Save Incremental Encoder Position During Unidrive SP Power Loss

1. Problem Statement: Incremental Encoder Position Loss on Drive Power-Down

A four-axis servo system built around a Siemens SIMATIC S7-300 CPU 317F-2DP and four Control Techniques Unidrive SP drives on Profibus DP-V1 loses the absolute position of every axis when the drive's main power is cycled. The PLC retentive data block still holds the last commanded preset, but the drive's actual position register reads 0 on the first Profibus scan, the application overwrites the stored value with that 0, and the machine is sent to the wrong home.

Two failure modes appear in the field:

• The drive latches an "Under Voltage" (UU) trip after power restore. Actual position reads 0 mm. Preset value (e.g. 1000 mm) and actual position diverge.
• The S7-300 saves the value to a retentive DB and survives drive power-down, but the value is overwritten by the next Profibus input update before OB 100 (restart) finishes.

The root cause is fundamental: an incremental encoder reports only changes, never absolute position, and its count is meaningless the moment it loses power. Anything the system relies on after a power cycle must be stored elsewhere - in the drive, in the PLC, or in the encoder itself.

2. Root Cause Analysis: Why the PLC DB Is Being Overwritten

The fault path is straightforward but easy to miss when the application reads actual position rather than commanding it. The sequence below describes what happens on a CPU 317F-2DP when drive mains is removed and then restored:

1. Drive mains is removed. The DC bus discharges, the inverter disables, the drive latches "UU" (under-voltage) or a configurable trip.
2. If the drive is fed only from the three-phase supply (no 24 Vdc backup), the control board powers down. The Unidrive SP cannot write to its own NVM in this state.
3. The S7-300 continues to run on its own power supply. Its retentive DB still holds the last good preset value (e.g. 1000).
4. When mains is restored, the Unidrive SP re-initialises its encoder interface. The incremental counter is 0, because the encoder has no absolute datum.
5. Profibus DP data exchange resumes as soon as the drive enters the data-exchange state. The PZD word for actual position reads 0.
6. The S7-300 application runs OB 1 (or a cyclic FB). It reads actual position, and - depending on the code - either writes the drive's 0 into the DB, or reads the DB and writes the old preset to the drive, and the drive moves the motor to chase the 0 it was just sent.

The result is the same in both cases: the saved value is destroyed or contradicted before the operator can react. The fix is to make the saved value the master on power-up, not the live readback. Specifically:

• The first Profibus input cycle after drive power-up must be discarded, or the DB value must be written to the drive's position-reset register before the application accepts the drive's actual position.
• The save must be triggered at the moment of trip detection, not on a periodic timer that may not fire in time.

3. Safety Constraint: Motor Must Not Rotate After Power Removal

Before discussing the save mechanisms, confirm that the load cannot move between power-down and power-up. Any solution that saves a position and restores it later assumes the mechanical position is unchanged in the interval. If the load can sag, drift, or be moved by an external process, the restored value is wrong and the drive will command a step to "correct" the error.

For the rest of this article, assume a holding brake is fitted and verified, or that an external reference (limit switch, datum) is available at the next enable.

4. Solution 1 - Replace the Incremental Encoder with an Absolute Encoder

The cleanest fix is hardware. An absolute encoder reports a unique value at every position in its range, and a multi-turn variant retains that value across a complete power-down with no battery, no UPS, and no PLC intervention.

When the drive is fitted with an absolute encoder, the position is available in Menu 13 the moment the encoder interface powers up. The PLC reads the correct value in the first PZD cycle and the issue disappears without any code changes.

5. Solution 2 - Use the Unidrive SP Onboard PLC (DPL) to Save the Position

If the encoder cannot be changed - for cost, lead-time, or mechanical reasons - the next best option is to run a small DPL program inside the Unidrive SP. DPL is the Control Techniques equivalent of Siemens' DCC (Drive Control Chart) used on SINAMICS. The Unidrive SP has an Application Menu (Menu 18-21 depending on firmware) that hosts a DPL runtime with non-volatile RETAIN variables. The drive holds the last position in its own NVM and pushes it back into the position controller on power-up, independent of the S7-300.

For a control engineer familiar with STEP 7, the mental model is:

• Tasks are like OBs: @INIT runs once at power-up, @FAST runs every 4 ms, @BACKGROUND is a low-priority loop, @EVENT runs on a parameter event.
• RETAIN variables are like the S7-300 retentive MB/DB - their value survives a power cycle.
• Drive parameters are addressed with a leading @ in DPL; the same values appear as Pr 13.xx on the keypad and in the GSD configuration.

5.1 Saving on a Trip Event

The DPL program below is a representative position-save routine. It uses a RETAIN variable to hold the last position and writes it into the position controller on power-up. The exact Menu 13 parameter addresses vary between Closed-Loop Vector and Closed-Loop Servo modes and between Unidrive SP firmware revisions; consult the Unidrive SP parameter reference guide for your specific firmware.

VAR
  pos_nv   : DINT RETAIN    // last saved position, non-volatile
  pos_act  : DINT           // live position feedback
  b_save   : BOOL           // one-shot save trigger
ENDVAR

// @INIT block: power-up restore
BEGIN
  @13.xx = pos_nv;          // position preset/offset (firmware-dependent)
  @13.yy = 1;               // issue preset command
END

// @BACKGROUND block: continuous save
BEGIN
  pos_act = @13.zz;         // read live position feedback
  IF b_save OR @10.01 = 0 THEN  // @10.01 = drive healthy bit
    pos_nv = pos_act;
    @0.00 = 1000;           // commit NVM by writing 1000 to Pr 0.00
    b_save = FALSE;
  ENDIF
END

5.2 Why DPL Beats the S7-300 for This Application

1. The drive has visibility into its own trip state. The Unidrive SP sets the "drive healthy" bit (e.g. Pr 10.01) to 0 within a few milliseconds of detecting under-voltage. A DPL task can read this and write to NVM at the moment of the trip, before the control board loses its auxiliary supply.
2. For multi-axis synchronisation, the drives exchange position data among themselves. A drive-resident save can use the same path to keep the four axes coherent. A PLC-resident save can only see one drive at a time over Profibus.

6. Solution 3 - S7-300 Retentive Data Block with Power-Loss Trigger

If the application must use a Siemens-native solution, the next option is a retentive DB on the S7-300 with a save trigger derived from the drive's healthy signal. STEP 7 / SIMATIC Manager and the HW Config tool are part of the standard Siemens Industry Online Support documentation set.

6.1 Configuring the Retentive DB

1. In STEP 7 / SIMATIC Manager, open the DB and select "Properties".
2. On the "General - Part 2" tab, clear the "Non-Retain" checkbox. With the box cleared, all tags in the DB are retentive; with the box set, no tags are.
3. Confirm the DB is loaded into the work memory, not just the load memory. Retentive tags need to be in the work-memory image.

6.2 Retentive Memory Sizing on the CPU 317F-2DP

Retentive data on the CPU 317F-2DP is divided into three areas: bit memory, counters, and timers. The default is 0-15 MB, 0-7 counters, 0-7 timers. To increase the retentive area, open HW Config > CPU properties > "Retentive Memory" and enter the new byte/count ranges. The remainder of the work memory is allocated to the DBs.

6.3 Save Trigger on a Loss-of-Drive-Healthy Event

Wire the Unidrive SP's status word into the cyclic PZD input. Most Unidrive SP Profibus configurations expose the drive status word in PZD1 and the actual position in PZD2-PZD3 (the exact slot depends on the GSD file). The bit "drive healthy" is bit 0 in some status words, but the Unidrive SP uses a non-standard mapping - consult the Unidrive SP Profibus manual for the slot assignment.

In STEP 7, the save is implemented as a falling-edge trigger on the drive's healthy bit:

// SCL example for the position-save FB
FUNCTION_BLOCK FB_PosSave
VAR
  DriveHealthyOld : BOOL;       // edge-detection memory
END_VAR
BEGIN
  // Read drive healthy bit (slot depends on Unidrive SP GSD)
  // Common mapping: bit 0 of the drive status word
  IF NOT "Drive".Status.Healthy AND DriveHealthyOld THEN
    "DB_Pos".SavedValue := "Drive".ActualPosition;
    "DB_Pos".SavedTimestamp := WORD_TO_INT("SysTime");
  END_IF;
  DriveHealthyOld := "Drive".Status.Healthy;
END_FUNCTION_BLOCK

6.4 Restore on Drive Power-Up

Restore runs in OB 100 (warm restart) and in OB 1 on a rising edge of the drive's healthy bit. The restore writes the saved value into the drive's position-reset register and issues a reset command before the application accepts the drive's actual position.

// OB 100 - warm restart
// Read the saved value, push it to the drive, mark the DB as "applied"
IF "DB_Pos".SavedValue <> 0 AND NOT "DB_Pos".Applied THEN
  "Drive".Command.Word := "Drive".Command.Word OR 16#0020;  // preset bit (slot-dependent)
  "Drive".Command.Position := "DB_Pos".SavedValue;
  "DB_Pos".Applied := TRUE;
END_IF;

7. Solution 4 - Unidrive SP SMARTCARD and Parameter Save

The Unidrive SP supports a SMARTCARD slot that can store a complete parameter set, including user-defined parameters. The SMARTCARD is hot-pluggable, but for the position-save application the most useful mode is the "auto-restore on power-up" mode.

1. Insert a SMARTCARD with the write-protect tab in the "enabled" (write) position.
2. From the keypad, navigate to Pr 0.00 and enter the save value. On most Unidrive SP firmware revisions, writing 1000 to Pr 0.00 copies the active user menu to the card. The drive returns Pr 0.00 to 0 on success.
3. Set the "load on power-up" parameter (often in Menu 11, slot-dependent) to enable automatic restore.
4. Remove the SMARTCARD if no further changes are needed, or leave it in place if the application is in a shipping state.

This is the simplest method for a static commissioning, but it has three limitations for the position-save problem:

• The SMARTCARD restore happens on drive power-up, but the position counter on the Unidrive SP is a runtime value, not a parameter that can be written by the SMARTCARD in all firmware versions. Whether the position is part of the saved set is firmware-dependent.
• Writing the SMARTCARD is slow. Each write takes hundreds of milliseconds, which is longer than the time the drive's control board is alive on a mains-loss event.
• SMARTCARD endurance is lower than the drive's internal NVM. Use it for static configuration, not for cycle-by-cycle position saves.

8. Solution 5 - UPS-Backed Power-Fail Save Sequence

For applications where neither the encoder nor the drive's NV storage is reliable, the last resort is to keep the drive and the PLC alive long enough to perform a complete save. This requires a UPS on the 24 Vdc control rail of the Unidrive SP, on the S7-300 PS 307, and on any encoder interface electronics.

8.1 Power-Fail Detection and Sequence

1. The UPS monitors the AC mains. On mains loss, it signals the PLC via a digital input or via a serial/Profibus UPS module.
2. OB 40 (hardware interrupt) is triggered by the UPS digital input. The OB 40 code performs the save: reads the current actual position from each of the four Unidrive SP drives via the acyclic SFB 52 (RDREC) interface, and stores the values in the retentive DB.
3. The UPS holds the system up for at least 5 seconds, comfortably more than the S7-300 OB 40 execution time and the Profibus acyclic read time (typically < 50 ms per drive).
4. On power restoration, OB 100 (warm restart) reads the retentive DB and pushes the saved value to each drive before the application code reads the drive's actual position.

8.2 Acyclic Profibus Read for Position

For a CPU 317F-2DP, the acyclic read is implemented with SFB 52 (RDREC) and SFB 53 (WRREC). The Unidrive SP supports the standard PKW parameter channel. To read the actual position from drive 3:

// SCL - read parameter from Unidrive SP over DP-V1 acyclic channel
// The slot/index combination is firmware-dependent
CALL SFB 52, DB52
  REQ    := TRUE
  IOID   := B#16#54              // input/output identifier (read)
  LADDR  := W#16#03E8            // Profibus I/O address of the drive
  RECNUM := 0                    // record number for PKW channel
  RET_VAL := "Rd_RetVal"
  BUSY   := "Rd_Busy"
  RECORD := "ParamChannel"
  LEN    := 8;
IF NOT "Rd_Busy" AND "Rd_RetVal" = 0 THEN
  "DB_Pos".SavedValue[3] := DWORD_TO_DINT("ParamChannel".PKW_Value);
END_IF;

9. Implementation: Position Restore Sequence After Drive Power-Up

Once the save mechanism is in place, the restore sequence must be deterministic. The recommended order on a CPU 317F-2DP is:

1. OB 100 (warm restart): the PLC has just powered up. For each drive, read the saved value from the retentive DB and issue a position-preset command. The drive's status word should not be read yet - the drive may still be initialising.
2. OB 82 / OB 86 (diagnostics): as the Profibus slaves re-enter data exchange, the diagnostic OBs are called. Do not use the actual position from the drive here. The drive has not yet received the preset.
3. OB 1 cycle 1: the drive status word is now valid. Confirm the preset was accepted (drive returns "preset complete" or "no fault").
4. OB 1 cycle 2 onwards: the actual position word is now valid and matches the saved value (within one encoder line per motion step during the power-down). The application can resume normal control.

10. Verification and Commissioning Checklist

Before returning the system to production, run the following verification on each of the four axes:

1. Set the drive's command word to "disable" and remove mains. Confirm the drive enters "UU" or the configured trip.
2. Restore mains. Confirm the drive returns to the "ready" state without operator intervention.
3. Read the actual position on the S7-300 HMI or via Pr 13.xx on the Unidrive SP keypad. Confirm the value matches the value before the power cycle.
4. Issue a small motion command (e.g. 10 mm) and confirm the drive responds without a step or stall.
5. Repeat steps 1-3 ten times. Verify the value is consistent on every cycle.
6. Inspect the Unidrive SP NVM write count, if exposed, to confirm the save trigger is firing on every cycle and only on every cycle.
7. Inspect the S7-300 retentive DB on the PG. Confirm the saved value is updated on every power cycle.

11. Troubleshooting Matrix

Frequently Asked Questions