Tuning Motion Systems and Avoiding Resonance

Why a motion axis misbehaves

A motion axis tries to follow a command, and it never does so perfectly. When it works well, the load moves smoothly and stops where it should. When it works badly, it overshoots the target, swings back and forth around it, buzzes at standstill, or shakes at certain speeds. These symptoms usually come from the same few causes: how aggressive the controller is, how heavy and flexible the mechanics are, and how fast and clean the feedback is.

The feedback loop in brief

A servo drive compares where the load should be with where it is and applies a correction. The strength of that correction is set by gains. In a typical position loop:

• Proportional gain pushes in proportion to the error. Higher values respond faster and hold position more stiffly, but a value that is too high causes overshoot and oscillation.
• Integral gain accumulates small errors over time and removes steady offsets, such as those caused by friction or gravity. Too much can cause slow, wallowing oscillation and overshoot.
• Derivative gain reacts to the rate of change of error and acts like damping, calming overshoot. It also amplifies noise, so too much can cause buzzing.

Many drives also add feed-forward, which supplies the expected speed or torque based on the command rather than waiting for an error to develop. Good feed-forward lets the loop work with a smaller error, which lets the gains stay lower and the system quieter.

Loops are nested, with a fast current loop inside a speed loop, inside a position loop. The usual advice is to tune from the inside out, and to keep each outer loop slower than the one inside it.

Overshoot and oscillation

A system with too little damping behaves like a mass on a spring. It reaches the target with speed and carries on past it, then swings back. Overshoot is the first sign. If it grows or never dies away, the loop is becoming unstable.

Typical causes include gains set too high for the load, delay in the feedback signal, and a heavy load with a lightly sized motor. Reducing gain, adding damping or slowing the commanded profile with gentler acceleration usually helps. There is always a trade-off: lower gains give a calmer axis but a softer response and larger following error.

Inertia matching

The motor feels the load’s inertia through any gearing. If the load’s inertia is much larger than the rotor’s, the motor has trouble controlling it, and the loop is sensitive to small disturbances. Guidance on acceptable ratios varies by manufacturer and by how stiff the mechanics are, so check their documentation instead of relying on a general number. A gear reducer can lower the inertia the motor sees, which is one reason gearboxes are used even where speed is not an issue.

What resonance is

Every mechanical structure has natural frequencies at which it vibrates easily. A shaft, coupling, belt, frame or ball screw is not perfectly rigid. When the motor drives the load through a springy link, the motor and load can oscillate against each other at a particular frequency. If the controller’s response includes energy at or near that frequency, the vibration grows and the axis rings, hums or squeals.

Common sources of resonance:

• Flexible couplings or long thin shafts that twist under load.
• Belts that stretch and rebound.
• Long screws that flex, or screws that approach their critical speed.
• Machine frames and mounts that are not stiff.
• Backlash, which creates a “bump” each time torque changes direction.

Stepper motors have their own resonance problem. At certain speeds, the stepping pulses line up with the rotor’s natural frequency, producing vibration, noise and sometimes lost steps. It is often most noticeable at low speeds and can be reduced with microstepping, mechanical damping, drive features that add electronic damping, or changes in acceleration so the motor does not linger in a troublesome speed range.

How tuning commonly proceeds

Procedures differ between drives, and the maker’s tuning guide should lead. In general terms, the process often looks like this:

1. Check the basics. Confirm that the mechanics are tight, couplings are secure, belts are properly tensioned, the load is free to move, and the feedback signal is clean. Many “tuning problems” are loose parts.
2. Set safe limits. Torque limits, speed limits and travel limits keep a bad test from damaging equipment.
3. Start with conservative gains. Many drives offer automatic tuning or an inertia estimate as a starting point. These are good for a first pass, though not always the final answer, and they need the axis to move, so make sure it is safe to do so.
4. Tune the speed loop first. Increase gain until the response is quick, then back off when ringing starts.
5. Then the position loop. Raise the gain until moves are crisp, and stop short of overshoot.
6. Add feed-forward where available to reduce following error.
7. Use filters. Notch filters remove a narrow band of frequency where a resonance lives, and low-pass filters soften noisy signals. They add delay, so they are used sparingly.
8. Test across the working range. Check at full and light load, at different positions and speeds, since mechanics can behave differently along the stroke.

Many drives include scopes or trace tools that record command, actual position, speed and torque. Looking at a plot of a step move is often the quickest way to tell overshoot from noise from resonance.

Reducing the problem at the source

Software is not the only answer. Often the best improvements are mechanical:

• Use stiffer couplings and shafts where precision matters, or well-sized couplings that absorb misalignment without adding springiness.
• Reduce backlash with preloaded components or better gearing.
• Make sure shaft alignment is correct, since misalignment adds vibration and wear.
• Check bearings, since worn parts add play. Bearing wear patterns that appear early often show up as noise before they affect accuracy.
• Mount motors and frames rigidly and add damping where needed.
• Use feedback with enough resolution and low delay.

Vibration analysis tools used in maintenance can help identify which frequencies are present and where they originate.

Safety

Tuning involves moving axes, sometimes at unexpected speeds or with oscillation that can build quickly. Keep people clear, use limits and stop circuits, follow the manufacturer’s procedures, and let only trained, authorized personnel carry out tuning on real machines. Never defeat safety devices to make testing easier. This article is general information, not engineering advice.