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One of the most crucial decisions in the initial design phase of a machine is the selection of the control system. Since programmable logic controllers (PLCs) were first introduced in the 1970s, they have dominated the process and automation markets. For years, the PLC has led the way for advances in machine automation control. From small controls used in the automotive industry to large-scale controllers running entire factories, the PLC was the primary controller of choice.

 

Ten years ago, deciding what type of controller to use might have been a clear choice; today, that is no longer the case.

 

The PLC was developed as a more streamlined, flexible, and reliable alternative to switch boxes and relay panels. It was dedicated only to specific tasks in the factory, and its language and structure were modeled around the switches and relay panel circuits it was replacing.

 

The PC, on the other hand, served a higher level role in the scope of the machine. It was used primarily for complex calculations, monitoring, measuring, and factory networking as well as a user-interface to the PLC. It was usually housed in a more environmentally controlled location because it couldn’t operate as reliably as a PLC in harsh factory conditions.

 

So, how does one choose between PLC or PC-based architecture? To simplify the decision-making process, it is important to first analyze and compare differentiating characteristics. Some major areas to consider are:

 

 • Operation

 • Robustness

 • Serviceability

 • Hardware integration

 • Security

 • Safety

 • Programming

 

After analyzing the technical requirements and comparing the design options for your machine, one more topic needs to be considered: cost. Many applications can be solved by either a PC-based or PLC-based solution, but for some applications, the cost of those solutions can differ widely. A comparison of the relative cost of PC- and PLC-based architectures centers on four areas: performance, expandability, environment, and development effort. With extensive expertise and intimate knowledge of the system, there should be little or no cost difference between a PC- or PLC-based solution.

 

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