The modular principle for digital twins

Virtual commissioning of modular plants

Customized products are becoming increasingly important for customers. The factory of the future will have to be flexible and able to fulfill customers’ individual demands. That’s why plants are becoming more and more modular, meaning that they’re constructed out of a number of individual components that can be combined in many different ways like building blocks. However, a commissioning test has to be performed after each conversion in order to verify that the plant is operating correctly. To save time, virtual commissioning based on the modular principle could become the standard. But the fact is, it won’t work without new standards to which all parties conform.

 “Modular plants are the future of production, because they can be converted and adapted very quickly,” explains Tim Schenk, a simulation expert at Siemens Corporate Technology. “But if production has to be interrupted to perform long commissioning tests, there’s ultimately no gain. The only way to fully exploit the benefits of modularity is with virtual commissioning!”

Virtual commissioning means performing the tests on the digital twin of a plant instead of on the plant itself. For a plant that’s frequently reconfigured, the new setup can be tested virtually while the actual plant continues to operate. As an absolute prerequisite, it has to be possible to quickly create a digital twin of every possible plant configuration. This is where Corporate Technology’s development work comes in. “As part of the government-funded  ENPRO ORCA project, we worked with colleagues from Siemens Digital Industries to develop a system for generating and testing digital twins that’s just as modular as the plants themselves,” says Andrés Botero, ORCA Project Manager at Siemens Corporate Technology.

Demonstrator in Munich-Perlach

In the simulation lab in Munich-Perlach, Germany, a small demonstration plant illustrates the approach. Liquids from three tanks are combined at the specified ratio, stirred, and/or heated or cooled, and then bottled. “It’s a small but typical modular plant,” explains Botero. “The three modules – the mixing module, reactor module, and filling module – are products from different manufacturers. They can be combined on the automation level because they all conform to an unambiguously defined interface standard (MTP).”

Der Schnittstellenstandard MTP (Module Type Package)

Uniform interface standards are the prerequisite for all modular architectures.  The modules are like Lego blocks that fit together because they all have the same connectors. To make modular production plants possible, Siemens has been cooperating with well-known partners for several years on the development of the MTP (module type package) interface standard. MTP defines all the relevant information that’s necessary for operating a module, including the module’s user interface (module HMI), the data to be exchanged and, most importantly, the automation functions.

Even though a great deal of progress has been made on the MTP standard, the partners’ work continues. “Virtual commissioning is possible only if we also have a digital twin of each module. Ideally, the module manufacturer supplies it with their module. We’re working on making that standard practice because, generally speaking, it’s fairly complex and can even be impossible to generate the digital twin of a finished module from a third-party vendor,” says Schenk.

A module as a logical black box

It’s impossible because modules purchased from third-party vendors are a sort of black box. The specification describes the variables and their meanings and defines the functions that the module performs, but what is undefined and invisible – and, therefore, can’t be precisely simulated – is how the module was implemented internally.

The commissioning test of a modular plant verifies that the functions of the individual modules have been orchestrated and parameterized in such a way that the desired production results are achieved. It’s quite conceivable that all the individual modules could be functioning correctly but could still not work together because something has been incorrectly orchestrated. Virtual commissioning tests can reliably detect this type of error only if the digital twins of the modules behave exactly like the real modules.

A twin made up of virtual modules

Thanks to the modular principle, once this hurdle has been cleared and there’s a digital twin for each individual module, it’s possible to generate the digital twin of any plant configuration with almost no additional effort using a suitable co-simulation environment. The methodology was developed by experts from Corporate Technology and is still currently being implemented in a prototype environment based on the SIMIT simulation platform.

“This enables us to perform virtual commissioning tests and supplied the missing puzzle piece that we needed for efficiently converting modular plants and putting them back into operation. We built our demonstrator plant in spring 2020 and had to virtually commission it ourselves,” says Botero. “At that time, no one could work in our simulation lab due to COVID-19 restrictions. But because we could access our simulators from home, we were able to continue working effectively.” 

Aenne Barnard  - September 2020

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