Control method lets robot arms change stiffness quickly without losing passivity

Researchers led by Huseyin Tugcan Dinc have proposed a way for robot arms to vary their impedance while staying passive, in a paper submitted to Control Engineering Practice. Changing impedance parameters quickly can inject energy into the system and break passivity, the authors write, and current fixes add damping or limit stiffness changes, which often slows the response.

Their method uses quadratic programming to adjust the desired inertia so that it offsets the energy added by changes in stiffness, while keeping the system critically damped. They extend it to compliance control, where the desired inertia is linked to the robot's own inertia matrix, using an eigendecomposition based on Cholesky factorisation.

Experiments with people pushing a Kinova Gen3 arm with seven degrees of freedom validated both versions. Compared with an energy tank approach and a method not based on passivity, the framework tracked stiffness less conservatively without needing an energy reservoir or extra damping, the authors report.