GB2098007A - Electrical generators - Google Patents

Abstract

An electrical generator comprises a permanent magnet (11) and a coiled electrical conductor (12) arranged in the magnetic field of the magnet (11). Upon linear reciprocating movement between the magnet and the coiled conductor an e.m.f. is induced in the coiled conductor which is proportional to the relative velocity therebetween. The coil is of a non-uniform or stepped construction in order to compensate for end effects due to the asymmetry of the magnetic field (see Figs. 3 (b, e, f) 5 and 6 for suitable stepped profiles). The induced e.m.f. is thus substantially constant at a predetermined relative velocity for predetermined relative positions between the magnet and the coiled conductor. The generator is applicable to monitoring the condition of shock absorbers, for which it can act as a power source and/or a velocity transducer, and it can also be similarly employed with other members which execute relative reciprocating linear movement. <IMAGE>

Description

SPECIFICATION Electrical generators This invention relates to electrical generators and in particular to devices in which there is linear reciprocating movement between a magnet and an electrical conductor, in the form of a coil, arranged in the magnetic field of the magnet.

During the relative movement an electrical current is caused to flow in the conductor.

Such a linear electromagnetic generator device may be employed in a velocity sensor, which may be used, for example, in a shock absorber monitoring arrangement. The induced e.m.f. in the coil is directly proportional to the relative velocity between the coil and the magnet. However, in a linear device the geometry introduces "end effects" which result in a non-symmetrical flux distribution and this gives rise to a non-linearity in the generated e.m.f.

One method of overcoming this non-linearity is described in our co-pending British Patent Application No. 7939228 (Serial No. 2063574) (M.E. Steele 2).

According to one aspect of the present invention there is provided an electrical generator including a permanent magnet and a coiled electrical conductor arranged in the magnetic field of the magnet, wherein upon linear reciprocating relative movement between the magnet and the coiled conductor an e.m.f. is induced in the coiled conductor, and wherein the coiled conductor is of a non-uniform thickness such that the induced e.m.f. at a predetermined relative velocity between the magnet and the coiled conductor is substantially constant for different relative positive between the magnet and the coiled conductor.

According to another aspect of the present invention there is provided a velocity transducer, for use with two members which linearly reciprocate relative to one another, including an electrical generator comprising a permanent magnet and a coiled conductor arranged in the magnetic field of the permanent magnet according to the preceding paragraph.

The magnet may be annular and movable with a cylinder of a shock absorber, for example, whilst the coil may be movable with the piston of the shock absorber. The e.m.f. induced in the coil during an excursion is related to the relative velocity between the cylinder and the piston and thus the condition of the shock absorber, which can be monitored by monitoring of the induced e.m.f. The non-uniform nature of the coil compensates for the "end effects" and thus linearises the induced e.m.f., which improves the latter's suitability to act as a power source of electronic shock absorber monitoring equipment.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a known linear electromagnetic generator device employed as a velocity sensor for a shock absorber and/or as a power source for an electronic circuit, such as a shock absorber condition monitoring circuit; Fig. 2 shows an enlarged detail of Fig. 1 and the flux paths; Fig. 3 shows schematically and on a different scale various profiles of windings; Fig. 4 shows a graph of the relationship between the output voltage and position for a simple d.c. winding and for the shaped windings, and Fig. 5 and 6 show two alternative windings.

For the purposes of describing the present invention specific reference will be made in the following to linear electrical generators applied to shock absorbers. This is not, however, to be taken as a limitation of the use of the present invention, which is applicable also to other systems involving linear reciprocating movement where an output related to velocity is required, for example in the field of machine tools, and/or where a power source is required.

Fig. 1 shows a shock absorber consisting essentially of a piston 1 which moves in a cylinder 2. The piston 1 is connected by a piston rod 3 to the sprung mass of a vehicle (not shown) and the cylinder 2 is rigidly supported in an outer housing 5 connected to the unsprung mass of the vehicle.

Movement of the piston 1 within the cylinder 2 forced fluid through pressuree valves 7 located in a sealed piston-rod guide 8 into a reservoir space 9 between the cylinder 2 and the housing 5.

During extension, fluid is also drawn from the reservoir into the cylinder 2 via footvalve assembly 4. During compression, the compressed fluid in the cylinder 2 is forced through valves 10 in the piston 1.

An annular magnet 11 is mounted on the top of the piston-rod guide 8, and an annular (cylindrical) coil 12 affixed to the inside of a dust cover 13. The dust cover 15, piston rod 3 and a pole piece 14 provide magnetic flux paths 1 5 (shown by dotted lines). The flux paths are such as to yield a net useful flux intersecting coil 1 2 and the rate of cutting by the windings of the coil produces a voltage indicative of the relative velocity of the sprung and unsprung parts of the shock absorber.

For a simple d.c. winding comprising coil 1 2 as shown, the output voltage, generated at a given velocity, varies with the position of the magnet relative to the coil according to curve a of Fig. 5 (extension and compression referring to the shock absorber). The output voltage is directly proportional to the velocity for small movements about any given position but for a given velocity the output in the compressed position is generally lower than the output in the extended position.

Particularly if the device is for use as a power source, it is desirable to reduce this position dependence to within acceptable limits, that is to linearise the curve a at least over a specified range of positions.

For a simple cylindrical d.c. winding the induced e.m.f. e in the coil is equal to a constant C times the velocity. C is a velocity constant and is the rate of change of flux linkage with distance (position).

Referring to Fig. 2, the e.m.f. induced in the winding will be mainly due to the flux 2. However, the leakage flux L3 generates a voltage that directly opposes that produced by the main flux 2.

The magnitude of the leakage flux (pL3 depends on the relative position between the coil and the magnet, that is it is position dependent and results in variations in the velocity constant C, thus causing the non-linearity in the output voltage characteristic. , is another leakage flux but this is not position dependent, and is constant since there is no relative movement associated therewith.For low values of R2, the reluctance to the main flux path ((p2) through the dust cover 13, the extent of the position dependence effect can be estimated from the ratio of L3 to 2. Thus it is desirable to reduce ,3 by, for example, having iarge values of RL3, the reluctance to the leakage flux path (vL3) through the guide 8 and the lower part of the dust cover 13, and low values for R2.

A simple straight (cylindrical) d.c. winding produces an output voltage that varies with position (curve a of Fig. 4). However, we have found that by modifying the winding it is possible to compensate for the non-linearity and in fact produce a voltage/position curve which is substantially level, between limits. A tapered winding will introduce a position dependent effect that can aid or oppose the existing position dependence, and by a suitable choice of the taper, so that both effects are "equal and opposite", a level curve may be obtained. Since, however, neither effect will vary linearly with position the "ideal" profile will not be a uniform taper, although a specific taper may produce a characteristic that is sufficiently level.A close approximation to a uniformally tapered profile is a stepped profile, and by winding onto a stepped cross-section former, for example, a stepped winding may be achieved in a simpler manner than that required to produce a tapered winding.

We have found that a "level" profile can be obtained with a non-uniform or stepped winding comprising only two sections, although for the "level" part of the profile to extend over a sufficient length of the stroke of the shock absorber piston in practice a minimum of three sections are desirable, although more than four sections are not necessary. The overall "smoothness" of the characteristics produced is related to the axial thickness of the pole piece. A thicker pole piece produces a less concentrated pattern of pole flux and thus a smoother characteristic. The stepped winding is generally thicker at its end adjacent the top of the dust cover.

The six different winding sections a to f schematically illustrated in Fig. 3 (only one half of each winding is shown) produce the corresponding characteristics a to fshown in Fig.

4. Each of the windings is shown as comprised of nine elements with equal axial length, those of windings b to fhave differing radial thicknesses along their lengths. Only one half of each winding is shown, as stated above, the other half may be such that the stepped surface is "inside" or "outside" as illustrated in Figs. 5 and 6 without affecting the characteristic. The stepped winding profile b is provided with a widened portion adjacent its lower end in order to compensate for end effects thereat and as can be seen from Fig. 4 this produces a profile with the longest linear portion.

A winding as illustrated in Fig. 5 may be made on a stepped former (not shown) winding onto the smaller diameter thereof until level with the next largest diameter and so on. The winding illustrated in Fig. 6 may be wound on a plain mandrel (not shown), the winding being automatically controlled such that it first winds over the whole mandrel to a predetermined diameter, then over only a portion to a predetermined diameter and then over a reduced portion to a predetermined diameter.

As can be seen from Fig. 5, tapered windings with only a small number of different radial thickness sections, b, e and f, result in a linear output profile over varying lengths of the characteristics, b having a longer linear portion than f, which in turn has a longer linear portion than e. Fig. 4 also shows that in the extreme cases tapering the winding over its entire length (c and d) does not in fact result in a linear profile portion, whereas the simple two sectioned stepped profile e does.

Thus a specially constructed coil (non-uniform or stepped) inherently compensates for the position dependent effects that would be present with a conventional coil (cylindrical) due to the asymmetrical surrounding magnetic circuit. This stepped coil and magnet arrangement can be used as an electromagnetic velocity transducer and/or a power source, such as described in our British Patent Specification No. 1508527 (K.L. Ellington 7) and our co-pending British Patent Application No. 8106047 (K.L. Ellington -- M.E. Steele P.S. Dayan 9-4-1). or in any other suitable arrangement in which there is relative linear movement between two members. Whereas the magnet has been described as annular, in certain applications other shapes would be applicable, such as cylindrical shapes.

Claims (9)

1. An electrical generator including a permanent magnet and a coiled electrical conductor arranged in the magnetic field of the magnet, wherein upon linear reciprocating relative movement between the magnet and the coiled conductor an e.m.f. is induced in the coiled conductor, and wherein the coiled conductor is of a non-uniform thickness such that the induced e.m.f. at a predetermined relative velocity between the magnet and the coiled conductor is substantially constant for different relative positions between the magnet and the coiled conductor.

2. An electrical generator as claimed in claim 1, wherein the magnet is arranged within the coiled conductor for all relative positions therebetween, and wherein the coiled conductor comprises an elongate stepped annular winding having at least two sections of differing radial thickness and having a greater radial thickness adjacent one axial end of the winding.

3. An electrical generator as claimed in claim 2, and including first and second and third sections of differing radial thickness such that the winding tapers towards the other axial end.

4. An electrical generator as claimed in claim 3 and including a fourth section adjacent the third section and the other axial end, which fourth section is of radial thickness greater than that of third section.

5. An electrical generator as claimed in any one of claims 2 to 4, wherein the magnet is annular and movable with a cylinder of a shock absorber and coil is movable with a piston of the shock absorber.

6. An electrical generator as claimed in claim 5, wherein the coil is mounted to a dust cover movable with the piston and the magnet is mounted to a piston guide and outer housing movable with the cylinder, wherein the induced e.m.f. is mainly due to a magnetic flux path extending through an innermost end of the coil in the dust cover, a piston rod and the piston guide, the innermost end of the winding comprising the one axial end.

7. An electrical generator as claimed in any one of claims 2 to 6, wherein the winding presents a cylindrical outer surface and a stepped inner surface.

8. An electrical generator substantially as herein described with reference to and as illustrated in Figs. 1 and 2 as modified by winding profiles b, e or fof Fig. 3 of the accompanying drawings.

9. A velocity transducer, for use with two members which linearly reciprocate relative to one another, including an electrical generator as claimed in any one of the preceding claims.

1 0. A shock absorber having an electrical generator as claimed in any one of claims 1 to 8 and a monitor for monitoring the condition/performance of the shock absorber, said generator providing a signal representative of velocity to the monitor.