What Programmable Logic Controllers (PLCs) Can & Can’t Do

Programmable logic controllers (PLCs) are valuable tools for warehouse automation. They help improve the operational efficiency of a warehouse and ensure everything is timed correctly so the right item arrives at the right location at the right time.

Since PLCs are computers, they can accomplish a large variety of tasks that can increase your company’s efficiency. However, different experts will have varying opinions on the proper use of PLCs, depending on their position and expertise.

Ultimately, the question warehouse managers should ask isn’t what a PLC can’t do, but what a PLC should or shouldn’t be used for—at least not without additional modules or interfacing with software.

Not sure which strategy makes the most sense for your operation? Book a meeting with a member of the Conveyco Team at Modex!

What are PLCs?

PLCs have come a long way from their humble beginnings. They were initially created to address the problems that arose from using relay-powered machines. These systems used an abundance of wires and took up lots of space. Furthermore, they were difficult to hard-wire, and any changes that needed to be made necessitated rewiring the entire system.

PLCs were a response to this situation. They accomplished the same task, but provided a much easier method. They’re modular devices that don’t require extensive wiring, which saves space and makes them much easier to program.

In simpler terms, PLCs take inputs and convert them into outputs. They have three primary components: the input, CPU, and output. Their function in a warehouse setting is monitoring inputs from machines in a warehouse and transferring the data to an interface. The interface processes this data using logic such as ladder logic or other programming languages, then outputs the data to any devices connected to the PLC.

Consider how a PLC is used alongside a Pick and Pass conveyor. An automated device scans a container’s barcode and sends that data to a PLC as an input. The PLC will then process that barcode and transfer it to a Warehouse Execution System (WES). This software responds by directing the container to the proper picking zone.

What is Warehouse Software?

Order fulfillment centers need key pieces of software to operate smoothly, particularly when automation is involved. Most warehouses and distribution centers need a warehouse execution system (WES), warehouse management system (WMS) and enterprise resource planning (ERP) software to run efficiently.

There are multiple levels of software, but here are the four most common.

1. Enterprise Resource Planning (ERP) Software
2. Warehouse Management System (WMS)
3. Warehouse Controls System (WCS)
4. Warehouse Execution System (WES)

Historically, automated warehouses have utilized ERP, WMS, and WCS software. Over time, however, WES software began to take the place of WCS software, as it incorporated the functionality of the latter as well as additional features such as labor and order management capabilities.

PLCs vs. WES Software

PLCs Strengths

PLCs are phenomenal real-time devices. They run the motors on conveyors, receive inputs from scanners, time how long it takes before a divert needs triggering, and more. Any function that requires running inputs through a set of rules to determine whether they need to be changed to outputs can be effectively accomplished by a PLC. These inputs can include scanners, physical buttons and switches, and more.

PLCs Weaknesses

While PLCs were initially created to reduce the risk of reworking a system built entirely on wires, at its core, a PLC is a real-time device that translates inputs into outputs. They’re powerful computers with extensive functionality that gives them the ability to accomplish a variety of tasks.

However, when they aren’t integrated with software and other forms of warehouse control, PLCs run the risk of causing the exact situation they were created to avoid. Using a PLC to complete tasks better suited for WCS and WES software can lead to time-consuming and expensive errors. In some cases it can even necessitate a rework of the system.

Here are some things PLCs can do, but would be more effective when used in conjunction with WES software.

• Print and Apply (PandA) applications: PLCs can definitely be used in PandA applications, but require WES software to ensure everything runs smoothly. Ladder logic is unlikely to effectively accomplish this task on its own.
• Tracking historical data: While there are modules you can add to make this possible, PLCs are better suited for real time applications.
• Speak to other systems within the warehouse: PLCs typically speak one of 5 languages. Without a software interface to translate that language, your PLCs will have difficulty communicating with other systems within the warehouse.

Software Strengths

While PLCs shouldn’t replace software in a warehouse, they have the ability to integrate to maximize efficiency. For example, PLCs that work in tandem with WES software like Horizon WES can mitigate the risk of programming disasters and achieve the best of both worlds.

Integrations of WES software with PLCs aids communication with the hardware and ensures it effectively completes its tasks. Consider the following sequence of events

• The barcode on a carton gets scanned and sent to a PLC
• The PLC detects it’s a carton and interfaces with WES software
• The software tells the PLC where to direct the carton
• The PLC directs it there by triggering the right diverts at the right time

As demonstrated above, WES software can function as a bridge between PLCs and other systems or hardware in a warehouse. It ensures everything is performing properly and each system is effectively communicating with one another.

Another benefit of WES software is its ability to test ahead of implementation. If a warehouse relies on hardware like PLCs to complete a function that software would be better suited for, it won’t have the ability to emulate the implementation ahead of time.

WES Software like Horizon WES, however, has the unique ability to test the software (emulate) using real world conditions before it goes live. This emulation helps ensure the software is ready to run on day one of implementation, which significantly reduces risk of failure, lowers costs, and decreases installation time.

Software Weaknesses

While WES software integrates with PLCs very effectively, some forms of software that are intended to replace PLCs aren’t as effective. Replacing PLC hardware with PC-based controls software is likely to encounter some issues. While each have their advantages and disadvantages, PC-based controls fall short of PLCs in several ways.

• Computers often need updates to their operating systems. These can cause issues with PC controls, and can sometimes break them.
• The hardware life cycle of a PC is short. They typically need replacement parts within 3-5 years, whereas PLCs often have a shelf life of 15-20 years.
• PLCs are sturdier when it comes to weathering a brownout, as they typically have larger capacitors than PCs. While there are ways of remedying this for a PC, PLCs have a more robust infrastructure to better handle these circumstances.

It’s important to note that every warehouse is different and there may be some situations in which PC-based controls have the edge over PLCs. However, generally speaking, in the same way as you shouldn’t use PLCs to do the job of software, it’s not recommended to use software to do jobs intended for PLCs. Again, it’s not about each component’s technical ability, but rather what reduces risk and optimizes efficiencies.

How To Determine Which to Use

Ultimately, you want to consider the best use case of PLCs rather than what they can or cannot do. A highly trained engineer can generally successfully adapt PLCs to a variety of functions outside its original scope depending on your goals. Consider which option best optimizes efficiency and reduces risk. Here are some questions you should ask when evaluating whether software or PLCs should be used in a given situation:

1. Was the PLC designed for this purpose? Does this situation align with the fundamental purpose of a PLC, or does it push the hardware to the outer limits of what it was designed to do?
2. What offers the least risk? At the end of the day, it’s important to do what you can to reduce the risk of system failure to the best of your ability.
3. Do you need to track historical data? PLCs have the capability to track historical data and inventory, but are much better suited as real-time devices. Integrating the hardware with software is a better way to track inventory.
4. What other systems does the PLC have to talk to? PLCs can be adapted to talk to a variety of systems, and the right software can help. Fundamentally, however, the more complex a situation, the harder it will be for a PLC to resolve the situation without adaptors or software integration. This is because most PLCs use ladder logic, which is well-suited for Boolean tasks. It’s less suited, however, for more complicated implementations involving analog sensors, feedback loops, or PIDs.

Choosing the Right Solution For Your Warehouse

At the end of the day, there’s no “one size fits all” approach to warehouse automation. What works well for one fulfillment center doesn’t necessarily work for another. Finding a warehouse integrator who can customize a solution that fits your needs is crucial for your success.

If you’re interested in learning more about how implementing and modifying systems using PLCs and/or WES software can benefit your facility, or how a warehouse systems integrator can benefit you, schedule a consultation now.