Rendering viscoelasticity with Series Elastic Actuators using cascade control

Rotary Series Elastic Actuators (SEAs) are largely adopted to safely and accurately modulate interaction torques when robots operate in close contact with humans. Torque control is often based on a cascade scheme including PI regulators for a velocity controller nested in a torque controller. This solution is simple, robust and can potentially guarantee coupled stability. A high-level impedance control loop is also commonly added to regulate the behavior of the interaction port as a desired virtual viscoelastic body. In the present work, passivity is analyzed when a cascade controlled SEA is employed to display a virtual parallel spring-damper system. The case of a null desired impedance and of a pure spring are also tackled. The range of renderable mechanical impedance and guidelines for the selection of the control gains are derived.