April 2008
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Typical Controllers in DDC

Understand the differences in controller types and applications

Part 1 of 3

Steven R. Calabrese

Steven R. Calabrese
Automated Logic Chicago

Contributing Editor

This is the first of a three-part series, and covers (in broad fashion) the different types of controllers that any particular Direct Digital Controls (DDC) manufacturer offers as part of its family of products. The types of controllers described herein are based on the “traditional model”, and specific controller types may vary considerably, from manufacturer to manufacturer. Part two of this series will go more in depth on unit level controllers, and part three will endeavor to portray a typical Building Automation System, showing all types of controllers as they are applied to the equipment that they are specifically designed for. Watch for parts two and three in the upcoming months!

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In general, there are four types of DDC controllers, at least as far as how this writing categorizes them (again, the “traditional model”). These are classified as follows: Unit Level, Equipment Level, Plant Level, and Building Level. Please again bear in mind that, even though when all is said and done they all accomplish the same results, controller styles vary considerably among manufacturers, and a particular manufacturer’s product line may not fall so neatly into the above categories. However we can still observe some similarities and establish some generalizations, which transcend the individualities that distinguish between the myriad of manufacturers and their product lines.

Unit Level Controllers

Unit level, unitary, zone level, application-specific, what have you. These are all terms used to describe a controller that is designed to take control of a “unitary” piece of equipment. By unitary I mean equipment manufactured to serve a single zone, or area of temperature control. Zone level equipment includes single-zone rooftop units, fan coil units, and terminal units (VAV and fan-powered boxes).

These controllers have a small footprint, and are able to typically reside in the controls compartment of the unitary piece of equipment that they’re controlling. They have a limited point count, and are non-expandable. The digital outputs, in lieu of being true “dry contact” outputs, may be triac type outputs, which are electronic, transistor-based DC switches. The analog outputs may or may not be true “voltage varying” analog signals. A unit level controller may accomplish proportional control via floating control or PWM (pulse-width modulation). There is no realtime clock to speak of, yet the controller may keep time via a software clock that needs to be periodically synchronized with the system timekeeper, via the network connection. Finally, programming of these controllers is limited. As they are designed to perform specific functions, the programming is not so much free-style, “anything goes” programming as it is configuration of a “canned” program. As such, the controllers will typically have a library of “personalities” (for lack of a better term), that the technician can choose from to set up the controller to perform the methods of control best suited to the particular piece of equipment.

Unit level controllers are designed and manufactured to keeps cost down. A typical office building project may consist of dozens and dozens of this type of controller. Cost matters! And in the end the minute differences in cost between devices that are numerous in quantity make a big difference in the overall components cost of a project. We’ll get into these types of controllers and their applications in part two of this series. For now we forgo any further discussion on unit level controllers. Which brings us to…

contemporary Equipment Level Controllers

When we think of equipment level applications, we are generally thinking about built-up air handlers, however that’s certainly not the extent of it. That aside, the controllers for these types of applications will be larger in size than unit level controllers, and will have an adequate point count for a range of applications. They will typically reside in a separately furnished enclosure, however they may be able to fit inside the controls compartment of the equipment they are serving, as would be the case with a large VAV rooftop unit. They may have an extended ambient operating temperature range, making them suitable for outdoor use, as long as they are protected from the elements. They’re generally expandable, with add-on modules available for point expansion. The digital outputs may be true contact closures (relay “dry contact” outputs), and the analog outputs will likely be true analogs (voltage outputs). They will have a battery backed realtime clock, that will keep time in the event of a loss of power to the controller. Programming is normally more flexible than with unit level controllers. Applications can be built from scratch, or libraries of previously built applications can be called upon and customized as required. These types of controllers can accommodate an extremely wide range of equipment level applications, and depending upon their expandability, may even extend into plant level applications, of which we now discuss.

Plant Level Controllers

Plant level apps aren’t much different than equipment level apps, in as much as they’re both typically handled by the same basic type of controller. The difference really lies in the size and point count of the particular system. Also, whereas an equipment level application is generally considered to be an air handler or some other single-equipment system, a plant level application is considered as a system that encompasses more than one piece of major equipment, and often several pieces of equipment, all to be controlled as a single system. Boiler and chiller plants are first to come to mind, however there are other types of systems that can fall into this category.

As implied above, a plant level application will have a large point count, which will be accommodated by a single plant level controller, or by multiple modules (base module plus expansion modules). The controller will in most cases reside in an appropriately located enclosure, perhaps central to the various equipment that it’s serving. As with the equipment level controllers, the digital outputs may be true dry contact closures, and the analog outputs will likely be true analogs. There will most definitely be a battery backed realtime clock on board. And programming capabilities will allow for virtually anything that can be thought up, including the impossible, so be careful with your logic!

Building Level (Network) Controller

Last on the list to discuss is the venerable building level, or network, controller. This really is not controller in the traditional definition of the word, however it’s still an essential component of most Building Automation Systems (BAS). This device is more so a system level router than a controller, as its responsibilities are geared toward the network rather than any particular system of control. As such, the building level controller is basically a network “traffic cop”, as it handles the sharing of information between controllers on the network, oversees the distribution of global data, and broadcasts important system-wide information, such as building schedules and true time-of-day. Outside access to the BAS is also typically via this controller, through a dial-up connection to it, or more commonly nowadays, through a high speed internet connection.

Tip of the Month: I suppose that the most important thing to take away from this month’s column is to simply be aware that there are notable differences between any given manufacturer’s line of controllers. For instance, while you may have the point count available on a unit level controller to operate that large VAV rooftop unit you’re specified to control, there may be other limitations that prevent you from utilizing that controller in that particular application (ambient temperature restrictions, programming limitations, etc.) So in lieu of a real “tip of the month”, I simply offer some good advice. Know your product line’s controllers, their features, specifications, differences, practical limitations, and so on, so that your applications and your controllers are all on the same page!



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