JPH08278336A - Electrostatic sensor device - Google Patents

Abstract

PURPOSE: To detect a little quantity of electrostatic capacitance by a method wherein a phase transition circuit outputs a phase transition signal representing the phase change of a resonance circuit in accordance with the change of electrostatic capacitance of an electrostatic capacitance detection element and the signal is distinguished by a phase. CONSTITUTION: An oscillator 3 oscillates at a frequency (f) equal to a resonance frequency of a resonance circuit of a phase transition circuit 2. A reference signal V1 of the frequency (f) of the oscillator 3 is supplied to the circuit 2. The circuit 2 outputs a signal V2 of which phase is shifted in accordance with the change of electrostatic capacitance designated by an electrostatic capacitance detection element 1. The signal V2 is introduced to a phase distinction circuit 5 via a circuit 4A having a buffer function. The signal V1 of the oscillator 3 is introduced to a circuit 5 via a circuit 4B having a buffer function. Circuits 4A, 4B obtain signals having desired voltages by virtue of the buffer functions. The circuit 5 executes the phase detection between the signals V2 and V1 to output a phase differential signal. The signal becomes an analog signal corresponding to the change of the electrostatic capacitance of the element 1. It is possible to detect the phase change from the analog signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic sensor device for detecting a change in micro capacitance.

[0002]

2. Description of the Related Art In recent years, electrostatic sensors have been used in various fields. For example, it is a minute displacement measurement in fields such as speed, pressure, and vibration. Furthermore, when focusing on the capacitance sensor as a sensing element, the measurement target is the detection of changes in the facing area of the electrode plates, the detection of changes in the distance between the electrode plates, and the foreign substances inside the dielectric, as the detectability improves. Detection, detection of change in dielectric constant due to food deterioration, measurement of paper thickness, measurement of print density, detection of adhesive layer thickness of adhesive tape, detection of electrode thickness of electrical parts, engraved on information medium Tracking signal detection, AV
In the field, such as camera shake detection, food, printing, finance, information processing,
It is used and expected to be used in a wide range of industrial fields such as consumer equipment.

As a connection circuit for the electrostatic sensor, a so-called oscillation type sensor circuit is provided in which an electrostatic sensor is coupled to a resonator of an oscillation circuit to take out a detection signal with a change in capacitance as a frequency change.
A capacitor is configured with a bridge, and a static capacitance sensor is connected to a bridged capacitance circuit that connects an electrostatic sensor to one side of it, a well-known wing bridge circuit, or a detection resonance circuit that uses a ceramic resonator. There is a circuit for detecting the capacitance.

[0004]

In recent years, an electrostatic sensor device utilizing a change in fine electrostatic capacitance is being developed by using a semiconductor fine processing technique, so-called micromachining technique. These electrostatic sensor devices are extremely small, 1 × 1 mm or less, and the electrode area showing the change in capacitance is minute, and the obtained specific capacitance is, for example, about several pF. Is about 10% at the maximum. For this reason, the configuration of the electrostatic sensor has to be carefully selected, and it has been difficult to apply a conventionally known signal processing circuit as it is.

The present invention provides an electrostatic sensor device capable of detecting a very small capacitance and having excellent detectability.

[0006]

In order to solve the above-mentioned problems, according to the invention of claim 1, a capacitance detecting element, a phase shift circuit having a resonance circuit including the capacitance detecting element, and a reference. The phase transition circuit outputs a phase transition estimation signal indicating the phase transition of the resonance circuit according to the capacitance change of the capacitance detection element, and the phase transition estimation circuit outputs the phase transition estimation signal. It is characterized in that the signal is phase-discriminated by the phase discrimination circuit by the reference signal from the oscillation circuit to obtain an output signal corresponding to a slight change in electrostatic capacitance of the electrostatic capacitance detection element.

Further, according to the invention of claim 2, it is provided with an electrostatic capacitance detection element, a phase shift circuit having a resonance circuit including the electrostatic capacitance detection element, and an oscillation circuit for generating a reference frequency signal. , The phase transition circuit outputs a phase transition estimation signal indicating the phase transition of the resonance circuit according to the capacitance change of the capacitance detection element, and the phase transition estimation signal and the reference signal are supplied to independent frequency conversion circuits. It is characterized in that a frequency signal lower than the oscillation frequency of the guiding and oscillating circuit is used, and these signals are phase discriminated by a phase discriminating circuit to obtain an output signal having a pulse width corresponding to a phase shift.

Further, in the invention of claim 3, the phase discrimination circuit is composed of two zero-cross comparator circuits and a logical product circuit, the phase transition signal is inputted to one zero-cross comparator circuit, and the other zero-cross comparator circuit is inputted to the other zero-cross comparator circuit. It is characterized in that a reference signal is input and the output signals of these zero-cross comparator circuits are input to an AND circuit.

Further, in the invention of claim 4, the second AND circuit is provided with a second AND circuit and a clock circuit for generating a clock pulse having a cycle higher than the pulse output of the phase discrimination circuit. A phase counting signal is obtained by inputting the pulse output of the phase discrimination circuit and the clock pulse of the clock circuit.

Further, according to the invention of claim 5, a capacitance detecting element, a phase shift circuit having a resonance circuit including the capacitance detecting element, a main oscillator for generating a reference frequency signal, With a local oscillator that generates a frequency signal of a higher frequency than the main oscillator, the phase transition circuit outputs a phase transition estimation signal indicating the phase transition of the resonance circuit according to the capacitance change of the capacitance detection element. Then, the phase transition estimation signal and the local signal of the local oscillator are guided to the first frequency conversion circuit, and the reference signal of the main oscillator and the local signal of the local oscillator are guided to the second frequency conversion circuit and the main oscillator From a phase discriminating circuit that converts into a frequency lower than the oscillation frequency and discriminates these signals to obtain an output signal having a pulse width corresponding to a phase shift; and a digital phase circuit that converts the pulse width output signal into the number of pulses. To configure The one in which the features.

[0011]

According to the first aspect of the invention, when the inherent capacitance changes due to the change in the physical quantity of the environment in which the capacitance detecting element is installed, the phase of the resonance frequency of the resonance circuit of the phase shift circuit changes. To do. The phase shift circuit outputs a phase shift estimation signal having a phase as a variable. The oscillation frequency of the oscillator circuit is
It matches the resonance frequency of the resonance circuit before the capacitance of the capacitance detection element changes, and becomes the reference frequency. The phase transition estimation signal is phase discriminated by the phase discrimination circuit by the reference signal from the oscillation circuit. From the phase discriminating circuit, an output signal corresponding to the minute electrostatic capacitance change of the electrostatic capacitance detecting element is obtained.

According to the second aspect of the invention, the phase transition estimation signal and the reference signal of the oscillating circuit, which are phase-shifted according to the change in the capacitance of the capacitance detecting element, have a frequency lower than the frequency of the reference signal in the frequency conversion circuit. Signal, for example, a beat signal or a split-frequency signal. The converted signal is subjected to phase discrimination by the phase discrimination circuit and converted into a signal having a pulse width corresponding to the phase shift.

According to the third aspect of the invention, the phase shift signal is converted into a pulse waveform by the zero cross comparator circuit, and
The reference signal is converted into a pulse waveform by another zero cross comparator circuit. These pulse waveforms are input to a logical product circuit, that is, an AND gate circuit to obtain a pulse output having a pulse width that is the phase difference between the phase transition signal and the reference signal.

According to the invention of claim 4, the pulse waveform showing the phase difference and the clock pulse added from the clock circuit are input to the AND circuit, in other words, the AND gate circuit, and the number of clock pulses corresponding to the pulse width is inputted. Obtained as a phase counting signal.

According to a fifth aspect of the present invention, when the capacitance inherent to the capacitance detecting element changes due to a change in the physical quantity of the environment in which the capacitance detecting element is installed, the phase of the resonance frequency of the resonance circuit of the phase shift circuit changes. To do. The phase shift circuit outputs a phase shift estimation signal having a phase as a variable. The oscillation frequency of the main oscillator coincides with the resonance frequency of the resonance circuit before the capacitance of the capacitance detection element changes, and becomes the reference frequency. The phase transition estimation signal and the reference signal are frequency-converted into a low frequency by a frequency conversion circuit by a local signal from a local oscillator. These signals are phase discriminated by a phase discriminating circuit and converted into an output signal having a pulse width corresponding to the phase shift. This output signal is converted into the number of clock pulses by the digital phase circuit. The number of pulses indicates an amount corresponding to a slight change in electrostatic capacitance of the electrostatic capacitance detection element.

[0016]

1 is a block diagram showing the principle of the electrostatic sensor device of the present invention. In FIG. 1, the phase shift circuit 2 is a reactance circuit such as an induction M circuit or a parallel resonance circuit that causes a jumped phase shift. Such a reactance circuit is composed of an LC, mostly a series resonance circuit, a parallel resonance circuit, and a series-parallel resonance circuit. The phase shift circuit 2 includes the capacitance detection element 1 as its circuit element. The capacitance detecting element 1 changes its own capacitance by changing the physical quantity of the environment in which it is installed.

The oscillator 3 is composed of a well-known high precision oscillator called TCXO or DCXO, and the precision of the oscillation frequency is 10 ^-6 . The oscillator 3 oscillates at an oscillation frequency f that matches the resonance frequency of the resonance circuit of the phase shift circuit 2. That is, the resonance frequency of the parallel resonance circuit including the intrinsic capacitance of the capacitance detection element 1 as a circuit constant is set to match the oscillation frequency f of the oscillator 3. The reference signal V1 having the oscillation frequency f of the oscillator 3 is supplied to the phase shift circuit 2.

The phase shift circuit 2 is a signal V2 whose phase shifts in response to a change in electrostatic capacitance indicated by the electrostatic capacitance detection element 1.
(Phase shift signal) is output.

The phase shift signal V2 of the phase shift circuit 2 is guided to the phase discrimination circuit 5 through the circuit 4A having a buffer function, and the reference signal V1 of the oscillator 3 is passed through the circuit 4B having a buffer function. It is guided to the phase discrimination circuit 5. The circuits 4A and 4B having a buffer function are, for example, buffer circuits, and obtain a signal voltage of a desired potential according to the buffer function. The phase discrimination circuit 5 performs phase detection between the phase shift signal V2 and the reference signal V1 and outputs a phase difference signal. This phase difference signal becomes an analog output corresponding to the electrostatic capacitance change of the electrostatic capacitance detection element 1.

If this analog output is converted into a rectangular wave at the zero crossing point by using, for example, a waveform shaping circuit, the phase change can be detected from the width of the rectangular wave.

A low-pass filter 6 and an amplifier circuit 7 are connected to the phase discrimination circuit 5 as needed. Even in this case, an analog output signal corresponding to the electrostatic capacitance change of the electrostatic capacitance detection element 1 is sent from the sensor device. Also,
Instead of the phase discrimination circuit 5, a frequency conversion circuit, for example, a beat signal generation circuit, in other words, a heterodyne detection circuit that generates a beat frequency signal, may be used. Alternatively, a frequency divider may be used to obtain the divided analog signal.

FIG. 2 shows the principle of the basic reactance circuit of the phase shift circuit 2 of FIG. 1. The phase shift circuit is composed of a parallel circuit of capacitance and inductance.

In FIG. 2, the capacitor 10 has an inherent electrostatic capacitance C before the capacitance detecting element 1 is changed. The capacitor 10 has a coil 11 having an inductance L.
Are connected in parallel to form a parallel resonant circuit. A capacitor 12 having a fixed capacitance C ₀ of the phase shift circuit 2 is connected in series to this resonance circuit to form a basic reactance circuit. The reference signal V1 of the frequency f is supplied from the oscillator 3 to the basic reactance circuit, and the electrostatic capacitance change of the electrostatic capacitance detection element 1 from between the LC parallel resonance circuit including the capacitor 10 and the coil 11 and the capacitor 12. The phase shift signal V2 corresponding to ΔC is taken out.

Here, when the voltage (phase shift signal) V2 appearing at both ends of the LC parallel circuit is expressed with respect to the output voltage (reference signal) V1 of the main oscillator 3, the following expression is obtained.

[0025]

[Equation 1]

Focusing on the denominator of the equation 1, the following equation is obtained.

[0027]

[Equation 2]

Since the denominator of the equation (2) becomes zero under the condition that this equation is satisfied, it can be understood that the value of the phase shift signal V2 becomes extremely large and resonance occurs. Furthermore, when the magnitude of the electrostatic capacitance C increases or decreases around the resonance point, the sign of Equation 1 is inverted before and after the resonance point, so that the phase can be expected to change significantly.

However, the fact that the denominator becomes zero, the value becomes infinite, and the phase is instantaneously inverted is only possible in a superconducting environment, and it occurs in an ideal circuit at room temperature. Nothing more. In the working circuit, Q of the LC parallel resonance circuit has a finite value, and the equivalent parallel resistance R exists.

FIG. 3 shows the principle of the basic reactance circuit when the equivalent parallel resistance is taken into consideration. The equivalent parallel resistor 13 having a resistance value R is connected in parallel with the coil 11. Other configurations are the same as those in FIG.

This resonance circuit becomes anti-resonance when the resistance value R does not exist, and a very large phase transition jump occurs from the negative direction to the positive direction even if the electrostatic capacitance C slightly changes. This state means that the detectability becomes infinite,
Although the sensitivity is remarkably increased, on the contrary, since the stability is poor, the relationship between the equivalent resistance R and the detectability becomes important.

The voltage (phase shift signal) V2 appearing at both ends of the LCR parallel resonance circuit is expressed with respect to the output voltage (reference signal) V1 of the oscillator 3 as follows.

[0033]

(Equation 3)

In Equation 3, the real part becomes zero under the condition that Equation 2 is satisfied. That is, resonance occurs. The equivalent resistance R has no effect.

When the magnitude of the electrostatic capacitance C increases or decreases around this resonance point, the phase difference θ between the voltage V1 and the voltage V2 becomes C
When> 1 / 4π ² f ² L−C ₀ , the following equation holds.

[0036]

[Equation 4]

When C <1 / 4π ² f ² L-C ₀ , the following equation holds.

[0038]

(Equation 5)

From the equations (4) and (5), the following equation is obtained at θ = 90 °.

[0040]

(Equation 6)

That is, the equation (6) is an equation showing R and basic detectability. Further, this equation shows that the output characteristic of the phase shift circuit 2 is not affected by the power supply at all.

As described above in detail, since the minute capacitance change ΔC of the capacitance detecting element of the present invention is taken out as the change of the phase difference θ, a very sensitive electrostatic sensor device can be provided and the micromachining technique can be used. It becomes suitable for the minute electrostatic capacitance change in the constituted minute sensor.

In the above, the two signals V1 and V2 are directly phase-detected to obtain an analog output. However, in analog signal processing, the difference in amplitude between the reference signal and the phase shift signal causes an error, and in analog signals, the interface
It is conceivable that it is easily affected by the noise that is superimposed between spaces. Therefore, a device that obtains a digital output is also desired.

FIG. 4 shows an embodiment for obtaining a digital interface output in the device of the present invention.

The phase shift circuit 22 shown in FIG.
Similarly to the above, it is a reactance circuit such as an inductive M circuit or a parallel resonance circuit that causes a jump in phase transition. The phase shift circuit 22 includes the capacitance sensor 21. The main oscillator 23 is composed of a well-known high-precision oscillator called TCXO or DCXO, and oscillates at an oscillation frequency f1 that matches the resonance frequency of the reactance circuit of the phase shift circuit 22. The oscillation frequency f1 has a precision of 7 digits, and for example,
It is 4.900000 MHz. The main oscillator 23 supplies a reference signal V1 having an oscillation frequency f1 to the phase shift circuit 22. On the other hand, the phase shift circuit 22 outputs a signal V2 (phase shift signal) in which the phase shifts in response to a minute change in electrostatic capacitance indicated by the electrostatic capacity sensor 21.

The phase shift circuit 22 supplies the phase shift signal V2 to the mixer 27A via a buffer circuit 24A, for example, a buffer circuit. Further, the main oscillator 23 supplies the reference signal V1 having the oscillation frequency f1 to the mixer 27B via the buffer circuit 24B, for example, the buffer circuit.
The buffer circuit 24A and the buffer circuit 24B have a buffer function and obtain a signal voltage of a desired level.

A second oscillator circuit 25, that is, a local oscillator is provided. The oscillator circuit 25 outputs a local signal V3 having an oscillation frequency f2 slightly higher than the frequency f1 oscillated by the main oscillator 23. The oscillation frequency f2 of the oscillation circuit 25 is
It is 4.914677 MHz and has a precision of 7 digits. The local signal V3 of the frequency f2 of the oscillating circuit 25 passes through the buffer circuits 26A and 26B and the mixers 27A and 2A, respectively.
7B.

The mixer 27A, to which the phase shift signal V2 and the local signal V3 are input, is divided into a signal V having a low frequency fa.
2a is generated. The mixer 27B, to which the reference signal V1 and the local signal V3 are input, generates the divided signal V1a having the low frequency fb. These signals V1a and V2a
Contains a lot of harmonic components, the harmonic components are removed by passing through the low-pass filters 28A and 28B, respectively. Even if the above operation is performed, the mixers 27A and 27B
In the output signals V1a and V1b of 1., the relationship of the phase difference corresponding to the change in the capacitance of the capacitance sensor between the phase transition signal V2 and the reference signal V1 is present without being impaired.

The signals that have passed through the low pass filters 28A and 28B are input to the digital phase discrimination circuit. The digital phase discrimination circuit has two zero-cross comparators 2.
9A, 29B and a logical product circuit 30 for obtaining a logical product of the zero-cross rectangular wave of this comparator. The signal V1a introduced to the zero cross comparators 29A and 29B,
V2a is zero cross shaped.

This zero-cross comparator 29A, 29
Since the signals V1b and V2b output from B have pulse waveforms, they are pulse output signals from which errors in the amplitude direction of the analog signals from the mixers 27A and 27B have been completely removed. These signals are simultaneously input to the AND circuit 30, that is, the AND circuit, and the AND between the signals is taken. As a result, from the AND circuit 30, the phase difference between the phase shift signal V2 and the reference signal V1, in other words, the capacitance sensor 21.
It is possible to obtain a pulse output in which the duty ratio of the pulse width changes in accordance with the change in electrostatic capacity. This pulse output is guided to the digital phase circuit 31.

The digital phase circuit 31 is an AND circuit 3.
3, a clock oscillator 34, a timing circuit 35, a binary counter 36, and a storage element 37. The clock oscillator 34 outputs, for example, a clock signal of 15.0000 MHz to output the AND circuit 33 and the timing circuit 3.
Give 5 The AND circuit 33 ANDs the clock signal from the clock oscillator 34 as shown in FIG. 7B and the pulse output from the AND circuit 30 as shown in FIG. The number of pulses is output according to the pulse width of the pulse output.

This pulse is given to the timing circuit 35 and the binary counter 36 of the synchronous type. Timing circuit 3
In synchronization with the timing of 5, the binary counter 36 can obtain, for example, a 9-bit digital parallel output.
By counting the number of clock pulses that have passed through the AND circuit 33 during one cycle of the AND circuit 30 with the counter 36, the change in the pulse width can be converted into the number of pulses and easily counted.

The storage element 37 stores the binary signal of the binary counter 36 with the timing of the signal of the timing circuit 35 and stores the capacitance value of the capacitance sensor directly on the display device 32. That is, since the pulse output indicates the electrostatic capacity of a slight change obtained from the electrostatic capacity sensor, the capacity change of the electrostatic capacity sensor can be directly viewed by sending it to the display device 32. Further, by using the parallel output of the binary counter 36, the minute displacement of the electrostatic capacity of the electrostatic capacity sensor can be used as digital information.

Phase shift circuit 2 including capacitance sensor 21
The principle of No. 2 is the same as that described with reference to FIGS.

Here, the principle of obtaining pulse-width-converted pulse outputs from the analog signals V1a and V2a having a phase difference from the mixers 27A and 27B will be described with reference to FIGS. FIG. 5 is a basic circuit of a digital phase difference discriminating circuit, and FIG. 6 shows an operation principle of phase difference pulse width conversion.

Assume that the analog signals V1a and V2a from the mixers 27A and 27B are sinusoidal signal waves. The analog signals V1a and V2a are input to the positive input terminals of the comparators 40 and 41, respectively. Comparator 4
The negative input terminals of 0 and 41 fix the reference voltage to 0V. That is, a zero cross comparator is configured.

With this configuration, the analog signal V1
a, V2a only during the positive half cycle, the comparators 40, 4
The output of 1 becomes high level, and two independent pulse outputs V1b and V2b with a duty ratio of 50% are obtained.
Since the phase difference between the pulse outputs V1b and V2b changes according to the slight capacitance displacement of the capacitance sensor 21, the rising and falling of the output waveform between the outputs of the zero-cross comparators are the capacitance sensor. 21 has a time difference corresponding to the change in electrostatic capacity.

The outputs of the comparators 40 and 41 are input to the input terminal of the AND gate circuit 42. By passing these two pulse outputs V1b and V2b through the AND gate circuit 42 to obtain a logical product, the output becomes a signal whose pulse width changes in accordance with a minute change in electrostatic capacitance of the capacitance sensor 21. This output cancels out a change in amplitude due to a disturbance such as noise or temperature change, in other words, an error due to a disturbance with respect to a signal or a power supply is canceled, and theoretically, the output signal is not affected by the disturbance at all.

As described above, the pulse width can be easily converted by a well-known gate circuit process if the minute change in electrostatic capacitance of the capacitance sensor 21 can be converted into the pulse width as described above. It

However, as in the present invention, when the change in the minute capacitance is targeted, it is necessary to use a clock oscillator that oscillates an accurate frequency like the oscillators 3, 23 and 25 described above. Therefore, the method of multiplying the frequencies of the highly accurate oscillators 3, 23 and 25 to obtain the clock pulse signal is an effective means for improving the accuracy and the detectability.

In the circuits shown in FIGS. 1 and 4, it is necessary to consider the power supply circuit. In other words, in a power supply circuit that supplies power to a circuit in which a digital circuit and an analog circuit coexist, the noise generated by the digital circuit mixes into the analog circuit, and the noise in each circuit has a random phase difference and S / N. Often worse. In such a case, it is preferable to take measures such as inserting a three-terminal regulator into either or both of the digital circuit and the analog circuit, or supplying independent power from an external circuit. Is particularly important in order to secure a better S / N and to detect a minute electrostatic capacitance with high accuracy.

The principle of the circuit of FIG. 5 can be used instead of the phase discrimination circuit 5 of FIG. In that case, the reference signal V1 may be input to the positive input terminal of the comparator 40, and the phase shift signal V2 may be input to the positive input terminal of the comparator 41.

The detectability of the electrostatic sensor device of the present invention will be described with reference to the specific circuit example of FIG.

As the capacitance detecting element 10, a steatite type air variable capacitor was used in a pseudo manner, and as the capacitor 12, a dip type tantalum capacitor was used. Coil 1
In 1, a high frequency coil for wireless equipment was used. The operating frequency f of the reactance circuit was set to 4.9 MHz. The dip-type tantalum capacitor of the capacitor 12 was 4.7 pF. Further, 1.971 pF was used as the initial capacity of the steatite type air variable capacitor. When the value of the inductance L of the coil 11 satisfying the condition of the equation 2 is calculated, it is about 158 μH, and the impedance of the coil 11 during operation is about 4.9 kΩ. Under this condition, by recording the output value of the phase difference when the electrostatic capacitance of the air variable capacitor is changed, the characteristic can be measured.

In the experiment, the phase difference is 0 to 1 at full scale.
Since the digital output from the AND circuit 33, that is, the number of pulses changes from 0 to 511 when changing to 80 °, θ per digit is the phase difference θ = 180−180 / 511 × d [degree ] Can be obtained as. However, d is the number of pulses.

FIG. 8 shows a characteristic curve of the digital output measured value and the calculated phase difference value with respect to the capacitance of the capacitance sensor, showing the capacitance change characteristic in comparison with the experimental value and the theoretical value. ing. The solid line shows the number of pulses, and the dotted line shows the calculated phase difference.

From this result, it is shown that the capacitance of the capacitance sensor has the highest sensitivity in the vicinity where the resonance condition of the basic reactance circuit is satisfied. The high sensitivity referred to here means that the phase difference calculation value and the digital output measurement value greatly change due to a minute change in the capacitance C.

Here, the detectability in the vicinity of the electrostatic capacitance value showing high sensitivity is obtained. In FIG. 8, the value of the capacitance C is 1.868.
When changing from 6 to 2.063 pF, the digital output value changes from 236 to 273. In addition, the average rate of change in capacitance per digit corresponding to this value Δ
When C is calculated, ΔC = (2.063-1.8686) / (273-236) = 5.25 × 10−
It becomes 3 pF. That is, the change of C near the most sensitive characteristic curve is 5.25 × 1 for 1 digit of the digital output value.
It becomes 0 ^-3 pF. Also, this value corresponds to a phase angle of 0.352 degrees.

This is not the electrostatic capacity itself but the detection ability for a slight change in the electrostatic capacity.
Considering the value of capacitance and its allowable error, which should be a problem in the operation of general electronic equipment, the detectability obtained by prototyping and verifying such a simple circuit is 5.25 × 10.
It can be seen that the sensitivity is as high as ^-3 pF.

However, the result obtained by this verification does not consider the equivalent resistance component of the coil 11. Actually, the coil 11 has an equivalent resistance component, and the equivalent series resistance component of the coil 11 is also considerably large, so that the equivalent parallel resistance R is unexpectedly small. That is Q
Becomes smaller.

These equivalent resistance components particularly affect the slope of the curve in FIG. In addition to this, there is room for improvement in the stray capacitance of the board pattern and lead wires of each circuit element in this prototype device, and it is expected that ωR in the formula 7 will be improved by about two digits. If the items are improved, the detectability of two digits or more can be easily improved.

Further, by further increasing the speed of the digital interface output and the clock pulse signal of the clock oscillator 34, the detectability is greatly improved.

As described above, the characteristic of the specific example shown here is not the best value, and the detectability of about 10 ^-3 pF is simple in spite of having many improving elements including the equivalent resistance component. Obtained with a prototype circuit.

[0074]

[Effect] As described above, the electrostatic sensor device of the present invention has the following effects because it is constructed by focusing on the phase jump phenomenon of the resonance circuit.

(1) Since the phase of the resonance circuit is changed by a slight change in the capacitance of the capacitance detecting element and detected as a phase difference, a minute change in the capacitance can be clearly discriminated by the phase angle. be able to. Therefore, it is possible to obtain a detectability of 10 ⁻⁵ pF or more with respect to the minute change ΔC of the electrostatic capacitance.

(2) Since a minute change in the capacitance of the capacitance detecting element is detected by the phase difference and converted into a digital amount,
The signal is not affected by the amplitude value of, and is less affected by external noise or fluctuations in the power supply voltage.

(3) Since the phase shift signal indicating the phase difference and the reference signal are completely matched in frequency by using the reference frequency of the same oscillator, they are almost equally affected by noise and temperature change. The phase difference will be detected, and the effects of external disturbances will cancel each other out.
/ N circuit can be configured.

[Brief description of drawings]

FIG. 1 is a block diagram showing the principle of an electrostatic sensor device of the present invention.

FIG. 2 shows the principle of the phase shift circuit according to the present invention.

FIG. 3 shows the principle of another configuration of the phase shift circuit according to the present invention.

FIG. 4 is a block diagram showing another configuration of the electrostatic sensor device of the present invention.

FIG. 5 shows a basic phase difference discriminating circuit according to the present invention.

FIG. 6 shows an operation principle of a phase difference pulse width conversion circuit according to the present invention.

FIG. 7 is a conversion circuit for converting a pulse width into a pulse number according to the present invention.

FIG. 8 is a graph showing a pulse number and a phase difference calculation value with respect to capacitance according to the present invention.

[Explanation of symbols]

 1, 21 Capacitance detection element (sensor) 2, 22 Phase shift circuit 3, 23, 25 Oscillator 5 Phase discrimination circuit 27A, 27B Mixer 29A, 29B Zero cross comparator 30, 33 AND circuit 34 Clock oscillator

Claims (5)

[Claims] 1. A capacitance detection element, a phase shift circuit having a resonance circuit including the capacitance detection element, and an oscillator circuit for generating a reference frequency signal, wherein the phase shift circuit is A phase transition estimation signal indicating the phase transition of the resonance circuit is output according to the capacitance change of the capacitance detection element, and the phase transition estimation signal is phase discriminated by the phase discrimination circuit by the reference signal from the oscillation circuit. An electrostatic sensor device, wherein an output signal corresponding to a slight change in electrostatic capacitance of the electrostatic capacitance detection element is obtained. 2. A capacitance detecting element, a phase shift circuit having a resonance circuit including the capacitance detecting element, and an oscillator circuit for generating a reference frequency signal, the phase shift circuit comprising: Outputting a phase transition estimation signal indicating the phase shift of the resonant circuit according to the capacitance change of the capacitance detection element,
The phase transition estimation signal and the reference signal are respectively guided to independent frequency conversion circuits to be frequency signals lower than the oscillation frequency of the oscillation circuit, and these signals are phase discriminated by a phase discrimination circuit and have a pulse width corresponding to a phase shift. An electrostatic sensor device characterized by obtaining an output signal. 3. A phase discriminating circuit is composed of two zero-cross comparator circuits and a logical product circuit. One of the zero-cross comparator circuits receives a phase transition signal and the other zero-cross comparator circuit receives a reference signal. The electrostatic sensor device according to claim 1, wherein an output signal of a zero-cross comparator circuit is input to the AND circuit. 4. A second AND circuit and a clock circuit for generating a clock pulse having a cycle higher than the pulse output of the phase discrimination circuit, wherein the second AND circuit has a pulse output of the phase discrimination circuit. 4. The electrostatic sensor device according to claim 3, wherein a phase counting signal is obtained by inputting a clock pulse of a clock circuit. 5. A capacitance detecting element, a phase shift circuit having a resonance circuit including the capacitance detecting element, a main oscillator for generating a reference frequency signal, and a frequency higher than that of the main oscillator. With a local oscillator that generates a frequency signal,
The phase transition circuit outputs a phase transition estimation signal indicating a phase transition of the resonance circuit according to a capacitance change of the capacitance detection element, and outputs the phase transition estimation signal and the local signal of the local oscillator as a first signal. 1 frequency conversion circuit, the reference signal of the main oscillator and the local signal of the local oscillator are guided to a second frequency conversion circuit to convert to a frequency lower than the oscillation frequency of the main oscillator, and these signals An electrostatic sensor device comprising: a phase discriminating circuit for discriminating a phase to obtain an output signal having a pulse width corresponding to a phase shift; and a digital phase circuit for converting the pulse width output signal into a pulse number.