GB1595680A - Wheel speed transducer - Google Patents

Description

(54) WHEEL SPEED TRANSDUCER (71) We, KELSEY-HAYES COM PANY, a Corporation organised and existing under the laws of the State of Delaware, United States of America, of 38481 Huron River Drive, Romulus, Michigan 48174, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a speed sensing device and more particularly an improved, compact speed sensing device for a vehicle wheel or other rotatable member.

Speed sensing devices have a variety of applications. One specific application for such devices is in combination with a vehicular skid control wherein the speed sensing device provides a signal indicative of wheel rotation to a computer for controlling brake pressure relief. It is essential that such devices be relatively compact so as to permit convenient assembly into existing vehicles without substantial modification but must nevertheless provide a strong signal. Somewhat inconsistent with this requirement is that the device must have a high rejection of unwanted signals, commonly called noise, as occurs due to dynamic variations in the air gap.

One attempt at providing a relatively compact speed sensing device that is insensitive to variations in air gap which cause noise is described in the Specification of United States Patent No. 3,854,556. The device shown in this patent, however, requires three pole faces for the stator and two separate coil windings. This device operates on a reversing flux principle and variations in spacing are cancelled out by subtracting the electrical signals generated through the respective windings. Although this device has considerable advantage, it is not compact in size, and, because of its complexity, it is expensive.

According to the invention there is provided a speed sensor for a vehicle wheel or other rotatable member comprising a rotatable tone wheel formed of magnetic material and having a plurality of teeth separated by gaps spaced circumferentially of the wheel, and a stator juxtaposed to said tone wheel, said stator having a pair of spaced legs juxtaposed to said tone wheel, the spacing of the legs of the stator being greater than the distance of the spacing of two adjacent teeth on the tone wheel and less than the distance spanning three of the teeth on the tone wheel, a core extending between said legs, a coil encircling the core, and means for magnetizing the legs which exerts like pole faces on each of the legs, the arrange ment being such that upon rotation of the tone wheel relative to the stator a reversing flux is caused to flow through said core.

The device of this invention can be so constructed as to be compact in construction while, nevertheless, being relatively insensitive to those dynamic variations in air gap which cause unwanted signals (noise).

The invention will be further described, by way of example, with reference to the accompanying drawings, wherein: FIGURE 1 is a cross sectional view taken through a speed sensing device embodying the invention; and FIGURES 2 to 4 are schematic views of the speed sensing device shown in Figure 1, each representing a different condition during the operation of the device.

In the drawings a speed sensing device 11 is shown which is particularly adapted to sense the rotational speed of an associated wheel for providing a signal indicative of wheel speed to a skid control module, or computor (not shown) for processing the signals from the speed sensing device 11.

It is to be understood, however, that there are various other uses for the speed sensing device 11.

The speed sensing device 11 includes a tone wheel 12 that is comprised for ferromagnetic material and which is adapted to be mounted for rotation with the associated wheel. The tone wheel 12 has a pluralitv of radially extending teeth, only three of which are shown in the drawings and are identified by the reference numerals 13, 14 and 15. The respective teeth are separated by air gaps 16 and 17.

A stator or transducer 18 is mounted in a fixed manner relative to the tone wheel 12. The transducer 18 has a non-magnetic outer housing 19, which may be formed from aluminium or the like, and which has a pair of outwardly extending mounting flanges 21 and 22. Mounted within a central cavity of the housing 19 are a pair of spaced legs 23 and 24, which are formed of a ferrornagnetic material. A ferromagnetic core 25 has projections 26 and 27 that are received in central openings in the legs 23 and 24 to form a generally H-shaped sensor assembly. A winding 28 encircles the core 25 and is electrically connected to a pair of terminals 31 and 32. A permanent magnet 33 is also supported in the housing 19 and is juxtaposed to the portion of the legs 23 and 24 that extend away from the tone wheel 12. As will be described, the permanent magnet 33 is magnetized in such a way that like poles are juxtaposed to the inner ends of the legs 23 and 24. Alternatively, two separate permanent magnets may be provided, one cooperating with each of the legs 23 and 24.

The operation of the speed sensing device 11 may be best understood by reference to the schematic views of Figures 2 to 4.

In referring to these views, it should be apparent that the spacing of the legs 23 and 24 is slightly greater than the spacing of two adjacent ones of the teeth 13, 14 or 15 but less than the distance spanning three of these teeth. For this reason, under nonnal circumstances only one of the legs 23 or 24 will be juxtaposed to one of the tone wheel teeth 13, 14 or 13 during operation. In Figures 2 to 4, the direction of rotation of the tone wheel 12 is identified by the arrow 35. As seen in Figure 2, initially the leg 24 will be juxtaposed to the tone wheel tooth 15, while the leg 23 is juxtaposed to the tone wheel gap 16.

As has been previously noted, the permanent magnet 33 is designed so that the legs 23 and 24 will experience like poles.

The poles have been indicated in Figures 2 to 4 and, in the illustrated embodiment, the legs 23 and 24 are both under the influence of the north poles of the magnet 33.

Considering the instantaneous condition as existing in Figure 2, a first magnetic path, indicated by the solid line 36 extends from the north pole of the magnet 33 through the leg 24 across the small axial air gap to the tone wheel tooth 15 and back to the south pole of the magnet 33. Another magnetic flux path extends from the north pole of the magnet 33 through the leg 23, core 25 and leg 24 across to the tooth 15 and back to the south pole of the magnet.

This flux path is indicated by the solid line 37. At this instant the rate of change of the magnetic flux in the core 25 is zero and no voltage is induced in the winding 28.

As the rotation of the tone wheel 12 continues, the leg 23 will register slightly with the tooth 13 and the leg 24 will register slightly with the tooth 15. Magnetic flux paths then extend from the magnet 33 through the individual legs 23 and 24 to the teeth 13 and 15 and back to the magnet 33.

These flux paths are shown by the solid lines 38 and 39 in Figure 3. Under this condition, there will be no magnetic flux path through the core 25 but the rate of change in flux through the core 25 is at its maximum, and maximum voltage will be induced in the winding 28 and sensed at the terminals 31 and 32.

Upon continued rotation (Figure 4) the tone wheel tooth 13 will register with the sensor leg 23. In this condition, a flux path will extend from the magnet 33 through the leg 23 to the tooth 13 and back to the magnet 33. This flux path is indicated by the solid line 41 in Figure 4. In a like manner, a magnetic path will extend from the magnet 33 through the leg 24, along the core 25 and through the leg 23 to the tooth 13. This flux path is indicated by the solid line 42 in this Figure. The rate of change in magnetic flux in the core 25 will again be zero and no voltage will be induced in the winding thus completing one half of a cycle of the device.

It should be readily apparent from the foregoing description, that relative rotation between the tone wheel 12 and the sensing device 11 will cause alternating flux paths through the core 25 that provide a voltage signal at the terminals 31 and 32 that is indicative of the speed of rotation of the associated wheel.

The construction of the speed sensing device 11 is such that it provides a high magnitude of signal for its size and also has a high rate of reduction of noise such as may be generated by vibrations in the axial distance between the speed sensing device 11 and the tone wheel 12 as are encountered during operation.

In order to provide a compact sensing device it has been previously proposed to use what has been called a single point or single pole sensor. By using the bipole sensor having two poles which 180 out of phase with each other it is possible to magnify the signal strength for a given number of turns of the winding 28 or for a given strength in the magnetic field. The same construction also has the effect of improving the rejection of noise signals, which is further aided by the use of the common core for the two magnetic flux paths. By reducing the strength of the magnetic field or the number of turns in the winding from those used in a single point sensor providing the same strength output signal, noise rejection is increased. However, the use of the common core further increases the noise rejection due to the fringing flux present in the circuit including the pole which is positioned adjacent the air gap in the tone wheel 12.

WHAT WE CLAIM IS:- 1. A speed sensor for a vehicle wheel or other rotatable member comprising a rotatable tone wheel formed of magnetic material and having a plurality of teeth separated by gaps spaced circumferentially of the wheel, and a stator juxtaposed to said tone wheel, said stator having a pair of spaced legs juxtaposed to said tone wheel, the spacing of the legs of the stator being greater than the distance of the spacing of two adjacent teeth on the tone wheel and less than the distance spanning three of the teeth on the tone wheel, a core extending between said legs, a coil encircling the core, and means for magnetizing the legs which exerts like pole faces on each of the legs, the arrangement being such that upon rotation of the tone wheel relative to the stator a reversing flux is caused to flow through said core.

2. A speed sensor as claimed in claim 1, wherein the legs and core together have a generally H-shape.

3. A speed sensor as claimed in claim 2, wherein the means for magnetizing the legs comprises a permanent magnet extending between the portions of the legs lying on the opposite side of the core from the tone wheel.

4. A speed sensor as constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. strength in the magnetic field. The same construction also has the effect of improving the rejection of noise signals, which is further aided by the use of the common core for the two magnetic flux paths. By reducing the strength of the magnetic field or the number of turns in the winding from those used in a single point sensor providing the same strength output signal, noise rejection is increased. However, the use of the common core further increases the noise rejection due to the fringing flux present in the circuit including the pole which is positioned adjacent the air gap in the tone wheel 12. WHAT WE CLAIM IS:-

1. A speed sensor for a vehicle wheel or other rotatable member comprising a rotatable tone wheel formed of magnetic material and having a plurality of teeth separated by gaps spaced circumferentially of the wheel, and a stator juxtaposed to said tone wheel, said stator having a pair of spaced legs juxtaposed to said tone wheel, the spacing of the legs of the stator being greater than the distance of the spacing of two adjacent teeth on the tone wheel and less than the distance spanning three of the teeth on the tone wheel, a core extending between said legs, a coil encircling the core, and means for magnetizing the legs which exerts like pole faces on each of the legs, the arrangement being such that upon rotation of the tone wheel relative to the stator a reversing flux is caused to flow through said core.

2. A speed sensor as claimed in claim 1, wherein the legs and core together have a generally H-shape.

3. A speed sensor as claimed in claim 2, wherein the means for magnetizing the legs comprises a permanent magnet extending between the portions of the legs lying on the opposite side of the core from the tone wheel.

4. A speed sensor as constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.