Introduction to PLCs and Arduino
Programmable Logic Controllers (PLCs) have been the backbone of industrial automation systems since the 1970s. Modern PLCs provide robust, reliable, and scalable process control capabilities. However, at price points starting in the several hundred dollar range, commercial PLC solutions remain cost-prohibitive for smaller scale systems.
Arduino emerged in 2005 as an open-source microcontroller platform aimed at non-engineers. Hobbyists quickly realized Arduino‘s potential for basic automation and interfacing capabilities at a mere fraction of the cost of PLCs.
This guide offers an in-depth examination of Arduino‘s suitability as an alternative to commercial PLCs for automation professionals and system integrators.
Detailed Technical Comparison
While Arduinos share conceptual similarities with PLCs, even top-of-the-line Arduino boards pale in capabilities compared to entry-level commercial PLCs.
Processing and Memory
The flagship Arduino Mega2560 utilizes an 8-bit ATmega2560 MCU operating at 16MHz with 256KB flash storage and 8KB SRAM. In contrast, compact PLCs like the Allen-Bradley Micro800 contain 32-bit ARM processors exceeding 100MHz with MBs of memory.
Table 1 – Comparison of Arduino Mega vs. Allen-Bradley Micro810 PLC
| Specification | Arduino Mega2560 | Allen-Bradley Micro810 |
|---|---|---|
| CPU Type | 8-bit AVR | 32-bit ARM9 |
| Clock Speed | 16 MHz | 100 MHz |
| Flash Memory | 256 KB | 3 MB |
| RAM | 8 KB | 20 KB |
The considerably faster processor and extra memory of the PLC allows for more sophisticated and optimized logic and HMI capabilities.
Available I/O
The Arduino Mega itself can interface with only 70 GPIO pins without additional hardware. Basic PLCs offer upwards of a dozen modular expansion I/O cards supporting hundreds to thousands of I/O in various configurations:
Table 2 – I/O Capabilities Comparison
| Device | Digital I/O | Analog I/O | Specialty I/O |
|---|---|---|---|
| Arduino Mega | 70 pins | 16 pins | Minimal |
| Allen-Bradley Micro810 | 1440 points | 180 points | Hundred modules available including high speed counters, thermocouples, RTD, etc. |
This extensive and flexible I/O allows PLCs to collect data and directly control a wider array of sensors, actuators, drives, and other equipment than possible with an Arduino alone.
Communication and Interoperability
PLCs support various industrial communication protocols (EtherNet/IP, Modbus TCP/RTU, etc.) enabling seamless data exchange in smart factories. An Arduino CAN typically interface with common protocols using addon shields. But implementations lack many advanced features for managing complex networks.
Likewise, PLCs readily integrate with SCADA, MES, and ERP systems. Interfacing an Arduino with higher level systems poses custom integration challenges.
Programming Experience Comparison
Arduinos use a C/C++ based Integrated Development Environment (IDE) that allows writing logic via familiar general purpose programming constructs. PLCs instead rely on ladder logic – a language resembling electrical control diagrams.
Ladder logic builds on the concepts electricians are familiar with. But those without an electrical background often find software-based PLC programming more intuitive.
The sequential nature of ladder logic programs also makes the order and dependencies between rungs unambiguous. Complex Arduino code can be harder to structurally organize and debug.
Certain techniques used in software programming like interrupts and overrides can wreak havoc on timing in deterministic automation systems. PLC languages simplify logic development avoiding such issues altogether.
Applications Well-Suited for Arduino PLCs
For non-critical processes with minimal I/O needs, Arduino PLCs present a compelling lower-cost system. Example applications include:
Academic/Training Test Rigs
- Low/no risk if control failures occur
- Tight budgets
- Require flexibility to readily incorporate sensors, displays, etc.
Monitoring and Supervisory Roles
- Redundancy for PLCs
- Light collecting data to send upstream
- Alerting operators of issues
Single Purpose Small-Scale Machine Control
- Self-contained systems
- Less than 20 I/O
- Process equipment costs dwarf controls expenses
Rapid Prototyping and Pilot Projects
- Business and technical unknowns necessitate flexibility
- Minimal standards and integration requirements
- Primary goal is demonstrating basic functionality
Specialized Arduino PLC Products
Various manufacturers now produce Arduino form factor boards tailored specifically as low-cost PLC replacements:
Controllino PLCs
The Controllino Mini through Maxi lineup packs Arduino Leonard or Mega cores into compact, DIN rail mountable enclosures with screw terminal I/O blocks familiar to commercial PLCs. Available discrete, analog, and special function I/O ranges from 13 points on the Mini up to over 500 points on the Maxi. The Controllino Mega at under $200 with ruggedized base hardware represents an economical intermediate between DIY microcontrollers and commercial PLCs for light industrial control tasks.
Industrial Shields
This company offers stackable shields adding 4-20mA, relays interfaces, Ethernet, and other enhancements to Arduino Uno, Leonardo, and Mega development boards. For example, the 14 point all-relay IRYD shield combined with the PoE enabled ESP210 Ethernet and WiFi shield creates budget PLC server. With modules costing between $30-$100 each, buyers can scale capabilities while still saving considerably over a traditional PLC.
PLCFeatures Board
The PLCFeatures board interfaces directly with 24V systems human-machine interfaces (HMIs) commonly used with industrial PLCs without any additional power supplies or adapters needed. Onboard isolated 12 and 24V DC power supplies replace wall warts otherwise needed for standalone Arduino boards. The PLCFeatures board starts under $90, costs comparable a commercial PLC CPU alone before adding any I/O modules.
Enhancing Reliability for Industrial Usage
While lacking the rigorous testing and component ratings for harsh plant conditions, several tactics help boost Arduino survival:
Strategic Placement
Avoiding high vibration areas and locating boards in electrical enclosures away from EM/RF noise sources reduces stresses.
Conformal Coating
Applying protective plastics coatings on the boards adds moisture and corrosion resistance.
Filtered Power Supplies
Power line conditioners and ferrite chokes filter noise that could disrupt microcontroller operation.
DIP Duplication
Using two microcontrollers running duplicate programs with cross-checking allows system self-diagnostics.
Watchdog Timers
External clock monitors detect lockups forcing controller resets if normal logic scan cycles exceed thresholds.
Cost Analysis
A Controllino Mega with 16 discrete inputs, 16 discrete outputs, 2 analog inputs, and an Ethernet port carries a list price around $160. An Allen-Bradley Micro810 PLC with equivalent base capabilities would cost over $730 just for the CPU and expansion I/O modules.
Factoring in average hourly rates for integration labor, highly affordable Arduino PLCs can easily save thousands of dollars in smaller automation system costs. Within limitations, Arduino PLCs economically address applications considered previously out of reach budget-wise for programmable control.
Challenges Replacing Traditional PLCs
Attempting to substitute Arduino solutions into larger sites with existing stand PLC standards poses difficulties including:
IT Security Policies
Lack of audit controls, access management, and patching hinders authorization for plant floor deployment.
Data Historian Integration
Inconsistent data formats complicate aggregating for trend analysis and other continuous improvement use cases.
Supporting Infrastructure
Unfamiliar tools to maintenance staff and absence of PLC vendor support channels increase downtime risks.
Regulatory Compliances
Validation documentation and rigorous testing required by sectors such as pharma may be impractical.
Forward thinking operations teams however recognize smartly incorporating Arduino class technologies in appropriate situations stands to provide value.
The Future of the Programmable Logic Controller
Once dominated completely by a handful of vendors, PLC manufacturers now must contend with rising open source hardware competitors. Following the path blazed by Linux and Raspberry Pi in IT circles, Arduinos represent the leading edge of similar democratization in industrial controls.
Enhancements like machine learning optimized neural network libraries for logic design combined with IoT connectivity position Arduinos as legitimate programmable automaton alternatives. Technology commentators foresee embedded electronics disrupting traditional vendors over the next decade like personal computers upended mainframes.
Adjustment remains ongoing as promising Arduino PLC ports face market headwinds related to general industry unfamiliarity and inertia around updating legacy systems. Nevertheless, the long term outlook points to open-source solutions claiming significant automation market share.
Conclusion
Arduino microcontrollers originally intended strictly for prototyping now seriously challenge perceptions around capabilities for direct industrial control implementations. Purpose built Arduino PLC offerings guarantee continued blurring between commercial and hobbyist automation. While full replacement of traditional PLCs remains unrealistic in most situations, Arduinos clearly demonstrate vulnerabilities in customer value propositions. Savvy technical investors look towards emerging third party ecosystems as bellwethers for potentially reimagining automation infrastructure strategies.
