Friction Compensation and Stiffness Evaluation on a Variable Torsion Stiffness P. Beckerle *, F. Stuhlenmiller, J. Schuy *, J. Wojtusch **, S. Rinderknecht *, and O. v. Stryk ** * Institute for Mechatronic Systems (IMS) — ** Simulation, Systems Optimization and Robotics (SIM) {lastname}@{ims/sim}.tu-darmstadt.de — Technische Universit¨ at Darmstadt, Germany 1 Introduction With safety aspects due to closer human-robot inter- action and increased requirements in energy effiency due to mobile applications, series elastic joint concepts receive high priority in contemporary robotics. Beyond making robots more safe and flexbile, such concepts can store en- ergy and thus optimize the energetic efficiency of the robot’s motion. Therefore adjusting the stiffness is advantageous, since the natural frequency of the drive train and the fre- quency of the desired trajectory can be matched [1, 2]. 2 State of the Art Introduced in the 1990s, the Series Elastic Actua- tor (SEA) [1] and the Mechanical Impedance Adjuster (MIA) [3] pathed the way for series elastic actuation and variable compliance in robotic joints. The majority of concepts developed since then can be categorized in four groups considering the principle of stiffness variation [4]. Those are equilibrium-controlled, antagonistic-controlled, structure-controlled and mechanically controlled stiffness. As the original SEA changes the equilibrium position of a spring, it belongs to the first group. Antagonistic-controlled approaches utilize actuators working against each other as in AMASC [5]. Although both groups allow for stiffness variation, energy is dissipated during operation in both: The equilibrium-controlled solutions require power to simulate a virtual spring while the actuators work against each other in the antagonistically-controlled ones. Thus, many contempo- rary variable stiffness designs belong to structure-controlled and mechanically controlled solutions. Strucutre-controlled devices change stiffness by a modification of an elastic el- ement’s structure as in MIA. Mechanically controlled ones like MACCEPA [2] adjust the system stiffness by preten- sion. 3 Concept and Implementation The authors’ approach based on variable torsion stiffness (VTS) aims at biomechanically inspired robotic joints [6]. As described there and shown in Figure 1, actuator 1 ap- plies an input torque τ i to the torsional elastic element to move the link. For the the adjustment of the torsional stiff- ness k vts (x), the active length x of an torsional elastic el- ement is varied by changing the position of counter bear- Elastic Element Actuator 2 Actuator 1 t o  o x min x max x Counter Bearing l t i  i Figure 1: Concept of variable torsion stiffness [6] ing using actuator 2. Hence, this concept belongs to the structure-controlled group. Since the joint driving and the stiffness control actuator are seperated, the adjustment of the stiffness does not depend on the joint position. For the im- Figure 2: Implementation of the elastic element plementation of the elastic element, a hollow cylinder with outer radius R = 11.0 mm and inner radiues r = 8.7 mm and a length of l = 0.16m is chosen due to [6]. The realized version manufactured from polyamide is presented in Fig- ure 2. A flange on the left connects the element to actu- ator 1, while six evenly distributed fitting rails are used to transmit the torque from the tube to the output side. Due to the deviations in material and geometry compared to the ideal cylinder proposed in [6], the stiffness characteristics might not fit perfectly the requirements given there. Thus, the influence of those deviations is investigated experimen- tally in this paper. To realize the experimental evaluation Figure 3: Test rig for experimental investigations of the stiffness characteristics, a friction compensation is This is a preprint of a paper that appeared in the Proceedings of International Symposium on Adaptive Motion of Animals and Machines, 2013, Darmstadt