JP2002090401A - Capacitance sensor circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a suitable sensor circuit for using as a burglar alarm, which is reliable with no malfunction caused by noise that is input from a capacitance sensor and a cable due to deterioration of a recent electromagnetic environment, and can detect a change of a capacitance with no influence caused by the capacitance temperature of the capacitance sensor and a temporal capacitance deviation. SOLUTION: The capacitance sensor circuit is provided with a phase shifting type of sinusoidal oscillator 1, a two-phase buffer 2 with the difference of 180 degrees in phase that receives a signal from the oscillator 1 to transmit a capacitance adjusting part 3, the capacitance adjusting part 3, an I-V converting part 4 that is connected to the adjusting part 3, a BPF(band-pass filter) 5 that is connected to the I-V converting part 4, an alternating current amplification 6 that is connected to the BPF 5, a synchronizing detecting part 7 that is connected to the amplification 6, a synchronous signal generating part 8 that is connected to the detecting part 7 and transmits a signal to the detecting part 7, an LPF(low-pass filter) 9 that is connected to the detecting part 7, a direct current amplification 10 that is connected to the LPF 9, a DC servo 11 that is attached to the amplification 10, and a voltage comparing driver 12 that is connected to the amplification 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance sensor circuit and, more particularly, to a capacitance sensor suitable for use in a security alarm or the like by mounting the capacitance sensor in a place where electromagnetic noise or temperature fluctuation is large. Circuit.

[0002]

2. Description of the Related Art As a conventional capacitance measuring circuit, a capacitance meter (transistor technology, September 1983, P318-P) capable of obtaining a voltage proportional to the capacitance is used.
322). FIG. 6 shows a configuration diagram of this circuit. That is, as shown in FIG. 6, the signal of the phase-shift type sine wave oscillator 1 is applied to the capacitor under test 15, and the current flowing through the capacitor 15 under test is IV-converted by the IV converter 4. To generate a detection output synchronized with the signal of the synchronization signal creation unit 8 created by the phase shift type sine wave transmitter 1 by the synchronization detection unit 7, and convert this signal into DC by the LPF 9 (low-pass filter). The voltage is converted and amplified to a predetermined voltage by the DC amplifier 10 to obtain an output voltage of a voltage proportional to the capacitance of the capacitor 15 to be measured. Here, the output signal amplitude of the phase shift type sine wave transmitter 1 is detected by the synchronous detector 7 and then converted into a DC voltage by the LPF 9s, and compared with a reference voltage inside the phase shift type sine wave transmitter 1. The stabilization is achieved by setting the limiter voltage of the diode limiter circuit by the error voltage.

FIG. 7 shows a circuit for explaining the basic principle of the conventional circuit. Assuming that the voltage of the sine wave oscillator OSC1i-201 is Vri and the angular velocity is ω, the output voltage Vout of the operational amplifier 204 can be calculated by using the reference capacitor Cri-203 and the measured capacitance (capacitor) Cxi-202 according to the equations (1) and (2). It is expressed by a relational expression as in 2).

[0004]

(Equation 1)

[0005]

(Equation 2)

As described above, the measured capacitance Cxi-202
And an output voltage Vout proportional to the capacitance is obtained. The output voltage Vout is not related to the angular velocity ω, that is, not related to the oscillation frequency of the sine wave oscillator OSC1i-201.
, It is in a virtual ground state, so even if it is connected using a shielded wire, the influence on the measurement result is reduced.

The output amplitude of the phase-shifted sine wave oscillator 1 is controlled by applying a restriction to the capacitor under test 15 by synchronously detecting a signal, converting the signal to a DC voltage by the LPF 9, and comparing this voltage with a reference voltage. This is performed by limiting the amplitude of the circuit of the phase-shift type limited wave oscillator 1 using a limiter.
This limiter circuit is a limiter using a diode and is not provided with feedback by an operational amplifier.

[0008]

In such a conventional measuring method, as shown in the equation (1), the output voltage V
Although out has a relationship proportional to Cxi / Cri, the output voltage of the operational amplifier is a finite value limited by the power supply voltage or the like. Therefore, if Cxi has a large capacitance when Cri is fixed, the output voltage becomes large. Is saturated, and when Cri is increased so as not to saturate, the gain decreases. When Cxi has a large capacitance, the change value Δ
There was a problem that Cxi could not be detected with high sensitivity. Also, when the capacitor under test is connected to the operational amplifier via a long cable, there is an increase in capacitance due to an increase in the length of the cable and an increase in noise induced into the cable. There was a problem that it could not be obtained. For this reason, there is also a problem that when the capacitance to be measured (capacitor) is a capacitance sensor, it cannot be used in parallel connection.

As in the case where the capacitance to be measured is a capacitance weight sensor, even if the temperature of the environment in which the sensor is installed or a relatively gradual change such as rainfall is relatively small over time, the capacitance of the sensor is not affected. When the change in the capacitance is large, the output voltage fluctuates greatly, and there is a problem that the capacitance weight sensor cannot identify a human and a small animal without malfunction.

[0010] Feedback is applied to the circuit for stabilizing the output amplitude of the phase-shifted sine wave oscillator 1 using the synchronous detection output, and the circuit is complicated and the number of parts is large.
Since a diode limiter circuit is used to control the amplitude, there is a problem that the output fluctuates greatly due to temperature and the offset voltage of the oscillation output is large. For this reason, in the circuit for synchronizing signal generation 8, the duty of the waveform of the synchronizing signal of the synchronizing detection unit did not match the phase-shift sine wave oscillator 1, and the synchronizing detection output voltage became unstable with respect to temperature.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is intended to prevent malfunction due to noise input from a capacitance sensor, a cable, or the like due to recent deterioration of an electromagnetic environment or the like. Not to be used for security alarm, which can detect the change of capacitance without being influenced by the temperature of the capacitance of the capacitance sensor and the deviation of the capacitance over time. To provide a sensor circuit suitable for

[0012]

A capacitance sensor circuit according to the present invention includes a phase shift type sine wave transmitter 1 and a capacitance adjustment unit 3 which receives a signal from the phase shift type sine wave transmitter 1 and receives the signal. 1 to communicate
Two-phase buffer 2 having a phase difference of 80 degrees, its capacitance adjustment unit 3, and IV conversion unit 4 connected to capacitance adjustment unit 3
, A BPF (bandpass filter) 5 connected to the IV converter 4, an AC amplifier 6 connected to the BPF 5, a synchronous detector 7 connected to the AC amplifier 6, and a phase-shift sine wave oscillator 1
, And a synchronizing signal generator 8 for transmitting a signal to the synchronous detector 7, an LPF (low-pass filter) 9 connected to the synchronous detector 7, a DC amplifier 10 connected to the LPF 9, and a DC amplifier 10. DC servo 11 and DC amplification 10
And a voltage comparison driver 12 connected to the power supply. With such a configuration, a change in capacitance can be detected regardless of the size of the capacitance of a sensor or the like, and at the same time, the signal-to-noise ratio (S / N)
Can be improved.

In the above-described DC output, a DC voltage corresponding to the capacitance of the sensor, which fluctuates gradually with time, is generated. In order to eliminate this, a circuit comprising a DC amplifier 10 and a DC servo 11 is provided. Provided. This circuit integrates the output voltage of the DC amplifier 10 and feeds it back to the reference voltage input of the DC amplifier 10 to remove a temporally gradual DC voltage. Is composed of a circuit (DC servo 11) for limiting the output of the DC amplifier 10 from entering the input of the integration circuit, so that a DC output that is fast in time and has only a large change can be obtained. Therefore, by comparing the magnitudes of the output voltages, the magnitude of the change in the capacitance of the sensor can be identified and detected without being affected by the temperature of the sensor or the deviation of the capacitance with time.

Further, in order to stabilize the output amplitude of the above-mentioned phase-shift type sine wave oscillator 1, a limiter circuit using an operational amplifier is used, and the limiter voltage is changed to a simple circuit configuration which is set from a precise reference voltage. did. As a result, the number of components can be reduced, and the output amplitude of the phase-shift type sine wave oscillator 1 and the temperature stability of the offset voltage can be improved. As a result, the duty of the signal output of the circuit of the synchronizing signal generator 8 is improved, and the synchronizing detector operates stably with high accuracy.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the capacitance sensor circuit of the present invention will be described in detail with reference to the drawings. According to the present invention, it is possible to use the capacitance sensor regardless of the capacitance of the capacitance sensor and the capacitance of the cable connecting the capacitance sensor and the sensor circuit, and at the same time, to use the signal-to-noise ratio (S / To improve N), as shown in FIG.
A cable 14, a phase-shifted sine wave oscillator 1, a two-phase buffer 2 having a phase difference of 180 degrees, a capacitance adjusting unit 3,
A circuit having a configuration including the -V conversion unit 4 is provided. Then, from the sine wave signal generated by the phase shift type sine wave
A two-phase buffer 2 having two sinusoidal oscillation outputs having different phases of 80 degrees is created, and one of these signals is connected to a cable 14.
And the other signal is connected to the variable capacitor, and the other electrode to which the variable capacitor is not connected is connected to the coupling capacitor. The output of the capacitance adjusting unit 3 having this configuration is converted by the IV converter 4 into a current flowing through the coupling capacitor into a voltage. With the circuit having the above configuration, an AC signal that does not saturate due to the magnitude of the capacitance of the sensor can be obtained by adjusting the capacitance of the variable capacitor of the capacitance adjusting unit 3. The operation principle of the above circuit will be described with reference to FIG. The capacitances Cx... 103 composed of a capacitance sensor and a cable are connected to the sine wave oscillators OSC1. Capacitor Cvari… 1
04 and the other of the capacitances Cx... 103 to be measured and the variable capacitors Cvari. The connection point A and the negative-phase input of the operational amplifier 107 are connected by coupling capacitors Cc 105, and the reference capacitors Cr 106 are connected to the feedback loop of the operational amplifier. However, the coupling capacitor Cc and the measured capacitance Cx
Shall satisfy the relationship of Cx> Cc.
In this circuit configuration, the current flowing through the measured capacitance Cx is Ix, and the current flowing through the variable capacitor Cvari is Ivar.
i, assuming that the current flowing through the coupling capacitor Cc is Ic, the following is obtained.

(Equation 3)

(Equation 4)

(Equation 5) Here, the current flowing through the reference capacitor Cr is
It is equal to the current flowing through the capacitor Cc, and is as follows.

(Equation 6) Since the equations (5) and (6) are equal, between Vr and Vout
The following relational expression holds.

(Equation 7) From the equation (5), the measured capacitance Cx and the variable capacitor C
variable capacitor Cvari so that the capacitance of vari
Is varied, the current flowing through the coupling capacitor Cc becomes 0
(Zero). At this time, the reference capacitor
The current flowing through Cr also becomes 0, and from the relational expression of Expression (7),
The output voltage Vout of the pair amplifier 107 becomes 0. This
Variable capacitors for various measured capacitances Cx
Adjust the output voltage to 0 by making Cvari variable
The capacitance of the capacitance sensor and the capacitance
Large capacitance of cable connecting sensor and sensor circuit
Make it usable regardless of size. Further, equation (7)
Transform

(Equation 8) Here, the variable capacitor Cva is set so that Cx = Cvari.
Adjust ri. Where the measured capacitance of the sensing capacitor
When Cx increases by ∇Cx, Cx−Cvari = ∇Cx
Equation (8) gives the measured capacitance of the sensing capacitor.
Using the change ΔCx of

(Equation 9) Here, the condition that Cvari> Cc and ∇Cx << Cc holds
Then, equation (9) can be approximated as follows.

(Equation 10) Thus, the change in the capacitance of the detection capacitor ∇Cx
A proportional output voltage Vout can be obtained. Also positive
Adjust Cvari / Cc even if the sinusoidal voltage is set to the maximum value
By adjusting, Vout can be prevented from being saturated. one
In the case of the circuit shown in Fig. 7, the noise
Output voltage Vouti is equal to the reference
Capacitor C i, the measured capacitance Cx of the detection capacitor
i, and can be expressed as follows using the sine wave signal voltage Vri.

[Equation 11] In the case of the circuit of FIG.
From (10)

(Equation 12) In the equation (12), the relationship of Cvari> Cc is satisfied,
If 1 + Cvari / Cc = 4, then the output voltage Vout
The appearing noise voltage ΔVn is the output voltage shown in equation (8).
The effect of the noise voltage ΔVn appearing at Vouti about 1/16 of
Become. Thus, the relation of Cx> Cc or Cvari> Cc
Coupling capacitor Cc and a large sinusoidal signal voltage
S / N is improved by the parameter setting condition of Vr. What
In the equation (12), Cvari / Cc is the gain of the circuit.
Influences the resistance of the variable capacitor Cvari.
First, the coupling capacitor Cc is also changed, and Cvari / Cc =
Adjust it to be a constant or change it in conjunction
Circuit.

FIG. 8 shows a specific embodiment of the circuit of FIG.
In FIG. 8, the two-phase buffer 2 is a phase-shift type sine wave oscillator 1
And a circuit for inverting and amplifying the above signal using an operational amplifier 401, and an operational amplifier 402 for amplifying the signal in the normal phase.
The gain of the circuit 01 can be adjusted by the resistance values of the resistors R1 and R2. The operational amplifier 40 used here
Reference numeral 1402 denotes an IC such as OP279 capable of driving a large capacitive load. The output of the positive-phase amplification operational amplifier 402 is connected to the coupling capacitors Cc... 105 via the capacitance sensor 13 via the cable 14. The output of the inverting amplification operational amplifier 401 passes through the variable capacitors Cvari... 104 via the cable 14 and the coupling capacitors Cc.
Connected to. In the cable 14, the output of the operational amplifier 401 and the output line of the operational amplifier 402 are twisted in order to reduce the influence of external induction noise and cancel the influence of the capacitance of the cable. The electrostatic capacity adjustment unit 3 and the IV converter 4 are connected via coupling capacitors Cc... 105, and a reference capacitor Cr. R3, R4, R5, R6, and C1 perform DC feedback in AC amplification by the operational amplifier 403,
This is provided to minimize the output offset.
This DC feedback circuit suppresses fluctuations in the output voltage due to low frequency noise such as a commercial frequency induced through the cable 14, the capacitance sensor 13, and the like. In the embodiment, 10 KHz is used as the oscillation frequency of the sine wave transmitter 1 because it is advantageous in terms of circuit design, and 50 Hz and 60 Hz of commercial frequency and 500 KH of radio broadcast frequency are used.
While it is not affected by inductive interference from frequencies above z, other frequencies may be used. The adjustment method of the circuit shown in FIG. 8 is as follows. When the output voltage of the IV converter 4 is observed with an oscilloscope, and the capacitance of the variable capacitors Cvari.
The amplitude of the output voltage of the IV converter 4 is reduced and adjusted to the minimum. Thereafter, when a weight capacitance sensor is used as the capacitance sensor 13, the coupling capacitors Cc... 105 are connected to prevent the output voltage of the IV converter 4 from saturating into a pulse waveform by applying the maximum weight. adjust.
When a plurality of capacitance sensors 13 are connected in parallel, or when the capacitance sensors 13 are replaced with those having a large capacitance, the capacitance of the variable capacitors Cvari... At this time, the coupling capacitor Cc 105
Also, care should be taken to increase the capacitance of C.sub.variable so that Cvari / Cc = constant as much as possible. In the case of this embodiment, the capacitance of the capacitor is varied by switching the parallel connection of a fixed capacitor having good temperature characteristics as a variable capacitor using a switch. In the circuit of FIG. 8, the variable capacitors Cvari... 104 of the capacitance adjusting unit 3 are installed at the same location as the capacitance sensor 13 so that the variable capacitors Cvari.
FIG. 9 shows an embodiment designed to improve the temperature characteristics by using the same temperature characteristics as the capacitor 104 and the capacitor of the capacitance sensor 13. In this case, the adjustment is performed on the side of the capacitance sensor outside the substrate of the sensor circuit, but the adjustment method is the same as in FIG. However, the adjustment in FIG. 8 and FIG.
It is necessary to confirm again when the gain of the DC amplifier 10 is adjusted.

The AC voltage output from the IV converter 4 further removes low-frequency noise and high-frequency noise induced by a band-pass filter (BPF... 5) circuit whose center frequency is the frequency of the sine wave signal Vr. I do. After that, the signal is amplified by the AC amplifier 6 to increase the sensitivity, and then detected by the synchronous detection unit 7. This synchronous detection circuit generates a sine wave signal V
The detection is performed in synchronization with the signal r, and the synchronization signal generator 8 adjusts only the capacitance component to be the detection output by the phase adjustment signal synchronized with the sine wave signal Vr. This detection output is converted into a DC voltage by LPF. The S / N can be improved by the operation of the synchronous detector 7 and LPF. FIG. 11 shows a specific embodiment of these circuits.

According to the present invention, a change in the capacitance of a capacitance weight sensor or the like due to a relatively time-dependent change in the environment such as a temperature change in the environment in which the sensor is installed or a rainfall is output as an output voltage. In order to suppress, the DC amplification 1 of FIG.
0 is provided with a DC servo... 11 and a circuit for obtaining a DC voltage output from which a gradual temporal voltage fluctuation is suppressed and removed.
The operation principle of this circuit is as shown in FIG.
01 is provided with an LPF (integrating circuit) at the output and its output is connected to the REF input of the differential amplifier 301. … 301, a gradual voltage fluctuation is temporally added by adding a limiter circuit that limits the input voltage of the LPF attached to the output of the differential amplifier... 301 to a limiter voltage close to the reference voltage. The suppressed DC voltage output can be obtained from the output of the differential amplifier 301. The limiter voltage and the limiter circuit are set to a positive value when the output of the differential amplifier 301 detects a positive voltage change when the input of the differential amplifier 301 A limiter circuit is used that sets a limiter voltage, and a limiter circuit that limits on the plus side is used. When a change in voltage in the minus direction is detected, a limiter circuit that sets a minus limiter voltage and uses a limiter on the minus side is used. Here, the limiter voltage is selected to be about 1/50 of the maximum output voltage of the differential amplifier 301. This is determined by the experimental fact that the output voltage of the differential amplifier 301 is servo-operated when there is a gradual change in capacitance with time and falls within a range of about 1/50 of the maximum change output voltage. The voltage value is about 100 mV when the maximum output is 5V. Here, the reference voltage is usually set to 0 V, but may be set to other voltages close to 0 V. The limiter circuit shown in FIG. 3 includes a limiter voltage, an operational amplifier 303, a diode 308, and a resistor.
307, 304, and analog switch 309. The analog switch 309 turns on the limiter function.
-Used to turn off, and turn off when adjusting the variable capacitors Cvari ... 104 of the sensor circuit. When the power supply voltage of the circuit is activated, a circuit is provided in which the function of the zero point correction circuit works first and the limiter circuit is turned on after the output voltage becomes close to the reference voltage. The circuit of FIG. 3 shows an example of a circuit configuration in which the output of the differential amplifier 301 detects a change in the voltage in the plus direction, and uses a limiter circuit that limits the voltage on the plus side. In FIG. 3, the limiter voltage is set to a negative value and a diode is used.
If the anode and the cathode of 308 are reversed, a limiter circuit for limiting on the minus side is provided, and a differential amplifier 301
Is a circuit configuration for detecting a change in voltage in the negative direction. FIG. 4 shows a circuit obtained by replacing the limiter circuit of the circuit of FIG.
In this case, the limiter start voltage is the limiter voltage, but the input voltage of the integration circuit using the operational amplifier 302 matches the reference voltage. FIG. 5 shows a differential amplifier of the circuit of FIG.
1 is realized by using an operational amplifier 316, and a differential amplifier is constituted by resistors 312, 313, 314, and 315. In this circuit, the output of the operational amplifier 302 is connected to the resistor 315, and the output of the operational amplifier 302 shown in FIG.
It is characterized in that it is not connected to the part corresponding to the F input. FIG. 12 shows an example of the specific embodiment of FIG. In the DC servo 11 circuit of FIG. 12, S1 is normally set to ON, and is turned OFF at the time of capacitance adjustment or the like.
Return to the original ON state. Since a power-on reset circuit is connected to the circuit in FIG. 12 when the power supply is momentarily interrupted or when the power supply is started, the limiter circuit remains off even when S1 is ON.
It becomes F.

The output of the above-described differential amplifier 301 is inputted to a COMP (voltage comparison) driver 12 shown in FIG. 1 and is compared with a reference voltage which can be arbitrarily adjusted to output a signal which can be used for an alarm or the like. And a detection signal level for detecting a change in capacitance of the sensor. Thus, when a weight sensor is used as the capacitance sensor, it is possible to discriminate and detect a person and a small animal by adjusting the detection level, which is excellent in applications such as a security alarm. The detected sensor signal output has an interface function of outputting a predetermined time using an LED display or a timer, and a setting function such as reset.

FIG. 10 shows a specific embodiment of the phase shift type sine wave transmitter 1. The phase-shifted sine wave oscillator 1 includes a Butterworth filter circuit 501, an integrating circuit 502, a limiter circuit 503, and a Butterworth filter circuit 501
And an integration circuit 502, a phase-shift type sine wave transmission circuit is obtained. The output amplitude of this oscillator is stabilized by positive and negative limiter voltages of a limiter circuit... 503 using an operational amplifier. The limiter circuit using this operational amplifier is characterized in that the effect of the forward voltage of the diode is very small, and as a result, there is almost no change in the limiter voltage due to a temperature change.The offset voltage of the oscillation waveform is small and the temperature stability is good. A circuit can be realized. The sine wave oscillation output includes an output of OUTPUT1 and an output of OUTPUT2.
Since the phases are different from each other by 90 degrees, it is advantageous to configure the phase adjustment circuit for synchronizing signal generation 8 using this phase difference because the circuit can be simplified.

[0021]

The present invention provides a capacitance adjusting circuit 3 for subtracting and compressing the capacitance of a capacitance sensor and a cable.
, It is possible to use capacitance sensors and cables with various capacitances, detection by connecting a large number of capacitance sensors in parallel, and detection using a cable with a length of 100 m or more. Is possible. Also, since a relatively large voltage can be applied to the capacitance sensor 13 by the capacitance adjustment circuit 3 and the two-phase buffer 2, the S / N (signal-to-noise ratio) can be improved. In addition, since the BPF 5 is provided, external noise from the sensor can be removed, and the function of the synchronous detection unit 7 and the LPF 9 having a noise removal capability makes the circuit resistant to noise. In addition, when the capacitance sensor changes gradually over time due to a gradual environmental change such as temperature, it can be suppressed and removed, so a pressure-sensitive weight sensor is used as the capacitance sensor. Then, it is possible to realize a sensor circuit output capable of discriminating a small animal from a human or the like without malfunction due to a temperature change, rainfall, gentle snowfall, or the like. Further, since the temperature stability of the output amplitude of the phase-shift type sine wave transmitter 1 is improved, the operating temperature range in which the sensor circuit can be used can be widened, and the temperature change in the installation location of the sensor circuit can be detected by the sensor. No malfunction is caused by affecting the output.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of the present invention.

FIG. 2 shows a capacitance adjustment 3 of the present invention and a two-phase buffer 2
FIG. 4 is a circuit diagram illustrating the operation of the IV converter 4.

FIG. 3 is a circuit diagram illustrating the operation of the DC amplifier 10 and the DC servo 11 of the present invention.

FIG. 4 is a circuit diagram in which the limiter circuit of the circuit of FIG. 3 is replaced using a comparator.

FIG. 5 is a circuit diagram showing a state where the differential amplifier of the circuit of FIG. 3 is realized using an operational amplifier.

FIG. 6 is a configuration diagram of a conventional technique.

FIG. 7 is a circuit diagram illustrating the basic principle of a conventional circuit.

FIG. 8 is a diagram showing a specific example of the circuit of FIG. 2;

FIG. 9 is a circuit diagram showing an embodiment designed to improve temperature characteristics.

FIG. 10 is a circuit diagram showing a specific embodiment of the phase-shifted sine wave transmitter 1.

FIG. 11 is a circuit diagram showing a specific example of a BPF 5, an ACAMP (AC amplification) 6, a synchronous detector 7, and an LPF 9.

FIG. 12 is a circuit diagram showing a specific example of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 sine wave transmitter 2 two-phase buffer 3 capacitance adjustment unit 4 IV converter 5 BPF 6 AC amplification 7 synchronization detection unit 8 synchronization signal generation 9 LPF 10 DC amplification 11 ... DC servo 12 ... Voltage comparison driver 13 ... Capacitance sensor 14 ... Cable 15 ... Capacitor to be measured 101 ... Sine wave transmitter 1 (OSC1) 102 ... Sine wave transmitter 2 (OSC2) 103 ... Capacitance to be measured Cx 104: variable capacitor Cvari 105: coupling capacitor Cc 106: reference capacitor Cr 107: operational amplifier 201: sine wave oscillator (OSC1i) 202: capacitance to be measured Cxi 203: reference capacitor Cri 204: operational amplifier 301: differential for instrumentation Amplifier 302: Operational amplifier (for integration) 303: Operational amplifier (for limiter) 304: Resistance (limiter) 305 resistor (for integration) 306 capacitor (for integration) 307 resistor 308 diode 309 switch 311 comparator 312 resistor 313 resistor 314 resistor 315 resistor 316 operational amplifier 401 inverting operational amplifier 402 Normal-phase amplification operational amplifier 403: AC amplification operational amplifier for IV conversion 501: Butterworth filter circuit 502: Integration circuit 503: Limiter circuit

Continued on the front page F term (reference) 2G028 AA01 CG07 DH05 DH21 FK01 FK07 FK09 GL03 GL12 LR04 5C084 AA02 AA07 BB05 BB34 DD10 GG22 GG23 GG35 GG38 GG56 GG63 GG73

Claims (7)

[Claims] 1. A phase-shifted sine wave transmitter 1, a two-phase buffer 2 receiving a signal from the phase-shifted sine wave transmitter 1, and transmitting the signal to a capacitance adjusting unit 3 having a 180-degree phase difference. A capacitance adjustment unit 3; an IV conversion unit 4 connected to the capacitance adjustment unit 3; a BPF (bandpass filter) 5 connected to the IV conversion unit 4; and an AC amplifier 6 connected to the BPF 5 , A synchronous detector 7 connected to the AC amplifier 6, a synchronous signal generator 8 connected to the phase-shifted sine wave transmitter 1 for transmitting a signal to the synchronous detector 7, and an LPF connected to the synchronous detector 7. (Low-pass filter) 9, DC amplification 10 connected to LPF 9,
A DC servo 11 attached to the DC amplifier 10;
A capacitance sensor circuit comprising a voltage comparison driver 12 connected to 0. 2. The capacitance adjustment unit 3 and the IV conversion unit 4 according to claim 1, wherein one of the outputs of the two-phase buffer 2 has a capacitance to be detected comprising a capacitance sensor 13 and a cable 14. Connect the electrode of the variable capacitor to the other,
The capacitance to be detected and the other electrode of the variable capacitor are connected and connected to the electrode of the coupling capacitor.The other electrode of the coupling capacitor is connected to the inverting input of the operational amplifier. The capacitance sensor circuit according to claim 1, wherein a capacitor is connected. 3. A DC amplifier according to claim 1,
Reference numeral 1 designates the output of the instrumentation differential amplifier as L through a limiter circuit.
This LPF is connected to the input of a PF (low-pass filter).
Is connected to the REF (voltage reference input) of the instrumentation differential amplifier, and the limiter voltage of the limiter circuit is 1/5 of the maximum change output voltage of the DC amplifier 10.
It is about 0, and is used as a positive limiter voltage when detecting a change in the voltage in the positive direction, and as a negative limiter voltage when detecting a change in the voltage in the negative direction.
2. The capacitance sensor circuit according to claim 1, wherein the capacitance sensor circuit has a function of canceling a limiter function of the limiter circuit when adjusting the capacitance adjustment unit 3 and when the power supply voltage of the circuit is activated. 3. 4. A limiter circuit comprising a comparator and an analog switch, wherein the limiter circuit according to claim 3 is replaced by a limiter circuit for detecting a change in voltage in the positive direction, wherein a positive limiter voltage is used. If a change in the voltage in the direction is detected, a negative limiter voltage is used.
Is used as a reference voltage, and the limiter voltage is 1/50 of the maximum change output voltage of the DC amplifier 10.
And a function of releasing the limiter function of the limiter circuit when the capacitance adjusting unit 3 adjusts and when the power supply voltage of the circuit is activated.
3. The capacitance sensor circuit according to claim 1. 5. A phase-shifted sine wave oscillator 1 according to claim 1, wherein an input of a primary integration circuit is connected to an output of a secondary Butterworth filter using an operational amplifier, and an output of the integration circuit is limited by an operational amplifier. The capacitance sensor circuit according to claim 1, wherein the capacitance sensor circuit has an oscillation amplitude adjustment function and oscillates at a fixed frequency by connecting the circuit. 6. The BPF 5 according to claim 1, wherein the BPF 5 is a band-pass filter having a transmission frequency band of a transmission frequency of the phase-shift type sine wave transmitter 1, and the AC amplifier 6 has an AC amplifier circuit having a gain adjustment function. 2. The method according to claim 1, wherein
3. The capacitance sensor circuit according to claim 1. 7. A synchronizing signal generator according to claim 1, further comprising a phase adjusting circuit for the signal from the phase-shifted sine wave oscillator and an interface circuit for converting the signal level into a pulse. Numeral 7 is characterized in that it has a function of detecting the signal from the AC amplifier 6 by a switch that switches with a signal from the synchronizing signal generator 8, and the LPF 9 has a function of converting the signal detected by the synchronizing detector 7 into DC.
The capacitance sensor circuit according to claim 1, wherein the capacitance sensor circuit is a PF.