Method and arrangement for detecting a fault in the context of measurement quantities in a motor vehicle

BACKGROUND OF THE INVENTION

An example of a method and an arrangement for detecting an error in the context of measurement quantities in a motor vehicle is disclosed, for example, in German patent publication 4,204,623. The method and arrangement proceed from a measurement value detection system, especially for detecting the position of an actuating element determining the power of an internal combustion engine, such as a throttle flap, wherein the measurement quantity is detected by at least two redundant measuring devices. The measurement signals are supplied to a microcomputer without pull-up or pull-down resistors being present on the signal lines with these pull-up and pull-down resistors having been used up to that time point. For interruptions in the signal line, these resistors serve to pull the signal level at the input of the microcomputer to a defined value which lies outside of the pregiven signal value range. The defective performance of at least one measuring device can be determined by means of a conventional signal range check. Possibly unwanted fault detection could occur for the dimensioning of these resistors which is correct for the purpose intended. To avoid this unwanted fault detection, in the known solution, it has been suggested to do without these resistors for both measuring devices. To detect faults, a controllable electrical switch is inserted in at least one supply line of the measuring devices. This switch is driven in the context of a testing operation to determine interruptions of the signal line and similar fault conditions. However, the complexity of the arrangement is increased with such an electrically actuable switch. Furthermore, the function of the control, which is carried out in dependence upon the measurement quantities, is possibly negatively affected.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for fault detection in measurement quantities in a motor vehicle which makes possible a reliable fault detection without requiring the additional complexity necessitated in the known solution. Furthermore, it is an object of the invention to provide an arrangement for detecting a fault in measurement quantities in a motor vehicle which makes possible a reliable fault detection while avoiding the above-mentioned disadvantages.

The method of the invention is for detecting a fault in the context of a measurement quantity in a vehicle. The method includes the steps of: providing redundant first and second signal values for the measurement quantity; comparing the time-dependent changes of the two signal values; and, detecting a fault condition when the time-dependent changes do not correlate with each other.

The method of the invention permits a reliable fault detection to be undertaken for measurement quantities in motor vehicles without additional complexity and especially without the complexity associated with switches. It is especially advantageous that the plausibility comparison of two redundant signals of the measurement quantity is undertaken on the basis of the relative signal change and not on the basis of absolute signal values. In this way, lower tolerances can be achieved because the linearity tolerances of the measuring devices are substantially eliminated because of the differential consideration.

With the use of potentiometers as measuring devices, it is especially advantageous that the monitoring can even be carried out when the signals are incorrect because of possible contact resistances in one of the potentiometers. The measuring devices can, for example, be for detecting the position of a power actuating element or an operator-controlled element. This incorrectness does not lead to an unwanted fault detection in the relative plausibilization which is in contrast to the plausibility comparison on the basis of absolute values.

In an especially advantageous manner, pull-up and/or pull-down resistors are omitted in the signal line of the measuring device or very high ohmage resistors are used (much greater than 500 KOhms). The measurement quantities of the measuring devices are evaluated when carrying out a control function. For the redundant measuring device for the above, such resistors are used in a manner known per se in order to reliably detect a defective performance of this measuring device in the context of a signal-range check. This has the advantage that falsifications of the signal of the first measuring device do not lead to an unwanted fault detection in the context of the signal range check because this signal range check is replaced by the differential monitoring of the signal values. Securing this differential plausibilization takes place, inter alia, via the signal-range check in the signal of the second measuring device which reliably detects faults in the region of the signal line and of the second measuring device in a conventional manner because of the pull-up and/or pull-down resistors.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings wherein:

FIG. 1

is a block circuit diagram of a control arrangement for controlling an actuating element of a vehicle;

FIG. 2

is a sequence diagram for controlling the actuating element in dependence upon the measurement quantities which are monitored for faults; and,

FIG. 3

is a flowchart showing a preferred embodiment of the method for detecting a fault.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1

shows, in the context of a preferred embodiment, a control apparatus

10

for controlling an actuating element

12

of an internal combustion engine such as a throttle flap. The control apparatus

10

includes at least an input circuit

14

, a microcomputer

16

and an output circuit

18

which are connected to each other via a communication system

20

.

Various input lines are connected to the input circuit

14

. A first input line

25

leads from a first measuring device

26

to the input circuit

14

and this measuring device

26

is for detecting the position (DK

1

) of the actuating element

12

. A second input line

27

leads from a second measuring device

28

to the input circuit

14

. The measuring device

28

is redundant to the first measuring device

26

and also detects the position (DK

2

) of the actuating element

12

. A third input line

22

leads from a measuring device

24

to the input circuit

14

. The measuring device

24

is provided for detecting the air supply (HFM) to an internal combustion engine.

Furthermore, additional input lines

30

to

34

are provided which originate from measuring devices

36

to

40

for detecting additional operating variables of the engine and/or of the vehicle. operating variables of this kind include, for example, the position of an operator-controlled element actuated by the driver, the temperature of the engine, the rpm of the engine, et cetera. These operating variables are evaluated in the control apparatus of the actuating element. The electrically actuable actuating element

12

is actuated by the control apparatus

10

via at least one output line

42

which leads from the output circuit

18

to the actuating element. The use of pull-up and/or pull-down resistances is a feature of the invention in connection with the detection of measurement quantities and the supply of these quantities in the context of the measuring devices

26

and

28

.

The signal of the measuring device

28

is only available in the case of an emergency for evaluation in connection with the control of the actuating element; otherwise, the signal of the measuring device

28

is evaluated for monitoring the signal value of the first measuring device

26

in the context of plausibility comparisons. A resistor

44

is provided in the signal line

27

of the second measuring device

28

and is switched from the signal line

27

to ground (⊥) or to a supply voltage (+). The magnitude of the resistance of this resistor is so selected that the potential at the input of the input circuit

14

lies reliably outside of the normal signal value range of the measuring device

28

when there is an interruption of the signal line. Such a fault condition is therefore detected on the basis of a signal-range check.

In one embodiment, the measuring device

28

is configured as a potentiometer and the magnitude of this resistance of approximately 470 KOhm has been shown to be suitable. In contrast, a resistance of this kind is dispensed with in the signal line

25

or at least a very high ohmage value of this resistance is pregiven in the signal line

25

of the measuring device

26

(see the phantom outline of resistor

46

which is likewise switched from the signal line to ground (⊥) or to the supply voltage (+)). This value of the resistance

46

is significantly greater than the value of the resistance

44

on the signal line

27

so that erroneous signals because of this resistance under special operating conditions cannot occur. These special conditions include, for example, the situation where there is a very high contact resistance between the potentiometer track and the wiper contact. A reliable signal-range check is, however, no longer ensured because, under circumstances, the potential on the line lies in the plausible range notwithstanding the error.

For the above reasons, the signal-range check in combination with the measurement quantity of the measuring device

26

is not used. The measurement quantity of the measuring device

26

is evaluated for controlling the actuating element, for example, in the context of a position control. In lieu of the signal-range check, the time-dependent change of the measurement quantity of the measuring device

26

(or a measurement value derived therefrom) and the time-dependent change of the corresponding value of the measuring device

28

are compared to each other.

The differential plausibility comparison is also carried out in the idle range. The differential plausibilization of the two signals would announce an unplausible condition for a possible incorrectness of a signal because of an occurring contact resistance at the potentiometer. In the next run-through, the condition would again be plausible because of the differential consideration. No unwanted fault reaction occurs because of the debounce circuit provided.

In the preferred embodiment, the measuring devices are potentiometers which detect the position of a power actuator element of an internal combustion engine. The method and arrangement according to the invention are, however, not limited to an arrangement of this kind. Rather, the invention can be applied in combination with contactless position transducers as well as for the position of the accelerator pedal or brake pedal and in combination with measuring devices for other operating variables of the drive unit or vehicle which detect the same quantities, in a different manner, as needed. Accordingly, the application of the invention is not limited to the drive unit but can be used also in combination with other actuator elements of the vehicle such as the wheel brakes.

In the preferred embodiment, the above-mentioned procedure is realized as a program of the microcomputer

16

. The change of the signal of the first measuring device

26

and the change of the signal of the second measuring device

28

are formed at fixed time intervals and, in each case, referred to a previous value, preferably the most recent value. If the two signals are plausible with respect to each other, then both signals would have to show the same change except for the deviations with respect to linearity and slight disturbances caused by the contact resistances in the permissible range.

The measuring devices can have a different dependency of the signal magnitude from the measurement quantity. In the above comparison, and depending upon the configuration of the measuring devices, it can be necessary to convert the detected quantities to a common physical value (for example, where there is redundant detection of positions when the deflection and the actuating force are measured). In this case, the differential plausibility check takes place on the basis of this physical value. If the signal line, for example, of the measuring device

26

, is interrupted, the voltage on this signal line drifts to a value which has nothing to do with the actual operating variable (for example, the throttle flap position). This drift depends upon the actual shunts and leakage currents.

In the preferred embodiment of a position control, the position controller controls the actuating element either in a direction of closing or in a direction of opening because the desired value and the transmitted actual value no longer correlate. From this, a signal change of the signal value of the measuring device

28

results which operates correctly. The signal change of the measuring device

28

is uncorrelated to the signal change of the signal of the measuring device

26

. Accordingly, a fault condition is recognized. The actual value of the measuring device

26

is switched over to the measuring device

28

as a fault reaction after an adjustable debounce time, that is, a pregiven number of program run-throughs with fault recognition. However, this measure is only performed when the signal on the measuring device

28

is in order, that is, for example, no signal-range check faults are shown and, in the preferred embodiment, the signal is plausible with a signal determined by a measuring device for detecting the air supply to the engine.

A preferred embodiment of the above-described procedure is shown in the sequence diagram of FIG.

2

and the flowchart of FIG.

3

. In

FIG. 2

, a position controller

100

is shown which receives a desired value DES on input line

102

. The desired value DES is for the position of the actuator element to be driven. In the preferred embodiment, this desired value is determined at least on the basis of the accelerator pedal position. Furthermore, the position controller

100

receives the actual value ACT via the line

104

. The actual value ACT is derived, in normal operation, from the signal value DK

1

of the measuring device

26

. In accordance with a pregiven control strategy, the position controller

100

determines a drive signal ⊥ from the deviation between desired and actual values. The drive signal ⊥ is outputted via the line

42

for actuating the actuating element

12

.

A positioning signal, which is redundant to the signal DK

1

, is derived from the signal DK

2

of the measuring device

28

. This positioning signal is supplied and both of the signals DK

1

and DK

2

are logically coupled to each other in the fault recognition stage

106

as described above. In the case of a fault, and under the precondition that the signal DK

2

is in order, a switching signal is generated via the line

108

and switches the switching element

110

into the position shown in phantom outline. This means that, in the case of a fault, the correctly operating signal DK

2

serves as the basis of the actual value ACT in lieu of the signal DK

1

.

In the preferred embodiment, the signal HFM for the air supplied to the engine is also evaluated for fault recognition in the fault recognition stage

106

.

The elements shown in

FIG. 2

are realized as programs of the microcomputer

16

of the control apparatus

10

. Such a program is shown in

FIG. 3

in the context of a flowchart for the fault recognition stage

106

.

The program is run through at pregiven time intervals, for example, 10 milliseconds. In the first step

200

, the measurement quantities DK

1

, DK

2

and HFM are read in. Thereupon, a comparison takes place in step

202

of the value DK

2

with the upper and lower limits (signal-range check). In the next step

204

, a check is made as to whether a fault is present in the signal DK

2

. If this is the case (that is, if this signal exceeds or drops below the pregiven limit values) then, and in accordance with step

206

, an emergency operation of the vehicle is initiated in that, for example, the power of the drive unit of the vehicle is limited. Thereafter, the program is ended and is only run through again when the control apparatus

10

is again taken into operation. If it had resulted in step

204

that no fault is present in the signal DK

2

, then in the next step

206

, a check is made as to whether the signals DK

2

and HFM are plausible with respect to each other. This takes place via a comparison of the two signals. At least one signal is so converted that it can be compared to the other. If no plausibility is given, then the program continues with step

206

and the emergency operation of the control. If plausibility is present, one can assume in combination with the result of step

204

that the signal DK

2

is okay. This means that the signal changes &Dgr;DK

1

and &Dgr;DK

2

are determined in step

208

on the basis of the actual measurement quantities and a previously occurring measurement quantity and preferably the measurement quantity read in from the last program run-through. Thereupon, in step

210

, a check is made as to whether both signal changes are correlated to each other.

In the preferred embodiment, the above is carried out in such a manner that the algebraic sign of the change as well as the magnitude of the change are compared to each other and the correlation of the signal changes is assumed when the signs are the same and the magnitude of the changes lie in a pregiven tolerance range with respect to each other. If this is the case, then, in step

212

, the counter Z is set to the value

0

which serves for fault debouncing. After step

212

, the program is repeated with step

200

.

If step

212

had yielded that the signal changes do not correlate with each other in the context of the pregiven tolerances, then, the counter Z is incremented by 1 in step

214

. Thereupon, in step

216

, the count of counter Z is compared to a maximum value. If the maximum value is not reached, then the program is continued with step

200

. On the other hand, if the maximum value is reached, the program is continued with step

218

in which the actual value is now formed on the basis of the measurement quantity DK

2

and no longer on the basis of the measurement quantity DK

1

. This is so because one can assume a fault condition in the area of the measurement quantity DK

1

.

Thereafter, the program is ended and the monitoring of measurement quantity DK

2

is carried out only on the basis of the steps

204

and

206

and, if required, while limiting the power of the drive unit.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.