Electrical generator or motor structure – Dynamoelectric – Linear
Reexamination Certificate
2002-07-02
2004-08-03
Lam, Thanh (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Linear
Reexamination Certificate
active
06770988
ABSTRACT:
The present invention relates generally to a linear electromagnetic machine and particularly, although not exclusively, to apparatus for producing linear motion.
Although such machines are frequently referred to as “linear actuators” the present invention also comprehends a machine which, although capable of motion, is adapted to maintain a fixed position against a varying force.
A number of designs of linear electromagnetic actuator, (sometimes called a linear motor), are known. Various configurations of previously known machines are described in WO93/01646, which relates to a linear electromagnetic device having axial symmetry and formed as a piston-in-cylinder machine. The principal advantage of that form of construction is that the strong attractive forces between the magnetic elements of the stator and the magnetic elements of the armature are balanced about a central axis, so that the bearings of the machine do not need to withstand resultant magnetically created forces, as is the case with a unilateral so-called linear motor.
A further advantage of an axially-symmetric construction is that the magnetic fields within the machine are contained within an outer casing (which may be steel or other suitable ferromagnetic material) and they can be so arranged as to intersect electrical coils of the machine with a high degree of efficiency. Moreover, in an axially-symmetric construction of cylindrical form it is convenient and practical for the moving member, or armature, to carry a sliding seal between it and the stationary member, or stator of the actuator so as to be able to contain a volume of fluid, at least to one side of the armature, which can act as a supplementary fluid actuator or damper.
For many purposes the most appropriate form of such a machine is that using a permanent-magnet armature and a surrounding stator carrying coils through which electrical currents pass. Preferably the electrical currents are controlled in magnitude and sign by an electronic control system. This configuration allows the heat generated in the windings to pass easily to the external environment and it simplifies the electrical connections to the control system. Important design parameters of such machines are the conformation of the magnetic fields of the armature and the stator, and the choice of the most suitable diameter of the machine for a given stroke and thrust rating.
The thrust generated by an electromagnetic machine may be described by:
T=B*I*L
where T is the thrust in Newtons, B is the flux density in Tesla, I is the current in the wire in Amps and L is the length of the wire in Meters. It follows directly from this relationship that the volume of copper that must be intersected by the magnetic flux is given by
V=T
/(
B*d
)
where V is the volume in cubic meters and d is the current density in the wire in amps per square meter. It also follows that the electrical power required to produce this thrust when the machine is stationary is given by
W=T*r*d/B
where W is the power in Watts and r is the resistivity in ohm meters.
It will be appreciated, therefore, that the performance of an actuator can be improved by ensuring that the magnetic flux is concentrated as much as possible in the regions where it intersects the coils.
According to one aspect of the present invention, therefore, there is provided a linear electromagnetic machine for generating a force acting between two members relatively movable along an axis, one of which is capable of producing a magnetic field the direction of at least part of which extends substantially radially of the said axis and the other of which comprises or includes a conductor in which an electric current can flow substantially perpendicularly with respect to the said magnetic field over at least part of its path whereby to generate a force between the said two members substantially parallel to the said axis, characterised in that the said means for generating the radial magnetic field comprises a permanent magnet orientated with its direction of magnetisation substantially parallel to the said axis associated with pole pieces for its north and south poles from circumferential surfaces of which the generally radial magnetic field extends.
Early forms of linear electromagnetic machine were of the “ironless” type. That is to say, the copper windings were located directly in the air gap of the magnetic circuit. The air gap had therefore to be large in order to contain, in an acceptable length, the minimum copper volume necessary to produce the required thrust. In order to force the flux across such a large air gap the magnetic length of each associated permanent magnet was also large, so that the volume, weight and cost of the permanent magnetic material was significant. In embodiments of the present invention it is preferred that the axial length of the polepieces should be kept as short as possible in order to maximise the flux density. However, for reasons which will become clearer below, the axial length of the magnets should preferably be less than, and in any event, not substantially greater than the axial length of the polepieces. In attempting to minimise the polepiece axial length, however, a constraint is imposed by the condition that the radial thickness of the air gap (including the copper in an ironless machine) must be approximately half the length of the magnets in order for these to operate at maximum efficiency. There is a limit to the radial “thickness” to which the coils can be reduced and this has to be balanced against the need to minimise the axial length of the polepieces. A particularly effective compromise is achieved when the axial length of the or each magnets, the axial length of each polepiece and the radial “thickness” of the air gap are all substantially the same.
In one embodiment of the present invention, the stator of the linear electromagnetic machine surrounds the armature, and has conductor coils housed within circumferential slots in a ferromagnetic stator body. The hoop resistance of the ferromagnetic stator body, that is the circumferential resistance with respect to induced electrical currents, is increased by means of at least one axially extending channel the depth of which is less than the thickness of the material of the stator body. Preferably the circumferential slots have a symmetrical cyclic contour in the axial direction so as to maintain a constant radial reluctance. The armature is preferably close-fitting to the stator and provided with suitable bearings.
In another aspect the present invention provides a linear electromagnetic machine according to any preceding claims, characterised in that the stator is located radially within the armature and the said ferromagnetic sleeve is located on the radially outer surface of the armature.
The axial spacing, electrical connections and impedances of the coils are so adapted as to permit the machine to be controlled by a suitable electronic drive unit as will be described in more detail below. The means for producing a radial magnetic field may comprise plane discs or rings of magnetic material permanently magnetised in a direction orthogonal to their flat surfaces. The rings are assembled in series along the central axis of the machine with like poles facing one another and having ferromagnetic polepieces interposed so as to concentrate the flux from the opposing magnets and to conduct that flux in a desired radial direction towards the electrical coils. A sleeve of radially-magnetised magnetic material may be positioned at the opposite radial periphery of the magnet rings and polepieces so as to oppose the conduction of the flux in the unwanted direction, there being a ferromagnetic sleeve fitted to said radially-magnetised material on the side thereof remote from the said rings and polepieces.
In another aspect the present invention provides a linear electromagnetic machine according to any preceding claim, characterised in that the coils follow circilinear or rectilinear axially sinuous path circumferentially around the said axi
Advanced Motion Technologies Inc.
Darby & Darby
Jones Judson H.
Lam Thanh
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