Patent Publication Number: US-2012035824-A1

Title: Abnormality detection device for detection circuit and electric circuit, and detection system and electronic system which uses abnormality detection device

Description:
TECHNICAL FIELD 
     The present invention relates to an abnormality detection device for detecting abnormality in an electric circuit, and more particularly abnormality in a detection circuit, and a detection system or an electronic system which uses the abnormality detection device. 
     BACKGROUND ART 
     There has been known a detection system which detects a pressure of a negative pressure booster which assists a braking force of a braking device (brake) of a vehicle. The detection system is constituted of a pressure sensor which detects a negative booster pressure and a processing device (for example, ECU) which processes an output from the pressure sensor. In such a detection system, there may be a case where even when abnormality exists in a sensor circuit, a detection signal level falls within a normal range thus giving rise to a case where the detection of abnormality in the sensor circuit is difficult. As a method of detecting abnormality in a sensor circuit, there has been proposed a technique which uses test pulses described in patent documents 1 and 2. 
     Patent document 1 discloses a brake system which executes an antilock operation of a vehicle using a detected value of a vehicle speed sensor, wherein when the detected value of the vehicle speed sensor indicates a value less than a predetermined value, a defect of an electric circuit including the vehicle speed sensor is detected. In the system, a vehicle speed sensor  18  is connected to a sensor signal condition imparting circuit  36  through an electric circuit  22 , and the sensor signal condition imparting circuit  36  outputs a signal to a microprocessor  37  when a value of a detection signal from the vehicle speed sensor  18  exceeds a predetermined value. The electric circuit  22  includes two signal lines which connect two terminals on an output side of the vehicle speed sensor  18  and two terminals on an input side of the sensor signal condition imparting circuit  36 , and input impedance  35  which is connected between these two signal lines. In the system, when an output of the sensor signal condition imparting circuit  36  becomes 0, a DC current (test pulse) is supplied between two signal lines of the electric circuit  22  from a conduction test circuit  38  thus performing a conduction test of the sensor  18  or the electric circuit  22 . When the sensor  18  or the electric circuit  22  is conductive (normal), a relatively small voltage step-down occurs between both terminals of an input of the sensor signal condition imparting circuit  36  and there is no output from the sensor signal condition imparting circuit  36 . On the other hand, when the sensor  18  or the electric circuit  22  is not conductive (abnormal), impedance viewed from the conduction test circuit  38  is high so that a voltage step-down of a predetermined value or more occurs between both terminals of an input of the sensor signal condition imparting circuit  36  whereby an output is generated from the sensor signal condition imparting circuit  36 . Accordingly, in this system, a test pulse is supplied between two signal lines of the electric circuit  22 , wherein it is determined that the sensor  18  or the electric circuit  22  is in a normal state when the output of the sensor signal condition imparting circuit  36  is 0, while the sensor  18  or the electric circuit  22  is in an abnormal state when the output of the sensor signal condition imparting circuit  36  is not 0. 
     Patent document 2 relates to a diagnosis device which diagnoses a failure of an electric system of an automobile. With respect to this diagnosis device, there is described a device which diagnoses abnormality in parts which are subject to diagnosis by outputting a test pulse signal to the part which is subject to diagnosis from a pulse generator (see  FIG. 4 ) and by detecting a response of the part which is subject to diagnosis to the test pulse (see  FIG. 1 ). 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent document 1: JP-T-5-503779 
         Patent document 2: JP-A-4-231838 
       
    
     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     However, in the pressure detection system for the negative pressure booster, there may be a case where a residual pressure remains even when an engine is stopped and hence, a pressure value cannot be specified whereby a sensor circuit output which becomes a determination standard cannot be specified. Accordingly, unlike the devices disclosed in the patent documents 1 and 2, the pressure detection system for the negative pressure booster cannot detect abnormality in a sensor circuit using a test pulse. 
     On the other hand, in the method described in patent documents 1 and 2, additional parts for a circuit which generates a test pulse, a circuit which evaluates an output based on the test pulse and the like become necessary thus giving rise to a possibility that a cost is pushed up. 
     It is an object of the present invention to provide, in an electric circuit whose behavior is changed corresponding to a peripheral environment, a technique which can surely detect abnormality in an electric circuit even in a state where a value of the peripheral environment cannot be specified. 
     It is also another object of the present invention to provide a technique which can easily and surely detect abnormality in an electric circuit whose behavior is changed corresponding to a peripheral environment. 
     Means for Solving the Problem 
     One mode for carrying out the present invention relates to an abnormality detection device which detects abnormality in a detection circuit ( 112 ) which detects a specific kind of physical quantity. The abnormality detection device includes an abnormality detection part ( 220   a ) which changes magnitude of a power source voltage (Vcc′) which is supplied to the detection circuit ( 112 ), and detects abnormality in the detection circuit based on an output signal (Vo 2 ) from the detection circuit at a power source voltage (Vc 2 ) after the change of the magnitude of the power source voltage (Vcc′). Here, specific kinds of physical quantities include values of pressure, temperature, speed, acceleration and humidity. However, the physical quantities are not limited to these physical quantities. 
     The abnormality detection device for the detection circuit ( 112 ) can detect the abnormality in the detection circuit by changing the magnitude of the power source voltage which is supplied to the detection circuit and by determining whether or not the output signal (Vo 2 ) of the detection circuit with respect to the power source voltage (Vc 2 ) after the change conforms to predetermined input/output characteristic. That is, even when the current physical quantity is unknown, provided that the input/output characteristic of the detection circuit is known in advance, the abnormality in the detection circuit can be detected. Here, the input/output characteristic is the relationship between input/output values of the detection circuit, and means the relationship between the power source voltage (input) and an output signal. 
     In one mode for carrying out the present invention, the abnormality detection part ( 220   a ) detects the abnormality in the detection circuit ( 112 ) based on whether or not output signals (Vo 1 , Vo 2 ) from the detection circuit ( 112 ) before and after the change of the power source voltage are on an input/output characteristic curve with respect to the same physical quantity (P). 
     In this case, the abnormality detection part ( 220   a ) obtains and stores input/output characteristic curves corresponding to respective physical quantities in advance, detects the output signals (Vo 1 , Vo 2 ) of the detection circuit ( 112 ) before and after the change of the power source voltage within a short time which does not cause a change of the physical quantities, determines that the detection circuit is in a normal state when the output signals (Vo 1 , Vo 2 ) are on an input/output characteristic curve with respect to the same physical quantity and determines that the detection circuit is in an abnormal state when the output signals (Vo 1 , Vo 2 ) are not on the input/output characteristic curve with respect to the same physical quantity. In a state where the input/output characteristic curve of the detection circuit can be calculated, when the input/output characteristic is calculated during abnormality detection processing, it is unnecessary for the abnormality detection part ( 220   a ) to store input/output characteristic curves corresponding to respective physical quantities in advance. When the input/output characteristic of the detection circuit is formed of a straight line, it is unnecessary for the abnormality detection part ( 220   a ) to store input/output characteristic curves corresponding to respective physical quantities in advance, and the abnormality detection part ( 220   a ) also can detect abnormality in the detection circuit based on whether or not a ratio of output signals before and after the change agrees with a ratio of input signals before and after the change. 
     In one mode for carrying out the present invention, the abnormality detection part ( 220   a ) detects the abnormality in the detection circuit ( 112 ) based on whether or not the ratio of output signals (Vo 1 , Vo 2 ) from the detection circuit ( 112 ) before and after the change of the power source voltage agrees with the ratio of power source voltages (Vc 1 , Vc 2 ) before and after the change of the power source voltage. When the input/output characteristic of the detection circuit is formed of a straight line, the abnormality detection part ( 220   a ) can detect the abnormality in the detection circuit based on whether or not a ratio of output signals before and after the change agrees with a ratio of input signals before and after the change. 
     In one mode for carrying out the present invention, the abnormality detection part ( 220   a ) changes the power source voltage to a plurality of voltages (Vc 2 , Vc 3 ) which differ from each other, and detects the abnormality in the detection circuit ( 112 ) based on output signals (Vo 2 , Vo 3 ) from the detection circuit ( 112 ) at the plurality of power source voltages (Vc 2 , Vc 3 ) after the change. 
     In such a mode for carrying out the invention, the abnormality detection part ( 220   a ) can detect the abnormality in the detection circuit by determining whether or not the output signals (Vo 2 , Vo 3 ) at the plurality of power source voltages (Vc 2 , Vc 3 ) after the change conform to the predetermined input/output characteristic. Further, the abnormality detection part ( 220   a ) may determine whether or not output signals (Vo 1 , Vo 2 , Vo 3 ) corresponding to three or more kinds of power source voltages (Vc 1 , Vc 2 , Vc 3 ) including the power source voltage (Vcc) before the change and a plurality of power source voltages (Vc 2 , Vc 3 ) after the change conform to the predetermined input/output characteristic. In this case, when the input/output characteristic is formed of a curve, the abnormality detection part ( 220   a ) can determine with high accuracy whether or not the output signal conforms to the predetermined input/output characteristic. 
     In one mode for carrying out the present invention, the abnormality detection part ( 220   a ) measures an output signal from the detection circuit ( 112 ) with respect to the power source voltage value (Vc 1 ) before the change at least twice at a predetermined time interval, and when the output signals (Vo 1 , Vo 1 ′) agree with each other at least twice, the abnormality detection part ( 220   a ) detects the abnormality in the detection circuit based on the output signal (Vo 2 ) from the detection circuit at the power source voltage (Vc 2 ) after the change. The abnormality detection part ( 220   a ) determines whether or not the detection circuit is in a situation where a physical quantity which is an object to be detected is not changed within a short time based on whether or not the plurality of measured values of the output signal with respect to the power source voltage value (Vc 1 ) before the change agree with each other, and executes the abnormality detection processing in the situation where the physical quantity is not changed within the short time. 
     In one mode for carrying out the present invention, the abnormality detection device includes a power source voltage control part ( 230 ) which changes magnitude of a power source voltage (Vcc′) which is supplied to the detection circuit ( 112 ) in response to a control by the abnormality detection part ( 220   a ). For example, the detection of abnormality in the detection circuit can be surely performed with the simple constitution by simply adding the power source voltage control part consisting of a resistance and a switch. 
     In one mode for carrying out the present invention, the detection circuit ( 112 ) is a pressure sensor which detects pressure in a negative pressure booster which assists a braking device of a vehicle. There may be a case where a residual pressure remains in the inside of the negative pressure booster even after an engine is stopped. In this case, a pressure value cannot be determined. Accordingly, in a conventional method which uses a test pulse, abnormality in the pressure sensor cannot be detected. To the contrary, according to the present invention, even when a pressure value at the time of diagnosis is unknown, provided that an input/output characteristic of the detection circuit is known in advance, abnormality in the detection circuit can be detected. 
     One mode for carrying out the present invention relates to an abnormality detection device which detects abnormality in the electric circuit ( 112 ) whose behavior is changed corresponding to a peripheral environment. The abnormality detection device includes an abnormality detection part ( 220   a ) which changes magnitude of a power source voltage (Vcc′) supplied to the electric circuit ( 112 ), and detects abnormality in the electric circuit based on the behavior (Vo 2 ) of the electric circuit at a power source voltage (Vc 2 ) after the change. Here, the peripheral environment means a state such as pressure, temperature, speed, acceleration, temperature or humidity around the electric circuit. 
     One mode for carrying out the present invention relates to a detection system. The detection system includes a detection circuit ( 112 ) which detects a specific kind of physical quantity, a processing device ( 200 ) which processes an output from the detection circuit ( 112 ), conductive lines (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) which electrically connect the detection circuit and the processing device, and monitoring conductive lines (L 1   a ,  101   a ; L 2   a ,  102   a ; L 3   a ,  103   a ) which are electrically connected with the conductive lines on the detection circuit ( 112 ) side, wherein a resistance state of the conductive line is detected by detecting a potential at a connection point between the conductive line and the monitoring conductive line using the monitoring conductive line. Further, the detection system includes an abnormality detection part ( 220   a ) which changes magnitude of a power source voltage (Vcc′) supplied to the detection circuit ( 112 ), and detects abnormality in the detection circuit ( 112 ) based on an output signal (Vo 2 ) from the detection circuit at a power source voltage (Vc 2 ) after the change. 
     One mode for carrying out the present invention relates to an electronic system. The electronic system includes a first electric circuit ( 112 ) whose behavior is changed corresponding to a peripheral environment, a second electric circuit ( 200 ), conductive lines (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) which are electrically connected between the first electric circuit and the second electric circuit, and monitoring signal lines (L 1   a ,  101   a ; L 2   a ,  102   a ; L 3   a ,  103   a ) which are electrically connected to the conductive lines on the first electric circuit side, wherein a resistance state of the conductive line is detected by detecting a potential at a connection point between the conductive line and the monitoring signal line through the monitoring signal line. Further, the electronic system includes an abnormality detection part ( 220   a ) which changes magnitude of a power source voltage (Vcc′) supplied to the first electric circuit ( 112 ), and detects abnormality in the first electric circuit ( 112 ) based on the behavior (Vo 2 ) of the first electric circuit at a power source voltage (Vc 2 ) after the change. 
     In one mode for carrying out the present invention, an abnormality detection device for a detection circuit ( 112 ) which includes a detection part ( 151 ) for detecting a specific kind of physical quantity includes an abnormality detection part ( 220   b ) which changes magnitude of a power source voltage (Vcc′) supplied to the detection circuit ( 112 ), detects an output signal (Vout) of the detection circuit ( 112 ) when the power source voltage (Vcc′) is less than the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects abnormality in the detection circuit ( 112 ) based on the detected value. 
     In one mode for carrying out the present invention, the abnormality detection part ( 220   b ), further, detects a resistance state of a conductive line which connects the detection circuit ( 112 ) with the outside based on the power source voltage (Vx) at which the detection part ( 151 ) is stopped. 
     In one mode for carrying out the present invention, an abnormality detection device for a detection circuit ( 112 ) including a detection part ( 151 ) which detects a specific kind of physical quantity includes an abnormality detection part ( 220   b ) which changes magnitude of a power source voltage (Vcc′) supplied to the detection circuit ( 112 ), detects the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects a resistance state of a conductive line which connects the detection circuit ( 112 ) with the outside based on the detected value. 
     In one mode for carrying out the present invention, an abnormality detection device for an electric circuit ( 112 ) includes an abnormality detection part ( 220   b ) which changes magnitude of a power source voltage (Vcc′) supplied to the electric circuit ( 112 ), detects an output signal (Vout) of the electric circuit ( 112 ) when the power source voltage (Vcc′) is less than the power source voltage (Vx) at which a portion ( 151 ) included in the electric circuit ( 112 ) is stopped, and detects abnormality in the electric circuit ( 112 ) based on the detected value. 
     In one mode for carrying out the present invention, an abnormality detection device for an electric circuit ( 112 ) includes an abnormality detection part ( 220   b ) which changes magnitude of a power source voltage (Vcc′) supplied to the electric circuit ( 112 ), detects the power source voltage (Vx) at which a portion ( 151 ) of the electric circuit ( 112 ) is stopped, and detects a resistance state of a conductive line which connects the electric circuit ( 112 ) with the outside based on the detected value. 
     In one mode for carrying out the present invention, an abnormality detection device for a detection circuit ( 112 ) which includes a detection part ( 151 ) for detecting a specific kind of physical quantity includes a first abnormality detection part ( 220   a ) and a second abnormality detection part ( 220   b ). The first abnormality detection part ( 220   a ) changes magnitude of a power source voltage (Vcc′) supplied to the detection circuit ( 112 ), detects an output signal (Vo 2 ) from the detection circuit with respect to a power source voltage (Vc 2 ) after the change within a range of not less than the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects abnormality in the detection circuit based on the detected value. The second abnormality detection part ( 220   b ) detects the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects a resistance state of a conductive line which connects the detection circuit ( 112 ) with the outside based on the detected value. 
     In one mode for carrying out the present invention, an abnormality detection device which detects abnormality in an electric circuit ( 112 ) including a circuit part ( 151 ) whose behavior is changed corresponding to a peripheral environment includes a first abnormality detection part ( 220   a ) and a second abnormality detection part ( 220   b ). The first abnormality detection part ( 220   a ) changes magnitude of a power source voltage (Vcc′) supplied to the electric circuit ( 112 ), detects an output signal (Vo 2 ) from the detection circuit with respect to a power source voltage (Vc 2 ) after the change within a range not less than the power source voltage (Vx) at which the circuit part ( 151 ) is stopped, and detects abnormality in the electric circuit based on the detected value. The second abnormality detection part ( 220   b ) detects the power source voltage (Vx) at which the circuit part ( 151 ) is stopped, and detects a resistance state of a conductive line which connects the electric circuit ( 112 ) with the outside based on the detected value. 
     One mode for carrying out the present invention relates to a detection system. The detection system includes a detection circuit ( 112 ) including a detection part ( 151 ) which detects a specific kind of physical quantity, a processing device ( 200 ) which processes an output from the detection circuit ( 112 ), a conductive line (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) which electrically connects the detection circuit and the processing device, and a monitoring conductive line (L 1   a ,  101   a ; L 2   a ,  102   a ; L 3   a ,  103   a ) which is electrically connected with the conductive line on the detection circuit ( 112 ) side, wherein a resistance state of the conductive line is detected by detecting a potential at a connection point between the conductive line and the monitoring conductive line through the monitoring conductive line. Further, the detection system further includes an abnormality detection part ( 220   b ) which changes magnitude of a power source voltage (Vcc′) supplied to the detection circuit ( 112 ), detects an output signal (Vout) of the detection circuit ( 112 ) when the power source voltage (Vcc′) is less than the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects abnormality in the detection circuit ( 112 ) based on the detected value. 
     One mode for carrying out the present invention relates to an electronic system. The electronic system includes a first electric circuit ( 112 ), a second electric circuit ( 200 ), a conductive line (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) which electrically connects the first electric circuit and the second electric circuit, and a monitoring signal line (Lla,  101   a ; L 2   a ,  102   a ; L 3   a ,  103   a ) which is electrically connected to the conductive line on the first electric circuit side, wherein a resistance state of the conductive line is detected by detecting a potential at a connection point between the conductive line and the monitoring signal line through the monitoring signal line. Further, the electronic system includes an abnormality detection part ( 220   b ) which changes magnitude of a power source voltage (Vcc′) supplied to the electric circuit ( 112 ), detects an output signal (Vout) of the electric circuit ( 112 ) when the power source voltage (Vcc′) is less than a stop power source voltage (Vx) at which a part ( 151 ) of the electric circuit is stopped, and detects abnormality in the electric circuit ( 112 ) based on the detected value. 
     In one mode for carrying out the present invention, an abnormality detection device for a detection circuit ( 112 ) which including a detection part ( 151 ) for detecting a specific kind of physical quantity includes a first abnormality detection part ( 220   a ) and a second abnormality detection part ( 220   b ). The first abnormality detection part ( 220   a ) changes magnitude of a power source voltage (Vcc′) supplied to the detection circuit ( 112 ), detects an output signal (Vo 2 ) from the detection circuit with respect to a power source voltage (Vc 2 ) after the change within a range not less than the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects abnormality in the detection circuit based on the detected value. The second abnormality detection part ( 220   b ) detects an output signal (Vout) of the detection circuit ( 112 ) when the power source voltage (Vcc′) is less than the power source voltage (Vx) at which the detection part ( 151 ) is stopped, and detects abnormality in the detection circuit ( 112 ) based on the detected value. 
     In one mode for carrying out the present invention, an abnormality detection device which detects abnormality in an electric circuit ( 112 ) including a circuit part ( 151 ) whose behavior is changed corresponding to a peripheral environment includes a first abnormality detection part ( 220   a ) and a second abnormality detection part ( 220   b ). The first abnormality detection part ( 220   a ) changes magnitude of a power source voltage (Vcc′) supplied to the electric circuit ( 112 ), detects an output signal (Vo 2 ) from the detection circuit with respect to a power source voltage (Vc 2 ) after the change within a range not less than the power source voltage (Vx) at which the circuit part ( 151 ) is stopped, and detects abnormality in the electric circuit based on the detected value. The second abnormality detection part ( 220   b ) detects an output signal (Vout) of the electric circuit ( 112 ) when the power source voltage (Vcc′) is less than the stop power source voltage (Vx) at which the portion ( 151 ) of the electric circuit is stopped, and detects abnormality in the electric circuit ( 112 ) based on the detected value. 
     In one mode for carrying out the present invention, an electric system includes a first electric circuit ( 112 ), a ground line (L 3 ,  103 ) which is connected to a ground terminal of the first electric circuit ( 112 ), and a monitoring conductive line (L 3   a ,  103   a ) which is electrically connected to the ground line and detects a potential (V 2 ′) at a connection point with the ground line, and corrects a behavior (Vout) of the first electric circuit ( 112 ) based on a detection voltage (V 2 ) through the monitoring conductive line. 
     In one mode for carrying out the present invention, the monitoring conductive line (L 3   a ,  103   a ) is connected to a power source voltage (Vcc) through a first resistance (R 5 ), and is also connected to a ground potential (GND) through a second resistance (R 6 ), and detects abnormality in the monitoring conductive line per se by detecting a voltage of the second resistance as the detection voltage (V 2 ). 
     In one mode for carrying out the present invention, when the detection voltage (V 2 ) through the monitoring conductive line is smaller than a first threshold value, the correction of the behavior (Vout) of the first electric circuit ( 112 ) is executed using the detection voltage (V 2 ) through the monitoring conductive line, and it is determined that the monitoring conductive line is disconnected when the detection voltage (V 2 ) through the monitoring conductive line becomes the first threshold value or more. 
     In one mode for carrying out the present invention, when the detection voltage (V 2 ) through the monitoring conductive line is smaller than a second threshold value which is smaller than the first threshold value, it is determined that the ground line is in a normal state and the correction of the behavior (Vout) of the first electric circuit ( 112 ) is not executed, when the detection voltage (V 2 ) through the monitoring conductive line is not less than the second threshold value and less than the first threshold value, the correction of the behavior (Vout) of the first electric circuit ( 112 ) is executed using the detection voltage (V 2 ) through the monitoring conductive line, and when the detection voltage (V 2 ) through the monitoring conductive line becomes not less than the first threshold value, it is determined that the monitoring conductive line is disconnected. 
     In one mode for carrying out the present invention, the first electric circuit ( 112 ) is a detection circuit which detects a specific kind of physical quantity, and corrects an output voltage (Vout) of the detection circuit ( 112 ) using the detection voltage (V 2 ) through the monitoring conductive line. 
     In one mode for carrying out the present invention, the correction of a physical quantity to be detected is executed by correcting the output voltage (Vout) of the detection circuit ( 112 ) and an upper limit value (VDD) of the output voltage (Vout) using the detection voltage (V 2 ) through the monitoring conductive line. 
     In one mode for carrying out the present invention, the detection circuit ( 112 ) is a pressure sensor which detects pressure in a negative pressure booster which assists a braking device of a vehicle, and the output voltage (Vout) is a pressure detection signal. 
     In one mode for carrying out the present invention, the abnormality detection device further includes a third resistance (R 7 ) which is interposed on the monitoring conductive line (L 3   a ,  103   a ), one end of the third resistance (R 7 ) is connected to the first resistance (R 5 ), and the other end of the third resistance (R 7 ) is connected to the second resistance (R 6 ). 
     In one mode for carrying out the present invention, the monitoring conductive line is further connected to a ground potential through a first capacitor (C 1 ) which is connected parallel to the second resistance (R 6 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a detection system according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram of the detection system according to the first embodiment when a ground line is brought into a high resistance state in the circuit diagram; 
         FIG. 3  is a circuit diagram when a monitoring line and the ground line are connected to each other outside a sensor chip in the circuit diagram of the detection system according to the first embodiment; 
         FIG. 4  is a circuit diagram of a detection system according to a second embodiment of the present invention; 
         FIG. 5  is a circuit diagram when a ground line is brought into a high resistance state in the detection system according to the second embodiment of the present invention; 
         FIG. 6  is a view showing a modification in which, in the detection system of the second embodiment of the present invention, a resistance for adjusting a voltage is arranged in a detection device; 
         FIG. 7A  is an equivalent circuit diagram when the ground line is brought into a high resistance state in the circuit diagram of the detection system according to the first embodiment; 
         FIG. 7B  is an equivalent circuit diagram when the ground line is brought into a high resistance state in the circuit diagram of the detection system according to the second embodiment; 
         FIG. 8  is a circuit diagram when one embodiment of the present invention is applied to a power source line and a detection signal line; 
         FIG. 9  is a circuit diagram of a detection system according to a third embodiment; 
         FIG. 10  is an input/output characteristic curve which expresses the behavior of a circuit which is subject to diagnosis with respect to respective values of a peripheral environment; 
         FIG. 11  is a view for explaining an allowable range for determining that the behaviors of circuit before and after the change of an input voltage are on the same input/output characteristic curve; 
         FIG. 12  is a flowchart for explaining abnormality detection processing of a detection circuit  112  according to the third embodiment; 
         FIG. 13  is a view showing a case where the input/output characteristic is formed of a straight line in  FIG. 10 ; 
         FIG. 14  is a flowchart for explaining the abnormality detection processing of the detection circuit  112  according to the third embodiment when the input/output characteristic is formed of a straight line; 
         FIG. 15  is a view showing a constitutional example of a power source voltage control circuit; 
         FIG. 16  is a circuit diagram of a detection system according to a fourth embodiment of the present invention; 
         FIG. 17  is a block diagram showing the constitution of the detection circuit; 
         FIG. 18  is a view showing a range of an output signal value of the detection circuit; 
         FIG. 19  is a view showing an input/output characteristic curve which expresses the behaviors of a circuit which is subject to diagnosis with respect to respective values of a peripheral environment; 
         FIG. 20  is a view showing a change of an input/output characteristic curve when a conductive line is brought into a high resistance state; 
         FIG. 21  is a view showing a change of an input/output characteristic curve when an abnormality occurs in the detection circuit; 
         FIG. 22  is a flowchart for explaining the abnormality detection processing of a detection circuit according to a fourth embodiment; 
         FIG. 23  is a flowchart for explaining the abnormality detection processing of a detection circuit according to a fifth embodiment; 
         FIG. 24  is a circuit diagram of a detection system according to a sixth embodiment; 
         FIG. 25  is an explanatory view for explaining a potential of a monitoring point in the detection system according to the sixth embodiment; and 
         FIG. 26  is an explanatory view for explaining the correction of an output of a detection circuit according to the sixth embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
       FIG. 1  shows a circuit diagram of a detection system according to the first embodiment of the present invention. Here, the explanation is made by taking a detection system which is used for detecting a pressure of a negative pressure booster for assisting a braking device of a vehicle as an example. However, this embodiment is not limited to the detection system and is applicable to an arbitrary electric system provided that the system is configured to perform the supply of power source or the transmission/reception of signals among a plurality of circuits. 
     [Circuit Constitution] 
     A detection system  1  shown in  FIG. 1  includes a detection device  100  and a processing device  200 , and the detection device  100  and the processing device  200  are electrically connected with each other through signal lines (conductive lines) L 1  to L 3 , L 3   a . The detection device  100  is a pressure detection device, and is a detection device which is mounted on a negative pressure booster (not shown in the drawing) for assisting a braking device of a vehicle and detects a pressure (negative pressure) in the negative pressure booster. The processing device  200  is, for example, an electronic control device (ECU) mounted on the vehicle. The processing device  200  supplies a power source voltage (input signal) to the detection device  100 , receives a pressure detection signal (output signal) from the detection device  100  and uses the pressure detection signal for various controls of the vehicle. 
     The signal line L 1  is a detection signal line through which a pressure detection signal is outputted from the detection device  100  to the processing device  200 , and is connected to a detection signal electrode P 1  of the detection device  100  and a detection signal terminal T 1  of the processing device  200 . The signal line L 2  is a power source line through which a power source voltage Vcc (for example, 5V) is supplied to the detection device  100  from the processing device  200 , and is connected to a power source electrode P 2  of the detection device  100  and a power source terminal T 2  of the processing device  200 . The signal line L 3  is a ground line through which a ground potential (GND) is supplied to the detection device  100  from the processing device  200 , and is connected to a ground electrode P 3  of the detection device  100  and a ground terminal T 3  of the processing device  200 . The signal line L 3   a  is a monitoring line for monitoring and detecting an abnormality in the ground line L 3 , and a potential V 2 ′ on a detection device  100  side of the ground line L 3  is supplied to the processing device  200  through the signal line L 3   a.    
     The detection device  100  includes a housing  110  formed by resin molding, and a sensor chip  111  arranged in the inside of the housing  110 . The sensor chip  111  includes a pressure detection circuit  112 , and the pressure detection circuit  112  is provided with, for example, a pressure sensor constituted of a diaphragm and a resistance bridge, an amplifier circuit and the like. The sensor chip  111  and the signal lines L 1  to L 3  are connected with each other by wires  101  to  103 ,  103   a  which constitute a detection signal line  101 , a power source line  102 , a ground line  103 , and a monitoring line  103   a  respectively, while the pressure detection circuit  112  is connected with the signal lines L 1  to L 3  by way of the wires  101  to  103 ,  103   a . The pressure detection circuit  112  detects a pressure in the negative pressure booster, and outputs a pressure detection signal to the detection signal terminal T 1  of the processing device  200  through the detection signal line  101 , the detection signal electrode P 1  and the detection signal line L 1 . The pressure detection signal is inputted to a detection signal terminal  211  of an ADC  210  through a detection signal line  201 . A power source voltage Vcc is supplied to the pressure detection circuit  112  from a power source Vcc of the processing device  200  through a power source line  202 , the power source terminal T 2 , the power source line L 2 , a power source electrode P 2  and the power source line  102 . A ground potential GND is supplied to the pressure detection circuit  112  from the ground line  203  of the processing device  200  through a ground terminal T 3 , the ground line L 3 , a ground electrode P 3  and the ground line  103 . 
     A recessed portion which receives connectors (not shown in the drawing) mounted on one ends of the signal lines L 1  to L 3 , L 3   a  is formed on the housing  110  at the time of forming the housing  110  by resin molding, and comb-teeth-shaped electrodes P 1  to P 3 , P 3   a  which correspond to the respective signal lines L 1  to L 3 , L 3   a  are provided in a state where the electrodes P 1  to P 3 , P 3   a  penetrate a bottom surface of the recessed portion from the inside to the outside of the housing. The recessed portion and the electrodes P 1  to P 3 , P 3   a  constitute a connector on a detection device  100  side. When the connector on a signal lines L 1  to L 3 , L 3   a  side is fitted in the recessed portion of the housing  110 , the signal lines L 1  to L 3 , L 3   a  are electrically connected to the electrodes P 1  to P 3 , L 3   a  respectively. Inside the housing  110 , the electrodes P 1  to P 3 , P 3   a  are respectively formed into a shape for receiving distal end portions of the wires  101  to  103 ,  103   a , and distal ends of the wires  101  to  103 ,  103   a  are fitted into and are connected to the respective electrodes P 1  to P 3 . Due to such a constitution, the wires  101  to  103 ,  103   a  are respectively made electrically conductive with the signal lines L 1  to L 3 , L 3   a  through the electrodes P 1  to P 3 . Further, the ground lines L 3 ,  103  and the monitoring lines L 3   a ,  103   a  are made electrically conductive with each other in the inside of a sensor chip  111 . 
     In the processing device  200 , the analog/digital converter (ADC)  210  is provided. The ADC  210  includes the detection signal terminal  211  to which a pressure detection signal is inputted, a reference terminal  212  to which a power source voltage Vcc is supplied through the power source line  202  in the processing device  200 , a signal terminal  213  to which a potential on a processing device  200  side of the ground lines L 3 ,  103  (ground potential V 1  of the processing device  200  (V 1 =GND)) is inputted, and a monitoring terminal  213   a  to which a potential V 2  on a detection device  100  side of the ground lines L 3 ,  103  is inputted through the monitoring lines  103   a , L 3   a ,  203   a.    
     Further, the detection signal terminal T 1  of the processing device  200  is connected to the detection signal terminal  211  of the ADC  210  through the detection signal line  201 , and the detection signal terminal T 1  is also connected to a power source VA through a pull-up resistance R 2 . For example, assume that the resistance R 2  is set to 680 kΩ (R 2 =680 kΩ) and the power source voltage Va is set to a fixed voltage of 5.5 to 16V (VA=5.5 to 16V). When the detection signal terminal T 1  is brought into an open state due to the disconnection of the detection signal line L 1  or the like, the voltage VA is inputted to the detection signal terminal  211  of the ADC  210  through the resistance R 2  and the detection signal line  201 . For example, when the signal lines L 1  to L 3  are in a normal state, a voltage of the pressure detection signal which is inputted to the ADC  210  is changed within a range from 0.25V to 4.75V, while when the detection signal line L 1  is disconnected (when the detection signal terminal T 1  is brought into an open state), a voltage which is inputted to the ADC  210  from the power source VA through the resistance R 2  becomes not less than 5V. Based on the difference between the input voltages supplied to the ADC  210 , the disconnection of the detection signal line L 1  can be detected. 
     In such a detection system  1 , a power source voltage Vcc is supplied to the pressure detection circuit  112  in the sensor chip  111  from the power source Vcc of the processing device  200  through the power source line  202  and the power source lines L 2 ,  102 . Further, a pressure detection signal from the pressure detection circuit  112  is supplied to the detection signal terminal  211  of the ADC  210  through the detection signal lines  101 , L 1 ,  201 . Further, the ground potential GND of the processing device  200  is supplied to the pressure detection circuit  112  of the detection device  100  from the ground line  203  through the ground lines L 3 ,  103 . Further, a ground potential GND of the processing device  200  is inputted to the signal terminal  213  of the ADC  210  from the ground line  203 , and a potential V 2 ′ at a connection point (monitoring point) between the ground line L 3 ,  103  and the monitoring line L 3   a ,  103   a  is inputted to the monitoring terminal  213   a  of the ADC  210  through the monitoring line  103   a , the monitoring electrode P 3   a  and the monitoring lines L 3   a ,  203   a.    
     [Abnormality Detection Processing] 
     Hereinafter, the abnormality detection processing of the ground line in the detection system  1  is explained. In this abnormality detection processing, a resistance state of the ground line is detected based on a potential V 2 ′ at the connection point between the ground line and the monitoring line on the pressure detection circuit  112  side. In the explanation made hereinafter, a potential (V 1 =GND) of the ground line  203  is used as a reference value, and the potential V 1  is set to 0 (V 1 =0). 
     When the ground lines L 3 ,  103  are in a normal state, the ground line  103  of the detection device  100  is connected to the ground line  203  (V 1 =0) of the processing device  200  through the ground line L 3  in a low resistance state, and a potential V 2 ′ at the connection point agrees with a ground potential V 1 (=0) of the processing device  200  (V 2 ′=0). 
     On the other hand, as shown in  FIG. 2 , there may be a case where contact at a connection portion between the ground electrode P 3  and the ground line L 3  or contact at a connection portion between the ground electrode P 3  and the ground line  103  of the detection device  100  becomes defective due to vibrations caused by an operation of the negative pressure booster so that contact resistance is increased whereby resistance RX is generated on the ground lines L 3 ,  103 . In such a case, a potential V 2 ′ at the connection point is connected to the ground line  203  (V 1 =0) of the processing device  200  through the resistance RX. Here, as shown in an equivalent circuit in  FIG. 7A , a power source voltage Vcc is divided by a resistance value R 0  of the sensor chip  111  (a resistance value from an input part of the sensor chip  111  for the power source line  102  to the connection point (V 2 ′)) and the resistance RX. Accordingly, the resistance RX can be calculated by the following formula (1) using the potential V 2 ′ at the connection point. 
         RX=V 2′/( Vcc−V 2′)* R 0  (1)
 
     A resistance state of the ground line L 3  can be evaluated using the resistance value RX. Further, the potential V 2 ′ at the connection point corresponds to the resistance value RX on a one-to-one basis and hence, a resistance state of the ground line L 3  can be also evaluated using the potential V 2 ′ at the connection point. 
     Assuming that the power source voltage Vcc is 5V (Vcc=5V) and the resistance value R 0  is 500Ω (R 0 =500Ω), the resistance RX(=10Ω) is connected between the ground electrode P 3  and the ground line L 3  (or between the ground electrode P 3  and the ground line  103 ), and a potential V 2 ′(=V 2 ) at the connection point is measured. The result shows that V 2 ′=V 2 =0.098V. Substituting this value into the formula (1), the resistance RX becomes 9.996Ω (RX=9.996Ω). When a resistance RX(=300Ω) is connected between the ground electrode P 3  and the ground line L 3  (or between the ground electrode P 3  and the ground line  103 ), the potential V 2 ′(=V 2 ) at the connection point becomes 1.894V. Substituting this value into the formula (1), the resistance RX becomes 304.9Ω (RX=304.9Ω). As a result, it is understood that a resistance state of the ground line can be accurately evaluated by measuring the potential V 2 ′ at the connection point. 
     When the resistance value RX caused by a contact failure or the like is increased, a potential V 2 ′(=V 2 ) at the connection point is also increased and hence, the determination of a resistance state of the ground line is preferably performed as follows using the potential V 2 ′ at the connection point or an input V 2  supplied to the ADC  210 . By determining that “the ground line is in a normal state” when the relationship of V 2 ′=V 2 =0 (−10 mV&lt;V 2 ′=V 2 &lt;10 mV in this embodiment) is established, and by determining that “the ground line is in a high resistance state” when the relationship of 10 mV≦V 2 ′=V 2  is established, abnormality that the ground lines L 3 ,  103  is brought into a high resistance state can be detected. Although it is determined that V 2 ′(=V 2 ) agrees with a predetermined value (0V) when V 2 ′(=V 2 ) falls within a range of −10 mV≦V 2 ′=V 2 &lt;10 mV in this embodiment, this range is suitably decided corresponding to the resolution of the ADC  210 . 
     Specific processing executed by the processing device  200  is as follows. A potential V 1 (=0) (reference value) on a processing device  200  side of the ground line L 3  is inputted to the signal terminal  213  of the ADC  210 , and a potential V 2 ′ (=V 2 ) at the connection point is inputted to the monitoring terminal  213   a  of the ADC  210  through the monitoring line  103   a , the monitoring electrode  3   a  and the monitoring lines L 3   a ,  203   a . The ADC  210  converts the potential V 1  (reference value) on a processing device  200  side of the ground lines L 3 ,  103  and the potential V 2  at the connection point into digital signals respectively and the ADC outputs the respective digital signals to a processing part  220 . The processing part  220  calculates a potential difference ΔV (=V 2 −V 1 ). When the potential difference ΔV falls within a range of −10 mV&lt;ΔV&lt;10 mV, the processing part  220  determines that “the ground line is in a normal state”. On the other hand, when the potential difference ΔV satisfies the relationship of 10 mV≦ΔV, the processing part  220  determines that “the ground line is in a high resistance state” and performs an alarm display indicating that “the ground line is in a high resistance state”. 
     In the processing part  220 , the resistance value RX of the ground line may be calculated using a detected value of V 2 ′ and the formula (1). In this case, a change in resistance value of the ground line and abnormality that the ground line is brought into a high resistance state can be detected by monitoring the resistance value RX. 
     According to the above-mentioned first embodiment of the present invention, by monitoring a potential V 2 ′ at the connection point between the ground line L 3 ,  103  and the monitoring line L 3   a ,  103   a  through the monitoring line L 3   a ,  103   a , a resistance state of the ground line can be detected. Accordingly, abnormality that the ground line L 3 ,  103  is brought into a high resistance state can be surely detected. To consider a case where the detection device  100  is mounted on a negative pressure booster, when vibrations caused by an operation of the negative pressure booster are transmitted to the detection device  100  so that contact between the ground line L 3 ,  103  and the electrode P 3  becomes poor whereby a contact resistance is increased, abnormality that the ground line L 3 ,  103  is brought into a high resistance state can be surely detected. According to the above-mentioned first embodiment of the present invention, a resistance value RX of the ground line can be also detected. 
     Further, according to the above-mentioned embodiment, by monitoring a potential V 2 ′ at the connection point between the ground lines L 3 ,  103  and the monitoring lines L 3   a ,  103   a  through the monitoring lines L 3   a ,  103   a , a resistance state of the ground lines L 3 ,  103  can be directly detected and hence, abnormality can be surely detected with the simple constitution. 
     [Modification] 
     Although the detection system has been explained heretofore by taking the case where the ground lines L 3 ,  103  and the monitoring lines L 3   a ,  103   a  are made electrically conductive with each other in the inside of the sensor chip  111  as an example, as shown in  FIG. 3 , the monitoring line L 3   a  and the ground line L 3  may be made electrically conductive with each other by making the monitoring electrode Pia and the ground electrode  103  electrically conductive with each other. 
     In the explanation made heretofore, a resistance state of the ground lines L 3 ,  103  is detected by providing the monitoring lines L 3   a ,  103   a  which are made electrically conductive with the ground lines L 3 ,  103  on a detection device  100  side. However, as shown in  FIG. 8 , this embodiment is also applicable to the power source lines L 2 ,  102  and the detection signal lines L 1 ,  101 . When this embodiment is applied to the power source line, a resistance state of the power source line is detected by using a power source potential Vcc inputted to the terminal  212  of the ADC  210  as a reference value and by monitoring a potential at a connection point between the power source lines L 2 ,  102  and the monitoring lines L 2   a ,  102   a . When this embodiment is applied to the detection signal line, for example, a resistance state of the detection signal line is detected by using a detection signal voltage at the time of calibration which releases a pressure in the negative pressure booster to the atmosphere (a voltage inputted to the terminal  211  of the ADC  210 ) as a reference value, by detecting a potential at a connection point between the detection signal lines L 1 ,  101  and the monitoring lines L 1   a ,  101   a  and by comparing the potential and the reference value with each other. 
     Here, although the detection system has been explained in this embodiment with respect to the case where the ADC  210  and the processing part  220  are arranged in the inside of the processing device  200 , the processing part  220  may be arranged outside the processing device  200 , or both the ADC  210  and the processing part may be arranged outside the processing device  200 . 
     Further, in the detection device  100 , a device which is constituted by arranging the sensor detection circuit  112  on a printed circuit board may be used in place of the sensor chip  111 . 
     Second Embodiment 
       FIG. 4  shows a circuit diagram of a detection system according to the second embodiment of the present invention. In this embodiment, while constitutions identical with the constitutions of the first embodiment are given the same symbols, and the repeated explanation of these constitutions is omitted. Parts which make this embodiment different from the first embodiment are explained hereinafter. 
     In the first embodiment, when the monitoring line L 3   a  per se is disconnected, the monitoring terminal  213   a  of the ADC  210  is brought into an open state and hence, V 2  becomes a ground potential V 1  (V 1 =0) whereby the relationship of V 2 =ΔV=0 is established. On the other hand, as described above, when the ground line is in a normal state, the relationship of V 2 =ΔV=0 is established and hence, this normal case cannot be distinguished from the abnormal case where the monitoring line L 3   a  per se is disconnected (V 2 =ΔV=0). That is, in the first embodiment, abnormality that the monitoring line L 3   a  per se is disconnected cannot be detected. In view of the above, the second embodiment provides a detection system which can detect the abnormality that the monitoring line L 3   a  per se is disconnected. 
     As shown in  FIG. 4 , in the second embodiment, a resistance R 3  (10Ω) for correcting a potential is interposed on the ground line  203  in the detection device  100 . 
     Hereinafter, the abnormality detection processing of the ground line in the detection system  1  according to the second embodiment is explained. When the ground lines L 3 ,  103  are in a normal state, a potential V 2 ′(=V 2 ) at a connection point between the ground lines L 3 ,  103  and the monitoring lines L 3   a ,  103   a , due to the connection with the ground line  203  through the resistance R 3 , becomes higher compared to a case where V 1  is 0 (V 1 =0) by an amount of voltage step-down generated by a resistance R 3  (99 mV at R 3 (=10Ω)). Accordingly, in the second embodiment, V 2 ′(=V 2 =99 mV (ΔV=V 2 −V 1 =0.1V)) becomes the reference for determining that “The ground line is in a normal state”. This corresponds to the correction of the reference value V 2 ′(=V 2 =0 (ΔV=0)) for determining that “The ground line is in a normal state” in the first embodiment by +99 mV using the resistance R 3 . 
     On the other hand, as shown in  FIG. 5 , when a contact resistance at a connecting portion between the electrode P 3  and the ground line L 3  of the detection device  100  (or a connecting portion between the electrode P 3  and the ground line  103 ) is increased so that a resistance RX is generated on the ground lines L 3 ,  103 , a power source voltage Vcc is, as shown in an equivalent circuit in  FIG. 7B , divided by a resistance value R 0  of the sensor chip  111 , the resistance R 3 , and the resistance RX. Accordingly, the resistance RX can be calculated by a following formula (2) using the potential V 2 ′ at the connection point. 
         RX=V 2′/( Vcc−V 2′)* R 0 −R 3  (2)
 
     As a result of measurement of a potential V 2 ′(=V 2 ) at the connection point in a state where R 0  is set to 500Ω (R 0 =500Ω), R 3  is set to 10Ω (R 3 =10Ω), Vcc is set to 5V (Vcc=5V) and there is no resistance RX (RX=0), the relationship of V 2 ′=V 2 =99 mV is established. Substituting this value into the formula (2), RX becomes 0.100Ω (RX=0.100Ω) so that RX substantially agrees with 0Ω. Further, when the resistance RX(=300Ω) is connected between the ground electrode P 3  and the ground line L 3  (or between the ground electrode P 3  and the ground line  103 ), a measured value of a potential at the connection point satisfies the relationship of V 2 ′=V 2 =1.927V, and the RX becomes 303.5Ω (RX=303.5Ω). Accordingly, it is found that the value of the resistance RX can be obtained based on the potential V 2 ′ at the connection point. 
     On the other hand, when the monitoring line L 3   a ,  103   a  is disconnected, V 2  becomes 0 (V 2 =0) so that the relationship of V 1 =V 2 =0, that is, the relationship of ΔV=V 2 −V 1 =0 is always established. As described above, in this embodiment, when “the ground line is in a normal state”, the relationship of V 2 =ΔV=99 mV is established. Accordingly, it is possible to distinguish the case where the monitoring line is disconnected (ΔV=0) from the case where “the ground line is in a normal state” (ΔV=99 mV). Accordingly, when ΔV is 0, it is determined that “the monitoring line is disconnected”. 
     The determination of a resistance state of the ground line is performed as follows using a potential V 2 ′ at the connection point or an input V 2  of the ADC  210 . When the relationship of 99 mV−10 mV&lt;V 2 (ΔV)&lt;99 mV+10 mV is established, it is determined that “the ground line is in a normal state”. When the relationship of 99 mV+10 mV≦V 2 (ΔV) is established, it is determined that “the ground line is in a high resistance state”. When the relationship of −10 mV&lt;V 2 (ΔV)&lt;10 mV is established, it is determined that “the monitoring line is disconnected”. In this embodiment, when V 2 (ΔV) falls within a range of a predetermined value (0V, 99 mV) ±10 mV, it is determined that V 2 (ΔV) agrees with the predetermined value (0V, 99 mV). However, this range is suitably decided corresponding to the resolution of the ADC  210 . 
     The specific processing executed by the processing device  200  is as follows. The ADC  210  outputs a digital signal corresponding to inputted analog signals V 1 , V 2 . The processing part  220  calculates ΔV(=V 2 −V 1 ) based on the digital signals V 1 , V 2 . The processing part  220  determines that “the ground line is in a normal state” when the potential difference ΔV falls within the range of 99 mV−10 mV&lt;ΔV&lt;99 mV+10 mV, determines that “the ground line is in a high resistance state” when the potential difference ΔV falls within the range of 99 mV+10 mV≦ΔV, and determines that “the monitoring line is disconnected” when the potential difference ΔV falls within the range of −10 mV≦ΔV&lt;10 mV. 
     Although the resistance R 3  is interposed on the ground line  203  in the inside of the processing device  200  in this embodiment, as shown in  FIG. 6 , the resistance R 3  may be interposed on a ground line  103  side in the inside of the sensor chip  111 . Further, the resistance R 3  for potential correction may be arranged at any position on the ground line provided that the resistance R 3  is electrically connected to the ground lines  103 ,  203  in series. 
     According to the second embodiment of the present invention described above, the second embodiment can obtain the substantially same manner of operation and advantageous effects as the first embodiment. Further, in the second embodiment, by interposing the resistance R 3  for voltage correction on the path formed of the ground lines  203 , L 3  and  103  in series, an input voltage V 2  of the ADC  210  at which it is determined that “the ground line is in a normal state” is corrected by an amount of voltage step-down generated by the resistance R 3  (99 mV) and hence, the case where the “the ground line is in a normal state” and the case where the monitoring line L 3   a ,  103   a  per se is disconnected (ΔV=0) can be distinguished from each other. Accordingly, abnormality that “the monitoring line is disconnected” can be detected. 
     Third Embodiment 
       FIG. 9  shows a circuit diagram of a detection system according to the third embodiment of the present invention. This embodiment has the substantially same constitution as the detection system according to the first embodiment shown in  FIG. 1  except for a point that a power source voltage control circuit  230  is added to the power source line. Hereinafter, parts having the substantially same constitution as the corresponding parts of the first embodiment are given the same symbols, while the constitution which makes the third embodiment different from the constitution of the first embodiment is explained in detail. 
     [Circuit Constitution] 
     The detection system  1  shown in  FIG. 9  is configured such that, in the detection system  1  shown in  FIG. 9 , the power source voltage control circuit  230  is interposed on the power source line  202  of the processing device  200 , and an abnormality detection part  220   a  which detects abnormality in the detection circuit  112  based on a pressure detection signal (output signal) Vout after a power source voltage is changed is provided to the processing part  220 . Although the explanation is made with respect to a case where the power source voltage control circuit  230  is arranged in the inside of the processing device  200  in this embodiment, the power source voltage control circuit  230  may be arranged outside the processing device  200 . 
     The power source voltage control circuit  230  is interposed on the power source line  202  in series and includes a resistance RL for voltage step-down and a switching circuit  231 . In the explanation made hereinafter, a supply path of a power source voltage Vcc′ which does not go through the resistance RL is referred to as a path I, and a supply path of the power source voltage Vcc′ which passes the resistance RL is referred to as a path II. The resistance RL steps down a power source voltage Vcc by an amount of a predetermined voltage ΔVcc, and outputs a power source voltage Vcc′(=Vcc−ΔVcc) to the detection circuit  112 . The switching circuit  231  outputs, in a conductive state, a power source voltage Vcc′(=Vcc) to the detection circuit  112  through the path I which does not go through the resistance RL, and outputs, in an open state, the power source voltage Vcc′(=Vcc−ΔVcc) to the detection circuit  112  through the path II which passes the resistance RL. The switching circuit  231  is constituted of, for example, a switch which has a mechanical contact or a semiconductor switch. The switching circuit  231  may have any constitution provided that the switching circuit  231  is an element or a circuit which can change over a supply path of the power source voltage Vcc between the path I and the path II. The switching circuit  231  is connected to an abnormality detection part  220   a  of the processing part  220  through a control line  232 , and is changed over between a conductive state and an open state in response to a control signal from the abnormality detection part  220   a.    
     For example, assuming that a power source voltage Vcc is 5V, RL is 125Ω, a resistance value of the detection circuit  112  (a resistance value between a connecting portion with the power source line  102  and a connecting portion with a ground line  103 ) is 500Ω, when the switching circuit  231  is in a conducive state, Vcc′(=Vcc=5V) is supplied to the detection circuit  112 . On the other hand, when the switching circuit  231  is in an open state, 5V is divided by 125Ω(RL) and 500Ω (detection circuit  112 ) so that the power source voltage Vcc′ (=4V) is supplied to the detection circuit  112 . 
     The processing part  220  is constituted of, for example, a CPU or a microprocessor, and executes abnormality detection processing in which a high resistance state of the signal lines (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) explained in conjunction with the first embodiment is detected. The processing part  220  further includes the abnormality detection part  220   a  which detects abnormality in the detection circuit  112  based on a detection signal (output signal) Vout after a change of the power source voltage. The abnormality detection part  220   a  changes a power source voltage Vcc′ which is outputted to the detection circuit  112  by controlling the switching circuit  231  and executes abnormality detection processing (described later in conjunction with a flowchart shown in  FIG. 12 ). 
     [Principle of Detection of Abnormality] 
     The abnormality detection processing according to this embodiment is explained in conjunction with  FIG. 9  and  FIG. 10 . 
       FIG. 10  shows an input/output characteristic curve which expresses the behavior of a circuit which is subject to diagnosis with respect to respective values P of peripheral environment. In this embodiment, assume that the circuit which is subject to diagnosis is the detection circuit  112  of the detection device  100 , and a value of the peripheral environment is a pressure value (a value of a negative pressure of a negative pressure booster) which is an object to be detected by the detection circuit  112 . Further, assume that the behavior of the circuit which is subject to diagnosis indicates the relationship (Vcc′, Vout) between an input (power source voltage Vcc′) and an output (detection signal Vout) of the detection circuit  112  with respect to respective pressure values P. Further, assume that an input/output characteristic curve indicates the behavior (Vcc′, Vout) of the detection circuit  112  with respect to a value (pressure value) P of the same peripheral environment by a curve (also including a straight line). Here, the present invention is not limited to the detection circuit and is applicable to an arbitrary electric circuit, an arbitrary electric element and an arbitrary electronic element whose behavior is changed corresponding to a value of the peripheral environment. Further, the value of the peripheral environment is not limited to the pressure value and may be an arbitrary physical quantity such as temperature, speed, acceleration, humidity. Further, the behavior of the circuit which is subject to diagnosis with respect to the respective values of the peripheral environment is not limited to input and output voltage values, and at least one of the input and the output may be an electric current. 
       FIG. 10  shows the input/output characteristic curves CA, CB, CC indicative of the behaviors (Vcc′, Vout) of the detection circuit  112  when the pressure P is PA, PB, PC (PA&lt;PB&lt;PC). In  FIG. 10 , a point s 1  indicates the behavior (Vc 1 , Vo 1 ) of the detection circuit  112  when a power source voltage Vcc′ (=Vcc=Vc 1 ) (for example, 5V) is supplied in the detection system shown in  FIG. 9  under the pressure P (=PB). A point s 2  indicates the behavior (Vc 2 , Vo 2 ) of the detection circuit  112  when a power source voltage Vcc′ (=Vc 2 ) (for example, 4V) is supplied under the pressure P (=PB). Points s 21  and s 22  indicate the behaviors (Vc 2 , Vo 21 ) and (Vc 2 , Vo 22 ) of the detection circuit  112  when a power source voltage Vcc′ (=Vc 2 ) (for example, 4V) is supplied under the pressure P (=PA and PC). 
     Here, assume a case where a power source voltage Vcc′ is changed from Vc 1  to Vc 2  and an output signal Vout is detected within a time during which a pressure value in the negative pressure booster (value of the peripheral environment) is not changed, for example, within 100 msec (preferably 10 msec). In this case, when the detection circuit  112  is in a normal state, the value of the output signal Vout, that is, the behavior of the detection circuit  112  changes on the same input/output characteristic curve (CB) from the point s 1  to the point s 2  as shown in  FIG. 10 . On the other hand, when abnormality is present in the detection circuit  112 , the value of the detection signal Vout, that is, the behavior of the detection circuit  112  changes and deviates from the same input/output characteristic curve (CB) such that the behavior deviates from the point s 1  to the point s 21  or the point s 22  as shown in  FIG. 10 , for example. 
     Accordingly, the abnormality detection processing of the detection circuit  112  is executed in such a manner that in the case where the power source voltage Vcc′ is changed from Vc 1  to Vc 2 , it is determined that “the detection circuit  112  is in a normal state” when the behavior s of the detection circuit  112  (or the output signal Vout) moves on the same input/output characteristic curve before and after the change of the power source voltage, and it is determined that “the detection circuit  112  is in an abnormal state” when the behavior changes and deviates from the same input/output characteristic curve before and after the change of the power source voltage, and the abnormality detection processing is executed. 
     In an actual operation, to enable the detection of the pressure (value of the peripheral environment) before and after the change of the power source voltage (input) even when the pressure (value of the peripheral environment) is slightly fluctuated, a predetermined tolerance range may be provided as a criterion for determining that the behaviors s 1 , s 2  before and after the change of the power source voltage are on the same input/output characteristic curve (or a criterion for determining that the detection circuit  112  is in a normal state). For example, as shown in  FIG. 11 , in the case where the P is PB before the input is changed, when a behavior s 2 ′ after the input is changed is present between input/output characteristic curves (a curve P=PB+, a curve P=PB−) with respect to physical quantities P=PB±predetermined errors (for example, 2%), it is determined that the behavior of the detection circuit  112  is on the same input/output characteristic curve (the detection circuit  112  is in a normal state) before and after the change of the power source voltage. That is, when an output signal Vo 2 ′ after the change of the power source voltage falls within a range of Vo 2 ±predetermined error (for example, 1%), it is determined that the behavior of the detection circuit  112  is on the same input/output characteristic curve (the detection circuit  112  is in a normal state). 
     Abnormality Detection Processing 
     Example 1 of Abnormality Detection Processing 
       FIG. 12  is a flowchart for explaining abnormality detection processing executed by the detection circuit  112  according to this embodiment. 
     In step S 10 , in a state where the power source voltage Vcc′(=Vc 1 ) (for example, 5V) is supplied to the detection circuit  112  (in a state where the power source voltage supply path I which does not go through the resistance RL is selected in the power source control circuit  230 ), the abnormality detection part  220   a  of the processing part  220  acquires a value of the power source voltage Vcc′ (=Vc 1 ) inputted to the reference terminal  212  of the ADC  210 , and also acquires a value of a detection signal (output signal) Vout (=Vo 1 ) (behavior s 1  (Vc 1 , Vo 1 )) inputted to the detection signal terminal  211 . 
     In step S 11 , the abnormality detection part  220   a  selects the power source voltage supply path II which passes the resistance RL by controlling the power source voltage control circuit  230 , and allows the detection circuit  112  to output Vcc′(=Vc 2 ) (&lt;Vc 1 ) (Vcc′=Vc 1 →Vc 2 ). Then, the abnormality detection part  220   a  acquires a value of the power source voltage Vcc′(=Vc 2 ) which is inputted to the reference terminal  212  of the ADC  210 , and also acquires a value of the output signal Vout(=Vo 2 ) (behavior s 2  (Vc 2 , Vo 2 )) after the change of the power source voltage. 
     In step S 12 , the abnormality detection part  220   a  selects again the power source voltage supply path I which does not go through the resistance RL by controlling the power source voltage control circuit  230 , and supplies the power source voltage Vcc′(=Vc 1 ) to the detection circuit  112  (Vcc′=Vc 2 →Vc 1 ). Then, the abnormality detection part  220   a  acquires a value of the power source voltage Vcc′(=Vc 1 ) which is inputted to the reference terminal  212  of the ADC  210 , and also acquires a value of the output signal Vout(=Vo 1 ′) (behavior s 1  (Vc 1 , Vo 1 ′)) inputted to the detection signal terminal  211 . 
     In step S 13 , the value of the output signal Vout(=Vo 1 ) which is acquired in step S 10  and the value of the output signal Vout(=Vo 1 ′) which is acquired in step S 12  are compared to each other. When both values agree with each other, that is, when a pressure value (a value of a peripheral environment) is not changed between step S 10  and step S 12 , the processing advances to step S 14  and the presence or non-presence of abnormality in the detection circuit  112  is determined. On the other hand, when it is determined that Vo 1  and Vo 1 ′ differ from each other in step S 13 , the processing returns to step S 10 , and the acquisition of the output signal Vout before and after the change of the power source voltage is executed again. 
     In step S 14 , it is determined whether or not the output signal Vo 1  of the detection circuit  112  before the change of the power source voltage (behavior s 1  (Vc 1 , Vo 1 )) and the output signal Vo 2  of the detection circuit  112  after the change of the power source voltage (behavior s 2  (Vc 2 , Vo 2 )) are on the input/output characteristic curve at the same pressure value. When both output signals are on the same input/output characteristic curve, it is determined that the detection circuit  112  is in a normal state (step S 15 ), while when both output signals are not on the same input/output characteristic curve, it is determined that the detection circuit  112  is in an abnormal state (step s 16 ). 
     The reason the output signal Vo 1  (behavior s 1  (Vc 1 , Vo 1 )) with respect to the power source voltage value Vc 1  before the change of the power source voltage is measured twice in steps S 10  and S 12  in the above-mentioned abnormality detection processing is that the presence and the non-presence of abnormality are determined by comparing the behaviors of the detection circuit  112  before and after the change of the power source voltage under the condition where the pressure value (value of the peripheral environment) is not changed. Even when there exists the tolerance of a predetermined range (for example, approximately 1%) in the measured values of the detection signal Vout obtained by carrying out the measurement twice with respect to the power source voltage Vc 1  before the change of the power source voltage, it may be determined that the measured values obtained by carrying out the measurement twice agree with each other. 
     Further, in step S 14 , for example, values of output signals Vout(=Vo 1 , Vo 2 ) (reference values) with respect to the power source voltages Vcc′(=Vc 1 , Vc 2 ) are stored in advance with respect to respective physical quantities P (for example, PA, PB, PC). Then, the physical quantity P (PA, PB, PC) is decided corresponding to Vout(=Vo 1 ) (detected value) acquired in step S 10 , and it is determined whether or not Vout(=Vo 2 ) (detected value) acquired in step S 11  agrees with Vo 2  (reference value) with respect to the decided physical quantity P. Here, even when there is a predetermined tolerance (for example, 1%) in Vo 2  (detected value), it may be determined that Vout(=Vo 2 ) (detected value) agrees with the Vo 2  (reference value). 
     Here, in the above-mentioned processing, it is determined that “the detection circuit  112  is in a normal state” when the output signal Vo 1  (behavior s 1 ) of the detection circuit  112  with respect to the power source voltage Vc 1  before the change of the power source voltage and the output signal Vo 2  (behavior s 2 ) of the detection circuit  112  with respect to the power source voltage Vc 2  after the change of the power source voltage are on the same input/output characteristic curve. However, as shown in  FIG. 10 , it may be determined that “the detection circuit  112  is in a normal state” when the power source voltage is changed from Vc 1  to two or more different power source voltages (for example, Vc 2 , Vc 3 ) and an output signal Vo 2  (behavior s 2 ) with respect to the power source voltage Vc 2  after the change of the power source voltage and an output signal Vo 3  (behavior s 3 ) with respect to the power source voltage Vc 3  after the change of the power source voltage are on the same input/output characteristic curve. 
     Further, as shown in  FIG. 10 , it may be determined that “the detection circuit  112  is in a normal state” when all of the output signal Vo 1  (behavior s 1 ) with respect to the power source voltage Vc 1  before the change of the power source voltage, the output signal Vo 2  (behavior s 2 ) with respect to the power source voltage Vc 2  after the change of the power source voltage, and the output signal Vo 3  (behavior s 3 ) with respect to the power source voltage Vc 3  after the change of the power source voltage are on the same input/output characteristic curve. When the input/output characteristic with respect to a pressure value is formed of a curve, the presence or the non-presence of the abnormality in the detection circuit  112  can be determined with higher accuracy by determining whether or not three or more points are on the same input/output characteristic curve. In this case, for example, by constituting the power source voltage control circuit  231  as shown in  FIG. 15  thus enabling the selection of a path through which a power source voltage is supplied via any one of the plurality of different resistances values RL, RL 1 , the power source voltage can be changed among a plurality of different values (for example, Vc 2 =4V, Vc 3 =3V). 
     Example 2 of Abnormality Detection Processing 
       FIG. 13  shows, in the input/output characteristic curves shown in  FIG. 10 , input/output characteristic curves (input/output characteristic straight lines) when the relationship between an input (Vcc′) and an output (Vout) of the detection circuit  112  is formed of a straight line with respect to the respective pressure values P. 
     When the relationship between the input and the output of the detection circuit  112  is formed of the straight line, it is determined whether or not an output signal Vo 1  (behavior s 1  (Vc 1 , Vo 1 )) of the detection circuit  112  before the change of the power source voltage and an output signal Vo 2  (behavior s 2  (Vc 2 , Vo 2 )) of the detection circuit  112  after the change of the power source voltage are on the same characteristic straight line by studying whether or not the output signals Vo 1 , Vo 2  (behaviors s 1 , s 2 ) of the detection circuit  112  before and after the change of the power source voltage are changed at a predetermined ratio. 
     For example, in  FIG. 13 , when a ratio Vo 1 /Vo 2  of the outputs (Vout) before and after the change of the power source voltage agrees with a ratio Vc 1 /Vc 2  of the inputs (Vcc′) before and after the change of the power source voltage, it is possible to determine that the outputs (behaviors) before and after the change of the power source voltage are on the same straight line (LB). When the ratio Vo 1 /Vo 2  and the ratio Vc 1 /Vc 2  do not agree with each other, it is possible to determine that the outputs (behaviors) before and after the change of the power source voltage are not on the same straight line (LB). When Vc 1  is 5V (Vc 1 =5V) and the Vc 2  is 4V (Vc 2 =4V), by determining whether or not a ratio Vo 1 /Vo 2  of the outputs before and after the change of the power source voltage agrees with a ratio 5/4 of the inputs (Vcc′) before and after the change of the power source voltage, it is possible to determine whether or not the outputs (behaviors) before and after the change of the power source voltage are on the same characteristic straight line (LB) and whether or not the detection circuit  112  is in a normal state. 
       FIG. 14  is a flowchart for explaining the abnormality detection processing of the detection circuit  112  according to this embodiment when the input/output characteristic curve of the detection circuit  112  is formed of a straight line. In this flowchart, the steps other than step S 14   a  are substantially equal to the steps in the flowchart shown in  FIG. 12 . 
     In step S 14   a , it is determined whether or not a ratio Vo 1 /Vo 2  of output signals Vout before and after the change of the power source voltage agrees with a ratio Vc 1 /Vc 2  of the power source voltages before and after the change of the power source voltage. When both ratios agree with each other, it is determined that the detection circuit  112  is in a normal state (step S 15 ), while when both ratios do not agree with each other, it is determined that the detection circuit  112  is in an abnormal state (step S 16 ). 
     For example, assume that the power source voltage Vcc′ before the change of the power source voltage is 5V (Vcc′=5V) and the power source voltage Vcc′ after the change of the power source voltage is 4V (Vcc′=4V). By setting the ratio Vc 1 /Vc 2  of the power source voltages before and after the change of the power source voltage to 5/4 (Vc 1 /Vc 2 =5/4) (fixed value) in step S 14   a , it may be determined whether or not the ratio Vo 1 /Vo 2  agrees with 5/4 (fixed value). Further, it may be determined whether or not the ratio Vo 1 /Vo 2  agrees with the ratio Vc 1 /Vc 2  (detected value) using Vc 1  which is detected at the reference terminal  212  of the ADC  210  in step S 10  and Vc 2  which is detected at the reference terminal  212  of the ADC  210  in step S 11 . 
     Further, when the ratio Vo 1 /Vo 2  of the outputs (Vout) before and after the change of the power source voltage falls within a range of Vc 1 /Vc 2 ±predetermined tolerance (for example, 1%) in step S 14   a , it may be determined that “Vo 1 /Vo 2  agrees with Vc 1 /Vc 2 ”. 
     Further, when the behavior of the detection circuit  112  is formed of a straight line as shown in  FIG. 13 , a formula expressing a straight line which passes a point s 1  (Vc 1 , Vo 1 ) obtained in step S 10  and an origin (0, 0) is calculated, and a power source voltage Vcc′(=Vc 2 ) after the change of the power source voltage is substituted into the formula of the straight line thus calculating an output Vout(=Vo 2 ) (theoretical value) after the change of the power source voltage. This processing is executed between step S 13  and step S 14   a , for example. Then, in step S 14   a , abnormality in the detection circuit  112  may be detected by determining whether or not both the detected value Vo 2  acquired in step S 11  and the theoretical value Vo 2  agree with each other by comparing the detected value Vo 2  and the theoretical value Vo 2  to each other. Here, also in this case, it may be determined that both the detected value Vo 2  and the theoretical value Vo 2  agree with each other when the detected value Vo 2  falls within a range of predetermined tolerance (for example, 1%) of the theoretical value Vo 2 . 
     In  FIG. 13  and  FIG. 14 , the explanation has been made by taking the case where the input/output characteristic curve is formed of the straight line which passes the origin as an example. However, in a case where the input/output characteristic curve does not go through the origin (when Vout is not 0 when Vcc′ is 0 (Vcc′=0) and there exists an offset), an offset amount may be corrected based on the detected values of Vo 1  and Vo 2  obtained in steps S 10  to S 11 , and the processing in step S 14   a  may be executed. 
     According to the third embodiment described above, a resistance state of the signal line which is connected to the detection circuit  112  can be monitored using the monitoring line as described in detail in the first embodiment, and also abnormality in the detection circuit  112  per se can be surely detected with the simple constitution by evaluating the output signal Vout after the change of the power source voltage. That is, according to the third embodiment, the systematic abnormality detection with respect to the detection device  100  can be easily and surely executed with the simple constitution. 
     Further, in a method using a test pulse, it is necessary that a value of a peripheral environment (a pressure value or the like) of the detection circuit  112  is known at the time of performing abnormality detection processing. In a case of a pressure detection system of a negative pressure booster, there may be a case where a residual pressure remains in the negative pressure booster even at the time of stopping the engine. In this case, it is not possible to determine whether a pressure value at the time of stopping the engine is under an atmospheric pressure state or in a negative pressure residual state and hence, an abnormality diagnosis using a test pulse cannot be performed. To the contrary, according to the abnormality detection method of the detection circuit of this embodiment, it is unnecessary that a value of the peripheral environment (a pressure value or the like) per se is known. That is, provided that an input/output characteristic of the detection circuit  112  with respect to each pressure value is known in advance, abnormality in the detection circuit  112  can be detected by determining whether or not an output of the detection circuit  112  before and after the change of the power source voltage follows a known input/output characteristic (whether or not the output of the detection circuit  112  is on the same input/output characteristic curve). 
     [Modification] 
     Here, the monitoring lines described in the first and second embodiments and electrodes for connecting the monitoring lines may be omitted. For example, in  FIG. 9 , the monitoring lines  103   a , L 3   a ,  203   a , the electrode P 3   a  and the terminal T 3   a  may be omitted. In this case, with respect to the detection device  100  and the processing device  200 , it is possible to surely detect abnormality in the detection circuit  112  with the simple constitution without requiring additional signal lines for connecting both the detection device  100  and the processing device  200 , and additional electrodes and terminals for connecting the additional signal lines. Parts to be added in the processing device  200  are only the power source voltage control circuit  231  which is constituted of the resistance RL, the switch  231  and the like. Accordingly, abnormality in the detection circuit  112  can be detected in accordance with software processing executed by the processing part  220   a  using the minimum number of parts to be added. 
     Further, when the processing device  200  is originally configured such that power source voltages of a regulator, DC/DC converter and the like are variable, it is unnecessary to add the power source voltage control circuit  231 , and abnormality in the detection circuit  112  can be detected only in accordance with the software processing executed by the processing part  220   a.    
     Further, although the explanation has been made with respect to the case where the power source voltage control circuit  230  is constituted of the resistance RL and the switch  231 , the power source voltage control circuit  230  may be constituted of a regulator and a DC/DC converter. 
     Further, the abnormality detection processing of the detection circuit  112  according to this embodiment is applicable not only to the circuit shown in  FIG. 1  but also to the circuits shown in  FIG. 3 ,  FIG. 4 ,  FIG. 6  and  FIG. 8  and modifications of the respective circuits. That is, by combining the abnormality detection processing according to the third embodiment which uses the input/output characteristic of the detection circuit to the abnormality detection processing of the signal line according to the first and second embodiment which use the monitoring line, a resistance state of a signal line connected to the detection circuit  112  can be monitored, and abnormality in the detection circuit  112  per se can be also surely detected. 
     Fourth Embodiment 
       FIG. 16  shows a circuit diagram of a detection system according to a fourth embodiment of the present invention. This embodiment has the substantially same constitution as the detection system of the third embodiment shown in  FIG. 9  except for a point that, in the detection system of the third embodiment shown in  FIG. 9 , a power source voltage control circuit  240  is provided in place of the power source voltage control circuit  230 , an abnormality detection part  220   b  is provided in place of the abnormality detection part  220   a , and the monitoring lines  103   a , L 3   a  and  203   a  are omitted. 
     [Circuit Constitution] 
     The power source voltage control circuit  240  receives inputting of a power source voltage Vcc, continuously changes a power source voltage Vcc′ and outputs the power source voltage Vcc′ to a detection device  100 . The power source voltage control circuit  240  is, for example, a circuit which continuously changes the power source voltage Vcc′ by controlling a switching element such as a transistor, and is a regulator circuit such as a DC/DC converter, for example. The power source voltage control circuit  240  is connected to a processing part  220  through a control line  241 , and a value of an output power source voltage Vcc′ is controlled by the abnormality detection part  220   b  of the processing part  220 . 
       FIG. 17  is a block diagram showing the constitution of a detection circuit  112 . The detection circuit  112  includes a detection part (sensor)  151  which detects a physical quantity such as a pressure, a correction circuit  152  which adds a predetermined correction Δv to a detection signal vo outputted from the detection part  151 , and an amplifier circuit  153  which amplifies a detection signal vo+Δv obtained by correction performed by the correction circuit  152  at a predetermined amplification ratio α and outputs an output signal Vout (=α(vo+Δv)). The output signal Vout is inputted to the processing device  200  through detection signal lines  101 , L 1 . 
     The detection part  151  is a pressure sensor constituted of a diaphragm and a resistance bridge, for example, and outputs an electric signal (detection signal) vo indicative of a change in resistance caused by the deformation of the diaphragm. The correction circuit  152 , for example, adds a predetermined correction value Δv to a detection signal vo obtained by the detection part  151  in such a manner that, as shown in  FIG. 18 , when the detection circuit  112  is used in a state where an output signal Vout falls within a range of 0.5V to 4.5V with respect to a detection pressure P (0 to Pmax) with the power source voltage Vcc′ set to 5V (Vcc′=5V), the output signal Vout of the amplifier circuit  153  takes Vmin(=0.5V) at P(=0) and the output signal Vout takes Vmax(=4.5V) at P (=Pmax (maximum detection pressure)). The correction value Δv is adjusted corresponding to a value of a power source voltage Vcc′ which is supplied to the detection circuit  112  and is proportional to the power source voltage Vcc′. For example, when the power source voltage Vcc′ is 3V, the correction value Δv is adjusted in the decreasing direction so as to allow an output signal Vout which falls within a range of 0.3V to 2.7V to be outputted from the amplifier circuit  153 . Although the range of the output signal Vout with respect to the power source voltage Vcc′(=5V) is set to 0.5V to 4.5V as one example, there may be also a case where the above-mentioned operation is carried out with the output signal Vout set within a range of 0.25V to 4.75V or the like. 
     Principle of Abnormality Detection] 
     The abnormality detection processing according to this embodiment is explained in conjunction with  FIG. 19  to  FIG. 21 . 
       FIG. 19  shows input/output characteristic curves of the detection circuit  112  with respect to respective pressure values (here, straight lines), and shows a change in the output signal Vout when the power source voltage Vcc′ supplied to the detection circuit  112  is swept from Vcc (for example, 5V) to 0V with respect to a plurality of detection pressures P=PA, PB, PC (PA&lt;PB&lt;PC). 
     In  FIG. 19 , when the power source voltage Vcc′ falls within a range from Vcc to Vx 0  (an area I), the detection circuit  112  exhibits a characteristic that the output signal Vout is linearly decreased along with the decrease of the power source voltage Vcc′ with respect to the respective pressure values P. This characteristic corresponds to a change of the output signal Vout when the power source voltage Vcc′ is continuously decreased in the input/output characteristic shown in  FIG. 13 . Here, although the explanation is made by taking the case where the input/output characteristic of the detection circuit  112  is formed of a straight line as an example, the input/output characteristic may be formed of a curve in the same manner as the case of the third embodiment. 
     Further, this embodiment makes use of the characteristic that the detection part  151  of the detection circuit  112  shown in  FIG. 17  is stopped when the relationship of Vcc′&lt;Vx 0  (minimum operation power source voltage) is established so that vo becomes 0 (vo=0) and the output signal Vout becomes αΔv (Vout=αΔv) (Δv being proportional to Vcc′) whereby the characteristic is irrelevant to the pressure. To explain the embodiment in conjunction with  FIG. 19 , it is understood that the detection circuit  112  exhibits the characteristic that the output signal Vout changes on separate curves or straight lines for respective pressure values P in an area I (in the same manner as the change shown in  FIG. 13 ), the output signal Vout changes on the same curve (or straight line) C irrelevant to the pressure values PA to PC in an area II (Vcc′&lt;Vx 0 ) using Vcc′(=Vx 0 ) as a boundary. In this manner, the input/output characteristic of the detection circuit  112  according to this embodiment includes the area I where the output signal Vout changes corresponding to the power source voltages Vcc′ for the respective pressure values P, and the area II where the output signal Vout changes corresponding to the power source voltage Vcc′ irrelevant to the pressure value P. 
     In this embodiment, by making use of such an input/output characteristic constituted of the area I and the area II, abnormality in a conductive line which connects the detection circuit  112  and an external circuit to each other and abnormality in the detection circuit  112  per se (abnormality in the inside of the detection circuit  112 ) are detected. 
     Firstly, as shown in  FIG. 20 , based on a change of the minimum operation power source voltage Vx (Vx 0 →Vx 1 ), abnormality that any one of the conductive lines (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) is in a high resistance state is detected. In  FIG. 16 , any one of the conductive lines (L 1 ,  101 ; L 2 ,  102 ; L 3 ,  103 ) is brought into a high resistance state because of a contact resistance or the like in the terminal P 1  to P 3  so that a resistance RX is generated on the conductive line L 3  (see  FIG. 2  or the like), for example, ΔVcc which is a part of Vcc′ supplied from the power source voltage control circuit  240  is consumed by the resistance RX so that Vcc′−AVcc is supplied to the detection circuit  112 . As a result, when a power source voltage Vcc′(=Vx 0 +ΔVcc) is outputted from the power source voltage control circuit  240 , Vx 0  is supplied to the detection circuit  112 , and the detection part  151  is stopped when Vcc′ is less than Vx 0 +ΔVcc. That is, the power source voltage Vcc′ (minimum operation power source voltage Vx) at a point of time that the detection part  151  is stopped is increased from Vx 0  to Vx 0 +ΔVcc and, as shown in  FIG. 20 , the input/output characteristic curve (CB) is shifted leftward as indicated by a curve CB′. Accordingly, by detecting a change in the minimum operation power source voltage Vx, it is possible to detect abnormality that any one of the conductive lines is in a high resistance state. 
     Secondary, as shown in  FIG. 21 , abnormality in the detection circuit  112  is detected based on whether or not an input/output characteristic (C) of the detection circuit  112  when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx agrees with the input/output characteristic curve C 0  in a normal state which becomes the reference. When the power source voltage Vcc′ is less than the minimum operation power source voltage Vx 0 , the detection part  151  ( FIG. 17 ) is not operated, and an input/output characteristic of the detection circuit  112  is determined irrelevant to the pressure value P and hence, it is possible to detect abnormality in the detection circuit  112  irrelevant to a pressure value. 
     When abnormality occurs in the correction circuit  152  or in the amplifier circuit  153 , for example, when a correction value Δv of the correction circuit  152  becomes abnormal or an amplification ratio α of the amplifier circuit  153  becomes abnormal, as shown in  FIG. 21 , an input/output characteristic of the detection circuit  112  when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx 0  is shifted upward or downward from a curve or a straight line (C 0 ) in a normal state which becomes the reference (curve C+ or C−). That is, by comparing an input/output characteristic or an output signal Vout of the detection circuit  112  when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx 0  with the input/output characteristic C 0  or the output signal in a normal state which becomes the reference, abnormality in the detection circuit  112  can be detected. Here, when the power source voltage Vcc′ is 0V (Vcc′=0V), the output signal Vout becomes 0 (Vout=0) and hence, the input/output characteristic curve passes the origin ((Vcc′, Vout)=(0, 0)). 
     [Abnormality Detection Processing] 
       FIG. 22  is a flowchart for explaining the abnormality detection processing of the detection circuit  112  according to the fourth embodiment. 
     The abnormality detection part  220   b  sweeps the power source voltage Vcc′ from Vcc to 0 as indicated on an axis of abscissas of a graph in  FIG. 19  by controlling the power source voltage control circuit  240  (step S 20 ), and based on a change in an output signal Vout, detects a minimum operation power source voltage Vx at which the detection part  151  stops (step S 21 ), and detects an input/output characteristic curve C or an output signal Vout of the detection circuit  112  when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx is detected (step S 22 ). Although the explanation is made with respect to the case where the power source voltage Vcc′ is swept from Vcc to 0V in this embodiment, the power source voltage Vcc′ may be changed from a voltage lower than Vcc within a range where the minimum operation power source voltage Vx can be detected. Here, the minimum operation power source voltage Vx(=Vx 0 ) (reference value) when a conductive line is in a normal state and the input/output characteristic curve C 0  or a value of the output signal Vout with the power source voltage Vcc′ being less than Vx 0  when the detection circuit  112  is in a normal state are stored in the detection circuit  112  in advance. 
     In step S 23 , the minimum operation power source voltage Vx which is detected in step S 21  and Vx 0  (reference value) are compared with each other. When both of Vx and Vx 0  agree with each other, it is determined that the conductive line is in a normal state (step S 24 ). On the other hand, when the minimum operation power source voltage Vx and Vx 0  (reference value) are different from each other in step S 23  (see  FIG. 20 ), it is determined that the conductive line is in an abnormal state (step S 25 ). 
     In step S 26 , the input/output characteristic curve C which is detected in step S 22  is compared with the reference curve C 0 . When both the input/output characteristic curve C and the reference curve C 0  agree with each other, it is determined that the detection circuit  112  is in a normal state (step S 27 ). On the other hand, when the input/output characteristic curve C is different from the reference curve C 0  in step S 26  (see  FIG. 21 ), it is determined that the detection circuit  112  is in an abnormal state (step S 28 ). 
     To consider a case where the input/output characteristic curve C 0  when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx 0  is formed of a straight line, a value of Vout when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx is detected at one point in step S 22 , and a detected value of Vout may be compared with the reference value in step S 26 . 
     According to the abnormality detection processing of this embodiment described above, based on the minimum operation power source voltage Vx of the detection part  151  which is decided irrelevant to a pressure value, abnormality caused by a high resistance state of the conductive line which connects the detection circuit  112  with the external circuit can be detected, and also based on the input/output characteristic of the detection circuit  112  when the power source voltage Vcc′ is less than the minimum operation power source voltage Vx, abnormality in the detection circuit  112  per se can be detected. 
     According to the abnormality detection processing of this embodiment, by making use of the area of the input/output characteristic irrelevant to the pressure value, abnormality detection processing of the detection circuit  112  can be also executed even when a value of the peripheral environment per se (pressure value and the like) is not known. 
     According to the abnormality detection processing of this embodiment, also even when a pressure value is frequently changed, the abnormality detection processing of the detection circuit  112  and the conductive line can be executed irrelevant to the pressure value. Further, abnormality caused by a high resistance state of the conductive line can be detected without adding a monitoring line. 
     Although the explanation has been made heretofore by taking the detection circuit which detects a pressure as an example, this embodiment is applicable to an arbitrary detection circuit such as a detection circuit for detecting a temperature, a speed, acceleration, humidity or the like. 
     Embodiment 5 
     According to a method of detecting abnormality of the fourth embodiment, the minimum operation power source voltage Vx at which the detection part  151  ( FIG. 17 ) is stopped is used and hence, in an actual operation, the abnormality of the correction circuit  152  and the amplifying circuit  153  in the detection circuit  112  excluding the detection part  151  is detected. To the contrary, the abnormality detection processing of the detection circuit  112  according to the third embodiment corresponds to the execution of the abnormality detection processing using an input/output characteristic of the region (I) of not less than the minimum operation power source voltage Vx in  FIG. 19  and hence, the presence or the non-presence of the abnormality in the whole detection circuit  112  can be detected. Accordingly, in this embodiment, the abnormality detection processing for the detection circuit  112  per se is executed by the method of detecting abnormality according to the third embodiment, and the abnormality detection processing for a conductive line which connects the detection circuit  112  to an external circuit is executed by the method of detecting abnormality according to the fourth embodiment. 
       FIG. 23  is a flowchart for explaining the abnormality detection processing of the detection circuit according to the fifth embodiment. In the flowchart, the abnormality detection processing of the detection circuit  112  per se is executed by the method of detecting abnormality according to the third embodiment, and the abnormality detection processing of the conductive line is executed by the method of detecting abnormality according to the fourth embodiment. In the explanation made hereinafter, an abnormality detection function of a processing part  220  of this embodiment is referred to as an abnormality detection part  220   c  (not shown in the drawing). 
     The abnormality detection part  220   c  controls a power source voltage control circuit  240 , sweeps a power source voltage Vcc′ from Vcc to 0 as indicated on an axis of abscissas in a graph shown in  FIG. 19  (step S 30 ), and detects output signals Vout (=Vo 1 , Vo 2 ) at the power source voltage Vcc′(=Vc 1 ,Vc 2 )(step S 31 ) and also detects a minimum operation power source voltage Vx at which the detection part  151  stops (step S 32 ). The processing in step S 31  corresponds to step S 10  and step S 11  shown in  FIG. 12  and  FIG. 14 . Further, the diagnosis of the detection circuit  112  may be executed after confirming a state where a pressure value is not changed by adding processing in step S 12  and step S 13  shown in  FIG. 12  and  FIG. 14 . 
     In step S 33 , the minimum operation power source voltage Vx detected in step S 32  and Vx 0  (reference value) are compared with each other, and it is determined that a conductive line is in a normal state when both power source voltages agree with each other (step S 34 ) and it is determined that the conducive line is in an abnormal state when both power source voltages do not agree with each other (step S 35 ). 
     In step S 36 , the abnormality detection part  220   c  determines whether or not the output signals Vout (=Vo 1 , Vo 2 ) detected in step S 31  are on the same input/output characteristic curve, for example, whether the relationship of Vo 1 /Vo 2 =Vc 1 /Vc 2  is satisfied (see steps S 14 , S 14   a  in  FIG. 12  and  FIG. 14 ). When the output signals Vout (=Vo 1 ,Vo 2 ) are on the same input/output characteristic curve, the abnormality detection part  220   c  determines that the detection circuit  112  is in a normal state (step S 37 ), while when the output signals Vout (=Vo 1 ,Vo 2 ) detected in step S 31  are not on the same input/output characteristic curve, the abnormality detection part  220   c  determines that the detection circuit  112  is in an abnormal state (step S 38 ). 
     According to the abnormality detection processing of this embodiment, without adding a monitoring line, abnormality caused by a high resistance state of the conductive line can be detected, and also the presence or non-presence of abnormality in the whole detection circuit  112  including the detection part  151  can be detected based on the input/output characteristic of the detection circuit  112  when the power source voltage Vcc′ is not less than Vx. 
     Other Embodiments 
     The abnormality detection processing of the detection circuit  112  per se may be executed using the method of the fourth embodiment, and the abnormality detection processing of the conductive line may be executed using the method through the monitoring line of the first and second embodiments. In this case, using the method through the monitoring line, it is possible to specify the conductive line on which abnormality occurs, and it is also possible to execute the abnormality detection processing of the detection circuit  112  in a peripheral environment where pressure is frequently changed. 
     Further, both the abnormality detection processing of the detection circuit  112  per se according to the third embodiment and the abnormality detection processing of the detection circuit  112  per se according to the fourth embodiment may be executed. In this case, abnormality in the whole detection circuit including the detection part can be detected by the abnormality detection processing of the third embodiment in a state where no pressure change takes place, and abnormality in the detection circuit excluding the detection part can be detected by the abnormality detection processing of the fourth embodiment in a state where the pressure is frequently changed. Accordingly, it is possible to more surely detect abnormality in the detection circuit  112 . 
     By combining the abnormality detection processing of the conductive line by the fourth embodiment with the abnormality detection processing of the detection circuit  112  per se by the third embodiment and the abnormality detection processing of the detection circuit  112  per se by the fourth embodiment, it is possible to easily and surely perform systematic abnormality detection of the detection circuit  112  including the detection of abnormality in the conductive line which connects the detection circuit  112  with the external circuit. 
     By combining the abnormality detection processing of the detection circuit  112  per se by the third embodiment and the abnormality detection processing of the detection circuit  112  per se by the fourth embodiment with the abnormality detection processing of the conductive line by the first and second embodiments and the abnormality detection processing of the conductive line by the fourth embodiment, it is possible to further surely perform the systematic abnormality detection of the detection circuit  112  including the abnormality detection of the conductive line which connects the detection circuit  112  and the external circuit with each other. 
     Further, the technical concept of the present invention is not limited to the abnormality detection of an electric circuit such as a detection circuit and is also applicable to the abnormality detection of other electric devices such as an electrically-operated motor or an electronic device. For example, in a case where an electrically-operated motor is rotated at a high speed with the supply of a predetermined power source voltage Vcc′(=Vcc), that is, a sufficiently high power source voltage, even when the electrically-operated motor is brought into a high frictional state because of a malfunction of a bearing or the like, a change of an output rotational speed of the electrically-operated motor is small and hence, it is difficult to detect the malfunction. On the other hand, when the supplied power source voltage Vcc′ is decreased (when the rotational speed of the electrically-operated motor is decreased), the rotational speed of the electrically-operated motor is remarkably lowered in a high frictional state compared to a normal state. Accordingly, a detected value of the rotation speed is compared to a reference value by decreasing the power source voltage Vcc to a power source voltage Vy (&lt;Vcc) at which the rotational speed of the electrically-operated motor remarkably differs between a normal state and a high frictional state. In this case, a value of the power source voltage Vy(&lt;Vcc) at which the rotational speed of the electrically-operated motor remarkably differs between a normal state and a high frictional state and a value of the rotational speed R 0  (reference value) with respect to Vcc′(=Vy) when the electrically-operated motor is in a normal state are measured in advance. The abnormality detection processing is performed as follows. During the operation of the electrically-operated motor, the power source voltage is decreased from Vcc to Vy and a rotational speed R of the electrically-operated motor is detected. When the detected value R agrees with the reference value R 0 , it is determined that “the electrically-operated motor is in a normal state”, while when the detected value R does not agree with the reference value R 0 , it is determined that “the electrically-operated motor is in an abnormal state”. 
     The abnormality can be also detected by detecting a value of the power source voltage Vcc′ at which the electrically-operated motor starts, that is, a value of the power source voltage Vcc′ at which the rotation of the electrically-operated motor is started. When the electrically-operated motor is brought into a high frictional state because of a malfunction of a bearing or the like, the power source voltage at which the electrically-operated motor is started (starting voltage) is increased from a value Vcc at a normal state (Vcc→Vcc+ΔVcc). Accordingly, when a starting voltage Vst of the electrically-operated motor is detected and the detected value Vst agrees with the reference value Vcc, it is determined that “the electrically-operated motor is in a normal state”. On the other hand, when the detected value Vst does not agree with the reference value Vcc, it is determined that “the electrically-operated motor is in an abnormal state”. Abnormality in the electrically-operated motor can be detected in this manner. 
     Sixth Embodiment 
       FIG. 24  shows a circuit diagram of a detection system according to a sixth embodiment. In this embodiment, constitutions identical with the constitutions of the above-mentioned embodiments are given same symbols, and the detailed explanation of these constitutions is omitted. Hereinafter, parts having the constitution different from the constitution of the above-mentioned embodiments are explained in detail. 
     The detection system includes a disconnection detection circuit  250  which is interposed on a monitoring line  203   a  connected to a monitoring line  203  in the inside of a processing circuit  200 . 
     The detection circuit  250  includes: a resistance R 5  which is interposed between the monitoring line  203   a  and a power source voltage Vcc, a resistance R 6  which is interposed between the monitoring line  203   a  and a ground potential GND, a capacitor C 1  which is interposed between the monitoring line  203   a  and the ground potential GND parallel to the resistance R 6 , and a resistance R 7  which is interposed on a middle portion of the monitoring line  203   a  and is connected to the resistance R 5  and the resistance R 6  in series. That is, in the inside of the processing circuit  200 , the monitoring line  203   a  is connected to the power source voltage Vcc through the resistance R 5 , and is connected to the ground potential GND through the resistance R 6 . The monitoring line  203   a  is also connected to the ground potential GND through the capacitor C 1  connected to the resistance R 6  in parallel. The resistance R 7  is interposed on the monitoring line  203   a  in series, and one end of the resistance R 7  is connected to the first capacitor C 1  while the other end of the resistance R 7  is connected to the resistance R 6 . In the disconnection detection circuit  250 , the power source voltage Vcc is connected to the ground potential GND through the resistance R 5 , the resistance R 7  and the resistance R 6 , and is connected to the ground potential GND through the resistance R 5  and the capacitor C 1 . Here, the capacitor C 1  is provided for stabilizing a potential of the monitoring line  203   a . The resistance R 7  is provided for limiting an electric current which flows into a ground terminal  213   a  of the ADC 210 . 
     In  FIG. 24 , symbols RC 1 , RC 2  indicate contact resistances at electrodes or terminals and resistance components of conductive lines. The RC 1  includes a contact resistance between a ground electrode P 3  of a detection device  100  and a ground line  103 , a contact resistance between the ground electrode P 3  of the detection device  100  and a ground line L 3 , a resistance component between the ground electrode P 3  of the detection device  100  and a connection point (V 2 ′), and a resistance component on the ground line L 3 . The RC 4  includes a contact resistance between a ground terminal T 3  of the processing device  200  and the ground line L 3 , a contact resistance between the ground terminal T 3  of the processing device  200  and the ground line  203 , and a resistance component on the ground line  203 . 
     Assuming a case where the contact resistances RC 1 , RC 2  are 0 (when it is considered that resistance values of paths formed of the ground lines  203 , L 3 ,  103  from the ground potential GND to the connection point (V 2 ′) are substantially 0), in the disconnection detection circuit  250  of the processing circuit  200 , the capacitor C 1  is charged with an electric current from the power source voltage Vcc through the resistance R 5  and, after the charge of the capacitor C 1  is finished, the electric current from the power source voltage Vcc flows into the ground potential GND through the monitoring lines  203   a , L 3   a ,  103   a  and the ground lines  103 , L 3 ,  203 . In this case, the electric current from the power source voltage Vcc does not flow into a resistance R 7  side so that a detection voltage V 2  which is inputted to a monitoring terminal  213   a  of an ADC 210  from the monitoring line  203   a  assumes the same potential (0V) as the ground potential GND. 
     Assuming a case where the contact resistances RC 1 , RC 2  are not 0 (when it is not considered that resistance values of paths formed of the ground lines  203 , L 3 ,  103  from the ground potential GND to the connection point (V 2 ′) are substantially 0), the potential difference is generated between a potential V 2 ′ at the connection point and the ground potential GND of the processing circuit  200 . Accordingly, the potential difference is also generated between a potential at a ground terminal  112   a  of the detection circuit  112  (the same potential as the potential V 2 ′ at the connection point) and the ground potential GND of the processing circuit  200 . In this embodiment, based on the detection voltage V 2  indicative of a resistance state of the ground line ( 103 , L 3 ,  203 ), the influence which a high resistance state of the ground line exerts on the behavior of the detection circuit  112  (output voltage Vout) is corrected. 
     As shown in  FIG. 25 , when the contact resistances RC 1 , RC 2  are not 0, the potential V 2 ′ at the connection point is expressed by following formulae (3) to (5) using an electric current Ids which flows through the ground lines ( 103 , L 3 ,  203 ) from the detection circuit  112  and an electric current Icc which flows through the resistance R 5 , the monitoring lines ( 203   a , L 3   a ,  103   a ), the ground lines ( 103 , L 3 ,  203 ) from the power source voltage Vcc. 
         V 2=( Ids+Icc )*( RC 1 +RC 2)  (3)
 
         Ids= 10 mA  (4)
 
         Icc=Vcc /( R 5 +RC 1 +RC 2)  (5)
 
     Here, Ids is set to 10 mA (a constant value) which is a value of an electric current which typically flows into the detection circuit  112 . 
     As indicated in the formulae (3) to (5), the potential V 2 ′ at the connection point is a value proportional to RC 1 +RC 2 . Assuming the resistance values of the resistances R 5 , R 7 , R 6  as R 5 =1 [kΩ], R 7 =10 [kΩ], R 6 =15 [kΩ] respectively, for example, when the contact resistance RC 1 +RC 2  is sufficiently small compared to the resistance value of the resistance R 7 , an electric current I 7  which flows through the resistance R 7  is sufficiently small compared to the electric current Ids and hence, the electric current I 7  can be ignored. In one example, the electric current Icc is several mA, and the electric current I 7  which flows through the resistance R 7  is approximately 1 μA. Accordingly, a voltage step-down amount at the resistance R 7  can be ignored and hence, it is considered that the detection voltage V 2  which is inputted to the monitoring terminal  213   a  of the ADC  210  agrees with the potential V 2 ′ at the connection point (V 2 =V 2 ′). As a result, a state of the resistance component RC 1 +RC 2  of the ground line can be monitored using the detection voltage V 2 . 
     Since the detection voltage V 2  agrees with the potential V 2 ′ at the connection point, the deviation of the potential at the ground terminal of the detection circuit  112  can be corrected using the detection voltage V 2 . To be more specific, as described later, an output voltage Vout and an input voltage Vcc of the detection circuit  112  are corrected using the detection voltage V 2  (ΔV=V 2 −V 1 ). 
       FIG. 26  is an explanatory view for explaining the processing which corrects the output voltage Vout of the detection circuit  112  using the detection voltage V 2 .  FIG. 26(   a ) shows a characteristic curve indicating the relationship between the output voltage Vout and the detection pressure (negative pressure) when the potential V 2 ′ at the connection point agrees with the ground potential (0V).  FIG. 26(   b ) shows the characteristic curve when the voltage V 2 ′ at the connection point is elevated because of the contact resistance RC 1 +RC 2  of the ground line. 
     Here, the detection circuit  112  is constituted of a pressure detection circuit, and the explanation is made by taking a type of detection circuit in which the output voltage Vout is linearly lowered corresponding to the increase of a value of a negative pressure as an example. A pressure detection range of the pressure detection circuit  112  is, as shown in  FIG. 26(   a ) and (b), approximately from −100 kPa to −5 kPa. When Vcc(=5[V]) is supplied to the pressure detection circuit  112 , the pressure detection circuit  112  outputs the output voltage Vout which falls within a range from 0.3[V] to 3.3[V]. 
     When the potential V 2 ′ at the connection point agrees with the ground potential GND, the characteristic curve of the pressure detection circuit  112  takes a curve shown in  FIG. 26(   a ). In the characteristic curve in  FIG. 26(   a ), a zone where the output voltage Vout is linearly lowered is expressed by a formula (6). 
         V out=( c 1 *pe+c 0)* VDD   (6)
 
     Here, Vout is an output voltage [V] of the pressure detection circuit  112 , and pe is a detection pressure (negative pressure: [kPa]). c 1 , c 0  are constants which are decided depending on the specification of the pressure detection circuit  112 . VDD is a reference voltage which decides the inclination of the characteristic curve and corresponds to an upper limit value [V] of a pressure detection range of the pressure detection circuit  112  (3.3[V] in this example). The reference voltage VDD is set in advance corresponding to a kind of the pressure detection circuit  112  and corresponding to a magnitude of a value of the power source voltage Vcc. 
       FIG. 26(   b ) shows a characteristic curve of a case where the potential V 2 ′ at the connection point (detection voltage V 2 ) is elevated because of the contact resistance Rc 1 +RC 2  of the ground line. Here, the output voltage Vout of the pressure detection circuit  112  is detected as a value which is offset in the increasing direction by V 2 ′(=V 2 ) compared to the case where V 2 ′ is 0 (V 2 ′=0). In the same manner, the reference voltage VDD is also detected as a value which is offset in the increasing direction by V 2 ′(=V 2 ) compared to a case where V 2 ′ is V 2  and 0 (V 2 ′=V 2 =0). Accordingly, the processing which corrects the output voltage Vout and the reference voltage VDD by only the offset amount V 2 ′(=V 2 ) is executed. As described previously, since the potential V 2 ′ at the connection point can be considered as equal to the ground potential detection voltage V 2 , the output voltage Vout and the reference voltage VDD are corrected as expressed by a formula (7) using the correction amount V 2 . The output voltage Vout and the reference voltage VDD after the correction are respectively assumed as an effective output voltage Vout_eff and an effective reference voltage VDD_eff respectively. 
         V out_eff= V out− V 2
 
         VDD _eff= VDD−V 2  (7)
 
     By using Vout_eff and VDD_eff in the formula (7) in the formula (6) in place of Vout and VDD, the relationship expressed by a formula (8) is established. 
         V out_eff=( c 1 *pe+c 0)* VDD   —   ef   (8)
 
     Accordingly, by calculating the detection pressure pe by the formula (8) using the effective output voltage Vout_eff and the effective reference voltage VDD_eff which are obtained by correcting the output voltage Vout and the reference voltage VDD of the pressure detection circuit  112  using the detection voltage V 2  respectively, it is possible to calculate the detection pressure pe which compensates for a contact resistance amount of the ground line. Due to this processing, even when the unexpected elevation of the resistance value occurs on the ground line, it is possible to compensate for the influence which the resistance value on the ground line exerts on the detection pressure pe. 
     Assuming a case where the monitoring lines  103   a , L 3   a ,  203   a  are disconnected so that the monitoring lines  103   a , L 3   a ,  203   a  are brought into an excessively high resistance state, in the disconnection detection circuit  250 , an electric current from the power source voltage Vcc flows toward only a resistance R 7  side (Icc=0). Accordingly, a detection voltage V 2  which is inputted to the monitoring terminal  213   a  of the ADC  210  becomes a voltage applied to the resistance R 6 . The voltage of the resistance R 6  is a voltage obtained by dividing the power source voltage Vcc by the resistance R 5 +R 7  and the resistance R 6 , and is expressed by a formula (9). 
         V 2 =Vcc*R 6/( R 5 +R 7 +R 6)  (9)
 
     For example, V 2  becomes 2.88[V] (V 2 =2.88[V]) when Vcc, R 5 , R 7  and R 6  are set such that Vcc=5[V], R 5 =1 [kΩ], R 7 =10 [kΩ] and R 6 =15 [kΩ]. In this case, a value of V 2  may be monitored by setting a threshold value of V 2  when the monitoring line is disconnected to 2.88[V] (Vth=2.88[V]), for example. 
     The specific processing in the processing device  200  is as follows. The ADC  210  outputs a digital signal corresponding to analogue signals V 1 , V 2  which are inputted to the ADC  210 . A processing part  220  calculates ΔV(=V 2 −V 1 ) based on the digital signals V 1 , V 2 . When ΔV falls within a range of −10 mV&lt;ΔV&lt;10 mV, the processing part  220  determines that “The ground line is in a normal state”, when ΔV falls within a range of 10 mV≦ΔV≦Vth, the processing part  220  determines that “The ground line is in a high resistance state”, and when ΔV satisfies the relationship of Vth&lt;ΔV, the processing part  220  determines that “The monitoring line is disconnected”. In this embodiment, although it is determined that V 2 ′(=V 2 ) agrees with a predetermined value (0V) when ΔV falls within a range of −10 mV&lt;ΔV&lt;10 mV, the range may be suitably decided corresponding to the resolution of the ADC 210 . 
     When ΔV satisfies the relationship of ΔV≦Vth, the processing part  220  corrects an output voltage Vout and a reference voltage VDD by only ΔV(=V 2 ) by the formula (7) and calculates the effective output voltage Vout_eff and the effective reference voltage VDD_eff. Then, the processing part  220  calculates a detection pressure pe by the formula (8) using the effective output voltage Vout_eff and the effective reference voltage VDD_eff. 
     When the relationship of −10 mV&lt;ΔV&lt;10 mV (ΔV being approximately 0) is established, the detection pressure pe may be calculated by the formula (6) without correcting the output voltage Vout and the reference voltage VDD. 
     According to the above-mentioned sixth embodiment of the present invention, the detection pressure pe which compensates for the contact resistance amount of the ground line can be calculated by correcting the output voltage Vout and the reference voltage VDD using the potential V 2 ′(=V 2 ) at the connection point. Due to this processing, even when an unexpected elevation of the resistance value occurs on the ground line, it is possible to compensate for the influence which the resistance value on the ground line exerts on the detection pressure pe. Accordingly, when a high resistance state of the ground line is detected, the detection pressure Pe can be corrected corresponding to a resistance state of the ground line so that the accurate detection of pressure can be continued while preventing the system from being stopped. 
     Further, by arranging the disconnection detection circuit  250  which includes the resistances R 5 , R 6  interposed between the monitoring line and the power source voltage Vcc and between the monitoring line and the ground potential GND respectively in the inside of the processing device  200 , the disconnection of the monitoring line per se for monitoring a resistance state of the ground line can be detected without adding the special constitution to the detection device  110 . 
     The above-mentioned correction processing of the output voltage Vout and the reference voltage VDD is also applicable to the above-mentioned first embodiment. To be more specific, the output voltage Vout and the reference voltage VDD may be corrected using the AV which is calculated in the first embodiment as a correction amount. 
     In the above-mentioned first to sixth embodiments, the explanation has been made mainly with respect to the detection system. However, the present invention is not limited to the detection system and is applicable to an arbitrary electric system provided that the electric system has the constitution where the power source supply or the signal communication is performed among a plurality of circuits. 
     The present application is the application which claims the priority from the international patent application PCT/JP2009/57606, and is filed as the priority claiming application where claims  9  to  29  of the present application correspond to claims  1  to  21  of the basic application and claims  1  to  8  are newly added. 
     EXPLANATION OF SYMBOLS 
     
         
           1 : detection system 
           100 : detection device (sensor device) 
           200 : processing device (ECU) 
           101 , L 1 ,  201 : detection signal line 
           102 , L 2 ,  202 : power source line 
           103 , L 3 ,  203 : ground line 
           101   a  to  103   a , L 1   a  to L 3   a ,  201   a  to  203   a : monitoring line 
         P 1  to P 3 , P 1   a  to P 3   a : electrode 
         T 1  to T 3 , T 1   a  to T 3   a : terminal 
           110 : housing 
           111 : sensor chip 
           112 : detection circuit 
           112   a : ground terminal 
           151 : detection part 
           152 : correction circuit 
           153 : amplifying circuit 
           210 : analogue digital converter (ADC) 
           220 : processing part 
           220   a ,  220   b : abnormality detection part 
         R 2 , R 3 : resistance 
         Vcc: power source voltage source, power source voltage 
           230 ,  240 : power source voltage control circuit 
           231 : switching circuit 
           232 ,  241 : control line 
           250 : disconnection detection circuit 
         RL: voltage step-down resistance