A Faye-ry try hard blog

How to PLC

Posted on 02 August, 2021

Here is a short little guide I have put up for when you find yourself asking these questions:

“What should I use?”  “What’s are available?” “Why programmable controllers? Are there alternative?”

Why? Because #beentheredonethatbutwithcountlessoftabsidontunderstand

Assuming you do know what a Programmable Logic Controller (PLC) is and not only look at it as a box with many holes, or its documentations as papers with many scribbles, symbols and lines, read on! Else, please refer to here and then this.

PLC goes a long way back, from the very first model Modicon 084 (Or known as Modular Digital Controller 084) emerge to eliminate the large cost involved in replacing the complicated relay-based machine control systems. Change was expensive, and it is still (relatively) expensive, especially if its frequent, and that’s why I cannot empathise enough how important it is to refine and identify the right processes to automate. Still confused? Read here and here.

And now back to where we left off, PLC technologies are so advance now with lasers, industrial robots, IOT and automatic identification systems. PLC are used in so MANY areas where there are activities that require high control, process fault analysis and ease of programming. PLCs are so highly reliable and standardized languages have been set (IEC 61131-3 *accurate as to time of the blog post, please follow PLCopen to stay ahead)

In any automation system design: cost, functionality, space and interoperability is always the top few matrix of consideration.  While PLCs are often compared to Microcontrollers which you might be interested to read about here or here.

Tldr; In short, if you are looking for something to interact with an ecosystem of brands/scalability/reliable/require integrated safety with not much budget constraints, or something that is simply extremely rugged and robust, PLC is the way to go- these guys can operate in very harsh environments and 24/7/365 without fail for at LEAST a good 10 years. Else, Microcontrollers are great if you are only looking at simple control architecture, mobility and maximum cost effectiveness, many IoT (Internet-Of-Things) works are also done on Arduino boards.

Okaaaaay, so what do I look out for when it comes to choosing a PLC that’s suitable for my application/ system?

1. Start with an initial sequence of operations. As in what does this thing have to do. Remember to take scalability in consideration too.

2. And develop an initial I/O list.

3. Develop a list of field devices as part of your I/O list.

4. Determine your communication protocols (e.g., DeviceNet, ControlNet and Ethernet IP as well as DH+ and DH485, Profibus, ASI, MODBUS or MODBUS TCP/IP)

5. Know your budget! Evaluate what kind of processing speed/response you need or critical parameters to be controlled. 6. Choose a PLC and required I/O.

7. And decide what your user interface is going to be and the language you are most comfortable in programming. (Don’t worry, even the trusty Ladder Diagram (LD) can support many functionality and is used in about 70% of automation projects today) This will probably play a part in your software selection as well.

8. Figure out what you need for safety configuration. With moving parts you'll likely need some guards as well as Estops - and a safety relay.

9. Review your power requirements.

10. Figure out what kind of a box you're going to need to put all this stuff in. Does it need to be watertight? Does it need to be cooled? Will you be working with chemicals? What kind of space is available for a control cabinet.

11. Once you have a good general idea of what hardware you'll be using, start a backpanel layout.

12. As you go through this process you can identify long lead time items, and low-cost items - and start ordering parts as you go.

Some other tips and tricks i can think off at the top of my head would be:

1. Always good to have a ~15% buffer capacity (there’s are always add-ons and improvements!)

2. Modularity! Instead of tackling the system as one whole, think of how you can break them apart into smaller puzzle pieces to fit.

3. PROFIBUS or serial communications can be good for brownfield projects but with the evolving technologies, PROFINET or parallel communications are encouraged in greenfield projects

4. Lifecycle- hardware failure can manifest to total failure and potentially cause the company to be making huge losses

5. Follow the tried and tested coding methods are structural techniques! Try not to create overly complex code! Look up Reusable codes! PLC is an established market, there are many readily available resources online in IEC-compliant PLC languages to help make platforms much more powerful, consistent, and enabling “rapid-development”.

6. Anticipate issues and create fail-safes. This is help greatly in preventing human errors and overrides to critical system functions

7. Unit testing!! A good practice is to regularly check your variable values, functions, blocks or code segments. This could help you spot potential errors or just tiny typos that could potentially screw up your whole systems.

Some other things to look out for/ good practice:

I hope this guide has been helpful and if I welcome any feedbacks or discussion on nhyfaye@hotmail.com 😊