Architecture and Advantages of Direct Digital Controls
So far we have considered the controls of a single, simple system connected to a single DDC panel. In many buildings, there will be several systems, often with many more points controlling air-handlers, VAV boxes, heating valves, pumps, boilers and chillers. Wiring from a single huge DDC panel is not a practical option for two reasons. First, failure of the unit means failure of the entire system, and secondly, the wiring becomes very extensive and expensive. Instead, the system is broken down into smaller panels that are linked together on a communications cable, called a “communications network.”
It sounds simple, and it is if the system uses equipment from only one manufacturer. However, when more than one manufacturer is involved, it is not as simple. There are three communication issues that create problems. Let us identify them in terms of human communication first.
Languages
The problem is very similar to the problems of human language. In order for people from different countries to communicate, interpretation or language translation is required.
Similarly, in the controls world, different companies have worked up different languages. The languages differ both in terms of the words and in terms of sentence structure. There are two ways of enabling communication so that one manufacturer’s equipment can communicate with another manufacturer’s equipment that uses a different programming language. The first is to have an interpreter, called a “gateway,” between the two units. The second way is to program an additional, common language into both manufacturers’ units.
Vocabulary and Idea Complexity
Different people learn different sets of words in the same language. For simple, everyday things, like bread and water, everyone learns the words in each language. In addition, different people are trained in different skills. Consider, for example, when an engineer and an accountant want to discuss the long-term value of a project. They can find themselves having great difficulty communicating, because they have different vocabularies and different thinking skills in the same language.
Transmission Method and Speed
Finally, people send messages over long distances by a variety of methods at various speeds. For example, consider a letter being faxed to a remote recipient. It first goes through the fax machine (gateway) to be converted into telephone data. The telephone data is routed through various telephone exchanges (routers) till it reaches the receiving fax machine (gateway) that converts the data back into the original text letter.
In addition to the method, there is an issue of speed. Faxing is a quick and easy way of sending a letter, but if a whole book of text is to be sent, the much higher speed available on the Internet is considerably more attractive.
The issues of language, vocabulary and idea complexity, and transmission method and speed are very much the same in DDC systems.
Typically, a DDC panel includes software that provides the sequence of control activities and software for communicating with other panels. The internal software is generally proprietary to each manufacturer, and the communications software can be proprietary or public. There are several good, reliable communication languages, called “protocols” for simple information such as ‘the temperature is 100F’, ‘open to 60%’. The problems arise as soon as higher level communications, including any form of logic, are required.
In an attempt to eliminate the cost and challenges of no communication or expensive and limited gateways, ASHRAE produced a communications standard called “BACnet.” This is a public communications protocol that is designed to allow communication at all levels in a DDC system. It is documented in ASHRAE Standard 135-2004 A Data Communication Protocol for Building Automation and Control Networks2.
BACnet is particularly aimed at facilitating communications between different vendors’ products at all levels. This allows buyers to have more vendor choice. It is important to note, though, that while the BacNet standard estab- lishes rules, the designer still has to be very careful, since the number of rules used by different manufactures can make ‘BACnet compatible’ systems and components unable to communicate. However, with careful specification, one can obtain units and components from a variety of manufacturers that will communicate with each other.
The ability of different manufacturers’ equipment to work together on a network is called “interoperability.” To assist in ensuring interoperability and the use of BACnet, a BACnet interoperability association has been formed to test and certify products.
System Architecture
Let us now consider a DDC system and how it might be arranged—the system architecture. Consider the system illustrated in Figure 11.10.
Across the top of the figure is a high-speed network connecting main standalone panels and the operator terminal. In this example, the standalone panel on the left uses a different communication protocol (language) from the protocols used by the other two panels and the operator workstation. Therefore, a gateway (translator) connects the standalone panel on the left to the network. A “gateway” is a processor specifically designed to accept specific information in one protocol and send out the same information in another protocol.
Note that gateways are specific in terms of ‘protocol in’ and ‘protocol out’ and are often not comprehensive. By “not comprehensive,” we mean that only
specifically chosen information, not all information, can be translated (think of it as a translator with a limited vocabulary and limited intelligence).
The standalone panel on the left has a lower speed network of devices connected to it. The sub-panels might be small processors dealing with an air-handling unit, while the “data gathering panel” DGP, may be simply gathering outside temperature and some room temperatures and transmit- ting them to the other panels.
The central standalone panel does all the processing for its branch of the system, with remote DGPs to collect inputs and drive outputs. A laptop is temporarily connected to one of the DGPs to allow the operator/maintenance staff to interrogate the system. The use of a laptop allows the operator/maintenance staff to have access to every function on that network branch, but it may not allow access through the standalone panel to the rest of the system.
The right-hand standalone panel is shown as having numerous VAV box custom controllers connected to it. These controllers are factory-produced, with fixed software routines built in to them. Programming involves setting setpoints and choosing which functions are to be active. These custom controllers are attractive because they are economical, but they are restrictive, in that only the pre-written instructions can be used.
In Figure 11.10 there are a variety of devices in various arrangements with an operator PC as the local human interface. In addition, a phone “modem” is shown allowing communications with the system via a telephone from any- where in the world. The modem is a device that converts the digital signals from the PC to audio signals, to allow them to travel on the telephone lines. There are three strikes against modems: they are slow, telephone charges can be prohibitive, and only one connection can be made to the modem.
These restrictions are now being removed by adding a “web server.” A web server is another computer! The web server connects between the high-speed network and the Internet. It is programmed to take information from the DDC system and to present it, on demand, as web pages on the Internet. This enables anyone who has the appropriate access password to access the system, via the Internet, from anywhere in the world, at no additional cost.
Within the facility, web access allows any PC with web access to be used as an operator station, instead of only specifically designated operator stations. This is much more flexible than having to go to the operator’s terminal to access the system. For example, the energy manager can use an office PC to access energy data on the machine that is used for normal day-to-day office work.
This chapter has done no more than introduce you to some of the basics and general ideas of DDC. The system has advantages including:
e Increased accuracy and control performance
e System flexibility and sophistication that is limited only by your ingenuity
and the available financial resources.
e The system ability to store knowledge about the internal behavior over time
and to present this information in ways that assist in energy saving, monitoring, and improved maintenance.
e Remote access to the entire system to modify software, alter control settings, adjust setpoints and schedules via phone or via the Internet.
e With increased use and the falling price of computer systems in general, DDC is often less expensive than conventional controls.
Then, there are the disadvantages:
e DDC systems are not simple. Qualified maintenance and operations people are critical to ongoing success. They must be trained so that they understand how the system is designed to operate.
e Extending an existing system can be a really frustrating challenge due to the frequent lack of interoperability between different manufacturers’ products and even between upgrades of the same manufacturer’s products.
For fairly detailed information on the specification of DDC systems ASHRAE Guideline 13-2000 Specifying Direct Digital Control Systems4 is available.
The Next Step
In Chapter 12 we move on to consider energy conservation. We will review the subject in general before a brief discussion of the ASHRAE/IESNA Standard 90.1-2001 Energy Standard for Buildings Except Low-Rise Residential Buildings and some heat recovery and evaporative cooling energy saving methods.
Related posts:
Incoming search terms:
- Advantage and disadvantage of direct digital control
- advantage of direct digital control
- advantages of direct digital control
- advantages and disadvantages of direct digital control
- direct digital control advantage
- direct digital control architecture
- disadvantages and advantages and applications of direct digital control
- list the advantages of direct digital control