GB2123558A - Tacho-generators - Google Patents

Abstract

Rotational speed of a non- magnetic spindle 8 is converted to an alternating electrical signal, by driving an alternating multipole permanent magnet rotor 18 close to the interleaved pole pieces 32, 33 of two magnetic combs 20, 22. The combs comprise extensions parallel to the rotor axis from the outer peripheries of two radially broad rings. Their inner edges may be magnetically connected together. Each comb is an integral mild steel magnetizable spider. The rings are axially spaced and a pick-up coil 23 lies between the pole pieces to generate an alternating electrical signal which is sinusoidal, but which can be converted to a clipped square waveform. <IMAGE>

Description

SPECIFICATION Rotational speed sensor This invention relates to a sensor of rotational speed able to give an indication of a rotational speed or revolution count. The indication may be used to control some further function or operation, or it may be used to keep the speed constant. The invention gives an alternating electrical waveform output, with the frequency giving the required indication of speed. Preferably, the approximately sinusoidal ac output may be electronically converted to a square wave between constant positive and negative voltage levels for low distortion despatch or distribution of the speed sensing signal. Such conversion may involve voltage amplitude limitation at both signal polarities.

This signal may be useful for speed control as aforesaid, in a vehicle "cruise" control system, and/or simply as its electronic speedometer. Also the signal may be used in the more sophisticated on-board vehicle systems such as fuel injection control, on-board computations or electronic ignition systems. Often the rotational speed sensed will then be that of the gearbox or transmission output shaft, or that of the vehicle engine shaft. The sensor instrument may be mounted on the housing adjacent the shaft whose speed is to be sensed or controlled; alternatively it may be mounted some way along a flexible drive, such as a speedometer cable, as a series element.

More likely, however, the speedometer would use the electrical output of the instrument of the invention.

Main and preferred features of the invention as typically embodied are (in combination or separately) a two-part resilient e.g. plastics housing with snap engagement between the parts; a plastics axle carrying a sixteen-pole ferrite permanent magnet rotor; a pair of low reluctance, e.g. soft iron, polar rings to surround the axle and each carrying eight equi-angularly spaced barshaped stator pole-pieces; the polar rings being an "upper" and a "lower" ring mounted axially spaced such that the sixteen pole pieces are parallel with the axle and such that the eight polepieces of one polar ring interleave or symmetrically alternate with the eight pole-pieces of the other ring; the rings being axially spaced to sandwich an electrical pick-up coil and former; at least some of the pole-pieces being held against rotations by peripheral grooves parallel to the axis in the inside wall of the instrument housing; some space economy being achieved, if desired, by the threading of one pole-piece of the first-inserted ("lower") polar ring through a close-fitting aperture in the insulating non-magnetic former of the coil; the thus threaded pole-piece being one not held in a said peripheral groove of the housing; the lower polar ring having longer pole pieces than the upper, so that the distal ends of the pole pieces all terminate mutually flush.

The terms upper and lower for the polar rings, and other references to "above" etc. are generally for ease in interpreting the first illustrated embodiment. In fact the rotation sensors of the invention will usually be operable in any disposition.

The invention aims at compactness and reliability, and the low cost furnishing of an electrical indication of rotational speed having hiyh integrity.

Further objects, features and advantages of the present invention will appear from the following description of embodiments thereof, given by way of example in conjunction with the drawings, in which Figures 1 and 2 show a perspective view and a sectional elevational view, of a sensor unit.

Figure 3 shows the coil and former of this unit; and Figure 4 shows in perspective an exploded view of a similar but not identical speed sensor unit Referring to Figures 1 and 2, a sensor unit 1 has an upper housing part 2 snap engaged with a housing part 3 by means of four snap fittings, three of which can be seen at 4, 5 and 6 in Figures 1 and 2 to feature resilient legs 6 with barbed detect ends 7. Protruding from the upper housing 2 is a square-shanked mechanical output drive 8 if wanted, and twin electrical output terminals 9, 10, to deliver the sensing signal, each within a respective tubular extension 11, 12 of the upper insulating housing 2.The mechanical input drive comprises a square-sectioned axial hole 1 3 in what can advantageously be shown the lower end 14 of the same glass-reinforced plastic shaft providing the output drive at its upper end 8. The square-sectioned input hole and output spindle are of standard M 12 size for vehicles. The upper and lower housing give fluid tight closure by means of the above mentioned snap engagements sandwiching an annular gasket 1 5. The lower housing 3 has a self-lubricating sintered 30% oilsaturated bronze plain bearing 1 6 (of bronze BP25) and a thrust washer 1 7 may space the rotor permanent magnet part 1 8 from the apertured part of the upper housing 2.

The permanent magnet part 1 8 comprises a continuous annulus magnetized to have sixteen poles. They pass close to sixteen vertical pole pieces details of which cannot be seen in Figure 2, but two diametrically opposite ones being positioned at 19 and 20. The pole pieces may be parallel equispaced extensions of two zinc-plated soft iron rings 21, 22 providing interleaving magnetic "comb" stators. Between the washershaped rings 21, 22 is a coil 23 comprising a single winding.

The interaction between the two soft magnetic eight-pole combs and the 1 6-pole permanent magnet, and the two soft iron annuli 21, 22 provide a fluctuating magnetic field through the coil 23 sandwiched betweeen the annuli, such as to cause an alternating (ac) voltage at eight times the frequency of revolution.

The frequency of the ac may be used as such, or be converted to an anaiogue voltage or current whose level exerts a control function. The way the recovered frequency may be used can be state of the art technology, not further considered herein.

The ac is derived at terminals 9, 10.

The coil 23, its former 24 and anchoring of the coil ends 26 at two soldering sites 25 can be seen in the perspective view of Figure 3 which shows two metal terminal pieces 27 attached to an insulating plastics extension 28 of the former 24.

The two terminal pieces 27 have extensions providing terminals 9, 10 and further extensions providing anchoring points 25. The coil former has its axial aperture 29 threaded by the nonmagnetic part of the rotor at 28 (Figure 2), and has another aperture 30 e.g. somewhere in the insulating extension 27. Aperture 30 is used to locate and hold the coil, its former and its terminals, by its being threaded over one of the magnetic pole pieces 20. Some or all of the magnetic pole pieces 1 9, 20 may be located in vertical recesses (not shown in Figure 2) in the internal wall of lower housing 3. Thus the coil and the static magnetic combs and rings are indeed held static against the tsndency to turn in response to the magnetic pull of the rotor.

The above extremely compact and sturdy structure may be appreciated even more fully from an examination of the sirnilar instrument shown in exploded view in Figure 4. The latter instrument is shown with a different shaped coil former 24 and lacks the snap fixing arrangements 4-7, but otherwise closely resembles the embodiment of Figure 1-3, and indeed analogous items bear like reference numerals.

In Figure 4 the permanent magnet rotor 18 is fixed on a plastics carrier having an input spindle pcrtion 31 rotating in sintered self-lubricating bearing 16, a magnet attachment portion 32 of greater diameter, and an output drive portion 8 if required, threading a thrust washer 1 7. The electrical output terminals 9 and 10 are not integral with the coil former 24 but are separate items held projecting through apertures 35, 36 in lower body portion 3. The former 24 as before has one or more apertures 30 by which it is held on one magnet pole-piece of e.g. the longer magnet comb 33. The coil is terminated at terminals 38 and 39 carried by the coil former, and these have soldered connections, not shown, to instrument output terminals 9 and 10.

The forms of the longer and shorter magnetic soft iron combs 33 and 32 (which again sandwich the coil 23 and former 24) can be more clearly appreciated from Figure 4, as can the locating grooves 34 in the interior wall of the lower housing 3. The lower housing also has riveted or otherwise fixed therein a soft steel sleeve 37 terminating in a standard threaded union portion 38, for attaching to a cable or engine gearbox casing etc. Sleeve 37 also forms a magnetizable bridge between the spiders 32, 33 and surrounds bearing 1 6.

Both embodiments can be seen to incorporate a rotating annular multi-polar permanent magnet, the poles passing close to interleaved soft steel stator poles of like number. The ac generated may then alternate at e.g. eight times the revolution rate of the rotor. The interleaving of the stator poles may be achieved by magnetic "combs", vertical pole-pieces such as 1 9, 20 projecting from horizontal washer-like soft iron rings 22, 21.

The rings are threaded by the rotor spindle and are located in close contact with, above and below the flanges of the former of the pick-up coil 23. The stator poles are held by multiple vertical grooves within the instrument, e.g. in the casing wall. The coil may be held by an aperture 30 in the former thereof being threaded by one of the stator poles.

The coil of Figure 4 may be replaced by one arranged as in Figure 3.

Some more detail preferences in the above embodiments are; use of low carbon steel for the stator poles, and apertured discs i.e. for the magnetic spiders 32, 33 as embodied; spacings of about 0.75-1.00 mm between the rotor and stator poles; the aperture 30 in the coil former being a close fit on the width of the accommodated pole piece and just less than the distance between the two terminating leads of the coil; the use of Bowden-type cable mountings; the use of sintered bronze e.g. 30% oil saturated by volume for plain bearing 16; also the insertion for bearing 1 6 inside a magnetizable mild steel jacket e.g. 37 in Figure 4 in order both to strengthen the bearing and to provide a component of the lowreluctance magnetic circuit, which magnetically bridges the two ring portions 21 and 22.

Other changes will readily occur to the skilled man and be appreciated as within the scope and spirit of the invention.

These embodiments deliver an approximately sinusoidal output wave, which is preferably squared for many purposes. Such signal processing, typicaliy involving symmetrical clipping or voltage limiting, is commonplace and needs no further description herein.

Claims (9)

1. A rotational speed sensor comprising a housing for mounting on a rigid or flexible nonrotating base, an input spindle adapted or standardized for responding to a shaft rotating in relation to said base, a magnetic rotor rigidly rotatable with said input spindle in said housing, said rotor being permanently magnetized to present poles of magnetic polarity alternating around its periphery, a number of first magnetizable pole pieces located in said housing in proximity to the periphery of said rotor and magnetically commoned to each other, a like number of commoned together second magnetizable pole pieces being also located in said housing and in proximity to the rotor periphery, and being interleaved with the first magnetizable pole pieces, a pick-up coil arranged to magnetically interact with the pole-pieces, and means to terminate the coil externally of housing.

2. A rotational speed sensor according to claim 1 comprising first and second axially spaced magnetizable rings threaded by a non-magnetic extension of the rotor, the peripheries of the rings having axial attachments to the respective first and second pole pieces, the pick-up coil also being threaded by said non-magnetic extension of the rotor and being sandwiched by the first and second rings.

3. A rotational speed sensor according to claim 2 comprising a magnetizable cylinder surrounding said non-magnetizable extension of the rotor and threading the two rings for their magnetic bridging.

4. A rotational speed sensor according to claim 3 comprising a bearing for the rotor having said magnetizable cylinder as a jacket.

5. A rotational speed sensor according to claim 1 comprising upper and lower plastics housing shells, resilient snap engagement means around the peripheries of said shells, and a gasket conforming to said peripheries to enable their fluid tight snap engagement.

6. A rotational speed sensor according to claim 5 comprising recesses parallel to the axis in the lower housing shell for receiving and locating said pole pieces against rotations by magnetic pull.

7. A rotational speed sensor according to claim 6 comprising a former on which the coil is wound, said former being apertured at a region of small magnetic interaction to receive one of the magnetic pole pieces, thus to be anchored thereby.

8. A rotational speed sensor comprising upper and lower snapped together housing shells, a vertical magnetic rotor assembly, said assembly comprising a permanent magnet annulus having 2N alternating magnet poles, a non-magnetic input spindle, and an end portion shaped to be driven from an output shaft or cable inner drive, said lower housing having attachable means for a drive box or a cable, a rotor bearing for the nonmagnetic spindie, a mild steel jacket for the bearing, first and second magnetizable combs non-rotatably located in the lower housing so that N pole piece portions of each of the combs alternate at the same spatial frequency as the permanent magnet rotor poles and interact therewith, ring portions of the combs being axially spaced and closely surrounding said mild steel jacket, an apertured coil former having one aperture surrounding said mild steel jacket and having another aperture threaded by and anchored by one pole piece of the lower of the two ring portions, a pick-up coil mounted on the former and terminated outside the housing, and a utilization device for the picked-up alternating signal, connected to the termination.

9. A rotational speed sensor substantially as hereinbefore described with reference to Figures 1,2 and 3; or to Figure 4.