January 2013

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The Evolution of Electronic Controls – Part Two

And then came DDC

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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The Evolution of Electronic Controls – Part One From prehistory to the present-day

The birth of the computer paved the way for processor-based control as we know it today. However, DDC wasn’t created overnight. It has evolved over time, just as with any technology, to where it’s at now. The present-day meaning of the term DDC has become synonymous with computer-based control or even more specifically, microprocessor-based control. However, the basic definition of the term has more esoteric connotations.

Defining DDC

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) defines DDC as follows:

Direct Digital Control – Closed loop control of a condition applied directly at application using a digital loop with feedback to sense changes in condition.

If we try to dissect this definition, we find that it basically describes a typical controller. Closed loop control means that a controller is controlling a process and at the same time sensing the process (feedback) in order to determine subsequent actions to be taken on the process. The definition distinguishes “digital control” from other types of control by simply stating that the control process is done using a “digital loop”. The definition doesn’t really offer any true insight into what is meant by that.

The term digital is an extension of the word “digit”. In modern electronics, digital refers to the notion that a piece of electronic information, or bit, can assume a digital value of either a zero (absence of voltage) or a one (presence of voltage). By stringing a series of bits together, a real world value can be represented, electronically. This digital representation of a real world value can be understood and processed by a computer, which only understands information in this digital format. A simple example that relates digital electronics to HVAC controls is temperature. A temperature reading can be taken from a sensor, yet the temperature value must be converted from its continuous, real world format, to something a microprocessor can understand. A microprocessor-based controller can accept this temperature as a real world, or “analog” input, convert it to digital information (zeros and ones), process it, and perform an appropriate control action on some end device in response to the sensed temperature. Yet in the case of the proportional end device, such as the radiator control valve that we discussed in Part One of this series, the control action must be in the form of a real world value, and not in a digital format. The controller will calculate the appropriate action to be taken on the end device, digitally, and then convert the digital result into an analog control signal, one that can be accepted by the electronic actuator of the control valve.

We begin to gain a little bit of an understanding of what is meant by digital control. First and foremost, we can say that digital control is microprocessor-based. A computer is at the center of the process. This computer is supported by converters that allow the computer to interact with ”the real world”: input signals that represent real world values, and end devices that can accept real world control signals. The computations are performed digitally, by the computer. This is the fundamental concept of DDC.

The term DDC has been broadened to mean many things to many people, yet the above explanation is the “technically correct” version. In today’s market, other terms are often used interchangeably with the term DDC. Terms such as Building Automation System (BAS) and Facility Management System (FMS) typically refer to fully networked control systems, systems that utilize digital controllers to perform the control of HVAC equipment, all networked together and to some “front end” computer workstation. While there is no dispute that these fully networked control systems are DDC systems, sometimes the term DDC is (incorrectly?) used to express networking. A single digital controller operating a piece of air handling equipment can be thought of as a DDC system. A building full of “stand-alone” digital controllers (sans the network cable) can also be referred to as a DDC system. Run the network amongst all of the controllers, and you have yourself a networked DDC system, or a BAS. The point is, the term DDC should not automatically imply that the system is networked. This common “misinterpretation” shows, in one way, how the basic term has been stretched and extended to cover more than perhaps what it originally was coined to signify.

Centralized DDC

“Back in the old days…” The first versions of digital control systems were what are referred to now as centralized systems. These systems consisted of a main central processing unit (CPU), typically located in the maintenance engineer’s office, or somewhere else suited to its mammoth presence. All inputs and outputs were located at this central location. Inputs and outputs took the form of printed circuit cards that occupied space within the enclosure that housed the CPU. The more inputs and outputs that were needed, the more cards would reside in the “card cage”.

All HVAC equipment control points (sensors, switches, end devices, etc.) were wired back to the central control location, terminated as required at the appropriate input/output cards. These cards were electronically interfaced with the CPU, of which was custom programmed to perform all of the appropriate control functions of all connected HVAC equipment. You could imagine how easily a system such as this could become a virtual “rat’s nest” of wires. Suffice it to say that neat and orderly installation practices were of the utmost importance for these systems. Of course only to be negated when it was time to revise existing controls or add new equipment.

In the present century, there are likely very few (if any) of these dinosaurs still out there and in operation. There would possibly be some still in existence, were it not for the whole Y2K thing at the turn of the century. The concern was that the turnover of the calendar from the 1900s to the year 2000 would have adverse effects on many computer-based systems that weren’t originally designed to take this into consideration. Along with this concern came an immense onrush of upgrades, software rewrites, and system replacements. Thus, it is likely that many of these centralized digital control systems, having surely outlived their useful lives at this point anyway, were torn out and replaced with Y2K-conforming “distributed control systems”.

Distributed DDC

The centralized digital control systems gave way to what we call distributed digital control systems. With distributed DDC, the computing power is de-centralized, and not concentrated in one location via a single CPU. The logic is spread or distributed among digital controllers, each equipped with microprocessors, which are selected and configured to perform the control of a specific piece of equipment or subsystem. Distributed DDC is the present “state of the art”.

To put forth an example of a distributed DDC system in a typical HVAC application, consider a four-story facility served by a central station VAV air handling unit. The air handler is located in a mechanical room, and consists (minimally) of a supply fan, return fan, economizer section (outside, return, and exhaust air dampers), hot water coil, and refrigeration (DX) coil, piped to an outdoor air-cooled condensing unit. The air handler serves terminal units on all floors: cooling-only VAV boxes for the interior zones, and VAV boxes with hot water reheat coils for the perimeter zones. A hot water system, consisting of two hot water boilers, two system pumps, and distribution piping, serves the air handler’s hot water coil, as well as the perimeter VAV box reheat coils. The boilers and pumps are located in the basement of the facility.

In the old days, to control all of this equipment digitally basically meant wiring out from a central control panel to all of the various equipment in the facility. Yet now with distributed digital control at our disposal, we would be inclined to implement such control. For instance, the central station air handler would be equipped with a local digital controller, that would accommodate all required points of control for that piece of equipment. This includes (but is not limited to), start/stop control of the supply and return fans, proportional control of the economizer damper actuators, proportional control of the hot water valve, and staged control of the (remote) air-cooled condensing unit. For the terminal units, each unit would be equipped with a digital controller capable of operating the primary air damper as a means to maintain zone temperature setpoint, and for those with hot water coils, capable of modulating the hot water control valve as well. For the boiler plant, A digital controller would be located in the basement, in proximity to the boilers and pumps, and be capable of performing all facets of boiler plant control, including the staging of the boilers, pump start/stop control, and the like.

For each piece of equipment or subsystem operated by a digital controller, the logic for that particular equipment/subsystem is resident within that particular controller, and provided that each controller is programmed or configured correctly, the controllers will operate their respective equipment and subsystems in stand-alone fashion, with no dependency upon anything else for the operation of said equipment. The logic for the overall HVAC system (air handler, terminal units, and boiler plant) is distributed among decentralized controllers, each performing their own tasks. This in essence is a DDC system. A bunch of stand-alone digital controllers operating various HVAC equipment within a facility, all properly configured so as to be able to work together and provide the functionality of a single HVAC system. Now, tie all the controllers together with a network cable, and you have a networked distributed DDC system, or a Building Automation System (BAS). The controllers can be set up to share information over the network, so as to coordinate tasks, operate via a single time-of-day schedule, etc. Add a personal computer to the network, as a “front end”, and you complete the BAS, by gaining access to all of the various operating parameters and monitoring functions.

Tip of the Month: Words of Wisdom – Although technically around for decades, DDC as we know it today is a relative child in this industry, as compared to its mechanical counterparts. This century will bring about massive changes in our industry, and we must keep up with the technology, or be left behind to design control systems using the old school methods of yesteryear.


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