On the Stability of Robotic Systems Worn by Humans

An extender is defined to be an active manipulator worn by a human to increase his strength [6]. The human, in physical contact with the extender, exchanges power and information signals with the extender. The human arm, the extender, and the environment comprise the dynamic model of the extender system. In this paper, an expression for system performance is derived to determine the force augmentation of the extender. The stability and the performance of the extender are proven via simulation and via experiments with a single-degree-of-freedom hydraulic extender. The trade-off between performance and stability is discussed: the better the required performance (larger force amplification in this experiment), the narrower the stability range is. An experimental extender is shown in Figure la. The human arm, wrapped in a cylinder of rubber for a snug fit, is located in the inner tube. A piezoelectric load cell, placed between the tubes, measures the interaction force between the human arm and the extender. Another piezoelectric force cell, set be ween the extender and the environment, measures the interaction force between the extender and the environment. A rotary hydraulic actuator, mounted on a solid platform, powers the outer tube of the extender. The actuator shaft, supported by two bearings, is connected to the outer tube to transfer power. This article establishes experimentally-verified ground rules for control of human-machine interaction in the sense of transfer of power and information signals. Human-extender interaction and its dynamic behavior are described. A mathematical description of extender perfo~ance is derived. The stability of the human-extender-load system is.analyzed. The tradeoffs between perform~ce and stability are exlftl:lined. 2. DYNAMIC BEHAVIOR OF THE EXTENDER If an extender has elements with significant dynamic behavior, such as hydraulic actuators or transmission systems, rigid-body dynamics [5] can not sufficiently represtlnt the extender dynamic behavior. In the modeling approach taken here, the dynamics of the extender elements are implicitly included in an unstructured model by focusing on the input and output properties of the extender. The extender position,. Y., is an nx1 function of two variables: the electronic command to the extender drive system and the external forces imposed on the extender. It is assumed that the extender has either a closed-loop velocity or closed-loop position controller. This controller, called a primary stabilizing compensator in this article, is chosen because the extender must be stable when no one is wearing it and because robustness in the extender can be created without considering human and object dynamics. Regardless of the type of primary stabilizing compensator, it is assumed that the extender position Y. is the extender model output, u. is the input electronic command to the primary compensator!, fell is the force imposed by the human on the extender, and fen is the