The control of kinematically redundant robots is often approached using nullspace projection, which requires precise models and can be computationally challenging. Humans have many more degrees of freedom than are required to accomplish their tasks, but given neuro-mechanical limitations, it seems unlikely that biology relies on precise models or complex computation. An alternative biologically-inspired approach leverages the compositionality of mechanical impedance. In theory, nullspace projection eliminates any conflict between two tasks. In contrast, superposition of task-space impedance and a full-rank joint-space impedance may impose a task conflict. This work compared nullspace projection with impedance superposition during unconstrained motion and forceful physical interaction. In practice, despite their theoretical differences, we did not observe a substantial influence of the nullspace projector weighting matrix. We found that nullspace projection and impedance superposition both resulted in measurable task conflict. Remarkably, when the dimensionality of the nullspace was increased, impedance superposition was comparable to nullspace projection.
To this day, most robots are installed behind safety fences, separated from the human. New use-case scenarios demand for collaborative robots, e.g. to assist the human with physically challenging tasks. These robots are mainly installed in work-environments with limited space, e.g. existing production lines. This brings certain challenges for the control of such robots. The presented work addresses a few of these challenges, namely: stable and safe behaviour in contact scenarios; avoidance of restricted workspace areas; prevention of joint limits in automatic mode and manual guidance. The control approach in this paper extents an Energy-aware Impedance controller by repulsive potential fields in order to comply with Cartesian and joint constraints. The presented controller was verified for a KUKA LBR iiwa 7 R800 in simulation as well as on the real robot.
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