JP2003042879A - Sensor signal processing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve sensor measurement accuracy and obtain quick responsivity. SOLUTION: This sensor signal processing circuit is provided with a capacitance-type sensor part with characteristics which changes as a whole according to changes in a physical quantity to be measured, an electric power source part with at least two types of A.C. power sources with different polarities or phases, and an arithmetic part connected to the output side of the sensor part for performing arithmetic processing for obtaining the difference, ratio, etc., of signals due to sensor capacitance. The sensor signal processing circuit is provided with a part for synchronously detecting output signals of the arithmetic part by two types of A.C. signals with different polarities or phases, and measurement and responsivity in measuring physical quantities are heightened by the sensor signal processing circuit.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor signal processing circuit for extracting a signal from a sensor section as a function of a physical quantity to be measured. The present invention relates to a sensor signal processing circuit including high-speed arithmetic means for removing a circuit error such as a gain of an operational amplifier by signal processing. 2. Description of the Related Art In recent years, in the field of pressure measurement, replacement from a mechanical type to an electronic type has been rapidly progressing. The electronic pressure gauges can be roughly classified into a resistance type that converts a change in stress of the pressure-sensitive diaphragm into a change in electric resistance and a capacitance type that converts displacement of the pressure-sensitive diaphragm into change in capacitance. Among them, the capacitance type pressure sensor has a feature that it is excellent in measuring a minute pressure. FIG. 1 is a sectional view showing a structure of a sensor section of a conventional latest capacitive pressure sensor. FIG. 2 is a sectional view of the sensor section shown in FIG. 1 taken along the line II-II '. As shown in FIG. 1, a first recess is formed in the center of one surface of the pedestal substrate 1, and a second recess is formed in a groove shape so as to surround the first recess. I have. A thin diaphragm substrate 2 is joined to the surface of the pedestal substrate 1 in which these concave portions are formed. Spaces surrounded by the first and second concave portions of the pedestal substrate 1 and the inner surface of the diaphragm substrate 2 are capacitance chambers 3a and 3b, respectively. As shown in FIG. 2, an electrode 5a is disposed on the pedestal substrate 1 side of the capacitance chamber 3a, and an electrode 5b is disposed on the diaphragm substrate 2 side so as to face the electrode 5a. The electrode 5a is taken out by a lead 5c.
The same applies to the electrode 5b. These electrodes 5a, 5
The sensor capacitance Cx is constituted by b and air as a dielectric. FIG. 2 shows that the capacity chamber 3b is on the pedestal substrate 1 side.
As shown in FIG. 5, the electrodes 6a are arranged in a strip shape, and the electrodes 6b are arranged on the diaphragm substrate 2 side so as to face the electrodes 6a. The electrode 6a is taken out to the outside by the lead 6c. The same applies to the electrode 6b. These electrodes 6a and 6b and air as a dielectric form a reference capacitance Cr. The capacity chambers 3a,
3b, so that the air in 3b is easily mixed.
The partition 4 between 3b is partially removed. The portion of the diaphragm substrate 2 that constitutes the capacity chamber 3a acts as a pressure-sensitive diaphragm 2a. For example, when a positive pressure P is applied to the upper side of the diaphragm substrate 2 as shown in FIG. The diaphragm 2a bends downward. Since the electrode 5b is displaced in conjunction with the pressure-sensitive diaphragm 2a, the gap between the electrodes 5a and 5b is reduced, and the capacitance value of the sensor capacitance Cx increases. On the other hand, the portion constituting the capacity chamber 3b of the diaphragm substrate 2 hardly bends even when the pressure P is applied, so that the capacitance value of the reference capacitor Cr does not change. The reference capacitor Cr is provided for removing a measurement error (this is called an element error) based on a temperature change inside and outside the sensor section and a humidity change inside the capacity chamber 3a. That is, the capacitance value of the sensor capacitance Cx and the capacitance value of the reference capacitance Cr
From the calculation, the pressure P from which the element error has been removed can be obtained. That is, the dielectric constant of air in the capacitance chambers 3a and 3b is ε, the normal gap between the electrodes 5a and 5b in the sensor capacitance Cx and the gap between the electrodes 6a and 6b in the reference capacitance Cr are d, and the pressure-sensitive diaphragm is 2a
Let Δd be the pressure sensitivity displacement of the electrodes 5a,
Assuming that the opposing area of 5b and the opposing areas of the electrodes 6a and 6b are both S, the capacitances Cx and Cr are respectively Can be expressed as Substituting these equations (1) and (2) into the following equation (3) gives Therefore, it can be understood that Δd indicating the pressure P can be obtained from the equation (3). FIG. 5 is a conventional example of a circuit diagram of a sensor signal processing circuit for extracting a signal from the sensor section 91 shown in FIG. 1 as a function of pressure P. The input side of the sensor capacitance Cx of the sensor section 91 is connected to a sine-wave AC power supply 100 having a frequency f of the sensor driving power supply section 90. Similarly, an AC power supply 100 is also connected to the input side of the reference capacitor Cr. A calculation unit 92 is connected to the output side of the sensor unit 91, and a synchronous detection unit 93 that detects an AC signal to rectify it into a DC signal and obtains a sensor signal value is connected to the output side of the calculation unit 92. Have been. The operation unit 92 includes an operational amplifier 110,
111 and a feedback calculation capacitor Cf. In the operational amplifier, the non-inverting input terminal (+) is connected to the ground, and the inverting input terminal (-) is connected to the capacitor C.
x, Cr, and the output of 110 is directly synchronized with the synchronous detector 9
3, the output of 111 is connected to a subtractor 112 to subtract from the output of 110. Here, the output at point A and the output at point B, which are the outputs of the operational amplifiers 110 and 111, are given by the following equations (4), respectively.
(5). The output at point D, which is the difference between these two signals, is given by the following equation (6). The signals at points A and D are synchronized with the sine wave of the drive signal,
The signal is switched between W1 and SW2 and applied to a low-pass filter (LPF) for a half cycle. That is, it is equivalent to integrating and averaging in a half cycle period, and the signal thus smoothed
The points E and F are given by the following equations (7) and (8). Furthermore, by dividing the signal at point F by the signal at point E,
The signal of the following equation (9) can be measured. The division in this case is considered to be performed after conversion into analog or digital. Next, a conventional technique for signal processing of a capacitance type pressure sensor which detects a difference between pressures applied from two directions will be described. FIG. 3 is a cross-sectional view showing the structure of a sensor section of this type of pressure sensor. FIG. 4 is a circuit diagram of the sensor unit shown in FIG. The sensor section 34 has pressure-sensitive diaphragms 22a and 22a 'on both upper and lower sides. In the case of the sensor section 34, a concave portion is formed over the central portion of the diaphragm substrates 22, 22 ', and the thinned portion formed by the concave portion is formed by the pressure-sensitive diaphragms 22a, 22a'.
Function as The diaphragm substrates 22, 22 'including the pressure-sensitive diaphragms 22a, 22a' are joined to both ends of an opening of a frame 21 formed in a cylindrical shape. The space surrounded by these diaphragm substrates 22 and 22 ′ and the frame body 21 is a capacity chamber 23. Pressure-sensitive diaphragms 22a and 22
a ′ is connected to a support column 27 disposed in the capacity chamber 23. The support 27 is parallel to the diaphragm substrates 22 and 22 ', and the diaphragm substrate 2
A central support plate 28 is attached so as not to come into contact with any of the frames 2 and 22 'and the frame 21. The above-mentioned diaphragm substrates 22, 22 ', frame 21, support column 27, and center support plate 28 are formed of an insulating member such as sapphire glass. Further, electrodes 25a, 26 made of a conductive thin film are provided on the inner surfaces of the peripheral portions of the diaphragm substrates 22, 22 '.
a are respectively arranged. On the upper and lower sides of the peripheral portion of the central support plate 28, electrodes 25b and 26b, which are also made of a conductive thin film, are arranged. A first sensor capacitance 34a is formed by the electrodes 25a and 25b facing each other and the air in the capacitance chamber 23. Similarly, the opposing electrodes 26 a and 26 b and the air in the capacity chamber 23
The second sensor capacitor 34b is configured. The pressure HP is applied to the sensor unit 34 configured as described above from above the pressure-sensitive diaphragm 22a, and the pressure LP is applied from below the pressure-sensitive diaphragm 22a '. Here, for example, HP <LP. At this time, according to the difference between the pressures HP and LP, the pressure-sensitive diaphragms 22a, 22a ', the column 27, and the central support plate 28 are integrally displaced upward. Since the electrode 25b disposed on the central support plate 28 is displaced upward together with the central support plate 28, the gap between the electrodes 25a and 25b is reduced, and the capacitance Cx of the first sensor capacitor 34a is increased. Similarly, since the electrode 26b is also displaced upward together with the center support plate 28, the gap between the electrodes 26a and 26b increases, and the capacitance Cy of the second sensor capacitor 34b decreases. The first sensor capacity 34a and the second sensor capacity 34b can be obtained by calculating Δd by calculating the equations (10), (11) and (12), so that the differential pressure between LP and HP can be obtained. I can do it. As described in the above-described conventional example, it is possible to remove an element error based on a temperature characteristic or the like from a measurement result in calculation. FIG. 6 is a circuit diagram of a sensor signal processing circuit for extracting a signal from the sensor section shown in FIG. 3 as a function of a differential pressure. (Parts notation for simplification of the figure (90 to 9
3) is omitted, but is common thereafter. The sensor capacitance Cx is connected to a sine-wave AC power supply 100 of a power supply for driving the sensor. Similarly, an AC power supply 10
0 is connected. A calculation unit is connected to the output side of the sensor unit, and a synchronous detection unit that filters the AC signal to rectify it into a DC signal and obtains the value of the sensor signal is connected to the output side of the calculation unit. The operation units are operational amplifiers 110 and 11 respectively.
1 and a feedback calculation capacitor Cf. Of these, the operational amplifier has a non-inverting input terminal (+)
Are connected to ground, and the inverting input terminal (-) is connected to the capacitors Cx and Cx.
y, and the output of 110 and the output of 111
13 and the output of 110
The result of subtraction by the subtractor 112 based on the output of 1 is connected to the synchronous detector. Here, the same calculation as in the above-described example is performed, and the pressure displacement Δd is finally obtained by the equation (19). The outputs at points C and D, which are the sum and difference of these signals, are given by the following equations. Further, the following is required as a result of the synchronous detection. However, the circuits shown in FIG. 5 or FIG. 6 include the gain characteristics of the operational amplifiers 110 and 111 and the capacitance values of the feedback capacitors Cf used in the operational amplifiers. Because of the variation,
There was a problem that the relationship between the expressions (9) and (19) could not be obtained accurately. Even if the tuning is once performed to remove the error of the element, there is a problem that an error occurs due to aging. The present invention has been made to solve such a problem, and has as its object to improve the measurement accuracy of a sensor. Further, the present invention realizes a high-speed operation in which calculation for improving accuracy and execution of synchronous detection are performed within a time period of about one to one cycle of the driving power supply. In order to achieve the above object, the present invention provides a sensor section whose characteristics change as a whole in response to a change in a physical quantity to be measured, and a sensor section having different polarities or phases. It is characterized by comprising a power supply unit having more than two types of power supplies, an operation unit connected to the output side of the sensor unit and consisting of only an operational amplifier and a feedback capacitor, and a synchronous detection unit using two types of AC power supplies having different polarities or phases. And
As a result, if the amplitude and phase of the power supply can be set accurately, an element error of the operational amplifier or the feedback capacitor due to a temperature change or the like does not cause an error in measurement. Embodiments of the present invention will be described below in detail with reference to the drawings. (Embodiment of First Sensor Type) An embodiment in which the sensor signal processing circuit according to the present invention is applied to a capacitance type pressure sensor will be described with reference to FIG. The target capacitance type pressure sensor is related to FIG. 1 which is a cross-sectional view showing the structure of the sensor unit. FIG. 7 is a circuit diagram of a sensor signal processing circuit for extracting a signal from the sensor section 14 shown in FIG. 1 as a function of the pressure P. The power supply unit 90 includes three types of AC power supplies 100, 101, and 102 having different polarities and phases. The sensor unit uses the one shown in FIGS. 1 and 2, and is composed of a sensor capacitor Cx and a reference capacitor Cr. Each of these capacitors has a temperature characteristic and a humidity characteristic based on the dielectric constant ε, but the dielectric constant ε of the numerator denominator is obtained by the effect of the equation (3).
Is removed, so the effect is negligible. On the output side of the power supplies 100 and 101, an adder 103 is provided. Therefore, the sensor capacity Cx
Is connected to the sum of the sine wave 100 and the cosine wave 101. The reference capacitor Cr is connected to the sine wave 102 whose polarity is inverted. A negative feedback circuit is formed by the operational amplifier 114 and the feedback capacitor Cf. The output at point A is given by the following equation. The signal at point A is detected in synchronization with the cosine wave of the drive signal, and the smoothed signal point E is given by the following equation (21). That is, in order to detect in synchronization with the cosine wave, 1/4 to 3/4 period is integrated to set the half period of one peak of the cosine wave, and an average is obtained by dividing by the integration time. I have. Similarly, the signal at point A is detected in synchronization with the sine wave of the drive signal, and the smoothed signal point F is given by the following equation (22). By dividing the signal at the point F by the signal at the point E by the divider 122, the signal of the following equation (23) can be measured. This gives the pressure. FIG. 8 shows another embodiment of the sensor. (The notation of parts (90 to 93) is omitted for the sake of simplicity of the figure, and is common hereinafter.) The power supply unit is constituted by AC power supplies 100 and 101 of sine and cosine waves. The sensor section is the same as that shown in FIGS. 1 and 2, and is composed of a sensor capacitor Cx and a reference capacitor Cr. The output sides of the power supplies 100 and 101 are directly connected to the sensor capacitance Cx and the reference capacitance Cr, respectively.
Are connected, the operational amplifier 114 and the feedback capacitor C
f forms a negative feedback circuit. The output at point A is given by the following equation (24). The signal at point A is detected in synchronization with the sine wave of the drive signal, and the smoothed signal point E is given by the following equation (25). The signal at point A is detected by synchronizing and detecting the signal at + 45 ° from the sine wave of the drive signal, and the smoothed signal point F is given by the following equation (26). By dividing the signal at point F by the signal at point E, the following equation (2)
The signal of 7) can be measured. (Embodiment of Second Sensor Type)
Next, an embodiment in which the present invention is applied to a capacitance type pressure sensor for detecting a difference between pressures applied from two directions will be described. FIG. 9 shows an embodiment of such a sensor signal processing circuit. The structure of the pressure sensor is shown in the sectional view of FIG. 3, and FIG. 4 is a circuit diagram thereof. The power supply unit shown in FIG. 9 includes AC power supplies 100, 101, and 1 for sine waves, cosine waves, and sine waves of inverted polarity.
02. The sum of the sine wave and the cosine wave by the adders 103 and 104, and the sum of the cosine wave and the inverted sine wave are connected to two sensor capacitors Cx and Cy, respectively. And the operational amplifier 1
14 and a feedback capacitor Cf form a negative feedback circuit. The output at point A is given by the following equation (28). The signal at point A is detected in synchronization with the cosine wave of the drive signal, and the smoothed signal point E is given by the following equation (29). The signal at point A is detected in synchronization with the sine wave of the drive signal, and the smoothed signal point F is given by the following equation (30). By dividing the signal at point F by the signal at point E, the following equation (3)
The signal can be measured in 1). Next, FIG. 10 shows another embodiment of the sensor. The power supply unit includes sine and cosine wave AC power supplies 100 and 101. The sensor section is constituted by sensor capacitances Cx and Cy. Power supply 10
Sensor outputs Cx and Cy are directly connected to the output sides of 0 and 101, respectively, and a negative feedback circuit is formed by the operational amplifier 114 and the feedback capacitor Cf. The output at point A is given by the following equation (32). The signal at point A is detected by synchronizing and detecting the signal shifted by + 45 ° from the cosine wave of the drive signal, and the smoothed signal point E is given by the following equation (33). The signal at point A is detected by synchronizing and detecting the signal shifted by + 45 ° from the sine wave of the drive signal, and the smoothed signal point F is given by the following equation (34). By dividing the signal at point F by the signal at point E, the following equation (3)
5) The signal can be measured. Although the embodiment in which the present invention is applied to the pressure sensor has been described above, the present invention can be applied to a sensor for measuring various physical quantities such as temperature, humidity, displacement, variable, and acceleration in addition to pressure. (Explanation of Synchronous Detection Circuit) The smoothing by the synchronous detection circuit described in the above embodiment means that the signal from the arithmetic section is integrated as it is while the AC signal for detection is positive.
When the AC signal for detection is negative, this means performing an operation of inverting the polarity of the signal from the arithmetic unit and integrating the signal, and averaging during the integration period. For example, FIG. 11A shows a case where the signal from the arithmetic unit which is an input to the synchronous detection unit is a sine wave, and FIG. 11B shows an output diagram when the sine wave is synchronously detected with the same sine wave.
Shown in If the output of FIG. 11b is smoothed for a half period, an input amplitude × 2 / π is obtained. FIG. 12A shows a case where the signal from the arithmetic unit is a cosine wave, and FIG. 11B shows an output diagram when the signal is synchronously detected with the sine wave of FIG. 11A. FIG.
When the output of 1b is smoothed for a half cycle, it becomes zero. Similarly, when the cosine wave is synchronously detected with the cosine wave, the input amplitude × 2 / π
Similarly, when the sine wave is synchronously detected with the cosine wave, the value becomes zero. The operation of the synchronous detector of FIG. 7 in the first embodiment will be described based on this basic concept. Since the point E in FIG. 7 is synchronously detected by a cosine wave, the input component of the sine wave disappears. , Only the input component of the cosine wave remains. Therefore, only CX signals can be extracted. Since the point F is synchronously detected with a sine wave, the input component of the cosine wave disappears, and only the input component of the sine wave remains. Therefore, the signal of the difference between CX and CR can be extracted.
The same applies to other embodiments. Further, at least a half period of the trigonometric function wave is smoothed, and the period can be extended by one or more periods to improve stability. As described above, according to the present invention, if two or more types of driving AC power sources and two types of synchronous detection power sources are prepared, an accurate circuit that eliminates both circuit errors and element errors can be obtained. Sensor results can be obtained. In addition, the calculation is a simple one consisting of only one feedback capacitor and an operational amplifier, and does not require element selection and tuning, and can be realized at low cost. Furthermore, since the operation of the synchronous detection circuit is sufficient for about half cycle to one cycle of the AC power supply frequency, there is an effect that a physical quantity to be measured can be quickly obtained from sensor measurement values.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing a structure of a sensor unit of a capacitance type pressure sensor for detecting a pressure applied from one direction. FIG. 2 is a sectional view taken along the line II-II 'of the sensor unit shown in FIG. FIG. 3 is a cross-sectional view illustrating a structure of a sensor unit of a capacitive pressure sensor that detects a difference between pressures applied from two directions. FIG. 4 is a circuit diagram of a sensor unit shown in FIG. FIG. 5 is a conventional circuit diagram of a sensor signal processing circuit that extracts a signal from the sensor unit shown in FIG. 1 as a function of pressure. 6 is a conventional circuit diagram of a sensor signal processing circuit for extracting a signal from the sensor unit shown in FIG. 3 as a function of pressure. FIG. 7 is a circuit diagram of a sensor signal processing circuit according to an embodiment of the present invention for extracting a signal from the sensor unit shown in FIG. 1 as a function of pressure. 8 is a circuit diagram of a sensor signal processing circuit obtained by partially improving the sensor signal processing circuit shown in FIG. 7 according to an embodiment of the present invention. FIG. 9 is a circuit diagram of a sensor signal processing circuit according to an embodiment of the present invention for extracting a signal from the sensor unit shown in FIG. 3 as a function of pressure. 10 is a circuit diagram of a sensor signal processing circuit obtained by partially improving the sensor signal processing circuit shown in FIG. 9 according to an embodiment of the present invention. FIG. 11A shows a case where the signal input from the arithmetic unit is a sine wave, and FIG. 11B shows an output diagram when a sine wave is synchronously detected with the same sine wave. FIG. 12A shows a case where the signal input from the arithmetic unit is a cosine wave, and FIG. 12B shows an output diagram when the signal input is synchronously detected with the sine wave of FIG. 11A. DESCRIPTION OF SYMBOLS 1 ... pedestal substrate 2, 22, 22 '... diaphragm substrate 2a, 22a, 22a' ... pressure-sensitive diaphragm 3a, 3b, 23 ... capacity chamber 4 ... partition walls 5a, 5b, 6a, 6b, 25a, 25b , 26a, 2
6b ... electrodes 5c, 6c ... leads 14, 34 ... sensor configuration 34a, 34b ... sensor capacity configuration 21 ... frame body 27 ... support 28 ... central support plate 90 ... power supply unit 91 ... sensor unit 92 ... calculation unit 93 ... synchronous detection parts 100, 101, 102 ... driving sensor AC power source 110,111,114 ... op 112 ... subtractor 113 ... adder 120, 121 ... integrator or low pass filter 122 ... divider Cx, C Y _... sensor capacitance capacitance _{C R} ... Reference capacitance capacitance Cf ... Feedback calculation capacitance capacitance SW1, SW2 ... Synchronous detection signal changeover switch

Claims (1)

Claims: 1. An electrostatic capacity sensor unit whose characteristics change as a whole according to a change in a physical quantity to be measured, and an AC power supply for driving at least two types of sensors having different polarities or phases. A power supply unit, a calculation unit including an operational amplifier connected to the output side of the sensor unit for performing a signal calculation process based on a sensor capacitance value change, and a synchronous detection unit for the output signal of the calculation unit using two types of AC signals having different polarities or phases. And a sensor signal processing circuit.