CA1176732A - Product-to-frequency converter - Google Patents

Abstract

PRODUCT-TO-FREQUENCY CONVERTER

ABSTRACT OF THE DISCLOSURE

The converter circuit is illustrated as a feed-rate control circuit where a DC weight-per-unit length signal is multiplied by a pulse signal proportional to rate of flow of material. This product is further multiplied by a scaler signal to accommodate material delivery systems of various sizes. This product of three quantites is converted into a feedback frequency which is fed back to increase circuit response and linearity. The circuit is independent of any clock frequency and reference voltage variations by using the frequency and reference voltage in both the main input signal and the negative feedback signal. The foregoing abstract is merely a resume of one general application, is not a complete discussion of all principles of operation or applications, and is not to be construed as a limitation on the scope of the claimed subject matter.

Description

PRODUCT-TO-FREQUENCY CONVERTER

BAC~GROUND OF THE INVE NTION

Control circuits have previously been utilized for controlling the rate of feed of material to a utilization de-vice~ As one example, coal on a conveyor belt ~ay be fed at a variable rate by variable speed of the motor driving the conveyor belt and the actual coal per-unit-length of conveyor belt may vary according to the amount of coal dropping out of a bunker or chute onto the conveyor belt. Accordingly~ the rate of feed is the multiplication product of the we;ght-o ~er-unit of belt length times the speed of the con~eyor belt.
The weight signal may be generated by a transducer, for example~ a load cell, which converts the force or weight of material into an electrical signal. Belt travel may be obtained by an odometer or tachometer that generates a pulse per unit of belt travel or generates a freauency proportional to belt speed. A prior art system for performing this multi-plying product is to transmit the load cell signal and odo~e-ter signal to a distant electrical cabinet whereat the 102d cell signal is amp,lified, converted into a digital signal, and then multiplied by the belt speed signal. This prior art system has at least three disadvantages:
~ 1 ) It requires the transmission of the load cell output signal, which is a low level signal of ~enerally a few millivolts, over a lQng conductor. For this reason, the load cell wiring requires special preca~tions to eliminate ~he noise induced by electroma~netic radiation. Also, errors are introduced by the thermocoupie effect bet~een wire connec-tions.

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(2) The electronics require considerable program-ming to scale the system to the required demand and to pro-vide the correct feedback signal. Usually the prior art sys-tems reverted to a scaling of both the ~eighing signal and the belt speed signal into a com~ined percentage signal in order to accommodate system variations.

(3) The circuit requires the use of an analog-to-digital converter to dig;tize the ~eighing signal. These converters are expensive and introduce errors for which com-pensation is extremely difficult.

SU~ARY OF THE INVENTION

The problem to be solved, therefore, is how to achieve a product-to-fre~uency converter which is more accu-rate, which may be utilized at remote locations, which is compensated for variables, and which may be used in utiliza-tion devices of a wide range of maximum feed rates.
The problem may be solv~d by a product-to-frequency converter comprising first signal-generating means providing a continuous DC signal with a varyin~ amplitude constituting a multiplicand value; second signal-generating means provid-ing a first periodic pulse signal whose frequency constitutes a multiplier value, the pulses constituting said first peri-odic pulse signal each being of predetermined duration; sum-ming means providin~, in response to said DC signal and said first periodic pulse signal, a product value constituted by a second periodic pulse signal haviny a frequency equivalent to said first periodic pulse signal, a peak amplitude equivalent to said DC signal, and a pulse duration equival~nt to said predetermined duration; and a voltaye-controlled oscillator .:
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means having an input responsive to said second periodic pulse signal, and an output providing a third periodic pulse signal of a frequency proportional to said product value, said third periodic pulse signal remaining constant when said DC amplitude varies in inverse proportion to a change in the frequency of said second periodic pulse signal.
The problem may further be solved by a feed rate control circuit comprising, in combination, a first multi-plier having an output and having firstl second, and third lo inputs, means supplying a material ~Pight signal to said first input of said first multiplier, means supplyin~ a material delivery speed signal to said second input of said first multiplier, means supplying a scaler signal to said third input of said first multiplier, an amplifier connected to amplify the output of said first multiplier, and a volts-to-frequency converter connected to the output of said ampli-fier to supply an output frequency signal with the frequency dependent upon said amplifier voltage output and with said output frequency signal being a scaled feed rate signal of material weight times material delivery speed.
The problem may further be sol~ed by a feed rate control circuit comprising, in combination, first and second multipliers each having an output ~nd each havin~ first and second inputs, means supplying a material weight signal to said first input of said first multiplier, means supplying a material delivery speed signal to said second input o~ said . first multiplier, an amplifier connected to amplify the dif-ference between the outputs of said first and second multi-pliers, a volts-to-freyuency converter connected to the out-30 put of said amplifier to supply an output frequency signal with the frequency dependent upon said amplifier voltage o~t-put, and feedback means connecting said output frequency signal to an input of said second multiplier to reduce the vol tag e appl i ed to sa i d ampl i f i er .

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~ :~7~732 Accordingly, an object of the invention is t~ pro-vide a product-to-frequency converter which obtains a product of speed times the unit wei~ht of the material and scales this to maximum capacity of a particular system.
Another object of the invention is to provide a co~-trol circuit which multiplies the product of three different inputs, speed, unit weight, and a scaling factor.
Another object of the invention is to provide a con-trol circuit which multiplies together three inp~t signals 0 and then provides a feedback to compensate for possible errors in two of those input signals, components, and cir-cuits.
Another object of the invention is to provide a feed rate control circuit which has at least a 100:1 range with the same accuracy at the lower scale as at full scale.
Another object of the invention is to provide a feed-rate control circuit wherein the scaling for different capacities of systems may be accomplished with a scaling of a single input signal.
Another object of the invention is to provide a con-trol circuit which is independent of both reference voltage and clock frequency variation.
Other objects and a fuller understanding of the in-vention may be had by referring to the following description ,and claims, taken in conjunction with ~he accompan~ing draw-ing.

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B~IEF DE~RIP~I~N OP THE DRA~IN~S
~ igures 1 and 2, placed from left to right, toge~her represent the schematic diagram of t~e circuit embodying the invention; and Figure 3 is a graph of signals versus time to illustrate the operation of the circuit of Figures 1 and 2.
DES'CRIPTI'ON' OF' THE' PREFERRED EMBOD'IMENTS
Figures 1 and 2 r positioned side by side, show schematically a circuit 11 which is a multiplier circuit with a product-to-frequenc~ converter. This mllltiplier circuit may be used in anumbar of different ways, and is illustrated as a feed rate circuit as one example of utility. Material, such as coal 12, may be delivered by some means such as a conveyor 13 to a utilization device (not shown~ such as a steam boiler. The amount vf material on the conveyor may vary per-unit-length~ due ~o irregularities of density or feeding the material onto the conveyor, so the weight of the material per-uni~length of ~he conveyor is a weight signal. A
multiplication of the weight per unit of conveyor belt length times the speed of the conveyor belt will e~ual the feed rata in weight or mass per unit of time. To illustrate a way of obtaining the weight signal and the speed signal, the circuit 11 illustrates a weight span 14 over which the conveyor passes, and this acts on a load transducer, such as a Wheatstone bridge load cell 15 which is supplied by a reference voltage source 16 and the output supplied to a precision or instrument amplifier 17 to obtain a weight signal on conductor 18. In this embodiment, this is an analog signal, which is a variable DC signal of a few volts.

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A motor 21 is connected to a drive wheel 22 to drive the conveyor to ~eed the material 12 to the utili~ation de-vice. This feed signal may be taken from the drive wheel 22 or, as shown, from a tachometer or generator 23 connected to the drive shaft. In this preEerred embodiment, the tacho~e-ter ~3 is a pulse generator, generating one pulse for each increment of conveyor belt travel. The particular pulse generator shown has two outputs, so that either a given speed frequency F may be obtained on a conductor 24, or a half-o speed signal 2- may be obtained on a con~uctor 25. The circuit 11 of FIGS~ 1 and 2 accomplishes the multiplication of the weight signal on conductor 18 by the speed signal on conduc-tor 24 or 25. This is aecomplished principally in a first multiplier 28. More importantly, the circuit accomplishes a scaled product of unit weight times feed s~eed by also multi-plying by a scaling factor from a scaler 29.
FIG~ 2 shows another portion of the circuit 11, and it inc~udes a clock 30 which establishes a reference frequen-cy or multiples thereof for operation of circuit 11. The scaler 29 scales a frequency from this clock 30 so that this scaled clock frequency is multiplied times the weight signal, which is multiplied by the speed signal in the first multi-plier 28. The FIG. ~ portion of circuit 11 also shows a second multiplier 31 which is used in a feedback circuit 32.
Another part of the feedback circuit 32 is a voltage con-trolled oscillator circuit 33 having an output 34.
The first multiplier 28 has an output on a conductor 36 on which appears an average input voltage to a first input resistor 41. A second input resistor 42 from the feedback circuit 32 is connected as a negative feedback, together with the irst input resistor 41, to an error ar.pliier 43. A
signal conditioning circuit ~4 conditions this output so that i 7 3 ~

a motor control signal appears on the output 45 of this con-ditioning circuit 44. This motor control signal is supplied back to a motor control circuit 46, which is connected to control the s?eed of the motor 21 and ~hich may have a manual speed control 47. Once the conveyor speed is set by the speed control ~7, then the circuit 11 establishes the preset feed rate. If the material is coal being àelivered to a steam boiler, and if the coal becomes partially blocked in the bunker from which it drops onto the conveyor 13, since 0 the amount of coal per-unit-length of conveyor becomes rater-ially smaller, then the circuit 11 controls the conveyor speed such that the motor 21 increases the speed of the con-veyor 13 so as to maintain constant the rate of feed of the coal mater ial to the boiler.
The same circuit 11 may be provided with many dif-ferent sizes of steam boilers or other utilization devices, so the scaler 29 scales the output of the first multiplier 28 in accordance with the total capacity of the utili~ation de-vice. If this device is a steam boiler, then, for example, 20 the maximum capacity of the system might be 100 tons of coal per hour bein~ delivered. However, the utilization device mi~ht easily be of smaller capacity, for example 20 tons, 40 tons, or 60 tons per hour maximum, in hhich case the scaler 29 would be set at 20~ 40, or 60, respectively.
The circuit 11 multiplies together two signals. In the preferred embodiment, this circuit multiplies a variable DC or analog voitage, shown as the weight signal on conductor 18, by a requency, shown as the conveyor speed signal on conductor 24 or 25. The first multiplier 28 multiplies to-gether these two voltages to generate an output signal onconductor 36 which is proportional to the product o~ these two signals. Additionally, the circuit 11 produces first and second control signals. The firs~ control signal appears on the output conductor 45 and is used to operate the r,otor 21 ' 3 '~

via the motor control 46, and the second control signal is an output ~requency on conductor 34 proportional to the prod-uct of the multiplied voltage and frequency. From this sec-ond control signal, a feed rate indicator 48 may be supplied to indicate the rate of material 12 being delivered, and also a totalizer 49 may be supplied which indicates the total ~uantity of material delivered. In the preferre~ e~bodi~ent, the second control signal on conductor 34 is afected direct-ly by the scaler 29~ to represent the percentage of the capa-city of the system with which the circu~t 11 is used relativeto the maximum capacity of circuit 11. For exampler a 4-20 milliamp output at conductor 45 might indicate ~ delivery rate of the conveyor at 4 milliamps, and maximum delivery rate at 20 milliamps. However, in two different material delivery systems, the 20 milliamp maximum signal may estab-lish a feed rate of 20 tons per hour or 60 tons per hour, depending upon the scaling by the scaler 29, described in detail below.
In more detail, the circuit 11 includes a pair of analog switches 51, and in the preferred embodiment these are paired for current carrying capacity and to lower the on-state resistance. An analog or variable DC voltage is applied on the conductor 18 to t~e analog switches 51. The on or conduction time of these switches 51 is controlled by an input precision pulse generator 52. This input pulse generator includes a divider or counter 53 and a flip-flop 54. The counter 53 counts a certain number of pulses, e.g., 128 pulses, from an input reference frequency on a conductor 55. Ori~inally, this reference frequency co~es from the clock 30, but is a scaled frequency as scaled by the scaler 29. The flip-flop 54 and hysteresis gate 57 are used as a synchronizing circuit to synchronize the start o a pulse on speed frequency conductor 56, with a pulse on the reference conductor 55. The inco.~ing fre~uency, which is the CGnveyor .

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speed signal, is controlled by a range selector 58 which minimi~es propagation delay errors in the circuit. This range selector includes a multiplexer S9 and a mag~itude comparator 60. The function of this range selector 58 will be described later, and for simplicity, let it be 2ssumed that a square wave proportional to the speed signal in fre-quency exists at the output 61 of the multiplexer 59. This may be illustrated by curve 61A in FIG. 3A. The falling edge of the square wave is converted into a pulse by resistor 62, lo capacitor 63, and hysteresis gate 64. This is a narrowing of the pulse for sharp rise and fall times of the pulse. This is illustrated by pulse 56A in FIG. 3B. This pulse 56A

resets the flip-flop 54, and, on the next rising edge 55A

(see FIG. 3C) of the input reference frequency on conjductor 55, it toggles the flip-flop 54. The action of the ~ output 65 of flip-flop 54 is shown by the pulse output 65A in FIG.

3D. This aotion generates a narrow pulse 67A ~see FIG. 3E) on the reset input 67 of the divider 53. This pulse 67A is a narrow pulse generated by the action of a resistor 68, capa-citor 69, and a hysteresis gate 70. The pulse 67A resets the divider 53, causing its output 71 to go lo~, which turns on the input analog switches 51 and applying the magnitude of the input voltage or weight signal to the first input resist-or 41. The output 71 of the divider 53 remains a logic lo~

(see curve 71A in FIG~ 3F) until 1,28 pulses ~rom the input reference frequency on conductor 55 are counted. At this timet the output 71 goes to a logic 1, turning off the input analog switches 51 and the divioer 53 stops counting. Thus, for an input speed signal pulse 56A on conductor 56, the analog switches Sl remain conducting for 128 pulses rom the input reference frequency on conductor 55. This produces a pulse 41A ~FIG. 3G) on the first input resistor 41 ~hich is equal to the width of the pulse 71A. The action of generat-ing a pulse 71A of fixed ~idth for every input freouency ~ ~673~

pulse of the speed frequency on conductor 56 generates an average voltage on ~he first input resistor 41 ~hose average value is directly proportional to the input speed requency times the amplitude of the input analog voltage or ~eight signal on conductor 18. Therefore, the average voltage ap-plied on ~he first input resistor 41 is the product of both the input analog signal 18 and a speed frequency signal ~
conductor 56. Still further, the average voltage applied at this first input resistor 41 is a product of three things~
lo the weight signal on conductor 18, the s~eed freauency signal on conductors 24 or 25, and a scaled clock signal.
FIG. 2 shows that the first input resistor 41 is an input to the error amplifier 43~ The error amplifier 43 has the feedbaok capacitor 38 to make it act as an integrator, and has high impedance resistors 39 on the input which pro~
vide a path to ground for the op amp bias current when both inp~t resistors 41 and 42 momentarily provide no input. The error amplifier 43 has no resistive feedback, so that it acts not only as an integrator b~t also with practically complete open loop gain of, for example, S0,000 or 100,000. This amplifier amplifies the difference between the average input voltage applied at the first input resistor 41 and the aver-age feedback voltage applied at the second input resistor 42. These resistors are precision resistors in order to minimize any errors ;n the circuit. The feedback voltage applied at the second input resistor 42 is generated by a circuit similar to the one used to generate the input voltage for the first input resistor 41. The output o~ the error amplifier 43 is connected to a two-pole, non-inverted, low pass filter made up of resistors 75,76, and 77, capacitors 78 and 79, and op amp 80. This low pass ilter, ~hich has a roll-off point of approximately 20 hertz in one circuit con-structed according to the invention, is used to eliminate the ripple ~hich is present at the output of the error amplifier 43. The output of the two-pole filter 44 is connected through a resistor Bl to the voltage-controlled oscillator circuit 33 which has a conversion ratio of approximately 2000 hertz per volt~ The voltage-to-frequency conversion is per-formed by a volt-to-freq~ency converter 82. The entire cir-cuit 11 is scaled such that, ~hen the average input voltage on input resistor 41 is at a maximum, the output 34 of the VT~C circuit 82 is 20 kilohertz frequency, as an example of a practical circuit 11. This is fed to a divider 84, which has two outputs 85 and 86. These outp~ts divide do~n the output frequency, with the output 85 going to supply the indicator 48 and the totalizer 49. The output 86 is divided still fur-ther, for example divided by 8, to eliminate errors created by the variation in propagation delays. This output frequen-cy is used in conjunction with a negative voltage reference on a reference conductor 88 to generate the average feedback voltage on the second input resistor 42.
The feedback frequency at the VTFC output 34 and divider output 86 is converted into a pulse by the network of resistor 89, capacitox 90, and hysteresis gate 91. This is a narrow pulse with sharp rise and fall times. This feedback pulse resets a flip-flop 94, similar to flip-flop 54, and, with hysteresis gate 93, is used to synchronize the frequency of the clock 30 and the feedback pulse~ After the flip-flop 94 has been reset, then the next pulse from the feedback ref-erence frequency on conductor 95 clocks the flip-f,lop 94 and a 2ulse is generated by the network of resistor 96, capacitor 97, and hysteresis gate 98. This pulse resets a counter or a divider 99, similar to the divider ~3. In one practical cir-cuit made in accordance with this invention, this divider did not divide by 128; rather, it divided by 4. hs soon as the divider 99 is reset by the pulse from the hystercsis gate 98, this immediately turns on a pair o~ ~eedback analog s~itches 100 via a conductor lOlo This action connects an input from ~ o ~
the reference~conductor 88 through the analog switches 100 to 1 ~ 7~732 the second input resistor 42. In a practical circuit made in accordance with thi invention, this reference voltage was -10 volts. The divider or counter 99 counts the predeter-mined number of pulses (from conductor 95), four in thi~
case, and then turns off the analog shitches 100. Therefore, whenever ~he system is operating at its programmed maximum capacity, the average voltage applied at the second input resistor 42 is always the same.
In order to scale the circuit 11 correctly h~hen a lower maximum input frequency on conductor 56 is desired to generate the maximum feed rate frequency on divider output 85, the pulse width out of the input pulse generator 52 must be increased in order to apply the same average voltage at the first input resistor 41, keeping the circuit 11 on the same scaling. The scaling of the average input voltage is achieved by the scaler 2~, and will be described below.
The feedback circuit 32 includes the voltage-controlled oscillator 33. This circuit includes the volt~to-frequency~converter 82, which has an op amp 104 connected to conduct current from the current output 111 of the VTFC 82 to the input terminal. Also, a diode 105 is connected to limit the negative voltage across the input and output 112 of the op amp 104. A feedback capacitor 106 is connected from the output to the input of the op amp 104. The threshold input of the VTFC 82 is connected to the junction of resistors 107 and 108, which are connected hetween positive operational voltage and ground. The ON RC input of the VT~C 82 is con-nected to the junction between a resistor 109 and a capacitor 110, which are connected between positive operational voltage and ground.
This voltage-controlled oscillator circuit 33 acts as follows. The positive voltaqe ap21ied by conductor 112 to the input pin of the VTFC 82 is compared to the voltage at the threshold input as set by the value of resistors 107 and 108. If the input voltage is higher, the input comparator ~ 17'o732 fires a one-shot multivibrator, whose output is connected to both the logic output at conductor 34 and a precision switched current source internal of the VTFC 82. The logic output at conductor 34 goes low, and the internal current source produces a current pulse at the current output con-ductor 111. ~he time on for the one-shot is determined by the resistor-capacitor network 109, 110 connected to the O~-RC terminal. The op amp 104 acts as an error ampl;fier whose output is proportion~l'to the error bet~een the current lo generated by the output voltage of the two-pole filter 44 divided by the output resistor 81 and the current pulse gen-erated at conductor 111 of the VTFC 82. The use of the ca-pacitor 106 makes the error amplifier 104 an integrator, and this improves the linearity~ of the voltage-controllecl oscil~
lator circuit 33 because it keeps the output of the ~urrent source at conductor 111 at a constant voltage of practically zero. Actually, this voltage might be 1 millivolt, which, multiplied by the high gain of the amplifier 104, produces just enough voltage on conductor 112 to maintain the circuit in balance. This eliminates the linearity error due to the current source output conductance.
The logic ou put of the VTFC 82, which is on con-dvctor 34, is connected by a resistor 12~ to posit;ve operat-ing voltage, and, is 20 kilohertz in one practical circuit made in accordance with the invention, whenever the ~ircuit is operating at'i~s maximum feed rate. This 20 kilohertz fre~uency is divided by 2 and applied to the output conductor 85 in order to generate a syr~etrical 10 kilohertz signal, which is the output of the circuit 11. The 10 kilohertz sig-nal on conductor 85 is transmitted by the hysteresis gate 114 and line driver 115 to one ;transmission line 117, and, by a line driver 116, to anothe~ transmission line 11~. The devices 115 and 116 are line driver buffers to drive these transmission lines so that the output frequency, at a mzximum -.

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of 10 kilohertz frequency, may be transmitted over long dis tances, for example, some remote location whereat the total-izer 49 and indicator 48 are mounted. The two transmission lines transmit two square wave signals 180 degrees out of phase and they are received at a split phase receiver 119, ~hich passes thë signal to a scaler 120/ ~hich may be a binary rate multiplier and which may be essentially the same as the scaler 29, and from there to a divider 121. The out-put of the scaler 120, which multiplies the inco~ing frequen-lo cy by N/100, supplies the feed rate indlcator 48 and the out-put of the divider 121 supplies the totalizer 49, N being the number on scaler 29.
The scaler 29 establishes the scaling of the average input voltage to the first input resistor 41. The reason is that it is desired that the output frequency at the conductor 34 be 20 kilohertz whenever the circuit 11 is operating at its maximum feed rate. This scaling is accomplished by changing the pulse width out of the input precision pulse generator 52 to accommodate changes in the desired maximum input frequency on conductor 56. The scaler 29 accomplishes this function and-it includes a phase lock loop circu;t 126 and a divider 127. A capacitor 12g is connected between the VDD and Vss inputs of the phase lock loop 126 for noise suppression and a capacitor 130 is connected across the capacitor terminals of this phase lock loop. A resistor 131 is connected between the resistor terminal and ground of this phase lock loop. Resistors 132 and 133, together ~ith capa-citors 134 and 13S, provide compensation and filter the out-put of the phase comparator and are connected to VIN, which is the input to the voltage-controlled oscillator of the phase lock loop 126.
The divider 127 may be one of several types, but in this case includes two dividers 137 and 138 and two switches 139 and 140. The dividers 137 and ~38 may be dec;mal 1 ~ 7 3 2 divide-by-~ counters and the switches 139 and 140 may be manually operable switches, such as thum~ ~heel switches. By using two of these dividers and two s~i~ches~ two different decimal numerals may be selected as ~he letter N so that this divider divides by any integer from zero to gs. The switch 140 sets the least significant ~it and the switch 139 ~ets the most significant bit.
In a circuit made in accordance with the invention, the circuit 11 was designed to supply a maximum of 20 kilohertz feed rate frequency on conductor 34, and one system for which the circuit 11 was designed was intended to supply 100 tons per hour of coal via the conveyor 13 to a utilizat~on device such as a steam boiler.
The circuit 11 may also be used with systems of smaller capacity, for example, 20, 40, or 60 tons per hour. In such case, the scaler 29 permits the ready scaling of the circuit 11 to this lower capacity system. In such case, the thumb wheel switches 139 and 140 would be set at 20, 40, or 60, respectively. This scales the circuit 11 at 20%, 4a~ ~ or 60% or the maximum capacity~ For a 20-ton per hour sy~tem, for example, one could then ~till have 20 kilohertz maximum ~eed rate frequency at the conductor 34 whenever 2Q the conveyor 13 was delivering coal to the steam boiler at the maximum feed rate for that size system.
The scaler 29 u~ilizes the divider 127 to divide by a number N, and this is supplied on a conductor 141 to the comparator-in terminal of the phase lock loop 126, The clocked frequency or a multipla thereof is applied on a conductor 143 to the frequency-in terminal of the phase lock loop 126. The voltage-out terminal of the phase lock loop is connected to the input xeference frequency conductor 55 to supply it with a scaled or multiplied frequency.
The phase lock loop 126 will normally track an input frequency applied at the frequency-in terminal at conductor 143. However, with the divide-by-N counter connected between the comparison-in , .

terminal and the voltage-out terminalp the phase lock loop 126 will operate at N times the input frequency applied to conductor 141. Thus, the ef~ect is that wi~ the divider set at some integer N, then the phase lock loop runs with an outpu~ at N times the incoming frequency on conductor 143.
An al~ernative position for the scaler 29 is to position it between t,he generator 23 and the conductor 61, where it will scale the incoming frequency rather than the pulse width.
The range selector 58 is provided ~o minimize circuit errors. The phase lock loop 126 will operate over a wide frequency range, for example, 1000:1. However, the range selector 58 narrows the capture range of this phase lock loop to about 50:1, so that it is stable and easier to compensate. Further, the range selector 58 maintains the pulse width out of the input precision pulse generator 52 as wide as possible in order to minimize propagation delay errors. The range selector S8 includes the multiplexer 59 and the magnitude comparator 60~ Diodes 146 and 147, together with resistor 149, form a discreet ~ND gate to conduct the output from the A = B out terminal and A ~ B out terminal by a conductor 148 2~ to the A terminal of the multiplexer 59, which is a one-of-four switch.
The clock 30 is controlled by a crystal 151 which is con-nected to the crystal terminals of a divider or counter 152. In this particular instance, the divider 152 is a binary ripple counter which has 14 stages for maximum division of 214 = 16,384.
A resistor 153 is connected across the crystal lSl and a capacitor 154 is connected from one side of the crystal to ground. A
capacitor 155 is connected between the VDD terminals and Vss terminals for noise suppression. The operating frequency of the `` ~ 17~73~

clock is not critical; and in a circuit made in accordance with the in~ention the cry~tal 151 operated at 4 megahertz. At such fre-quency of oscillation, Q7 output on conduc~or 95 was 31.25 kilo-hertz, the Q9 outpu~ on a clock conduc~or 157 was 7.8125 kilohertz, and the Q10 output on a clock conductor 158 was 3.90625 kilohextz.
.: The range selector 58 selects either ~he clock frequency o 7.8 kilohertz or 3.9 kilohertz, and also selects the incomin.g speed frequency vf F on conductor 24 or ~ on conductor 2S. Since . the scaler 29 has a 1 to ~9 range of scaling, the numeral 50 is preset on the magnitude comparator 60 by making the B~ and B2 terminals high and the Bl and B3 terminals grounded. This numeral 50, or numeral 5 of ~he most signiicant bit, is passed by the conductors 160 from the magnitude comparator to the most significant bit switch 139. Accordingly, if the scaler 29 is set at less than 50, then the magnitud~ comparator 60 selects the higher clock frequency of 7.8 kilohertz, and selects the higher speed frequency of F on conductor 24. If, on the other handt the scaler 29 i5 set at 50 or greater, then the opposite is true, with the magnitude comparator 6~ selecting the lower clock frequency of 3.9 kilohertz and ~he lower speed frequency of ~ on conductor 25. Therefore, the larger the number programmed on the digit switches 139 and 140, the higher the ou~put frequency of the phase lock loop 126. By this means, the relationship between the input speed frequency and the input referenc~ frequency on conductor 55 remains the same regardless of the position of the switches 139 and 140. The purpose of this circuit feature i6 to keep the pulse width out of the input precision pulse generator 52 as wide as possible to minimize errors introduced by variations in propagation delay.

.;. ;,r The feed rate indicator 48 and totalizer 49 may be at a remote location~ The scaler 120, which may be a binary rate multi-plier, is set at the same multiplier as the scaler 29. If the scaler 29 is set at the numeral 20, for example, then the scaler 120 would also be set at the numeral 20, and if the frequency, for example, at the output conductor 85 is 10 kilohertz, then this will indicate 20 tons per hour delivered by conveyor 13, in the example set forth aboveO I~ the output frequency at conductor 85 is only ~ kilohertz, the fead rate indicator will indicate 18 tons per hour being delivered.
The divider 121 further scales down the output signal based upon a fixed convexsion ~actor to obtain a signal which represents pounds of material 12 being delivered.
In a circuit constructed in accordance with this invention, tha circuit components and values thereof were as follows:
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17 instrument amplifier, automatic zero reset once per second 43 amplifier LM 208 51, 100 analog switch HI 201-5 53,99 Multiplexer 4520 54,94 Flip~Flop 4027 59 Multiplexer 4052 Magnitude Comparator 4585 64,70 Hysteresis Gate 40106 80,lG4 op amp LM 201 84 Divider 4520 91~98 Hysteresis Gate4Q106 114 Hysteresis Gate40106 115,116Line driver buffer 9668 126 Phase Lock Loop4046 137,138 Divider 4522 152 Binary Ripple Counter 4060 57, 93 Hysteresis Gate 40106 Resis_ors ~5% normally Capacitors n microfarads, except as noted 10 39 1 Megohm 38 1. 50 v.
41l42 20K Ool~ SPPM/degree C 63 100 pf 62 6.8K S9 100 pf 68 506K 7~ ,047 100K 79 .1 76 lOOK 90 100 pf 77 200K 96 100 pf 89 10~ 106 .0047 97 5.6K 110 .001 103 11.3K 129 ,1 20 107 4.99K 130 100 pf 108 lOK 134 .047 109 27. 4K 135 .1 113 lOOK 154 33 pf 122 lOK 155 .1 131 lOK
132 lOX
133 4.7K

153 22 Megohms 30Re~erring again to Figure 3, the square wave 42A shown at Figure 3H is the voltage pulse obtained across the second 3 ~

input resistor 42. This voltage pulse is negative, whereas, the pulse 41A is positive, 50 that t~ese two signals are com-bined and only the difference, or error, between the two is that which is amplified by the error amplifier 43. This error might be only about 1 millivolt, and ~hen multiplied by the high gain amplifier 43, provides a maximum output of, for example, 10 volts supplied to resistor 75. l~hen filtered and supplied as a DC signal, this is about 10 volts DC at the conductor 45. This is returned to the motor control circuit lo 46 to control the conveyor motor 21 to maintain the stable speed unless the amount of coal per unit of length on the conveyor 13 should change, in which case, the motor speed will change inversely to maintain a constant feed rate.
Referring to FIG. 3G, the height of the pulse 41A is proportional to the weight of material on the conveyor 13~
The frequency of the pulses 41A is directly proportional to the conve~or s~eed rate on conductors 24 or 25, 50 the period Gf the fr-quency between pulses 41A is inversely proportional to the speed rate. The width of each pulse 41A is the scaled clock signal proportional to the numeral set on the scaler s~itches 139 and 140. Thus, this signal available on the first input resistor 41 is a product of three ~uantities. A~
., the same time, the second input resistor 42 has a signal which is a feedback signal almost completely canceling the voltage across the first inp~t resistor, except for the small error, for example 0.1 millivolt. This feedback signal, represented by pulse 42A in FIG. 3H, is one wherein the height of the pulse 42 is dependent on the reference voltage from the reference voltage source 16. ~he period ~etween pulses is inversely proportional to the feedback frequency, and the width of each pulse 42A is proportional to the clock frequency. Accordingly, the feedback arr2nge~ent is such that the variations, if there are any due to temperature changes or the like in the reference Yoltage and in the clock . . ~

7 ~ ~

fre~uency, are bala~ced out because the input voltage at 18 is proportional to the reference voltage. The clock frequen-cy and the re~erence voltage appear in the same manner in both the pulses 41A and 42A, so that it is only the ratio of the reference voltage which appears on the input resistor 41 versus that on ~he input resistor 42. Also it is only the ratio of the olock frequency which appears on the input resistor 41 versus that on the input resistor 42. The motor speed signal at the conductor 45 is therefore a very accurate lo signal proport;onal to the ~eight sign~l on conductor 18 times the speed rate si~nal on conductor 24 or 25. The transfer function for the circuit is~fout = ~ N x V VINRX R42^

The circuit 11 provides a product-to-frequency con-verter which has a continuous DC signal on the conductor 18 of varying amplitude which constitutes a multiplicand value.
Also, this circuit 11 provides the tachometer generator 23 which generates a first periodic pulse signal on conductors 24 or 25 whose frequency constitutes a ~ultiplier value. In one typical circuit; for example, this might be a maximum of 2 kilohertz at maximum speed of the conveyor 13. The pulses of this first periodic pulse signal are controlle~ by the signal ~rom the clock 30 or a scaled clock signal from the scaler 29, so that at the outp~t of the divider 53, these pulses are each of a predetermined duration. The analog switches Sl and the first input resistor 41 may be considered summing means which act in response to the DC signal on con-ductor 18 and the first periodic pulse signaI on conductor 71 to establish a product value constituted by a second periodic pulse signal across resistor 41, ~hich has a frequency equiv-alent to the first periodic pulse signal; a peak amplitude eauivalent to the DC signal on conductor 18, and a pulse du-ration equivalent to the predetermined duration established by divider 53 and scaler 29. The circuit 11 also includes ~2 the voltage-controlled oscillator means 33, which has an input from the~input resistor 41'via the error a~plifier 43 and filter 44, and is responsive to this second periodic pulse signal. The voltage-controlled oscillator 33 also has an output providing a third periodic pulse signal on conduc-tor 86 at a frequency propoxtional to said product value. 0 importance is the faot that the third periodic pulse signal remains constan~ when the DC amplitude on conductor 1~ varies in inverse proportion to a change in the frequency of the lo second periodic pulse signal on conductor 71. Still further, the circuit 11 includes the scaler 29 which scales the prede-termined duration of the pulse appearing on conductor 71.
Also, this circuit 11 includes the clock 30, which is deter-minative of the pulse duration provided by this scaler 29.
The error amplifier 43 and filter 44 establish that the voltage-controlled oscillator 33 has an input responsive to the average DC value of this second periodic pulse signal.
Another important feature of the circuit 11 is that it includes a feedback circuit from the output of the voltage-controlled oscillator 33 to the input of the voltage-controlled oscillator via the second input resistor 42, error amplifier ~3, and filter 44. This feedback circuit is responsive to ~ny changes'in the clock'frequency and any changes in the value of the reference source 16 to maintain the third frequency signal at a c,onstant valu,e upon changes in the DC amplitude on conductor 18 in inverse proportion to a change in the fre~uency of the second periodic pulse signal on conductor 24 or 25.- ' ' It will also be noted that the circuit 11 is a-feed-rate control circuit which controls one of the quantity of coal delivered to the conveyor 13 or the speed of the con-veyor 13 to maintain the predetermined rate o feed of the coal or othër material 12 to a utilization device. In the circuit as illustrated, this control is of the rate of speed 1 ~7~732 of the conveyor 13~ The material weigh~ signal on conductor 18 is a combination of the output from the material weighing trans-ducer 15 and the reference voltage source 16, The feedback circuit 32 includes a means to compensate for any variations in the reference voltage By having this same reference voltage supplied on conductor 88 to the analog switches 100 to determine the height of the pulse 42A in Figure 3H. Also, it will be noted in the circuit 11 that the scaler signal on the conductor 71 is a product of the multiplying factor set by the switches 139 and 140 times the signal from the clock 30. The feedback circuit 32 further includes a means to compensate for any variations in the clock signal by having this same clock signal fed back on the conductor 95, and thus affect the output duration of the pulse from divider 99 on the conductor 101 which is applied to the feed-back analog switches 100.
The circuit 11, as constructed in the preferred embodiment, provides a feed-rate control circuit which has a 100:1 range in the maximum feed rate of the material flow system being controlled, yet with the same high accuracy at the lower scale as at full scale.
2Q The present disclosure includes that contained in the appended claims, as well as that of the foregoing description.
Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of the circuit and the combination and arrangement of circuit elements may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A product-to-frequency converter comprising:
first signal-generating means providing a continuous DC
signal with a varying amplitude constituting a multiplicand value;
second signal-generating means providing a first periodic pulse signal whose frequency constitutes a multiplier value, the pulses constituting said first periodic pulse signal each being of predetermined duration;
multiplying means providing, in response to said DC
signal and said first periodic pulse signal, a product value con-stituted by a second periodic pulse signal having a frequency equivalent to said first periodic pulse signal, a peak amplitude equivalent to said DC signal, and a pulse duration equivalent to said predetermined duration; and a voltage controlled oscillator means having an input responsive to said second periodic pulse signal, and an output providing a third periodic pulse signal of a frequency proportional to said product value, said third periodic pulse signal remaining constant when said DC amplitude varies in inverse proportion to a change in the frequency of said second periodic pulse signal.

2. A product-to-frequency converter according to claim 1, including scaling means for varying said predetermined duration.

3. A product-to-frequency converter according to claim 2, including clock means determinative of the pulse duration provided by said scaling means.

4. A product-to-frequency converter according to claim 3, including feedback means connected from the output of said voltage-controlled oscillator to the input of said voltage-controlled oscillator to maintain said third frequency signal at a constant value when said DC amplitude varies in inverse proportion to a change in the frequency of said second periodic pulse signal.

5. A product-to-frequency converter according to claim 4, including a reference source providing a reference value, one of said multiplicand and multiplier varying directly in accordance with said reference value, and said feedback means including a feedback of said reference value.

6. A product-to-frequency converter according to claim 4, including clock means determinative of said predetermined pulse duration, and said feedback means including a feedback of a signal from said clock means.

7. A product-to-frequency converter according to claim 1, wherein said oscillator means has an input responsive to the average DC value of said second periodic pulse signal.

8. A feed rate control circuit comprising, in combination, a first multiplier having an output and having first, second, and third inputs;
means supplying a material weight signal to said first input of said first multiplier;
means supplying a material delivery speed signal to said second input of said first multiplier;
means supplying a scaler signal to said third input of said first multiplier;
an amplifier connected to amplify the output of said first multiplier; and a volts-to-frequency converter connected to the output of said amplifier to supply an output frequency signal with the frequency dependent upon said amplifier voltage output and with said output frequency signal being a scaled feed rate signal of material weight times material delivery speed.

9. A control circuit as set forth in claim 8, wherein one material signal is a direct current signal and the other material signal is an alternating current signal.

10. A control circuit as set forth in claim 8, wherein one signal is a direct current signal and the other two signals are alternating current signals.

11. A control circuit as set forth in claim 10, wherein said amplifier is connected to produce a pulse train of variable height, width, and period.

12. A control circuit as set forth in claim 8, wherein with a constant feed rate said converter has an output to vary the frequency of said delivery speed signal inversely proportional to variations in said weight signal.

13. A control circuit as set forth in claim 8, wherein said weight signal is proportional to a combination of a material weighing transducer output and a reference voltage, and means to compensate for any variations in said reference voltage.

14. A control circuit as set forth in claim 8, wherein said scaler signal is proportional to a combination of a clock signal and a multiplying factor, and means to compensate for any variations in said clock signal.

15. A feed rate control circuit comprising, in combination, first and second multipliers each having an output and said first multiplier having first and second inputs;
means supplying a material weight signal to said first input of said first multiplier;
means supplying a material delivery speed signal to said second input of said first multiplier;
an amplifier connected to amplify the difference between the outputs of said first and second multipliers and to have an output connected to control the rate of material feed;
a volts-to-frequency converter connected to the output of said amplifier to supply an output frequency signal with the frequency dependent upon said amplifier voltage output; and feedback means connecting said output frequency signal to an input of said second multiplier to reduce the voltage applied to said amplifier.

16. A feed rate control circuit as set forth in claim 15, including a reference voltage source, the output of said first multiplier being proportional to said reference voltage and being connected to one of said supplying means.

17. A feed rate control circuit as set forth in claim 16, wherein said feedback means includes a feedback of said reference voltage to a second input of said second multiplier.

18. A feed rate control circuit as set forth in claim 15, including a clock signal, means to scale said clock signal, and said first multiplier having a third input connected to receive said scaled clock signal.

19. A feed rate control circuit as set forth in claim 18, wherein said feedback means includes a feedback of said clock signal to a third input of said second multiplier.

20. A product-to-frequency converter according to claim 1, including scaling means for varying either said predetermined duration or said peak amplitude.