The heart of any automated system is its controller. This is where the decisions are made to signal actuators in the system to launch into action based on feedback supplied by sensors.
But implementing a controller requires a series of decisions by the integrator or end user based on the application. A key controller decision revolves around which type of controller to use—a programmable logic controller (PLC), a programmable automation controller (PAC), or industrial PC (IPC).
Typical control panel
The primary difference between a PLC and PAC is that a PAC is similar to a PLC but with additional features. An IPC can run the same software found on a PAC, but with the full features of a PC.
Allen-Bradley ControlLogix PLC.
PLCs are typically used to control a relatively small number of I/O, analog or discrete, and in a PLC the physical I/O is often tightly coupled both to the programming language and to the PLC hardware itself. They can communicate with network devices such as drives, but it often requires adding on additional modules to expand their capabilities.
Opto 22 SNAP PAC R1-B.
A feature typically associated with a PAC is its ability to be programmed in languages other than ladder logic. Languages such as structured text, function block diagrams, and flowcharts can be used to program a PAC. The memory is typically tag-based, whereas in a PLC the memory structure is often address-based. PACs also inherently use standard communication protocols so they can efficiently communicate with a wide variety of network devices. PACs also tend to be modular, so they can communicate with remote I/O, remote panels, and devices like drives. They can also handle complex applications like motion, advanced process control, and integrated safety.
Address-based structures, which can be found in most major PLCs, such as those from Allen-Bradley, Siemens, and Mitsubishi, come with a predefined range of integers, timers‚ or Boolean addresses. A tag-based controller is not restricted to using only the predetermined address ranges. You can give an address any name you want. It more closely resembles higher level programming languages like C, where you create variables as needed.
An IPC can be programmed to run the same control software used on a PAC, but it runs on a full-blown industrial computer; and with that comes an operating system familiar to most end users and the IT department, such as Windows or Linux.
Making the determination
Ultimately, the application should help dictate which type of controller you choose. PLCs are well-suited for standalone machines because they’re robust and simple, which makes it easier for maintenance personnel or technicians familiar with technical drawings—the basis for ladder logic commonly used in PLC programming— to troubleshoot rather than the PC programming languages often used in PACs and IPCs.
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