Exploiting Inate Motor Dynamics for High Stiffness Haptic Display on Brushless DC Motors

The maximum stiffness that can be rendered by impedance-controlled haptic devices has traditionally been limited by quantization, discretization, and delay in the digital loop. Recent research has shown that performance can be improved by utilizing the natural inductive stiffness inherent in brushed DC motors. This study extends the concept of exploiting a motor’s dynamics to the three phase brushless DC (BLDC) motor. It is analytically shown that the inductances of a BLDC motor’s windings map to a high physical stiffness. This stiffness is made available at frequencies important to haptic interaction by cancelling the resistance in each winding with analog feedback, effectively slowing the motor’s electrical dynamics. Experimental verification is obtained by implementing the proposed spring drive in analog circuitry in combination with a digital position feedback loop. The final results support the analytic solution and compare the spring drive favorably with traditional current and voltage drives for haptic applications.