Micro-controllers versus PLCs: Which one belongs in your plant?

Cover Story: Single-board computers and micro-controllers offer opportunities for automation applications, but they may not be the best option for critical production applications.


Figure 1: There are dozens of micro-controller developer board designs (Arduino, BeagleBone, Raspberry Pi, and others) available from a variety of manufacturers. Courtesy: AutomationDirectThe variety of micro-controllers emerging has been growing rapidly, with no signs of abating. These devices offer many capabilities, along with whole ecosystems of accessories that are also cost-effective (see Figure 1). Consequently, interest in these products has moved from automation clubs and basement robot builders, to the point where some are considering their use in manufacturing.

When an engineer is looking at solving a small industrial automation challenge, the traditional approach has been to use a programmable logic controller (PLC) (Figure 2), but some engineers now may consider a micro-controller; however, certain factors need to be considered before a decision is made.

For the purpose of this discussion, some generalizations will be made due to the number of possible open-source board-level products resulting from the do-it-yourself electronics industry (the maker world). Micro-controllers, field programmable gate array (FPGA) boards and single-board computers have varying capabilities and limits. However, for purposes of this discussion, we'll lump them together under the heading of micro-controllers.

Similarly, the characteristic attributions of PLCs and other industrial controllers do not apply to every model and manufacturer, although there is a high degree of consistency across the range of companies within this space. 

Industrial automation example

An engineer may be considering a small automated task involving two or three sensors, an actuator for the output, and a reporting function to the larger control system. It will require a basic program to make it operate.

This is simple for many small PLCs, with prices starting at a few hundred dollars, but there are factors that should be considered first. A small open-source board-level micro-controller might be tempting to try at a fraction of the cost.

The first obstacle the engineer is likely to hit is the input/output (I/O) compatibility: does the prospective micro-controller offer the required I/Os? It isn't difficult to find a micro-controller with the correct number of discrete and analog I/Os, but they may not be the right type.

Some are relatively easy to convert, such as a 4-20 mA current loop to a 0-5 V voltage loop. Others are more difficult to convert to anything, such as an analog output using pulse-width modulation (PWM), common for micro-controllers. Some signal converters are available as standard products, but they add to the overall cost. An engineer insisting on a full do-it-yourself experience may try to create the converter internally, but such an undertaking can be complex and require extensive development time.

On the other hand, PLCs are designed to work with industrial sensors, and therefore offer a wide range of I/O choices, so there should be little or no need for external conversion. The ability to wire directly to a PLC or I/O module will be easier since it is designed for this purpose. A PLC also will ensure a high degree of protection for the devices and circuits by building in isolation for relevant I/O points. An end user may be able to do the same, but this requires additional knowledge and increases complexity.

Figure 2: PLCs, like this AutomationDirect BRX, have been the workhorse in industrial automation applications for decades. Courtesy: AutomationDirectThere's also the matter of mounting and housing a micro-controller, since it likely will be a naked board with pins for connections (Figure 3). The end user also must supply power and create terminals to attach external devices. These are manageable tasks, but take time.

These physical matching and mounting challenges may seem to be major elements of the discussion, but they only scratch the surface of the differences between the two major platform groups, with other less visible considerations being more important.

The OS supporting the application

Micro-controllers are bare-bones devices, and that includes the operating system. After all, a single-board computer selling for $40 is not going to have much in the way of built-in software routines, so the user is left to code everything except the most basic capabilities. This isn't necessarily a problem since most micro-controllers use common programming environments such as Linux and C and are typically used for relatively simple applications.

Similarly, writing an application for a PLC also may be simple, but much happens below the surface that is not visible to the programmer or user. PLCs have many housekeeping functions watching the program and associated equipment.

Figure 3: Most micro-controllers only provide pin type connectors as shown on the bottom right of this board, and thus require the addition of basic terminal strips for I/O connections. Courtesy: AutomationDirectSoftware watchdogs keep an eye on the program to make sure it is executing as it should. For example, say there is a problem with a for/next loop, and the program gets stuck. If it's stuck, it can't carry out its function, which could cause a harmful and potentially dangerous situation. The software watchdog times each scan of the program. If a given scan is not completed in the allowable time, the dog will bark, fault the PLC and put it into a safe mode while alerting the operator.

Hardware watchdogs keep an eye on the devices connected to the PLC, particularly I/O modules or individual devices such as switches, sensors, and actuators. The PLC always is exchanging handshakes with the devices, and counts each scan of the program as it operates. If the scan count for any individual device begins to lag, the PLC will assume something is wrong with the device or wiring. Depending on the level of sophistication and the program set up, it may fault and go into a safe mode, or it may continue to operate while informing operators of the problem.

Data from devices is verified using a cyclic redundancy check to ensure there are no communication errors. Again, if a problem develops, the PLC can go into a safe mode. All these functions are designed to warn users if the PLC is not functioning as expected, and therefore cannot control the machine or process as desired.

Theoretically, any of these capabilities could be added to a micro-controller's programming, but the user would either have to write the routines from scratch, or find existing software modules to reuse. Naturally, these would have to be tested and verified for the application, which would be a major undertaking, at least the first time around. An engineer writing multiple programs for the same controller could probably reuse proven code blocks, but these capabilities are included with the operating system for virtually any PLC. 

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