first and second pole pieces on a first part of a motor, the first and second pole pieces having respective sets of teeth spaced regularly on respective faces of the first and second pole pieces;

at least one motor coil to at least in part effect motor motion between the first part of a motor and a second part of the motor when the at least one motor coil is driven, the respective sets of teeth of the first and second pole pieces magnetically interacting with corresponding sets of regularly spaced teeth on the second part of the motor when the at least one motor coil is driven;

first and second sensing coils coupled to the first and second pole pieces and arranged to detect changes in magnetic flux coupling between the teeth of the first pole piece and the second part of the motor, on the one hand, and between the teeth of the second pole piece and the second part of the motor, on the other hand; and

third and fourth sensing coils coupled to the first and second pole pieces and arranged to detect changes in magnetic flux coupling between the teeth of the first pole piece and the second part of the motor, on the one hand, and between the teeth of the second pole piece and the second part of the motor, on the other hand;

wherein the teeth on the first and second parts of the motor are arranged to produce a different variation period in the magnetic flux coupled through the third and fourth sensing coils than the magnetic flux coupled through the first and second sensing coils as a function of the relative positioning of the first part of the motor to the second part of the motor.