Explicit compliance and safety on torque controlled robots for physical interaction
Résumé
This paper introduces a new paradigm of compliant control for torque-controlled robots through the notion of explicit compliance, which enables selective and tunable compliance for each task or joint in a control hierarchy. We reformulate the motion generation quadratic program (QP) to incorporate this explicit compliance model, allowing the robot to adaptively respond to external forces while preserving task priorities. Our formulation also integrates safety and feasibility constraints—such as torque, velocity, and self-collision limits—at the highest level of the control hierarchy. To improve robustness near constraint boundaries, we propose a second-order velocity damper expressed in acceleration, which ensures stable constraint enforcement without dependency on the control loop frequency. In addition, we enhance external force estimation through a lag-free sensor fusion strategy that combines high-frequency force/torque sensor measurements with low-frequency residual-based estimates. This complementary filter achieves accurate external torque estimation across contact scenarios, reducing the RMS estimation error by about 40% from 11.168 N (residual only) to 6.949 N. The proposed framework is deployed on a Kinova Gen3 robot and validated through experiments with various compliance configurations. Using the compliance parameter Γ, we demonstrate three distinct behaviors: full stiffness, null-space compliance, and full-body compliance. Our results show that the proposed approach preserves safety under contact while offering precise task execution and flexible compliance, enabling safe and adaptable physical interaction in dynamic environments.
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