Using locally adjustable hand-eye calibrations to reduce robot localization error
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Using locally adjustable hand‑eye calibrations to reduce robot localization error Marek Franaszek1 · Geraldine S. Cheok1 Received: 5 November 2019 / Accepted: 30 March 2020 © This is a U.S. Government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020
Abstract Accurate pose of a robot end effector is required in many applications. Typically, this is achieved by robot calibration and then, registration of the robot frame to the world frame. In this paper, the registration of a poorly calibrated robot to a world frame was performed using many local hand-eye registrations and using one global registration. Both approaches were evaluated using a set of target poses. The use of the properly chosen local transformation applied to each target led to a tenfold reduction in the orientation and position error in comparison with a use of one global transformation. The median orientation error was reduced to 0.029° and the median position error was reduced to 0.221 mm which is approximately four times larger than robot specified position repeatability. Keywords Robot localization error · Error compensation · Forward kinematic · Hand-eye calibration · Accuracy · Repeatability
1 Introduction Robot manipulators are characterized by very good repeatability. For example, the unidirectional position repeatability of the non-calibrated industrial robot was found to be better than ± 37 μm and the orientation repeatability was at worst ± 87 μrad [1]. Unfortunately, the accuracy of robots is frequently larger than the repeatability by about two orders of magnitude. This negatively impacts the ability of these robots to perform manufacturing tasks which require accurate knowledge of the pose of the end effector. For example, in the aerospace industry, tasks such as automated drilling require errors less than 0.25 mm [2]. The localization error of industrial robots can be traced back to two root causes: (1) kinematic error caused by incorrect values of the Denavit–Hartenberg (DH) parameters in the kinematic model of the serial manipulator; (2) non-kinematic errors such as elastic deformation under gravity, thermal effects, backlash, joint compliance,
encoder resolution, or wear and tear of motors [3]. Out of four DH parameters characterizing each revolute joint, incorrect values of the joint angle zero offsets are responsible for the largest component of the total kinematic error [4]. This type of error can be substantially reduced by remastering the robot and many different calibration techniques using different sensors have been developed [5–11]. The remaining residual part of the total localization error is usually attributed to non-kinematic errors. These errors are difficult to reduce [12], and they degrade robot performance. Different approaches have been used to compensate for the localization error and to improve robot performance. Whenever conditions allow (e.g., space and visibility) visual servoing offers great improvement in robot accuracy [13, 14]. This type of technique is
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