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The Problem

A customer noticed that an anomaly was being introduced at some point during production. An initial investigation into the problem suggested that the anomaly was related to a spin cycle at the end of the production cycle. The process called for a motor velocity stability of 1 RPM. Although the servo controller driving the motor reported that the motor was spinning at a constant RPM, the anomaly remained.

Measurement

To certify that the motor was operating as programmed, the SensorPlot™ was connected to the encoder attached to the servomotor. As indicated to the below, the SensorPlot™ was programmed to acquire a measurement every 500uS for 5 seconds. The motor was programmed to operate at 1000 RPM, and a single block of data was acquired.

Analysis

Initially, the measured velocity appeared to be noisy. Upon zooming the graph, the customer observed that noise was in fact a true variation in velocity with a peak to peak variation of about 30 RPM! Evidently, the servo controller was reporting measured velocities that had been severely averaged.

Using the cursors, the period of the fluctuations was measured to be about 60mS corresponding to one revolution of the motor (60 / 1000 RPM = 60mS / Revolution). Further exploration using the spectral analysis tools indicated that the servo system exhibited a 16.7Hz periodic wander with harmonics.

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