SINAMICS V90 Position Loss After Power-Off with S7-1200 SINA_POS

1. Problem Statement: Position Reference Lost on Drive Power-Off

When a SINAMICS V90 servo drive is controlled from a SIMATIC S7-1200 CPU using the SINA_POS (FB284) library block in EPOS (Basic Positioner) mode, the application-level position is lost every time the 24 V control and power to the drive is removed. On the next power-on, the drive boots, runs through its initialization, and the EPOS state machine reports not referenced regardless of the absolute encoder's internal retention. The PLC cannot issue a single MC_MoveRelative or MC_MoveAbsolute until a homing run has been executed.

For a conveyor that indexes in fixed 76.2 mm steps, the re-homing adds 2-6 s of unproductive machine time per shift and, more importantly, breaks the assumption that the HMI position display matches the mechanical position. Operators are forced to run a manual reference procedure after every E-stop reset or night-shift power-down.

2. Root Cause Analysis

Three interacting factors cause the symptom:

1. EPOS owns the position. When SINA_POS is selected as the control method, the drive's EPOS function module is the source of truth for actual position. The PLC only mirrors that value through the ActPosition (LREAL) output. The PLC does not own a copy that survives a power cycle, and SINA_POS does not write ActPosition back into a retentive tag.
2. EPOS state machine is non-retentive across SINA_POS sessions. On every drive power-up, EPOS initializes in the wait for homing state. The drive-internal absolute position retained by the multi-turn encoder is available to the encoder interface, but EPOS requires an explicit set reference point command before the actual position becomes a valid mechanical coordinate for SINA_POS.
3. SINA_POS expects the drive, not the PLC, to define the coordinate system. There is no SINA_POS input that lets the PLC push a stored offset to the drive on startup. The only homing mode that can be triggered without mechanical motion is direct-setpoint homing (ModePos = 6), and the application must explicitly issue it.

3. Hardware and Software Configuration in Scope

4. Confirming the Diagnosis

Before changing the architecture, verify that the symptom is not caused by a misconfigured encoder, a missing parameter set, or a wrong PROFINET device name.

4.1 Encoder type in V-Assistant

1. Connect V-Assistant to the V90 over the service port (mini-USB) or PROFINET.
2. Navigate to Parameters → Encoder and read p0400[0]. A value of 9999 (unconfigured) or 1 (incremental) means the drive has no absolute feedback and the EPOS "not referenced" behavior is by design.
3. For a multi-turn absolute encoder (EnDat 2.1/2.2), confirm p0421[0] matches the encoder family (e.g. 1003 for the V90 ASI option).

4.2 Mechanical reference configuration

1. Open V-Assistant → Control / Status of setpoints → Basic positioner → Mechanical system.
2. Confirm linear scaling active: p29240 = 1.
3. Confirm load gear / spindle pitch values: p2503 = LU per motor revolution, p2504 = motor revolutions per load revolution, and the load distance per motor revolution in the assistant (default 10 mm for a 10 mm pitch leadscrew, or 76.2 mm for a direct-drive belt index).
4. For a non-modulo linear axis, confirm p2507 = 0 and p2599 = 0 (absolute encoder adjustment value cleared).

4.3 PLC tag classes used by SINA_POS

Open the SINA_POS instance DB in TIA Portal and inspect the I/O. The actual position ActPosition (LREAL) is an output only. SINA_POS does not write it back into a retentive tag, so the HMI value is regenerated from the drive's status word on every PLC restart - and the drive reports "not referenced" until a homing run is executed.

5. Solution A - Use the S7-1200 Technology Object (TO) with the Drive in Speed Control

The most reliable long-term solution is to remove the drive from EPOS mode and let the SIMATIC S7-1200 technology object TO_PositioningAxis handle the absolute position. The drive then operates as a closed-loop speed/torque follower (servo with controlled velocity), and the PLC stores the absolute position in a retentive DB. After a CPU restart, the TO automatically restores the actual position from its remanent actual value; the drive accepts the next setpoint without requiring a re-home.

5.1 Architecture changes

1. Change the V90 control mode in V-Assistant from Basic positioner (EPOS) to Speed control (S) or Position control (PTI) without EPOS. Save and reboot the drive.
2. In TIA Portal, add a new Technology object → PositioningAxis under the S7-1200 PLC.
3. Select the drive as a PROFINET device with telegram and map Telegram 3 to the TO's "Drive" interface. The TO uses STW1/STW2, ZSW1/ZSW2, NIST (setpoint speed) and the encoder evaluation channel G1_ZSW/G1_STW.
4. Configure the axis as Linear, Non-modulo (continuous) with the load distance per motor revolution set to 76.2 mm (or whatever the conveyor's belt travel per motor revolution is).
5. Set the homing method in the TO to Active homing via digital input or Passive homing as appropriate to your mechanical reference marker.
6. In the TO configuration, set "Restore position after CPU restart" to Restore position from remanent actual value. The TO writes the absolute position to a system-side retentive DB on every actual-value update.

5.2 Program structure

1. Call MC_Power (axis enable). Re-assert Enable after a CPU restart; the TO does not auto-re-enable the drive stage.
2. Call MC_Home once during initial commissioning (Mode = 0 for direct absolute home, or Mode = 3 for passive homing).
3. Call MC_MoveAbsolute or MC_MoveRelative for the 76.2 mm index step.

Reference project: see the Siemens FAQ S7-1200 Motion Control - SINAMICS V90 with technology object for a working sample program. The companion FAQ S7-1200 Motion Control - SINAMICS V90 with SINA_POS (EPOS) covers the SINA_POS variant for comparison.

6. Solution B - Keep SINA_POS but Store and Restore the Position Offset

If the application is already written around SINA_POS and a full migration to a TO is not feasible, you can wrap SINA_POS with a small helper block that:

1. After a successful homing run, reads the drive's current absolute position from ActPosition and writes it into a retentive LREAL tag (e.g. "DB_Homing".lr_HomeOffset_LU). Mark the instance DB as Retain in TIA Portal so the value survives a CPU restart.
2. On the next PLC startup, before enabling SINA_POS, force the drive to adopt the stored offset by triggering a one-shot direct homing (ModePos = 6, direct setpoint) with the value coming from the retentive tag. Then enable the axis.

6.1 Helper block pseudo-code (SCL)

// On first scan after PLC restart
IF "FirstScan" THEN
    "SINA_POS".ModePos := 6;            // Direct setpoint homing
    "SINA_POS".Position := "DB_Homing".lr_HomeOffset_LU;
    "SINA_POS".ExecuteMode := TRUE;     // Edge trigger
    WAIT_FOR "SINA_POS".StatusDone;
    "SINA_POS".ModePos := 1;            // Restore absolute mode
END_IF;

// On homing complete (any homing mode)
IF "SINA_POS".StatusHomingDone AND "HomingDoneOneShot" THEN
    "DB_Homing".lr_HomeOffset_LU := "SINA_POS".ActPosition;
    "HomingDoneOneShot" := FALSE;
END_IF;

7. Solution C - Re-enable Absolute Encoder Context in the Drive

The V90 supports absolute encoder adjustment via p2599 and the related encoder parameters. If the drive is configured correctly and the encoder backup is functional, the drive retains the absolute position across power cycles. However, the EPOS state machine still requires a one-time Set reference point command after power-on; this can be done without mechanical movement by issuing the direct-setpoint homing with the saved value (Solution B). Verify:

1. Encoder backup OK: r0471[0] reports no warning, r0480[0] shows the multi-turn revolution counter incremented correctly across power cycles.
2. Absolute encoder range: p0418 = 0xFFFF (full 4096-rev multi-turn range) and p0421.0 matches the connected encoder family.
3. After power-on, the drive's ZSW1.12 (referenced) is 0 and ZSW1.11 (setpoint acknowledgment) clears. The SINA_POS block mirrors this through StatusWord.11 = 0 and StatusWord.12 = 0.
4. Use a small jog to confirm the encoder increments correctly: enter p2585 = 1 in V-Assistant, jog the motor 1 LU, then read r2521[0]. The value must increase by exactly 1 LU per 0.1 µm of mechanical travel.

8. SINA_POS Key I/O Reference

None of these are persisted by SINA_POS; a PLC restart always reverts the controller to a "not referenced" state with respect to the drive.

9. Conversion Formulae for Conveyor Indexing

For a linear axis driving a conveyor with index step L_step in millimetres:

Belt travel per motor revolution (pinion of diameter D):

L_motor = π × D

LU per motor revolution (1 LU = 0.1 µm default, configured via p2502 linear scaling):

N_LU = L_motor × 10000

Set this value in the drive: p2503 = NLU.

Index step in LU:

N_{step_LU} = (L_step / L_motor) × N_LU

For the application (76.2 mm step, 30 mm pinion):

• Circumference = π × 30 mm = 94.2478 mm
• LU per rev = 94.2478 × 10000 = 942478 LU
• Step in LU = (76.2 / 94.2478) × 942478 = 762000 LU

Note that 76.2 mm corresponds to 762000 LU at 0.1 µm/LU. The SINA_POS input Position is given in LU; verify scaling by reading r2521[0] (actual position LU) in V-Assistant while jogging.

10. Verification and Commissioning Checks

1. Power down the drive and CPU for at least 30 seconds.
2. Power on, wait for the PROFINET connection (X1 LED green) and the drive to reach the Ready state.
3. Read the HMI position display - it should match the mechanical position to within the encoder resolution (1 LU = 0.1 µm).
4. Send a relative 76.2 mm index move; the conveyor should step exactly one increment, with no re-homing message in the HMI.
5. Repeat the power cycle five times to confirm reproducibility.
6. Verify the drive's status bits: ZSW1.0 = 1 (ready), ZSW1.12 = 1 (referenced) within 500 ms of MC_Power.Enable = TRUE.

11. Troubleshooting Matrix

12. Field-Engineering Notes and Common Pitfalls

• Even with a multi-turn absolute encoder, the V90 in EPOS mode requires a homing run after each power-on if SINA_POS is the controller. The encoder's "absolute" property refers to drive-internal position retention across short power-off events (typically < 5 s on the V90, longer with battery-backed SSI), not to the application-level coordinate system.
• For machines with a mechanical holding brake (vertical axis or stopped conveyor), the TO approach is always preferred; the absolute encoder is only used to detect any inadvertent motion while the drive is off.
• When using SINA_POS with PROFINET, ensure that the PROFINET device name of the V90 is consistent across the V-Assistant export and the TIA Portal project. Mismatches cause the drive to drop the connection silently and SINA_POS to report 0x0010 (no PROFINET link).
• The V90 firmware should match the GSDML file imported in TIA Portal. For TIA Portal V18, the recommended GSDML is GSDML-V2.34-Siemens-Sinamics_V90PN-... shipped with the SINAMICS V90 HSP.
• The Siemens S7-1200 Motion Control manual (S7-1200 Motion Control V18 function manual) describes the TO retentive actual-value mechanism in detail; the section on Axis behavior after CPU restart is the authoritative source.
• For SINA_POS, the official library documentation is the SINA_POS (FB284) library description; in particular, the section on Configuration of homing explains why ModePos = 6 (direct setpoint) is the only mode that can be triggered without mechanical motion.
• Avoid MC_Reset on a TO that has a retentive actual value - it clears the actual position and forces a re-home. Use the TO's Acknowledge error bit instead.