JPH11237423A - Electric potential sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately measure an electric potential of an accurate object to be measured even when an electric potential sensor is made compact by suppressing influences of a stray capacity generated between a sensor electrode and a vibrator. SOLUTION: When a mechanically vibrating fork is closed, base parts 11a, 12a move in a direction to come close to each other, whereby effective opposite faces 11b, 12b between a sensor electrode 120 and a target are expanded. When the fork is opened, the base parts 11a, 12a move in a direction to be away from each other, and the effective opposite faces 11b, 12b of the sensor electrode 120 and target are narrowed. A capacitance C(t) between the sensor electrode 120 and target and a stray capacity Cx(t) between the sensor electrode 120 and fork change at the same phase. A current flowing in the C(t) can be sent to a detection resistor Rs. Accordingly, a target potential Vt can be detected highly accurately.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a potential sensor capable of measuring a potential of a surface to be measured in a non-contact manner, for example, a potential suitable for measuring a potential of a photosensitive drum surface of an electrophotographic copying machine, a printer or the like. It concerns a sensor.

[0002]

2. Description of the Related Art In electrophotographic copying machines, printers, and the like, in order to obtain stable print output image quality, the charged potential of a photosensitive drum is measured and feedback is applied.
It is necessary to charge uniformly even in various environments. Therefore, a potential sensor for measuring the potential of the photosensitive drum is used.

Conventionally, this kind of potential sensor is generally shown in FIG.
Was provided.

That is, a shielding plate 110 called a fork having a shape as shown in the figure is arranged between a sensor electrode 120 and an object to be measured (hereinafter, referred to as a “target”) 200. For example, the structure shown in FIG. 7 is adopted for the facing portions 111 and 112 of the target surface 200 of the fork 110 to be measured.

A drive piezoelectric element 130 and a vibration detecting piezoelectric element 140 are attached to both arms of the fork 110. A sine wave is applied to the drive piezoelectric element 130, and a square wave is applied to mechanically oscillate. The vibration is converted into an electric signal by the vibration detecting piezoelectric element 140, and the frequency of the waveform applied to the driving piezoelectric element 130 is controlled so that the amplitude of the electric signal is maximized. For example, a method of causing the same is used.

FIG. 6 shows a specific electric circuit of a detecting section of a conventional potential sensor.
Vibration by 0, etc., the sensor electrode 120 and the target 2
It has a structure in which the effective facing area between 00 is periodically changed.

In the above-described configuration, the fork 110 is vibrated, and the target surface 20 of the fork 110 is measured.
The zero opposing portions 111 and 112 are vibrated as indicated by arrows, and the effective opposing area between the sensor electrode 120 and the target 200 is periodically changed. This is electrically equivalent to periodically changing the capacitance between the sensor electrode 120 and the target 200.

An example of the control of measuring the potential of the surface of the photosensitive drum to be measured by this type of potential sensor will be described below with reference to the example of FIG.

In the example shown in FIG.
Is Vt, and the potential of the sensor electrode 120 is Vs.

Then, the fork 110 is vibrated as shown below.

[0011]

(Equation 1)

X (t) = X * sin (2πf * t) That is, the vibration frequency is f, the vibration amplitude is X, and the time is t.

The capacitance between the sensor electrode 120 and the target 200 is represented by C (t).

When the fork 110 vibrates, the following I is set between the target 200 and the sensor electrode 120.
The current (t) flows.

[0015]

(Equation 2)

I (t) = (Vt−Vs) * {dC (t) / dt} Here, {dC (t) / dt} is a time derivative of C (t).

For simplicity,

[0018]

(Equation 3)

{DC (t) / dt} = Y * sin (2πf * t + θ) Here, Y and θ are constants.

As a result, the sensor electrode potential Vs can be easily detected by detecting I (t) using the detection resistor Rs.

The detected voltage has a repetitive waveform having a frequency synchronized with the vibration of the fork 110. For example, the (peak)-(peak) voltage

[0022]

(Equation 4)

It is easy to detect Vpp = Rs * Y * (Vt-Vs).

Only when Vs = Vt, since I (t) = 0, that is, Vpp = 0, the control was performed by controlling the value of Vs so that Vpp = 0 using a DC voltage source. The Vs voltage itself becomes equal to Vt, and the value of Vt can be detected without contact.

[0025]

In recent years, as the diameter of the photosensitive drum has been reduced and the density around the photosensitive drum has been increased, the potential sensor has also been required to be reduced in size and thickness. Accordingly, a small fork has been used for the shape of the sensor fork. However, as the size of the sensor is relatively reduced (the ratio of the size of the sensor electrode to the fork), the gain of the sensor decreases, and the Vpp becomes smaller even when Vt ≠ Vs, and S
/ N ratio deteriorates.

When the electrode area is increased to increase the gain, there is a problem that the target potential is detected as an offset value as described later, and the offset adjustment is performed for each sensor. is necessary.

The principle of generation of this offset voltage will be described with reference to FIGS. FIG. 9 is a diagram for explaining the stray capacitance of the potential sensor, and FIG. 10 is a diagram showing an example of an equivalent circuit of the potential sensor.

As shown in FIG. 9, a stray capacitance Cx exists between the sensor electrode 20 and the fork 10. The stray capacitance Cx is equal to the measured surface opposing portions 111 and 11 of the fork 110.
When 2 vibrates, the distance between the sensor electrode 120 and the measurement surface facing portions 111 and 112 of the fork 110 changes according to the vibration. As a result, the capacitance Cx (t) changes electrically.

Here, it should be noted that the sensor electrode 12
0 and the capacitance C (t) between the target 200 and the stray capacitance Cx (t) between the sensor electrode 120 and the fork 110.
Is that they change in opposite phases. That is, Cx (t) becomes smaller at the timing when C (t) becomes larger due to the opening between the measured surface facing portions 111 and 112 of the fork 110, and the measured surface facing portions 111 and 11 of the fork 110 are increased.
At the timing when C (t) becomes smaller as the interval between the two closes, Cx
(T) increases.

This can be expressed by the following equation.

[0031]

(Equation 5)

Cx (t) = Z * C (t + π) Z is a constant. That is, Cx (t) is out of phase with C (t) by π.

Therefore, FIG. 6 is an equivalent circuit of the potential sensor.
The current flowing through the sensor electrode 120 flows through the detection resistor Rs. Therefore, as described with reference to FIG. 6, if no current flows through the detection resistor Rs, the sensor electrode potential Vs (t) and the fork potential Vf are equal.

Based on the above, FIG. 6 described above is rewritten as shown in FIG. Cx (t) is C (t)
When there is an offset voltage Vfst between the target potential Vt and the fork potential Vf when the phases are shifted by π and π, the current flowing through C (t) does not entirely flow through Rs but partially flows through Cx (t). Flows. And C
When the offset voltage balances (t) and Cx (t), no current flows through the detection resistor Rs.

At this time, when Vfst ≠ 0, the sensor electrode potential Vs (t) = constant AC voltage component at both ends of Rs = 0 Vpp = 0, and Vt is detected as a value having an error in the detected offset voltage Vfst. .

When an N-channel J-FET is used for amplifying the voltage across the detection resistor Rs (including impedance conversion such as source follower), Vt = Vf-Vfst, and a positive detection offset voltage always occurs.

The offset voltage Vfst is determined by the vibration state of the fork, an error in the mounting position, and an error in the mounting position of the sensor electrode, and varies in various ways depending on the manufacturing and use environment.

Here, a similar error should have existed even when the potential sensor shape was large, but Cx (t) was
Since it was sufficiently smaller than (t), it was conventionally sufficiently accepted. However, when the size of the potential sensor is reduced, Cx (t) becomes a non-negligible value, and the target voltage cannot be measured accurately due to the influence of the offset voltage. In addition, it is difficult to realize a mechanical shape and arrangement in which Cx (t) does not exist.

[0039]

SUMMARY OF THE INVENTION The present invention has been made for the purpose of solving the above-mentioned problems, and solves the above-mentioned problems.
An object of the present invention is to provide a potential sensor that does not require offset adjustment. For example, the following configuration is provided as one means for achieving such an object.

That is, a vibrator that vibrates mechanically, a sensor electrode disposed at a predetermined distance from a surface to be measured of a portion of the vibrator facing the surface to be measured, and a current flowing through the sensor electrode A current detection element for detecting, and at least the portion of the vibrator facing the surface to be measured includes a stray capacitance Cx (t) between the sensor electrode and the vibrator and a capacitance C (between the sensor electrode and the surface to be measured. t) has a shape in which the phase of the sensor fluctuates in the same phase, and vibrates the portion of the vibrator facing the surface to be measured, thereby varying the effective facing area between the sensor electrode and the surface to be measured. The potential of the surface to be measured can be measured in a non-contact manner by measuring the AC current flowing through the sensor electrode while changing the capacitance of the capacitor between the electrode and the surface to be measured.

[0041] For example, the shape of the portion to be measured facing the vibrator includes a portion to be measured which is separated from at least two of the surfaces to be measured by a predetermined distance, and the shape when the portion to be measured is closed by vibration. The effective facing area between the sensor electrode and the measured surface is increased, and the effective facing area between the sensor electrode and the measured surface is reduced when the measured surface facing portion of the vibrator is opened by vibration. Features.

Further, for example, the shape of the portion to be measured facing the vibrator is characterized in that a punched portion is formed near the center portion of the portions which overlap with each other.

Also, a vibrator that vibrates mechanically, a sensor electrode disposed at a predetermined distance from the surface to be measured of the surface to be measured of the vibrator, and a current flowing through the sensor electrode A current detection element for detecting, and at least the portion of the vibrator facing the surface to be measured includes a stray capacitance Cx (t) between the sensor electrode and the vibrator and a capacitance C (between the sensor electrode and the surface to be measured. t) has a shape in which the phase of the sensor fluctuates in the opposite phase, and vibrates the portion of the vibrator facing the surface to be measured, thereby changing the effective facing area between the sensor electrode and the surface to be measured. The potential of the surface to be measured can be measured in a non-contact manner by measuring the alternating current flowing through the sensor electrode while changing the capacitance of the capacitor between the electrode and the surface to be measured.

Further, a vibrator which vibrates mechanically, a sensor electrode disposed at a predetermined distance from a surface of the vibrator facing the surface to be measured, and a current flowing through the sensor electrode A current detection element to be detected, and an additional electrode disposed at a predetermined distance in the external direction of the vibrator having the same potential as the sensor electrode, and the sensor electrode and the additional electrode are provided between the vibrator and the additional electrode. Stray capacitance Cx between oscillators
It is characterized in that a stray capacitance Cx ′ (t) having a phase opposite to that of (t) is generated to cancel the Cx (t).

Further, a vibrator which vibrates mechanically,
A sensor electrode disposed at a predetermined distance from the measured surface facing portion of the vibrator with respect to the measured surface, a current detecting element for detecting a current flowing through the sensor electrode, and a sensor electrode having the same potential as the sensor electrode. An additional electrode disposed at a predetermined distance from the vibrator, wherein the vibrator has a protruding portion having a predetermined area serving as a predetermined gap with the additional electrode at a position facing the additional electrode; And the stray capacitance C between the oscillator
A stray capacitance Cx ′ (t) having a phase opposite to that of x (t) is generated between the ear portion and the additional electrode, so that the Cx (t) can be canceled.

[0046]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

[First Embodiment] In one embodiment of the present invention according to the present invention, the above-mentioned Cx (t) is not completely eliminated, but the fork shape is appropriately set. , C (t) and Cx (t) have the same phase, to reduce the influence of the offset voltage Vfst (eliminate the influence).

As a configuration in which the phases of C (t) and Cx (t) match, the fork shape of one embodiment of the present invention according to the present invention in which the influence of the offset voltage Vfst is reduced is shown in FIG.
Also in the present embodiment, the basic configuration of the potential sensor unit is the same as the configuration shown in FIGS. 6 and 8, but the shape of the portion of the tip of the fork facing the target 200 is different.
The shape shown in FIG. 1 is used to reduce the influence of the offset voltage Vfst.

That is, conventionally, the front shape of the fork is shown in FIG.
The shape shown in FIG. 1 is changed to the front shape shown in FIG.

The fork having the shape shown in FIG. 1 is hereinafter referred to as a cross-type fork. In the cross-type fork, the facing area of the portion to be measured between the sensor electrode 120 and the target 200 increases when the interval between the forks is closed, and the facing area between the sensor electrode and the target decreases when the fork is opened. ,
It has a fork shape such that the phases of Cx (t) and C (t) match.

That is, when the fork is closed, the base 11a,
12a move in the direction of approaching (closing) each other, and the effective opposing surfaces 11b, 12b between the sensor electrode 120 and the target.
b spreads. Also, when the fork is opened, the bases 11a, 1
2a move in a direction away from each other (open), and the effective facing surfaces 11b and 12b between the sensor electrode 120 and the target become narrower.

Therefore, the capacitance C (t) between the sensor electrode 120 and the target and the stray capacitance Cx (t) between the sensor electrode 120 and the fork change in the same phase. That is, even if the fork is opened, the effective facing surfaces 11a and 12a of the measured surface facing portions 11 and 12 are closed, so that Cx (t) decreases at the timing when C (t) decreases, and the fork faces the measured surface. Even if the portion between the portions 11 and 12 is closed, the portion between the effective facing surfaces 11a and 12a of the portions to be measured facing 11 and 12 is open.
Cx (t) increases at the timing when C (t) increases.

Therefore, according to the first embodiment, C
Part of the current flowing through (t) does not flow into Cx (t) (or greatly decreases), and all (or most) of the current flowing through C (t) is detected by the detection resistor Rs.
And the target potential Vt can be detected with high accuracy.

[Second Embodiment] In the above description, an example in which the cross type fork is used to prevent the generation of the offset voltage has been described. However, the present invention is not limited to the above-described example.
The configuration in which the phases of (t) and Cx (t) coincide with each other is only required to reduce (eliminate) the influence of the offset voltage Vfst.

A second embodiment according to the present invention will be described below with reference to FIG. 2 in which the phases of C (t) and Cx (t) are matched by the shape of a forked hole. .
FIG. 2 is a front view of a fork shape of a potential sensor according to a second embodiment of the present invention. Also in the second embodiment, the basic configuration of the potential sensor section is the same as the configuration shown in FIGS. 6 and 8, but the shape of the portion of the tip of the fork facing the target 200 is different, and the influence of the offset voltage Vfst is different. The shape shown in FIG.

The forked type fork shown in FIG. 2 has a sensor electrode 120 and a target 2 when the fork interval is closed.
The opposing area of the portion to be measured between 00 increases, the opposing area between the sensor electrode and the target decreases when the fork is opened, and the phases of Cx (t) and C (t) match. It has a fork shape.

That is, when the fork is opened, the portions 21 and 22 to be measured move outward from each other from the state shown in FIG. 2, so that the area where the windows 21a and 22a overlap with each other increases. The effective opposing surface becomes narrower. When the fork is closed, the state shown in FIG. 2 is obtained, and the area where the windows 21a and 22a overlap with each other is reduced, so that the effective facing surface 11 between the sensor electrode 120 and the target is reduced.
b and 12b spread.

Accordingly, the capacitance C (t) between the sensor electrode 120 and the target and the stray capacitance Cx (t) between the sensor electrode 120 and the fork change in the same phase. That is, Cx (t) decreases at the timing when C (t) decreases, and Cx at the timing when C (t) increases.
(T) increases.

Therefore, according to the second embodiment, C
Part of the current flowing through (t) does not flow into Cx (t) (or greatly decreases), and all (or most) of the current flowing through C (t) is detected by the detection resistor Rs.
And the target potential Vt can be detected with high accuracy.

[Third Embodiment] In the above description, the cross-shaped fork or the hole-shaped fork is used to prevent the generation of the offset voltage. However, the present invention is not limited to the above example, and Cx
If the occurrence of (t) is prevented, the influence of the offset voltage Vfst can be reduced (eliminated).

A third embodiment of the present invention in which the generation of Cx (t) is prevented will be described below with reference to FIG.
FIG. 3 is a diagram showing a fork structure of a potential sensor according to a third embodiment of the present invention. In the third embodiment, regardless of the shape of the portion of the tip of the fork facing the target 200, the structure shown in FIG. 7 may be used.

In the fourth embodiment, a second electrode 30 for positively canceling Cx (t) is added outside the fork 110. Then, the second electrode 30 is connected so as to have the same potential as the sensor electrode 120. As a result, a stray capacitance Cx ′ (t) is generated between the fork 110 and the second electrode 30. This stray capacitance C
Since x ′ (t) has an opposite phase to Cx (t), they cancel each other. Therefore, generation of an offset voltage can be prevented.

As described above, according to the fourth embodiment, Cx (t) can be canceled by positively generating Cx '(t) having a phase opposite to that of Cx (t). And part of the current flowing through C (t) does not flow into Cx (t) (or greatly decreases),
All (or most) of the current flowing through (t) can flow through the detection resistor Rs, and the target potential Vt can be detected with high accuracy.

[Fourth Embodiment] In the third embodiment described above, the second electrode 30 for positively canceling Cx (t) is added, and Cx (t) is reduced. An example of preventing occurrence has been described. However, Cx (t)
Is not limited to the above example, and various structures and methods can be adopted as long as Cx (t) can be positively canceled.

For example, an “ear” protruding from the fork by a certain area is separately added to the fork, and an electrode connected to be at the same potential as the sensor electrode 120 on the lower surface thereof, for example, a conductive metal pattern disposed on a printed circuit board is provided. Configuration. Then, similarly to the third embodiment, a stray capacitance Cx ′ (t) having a phase opposite to that of Cx (t) generated between the sensor electrode 120 and the fork 110 is positively applied between the “ear” of the fork and the pattern on the lower surface. And Cx (t) may be cancelled.

FIGS. 4 and 5 show a fourth embodiment of the present invention having the above-described structure. FIG. 4 is a view for explaining the structure of a potential sensor according to a fourth embodiment of the present invention, and FIG. 5 is a view showing a specific configuration example of the potential sensor shown in FIG. Hereinafter, an embodiment of the fourth invention according to the present invention will be described in detail with reference to FIGS.

In the fourth embodiment, the "ear" portion 40 protrudes from the fork portion by a predetermined amount on the fork 110.
Are additionally provided, and the sensor electrode 1
For example, a conductive metal pattern (additional electrode) 45 provided on the surface of the printed board 46 so as to have the same potential as that of the printed circuit board 46 is provided.

For example, as shown in FIG. 4, the ear portion 40 has a rectangular shape projecting below the fork 110, and the additional electrode 4
5 is an electrode pattern having a larger area toward the outside of the fork 110 (the lower side in FIG. 4), and a floating capacitance Cx ′ (t) having a phase opposite to that of the floating capacitance Cx (t) generated between the sensor electrode 120 and the fork 110. Is positively generated between the ear portion 40 and the additional electrode 45 to cancel Cx (t).

In this way, for example, a printed board pattern connected to the sensor electrode 120 is formed. The stray capacitance Cx ′ (t) is placed between the “ear” of the fork and the pattern on the lower surface.
Is positively formed to cancel Cx (t), so that a part of the current flowing through C (t) becomes Cx (t).
Does not flow (or greatly decreases), all (or most) of the current flowing through C (t) can flow through the detection resistor Rs, and the target potential Vt can be detected with high accuracy.

In the above description, the additional electrode is formed by the conductive pattern disposed on the printed circuit board 46. However, the additional electrode may be formed by a conductive metal plate.
Further, the shape of the ear portion and the shape of the additional electrode are not limited to the above shapes, and various shapes can be used as long as Cx ′ (t) that cancels or reduces Cx (t) is generated.

[0071]

According to the present invention as described above,
Even if the potential sensor is reduced in size by suppressing the effect of the stray capacitance generated between the sensor electrode and the vibrator, the potential of the measurement target can be measured with high accuracy.

Further, it is possible to provide a potential sensor that can realize this with a simple configuration such as optimizing the shape of the vibrator.

That is, the stray capacitance Cx (t) generated between the vibrator and the sensor electrode is canceled or Cx (t) is made in phase with the stray capacitance C (t) generated between the sensor electrode and the object to be measured. As a result, a potential sensor that does not generate (very little) offset voltage and does not require offset adjustment can be realized at low cost.

[0074]

[Brief description of the drawings]

FIG. 1 is a front view showing a fork shape of a potential sensor according to an embodiment of the present invention;

FIG. 2 is a front view showing a fork shape of a potential sensor according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a fork structure of a potential sensor according to a third embodiment of the present invention.

FIG. 4 is a diagram showing a fork structure of a potential sensor according to a fourth embodiment of the present invention.

FIG. 5 is a view showing a fork structure of a potential sensor according to a fourth embodiment of the present invention.

FIG. 6 is a diagram for explaining the principle of a potential sensor.

FIG. 7 is a diagram showing a structure of a portion of a conventional fork opposed to a surface to be measured.

FIG. 8 is a substantial diagram of a potential sensor.

FIG. 9 is a diagram for explaining the stray capacitance of the potential sensor.

FIG. 10 is a diagram illustrating an example of an equivalent circuit of a potential sensor.

[Explanation of symbols]

11, 12, 21, 22, 111, 112 Surface to be measured 30, 45 Cx (t) canceling electrode 40 Ear 110 Shielding plate (fork) 120 Sensor electrode 130 Drive piezoelectric element 140 Vibration detection piezoelectric element 150 Fulcrum 200 Object to be measured (target)

Claims (6)

[Claims] A vibrator that vibrates mechanically; a sensor electrode disposed at a predetermined distance from a surface of the vibrator facing the surface to be measured; and a current flowing through the sensor electrode. A current detection element for detecting, and at least the portion of the vibrator facing the surface to be measured includes a stray capacitance Cx (t) between the sensor electrode and the vibrator and a capacitance C (between the sensor electrode and the surface to be measured. t) having a shape that fluctuates in phase with the same phase, and vibrating the portion of the vibrator facing the surface to be measured, thereby changing the effective facing area between the sensor electrode and the surface to be measured. A potential sensor wherein the potential of the surface to be measured can be measured in a non-contact manner by measuring the alternating current flowing through the sensor electrode while changing the capacitance of the capacitor between the electrode and the surface to be measured. 2. The shape of the vibrator surface-to-be-measured portion includes a surface-to-be-measured portion that is separated from at least two of the surface-to-be-measured by a predetermined distance, and the sensor when the surface-to-be-measured portion is closed by vibration. The effective facing area between the electrode and the measured surface is increased, and the effective facing area between the sensor electrode and the measured surface is reduced when the measured surface facing portion of the vibrator is opened by vibration. The potential sensor according to claim 1, wherein 3. The potential sensor according to claim 2, wherein the shape of the portion to be measured facing the transducer overlaps with each other, and a punched portion is formed near a substantially central portion. 4. A vibrator that vibrates mechanically, a sensor electrode disposed at a predetermined distance from a surface to be measured of a portion facing the surface to be measured of the vibrator, and a current flowing through the sensor electrode. A current detection element for detecting, and at least the portion of the vibrator facing the surface to be measured includes a stray capacitance Cx (t) between the sensor electrode and the vibrator and a capacitance C (between the sensor electrode and the surface to be measured. t) has a shape that fluctuates in the opposite phase, and vibrates the portion of the vibrator facing the surface to be measured, thereby changing the effective facing area between the sensor electrode and the surface to be measured. A potential sensor characterized in that the capacitance of a capacitor between an electrode and the surface to be measured is made variable and an alternating current flowing through the sensor electrode is measured so that the potential of the surface to be measured can be measured in a non-contact manner. 5. A vibrator that vibrates mechanically, a sensor electrode disposed at a predetermined distance from a surface to be measured of a portion facing the surface to be measured of the vibrator, and a current flowing through the sensor electrode. A current detecting element to be detected, and an additional electrode disposed at a predetermined distance in an external direction of the vibrator having the same potential as the sensor electrode, the sensor electrode being provided between the vibrator and the additional electrode. A stray capacitance Cx ′ having a phase opposite to that of the stray capacitance Cx (t) between the vibrators.
A potential sensor capable of generating (t) and canceling Cx (t). 6. A vibrator that vibrates mechanically, a sensor electrode disposed at a predetermined distance from a surface to be measured of a portion facing the surface to be measured of the vibrator, and a current flowing through the sensor electrode. A current detecting element to be detected; and an additional electrode disposed at a predetermined distance from the vibrator having the same potential as the sensor electrode. The vibrator is provided at a position facing the additional electrode and at a predetermined gap from the additional electrode. And a stray capacitance Cx ′ (t) having a phase opposite to that of the stray capacitance Cx (t) between the sensor electrode and the vibrator is generated between the ear portion and the additional electrode. A potential sensor capable of canceling the Cx (t).