Identification and Control of a Soft-Robotic Bladder Towards Impedance-Style Haptic Terrain Display

This letter evaluates the capabilities of a soft robotic pneumatic actuator derived from the terrain display haptic device, the "Smart Shoe." The bladder design of the Smart Shoe is upgraded to include a pressure supply and greater output flow capabilities. Forcing characteristics are identified and incorporated as feedforward terms in a sliding-mode tracking controller to provide impedance control. A bench top setup is created to rigorously test this new type of actuator. The bandwidth and stiffness capability are evaluated relative to forces and displacements encountered during human gait. Four force vs. displacement profiles relevant to haptic terrain display are proposed and evaluated. It was found that the actuator could sustain a stiffness similar to a soft-soled shoe on concrete, as well as other terrain (sand, dirt, etc.). Compressions of the bladder done at 20 mm/s, which is similar to the speed of human gait, showed promising results in tracking a desired force trajectory. The results in this letter show this actuator can display haptic terrain trajectories, providing a basis for future wearable haptic terrain display devices.