Direct Answer
Servo motor tuning on a paper cup machine is a load-matching job, not a one-time factory setting. Three numbers decide whether a forming, curling, or bottom-heat axis holds its position at speed: the load-to-motor inertia ratio, the following error under load, and the settling time after each index. Tune the mechanical path first, set the inertia ratio below 10:1, then raise the position loop from a stable velocity loop and add feedforward instead of brute-force gain. A tuned axis repeats side-wall height and seam overlap without operator babysitting; an over-gained axis hunts, overheats, and throws reject cups at high cycle rates.
Opening Hook
A cup plant in Vietnam replaced a burnt servo on its side-seal axis, used the drive's auto-tune routine, and shipped the next batch — within two days the line was stopping on following-error alarms every few hundred cups and the reject bin filled with misaligned seams. The new motor had a different rotor inertia than the old one, and auto-tune had matched gains to a load it had not measured. The fix was not a bigger motor but a proper tune: verify the mechanical path, set the real inertia ratio, then walk the loops up in order. At yoco-group, we commission every cup machine with a documented tuning record — here is the procedure we use and the numbers we hand over in the machine file.
What Servo Tuning Actually Controls
A cup machine servo runs a nested control structure, and tuning means setting the outer loops without destabilizing the inner ones.
| Control Layer | Parameter | What It Fixes | Symptom If Wrong |
|---|---|---|---|
| Current loop | Motor constants | Torque response | Rough, noisy motion |
| Velocity loop | Velocity gain, integral | Speed stability | Hum, hunting, overshoot |
| Position loop | Position gain | Final accuracy | Following error, soft dwell |
| Feedforward | Velocity/accel gain | Error under acceleration | Lag during index moves |
| Filters | Notch, low-pass | Resonance | Vibration at one speed |
The current loop is set by the drive against the motor and rarely changed. The work a technician does on site is velocity gain, position gain, feedforward, and the notch filter — in that order, because position gain on an unstable velocity loop amplifies noise instead of accuracy.
Data: ISO's machinery risk-assessment standard (ISO 12100) requires that unintended motion and stored energy in a drive system be analyzed as design hazards, with predictable stopping and controlled access before commissioning.
Judgment: Treat a re-tune as a safety event, not a maintenance chore: raising gains changes how the axis stops under fault, so the stopping performance and guarding interval must be re-checked after any tuning change, not only after a mechanical rebuild.
Source: ISO — ISO 12100 Risk Assessment for Machinery (2023)
The Pre-Tune Mechanical Check
No gain value fixes a loose mechanical path, so the mechanical check comes before the software.
- Coupling and keyway: check for play by hand and with a dial indicator; any rock means the feedback no longer reflects the table or belt position.
- Belt tension and pulley runout: a slack belt adds compliance that raises the effective inertia ratio and makes gains unstable.
- Gearbox backlash: measure at the output, not the input; backlash above a few arc-minutes turns every index into a settling problem.
- Encoder mounting and cable: a loose encoder couples vibration into the position signal and shows up as phantom hunting.
- Lubrication and drag: dry ways or a stuck bearing add friction load that makes gains behave differently hot and cold.
| Check | Target | Tool |
|---|---|---|
| Coupling play | Near zero | Dial indicator |
| Belt tension | Per OEM spec | Tension gauge |
| Gearbox backlash | Minimum for the axis | Dial indicator |
| Encoder runout | Within spec | Indicator on shaft |
Only when the mechanical path is tight does the inertia ratio mean anything — and that ratio is the single most useful number in the whole tune.
Tuning Sequence From Inertia Ratio to Following Error
Follow the order; skipping a step is what produces a machine that passes auto-tune and fails in production.
- Set the inertia ratio. Measure or estimate load-to-motor inertia and keep it under 10:1, targeting 3:1 to 5:1 on fast forming and curling axes.
- Run auto-tune as a baseline. Let the drive estimate inertia and set initial gains — then verify against a step response, do not trust it blindly.
- Raise velocity gain until the drive hums, then back off. Find the stability edge, then drop to roughly 60–70% of it.
- Add position gain for accuracy. Increase until following error under a real index drops and settling stays clean.
- Add feedforward. Feedforward removes lag during acceleration without pushing gains toward instability.
- Place the notch filter. Where one speed rings, tune the notch to that frequency rather than lowering gain globally.
| Loop | First Move | Stop When |
|---|---|---|
| Velocity | Raise until hum, back off 30% | Motion is crisp, no hum |
| Position | Raise for tighter error | Following error within spec |
| Feedforward | Add fraction of accel signal | Lag at index drops |
| Notch | Sweep to ring frequency | Vibration gone across the speed range |
Reading the Diagnostics — Faults and Fixes
The drive's diagnostics name the fault; the cause is usually mechanical or gain-related.
| Fault | Likely Cause | First Fix |
|---|---|---|
| Following-error alarm | Gains too low for load, or mechanical drag | Re-check mechanics, then feedforward |
| Hunting at standstill | Position gain high on soft load | Notch filter, lower velocity gain |
| Overshoot at index | Inertia ratio high, integral too strong | Reduce integral, tighten belt |
| Overheat / overload | Re-tuning exposed a binding axis | Clear the drag, verify current limits |
| Rejects at high speed | Settling too slow for cycle | Shorten dwell or raise response |
A tune that holds at 60 cycles per minute but fails at 120 is a settling problem, not a strength problem. The machine is telling you the axis needs to reach position sooner, and the answer is a tighter mechanical path with feedforward — not a heavier motor.
Data: UL Solutions' industrial control panel certification evaluates the electrical, fire, and operator-safety risks of a control cabinet before listing, giving buyers third-party evidence that the electronics driving a servo axis meet a recognized safety standard.
Judgment: Because servo tuning lives in the same panel that handles faults and emergency stops, request the panel certificate and the tuning record as one document set; an untuned cabinet and an uncertified cabinet are both latent risks hidden behind a running machine.
Source: UL Solutions — Industrial Control Panel Safety Certification (2024)
Tuning in Production — Verification and Safe Access
A tune is finished only when it survives a real production run with the guards closed and the operators in place.
- Run at target speed and log following error on every axis — it should sit inside spec for a full shift, not just a sample.
- Check cup outputs: side-wall height, seam overlap, and bottom roundness against the drawing.
- Let the machine idle between jobs and listen for hum that only appears at one speed.
- Re-check stopping time under emergency stop, since new gains change the deceleration profile.
- Write the gains, inertia ratio, and filter values into the machine file so the next technician starts from data.
Data: OSHA's control of hazardous energy rules require the machine to be isolated and verified at zero energy before anyone reaches into the drive area, which includes the window where a technician re-tunes an axis and must be near moving members.
Judgment: Bake lockout into the tuning routine: a tune done with guards open and power live is how technicians lose fingers, and a documented isolation step in the procedure keeps a fast fix from becoming an incident.
Source: U.S. OSHA — Control of Hazardous Energy (Lockout/Tagout) (2023)
Keep the spare-drive and spare-coupling stock aligned with the tuning cycle, because an axis re-tuned on a fresh coupling behaves differently from the one it replaced — our paper machinery spare parts inventory planning guide maps that link between parts and uptime.
Data: Lean Enterprise Institute's lexicon frames standard work as the documented current best method, and SMED as the discipline of moving setup work off the stopped machine — the same logic that turns tuning from a heroic intervention into a repeatable recipe.
Judgment: Record the tuning sequence as standard work with a target following-error figure: when every technician tunes to the same numbers, machine behaviour stops depending on who touched the panel last, and changeover and fault recovery both get faster.
Source: Lean Enterprise Institute — Lean Lexicon: Standard Work and SMED (2023)
Drive architecture also decides how much tuning work falls on you — the trade-offs are set out in our paper cup machine servo vs cam technology guide, and the efficiency payoff is covered in our paper cup machine OEE guide.
The Bottom Line
Servo motor tuning on a paper cup machine is a sequence, not a single setting: tighten the mechanical path, keep the inertia ratio under 10:1, build a stable velocity loop before adding position gain, use feedforward for lag, and notch out resonance. Verify against follow-up error, cup dimensions, and emergency-stop time on a live shift, then write the numbers into the machine file as standard work. Three deliverables prove a tune is real — the inertia ratio, the following error at speed, and the tuning record.
At yoco-group, every cup machine ships with a commissioning record that lists the tuned gains, the measured inertia ratio, and the verified following error per axis — so the machine you receive is a machine you can hold to a number.