JP3383240B2 - Capacitive displacement sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitive displacement sensor for measuring displacement of a water surface or displacement of a vibrating or shaking animal as a change in capacitance. 2. Description of the Related Art An example of a conventional capacitive displacement sensor is shown in FIG.
This will be described with reference to the block circuit diagram shown in FIG. In FIG. 5, reference numeral 1 denotes an object to be measured for displacement, for example, for measuring a change in the water surface 2 of a water tank. Reference numeral 3 denotes a capacitance sensor including a capacitance electrode 4 and an LC oscillator 5. When the distance H between the water surface 2, which is the surface to be measured, and the measurement surface 6 of the capacitance electrode 4 is reduced, the capacitance between the measurements is increased, and the LC oscillator is increased. 5, the oscillation frequency Fh decreases, and as the distance H increases, the capacitance between the measurements decreases and L
The oscillation frequency Fh of the C oscillator 5 increases. Reference numeral 7 denotes a reference frequency oscillator which can oscillate a required reference frequency Fr related to the oscillation frequency Fh of the LC oscillator 5,
The frequency is set to be slightly higher than the oscillation frequency Fh of the LC oscillator 5 corresponding to the maximum distance H of the measurement range. A detection circuit 8 outputs a beat frequency Fb, which is a signal representing a difference between the oscillation frequency Fh of the LC oscillator 5 and the reference frequency Fr of the reference frequency oscillator 7. Reference numeral 9 denotes a frequency / voltage converter (FV converter). When the beat frequency Fb becomes lower than the reference frequency, the DC output voltage Vf also becomes lower than the reference voltage, and when the beat frequency Fb becomes higher, the DC output voltage Vf becomes higher than the reference voltage. Will also be higher. Reference numeral 10 denotes a voltage-to-distance converter (V) that converts the output voltage of the FV converter 9 to a distance H.
-H converter). Therefore, when the position of the water surface 2 is located on the reference surface, the distance H between the water surface 2 and the capacitance sensor 3
If the reference beat frequency Fb is set with reference to 0, if the position of the water surface 2 is lower than the reference surface, the distance H becomes wider H + and the oscillation frequency Fh becomes higher, so that the beat frequency Fb becomes lower, DC output voltage V of -V converter 9
f decreases. On the other hand, if the position of the water surface 2 is higher than the reference surface, the distance H becomes shorter and becomes H-, the oscillation frequency Fh becomes lower, the beat frequency Fb becomes higher, and the DC output voltage Vf of the FV converter 9 becomes higher. Will be higher. Therefore, this FV
The change in the DC output voltage Vf of the converter 9 is
, The vertical change (displacement) of a predetermined position on the water surface can be measured. However, the distance H
Since the change in the capacitance with respect to the change is non-linear, it is necessary to read the change by a required conversion process.
When the DC output voltage Vf of the FV converter 9 changes, the servo sensor moves the capacitance sensor 3 up and down so that the DC output voltage Vf follows the reference voltage corresponding to the distance H0. (Distance) can also be configured to be read. [0005] In the conventional capacitive displacement sensor shown in FIG. 5, when the distance H between the capacitive electrode 4 and the measuring surface 6 is small, for example, the diameter of the capacitive electrode 4 is 20 mm. ,
When the measurement range (displacement) is 2 mm or less, the capacitance between the capacitance electrode 4 and the measurement surface 6 is approximately proportional to 1 / H, so that the capacitance is sufficiently large and the measurement accuracy can be maintained. However, when the distance H is equal to or greater than the diameter of the capacitance electrode 4, for example, when the diameter of the capacitance electrode 4 is 20 mm and the measurement range is 20 mm or more, the change in the capacitance with respect to the change in the distance H It becomes extremely small in proportion to approximately 1 / H, the change in the LC oscillator frequency Fh becomes small, and the frequency drift due to the temperature characteristic and the aging change of each component constituting the capacitive sensor 3 occurs. And there was a problem in reliability. The present invention provides a highly reliable capacitive displacement sensor that is not affected by the drift of the capacitive sensor 3 and that can maintain measurement accuracy even for a measurement object having a large displacement. A capacitive displacement sensor according to the present invention detects a change in the distance between a displacement measurement target surface and a capacitive electrode as a change in capacitance and changes the oscillation frequency of an LC oscillator. A variable capacitance sensor, a reference frequency oscillator that oscillates a reference frequency having a predetermined relationship with the oscillation frequency of the capacitance sensor, and a beat frequency that is the difference between the oscillation frequency of the capacitance sensor and the reference frequency of the reference frequency oscillator In a capacitive displacement sensor comprising a detecting circuit to be operated and an FV converter for converting a beat frequency output of the detecting circuit into a voltage, the capacitive sensor has a constant amplitude with respect to the displacement measurement target surface and A micro-vibration generator that causes micro-vibration at a low frequency; and a change in capacitance based on the micro-vibration superimposed on the beat frequency output from the FV converter. A low-frequency AC amplifier that converts a low-frequency amplitude into a voltage and detects the change, thereby detecting a change in capacitance based on the displacement of the displacement measurement target surface as a change in low-frequency amplitude based on the minute vibration Then, the amplitude is converted into a voltage value, and a displacement amount is obtained from the converted voltage value. FIG. 1 is a block circuit diagram showing an embodiment of a capacitive displacement sensor. Note that the same parts as those of the conventional example shown in FIG. 5 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 1, reference numeral 11 denotes a fine movement signal generator, for example, 20.
It generates a fine movement signal having a low cycle of about Hz.
Reference numeral 12 denotes a fine movement driving unit which converts the signal of the fine movement signal generator 11 into mechanical minute vibration having a constant amplitude. The fine movement signal generator 11 and the fine movement driving unit 12 are integrated, for example, a minute vibration of a vibrator or the like. It is configured as a generator 13.
The minute vibration generated by the minute vibration generator 13 is
Is added to 14 is a low-frequency AC amplifier, 15 is a linearization circuit, and 16 is an output amplifier. Next, the operation of the capacitive displacement sensor will be described with reference to FIG. The LC oscillator 5 generates an oscillation frequency Fh based on the capacitance between the measurement surface 6 of the capacitance sensor 3 and the surface 2 of the displacement measurement object 1, for example, a water tank. At this time, the capacitance sensor 3 performs minute vibration at a low frequency of about 20 Hz.
Therefore, the oscillation frequency Fh of about 4 MHz of the LC oscillator 5
Is superimposed on the frequency change due to the capacitance change due to the low frequency minute vibration, and output to the detection circuit 8. The low-frequency amplitude output in response to the low-frequency micro-vibration is larger than the standard amplitude if the distance H is short, and the standard amplitude if the distance H is wide, in relation to the capacitance determined by the distance H. It becomes smaller than the amplitude. The beat frequency is output from the detection circuit 8 by the reference frequency Fr of the reference frequency oscillator 7, and F-V
The DC output voltage Vf converted by the converter 9 based on the level of the beat frequency Fb is, for example, as shown in FIG. 2, when the distance H = 0, which is a displacement value 0, is 3 V and the distance H is plus 10 mm. When the distance H is about 1.4 V and the distance H is minus 10 mm, the H-Vf characteristic chart of about 8.7 V is shown. When the DC output voltage Vf is superimposed with the low frequency of about 20 Hz, the specific DC output voltage Vf of the FV converter 9 has a waveform as shown in FIG. 3, for example. That is, the waveform a is a waveform at a distance H0 where the distance H to the capacitance sensor 3 is a reference when the water surface 2 is located on the reference surface, and the AC voltage superimposed on the output voltage of about 3 V corresponding to the beat frequency. Output as a waveform. A waveform b is a waveform in a state in which the water surface 2 is lower than the reference surface and the distance H from the capacitance sensor 3 is widened, and is similarly output as an AC voltage waveform superimposed on the output voltage of about 2.1 V. However, the amplitude of the low-frequency waveform is smaller than the amplitude of the waveform a. The waveform c is a waveform in a state where the water surface 2 is higher than the reference surface and the distance H from the capacitance sensor 3 is narrow, and similarly, the output voltage 5.5
It is output as an AC voltage waveform superimposed on V, and the amplitude of this low-frequency waveform is larger than that of waveform a. Next, the HA characteristic diagram of FIG. 4 shows the relationship between the output differential voltage A obtained by processing the output of the FV converter 9 by the low-frequency AC amplifier 14 and the distance H. The amplitude of the AC waveform corresponding to the micro-vibration included in the output of the FV converter 9 shown in FIGS. 2 and 3 is calculated by changing the amplitude of each distance (displacement) H (10 mm to 0 to -10 mm) by 1 mm. Is shown as a change in the output voltage with respect to. That is, as shown in the H-Vf characteristic diagram of FIG. 2, the change of the voltage of 1 mm when the distance H is 0 mm is 0.2%.
As shown in FIG. 7, a change in a voltage of 1 mm at a distance H of −6 mm is 0.6 V, and a change of a voltage of 1 mm at a distance H of 4 mm is 0.17 V.
FIG. 9 is a characteristic diagram showing a change in output voltage with respect to a displacement of mm. Accordingly, the distance H can be read by detecting the output differential voltage A of the low-frequency AC amplifier 14. Since the output differential voltage A is non-linear as shown in the HA characteristic diagram, a linearization circuit 15 is provided in order to easily detect the displacement (distance H) from the output differential voltage A. An output amplifier 16 may be provided so that the displacement amount (distance H) can be directly read. Also,
As described in the conventional example, the capacitance sensor 3 is moved up and down by the servo mechanism so that the output differential voltage A of the low-frequency AC amplifier 14 follows the reference voltage corresponding to the distance H0 at the displacement amount 0, and the change amount (Distance) may be configured to be read. In the above-described embodiment of the present invention, an example of measuring the displacement of a horizontal surface such as a water surface has been described. However, it is natural that the displacement measurement target surface 2 can be measured whether it is a vertical surface or an inclined surface. Also, the size of the area of the capacitance electrode 4, the distance H between the displacement measurement target surface 2 and the capacitance electrode 4 and the capacitance between them, the characteristics of the LC oscillation circuit, the transmission frequency of the reference frequency oscillator, The vibration amplitude and vibration period
It can be appropriately selected according to the characteristics of the displacement measurement target and the measurement environment. As described above in detail, the capacitive displacement sensor according to the present invention detects a change in the distance between the displacement measurement target surface and the measurement surface of the capacitance electrode as a change in the capacitance. Micro-vibration at a low frequency, and detects the amplitude of the change in capacitance due to this micro-vibration as a voltage value.
Since this voltage value is converted into a distance to obtain the displacement of the displacement measurement target surface, even if there is a drift due to the temperature characteristics of the capacitive sensor or a drift due to aging of components, the amount of displacement is low frequency. Is detected as the amplitude value of the AC voltage, the displacement is hardly affected by the drift described above, and the displacement amount can be measured accurately and with high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block circuit diagram showing one embodiment of a capacitive displacement sensor according to the present invention. FIG. 2 is an H-Vf characteristic diagram showing an example of a relationship between a beat frequency corresponding to a displacement amount and an output voltage thereof in the capacitive displacement sensor of the present invention. FIG. 3 is a waveform chart showing some examples of output voltage waveforms corresponding to minute vibrations included in the output of the FV converter in the capacitive displacement sensor of the present invention. FIG. 4 is an HA characteristic diagram showing a relationship between an output differential voltage A, which is an output of a low-frequency AC amplifier, and a distance H in the capacitive displacement sensor of the present invention. FIG. 5 is a block circuit diagram showing an example of a conventional capacitive displacement sensor. [Description of Signs] 1 Displacement measurement object 2 Measurement surface of displacement measurement object 3 Capacitance sensor 4 Capacitance electrode 5 LC oscillator 6 Measurement surface of capacitance electrode 7 Reference frequency oscillator 8 Detection circuit 9 Beat frequency-voltage converter (F- V converter) 10 Voltage-distance converter (VH converter) 11 Fine motion signal generator 12 Fine motion driver 13 Micro vibration generator 14 Low frequency AC amplifier 15 Linearization circuit 16 Output amplifier

Claims (1)

(57) [Claim 1] A capacitance sensor for detecting a change in the distance between a displacement measurement target surface and a capacitance electrode as a change in capacitance and varying the oscillation frequency of an LC oscillator; A reference frequency oscillator that oscillates a reference frequency having a predetermined relationship with the oscillation frequency of the capacitance sensor; a detection circuit that generates a beat frequency that is the difference between the oscillation frequency of the capacitance sensor and the reference frequency of the reference frequency oscillator; FV to convert beat frequency output of circuit to voltage
A displacement type vibration sensor comprising: a transducer; and a micro-vibration generator for causing the capacitance sensor to micro-vibrate with a constant amplitude and a low cycle with respect to the displacement measurement target surface, and an output from the FV converter. A low-frequency AC amplifier that converts a low-frequency amplitude due to a change in capacitance based on the minute vibration superimposed on the beat frequency to be detected into a voltage, and detects the voltage. Detecting a change in capacitance as a change in low-frequency amplitude based on the minute vibration,
A capacitance type displacement sensor wherein the amplitude is converted into a voltage value, and a displacement amount is obtained from the converted voltage value.