Power-optimized stiffness and nonlinear position control of an actuator with Variable Torsion Stiffness

Abstract

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.

Stiffness Control of Variable Serial Elastic Actuators: Energy Efficiency through Exploitation of Natural Dynamics

“…To this end, power is integrated over the period t = 1 f d to determine the consumed energy per oscillation. The considered parameters of the variable torsion stiffness (VTS) prototype are determined in [13,22,24] and summarized in Table 1. The serial stiffness K s is varied.…”

“…The experimental evaluation is performed using the VTS actuator introduced in [13] and presented in Figure 8. The actuator-gearbox unit in the lower left of the system drives a pendulum load located on the right.…”

“…The rotational inertial parameters of actuator and link are denoted by I l and I a , while m l represents the mass of the pendulum. Stiffness adjustment is performed by Actuator 2 that relocates a counter bearing and thereby changes the effective length q s and hence the stiffness K s of the elastic element [13].…”

“…Besides reducing power consumption [12,13], approaches aim at setting end effector stiffness [14], ensuring user-safety [15], and improving dynamic behavior [16,17]. A simple but efficient class of stiffness controllers rely on PID-type feedback [17,18].…”

“…Yet, for improved energy efficiency, the combination of PID-type feedback with system dynamics models appears promising since it facilitates the exploitation of antiresonance operation [8,13,21]. This paper presents analytical and experimental investigations of the model-extended PID control approach suggested in [21,22] on a variable stiffness actuator.…”