Implementing advanced virtual commissioning strategies significantly reduces the risks involved in complex automation upgrades. Simulating the production line allows the system’s operating logic to be verified in a risk-free environment, preventing most potential onsite implementation issues and laying the groundwork for the creation of digital twins.

Digital twins insights
- Virtual commissioning models lay the groundwork for the development of a real-time digital twin of the production line.
- Virtual commissioning paired with a phased upgrade approach is a powerful strategy that reduces the risks of completing complex upgrades.
- A recent project completed by Patti Engineering for a renowned automotive manufacturer utilized virtual commissioning.
Manufacturers often face the dilemma of deciding between maintaining legacy equipment or undertaking the risks associated with system upgrades. These risks are primarily due to uncertainties in transitioning from a legacy programmable logic controller (PLC) to a newer system. A poorly executed upgrade can leave unresolved compatibility issues within the system, leading to lingering operational inefficiencies and complex troubleshooting challenges. This often results in greater expense and more downtime than initially planned for, as further efforts are made to resolve these persistent problems. On the other hand, continuing to maintain legacy equipment inevitably results in increased maintenance costs and frequent, unpredictable downtime with few diagnostic capabilities to identify the underlying causes.
Virtual commissioning, particularly when paired with a phased upgrade approach, is a powerful strategy that dramatically reduces the risks of completing complex upgrades. Tasked with upgrading a major engine assembly line, advanced virtual commissioning strategies were for a recent project Patti Engineering undertook for an automative manufacturer to rewrite and verify the control software and robotics routines in a simulation environment.
As a result, onsite commissioning was completed on schedule, on budget, with minimum prototype parts, and without unforeseen complications – a true high-quality outcome by all measures. Furthermore, the simulation of the production line has the potential to serve as the foundation for building a visualized real-time virtual replica of the system – a digital twin.


How virtual commissioning leads to a digital twin
By simulating the operation of a production line, virtual commissioning allows a lot of software design and verification to be completed without any physical hardware. Computer models of each line component are instantiated within the simulation environment, including PLCs, robots, sensors, variable frequency drives (VFDs) and conveyor system motors. The controls software can be developed, exercised and verified within a virtual environment, significantly reducing the time, risk and need for prototype parts during final onsite commissioning.
Beyond the initial upgrade planning and verification, the completed system simulation offers the potential for further use as the foundation of a digital twin – a fully visualized model of the actual system in operation. By integrating real-time feedback from the PLC and on-floor sensors into their respective models within the virtual environment, the behavior of the production line is continuously updated into the digital twin such that its behavior mirrors that of the actual equipment.
A legacy production line full of challenges
As a control system ages, it becomes increasingly difficult to maintain its operation. Like many in the industry, this automotive manufacturer faced several of the common issues associated with legacy equipment:
- Scarcity and expense of spare parts
- Reduced reliability
- Increased downtime of unknown origin
- Limited diagnostics to discern root causes of failure
- A long, arduous system restart process
The line, controlled by an outdated Allen-Bradley PLC system and operating around the clock, included over 100 independent conveyor sections with their own associated motors controlled by VFDs and motor starters, 12 robotic stations, over 300 sensors, multiple lift and locate units (LLUs), lift and transfer units (LTUs) and elevators and a hardwired safety system. This important production line would frequently go down with difficult-to-identify underlying causes and challenging restarts. The manufacturer made the important decision to invest in its upgrade.

Complex upgrades require a phased approach and significant simulation
When the target line to be upgraded is particularly complex and heavily utilized, downtime availability for upgrades is typically short and sparse. The integrator planning the upgrade is responsible for dividing the project into appropriately sized phases, ensuring each segment of the project can be completed within the allocated downtime and returning the line to continued operation between installation phases.
Use of virtual commissioning tools is a particularly powerful way to maximize verification of the system’s overall operation without physical equipment. It allows the new PLC code to be debugged and verified in a simulated environment. This dramatically reduces onsite code debug and provides the high level of confidence needed to complete the onsite work within the scheduled downtime.
Planning a complex upgrade
This particular project’s overall scope was to replace the legacy Allen-Bradley system that included two Allen Bradley Control Logix PLCs with a single Siemens S7-1500 PLC, rewrite and standardize the code structure, optimize cycle times in the robotics cells and transition to a software-based safety system managed directly by the new PLC.
With downtime confined to intermittent three- to five-day windows, the upgrade was broken into 15 different installation phases. To maintain normal operations between upgrade phases, the legacy PLC continued to control untouched sections of the line, while the new Siemens PLC took over each upgraded section. Communication gateways were used to establish a handshake between the new and legacy control systems after completion of each installation phase, allowing the line to remain operational under mixed-PLC control.
Advanced simulation tools streamline code verification
Used effectively, modern simulation tools create a close replica of complex manufacturing operations.
For the assembly line upgrade, engineers used multiple software tools to model individual sections of the line targeted for migration during the given phase of the upgrade. This approach allowed the majority of debug to occur within simulation, maximizing the efficiency of the time spent onsite and reducing the need for prototype parts.
Here are the major components used in the upgrade:
Siemens Process Simulate
Siemens Process Simulate is designed to integrate digital models of each line component into its virtual environment. The application was used to accurately model the movement of engine blocks on pallets through the stations based on the new code written for the upgraded PLC. The following device models were integrated into Process Simulate:
Virtual Siemens S7 PLC: Process Simulate was connected to a virtual Siemens PLC environment through TIA Portal, allowing the actual control logic to run the simulation and control the component models. This allowed engineers to dynamically view and assess the PLC’s input and output (I/O) signals during production simulation.
Simulated VFDs for conveyor sections: Because each conveyor section operates independently with its own associated motor and VFD, the simulation environment needed to incorporate a model of this equipment to accurately simulate part movement along the conveyors.
High-level scripts were written in Siemens-native Structured Control Language (SCL) to emulate the behavior of the VFDs and associated motors. This virtual motor element allowed information sent between each virtual VFD (including current, voltage and speed) and the virtual PLC to be mimicked and evaluated within Process Simulate.
Photo-eye sensors: Used to monitor part position, virtual sensor models were implemented in Process Simulate at their appropriate real-world location along the conveyors.
The newer PLC, capable of handling faster and more complex processing tasks, significantly improved control and communication with the robotic cells, leading to opportunities to improve the speed and efficiency of the robots. As a result, they were evaluated for cycle time optimization.

FANUC’s ROBOGUIDE
ROBOGUIDE was used to simulate the robotic cells within the line. ROBOGUIDE includes mature models of all FANUC robots that closely mimic their real-world counterparts. Actual robot routines can be run on the robot models, allowing them to be debugged and optimized. In some cases, end-of-arm tooling changes were simulated using ROBOGUIDE and subsequently updated to further capitalize on time availability and increase throughput.
The virtual PLC implemented using TIA Portal in Process Simulate can also communicate directly with ROBOGUIDE to control robotic operations. By integrating the ROBOGUIDE models into the broader Process Simulate simulation, the PLC’s communication with the robots could be monitored and verified within the broader system.
Virtual commissioning best practices
While virtual commissioning strategies can increase confidence and minimize downtime for system upgrades, its effectiveness requires employing proven strategies and focusing efforts where they make the biggest impact.
Virtual commission refinement over time
The first sections targeted for virtual commissioning inevitably require the greatest time investment. The entire virtual environment must be built, such as motor emulation scripts, PLC tag mapping and sensor instantiation. Once established, subsequent sections can be set up much faster as a significant amount of the initial work is reusable.
Integrating only what makes sense
It’s often the case that not every mechanical device needs to be fully simulated. For the automotive project, modeling of turntables and lift tables was particularly challenging and offered limited value. Instead, the focus remained on the core logic for conveyors, robotics and safety.
Planning for onsite adjustments and tuning
Patti Engineering’s approach to onsite commissioning was to anticipate some discrepancies between the simulation of the line and the physical equipment’s actual operation. Some parameters that required onsite tuning included sensor offsets, actual motor speeds and other mechanical tolerances.
Laying the groundwork for a digital twin
While the immediate goal of virtual commissioning is to reduce both downtime and risk during an upgrade, the completed simulation models can also serve as the foundation of a digital twin. By continually feeding real-time operational data from the upgraded PLC and other sensors back into the models, the line’s virtual replica can visualize the current operational state of the system.
Beyond a visual replica of the production line, some of the potential ongoing use cases for digital twins include implementation of predictive maintenance analysis platforms, continued production optimization and a risk-free training environment.
How virtual commissioning addresses unknowns
Virtual commissioning simulation tools allow integrators to uncover and address most unknowns involved in upgrading a complex line in a low-stakes virtual environment and lay the foundation for a digital twin model of the system. Used in combination with a phased onsite upgrade strategy, complex projects can be approached with high confidence of a successful completion, even with significant downtime constraints. In turn, manufacturers can feel confident that their struggling legacy systems can be revitalized and optimized for throughput and quality, futureproofing them for years to come.
Aryan Abbott is a controls engineer and Nikhil Niphadkar is the engineering team lead and senior controls engineer II with Patti Engineering.