On the Analysis of Stability, Sensitivity and Transparency in Variable Admittance Control for pHRI Enhanced by Virtual Fixtures

The interest in Physical Human-Robot Interaction (pHRI) has significantly increased over the last two decades, thanks to the availability of collaborative robots that ensure user safety during force exchanges. For this reason, stability concerns remain a key issue in the literature whenever new control schemes for pHRI applications are proposed. Due to the nonlinear nature of robots, stability analyses generally rely on passivity concepts and consider ideal models of robot manipulators. Therefore, the first objective of this paper is to conduct a detailed analysis of the sources of instability in proxy-based constrained admittance controllers for pHRI applications, taking into account parasitic effects such as transmission elasticity, motor velocity saturation, and actuation delay. The second objective of this paper is to perform a sensitivity analysis, supported by experimental results, to identify how the control parameters affect the stability of the overall system. Finally, the third objective is to propose an adaptation technique for the proxy parameters, with the goal of maximizing transparency in pHRI. The proposed adaptation method is validated through simulations and experimental tests.