JPH02107936A - Pressure detection circuit - Google Patents

Abstract

PURPOSE:To enable a better detection with a stable accuracy by providing a capacitor connected to a detection electrode capacitance at one end and an earth at the other end to make a charged/discharged current flow to the capacitance from both positive and negative power sources of a saw-tooth wave generation circuit. CONSTITUTION:One end of a capacitor 7 is connected to an output end of a saw-tooth wave generation circuit 1 using an operational amplifier while the other end thereof is connected to an earth. The operational amplifier is operated with both positive and negative power sources + or -B. With such an arrangement, since, if a change takes place in a floating capacitance of a circuit, charged or discharged current flows through the capacitor 7 from the power source against the stable frequency of the saw-tooth wave can be maintained. Only connecting the capacitor 7 with a slight capacity value can suppress the change to about one-tenth thereby enabling increase in resolving power by one digit as compared with that in the past, and as a consequence, can expand a measuring range as much. However, considering that an abnormal oscillation occurs as the capacity of the capacitor 7 increases, there exists an optimum range for the capacity of the capacitor 7.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a pressure detection circuit that uses a change in capacitance to detect pressure applied to a sensor.

2. Description of the Related Art A large number of capacitive pressure sensors have already been put into practical use, in which two-page electrodes are placed facing each other with a predetermined gap maintained, and the pressure applied thereto is detected as a change in capacitance.

FIG. 4 shows a detection circuit using such a conventional capacitance pressure sensor. A method in which a CR oscillation circuit is formed by the capacitance C of the electrode and the resistor R, and the change in capacitance is read out as a frequency has an advantage in terms of resolution and has been widely used.

Further, such a CR oscillation circuit is often formed by a combination of a sawtooth wave generation circuit 1 using an operational amplifier and an amplifier circuit 2 for waveform shaping. This is because the oscillation operation of this oscillation circuit is stable, and an abnormality in which oscillation stops for some reason is unlikely to occur.

Problems to be Solved by the Invention However, in such a conventional configuration, the capacitance value C of the capacitive pressure sensor is usually very small, about 10 to 50 pF, and is therefore easily affected by the stray capacitance of the circuit.

Page 3 For this reason, when the output frequency f was selected to be between 7 and 7, the frequency fluctuated between 7 and 7.

Therefore, if the resolution (g/Hz) is not reduced, the weight variation will increase and it will become impractical.

For this reason, the weighing range often becomes narrow, measurement accuracy deteriorates, and measurement reproducibility is impaired.

Means for Solving the Problems In order to solve the above problems, the present invention provides a capacitive pressure sensor formed from a pair of flat plates having detection electrodes facing each other with a predetermined gap therebetween, and a capacitance pressure sensor formed by the detection electrodes. a sawtooth wave generation circuit that includes a capacitance to be generated, an amplifier circuit that shapes the sawtooth wave, a certain resistor that returns the output of the amplifier circuit to the input terminal of the sawtooth wave generation circuit, one end of which is a detection electrode capacitor, and the other end of which is a Consisting of a capacitor connected to ground, the sawtooth wave generating circuit is configured to operate with both positive and negative power supplies.

Operation With the above configuration, in the pressure detection circuit of the present invention, charge/discharge current flows from both the positive and negative power supplies of the sawtooth wave generation circuit into the capacitor whose one end is connected to the detection electrode capacitance and the other end is connected to the ground, thereby preventing fluctuations in stray capacitance. Absorbs the output frequency patter caused by

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

FIG. 2 shows a cross section of a capacitive pressure sensor according to the present invention.

The substrate 3 and the diaphragm 4 are formed of insulating flat plates such as alumina, and are surrounded by a sealing material 5 while maintaining a certain gap d.
It is sealed with. A detection electrode 6 is formed on the substrate 3 and diaphragm 4 .

When a load P is applied to the diaphragm 4, the diaphragm bends as shown, and the capacitance formed by the detection electrode changes.

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

One end of a capacitor 7 is connected to the output end of a sawtooth wave generating circuit 1 using an operational amplifier, and the other end is connected to ground. The operational amplifier is operated with both positive and negative power supplies ±B.

In such a configuration, even if the stray capacitance of the circuit fluctuates, a charging/discharging current flows from the power supply through the capacitor 7 in response to the fluctuation, so that the frequency of the sawtooth wave can remain stable.

FIG. 3 is a diagram showing the oscillation frequency of such a circuit, where the horizontal axis shows the capacitance value of the capacitor 7, and the vertical axis shows the amount of frequency fluctuation.

From this diagram, it can be seen that by simply connecting a capacitor 7 with a small capacitance value, the fluctuation can be suppressed to about 1/10. That is, the resolution can be increased by an order of magnitude compared to the conventional method, and the measurement range can be expanded by one order of magnitude.

However, it was observed that when the capacitance of capacitor 7 was further increased, abnormal oscillation occurred.

In one example, when the capacitance of the sensor was 20 pF, a TLO62 was used as the operational amplifier, and a ceramic capacitor was used as the capacitor, abnormal oscillation occurred when the capacitance exceeded 1000 PF. I believe this is because the frequency characteristics of capacitor 7 cannot be ignored (see page 6). In other words, there is an optimal range for the capacitance of the capacitor.

Effects of the Invention As described above, the pressure detection circuit of the present invention has less variation in output frequency than the conventional one, and can stably detect pressure. Furthermore, the reproducibility of measurements is high, detection can be performed with good accuracy, and the measurement range can be greatly expanded.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing the configuration of a pressure detection circuit according to an embodiment of the present invention, Figure 2 is a sectional view showing the configuration of the capacitance type sensor, and Figure 3 is the capacitance and oscillation frequency of the capacitor. FIG. 4 is a circuit diagram of a conventional example. 1- sawtooth wave generation circuit, 2- waveform shaping amplifier circuit, 7- capacitor. Name of agent: Patent attorney Shigetaka Awano (1 person) Figure 3 (Desk Figure 1) - Figure 2 Figure 4

Claims (1)

[Claims] A capacitance type pressure sensor formed of a pair of flat plates having detection electrodes facing each other with a predetermined gap therebetween; a sawtooth wave generation circuit including a capacitance formed by the detection electrode; and a sawtooth wave generation circuit for shaping the sawtooth wave. It consists of an amplifier circuit and a resistor that returns the output of the amplifier circuit to the input terminal of the sawtooth wave generating circuit. A capacitor is connected to one end of the detection electrode capacitor, the other end is connected to ground, and the output of the sawtooth wave generator is connected to the capacitor. The generation circuit is a pressure detection circuit configured to operate with both positive and negative power supplies.