JPH11237424A - Electric potential sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive potential sensor which can highly accurately and stably measure an electric potential of an object to be measured. SOLUTION: An electric potential sensor is composed of a detection electrode 2, a modulation device 13, a grid electrode 14 at a position separated by a distance Dg from the detection electrode 2, a grid power source device 15 for supplying a grid voltage Vg to the grid electrode 14, a power source control device 16 for controlling the grid voltage Vg supplied from the grid power source device 15 to the grid electrode 14 so as to make zero an amplitude of an a.c. signal resulting from an object to be measured and induced to the detection electrode 2, and an output circuit 17 for outputting a potential of the object to be measured according to V1=Vg (Ds/Dg) on the basis of the grid voltage Vg when the a.c. signal becomes zero. The modulation device 13 comprises a piezoelectric tuning fork 11 which periodically vibrates the detection electrode 2 to change a distance Ds thereby periodically modulating a capacitance generated between the object 1 to be measured and the detection electrode 2 and a tuning fork-driving circuit 12 for driving the tuning fork.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a potential sensor and, more particularly, to a potential sensor for measuring a potential of an object to be measured such as a photosensitive drum of an electrophotographic copying machine in a non-contact manner.

[0002]

[Prior art]

Conventionally, as this kind of potential sensor,
What is called a passive type and what is called an active type are generally known, and the passive type is further classified into a chopper type and a vibration capacitance type. The active type is also called a feedback type. Both potential sensors,
The common point is that the capacitance between the object to be measured and the detection electrode is changed by using a mechanical means such as a piezoelectric tuning fork, and the charge fluctuation induced on the detection electrode is extracted as a detection signal.

FIG. 6 shows the principle of potential detection by a conventional passive potential sensor. As can be seen from FIG. 6, the capacitance Cv formed between the DUT 1 and the detection electrode 2
Is periodically changed by vibrating the detection electrode 2 by mechanical means, so that electric charges are induced in the detection electrode 2 and an alternating current is is generated. The passive-type potential sensor detects the potential V1 of the DUT 1 by utilizing that the alternating current is proportional to the potential V1 of the DUT 1.
That is, the AC current is is converted into the output voltage V2 by the resistor Rg, and the potential V1 of the device under test 1 is detected based on the output voltage V2.

On the other hand, the principle of potential detection by an active potential sensor is shown in FIG. The active-type potential sensor applies a voltage Vv from the variable voltage source 4 to the detection electrode 2 through the resistor Rg, and outputs the potential V1 of the DUT 1 and the variable voltage source 4.
When the voltage Vv becomes the same potential, the output voltage V2 becomes zero. That is, it monitors that the potential difference between the DUT 1 and the detection electrode 2 becomes zero, and detects the potential V1 of the DUT 1 from the voltage Vv of the variable voltage source 4 at that time.

[0006]

The passive type potential sensor shown in FIG. 6 has an output voltage V
2 has a characteristic that it depends on a change in the distance Ds between the DUT 1 and the detection electrode 2. This is because the capacitance Cv generated between the DUT 1 and the detection electrode 2 depends on the distance Ds. Therefore, in the passive type potential sensor, the detection accuracy of the potential V1 of the DUT 1
There is a problem that the temperature is reduced by a change in the distance Ds between the DUT 1 and the detection electrode 2 due to thermal expansion or vibration of the member supporting the sensor. Further, the change ΔCv of the capacitance Cv
Is also greatly affected by the stability of the vibrations of the mechanical means for generating

On the other hand, the active type potential sensor shown in FIG. 7 monitors that the potential difference between the DUT 1 and the detection electrode 2 becomes zero, and monitors the voltage V of the variable voltage source 4 at that time.
Since the potential V1 of the DUT 1 is detected from v, the measured value of the potential V1 of the DUT 1 does not depend on the distance Ds between the DUT 1 and the detection electrode 2. Therefore, the active-type potential sensor can stably detect the potential V1 of the device under test 1 with high accuracy. However, since an expensive variable voltage source 4 that can generate a high voltage equal to the voltage V1 of the device under test 1 is required, there is a problem that the cost of the potential sensor increases.

An object of the present invention is to provide an inexpensive potential sensor capable of measuring the potential of an object to be measured with high accuracy and stability.

[0009]

To achieve the above object, a potential sensor according to the present invention comprises: (a) a detection electrode facing an object to be measured; and (b) a detection electrode between the detection electrode and the object to be measured. A modulator that mechanically modulates the capacitance generated in the
(C) a grid electrode arranged between the object to be measured and the detection electrode; (d) a grid power supply for applying a grid voltage to the grid electrode; and (e) a grid electrode induced by the detection electrode. A grid power supply control device for controlling a grid voltage output from the grid power supply device so that a signal due to a measured object becomes zero; and (f) a device to be measured based on the grid voltage controlled by the grid power supply control device. And an output circuit for outputting a measured value of the potential of the object.

[0010]

With the above arrangement, the grid electrode is incident on the detection electrode (when the potential of the device under test is positive) or emitted from the detection electrode (when the device under test is in contact with the grid voltage) supplied from the grid power supply device. An electric field generated when the potential is negative) is controlled so that a state in which an electric field due to the measured object does not enter the detection electrode is formed. The grid voltage at this time is proportional to the potential of the device under test, and the proportional coefficient is approximately the value of the ratio of the distance between the device under test and the detection electrode to the distance between the grid electrode and the detection electrode. equal. The value of this ratio is greater than 1 because the grid electrode is between the device under test and the detection electrode, and the grid voltage is lower than the potential of the device under test.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a potential sensor according to the present invention will be described below with reference to the accompanying drawings.

One embodiment of a potential sensor according to the present invention is shown in FIGS. The potential sensor includes a detection electrode 2, a modulation device 13, a grid electrode 14 disposed between the DUT 1 and the detection electrode 2, a grid power supply 15 for supplying a grid voltage Vg to the grid electrode 14, and a grid. The power supply control device 16 includes an output circuit 17 that outputs a measured value of the potential V1 of the device under test 1. The modulation device 13 periodically vibrates the detection electrode 2 and
To be measured 1 and the detection electrode 2
And a tuning fork drive circuit 12 for driving the piezoelectric tuning fork 11 for periodically modulating the capacitance generated between the piezoelectric tuning fork 11 and the tuning fork driving circuit 12.

The detection electrode 2 is fixed to one leg 11a of the piezoelectric tuning fork 11 via an insulator 18 as shown in FIG. The piezoelectric tuning fork 11 is fixed together with the grid electrode 14 on a printed circuit board 22. Further, on the printed circuit board 22, a tuning fork driving circuit 12, which is constituted by an amplification circuit constituting one oscillation circuit using the piezoelectric tuning fork 11 as a feedback element, is indicated by reference numeral 21 in FIG.
The grid power supply device 15 and the grid power supply control device 16
And an output circuit 17 are mounted. In the potential sensor shown in FIG. 1, the printed circuit board 22 is housed in one case 23 made of metal having an electrostatic shielding function and is unitized.

The case 23 is provided with a window 24 for inputting an electric field from the device under test 1. The piezoelectric tuning fork 11 is fixed to a printed circuit board 22 with its detection electrode 2 facing a window 24 with a grid electrode 14 therebetween. The detection electrode 2 and the grid electrode 14 face the DUT 1 through the window 24 of the case 23. A cable 25 for power supply, grounding, and signal derivation is led out from the printed circuit board 22.

The tuning fork drive circuit 12 outputs the output of the tuning fork driving circuit 12 to a piezoelectric vibrator 26a fixed to one leg 11a of the piezoelectric tuning fork 11.
To generate an electrostrictive vibration, which causes the piezoelectric tuning fork 11 to vibrate at its natural frequency. Then, a piezoelectric signal having a frequency equal to the natural frequency is generated on the piezoelectric vibrator 26 b fixed to the other leg 11 b of the piezoelectric tuning fork 11 by the vibration, and the piezoelectric signal is fed back to the tuning fork driving circuit 12. The tuning fork drive circuit 12 causes the piezoelectric tuning fork 11 to self-oscillate at a predetermined oscillation frequency.

The grid power supply control device 16 includes the detection electrode 2
The grid voltage Vg supplied from the grid power supply device 15 to the grid electrode 14 is controlled so that the amplitude of the AC signal induced in the grid electrode becomes minimum (ideally zero).
As shown in FIG. 1, the grid power supply control device 16 includes an impedance conversion circuit 31, an AC amplification circuit 32, a phase detection circuit 33, and a zero output detection circuit. The impedance conversion circuit 31 is a circuit for impedance-converting the detection signal generated at the detection electrode 2 and supplying the converted signal to the AC amplification circuit 32. An example of the configuration is shown in FIG. The impedance conversion circuit 31 includes a source follower circuit including a field effect transistor (FET) 35 and a resistor Rs. The field effect transistor 35 has a resistor Rs connected between the source and the ground, a gate connected to the detection electrode 2, and a drain connected to the power supply Vcc. Between the detection electrode 2 and the ground, a resistor Rg for converting a current is due to a change in charge induced in the detection electrode 2 into a voltage is connected.

The AC amplifier circuit 32 amplifies the signal output from the field effect transistor 35 of the impedance conversion circuit 31 to a level suitable for the subsequent processing, and supplies the signal to the phase detection circuit 33. The phase detection circuit 33 performs phase detection on the output of the DC amplification circuit 32 in synchronization with the timing signal supplied from the tuning fork drive circuit 12, and converts the output into a DC signal. The DC signal output from the phase detection circuit 33 is input to the zero output detection circuit 34. Zero output detection circuit 34
Is composed of a differential amplifier having an inverting input terminal and a non-inverting input terminal. The output signal of the phase detection circuit 33 input to the inverting input terminal and the potential of the non-inverting input terminal which is grounded and is at the ground potential. Amplify the difference.

The output of the zero output detection circuit 34 is input to the grid power supply 15. The grid power supply 15 responds to the output of the zero output detection circuit 34 by
Is changed.

The above-described detection electrode 2, impedance conversion circuit 31, AC amplification circuit 32, phase detection circuit 3
3. The loop from the zero output detection circuit 34 and the grid power supply 15 to the grid electrode 14 forms one negative feedback loop. Thus, the grid voltage Vg is controlled so that the output of the phase detection circuit 33 is minimized (ideally zero), that is, the potential of the detection electrode 2 is minimized (ideally zero). Ideally, only the electric field caused by the DUT 1 and the grid electrode 14 is incident on the detection electrode 2. However, actually, an electric field due to a printed circuit board or the like is incident on the detection electrode 2 in addition to the above. Therefore, an electric field due to the DUT 1 is applied to the detection electrode 2 by the action of the grid voltage Vg. Even when the light does not enter, the potential of the detection electrode 2 does not become zero. A detection output of the potential V1 of the DUT 1 is output from the output circuit 17 based on the grid voltage Vg at this time.

In such a configuration, the DUT 1 has, for example, a positive (plus) potential V1, and the potential of the detection electrode 2 is controlled to zero by the negative feedback loop of the potential sensor shown in FIG. It is assumed that it is. At this time, the grid power supply device 15 sets the potential of the device under test 1 to zero,
A negative (minus) grid voltage g is applied to the grid electrode 14. Thereby, as shown in FIG.
An electric field Es is generated between the detection electrode 2 and the detection electrode 2, and an electric field Eg is generated between the detection electrode 2 and the grid electrode 14.

The distance between DUT 1 and detection electrode 2 is D
s, the distance between the detection electrode 2 and the grid electrode 14 is Dg
Assuming that the potential of the DUT 1 is V1 and the grid voltage of the grid electrode 14 when the potential of the detection electrode 2 is zero is Vg, the electric field Es and the electric field Eg are expressed by the following equations (1) and (2).
It is displayed in (2). Es = V1 / Ds (1) Eg = Vg / Dg (2)

When the potential of the detection electrode 2 is controlled to be zero, the electric field Es and the electric field Eg cancel each other between the detection electrode 2 and the grid electrode 14 (Es = E).
g). Therefore, from the equations (1) and (2), the following equation (3)
Is obtained. The same applies to the case where the DUT 1 is negative (minus). V1 / Ds = Vg / Dg V1 = Vg (Ds / Dg) (3)

If the grid voltage Vg when the potential of the detection electrode 2 is controlled to zero based on the equation (3) is known, the potential V1 of the DUT 1 can be detected.
At this time, the distance Dg between the detection electrode 2 and the grid electrode
Is smaller, the value of Ds / Dg increases, so that the grid voltage Vg applied to the grid electrode 14 can be lowered. In FIG. 5, Ds / Dg = 2 (see solid line 41), Ds / Dg = 2.5 (see solid line 42), Ds / D
g = 3 (see solid line 43), Ds / Dg = 3.5 (solid line 4)
4), Ds / Dg = 4 (see solid line 45), Ds / D
The graph shows the measured value of the grid voltage Vg of the grid electrode 14 with respect to the potential V1 of the DUT 1 when g = 4.5 (see the solid line 46) and Ds / Dg = 5 (see the solid line 47).

Thus, an inexpensive grid power supply 15 having a low generated voltage can be used. Further, since the detection of the potential V1 of the DUT 1 is performed by detecting the potential Vg of the grid electrode 14 fixed at a fixed position, the potential Vg of the DUT 1 can be stably and highly accurately detected. Can be detected.

Although the basic embodiment of the present invention has been described above, the potential sensor according to the present invention is not limited to the above embodiment, but can be variously modified within the scope of the gist. For example, instead of the so-called vibration capacitance type using the piezoelectric tuning fork 11 as the modulation device 13, a so-called chopper type that changes the effective area of the detection electrode 2 can be used.

The grid electrode 14 may be of a vertical bar shape, a horizontal bar shape, or the like instead of the mesh electrode. Furthermore, the grid voltage Vg applied to the grid electrode 14 can also be reduced by reducing the mesh size of the grid electrode 14 to reduce the opening area of the electrode.

[0027]

As is apparent from the above description, according to the present invention, a grid electrode is disposed between an object to be measured and a detection electrode, and a grid voltage is applied to the grid electrode to generate a voltage from the detection electrode. By controlling the output voltage to be zero, it is possible to use an inexpensive grid power supply that generates a low voltage as the grid power supply that applies the grid voltage. As a result, the cost of the potential sensor can be significantly reduced. Further, since the potential of the device under test is detected by detecting the potential of the grid electrode fixed at a fixed position, the potential of the device under test can be stably detected with high accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of one embodiment of a potential sensor according to the present invention.

FIG. 2 is a configuration diagram showing a specific configuration of the potential sensor shown in FIG.

FIG. 3 is an explanatory diagram for explaining the operation principle of the potential sensor shown in FIG. 1;

FIG. 4 is a graph showing a relationship between a potential of a grid electrode and a potential of an object to be measured with respect to a distance from a detection electrode.

FIG. 5 is a graph showing a relationship between a measured object potential and a grid voltage.

FIG. 6 is an explanatory diagram for explaining the operation principle of a conventional potential sensor.

FIG. 7 is an explanatory diagram for explaining the operation principle of another conventional potential sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Measurement object 2 ... Detection electrode 11 ... Piezoelectric tuning fork 12 ... Tuning fork drive circuit 13 ... Modulation device 14 ... Grid electrode 15 ... Grid power supply device 16 ... Grid power supply control device 17 ... Output circuit 31 ... Impedance conversion circuit 32 ... AC amplification Circuit 33: Phase detection circuit 34: Zero output detection circuit Rg: Resistance Vg: Grid voltage V1: Potential of DUT Ds: Distance between detection electrode and DUT Dg: Distance between detection electrode and grid electrode distance

Claims (1)

[Claims] 1. A detection electrode facing a device under test, a modulation device that mechanically modulates a capacitance generated between the detection electrode and the device under test, A grid electrode disposed between the grid electrode, a grid power supply for applying a grid voltage to the grid electrode, and an output from the grid power supply such that a signal caused by the device under test induced on the detection electrode becomes zero. A grid power supply control device that controls a grid voltage to be applied; and an output circuit that outputs a measured value of the potential of the device under test based on the grid voltage controlled by the grid power supply control device. Potential sensor.