Troubleshooting Siemens SINUMERIK 810D / 840D sl Alarm 25080: Axis Positioning Monitoring

Alarm 25080 Axis %1 positioning monitoring on a Siemens SINUMERIK 810D (and the related 840D sl / SINAMICS S120 platform) indicates the actual position of the named axis failed to reach the position window within the time delay configured for positioning completion. On a DMC 63V vertical machining center with a heavy spindle head on the Z slide, this alarm almost always couples to a mechanical inability to move (binding, brake drag, gravity drop), a drive-side issue (SINAMICS IGBT module, motor, encoder feedback), or a control-side mis-configuration (machine data 36000/36010/36020, Kv factor MD 32200). The diagnostic and remediation procedure below mirrors the field-proven escalation path used on Deckel-Maho DMC 63V retrofit cells.

1. Alarm 25080 Definition and Trigger Conditions

According to the Siemens SINUMERIK 840D sl / SINAMICS S120 alarm documentation (SINUMERIK 840D sl SINAMICS S120 Alarms - ID 109777872), alarm 25080 belongs to the positioning-monitoring class. It is raised when:

• The actual position value of axis %1 does not reach the exact-stop fine window MD 36010 within the positioning-monitoring time MD 36020; or
• The axis leaves the exact-stop coarse window MD 36000 after reaching it, signaling that the loop cannot hold position.

On a Z axis that shows drift between +286.864 mm and -114.388 mm while the operator holds RESET — the classic symptom reported on the DMC 63V — the actual value is being moved by gravity against a commanded setpoint of 0 mm. The drive is energised, the controller is demanding "hold position", the brake (mechanical or counterbalance) has lost its static holding force, and the head is sliding down the ballscrew. The drive eventually cannot close the loop, exits the fine window, and 25080 latches.

2. Affected Systems, Firmware, and Cross-Reference

The alarm number is unchanged across 810D/840D/840D sl; only the drive-side hardware differs. The DMC 63V with SINUMERIK 810D in the field is most commonly fitted with a SIMODRIVE 611D 2-axis power section or a later SINAMICS S120 retrofit. Both expose the same axis-MD interface described below.

3. Machine Data That Define 25080

Three MDs set the exact-stop window; one MD sets the loop bandwidth. All four are tunable in JOG / MDI / AUTO via the standard Siemens Start-up > Machine Data tree, or with the SinuTrain / SinuCom commissioning tool.

4. Field Symptom Pattern (DMC 63V, Z axis)

The original report describes a textbook progression:

1. Head falls on Z from power-on, both in JOG and in program — the brake is not holding.
2. Reset clears the alarm, motors attempt to move, alarm returns in <1 s.
3. JOG in Z+ is impossible; JOG in Z- produces a grinding / rattling noise — the head is being pushed into the direction gravity is already pulling it, the drive cannot develop enough torque to lift, and the current limit is hit.
4. Position display drifts between +286.864 mm and -114.388 mm — the encoder sees the gravity drop; the controller is commanding 0 mm.
5. Encoder, when manually turned with the screw (ball-screw free of the nut), shows correct linear displacement — encoder is functional.

The 25080 in this scenario is a symptom, not the disease. The disease is either:

• Pneumatic / hydraulic counterbalance pressure lost (machine reports "air low pressure" — verified separately);
• Motor holding brake sticking / dragging / electrically released but mechanically frozen;
• Motor power module (SIMODRIVE 611D or SINAMICS S120 Motor Module) with one failed IGBT or current-sensor drift;
• Encoder cable intermittent — eliminated by manual-displacement test;
• Mechanical bind — eliminated by decoupling the ball-screw and turning the screw by hand.

5. Diagnostic Procedure: Cross-Cable Swap (Axis-X ↔ Axis-Z)

The fastest discriminator between the drive and the mechanics is the cross-swap. Performed correctly, it localises a fault to the controller slot, the power module, the motor, the encoder, or the mechanics within 30 minutes.

5.1 Prerequisites

• Mechanical lock on the Z head (service prop or wooden blocking rated for head mass).
• Both motor power and encoder cables for X and Z are physically accessible; connectors keyed and labelled.
• Schematic for the DMC 63V cabinet (S1/S2 numbering, X axis on slot 1, Z axis on slot 2 typical on 611D).
• 24 VDC bench supply for the brake release.

5.2 Procedure

1. Lock the Z head mechanically. Power off the cabinet. Wait 5 minutes for DC-link discharge (LED on the 611D pulse resistor module / SINAMICS Active Line Module).
2. Disconnect the Z motor power cable at the module side. Disconnect the Z motor encoder cable at the controller / SMC module side.
3. Connect the X-axis power cable to the Z-axis motor terminals. Connect the X-axis encoder cable to the Z-axis encoder.
4. Connect the Z-axis power cable to the X-axis motor terminals. Connect the Z-axis encoder cable to the X-axis SMC.
5. Apply 24 VDC to the Z motor holding brake. Verify the brake releases (clicks, head is free to be pushed by hand against the still-locked slide).
6. Power on, clear all alarms, set MD 36010 = 0.5 mm and MD 36020 = 5 s, press MD active.
7. JOG the X axis label on the HMI. The motor physically wired to the Z slide should now move. Note the smoothness, current display, and any new alarm.
8. JOG the Z axis label on the HMI. The motor physically wired to the X slide should now move.

5.3 Interpretation

On the DMC 63V case documented, "JOG X / motor-Z" worked perfectly and "JOG Z / motor-X" tripped the same 25080-class alarms. That isolates the fault to the Z-axis power module slot (motor module / 611D power block), not to the motor and not to the mechanics.

6. Power Module / IGBT Inspection

With the cabinet de-energised and DC-link confirmed discharged (verify < 5 VDC across the DC-link test points), perform the following on the suspected Motor Module.

6.1 Static IGBT Test

Use a digital multimeter on diode-test mode. Measure between each of the three motor output terminals (U2 / V2 / W2) and the DC+ bus; then between each motor output and the DC- bus. A healthy IGBT pair shows ~0.3–0.7 V in one direction and OL in the other, with all three phases showing closely matching values. Any phase where the forward voltage is significantly lower (e.g. 0.05 V) indicates a shorted IGBT; a phase that reads OL in both directions indicates an open emitter.

6.2 Gate-Board Check

On a SINAMICS S120 Motor Module (e.g. 6SL3120-1TE21-8AA3, 6SL3120-1TE23-8AA3 for 18 A / 38 A class), remove the front cover and inspect the gate-driver board for the affected axis. Look for:

• Burnt gate-resistor arrays (small black SMD pads near the optocouplers).
• Discoloured / cracked optocouplers (white or yellow plastic).
• DC-link snubber capacitors with domed tops.
• Cold-solder joints on the current sensors (Hall-effect transducers).

6.3 Current Sensor Drift

A current sensor with offset drift will produce a torque offset that the position loop cannot null without saturating, and 25080 will appear only on the affected axis with no obvious IGBT short. Substitute the Motor Module with a known-good spare to confirm. On 611D, exchange the axis-specific power block (1FT5 / 1FK6 motor + matched 611D module pair is usually 1-to-1 per axis slot).

7. Encoder, Cable, and SMC/SME Check

If the cross-swap points to the encoder side, perform the following with the motor mechanically free and brake released:

1. With the motor free, turn the shaft by hand at < 1 rev/s. The actual position display in Service > Axis > Position should follow smoothly. If it jitters, stutters, or shows a hysteresis, the encoder signals are degraded.
2. Measure the encoder supply voltage at the motor-end connector: 5 VDC ± 5 % for sin/cos encoders, 10–30 VDC for HTL. Ripple < 50 mV peak.
3. Inspect the encoder cable for chafing, kinks, oil ingress, and tight bend radii < 6 × cable diameter. Replace if any of these are present; do not splice encoder cables.
4. Check the shield bonding: the shield must be 360° bonded to the connector backshell at both ends on SINAMICS S120 with SMC30, and only at the cabinet end on resolvers. A broken shield is a common intermittent cause of 25080 after warm-up.

8. Brake, Counterbalance, and Mechanical Chain

8.1 Motor Holding Brake

Apply 24 VDC to the brake release input (typical on 1FK6 / 1FT6 motors, 24 V ± 10 %, inrush < 1 A, holding < 0.5 A). Listen for the click. Measure brake release time < 60 ms. A dragging brake adds a constant torque load that the position loop must overcome every move — for a heavy Z head this can exceed the motor's continuous current and trip 25080 on the coarse window.

8.2 Pneumatic / Hydraulic Counterbalance

The DMC 63V typically uses a nitrogen-charged hydraulic accumulator on the Z slide. The control monitors accumulator pressure (6 bar / ~87 psi on the field-reported case). A bleed-down through a leaking pilot-operated check valve will present as "air low pressure" alarms and 25080 simultaneously. The accumulator pre-charge should be checked with the system depressurised against the manufacturer's spec; the working pressure should be set with the slide at mid-stroke and verified at both ends.

8.3 Ballscrew, Coupling, and Bearings

With the motor decoupled, the ballscrew should turn smoothly by hand for one full revolution in each direction. Any tight spot, roughness, or backlash > 0.05 mm indicates a ballscrew nut failure or bearing damage. The belt coupling between motor and ballscrew should be checked for tension and tooth wear.

9. Machine Data Tuning (Post-Repair Verification)

Once the hardware fault is repaired, restore the diagnostic overrides:

1. Set MD 36010 back to its machine-specific fine value (0.010 mm is a common default but not universal — use the OEM's commissioning sheet).
2. Set MD 36020 back to the standard value (1.0 s).
3. Set MD 32200 back to 1.0 1/s. A Kv < 1.0 is a diagnostic-only setting; do not leave the machine in production with a reduced gain.
4. Set MD 36000 to the coarse window (0.040 mm typical). Verify exact-stop coarse is reachable from a JOG move at rapid traverse.
5. Run a full-reference / G74 on the affected axis to re-establish the machine zero.

10. Verification Procedure

1. Power on, clear all alarms. Verify 25080 does not reappear at idle.
2. JOG Z in INC 0.001 mm / 0.01 mm / 0.1 mm / 1 mm / 10 mm / 100 mm variants. The position display must follow the commanded increment to within MD 36010.
3. Run a positioning accuracy test: command a move of 100 mm, measure the actual with a dial indicator or laser interferometer; deviation < 0.010 mm.
4. Run a circularity test (G02/G03) and compare against the recorded ball-bar plot from the last known-good state.
5. Run a full program in DRY RUN for 30 minutes, then a full production program for 1 hour. Monitor the drive current on the service display — it should not exceed the motor's continuous rating.

11. Troubleshooting Matrix (Compact Field Reference)

12. Common Pitfalls

• Leaving MD 32200 at 0.6 after the test. A reduced Kv will pass a low-speed move and fail a rapid — the alarm will return in production, usually with a heavier workpiece.
• Releasing the brake without a mechanical lock. The DMC 63V Z head weighs > 200 kg. The screw will self-destruct, the nut will be damaged, and the spindle will hit the table.
• Substituting a higher-rated Motor Module. Current-loop tuning, IGBT switching, and the motor's thermal model are matched; a 38 A module on a 25 A motor can trip 25080 from current-loop saturation, not from a real positioning error.
• Ignoring the "air low pressure" alarm. The counterbalance is part of the positioning system on a vertical Z axis. Low pressure + brake release is a guaranteed 25080.

13. Reference Documentation

For commissioning and parameter detail, the canonical Siemens references are:

• SINUMERIK 840D sl SINAMICS S120 Alarms (ID 109777872) — alarm 25050, 25070, 25080, 25100, 25105 reference.
• SINUMERIK 840D sl / 810D Lists Manual (BA) — MD 36000, 36010, 36020, 32200 definitions.
• SINAMICS S120 Function Manual — Motor Module diagnostics and replacement.