What Exactly Is a PAC?

Does It Matter Whether You Use a PAC From a PLC Vendor vs. a PC-Based Control Supplier?

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"We've used basic PLCs and I/O for machine control for a long time. Our machines have to do more than before, and we need more connectivity options and a more COTS, standards-based approach to hardware and programming. Some customers are on the PAC bandwagon and think it's the next step for them and us. There are different opinions of what a PAC is. PLC vendors say they make PACs. So do PC-based control suppliers. Does it matter?"

—From August '12 Control Design

Answers

Mashup Control
A programmable automation controller (PAC) can be described as a "mashup" between a PC and a PLC in that it typically offers the benefits of both in a single package. Therefore, it's becoming more common that PLC vendors position their higher-end controllers as PACs — largely because their higher-end products incorporate more connectivity options and broader control capabilities than their PLC lines.

In your situation, the key point is what you state in your question: connectivity options and commercial off-the-shelf (COTS) technology. PACs offer both out of the box. Connectivity options like Ethernet are standards-based and therefore deliver on the COTS promise. For example, standard, readily available components like networking hardware will more easily and cost-effectively interface with a PAC.

In contrast to PC-based control, more often than not, a PAC will have lower running and maintenance costs. One advantage of a PC-based controller is faster computing speed and greater data storage area, but not necessarily faster I/O access. Will your machine accommodate a larger PC-based controller? Is the environment around the machine harsh in any way? PACs are usually smaller and more robust.

Taking all this into account, you must consider your customer requirements, weigh in your machine constraints, and choose the best upgrade. Many options certainly exist, and ideally, you should choose a vendor that provides multiple controller options (both PAC and PC-based for example).

Given the rising use of smartphones and tablets in industrial automation, you should also be looking at a platform that will ease adoption of this technology into your machine. Internet, wireless and cloud-based storage solutions are all growing in use. Select a PAC, PLC or PC that can support these technologies and protocols in a secure manner.

So, does it matter? Yes, I believe so. But, it's your current and future customers that will ultimately help you decide.

Ben Orchard,
systems engineer,
Opto 22, www.opto22.com

The One That Suits Your Needs
PLC is a term generally used to describe a general-purpose controller ideal for controlling standalone, discrete machinery or processes. A PAC refers to a controller that offers multi-discipline control. For example, a PAC might offer the ability to execute complex motion instructions as well as possess integrated safety functionality, while a PLC typically would offer only logic control.

Some PC-based control suppliers might offer PACs, but not all PC-based controllers are PACs. The terminology is often interchanged, even among the most savvy machine builders.

Here are some questions to help guide you:

1. Does your application require multi-discipline control (motion, safety, drive, process, etc.)?

2. Do you have panel size constraints that should be considered?

3. What is the expected lifecycle of the machine?

4. Is rugged packaging required?

5. What networks will you be using for your machine design?

Overall, the terminology is less important than selecting the controller and vendor partner that best meet your specific needs.

Dexter Leong,
product manager, CompactLogix,
Rockwell Automation, www.rockwellautomation.com

A Controller by Any Other Name…
I think the concept of a PAC, while technically defined, is more of an all around automation "solution" piece of hardware, rather than the simple I/O controller we're used to in PLCs. It used to be that the control system ended with the PLC, and users interacted with an HMI to report this to the office, where the numbers were punched in again. Then we added SCADA, and had to move information out of the PLC and into a more PC-based environment.

It's inevitable that the next step of control systems would be to integrate a more tight-knit linkage with the systems where the information needs to flow, and often that information needs to flow to an IT-controlled system. So really, I think PLCs are becoming the less-expensive alternative to a PAC and becoming PACs in all but name, and PC-based control systems are "hiding" their PC background and moving toward a form factor and user interface much more akin to the traditional interfaces we're used to in the controls industry. Really, I don't think it matters.

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