JPH0545241A - Pressure sensing circuit - Google Patents

Abstract

PURPOSE:To secure a pressure sensing circuit with ability of stable and certain sensing of pressure, and enhance the reliance upon the foreign matter pinch sensing. CONSTITUTION:A first amplifier 1 has a pressure sensor 2, which is connected between the inversive input end and the ground and whose resistance value Rs varies in compliance with the pressurization force P, and also a feedback resistance 3 in connection between the inversive input end and the output end, which is connected with the output terminal 5 and a second amplifier 6 inversive input end. At the non-inversive input end of this second amplifier 6, a reference voltage source 7 is connected which generates a constant reference voltage Eref, and the output end is connected with a capacitor 10 and a voltage follower 11 through a resistance 8 and an analog switch 9. The other end of this capacitor 10 is grounded, and the output end of the voltage flower 11 is connected with the non-inversive input end of the No.1 amplifier 1. A voltage holding means to hold the input voltage Ei of the No.1 amplifier 1 is composed of these named voltage follower 11, capacitor 10, and analog switch 9 opened and closed at certain timings in conformity to switch control signals SW.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure detecting device suitable for use in an automatic opening / closing device such as a power window device of an automobile.

[0002]

2. Description of the Related Art In an automatic opening / closing device (for example, a power window device for an automobile) in which a moving body is moved by driving a motor, a motor of a motor is used when a moving body such as a window glass is closed when foreign matter is caught. To stop the drive,
Detection of entrapment of foreign matter. To detect such entrapment of foreign matter, the pressure applied to the foreign matter is detected by a pressure-sensitive sensor using pressure-sensitive conductive rubber or pressure-sensitive conductive paint.

The electrical characteristics of a pressure-sensitive sensor using pressure-sensitive conductive rubber or pressure-sensitive conductive paint used for pressure detection are generally such that the resistance value corresponding to the applied pressure P is Rs. As the pressure P increases, the resistance value Rs decreases. Usually, when K and N are positive constants, Rs∝KP ^-N ............ (1) On the basis of the constant relationship represented by the above formula (1) between the pressing force P and the resistance value Rs of the pressure sensitive sensor, the conventional pressure detection circuit using this type of pressure sensitive sensor A voltage corresponding to the resistance value Rs of the pressure sensor is output as a pressure detection signal.

[0004]

By the way, the resistance value of the pressure-sensitive sensor not only varies due to manufacturing errors among products, but also changes with ambient temperature even in one pressure-sensitive sensor, and further, with the passage of time. Also changes. Therefore, according to the above equation (1), the values of K and P vary or change.

Further, in the power window device of an automobile, when the temperature decreases, the torque of the drive motor decreases,
Due to deformation of the window frame or adhesion of frost to the glass, the load during operation of the power window becomes heavy, and the closing speed of the window becomes slow. In that case, even if a foreign object is caught between the window and the window frame, the pressure is slowly applied to the pressure sensor and the resistance value of the pressure sensor changes slowly. Reliability has decreased.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a pressure detection circuit which solves the above-mentioned problems, guarantees stable and reliable pressure detection, and improves the reliability of foreign matter entrapment detection.

[0007]

In order to achieve the above object, the pressure detecting circuit of the present invention is such that a pressure sensitive sensor whose resistance value changes in accordance with a pressing force is connected between an inverting input terminal and ground.
1st which connected the negative feedback resistance between the inverting input terminal and the output terminal
, An output terminal of the first amplifier connected to the inverting input terminal and a reference voltage source connected to the non-inverting input terminal, and an output terminal of the second amplifier and the first amplifier. A voltage holding means provided between the non-inverting input terminal of the amplifier and for holding the voltage of the non-inverting input terminal of the first amplifier at a predetermined timing, A corresponding detection signal is obtained from the output terminal of the first amplifier.

[0008]

According to this structure, the first amplifier in the steady state is connected to the non-inverting input terminal so that the output voltage becomes equal to the reference voltage even if the resistance value of the pressure-sensitive sensor when the pressure is not applied varies. The applied input voltage is automatically adjusted. At the time when a moving body such as a power window starts to close, the voltage holding means operates at the same time, and the first amplifier applies a substantially constant hold voltage to the non-inverting input terminal, and the first amplifier receives a pressure-sensitive signal from the output terminal. A voltage that is dependent on the resistance value of the sensor when not pressurized, that is, a voltage equal to the reference voltage, is output.

When a foreign object is caught in the closing window, pressure is applied to the pressure sensor, and as a result, the resistance value of the pressure sensor decreases. Since the first amplifier has a large amplification factor and the input voltage is constant, the output voltage rises. When the pressure applied to the pressure-sensitive sensor increases, the resistance value of the pressure-sensitive sensor further decreases, and the first
The output voltage of the amplifier exceeds the predetermined foreign matter detection voltage (set value), and as a result, the power window device is stopped.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the pressure detecting circuit of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of an embodiment of the pressure detection circuit of the present invention. FIG. 2 is a voltage waveform diagram for explaining the pressure detecting operation according to this embodiment.

In FIG. 1, the first amplifier 1 is connected with a pressure-sensitive sensor 2 whose resistance value Rs changes according to the applied pressure P between the inverting input terminal and the ground, and connects the inverting input terminal and the output terminal. A negative feedback resistor 3 is connected in between, and the output terminal is connected to the output terminal 5 of the pressure detection circuit according to the present invention and the inverting input terminal of the second amplifier 6. A reference voltage source 7 for generating a constant reference voltage Eref is connected to the non-inverting input terminal of the amplifier 6.

The output terminal of the second amplifier 6 is connected to one end of the capacitor 10 and the input terminal of the voltage follower 11 via the resistor 8 and the analog switch 9, and the other end of the capacitor 10 is grounded. Voltage Follower 1
The output of 1 is connected to the non-inverting input of amplifier 1. The resistors 8 and the capacitors 10 form a time constant circuit having a time constant τ where τ = C · R, where R and C are values, and a time constant τ of 10 minutes or more is set.

The analog switch 9 is opened and closed by a switch control signal SW from a control circuit (not shown). When the pressure detection circuit of the present embodiment is applied to a power window device, the analog switch 9 is turned off (open) during the closing operation of the power window, and is turned on (closed) during other times. Controlled.

The voltage follower 11 is an operational amplifier having an extremely high input impedance and an amplification factor of 1, and outputs a voltage always equal to the input voltage to the output end. Therefore, the first
The voltage Ei which is always equal to the voltage across the capacitor 10 is applied to the non-inverting input terminal of the amplifier 1. In this embodiment, the input voltage E of the operational amplifier 1 is set by the analog switch 9, the capacitor 10 and the voltage follower 11.
A voltage holding means for holding i is configured.

In the first amplifier 1, the resistance value of the pressure sensitive sensor 2 is Rs, the resistance value of the negative feedback resistor 3 is Rf, the voltage input to the non-inverting input terminal is Ei, and the output voltage of the output terminal is Eo.
Then, the following equation is obtained. Eo = [(Rs + Rf) / Rs] × Ei …………………… (2)

On the other hand, in the second amplifier 6, in a steady state in which the analog switch 9 is closed, for example, the resistance value of the pressure sensitive sensor 2 is decreased, and the output voltage Eo applied to the inverting input terminal is changed to the reference voltage Eref. When the voltage is forcibly increased, the voltage at the output end drops from a predetermined equilibrium voltage, the capacitor 10 is discharged via the resistor 8, and the input / output voltage of the voltage follower 11 is lowered. .. Therefore, the first amplifier 1 lowers the output voltage Eo at the output end by lowering the input voltage Ei, and finally converges the output voltage to Eref. This means that in the steady state, the output voltage Eo of the first amplifier 1 becomes equal to the reference voltage Eref regardless of the resistance value of the pressure sensitive sensor 2 when the pressure sensor 2 is pressed or unpressurized. Like the second
It means that the output voltage of the amplifier 6 is automatically adjusted, and thus the input voltage of the voltage follower 11 is automatically adjusted after a predetermined time delay.

Further, since a constant reference voltage Eref is constantly supplied from the reference voltage source 7 to the non-inverting input terminal of the amplifier 6, the output voltage E supplied to the inverting input terminal in a steady state.
o converges to a value equal to the reference voltage Eref. Therefore, in the steady state, the voltage Ei obtained at the non-inverting input terminal of the first amplifier 1 is Ei = Eref × Rs / (Rs + Rf) (3). In the steady state, while the analog switch 9 is closed, the average value voltage from the output terminal of the second amplifier 6, the voltage across the capacitor 10, and the output voltage of the voltage follower 11, that is, the first amplifier. The input voltage Ei of 1 is the same voltage.

Next, the pressure detecting operation in this embodiment will be described with reference to FIG. In FIG. 2, when the closing operation of the power window is started at time t0, the analog switch 9 which has been in the on state until then is simultaneously turned off. As a result, the ground capacitor 10 is cut off from the output end of the amplifier 6 and connected to the input of the voltage follower 11. However, since the input impedance of the voltage follower 11 is very high, almost no charge is discharged. do not do. As a result, the voltage across the analog switch 9
Is held at a steady value immediately before is turned off, that is, until the closing operation of the power window is started. Therefore, the voltage
The input voltage E of the first amplifier 1 from the output terminal of the follower 11
i is also held at the steady value of the equation (3), that is, the reference value.

Now, in FIG. 2, at time t ₁ when the power window is closed, foreign matter is caught between the window and the window frame, and when pressure starts to be applied to the pressure sensor 2, the pressure sensor 2 moves. The resistance value Rs decreases. As a result, the amplification factor of the first amplifier 1 increases, and the input voltage Ei held at the steady value is constant, so that the time t
_The output voltage Eo, which was kept at the steady value Eref up to ₁ before, rises. When the output voltage Eo rises, in the second amplifier 6, the voltage at the output end becomes lower than the reference voltage Eref, but since the analog switch 9 is in the off state,
It has no effect on the voltage across the capacitor 10.

As described above, when the output voltage Eo rises as the pressure applied to the pressure sensitive sensor 3 increases, and eventually reaches the set value Es (time t ₂ ), the control circuit stops the closing operation of the power window. Then, the power window is opened to release the entrapment of the foreign matter, and after a while, the analog switch 9 is turned on. As a result, the capacitor 10 is supplied from the output terminal of the second amplifier 6 via the resistor 8 with the electric charge corresponding to the voltage drop consumed by the self-leakage current or the bias current of the voltage follower 11.

On the other hand, when the power window is opened, the pressure-sensitive sensor 3 is gradually released from the pressing force, and the resistance value Rs rises toward the value before the pressurization, so that the first amplifier 1 is operated. The amplification factor returns to the original value. Eventually, when the resistance value Rs of the pressure sensor 3 returns to the value before pressurization, the first amplifier 1
The output voltage Eo returns to the steady value (reference value) Eref, and the input voltage Ei also returns to the steady value represented by the equation (3).

At the time when the output voltage Eo reaches the set value Es (time t ₂ ), the closing operation of the power window is simply stopped, that is, the foreign matter is trapped (the pressure sensitive sensor 3
When the analog switch 9 is turned on with the pressure still applied to the reference voltage E, the output voltage of the first amplifier 1 becomes
There is a risk that it will gradually decrease to ref. Therefore, the analog switch 9 is controlled so as not to be turned on unless the power window is opened.

As described above, in the pressure detecting circuit of the present embodiment, regardless of the magnitude of the resistance value Rs of the pressure-sensitive sensor 2, that is, even if the resistance value Rs varies, it depends on the ambient temperature or with the passage of time. Even if the resistance value Rs changes, the output voltage Eo is held at a value equal to the reference voltage Eref when the pressure-sensitive sensor 2 is not pressed or is held at a constant pressure. When pressure is applied to the pressure-sensitive sensor 2 due to the entrapment of a foreign substance, the output voltage Eo changes from the value of the reference voltage Eref, so the pressure can be detected from this voltage change. Therefore, even if the resistance value Rs of the pressure-sensitive sensor 2 varies due to manufacturing, temperature change, or time-dependent change, a constant pressure detection signal that is not affected by the change can be obtained. As a result, highly reliable foreign matter entrapment is possible. Detection is done.

Further, in the pressure detecting circuit of this embodiment, the analog switch 9 is turned off at the same time when the power window closing operation is started, and the input voltage Ei of the first amplifier 1 is held at a steady value. Even if the closing speed of the power window becomes slow due to a decrease or the like, the pressure detection signal can be obtained early and reliably.

In this case, the differential value of the output voltage Eo may be detected. That is, when the closing speed of the window becomes slow, even if a foreign object is sandwiched between the window and the window frame, the pressure sensor 2 is slowly pressed, and the resistance value Rs of the pressure sensor 2 is slowly decreased. The amplification factor of the amplifier 1 slowly increases, but since the input voltage Ei is constant, the voltage increase rate of the output voltage Eo changes. When the rate of increase in voltage exceeds a predetermined value, entrapment of foreign matter can be reliably detected and the power window opening control device can be operated.

In this embodiment, as the first amplifier 1, a FET-type operational amplifier having a considerably high input impedance is used, the voltage follower 11 is omitted, and the capacitor 10 is connected to the non-inverting input terminal of the amplifier 1. May be directly connected to. Further, the pressure detection circuit of the present embodiment is shown as an unbalanced type, but two circuits shown in FIG. 1 may be combined to form a balanced type. In this case, the pressure-sensitive sensor 2, the reference voltage source 7, and the capacitor 10 are composed of the inverting input terminal of the first amplifier, the non-inverting input terminal of the second amplifier, and the voltage follower corresponding to the ground terminal in the unbalanced type. Are respectively connected to the input ends of. In addition, balanced output voltages that are strong against external noise are obtained from the output terminals of the two sets of first amplifiers.

[0027]

Since the pressure detection circuit of the present invention is constructed as described above, even if the resistance value of the pressure-sensitive sensor varies due to manufacturing, temperature change or aging, it is affected. It is possible to obtain a stable pressure detection signal. Further, even if the closing speed of the moving body such as the power window becomes slow, the pressure detection signal can be reliably obtained. Therefore, the reliability of entrapment of a foreign substance can be improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of a pressure detection circuit of the present invention.

FIG. 2 is a signal waveform diagram showing a pressure detection operation of the circuit shown in FIG.

[Explanation of symbols]

 1 First Amplifier 2 Pressure Sensitive Sensor 3 Negative Feedback Resistor 6 Second Amplifier 7 Reference Voltage Source 9 Analog Switch 10 Capacitor 11 Voltage Follower

Claims (1)

[Claims] 1. A first amplifier in which a pressure-sensitive sensor whose resistance value changes according to a pressing force is connected between an inverting input terminal and a ground, and a negative feedback resistor is connected between the inverting input terminal and an output terminal. A second amplifier having an output terminal of the first amplifier connected to the inverting input terminal and a reference voltage source connected to the non-inverting input terminal, and an output terminal of the second amplifier and a non-input terminal of the first amplifier. A voltage holding unit that is provided between the pressure sensor and a non-inverting input terminal of the first amplifier and holds the voltage of the non-inverting input terminal of the first amplifier at a predetermined timing. A pressure detecting circuit, wherein a signal is obtained from the output terminal of the first amplifier.