Capacitive Sensor

Abstract

To reduce circuit mounting area and cost of a circuit.SOLUTION: A capacitive sensor comprises: a sensor unit 100 having capacitance changed according to physical quantity of a measurement object; a circuit unit 110 detecting a capacitance signal from an output of the sensor unit 100; a microprocessor 106 converting the capacitance into a measured value of the physical quantity; and a filter 107 outputting as a drive signal to the sensor unit 100, a signal of a specific frequency extracted from a square wave signal output from the microprocessor 106.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a capacitance type sensor used as a pressure sensor or the like. [Background technology]

[0002] Conventionally, capacitance sensors in which the capacitance varies according to a physical quantity have been used as, for example, pressure sensors (see Patent Document 1). The circuit of a capacitance sensor is shown in Fig. 8. In a capacitance sensor, a sine wave is applied from a signal generator 101 to a sensor unit 100 whose capacitance CX varies according to the physical quantity (e.g., pressure) to be measured. A charge amplifier circuit 102 consisting of a capacitance CF and an operational amplifier A1 converts the current output from the sensor unit 100 into a voltage and amplifies it.

[0003] The detection circuit 103 full-wave rectifies the signal amplified by the charge amplifier circuit 102. A low-pass filter (LPF) 104 smoothes the output of the detection circuit 103. In this way, a capacitance signal based on the capacitance CX is obtained. This capacitance signal is converted into a digital signal by an AD converter 105 (for example, a ΔΣ type AD converter). A microprocessor 106 converts the digital signal into a pressure measurement value.

[0004] The signal generator 101 has various configurations, for example, the following examples (I) and (II). (I) An analog oscillator circuit using an operational amplifier. (II) Digital sine wave generation IC (an IC that performs digital-to-analog conversion of digital signals and outputs them).

[0005] In the above configuration (I), there was a problem that the constants of passive elements such as resistors and capacitors fluctuate due to individual differences and temperature characteristics, resulting in large errors in the amplitude and frequency of the output waveform, which has a significant impact on pressure measurement errors. In the configuration of (II), although it is easy to control the amplitude and frequency of the output waveform, there is a problem that the mounting area and cost of the circuit are increased.

[0006] In recent years, mainly in semiconductor manufacturing processes, equipment has been made smaller in order to efficiently heat the manufacturing equipment. As a result, peripheral devices such as pressure sensors attached to the manufacturing equipment also need to be made smaller, and there is a demand to keep the mounting area of ​​the circuits small. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2005-331328 A Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made to solve the above-mentioned problems, and has an object to provide a capacitance type sensor that can reduce the circuit mounting area and costs. [Means for solving the problem]

[0009] The capacitance type sensor of the present invention includes a sensor unit configured to change capacitance in response to a physical quantity of a measurement target, a processor configured to convert the capacitance into a measurement value of the physical quantity, and a first periodic signal output from the processor. an analog switch configured to output a second periodic signal generated by switching a second power supply voltage separate from a first power supply voltage for the processor, the second periodic signal being a control input of the analog switch; and a filter configured to output a signal of a specific frequency extracted from the signal as a drive signal to the sensor unit. 。

[0010] In one configuration example of the capacitance type sensor of the present invention, the filter is a band-pass filter. In one configuration example of the capacitance type sensor of the present invention, the filter is a combination of a low-pass filter and a high-pass filter. Furthermore, one configuration example of the capacitance type sensor of the present invention is characterized in that it further includes a circuit unit configured to detect a capacitance signal from the output of the sensor unit driven by the drive signal, and the processor converts the capacitance signal into a measurement value of the physical quantity. Effect of the Invention

[0011] According to the present invention, a signal generator is realized by utilizing a microprocessor that is conventionally provided in a capacitance type sensor, so that the mounting area and cost of the capacitance type sensor circuit can be reduced. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a block diagram showing the configuration of a signal generator for a capacitance type sensor according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a waveform diagram showing an example of a square wave signal. [Diagram 3] FIG. 3 is a diagram showing frequency components of a square wave signal. [Figure 4] FIG. 4 is a circuit diagram showing an example of a filter of the signal generator according to the first embodiment of the present invention. [Diagram 5] FIG. 5 is a cross-sectional view showing the configuration of a main part of a sensor unit of a capacitance type sensor according to a first embodiment of the present invention. [Figure 6] FIG. 6 is a circuit diagram showing an example of a filter of a signal generator according to a second embodiment of the present invention. [Figure 7] FIG. 7 is a block diagram showing the configuration of a signal generator of a capacitance type sensor according to a third embodiment of the present invention. [Figure 8] FIG. 8 is a block diagram showing a circuit configuration of the capacitance type sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] [Principle of the invention] The microprocessor that converts the capacitance of the sensor section into a pressure measurement value has many output ports. A periodic signal (e.g., a square wave) is output from one of these output ports, and the periodic signal is filtered to extract only the sine wave, which is then applied to the sensor section. This eliminates the need for an IC to generate sine waves, making it possible to reduce the mounting area and cost of the capacitance sensor circuit.

[0014] [First Example] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a block diagram showing the configuration of a signal generator for a capacitance type sensor according to a first embodiment of the present invention, and the same components as those in Fig. 8 are given the same reference numerals. In Fig. 1, the charge amplifier circuit 102, detection circuit 103, low-pass filter 104, and AD converter 105 in Fig. 8 are shown as circuit section 110.

[0015] The signal generator 101a of this embodiment is composed of a microprocessor 106 that outputs a square wave signal (first periodic signal), and a filter 107 that outputs a signal of a specific frequency extracted from the square wave signal output from a port PO of the microprocessor 106 as a drive signal to the sensor unit 100.

[0016] As described above, the capacitance signal converted into a digital signal by the AD converter of the circuit section 110 is converted into a pressure measurement value by the microprocessor 106. The microprocessor 106 executes processing according to a program stored in an internal memory (not shown). In this embodiment, the program is set so that the function of outputting a square wave signal of fundamental frequency f0 is added as a function of the microprocessor 106. FIG. 2 is a waveform diagram showing an example of a square wave signal. In the example of FIG. 2, a square wave signal of fundamental frequency f0=10 kHz is used.

[0017] When the square wave signal with fundamental frequency f0=10 kHz shown in Fig. 2 is expanded into a Fourier series, it is found to have frequency components that are integer multiples of f0, as shown in Fig. 3. If frequency components other than f0 are applied to the sensor unit 100 together with a signal with fundamental frequency f0, noise will be included in the capacitance signal, which will cause pressure measurement errors. Therefore, only the fundamental frequency component (component shown by 30 in Fig. 3) is extracted by filter 107, and a sine wave signal with fundamental frequency f0 is applied to the sensor unit 100.

[0018] In this embodiment, a band-pass filter as shown in Fig. 4 is used as the filter 107. This filter 107 is composed of an operational amplifier U1, resistors R1 to R3, and capacitors C1 and C2.

[0019] 5 is a cross-sectional view showing the configuration of the main parts of the sensor unit 100 of the capacitance sensor. A recess is formed in the center of a base 200 of the sensor unit 100. A diaphragm 201 configured to be deformable in response to the pressure P of the medium to be measured is bonded to the surface of the base 200 on which the recess is formed. The recess of the base 200 forms a reference vacuum chamber 202 together with the diaphragm 201.

[0020] A fixed electrode 203 is formed on the surface of the base 200 facing the reference vacuum chamber 202, and a movable electrode 204 is formed on the surface of the diaphragm 201 facing the reference vacuum chamber 202 so as to face the fixed electrode 203. In this way, the fixed electrode 203 and the movable electrode 204 are arranged to face each other across a gap. When the diaphragm 201 is deflected by receiving the pressure P of the medium to be measured, the distance between the movable electrode 204 and the fixed electrode 203 changes, and the electrostatic capacitance CX between the movable electrode 204 and the fixed electrode 203 changes. The diaphragm 201 and the base 200 are made of an insulator such as sapphire.

[0021] The sinusoidal signal output from the signal generator 101a is applied to a first electrode (e.g., the fixed electrode 203) of the sensor unit 100. A second electrode (e.g., the movable electrode 204) of the sensor unit 100 is connected to a charge amplifier circuit of the circuit unit 110. The configuration of the capacitance sensor after the charge amplifier circuit is as described in FIG.

[0022] As described above, in this embodiment, the signal generator is realized by utilizing the microprocessor 106 that is conventionally provided in the capacitance type sensor, so that the mounting area and cost of the capacitance type sensor circuit can be reduced.

[0023] [Second Example] In the first embodiment, a band-pass filter is used as filter 107 of signal generator 101a, but a circuit combining a low-pass filter and a high-pass filter may also be used as shown in Fig. 6. In the example of Fig. 6, resistor R4 and capacitor C3 form a low-pass filter, and capacitor C4 and resistor R5 form a high-pass filter.

[0024] [Third Example] In the first and second embodiments, the filter 107 is directly connected to the output of the microprocessor 106. However, since noise caused by other digital ICs is superimposed on the power supply voltage VCC supplied to the microprocessor 106, the square wave signal output from the microprocessor 106 is also a noisy signal. Therefore, in this embodiment, an analog switch is inserted between the microprocessor 106 and the filter 107 to generate a square wave signal with low noise.

[0025] 7 is a block diagram showing the configuration of a signal generator for a capacitance sensor according to this embodiment. The signal generator 101b of this embodiment is composed of a microprocessor 106, a filter 107, and an analog switch 108 that uses a square wave signal output from the microprocessor 106 as a control input and outputs a square wave signal (second periodic signal) generated by switching a power supply voltage Van for an analog circuit, which is different from a power supply voltage VCC for a digital circuit, to the filter 107.

[0026] The analog switch 108 is turned on, for example, when the square wave signal from the microprocessor 106 is high, and is turned off when the square wave signal from the microprocessor 106 is low. In this way, the analog switch 108 uses the square wave signal output from the microprocessor 106 as a control input and switches (on / off) the power supply voltage Van for the analog circuit to convert it into a square wave signal. By using the power supply voltage Van for the analog circuit, which is separate from the power supply voltage VCC for the digital circuit, a square wave signal with low noise can be supplied to the filter 107. The filter 107 may have the configuration in either the first or second embodiment.

[0027] Thus, in this embodiment, a sinusoidal signal with less noise can be supplied from the filter 107 to the sensor section 100, making it possible to reduce pressure measurement errors. [Industrial Applicability]

[0028] The present invention can be applied to capacitance sensors such as pressure sensors. [Explanation of symbols]

[0029] 100: sensor section, 101a, 101b: signal generators, 102: charge amplifier circuit, 103: detection circuit, 104: low-pass filter, 105: AD converter, 106: microprocessor, 107: filter, 108: analog switch, 110: circuit section.

Claims

1. A sensor unit configured such that a capacitance changes in response to a physical quantity of a measurement target; a processor configured to convert the capacitance into a measure of the physical quantity; an analog switch configured to output a second periodic signal generated by switching a second power supply voltage different from a first power supply voltage for the processor, using a first periodic signal output from the processor as a control input; a filter configured to output a signal of a specific frequency extracted from the second periodic signal as a drive signal to the sensor unit.

2. 2. The capacitance type sensor according to claim 1, The capacitive sensor is characterized in that the filter is a band-pass filter.

3. 2. The capacitance type sensor according to claim 1, The capacitive sensor is characterized in that the filter is a combination of a low-pass filter and a high-pass filter.

4. 4. The capacitance type sensor according to claim 1, a circuit unit configured to detect a capacitance signal from an output of the sensor unit driven by the drive signal; The capacitive sensor according to claim 1, wherein the processor converts the capacitance signal into a measurement value of the physical quantity.

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