JPH0823514B2 - Pressure detector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure detection device of a type that detects a pressure applied to a sensor by utilizing a change in capacitance.

2. Description of the Related Art A large number of capacitance-type pressure sensors have been put to practical use, in which electrodes are opposed to each other with a predetermined gap therebetween and the pressure applied to the electrodes is taken out as a change in capacitance.

Since the capacitance value of such a sensor is usually as small as about 20 to 50 pF, a large error occurs due to the temperature characteristics of the detection circuit and the sensor itself. Therefore, how to solve the change in characteristics due to temperature was an important issue in this type of sensor.

For this reason, a capacitance is provided in the center of the substrate where the capacitance value changes sharply according to the applied pressure, and a reference electrode whose capacitance value does not change much depending on the pressure applied to the outer periphery Mold pressure sensors are generally known.

The capacitance type pressure sensor disclosed in Japanese Patent Laid-Open No. 58-198739 is an example of such a sensor. By charging and discharging the above two capacitances, it is possible to reduce the temperature characteristics of the sensor and the circuit. I'm trying.

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

Two electrodes are printed on the substrates 1 and 2 made of alumina, and the peripheral edges thereof are sealed with glass 3 so as to face each other with a constant gap. For this reason, the detection electrode 4 at the center changes sensitively to the external pressure, while the reference electrode 5 at the outer periphery is close to the glass and is not easily bent, and the pressure change due to the 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 almost in the same manner. Therefore, by comparing the two, it is possible to remove only the influence of temperature and obtain only pressure information.

FIG. 6 shows such a conventional circuit configuration. The capacitance C _p of the detection electrode 4 and the capacitance C _r of the reference electrode 5 are the resistances R ₁ and R ₂ respectively.
And a charge / discharge circuit is configured. Both capacitors are turned on and off by the transistors Q ₁ and Q ₂ , and charging and discharging are repeated.
The operation and the waveform of each part are omitted because they are detailed in the reference example, but if the power supply voltage is V _CC , the output voltage V _out of the low-pass filter 6 is V _out = V _CC (1-C _r / C _p ). … (1). Since the temperature characteristics of the sensor are almost the same as already described, the change in capacitance due to temperature ΔC _r (ppm /
C) and ΔC _p (ppm / ° C.) are canceled by the (1-C _r / C _p ) term to remove the temperature characteristic of the sensor from the output voltage V _out .

At this time, however, R ₁ = R ₂ , and it is necessary to select those having the same temperature characteristics as the pair 8 with the comparator 7 and the transistors Q ₁ and Q ₂ to form a pair.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in such a conventional configuration, the temperature characteristic of the sensor can be suppressed to a small value, but the temperature characteristic of the circuit cannot be completely removed.

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

Next, since R ₁ and R ₂ cannot usually be completely coincident with each other, the temperature characteristic is generated by the unbalanced amount.

It is very troublesome in actual mass production to select the comparators 7 and 8 and the transistors Q ₁ and Q ₂ that have the same temperature characteristics to form a pair, and to make the temperature characteristics completely the same. That is a difficult technique.

Further, the output frequency of the comparator cannot be handled as it is, and the temperature characteristic cannot be canceled unless it is once converted into a DC voltage by a low pass filter. Therefore, the circuit configuration becomes complicated and expensive.

Means for Solving the Problems In order to solve the above problems, the present invention is formed by a pair of flat plates facing each other with a predetermined gap, a detection electrode in the central part, and a flat plate having a reference electrode in its outer peripheral part. Capacitance type pressure sensor, single oscillation circuit that detects the capacitance of the detection electrode and reference electrode, switching means that switches the capacitance of the detection electrode and reference electrode to connect to the oscillation circuit, and measurement through the oscillation circuit In the pressure detection device of the present invention, the control unit counts the output frequency of the oscillation circuit corresponding to the capacitances of the detection electrode and the reference electrode by the counter unit and calculates the ratio by the calculation unit. At the same time, the pressure applied to the sensor is calculated based on the solution.

Example Hereinafter, a pressure detecting device according to the present invention will be described with reference to the drawings.

The configuration of the sensor is the same as that of the conventional one shown in FIG. 2 and has already been described. Therefore, the description is omitted here by avoiding duplication.

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

The single detecting means is composed of a CR oscillation circuit 9 including a reference capacitance C _r , a detection capacitance C _p and a resistor R. The switching means 10 is a switching gate signal control means built in the control unit 11.
Controlled by 12, the reference capacitance and the detection capacitance are switched and connected to the oscillation circuit 9, and the oscillation frequencies f _r and f _p are input to the counter means 13 in the control unit 11. Then, the output of the counter means 13 is stored in a predetermined address of the RAM 14 respectively, transferred to the arithmetic means 15 and subjected to division as arithmetic processing to obtain the ratio r.

_{r = f r / f p ......} (2) FIG. 2 is a graph showing the relationship between the frequencies and ratios such. As the pressure p increases, the deflection of the substrate increases and the distance between the electrodes decreases, so that the capacitance value increases from the following equation.

C = ε S / D (3) However, C: capacitance between electrodes S: electrode area d: distance between electrodes Since frequency f is obtained from the following equation, frequency f decreases conversely as pressure p increases. .

f = K / RC (4) where K: Circuit constant From the relationship between the ratio r and the pressure p, the pressure p can be obtained by calculating a high-order approximation formula, for example, the following quadratic formula. Recognize.

p = ar ² + br + c (5) However, a, b, c: constants Here, the reason why the ratio r is obtained by the equation (2) will be described. When the formula (2) is expanded, it is as follows.

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

Since the temperature characteristics of both 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 concrete circuit configuration of such a system. The control unit 11 is formed by a microcomputer, and is provided with an output E _o as a switching gate signal control means and a TC as an input terminal of a built-in counter means.

The detection means 9 is formed by a combination of a sawtooth wave generation circuit of an operational amplifier and a waveform shaping circuit. The switching means 10 is composed of an analog switch,
This can be achieved with other semiconductor switching means as well as with relays.

Reference numeral 16 is a level shift circuit that performs voltage conversion and waveform shaping, and may be appropriately added as necessary.

For example, analog switch is μPC4066, operational amplifier is
The TL082 and the microcomputer can be realized by MB88515, but it goes without saying that they can be used as long as they have a function equivalent to this.

FIG. 4 is a flow chart showing a control program of such a microcomputer.

When the pressure measurement is started, the gate signal E _o is first changed to the H level (a). And after a slight delay time is appropriately inserted (b), built-in counter which is connected to the TC terminal is started (c), the measurement of the reference frequency f _r is started.

Then, the gate time of the counter, for example, 1 second, is managed using a timer interrupt or the like (d), and when this predetermined time has elapsed, the counter is stopped (e). The counting result _fr is transferred to a predetermined address of the RAM and stored (f).

Then the gate signal E _o is turned to L level (g), the measurement of the f _p is carried out in exactly the same procedure as below f _r (h)
~ (L).

Through such processing, f _r and f _p stored in the RAM are then subjected to division processing to calculate r first (m), and then the pressure p is calculated by the quadratic approximation formula based on the ratio r. (N).

The pressure p obtained by the above procedure is not affected by temperature as described above.

EFFECTS OF THE INVENTION As described above, the pressure detection device of the present invention is not affected by the power supply voltage V _CC as in the conventional case, and it is not necessary to use a temperature-compensated complex and expensive power supply.

Further, since the detection capacitance C _p , the resistance R and the oscillation circuit 9 used for detecting the reference electrode C _r are exactly the same,
As in the past, there is no trouble of pairing those having the same temperature characteristics, and in principle, the temperature characteristics do not appear.

Furthermore, the output frequency of the oscillator circuit can be input to the counter as it is, and it is not necessary to convert it to DC voltage with a low-pass filter as in the past, so the circuit configuration is simple and inexpensive.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the configuration of a pressure detecting device according to the present invention, FIG. 2 is a diagram showing pressure and respective frequencies and ratios thereof according to the present invention, and FIG. A circuit diagram showing a specific example, FIG. 4 is a flowchart showing the structure of a control program, FIG. 5 (a) is a sectional view of a capacitance type sensor, FIG. 5 (b) is the same developed view, and FIG. It is a circuit diagram of a prior art example. 4 ... Detection electrode, 5 ... Reference electrode, 9 ... Detection means, 10
...... Switching means, 11 ...... Control section, 13 ...... Counter means, 15 ...... Computing means.

Continuation of front page (72) Inventor Kenzo Ochi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-58-198739 (JP, A) JP-A-57-168398 (JP) , A) Actually open 59-10014 (JP, U) Actually open 63-99217 (JP, U)

Claims (1)

[Claims] 1. A capacitance type pressure sensor comprising a pair of flat plates facing each other with a predetermined gap and having a detection electrode at a central portion and a reference electrode on an outer peripheral portion thereof, and the detection electrode and the reference electrode. The control unit includes a single oscillating circuit that detects a capacitance, a switching unit that switches the capacitances of the detection electrode and the reference electrode to connect to the oscillating circuit, and a control unit that measures through the oscillating circuit. A counter means for counting the output of the oscillation circuit is provided, the frequencies corresponding to the capacitances of the detection electrode and the reference electrode are counted, the ratio thereof is calculated by the arithmetic means, and the capacitance type pressure sensor is based on the ratio. A pressure sensing device configured to calculate the pressure applied to the.