Shorted turn effects in rotary voice coil actuators

The response of a voice coil motor (VCM) with a moving shorted turn is examined in both the frequency and time domains. The shorted turn modifies the low frequency behavior of both the impedance and the transfer function between coil current and VCM torque; the voltage to VCM torque transfer function is minimally affected. The model is validated with measurements of impedance frequency response functions and agreement is good in all but the lowest frequencies. Seek simulations show that the shorted turn acts like a brake and for a parameter set from an existing disk drive the net effect on the move time and power is small. A criterion based on the torque constants, coil resistances and mechanical time constant of the system is developed to assist in determining when shorted turn effects are large enough to be detrimental to the performance. The use of the moving shorted turn as an electromechanical filter for the current to torque frequency response function is also investigated.