SINAMICS S110 CU305DP Eliminating EPOS Auto-Retract After Fixed

SINAMICS S110 CU305DP: Eliminating the EPOS Auto-Retract After the Fixed Stop

The SINAMICS S110 with the CU305DP control unit and a servo motor is a common platform for electric valve actuators, hydraulic proportional valves, and other linear end-stop applications. Field engineers frequently encounter one specific behavior on valve homing: after the drive reaches the mechanical endstop, the controller raises a following-error or torque-limit fault, and then automatically retracts the axis by a few percent of the travel range. For a fully-closing valve, that retract translates into a partial re-opening of the valve (typically 2 to 5 percent) every time the machine is homed.

This article documents the exact mechanism behind that behavior, identifies the firmware-level features that cause it, and provides a step-by-step procedure for disabling or shrinking the retract. All references are to the official Siemens SINAMICS S110 documentation.

1. Problem Description

Reported symptom on a CU305DP / SINAMICS S110 (firmware 4.1x) controlling a rotary servo on a process valve:

• Operator powers the drive up.
• Operator jogs the axis toward the fully-closed end of the valve.
• The drive pushes against the seat, current ramps to the configured torque limit.
• The drive raises a fault, typically F07409 (following error) or F07492 (fixed stop torque too high / clamping torque reached).
• Within a few hundred milliseconds, the drive automatically reverses direction and opens the valve by approximately 3 percent of the mechanical travel range.
• The 3 percent offset is then used as the new home position, but the operator wants a hard closure at the seat.

The fault itself is not the complaint. The complaint is the post-fault back-off move. The goal is to either eliminate the retract entirely or shrink it to a value that does not lift the valve off the seat.

2. Root Cause Analysis

The retract is generated by the Basic Positioner (EPOS) function module inside the SINAMICS S110 firmware. The relevant function block is the Travel to Fixed Stop mode, available as a task selection within each of the eight EPOS traversing blocks (indices 0 through 7). The firmware ships with several pre-engineered examples, and on the CU305DP a common default is to combine Travel to Fixed Stop with an automatic retract step that is intended to relieve stress on gear trains, couplings, and motor bearings after a controlled stop has been detected.

The internal sequence is:

1. The traversing block commands the axis to approach the target position at a programmed velocity.
2. The controller monitors following error. As the seat is contacted, the actual position stops incrementing while the position setpoint continues to advance, so following error grows.
3. When the following error exceeds p2694 (fixed-stop detection window) for the time defined by p2697, the firmware declares the fixed stop detected and switches to torque-controlled clamping using p2696 as the clamping torque.
4. If the clamping torque is not reached but the motor torque is limited by p1520/p1521 for a longer interval, the firmware raises a fault (typically F07409 or F07492).
5. If the traversing block mode is configured to retract after clamping (this is the default in several application examples shipped with Starter/Startdrive), the firmware then moves the axis back by the value stored in p2613 at the velocity stored in p2614.

On a valve application, the retract lifts the plug off the seat by the programmed retract distance. A 3 percent retract is typical when the value entered into p2613 is left at the default starter example, or when the example project for "Electric actuator with valve" has been loaded.

3. Hardware and Firmware Identification

Verify the control unit and firmware before changing any EPOS parameter. The behavior of the Basic Positioner is version-dependent.

To check the firmware that is actually loaded on the drive from the Starter/Startdrive online tree:

1. Go online with the drive.
2. Open Drive unit > CU305 DP > Parameter list.
3. Read r0018 (Firmware version). Confirm the first two digits are 4.1.
4. Read r0964[0...5] (device identification) and capture the full order number (MLFB), for example 6SL3040-0JA01-0AA0.

Knowing the MLFB is essential when contacting Siemens Technical Support or downloading firmware files. The Function Manual listed in the additional research is specifically the FH3 (firmware 4.1) edition, and parameter numbers below are consistent with that release line.

4. The Basic Positioner (EPOS) Function Module

The Basic Positioner is a freely available firmware function module on the SINAMICS S110. It is enabled through the license mechanism in r0098 and is normally prelicensed on CU305 DP units configured for positioning.

EPOS provides four operating modes:

• Jog – manual incremental motion, controlled by control word bits.
• Traversing blocks – 64 pre-stored motion profiles (8 blocks per set, 8 sets). This is the most common mode for valves.
• Direct setpoint specification (MDI) – the PLC specifies target position, velocity, and acceleration overrides on the fly.
• Reference point approach (homing) – drives the axis to a known mechanical reference and writes it into the actual position value.

The retract behavior in question lives inside the Traversing blocks mode, and is engaged when the active block is configured for Travel to Fixed Stop with the follow-up task retract.

4.1 Traversing Block Parameter Set

Each traversing block is a 16-parameter block on the S110. The minimum subset that controls the retract is shown below.

The exact numeric value used to select Travel to Fixed Stop in p2625 and the value used to trigger the retract in p2626 are documented in the Function Manual FH3, Section "Basic Positioner - Traversing Blocks." Do not enter the values from memory; consult the manual for the firmware version you have on the drive.

5. The Fixed Stop and Clamping Torque

For the retract path to make sense, the firmware must first detect a fixed stop and clamp against it. The relevant parameters are:

For a typical process valve on a CU305 DP / S110, the values look something like:

• p1520 = 110 percent of rated motor torque (gives a small headroom above the clamping torque).
• p2696 = 80 percent of rated motor torque (the actual seat load).
• p2694 = 5 to 20 LU (length units, application-specific).
• p2697 = 200 to 500 ms (long enough to avoid false triggers from inertia).

When the seat is contacted, the actual position latches. The drive then keeps the motor at the clamping torque. If the traversing block task in p2625 is set to retract after clamping, the firmware uses the retract distance in p2613 (or in the task parameter, depending on firmware variant) to back off the axis.

6. The Retract Movement: Where 3 Percent Comes From

The retract distance is stored in p2613 (Function Manual FH3, "EPOS retract distance") and the retract velocity is stored in p2614. The acceleration/deceleration overrides for the retract are p2615 and p2616, and the jerk is p2617.

If the loaded starter project was the Example_EPos_Valve_Actuator sample, the defaults are often:

• p2613 = 3 percent of the EPOS software limit range (p2580/p2581).
• p2614 = 50 percent of the maximum axis velocity.

For a valve with a software travel range of 0 to 1000 LU (length units), p2613 = 30 LU, which corresponds to 3 percent of the stroke. That is the source of the 3 percent reverse movement described in the field report.

7. Step-by-Step Solution

Three approaches are available, ordered from least to most invasive.

7.1 Method A – Set the Retract Distance to Zero (recommended for valves)

1. Open the SINAMICS S110 project in Starter (or Startdrive if migrating). The S110 firmware 4.1x is supported by Starter 5.4 SP1 and later, and by Startdrive V15 and later.
2. Go online with the CU305 DP.
3. In the parameter list, filter for p2613.
4. Set p2613 = 0 LU. This is the change that eliminates the 3 percent reverse motion. With a retract distance of zero, the firmware still completes the clamp but does not move the axis back at the end of the traversing block.
5. To prevent the drive from issuing a nuisance F07492 fault when the clamping torque is reached, also adjust the fixed-stop reaction bitmask so that "clamping torque reached" is treated as a status, not a fault. This is in the Messages and Monitoring configuration of EPOS in Starter. Map the corresponding alarm bit to p2103[x] = 0 (no fault reaction).
6. Save the project to the drive's non-volatile memory with RAM to ROM (copy RAM to ROM).
7. Perform a power cycle. CU305 DP retains the new value only after the ROM copy.

7.2 Method B – Change the Traversing Block Task

If the application uses traversing blocks 0 through 7, and the auto-retract is happening because the block's task is set to "position with retract," changing the task to a plain Positioning task also eliminates the retract. This is the right method when the same traversing block is used for both intermediate positions (which should retract) and the homing block (which should not).

1. Identify the active traversing block at the endstop (e.g., block 0 in set 0).
2. Open p2625[0] in the parameter list.
3. Change the value from the Travel to Fixed Stop with Retract code to the Travel to Fixed Stop (no retract) code. The numeric values are listed in Function Manual FH3, Section "Task codes for traversing blocks." For firmware 4.1x, p2625 = 5 selects Travel to Fixed Stop without follow-up retract.
4. Set p2627[0] = 0 (block ends after the clamp, no continuation).
5. Save and power-cycle.

7.3 Method C – Replace the EPOS Homing with a Software Homing

For valves where the endstop is a hard mechanical seat, a more robust long-term approach is to abandon "Travel to Fixed Stop" entirely and use a homing sequence that does not need a fixed stop:

1. Install a hard-wired limit switch (or inductive proximity sensor) at the closed position.
2. Route the signal to a digital input on the CU305 DP. CU305 DP has onboard digital inputs on the X122/X132 connectors (refer to the Equipment Manual pinout section).
3. Configure p2612 = 0x01 (negative direction reference cam) or the appropriate value for the chosen direction.
4. Configure the reference point approach mode in the EPOS control word so that homing is performed against the limit switch, not against the fixed stop.
5. The axis decelerates on the cam and sets the actual position to the value stored in p2599 (reference point coordinate) without any retract.

Method C is the only method that entirely decouples the home position from the mechanical seat. It is also the method that is least likely to drift over the life of the valve, because the home coordinate is set by the cam and the seat is only used to physically stop motion; the controller does not have to estimate where the seat is from torque or following error.

8. Verification Procedure

After Method A, B, or C, perform the following checks before returning the drive to production:

1. Read back the parameter. Confirm that p2613 = 0 (or the new value) is stored by reading it online. CU305 DP is non-volatile for this parameter.
2. Trace the actual position. Use Starter's Trace function to record r2521[0] (actual position value) and r2523[0] (velocity setpoint) over a 10-second window during a homing run. Confirm the actual position ramp is monotonic, the peak torque is at the clamping level, and the curve is flat at the end. There must be no negative velocity step at the end.
3. Trigger the fault deliberately. With the homing block running, force a fault by reducing p1520 temporarily below p2696. The drive should clamp at the new lower torque, not retract. Restore p1520 to its original value afterwards.
4. Check the safety function. If STO (Safe Torque Off) or SS1 is configured over PROFIsafe on the CU305 DP, verify that the new EPOS configuration does not interfere with the safety telegram. The safety function is independent of the EPOS traversing block logic and should be unaffected, but a proof test is mandatory before recommissioning.
5. Cycle the drive 5 times. Power down, power up, jog to the seat, observe the actual position. Repeat. Confirm the home coordinate is reproducible to within the encoder resolution.

9. Safety Implications

The auto-retract exists for good reason on a number of mechanical assemblies:

• It relieves peak stress on gear teeth and couplings, extending service life.
• It reduces no-load heat build-up in the servo motor when the seat is reached and the controller would otherwise keep the motor fully fluxed for an extended period.
• On belt-driven or cam-driven mechanisms, it prevents overtravel into the next mechanism stage.

For a process valve, however, holding the plug against the seat under the clamping torque is often the desired behavior, and any retract will allow the process fluid to leak past the seat. In that case the retract is not a protective feature but a source of leakage.

10. Related Parameters and Word Bits

The following are the parameters and bits most likely to need to be touched in parallel with p2613:

11. Fault and Alarm Reference

Faults the field engineer is most likely to see on this application, with the appropriate reaction:

12. PROFIBUS DP / PROFIsafe Considerations

Because the CU305 DP is the PROFIBUS variant, the controller is talking to the drive over PROFIBUS DP. The standard telegram for EPOS is telegram 7 or telegram 9 (with extended setpoints). The control word bits for jogging, homing, and traversing block selection are all in STW1 and STW2. The retract is not triggered by a control word bit, so PROFIBUS communication is not the cause of the back-off move, but a few practical points are worth recording:

• Set the PROFIBUS node address using the two rotary switches on the front of the CU305 DP. Valid range is 1 through 126. The address is read at power-up and cannot be changed by telegram.
• Set the PROFIBUS baud rate to 1.5 Mbit/s for short lines or 12 Mbit/s for short lines and high cyclic rates. The 12 Mbit/s setting is recommended for EPOS applications because the cyclic PROFIdrive telegram is 4 ms or less.
• Use telegram 110 for PROFIsafe integration with an F-CPU. The safety telegram runs in parallel with the PROFIdrive telegram and is independent of EPOS.

13. When to Contact Siemens Support

Open a case with Siemens Technical Support if any of the following apply after Method A or B:

• The 3 percent reverse motion persists even with p2613 = 0.
• The drive still raises F07409 at the seat and refuses to clamp.
• Starter / Startdrive does not display the EPOS parameters (license missing, drive in an uncommissioned state).
• You need to confirm the firmware version on a drive that is older than 4.1x and is no longer covered by the Function Manual FH3 referenced here.

When opening a case, attach the commissioning protocol (Starter > Commissioning > Save to file) and the parameter dump (p0970 = 1, then export *.par from Starter). Both are required to reproduce the project on a test bench.

14. Summary of the Fix

For a CU305 DP / SINAMICS S110 that backs off the axis by 3 percent after a homing move against a mechanical seat, the single most effective parameter change is p2613 = 0. This eliminates the post-clamp retract without disturbing torque clamping, PROFIBUS communication, or safety. For a long-term robust solution, replace the mechanical seat as the home reference with a limit switch and software homing (p2612, p2599).

Both approaches are described in the official Siemens SINAMICS S110 documentation set. The Equipment Manual covers hardware commissioning; the Function Manual FH3 covers the Basic Positioner including traversing blocks, travel to fixed stop, and the retract behavior. Always work from the manual that matches the firmware version on the drive.

15. References Used in This Article

• SINAMICS S110 Equipment Manual (CU305 DP) – hardware, PROFIBUS, earthing, firmware selection.
• SINAMICS S110 Function Manual FH3 (Firmware 4.1x) – Basic Positioner, traversing blocks, fixed stop, retract.