Pressure detector circuit

Abstract: A pressure to resistance transducer or sensor (10) is connected to an amplifier (12). A three-state comparator (14) compares the amplifier output signal with a first reference voltage (15) and its comparison output is looped to another input of the amplifier (12) through a low-pass filter (16). A first controllable current source (41) in series to a first reference resistor (42) is provided in the loop path to detect a break condition of the pressure sensor (10). A second controllable current source (51) in series to a second reference resistor (52) and a voltage divider (54) are also provided in the loop path to detect a short circuit condition of the pressure sensor (10) in combination with the first current source (41). The low-pass filter (16) includes a switch (59) in parallel with a high resistor (58) having its resistance higher than that of a low resistor (17).

Description: The present invention relates to a pressure detector circuit comprising a pressure sensor having a resistance that changes in response to change in the pressure applied; an amplifier operatively connected to said pressure sensor and providing at its output an output signal; a three-state comparator connected to said output of said amplifier and for comparing said output signal with a first reference voltage to provide a comparison output signal from its comparison output; a low-pass filter connected to said comparison output; and a feedback means for said amplifier comprising said low-pass filter. Such a circuit is suitable for use in an automatic opening/closing apparatus, for example, a power window of an automobile to detect an obstacle between a window frame and a closing window glass thereof. A power window device mounted on a door of an automobile drives a window glass received in the door by a motor. For example, a controller controls the motor to open the window glass by supplying reversing power to the motor when an obstacle is put or clamped between a window frame and the window glass during closing the window glass. For this, a pressure sensor is mounted or embedded in the door frame to detect the obstacle. Fig. 1 shows a suitable pressure-to-resistance transducer or sensor 10 that comprises a metal core or electrode 1, a pressure sensitive conductive or resistive rubber 2 coaxially provided or disposed around the core 1, a metal sheath electrode (or a wire braid) 3 disposed around the rubber 2 and a dielectric cover 4 disposed around the sheath 3. The resistance of the pressure sensor 10 generally increases exponentially upon application of pressure. Assuming the applied pressure to be P, the resistance Rs of pressure sensor 10 is rendered as follows: Rs α KP -N where K and N are positive constants, respectively. Japanese Patent Application (JPA) No. 2-198224, filed July 26, 1990 and laid open for public inspection under JP-A-4-83136, discloses a pressure detector circuit which is illustrated by Fig. 2 of the accompanying drawings. It comprises a reference resistor 11 connected between an inverting input and an output of an operational (OP) amplifier 12. The inverting input of amplifier 12 is also grounded through the pressure sensor 10 as shown in Fig. 1. The output of amplifier 12 is connected to an output terminal 13 and an inverting input of a three-state comparator 14. A three-state comparator 14 has its noninverting input applied to a reference voltage from a reference voltage source 15 and its output looped or fed back to a noninverting input of OP amplifier 12 through a low-pass filter 16. The low-pass filter 16 includes a resistor 17 connected between the output of comparator 14 and the noninverting input of amplifier 12, and a capacitor 18 connected between the noninverting input of amplifier 12 and ground. Therefore, the pressure detector circuit under normal conditions provides an output voltage from the terminal 13 identical to the reference voltage even if the resistence of unpressured pressure sensor 10 upon manufacturing deviates from mean value and if the ambient temperature of the automobile equipped therewith is changed. When a force is abruptly applied to the pressure sensor 10, the output voltage is increased from the reference potential due to a time constant of the low-pass filter and then slowly returns to the reference potential. Thus, a sudden application of pressure to pressure sensor 10 will be detected by another comparator (not shown) having a threshold voltage higher than the reference voltage. Further pressure detector circuits are disclosed in Japanese Patent Applications Nos. 3-228389 to 3-228391 each filed on August 13, 1991. Fig. 3 shows a pressure detector circuit of the kind defined hereinbefore at the beginning. This is the pressure detector circuit which is proposed in JPA No. 3-228391 and is similar to that of Fig. 2 except that reference resistor 11 and pressure sensor 10 are connected to ground and the output of amplifier 12, respectively, and, additionally, a switch 19 and a voltage follower 20 are provided within the feedback loop. The switch 19 is controlled by a signal SW to turn off the feedback loop when the window glass is driven downwards by a motor (not shown). The voltage follower 20 comprises another operational amplifier 20 having its inverting input connected to its output and a noninverting input of amplifier 12, and its noninverting input connected to low-pass filter 16. The circuit of JPA No. 3-228389 is similar to that of Fig. 2 except that as witch 19 and a voltage follower 20 are provided within the feedback loop. The circuit of JPA No. 3-228390 is similar to that of Fig. 2 except that the reference resistor 11 and pressure sensor 10 are connected to ground and the output of amplifier 12, respectively. A controller (not shown) shuts down the power supply to the motor to stop the window glass moving after detection of an obstacle between a window frame and the window glass when the obstacle is put therebetween upon closing the window glass. A pressure sensor is used to detect the obstacle and comprises a conductive rubber or coating sensitive to pressure. However, the above conventional pressure detector circuit cannot detect breakage, and short circuit conditions of pressure sensor 10, which is less reliable than other components such as resistors, capacitors and OP amps. In the circuit shown in Fig. 3, the input voltage of voltage follower 20 will be changed, upon turning off switch 19, by a dielectric characteristic of a printed circuit board, by ambient moisture or drift in the OP amps. The input voltage may be decreased slightly by a leakage current through the printed circuit board during movement of the window glass upwards, so that the circuit acts to stop the window glass as a result of detecting an erroneous output voltage which is more than the predetermined threshold voltage. In contrast, the input voltage may be increased slightly by another leakage current to a positive supply through the printed circuit board, so that the sensitivity of pressure sensor 10 is degraded. It is a primary object of the invention to provide a reliable pressure detector circuit which detects a broken or a shortcut condition of a pressure sensor and wiring thereto. According to the present invention, a pressure detector circuit of the kind defined hereinbefore at the beginning is characterised in that the feedback means is connected to an inverting input of said amplifier and includes a reference resistor for detecting a condition of said pressure sensor and a transistor in series with said reference resistor. Preferably, a second comparator is coupled to the pressure sensor to detect breakage of the pressure sensor , and a third comparator is coupled to the pressure sensor to detect short circuiting of the pressure sensor. The invention will be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 shows a perspective view of a coaxial pressure cable for use in an embodiment of the invention; Fig. 2 shows a conventional pressure detector circuit with a temperature compensation; Fig. 3 shows another conventional detector circuit having a voltage follower and a switch; Fig. 4 shows a pressure detector circuit similar to that of Fig. 3 but having a high resistor in parallel with the switch; Fig. 5 shows a pressure detector circuit having a controlled current source and embodying the invention; Fig. 6 shows a second embodiment of the pressure detector circuit according to the invention having a controlled current source and window comparators; Fig. 7 shows a third pressure detector circuit according to the invention having two controlled current sources each followed by a comparator; Fig. 8 shows a fourth embodiment which is a modification of the pressure detector circuit in Fig. 7; and Fig. 9 shows a fifth pressure detector circuit according to the invention having two controlled variable resistor elements each followed by a comparator. Referring now to Fig. 4, a pressure detector circuit is shown which includes a high, for example, 1MΩ resistor 21 in parallel with the switch 19, its resistance being higher than that of resistor 17. In Fig. 4, the same reference numerals are used to denote the same parts or members as those used in the circuit of Fig. 3. Thus, the pressure detector circuit comprises an operational (OP) amplifier 12 having its inverting input connected to the pressure sensor 10 as shown in Fig. 3 and a reference resistor 11. The pressure sensor 10 has another end connected to an output of amplifier 12 while reference resistor 11 has another end connected to ground. The output amplifier 12 is connected to an output terminal 13 to provide an output voltage Vo, and an inverting input of a three-state comparator 14. The three-state comparator 14 compares the output voltage Vo with a reference voltage Vr applied from a reference voltage source or regulator 15. Therefore, the comparator provides three-state voltage Vc, that is, a logical "H" level substantially identical to a positive power supply voltage Vcc, a logical "L" level substantially identical to ground or a negative power supply voltage - Vcc, and a balance voltage when the output voltage is equal to the reference voltage. The output of three-state comparator 14 is looped or fedback to a noninverting input of OP amplifier 12 through a low-pass filter 16, a switch 19 and a voltage follower 20. The low-pass filter 16 includes a 10 KΩ resistor 17 connected between the output of comparator 14 and switch 19, and a 100 µF capacitor 18 connected between the noninverting input of voltage follower 20 and ground. The switch 19 may be a relay contact or Complementary Metal Oxide Semiconductor (CMOS) transmission gate and is controlled by an SW signal (not shown) to turn off the feedback loop when the window glass is driven upwardly by a motor (not shown). The voltage follower 20 includes another OP amplifier 20 having its inverting input connected to its output and a noninverting input of amplifier 12, and its noninverting input connected to the capacitor 18. A 1 MΩ resistor 21 is provided in parallel with the switch 19 to prevent the feedback loop from shielding or blocking. Therefore, the pressure detector circuit in ordinary conditions provides an output voltage from the terminal 13 identical to the reference voltage with low drift characteristics even if the resistance of the unpressured pressure sensor 10 upon manufacturing is deviated from its mean value and if the ambient temperature of the automobile equipped therewith is changed. When a force is applied abruptly to the pressure sensor 10, the output voltage is increased from the reference potential due to the time constant of the low-pass filter and then slowly returns to the reference potential. Then, sudden pressure to the sensor 10 will be detected by another comparator (not shown) having a threshold voltage higher than the reference voltage. Fig. 5 shows a first embodiment of a pressure detector circuit of the invention. It detects conditions of the pressure sensor 10 by monitoring terminal voltage at an end of the reference resistor 11 connected to the inverting input of amplifier 12. In Fig. 5, the same reference numerals are used to denote the same parts or members as those used in Fig. 4, and detailed descriptions are omitted. In this embodiment, the pressure detector circuit comprises OP amplifier 12 having a grounded noninverting input. The OP amplifier 12 and three-state comparator 14 are therefore connected to a positive supply line (not shown) and negative supply line (or -Vcc line) 22 to provide, for example, ± 6 volts. It further comprises an NPN transistor 23, as a controlled current source to resistor 11, having an emitter connected to negative line 22, a base connected to the low-pass filter, and a collector connected to reference resistor 11. The capacitor 18 is connected to the base of transistor 23 and ground or negative line 22. Then, an output current of three-state comparator 14 is supplied to the base of transistor 23 through resistor 17 of the low-pass filter 16 and then looped or fedback from its collector to the inverting input of OP amplifier 12 through resistor 11. The collector current is controlled to be proportional to the resistance of pressure sensor 10, so that the output voltage is equal to, for example, a 3 volts' reference voltage from voltage source or regulator 15. Therefore, the second pressure detector circuit in normal conditions provides a 3 volts' output voltage from output terminal 13 with low drift characteristics even if the resistance of the unpressured pressure sensor 10 upon manufacturing deviates from the mean value of the resistances and if the ambient temperature of the automobile equipped therewith is changed. When a force is abruptly applied to the pressure sensor 10, the output voltage is increased from the 3 volts due to the time constant of the low-pass filter. Then, sudden pressure to the sensor 10 will be detected by another comparator (not shown) having a threshold voltage higher than the reference voltage. It should be noted that the terminal voltage of reference resistor 11 is proportional to the resistance of pressure sensor 10 because the voltage drop across the pressure sensor 10 is constant or identical to the reference voltage, and the same current flows through pressure sensor 10 and reference resistor 11 since the input current of OP amplifier 12 is assumed to be negligible. If the resistance of pressure sensor 10 is identical to that of resistor 11, then the terminal voltage will be minus 3 volts. If pressure sensor 10 is shorted to provide zero ohm, then the terminal voltage will go minus 5.5 volts upon assuming the saturated collector to emitter voltage to be 0.5 volt. If pressure sensor 10 is broken to provide infinite resistance, then the terminal voltage will fall to zero volts. Therefore, a shortcircuit or broken condition of the pressure sensor can be monitored from the voltage at the output terminal 13. Fig. 6 shows a second embodiment of a pressure detector circuit of the invention in which upper and lower limits of resistance for sensor 10 are provided by using comparators 24 and 25 to detect breakage and shortcircuit conditions of pressure sensor 10. The second comparator 24 for detecting breakage has a noninverting input connected to the collector of transistor 23, and an inverting input connected to ground through a second reference voltage source 26 having a low voltage less than minus 0.6 volt. The voltage source 26 may be a Schottky or general use silicon or germanium diode and a pull-up resistor from -Vcc. To provide less than 0.1 v, a voltage divider or potentiometer may be paralleled to the diode 26. When pressure sensor 10 is broken, the sensor current is reduced to zero milliampere because the pressure detector circuit controls itself so that the output voltage maintains the reference potential. Therefore, the terminal voltage is reduced to zero volt. The third comparator 25 for detecting the shortcircuit condition has an inverting input connected to the collector of transistor 23, and a noninverting input connected to the second voltage source 26 through a third reference voltage source 27 of more than minus 1.25 V. The voltage source 27 may be a bandgap or shunt voltage regulator cascaded to diode 26 and another pull-up resistor from -Vcc. When pressure sensor 10 is shorted, the sensor current will be increased to the maximum rating current or about 20 milliamperes of OP amplifier 12 because the pressure detector circuit self-controls so that the output voltage maintains the reference potential. However, the sensor current is limited by reference resistor 11 and the terminal voltage increases to about minus 5.5 volts with saturation of transistor 23. Alternatively, the reference voltage sources 26 and 27 may include two diodes connected to ground and power supply line 22, respectively and a pull-up resistor connected between the diodes. In practice, the resistance of pressure sensor 10 is changed exponentially between, for example, 390 KΩ and 100Ω corresponding to linear unpressured to pressured conditions thereof, respectively. Fig. 7 shows a third pressure detector circuit according to the invention that has two controlled current sources and two reference resistors. In Fig. 7, components, parts or members corresponding to those of Fig. 6 have the same reference numerals, respectively. This pressure detector circuit comprises an OP amplifier 12 having an inverting input connected to an output through a pressure sensitive sensor 10. The amplifier 12 also has a noninverting input connected to ground, and the output connected to an output terminal 13 and an inverting input of three-state comparator 14. A noninverting input of the comparator 14 is connected to a reference voltage source 15 that applies a positive reference voltage thereto. Its output is connected to a base of an NPN transistor 23 through a low-pass filter 16 and a voltage divider 30. The transistor 23 is emitter-coupled to a negative supply line 22 for amplifier 12 and comparator 14, and has its collector connected to the inverting input of amplifier 12 through, for example, 100Ω reference resistor 11. The pressure detector circuit controls the collector current of transistor 23, or the sensor current of pressure sensor 10, so that the output voltage of amplifier 12 is equal to the reference voltage under the unpressured condition of pressure sensor 10. Conditions of pressure sensor 10 are detected by monitoring the collector current or a current flowing through 100 Ω reference resistor 11. Accordingly, connected to the collector of transistor 23 is a comparison input of the third comparator 25 that detects a shortcircuit condition of pressure sensor 10, and has its reference input connected to a second reference voltage source 31 that applies a negative reference voltage. The low-pass filter 16 includes a resistor 17 connected between the output of three-state comparator 14 and a junction J to voltage divider 30, and a grounded capacitor 18 connected to junction J to derive a time constant from the product of the resistance of resistor 17 and capacitance of capacitor 18. The voltage divider 30 includes a pair of resistors 32 and 33 in series to negative supply line 22 and the junction connected to the base of the transistor 23 to provide a first division ratio, for example, 1/3. Additionally connected to the inverting input of OP amplifier 12 is a collector of a second NPN transistor 35 through a second or, for example, 390 kΩ reference resistor 36 that detects a broken condition of pressure sensor 10. The second transistor 35 is also emitter-coupled to the negative supply line 22. On the other hand, junction J of low-pass filter 16 is connected to a base of second transistor 35 through a second voltage divider 37. Connected to the collector of transistor 35 is a comparison input of the second comparator 24, which detects the broken condition of pressure sensor 10, and has its reference input connected to the second reference voltage source 31. The second voltage divider 37 also includes a pair of resistors 38 and 39 in series to line 22 to provide a second division ratio, for example, 1/2. Then, the first division ratio is preferably set less than the second division ratio so that transistor 23 is active upon saturation of transistor 35, and transistor 23 is in the off state upon activation of transistor 35. Fig. 8 is a variation of Fig. 7. The only change is that second reference resistor 36 is connected to the collector of transistor 23, and resistor 32 is divided into two resistors 32a and 32b to provide an intermediate node to be connected to the base of transistor 35 through a 100 kΩ limit resistor 34. The resistance of the limit resistor 34 is chosen to not affect division ratios for maintaining the saturated condition of transistor 35. The pressure detector circuits shown in Figs. 4 to 8 have advantages to compensate resistance drifts of pressure sensor 10 by temperature characteristic, aging deterioration, and variations in manufacturing. For example, the output voltage of terminal 13 will be increased or go up from the reference potential when the change of ambient temperature affects the resistance Rs of pressure sensor 10. However, three-state comparator 14 goes low level to decrease the base current of transistor 23 through low-pass filter 16, and then its collector current or the sensor current, and finally returns the output voltage to reference potential. Then, the output voltage remains substantially the reference potential without its differential voltage because the rate of change of the ambient temperature is lower than the time constant of low-pass filter 16. Upon pressing of pressure sensor 10 by an obstacle, the rate of change of resistance Rs in higher than the time constant and results in a change of the output voltage that is detected by another comparator (not shown) which stops or reverses a drive motor (not shown) for the window glass. However, the pressure detector circuits shown in Figs. 4 and 8 may be subject to ambient noise because the pressure sensor 10 as shown in Fig. 1 has the same configuration as an ordinary coaxial cable and is disposed as a negative feedback resistor. Consequently, a drift in the output voltage of amplifier 12 is generated. If to suppress the drift, a noise suppression capacitor is placed in parallel with pressure sensor 10, the capacitance of the noise suppression capacitor is constant while the resistance of pressure sensor 10 is changed. When the resistance of pressure sensor 10 moves to higher value, the rate of change of the output voltage is small relative to that of resistance Rs at the higher value and causes a phase delay to the pressure response and the sensitivity to pressure of the detector circuit is degraded. The resistance Rs is changed widely by the ambient temperature, the sensor current flowing through pressure sensor 10, and the pressure P applied. Fig. 9 shows a fifth embodiment of a pressure detector circuit according to the invention giving highly reliable obstacle detection and noise tolerance. In Fig. 9 components, parts or members corresponding to those of Fig. 7 have the same reference numerals, respectively and detail descriptions are omitted. The pressure detector circuit according to the invention comprises a pressure sensor 10 as shown in Fig. 1 having its resistance responsive to pressure applied. The pressure sensor 10 is connected between an inverting input of amplifier 12 and ground, so that its end or core electrode 1 is connected to the inverting input and its sheath electrode 4 is connected to ground to shield the core electrode 1 to enhance the noise tolerance characteristic of the core electrode 1. The amplifier 12 receives power from a power supply unit providing, for example, 12 volts. A reference voltage source 40 that applies a positive reference voltage is connected between a noninverting input of amplifier 12 and ground. Alternatively, upon using positive and negative power supplies, the noninverting input is grounded and the other end or sheath electrode 4 of pressure sensor 10 is connected to a negative reference voltage source 40. In both cases, a first constant reference voltage is applied to pressure sensor 10. A Field Effect Transistor (FET) 41 has its source coupled to the inverting input of amplifier 12. The FET 41 has its drain connected to a detecting output of amplifier through, for example 390 KΩ resistor 42 for detecting a break in pressure sensor 10 and associated wiring. The detecting output of amplifier 12 is connected to an output terminal 13 and a n inverting or comparison input of a three-state comparator 14. The comparator 14 has its non-inverting or reference input connected to a second reference voltage source 15, and its comparison output connected to a gate of FET 41 through a low-pass filter 16 and a voltage follower 44. The second reference voltage source 15 supplies a second reference voltage twice the first reference voltage. Accordingly, the comparator compares the detecting voltage of amplifier 12 with the second reference voltage to obtain a resulting comparison signal to be applied to the gate of FET 41 through the low-pass filter 16 and voltage follower 44 to control the detecting output voltage to be equal to the second reference voltage or potential. Then, the detecting output or output voltage from terminal 13 detects the clamping of an obstacle in response to the pressure applied to pressure sensor 10 by an obstacle being clamped by closure of for example, a window glass of an automobile door. The comparison signal from the comparison output has three states or conditions; a balancing voltage when the output voltage is equal to the second reference voltage, a low level voltage when the output voltage is less than the second reference voltage due to pressing of pressure sensor 10, and a high level voltage when the output voltage is up from or more than the second reference voltage due to, for example, sudden heating of the pressure sensor 10. General use or low voltage type dual or quadruple operational amplifiers (or integrated circuits) are used for amplifier 12, comparator 14 and voltage follower 44. The drain of FET 41 connected to the break detecting resistor 42 is coupled or branched to a comparison input of a second comparator 24 that detects a first condition or break condition of pressure sensor 10. The second comparator 24 has its reference input connected to a third reference voltage source 31. The third reference voltage source 31 provides a third reference voltage midway between the first and second reference voltages. With the 390 KΩ resistor 42, pressure sensor 10 is deemed to be broken when an internal or drain-source resistance is higher than 390 KΩ. Therefore, the limit resistance of pressure sensor 10 considered as a break is 780 KΩ. A capacitor 43 is paralleled to resistor 42 to compensate drift changes and drift phase delays in the output voltage. On the other hand, a second FET 51 has its source coupled to the inverting input of amplifier 12. The FET 51 has its drain connected to the detecting output of amplifier 12 through, for example 100 Ω resistor 52 for detecting a short-circuit of pressure sensor 10 and associated wiring. A voltage divider circuit 54 is provided between a gate of FET 51 and voltage follower 44. The divider 54 includes two series resistors 55 and 56, or a 3-terminal variable resistor such as potentiometer. Its division ratio is predetermined so that FET 51 is active when the voltage applied to the gate of FET 41 holds FET 41 "ON" or in its saturated condition. Accordingly, during activation of FET 51, FET 41 becomes saturated or, for example, 50 Ω. The saturated FET 41 does not affect FET 51 because there is negligible current flowing through 390 KΩ resistor 42 in series with FET 41. With the 100 Ω resistor 52, pressure sensor 10 is deemed to be shorted when an internal or drain-source resistance is less than 100 Ω. Therefore, the limit resistance of pressure sensor 10 considered as a shortcircuit is 200 Ω Another capacitor 53 is paralleld to resistor 52 to compensate drift changes and drift phase delays of the output voltage. A 47 pF capacitor 57 is connected between the inverting input and output of amplifier 12 to absorb noises and low drifts of the output voltage. The low-pass filter 16 in this embodiment comprises a 51 Ω low resistor 17, a 1 MΩ high resistor 58 connected between the low resistor 17 and the output of comparator 14, and a switch 59 connected in parallel with the high resistor 58. The low resistor 17 has another end connected to a 22 µF capacitor 18 and an input of voltage follower 44. The voltage follower 44 prevents the output impedance of low-pass filter 16 from acting as an input impedance of voltage divider 54. Therefore, it applies a voltage identical to a terminal voltage of capacitor 18 to the gate of FET 41. Though an OP amplifier is used as voltage follower 44 in this embodiment, the voltage follower may be an emitter follower or source follower. The switch 59 is controlled by a control signal (not shown) to turn off during closing of the window glass. The switch 59 compensates for drifts in dielectric characteristics of the pressure detector circuit printed board caused by ambient moisture associated with resistor 17 upon turning on. The above embodiment uses variable resistance characteristics in low drain to source voltage regions of FETs 41 and 51 controlled by gate voltages thereof. These FETs can be replaced with NPN transistors having inherent controllable current regulated characteristics when emitter to collector voltages are more than 0.5 volt. An NPN transistor replacing FET 41 is emitter-coupled to the inverting input of amplifier 12, and has a collector connected to the detecting output of amplifier 12 through the break detecting resistor 42 and a base connected to the output of voltage follower 44 through an additional 1 KΩ protection resistor. Another NPN transistor replacing FET 51 is also emitter-coupled to the inverting input of amplifier 12, and has a collector connected to the detecting output of amplifier 12 through the shortcircuit detecting resistor 52 and a base connected to the voltage divider 54. The operation of the pressure detector circuit in this embodiment is described as follows. First, when power to the detector circuit is turned on, the three-state comparator 14 charges capacitor 18 through switch 59 and resistor 17 to provide a normal condition in which the output voltage of amplifier 12 is equal to the second reference potential even if the resistance of pressure sensor 10 is deviated. In the normal condition, the output voltage of amplifier 12 becomes equal to the second reference potential by the action of comparator 14, low-pass filter 16, voltage follower 44 and FET 51 even if the resistance of pressure sensor 10 is gradually changed by change of atmosphere or ambient temperature. Then, the internal resistance (drain to source resistance) of FET 41 is assumed to be R Q1 , the resistance of resistor 42 is assumed to be R 1 , the internal resistance of FET 51 is assumed to be R Q2 , and the resistance of resistor 42 is assumed to be R 2 . In the normal condition, the following equation is derived since the voltage difference between the inverting input and detecting output of amplifier 12 is identical that between the inverting input and ground. Rs = (R Q1 + R 1 ) ∥ (R Q2 + R 2 ) When the window glass is driven toward a closing direction by a motor (not shown), switch 59 is controlled to turn off synchronously. The voltage drop across resistors 58 and 17 is increased slightly since a leakage current to capacitor 18 and input current to voltage follower 44 are supplied through resistors 58 and 17. However, the output of comparator 14 will absorb such an increased component of the voltage drop to maintain the normal condition. When pressure sensor 10 is pressed by an obstacle between the window glass and the window frame, its resistance is abruptly decreased to increase the output voltage of amplifier 12 from a normal value identical to the second reference voltage. The comparison signal or output of comparator 14 goes also abruptly up to an upper saturated voltage near the positive power supply voltage to charge gradually capacitor 18 through resistors 58 and 17. The output voltage of the detecting output goes up within a predetermined period because the output voltage of voltage follower 44 or gate voltages of FETs are almost all maintained constant or on a gentle gradient. The capacitor 57 is necessary to suppress low drifts of the output voltage. As resistance Rs increases, drift in the output voltage to the resistor change decreases, and its response time increases. Though it results in deterioration of pressure detecting sensitivity, the addition of capacitors 43 and 53 resolves this problem. As the resistance of pressure sensor 10 becomes large (or small), effect of capacitor 43 and 53 became small (or large) to increase (or decrease) internal resistances of FETs to provide an appropriate response characteristic of the output voltage automatically. Accordingly, at application of pressure to pressure sensor 10, the output voltage goes above the predetermined threshold, a fourth comparator (not shown) detects this, and then further means latch or memorize its pressure detecting output. A control circuit (not shown)controls the window glass to open it based on bit data already latched and allow removal of the obstacle from the window space. If the closing action of the window glass is simply stopped for a pressed or clamped condition of pressure sensor 10 in which a recessed portion of an obstacle is held or clamped between the window glass and window frame, the output voltage reached at the detecting level returns gradually to the second reference potential or voltage, and it is difficult to remove the recessed obstacle. Therefore, it is preferable to switch from the closing to the opening of the window glass when clamping of the obstacle is detected. After the window glass is opened completely, switch 59 is controlled to turn on to discharge quickly through resistor 17 and return to the normal condition. As described above, conditions of pressure sensor 10 are generally monitored under the normal condition in which the output voltage is equal to the second reference potential. The pressure sensor 10 has a resistance Rs which changes between 780 KΩ and 200 Ω. The resistance limits are detected by the drain voltage of FET 41 or 51. For the resistance Rs more than 780 KΩ, a comparison output of comparator 24 goes H logic level. Therefore, the break condition of pressure sensor 10 is indicated by outputting the H logic level. When pressure sensor 10 is broken completely, the output voltage is equal to the inverting input voltage of amplifier 12. For the resistance Rs less than 200 Ω, the comparison output of comparator 25 goes to the H logic level. Therefore, shortcircuit condition of pressure sensor 10 is indicated by outputting the H logic. level. When pressure sensor 10 is shorted completely, the output voltage is increased to about positive power supply voltage. Thus, it will be appreciated that various modification, alternatives, variations, etc., may be made without departing form the scope of the invention as defined in the appended claims, It is, of course, intended to cover by the appended claims all such modification as fall within the scope of the claims.

Claims: A pressure detector circuit comprising: a pressure sensor (10) having a resistance that changes in response to change in the pressure applied; an amplifier (12) operatively connected to said pressure sensor (10) and providing at its output an output signal; a three-state comparator (14) connected to said output of said amplifier (12) and for comparing said output signal with a first reference voltage (15) to provide a comparison output signal from its comparison output; a low-pass filter (16) connected to said comparison output; and a feedback means for said amplifier (12) comprising said low-pass filter (16), characterised in that the feedback means is connected to an inverting input of said amplifier (12) and includes a reference resistor (11) for detecting a condition of said pressure sensor (10) and a transistor (23) in series with said reference resistor (11). A pressure detector circuit according to claim 1, characterised in that a second comparator (24) is coupled to the pressure sensor (10) via said reference resistor (11) to detect breakage of the pressure sensor (10), and a third comparator (25) is coupled to the pressure sensor (10) via said reference resistor (11) to detect short circuiting of the pressure sensor (10). A pressure detector circuit according to claim 2, characterised in that said low-pass filter (16) comprises a low resistor (17) connected between said output of said three-state comparator (14) and said amplifier (12) through a switch (59), and said switch (59) is provided in parallel with a high resistor (58) having its resistance higher than that of said low resistor (17). A pressure detector circuit according to claim 2, characterised in that said second comparator (24) is connected to a node of said reference resistor (11) and said transistor (23) and to a second reference voltage (26), and said third comparator (25) is connected to both said node and a third reference voltage (27). A pressure detector circuit according to claim 2, characterised in that said feedback means further comprises: a second reference resistor (36) for detecting another condition of said pressure sensor (10) and a second transistor (35) in series with said second reference resistor (36), said second transistor (35) being connected to said low-pass filter (16) through a voltage divider (38, 39); and in that said second comparator (24) is connected both to a node of said second reference resistor (36) and said second transistor (35) and to a second reference voltage (31), and said third comparator (25) is connected both to a node of the first reference resistor (11) and the first transistor (23) and to said second reference voltage (31). A pressure detector circuit according to claim 5, characterised in that said pressure sensor (10) is connected to the inverting input of said amplifier (12); said first transistor (23) being a bipolar transistor with its emitter coupled to a power supply line (22), and having its base connected to said low-pass filter (16) through another voltage divider (32,33) and its collector connected to said inverting input of said amplifier (12) through said first reference resistor (11); and said second transistor (35) being also a bipolar transistor with its emitter coupled to said power supply line (22), and having its base connected to said low-pass filter (16) through the first voltage divider (38, 39) and its collector connected to said inverting input of said amplifier (12) through said second reference resistor (36). A pressure detector circuit according to claim 2, characterised in that said pressure sensor (10) is connected to the inverting input of said amplifier (12); and the first transistor is a field effect transistor (51) with its source coupled to said inverting input, and has its gate connected to said low-pass filter (16) through a voltage divider (54), and its drain connected to said output of said amplifier (12) through the first reference resistor (52); in that a second transistor which is also a field effect transistor (41) with its source coupled to said inverting input has its gate connected to said low-pass filter (16), and its drain connected to said output of said amplifier (12) through a second reference resistor (42); and in that said second comparator (24) is connected both to the drain of the second transistor (41) and to a second reference voltage (31), and said third comparator (25) is connected both to the drain of the first transistor (51) and to said second reference voltage (31). A pressure detector circuit according to claim 5, characterised in that said pressure sensor (10) is connected to said inverting input of said amplifier (12); said first transistor being a bipolar transistor (23) with its emitter coupled to a power supply line (22), and having its base connected to said low-pass filter (16) through said voltage divider (32a, 32b, 33) and its collector connected to said inverting input of said amplifier through said first reference resistor (11); and said second transistor being also a bipolar transistor (35) with its emitter coupled to said power supply line (22), and having its base connected to said low-pass filter (16) through a limit resistor (34) and said voltage divider (32a,32b,33) and its collector connected to said inverting input through said first and second reference resistors (11,36),said second reference resistor (36) being in series with said first reference resistor (11). A pressure detector circuit according to claim 7, characterised by a voltage follower (44) connected between said low-pass filter (16) and said voltage divider (54). A pressure detector circuit according to claim 2 or 7 or 9, characterised in that the low-pass filter (16) connected between the comparison output of said three-state comparator (14) and said amplifier (12) includes a low resistor (17) which is connected between said comparison output of said three-state comparator (14) and said amplifier (12) through both a switch (59) and a high resistor (58) having its resistance higher than that of said low resistor (17) and the switch (53) being in parallel with the high resistor (58).

Inventors: YAGUCHI OSAMU, YAGUCHI, OSAMU, YAGUCHI, OSAMU, C/O KABUSHIKI KAISHA RIKEN
Applicants: RIKEN KK, KABUSHIKI KAISHA RIKEN
Application year: 1993
Main classification: G01R31
IPC codes: E05F15, G01L1, G01R31, B60J7