Selecting the best PLC is a complex process
Selecting the “best” PLC is not a trivial decision as the PLC has a complex lifecycle with an operational lifetime measured in decades. Alos, there are many people who will interact with the PLC and associated hardware/software. For example:
- the system integrator or machine designer will select a PLC
- engineers, programmers or technicians will code the PLC
- technicians and engineers will commission the PLC-based equipment
- technicians will service the equipment
- engineers, programmers or technicians will modify the equipment and software
- engineers, programmers or technicians may add a remote interface or industry 4.0 data sharing capabilities
- engineers, programmers or technicians may replace the PLC in the distant future.
The number of people involved with this process varies by the complexity of the system. Sometimes a single skilled person will perform all tasks within a factory. The most complex system will have a contracted team working with in-house personnel to design, implement, and maintain PLC based systems. The selection process is further complicated when we consider turnover and promotions. It is unlikely that the team will remain unchanged over the decade’s long lifecycle of the PLC.
What are the soft requirements for PLC selection?
PLC and associated components are used in industrial control of automation processes. System downtime an ever-present concern as the cost can range from hundreds to thousands of dollars per minute in idle workforce, wasted product, loss of goodwill, and overtime to boost product numbers. In other words, the initial cost of the PLC is insignificant when compared to the associated lifecycle cost.
Selecting the best PLC is a holistic process that involves the hard PLC requirements along with many in-house considerations. Here are a few of those soft requirements:
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Some factories will select a single PLC or family of PLC for use on the factory floor. This is beneficial as technicians can focus on a single PLC and associated software package. Logistics is also simplified as the PLC family will likely share many accessories such as a plug-in or expansion modules.
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Software is one of the most contentious arguments surrounding the PLC and PCL selection. On one side we find purists who insist that all programs be written in ladder logic. On the other we find people who will write in the latest python, C++, Node-RED, MATLAB etc., and then use docker to push the changes to the production system. Both are correct, provided they account for the skill and knowledge level of their team with the understanding of the long lifecycle and the high cost system downtime. DigiKey has solutions to fit both camps.
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Future expansion is an important consideration for PLC selection. It’s not unusual to expand the PLC to include advanced control modes, additional sensors/actuators, or to add networked remote-control and data monitoring capabilities. This soft requirement implies that a PLC should be over specified in terms of computational power and memory. It should also have ample resources for expanded hardware and networking capabilities.
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Modifications of PLC-based equipment carries a high cost. This is a reflection of the highly specialized nature of the work as the programmer must understand the PLC software and the process/machine that is being controlled. This includes all of the corner cases such involved with startup, shutdown, and error handling. As a result, manufacturers are hesitant to change PLCs for operational equipment. While they may think nothing about changing a workstation every few years, the PLC will likely remain unchanged for the decades’ long life of the machine.
The natural implication is that PLCs tend to be selected on popularity as a hedge against future obsolescence. This includes an expectation that the PLC and associated parts will be available into the next decade. It also includes an expectation that a thriving PLC repair industry will be available to service the equipment. For example, the Rockwell Automation introduced the SLC 500 over 30 years ago and then discontinued it about 10 years ago. Second hand or repaired modules such as the venerable 1746-0V16 (16 semiconductor output) are likely to be available for another century to support the many SLC500 still in service.
- The final soft specification is a recognition of support offered by manufacturers, trade schools, engineering schools, textbook publishers, and the larger community. This includes contributions to social media in the form of how-to videos. PLCs with a larger support system are likely to be more popular.
TL:DR; Selecting a PLC is a complex undertaking. There are many different soft specifications beyond the hard specification contained in the next section. The final selection is a holistic decision that considers the PLC specifications and the PLC’s lifecycle in terms of who will use the PLC in the future.
Looking forward to continuing the conversation on this forum.
Best wishes,
Aaron Dahlen
LCDR USCG (Ret.), MSEE
DigiKey Applications Engineer
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.