A virtual factory acceptance test (virtual FAT) is a formal test of a machine's or line's control software against a simulation model, witnessed remotely by the buyer before the equipment ships or before the software goes to site. The buyer's engineers run the agreed test protocol on a shared screen: start the line, inject faults, change recipes, check alarms and HMI behavior. It does not replace every physical check, but it moves the software part of acceptance earlier, when changes are cheap.
FAT, SAT and where virtual fits
A traditional FAT tests equipment at the supplier's facility before shipment. A site acceptance test (SAT) repeats key checks after installation. ISA's InTech has warned against rushing to startup by skipping a proper FAT, and describes typical checks such as testing analog inputs at 4, 12 and 20 mA, with the integrator writing the procedure and the client's engineer approving it before the test (ISA InTech, March-April 2017).
| Test | What it proves | Can be virtual? |
|---|---|---|
| Software functions | Sequences, modes, recipes, interlocks | Yes, fully |
| Alarms and HMI | Every alarm, every screen, user levels | Yes, fully |
| Interfaces | Handshakes with other machines and MES | Yes, with simulated partners |
| I/O and wiring | Each point wired and scaled correctly | Partly: panel FAT still needed |
| Mechanical performance | Rate, quality, product handling | Partly: simulation estimates only |
| Safety validation | Safety functions on real devices | No: final validation on real equipment |
What a virtual FAT gains you
Plant Engineering, writing with Applied Manufacturing Technologies, notes that removing travel cuts stakeholder time from days to hours, and that sessions can be recorded as documentation (Plant Engineering, May 2021). In our experience the bigger benefit is timing. The software can be accepted while the machine is still being built, so the physical FAT focuses on mechanics and wiring rather than logic.
It also lets more people take part. The operators who will run the line, the maintenance lead and the quality engineer can each join for the hour that matters to them. On a physical FAT, usually one or two engineers travel and everyone else sees a report.
Empowering Machine Builders for Virtual Factory Acceptance Tests
Siemens Software
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Watch on YouTubeHow to run a virtual FAT
- Agree the protocol early. Write test cases from the functional specification, each with steps and pass criteria. Sign it before the test.
- Freeze versions. Record PLC, HMI and model versions, and don't change them during the session without logging it.
- Set up the session. A shared view of the model and the HMI, a separate camera or view for the PLC online, and a clear owner of the controls.
- Let the buyer drive. The client's engineers press the buttons and choose the faults. Watching a supplier demo is not a test.
- Log everything. Each step passed, failed or deferred, with a punch list owner and a date.
- Retest failures. Fixed items are retested in a follow-up session and signed off.
Practical guidance from integrators includes using several views, one fixed on the HMI and a roving one for the equipment (Plant Engineering). For a virtual FAT the equivalent is a shared HMI screen, the 3D model view and the PLC program online, all visible at once.
Who should attend
A virtual FAT works best with a small, empowered group from the buyer: the controls engineer who will own the code, a lead operator who knows how the process is really run, and someone from maintenance who will troubleshoot it at night. Quality and validation staff join for the sessions that touch their requirements. On the supplier side, the engineer who wrote the code drives the model and answers questions, and a second engineer logs results and fixes.
Plan sessions of two to three hours rather than full days. Screens and attention both tire, and short sessions spread over a week give time to fix punch items between them. A test protocol of a few hundred steps is normal for a packaging line. Split it by area and by mode, and run the hardest cases, such as recovery after an emergency stop, early in the week.
Keep the model honest. Before the session, write down what the simulation does not represent, for example product slip, real valve dynamics or a drive's internal faults. Testers then know which results are proof and which are only indications.
What still needs a physical test
A physical panel or machine FAT is still the right place to verify every I/O point, device settings and wiring. An experienced engineer writing in Plant Engineering describes a protection relay that passed office testing with correct settings, but whose triggering logic had not been enabled (Plant Engineering, February 2022). That kind of issue lives in real devices. Safety functions also need final validation on the real equipment.
We run virtual FATs for U.S. clients on our own models and on models supplied by OEMs. The approach fits our delivery model: engineering in our India center, accepted remotely by your team, then commissioned on site by your team or a local partner with our engineers supporting. Learn about our virtual commissioning service, the tools in virtual commissioning software compared, or contact us to plan a virtual FAT. Our controls engineering team writes the code under test, and explains how test depth affects price in PLC programming cost.
