PACs—The Next PlatformSeptember 1, 2005 By: Greg Wempe, National Instruments Sensors
Programmable automation controllers combine the best of PLCs and PCs in a new control platform to deliver greater functionality, scalability, and flexibility.
For decades, engineers have developed industrial automation solutions with the help of programmable logic controllers (PLCs). As the challenges evolved, so have PLCs, incorporating analog I/O, network communications, and programming standards. Those whose needs exceeded the capabilities of PLCs turned to PCs that featured advanced processors, high-speed I/O, and advanced software development tools. However, the increased flexibility of the PC environment came at a cost because PCs were not designed for rugged industrial environments and standard PC operating systems did not provide the reliability and stability required for industrial control applications. Hybrid solutions began to appear, and engineers began using PLCs for closed-loop control and PCs for more advanced functionality, such as vision integration, high-speed I/O, and report generation.
The nature of a hybrid solution, a solution containing multiple hardware and software platforms from various vendors, ushered in new challenges of its own, primarily integration and maintenance. Engineers sought a single-platform system with multidomain functionality and an open, modular architecture, supported with full-feature programming environments.
The answer to this need came in the form of the programmable automation controller (PAC). Engineers now have a new class of controller that melds the processing power, data collection speeds, and communication capabilities of the PC with the reliability and industrial form factor of a PLC (see Figure 1).
Parallel to the advances in automation controller technology, the ongoing pursuit of increased efficiency in automation systems coupled with advances in new system development has spurred significant innovation in industrial measurement technology. Although the types of industrial measurements have not changed much, sensor vendors have significantly improved measurement technology. Today's sensors offer increased sensitivity, faster response, decreased hysteresis, and longer-term stability and durability, resulting in more accurate and more dependable sensors. Additionally, there are now sensors that take multiple types of measurements and sensors with onboard ADCs and DACs that reduce the need for external linearization circuitry.
PACs let you take full advantage of enhanced sensor accuracy through increased measurement resolution and signal conditioning. PAC-based acquisition hardware offers as much as 18 bits of resolution and signal conditioning for a variety of sensor inputs, including strain gauges and LVDTs, which traditionally require extremely precise signal conditioning to ensure measurement accuracy.
Another area of evolution for industrial measurements is analog loop rates. Traditional industrial applications scan analog inputs at or below 100 Hz. For many applications, including vibration and rota- tion monitoring, this frequency is far too slow and can result in missing critical transients. Because of PAC's greater processing power and high-throughput backplane architecture, you can take high-accuracy measurements at hundreds of thousands or millions of samples per second. The increased data rate ensures that even minute changes are detected. Increased input rates, combined with increased power for data processing, also translate to improved system control. Users have successfully integrated PACs into power-control applications, where tighter control-loop rates provided by the increased input measurement speed and real-time data analysis capabilities resulted in significant improvements in efficiency.
PACs' open architecture also simplifies the upgrade path for users with existing industrial automation and control systems. In many instances, users have deployed PACs in parallel with existing control systems, introducing additional functionality, such as online data logging, independent of the existing automation or control system.
To further integrate the PAC into an established system, you can make simple digital I/O connections between a PAC and PLC. In this configuration, the PAC provides the high-speed acquisition and analysis capabilities and passes a digital output to the PLC for integration into the overall control routine. Also, standard communications buses and bus gateways let you add PAC functionality, such as machine vision and image processing, to an existing architecture by simply plugging the PAC into the system's main communications bus, requiring little or no rework to the existing system (see Figure 2).
Figure 2. PACs provide hardware and software flexibility, integrating machine vision, motion, and high-speed analog I/O with advanced software functionality.
The merging of the PLC and PC into the PAC platform also brings advanced functionality and flexibility in terms of software. Most PLC users have historically been restricted to developing their applications using ladder logic, an easy-to-use programming language that abstracts much of the underlying housekeeping functions, leaving the user to focus on implementing the control code. While you can learn ladder logic relatively quickly, the abstraction that simplifies the ladder logic development experience comes at the expense of functional extensibility, such as communications capabilities or advanced control algorithms. PLC vendors have invested significant amounts of capital into the development of features that help overcome some of these issues, such as the introduction of function block diagrams. Unfortunately, their efforts remain disparate and a unified effort for standardizing vendor-specific ladder languages has yet to be fully adopted by the industry.
On the other hand, PACs inherit the software flexibility afforded to PC users, placing the software architecture decision in the hands of the user, not the hardware vendor. You can choose to develop with a single programming, simplifying code integration across a team and throughout the design cycle, or opt for a modular software architecture. This type of architecture, built on an open, general-purpose language that accepts individual nodes exported from niche development environments, offers maximum extensibility and system scalability.
The software flexibility granted to PAC users also positions them well for adoption of additional technology. From an enterprise standpoint, with the PAC open software architecture, you can easily incorporate database connectivity, report generation, and network communications, as well as host Web pages. PACs can use open software to easily incorporate with existing automation systems, serving as an additional node on a standard communications bus alongside PLCs or PCs.
Looking at the future of industrial control software, you can often take cues from embedded-control engineers in the automotive and aerospace industries. Advanced control techniques, such as m-synthesis and H-infinity, are used in the design and implementation of robust controllers for high loop rate systems, and significant progress has been made in these areas with PC software. As the industrial automation and control market pushes beyond the limits of PID algorithms and into the realm of these advanced control techniques, PAC users will be poised to easily absorb them into their open, flexible software systems.
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