Servo parameter tuning is the area where Chinese-language references contradict each other most severely: for the same 1825, some say set 5000, some say 50–80, and others say 80. These statements leave people at a loss, and following them may even tune the machine into vibration.
This article follows the FANUC official “Series 0i-MODEL D / 0i Mate-MODEL D START-UP MANUAL” (B-64304EN-3), chapter “PARAMETERS RECOMMENDED TO BE SET,” to clearly explain the parameter numbers, starting values, adjustment direction, conversion formulas, and waveform interpretation, and points out the common errors in Chinese references one by one.
Safety note: Servo parameters directly affect the stability and accuracy of machine motion. You must complete a parameter backup before tuning, and verify step by step under non-production, low-speed, single-axis conditions. Too-high gain causes severe axis vibration and even crash into the limit. Before changing safety-related limits or soft limits, confirm you have a rollback plan.
1. Relationship of the Three Loops (adjustment order cannot be reversed)
Servo control is a three-layer nested loop, from inner to outer:
| Loop | Role | Who owns it |
|---|---|---|
| Current loop (innermost) | Improves system responsiveness, limits maximum current, gives enough acceleration torque | Usually preset by the drive at factory, generally not user-adjusted |
| Speed loop (middle) | Improves resistance to load disturbance, suppresses speed fluctuation | One of the main user adjustment objects (corresponds to 2021 related) |
| Position loop (outermost) | Guarantees static accuracy and dynamic following performance | One of the main user adjustment objects (corresponds to 1825) |
Why the order cannot be reversed: the inner loop is the foundation of the outer loop. If the inner loop is not tuned, no matter how you tune the outer loop it is built on sand — the speed loop is still oscillating, and lowering the position-loop gain can’t suppress it; conversely, a position-loop problem may be misjudged as a speed-loop problem.
Correct adjustment order: current loop → speed loop → position loop.
A practical judgment: servo parameters have high matching requirements with mechanical moving parts. The same set of parameters behaves completely differently on machines with different rigidity and different inertia — so there is no “universal optimal value,” only “the suitable value for your machine.”
2. Parameter Quick-Reference Card
The table below is the original organization of “parameters to adjust for finding the optimal value” from the official start-up manual (0i-D / 0i Mate-D):
| Parameter No. | Debug starting value | Official description | Official adjustment method |
|---|---|---|---|
| 2021 | 128 | Load inertia ratio (LDINT) / velocity gain | If vibration occurs when the axis moves, decrement in order 128 → 64 → 0 |
| 1825 | 5000 | Position gain | If the inertia ratio is already down to 0 and vibration still persists, decrement all axes’ 1825 in order 5000 → 4000 → 3000 |
| 2048 | 100 | Backlash acceleration | If a protrusion appears at axis reversal, add 50 each time; if a depression appears, subtract 50 each time |
About the contradiction between 2020 and 2021 (key point)
This is the most chaotic spot in Chinese references; it must be clarified:
- The official 0i-D start-up manual writes 2021 = Load inertia ratio (LDINT), and in parentheses notes it also corresponds to velocity gain.
- Many Chinese references separately explain “2021 = speed-loop gain, 2020 = inertia ratio,” which is inconsistent with the official manual.
- My suggestion: don’t rely on memorizing the parameter number; instead look at the parameter name shown on the parameter screen. On the servo setting / servo adjustment screen, the parameter name is explicit (e.g. “load inertia ratio”); follow the screen display. Parameter numbers may differ across system models (0i-C / D / F, and 15i/16i/18i/21i/30i series), which is one reason references don’t line up.
Other common parameters (treat with caution)
- Feed-forward related (some references call it 2068 / 2069): used to reduce position following error. This article gives no specific value suggestion because references differ; follow your machine’s manual.
- HRV control: can eliminate low-rigidity-machine low-frequency vibration; some references mention enabling it via HRV-related bits (e.g. No.2013 #0). For function enablement, check your machine’s manual and confirm the option exists.
- 2205: this article does not list its specific meaning — I could not find a sufficiently reliable official source; rather than give a possibly wrong value, let it be your manual lookup. This is the site’s consistent principle: rather omit than write without a reliable source.
3. Conversion Formula: Inertia Ratio and Speed-Loop Gain
The official manual gives a clear conversion relationship (NOTE 1):
Speed-loop gain (%) = (1 + load inertia ratio / 256) × 100
| Speed-loop gain | Load inertia ratio |
|---|---|
| 150 % | 128 |
| 200 % | 256 |
| 250 % | 384 |
| 300 % | 512 |
How to use this formula:
- Knowing the inertia ratio → you can calculate the current speed-loop gain, to judge whether the setting is reasonable (e.g. 2021 = 128 corresponds to 150%).
- Conversely: if you want the speed-loop gain to reach a certain percentage, you can reverse-calculate the inertia ratio setting.
- Practical meaning: it explains why “lowering 2021 from 128 to 0” can kill vibration — because the speed-loop gain drops from 150% to 100%, the system response slows and the vibration is suppressed. The cost is slower response and larger following error.
So the essence of tuning is trade-off: higher gain → faster response and higher accuracy, but prone to vibration; lower gain → stable, but larger following error and poorer contour accuracy. There is no best of both worlds, only the balance point for this machine.
4. Practical Adjustment Flow (two routes)
Applicable: initial commissioning, or quick usable parameters after replacing motor / drive.
Route A: Auto tuning (AUTO TUNE / servo adjustment screen)
- Enter the servo setting / servo adjustment screen (on some systems via SERVO TUNING in the parameter setting support screen).
- Use the auto-tuning function to let the system automatically calculate a set of initial values based on mechanical characteristics.
- Auto-tuning done does NOT mean finished — you must actually run and observe, then manually fine-tune per section 3.
The official start-up manual mentions: the servo tuning tool (Servo Guide) can be used to observe vibration state and the bump/dip at reversal. Where available, use it to view the waveform — far more reliable than judging by feel.
Route B: Manual fine-tuning (the order officially recommended)
Follow the official table order; do not skip steps:
- Step 1: first touch 2021 (inertia ratio).
- Step 2: 1825 is the backstop, not the first choice.
- Step 3: 2048 handles the shape at reversal.
- Step 4: verify.
General idea (specific menu follows your machine): starting value 128; if vibration when the axis moves: 128 → 64 → 0 decrement. After each change, run and observe whether the vibration disappears. Note the order: the official writes “only touch 1825 if the inertia ratio is already down to 0 and vibration still persists.” In other words: lower the inertia ratio first; only lower the position gain when the inertia ratio is bottomed out and it still vibrates.
5. Semi-closed Loop vs Full-closed Loop: What Should 1825 Actually Be?
This is the real origin of the “5000 or 80” contradiction; let’s clarify now.
- Official 0i-D start-up manual: 1825 debug starting value = 5000; when vibrating, decrement 5000 → 4000 → 3000.
- This exactly matches the official decrement sequence: start 5000, if unstable drop to 4000, then 3000.
Semi-closed / full-closed difference
- Semi-closed loop: position is fed back by the motor encoder. The mechanical transmission-chain error (backlash, pitch error) is outside the feedback loop, so the system “can’t see” the mechanical error and can tolerate a relatively high position gain. Common suggested value about 5000 (0.01/s unit).
- Full-closed loop: position is directly detected by an external detector such as a scale. Mechanical backlash and transmission elasticity are brought into the feedback loop, and the system easily vibrates due to mechanical issues, so the position gain usually needs to be lowered. Common suggested value about 3000.
Key conclusion: 5000 and 50–80 are very likely the same thing in two units.
- The official 1825 starting value 5000 has unit 0.01/s.
- Converted to /s: 5000 × 0.01 = 50 /s.
- The “50–80” (or “30–80 /s”) in Chinese references uses the /s unit.
So they do not contradict — 5000 (0.01/s) = 50 /s. The real problem is:
- Many references mix units without stating them, making readers think they are two completely different conclusions.
- Some references mixed in other brands’ parameter numbering (e.g. Pn. prefix) into FANUC’s parameter numbers — for example writing “load inertia ratio” as Pn.0.12, “acceleration/deceleration time” as Pn.0.02, initialization as P900=1 — these are not FANUC’s parameter system, a clear case of misattribution; do not copy them.
So a clear suggestion: fill in per the official manual’s setting-value format (e.g. 1825 = 5000), and adjust per the official decrement sequence (5000 → 4000 → 3000). If your reference gives 50–80 type values, first confirm its unit, don’t directly fill them into the parameter screen.
Additional hard rule: the servo axes doing interpolation must have their position-loop gains set consistently. Only servo axes doing pure positioning control may differ. Inconsistent interpolation-axis gain causes trajectory error in simultaneous machining.
6. Acceleration/Deceleration Time and Backlash Compensation (2048)
Backlash acceleration 2048
- 2048 (backlash acceleration) is an electrical compensation parameter handling acceleration compensation at reversal.
- Mechanical backlash (the gap between screw and nut) physically exists; parameters can only compensate, not eliminate.
- Protrusion at axis reversal → 2048 add 50 each time; depression → 2048 subtract 50 each time. Starting value 100.
About mechanical backlash
Vibration looks at the inertia ratio (inner loop); shape looks at the backlash compensation; only as a backstop touch the position loop.
“Over” and “under” have opposite directions: too-large inertia ratio = high-frequency vibration; too-small inertia ratio = poor circle, large quadrant protrusion. Don’t blindly raise gain the moment you see a protrusion.
Acceleration/deceleration time
Extending acceleration/deceleration time reduces mechanical shock, suitable for heavy-load conditions. The specific parameter number varies by system model; follow your machine’s manual. This article does not list uncertain numbers.
7. Waveform Interpretation: Phenomenon → Corresponding Parameter
The official start-up manual gives the correspondence between waveform and parameter setting — the most valuable table:
| Observed phenomenon | What it means | Which parameter / how |
|---|---|---|
| High-frequency vibration | Load inertia ratio (velocity gain) too large | Lower 2021: 128 → 64 → 0 |
| Poor circle shape, large quadrant protrusion | Load inertia ratio (velocity gain) too low | Raise 2021 (note: don’t add too much at once) |
| Protrusion at reversal | Backlash acceleration too small | 2048 add 50 each time |
| Depression at reversal | Backlash acceleration too large | 2048 subtract 50 each time |
| Inertia ratio already down to 0 and still vibrating | Need to touch position loop | 1825 decrement: 5000 → 4000 → 3000 (all axes) |
Judgment point: the handling directions for protrusion and depression are opposite; doing it reversed makes it worse. Observe the shape at the reversal instant, not the steady-state segment.
8. Must-Do Before Tuning: Parameter Backup
This is the only item with “don’t do it and don’t start.”
Three things you must do:
- Back up the current servo parameters and NC parameters completely, and confirm the backup file is readable (a backup never verified is as good as no backup).
- Record what you changed and what the before value was — suggest a table, item by item, for easy rollback.
- Keep at least one copy off-site.
On backup file format: different systems and backup methods output different formats (e.g. via memory card the system may output a packaged file). Follow the file your system actually outputs; don’t look for a file by the suffix written in a reference. The key is to verify it can be read back.
Backup operations and common accidents: see the “backup five-piece set” principle in FANUC PMC Backup and Restore — servo parameters are only one of them; don’t back up only the ladder.
9. Is Servo Vibration Always an Electrical Problem?
No, and mechanical causes are very common.
First assign responsibility, then act:
| Possible source | Typical manifestation |
|---|---|
| Mechanical | Screw wear / large backlash, poor guide lubrication, loose coupling, bearing damage, insufficient mechanical rigidity, very large load inertia |
| Electrical / parameter | Gain too high, inertia ratio set wrongly, feedback signal interfered, wiring problem |
| Installation | Motor and screw not concentric, coupling installed badly |
Troubleshooting order (cheap to expensive):
- First confirm mechanical state: manually jog / run at low speed and feel for sticking, abnormal noise, backlash. If mechanical problems are not solved, tuning only covers them up.
- Then check feedback and grounding: encoder cable, shielding, grounding.
- Only last is parameter adjustment.
A field lesson: servo parameters are the easiest to go wrong and the hardest to tune, with high matching requirements to mechanical moving parts. When vibration occurs, suspecting the machine first often finds the root cause faster than changing parameters.
For servo-alarm troubleshooting see FANUC Servo Alarm Code Cross-Reference SV0301-SV0441: SV0430 (servo overload) mechanical priority over electrical, same idea.
Closing
Take these with you:
- Order cannot be reversed: current loop → speed loop → position loop; lower the inertia ratio first, only as backstop touch the position loop.
- Official starting values: 2021 = 128, 1825 = 5000, 2048 = 100; decrement sequences 128 → 64 → 0, 5000 → 4000 → 3000.
- Formula: speed-loop gain (%) = (1 + load inertia ratio / 256) × 100.
- 1825’s 5000 and “50–80” are a unit difference (0.01/s vs /s), not two conclusions; but beware of references mixing in other brands’ numbering.
- Semi-closed 5000, full-closed usually drop to 3000; interpolation-axis position-loop gains must be consistent.
- Vibration: check mechanical first, then parameters. Backup before tuning.
Disclaimer: This article is compiled from the FANUC official “Series 0i-MODEL D / 0i Mate-MODEL D START-UP MANUAL” (B-64304EN-3) and public technical materials. Parameter numbers, setting values, and units vary by system model (0i-C/D/F, 16i/18i/21i/30i etc.) and software version; follow your machine’s parameter manual and parameter screen display. Servo parameters directly affect motion stability; complete a backup before tuning and verify step by step under non-production, low-speed conditions.
Zero-base learning entry: PMC Ladder Video Course From Zero.
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