PLCs and PACs: Know when and where to use them
Programmable logic controllers have been around for decades, and programmable automation controllers for not quite so long, but each has its special role in the manufacturing hierarchy.
Both PLCs and PACs can come in small or large packages and range in price from about a hundred dollars or more to thousands of dollars. Knowing which to specify for an application depends on your needs and may include, for example, cost, performance, bandwidth (number and type of I/O and processing speed), type of process (discrete, analog/PID or a combination thereof), data sharing and manipulation, and/or maintenance—keeping track of sensor and actuator health in a machine—or monitoring motor and bearing wear on pumps, compressors and the like.
No one article can pretend to have all the answers, but it can provide some bullet points to consider how much technology you need to solve an automation problem. When you purchase a machine, it already includes the necessary controllers to meet its needs. (For more on PLCs and PACs in machine control, see FE, September 2016.) However, unless you buy all of the equipment on a single line from the same manufacturer, there may be some additional controls that need to be added, for example, conveyor controls that link to a machine controller. Often, this might be the role of a “cell controller,” a device taking the appearance of a PAC with some kind of PC-based framework. No doubt, this will require additional system integration.
Fortunately, if you don’t have the engineering manpower on staff, you can enlist the help of a system integrator, or you’ll find that some A&E/C firms have knowledgeable engineering staffs that can point you in the right direction or actually do the integration job.
Classic definitions: PLCs and PACs
Simply put, a programmable logic controller (PLC) is a single microprocessor-based device used to automate equipment operation, says Laura Mann, marketing coordinator at MartinCSI, a member of the Control System Integrators Association (CSIA). The PLC was designed over 40 years ago to replace hardware-level logic used in manufacturing machinery, infrastructure and transportation control.
“Designed to replace relays and timers, it uses electrician- and maintenance-friendly ladder logic to program a sequence of events and reaction via real-world input and output devices,” says Mann.
Sloan Zupan, Mitsubishi senior product manager, describes a PLC as a sequence controller that can be programmed using a variety of languages that conform to the IEC61131-2 standard. With the appropriate I/O modules, today’s PLC works with both digital and analog inputs and outputs.
“PLCs were designed for fast digital on/off processing and were the first step in automating applications such as automotive manufacturing,” says Matt Newton, director of technical marketing at Opto 22, a manufacturer of PACs available for rack mounting, standalone operation or as a SoftPAC (runs in software on a Windows PC).
The term programmable automation controller, or PAC, was first coined by ARC Advisory Group in 2001. PACs are based on two or more processors, similar to a PC merged with a PLC capable of automating control of one or more pieces of equipment. Although the PAC includes PLC capabilities, its hardware architecture and software are designed to be more user friendly to the IT/computer programmer.
“Differences include multitasking, multiple processor modules per rack, multiple programming languages like structured text, distributed control system [DCS] [functionality], tag-based addressing, motion control, standard PC networking like Ethernet—to name a few,” says Mann. These differences allow PACs to offer more flexibility and interoperability than PLCs. Because PACs use standard Ethernet networking, plus handle a variety of industrial protocols, they “foster internet and network data sharing by leveraging open, standards-based communication and data processing technologies such as TCP/IP and RESTful [representational state transfer] APIs for programming,” says Newton.
“The line between the [PLC and PAC] is blurred and becoming ever more so, as PLCs add more and more features formerly associated with PACs,” says Don Pham, product manager at IDEC, which makes both PLCs and PACs. “At this point in the evolution of the industry, IDEC feels a PAC is a controller with the capability to directly control complex motion and vision systems—in addition to real-time control of a machine.”
There are alternatives to PACs and PLCs, which offer similar housing styles with DIN rail mounting, and Beckhoff Automation refers to small PC-based controllers as embedded PCs, according to Eric Reiner, industrial PC market specialist. These devices can use basic ARM CPUs and go all the way up to quad-core processors, making them capable of controlling entire machines and even those with attached robot arms.
PLC’s, PAC’s role in processing/ packaging
Fitting the right controller for a task is often the job of a machine builder or system integrator. Malisko Engineering, also a CSIA member, designs, implements and commissions automation solutions for various process industries where PLCs and PACs play a vital role in the control systems it designs and builds.
“PLCs are optimal for controlling simple applications with minimal I/O requirements and basic control schemes not needing complex analog control or motion control,” says Tim Malyszko, director of strategic development. “There is no need to pay the premium price for PAC functionality when the application does not require the advanced functionality.”
While PACs are optimal for large applications, they also can be implemented on small systems due to their scalability. However, solutions requiring complex calculations or advanced functionality should use a PAC, advises Malyszko.
“Plant owners typically benefit from maximized automation throughout the line,” says Scott Mark, Stellar vice president of operations, food and beverage. “However, it all comes down to cost. Many plant owners must decide where a single piece of automation equipment will have the most impact, as replacing all operations with automation technology is expensive.”
Processing and packaging are areas that can benefit from automation, especially in automated robotics like box erectors or palletizers. Both PLC and PAC technology can improve efficiency and reduce labor, creating low-hanging fruit opportunities for food plant optimization, says Mark. “PLCs have their place when it comes to controlling individual machines, as they will generally be much more cost effective and compact than a PAC,” says IDEC’s Pham. A PAC could be used to gather data from all of the individual PLCs in a system and coordinate operation among them. For example, a production machine might be controlled by a PLC and feed into a conveying system controlled by another PLC, with a PAC controlling the interaction between the two. The PAC could also act as an interface to higher-level computing platforms, such as MES and ERP systems, adds Pham.
“PACs are often used for process control and integration between systems because of their interoperability and ability to share data using a variety of methods,” says Opto 22’s Newton. A PAC is more compatible with company databases and computer networks. Because it can multitask, a PAC can efficiently run a process and simultaneously share specific data using a separate protocol, often on a separate network segment, says Newton.
What’s new and newer?
Today, a PAC has the ability to control complex motion and vision systems—and control a machine in real time, says Pham. The most advanced PLCs can perform real-time control of a machine, store and manipulate data, act as a web server and perform many other functions.
“In the past, [engineers] would typically use a PLC for a discrete packaging solution and a DCS [distributed control system] for process applications, resulting in multiple disparate systems,” says Julie Robinson, Rockwell Automation marketing manager. “Usually, a PAC allows for true plant-wide control, where discrete, process, motion, batch and safety control are all in a common platform with a single integrated development environment [IDE].”
Today’s PACs do offer advantages over PLCs, says Malyszko, such as the ability to handle not only ladder functionality, but also other programming functionality like block diagrams, structured text and sequential function charts. In addition, the same PAC can execute advanced process control, such as model-predictive control (MPC), motion and PID control. PACs are optimal for clients that standardize on one processor and I/O type, as a PAC can handle typically any application, he adds. “A newer trend is to go a step further and begin integrating the power of third-party applications alongside real-time control,” says Robinson. For example, having the ability to run Windows-based applications in parallel to the real-time, multi-disciplined PAC offers maximum flexibility and integration of control and information, what Rockwell Automation defines as its “Connected Enterprise.” Other use cases that are enabled by this technology include advanced control algorithms, cloud gateway, data concentrator, predictive computations and integrated control and visualization.
While PACs have benefitted from expanded communication and programming capabilities, the Industrial Internet of Things (IIoT) provides an impetus for engineers to provide tighter coupling of information technology and operations/manufacturing technology to increase an enterprise’s competitive advantages. One stumbling block in reaching this tightened connectivity, according to Opto 22’s Newton, has been the difficult and required complicated layers of middleware, such as OPC servers and protocol converters, to move data from the plant to enterprise level.
“As PACs advance in technology adoption, we’re starting to see more information technologies, such as TCP/IP and Ethernet, become standard features in PACs,” adds Newton. “PACs are moving up the IT technology stack with the adoption of open-standards-based forms of programming. For example, Opto 22 PACs now have a RESTful API and web server built into them. This eliminates the need for protocol converters and OPC servers, allowing IT resources, such as databases and cloud applications, to request information directly from the PAC—no middleware required.”
Safety, security, energy management and other advanced needs
Today’s safety systems no longer require the use of a separate PLC or other controller to manage them. In fact, safety systems can be run on the same PLC or PAC, if necessary. Also, several high-end PLCs already have safety built in, says MartinCSI’s Laura Mann.
“Integrating safety functions to meet safety integrity levels has become commonplace with most PACs,” says Mitsubishi’s Zupan. “Many applications that utilize PLCs are still better served by standalone, independent safety offerings, which are often less expensive.”
When it comes to security, this has also been added to some PLCs, but, Mann adds, “I expect a third-party device to always be used to isolate the plant floor from [manufacturing/enterprise networks].”
“Cybersecurity and intellectual property protection are easy to implement through the use of sophisticated hardware keys and software functionality, which protect against unauthorized access,” says Zupan. Regarding password protection, Zupan feels this, used by itself, is an outdated technology and needs additional support of usernames/access levels/passwords, IP address filtering and hardware keys.
“According to a recent Kaspersky Lab study, 21 percent of manufacturers reported a loss of intellectual property due to a breach in security,” says Rockwell’s Robinson. “In order to help companies protect their intellectual property [IP], it has become important to offer IP protection at the PAC level. Built-in security in the PAC is just one layer of protection that is part of a larger defense-in-depth approach to industrial network security.”
Security is a major concern as more information makes its way from the plant floor to the enterprise system. More end-users expect plant floor systems to pass information to and from the enterprise system without compromising enterprise or plant floor security. To ensure enterprises are not left open to vulnerabilities, automation assets, such as PLCs and PACs, need to have cybersecurity capabilities, such as data encryption and authentication, built in from design inception. Opto 22 PACs use SSL/TLS encryption and authentication to ensure data privacy and integrity, says Newton.
“Standards are critical,” adds Beckhoff’s Reiner. “In our hardware and software, Beckhoff has leveraged secure standards for transmitting data, such as MQTT and OPC-UA. In fact, all Beckhoff PC-based controllers can be used as an OPC-UA server and client, providing built-in data encryption and security.”
“PLC/PAC manufacturers continue to integrate security measures that tie directly into Microsoft User and Group Security Policy,” says Malyszko. “The additional security goes beyond password protecting programs by now giving administrators the ability to set different levels of access for different users.”
The use of PLCs/PACs in energy management systems may be a consideration for today’s plants. Because they regularly collect operations data, PLCs and PACs can increase a plant’s energy management, says Stellar’s Mark. For example, meters on water lines can track usage based on the line, day of the week or time of day. Since utilities are the second-biggest cost behind labor, understanding where these resources are going can greatly impact facility costs. However, meter installation can be expensive. It is up to the plant owner to balance the cost of installation versus the long-term benefits of utilizing this data, adds Mark.