Comparison of passivity based impedance controllers without torque-feedback for Variable Stiffness Actuators

Making robotic joints compliant in their interaction with the environment is an important objective. Controlling contact forces ensures safety as required for human-robot interaction. One issue is to achieve this along with position tracking for Variable Stiffness Actuators (VSA). Since VSAs are compliant themselves, they can contribute to safety requirements in terms of hardware. Beyond this, impedance controllers can additionally alter system output characteristics in software. In this paper, impedance control of VSAs without torquefeedback is investigated. Therefore, two algorithms, which are both based on the passivity principle but differ in the calculation of the desired actuator trajectory, are compared. Implementation without torque-feedback becomes possible by calculating the torque in the elastic element model-based. Simulation and experiments show good position tracking that is comparable for both algorithms. Experimental investigations of interaction with the environment show a compliant behavior which matches the desired compliance of the system for the first algorithm. Using second algorithm leads to a more complex closed loop system and therefore, the compliant behaviour could not be reproduced exactly.