In the case where an actuator is installed in a location where access is limited or where downtimes may be costly, there is a need for redundancy in a voice coil. A redundant voice coil comes with multiple coils and is designed to operate with equal performance in each of coil/magnet pairs. Each coil operates independently of the other and will have little to no effect on the other coil while in operation.
Each coil pair operates as its own voice coil.
Redundancy is a requirement on many flight systems in a special case when one coil faults, the other can be used to make the motor “fault-resistant”.
Other systems require simplified circuitry which would not allow the change of current polarity in order to change the direction, in these cases each coil can be used to drive the coil in a single direction.
The challenge in manufacturing a redundant coil is ensuring the same performance no matter the coil, which requires precise manipulation of the magnetic circuit design, and the precise coil layout to ensure the identical operation and constants for each coil. This will allow the controls to seamlessly switch between the two coils.
There are two ways of accomplishing this coil redundancy. The most common way would be to generate multiple coils operating on individual magnetic circuits. This requires a longer coil and multiple magnetic circuits; however this is the most robust design and the least vulnerable to damage or interference from the redundant coil. The NCC08-18-025-2R2C is an example of the multiple coil design. An alternate method, which allows for the most compact solution, would be a multi-filar wind. This method only requires one magnetic circuit, which is why the length is shorter. In this case multiple coils are wound in the same winding space.