Configuring SIMOTION Position Control with a Load-Side Encoder

Overview: Dual-Encoder Position Control in SIMOTION

Many high-precision machine applications require the position control loop to close on the load-side encoder while the velocity control loop continues to operate on the motor-side encoder. This configuration compensates for mechanical compliance, backlash, belt stretch, gearbox windup, and torsional deflection that would otherwise degrade accuracy if only motor-side feedback were used. SIMOTION supports this scenario natively through the Technology Object (TO) Axis, which accepts up to three encoder channels, and through Axis Data Sets (ADS) that allow runtime selection of the active encoder for the position controller.

This reference covers the engineering steps in Siemens Industry Online Support documentation and SCOUT/SCOUT TIA, the system-function programming required for deterministic runtime switching (_setAndGetEncoderValue() with CommandID), and the commissioning checks used to verify that the load-side position loop is stable, accurate, and free of following-error excursions when the data set is switched.

System Architecture and Component Model

A typical load-side controlled axis consists of the following elements:

• SIMOTION controller (SIMOTION D4x5/D4x5-2, P320/420, or C240/C240 PN) running the motion control runtime.
• SINAMICS S120 drive line module and motor module connected over PROFINET IRT or PROFIBUS DP with PROFIdrive telegram (typically telegram 3, 5, or 103/105).
• Motor-side encoder (Encoder 1): incremental, sin/cos 1 Vpp, EnDat 2.1/2.2, resolver, or SSI; processed in the Sensor Module (SMCxx or SME2x) of the SINAMICS.
• Load-side encoder (Encoder 2): mounted on the machine output, coupled directly to the load (ball screw, rack, conveyor, robot link, etc.). This encoder is routed to a free SMC/SME slot and exposed as a second encoder word in the PROFIdrive telegram, or to a TM (Terminal Module) connected to the SIMOTION.

The transmission ratio between motor and load is stored in the Axis TO configuration under LoadGear.Numerator / LoadGear.Denominator and LoadGear.Reverse. For a 1:1 ratio the value is 1:1 with reverse direction set as required by the mechanical layout.

Prerequisites

Before starting the configuration, confirm the following:

1. SCOUT (or SCOUT TIA) is installed at a version compatible with the SIMOTION runtime. SIMOTION V4.5/V5.x engineering is required for the latest Axis TO features.
2. The SIMOTION project compiles cleanly and the Axis TO already exists with the motor-side encoder configured and tuned.
3. Encoder 2 is wired to an SMC/SME or TM and visible in the topology (online diagnostics should show the encoder as OK).
4. The PROFIdrive telegram configured on the drive has two encoder words (telegrams 5, 105, 6, 106, or explicit 860/861 configuration) so that G1_XIST2 and G2_XIST2 are both available.
5. The PLCopen or MCC program for the axis is paused or in a safe state so that configuration changes can be downloaded.

Step 1: Add the Load-Side Encoder in the Axis TO

Open the Axis TO configuration in SCOUT and navigate to the Encoder subsection. Three encoder slots are available per axis.

1. Select Encoder 2 in the list view.
2. Assign the encoder type (Incremental, Absolute SSI, EnDat, etc.) and the corresponding Sensor Module address.
3. Set the resolution to the load-side encoder's actual line count or singleturn bits (e.g. 20-bit singleturn EnDat → 1048576 increments/rev).
4. Enter the mechanical adaptation values: Adaptation factor for line counts per motor revolution when gearing differs, or set to 1 for 1:1 load transmission.
5. Configure the reference mark handling, error reaction, and zero-mark evaluation per application requirements.
6. Download the configuration to the SIMOTION and verify that Encoder 2 reports a non-zero, stable actual value while manually jogging the axis slowly.

Step 2: Create a Second Axis Data Set (ADS)

An Axis Data Set contains a complete set of axis parameters including the active encoder number. The default data set (typically ADS 0) uses Encoder 1; create ADS 1 (or ADS 2) for load-side control.

1. Open the Axis Data Sets dialog of the Axis TO.
2. Click Add to create a new ADS. The new data set inherits the parameters of the active data set; review and adjust.
3. Set EncoderNumber := 2 in the new ADS.
4. Confirm the controller data set (CDS) and motor data set (MDS) selections are appropriate. Typically MDS is unchanged because the motor is the same; CDS may differ to allow different controller gains for the load-side loop.
5. Repeat the following error limits, positioning window, and position controller Kp/Kv values. For a load-side loop the proportional gain is usually slightly lower and the integral action slower, because the mechanical plant has more inertia and compliance.
6. Set the active ADS to the desired start-up data set (typically ADS 0 with motor-side encoder) before download.

Step 3: Configure the Control Loop Structure

SIMOTION Axis TO uses a cascaded structure: current loop → velocity loop → position loop. The position loop accepts only one encoder at a time (selected by EncoderNumber in the active ADS), while the velocity loop can be configured to use either the same encoder or, alternatively, a different encoder through the VelocityEncoder configuration (available in SIMOTION V4.4 and later).

For the case described in the source — speed control on motor encoder, position control on load encoder — set the following in the Axis TO configuration:

• VelocityEncoder = ENC_MOTOR (Encoder 1) — set in the Axis TO's velocity control section. The velocity controller reads from Encoder 1 regardless of the active ADS.
• PositionEncoder = ENC_ACTUAL_ADS — position controller reads from the encoder selected by the currently active ADS (Encoder 1 in ADS 0, Encoder 2 in ADS 1).

Step 4: Programmatic Runtime Switching

Switching ADS at runtime is the recommended path. The function block _setAndGetEncoderValue() from the SIMOTION system library provides a controlled, command-oriented transition.

ST Example: Switch to Load-Side Encoder

// Switch position control to load-side encoder
VAR
    fbEncoderSet : _setAndGetEncoderValue;
    udCommandId  : UDINT;
    dActualPos   : LREAL;
    bDone        : BOOL;
    bBusy        : BOOL;
    bError       : BOOL;
    wErrorId     : WORD;
END_VAR

// Trigger the switch
fbEncoderSet(
    Axis := Axis1,                     // axis reference
    EncoderNo := 2,                    // 2 = load-side
    SetValue := FALSE,                 // do not set absolute value
    GetValue := FALSE,                 // no read back of value
    CommandId := udCommandId,          // out: tracking ID
    Done := bDone,
    Busy := bBusy,
    Error := bError,
    ErrorId := wErrorId
);

IF bDone THEN
    // Switch ADS to 1 (load-side data set)
    Axis1.AxisDataSetControl := ADS_1;  // user-defined enum
    Axis1.AdsSwitch();                  // explicit switch FB call
END_IF;\code>

Why CommandID Is Required

The field report explicitly notes that without a unique CommandID the caller cannot reliably determine when _setAndGetEncoderValue() has completed. SIMOTION system functions are asynchronous; the busy/done bits must be observed until Done = TRUE. Persist the latest CommandId in a project-global UDINT tag so that re-entrant calls do not collide.

Recommended Pre-Switch Sequence

1. Stop the axis: Axis1.Stop() and wait for Standstill = TRUE.
2. Set VelocityEnable = FALSE to drop the speed controller output.
3. Call _setAndGetEncoderValue(EncoderNo := 2, ...) to inform the Axis TO of the upcoming encoder swap.
4. Wait for Done = TRUE (or Error = TRUE with evaluation of ErrorId).
5. Switch ADS via the Axis Data Set control mechanism.
6. Re-enable the axis: VelocityEnable := TRUE and clear any following-error latches.
7. Resume motion commands.

Step 5: Verify Position Reference Between Encoders

When switching from motor-side to load-side feedback, any difference in absolute position reference manifests as a step in following error at the instant of the switch. Eliminate the step through the following procedure:

1. Reference both encoders to a common machine zero (use the homing routine once with Encoder 1 active, then switch ADS and re-reference with Encoder 2 active, comparing absolute positions).
2. Compute the offset: dOffset = ActualPosEncoder1 - ActualPosEncoder2.
3. Configure this offset in the Axis TO under PositionOffset.Encoder1ToEncoder2 (parameter name varies by runtime; in SCOUT it appears in the Encoder configuration under Adaptation/Offset).
4. Verify by switching ADS at standstill and confirming the following error remains within the positioning window without a visible step in the trace.

Step 6: Commissioning and Verification

After the project has been downloaded and the controller is running:

1. Open the Axis Trace in SCOUT for the affected axis. Record the following signals simultaneously: PositionSetpoint, PositionActualEncoder1, PositionActualEncoder2, VelocitySetpoint, VelocityActual, FollowingError, AxisDataSetActive.
2. Jog the axis at a low velocity (e.g. 5% of Vmax) in both directions; confirm Encoder 1 and Encoder 2 track each other within the configured mechanical tolerance (≤ 0.01 rev for direct-drive 1:1 axes).
3. Run a positioning profile (typical S-curve with 100 ms jerk time) and record the following error peak. Compare against the warning and shutdown limits.
4. Perform a forced ADS switch at mid-motion (a transient switching test). The following error should show a controlled step that decays within 200-500 ms depending on the Kv setting.
5. Re-tune the position controller Kp and Kv on the load-side ADS using the trace as the response reference. SIMOTION's autotuning tool can be used but verify manually with the trace for the final gain values.

Fault Codes and Error Reaction

Encoder-related faults in SIMOTION Axis TO use the 50000-59999 range. Relevant entries:

Troubleshooting Matrix

Best Practices and Field-Proven Notes

• Always stop the axis and disable the controller before switching encoders. SIMOTION will reject an ADS switch in Motion state with fault 50100.
• Maintain a single UDINT counter for CommandID increments across all axes to keep audit trails simple.
• Use the EncoderData diagnostic function in SCOUT to compare raw counts of Encoder 1 and Encoder 2 during commissioning; differences outside tolerance indicate mechanical play or scaling errors.
• Keep motor and load encoder cable screens grounded at the cabinet entry; floating screens are a leading source of intermittent 50110 errors.
• For axes with significant backlash or torsional compliance, prefer a load-side position loop with a position feed-forward from the motor encoder. SIMOTION supports this through the PositionFeedforward configuration in the Axis TO.
• Document the Encoder1ToEncoder2 offset as a startup parameter; if either encoder is replaced, the offset must be re-measured.
• For safety-related axes (Safety Integrated), coordinate the encoder change with the safety configuration: the safe encoder is typically the motor-side encoder and must remain in use for SIL monitoring.

Summary

Implementing load-side position control in SIMOTION requires three coordinated steps: (1) configure the load encoder as Encoder 2 in the Axis TO, (2) create a second Axis Data Set that selects EncoderNumber = 2, and (3) implement a controlled runtime transition using _setAndGetEncoderValue() with a tracked CommandID followed by an explicit ADS switch. With the velocity loop tied to the motor encoder (or computed from the active position encoder) and the position loop tied to the load encoder, the axis achieves the precision of a load-referenced system while retaining the dynamic stiffness of motor-side velocity control.