Independently Commanding Size, Shape and Orientation of Robot Endpoint Stiffness in Tele-Impedance by Virtual Ellipsoid Interface

The existing state-of-the-art interfaces for commanding a remote robot's endpoint stiffness ellipsoid in tele-impedance lack the ability to independently control its size, shape and orientation or they are not easily to implement due to the use of physiological signals, such as electromyography, to control the endpoint stiffness. We propose a novel method that can command size, shape and orientation independently and simultaneously through a virtual stiffness ellipsoid generated on a touchscreen device. The human operator controls size, shape and orientation of the virtual ellipsoid using his/her index and thumb fingers of one hand. This virtual ellipsoid is then mapped to the Cartesian stiffness ellipsoid of a remote robot endpoint in real-time. The other hand holds the haptic device to control the pose of the remote robotic arm. Compared to the state-of-the-art methods to control the robot stiffness in tele-impedance, the main advantages of the proposed method are its relatively simple implementation and ability of independent control over various aspects of the robot endpoint stiffness ellipsoid. To provide a proof-of-concept and demonstrate the main features of the proposed approach, we performed several experiments on a tele-impedance setup with a Kuka LBR iiwa robotic arm and a Force Dimension Sigma7 haptic device. We examined two principal types of tasks, in which changing stiffness parameters of the remote robot is important for successful task execution: counteracting external perturbations and establishing contact with unknown objects. The results indicate that our proposed approach can successfully deal with these tasks. A human subject study showed that the touchscreen interface is faster in commanding the desired stiffness compared to another state-of-the-art input method, while showing similar workload ratings.