Control System Evolution
The recent history of control technology can be seen in the transition from purely discrete devices to the use of programmable logic controllers and distributed control systems. Now, a new model—the distributed processor-based system-—has appeared on the horizon boasting the best features of earlier systems while providing new capabilities.
Traditionally, manufacturers have used programmable logic controllers (PLCs) to automate simple, discrete applications and distributed control systems (DCSs) to automate large-scale process applications.
Today, however, manufacturers must run complex processes employing tighter enterprise integration of control, production, and front office applications while offering customers the opportunity for greater product customization. Crossing the control technology boundaries that separate traditional methods of operation and the demands of the current business climate requires manufacturers to possess a high degree of flexibility.
To better understand how to make this leap, let's take a look at where control system technology has been...and where it's going.
PLCs have the distinct advantage of being low cost, very compact, tolerant of rough manufacturing environments, and having excellent logic-handling capabilities. However, they cannot support fast loop times easily. As the number of controllers and control variables increase, opting for a PLC-based solution implementation becomes increasingly difficult. Even when using HMI (human-machine interface) over a network for ease of PLC configuration—and to lower the cost even further—each PLC must still be configured separately. Furthermore, if you choose an HMI product from a vendor other than the manufacturer of the PLC, another layer of configuration will be required. Because PLCs are sold at the device level, independent system integrators are often needed to configure the complex application of control, PLC, and HMI systems. This added cost of configuration can easily offset the lower cost advantage typically associated with PLCs.
DCS architecture allows the use of a single, unified environment, often called an integrated development environment, in which to create applications. With a DCS, supervisory applications such as database, alarms, trends, displays, and system management emanate automatically from the control strategy. The distributed nature of the system also enables common creation of control strategies from a function block library that simplifies creating, testing, and validating control applications. It is also much easier to implement redundancy at both the control and supervisory level. However, a DCS is not well suited for the rough factory floor environment or high-speed switching. Also, the cost of implementing a DCS is only effective for applications involving numerous control loops.
Things to Come
New, emerging system configurations link micro-processor equipped controllers together to form mini-DCSs. Using this method, a very small operation can enjoy multi-loop control and a large organization can enjoy greater flexibility in deploying control strategies throughout its operations. These solutions include HMI, such as Contec's IPC-PT/H630X(PCI)CP300, through which users can implement continuous control, sequential control, batch control, set point control, trending and logging, touch-screen control, and recipe management.
The cost of implementing a distributed processor-based system is 30-50% lower than that of a traditional DCS. Also, programming elements of the graphical interface, such as function blocks, sequential function charts, and structured text, make tasks using ladder logic control strategies much simpler and less expensive to configure. The single database used in distributed processor-based systems allows users to manage the control strategy and the tag data without mistakes.
Benefits of this emerging system include the uniformity of quality assurance that comes from operating a system as a unit and uniform documentation, which makes the implementation and enforcement of standards much easier to accomplish. Maintenance is also more consistent—not only as a result of the system-wide diagnostics, but the ease of troubleshooting and ability to review the control strategy configuration.
The scalable and affordable functionality offered by these new systems means that DCS capability can be delivered to applications that previously used PLCs or were not automated at all.
Contec's NEMA 4-rated HMIs are highly reliable and can withstand rough factory floor environments. In manufacturing facilities where a failed controller can result in huge losses in terms of wasted man-hours and scrap material, it is of utmost importance for these HMIs to be highly reliable. Taking the next logical step in system toughening by replacing the HMI hard-drive with a Contec solid state disk drive eliminates all moving parts from the system making it able to withstand high shock and vibrations.
Return to top