ELECTROMAGNETIC ACTUATOR AND METHODS OF OPERATION THEREOF

Abstract

A rotary electromagnetic actuator includes a biasing assembly for applying a torque to its rotor. Such an actuator may be used to operate a poppet valve of an internal combustion engine. The rotor defines a cam surface and the biasing assembly includes a cam follower in engagement with the cam surface, and the magnitude of the torque exerted on the rotor by the biasing assembly is dependent on the magnitude of the displacement of the cam follower by the cam surface. The cam surface defines at least one detent for receiving the cam follower.

Claims

1. An electromagnetic actuator comprising: a stator; a rotor which is rotatable relative to the stator over a range of rotation of the rotor; and a biasing assembly for applying a torque to the rotor over at least part of the range of rotation of the rotor, wherein the rotor defines a cam surface and the biasing assembly includes a cam follower in engagement with the cam surface, and the magnitude of the torque exerted on the rotor by the biasing assembly is dependent on the magnitude of the displacement of the cam follower by the cam surface, and wherein the cam surface defines at least one detent for receiving the cam follower.

2. The actuator of claim 1, wherein the cam follower is displaced between minimum and maximum displacement positions over the range of rotation of the rotor, and the displacement of the cam follower when it is in engagement with the detent is greater than the minimum displacement.

3. The actuator of claim 1, wherein the at least one detent comprises portions of the cam surface where its radius of curvature decreases and then increases, when moving along the cam surface in a circumferential direction relative to an axis of rotation of the rotor.

4. The actuator of claim 1, wherein a profile of a part of the cam surface which defines the at least one detent is symmetrical about a line which passes through a point of a profile of minimum radius and the axis of rotation of the rotor, in a plane which is perpendicular to the axis.

5. The actuator of claim 1, wherein a profile of a part of the cam surface which defines the at least one detent is asymmetrical about a line which passes through a point of a profile of minimum radius and the axis of rotation of the rotor, in a plane which is perpendicular to the axis.

6. The actuator of claim 1, wherein the biasing assembly is a mechanical assembly and comprises a resilient mechanical component.

7. The actuator of claim 6, wherein part of the resilient mechanical component forms or is coupled to the cam follower and moves in response to movement of the cam follower.

8. The actuator of claim 7, wherein the resilient mechanical component is a leaf spring.

9. The actuator of claim 1, wherein the cam follower comprises a roller.

10. An internal combustion engine including at least one cylinder having at least one valve and the actuator of claim 1, wherein the actuator is arranged to actuate the at least one valve.

11. A method of operating an electromagnetic actuator comprising: a stator; a rotor which is rotatable relative to the stator over a range of rotation of the rotor; and a biasing assembly for applying a torque to the rotor over at least part of the range of rotation of the rotor, wherein the rotor defines a cam surface and the biasing assembly includes a cam follower in engagement with the cam surface, and the magnitude of the torque exerted on the rotor by the biasing assembly is dependent on the magnitude of the displacement of the cam follower by the cam surface, and wherein the cam surface defines at least one detent for receiving the cam follower, the method comprising the steps of: passing current through the stator so as to cause rotation of the rotor relative to the stator such that the detent moves towards and into engagement with the cam follower, then, after a time delay, passing current through the stator so as to cause rotation of the rotor relative to the stator such that the detent moves away from the cam follower.

12-13. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:

[0037] FIG. 1 is a perspective view of a pair of rotary electromagnetic actuators, with one of the actuators embodying the invention; and

[0038] FIG. 2 is a diagram illustrating a cam surface profile and cam follower according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0039] A rotary electromagnetic actuator 2 embodying the invention is shown in FIG. 1. It includes a rotor 4 which is rotatably mounted in a stator 6. In the embodiment shown, the stator 6 is shared with a second actuator 8. The stator includes eight coils 10 which are evenly circumferentially spaced around the rotor, with respect to the rotational axis 12 of the rotor. In operation of the actuator, a magnetically generated torque is exerted on the rotor by selectively energising the stator windings. The rotor of actuator 8 is omitted for clarity in the drawing.

[0040] A cam surface 14 is formed on the rotor. A cam follower in the form of a roller 16 is in engagement with the cam surface. The cam follower 16 is rotatably mounted at one end of an arm 18. The other end of the arm is rotatably mounted on a shaft 20. Shaft 20 is supported by a bearing housing for the rotor 4. This bearing housing is omitted for clarity in FIG. 1. The exposed part of the shaft 20 is a press fit into a bore in the bearing housing.

[0041] The cam follower 16 is urged into engagement with the cam surface 14 by a biasing assembly 30. This assembly includes a leaf spring 32. The leaf spring is pivotably mounted on the stator 6 at a first end 34. A second, opposite end 36 of the leaf spring bears against the cam follower arm 18, urging it downwardly, towards the cam surface 14. This downward force acts to one side of the rotor axis, rather than towards it, so that it generates a torque around this axis.

[0042] The biasing assembly also includes a constraining member in the form of a locking cylinder 40. The locking cylinder 40 is mounted in use for rotation about its central, longitudinal axis 42, by means not shown in FIG. 1. When the locking cylinder is orientated as shown in FIG. 1, its cylindrical circumferential surface 44 is in engagement with the upper surface of the leaf spring 32. The circumferential surface of the locking cylinder also includes a flattened, planar portion 46, which extends in a plane parallel to the rotational axis 42 of the locking cylinder and perpendicular to a plane containing that axis. This aspect is the subject of a co-pending UK patent application filed by the present applicant.

[0043] The biasing assembly 30 is rotatable between a configuration (as shown in FIG. 1) in which upwards movement of the leaf spring in response to the interaction between the cam surface 14 and cam follower 16 is constrained by the constraining member 40, and a second configuration in which planar portion 46 is facing the leaf spring. In this second configuration, biasing effect of the assembly is reduced or removed entirely, as the upwards movement of the leaf spring is less constrained or unconstrained by the locking cylinder 40 when its second end 36 is moved in response to the interaction between the cam surface 14 and the cam follower 16. This aspect of the actuator is the subject of a co-pending UK patent application.

[0044] The cam surface 14 includes a detent 50. The opposite ends of this section of the cam surface are defined by locations where the profile of the cam surface has a change of curvature. The cam surface profile changes with circumferential position such that, when approaching the detent from either circumferential direction, the radius changes at the edge of the detent from being constant or increasing to decreasing. Continuing along the detent in the same circumferential direction, the radius then increases towards its opposite end. A minimum radius of the detent is therefore defined within the detent. The biasing force on the cam follower towards the cam surface means that when the cam follower is in engagement with the detent, it will tend towards this minimum radius location within the detent.

[0045] In the embodiment of FIG. 1, the position defined by the detent is at a location where spring 30 is deflected by the interaction between the cam surface and the cam follower. Thus, potential energy is stored in the form of strain energy within the leaf spring 30 of the biasing assembly at this point. This energy is therefore available for release at a subsequent time.

[0046] The provision of the detent reduces the level of precision required in the control system for it to be able reliably to select this position. Once the rotor is orientated such that the cam follower is in contact with the section of the cam surface defining the detent, and providing the angular velocity of the rotor is below a predetermined threshold, the rotor will acquire the rotational position defined by the minimum radius within the detent without requiring more precise control by the control system. This enables the rotor to be reliably parked in a predefined rotational position.

[0047] FIG. 2 is a diagram showing a cam surface profile and cam follower in a plane perpendicular to the rotational axis 12 of the rotor.

[0048] The cam follower 16 is shown in contact with a detent 50. The detent 50 is symmetrical about a line 60 which extends from the detent to the axis 12 and lies in a plane perpendicular to that axis.

[0049] The detent 50 is a section of the cam surface 14 which extends between opposite ends 62 and 64. At each end, the radius of the cam surface changes from increasing to decreasing when moving along the cam surface in the circumferential direction towards the detent.

[0050] The cam surface shown in FIG. 2 has a minimum radius r and a maximum radius R. The maximum radius is reached at each of the ends 62, 64 of the detent 50.

[0051] The detent is defined within a portion 66 of the cam surface over which the radius of the cam surface is greater than the minimum value, r. The detent is defined between the points 62 and 64 of maximum radius over portion 66.

[0052] Portion 66 subtends an angle A of about 72 degrees at the central axis 12 of the rotor. To increase the ability to reliably select the detent, the two edges of the detent (that is the outer edges in the circumferential direction) may subtend a substantial angle B at the axis 12 of the rotor. Preferably, this angle is in a range of around 8 to 15 and more preferably this angle is around 11.