Role of SIMIT Simulation Platform in Mechatronics Concept Designer (MCD) and Digital Twins

While NX Mechatronics Concept Designer (MCD) can connect directly to a simulated controller, integrating SIMIT between MCD and the controller is essential for complete Virtual Commissioning. MCD specializes exclusively in 3D kinematic physics and mechanical collision modeling. SIMIT provides the comprehensive behavioral and process simulation layer: modeling process thermodynamics, sensor electrical dynamics, valve flow characteristics, PROFIsafe/HART telegram packaging, and fieldbus diagnostics. SIMIT also acts as the central simulation orchestrator, controlling synchronized clock stepping, snapshots, and fault injections.

Executive Summary

Virtual Commissioning (VC) of manufacturing machinery and process systems is revolutionizing automation engineering by enabling software testing prior to physical hardware fabrication. In Siemens Digital Enterprise architectures, engineers frequently encounter hybrid toolchains combining NX Mechatronics Concept Designer (MCD), SIMIT Simulation Platform, and SIMATIC S7-PLCSIM / SIMIT Virtual Controller. A common question during architecture design is why SIMIT is needed if MCD can communicate directly with the PLC. This article defines the specialized roles of mechanical vs behavioral simulation, explains why SIMIT is indispensable, and details the co-simulation dataflow.

Architecture and Fundamental Mechanics

Three-Tier Virtual Commissioning Architecture and Co-Simulation Mechanics:

Division of Responsibilities Across the Digital Twin Hierarchy:
1. Tier 1: Mechanical & Kinematic Simulation (NX MCD):
- Models 3D rigid bodies, mechanical joints (hinges, sliding rails), motor torques, conveyor belt friction, gravity, and part collisions.
- Output: Physical spatial positions ($X, Y, Z$) and geometric proximity triggers.
- Limitation: MCD has no native concept of process thermodynamics, analog transducer calibration, electrical noise, or fieldbus protocol framing.
  2. Tier 2: Behavioral & Process Simulation (SIMIT):
- Models the physical behavior of process equipment and electrical automation components:
- Continuous process physics: pipe flows, heat exchangers, fluid mixing, reaction kinetics.
- Actuator and sensor dynamics: valve opening curves, motor inertia, thermocouple lag.
- Fieldbus and I/O Emulation: PROFIsafe telegram formatting, channel passivation, HART communication, and ET 200 module diagnostic addresses.
- Acts as the Co-Simulation Bridge: SIMIT maps spatial coordinates from MCD into realistic industrial sensor signals and translates controller outputs into physical forces for MCD.
  3. Tier 3: Control Logic Execution (PLC / Controller):
- S7-PLCSIM, PLCSIM Advanced, or SIMIT Virtual Controller executing unmodified PCS 7 / TIA Portal machine code.
Master Simulation Orchestration in SIMIT:
SIMIT provides centralized simulation lifecycle control:
- Simulation Snapshots: Save the exact state of the entire virtual plant (MCD mechanical positions + SIMIT process variables + PLC memory) and restore it instantly for regression testing.
- Clock Synchronization & Time Scaling: Synchronize mechanical and control loops; run simulations in fast-forward (e.g. 10x real-time for burn-in) or slow-motion for detailed fault analysis.
- Fault Injection: Emulate wire breaks, short circuits, stuck valves, and sensor drift to validate operator response and safety interlocks.

Step-by-Step Implementation Procedure

Step-by-Step Procedure to Integrate NX MCD, SIMIT, and the Controller:

Build Kinematic Mechanical Model in NX MCD:
- Model machine 3D geometry in Siemens NX.
- In Mechatronics Concept Designer, define Rigid Bodies, Collision Bodies, and Joints (e.g. Sliding Joint, Cylindrical Joint).
- Define Signal Adapters in MCD for actuators (position controls, velocity controls) and position sensors.
Configure SIMIT Coupling to MCD:
- Open SIMIT Simulation Platform.
- In the Couplings tab, add a new coupling: MCD Coupling (or Shared Memory / OPC UA).
- Import the signal exchange table exported from NX MCD.
- Map kinematic positions (e.g. Cylinder1_Position) and actuator commands.
Configure SIMIT Process and Behavioral Models:
- In SIMIT, create simulation charts modeling pneumatic pressure supply, valve delay timers, and proximity switch debounce.
- Interconnect MCD position signals with SIMIT behavioral blocks (e.g. converting cylinder stroke into inductive sensor feedback B1_Extended = TRUE when stroke $\ge 98\%$).
Configure SIMIT Controller Coupling:
- In SIMIT Couplings, add the controller coupling: PLCSIM Advanced or SIMIT Virtual Controller.
- Import hardware configuration from STEP 7 / PCS 7 HW Config.
- Map SIMIT behavioral signals directly to controller peripheral input addresses (IWxxx, I 0.0).
Execute Co-Simulation:
- Start the controller in PLCSIM.
- Start SIMIT simulation (orange toolbar background indicates active simulation).
- Start MCD simulation; verify 3D mechanical movement corresponds to controller execution.

Verification

Verification Procedure:
1. Signal Dataflow Check: Force an output in the PLC logic. Verify that SIMIT receives the output, applies behavioral delay, commands MCD to move the 3D mechanism, and returns the simulated sensor feedback to the PLC.
2. Snapshot Verification: While the 3D machine is mid-cycle, save a SIMIT Snapshot. Let the cycle complete. Restore the snapshot; verify that both MCD mechanical positions and PLC memory return to the exact saved state.
3. Fault Injection Test: In SIMIT, inject a simulated wire break on an analog pressure sensor. Confirm that the PLC channel driver detects passivation (QBAD = 1) and trips the safety interlock, halting the MCD 3D model.

Limitations and Operational Precautions

Operational Precautions and Simulation Boundaries:
- Computational Sizing: Running high-polygon 3D CAD simulation (MCD) alongside multi-node PLC emulation and SIMIT requires a high-performance engineering workstation with a certified discrete GPU and multi-core CPU.
- Time Step Sizing: Co-simulation requires aligned calculation cycles; typically 10ms to 50ms in SIMIT matching the MCD physics step time to prevent simulation lag or jitter.
- Commercial Licensing: SIMIT and NX MCD require separate software licenses; SIMIT licensing scales by component packages and coupling options.

Engineer FAQ

Why can't I connect NX MCD directly to the PLC without SIMIT?

You can for simple mechanics, but MCD cannot simulate continuous process physics, sensor electrical characteristics, valve curves, PROFIsafe telegrams, or module diagnostics. SIMIT provides the necessary behavioral simulation layer.

What is the primary role of SIMIT in a digital twin?

SIMIT models the behavioral and process layer (thermodynamics, fluid mechanics, electrical signals, and fieldbus communication) and orchestrates co-simulation.

Can SIMIT simulate without NX MCD?

Yes. If 3D mechanical visualization is not required, SIMIT can simulate the complete plant process and directly validate PLC logic independently of MCD.

What is a SIMIT Snapshot?

A snapshot is a complete memory capture of the simulation state, enabling engineers to instantly save and restore complex plant operating points for regression testing.

Which controllers can connect to SIMIT?

SIMIT connects to S7-PLCSIM, PLCSIM Advanced, SIMIT Virtual Controller (CPU 410 / S7-400 / S7-1500), and physical controllers via PROFINET/PROFIBUS interface units.