Written by: Mehdi Hassanzadeh, Content Specialist – Radonix
Short answer: Servo gain is the factor a servo loop uses to turn the error between command and feedback into a corrective output. Higher gain makes an axis stiffer and faster to respond, but pushing it beyond the stability margin causes vibration, noise, overshoot and overheating. The right gain is the highest value that stays stable with the real load.
In servo drive systems, several nested control loops operate at once: the current (torque) loop, the velocity loop and the position loop. What is labelled gain in servo settings is the responsiveness of each of those loops: how aggressively the controller converts error into a torque, speed or position command.
Higher gain values mean faster error correction and stiffer axis behaviour. Exceeding the stability margin leads to oscillation, audible noise, vibration, overheating and following errors. Setting gain correctly is a balance between dynamic performance and mechanical stability, and this guide explains how each gain works so you can set it with confidence. If you need the full commissioning sequence, follow our step-by-step guide to servo motor tuning; this page focuses on the gain parameter itself.
What Does Servo Gain Mean?
From a control theory perspective, an error is the difference between the commanded value and the actual value. The controller multiplies this error by a gain to generate its output.
Low gain means:
- weak corrective action,
- sluggish motion,
- persistent following error.
High gain means:
- aggressive correction,
- fast response,
- risk of oscillation and instability.
In practical servo terms, gain defines how stiff or aggressive an axis behaves. It directly affects settling time, overshoot, vibration, motor temperature and following error.
Servo Gain in Each Control Loop
Current (Torque) Loop Gain
This is the fastest loop, typically running at several kHz. It is usually factory-tuned and not exposed directly. Incorrect settings here can cause current noise, overheating or an audible hum, so leave it at the drive’s calculated values unless you have a measured reason to change it.
Velocity Loop Gain
Common parameters:
- Velocity proportional gain (Kvp): increases speed-response stiffness.
- Velocity integral gain (Kvi): eliminates steady-state speed error.
Excessive Kvp leads to vibration or high-frequency noise, while excessive Kvi causes low-frequency oscillation or “wave-like” motion.
Position Loop Gain
The typical parameter is the position proportional gain (Kpp), also called axis stiffness or servo gain. Higher values reduce following error but may cause hunting, vibration or instability during stops.
Feedforward Parameters
- Velocity feedforward (VFF)
- Acceleration feedforward (AFF)
Feedforward reduces tracking error during acceleration and deceleration without increasing gain beyond its stability limit. It is the safest way to improve CNC path accuracy.
Gain and P / I / D Control
Many drives expose gain through a PID structure:
| Term | Role | Too much |
| P (proportional) | Immediate error-correction strength | Oscillation, noise |
| I (integral) | Eliminates steady-state error | Overshoot, slow wave-like motion, heating |
| D (derivative / damping) | Suppresses overshoot and oscillation | Amplified noise, rough motion |
In industrial servo drives, D-action is often hidden under names such as damping gain, derivative filter or stability control, so check the drive manual for the equivalent parameter.
How Position Gain Sets Following Error: A Worked Example
For an axis without feedforward, the steady-state following error during a constant-speed move is approximately the feedrate divided by the position-loop gain (often written Kv):
Following error ≈ feedrate ÷ position-loop gain
Take a feedrate of 6 m/min, which is 0.1 m/s. With a position-loop gain of 30 s⁻¹:
- Following error ≈ 0.1 ÷ 30 ≈ 0.0033 m, or about 3.3 mm.
- Doubling the gain to 60 s⁻¹ halves it to about 1.7 mm.
Three practical conclusions follow:
- Following error grows linearly with feedrate, which is why an axis that looks fine when jogging can lag badly at cutting speed.
- Raising gain is not the only way to shrink the error. Velocity feedforward removes most of the speed-dependent part without touching stability.
- Gain units differ between drives (1/s, Hz, rad/s or a scaled integer), so confirm the unit in your drive manual before comparing numbers.
Match Gain Across Axes
When two or more axes interpolate together, such as in circles and corners, their position-loop gains and dynamic responses should be matched. If one axis lags more than the other, a programmed circle comes out as an ellipse and corners are distorted, even if every axis is individually stable.
Is High Gain Good or Bad?
Neither, inherently. Correct gain achieves fast response, minimal following error, stable motion and acceptable noise and temperature.
Symptoms of low gain:
- slow axis response,
- high following error under acceleration,
- poor path accuracy under load.
Symptoms of excessive gain:
- high-pitched noise or vibration,
- oscillation near zero speed,
- overshoot and correction loops,
- overcurrent or overheating alarms.
Why Small Gain Changes Can Destabilise an Axis
Gain is not purely an electronic setting. The mechanical system dominates stability:
- backlash, friction or a dry ball screw,
- structural resonance,
- loose couplings or mounts,
- load-to-motor inertia mismatch,
- sampling rate and feedback resolution.
Softer or more resonant mechanics reduce the allowable gain margin. This is why the same gain value that is safe on one machine can make another one howl.
A Safe Way to Raise Gain
You do not need the full commissioning procedure to adjust gain safely, but the order matters:
- Verify mechanics and feedback first. Secure couplings and mounts, confirm smooth ball-screw motion, check clean encoder signals and correct resolution and scaling.
- Reduce risk. Use reduced speed, acceleration and torque limits, and keep tooling disengaged.
- Stabilise the velocity loop. Raise speed P gradually until just below high-frequency oscillation, then add speed I carefully.
- Raise position gain. Reduce following error while watching for hunting at stops.
- Add feedforward for contour accuracy instead of more gain.
- Validate with following error against time, speed command against feedback, and torque and current traces.
Auto-tuning provides a stable baseline but rarely the best contour quality. For the complete procedure, with test profiles and verification targets, see the guide to servo motor tuning.
Filters and Settings That Work With Gain
- Notch filters: suppress mechanical resonance frequencies.
- Low-pass filters: reduce noise at the cost of delay.
- Damping and derivative filters: improve stability.
- Jerk and S-curve control: reduce resonance excitation.
- Torque and current limits: prevent damage during adjustment.
Common CNC Gain Problems: Cause and Solution
| Problem | Cause | Solution |
| Distorted circles or corners | Low gain or insufficient feedforward | Increase position gain and VFF/AFF, reduce jerk |
| Vibration at specific speeds | Mechanical resonance | Notch filtering or mechanical stiffening |
| Axis hunting at standstill | Excessive position gain or noisy feedback | Reduce gain, improve damping and filtering |
| Following error under heavy load | Low speed I or torque limits | Increase speed I carefully, review limits |
| Circle turns into an ellipse | Mismatched gain between axes | Match position-loop gain and response across axes |
Practical Rules of Thumb
- Stability comes before response speed.
- Never use gain to mask mechanical resonance.
- Feedforward improves CNC path quality more safely than gain.
- Rising motor temperature often indicates a gain problem.
- Change one parameter at a time and test consistently.
FAQ
What does gain mean on a servo motor?
Gain defines how strongly a servo control loop reacts to the difference between command and feedback. Higher gain increases response speed and stiffness but reduces stability margins.
Which servo loops have gain parameters?
The current (torque) loop, the velocity loop and the position loop each have gain parameters. They influence system behaviour at different dynamic levels, and the inner loops must be stable before the outer ones are raised.
Is higher gain always better for CNC axes?
No. Excessive gain can cause oscillation, acoustic noise, vibration, overheating and following errors. Good setup balances responsiveness and mechanical stability.
How does position gain affect following error?
For a constant-speed move without feedforward, following error is roughly feedrate divided by position-loop gain. Doubling the gain roughly halves the error, and velocity feedforward reduces it further without adding instability.
What is the safest way to increase servo gain?
Fix the mechanics first, stabilise the velocity loop, raise position gain gradually, use feedforward to reduce tracking error, and verify the result by monitoring following error and torque or current.
Set Gain With Confidence
Setting gain is a multidisciplinary task combining control theory, mechanical integrity and real-world diagnostics. The best CNC performance comes not from maximum gain but from stable loop settings combined with feedforward, filtering and sound mechanical design.
Contact Radonix or use the chatbot in the bottom right corner to talk to our technical team about your drives and axes. For machine integration, compare Radonix CNC controller hardware for independent multi-axis motion, pulse/direction signals and industrial I/O.
For machine integration, compare Radonix CNC controller hardware for independent multi-axis motion, pulse/direction signals and industrial I/O. Radonix CNC controller hardware.
