Electricity: measuring and testing – Electrostatic field
Reexamination Certificate
1999-06-03
2001-11-13
Nguyen, Vinh P. (Department: 2858)
Electricity: measuring and testing
Electrostatic field
C324S458000, C324S109000, C324S072000
Reexamination Certificate
active
06316942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric potential sensor, and more particularly to an electric potential sensor to detect, in a non-contact manner, the charge on a photosensitive drum of an electrophotographic device such as a photocopy machine.
2. Description of the Related Art
The construction of a conventional oscillation-capacity type electric potential sensor is illustrated in FIG.
3
.
An electric potential sensor
100
includes a piezoelectric tuning fork
11
which acts as a mechanical oscillator. The piezoelectric tuning fork
11
includes a vibrating body
11
a
of tuning fork type formed of a metal such as elinvar, a driving piezoelectric element
11
b
for driving the vibrating body lia is provided a first arm of the vibrating body
11
a
, and a feedback piezoelectric element
11
c
for feeding back the signal to the driving piezoelectric element
11
b
is provided on a second arm of the vibrating body
11
a
. A detection electrode
12
is formed on the surface of the second arm of the vibrating body
11
a
, i.e., on the surface of the second arm of the piezoelectric tuning fork
11
, and the piezoelectric tuning fork
11
is arranged so that the detection electrode
12
is arranged opposite to a object
13
to be measured. In
FIG. 3
, the detection electrode
12
is shown separate from the piezoelectric tuning fork
11
to aid in easy understanding of the figure.
The driving piezoelectric element
11
b
is connected to an output end of a self-oscillation circuit
15
, and the feedback piezoelectric element
11
c
is connected to an input end of the self-oscillation circuit
15
. The detection electrode
12
is connected to a signal-processing circuit
20
comprising an impedance converter
21
, an AC amplifier
22
, a synchronous detection and smoothing circuit
23
, and a DC amplifier
24
.
The operation of the electric potential sensor
100
is as follows.
The drive signal is output from an output end of the self-oscillation circuit
15
. The drive signal is applied to the driving piezoelectric element
11
b
to cause the driving piezoelectric element
11
b
to be distorted and a first arm of the vibrating body
11
a
to be vibrated.
Due to the vibration of the first arm and the tuning fork shape of the vibrating body
11
a
, a vibration having an opposite phase to that of first arm is generated in the second arm of the vibrating body
11
a
. The vibration of the second arm of the vibrating body
11
a
causes the feedback piezoelectric element
11
c
to be distorted. A feedback signal is therefore generated from the feedback piezoelectric element
11
c
and the piezoelectric tuning fork
11
is self-oscillated by the application of the feedback signal to the input end of the self-oscillation circuit
15
.
The object
13
to be measured is charged to the electric potential V
HV
, and an electric field E is generated between the object
13
measured and the detection electrode
12
. When the piezoelectric tuning fork
11
is caused to vibrate, the distance between the detection electrode
12
formed on the piezoelectric tuning fork
11
and the object
13
fluctuates periodically and the electrostatic capacitance generated between the detection electrode
12
and the object
13
is changed periodically. This induces a charge at the detection electrode
12
, thereby generating an AC signal. Because the AC signal is proportional to the electric potential V
HV
of the object
13
, the detection output signal corresponding to the electric potential V
HV
of the object
13
can be obtained by applying the AC signal in the signal-processing circuit
20
.
The above-mentioned conventional electric potential sensor has a problem that it is difficult to correctly measure the charged electric potential of the object to be measured. This is because the amplitude of the vibration of the piezoelectric tuning fork is changed due to the temperature characteristic of the piezoelectric tuning fork, the vibration leakage from a support member to support the piezoelectric tuning fork, etc., with the result that the output signal of the electric potential sensor is not stable.
An electric potential sensor of oscillation-capacity type designed to solve this problem is disclosed in Japanese Unexamined Patent Publication No. 60-29673. The electric potential sensor detects the amplitude of the piezoelectric tuning fork using a photosensor and maintains the amplitude of the piezoelectric tuning fork to be constant accordingly. However, since the electric potential sensor requires a photosensor, a new problem is raised that the wiring in the vicinity of the piezoelectric tuning fork becomes complicated.
Another electric potential sensor intended to solve the above-mentioned problem includes an electric potential sensor of chopper type disclosed in Japanese Examined Utility Model Publication No. 5-2865. This electric potential sensor eliminates the signal attributable to the amplitude of the piezoelectric tuning fork from the output electric potential by dividing the output voltage of the electric potential sensor and the output voltage of the chopper part.
However, the sensitivity of the detection signal is degraded because the output voltage is divided. Further, because the output voltage of the electric potential sensor and the output voltage of the chopper part both include noise, the noise components are not eliminated even through the division and, indeed, are further increased through the division. In addition, a new problem is raised that the S/N ratio of the detection signal is reduced.
SUMMARY OF THE INVENTION
An electric potential sensor of the present invention can solve the aforementioned problems associated with the conventional electric potential sensors and generates a stable output signal by stabilizing the vibrator amplitude of the piezoelectric tuning fork. It is also free from degradation of the S/N ratio and has a simple structure.
The electric potential sensor according to one embodiment of the present invention comprises an piezoelectric tuning fork provided with a vibrating body of tuning fork type, a drive piezoelectric element provided on a first arm of the vibrating body, and a feedback piezoelectric element provided on the second arm of the vibrating body, a detection electrode provided on the piezoelectric tuning fork, a signal-processing circuit connected to the detection electrode and detecting a change in the electric signal generated in the detection electrode as a result of the charge on the surface of a work to be measured, a self-oscillation circuit connected between the feedback piezoelectric element and the drive piezoelectric element, the self-oscillation circuit includes a gain control circuit, and a comparator connected between the feedback piezoelectric element and the gain control circuit.
The electric potential sensor according to another embodiment of the present invention comprises a piezoelectric tuning fork provided with a vibrating body of tuning fork type, a drive piezoelectric element provided on a first arm of the vibrating body, and a feedback piezoelectric element provided on the second arm of the vibrating body, a detection electrode provided on the piezoelectric tuning fork, a signal-processing circuit connected to the detection electrode and detecting a change of the electric signal generated in the detection electrode which is attributable to the charge on the surface of a work to be measured, a gain control circuit provided on the signal-processing circuit, a self-oscillation circuit connected between the feedback piezoelectric element and the drive piezoelectric element, and a comparator connected between the feedback piezoelectric element and the gain control circuit.
According to a preferred embodiment of the present invention, the feedback signal corresponding to the amplitude of the piezoelectric tuning fork is compared with the reference signal by the comparator, the obtained signal through comparison is received by the gain control circuit in the self-oscillation cir
Murata Manufacturing Co. Ltd.
Nguyen Vinh P.
Ostrolenk Faber Gerb & Soffen, LLP
Patel Paresh
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