Abstract:
A sensor device includes a sensor circuit for measuring a physical amount such as pressure, a voltage detector for detecting a voltage actually supplied to the sensor circuit, and an oscillator for generating an oscillating signal when the voltage detector finds the voltage supplied to the sensor circuit is abnormally low. The sensor signal and the oscillating signal are selectively supplied to a controller. The controller controls various devices connected thereto based on the sensor signal, while it detects a malfunction of the sensor device based on either a high level signal or a low level signal in the oscillating signal. Thus, the malfunction of the sensor device is automatically detected without fail.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-366459 filed on Dec. 24, 1999, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sensor device that includes a function in itself to detect whether it is normally functioning or malfunctioning. The sensor device is used for measuring a physical amount such as pressure, acceleration, yaw rate, or the like. 
     2. Description of Related Art 
     A sensor device for measuring various pressures in an automobile vehicle, such as a braking pressure or a fuel pressure, is connected to a control device such as an electronic control unit (ECU) that controls operation of various devices based on outputs of the sensor device. The sensor device is powered by a power source voltage. A signal representing a detected physical amount is amplified and adjusted in the sensor device and then supplied to the control device as a voltage proportional to the measured physical amount. 
     The sensor device is connected to the control device via a power source line for supplying power to the sensor device from the control device, a ground line, and an output line through which the sensor outputs are fed to the control device. Those connecting lines are usually connected by connectors, couplers, soldering, welding or the like. Accordingly, there is a possibility that such connections become loose or incomplete, thereby increasing contact resistance at junctions. If resistance in the power source line or the ground line increases, a power source voltage supplied to the sensor device decreases. This results in a malfunction of the sensor device, that is, the outputs of the sensor device do not correctly represent the physical amount to be measured. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved sensor device, in which the sensor malfunction due to the contact resistance increase in the connecting lines is automatically detected. 
     The sensor device of the present invention includes a sensor circuit, a voltage detector and an oscillator. Preferably, those components are all formed on a single semiconductor substrate. The sensor circuit measures a physical amount such as pressure, acceleration, yaw-rate or the like and outputs a sensor signal (a first signal) representing a measured physical amount. The voltage detector detects a voltage actually supplied to the sensor circuit and outputs a detector signal when the voltage supplied to the sensor circuit is lower than a predetermined level. The oscillator generates a signal (a second signal) oscillating between a high level and a low level, triggered by the detector signal fed from the voltage detector. The second signal indicates that the sensor circuit is malfunctioning because the sensor circuit is operating under a voltage lower than a predetermined level. 
     The first signal, which is a normal sensor signal, and the second signal indicating the malfunction of the sensor device are selectively supplied to a control device connected to the sensor device. The control device controls various devices connected thereto based on the first signal, while it detects the malfunction of the sensor device based on the second signal. 
     The maximum level of the first signal is set to a level (e.g., 4.5 V) lower than a normal power source voltage level (e.g., 5.0 V), and its minimum level (e.g., 0.5 V) is set to a level higher than a ground level. The high level of the second signal is set to a level higher than the maximum level of the first signal, and the low level is set to a level lower than the minimum level of the first signal. 
     The control device connected to the sensor device normally receives the first signal (the sensor signal), while it receives the second signal (the oscillating signal indicating the malfunction of the sensor device) when the voltage actually supplied to the sensor device is lower than the predetermined level. 
     The selection between the first signal and the second signal may be made by a switching circuit formed in the sensor device. Alternatively, a circuit having a low-pass filter that allows only the first signal having a low frequency to pass through and a high-pass filter that allows only the second signal having a high frequency to pass through may be used for selecting the signals. 
     The second signal is not fixed to one voltage level, but it oscillates between the high level and the low level, and the malfunction of the sensor device is detected when either the high level or the low level signal is detected. Therefore, the malfunction of the sensor device caused either by a voltage decrease in the power supply line or by a voltage increase in the ground line is surely detected without fail. 
     Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a sensor device connected to a control device, as a comparative example to embodiments of the present invention; 
     FIG. 2 is a block diagram showing a sensor device connected to a control device, as a first embodiment of the present invention; 
     FIG. 3 is a circuit diagram showing a voltage detector used in the sensor device shown in FIG. 2; 
     FIG. 4 is a circuit diagram showing an oscillator used in the sensor device shown in FIG. 2; 
     FIG. 5 is a graph showing waveforms of a sensor output and an oscillator output; and 
     FIG. 6 is a graph showing waveforms of a low-pass filter output and a high-pass filter output in a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to realize the object of the present invention, a prototype of a sensor device shown in FIG. 1 is made and tested. This prototype is shown and described here as a comparative example to the embodiments of the present invention. A sensor device  20  is connected to a control device  10  through a power source line P, an output line O and a ground line G. A resistance RP represents a contact resistance in the power source line P, and a resistance RG represents a contact resistance in the ground line G. 
     A power source terminal PS of the sensor device  20  is connected to a power source terminal PE of the control device  10  via the power source line P through which power is supplied from the control device  10  to the sensor device. An output terminal OS of the sensor device  20  is connected to an input terminal OE of the control device  10  via the output line O through which an output voltage of the sensor device  20  is fed to the control device  10 . A ground terminal GS of the sensor device  20  is connected to a ground terminal GE of the control device via the ground line G through which both the sensor device  20  and the control device  10  are grounded. A power source voltage Vcc is supplied to the power source terminal PE through a regulator  11 . 
     The sensor device  20  is composed of a sensor circuit  21 , amplifier circuit  22 , a switching circuit  23  and a voltage detector  24 . The sensor circuit  21  is connected between the power source terminal PS and the ground terminal GS. The voltage detector  24  is also connected between PS and GS to detect a voltage ΔV (a difference between a voltage Vps at PS and a voltage Vgs at GS) supplied to the sensor circuit  21 . The output of the sensor circuit  21  is fed to the amplifier circuit  22  through the switching circuit  23 . When the voltage detector  24  detects that the voltage ΔV supplied to the sensor circuit  21  is lower than a predetermined level, the switching circuit  23  switches the output signal from the sensor circuit  21  to a signal level which is outside a sensor signal range, based on the signal fed from the voltage detector  24 . Thus, a malfunction of the sensor device  20  is notified to the control device  10 . 
     For example, assuming that the power source voltage Vcc is 5 V, and the normal sensor output is set in a range of 0.5 V-4.5 V, the switching circuit  23  switches the sensor signal to a low level signal LL (e.g., 4% of ΔV) or to a high level signal HL (e.g., 96% of ΔV) when ΔV becomes lower than a predetermined level. In other words, the LL or HL signal appears at the output terminal OS when the malfunction in the sensor device  20  is detected. The control device  10  detects the malfunction upon receipt of the LL or HL signal. 
     The voltage detector  24  determines that a malfunction has occurred if ΔV is out of a predetermined voltage range. In this case, however, whether the voltage Vps has decreased due to an increase of the contact resistance RP, or Vgs has increased due to an increase of the contact resistance RG cannot be determined. This brings about the following problem. For example, if the malfunction signal level is set to HL (96% of ΔV), and the voltage ΔV becomes lower than the predetermined level (e.g., 4.5 V) due to increase of the contact resistance RP, the output terminal voltage Vos relative to the ground level (Vge) becomes 4.32 V (=4.5 V×96%) which is within the normal sensor output range. If the voltage ΔV becomes lower than the predetermined level due to increase of the contact resistance PG, the output terminal voltage Vos relative to the ground level (Vge) becomes 4.82 V (=4.5 V×96%+0.5 V) which is outside the normal sensor output range, because the voltage Vgs increases by 0.5 V due to increase of the contact resistance RG. This means that the terminal voltage Vos cannot indicate the malfunction when Vps decreases, while it correctly indicates the malfunction when Vgs increases. 
     On the other hand, if the malfunction signal level is set to LL (4% of ΔV), and the voltage ΔV becomes lower than the predetermined level (e.g., 4.5 V) due to increase of the contact resistance RP, the output terminal voltage Vos relative to the ground level (Vge) becomes 0.18 V (=4.5 V×4%) which is outside the normal sensor output range. If the voltage ΔV becomes lower than the predetermined level due to increase of the contact resistance RG, the output terminal voltage Vos relative to the ground level (Vge) becomes 0.68 V (=4.5 V×4+0.5 V) which is within the normal sensor output range, because the voltage Vgs increases by 0.5 V due to increase of the contact resistance RG. This means that the terminal voltage Vos cannot indicate the malfunction when Vgs increases, while it correctly indicates the malfunction when Vps decreases. 
     A first embodiment of the present invention shown in FIGS. 2-5 has been made to overcome the problem found in the above-described comparative example. The sensor device  20  is for use in an automobile and is designed for measuring an oil pressure in a braking system, a fuel pressure in a fuel injection system or the like. The sensor device  20  shown in FIG. 2 is similar to the sensor device shown in FIG. 1, except that an oscillator  25  is added between the voltage detector  24  and the switching circuit  23 . The sensor device  20  is connected to the control device  10  in the same manner as in the comparative example shown in FIG.  1  and functions similarly. Therefore, only the structure and function different from those of the comparative example will be described below. 
     The power source voltage Vcc (e.g., 12 V) is supplied to the control device  10  through a regulator  11  that converts Vcc to a sensor power source voltage Vps (e.g., 5 V). The sensor circuit  21  outputs a first signal representing a measured pressure, and the oscillator  25  outputs a second signal indicating a malfunction of the sensor device  20  caused by decrease of the voltage ΔV imposed on the sensor circuit  21 . The first and the second signal are selectively switched by the switching circuit  23  and are led to the output terminal OS through the amplifier  22 . 
     The sensor circuit  21  is composed of a Wheatstone bridge including four gauge resistors such as diffused resistors formed on a thin diaphragm (not shown) of a silicon semiconductor substrate. The oscillator  25  outputs an oscillating signal triggered by the output of the voltage detector  24  that indicates the voltage ΔV has become lower than a predetermined level. The sensor circuit  21 , the amplifier circuit  22 , the voltage detector  24  and the oscillator  25  are all formed on a single semiconductor substrate. 
     FIG. 3 shows details of the voltage detector  24  connected to the oscillator  25 . The voltage detector determines that a malfunction occurred if the voltage ΔV (Vps−Vgs) becomes lower than a predetermined level (e.g., 4.5 V). As shown in FIG. 3, the voltage ΔV is divided by resistors to obtain a divided voltage Vd. The divided voltage Vd is compared with a standard voltage Vs which is independent from the voltage ΔV in a comparator  24   a . The comparator  24   a  outputs a malfunction signal to trigger the oscillator  25  when the voltage ΔV becomes lower than the predetermined level. 
     Referring to FIG. 4, operation of the oscillator  25  will be described. A transistor  25   a  is turned on by the malfunction signal fed from the voltage detector  24 , and current is supplied to a capacitor  25   c  through a resistor  25   b . The capacitor  25   c  is charged with a time constant determined by the resistor  25   b  and the capacitor  25   c . A comparator  25   d  compares a terminal voltage of the capacitor  25   c  with a standard voltage  25   f . When the terminal voltage of capacitor  25   c  is higher than the standard voltage  25   f , the comparator  25   d  outputs a high level signal HL (e.g., 96% of ΔV) which exceeds a normal sensor output range (e.g., 0.5 V-4.5 V). A transistor  25   e  is turned on by the high level signal HL, and thereby the capacitor  25   c  is discharged through a resister  25   g  with a time constant determined by the capacitor  25   c  and the resistor  25   g . When the terminal voltage of the capacitor  25   c  is lower than the standard voltage  25   f , the comparator  25   d  outputs a low level signal LL (e.g., 4% of ΔV) which is lower than the normal sensor output range. 
     The oscillator  25  functions as described above and outputs the second signal oscillating between the high level signal HL and the low level signal LL. The frequency and waveform of the oscillating signal can be arbitrarily set by selecting the capacitance of the capacitor  25   c  and the resistance of the resistors  25   b ,  25   g.    
     The switching circuit  23  normally outputs the sensor signal (the first signal) fed from the sensor circuit  21 , while it outputs the oscillating signal (the second signal) when the oscillating signal is fed from the oscillator  25 . The outputs from the switching circuit  23  are shown in FIG.  5 . As seen from the graph of FIG. 5, when the voltage ΔV is within the predetermined range, the switching circuit  23  outputs the normal sensor output as the first signal. When the voltage ΔV is out of the predetermined range (the sensor device is malfunctioning), the switching circuit  23  outputs the oscillating signal as the second signal. 
     If the second signal indicating the malfunction of the sensor device  20  is fixed to either the high level HL or the low level LL, the malfunction cannot be detected under certain conditions, as described above in connection with the comparative example. As the second signal in the first embodiment is a signal oscillating between HL and LL, the malfunction of the sensor device  20  is always detected without fail. That is, when the high level signal HL, with which the malfunction cannot be detected because it falls within the normal sensor output range, is generated due to the Vps decrease, the malfunction is detected by the low level signal LL that immediately follows the high level signal HL. Similarly, when the low level signal LL, with which the malfunction cannot be detected because it falls within the normal sensor output range, is generated due to the Vgs increase, the malfunction is detected by the high level signal HL that immediately follows the low level signal LL. In short, the malfunction is always detected by the second signal oscillating between HL and LL, not depending on the causes of the ΔV decrease. 
     The control device  10  performs periodical sampling (e.g., with an interval of 5-10 m sec) of the signal fed from the sensor device  20 . Accordingly, if an oscillating period of the second signal is set to about 1.5 times the sampling interval, at least one high level signal HL or one low level signal LL can be detected during three times of the sampling operation. 
     A second embodiment of the present invention will be described with reference to FIG.  6 . In this embodiment, a low-pass filter  12  as a first filter and a high-pass filter  13  as a second filter is connected to the output terminal OS of the sensor device  20 . Outputs of the respective filters are connected to the input terminal of the control device  10 . As shown in FIG. 6, only the sensor output signal (the first signal) having a low frequency passes through the low-pass filter  12 . On the other hand, only the malfunction signal (the second signal) having a high frequency passes through the high-pass filter  13 . 
     In other words, the sensor signal is fed to the control device  10  when the sensor device  20  is normally operating, while the malfunction signal is fed when the sensor device is malfunctioning. Therefore, the first signal passed through the low-pass filter  12  is utilized to control various devices connected to the control device  10 , and the second signal passed through the high-pass filter  13  is utilized to detect the malfunction of the sensor device  20 . 
     The switching circuit  2 . 3  may be replaced with a circuit composed of the low-pass filter  12  and the high-pass filter  13 . 
     The gauge resistors used in the Wheatstone bridge constituting the sensor circuit  21  may be replaced with other resistors. For example, thin film resistors made of a temperature-insensitive metal such as CrSi, or other sensors such as capacitor-sensors may be used. Though the sensor device  20  is described as a pressure sensor, it can be used also as an acceleration sensor, a yaw-rate sensor or the like. 
     While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.