There are many industrial applications where a micro PLC will give you better bang for your buck.
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It’s no secret that two of the keys to success for a modern manufacturing company are excellent maintenance and a culture of continuous improvement. From improving quality verification at the machine, speeding throughput at a robotic pick-and-place cell or unclogging a pesky bottleneck, the incremental gains from tweaking a process can add significant long-term value for a company’s profit margins.
What all these adjustments have in common—aside from adding value—is they all require some sort of control and measurement to have the greatest impact. And while the number of choices for potential solutions can be overwhelming, if the task is localized in a small area of the plant and the communication requirements are small, a micro PLC is likely going to be all you need.
A micro PLC isn’t just a cheaper version of a standard PLC—though they are indeed less costly. It’s more like an optimized version for jobs that don’t require as many outputs to complete the task. Micro PLCs are better suited for smaller systems with around eight output devices because there are a limited number of instruction steps available. Trying to control too many outputs can use up a lot of those steps and leave little room for actual logic.
“Micro PLCs are the added functionality to the world. If you need to add functionality to whatever process you’re manipulating, that’s where a micro PLC really shines.” says Jeff Clonts, Technical Support Manager at RS Americas. “A micro PLC is far less of a technical implementation than a full-blown chassis PLC would be, primarily because there are fewer moving parts,” he says, adding that you don’t have to specify as many outputs, communications protocols or software.
“Plant managers, maintenance managers and continuous improvement managers will often contact us asking if a certain functionality is feasible to do on a micro PLC, and we can help them break it down to make sure that they are planning a reasonable number of outputs and that they’re only monitoring a normal number of inputs,” says Clonts.

Applications for Micro PLCs
Clonts says that when engineers contact him, he hears a lot of similar challenges, often involving how to add a new wrinkle to a machine or process that’s already in operation on the shop floor.
“Maybe they want to be able to start and stop the operation from somewhere else in the process or add a second axis, on something like a crane or an overhead gantry system,” he says. “They’ve got the hardware but need a way to control the new function.”
A common first step to solving this problem is to simply try to integrate the new function into the existing rackmount PLC, but Clonts says that’s where engineers encounter some difficulties.
“The reason we go down the micro PLC path rather than just using their other PLC is they may not have much access to the software on that unit—they need to use a micro PLC to take over some of the processing to make it work.”
Often companies don’t own the software that runs a PLC, which usually has source code protection on it. Clonts compares it to adding an extra step in Windows to send an email containing everything you wrote when you close out of Word. “You’re probably going to do it through a third-party app, not by going into Word’s source code and change it. Working with PLCs is the same concept,” says Clonts.

Open Architecture
Micro PLCs, don’t necessarily operate strictly in open architecture, but many of them do, which gives the user more control over that part of the code with the micro PLC feeding one bit or one string of bits back to the main PLC.
Clonts says you need to understand the overall flow of your data before you really get too far down this process because micro PLCs have limited memory space. Until about a decade ago PLCs had limited operational memory to handle the logic, so engineers had to get creative in how they built their code. Micro PLCs are no different—they still have the memory requirements of older PLCs. When an engineer gets into the architecture, they need to make sure they understand the data flow and if the micro PLC can fit that data before they choose the hardware.
“That’s not new to an engineer—they’re pretty good at that. And then the other side of the architecture is to figure out if this device will send data over a network back to a bigger device. Do they need rungs for signaling or rungs for communication? Those take up different amounts of space and to need to be accounted for,” he says.
In terms of communication protocols, Clonts says most micro PLCs use Ethernet IP or Modbus TCP, with a few exceptions primarily from Siemens that are loaded with Profinet. The reason he usually specifies micro PLCs that use Modbus is because it’s an open architecture which is completely free. “Anybody can use it; anybody can put it on their machines. I’m not going to say its user-friendly because it’s a communication protocol, but it’s cost friendly,” says Clonts. If you already use a certain architecture, you may have to choose a compatible micro PLC. “How do you want program it? Is it through a ladder-like diagram set of instructions, a structured tech set of instructions, or a function block diagram set of instructions? That defines the brands you need to consider.”
Clonts says that decision should be based strictly on how familiar and comfortable you are with that function. “There are so many options out there and so many things that can do the job, there’s no reason to relearn how to use it. Buy something that fits the protocols you already use unless there’s a compelling reason.”

Who uses Micro PLCs
Maintenance managers like micro PLCs because they can add functionality on top of whatever they already have. Continuous improvement managers—the people who are trying to continuously improve a cycle time or process—use Micro PLCs to focus on a very small subset of a process to save time, energy or to prevent injuries.
If you are investigating the reasons processes or machines fail, such as bearings on a motor or a jam that’s always occurring at one location, micro PLCs should be your first choice. Clonts says they provide preventative maintenance data by facilitating a sensor in line to get better clarity on where and when things are jamming. And they are much easier to implement than other options. “Trying to run cables back to your main control cabinet or access the main PLC architecture would take hours. Instead, use a small junction box, the micro PLC and just the ethernet cable to save time and get a little bit better bang for the buck.”
Visit RS to learn more about micro PLCs from Omron, Schneider, Eaton and more.