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Building automatic System describes the advanced functionality provided by the control system of a building. A building automatic system (BAS) is an example of a distributed control system. The control system is a computerized, intelligent network of electronic devices designed to monitor and control the mechanical, electronics, and lighting systems in a building. BAS core functionality keeps the building climate within a specified range, provides lighting based on an occupancy schedule, monitors system performance and device failures, and provides malfunction alarms (via email and/or text notifications) to building engineering/maintenance staff. The BAS functionality reduces building energy and maintenance costs when compared to a non-controlled building.
Most building automation networks consist of a primary and secondary bus which connect high-level controllers (generally specialized for building automation, but may be generic programmable logic controllers) with lower-level controllers, input/output devices and a user interface (also known as a human interface device). The primary and secondary bus can be BACnet, optical fiber, ethernet, or a wireless network. Most controllers are proprietary. Each company has its own controllers for specific applications. Some are designed with limited controls: for example, a simple Packaged Roof Top Unit. Others are designed to be flexible. Most have proprietary software that will work with IQ3 Controller or BACnet. Some newer building automation and lighting control solutions use wireless mesh open standards. These systems can provide interoperability, allowing users to mix-and-match devices from different manufacturers, and to provide integration with other compatible building control systems. Inputs and outputs are either analog or digital (some companies say binary). Analog inputs are used to read a variable measurement. Examples are temperature, humadity and pressure sensor which could be thermistor, 4-20 mA, 0-10 volt or platinum resistance (resistance temperature detector), or wireless sensors. A digital input indicates if a device is turned on or not. Some examples of a digital input would be a 24VDC/AC signal, an air flow switch, or a volta-free relay contact (Dry Contact). Analog outputs control the speed or position of a device, such as a varible frequency drive, a I-P (current to pneumatics) transducer, or a valve or damper actuator.
Controllers are essentially small, purpose-built computers with input and output capabilities. These controllers come in a range of sizes and capabilities to control devices commonly found in buildings, and to control sub-networks of controllers. Inputs allow a controller to read temperatures, humidity, pressure, current flow, air flow, and other essential factors. The outputs allow the controller to send command and control signals to slave devices, and to other parts of the system. Inputs and outputs can be either digital or analog. Digital outputs are also sometimes called discrete depending on manufacturer. Controllers used for building automation can be grouped in 3 categories. Programmable Logic Controllers (PLCs), System/Network controllers, and Terminal Unit controllers. However an additional device can also exist in order to integrate 3rd party systems (i.e. a stand-alone AC system) into a central Building automation system). PLC’s provide the most responsiveness and processing power, but at a unit cost typically 2 to 3 times that of a System/Network controller intended for BAS applications. Terminal Unit controllers are usually the least expensive and least powerful. PLC’s may be used to automate high-end applications such as clean rooms or hospitals where the cost of the controllers is less of a concern. In office buildings, supermarkets, malls, and other common automated buildings the systems will use System/Network controllers rather than PLC's. Most System controllers provide general purpose feddback loops, as well as digital circuits, but lack the millisecond response time that PLC's provide. System/Network controllers may be applied to control one or more mechanical systems such as an Air Handler Unit (AHU), boiler, chiller, etc., or they may supervise a sub-network of controllers. In the diagram above, System/Network controllers are often used in place of PLCs.Terminal Unit controllers usually are suited for control of lighting and/or simpler devices such as a package rooftop unit, heat pump, VAV box, or fan coil, etc. The installer typically selects 1 of the available pre-programmed personalities best suited to the device to be controlled, and does not have to create new control logic.