PLC vs. programmable relay

Can a simple application still benefit from a programmable logic controller, or is a programmable relay all it needs?

By Mike Bacidore, editor in chief

A Control Design reader writes: I have a small test system that I want to put error proofing on. The sensors are actuated in a very simple extend-and-retract sequence. The application only has four inputs and three outputs. Because it’s such a simple application, how do I decide whether to use a PLC or a programmable relay?


Expand the comments for responses from our panel of experts.

REAL ANSWERS: What should you use — PLC or programmable relay?

Mini-PLCs—more than just a smart relay

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  • <p>PLCs will allow your process expansion capability that a programmable relay cannot. If you believe that your process will never need more than four inputs and three outputs a programmable relay is just fine and much cheaper. However, if you think you may be expanding this machine to include more than just this process, then you may want to consider a PLC sized to meet your current and possible near expansion needs.</p>


  • <p>Today’s programmable relays offer a significant cost reduction in signal to relay movement configuration. No training is required. Many such industrial programmable relays provide upwards of four relay outputs corresponding to input analog signal thresholds. These decisions also include timing delays and ‘dead zones’ and even include a real time display of signals. Thus consider the simplicity and cost of programmable relays should the number of variables of your test allows. If more complex logic is required, then the next step up in cost and setup complexity is the PLC.</p> <p>Jim McConahay, PE, senior field applications engineer, Moore Industries International,</p>


  • <p>Modern programmable relays offer multiple communication possibilities like MODBUS RS485 or TCP/IP and are expandable. To draw a line between PLCs and programmable relays getting harder. You even get analog inputs as standard. There are different vendors which offer advanced programmable relays. Programming is done by powerful easy to use function blocks.</p>


  • <p>I would also add previous knowledge to your decision. Since you are not producing hundred's of these systems, the cost and complexity is not just hardware, but time to implement the solution. This includes installing software and learning to programing the device. If you or your facility has already used a basic PLC for test stations 1,2 and 3, then your best choice could be to use the same hardware - you already have the programming tools, implementation knowledge, etc. Of course, if there are no previous systems - this does not apply. PS - don't forget about data logging of test results! Is it needed???</p>


  • <p>With simple applications, a PLC is often the best solution. A PLC offers a superior experience to an end user. With a wide variety of HMI options including color touch displays, a PLC can offer user interaction that a programmable relay cannot. Advanced troubleshooting options via the PC based software reduces development time. Data logging, communications, and other advanced features may enhance the system. With a PLC, there is always room for system expansion and to allow you to bring your application into the 21st century. </p>


  • <p>For this application, we’d definitely recommend a programmable relay as current models of these products can easily accommodate your application needs. For example, our model FL1F SmartRelay can handle up to 24 discrete inputs, 20 discrete outputs, 8 analog inputs and 8 analog outputs. This is more than sufficient to meet your current needs, and can also handle future additions or modifications. While a PLC would certainly work, it would be much more expensive in terms of not only hardware, but also required programming software. With programmable relays, the program can be entered from the built-in front panel display. Alternately, the program can be created on a PC with programming software that’s typically free, and then downloaded to the relay. Another cost savings will be realized in terms of the operator interface. Most PLCs don’t include a built-in, integral operator interface, so one has to be purchased, programmed and integrated with the PLC. By contrast, most smart relays have a built-in front panel display, which should be more than sufficient to meet the needs of your application. </p>


  • The number of I/O is one consideration, but it's not the only one. Clearly, the I/O count you are dealing with is well within reach of even the smallest smart relays and PLCs. Consider the features available in the programming software, serial communication options, connectivity to HMIs, available programming languages, physical size, expandability and price. Micro/Pico PLCs and Smart Relays continue to become more capable. Most automation suppliers offer both Smart Relays and small PLCs. One way to think of the differences between Smart Relays and PLCs is their targeted applications. Generally, smart relays are a great way to replace and upgrade from traditional control panels that previously were built using dozens of relays, timers and counters and complex wiring with a single programmable control device. Small PLCs offer more complex programming and communication capability and are very good at performing machine automation. With both smart relays and micro PLCs you can download free or trial software. This will let you see what's possible, run simulations and make your choice before purchasing the hardware.


  • Newer technology has allowed us to introduce smaller and less expensive PLCs that definitely cross into the Programmable Relay category. The CLICK PLC from AutomationDirect has only 21 instructions for simple to moderate applications and is very simple to program. With a beginning price tag of $69 for 8 digital inputs and 6 digital outputs, and expandable up to 142 I/O, you will be hard pressed to find a programmable relay with greater functionality at a lower price and easier to use. The various built-in communications options make connecting a low cost HMI a snap. In general, a PLC will give you a more scalable product line for better initial selection of options, as well as room for future expansion. With a little homework you can find a PLC or a Programmable Relay that will accomplish the job at a reasonable price, the question is whether or not you want a solution for now or a solution that will benefit you in the future. – Jeff Payne, Automation Controls Group,


  • For this specific case, I would not recommend the purchase of a standard PLC as it would be overkill for this application. A programmable relay would be a better solution because of the sequence simplicity and limited I/Os. TURCK is also introducing I/O block technology called ARGEE, which enables users to program the device through an HTML5-enabled browser simply by using the device’s IP address. A built-in HMI interface allows ARGEE to communicate back and forth with a user’s browser-enabled device. The ease of this programming environment allows for simple programming of these divides in a minimal amount of time, which could be well-suited for your application. ARGEE can also execute features such as assigning of variable names, assigning of register values, timers, counters and math functionality, and even communicating to an upper level PLC. Noah Glenn, TURCK Product Manager – Fieldbus Technology


  • Smart relays are usually all-inclusive of power supply, I/O, and a simple display in a small and affordable footprint. Often you can program it from the built-in display or through a free software package. If the project scope were to grow in the future, Mitsubishi Electric Automation’s Alpha2 smart relays even offer expansion options in the form of communication boards or additional digital and analog I/O. All of these features make the smart relay a suitable solution for the simple application being discussed here. But, besides accomplishing the application at hand, are there other pain points or goals to address? For instance, by using the FX3S programmable controller, you would benefit from more customizable programming, robust diagnostics, and advanced security features. Do you have other applications that are already using PLCs? If so, would it be beneficial to standardize on one control platform and reduce learning curve and spare part inventory? – Deana Fu, Product Manager, Mitsubishi Electric Automation


  • With PLCs there is a commitment to learning the programming software. The PLC hardware is expensive and probably overkill for this small application. Micro controllers are limited to the specific input and output parameters of each controller. These micro controllers could be equipped with integrated relays, with fixed voltage and current ratings. The relays are soldered on to the board and cannot be replaced. Therefore, if the relays fail, the system has to be shut down and the controller has to be replaced. PLC Logic, is a smart relay system developed by Phoenix Contact to help solve the problems listed above. PLC Logic modules are inserted directly into the contacts of our slim 6mm wide PLC relays. The relays are pluggable and can be replaced if needed, without having to shut the system down. Each of the relay channels can be configured as an input or an output, from 16 I/Os up to 48 I/Os, with the use of expansion modules. PLC Logic is programmed using LOGIC+ software. The software is free and only requires a micro USB cable for programming. LOGIC+ is easy to use, drag and drop, software which uses common function block programming. PLC Logic was made for simple standalone automation applications. Traci Bretz Relay Product Marketing Specialist Phoenix Contact


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