JP3056294B2 - Pressure detection circuit - Google Patents

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 in which a moving body is moved by driving a motor (for example, a power window device of an automobile), when a moving object such as a window glass is closed by a foreign object when the moving body is closed. To stop driving,
Detects foreign object entrapment. In order to detect such foreign matter being caught, a pressure applied to the foreign matter is detected by a pressure-sensitive sensor using a pressure-sensitive conductive rubber or a pressure-sensitive conductive paint.

The electrical characteristics of a pressure-sensitive sensor using a pressure-sensitive conductive rubber or a pressure-sensitive conductive paint used for detecting pressure as described above generally indicate that a resistance value corresponding to a pressure P is Rs. As the pressure P increases, the resistance value Rs decreases. Normally, when K and N are each a positive constant, the resistance value is represented by Rs∝KP− ^N (1). Based on the fact that there is a certain relationship expressed by the above equation (1) between the pressure P and the resistance value Rs of the pressure sensor, in a conventional pressure detection circuit using this type of pressure sensor, A voltage corresponding to the resistance value Rs of the pressure sensor is output as a pressure detection signal.

[0004]

Incidentally, the resistance value of the pressure-sensitive sensor not only varies from product to product due to manufacturing errors, but also varies with one pressure-sensitive sensor depending on the ambient temperature. Also change. Therefore, when looking at the above equation (1), the values of K and P vary or change.

Further, in a power window device of an automobile, when the temperature decreases, the torque of the drive motor decreases,
Due to the deformation of the window frame or the adhesion of frost to the glass, the load during the operation of the power window becomes heavy, thereby slowing down the closing speed of the window. In this case, even if a foreign object is caught between the window and the window frame, the pressure is slowly applied to the pressure-sensitive sensor, and the resistance value of the pressure-sensitive sensor changes slowly, making it difficult to reliably detect the pressure. Reliability decreased.

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

[0007]

In order to achieve the above object, a pressure detecting circuit according to the present invention comprises a pressure-sensitive sensor having a resistance value which varies according to a pressing force, connected between an inverting input terminal and ground,
A first feedback resistor connected between the inverting input terminal and the output terminal;
, An output terminal of the first amplifier is connected to an inverting input terminal, a reference voltage source is connected to a non-inverting input terminal, and an output terminal of the second amplifier is connected to the first amplifier. A voltage holding means provided between the non-inverting input terminal of the amplifier and the non-inverting input terminal of the first amplifier at a predetermined timing; The corresponding detection signal is obtained from the output terminal of the first amplifier.

[0008]

According to this configuration, 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 sensor is not pressurized varies. The applied input voltage is automatically adjusted. At the time when the moving body such as the 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. A voltage dependent on the resistance value of the sensor when the sensor is 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-sensitive sensor, and as a result, the resistance value of the pressure-sensitive sensor decreases. In the first amplifier, the amplification factor is large and the input voltage is constant, so that the output voltage increases. As the pressure applied to the pressure sensor increases, the resistance of the pressure sensor further decreases, and
Output voltage of the amplifier exceeds a predetermined foreign object 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 according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of one embodiment of the pressure detection circuit of the present invention. FIG. 2 is a voltage waveform diagram for explaining the pressure detection operation according to this embodiment.

In FIG. 1, a first amplifier 1 is connected between an inverting input terminal and a ground, a pressure-sensitive sensor 2 whose resistance value Rs changes in accordance with a pressing force P, and is connected between an inverting input terminal and an output terminal. A negative feedback resistor 3 is connected therebetween, 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 a non-inverting input terminal of the amplifier 6.

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

The analog switch 9 opens and closes in response to 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 a very high input impedance and an amplification factor of 1, and outputs a voltage always equal to the input voltage to an output terminal. Therefore, the first
A non-inverting input terminal of the amplifier 1 is supplied with a voltage Ei that is always equal to the voltage across the capacitor 10. In the present embodiment, the input voltage E of the operational amplifier 1 is controlled by the analog switch 9, the capacitor 10, and the voltage follower 11.
Voltage holding means for holding i is configured.

In the first amplifier 1, the resistance of the pressure-sensitive sensor 2 is Rs, the resistance 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 reduced, and the output voltage Eo applied to the inverting input terminal is reduced to the reference voltage Eref. When the voltage is more forcibly increased, the voltage at the output terminal falls from a predetermined balanced voltage to discharge the capacitor 10 via the resistor 8 and decrease the input / output voltage of the voltage follower 11 respectively. . Therefore, the first amplifier 1 lowers the output voltage Eo at the output terminal when the input voltage Ei decreases, 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 sensor 2 when the pressure sensor 2 is pressurized or not. So the second
Means that the output voltage of the amplifier 6 is automatically adjusted, and accordingly, the input voltage of the voltage follower 11 is automatically adjusted after a predetermined time delay.

Further, since the amplifier 6 always receives a constant reference voltage Eref from the reference voltage source 7 to the non-inverting input terminal, the output voltage Eref 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 as follows: Ei = Eref × Rs / (Rs + Rf) (3) In the steady state, while the analog switch 9 is closed, the average 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 1 is equal to the input voltage Ei.

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 that has been on until then is simultaneously turned off. As a result, the grounding capacitor 10 is cut off from the output terminal of the amplifier 6 and is 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 turning off, that is, until the closing operation of the power window starts. 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, that is, the reference value of the above equation (3).

In FIG. 2, when a foreign object is caught between the window and the window frame at time t ₁ while the power window is closed, and pressure is started to be applied to the pressure sensor 2, the pressure sensor 2 The resistance value Rs decreases. As a result, the first amplifier 1 increases the amplification factor, and the input voltage Ei held at the steady value is constant, so that the time t
_The output voltage Eo, which has been kept at the steady value Eref until before _1, rises. When the output voltage Eo increases, the voltage at the output terminal of the second amplifier 6 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 capacitor 10.

As described above, the output voltage Eo increases as the pressure applied to the pressure-sensitive sensor 3 increases, and when the output voltage Eo 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 so as to release the foreign substance from being caught, and the analog switch 9 is turned on after a while. As a result, the capacitor 10 is supplied with a charge corresponding to the voltage drop consumed by the self-leakage current or the bias current of the voltage follower 11 from the output terminal of the second amplifier 6 via the resistor 8.

On the other hand, when the power window is opened, the pressure-sensitive sensor 3 gradually releases the pressure, the resistance value Rs increases toward the value before pressurization, and the pressure of the first amplifier 1 is increased. The amplification factor returns to the original value. Eventually, when the resistance value Rs of the pressure-sensitive 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 above equation (3).

Incidentally, 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 state where the foreign matter is caught (the pressure-sensitive sensor 3
When pressure is applied to the analog switch 9) and the analog switch 9 is turned on, the output voltage of the first amplifier 1 becomes equal to the reference voltage E.
There is a possibility that the ref gradually decreases. Therefore, control is performed so that the analog switch 9 is not 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, or depending on the ambient temperature or over time. Even if the resistance value Rs changes in magnitude, the output voltage Eo is maintained at a value equal to the reference voltage Eref when the pressure sensor 2 is not pressurized or when the pressure sensor 2 is maintained at a constant pressure. When pressure is applied to the pressure-sensitive sensor 2 due to pinching of a foreign object, the output voltage Eo changes from the value of the reference voltage Eref, so that the pressure can be detected from this voltage change. Therefore, even if the resistance value Rs of the pressure-sensitive sensor 2 has a manufacturing variation, a temperature change, or a temporal change, a constant pressure detection signal that is not affected by the variation can be obtained. Detection is performed.

Further, in the pressure detection circuit of this embodiment, the analog switch 9 is turned off at the same time as the closing operation of the power window 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 drop or the like, a 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 movement speed of the window decreases, even if a foreign object is caught between the window and the window frame, the pressure-sensitive sensor 2 is slowly pressurized, and the resistance value Rs of the pressure-sensitive sensor 2 decreases slowly. Although the amplification factor of the amplifier 1 gradually increases, the voltage increase rate of the output voltage Eo changes because the input voltage Ei is constant. When the voltage rise rate exceeds a predetermined value, the entrapment of foreign matter is reliably detected, and the power window opening control device can be operated.

In this embodiment, an FET-type operational amplifier having a considerably high input impedance is used as the first amplifier 1, the voltage follower 11 is omitted, and the capacitor 10 is connected to the non-inverting input terminal of the amplifier 1. May be connected directly. Although the pressure detection circuit of the present embodiment is of the unbalanced type, the pressure detection circuit may be of a balanced type by combining two circuits shown in FIG. In this case, the pressure-sensitive sensor 2, the reference voltage source 7, and the capacitor 10 are connected to the inverting input terminal of the corresponding first amplifier, the non-inverting input terminal of the second amplifier, and the voltage follower. Are respectively connected to the input terminals. From the output terminals of the two first amplifiers, a balanced output voltage resistant to external noise is obtained.

[0027]

Since the pressure detecting circuit of the present invention is constructed as described above, even if the resistance value of the pressure-sensitive sensor has a manufacturing variation, a temperature change or a temporal change, it is affected by the variation. And a stable pressure detection signal can be obtained. Further, even if the closing movement speed of a moving body such as a power window becomes slow, a pressure detection signal can be reliably obtained. Therefore, the reliability of the foreign matter trapping can be improved.

[Brief description of the 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]

 DESCRIPTION OF SYMBOLS 1 1st amplifier 2 Pressure sensor 3 Negative feedback resistor 6 2nd amplifier 7 Reference voltage source 9 Analog switch 10 Capacitor 11 Voltage follower

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01L 1/20 B60J 1/00 E05F 15/16

Claims (1)

(57) [Claims] A first amplifier connected between an inverting input terminal and a ground, and a negative feedback resistor connected between the inverting input terminal and the output terminal; A second amplifier having an output terminal connected to the inverting input terminal and a reference voltage source connected to the non-inverting input terminal; an output terminal of the second amplifier and a non-inverting terminal of the first amplifier; Voltage holding means provided between the non-inverting input terminal of the first amplifier and the non-inverting input terminal of the first amplifier at a predetermined timing; and detecting the voltage corresponding to the pressure applied to the pressure-sensitive sensor. A pressure detection circuit, wherein a signal is obtained from an output terminal of the first amplifier.