Actuator

1. A method of making an actuator comprising a plurality of force elements, the method comprising:

providing a coupler for coupling force from the force elements to a load;

calculating a transfer function for each force element wherein the transfer functions allow the force provided to the load to be predicted;

determining an error functional which is indicative of dips in the predicted force;

calculating the parameters of the force elements which minimise the error functional, and

coupling force elements having the calculated parameters to the coupler to make the actuator.

2. A method according to claim 1 , wherein the parameters of the force elements include their location on the coupler and the force applied by each force element.

3. A method according to claim 1 , comprising providing the coupler in the form of a resonant element having a plurality of resonant bending wave modes in the operative frequency including at least one anti-resonance and the error functional is indicative of the presence of the at least one anti-resonance.

4. A method according to claim 3 , comprising coupling the force elements to the resonant element at or near the nodal lines of the at least one anti-resonance of the resonant element.

5. A method according to claim 3 , comprising coupling the force elements to the resonant element at locations which suppress at least one of the second and third modes of the resonant element.

6. A method according to claim 5 , comprising coupling first and second force elements to the resonant element on the nodal lines of the third mode and coupling a third force element to the resonant element on the nodal line of the second mode.

7. A method according to claim 1 , comprising obtaining the transfer functions by measurement.

8. A method according to claim 1 , comprising obtaining the transfer functions by numerical analysis.

9. A method according to claim 1 , comprising obtaining the transfer functions by knowledge of a mathematical expression for the behaviour of the system.

10. An electromagnetic actuator made by the method of claim 1 , the actuator comprising a magnet assembly having a plurality of magnets and a voice coil assembly having a plurality of voice coils which are supported on the coupler and which cooperate with the magnets to form the force elements.

11. An actuator according to claim 10 , wherein the coupler is annular and the voice coils are mounted at relative locations 0.43, 0.56 and 0.69 along the radius of the coupler.

12. An actuator according to claim 10 , wherein the forces applied by each voice coil are 0.05 N for the outer Voice coil, 0.17 N for the centre voice coil and 0.15 N for the inner voice coil.