GB2056073A - DC tachogenerators - Google Patents

Abstract

Arcuate magnetic poles (22, 24) are coaxially arranged with respect to coils (N1 to N4) wound on a toroid (36) rotatable with a shaft (12). The flux from the poles (22, 24) in the air gap between them and the toroid (36) is uniform around the angular extent of each pole, the arrangement being such that each coil pair (N1, N3 and N2, N4) produces one of alternating waveforms (V1, V2) that have flat peaks of identical amplitude and overlapping in time. Switching between the waveforms (V1, V2) is effected by a commutator (34) and brushes (42) during the overlap intervals (X) to produce a substantially ripple-free output voltage (V0) of amplitude proportional to the relative speed of rotation of the poles (22, 24) and toroid (36), i.e. the speed of rotation of the shaft. The poles can alternatively be rotated with the coils stationary, in which case brushless switching and rectification is used. <IMAGE>

Description

SPECIFICATION DC tachogenerators This invention relates to DC tachogenerators, that is to say to tachogenerators operative to produce a DC output signal having an amplitude proportional to a measured speed.

Known DC tachogenerators produce their DC output signals simply by rectifying (generally by means of a commutator) an alternating signal or the like. This leads to the DC output signal having a substantial ripple content, which can be a disadvantage, particularly in certain applications.

According to the present invention there is provided a DC tachogenerator comprising a pair of magnetic poles of opposite polarity each extending part way around an axis of rotation, a plurality of coils spaced from the poles and each toroidally wound with respect to said axis, the coils and poles being relatively rotatable about said axis whereby the coils cut magnetic flux produced by the poles between the poles and coils and generate a plurality of alternating waveforms of like amplitude, and output means to rectify and switch between said waveforms to produce a DC output signal, wherein said magnetic flux is substantially uniform around the angular extent of each pole and the arc occupied by each coil is less than that occupied by each pole whereby said waveforms have flat peaks and troughs, the relationship of said arcs is such that the end of each flat region of each waveform overlaps with a flat region of another waveform, and the output means is operative to switch between the waveforms during such overlaps to produce a continuous DC output signal.

The fact that switching between the waveforms is effected during the above-mentioned overlaps, and that the amplitude of the waveforms switched between are identical, leads in theory to the DC output signal having no ripple component. In practice, of course, the output signal will generally have a small ripple component, though embodiments of the invention can be designed so that the ripple component is considerably smaller than is usually experienced in DC tachogenerators.

In an embodiment of the invention illustrated in the accompanying drawings and disclosed in more detail hereinbelow there is a single pair of magnetic poles and two pairs of coils, the coils of each pair being diametrically opposed with respect to the axis of rotation and each coil pair being connected in series to produce one of a pair of said waveforms. However, none of these features is limiting. There may, for example, be a further pair of magnetic poles. When, as in the illustrated embodiment, the tachogenerator is of a generally disc-like form rather than of a drum-like form, the further pair of magnetic poles may be disposed on the opposite side of the coils to the first-mentioned pair.

As just mentioned, the illustrated embodiment is of a generally disc-like form. Both a first structure comprising the coils and a second structure comprising the poles are generally disclike and are spaced apart along the axis of rotation. Alternatively, however, as will be more fully described hereinbelow, the tachogenerator may be embodied in a generally drum-like form, with a structure comprising the magnetic poles and a structure comprising the coils being spaced apart radially of the axis of rotation.

In the illustrated embodiment the output means comprises a commutator and brush arrangement.

The commutator and coils form part of an armature assembly which rotates with respect to the magnetic field producing means, which is stationary. However, the rectifying and switching could be performed in a brushless manner, in which case the coils would be stationary and the magnetic field producing means would rotate.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is an axial section through a DC tachogenerator constituting the above-mentioned embodiment of the invention; Figure 2 is an end view of a magnet assembly of the tachogenerator of Figure 1, as viewed from the right in Figure 1; Figure 3 is an end view of an armature of the tachogenerator of Figure 1; and Figures 4A to 4C show various waveforms present in the tachogenerator of Figure 1.

Figure 1 shows a DC tachogenerator embodying the invention fitted to a motor 10 having an output shaft 12 to measure the speed of rotation thereof. The tachogenerator includes a housing 14 that is fitted to the motor 10 by means of an adapter plate 1 6.

Within the housing 14, the tachogenerator includes a generally disc-like stator or stationary permanent magnet assembly 1 8 comprising a toroidal member 20 of magnetic material and, secured to the member 20, a pair of magnetic poles or magnets 22, 24 of opposite polarity. The poles 22, 24 may be formed from any suitable permanent magnet material. As can best be seen from Figure 2, each pole 22, 24 extends part of the way round an axis 0--0' of rotation of the tachogenerator, which is coincident with that of the shaft 12. The arc 0 occupied by each pole 22, 24 is hereinafter referred to as the 'pole arc'.

Spaced from and confronting the permanent magnet assembly 1 8 is an armature assembly or rotor 30 which is rotatable with the shaft 12. The assembly 30 is generally disc-like and is spaced from the assembly 18 along the axis 0--0'. The armature assembly 30 includes a body member 32 that mounts a commutator 34 at one end thereof and a toroidal armature 36 at the other end thereof. The armature 36 is of a magnetic material, for example, as shown in Figure 1, comprising a spiral of ferromagnetic laminations.

As can best be seen from Figure 3, the armature 36 has four like coils N1, N2, N3 and N4 wound toroidally thereon. The arc ru occupied by each coil is hereinafter referred to as the 'coil span'. It is desirable to make the coil span a as large as possible to accommodate as many turns as possible. Further, the angle (6 - a) should be greater than 90 , for the reason explained below.

Accordingly, the pole arc 0 is desirably as near as possible to 1800, without excessive leakage being incurred, bearing in mind that as the pole arc approaches 1800 there may be significant flux leakage between the poles which can affect the flatness of the output waveforms (see below) in the overlap regions X (see below).

The tachogenerator further comprises a pair of diametrically opposed brush assemblies 40. Each brush assembly 40 comprises a brush 42 which is spring urged against the commutator 34.

The tachogenerator operates in the following manner. Magnetic flux produced by the permanent magnet assembly 18 leaves the magnetic north pole 22 and travels axially to the toroidal armature 36 across the air gap between them. The flux then flows circumferentially around the armature 36 and then axially from the armature to the magnetic south pole 24. The flux path is shown in Figure 1 by dotted lines.

The flux in the air gap between each pole 22, 24 and the armature assembly 30 is substantially uniform around the angular extent of each pole, i.e. circumferentially of the axis 0--0'.

On rotation of the shaft 12 the abovementioned magnetic flux is cut by the coils N1 to N4. The coils N, and N3 are connected in series with each other, as also are the coils N2 and N4.

This leads to the generation in the coil pairs of respective waveforms V1, V2 shown in Figure 4A and Figure 4B, respectively, the two waveforms being in phase-quadrature relationship, that is to say phase displaced by 900.

As will be apparent, when each pair of opposed coils N1, N3 or N2, N4 is entirely within the pole arcs of the opposed pair of poles 22, 24, the output voltage induced across the pair of coils is of constant amplitude (provided the shaft 12 is not changing speed) since the magnetic flux in the portion of the air gap adjacent each pole is substantially uniform circumferentially of the pole arc. The amplitude is proportional to the speed of rotation of the shaft 12. This explains why the waveforms V, and V2 have, as shown in Figures 4A and 4B, flat peaks and troughs. Such flat regions, which are of angular extent (0 - a), are separated by linear regions where the waveform changes polarity as the coil pairs traverse the space between the poles 22, 24.

The values of 0 and a are so chosen that the end of each flat region of each waveform overlaps with a flat region of the other waveform as shown by the intervals designated X in Figure 4. In the present case, to accomplish this, (0 - a) must be greater than 900. The commutation is so arranged as to be completed during the overlap intervals X (i.e. four times per revolution of the shaft 12) to produce a constant output voltage V0 (Figure 4C) between the brushes 42, V, and V2 being represented by full and dotted lines, respectively, in Figure 4C so that the contribution each makes to V0 can be more readily appreciated.That is to say, during a first quarter of a revolution of the shaft 12 the flat peak of the first positive half cycle of the waveform V, from a first of the coil pairs shown in Figure 4A is passed to the brushes 42.

During the overlap interval X at the end of such flat peak, that is to say at the end of the first quarter revolution, the brushes 42 are switched by the commutator 34 to the other coil pair so that the flat peak of the waveform V2 is connected to the brushes 42. During the next overlap interval X, that is to say at the end of the second quarter revolution of the shaft, the brushes 42 are connected back to the first coil pair by the commutators 34, but in the opposite sense, whereby during the third quarter revolution the brushes receive the flat trough of the next, negative half cycle of the waveform produced by the first coil pair after rectification to have the same sense as the voitages produced during the first two quarter revolutions. During the fourth quarter revolution the rectified flat trough of the negative half-cycle of the waveform V2 is supplied to the brushes 42.The commutator 34 continues to switch the voltage applied to the brushes 42 for each quarter revolution in the manner set forth, whereby a continuous DC output voltage V0 is provided. Since the amplitudes of the waveforms V, and V2 are identical and since switching is effected during the overlap intervals X, there is only a small amount of ripple present in the output voltage VO.

The invention can, of course, be embodied in other manners than that described above by way of example.

For example, the tachogenerator could be embodied in a generally drum-like form rather than in the generally disc-like form disclosed above. In this case, a construction using magnets which are radially positioned could be employed and the four armature coils could be toroidally wound on a laminated hollow cylinder. A drumlike version of the tachogenerator would suffer the disadvantage of being longer axially than the above disclosed version, but may have a lower inertia.

It would also be possible to modify the tachogenerator disclosed above to be of a brushless form. In such a device, the magnets would rotate and the coils would be stationary.

The outputs from the stationary coils would be fed to sensing and switching means which would replace the commutator, brushes (and possibly slip rings) of the version disclosed above. In its simplest form the device would be suitable for unidirectional operation only, but with some complexity bidirectional operation could be catered for.

The tachogenerator disclosed above could be modified by the provision of at least one additional pair of magnets. The additional pair of magnets could be disposed on the opposite side of the armature disc, thereby doubling the number of active conductors, but also increasing mechanical complexity.

Claims (5)

1. A DC tachogenerator comprising a pair of magnetic poles of opposite polarity each extending part way around an axis of rotation, a plurality of coils spaced from the poles and each toroidally wound with respect to said axis, the coils and poles being relatively rotatable about said axis whereby the coils cut magnetic flux produced by the poles between the poles and coils and generate a plurality of alternating waveforms of like amplitude, and output means to rectify and switch between said waveforms to produce a DC output signal, wherein said magnetic flux is substantially uniform around the angular extent of each pole and the arc occupied by each coil is less than that occupied by each pole whereby said waveforms have flat peaks and troughs, the relationship of said arcs is such that the end of each flat region of each waveform overlaps with a flat region of another waveform, and the output means is operative to switch between the waveforms during such overlaps to produce a continuous DC output signal.

2. A tachogenerator according to claim 1, comprising a single pair of magnetic poles and two pairs of coils, the coils of each pair being diametrically opposed with respect to the axis of rotation and each coil pair being connected in series to produce one of a pair of said waveforms.

3. A tachogenerator according to claim 1 or claim 2, wherein the coils are toroidally wound on a common armature.

4. A tachogenerator according to claim 1, claim 2 or claim 3, wherein a first structure comprising the coils and a second structure comprising the magnetic poles are each generally disc-like and are spaced apart along the axis of rotation.

5. A DC tachogenerator substantially as herein described with reference to the accompanying drawings.