Understanding the basics of a voice coil actuator

A voice coil actuator, also known as a non-commutated DC linear actuator, is a type of direct drive linear motor that gets its name from one of its first applications: vibrating the paper cone of a loudspeaker. It consists of a permanent magnetic field assembly – permanent magnets and ferrous steel – and a coil assembly. Current flows through the coil assembly and interacts with the permanent magnetic field, generating a force vector perpendicular to the direction of the current. The force vector can be reversed by changing current polarity.

A voice coil actuator is capable of a displacement of up to 5", whereas most actuators have displacements up to 2". Voice coils come in various sizes for generating few ounces of force to several hundred pounds. In addition, voice coil actuators can move bi-directionally, have a constant force across the stroke, and can be used for either open-loop applications or closed-loop position or force applications.

Force is based on an interaction of current-carrying conductors in a permanent magnetic field. The generated force is proportional to the cross product of the current flowing through the coil and the magnetic flux in the permanent magnetic field, as dictated by Lorentz’ force equation, F = B x I: Force (F) = Flux density in Teslas (B) x Current in amps (I)

The generated force is relatively constant throughout the stroke of the actuator, with minor decreases at the beginning and end of the stroke. Either the coil assembly or permanent magnetic field assembly can be used as the moving member in a voice coil actuator.

Moving coil

While available in various packages, the standard voice coil actuator is a moving coil actuator. These typically involve a coil wound around a bobbin, made from many non-magnetic materials, that moves in and out of a permanent magnetic field assembly consisting of a steel housing with a concentric permanent magnet assembly in the middle. (See figure 1, above)

Moving magnet

The other common type of actuator is the moving magnet design, where the coil is fixed and the magnet assembly moves. This construction change prevents moving leads during operation. The package operates similarly to moving coil, but instead of an exposed coil moving in and out of the magnet assembly, the moving magnet uses a permanent magnetic field assembly piston moving inside a cylindrical coil tube. This style often comes with the permanent field assembly attached to a shaft and end caps containing bearings, so it is commonly supplied with an integrated bearing system. (See figure 2, above)

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