PLCs Vs. Microcontrollers – Which Should I Use?

In any automation system design: cost, functionality, space, and interoperability are always on the mind of the engineer designing the system. This is true of small control builds and complex, factory-wide control environments with numerous machines connected to the system. For many decades, PLCs have been the workhorse of such automation and control systems, providing an effective and reliable way for engineers to get the job done.

But within the last decade, microcontrollers have come to the forefront of numerous applications leading many to wonder if they can be considered a more cost-effective and smaller substitute for PLCs. As the “maker” culture arose with small automation applications such as desktop 3D printers, an entire ecosystem of accessories grew with them until many have reached a comparable feature set and list of capabilities of PLCs.  

Why then can’t they be used in place of traditional PLCs? The answer lies in a few important distinctions between the two.

Benefits of a PLC

PLCs have been widely used since the 1970s. They are used in industrial control and automation systems and are designed to withstand the conditions of modern factory environments. They monitor the inputs and outputs of a programmed control system to make decisions based on information and data received from sensors, switches, and other devices.

Many are even ruggedized or offer extended safety features for specific applications within hazardous industries.  Ruggedized PLCs like the Siemens Simatic Plus PLCs offer units that can withstand extreme conditions, while safety PLCs are available in PLCs within Allen Bradley’s ControlLogix and CompactLogix family of products offering added safety of SIL2/PLd and SIL3/PLe. 

PLCs also have generational dependability. As each model nears the end of its lifecycle, its replacement is clearly identified by the OEM. Extensive data is also published to ensure proper migration from generation to generation.

Because they are specifically designed for control systems, PLCs are built with a wide range of I/O connections that can be tied directly to the communication system. They also have programming environments that are designed to ensure interoperability between components in the system and even third-party components.

Benefits of Microcontrollers

Microcontrollers are essentially single-board computers. They work well in applications where computing requirements are low, such as appliances and exceedingly small automation tasks. This is a key reason they have been championed by the maker and DIY community for those kinds of projects.  Microcontrollers can, however, perform complex mathematical equations and process both data and logic as well as control motion on some devices. They are also able to handle wireless communication.

One of the most significant benefits of microcontrollers is their cost. Compared to PLCs, microcontrollers are considerably less expensive. They are also very compact, making them tempting for control solutions where space matters.

What then are the differences that would help engineers and designers choose what to use? Here is a look at several issues side by side for comparison.

PLCs vs. Microcontrollers

  1. Architecture – PLCs consist of a CPU, a power supply, and various I/O connections. The CPU will also have a programming interface such as USB, EtherNet, or others. I/O modules may include motion control functions and counters for high-speed applications as well as both discreet and analog functionality. On the other hand, place microcontrollers, the CPU, the I/O, and the power supply on a single chip. This means a more straightforward design and lower cost. However, microcontrollers may suffer from a lack of interoperability. PLCs are more likely to be used within a hybrid OEM component control system due to their adherence to rigid industry standards.
  2. Interface Capability: PLCs can interfacing with sensors, actuators, HMIs, robotic components, and communication modules. And because they can communicate over many different communication protocols, they are open to programming from a single software platform such as Allen Bradley’s Studio 5000 Logix Designer or similar programs from other OEMs. This means that they have a broader range of use across many different industries and equipment types. Microcontrollers may also have onboard sensors, actuators, and other components comparable to those controlled by a PLC. Still, they are often designed for more specific uses making them difficult or impossible to link to PLCs.
  3. I/Os – Microcontrollers have limited I/O capability compared to PLCs. Most are capable only of a few hundred I/Os, while PLCs have a much larger I/O capacity. Microcontrollers used in a PLC may require conversion of their I/O for voltage and other factors, whereas PLCs are designed for standard industrial voltages. It is usual for PLCs to isolate I/O circuits for protection, a feat that could be accomplished manually on a microcontroller. However, this would add cost, time, and complexity.
  4. Mounting:  One of the most significant perceived benefits of microcontrollers is their size. Their computational power and small size make them desirable for those facing small spaces or who wish to save space within an enclosure. However, other factors make the PLC a better choice. PLCs are designed for use in cabinets and enclosed to further protect the inside components from `dust, moisture, and other conditions found in harsh manufacturing conditions and are essentially “plug-and-play” within the cabinet. Microcontrollers are often open-faced with pins for connection. It may be possible to connect them manually within a system, but again, it adds complexity, time, and labor. 
  5. Operating System: Microcontrollers by design operate on common programming languages. Such as Linux, C, and C++. That is because the applications where they are deployed, such as appliances, can be controlled with such programming. PLCs are also simple to program but are designed to work within an ecosystem of other devices that must operate in synch, or the system would fail to cause a stoppage, equipment damage, or safety issues. But PLCs also have deeper design features to work within a complex control system as well. This includes the presence of both hardware and software safeguards built into the PLC. Software safeguards make sure the program is executing at the correct timing, while hardware safeguards ensure that devices are performing correctly through “handshakes.” Detection of errors in either safeguard results in the equipment being placed in safe mode or an alert sent to an operator or technician. This makes PLCs inherently safer than off-the-shelf microcontrollers.
  6. Programming – PLCs can be programmed by a standard programming environment within a platform created by PLC manufacturers. It ensures that common programming languages can be used to keep devices operating together correctly. These platforms also utilize natural language algorithms meaning that less skill is involved in programming a PLC. By contrast, a microcontroller can be programmed to work with other components, including safety notifications and other alerts. But the programming skillset required is more advanced and takes more time and effort.  
  7. Environment:  As mentioned, microcontrollers are often open-faced with pin connections compared to a PLC that is encased and is plug-and-play. This makes PLCs more durable in a manufacturing or warehouse environment. Control system components must withstand shock, temperature extremes, vibration, noise, moisture, dust, corrosion, and other conditions. PLCs are designed for this, and many are even ruggedized for the most extreme applications. Microcontrollers are less durable and are exposed to the conditions mentioned above, meaning they are more likely to fail in a much shorter amount of time.
  8. Support:  PLCs have a generational evolution supported by an OEM. This means that as each PLC nears the end of its lifecycle, the replacement has been identified and produced. For PLC OEMs, considerable effort goes into the training and documenting of migration parameters so system engineers can seamlessly upgrade to newer generations without shutting down the system.  Microcontrollers are often more narrowly designed, and future generations may not be compatible as technology within their original industry may have made significant advances. I/Os and programming can be changed on a microcontroller. However, once this is done for that specific application, the microcontroller becomes unique. Changes at the end of life may result in a system failure if programming and modification are not available or if replacement units are not compatible. Microcontrollers also do not have the diligence and technical support ecosystem behind them, as do PLCs.

Making Choices

So, can a microcontroller be used in place of a PLC? The answer is in a very narrow set of circumstances, such as straightforward control requirements in a low duty environment, then the answer is yes. However, the differences above highlight that PLCs are designed for industrial and high-volume service applications that require safety, redundancy, ease of programming, and a host of other features that microcontrollers cannot provide. In the end, outside that narrow set of circumstances, PLCs remain the best choice for industrial automation, control systems, robotic manufacturing, and motion control.

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