Conference Paper

Optimization and Fail-Safety Analysis of Antagonistic Actuation for pHRI.

DOI: 10.1007/11681120_9 Conference: First European Robotics Symposium 2006, EUROS 2006, Palermo, Italy
Source: DBLP


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 systems 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 between actuators. 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.

Full-text preview

Available from:
  • Source
    • "The most recent research in pHRI calls for the use of variable stiffness actuation (VSA), in which each joint is driven by two independent actuators that allow to control link motion as well as device stiffness [13] [14] [15] [16] and to shape the compliant interaction with the environment. Actuators are typically arranged in antagonistic mode [17], with both motors of each joint being involved in robot motion and stiffness variation. Other systems use a separate actuation [18] [19] for stiffness control. "
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider the problem of perfect cancellation of gravity effects in the dynamics of robot manipulators having flexible transmissions at the joints. Based on the feedback equivalence principle, we aim at designing feedback control laws that let the system outputs behave as those of the same robot device when gravity is absent. The cases of constant stiffness (elastic joints), nonlinear flexible, and variable nonlinear flexible transmissions with antagonistic actuation are analyzed. As a particular case, antagonistic actuation with transmissions having constant but different stiffness is also considered. In all these situations, viable solutions are obtained either in closed algebraic form or by a simple numerical technique. The compensated system can then be controlled without taking into account the gravity bias, which is particularly relevant for safe physical human-robot interaction tasks where such compliant manipulators are commonly used. Moreover, dynamic gravity cancellation allows to design new PD-type regulation controllers and to show their global asymptotic stability without the need of any positive lower bound neither on the stiffness nor on the proportional control gain. A Lyapunov-based proof is provided for the case of robots with elastic joints. Simulation results are reported to illustrate the obtained performance in the various robotic systems with flexible transmissions.
    Full-text · Article · Jan 2010
  • Source
    • "In particular, VSA devices may either have a passive variation of joint stiffness [7], or actively modify it, with an antagonistic arrangement of two motors [8]–[10] or with separate actuation for motion and stiffness [11], [12]. All the above mechanical/actuation choices allow to reduce intrinsically the risk of user injuries resulting from possible unexpected collisions of a robot that closely cooperates with humans [13], [14]. From the point of view of performance, control design should be aimed at compensating for the static deflection and dynamic vibrations associated to the presence of (constant or time-varying) compliant transmissions, so as to accurately execute fast motions as in the rigid case. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Variable stiffness actuation (VSA) devices are being used to jointly address the issues of safety and performance in physical human-robot interaction. With reference to the VSA-II prototype, we present a feedback linearization approach that allows the simultaneous decoupling and accurate tracking of motion and stiffness reference profiles. The operative condition that avoids control singularities is characterized. Moreover, a momentum-based collision detection scheme is introduced, which does not require joint torque sensing nor information on the time-varying stiffness of the device. Based on the residual signal, a collision reaction strategy is presented that takes advantage of the proposed nonlinear control to rapidly let the arm bounce away after detecting the impact, while limiting contact forces through a sudden reduction of the stiffness. Simulations results are reported to illustrate the performance and robustness of the overall approach. Extensions to the multidof case of robot manipulators equipped with VSA-II devices are also considered.
    Full-text · Conference Paper · Nov 2009
  • Source
    • "where a 1 , a 2 , b 1 , and b 2 are suitable constants and f i (s i ) are the elements of the diagonal of the matrix F (s). For the variable stiffness actuation joint (VSA), described in [3], the third-order polynomial approximation of the transmission model reported in [20] can be used to transform the system in the desired form: "
    [Show abstract] [Hide abstract]
    ABSTRACT: Physical human-robot interaction requires the development of safe and dependable robots. This involves the mechanical design of lightweight and compliant manipulators and the definition of motion control laws that allow to combine compliant behavior in reaction to possible collisions, while preserving accuracy and performance of rigid robots in free space. In this framework, great attention has been given to robots manipulators with relevant elasticity at the joints/transmissions. While the modeling and control of robots with elastic joints of finite but constant stiffness is a well- established topic, few results are available for the case of robot structures with variable joint stiffness -mostly limited to the 1-dof case. We present here a basic control study for a general class of multi-dof manipulators with variable joint stiffness, taking into account different possible modalities for changing the joint stiffness on the fly by an additional set of commands. It is shown that nonlinear control laws, based either on static or dynamic state feedback, are able to exactly linearize the closed- loop equations and allow to simultaneously impose a desired behavior to the robot motion and to the joint stiffness in an decoupled way. Illustrative simulations results are presented.
    Full-text · Conference Paper · Jun 2008
Show more