Shaft Compliance as a Soft Sensor to Eliminate Stiction in Hybrid Haptic Devices

Haptic devices that rely on electric motors to generate force feedback suffer from an inherent trade-off between performance and stability. A newer class of devices use hybrid actuation with brakes and motors to improve force rendering. However, while brakes are intrinsically stable, they can only oppose an input torque. When the brake and motor are activated at the same time, the brake blocks both the motor’s and the user’s input torque and can create stiction - an unwanted resistance to motion that negatively affects simulation realism. In this paper, we propose the use of shaft compliance as a soft torque sensor in a hybrid actuator integrating a DC motor and a brake. The angular displacement of the compliant element is used in a controller to infer the user’s input torque from readings of a single position encoder. The controller shares a desired torque between each actuator while preventing stiction. Experimental evaluation of the proposed sensor confirms the sensor’s ability to detect torque and the controller’s ability to simulate virtual walls, despite sensor accuracy limitations and nonlinearities. The proposed sensor concept offers a reliable and low-cost alternative for torque sensors in portable haptic devices.