Passive Multi-Task Compliance Control with Strict Priority through Energy Tanks
Erling Tveter, Bjørn Kåre Sæbø, Christian Ott, Kristin Y. Pettersen, Jan Tommy Gravdahl
AI summary
Problem
Classical hierarchical impedance controllers lose passivity due to null-space projections, risking instability during physical contact. Previous energy-tank passivation methods degrade lower-priority task performance and introduce steady-state errors when the tank depletes.
Approach
The method dynamically removes null-space projections when the virtual energy tank runs out, switching to a passive control law that maintains stability without altering task velocity signals.
Key results
- Proven output strict passivity regardless of tank energy levels
- Elimination of steady-state errors in lower-priority tasks upon tank depletion
- Validation through double pendulum simulations and Franka Panda experiments
- Superior lower-priority task performance compared to classical and prior passivation methods
Why it matters
Provides a robust alternative for safe, compliant multi-task robot control in contact-rich scenarios where stability and task accuracy must be balanced.
Abstract
A robot with kinematical redundancy with respect to a main task may perform additional tasks simultaneously with the main one. Often, it is desirable to prioritize the performance of some tasks over that of others. To create a strict priority between the different tasks, meaning the performance of higher-prioritized tasks is unaffected by lower- prioritized tasks, null-space projections are often used. Null- space projections may, however, cause the closed-loop system to lose the desirable passivity property, which is necessary to ensure stable interactions with passive environments. In previous works, an energy tank has therefore been introduced to compensate for the potential activity stemming from the null- space projections. However, if the energy tank becomes empty when using these previous methods, the performance of the lower-prioritized tasks suffers more than when using a classical, non-passive hierarchical control scheme. Thus, a new approach to handling this case is proposed in this work. In the event of the energy tank becoming empty and unable to compensate for any null-space projection-induced activity, the hierarchy is ceded to preserve the passivity of the system, leading to better performance of the lower-prioritized tasks compared to previous passivation schemes. Output strict passivity of the closed-loop system is proven irrespective of the amount of energy available from the energy tank, and the performance of the proposed method is validated and compared to that of a classical hierarchical impedance controller and that of an earlier passivation method through simulation and experiments of redundant robotic manipulators.