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ROBOCRANE Robot Calibrator The Robot Calibrator is a measurement system that includes three string-encoders mounted to a metal base plate and used to measure the position of a robot in three space. Combining the three string ends and attaching them to a robot arm provides an inexpensive and relatively accurate position sensor with respect to the calibrator base. Using a quick-change plate attached to the three string ends, the robot can attach to the plate, maneuver the plate through a calibration routine, and replace the plate completing an in-situ calibration during manufacturing. Errors that can build during repetitive robot motions are thereby compared to the Robot Calibrator measurements and canceled out. String encoders or potentiometers can be used to provide relatively accurate length measurements with respect to their housings and combined as in the Robot Calibrator to triangulate the position of their combined lengths. Since the string-encoders allow only perpendicular string-extension/retraction, steel toriods were machined and attached to the string-encoder housings allowing the string to exit at angles within a hemisphere, as needed for a robot arm.
Here, the robot tested was a 7-axis arm that ran through a series of tests to "calibrate" the calibrator. Calibration included the simultaneous use of a laser tracker with the calibrator providing a direct calibration of three-space points for this robot.
a. Calibrator attached to the robot armb, d, e, f. Laser tracker (black object to the right) tracking the robot position simultaneously with the Robot Calibrator.c. Close-up of the retro-reflector (laser tracker) and strings (Robot Calibrator) attached to the robot.g. h. Center stand for the calibrator attachment plate with attached strings. The strings attached to the robot attachment plate such that the robot could quick-change to it and replace it on the stand.
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