JP2000180490A - Potential sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a potential sensor in which the adjustment of the timing or the like of a piezoelectric tuning fork is reduced and in which a stable vibration characteristic is obtained. SOLUTION: A detecting electrode 3 which is arranged so as to face the surface of a photoconductor drum 2, a piezoelectric tuning fork 4, a tuning-fork drive circuit 20 which drives the piezoelectric tuning fork 4, an impedance conversion circuit 21, a rectifying circuit 22 and a DC amplifier circuit 23 constitute this potential sensor 1 roughly. In this piezoelectric tuning fork 4, a piezoelectric vibrator 12a and a piezoelectric vibrator 12b are pasted respectively on both arm parts 4a, 4b whose lengths are different. The detecting electrode 3 is fixed, via an insulator 5, to the shorter-length arm part 4a at the piezoelectric tuning fork 4.

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]

2. Description of the Related Art Conventionally, as this kind of potential sensor, a sensor using a piezoelectric tuning fork 34 as shown in FIG. 3 is known. The piezoelectric tuning fork 34 has arms 34a,
The piezoelectric vibrators 32a and 32b are attached to 34b, respectively. A detection electrode 33 is fixed to one arm 34 a of the piezoelectric tuning fork 34 via an insulator 35. The detection electrode 33 is arranged so as to face the surface of the photosensitive drum 2 which is an object to be measured.

[0003] When the piezoelectric tuning fork 34 vibrates, the distance between the surface of the photosensitive drum 2 and the detection electrode 33 changes, and the capacitance C between the photosensitive drum 2 and the detection electrode 33 changes. Thereby, the charge induced in the detection electrode 33 is charged and discharged, and an electric signal based on the charge is extracted as a detection signal.

[0004]

However, the conventional piezoelectric tuning fork 34 has a piezoelectric vibrator 32a on one arm 34a.
, Insulator 35 and detection electrode 33 are fixed, and piezoelectric vibrator 32b is fixed to the other arm 34b. Therefore,
In the piezoelectric tuning fork 34 as it is, the natural frequency balance between right and left is poor, so that the loss of vibration energy is large, and stable vibration characteristics cannot be obtained.

[0005] For this reason, the conventional piezoelectric tuning fork 34 has arms 3a and 3a so that the natural frequency balance between the left and right is improved.
It was necessary to adjust 4b by trimming or the like. In addition, this adjustment has caused a variation in vibration characteristics.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a potential sensor which has a small adjustment such as trimming of a piezoelectric tuning fork and can obtain a stable vibration characteristic.

[0007]

In order 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) mechanically vibrating the detection electrode. A piezoelectric tuning fork that changes a distance between the object to be measured and the detection electrode to change a capacitance between the object to be measured and the detection electrode; and (c) a piezoelectric tuning fork. The two arms have different lengths.

[0008]

According to the above configuration, the natural frequency balance between the left and right sides of the piezoelectric tuning fork is improved, the loss of vibration energy is small, and stable vibration characteristics can be obtained.

[0009]

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.

As shown in FIG. 1, a potential sensor 1 generally includes a detection electrode 3, a piezoelectric tuning fork 4, a tuning fork driving circuit 20 for driving the piezoelectric tuning fork 4, and an impedance disposed opposite to the surface of a photosensitive drum 2. It comprises a conversion circuit 21, a rectifier circuit 22, and a DC amplifier circuit 23.

The piezoelectric tuning fork 4 has two arm portions 4a having different lengths.
The piezoelectric vibrators 12a and 12b are attached to 4b, respectively. The detection electrode 3 is fixed to the arm 4 a having the shorter length of the piezoelectric tuning fork 4 via an insulator 5. The detection electrode 3 is disposed so as to face the surface of the photosensitive drum 2 which is an object to be measured. The detection electrode 3 is connected to the impedance conversion circuit 21. When the piezoelectric tuning fork 4 vibrates, the distance between the surface of the photosensitive drum 2 and the detection electrode 3 changes, and the capacitance C between the surface of the photosensitive drum 2 and the detection electrode 3 changes.

The tuning fork drive circuit 20 and the piezoelectric tuning fork 4 constitute an oscillation circuit 9. That is, the output of the tuning fork drive circuit 20 is applied to the piezoelectric vibrator 12b provided on the longer arm 4b of the piezoelectric tuning fork 4 to generate electrostrictive vibration, and the piezoelectric tuning fork 4 is caused to have its natural frequency by the electrostrictive vibration. To vibrate. Then, by the vibration of the piezoelectric tuning fork 4, a piezoelectric signal having a frequency equal to the natural frequency is generated on the piezoelectric vibrator 12 a provided on the shorter arm 4 a and is fed back to the tuning fork driving circuit 20. Thus, the oscillation circuit 9 oscillates at a predetermined oscillation frequency.

Here, in the piezoelectric tuning fork 4, the left and right arms 4a and 4b can be regarded as individual vibrating reeds at the node position. And the left and right arms 4a,
4b are equal to each other,
It will function as a well-balanced tuning fork. The natural frequency ω is represented by the following equation (1). ω = α _n {(EI) / (l ³ M)} ^1/2 (1)

In equation (1), α _n is a constant, E is Young's modulus, I is the elastic moment, l is the length of the vibrating bar, and M is the mass of the vibrating bar. Since the constant α _n and the elastic moment I are constant, the natural frequency balance of the left and right arms 4a, 4b is determined by the weight M, the length 1, and the Young's modulus E.

In the piezoelectric tuning fork 4, the arm 4a has the detection electrode 3 and the insulator 5 in addition to the piezoelectric vibrator 12a, and is softer than the arm 4b. The upper Young's modulus becomes lower. Therefore,
By reducing the length 1 of the arm 4a (the mass M also decreases), the natural frequency ω of the arm 4a increases, and matches the natural frequency ω of the arm 4b.

Here, the piezoelectric tuning fork 4 has different lengths of the arms 4a and 4b to balance the right and left natural frequencies when the piezoelectric vibrators 12a and 12b, the detection electrode 3 and the insulator 5 are assembled. Since it is better, only a slight fine adjustment of the natural frequency balance is required, and adjustment such as trimming of the arms 4a and 4b can be reduced. In addition, since the trimming adjustment, which is one of the causes of the variation in the vibration characteristics, is hardly required, the vibration characteristics of the piezoelectric tuning fork 4 are stabilized.

The impedance conversion circuit 21 is a circuit for converting the detection signal generated at the detection electrode 3 into an impedance and supplying the converted signal to the rectification circuit 22. The rectifier circuit 22
The AC detection signal output from the impedance conversion circuit 21 is converted into a DC signal. The DC amplifying circuit 23 amplifies the DC signal output from the rectifier circuit 22 into DC.

In the potential sensor 1 having the above-described configuration, when a high voltage is applied to the surface of the photosensitive drum 2, electric lines of force from the surface of the photosensitive drum 2 reach the detection electrode 3, and the electric field is generated by electrostatic induction. In addition, electric charges equal to the electric charges generated on the surface of the photosensitive drum 2 are also induced on the detection electrodes 3. In this state, when the piezoelectric tuning fork 4 vibrates due to the oscillation of the oscillation circuit 9, the distance between the surface of the photosensitive drum 2 and the detection electrode 3 changes, and the capacitance C changes. As a result, the charge induced in the detection electrode 3 is charged and discharged, and an electric signal based on the charge is input to the impedance conversion circuit 21 as a detection signal.

The detection signal is supplied to the impedance conversion circuit 21
, And then converted into a DC signal by the rectifier circuit 22. The DC signal output from the rectifier circuit 22 is amplified by the DC amplifier circuit 23 and
Is output as the surface potential signal. The surface potential signal output from the DC amplification circuit 23 is supplied to the high voltage control system of the electrophotographic copying machine as a feedback signal for controlling the surface potential of the photosensitive drum 2.

Since the potential sensor 1 has a good natural frequency balance between the left and right of the piezoelectric tuning fork 4, the loss of vibration energy is small and stable vibration characteristics can be obtained. Arm 4
Since the lengths a and 4b can be freely changed, the degree of freedom in designing the shapes and materials of the detection electrodes 3 and the piezoelectric vibrators 12a and 12b increases.

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. In particular, it is not necessary to provide a piezoelectric vibrator on each of the two arms 4a and 4b. As shown in FIG. 2, one piezoelectric vibrator 12 is provided on only one arm 4b to vibrate the piezoelectric tuning fork 4. It may be.

[0022]

As is apparent from the above description, according to the present invention, the use of the piezoelectric tuning forks having different lengths of the two arms improves the right and left natural frequency balance of the piezoelectric tuning fork. In addition, adjustment such as trimming of the piezoelectric tuning fork is reduced. As a result, it is possible to obtain a potential sensor with small vibration energy loss and stable vibration characteristics.

[Brief description of the drawings]

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

FIG. 2 is a front view of a piezoelectric tuning fork showing another embodiment.

FIG. 3 is a front view showing the configuration of a conventional potential sensor.

[Explanation of symbols]

 3: Detection electrode 4: Piezoelectric tuning fork 4a, 4b: Arm 12a, 12b, 12: Piezoelectric vibrator

Claims (1)

[Claims] A detecting electrode opposed to the object to be measured; mechanically vibrating the detecting electrode to change a distance between the object to be measured and the detecting electrode; A piezoelectric tuning fork for changing a capacitance between the electrode and an electrode, wherein two arms of the piezoelectric tuning fork have different lengths.