Towards Teleoperation with Human-like Dynamics: Human Use of Elastic Tools

Variable stiffness actuators undergo lower peak force in contacts compared to their rigid counterparts, and are thus safer for human-robot interaction. Furthermore, they can store energy in their elastic element and can release it later to achieve human-like dynamic movements. However, it is not clear how to integrate them in teleoperator systems so that they can be controlled intuitively by a human. We performed an experiment to study human use of elastic tools to determine how a teleoperator system with an elastic slave would need to be designed. For this, we had 13 untrained participants hammer with an elastic tool under different stiffness conditions, asking them to try to find the best timing for a backward-forward swing motion in order to achieve the strongest impact. We found that the participants generally executed the task efficiently after a few trials and they converged to very similar solutions. The stiffness influenced the performance slightly, a stiffness between 2.3 Nm/rad and 4.1 Nm/rad showing the best results. We conclude that humans intuitively know how to efficiently use elastic tools for hammering type tasks. This could facilitate the control of teleoperator systems with elastic slave manipulators for tasks requiring explosive movements like hammering.

Pushing the limits of the CyberGrasp for haptic rendering

The CyberGrasp is a well known dataglove-exoskeleton device combination that allows to render haptic feedback to the human fingers. Its design, however, restricts its usability for teleoperation through a limited control bandwidth and position sensor resolution. Therefore the system is restricted to low achievable contact stiffness and feedback gain magnitudes in haptic rendering. Moreover, the system prohibits simple adaption of its controller implementation. In this paper, the ExHand Box is presented, a newly designed back-end to widen the CyberGrasp's bandwidth restrictions and to open it up for fully customized controller implementations. The ExHand Box provides a new computer, interface electronics and motor controllers for the otherwise unmodified CyberGlove and CyberGrasp hand systems. The loop frequency of the new system can be freely varied up to 2 kHz and custom controllers can be implemented through an automatic code generation interface. System performance identification experiments are presented that demonstrate improved behavior in hard contact situations over a range of sampling periods. Maximum contact stiffnesses of up to 50kN/m in a stable condition are demonstrated, which is significantly higher than what could be achieved with the non-customized original system version. Moreover, a bilateral control experiment is conducted to demonstrate the new system's usability for generic teleoperation research. In this experiment a raycasting algorithm is introduced for pre-contact detection in order to compensate for high delay and jitter communication links between master and slave as they appear in an Ethernet network. It is demonstrated that the contact stiffness can be maintained in the order of magnitude of the system performance identification with a demonstrated stiffness of 41kN/m in a stable condition.