US3926145A

US3926145A - Toner concentration detector

Info

Publication number: US3926145A
Application number: US447701A
Authority: US
Inventors: Robert M Muth
Original assignee: Honeywell Information Systems Italia SpA
Current assignee: BULL PRINTING SYSTEMS Inc A CORP OF DELAWARE; Bull HN Information Systems Italia SpA; Bull HN Information Systems Inc; Delphax Systems Inc
Priority date: 1974-03-04
Filing date: 1974-03-04
Publication date: 1975-12-16
Anticipated expiration: 1992-12-16

Abstract

An improved apparatus for applying toner solution to a charged recording medium wherein the concentration of toner particles in the solution is continuously monitored and adjusted is disclosed. The apparatus includes means to measure toner concentration comprising a light source, a transparent chamber receiving a continuously flowing stream of toner solution and means for measuring light intensity from the light source as passed through the transparent chamber. Preferably the means for measuring light is particularly sensitive to light in the near infrared region (about 8500 Angstroms). The output signal from the means for measuring light is a measure of the concentration of toner particles in the toner solution and is used to add toner concentrate to the toner solution as the concentration of toner particles in the solution is depleted. In preferred embodiments, means for controlling the output from the light source and for measuring toner concentration independent of the position of the light source are disclosed.

Description

United States Patent Muth 5] Dec. 16, 1975 TONER CONCENTRATION DETECTOR Primary Examiner-Mervin Stein Assistant Examiner-Steven Hawkins [75] Inventor. 315:? M. Muth Oklahoma Clty Attorney, Agent, or FirmRonald T. Reiling [73] Assignee: Honeywell Information Systems Inc., 57 ABSTRACT Waltham, Mass. An improved apparatus for applying toner solution to [22] Filed: Mar. 4, 1974 a charged recording medium wherein the concentration of toner particles in the solution is continuously [21] Appl' 44770l monitored and adjusted is disclosed. The apparatus includes means to measure toner concentration com- [52] U.S. Cl 118/7; 1 18/637 prising a light source, a transparent chamber receiving [51] Int. Cl. B05B 5/02 a continuously flowing stream of toner solution and [58] Field of Search 118/637, DIG. 23, 7, 9, means for measuring light intensity from the light 118/10, 11; 117/ 17.5 source as passed through the transparent chamber.

Preferably the means for measuring light is particu- [56] References Cited larly sensitive to light in the near infrared region UNITED STATES PATENTS (about 8500 Angstroms). The output signal from the 3,430,606 3/1969 Pease et a1. 118/7 means for measuring light is a measure of h 3,494,328 2/1970 Maloney H 118/637 tration of toner particles in the toner solution and 15 3,635,373 1/1972 Kuhl et a1? H 222/57 used to add toner concentrate to the toner solution as 3,712,203 l/l973 Kishi et al l A 7. 95/89 R the concentration of toner particles in the solution is 3,727,065 4/1973 Maksymiak 250/218 depleted. In preferred embodiments, means for con- 3,752,119 8/l 73 Matkan 1 1 11 trolling the output from the light source and for mea- 3-756v192 9/1973 Locklar et 4 1 18/7 suring toner concentration independent of the position 3,777,173 12/1973 Landrith 118/637 of the light Source are disclosed 3,819,948 6/1974 Lljlma et a1 250/559 8 Claims, 6 Drawing Figures law US. Patent Dec. 16,1975 Sheet20f4 3,926,145

U.S. Patent Dec. 16, 1975 Sheet3of4 3,926,145

U.S. Patent Dec.16, 1975 Sheet4of4 3,926,145

TONER CONCENTRATION DETECTOR BACKGROUND OF THE INVENTION This invention pertains to an improved apparatus for printing upon a recording medium and particularly, to printing permanent images on a charged paper, such as an electrographic paper, at high speeds.

High speed, electrographic printing is used to pro duce a computer printout by non-impact printing tech niques. Generally, electrographic printing utilizes an electrographic paper medium composed of a conductively treated base that supports a plastic dielectric coating. Latent images are formed on the paper by positioning the electrographic paper between an electrode that contacts the conductive base and a second electrode. The surface of the second electrode conforms to the shapes to be printed or, in an alternative embodiment, the second electrode can be selectively activated to form a predetermined image. Thus, this second electrode can have a fixed format and/or a variable format to produce the shapes to be printed on the dielectric paper. A high voltage (i.e., 500 to 800 volts) applied between the two electrodes excites the paper medium, thereby establishing an electrostatic field across the dielectric coating. The dielectric coating retains a residual electrostatic field that constitutes a charged latent image of the shapes to be printed.

Latent images on the charged paper are developed by contacting the paper with a toning liquid composed of charged carbon-resin particles suspended in a liquid carrier. The liquid carrier is preferably a highly parrafinic solvent such as Isopar L which also functions to soften the charged carbon-resin particles. The residual electrostatic field on the dielectric surface of the charged paper attracts the carbon-resin particles and holds them in position. This makes the latent image visible. The visible image is then fixed, i.e., made permanent, by drying and applying heat to remove the liquid carrier and to solidify the carbon-resin particles and bond them to the paper. A suitable method and apparatus for high speed electrographic printing is described in US. Pat. No. 3,701,337, the teachings of which are incorporated by reference herein.

In a high speed electrographic printing operation such as illustrated in US. Pat. No. 3,701,337, the charged printing medium is contacted with a recirculating stream of toner solution. The excess toner solution is recovered and recycled for future use. However, since toner particles are removed from the toner solution in developing the latent images, the excess toner liquid recovered after application of the toner solution to the excited paper contains a lesser amount of toner particles. As a consequence, when the toning solution is continuously recycled, additional toner particles must be added to the circulating solution to maintain a constant, high quality printout.

In the prior art, this was accomplished by the operator visually determining the quality of the printout. When the operator noticed that the printout quality was declining and the developed characters were not sharp, he would, at his discretion, manually add additional toner particles, typically in the form of a concentrated solution or suspension, to the circulating toner solution. As a consequence, in the manual prior art method of addition, printout quality frequently varied. Further, in high speed electrographic printing wherein printing speed in excess of 1,000 lines per second commonly occurs, the operator would frequently have to add toner concentrate to the system. Thus, when the machine operated at a high printout rate with a large amount of printing per page, it was difficult for the operator to maintain a constant print quality because of the frequent need to add toner concentrate to the circulating toner solution.

SUMMARY OF THE INVENTION It is an object of this present invention to provide a printing apparatus for developing latent charged images on a recording medium with a toner solution wherein the concentration of toner particles in the toner solution is continually monitored and controlled so as to guarantee the production of uniformly high quality prints.

It is another object of this invention to provide a printing apparatus which will automatically add a con centrated toner solution to a working toner solution so as to maintain a relatively constant toner particle concentration in the working toner solution.

It is another object of this invention to provide a printing apparatus for producing high quality images wherein light is passed through a toner solution, the resultant light intensity is measured, and this measured light intensity is used as a means for determining when to add toner concentrate to the toner solution.

It is a specific object of this invention to provide a printing apparatus which measures the concentration of toner particles in a toner solution by measuring the intensity of light passed through the toner solution wherein means are employed for providing a constant intensity light source so as to insure an accurate mea surement of the toner particle concentration.

Although a typical toner solution is substantially opaque, it has been discovered that the intensity of light passed through a thin layer of toner solution is inversely proportional to the concentration of toner particles in the toner solution. In other words, a higher concentration of toner particles in the toner solution produces a lower light intensity. This light intensity, I, can be represented by the general formula wherein K and K are constants depending on the type of toner, the intensity of the light source, and the sensors used to sense the light intensity; and C is the concentration of toner particles in the toner solution being measured. This light intensity can be determined by measuring the voltage or current emitted by a conventional photoelectric or solar cell. This current or voltage, in turn, can be electronically processed to provide a measurement of concentration and a command to add toner concentrate to the circulating toner solution when the concentration of toner particles falls below a predetermined value.

In a broad embodiment, therefore, the present invention relates to an improvement in an apparatus for applying a working toner solution comprising a suspension of black or colored toner particles in a volatile carrier to a charged recording material containing a latent image in a toning station to render the latent image visible. The particular improvement of the present invention includes means for continuously measuring the concentration of toner particles in the working toner solution, transfer means for transferring concentrated toning solution from a reservoir to tlie wfirking solution, and operating means for operatiiiitlie transfer means when the toner concentration fa ls b elow a predetermined value as determined by the measuring means and for terminating the operation of the transfer means when the toner concentration returns to the predetermined value.

A preferred apparatus for measuring the toner concentration according to the broad embodiment specified above includes a light source, a transparent chamber spaced-apart from the light source to receive a continuously flowing stream of working toner solution, means for measuring the light intensity from the light source after the light passes through the chamber and means for focusing the light from the light source through the transparent chamber. Preferably, the means for measuring the light intensity is particularly sensitive in the near infrared range; a conventional solar cell having optimum sensitivity in the range of 8500 Angstroms is particularly preferred. The use of light in the infrared region produces particularly accurate, reliable and reproducible results when working with solutions of high opacity, such as a working toner solution. The thickness of the measuring chamber is typically about 0.01 to 0.05 inches thick when dealing with relatively high opacity solutions. However, for different solutions, the width of the chamber can be up to 0.25 inches.

In a more limited embodiment, the apparatus of the present invention further includes means for detecting the amount of light passed to the transparent chamber, means for providing current to the light source and means for adjusting the current to the light source in response to the amount of light passed to the chamber to maintain a predetermined constant light level.

In another limited embodiment, the apparatus of the present invention includes means for detecting the amount of light passed to the chamber from the light source and means for comparing the light intensity on either side of the chamber, as determined by the detection'means, said comparison means providing an accurate measurement of toner concentration irrespective of the output from the light source.

In another specific alternate embodiment of the present invention, the apparatus is equipped with means for measuring the uninterrupted light intensity from the light source at a distance from the light source equal to the distance between the measuring means positioned behind the chamber and the light source. This distance is measured by following the light path from the light source to the specified measuring means. Also included is means for comparing the uninterrupted light inten sity in front of the chamber to the light intensity behind the chamber, thereby providing an accurate measurement of toner concentration irrespective of lamp placement. This embodiment is particularly useful since the dimensions of lamps and filaments within lamps can vary substantially. In a conventional device in which the length of the light paths from the light source to the means for measuring are unequal, the substitution of one lamp for another can vary the ratio of the light intensities received by the means for measuring. When the two means for measuring, however, are equidistant from the light source, any effect due to variations of the lamp dimensions is removed. Typically, the means for measuring uninterrupted light intensity is positioned in front of the chamber and includes means for reflecting of the light from the light source to this light intensity measuring means.

Other objects and embodiments and a more detailed description of the foregoing embodiments will be found in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram of a nonimpact printer employing a preferred embodiment of the present invention which automatically adjusts the concentration of toner particles in the working toning solution;

FIG. 2 is a detailed cross-sectional view of a specific device to measure the concentration of toner particles in a toning solution according to the present invention;

FIG. 3 is a schematic diagram illustrating the operation of the device illustrated in FIG. 2;

FIG. 4 is a schematic diagram illustrating the operation of an alternate device for measuring toner concentration in FIG. 1, wherein an accurate measurement of toner concentration is obtained irrespective of the position of the light source; and

FIGS. 5A and 5B are circuit diagrams of the operating circuit schematically illustrated in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, there is schematically illustrated an apparatus for applying toning solution to a charged electrographic medium. This apparatus includes a storage tank 2 containing a working toning solution which is passed through a line 30, a toner concentration detector 32 and a line 4 to toning station 6. Within detector 32, the bulk of the toning solution passes through

cavities

35 and 37. Within toning station 6, working toning solution is applied to a charged electrographic paper 8 containing a latent image to produce a wet paper 10 containing a visible image. This paper in turn, is dried by application of heat in drying station 14 to produce a dried electrographic paper 16 containing a fixed visible image. The evaporated carrier is removed from drying station 14 via line 18 and recycled back to storage tank 2, or is removed from the process via line 20. The toning solution passed to toning station 6 that is not deposited on electrographic paper 8 is removed from toning station 6 via line 12 and is returned to storage tank 2 with a depleted toner particle concentration by any conventional means, such as a pump (not shown).

To maintain a constant predetermined level of toning particles in the working toning solution, a slip stream from

cavities

35 and 37 that flows through a chamber 64 is continuously monitored. When the concentration of toner particles in the working solution falls below a predetermined value, concentration detector 32 energizes pump 26 via control line 34 and a concentrated toner solution withdrawn from concentrate tank 22 via line 24 is passed to storage tank 2 via line 28.

Pump 26 comprises a plunger P which reciprocates in a cylinder C. Each time plunger P-moves to the right as shown in FIG. 1, a charge of toner concentrate spurts into tank 2 through line 28. Plunger P is moved to the right by conducting a current pulse through a conventional solenoid coil 29. As soon as the current is terminated, plunger P is returned to the normal position shown in FIG. 1 by resilient means not shown. By increasing the repetition rate of the pulses applied to coil 29, the rate at which toner concentrate is added to tank 2 is likewise increased.

The addition of toner concentrate via pump 26 continues until the concentration level of toner particles in the working solution circulated through line 30 reaches a desired value. In a typical embodiment, the concentration level which actuates pump 26 is the same concentration level at which pump 26 is deactivated. The same concentration level or set point can be used for actuating and deactuating pump 26 because of the large inventory of toning solution contained in tank 2 and circulated through a typical non-impact printer apparatus. Actuating pump 26 when the toner concentration level is falling below the desired value does not immediately result in an increased concentration level at toner concentration detector 32. Because of the large inventory of toning solution and the physical placement of the tank 2 relative to toner concentration detector 32, the toner concentration level may actually continue to decline at detector 32 for a time before it begins to increase due to the toner concentrate added at tank 2. It will take a period of time for the concentration of toner particles in line 30 to build up to the desired value after pump 26 has been actuated.

Preferably, toning concentration detector 32 in cludes means to increase the addition rate via pump 26 when the concentration of toning particles in the working solution falls below a second predetermined value less than the desired value. This feature allows the apparatus of the present invention to rapidly compensate for any large imbalances in the concentration of toner particles in the toning solution and to rapidly correct the concentration so as to ensure a constant quality printout.

In a typical embodiment, the working toner solution is maintained at 4.0 :t 0.5 wt. total concentrate, wherein the concentrate contains 17% solids. In other words, the working solution contains about 0.595 to about 0.765 wt. solids. However, the amount of solids in the working solution is a function of the identity of the specific toner solution being used and the degree of clarity desired on the final developed electrographic paper.

Referring to FIG. 2, there is illustrated a preferred toner concentration detector 32. Toner concentration detector 32 comprises an incandescent light bulb 38 containing a filament 40 positioned in light bulb socket 36 located within housing 33. The light from light bulb 38 is directed through

lenses

42 and 44 mounted in lens holder 46, through

glass plates

58 and 60 before impinging on light intensity detector 68. Light intensity detector 48 is positioned immediately in back of forward lens 44 and measures the light intensity that is passed to glass plate 58.

Light intensity detectors

48 and 68 are conventional silicon solar cells of the type manufactured by Sensor Technology, Inc. These cells are short-circuited and the current generated therein is indicative of the light intensity falling upon the detec tor. The current generated in detector 48 is passed via lead 50 to standoff 52 and the current generated in detector 68 passed via lead 72 to terminal 70. The function of these current values in the apparatus of the present invention will be explained in further detail by reference to FIGS. 3 and 4 hereinafter.

The light passing from lens 44 passes through a predetermined distance 54 before contacting front glass plate 58. The spacing between glass plate 58 and lens 44 is adjusted by washers 56. Second glass plate 60 is mounted in fixed end piece 66 and is spaced apart from first glass plate 58 to define a space or chamber 64 having a predetermined thickness. In accordance with the present invention, working toning fluid is removed as a slip stream from

cavities

35 and 37 and is passed through chamber 64 to measure the concentration of toning solids in the toner solution. Seals 62 provides a tight fit between glass plate 58 and housing 33 and between glass plate 60 and end piece 66. O-ring 67 provides a leak proof fit between end piece 66 and housing 33.

In operation, the intensity of the light emitted from light bulb 38 and focused by

lenses

42 and 44 is determined by light intensity detector 48. The light then passes through transparent glass plate 58 and the working fluid contained within chamber 64, thereby reducing the intensity of the light passing through plate 60. The light striking light intensity detector 68 is compared with the original light intensity determined by detector 48 to provide a reliable, accurate measurement of the concentration of toning particles in the toner solution passed to chamber 64.

Referring to FIG. 3, there are illustrated two embodiments for accurately measuring the concentration of toning particles in the toner solution. In one embodiment, to ensure that light bulb 38 continues to emit a light of a predetermined constant intensity, the light intensity is continuously monitored by detector 48 that is connected by

conductors

48A and 48B to an amplifier 78 which forms part of an operating circuit 79. The current produced by detector 48 is amplified by ampli fier 78 to provide a control signal. In one embodiment, this control signal is passed via circuit 80 to current regulator 74 which controls the input of current to light bulb 38 from a current source 76. This configuration provides a constant intensity emission of light from light bulb 38.

In FIG. 3, detector 68 produces a DC current proportional to the intensity of the light that passed through glass plate 58, the toner solution contained in chamber 64 and glass plate 60. Detector 68 is connected to an amplifier 82 by conductors 68A and 68B. The resultant current produced by detector 68 is amplified by current amplifier 82 to produce another control signal on a conductor 84. Conductors 84 and 86 are connected to a control circuit 88 which produces a pulsating voltage that drives plunger P at a predetermined rate when the concentration of the toner particles in the toner decreases below a predetermined value. The embodiment illustrated in FIG. 3 provides a reliable and accurate method of determining toner concentration that is not sensitive to the efficiency of lamp 38.

A preferred form of operating circuit 79, including

amplifiers

78 and 82, as well as control circuit 88, is shown in more detail in FIG. 5, consisting of FIGS. 5A and 5B. In FIG. 5,

detectors

48 and 68 are represented as current generators having

positive terminals

92, 93 and

negative terminals

95, 96. In response to light, the detectors generate an electrical current that flows in the direction of arrows I.

Amplifier 78 comprises a type 747 operational amplifier having an inverting input 102, a noninverting input 104 and an output 105. The operational amplifier is controlled by resistors 106-108, a potentiometer 110 having a slider arm 111 and a capacitor 114, all connected as shown.

Amplifier 82 comprises a type 741 operational amplifier 120, having an inverting input 122, a noninverting input 124 and an output 126. Amplifier is controlled by resistors 128432, potentiometers 134 and 135 having

slider arms

136 and 137, respectively, and capacitors 140143, all connected as shown.

A power supply (not shown) furnishes plus 12 volts DC over conductors 144-147 and minus 12 volts DC over conductors 150-155. A plus volt DC signal is transmitted by the power supply to conductors 158 and 159.

Control circuit 88 comprises a bipolar transistor 166 having a base 168, an emitter 169 and a collector 170. Circuit 88 also includes another bipolar transistor 172 having a base 174, an emitter 175 and a collector 176, as well as a unijunction transistor 180 having a base two 182, a base one 183, and an emitter 184. Circuit 88 is controlled by diodes 188-192, resistors 196-208 and a capacitor 210. A NAND gate 211 supplies pulses to a one shot multivibrator 212 that, in turn, supplies current pulses through conductor 34 to solenoid coil 29. A pump inhibit conductor 215 may be switched to a logical 0 state to prevent the operation of pump 26.

Control circuit 88 is functionally divided into

analog circuits

213, 214; switching circuits 215, 216; and a signal generator 218:

Analog circuit 213 comprises

resistors

196, 197 which algebraically add and scale the amplified signals on output conductors 84 and 86 to produce a switching signal that is transmitted to the base of transistor 172. Analog circuit 214 comprises

resistors

198, 199 which algebraically add and scale the amplified signals on output conductors 84 and 86 to produce another switching signal that is transmitted to the base of transistor 166. Since algebraic addition also includes subtraction,

circuits

213, 214 can accommodate both neg ative and positive voltages on conductors 84 and 86. Since

transistors

166 and 172 are switched from their conductive to their nonconductive states by the application of voltages to their bases near ground potential, the ratio of the voltage on conductor 86 to the voltage on conductor 84 at which transistor 166 and transistor 172 switch state is nearly constant irrespective of changes in the intensity of lamp 38, so that the detection of toner concentration is independent of lamp intensity.

Switching circuit 215 disables signal generator 218 when the concentration of toner particles becomes too great, and switching circuit 216 quadruples the repetition rate of singal generator 218 when the concentration of toner particles becomes too dilute.

A lamp indicator circuit 220 may be used in order to monitor the condition of light bulb 38. The circuit comprises a type 747 operational amplifier 222 having an inverting input 224, a noninverting input 226 and an output 228. Output 228 drives a bulb 231 through an amplifier 229 having its input voltage controlled by a 4.7 volt Zener diode 230. The operational amplifier is also controlled by resistors 232-237 and by capacitors 240-242.

A dilute toner indicator circuit 250 may be used in order to indicate to an operator the condition of the toner concentration. The circuit comprises a flip-flop 252 consisting of

NAND gates

254 and 255. The circuit also includes another NAND gate 258, as well as

diodes

260, 261, resistors 264, 265 and

capacitors

268, 269.

An output conductor 270 is connected through an' amplifier 273 to an indicator bulb 271.

In order to adjust amplifier 78, lamp 38 is turned on the slider arm 111 is moved until amplifier 100 produces a plus 8 volt DC signal on conductor 86. In order to achieve this result, detector 48 should produce 0.16 to 0.01 milliamps of current. In order to adjust amplifier 82, tank 2 is loaded with a toner solution of normal concentration and potentiometer slider arm 136 is moved until amplifier produces a 2 volt DC signal on conductor 84. The adjustment of slider arm 136 will accommodate types of detector 68 capable of producing between 8.0 and 0.4 microamps of current. If detector 68 produces less than 0.1 microamps of current, slider arm 137 also may need to be adjusted.

Because of the reverse polarity connection of

detectors

48 and 68 to

amplifiers

78 and 82, the amplifiers produce amplified signals on conductors 84 and 86 which represent the detector current signals inverted with respect to each other.

When

amplifiers

78 and 82 are adjusted to produce the normal conditions on conductors 84 and 86 described above, the base of transistor 172 is biased near ground potential so that the transistor is switched to its nonconductive state (i.e., turned off). However, the base of transistor 166 is biased slightly above ground potential so that the transistor is turned on and driven into saturation. As a result, diode 189 is forward biased and diode 190 is reversed biased so that capacitor 210 is charged from 12 volt supply conductor 146 through a 270 K resistor 205. The values of resistor 205 and capacitor 210 are arranged so that capacitor 210 charges to the emitter firing voltage of unijunction every 2 seconds. When unijunction 180 fires, current is drawn through emitter 184 and

resistors

207, 208, so that a positive pulse of voltage is transmitted to NAND gate 211, and multivibrator 212 produces a current pulse having a predetermined duration. In this mode of operation, unijunction 180 operates as a pulse oscillator or generator having a period of 2 seconds. As a result, once every 2 seconds, plunger P of pump 26 (FIG. 1) pumps a charge of toner concentrate into tank 2, thereby increasing the concentration of toner particles in the working toner solution. If the concentration of the toner particles in tank 2 increases slightly above the normal concentration, the voltage on conductor 84 increases slightly above minus 2 volts DC, thereby switching transistor 172 to its conductive state. At this point in time, diode 192 is forward biased so that emitter 184 of unijunction 180 is held to a low voltage, thereby preventing the unijunction from firing. In this mode of operation, no signals are transmitted to pump 26 and the concentration of toner particles cannot increase. At this same time, diode 261 is forward biased, thereby transmitting a logical 0 signal to flip-flop 250 over conductor 272. This signal forces NAND gate 255 to produce a logical 1 signal of plus 5 volts on conductor 270. The plus 5 volt signal causes amplifier 273 to light bulb 271, thereby indicating to an operator that the toner particle concentration is within an appropriate range of values.

As toner solution is applied to recording medium 8 in toner station 6, the toner particle concentration in tank 2 decreases, and the voltage on conductor 86 also decreases to minus 2 volts DC. At this point in time, transistor 172 again is switched to its nonconductive state and diode 192 is reverse biased, thereby enabling the production of pulses having a period of 2 seconds from unijunction 180. At the same time, diode 261 is reverse biased so that a logical 1 signal is transmitted to conductor 272. However, a logical 1 signal continues to be produced on conductor 270 and bulb 271 continues to glow, so that an operator will know that the toner particle concentration is adequate.

If the concentration of toner particles in tank 2 continues to decrease below the normal concentration in spite of the operation of generator 218 at 2 second intervals, the voltage on conductor 84 continues to decrease. When the concentration decreases to a too dilute value at which the voltage on conductor 84 is about minus 7 volts DC, transistor 166 is switched to its nonconductive state (i.e., turned off). At this point in time, diode 189 is reverse biased and diode 190 is forward biased to place resistor 204 (having a value of 47 K ohms) in parallel with resistor 205. This mode of operation decreases the time constant of the charging circuit for emitter 184 so that unijunction 180 fires approximately every one-half second. As a result, pump 26 operates 4 times as fast as the normal rate. That is, the plunger operates every half second instead of every 2 seconds.

When transistor 166 is turned off, diode 260 is reverse biased, thereby transmitting a logical 1 signal to NAND gate 258. NAND gate 258, in turn, transmits a logical signal over conductor 274 which causes flipflop 252 to change state, thereby switching conductor 270 to ground potential and turning off bulb 271.

After the 2 stroke per second pumping rate of pump 26 increases the toner particle concentration so that the voltage on conductor 84 increases above minus 7 volts DC, transistor 166 is turned on, thereby decreasing the pump rate to one stroke in 2 seconds. At this point in time, diode 260 is forward biased so that a logical 1 signal is transmitted to conductor 274. However, flipflop 252 continues to hold conductor 270 at ground potential and bulb 271 remains off. Bulb 271 remains off until the toner particle concentration increases slightly above the normal concentration, at which time transistor 172 is turned on in the manner previously described. Bulb 271 exhibits hysteresis due to the operation of flipflop 252 which provides a reliable indication of toner particle concentration.

Lamp indicator circuit 220 is adjusted so that output 228 is at ground potential and bulb 231 is turned off when lamp 38 has at least average intensity. However, if the intensity of lamp 38 decreases below a predetermined value, the voltage on conductor 86 will decrease. When the voltage reaches about 4.7 volts, amplifier 222 produces a plus 4.7 volt signal onoutput 228. The signal causes amplifier 229 to turn on lamp 231, thereby indicating that lamp 38 requires replacement.

Resistors

234 and 235 provide about 0.21 volts of the hysteresis to prevent flickering of lamp 231 when conductor 86 slowly changes voltage near the plus 4.7 volt level.

The embodiment illustrated in FIG. 3 is sensitive to the position of filament 40 relative to

detectors

48 and 68. As can be seen in FIG. 3,

detectors

48 and 68 are each positioned at different distances from filament 40. Although the socket for light bulb 38 is fixed, the position of the filament within the light bulb can vary. If such variance regularly occurs,

detectors

48 and 68 would have to be recalibrated each time light bulb 38 is replaced. By use of the apparatus illustrated in FIG. 4, these deficiencies are avoided. In particular, detector 48 is positioned at the same distance from filament 40 as detector 68 as measured along the light path from filament 40. More particularly, a portion of the light passing through lens 42 is deflected downward by a beam splitter 90 onto detector 48. However, since the distance from the center point 92 of beam splitter 90 to detector 48 is the same as the distance from center point 92 to detector 68, the light which reaches detector 48 must pass the same distance as the light that 10 reaches detector 68. Accordingly, the apparatus schematically illustrated in FIG. 4 is a further improvement over the apparatus illustrated in FIG. 3 in that light bulb 38 can be changed and an accurate measurement of toner concentration obtained irrespective of the position of filament 40.

Those skilled in the art will recognize that the specific embodiments described herein may be altered and modified without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In a printing apparatus for rendering visible a latent electrostatic image carried on a recording medium by applying to the medium at a toning station a working toner solution comprising a suspension of toner particles in a volatile carrier, improved means for maintaining a predetermined concentration of toner particles in the working toner solution comprising, in combination:

a reservoir of toner particles;

measuring means for continuously measuring the concentration of toner particles in the working toner solution, said measuring means including a light source,

a transparent chamber spaced apart from the light source to receive a continuously flowing stream of working toner solution,

first light detection means sensitive to light in the near infrared region of the light spectrum for measuring light intensity from the light source passing through the chamber, and means for transmitting light from the light source through the transparent chamber to the first light detection means along a first path having a first predetermined length;

second light detection means for detecting the intensity of light passed to said chamber from said light source;

means for providing current to the light source;

means for adjusting the current to the light source in response to said second light detection means to maintain a predetermined constant light intensity from the light source;

transfer means for transferring toner particles from the reservoir to the working toner solution at a predetermined rate; and

operationmeans responsive to the measuring means for operating the transfer means when the toner particle concentration decreases below a first predetermined value and for terminating the operation of the transfer means when the toner particle concentration increases to the first predetermined value.

2. In a printing apparatus for rendering visible a latent electrostatic image carried on a recording medium by applying to the medium at a toning station a working toner solution comprising a suspension of toner particles in a volatile carrier, improved means for maintaining a predetermined concentration of toner particles in the working toner solution comprising in combination:

a reservoir of toner particles;

measuring means for continuously measuring the concentration of toner particles in the working toner solution, said measuring means comprising a light source, a transparent chamber holding a portion of the working toner solution, first light detection means having a positive terminal and a negative terminal for generating a first current signal flowing from the negative terminal to the positive terminal in response to light, first positioning means for positioning the first light detection means to receive light from the light source that is passed through the chamber, second light detection means having a positive terminal and a negative terminal for generating a second current signal flowing from the negative terminal to the positive terminal in response to light, and second positioning means for positioning the second light detection means to receive light from the light source before the light has passed through the working toner solution;

operating means responsive to the measuring means for operating the transfer means when the toner particle concentration decreases below a first predetermined value and for terminating the operation of the transfer means when the toner particle concentration increases to the first predetermined value, said operating means comprising amplifier means having a first output and a second output for amplifying and inverting the first current signal with respect to the second current signal to produce a first amplified signal having a first voltage on the first output corresponding to the first current signal and to produce a second amplified signal having a second voltage on the second output corresponding to the second current signal so that the ratio of the first voltage to the second voltage remains constant irrespective of the changes in the intensity of the light produced by the light source, said amplifier means comprising a first operational amplifier having an inverting input, a noninverting input and an output comprising said first output; a

means for operatively connecting the positive termi- 1 nal of the first light detection means to the noninverting input of the first operational amplifier and for operatively connecting the negative terminal of the first light detection means to the inverting input of the first operational amplifier;

a second operational amplifier having an inverting input, a noninverting input and an output comprising said second output; and

means for operatively connecting the positive terminal of the second light detecting means to the inverting input of the second operational amplifier and for operatively connecting the negative tenninal of the second light detecting means to the noninverting input of the second operational amplifier.

3. In a printing apparatus for rendering visible a latent electrostatic image carried on a recording medium by applying to the medium at a toning station a working toner solution comprising a suspension of toner particles in a volatile carrier, improved means for maintaining a predetermined concentration of toner particles in the working toner solution comprising in combination:

a reservoir of toner particles;

transfer means for transferring toner particles from the reservoir to the working toner solution at a predetermined rate; and 7 operating means responsive to the measuring means for operating the transfer means when the toner particle concentration decreases below a first predetermined value and for terminating the operation of the transfer means when the toner particle concentration increases to the first predetermined value, said operating means comprising amplifier means having a first output and a second output for amplifying and inverting the first current signal with respect to the second current signal to produce a first amplified signal having a first voltage on the first outputcorresponding to the first current signal and to produce a second amplified signal having a second voltage on the second output corresponding to the second current signal so that the ratio of the first voltage to the second voltage remains constant irrespective of the changes in the intensity of the light produced by the light source,

first analog means for algebraically adding and sealing the first amplified signal and the second amplified signal to produce a first switching voltage which varies with the toner particle concentration and which has a first predetermined voltage value corresponding to said first predetermined value of the toner particle concentration;

signal generating means for operating the transfer means; and

first switching means for enabling the signal generating means in response to a first switching voltage less than the first predetermined voltage value and for disabling the signal generating in response to a first switching voltage greater than the first predetermined voltage value..

4. An apparatus according to claim 3 wherein the operating means further comprises:

second analog means for algebraically adding and scaling the first amplified signal and the second amplified signal'to produce a second switching signal having a voltage which varies with the toner particle concentration and which has a second predetermined voltage value corresponding to a second predetermined value of the toner particle concentration less than the first predetermined value of toner particle concentration; and

second switching means for altering the signal generating means to operate the transfer means at an increased rate in response to a second predetermined voltage less than the second predetermined voltage value. I

5. An apparatus according to claim 4 wherein the first analog means comprises a first resistor connected to h fi t Output and a Second resistor Connected to first switching means comprises a transistor connected between the first analog means and the control gate.

the Second Output 8. An apparatus according to claim 7 wherein the 6. An apparatus according to claim 4 wherein the second switching means comprises means for IHCI'EQS' signal generating means Comprises a pulse generator 5 ing the predetermined repetition rate of the pulses produced by the pulse generator. having a control gate for producing pulses at a predetermined repetition rate.

7. An apparatus according to claim 6 wherein the

Claims

1. In a printing apparatus for rendering visible a latent electrostatic image carried on a recording medium by applying to the medium at a toning station a working toner solution comprising a suspension of toner particles in a volatile carrier, improved means for maintaining a predetermined concentration of toner particles in the working toner solution comprising, in combination: a reservoir of toner particles; measuring means for continuously measuring the concentration of toner particles in the working toner solution, said measuring means including a light source, a transparent chamber spaced apart from the light source to receive a continuously flowing stream of working toner solution, first light detection means sensitive to light in the near infrared region of the light spectrum for measuring light intensity from the light source passing through the chamber, and means for transmitting light from the light source through the transparent chamber to the first light detection means along a first path having a first predetermined length; second light detection means for detecting the intensity of light passed to said chamber from said light source; means for providing current to the light source; means for adjusting the current to the light source in response to said second light detection means to maintain a predetermined constant light intensity from the light source; transfer means for transferring toner particles from the reservoir to the working toner solution at a predetermined rate; and operation means responsive to the measuring means for operating the transfer means when the toner particle concentratiOn decreases below a first predetermined value and for terminating the operation of the transfer means when the toner particle concentration increases to the first predetermined value.

2. In a printing apparatus for rendering visible a latent electrostatic image carried on a recording medium by applying to the medium at a toning station a working toner solution comprising a suspension of toner particles in a volatile carrier, improved means for maintaining a predetermined concentration of toner particles in the working toner solution comprising in combination: a reservoir of toner particles; measuring means for continuously measuring the concentration of toner particles in the working toner solution, said measuring means comprising a light source, a transparent chamber holding a portion of the working toner solution, first light detection means having a positive terminal and a negative terminal for generating a first current signal flowing from the negative terminal to the positive terminal in response to light, first positioning means for positioning the first light detection means to receive light from the light source that is passed through the chamber, second light detection means having a positive terminal and a negative terminal for generating a second current signal flowing from the negative terminal to the positive terminal in response to light, and second positioning means for positioning the second light detection means to receive light from the light source before the light has passed through the working toner solution; transfer means for transferring toner particles from the reservoir to the working toner solution at a predetermined rate; and operating means responsive to the measuring means for operating the transfer means when the toner particle concentration decreases below a first predetermined value and for terminating the operation of the transfer means when the toner particle concentration increases to the first predetermined value, said operating means comprising amplifier means having a first output and a second output for amplifying and inverting the first current signal with respect to the second current signal to produce a first amplified signal having a first voltage on the first output corresponding to the first current signal and to produce a second amplified signal having a second voltage on the second output corresponding to the second current signal so that the ratio of the first voltage to the second voltage remains constant irrespective of the changes in the intensity of the light produced by the light source, said amplifier means comprising a first operational amplifier having an inverting input, a noninverting input and an output comprising said first output; means for operatively connecting the positive terminal of the first light detection means to the noninverting input of the first operational amplifier and for operatively connecting the negative terminal of the first light detection means to the inverting input of the first operational amplifier; a second operational amplifier having an inverting input, a noninverting input and an output comprising said second output; and means for operatively connecting the positive terminal of the second light detecting means to the inverting input of the second operational amplifier and for operatively connecting the negative terminal of the second light detecting means to the noninverting input of the second operational amplifier.

3. In a printing apparatus for rendering visible a latent electrostatic image carried on a recording medium by applying to the medium at a toning station a working toner solution comprising a suspension of toner particles in a volatile carrier, improved means for maintaining a predetermined concentration of toner particles in the working toner solution comprising in combination: a reservoir of toner particles; measuring means for continuously measuring the concentration of toner particles in the working toner solution, said measuring means comprising a light source, a transparent chamber holding a portion of the working toner solution, first light detection means having a positive terminal and a negative terminal for generating a first current signal flowing from the negative terminal to the positive terminal in response to light, first positioning means for positioning the first light detection means to receive light from the light source that is passed through the chamber, second light detection means having a positive terminal and a negative terminal for generating a second current signal flowing from the negative terminal to the positive terminal in response to light, and second positioning means for positioning the second light detection means to receive light from the light source before the light has passed through the working toner solution; transfer means for transferring toner particles from the reservoir to the working toner solution at a predetermined rate; and operating means responsive to the measuring means for operating the transfer means when the toner particle concentration decreases below a first predetermined value and for terminating the operation of the transfer means when the toner particle concentration increases to the first predetermined value, said operating means comprising amplifier means having a first output and a second output for amplifying and inverting the first current signal with respect to the second current signal to produce a first amplified signal having a first voltage on the first output corresponding to the first current signal and to produce a second amplified signal having a second voltage on the second output corresponding to the second current signal so that the ratio of the first voltage to the second voltage remains constant irrespective of the changes in the intensity of the light produced by the light source, first analog means for algebraically adding and scaling the first amplified signal and the second amplified signal to produce a first switching voltage which varies with the toner particle concentration and which has a first predetermined voltage value corresponding to said first predetermined value of the toner particle concentration; signal generating means for operating the transfer means; and first switching means for enabling the signal generating means in response to a first switching voltage less than the first predetermined voltage value and for disabling the signal generating in response to a first switching voltage greater than the first predetermined voltage value.

4. An apparatus according to claim 3 wherein the operating means further comprises: second analog means for algebraically adding and scaling the first amplified signal and the second amplified signal to produce a second switching signal having a voltage which varies with the toner particle concentration and which has a second predetermined voltage value corresponding to a second predetermined value of the toner particle concentration less than the first predetermined value of toner particle concentration; and second switching means for altering the signal generating means to operate the transfer means at an increased rate in response to a second predetermined voltage less than the second predetermined voltage value.

5. An apparatus according to claim 4 wherein the first analog means comprises a first resistor connected to the first output and a second resistor connected to the second output.

6. An apparatus according to claim 4 wherein the signal generating means comprises a pulse generator having a control gate for producing pulses at a predetermined repetition rate.

7. An apparatus according to claim 6 wherein the first switching means comprises a transistor connected between the first analog means and the control gate.

8. An apparatus according to claim 7 wherein the second switching means comprises means for increasing the predetermined repetition rate of the pulses produced by the pulse generator.