PLC vs. Microcontroller: How to Choose the Right Control System

PLCs and microcontrollers are essential components in modern control systems. While both are used for automation and control purposes, they have distinct differences in terms of design, functionality, and application.

Choosing the right control system, whether it's a PLC or a microcontroller, is crucial for the success of any automation project. The choice depends on various factors, such as the complexity of the task, speed requirements, cost considerations, and the environment in which the system will operate.

Over the years, PLCs and microcontrollers have evolved significantly, driven by advancements in technology and the increasing demand for automation. PLCs have become more powerful and feature-rich, offering enhanced capabilities for complex control tasks. Similarly, microcontrollers have become more efficient, with higher processing power and improved peripherals, making them suitable for a wide range of applications. In this article, we will dive into the differences between PLCs and microcontrollers to help you choose the right control system for embedded applications.

Overview of PLC

A programmable logic controller (PLC) is a rugged computer designed for industrial automation. Like microcontrollers, PLCs monitor inputs and outputs to make logic-based decisions, automate processes based on custom programs, and control machine functions and entire production lines. A PLC can be seen as a more robust version of a microcontroller. It is an industrial-grade digital computer designed to withstand harsh manufacturing environments and facilitate complex manufacturing processes.

PLC offers significant enhancements to production lines, machine functions, and processes. Its key advantage lies in the ability to modify and replicate operations or processes while collecting and communicating critical information.

Another benefit of PLC systems is their modularity. This means you can mix and match input and output devices to suit your application best.

Overview of Microcontroller

A microcontroller is a small integrated circuit created to manage a particular function within an embedded system. It usually consists of a processor, memory, and input/output (I/O) peripherals, all on a single chip.

Also known as an embedded controller or microcontroller unit (MCU), these devices are used in a variety of applications, such as vehicles, robots, office equipment, medical devices, mobile radios, vending machines, and household appliances. They act as simplified miniature computers, controlling specific aspects of a larger system without the need for a complex operating system (OS).

PLC vs. Microcontroller: Architecture

PLCs are comprised of a CPU, a power supply, and various I/O connections, often with a programming interface like USB or EtherNet. Their I/O modules can include motion control functions, high-speed application counters, and discreet and analog functionality. In contrast, microcontrollers integrate the CPU, I/O, and power supply on a single chip, offering a simpler design and lower cost. However, they may lack interoperability. PLCs, known for their adherence to industry standards, are preferred in hybrid OEM component control systems.

PLC Architecture

8051 Microcontroller Architecture

PLC vs. Microcontroller: Characteristics

PLC Characteristics

What distinguishes a PLC from a microcontroller, larger-scale industrial PLC, or any other industrial control solution? Let's examine some key characteristics.

Inputs/Outputs

A PLC contains a CPU that stores and processes program data. However, it relies on input and output modules to connect it to the rest of the machine. These I/O modules send information to the CPU, triggering specific results.

PLC I/O controls can be analog or digital. Inputs can range from sensors and switches to meters, while outputs include relays, valves, drives, and lights. One advantage of PLCs is that users can customize their I/Os to meet configuration requirements for their application.

Human Machine Interface

When humans need to interact with a PLC in real time, they use a Human Machine Interface (HMI). These interfaces can vary from a simple display with a keypad and text to a smart touchscreen panel similar to modern consumer electronics.

Communications

While I/O devices are important, the PLC may also need to connect to different systems. For example, a user might need to export recorded application data from the PLC and send it to a Supervisory Control and Data Acquisition (SCADA) system.

To enable this function, most PLCs include multiple ports and communication protocols to ensure they can communicate with other systems.

Microcontroller Characteristics

Uncertain whether a device is a microcontroller or another type of computer. Let's explore some distinctive features.

Embedded

A microcontroller is embedded inside another device to help control its actions or features. This is why it's often referred to as an "embedded microcontroller."

Task-Specific

Microcontrollers are designed to execute a single specific task, running a program stored in Read-Only Memory (ROM), unlike RAM, which can change.

Low-Power

While a typical general-purpose computer consumes around 50 watts of electricity when plugged into a wall socket, a microcontroller uses only about 50 milliwatts.

Dedicated Input/Output

A microcontroller typically has dedicated input and often output devices (like an LCD or LED display).

It accepts input from the controlled device and sends signals to other components within it for control purposes.

Consider a television's microcontroller, which takes input from the remote control and displays output on the screen. Internally, it manages the speaker system, channel selection, and picture tube electronics, such as brightness.

Or, think of a microwave oven. Its controller receives input from a keypad and displays output on an LCD. It also controls the relay that switches the microwave generator on or off.

Economical and Compact

Microcontrollers are cost-effective. Users usually seek the smallest, most affordable microcontroller that can fulfill the required task.

Rugged

Many microcontrollers can endure high temperatures and harsh conditions better than general-purpose computers.

For instance, the microcontroller in a car's engine must withstand extreme temperatures ranging from freezing in Alaska to over 100 degrees Fahrenheit in Nevada.

While some applications demand ruggedized microcontrollers, others do not. The microcontroller in a Blu-ray Disc player, for example, does not require this level of durability.

PLC vs. Microcontroller: Working Principle

PLC Working Principle

PLCs are made up of a processor, I/O modules, a power supply, and an external programming device. The processor acts as the PLC's brain, executing pre-programmed control functions based on input from connected devices like sensors or switches. For instance, if a sensor detects low temperature, the processor can activate a heating element.

I/O modules are physically connected to field devices, providing input data to the processor and sending commands to output devices. These modules can be analog or digital and can be mixed and matched as needed.

The external programming device, typically a desktop or laptop computer, is used to write and download the PLC program. There are various programming methods, including ladder logic, function block diagrams, structured text, instruction lists, and sequential flow charts.

Microcontroller Working Principle

Microcontrollers comprise a processor (CPU), data memory, program memory, input/output (I/O) control, and supporting circuitry. The I/O of the microcontroller receives data from the various inputs of a device, which is temporarily stored in memory. The processor analyzes this data based on pre-programmed instructions stored in the microcontroller's program memory to determine the appropriate response. The processor then uses the I/O to communicate the response and perform an output function.

PLC vs. Microcontroller: Interface Capability

PLCs can interface with a variety of components, such as sensors, actuators, HMIs, robotic components, and communication modules. They can communicate over various protocols, allowing programming from a single software platform. This versatility enables their use across many industries and equipment types. Microcontrollers may also include onboard sensors, actuators, and components similar to those controlled by PLCs. However, they are often tailored for specific uses, which can make linking them to PLCs challenging or impossible.

PLC vs. Microcontroller: I/O Compatibility

Once you've determined the required number of inputs and outputs, check if the microcontroller you're considering has a sufficient number of I/Os. It's important to consider the type of I/Os as well. While a microcontroller may have the right number of discrete and analog I/Os, there may be a different type for your application.

In some cases, you can convert them to match your needs. For example, you can convert a 4-20 mA current loop to a 0-5 V voltage loop relatively easily. However, if the microcontroller includes a more specific type of I/O, such as an analog output with pulse-width modulation (PWM), it may take more work to convert.

This is where a PLC can be advantageous. Designed to work with industrial sensors, most PLCs offer a wider range of I/O choices, reducing the need for external conversion. Additionally, most PLCs include built-in isolation for I/O points, which helps protect devices and circuits.

PLC vs. Microcontroller: Mounting

One of the most notable advantages of microcontrollers is their compact size. Their computational power and small footprint make them ideal for applications with limited space or where space-saving is crucial. However, other factors make PLCs a more suitable choice in certain scenarios. PLCs are specifically designed to be used in cabinets, providing additional protection from dust, moisture, and other harsh manufacturing conditions. They are essentially "plug-and-play" within the cabinet environment. In contrast, microcontrollers are often open-faced with pins for connection. While it is possible to connect them within a system manually, this process adds complexity, time, and labor.

PLC vs. Microcontroller: Operating Systems and Watchdogs

A microcontroller may come at a lower cost, but it comes with trade-offs. Due to its basic nature, users are responsible for programming every aspect beyond the most basic functionalities.

In most cases, this is manageable, as most microcontrollers operate within common programming environments like Linux and C.

On the other hand, a PLC also requires application programming, but additional processes are running behind the scenes that are not visible to the programmer or user. Housekeeping programs and watchdogs continuously monitor the systems and equipment to ensure safety and data integrity.

For example, consider a for/next loop within a PLC. If this loop encounters an issue and the program stalls, it could impact performance and pose a safety risk.

In such cases, the software watchdog activates, timing each scan of the program. If a scan exceeds the allocated time, an alert is triggered. This puts the PLC into safe mode and notifies the user.

Additionally, there are hardware watchdogs that monitor the PLC's connected devices. As the PLC communicates with its I/O modules or external devices like switches, sensors, or actuators, the watchdog keeps track of each scan.

If there's a delay in any scan count, it indicates an issue with the PLC. This usually prompts the system to enter safe mode and notifies the user.

While it's possible to add this functionality to a microcontroller, it could be more straightforward. Users would need to develop these programs from scratch or reuse existing software modules, both of which are complex tasks. Furthermore, extensive testing and verification of the software for the specific application are necessary, adding to the complexity and labor involved.

PLC vs. Microcontroller: Programming

PLCs can be programmed using a standard programming environment provided by PLC manufacturers. This allows for the use of common programming languages to ensure that devices operate together smoothly. These platforms also employ natural language algorithms, making PLC programming less skill-intensive. In contrast, programming a microcontroller to interact with other components, such as safety notifications and alerts, requires a more advanced skill set and involves more time and effort.

Programmable Components in Microcontroller and PLC

PLC vs. Microcontroller: Environment

As noted, microcontrollers typically have exposed pin connections, whereas PLCs are enclosed and designed to be plug-and-play. This design makes PLCs more durable in environments like manufacturing or warehouses where components must endure shock, temperature extremes, vibration, noise, moisture, dust, corrosion, and other harsh conditions. PLCs are specifically engineered to withstand these challenges, with many models ruggedized for extreme applications. In contrast, microcontrollers are less durable and more susceptible to these conditions, increasing the likelihood of failure in a shorter period.

PLC vs. Microcontroller: Support

PLCs undergo a generational evolution with support from the OEM. This means that as each PLC approaches the end of its lifecycle, a replacement is identified and produced. PLC OEMs invest significant effort in training and documenting migration parameters so system engineers can seamlessly upgrade to newer generations without system shutdowns.

On the other hand, microcontrollers are often more narrowly designed, and future generations may need to be more compatible as technology within their original industry advances. While I/Os and programming can be changed on a microcontroller, once this is done for a specific application, the microcontroller becomes unique. Changes at the end of life may lead to system failure if programming and modification are not available or if replacement units are not compatible. Additionally, microcontrollers need a more robust technical support ecosystem than PLCs have.

PLC vs. Microcontroller: Benefits

PLCs Benefits

PLCs have been extensively utilized since the 1970s for industrial control and automation systems, engineered to endure the rigors of modern factory environments. They manage the inputs and outputs of a programmed control system, making decisions based on data from sensors, switches, and other devices.

1. PLCs are purpose-built and rigorously tested to operate reliably in industrial settings, enduring shock, vibration, noise, corrosive substances, and extreme temperatures. Some models, like Siemens Simatic Plus PLCs, are ruggedized to withstand harsh conditions. Safety PLCs, such as those in Allen Bradley's ControlLogix and CompactLogix range, offer SIL2/PLd and SIL3/PLe safety levels for hazardous industries.
2. PLCs are designed for long-term use, with many running in facilities for over a decade. Manufacturers typically support PLCs for years, ensuring ease of maintenance and availability of replacement parts. If a PLC model is discontinued, clear migration paths to newer hardware are usually provided, avoiding the need for complete program rewrites. IEC61131 standards for all PLC programs ensure consistency across systems and simplify troubleshooting.
3. PLCs feature a wide range of I/O connections tailored for control systems, directly integrating with communication systems. Their programming environments are designed for interoperability between system components and third-party devices.

Microcontroller Benefits

Microcontrollers are adept at handling tasks with modest computing requirements. Yet, they can execute a wide array of functions, including mathematical calculations, logic and data processing, device control, and serial and wireless communications. Moreover, microcontrollers are typically small in size and cost-effective, with some models available for under a hundred dollars.

Functioning as single-board computers, microcontrollers are well-suited for applications with low computing demands, such as household appliances and small-scale automation tasks. This makes them popular in the maker and DIY community for various projects. Despite their modest size, microcontrollers can tackle complex mathematical calculations, process data and logic, and even control motion in certain devices. They also support wireless communication.

Overall, one of the major advantages of microcontrollers is their affordability. Compared to PLCs, microcontrollers are significantly cheaper. They are also highly compact, making them attractive for control solutions where space is a critical factor.

PLC vs. Microcontroller: Industrial Applications

When comparing PLCs and microcontrollers, it's important to consider their performance in specific industrial conditions. Here's a comparison of various environments:

Shock and Vibration

PLCs are designed to withstand the strong shock and vibration often present in warehouse environments. While microcontrollers can also function in such conditions, they require special mounting and connection considerations for durability.

Corrosion

In environments with corrosive vapors and fumes, wiring and components can corrode. PLCs are typically equipped with coatings and corrosion-resistant materials to mitigate these effects, whereas microcontrollers may lack such protection.

Noise

Industrial environments often have electronic noise and magnetic fields that can affect equipment. While a microcontroller may fault or lose its program under significant interference, PLCs are more robust and can withstand standard electronic noise.

Temperature Levels

Microcontrollers generally operate best in controlled temperature environments. Some ruggedized microcontrollers can handle wider temperature ranges, but PLCs can be installed in outdoor enclosures subjected to extreme temperatures with the right components.

Industry Standards for Testing

PLCs undergo rigorous testing according to standards set by organizations like the International Electrotechnical Commission (IEC) and Underwriters Laboratories (UL). Documentation accompanying PLC systems details the tests conducted and methodologies used. In contrast, microcontrollers often need more extensive testing, making it harder to assess their capabilities. Additionally, there are differences between generic and brand-specific microcontroller boards.

PLC vs. Microcontroller: What's the Difference?

A programmable logic controller (PLC) can be thought of as a larger, faster, and more dependable version of a microcontroller. The following is the PLC and microcontroller comparison table:

PLC vs. Microcontroller: What are the Similarities?

Microcontroller and PLC Code Execution

In Figure, Ladder Logic is illustrated with "rungs," akin to statements in C programming. These rungs execute sequentially from top to bottom, similar to C language statements. Both Ladder Logic and C firmware feature an infinite loop concept, allowing rungs or statements to repeat execution. Ladder Logic also allows configuring the number of times a rung executes within an infinite loop. The "Init" rungs in the PLC program above are set to execute only once, behaving like statements inside the Initialization block preceding the microcontroller program's infinite loop.

Next, we will explore how rungs comprise instructions that handle signals Input to the PLC and control signals Output from the PLC. We will demonstrate this using switches and a light bulb to illustrate a simple application interfacing with these PLC Inputs and Outputs. Additionally, we will introduce symbols representing common examples of these PLC instructions.

PLC vs. Microcontroller: Which one is Better?

Depending on your specific needs, budget, and program expectations, you may consider using either a microcontroller or a PLC.

It's important to note that while microcontrollers offer cost savings, they are more limited compared to PLCs. This means that if you require a system for a larger industrial network, more than a microcontroller may be required. However, for simpler projects like DIY endeavors or educational tools, a microcontroller could be ideal.

So, can a microcontroller replace a PLC? In very specific circumstances, such as simple control needs in a low-demand environment, the answer is yes. However, the distinctions mentioned above emphasize that PLCs are specifically designed for industrial and high-volume service applications, which demand features like safety, redundancy, and ease of programming. These are aspects that microcontrollers generally cannot match. Therefore, in most cases, PLCs remain the superior choice for industrial automation, control systems, robotic manufacturing, and motion control applications.

Why PLCs are Better?

1. User-Friendly: PLCs are easy to learn and use, with a short learning curve. They come with comprehensive user manuals and require minimal electronics knowledge compared to microcontrollers.
2. Ease of Programming: PLC programming is graphical and easier to understand, even for non-technical staff. Microcontrollers, on the other hand, often use lower-level languages like C, which can be more complex.
3. Modular Design: PLCs offer modular designs, allowing for easy expansion and troubleshooting. Microcontrollers are limited in I/O units and need to be more scalable.
4. Safety Systems: PLCs, especially safety PLCs, come with integrated safety functions that ensure maximum uptime and safety in case of failures.
5. Robustness: PLCs are designed to withstand industrial conditions, including electromagnetic interference, high temperatures, and power fluctuations, making them more reliable than microcontrollers.
6. Maintenance: PLCs require low maintenance due to their robust design and standardized programming.
7. Historical Inertia: PLCs have a proven track record of reliability and compatibility, making them a preferred choice for many industries.
8. Cost Efficiency: While PLCs may have higher upfront costs, their lower maintenance costs and longevity make them more cost-effective in the long run compared to microcontrollers.
9. Standardization: PLCs follow IEC61131 standards, ensuring standardization and simplifying troubleshooting and programming.

Therefore, while microcontrollers have their advantages, PLCs remain the preferred choice for industrial applications due to their reliability, durability, and comprehensive features.

When to Choose a Microcontroller over a PLC?

In many cases, a custom-embedded system is preferred over a ready-made controller. This is especially true in two key scenarios:

• Unique requirements: Some industrial processes need features that off-the-shelf controllers don't provide. In these situations, a custom solution with a microcontroller is often more practical.
• High volume needs: PLCs can be expensive, making them less ideal for applications with lower deployment volumes. While developing with microcontrollers may initially cost more than using PLCs, the overall expense of a custom solution can be significantly lower for high-volume deployments. This is because the hardware cost per unit can be as low as one-tenth the price of a PLC.

Conclusion

Microcontrollers and other development boards serve as excellent educational aids and for experimental purposes. They are cost-effective and simplify the learning process for programming and automation concepts. They are wonderful tools if you have the time to invest.

On the contrary, when it comes to ensuring the effective, efficient, and safe operation of manufacturing processes, PLCs offer a wide range of capabilities with a proven track record of reliability over decades. In manufacturing environments where continuous operation is crucial, reliability and safety take precedence over other considerations.

In summary, PLCs remain the preferred choice for industrial automation and control systems. While microcontrollers have their place in specialized systems, they need to be equipped to handle the demands and challenges faced by PLCs in industrial settings. PLCs are favored for their simplicity, cost-effectiveness, and robustness in industrial control applications.

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