IE44374B1 - Apparatus for measuring the rate of change in speed of a moving body - Google Patents

Abstract

1512118 Measuring acceleration electrically PCB CONTROLS Ltd 21 July 1976 [19 July 1976] 30391/76 Heading G1N The acceleration or deceleration of a rotating metal disc 1 is measured by the e.m.f. induced in a coil 4 by the change in flux produced by the eddy currents developed in the disc by two permanent magnets 2 located near the periphery of the disc on either side of the coil 4, which is wound on a laminated core 3.

Description

The invention relates, to an apparatus or device for measuring rate of change in the speed of a moving body, i.e. an acceleration or a deceleration, Xn some vehicle anti-skid control systems, it is necessary to measure the rotational deceleration of a braked wheel. This deceleration is constant during underbraking, with a constant brake pressure, but rises rapidly during over-braking. This causes the braked wheel to look while the vehicle Is in motion, prior art systems for measuring the deceleration of a wheel use disks provided with dots or holes, the acceleration or ..1 deceleration being derived from the change in the number of dots or holes·’which, in a given time, pass by a counting device which is positioned adjacent to the disk. These known systems require electronic means for calculating the deceleration or acceleration from differences in the number of dots or holes counted per unit time.

It is an object of the present invention to provide a device, which while it may be simple in construction directly monitors an acceleration or deceleration.

According to the invention there is provided apparatus for producing electrical signals relating to the rate of change in speed of a moving body, comprising a rotatable electrically conducting 'member which is part of said body or is connected or connectible to said body · so as to rotate at a speed corresponding to the speed of said body, a plurality of magnets arranged to provide magnetic fields penetrating said member with the field -244374 excitation and the field position being at least partially Independent of the rotation of the member whereby eddy currents are induced in the member, said magnets being spaced from each other circumferentially around said member and each having its opposite poles located adjacent the member and respectively on opposite axial sides of the member at substantially equal distances from the centre of rotation, and further comprising a magnetic induction sensor, arranged at a position at least partially independent of the rotation of said member so that variation of the eddy currents causes induction of an electrical current in the sensor.

Preferably, the said member is an electrically conducting disk having at least an annular portion in which said eddy currents are induced.

Preferably also, the magnets and sensor are in fixed positions relative to the centre of rotation of the said member.

When a conducting member, e.g. a disk, moves in a stationary magnetic field, the electrons of the disk material see an electric field which is proportional to the vector product Ev Bj where v is the speed of the disk at the site of the electron and B the magnetic flux density at the same site. Since the material of the disk is conducting and thus contains free electrons, the electric field will give rise to eddy currents in the disk which produce a magnetic field of their own. This magnetic field is found to be proportional to the angular speed of the disc. The magnetic field generated by the eddy currents will change as the eddy currents change and, consequently, every change in the rotational -344374 Speed of the disk, since it gives rise to a change in the magnitude of the eddy currents, will cause the magnetic field created by the eddy currents to vary correspondingly. A voltage is induced in the inductive sensor means, which is proportional to the rate of change of this magnetic field and hence is proportional to the amount of acceleration or deceleration of the disk. With the disk connected to a wheel of a vehicle and its rotational speed being caused to be equal or at least to be in definite relationship to the rotational speed of the wheel, the instantaneous deceleration of the latter can be measured using the device of the invention. Alternatively, the felloe or other part of the wheel itself could serve as the disk.

The rotatable members is preferably made of a material having poor magnetic properties (relative permeability yal) . As the eddy currents in the member will increase as its electric conductivity increases, metals such as copper or aluminium are especially suitable.

Especially high voltage output from the induction means is obtainable when the eddy current inducing the magnetic field is arranged to penetrate the member primarily in a peripheral region thereof, because the portion of the member in this area has the highest rotational speed of the whole member and thus gives rise to relatively higher eddy currents.

The preferred induction sensor is a coil. The magnets may be electromagnets or permanent magnets, e.g. of horse-shoe shape, enclosing the periphery of the -44437a rotatable member and generating a magnetic field which is basically at right angles to the surface of the member, which is preferably planar. This is preferred because the eddy currents which are produced in a plane parallel to the plane of the member give rise to a magnetic field which is basically perpendicular to the plane of the member and have an optimal direction to traverse an induction coil adjacent the member.

In an especially preferred embodiment, the apparatus has two said magnets spaced from each other circumferentially around said member, the sensor being a coil located circumferentially between the two magnets. The induction sensor may be a multi-turn coil located on a laminated former of about the same shape as the magnets which are horse-shoe-shaped permanent magnets. This arrangement may produce the maximum induced voltage for a given deceleration.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying schematic drawing.

Referring now to the drawing, a cut-away portion of a rotatable disk 1 made of aluminium or copper is shown. The disk is arranged to rotate at the same speed as the wheel of a vehicle. Two permanent horse-shoe-shaped magnets 2 are positioned so that they enfold the outer edge of the disk, with the N pole of one magnet on the same side of the disk as the S pole o,f the other magnet, i.e. so that the two fields produced are in opposite directions. The two poles of each magnet are closely adjacent and on opposite axial sides of the disk 1, at the same distance from the centre of -544374 rotation. The magnets 2 are at some circumferential distance from each other.

Halfway between the magnets, a laminated ferromagnetic core 3 of about the same shape as the magnets is arranged, this being provided with a multi-turn coil 4 on both of its legs, to form a magnetic induction sensor.

As the disk 1 rotates, circulating eddy currents are induced in the disk by the magnets 2. The eddy currents generate a magnetic field which, as long as the rotational speed of the disk is constant, will be constant as well and thus induce zero voltage in the coil 4, which is linked by an enhanced magnetic flux due to the core 3. Deceleration tor acceleration) of the disk, however, gives rise to a change in the eddy currents, and therefore a change in the corresponding magnetic flux. The changing flux induces a voltage in the coil 4 which is a measure of the instantaneous rate of change in the rotational speed of the disk.

Claims (10)

1. CLAIMS:1. Apparatus for producing electrical signals relating to the rate of change in speed of a moving body, comprising a rotatable electrically conducting member which is part of said body or is connected or connectible to said body so as to rotate at a speed corresponding to the speed of said body, a plurality of magnets arranged to provide magnetic fields penetrating said member with the field excitation and the field position being at least partially independent of the rotation of the member whereby eddy currents are induced in the member, said magnets being spaced from each other circumferentially around said member and each having its opposite poles located adjacent the member and respectively on opposite axial sides of the member at substantially equal distances from the centre of rotation, and further comprising a magnetic induction sensor, arranged at a position at least partially independent of the rotation of said member so that variation of the eddy currents causes induction of an electrical current in the sensor.

2. Apparatus according to Claim 1, wherein the said member is an electrically conducting disk having at least an annular portion in which said eddy currents are induced.

3. Apparatus according to Claim 1 or Claim 2, wherein the rotational speed of said member has a fixed relationship to the speed of said body.

4. Apparatus according to any one of Claims 1 to 3, wherein the magnets and the sensor are in fixed positions relative to the centre of rotation of the said member. -744374 ' 5. Apparatus according to any one of the ’ preceding claims, wherein the said member, at lesrt where said eddy currents are induced, has poor magnetic properties.

5

6. Apparatus according to Claim 5, wherein said member, at -least where said eddy currents are induced, has a relative magnetic permeability (.μ) of about 1.

7. Apparatus according to any one of the preceding claims, wherein the magnets are arranged so 10 that their magnetic field penetrates the.said member substantially perpendicular to its surface.

8. Apparatus according to any one of the preceding claims, having two said magnets, the sensor being a coil located circumferentially between the two magnets. 15

9. Apparatus for producing electrical signals relating to the rate of change of speed of a moving body substantially as herein described with reference to and as shown in the accompanying drawing.

10. A vehicle anti-skid control arrangement including 20 at least one apparatus according to any one of the preceding claims.