Troubleshooting S120 EPOS Axis Auto-Movement After Power

1. Problem Description

On a grinding machine retrofit, the U-axis driven by a SINAMICS S120 (with Basic Positioner / EPOS activated) exhibited uncontrolled motion immediately after the 24 V control section and DC link were re-applied. The motor would travel a short distance without any enable from the operator panel, then either stop on its own or trip a position-monitoring alarm. Configuration details observed on site:

• Position control active, EPOS function module selected (r108.1, r108.2 status bits reported by the drive).
• Closed-loop control of the position using a motor-mounted absolute encoder and a linear encoder at the load, with a cross-check of the two actual position values.
• Setpoint source is an external CNC industrial computer connected via PROFIBUS. The S7-300 PLC handles peripheral I/O and interlocks but does not issue the position setpoint.
• An alarm was reported by the CNC screen, but the drive itself did not immediately flag a unique S120 fault code - the alarm was generated because the load position versus the expected CNC setpoint no longer matched.

When the technician inspected the drive in SINAMICS STARTER (or the modern TIA Portal-based SINAMICS Startdrive), the load-side position actual value (r2521) was shown as a large positive number when the machine was mechanically sitting at a clearly negative datum point. In other words, the drive's absolute zero reference was missing or had been incorrectly set on a previous commissioning pass, so the internal model position was not aligned with the real axis position. The EPOS function therefore applied a corrective setpoint step the moment it was enabled, which is what made the axis jerk at power-on.

2. Root Cause Analysis

The motion at power-on is a position reference mismatch, not a hardware failure. Three different mechanisms can produce the symptom, and the diagnostic procedure must confirm which one applies before any parameter is changed.

In the field case, the technician identified the first cause. The EPOS function retained the encoder position but the load-side zero reference (mechanical zero of the grinder table) had not been written. After re-establishing the zero reference, the drive correctly recognized the stationary axis, the position actual value stabilized at the correct datum, and the spurious motion disappeared.

3. SINAMICS S120 EPOS Architecture Recap

EPOS in SINAMICS is a software positioner that closes the position loop on top of a velocity-controlled drive. The position loop is closed inside the Control Unit (CU320-2 DP/PN or CU310-2) using the position actual value derived from one of two encoder channels. Per the official SINAMICS S120 Function Manual - Basic Positioner (EPOS):

1. Encoder acquisition (motor / load) - parameters p0400..p0430 select the type (resolver, EnDat, SSI, HTL/TTL, DRIVE-CLiQ).
2. Measuring gear / load gear - p0410..p0413 scale the rotary sensor to load units.
3. Position actual value preprocessing - p2540..p2546 select between motor encoder, load encoder, or both.
4. Mechanical zero reference / position offset - p2520..p2526, p2590..p2595 define the absolute datum and the home position offset.
5. Position setpoint processing - p2610..p2640 (modes: jog, MDI, traversing blocks, direct setpoint from PROFIdrive).
6. Position controller - p2530..p2536 (gain, pre-control, output limits, following-error filtering).
7. Monitoring - p2540 / p2542 / p2543 (following error, position limit, setpoint limit reached).

The setpoint source in this case is the external CNC. The drive is configured as a PROFIdrive slave (positioning mode, telegram 110 or telegram 7 with extended setpoints). Telegram selection is in p0922, and the PZD wiring of the setpoint and actual-value words is in p2050/p2051 according to the SINAMICS S120/S150 List Manual.

4. Absolute Encoder and Linear Encoder Setup

Because the field case uses a redundant encoder pair, both p2540 (encoder source) and p2544 (position-actual-value processing) must be set correctly. The standard wiring uses one of the Sensor Modules SMC30 / SME25 / SME120 to read each encoder.

After these are set, the absolute encoder should report a stable value at r0480[0..1] / r0481[0..1] and the linear counter at r0482[0..1] / r0483[0..1]. The position tracking window (p0413) is what allows the absolute value to remain valid across an OFF/ON cycle within a limited number of motor revolutions - a critical detail because if the table is moved past the window while the drive is unpowered, the absolute value becomes invalid and the drive demands a new reference run.

5. Position Reference and Zero-Point Parameters

The EPOS zero reference is the most likely source of a power-on auto-motion. The following parameters must be set coherently. These are the parameters that the field technician corrected via STARTER.

When the axis is correctly referenced, bit 0 of r2504 reads "1". When the zero reference is lost or was never set, the bit reads "0" and the drive treats the position as uncalibrated. If the drive receives a setpoint under those conditions, EPOS will perform a homing-run to recover the datum, and on some configurations it will simply step to the last known setpoint - which is what produced the field symptom.

6. CNC-PLC Communication over PROFIBUS

The setpoint in this application is generated by an external CNC computer. The drive is a PROFIdrive positioning slave. The relevant commissioning items are:

A common error pattern is that the CNC immediately begins writing the MDI position setpoint as soon as the bus goes cyclic, before the drive has completed the homing or the absolute reference check. The EPOS function then acts on the new setpoint - either moving to a "homing" coordinate or to the last MDI target. The fix is one of:

1. Make the CNC gate the setpoint on the drive's "axis referenced" bit (ZSW1 bit 13 = closed-loop, ZSW2 bit 4 = referenced/homing done).
2. Set EPOS mode to "no setpoint until referenced" via p2617 / p2618 and the configuration of p2650/p2660.
3. Use the drive's own p2640 to retain a safe intermediate value until the reference is set.

7. Commissioning with STARTER / Startdrive

The corrective sequence described below is what produced the working result in the field. Tools and preconditions are listed first.

7.1 Prerequisites

• SINAMICS STARTER (legacy) or TIA Portal with SINAMICS Startdrive, version matched to the drive firmware (e.g. Startdrive V17 for V5.2 firmware, V18 for V5.2 SP1, V19 for V5.2 SP3).
• Drive firmware version known: SINAMICS S120 firmware V5.2 SP3 or V5.3 is typical for current retrofits. V4.x firmware remains in service on older CU320-2 units.
• Project backup taken (drive CF card copy, or STARTER "Upload from device").
• Online connection established via PROFIBUS or PROFINET; STARTER online mode shows r parameters live.
• Safe state established; STO engaged; motor uncoupled or axis mechanically locked if possible.

7.2 Step-by-Step Corrective Procedure

1. Read the actual position - In STARTER's Control Unit > Commissioning > Position actual value view, read r2521. With the machine sitting at a known datum, this should equal the mechanical zero. If it does not, the zero reference is missing.
2. Disable all setpoints - Block the CNC MDI setpoint (p2640 = 0) and clear p2650 / p2660 traversing block position. Set p0840 = 0 (OFF1), p0852 = 0 (operation enable). The drive will coast down.
3. Force an active homing run - Set p2590 = 1 (use reference cam + zero mark) or 4 (passive homing using the existing zero) depending on the machine. Pulse p2591 (rising edge) via STARTER's Control > Signals panel. Observe r2521 climbing as the axis runs to the cam and settles at the zero mark.
4. Set the home position offset - Once at the mechanical zero, write the desired value of r2521 into p2595 (home position offset) and accept with "Set home position". The drive writes p2522 (position reference mechanical) to the current r2521 plus p2595.
5. Mark as referenced - In STARTER, under Position Control > Homing, click "Set absolute value" / "Set reference point". This sets r2505.0 = 1 (control word position reference enable) and r2504.0 = 1 (axis referenced).
6. Save to ROM - Drive > Copy RAM to ROM. Without this step, the corrected zero reference is lost on the next power-off.
7. Power cycle - Turn the 24 V off and back on (the drive will run-up, recognize the absolute encoder, validate against p0413, find the new reference, and not move).
8. Verify - Re-enable the drive (p0840, p0852), bring the CNC into cyclic mode, and check that the axis does not move until a deliberate setpoint is issued. Use r2521 (actual) and r2520 (setpoint) side by side; they must match within the standstill window (p2542).
9. Re-enable the CNC setpoint source - Restore p2640 / p2650 / p2660 to their process values and clear the gate in the CNC application.

8. Verification and Acceptance Test

After the corrective procedure, run the following acceptance test:

1. Standstill test - With the drive enabled and zero setpoint, monitor r2520, r2521, r2561, r2562 (following error / tracking deviation) for 10 minutes. They must remain within +/- 1 LU.
2. Power-cycle test (hot) - Remove and re-apply the 24 V control voltage. The drive must come up without any axis motion and r2521 must read the previously established value.
3. Power-cycle test (cold) - Remove the DC-link enable and the 24 V supply simultaneously, wait 30 s, reapply. The drive must not move. The CNC must report "axis referenced" on its status page before issuing a motion command.
4. Absolute value tracking - Move the axis 50 mm by manual mode (JOG) and back. The drive must remain referenced (r2504.0 = 1) and r2521 must track.
5. Encoder difference test - Read r2540 / r2541 (position difference between the two encoders). It must be below p2543 (encoder difference threshold) at all times. If the linear encoder reports motion that the absolute encoder does not, mechanical slip is present and the differential check is the more critical safeguard.
6. Fault injection - Briefly pull the home-cam input low during a homing run. The drive must abort with F07457 (homing error) and not move into the end-stop. Restore and re-home to confirm normal operation.

9. Common Related Faults and Alarms

The S120 EPOS fault set documented in the SINAMICS S120 List Manual includes the following codes that are likely to be raised on this class of problem:

None of these are typically the root cause of "axis moves automatically at power on". The root cause is the missing reference; the fault is the consequence. Confirm the reference first, then investigate any remaining alarms.

10. Field-Proven Tips and Best Practices

1. Always read r2521 against a known mechanical datum on first commissioning. A positive value when the axis is sitting at a negative side of zero is the classic sign that the absolute value was set in the wrong direction.
2. Use passive homing (p2590 = 4) on retrofits that already have a known datum, instead of active homing. It does not move the axis; it merely assigns the current position to the configured home.
3. Set p0413 (position tracking tolerance window) smaller than the maximum axis travel between power-off and power-on. If the operator can move the table by hand more than the window allows, the absolute value will be invalidated and a re-reference will be required.
4. Always set p2640 (MDI position setpoint) to a safe value - typically the current r2521 - before the CNC starts streaming setpoints. This eliminates the first-cycle setpoint jump.
5. Configure ZSW2 bit 4 ("referenced/homing done") in p2084[0..15] and map it into the PROFIdrive telegram. The CNC will then know when it is safe to start motion commands.
6. On machines with multiple encoders (motor + load), configure p2544 = 1 to use the load encoder as the position reference. The motor encoder is used for speed control; the load encoder is used for absolute position. This is the most common mis-configuration leading to drift between the two encoders.
7. Save parameters to the CF card as well as to ROM. The CU320-2 supports both; the CF card survives CU replacement and is the master record for service engineers.
8. After every parameter save, do a power-cycle test before returning the machine to production. Many parameters only become active on the next ramp-up.
9. Document the values of p2522, p2523, p2524, p2595, p0412, p0413, p2540, p2543, p2544, p2590 in the machine logbook. These are the parameters that field service will need to recover the configuration after a CU replacement.
10. If the machine is part of a Safety Integrated system, verify that the homing run is allowed only in a safe state (SOS or reduced speed monitoring). The S120 Safety Integrated Function Manual describes how to gate the EPOS enable on the safety state.

11. Diagnostic Matrix - Power-On Auto-Motion

12. Related Standards and Documentation

The drive and EPOS function are documented in Siemens' official manuals, which are the authoritative source for parameter values and acceptance criteria. Engineers commissioning or repairing S120 EPOS axes should keep the following documents at hand:

• SINAMICS S120 - Function Manual, Basic Positioner (EPOS)
• SINAMICS S120 - Function Manual (FH1) Drive Functions
• SINAMICS S120/S150 - List Manual (parameter list and fault codes)
• SINAMICS Startdrive in TIA Portal - Programming and Operating Manual
• SINAMICS S120 - Safety Integrated Function Manual
• SINAMICS S120 - Commissioning Manual with STARTER

These manuals are the contract for the parameter numbers quoted above; verify any value in the live List Manual for the firmware version installed in your drive before saving to ROM.