Troubleshooting

Servo Following Error Fault: Causes, Checks and Tuning

The axis faults with "following error exceeded" at the end of every fast move. The drive is telling you the motor cannot keep up — find out why.

· 9 min read · By TryPLC Team

  • Servo
  • Motion Control
  • Following Error
  • Tuning

What following error means

The motion controller calculates where the axis should be at every instant (the set position). The encoder reports where it actually is. The difference is the following error (lag). When it exceeds a configured window, the axis faults to protect the machine, because an axis that cannot follow its profile might collide with something.

Step 1: when does it happen?

Trace set position, actual position, following error, motor current and velocity in the drive or PLC scope. When the error appears tells you most of what you need:

  • During acceleration, the motor is running out of torque. Check the current: if it sits at the limit, the profile asks too much.
  • At constant speed, with the error growing slowly, there is too little gain or the friction is increasing (lubrication, wear).
  • At the same position every time, the cause is mechanical: a tight spot on a ballscrew, a cable chain catching, a misaligned guide.
  • At random, after a collision or jam, the fault is doing its job.

    • Step 2: is the profile realistic?

    Torque needed = inertia × angular acceleration + load torque + friction. If the program asks for an acceleration the motor and gearbox cannot deliver, no tuning will help. Reduce acceleration and jerk, or confirm the motor sizing.

    Step 3: torque and current limits

    A torque limit set low during commissioning and never restored causes lag on every fast move. Compare limits with the motor's peak rating.

    Step 4: mechanics

    On a vertical or gravity-loaded axis, block or support the load first: with the drive disabled and the brake released it can fall. With the drive disabled and power isolated, move the axis by hand over its full stroke and feel for tight spots. Check the coupling for slip as well. A slipping coupling creates a position difference that the motor encoder cannot see at all; only a secondary encoder on the load can.

    Step 5: tuning

    Most modern drives offer auto-tuning that measures the load inertia. After a mechanical change (a heavier tool, a new gearbox) the old tuning no longer fits, so run it again.

  • Velocity feedforward removes most of the steady lag at constant speed.
  • Acceleration (torque) feedforward reduces lag during ramps.
  • Raising the position gain on its own also reduces lag, but it can cause oscillation, so feedforward is usually the better first step.

    • Step 6: the error window

    Only once you have worked through the steps above should you check whether the following error window suits the application. A window that is too tight causes nuisance trips; a window that is too wide hides real problems and reduces collision protection. Never widen it just to make the fault go away.

    Practise motion concepts in the motion control article and the hardware chapter on motion feedback.

    Frequently asked questions

    What is a servo following error?

    The difference between the commanded position and the actual encoder position. When it exceeds the configured window the axis faults to prevent collisions.

    Should I increase the following error window?

    Only after checking the motion profile, torque limits, mechanics and tuning. Widening it to hide the fault reduces machine protection.