Abstract:
A testing and calibrating device for an evaluation circuit of a linear oxygen probe (S) of an internal combustion engine is disclosed. Said device comprises a probe equivalent circuit (SES) having the same terminals (Vs+, Vs−/Vp−, Vp+ and Rc) as the oxygen probe (S). The probe equivalent circuit can largely emulate the electrical and chemical behaviors of the oxygen probe (S) and simulate probe faults and, at least during a testing and calibrating process, is connected to the evaluation circuit in place of the oxygen probe (S) or is connected parallel to said oxygen probe.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE01/01974 filed May 22, 2001, which designates the United States, and claims priority to German application number 10025578.7 filed May 24, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a testing and calibrating device for an evaluation circuit for a linear oxygen probe (referred to below as lambda probe or probe) in an internal combustion engine, particularly in a motor vehicle internal combustion engine. 
     The production process for engine control circuits for internal combustion engines requires that the probe evaluation circuit be tested and calibrated in the fitted state. This requires that all relevant parameters be verified under operating conditions—various supply voltages and temperatures for the engine control circuit, but also various operating states (lambda values) for the probe. It is also necessary to test whether the evaluation circuit identifies particular probe faults. 
     Furthermore, it is desirable to be able to perform not only the legally required OBD (on board diagnostics) for the probe but also calibration of the system (probe and evaluation circuit) when the internal combustion engine is operating. 
     The probe and the evaluation circuit are a closed control system. Simple measurement of the electrical properties of the evaluation circuit (for example offset or gain) is therefore not very revealing. The test needs to be performed when the control loop is closed and stable. 
     Although a test using a connected lambda probe allows measurement in the operating state, it is time-consuming and imprecise. The addition of a fault to demonstrate the diagnostic function is done using switches in the probe supply lines, for example, which allows short circuits and interruptions to be simulated. This is very time-consuming and susceptible to error. In addition, various operating points of the probe can be measured only by altering the oxygen concentration around the probe. This requires a very complex gas-changing device which regularly needs to be calibrated. Since a gas change, for technical reasons, proceeds comparatively slowly, it is not possible to assess the control stability of the system. 
     The system (probe and evaluation circuit) is calibrated during engine operation at two operating points: 
     a) when λ=1. In this case, no or only a minimal pump current should flow, since the oxygen concentrations in the exhaust from the internal combustion engine and in the measuring cell are balanced; 
     b) when λ=∞, that is to say with no fuel, i.e. when a motor vehicle is in overrun mode. In this case, the (maximum) pump current required is measured. 
     The values obtained for these two measurements can be used to determine the offset and gradient of the transfer function. The arithmetic values are stored in a correction table. 
     Overall, the method is extremely complex and has only limited reliability (on account of possible residual exhaust or cooling of the probe in overrun mode). 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a testing and calibrating device for an evaluation circuit for a linear lambda probe in an internal combustion engine which provides a simple way of testing and calibrating the evaluation circuit and of adding various faults to demonstrate the diagnostic function during engine operation at prescribed operating points. 
     The invention achieves this object by a testing and calibrating device for an evaluation circuit for a linear oxygen probe in an internal combustion engine, comprising a probe equivalent circuit having the same connections as the oxygen probe, which largely emulates the electrical and chemical behaviour of the oxygen probe and can simulate probe faults, and which is connected to the evaluation circuit instead of the oxygen probe, or in parallel with it, at least during a testing and calibrating process. 
     The probe equivalent circuit may comprise an inverting integrator with integral-proportional-integral response, whose non-inverting input is connected to the connection and is connected to the connection via a series circuit comprising a resistor and a resistor, whose inverting input is connected firstly via a resistor to the junction point between the two resistors and to the connection, and secondly via a resistor to a further connection, whose output is connected to its inverting input via a capacitor which has a series circuit comprising a capacitor and a resistor connected in parallel with it, and wherein the inverting integrator has an inverter connected downstream of it, whose inverting input is connected to the output of the integrator via a resistor, whose non-inverting input is connected to the connection, and whose output is connected to its inverting input via a resistor and to the connection via a resistor. A switch can be arranged between the output of the inverter and the resistors, wherein selected potentials can be applied to the connection, for example via an external switch. Furthermore, a switch can be arranged between the output of the integrator and the capacitor, on the one hand, and the resistor on the other. Moreover, a switch can be arranged between the first probe connection and the second probe connection. An external computer may be provided which controls the testing and calibration processes and checks the response of the evaluation circuit under the various operating conditions at the end of production of the evaluation circuit (automated test system). During operation of the oxygen probe, the probe equivalent circuit can be connected in parallel with the oxygen probe at a probe temperature of below 200° C. during the probe&#39;s heating phase. The measurement accuracy can be checked and age-related tolerance discrepancies in the evaluation circuit can be equalised by using the probe equivalent circuit in the heating phase of the oxygen probe to set various operating points and by storing the evaluation circuit&#39;s pump-current measured values Ip measured in the process for the purpose of further processing. 
     A method for operating a testing and calibrating device for an evaluation circuit for a linear oxygen probe in an internal combustion engine, comprises the step of: 
     coupling a probe equivalent circuit having the same connections as the oxygen probe, which largely emulates the electrical and chemical behaviour of the oxygen probe and can simulate probe faults, with the evaluation circuit instead of the oxygen probe during a testing and calibrating process. 
     The method can further comprise the step of applying selected potentials to the connection, for example, via an external switch. The method can further comprise the step of providing an external computer which controls the testing and calibration processes and checks the response of the evaluation circuit under the various operating conditions at the end of production of the evaluation circuit. During operation of the oxygen probe, the probe equivalent circuit can be connected in parallel with the oxygen probe at a probe temperature of below 200° C. during the probe&#39;s heating phase. The measurement accuracy may be checked and age-related tolerance discrepancies in the evaluation circuit can be equalised by using the probe equivalent circuit in the heating phase of the oxygen probe to set various operating points and by storing the evaluation circuit&#39;s pump-current measured values Ip measured in the process for the purpose of further processing. 
     Another method for operating a testing and calibrating device for an evaluation circuit for a linear oxygen probe in an internal combustion engine, comprises the step of: 
     coupling a probe equivalent circuit having the same connections as the oxygen probe, which largely emulates the electrical and chemical behaviour of the oxygen probe and can simulate probe faults, with the evaluation circuit instead of the oxygen probe in parallel with the oxygen probe, at least during a testing and calibrating process. 
     The method can further comprise the step of applying selected potentials to the connection, for example, via an external switch. The method can further comprise the step of providing an external computer which controls the testing and calibration processes and checks the response of the evaluation circuit under the various operating conditions at the end of production of the evaluation circuit. The measurement accuracy can be checked and age-related tolerance discrepancies in the evaluation circuit may be equalised by using the probe equivalent circuit in the heating phase of the oxygen probe to set various operating points and by storing the evaluation circuit&#39;s pump-current measured values Ip measured in the process for the purpose of further processing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below with reference to a schematic drawing, in which: 
         FIG. 1  shows a known, electrical circuit diagram of a lambda probe having an evaluation circuit, and 
         FIG. 2  shows an electrical circuit diagram of a probe equivalent circuit in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a known, electrical circuit diagram of a lambda probe S (in a dotted frame) having an evaluation circuit for operating a linear lambda probe in an internal combustion engine. 
     The lambda probe S comprises 
     a) the “reference cell”, i.e. the electrodes between measuring chamber and air, shown in the drawing by the Nernst voltage Vs which can be measured between the electrodes and the internal resistance Ris of the diffusion barriers between them, 
     b) the “pump cell”, i.e. the electrodes between measuring chamber and exhaust, shown by the voltage Vp drop between them and the (reference) resistance Rip between these electrodes, and 
     c) the calibration resistor Rc in the probe connector. 
     The electrodes are fitted to the ceramic body of the probe. The ceramic material between the electrode pairs is conductive at high temperatures and serves as a solid electrolyte. 
     Since the resistor Rc is exposed to considerable environmental stresses on account of its installation position in the probe connector, a further resistor Rp is connected in parallel with it in the control unit. This reduces the influence of Rc on the overall accuracy. Four connections Vs+, Vp−/Vs−, Vp+ and Rc emerge from the probe S and are connected to the evaluation circuit. 
     The inverting input R− of a differential amplifier or controller R is connected to the connection Vs+ of the probe S, and its non-inverting input R+ is connected to the mid-voltage (Vm) via a reference voltage Vref, where Vm=Vcc/2 and Vcc (normally 5 V) is the supply voltage of the circuit. 
     The mid-voltage Vm also has the inverting input P− of a pump current source P connected to it, the non-inverting input P+ of said pump current source being connected to the output of the differential amplifier R. 
     The output of the pump current source P is connected to the input Rc of the probe S. 
     The differential amplifier/controller R compares the Nernst voltage Vs of the probe S (between external air and measuring cell) with the reference voltage Vref (450 mV) and generates an output voltage which is proportional to the difference and is converted by the pump current source P into a proportional pump current Ip which flows through the pump cell (Rip and Vp) to Vm. The pump current Ip results in a change in the oxygen concentration in the probe&#39;s measuring cell (not shown), which in turn results in a change in the Nernst voltage Vs. 
     The oxygen concentration in the exhaust (lambda) is ascertained by measuring the pump current. To this end, the voltage drop brought about by the pump current Ip across the parallel circuit comprising Rc and Rp is measured using a differential amplifier (not shown). 
     In a stable control state, the Nernst voltage is Vs=Vref=450 mV: there is a state of equilibrium between the oxygen flow through the diffusion barrier and the oxygen-ion flow, caused by the pump current Ip. 
     In line with the invention, during the testing and calibration operation of the evaluation circuit, the lambda probe is replaced by an electronic probe equivalent circuit SES whose electrical circuit diagram is shown in  FIG. 2 , in a dotted frame. 
     This probe equivalent circuit SES has the same connections Vs+, Vp−/Vs−, Vp+ and Rc as the lambda probe S shown in FIG.  1  and largely emulates the electrical and chemical behavior thereof. 
     If the connections of the probe equivalent circuit SES are connected to the corresponding connections of the evaluation circuit, the control loop is closed and a stable operating state (normal state) of the evaluation circuit normally becomes established. 
     The probe equivalent circuit SES has an inverting integrator with IPI response (Integral-Proportional-Integral response) which is constructed from an operational amplifier OP 1 , resistors R 2  and R 3  and capacitors C 1  and C 2 . This integrator emulates the transfer function of the probe in a relevant frequency range. 
     From the connection Rc, a series circuit comprising a resistor Rc and a resistor Rip is connected to the non-inverting input OP 1 + of the operational amplifier OP 1 . A further resistor R 2  is connected between the junction point between the two resistors Rc, Rip and the inverting input OP 1 − of the operational amplifier OP 1 . The output of OP 1  is connected to a—normally-on—switch S 3 . Connected between the inverting input OP 1 − and the other connection of the switch S 3  is a capacitor C 2  which has a series circuit comprising a capacitor C 1  and a resistor R 3  connected in parallel with it. The junction point between the two resistors Rc and Rip is connected to the connection Vp+. 
     The inverting integrator OP 1  is followed by an inverter constructed from an operational amplifier OP 2  and resistors R 4  and R 5 . It produces the correct phase for the transfer function. 
     The resistor R 4  connects the inverting input OP 2 − of the operational amplifier OP 2  to the output of the operational amplifier OP 1 . The output of the operational amplifier OP 2  is fed back to its inverting input via a—normally-on—switch S 2  and the resistor R 5 . The non-inverting inputs of the two operational amplifiers OP 1  and OP 2  are connected to one another and to the connection Vp−/Vs−. Connected between the connections Vp−/Vs− and Vs+is a further—normally-off—switch S 4 . 
     The output of the operational amplifier OP 2  is connected to the connection Vs+ via the switch S 2  and the resistor Ris. The inverting input OP 1 − of the operational amplifier OP 1  is connected via a resistor RI and a further connection In to an external changeover switch S 1  which can be used to apply selected potentials to the resistor R 1 . 
     The probe equivalent circuit SES can thus be inexpensively produced from standard components. 
     If the probe equivalent circuit SES shown in  FIG. 2  is connected to the evaluation circuit shown in  FIG. 1  instead of the probe S, then a closed control loop is obtained. The integrator OP 1  will change its output voltage such that its input voltage becomes zero. 
     If switch S 1  is changed to its center position  2 , then resistor R 1  has no current and the voltage on the inverting input OP 1 − corresponds to the mid-voltage Vm, for example 2.5 V. Accordingly, the voltage on the non-inverting input OP 1 + will also become Vm. No pump current Ip flows and the circuit sets itself to the value λ=1. 
     If switch S 1  is set to the position  1 =0 V (ground), then a voltage divider comprising the resistors R 1  and R 2  is obtained: the voltage on the inverting input OP 1 − falls and the inverting integrator OP 1  readjusts the pump current Ip via the evaluation circuit. The voltage drop across the resistor Rip causes the voltage on the connection Vp+ to rise Equilibrium has been reached again when the voltage drop across the resistor Rip corresponds to that across resistor R 2 , and the inverting input OP 1 − has reached the voltage Vm again. 
     With appropriate proportioning of the resistors R 1  and R 2 , an operating point λ=∞ (air) can thus be set. 
     If switch S 1  is set to position  3 =+5 V, then the voltage on the inverting input OP 1 − will rise, and the inverting integrator OP 1  readjusts the pump current Ip via the evaluation circuit, but this time in the other direction. The voltage drop across the resistor Rip causes the voltage on the connection Vp+ to fall. Equilibrium has been reached again when the voltage drop across the resistor Rip corresponds to that across resistor R 2 , and the inverting input OP 1 − has reached the voltage Vm again. 
     With appropriate proportioning of the resistors R 1  and R 2  (which proportioning can be different than in the case of switch position  1 , but does not have to be), an operating point can thus be set which corresponds to a mixed value of, by way of example, λ=0.6 (rich). 
     In this way, any operating points from λ=0.6 (rich) through λ=1 to λ=∞ (air) can be set. 
     When an AC voltage signal is applied to the input In, it is even possible to ascertain the dynamic response of the control loop comprising evaluation circuit and probe equivalent circuit SES, which has not been possible to date. 
     Various probe faults can be simulated by operating the (CMOS) switches S 2 , S 3  and S 4 . In this case, the turning-off of switch S 2  corresponds to a faulty (ineffective) pump cell, the turning-off of switch S 3  corresponds to an interruption in the measuring cell or in its supply line, and the turning-on of switch S 4  corresponds to a short circuit between the probe connections Vp−/Vs− and Vs+. Other faults can be simulated in a similar manner by adding and operating further switches. 
     In each of these cases, the probe equivalent circuit SES will assume an impermissible operating point which then needs to be identified by a diagnostic circuit (not shown) monitoring the evaluation circuit. This provides a simple way of checking the evaluation circuit&#39;s diagnostic function completely. 
     During production, the probe equivalent circuit SES is controlled by a computer which simultaneously measures the response of the evaluation circuit under various operating conditions (automated test system). 
     A linear lambda probe has a very high impedance at low temperatures (&lt;200° C.). At the start of the heating phase, the probe is virtually non-existent. It is thus possible to connect the probe equivalent circuit SES to the evaluation circuit in parallel with the probe at this time. The control loop then becomes stabilised via the probe equivalent circuit, so that it is possible to check and calibrate the evaluation circuit during operation. If various operating points are now set (for example λ=0.6, 1, ∞) and the evaluation circuit&#39;s associated measured values are stored, it is possible to check the measurement accuracy and to equalise age-related tolerance discrepancies. In this case, the probe equivalent circuit SES can be a fixed (integrated) part of the evaluation circuit. To this end, it merely needs to be isolated from the evaluation circuit by means of (CMOS) switches during normal operation with the real probe.