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Programmable Automation Controllers

Programmable Automation Controllers Bring Together the Best of PLCs and PCs

In a single compact controller, PACs offer many of the advanced control features, network connectivity, device interoperability and enterprise data integration capabilities found in most PLC- and PC-based automation controllers.

BY DAVID CRUMP & DAVID HILL, OPTO22

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Implementing a modern industrial application can present a challenging and sometimes daunting mix of requirements. For example, it is well understood that a typical control system must interface with signals from simple sensors and actuators. Yet for many modern applications this is merely the starting point. Advanced control features, network connectivity, device interoperability and enterprise data integration are all capabilities increasingly demanded in a modern industrial application. These modern requirements extend far beyond the traditional discrete-logic-based control of input/output (I/O) signals handled by a programmable logic controller (PLC).

Most PLCs are programmed using ladder logic, which has its origins in the wiring diagrams used to describe the layout and connections of discrete physical relays and timers in a control system. Applications that diverge from or expand beyond this model become increasingly hard to program in ladder logic. For example, mathematically complex applications such as proportional-integral-derivative (PID) loops used for temperature control involve floating-point arithmetic. To perform these calculations, PLCs must often be enhanced with separate—and separately programmed—hardware cards.

Using a PLC to meet modern application requirements for network connectivity, device interoperability and enterprise data integration presents other challenges. These types of tasks are usually more suited to the capabilities of a computer (PC). To provide these capabilities in a PLC-based application, additional processors, network gateways or converters, “middleware” software running on a separate PC, and special software for enterprise systems must often be integrated into the system.

On the other hand, a PC packaged for industrial environments can provide many of the capabilities sought in modern applications, particularly those needed for networking and data communication. However, just as it is necessary to augment a PLC to accomplish PC-like tasks, an industrial PC that needs to perform PLC-like tasks, such as machine or process control, also requires expansion. For example, a PC may be using an operating system that is not optimized for high-performance and deterministic industrial applications. Additional I/O expansion cards or special extensions may need to be integrated into the PC’s operating system to provide the high-performance, deterministic or near-deterministic operation.

Introducing the PAC

Automation manufacturers have responded to the modern industrial application’s increased scope of requirements with industrial control devices that blend the advantages of PLC-style deterministic machine or process control with the flexible configuration and enterprise integration strengths of PC-based systems. Such a device has been termed a programmable automation controller, or PAC. 

While the idea of combining PLC and PC-based technologies for industrial control has been attempted previously, it has usually only been done through the “add-on” type of approach described earlier, where additional middleware, processors, or both are used in conjunction with one or more PLCs. A PAC, however, has the broader capabilities needed built into its design. For example, to perform advanced functions like counting, latching, PID loop control and data acquisition and delivery, a typical PLC-based control system requires additional, and often expensive, processing hardware. Many PACs have these capabilities built in. 

PACs are also notable for their modular design and construction, as well as the use of open architectures to provide expandability and interconnection with other devices and business systems. In particular, PACs are marked both by efficient processing and I/O scanning, and by the variety of ways in which they can integrate with enterprise business systems.

Characteristics of a PAC

Most agree that industrial analyst ARC Advisory Group created the term “PAC.” ARC coined the term for two reasons: to help automation hardware users better define their application needs, and to give automation hardware vendors a term to more clearly communicate the capabilities of their products (Figure 1). According to ARC, a programmable automation controller must fulfill the following requirements:

Figure 1
Figure 1: The Opto22 Snap Pac system is an example of a modular, integrated system of hardware and software for industrial control, remote monitoring and data acquisition applications.

• Operate using a single platform in multiple domains, including logic, motion, drives and process control.

• Employ a single development platform using common tagging and a single database for development tasks across a range of disciplines.

• Tightly integrate controller hardware and software.

• Be programmable using software tools that can design control programs to support a process that “flows” across several machines or units.

• Operate on open, modular architectures that mirror industry applications, from machine layouts in factories to unit operation in process plants.

• Employ de facto standards for network interfaces, languages and protocols, allowing data exchange as part of networked multi-vendor systems.

• Provide efficient processing and I/O scanning.

 Development and Functional Benefits of a PAC

The characteristics that define a PAC also describe the key advantages of deploying a PAC in an industrial application. These advantages include being able to independently meet the complex requirements that PLCs require extra components to do, and improved control system performance due to tightly integrated hardware and software.

The integrated hardware and software also provides advantages when programming. The integrated development environment (IDE) used to program a PAC includes a single tagname database shared by all development tools. PACs use one software package to address existing and future automation needs, instead of multiple software packages and utilities from various vendors.

Another benefit is the ease with which PAC-based control systems can be upgraded. Modular processor hardware can be replaced without having to rip out existing sensor and actuator wiring. Plus, due to their compact size, PACs are able to conserve valuable cabinet space (Figure 2). Additionally, with their modern networking and communication capabilities, PACs make production information available in or near real time. This in turn makes the data collected more accurate and timely, and thus more valuable for business use.

Figure 2
Figure 2: PACs are flexible, multidisciplinary devices suitable for many types of industrial control.

PACs offer multiple financial advantages. The overall cost of the control system is lowered because hardware is less expensive, and less development and integration time is required. Purchasing a PAC is often more affordable than augmenting a PLC to give it similar capabilities. There is also a lower total cost of ownership due to PACs adaptability to a broader range of applications.

The single PAC in Figure 3 is operating in multiple domains to monitor and manage a production line, a chemical process, a test bench and shipping activities. To do so, the PAC must simultaneously manage analog values such as temperatures and pressures; digital on/off states for valves, switches and indicators; and serial data from inventory tracking and test equipment. At the same time, the PAC is exchanging data with an OLE for Process Control (OPC) server, an operator interface and a SQL (Structured Query Language) database. Simultaneously handling these tasks without need for additional processors, gateways, or middleware is a hallmark of a PAC.

Figure 3
Figure 3: Modern industrial applications often include multiple tasks requiring I/O point monitoring and control, data exchange via OPC, and integration of factory data with enterprise systems.

In the factory example in Figure 3, the PAC, operator and office workstations, testing equipment, production line and process sensors and actuators, and barcode reader are connected to a standard 10/100 Mbit/s Ethernet network installed throughout the facility. In some instances, devices without built-in Ethernet connectivity, such as temperature sensors, are connected to I/O modules on an intermediate Ethernet-enabled I/O unit, which in turn communicates with the PAC.

Figure 3
Figure 3: Modern industrial applications often include multiple tasks requiring I/O point monitoring and control, data exchange via OPC, and integration of factory data with enterprise systems.

Using this Ethernet network, the PAC communicates with remote racks of I/O modules to read/write analog, digital and serial signals. The network also links the PAC with an OPC server, an operator interface and a SQL database. A wireless segment is part of the network, so the PAC can also communicate with mobile assets like the forklift and temporary operator workstation. The PAC can control, monitor and exchange data with this wide variety of devices and systems because it uses the same standard network technologies and protocols that they use. This example includes wired and wireless Ethernet networks, Internet Protocol (IP) network transport, OPC and SQL. In another control situation, common application-level protocols such as Modbus, SNMP (Simple Network Management Protocol) and PPP (point-to-point protocol) over a modem could be required. The PAC has the ability to meet these diverse communication requirements.

In the factory example, the PAC exchanges manufacturing, production and inventory data with an enterprise SQL database. This database in turn shares data with several key business systems, including an enterprise resource planning (ERP) system, operational equipment effectiveness (OEE) system and supply chain management (SCM) system. Because data from the factory floor is constantly and automatically updated by the PAC, timely and valuable information is continually available for all business systems.

A variety of industrial automation vendors now offer PAC or PAC-like products. In some cases, the product is more PLC-like, while in other cases the offering is more like an industrial PC. As described earlier, PACs integrate capabilities from both of these devices, so a device that emphasizes PLC or PC capabilities may or may not fit the application requirements. As with any product, some vendors have been in the game longer than others. While many vendors have recently introduced their new PAC or PAC-like offerings, a select few companies have demonstrated a successful track record of providing PAC functionality several years before the term “PAC” entered the mainstream.

Opto22
Temecula, CA.
(951) 695-3000.
[www.opto22.com].