Menu Bar Wafer Handler End-point Position Analysis

Robot Arm

The Problem

A customer noticed that one of many robotic arms occasionally failed to correctly grasp a silicon wafer after a programmed move.

The Measurements

To identify and rule out any differences between what the servo system believed to be the correct position and the actual arm position, a laser distance sensor was employed to monitor the end of the arm. This particular sensor produced an analog voltage that was proportional to distance. Connecting both the encoder of the servomotor and the output of the laser to the SensorPlot™, the customer programmed the arm to repeatedly move horizontally back and forth 7.000 inches.


After a few runs the customer immediately saw that the robot arm was resonating with a peak value of 0.068" at the end of the move. As indicated by the graph above, it took the end of the arm nearly a full second to settle.

Comparing the position as measured by the encoder to the position as measured by the laser, indicated that the resonance was not a servo loop-tuning problem. Instead, the resonance was caused by the mechanics of the arm (couplings, mountings, belts...). Subsequently, the problem required altering the velocity profile and adding mechanical damping to solve the problem.


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