Choosing Between PLC Platforms: What Should You Consider?
Choosing a PLC platform is a decision that can affect an industrial operation for many years.
At first glance, the choice might appear to come down to technical specifications or price.
In reality, selecting a Programmable Logic Controller (PLC) involves much more than comparing processor speeds, I/O capacities or individual hardware costs.
The PLC becomes part of the wider automation environment. Engineers need to maintain it, replacement components need to remain available, software needs to be supported and the controller may need to communicate with drives, HMIs, SCADA systems and other equipment throughout its operational life.
For many industrial businesses, the better question is therefore not simply “Which PLC is best?”
It is “Which PLC platform is best suited to our machinery, people and long-term automation strategy?”
There is no single best PLC platform
Siemens, Allen-Bradley and Mitsubishi are all well-established PLC manufacturers, and each has a substantial presence within industrial automation.
Trying to declare one universally better than the others misses an important point.
A PLC that is an excellent choice for one manufacturing environment may be unnecessarily difficult or expensive to support in another.
For example, an organisation operating a site predominantly using Siemens automation may have good reasons to continue with Siemens. Its engineers may already understand the platform, the programming software may already be available and spare components may already be held on site.
Introducing a completely different PLC platform for one new machine could create additional training, software, support and spares requirements.
The existing automation environment should therefore be one of the first considerations.
Start with what is already installed
Before selecting a PLC for a new machine or automation project, it is worth understanding what already exists across the site.
Which PLC manufacturers are currently used?
Which platforms do the maintenance and engineering teams understand?
What programming software and licences are already available?
Which replacement components are held as critical spares?
What communication networks are being used?
If most of the facility is standardised around one PLC family, there may need to be a compelling reason to introduce another.
Standardisation can make fault finding, training, software management and spare-part strategies considerably simpler.
That does not mean a business should remain tied to an unsuitable platform indefinitely. It means that changing platform should be a deliberate engineering decision rather than something that happens simply because a particular PLC was specified with a new machine.
Consider the people who will maintain the system
One of the most overlooked factors in PLC selection is the engineering team that will eventually have to support it.
A technically sophisticated control system provides little operational advantage if nobody on site understands how to diagnose it when production stops at 2am.
Ask practical questions.
Does the maintenance team have experience with the platform?
Can engineers connect to the PLC and diagnose faults?
Is specialist external support readily available?
Will additional training be required?
The cost of developing these capabilities should be considered alongside the cost of the PLC hardware itself.
For production-critical machinery, supportability can be more important than a relatively small difference in initial equipment cost.
Think beyond the PLC itself
Modern PLCs rarely operate as isolated controllers.
They may need to communicate with HMIs, SCADA systems, variable speed drives, servo systems, robots, remote I/O, instrumentation and other controllers.
Industrial communication requirements therefore need to be considered early.
A machine may need to integrate with an existing PROFINET, PROFIBUS, EtherNet/IP, Modbus TCP or other industrial network. There may also be requirements for communication with supervisory systems or production data platforms.
Choosing a PLC without considering these wider integration requirements can create unnecessary complexity later in the project.
The objective should be to select a controller that fits naturally into the overall automation architecture.
Siemens PLCs
Siemens PLCs are widely used across industrial automation, particularly in European manufacturing environments.
The Siemens SIMATIC family covers applications ranging from relatively small machines through to complex industrial automation systems. Siemens' TIA Portal engineering environment also brings PLC programming together with other automation technologies within the Siemens ecosystem.
For organisations that already operate Siemens PLCs, HMIs, drives and associated automation equipment, remaining within that ecosystem can provide advantages in engineering consistency and integration.
However, the suitability of Siemens still depends on the application, existing site standards and available engineering expertise.
A familiar platform that can be efficiently supported is often more valuable than selecting technology purely because it offers additional capabilities that the application may never use.
Allen-Bradley PLCs
Allen-Bradley, part of Rockwell Automation, is another major PLC platform used across industrial environments.
Its ControlLogix and CompactLogix controller families are widely used for machine and process control applications, with the wider Rockwell automation environment providing integration with other control and information technologies.
Allen-Bradley is particularly well established within North American industrial environments, although its controllers are used globally.
For businesses already operating Rockwell-based systems, selecting Allen-Bradley for additional machinery may provide advantages in platform consistency, engineering familiarity and integration.
Again, the important consideration is not simply the name on the PLC. It is how effectively the platform fits into the organisation's wider automation environment.
Mitsubishi PLCs
Mitsubishi Electric PLCs are also widely used in machine automation and manufacturing applications.
They are commonly encountered in OEM machinery and production equipment, with controller options covering everything from compact machine applications to larger automation systems.
For businesses with existing Mitsubishi equipment, maintaining platform consistency can simplify engineering support and machine maintenance.
Mitsubishi may also be an appropriate choice where the wider machine architecture incorporates other Mitsubishi automation products.
As with Siemens and Allen-Bradley, however, the decision should be driven by the application and long-term support requirements rather than brand preference alone.
How complex is the machine?
Not every industrial machine needs the most powerful PLC available.
A relatively straightforward machine with a limited number of inputs and outputs may require a very different controller from a large automated production system involving multiple networks, motion control, remote I/O and hundreds of field devices.
When selecting a PLC, engineers need to consider both current and future requirements.
These can include I/O capacity, processing requirements, communication interfaces, motion control, safety integration, data requirements and potential future expansion.
Overspecifying a controller can add unnecessary cost and complexity.
Underspecifying it can be worse.
If the machine later needs additional equipment, instrumentation or communication capability, an unsuitable controller can restrict future development.
Consider obsolescence before it becomes a problem
PLCs can remain in service for decades.
That is good for industrial reliability, but it also means today's platform choice can become tomorrow's obsolescence problem.
Before selecting a controller, consider the maturity of the product family and the manufacturer's longer-term support arrangements.
For an existing machine, the decision may be slightly different.
If the current PLC platform is becoming obsolete, replacing it like-for-like with ageing technology simply because it is familiar may postpone rather than solve the underlying problem.
A planned PLC migration can provide an opportunity to move the machinery onto a supported platform while reviewing the control software, documentation and wider automation architecture at the same time.
Don't overlook software and licensing
The physical PLC is only part of the cost.
Engineering software, licences, communication hardware, training and ongoing support can all contribute to the total cost of ownership.
If a site already has the appropriate engineering environment for one PLC manufacturer, introducing another platform can mean purchasing additional software and developing another set of engineering skills.
Software version compatibility also matters.
Older machinery may require legacy programming environments that are increasingly difficult to run on modern engineering laptops. This is one reason why maintaining current PLC program backups and accurate documentation is so important.
The cheapest controller on a quotation is therefore not necessarily the cheapest platform to own for the next ten or fifteen years.
What about spare parts?
Production downtime has a habit of exposing decisions that seemed insignificant when machinery was originally purchased.
If a PLC component fails, how quickly can it be replaced?
Does the site hold compatible spares?
Can replacement hardware be sourced locally?
Will the replacement work with the existing PLC program and firmware?
A standardised PLC strategy can make managing critical spares considerably easier, particularly across facilities operating multiple production lines.
By contrast, a site containing numerous PLC brands and generations can gradually develop a substantial collection of different processors, I/O modules, cables, software packages and engineering tools that need to be maintained.
New machinery can introduce an unexpected PLC problem
PLC selection does not always happen during an internal automation project.
Sometimes a new machine arrives from an OEM with its PLC platform already selected.
That can introduce an entirely new automation ecosystem into a facility.
Before purchasing production machinery, it is therefore worth asking what controller platform will be supplied and whether it aligns with existing site standards.
Where appropriate, PLC and automation requirements can be included within the machine specification supplied to the OEM.
This can help prevent a situation where engineering teams discover only after installation that the new equipment requires unfamiliar software, specialist programming cables or external support for even relatively straightforward fault finding.
Should you standardise your PLC platforms?
For many industrial facilities, some level of PLC standardisation makes sense.
Standardisation can reduce the number of software environments, spare components and specialist skills required across the site.
However, standardisation should not become so rigid that an unsuitable controller is selected simply to comply with a preferred-brand policy.
A useful PLC standard can define preferred platforms while still allowing exceptions where there is a genuine engineering justification.
The objective is consistency without unnecessarily restricting the solution.
Questions to ask before choosing a PLC
Before committing to a PLC platform, it is useful to consider the decision from several perspectives.
What PLC platforms are already installed across the site?
What experience does the internal engineering team have?
What equipment must the PLC communicate with?
How many inputs and outputs are required?
Will the machine require motion, safety or advanced process control?
How easy will replacement components be to source?
What software and licences will be required?
Is the platform appropriate for the expected lifetime of the machinery?
How easily can external PLC support be obtained?
Could the machine or production process expand in the future?
These questions often reveal far more about the right PLC choice than comparing hardware specifications alone.
Choose a platform for the life of the machine, not just the project
A PLC may represent a relatively small proportion of the overall cost of an industrial machine, but it can have a disproportionately large influence on how that machine is maintained and supported.
The right PLC platform should meet the technical requirements of the application while also fitting the organisation's existing automation infrastructure, engineering capability and long-term maintenance strategy.
Siemens, Allen-Bradley and Mitsubishi can all be excellent choices in the right environment.
The important part is understanding why a particular platform is being selected.
At Stratos Control Systems, we work across Siemens, Allen-Bradley and Mitsubishi PLC platforms, supporting businesses with PLC programming, system integration, fault finding and PLC upgrades and migration.
Whether you are specifying a new control system or deciding how to replace an obsolete PLC, considering the complete automation environment before choosing the hardware can prevent unnecessary complexity later.
Speak to Stratos Control Systems about PLC programming, platform selection, migration and industrial automation support.
