JPH0658824A - Pressure detection circuit - Google Patents

Abstract

PURPOSE:To obtain a pressure detection circuit which enables the detecting of a change in the resistance value alone of a pressure sensitive sensor at a high sensitivity as caused by the galling of foreign matters without being affected by any change in the resistance value of the pressure sensitive sensor owing to a change or the like in the ambient temperature, scattering of the resistance value of the pressure sensitive sensor existing during a production or the like. CONSTITUTION:A first amplifier 12 subjected to a negative feedback with a transistor 10 which changes in a resistance value RQ according to a control signal allowed to be supplied to a gate G and an output end thereof 2 are connected to a non-inversion input end while a second amplifier 12 in which Eref is applied to an inversion input end to generate a control signal at an output end is provided. Moreover, a time constant circuit 19 is provided to set time until a potential of the gate G reaches a balance value after the control signal chances and a pressure sensitive sensor 13 is connected to a non-inversion input end of the first amplifier 12 to has a resistance value thereof changing according to a pressurizing force. A pressure is detected based on an output voltage Eout of the first amplifier 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure detecting circuit of a pressure sensitive sensor using a variable resistance type pressure sensitive conductive rubber.

[0002]

2. Description of the Related Art A flat pressure-sensitive sensor is used in an automatic power window and sunroof opening / closing device for an automobile to prevent foreign matter from being caught. This pressure-sensitive sensor has a structure in which a pair of electrodes having a predetermined shape sandwich a pressure-sensitive conductive rubber, the resistance value of which changes by applying pressure, and a cover from above. Assuming that the resistance value of the pressure-sensitive sensor is R _s and the pressure applied to the pressure-sensitive sensor is P, the expression R _s ∝P ^−N (1) is established. However, N in the equation (1) is a positive constant.

A pressure detection circuit is constructed so that the output voltage changes when the resistance value R _s of such a pressure sensitive sensor changes. Then, by comparing the output voltage of the pressure detection circuit with a predetermined voltage, it is possible to detect a rapid change in the output voltage due to a rapid change in R _s due to the entrapment of foreign matter. When a controller (not shown) detects this, the power window drive motor is stopped to try to further close the power window with foreign objects trapped, destroying the trapped foreign objects or driving the power window. It is possible to prevent the mechanism from being damaged.

[0004]

However, the resistance value R _s of the pressure-sensitive sensor is not only changed by the pressurization, but is also changed by the sensor current I _s flowing in the pressure-sensitive sensor as shown in FIG. . Further, as shown in FIG. 2, it also varies depending on the ambient temperature T. Furthermore, it is known that the resistance value R _s varies to some extent even at the time of manufacturing the pressure-sensitive sensor, and also varies depending on the years of use.

As described above, the resistance value R _s of the pressure-sensitive sensor itself varies due to various factors, and the output of the pressure detection circuit using the pressure-sensitive sensor is not affected by the variation of R _s due to the ambient temperature. If it is received, it is impossible to detect the entrapment of the foreign matter with extremely high reliability. In order to solve the above problems, the present invention is not affected by fluctuations in the resistance value of the pressure-sensitive sensor due to changes in ambient temperature and the like, and variations in the resistance value of the pressure-sensitive sensor existing during manufacturing, and An object of the present invention is to provide a pressure detection circuit that can detect only a change in resistance value of a pressure-sensitive sensor due to pinching with high sensitivity.

[0006]

In order to achieve the above object, the pressure detecting circuit of the present invention uses a variable resistance element whose resistance value changes in accordance with a control signal that can be supplied to the control end to provide negative feedback. A first amplifier, a second amplifier connecting the output terminal of the first amplifier to a non-inverting input terminal, a reference power supply connected to the inverting input terminal, and generating the control signal at the output terminal; It is connected to a time constant circuit that sets the time from the change of the signal to the time when the potential at the control end reaches the equilibrium value, and the resistance value changes according to the applied pressure, which is connected to the inverting input end of the first amplifier. A pressure sensitive sensor is provided, and the pressure is detected based on the output signal of the first amplifier.

[0007]

According to the above, when the resistance value of the pressure-sensitive sensor decreases due to a decrease in ambient temperature, the output signal of the first amplifier tends to increase. At this time, since the output signal of the first amplifier becomes higher than the reference signal supplied from the reference power source, the second amplifier changes the control signal to the high level.
The level of the control signal at the control end of the variable resistance element reaches the equilibrium value after a delay set by the time constant circuit. Even before the level of the control signal at the control end reaches the equilibrium value, the resistance value of the variable resistance element decreases according to the level of the control signal at the control end, and the output signal of the first amplifier decreases. Go. Then, when the level of the control signal at the control end reaches the balanced value, the output signal of the first amplifier is restored to the same level as the reference signal. After that, this equilibrium state is maintained.

On the contrary, when the resistance value of the pressure sensitive sensor rises due to the rise of the ambient temperature, the output signal of the first amplifier tends to fall. At this time, since the output signal of the first amplifier becomes lower than the reference signal supplied from the reference power source, the second amplifier changes the control signal to the low level. The level of the control signal at the control end of the variable resistance element reaches the equilibrium value after a delay set by the time constant circuit. Even before the level of the control signal at the control end reaches the equilibrium value, the resistance value of the variable resistance element rises according to the level of the control signal at the control end, and the output signal of the first amplifier rises. go. Then, when the level of the control signal at the control end reaches the balanced value, the output signal of the first amplifier is restored to the same level as the reference signal. After that, this equilibrium state is maintained.

In the equilibrium state, if a foreign object is caught, the resistance value of the pressure sensitive sensor sharply decreases and the output signal of the first amplifier sharply increases. As a result, the control signal generated by the second amplifier also rises sharply, but the level of the control signal at the control end of the variable resistance element cannot rise sharply to the equilibrium value due to the existence of the time constant circuit. . Therefore, the output signal of the first amplifier maintains a level higher than at least the reference signal within the time set by the time constant circuit. By detecting this high-level output signal, pressure detection, that is, detection of entrapment of foreign matter can be performed. An embodiment of the present invention will be described below with reference to the drawings.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 3, an N-channel junction field effect transistor (J-FET) 1 is used as a first amplifier 12.
Negative feedback is applied by 0 and the resistor 11 having the resistance value R ₁ . A voltage of −V _c is applied to the inverting input terminal of the first amplifier 12 via the pressure sensitive sensor 13 having a resistance value R _s . The non-inverting input terminal of the first amplifier 12 is grounded, and the output terminal is connected to the non-inverting input terminal of the second amplifier 14. The resistor 11 is provided to protect the transistor 10 and the first amplifier 12 when the transistor 10 is short-circuited.

A reference voltage source 16 for generating a reference voltage E _ref is connected to the inverting input terminal of the second amplifier 14, and its output terminal is connected through the resistor 17 having a resistance value R _{2 to} the transistor 1 described above.
It is connected to the gate G of 0. The output voltage of the second amplifier 14 switches between two levels of high level and low level. Further, the gate G of the transistor 10 is connected to the ground capacitor 18 having a capacitance value C ₁ . The resistor 17 and the capacitor 18 form a time constant circuit 19. The electrode on the gate G side of the capacitor 18 is
Negative charges are accumulated when the power is turned on. As a result, the potential of the gate G is constantly set to a level lower than zero volt. The potential of the source S of the transistor 18 is always held at approximately zero volt by the second amplifier 12.
Hereinafter, the operation of the pressure detection circuit 20 of the present embodiment having the above configuration will be described.

In the balanced state, the output voltage E _out of the first amplifier 12 is at the same level as the reference voltage E _ref , so E _out = E _ref・ (2) is established. If the resistance value between the drain and the source of the transistor 10 is R _Q , then E _out = −V _c [− (R ₁ + R _Q ) / R _s ]. Further, from the above equations (2) and (3), R ₁ + R _Q = R _s E _ref / V _c . Since R ₁ , E _ref , and V _c are constants, in order to maintain the equilibrium state, the change on the right side of the above equation (4) obtained by multiplying the change amount of R _{s by} the constant (E _ref / V _c ) is on the left side. It will be absorbed by the change in R _Q of.

In the equilibrium state as described above, when R _s decreases due to a decrease in ambient temperature T or the like, E _out tends to increase in proportion to the decrease amount of R _s according to the equation (3).
When the second amplifier 14 detects that E _out becomes higher than E _ref , the second amplifier 14 outputs a high level.
As a result, the voltage applied to the gate G of the transistor 10 rises, and the potential of the gate G changes toward zero volt, so that the resistance value R _Q drops and the output E _out drops. In this way, the potential of the gate G of the transistor 10 reaches the equilibrium value after the elapse of the predetermined time set by the time constant circuit 19 based on the time constant τ = C ₁ R _2, and thus the output voltage E _out is restored to the same level as the reference voltage E _ref .

On the contrary, when R _s rises due to the rise of the ambient temperature T or the like, the above E _{out tends} to fall. When the second amplifier 14 detects that E _out becomes lower than E _ref , the second amplifier 14 outputs a low level. As a result, the voltage applied to the gate G of the transistor 10 decreases, and the potential of the gate G decreases, so that the resistance value R _Q increases and the output voltage E _out increases. Then, after the elapse of a predetermined time set by the time constant circuit 19 based on the time constant τ, the potential of the gate G of the transistor 10 reaches the equilibrium value, so that the output voltage E _out is equal to the above E.
Restored to the same level as _ref . After that, whenever the resistance value R _s changes due to the change of the ambient temperature T or the like, R _Q is changed at any time to maintain the equilibrium state.

In such an equilibrium state, if a foreign object is caught during the closing of the power window, the pressure sensitive sensor 11
Is abruptly pressed and the resistance value R _s abruptly drops, so that the output voltage E _out of the first amplifier 12 abruptly rises. If E _out at this time is written as E _out ^′, it can be written from the above equation (3) as E _out ^′ = −V _c [− (R ₁ + R _Q ) / R _s ^′ ] (5). Therefore, the relationship of E _out ^' / E _out = R _s / R _s ^' (6) holds. Within the predetermined time set based on the time constant τ, the output voltage E _out maintains a value higher than at least the reference voltage E _ref , and the relationship shown in the equation (6) is established. A controller (not shown) that has detected a sudden increase in the output voltage E _out performs processing such as stopping the drive motor of the power window. Since the pressure-sensitive conductive rubber which is the main constituent of the pressure-sensitive sensor 11 has the same resistance change rate for the same applied pressure regardless of the resistance value R _s of the rubber, the output voltage E of the first amplifier 11 is the same. By capturing the change in _out, the same pressure sensitivity can always be obtained.

The drain-source voltage is several tens mV.
In the following range, it is known that the area between the drain and source of the FET can be regarded as pure resistance with almost no strain. Therefore, by setting the value of E _ref to about several tens of mV, the reliability and stability of the circuit operation when the potential of the gate G of the transistor 10 is near the equilibrium value is extremely high. Can be Also,
In the above embodiment, a junction field effect transistor (J-FET) is used as the transistor 10, but a MOS is used.
-FET or bipolar transistor can also be used.

[0017]

As described above in detail, in the present invention, the time constant circuit is provided and the negative feedback is applied to the first amplifier by the variable resistance element. Therefore, by changing the resistance value of the variable resistance element, fluctuations in the resistance value of the pressure-sensitive sensor due to changes in the ambient temperature and sensor current, and variations in the resistance value of the pressure-sensitive sensor that existed at the time of manufacturing, etc. It is possible to stabilize the output of the first amplifier in the balanced state without receiving it. Further, by providing the time constant circuit, it is possible to detect a rapid change in the resistance value of the pressure-sensitive sensor due to pressurization based on the output signal of the first amplifier. This makes it possible to perform highly reliable detection of entrapment of foreign matter.

[0018]

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an I _s -R _s characteristic of a pressure-sensitive sensor.

FIG. 2 is a schematic diagram showing a T-R _s characteristic of a pressure-sensitive sensor.

FIG. 3 is a circuit diagram showing a pressure detection circuit according to an embodiment of the present invention.

[Explanation of symbols]

10 Transistor (Variable Resistance Element) 12 First Amplifier 13 Pressure Sensitive Sensor 14 Second Amplifier 16 Reference Current (Pressure) Source 19 Time Constant Circuit 20 Pressure Detection Circuit G Gate (Control End) R _s (Pressure Sensitive Sensor) Resistance Value R _Q (variable resistance element) resistance value E _out (first amplifier) output voltage

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location B60J 10/12 G01L 1/18

Claims (1)

[Claims] 1. A first amplifier negatively fed back by a variable resistance element whose resistance value changes according to a control signal that can be supplied to the control terminal, and an output terminal of the first amplifier is connected to a non-inverting input terminal. At the same time, the reference power source is connected to the inverting input terminal, the second amplifier that generates the control signal at the output terminal, and the time from the change of the control signal until the potential of the control terminal reaches the equilibrium value. A time constant circuit to be set and a pressure-sensitive sensor that is connected to the inverting input terminal of the first amplifier and whose resistance value changes according to the applied pressure are provided, and pressure detection is performed based on the output signal of the first amplifier. The pressure detection circuit characterized in that.