EP0445127B1 - Process and device for establishing at least one threshold voltage for a lambda closed-loop control - Google Patents

Abstract

In the process disclosed, the threshold voltage is not fixed at an accurately predetermined value using a high-precision reference voltage source. Instead, a countervoltage connected in opposition to the sensor voltage is measured and the threshold voltage(s) is(are) correlated with the measured value the absolute value of which need not be known. This process makes it possible to dispense with a high-precision reference voltage source and with high-pression resistors. These are replaced in the device according to the invention by a means for measuring the countervoltage and a means for correlating threshold voltages with the countervoltage.

Description

The invention relates to a method for determining at least one threshold voltage in lambda one control, with a lambda probe, to which a voltage is connected, which essentially corresponds to the desired switching voltage. The invention also relates to a device for performing such a method.

State of the art

Such a structure is known from DE 33 19 432 A1 and EP-A-81759.

In closed-loop control mode, control is carried out in the lean direction as long as the measured voltage is above a changeover threshold voltage, and control is carried out in the rich direction if it is above it. The switching threshold voltage is usually close to the counter voltage, e.g. B. 10 mV above this.

In the case of the example mentioned, the counter voltage is 450 mV, the upper switch-on threshold voltage is 500 mV, the lower switch-on threshold voltage is 400 mV and the switchover threshold voltage is 460 mV. A digital value is permanently assigned to each voltage value. In the example, these are the values 130, 143, 117 and 132.

In order to be able to precisely set the voltage values mentioned, a highly precise reference voltage source is used, that is, not the usual voltage stabilizer, which u. a. provides the usual logic voltage of + 5 V for electronic components. The voltage of the reference voltage source is divided with high-precision, temperature-stable resistors in such a way that the desired counter voltage is set. The threshold voltages are related to this precisely set counter voltage. In addition to tuning with the help of the exact resistors, software tuning can also take place in such a way that exactly the predetermined digital value for the counter voltage is set.

The invention has for its object to be able to do without the highly accurate reference voltage source and without high-precision resistors. For this purpose, a method and a device are to be specified.

Advantages of the invention

The invention is for the method by the features of claim 1 and for the device by the features of Claim 6 given. Advantageous further developments and refinements of the method are the subject of subclaims 2-5.

The method according to the invention is characterized in that the counter voltage is measured and threshold voltages are related to the measured counter voltage. Is the measured counter voltage z. B. 460 mV instead of 450 mV, the upper switch-on threshold voltage U _{O is set} to 510 mV instead of 500 mV. It is pointed out that, strictly speaking, it is not known whether the counter voltage is 455 mV or 465 mV, since there is no high-precision reference voltage source for comparison. Exact knowledge is also not important, since other tensions that are important, e.g. B. the switch-on threshold voltages are related to the measured voltage. This ensures that, for. B. the two switch-on threshold voltages are always symmetrical to the counter voltage, regardless of what the exact value of the counter voltage is. For the digital value of the measured counter voltage z. For example, 13 units are always added to obtain the value for the upper switch-on threshold voltage, or 13 units are subtracted in order to arrive at the lower switch-on threshold voltage. Other distances or asymmetrical distances can be set accordingly.

In addition to the switch-on threshold voltages, the switchover threshold voltage is advantageously also related to the measured voltage.

Because threshold voltages are related to the measured countervoltage, it is no longer necessary to set the countervoltage to a precisely specified value with the aid of a highly precise reference voltage source and with the aid of highly accurate resistors. Rather, the voltage from the usual voltage stabilizer can be used and resistors with normal accuracy can be used.

If only the switch-on threshold voltages are corrected, it is sufficient to measure the counter voltage when the lambda probe is cold. This is the case in particular when starting an internal combustion engine on which the lambda probe is used, but also after extended overrun phases.

Should changes in the switching threshold voltage be detected, such as z. B. are due to temperature or aging-related changes in resistance values or gain factors, it is recommended to measure the counter voltage at regular intervals and to relate the switching threshold voltage to the measured counter voltage.

An apparatus for executing the above-mentioned method has a means for measuring the counter voltage and a means for relating threshold voltages to the counter voltage.

drawing

• Fig. 1 ein Schaltbild einer mit einer Gegenspannung belasteten Sonde, wobei die Gegenspannung mit Hilfe der Spannung eines üblichen Spannungsstabilisators erzeugt wird;
• Fig. 2 ein Flußdiagramm zum Erläutern eines Verfahrens, gemäß dem Schwellspannungen auf eine gemessene Gegenspannung bezogen werden; und
• Fig. 3 und 4 Schaltbilder zum Erläutern von Möglichkeiten zum Messen der Gegenspannung.

The invention is explained in more detail below on the basis of exemplary embodiments illustrated by figures. Show it:

• 1 shows a circuit diagram of a probe loaded with a counter voltage, the counter voltage being generated with the aid of the voltage of a conventional voltage stabilizer;
• 2 shows a flowchart for explaining a method according to which threshold voltages are related to a measured countervoltage; and
• 3 and 4 circuit diagrams for explaining possibilities for measuring the counter voltage.

Description of exemplary embodiments

1 has, inter alia, a lambda probe 10, which is represented by its equivalent circuit diagram, namely by a DC voltage source 11 with the probe voltage U _S and a resistor 12 with the internal resistance value R _S. A counter voltage U _G is connected across the probe voltage via a load resistor 13 with the resistance value R _L. This is generated by dividing the + 5 V voltage of a voltage stabilizer 14. In the exemplary embodiment, it should be 450 mV. However, this value fluctuates by a good 5% up and down, since the output voltage from the voltage stabilizer 14 depends on its input voltage, that is to say the battery voltage, on the load on the stabilizer and on the effects of temperature and aging. In addition, there is a spread from circuit to circuit with respect to the output voltages of the voltage stabilizers due to manufacturing tolerances. Further causes for the fluctuation range of the counter voltage U _G are manufacturing tolerances of the voltage divider resistors 15.1 and 15.2 and changes in their resistance values due to temperature and aging effects.

The probe voltage U _S and the counter voltage U _G are composed of the input voltage U _E at a differential amplifier 16 as follows: $${\text{U}}_{\text{E}}{\text{= U}}_{\text{G}}{\text{+ R}}_{\text{L}}{\text{(U}}_{\text{S}}{\text{- U}}_{\text{G}}{\text{) / (R}}_{\text{L}}{\text{+ R}}_{\text{S}}\text{)}$$

This voltage is fed to an A / D converter 17, the digital output values of which are processed by a microcomputer 18 for lambda control purposes.

The microcomputer 18 checks in particular whether the input voltage U _{E has} exceeded an upper switch-on threshold voltage U _O or has fallen below a lower switch-on threshold voltage U _U. These threshold voltages are in the embodiment no longer at precisely predefined values, but rather they lie by predefined count values above or below the count value which arises at the output of the A / D converter 17 when the counter voltage U _{G is} measured with the aid of the differential amplifier 16.

The process just mentioned is shown in the flow chart of FIG. 2. The counter voltage U _{G is} measured in a step s1. In a step s2, a difference value ΔU _{O is} counted for the measured voltage in order to achieve the value for the upper switch-on threshold voltage U _O , a difference ΔU _{U is} subtracted in order to obtain the lower switch-on threshold voltage U _U , and a voltage ΔU _UM added to provide a switching _threshold voltage U _UM . If the latter threshold voltage is exceeded or undershot by the input voltage U _E , the control direction is reversed.

A customary control method is carried out in a subroutine in accordance with step s3. The subroutine in step s3 is executed repeatedly. This repeated execution can be preceded by a new measurement of the counter voltage and the setting of threshold voltages. This is shown by the dashed return line in FIG. 2. You can return to step s1 after each step of step s3. However, it is not necessary to go through steps s1 and s2 so often, since the counter voltage changes only slowly due to temperature and aging effects and load-dependent effects hardly play a role when the internal combustion engine is running. It is therefore normally sufficient to measure the countervoltage only once when the internal combustion engine is started up. In order to increase the accuracy, however, can also be at predetermined times, eg B. at intervals of a few seconds, or when predetermined operating conditions occur. An operating state which triggers the measurement can be overrun operation which lasts for a few seconds. In this case it cools the probe off, and so far with extended overrun that it is even no longer ready for control when the overrun is ended. Sufficient time is available to carry out steps s1 and s2 during overrun operation, since no control processes take place. If the overrun mode ends, the values determined in step s2 can be used to check whether the probe is ready for control.

It can be seen from the equation given above that if the internal resistance R _{S of} the probe is very high, the input voltage U _E corresponds to the counter voltage U _G. At a probe temperature of around 250 ° C, the internal resistance value R _{S is} over 1 MOhm, while the load resistance value R _{L is} only in the order of a few 100 ohms. At the start of the internal combustion engine, that is, when the probe is still completely cold, or during extended overrun operation when the probe temperature falls below approximately 300 ° C., the counter voltage can thus be measured in the simplest way by measuring the measured input voltage U _E is equated.

If the probe is warm, the measurement procedure just mentioned fails. The voltage can then be measured, however, as illustrated in FIGS. 3 and 4.

3, the probe voltage is disconnected by opening a switch 19 so that the input voltage U _E is identical to the counter voltage U _G to measure the counter voltage. In the variant according to FIG. 4, there is a tap directly at the positive pole of the counter voltage source and the voltage value present at this tap is applied to the positive input of the differential amplifier 16 by a changeover switch 20. 3 and 4, the counter voltage is generated by a counter voltage source 21. How this is designed is irrelevant.

The switch 19 in the circuit diagram according to FIG. 3 and the changeover switch 20 in the circuit diagram according to FIG. 4 together with the differential amplifier 16, the A / D converter 17 and the microcomputer 18 serve as means for measuring the counter voltage. In the variant according to FIG. 1, the measurement of the counter voltage is controlled in that the microcomputer 18 triggers the measuring process when a predetermined condition occurs, e.g. B. at the start of the internal combustion engine or after prolonged overrun or when the internal resistance of the probe is measured, when this internal resistance exceeds a certain threshold. It is pointed out that if the measurement is taken when the internal combustion engine starts, it is expedient to wait until it has completed a few revolutions. If measurements were taken earlier, in the worst case when the starter was still actuated, a counter voltage could be measured that no longer exists in later operation. This is because during the starting process and shortly thereafter, the output voltage from the voltage stabilizer 14 has dropped to a value which is no longer present during the entire further operation.

The microcomputer 18 is not only part of the means for measuring the counter voltage, but it is also a means for relating threshold voltages to the measured counter voltage. It also has a memory that either stores the measured counter-voltage or the threshold voltages calculated with the aid of it.

Claims (6)

1. Method for specifying at least one threshold voltage for lambda = 1 control, having a lambda probe to which a countervoltage is connected in opposition, characterised in that the countervoltage is measured and threshold voltages are related to the measured countervoltage.
2. Method according to Claim 1, characterised in that the countervoltage is measured when the lambda probe is cold.
3. Method according to Claim 2, characterised in that the switch-on threshold voltages which serve to detect the ready-to-control state of the probe are related to the measured countervoltage.
4. Method according to one of Claims 1 to 3, characterised in that the switch-over threshold voltage which serves to switch over the control direction is related to the measured countervoltage.
5. Method according to one of Claims 1 to 4, characterised in that the countervoltage is measured at regular time intervals.
6. Device for specifying at least one threshold voltage for lambda = 1 control, having a lambda probe to which a countervoltage is connected in opposition, characterised by

   - a means (19, 20, 16, 17, 18) for measuring the countervoltage, and

   - a means (18) for relating threshold voltages to the measured countervoltage.

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