1. Overview
Siemens Sinumerik 840D systems equipped with SIMODRIVE 611 digital drives and a PCU 50 HMI can report Contour Monitoring faults on individual or multiple axes during a contouring move. The fault indicates that the actual position lag of one or more axes has exceeded the configured contour deviation threshold while the interpolator was generating a synchronized path. Unlike a pure position-lag alarm triggered by a single axis, a contour monitoring alarm is a path-relative check between the commanded and the actual contour.
This reference consolidates the mechanical, drive-side, and control-side root causes encountered in field service and walks through the diagnostic path, the relevant machine data, the 611D drive parameters, the HMI axis-optimization procedure (Setup → optim./test), and the verification checks required to clear and prevent the alarm.
2. Problem Detail: How the Alarm Is Raised
On a Sinumerik 840D, the NCK monitors the contour deviation for every geometry axis pair participating in a programmed contour. The contour deviation is the difference between the commanded contour derived from the interpolator and the actual contour reconstructed from the measuring-system feedback of all participating axes. When the deviation exceeds the threshold defined in the machine data, the NCK raises:
- Alarm 10880 — "Channel %1 axis %2 contour monitoring" (channel-level contour deviation).
- Alarm 25080 — "Axis %1 contour monitoring" (axis-specific contour deviation).
- Alarm 25050 — "Axis %1 contour monitoring" combined with position-lag / following-error faults (referenced by PLC/DRF monitoring blocks).
The alarm is typically followed by an NC STOP / feed-hold or, with MD 36610 $MA_AX_EMERGENCY_STOP_TIME configured for fast-stop behavior, a hard ramp-down of the affected axis. The HMI displays the alarm banner and writes the alarm log to the active commissioning or operator logbook.
Contour monitoring can be:
- Intermittent — appears only on certain contour segments, on direction reversal, or when the auxiliary axis performs an index move (for example a B axis rotating ±90° between operations).
- Persistent on a single axis — appears whenever the horizontal axis (X) is commanded to a position, even at modest feed rates.
- Multi-axis — appears on the X axis, the Y axis, and the B axis in turn, often pointing at a common denominator rather than a per-axis fault.
3. Root Cause Matrix
| Domain | Symptom pattern | Typical root cause | Quick verification |
|---|---|---|---|
| Mechanical | Alarm on direction reversal, on rapid traverse, or on the first move after lubrication | Stiction in the slide, dry guideway, lubrication block, defective roller bearing, preloaded guide bind | Move axis manually; check the slide force, listen for irregularity, check the central lubrication distribution block |
| Mechanical (B axis) | Fault during ±90° index moves; manually lubricated axis | Worm/worm-wheel preload too high, insufficient lubrication film, swivel bearing wear | Rotate B axis manually; check torque; confirm the lubricant path is open at the bearing |
| Coupling | Fault on acceleration, especially on horizontal axis under load | Loose motor-to-spindle coupling, worn flexible coupling, key fretting | Setpoint/actual comparison with the drive in torque-controlled service; visual inspection of coupling |
| Brake | Fault on the first move after release | Motor holding brake not fully releasing, brake drag, air-gap out of tolerance | Check the brake current, listen for the release click, measure the rotor for residual torque |
| Drive | Fault on the first heavy pass, no mechanical cause | 611D speed controller poorly tuned, P gain too high, integral time too short, no adaptation | Run the drive's autotune; check p1460, p1462, p1414, p1415 |
| Control | Fault appears on every contour regardless of feed | Position controller gain (Kv) set too high, contour tolerance too tight, feedforward not active | Inspect MD 32200, MD 36400, MD 32800/32810, MD 32900/32910 |
| Measuring system | Fault only on a specific axis at a specific position | Encoder contamination, cable screening, dirt on the linear scale | Check the axis diagnostic screen for the position-act-value ripple |
4. Diagnostic Procedure
- Capture the alarm context. Before resetting, record the alarm number, the channel, the axis, and the program block. On PCU 50, navigate to Diagnostics → Alarms and export the alarm log.
- Reproduce in JOG. Move the affected axis in JOG at progressively higher feed override. If the contour monitoring fault appears in JOG, the issue is in the drive or the axis mechanics. If it does not, the issue is in the program (block transitions, G64/G642 setting, ACC settings).
- Inspect the lubrication system. On horizontal X and Y slides, check the central lubrication distribution block for blocked outlets, blown seals, and correct pump cycle. A dry slide is the most common cause of intermittent contour monitoring faults on the horizontal axis.
- Check the measuring system. Open Diagnostics → Axis diagnostics and look at the position-act-value signal at standstill and during movement. A noisy or jittery signal indicates a contaminated scale or a damaged encoder cable.
- Check the drive. On the 611D module, read out the warnings in Drive → Diagnostics. Fault 790D (measuring-circuit errors) or 608 (speed-controller deviation) confirms a drive-side problem.
- Rule out mechanical binding. With the drive disabled and the brake released, rotate the affected axis by hand and feel for tight spots. For a B axis rotating ±90°, this is especially important because the swivel bearing can be under-greased.
5. Machine Data for Contour Monitoring
The following machine data are the most frequently involved in a contour monitoring alarm on 840D. The exact value at which the alarm is raised depends on the machine builder's commissioning profile; only adjust these after a mechanical and drive check has been completed.
| MD | Identifier | Function | Effect on contour monitoring |
|---|---|---|---|
| MD 32200 | $MA_POSCTRL_GAIN | Position-controller gain (Kv factor) | Higher Kv = stiffer following but greater overshoot and contour deviation on acceleration peaks |
| MD 32300 | $MA_MAX_AX_ACCEL | Maximum axis acceleration | Too low = axis cannot follow the path; too high = drive saturation and contour lag |
| MD 32400 | $MA_AX_JERK_ENABLE / filter | Jerk limiting | Disable or widen jerk limit only after the contour accuracy is verified |
| MD 32800 / 32810 | $MA_EEC_ENABLE / $MA_EEC | Speed feedforward control | Activates feedforward to reduce contour lag at constant velocity |
| MD 32900 / 32910 | $MA_DYN_MATCH_ENABLE / $MA_DYN_MATCH_TIME | Dynamic response adaptation | Equalises the dynamic response of axes participating in the contour |
| MD 36030 | $MA_STANDSTILL_POS_TOL | Standstill position tolerance | Position window for the standstill check |
| MD 36050 | $MA_STOP_LIMIT_FACTOR | Stop-limit factor | Multiplied with the contour tolerance for the deceleration phase |
| MD 36400 | $MA_CONTOUR_TOL | Contour tolerance | Direct threshold for the contour deviation alarm; widening reduces false trips but degrades accuracy |
| MD 36600 | $MA_BRAKE_MODE_CHOICE | Brake control | Defines when the holding brake is applied; mis-configuration can hold the axis at start-up |
6. SIMODRIVE 611 Digital Drive Parameters
For an 840D with 611D drives, the contour monitoring alarm is usually preceded by a drive-side warning that the speed controller cannot follow the setpoint. The relevant 611D parameters are:
| Parameter | Function | Diagnostic hint |
|---|---|---|
| p1460 / p1461 | Speed controller P gain (positive/negative direction) | Too high = controller oscillation; too low = following error |
| p1462 / p1463 | Speed controller integral time | Too short = oscillation; too long = static lag |
| p1414 | Reference model natural frequency | Set close to the mechanical resonance, but below the encoder limit |
| p1415 | Reference model damping | 1.0 = critical damping; lower = faster but more overshoot |
| p1407.0 | Speed controller adaptation | Enable to switch controller parameters in the low-speed / high-speed range |
| p1409 / p1410 | Torque setpoint smoothing | Reduces the excitation of mechanical resonances |
The speed gain referenced in the field discussion is the combination of p1460/p1461 and p1462/p1463. The Drive MD set described in the original session (speed gain in Drive MD) maps to these parameters. The correct adjustment procedure is:
- Run the 611D automatic speed-controller tuning (drive-side autotune).
- Verify the result by commanding a step response and reading the speed-act-value transient.
- Adjust p1460 (gain) and p1462 (integral time) iteratively. Increase p1460 by 10–20 % and reduce p1462 by 10 % to start; re-run the contour move.
- After the drive is stable, adjust the position controller in the NCK (MD 32200 $MA_POSCTRL_GAIN).
7. Step-by-Step: HMI Axis Optimization on PCU 50
On a PCU 50 HMI, the axis-optimization procedure is the standard, manufacturer-blessed way to retune a single axis end-to-end (drive + position controller). Procedure:
- From the Setup operating area, press the optim./test softkey.
- Select the axis to be tuned from the axis list (X, Y, or B in the reported case).
- The HMI will guide the operator through several steps. The third step requests the brake status:
- Confirm that the motor is rigidly coupled to the mechanical axis (no looseness, no slip in the coupling). A loose coupling during the optimization step will produce an artificially low inertia and a Kv factor that is too high.
- Let the optimization run to completion. Do not abort early.
- Save the results when prompted. The HMI writes the new Kv, feedforward, and jerk values to the corresponding machine data.
- Repeat for each affected axis.
8. Mechanical Verification Checklist
Before any tuning, complete this mechanical checklist. It is the fastest way to rule out a non-electronic root cause.
- Guideway lubrication — confirm the central lubrication pump is running, the distributor block is delivering oil to every outlet, and the guideways are not dry. A dry X-axis slide is the single most common cause of contour monitoring faults on horizontal moves.
- B axis lubrication — on a manually lubricated B axis, grease the swivel bearing and the worm-wheel. Run a ±90° index move after greasing to confirm the fault is gone.
- Coupling — visually inspect the motor-to-spindle coupling for fretting, backlash, and tightness. For a toothed belt, check the tension. For a bellows coupling, check the bellows for cracks.
- Brake — with the drive inhibited, command the holding brake from the HMI (Diagnostics → Axis → Brake test) and listen for the full release. A dragging brake will create a continuous position lag that always exceeds the contour tolerance.
- Backlash — measure the mechanical backlash on the affected axis. Excessive backlash produces a contour error on direction reversal even when the drive is correctly tuned.
- Preload — check the preload on the recirculating bearing and the worm/worm-wheel. A binding axis will trip the contour monitor on every direction reversal.
9. Drive Tuning Procedure for 611D
If the mechanical checks pass and the HMI optimization is not available (no PG/PC, no HMI Advanced, or PCU 50 with restricted softkey access), the drive can be tuned directly with SimoCom U or with the drive's integrated autotune.
- Open the drive in SimoCom U (online via PROFIBUS or RS-232 on the 611D module).
- Execute Commissioning → Controller data → Automatic speed controller setting. The drive performs a current-loop and speed-loop identification.
- Save the new controller parameters to the drive.
- Reset the NCK (NCK reset) to re-read the drive configuration.
- Run a JOG move on the tuned axis with progressive feed override. Confirm that the speed-act-value reaches the setpoint without oscillation.
- Run a real contouring block and observe the contour deviation in the HMI service screen.
10. Verification
After the corrective action, run the following verification sequence. The alarm is considered cleared only when all steps pass.
- JOG move — traverse the affected axis in JOG at 25 %, 50 %, 75 %, and 100 % feed override. No alarm.
- Reference move — run a G74 reference-point approach. No alarm.
- Index move — for the B axis, run a ±90° index several times. No alarm.
- Contour move — run a representative contour program (a circle, a spline, an inclined plane) at the maximum programmed feed. No alarm.
- Service screen — open the HMI service screen for the contour and confirm that the contour deviation stays within the configured tolerance during the entire move.
- Alarm log — export the alarm log and confirm that no new contour monitoring alarm has been written since the corrective action.
11. Best Practices and Prevention
- Lubrication schedule — respect the central lubrication interval. A blocked distributor will fail one slide and produce a contour monitoring alarm that looks like a drive fault.
- Coupling inspection — include the motor coupling in the preventive-maintenance check. A worn coupling is the most common mechanical cause of intermittent contour monitoring.
- Drive firmware — keep the 611D firmware in line with the NCK software version. Mismatched versions can produce spurious contour monitoring faults at direction reversal.
- Spare parameter set — export the drive and axis parameters to a backup file after every successful commissioning. A parameter corruption after a power loss can manifest as a contour monitoring fault.
- Feedforward — keep MD 32800 $MA_EEC_ENABLE active for every geometry axis. The feedforward term is the single most effective parameter for reducing the steady-state contour deviation.
- Dynamic matching — configure MD 32900 / 32910 $MA_DYN_MATCH for every contour group. A mismatch in acceleration capability between two contouring axes is a frequent cause of a contour monitoring alarm that "moves" between axes (X, then Y, then B) depending on the programmed move.
- Contour tolerance — never widen MD 36400 $MA_CONTOUR_TOL as a first response. Widen it only after the mechanical, drive, and control root causes have been eliminated, and only to the value documented in the machine builder's commissioning record.
12. Quick Reference: Alarm → Action Map
| Alarm | Most likely cause | First action | Second action | Tuning step |
|---|---|---|---|---|
| 10880 on X during positioning | Dry X-axis slide or loose coupling | Check central lubrication; check coupling | Run HMI optim./test on X | Step 3 with brake applied if X motor is brake-motor |
| 25080 on Y during interpolation | Drive-side speed controller too soft | Drive autotune on Y axis | Check p1460, p1462, Kv factor | Re-run HMI optim./test on Y |
| 25080 on B during ±90° index | Swivel bearing dry or binding | Manually lubricate B axis | Check worm-wheel preload | Re-run HMI optim./test on B |
| Multi-axis, intermittent | Dynamic response mismatch | Enable MD 32900/32910 | Check feedforward MD 32800/32810 | Re-run optimization on all axes |
What does "contour monitoring" mean on a Sinumerik 840D?
It means the NCK has detected that the actual contour reconstructed from the axis feedback deviates from the commanded contour by more than the configured threshold. The relevant alarm is 10880 (channel) or 25080 (axis), and the threshold is defined primarily by MD 36400 $MA_CONTOUR_TOL together with the position-lag monitoring parameters.
Can a lubrication problem really cause a contour monitoring alarm?
Yes. A dry X-axis slide increases the friction torque and the static friction at direction reversal, which the drive cannot follow. The NCK sees this as a position-lag spike on the path and raises a contour monitoring alarm. It is the most common mechanical cause on horizontal axes.
How do I tune an axis on a PCU 50 with no PC/PG available?
Use the HMI axis optimization under Setup → optim./test. The HMI guides you through the identification steps. For a brake-motor, apply the brake in step 3, confirm the coupling is tight, and let the sequence finish without aborting.
Is it safe to widen MD 36400 $MA_CONTOUR_TOL to clear the alarm?
It is safe only as a temporary diagnostic. Widen the tolerance by 20–30 % and re-run the contour. If the alarm disappears only when the tolerance is widened, the real fault is in the drive, the position controller, or the mechanics. Widen the tolerance as a permanent fix only with the machine builder's approval.
Why does the alarm move between X, Y, and B axes?
It is usually a dynamic-response mismatch. If one axis is slower than the rest of the contour group, the faster axes will exceed the contour tolerance and the alarm will appear to follow whichever axis is the limiting one at the time. Enable MD 32900/32910 $MA_DYN_MATCH and the speed feedforward MD 32800/32810 $MA_EEC on every geometry axis, then re-run the HMI optimization on all axes.