SINUMERIK 840D Pl Spindle: Fixing Reference Drift and RPM Error

SINUMERIK 840D Pl Spindle: Fixing Reference Drift and RPM Error

Two distinct but co-occurring anomalies are routinely observed on SINUMERIK 840D Pl (Powerline) and 840D sl (Solution Line) installations where the speed/position feedback encoder is mounted on the motor rather than on the spindle: (a) the actual spindle position displayed on the HMI rotates more or less than 360 ° per mechanical turn, and (b) the spindle rotates at an RPM that does not equal the programmed S-word, even with rapid/feedrate override = 100 %, no M40/M41–M45 gear-stage change active, and no spindle override. A third, more subtle symptom is that the position at which the spindle becomes "referenced" after power-on or NCK-RESET varies by 60 °, 180 ° or some other non-multiple of 360 ° from cycle to cycle. All three symptoms have a common, frequently misdiagnosed root cause: the controller's load-gear and drive-encoder machine-data pairs (MD31050/MD31060 and MD31070/MD31080) are tuned independently, while the S-word to speed setpoint calculation uses the result of their product. This reference compiles the field-validated parameter set for an Heidenhain ERN 1387 mounted on the motor shaft with a toothed belt (3 motor revolutions for 2 spindle revolutions), homing via a separate BERO (proximity) sensor wired to the drive module connector X432, on a machine where the NCU was upgraded from NCU 571 to NCU 573.2. Cross-check the resolver at Siemens Industry Online Support and the encoder catalog page at Heidenhain ERN 138x encoder series.

1. System Topology and Observed Symptoms

The failing machine exposes the following architecture, derived from the controller's actual MD values and the drive module wiring diagram:

The reported symptoms in chronological order are:

1. Spindle HMI display shows < 360 ° per actual spindle turn (mechanical angle reading is foreshortened).
2. After MD31070/MD31080 are changed to ratio 3/2, the position display becomes correct, but M3 S1500 drives the spindle at only 1000 RPM (with override at 100 %).
3. The position at which Spindle referenced appears on the HMI after each NCK reset is inconsistent — sometimes 60 ° after the BERO signal, sometimes 180 ° after the BERO signal.
4. Pressing RESET on the MCP yields the HMI status Channel interrupted for 10–12 s before the channel is operational again, while the same operation typically clears in < 2 s on a healthy 840D Pl machine.

Each symptom is independent but the root causes interact; treat them as a single commissioning cluster.

2. Mechanical Transmission: Encoder-to-Spindle Ratio (3 : 2)

The kernel of every MD decision is the ratio between motor shaft revolutions and spindle (load) revolutions:

n_spindle = n_motor × (MD31050 / MD31060)

With the belt giving 3 motor revolutions per 2 spindle revolutions, the algebraic ratio is 2/3. Therefore the canonical machine data values are:

• MD31050 $MA_LOAD_GEAR_RATIO_NUMERA = 2
• MD31060 $MA_LOAD_GEAR_RATIO_DENOM = 3

This ratio is multiplied by the drive encoder gear ratio, MD31080/MD31070, to obtain the controller's effective feedback scaling:

displayed_position_mech = encoder_counts × (MD31080/MD31070) × (MD31050/MD31060)

Because the ERN 1387 is mounted on the motor, the encoder turns 1:1 with the motor, so MD31070 and MD31080 must both be 1. Setting them to non-unity values, as the original commissioning did with 3 and 2, double-applies the ratio and gives the exact "less than 360 ° per turn" symptom.

3. Machine Data for Load Gear (MD31050 / MD31060)

Behavioural test:

1. Jog the spindle in M3 at very low speed (≤ 50 RPM actual) so the human eye can verify rotation.
2. Read the position in the service menu Axis → Spindle → Display position.
3. Take a registration mark on the spindle housing with a marker.
4. Confirm that exactly 360.000 ° of displayed position corresponds to one mechanical spindle turn.

If the displayed position is 540° per spindle turn, the ratio is inverted (MD31050 > MD31060). If the displayed position is 240° per spindle turn, the ratio is over-divided; halve MD31050 or double MD31060. If it is 360° × (31050/31060), the load ratio is correct and the discrepancy is in MD31070/MD31080 or in MD34200's homing behaviour.

4. Machine Data for Drive Encoder (MD31070 / MD31080)

Per Siemens documentation the pair MD31080/MD31070 represents the ratio of motor revolutions to encoder revolutions (or vice-versa depending on convention). Because the ERN 1387 is mechanically fixed to the motor, the correct values are 1, 1. The original pairing of 31070=3, 31080=2 forces the controller to interpret each spindle revolution as the encoder having turned 2/3 of a motor revolution, which compounds the belt ratio and yields exactly the "display less than 360 °" symptom. Reset both MDs to 1 before continuing.

5. Index Pulse and BERO Polarity (MD34120)

The Heidenhain ERN 1387 outputs one zero reference pulse per revolution. The BERO sensor delivers one falling (negative) edge per spindle revolution when the spindle marker passes its face. Both pulses are required for robust homing:

• Index pulse: gives absolute position modulo 360 °/encoder resolution within one revolution.
• BERO: gives the absolute position within one spindle revolution, and is used as a coarse gate so the controller does not have to rotate the spindle through many index pulses during homing.

MD34120 selects the active edge of the BERO. For a PNP BERO with NC contact a negative edge corresponds to REFP_BERO_LOW_ACTIVE = 1. If the polarity is reversed the controller will home against the opposite side of the spindle index and the displayed reference will be systematically shifted by 180 °.

Verify polarity with a scope (channel 1 = BERO at X432, channel 2 = spindle marker as observed on the spindle housing): the falling edge of the BERO should coincide with the spindle marker within ± 5 mechanical degrees. Anything outside this window indicates either a faulty BERO adjustment, a worn-tooth belt, or an axis-swapped cable.

6. Referencing Mode MD34200 — Selection and Behaviour

MD34200 selects which signal(s) are used to synchronize the controller's internal zero with the machine's mechanical zero. For belt-driven spindles with both an encoder index pulse and a BERO, the recommended mode is 5.

For a spindle with a BERO that produces one falling edge per spindle revolution and an incremental encoder with one zero pulse per motor revolution, MD34200 = 5 is the canonical choice. The controller rotates the spindle until the BERO edge is detected, then waits one encoder period for the next zero pulse and latches position. The result is a deterministic reference position.

7. Speed Setpoint Mismatch: Why M3 S1500 Reads 1000 RPM

The reported symptom of M3 S1500 producing a measured 1000 RPM spindle speed with override at 100 % is almost always not a gear-ratio problem; it is almost always a default ceiling on one of three machine-data parameters that clamp the programmed S-word to a maximum.

The actuator control loop in the SINUMERIK performs:

n_motor_commanded = (S × min{MD35100, MD35130[gear]}) × (MD31060/MD31050) × drive scaling

The command path is governed by:

The Siemens factory default for MD35100 = 1000 RPM (in the v1.x default set) exactly explains the symptom "S1500 yields 1000 RPM". Even if the S-word is 1500 and the spindle override is 100 %, the controller clamps the spindle at 1000 RPM. The remedy:

1. Set MD35100 $MA_SPIND_VELO_LIMIT to a value greater than or equal to the S-word maximum +10 % margin. For this machine use MD35100 = 2000 if the encoder's mechanical limit allows.
2. Verify that MD35130[1..5] for each configured gear stage also accommodates the S-range; for a single-gear-stage (gearing-less) machine set MD35130[1] = 5000 to remove stage limitations, or use the values below.
3. For an ERN 1387 motor with 3000 RPM nominal, the achievable spindle speed is: n_spindle_max = 3000 × (MD31050/MD31060) = 3000 × 2/3 = 2000 RPM. Therefore MD35100 must be between 1500 and 2000.

8. Spindle Speed Limits MD35100 / MD35110 / MD35130

An independently commissioned parameter set for a single-stage spindle with the ERN 1387 and 3/2 belt is:

If the spindle is configured for two or more gear stages (M40/M41–M45 in the part program), repeat the gear-stage entries for MD35130[2..5] with the values appropriate to that stage. Stage 1 is for the low-speed, high-torque end of the spindle speed range; subsequent stages multiply by belt/planetary ratios. Without M40, only [1] is active.

9. Inconsistent Reference Position (60 ° vs 180 ° Drift)

With MD34200 = 5, every homing cycle should land at the same mechanical angle relative to the BERO edge. Observed drift between 60 ° and 180 ° indicates the controller is latching two different zero-pulse candidates. The possible causes, in order of likelihood, are:

The most prevalent cause after an NCU swap (NCU 571 → NCU 573.2) is firmware default-state divergence: the older NCU 571 may have used a different homing state machine. Reload the machine manufacturer's commissioning archive; if unavailable, manually reset MD34200 to 5 and explicitly compute MD34102 REFP_CAM_DIR_IS_MINUS = 0 and MD34104 REFP_DRIVE_NEGATED = 0 before homing, then save via SAVE /CFGINIT.

10. NCU Migration 571 → 573.2 Compatibility

The NCU 571 (6FC5 271-0AA00-0AA0) is the original 840D Pl control unit. The NCU 573.2 (6FC5 573-2AY00-… ) is a 840D sl Solution Line NCU. They are not directly interchangeable; they require a different CF card, different MLFB firmware (different $NC_VERSION default range), and different PLC firmware. Typical side-effects of an unconditional drop-in replacement are:

• Spindle MD defaults do not load; controller boots with HMI error 600D/F "PLC user program version mismatch".
• PLC cycle is forced to 4 ms (NCU 571 default) but actual achievable is 8 ms or more under NCU 573.2 if the PLC OB1 was not refactored, slowing the reset handshake.
• Drive objects (DO) for the spindle may not be assigned to logical axis index 4 / 5, leading to Channel interrupted until re-mapping completes.

Recover the 840D Pl behaviour on NCU 573.2 by:

1. Confirm NCU software release matches the CF card image (boot the NCU in Service mode, check Display info → SW release).
2. Load the original machine vendor's commissioning backup: Enable HMI → Startup → Commissioning → Read-in archive.
3. Verify PLC $PLC_VERSION and toolbuild messages on the HMI are clean.
4. Reset the spindle axis with GETSEMAPHORE / SETSEMAPHORE in the PLC and clear axis alarms with RESET from the NCK.

11. Reset Behaviour and DB21.DBX35.6 "Channel Interrupted"

The HMI status Channel interrupted is set from the controller's notification interface. The corresponding signal is on the user-program interface:

DB21.DBB35 Bit 6 = channel interrupted

Under normal conditions this bit clears within < 2 seconds after MCP-RESET. The 10–12 second dwell observed on this machine indicates the controller is waiting on one or more asynchronous handshakes that fail to complete in their nominal window. Typical causes:

A typical reset-time of 2 s rises to 10–12 s if the controller is stuck on a hung PROFIBUS-DP cyclic exchange on the spindle DO. The PROFINET or PROFIBUS scan time is determined by the bus configuration in HW-Config and is normally ≤ 4 ms. The error pattern observed aligns with PROFINET sync retries on drive-object DOWN-link.

12. Recommended Parameter Set and Commissioning Procedure

The consolidated MD set recommended for this machine is shown below. Adjust gear stage indexes ([1..5]) to suit the actual gearbox; the values below assume a single-gear-stage spindle (M40 not in part program).

Apply the following procedure after writing these values:

1. Run NCK reset (cold restart, NOT a power-cycle) so the new MDs are picked up.
2. Confirm the HMI reports no axis alarms on the spindle.
3. Run the spindle in M3 S1000 and read actual RPM via tacho on the spindle housing. Compare to M3 S500 and M3 S1500. The actual RPM should be a linear function of S, scaled only by spindle override and MD35100/MD35130 ceilings.
4. Set S = 0 and MC spindle: read "spindle stopped" within 1 s of S0.
5. Issue SPC(0) or DB21.DBX30.0 = 1 via PLC to trigger spindle parameter set switch.
6. Issue a positioning check: SPOS=0 then SPOS=180. The spindle should align at the programmed angle without slipping the BERO mark.
7. Power off / power on, and verify referencing lands at the same mechanical angle each cycle (within ± 1 encoder count).

13. Field Validation and Verification Checklist

Run the checklist twice — once before and once after a power-cycle — to capture any non-volatile MD persistency issues that would indicate a CF card / NCU backup mismatch.

14. Troubleshooting Matrix

15. Frequently Asked Questions