SIMOTION Scout Tutorial: ST Programming, SIZER, and S120 Drives

SIMOTION System Architecture Overview

SIMOTION is Siemens' modular motion control system designed for high-dynamic, multi-axis production machinery such as printing presses, packaging lines, converting equipment, and textile machines. Unlike a PLC that handles logic first and motion as a peripheral task, SIMOTION is a true motion controller that treats I/O, logic, and synchronized axis motion as first-class objects. The runtime executes three programming languages side-by-side - ST (Structured Text), MCC (Motion Control Chart), and LAD/FBD - all on the same task system, with deterministic cycle times typically ranging from 1 ms to 4 ms for the IPO (interpolator) and servo tasks.

The platform splits into three controller families:

All three families compile to the same runtime, which is why a project written in Scout can be deployed to a C240, a D435, or a P320 with only the device assignment changed. This is critical for retrofit work: an existing D435 commissioning on a press line can be cloned and re-flashed to a C240 PN when the cabinet is rebuilt without rewriting application code.

Scout TIA V4.4 Engineering Environment

Scout (SCOUT) V4.4 is the standalone engineering tool - independent of the TIA Portal - that configures hardware, programs in ST/MCC/LAD, and performs online diagnostics. TIA Portal integration is achieved through the Scout TIA add-in and the TIA-SCOUT plug-in for SIMATIC, but Scout itself remains the primary motion engineering tool, even when S7-1500 logic is configured in TIA Portal next door.

Project structure inside a Scout V4.4 project follows a fixed hierarchy:

1. Device (SIMOTION C/D/P) - hardware target, firmware version, and PROFINET/PROFIBUS interfaces.
2. Drive assignment - SINAMics S120 components added under the device, including Line Module, Motor Module, and motor/encoder via Drive-CLiQ.
3. TO (Technology Objects) - Axis, Cam, CamDisc, Following Axis, Synchronous Axis, External Encoder, Path Object, Cam_ext.
4. Program organization units (POUs) - ST sources, MCC charts, LAD/FBD programs, organized under Execution Levels (BackgroundTask, MotionTask, IPOsynchronousTask, ServoTask, IPO-Task 1/2, Watchdog).
5. Connections - PROFINET IRT topology, PROFIBUS DP slaves, HMI tags, OPC UA server settings.

Configuration of the SINAMICS S120 follows the same offline/online pattern as STARTER: the topology is assembled under Drive-CliQ (Smart Line Module -> Motor Modules -> motors with DRIVE-CLiQ interface), parameters are pushed offline, and the device is commissioned online. Scout V4.4 also supports topology-based commissioning where the actual physical wiring is read back from the drive and compared to the offline project; mismatches are flagged with a per-component error.

Structured Text (ST) Programming in Scout

ST in SIMOTION is the IEC 61131-3 language binding, very close to the same ST used in CODESYS-based controllers. SCL in the S7-300/400/1500 world is a Siemens-specific name for an ST dialect. STL (Statement List) is the Siemens dialect of IEC 61131-3 IL (Instruction List). SCL and ST are functionally very close; STL has no direct equivalent in SIMOTION, and any IL-style logic must be ported to ST or MCC when migrating.

Typical ST unit in SIMOTION - a unit is the compilation container that holds programs, function blocks, functions, and data types:

UNIT PressControl;
INTERFACE
  PROGRAM Main;
END_INTERFACE

IMPLEMENTATION
  PROGRAM Main
    VAR
      fWebSpeed    : LREAL;     // m/min master speed setpoint
      bRun         : BOOL;      // line run permit
      fTorqueLimit : LREAL;     // Nm cap per drive
    END_VAR
    
    // Read operator setpoint from HMI tag
    fWebSpeed := gp_axMaster.SetpointSpeed;
    
    // Enable drive only when guards are satisfied
    IF bRun AND (fWebSpeed > 0.0) THEN
      _enableAxis := TRUE;
    ELSE
      _enableAxis := FALSE;
    END_IF;
    
    // Apply torque limit cyclically
    Axis1.TorqueLimit := fTorqueLimit;
  END_PROGRAM
END_IMPLEMENTATION

System functions are exposed as _-prefixed function calls (for example, _enableAxis, _pos, _velocity, _cam) and as method calls on the technology objects (e.g. Axis1._enable(), Cam1._enable()). The Scout editor highlights unknown function names and inserts the correct import directive on confirmation, so unlearning an API is largely a problem of typing accuracy rather than remembering include files.

For control engineers fluent in SCL, the transition is essentially zero. Two semantic differences to internalize:

• SIMOTION ST runs on a deterministic task system. Writing to a drive setpoint from a slow BackgroundTask will not cause a compile error, but the IPO task will sample the value at its own tick and ignore intermediate updates. Always assign motion-relevant variables inside the IPO-synchronous or Servo task.
• Variables declared in the unit interface (under INTERFACE) are unit-global; variables declared inside a POU are local. This is the opposite convention from SCL where the static section of an FB is implicitly global to instances.

SINAMICS S120 Drive Integration

SINAMICS S120 is the modular drive system that pairs natively with SIMOTION. A typical S120 cabinet section contains three subsystems:

Drive-CLiQ is the digital, point-to-point connection that carries encoder feedback, motor nameplate, and diagnostic data on a single shielded cable. When a Motor Module powers up, the electronic rating plate (DRIVE-CLiQ) is read and the firmware configures itself - no manual motor parameter entry is required for SIMOTICS 1FK7, 1FT7, 1PH8, or 1FW6 motors.

PROFINET IRT with a SIMOTION C240 PN or D4x5 PN delivers deterministic isochronous communication between controller and drive; the IPO cycle and PROFINET send clock are usually locked to 1 ms. With PROFIBUS DP, isochronous PROFIBUS achieves the same with master cycle / DP cycle of 1.5 ms to 4 ms depending on slave count. For new builds, IRT on PROFINET is preferred because the topology editor in Scout can verify cable lengths and switch port assignments against the planned cabinet.

Electronic Line Shaft and Virtual Master

Replacing a mechanical line shaft (a single physical shaft running through all units) with an electronic equivalent is the principal reason to use SIMOTION on a printing press. The mechanical shaft is replaced by a virtual master - a TO of type Axis configured as the master, often driven by a position-controlled external encoder (TO ExternalEncoder) at the draw rolls. Each printing unit is then a Following Axis or Synchronous Axis linked to that master with a configurable gear ratio and a cam for phase correction.

A 6-unit press with a master draw encoder typically configures as:

The register cam is the critical piece for color-to-color register on a press. By adjusting the cam's leading edge, the operator shifts the unit's print phase without changing gear ratio - a far more precise correction than the mechanical phase adjustment on an old DC unit. The cam is calculated in millimeters of web travel and is interpolated linearly between supporting points, which is why a typical press registers in less than ten web meters from a cold start.

SIZER Dimensioning Workflow

SIZER is the Siemens engineering tool used to size SINAMICS S120 line modules, motor modules, motors, and SIMOTION controllers from a mechanical description of the load. It is a free download and integrates with the Siemens Product Configurator for part-number generation.

1. Define the mechanical system - for each axis enter: load mass, load inertia, leadscrew pitch (or wheel diameter), gear ratio, friction coefficient, and motion profile (accel, constant vel, decel, dwell).
2. Select drive family - SIMOTION typically pulls SINAMICS S120 Booksize or Chassis format. SIZER auto-selects the smallest matching Line Module and Motor Module.
3. Verify duty cycle - SIZER plots the torque/speed trace against the motor's S1 (continuous) and S6 (cyclic) curves. Overload margin is reported as a percentage; aim for at least 20% on peak torque and 15% on RMS torque.
4. Export BOM - the bill of materials includes 6SL3xxx part numbers, plus firmware versions, braking resistors (if any), line filters, and recommended cable cross-sections.

For a printing press retrofit where each unit is replaced by a 1FK7 synchronous servo, SIZER accepts the cylinder diameter, ink/water mass, and impression load to compute required torque. Typical 1FK7 sizes on a 6-unit press run from 1FK7042 (4 Nm, ~1.5 kW) for nipping/ink-form rollers up to 1FK7060/1FK7080 (12-30 Nm) for the impression cylinder drive.

Regenerative Smart Line Modules

A non-obvious benefit of swapping an old DC drive for SIMOTION + SINAMICS S120 is energy recovery during deceleration. Old DC drives dissipate braking energy in resistor banks as heat. SINAMICS S120 with a Smart Line Module (SLM) or Active Line Module (ALM) feeds braking energy back to the line through the IGBT bridge. The choice between them depends on line quality:

For a press with frequent ramp-down cycles (web splices, speed changes, register corrections), the recovered energy is substantial. A six-unit press cycling between 300 m/min and 50 m/min every 30 seconds can regenerate 3-5 kWh per cycle into a 480 V line; over a year of three-shift operation, that is several thousand euros of recovered line energy - often enough to pay back the SLM premium over a BLM in 18-24 months.

Migration Path: DC to Servo on a Printing Press

Replacing a single large DC motor and mechanical line shaft with one servo per unit is the most common SIMOTION retrofit on a 6-unit press. The mechanical part is non-trivial and must precede the electrical part of the upgrade:

1. Mechanical survey - measure journal wear, draw-roll parallelism, and web-tension uniformity unit-by-unit. Worn journals make register impossible to hold even with perfect servos.
2. Decide gear ratio and servo frame - use SIZER with the impression cylinder as the driven element. Confirm the existing gear-box input speed (typically 1500-2000 rpm for the old DC) is reachable by a 1FK7 in the available frame size.
3. Run the new line shaft shafts empty first - bench-couple each servo to its gear-box and verify direction, encoder polarity, and the absence of mechanical bind before integrating with the master.
4. Wire and configure Drive-CLiQ - the SIMOTION D4x5 sits in the cabinet with the SINAMICS S120 line and motor modules; one Drive-CLiQ cable from each Motor Module to its motor completes encoder + power return paths.
5. Configure virtual master - replace the old mechanical shaft with a virtual axis driven by the draw-roll encoder. Tie each unit's Following Axis to the virtual master with a 1:1 gear and a register cam.
6. Tune each axis with the auto-tuning routine - Scout's auto-tuning measures the mechanical response with the Bode plot method and writes Kp, Tn, and filter coefficients. Run with the web unloaded first, then re-tune loaded.
7. Validate register - run cold-start register tests with the press at production speed; target < 0.05 mm lateral register at the output.

It is also valid to consider keeping a small DC motor with an encoder on each unit rather than converting to AC servo. This is cheaper (no S120 hardware per unit) and simpler mechanically, but it loses the regen benefit and the precision of the SIMOTION TO for camming. For a press with light register requirements and a low duty cycle of speed changes, the DC-with-encoder option remains competitive.

Energy Savings: DC Shunt vs. Permanent-Magnet Servo

The energy comparison between an old DC shunt motor and a 1FK7 permanent-magnet servo is two-part: motor efficiency and drive efficiency.

• Motor efficiency: A 1980s DC shunt motor at 75% load typically runs at 82-87% efficiency. A 1FK7 servo at the same load point runs at 93-96%. The 6-12 percentage-point gap is the headline savings.
• Drive efficiency: Old analog thyristor DC drives (e.g. Siemens 6RA23) convert at 92-95% line-side efficiency. A SINAMICS S120 Motor Module converts at 96-98%. Smaller, but cumulative.
• Regeneration credit: Not applicable to a DC system without regen; SLM-enabled servo recovers 30-70% of braking energy depending on cycle.

Approximation for a six-unit press operating at average 60% load with the old DC system drawing 360 A at 460 V (three-phase line current):

P_old,3ph (kVA) = sqrt(3) * V_LL * I / 1000
                = 1.732 * 460 * 360 / 1000
                = 286.7 kVA

If 360 A is single-phase current (interpret as the DC armature feed on a single-phase front end, which is common for older thyristor DC drives):

P_old,1ph (kVA) = V * I / 1000
                = 460 * 360 / 1000
                = 165.6 kVA

Both cases should be evaluated against a power-quality measurement, because the DC drive's input is non-sinusoidal and the true RMS is often 5-10% above the nameplate current. Apply the assumed efficiency stack to convert kVA to kW:

At an industrial electricity price of 0.18 EUR/kWh, the 385 MWh savings equates to roughly 69,000 EUR/year. Capital payback for a full SIMOTION D445 + SINAMICS S120 + SIZER-engineered BOM typically falls in the 18-30 month range when both energy and scrap-recovery (better register = less waste) are credited.

Self-Learning Resources and Structured Training

Formal training is the fastest path; Siemens SITRAIN offers role-based courses covering both programming and commissioning. For engineers evaluating an in-house adoption of SIMOTION, the relevant catalog entry is the SIMOTION programming and commissioning course, available both as classroom and as online training:

• SITRAIN catalog - SIMOTION training courses (Siemens official)
• Siemens Motion Control Training hub (SIMOTION, SIMATIC Motion Control)
• Online Training - SIMOTION Programming and Commissioning (Siemens SITRAIN)

For self-study without enrolling, three official resources carry the bulk of the value:

1. Scout Online Help "First Steps" - ships with the Scout installation and walks through hardware configuration, axis setup, and a simple positioning task.
2. SIMOTION Utilities & Applications DVD - the part-number media included with Scout delivery and documented under Siemens Industry Online Support entry 26340545. Contains ready-to-compile sample projects (conveyor, cam, electronic gearbox, winder).
3. SCOUT application examples in the Siemens Industry Online Support portal - search "SIMOTION application example" for FAQ-style articles with downloadable ZIP projects.

Commissioning Checklist for a Press Retrofit

A condensed pre-power-up, first-power-up, and production-release checklist, in the order it should actually be executed on site:

Troubleshooting Matrix

Frequently Asked Questions