Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-10-01
2001-08-21
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
Reexamination Certificate
active
06278283
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a capacitance detecting circuit for a capacitance-type sensor used for measurement of pressure, acceleration, angular velocity and the like.
2. Description of Related Art
As a sensor for detecting pressure of fluid, acceleration or angular velocity of a moving object or the like, the capacitance-type sensor is attracting attention which can detect the pressure, acceleration or the angular velocity by detecting change of capacitance of a capacitor. In particular, the sensor implemented by resorting to semiconductor micromachining techniques provide advantages such as miniaturized implementation of the device incorporating the sensor, enhanced manufacturability on a mass-production basis, realization of high precision and high reliability and so forth.
For a better understanding of the invention, background techniques will first be described in some detail.
FIG. 7
is a cross-sectional view showing a typical capacitance-type acceleration sensor manufactured through a semiconductor micromachining process. As can be seen in the figure, the capacitance-type acceleration sensor is implemented such that a silicon inertial mass member
1
serving as an electric conductor is supported on a anchor portion
2
by a cantilever portion
3
. stationary electrodes
4
and
5
formed on a glass or silicone plate
6
are disposed, respectively, above and below the inertial mass member
1
. As can ready be understood, the inertial mass member
1
and the stationary electrodes
4
and
5
constitute capacitors
7
and
8
, respectively, as shown in
FIG. 8
, which is an equivalent circuit of the capacitance-type acceleration sensor shown in FIG.
7
.
The capacitors
7
and
8
constitute a sensor element
9
. When an inertial force brought about by acceleration acts on the inertial mass member
1
in the x-direction, the inertial mass member
1
is displaced by u in the x-direction. Due to this u, one of the difference voltages between the inertial mass member
1
and the stationary electrodes
4
and
5
increases by &Dgr;C to a value (C−&Dgr;C). In this manner, when the capacitance-type acceleration sensor is subjected to acceleration, differential capacitance changes take place.
A method of converting the differential capacitance change brought about by the displacement of the inertial mass member
1
may be realized by using an impedance-conversion circuit, as already proposed by the applicant of the present application.
FIG. 9
shows, by way of example, a hitherto known circuit capable of outputting output voltage in proportion to change in an unknown capacitance Cx and shows a timing chart for illustrating operation of the capacitance detecting circuit.
Referring to
FIG. 9
, a capacitance detecting circuit
10
includes an operational amplifier OP, wherein a feedback capacitance Cf is connected between input and output terminals of the operational amplifier OP. The feedback capacitance Cf is short-circuited during a time period T
1
by a switch S at a time point or timing &phgr;
1
. The unknown capacitance Cx is connected to the non-inverting input terminal of the operational amplifier OP. A supply voltage Va is applied to the unknown capacitance Cx during the period T
1
at the timing &phgr;
1
. After lapse of the period T
1
, the unknown capacitance Cx is coupled to the ground potential during a time period T
2
by the switch S at a time point or timing &phgr;
2
. The output terminal of the operational amplifier OP is connected to a sample-and-hold circuit
11
by means of the switch S during the period T
2
at a timing &phgr;
3
.
In the capacitance detecting circuit
10
shown in
FIG. 8
, the supply voltage Va is applied to the unknown capacitance Cx during the period T
1
. Since the inverting input terminal of the operational amplifier is connected to a virtual ground potential by way of the non-inverting input terminal due to imaginary shorting of the operational amplifier, electric charge is stored in the unknown capacitance Cx and stored in the feedback capacitance Cf is discharged by way of the switch S.
After lapse of the period T
1
, the unknown capacitance Cx is connected to the ground potential by means of the switch S at the timing &phgr;
2
. As a result, the electric charge stored in the unknown capacitance Cx migrates to the feedback capacitance Cf, whereby the reference voltage Vc is realized. At the timing &phgr;
3
, a saturation output voltage Vout generated by the sample-and-hold circuit
11
can be given by the undermentioned expression (1):
Vout=(Cx/Cf)·Va (1)
As can be seen from the expression (1), the saturation output voltage Vout assumes a value which is in proportion to the unknown capacitance Cx.
The conventional capacitance detecting circuit implemented in the structure described above suffers however problems which will be mentioned below.
(1) The mean external force acting between pole plates of the unknown capacitance Cx over one clock period is determined only by the supply voltage Va. Thus, the external force can not be reduced to zero unless the supply voltage Va is set to zero.
(2) The saturation output voltage Vout is forcibly set in phase with the change of the unknown capacitance Cx.
(3) The differential capacitance type sensor can not be applied intact to the conventional capacitance detecting circuit known heretofore. More specifically, in the differential capacitance type sensor, the terminal
3
of the equivalent circuit shown in
FIGS. 7 and 8
corresponds to the inertial mass member
1
of the sensor. By adjusting the potential difference between the terminals
1
and
3
and the potential difference between the terminals
2
and
3
, an external force acts to cancel out the displacement of the inertial mass member
1
in the x-direction due to acceleration. However, the circuit structure such as shown in
FIG. 9
can not be applied to the differential capacitance/servo-type sensor.
As is apparent from the above, the conventional capacitance detecting circuit lacks flexibility in respect to the utilization of the output signal as well as application to the sensors.
SUMMARY OF THE INVENTION
In the light of the state of the art described above, it is an object of the present invention to provide a capacitance detecting circuit which can be applied to as many various types of capacitance-type sensors as conceivable, such as a single-capacitance-type sensor, differential capacitance type sensor, differential capacitance/servo-type sensor and the like.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a capacitance detecting circuit, which includes an operational amplifier having an inverting input terminal and an output terminal between which a feedback capacitance component is connected, a capacitance-type sensor having an electrostatic capacitance subjected to change under action of an external force, a charge/discharge control means for electrically charging the capacitance component of the capacitance-type sensor by connecting a charge/discharge terminal of the capacitance component to a reference voltage source at a first clock timing for discharging the feedback capacitance component and changing over the charge/discharge terminal to the feedback capacitance component at a second clock timing to thereby transfer electric charge, and a voltage converting means for converting the transferred electric charge to a voltage to thereby output the voltage as a sensor output voltage.
In an exemplary preferred mode for carrying out the invention, the capacitance-type sensor may be implemented as a differential capacitance type sensor having first and second capacitances, one of which is caused to increase with the other being caused to decrease under action of a same external force, wherein the charge/discharge control means connects the charge/discharge terminal of the first and second capacitances to the reference voltage
Leroux E P
Leydig , Voit & Mayer, Ltd.
Metjahic Safet
Mitsubishi Denki & Kabushiki Kaisha
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