Analysis of Coupling Effect in Human-Commanded Stiffness During Bilateral Tele-Impedance

Abstract

Tele-impedance augments classic teleoperation by enabling the human operator to actively command remote robot stiffness in real-time, which is an essential ability to successfully interact with the unstructured and unpredictable environment. However, the literature is missing a study on benefits and drawbacks of different types of stiffness command interfaces used in bilateral tele-impedance. In this article, we introduce a term called coupling effect, which pertains to the coupling between human-commanded stiffness going to the remote robot and force feedback coming from the remote robot. We hypothesize that, whenever the operator's commanded stiffness and force feedback are subject to coupling effect (e.g., muscle activity based stiffness command interfaces), force feedback can invoke involuntary 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. We examined the coupling effect in an experimental study with 16 participants, who performed position and force tracking tasks by using both coupled type (muscle activity based) and decoupled type (external device based) of interface. The results demonstrate a benefit of the coupling effect when the remote robot is operating in presence of unexpected force perturbations, where lower absolute error in position tracking task was observed. On the other hand, the decoupled type of interface is beneficial for force tracking tasks on the remote robot side, such as establishing or maintaining a stable contact with objects. However, the coupling effect negatively influences the commanding of reference stiffness to the remote robot in both position and force tracking tasks for the coupled type of interface, compared to the decoupled type of interface, which is not affected.

A Semi-Autonomous Tele-Impedance Method based on Vision and Voice Interfaces

“…Unlike the state-of-the-art interfaces based on EMG [1], grip force [2] and button [5], the voice-based impedance command interface does not require a limb to operate it and is essentially a hand-free approach. It was shown that the EMGbased interfaces have a coupling effect between between the force feedback (if implemented) and the commanded stiffness; i.e., physical interaction with the limb of the human operator can affect the commanded impedance [26]. Since the proposed method is based on vision and voice, there is no direct physical contact between the human limb and the impedance-command interface and such effect is not present.…”

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

“…A potential disadvantage of the proposed interface is that it could require more attention from the human operator and might therefore be less intuitive compared to some of the existing interfaces. Using EMG [5], [9], EIT [15] or grip-force [11] based interfaces, the human might naturally contract its muscles based on the situation in the remote environment, which in turn would change the remote robot's endpoint stiffness, as shown in [21]. Therefore, the proposed method might be more suitable for application in space or industrial robotics, where changes tend to be slower, while the above-mentioned methods might be more suitable for application in robotic surgery, where quick stiffness changes are crucial.…”

“…Therefore, the ability to command the size, shape and orientation of the stiffness ellipsoid to the robot is limited. In addition, it has been shown that there is a coupling effect between the interaction force at the haptic interface and human commanded stiffness in EMG-based tele-impedance [21]. Furthermore, these approaches typically involve a complex setup (including issues related to wearable devices [22], [23]) and a time-consuming calibration process, which is another key disadvantage in terms of applicability.…”

Human guided trajectory and impedance adaptation for tele-operated physical assistance

“…Besides, the changes in EMG and grip force are not only due to the limb impedance (i.e., to the stiffness and damping parameters) but are person specific, and also due to: limb motion trajectory, body posture and time (fatigue). Therefore, while EMG or grip force may still be good methods to estimate impedance in the absence of perturbations, they require user-specific calibration [21], [22]. On the other hand, in the presence of external perturbations, particularly continuous and non-repetitive ones, EMG and grip force signals include muscle reflexes, which are characterized by their own temporal and state dynamics [22] making impedance estimation non trivial.…”

“…Therefore, while EMG or grip force may still be good methods to estimate impedance in the absence of perturbations, they require user-specific calibration [21], [22]. On the other hand, in the presence of external perturbations, particularly continuous and non-repetitive ones, EMG and grip force signals include muscle reflexes, which are characterized by their own temporal and state dynamics [22] making impedance estimation non trivial.…”