Servo Drive Following Error: Common Causes and Troubleshooting Steps

Servo drive following error is one of the most common axis faults on packaging machines, CNC routers, printing lines, and robotic cells. The drive is not reporting a simple motor failure. It is telling you that the commanded position and the actual feedback position have separated by more than the configured error window. That distinction matters. A technician who treats every following error as a tuning issue can spend hours changing gains, replacing encoders, and swapping drives while the real fault remains in a coupling, cable shield, or controller scan time. This guide breaks down the fault into measurable mechanical, electrical, wiring, and control-loop checks. It is written for maintenance teams, controls engineers, and OEM field service staff who need a repeatable way to isolate a following error without guessing.

How the Drive Calculates a Following Error

Every servo cycle, the drive compares the position command from the motion controller or internal profile generator with the actual position reported by the motor encoder, linear scale, or resolver. The difference is the following error. It is usually displayed in encoder counts, motor revolutions, millimeters, or degrees, depending on how the axis is scaled. When the difference exceeds the drive’s following error limit, the axis trips and the drive logs the event in its fault history.

The loop update rate has a direct effect on how large that error can become. Many industrial servo systems update the position loop at 1 kHz to 4 kHz, while the current loop runs faster at roughly 8 kHz to 16 kHz. Those are typical industry ranges, not a fixed rule. An axis traveling at 1 m/s with a 1 ms position loop moves 1 mm per loop. If the following error limit is set to 0.5 mm, a brief command delay may trip the drive even when nothing is mechanically wrong. This is often a scaling problem, not a hardware defect.

The table below separates common causes by symptom and first check.

Read the Fault Record Before Touching Hardware

Do not clear the fault immediately. The stored trace can show whether the following error developed slowly or jumped in one cycle, which is often enough to separate a mechanical bind from an electrical noise event. Export the drive parameter file and trend buffer before resetting anything. If the machine uses a SLANVERT servo drive range or a similar multi-axis system, copy the axis parameters, electronic gear ratio, following error limit, and gain settings first.

A useful field kit includes a digital multimeter, a true RMS clamp meter, a dial indicator, a torque wrench, and a commissioning laptop with drive software. If the error is intermittent, add a portable oscilloscope or a high-speed data logger. Mechanical drawings of the coupling, gearbox, and belt drive also save time because they give you the expected ratios and backlash limits.

Mechanical Causes That Look Like a Servo Problem

Several pure mechanical faults arrive as following error alarms. The first suspect is backlash in a gearbox, ball screw, or coupling. Under direction reversal, the motor turns briefly while the load does not move. The feedback from the motor encoder says the motor has moved, but the load position has not caught up. The drive interprets that as a position error. On linear axes, backlash above 0.05–0.1 mm is often enough to create an unstable loop or a repeatable following error spike.

Coupling windup is another frequent cause. A coupling rated below the motor’s peak torque can twist and release during acceleration, producing a following error that appears only at high torque. Check for cracked jaws, loose set screws, or keyway wear. Belt drives introduce their own problems: low belt tension allows tooth skip, while excessive tension overloads the motor bearing and increases friction. If the fault appears only during a specific travel range, inspect the guide rails and ball screw for a localized tight spot or contamination.

Inertia mismatch also behaves like a tuning problem. When the reflected load inertia is much larger than the motor rotor inertia, the motor can overshoot or lag because the position loop is fighting a load it cannot accelerate quickly enough. A mismatch above 10:1 is usually worth investigating with the machine builder or drive supplier. Some applications can tolerate higher ratios with careful gain reduction, but stability margins shrink.

Electrical, Tuning, and Wiring Causes

Tuning parameters are a common cause, but they are not always the root cause. If the proportional gain is too low, the axis follows with a steady lag and the following error increases with speed. If the gain is too high, the axis may buzz or oscillate. Feedforward can compensate for velocity and acceleration lag, but only if the mechanical system is linear. Feedforward on a badly worn ball screw can make the error look normal in one direction and poor in the other.

Encoder resolution mismatch is another frequent source. If the drive’s electronic gear ratio or encoder configuration does not match the physical encoder counts per revolution, the axis may move a different distance than the controller expects. Incremental encoders often require a 5 V DC supply, and many units operate within a 5% tolerance. When the supply drops below roughly 4.75 V, encoder channels may become unstable. Cable routing can make this worse. High-voltage motor cables should be separated from feedback cables by at least 200 mm in cable trays, and feedback cable shields should be terminated with a 360-degree connection to the connector housing.

Grounding remains one of the least expensive and most effective fixes. A ground resistance below 1 Ω from the drive protective earth to the motor frame is a practical field target. If the machine shares a panel with SLANVERT low and medium voltage inverters or other power equipment, check supply dips and shared DC bus behavior when contactors or large loads switch. Electrical installation rules in IEC 61800-5-1 and IEC 60204-1 provide the framework for safe drive and machine wiring. They do not set the following error limit, but they address many of the grounding, shielding, and separation practices that prevent noise-related position faults.

A Practical Fault Elimination Sequence

The fastest field method is to remove variables one at a time. Use this sequence before changing gain values.

• Capture the fault trace. Record commanded position, actual position, torque reference, and DC bus voltage at high speed. The trace shows whether the error is monotonic, oscillatory, or a single spike.
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