Servo “Following Error” (Position Deviation) error in plc
Description
In precision motion control, the PLC sends a command position to the servo drive, and the servo sends back its actual feedback position. A “Following Error” or “Position Deviation Fault” occurs when the difference between where the axis should be and where it actually is exceeds a programmed tolerance limit. The machine stops instantly to prevent crashing. This is distinct from an encoder failure; the encoder is working, but the motor simply cannot keep up with the command profile.
Cause
Mechanical Binding: The most common cause is increased friction. A bearing might be seizing, a linear rail might lack grease, or a product jam might be physically resisting the movement. The motor applies torque, but the load drags behind.
Aggressive Tuning vs. Inertia: If the PID tuning is too soft (low proportional gain), the axis will lag during acceleration. Conversely, if the machine load changes (e.g., a robot arm picking up a heavy weight), the inertia ratio changes. If the servo drive isn’t tuned to handle this extra mass, the motor will fall behind the command trajectory during rapid moves.
Torque Saturation: If the requested acceleration requires 110% of the motor’s peak torque, the motor will output 100%, causing it to inevitably lag behind the command.
Solution
Step 1: Check the Mechanics. Disconnect the motor from the load. Move the mechanism by hand. It should move smoothly without “tight spots.” If it is hard to move, fix the mechanics before touching the PLC.
Step 2: Increase Following Error Window. If the error is nuisance tripping during non-critical moves (like homing), slightly increase the tolerance in the motion profile configuration. However, do not make it too large, or you risk the axis crashing into end stops.
Step 3: Tune the Loop. Perform an “Auto-Tune” on the servo drive with the full load attached. If possible, use “Feed Forward” gains (Velocity Feed Forward or Acceleration Feed Forward). These predictive gains tell the drive to apply torque before the error accumulates, dramatically reducing lag during acceleration.