IEEE Robotics & Automation Magazine - December 2015 - 50

Discussion and Conclusions
In this article, we generalize torque control methods widely used
for flexible joint robots to variable stiffness robots. Popular control approaches include energy shaping techniques, which constitute a solid basis for VSA robot control. Therefore, a crucial
factor is the joint torque estimation capability of VSAs. This
capability is a result of the mechanical design concept to reduce
the mechanical damping in the joints and enables the use of the
collocated, static link-side equivalent coordinates. Therefore, a
high positioning accuracy can be achieved based only on the
collocated variable. The
flexibility of the energy
shaping approach to define
The formulation of
arbitrary output coordinates allows for intuitive
collocated variables by
task formulations. The passivity-preserving controller
potential energy shaping is formulation ensures the
stability and seamless intean important cornerstone.
gration of the controller,
and enables the use of
advanced controllers.
The presented control method extensions for VSA robots
show promising performance. The active/passive Cartesian
impedance scheme combines the best of active impedance
control with the beneficial mechanical properties of passive
compliant elements.
The handling of task hierarchies helps to cope with the
kinematic redundancy. Again, the formulation of collocated variables by potential energy shaping is an important
cornerstone. Based on the presented method, results from
research on the hierarchical control design for torque-controlled robots can be transferred with ease.
The easily excitable oscillatory robot dynamics, which is
designed for energy-efficiency purposes, has been of great
concern for the general applicability of VSA robots. Furthermore, these concerns have been stated in general elastic joint
context. The damping control algorithm presented here
shows excellent performance and functionality, and it proves
the validity of the concept. The controller allows for link-side
position regulation, thus rigid body planning can be applied.
Well-damped robot motions can be executed with high precision and repeatability.
Some approaches presented here exploit the capabilities of
VSA robots to full extent, while others do not necessarily
address every aspect of the VSA technology. The approaches
are chosen to use VSA robots at a similar performance level
or higher than classically actuated robots. In return, some of
the presented methods can be applied to different joint types,
such as SEA joints as well. The discussed robot model can
describe various classical elastic joint robots. A comparison
of different joint technologies and the applicability of the discussed control methods are given in Table 2.
Further development continues in the direction of integrating the developed controllers in a general and intuitively
applicable control framework. Noncollocated feedback is an
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important premise and content of further research. Also, the
execution of tasks in awareness and accordance of the robot's
dynamics promises further potential for improvement.
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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2015
