Null Space Exploration for Enhanced Transparency Dissipation in TDPA-based Teleoperation with Redundant Manipulators

In teleoperation, it is fundamental to achieve stability and transparency to accomplish a task successfully. There are several sources of instability, the worst is time delay over the communication channel. To face this instability, Time-Domain Passivity Approch (TDPA) is one of the most studied control techniques: it guarantees stability by passivating the active elements of the system. However, ensuring stability with TDPA largely degrades transparency by introducing artifacts such as position drift and force jittering. In a teleoperation with redundant manipulators, it is possible to decrease these effects by exploiting the null space motion. Accordingly, the redundant Time-Domain Passivity Approach (rTDPA) guarantees stability using redundancy to dissipate energy through null space motion. However, this method does not exploit efficiently the null space because of dissipation that may lead to configurations with poorly usable null space for further dissipation movements.This paper presents a new dissipation strategy that aims to maximize efficiently the dissipated power in null space. The method is based on a new index, called Nullability, which measures the capability of the manipulator to move in null space. The proposed dissipation policy ensures stability while maximizing the Nullability index, thus exploiting efficiently the dissipation in the null space. The proposed method, called Nullability-based rTDPA (NrTDPA), is empirically proven to perform noticeably better than the rTDPA with an experimental set-up made by a leader robot and a follower robot contacting a stiff wall in presence of time delay. In particular, the experiments pointed out the higher efficiency of NrTDPA in the null space dissipation with respect the rTDPA, leading to fewer artifacts in the task space: both the position and force drift errors were diminished by almost an order of magnitude.