JP2001330638A - Surface potential detecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface potential detecting device capable of making a circuit simple, compact, and lightweight and reducing cost. SOLUTION: A switching circuit 2 selects and outputs signals S11-S41 and S12-S42 supplied from surface potential sensors 11-14 at different temporal timing for every one of the surface potential sensors 11-14. One signal processing circuit 3 is connected to the surface potential sensors 11-14 via the switching circuit 2. A first bias circuit 36 impresses a positive bias voltage to raise the level of a surface potential detection signal Z. A second bias circuit 39 impresses a negative bias voltage to offset the positive bias voltage impressed by the first bias circuit 36 on the basis of an analogue voltage signal supplied from a microcomputer 42 and D/A converting circuit 41.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface potential detecting device for detecting a surface potential in a non-contact manner.

[0002]

2. Description of the Related Art This type of surface potential detecting device is, for example,
It is used as a means for detecting the surface potential of a photosensitive drum in a copying machine, a laser beam printer, or the like. As disclosed in Japanese Patent Publication No. 3-6467 and the like, this type of surface potential detection device mechanically interrupts an electric field between a detection electrode and a photosensitive drum with a tuning fork, thereby forming a photosensitive drum. An AC signal corresponding to the surface potential is obtained. Then, the AC signal is amplified by a preamplifier, guided to a synchronous detection circuit via an isolator, and detected by a signal synchronized with the mechanical interruption. The synchronous detection output signal output from the synchronous detection circuit is converted to a direct current by the integration circuit. The DC signal obtained by the integration circuit is input to a high voltage amplifier.

[0003] Each circuit has a common ground line. The high-voltage amplifier controls the potential of the common ground line based on the input DC signal so that the potential of the common ground line becomes the same as the potential of the surface of the photosensitive drum that is the surface to be measured. By extracting the potential of the common ground line via an attenuator, a buffer amplifier, or the like, a surface potential signal can be obtained. The common ground line has a floating relationship with respect to the ground potential or the frame ground potential.

The greatest advantage of this method is that even if the distance between the detection electrode included in the surface potential sensor and the surface of the photosensitive drum, which is the surface to be measured, changes, the distance dependency is very small and high accuracy is achieved. That is, an important surface potential detection signal can be obtained.

[0005] The above-mentioned surface potential detecting device usually includes a surface potential sensor and a signal processing device. The surface potential sensor is configured as a kind of probe including a detection electrode, a tuning fork, a drive circuit, a preamplifier, and the like. The signal processing device
It includes the remaining circuit parts necessary for the surface potential detecting device.

Recently, high-speed tandem-type copying machines and laser beam printers have been proposed and put to practical use. In these image forming apparatuses, four (cyan,
(Magenta, yellow, and black) photosensitive drums, and the surface potentials of the four photosensitive drums must be measured.

However, conventionally, it is necessary to provide one surface potential sensor and one signal processing device for one photosensitive drum, and therefore, for four photosensitive drums,
It was necessary to provide four sets of the surface potential sensor and its signal processing device. Among the components of the surface potential detecting device, the signal processing device has a complicated circuit configuration and has two transformers that cause an increase in size, weight, and cost. Therefore, in the prior art in which each of the four photosensitive drums must be provided with the above-described signal processing device, the surface potential detecting device as a whole is significantly increased in shape, weight, and cost, and the solution is solved. Has become a very big issue.

In order to realize a highly accurate surface potential detection signal, it is important to adjust the offset of the surface potential detection signal. In the surface potential sensor, when the difference voltage between the surface potential of the surface to be measured and the potential of the common ground line is zero, the surface potential detection signal should be zero.

However, in many surface potential sensors, since the periphery of the detection electrode is slightly charged positively or negatively, an AC signal is output from the surface potential sensor, and an offset voltage is generated in the surface potential detection signal. For example, when the periphery of the detection electrode is negatively charged, even when the surface potential of the surface to be measured is 0 volt, a surface potential detection signal is output as if the surface potential of the surface to be measured is positive.

Japanese Unexamined Patent Publication No. 6-242166 discloses a surface potential detecting device provided with an offset adjusting means. This surface potential detecting device includes a power supply circuit (DC / DC converter), and a resistance voltage dividing circuit is inserted between a positive power supply line and a negative power supply line led from the power supply circuit. With this resistance voltage dividing circuit, the voltage difference between the positive power supply voltage and the negative power supply voltage from the power supply circuit is divided by resistance, and the divided voltage is adjusted near the common ground line potential by a variable resistor. The adjusted voltage is applied as a bias voltage to the integration circuit to cancel the offset voltage. Specifically, the offset adjustment is performed so that the surface potential detection signal becomes zero when the surface potential to be measured is zero.

However, in the above-described offset adjusting means, since the bias voltage is generated based on both the positive power supply voltage and the negative power supply voltage of the power supply circuit, when the positive power supply voltage or the negative power supply voltage changes, the bias voltage also increases. And the offset adjustment is difficult. In order to stabilize the bias voltage, the power supply circuit must be configured to stabilize both the positive power supply voltage and the negative power supply voltage with high accuracy.

In the surface potential detecting device disclosed in the above-mentioned publication, as a technique for stabilizing both the positive power supply voltage and the negative power supply voltage, a stabilized power supply having high accuracy and good temperature characteristics is replaced by a positive power supply. It is provided for each voltage and negative power supply voltage. In this method, two stabilized power supplies are required, and the cost of the power supply circuit increases.

Moreover, a dropper-type stabilized power supply (three-terminal regulator) typically used as a stabilized power supply
Has a low power efficiency of about 50%. The use of a dropper-type stabilized power supply reduces the overall power efficiency of the surface potential detection device.

When a stabilized power supply such as a dropper-type stabilized power supply is not used, it is necessary to balance the positive power supply voltage and the negative power supply voltage. For this reason, a transformer having a bifilar wound secondary winding is used as a power transformer of the power circuit. However, this type of transformer is expensive, which increases the cost of the power supply circuit.

[0015]

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface potential detecting device capable of simplifying the circuit, reducing the size, reducing the weight, and reducing the cost.

Another object of the present invention is to provide a surface potential detecting device capable of eliminating an offset generated in a surface potential detection output signal.

Still another object of the present invention is to provide an inexpensive surface potential detecting device.

Still another object of the present invention is to provide a surface potential detecting device capable of realizing high total power efficiency.

[0019]

In order to solve the above-mentioned problems, a surface potential detecting device according to the present invention includes a plurality of surface potential sensors, a switching circuit, and a signal processing circuit. Each of the plurality of surface potential sensors is independent of each other.
The switching circuit selects and outputs a signal supplied from the surface potential sensor at a different temporal timing for each of the surface potential sensors. One signal processing circuit is connected to the plurality of surface potential sensors via the switching circuit, and is shared by the plurality of surface potential sensors.

The signal processing circuit includes a first bias circuit, a microcomputer, a digital-to-analog conversion circuit, and a second bias circuit. The first bias circuit is a circuit for applying a positive bias voltage for increasing the level of the surface potential detection signal to the detection signal.

The microcomputer has digital bias voltage information preset and stored for each of the plurality of surface potential sensors, drives the switching circuit, and supplies the digital data to the surface potential sensor selected by the driven switching circuit. The assigned digital bias voltage information is output.

The digital-analog conversion circuit converts digital bias voltage information supplied from the microcomputer into an analog voltage signal and outputs the analog voltage signal.

The second bias circuit applies a negative bias voltage for canceling a positive bias applied by the first bias circuit based on an analog voltage signal supplied from the digital-analog conversion circuit.

As described above, the surface potential detecting device according to the present invention includes a plurality of surface potential sensors, and each of the plurality of surface potential sensors is independent of each other. Potential sensor in an image forming apparatus such as a printer or a laser beam printer.
The surface potentials can be individually detected corresponding to the book (cyan, magenta, yellow, and black) photosensitive drums. In addition, even when an extremely large number of surface potential sensors are provided in a distributed manner, such as when detecting the electrification of a film to be conveyed, the surface potential can be individually detected at different portions of the film. .

The switching circuit selects and outputs signals supplied from the plurality of surface potential sensors at different timings for each surface potential sensor. Therefore, signals output from the plurality of surface potential sensors can be temporally separated.

Since the signal processing circuit is connected to the plurality of surface potential sensors via the switching circuit, the signal processing circuit receives signals for each surface potential sensor in a time-separated state. . Then, necessary signal processing can be performed within the time allocated to each surface potential sensor.

The signal processing circuit is shared by a plurality of surface potential sensors. Therefore, only one signal processing circuit is required. Therefore, for example, for four photosensitive drums,
Compared with the prior art that required four sets of the surface potential sensor and its signal processing device, the circuit configuration is significantly simplified, and the shape, weight and cost are significantly reduced. This advantage becomes even more pronounced as the number of detected locations increases.

Next, the signal processing circuit includes a first bias circuit and a second bias circuit. The first bias circuit applies a positive bias voltage for increasing the level of the surface potential detection signal to the detection signal. Due to this positive bias voltage, the relationship (detection characteristic) of the surface potential detection signal to the surface potential to be measured moves in the direction in which the value of the surface potential detection signal becomes higher with respect to the same surface potential to be measured. Is zero, the surface potential detection signal has a positive value (referred to as a positive offset voltage). Therefore, if the potential of the surface to be measured is equal to or higher than zero, a surface potential detection signal is always generated, so that a dead zone does not occur.

Further, since the power supply circuit of the surface potential detecting device does not require a high degree of voltage stability, a surface potential detecting device with a reduced power supply circuit can be obtained. A dropper-type stabilized power supply (three-terminal regulator) that has been conventionally required can be omitted, and a surface potential detecting device with high overall power efficiency can be obtained.

The second bias circuit applies a negative bias voltage for canceling the positive bias voltage applied by the first bias circuit in the surface potential detection signal. According to this configuration, by adjusting the magnitude of the negative bias voltage, it is possible to cancel the positive offset voltage generated by the first bias circuit and to match the ideal characteristics. The ideal characteristic means that the relationship between the surface potential to be measured and the surface potential detection signal is a linear relationship passing through the origin on the graph.

In the case where a plurality of surface potential sensors are provided, the amount of charge in the vicinity of the detection electrode usually differs from one surface potential sensor to another, so that a different negative bias voltage is applied to each of the surface potential sensors. Must be applied. For example, for each of the surface potential sensors,
If a second bias circuit is provided and a different negative bias voltage is applied to each of the surface potential sensors, a number of second bias circuits corresponding to the number of the surface potential sensors are required. This leads to a complicated circuit configuration, an increase in the number of parts, an increase in the size of the circuit, and an increase in cost.

As means for avoiding such a problem,
In the present invention, the signal processing circuit includes a microcomputer and a digital-analog conversion circuit. The microcomputer has digital bias voltage information preset and stored for each of the plurality of surface potential sensors, and outputs digital bias voltage information assigned to the surface potential sensor selected by the switching circuit. The digital-analog conversion circuit converts digital bias voltage information supplied from the microcomputer into an analog voltage signal and outputs the analog voltage signal. The second bias circuit is based on an analog voltage signal supplied from the digital-analog conversion circuit,
In the surface potential detection signal, a negative bias voltage in a direction to cancel the positive bias voltage applied by the first bias circuit is applied.

According to this configuration, although there are a plurality of surface potential sensors, one microcomputer,
One digital-to-analog conversion circuit and one second
Only need to be provided. For this reason, the circuit configuration is simplified, the size is reduced, the number of circuit components is reduced, and the cost is reduced. This advantage becomes more pronounced as the number of surface potential sensors increases.

The microcomputer is also used to drive the switching circuit. Accordingly, a dedicated circuit used only for driving the switching circuit is not required. In addition, it is possible to reliably synchronize the driving operation of the switching circuit and the offset canceling operation for the surface potential sensor selected by the switching circuit.

Each of the plurality of surface potential sensors may have a conventionally known structure. Each of the plurality of surface potential sensors typically generates an AC signal corresponding to the surface potential of the measured surface by interrupting an electric field between the detection electrode and the measured surface. In this case, the switching circuit selects and outputs a synchronization signal synchronized with a detection signal supplied from the surface potential sensor and a drive signal for intermittent operation at different timings for each surface potential sensor.

In the surface potential detecting device according to the present invention,
Typically, the plurality of surface potential sensors and signal processing circuits have a common ground line. The signal processing circuit controls the potential of the common ground line so that the potential of the common ground line is substantially equal to the potential of the surface to be measured. As a result, even if the distance between the surface potential sensor and the surface of the photosensitive drum that is the surface to be measured changes, a highly accurate surface potential detection signal with very little distance dependency can be obtained.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram schematically showing a configuration in the case where the surface potentials of four photosensitive drums arranged in tandem are detected using the surface potential detecting device according to the present invention. In the figure, along the running direction W of the transfer belt V,
Four photosensitive drums C, M, Y, and K are arranged in tandem. The photosensitive drum C is for cyan, the photosensitive drum M is for magenta, the photosensitive drum Y is for yellow, and the photosensitive drum K is for black. Each of the photosensitive drums K to C has a charging corotron U1, a transfer corotron U2, and a developing machine U
3 are provided.

The surface potential detecting device 1 according to the present invention includes a plurality of surface potential sensors 11 to 14, a switching circuit 2, and a signal processing circuit 3.

The illustrated embodiment shows a case where the surface potentials of four photosensitive drums arranged in tandem are detected. Therefore, the number of surface potential sensors 11 to 14 is four. It is increased or decreased according to the number. Each of the four surface potential sensors 11 to 14 is independent of each other. Each of the surface potential sensors 11 to 14 is individually provided for each of the photosensitive drums K to C, and is fixedly arranged at a distance of, for example, 2.5 mm from the surface of the photosensitive drums K to C. I have. Surface potential sensors 11-1
4, the signals S11 to S41, S
12 to S42 are switching circuits 2 via a coaxial cable or the like.
Sent to

The switching circuit 2 includes surface potential sensors 11 to 14
Are selected and output at different timings for each of the surface potential sensors 11 to 14.

One signal processing circuit 3 is connected to the four surface potential sensors 11 to 14 via the switching circuit 2 and is shared by the four surface potential sensors 11 to 14.

As shown in the embodiment, since each of the four surface potential sensors 11 to 14 is independent of each other, in an image forming apparatus such as a tandem type copying machine or a laser beam printer for speeding up, Surface potential sensor 1
1 to 14 correspond to the four photosensitive drums K to C, and their surface potentials can be individually detected.

The switching circuit 2 includes four surface potential sensors 11
Signals S11 to S41, S12 to S supplied from.
42 is selected and output at different timings for each of the surface potential sensors 11 to 14. Therefore, signals output from the four surface potential sensors 11 to 14 can be temporally separated.

FIG. 2 is a time chart showing a specific example of the signal selection operation of the switching circuit. First, as shown in FIG. 2A, only the signal output from the surface potential sensor 11 provided on the photosensitive drum K is selected from the time t1 to the time t2.

Next, as shown in FIG. 2B, only the signal output from the surface potential sensor 12 provided on the photosensitive drum Y is selected from the time t3 to the time t4.

Further, as shown in FIG. 2C, only the signal output from the surface potential sensor 13 provided on the photosensitive drum M is selected from the time t5 to the time t6. Further, as shown in FIG. 2D, between time t7 and time t8, the surface potential sensor 14 provided on the photosensitive drum C is used.
Only the signal output from is selected.

The signal processing circuit 3 passes through the switching circuit 2
It is connected to four surface potential sensors 11 to 14. Therefore, the signal processing circuit 3 receives the signals of the surface potential sensors 11 to 14 in a time-separated state. Then, the signal processing circuit 3 performs necessary signal processing within a time allocated to each of the surface potential sensors 11 to 14 and outputs a surface potential detection signal Z corresponding to each of the surface potential sensors 11 to 14.

The signal processing circuit 3 is shared by the four surface potential sensors 11 to 14. Therefore, only one signal processing circuit 3 is required. Therefore, the four photosensitive drums K to
For C, the circuit configuration is significantly simplified as compared with the prior art, which requires the provision of four sets of the surface potential sensors 11 to 14 and their signal processing devices.
Weight and cost are significantly reduced.

Each of the four surface potential sensors 11 to 14 may have a conventionally known structure. Typically, the four surface potential sensors 11 to 14 periodically change an electric field between the detection electrode 15 and the surface to be measured, thereby forming an AC signal corresponding to the surface potential of the photosensitive drums K to C. Generate In this case, the switching circuit 2
A synchronization signal synchronized with the detection signal and the drive signal supplied from the surface potential sensors 11 to 14 is transmitted to the surface potential sensors 11 to 14.
For each of them, they are selected and output at different timings.

FIG. 3 is a block diagram more specifically showing the structure of the surface potential detecting device according to the present invention. In the figure,
Since each of the four surface potential sensors 11 to 14 has the same configuration, the surface potential sensor 11 will be representatively described here. The surface potential sensor 11 includes a detection electrode 15, a chopper 16, a preamplifier 17, and a chopper drive circuit 18. The detection electrode 15 generates an electric field for measuring the surface potential of the photosensitive drum K in a non-contact manner.

The chopper 16 periodically chops the electric field between the surface to be measured, which is the surface of the photosensitive drum K, and the detection electrode 15. Its specific structure is already known.
For example, the tuning fork is excited by a piezoelectric vibrator, and a metal piece attached to the tuning fork is vibrated between the surface of the photosensitive drum K and the detection electrode 15.

The preamplifier 17 is a circuit for converting the impedance of the AC signal detected by the detection electrode 15 into a low impedance. The AC signal (detection signal) S11 that has passed through the preamplifier 17 is supplied to the switching circuit 21.
The chopper drive circuit 18 excites the chopper 16. Specifically, a driving signal having a predetermined frequency is supplied to the piezoelectric vibrator constituting the chopper 16 to excite it. In the surface potential sensors 12 to 14 as well, the AC signals S21 to S41 passed through the preamplifier 17 are supplied to the switching circuit 21.

FIG. 4 is a diagram showing a specific circuit configuration of the surface potential sensors 11 to 14. For simplicity of description, the surface potential sensor 11 provided on the photosensitive drum 11
This will be described representatively. In the figure, a chopper 16 attaches a piezoelectric vibrator 162 to a tuning fork 161, and excites the tuning fork 161 at a predetermined frequency by the piezoelectric vibrator 162.
The vibration of the tuning fork 161 is detected by the piezoelectric vibrator 163 and input to the chopper drive circuit 18 as a feedback signal.

At the free end side of the tuning fork 161, a metal piece 164,
165 are attached. These metal pieces 164, 16
Reference numeral 5 denotes the surface of the photosensitive drum K (see FIG. 1) and the detection electrode 1
5 is arranged. Therefore, when the metal pieces 164 and 165 are excited by the vibration of the tuning fork 161, the electric field between the surface to be measured as the surface of the photosensitive drum K and the detection electrode 15 is periodically chopped.

The preamplifier 17 includes an amplifying element Q0 composed of an FET (field effect transistor) and a gate resistor R1.
And the source resistance R2, and converts the impedance of the AC signal detected by the detection electrode 15 into low impedance. More specifically, in the preamplifier circuit 17,
The source of the FET constituting the amplifying element Q0 is connected to the common ground line C.P. It is connected to GND. Common ground line C. GND is the frame ground GN
Floating from D (floating).

When the AC signal appearing on the detection electrode 15 is applied to the gate of the amplifier Q0 and the amplifier Q0 operates,
The amplified signal causes the source resistor R2 to negatively bias the amplifying element Q0. The high-impedance signal seen on the input side of the amplifying element Q0 is impedance-converted into a low-impedance signal, and the signal is supplied to the drain D of the amplifying element Q0. appear.

The chopper drive circuit 18 is an operational amplifier IC
5, including the resistors R18, C12, R17, R16 and the capacitor C12. When a drive signal is supplied from the operational amplifier IC5 to the piezoelectric vibrator 162, the piezoelectric vibrator 162
As a result, the tuning fork 161 is excited. Due to the vibration of the tuning fork 161, a feedback signal is generated in the piezoelectric vibrator 163, and the resistance R 1
8, positive feedback is applied to the operational amplifier IC5 by the capacitor C12 and the resistors R17 and R16, and the next drive pulse is applied to the piezoelectric vibrator 162. By repeating this operation, the tuning fork 161 continues to vibrate at a specific frequency (for example, 680 Hz) at its mechanical resonance point.

The vibration of the tuning fork 161 causes the metal pieces 164 and 165 attached to its free ends to vibrate, and the electric field between the detection electrode 15 and the surface of the photosensitive drum K to be measured is chopped. As a result, the capacitance between the detection electrode 15 and the surface to be measured of the photosensitive drum K increases or decreases in a substantially sinusoidal manner around the capacitance Co at the time of non-excitation, and correspondingly,
An AC detection signal S11 is obtained.

The drive signal generated by the chopper drive circuit 18 or the signal S12 synchronized therewith is supplied to the switching circuit 2. Although the switching circuits 21 and 22 are shown as different blocks from each other in the drawing, the switching circuit 2 may be integrally configured as the switching circuit 2. The switching circuit 2 performs an interlocking selection operation so that the detection signal S11 and the synchronization signal S12 of the surface potential sensor 11 provided on the photosensitive drum K are simultaneously selected.

The surface potential sensors 12 to 14 provided on the other photosensitive drums Y, M, and C also perform the operation described with reference to the surface potential sensor 11. First, the switching circuit 2 detects the detection signal S21 of the surface potential sensor 12 provided on the photosensitive drum Y.
And an interlocking selection operation such that the synchronization signal S22 is simultaneously selected. Further, an interlocking selection operation is performed so that the detection signal S31 and the synchronization signal S32 of the surface potential sensor 13 provided on the photosensitive drum M are simultaneously selected. Further, an interlocking selection operation is performed so that the detection signal S41 of the surface potential sensor 14 provided on the photosensitive drum C and the synchronization signal S42 are simultaneously selected.

Description will be made again with reference to FIGS. The signal processing circuit 4 is one, and includes an amplification circuit 30, a synchronous detection circuit 31, an integration circuit 32, and a high-voltage amplification circuit 33. The amplification circuit 30 amplifies and outputs the detection signals S11 to S14 supplied from the surface potential sensors 11 to 14.
The synchronous detection circuit 31 detects the signal a supplied from the amplification circuit 30 in synchronization with the signal b supplied from the switching circuit 2. The integration circuit 32 converts the detection output signal c supplied from the synchronous detection circuit 32 into a direct current and outputs the direct current.

The high voltage amplifier 33 boosts the signal d supplied from the integrating circuit 32. The signal boosted by the high voltage amplifier 33 is fed back to the tuning fork 16, the preamplifier 17, and the drive circuit 18 as a feedback voltage Vf. Thereby, the common ground line C.I. Feedback control is applied so that the potential of GND is substantially equal to the potential of the measured surfaces of the photosensitive drums K to C. The signal boosted by the high-voltage amplifier circuit 33 is supplied to the detection signal output circuit 35. Then, the detection signal output circuit 3
5, a DC surface potential detection signal Z is output.

The signal processing circuit 3 includes a DC / DC converter 34. In the embodiment, the DC / DC converter 34 generates DC voltages + Vcc and -Vcc.
The DC voltages + Vcc and -Vcc are supplied as operating voltages of the respective components. DC voltages + Vcc and -Vcc are common ground lines C.V. This voltage is based on GND.

The signal processing circuit 3 further includes a first bias circuit 36, a second bias circuit 39, and a digital-analog conversion circuit (hereinafter, referred to as a D / A conversion circuit) 41.
And a microcomputer 42. The first bias circuit 36 applies a positive bias voltage for increasing the level of the surface potential detection signal Z to the detection signal.
Due to this positive bias voltage, the relationship (detection characteristic) of the surface potential detection signal Z to the surface potential to be measured moves in the direction in which the value of the surface potential detection signal Z increases with respect to the same surface potential to be measured. When the potential of the measurement surface is zero, a positive offset voltage is generated in the surface potential detection signal Z. Therefore, if the potential of the surface to be measured is equal to or higher than zero, the surface potential detection signal Z is always generated, so that a dead zone does not occur.

The second bias circuit 39 cancels the positive bias voltage applied by the first bias circuit 36 in the surface potential detection signal Z based on the analog voltage signal supplied from the D / A conversion circuit 41. Apply a negative bias voltage in the direction.

According to this configuration, by adjusting the magnitude of the negative bias voltage, the positive offset voltage generated by the first bias circuit 36 can be canceled, and the ideal characteristics can be obtained. The ideal characteristic means that the relationship between the surface potential to be measured and the surface potential detection signal Z is a linear relationship passing through the origin on the graph.

In the case where a plurality of surface potential sensors 11 to 14 are provided, the charge amount around the detection electrode 15 is generally different from each other in each of the surface potential sensors 11 to 14. For each of the fourteen different negative bias voltages must be applied.

It is assumed that a circuit configuration is provided in which a second bias circuit 39 is provided for each of the surface potential sensors 11 to 14, and a different negative bias voltage is applied to each of the surface potential sensors 11 to 14. For example, the number of the second bias circuits 39 corresponding to the number of the surface potential sensors is required, and the circuit configuration becomes complicated, the number of parts increases, the circuit becomes large, and the cost increases.

As means for avoiding such a problem,
In the present invention, the signal processing circuit 3 includes a microcomputer 42 and a D / A conversion circuit 41. The microcomputer 42 has digital bias voltage information preset and stored for each of the plurality of surface potential sensors 11 to 14, and the surface potential sensor 1 selected by the switching circuit 2.
It outputs digital bias voltage information assigned to 1 to 14.

The microcomputer 42 includes an arithmetic processing unit 43, a memory 44, and an I / O unit 45. The digital bias voltage information is stored in the memory 44. The digital bias voltage information stored in the memory 44 is read out based on a command from the arithmetic processing unit 43, supplied to the I / O unit 45, and passed through the output port unit provided in the I / O unit 45. It is supplied to the D / A conversion circuit 41. The digital bias voltage information stored in the memory 44 is read out as, for example, an 8-bit digital signal and supplied to the D / A conversion circuit 41.

The D / A conversion circuit 41 converts the digital bias voltage information supplied from the microcomputer 42 into
Convert to analog voltage signal and output. Based on the analog voltage signal supplied from the D / A conversion circuit 41, the second bias circuit 39 uses the surface potential detection signal Z to generate a negative bias in the direction to cancel the positive bias voltage applied by the first bias circuit 36. Is applied.

According to this configuration, the surface potential sensors 11 to 11
Although there are a plurality of 14, it is only necessary to provide one microcomputer 42, one D / A conversion circuit 41, and one second bias circuit 39. For this reason, the circuit configuration is simplified, the size is reduced, the number of circuit components is reduced, and the cost is reduced. The advantage is that as the number of surface potential sensors 11 to 14 increases,
Become noticeable.

The microcomputer 42 is used for driving the switching circuit 2. Specifically, the microcomputer 42 sends the drive signal CS1 to the switching circuit 2.
To CS4. In this case, the surface potential sensors 11 to
The switching circuit 2 is driven so that a detection signal of the surface potential sensor corresponding to the digital bias voltage information read from the memory 44 is selected from among the fourteen signals. Therefore, a dedicated circuit used only for driving the switching circuit 2 is not required. In addition, it is possible to reliably synchronize the driving operation of the switching circuit 2 and the canceling operation for the surface potential sensors 11 to 14 selected thereby.

FIG. 5 is a diagram showing a more specific circuit configuration of the signal processing circuit 3 included in the surface potential detecting device according to the present invention. The amplifier circuit 30 includes operational amplifiers IC4 and IC6,
Resistors R8, R13, R14, R85, R87, R89,
The switch includes the capacitors C10 and C11, and amplifies the detection signals S11 to S41 supplied through the switches included in the switching circuit 2 and the capacitor C11.

The signal amplified by the amplifier circuit 30 is
The signal is supplied to the synchronous detection circuit 31. The synchronous detection circuit 31 includes an operational amplifier IC3, resistors R9, R10, R11, R12, and an FET (field effect transistor) Q5 forming a switch element. The synchronous detection circuit 31 receives a signal from the amplifier circuit 30 based on the synchronization signals S12 to S42 supplied to the gate of the FET Q5 from the drive circuit 18 (see FIGS. 3 and 4) of the surface potential sensors 11 to 14 via the resistor R86. The supplied signal is synchronously detected.

The synchronously detected signal is supplied to an integrating circuit 32 and converted into a direct current. The illustrated integration circuit 32 includes an operational amplifier IC2, a capacitor C6, a diode D3, and an output resistor R6. The transistor Q4 and the light emitting diode PCA are connected to the output resistor R6, and the light emitting diode PCA emits light by the output of the integrating circuit 32.

The high voltage amplifier circuit 33 includes an oscillation circuit, a transformer T2, and a triple voltage rectifier circuit. The oscillation circuit is
Transistor Q2, Q3, primary winding Np of transformer T2
2. It includes an auxiliary winding Nb2 provided in the transformer T2, a capacitor C3, and an inductor L1. Transistor Q
2, the switching operation of Q3 excites the primary winding Np2 of the transformer T2 and, through the auxiliary winding Nb inductively coupled to the primary winding Np2,
2. Supply a feedback signal to the base of Q3. The transistors Q2 and Q3 are driven by the above-described feedback signal and the resonance phenomenon of the LC resonance circuit including the capacitor C3 and the inductor L1.
Continue the self-oscillation operation.

The triple voltage rectifier circuit is connected to the secondary winding NS of the transformer T2 and triple rectifies the AC voltage generated in the secondary winding NS with the oscillating operation of the oscillation circuit.
The rectified voltage is applied to a common ground line C. Supply to GND. Thereby, the common ground line C.I. The potential of GND is controlled. The illustrated triple voltage rectifier circuit includes capacitors C7 to C9 and diodes D4 to D6.

The input side of the high-voltage amplifier circuit 33 is connected to an input circuit composed of a phototransistor PCB and a transistor Q4. The phototransistor PCB is optically coupled to a light emitting diode PCA driven by the output of the integration circuit 32. Therefore, a voltage controlled according to the output signal of the integration circuit 32 is supplied to the input side of the oscillation circuit constituting the high-voltage amplification circuit 33.

Common ground line C. The potential of GND is converted to an appropriate potential by the detection signal output circuit 35 and output as a surface potential detection signal Z.

The DC / DC converter 34 includes a transformer T
DC input voltage V supplied through the primary winding NP1
IN is switched by the switching element Q1. The voltage generated in the secondary windings NS1 and NS2 of the transformer T1 due to the switching operation is changed to diodes D1 and D2.
2 and smoothed by capacitors C4 and C5 to generate DC voltages + Vcc and -Vcc. The DC voltage is stabilized by the Zener diode ZD2, and the amplifier circuit 30, the synchronous detection circuit 31,
It is supplied to the integrating circuit 32 and the light emitting diode PCA.

As described above, the surface potential detecting device according to the present invention is characterized in that the signal processing circuit 3
The bias circuit 36 of FIG. First bias circuit 36
Is a common ground line C. Based on GND, the positive power supply voltage (+ Vcc) supplied from the power supply circuit 34 is divided by the resistor R81 and the Zener diode D81. The constant voltage appearing at both ends of the Zener diode D81 is divided by the resistors R82 and R83, and the divided positive voltage V
3 is supplied to a non-inverting input terminal (+) of the integrating circuit 32.

FIG. 6 is an electric circuit diagram showing a specific example of the switching circuit. The illustrated switching circuit 2 includes a first switching circuit 201 to a fourth switching circuit 204. The first switching circuit 201 includes a detection signal S11 and a synchronization signal S12 supplied from the surface potential sensor 11 provided on the photosensitive drum K.
Switch SW11 (K) and SW12 (K) for selecting SW11 (K) and SW12 (K) are C
Driven simultaneously by drive circuits DR11 and DR12 composed of MOS. That is, an interlocking operation is performed. From the microcomputer 42 to the first switching circuit 201, the photodiode PD1 and the phototransistor P
The drive signal CS1 is supplied via a photocoupler based on T1.

The second switching circuit 202 includes a switch SW21 for selecting a detection signal S21 and a synchronization signal S22 supplied from the surface potential sensor 12 provided on the photosensitive drum Y.
(Y) and SW22 (Y). These SW21
(Y) and SW22 (Y) are simultaneously driven by drive circuits DR21 and DR22 composed of CMOS. The drive signal CS2 is supplied from the microcomputer 42 to the second switching circuit 202 via a photocoupler including the photodiode PD2 and the phototransistor PT2.

The third switching circuit 203 includes a switch SW31 for selecting a detection signal S31 and a synchronization signal S32 supplied from the surface potential sensor 13 provided on the photosensitive drum M.
(M) and SW32 (M). These switches S
W31 (M) and SW32 (M) are simultaneously driven by drive circuits DR31 and DR32 composed of CMOS. From the microcomputer 42 to the third switching circuit 20
3, the drive signal CS3 is supplied via a photocoupler including the photodiode PD3 and the phototransistor PT3.

The fourth switching circuit 204 includes a switch SW41 for selecting a detection signal S41 and a synchronization signal S42 supplied from the surface potential sensor 14 provided on the photosensitive drum C.
(C) and SW42 (C). These switches S
W41 (C) and SW42 (C) are simultaneously driven by drive circuits DR41 and DR42 composed of CMOS. From the microcomputer 42 to the fourth switching circuit 20
4 is supplied with a drive signal CS3 via a photocoupler including a photodiode PD4 and a phototransistor PT3. Reference numerals R51 to R54 are resistors.

FIG. 7 shows a microcomputer 42, D / A
The specific circuit configuration of the conversion circuit 41, the second bias circuit 39, and the detection signal output circuit 35 is shown. The microcomputer 42 is a so-called one-chip microcomputer.

As the D / A conversion circuit 41, various types can be used. The illustrated D / A conversion circuit 41 is called an R-2R ladder type.
A set of the resistor R and the resistor 2R is connected to form a ladder resistor circuit. As described above, the memory 44 of the microcomputer 42 (FIG. 3)
) Stores negative bias voltage information to be applied in the second bias circuit 39 for each of the surface potential sensors 11 to 14. The negative bias voltage information read from the microcomputer 42 is D /
The signal is supplied to the A conversion circuit 41 and is converted into an analog amount of a negative voltage signal. The voltage signal converted into an analog amount by the D / A conversion circuit 41 is supplied to the second bias circuit 39 via the buffer circuit 47.

In the embodiment shown in FIG. 7, the second bias circuit 39 is constituted by an operational amplifier IC7 having an inverting input terminal (-) and a non-inverting input terminal (+). An input resistor R66 is connected to the inverting input terminal (-) of the operational amplifier, a resistor R67 is connected between the inverting input terminal (-) and the output terminal, and the non-inverting input terminal (+) is connected to the frame ground GND. Has become. The gain G of the second bias circuit 39 is (-R67 / R66).

As an example, the microcomputer 42
Is set so that the bias voltage can be adjusted, for example, in the range of 0 to 5V. Therefore, in this case, the analog bias voltage output from the D / A conversion circuit 41 changes, for example, in the range of 0 to 5V. When the analog voltage output from the D / A conversion circuit 41 changes in a range of 0 to 5 V, the second bias circuit 39 outputs an analog voltage signal that changes in a range of 0 to 250 mV. Can be.

The signal processing circuit 3 further includes a detection signal output circuit 35. The detection signal output circuit 35 includes an operational amplifier I having an inverting input terminal (-) and a non-inverting input terminal (+).
C8. The inverting input terminal (−) of the detection signal output circuit 35 is guided to the middle point of the power supply circuit 34. The inverting input terminal (-) of the operational amplifier IC8 has an input resistor R68.
Is connected, and a resistor R69 is connected between the inverting input terminal (−) and the output terminal. The output signal of the second bias circuit 39 is supplied to the non-inverting input terminal (+). The gain G of the detection signal output circuit 35 is (−R6
9 / R68).

The microcomputer 42 further includes a photocoupler (PD1, P2) constituting an input circuit of the switching circuit 2.
T1) to (PD4 to PT4), drive signals CS1 to CS
4 are supplied at different timings.

FIG. 8 is a time chart showing a specific example of the timing for supplying the drive signals CS1 to CS4. First, as shown in FIG. 8A, the surface potential sensor 11 provided on the photosensitive drum K is between time t1 and time t2.
Is supplied to the photocouplers (PD1, PT1) in order to select only the signal output from. At this time, a digital amount of negative bias voltage information corresponding to the surface potential sensor 11 is read from the microcomputer 42. This digital amount of negative bias voltage information is supplied to the D / A conversion circuit 41 and converted into an analog amount of negative voltage signal. The analog voltage signal output from the D / A conversion circuit 41 is
Is supplied to the second bias circuit 39 via the

Next, as shown in FIG. 8 (b), in order to select only the signal output from the surface potential sensor 12 provided on the photosensitive drum Y between t3 and t4.
A drive signal CS2 having a logical value of 0 is converted to a photocoupler (PD2,
PT2). At this time, the microcomputer 4
2, the digital amount of negative bias voltage information corresponding to the surface potential sensor 12 is read. This digital amount of negative bias voltage information is supplied to the D / A conversion circuit 41 and converted into an analog amount of negative voltage signal. D / A
The voltage signal converted into the analog amount by the conversion circuit 41 is supplied to the second bias circuit 39 via the buffer circuit 47.

As shown in FIG. 8C, from time t5 to time t6
During the time, in order to select only the signal output from the surface potential sensor 13 provided on the photosensitive drum M, the drive signal CS3 having the logical value 0 is converted to the photocoupler (PD3, PT3
Supply to 3). At this time, a digital amount of negative bias voltage information corresponding to the surface potential sensor 13 is read from the microcomputer 42. This digital amount of negative bias voltage information is supplied to the D / A conversion circuit 41 and converted into an analog amount of negative voltage signal. The voltage signal converted into the analog amount by the D / A conversion circuit 41 is supplied to the second bias circuit 3 via the buffer circuit 47.
9.

Further, as shown in FIG. 8D, in order to select only the signal output from the surface potential sensor 14 provided on the photosensitive drum C between time t7 and time t8,
A drive signal CS4 having a logical value of 0 is converted to a photocoupler (PD4,
PT4). At this time, the microcomputer 4
From 2, the digital amount of negative bias voltage information corresponding to the surface potential sensor 14 is read. This digital amount of negative bias voltage information is supplied to the D / A conversion circuit 41 and converted into an analog amount of negative voltage signal. D / A
The voltage signal converted into the analog amount by the conversion circuit 41 is supplied to the second bias circuit 39 via the buffer circuit 47.

In the surface potential detecting device of the present invention, as described above, the signal processing circuit 3 includes the first bias circuit 36 and the second bias circuit 39. Next, the operation of the first bias circuit 36 and the second bias circuit 39 will be described with reference to FIG.
This will be described with reference to FIG.

FIG. 9 is a graph showing the relationship between the surface potential Vsu of the surface to be measured and the surface potential detection signal Z when a bias is applied by applying a positive voltage (+ V3) by the first bias circuit 36. In FIG. 9, a characteristic L01 is an ideal characteristic, a characteristic L03 is a characteristic when no bias is applied by the first bias circuit 36, and a characteristic L02 is a characteristic when the bias is applied by the first bias circuit 36. Characteristic L
03 has a dead zone where the surface potential detection signal Z = 0 until the surface potential Vsu of the surface to be measured exceeds Vsu = Vsu1.
By applying a positive voltage (+ V3) from the first bias circuit 36 to the non-inverting input terminal (+) of the integration circuit 32, a characteristic L02 obtained by adding a voltage △ Z1 to the characteristic L03.
Can be obtained. In the characteristic L02, when the surface potential Vsu of the surface to be measured is zero, the surface potential detection signal Z becomes
It becomes a positive offset voltage Vos. Therefore, a dead zone does not occur.

Further, the positive voltage (+ V3) applied from the first bias circuit 36 to the non-inverting input terminal (+) of the integration circuit 32 is applied to the common ground line C. It is based on the potential of GND. Therefore, a stable offset voltage Vos
Is obtained.

Further, since the power supply circuit 34 is not required to have a high degree of voltage stability accuracy, a surface potential detecting device in which the power supply circuit 34 is reduced in cost can be obtained. In the embodiment, a dropper-type stabilized power supply (three-terminal regulator) that was required in the past
Is omitted. Therefore, a surface potential detecting device with high overall power efficiency can be obtained.

The offset voltage Vos generated by the first bias circuit 36 is canceled by the second bias circuit 39.

FIG. 10 is a graph showing the relationship between the surface potential Vsu of the surface to be measured and the surface potential detection signal Z when a negative voltage (-V5) is applied by the second bias circuit 39. As shown in FIG. 10, the surface potential V
The characteristic of the surface potential detection signal Z with respect to su can be shifted in a direction in which the value of the surface potential detection signal Z becomes lower with respect to the surface potential Vsu of the same measured surface. Movement amount ΔZ
2 corresponds to the magnitude of the negative voltage (-V5). Therefore,
By adjusting the magnitude of the negative voltage (−V5), the first
Offset voltage Vo generated by the bias circuit 36 of FIG.
s can be canceled, and the relationship between the surface potential Vsu of the surface to be measured and the surface potential detection signal Z can be set on the graph to the relationship of the primary straight line L01 passing through the origin (0, 0). As a result, a surface potential detection signal Z having no offset voltage Vos can be obtained. When the magnitude of the negative voltage (−V5) is adjusted to be equal to the magnitude of the offset voltage Vos of the characteristic L02, the characteristic L02 is changed to the ideal characteristic L2.
01 can be set.

The offset voltage canceling operation shown in FIG. 10 is executed for each detection signal output from the surface potential sensors 11 to 14 in accordance with the selection timing of the surface potential sensors 11 to 14. Therefore, the surface potential sensors 11 to 14
, Only one microcomputer 42, one D / A conversion circuit 41, and one second bias circuit 39 need be provided. For this reason,
The circuit configuration is simplified, the size is reduced, the number of circuit components is reduced, and the cost is reduced. This advantage becomes more remarkable as the number of surface potential sensors 11 to 14 increases.

[0104]

As described above, according to the present invention, the following effects can be obtained. (A) It is possible to provide a surface potential detecting device capable of simplifying a circuit, reducing the size, reducing the weight, and reducing the cost. (B) It is possible to provide a surface potential detection device capable of eliminating an offset generated in a surface potential detection output signal. (C) An inexpensive surface potential detecting device can be provided. (D) It is possible to provide a surface potential detecting device capable of realizing high total power efficiency.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration in a case where surface potentials of four photosensitive drums arranged in tandem are detected by using a surface potential detecting device according to the present invention.

FIG. 2 is a time chart showing a specific example of a signal selecting operation of a switching circuit included in the surface potential detecting device according to the present invention.

FIG. 3 is a block diagram more specifically showing the configuration of the surface potential detecting device according to the present invention.

FIG. 4 is a diagram showing a specific circuit configuration of a surface potential sensor included in the surface potential detection device according to the present invention.

FIG. 5 is an electric circuit diagram showing a specific example of the surface potential detecting device according to the present invention.

FIG. 6 is a diagram showing a specific circuit configuration of a switching circuit included in the surface potential detecting device according to the present invention.

FIG. 7 is a diagram showing a more specific circuit configuration of a signal processing circuit included in the surface potential detecting device according to the present invention.

FIG. 8 is a time chart showing a specific example of timing for supplying a drive signal in the configuration shown in FIGS.

FIG. 9 is a graph showing a relationship between a surface potential detection signal and a surface potential of a surface to be measured when a positive voltage is applied and biased by a first bias circuit.

FIG. 10 is a graph illustrating a relationship between a surface potential of a surface to be measured and a surface potential detection signal when a negative voltage is applied by a second bias circuit.

[Explanation of symbols]

 11-14 Surface potential sensor 2 Switching circuit 3 Signal processing circuit K, Y, M, C Photosensitive drum

Claims (8)

[Claims] 1. A plurality of surface potential sensors, a switching circuit,
A surface potential detection device including a signal processing circuit, wherein each of the plurality of surface potential sensors is independent of each other, and the switching circuit outputs a signal supplied from the surface potential sensor to the surface potential sensor. For each, select and output at different time timings, the signal processing circuit is one, is connected to the plurality of surface potential sensors via the switching circuit, and passes through the switching circuit to a plurality of surfaces Each detection signal supplied from the potential sensor is processed to generate a DC surface potential detection signal corresponding to each detection signal. The signal processing circuit includes a first bias circuit, a microcomputer, a digital-analog A conversion circuit;
Wherein the first bias circuit is a circuit for applying a positive bias voltage for increasing the level of the surface potential detection signal to the detection signal, wherein the microcomputer The digital bias voltage information set in advance for each surface potential sensor, and having stored digital bias voltage information, driving the switching circuit, and assigning the digital bias voltage information assigned to the surface potential sensor selected by the driven switching circuit. Outputting the digital-analog conversion circuit, digital bias voltage information supplied from the microcomputer,
The second bias circuit is converted into an analog voltage signal and output. The second bias circuit is added by the first bias circuit in the surface potential detection signal based on the analog voltage signal supplied from the digital-analog conversion circuit. A surface potential detecting device that applies a negative bias voltage that cancels out a positive bias voltage. 2. The surface potential detection device according to claim 1, wherein the surface potential sensor generates an AC signal by periodically changing an electric field between a detection electrode and a surface to be measured. The switching circuit, a synchronization signal synchronized with the drive signal for generating the AC signal and the AC signal generated in the surface potential sensor, for each surface potential sensor,
A surface potential detection device that selects and outputs the signals at different timings, and wherein the signal processing circuit processes the detection signal and the synchronization signal supplied via the switching circuit for each of the surface potential sensors. 3. The surface potential detection device according to claim 1, wherein the plurality of surface potential sensors and the signal processing circuit have a common ground line serving as an operation reference potential. The common ground line is at a floating potential with respect to a ground potential, and the signal processing circuit is configured such that the potential of the common ground line is
A surface potential detecting device for controlling the potential of the common ground line so as to be substantially equal to the potential of the surface to be measured. 4. The surface potential detecting device according to claim 3, wherein the signal processing circuit further includes a synchronous detection circuit, an integration circuit, and a high-voltage amplifier circuit. Detects the AC signal supplied from the surface potential sensor in synchronization with the periodic operation of the surface potential sensor, the integration circuit includes an operational amplifier having an inverting input terminal and a non-inverting input terminal, A detection output signal from a synchronous detection circuit is supplied to the inverting input terminal, and the supplied detection output signal is converted into DC and output, and the high-voltage amplifier circuit is supplied with a signal from the integration circuit, and outputs the common signal. A high DC voltage that makes the potential of the ground line substantially equal to the potential of the surface to be measured is supplied to the common ground line, and the first bias circuit is configured to generate a positive voltage based on the potential of the common ground line. , Said integration circuit of said non-inverting surface potential detecting apparatus for applying to the input terminal. 5. The surface potential detecting device according to claim 4, wherein the first bias circuit includes a Zener diode and a resistance voltage dividing circuit, and a cathode of the Zener diode has the common ground. A surface potential detecting device which is guided to a line, wherein the resistance voltage dividing circuit is connected in parallel to the Zener diode, and a divided voltage is supplied to a non-inverting input terminal of the integrating circuit. 6. The surface potential detecting device according to claim 1, wherein the signal processing circuit further includes a detection signal output circuit, wherein the detection signal output circuit has an inverting input terminal. And an operational amplifier having a non-inverting input terminal, wherein the inverting input terminal is guided to the common ground line, and outputs a surface potential detection signal. The second bias circuit includes the non-inverting input of the detection signal output circuit. Surface potential detector that applies a negative voltage to terminals. 7. The surface potential detection device according to claim 1, wherein each of the plurality of surface potential sensors includes a detection electrode,
A chopper, a preamplifier, and a chopper drive circuit, wherein the detection electrode generates an electric field for measuring a surface potential of the surface to be measured in a non-contact manner, and the chopper includes the surface to be measured and the chopper. The preamplifier is a circuit for periodically chopping an electric field between the detection electrode and the electric field, and the preamplifier is a circuit for converting an impedance of an AC signal detected by the detection electrode, and the chopper driving circuit excites the chopper. Surface potential detection device that is a circuit. 8. The surface potential detection device according to claim 1, wherein the switching circuit selects the detection signal and the synchronization signal simultaneously for each of the surface potential sensors. A surface potential detecting device that performs an interlocking selection operation.