MIL, SIL, PIL, HIL..... How is the "in the loop" term used.

Hello all!

It's been a while. I read a while ago that one shouldn't apologise for not writing and just keep writting as if nothing happen, so I'll take that advice.

Often in my customer interactions I hear and use the term "HIL". As you might know, HIL stands for "Hardware In the Loop" and is a testing methodology to verify and/or validate and embedded system design. I heard it was developed around 30 years ago by the automotive industry (feel free to correct me!) and had evolved much since. You can learn more from the wikipedia entry.

Interesting enough, there are a number of different "In the Loop" variations, depending on methodology used, equipment and even culture inside a company. By no means the terms I will refer below are anything like industry standards, but that's the way I have them in my head and make quite sense.

To simplify the explanation, let's assume we are developing an embedded control algorithm that will act over a physical system. The plant execution is needed to provide the information needed to close the control loop on the embedded device.  For example, let's assume the control system for an automatic transmission.

MIL ("Model in the Loop"):  In this instance, both the control algorithm and the plant model (transmission and car) are implemented as simulation models. Here is one example that ships with LabVIEW

SIL ("Software in the Loop"): Here we have created the optimized code for the embedded target but still run it against the plant model on a computer environment.

PIL ("Processor in the Loop"): The embedded code is running on an embedded target. The plant model. Here is great video below about simulation, control theory and a PIL system

HIL(or HWIL "Hardware in the Loop"): Embedded target is finished (or nearly so) and plant model is executed on an external device.

Test Cells: This is an extension of the methodology where both the embedded system and the plant are physically present and external elements are simulated. In the case of our automatic transmission, both transmission and TCU (Transmission ECU) exist and the engine input torque and road/car  load are simulated by electrical motors. Here is an example for a transmission from a customer.

There are many more terms I've heard like "Person in the Loop", "Sensor in the Loop", etc, but thought it would be good just somewhere to start on.

At the end of they day, when I have to explain my kids what I do, I tell them I help people build "simulators"