ELECTRIC MOTOR

Abstract: An electric motor comprising: a magnetic circuit formed by a permanently magnetised element (1) with first and second soft magnetic pole-pieces (2, 3); a magnetic return element (4) providing a path via which flux flows between the pole-pieces (2, 3), the said pole-pieces respectively defining, in conjunction with the magnetic return element (4), first and second radial gaps across which the permanent magnet flux flows; and a system of windings (W1-W4) positioned in the radial gaps and arranged in two groups of two windings. Each group of winding is positioned symmetrically a bout the nominal geometric centre and hence nominal centre of magnetic saliency of a respective one of the first and second pole-pieces.

Description: ELECTRIC MOTOR The present invention relates to an electric motor, for instance ofthe linear electromagnetic type, comprising an integral position measurement system for providing force or thrust in response to an electrical current. The provision of an integral position measurement system greatly enhances the applications of such motors, allowing them to provide information on the output movement produced and to be servo position controlled. Applications for such motors include industrial automation, instrumentation, proportional valve control, vibration testing, pumping and metering. Figure 1 ofthe accompanying drawing shows a linear motor ofthe type particularly relevant to this invention, this being described in a prior GB Patent application 8801924. The main elements ofthe motor are a moving assembly comprising a permanent magnet 1, pole-pieces 2 and 3, split stationary windings Wl and W2, and a soft iron magnetic return circuit formed by an outer shell 4. Magnetic flux due to the permanent magnet 1 is forced to flow radially between the pole-pieces 2, 3 and the outer shell 4 and thus to traverse the windings W 1, W2 substantially at right angles to the direction of current flow in the windings W 1, W2. It can be appreciated that the direction of magnetic flux is opposite for the two pole-pieces 3, 4, these being arranged at the North and South polarity ends ofthe magnet 1. Thus in order for the current flow in the windings W 1, W2 to produce a consistent or net force on the moving assembly it is necessary that the direction of current flow in the two windings W I, W2 surrounding the pole-pieces 3, 4 be mutually opposite. It is for this reason the winding in the motor of figure 1 is split or comprises two separate windings Wl, W2 surrounding the pole-pieces 3, 4. Providing the correct direction of current flow is achieved, the windings W 1, W2 may be series or parallel connected to provide a two terminal connection to the driving source. To make efficient use ofthe permanent magnet 1 and winding material, the windings W I, W2 must be arranged substantially as shown in figure 1. Here the windings are arranged to fill a significant portion ofthe radial gap around each pole-piece 3, 4 and to overlap each pole-piece 3, 4 by a given amount in the axial direction. In this way the flux produced by the permanent magnet 1 is forced to flow across the windings W 1, W2 and to contribute to the production of output thrust. The overlap of windings Wl, W2 is used to encompass leakage or stray flux but is also necessary to allow movement ofthe magnet and pole-piece assembly without substantial change in the amount of flux linked by the windings Wl, W2. For short stroke linear motors of this type it can be seen that the windings are very effectively used in linking the working permanent magnet flux of the machine and thus contributing to an overall high electrodynamic efficiency. Any reduction in the volume ofthe windings or the consistency of flux linking due to the introduction of additional windings or electrodes for the provision of position measurement ofthe moving assembly, will reduce the effective working volume ofthe windings and thus degrade the force output and efficiency ofthe machine. These features ofthe motor make it difficult to use the simple split winding arrangement to perform the additional function of position measurement. In GB Patent application 8801924 the windings are shown series connected, this resulting in a centre connection node. The winding ends are driven, in addition to the thrust producing current, by an AC energising signal, this being applied as anti-phase EMFs impressed on the ends ofthe windings. Here the moving assembly is intended to function as a magnetically salient object, the movement of which produces differential changes in inductance ofthe two windings. This differential inductance change results in an imbalance ofthe impedance ofthe windings and a corresponding output at the centre tap ofthe windings of a fraction of the AC energising voltage. With this arrangement there is a conflict between the requirements for production of optimum output force with high efficiency and for establishing a useable differential inductance to determine movement ofthe motor armature. The symmetrical positioning ofthe windings about the pole-pieces in the axial direction and the necessary overlap ofthe pole-pieces make the inductance change negligible. Without significantly offsetting the windings from the position and configuration for optimum electrodynamic efficiency, the attainable transducer signals are of minimal amplitude. The present invention sets out to provide a motor and winding arrangement in which a useable differential inductance can be achieved whilst maintaining optimum electrodynamic efficiency. According to the invention there is provided an electric motor comprising: a magnetic circuit formed by a permanently magnetised element with first and second soft magnetic pole-pieces; a magnetic return element providing a path via which flux flows between the pole-pieces, the said pole-pieces respectively defining, in conjunction with the magnetic return element, first and second radial gaps across which the permanent magnetic flux flows; and a system of windings positioned in the radial gaps and arranged in two groups of two windings; such that each group is positioned symmetrically about the nominal geometric centre and hence nominal centre of magnetic saliency ofthe first and second pole-piece elements. It will therefore be seen that, by an embodiment ofthe present invention, the above described compromise is avoided altogether. An alternative winding configuration is used to satisfy jointly the requirements of efficient and optimised force output and of effective position measurement. This arrangement takes account ofthe fact that the material comprising the permanent magnet element possesses a relatively low incremental permeability compared to the material ofthe pole-pieces. The second division of windings about the centre ofthe pole-pieces allows this magnetic saliency ofthe individual pole-pieces to cause significant differential inductance change between selected members ofthe winding elements with armature movement. The overall volume and disposition ofthe windings about the pole-pieces is not significantly changed by this arrangement and providing, as is the case, the steady or DC current flow in the windings is arranged to provide consistent force output, the efficiency and thrust output ofthe machine is not degraded. It is thus possible to provide a completely integral motor and position measurement system without compromise of thrust output or of electrodynamic efficiency and without significant compromise of position measurement sensitivity. A further benefit ofthe present invention is that, in addition to providing a combined motor and position measurement function, it may alternatively be employed as a combined linear tachogenerator and position measurement device. Throughout this specification it is intended that the term "motor" should be taken to refer to both a combined linear motor and position sensor or a combined linear tachogenerator and position sensor device. Preferably the magnetic return element is tubular in shape and surrounds the permanent magnetised element and the windings. The permanently magnetised element may be disposed on the axis ofthe cylindrical soft magnetic element. A further pair of windings may be located radially adjacent the said two groups of windings. Each ofthe further windings is preferably located adjacent a respective one ofthe said two groups of windings. The further windings may be located radially outside the said two groups of windings or radially inside the said two groups of windings. In an alternative embodiment, the magnetic return element is surrounded by the permanently magnetised element, which is tubular in shape. The pole-pieces are preferably also tubular in shape and coaxial with the permanently magnetised element. The invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross sectional diagrammatic view of an electric motor and position measurement system in accordance with the prior art; Figure 2 is a cross sectional diagrammatic view of an integral electric motor and position measurement system constituting a first embodiment ofthe invention and Figure 2 A is a schematic, perspective view ofthe coil windings of Figure 2; Figure 3 is a schematic circuit diagram showing a series connection ofthe windings to achieve position measurement and output thrust; Figure 4 is a schematic circuit diagram showing a parallel-series connection ofthe windings to achieve position measurement and output thrust; Figure 5 is a schematic circuit diagram showing a bridge connection ofthe windings to achieve position measurement and output thrust; Figure 6 is a cross sectional diagrammatic view of an integral electric motor and position measurement system constituting a second embodiment ofthe invention and Figure 6A is a schematic, perspective view ofthe coil windings of Figure 6; Figure 7 is a cross sectional diagrammatic view of an integral electric motor and position measurement system constituting a third embodiment ofthe invention, Figure 7 A is a schematic, perspective view ofthe position measurement windings 5 of Figure 7, and Figure 7B is a schematic, perspective view ofthe thrust producing windings of Figure 7. The diagrammatic representation of figure 2 shows a permanent magnet 1 whose poles are extended by pole-pieces of high permeability material 2 and 3 to provide the armature or moving element ofthe assembly. The moving element is cylindrical in shape and is disposed co-axially within a sleeve comprising winding elements Wl and W4. Surrounding the windings and mechanically secured to them is an outer sleeve of high permeability material, 4. Magnetic flux generated by the magnets flows radially between the pole pieces and the surrounding outer sleeve thus intersecting the windings substantially at right angles. The flux generated by the magnet 1 leaves the moving assembly by way of one ofthe pole-pieces and thus enters by the other. Current flow in the windings results in a force acting axially on them with a corresponding thrust generated between the outer winding and sleeve assembly and the moving magnet and pole-piece assembly. In order for the forces generated on the windings to act in unison, the reversal of flux flow must be taken into account in the connection ofthe windings. Windings Wl to W4 are arranged in two groups, each group being symmetrically displaced in the axial direction about the pole-piece elements 2, 3. The inductance of each winding, being determined by the reluctance of its local magnetic circuit, is influenced by the movement ofthe pole-pieces particularly since they are displaced from the geometric centre of each winding. Thus for the movement ofthe magnet and pole-piece assembly shown in figure 2 to the right, it can be seen that the inductance of windings W2 and W4 increases whilst that of windings W 1 and W3 decreases. Figure 2A shows the relative winding directions and terminal arrangements for windings Wl to W4 in the embodiment of Figure 2. Figures 3 to 5 show how the windings are interconnected to provide simultaneous motor action in response to DC current in the windings, and position measurement in response to an impressed AC energising voltage across the windings. Figure 3 shows a series connection ofthe windings to form a three terminal connection. Windings Wl and W3 are series connected in one branch and windings W2 and W4 are in series connected in the other. Anti-phase energising voltages and thrust producing current is impressed on the windings ends from sources e 1, e2 and the position signal e3 appears at the junction. This signal varies in magnitude and phase with respect to the AC component ofthe energising sources e 1, e2 as the motor moving assembly is displaced about its centre position. The relative phase ofthe signal gives the direction of movement and the amplitude represents the magnitude of movement from the centre position. The relative connections ofthe starts and finishes ofthe windings ensure that current flow in the windings gives rise to mutually contributing thrust. Figure 4 shows a parallel-series connection. Here windings W 1, W3 and W2, W4 are grouped into paralleled pairs and then series connected to provide a three terminal connection. Figure 5 shows a bridge arrangement where windings W 1, W2 and windings W3, W4 are connected in series to form the arms of a bridge or 4 terminal arrangement. The two series arms ofthe bridge are then paralleled to provide two terminals for application of driving current and excitation voltage from source e 1, and the position information appears as a differential signal e2 between the two junctions ofthe bridge arms. As for the previous cases of figures 3 and 4 the starts and finishes ofthe windings are arranged to ensure consistent contribution of force from each winding in response to the dc driving current flowing in the windings. Figure 6 depicts an alternative configuration ofthe integral motor and position measurement system. Here the soft iron element 4 is cylindrical in shape and is arranged at the centre ofthe assembly. Windings Wl to W4 are positioned on the outer cylindrical surface ofthe element 4 and are mechanically attached to it. Permanent magnet 1 is now tubular in shape and is arranged with tubular shaped pole-pieces 2, 3 at each end. Flux generated by the permanent magnet flows radially from the pole-pieces to the central cylindrical element 4 thus intersecting the windings at right angles. This function ofthe motor end ofthe position measurement system is identical to that ofthe alternative arrangement described earlier. The interconnections of windings of figures 3 to 5 may also be used with this configuration. Figure 6A shows the relative winding directions and terminal arrangements for windings Wl to W4 in the embodiment of Figure 6. Figure 7 depicts a further alternative configuration ofthe integral motor and position measurement system. Here the windings are separated to perform the function of position measurement and generation of output thrust. Windings W 1 to W4 are the position measurement windings which are positioned about the centre magnet and pole-piece assembly as described previously, with the exception that they share the annular flux gap with a system of thrust producing windings W5, W6. The windings Wl to W4 are shown positioned concentrically inside the winding layer W5, W6, but they could equally well be positioned as the outer layer or be radially interleaved with the thrust windings. It can be appreciated that this configuration of functionally separate windings could be applied also to the alternative arrangement of figure 6. Figures 7A and 7B show the relative winding directions and terminal arrangements for the windings Wl to W4, and W5 and W6 respectively. In addition to the benefit of providing a combined motor and position sensor, it can be appreciated that the invention offers the option for a device capable of combined linear velocity and linear position sensing. This is so because the high degree of electrodynamic efficiency afforded by the configurations described above applies equally to the case of a permanent magnet machine if it is used as a generator or as a motor; that is if the device is used to generate an EMF in response to an applied velocity or to generate an output thrust in response to an applied driving current. It is well known that the constant or factor relating induced EMF to the relative velocity between the coil assembly and the magnet/pole piece assembly in a given configuration of machine (Volts/Metre/Second), is dimensionally and numerically the same as that relating output thrust to applied current (Newtons/Amp), given of course a consistent set of units such as the SI system. Many further modifications and variations will suggest themselves to those versed in the art upon reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the present invention, the scope of which is determined by the appended claims.

Claims: CLAIMS 1. An electric motor comprising: a magnetic circuit formed by a permanently magnetised element with first and second soft magnetic pole-pieces ; a magnetic return element providing a path via which flux flows between the pole-pieces, the said pole-pieces respectively defining, in conjunction with the magnetic return element, first and second radial gaps across which the permanent magnet flux flows; and a system of windings positioned in the radial gaps and arranged in two groups of two windings; such that each group is positioned symmetrically about the nominal geometric centre and hence nominal centre of magnetic saliency of a respective one ofthe first and second pole-pieces. 2. An electric motor according to Claim 1, wherein the magnetic return element is tubular in shape and surrounds the permanently magnetised element and the windings. 3. An electric motor according to Claim 1 or 2, wherein the permanently magnetised element is disposed on the axis ofthe magnetic return element. 4. An electric motor according to Claim 1, wherein the permanently magnetised element is tubular in shape and surrounds the magnetic return element. 5. An electric motor according to Claim 4, wherein the pole-pieces are tubular in shape and coaxial with the permanently magnetised element. 6. An electric motor according to any preceding claim, wherein a further pair of windings is located radially adjacent the said two groups of windings. 7. An electric motor according to Claim 6, wherein each ofthe said further windings is located adjacent a respective one ofthe said two groups of windings. 8. An electric motor according to Claim 6 or 7, wherein the further windings are located radially outside the said two groups of windings. 9. An electric motor according to Claim 6 or 7, wherein the further windings are located radially inside the said two groups of windings. 10. An electric motor substantially as hereinbefore described with reference to Figures 2 and 3; or Figures 2 and 4; or Figures 2 and 5; or Figures 6 and 3; or Figures 6 and 4; or Figures 6 and 5; or Figures 7 and 3; or Figures 7 and 4; or Figures 7 and 5; or Figure 2; or Figure 6; or Figure 7 ofthe accompanying drawings.

Inventors: SIDEY ROGER C H, SIDEY, ROGER, C., H.
Applicants: SIDEY ROGER C H, SIDEY, ROGER, C., H.
Application year: 1996
Main classification: H02K33
IPC codes: H02K11, H02K33, H02K41