Feature: Data acquisition


Today’s PLCs can be very powerful controllers, and remain the basic building blocks for small automation projects


Data acquisition with PACs and PLCs By Neil Ballinger, Head of EMEA, EU Automation


P


rogrammable logic controller (PLC), programmable automation controller (PAC) or the most recent industrial PC (IPC)? With so many


options available, it can be hard for manufacturers to make an informed decision. PLCs, PACs and IPCs are all used for


the same purpose, that is to reliably control automation equipment in a manufacturing plant. However, assessing an application’s needs in terms of size, capacity and compatibility is crucial to choosing the right model to fulfil the plant’s automation needs. PLCs first appeared in the late 1960s


in automotive manufacturing, and soon spread to other sectors where continuous processes are required, such as electronics assembly, food and beverage, and the pharmaceutical industry. PLCs were introduced to replace


hardware-based relay racks, which took up a lot of space, consumed massive amounts of electricity and were very expensive to set up and maintain. In fact, before the introduction of PLCs, whenever the production process had to be modified, thousands of relays were manually re-wired – an expensive and time-consuming process. Te first PLC, the Modular Digital


24 April 2021 www.electronicsworld.co.uk


Controller (Modicon), replaced manual rewiring with soſtware program changes. It was designed to be programmed in a ladder logic, similar to the diagrams of relay logic it was replacing, making the transition from relays to PLCs much easier. PLCs became common used in industrial


applications during the 1970s and early 1980s. Since then, they have undergone substantial improvements, such as reduced size, expanded memory and increased processing power. PACs are a more recent addition to the


automation market and are generally more complex systems than PLCs, although advancements in modern technology have lessened the gap between the two. In the 1990s another option entered the


market – industrial PCs (IPC). Tese allow users to run PLC-style applications on a regular PC’s operating system. While the first models were not rugged enough to withstand harsh industrial environments and had serious stability issues, more recent versions are now successfully used in larger applications – alone or in combination with one or more PLCs.


PAC/PLC comparison Te main difference between PLCs and PACs is the way they are programmed. PLCs are programmed using ladder logic – a programming language that


uses symbols representing a relay-based electrical schematic. Tis is a sequential form of programming, making PLCs largely inflexible. On the other hand, PACs are programmed using C and C++, making them more adaptable and efficient, since an engineer can reprogram the PAC by writing a new set of code. Te ways the two computers are programmed create distinct capabilities for each. PLCs have a single microcontroller


and therefore limited logic capacity. Tis makes them unsuitable for complex applications that require motion control, which can be achieved only by adding separate modules, or by using a hybrid system with one or more PLCs and an industrial PC. As a consequence, PLCs are ideal for smaller automation projects performing simple tasks. PACs normally use two or more


processors and have more sophisticated logic than PLCs. As a result, they can operate in multiple domains such as motion and process control using a single platform. In this sense, using a PAC can be likened to using a PC and a PLC together. As a result, PACs offer the advantages of a hybrid system but without the added hardware. Tey also provide better integration with SCADA (Supervisory Control and Data Acquisition), which allows them to manage complex industry-specific instructions.


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