A prototype arm signature identification system

The S-Model identification algorithm described in [6,7] is a technique which can be used to accurately identify the actual kinematic parameters of serial link robotic manipulators. The actual kinematic parameters of a manipulator differ from the design parameters due to the presence of random manufacturing errors. The set of identified kinematic parameters is called the arm signature. Accurate arm signatures are needed to control and improve the end-effector positioning accuracy of robotic manipulators for a variety of important tasks. This paper describes the hardware and software implementation of a prototype arm signature identification system. This system uses an external ultrasonic range sensor to measure the Cartesian position of target points placed on the links of the robot. Algorithms to compensate the primary range measurements for spatial variations in air temperature and humidity are also incorporated. The relative Cartesian positioning accuracy of the sensor system is ± .02cm. The general characteristics of our sensor design and the overall system design which exploits averaging over many sensor readings offer numerous advantages for arm signature identification. The prototype system has been applied in [6] to improve the kinematic performance of seven Puma 560 robots. For these robots relative positioning accuracy was improved by a factor of 10 on straight'line positioning tasks. Analysis and simulation of systematic errors confirms that the resolution of our sensor system should provide kinematic performance close to the limitations of the joint encoders. Our experimental studies show that sensor bias ultimately limits kinematic performance using this arm signature system. Experience with this prototype system has demonstrated that the S-Model identification algorithm is a practical and viable method for improving the kinematic performance of robotic manipulators.