the output of the AND circuit 72 and a second input connected to the conductor 49. With this arrangement, the heavy rain output circuit 17 is energized in response to the same conditions which energized the rain output circuit 15, provided there is also present a signal on the conductor 49. As described above, a signal is developed on the conductor 49 when there is a large amount of precipitation.

(8) Snow Indication The input of the snow output circuit 18 is connected to the output of an OR circuit 76 having a first input connected to the output of an AND circuit 77 and a second input connected to the output of an AND circuit 78. The OR circuit 76 develops an output when there is an output from either of the circuits 77, 78. The AND circuit 77 is connected to develop an output signal in the presence of snowflakes while the AND circuit 78 is connected to develop an output signal in response to the presence of snow pellets.

In particular, circuit 77 has four inputs connected to conductors 54, 58, 60 and 63. Thus an output signal is developed by circuit 77 when there is an absence of rebound, when the reliection of light from the precipitation is above a relatively high value, when there is an absence of thunderstorm conditions, and When the temperature is below a certain value which may be 40 F. Such conditions prevail when the precipitation is in the form of snowflakes.

The circuit 78 has four inputs. The first three inputs are connected to conductors 50, 60 and 63. The fourth input is connected to the output of an OR circuit 79 having inputs connected to conductors 57 and 58. Thus the circuit 78 develops an output when the precipitation produces impacts having amplitudes above a certain relatively low value, when there is an absence of thunderstorm conditions, when the temperature is below a certain value which may be 40 F., and when the reflection of light from the precipitation is either in an intermediate range or above a relatively high value. Such conditions prevail when the precipitation is in the form of snow pellets.

It should be noted that the snowflake indicating circuit 77 and the snow pellet indicating circuit 78 may be connected to separate output circuits, rather than through the OR circuit 76 to the single output circuit 18.

It may further be noted that the specic design of the system was based upon limited experience with respect to weather conditions which prevail in connection with the various forms of precipitation in the Cleveland, Ohio, area. In other parts of the country, and with additional experience, the exact conditions of operation may have to be varied to some degree. However, the general principle of operation remains the same.

Mass Accumulation Sensor FIGURE 2 shows the mass accumulation sensor circuit 20 and also shows the construction of the apparatus 26, in a plan View of the apparatus with the cover plates of the tuning fork assembly 29 being removed. As shown, the tuning fork assembly 29 comprises a tuning fork 80 having a pair of leg portions 81 and 82 and a base portion 83 secured by a bolt 84 to a support 85. The plates 27 and 28 are secured to the ends of the legs 81 and 82 through a pair of coupling members 86 and 87. The coupling members 86 and 87 are preferably of a lightweight rigid material such as plastic. The legs 81 and 82 vibrate horizontally, in a direction parallel to the plane of the plates 27 and 28, the natural resonant frequency of vibration being determined in part by the mass presented by the plates 27 and 28 together with the coupling members S6 and 07. It will be appreciated that as ice accumulates on the plates 27 and 28, the natural resonant frequency of vibration will be lowered.

To vibrate the legs 81 and 82 of the tuning fork S0 and to derive electrical signals therefrom, a pick-up coil 88 is disposed adjacent the outer surface of the leg 81 While a drive coil 89 is disposed adjacent the outer surface of the leg S2. The coils 58 and S9 are supported on uprights 90 and 91, secured to the support 85.

The assembly is mounted within a generally rectangular housing 92 which includes an intermediate generally vertical wall 93 approximately in the plane of the inner sections of the plates 27, 28 and coupling members 86 and 07. The wall 93 has openings just large enough to allow passage of the plates 27, 2S and the ends of the coupling members 36, 37 therethrough and to allow the required movement thereof, so that the Wall 93 serves to minimize the entry of precipitation into the tuning fork chamber.

As above described, a blower is mounted within the section 30 of the apparatus 26. The blower may be of any desired construction. An outlet therefrom is provided in a Wall portion 94 of the housing 92 with a dellector plate 95 being located on the wall 94 above the opening therein and above the plane of the plates 27, 28. The blower directs a stream of air across the plates with suflicient force to keep snow from collecting on the plates during time intervals when the apparatus is used to measure ice accumulation. The blower also serves to aid in removing accumulated ice and in cooling the plates after they are heated to remove accumulated ice. An electrical heater is preferably incorporated in the blower.

Heating of the plates 27 and 2S is desirable for rapid removal of ice therefrom. It will be appreciated that it is necessary to periodically remove ice from the plate, to keep the output of the sensor up to date.

As shown in the cross-sectional view of FIGURE 3, the plate 27 preferably comprises a thin electrical heating element 96 sandwiched between two thin metal plates 97' and 98, preferably aluminum plates. With this construction, heat may be applied uniformly over the entire surface of the plate, to rapidly melt ice. The plate 28 preferably has the same construction as the plate 27.

The pick-up and drive coils 80 and 89 are connected through conductors 99 and 100 to terminals of driver and amplifier circuits 101. The construction of such circuits, as well as the construction of the tuning fork assembly with its coils, is known in the art. The circuits 101 develop an output electrical signal having a frequency corresponding to the natural resonant frequency of the tuning fork with the attached plates 27, 28 and coupling members 86, 87. The frequency of the signal decreases as ice accumulates on the plates 27, 28. An A.C. source 102 develops a reference signal whose frequency is equal to that developed by circuits 101 when there is no ice on plates 27, 28. Such signals are applied to squaring circuits 103 and 104 to develop square Wave signals. When the two frequencies are the same but of arbitrary phase relationship, they may have a form such as illustrated by lines 105 and 106 in FIGURE 4. However, when the ice accumulates on the plate 27, the natural resonant frequency is lowered, and the outputs of the squaring circuits may have forms as indicated by reference numerals 107 and 100 in FIGURE 5, the signal 10S being of the same form as 106 While the signal 107 is of a lower frequency than the signal 105.

The outputs of the squaring circuits 103 and 104 are applied to a flip-flop circuit 109, the output of which is applied to an averaging circuit 110. Referring to FIG- URE 4, the flip-flop circuit may be set by each leading edge in the signal 105 and reset by each leading edge in the signal 106 to develop an output signal having a form as indicated by reference numeral 111 in FIGURE 4. It will be noted that a series of pulses of equal amplitude are produced. When such pulses are applied to the averaging circuit 110, a substantially constant output signal 112 is produced. It is noted that FIGURE 4 illustrates the operation with a phase relationship a1'- bitrarily assumed to be about With a different phase relation, the width of the output pulses would be changed, and the average output would be changed, but would remain at a substantially constant value.

Referring to FIGURE 5, when the flip-flop circuit 109 is set by each leading edge of the signal 107 and reset by each leading edge of the signal 108, the flip-dop circuit 109 produces an output signal having a form as indicated by reference numeral 113 in FIGURE 5. lt will be noted that the width of the pulses varies in a certain fashion, dependent upon the difference in frequency. When the output signal 113 from the flip-flop circuit 109 is applied to the averaging circuit 110, an output signal is developed having a form such as indicated by reference numeral 114 in FIGURE 5.

The output of the averaging circuit is applied to a multivibrator 115, the output of which is applied to an averaging circuit 116. Multivibrator 115 is triggered by output signals from the averaging circuit 110 having a certain amplitude and rate of change, and the multivibrator 115 stays on for a certain time intermal after triggering thereof.

When the output of the averaging circuit 110 is substantially constant, as is true under equal frequency conditions of operation as illustrated in FlGURE 4, the multivibrator 115 is not triggered and its output stays at zero, as indicated by the straight line 117 in FIGURE 4. The output of the averaging circuit 116 is also Zero, as indicated by the straight line 118 in FIGURE 4. When, however, there is a difference in the frequencies, as illustrated in FIGURE 5, the multivibrator 115 is periodically triggered at a rate corresponding to the difference in frequencies to produce an output signal of a form such as indicated by reference numeral 119 in FIGURE 5. The averaging circuit 116 then produces an output signal such as indicated by reference numeral 120 in FIGURE 5. It will be appreciated that since the frequency of the output pulses from the multivibrator 115 increases with the difference in frequency of the applied signals, the output of the averaging circuit 116 also increases in proportion to the difference in frequency.

The output of the averaging circuit 116 is applied to an amplitude discriminator 121 which produces an output signal when the output from the averaging circuit 116 exceeds a certain value. The output of the averaging circuit 116 may also be connected to `a terminal 122 which is not used in the illustrated system, but may be used Where an analog signal of mass accumulation is desired. In the event a digital approach to the measurement of mass accumulation is desired, the output of the multivibrator 115 may be applied to a counter which counts for a fixed time interval. The count at the end of the timing interval would then be a digital representation of the difference in frequency between the two tuning forks and thus an indication of the actual mass accumulation. The rate of mass accumulation may be measured by measuring the rate of change of the frequency appearing at the output of the one-shot or monostable multivibrator 115. In such cases, it may be desirable to use a relatively long accumulation period on the order of hours, rather than minutes as is preferably used in the system 10.

The output of the amplitude discriminator 121 is applied to one input of a dual input AND circuit 123, the output of which is applied to a storage circuit in the form of a flip-flop circuit 124. The output of the flipop circuit 124 constitutes the output of the mass accumulation sensor circuit, and is connected to the conductor 46, shown in FIGURE l.

The second input of the AND circuit 123 is connected to the output of an amplitude discriminator 125, the input of which is applied to a at surface temperature sensor 126. The purpose of this arrangement is to avoid the possibility of an erroneous indication, which may be produced if the temperature of the collector plates 27, 28 is lower than freezing while the temperature of a standard reference surface is above freezing. The plates 27 and 28 may have a temperature somewhat lower than that of a standard reference surface due to the operation of the blower, and due to the material (metal) thereof. To prevent the erroneous indication, a temperature sensor device is associated with a reference surface, as diagrammatically illustrated by the block 126, and is connected to the amplitude discriminator 125 to produce an output signal only when the temperature of the reference signal is below freezing temperature. Thus a signal can be applied to the ip-op circuit 124 only under such conditions.

A timer and control circuit 127 is provided for cyclic energization of the various components of the system. The circuit 127 includes a terminal 128 connected to a terminal 129 of the driver and amplier circuits 101 and a terminal 130 of the A.C. source 102, a pair of terminals 131 and 132 connected to terminals of the heating elements of plates 27 and 28, a terminal 133 connected to the other terminals of the heating elements, a pair of terminals 134 and 135 connected to the blower within the housing 30 and a terminal 136 connected to the flip-flop circuit 124. The circuit 127 also has a pair of terminals 137 and 138 for connection to a suitable power source.

In operation of the circuit 127, a power is applied to the terminals 134 and 135 to energize the blower. At this time, no power Aor signals are applied from the remaining terminals 128, 131-133 and 136, so that the tuning fork system, the heating elements, and the ip-op circuit 124 are inactive. Freezing precipitation may then be accumulated on the plates 27 and 28 for a certain time interval, preferably on the order of five minutes.

Control signals are then applied from terminals 128 and 135 to energize the tuning fork and measuring system, including the flip-flop circuit 124, and to produce an output signal from the flip-flop circuit 124 if the ice accumulation exceeds a value determined by the amplitude discriminator 117 and if the temperature of the reference surface is below freezing to produce an output from the amplitude discriminator 125. This operation may take place in a very short time interval.

The control signals from terminals 128 and 136 are then cut off, the supply of power to the blower is cut off, and power is applied to the terminals 131-133 to heat the collector plates for a certain time interval which may preferably be on the order of 2.5 minutes.

Power is then cut olf from terminals 131-133 to deenergize the heating elements, while power is again applied to the terminals 134, 135 to energize the blower for a certain time interval, preferably on the order of 2.5 minutes. The plates 27, 28 are then cooled.

The cycle is then restarted. With this operation, the output of the system is cyclically brought up to date, with the duration of each cycle being on the order of ten minutes. Certain features of the construction of the mass accumulation sensor system are important and should be mentioned. In the illustrated arrangement, the collector plates 27, 28 are mounted parallel to the plane of vibration of the tuning fork members. This is very important in minimizing the sensitivity to wind blasts. It has been found that with the collector plates mounted perpendicular to the plane of vibration, the oscillations are quite sensitive to wind gusts, and small gusts could damp out oscillation entirely. With the plates mounted parallel to the plane of vibration, however, virtually no effect can be discerned by wind blasts from any direction. The plates 27 and 28 should preferably be oriented in a plane parallel to the ground which allows ice to build up much more evenly than would be possible if the plates were mounted vertically or at some intermediate angle. The collector plates should have an area large enough to collect a measurable amount of trace precipitation but small enough to avoid adding appreciable weight to the tuning fork and to remain rigid. If the plates are too large, undesirable modes of vibration may be excited, particularly with uneven ice distribution. It has been found that an area of about three square inches produces a high degree of sensitivity without producing undesired vibrational modes or other problems. The plates should be as thin as possible to present very small areas in the plane of vibration and to avoid wind effects. A slight crown or peak in the center of the plates may be provided to aid in ice removal, but the slope should be very slight.

It may be noted even with the blower operative, snow may accumulate on the plates 27 and 28 to provide an output indication. However, thte indication of the existence of freezing rain or freezing drizzle does not depend solely upon the mass accumulation sensor system, since the output of the system is combined logically with information gathered by the other sensors. Accordingly, if there is mass accumulation below 32 F. but the other sensors indicate that no rain or drizzle is present, the system output will not indicate such types of precipitation.

IMPACT SENSOR FIGURES 6 and 7 show the construction of the impact sensor device 31, while FIGURE 6 also shows a block diagram of the impact sensor circuit 21.

The impact sensor system is highly important in that the various forms of precipitation can be categorized by sensing the kinetic energy that precipitation particle possesses upon impact. In general, the system comprises the device 31 which includes a generally hemispherical plate 32, on which precipitation particles may fall, mechanically coupled to an electro-mechanical transducer 14) which produces an electrical signal corresponding to the kinetic energy of the particle. The output of the transducer is applied to the circuit 21 which develops output signals at conductors 49, 50, 51 and 52. As indicated above, a signal is developed on conductor 49 in accordance with the amount of precipitation falling in a certain time interval, the signal being developed when the amount exceeds a certain value. This signal is used in indicating a heavy precipitation condition.

As also indicated above,.the circuit develops signals on conductors 50, 51 and 52 in accordance with the kinetic energy of the precipitation particles. The signal being developed on the conductor 50 when the energy is within a certain relatively low range, a signal is developed on conductor 51 when the energy is in an intermediate range, and a signal is developed on the conductor 52 when the energy is above a relatively high value.

The transducer 140 comprises output teminals 141 electrically connected to a crystal therewithin, the crystal being mechanically coupled to a shaft 142, to generate electrical signals at the output terminals 141 in response to angular movement of the shaft 142 about the axis thereof. The shaft 142 has a transverse opening therein to receive one end of a pin 143 which is locked in position by means of a thumb screw 144. The pin 143 forms a lever arm with the plate 32 being supported from the free ends thereof. The support comprises a vertical strut 145 secured at its lower end to the free end of the pin 143 and secured at its upper end to the center of the plate 32. Four additional struts 146 are secured at their lower ends to the free end of the pin 143 and extend angularly outwardly and upwardly to points spaced 90 on the plate 32, where they are secured thereto. This arrangement provides a lightweight and yet rigid connection between the plate 32 and the pin 143.

It will be appreciated that as precipitation particles impinge on the plate 32, the shaft 142 is rotated about its 1 1 axis to stress the crystal of the transducer 140 and gencrate an electrical signal at the terminals 141.

The magnitude of the output voltage is, of course, dependent upon the type of precipitation. Liquid drops produce smaller output voltages, whereas solid particles produce relatively high voltage outputs, the impact of a small solid producing an output voltage substantially larger than that produced by a large drop of water. To eliminate injury to the crystal from the impact of hailstones or the like, a mechanical stop is provided to limit movement of the plate 32. In the illustrated arrangement, the housing 33 has a cylindrical portion 147, the upper edge of which is in closely spaced relation to `the lower peripheral edge of the plate 32, to limit the movement thereof.

With regard to the response to liquid drops of various sizes, it is found that the electrical signal is in the form of a dampened sinusoidal burst having an amplitude approximately proportional to the kinetic energy of the impinging drop. lt has further been found that the kinetic energy of the drop is an exponential function of the mass of the drop alone, there being a distinct impact Velocity for each drop diameter. Thus the voltage output of the transducer depends only on the mass of the impinging drop, according to a function which is exponential in nature.

To obtain short duration, high voltage output bursts, the resonant frequency of the mechanical system should be as high as possible, to obtain a quick damp-out of the burst and at the same time to obtain a high Q, i.e. a high ratio of energy stored to energy dissipated in one cycle. Thus the system should have the lowest possible mass. However, the largest possible area should be provided to obtain a rate of impact high enough to permit measurement even when the rate of precipitation is low. It is found that even in a heavy rain, the rate of impact is relatively small, on the order of one impact per second per square inch.

With the arrangement as illustrated, a large area is obtained with a relatively small mass and at the same time, the strut arrangement provides a very rigid structure and eliminates undesired vibrational modes. The rigidity is further improved by the use of the plate 32 of generally hernispherical shape. This shape is also important in that it is comparatively insensitive to wind induced errors. If desired, a wind screen may be disposed around the sensing unit.

To prevent the build-up of ice from freezing rain and drizzle, an electrical heating element 148 is disposed within the cylindrical portion 147 of the housing 33, below the plate 32.

The impact sensor circuit 21 comprises a shaping and filter circuit 149 which includes a non-linear shaping network such as to produce an output signal Which is a linear function of the mass of the'impacting drop. As noted above, the voltage output of the transducer 140 depends only on the mass of the impinging drop but varies according to a function which is exponential in nature. The circuit 149 further includes filter means for attenuating high frequency components and particularly components in the frequency ranges generated by aircraft engines, both pistons and jets.

The output of the circuit 149 is fed through an amplitude discriminator 150 to a peak reading circuit 151. The amplitude discriminator passes only those signals above a certain background noise level. The peak reading circuit 151 responds to each pulse signal applied thereto (in response to the impact of a particle on the plate 32) to become charged and to develop at its output a voltage proportional to the amplitude of the pulse. The circuit 151 then holds its peak charge until another input pulse causes it to charge or discharge to a new level.

The peak readingrcircuit 151 is periodically reset by a reset timer 152 having an input connected to the output of a monostable vibrator 153 which is triggered by pulses from the output of the amplitude discriminator 150. In operation, pulse signals from the amplitude discriminator trigger the monostable multivibrator 153 to generate pulses used to trigger the reset timer 152. The reset timer remains in a triggered state so long as pulses are applied within short time intervals from the multivibrator 153. However, when no input pulse is received in a certain time interval, the reset timer 152 delivers an output pulse to the peak reading circuit 151, whereupon the output of the peak reading circuit 151 falls to zero.

|The output of the peak reading circuit 151 is applied to one input of a pulse width modulator 154, to control the width or duration of pulses generated thereby. The pulse width modulator 154 has a second input connected to the output of a multivibrator 153 for application of triggering signals thereto. Accordingly, each pulse signal developed at the output of the amplitude discriminator 1519 triggers the monostable multivibrator 153 which, in turn, triggers the pulse width modulator, which generates a pulse whose width is determined by the output of the peak reading circuit 151 which, in turn, is determined by the amplitude of the pulse signal. The output of the pulse width modulator 154 is applied to an averaging circuit 155 which develops an output proportional to the integrated or average value of the square wave pulses developed by the pulse width modulator 154. The output of the averaging circuit 155 is thus proportional to the rate of precipitation and, although not used in the system 1l), the signal therefrom may be usable directly or in other systems. For this purpose, an output terminal 156 is connected to the output of the averaging circuit 155.

The output of the averaging circuit 155 is also applied through an amplitude discriminator 157 to a squaring circuit 158 having its output connected to the conductor 49. When the precipitation rate reaches a predetermined relatively high value, the amplitude discriminator 157 applies a signal to the squaring circuit 158 to develop a control signal at the conductor 49, for use in the system 10 as above described.

The output of the peak reading circuit 151 is also applied through three amplitude discriminators 159, and 161 to squaring circuits 162, 163 and 164. Amplitude discriminator 159 is operative when the kinetic energy of the precipitation particles exceeds a certain low value, discriminator 160 is operative when the kinetic energy 0f the particles exceeds a certain medium value, and discriminator 161 is operative when the kinetic energy of the particles exceeds a certain relativelly high value. The output conductor 50 is connected to the output of an AND circuit 155 having a first input connected to the output of the squaring circuit 162, a second input connected through an inverter 166 to the output of the squaring circuit 163, and a third input connected through an inverter 167 to the output of the squaring circuit 164. With this arrangement, an output signal is developed on the conductor 50 when there is concurrently an output from the low amplitude discriminator 159, and no outputs from the medium and high discriminators 161) and 161.

The output conductor 51 is connected to the output of an AND circuit 163 having a tirst input connected to the output of the squaring circuit 163 and a second input connected through the inver-ter 167 to the output of the squaring circuit 164. Thus an output is developed on the conductor 51 when there is concurrently an output from the medium amplitude discriminator 160 and no output from the high amplitude discriminator 161.

Conductor 52 is connected directly to the output of the squaring circuit 164, to have arl output signal developed thereon when the high amplitude discriminator 161 is operative.

REBOUND SENSOR FIGURE 8 shows the construction of the rebound sensing apparatus 34 and also shows a block diagram of the 'rebound sensor circuit 22. As noted above, the apparatus 34 comprises inclined plates 35, 36, 37 and 38 on which precipitation falls and from which it may rebound against lower surface portions of an impact sensing structure 39, when the precipitation is of solid form, such as sleet or hail. The circuit 22 functions to produce an output signal on conductor 53 when there is rebound, and to produce no output signal when there is no rebound.

The impact sensing structure 39 comprises a sensing member 170 against which the rebounding particles may impinge. Preferably, the member 170 has four inclined surfaces generally parallel to the surfaces of the plates 35-38, so as to provide an inverted pyramidal shape. To develop an electrical signal in response to movement of the member 170, an electro-mechanical transducer is provided in the form of four piezoelectric crystals 171, the lower faces of the crystals 171 being secured to the upper side of the member 170. The upper faces of the crystals 171 are secured to a mounting plate 172 which, in turn, is secured within a support block 173. A cushion 174 is provided between the lower face of the block 173 adjacent its peripheral edge and the upper face of the member 170.

It will be appreciated that when a solid precipitation particle strikes one of the plates 35-38 with sufficient velocity to rebound against the lower surface of the member 170, the crystals 171 will be compressed to generate an electrical voltage between the upper and lower faces thereof. To transmit the electrical signal to the circuit 22, the lower faces are electrically connected to the center conductor of a coaxial or shielded transmission line 17S while the upper faces of the crystals 171 are electrically connected to the block 173 which, in turn, is connected electrically to the outer or shield conductor of the line 175.

The support block 173 is resiliently suspended from a plate 176 by means of stud bolts 177, nuts 178 and resilient washers 179, disposed between the nut 178 and the upper surface of the plate 176, and also disposed between the lower surface of the plate 176 and the upper surface of the block 173.

The plate 176 is secured at its opposite ends to the lower surfaces of the central portions of a pair of horizontal support bars 180, supported by means of uprights 181 from a base plate 182. The structure 39 may further include a top cover plate 183 and four shield plates 184 disposed adjacent the four sides of the block 173, preferably with the lower edges of the shield plates 184 being disposed slightly below the peripheral edges of the member 170. The rebound plates 35-38 are supported from the baseplate 182 through upright walls 185 and spacer members 186 between the upper inside edge portions of the walls 185 and upper edge portions of the plates 35- 38. The adjoining edges of the plates 35-38 are preferably secured together as by welding.

To prevent formation of ice on the plates 35-38, electrical heaters 187 are insalled on the lower surfaces thereof. A pair of electrical heaters 188 are preferably installed in the block 173, to prevent formation of ice on the member 17 0 and associated structure. The heaters 187 and 188 may be connected to a suitable source of electricity, preferably through thermostat means operative when the temperature drops below a certain value.

The circuit 22 comprises a peak reading circuit 189 having an input connected through an amplitude discriminator 190 and a noise filter 191 to the output of the crystal array 171, and having an output connected through an amplitude discriminator 192 and a squaring circuit 193 to the output conductor 53 when the peak value of an input signal exceeds a certain value, as determined by the amplitude discriminator 192. The amplitude discriminator 190, in the input circuit of the peak reading circuit 189, functions to eliminate background noise signals, while the noise iilter 191 may attenuate high frequency signals, such as may be caused by jet or piston-type aircraft.

The peak reading circuit 151 responds to each pulse signal applied thereto to become charged and to develop at its output a voltage proportional to the amplitude of the pulse. The circuit 189 then holds its peak charge until another input pulse causes it to charge or discharge to a new level. To prevent the circuit from maintaining an output signal when rebounds are no longer detected, the circuit 189 is periodically reset by a reset timer 194 having an input connected through a monostable multivibrator 195 to the output of the sensing apparatus 34. In operation, pulse signals from the sensing apparatus trigger the monostable multivirbator 195 to generate pulses which trigger the reset timer 194. The reset timer 194 remains in a triggered state so long as pulses are applied within short time intervals from the multivibrator 195. However, when no input pulse is received in a certain time interval (which may be on the order of 10 seconds), the reset timer 194 delivers an output pulse to the peak reading circuit 189, whereupon the output of the peak reading circuit 189 falls to Zero.

PHOTO-OPTICAL SENSOR FGURE 9 shows the construction of the photo-optical sensing apparatus 40 and also shows a block diagram of the photo-optical sensor circuit 23. As described above, the precipitation falls through a central portion 41 of the apparatus 40, between portions 42 and 43 thereof. Portion 42 houses a light source 197 which projects light through the portion 41 toward the portion 43, and also houses a photoelectric detector, generally designated by reference numeral 198, which develops electrical signals Vin response to the reflection of light from particles of precipitation falling through the portion 41. The output of the detector 198 is applied to the sensor circuit 23 which develops output signals on the conductors 56, 57 and 58, a signal being developed on conductor 56 when the reflection of light is in a relatively low range, a signal being developed on conductor 57 when the reilection of light is in a certain intermediate range, and a signal being developed on conductor 58 when the reflection of light exceeds a relatively high value.

The photoelectric detector 198 comprises a parabolic mirror mounted within a housing 200, and a suitable photoelectric cell 201, preferably a photo-diode, supported at the focal point of the parabolic mirror by means of a pair of supports 202 extending from the housing 200. The supports 202 may be hollow to carry connection wires which extend from the housing 200 to the circuit 23 through a suitable cable 203.

The photoelectric detector 198 and the light source 197 are supported within a housing which includes a bottom wall 204, a top wall 205, a pair of side walls 206 and an end wall 207. A wall 208 extends downwardly from the forward edge of the top wall 205 part way toward the bottom wall 204, while a plate 209, having a window 210 therein, extends from the lower inside edge of the plate 288 downwardly and inwardly, and then downwardly and outwardly to the forward edge of the bottom wall 284. A plate 211 is disposed between the side walls 206 over the photoelectric detector 198, and another plate 212 is disposed between the side walls 206, between the light source 197 and the photoelectric detector 198. The surfaces of the plates 209, 211 and 212, especially those which face the photoelectric detector are preferably dull, light-absorbant surfaces, to minimize transmission of undesired light to the photoelectric detector 198.

The portion 43 of the apparatus 40 functions to provide a dark background, to obtain maximum contrast. Portion 43 comprises a top wall 213, a bottom wall 214, an end wall 216 and an end wall 217 having an opening 218 therein.

To provide the dark background, a plate 219 having a dull surface is mounted within the portion 43 in alignment with the axis of the photoelectric detector 198 and at an angle such that any light reflected therefrom toward the photoelectric detector 198 comes from the direction of another plate 220. Plate 220 also has a dull surface and is located at an angle such that only light coming from directly above could be reflected to the plate 219 and thence to the photoelectric detector 198. The lower surface of the top wall 213, as well as the other internal surfaces of the housing should, of course, have dull surfaces. With this arrangement, the reflection of light to the photoelectric detector 198 is minimized.

It has been found that reection of light from particles of precipitation falling through the portion 41 of the apparatus is increased by providing a mirror 221 disposed above the plate 219 and in alignment with the light source 197, at such an angle as to cause reflection of light from the upper surfaces of precipitation particles to the photoelectric detector 198.

It is important that condensation and freezing of moisture on the various surfaces be minimized, particularly on the plate 219, since such condensation and freezing can greatly increase the coetcient of reflectivity. For this reason, an electrical heater 219a is preferably installed on the back side of the plate 219.

The circuit 23 comprises an amplifier 222 having an input connected to the output of the photoelectric detector 198 and having an output connected to three amplitude discriminators 223, 224 and 225. Each of the amplitude discriminators 223-225 is in the form of a switching circuit whose output remains in one state until the input level reaches some preset value at which time the output switches abruptly to a different state. The output then remains in the second state until the input returns to a value just below the preset level, and the output then switches back to the first state. Amplitude discriminator 223 triggers at a relatively low input pulse amplitude, discriminator 224 triggers at an intermediate input pulse amplitude and discriminator 225 triggers only at a relatively high input pulse amplitude.

The outputs of the amplitude discriminators 223, 224 and 225 are respectively applied to flip- flop circuits 226, 227 and 228 and are also applied to first inputs of three OR circuits 229, 23) and 231 having second inputs connected to the outputs of the ilip- flop circuits 226, 227 and 228. The output of the OR circuit 229 is connected to one input of an AND circuit 232 having its output connected to the conductor 56, having a second input connected through an inverter 233 to the output of the OR circuit 230 and having a third input connected to the output of the OR circuit 231 through an inverter 234. The output of the OR circuit 230 is connected to one input of an AND circuit 235 having its output connected to the conductor 57 and having a second input connected through the inverter 234 to the output of the OR circuit 231. The output of the OR circuit 231 is directly applied to the conductor 58.

The Hip- flop circuits 226, 227 and 228 are arranged to be reset at certain times by pulses from reset timers 236, 237 and 238 having inputs connected to the outputs of the amplitude discriminators 223, 224 and 225. Each of the reset timers 236438 is triggered in response to a pulse received from the respective one of the amplitude discriminators 223-225 and remains in a triggered state so long as pulses are applied within short time intervals. However, when no pulse is applied within a certain time interval, each reset timer delivers an output pulse to the respective ip-ilop circuit, to reset the flip-flop circuit.

In operation, if only the low amplitude discriminator 223 is triggered, a signal is applied to the OR circuit 229 and also a set signal is applied to the ip-op circuit 226, to apply a second signal to the OR circuit 229. The signal from the output of the OR circuit 229 is applied to one input of the AND circuit 232 and if no signals are developed from the OR circuits 230 and 231, the AND circuit 232 will be energized through the inverters 233 and 234, developing an output signal on the conductor 56.

When the pulse signal is developed at the output of 16 the low amplitude discriminator 223, the reset timer 236 is triggered, and it will remain triggered if additional pulses are received within a certain predetermined time interval. However, if no input pulses are applied within the predetermined time interval, the reset timer applies an output pulse to the hip-flop circuit 226, to operate the flip-flop circuit 226 to its reset condition. The OR circuit 229 will no longer function to apply a signal to the AND Y circuit 232, and the signal output on the conductor 56 will disappear.

The hip-flop circuits 227 and 228 are operated in a similar way through signals from the medium and high amplitude discriminators 224 and 225, and by signals from the reset timers 237 and 238. If the amplitude discriminator 224 delivers a pulse, the flip-flop 227 will be triggered to its set condition, to apply a signal through the OR circuit 238 and the AND circuit 235 to the conductor 57, assuming that the ip-tlop 228, associated with the high amplitude discriminator 226, is in its reset condition. At the same time, no signal is applied to the middle input of the AND circuit 232, by operation of the inverter 233, and thus there will be no signal on the conductor 56 and only the conductor 57 will be energized.

If the high amplitude discriminator 225 delivers an output pulse, the conductor 58 is energized directly from the hip-flop 228. Through the inverters 233 and 234 operative on the AND circuits 232 and 235, there will be no output signal on the conductors 56 and 57. Hence only the conductor 58 will have an output signal thereon.

AND CIRCUIT The AND circuits 66-75, 77, 78, 123, 265, 168, 232 and 235 referred to above may be of any desired construction, a preferred construction being illustrated in FIGURE l0. Referring thereto, an AND circuit is illustrated having four input terminals 241, 242, 243 and 244 in addition to a common grounded terminal 245, and having a pair of output terminals 246 and 247, terminal 247 being grounded.

The circuit is arranged to produce a positive output signal at the terminal 246 when positive input signals are concurrently applied to all of the input terminals 241-244. For this purpose, input terminals 241-244 are respectively connected to a circuit point 248 through diodes 249-252, circuit point 248 being connected through a resistor 253 to a terminal 254 arranged for connection to the positive terminal of a D.C. source having its negative terminal connected to ground, terminal 254 being preferably at plus 25 volts relative to ground. With this arrangement, the circuit point 248 is at a potential determined by the least positive one of the input terminals 241-244. If any one of the input terminals 241-244 is placed at ground potential the circuit point 248 will be at approximately ground potential. Thus a positive potential is developed at circuit point 248 only if all of the terminals 241-244 are concurrently at positive potentials. Circuit point 248 is connected through a diode 255 to the output terminal 246 which is connected through a resistor 256 to a terminal 257 arranged for connection to the negative terminal of a D.C. power supply, terminal 257 being preferably at minus 25 volts relative to ground. The diode 255 normally conducts to place terminal 246 at a potential approximately equal to that of the circuit point 248. However, diode 255 serves as an isolation device, to permit the output terminal 246 to be at a positive potential, through its interconnection with other circuits, even when one of the input terminals 241-244 is at ground potential. It will be appreciated, of course, that the AND circuit of FIGURE 10 may have any desired number of inputs.

OR CIRCUIT The OR circuits 76, 79 and 229-231 may have any desired construction, a preferred circuit being illustrated in FIGURE ll. Referring thereto, an OR circuit is illustrated having three input terminals 258, 259 and 260, in addition to a common grounded input terminal 261, and having output terminals 262 and 263, terminal 263 being grounded. The circuit is arranged to produce a positive output signal at terminal 262 when a positive signal is applied to any one or more of the input terminals 258-260. For this purpose, input terminals 258- 260 are connected through diodes 264-266 to a circuit point 267 which is connected through a resistor 268 to a terminal 269 arranged for connection to the negative terminal of a D.C. power supply having its positive terminal connected to ground, terminal 269 being preferably at minus 25 volts relative to ground.

In operation, when a positive voltage is applied to any of the input terminals 258-260, the corresponding one of the diodes 264-266 will conduct to place circuit point 267 and the output terminal 262 at a positive potential closely approaching that of the applied input signal. However, if all of the input terminals 258-260 are at ground potentials, the output terminal 262 will likewise be at ground potential.

INVERTER AMPLIFIER CIRCUIT The circuits 48, 55, 61, 64, 166, 167, 233 and 234 as above described, may have any desired construction, a preferred inverter circuit being illustrated in FIGURE l2. Referring thereto, the circuit has a pair of input terminals 270 and 271 and a pair of output terminals 272 and 273, terminals 271 and 273 being grounded. The circuit is arranged to produce a positive output signal at output terminal 272 when the input terminal 270 is at ground potential, and to produce substantially no output signal at the output terminal 272 when the input terminal 270 is at a positive potential.

In particular, terminal 270 is connected through the parallel combination of a resistor 274 and a capacitor 275 to the base of a transistor 276, the base being connected through a biasing resistor 277 to a terminal 278 arranged for connection to a negative terminal of a D.C. power supply having a positive terminal connected to ground. The emitter of the transistor 276 is connected to ground while the collector thereof is connected to the output terminal 272 and also to a resistor 279 to a terminal 280, arranged for connection to the positive terminal of a D.C. power supply having its negative terminal connected to ground.

In operation, if the input terminal 270 is at ground potential, there is substantially no conduction through the transistor 276, and the output terminal 272 is at a positive potential approaching that of the terminal 280. When a positive potential is applied to the terminal 270, the transistor 276 is rendered conductive, and the output terminal 272 is placed at a low potential, approaching ground potential. Resistor 274 serves fto limit the baseemitter current of the transistor 276, while the capacitor 275 improves the speed of response of the circuit.

F LIP-FLOP CIRCUIT The ip-op circuits 109 and 124 of FIGURE 2 and the flip-flop circuits 226-228 of FIGURE 9 may likewise have constructions such as are well known in the art, a preferred flip-flop circuit being illustrated in FIG- URE 13.

Referring to FIGURE 13, a pair of output terminals 281 and 282 are connected to the collectors of a pair of transistors 283 and 284 which are so connected that one is conductive while the other is cut H. The emitters of the transistors are connected to ground through resistors 285 and 286 in parallel with capacitors 287 and 288. The collectors are connected through resistors 289 and 290 to a terminal 291 arranged for connection to the positive terminal of a D.C. power supply. The base electrodes of the transistors 283 and 284 are connected to a terminal 292, arranged for connection to the negative terminal of a power supply whose positive terminal is con- 18 nected to ground, through biasing resistors 293 and 294.

To render one transistor conductive while the other is cut olf, cross-connections are provided between the base and collector electrodes thereof. In particular, the parallel combination of a resistor 295 and a capacitor 296 is connected between the collector of transistor 283 and the base of transistor 284 while the parallel combination of a resistor 297 and a capacitor 298 is connected between the collector of the transistor 284 and the base of the transistor 283.

With such cross-connections and with the bias obtained through connection of the base electrodes to the negative terminal 292 through the resistors 293 and 294, one transistor will be cut off while the other is conductive and the circuit will remain stably in such a condition, until a switching signal is applied to one of the conductors, whereupon the reverse action takes place.

The switching signals may be applied in various ways. In particular, an A.C. switching signal may be applied to one or the other of a pair of input terminals 299 and 300. Terminal 299 is connected through a capacitor 301 to a circuit point 302 which is connected to ground through a resistor 303, circuit point 302 being connected to the base electrode of the transistor 283 through a diode 304. Similarly, terminal 300 is connected through a capacitor 305 to a circuit point 306 connected to ground through a resistor 307 and connected to the base electrode of transistor 284 through a diode 308.

If it is assumed that the transistor 283 is cut off while the transistor 284 is conducting, a positive-going signal may be applied to the terminal 299 to develop a positive pulse at the circuit point 302 which is applied through the diode 304 to the base of the transistor 283. Transistor 283 then starts to conduct whereupon the potential of its collector moves in a negative direction, causing the potential of the base of the transistor 284 to move in a negative direction to decrease conduction through the transistor 284. As conductance through the transistor 284 is decreased, the potential of its collector moves in a positive direction thus applying a positive signal to the base of the transistor 283 through the capacitor 298. As a result, the circuit is rapidly switched to` a condition in which the transistor 283 conducts heavily while the transistor 284 is cut off. A positive-going signal may then be applied to the input terminal 300 whereupon the circuit will be switched back to its initial condition.

Control signals may also be applied to either of a pair of terminals 309 and 310, connected through diodes 311 and 312 to the base electrodes of the transistors 283 and 284. When a negative potential is applied through the terminal 309, the transistor 283 can be cut oi, or can be kept from becoming conductive. Similarly, the negative potential may be applied to the terminal 310 to cut off the transistor 284 or to prevent it from becoming conductive.

Although not so used in the circuits thus far described, the flip-flop circuit of FIGURE 13 may also be used as a binary counter device. For this purpose, pulse signals may be applied to an input terminal 313 connected through capacitors 314 and 315 to circuit points 316 and 317 which are connected to ground through resistors 318 and 319 and which are connected to the base electrodes of the transistors 283 and 284 through diodes 320 and 321. When a positive pulse is applied to the terminal 313 one or the other of the transistors 283 or 284 will be rendered conductive while the other will become cut oi. The next positive pulse applied to the terminal 313 will cause the circuit to reverse to its previous condition.

When the hip-flop of FIGURE 13 is used as the circuit 109 of FIGURE 2, the output of the squaring circuit 103 may be connected to the terminal 299 while the output of the squaring circuit 104 may be connected to the terminal 300. The output terminal 282 may then be connected to the averaging circuit 110. In response to the leading edges of the signals from the squaring` circuit 103,

i.e. the leading edges of the signal 105 shown in FIGURE 4v or the signal 107 shown in FIGURE 5, the transistor 283 may be shifted from a non-conductive state to a conductive state while the transistor 284 is shifted from a conductive state to a non-conductive state. A positive signal is then developed at the output terminal 282. It is noted that the capacitor 301 together with the resistor 303 should have a comparatively short time constant, to provide a differentiating action.

Similarly, when the leading edges of the signals from the squaring circuit 104, i.e. Vthe leading edges of the signal 106 of FIGURE 4 o1' the signal 108 of FIGURE 5, are applied to the terminal 300, the transistor 284 is shifted from anon-conductive state to a conductive state while the transistor 283 is shifted from a conductive'state to a non-conductive state. When transistor 284 is conductive, the potential of the output' terminal 282 is reduced to a value approaching ground potential. Accordingly, waveforms are developed such as indicated by the reference numerals 111 and 113 in FIGURES 4 and 5.

With the flip-flop circuit of FIGURE 13 usedv as the circuit 124 of FIGURE 2, the output of the AND circuit 123 may be connected to the terminal 299, while the terminal 136 of the timer and control circuit 127 may be connected to the terminal 309, terminal 282 being connected to the output conductor 46. In'operation, a negative signal may be applied from the terminal 136 to the terminal 309 to cause they transistor 283 to be non-conductive and toprevent conduction thereof. At certain times, as described above, a signal may be applied from the terminal 136 to render the flip-flop circuit 124 operative. This signal may be a positive signal, applied to the terminal 309. If, then, a positive signal is applied from the AND circuit 123 to the terminal 299, the transistor 283 is shifted from a non-conductive state to a conductive state while the transistor 284 is shifted from a'conductive state to a non-conductive state thereby developing a'positive voltage at the output terminal 282, connected to the conductor 46.

With the ip-op circuit4 of FIGURE 13 used for the circuits 226, 227 and 228 of FIGURE 9, the outputs of the amplitude discriminators 223-225 may be connected to the terminal 299. The terminal 300 may then be connected to the output of the associated one of the reset timers 236-238. The output may be connected then to the output terminal 282. Under such'circumstances, when a positive pulse is applied from the amplitude discriminator to the terminal 299, the transistor 283 is shifted from a non-conductive state to a conductive state while the transistor 284 is shifted from a conductive state to a non-conductive state, thereby developing a positive signal at the output terminal 282. When a pulse is applied from the reset timer to the terminal'300, the circuit is then shifted back to its initial condition with the transistor 284 conductive and with the transistor 283 cut olf.

MULTIVIBRATOR CIRCUIT conductive state to a conductive state while'the transistor 324 is shifed from a conductive state to a non-conductive state. A positive signal is then developedat the collector of the transistor 324, which is connected to an output terminal 327. The positive signal is also applied through' a resistor 328 to the emitter ofthe unijunction transistor 325, a capacitor 329 being connected between the emitter of the transistor S-and ground. When the positive sig- Whena positive pulse is applied to an inputV terminal 326, the transistor 323 is shifted from a non-V nal isapplied from the collector of the transistor 324, the capacitor 329 starts -to charge up and after a certain time interval, the voltage across the capacitor 329 exceeds the breakdown voltage between the emitter of the unijunction transistor 325 and the first base 330 thereof, which is connected to a resistor 331 to ground. A positive pulse is then developed across the resistor 331 which is applied through'a capacitor 332 to a circuit point 333 connected through a diode 334 to the base of the transistor 324, circuit point 333 being connected through a resistor 335 to ground. Thus a positive pulse is applied to the basevof the transistor 324. This causes the transistor 324 to shift from a non-conductive state to a conductive state, while the transistor 323 is shifted from a conductive state to a non-conductive state. Thus the circuit is in its initial condition. The output voltage, at terminal 327, drops to a low value.

Accordingly, a positive pulse is developed at the output terminal 327 of a certain duration, determined primarily by the time constant of the circuit including resistor 328 and capacitor 329.

As is conventional, the second base of the unijunction transistor 325 is connected through a resistor 336 to a terminal 337 arranged for connection to the positive terminal of a D.C. power supply having its negative terminal connected to ground.

It will be recognized that the transistors 323 and 324 are connected in a manner similar to the connection of transistors 283 and 284 in the flip-flop circuit of FIGURE 13. The collectors are connected through resistors 339 and 340 to the power supply terminal 337. The emitters are connected to ground through resistors 341 and 342 and capacitors 343 and 344. The collector of transistor 323 is connected to the base of the transistor 324 through the parallel combination of a capacitor 345 and a resistor 346, while the collector of the transistor 324 is connected to the base of the transistor 323 through the parallel combination of a capacitor 347 and a resistor 348. The bases of the two transistors are connected to resistors 349 and 350 to a terminal 351 arranged for connection to the negative terminal of a D.C. power supply having its positive terminal connected to ground.

To apply the switching pulse from the input terminal 326, it is connected through a capacitor 352 to a circuit point 353 connected to ground through a resistor 354 and connected to the base of the transistor 323 to a diode 355. Preferably, the capacitor 352 together with the resistor 354 should have a comparatively low time constant, to provide a differentiating action.

In the event that a negative output pulse is desired, rather than a positive output pulse, an ouput terminal 356 may be used, connected to the collector of the transistor 323.

RESET TIMER CIRCUIT The reset timer Vcircuits 152, 194 and 2364238, discussed above, may preferably have a construction such as illustrated in FIGURE 15. Referring thereto, the circuit is arranged to be triggered by a pulse applied to an tinput terminal 368 and to remain triggered so long as additional pulses are applied, with the time intervals between pulses not exceeding a certain time interval. However, when no input pulse is applied to the terminal 360 within a fcertain time interval, the circuit delivers an output pulse at an output terminal 361.

The input terminal 360 is connected to the input of a pulse ampliier 362 having output terminals 363 and 364. When the pulse is applied to the input of the pulse amplifier 362, an output pulse is immediately developed at terminal 363 while a pulse is developed at the terminal 364 after a certain delay time.

Terminals 363 and 364 are connected to input terminals of AND circuits 365 and 366 having outputs connected to monostable multivibrators 367 and 368, which may be constructed in the manner as illustrated in FIG- URE 14, and as described above. The outputs of the multivibrators 367 and 368 are connected to inputs of the AND circuits 365 and 366 and are also connected through pulse amplifiers 369 and 378 to inputs of another pair of AND circuits 371 and 372. In addition, the output of the multivibrator 367 is connected to another input of the AND circuit 372, while the output of the multivibrator circuit 368 to another input of the AND circuit 371 and also through an inverter 373 to a third input of the AND circuit 365. The outputs of the AND circuits 371 and 372 are connected to inputs of an OR circuit 374, having an output connected to the output terminal 361.

In operation, before any input pulse is applied to the input terminal 360, the outputs of both multivibrators 367 and 368 are postive. Assuming that the multivibrator circuit of FIGURE 14 is used, the output may be taken at the output terminal 356, the transistor 323 being normally non-conductive.

When a pulse is applied to the pulse amplifier 362, a pulse is immediately developed at the output terminal 363 and is applied to the AND circuit 365. However, no pulse is transmitted to the multivibrator 367, since the AND circuit 365 is inhibited by a negative signal applied from the inverter 373. After a certain delay time, a pulse is applied from the terminal 364 to the AND circuit 366 which applies the pulse t the input of the multivibrator 368. When the multivibrator 368 is triggered, an inhibiting signal is applied to the AND circuit 366 and through the inverter 373, an enabling signal is applied to the AND circuit 365.

If it is assumed that no further input pulses are applied to the terminal 360, the monostable multivibrator 368 will, after a certain time interval, switch back to its initial condition. It then applies a pulse to the pulse amplifier 37) which applies a pulse through the AND circuit 372 and the OR circuit 374 through the output terminal 361. The AND circuit 372 is enabled,` since the multivibrator 367 is in its initial condition with a positive output therefrom. Accordingly, a positive output pulse is developed at the terminal 361 if no further input pulse is applied within the time interval of operation of the multivibrator 368.

If, however, a second input pulse is applied before the multivibrator 368 times out, it will develop an output at the terminal 363 of the pulse amplifier 362, which is applied through the AND circuit 365 to the multivibrator 367 to trigger the same. The multivibrator 368 may subsequently time out to apply a pulse to the pulse amplifier 370. However, the pulse will not be applied through the AND circuit 372, since the AND circuit 372 will be inhibited through a signal applied from the multivibrator 367. If no further input pulse is applied, the multivibrator 367 will time out after a certain time interval and will deliver a pulse to the pulse amplifier 369 and through the AND circuit 371 and the OR circuit 374 to the output terminal 361. The AND circuit 371 will be enabled, since a positive signal is at this time applied from the output of the multivibrator 368.

However, if it is assumed that a third pulse is applied before the multivibrator 367 times out, it will develop a delayed pulse at the terminal 364 which will trigger the multivibrator 368. Then when the multivibrator 367 times out, no pulse will be applied to the output terminal 361, since the AND circuit 371 will be disabled through the signal applied from the triggered multivibrator 368. If no further pulse is applied, the multivibrator 368 may time out to deliver a pulse to the output terminal 361 in the manner as above described. However, if another pulse is received, the multivibrator 367 may be triggered.

Accordingly, one or the other of the multivibrators 367 and 368 will be triggered so long as input pulses are received with a time delay therebetween not exceeding a certain time interval, and no output pulse will be developed at the terminal 361. However, when no input pulse is received in a certain time interval, an output pulse will be developed at the terminal 361.

2.2 AMPLITUDE DISCRIMINATOR-SQUARING CIRCUIT The circuit of FIGURE 16 may preferably be used for the various amplitude discriminator and squaring circuits referred to above. This circuit comprises a pair of transistors 377 and 378 having emitters connected together and through a common resistor 379 to ground and having collectors connected to a positive power supply terminal 388 through resistors 381 and 382.

The base of the transistor 378 is connected to ground through a resistor 383 and is connected through the parallel combination of a resistor 384 and a capacitor 385 to the collector of the transistor 377, the potential of the base of the transistor 378 being thereby controlled by the potential of the collector of the transistor 377. The collector of the transistor 378 is connected to an output terminal 386, while the base of the transistor 377 is connected through a resistor 387 to an input terminal 388, input terminal 388 being connected through a biasing resistor 389 to a terminal 390 arranged for connection to a suitable source of biasing potential.

In operation, it may be assumed that the potential of the bias terminal 398 is suiiciently negative to prevent conduction through the transistor 377. The collector thereof will then be at a highly positive potential and through the resistor 384, the base of the transistor 378 will be biased positively and there will be a high current flow through the transistor 378. The output terminal 386 will then be at a low potential, approaching ground potential.

When the input signal applied to the terminal 388 has a potential exceeding a certain value, the circuit will be rapidly switched to a condition wherein the transistor 377 will conduct heavily while the transistor 378 will be cut oit, thus producing a highly positive potential at the output terminal 386. Two features of the circuit contribute to the rapid switching operation. First, as the transistor 377 starts to conduct, the current ow through the resistor 379 will be increased, to decrease the base-to-emitter voltage of the transistor 378. Secondly, as the transistor 377 starts to conduct, the potential of the base of the transistor 378 will be dropped through the capacitor 385. In any event, as a result of a comparatively slight elevation in the potential of the input terminal 388, the circuit is shifted from a condition in which the transistor 378 conducts heavily to a condtion in which it is cut olf, to produce a highly positive output voltage.

lVhen the input voltage is then dropped below a certain value, the reverse action takes place and the circuit will be rapidly switched to a condition in which the transistor 378 again conducts heavily while the transistor 377 will be cut olf. The connection of the emitters to ground through a common resistor 379 contributes to the rapid switching operation.

It will thus be appreciated that the circuit of FIGURE 16 provides a squaring action and is also usable as an amplitude discriminator. The potential of the terminal 398 may be adjusted by any suitable means, to control the point at which the switching action takes place.

PULSE AMPLIFIER FIGURE 17 illustrates a preferred circuit for a pulse amplifier which may be used as the amplifier 362 and amplifiers 369 and 370 in the circuit of the reset timer, as illustrated in FIGURE l5.

The pulse amplifier circuit of FIGURE 17 comprises a transistor 391 having an emitter connected to ground through the parallel combination of a resistor 392 and a capacitor 393 and having a collector connected through a primary winding 394 to a positive power supply terminal 396. The base of the transistor 391 is connected through a resistor 397 and a diode 398 to a circuit point 399 which is connected to ground through a resistor 400.

The hase is also connected through a biasing resistor 401 to a negative power supply terminal 402.

As a result, the transistor 391 is normally nonconductive or conducts a comparatively small amount of current. Conduction thereof may be increased, however, by application of a pulse to the base through a capacitor 403 connected to a pulse input terminal 404. When a positive pulse is applied to the terminal 404, there is a build-up of current iiow through the inductive impedance formed by the transformer primary 394. A voltage is then developed across a secondary winding 405 of the transformer 395, which is applied through a capacitor 406 to the circuit point 399, and thence through resistor 397 and diode 398 to the base of the transistor 391. It should be noted that the lower terminal of the winding 405 is grounded.

As a result of the application of this feedback voltage, the build-up of current through the transistor 391 is increased, thus further increasing the voltage developed across the secondary winding 405. As a result, a comparatively high voltage is rapidly developed across the winding 405. This voltage is applied through a diode 407 in parallel with a resistor 408 to the base of a transistor 409 having its emitter connected to an output terminal 410 and also connected through a resistor 411 to the positive power supply terminal 396. The base of the transistor 409 is additionally connected through a biasing resistor 412 to the negative power supply terminal 402. The collector of the transistor 409 is grounded.

The transistor 409 operates as an emitter-follower. Normally, it conducts heavily and the potential of the output terminal 410 is approximately at ground potential. When the positive pulse is applied from the transformer secondary winding 405 through diode 407 and resistor 403 to the base of the transistor 409, its conduction is decreased to zero or a low value, producing a large positive output voltage at the output terminal 410.

To produce a delayed output' signal, a second winding 413Y is provided on the transformer 395. The upper terminal of the winding 413 is connected to ground while the lower terminal thereof is connected through a diode 414 to another output terminal 415 which is connected to ground through a resistor 416.

In operation, the current through the transformer primary 394 builds up very rapidly in response to application of an input pulse, in a manner as above described, and the 1ield in the core of the transformer increases correspondingly. After the current and eld reach peak values, they then decrease rather rapidly producing a voltage of the opposite polarity in the winding 405 and also in the winding 413. This opposite polarity voltage in the winding 413 is applied through the diode 414 to the output terminal 415. Accordingly, a slightly delayed output pulse is obtained.

It will be appreciated that when the circuit of FIGURE 17 is used for the pulse amplifier 362 of FIGURE l5, the input terminal 360 is connected to the input terminal 404, the output terminal 363 corresponds to the output terminal 410, and the output terminal 364 corresponds to the output terminal 415.

In the reset timer circuit of FIGURE as above described, the pulse amplifiers 309 and 370 respond to a sudden change in the outputs of the multivibrators 367 and 368, from a low value to a highly positive value, occurring when the multivibrators time out. A somewhat different input circuit is desirable for the pulse amplifiers 369 and 370. In particular, an input terminal 417 may be provided in the circuit of FIGURE 17 connected through a diode 418 and a resistor 419 to the pulse input terminal 404 which is connected through the capacitor 403 to the base of the transistor 391.

It is believed that the construction of the remaining circuits is well known in the art and, accordingly, they are not illustrated and described in detail. The peak reading circuit 151 of FIGURE 6 may comprise a capacitor connected to the output of a transmission gate circuit, various forms of which are well known in the art, in which the output signal corresponds to the input signal during a selected time interval which is controlled by a gating signal, and in which the output is zero and the output impedance is high except during application of the gating signal. The pulse whose peak amplitude is to he registered and may be applied to the input of the transmission gate circuit and rnay also act as a gating signal so that the capacitor is charged or discharged during the applied pulse, to a value proportional to the peak amplitude of the pulse.

The pulse width modulator 154 may comprise a monostable multivibrator triggered by signals from the multivibrator 1753 with the signal from the peak reading circuit 151 being applied to the timing circuit of the monostable multivibrator to control the duration of the pulses generated thereby. For example, the multivibrator circuit of FIGURE 14 may be used with the input terminal 326 being connected to the output of the multivibrator 153. The voltage output of the peak reading circuit 151 may then be applied in the timing circuit, for example between ground and the lower terminal of the capacitor 329.

It will be apparent to those skilled in the art that various changes in the system and the components thereof may be made. For example, the system might include an additional output circuit or circuits, to indicate extremely light rain, or rain in a range lying between drizzle and normal rain; In this respect, any number of output circuits could be added, provided of course that additional amplitude discriminators are provided in the impact sensor circuit. And, if desired, a continuous signal, proportional to the impact energy, may be used. Similarly, additional outputs may be taken from the mass accumulation sensor circuit, the rebound sensor circuitY and the photo-optical circuit, provided additional amplitude discriminators are used.

It is also possible to simplify the system, particularly where it is not necessary to provide output indications of all types of precipitation, or where a high degree of accuracy is not required. For example, the mass accumulation sensor arrangement need not be used where an indication of freezing drizzle or freezing rain is not required.

It will be understood that other modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

We claim as our invention:

l. In a precipitation sensing system, impact sensing means including a surface exposed to precipitation and electro-mechanical transducer means for developing control signals in response to impacts of particles on said surface, rebound sensing means including a surface exposed to precipitation and an electro-mechanical transducer device in spaced relation to said surface for developing control signals in response to rebounds of particles from said surface, photo-optical means including a light source for illuminating a certain region through which precipitation falls and photocell means for deriving control signals in response to light impulses arising from reflection of light from particles falling through said region, mass accumulation sensing means including a vibration device having a surface exposed to precipitation and means for developing control signals in response to a lowered natural resonant frequency of vibration of said device caused by accumulation of freezing precipitation on said surface, a plurality of output circuits corresponding to various forms of precipitation, and logic circuitry responsive to said control signals for selectively energizing said output circuits.

2. In a precipitation sensing system, mass accumulation sensing means including a vibration device having a surface exposed to precipitation and means for developing control signals in response to a lowered natural

Claims (1)

1. IN A PRECIPITATION SENSING SYSTEM, IMPACT SENSING MEANS INCLUDING A SURFACE EXPOSED TO PRECIPITATION AND ELECTRO-MECHANICAL TRANSDUCER MEANS FOR DEVELOPING CONTROL SIGNALS IN RESPONSE TO IMPACTS OF PARTICLES ON SAID SURFACE, REBOUND SENSING MEANS INCLUDING A SURFACE EXPOSED TO PRECIPITATION AND AN ELECTRO-MECHANICAL TRANSDUCER DEVICE IN SPACED RELATION TO SAID SURFACE FOR DEVELOPING CONTROL SIGNALS IN RESPONSE TO REBOUNDS OF PARTICLES FROM SAID SURFACE, PHOTO-OPTICAL MEANS INCLUDING A LIGHT SOURCE FOR ILLUMINATING A CERTAIN REGION THROUGH WHICH PRECIPITATION FALLS AND PHOTOCELL MEANS FOR DERIVING CONTROL SIGNALS IN RESPONSE TO LIGHT IMPULSES ARISING FROM REFLECTION OF LIGHT FROM PARTICLES FALLING THROUGH SAID REGION, MASS ACCUMULATION SENSING MEANS INCLUDING A VIBRATION DEVICE HAVING A SURFACE EXPOSED TO PRECIPITATION AND MEANS FOR DEVELOPING CONTROL SIGNALS IN RESPONSE TO A LOWERED NATURAL RESONANT FREQUENCY OF VIBRATION OF SAID DEVICE CAUSED BY ACCUMULATION OF FREEZING PRECIPITATION ON SAID SURFACE, A PLURALITY OF OUTPUT CIRCUITS CORRESPONDING TO VARIOUS FORMS OF PRECIPITATION, AND LOGIC CIRCUITRY RESPONSIVE TO SAID CONTROL SIGNALS FOR SELECTIVELY ENERGIZING SAID OUTPUT CIRCUITS.

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