- Direct-digital control has replaced conventional control technology in energy management systems.
- The system architecture maps out the system and allows controllers to share data.
- The advent of Internet-based control has increased the scalability and capabilities of EMS systems.
Energy management systems (EMS) have long been used to control energy use in heating, ventilating, and air conditioning (HVAC), lighting, process equipment, and other building systems and equipment. Recent advances in digital controls, system architecture, and integration with Internet-based systems has increased the scalability and capabilities of these systems.
The controls for energy management systems (EMS) can be broken down into three functions:
- Measure a variable and collect data
- Process the data
- Initiate a control action
Another way to think of the control loop is to consider the main components: sensor, controller, and controlled device. Direct digital control (DDC) systems have replaced pneumatic and distributed electronic controls. DDCs are networked microprocessor-based controllers to provide sensing or control functions. These controls operate within heating, ventilating, and air-condition ing (HVAC) systems, lighting systems, and process applications to achieve significant energy savings.
Most EMS systems include standalone or remote controllers with software to eliminate the need for continuous communication. A personal computer can monitor status, record alarms and trending functions, and store data. Relatively complex strategies and energy management functions are typically available at low levels in the system architecture.
Sensors can measure temperature, time of day, electrical demand condition, pressure, airflow, water flow, and other variables that affect the controller logic. The controller compares input from these sensors with a set of instructions such as set point, throttling range, and action; then produces an output signal to operate devices such as lights, valve operators, damper operators, electric relays, fans, pumps, compressors, and variable speed drives. How the controller functions is referred to as the control response.
Control responses are characterized as two-position, floating, or proportional. In a two-position control sequence, the upper or lower limits are defined and the output changes value as the input crosses the limits. Two-position control functions as a simple switch, which can be used in basic control loops such as temperature control.
With floating control, the input is allowed to change. The user may want to track the midpoint of static pressure in a variable air volume system with a variable frequency drive. In this case, the upper and lower limit are changing, thereby changing the input parameter.
Proportional control responses produce an analog or variable output change in proportion to a varying input. This relationship is linear, and can be direct acting (output will rise with an increase in the measured variable) or reverse acting (output will decrease as the measured variable increases). There are variations of proportional control called proportional plus integral (pi) and proportional plus integral plus derivative (pid) that add capability and cost to these controllers.
Knowing the difference between these controllers is important because the appropriate controller is application dependent. Many specifications do not distinguish between the various types of controllers.
System architecture is the map or layout of the system used to describe the overall local area network (LAN) structure. The most basic task of the system architecture is to connect the DDC controllers so that information can be shared between them. Communications between devices on a network can be characterized as peer-to-peer or polling. On a peer-to-peer LAN, each device can share information with any other device on the LAN without going through a communication manager. In a polling controller LAN, the individual controllers cannot pass information directly to one of the other controllers. Data flows from one controller to the interface and then from the interface to the other controller.
These peer-to-peer primary controllers have more memory, more sophisticated processing units, higher resolution A/D converters, more accurate clocks, and can store more complex control strategies as well as trends, schedules, and alarms. Polling controllers have more limited memory and processing capabilities and must use a supervisory interface device to communicate with all other devices. Since these controllers have more limited memories, they usually do not store data and rely on the supervisory interface for this function.
The secondary polling networks are configured such that one supervisory interface can monitor a limited number of controllers. This limitation varies by manufacturer. A large number of controllers on a secondary controller network can negatively affect the number of trends that can be practically used, the amount of data that can be processed, and the speed of transmission over the network.
The Growth of Internet-Based Control
The proprietary nature of control software has limited the scalability of some EMS systems, but this is changing with the advent of Internet-based control systems based on standard Web browser software. Building control is entering the mainstream of information technology. Once separate control systems are now seen as part of a larger corporate or global information network. The advent of open protocols, the increased availability and use of site networks, and the interfacing of DDC systems to the Internet have increased the capabilities of these systems.