JP2004028719A - Electrostatic capacity sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrostatic capacity sensor capable of reducing an error in measuring an electrostatic capacity and improving the sensitivity. <P>SOLUTION: The sensor is provided with a sinusoidal wave oscillating circuit 1; a circuit means 2 converting a sinusoidal wave signal of the sinusoidal wave oscillating circuit 1 to a signal of the same phase and a phase inversion signal having the phase sifted by 180°; an impedance conversion means 4 for extracting a difference detection signal from a series circuit 3, which includes an electrostatic capacity Cx to be measured and a comparison electrostatic capacity as a reference in which the signal of the same phase is applied to one end thereof and the phase inversion signal is applied to the other end thereof, and a connection point between the electrostatic capacity Cx to be measured and the comparison electrostatic capacity, and applies an impedance conversion to the extracted signal; an adding means 5 for adding a part of the sinusoidal wave signal of the oscillating circuit 1 having the phase shifted by nearly the 90-degree, to the output of the impedance conversion 4 as an adding signal; an amplification means 6 for amplifying the output with the adding signal added thereto; a phase detecting means 7 for detecting the phase of the output from the signal of the oscillating circuit 1; and a smoothing means 8 for smoothing the output. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a capacitance sensor for detecting a change in capacitance, and is particularly suitable for a minute change in capacitance, for example, object detection, moisture detection, humidity detection, or monitoring of an approaching object. The present invention relates to a simple capacitance sensor.
[0002]
[Prior art]
Conventionally, there are two pairs of capacitances, one of which is the capacitance to be detected. Usually, the capacitance of two appropriately arranged electrodes and the minute static capacitance due to the proximity of the target object. The capacitance is added, and the others are formed of a constant reference capacitance. In order to detect the difference between the two capacitances, there is known a method of detecting the difference by using differential amplification as disclosed in Japanese Patent No. 3036680 and Japanese Patent Application Laid-Open No. 6-222032.
[0003]
[Problems to be solved by the invention]
In order to capture the difference between the two capacitances, one reference signal is added to the one obtained by connecting the resistance and the two capacitances in series, and the difference between the first-order lag signals generated in the respective capacitances is subtracted. Generally, the output is output from a dynamic amplifier. However, since the change in the capacitance is very small, it is affected by the temperature fluctuation and drift of the circuit, and the accuracy is not very high.
[0004]
Normally, when the impedance of the resistor and the capacitance is set to be substantially the same in order to obtain the optimum sensitivity, the phase of the comparison reference side and the detection capacitance side is 45 degrees behind the original reference signal, and the capacitance of the detection capacitance side is When it increases, the difference in amplitude due to a slight phase shift is amplified by an operational amplifier (op-amp), so that the product of the amplification degree of the operational amplifier and the change due to the phase shift appears in the output.
[0005]
However, this conventional method is greatly affected by the magnitude of the original reference signal and the external AC noise signal, and thus has a problem in capturing a minute change in capacitance.
[0006]
In view of the above, an object of the present invention is to provide a capacitance sensor capable of reducing a measurement capacitance error and improving sensitivity.
[0007]
Other objects and novel features of the present invention will be clarified in embodiments described later.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, a capacitance sensor according to the present invention includes a sine wave generation signal source,
Circuit means for converting the sine wave signal of the sine wave generation signal source into a signal having the same phase and a phase inversion signal whose phase is shifted by 180 degrees;
A series circuit of the measured capacitance and a reference capacitance that is a reference, wherein the signal having the same phase is applied to one end, and the phase inversion signal is applied to the other end,
Extracting a differential detection signal from a connection point between the measured capacitance and the comparison capacitance, and impedance conversion means for performing impedance conversion,
Adding means for adding, as an additional signal, a part of the sine wave signal of the sine wave generation source whose phase is shifted by about 90 degrees to the output of the impedance converting means;
Amplifying means for amplifying the output after adding the additional signal,
Phase detection means for performing phase detection on the output of the amplification means by a signal from the sine wave generation source,
And a smoothing means for smoothing the output of the phase detection means to obtain a DC output.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the capacitance sensor according to the present invention will be described with reference to the drawings.
[0010]
FIG. 1 shows an embodiment of a capacitance sensor according to the present invention. In this figure, a DC power supply voltage is supplied between a power supply input terminal Vin and a common terminal COMMON (ground terminal). The self-excited oscillator 1 as a sine wave generating signal source has an exclusive OR circuit EX1, a coil L1, capacitors C1 and C2, and resistors R1 and R2, and has a frequency determined by a resonance circuit of the coil L1, capacitors C1 and C2. Oscillate at, for example, about 500 kHz.
[0011]
The signal waveform of the self-excited oscillator 1 is shown in FIG. 2. In the voltage waveform diagram of FIG. 2, a waveform L1 (2) is a voltage waveform on the input terminal (2) side of EX1 to which the coil L1 is connected, and a waveform R1 ( 3) is a voltage waveform on the output terminal (3) side of EX1 to which the resistor R1 is connected, all of which are based on the terminal COMMON.
[0012]
The in-phase and inverted signal conversion circuit means 2 is a circuit means for receiving a sine wave signal of the self-excited oscillator 1 as a sine wave generation signal source and converting it into a signal of the same phase and a phase inverted signal whose phase is shifted by 180 degrees. It has a transistor Q2, base-side resistors R3 and R4, a collector-side resistor R5, and an emitter-side resistor R6. The sine wave signal of the self-excited oscillator 1 is applied to the base of the transistor Q2 via the resistor R2, and the same as the sine wave signal between the connection point between the emitter of the transistor Q2 and the resistor R6 and the terminal COMMON (that is, both ends of the resistor R6). An in-phase signal is obtained, and an inverted signal whose phase is shifted by 180 ° from the sine wave signal is obtained between a terminal Vin and a connection point between the resistor R5 and the collector of the transistor Q2.
[0013]
In the voltage waveform diagram of FIG. 3, waveform L1 (2) is a voltage waveform on the input terminal (2) side of EX1 to which the coil L1 is connected, and waveform R4 is a voltage waveform at a connection point between the base of the transistor Q2 and the resistor R4. Yes, based on terminal COMMON. Both voltage waveforms are in phase. Also, in the voltage waveform diagram of FIG. 4, waveform R5 is a voltage waveform at a connection point between the collector of transistor Q2 and resistor R5, and waveform R6 is a voltage waveform at a connection point between the emitter of transistor Q2 and resistor R6 (FIG. Waveform L1 (2)), both of which are based on the terminal COMMON. It can be seen that both phases are shifted by 180 degrees from each other.
[0014]
A series circuit 3 of a measured capacitance Cx and a capacitor C5 as a reference comparative capacitance (constant capacitance) is connected between the collector and the emitter of the transistor Q2. That is, the in-phase signal is applied to one end of the series circuit 3 (one end of the capacitor C5), and the phase inversion signal is applied to the other end (one end of the measured capacitance Cx).
[0015]
The impedance conversion means 4 has a transistor Q1 and resistors R7, R8, and R9, extracts a differential detection signal from a connection point between the measured capacitance Cx and the capacitor C5, and performs impedance conversion (when the input side is high). Impedance so that the output side has low impedance).
[0016]
Here, if the amplitude of the in-phase signal on the emitter side of the transistor Q2 and the phase-inverted signal on the collector side are made equal and the capacitance to be measured Cx and the capacitance of the capacitor C5 are made to match in the initial setting, then Since the differential detection signal at the connection point between the measured capacitance Cx and the capacitor C5 becomes zero, it is most preferable to set such a setting.
[0017]
The adding means 5 for adding a part of the sine wave signal of the self-excited oscillator 1 whose output is shifted by about 90 degrees to the output of the impedance converting means 4 as an additional signal is constituted by a series circuit of a resistor R2 and a capacitor C6, and an emitter of the transistor Q1. The original sine wave signal is added (superimposed) to the low-impedance resistor R9 via the resistor R2 and the capacitor C6 as an additional signal delayed by about 90 degrees to the output signal extracted from the side.
[0018]
The amplifying means 6 for amplifying the output after the addition of the additional signal has an exclusive OR circuit EX2, a capacitor C4 and a resistor R10. The output after the addition of the additional signal via the capacitor C4 is the input of the EX2. Applied to terminal (2). The DC supply voltage of the power input terminal Vin is applied to the input terminals (1) of the exclusive OR circuits EX1 and EX2. The amplifying means 6 outputs an output signal obtained by shaping the waveform of the input signal to the output terminal of EX2.
[0019]
The phase detection means 7 for phase-detecting the output of the amplifying means 6 by the signal of the self-excited oscillator 1 is constituted by an exclusive OR circuit EX3, and the waveform-shaped signal of the self-excited oscillator 1 (the output terminal (3) of the EX1) The signal () is applied to the input terminal (1) of EX3, and the waveform-shaped output of the amplifying means 6 (the output of the output terminal (3) of EX2) is applied to the input terminal (2) of EX3. The phase detection output of the phase detection means 7 is obtained at the output terminal (3) of EX3.
[0020]
The smoothing means 8 for smoothing the output of the phase detection means 7 to obtain a DC output is composed of a resistor R11 and a capacitor C7, and the output of EX3 is smoothed (averaged) by the resistor R11 and the capacitor C7. Output to the sensor output terminal Vout1.
[0021]
Next, the operation of this embodiment will be described. Here, the amplitude of the in-phase signal on the emitter side of the transistor Q2 of the in-phase and inversion signal conversion circuit means 2 and the amplitude of the phase inversion signal on the collector side are made equal, and the capacitance to be measured Cx and the capacitor C5 (fixed) It is assumed that the capacitance values of the comparative capacitances are matched so that the differential detection signal at the connection point between the measured capacitance Cx and the capacitor C5 is initially set to zero.
[0022]
If there is no change in the measured capacitance Cx, the differential detection signal serving as an input signal to the impedance conversion means 4 is zero, and only the additional signal obtained by delaying the phase of the original sine wave signal by about 90 degrees is included. The waveform is shaped by the amplifying means 6, and the phase is detected by the phase detecting means 7 using the signal waveform R1 (3) on the output terminal (3) side of EX1 in FIG. Therefore, if there is no change in the measured capacitance Cx, a DC voltage corresponding to a phase delay of 90 degrees is output to the sensor output terminal Vout1.
[0023]
When the measured capacitance Cx increases from an initial set value due to any cause (approach or separation of an object, adhesion or disappearance of moisture, change in humidity, etc.), a phase inversion signal having a phase shifted by 180 degrees as the differential detection signal. As shown in the waveform diagram of FIG. 5, a signal waveform R9a shifted 180 degrees from the signal waveform L1 (2) on the input terminal (2) side of EX1 is obtained at the output of the impedance conversion means 4. This signal waveform R9a is one input signal of the phase detection means 7 (a signal of the output terminal (3) of EX1 and a waveform obtained by shifting the signal waveform L1 (2) of the input terminal (2) of EX1 by 180 degrees). As a result, the detection output of the phase detection means 7 is higher than that of the case of only the additional signal obtained by delaying the phase of the original sine wave signal by about 90 degrees as shown in FIG. (The voltage waveform of the output terminal (3) of EX3), and the DC voltage of the sensor output terminal Vout1 smoothed by the smoothing means 8 of the resistor R11 and the capacitor C7 increases. That is, as the degree of increase in the measured capacitance Cx increases, the DC voltage at the sensor output terminal Vout1 also increases.
[0024]
Conversely, if the measured capacitance Cx decreases below the initial set value due to some cause (approach or separation of an object, adhesion or disappearance of water, change in humidity, etc.), the sine wave having the original phase as the differential detection signal An in-phase signal coincident with the signal appears, and a signal waveform R9b in phase with the signal waveform L1 (2) on the input terminal (2) side of EX1 is obtained at the output of the impedance conversion means 4, as shown in the waveform diagram of FIG. . This signal waveform R9a is one input signal of the phase detection means 7 (a signal of the output terminal (3) of EX1 and a waveform obtained by shifting the signal waveform L1 (2) of the input terminal (2) of EX1 by 180 degrees). Since the phase of the signal is shifted by 180 degrees from that of the original sine wave signal, the detection output of the phase detection means 7 is smaller than that of the case of only the additional signal obtained by delaying the phase of the original sine wave signal by about 90 degrees. (The voltage waveform of the output terminal (3) of EX3), and the DC voltage of the sensor output terminal Vout1 smoothed by the smoothing means 8 of the resistor R11 and the capacitor C7 decreases. That is, as the degree of decrease in the measured capacitance Cx increases, the DC voltage at the sensor output terminal Vout1 also decreases.
[0025]
FIG. 9 is a detection capacitance-output voltage characteristic diagram showing the relationship between the measured capacitance Cx and the DC voltage at the sensor output terminal Vout1. However, the comparative capacitance C5 was set to 3 pF.
[0026]
The characteristic of the circuit of FIG. 1 is that the detection sensitivity for amplifying the change in the measured capacitance Cx is determined by the parameters of Cx, C5, R2, and C6, that is, the magnitude of the differential detection signal obtained at the emitter of the transistor Q1, It is determined by the ratio to the magnitude of a signal with a phase delay of about 90 degrees, and has a feature that it is hardly affected by the magnitude of the original signal or external noise because it employs a phase detection method.
[0027]
In the series circuit of the capacitance Cx to be measured and the capacitor C5, one end of the capacitance Cx to be measured may be connected to the emitter of the transistor Q2, and one end of the capacitor C5 may be connected to the collector. When the capacitance Cx increases, a change in capacitance can be detected by reducing the DC voltage of the sensor output terminal Vout1.
[0028]
Although the embodiments of the present invention have been described above, it will be obvious to those skilled in the art that the present invention is not limited to the embodiments and various modifications and changes can be made within the scope of the claims.
[0029]
【The invention's effect】
As described above, according to the capacitance sensor according to the present invention, a minute change in capacitance can be detected stably and with high accuracy, it is strong against the influence of external noise, and the S / N ratio is large. There are advantages to take. In addition, the operation is stable, and no circuit shield is required. Further, the circuit configuration is simple, and cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of a capacitance sensor according to the present invention.
FIG. 2 is a voltage waveform diagram showing an input-end-side voltage waveform {L1 (2)} and an output-end-side voltage waveform {R1 (3)} of an exclusive OR circuit EX1 of the self-excited oscillator according to the embodiment. .
FIG. 3 is a voltage waveform diagram showing an input-end-side voltage waveform {L1 (2)} of an exclusive OR circuit EX1 and a voltage waveform (R4) at a connection point between a base of a transistor Q2 and a resistor R4 in the embodiment. is there.
FIG. 4 is a voltage waveform diagram showing a voltage waveform (R5) at a connection point between the collector of the transistor Q2 and the resistor R5 and a voltage waveform (R6) at a connection point between the emitter of the transistor Q2 and the resistor R6 in the embodiment. is there.
FIG. 5 is impedance conversion shifted by 180 degrees with respect to the input-side voltage waveform {L1 (2)} of the exclusive OR circuit EX1 and the signal waveform L1 (2) of the EX1 on the input terminal {circle around (2)} of the embodiment. It is a voltage waveform diagram which shows the output voltage waveform R9a of a means.
FIG. 6 shows the output of the impedance conversion means in phase with the input terminal side voltage waveform {L1 (2)} of the exclusive OR circuit EX1 and the signal waveform L1 (2) on the input terminal {2} side of the EX1. FIG. 11 is a voltage waveform diagram showing a voltage waveform R9b.
FIG. 7 is a voltage waveform showing an input-end-side voltage waveform {L1 (2)} of the exclusive OR circuit EX1 and a phase detection output voltage waveform (R11a) when the measured capacitance Cx increases in the embodiment. FIG.
FIG. 8 is a voltage waveform showing an input-end-side voltage waveform {L1 (2)} of the exclusive OR circuit EX1 and a phase detection output voltage waveform (R11b) when the measured capacitance Cx decreases in the embodiment. FIG.
FIG. 9 is a detection capacitance-output voltage characteristic diagram showing a relationship between a measured capacitance Cx (detection capacitance) and a DC voltage (output voltage) of a sensor output terminal Vout1 in the embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Self-excited oscillator 2 In-phase and inverted signal conversion circuit means 3 Series circuit 4 Impedance conversion means 5 Addition means 6 Amplification means 7 Phase detection means 8 Smoothing means C1, C2, C4, C5, C6, C7 Capacitor Cx Capacitance to be measured EX1, EX2, EX3 Exclusive OR circuit L1 Coil Q1, Q2 Transistors R1-R11 Resistance

Claims (1)

A sine wave generating signal source,
Circuit means for converting the sine wave signal of the sine wave generation signal source into a signal having the same phase and a phase inversion signal whose phase is shifted by 180 degrees;
A series circuit of the measured capacitance and a reference capacitance that is a reference, wherein the signal having the same phase is applied to one end, and the phase inversion signal is applied to the other end,
Extracting a differential detection signal from a connection point between the measured capacitance and the comparison capacitance, and impedance conversion means for performing impedance conversion,
Adding means for adding, as an additional signal, a part of the sine wave signal of the sine wave generation source whose phase is shifted by about 90 degrees to the output of the impedance converting means;
Amplifying means for amplifying the output after adding the additional signal,
Phase detection means for performing phase detection on the output of the amplification means by a signal from the sine wave generation source,
And a smoothing means for smoothing the output of said phase detection means to obtain a DC output.

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