Electricity: measuring and testing – Electrostatic field
Reexamination Certificate
2000-09-15
2003-12-09
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Electrostatic field
C324S072500, C324S109000, C310S319000
Reexamination Certificate
active
06661232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric potential sensor and an electronic apparatus using the same, and more particularly to an electric potential sensor employing a piezoelectric tuning fork and an electronic apparatus using the same.
2. Description of the Related Art
In an electronic apparatus such as an electrophotographic copying machine or a laser printer, a photosensor drum must be electrified to a desired level. To this end, an electric potential sensor is used to perform non-contact measuring of the surface potential of the photosensor drum.
FIG. 5
is a block diagram of a conventional electric potential sensor. As shown in
FIG. 5
, an electric potential sensor
1
is provided with a piezoelectric tuning fork
2
′, an oscillation circuit
9
, pre-amplifier
10
, an amplifier
11
, a detecting circuit
12
, a low-pass filter
13
, a DC amplifier
14
, and an output terminal
15
. The piezoelectric tuning fork
2
′ comprises a tuning fork
2
having two legs
3
and
4
(generally made of a metal), a driving piezoelectric member
5
provided on one of the legs
3
, and a feedback piezoelectric member
6
provided on the other leg
4
. A detecting electrode
7
is provided on the leg
3
opposedly facing a surface
8
to be detected, and a capacitance C
1
is formed between the detecting electrode
7
and the surface
8
. The surface
8
indicates, for example, the surface of the photosensor drum.
An output from the oscillation circuit
9
is connected to the driving piezoelectric member
5
, and the feedback piezoelectric member
6
is connected to an input of the oscillation circuit
9
.
The detecting electrode
7
is connected, through the pre-amplifier
10
and the amplifier
11
, to the synchronous detecting circuit
12
. The output of the feedback piezoelectric member
6
is also connected to the synchronous detecting circuit
12
. The output from the synchronous detecting circuit
12
is connected to the output terminal
15
via the low-pass filter
13
and the DC amplifier
14
.
FIGS. 6A
to
6
D show the waveforms of the signal in respective parts of the electric potential sensor
1
described above. An operation of the electric potential sensor
1
is explained with reference to
FIGS. 5 and 6
.
When the signal generated by the oscillation circuit
9
is applied to the driving piezoelectric member
5
provided on one leg
3
of the tuning fork
2
, the driving piezoelectric member
5
mechanically vibrates to cause the piezoelectric tuning fork
2
to vibrate in a reverse-phase mode in which the two legs of the tuning fork
2
vibrate 180° out of phase—that is, as the left leg
3
moves to the left, the right leg
4
moves to the right and vice versa. When the piezoelectric tuning fork
2
vibrates, the feedback piezoelectric member
6
on leg
4
vibrates mechanically and outputs an electrical signal indicative of the frequency, phase and magnitude of the vibration of leg
4
. The electrical signal is fed back to the oscillation circuit
9
to form a closed loop comprising the oscillation circuit
9
, the driving piezoelectric member
5
and the feedback piezoelectric member
6
. This feedback circuit causes self-oscillation to occur at a desired frequency.
When the tuning fork
2
vibrates, the distance between the detecting electrode
7
and the surface
8
(and along with it the capacitance C
1
) changes as a function of the vibration of the tuning fork
2
. The signal output from the detecting electrode
7
changes in accordance with both the change in capacitance C
1
and the electrical potential of the surface
8
to be detected. Particularly, the frequency of the output signal varies as a function of the vibration of the tuning fork
2
and the amplitude of the output signal varies substantially proportionally with any change in the electrical potential of the surface
8
. The signal output from the detecting electrode
7
is amplified by the pre-amplifier
10
and the amplifier
11
and is input into the synchronous detecting circuit
12
.
FIG. 6A
shows the sinusoidal waveform of the signal V
out
output from the detecting electrode
7
and input to the detecting circuit
12
.
The signal output from the feedback piezoelectric member
6
is also input to the synchronous detecting circuit
12
as a reference signal. The signal input from the amplifier
11
is synchronously detected by cross correlating it with the reference signal. As shown in
FIG. 6B
, the signal output from the feedback piezoelectric member
6
is a sine wave having the same frequency and phase as the signal output from the detector electrode
7
. The difference between these signals is their amplitude (the amplitude of the output of amplifier
11
changing as a function of the amplitude of the change on the surface b).
FIG. 6C
shows the waveform of the signal output from the detecting circuit
12
. The signal output from the detecting circuit
12
is preferably a signal which is formed by full-wave rectification of the cross correlated signals input to the detecting circuit
12
.
The DC component in the signal detected by the detecting circuit
12
is extracted by the low-pass filter
13
, and the signal is amplified by the DC amplifier
14
to be output from the output terminal
15
. This signal, shown in
FIG. 6D
, is substantially proportional to the potential of the surface
8
.
Due to external factors, the piezoelectric tuning fork will sometimes include unwanted vibration components which are not in reverse-phase with each other. A typical vibration component of this type is an in-phase vibration component wherein the two legs of turning fork
2
move in unison (in-phase).
FIG. 7A
to
7
D show the waveforms of respective parts of the piezoelectric tuning fork
2
when a non-reverse-phase vibration components having a frequency different from the vibration in the reverse-phase mode component is added to the tuning fork
2
in a conventional electric potential sensor.
In the electric potential sensor
1
, when the piezoelectric tuning fork
2
vibrates with non-reverse-phase components in addition to reverse-phase components, a signal (
FIG. 7A
) is output from the synchronous detecting electrode
7
and is then input to the detecting circuit
12
. This signal has a waveform in which two or more sine waves having different waveforms are combined. The synchronous detecting circuit also receives the reference signal (
FIG. 7B
) generated by the feedback piezoelectric member
6
. This signal also has a waveform in which non-reverse-phase vibrations are added to reverse-phase vibrations. As described above, this signal has the same waveform and phase (but a different magnitude) as the signal output from the detecting electrode
7
.
In the detecting circuit
12
, a signal from amplifier
11
which has both reverse-phase mode and non-reverse-phase mode components is cross correlated with the reference signal from piezoelectric member
6
having the same phase in which the vibration in the reverse phase mode is added to the in-phase mode.
FIG. 7C
shows the waveform of the signal output from the detecting circuit
12
. This signal is formed by full-wave rectification of the signal input to the detecting circuit
12
.
The DC component in the signal detected by the detecting circuit
12
is extracted by the low-pass filter
13
, amplified by the DC amplifier
14
and output to the output terminal
15
.
FIG. 7D
shows the waveform of the signal output from the output terminal
15
. As described above, a DC voltage having the predetermined value is output from the output terminal
15
. By comparing the voltage value of the signal in
FIG. 7D
with that in
FIG. 6D
, it can be seen that the signal output from the output terminal
15
is larger for the case in which the non-reverse-phase mode vibration is added to the reverse-phase mode vibration.
As described above, the conventional electric potential sensor
1
has had a problem that the signal output by the sensor becomes unstable when the non-reverse-phase mo
Deb Anjan K.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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