US3830401A

US3830401A - Toner concentration monitoring apparatus

Info

Publication number: US3830401A
Application number: US00384670A
Authority: US
Inventors: H Siebenrock; B Benwood; T Morse
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1971-12-13
Filing date: 1973-08-01
Publication date: 1974-08-20
Anticipated expiration: 1991-08-20

Abstract

Apparatus for continuously monitoring the concentration of toner in an electrographic developer mixture carried on a development mechanism by sensing the reflectivity of the mixture. A source of radiant energy, periodically energized at a selected frequency, is directed at the developer mixture and the reflectance thereof is monitored by a photoelectric transducer which produces a first output signal representative of the intensity of such reflectance. A second photoelectric transducer illuminated directly by the source produces a second output signal representative of the intensity of the radiation emanating from the source as modulated by the surrounding environment. The first and second output signals of the two transducers are coupled to high pass filters which eliminate signals generated by the transducers representative of ambient light and electrical noise. The a.c. output signals from the filters are converted to d.c. signals corresponding to the first and second output signals of the transducers, which d.c. signals are used to control the concentration of toner in the mixture in two alternative ways. In one embodiment, the d.c. signals are fed to a mechanism which produces a control signal proportional to the ratio of their amplitudes, which ratio is proportional to the proportion of the mixed materials. In a second embodiment, a toner replenisher is activated in response to a predetermined change in amplitude of the first d.c. signal. The second d.c. signal is used in a feedback loop for stabilizing the output of the radiation source.

Description

United States Patent 1191 Benwood et al.

[ TONER CONCENTRATION MONITORING APPARATUS [75] Inventors: Bruce R. Benwood, Spencerport;

Theodore H. Morse; Howard D. Siebenrock, both of Rochester, all of N.Y.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

221 Filed: Aug. 1,1973.

21 App1.No.:384,670

Related U.S. Application Data [63] Continuation-impart of Ser. No. 207,226, Dec. 13,

1971', abandoned.

[52] US. Cl 222/57, 222/DIG. 1, 118/637, 250/205 [51] Int. Cl B67d 5/08 [58] Field of Search 222/52, 57, 59, 76, DIG. 1; 118/637; 250/205, 211, 212, 217 R, 219 SS,

[451 Aug. 20, 1974 [5 7 ABSTRACT Apparatus for continuously monitoring the concentration of toner in an electrographic developer mixture carried on a developmcnt'mechanism by sensing the reflectivity of the mixture. A source of radiant energy, periodically energized at a selected frequency, is directed at the developer mixture and the reflectance thereof is monitored by a photoelectric transducer which produces a first output signal representative of the intensity of such reflectance.

A second photoelectric transducer illuminated directly by the source produces a second output signal representative of the intensity of the radiation emanating from the source as modulated by the surrounding environment. The first and second output signals of the two transducers are coupled to high pass filters which eliminate signals generated by the transducers representative of ambient light and electrical noise. The ac. output signals from the filters are converted to d.c. signals corresponding to the first 220 C, 574 and second output signals of the transducers, which d.c. signals are used to control the concentration of References Cited toner in the mixture in two alternative ways. In one UNITED STATES PATENTS embodiment, the dc. signals are fed to a mechanism 3,053,985 9/1962 Grammer et al 2501212 which Produces a Control Signal Proportional to the 3,233,781 2/1966 Grubbs 222/57 ratio of their amplitudes, which ratio is Proportional to 3,413,480 11/1968 Biard etal. 250/211 the proportion of the mixed materials. In a second 3,495,089 2/1970 Brown 250/226 X embodiment, a toner replenisher is activated in 3.610.205 10/1971 y 222/57 X response to a predetermined change in amplitude of 3,736,431 5/1973 Childs 250/205 h fi d SignaL h second Signal is used in a feedback loop for stabilizing the output of the Primary Examiner-Robert B. Reeves radiation Source Assistant Examiner-Joseph J. Rolla Attorney, Agent, or FirmDouglas 1. Hague 11 Clalms, 5 Drawlng Flgul'es l H W [1 H5 ran 15/ TONE; I? M. DETECTOR REPLE/V/SHER PAIENTEumszoasu -suzmors k f l 3 3$ 953% 95? SM Nu PATENIEQ ms 2 0 1914 FIG. 2

DETECTOR REPLE/V/SHER F/LTER GENERATOR PEAK DETECTOR TONER CONCENTRATION MONITORING APPARATUS CROSS-REFERENCE TO RELATED APPLICATIONS Reference is made to commonly assigned US Pat. application Ser. No. 207,226, now abandoned, entitled TONER CONCENTRATION MONITORING APPA- RATUS, filed on Dec. 13, 1971, in the names of Bruce R. Benwood, Theodore H. Morse and Howard D. Siebenrock of which the present application is a continuation in part.

BACKGROUND OF THE INVENTION The present application is a continuation-in-part of US. Pat. Ser. No. 207,226, now abandon, filed on Dec. 13, 1971 in the names of Bruce R. Benwood, Theodore H. Morse and Howard D. Siebenrock.

This invention relates to electrographic development apparatus for controlling the concentration of electroscopic toner particles in an electrographic developer. More specifically, this invention relates to improvements in toner concentration monitoring apparatus of the type which sense toner concentration by sensing variations in reflectivity of the developer.

In the electrographic reproduction process, the surface of a radiation-sensitive plate, generally comprising a layer of photoconductive material disposed on a conductive backing, is given a uniform electrostatic charge and is then imagewise exposed to a pattern of actinic radiation corresponding to the indicia on a document or the like being reproduced. Such exposure serves to selectively dissipate the uniform charge on the surface, leaving behind a latent electrostatic image which can then be developed by contacting it with an electrographic developer.

In general, electrographic developers comprise a mixture of suitably pigmented or dyed resin-based electroscopic particles, known as toner, and a granular carrier material which functions to carry such toner by generating triboelectric charges thereon. As mentioned above, development of the latent electrostatic image occurs when the developer mixture is brought into contact with the electrostatic image-bearing surface. Such contact is commonly effected by either cascading the mixture over such surface or, as is becoming increasingly prevalent, subjecting the surface to the periphery of one or more rotating magnetic development brushes, the bristles of which comprise chain-like arrays of toner-coated carrier particles. Upon contacting the electrostatic image-bearing surface, the toner particles, being charged to a polarity opposite to that of the electrostatic image, are separated from the carrier particles and are selectively deposited on the surface to form a developed or toner image which may thereafter be transferred to a paper receiving sheet and fixed thereto by any suitable means, such as heat, to form a copy of the original document.

Obviously, as toner images are repetitively formed, toner particles are continuously depleted from the developing mixture, requiring subsequent replenishment to avoid a gradual reduction in image density.

To avoid the necessity of manual replenishment and the operating difficulties often encountered as a result of overreplenishment, a variety of devices has been heretofore proposed for automatically replenishing toner particles after a predetermined number of copies are made or, alternatively, after the concentration of toner particles in the developer mixture drops below a predetermined level. One such device, disclosed in US. Pat. No. 3,233,781, issued to W. J. Grubbs, utilizes the difference in reflectivity exhibited by toner and carrier particles as a means for monitoring the concentration of toner particles in the developer mixture. Toner particles, usually being black and possessing highly absorbing surfaces, reflect less radiant energy than the carrier particles. Thus, the reflectivity of the developer mixture depends upon the relative proportions of the mixed particles, the higher the concentration of carrier particles, the higher the reflectivity of the mixture, and vice versa. According to the Grubbs disclosure, the reflectance of the developer mixture is monitoring by directing energy emanating from an incandescent lamp toward the mixture and detecting the energy reflected by the mixture with a photoconductive cell. Such photocell, together with a similar photocell which is illuminated directly by. the lamp and thereby acts as a reference signal, is employed as a variable resistance arm of a bridge circuit which is capable of activating a toner replenishing device in response to a predetermined change in the ratio of photocell outputs, such change being characterized by an unbalance in the circuit.

While operable, photoelectric toner monitoring devices of the type described above have not proven entirely satisfactory in operation, especially over extended periods of time. A principal cause of unsatisfactory performance is the airborne toner particles circulating in the environment in which the monitoring apparatus is employed. These airborne toner particles accumulate on the external surface of the various components of the monitoring apparatus, including the lamp, thereby gradually reducing the quantity of light received by the detecting photocell. The low level of received energy causes the photocell to produce a false control signal indicating a greater concentration of toner than is actually present in the mixture. To assure freedom from erroneous measurements of this type, such monitoring apparatus requires frequent cleaning maintenance.

Other causes of difficulty can be attributed, at least in part, to the incandescent lamp used for illuminating the development mixture, a luminous energy source of relatively low output, unstable intensity and short life, and the fact that spectral characteristics of the lamp are closely akin to the background light, thereby making background discrimination difficult. The reference photocell and bridge circuit of the Grubbs apparatus provide a means to compensate for fluctuations in the intensity of the output of the lamp itself. The reference photocell and bridge circuit cannot, however, compensate for the gradually decreasing output detection caused by toner particles accumulating on its exterior surface as discussed in the preceding paragraph.

Detection of ambient light is partially controlled in the Grubbs apparatus by enclosing the reference photocell and lamp within a housing. The problem is not eliminated, however, because the detecting photocell is not similarly enclosed and will, therefore, sense ambient light present in the development mechanism.

Another source of uncompensated error in the Grubbs apparatus is the random electrical perturbations, commonly called noise, which are received and generated by both the detecting and reference photocells.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to improve photoelectric devices adapted for monitoring and maintaining substantially constant the relative proportion of mixed materials having divergent reflectance characteristics.

Another object of the invention is to provide an optical electrographic toner monitoring apparatus which is insensitive to ambient environmental conditions.

These and other objects of the invention are achieved by the provision of a substantially improved apparatus for monitoring, during the operation of an electrographic copier, the reflectivity of the developer used therein.

Developer reflectance monitoring is accomplished by directing radiation emanating from a source, periodically energized at a selected frequency, at a portion of the developer wherein the toner concentration is characteristic of that of the overall developer mixture. A first photoelectric transducer is positioned to be irradiated by the pulsed radiation reflected from the developer mixture and provides an output signal representative of the intensity of the reflected radiation.

A second photoelectric transducer, positioned to receive directly the radiation from the source, provides an output signal representative of the intensity of the emitted radiation as modulated by the surrounding environment. In one embodiment, the outputs of both photoelectric transducers, after being passed through amplifiers, high pass filters and peak detectors serve as the inputs to a divider network, the output of which is representative of the ratio of the transducer outputs and can be used as a control signal to activate a toner replenishment device when the amplitude of the control signal is outside a preset range. Alternatively, the second transducer and associated amplifier, filter and peak detector are used as a feedback loop that corrects and maintains the intensity of the output of the radiation source at a constant value. The outputs from the first transducer, amplifier, fllter and peak detector which are inversely proportional to the toner concentration in the developer mixture, assuming the toner particles have a higher optical reflection density than the carrier particles, are used to activate the toner replenisher whenever the amplitude of such signals rises to a predetermined value.

The apparatus of the invention has been found to be particularly well adapted for use with conventional magnetic brush developing assemblies, although it is contemplated that such apparatus can be adapted for use with other types of developing assemblies, such as the cascade type.

The objects and various advantages of the present invention can best be understood from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation view of an automatic electrographic copier wherein the present invention has utility;

FIG. 2 is a plan view of a magnetic brush development station embodying the invention, including a block diagram of toner concentration monitoring apparatus according to a preferred embodiment;

FIG. 3 is a cross sectional view of the magnetic brush developing station depicted in FIG. 2 taken along the section line 33;

FIG. 4 is a detailed electrical schematic of the circuitry depicted in FIG. 2; and

FIG. 5 is a block diagram of another embodiment of the toner concentration monitoring apparatus of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To assist the understanding of the present invention, the operation of an electrographic copying machine in which the invention may be used will be briefly described. It is to be understood, however, that the apparatus of the present invention could be used with equal facility and advantage in other copying machines and, therefore, that the following description of apparatus related to but not forming part of the invention is pro vided for illustrative purposes only.

Referring now to the drawings and in particular to FIG. 1, an electrophotographic copying machine wherein the invention is particularly useful is shown to comprise an endless photoconductive belt 2, driven about rollers 3 and 4 along a predetermined path by motor 5 which is operably coupled with one of the rollers. Disposed along the path are the various electrophotographic stations which serve to form a toner image of the document being reproduced on the outer surface of belt 2. As belt 2 passes charging station 6, its outer surface receives a uniform electrostatic charge from a corona source or the like. Upon being uniformly charged, the belt is advanced past an exposure station 7 where it is imagewise exposed to actinic radiation in accordance with the light and dark areas of the original document. Such imagewise exposure serves to selectively dissipate the uniform charge on the belt to form an electrostatic latent image corresponding to the indi cia on the original document. Development of the electrostatic image is accomplished as belt 2 is advanced past development station 8. The latter generally comprises a reservoir for containing an electrographic developer, and means for applying the developer to the electrostatic image so as to render the image visible. By employing appropriately charged toner particles, it is possible to produce a positive or negative toner image of the original document. In order to reuse that portion of the photoconductive belt bearing the toner image, the toner image is transferred to a paper receiving sheet 11 on which the toner image can be permanently fused. Such a transfer is commonly effected by a paper feeding device 12 which feeds sheets of paper from a paper supply 13 to a transfer station 14 simultaneously with the passage therepast of the toner image bearing belt 2. A shift register R serves to control the timing of the electrophotographic operations and to synchronize the feeding of the paper receiving sheets with the movement of the photoconductive belt. The shift register R includes a rotatable segmented and slotted cylinder 20 which is driven by suitable means, such as belt 22 ex tending from a pulley around roller 3 so that movement of the shift register is in direct response to movement of the photoconductive belt 2.

Transfer station 14 commonly comprises means for electrostatically charging the paper receiving sheet so as to attract toner particles from belt 2 thereto. After the toner image is transferred to the paper receiving sheet, the sheet is peeled away from the belt as the latter passes over small roller 4. The toner-bearing receiving sheet is then attracted by endless mesh belt transport 24, traveling about rollers 25 in a clockwise direction and at the same speed as belt 2, and is advanced thereby past a fusing station 30 where the toner image is permanentized. The paper receiving sheet, with its toner-bearing surface facing downward, is caused to adhere to transport by a source of negative pressure on the rear surface of the lower leg of the transport. After fusing, the receiving sheet is dropped in a receptacle 31.

Referring now to FIGS. 2 and 3, the developing station 8 of the electrophotographic apparatus described hereinabove may, for instance, be of the magnetic brush variety, commonly comprising a trough 50 for containing an electrographic developer 51, and a pair of conventional magnetic development brushes 52 and 53 for applying the developer to the electrostatic image-bearing surface of belt 2. Each brush generally includes a rotatably mounted aluminum cylinder 54 having at least one magnetic pole piece 55, shown in FIG. 3, interiorly disposed in a fixed position along the longitudinal axis thereof. As previously mentioned, electrographic developer 51 generally comprises a mixture of toner and carrier particles which adhere to each other under the influence of triboelectric forces. In magnetic brush development, the carrier particles are fabricated from a magnetically attractable material so as to be attracted to the surface of cylinder 54 by the magnetic field produced by the internal pole pieces 55 to form chain-like arrays 57 which resemble the bristles of a brush. A pair of rotating mixing augers 58 and 59 serve to continously circulate the developer laterally through trough 50 and maintain the relative concentrations of the developer components substantially constant throughout the trough. Typically, drive means (not shown) are provided for rotating the development brushes in a direction such that the movement of the brushes in the vicinity of contact with belt 2 is in a direction opposite the direction of travel of belt 2.

' As cylinders 54 rotate, developer 51 collects on the outer surfaces thereof under the influence of the magnetic field produced by the internal pole pieces 55. As

the electrostatic image-bearing surface of belt 2 tangentially contacts the developer-bearing cylinders,

toner is stripped from the carrier particles, due to the stronger electrostatic forces, and selectively deposited on the belt surface to form toner images. As the cylinders continue to rotate, the partially denuded carrier particles used in forming the toner images are moved beyond the influence of pole pieces 55 and fall back into the main body of the developer to be recoated with toner. Obviously, as toner images are continuously formed, the concentration of toner in the development mix gradually diminishes, the ultimate result being toner images of gradually decreasing density.

Turning now to a more specific description of the subject matter of the present invention there is provided improved apparatus to continuously monitor the concentration of toner in the electrographic development mix and provide a signal whereby a conventional toner replenisher is activated to maintain the concentration of toner at or above the level required for high quality copies. Since the carrier and toner components of the development mixture commonly possess divergent reflectance characteristics, toner having a lower reflectivity than the carrier component, the reflectance of the development mix is an accurate measure of the relative concentrations of its two major components. The higher the concentration of the carrier particles, the higher the reflectance of the development mix. Stated otherwise, the developer reflectance is inversely proportional to the toner concentration.

Referring now to FlGS. 2 5 illustrating one embodiment of the invention, monitoring of the reflectance of the development mix 51 is accomplished by directing the output of radiant energy source 60, preferably a light emitting diode, toward a portion of the developer where the concentration is representative of the average toner concentration throughout the development mix. In a magnetic brush development station, the toner concentration on the downstream development brush (i.e., brush 53) between the points where the brush surface energes from the development mix and first contacts the surface of belt 2 is usually characteristic of the average toner concentration. Between such points, the surface of the development brush comprises fresh developer which has not yet been subjected to the localized depletion of toner which results from development of the electrostatic image. Alternatively, toner concentration can be monitored on the upstream development brush (i.e., brush 52) between the points where the brush surface loses contact with the surface of belt 2 and reenters the development mixture. To

agitate the developer carried by the rush before being subjected to the toner monitoring operation.

The use of a light-emitting diode as the source of radiation from which developer reflectance is determined offers the advantages over conventional sources of long-term stability, collimated output, fast response time, compactness and physical durability. Because of its fast response time, the light-emitting diode can be electronically pulsed at electronic speeds (i.e., faster than several hundred Hertz), thereby offering a means for readily discriminating against electrical interference, commonly called noise, and normal environmental lighting which might illuminate the receiving element.

The periodic energization of the light-emitting diode is accomplished by means of a conventional pulse generator 67, comprising for example a unijunction oscillator and pulse shaping multivibrator (not shown), operating at' 1,000 Hertz.

'A lens 62 serves to concentrate the energy reflected from the developer mixture on the brush surface onto the radiation sensitive surface of a photoelectric transducer 61, preferably a silicon photodiode operating in the zero-bias photovoltaic mode. The use of a photodiode as a receiving element offers the advantages over conventional photoconductive and photovoltaic cells of temperature stability and linearity of operation, particularly when used in the photovoltaic zero-bias mode.

When a silicon photodiode is used as the receiving element, a gallium arsenide light-emitting diode is particularly preferred since the wavelength at which thediodes output peaks is in the near infrared spectral region which closely matches the peak spectral response of a silicon photodiode.

The special region wherein developer reflectance is monitored is chosen with availabity and cost requirement of commercial light-emitting diodes and silicon photodetectors in mind; however, the most important requirement is that of choosing the wavelength at which the reflectance disparity between toner and carrier is maximum, thereby producing the greatest system sensitivity possible. Since the reflectance disparity between toner and carrier particles was found to be substantially constant throughout the visible and near infrared spectral regions, efforts were restricted to matching the spectral output of the light-emitting diode with the spectral response of the photodetector, and also to minimizing the sensitivity of the reflectance monitoring apparatus to normal environmental lighting. To maximize the use of available energy, the lightemitting diode 60 and photodiode 61 are preferably arranged side by side along and below the edge of belt 2, as shown in FIG. 3.

The a.c. output of the reflectance monitoring photodiode 61 is fed to

amplifiers

80 and 82 and then to a high pass filter 91 comprising capacitor C4, resistor R18 and diode D8. The filter 91 filters the components of the signals generated by the photodiode 61 which are representative of the electrical interference of the circuitry and the detected level of ambient light. The output of filter 91 is fed to a peak detector 63 which converts its input to a d.c. signal having an amplitude directly proportional to the reflectance of the developer mixture 51 and, hence, inversely proportional to the instantaneous toner concentration. Peak detector 63 also functions to prevent isolated input signals from activating a conventional toner replenisher 64 such as disclosed in US. Pat. No. 3,409,901. Consequently, the detector 63 activates the toner replenisher 64 only when the amplitude of a predetermined number of input signals is outside a preset range.

Since the atmospheric environment wherein the re flectant monitoring apparatus of the invention is employed is normally contaminated by toner particles circulating in the air in the vicinity of the development station 8, there is a tendency for toner to gradually accumulate on the external surfaces of the various elements of the monitoring apparatus, including the lightemitting diode. Such accumulation would, if not compensated for, gradually reduce the output from diode 60, thereby gradually reducing the energy received from photodiode 61. To stabilize the output of the light-emitting 60, an electro-optical feedback loop is provided which serves to gradually adjust the peak-topeak amplitude of the pulsed output of generator 67 as the modulated radiant output of the light-emitting diode gradually changes. Such a feedback loop obviates the necessity of a periodic cleaning operation to maintain the optics of the monitoring apparatus in the same condition as existed at the time of callibration. The feedback looping includes a second photodiode 68 disposed to receive modulated energy directly from light-emitting diode 60, an a.c. amplifier 81, a high pass filter 92, a peak detector 69 and a d.c. amplifier 70. Like reflectance monitoring photodiode 61, the feedback photodiode 68 is preferably operated in the zerobias photovoltaic mode. Similarly, filter 92 serves to filter the components of the signals generated by the photodiode 68 representative of electrical interference and ambient light. Peak detector 69 serves to convert the pulsed a.c. output from the feedback photodiode 68 to a d.c. signal having an amplitude proportional to the peak-to-peak amplitude of such output and to prevent spurious signals from effecting the adjustment of the output of light emitting diode 60. The output of the peak detector 69 is amplified by the d.c. amplifier 70 and is fed back to pulse generator 67 to control the peak-to-peak amplitude of the pulses provided thereby.

By operating the

photodiodes

61 and 68 in the photo voltaic, zero-bias mode, the dark current is exactly zero and the response is perfectly linear. Since the dark current is zero, the photodiodes are practically insensitive to the temperature variations normally associated with the interior of the electrographic copying machine. Thus, temperature-compensation is unnecessary in the toner monitoring apparatus of the invention, unlike the apparatus disclosed in US. Pat. No. 3,399,652 wherein a thermistor is employed to stabilize system sensitivity. However, operation at zero-bias necessitates the use of a high gain, amplifier, such as a conventional operational amplifier. Thus, the outputs of

photodiodes

61 and 68 are applied to the inputs of operational amplifers and 81, respectively, to amplify their respective photocurrent outputs before being peak detected.

In FIG. 4 there is provided an electrical schematic which illustrates preferred circuits for maintaining the output of the light-emitting diode 60 substantially constant and for deriving an electrical signal from the output of photodiode 61 by which the toner replenishing apparatus can be activated. As mentioned above, stabilization of the output of the light-emitting diode 60 is accomplished via an electro-optical feedback which includes silicon photodiode 68 positioned to continuously sample the radiant output of light-emitting diode 60 as modulated by the airborne toner particle cloud surrounding the development brushes 52 and 53. Operation of the photodiode 68 in the zero-bias photovoltaic mode is made possible through the use of operational amplifier 81 which functions as a currenttovoltage transducer. The gain of amplifier 81 is determined by feedback resistor R1. The output of photodiode 68 constitutes a series of current pulses, the frequency and amplitude of which are proportional to the modulation frequency and the environmentally modified irradiation intensity of the light-emitting diode 60, respectively. Operation amplifier 81 serves to amplify and convert such current pulses to voltage pulses of proportional amplitude. The a.c. output of amplifier 81 is coupled' to high pass filter 92 comprising capacitor C1 and diode D2. High pass filter 92 functions to filter the d.c. components of the signals generated by photodiode 68 representative of undesired electrical interference as well as low frequency a.c. signals representative of the level of ambient light detected by the photodiode. The output of high pass filter 92 is fed to peak detecting circuit 69 which comprises diode D1, resistors R1 and R3, and capacitor C2. The d.c. output of the peak detector 69, being directly proportional to the radiant output of the light-emitting diode 60, is subsequently applied to the non-inverting input of .the high gain differential d.c. amplifier 70. A control voltage derived from resistors R4, R5 and R6 is supplied to the inverting output of amplifier 70. The control voltage serves to balance out the peak detector signal voltage so that the high-gain amplifier can be utilized. Variable resistor R6 serves to provide an external control on the operating point of the light-emitting diode 60. The values of resistors R4 and R7 determine the gain of amplifier 70. Resistors R8 and R9 and diode D3 serve to prevent amplifier 70 from saturating on initial startup. The output of amplifier 70 is a very sensitive indicator of the output of the light-emitting diode 60. That is to say, when the output of the light-emitting diode 60 changes slightly, the output of amplifier 70 will reflect this change with an amplification equal to its gain. The output of amplifier 70 is fed back and combined with the input from pulse generator 67. Diodes D4 and D comprise a summing network to combine the feedback voltage and the pulse generator input. This summation serves to adjust the pulse amplitude that appears at the base of transistor Q1, which in turn drives the light emitting diode 60. Resistor R10 is a load resistor for amplifier 70 and the pulse generator. Transistor Q1 amplifies and inverts its pulse train input which is sub sequently capacitively coupled through capacitor C3 to the Darlington current amplifier Q2 and Q3. Resistors R11 and R12 determine the bias level and amplification factor of transistor Q1. Diode D6 provides a discharge path for capacitor C3 through transistor Q1. Resistors R13 and R22 act to control the amplitude of the current pulses supplied to light-emitting diode 60.

Also shown in FIG. 4 is circuitry for processing the output of the reflectance-monitoring photodiode 61. Operational amplifier 80 serves as a current-to-voltage transducer for converting the signal current from the photodiode 61 to a signal voltage. The signal voltage is amplified by voltage amplifier 82. Resistors R14, R15 and R16 are used to set the gain of

amplifiers

80 and 82. Resistor R17 is used to balance the input bias current for amplifier 82. The output of amplifier 82 is then coupled to high pass filter 91 comprising capacitor C4 and diode D8. High pass filter 91 filters the dc. components of the signals generated by photodiode 61 representative of undesired electrical interference as well as low frequency a.c. signals representative of the level of ambient light detected by the photodiode. The output of filter 91 is fed to the peak detector 63 comprising resistors R18 and R19, diode D7, and capacitor C5 which functions to convert its pulsed a.c. input to a do. output and to prevent isolated peak signals from activating the toner replenisher 64. Transistor Q4 and associated biasing resistors R20 and R21 are used as a buffer stage to obtain impedance matching to the toner replenishing circuitry. The output of transistor Q4 is proportional to the instant radiation on the photodiode 61, which in turn is proportional to the reflectivity of the electrographic developer. When the output signal from transistor Q4 exceeds a predetermined level, thereby indicating that the toner concentration has dropped below a desired level, the toner replenisher is activated.

Referring now to FIG. 5, circuitry is diagrammatically illustrated for continuously providing an electrical signal having an amplitude proportional to the ratio of the outputs of

photodiodes

61 and 68. As shown, the output of the light-emitting diode 60 is used to directly illuminate the photodiode 68 through the airborne toner cloud surrounding the development brushes 52 and 53. The ac. output of photodiode 68 is amplified by operational amplifier 81 and filtered by high pass filter 92. The output of filter 92 is then peak detected by peak detector 69, and the dc. output of the peak detector is fed as a reference signal to one of the inputs of a conventional divider network 86. The ac. output of the reflectance-monitoring photodiode 68, upon being amplified by

amplifiers

80 and 82, filtered by high pass filter 91 and peak detected by peak detector 63 serves as the other input to the divider network. Such circuitry obviates the necessity for an optical feedback loop to maintain the output of the light-emitting didode substantially constant. It achieves long-term stability by always normalizing the output of photodiode 61 to the radiant output of the light-emitting diode as modified by the toner cloud in the surrounding atmospheric environment. By this arrangement, any changes in the output of the light-emitting diode 60 and any changes 5 in the density of the toner cloud or in the accumulation of toner particles on the external surfaces of the optical components of the system will be automatically eliminated through the division provided by the divider network. Since this system is completely symmetric, its stability depends on the ratio of the errors in the two photodiodecircuits and on the stability of the divider network. In this mode of operation, any errors in the two detector circuits tend to cancel each other and the system stability has been found to depend primarily on vider network is the Philbrick-Nexus Model 4452 divider module. This device is hermetically sealed and has a temperature coefficient of only 3mvl C for a 25 C shift in temperature, there is less than 1 percent change in a 10 volt output signal. Alternatively, the divider network may comprise a digital computer in which case, of course, the detected amplified and filtered signals of the photodiodes are converted to a digital format prior to processing by the computer. The computer performs the required calculation and outputs a signal to drive the toner replenisher 64.

The reflectance monitoring systems disclosed herein have been found to be extremely sensitive to minute changes in the reflectivity of the mixture being monitored, and exceptionally stable over extended periods of operation. For these reasons, they have been found useful in maintaining the relative proportions of mixed materials substantially constant over extended periods even when the reflectance disparity of such materials is very small. For instance, a change in toner concentration of from 0 to 7 percent (7 percent toner concentration producing total saturation or coverage of the carrier particles with toner) results in a 25 percent change in the reflectivity of the developer mixture. Since, for many developers, it is desirable to maintain the toner concentration within the range of 4.75 to 5.25 percent, it may be appreciated that the system must be sufficiently sensitive to detect changes in the reflectivity of the developer of the order of 1 percent. By taking into account the changes in the density of the atmospheric environment in which the monitoring apparatus is employed as well as the gradual accumulation of mixed materials on the external surfaces of such apparatus, the reflecting monitoring systems have also been found to possess the stability required for maintaining the toner concentration within the required range for several months of continued use.

The invention has been described in detail with reference to preferred embodiments thereof but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

1 claim:

that of the divider network. A particularly useful di-' 1. ln an electrographic machine having a development mechanism which brings a developer mixture of carrier particles and toner particles having divergent reflectance characteristics into contact with a latent electrostatic image, the relative motion between the developer mixture and the latent image creating a cloud of airborne toner particles surrounding the development mechanism and an apparatus for regulating the relative proportion of the carrier and toner particles in the developer mixture, the regulating apparatus including a source of radiant energy directed toward the mixture, a first photo-electric transducer illuminated by the radiation reflected from the developer mixture, a second photoelectric transducer illuminated directly by the radiant energy source, and means responsive to the transducer outputs for producing a control signal representative of the proportion of toner and carrier particles in the mixture, the improvement wherein the second photoelectric transducer is positioned so as to receive radiation from the radiant energy source which passes through the airborne toner cloud and to provide an output signal representative of the intensity of the radiation produced by the radiant energy source as modulated by such toner cloud.

2. Apparatus for monitoring the proportion of a mixture of carrier and toner particles having divergent reflectance characteristics, the apparatus being used in an environment including a circulating airborne cloud of such particles, said apparatus comprising:

a. a radiation source for projecting radiant energy upon a portion of the development mixture;

b. means for periodically energizing said source to produce pulses of radiation at a selected frequency;

c. first photosensitive means positioned to receive pulses of radiation reflected from the development mixture, said first photosensitive means being adapted to provide an output signal representative of the reflectivity of the mixture as modulated by the particle cloud;

d. second photosensitive means positioned to receive pulses of radiation from said source which pass through the particle cloud, said second photosensitive means being adapted to provide an output signal representative of the intensity of the radiation produced by said source as modulated by the particle cloud;

e. first and second filters coupled with the output signals from said first and second photosensitive means for filtering signals of non-selected frequencies; and

f. means coupled to said first and second filters and responsive to the output signals therefrom for producing a signal representative of the proportion of toner particles in the mixture.

3. The invention as defined in claim 2 wherein said source of radiant energy comprises a light emitting diode.

4. The invention as defined in claim 2 further comprising a pair of peak detectors operatively connected to said first and second photosensitive means and adapted to provide d.c. signals proportional to the peak to peak outputs of said first and second photosensitive means.

5. Apparatus for monitoring the proportion of mixed materials having divergent reflectance characteristics, said apparatus comprising:

a. a container for the mixed materials;

b. a luminous diode capable of emitting infrared energy when energized;

c. means for periodically energizing said diode to produce pulses of infrared energy at a selected frequency;

d. means for directing said pulses of infrared energy toward the mixed materials;

e. a first photoelectric transducer responsive to said infrared energy positioned to receive said pulses of infrared energy after reflection by the mixed materials, said first photoelectric transducer being adapted to produce an output signal having an informational content representative of the reflectivity of the mixture as modulated by the interior environment of said container;

a second photoelectric transducer responsive to said infrared energy positioned to receive said pulses of infrared energy directly from said diode, said second photoelectric transducer being adapted to produce an output signal having an informational content representative of the intensity of radiation emitted from said diode as modulated by the interior environment of said container;

g. a pair of high pass filters coupled to the outputs of said first and second photoelectric transducers and adapted to pass signals only of said selected frequency;

h. a pair of peak detectors coupled to the outputs of said first and second photoelectric transducers through said filters, said peak detectors being adapted to provide d.c. signals proportional to the peak to peak outputs from said photoelectric transducers; and

i. means coupled to the outputs of said peak detectors and adapted to provide a signal proportional to the ratio of said peak detector outputs, said ratio being proportional to the proportion of the mixed materials;

6. The invention as defined in claim 5 wherein said means coupled to the outputs of said peak detectors comprises a divider network.

7. Apparatus for monitoring the proportion of a development mixture of carrier and toner particles having divergent reflectance characteristics, the apparatus being used in an environment including a circulating airborne cloud of such particles, said apparatus comprising:

d. a first high pass filter coupled to said first photosensitive means for filtering output signals of nonselected frequencies;

e. means coupled to said first high pass filter for regulating the proportion of the particles of the development mixture in response to a predetermined change in the amplitude of the output signals from said first high pass filters;

f. second photosensitive means positioned to receive pulses of radiation from said source which pass through the particle cloud, said second photosensitive means being adapted to provide an output signal representative of the intensity of the radiation produced by said source as modulated by the particle cloud; and

g. a second high pass filter coupled to said second photosensitive means for filtering output signals of non-selected frequencies, the output signals of said second filter being coupled to said radiation source to maintain the intensity of the radiation output produced by said source at a constant value.

8. The invention as defined in claim 7 further comprising first and second peak detectors coupled to said first and second filters, respectively, and adapted to provide d.c. signals proportional to the outputs of said first and second filters.

9. For use in an electrographic machine in which a surface carrying an electrostatic charge pattern is contacted by the bristles of an electrographic magnetic development brush, the bristles comprising an electrographic development mixture of carrier particles and toner particles having divergent reflectance characteristics, the contact of the charge pattern by the brush bristles producing an airborne toner particle cloud surrounding the development brush, an apparatus for regulating the proportion of such particles in the mixture, said apparatus comprising:

a. a source of radiant energy; b. means for periodically energizing said source to produce pulses of radiation at a selected frequency;

c. means for directing said pulses of radiation emanating from said source toward the development brush to irradiate a portion of the development mixture thereon;

d. a first photoelectric transducer positioned to receive said pulses of radiation after reflection by the development mixture, said first photoelectric transducer being adapted to provide a first output signal representative of the reflectivity of the mixture on the development brush as modulated by the surrounding toner particle cloud;

e. a second photoelectric transducer positioned to receive said pulses of radiation from said source which pass through the toner particle cloud, said second photoelectric transducer being adapted to provided a second output signal representative of the intensity of radiation emanating from said source as modulated by the surrounding toner par ticle cloud;

f. first and second high-pass filters connected to the outputs of said first and second photoelectric transducers, respectively, for filtering signals of nonselected frequencies; and

g. means responsive to said filtered first and second output signals and adapted to produce a third output signal representative of the proportion of toner particles in the mixture.

10. The invention as defined in claim 9 further comprising converter means coupled to said first and second filters and adapted to provide d.c. signals proportional to the outputs of said first and second filters.

11. The invention as defined in claim 9 further comprising means extending into the brush bristles for agitating the development mixture before the mixture is subjected to said pulses of radiation emanating from said source.

i233? v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 3 A Dated August 97 Inventor(s) BenWOOd 6t e11 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

1, line 17, delete "Pat.".

I 2, line 35, "surface" should read ii surfaces Column 6, line 36, "rush" should read -brush-.

Column 0, line 8, 'didode" should read -diode--.

Signed and sealed this- 11th day of February 1975.

(SEAL) I Att St:

e C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 73 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 3 A D d August 20, 197

Inventor(s) BenWOOd et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I- 1| H I q Column 1, llne l7, delete Pat. Column 2, line 35, "surface" should read -I SurfaCeS Column 6, line 36, "rush" should read --brush-. Column 10, line 8, "didode" should read -diode--.

Signed and sealed this 11th day of February 1975-.

(SEAL) I A t t:

t es C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims

1. In an electrographic machine having a development mechanism which brings a developer mixture of carrier particles and toner particles having divergent reflectance characteristics into contact with a latent electrostatic image, the relative motion between the developer mixture and the latent image creating a cloud of airborne toner particles surrounding the development mechanism and an apparatus for regulating the relative proportion of the carrier and toner particles in the developer mixture, the regulating apparatus including a source of radiant energy directed toward the mixture, a first photo-electric transducer illuminated by the radiation reflected from the developer mixture, a second photoelectric transducer illuminated directly by the radiant energy source, and means responsive to the transducer outputs for producing a control signal representative of the proportion of toner and carrier particles in the mixture, the improvement wherein the second photoelectric transducer is positioned so as to receive radiation from the radiant energy source which passes through the airborne toner cloud and to provide an output signal representative of the intensity of the radiation produced by the radiant energy source as modulated by such toner cloud.

2. Apparatus for monitoring the proportion of a mixture of carrier and toner particles having divergent reflectance characteristics, the apparatus being used in an environment including a circulating airborne cloud of such particles, said apparatus comprising: a. a radiation source for projecting radiant energy upon a portion of the development mixture; b. means for periodically energizing said source to produce pulses of radiation at a selected frequency; c. first photosensitive means positioned to receive pulses of radiation reflected from the development mixture, said first photosensitive means being adapted to provide an output signal representative of the reflectivity of the mixture as modulated by the particle cloud; d. second photosensitive means positioned to receive pulses of radiation from said source which pass through the particle cloud, said second photosensitive means being adapted to provide an output signal representative of the intensity of the radiation produced by said source as modulated by the particle cloud; e. first and second filters coupled with the output signals from said first and second photosensitive means for filtering signals of non-selected frequencies; and f. means coupled to said first and second filters and responsive to the output signals therefrom for producing a signal representative of the proportion of toner particles in the mixture.

5. Apparatus for monitoring the proportion of mixed materials having divergent reflectance characteristics, said apparatus comprising: a. a container for the mixed materials; b. a luminous diode capable of emitting infrared energy when energized; c. means for periodically energizing said diode to produce pulses of infrared energy at a selected frequency; d. means for directing said pulses of infrared energy toward the mixed materials; e. a first photoelectric transducer responsive to said infrared energy positioned to receive said pulses of infrared energy after reflection by the mixed materials, said first photoelectric transducer being adapted to produce an output signal having an informational content representative of the reflectivity of the mixture as modulated by the interior environment of said container; f. a second photoelectric transducer responsive to said infrared energy positioned to receive said pulses of infrared energy directly from said diode, said second photoelectric transducer being adapted to produce an output signal having an informational content representative of the intensity of radiation emitted from said diode as modulated by the interior environment of said container; g. a pair of high pass filters coupled to the outputs of said first and second photoelectric transducers and adapted to pass signals only of said selected frequency; h. a pair of peak detectors coupled to the outputs of said first and second photoelectric transducers through said filters, said peak detectors being adapted to provide d.c. signals proportional to the peak to peak outputs from said photoelectric transducers; and i. means coupled to the outputs of said peak detectors and adapted to provide a signal proportional to the ratio of said peak detector outputs, said ratio being proportional to the proportion of the mixed materials;

7. Apparatus for monitoring the proportion of a development mixture of carrier and toner particles having divergent reflectance characteristics, the apparatus being used in an environment including a circulating airborne cloud of such particles, said apparatus comprising: a. a radiation source for projecting radiant energy upon a portion of the development mixture; b. means for periodically energizing said source to produce pulses of radiation at a selected frequency; c. first photosensitive means positioned to receive pulses of radiation reflected from the development mixture, said first photosensitive means being adapted to provide an output signal representative of the reflectivity of the mixture as modulated by the particle cloud; d. a first high pass filter coupled to said first photosensitive means for filtering output signals of non-selected frequencies; e. means coupled to said first high pass filter for regulating the proportion of the particles of the development mixture in response to a predetermined change in the amplitude of the output signals from said first high pass filters; f. second photosensitive means positioned to receive pulses of radiation from said source which pass through the particle cloud, said second photosensitive means being adapted to provide an output signal representative of the intensity of the radiation produced by said source as modulated by the particle cloud; and g. a second high pass filter coupled to said second photosensitive means for filtering output signals of non-selected frequencies, the output signals of said second filter being coupled to said radiation source to maintain the intensity of the radiation output produced by said source at a constant value.

9. For use in an electrographic machine in which a surface carrying an electrostatic charge pattern is contacted by the bristles of an electrographic magnetic development brush, the bristles comprising an electrographic development mixture of carrier particles and toner particles having divergent reflectance characteristics, the contact of the charge pattern by the brush bristles producing an airborne toner particle cloud surrounding the development brush, an apparatus for regulating the proportion of such particles in the mixture, said apparatus comprising: a. a source of radiant energy; b. means for periodically energizing said source to produce pulses of radiation at a selected frequency; c. means for directing said pulses of radiation emanating from said source toward the development brush to irradiate a portion of the development mixture thereon; d. a first photoelectric transducer positioned to receive said pulses of radiation after reflection by the development mixture, said first photoelectric transducer being adapted to provide a first output signal representative of the reflectivity of the mixture on the development brush as modulated by the surrounding toner particle cloud; e. a second photoelectric transducer positioned to receive said pulses of radiation from said source which pass through the toner particle cloud, said second photoelectric transducer being adapted to provided a second output signal representative of the intensity of radiation emanating from said source as modulated by the surrounding toner particle cloud; f. first and second high-pass filters connected to the outputs of said first and second photoelectric transducers, respectively, for filtering signals of non-selected frequencies; and g. means responsive to said filtered first and second output signals and adapted to produce a third output signal representative of the proportion of toner particles in the mixture.