Bilateral Impedance Control for Telemanipulators

The research investigates a new method of control for telemanipulators called bilateral impedance control. This new method differs from previous ap"roaches in that interaction forces are used as the communication signals betWeen the master and slave rooots. The main advantage of bilateral impedance control is that it permits the arbitrary specification of desired system performance characteristics. Performance c!'n be described by a set of three independent parameters that relate the robot forces and positions. nis set of parameters may includ.: the force ratio, the position ratio, .lid the robot impedances. The s)stem stability is analyzed with the Nyquist Criterion to obtain two conditions that are sufficient to guarantee closed-loop stabilit)'. This control architecture is implemented on a telemanipulator having seven degrees of freedom. The theoretical predictions for performance and stability are experimentally verified. Teleoperation is greatly C Ihanced if the forces acting on the slave robot are fed back to the op,~rator. This gives the operator the feeling that she is manipulating the remote environment directly. Systems that provide force reflxtion from the environment are called bilateral because infom ation travels in both directions between the master and slave. In c~ntly used telerobotic control methods, the slave robot is driven by position or velocity s.gnals from the master robot [1-4]. Force reflection is implemented i'1 several ways. The master robot may be back driven by position signals from the slave robot. The position error generated when the slave contacts the environment allows the operator to feel the interaction [2]. Alternately, the interaction force may be sensed directly by a force sensor on the slave, and the resulting force signal is used to back drive the master [2]. A comparison of common control methods is given in [5:. This paper proposes a new method of telerobotic contr:>l that is based solely on the exchange of force signals. The prc')Osed control architecture has several advantages over pre lious approaches. It permits the arbitr.u-y specification of desired system performance characteristics. The relationship between force and position can be modulated at bC'th ends of the system to suit the requirements of the task. The master and slave robots can be stabilized independently without becoming involved in the overall system dynamics. Finally, thr: new control method allows the human to overcome the master rLbot's resistance to motion if it has high friction, inertia, or gear reduction. Introduction A telerobotic system consists of a master robot that is manipulated by a human operator, and a slave robot that performs tasks at a remote location. The t"o robots are electronically coupled so that the slave robot moves in response to commands from the master obot. Figure I shows th,' elements of a teleroboric system. The slave robot is shown interacting with an environment. In this paper "environment" refers to th.: object that the slave robot moves or manipulate through space. slave Dynamic Models The dynamic behavior 01 a telerobotic system results from the interaction of its components: the master and slave robots, the human, and the environment. Linear dynamic models" ill be developed separately for each of these components. The models will then be assembled to form a telerobotic control architectu': that describes overall system behavior. A single-degree-of-frtedom telerobotic system is discussed in this paper; the multivariable control technique is described in 12. It is assumed that both rol)Qts are stabilized by independent, closed-loop position controllers t'tat keep them Stationary when the human is not interacting with the ;ystem. The stabilizing controllers may include velocity feedback, but closed-loop velocity control by itself cannot guarantee that the s!ave will always track the master