A data center has unique and demanding HVAC requirements. If its control system does not respond quickly and appropriately, a data center may experience a destructive and rapid failure -- even if redundant chillers, air handlers and power sources have been installed.
Yet in spite of these stringent requirements and the serious consequences of failure, most data centers are built with the same commercial DDC (Direct Digital Control) style control systems used in office buildings. This is in contrast to other mission-critical environments (semiconductor cleanrooms, pharmaceutical labs), where industrial controls, such as a combination of PLCs (Programmable Logic Controllers) with SCADA (Supervisory Control and Data Acquisition) computers or even DCS (Distributed Control Systems) systems, perform many of the same functions.
The following discussion is intended to provide an overview of the main areas where industrial and commercial style controls differ, and to help data center owners and system designers understand the value to be gained from industrial PLC/SCADA control systems.
PLC systems offer more robust options
Compared to commercial systems, industrial control systems feature more accurate and rugged sensors and devices, signal types and wiring methods. Industrial controllers are more robust, have higher performance, faster networks and more flexible programming capability. Redundancy options with industrial controls can address the most difficult control issues without relying on "passive automation."
Passive automation involves providing distributed control in which small, inexpensive controllers can be dedicated to individual machines or processes. In this case, the loss of a single controller cannot shut down the entire facility if there are redundant pieces of equipment installed each with their own controller.
Commercial systems typically use a mix of "unitary" controllers to control a single piece of equipment, with larger building controllers used for facility-wide programming tasks or monitoring general I/O points. Industrial systems use PLCs, which also come in a range of sizes and intended applications. The differences between these controllers can be discussed in terms of form factor and physical robustness, I/O type and capacity, and processor programming capability and flexibility.
Performance, flexibility and higher cost characterize PLC systems
The difference between PLC and DDC programs is essentially one of flexibility. The programming functions in a PLC are more numerous and powerful. There is a richer instruction set for math, logic and bit manipulation. Many PLCs allow encapsulation of instructions to create user-defined function blocks. This is a powerful tool that sophisticated users leverage to create simple, re-usable code. These differences allow creation of more sophisticated and powerful programs. Finally, modification of PLC programs can be done "on-line," which means the controllers do not need to be stopped if the program needs to be changed.
The two types of systems conceptually can look very similar. The distinction, in a word, is performance. Industrial systems are designed for "real-time" control. Like a DDC, a PLC program looks at sensor data input, performs logic or calculations and writes outputs. However, the speed of processing and communication in PLC systems allows inputs to be read from anywhere in the system, logic solved, and outputs to be written to anywhere else in the system in real-time. Scan rates for PLCs, even in large programs with distributed I/O, are generally measured in milliseconds. DDCs have program execution times measured in seconds.
There is a cost premium for industrial control systems. A rule of thumb for control systems is this: Industrial controls total installed cost is approximately $2000/point. Commercial systems cost approximately $1000/point. For reference, a recent data center project was completed with 500 I/O points. This represents a difference of $1M versus $500K. This estimate does not take into account the difference in maintenance and service contract costs (which is typically higher for commercial controls) but is a reasonable idea of the difference in up-front costs.
Owners and system designers should not expect to achieve industrial control system performance on a commercial control system budget. But consider: The control system represents just 1-2% of the total facility cost. With today's ever more demanding environments, it pays to consider the long-term value represented by the increased performance, flexibility and reliability of PLC/SCADA systems.
ABOUT THE AUTHOR: Steve Blaine is an instrumentation and controls specialist with IDC Architects, a CH2M HILL company. He has more than 25 years experience designing control systems for processes and facilities -- including data centers.
