Troubleshooting SINUMERIK 840C Alarms 1161 and 1121 on Z-Axis

1. Problem Overview

This article documents a field procedure for resolving simultaneous SINUMERIK 840C alarms 1161 (Z-axis contour monitoring) and 1121 (Z-axis zero speed control) on a vertical axis driven by a Baumuller Nurnberg BUG2 analog servo drive, position-closed through a Fagor CX-1345 linear glass scale. The fault appears after an extended machine shutdown (in this case approximately four months of idle storage following pandemic downtime) once the control and drives are re-energized. Both alarms are classified as following-error faults and are generated when the actual position reported by the measuring system lags the commanded position beyond a configured threshold inside the NCK's position-controller cycle.

Key observed symptoms:

• Pressing the Z+ jog button produces visible motion up to approximately 1 mm of travel, then the NC drops into a follow-up error stop.
• Alarm 1161 (contour monitoring) is dominant; alarm 1121 (zero speed / standstill monitoring) appears intermittently, usually after a few cycles of 1161.
• Swapping the motor, drive, NCK CPU, PLC CPU, and the measuring-circuit card does not clear the fault.
• Moving the linear scale head by hand updates the HMI position display correctly, indicating the head, cable, and reading electronics are not fully dead.
• Initial power-up produced PLC_CPU not ready for operation and No communication to NCK, both of which were cleared by replacing the PLC CPU and the NCK CPU modules. The 1161/1121 fault appeared only after the controls were running cleanly.

2. System Configuration and Affected Hardware

The combination of an 840C control with a non-Siemens analog drive is significant: with a 611A Siemens drive and a motor-mounted encoder, the position loop and the speed loop are co-located inside the drive, and the NCK only has a simple lag check. With a BUG2, the position loop is closed entirely in the NCK, the speed loop is closed in the drive, and the two halves are coupled only by the analog setpoint cable and the resolver cable. Anything that breaks the coupling - a wire, a ground bond, a DC bus, an enable - looks identical to the NCK as a following error.

3. Alarm Definitions and Trigger Conditions

Both alarms are produced by the NCK's position controller; the drive itself is not signalling a fault. The NCK reports them because the resolver-driven speed loop (inside the BUG2) is supposed to be tracking the analog command closely enough that the resolver position agrees with the NCK's setpoint within the configured window. When 1161 triggers within a fraction of a millimetre of commanded motion, the most likely root cause is one of:

1. Analog setpoint (DAC) is not arriving at the drive (open, short, ground loop, wrong pin).
2. Drive enable is dropping or never being asserted (24 V missing or PLC output not latched).
3. DC bus to the drive is missing or sagging (motor stalls on the first pulse of torque).
4. Resolver is healthy to the drive but the NCK measuring-circuit card is not seeing valid signal from the linear scale (so position actual value stays at zero while the NCK integrator climbs).
5. Mechanical jam (ballscrew nut, brake not released, counterbalance cylinder stuck).

4. Diagnostic Strategy - Position Loop vs Speed Loop

The fastest path to root cause on a non-Siemens analog drive is to physically separate the two control loops:

When component swaps do not move the fault, the suspect shifts to the interface between loops - the setpoint cable, the enable line, the DC bus, or the mechanical coupling - not to a single component. The diagnostic ladder is therefore:

1. Confirm the drive's internal speed loop is healthy in isolation.
2. Confirm the analog setpoint is reaching the drive with the correct polarity, amplitude, and ground reference.
3. Confirm the drive enable is held throughout the motion.
4. Confirm the NCK is seeing valid direct-measuring-system feedback from the linear scale.
5. Confirm the mechanical loop is free.

5. Hardware Elimination Steps Already Performed (and what they rule out)

6. Drive Speed Loop Isolation (Bumpless / Battery-Box Test)

The BUG2 is an analog thyristor/DC servo drive that accepts a +/-10 V velocity command and a 24 V enable. To verify the drive and motor in isolation from the NCK, decouple the command and run the drive from a temporary source.

6.1 Prerequisites

• Machine in E-STOP, Z-axis brake (if fitted) released or backed off.
• DC bus confirmed present at the drive (200/220/440 V DC class, depending on the Baumuller motor/line).
• Motor mechanically decoupled from the ballscrew (coupling removed).
• Reference to the BUG2 wiring diagram for terminal assignment of the analog setpoint and enable.

6.2 Procedure

1. Open the drive's command terminal block; lift the +/-Setpoint wires and the Enable wire from the NCK side.
2. Wire a temporary battery box (4x 1.5 V cells with a potentiometer, or a 0-10 V lab supply) into the drive's setpoint input.
3. Apply drive enable manually (24 V to the enable input from a clean source).
4. Ramp the setpoint slowly from 0 to +/-2 V; observe motor rotation, current, and any drive alarm.
5. Increase to +/-8 V; motor should reach rated speed smoothly.
6. Reduce to 0 V; motor should coast down with no drive alarm.
7. Reverse polarity of the setpoint; motor must reverse direction without alarm.

6.3 Interpretation

7. Reconnecting to the 840C - Verifying the Position Loop

Once the speed loop test passes, re-integrate the drive with the NCK:

1. Re-couple the motor to the ballscrew.
2. Reconnect the analog setpoint wires to the NCK-side terminal block.
3. With the NCK in follow-up (axis not enabled), verify on an isolated scope that the setpoint is 0 V and the enable is 0 V.
4. Enable the axis from the HMI/PLC. The enable line should rise to 24 V, and the setpoint should reflect the position-controller output.
5. Issue a small jog (e.g. +0.5 mm Z+). On the scope, observe:
   • Setpoint voltage ramping.
   • Actual position (resolver) reading updating on the HMI.
   • Lag (following error) on the service display.
6. If 1161/1121 reappear immediately, the lag exceeds threshold even at standstill. The remaining suspects are:
   • Wrong sign of resolver feedback (motor turns but NCK integrator climbs in the wrong direction).
   • Wrong MD setting for measuring-system increments per mm (840C MD family around MD 3600-3800, exact number per software version - confirm against the 840C documentation for the installed SW).
   • Drive enable dropping intermittently (check 24 V hold by the PLC, not just initial pick-up).
   • Mechanical binding returning when the motor re-engages the screw.

8. Linear Scale Signal Verification

The Fagor CX-1345 is an exposed linear glass or steel scale with optical incremental readout; it is typically 20 um or 1 um selectable at the measuring-circuit card. The Siemens 6FC5111-0BA01-0AA0 is the interpolation / signal-conditioning card that converts the scale's sinusoidal signal into TTL square waves for the NCK. A manual test (move head by hand) only proves DC continuity; it does not prove the signal is clean under servo motion.

8.1 Cable and grounding checks

• Scale cable shield must be terminated on the EMC rail at the cabinet entry; a missing shield bond injects common-mode noise that the NCK interprets as position jumps and that the contour monitor then trips on.
• Scale cable must not run parallel to power cables; cross at 90 degrees if routing requires it.
• Connector pins on the 6FC5111-0BA01-0AA0 must be free of oxidation; reseat the connector and re-test.

8.2 Card-level test points

The 6FC5111-0BA01-0AA0 has test points (TP1, TP2, referenced to the card manual) where the sinusoidal amplitudes from the scale head can be observed. The signals must be:

• Amplitude > 1 Vpp (consult the card manual for the exact threshold; typical acceptance is 0.8 - 1.2 Vpp with a 1.0 Vpp nominal).
• Balanced between the two phases (within ~10%).
• Phase-quadrature (90 degrees +/- 10 degrees).

If amplitude is below threshold, the card drops into error and the NCK sees no count increments - the symptom looks like a dead scale even though the head is reading. This is the most common "passive hand test ok, dynamic test fails" failure mode after long storage.

8.3 Fagor test kit / factory evaluation

A Fagor linear scale test kit is the correct tool to capture the head output waveform and confirm signal integrity in motion. Where this is not available on site, sending the head to a Fagor service center for evaluation is the next-best step.

9. Can the Linear Scale Be Disabled on a SINUMERIK 840C?

The 840C was designed for either a semi-closed loop (motor-mounted encoder) or a closed loop (linear scale on the load), configured at machine data level - not by PLC program. The configuration is set in the NCK's machine data, not in the PLC.

To switch the Z axis from the direct measuring system (linear scale) to the indirect measuring system (motor resolver), the relevant machine data must be flipped. In the 840C MD tree this typically lives in:

• MD 36xx / 38xx family - axis configuration and measuring-system assignment (exact MD number depends on the 840C software version installed; verify against the SINUMERIK 840C documentation for the specific SW release).
• MD 56xx family - position-controller gain (Kv factor); must be re-tuned when the feedback source changes.

9.1 Procedure to move the axis on motor resolver only

1. Back up all current machine data before any change.
2. Modify the MD that assigns the direct measuring system for the Z axis so the resolver is the active feedback source. The linear scale remains wired but is ignored by the NCK.
3. Cold-restart the NCK (general reset, full power-cycle).
4. Reference the Z axis on the resolver marker.
5. Jog the Z axis; it should now move on resolver feedback only.

9.2 Caveats

• Positioning accuracy is now limited to ballscrew lead error; thermal compensation (leadscrew-error compensation, SSFK) is the only correction available, and it requires a calibrated reference (the linear scale) to build the table.
• Mechanical backlash (Hirth coupling, brake backlash) becomes visible on the HMI and may need to be re-compensated.
• The machine builder's PLC may be written assuming the linear scale is the master; if the PLC references scale status bits (e.g. refpoint reached, position reached), the PLC may need to be re-compiled for the new feedback source.

9.3 What this proves

If the axis now moves cleanly on resolver feedback only, the linear scale signal path is the fault. The chapter 8 verification then becomes the active diagnostic path. The PLC program change is not the right tool for this reconfiguration - the change must be at machine data level.

10. DC Bus and Setpoint Voltage Checks

11. Mechanical Hand-Wheel Test Under E-Stop

With the machine in E-STOP and the drive un-powered:

1. Release the Z-axis brake (manually or with shop air, if fitted).
2. Push the Z axis by hand and observe the HMI position display.
3. The display must update in the correct direction at the correct ratio (1 mm of hand motion ≈ 1 mm of HMI travel).

This is the cleanest mechanical-only verification on the machine and should be done before any drive swap.

12. Baumuller BUG2 Drive Interface to SINUMERIK 840C

The BUG2 is a fully analog drive. The 840C must supply:

• +/-10 V velocity setpoint (DAC output of the NCK, often via a Siemens 6FC9... analog output module).
• Drive enable (24 V from the PLC, often through the same interface module).
• Drive ready / fault feedback (potential-free contact or 24 V from drive to PLC).

12.1 Common failure modes specific to a non-Siemens drive on 840C

• Setpoint wiring reversed (motor runs in wrong direction - usually caught at commissioning; a swap of motor phases after long storage can reintroduce the same symptom if the resolver was re-landed in the wrong sequence).
• Setpoint ground reference not the same as the NCK ground - creates a ground loop, 50/60 Hz noise on setpoint.
• Enable line wired to the wrong PLC output or to a non-retentive output that drops during NCK cycle.
• Drive trips on first enable after long storage (DC bus capacitors need re-forming, see chapter 13).
• Resolver cable shield not bonded at cabinet entry - injects common-mode noise into the drive's resolver input.

13. Long-Storage Considerations After 4+ Months Down

After 3+ months of de-energized storage, electrolytic capacitors in the NCK power supply and in the drive's DC bus lose their dielectric conditioning. Re-energizing without a controlled re-form procedure can:

• Cause the drive to trip on DC bus overvoltage or undervoltage.
• Cause the NCK to fail to boot (the initial PLC_CPU not ready for operation error is consistent with this).
• Stress resolver insulation (resolver cable may be fine for DC resistance but marginal for insulation).

13.1 Recommended re-form procedure

1. Apply 3-phase line through a variac at 30 % of nominal for 30 min.
2. Raise to 60 % for 30 min.
3. Raise to 100 %.
4. Allow 24 h of standby before loading the DC bus with motion commands.
5. Verify all internal supplies (5 V, 15 V, 24 V) with an isolated scope before connecting the NCK to the drive.

The original PLC_CPU and NCK failures are consistent with capacitor-related start-up stress, not with field wiring. If the BUG2 itself was powered up cold after 4 months, the same applies to its DC bus capacitors - a drive that faults on first enable after long storage is often healthy once the bus is re-formed.

14. Final Verification Procedure

After the fault is cleared, run a structured verification before returning the machine to production:

1. Reference all axes via the NC reference-point cam and encoder marker. The Z axis must reference without re-triggering 1161.
2. Handwheel test at 100 um, 1 mm, 10 mm, 100 mm increments in both directions. Monitor the lag value on the service display; it should remain < 0.5 mm at 100 mm/min feed and < 2 mm at 5000 mm/min (verify against the contour window MD for the installed SW).
3. G01 test block - run a 10 mm x 10 mm square in XZ at 2000 mm/min with a sharp corner. Alarms 1161/1121 must not trigger.
4. Thermal test - run the Z axis continuously for 30 min at 100 % rapid. Lag should remain stable; any drift must be compensated by the leadscrew-error compensation table (SSFK).
5. Service display dump - record all current machine data and compare to the pre-fault backup to confirm no unintended changes.

15. Preventive Recommendations