Configuring SIMOTION External Encoder for Positioning

Configuring SIMOTION External Encoder for Positioning Applications

An external encoder on a SIMOTION controller does not, by itself, perform positioning. It only senses the actual mechanical position of a load or axis and feeds that value back to the controller. A closed-loop positioning system still requires a SIMOTION axis with a configured drive (SINAMICS S110/S120, hydraulic axis, virtual axis, etc.), a motion control profile, and a feedback path. The external encoder becomes the actual-value source — typically encoder_2 — for a second feedback channel or a load-side position reference. Misreading this architecture is the single most common reason SIMOTION commissioning fails.

1. SIMOTION Platform Selection

Identify the controller before selecting the encoder wiring path. SIMOTION platforms differ in onboard encoder interfaces, DRIVE-CLiQ topology support, and PROFINET performance.

For SIMOTION D410 the typical topology is: SIMOTION D410 → integrated SINAMICS S120 → SINAMICS Sensor Module SMC30 (DRIVE-CLiQ) → external encoder. The external encoder can also connect directly to the D410 onboard input if it is a TTL/HTL incremental encoder and the resolution requirement matches the onboard counter (typically 32-bit, hardware-dependent).

For SIMOTION D425/D435/D445/D455 the encoder must connect through a SINAMICS drive or sensor module because these platforms have no onboard encoder input. The path is D4x5 → DRIVE-CLiQ → SMC20/SMC30/SME120/SME125 → encoder, or D4x5 → PROFINET/PROFIBUS → SINAMICS drive with configured telegram.

2. Encoder Connection Interfaces

Choose the encoder interface based on the encoder type, distance, and required signal level.

2.1 DRIVE-CLiQ via SINAMICS Sensor Module

The DRIVE-CLiQ path is the cleanest because it carries encoder data, diagnostics, and electronic nameplate over one cable. Siemens sensor modules accept the following encoders:

• SMC20: Incremental encoders sin/cos 1 Vpp without EnDat interface (e.g., ERN 1387, Heidenhain LC 183).
• SMC30: Incremental encoders TTL/HTL with or without commutation signals (RS422, push-pull).
• SME120/SME125: Absolute encoders with EnDat 2.1 / EnDat 2.2 interface (e.g., Heidenhain ECN/EQN/ECI/EQI).
• SMC40: SSI absolute encoders.

Each sensor module occupies one DRIVE-CLiQ port and appears in SCOUT topology as a sub-slot of the SINAMICS drive or as a standalone DRIVE-CLiQ participant.

2.2 PROFINET / PROFIBUS via SINAMICS Telegram

When the encoder connects to a remote SINAMICS drive or a third-party controller (e.g., ET 200S 1Count24V), the encoder data travels over PROFINET or PROFIBUS inside a Siemens telegram. Telegram 105/106 are the relevant standard telegrams for SINAMICS S120 with two encoders:

Selecting Telegram 106 is the typical way to forward encoder_2 (the external encoder) data into the SIMOTION cyclic interface. If the drive communication only shows Telegram 3 or 105, the SCOUT axis configuration wizard will not expose encoder_2 selection.

2.3 Onboard Encoder Input (D410 Only)

SIMOTION D410 has one onboard encoder input (X120) supporting TTL (RS422) and HTL incremental encoders. Resolution and maximum frequency are hardware-defined; for high-resolution encoders use a sensor module instead.

3. External Encoder Technology Object (TO externalEncoder)

The TO externalEncoder holds the configuration of one physical encoder that is not the motor encoder of an axis. It exposes the actual position, velocity, and status to other technology objects via the SIMOTION axis configuration.

The Siemens TIA Portal documentation for S7-1500T (the successor platform with a similar concept) describes the external encoder technology object as follows:

"The external encoder technology object detects a position and makes this available to the controller. The actual position detected by the external encoder can be used as an actual value for an axis or for position-controlled processes."

The full documentation set is available at the Siemens TIA Portal V20 External Encoder Technology Object documentation page. While SIMOTION SCOUT uses a different engineering tool, the conceptual model (encoder as a sensor, not as a stand-alone positioning element) is identical.

3.1 External Encoder Roles in an Axis

4. Encoder Type Selection and Wiring

External encoders on SIMOTION projects fall into four families. Choose the sensor module before you wire.

For background on why load-side (external) encoders are preferred for true position accuracy — versus motor encoders that measure indirectly through the gearbox — see the SEW Eurodrive encoder fundamentals article. The mechanical principle is identical across vendors: an external encoder measures motion directly at the load, eliminating backlash and torsional windup errors.

5. Dynamic Servo Control (DSC) Considerations

DSC is a Siemens-specific control loop topology where the position controller runs in the SINAMICS drive (closed at the 125 µs current-controller cycle) and SIMOTION only provides the position setpoint with an additional gain factor (Kp) over the PROFINET telegram. DSC dramatically reduces following error at high dynamics but requires:

1. Telegram 105, 106, 116, 136, or 138 selected on the drive.
2. Position controller enable bit set in the drive.
3. The DSC Kp parameter configured on the SIMOTION axis (typically 1.0 × motor Kp for first try).
4. Encoder_2 mapped through the telegram if DSC uses the external encoder as actual value.

If DSC is enabled but the telegram only carries encoder_1, SIMOTION will report a configuration inconsistency at axis start-up.

6. Step-by-Step Configuration in SIMOTION SCOUT

6.1 Prerequisites

• SIMOTION SCOUT TIA or SCOUT V5.x with matching firmware support package (e.g., SIMOTION D410 V5.x supports V5.4 SP1+).
• SINAMICS drive commissioned in STARTER or SCOUT with the motor identified.
• Sensor module (SMC20/30, SME120/125, SMC40) visible in DRIVE-CLiQ topology.
• External encoder wired and powered; mechanical mounting verified.
• SIMOTION project online to the controller.

6.2 Configure the Encoder at the Sensor Module

1. Open the SINAMICS drive in the project tree.
2. Drill into Commissioning → Encoder interface (for SMC30 it is p0400 family selector; for SME120 it is p0421).
3. Set the encoder type to match the physical device (e.g., p0400 = 9999 user-defined, then p0421 = binary code matching the encoder datasheet).
4. For incremental encoders: set p0425 (lines per revolution) and p0411 (gear ratio mechanical if any).
5. Save and download to drive. Verify in Diagnostics → Actual values that the encoder count increments when the shaft rotates manually.

6.3 Select the Right Telegram on the Drive

1. In SCOUT, double-click the SINAMICS drive, go to Communication → Message frame configuration.
2. Change the setpoint/actual-value telegram from default to Siemens telegram 106 (or 136/138 for D410, or 116 if higher dynamics required).
3. Confirm the PZD length and slot assignment in the PROFINET device properties (or PROFIBUS slave properties).
4. Save, compile, download.

6.4 Add the TO externalEncoder

1. In the project tree, right-click the SIMOTION device → Add new technology object → External encoder.
2. Assign a name (e.g., to_external_enc_load).
3. In the configuration dialog, select the encoder interface source. Options:
   • Sensor Module on DRIVE-CLiQ (recommended; the wizard shows discovered modules).
   • Encoder from PROFINET/PROFIBUS device.
   • Onboard encoder input (D410 only).
4. Set the resolution and any fine interpolation (e.g., 2^n fine resolution on top of line count for sin/cos).
5. Configure modulo or linear range depending on mechanical travel.
6. Save and download.

6.5 Bind the External Encoder to an Axis

1. Open the axis configuration (TO axis) for the positioning axis.
2. Go to Encoder selection.
3. Change the encoder from Motor encoder to External encoder, then point it at the TO externalEncoder just added.
4. If DSC is used, verify Drive configuration → Encoder DSC matches the same encoder.
5. Configure the load-side gear ratio under Mechanical system.
6. Compile and download the SIMOTION project.

6.6 Commissioning Sequence

1. Bring axis to safe standstill state.
2. Use Control panel in SCOUT to jog the axis slowly. Verify encoder count increments match the mechanical travel.
3. Run Axis → Commissioning → Homing to set the reference point. For incremental encoders, this is mandatory each power-up; for absolute EnDat, homing is required only once.
4. Run a small MC_MoveRelative command (e.g., 10 mm or 360°) at low velocity. Verify actual position equals commanded position within the configured tolerance.
5. Run MC_MoveAbsolute through the full travel to confirm modulo and limits.
6. Capture a trace with setpoint, actual position, following error, and drive current. Following error should remain below the configured tolerance (default is dynamic following error warning).

7. Verification Checklist

8. Common Fault Codes and Remedies

9. Troubleshooting Matrix

10. Cross-Platform Notes (SIMOTION vs S7-1500T)

SIMOTION is gradually being replaced by S7-1500T for new machine builds. The conceptual model of the external encoder technology object is preserved. In S7-1500T (with TIA Portal V17 or higher), the equivalent object is TO_ExternalEncoder in the Technology objects → Motion Control tree. The same configuration steps apply: assign encoder interface, configure resolution, configure modulo, then assign as actual value for a positioning axis. The official Siemens documentation for S7-1500T is at the External Encoder Technology Object documentation.

If you have an existing SIMOTION project that you migrate to S7-1500T, you must re-create the external encoder object because the underlying tool, configuration data, and program blocks are different. Telegram concepts (105/106/116/136/138) do not apply on S7-1500T in the same way; instead, telegram selection happens in the device configuration of the SINAMICS drive under Properties → Telegram configuration.

11. Safety and Wiring Considerations

• Functional safety: A standard external encoder cannot be used as a SIL/PL position sensor unless it is a certified encoder (e.g., Heidenhain ECN/EQN with Safety-Lock or SinCos with Siemens Safety Integrated). SIMOTION safety functions require SINAMICS Safety Integrated and a safety-licensed encoder channel.
• Shielding: Run encoder cables in a separate conduit, minimum 200 mm from VFD power cables. Ground the shield at the cabinet entry point only.
• Bending radius: Observe minimum 6× cable diameter for Heidenhain and Siemens prefabricated cables.
• DRIVE-CLiQ cable length: Maximum 70 m between devices; maximum 100 m total ring length.
• Encoder cable length: Heidenhain specification per cable type; for incremental RS422, typically < 100 m; for EnDat 2.2, < 40 m.
• 24 V supply: Sensor modules must have a separate, regulated 24 V supply (typically from SITOP) to avoid encoder count glitches during drive DC-bus transients.