A conventional disc brake comprises a disc which is fixedly mounted on a hub to rotate with the hub, a first brake pad mounted on one side of the disc for movement towards or away from said disc, and a second brake pad mounted on the opposite side of the disc. Each brake pad comprises a support (often called a"back plate") and friction material fixed to the support. The friction material is arranged to engage a portion of the surface of the disc to apply frictional forces to the disc to brake the disc and hence the hub. In the caliper, type of disc brake, the caliper straddles the edge of the disc and the first pad is mounted for movement relative to the caliper and the second pad is fixedly mounted on the caliper. The caliper itself is mounted on a slide so that when an actuator (usually a hydraulic piston and cylinder assembly) is used to press said first pad against the disc, the caliper slides to bring the second pad into engagement with the opposite side of the disc. This invention is applicable to caliper type disc brakes, and to other types of disc brake for example that disclosed in WO 98/25804.

Disc brakes are subject to wear of both the friction material and the disc so that it is desirable to provide a method of measuring such wear which can alert the driver of a vehicle or a mechanic to the desirability of changing the brake pads and/or the disc. Many such proposals have been made but none is entirely satisfactory.

It is an object of the present invention to provide an improved method of measuring wear in a disc brake.

The invention provides a method of measuring wear in a disc brake which comprises at least one disc, a first brake pad mounted on one side of the disc for movement towards or away from said disc, and a second brake pad mounted on the opposite side of the disc, each brake pad comprising a support and friction material fixed to the support, the friction material being arranged to engage a portion of the surface of the disc to apply frictional forces to the disc, characterised in that the method comprises, when the friction material of both pads is engaging the disc, measuring a distance from a point on said first pad to a point whose distance from said first pad, at that time, depends on the wear of the friction material and/or the disc.

In a method according to the invention the difficulties of measuring the friction material directly are avoided.

Said distance may be between a point on said first brake pad and the portion of the surface of the disc which is engaged by the friction material of the first pad. Alternatively, said distance may a distance which is dependant on the distance between the supports for the friction material of said pads. For example, the distance may be between the supports of the two pads, or distance is between said first pad and an actuator of the brake operable to move said first pad towards said disc.

Where said distance is between a point on said first brake pad and the portion of the surface of the disc which is engaged by the friction material of the first pad, this gives a measurement of the total wear of the friction material of the first pad and of the side of the disc facing the first pad. Preferably, the method also comprises, when the friction material of both pads is engaging the disc, measuring a second distance which is between a point on said second brake pad and the portion of the surface of the disc which is engaged by the friction material of the second pad. This enables the total wear of the friction material of the second pad and of the side of the disc facing the second pad to be measured and, by a simple calculation, the total wear of the brake can be determined.

Preferably, a method according to the invention also comprises measuring a third distance which is between a fixed point and the portion of the surface of the disc which is engaged by the friction material of the first pad, and wherein the method also comprises measuring a fourth distance which is between a fixed point and the portion of the surface of the disc which is engaged by the friction material of the second pad. In this case, the wear on the disc can be determined by a simple calculation combining the measurements of the third and fourth distances and, by including the measurements of the friction material and disc wear, the friction material wear can be calculated. The fixed points referred to are fixed relative to the vehicle on which the brake is mounted.

It is a further object of the present invention to provide an improved wear sensor for measuring wear in a disc brake.

The invention also provides a wear sensor for use in measuring wear in a disc brake comprising at least one disc, a first brake pad mounted on one side of the disc for movement towards or away from the disc, and a second brake pad mounted on the opposite side of the disc, each brake pad comprising a support and friction material fixed to the support, the friction material being arranged to engage a portion of the surface of the disc to apply frictional forces to the disc, characterised in that the wear sensor comprises a first distance measurer mounted on said first brake pad and operable to measure the distance between a point on said first brake pad and said portion of the surface of the disc which is engaged by the first pad.

A wear sensor according to the invention enables the wear of the friction material of the first pad plus the wear on the facing side of the disc to be determined.

Preferably, a wear sensor according to the invention also comprises a second distance measurer mounted on the second brake pad and operable to measure the distance between a point on said second brake pad and said portion of the surface of the disc which is engaged by the second pad. This enables the wear of the friction material of the second brake pad plus the wear on the facing side of the disc to be determined and, by calculation the total wear to be determined.

Preferably, a wear sensor according to the invention also comprises a third distance measurer mounted on a fixed mounting on one side of the disc, the third distance measurer being operable to measure the distance between a fixed point and said portion of the surface of the disc which is engaged by the first pad, and said wear sensor also comprises a fourth distance measure mounted on a fixed mounting on the opposite side of the disc, the fourth distance measurer being operable to measurer the distance between a fixed point and said portion of the surface of the disc which is engaged by the second pad. This enables the total wear of the disc to be determined and also, by calculation, the total wear of the friction material to be determined.

A wear sensor according to the invention may comprise a distance measurer operable to measure a distance which is dependant on the distance between supports for the friction material of said pads. This distance may be between the supports of the first and the second brake pad or may be between the first brake pad and the actuator therefor.

Each distance measurer comprises an inductive coil which is supplied with an oscillating electrical signal. This induces an electrical field extending from the measurer. The proximity of the metal disc causes a change in the inductance which varies in accordance with the distance to the disc. An electronic system comprising a demodulator, positioned remotely from the brake, is used to convert changes in the inductance to a D. C. voltage proportional to the distance to the disc. Preferably, the electrical field extends through gaps in the friction material.

Alternatively, in a wear sensor according to the invention, said first distance measurer comprises a plurality of sensors contained within the friction material of the pad. Each sensor may be arranged to detect contact between itself and the disc. The sensors may be positioned at differing distances from the surface of the friction material which engages the disc. In this case, the first distance is only measured when wear of the friction material causes one of the sensors to be exposed to the disc. The sensors may be of the type which, on exposure to the disc, have an electrical conductor which is broken by contact with the disc. This causes a loss of continuity in an electrical circuit.

There now follows a detailed description, to be read with reference to the accompanying drawings of five illustrative examples of methods and wear sensors according to the invention.

In the drawings: Figure 1 is a diagrammatic vertical cross-sectional view taken through the first illustrative wear sensor; and Figures 2 to 5 are similar views to Figure 1 but of the second, third, fourth and fifth illustrative wear sensors, respectively.

The first illustrative wear sensor 10 is shown in Figure 1 and is for use in measuring wear in a spot type disc brake 11 comprising a disc 12 which is of the conventional"top hat"shape and is fixed to a hub (not shown) which rotates about an axis 14. The disc 12 has two opposite facing annular surfaces 12a and 12b which are arranged so that they face in opposite directions and extend normally to the axis 14. The disc brake 11 also comprises a first brake pad 16 mounted on one side of the disc 12 for movement towards or away from the disc.

The pad 16 is mounted for sliding movement parallel to the axis 14 on a caliper 11a of the disc brake 11, the caliper 11a being itself mounted for sliding movement parallel to the axis 14. The first pad 16 is moveable on the caliper 11a by actuation of a hydraulic piston and cylinder assembly 18 (only the cylinder being shown). The disc brake 11 also comprises a second brake pad 20 mounted on the caliper 11a on the opposite side of the disc 12 to the pad 16.

The pad 20 is fixedly mounter on the caliper 11 a. Each brake pad 16 and 20 comprises a support 16a and 20a in the form of a back plate and friction material 16b and 20b fixed to the support. The friction material 16b and 20b is, respectively, arranged to engage a portion of the surfaces 12a and 12b of the disc 12 to apply frictional forces to the disc.

The wear sensor 10 also comprises a first distance measurer 22 mounted on the support 16a of the first brake pad 16. The measurer 22 is operable to measure the distance between a point on said first brake pad 16 and said portion of the surface 12a of the disc 12 which is engaged by the first pad 16.

Specifically, the measurer 22 is mounted on the support 16a within a passage 24 through the friction material 1Gb, the passage 24 extending to the surface of the friction material 16b which engages the disc 12. The measurer 22 operates by inducing an electrical field in the passage 24 and this is modified by the surface 12a of the disc 12 and changes are detected by the measurer 22. The magnitude of the change is converted, remotely, to give a DC voltage proportional to the length of the passage 24 and hence of the thickness of the friction material 16b. The measurer 22 acts to measure the length L, of the passage 24 which is equivalent to measuring the thickness of the friction material 16b.

The wear sensor 10 also comprises a second distance measurer 26 mounted on the second brake pad 20 and operable to measure the distance between a point on said second brake pad 20 and said portion of the surface 12b of the disc 12 which is engaged by the second pad. 20. Specifically, the measurer 26 is mounted on a support 20a within the passage 28 in the friction material 20b, the passage 28 being similar to the passage 24. The measurer 26 acts in the same way as the measurer 22 to measure the length L2 of the passage 28 which is equivalent to measuring the thickness of the friction material 20b.

The wear sensor 10 also comprises a third distance measurer 30 mounted on a fixed mounting 32 on one side of the disc 12. The third distance measurer 30 is positioned opposite the portion of the surface 12a of the disc 12 which is engaged by the first pad 16 but at a circumferentially spaced position relative to the pads 16 and 20. The third distance measurer 30 is operable in the same way as the measurers 22 and 26 to measure the distance S, between a fixed point (the position of the measurer 30) and said portion of the surface 12a of the disc 12 which is engaged by the first pad 16. Said wear sensor 10 also comprises a fourth distance measurer 34 mounted on a fixed mounting 36 on the opposite side of the disc 12 to the mounting 32. The fourth distance measurer 34 is operable to measure the distance S2 between a fixed point and said portion of the surface 12b of the disc 12 which is engaged by the second pad 20.

The first illustrative wear sensor 10 is used in the first illustrative method of measuring wear in a disc brake. This method comprises, when the friction material 16b and 20b of both pads 16 and 20 is engaging the disc 12, measuring a distance L, from a point on said first pad 16 to a point on the surface 12a whose distance from said first pad, at that time, depends on the wear of the friction material 16b and/or the disc 12.

The first illustrative method also comprises, when the friction material 16b and 20b of both pads 16 and 20 is engaging the disc 12, measuring a second distance L2 which is between a point on said second brake pad 20 and the portion of the surface 12b of the disc 12 which is engaged by the friction material 20b of the second pad 20.

The first illustrative method also comprises measuring a third distance S, which is between a fixed point and the portion of the surface 12a of the disc 12 which is engaged by the friction material 16b of the first pad 16. The method also comprises measuring a fourth distance S2 which is between a fixed point and the portion of the surface 12b of the disc 12 which is engaged by the friction material 20b of the second pad 20.

From the distances Li, L2, S, and S2, it is possible to calculate various changes in thickness of the disc 12 and the friction material 16b and 20b. These calculations are preferably performed continuously by a computer mounted in the vehicle in which the disc brake 11 is mounted. L, gives a measure of the wear of the disc 12 on its side which faces the pad 16 plus the wear of the friction material 16b. L2 gives the same measurement as L, but in relation to the second pad 20. The wear of the friction material 16b is given by L, minus the change in S, since the disc 12 was new and a similar calculation can be carried out to give the wear of the friction material 20b.

The second illustrative wear sensor 40 is illustrated by Figure 2 in relation to a disc brake which is identical to the disc brake 11. The same reference numerals are used in Figure 2 for parts of the disc brake as are used in Figure 1.

The wear sensor 40 differs from the wear sensor 10 in that the first and the second distance measurers 22 and 26 are replaced by a first distance measurer 42 and a second distance measurer 44 which operate on a different principle.

The wear sensor 40, however, retains the third and fourth distance measurers 30 and 34.

Said first distance measurer 42, specifically comprises a plurality of sensors 42a contained within the friction material 16b of the first pad 16. Each sensor 42a is arranged to detect contact between itself and the disc 12 which occurs when the friction material 16b has worn sufficiently to expose the sensor 42a to the disc 12. The sensors 42a contain an insulated electrical conductor which normally carries a small current to indicate that the sensor is intact and wear has not reached that position. When the friction material wears sufficiently to bring the disc 12 into contact with the sensor 42a, the conductor will be broken by the rubbing friction, thus breaking the circuit and thus indicating that wear has reached a certain level. The sensors 42a are positioned at different distances from the surface of the friction material 16b which engages the disc 12 so that different amounts of wear of the friction material 16b brings different sensors 42a into operation. In other words, the distance L, from a point on the first pad 16 to the disc is measured at periods determined by the wear of the friction material 16b bringing the sensors 42a into operation.

In the wear sensor 40 the distance L2 is measured by a distance measurer 44 which is similar to the measurer 42 but acts on the second pad 20. Said second distance measurer 42, specifically comprises a plurality of sensors 44a contained within the friction material 20b of the second pad 20. Each sensor 44a is arranged to detect contact between itself and the disc 12 which occurs when the friction material 20b has worn sufficiently to expose the sensor 44a to the disc 12.

The wear sensor 40 is used in the second illustrative method which is the same as the first illustrative method except that L, and L2 are not measured continuously but instead are measured at specific amounts of wear when the sensors 42a and 44a make contact with the disc 12.

The third illustrative wear sensor 50 is illustrated by Figure 3 in relation to a disc brake which is identical to the disc brake 11. The same reference numerals are used in Figure 3 for parts of the disc brake as are used in Figure 1. The wear sensor 50 differs from the wear sensor 10 in that the distance X measured by the wear sensor 50 is a distance which is dependant on the distance between the supports 16a and 20a of the first and second pads 16 and 20.

The sensor 50 comprises a distance measurer 52 which operates in the same way as the distance measurers 22,26,30 and 34. The distance measurer 52 is mounted on the support 16a in similar fashion to the measurer 22 but faces in the opposite direction so that it can measure the distance between the support 16a and a fixed point on the caliper 11a, the specific fixed point being a surface of the cylinder 18 of the actuator for the pad 16.

The third illustrative wear sensor 50 is used in the third illustrative method of measuring wear in a disc brake. This method comprises, when the friction material 16b and 20b of both pads 16 and 20 is engaging the disc 12, measuring the distance X. The distance X increases with wear of the disc 12 or of the friction material 16b and 20b so that a measure of the total wear on the brake is obtained. If desired, the distance measurers 32 and 34 may be incorporated into a modification of the wear sensor 50 so that a measurement of disc wear can be subtracted from the total wear given by X to give a measurement of the friction material wear.

The fourth illustrative wear sensor 60 is illustrated by Figure 4 in relation to a disc brake which is identical to the disc brake 11. The same reference numerals are used in Figure 4 for parts of the disc brake as are used in Figure 1.

The wear sensor 60 differs from the wear sensor 10 in that the first and the second distance measurers 22 and 26 are retained but the distance measurers 30 and 34 are omitted. The wear sensor 60 also comprises distance measurers 42 and 44 as described in relation to the second illustrative wear sensor 40.

The fourth illustrative wear sensor 60 is used in the fourth illustrative method of measuring wear in a disc brake. In this method, the measurers 22 and 26 are used to measure L, and L2 respectively which gives the total wear of the friction material 16b or 20b and of the disc 12. The measurers 42 and 44 are used to periodically measure the wear of the friction material 16b or 20b which can be subtracted from the total wear Li or L2 to give the wear of the disc.

The fifth illustrative wear sensor 70 is illustrated by Figure 5 in relation to a disc brake which is identical to the disc brake 11. The same reference numerals are used in Figure 5 for parts of the disc brake as are used in Figure 1. The wear sensor 70 is similar to the wear sensor 50 shown in Figure 3 in that it has a distance measurer 52 mounted on the support 16a to measure the distance X to the cylinder 18. However, the wear sensor 70 also comprises distance measurers 72 and 74 arranged to give an alarm when the thickness of the friction material 16b or 20b reaches a minimum safe thickness.

The distance measurer 72 comprises a plurality of sensors 72a which are similar to the sensors 42a and 44a and are mounted in the friction material 16b of the first pad 16. The sensors 72a, however, differ from the sensors 42a and 44a in that all the sensors 72a are positioned at the same distance from the surface of the friction material 16b which engages the disc 12. The distance at which the sensor 72a are positioned is selected to ensure that when the sensor 72a are operated by contact with the disc 12 a minimum quantity of friction material M is still present. The distance measurer 74 has a similar array of sensors 74a to the sensors 72a but mounted in the friction material 20b of the second pad 20.

The fifth illustrative wear sensor 70 is used in the fifth illustrative method of measuring wear in a disc brake. This method is the same as the third illustrative method except that the distance measurers 72 and 74 provide insurance that a minimum thickness of friction material 16b and 20b will remain present.

It will be understood that the distance measurers 22,26,30,34 and 52 may be replaced by other types of distance measurer. Similarly, the sensors 42a, 44a, 72a and 74a may be replaced by other types of sensor. It is also possible to include measurers 72 and 74 in the wear sensors 10,40,50 and 60 and to incorporate measurers 30,34,42 and 44 in the wear sensor 70.