Optimization and Fail–Safety Analysis of
Antagonistic Actuation for pHRI
Gianluca Boccadamo, Riccardo Schiavi, Soumen Sen, Giovanni Tonietti, and
Interdepartmental Research Centre “E. Piaggio” via Diotisalvi 2, Faculty of
Engineering University of Pisa, Italy
Summary. In this paper we consider some questions in the design of actuators for
physical Human-Robot Interaction (pHRI) under strict safety requirements in all
circumstances, including unexpected impacts and HW/SW failures.
We present the design and optimization of agonistic-antagonistic actuation sys-
tems realizing the concept of variable impedance actuation (VIA). With respect to
previous results in the literature, in this paper we consider a realistic physical model
of antagonistic systems, and include the analysis of the effects of cross–coupling be-
We show that antagonistic systems compare well with other possible approaches
in terms of the achievable performance while guaranteeing limited risks of impacts.
Antagonistic actuation systems however are more complex in both hardware and
software than other schemes. Issues are therefore raised, as to fault tolerance and
fail safety of different actuation schemes. In this paper, we analyze these issues and
show that the antagonistic implementation of the VIA concept fares very well under
these regards also.
One of the goals of contemporary robotics research is to realize systems which
operate with delicacy in environments they share with humans, ensuring their
safety despite any adverse circumstance . These may include unexpected
impacts, faults of the mechanical structure, sensors, or actuators, crashes or
malfunctional behaviours of the control software [2, 3, 4, 5].
A recent trend in robotics is to design intrinsically safe robot arms by
introducing compliance at their joints. The basic idea of this approach is that
compliant elements interposed between motors and moving links help prevent
the (heavy) reflected inertia of actuators from concurring to damage in case of
impacts. Introducing compliance, on the other hand, tends to reduce perfor-
mance of the arm. Some approaches in the direction of minimizing the perfor-
mance loss while guaranteeing safety in case of impacts have been presented
2G. Boccadamo, R. Schiavi, S. Sen, G. Tonietti, and A. Bicchi
in the recent literature (see e.g. ). Among these, a method was proposed
in  consisting in varying the compliance of the joint transmission mech-
anism while moving the arm. This so-called Variable Stiffness Transmission
(VST) technique, and its generalization in the Variable Impedance Actuation
(VIA) concept, have been shown to be capable in theory of delivering better
performance than purely passive compliance and other techniques.
In its formulation, however, the VIA concept in  used a rather abstract
model of actuator and transmission, whereby the impedance could be directly
controlled to desired values in negligible time. In this paper, we consider a
more realistic model of an actuation system implementing the idea, which is
based on the use of two actuators and nonlinear elastic elements in antag-
onistic arrangement. The antagonistic solution has several advantages, and
has been used in many robotic devices before (see e.g. [8, 9]), in some cases
because of biomorphic inspiration. However, to the best of our knowledge
the introduction of nonlinear springs to achieve variable stiffness was not a
motivation for earlier work.
In this paper, we consider the implementation of the VIA concept by means
of antagonistic actuation, discuss the role of cross-coupling between antago-
nist actuators, and apply optimization methods to choose parameters which
are crucial in its design. We show that antagonistic systems can implement
effectively the VIA concept, and their performance compares well with other
Antagonistic actuation systems however are more complex in both hard-
ware and software than other schemes. Issues are therefore raised as to whether
safety is guaranteed under different possible failure modes. In the paper, we
also analyze these issues and show that the antagonistic implementation of
the VIA concept fares very well under these regards also.
2 Antagonistic actuation as a VIA system
In  it was shown that an ideal VIA mechanism (depicted in fig. 1-a) can
effectively recover performance of mechanisms designed to guarantee safety of
humans in case of impact. The basic idea is that a VIA mechanism can be
controlled according to a stiff-and-slow/fast-and-soft paradigm: namely, to be
rather stiff in the initial and final phases of motion, when accuracy is needed
and velocity is low, while choosing higher compliance in the intermediate,
high-velocity phase, where accuracy is typically not important. Low stiffness
implies that the inertia of the rotors does not immediately reflect on the link in
case of impacts, thus allowing smoother and less damaging impacts. Such ar-
guments where supported in  by a detailed mechanism/control co-design op-
timization analysis, based on the solution of the so-called safe brachistochrone
problem, i.e. a minimum time control problem with constraints on the max-
imum acceptable safety risk at impacts. The model considered fig. 1-a, how-
ever, uses direct variations of impedance, which is not physically realizable.
Optimization and Fail Safety of Antagonistic Actuation for pHRI9
Work partially done with the support of the EU Network of Excellence EU-
RON - European Robotic Network, under the Prospective Research Project
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