Hybrid Digital Twin for Virtual Commissioning

Virtual commissioning with a digital twin is a valuable approach to test and verify software for production systems. It allows engineers to digitally test their software early in the development process to proactively find and fix any issues, long before they receive hardware. Additionally, this approach allows engineers to explore more options, letting them optimize production systems to best match the company’s business requirements over a wider range of scenarios. Overall, virtual commissioning is a highly effective tool with many real-world benefits, helping to minimize production facility downtime and optimize systems by finding the right software tools for the job.

Virtual commissioning comprises three main stages; model-in-the-loop (MiL), software-in-the-loop (SiL) and hardware-in-the-loop (HiL). In this blog post, we’re going to look at the three different stages of virtual commissioning in greater detail.

Model-in-the-Loop

The first stage in virtual commissioning is called model-in-the-loop (MiL) testing.

The main idea is to create a block diagram model of the software logic for use by programmable logic controllers (PLCs) and human-machine interfaces (HMIs). That model connects to a simulation model of the production cell or line. These two models run in a real-time, coupled, interactive fashion. The logic model representing the PLCs and HMIs sends commands to the model representing production cell or line. The results feed back into the logic model, closing the loop.

If the simulation produces an error, simply change the logic model and rerun the simulation. Continue tweaking the logic model until the desired result occurs. Additionally, engineers can change the model to explore new options for the logic of the PLCs and HMIs.

This approach enables testing, simulation, and verification of the ‘logic model’ by comparing it to a model of the behavior of the production line or cell under development. This is a fast and easy digital verification method. Engineers don’t have to compile software, get it on the physical PLCs and HMIs, nor set up the production system. Everything is done digitally.

Software-in-the-Loop

The MiL stage enables verification that the behavior of the logic for PLCs and HMIs. The next step is to verify that the logic in the model is the same once it has been compiled into software. This is the basic premise of the following stage of the virtual commissioning process, which is called software-in-the-loop (SiL).

The steps to setting up and running a SiL simulation is a progression from the MiL simulation. First, engineers generate compiled code from the logic model. Such code can be auto-generated and run on an emulator that mimics the electronic hardware in the target PLC or HMI. The software and the emulator are the digital equivalents of the software running on a physical PLC or HMI. This connects to the model of the production cell or line, creating a closed-loop, real-time, coupled simulation.

The objective here is the same as the MiL simulation: uncover any errors in the compiled software. While the logic model may, to all intents and purposes, be right, the translation from model to software can result in previously undetected issues. This is a simple progression from a logic model to compiled software. If the MiL simulation worked and the SiL simulation failed, then engineers know that the source of the failure was the conversion of logic from model to compiled software. Isolating such issues allows engineers to address them more directly.

Hardware-in-the-Loop

The SiL stage permits verification that the compiled software works on a PLC or HMI emulator. The next and final stage, hardware-in-the-loop (HiL), lets users verify that the compiled software runs on the actual, physical PLC or HMI.

Setting up a HiL simulation is a progression from the SiL activity. Here, engineers run compiled software on the real PLC or HMI. Yet, they may still not be ready or may not have the physical production cell or line set up. Instead, they connect the physical PLCs and HMIs up to the digital simulation of the production environment. These components run in an interactive, real-time, coupled analysis.

This progression verifies that the compiled software runs on the real PLCs and HMIs without waiting for the setup of the production system. A failure at this point indicates an issue with the compiled software running on the physical controllers as opposed to the emulators used in the SiL stage. Engineers know the root cause lies in that difference, so they can focus on uncovering and fixing that issue.

This HiL stage allows the testing of controllers before moving into a live environment. That way, engineers can identify and implement any design changes before moving to physical validation. So, they only need to deal with failures in a virtual simulation and test environment, long before they use any production hardware.

This also permits testing to see what happens during extreme events. While such events are highly unlikely to occur, they could have a devastating impact on the production environment. For example, how would a controller react during a full power outage? By performing HiL tests, engineers can come up with reliable and accurate contingency plans for such improbable yet potentially catastrophic events.

Summary and Recap

• Virtual commissioning is a model-based approach to test and verify software designed to run on PLCs and HMIs. It is a highly cost-effective method and comprises three main steps.
• Model-in-the-loop: where engineers test and verify the control and behavior of the logic intended for controllers.
• Software-in-the-loop: the software model generates the compiled code, which then runs on PLC and HMI emulators.
• Hardware-in-the-loop: the compiled software runs on actual, physical PLCs and HMIs.
• A failure at any of these stages isolates the root cause, allowing engineers to focus on identifying the problem in a narrow set of issues.