PACs versus PLCs

The consensus among both users and automation suppliers isthat use of PACs is on the rise, driven by the move to multidisciplinarycontrol. But the PLC is still findingapplication in systems design by adding advanced features and capitalizing onits roots as a low-cost controller.

"Fiveyears ago, the line was more distinctive, but today PLC makers are adding moreand more functionality to their hardware to make PACs and PLCs more similarthan different," says Ben Orchard, an application engineer with Opto 22.

Hesays a good example of this functionality that once separated PACs from PLCs iscommunications. When PACs arrived on the scene over the course of the lastdecade, they utilized standard network interface protocols like TCP/IP, OPC andSMTP to achieve a high degree of enterprise connectivity. This included, forexample, the ability to communicate data across a networked plant floor controlsystem and even up to enterprise applications and databases residing on thecorporate network.

"Despitethe fact that some PLCs now incorporate this level of communicationstechnology, some distinctions between PACs and PLCs persist. For example, PACscontinue to be better multidisciplinary devices and more naturally suited for awider variety of uses, including process control, sequential logic, stringhandling and data acquisition," Orchard says.

"Themain distinction is the PAC's ability to easily integrate traditional PLC logicand I/O control with higher performance features of purpose-built motioncontrollers, along with data management and networking capabilities of PC-basedcontrol systems," says Paul Ruland, automation systems marketing manager for SiemensIndustry Inc.

ControllerCPUs with more built-in communication ports for both Ethernet and fieldbus connectivitycan reduce the system cost by eliminating separate communication add-onmodules. Additionally, upgrading to a controller that includes a built-in Webserver for Internet connectivity saves on an additional PC in the system as aWeb server.

"Thetrend we are noticing is more of an emphasis on advanced software programmingpackages for controllers that offer reduced engineering costs, along witheasier machine connectivity for automation networks, enterprise reporting andremote troubleshooting," says Ruland. "The programmable controller can nolonger be a silent black box."

He says Siemens is seeing greater demand for advancedprogramming software capabilities from the OEM market that are scalable for afull range of controllers, regardless if they are categorized as a PLC or aPAC. Machine builders are always looking for ways to improve efficiencies intheir software development tasks.

Thedemands automation suppliers typically see in the market include unifiedengineering tools to develop system-wide automation software projects with oneuser program, and one engineering software tool for machine logic, motioncontrol, safety, process control, HMI screens and network management. Thesetools enable automation engineers to spend less time on repetitive softwarecoding and focus more on improving their machine design. It also preserves theintellectual property that keeps them competitive.

End-users and Designers Speak Out

On the LinkedIn"Automation & Control Engineering" group, a discussion among PLC and PAC users and systems designers focused on selection ofappropriate controllers for specific applications and concerns about technologyobsolescence.

Joseph Stevenson, president of ICA Engineering, writesthat he thinks PAC is merely a marketing term for the more capable PLCs. Headds that the selection of one platform or another is highly contingent on thesize of the process (I/O count, required scalability) and what the controlsystem needs to do with that process.

"In a manufacturing setting, compatibility with the restof the facility is a key feature," says Mark B. Strube, PE. "I personally thinkthat PACs are the next logical step to keep systems up to date and allow you totake advantage of the latest control technology."

Paul Sinclair, electrical general foreman at PotashCorp.,writes that his preference is to go 100 percent PAC if possible, but that smallamounts of I/O can be handled effectively by lower-end controllers. He says he likes to keep logic continuity andstructure, one programming language and one logic style such as Ladder sincethe average electrician understands it. He also looks at market penetration inselection of the controller of choice. "Cheap is nice but a euro ~here today, gonetomorrow' is not nice."

"I have been using the Rockwell PACs for a number ofyears now, and have not found an application where a traditional PLC would do abetter job," writes David Kaylor, controls engineer at Red Gold LLC. He saysthat machine OEMs have been a little slow adopting the newer technology and, asa former OEM and integrator, thinks they are missing out on some advantages inease of reuse. Along with specifying the Logix5000 platform, he has had toconvert OEM SLC programs in order to get the desired platform. But hopefullythis will change in the future, since it is still valuable to have a commonplatform wherever possible.

"Many automation suppliers offer both PLC and PACcontrollers because there are distinct differences between the two, but theapplication will dictate whether a PLC or a PAC is best suited," writes SloanZupan, product manager, controllers and HMIs, for Mitsubishi Electric America.

He says a PAC really refers to automation platforms thattypically combine multiple different control disciplines onto a single rack. Asan example, Mitsubishi Electric offers a product called the iQ Platform, whichenables users to select any combination of the following CPU types: sequence,motion, robot, CNC and C controller. TheCPUs can be mixed and matched to fit the requirements of any application. Byleveraging the same power supplies, racks, I/O and communication interfacesstartup, normal operation and maintenance becomes much easier. That is thevalue of a true PAC, according to Zupan.

Needs-Based Determination

Despite the clear advantages to PACs, many systemsdesigners find that a PLC is appropriate for stand-alone applications wheresequence and positioning control are all that is needed. PLCs are flexible intheir networking capabilities and the same software can be used for both thePLC and the PAC because the instruction sets are the same.

"APAC is a controller that looks similar to a traditional PLC system, but hasfeatures and capabilities that are typically associated with a PC-based controlsystem," says Jeff Payne, product manager for PLC, I/O and PC Control at AutomationDirect."These features include integration with corporate databases, tag basedaddressing, more memory, faster processors and more built-in communicationsoptions."

Paynesays that PLCs are still being utilized in the majority of the controlapplications at AutomationDirect. But there is an increasing demand for OEMcustomers to collect and obtain more data from the plant floor into theenterprise environment for analysis. The built-in capabilities of the PACallows for a seamless method of data collection without the need for additionalhardware or third-party software.

"Forthose needing more data exchange such as transferring data between the PAC anda corporate database, the PAC makes this much easier," says Payne. "Many times,the PAC will have preconfigured instructions that allow you to interface with athird-party server that handles the SQL, Microsoft Access or an ODBC-compliantdatabase."

According to Fabio Malaspina, marketing manager, componentdesign software, architecture & software - control & visualizationbusiness for Rockwell Automation, buying behaviors indicate that some customerspurchase integrated system solutions while others buy individual automationcomponents. Customers buying integrated system solutions prefer PACs. But someOEMs such as those producing simple, low-cost, stand-alone machines, prefer tobuy individual automation components because they are simple to use, reliableand, most importantly, cost effective.

There is also agreement that the PACs' use of standardnetworking interfaces and protocols, their powerful control features, extensivecommunications capabilities and compact size are increasingly making animpression on OEMs. Machine builders tend to find PACs easy to embed and usenot only for control, but for other purposes as well such as gathering machineor system data and monitoring the health, status and performance of themachines they build and deliver.

On the flipside, what many OEMs require is asmall-sized, basic controller that they can embed in a machine to performsimple control functions. The pricepoints on these devices can get quite low, so the OEM customer sometimes optsfor this in place of a more powerful and versatile PAC. What the OEM market isoften looking for is compactness, easy configuration and low cost. Often thefunctionality required is very limited and doesn't vary much from installationto installation.

ScottTenorio, marketing manager, controller & visualization business forRockwell Automation, says that the term "PAC" is often used to denote anevolution of the term PLC. He says a PACis more than just the controller, but rather is a complete system architecturethat encompasses controllers, networks and software all designed to communicateseamlessly. As a result, PACs provide a single multi-disciplined controlplatform for applications that span an entire plant floor.

Weighing PAC Benefits

A key benefit of the PAC platform is it provides a singlecommon development environment. This common development environment allows theintegration of varying applications that previously used multiple hardware andsoftware packages.

"PLCs primarily provide machine control logic capabilitiesusing a fairly standard instruction set, such as what is defined by IEC 61131,"says John Dart, program manager, global OEM solutions business for RockwellAutomation.

PACs have advanced capabilities, such as the ability tointegrate the programming of motion, robotics, safety, process and drives intoone common programming environment. This eliminates the need to have severaldisparate controllers and separate interfaces between them. For applications having multiple controldisciplines, a PAC can provide a lower-cost architecture than a PLC and alsoreduce complexity, improve performance and lower support costs.

Both PLCs and PACs are relevant in today's markets andwill be for the foreseeable future. The choice is typically driven by marketconditions and each machine builder's business model. Designers of lowerperformance, high-volume machine types will lean toward PLCs, whilehigher-performance, lower-volume machine types will lean toward PACs.

"There is demand, especially in emerging economies, forPLCs that have "just enough" PAC-type features to enable mid-range performanceat near the cost of low-end solutions," Dart says. "This demand is asignificant trend and could actually be considered an enabler to PAC adoptionrather than a constraint. As machine builders come to understand the benefitsof some PAC features that are included in a low-cost PLC, the tendency could beto move more toward PACs."