Resolving Sinumerik 840C Alarm 1121 Z Zero Speed Control

Resolving Sinumerik 840C Alarm 1121 "Z Zero Speed Control" on SIMODRIVE 611 Feed Axes

1. Alarm 1121 — Definition and Trigger Conditions

Sinumerik 840C alarm 1121 "Z zero speed control" (with the trailing axis identifier 1 = Z) is a follow-on position-monitor fault that the NCK raises when the actual position of the Z axis continues to move outside the configured zero-speed monitoring window while the axis is commanded to be stationary. The alarm belongs to the 1120–1129 family (1120 X, 1121 Z, 1122 Y, 1123 4th axis, and so on), one per feed axis; the final digit of the alarm number matches the axis channel identifier on the 840C.

The fault is a position-window monitor implemented in the NCK. When the axis reaches a programmed stop — at the end of a G0/G1 block, during a dwell, in single block, or in any state where the setpoint is being held — the NC expects the closed position loop to keep the actual value inside a tight tolerance around the last commanded position. If the actual value escapes that tolerance, typically by more than the value configured in machine data MD2121*, the NCK declares the loop unstable and trips alarm 1121 with a RESET / power-on clear classification.

The alarm is a symptom, not a root cause. The controller is reporting that something downstream — the SIMODRIVE 611 power section, the 6SN1118 control insert, the 1PH/1FT servo motor, the encoder, the tachogenerator, the ballscrew / nut / bearing, or the NC parameter file — is letting the axis walk. Clearing 1121 is a matter of finding the source of the drift and correcting it, not acknowledging the message. Pushing the reset button while the underlying drift is still present will either re-trip 1121 immediately or re-trip on the first commanded move.

The Siemens 840C classifies 1121 as a clearance class RESET alarm with a power-on clear. The recommended suppression sequence is: clear the E-stop / drive-enable chain (alarm 2000 ORD1 in particular), address the source of the drift, NCK reset, axis reset, and then run a positioning test under the original MD2121* window.

2. Affected Hardware and MLFB Identification

The Z-axis drive chain on the affected Hyundai HIT-15S and HIT-18 lathes uses the SIMODRIVE 611 analog feed drive system, with the following components that matter for this alarm:

• 6SN1123-1AA00-0BA0 — 2-axis SIMODRIVE 611 feed power module, 8 A continuous / 16 A peak per axis (submodule-dependent), used as the Z-axis power block in the drive group.
• 6SN1118-0AA11-0AA0 — single-axis SIMODRIVE 611 control insert for a feed drive with tachogenerator feedback and incremental encoder, fitted in the slot above the power module. This card carries the drift pot, the enable logic, the velocity-loop compensation components, and the tach input circuitry.
• 1FT or 1PH servo motor with incremental encoder (ERN 1387 / ROD 320 family, 1 Vpp A/B/Z signals, 2048 or 2500 periods/rev) and a brush-type tachogenerator (typically ±60 V / 1000 rpm, model-dependent).
• Sinumerik 840C NCU (any 6FC5x-series) with the SIMODRIVE 611 analog interface card and the X-axis / Z-axis command cable to the 6SN1118 inserts.

All MLFB numbers above are Siemens-original spare parts. Spares for the 6SN1 series are still orderable through the Siemens Industry Online Support SIMODRIVE 611 / SINUMERIK 840C legacy spare-parts catalog; the SIMODRIVE 611 platform was formally discontinued, but the manufacturer provides a long-term spare-parts commitment and the documentation archive remains active on the support portal.

3. Root Cause Matrix

Based on field history, alarm 1121 in this configuration almost always traces to one of the following five sources. Use this matrix to triage the fault before swapping parts. A common but unproductive move is to swap the 6SN1123 amplifier or swap the 6SN1118 control cards between axes — the most common cause is the drift pot on the original card (which moves with the card swap) or the motor / tach (which stays with the motor).

4. Pre-Checks Before Powering the Drive

Before any live work, work through this short list. Skipping a step here is the most common reason a 1121 returns after a "fix".

1. Confirm that alarm 2000 ORD1 Emergency stop is fully cleared and that all E-stop hardware interlocks (door, guard, hydraulics, chuck pressure) are latched. A residual E-stop keeps the drive disabled and produces a cascade of follow-on alarms, including 1121. Resolve the cascade first.
2. Read the actual MD21xx* values and the related MD22xx* / MD23xx* axis parameters from the HMI. Compare line-by-line against the green-covered parameter book shipped with the Hyundai machine. Do this in a spreadsheet, not on the screen — single-digit parameter changes that cause 1121 are easy to miss with the human eye.
3. Inspect the 6SN1118 control insert for the green "ready" LED and any red "fault" LED. Note the LED state before applying drive enable, so a status change after enable is meaningful.
4. Confirm that the SIMODRIVE 611's 24 V control supply, 600 V DC bus, and reference enable are all present. The velocity loop will not close without a fully charged DC link.
5. Verify that the motor power connector, the encoder connector, and the tach connector are fully seated, locked, and free of oil or coolant. Coolant ingress into a 6SN1118 or a 1FT/1PH connector is a common source of intermittent 1121 on turning centers.

5. Drift Pot Adjustment on the 6SN1118 Control Insert

The drift potentiometer on the front of the 6SN1118-0AA11-0AA0 control insert trims the analog offset of the velocity command channel. With the axis at standstill and the velocity setpoint at zero, the drift pot should drive the velocity command voltage to within tens of millivolts of zero. Any residual offset commands the motor to creep, the position drifts, the zero-speed monitor catches it, and 1121 raises.

5.1 Adjustment Procedure (drive enabled, axis stationary)

1. Enable the drive. Do not command a move.
2. Open the Service or Commissioning page on the HMI and display the Z-axis actual position in high resolution.
3. Insert a small non-metallic screwdriver into the drift-pot access hole on the 6SN1118 front panel. The pot is a single-turn trim, typically at the top of the card next to the test points.
4. Turn the pot in small increments and watch the actual position. If the position drifts in the negative direction, turn the pot one way; if positive, the other.
5. Stop when the actual position holds steady for at least 10 seconds at standstill.
6. Issue a G0 move of 10 mm, dwell, and re-check. If the actual position settles inside MD2121* and holds, the adjustment is correct.

5.2 Adjustment Procedure (drive will not stay out of alarm long enough)

If 1121 raises so quickly that the position cannot be read, perform the trim with the drive temporarily disabled, then re-enable and verify:

1. Remove drive enable (or pull the enable signal at the 6SN1118 connector).
2. Open the 6SN1118 front cover to access the test points for the velocity command (typically labeled I-IST or n-IST).
3. Connect a DMM between the velocity command test point and signal ground.
4. Power the drive with enable removed; the velocity command should already be 0 V.
5. Turn the drift pot in small increments and watch the DMM. Adjust until the voltage is as close to 0 mV as the pot resolution allows (typically < ±20 mV).
6. Re-apply drive enable and verify as in 5.1.

6. MD2121* and the Zero-Speed Monitoring Window

MD2121* on the Sinumerik 840C is the zero-speed tolerance window for the position monitor. It defines the position-error threshold inside which the NC considers the axis to be at zero speed. The factory value for a lathe Z axis is typically 0.1 mm. The field report described enlarging this to 10 mm in an attempt to silence 1121, which is a workaround and is not a fix.

Setting MD2121* to 10 mm allows the axis to walk 10 mm before the monitor reacts. On a turning center, that is enough distance to crash a tool into the chuck or a workpiece into the turret, and to scrap the part. The correct sequence is to restore MD2121* to the factory value and address the underlying drift source.

The relevant MD21xx* parameters on the 840C for the Z-axis zero-speed path are:

Restore the green-book values for all of the above. A change in any one of them can shift 1121's behavior, even if MD2121* itself looks correct on the screen.

7. Encoder Feedback Verification

The Sinumerik 840C closes the position loop in the NCK based on the encoder feedback from the 1PH/1FT motor. For 6SN1118-0AA11-0AA0, the encoder is typically an ERN 1387 (or compatible ROD 320) incremental encoder with sinusoidal 1 Vpp A/B/Z signals, 2048 or 2500 periods/rev.

7.1 Static Check (drive disabled)

1. With the drive disabled, turn the ballscrew by hand and read the actual position on the HMI. The position should update smoothly in both directions with no skips, jumps, or sign inversions.
2. If the position updates correctly, the encoder path is good. The field report confirmed this and concluded that the encoder was not the source.
3. If the position display is frozen, shows a sign error, or jumps in increments larger than one quadrature count, the encoder cable, connector, or the encoder itself is suspect.

7.2 Dynamic Check (encoder signals at the NCU)

For deeper diagnosis, scope the A/B/Z signals at the NCU's axis-encoder connector with the drive disabled:

• A and B: 1 Vpp sinusoidal, 90° quadrature, equal amplitude (within 10%), driven from the encoder's 5 V supply.
• Z (index): one pulse per motor revolution, centered between the A and B peaks.
• Supply: 5 V ± 5%, ripple < 50 mV.

Twist the connector and flex the cable while scoping — a marginal cable or pin will show dropouts and is the most common source of intermittent 1121 in plants with coolant splash.

8. Tachogenerator Verification

On a SIMODRIVE 611 analog drive, the velocity loop is closed in the 6SN1118 control insert using the tachogenerator signal. The encoder is a position feedback device for the NCK; the tach is the velocity feedback for the drive. The two paths are independent. A working encoder with a bad tach will still produce 1121, because the velocity loop cannot regulate without the tach signal.

8.1 Tach Test (motor decoupled)

1. Decouple the motor from the ballscrew so it can spin freely.
2. Disable the drive and isolate the tach pins on the 6SN1118 connector.
3. Spin the motor shaft by hand at a moderate speed.
4. Measure the tach output on a DMM in mV DC range. A healthy tach will produce a few hundred mV per 100 rpm; the polarity must reverse when the direction of spin reverses.
5. If the tach reads 0 mV in both directions, the tach windings are open, the brushes are worn past their spring travel, or the commutator is open. Replace the motor or the tach subassembly.

8.2 Tach Test (in-circuit)

1. With the drive disabled, measure the tach voltage at the 6SN1118 tach test point (TP). It should be < 50 mV at standstill.
2. Enable the drive with the axis stationary. The tach voltage should remain at < 50 mV.
3. Command a slow JOG move in the +Z direction. The tach voltage should go positive.
4. Command a slow JOG move in the -Z direction. The tach voltage should go negative.
5. If the tach voltage stays at 0 mV in JOG, the tach signal is not reaching the 6SN1118; the loop is open and the drift pot cannot compensate.

9. Mechanical Isolation Test

If the electrical chain (drift pot, encoder, tach) is clean, the residual cause is mechanical. To prove or eliminate a mechanical source:

1. Loosen the motor-to-ballscrew coupling so the motor and ballscrew are decoupled.
2. Issue a small G0 move (1 mm) on Z. The motor should move smoothly and stop without 1121.
3. If 1121 still fires with the ballscrew decoupled, the fault is electrical (drive, NC, encoder, or tach).
4. If 1121 clears with the ballscrew decoupled, the mechanical chain is the source. Inspect:
   • Ballscrew nut preload — a worn nut binds under load and the servo cannot hold position.
   • Bearings — worn thrust or angular-contact bearings on the ballscrew create a stiction pattern that the velocity loop cannot overcome.
   • Coupling — a misaligned or seized coupling holds the axis against servo commands and creates a position error.
   • Lubrication — starved lubrication on the nut or bearings is a frequent root cause of mechanical binding on production lathes.
5. Hand-turn the ballscrew through its full travel with the motor decoupled. It should turn smoothly with no tight spots. Any tight spot must be eliminated before re-coupling the motor.

10. Parameter Book Restoration (Green-Covered Book Method)

On Hyundai HIT lathes, the factory axis parameter set is delivered in a green-covered parameter book. This book lists the MD21xx*, MD22xx*, MD23xx* (drive-interface) and the machine-specific axis values from Hyundai commissioning. Field experience shows that after a PLC reload, NCK reset, or battery replacement, the MD21xx* set can be overwritten by the loader image and not the green book. Restoring from the green book is the most common path to clearing 1121 after a service intervention.

10.1 Procedure

1. Print the current MD21xx* values from the HMI in ASCII via RS-232, or write them down by hand. Always capture the "as found" values for traceability in the service log.
2. Open the green book to the Z-axis section. Compare the values to the current set, line by line. Pay special attention to "YES/NO" flags — the field report specifically described a "YES" flag that was set to "NO" and caused 1121 to clear once restored.
3. Note the discrepancies. Use a spreadsheet to track the deltas rather than relying on the HMI page.
4. Restore each discrepant MD to the green book value, one axis at a time, with NCK reset between major changes.
5. Once all values match, run the Z axis through a positioning test and confirm that 1121 does not raise.
6. Save the restored parameter set to a backup file on a PCMCIA card or via the RS-232 port, and label the backup with the date, machine serial, and the technician's initials.

11. Verification and Commissioning

After the corrective action, run the following verification sequence before returning the machine to production:

1. Power cycle the NC and the SIMODRIVE 611. The 600 V DC link must charge fully before the drive enables.
2. Clear all alarms and run an NCK reset.
3. JOG the Z axis in both directions in 1 mm increments. The axis must move smoothly and stop without position drift.
4. Issue a G0 move of 10 mm. After the move, read the actual position and the following error on the HMI. The following error must settle to < MD2121* within the dwell time defined in MD2122* / MD2125*.
5. Repeat step 4 fifty times consecutively. No 1121 should raise during the sequence.
6. Run a full part program with the original Z-axis feed and speed. Monitor the HMI for any 1121 occurrences during the cycle.
7. Re-check the MD21xx* set against the green book to confirm that no further parameter drift has occurred.
8. Document the new drift-pot setting, the MD21xx* deltas, and the parameter-book restoration in the machine's service log.

If 1121 raises during step 5, return to Section 3 and re-triage. The most common reason for a return is a drift pot that was set to null with the drive disabled but has shifted once the drive is enabled and the DC bus is loaded.

12. Common Pitfalls and Field Notes

• Masking 1121 with MD2121* = 10 mm — this silences the alarm, not the fault. The axis can still walk 10 mm and crash. Always restore MD2121* to the factory value and fix the source.
• Replacing the 6SN1123 power module or swapping the 6SN1118 control insert — if the source is the motor, the tach, or the drift pot (which moves with the card swap), the swap will not fix the issue. Verify the motor chain and the drift pot before swapping the drive.
• Skipping the tach test because the encoder is good — the encoder feeds the NC; the tach feeds the drive. They are independent paths and must both be verified.
• Performing the drift-pot adjustment with the motor coupled to a binding mechanical load — the load masks the adjustment. Decouple the motor first, trim, then re-couple.
• Re-loading the PLC and NCK reset without checking the parameter book — a fresh loader image can overwrite the MD21xx* set to a generic value. Always cross-check against the green book after a PLC load.
• Coolant ingress in the 6SN1118 connector or the motor feedback connector — a frequent root cause in turning centers. Inspect connectors for coolant tracks; replace the connector or the cable if the seal is compromised.
• Battery-induced parameter loss — the 840C has a buffer battery; if the battery is dead, the MD21xx* set can be lost or corrupted. Always replace the battery with the NC powered, and verify the MD21xx* set against the green book after the swap.

13. Related Alarms and Cascade Faults

1121 is part of a family of "zero speed control" alarms on the 840C. The full set, with the affected axis identifier in the alarm code, is:

The diagnostic procedure in this article applies unchanged to 1120, 1122, and 1123 — only the axis identifier and the corresponding 6SN1118 control insert change. When 1121 follows a 2000 ORD1, always clear the E-stop chain and the drive enable before assuming a 1121-specific root cause; a latched ORD1 will keep the drive disabled and 1121 will re-trip on the first enable.

14. Quick Reference — Symptom-to-Action Map