The Force-Feedback Coupling Effect in Bilateral Tele-Impedance

In this paper, we introduce and explore a concept called coupling effect, which pertains to the influence of force feedback on the commanded stiffness that is voluntarily controlled by the operator through the stiffness interface during bilateral tele-impedance. The degree of coupling effect depends on the type of interface used to control the impedance of the remote robot. In case of muscle activity based stiffness command interfaces, the force feedback can invoke involuntary changes in the commanded stiffness due to human reflexes. These involuntary changes can be either beneficial (e.g., during position tracking) or detrimental (e.g., during force tracking) to the task performance on the remote robot side. To investigate the coupling effect in different types of stiffness command interfaces (i.e., coupled and decoupled), we conduct an experimental study in which participants are asked to perform position and force tracking tasks. The results show that in both position and force tracking tasks a lower tracking error of the reference stiffness is obtained with a decoupled interface (p<0.001). However, the unexpected force perturbation yields lower absolute position error when using a coupled interface (p=0.0091), which indicates a specific benefit of the coupling effect. Finally, a lower absolute force error is found in the force tracking task by using the decoupled interface (p<0.001), which indicates a specific downside of the coupling effect.