Abstract:
The resistances of resistors arranged in a matrix form in a pressure-sensitive resistor mat are measured with high accuracy and using simple circuitry by the fact that the output of an operational amplifier is connected to each row conductor and each column conductor, which are connected to the resistor. A voltage is selectively applied to the individual resistors by appropriately activating the operational amplifiers. Each operational amplifier associated with the row or column conductors is equipped with a current balancing circuit which detects the output current of the operational amplifier connected to the selected resistor which flows through this resistor. A processor determines the individual resistance values from the currents flowing through the individual resistors and the voltage drops across those resistors.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a circuit arrangement for measuring the resistance of resistors arranged in a matrix form in a pressure-sensitive resistor mat arranged in a vehicle seat for detecting seat occupancy. 
   BACKGROUND INFORMATION 
   A resistor mat having a plurality of pressure-sensitive resistor elements is described in German Patent No. 42 37 072. Such a resistor mat is integrated in the vehicle seat, for example in the front passenger seat, in order to automatically detect seat occupancy. Whether or not the airbag belonging to the front passenger seat is to be deployed in the event of a crash or what inflating intensity is desirable basically depends on the occupancy of the front passenger seat. If no occupancy or a child&#39;s seat is sensed on the vehicle seat by the resistor mat, the deployment of the airbag is to be completely suppressed. The same is true if the front passenger seat is occupied not by a person but by an object (e.g., a piece of baggage). The intensity of inflation depends on the size of the person occupying the vehicle seat, which is expressed by his or her weight, which can be measured using the resistor mat. The resistor mat can also provide information on the seating position of the person, which should affect the inflation intensity of the airbag. The more accurately the resistances of the pressure-sensitive resistors arranged in the resistor mat in a matrix form can be measured, the more accurate the information on the type of occupancy or seating position of a person on the vehicle seat. 
   An object of the present invention is to provide a circuit arrangement which performs an accurate resistance measurement of the individual resistors arranged in a matrix form in a pressure-sensitive resistor mat with the lowest possible circuit complexity. 
   In an article by T. D&#39;Alessio, “Measurement Errors in the Scanning of Piezoresistive Sensor Arrays,” Sensors and Actuators A, CH, Elsevier, Lausanne, Vol. 72, No. 1, Jan. 8, 1999, pp. 71-76, different circuits are shown for activating piezoresistive sensors in a matrix. The outputs of buffers, for which operational amplifiers can be used,are connected to the columns and rows. 
   European Patent Application No. 791 834 describes a method for determining the resistance of a resistor arrangement, in which leakage current is generated by applying voltage potentials of the same value. Furthermore, a measurement of the quantities required for determining the resistance value is performed by applying a first and a second voltage potential having different potential values to obtain a test current flowing through the resistor. These voltage potentials are selected so that the direction of the test current coincides with the direction of the cover current. Subsequently, the adjusted terminal voltages and a test current are measured. The resistance value is calculated using the measured values of the leakage current, test current, and terminal voltage. 
   SUMMARY 
   In accordance with an example embodiment of the present invention, a resistor matrix includes row conductors and column conductors, each row conductor being connected to each column conductor via a resistor. Each row conductor and each column conductor is connected to the output of an operational amplifier, so that a voltage can be selectively applied to the individual resistors in the rows and columns by appropriately activating the operational amplifiers. Each operational amplifier associated with the row conductors or column conductors is equipped with a current balancing circuit which detects the output current of the operational amplifier connected to the selected resistor which flows through this resistor. A processor determines the resistance values from the currents flowing through the individual resistors and the voltage drops across these resistors. The current balancing circuits in the operational amplifiers allow the current flowing through the individual resistors to be measured very accurately without requiring extremely narrow tolerances for the circuit elements. 
   The current balancing circuit may include, for example, an adder which adds the current flowing from an output stage of the operational amplifier to its supply voltage source and the current flowing from the output stage to ground, so that the output current of the operational amplifier flowing through the selected resistor can be picked up at the output of the adder. A circuit arrangement is advantageously provided in the current balancing circuit for transforming the two currents supplied to the adder into another measured value range. It is advantageous for the evaluation of the measured currents, which may scatter very widely due to the large changes in resistance, if the measured currents are transformed into a narrow measured value range. 
   It is expedient that the resistors in one column and in one row of the resistor matrix have fixed, pressure-independent resistances to allow defects of the row conductors and column conductors to be diagnosed. These resistors having pressure-independent resistances are preferably arranged in one column and one row at the edge of the resistor mat, since pressure-sensitive resistors are less needed at the edge of the vehicle seat than on the seat surface, which is the actual measuring surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a resistor matrix. 
       FIG. 2  shows an operational amplifier which detects the currents flowing through the resistors. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a circuit diagram of a resistor matrix as it is used in a pressure-sensitive resistor mat arranged in a vehicle seat for detecting seat occupancy. The resistor matrix has row conductors and column conductors, which are all connected to one another via pressure-sensitive resistors. In the embodiment illustrated, there are four row conductors LZ 1 , LZ 2 , LZ 3 , LZ 4  and five column conductors LS 1 , LS 2 , LS 3 , LS 4 , LS 5 . There are resistors Rsz, s being the column index (s=1, 2, 3, 4, 5) and z being the row index (z=1, 2, 3, 4). For example, resistor R 11  connects first column conductor LS 1  to first row conductor LZ 1 ; resistor R 32  connects third column conductor LS 3  to second row conductor LZ 2 ; resistor R 54  connects fifth column conductor LS 5  to fourth row conductor RZ 4 , etc. Thus, a matrix of s*z resistors is obtained, which are distributed over the entire seat surface and respond to pressure exerted in the zones of the seat surface assigned to the individual resistors with a change in resistance. In order to detect the change in resistance which is proportional to the pressure for all resistors in the matrix, the measuring circuit described below is used. 
   Each column conductor LS 1 , LS 2 , LS 3 , LS 4 , LS 5  is connected to the output of an operational amplifier OS 1 , OS 2 , OS 3 , OS 4 , and OS 5 . Similarly, each row conductor LZ 1 , LZ 2 , LZ 3 , LZ 4  is connected to the output of an operational amplifier OZ 1 , OZ 2 , OZ 3 , OZ 4 . Each of these operational amplifiers OS 1 , . . . , OS 5  and OZ 1 , . . . , OZ 4  has a first input feedback connected to the output and a second input which can be connected to a voltage U 1  or a voltage U 2  via a switch. The two voltages U 1  and U 2  are different; for example, voltage U 1 =5 V and U 2 =2.5 V. There is a processor PR, which switches operational amplifiers OS 1 , . . . , OS 5  for the columns and operational amplifiers OZ 1 , . . . , OZ 4  for the rows consecutively to voltage U 1  or U 2  so that a voltage is applied consecutively to all resistors Rsz of the resistor matrix. In the initial condition, operational amplifiers OS 1 , . . . , OS 5  for column conductors LS 1 , . . . , LS 5  as well as operational amplifiers OZ 1 , . . . , OZ 4  for row conductors LZ 1 , . . . , LZ 4  are switched to the same voltage U 2 . Then, both terminals of each resistor Rsz are at the same potential, and there is no voltage drop across them, and therefore they are conducting no current. Then, operational amplifier OS 1  for first column conductor LS 1  is switched over to the other voltage Ul. All the other operational amplifiers continue to be at voltage U 2 . Due to the voltage switch-over . . . , of operational amplifier OS 1 , both terminals of resistors R 11 , R 12 , R 13 , and R 14  in the first column are at different potentials, namely U 1  and U 2 , so that there is a voltage drop across them and they also conduct a current. After the currents flowing through these resistors and the voltage drop across them have been measured, as will be described in more detail below, and the resistance values have been calculated therefrom by processor PR, operational amplifier OS 1  of first column conductor LS 1  is switched over again to voltage U 2  and operational amplifier OS 2  of second column conductor LS 2  is switched over to voltage U 1 . Then, the resistance values of resistors R 21 , R 22 , R 23 , and R 24  can be determined. Thus, the resistances are measured in all columns. 
   In order to measure the individual resistances in the matrix, the voltage applied to the respective resistor and the current flowing through it is measured. The voltage drop across each resistor is determined by processor PR, to which is connected outputs S 1 , S 2 , S 3 , S 4 , and S 5  of column conductors LS 1 , LS 2 , LS 3 , LS 4 , and LS 5  and outputs Z 1 , Z 2 , Z 3 , and Z 4  of row conductors LZ 1 , LZ 2 , LZ 3 , and LZ 4 . The currents through the individual resistors are detected by operational amplifiers OZ 1 , OZ 2 , OZ 3 , and OZ 4  connected to row conductors LZ 1 , LZ 2 , LZ 3 , and LZ 4 . The currents through the resistors could also be detected via operational amplifiers OS 1 , OS 2 , OS 3 , OS 4 , and OS 5  associated with column conductors LS 1 , LS 2 , LS 3 , LS 4 , and LS 5 . The current that flows through a resistor flows through the row conductor connected to the respective resistor and appears as an output current at the operational amplifier connected to this row conductor. 
   Each operational amplifier OZ 1 , OZ 2 , OZ 3 , and OZ 4  associated with the row conductors is provided, according to the embodiment illustrated in  FIG. 2  of an operational amplifier OP, with a current balancing circuit which is capable of detecting output current IA of operational amplifier OP, which is exactly equal to the current flowing through the resistor which is about to be measured. Output current IA flows into output stage ES of operational amplifier OP. The design of output stage ES will not be described here in detail, since it is a conventional circuit for operational amplifiers that is well known to those skilled in the art. Output stage ES is normally connected to a positive supply voltage +U and to ground. The current balancing circuit operates by tapping both current I 1  flowing from output stage ES to supply voltage source +U and current I 2  flowing from output stage ES to ground. If current IA flowing into output stage ES is positive, the equation I 2  =−(I +IA) applies to the current flowing to ground, I being the cross current flowing through the output stage. The equation I 1  =I applies to the current flowing to supply voltage source +U. If current IA flowing at the output of output stage ES has a negative sign, the equation I 1  =I +IA applies to current I 1  and the equation I 2 =−I applies to current I 2 . In order to detect output current IA alone, which corresponds to the current flowing through the resistor to be measured, independently of cross current I in output stage ES, the two currents I 1  and I 2  are supplied to an adder SU. By adding the two currents I 1  and I 2 , cross current I of output stage ES is eliminated and only desired output current IA appears at output A of adder SU. 
   Since the values of the pressure-sensitive resistors fluctuate between 1 kΩand 2 MΩ, current IA flowing through the individual resistors can also change over a very wide measurement range of approximately 5 μA to 5 m A. If this extremely wide range of currents is to be transformed into a narrower current range, which would facilitate the evaluation of the measured currents in processor PR, circuit arrangements MU 1  and MU 2  are provided, using which the two currents I 1  and I 2  supplied to adder SU are transformed into a limited defined current range of approximately  250  μA. Such conventional circuit arrangements for measured value transformation are known per se and include a plurality of attenuating and amplifying stages. 
   As  FIG. 1  shows, outputs A 1 , A 2 , A 3 , and A 4  of operational amplifiers OZ 1 , OZ 2 , OZ 3 , and OZ 4  associated with the rows, which provide the currents through the resistors, are connected to processor PR. Instead of currents at outputs A 1 , A 2 , A 3 , and A 4 , appropriate measuring voltages can also be supplied to processor PR. The measuring voltage of each operational amplifier can be obtained through a resistor connected to the output of adder SU and traversed by output current IA. 
   In order for defects in the resistor mat, for example, broken conductors, to be diagnosed, the resistors in one column and in one row have fixed, pressure-independent resistances. These pressure-independent resistors are preferably in one column and in one row at the edge of the resistor mat. In the embodiment shown, these are resistors R 11 , R 12 , R 13 , R 14 , R 24 , R 34 , R 44 , R 54 . Due to the fact that the pressure-independent resistors are located at the edge of the resistor mat, i.e., outside the actual seat surface, the pressure-sensitive measuring surface is not reduced.