Resolving SIMODRIVE 611U Drive Vibration and A1106 Alarm

Symptom Definition

On a 3-axis CNC vertical machining centre controlled by an ACRAMATIC controller and equipped with SIMODRIVE 611 universal (611U) digital converters, two axes begin to produce an audible high-frequency hum or whistle immediately after the drive enable is set. The control panel may also raise Alarm A1106 on the affected axes (X and Y) before any movement occurs. The third axis (Z) continues to operate normally with identical motor and encoder part numbers, leaving the maintenance engineer to assume that drives are interchangeable. Under this false assumption, a backup file from the Z axis is loaded into the X and Y control modules through SimoCom U. The alarm disappears, but the audible hum persists on every enable and the drives cannot be released for production cuts.

The four failure modes that converge in this scenario are:

• Sudden loss of the 6SN1118-0NH01-0AA0 control module's parameter memory on X/Y axes.
• Cross-loading parameter sets from a different axis whose Line Module (LT-MODULE) has a different current rating.
• Skipping the automatic four-step commissioning flow within SimoCom U that establishes the current loop, speed loop and commutation data.
• Manually decreasing speed and current-controller gains as a first response, which masks the underlying parameter mismatch but does not stabilise the velocity loop.

Hardware Identification Matrix

The first diagnostic step on any SIMODRIVE 611U tremor complaint is to read every MLFB label on the power section and the control module. Several differentiators between the data-card and its sibling are invisible without a parts list:

The two control modules end in different suffix letters (-0AA1 vs -0AA0). The trailing digit is not cosmetic; on Siemens 611U control boards it indicates board revision and corresponding firmware compatibility. Loading Version B firmware data into a Version A board corrupts three parameter groups: P100..P199 (drive/motor), P200..P299 (speed controller) and P700..P799 (position controller monitoring).

Root Cause Analysis

The mechanical plant on X, Y and Z is similar enough that the engineer reasonably assumed "a param file is a param file." On the 611U platform that assumption is invalid for four independent reasons:

1. LT-module current class — the 2 x 25 A module imposes a different DC-bus undervoltage threshold (P1594) and a different maximum short-term current clamp (P1114) than the 2 x 50 A module. Z-axis parameters trip these clamps on the smaller X/Y module the moment the spindle-brake release pulse is requested.
2. Regulator-default tables — P200 series defaults are populated from the LT module's current class. The same proportional and integral terms that damp the 50 A bus produce a saturated integrator on the 25 A bus. The motor then oscillates at the speed-loop crossover and the audible result is the 4–8 kHz whistle documented as "humming."
3. Position monitoring windows — the following-error trip windows in P700..P730 are set for the heavier Z-axis mechanical inertia. When applied to lighter X/Y mechanics the trip fires (or nearly fires) on every rapid traverse; if it does fire, the firmware raises A1106.
4. Loss of pole-pair identification — the hardware data card normally stores the result of the last pole-position identification run (P173). After data loss the drive has no commutation reference. With parameter data restored from another axis, the resolver offset is geometrically wrong, forcing the rotor to oscillate to find electrical zero on every enable.

The combination of points (2) and (4) produces the distinctive cold-start hum reported in the field: the drive has torque but cannot establish a stable commutation/torque angle, so it "chatters" against the resolver offset until thermal protection derates the current.

Alarm A1106 Decoded

Alarm A1106 in the SIMODRIVE 611 universal alarm schema is a position-controller monitoring event. It fires when the actual following error, measured between the position setpoint (from the setpoint channel) and the resolver-evaluated actual position, exceeds the dynamic threshold window set by:

• P700 — Following-error monitoring tolerance
• P702 — Coarse standstill window
• P703 — Positioning window

When A1106 is reported on a cold spare module, it is almost always a secondary symptom of either:

• Failed pole-position identification (commutation never stabilises), or
• An uncommissioned current loop (speed controller saturates, drives motor into torque limit, position loop cannot close).

Refer to chapter "Alarm and warning messages" in the SIMODRIVE 611 universal Function Manual (FBU 0510) for the exact reaction class (OFF1 / OFF2 / OFF3 stop response) and the corresponding P0xxx acknowledgement parameters. Do not treat A1106 as a stand-alone fault: clear the alarm, immediately begin a full four-step commissioning sequence, and confirm pole-Pair ID before any high-speed motion is commanded.

Parameter Copy Dangers Across Mixed MLFB

The SimoCom U "Download / Upload / Compare" workflow can copy parameters axis-to-axis at high speed. That convenience is the source of the problem. The following table documents the parameter families that must not be copied between an axis using a 2 x 50 A LT module and one using a 2 x 25 A LT module:

The correct workflow is to use SimoCom U only for the identical block group (board firmware defaults P0..P99) and then to commission on metal for every other block.

SimoCom U Environment Setup

Before connecting to the converter observe the following prerequisites:

1. Confirm the controlling ACRAMATIC is in E-stop / NC-Stop state so the drives cannot be enabled inadvertently during commissioning.
2. Connect a serial RS-232 cable between PC and the X471 service port on the front of the SIMODRIVE control module (or use PROFIBUS on X455 if the card supports it; verify with the device data block in SimoCom U).
3. Set the SimoCom U interface to COM1 / 9600 8N1 (default for 611U) or to the configured PROFIBUS node address.
4. Power-cycle the LT module (P-ON) and wait for the "DRV-OK" LED to settle to green before initiating online sessions.
5. From the menu Drive → Start-up → Automatic Configuration, enter the motor code for the installed 1FK6083-6AF71-1AG0 motor. The motor code is the order number suffix (e.g. 1FK6083-6AF71-1AG0) entered into parameter P110.

Always archive the present data set with File → Upload to PC (Drive) before loading any backup, even a known-good one. This is your only chance to capture any vendor-specific customer settings before they are overwritten.

Automatic Synchronization: Four-Step Commissioning

The permanent resolution of the humming is to run the four-step automatic synchronisation procedure that SimoCom U exposes under Start-up → Automatic Synchronisation. The drive moves the axis under controlled jerk during steps 2–4; on a vertical-machining-centre with a moving column this requires open safety clearance:

1. Step 1 — Parameter pre-configure
   The tool reads the connected motor/encoder pair from P110 and P115..P117 and pre-loads the safe starting values for P200 series (speed loop gain), P300 series (position loop gain), and current-limit clamps (P111/P112). Acknowledge the prompt "Parameter default values entered".
2. Step 2 — Pole-position identification
   The drive energises the motor with a low current pulse train to determine the absolute resolver position relative to the rotor. Result is written to P173. For 1FK6 series the typical final current value is 30 % of rated (P111 * 0.30). Wait for the green status before progressing; do not interrupt this step — an interrupted pole-ID leaves the resolver offset at its last value and may trip A1106 on the next enable.
3. Step 3 — Speed/current controller auto-tune
   The drive executes small amplitude oscillations to measure mechanical inertia and friction, then calculates K_P (P200), T_N (P201), current-controller K_P (P170/P171). The audible chirp during this step is normal. Acceptance criteria: settling time on the speed step response < 10 ms with zero overshoot.
4. Step 4 — Position controller / monitoring windows
   Sets the dynamic following-error window P700 based on the measured speed-loop bandwidth. Once accepted the A1106 monitoring becomes meaningful. The drive then performs a controlled 1-revolution homing to latch the encoder absolute reference.

Drive Tuning After Synchronization

If the hum persists after the four-step sequence it is no longer a parameter-corruption problem; it is a loop-tuning problem. Apply the following field-proven adjustments in SimoCom U under Diagnostics → Controller Tuning with the Trace window recording actual speed and torque:

• Reduce K_P speed controller (P200) by 15 % per iteration until oscillation ceases. Do not reduce below 0.3 of the auto-tuned value; below that the static stiffness of the X/Y axis becomes unacceptable for finishing cuts.
• Lengthen T_N integral time (P201) from the auto-tuned value by a factor of 1.5–2.0. Long integral times damp the resonance without losing steady-state accuracy.
• Confirm current limits that P111 + P112 do not exceed 1.5 x motor continuous current (M0I0). On 1FK6083-6AF71 (rated ~14 A continuous) the maximum motor-side peak must remain below 42 A. Higher clamp currents introduce audible magnetostriction on the stator stack.
• Enable P820 torque-smoothing filter with a fixed time constant around 800µs; this is the standard remedy for the 4–8 kHz whistle on 611U with 1FK motors.
• Check fieldbus cycle time. ACRAMATIC should update the PROFIBUS / setpoint channel at 4 ms or faster; a 12 ms cycle re-introduces position-loop jitter.

Record a final commissioning sheet containing P110, P173, P200, P201, P700, P820 with their final values; this sheet is the new machine reference for the X/Y axes.

Verification Procedure

1. With the NC in "Reference" mode, command a controlled jog of 10 mm on X and Y in each direction. The drive must accelerate and decelerate silently; any visible motor rock or audible tone indicates the regulator has not converged.
2. Open the Trace tool in SimoCom U and capture a 2 s window during the jog. Confirm the actual speed follows the ramp without oscillation and P701 (current actual) remains within ±25 % of the average. Save the trace as evidence.
3. Run a circular-interpolation program (G02/G03) with a 50 mm radius and a 5 000 mm/min feedrate. Look for a circularity error below 0.02 mm on the ACRAMATIC measurement screen; a high reading means the position-loop K_P (P300) is still under the optimum.
4. Cycle the LT-module control voltage (P-ON » P-OFF » P-ON) twice and verify that the drives re-establish ready state without alarm. This validates that the parameter set is persisted correctly into flash.
5. Confirm via Drive → Diagnostics → Alarm History that no A1106 / A1107 / A1108 events have been re-logged in the last 10 minutes of operation.

Troubleshooting Matrix

Spare-Part Strategy and Lessons Learned

Maintain a per-axis commissioning sheet for each SIMODRIVE 611U axis in the fleet. The sheet must include the LT module MLFB, the control module version, the motor order number, the resolver offset result P173, and the final K_P / T_N / K_V values. Without this sheet, a control-module failure will always result in a half-day of cross-loading attempts followed by a noise complaint. With the sheet, a maintenance engineer can perform four-step commissioning in roughly 25 minutes per axis.

For control modules on long lead-time (Version B has been discontinued for several years), order a 1-step "cold spare" that contains a known-good backup file for that axis burned into the FEPROM before the field swap. This converts a field service into a hot-swap and reduces the SIMODRIVE 611U exposure window from 25 min down to 5 min.

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