Absolute kinematic calibration of six-axis serial industrial robots using relative measurements

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The inherent lack of positioning accuracy in standard, un-calibrated industrial robots is a limiting factor for their use in the automation of manufacturing processes, such as those at Enclosures Direct Inc. (EDI). Robots utilized for manufacturing are generally designed to have excellent repeatability (ability to continually reach the same position and orientation, or pose, defined by a set of joint angles stored in the controller) but are lacking in accuracy (moving to a pose for which the joint angles must be computed by the controller). To overcome the accuracy shortcomings the robots are typically used in a "record and play" mode where poses are manually taught by a technician and repeated for long production runs. When dealing with shorter production runs of customized products, as is the case at EDI where over 3000 different products are offered, it is not viable to manually teach all of the required poses which are instead calculated based on part dimensions or a CAD model. This approach requires the robots to be calibrated in order to meet accuracy requirements. Existing kinematic calibration methods require accurate and precise knowledge of the absolute pose of the robot end effector with respect to the robot base. Achieving this requires an elaborate and expensive measurement system and often requires moving the robot offsite to a calibrated measurement cell. This research project will use recent advances in joint mastering to identify the errors in the six joint angles using relative measurements of tool point position using a novel combination of position sensitive devices and a linear encoder. Once the joint zero values have been identified, the encoder measurements will be used to identify the remaining parameter errors. Observations that the joint angle error is generally larger than the error in the remaining parameters has led to the development of a measurement system that will allow for the separation of the joint angle calibration from the remaining parameters while employing the same experimental data set. The goal of the proposed project is to develop a complete and general system for the kinematic calibration of serial industrial robots.