JPH05149802A - Pressure-sensitive circuit - Google Patents

Abstract

PURPOSE:To make pressure sensitivity constant irrespective of the resistance value of a pressure-sensitive sensor. CONSTITUTION:A negative feedback resistor 2 is connected between the inverted input end and the output end of an inversion amplifier 1 and a pressure-sensitive sensor 3 of which the resistance value Rs changes in accordance with an applied pressure is connected to this inverted input end. To this output end, the non- inverted input end of a second amplifier 6 is connected through a resistor 5 and the non-inverted input end of a comparator 8 through a resistor 7. A first reference voltage VR1 is impressed on the inverted input end of the second amplifier 6 and a voltage follower 18 is connected to the output end thereof through a time constant circuit 12, while the other end of the pressure-sensitive sensor 3 is connected to the output end of the follower. The time constant circuit 12 has a capacitor 13 connected to the non-inverted input end of the voltage follower 18 and a series circuit comprising a resistor 14 making a time constant in charging different from the time constant in discharging and a resistor 15 and a diode 16 connected in parallel to the resistor 14. A second reference voltage VR2 is impressed on the inverted input end of the comparator 8 and the result of comparison is obtained from an output terminal 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure-sensitive 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 For example, in an automatic opening / closing device such as an automobile power window device, a moving body such as a window glass is opened / closed by a motor. However, when a hand or finger is caught when the window glass is closed, the motor is It is necessary to stop or reverse the rotation so that the hand or finger is retracted. Therefore, it is necessary to detect the entrapment of foreign matter such as the hand or finger. A pressure-sensitive sensor using pressure-sensitive conductive rubber or pressure-sensitive conductive paint is used to detect the entrapment of the foreign matter.

Regarding the electrical characteristics of this type of pressure-sensitive sensor, the resistance value generally decreases exponentially as the applied pressure increases. That is, when the resistance value of the pressure sensitive sensor is Rs, the pressing force P is represented by Rs∝P ^−N . However, N is a positive constant. A pressure-sensitive circuit in consideration of variations in pressure-resistance characteristic of the pressure-sensitive sensor or secular change is disclosed in Japanese Patent Application No. 2-19.
No. 8224. As shown in FIG. 1, in this pressure sensitive circuit, a feedback resistor 2 is connected between the output end of the amplifier 1 and the inverting input end, and a pressure sensitive sensor 3 is connected between the inverting input end and ground. On the other hand, the second amplifier 6 controls the voltage supplied to the non-inverting input terminal of the time constant circuit so that the output voltage of the amplifier 1 becomes equal to the reference voltage in a steady state where the pressure sensor 3 is not pressurized. The time constant circuit is characterized in that the time constant during charging is different from the time constant during discharging.

[0004]

By the way, in the first amplifier 1, the resistance value of the pressure-sensitive sensor 3 in the unpressurized state is set to R.
Letting s be the resistance value of the feedback resistor 2 be Rf and the input voltage at the non-inverting input end be x, the output voltage E _O at the output end is expressed by the following equation. _{E O = (1 + Rf /} Rs) · x ......... (1)

Assuming that the resistance value of the pressure-sensitive sensor 3 during pressurization is Rs ', the output voltage E _O ' at the output end during pressurization is instantaneously expressed by the following equation. E _O '= (1 + Rf / Rs') · x (2)

Therefore, the rate of change of the resistance value of the pressure-sensitive sensor 3 when it is unpressurized or when it is pressurized is expressed by the following equation from equations 1 and 2. E _O '/ E _O = (1 + Rf / Rs') / (1 + Rf / Rs) ... (3)

When the resistance value Rs of the pressure sensor 3 is much smaller than the resistance value Rf of the feedback resistor 2, that is, 1 << Rf.
In the case of / Rs, E _O '/ E _O = Rs / Rs' (4) holds, but the pressure-sensitive sensor 3 determines which region of P when the increase amount of the pressing force P is equal. Since Rs '/ Rs also have the same characteristic, the change amount of E _O ' / E _O usually decreases as Rs increases. That is, there is a problem that the sensitivity is lowered. Further, it is ideal to remove "1" contained in the denominator or the numerator of Formula 3.

In view of the above-mentioned problems, it is an object of the present invention to provide a pressure sensitive circuit whose pressure sensitivity is constant regardless of the resistance value Rs.

[0009]

In order to achieve this object, the pressure-sensitive circuit of the present invention comprises a pressure-sensitive sensor whose resistance value changes in response to a pressing force, a first inverting input terminal, and a first inverting input terminal. A feedback resistor is connected between the output terminal and the first output terminal to output a predetermined output voltage from the first output terminal, and a second non-inverting input terminal to which the first output terminal is connected. A second amplifier including a second inverting input terminal to which the first reference voltage is applied and a second output terminal; and a time constant circuit whose input is connected to the second output terminal, A third non-inverting input terminal is connected to the output of the time constant circuit, and a third amplifier whose third output terminal is connected to the first amplifier via the pressure-sensitive sensor is provided. The constant circuit is characterized in that the time constant during charging is different from the time constant during discharging.

According to another aspect of the present invention, there is further provided a comparator which compares the output voltage of the first amplifier with a second reference voltage and outputs a detection signal for trapping a foreign substance. It is preferable to provide. The third amplifier has a gain of 1 when the third output end is connected to the third inverting input end.
Is equipped with a voltage follower.

According to such a configuration, the output voltage E _O of the first amplifier is equal to the reference voltage regardless of the resistance value of the pressure sensor when there is no load. Further, the voltage x of the voltage follower has a polarity different from that of the output voltage Eo, and in this case, a negative voltage is supplied to the pressure sensitive sensor. When pressure is applied to the pressure-sensitive sensor, the resistance value of the pressure-sensitive sensor sharply decreases, and the amplification factor of the first amplifier sharply increases, but the voltage at the inverting input terminal becomes substantially zero volt. Thus, the decrease in the resistance value of the pressure-sensitive sensor appears as the increase in the output voltage E _O. When the output voltage of the first amplifier exceeds a threshold value (voltage) higher than the reference voltage by a predetermined voltage value, the pressure-sensitive signal can be obtained.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the pressure sensitive circuit according to the present invention will be described below with reference to FIG. 2, parts corresponding to the parts shown in FIG. 1 are denoted by the same reference numerals.

In FIG. 2, an inverting amplifier 1 is connected with a feedback resistor 2 having a resistance value Rf between an inverting input terminal and an output terminal. The feedback resistance 2 is connected to the inverting input terminal according to a pressure P as will be described later. The pressure sensitive sensor 3 whose value Rs changes is connected. This inverting amplifier 1 has a non-inverting input terminal grounded via a grounding resistor 4, an output terminal connected to a non-inverting input terminal of a second amplifier 6 via a resistor 5, and further via a resistor 7. It is connected to the non-inverting input terminal of the comparator 8.

The inverting input terminal of the second amplifier 6 has a first
Reference voltage V _R1 is applied. This first reference voltage V _R1
Is obtained from a voltage dividing circuit provided between the positive voltage source Vcc and the ground, that is, three resistors 9 to 11 connected in series. In this case, the first reference voltage V _R1 is obtained from the first connection point connecting the grounded resistance 9 and the resistance 10, and the second connection point connecting the other end of the resistance 10 and the resistance 11 The second reference voltage V _R2 is obtained from the above, and the other end of the resistor 11 is connected to the constant voltage source.

The output terminal of the second amplifier 6 has a time constant circuit 12
Connected to the input of. The time constant circuit 12 includes a capacitor 13 having a capacitance C connected between the output and the ground, a resistor 14 that mainly determines a first time constant when the capacitor 13 is charged, and the capacitor 13. It is provided with a resistor 15 and a diode 16 that determine a second time constant when discharging. That is, a resistor 14 capable of charging and discharging the capacitor 13 is connected between the input and output of the time constant circuit 12, and a series circuit including a resistor 15 and a diode 16 that operates only when the capacitor 13 is discharged is formed. Connected. The resistor 15 is connected between the anode of the diode 16 and the output of the time constant circuit 12, and the cathode of the diode 16 is connected to the input of the time constant circuit 12. Resistance 14
For example, if a value 9 times larger than the value of the resistor 15 is used, the ratio of the first and second time constants is set to 10: 1. In this case, the forward voltage drop of the diode 16 is ignored.

The output of this time constant circuit 12 is connected to the non-inverting input of a voltage follower 18, which may consist of an operational amplifier. This voltage follower 18 has a resistor 1 at its inverting input end.
An amplifier having a gain of 1 and a high input impedance is connected to the output terminal via 9 and the output terminal is connected to the other end of the pressure-sensitive sensor 3 described above. on the other hand,
The second reference voltage V _R2 is applied to the inverting input terminal of the comparator 8. The output of the comparator 8 is the output terminal 2 of the pressure sensitive circuit.
Connected to 0.

Next, the operation of the pressure sensing circuit shown in FIG. 2 will be described. First, in the steady state, the output voltage E _O of the inverting amplifier 1 is operated to be equal to the first reference voltage V _R1 by the action of the second amplifier 6, the time constant circuit 12 and the voltage follower 18. That is, in the steady state, E _O = V _R1 ……………… (5). Further, from E _O = − (Rf / Rs) · x = V _R1 , the voltage x at the output end of the voltage follower 18 is x = − (Rs / Rf) · V _R1 …………………… (6 ) Is represented by.

Next, the pressure sensor 3 is turned on for a short time (or momentarily).
When the resistance value becomes Rs' by being pressurized to,
The output voltage E _O ′ at the output end at the time of pressurization is represented by the following equation: E _O ′ = − (Rf / Rs ′) · x = (Rs / Rs ′) · V _R1 (7) The rate of change of the resistance value of the pressure-sensitive sensor 3 when the pressure is not applied or when the pressure is applied is expressed by the following equations by the equations 6 and 7. E _O '/ E _O = (Rs / Rs') ……… (8)

Therefore, the pressure sensitivity of the pressure sensor 3 becomes constant regardless of the resistance value Rs of the pressure sensor 3. In the comparator 8, the output voltage E _O is supplied to the non-inverting input terminal and the second reference voltage V _R2 is supplied to the inverting input terminal. Since the output voltage E _O is equal to the first reference voltage V _R1 and lower than the second reference voltage V _R2 , the comparator 8 generates a binary logic level “L” at the output end. , Does not detect the entrapment of foreign matter. That is, even if the resistance value Rs of the pressure-sensitive sensor 3 fluctuates over a considerably long time due to temperature change, aging change, etc., the response of the voltage x can sufficiently follow and the condition of E _O <V _R2 is satisfied. The comparator 8 does not generate a signal indicating the entrapment of foreign matter.

On the other hand, in a transient state, when the resistance value Rs of the pressure sensitive sensor 3 is momentarily decreased by the applied pressure P, the amplification factor of the inverting amplifier 1 increases, and the voltage x at the output end of the voltage follower 18 is input. When the output voltage E _O rises and the condition of E _O > _VR2 is satisfied, the output of the comparator 8 changes from “L” to “H” when the output voltage E _O rises and the condition of E _O > V _R2 is satisfied. ”, And the entrapment of foreign matter is detected. On the other hand, the voltage at the output terminal of the second amplifier 6 instantaneously rises from the balanced voltage x to a value close to the positive power supply voltage,
The response of the voltage x at the non-inverting input terminal of the voltage follower 18 in the rising direction of the voltage x is delayed by the capacitor 13.

Further, in the above embodiment, when discharging from the capacitor 13 to the amplifier 6, it is carried out through two paths of the resistor 14 and the series circuit of the resistor 15 and the diode 16, but the capacitor 13 is charged. Done only through. For this reason, when the pressure applied to the pressure sensitive sensor 3 is removed, the rise of the voltage x is accelerated. In addition,
In the above embodiment, if the difference between the voltage V _R1 and the voltage V _R2 can be adjusted, the sensitivity of the foreign matter trapping detection can be controlled. Here, the capacitance value of the capacitor 13 is C, the resistance value of the resistor 14 is R1, and the resistor 15 is
If the resistance value of R is R2, the value of the first time constant τ is C · R
1 and the value of the second time constant is C · R1∥R2.

When this pressure-sensitive circuit according to the present invention is applied to a power window of an automobile, when the output voltage E _O rises to the second reference voltage V _R2 , the trapping of foreign matter is detected and the window glass is opened. Forward rotation of the motor to be closed is stopped or reversely driven by a predetermined control circuit.

Although the pressure sensitive circuit of this embodiment is an unbalanced type, it is also possible to form a balanced type by combining two circuits shown in FIG. In this case, the resistor 9 and the capacitor 13 are
The ground terminal in the case of the unbalanced type is connected to the non-inverting input terminal of the second amplifier and the non-inverting input terminal of the voltage follower, respectively, and the other end of the resistor corresponding to the resistor 11 has a negative power source. -Vcc is connected.

The inverting amplifier 1, the second amplifier 6, and the voltage follower 18 are general-purpose operational amplifiers such as 4558.
Alternatively, LF412 is used. A general-purpose operational amplifier may also be used as the comparator 8. When this operational amplifier is used for the inverting amplifier 1, the non-inverting input terminal is grounded,
The inverting input is defined as the input. Further, the pressure-sensitive circuit of the present invention uses positive and negative power supplies, but when a single positive power supply is used instead of this, the non-inverting input terminal of the operational amplifier corresponding to the inverting amplifier 1 has a single positive power supply. Is connected to a positive power supply having a predetermined voltage lower than the voltage.

[0025]

As described above, the pressure-sensitive circuit of the present invention can keep the pressure-sensitive sensitivity constant regardless of the resistance value Rs. Of course, it is possible to obtain a stable pressure-sensitive signal, that is, a voltage, which is not affected by variations in the resistance value of the pressure-sensitive sensor, variations in temperature, or changes with time.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an example of a conventional pressure sensing circuit by the applicant.

FIG. 2 is a circuit diagram showing a configuration of an embodiment of a pressure sensing circuit of the present invention.

[Explanation of symbols]

 1 Inverting Amplifier 2 Feedback Resistor 3 Pressure Sensitive Sensor 6 Second Amplifier 8 Comparator 12 Time Constant Circuit 13 Capacitor 14 Resistor 15 Resistor 16 Diode 18 Voltage Follower

Claims (3)

[Claims] 1. A feedback resistor is connected between a pressure-sensitive sensor whose resistance value changes according to a pressing force, and a first inverting input terminal and a first output terminal, and a feedback resistor is connected from the first output terminal. A first amplifier that outputs a predetermined output voltage; a second non-inverting input terminal to which the first output terminal is connected;
A second inverting input terminal to which the first reference voltage is applied;
A second amplifier including an output terminal, a time constant circuit whose input is connected to the second output terminal, and a third non-inverting input terminal connected to the output of the time constant circuit, A third amplifier whose output end is connected to the first amplifier via the pressure-sensitive sensor, wherein the time constant circuit has a different time constant during charging and a time constant during discharging. And a pressure sensitive circuit. 2. The comparator according to claim 1, further comprising a comparator that compares an output voltage of the first amplifier with a second reference voltage and outputs a detection signal for trapping a foreign substance. Pressure sensitive circuit. 3. The pressure sensitive circuit according to claim 1, wherein the third amplifier comprises a voltage follower having a unity gain, the third output end being connected to the third inverting input end.