Title: Reading Assignment
Course: WGF 105 DEVELOPMENT > Section: Content

Sub-Title | Preparation, Screen Check, Trending
Content
The basic control system monitors field devices and sensors that are connected to controllers or directly to a system headend. The data from the sensors and field devices are inputs to the control system. The data input is processed and compared to the rules and settings for the control system. Based on the input data, the control system will either determine the data is within an acceptable range and the system is operating properly or the data input is outside of an acceptable range triggering the control system to issue commands to change the status of the controlled device. A typical example would be the room temperature being higher or lower than the temperature setpoint. In this case the control system processes that data and commands or manages the equipment (in this case fans and dampers) to get the room temperature back to the setpoint.

Technician working on controls

(Copyright: O.C. McDonald Co., Inc.)

Each of the control systems is a network and the systems have some commonality. They all have data points that can be monitored and managed. The systems need cabling and cable pathways or wireless connectivity. The system will use a communications protocol to exchange data within the control system. Many of the systems will have a databases and system administration workstations. In addition, the systems will probably need IP addresses, power, and equipment room space and will generally require web access.

Sensors and transmitters that are part of a control system may include thermostats, liquid differential pressure transmitters for pumps and chillers, differential pressure sensors for fluids and airflow, static pressure sensors, air pressure sensors, CO2 sensors, occupancy sensors, humidity sensors, etc.

These devices may communicate to each other or to the controller with analog or digital signals. Analog inputs to a controller can be a continuously changing signal from an external device or sensor, such as a temperature sensor. Digital inputs to a controller are simply a two-state, on-off signal from external devices or sensors, such as a switch.

In much the same way, analog outputs from a controller are "proportional variable" signals sent by the controller to adjust an actuator or external control device, such as a valve actuator. Digital output from a controller is a two-state or two-position signal from the controller to an actuator, such as control fan relay start-stop switch.

Most field devices and equipment of building automation systems communicate at low network speeds, typically transmitting at rates of less than 1 Mbps. The communications network for a building automation system is typically in a physical star or bus topology from the controller.
Links/Uploads | Facility Performance Evaluation http://www.wbdg.org/resources/fpe.php?r=meet_performance Smart Controls  http://www.wbdg.org/resources/smartcontrols.php
Attributions | Image and content courtesy of Whole Building Design Guide – National Institute of Building Sciences