MOTION SYSTEMS HANDBOOK
Programmable automation controllers (PACs) and industrial PCs
A programmable automation controller (PAC) is an industrial controller that combines the functionality of a PLC with the processing capability of a PC. The term programmable automation controller is generally accepted as having been coined by the ARC Advisory Group, which specified five characteristics that define a PAC: • • • • •
Multi-domain functionality A single, multi-discipline development platform Flexible software tools that maximize process flow across machines or processes An open, modular architecture Compatibility with enterprise networks
But with no industry-standard definition of a PAC, the distinction between PACs and PLCs is blurry. Higher-end PLCs now incorporate some of the characteristics described above and are encroaching on what was once considered PAC territory. In fact, many PLCs now include standard programming languages, the ability to expand functionality through add-on modules, and connectivity to various bus systems. PACs still differentiate themselves from PLCs by employing a more open architecture and modular design. They’re also more capable than PLCs at monitoring and controlling a large number of I/O, such as in a large processing plant or a complex automation system. They do this because data can be exchanged between devices and applications in different domains, such as motion and process control. In addition, a programmable automation controller can send and receive data to and from other PACs, creating a distributed control system of PACs. Programmable automation controllers (PACs) excel in commanding complex automation setups that involve PC-based and HMI functions as well as process control (largely because of the way they handle I/O).
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DESIGN WORLD — MOTION
Controllers • PC & PAC — Motion Control HB 08-19 V3.indd 54
Programmable automation controllers integrate multiple components MAIN CONTROL UNIT (PROCESSOR) KEYPAD and CONTROL LED STATUS INDICATORS MODULES
POWER SUPPLY (on main unit or a module)
COMMUNICATIONS (on main unit or a module)
I/O MODULES
Programmable automation controllers (PACs) integrate multiple subcomponents. All include a base unit with CPU and power supply; this base unit or complementary modules can also include analog, pulse, and other I/O; positioning, high-speed counter, motion, energy and power functions; and safety communications, controls, and relays. PACs are also increasingly common for motion applications for machining or handling discrete product thanks to the flexibility and interoperability they offer machine designs. Today’s PACs evolved as an option for complex control when microprocessors with significantly more performance became affordable and commonly available. PACs differ from the still-dominant form of control for motion — the programmable logic controller (PLC) — in that all PACs can perform as PLCs but not vice versa. That’s because PACs serve multiple channels of communication; high-data traffic; and coordination with intelligent subsystems. Most performance PLCs can host intelligent processors in their backplanes — such as Ethernet modules with multiple ports for expanded data and communications, for example. But such setups can be expensive where vendors’ proprietary backplanes and operating systems are costly. Consider how PACs emulate the behavior of electric-relay controls. Relay logic executes sequentially with repeatability and reliability — on hardware rugged enough 8 • 2019
motioncontroltips.com | designworldonline.com
8/20/19 3:49 PM
