JPS6148668B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved surface electrometer for detecting latent image potential on the surface of a photosensitive drum of, for example, an electronic copying machine.

Figure 1 shows the characteristics of a conventional electrometer. In conventional surface electrometers, the relationship between the actual surface potential and the detection output of the electrometer does not pass through the origin near zero potential as shown in Figure 4. Therefore, in order to cancel this offset phenomenon, the measurement circuit of the electrometer was devised. Was. However, there were drawbacks such as a complicated circuit configuration and the need for calibration.

The present invention eliminates these drawbacks and facilitates the handling of the electrometer by providing a grid in the detection window of the potential probe and applying a bias to the grid, thereby making it possible to compare the actual surface potential with the detection output. It is characterized in that the relationship passes through the origin.

FIG. 1-1 is a plan view of a window portion of the surface electrometer, and FIG. 1-2 is a sectional view of that portion. In the figure, 1 is a metal case of the surface electrometer, 2 is an opening window for potential detection, 3 is a window frame made of glass epoxy resin for supporting the grid, and 4 is a thin metal wire made of gold-plated tungsten. A grid consisting of
5 is a metal blade that rotates at a constant speed for opening and closing the opening window 2, and 6 is a fixed electrode for potential detection.
The main body 1 and the rotating blade 5 are electrically grounded.

FIG. 2 shows an example of a potential measuring circuit, in which the same components as in FIG. 1 are designated by the same numbers. The other 7 is the rotating blade 5
motors 8, 9, and 10 for increasing the input impedance to the electrode 6;
In the buffer by FET, 9 is a resistor with a resistance value of 100MΩ, 8 is a FET, and 10 is a source resistance. 11
16 is an AC amplifier for amplifying the source output of FET 8 to a required magnitude; 16 is a synchronous clamp circuit that performs synchronous clamping using the output of the synchronous signal amplifier 15; Note that the synchronization signal is transmitted through the light receiving element 13 provided opposite the light emitting element 12 and the optical path 14 provided in the rotary blade 5.
I am getting it. That is, the optical path 14 is opened when the rotary blade 5 closes the aperture window 2, and the light from 12 is received by 13 and a timing signal is output. 17 is a circuit that integrates the clamp output; 18
is the output terminal.

Now, when the opening 2 of the case 1 of the surface electrometer is brought close to the surface of the object to be measured, an electric field is induced between the electrode 6 and the object to be measured. When the electric field is interrupted by the rotating blade 5, a weak alternating current signal whose amplitude is proportional to the strength of the electric field is induced in the electrode 6. In order to determine whether the electric field is positive or negative, the moment when the rotary blade 5 closes the aperture window 2 is defined as zero potential. For this purpose, a synchronous signal is generated by the optical path 14 which opens at the moment when the rotary blade 5 closes the aperture window 2, and the synchronous clamp circuit 16 momentarily clamps the alternating current signal from the amplifier 11 to zero. If the waveform is integrated by the integrator 17 using the zero as a reference, a DC output proportional to the surface potential of the object to be measured is obtained at the output terminal 18.

However, in FIG. 2, even when the surface potential of the object to be measured is zero, the waveform does not disappear and a slight residual waveform remains on the electrode 6, so that the output 1
8 cannot be zero. The cause of this is thought to be the gate leakage current of FET8.

Now, the maximum value of capacitance between electrode 6 and case 1 is C
_MAX , the minimum value is C _MIX , and the resistance value of resistor 9 is R ₉
If the gate leakage current of FET 8 is I _G , then the offset potential E ₀ of the electrode 6 is E ₀ = I _G・R ₉ (1) The peak amplitude value of the residual potential appearing at the electrode 6 △
E is △E=C _MAX - C _MIN / (C _MAX + C _MIN )/
2・E ₀ (2) = C _MAX − C _MIN / (C _MAX + C _MIN )/2
・I _G・R ₉ (3).

As concrete numbers, C _MIN = 1.793PF, C _MAX =
1.896PF, I _G = 1nA, R ₉ = 100MΩ, △E = 1.896-1.793/(1.896+1.
793)/2 × 1 × 10 ^-9 × 100 × 10 ^-6 = 5.6 mV _pp (4), and a residual voltage of about 5.6 mV _pp will be applied to the electrode 6.

Therefore, in the circuit of FIG. 2, the potential offset of the electrode 6 due to the gate leakage current of the FET 8 is positive. In this example, the residual voltage could be canceled by applying a negative potential to the grit 4. The size of the opening window 2 of the outer case 1 is as follows:
A value may be applied from the electrode 6 such that the space potential of the surface corresponding to the inner surface of the outer case 1 becomes zero. In the above example, the voltage applied to grid 4 is -
At 2.2V, the relationship between the surface potential and the output voltage showed good results as shown in FIGS. 4 to 3, and the residual voltage was reduced to about one-fourth.

In this example, the diameter of the grid wire 4 is 60 microns.
Good results were obtained with a line pitch of 1 mm, but
The line pitch does not necessarily need to be 1 mm, and the same effect can be obtained even if a mesh conductor is used instead of parallel thin lines.

FIG. 5 is a diagram showing the relationship between the distance between the grid and case 1 and the optimum grid bias voltage for zero correction. If the grid is provided on the inner surface of the case 1, the offset phenomenon can be eliminated by placing the grid at zero potential, that is, earth.

By providing a grid in the detection window of the electrometer and applying a bias voltage to it as described above, the offset phenomenon can be removed and the relative relationship between the actual surface potential and the detected potential can be accurately determined.

[Brief explanation of the drawing]

Figure 1-1 is a plan view of the electrometer, Figure 1-2 is a cross-sectional view of the electrometer, Figure 1-3 is a perspective view of the electrometer, Figure 2 is a potential measurement circuit diagram, and Figure 3 is a bookmark. The measurement graph according to the invention, Fig. 4, is the conventional measurement graph, Fig. 5
The figure is a bias voltage graph, in which 1 is an electrometer case, 2 is a detection window, 4 is a grid, and 16 is a clamp circuit.

Claims (1)

[Claims] 1. A surface electrometer characterized in that an offset phenomenon is eliminated by providing a grid in the opening window for potential detection in a rotary electrometer and applying a predetermined potential of zero, positive or negative to the grid.