WO2010005379A1

WO2010005379A1 - Method and system for monitoring the lining clearance in a disc brake or drum brake

Info

Publication number: WO2010005379A1
Application number: PCT/SE2009/050856
Authority: WO
Inventors: Magnus GRANSTRÖM; Roberth Harju
Original assignee: Scania Cv Ab
Priority date: 2008-07-10
Filing date: 2009-07-02
Publication date: 2010-01-14
Also published as: JP2011527266A; SG192484A1; SE0801662L; KR101593106B1; KR20110039341A; EP2310240A4; SE532652C2; EP2310240A1; MY156907A; JP5253572B2

Abstract

Method and system for monitoring the lining clearance of a disc brake or drum brake (10) which is provided with one or more brake linings (11a, 11b) and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder (12). The relationship between the stroke of the brake cylinder, or of some portion of a brake mechanism (16), and the brake pressure of the brake cylinder, is determined at the time of a braking operation on the basis of measured values representing said stroke and measured values representing said brake pressure. The position where thebrake linings under the action of the brake cylinder come into initial contact with the brake disc or brake drum at the time of the respective braking operation is determined by analysis of the gradient of the relationship between said stroke and said brake pressure, after which a calculation value representing the prevailing lining clearance is determined on the basis of the difference between the measured value of said stroke registered in said determined position and a measured value of said stroke registered at a time when said brake pressure is nil.

Description

Method and system for monitoring the lining clearance in a disc brake or drum brake

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method and a system for monitoring the lining clearance of a disc brake or drum brake which is provided with one or more brake linings and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder. The invention relates also to a computer programme product comprising computer programme codes for implementing a method according to the invention, and to an electronic control unit.

The lining clearance of a disc brake or drum brake of a motor vehicle, i.e. the clearance between the brake linings and the brake disc or drum disc which rotates jointly with one of the vehicle's wheels, depends inter alia on the progressive wear of the brake linings. For the brake to function correctly, it is important that the lining clearance be kept within a suitable range, normally of the order of about 0.5 to 1 mm. If the lining clearance is too great, the brake will take an undesirably long time to be applied after the driver or an electronic brake control system has initiated a braking operation, while at the same time the braking effect applied risks being less than what is expected. If the lining clearance is too little, there is risk of undesirable development of heat in the brake, which in the worse case might lead to fire. A disc brake or drum brake of a heavy motor vehicle, e.g. a bus, a truck or a tractor unit, is often provided with means for automatic adjustment of the lining clearance. However, it would also be desirable to be able to monitor the lining clearance when the vehicle is in operation in order to be able, for example, to warn the vehicle's driver if the lining clearance suddenly goes outside the appropriate range. OBJECT OF THE INVENTION

The object of the present invention is to provide a new and advantageous way of monitoring the lining clearance of a disc brake or drum brake which is provided with one or more brake linings and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder.

SUMMARY OF THE INVENTION

According to the present invention, said object is achieved by a method having the features indicated in claim 1 and a system having the features defined in claim 7.

The relationship between the stroke of the brake cylinder, or of some portion of a brake mechanism which is adapted to transmitting to the brake linings a pushing force exerted by the brake cylinder, and the brake pressure which creates the pushing force of the brake cylinder, is determined according to the invention at the time of a braking operation on the basis of measured values representing said stroke and measured values representing said brake pressure. The position where the brake linings under the action of the brake cylinder come into initial contact with the brake disc or drum disc at the time of the respective braking operation is determined by analysis of the gradient of the relationship between said stroke and said brake pressure. A calculation value representing the prevailing lining clearance is thereafter determined on the basis of the difference between the measured value of said stroke registered at said determined position and a measured value of said stroke registered at a time when said brake pressure is nil.

The solution according to the invention utilises the fact that the relationship between the stroke and the brake pressure is of a markedly different character before and after the position where the brake linings under the action of the brake cylinder come into initial contact with the brake disc or drum disc at the time of a braking operation. A significantly larger increase in the brake pressure is required in order to cause the brake linings to move a given distance in the application direction after they have come into initial contact with the brake disc/drum as compared with what is the case before they have come into contact with the brake disc/drum. By examining the gradient of the relationship between the stroke and the brake pressure during a braking operation it is therefore possible to determine the position at which the initial contact between the brake linings and the brake disc/drum takes place. The distance which the brake linings travel to reach this contact position from the position where the brake pressure is nil will then correspond to the lining clearance.

The aforesaid brake pressure provides a measure of the brake application force.

The solution according to the invention provides a simple and reliable way of determining the lining clearance of a disc brake or drum brake. An electronically controlled brake system will already have measured values of the brake pressure of the brake cylinder available, making it particularly easy to implement the solution according to the invention in such a brake system.

According to an embodiment of the invention, said calculation value is compared with a given upper limit value, and a warning, preferably in the form of an acoustic or visual signal, is delivered when the calculation value is above this upper limit value. This makes it possible, for example, for a driver in a vehicle to be made aware that the lining clearance is inappropriately large.

According to another embodiment of the invention, said calculation value is compared with a given lower limit value, and a warning, preferably in the form of an acoustic or visual signal, is delivered when the calculation value is below this lower limit value. This makes it possible, for example, for a driver in a vehicle to be made aware that the lining clearance is inappropriately small. Other advantageous features of the method and the system according to the invention are indicated by the dependent claims and the description set out below.

The invention relates also to a computer programme product having the features defined in claim 10 and to an electronic control unit having the features defined in claim 12.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below on the basis of embodiment examples with reference to the attached drawings, in which:

Fig 1 depicts a skeleton diagram of a system according to an embodiment of the present invention,

Fig 2 depicts a schematic diagram illustrating a relationship between the stroke and the brake pressure of a disc brake or drum brake,

Fig 3 depicts a schematic perspective view of a tractor unit provided with a system according to an embodiment of the present invention,

Fig 4 depicts a skeleton diagram of an electronic control unit for implementing a method according to the invention, and

Fig 5 depicts a flowchart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig 1 illustrates schematically a system 1 according to an embodiment of the present invention for monitoring the lining clearance of a disc brake 10 which is provided with brake linings 1 1 a, 1 1 b and which is applied by pneumatic brake pressure imparted by a brake cylinder 12. In the example illustrated, the disc brake 10 is a wheel brake of conventional type for a motor vehicle and comprises a brake disc 13 connected to, for joint rotation with, a wheel of the motor vehicle and is thus adapted to rotating together with said wheel. Friction surfaces 14a, 14b are arranged on the respective sides of the brake disc 13. The disc brake comprises an outer brake block 15a arranged on one side of the brake disc 13 and provided with one or more brake linings 1 1 a on its side which faces towards the brake disc. The disc brake further comprises an inner brake block 15b arranged on the opposite side of the brake disc 13 and provided with one or more brake linings 1 1 b on its side which faces towards the brake disc. The brake blocks 15a, 15b are adapted to responding to application of the brake by moving towards the brake disc 13 so that the brake linings 1 1 a, 1 1 b come into frictional engagement with the friction surfaces 14a, 14b and thereby brake the rotation of the brake disc and of the wheel connected to the brake disc.

In the embodiment illustrated in Fig. 1 , the brake comprises a brake mechanism 16 adapted to transmitting to the brake blocks 15a, 15b and hence to the brake linings 1 1 a, 1 1 b a pushing force exerted by the brake cylinder 12. This brake mechanism 16 is fitted, together with the brake blocks 15a, 15b, in a brake yoke 17 and comprises a lever arm 18 supported for pivoting in the brake yoke, an adjusting device 19 supported for linear movement in the brake yoke, and one or more pistons 20 supported for linear movement in the brake yoke. The brake yoke 17 is itself capable of linear movement relative to the brake cylinder 12 and the brake disc 13 perpendicular to the friction surfaces 14a, 14b of the brake disc. The adjusting device 19 is adapted in a conventional manner to automatically regulating the lining clearance of the brake, i.e. the clearance between the brake linings 1 1 a, 1 1 b and the brake disc 13. In the example illustrated, the brake cylinder 12 comprises a pushrod 21 , a return spring 22, a chamber and a membrane 24 which divides the chamber into a first portion 23a and a second portion 23b. The second portion 23b of the chamber is connected to a compressed air source 9 via a compressed air line 2 and the supply of compressed air to this portion of the chamber is regulated by a regulating valve 3 arranged in the compressed air line. The pushrod 21 is fastened to the membrane 24 and adapted to acting upon one end of the lever arm 18. The pushrod 21 is movable towards the lever arm 18 against the action of the return spring 22 arranged inside the brake cylinder. When a braking operation is ordered, e.g. by the vehicle's driver via a brake lever 4, the regulating valve 3 opens so that compressed air flows into the second portion 23b of the chamber. The pneumatic brake pressure thus generated in the second portion 23b of the chamber is converted by the membrane 24 and the pushrod 21 to a pushing force acting against the lever arm 18. When this pushing force has become great enough, it moves the pushrod 21 towards the lever arm 18 against the action of the return spring 22. The result is that the lever arm 18 pivots and not only exerts upon the inner brake block 15b, via the adjusting device 19 and the pistons 20, a pushing force which presses the inner brake block 15b against the brake disc 13, but also exerts upon the brake yoke 17 a force in the opposite direction which creates, via the brake yoke's slide bearing, a pushing force which acts upon the outer brake block 15a and pushes it towards the brake disc 13.

When the brake is applied, the brake blocks 15a, 15b first move, under the action of the brake cylinder 12, towards the brake disc

13 to a position where the brake linings 1 1 a, 1 1 b come into initial contact with the friction surfaces 14a, 14b of the brake disc. This initial movement corresponds to the lining clearance and is normally about 0.5 to 1 mm. After the brake linings 1 1 a, 1 1 b have come into initial contact with the friction surfaces 14a, 14b of the brake disc, the brake blocks 15a, 15b move further, under the action of the brake cylinder 12, towards the brake disc 13 in order to increase the friction forces between the brake linings 1 1 a, 1 1 b and the friction surfaces 14a, 14b of the brake disc to such a level that a desired braking effect is achieved.

Fig. 2 depicts a diagram illustrating the relationship during a braking operation between the movement s of the brake cylinder's pushrod 21 , or of some portion of the brake mechanism 16 which is mechanically connected to and manoeuvred by the pushrod, and the pneumatic pressure p in the second portion 23b of the brake cylinder's chamber which creates the pushing force of the brake cylinder. In this description and the claims set out below, this movement s is referred to as the "stroke" and the pressure p as the "brake pressure". Fig. 2 shows that the curve 25 representing the relationship between the stroke s and the brake pressure p has a clearly different slope before and after the position s<_\ where the brake linings 1 1 a, 1 1 b come into initial contact with the friction surfaces 14a, 14b of the brake disc 13. By analysis of the gradient of the relationship between the stroke s and the brake pressure p it is therefore possible to determine the position s<_\ where the brake linings under the action of the brake cylinder come into initial contact with the brake disc. The distance travelled to reach this position Si from a position S₀ where the brake pressure p is nil will correspond to the lining clearance.

Measured values Vp representing the brake pressure p are determined by a pressure sensor 5 which in the example illustrated is adapted to detecting the pressure in the compressed air line 2 between the regulating valve 3 and the brake cylinder 12. Measured values Vs representing the stroke s are determined by a sensor 6 which in the example illustrated is adapted to detecting the movement of the pistons 20. The sensor used for determining the stroke s might alternatively be adapted to detecting the movement of some other portion of the brake mechanism 16 or the movement of the brake cylinder's pushrod 21 . In the example illustrated, the system 1 according to the invention is adapted to cooperating with an electronic brake system whereby the regulating valve 3 is controlled by an electronic control unit 7 on the basis of measured values from a sensor 8 adapted to detecting operating movements of a brake lever 4 effected by the vehicle's driver. The control unit 7 is also adapted to receiving information from the pressure sensor 5 concerning the brake pressure p. In this case, the measured values Vp representing the brake pressure p are thus also provided by this control unit 7. The regulating valve 3 and hence the brake pressure p might alternatively be controlled directly by means of a brake lever, in which case the measured values Vp representing the brake pressure p might be determined on the basis of the position of the brake lever.

The system 1 according to the invention comprises processing means 31 adapted to:

- determining at the time of a braking operation the relationship between the stroke s and the brake pressure p on the basis of measured values Vs representing the stroke and measured values Vp representing the brake pressure;

- determining by analysis of the gradient of the relationship between the stroke s and the brake pressure p the position where the brake linings 1 1 a, 1 1 b under the action of the brake cylinder 12 come into initial contact with the brake disc 13 at the time of the respective braking operation; and

- determining a calculation value BV which represents the prevailing lining clearance on the basis of the difference between the measured value Vsi of the stroke s registered at said determined position and a measured value Vs₀ of the stroke s registered at a time when the brake pressure p is nil.

The processing means 31 might be implemented by means of the aforesaid control unit 7, but in the example illustrated the processing means 31 is implemented by means of another electronic control unit 30 which is connected to the first- mentioned control unit 7 in order to receive from it measured values Vp representing the brake pressure p, and to the sensor 6 in order to receive from it measured values Vs representing the stroke s.

The processing means 31 might be adapted to comparing the determined calculation value BV with a given upper limit value GV1 representing a magnitude which the lining clearance should not be greater than, and to initiating the generation of a warning to warn the vehicle's driver when the calculation value BV is above this upper limit value GV1 . The processing means 31 might also be adapted to comparing the determined calculation value BV with a given lower limit value GV2 representing a magnitude which the lining clearance should not be smaller than, and to initiating the generation of a warning to warn the vehicle's driver when the calculation value BV is below this lower limit value GV2. The warning is presented in the form, for example, of an acoustic signal and/or a visual signal, e.g. via a display unit or some other warning indicator in the vehicle's driving cab.

Fig. 3 illustrates a motor vehicle 40 in the form of a tractor unit provided with drum brakes 1 10 of a conventional type applied by pneumatic brake pressure imparted by a brake cylinder 1 12. Each drum brake 1 10 comprises a brake drum 1 13 connected to, for joint rotation with, a wheel of the motor vehicle and hence adapted to rotating together with said wheel. A friction surface is arranged on the inside of the brake drum. The drum brake 1 10 further comprises two brake blocks 1 15a, 1 15b arranged inside the brake drum 1 13 and each provided with a brake lining 1 1 1 a, 1 1 1 b on its side which faces towards the brake drum. The brake blocks 1 15a, 1 15b are adapted to responding to application of the brake by moving outwards towards the brake drum 1 13 so that the brake linings 1 1 1 a, 1 1 1 b come into frictional engagement with the friction surface of the brake drum and thereby brake the rotation of the brake drum and of the wheel 100 connected to the brake drum. In the embodiment illustrated in Fig. 3, the brake comprises a brake mechanism 1 16 adapted to transmitting to the brake blocks 1 15a, 1 15b and hence to the brake linings 1 1 1 a, 1 1 1 b a pushing force exerted by the brake cylinder 1 12. This brake mechanism comprises a adjusting device 1 19 and a camshaft 1 17. The adjusting device 1 19 is adapted to automatically regulating in a conventional manner the lining clearance of the brake, i.e. the clearance between the brake linings 1 1 1 a, 1 1 1 b and the brake drum 1 13. The configuration and function of the brake cylinder 1 12 are similar to those of the brake cylinder described above with reference to Fig. 1 . The brake cylinder 1 12 comprises inter alia a pushrod which is adapted to acting upon the adjusting device 1 19 and which is capable of linear movement towards the latter against the action of a return spring arranged inside the brake cylinder. The adjusting device 1 19 is also adapted, in addition to its adjusting function, to serving as a lever arm for converting a linear movement of the pushrod to a rotary movement of the camshaft 1 17. When a braking operation is ordered, e.g. by the vehicle's driver via a brake lever, compressed air is caused to flow into a chamber of the brake cylinder 1 12. The pneumatic brake pressure thus generated in said chamber is converted via a membrane arranged inside the brake cylinder and via the pushrod to a pushing force acting against the adjusting device 1 19. When this pushing force becomes great enough, it causes the pushrod to move towards the adjusting device 1 19 against the action of said return spring. The adjusting device 1 19 will thus be pivoted and cause rotation of the camshaft 1 17 which will itself, via a control cam arranged on the camshaft, cause the brake blocks 1 15a, 1 15b to be pushed outwards so that they are pressed against the brake drum 1 13.

When the brake is applied, the brake blocks 1 15a, 1 15b first move, under the action of the brake cylinder 1 12, towards the brake drum 1 13 to a position where the brake linings 1 1 1 a, 1 1 1 b come into initial contact with the friction surface of the brake drum. This initial movement corresponds to the lining clearance. After the brake linings 1 1 1 a, 1 1 1 b have come into initial contact with the friction surface of the brake drum, the brake blocks 1 15a, 1 15b move further, under the action of the brake cylinder 1 12, towards the brake drum 1 13 in order to increase the friction forces between the brake linings 1 1 1 a, 1 1 1 b and the friction surface of the brake drum to such a level that a desired braking effect is achieved. According to the present invention, the lining clearance of this drum brake 1 10 is determined in a manner corresponding to that described above.

A method and a system according to the present invention might also be used for monitoring the lining clearance of a disc brake or drum brake applied by hydraulic brake pressure imparted by a brake cylinder, in which case the aforesaid brake pressure p would constitute the hydraulic pressure in the brake cylinder which creates the latter's pushing force.

Fig. 5 depicts a flowchart illustrating an embodiment of a method according to the present invention for monitoring the lining clearance of a disc brake or drum brake which is provided with one or more brake linings and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder 12, 1 12. At a first step S1 , at the time of an initiated braking operation, the relationship between the stroke of the brake cylinder 12, 1 12, or of some portion of a brake mechanism 16, 1 16 which is adapted to transmitting to the brake linings a pushing force exerted by the brake cylinder, and the brake pressure which creates the pushing force of the brake cylinder 12, 1 12, is determined on the basis of measured values Vs representing said stroke and measured values Vp representing said brake pressure. At a second step S2 the gradient of the relationship between said stroke and said brake pressure is analysed to determine the position where the brake linings 1 1 a, 1 1 b, 1 1 1 a, 1 1 1 b under the action of the brake cylinder 12, 1 12 come into initial contact with the brake disc 13 or the brake drum 1 13 at the time of the respective braking operation. Thereafter, at a third step S3, a calculation value BV representing the prevailing lining clearance is determined on the basis of the difference between the measured value Vsi of said stroke registered at said determined position and a measured value Vs₀ of said stroke registered at a time when said brake pressure is nil.

Computer programme codes for implementing a method according to the invention are with advantage included in a computer programme which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control system of a motor vehicle. Such a computer programme is with advantage provided via a computer programme product comprising a data storage medium which is readable by an electronic control unit and which has the computer programme stored on it. Said data storage medium is for example an optical data storage medium in the form of a CD ROM disc, a DVD disc etc., a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc., or a memory of the ROM, PROM, EPROM or EEPROM type or a flash memory.

A computer programme according to an embodiment of the invention comprises computer programme codes for causing an electronic control unit of a motor vehicle comprising a disc brake or drum brake which is provided with one or more brake linings and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder to perform the following:

- determine or receive measured values Vs representing the stroke of the brake cylinder or of some portion of a brake mechanism which is adapted to transmitting to the brake linings a pushing force exerted by the brake cylinder; - determine or receive measured values Vp representing the brake pressure which creates the pushing force of the brake cylinder;

- determine on the basis of said measured values Vs, Vp the relationship between said stroke and said brake pressure at the time of a braking operation,

- determine by analysis of the gradient of the relationship between said stroke and said brake pressure the position where the brake linings under the action cylinder come into initial contact with the brake disc or brake drum at the time of the respective braking operation; and

- determine a calculation value BV which represents the prevailing lining clearance on the basis of the difference between the measured value Vsi of said stroke registered at said determined position and a measured value Vs₀ of said stroke registered at a time when said brake pressure is nil.

Fig. 4 illustrates very schematically an electronic control unit 30 comprising an execution means 34, e.g. a central processor unit (CPU), for execution of computer software. The execution means 34 communicates with a memory 36, e.g. of the RAM type, via a databus 35. The control unit 33 comprises also a data storage medium 37, e.g. in the form of a memory of the ROM, PROM, EPROM, EEPROM type or a flash memory. The execution means 34 communicates with the data storage medium 37 via the databus 35. A computer programme comprising computer programme codes for implementing a method according to the invention, e.g. in accordance with the embodiment illustrated in Fig. 5, is stored on the data storage medium 37.

This invention is particularly intended for use in a heavy motor vehicle, e.g. a bus, a tractor unit or a truck.

The invention is of course in no way limited to the embodiments described above, since a multitude of possibilities for modifications thereof are likely to be obvious to a specialist in the field without thereby having to deviate from the basic concept of the invention such as defined in the attached claims.

Claims

1 . A method for monitoring the lining clearance of a disc brake or drum brake (10; 1 10) which is provided with one or more brake linings (1 1 a, 1 1 b; 1 1 1 a, 1 1 1 b) and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder (12; 1 12), characterised in:

- that the relationship between the stroke of the brake cylinder (12; 1 12), or of some portion of a brake mechanism (16; 1 16) which is adapted to transmitting to the brake linings a pushing force exerted by the brake cylinder, and the brake pressure which creates the pushing force of the brake cylinder, is determined, at the time of a braking operation, on the basis of measured values (Vs) representing said stroke and measured values (Vp) representing said brake pressure;

- that the position where the brake linings (1 1 a, 1 1 b; 1 1 1 a, 1 1 1 b) under the action of the brake cylinder (12; 1 12) come into initial contact with the brake disc or the brake drum at the time of the respective braking operation is determined by analysis of the gradient of the relationship between said stroke and said brake pressure; and

- that a calculation value (BV) which represents the prevailing lining clearance is determined on the basis of the difference between the measured value (Vsi) of said stroke registered at said determined position and a measured value (Vs₀) of said stroke registered at a time when said brake pressure is nil.

2. A method according to claim 1 , in which said brake pressure is regulated by an electronic control unit (7), characterised in that the measured values (Vp) representing said brake pressure are provided by said electronic control unit (7).

3. A method according to claim 1 , in which said brake pressure is regulated via a brake lever (4), characterised in that the measured values (Vp) representing said brake pressure are determined by detection of the position of the brake lever (4).

4. A method according to any one of claims 1 -3, characterised in that the measured values (Vs) representing said stroke are determined by a sensor (6) adapted to detecting the position of the brake cylinder's pushrod (21 ) or the position of some portion of said brake mechanism (16; 1 16).

5. A method according to any one of claims 1 -4, characterised in that said calculation value (BV) is compared with a given upper limit value (GV1 ), and a warning, preferably in the form of an acoustic or visual signal, is delivered when the calculation value (BV) is above that upper limit value (GV1 ).

6. A method according to any one of claims 1 -5, characterised in that said calculation value (BV) is compared with a given lower limit value (GV2), and a warning, preferably in the form of an acoustic or visual signal, is delivered when the calculation value (BV) is below that lower limit value (GV2).

7. A system for monitoring the lining clearance of a disc brake or drum brake which is provided with one or more brake linings and which is applied by pneumatic or hydraulic brake pressure imparted by a brake cylinder, characterised in that the system (1 ) comprises processing means (31 ) adapted to:

- determining at the time of a braking operation the relationship between the stroke of the brake cylinder, or of some portion of a brake mechanism which is adapted to transmitting to the brake linings a pushing force exerted by the brake cylinder, and the brake pressure which creates the pushing force of the brake cylinder, on the basis of measured values (Vs) representing said stroke and measured values

(Vp) representing said brake pressure;

- determining by analysis of the gradient of the relationship between said stroke and said brake pressure the position where the brake linings under the action of the brake cylinder come into initial contact with the brake disc or brake drum at the time of the respective braking operation; and - determining a calculation value (BV) which represents the prevailing lining clearance, on the basis of the difference between the measured value (Vsi) of said stroke registered at said determined position and a measured value (Vs₀) of said stroke registered at a time when said brake pressure is nil.

8. A system according to claim 7, characterised in that the processing means (31 ) is adapted to comparing said calculation value (BV) with a given upper limit value (GV1 ) and to initiating the generation of a warning when the calculation value (BV) is above this upper limit value (GV1 ).

9. A system according to claim 7 or 8, characterised in that the processing means (31 ) is adapted to comparing said calculation value (BV) with a given lower limit value (GV2) and to initiating the generation of a warning when the calculation value (BV) is below this lower limit value (GV2).

10. A computer programme product comprising computer programme codes for causing an electronic control unit of a motor vehicle comprising a disc brake or drum brake which is provided with one or more brake linings and which is applied by pneumatic or hydraulic brake pressure imparted by the brake cylinder to: - determine or receive measured values (Vs) representing the stroke of the brake cylinder or of some portion of a brake mechanism which is adapted to transmitting to the brake linings a pushing force exerted by the brake cylinder;

- determine or receive measured values (Vp) representing the brake pressure which creates the pushing force of the brake cylinder;

- determine on the basis of said measured values (Vs, Vp) the relationship between said stroke and said brake pressure at the time of a braking operation, - determine by analysis of the gradient of the relationship between said stroke and said brake pressure the position where the brake linings under the action of the brake cylinder come into initial contact with the brake disc or brake drum at the time of the respective braking operation; and - determine a calculation value (BV) which represents the prevailing lining clearance on the basis of the difference between the measured value (Vsi ) of said stroke registered at said determined position and a measured value (Vs₀) of said stroke registered at a time when said brake pressure is nil.

1 1 . A computer programme product according to claim 10, characterised in that the computer programme product comprises a data storage medium which can be read by an electronic control unit, said computer programme codes being stored on the data storage medium.

12. An electronic control unit of a motor vehicle, comprising an execution means (34), a memory (36) connected to the execution means and a data storage medium (37) connected to the execution means, computer programme codes of a computer programme product according to claim 10 being stored on said data storage medium (37).