GB1588667A

GB1588667A - Devices for evaluating the output signal of a measuring probe which operates on the principle of ionic conduction in a solid electrolyte

Info

Publication number: GB1588667A
Application number: GB2331678A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1977-11-11
Filing date: 1978-05-26
Publication date: 1981-04-29
Also published as: DE2750478A1; FR2408835A1; JPS5476290A; DE2750478C2; JPS6228860B2; FR2408835B1

Description

(54) IMPROVEMENTS IN OR RELATING TO DEVICES FOR EVALUATING THE OUTPUT SIGNAL OF A MEASURING PROBE WHICH OPERATES ON THE PRINCIPLE OF IONIC CONDUCTION IN A SOLID ELECTROLYTE (71) We, ROBERT BOSCH GMBH, a German company of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to devices for evaluating the output signal of a measuring probe which operates on the principle of ionic conduction in a solid electrolyte.

The output voltage of a measuring probe which operates on the principle of ionic conduction in a solid electrolyte and has electrodes made from porous platinum exhibits an abrupt change of voltage at an air number of X = 1 when these probes serve to determine the OZ content of. for example, exhaust gases of an internal combusion engine, and, furthermore, the output voltage is greatly dependent upon temperature, the air number N being the actual air/fuel ratio divided by the stoichtrometric air/fuel ratio.

For this reason, various devices have been proposed by means of which this output voltage is monitored in the warming-up phase of the measuring probe and in which the signal is evaluated as a control quantity only when a utilizable probe output signal exists. Other devices render it possible to adapt the reference voltage of the comparator to the general level of the probe output voltage and thus to form a control signal which is substantially unfalsified by the range of temperature.

However. the range of application of the said devices essentially relates to the warming-up phases of the measuring probe.

According to the present invention there is provided a device for evaluating an output signal of a measuring probe which operates on the principle of ionic conduction in a solid electrolyte. comprising a comparator for comparing the probe output signal with a substantially constant reference voltage and for producing a control signal corresponding to the comparison result, and means for comparing an output voltage of the probe, when warmed to its operating state, with a derived voltage, such probe output voltage characterising an operating point of the medium measured by the probe, and for correcting the observed probe output voltage upon a departure from the desired voltage.

A device embodying the present invention can have the advantage that the satisfactory functioning of the measuring probe can be monitored even when the measuring probe is operating in a hot state, and malfunction, occurring only after a long time in operation, can be detected. Thus, it can be ensured that, by means of the output signal of the measuring probe, controlled in accordance with the invention, very accurate regulation can be obtained which is reliable over a long period of time.

It is particularly advantageous that the probe output voltage can be corrected and, in particular, the entire probe characteristic can be displaced. The comparing and connecting means can control a current fed to the measuring probe or drawn from the measuring probe when malfunction is detected. In a particularly advantageous manner, the current is controlled with the use of the forward voltage of a diode connected between a reference voltage source and the output voltage of the probe.

The invention will be further described by way of example with reference to the accompanying drawings in which: Fig. I shows. by way of example, a characteristic of an oxygen measuring probe output signal both when the measuring probe is in a new state and when it has been in operation for some time.

Fig. 2 is a circuit diagram of one embodiment of the invention. and Fig. 3 is a circuit diagram of a second embodiment of the invention.

For the purpose of accurately regulating the fuel/air ratio of an operating mixture fed to combustion chambers of an internal combustion engine, an oxygen measuring probe is used which is fitted in the exhaust gas system of the internal combustion engine and produces a control signal for a device for regulating the fuel/air ratio.

Measuring probes of this type operate on the principle of ionic conduction in a solid electrolyte, based on the difference between the partial pressure of oxygen in the measured mixture relative to a reference medium.

The solid electrolyte is in the form of a small tube coated with porous platinum which is exposed to the exhaust gas and which serves for contacting. The output voltage of the probe exhibits a voltage transient upon transition from a deficiency of oxygen to an excess of oxygen in the exhaust gas, which occurs when the fuel/air mixture fed to the internal combustion engine has an air number of X = 1. The characteristic of the probe output signal then substantially complies with the Nernst equation E = RT/K.ln (P1/P2). E is the output voltage in mV, R is a thermodyanic constant, T is the absolute temperature, K is an efficiency factor, P1 is the partial pressure of oxygen of the reference system, and P2 is the partial pressure of oxygen in the measured medium, that is to say in the exhaust gas.This equation shows the temperature-dependence of the probe output voltage. By way of example, when the probe is operating under hot conditions, and as is shown by the graph of Fig. 1, the output voltage can be less than 100 mV in the case of a lean mixture, and in excess of 900 mV in the case of a rich mixture.

The internal resistance is extremely high when the oxygen probe is in a cold state, and a voltage signal adequate for regulation cannot be obtained, particularly a perceptible abrupt change of voltage. Considerable importance is attached to accurate regulating means which are capable of operating at any time in all ranges of operation, since, in order to comply with the limits of emission when operating motor vehicles having internal combustion engines. it is important. in all ranges of operation, to keep the composition of the operating mixture in an optimum range with respect to the formation of injurious substances in the exhaust gas. An oxygen probe of the type described has proved to be most advantageous for this type of regulation.In order to minimize the disadvantage of the oxygen measuring probe in the warming-up phase, and to enable the oxygen measuring probe to come into operation as early as possible even before the normal operating temperature of oxygen probes and internal combustion engines has been attained, a number of monitoring and adap tation devices have been developed.

However, in addition to the abovementioned disadvantage, the output signal of the oxygen measuring probe is subjected, even in its hot operating state, to various influences which are attributable to ageing of the probe. Such ageing can reduce, for example, the utilizable abrupt change of voltage of the output signal of the probe, this being attributable, inter alia, to soiling of the platinum coating and to a deterioration in the action thereof. Furthermore, it has been found that the output signal is subjected to drift after the oxygen measuring probe has been in operation for some time. In the described oxygen measuring probe, the entire potential of the probe characteristic is shifted to negative values. This is shown by the broken line in Fig. 1.

Since the abrupt change in voltage of the oxygen measuring probe at an air number of A = 1 is generally detected by means of a threshold value switch whose threshold value voltage lies in the average range of the abrupt change in voltage, this drift can cause faulty formation of the control signal at the output of the threshold value switch depending on the magnitude of the drift of the characteristic. Thus, the regulating means processing this control signal can no longer operate accurately, or can even completely fail.

Fig. 2 shows. in a simplified form, a circuit diagram of a device according to a first embodiment of the present invention, for monitoring the probe output voltage. An equivalent circuit component for the oxygen measuring probe of the type described above is shown in the form of a voltage source 1 having a voltage Vs and an internal resistance Ri this being the internal resistance of the probe. One electrode of the oxygen probe is connected to a negative supply lead, and the other electrode is connected to the inverting input of an operational amplifier 3 arranged as a threshold value switch. A stabilized reference voltage determing the switching threshold is fed to the non-inverting input by way of a voltage divider comprising resistors 4 and 5. According as to whether the probe output signal lies above or below the switching threshold, there appears at the output of the operational amplifier a control signal which has a low or high voltage value and which. by means of a regulator 7 connected on the output side. is processed to form a control signal for a device 8, connected on the output side of the regulating device, for controlling the fuel/air ratio. The fuel/air mixture fed to the internal combustion engine 11 is correspondingly influenced.

Furthermore. a voltage-dependent resistor in the form of a diode 14 is connected to the output of the oxygen measuring probe.

The anode of the diode 14 is connected to a reference voltage source 15. The reference voltage source is also a stabilized voltage.

The level of the voltage source 15 is such that, taking into account the forward voltage of the diode 14, there is established on the cathode of the diode a voltage which is equal to the lowest oxygen measuring probe output voltage to be monitored. When the output voltage of the oxygen measuring probe drops below this lowest value as a result of a drift in the characteristic, a current is fed to the oxygen probe by way of the diode in conformity with the difference, so that the output voltage of the oxygen measuring probe is again brought to its original value. This applied to the entire characteristic.

It will be appreciated that any other voltage-regulated resistors or semiconductors can be used instead of the diode. These elements then serve as comparators which detect the drop in the probe output voltage and feed a current to the probe in conformity with the difference. Conversely, a characterisic at too high a voltage level can be reduced by drawing a current from the oxygen measuring probe. The reference voltage source then correspondingly has to be kept, by an amount equal to the value of the forward voltage of the diode, at a lower level than the highest desired voltage value at the probe output. Upon an increase in the output voltage, a current is taken, in conformity with the deviation from the desired maximum voltage value, from the probe by way of, for example, a diode reversed biassed relative to the diode shown in the embodiment of Fig. 2.

The probe output voltage or the entire characteristic is thereby brought to its desired original value again. The probe output voltage can be monitored in an ideal manner, and kept within desired limits, by means of the device described.

The embodiment of Fig. 3 constitutes a further improvement in the circuit. the temperature-dependence of the diode 14 being compensated for. Here also. the voltage source 1 and the resistor Ri constitute the equivalent circuit diagram of an oxygen measuring probe which is connected to the negative supply lead on the one hand and, on the other hand, to the inverting input of the operational amplifier 3. The regulating device 7 is connected to the output of the operational amplifier and produces the control signals for the device for controlling the fuel/air ratio. The said control device controls the supply of fuel and air for the internal combustion engine 11. The non-inverting input is connected to the central tapping of a voltage divider which comprises the resistors 4 and 5 and which is connected between the negative and positive supply leads.A Zener diode 17 is connected in parallel with the voltage divider for the purpose of voltage stablization. A voltage divider comprising resistors 18 and 19 is also fed by the voltage thus stabilized. the diode 14 being connected, in the direction of current flow to the oxygen measuring probe, between the central tapping 20 of the said voltage divider and the output of the oxygen measuring probe.

Furthermore, a forward biassed compensating diode 21 is connected between the central tapping 20 and the negative supply lead, thus ensuring that the temperature range of the forward voltage on the diode 14 is compensated for.

WHAT WE CLAIM IS: 1. A device for evaluating an output signal of a measuring probe, which operates on the principle of ionic conduction in a solid electrolyte, comprising a comparator for comparing the probe output signal with a substantially constant reference voltage and for producing a control signal corresponding to the comparison result, and means for comparing an output voltage of the probe, when warmed to its operating state, with a desired voltage, such probe output voltage characterising an operating point of the medium measured by the probe, and for correcting the observed probe output voltage upon a departure from the desired voltage.

2. A device as claimed in claim 1, in which the comparing and correcting means can shift the probe characteristic.

3. A device as claimed in claim 1 or 2, in which. for the purpose of correcting the probe output voltage, the comparing and correcting means controls a current fed to the measuring probe.

4. A device as claimed in claim 1 or 2, in which, for the purpose of correcting the probe output voltage. the comparing and correcting means controls a current which is drawn from the measuring probe.

5. A device as claimed in claim 3 or 4, in which the value of the current is dependent on the error difference between the observed voltage and the desired voltage to be complied with.

6. A device as claimed in any preceding claim. in which the comparing and correcting means include a voltage-dependent resistance element connected between a reference voltage source and the probe output.

7. A device as claimed in claim 6, in which the voltage-dependent resistance element comprises a diode.

8. A device as claimed in claim 7. in which the reference voltage differs from the desired observed voltage by the value of the forward voltage of the diode.

9. A device as claimed in any preceding claim. in which there is provided a compensating circuit for the temperaturedependence of the comparing and correcting means.

10. A device as claimed in claim 9. when appendant to claim 7 or 8 in which a compensating diode. biassed in the same forward direction as said diode acting as the voltage

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The level of the voltage source 15 is such that, taking into account the forward voltage of the diode 14, there is established on the cathode of the diode a voltage which is equal to the lowest oxygen measuring probe output voltage to be monitored. When the output voltage of the oxygen measuring probe drops below this lowest value as a result of a drift in the characteristic, a current is fed to the oxygen probe by way of the diode in conformity with the difference, so that the output voltage of the oxygen measuring probe is again brought to its original value. This applied to the entire characteristic. It will be appreciated that any other voltage-regulated resistors or semiconductors can be used instead of the diode. These elements then serve as comparators which detect the drop in the probe output voltage and feed a current to the probe in conformity with the difference. Conversely, a characterisic at too high a voltage level can be reduced by drawing a current from the oxygen measuring probe. The reference voltage source then correspondingly has to be kept, by an amount equal to the value of the forward voltage of the diode, at a lower level than the highest desired voltage value at the probe output. Upon an increase in the output voltage, a current is taken, in conformity with the deviation from the desired maximum voltage value, from the probe by way of, for example, a diode reversed biassed relative to the diode shown in the embodiment of Fig. 2. The probe output voltage or the entire characteristic is thereby brought to its desired original value again. The probe output voltage can be monitored in an ideal manner, and kept within desired limits, by means of the device described. The embodiment of Fig. 3 constitutes a further improvement in the circuit. the temperature-dependence of the diode 14 being compensated for. Here also. the voltage source 1 and the resistor Ri constitute the equivalent circuit diagram of an oxygen measuring probe which is connected to the negative supply lead on the one hand and, on the other hand, to the inverting input of the operational amplifier 3. The regulating device 7 is connected to the output of the operational amplifier and produces the control signals for the device for controlling the fuel/air ratio. The said control device controls the supply of fuel and air for the internal combustion engine 11. The non-inverting input is connected to the central tapping of a voltage divider which comprises the resistors 4 and 5 and which is connected between the negative and positive supply leads.A Zener diode 17 is connected in parallel with the voltage divider for the purpose of voltage stablization. A voltage divider comprising resistors 18 and 19 is also fed by the voltage thus stabilized. the diode 14 being connected, in the direction of current flow to the oxygen measuring probe, between the central tapping 20 of the said voltage divider and the output of the oxygen measuring probe. Furthermore, a forward biassed compensating diode 21 is connected between the central tapping 20 and the negative supply lead, thus ensuring that the temperature range of the forward voltage on the diode 14 is compensated for. WHAT WE CLAIM IS:

1. A device for evaluating an output signal of a measuring probe, which operates on the principle of ionic conduction in a solid electrolyte, comprising a comparator for comparing the probe output signal with a substantially constant reference voltage and for producing a control signal corresponding to the comparison result, and means for comparing an output voltage of the probe, when warmed to its operating state, with a desired voltage, such probe output voltage characterising an operating point of the medium measured by the probe, and for correcting the observed probe output voltage upon a departure from the desired voltage.

dependent element is connected between the reference voltage source and a voltage having a level comparable with the desired observed voltage.

11. A device as claimed in any preceding claim, in which the measuring probe is an oxygen measuring probe which is exposed to the exhaust gases of an internal combustion engine.

12. A device for evaluating an output signal of a measuring probe which operated on the principle of ionic conduction in a solid electrolyte, constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in Fig. 2 or Fig. 3 of the accompanying drawings.