Robot Calibration: Modeling Measurement and Applications

Most currently used industrial robots are still programmed by a teach pendant, especially in the automotive industry. However the importance of off-line programming in industry as an alternative to teach-in programming is steadily increasing. The main reason for this trend is the need to minimize machine downtime and thus to improve the rate of robot utilization. Nonetheless, for a successful accomplishment of off-line programming the robots need to be not only repeatable but also accurate. In addition to improving robot accuracy through software (rather than by changing the mechanical structure or design of the robot), calibration techniques can also minimize the risk of having to change application programs due to slight changes or drifts (wearing of parts, dimension drifts or tolerances, and component replacement effects) in the robot system. This is mostly important in applications that may involve a large number of task points. Theoretically, with the availability of robot calibration systems integrated with off-line programming systems, it would be possible to implement off-line programming in industry (currently they are not ready to be used in large scale), where multiple robots are used, by programming only one robot and copying the programs from one robot to the others. One of the most evident advantages of this type of programming is the reduction in maintenance expenditure, since the robot can return faster to service and easy reprogramming means the software becomes operational with little effort. Robot calibration is an integrated process of modeling, measurement, numeric identification of actual physical characteristics of a robot, and implementation of a new model. The calibration procedure first involves the development of a kinematic model whose parameters represent accurately the actual robot. Next, specifically selected robot characteristics are measured using measurement instruments with known accuracy. Then a parameter identification procedure is used to compute the set of parameter values which, when introduced in the robot nominal model, accurately represents the measured robot behavior. Finally, the model in the position control software is corrected. Many factors such as numerical problems, measurement system deficiencies, cumbersome setups and commercial interests have defied the confidence of the industry in such systems, especially if one realizes that a robot will not normally need full calibration more than once or twice a year.