Damping of Voice Coil Actuators

Voice coil actuators possess several characteristics that are highly desirable to implementing very precise control:

>> The force can be uniform over a significant displacement (several mm up to 10’s of cm)

>> The force developed is precisely proportional to excitation current

>> The actuator itself is almost frictionless (some friction will arise in the bearing system in most applications)

>> They have no commutation or ‘cogging’

>>They can be produced with low moving mass and inertia

In practical applications, one limitation that can arise with actuation based on Voice coil actuator devices is susceptibility to vibration. With negligible friction, moving parts can be excited into resonance, or external forces (such as arise in valves) can excite the system into vibration.

Damping can mitigate the effects of vibration in many systems and is typically implemented in voice coil actuators electromagnetically. The coil is wound on a former of conductive material – a ‘damping ring’ –  which behaves as a short circuited turn. As the coil moves, the currents generated in the damping ring produce a force opposing the movement. This force is proportional to the velocity of the moving coil, so resists rapid movements without friction and without impeding slower, controlled motion.

The influence of damping on behavior can be seen in the video below. The two voice coil actuators shown have identical magnet assemblies and coil windings; the actuator on the left incorporates a damping ring. The video is shot at 250fps to slow the motion 10x. As the devices are alternately energized and de-energized, the voice coil actuator on the right can be seen to bounce as it hits the lower position, and vibration of the flex-circuit termination is clearly seen.

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