JPH0460230B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a device for non-contact detection of the surface potential of an object to be measured, such as a device for detecting the surface potential of a photoreceptor used in a copying machine.

Conventional technology A vibrating capacitive surface potential sensor using a tuning fork places the tuning fork between the object to be measured and the output electrode.
The change in capacitance is extracted as a voltage waveform through the vibration, and the surface potential of the object to be measured is detected.

The principle diagram is shown in FIG. 1 and will be explained. 01 is an object to be measured, and its surface potential V ₀ is detected. 02 is an electrode, and a capacitance C ₁ is formed between it and the object to be measured 01, and a capacitance C ₂ is formed between it and the ground.

03 is a tuning fork in a grounded state, which is placed between the object to be measured 01 and the electrode 02, and when it vibrates, the capacitance C ₁ between the electrode 02 and the object to be measured 01 changes, and the electrode A waveform voltage is generated at 02.

04 is a casing, and has a hole at a position where the electrode 02 faces the object to be measured 01.

The voltages divided by capacitances C ₁ and C ₂ in the absence of tuning fork vibration are V ₁ and V ₂ , respectively.
Then, since the amount of induced charge is equal, C ₁ V ₁ = C ₂ V ₂ ……(1) holds true.

Also, since V ₀ = V ₁ + V ₂ , eliminating V ₁ from equation (1) and rearranging results in V ₂ = C ₁ / C ₁ + C ₂ V ₀ ...(2), and this V ₂ In other words, represents the voltage of electrode 02.

When the tuning fork vibrates, the capacitance C ₁ changes minutely, causing a waveform in the voltage of the electrode 02.

The surface potential V ₀ is detected by amplifying and measuring this voltage waveform.

However, since the amplitude of the voltage waveform of the electrode 02 is approximately proportional to the amplitude of the tuning fork, accurate surface potential V ₀ cannot be detected unless the amplitude of the tuning fork is fixed.

Since the amplitude of the tuning fork is determined by the driving voltage that drives the tuning fork, if a stable driving voltage is not supplied to the tuning fork, the amplitude of the tuning fork will be disturbed and the output voltage will also be affected.

Conventionally, no attempt has been made to stabilize the drive voltage by taking into account the amplitude of the tuning fork, which has caused problems in accurately detecting the surface potential.

Problems to be Solved by the Invention The present invention solves these drawbacks, and the problem to be solved lies in stabilizing the driving voltage to keep the amplitude of the tuning fork constant.

Means and Action for Solving Problems In a vibrating capacitive surface potential sensor using a tuning fork, a tuning fork driving circuit that converts the vibration of the tuning fork into an electrical signal and returns the signal to the tuning fork to vibrate the tuning fork is provided. This is a surface potential sensor equipped with a stabilizing means that stabilizes the output amplitude of the drive voltage to a constant value.

Embodiment An embodiment of the present invention illustrated in FIGS. 2 to 5 will be described below.

This embodiment is used to detect the surface potential of a photoreceptor in a copying machine.

A photoconductive material is vacuum-deposited on the surface of the photoreceptor to form a thin film that retains charge on the surface in the dark.

The surface potential formed by this charge determines the degree of toner adhesion and ultimately affects the copied image, so in order to obtain the best image, it is necessary to detect and control the surface potential. Therefore, surface potential sensors are used.

FIG. 2 is an external view of the surface potential sensor.

Reference numeral 1 denotes a casing, and a hole 2, which is a part for detecting the surface potential of the photoreceptor, is provided in a part of the top surface of the casing.

A tuning fork, an electrode, etc. are contained inside the casing 1, and the electrode is located at the opening of the hole 2.

3 and 4 are diagrams showing the circuits of this embodiment, in which FIG. 3 shows a tuning fork drive circuit, and FIG. 4 shows a surface potential detection circuit.

In FIG. 3, 3 is a tuning fork, which vibrates at a constant natural frequency due to the movement of ceramic piezoelectric elements 4 that are in contact with both sides of the tuning fork.

A low-pass filter consisting of a resistor 5 and a capacitor 6 is connected to one piezoelectric element, and a resistor 7
The output of the operational amplifier 8 is fed back to the other piezoelectric element of the tuning fork 3.

Further, the output of the operational amplifier 8 is negatively fed back via a resistor 9 and a parallel circuit of opposing Zener diodes 10 and 11, forming an amplifier circuit.

By feeding back the output from the amplifier circuit through the tuning fork and filter, the tuning fork 3 resonates at its natural frequency.

Here, the Zener diodes 10 and 11 inserted in parallel with the feedback resistor 9 were not conventionally provided in the amplifier circuit.

The Zener diodes 10 and 11 are limiters that keep the amplitude of the output voltage of the operational amplifier 8 constant, and the waveform of the output voltage of the operational amplifier 8 is shown in FIG. 5 and will be explained.

In the figure, the solid line shows the voltage waveform when a limiter is inserted, and the broken line shows the conventional waveform without the limiter.

The same voltage waveform shows the amplitude of the voltage Vcc supplied from the power supply to the operational amplifier 8 when the limiter is not used, so when the supply voltage from the power supply changes, it directly appears as a change in the amplitude of the output voltage of the operational amplifier, and as described above, This resulted in a change in the amplitude of tuning fork 3.

However, when a limiter is used, even if the power supply voltage changes, the output voltage is limited to a constant Zener voltage Vz of the Zener diode, so its amplitude is stabilized.

Therefore, the vibration of the tuning fork 3 always exhibits a constant amplitude regardless of changes in the power supply voltage.

Next, the surface potential detection circuit will be explained based on FIG. 4.

Reference numeral 12 denotes an electrode, and the voltage (V ₂ ) appearing across the high resistance 13 of the electrode is input to the gate of the field effect transistor FET 14 having a large input impedance.

The drain terminal of the FET 14 is connected to the power supply via a resistor 15, and the source terminal is grounded via a resistor 16 and connected to a non-inverting input terminal of an operational amplifier 19 via a capacitor 17 and a grounded resistor 18. .

The inverting input terminal of the operational amplifier 19 is connected to the output terminal of the operational amplifier 19, the feedback resistor 20, and the variable resistor 2.
1 and connected via capacitor 2
2. It constitutes an amplifier circuit that is grounded via a resistor 23.

The voltage is input to the rectifying and smoothing circuit 25 via the output capacitor 24 of the operational amplifier 19, and output as a DC voltage.

Under the above circuit configuration, the voltage appearing on the electrode 12 exhibits a voltage waveform due to the vibration caused by the tuning fork, and the voltage waveform is transmitted to the FET 1 which has a high input impedance.
The output voltage of the FET 14 is further amplified by the next stage amplifier circuit via the capacitor 17, and then converted into a DC voltage by the next rectifying and smoothing circuit and output.

Therefore, the surface potential V ₀ of the object to be measured is determined by the relationship shown in equation (2) above and the vibration caused by the tuning fork.
The DC voltage appears as a voltage waveform, is further amplified, rectified, and smoothed, and is output as a DC voltage, which is then detected.

By inserting a limiter to stabilize the drive voltage in the tuning fork drive circuit, the amplitude of the tuning fork is stabilized without being affected by fluctuations in the power supply voltage, so the voltage waveform appearing at electrode 02 is also stabilized regardless of the power supply voltage.
Accurate surface potential can be detected.

Two Zener diodes are used for the insertion and limiter, which is simple and inexpensive.

Although a voltage element is used to vibrate the tuning fork, an electromagnetic coil may also be used.

Effects of the Invention As described above, the present invention has a simple configuration in which a stabilizing means is provided in the circuit that drives the tuning fork, and the driving voltage of the tuning fork can be stabilized at a constant level without being affected by fluctuations in the power supply voltage. Surface potential can be detected accurately.

[Brief explanation of the drawing]

Fig. 1 is a principle diagram for explaining the principle of a surface potential sensor, Fig. 2 is an external view of a surface potential sensor according to an embodiment of the present invention, and Figs. 3 and 4 are diagrams showing a circuit of an embodiment. , FIG. 5 is a diagram showing the output waveform of the tuning fork driving voltage. 01...Object to be measured, 02...Electrode, 03...Tuning fork, 04...Casing, 1...Casing, 2
... Hole, 3 ... Tuning fork, 4 ... Piezoelectric element, 5 ... Resistor, 6 ... Capacitor, 7 ... Resistor, 8 ... Operational amplifier, 9 ... Resistor, 10, 11 ... Zener diode, 12 ... ...Electrode, 13...Resistance, 14...
...FET, 15, 16... Resistor, 17... Capacitor, 18... Resistor, 19... Operational amplifier, 20
...Resistor, 21...Variable resistor, 22...Capacitor, 23...Resistor, 24...Capacitor, 25...
... Rectifier smoothing circuit.

Claims (1)

[Claims] 1. In a vibrating capacitive surface potential sensor using a tuning fork, the output amplitude of the drive voltage is kept constant in the tuning fork drive circuit that converts the vibration of the tuning fork into an electrical signal and returns that signal to the tuning fork to vibrate the tuning fork. A surface potential sensor comprising a stabilizing means for stabilizing the surface potential.