JPH0674840A - Pressure detecting circuit - Google Patents

Abstract

PURPOSE:To provide the circuit capable of cheaply correcting a drop of the pressure sensitivity of a pressure sensor with a simple means. CONSTITUTION:The circuit comprises a first amplifier 12 given negative feedback by a pressure sensor 11 the resistance value RS of which changes according to pressure P; a second amplifier 16 for increasing a sensor current IS allowed to flow to the sensor 11 by changing the base potential of a transistor 14, when the output voltage Eout1 of the first amplifier 12 is dropped by the drop of the resistance value RS; a time constant circuit 19 for setting a time from the potential changing time by the second amplifier 16, till the attainment of the base potential to an equilibrium value; a second resistor 28 and a third resistor 29 for parting the potential difference between the potential EQ of a junction Q and the output voltage Eout1 to give potential Eint; and a voltage comparator 27 for comparing the potential Eint with a reference voltage Er and for inverting the output by pressurizing the sensor 11.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Automatic opening and closing equipment for power windows for automobiles
In order to prevent foreign matter from being caught, the flat pressure-sensitive
A sensor is used. This pressure-sensitive sensor is, for example,
Pressure-sensitive conductive rubber whose resistance changes with pressure
It is sandwiched between a pair of electrodes with a predetermined shape, and the cover is
It has a structure covered with a. The resistance value of the pressure sensor
R _s, Where P is the pressure applied to the pressure-sensitive sensor, R_s∝P^-N The formula (1) holds. However, N in equation (1) is a positive constant
Is. Conventional pressure used in such pressure sensitive sensors
As the detection circuit, there is a circuit 10 as shown in FIG.
In FIG. 1, negative feedback is applied by the pressure sensitive sensor 11.
A resistance value R is provided at the inverting input terminal of the first amplifier 12.₁Resistance 13
The collector of the transistor 14 is connected via
It -V is applied to the emitter of the transistor 14_cVoltage
Is being applied. Invert to the output terminal of the first amplifier 12
Connect the input terminal and connect the reference voltage E to the non-inverting input terminal.
_ref1The output terminal of the second amplifier 16 to which the_Four
Resistance 17 and capacitance value C₁Consist of the grounding capacitor 18 of
Through the time constant circuit 19
Connected to the base of. D of the transistor 14
The value of the mitter current depends on the voltage applied to the base.
Change.

By the way, the resistance value R _s of the pressure-sensitive conductive rubber, which is the main constituent of the pressure-sensitive sensor 11, does not change only by pressurization, but it also changes with changes in ambient temperature and years of use. In addition, there are some variations in the manufacturing process. In order to deal with this, in the pressure detection circuit 10, the second amplifier 16 and the transistor 1 are provided.
4 etc. are provided. The second amplifier 16 has a function of holding the output voltage E _out1 of the first amplifier 12 applied to its own inverting input terminal at the same value as the reference voltage E _ref1 . For example, if R _s rises due to changes in ambient temperature, the output voltage E _out1
Shows a tendency to increase, but the second
The amplifier 16 includes a transistor 14 via a time constant circuit 19.
The value of the emitter current is reduced by changing the voltage applied to the base of the. When the emitter current value decreases, the sensor current I _s decreases, so that the increasing tendency of E _out1 is suppressed. In this way, the value of E _out1 becomes equal to E _ref1 after the elapse of the time set by the time constant circuit 19, and thereafter this balanced state is maintained.

In such a state, when the user presses the drive switch 23, the controller 24 becomes an operating state and the motor 2 for opening and closing the power window (not shown).
6 is activated and the closing of the power window, for example, is initiated. If a foreign substance is trapped during the closing of the power window, the pressure sensor 11 is pressurized and its resistance value R _s sharply drops, so that the output voltage E _out1 of the first amplifier 12 sharply drops. When this lowered E _out1 falls below the predetermined voltage V _p , the voltage comparator 27 inverts its output. The controller 24 that detects this stops the motor 26. Therefore, it is possible to prevent the drive mechanism of the power window from being damaged or the foreign matter sandwiched by the power window to be further closed when the foreign matter is sandwiched between the power window and the power window.

[0005]

However, as is clear from the above equation (1), as the pressure P increases, the rate of decrease of the resistance value R _{s with} the increase of the pressure decreases. That is, compared with a case value becomes smaller when R _s of R _s by a reduction such as the ambient temperature is high, even if applied the same pressure P is R _s
Is small, and the pressure sensitivity of the pressure sensor 11 itself is reduced. This is due to the decrease in R
It is considered that not only _s decreases but also that the cover of the pressure-sensitive sensor 11 contracts.

As described above, the foreign matter entrapment detection sensitivity of the pressure-sensitive sensor 11 itself is lowered, whereas the threshold value of the voltage comparator 27 is a predetermined constant value, so that the foreign matter entrapment detection sensitivity of the entire apparatus is lowered. To do. Therefore, when R _s is reduced to a predetermined value or more, lowering of the even entrapment of foreign object E _out1 small E _out1 is above V _p
Therefore, the output of the voltage comparator 27 is not inverted, and it becomes impossible to notify that the foreign matter is caught.

Further, there is a problem that the foreign matter entrapment detection sensitivity is lower in the latter case depending on whether the pressure sensitive sensor 11 is not pressurized immediately before the power window is closed or when it is pressurized. For example, the pressure-sensitive sensor 1 is caused by a mischief of a child.
When 1 is pressurized just before the power window is closed, R _s is lowered by the pressure just before the power window is closed, and the pressure detection circuit 10 is in the equilibrium state when R _s is reduced. By stopping the pressurization, R _s rises temporarily and E _out1 also rises. In this way, if a foreign object is _trapped when E _out1 is increased, the threshold value becomes larger than the predetermined value of the voltage comparator 27 as a result. Therefore, the output of the voltage comparator 27 cannot be inverted even if the decrease amount of R _s due to the entrapment of the foreign matter is about normal. In order to solve the above problems, an object of the present invention is to provide a pressure detection circuit capable of compensating for a decrease in pressure-sensitive sensitivity of a pressure-sensitive sensor with a simple means at low cost.

[0008]

In order to achieve the above-mentioned object, the present invention provides a first amplifier which is negatively fed back by a pressure-sensitive sensor whose resistance value changes in response to a pressing force, and a first amplifier of the first amplifier. An inverting input terminal is connected to the output terminal, a first reference power source is connected to the non-inverting input terminal, and a second amplifier that generates a control signal from the output terminal and a first amplifier are connected to the inverting input terminal of the first amplifier. A constant current element connected via a resistor, the current value of which changes according to the control signal supplied to the control end, and the time from the change of the control signal until the potential of the control end reaches an equilibrium value. A time constant circuit for setting, a comparator in which one input end is connected to the output end of the first amplifier via a second resistor, and a second reference power source is connected to the other input end, and the first resistor And the connection point between the constant current element and
A third resistor connected to the one input end of the comparator is provided, and the pressure detection circuit is configured to detect the pressure based on the output signal of the comparator.

[0009]

According to the above, when the resistance value of the pressure sensitive sensor decreases in the equilibrium state due to a decrease in ambient temperature, for example, the output signal of the first amplifier tends to decrease. The second amplifier, which detects this, changes the control signal to the high level. As a result, the potential at the control end of the constant current element rises and the current value of the constant current element increases, so the sensor current flowing through the pressure sensitive sensor increases. Then, when the time set by the time constant circuit elapses, the potential at the control end of the constant current element reaches the equilibrium value, so that the output of the first amplifier has the same voltage level (or current) as the reference power source of the second amplifier. Value). At this time, the second amplifier lowers the control signal to a predetermined value, and the pressure detection circuit is in the equilibrium state again.

By increasing the current value of the constant current element,
The voltage level at the connection point between the first resistor and the constant current element drops. Therefore, the potential at one input terminal of the comparator drops according to the voltage division ratio of the second resistor and the third resistor, and the difference from the reference voltage (or current) supplied by the second reference power supply is reduced. Therefore, the threshold value for inverting the output of the comparator becomes small, and the sensitivity of the pressure detection circuit is increased. However, at this time, the resistance values of the second resistor and the third resistor are set so that the potential of the one input end of the comparator does not become lower than the potential of the other input end.

In the above, when the one input terminal of the comparator is a non-inverting input terminal and the other input terminal is an inverting input terminal, the comparator outputs a high level output in a balanced state to a low level. By reversing, it is notified that the foreign matter is caught. Conversely, when the one input terminal of the comparator is the inverting input terminal and the other input terminal is the non-inverting input terminal, the comparator should invert the low level output in the balanced state to the high level. Informs that a foreign object is caught. Less than,
An embodiment of the present invention will be described with reference to the drawings. However, components corresponding to those shown in FIG. 1 are designated by the same reference numerals.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 2, a resistance value R is applied to an inverting input terminal of a first amplifier 12 which is negatively fed back by a pressure sensitive sensor 11.
The collector of the transistor 14 via a first resistor 13 of ₁ is connected. For the emitter of the transistor 14,
A voltage source of V _c is connected. The non-inverting input terminal of the first amplifier 12 is grounded, and the output terminal of the first amplifier 12 is connected to the inverting input terminal of the second amplifier 16. The reference voltage E _ref1 is applied to the non-inverting input terminal of the second amplifier 16. The resistance value R ₄ connected to the output terminal of the second amplifier 16
A resistor 17 and a capacitor 18 having a capacitance value C _{1 form} a time constant circuit 19. The time constant of the time constant circuit 19 is τ = R ₄ C ₁ .

The output terminal of the first amplifier 12 has a resistance value R ₂
Is connected to the non-inverting input terminal of the voltage comparator 27 via the second resistor 28. The connection point Q between the first resistor 13 and the transistor 14 is connected to the non-inverting input terminal of the voltage comparator 27 via the third resistor 29 having the resistance value R ₃ . On the other hand, the cathode of the voltage source 31 of the voltage E _ref2 is connected to the inverting input terminal of the voltage comparator 27. The anode of this voltage source 31 is connected to the anode of the voltage source 32 that applies the reference voltage E _ref1 to the non-inverting input terminal of the second amplifier 16. Here, since E _ref1 > E _ref2 ,
The voltage E applied to the inverting input terminal of the voltage comparator 27
_r (= E _ref1 −E _ref2 ) indicates a positive value. Voltage source 31,
By connecting 32 as described above, even if the value of E _ref1 generated by the voltage source 32 fluctuates for some reason, the relative value between the output voltage E _out1 of the first amplifier 12 and the potential E _r. Since the relationship is maintained, the reliability of the operation of the pressure detection circuit 30 can be improved.

The operation of the pressure detecting circuit 30 of the present embodiment having the above structure will be described below. In the equilibrium state (hereinafter, referred to as “first equilibrium state”), when the resistance value R _s of the pressure sensor 11 decreases due to a decrease in ambient temperature or the like, the output voltage E _out1 of the first amplifier 12 tends to decrease. . The second amplifier 16 that has detected this is the time constant circuit 1
Since the potential of the base of the transistor 14 is raised via 9, the forward bias between the emitter and the base is increased, and the emitter current of the transistor 14 is increased. Since the sensor current I _s increases due to the increase in the emitter current, the output voltage E _out1 also increases. At the time when the time set by the time constant circuit 19 based on the time constant τ has elapsed, the potential of the base of the transistor 14 reaches the equilibrium value, and the drop tendency of the output voltage E _out1 is canceled and the above E _out1 is eliminated. Is restored to the same level as the reference voltage E _ref1 . After that, this equilibrium state (hereinafter, referred to as "second equilibrium state") is maintained. By the way, in the second equilibrium state, the resistance value R _s is lower than that in the first equilibrium state. Therefore, as is clear from the equation (1), the foreign matter entrapment detection sensitivity of the pressure sensor 11 itself is lowered. is doing. That is, even if the same amount of pressure is applied due to the entrapment of foreign matter, the amount of decrease in R _s is small, so
The amount of _out1 drop is small.

In the above-mentioned first and second equilibrium states,
The output voltage E _out2 of the voltage comparator 27 maintains the high level. At this time, the potential E _int of the non-inverting input terminal of the voltage comparator 27 is given by the following equation using the voltage division ratio based on the resistance values of the second resistor 28 and the third resistor 29. E _int = E _ref1 − (E _ref1 −E _Q ) R ₂ / (R ₂ + R ₃ ) = (E _ref1 R ₃ + E _Q R ₂ ) / (R ₂ + R ₃ ) ... (2) In (2), E _out1 = E _ref1 in the equilibrium state
Was used. On the other hand, since the sensor current I _s is increased in order to bring it to the second equilibrium state, the potential E _{Q at the} point Q drops according to the amount of increase. When the potential E _Q drops, the potential E _int at the non-inverting input terminal of the voltage comparator 27 drops according to the above equation (2). However, the level of E _{int at} equilibrium must be higher than at least E _r . For that purpose, E obtained by substituting the value of −V _c into E _Q of the above equation (2)
_The resistance values R ₃ , R ₂ and the reference voltage E _ref1 may be selected so that the value of _int satisfies the relationship of E _int > E _r . This is because the value of E _Q cannot fall below −V _c .

The drop of E _int in the second equilibrium state means that the threshold value for inverting the output of the voltage comparator 27 becomes smaller, and therefore,
The detection sensitivity of the pressure detection circuit 30 increases due to the drop of E _int . That is, the influence of the decrease in the detection sensitivity of the pressure-sensitive sensor 11 itself due to the decrease in R _s will be explained.
The compensation is made by increasing the detection sensitivity of 0. Second
If foreign matter is caught in the equilibrium state of
_{Although s} rapidly decreases, the amount of decrease is small, so the amount of decrease in E _out1 is also small. In the above formula (2), E _ref1
When E is replaced with E _out1 , E _int = (E _out1 R ₃ + E _Q R ₂ ) / (R ₂ + R ₃ ) ... (3)

This equation (3) is simply expressed by the potential E _out1 and the potential E _Q.
Since it indicates that and are divided by R ₂ and R ₃ ,
Applies even when not in equilibrium. In the formula (3), R
If we think about the time immediately after _s has dropped, the change in E _Q can be ignored, so E _int drops according to the amount of drop in E _out1 . At least at this time, the level of E _int is lower than the above E _r , so that the output of the voltage comparator 27 is inverted from the high level to the low level. The controller (not shown) that detects this performs the same operation as the conventional example shown in FIG. 1 such as stopping the closing of the power window. In this way, the pressure detection circuit 30 can detect the trapping of the foreign matter with substantially the same sensitivity as before the decrease of R _s .

On the other hand, when R _s rises due to a rise in ambient temperature, the above E _out1 tends to rise, and the second amplifier 16 which detects this rises the potential of the base of the transistor 14 to reduce the emitter current. By reducing the value, the sensor current I _s is reduced. As a result, E _out1 drops and after a lapse of the time set by the time constant circuit 19, the voltage level of E _out1 becomes equal to E _ref1 . At this time, the potential E _{Q at the} point Q rises, so the above equation
The value of E _int given by (2) rises, and the threshold value increases. In other words, the pressure detection circuit 30 can prevent the sensitivity of the pressure sensor 11 itself from being excessively increased due to the increase of R _{s and the} detection sensitivity of the pressure sensor 11 itself. Therefore, the pressure detection circuit 30 is capable of detecting the entrapment of a foreign substance with a desired and constant sensitivity.

When the pressure sensor 11 is pressurized just before the power window is closed, the sensor current I _s is increased according to the pressure P immediately before the power window is closed, and the potential E _Q is accordingly increased.
Goes down. Therefore, the E _int drops and the threshold becomes small. In this equilibrium state, consider the case where the pressurization is suddenly stopped and the closing of the power window is started. By stopping the pressurization, R _s rises and E _out1 rises, so E _int also rises according to the equation (3), and the threshold value also increases. When the time constant τ is sufficiently shorter than the closing speed of the power window, the base potential of the transistor 14 reaches the equilibrium value during the closing of the power window, and the E _out1 is restored to the same level as the E _ref1. In addition, the potential E _Q rises due to the decrease in the sensor current I _s . Therefore, E _int increases by an amount determined by the equation (2), and the threshold value increases accordingly. The larger R ₂ is relative to R ₃ , the larger the increase amount of E _int . Further, the smaller the value of E _ref1, the larger the increase amount of E _int . On the other hand, the sensitivity of the pressure-sensitive sensor 11 itself is increased by the increase of R _s as described above, so that the sensitivity of the pressure detection is kept substantially constant in the entire apparatus. Therefore, if a foreign object is caught during the closing, the pressure detection circuit 30 can detect it with a certain sensitivity. When the time constant τ is sufficiently short, the sensitivity of the pressure-sensitive sensor 11 itself increases in the conventional example shown in FIG. 1, whereas the threshold value recovers a predetermined constant value, so that the sensitivity of the entire apparatus is reduced. Will rise. Moreover, since the amount of increase in the sensitivity depends on the pressing force P immediately before closing, the entire apparatus becomes sensitized when the pressing force immediately before is strong.

On the other hand, if the time constant τ cannot be made sufficiently short for some reason, the rise of E _Q and the fall of E _out1 appear with a delay according to the time constant τ. In the course this case the power window closing, the base potential of the transistor 14 can not reach an equilibrium value, in the above formula (3), E _Q
Is slightly constant but is almost constant, and E _out1 is slightly decreased but still higher than E _ref1 . Therefore, E _int increases by an amount determined by the equation (3), and the threshold value also increases accordingly. In this case, the larger the above-mentioned V _c , the larger the increase amount of E _Q , and thus the increase amount of E _int is larger. On the other hand, the pressure sensor 1
Since the sensitivity of 1 itself is increased by the increase of R _s as described above, the sensitivity of pressure detection is maintained substantially constant in the entire device. In this case, compared with the conventional example shown in FIG. 1, since the voltage is divided by R ₂ and R ₃ ,
The amount of increase in _int is small. Furthermore, since the sensitivity of the pressure-sensitive sensor 11 itself increases due to the increase in R _s , the decrease in the pressure detection sensitivity of the entire device is very small, if any. Further, the amount of increase of E _int can be set arbitrarily by selecting the values of R ₂ , R ₃ and V _c .

In the above embodiment, the bipolar transistor is used as the transistor 14, but a unipolar transistor such as FET may be used. Also, the second
The voltage source for applying E _ref1 to the non-inverting input terminal of the amplifier 16 and the voltage source for applying E _r to the inverting input terminal of the voltage comparator 27 are completely separate without connecting as in the above embodiment. May be provided.

[0022]

As described in detail above, according to the present invention, the decrease in the sensitivity of the pressure sensitive sensor itself due to the decrease in the resistance value of the pressure sensitive sensor is compensated by the increase in the sensitivity of the pressure detection circuit. Even if the resistance value of the pressure-sensitive sensor decreases due to various causes, it is possible to detect the entrapment of the foreign matter with sufficiently high sensitivity. Conversely, if the resistance value of the pressure-sensitive sensor rises and the pressure-sensitive sensor itself becomes hypersensitive, the sensitivity of the pressure detection circuit decreases, so malfunctions and the like induced by the hypersensitivity of the pressure-sensitive sensor. The harmful effect of can be prevented. Therefore, the pressure detection circuit of the present invention enables stable detection of trapping a foreign substance with a desired constant sensitivity, which is neither too low nor too high. Moreover, such a circuit can be provided at low cost by using simple means.

[0023]

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a conventional example of a pressure detection circuit.

FIG. 2 is a circuit diagram showing a pressure detection circuit according to an embodiment of the present invention.

[Explanation of symbols]

11 Pressure Sensitive Sensor 12 First Amplifier 13 First Resistor 14 Transistor (Constant Current Element) 16 Second Amplifier 19 Time Constant Circuit 27 (Voltage) Comparator 28 Second Resistor 29 Third Resistor 30 Pressure Detection Circuit P Pressurization Q Connection Point R _s (pressure sensor) resistance

Claims (1)

[Claims] 1. A first amplifier in which a negative feedback is applied by a pressure-sensitive sensor whose resistance value changes according to a pressing force, an inverting input terminal is connected to an output terminal of the first amplifier, and a non-inverting input terminal is connected. Is connected to a first reference power source and is connected to a second amplifier which generates a control signal from an output end, and an inverting input end of the first amplifier via a first resistor, and is supplied to the control end. A constant current element whose current value changes in accordance with the above, a time constant circuit for setting the time from the change of the control signal until the potential of the control end reaches the equilibrium value, and the output end of the first amplifier. One input terminal of the comparator is connected to the other input terminal of the comparator by connecting the second reference power source to the other input terminal via the second resistor, and the one connection point of the first resistor and the constant current element is A third resistor connected to the input end of Pressure detection circuit, characterized in that the pressure detected based on the output signal of the comparator.