Voice Coil Actuator Stages

The voice coil actuator is a non-commutated, two terminal limited motion device. It has linear control characteristics, zero hysteresis, zero cogging and infinite position sensitivity. It has low electrical and mechanical time constants and a high output power to weight and volume ratio. It is nearly an ideal servomechanism. A voice coil actuator consists of two members - a coil wound on a non-ferrous base and a permanent magnet field/core assembly. When installed, the coil is situated in the gap between the field and core and one element is mechanically fixed to allow motion along the force (or torque) vector. When coil current flows, force or torque is generated. The magnitude and direction of the current flow determine direction and amplitude.
Voice coil actuators are available in two main configurations:  linear (cylindrical or rectangular) and rotary (cyclindrical or arc segment).

Motor Parameters
Force Constant: KNI N / Ampere-turn
The force developed per Ampere-turn of coil excitation. The force developed by the voice coil is the product of the Force constant, multiplied by the excitation in Ampere-turns.
Linear Range: mm
The range of movement for which the developed force is >90% of the peak force developed by the motor.
Thermal Resistance: Rth ºC / W
The temperature rise of the coil per watt excitation power with the pot assembly mounted on a massive aluminium block.
Max Temperature: Tmax ºC
Maximum permissible coil temperature for the motor.
Continuous Force: F100 N
The peak force developed by the motor when continuously energized at a power level such that the coil attains the maximum temperature when the pot assembly is attached to a massive aluminium block at 20ºC. This may vary slightly for different coil winding options.
Continuous Power Rating: P100 W
The continuous power input which results in the coil reaching maximum rated temperature when fully inserted in the pot,which is attached to a massive aluminium block at 20ºC.

Behaviour
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, 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 to the current flowing in the wire.

Hysteresis
Hysteresis is analogous to backlash in mechanical systems which can give rise to position or force errors. The graph shows how hysteresis is manifested in a positioning system - as current is varied to change the direction of a move it needs to change back significantly to achieve any movement. Hysteresis in solenoid devices can be as great as 10% or more of the developed force, whereas in voice coil motors it is typically <<1% of the developed force. Low hysteresis enables precise and repeatable position control to be realized.