Optimal torque and stiffness control in compliantly actuated robots

Anthropomorphic robots that aim to approach human performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators, used to drive many of these devices, are capable of modulating torque and passive impedance (stiffness and/or damping) simultaneously and independently. Here, we propose a framework for simultaneous optimisation of torque and impedance (stiffness) profiles in order to optimise task performance, tuned to the complex hardware and incorporating real-world constraints. Simulation and hardware experiments validate the viability of this approach to complex, state dependent constraints and demonstrate task performance benefits of optimal temporal impedance modulation.

Authors

• David J. Braun
• Florian Petit
• Felix Huber
• Sami Haddadin
• Patrick van der Smagt
• Alin Albu-Schäffer
• Sethu Vijayakumar

Keywords

• Actuators
• Torque
• Joints
• Bandwidth
• Modulation
• Robots
• Impedance
• Optimal Control
• Torque Control
• Stiffness Control
• Optimal Stiffness
• Optimal Torque
• Task Performance
• Damping
• Profiles In Order
• Control Input
• Control Problem
• Stiffness Matrix
• Limited Bandwidth
• State Trajectories
• Variable Stiffness
• Hard Constraints
• Soft Constraints
• Motor Position
• Control Constraints
• Servo Control
• Ball Throw
• Deformation Limit
• Pontryagin Maximum Principle
• Nonlinear Optimal Control
• Canonical State
• Critical Damping
• Main Motor
• Spring Length
• Actuator Limits
• Robot Joint
• Trajectory Optimization
• Variable-stiffness actuation
• physical constraints