PLCs or PACs: —It'’s all in the application

We focus on programmable controls for discrete machine applications. Although they still come with different names, what they do is becoming more and more the same.

By Patti Pool, Contributing Editor

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Patti Pool

Statistical research isn’t needed to know there are far more installed PLCs in industrial applications worldwide than programmable automation controllers (PACs). PLCs, the workhorses of machine automation, are the most understood technology by engineers worldwide.

According to various market sources, about 80% of industrial applications are solved by simple PLCs, such as microPLCs with digital I/O that use ladder logic. As users pushed the envelope of these PLCs, they weren’t always successful. Engineers said they needed more capabilities for complex applications. Enter the PAC, so named it seems by ARC. These new controllers include multi-domain functionality, a single multi-discipline development platform, software tools, open, modular architectures, and use of de facto standards for network interfaces, languages, etc.

Many vendors now offer PACs, but PLC vendors responded by adding multiple protocol support to connect with other devices in the process. “Another primary development that has literally revolutionized the way PLCs are programmed, communicate with each other, and interface with PCs for HMI, SCADA or DCS applications came from the [commercial] computing world,” says Jeff Payne, AutomationDirect’s product manager. “Ethernet communications on the plant floor have doubled in the past five years. While serial communications remain popular and reliable, Ethernet is becoming the media of choice with advantages that can’t be ignored, such as network speed, ease of use in setup and wiring, availability of off-the-shelf components, and built-in communications setups.”

The PLC now has been tasked with motion control, too. From simple open-loop to multi-axis applications, the trend has been to integrate this feature into PLC hardware and software.

A tutorial from National Instruments points out that “although PACs represent the latest in programmable controllers, the future of PACs hinges on incorporation of embedded technology. One example is to use software to define hardware. Field programmable gate arrays (FPGAs) are electronic components commonly used to create custom chips, allowing intelligence to be placed in the new device. FPGAs let engineers quickly create applications that incorporate custom control protocols or high-speed control loops.”

So is it PLCs or PACs for you? Vendor support is critical if you use PACs. One observer who has several PC-based control systems in his factory remarked that, “Computers are changing so quickly that most of what I just bought today might not be available in six months to a year. On the other hand, I know that if a SLC500 dies, I can get one just like the one I put on the machine eight years ago. This is a major hurdle in my eyes for the PACs.”


Product Roundup:

Programmable Controllers

Scalable Embedded Controllers
S-Max 50xx VLC and ILC 350 VLC embedded controller platforms using Steeplechase VLC flowchart programming combine secure PLC control functions with PC-based performance, capacity and connectivity options. Version 7.0 software offers sub-millisecond logic scans and flowchart programming. Phoenix Contact; 717/944-1300;

Fast and Furious
MicroSmart Pentra micro controller’s logic engine processes basic instructions in 0.056 µsec. It has more memory, field-upgradeable firmware to keep the controller updated, and seven communication ports for a barcode reader, Ethernet connection, modem or RS232C-compatible devices. IDEC; 800/262-4332;

Combo Touch Panel and PLC
EZTouchPLC combines the original EZTouch panel with the functionality of a micro-modular PLC. It integrates users control and visualization needs into one total device for a small to medium machine that requires a touch-type graphical display. EZAutomation; 563/359-7501;
On the I/O Rack Controllers

On the I/O Rack Controllers
Snap PAC R-series programmable automation controllers are designed for cell control, data acquisition and remote monitoring where the I/O needs are geographically contained. The Snap-PAC-R2 version supports 512 points of analog, digital, and serial I/O modules. It offers PID loop control, floating-point math and string handling and dual-Ethernet. Opto 22; 800/321-6786;
All Bundled Up

All Bundled Up
Box PC system is a fanless, dust-protected computer that operates with most commercial display units, and can be used for remotely operated applications. The system comes bundled with InTouch HMI and pre-installed ActiveFactory trending and analysis software. Wonderware; 949/727-3200;

A Strong Pulse
PCM-3240  four-axis stepping/pulse-type servo motor control card simplifies stepping and pulse-type servo motor control, providing added motor performance, and fits into a standard PC/104 board, eliminating  mechanical interference in the embedded controller. The card’s MCX341-motion ASIC provides  2/3-axis linear interpolation, two-axis circular interpolation, and T/S-curve acceleration/deceleration rate. Advantech, eAutomation Group; 800/205-7940;

16 MB More Memory
ControlLogix family features 16 MB of memory for data-intensive applications requiring higher memory capacity. It allows centralization of alarming, and for information-intensive applications that require recipe data, and it provides larger storage capacity to locally store more data, enabling users to switch recipes. Rockwell Automation; 800/223-5354 x1970;

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