JPH0267939A - Pressure detecting apparatus - Google Patents

Abstract

PURPOSE:To simplify the constitution of a circuit and to reduce a cost by switching the capacitance of a detecting electrode and the capacitance of a reference electrode with a control part through a detecting means, performing measurement, operating the measured results with an operating means, and computing a pressure that is applied on a sensor based on the solution of the operation. CONSTITUTION:A single detecting means 9 is composed of a reference capacitor Cr, a detecting capacitor Cp and a CR oscillating circuit 9 including a resistor R. A switching means 10 is controlled with a switching gate signal control means 12 which is built in a control part 11. The reference capacitor Cr and the detecting capacitor Cp are switched. The means 10 is connected to the oscillating circuit 9. Oscillating frequencies fr and fp are inputted into a counter means 13 in the control part 11. The output of the counter means 13 is stored in the specified address in a RAM 14 and transferred into an operating means 15. Dividing operation is performed, and a ratio (r) of the frequencies is obtained. The ratio (r) becomes a ratio between the detected capacitance Cp and the reference capacitance Cr. Since the temperature characteristics of both capacitors are approximately equal, the temperature characteristics can be removed by computing the pressure (p) based on the ratio (r).

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a pressure detection device that detects pressure applied to a sensor using changes in capacitance.

2. Description of the Related Art A large number of capacitance pressure sensors have already been put into practical use, in which electrodes are opposed to each other with a predetermined gap maintained, and the pressure applied thereto is extracted as a change in capacitance.

Since the capacitance value of such a sensor is usually very small, about 20 to 502 F, a large error occurs depending on the temperature characteristics of the detection circuit and the sensor itself. Therefore, how to resolve changes in characteristics due to temperature has been an important issue for this type of sensor.

For this reason, a detection electrode whose capacitance value changes sharply depending on the applied pressure is provided at the center of the substrate, and a reference electrode whose capacitance value does not change much depending on the applied pressure is placed at the outer periphery of the capacitance electrode. Type pressure sensors are known in -S.

The capacitive pressure sensor described in Japanese Patent Application Laid-open No. 198739/1987 is an example of such a sensor, and by charging and discharging the above two capacitances, the temperature characteristics of the sensor and circuit can be reduced. I'm about to try.

FIG. 5 shows the configuration of such a capacitive pressure sensor, with FIG. 5(a) showing a cross section and FIG. 5(b) showing a state in which two substrates are developed.

Two electrodes are printed on the alumina substrates 1 and 2,
Their peripheries are sealed with glass 3 so that they face each other with a constant gap. Therefore, the detection electrode 4 in the center changes sensitively to external pressure, while the reference electrode 5 on the outer periphery is bent because it is close to the glass, and the pressure change due to external pressure is very small compared to the former.

Since both capacitors are arranged close to each other on the same substrate, they are affected by temperature in almost the same way. Therefore, by comparing the two, only the influence of temperature can be removed and only pressure information can be obtained.

FIG. 6 shows such a conventional circuit configuration. The capacitance CP of the detection electrode 4 and the capacitance C of the reference electrode 5 are each a resistance R.
,,R, to form a charging/discharging circuit. Both capacitors are turned on and off by transistors Q, ,Q, and are repeatedly charged and discharged.The operation and waveforms of each part are detailed in the cited example, so they will be omitted.If the power supply voltage is Vcc, the output of the low-pass filter 6 is Voltage ■. uL is V,, L = Vcc (1-C, / C,)---(1
). As mentioned above, the temperature characteristics of the sensors are almost the same, so the change in capacitance due to temperature ΔCr
(ppm/'C) and ΔCp (ppa+/''C)
is canceled by the term (1-C, /Cp), and the output voltage V
The temperature characteristics of the sensor can be removed from ovL.

However, in this case, R,=R, and the comparators 7 and 8 and the transistors Q and Q2 must be bare by selecting those having the same temperature characteristics.

Problems to be Solved by the Invention However, with such a conventional configuration, although the temperature characteristics of the sensor can be kept small, the temperature characteristics of the circuit cannot be completely eliminated.

First, at a consumer level, the power supply voltage V cC usually has temperature characteristics. To avoid this, complex and expensive temperature compensated power supplies must be used.

Next, since it is impossible for R3 and Rz to be exactly the same, a temperature characteristic occurs due to the unbalance.

Selecting comparators 7 and 8 and transistors Q and Q8 that have the same temperature characteristics and making them bare is a huge effort in actual mass production, and it is difficult to completely match the temperature characteristics. is an extremely difficult skill.

Furthermore, the output frequency of the comparator cannot be treated as a frequency; the temperature characteristics cannot be canceled unless it is converted into a DC voltage using a low-pass filter. This makes the circuit configuration complicated and expensive.

Means for Solving the Problems In order to solve the above problems, the present invention provides a static static plate formed of a pair of flat plates facing each other with a predetermined gap and having a detection electrode in the center and a reference electrode on the outer periphery. The pressure sensor includes a capacitance type pressure sensor, a detection means for reading the capacitance of the detection electrode and the reference electrode, and a control section that has an arithmetic means and detects the capacitance of the detection electrode and the reference electrode using the detection means. .

In the pressure detection device of the present invention, the control section measures the capacitance of the detection electrode and the reference electrode while switching through the detection means, processes the measurement results using the calculation means, and calculates the solution. Calculate the pressure applied to the sensor.

Embodiments Hereinafter, a pressure detection device according to the present invention will be explained with reference to the drawings.

The configuration of the sensor is the same as the conventional one shown in FIG. 2, and since it has already been described, the explanation will be omitted here to avoid duplication.

FIG. 1 is an embodiment of a block diagram showing the system configuration of a pressure detection device according to the present invention.

The single detection means is constituted by a CR oscillation circuit 9 including a reference capacitor C1, a detection capacitor C2, and a resistor R. The switching means 10 is controlled by a switching gate signal control means 12 built in the control section 11, switches between the reference capacitance and the detection capacitance, connects it to the oscillation circuit 9, and controls the oscillation frequency f and . The outputs of the counter means 13 are input to the counter means 13 of the RAM.
14 is stored at a predetermined address, and transferred to the arithmetic means 15 where it is subjected to division as an arithmetic process to obtain the ratio r.

r=L/fp (2) FIG. 2 is a diagram showing the relationship between each frequency and ratio. As the pressure P increases, the deflection of the substrate increases,
Since the distance between the electrodes becomes smaller, the capacitance value becomes larger according to the following equation.

C=εS/d −・−(3) However, C: Capacity between electrodes S: Electrode area d: Distance between electric bridges The frequency f can be found from the following formula, so as the pressure p increases, the frequency r decreases. .

r = K/RC・-・・・・(4) However, K: From the relationship between the ratio r of each circuit constant and the pressure ρ, the pressure p is a high-order approximation formula,
For example, it can be seen that it can be obtained by calculating the following quadratic expression.

p=a r" + b r 0 c -・yama - (5) However, a, b, c: constants Here, we will explain why the ratio r was calculated using equation (2). Expanding equation (2), we get the following equation It is as follows.

Since a single oscillation circuit is used as the detection circuit, the circuit constants f and f are the same, and the resistance R is also common, so the frequency ratio r is detected as shown in equation (6). It is the ratio of the capacitance C, and the reference capacitance C, .

Since the temperature characteristics of the two are almost the same as described above, the temperature characteristics can be removed by calculating the pressure p based on the ratio r.

FIG. 3 shows an example of a specific circuit configuration of such a system. The control unit 11 is formed by a microcomputer and outputs an output signal as a switching gate signal control means.

However, TC is provided as an input terminal of the built-in counter means.

The detection means 9 is formed by a combination of a sawtooth wave generation circuit and a waveform shaping circuit of an operational amplifier.

Although the switching means 10 is constituted by an analog switch, it can also be realized by other semiconductor switching means or by a relay.

Reference numeral 16 denotes a level shift circuit for voltage conversion and waveform shaping, which may be added as appropriate.

For example, the analog switch can be implemented with a μPC4066, the operational amplifier can be implemented with a TLO82, and the microcontroller can be implemented with an M888515, but it goes without saying that any device with equivalent functions can be used.

FIG. 4 is a flowchart showing a control program for such a microcomputer.

When pressure measurement is started, the gate signal E is first changed to H level (a), and after some delay time is appropriately inserted (b), the built-in counter connected to the TC terminal is activated. (C) Measurement of the reference frequency f is started.

The gate time of the counter, for example 1 second, is managed using a timer interrupt (d), and when this predetermined time has elapsed, the counter is stopped (e).

The counting result r1 is transferred to a predetermined address in the RAM and stored).

Next, the gate signal E0 is changed to the L level (hereinafter referred to as f), and r is measured in exactly the same manner as (c) to (1).

Through such processing, rl and r stored in the RAM are then subjected to division processing to first calculate r (e), and then pressure p is calculated using a quadratic approximation formula based on the ratio. (n).

The pressure p determined by the above procedure is not affected by temperature at all, as described above.

Effects of the Invention As described above, the pressure detection device of the present invention does not require a power supply voltage of 1, as compared to the conventional one. temperature-compensated power supplies, eliminating the need for complex and expensive power supplies.

In addition, the resistor R and oscillation circuit 9 used for detecting the detection capacitor C9 and the reference voltage 8iC are exactly the same, so in principle, there is no need to use components with uniform temperature characteristics as in the past. No temperature characteristics appear.

Furthermore, the output frequency of the oscillation circuit can be directly input to the counter, and there is no need to convert it to a DC voltage using a low-pass filter as in the past (this makes the circuit configuration simple and inexpensive).

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the configuration of a pressure detection device according to the present invention, FIG. 2 is a diagram showing pressure, each frequency, and their ratio according to the present invention, and FIG. 3 is a diagram showing one embodiment of the configuration of a pressure detection device according to the present invention. A circuit diagram showing a specific example, FIG. 4 is a flowchart showing the structure of a control program, FIG. 5(a) is a cross-sectional view of a capacitance type sensor, FIG. FIG. 2 is a circuit diagram of a conventional example. 4... Detection electrode, 5... Reference electrode, 9... Detection means, 10... Switching means, 11... Control unit,
13...Counter means, 15...Calculation means. Name of agent: Patent attorney Shigetaka Awano and 1 other person9...
...Single detection means 10 ...Switching means 11 ...
・Control 71 part 13 ・・・・・ Counter means 15 −・・・・ Calculating means Fig. 2 M3 Fig. Hill power P Fig. 4 Fig. 5 (d)

Claims (1)

[Claims] A capacitive pressure sensor formed of a pair of flat plates facing each other with a predetermined gap and having a detection electrode in the center and a reference electrode on the outer periphery thereof, and a detection means for reading the capacitance of the detection electrode and the reference electrode. and a control section that switches and measures the capacitance of the detection electrode and the reference electrode via the detection means, and the control section has a calculation means and calculates the measured capacitance values of the detection electrode and the reference electrode. A pressure detection device configured to perform calculation processing using the calculation means and calculate the pressure applied to the capacitance type pressure sensor from the calculation result.