How To Specify a PLC
The controller is the core of any automated system, and must be able to give a reliable performance for many years. You don't want to be looking at an expensive upgrade because your PLC was under-specified and the system performance compromised. Nor do you want to over-specify the project requirements, and end up with lots of unnecessary features. Don't commission your PLC too soon in the system design process, but wait until you have a more precise idea of what it will need to do.
When deciding how to specify a PLC, these five tips will ensure that you always get exactly the controller you need, customised to the exact requirements of your application. In this way, you'll get the maximum performance return on your investment.
It will help to consult with experts before deciding on any particular make, model or platform to ensure that the system will perform to expectations.
Obviously, the capacity of your PLC must be dictated by what you need it to do, so you need to begin with an assessment of how many I/O connections you'll require. Modern PLCs vary widely, from the micro PLC with a maximum of 300 I/O, to the large modular devices that handle thousands. If you need only a low I/O count, then you'll get exceptional performance from a micro system, but unless you're running a complex operation, a mid-range compact device might suit you better.
Today's modular PLCs combine familiar programming with many more advanced options which can provide an ideal solution. These include extensive combinations of function sets and I/O counts which can be customised to your individual application. Bear in mind also that you may wish to add elements to your system in the future, so you should allow some capacity for expansion.
The most significant expenditure in system development nowadays is no longer hardware, but programming time. Designers need to develop a highly functional system in the least possible time, and that requires both hardware and software to be highly flexible. You'll want to be able to move seamlessly between a variety of programming languages, whether this is from one design project to another, or within a single project. You'll also want programming tools that allow you to design a system in a virtual environment, and pre-commission it before committing to any expensive physical commissioning time. Many of the best programming tools for PLCs now include powerful pre-coded function blocks, which will save you further time in development.
The performance capability of your PLC relates directly to your application, with more complex processes like packaging requiring a much higher-speed processor. There may also be connectivity and motion control options, and even simpler operations like temperature control can require the PLC to monitor or control numerous analogue I/O.
Calculate your instruction set to ensure that your PLC has sufficient power, perhaps including PID control and maths, as well as all the sensory control options now available. Don't cut it too close, either: you must be sure of having enough juice to run your applications, as well as any potential future upgrades.
Connectivity is one of the most important aspects of knowing how to specify a PLC, and your requirements must be carefully assessed. PLCs used to be designed as stand-alone systems and you may still want to do this, perhaps because it would be cheaper or more secure. Even compact PLCs, though, can now provide improved connectivity to a substantial number of open and proprietary network protocols. With sufficient connectivity you'll have the additional benefits of remote access, which provides monitoring flexibility and can vastly improve maintenance. You'll also be able to gather data sets which can help optimise system performance and reduce downtime.
Your PLC will be connected to the internet for many important monitoring and control processes, but this does leave you open to the threat of cyber attacks. PLC code, classified as intellectual property, is also being cloned and pirated by unauthorised users, so you need to be absolutely sure about the security of your suppliers. They should offer robust security, both within the PLC itself and within its programming tools, as well as providing fast and reliable customer support and a substantial warranty.
If you follow these golden rules on how to specify a PLC, you'll be able to avoid many of the common pitfalls in configuring your system.
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