Balancing command tracking and stable motion

In a servo system, position loop gain determines how strongly the controller responds to the difference between commanded and measured position. Often called Kp or position proportional gain, it helps an axis follow its commanded trajectory and settle at the target when correctly tuned.

For example, suppose a servo motor moves a table toward 100 mm. If the measured position is currently 99.8 mm, the position error is 0.2 mm. The gain influences the correction generated to close that gap; it does not increase motor power or encoder resolution.

How do the position, velocity and current loops interact?

In a common cascaded architecture, position control is the outer loop. It converts position error into a velocity correction command. The velocity loop requests torque to achieve that velocity, while the current loop regulates motor current to produce the required electromagnetic torque. The stability of the inner loops limits the performance of the outer loop.

Consequently, position gain and velocity gain are different settings. Overall servo loop tuning must consider their interaction. Depending on the velocity, torque or position control mode, the position loop may reside in the drive or in a higher-level motion controller. With both pulse/direction commands and EtherCAT servo systems, the selected operating mode determines where the loop closes.

The relationship between gain and following error

For a simple proportional position controller that generates a velocity command, v_correction = Kp × e, where e is position error. If position is measured in mm and velocity in mm/s, Kp has units of s⁻¹.

At constant velocity, with ideal velocity tracking, no feedforward and no active limits, this gives e ≈ v / Kp. At 100 mm/s with a gain of 100 s⁻¹, the following error is approximately 1 mm. At 200 s⁻¹, it becomes approximately 0.5 mm. These are illustrative calculations, not recommended drive settings.

This calculation describes tracking lag during motion. It does not predict the final error after stopping or the machine's absolute accuracy. Feedforward, friction, filters and controller architecture change the result.

What do insufficient and excessive gain look like?

Insufficient gain can leave the axis lagging behind its command and taking longer to settle. However, inadequate torque or excessive acceleration can produce similar symptoms. Check the load and transmission calculations alongside the drive's current and velocity limits.

Excessive gain can cause overshoot, hunting, vibration at standstill and audible noise. With flexible couplings, belts or a planetary gearbox, resonance and backlash limit usable gain. Vibration alone does not identify which control loop needs attention.

Why feedforward and mechanical accuracy matter

Feedforward uses planned velocity or acceleration to generate part of the required command before a large error develops. It can improve tracking without relying solely on increased Kp. Incorrect feedforward settings can increase overshoot, and feedforward does not replace feedback.

A motor encoder measures the motor shaft. With transmission backlash or compliance, the motor may appear on target while the table is elsewhere. Increasing gain does not remove mechanical backlash. Measuring load motion with a linear encoder requires an appropriate feedback architecture. Consider this requirement when selecting a servo motor and drive.

Evaluate adjustments with measured data

Back up the existing parameters first. Check connections, mechanical fastening, brake release and feedback direction. Use the manufacturer's autotuning procedure with a suitable load and available travel. During manual refinement, change one parameter at a time in small steps, using limited velocity and acceleration for test moves.

Compare traces of commanded position, actual position, following error, velocity and current/torque for the same move. Check both directions, different loads and the approach to the target. If vibration or overshoot appears, stop increasing gain; return to the previous stable setting and investigate the mechanics and loop balance.

Increasing the following-error alarm limit does not fix poor tuning. Widening the “in-position” window does not improve physical accuracy either. The objective is stable, repeatable motion within the required error tolerance. PID position control provides further background on alternative controller structures.