JPS6256985B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface electrometer for measuring the surface potential of an object to be measured, such as a photoreceptor of an electronic copying machine.

In conventional surface electrometers, the detection output fluctuates when the distance between the measurement electrode and the object to be measured changes, so vibrations of the device including the electrometer lead to fluctuations in the detection output. was required. In order to prevent such drawbacks, the present invention amplifies the detection signal with a high voltage amplifier circuit after DC regeneration, and returns it to the shielding means that shields the electric field between the measurement electrode and the object to be measured, thereby forming a negative feedback loop. Form. With this configuration, the potential difference between the electrometer and the object to be measured is always 0V, and the surface potential can be detected without being affected by the distance between the object to be measured and the electrometer.

However, in general, when the open gain of the high voltage amplifier circuit is very large, the negative feedback control system becomes unstable and tends to oscillate. For example, DC−
When a DC inverter circuit is used as an amplifier circuit, the input voltage decreases, and when it reaches a certain voltage, the oscillation of the inverter transformer suddenly becomes unstable and tries to stop. At this time, the input/output characteristics lose their linearity and the output drops rapidly. This is equivalent to a sudden increase in amplification gain. That is, if negative feedback is applied with this point as the operating point, the negative feedback control system becomes unstable and the output oscillates.

The object of the present invention is to provide a surface electrometer which eliminates the above-mentioned drawbacks. More specifically, the object is to provide a surface electrometer equipped with a limiter circuit that limits the input voltage to a high voltage amplifier circuit within the stable operating range of the amplifier circuit in order to prevent the negative feedback loop from breaking. It is said that

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1a is a sectional view of a copying apparatus to which the present invention can be applied.

The surface of the drum 47 is made of a three-layer seamless photoconductor using a CdS photoconductor, is rotatably supported on a shaft, and is rotated in the direction of the arrow by a main motor 71 activated when the copy key is turned on. Start.

When the drum 47 rotates at a predetermined angle, the original placed on the original platen glass 54 is moved to the first scanning mirror 44.
illuminated by an illumination lamp 46 integrally configured with
The reflected light is scanned by the first scanning mirror 44 and the second scanning mirror 53. The first scanning mirror 44 and the second scanning mirror 53 move at a speed ratio of 1/2, so that the original is scanned while the optical path length in front of the lens 52 is always kept constant.

The above reflected light image shows the lens 52 and the third mirror 55.
After passing through, an image is formed on a drum 47 at an exposure section.

The drum 47 is simultaneously neutralized by the pre-exposure lamp 50 and the pre-AC charger 51', and then corona-charged (for example, +) by the primary charger 51. Thereafter, the drum 47 is the exposure section, and the image irradiated by the illumination lamp 46 is slit-exposed.

At the same time, AC or corona charge removal with a polarity opposite to the primary one (for example -) is performed by a charge remover 69, and then a high-contrast electrostatic latent image is formed on the drum 47 by uniform surface exposure using the entire surface exposure lamp 18. The electrostatic latent image on the photosensitive drum 47 is then liquid-developed by the developing roller 65 of the developing device 62 and visualized as a toner image, and the toner image is transferred by the pre-transfer charger 61.

Upper cassette 10 or lower cassette 11
The transfer paper inside is fed into the machine by a paper feed roller 59, and is sent toward the photosensitive drum 47 with accurate timing by a register roller 60, so that the leading edge of the latent image and the leading edge of the paper are aligned at the transfer section. Can be done.

Next, while the transfer paper passes between the transfer charger 42 and the drum 47, the toner image on the drum 47 is transferred onto the transfer paper.

After the transfer is completed, the transfer paper is separated from the drum 47 by the separation roller 43, sent to the conveyance roller 41, guided between the hot plate 38 and press rollers 40, 41, and fixed by pressure and heat. The paper is discharged to the tray 34 by the discharge roller 37 via the paper detection roller 36 .

After the transfer, the drum 47 continues to rotate and its surface is cleaned by a cleaning device comprising a cleaning roller 48 and an elastic blade 49, and the process proceeds to the next cycle.

Here, a surface electrometer 67 for measuring the surface potential is connected to the drum 47 between the entire surface exposure lamp 18 and the developing device 62.
mounted in close proximity to the surface of the

As a cycle executed prior to the above copy cycle, there is a step of pouring a developer into the cleaning blade 49 while the drum 47 is stopped after the power switch is turned on. Hereinafter referred to as Priwetsu. This is to flush out the toner accumulated near the cleaning blade 49 and to provide lubrication to the contact surface between the blade 49 and the drum 47. After the prewetting time (4 seconds), the drum 47 is rotated, residual charges and memory on the drum 47 are erased using a pre-exposure lamp 50, a pre-AC static eliminator 51', etc., and the drum surface is cleaned using a cleaning roller 48 and a cleaning blade 49. There are steps. Hereinafter, this will be referred to as forward rotation. This is to make the sensitivity of the drum 47 appropriate and to form an image on a clean surface. The pre-wet time and pre-rotation time (number) are automatically changed depending on various conditions (described later).

Also, as a cycle after the set number of copy cycles are completed, the drum 47 is rotated several times.
There is a step in which residual charges and memory on the drum are removed using an AC charger 69 or the like, and the drum surface is cleaned. Hereinafter referred to as post-rotation LSTR. This is to leave the drum 47 electrostatically and physically clean.

FIG. 1b is a plan view of the vicinity of the blank exposure lamp 70 in FIG. Blank exposure lamp 70-1
70-5 is turned on when the drum is rotating and not during exposure to erase the drum surface charge and prevent excess toner from adhering to the drum. However, the blank exposure lamp 70-1 is
Since the drum surface corresponding to 7 is irradiated, it is turned off momentarily when measuring the dark area potential with the surface electrometer 67. Also, for B size copies, the image area is A4
The blank exposure lamp 70-5 is turned on for the non-image area even when the optical system is moving forward. The lamp 70-0 is called a sharp cassette lamp, and the separation guide plate 4
The portion of the drum that is in contact with the drum 3-1 is irradiated with light to completely erase the charge on that portion, thereby preventing toner from adhering to the drum and contaminating the width of the separation gap. This sharp cassette lamp is always lit while the drum is rotating.

How does the surface potential of the photosensitive drum change corresponding to bright areas (areas that reflect a lot of light) and dark areas (areas that reflect less light) of a document at the processing position of the copying process of such an electrophotographic copying device? Figure 2 shows how to do this. What is necessary for the final electrostatic latent image is the surface potential at the point in the figure, but the surface potentials in the dark and bright areas ○a and ○b will increase when the ambient temperature of the photosensitive drum 47 increases. The photosensitive drum 47 changes as shown in FIG.

In order to compensate for changes in surface potential due to temperature changes or aging, a surface electrometer may be used, and the exposure amount, charging voltage, and development bias may be controlled based on its detection output.

The surface electrometer of this example will be explained below.

Figure 5 is a side sectional view of the surface electrometer, and Figure 6 is a side sectional view of the surface electrometer.
Figure 7 is a cross-sectional view taken along line X-X' in the figure and viewed from the right side of the drawing, Figure 7 is a cross-sectional view taken along line X-X' in Figure 5 and viewed from the left side of the figure, and Figure 8 A is the electric potential. FIG. 8B, which is a perspective view of the meter, is a sectional view of the vibrator 82.

In Figures 5, 6, 7, and 8, the entire electrometer is
It is covered with a metal casing 81 and a metal base 95 to eliminate the influence of external electric fields.

The housing 81 has an opening for a measurement window 88, and the measurement window 88 is opposed to the portion to be measured of the drum 47 to measure the potential.

A tuning fork-shaped vibrator 82 is attached to the base 95 in an electrically conductive state, and a driving piezoelectric element 84-
1 and feedback piezoelectric element 84-2, and when a DC voltage is applied to the power supply terminal, self-excited vibration occurs at the mechanical resonance frequency of the vibrator 82. One tip of the vibrating piece of the tuning fork constitutes a chopper electrode 83, and the vibration of the vibrator causes the measurement window 88 to open and close at regular intervals. There is a printed circuit board 8 on the back side of the chopper electrode.
6 is fixed, and a measurement electrode 85 having the same shape as the window is formed at a position facing the measurement window side using a copper foil pattern on a printed board.

The electric lines of force based on the surface charge of the photosensitive drum 47 are:
The electric force enters the measuring electrode 85 through the measuring window 88, but the chopper electrode 83 located between the measuring window 88 and the measuring electrode interlinks and cuts the lines of electric force due to the vibration of the vibration 82. An alternating current voltage having an amplitude proportional to the voltage difference between the surface potential on the photosensitive drum 47 and the chopper electrode (same potential as the shield member potential) is induced in the measurement electrode.

The AC signal is passed through a preamplifier circuit 101 composed of a source follower built into the printed board 86.
After being converted into a low impedance signal, it is taken out as the output of the electrometer.

Reference numeral 89 in FIG. 8A is an internal shield member for preventing the drive signal of the piezoelectric element 84 from being guided to the measurement electrode 85.

As shown in FIG. 13, piezoelectric elements 84-1 and 84-2 are bonded to the fulcrum side of the vibrating element of the vibrator 82 at opposing positions in the length direction using a conductive adhesive.

84-1 and 84-2 are piezoelectric elements that generate distortion in the plane direction when an electric field is applied in the thickness direction.
When it is sandwiched between electrodes 99 and fixed with a conductive adhesive 98 to the vibrating piece of the vibrator 82 made of an elastic metal such as phosphor bronze, it forms a unimorph vibrator together with the vibrating piece. Since the shape of the piezoelectric element is elongated in the length direction of the vibrating piece, when an electric field is applied in the thickness direction, distortion occurs in the length direction of the vibrating piece. The drive circuit will be described later.

To attach the electrometer to the copying machine body, use the support base 95.
is fixed to a board 97, and this printed board is supported on the main body by a board connector 94 and a board guide 87.

On the connector insertion side of the printed circuit board 67, the connector contact terminal part is made of copper foil, and supplies power to the electrometer and takes out the output signal, making it possible to easily remove the electrometer. There is.

As described above, driving the tuning fork with a piezoelectric element does not require a high-precision small motor, compared to the conventionally proposed method of intermittent electric lines of force driven by a motor, resulting in lower costs. Since the resonant frequency of the piezoelectric element is constant, highly accurate detection and device control based on the detection are possible.

Next, the potential measuring circuit of such a surface electrometer will be explained with reference to the block diagram of FIG. 9 and FIG. 10 showing output waveforms of each part of FIG. 9.

When the electric field between the photosensitive drum 47 and the electrometer 67 is intermittently shielded by the tuning fork vibrator 82, a high impedance alternating current signal is induced in the electrode 85, which is converted into a low impedance signal _S1 by the preamplifier 101 and output. Ru. The signal S1 has noise removed by a bandpass filter 102, and is input to a clamp circuit 113 via an amplifier 103 and a photocoupler 104. Input signal S to clamp circuit 113
The phase of signal S2 changes somewhat compared to signal S1. on the other hand,
When a transducer drive signal is input to the terminal 120, the transducer drive circuit 105 operates via the photocoupler 107 and the transducer drive switch 106 to drive the transducer 8.
Vibrate 2. A feedback signal S3 from the vibrator is input to a synchronous circuit composed of a phase shift circuit 108a and a comparator 108b. Since the feedback signal S3 is different in phase from the signal S1 but synchronized with it, the phase is advanced by the phase shift circuit 108a, and the phase is advanced by the comparator 108b.
A signal S5 is obtained by comparing with
Furthermore, the synchronization signal S6 is obtained by differentiating it in a differentiating circuit 111.

This synchronization signal S6 is input to the clamp circuit 113 via the display circuit 112.

The clamp circuit 113 clamps the signal S2 using the synchronizing signal S6 to determine whether the surface potential of the photosensitive drum 47 is positive or negative, and obtains an output signal S7. Signal S
7 is integrated by an integrating circuit 114 and converted into a DC signal, which is then passed through an inverter drive circuit 115 and converted into a high voltage by a DC-DC inverter 116. DC−DC
The output voltage of the inverter 116 is fed back to the vibrator 82 and the electrometer housing 81 as a shield potential.

Here, from the preamplifier 101 to the inverter 116
If we consider the circuit up to this point as one amplifier, it works as an inverting amplifier. That is, when the surface potential of the drum is smaller than the shield potential, the inverting amplifier
It works to raise the shield potential, and when it is higher than the shield potential, lowers the shield potential, and finally works so that the sensor shield potential and the drum surface potential become the same.

Therefore, the output obtained by dividing the shield potential by the voltage dividing circuit 117 becomes the surface potential of the drum 47. All circuits surrounded by a broken line 100 use the output voltage of the inverter 116 as a reference voltage (ground), and are all insulated from external circuits by photocouplers to prevent the generation of noise.

That is, the output obtained by dividing the potential of the sensor by the voltage dividing circuit 117 measures the surface potential of the drum. All circuits within 100 surrounded by broken lines are circuits that use the output voltage of the DC-DC inverter 116 as the reference voltage (earth side potential), and are all insulated from external circuits by photocouplers to prevent noise generation. There is.

Each circuit in FIG. 9 will be explained in detail below. A detailed circuit diagram within the broken line 100 is shown in FIG. When a vibrator drive signal is input to the terminal 120, the photocoupler 107 consisting of a light emitting diode and a phototransistor connects the transistor Tr5.
0 turns on, and V _C is applied to transistors Tr51 and Tr52.
_C power is supplied. When the vibrator 82 vibrates, a feedback signal is output from the piezoelectric element 84-2, and is supplied to the base of the transistor Tr51 via the capacitor C50. The signal is current amplified by Tr51, and further amplified by transistor Tr52 via resistor R52 and capacitor C52. The output of transistor Tr52 drives piezoelectric element 84-1 via capacitor C54. As described above, the entire circuit forms an oscillation loop. The output of the transistor Tr51 is inputted via a capacitor C55 to a phase shift circuit including an operational amplifier Q50.

The phase shift circuit has constant amplitude characteristics over the entire frequency range, and only the phase characteristics are a function of frequency. The phase shift circuit of this embodiment is a linear type, and the phase can be changed between approximately 0° and 180° by changing the variable resistor VR51. It is used as a volume. By adjusting VR51, it is possible to compensate for differences in the frequency of the detection signal due to the shape of the vibrator or differences in phase shift due to variations in precision of the components of the bandpass filter. Therefore, it is possible to compensate for different phases for each surface electrometer, and it is possible to stably perform DC reproduction of the detected AC signal. In addition, conventional synchronization signals are disclosed in Japanese Patent Application Laid-Open No. 1983-67475,
As shown in Japanese Patent No. 67473, the position of the chopper was detected optically or magnetically. This required very high design precision for the chip, was expensive, and was difficult to adjust. In this embodiment, since the synchronization signal is obtained from the chopper drive signal, this problem does not occur, and accurate timing adjustment can be easily performed.

The output signal S4 of the phase shift circuit, which is an alternating current signal, is used as a comparator using an operational amplifier Q51 as a base, detects the zero cross point as an edge portion of the output square wave, and transmits and outputs it to the photocoupler 110. On the other hand, the high-impedance measurement signal induced in the measurement electrode 85 is converted into a low-impedance signal by the FETQ101, and is amplified via the capacitor C101 by a multiple feedback bandpass circuit with an operational amplifier Q12 as its base, and noise is removed. . A variable resistor VR101 adjusts the center frequency of the filter. The output of operational amplifier Q102 is connected to operational amplifier Q103 via capacitor C104.
After being amplified by the measurement signal, the light emitting diode LED 101 of the photocoupler 104 emits light with a volatility corresponding to the measurement signal. A variable resistor VR102 adjusts the amplification gain. An output S2 corresponding to the current flowing through the phototransistor Q104 of the photocoupler is input to the clamp circuit.

FIG. 11 shows a detailed circuit diagram outside the broken line 100 in FIG. 9.

The output of the photocoupler 104 is applied to a terminal P119, current amplified by an emitter follower Tr104, and then connected to a capacitor C207.

The opposite terminal of capacitor C207 is
FETTr105 source follower gate and Tr
It is connected to the drain of the FET switch of Tr106, and when the FET switch of Tr106 is cut off,
The gate of Tr105 and the drain of Tr106 are
Since the impedance becomes high, there is no place for the charge charged in C207 to escape, so Tr1 of C207
The terminal potential on the gate side of No. 5 changes in the same way as the terminal potential on the opposite side.

On the other hand, the output signal S5 of the photocoupler 110 is differentiated by a capacitor C228 and a resistor R228, and only a positive differential waveform is taken out by a diode D106 to obtain a synchronization signal S6. The transistor Tr107 is made conductive by the pulse output of the synchronization signal S6.

When the transistor Tr107 becomes conductive, the light emitting diode LED101 lights up. The LED 101 repeatedly lights up with short pulses only when the vibrator drive circuit and the vibrator are operating normally. accordingly
By looking at the LED 101, abnormalities in the vibrator drive circuit and the vibrator can be detected. Furthermore, a light emitting diode as a display element is connected in series to the synchronization signal transmission path, so no special display circuit is required for abnormality detection. Furthermore, since the circuits within the dashed line 100, such as the drive circuit 105, use a float power supply, a large display current cannot be passed at low voltage, and high voltage is applied, which affects the life of the element. A protection circuit is required for safety, but in this embodiment, a light emitting diode is provided outside the circuit using the float power supply, so a large current can be obtained with a low voltage.

The operational amplifier Q105 has an output of 0 to ±5V using Zener diodes ZD1 and ZD2 for clamping.
Therefore, when Tr107 becomes conductive, the cathode of diode D101 is biased to +12V, D101 is cut off, and the FET switch Tr
There is zero bias between the source and gate of 106,
The drain and source of the Tr 106 are brought into conduction. When Tr106 becomes conductive, operational amplifier Q1
The feedback loop of 05 is FET switch Tr106,
Since it is formed by the route of source follower Tr105 and resistor R221, the potential difference between the two input terminals of Q105 becomes zero, and considering that the input resistance of Q105 is high, the source potential of Tr105 becomes OV.
(ground potential), and the terminal voltage on the gate side of Tr105 of C207 changes from OV to the gate side of Tr105.
It comes to be biased to a potential shifted by the source-to-source voltage.

When the pulse output timing of the output signal S6 of the terminal P120 ends, the transistor Tr107 is cut off and the diode D101 becomes conductive.
Current flows through resistors R226 and R230, and Tr10
Conversely, the gate of Tr10 is deeply biased.
6 is the cutoff.

When Tr106 is cut off, as described above, the terminal voltage on the gate side of Tr105 of capacitor C207 shows the same change as the terminal potential on the opposite side.

S2 in FIG. 10 is a measurement signal applied to the terminal P119; the solid line indicates the case where the measurement potential is positive with respect to the chopper potential, and the broken line indicates the case where the measurement potential is negative with respect to the chopper potential.

As shown in S7, the negative peak of the output of the clamp circuit is clamped to OV (ground potential) when the measured potential is positive with respect to the chopper potential, and a waveform having the same shape as the signal S2 is obtained.

Further, as shown by the broken line S7, when the measured potential is negative with respect to the chopper potential, a waveform having the same shape as the signal S in which the positive peak is clamped to 0V is obtained. The output S7 is integrated by a resistor R231 and a capacitor C208 in an integrating circuit 114, and is connected to an inverter drive circuit 115.

Inverter transformer T101, transistor
Inverter 1 consisting of Tr110 and Tr111
16 is a variable inverter, and after multiplication rectification by diodes D102 and D103, the
A DC high voltage output of 1.5KV is taken out to both ends of the capacitor C211 and resistor R246, and the low voltage side terminal of the transformer T101 is connected to the -600V output terminal obtained from the power supply circuit (not shown), so D102 -300V to + on the cathode of
A variable output of 900V can be obtained. The output of the integrating circuit 113 is connected to an operational amplifier Q107, which amplifies the potential difference between the output and the DC potential selected by a volume VR102, which will be described later.
It is applied to the common terminal on the primary side of the inverter transformer T101 via Tr108 and Tr109, and the output of Q107 is boosted by about 100 times by the inverter 115. VR103 is a volume for correcting offset voltage, and the DC potential applied to the negative input terminal of Q107 is almost OV (ground potential).
becomes. The output, that is, the feedback voltage V _F boosted by the transformer T101 is transmitted from the terminal P123 to the chopper section 83 and the casing 81 via the terminal P52 in FIG.
Since the part to be measured and the electrometer form a negative feedback control system, the potential difference between the inputs of the operational amplifier Q107 is zero, that is, the potentials of the chopper 83 and the casing 81 are adjusted. , and the voltage to be measured, making the input voltage to the positive input terminal of Q107 the same as the input voltage to the negative input terminal.

When the potential to be measured V _P is within the range of -300 to +900 V, the output voltage of the inverter 116, that is, the feedback voltage V
_F is always equal to the potential to be measured V _P .

The output of the DC-DC inverter 116 is attenuated to 1/301 by the voltage dividing circuit 117 and taken out to the output terminal 124. The detection output taken out from the terminal P124 is transmitted to a control circuit (not shown) and used to control the output voltage of the primary charger 51, the AC static eliminator 69, the developing bias voltage, etc.

The feedback voltage V _F of the terminal P123 is the chopper 8
3. Since the voltage applied to the casing 81 changes so that the potential difference with the potential to be measured _VP is always zero, the output taken out to the terminal P123 is applied to the preamplifier circuit 1.
This provides a stable output that is not affected by offsets and errors of each circuit from 01 to inverter 116.

Here, when the input voltage of the DC-DC inverter circuit 116, that is, the voltage between the terminals of the capacitor C216, is reduced, the transistor Tr11
0, the oscillation by Tr111 and inverter transformer T101 becomes unstable and attempts to stop. At this time, as shown in the input/output characteristic diagram of the inverter transformer in FIG. 13, when the input voltage becomes 3.5V or less, the output decreases rapidly and the gain increases. If negative feedback is applied using this point as the operating point, the negative feedback control system becomes unstable, resulting in poor circuit response or even oscillation. In this embodiment, in order to prevent such a situation from occurring, a limiter circuit consisting of a diode D107 and a Zener diode ZD3 is provided. Zener diode ZD3 and diode D10 are connected through resistor R256, which causes the output voltage of operational amplifier Q107 to become lower than a predetermined voltage.
6 becomes conductive, and the cathode voltage of the diode D8 is suppressed to a predetermined voltage. Since the predetermined voltage determines the base potential of the transistors Tr108 and Tr109, the emitter output thereof, that is, the input voltage of the DC-DC inverter circuit 116 is a voltage at which the amplification gain of the inverter rapidly increases, that is, the inverter attempts to stop oscillation. The voltage will never drop.

As described above, since the input voltage of an amplifier circuit such as a DC-DC inverter is limited by the limiter circuit, breakage of the negative feedback loop can be prevented.

In this embodiment, a tuning fork type vibrator was used as the tipper, but a rotary vane type tipper shown in JP-A No. 54-67473, a squirrel-cage rotary tipper shown in JP-A-54-67475, etc. It is also possible to use Also, the amplifier circuit of this example is a DC
-The present invention is applicable to all amplifier circuits that use DC inverters but have an unstable region.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a copying apparatus to which the present invention can be applied. Figure 1b is a plan view of the vicinity of the blank exposure lamp, Figure 2 is a characteristic diagram showing the surface potential of each part of the photosensitive drum, Figures 3 and 4 are characteristic diagrams showing changes in surface potential, and Figure 5 is a characteristic diagram showing changes in surface potential. A side view of the surface electrometer, Figure 6 is a sectional view taken from the right side taken along the line X-X' in Figure 5, Figure 7 is a sectional view taken from the left side taken from the line X-X' shown in Figure 5, and Figure 8. A is a perspective view of the electrometer, FIG. 8B is a sectional view of the vibrator 82, FIG. 9 is a block diagram of the potential measurement circuit, FIG. 10 is a signal waveform diagram of each part of FIG. 9,
Figures 11 and 12 are detailed circuit diagrams of each part in Figure 9,
FIG. 13 is an input/output characteristic diagram of the DC-DC inverter circuit. In the figure, 47 is a photosensitive drum, 82 is a tuning fork type vibrator, 83 is a tipper, 85 is a measurement electrode, 108
a is a phase shift circuit, VR51 is a variable resistor for phase adjustment,
LED101 is a light emitting diode, ZD3 is a Zener diode, and D107 is a limiter diode.

Claims (1)

[Claims] 1. In a surface electrometer that detects the surface potential of a measured object by intermittently shielding the space between the measuring electrode and the measured object using a shielding means, the detection signal induced in the measuring electrode is converted into a direct current. a DC regeneration circuit for regenerating a signal; a high voltage amplification circuit for amplifying the output of the DC regeneration circuit to a high voltage and feeding it back to the shielding means; A surface potential meter characterized by being equipped with a limiter circuit that limits . 2. The surface electrometer according to claim 1, wherein a DC-DC inverter circuit is used as the high voltage amplifier circuit.