JPH10339754A - Surface electric potential detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a surface electric potential detector having a low cost and good temperature characteristics and capable of suppressing output fluctuation due to the rise of surrounding temperature. SOLUTION: Surface electric potential detector 31 is constitute of a detecting electrodes 3 arranged facing to the surface of a photosensor drum 2, a piezoelectric tuning fork 4 mechanically vibrating the detecting electrodes 3, an oscillation circuit 33 self-oscillating by feeding back the output of an operation amplifier 32 to the input side of the operation amplifier 32 via the piezoelectric tuning fork 4, etc. The oscillation circuit 33 is constituted of an operation amplifier 32, a piezoelectric tuning fork 4, an RC filter 34, a capacitor C21 and resistors R21 to R23 . The capacitor C22 of the RC filter 34 has a negative temperature coefficient by which electrostatic capacity decreases with the rise of surrounding temperature so as to give a temperature compensation function of piezoelectric tuning fork 4 to the RC filter 34.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface potential detecting device, and more particularly to a surface potential detecting device for non-contactly measuring the surface potential of a surface to be detected such as a photosensitive drum of an electrophotographic copying machine.

[0002]

2. Description of the Related Art As a surface potential detecting device of this type, for example, a device as shown in FIG. 2 is known. The surface potential detecting device 1 generally includes a detecting electrode 3, a piezoelectric tuning fork 4, an oscillating circuit 9, a detecting circuit 21, a band-pass filter 22
, A signal processing circuit 23, and a DC amplifier 24.

The piezoelectric tuning fork 4 has two legs 4a and 4b,
The piezoelectric vibrators 12a and 12b are adhered respectively.
An insulator 5 is provided at the tip of one leg 4a of the piezoelectric tuning fork 4.
, The detection electrode 3 is fixed. This detection electrode 3
Are arranged opposite to the surface of the photosensitive drum 2 which is the surface to be detected.
Forming _one . The detection electrode 3 is connected to the detection 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 between the surface of the photosensitive drum 2 and the detection electrode 3 changes. , i.e. the capacitance of the variable capacitor C ₁ is changed.

The oscillation circuit 9 comprises an operational amplifier 7 and a piezoelectric tuning fork 4
And The oscillation circuit 9 outputs the output of the operational amplifier 7.
Is applied to one piezoelectric vibrator 12b of the piezoelectric tuning fork 4 to
A distortion vibration is generated, and the piezoelectric tuning fork 4 is generated by the electrostriction vibration.
Vibration is performed at the natural frequency. Then, the vibration of the piezoelectric tuning fork 4
The other piezoelectric vibrator 12a is moved by the
Generates a piezoelectric signal with a new frequency,
R₁, R_Two, Capacitor C _TwoThe operation of the operational amplifier 7
Feedback to the input terminal. As a result, the oscillation circuit 9 is
Self-oscillates at a constant oscillation frequency. The operational amplifier 7 is
Input terminal is connected to ground, and the output terminal and inverted input terminal
Between the child and the resistor R_ThreeIs connected.

[0005] The detection circuit 21 amplifies the electric signal output from the detection electrode 3. The band-pass filter 22 converts the electric signal output from the detection circuit 21
Frequency components other than the above oscillation frequency are removed. The signal processing circuit 23 rectifies the electric signal output from the band pass filter 22. The DC amplifier 24 DC-amplifies the electric signal output from the signal processing circuit 23. In FIG.
Is a resistance element.

In the surface potential detecting device 1 having the above configuration, when a high voltage is applied to the surface of the photosensitive drum 2, the lines of electric force from the surface of the photosensitive drum 2 are applied to the detecting electrode 3.
And an electric charge equal to the electric charge generated on the surface of the photosensitive drum 2 is also induced on the detection electrode 3 by the electrostatic induction. In this state, when the oscillation of the oscillation circuit 9 piezoelectric tuning fork 4 vibrates, the capacitance of the variable capacitor C ₁ changes the distance between the surface and the sensing electrode 3 of the photosensitive drum 2 is changed. This charges and discharges the charge from the variable capacitor C ₁ , and an electric signal based on the charge is input to the detection circuit 21.

The electric signal is amplified by a detection circuit 21 to a level suitable for the subsequent processing, and then a band-pass filter 22 removes noise components other than a frequency component equal to the oscillation frequency of the oscillation circuit 9, and The signal is converted into a DC signal by the processing circuit 23. The DC signal output from the signal processing circuit 23 is amplified by the DC amplifier 24 and output as a surface potential signal of the photosensitive drum 2. The surface potential signal output from the DC amplifier 24 is supplied as a feedback signal for controlling the surface potential of the photosensitive drum 2 to a high voltage control system of the electrophotographic copying machine main body.

[0008]

The output characteristics of the conventional surface potential detecting device 1 fluctuate under the influence of temperature. The factors of the output fluctuation due to the temperature include stress at the bonding interface due to the difference in the coefficient of thermal expansion between the piezoelectric tuning fork 4 and the piezoelectric vibrators 12a and 12b affixed thereon, the shift of the resonance frequency due to the stress, the piezoelectric tuning fork 4 And piezoelectric vibrator 12
There is a change in physical characteristics due to heat of the adhesive material joining the a and 12b, and a temperature change in the piezoelectric vibrators 12a and 12b themselves. For this reason, the conventional surface potential detecting device 1
When the temperature rises, the resonance impedance increases due to these fluctuation factors, the input to the piezoelectric tuning fork 4 is limited, and the change in the distance between the surface of the photosensitive drum 2 and the detection electrode 3 decreases. , output taken from the variable capacitor C ₁ is lowered.

In order to solve such a problem, in the conventional surface potential detecting device 1, when the temperature rises between the output terminal of the operational amplifier 7 and the piezoelectric vibrator 12b as shown by a dotted line in FIG. A so-called negative characteristic thermistor (NTC) 28 having a low resistance value is inserted, and the negative characteristic thermistor 28 is inserted.
In order to prevent a decrease in output due to a rise in temperature. However, when the negative characteristic thermistor 28 is used in order to prevent the output from decreasing due to the temperature rise, there is a problem that the component cost of the surface potential detecting device 1 increases.

An object of the present invention is to provide a surface potential detecting device which can suppress output fluctuations due to an increase in ambient temperature, has low cost, and has good characteristics.

[0011]

In order to achieve the above object, a surface potential detecting device according to the present invention comprises: (a) a detecting electrode arranged to face a surface to be detected; and (b) the detecting electrode. A piezoelectric tuning fork that mechanically vibrates to change the distance between the detected surface and the detection electrode to change the capacitance between the detected surface and the detection electrode; and (c) An oscillation circuit that performs self-excited oscillation by feeding back the output of the amplifier to the input side of the amplifier via a feedback circuit including the piezoelectric tuning fork and a filter; and (d) statically controlling a capacitor constituting the filter. The capacitance has a negative temperature coefficient.

[0012]

With the above arrangement, the capacitance of the capacitor constituting the filter decreases due to the temperature change. As a result, the impedance of the filter decreases as the ambient temperature increases, and a decrease in output due to an increase in the resonance impedance of the piezoelectric tuning fork can be suppressed.

[0013]

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

As shown in FIG. 1, a surface potential detecting device 31
A detection electrode 3, a piezoelectric tuning fork 4, and an output of the operational amplifier 32, which are arranged to face the surface of the photosensitive drum 2;
, An oscillation circuit 33 that performs self-excited oscillation by feeding back to an input side of an operational amplifier 32, a detection circuit 21, a band-pass filter 22, a signal processing circuit 23, and a DC amplifier 24. In FIG. 1, parts corresponding to those in FIG. 2 are denoted by corresponding reference numerals, and redundant description will be omitted.

The oscillation circuit 33 includes an operational amplifier 32, a piezoelectric sound
4, RC filter 34, capacitor C_{twenty one}And resistance R_{twenty one}
~ R_{twenty three}It is composed of Piezoelectric tuning fork 4, RC filter
Reference numeral 34 denotes a feedback circuit of the operational amplifier 32. Calculation
R is provided between the output terminal of the amplifier 32 and the piezoelectric vibrator 12b.
Resistance R of C filter 34_{twenty four}Is connected. Resistance R _{twenty four}
Between the end on the connection side to the piezoelectric vibrator 12b and the ground
Is the capacitor C of the RC filter 34_{twenty two}Is connected
You. The non-inverting input terminal of the operational amplifier 32 is connected to the piezoelectric vibrator 1
2a, between the non-inverting input terminal and ground,
Resistance R_{twenty three}Is connected. Also, the inversion of the operational amplifier 32
A resistor R is connected between the input terminal and the output terminal. _{twenty one}And condensate
Sa C_{twenty one}Are connected in parallel between the inverted input terminal and ground.
Has a resistor R_{twenty two}Is connected. Output of operational amplifier 32
Since the power is a rectangular wave, the RC filter 34
By connecting between the output of P.32 and the piezoelectric tuning fork 4,
Exceeds the fundamental frequency corresponding to the natural frequency of the piezoelectric tuning fork 4
The high frequency signal is cut off, and the oscillation circuit 33
Prevents oscillation in higher modes other than the vibration mode.

The oscillation circuit 33 includes a piezoelectric vibrator 12 having the output of the operational amplifier 32 attached to one leg 4 b of the piezoelectric tuning fork 4.
b, and a signal having a phase of 360 degrees from the piezoelectric vibrator 12 a attached to the other leg 4 a of the piezoelectric tuning fork 4.
2 to the non-inverting input terminal.

Incidentally, in the surface potential detecting device 31,
And the capacitor C of the RC filter 34 _{twenty two}As ambient temperature
Use a capacitor with a constant capacitance regardless of the
Then, the same as the surface potential detecting device 1 described in FIG.
As the ambient temperature increases, the DC amplification
The surface potential signal of the photosensitive drum 2 output from the heater 24 is
Become smaller. Therefore, in this embodiment, the ambient temperature rises.
Has a negative temperature coefficient where the capacitance decreases as
Capacitor C_{twenty two}And a piezoelectric tuning fork in the RC filter 34
The temperature compensation function of No. 4 is provided.

By doing so, RC increases with increasing temperature.
The impedance of the filter 34 decreases, and the piezoelectric tuning fork 4
Of the output due to an increase in the resonant impedance of the output is suppressed. Incidentally, although it depends on the specifications of the individual piezoelectric tuning forks 4, the capacitor C ₂₂ of the RC filter 34 is −75.
When a material having a temperature coefficient of 0 ppm / ° C. was used, the change in the surface potential signal of the photosensitive drum 2 output from the DC amplifier 24 due to the temperature was greatly improved.

The surface potential detecting device according to the present invention is not limited to the above embodiment, but can be variously modified within the scope of the invention.

[0020]

As is apparent from the above description, according to the present invention, as the capacitors constituting the filter,
By using a capacitor having a negative temperature coefficient whose capacitance decreases as the temperature rises, the impedance of the filter decreases as the temperature rises, and it is possible to suppress a decrease in output due to an increase in the resonance impedance of the piezoelectric tuning fork. As a result, an inexpensive surface potential detecting device having good temperature characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a surface potential detecting device according to the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional surface potential detecting device.

[Explanation of symbols]

3 ... detection electrode 4 ... piezoelectric tuning fork 12a, 12b ... piezoelectric vibrators 21 ... detecting circuit 22 ... band-pass filter 23 ... signal processing circuit 24 ... DC amplifier 32 ... operational amplifier 33 ... oscillation circuit 34 ... RC filter C ₁ ... variable capacitor C ₂₂ ... capacitor R ₂₄ ... resistance

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[Procedure amendment]

[Submission date] September 26, 1997

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 1

[Correction method] Change

[Correction contents]

FIG.

Claims (1)

[Claims] A detection electrode disposed opposite to a detection surface; and mechanically vibrating the detection electrode to change a distance between the detection surface and the detection electrode, thereby detecting the detection target. A piezoelectric tuning fork for changing the capacitance between a surface and the detection electrode; and an output of the amplifier is fed back to the input side of the amplifier via a feedback circuit including the piezoelectric tuning fork and a filter to perform self-excited oscillation. A surface potential detection device comprising: an oscillation circuit; and a capacitance of a capacitor constituting the filter has a negative temperature coefficient.