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This paper proposes a novel variable torsion stiffness (VTS) aiming on biomechanical applications like prosthetic knee joints. By varying the effective length of a torsional elastic element via a relocatable counter bearing, the stiffness of a rotational joint is adjusted. This functional concept is described in detail by the authors as well as the design of such VTS joints. Additionally, analytical models for the transfer behaviour of drivetrain and stiffness control are derived. These are used for a simulative evaluation of a pendulum driven by a VTS unit. Based on the results of this simulation, the power requirements of VTS are analysed. Furthermore, an analysis of its structural strength is presented. For practical comprehensibility, the example of the design of a prosthetic knee joint is taken up for several times in this paper. Finally, the concept, modeling and design of VTS as well as the simulation results are concluded and discussed in a final assessment and in comparison to other contemporary concepts.This work was funded by Forum

Introducing compliant actuation to robotic joints is an approach to ensure safety in closer human-machine interaction. Further, the possibility to adjust stiffness can be benificial, considering energy storage and the power consumption required to track certain trajectories. The subject of this paper is the stiffness and position control of the Variable Torsion Stiffness (VTS) actuator for application in compliant robotic joints. For the realization of a variable rotational stiffness, the active length of a torsional elastic element in serial configuration between drive and link is adjusted in VTS. Based on a brief repetition of this basic concept and the deduction of an extended drive train model, this paper gives an advanced power analysis clarifying power-optimal settings from previous basic models and identifying addtional settings that allow for a more versatile operation. Based on these results that can be generalized to other variable elastic actuator concepts, an optimized strategy for setting stiffness is determined considering the whole system dynamics and natural frequencies as well as antiresonance effects. For position control of VTS in a prototypical imlementation, a nonlinear position controller is the designed by means of feedback linearization and the extended model. Adapting the stiffness of the model in the controller provides the posibility to ensure the required tracking performance although the system is modified significantly by changing the drive train stiffness. Further, notes on practical implemenation and a friction compensation are given.

This paper introduces a novel measuring approach for detecting relative movement between stump and socket in lower limb prostheses. The application of the motion capturing based measuring approach is shown at a single male trans-tibial amputee using a Patella Tendon Bearing (PTB) socket. It further investigates and assesses the feasibility of measuring the relative movement between stump and socket during level walking at different velocities and allocating it to the coinciding loads. Representative results for the two translational degrees of freedom in the sagittal plane are presented and discussed. For the proximodistal (pd) direction, a linear correlation between applied load and relative movement is found, while for the anteroposterior (ap) direction the stump movement is largely influenced by the motion sequence during the respective gait event. Additionally, the effect of walking speed is discussed.

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