TARGET

The invention relates to a wireless inductive brake pad wear sensor. Brake pad wear monitoring systems utilize a sensor including a resistive element that is clipped to the inner brake pad and wears as the brake pad wears. This resistive wear sensor typically has two levels of detection and is connected to a remote sensing unit via a wire harness. As the brake pad wears, the sensor also is worn due to contact with the brake rotor. In the aforementioned two level system, the sensor is used to indicate two levels of brake pad wear.

When the worn pads are replaced, the wear sensor must also be replaced as it has been worn away as well. In the known systems, this entails replacement of the entire brake pad wear sensor, which produces an undesirable extra expense that is incurred every time the brake pads are replaced. Current brake pad wear sensors are in the market, but they wear with pad wear, have a wire harness and must be changed with the pad. A wireless sensor that doesn't wear out with the brake pad is desired by the OEMs. Previous approaches include the use of an inductive sensor fixed to the caliper bracket and a wear indicator flag attached to the brake pad. A key issue in using this technology is that the brake pad will move in the caliper bracket guides that holds the pads, causes the target (which is used to sense distance) to move around with respect to the sensor mounted on the bracket. This shift in the target (flag) can cause very large errors in the distance measurement (i.e., the brake pad wear). Additionally, it is desirable from a design approach that the Flag and Pad do not have to touch or not attached, from a debris standpoint.

There are two established approaches for measuring the distance from a target (flag) to the sensor (coil antenna). The first approach is that the target and the coil are on parallel planes and the distance between those planes is the distance that is to be measured (axial sensing). The problem is that the magnetic field decreases as 1/d ³ , so while there may be good signal and measurement accuracy when the target is close to the sensor coil (say 1 mm) there is very poor range and lots of error when the sensor is far from the target (say 10 mm). This is a basic limitation of the first approach.

The second approach is to slide the target at a fixed distance across the plane of the sensor coil (lateral sensing). Different types of target shapes or coil designs can lead to a linear distance of travel with good accuracy across the whole range of motion. The key issue with the lateral approach is that the amplitude of the signal (and distance travel calculation) is highly dependent on the gap between the target plane of travel and the parallel sensor coil plane. An approach to handle this dependency by having offset dual coil structures and taking a ratio between the two opposing coil responses to determine the distance traveled. Figure 1 shows this type of sensor and target.

The sensor is in the form of two coils. Each coil extends is flat and extends in a plane, with both coils being arranged in a common plane. Each coil has rectangular windings from an electrically conductive material, with each winding being generally rectangular. Each coil is rectangular, with the length being significantly larger than the width. The direction of the length corresponds to the direction of relative movement between the coils and the target.

The rectangular windings of each coil differ in their length in as their length decreases (or increases) from winding to winding. To a smaller extent, their width also decreases (or increases). The target is made from metal and extends transversely over both windings such that its longer axis extends perpendicularly to the direction of the length of the coils.

Figure 2 shows the basics of obtaining a measurement signal based on the signals received from the two coils. While this arrangement has some advantages over the sensor types in which the distance of a target from the coil is measured, it still has some drawbacks. One drawback is that the dimension of the gap between the target and the coils has to be consistent during use. In the wireless brake pad sensor application, the brake pad will however shift dependent on road vibration and direction of rotational braking, resulting in a change of the gap. Another drawback is that this measurement method requires that someone must sweep the range of motion for the current gap. When a new brake pad is put on, or even during original OEM assembly, it is however not possible to sweep the range of travel.

Further, the area required for a sensing coil is twice that of a non-compensated approach. Therefore, the technique cannot be used for a wireless brake pad sensor, or any other application in which the gap between the metal target and sensor coil can change randomly without recalibration.

Thus, it is an object of the invention to provide a brake pad wear sensor which has a good accuracy, fits within the available installation space and does not require complicated calibration.

To this end, the invention provides a brake pad wear sensor having a coil and a target, one of these elements being adapted for being connected with a stationary part of the brake (such as a caliper) and the other one being adapted for being connected with the moveable part (then the brake pad), the target circumscribing the coil. The target circumscribing the coil, a shift in one direction (perpendicularly to the plane in which the coil extends) will move one side of the target closer to the coil such that is has a stronger effect on the sensor coil while the other side of the target will be moved further away from the coil so that it has less effect on the sensor coil. Thus, the combined normalize net effect will be mostly cancelled out so that the sensor signal is generally independent from a lateral displacement of the target with respect to the coil.

Preferably, the distance from each side of the target to the sensor coil is between 1 .0 and 3.5 mm, resulting in a mainly linear effect of a change of the distance between the target and the sensor coil on the inductance. Preferably, the coil extends in a plane so that the overall dimensions of the sensor are small. The coil preferably consists of rectangular windings in order to be able to very precisely detect changes of the position of the target relative to the coil in a longitudinal direction of the coil.

According to a preferred embodiment, the target has a rectangular cross- section so that the longer sides of the rectangle extend parallel to the plane of the coil. This results in a very good compensation of the effects of a change of the gap between the two sides of the target and the coil.

Preferably, the target has a length of approx. 10 mm. This is sufficient for obtaining a good sensor signal while at the same time keeping the overall dimensions low.

According to a preferred embodiment of the invention, the target has mounting lugs which are arranged at the shorter sides of the target. This allows mounting the target in a very simple manner.

In one embodiment, the target together with the mounting lugs has a height of approx. 22 mm. This is sufficient for obtaining a sensor signal over the entire way of brake pad wear.

Preferably, the mounting lugs are formed integrally with the target so that no step of connecting the target to a mounting structure is necessary.

The target can be made from sheet metal so that it can be punched and bent with low expenditure.

The coil has a length which is approx. 25 mm. This length has proven sufficient for detecting brake pad wear of the majority of brake pads.

Preferably, the coil is arranged on a carrier, the carrier also carrying sensor electronics so that no wiring or other electrical connection between the coil and the electronics is necessary.

The coil and the sensor electronics can be encapsulated in a common housing so that they are protected against water, dust and other contaminants.

The invention also provides a brake pad having friction material and a backing plate, the target of the brake pad wear sensor as described above being connected to the backing plate. The target is an element of low costs which can be automatically replaced together with the brake pad once the friction material has been consumed. The coil together with the sensor electronics does not need replacement and can operate together with the new target after the brake pad has been exchanged. The invention will now be explained with reference to the enclosed drawings. In the drawings,

Figure 1 shows a sensor arrangement of the prior art,

Figure 2 explains how a sensor signal can be obtained with the sensor arrangement of Figure 1 , Figure 3 shows a sensor arrangement according to the invention,

Figure 4 shows the effect of a change of the gap between the two longer sides of the target of the sensor arrangement of Figure 3 and the coil, and

Figure 5 shows a change of the impedance as a result of a change of the distance between the target and the coil. In Figure 3 on the left side, a brake pad wear sensor is shown. It comprises a coil 10 and a target 30.

Coil 10 is formed from electrically conductive material, which is formed on a carrier or substrate 12. The coil is rectangular and has a plurality of rectangular windings 14. Viewed in a longitudinal direction of the coil, each winding is shorter and more narrow than the next winding more outwardly (or longer and wider if compared with the adjacent inner winding.

On three of the four sides of the rectangular windings, the conductive portions are arranged very close to each other. On the remaining side (which is in Figure 3 on the lower side), the conductive portions are arranged with a distance to each other (e.g. approx. 2 mm).

A sensor electronics 16 is arranged on an extension of substrate 12 so that an integrated electronics/coil unit is formed. It is encapsulated in an insulating material so that it is protected against humidity, water and contaminants.

Substrate 12 is adapted for being connected to a brake caliper. Target 30 is made from (ferro-magnetic) metal. It has the form of a sleeve with a rectangular cross-section. Accordingly, two longer portions 32 and two shorter portions 34 are present.

Integrally with the target 30, mounting lugs 36 are formed which are intended for connecting the target with a backing plate 50 carrying the friction material 52 of a brake pad shown on the right side of Figure 3.

Target 30 together with mounting lugs 36 is formed from sheet metal.

The cross-section of target 30 is larger than the dimensions of substrate 12 with coils 10 so that there is some free space between each side of coil 10 and portion of target 30 arranged close thereto.

On the right side of Figure 3, target 30 is shown attached to backing plate 5. Coil 10 extends into target 30.

In the condition shown in Figure 3 on the right side, substate 12 extends into target 30 almost towards backing plate 50. This condition corresponds to a new, unworn friction pad.

If the friction pad is worn, backing plate 50 moves downward (in terms of the depiction in Figure 3) while coil 10 remains stationary. The change of the position of target 30 with respect to coil 10 can be detected by sensor electronics 16 which in turn sends a corresponding signal to e.g. a bus system of the vehicle fitted with the brake pad.

Figure 4 shows the effect of a change of the gap between the longer portions 32 of target 30 and coil 10 in the range highlighted by rectangle 60. In this range, the sensor coil and the sensor electronics are designed such that the effect of both sides of the target is generally linear. The more linear the area of operation for both halfs of the target, the less error in the net reponse do to a gap variation.

The advantages of the sensor arrangement according to the invention are as follows:

- Mostly independent of gap tolerances variations between the sensing plane and target plane without requiring recalibration each time the gap changes.

- Smaller sensor coil area needed and lower cost. - Complicated mechanical means to hold the target plane gap to sensor gap in tight tolerances is not needed.

- Target design with a differential effect on the inductive sensor coil, cancelling out gap variation effecting distance traveled measurement.