VOICE COILS
 

Voice coil motors offer excellent control characteristics where linear actuation is required over short distances with electronic control systems. Comparison of the force characteristics against displacement and against current for voice coil devices and solenoid actuators shows the difference between these devices. The flat force characteristic exhibited by the voice coil motor lends this to applications requiring precise control of force or position, such as control valves or lens and mirror positioning systems, whereas the sharp increase in developed force as the pole faces approach one another in a solenoid device makes these difficult to control.

The voice coil motor can develop force in either direction by reversing the polarity of the excitation. In a solenoid, a spring is typically required to produce force in the return direction. This spring force subtracts from the magnetic force developed, thus reducing force and speed in the energised direction. Combined with low inductance, this makes possible cycle times <1ms in certain cases, typically an order of magnitude faster than solenoid devices.

The behaviour of a voice coil motor can be explained by reference to the classical physics problem of a current-carrying wire supported in a magnetic field. Where the magnetic field strength is B, the current carried by the wire is I, and the length of the portion of wire cut by the field is l (= lower case "L"), a force F is developed according to the equation

   F = B x I x l

The force developed is perpendicular to both the magnetic field and the current flowing in the wire.