Series elasticity for free free-space motion for free

Series elastic actuators are used to significant advantage in many robot designs but have not found their way into the design of haptic devices. We use a pneumatic circuit to realize both a flexible power transmission as well as the elastic element in a series elastic actuator. The pneumatic circuit effectively hides the impedance of a high friction, high mass ball-screw actuator, while a low friction, low mass pneumatic cylinder is used at the end-effector. We offer a comparative study of an impedance-control device, admittance-control device, and a device incorporating a series elastic actuator, investigating both the open-loop and the closed-loop impedance displayed to the user. While all hardware and control designs offer an ability to shape the impedance within their operational bandwidths, the series elastic design has the particular advantage of low impedance (a very compliant spring) outside of that bandwidth. Thus, a haptic device featuring series elastic actuation is capable of providing both the low impedances required in free-space and the high impedance required for rendering stiff virtual walls.