The consensus among both users and automation suppliers is
that use of PACs is on the rise, driven by the move to multidisciplinary
control. But the PLC is still finding
application in systems design by adding advanced features and capitalizing on
its roots as a low-cost controller.
years ago, the line was more distinctive, but today PLC makers are adding more
and more functionality to their hardware to make PACs and PLCs more similar
than different," says Ben Orchard, an application engineer with Opto 22.
says a good example of this functionality that once separated PACs from PLCs is
communications. When PACs arrived on the scene over the course of the last
decade, they utilized standard network interface protocols like TCP/IP, OPC and
SMTP to achieve a high degree of enterprise connectivity. This included, for
example, the ability to communicate data across a networked plant floor control
system and even up to enterprise applications and databases residing on the
the fact that some PLCs now incorporate this level of communications
technology, some distinctions between PACs and PLCs persist. For example, PACs
continue to be better multidisciplinary devices and more naturally suited for a
wider variety of uses, including process control, sequential logic, string
handling and data acquisition," Orchard says.
main distinction is the PAC's ability to easily integrate traditional PLC logic
and I/O control with higher performance features of purpose-built motion
controllers, along with data management and networking capabilities of PC-based
control systems," says Paul Ruland, automation systems marketing manager for Siemens
CPUs with more built-in communication ports for both Ethernet and fieldbus connectivity
can reduce the system cost by eliminating separate communication add-on
modules. Additionally, upgrading to a controller that includes a built-in Web
server for Internet connectivity saves on an additional PC in the system as a
trend we are noticing is more of an emphasis on advanced software programming
packages for controllers that offer reduced engineering costs, along with
easier machine connectivity for automation networks, enterprise reporting and
remote troubleshooting," says Ruland. "The programmable controller can no
longer be a silent black box."
He says Siemens is seeing greater demand for advanced
programming software capabilities from the OEM market that are scalable for a
full range of controllers, regardless if they are categorized as a PLC or a
PAC. Machine builders are always looking for ways to improve efficiencies in
their software development tasks.
demands automation suppliers typically see in the market include unified
engineering tools to develop system-wide automation software projects with one
user program, and one engineering software tool for machine logic, motion
control, safety, process control, HMI screens and network management. These
tools enable automation engineers to spend less time on repetitive software
coding and focus more on improving their machine design. It also preserves the
intellectual 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 of
appropriate controllers for specific applications and concerns about technology
Joseph Stevenson, president of ICA Engineering, writes
that he thinks PAC is merely a marketing term for the more capable PLCs. He
adds that the selection of one platform or another is highly contingent on the
size of the process (I/O count, required scalability) and what the control
system needs to do with that process.
"In a manufacturing setting, compatibility with the rest
of the facility is a key feature," says Mark B. Strube, PE. "I personally think
that PACs are the next logical step to keep systems up to date and allow you to
take 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 small
amounts of I/O can be handled effectively by lower-end controllers. He says he likes to keep logic continuity and
structure, one programming language and one logic style such as Ladder since
the average electrician understands it. He also looks at market penetration in
selection of the controller of choice. "Cheap is nice but â€˜here today, gone
tomorrow' is not nice."
"I have been using the Rockwell
PACs for a number of
years now, and have not found an application where a traditional PLC would do a
better job," writes David Kaylor, controls engineer at Red Gold LLC. He says
that machine OEMs have been a little slow adopting the newer technology and, as
a former OEM and integrator, thinks they are missing out on some advantages in
ease of reuse. Along with specifying the Logix5000 platform, he has had to
convert OEM SLC programs in order to get the desired platform. But hopefully
this will change in the future, since it is still valuable to have a common
platform wherever possible.
"Many automation suppliers offer both PLC and PAC
controllers because there are distinct differences between the two, but the
application will dictate whether a PLC or a PAC is best suited," writes Sloan
Zupan, product manager, controllers and HMIs, for Mitsubishi Electric America
He says a PAC really refers to automation platforms that
typically combine multiple different control disciplines onto a single rack. As
an example, Mitsubishi Electric offers a product called the iQ Platform, which
enables users to select any combination of the following CPU types: sequence,
motion, robot, CNC and C controller. The
CPUs can be mixed and matched to fit the requirements of any application. By
leveraging the same power supplies, racks, I/O and communication interfaces
startup, normal operation and maintenance becomes much easier. That is the
value of a true PAC, according to Zupan.
Despite the clear advantages to PACs, many systems
designers find that a PLC is appropriate for stand-alone applications where
sequence and positioning control are all that is needed. PLCs are flexible in
their networking capabilities and the same software can be used for both the
PLC and the PAC because the instruction sets are the same.
PAC is a controller that looks similar to a traditional PLC system, but has
features and capabilities that are typically associated with a PC-based control
system," says Jeff Payne, product manager for PLC, I/O and PC Control at AutomationDirect
"These features include integration with corporate databases, tag based
addressing, more memory, faster processors and more built-in communications
says that PLCs are still being utilized in the majority of the control
applications at AutomationDirect. But there is an increasing demand for OEM
customers to collect and obtain more data from the plant floor into the
enterprise environment for analysis. The built-in capabilities of the PAC
allows for a seamless method of data collection without the need for additional
hardware or third-party software.
those needing more data exchange such as transferring data between the PAC and
a corporate database, the PAC makes this much easier," says Payne. "Many times,
the PAC will have preconfigured instructions that allow you to interface with a
third-party server that handles the SQL, Microsoft Access or an ODBC-compliant
According to Fabio Malaspina, marketing manager, component
design software, architecture & software - control & visualization
business for Rockwell Automation, buying behaviors indicate that some customers
purchase integrated system solutions while others buy individual automation
components. Customers buying integrated system solutions prefer PACs. But some
OEMs such as those producing simple, low-cost, stand-alone machines, prefer to
buy individual automation components because they are simple to use, reliable
and, most importantly, cost effective.
There is also agreement that the PACs' use of standard
networking interfaces and protocols, their powerful control features, extensive
communications capabilities and compact size are increasingly making an
impression on OEMs. Machine builders tend to find PACs easy to embed and use
not only for control, but for other purposes as well such as gathering machine
or system data and monitoring the health, status and performance of the
machines they build and deliver.
On the flipside, what many OEMs require is a
small-sized, basic controller that they can embed in a machine to perform
simple control functions. The price
points on these devices can get quite low, so the OEM customer sometimes opts
for this in place of a more powerful and versatile PAC. What the OEM market is
often looking for is compactness, easy configuration and low cost. Often the
functionality required is very limited and doesn't vary much from installation
Tenorio, marketing manager, controller & visualization business for
Rockwell Automation, says that the term "PAC" is often used to denote an
evolution of the term PLC. He says a PAC
is more than just the controller, but rather is a complete system architecture
that encompasses controllers, networks and software all designed to communicate
seamlessly. As a result, PACs provide a single multi-disciplined control
platform for applications that span an entire plant floor.
Weighing PAC Benefits
A key benefit of the PAC platform is it provides a single
common development environment. This common development environment allows the
integration of varying applications that previously used multiple hardware and
"PLCs primarily provide machine control logic capabilities
using a fairly standard instruction set, such as what is defined by IEC 61131,"
says John Dart, program manager, global OEM solutions business for Rockwell
PACs have advanced capabilities, such as the ability to
integrate the programming of motion, robotics, safety, process and drives into
one common programming environment. This eliminates the need to have several
disparate controllers and separate interfaces between them. For applications having multiple control
disciplines, a PAC can provide a lower-cost architecture than a PLC and also
reduce complexity, improve performance and lower support costs.
Both PLCs and PACs are relevant in today's markets and
will be for the foreseeable future. The choice is typically driven by market
conditions and each machine builder's business model. Designers of lower
performance, high-volume machine types will lean toward PLCs, while
higher-performance, lower-volume machine types will lean toward PACs.
"There is demand, especially in emerging economies, for
PLCs that have "just enough" PAC-type features to enable mid-range performance
at near the cost of low-end solutions," Dart says. "This demand is a
significant trend and could actually be considered an enabler to PAC adoption
rather than a constraint. As machine builders come to understand the benefits
of some PAC features that are included in a low-cost PLC, the tendency could be
to move more toward PACs."