A reach truck was experiencing violent, phantom faulting. The operator would drive down the aisle, extend the reach carriage into the rack, and suddenly the truck would lock up, dropping the load two inches before the hydraulic brakes engaged. The dash displayed a "Motor Position Encoder Fault." The shop replaced the expensive traction motor controller, but the drops continued.
We pulled the side cover off the reach motor. The motor has a spring-applied, electrically-released magnetic brake mounted on the tailshaft, with the encoder wheel sitting right next to it. As the brake wears over thousands of cycles, it generates a fine, black dust composed of friction material and iron particles.
Because the brake is magnetic, and the encoder disc is mounted mere millimeters away on the same shaft, that iron-laden dust is magnetically attracted to the encoder's sensor head. Over time, the dust built up into a thick crust on the sensor, completely blocking the optical or magnetic slots on the encoder wheel.
When the operator extended the reach, the motor spun rapidly. The controller needed the encoder to tell it exactly how fast the motor was rotating to apply the correct voltage. Because the sensor was blind due to the dust, the controller lost synchronization, instantly panicked, and fired the magnetic brake to stop the motor, causing the load to drop. We cleaned the encoder wheel with electronic contact cleaner, blew the dust out of the brake housing, and the phantom faults vanished. When an encoder faults on a motor with a magnetic brake, always suspect dust contamination before replacing the electronics.
