Evaluation of Robotic Force Control Strategies Using an Open Architecture Test Facility

Abstract

Industrial robots are currently employed in a large number of applications and are available with a wide range of configurations, drive systems, physical sizes and payloads. However, the numbers in service throughout the world are much less than predicted over twenty years ago (Engelberger 1980). This is despite major technological advances in related areas of computing and electronics, and the availability of fast, reliable and low-cost microprocessors and memory. This situation is mainly a result of historical and economic circumstances, rather than technical considerations. Industrial robots have traditionally performed a narrow but well-defined range of tasks to a specified degree of accuracy and whilst new robot arm designs are specified for many years of continuous operation, the technological development of their controllers has been slow in comparison with other computer-based systems. Traditionally, most industrial robots are designed to allow accurate and repeatable control of the position and velocity of the tooling at the device’s end effector. Increasingly, these systems are often also required to perform complex tasks requiring robust and stable force control strategies. In addition, task constraints sometimes require position or velocity control in some Degrees-Of-Freedom (DOF), and force control in others. Thus, to fulfil these extra demands, an important area of robotics research is the implementation of stable and accurate force control. However this is often difficult to achieve in practice, due to the technological limitations of current controllers, coupled with the demanding requirements placed upon them by the advanced control schemes that are needed in cases where robots are operating in unpredictable or disordered environments. This chapter describes a research project that has been undertaken to partly address these issues, by investigating algorithms and controller architectures for the implementation of stable robotic force control. The chapter is organised as follows. In Section 2, the fundamental concepts of robotic force control are introduced, and the problems inherent in the design of stable, robust controllers are described. This Section also describes some of the difficulties that are faced by developers when implementing force control strategies using traditional robot controllers. It is shown that linear, fixed-gain feedback controllers designed using conventional techniques can only provide adequate performance when they are tuned to specific task requirements. In practice the environmental stiffness at the robot/task