The coupling effect in bilateral tele-impedance

Abstract

Tele-impedance augments classical teleoperation by enabling the human operator to actively command remote robot stiffness. Hereby, an essential strategy used by humans to successfully interact with the unstructured environment complements remote robot-environment interaction. However, literature lacks awareness of benefits and disbenefits of currently used stiffness command interfaces (SCIs) in bilateral tele-impedance. In this paper, we introduce a term called the coupling effect. The coupling effect pertains to the coupling between human initiated commanded stiffness and force-feedback from the master robot. It is hypothesized that, whenever the operator’s commanded stiffness and the master device are coupled, like in muscle activity based SCIs, force-feedback can invoke changes in the commanded stiffness due to human reflexes. Although the coupling effect takes away some degree of the operator’s control over the commanded stiffness, 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. In an experimental study 16 participants perform position and force tracking tasks by using both an EMG based coupled type and an external device based decoupled type of SCI. Our results demonstrate a benefit of the coupling effect by showing lower absolute position error during the unexpected force perturbation when a coupled SCI was is in the position task. The coupling effect does not affect reference stiffness tracking in the force task during the process of establishing contact compared to maintaining reference force when a coupled SCI is used. The decoupled SCI is beneficial for tracking reference force in the force task and tracking reference stiffness for both tasks. We conclude that—when using bilateral tele-impedance—one should be aware of the coupling effect, which is beneficial for rejecting a disturbance in a position tracking task but detrimental for establishing contact in a force tracking task.