A logistics warehouse reported that a Linde E20C electric forklift had "twitchy" steering. When driving straight, the rear wheels would rapidly oscillate left and right (shimmy) by about 2 inches, making the truck unstable. The steering wheel felt stiff when turned.
The Linde E20C uses a "Steer-by-Wire" system (Linde Steer Control). A sensor on the steering column measures the driver's input, and an electric motor on the rear axle turns the wheels. There is also a position sensor on the rear axle to tell the computer where the wheels are.
The technician accessed the rear axle sensor. This sensor is a Hall-effect non-contact sensor. He measured the output voltage signal while manually moving the steering cylinder. The signal had "dead spots"-areas where the voltage fluctuated wildly or dropped to zero. This confused the electronic control unit (ECU). The ECU thought the wheels were in the wrong position and constantly tried to correct them, causing the twitching.
The "stiffness" was caused by the mechanical link between the steering column and the rear axle (a safety backup). The constant twitching was causing friction in the mechanism.
The repair involved replacing the rear axle position sensor (angle sensor). The new sensor was calibrated using the Linde diagnostic laptop, setting the "zero point" exactly in the middle of the cylinder stroke. After calibration, the twitching stopped, and the steering became smooth and responsive.
