Abstract:
In a circuit arrangement for measuring the resistance of a resistance sensor, for instance a wetness sensor, which is connected to an evaluation circuit, there is a galvanic separation, formed preferably by an isolating transformer, between the resistance sensor and the evaluation circuit.

Description:
This application is a continuation of my co-pending, now abandoned application Ser. No. 08/498,364 filed Jul. 5, 1995. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a circuit arrangement for measuring the resistance of a resistance sensor, for instance a wetness sensor, which is connected to an evaluation circuit. 
     By way of two example, sensor resistors in the form of strip-shaped electrodes which are interlaced in one another are used for measuring the wetness of the windshield of a motor vehicle. If drops of water contact the two electrodes, then the resistance decreases. The decrease in resistance can be used for automatically controlling a windshield wiper. For a measurement of the resistance, it is necessary fundamentally to apply a voltage to the resistance sensor. However, dc portions of the voltage to be applied can produce electrolysis phenomena between the electrodes, which phenomena leads finally to a limiting of the life of the resistance sensors. A device for controlling a drive means for a vehicle accessory having such a sensor is described, for instance, in WO 90/08680. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to prevent electrolysis phenomena on resistance sensors, particularly wetness sensors. 
     This object is achieved with the circuit arrangement of the invention, in the manner that a galvanic separation is present between the resistance sensor and the evaluation circuit. It is preferable for the galvanic separation to be provided by an isolating transformer. 
     The circuit arrangement of the invention has the advantage of a galvanic separation between the sensor itself and the evaluation circuit. Thus, no direct voltage acts on the sensor. Thereby, electrolytic deposits on so that electrolytic deposits on or removals from the electrodes of the sensor, or which may break off from the electrodes, are prevented. Although wetness sensors can be operated in a particularly advantageous manner as a result of the circuit arrangement of the invention, the circuit arrangement of the invention is also suitable for use with other resistance sensors, i.e. with sensors in which the resistance of a sensor element is dependent on a physical value which is to be measured. 
     In order to avoid lead-throughs for the wires in the window of the vehicle, the circuit arrangement of the invention can further be developed in the manner that the resistance sensor and the evaluation circuit are connected inductively through the window of the vehicle. 
     One advantageous development of the circuit arrangement of the invention is that the resistance sensor is connected to a first winding of an isolating transformer. A second winding of the isolating transformer is acted on by a pulse-like voltage in the manner that, during in each case of one pulse, there is a rise in the current through the second winding. Furthermore means are and in means being provided for measuring the length of time which passes between the start of the pulse and the current reaching a predetermined threshold in the second winding. 
     A further development of this embodiment provides that this second winding is connected, in series with a measurement resistor, to one pole of a source of operating voltage and, via a switching transistor, to the other pole of the source of operating voltage. Also, the junction of the second winding with the measurement resistor is connected to the one input of a difference amplifier the other input of which can be acted on by a predetermined potential. 
     In the case of this further development, it can be provided that the switching transistor can be controlled by a microcomputer and that the output of the difference amplifier is connected to an input of the microcomputer. This is particularly advantageous when the microcomputer, on the basis of the specific requirements present, is to control the measuring process and, in this connection, determines the period of time by a counting process. 
     With the aforementioned development it may, however, also be advantageous for the output of the difference amplifier to be connected furthermore, via a resistor, to the non-inverting input of the difference amplifier and, via an inverter, to the control input of the switching transistor, and for the output of the difference amplifier to be connected with an input of a microcomputer. In this case, the circuit arrangement of the invention is designed to be self-excited so that the microcomputer is programmed for measuring the frequency. Should this frequency be too high for the computer or because of other programs which are also to be run by the microcomputer, then, in accordance with a further development, the input of the microcomputer can be connected via a frequency divider to the output of the difference amplifier. 
     In order to avoid high negative voltage peaks upon the disconnecting of the current, it is provided, in accordance with another further development of the circuit arrangement of the invention, that a diode is connected in parallel to the second winding and the measurement resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With the above and other objects and other advantages in view, the present invention will become more clearly understood in connection with the detailed description of a preferred embodiment, when considered with the accompanying drawings of which: 
     FIG. 1 shows a first embodiment; 
     FIG. 2 is a time graph of the current through the second winding; 
     FIG. 3 shows a second embodiment; and 
     FIG. 4 shows a locating of transformer windings on opposite sides of a window. 
     Identical parts are provided with identical reference numerals in the figures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The wetness sensor 1 consists of electrodes 2, 3 of conductive material which are applied on the windshield (not shownin FIG. 1-3 ) of a motor vehicle. As soon as drops of rain fall on the windshield, the resistance between the electrodes 2, 3 is reduced, which is noted by means of the circuit arrangement of the invention. The electrodes 2, 3 of the wetness sensor are connected to the ends of a first winding 4 of an isolating transformer 5. 
     The second winding 6 is connected in series with a measurement resistor 7 and the collector-emitter path of a switching transistor 8 between ground potential and the positive pole 9 of a source of operating voltage which is otherwise not shown. In parallel to the second winding 6 and the measurement resistor 7, there is a diode 10 which, after the disconnecting of the switching transistor 8, briefly takes over the current from the second winding 6. 
     The junction between the second winding 6 and the measurement resistance 7 is connected to the inverting input of a difference amplifier 11 the non-inverting input of which is acted on by a bias voltage which is obtained from the operating voltage by means of a voltage divider comprising resistors 12, 13. The output of the difference amplifier 11 is connected to an input 14 of a microcomputer 15 which, via an output 16, controls the switching transistor 8. 
     FIG. 2 shows, in the from of a time graph, the current I through the measurement resistor 7 and the voltage drop Um over the measurement resistor 7, as well as the output voltage U14 of the difference amplifier 11. At the time t0, the switching transistor 8 is switched into the conductive state. The voltage applied by said switching to the second winding 6 of the isolating transformer 9 acts on an impedance having an ohmic component and an inductive component, the ohmic component being dependent on the value of the resistance of the wetness sensor 1. Therefore, at t0, the current first rises rapidly and then continues to rise gradually until it exceeds a threshold value S. The time when it exceeds said value is dependent on the resistance of the wetness sensor 1 and can be determined in the manner that the period of time between t0 and t1 is measured by a counting process in the microcomputer 15. At the time t2, the switching transistor 8 is then again brought into the non-conductive state, whereupon the current through the second winding 6 and the measurement resistor 7 drops relatively rapidly over the diode 10. 
     In the embodiment shown in FIG. 3, the output of the difference amplifier 11 is fed back via a resistor 21 to the non-inverting input. Furthermore, the switching transistor 8 is controlled by the output voltage of the difference amplifier 11 via an inverter 22. Thus, the circuit oscillates at a frequency which is dependent on the value of the resistance of the wetness sensor 1, which can be measured by the microcomputer 15. This can take place, for instance, in the manner that, for a predetermined period of time, the number of flanks of the output signal of the difference amplifier 11 are counted. Should the frequency, however, be too high with respect to the microcomputer 15 itself or with respect to the functioning of other programs in the microcomputer, a frequency divider 23 can be provided between the output of the difference amplifier 11 and the input 14 of the microcomputer 15. 
     In the embodiment shown in FIG. 4, the isolating transformer is not developed in one piece. Rather, the primary winding 16 and the secondary winding 17, including the core material possibly present are divided between the inner side 20 of the window of the vehicle and the outer side 21 thereof. The transfer of energy to the resistance sensor 1 thus takes place inductively. There are no wire lead-throughs between the inner side and the outer side of the window of the vehicle. The secondary winding 17 is connected to the evaluation circuit via wires 18, 19.