Improving Safety of Human-Robot Interaction Through Energy Regulation Control and Passive Compliant Design

Abstract

Modern production processes continuously require enhancement in production time and the quality of the products. The use of robots in this field of application has formed an increasingly important aspect of the drive for efficiency. These robots typically work in restricted areas to prevent any harmful interaction with humans and are designed for repeatability, speed and precision. However, new opportunities are arising in homes and offices that mean that robots will not be confined to these relatively restricted factory environments and this sets new demands in terms of safety and ability to interact with the environment. These new requirements make industrial heavy and stiff manipulators controlled with high gain PID controllers not suited to cooperate and work closely with humans. In order to cope with this, impedance control (Hogan, 1985; Ikeura and Inooka, 1995; Zollo, Siciliano et al., 2002; Zollo, Siciliano et al., 2003) for decreasing the replicated output impedance of the system to safe values and safety-oriented control strategies (Heinzmann and Zelinsky, 1999; Bicchi and Tonietti, 2004; Kulic and Croft, 2004) to react safely when a Human-Robot Interaction is detected have been introduced. The mentioned control algorithms work well for slow interaction transients and within specific frequency bands, however when the frequencies are above the closed loop bandwidth of the robot, these strategies are ineffective in reacting safely making the resulting system to be dangerous. When a sudden and fast impact occurs, the output impedance of the robot is dominated by the link and the rotor reflected inertia. This latter term is usually high due to the high reduction ratio of the gear making the overall robot output impedance large and dangerous meaning that the system’s safety is once again compromised. An alternative to this “active” approach is the incorporation of intrinsically safe structures particularly focusing on the actuation systems design. Several actuator prototypes have been developed embedding either passive compliant elements in the structure (Pratt and Williamson, 1995; Sugar, 2002; Yoon, Kang et al., 2003; Hurst, Chestnutt et al., 2004; Zinn, Khatib et al., 2004; Hollander, Sugar et al., 2005; Tonietti, Schiavi et al., 2005; Schiavi, Grioli et al., 2008; Tsagarakis, Laffranchi et al., 2009; Catalano, Grioli et al., 2010; Jafari, Tsagarakis et al., 2010; Tsagarakis, Laffranchi et al., 2010) or, more recently, clutches/damping devices (Lauzier and Gosselin, 2011; Shafer and Kermani, 2011) to decouple the link (i.e. the part usually interacting with the human) from the rotor during interaction with either the environment