Comparing and contrasting programmable logic controllers (PLCs), programmable automation controllers (PACs) and PC – based controllers has become a favorite pastime of the industry. However, this is a nuanced topic to say the least, and if the question isn’t positioned in a specific way, one may receive one answer from one source and a completely different answer from another. As a solutions provider, Valin is well versed in these complexities and has ample experience walking through the decision tree with customers in order to select what works best for them.
The biggest factor we look at is the programmer on the customer end. Who is ultimately going to program the system? It may be a mechanical engineer, but often is a controls engineer or contractor. This answer alone could steer the direction of the decision in one way or another.
Early in my career I was an application engineer for a motion-control manufacturer. At the time, I did not know what a PLC was or what it was capable of. I was assigned to support a customer that had been programming a PLC for 20 years. At first, I didn’t think this was a very big deal. The motion controller we were working with for the customer had a PLC scan mode. However, this motion controller used its scripting language in the PLC scan mode as opposed to the more common ladder logic that PLC programmers were used to. Additionally, it had to be compiled which slowed the whole process down compared to a typical PLC.
The customer’s lack of familiarity with this kind of controller led to some long support calls. His programming challenge seemed to be centered around ‘interlock’ and how to implement them in this programming architecture. I didn’t even understand that term at the time. The PLC scan mode wasn’t ladder logic, something the customer had been used to for 20 years. Ladder logic presents a visual representation that helps the programmer keep the logic organized. Scripting programming, conversely, makes logic programming a bit more confusing to track without experience.
It took a little longer to get the customer comfortable with the motion controller, but we did get there. This taught me a very valuable lesson, however. There are PLC programmers and there are motion control programmers. It can be rare to find someone well versed in both.
I broke down this entire interaction I had in Control Design Magazine months ago. If you’d like to read about it in more detail, click here.
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