Patent ID: 7227439

Claim:
A short stroke actuator comprising an annular field coil and a composite plunger for relative travel threrethrough, the composite plunger comprising two permanent magnets axially in line but spaced one from the other and with like poles facing, the length of the annular field coil and the corresponding spacing of the permanent magnets being so selected that the thrust experienced in use by the coil, when energised, relative to the plunger by interaction of the magnetic field produced by energization of the coil with the magnetic fields produced by the permanent magnets, is substantially constant over a pre-selected stroke regardless of displacement, the stroke spanning a range of relative displacements, at each end of which range the thrust is produced by interaction of the magnetic field of the coil predominantly with the magnetic field of a respective one of the permanent magnets and, intermediate said ends, the thrust is produced by interaction of the magnetic field of the coil with the magnetic fields of both magnets.