CONTROLLERS
THE RISE OF THE PAC
David Humphrey explains the difference today between a PLC and PAC and offers advice on which would best suit different applications within the manufacturing environment.
T
he term PAC is often used as a synonym for a programmable logic controller (PLC), but there are some important differences. A programmable automation controller (PAC) is an open control platform that supports and integrates multiple automation disciplines – typically logic and motion control – but also other functions. A common tag database to which all devices have access ties the platform together. Developed in the 1960s as a replacement for hard-wired relay systems, the PLC quickly became the machine control workhorse in factories around the world. Designed initially to solve bit-logic, PLC capabilities grew rapidly over the decades – Integer and floating-point variables were added together with high-level mathematical operations, and a slew of hardware modules were introduced to address specific manufacturing tasks, from weigh-scales through to machine vision. Five PLC programming languages were standardised under IEC 61131-3, and industrial networks started the trend to connected machines. Plant-floor software typically runs on industrial PCs (IPCs), ruggedised versions of their office brethren that can be installed on the plant-floor close to the process. As more data are collected, stored and processed locally, PLCs and IPCs are now often integrated into the same chassis. Alongside this journey, other devices for machine control – such as motion controllers – quickly evolved and needed to be integrated with PLCs, so a marriage of the two became inevitable. With the launch in the late 1990s of new PLC platforms such as ControlLogix and SIMATIC S7, the term PAC was coined by Craig Resnik, an ARC analyst, to define
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this new concept. These platforms are ‘open’ in that other devices and systems can communicate with them via industrial networks with ‘open’ protocols such as Profinet and EtherNet/IP. In the 1990s we also saw the dawn of industrial software, which has been transforming automation solutions ever since. Windows-based visualisation software was added to PLC solutions and CRT terminals were replaced by LCD operator panels with touchscreens. Manufacturing execution systems (MES) software also emerged as a layer between the PLC and enterprise software.
A software explosion The past two decades have witnessed an explosion in the market for industry software. Product lifecycle management (PLM) software brings together the worlds of product and process design and simulation, helping to shorten timeto-market with the introduction of the digital twin, a software rendition of a product or production equipment that accompanies the ‘real thing’ from design through operation. Today, the PLC is well integrated in the design process and PLM software can generate PLC code automatically. According to ARC market research, PACs now make up nearly two-thirds of the $12 billion PLC market,
and this segment is growing faster than traditional PLCs. So, why did the PAC become so popular? One reason is that when a vendor launches a new product today, it is much more likely to be a PAC than a traditional PLC. Machine builders and end users no longer want to spend time making dissimilar systems, instead preferring to buy integrated solutions. Today, most large and many small controllers qualify as PACs. Only lowcost micro and nano controllers are still considered to be PLCs. Another reason for the migration from PLC to PAC is the challenge from software-based and virtualised controllers. A software-based controller is often a good choice in applications that gather, store and process a lot of production data during manufacturing, because these data can be shared easily with other software running in the same environment. While PLC functionality has come a long way, data processing and storage are not among the PLC’s strengths. On the other hand, traditional PLCs are still the preferred solution for applications that require speed and repeatability, quick and simple deployment, and above all – low cost. ! David Humphrey is research director, Europe at ARC Advisory Group.
The market for PLC and PLC-based PAC (Millions of US Dollars) Source: ARC Advisory Group.
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