The role of supervisory control is to integrate all functions of a building into a common network. This networking allows the concept of "intelligent" building where a master control computer operates everything from the elevators, lighting and HVAC systems to the security, emergency power, and life- safety/fire protection systems. All these systems can be programmed to operate in an integrated manner. For example, suppose there is a smoke detector that alarms on the ninth floor. The computer might sound the fire alarm on that floor immediately, drive all available elevators to the ground floor for fire personnel use, open the exhaust dampers to remove smoke from that floor, and pressurize the stairwells and adjacent floors to prevent smoke from migrating to those areas. All this building
information is transferred through a pair of twisted wires like speaker cable. The electronic signals used in these systems are typically -5 to +5 volt direct current (VDC). This control signal is only used to control the actuator that actually controls the device. In other words, this voltage is used to set a
pneumatic or electrical signal that actually does the work. DDC computer systems are sensitive to high voltage transients such as lightning and current inrush when motors are started, and must be protected from these signals with varisters. Supervisory control systems have many names; such as BAS: Building automation system; BMS: Building management system; EMCS: Energy monitoring and control system; FMS: Facility management system and EMS: Energy management system. BAS is the most generic of these terms.
For more information on the subject refer to the ASHRAE journals.
Advantages of Using DDC
The benefit of direct digital control over past control technologies (pneumatic or distributed electronic) is that it improves the control effectiveness and increases the control efficiency. DDC systems for HVAC have many advantages.
1) Speed- Signals can be sent instantly.
2) Complex calculations- The computer can work out many complex calculations that a technician might not be able to do because of the time required.
3) Reliability- The computer always works a problem in the same way, so it is reliable and accurate as long as the information fed to it is accurate.
4) Central monitoring and control- A whole system can be controlled from a single location. This allows instant operator interaction with building's system or many systems. They can provide a picture of "what is going" on in the building via a computer screen. They can also change system operation from the same central location. This could include opening/closing valves,
starting/stopping fans and changing setpoints to name a few.
5) Complex adjustments- A computerized control system receives signals from all the sensors in the system and processes all the information at the same time. It instantly computes the best possible settings for the entire system and outputs many different signals continuously to control the system. In a non-computerized system, each controller reacts to only one or two conditions, rather than to the needs of the whole system.
6) Anticipation- A computerized control system anticipates what will be needed. For example, if the outside air temperature is extremely hot or cold, system settings will adjust for the additional time it will take to bring the space to the desired setpoint. It will also anticipate needs when the outside air temperature is changing rapidly.
7) Printed reports- A computerized system may generate all types of reports and graphs to show trends of the system. These can be printed as desired.
8) Improved accuracy- There is less chance of human error. Equipment is more accurate because there are no mechanical devices and no mass (weight) such as moving contact points or mercury bulb in the computer Automatic alarms.
9) More precise control- There is less time lag between setpoints and activation. DDC systems are electronic and use electronic circuits and devices for monitoring and control. Compared to a pneumatic sensor which is ± 2°F accurate a DDC sensor is ±0.3°F. As a result, improved comfort and IAQ by monitoring conditions close to a constant level.
10) Conservation of energy- Tighter control of the system. The hours of occupation can be refined and the temperature in the space remains at a more constant level. In some duct systems this means that many alternate heating and cooling cycles are eliminated. Strategies such as demand monitoring and limiting can be easily implemented with DDC systems. The overall demand for a facility can be monitored and controlled by resetting various setpoints based on different demand levels. By storing trends, energy consumption patterns can be monitored.
11) Sequencing- Provides the proper sequence of equipment start-up after a system shut-down or a power failure.
12) Feedback- Not only sends control signals to the operating systems but also feeds back and records data concerning the status, environmental conditions, and mechanical conditions. This data can be printed as reports, charts, or graphs.
13) Innovation - Allows the operator to invent additional strategies that can be carried out by a computerized system.
14) Flexibility - Since the logic of a control loop is in the software, this logic can be readily changed. In this sense, DDC is far more flexible in changing reset schedules, setpoints and the overall control logic. Users are apt to apply more complex strategies, implement energy saving features and optimize their system performance since there is less cost associated with these changes than there would be when the logic is distributed to individual components.
15) Operability & Maintainability -The alarming capabilities are strong and most system alarms to various locations on a given network. The trending capability technician or engineer to
troubleshoot system and control problems is visualized in various formats. This data can also be stored and measure equipments' performance over time. Run-times of various equipment alarms/messages can be generated when a lead/lag changeover occurs or if its time to conduct routine maintenance.