In one common configuration, the actuator consists of a cylindrical coil that is free to move axially in air gap. The air gap is formed between a cylindrical center pole and a permanent magnet that surrounds it. A soft iron shell houses both magnet and the pole. Figure 1 depicts this conventional construction.
( Figure 1 )
In some cases the axial length of the coil exceeds that of the magnet, by the amount of coil travel. In other cases the magnet is longer that the coil, by the travel length. The long-coil configuration provides a superior force-to-power ration and dissipates heat better, compared to the short-coil configuration. The short-coil, however, has a lower electrical time constant, smaller mas, and produces less armature reaction. Neither arrangement provides a perfectly linear force-vs-travel characteristic. An armature reaction results from current in the coil and alters the level of flux in air gap. Current through the coil in one direction decreases air gap flux, and current in the opposite direction increases it. Applications calling for a more linear force-vs-position characteristics may use two actuators working in concert. Here, one actuator pulls when the other pushes, and vice versa.
For applications requiring the highest force output in the smallest diameter possible, BEI Kimco offers its patented interleaved magnetic circuity technology. For comparable performance characteristics, this design tends to be longer axially. The interleaved design also contains a considerably less massive magnet assembly. The coil assembly tends to be heavier, because of the increased copper volume attainable in a given diameter.
A significant advantage of the interleaved magnetic circuit voice coil is the lower coil inductance. This characteristic results in an actuator with a very low electrical time constant (te =L/R).
Space-limited and weight-limited applications sometimes call for use of special materials for performance optimization. BEI Kimco’s Finite Element Analysis capability and extensive experience ensure the proper solution to your application requirements.