GB2157838A - Electrostatic voltmeter - Google Patents

Abstract

An electrostatic voltmeter (10), which can be used in the image reading device of a multi-mode copier/printer (Figs, 5, 6) comprises a microdeflector probe (13) for measuring the charge on a surface (35), e.g. the photoconductive surface in xerography. The probe comprises a flexible finger (14) cantilevered on a base (16) which is electrically biased to a predetermined potential Vre via connection (17). A sensing electrode (27) on the finger has a portion (33) which can be positioned in spaced relation adjacent surface (35), the capacitive relation thus established creating on the sensing electrode a charge representative of the charge on the surface (35). The potential difference between the finger and the base causes the finger to deflect. This deflection can be converted to a signal representing the charge on the surface (35) e.g. by directing a beam (41) of light and detecting the reflected beam with a detector (45) which can be arranged to receive the reflected light only when the finger undergoes deflection of predetermined amplitude. <IMAGE>

Description

SPECIFICATION Electrostatic voltmeter The invention relates to an electrostatic voltmeter incorporating a microdeflector-based probe for detecting charges on a surface, and further, to an improved multi-function copier/printer employing the aforementioned probe as an image reader.

Electrostatic voltmeters are utilized to measure charge on a suface, as for example the photoconductive surface of a xerographic system. There, it is often desirable to determine the charge on the photoconductive surface at one or more locations in the xerographic process to determine the operating condition of the system, and the need to adjust, service or replace system components. Indeed, in some xerographic system applications, an electrostatic voltmeter is incorporated into and made an integral part of the system with a feed back loop employed to enable automatic resetting of one or more of the system process components in accordance with the charge conditions detected by electrostatic voltmeter.

As will be understood, electrostatic type voltmeters are desirable in that no physical contact with the surface whose charge is being measured is required. In applications such as the xerographic system alluded to above, this is important in preventing damage or scratching of the relatively delicate photoreceptor surface. The electrostatic voltmeter probe, which is spaced opposite the surface whose charge is to be measured and out of contact therewith, operates on the basis of a capacitive relation established with the surface whose charge is being measured, the surface itself forming in effect one plate of a capacitor with the probe sensing electrode the second plate. Circuitry is provided to translate the charge accumulated on the probe electrode to a signal representing the charge detected.

According to a first aspect the invention provides an electrostatic voltmeter for measuring charges on a surface which comprises in combination: a probe having a microdeflector sensing unit, the sensing unit comprising a base, a flexible finger spacedly overlying the base so that a portion of the finger can deflect relative to the base on imposition of a potential thereacross, and a sensing electrode on the finger and movable with the finger on deflection of the finger, the sensing electrode including a side adapted to be spaced opposite and in capacitive relationship to the surface so that a charge representative of the charge on the surface is produced on the sensing electrode to cause deflecting movement of the finger relative to the base; and means for converting deflection of the finger to a signal representing the charge on the surface.

According to a further aspect the invention provides a multifunction copier/printer having a multi-function copier/printer having a photoreceptor and selectively operable in a first copy mode to produce latent electrostatic images from originals on said photoreceptor, in a second write mode to scan a beam of light across said photoreceptor and modulate said beam in accordance with image signals to produce latent electrostatic images represented by said image signals on said photoreceptor, and in a third read mode to scan latent electrostatic images on said photoreceptor and convert said images to images to image signals, including a microdeflector array comprising support means, and a plurality of flexible charge gathering fingers arranged in a linear array on said support means, each of said fingers having an unsupported portion capable of deflecting relative to said support means in response to a charge potential thereon, disposition of said array of flexible fingers in capacitive relation with said photoreceptor and the latent electrostatic images thereon resulting in discrete deflections of said fingers individually in response to the image charge levels on said photoreceptor opposite said fingers, the copier/printer further comprising means for scanning said beam across said array of fingers, collecting means for capturing light reflected from those of said fingers having a predetermined deflection, and means for converting light captured by said collecting means to image signals representative of said latent images on said photoreceptor.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an electrostatic voltmeter incorporating the microdeflector probe in accordance with the present invention; Figure 2 is a view showing a second embodiment of the present invention in which a feed back loop is provided; Figure 3 is an isometric view of a modified sensing electrode for use with the the microdeflector probe to enable the probe to be spacing insensitive; Figure 4 is a logic diagram of an electrostatic voltage follower circuit for operating the probe shown in Fig. 3; Figure 5 is a view showing details of the microdeflector probe modified for use as an image scanner; Figure 6 is a schematic view of an electrostatic copier/printer incorporating the microdeflector probe shown in Fig. 5.

Figure 7 is a schematic view of an alternate duel element microdeflector probe for detecting charges electronically witout reliance on the creation of an optical path between probe and detector; Figure 8 is an enlarged partial isometric view showing details of the dual element microdeflector probe of Fig. 7; Figure 9 is an isometric view of a second laternate dual element microdeflector probe in which compensation for the presence of any extraneous system is provided; and Figure 10 is a circuit schematic for the noise compensating dual element microdeflector probe shown in Fig. 9.

Referring to Fig. 1 of the drawings, an electrostatic voltmeter or ESV, designated generally by the numeral 10, and incorporating the microdeflector probe 1 2 in accordance with the present invention, is there shown. As will be understood by those skilled in the art, ESV's are typically employed to sense the charge level on the photoconductor surface, designated by the numeral 35 herein, of a xerographic type copier or printing machine (not shown). In such copiers or printing machines, the photoconductive surface 35 is uniformly charged to a predetermined level by suitable charging means such as a corotron, and thereafter exposed to the original being copied. The latent electrostatic image created on the photoconductive surface is thereafter developed and the developed image transferred to a copy sheet and fixed.The photoconductive surface 35 is cleaned and charged again to repeat the process.

In copiers or printing machines, it is often desirable or necessary to determine the charge on the photoconductive surface 35 at some point or points during the xerographic process, and for this purpose, the probe of an ESV is placed in predetermined operational relation to the photoconductive surface 35. The charge measured by the ESV may be indicated visually to the user or operator on a meter or scale, or may be input directly to the machine controller for use by the controller in setting or adjusting the operating level of one or more of the xerographic processing components, such as for example the charge corotron.

While the present invention is described in a copier or printing machine environment, the invention is not to be considered limited to these applications, but may be used in any application where the charge level on a surface is to be measured.

The probe 1 2 of ESV 10 employs a microdeflector in the form of a flexible finger 14 disposed in cantilever fashion opposite a recess 1 5 in a rigid base or chip 1 6. Preferably, base 1 6 is formed from silicon while flexible finger 14 comprises silicon dioxide. An electrode 20 may be provided on the surface 1 9 of recess 1 6 opposite finger 14, electrode 20 for example being formed by doping the surface 1 9 of recess 1 5 with boron. Alternately, electrode 20 may be dispensed with and the conductivity of base 1 6 alone relied upon.Base 1 6 (or electrode 20 where used) is coupled to a suitable reference potential (Vref) by line 1 7. As will appear, base 1 6 alone or in combination with electrode 20, forms what is hereinafter referred to as a reference electrode 21.

Microdeflector 1 3 may be formed by reacting one surface of a silicon chip with oxygen to form a silicon dioxide layer of desired depth. Selective etching may be used to remove the silicon material underlying the silicon dioxide layer to provide recess 1 5 and delineate finger 14.

An electrode 27, the outer surface 27' of which provides a mirror-like reflective surface, is provided on finger 14, electrode 27 preferably being formed by coating the outer surface 23 of finger 14 with a suitable metal such as Chromium and Gold. The lower portion of electrode 27 is enlarged at 30 to present a generally rectangular charge sensing or probe surface 33 disposable in charge sensing relation with the photoconductive surface 35 on use or installation of ESV 10. A probe surface 33 of 5 micrometres (0.002 in.) by 5 micrometres (0.002 in.) has been found to be satisfactory. Electrode 27 together with finger 1 4 forms what is hereinafter referred to as a flexible sensing electrode 29 in spaced overlying relation with the aforementioned reference electrode 21.

Microdeflector 1 3 operates on an electrostatic deflection principle, a difference in voltage between reference electrode 21 and sensing electrode 29 creating an electrostatic force which causes the flexible sensing electrode 29 to bend or deflect toward reference electrode 21. Since the degree of beding or deflection that sensing electrode 29 undergoes is dependent upon the voltage differential between electrodes 21 and 29 and the voltage (Vref) on reference electrode 21 is known, the degree of bending or deflection of sensing electrode 29 is a measure of the voltage on electrode 29. Accordingly, the charge on photoconductive surface 35 is a function of the deflection of sensing electrode 29.

To provide a visible indication or readout of the charge on photoconductive surface 35, a suitable light source 40 such as a Light Emitting Diode or LED is provided. LED 40 is located so as to project a beam of light 41 onto the outer reflective surface 27' of sensing electrode 29 proximate the finger unsupported end. A suitable light detector 45, which may for example comprise a photodiode, is positioned to intercept light reflected by electrode 29 when electrode 29 undergoes a predetermined deflection. Preferably, LED 40 and detector 45 are disposed relative to electrode 29 so that detector 45 responds to full deflection of electrode 29.For example, when the charge potential on surface 35 (Vp/r) minus the reference voltage (Vref) is approximately 500 volts and the spacing between probe surface 33 and the photoconductive surface 35 is approximately 1 5 micrometres (0.006 in.), full deflection of electrode 29 (i.e.

approximately 5 ) will occur.

As will be understood, ESV 10, during use, is calibrated so that the deflection of sensing electrode 29 to which detector 45 responds represents a known charge level.

The signal output of detector 45 is fed to line 46 which may be coupled to a suitable visual display such as a meter when a visible reading is desired. Where ESV 10 is used as a component or input to the copier or machine controller, the signal in line 46 is fed to the controller which in turn responds to adjust or reset one or more of the machine processing components such as the charge corotron (not shown) for charging the photoconductive surface 35.

The embodiment shown in Fig. 1 employs a single light detector 45. Where the reference voltage (Vref) is fixed, this arrangement may be used to detect when the charge on photoconductive surface 35 (Vp/r) minus the potential to which reference electrode 21 is biased (Vref.) is greater than a predetermined threshold potential (Vthres.), the latter being determined by the physical location of detector 45. In this case, the threshold level (Vthres.) is determined by the mechanical and electrical parameters of the probe. While for some applications such as image reading or assurance of minimum charging conditions, detection in relation to a threshold may suffice, in other applications it may be desirable to provide an output indicative of the actual magnitude of the charge on photoconductive surface 35 (Vp/r).

In the embodiment shown in Fig. 2, where like numbers refer to like parts, a feed back system for this purpose is shown. There, a second detector 45' is disposed at a predetermined location along the arcuate path followed by light reflected from sensing electrode 29 as electrode 29 undergoes a change in deflection.

The outputs of detectors 45, 45' are coupled by lines 47, 48 to the + and - inputs of a suitable differential amplifier 49. The output of amplifier 49, which represents the difference between the signal inputs from detectors 45, 45' to the + and - inputs thereof, is fed through a suitable resistor 50 to signal integrating circuit 52. Circuit 52 integrates the signal output of amplifier 49 with the reference voltage (Vref) to provide an adjusted reference voltage input to reference electrode 21 and a signal output to line 46.

In operation of the Fig. 2 embodiment, it is presumed that the current charge on the photoconductive surface 35 causes sensing electrode 29 to deflect to a null position where light reflected therefrom falls between detectors 45 and 45'. So long as surface 35 remains at that charge (or within a predetermined range as determined by the operating tolerance of ESV 10), no signal is output by detector 45 or 45'. Should the charge on photoconductive surface 35 undergo a predetermined change, which may be represented by either an increase or decrease in charge potential with corresponding increase or decrease in electrode deflection, light reflected from the surface 37' of sensing electrode 29 will impinge on the appropriate detector 45 or 45'.Amplifier 49 responds to the signal from the responding detector 45 or 45' to output a signal which is integrated by circuit 52 with the reference voltage (Vref) to provide an adjusted reference voltage to reference electrode 21 to restore sensing electrode 29 to the null position. Concurrently, the changed reference voltage is output through line 46 to a suitable meter (not shown) to provide a visual readout of the charge measured on the photoconductive surface 35.

Alternately, the signal in line 46 may be input to the machine controller for use in adjusting one or more of the machine xerographic processing components.

While a detector pair 45, 45' has been shown, it will be understood that additional detectors may be provided at suitable locations along the path of light reflections from sensing electrode 29. In that event, the output circuit arrangement shown for detectors 45, 45' would be suitably modified to accommodate the added inputs. Alternately, detectors 45, 45' may be dispensed with and a single detector having spaced light sensing elements such as a Charge Coupled Device (CCD) used instead.

As understood, the distance at which the probe charge sensing surface 33 is spaced from the photoconductive surface 35 is determinative of the capacitive coupling between probe 12 and photoconductive surface 35. For applications in which spacing independence is desired, probe 1 2 may be modified as shown in Figs. 3 and 4 to provide a modulated sensitive electrode. In this arrangement, the ability of sensing electrode 29 to physically respond to electrostatic forces is taken advantage of to provide the desired mechanical vibratory movement of electrode 29.

Referring to Figs. 3 and 4, where like numbers refer to like parts, sensing electrode 29 is modified by providing a second electrode 61 on finger 1 4 electrically isolated from electrode 20 by border area 64. Second electrode 61 is connected by line 62 to the output of oscillator 63 of voltage follower circuit 60 while the first electrode 20 is connected by line 65 to the preamplifier section 70 of circuit 60. The signal output of oscillator 63 to second electrode 61 causes sensing electrode 29 to oscillate, i.e. deflect toward and away from the photoconductive surface 35 in synchronization with the frequency of the oscillator signal.

A conductive ground plane member or bottom plate 66 having a sensitive aperture 67 is disposed opposite to and in predetermined spaced relation to photoconductive surface 35. Plate 66 is connected by line 68 to the circuit common 69 while the output of pre-amplifier section 70 is coupled to amplifier 73. The output of amplifier 73 is coupled to one input of phase sensitive demodulator 74 with the other input of demodulator 74 coupled to the output of oscillator 63. The output of demodulator 74 is passed through resistor 75 to integrator 76 having an output coupled to ground. A suitable meter 77 is connected across the output of integrator 76 and circuit common 69.

Where for example the charge on photoconductive surface 35 is at some positive potential, an a.c. signal, which varies inversely as the separation between sensing electrode 29 and the photoconductive surface 35 changes and which is proportional to the difference in d.c. potential between surface 35 and circuit common 69, is output by preamplifier section 70, amplified by amplifier 73, and input to demodulator 74. Demodulator 74 develops a positive d.c. voltage proportional to the in phase component of the amplifier output which is fed to integrator 76 causing the output (i.e. ground) to go negative relative to circuit common 69. Accordingly, circuit common 69 and plate 66 are driven positive with respect to ground.

Driving plate 66 positive reduces the signal output of preamplifier section 70 and this process continues until the output of preamplifier section 70 goes to zero corresponding to a zero voltage difference between circuit common 69 and surface 35. As in the Fig. 2 embodiment, the change in potential to circuit common 69 may be read on meter 77.

While a separate driving electrode, i.e. electrode 61, is used to impart oscillation to sensing electrode 29, a vibration source such as a piezo electric transducer, may instead be mechanically coupled to the sensing electrode 29 for this purpose. In this type of arrangement, the vibration source would desirably be at or near the natural resonant frequency of the flexible sensing electrode.

Referring to Figs. 5 and 6 of the drawings, there is shown a xerographic type multi-function copier/printer, designated generally by the number 100, which is operative in a COPY mode to produce copies of a document original, in a WRITE mode to write images on the photoconductive surface of the copier/printer photoreceptor in accordance with an image signal input, and in a READ mode to convert the latent electrostatic image on the photoreceptor to image signals.

For operation in the READ mode, copier/printer 100 utilizes a microdeflector array 102 to detect or read the latent electrostatic image charge pattern on the copier/printer photoreceptor and convert the image charge pattern to image signals representative of the image, the microdeflector array 102 being similar in construction and operating principle to the heretofore described microdeflector ESV probe.

Copier/printer 100 includes a photoreceptor 103 shown here in the form of a drum. Other photoreceptor configurations such as belt or web may however be envisioned. A charge station 104 is provided where photoreceptor 103 is charged in preparation for imaging. For operation in the COPY mode, a light/lens exposure station 106 is provided downstream of charge station 104. There, the charged photoreceptor 103 is exposed to a light image of the document original (not shown) being copied.

For operation in the WRITE mode, a write station 108 is provided downstream of exposure station 106 where photoreceptor 102 is raster scanned by a beam 110 of high intensity electromagnetic radiation derived for example from laser 11 2. Beam 110 is modulated by suitable means (not shown) in accordance with an image signal input, the modulated beam exposing the photoconductive surface of photoreceptor 103 to create the latent electrostatic image represented by the image signal input. For operation in the READ mode, a read station 109 is provided adjacent write station 1 08. There, the latent electrostatic image on the photoconductive surface of photoreceptor 103 is read by array 102 and converted to image signals as will appear.

A developer station 114 is provided downstream of read station 109 where the latent image created on photoreceptor 103 is developed followed by a transfer station 11 6 where the previously developed image is transferred to a copy sheet 11 8. Residual developer materials are removed from photoreceptor 103 at cleaning station 1 20 prior to charging at charge station 104.

Suitable means such as a polygon (not shown) are provided for scanning or sweeping beam 110 across photoreceptor 103, there being suitable optical elements such as a lens (not shown) for focusing beam 110 on photoreceptor 103 at write station 1 08. To enable selected operation in either the READ or WRITE mode, an optical deflector 1 26 is provided in the path of beam 110, deflector 126 when moved to the dotted line position shown in Fig. 6 directing beam 110 along a path to impinge on photoreceptor 103 at write station 1 08. When in the solid line position shown, optical deflector 1 26 directs the beam onto array 102 at read station 1 09. A tube like collector 1 30 is provided adjacent photoreceptor 103 to collect or capture light reflected from array 102 as will appear. A suitable light detector 131 in collector 130 converts light rays captured by collector 1 30 into electrical signals representative of the latent electrostatic image scanned.

Microdeflector array 102 comprises an array of discrete flexible fingers 1 40 disposed in side by side fashion on an elongated base 142, the longitudinal axis of base 142 extending in a direction substantially perpendicular to the direction of movement of photoreceptor 1 03. To accommodate deflection of fingers 140, an elongated recess 143 is formed in base 142 under the free end of fingers 140. As described earlier, base 142 is preferably silicon with fingers 1 40 silicon dioxide, the array of fingers 140 and recess 143 being formed on base 142 as by selective etching.

Each flexible finger 140 has a conductive sensing electrode 1 45 thereon, electrodes 145 being electrically isolated from one another. Electrodes 145, which comprise any suitable metal or metal combination such as Chromium and Gold, are formed on the outer surface of fingers 140 by any suitable process. Sensing electrodes 145 have a right angle shape when viewed in cross-section, with a charge sensing surface 149 provided on the depending leg 1 50 thereof.

On installation of microdeflector array 102, the charge sensing surfaces 1 49 of electrodes 1 45 are disposed facing the surface of photoreceptor 103 and in predetermined spaced capacitive relation thereto. The portion of base 1 42 opposite flexible fingers 140 may be provided with a conductive reference electrode 1 47 thereon, or alternately, electrode 147 may be dispensed with and the conductivity of base 142 relied upon instead. The overall width of the array of sensing electrodes 145 is preferably at least equal to the width of the photoconductive surface to be scanned.

During operation of copier/printer 100 in the READ mode, as the latent electrostatic image on photoreceptor 103, which may be created by operation of copier/printer 100 in either the COPY or WRITE mode, passes opposite microdeflector array 102, a charge representative of the image charge on the photoconductive surface of photoreceptor 103 is induced on the sensing electrodes 145 opposite thereto. As described earlier, where the charge differential between the sensing electrode 145 and reference electrode 147 undergoes a change, the degree of deflection of the affected sensing electrode 1 45 changes proportionally.

As each line of the latent electrostatic image on photoreceptor 103 passes under microdeflector array 102, the individual sensing electrodes 145 of the array assume various degrees of deflection depending upon the charge on the photoreceptor surface portion opposite the electrode charge sensing surface 149. Concurrently, beam 110 raster scans across the array of sensing electrodes, and light is reflected by the individual electrodes 1 45 in directions according to the degree of electrode deflection at that time. Collector 1 30 is disposed to intercept and collect light reflected by electrodes 145 having a predetermined deflection representative of a predetermined image charge level on the photoconductive surface of photoreceptor 103.

Detector 131 of collector 130 converts the collected light pulses to electrical signals representative of one image charge level (i.e. "1"). Where no light is collected, the image signal output of detector 131 represents a second charge level (i.e. "O").

To distinguish successive image signals from one another, suitable clock means (not shown) are provided for periodically enabling detector 131 in synchronism with the scanning of beam 110 across the array of sensing electrodes.

To protect against exposure to any spurious reflections of the high intensity scanning beam 110 during operation in the READ mode, a suitable light absorbing stop 150 is preferably provided to intercept light reflected from microdeflector array 102 at angles other than that which impinges on collector 1 30.

Referring to Figs. 7 and 8, probe 1 2 has a dual element microdeflector 21 3 having a finger 214, which is preferably silicon dioxide, with inner driven electrode 225 and outer variable capacitance electrode 226 thereon. Outer electrode 226 has a generally U-shape and is separated from inner electrode 225 by a space 227 to electrically isolate the electrodes 225, 226 from one another.

Electrodes 225, 226 are preferably formed by coating the outer surface of finger 214 in the desired electrode pattern with suitable metals such as Chromium and Gold. A microdeflector having a finger 214 with a free standing length (L) of 100 ym spaced a distance (D) of 10 ym from base 16, and having a thickness (T) of 0.5 iim with electrodes 225, 226 having a thickness of 1 5 nm chromium and 35 nm gold has been found suitable.

Probe 1 2 includes a pickup electrode 228, which may be formed from any suitable conductive material such as gold, electrically coupled to the parallel combination of a voltage division capacitor 221 and the driven electrode 225 of microdeflector 213 by conductor 229.

Pickup electrode 228 may be of any suitable shape such as rectangular and of a size selected to provide a charge sensing surface of desired area. During use of ESV 10, pickup electrode 228 is positioned in predetermined spaced capacitive relation to the surface 35.

Voltage division capacitor 221 serves two functions. Since VpR-V,ef can be of the order of hundreds of volts which may be higher than can normally be accommodated by microdeflector 213, it is necessary to divide the voltage VpR-Vref between electrode 228 and driven electrode 225. The rules of capacitive voltage division show that the smaller fraction of a voltage applied across two capacitors in series appears across the larger capacitance. In practice, the capacitance of driven electrode 225 (CD), when compared to the capacitance of pickup electrode 228 (CPE), iS relatively small. Thus, the capacitance of the voltage division capacitor 22l(CVD) is chosen greater than CPE to provide the desired division ratio.Since CVD > CPE > CDR, voltage division capacitor 221 also makes the voltage appearing across the driven electrode 225 insensitive to changes in the capacitance (CDR) which result from changes in the voltage on the photoreceptor VPR. Both the above affects are contained in the relationship between the voltage on the driven electrode VDR and the voltage on the photoreceptor VPR: where CPE is the capacitance of pickup electrode 228, CDR is the capacitance of driven electrode 225, CvD is the capacitance of voltage division capacitor 221, VPR is the voltage on photoconductive surface 35, and Vref. is the reference voltage 218 applied to base 216.

Since CVD CDR we have A shutter 230 is provided for periodic imposition between pickup electrode 228 and the surface 35. Preferably, shutter 230 comprises a plate like semi-circular shaped metal part supported for rotation on the shaft 231 of a suitable shutter drive motor 232. Energization of motor 232 rotates shutter 230 at a predetermined rate to periodically interpose shutter 230 between pickup electrode 228 and surface 35 as will appear. Conductor 34 couples shutter 230 to potential source 218.

A suitable signal processing circuit 239, which may for example comprise a capacitive voltage divider, is provided. Conductor 238 couples variable capacitive electrode 226 to the input of signal processing circuit 239. The output of signal processing circuit may be coupled to a suitable readout device, shown here as voltmeter 240, to provide a visual scale readout of the voltage measured by ESV 10 on photoconductive surface 35. Alternatively, ESV 10 may be integrated into the aforementioned machine controller, with the signal output of circuit 239 used by the machine controller to control one of more of the machine components in accordance with the voltage sensed.

In use and presuming shutter 230 to be retracted, probe 1 2 of ESV 10 is disposed in operative relation with the photoconductive surface 35 with pickup electrode 228 spaced a predetermined distance therefrom, and the reference voltage (Vref) applied to base 216 of microdeflector 21 3. Flexible finger 214 bends or deflects by an amount determined by the voltage on photoconductive surface 35(VpR). Signal processing circuit 239 outputs a signal representing the voltage on surface 35.

A change in the charge VDR on driven electrode 225 induces a change in the amount of deflection of finger 214 and a corresponding change in the capacitance between variable capacitance electrode 226 and base 216. Signal processing circuit 239 responds by outputting a signal reflecting the change in capacitance of electrode 226 as a measure of the voltage on the photoconductive surface 35. As discussed, the signal output of circuit 239 may be read by a suitable meter such as voltmeter 240. Alternately, the signal may be input to the machine controller and used by the controller to adjust one or more of the machine processing components.

Both preceding initial operation and periodically during operation, it is necessary to calibrate probe 1 2 to accommodate drift such as for example due to charge leakage. Accordingly, motor 232 is energized at start-up and periodically during operation to rotate shutter 230 at a relatively slow rate (i.e. once every 10 seconds) to interpose shutter 230 between pickup electrode 228 and the photoconductive surface 35 for calibration purposes. Disposition of shutter 230 opposite pickup electrode 228 results in there being no voltage across the capacitor formed by electrode 225 and base 216. The charge on variable capacitance electrode 226 may now be read using meter 240 to determine if ESV 10 is adjusted correctly.

In the embodiment of Figs. 9 and 10, there is shown a probe construction designed to compensate for the effects of extraneous noise, which may take the form of high frequency vibrations, ambient temperature conditions, etc. on probe 1 2. In this embodiment, a second microdeflector 213' is provided alongside microdeflector 21 3 on base 16, microdeflector 213' having a flexible finger 214' disposed in cantilever fashion opposite a second recess 15' in base 16. Flexible finger 214' is identical to the finger 214 described heretofore with inner driven electrode 225' and outer variable capacitance electrode 226'.

In this embodiment, microdeflector 213 functions as the charge sensing unit with driven electrode 225 thereof being coupled by conductor 229 to pickup electrode 228 while variable capacitance electrode 226 is coupled by conductor 238 to signal processing circuit 239 as described in connection with the Figs. 7 and 8 embodiment. Variable capacitance electrode 226' of microdeflector 213' is coupled by conductor 250 to a second signal processing circuit 251 while driven electrode 225' thereof is left uncoupled. As in the case of signal processing circuit 239, circuit 251 may comprise a capacitive type voltage divider.

The output terminals of signal processing circuits 239, 251 are coupled by conductors 254, 255 to the + and - terminals of a suitable differential amplifier. 256. The output terminal of amplifier 256 is connected to the user such as voltmeter 240 as described.

The operation of microdeflector 213 in the Figs. 9 and 10 embodiment is the same as that described in connection with the Figs. 7 and 8 embodiment, the capacitive relationship established between pickup electrode 228 and the photoconductive surface 35 producing a charge representative of the voltage on surface 35 on driven electrode 225 of finger 214. Any change in deflection of finger 21 4 changes the capacitive relation between variable capacitance electrode 226 and base 16, signal processing circuit 239 responding by outputting signal representing the voltage on photoconductive surface 35 to the + input terminal of amplifier 256.

Finger 214' of microdeflector 213', with application of the reference potential (Vref) to base 16, deflects and a charge is induced on both the driven and variable capacitance electrodes 225', 226' thereof in the manner described. The charge on variable capacitance electrode 226' is processed by signal processing circuit 251 and input to the-input terminal of amplifier 256.

Where the deflection of finger 214' of microdeflector 213' changes due for example to a change in ambient temperature conditions. mechanical vibrations, etc, (referred to generally as noise herein), the capacitance between finger 214' and base 216 changes with resulting change in the charge on variable capacitance electrode 226'. As a consequence, the signal output of singla processing circuit 251 to the-input terminal of amplifier 256 changes. Amplifier 256, which integrates the signal inputs from signal processing circuits 239, 251, responds by adjusting the output signal in compensation for the noise.

While a second recess 15' is shown and described, it will be understood that a common recess of suitable length may be provided in accommodation of both fingers 214 and 214'.

Further, the function of driven and variable capacitance electrodes (225, 226 in the Figs. 7 and 8 embodiment, and 225, 225' and 226, 226' in the Figs. 9 and 10 embodiments) may be reversed such that electrode 226 (226') function as the driven electrode while electrode 225 (225') function as the variable capacitance electrode.

Claims (22)

1. An electrostatic voltmeter for measuring charges on a surface, comprising (a) a probe having a microdeflector sensing unit, said sensing unit comprising a base, a flexible finger overlying and in spaced relationship with said base so that a portion of said finger can deflect relative to said base on imposition of a potential thereacross, and a sensing electrode on said finger and movable with said finger on deflection of said finger, said sensing electrode including a side adapted to be spaced opposite and in capacitive relationship to said surface so that a charge representative of the charge on said surface is produced on said sensing electrode to cause deflecting movement of said finger relative to said base; and (b) means for converting deflection of said finger to a signal representing the charge on said surface.

2. The voltmeter according to claim 1, comprising a reference electrode on said base; and means to bias said reference electrode to a predetermined reference potential to pre-deflect said finger together with said sensing electrode and enhance sensitivity of said probe.

3. The voltmeter according to claim 1 or claim 2, in which said converting means includes means for impinging a beam of electro-magnetic radiation on said finger, said beam being reflected by said finger; and detector means for detecting the beam reflected by said finger, said beam being reflected onto said detector on deflection of said finger to a predetermined position whereby the signal output of said detector represents a predetermined charge on said surface.

4. The voltmeter according to claim 3 in which said detector means comprises at least two detectors for detecting said beam on deflection of said finger to different predetermined positions whereby the signal output of said detectors represent different predetermined charges on said surface.

5. The voltmeter according to claim 3 or claim 4, in which said detector means comprises a first detector for detecting said beam on deflection of said finger to a first position whereby the signal output of said first detector represents a predetermined first charge on said surface, and a second detector for detecting said beam on deflection of said finger to a second position whereby the signal output of said second detector represents a predetermined second charge on said surface.

6. The voltmeter according to any of the preceding claims, comprising means to vibrate said finger whereby to make said sensing electrode relatively insensitive to the spatial relation between said sensing electrode probe and said surface.

7. The voltmeter according to claim 6 in which said vibrating means includes means to apply an oscillating reference potential to said base.

8. The voltmeter according to claim 7 in which said sensing electrode probe comprises a scanning aperture.

9. A multi-function copier/printer having a photoreceptor and selectively operable in a first copy mode to produce latent electrostatic images from originals on said photoreceptor, in a second write mode to scan a beam of light across said photoreceptor and modulate said beam in accordance with image signals to produce latent electrostatic images represented by said image signals on said photoreceptor, and in a third read mode to scan latent electrostatic images on said photoreceptor and convert said images to image signals, including a microdeflector array comprising support means, and a plurality of flexible charge gathering fingers arranged in a linear array on said support means, each of said fingers, having an unsupported portion capable of deflecting relative to said support means in response to a charge potential thereon, disposition of said array of flexible fingers in capacitive relation with said photoreceptor and the latent electrostatic images thereon resulting in discrete deflections of said fingers individually in response to the image charge levels on said photoreceptor opposite said fingers, the copier/printer further comprising means for scanning said beam across said array of fingers, collecting means for capturing light reflected from those of said fingers having a predetermined deflection, and means for converting light captured by said collecting means to image signals representative of said latent images on said photoreceptor.

10. The copier/printer according to claim 9 comprising a sensing electrode on each of said fingers, said sensing electrodes having a reflective surface to enhance reflection of said beam.

11. The copier/printer according to claim 9 or claim 10, comprising means to bias said support means to a predetermined reference potential whereby to uniformly pre-deflect said fingers.

1 2. The copier/printer according to any of claims 9 to 11, comprising means for selectively impinging said beam on said photoreceptor when in said second write mode or on said array of fingers when in said third read mode.

13. The voltmeter according to any of claims 1 to 8, in which said probe has separate driven and sensing electrodes on said finger, said base and said electrodes cooperating to form a capacitive type relationship between said base and each of said electrodes individually so that when said driven electrode is brought into capacitive relation with said surface, a charge representative of the voltage on said surface is produced on said driven electrode to cause said finger to deflect, delfection of said finger changing the capactive relation between said base and said sensing electrode, said deflection converting means converting a change in capacitance between said base and said sensing electrode to a signal representing the voltage on said surface.

14. The voltmeter according to claim 13, wherein said sensing electrode is generally Ushaped, said driven electrode is disposed within said sensing electrode and separated from said sensing electrode whereby said sensing electrode is electrically isolated from said driven electrode.

1 5. The voltmeter according to claim 1 3 or claim 14, in which said means for bringing said driven electrode into capactive relation with said surface comprises a pickup electrode disposable in predetermined spaced relation with said surface whereby a capactivie relationship is established betwen said surface and said pickup electrode, and means for electrically coupling said pickup electrode with said driven electrode.

1 6. The voltmeter according to claim 15, further comprising a shutter interposable between said pickup electrode and said surface, said shutter being coupled to a predetermined voltage for calibrating said probe, and means for periodically interposing said shutter between said pickup electrode and said probe, interposition of said shutter between said pickup electrode and said probe causing predetermined deflection of said finger for use in calibrating said probe.

1 7. The voltmeter according to any of claims 1 3 to 16, comprising capacitance means in parallel with said driven electrode to control charge levels of said driven electrode.

1 8. The voltmeter according to any of claims 1 3 to 16, comprising a second microdeflector on said base for providing a signal reflecting changes in capacitance due to noise; and means for adjusting the signal output of the first mentioned microdeflector with the signal output of said second microdeflector to correct the signal output of said first microdeflector for noise.

1 9. The voltmeter according to claim 18, in which said second microdeflector is disposed in side by side relation with said first mentioned microdeflector.

20. The voltmeter according to claim 18 in which said second microdeflector flexible finger being disposed in side by side relation with said first microdeflector flexible finger opposite a common elongated recess.

21. An electrostatic voltmeter substantially as herein described with reference to and as illustrated in Fig. 1, or Fig. 2, or Figs. 3 and 4, or Figs. 7 and 8, or Figs. 9 and 10 of the accompanying drawings.

22. An electrostatic copier/printer substantially as herein described with reference to and as illustrated in Figs. 5 and 6 of the accompanying drawings.