A high bandwidth low inertia motor for haptic rendering based on clutched eddy current effects

Abstract

The capacity of a conventional haptic device to render a mechanical impedance that approximates free space requires actuators with exceptionally low rotor inertia. Reducing rotor inertia in a DC motor design invariably means reducing the peak torque output, which in turn compromises the device's ability to render stiff, massive objects. To meet the large dynamic range requirement in haptic device applications without compromise, we present the design of an actuator that produces high torque on a low inertia rotor through the use of the eddy current braking effect. Eddy currents couple the rotor to a spinning, motorized stator through the action of a torque proportional to relative speed. In our design, permanent magnets are fixed to a two-part stator such that magnetic fields with alternating polarity transect a thin, aluminum disk-shaped rotor. By shifting the relative angular position of the two opposing stator parts under motor control, the magnetic fields traversing the rotor can be modulated between near zero and full intensity. Thus the torque output is effectively clutched under computer control. In this paper we analyze the limits to the speed of response of this design and present characterization results obtained using a prototype device.