GB2370866A

GB2370866A - Emergency control of an automatic clutch

Info

Publication number: GB2370866A
Application number: GB0208439A
Authority: GB
Inventors: Thomas Jager; Martin Zimmermann; Klaus Kupper; Michael Salecker
Original assignee: LuK Getriebe Systeme GmbH
Current assignee: LuK Getriebe Systeme GmbH
Priority date: 1997-05-09
Filing date: 1998-05-08
Publication date: 2002-07-10
Anticipated expiration: 2018-05-08
Also published as: GB2370867B; GB2370867A; GB0208439D0; GB2370866B; GB0208459D0

Abstract

An automatic clutch is controlled, via an actuator 13b, by a control unit 13 receiving signals from various sensors, eg engine speed sensor 16, gear determination sensors 19a, 19b, accelerator sensor 31, brake sensor 41, wheel sensors 99b-d etc. If a failure of a sensor indicating brake operation (such as a parking brake) is detected the control unit 13 enters an emergency mode in which a value representing an existing operation is used as a replacement. A brake light switch may be used as the replacement value.

Description

DEVICE FOR THE CONTROL OF AN ENGAGED STATE OF A CLUTCH

The invention relates to a device for the control or emergency operating control of an engaged state of an automatically operable clutch in the drive train of a motor vehicle with an engine and a gearbox, with a shift lever and a shift member on the gearbox side in active connection therewith for selecting the gear transmission ratios, with an operating unit controllable by a control unit with a drive, for operating, such as engaging or disengaging the clutch, wherein at least one sensor is provided which detects the position of the shift member on the gearbox side and a sensor is provided which detects the position the shift lever.

Devices of this kind are already known from DE OS 195 48 799. With the failure of a sensor or with a defect in or breakdown of a sensor for detecting the engaged gear or an operation by the driver of the shift lever as shift intent signal there may result uncomfortable breakdowns of the automated clutch.

If for example the sensor for detecting the position of the shift lever is defective then a shift intent can no longer be detected as a result of the lack of signal. If for example a sensor for detecting an engaged gear from a position of a shift member on the gearbox side is defective or damaged then the gear recognition cannot function properly and gear-dependent processes such as the engagement or disengagement of the clutch, for example during a driving off or shift process are not carried out with the desired comfort.

An object of the present invention is to provide a device of the kind mentioned above which in the event of a temporary or complete failure of a brake sensor guarantees a reliable control or regulation of the clutch engagement state. A further object of the invention is that despite a failure of such a sensor it should still be possible for the comfort of the automated clutch system to be maintained.

Furthermore it is an object of the invention in the event of a failure of a sensor or a signal to provide an emergency control by means of which the vehicle can be safely operated and as a far as possible without any great loss of comfort.

According to the invention there is provided a device for the control or emergency operating control of an engaged state of an automatically operable clutch in the drive train of a motor vehicle which has an engine, a gearbox with a shift lever and a shift member for selecting the gearbox transmission ratio and an operating unit controllable by a control unit with a drive for engaging and/or disengaging the clutch wherein upon a failure of a sensor indicating brake operation or upon a defect in a signal from such a sensor, a value representing an existing operation is used as the replacement value.

A signal from a brake light switch may be used as the replacement value. The brake in question may be a parking brake.

The invention will now be explained by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of a

vehicle ;

Figure la is a diagrammatic illustration of a drive train; Figure lb shows a shift selector gate; Figure 2a is a block circuit diagram; Figure 2b is a block circuit diagram; Figure 3a is a block circuit diagram and Figure 3b is a block circuit diagram.

Figure 1 shows diagrammatically a motor vehicle 1 with an engine 2, such as an internal combustion engine.

Furthermore a clutch 3 and a gearbox 4 are contained in the drive train of the motor vehicle. In this embodiment the clutch is mounted in the force flow between engine and gearbox wherein a drive torque of the engine is transferred through the clutch to the gearbox and from the gearbox 4 on the output side to an output shaft 5 and then to an axle 6 and wheels 6a.

Figure 1 shows further a device for controlling or regulating the engaged state of a clutch such as in particular an automated clutch. By engaged state is moreover meant the engagement position of the clutch between two end positions. In one end position there is substantially no torque transferred from the clutch and in the other end position maximum torque is transferred from the clutch. The engaged state can also be characterised by the torque transferable by the clutch.

The device can also be operated by an emergency operating control in the event of a failure of a sensor or another component part of the automated clutch wherein the control unit of the device detects the defect or failure and switches into an emergency operating mode.

The clutch can be formed as a friction clutch, such as dry friction clutch, multi-plate clutch, magnetic powder clutch or converter bridging clutch of a torque converter.

The clutch can be a self-adjusting wear-compensating clutch. The gearbox 4 is shown as a manual shift transmission, such as step-change gear.

The gearbox can be an automated shift gear which can be shifted automatically by means of one actor. Furthermore an automatic gearbox can be used wherein an automatic gearbox is a gearbox substantially without break in the pulling power during the shift processes and which as a rule is built up by planetary gear steps. Furthermore an infinitely adjustable gearbox such as for example cone pulley belt contact gearbox can be used. The automatic gearbox can also be formed with a torque transfer system 3 such as clutch or friction clutch mounted on the output side.

The clutch can furthermore be formed as a starting-off clutch and/or turning set clutch for reversing the direction of rotation and/or as a safety clutch with a deliberately controllable transferable torque. The clutch can be a dry friction clutch or a wet-running friction clutch which runs for example in a fluid.

The clutch 3 has a drive side 7 and output side 8 wherein a torque is transferred-from the drive side 7 to the output side 8 in that the clutch disc 3a is force-biased

by means of the pressure plate 3b, the plate spring 3c and a disengagement bearing 3e as well as the flywheel 3d. For this biasing the disengagement lever 20 is operated by means of an operating unit 90, such as actor 13b.

The control or regulation of the engaged state of the clutch 3 takes place by means of a control unit 13. The control unit can comprise the control electronics 13a and the operating unit 90. In another advantageous design the control unit can only comprise the control electronics 13a, the operating unit is housed in another housing. The control unit 13 can contain the control and power electronics for controlling the electric motor 12 of the actor 13b. It can thereby advantageously be reached that the system requires as a single structural space the structural space for the actor with electronics.

The operating unit 13b comprises basically a drive 12, such as electric motor, wherein the electric motor acts through a gearing 12a, such as worm gear or spur wheel gear or crank gear or threaded spindle gear on an output element. This output element can be a master cylinder 11. The output element can also be a rod linkage or other connection.

The movement of the output part of the actor or operating unit 13b such as master cylinder piston lia is detected by a clutch path sensor 14 which detects at least the position and/or the speed and/or the acceleration of a value which is proportional to the position or engagement position or speed or acceleration respectively of the clutch. The speed and/or acceleration can also be calculated from the position as a function of time. The engagement position or engagement state thus characterises the torque transferable by the clutch since this

transferable torque is coupled with the engaged position or engaged state through the clutch characteristic line. The master cylinder 11 is connected to the slave cylinder 10 by a pressurised medium line 9 such as hydraulic line.

The output element lOa of the slave cylinder is in active connection with the disengagement lever or disengagement means 20 so that a movement of the output part lOa of the slave cylinder 10 causes the disengagement means 20 likewise to move or tilt in order to control the torque transferable by the clutch 3.

The force biasing of the pressure plate or friction linings respectively can be deliberately controlled through the position of the disengagement means 20 such as disengagement fork or central disengagement member wherein the pressure plate can thereby be moved and fixed anywhere between the two end positions. The one end position corresponds to a fully engaged clutch position and the other end position to a fully disengaged clutch position. In order to control a transferable torque which is for example less or greater than the momentary arising engine torque a position of the pressure plate 3b can be controlled by way of example which lies in an intermediate range between the two end positions. The clutch can be fixed in this position by means of the deliberate control of the disengagement means 20. Transferable clutch moments can also be transferred however which lie defined above the engine moments momentarily arising. In such a case the engine moments actually arising can be transferred wherein the torque irregularities in the drive train in the form of for example torque peaks are damped and/or insulated.

In order to control such as govern or regulate the engaged

state of the clutch, sensors are also used which monitor at least temporarily the relevant values of the overall system and supply the condition values, signals and measured values which are necessary for the control and which are processed by the control unit wherein a signal connection with other electronics units such as for example engine electronics or an electronics unit of an anti-lock brake system (ABS) or an anti-slip regulating system (ASR) can be provided and can exist. The sensors detect for example speeds, such as wheel speeds, engine speeds, the position of the load lever, the throttle valve position, the gear position of the gearbox, a shift intent and further vehicle-specific characteristic values.

Figure 1 shows that a throttle valve sensor 15, an engine speed sensor 16, as well as a tacho sensor 17 are used and send measured values or information to the control unit.

The electronics unit, such as computer unit, of the control unit 13a processes the system input values and sends control signals to the actor 13b.

The gearbox is formed as a step change gearbox wherein the transmission ratio stages are changed by means of a shift lever or the gearbox is operated or controlled by this shift lever. Furthermore at least one sensor 19b is mounted on the shift lever 18 of the gearbox to detect the longitudinal path of the shift lever, the shift intent and/or the gear position and to send same to the control unit. The sensor 19a is attached to the gearbox and detects the actual gear position and/or shift intent. The shift intent recognition using at least one of the two sensors 19a, 19b can take place in that the sensor is a force sensor which detects the force acting on the shift lever-nor on a gearbox element. If a force threshold is exceeded then the signal is evaluated as a shift intent.

Furthermore the sensor can however also be formed as a path or position sensor wherein the control unit detects a shift inten-from the time change of the position signal.

Likewise a shift intent can be detected if a predefinable threshold value for the shift lever position is exceeded or if a difference path between the element on the gearbox side and the shift lever reaches or exceeds a predefinable boundary value. To this end a difference path is calculated as the difference between the sensor values on the side of the shift lever and those on the gearbox side.

The sensor 19b detects the position or movement of the shift lever. The sensor can be a path sensor such as for example a potentiometer or an echo sensor or another sensor which detects at least the longitudinal path of the shift lever during movement along the shift gates.

Furthermore a second sensor can also be provided (not shown) which detects the transverse path of the shift lever.

The sensor 19a detects the position or movement of a shift member 80 on the gearbox side, such as for example a central selector shaft or shift rods in the gearbox or coupling elements connected thereto. The sensor can be a path sensor such as a potentiometer or an echo sensor or another sensor which detects at least the longitudinal path of the shift member on the gearbox side during a movement along the shift gates. Furthermore a second sensor is advantageously provided which detects the transverse path of the shift member on the gearbox side.

The control unit is in signal connection at least at times with all the sensors and evaluates the sensor signals and system input values in the manner and way-where in dependence on the actual operating position the control

unit sends control or regulating commands to the at least one actor 13b. The drive element 12 of the actor, such as electric motor receives from the control unit which controls the clutch activation a setting value in dependence on measured values and/or system input values and/or signals of the attached sensor unit. To this end a control program is implemented in the control apparatus as hard and/or software which evaluates the incoming signals and calculates or determines the output values from comparisons and/or functions and/or characteristic fields.

The control unit 13 advantageously has an implemented torque determining unit, a gear position determining unit, a slip determining unit and/or an operating state determining unit or is in signal connection with at least one of these units. These units can be implemented by control programs as hardware and/or as software so that by means of the incoming sensor signals it is possible to determine the torque of the drive unit 2 of the vehicle 1, the gear position of the gearbox 4 as well as the slip which prevails in the area of the clutch and the actual operating state of the vehicle. The gear position determining unit detects the gear actually engaged at least from the signals of the sensors 19a and/or 19b. The sensors are thereby attached to the shift lever and/or to the setting means on the side or inside the gearbox such as for example a central selector shaft or shift rod and these detect for example the position and/or speed of the component parts.

Furthermore a load lever sensor 31 can be mounted on the load lever 30 such as accelerator pedal which detects the load lever position. A further sensor 32 can function as an idling--swit-ch, i. e. with an operated accelerator--pedal, such as load lever this idling switch 32 is switched on

and with a non-activated signal it is switched off so that through this digital information it is possible to recognise whether the load lever such as accelerator pedal is activated. The load lever sensor 31 detects the degree of activation of the load lever.

Figure 1 shows in addition to the accelerator pedal 30 such as load lever and the sensors in connection therewith a brake operating element 40 for operating the operating brake or the parking brake, such as brake pedal, hand brake lever or hand or foot operated operating element of the parking brake. At least one sensor 41 is mounted on the operating element 40 and monitors its operation. The sensor 41 is formed for example as a digital sensor such as switch wherein this detects that the operating element is activated or not activated. A signal device such as a brake light can be in signal connection with this sensor and signals when the brake is activated. This can take place both for the operating brake and for the parking brake. The sensor can however also be formed as an analogue sensor wherein such a sensor such as for example a potentiometer determines the degree of activation of the operating element. Also this sensor can be in signal connection with a signal device.

The control unit controls or regulates the clutch engagement position or the engaged state of the clutch, thus substantially the torque transferable by the clutch, through a control of the position of the clutch operating element such as disengagement bearing or the drive of the operating unit. The ideal position is controlled through this control and a substantially parallel position determination, such as calculation. From the data of the engaged state a-t--the time prior to adjustment of--t-he engaged state and from the drive power it is thus possible

to determine the relevant travel path and thus the relevant actual position or actual engaged state.

With an emergency operation of the operating unit it is advantageous if the engagement process and/or the disengagement process takes place with a lower drive power or force than in normal operation. With an engagement process and/or disengagement process it is expedient to reduce the drive power and/or current supply to the drive and it is also advantageous to carry this out during at least one time range so that the operating unit in the event of an emergency-controlled control does not move with full power or full force against a stop or end stop.

If this is the case then the service life of the operating system drops considerably since the design of the component parts of the operating system is not as a rule intended for such high loads and at least some individual component parts of the operating unit would be permanently damaged mechanically.

In order to determine the actual engagement position or the engaged state the actual position is calculated from the data of the relevant drive power in the relevant engagement position. The change in the actual position is thereby a function of the engagement position itself since the force of the clutch acting in each engagement position is substantially different. The drive power is thereby selected during an engagement and/or disengagement process as a function of the relevant engagement position.

If for example a sensor is provided for detecting the engaged state then the failure of or fault in the sensor can be recognised for example in that the sensor signal has left a plausible value'range which is usually occupied during normal operation. Likewise a fault in the sensor

can be recognised if for a predeterminable time such as for example the duration of at least two beat rates of the processor the data of the sensor are not renewed or the sensor produces no new data. It is thereby assumed that the processor records and/or receives sensor signals rhythmically. If for example the beat rate is 10 ms then it is expedient if the sensor is regarded as faulty if for at least 20 ms no signals or no new signals are received.

If a fault in a sensor is recognised then a flag or status bit is set in a memory of the control unit to show that a fault exists. If at a later time point the fault no longer exists then this relevant flag or status bit can be cancelled.

Figure la shows in a diagrammatic illustration a drive train 100 of a motor vehicle with an engine 101, a clutch 102 and a gearbox 102. The clutch is controlled by an operating unit 104, such as actor, wherein the operating unit 104 is controlled by a control unit 105 with internal micro processor with memory 13c, cf Figure 1. The gearbox has a shift member 106 on the gearbox side which is operated through a connection 107 by means of an operating element 108 such as a shift lever. The gearbox is shifted by means of the shift lever and a gear is engaged or a neutral area is selected. The shift lever and/or shift member on the gearbox side is moved or operated inside shift gates 150 or across the shift gates inside the selector path 151.

This is shown for example by Figure lb which shows a shift plan such as an expanded H-shift plan with the gear positions 1 to 5 and the reverse gear R.

The sensor 109 detects-at---least the position or the longitudinal path of the shift lever 108 substantially

along the shift gates 150. The sensor 110 substantially detects the position or the longitudinal path of a shift element 106 on the gearbox side along the shift gates 150. The sensor 111 detects the position or the transverse path of a shift member 106 on the gearbox side along the selection path 151. A sensor 109a detects preferably the transverse path of the shift lever.

The control unit 105 can contain a memory and a filter by means of which data and signals can be filtered for example by sensors or a data bus (CAN-bus) and where applicable can be further processed in another way. The filter is formed in one embodiment as an electronic component part. In a further embodiment the filter is formed as part of a control process wherein the data or signals are processed inside the microprocessor.

Figure la shows a gearbox speed sensor 112. With a failure of the engine speed sensor 16 of Figure 1 the control unit recognises this for example from the faulty signal or a sensor signal outside of a predeterminable plausibility range. In such a case the control unit 105 can calculate the engine speed by means of the gear speed and the gear transmission ratio engaged. The gear input speed is then equal to the gear output speed multiplied by the gear transmission ratio. This speed is then equal to the engine speed. The engine speed can also be calculated from a vehicle speed signal or wheel speed signal and the relevant drive rod overall transmission ratio.

Figure 2a shows a block diagram 200 for illustrating the action of the device for controlling an engaged state of the clutch. The process is started in block 201. In block 202 the sensor signal of thef-control unit is fed in and checked whether it is faulty or lies outside of a

predeterminable value range, such as plausibility range. If this is the case then the signal of the sensor or the sensor itself is regarded as faulty. This check can be carried out for example by comparing the signal with the boundary values of the plausible value area. If the signal is within these boundary values then the signal is not faulty. If no signal is present or over a predeterminable time span no signal is present or the signal is outside of the boundary values of the plausible value area then the signal is regarded as faulty.

The signal can be a sensor signal for recognising the engaged gear. The sensor signal can likewise be speed signal, such as engine speed or gear input seed or engine torque signal.

If the signal is not faulty then the method is ended at block 205 and the control of the engaged state of the clutch is continued by means of the non-faulty signals.

If the signal at 202 is faulty then in block 203 in an emergency operating control a replacement value or a replacement signal is retrieved from the memory of the control unit and used for controlling the engaged state of the clutch, see block 204. The process is then ended at 205. The emergency operating control is carried on further at least until the fault no longer exists. If for example the sensor for detecting the longitudinal path or transverse path of the shift element on the gearbox side is defective then a value can be used as replacement value which represents the shift gate of the gears 1-2. By switching over the control into emergency operating control the faulty signal is replaced by a replacement value. The entire system does not-t-hereby break down but the vehicle can be driven further but with possibly

reduced comfort. In the emergency operating control the faulty signals are not used.

The use of a replacement value for control can also be used with the breakdown of two sensors. In this case Figure 2a is modified so that with an input of sensor signals it is established that the signals of two sensors are faulty and then replacement values are retrieved from the memory for these sensor signals. It is thereby advantageous if these replacement values represent an engaged first gear.

Figure 2b shows a block diagram 210 for illustrating the development when using the device for controlling an engaged state of a clutch. The process is started in block 211. In block 212 the sensor signal Sensor-l of the sensor Sensor~1 is read by the control unit and checked whether it is faulty or lies outside of a predeterminable value range such as plausibility area. If this is the case then the signal of the sensor or the sensor itself is regarded as faulty. This check can be made for example through a comparison of the signal with the boundary values of the plausible value range. If the signal lies within these boundary values then the signal is not faulty. If there is no signal present or over a predeterminable time span no signal exists or the signal is outside of the boundary values of the plausible value area then the signal is regarded as faulty.

If the signal is not faulty then the process is ended at block 215 and it proceeds to the control of the engaged state of the clutch by means of the non-faulty signals.

If the'signal Ssensor-l is faulty at 212, then-in block 213 a replacement signal Sensor-2 of the sensor Sensor-2 is used

in an emergency operating control whereby this replacement signal stems from another sensor. This signal is used to control the engaged state of the clutch, see block 214.

The process is then ended at 215.

The sensor 109 on the shift lever is thereby used for example as the sensor Sensor-1 or sensor Sensor-2. If this sensor signal as Sensorsignal is faulty then it can be referred back to the sensor signal of the sensor 110,109 on the gearbox side as Sensor-2-signal or vice versa whereby the two sensors used each detect a longitudinal path of the shift lever and the shift member on the gearbox side.

Figure 3a shows a block circuit diagram 220 for using the device. The control process is started at block 221. In block 222 a sensor signal Sensor-l is retrieved by the control unit and filtered by means of the control unit.

The filter produces a filtered signal Ssensor-f.

In block 223 it is compared by the control unit by means of a comparison process whether the amount of [Ssensor-l Ssensor-f] is greater than a predeterminable boundary value.

If this is not the case then the clutch remains substantially in its actual engaged state and the process is ended at block 225. If the difference in the block 223 is greater than the predeterminable boundary value then an intended shift by the driver using a shift lever movement is detected and the clutch is disengaged in block 224 before the process is ended in block 225.

Figure 3b shows an embodiment for controlling the device in a block circuit diagram 230. A process is started at block 231"'In block 232 a sensor signal Gaspedal of a sensor of the accelerator (gas) pedal operation, such as

accelerator pedal operating path is retrieved. The sensor can be mounted or attached to the throttle valve or on the accelerator pedal or on a rod linkage between the throttle valve and accelerator pedal. A sensor of this kind can be a potentiometer or an echo sensor or another sensor.

In block 233 the clutch is controlled for engagement or disengagement. The rate or speed of the engagement or disengagement process is controlled in dependence on the accelerator pedal operation such as the amount of activation of the accelerator pedal and/or the changing speed of the accelerator pedal activation. If for example the return of the accelerator pedal activation takes place very quickly then the disengagement process is also relatively quick. During an engagement process the clutch can be closed up to the maximum torque transferable, thus completely.

It is advantageous if the transferable torque is a linear function of the accelerator pedal activation. The clutch can likewise be closed on exceeding a boundary value of the accelerator pedal activation and the clutch can be opened when a boundary value of the accelerator pedal activation is understepped.

According to Figures 2a and 2b in the event of a failure or defect of one wheel speed sensor 99a a replacement value of another wheel speed sensor 99b, 99c, 99d can be used. The same applies for the failure or defect of a sensor 98a which detects the activity of a secondary assembly 98. In this case a replacement value can be used which represents an active secondary assembly or a nonactive secondary assembly. A secondary assembly can be for example a--power steering pump or a climate control which can be driven on the motor side.

If the failed sensor is a brake activation sensor either a replacement value can be used or reference can be made to another sensor such as a brake light switch. The brake can be a parking brake, such as hand brake or operating brake.

Similarly in the event of a failure or defect of at least one sensor or a signal of the engine speed and/or of the engine torque and/or accelerator pedal activation and/or throttle valve position and/or at least some individual wheel speeds the torque transferable by the clutch can be increased through an emergency operating control when a gear is engaged and the brake not activated. It is thereby expedient if the torque transferable by the clutch is increased up to the maximum torque which can be transferred. It is likewise advantageous if the torque transferable by the clutch is increased constantly, linearly or in another functional connection.

The check on the engine torque can be carried out so that the signal sent by the engine control or the determined signal is checked as follows: * It is checked whether the engine torque signal lies above a maximum possible signal. In this case there has to be a fault in the torque value 'The engine torque is determined through a parallel conducted mathematical model using engine data, such as engine speed and throttle valve angle and where applicable the engine torque is determined by using engine characteristic fields. The data are then compared with the engine torque sent. With a deviation greater than a predeterminable value the engine torque

------ is regarded as faulty.

Correlations between the throttle valve signal and the

engine torque are carried out where applicable by using characteristic fields. If the values do not exist according to the characteristic field the engine torque signal is evaluated as faulty.

With a device for the control or emergency operating control of an engaged state or an automatically operated clutch in the drive train of a motor vehicle with an engine and a gearbox with a shift lever and a shift member on the gear side in active connection therewith for selecting the gear transmission ratio, with an operating unit controllable by the control unit with a drive for operating, such as engaging and/or disengaging the clutch and likewise with a failure or defect of at least one sensor or signal of the engine speed and/or the engine torque and/or the accelerator pedal activation and/or the throttle valve position and/or at least some individual wheel speeds, with the presence of a shift lever movement and/or a neutral area engaged in the gearbox and/or with an activated brake and/or with an engine speed less than a predeterminable boundary value it is possible for the torque transferable by the clutch to be reduced. It is thereby advantageous if the predeterminable boundary value of the engine speed is in the range from 800 to 1500 1/min, preferably 1000 1/min.

The invention is not restricted to the embodiments of the description. Numerous amendments and modifications are possible within the scope of the claims, particularly those variations, elements and combinations and/or materials which result from combinations or modifications of individual features or elements or process steps contained in the drawings and described in connection with the general description and embodiments and claims.

Claims

1. Device for the control or emergency operating control of an engaged state of an automatically operable clutch in the drive train of a motor vehicle which has an engine, a gearbox with a shift lever and a shift member for selecting the gearbox transmission ratio and an operating unit controllable by a control unit with a drive for engaging and/or disengaging the clutch wherein upon a failure of a sensor indicating brake operation or upon a defect in a signal from such a sensor, a value representing an existing operation is used as the replacement value.

2. Device as claimed in Claim 1, wherein a signal of a brake light switch is used as the replacement value.

3. Device as claimed in Claim 1 or Claim 2, wherein a value representing an existing operation is used as the replacement value upon a failure of a sensor indicating a parking brake operation or upon a defect in a signal from such a sensor.