Limited angle rotary actuator

Abstract: There is disclosed a limited angle rotary actuator comprising: an arcuate stator mountable on a platform, the stator being of predetermined length and predetermined radius so as to define a limited angle of rotation, the stator having a plurality of poles along its arcuate length which are energisable to alternate their polarity; and a rotor mountable on an element to be rotated at a position remote from the centre of rotation of the element, the rotor comprising a number of magnetic poles.

Description: THIS INVENTION relates to rotary actuators and more particularly to a limited angle rotary actuator and to a means of increasing the torque output of a rotary actuator. Rotary actuators are needed in many engineering applications, particularly in the automotive field. Rotary actuation is usually accomplished using a geared motor. There are numerous disadvantages in using geared motors such as the undesirable noise which is produced by the gears, the number of component parts required by such a motor leading to unreliability and the need for frequent servicing, and the increase in axial length caused by the need to include a transmission between the motor and gear box and the part to be rotated. Thus, it will be appreciated that geared motors are noisy, unreliable and bulky. Stepper motors provide an effective means for converting digital signals into precise mechanical rotary movement. Rotary movement in discrete steps allows precise control over moving parts but the use of a stepper motor to carry out a precise actuation of a rotatable part over a limited angle would be undesirable given the cost and complexity of stepper motors. Lack of torque is a common problem in stepper motors. JP 58 108960 (see also Patent Abstracts of Japan Volume 007,No. 218 (E-200), 28 September 1983) discloses a limited angle actuator where the limited angle of rotation is defined by the predetermined length of the stator. The article "Permanent magnet brushless drives using claw armatures", by G. W. McLean, and H. J. Al-Abadi, (Int. Conf. On Electrical Machines, Vol. II, E4/5-1, 13 September 1978) discloses a linear actuator and rotary actuator which each comprise a stator of predetermined length, a plurality of poles which are controllably energisable to alternate their polarity and drive rotors located on either side of the stator, the stator poles on the one and other side being adjacent to the rotor poles of the one and other rotor side respectively. It is an object of the present invention to provide a low cost limited angle rotary actuator which does not require gearing and which avoids the use of a conventional stepper motor. Accordingly, one aspect of the present invention provides a limited angle rotary actuator comprising: a rotor mounted on an elongate arm to be rotated at a position remote from the centre of rotation of the elongate arm, the rotor comprising a first and a second rotor part, each of the rotor parts comprising a number of magnetic poles; and an arcuate stator mounted on a platform, the stator being of predetermined length and having a predetermined radius so as to define a limited angle of rotation, the stator having a plurality of poles along its arcuate length which are controllably energisable to alternate their polarity and drive the rotor, wherein the stator is located between the first and the second rotor parts and the stator poles on one side of the stator are adjacent the poles of the first rotor part and the stator poles on the other side of the stator are adjacent the poles of the second part, characterised in that the stator comprises: an arcuate core; a winding wound around the core; and two arcuate stator plates each having two rows of pole teeth which define a C-section, the two stator plates interlocking with one another to sandwich the core and the winding. Conveniently, each rotor part comprises a permanent magnet which is magnetised into strips of poles of alternate magnetic polarity. Advantageously, each rotor part comprises a plurality of discrete permanent magnets having poles of alternate magnetic polarity. Preferably, each rotor part is part of an element to be rotated and is located at a position remote from the centre of rotation of the element. Conveniently, the stator is mounted to a fixed platform. Advantageously, the stator is energisable to provide alternating poles in accordance with a conventional 2 or 4 phase energisation sequence. Preferably, the pitch of the poles of each rotor part is defined by a desired step angle. Advantageously, the stator has a first set of poles on its inner radius and a second set of poles on its outer radius. Conveniently, the poles of each respective rotor part are located radially adjacent the poles of the stator, the profiles of the stator and the adjacent surface of each respective rotor part conforming with one another to minimise any air gap. Advantageously, the stator has a first set of poles on one side of the stator, which side extends between the stator inner radius and the stator outer radius, and a second set of poles on an opposite side of the stator, which side extends between the stator inner radius and the stator outer radius. Conveniently, the poles of each respective rotor part are located immediately adjacent the sides of the stator carrying the stator poles, the profiles of the sides of the stator and the adjacent surface of each respective rotor part conforming with one another to minimise any air gap. Advantageously, the stator comprises a pair of stators. In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic perspective view of a valve system incorporating an actuator embodying the present invention, parts of the valve system being shown in cutaway and parts of the actuator being shown in phantom; FIGURE 2 is a perspective cross-sectional view of a mid-section of a stator for use with the actuator of Figure 1; FIGURE 3 is a plan view of a stator part for use with the actuator of Figure 1; FIGURE 4 is a perspective view of the stator part of Figure 3 after a folding step; FIGURE 5 is a cross-section through the stator part of Figure 4 along line V-V; FIGURE 6 is a partial perspective view of a mid-section of two stator parts shown in Figure 4 interlocked with one another; FIGURE 7 is a perspective view of a rotor for use with the actuator of Figure 1; FIGURE 8 is a schematic perspective view of an actuator embodying the present invention; and FIGURE 9 is a schematic perspective view of another actuator embodying the present invention. Referring to Figure 1, an actuator 1 embodying the present invention is shown attached to a valve system 2. The valve system 2 is purely an illustration to show one use for the actuator of the present invention. In the present example, the valve system 2 is part of an air-moving conduit 3. Air flows along the conduit 3 and through an aperture 4 formed in a damper wall 5. A valve plate 6 is pivotally mounted to the conduit 3 on the downstream side of the damper wall 5 by a shaft 6A. The valve plate 6 is movable between a fully open position which allows the free flow of air through the aperture 4 and a fully closed position in which the valve plate 6 is sealed to the aperture 4 by a seal 7 provided on the valve plate 6. The actuator 2 comprises two main parts, a stator 8 and a rotor 9. The stator 8 is mounted on a fixed platform comprising a side wall of the conduit 3. The rotor 9 is formed as part of the valve plate 6, i.e. the element to be rotatably actuated. Preferably, the stator 8 is integrated in the conduit wall structure. The stator 8 and rotor 9 are only shown schematically in Figure 1 and are described in greater detail below. The stator 8 defines the predetermined limited angle through which the element to be rotated, the valve plate 6 in this example, may be rotated. Thus, the stator 8 has a predetermined length and a predetermined radius. In the example shown in Figure 1, the limited angle through which the valve plate 6 is rotatable is substantially 90 degrees. The stator 8 is manufactured as a pair of stators 8A,8B only one of which is shown in Figure 2. Each stator 8 comprises four parts: two stator plates 10 formed from plates of electrical steel to provide a housing for the stator and to provide the stator magnetic poles; a rectangular arcuate core 11; and a winding 12. Turning firstly to the stator plates 10, an example of one of these is shown in Figures 3, 4 and 5. Each stator plate 10 comprises an arcuate strip 13 formed with teeth 14 projecting radially from both the inner radius of the strip 13 and the outer radius of the strip. The teeth 14 are evenly spaced along the arcuate length of the strip 13. The pitch of the teeth in the assembled double stator is twice the step angle. In order to maintain the same number of teeth 14 on the inner radius of each stator plate 10 as there are on the outer radius, it is necessary for the teeth on the inner radius to be slightly narrower than the teeth on the outer radius and likewise for the gap between the teeth on the inner radius to be slightly smaller than the gap between the teeth on the outer radius. As shown in Figure 4, the teeth 14 are all folded so as to lie at 90 degrees to the plane of the plate 13 thereby defining an arcuate channel having a C-shaped section as shown in Figure 5 which is a section along the line V-V through Figure 4. Two stator plates 10 formed as described above can therefore be interlocked with one another as shown in Figure 2 to form an enclosed channel of rectangular section. The core 11 is elongate and arcuate so as to conform to the arcuate profile of the stator plates 10. The core is wound with numerous turns of wire 12 to produce a wound core which is inserted in the C-section of one of the stator plates 10. The other stator plate 10 is interlocked with the first stator plate 10 to sandwich the wound core 11,12 between the plates. The two pairs of interlocked teeth on the inner radius and outer radius of the stator 8 comprise two sets of poles for the stator 8. Two such stators 8A,8B which are substantially identical to one another are fixed adjacent one another as shown in Figure 6 to provide a double stator assembly in the form of an arc. The two stators 8A,8B are staggered or displaced circumferentially with respect to one another by half of one pole pitch as shown in Figure 6 by the phantom centre lines, the two outer centre lines being along the centres of two teeth 14 in one stator 8A and the inner centre line being along the centre of one tooth 14 in the other stator 8B. The winding 12 within the stators 8A,8B is controllably energised to provide alternating north and south poles in accordance with a conventional 2 or 4 phase energisation sequence. Turning now to the rotor 9 which is shown in isolation in Figure 7, the rotor 9 comprises two rotor parts being a pair of spaced apart permanent magnet blocks 15,16 which are linked by a non-magnetic plate 23 to form a C-section. The magnetic blocks 15,16 each comprise an individual rotor. The blocks 15,16 are spaced apart by a distance which is slightly greater than the radial depth of the stator 8. Thus, an upper block 15 fits alongside the outer radius of the stator 8 and a lower block 16 fits alongside the inner radius of the stator 8 thereby sandwiching the stator 8 between the two blocks 15,16. The lower surface 17 of the upper block 15 and the upper surface 18 of the lower block 16 conform to the radius of the stator 8 so that any air gaps between the stator 8 and rotor 9 are minimised, thereby avoiding any unnecessary loss of magnetic flux between the stator 8 and the rotor 9. The inner surfaces 17,18 of the rotor 9 radially adjacent the stator 8 are magnetised into strips 19 of alternate magnetic polarity, i.e. north and south poles. Each pole of the rotor 9 runs across the width of the rotor 9. It should be noted that the width of the rotor 9 is slightly narrower than the combined length of the teeth 14 (of the poles of the stator 8) so that, when the actuator is assembled, the ends of the stator poles protrude slightly past the edges of the rotor. The converse arrangement is also possible. As shown in Figure 1, the rotor 9 is formed as part of the element to be moved, in this example as part of the valve plate 6. Preferably, for torque considerations, the rotor 9 is formed at a location which is most distant from the centre of rotation of the element to be rotated (the valve plate 6) so as to provide the greatest moment arm thereby reducing the force requirements of the actuator 1. The provision of a rotor 9 having two parts 15,16 which couple with the stator poles on respective inner and outer radiuses of the stator 8 provides two advantages. Firstly, the torque of the actuator is doubled due to the coupling doubling between the two rotor parts 15,16 and the stator and secondly, the magnetic thrust experienced between the rotor and the stator is substantially cancelled out since substantially opposite thrusts are experienced by the two rotor parts 15,16. Thus, the only resultant force experienced by the rotor 9 is substantially radial, as required. As previously described, the poles of the stator 8 (which comprises a double stator assembly 8A,8B) are formed along the inner and outer radiuses of the stator 8 so as to allow the poles of the rotor 9 to couple with the stator poles on both the inner radius and outer radius of the stator. The stator 8 and rotor 9 are adapted to co-operate with one another, the top surface 18 of the lower rotor 16 being located immediately radially adjacent the inner radius of the stator 8 and the lower surface 17 of the upper rotor 15 being located immediately radially adjacent the outer radius of the stator 8. The strips 19 of the rotor poles being substantially parallel to the teeth 14 (stator poles) on the stator 8. The resultant actuator 1, shown in Figure 1 as part of a valve system 2, is therefore configured to operate as a stepper motor, the basic step of the motor being defined by the pitch of the stator poles 14 and rotor magnetic pole pitch. Thus, accurate control of the element to be rotated can be effected by sequential energisation of the winding 12. A stepper motor driver circuit (not shown) is required to complete the actuator to allow controllable energisation of the stator winding. The provision of such a circuit falls well within the capabilities of a person of ordinary skill in the art of stepper motors. Likewise, variations on the particular configurations of the rotor, the stator, the operation of the actuator and the energisation sequence for the winding are also well within the capabilities of a person of ordinary skill in the art of stepper motors. It should be appreciated that the actuator of the present invention is not limited in its application to valve systems but is also of use in general engineering applications where precise rotary control over a limited angle is required. In Figure 1, the actuator is shown as being built into the valve and housing assembly. The actuator can, however, be provided as a stand alone unit in its own housing as shown in Figure 8. Only a back plate 20 of the housing is shown in Figure 8. The basic elements of the actuator are identical to those in the previously described embodiment. The stator 8 is fixed to the housing back plate 20. The rotor 9 is mounted at one end of an elongate arm 21. The other end of the arm 21 carries a shaft 22. The shaft 22 is located at the centre of rotation defined by the radius of the stator 8. Actuation of the rotor 9 around the stator 8 causes angular movement of the arm 21 and thence rotation of the shaft 22. Rotation of the shaft 22 is limited to the angle defined by the stator 8. The housing can be hermetically sealed to prevent the ingress of dust or other contaminants. Whilst the stator is described as a double stator assembly, only a single stator is required. The rotor 9 need not be in the form of a pair of rotors 15,16 but could comprise just a single rotor block. The rotor block could be located radially adjacent either the inner radius or the outer radius of the stator 8. The use of a C-section rotor or other form of rotor which serves to sandwich the stator between two rotors (or two parts 15,16 of the rotor 9) is not restricted to the limited angle rotary actuator described above but can also be used in motors of a more conventional design. In a variation on the embodiments described above in which the magnetic fields coupling the rotor 9 and stator 8 together are substantially radially oriented (with respect to the stator radius), embodiments may also be configured in which the coupling magnetic fields are oriented normal to both the radial and tangential directions (with respect to the stator radius). A schematic example of such a configuration is shown in Figure 9, the stator poles being located on the sides of the stator 8 parallel to the radial direction. In this example, the rotor parts 15,16 of the rotor 9 need not be radiused as required in the embodiment shown in Figure 7.

Claims: A limited angle rotary actuator (1) comprising: a rotor (9) mounted on an elongate arm (21) to be rotated at a position remote from the centre of rotation of the elongate arm (21), the rotor (9) comprising a first (15) and a second (16) rotor part, each of the rotor parts (15, 16) comprising a number of magnetic poles; and an arcuate stator (8) mounted on a platform, the stator (8) being of predetermined length and having a predetermined radius so as to define a limited angle of rotation, the stator (8) having a plurality of poles along its arcuate length which are controllably energisable to alternate their polarity and drive the rotor (9), wherein the stator (8) is located between the first and the second rotor parts (15, 16) and the stator poles on one side of the stator (8) are adjacent the poles of the first rotor part (15) and the stator poles on the other side of the stator are adjacent the poles of the second part (16), characterised in that the stator (8) comprises: an arcuate core (11); a winding (12) wound around the core; and two arcuate stator plates (10) each having two rows of pole teeth (14) which define a C-section, the two stator plates (10) interlocking with one another to sandwich the core (11) and the winding (12). An actuator (1) according to Claim 1, wherein each rotor part (15, 16) comprises a permanent magnet which is magnetised into strips (19) of poles of alternate magnetic polarity. An actuator (1) according to Claim 1, wherein each rotor part (15, 16) comprises a plurality of discrete permanent magnets having poles of alternate magnetic polarity. An actuator (1) according to any preceding claim, wherein each rotor part (15, 16) is part of an element (6) to be rotated and is located at a position remote from the centre of rotation of the element (6). An actuator (1) according to any preceding claim, wherein the stator (8) is mounted to a fixed platform. An actuator (1) according to any preceding claim, wherein the stator (8) is energisable to provide alternating poles in accordance with a conventional 2 or 4 phase energisation sequence. An actuator (1) according to any preceding claim, wherein the pitch of the poles of each rotor part (15, 16) is defined by a desired step angle. An actuator (1) according to any preceding claim, wherein the stator (8) has a first set of poles on its inner radius and a second set of poles on its outer radius. An actuator (1) according to Claim 8, wherein the poles of each respective rotor part (15, 16) are located radially adjacent the poles of the stator (8), the profiles of the stator (8) and the adjacent surface of each respective rotor part (15, 16) conforming with one another to minimise any air gap. An actuator (1) according to any one of Claims 1 to 7, wherein the stator (8) has a first set of poles on one side of the stator (8), which side extends between the stator inner radius and the stator outer radius, and a second set of poles on an opposite side of the stator (8), which side extends between the stator inner radius and the stator outer radius. An actuator (1) according to Claim 10, wherein the poles of each respective rotor part (15, 16) are located immediately adjacent the sides of the stator (8) carrying the stator poles, the profiles of the sides of the stator (8) and the adjacent surface of each respective rotor part (15, 16) conforming with one another to minimise any air gap. An actuator (1) according to any preceding claim, wherein the stator (8) comprises a pair of stators (8A, 8B).

Inventors: SADLER GORDON, SADLER, GORDON, Sadler, Gordon, c/o NMB (UK) Ltd.
Applicants: MINEBEA KK, MINEBEA CO., LTD.
Application year: 1999
Main classification: H02K41
IPC codes: H02K33, H02K41, H02K7