We Don't Hate PLCs. Really.
We have a lot of conversations about PLCs with our clients. And there are many situations where we steer teams away from using PLCs after reviewing the design for a new system or going over the architecture of an existing system. In these situations, we advocate for the use of an industrial PC to replace the PLCs. In fact, we have these types of conversations often enough that our clients might think we hate PLCs and would never use them.
We swear, though, we don’t hate PLCs- we just think they are used too often.
The proliferation of PLCs makes sense, though. Successful integrators know that delivering a product on time requires taking “smart” risks and minimizing points of failure by using proven solutions. This is a wise strategy.
Unfortunately, we find many integrators are so comfortable with PLCs that they use them by default, often without giving alternatives serious consideration. As the saying goes, “When your only tool is a hammer, everything looks like a nail.”
So when is the right time to use a PLC? We’ve listed some of the criteria we recommend considering in the paragraphs below.
When to use a PLC
Where your processes involve direct motor control, microsecond control loops, or flow and pressure control, PLCs are your best option. While it is possible to modify industrial PCs to operate in a real-time (or close to real-time) manner with a real-time kernel, doing so typically introduces other limitations that effectively transform your industrial PC into a complex PLC.
There is a misconception that PLCs are lower level than industrial PCs but that simply isn’t the case. Industrial PCs can perform much of the same types of low-level operations, such as sending digital signals, as long as there are no real-time constraints. At the same time, this real-time constraint also makes PLCs a better solution even in cases where complex higher-level functions are required, such as managing motion in multiple virtual axes.
“ Industrial PCs can perform much of the same types of low-level operations such as electrical signals, which are very low level, as long as there are no real-time constraints. At the same time, this real-time constraint also makes PLCs a better solution even in cases of higher-level functions such as virtual axes.” - Gerry Ens, Go West Robotics CTO
When to use an Industrial PC
It’s worth clarifying that not all task dependencies require PLCs. PLCs are only critical when dependencies require milli- or micro-second coordination of moving parts. Systems that have no real-time requirements are simply tasked with performing a specific operation over a given time interval, and/or may only need to provide a start signal and receive progress data are great candidates for industrial PCs. Real-world examples range from machine tending systems to a robot arm on a mobile platform. Given the customizability of industrial PCs, as long as there is no hard real-time requirement, they are likely capable of being modified to suit your needs. To learn more about the benefits of industrial PCs read our article here.
While industrial PCs are great for reducing licensing costs and enabling more customization in your process, they aren’t the best option for everyone. To decide which solution would best fit your company, the first step is identifying if and where real-time constraints exist in your process. Once you’ve identified where these constraints are, you will be on your way to increasing the number of options available to you.
The “Goldilocks” Zone
Fortunately, the industrial PC versus PLC decision is not “either/or”. If you have an application that does need a PLC, adding an industrial PC can still be a great advantage. When using both, we recommend a bare minimum PLC, which can bring a significant price advantage, while retaining all the processing benefits (e.g., rapid development, source control, reusability, language and framework choice, etc.) of using an industrial PC to execute the majority of the code.
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