A building control system is a comprehensive system designed for the centralized monitoring, control, and management of electromechanical equipment within a building (or building complex), including electrical, lighting, air conditioning, transportation, disaster prevention, security, and broadcasting systems. Its purpose is to create a safe, healthy, comfortable, and welcoming living and working environment, while ensuring the system's economic efficiency and intelligent operation.
In modern large buildings, central air conditioning systems are generally used. The function of the air conditioning system is to process indoor air so that the air temperature, flow speed, freshness, cleanliness and other indicators meet the requirements of the place. For this purpose, it is necessary to cool or heat the air, dehumidify or humidify it, and filter it. The corresponding equipment includes refrigeration units, hot water boilers, fan coil systems, duct systems, water pipe systems, etc. For example, in the air conditioning system, the chiller is supplied as a complete set by the equipment manufacturer. It is generally automatically controlled by a microprocessor according to the principles and laws of air conditioning. The chiller consists of a compressor, a condenser and an evaporator. The compressor compresses the refrigerant. The compressed refrigerant enters the condenser and is cooled by the cooling water, becoming a liquid. The heat released is carried away by the cooling water and discharged into the atmosphere in the cooling tower. The liquid refrigerant enters the evaporator from the condenser to evaporate and absorb, cooling the chilled water. Then the chilled water enters the water-cooled fan coil to absorb the heat in the air. This cycle continues, carrying away the heat from the room. [1]
1. INTERBUS bus
INTERBUS is an industrial fieldbus provided by the German multinational corporation PHOENIX, and is one of the earliest bus technologies. It is now widely used in the electronics, automotive, tobacco, metallurgical, warehousing and conveying technology, papermaking, packaging, smart building, and food industries, among others.
INTERBUS, along with Modbus and PROFIBUS, is one of the eight major fieldbuses. It shares common characteristics with other fieldbuses, such as system openness and reliability; modular and intelligent field devices; and a highly distributed system structure with strong adaptability to field environments. Furthermore, INTERBUS offers long-distance transmission (up to 12.8 km), eliminating the need for repeaters and facilitating expansion. Various I/O modules and functional modules can be distributed and installed according to production needs. The controller and each module are connected via a single bus cable. Expansion simply requires connecting the module to the bus; no hardware changes are needed. Only the system configuration (automatically identified) and new functions need to be modified in the control software. During bus operation, the host computer can read and write process data and variables (writing is only possible to output variables) through the INTERBUS OPC Server, thereby enabling system monitoring and processing of collected information to meet the needs of production management informatization.
2. System Overview
This article uses the PHOENIX factory building automation system as an application example. This control system is mainly focused on the air conditioning system, and the control components include air conditioning units, chilled water systems, hot water systems, lighting systems, exhaust systems, and air compressor systems. The various control components are geographically dispersed, and there are more than 2,000 I/O nodes.
Based on the characteristics of the actual system, we designed the following system architecture using the INTERBUS bus.
BK module: Bus coupler
RFC: PLC controller, manufactured by PHOENIX
The RFC controller connects to the INTERBUS bus system via a 9-pin D-type connector and to an industrial Ethernet network via an Ethernet port on the RFC controller. RFC controllers can communicate with each other via Ethernet. The distance between two adjacent substations on the INTERBUS bus is 400 meters. Depending on the field installation requirements and changes in I/O points, substations can be flexibly added by adding an INTERBUS bus coupler BK. The INTERBUS bus does not require terminating resistors, and field I/O modules and devices do not require dedicated address settings. The INTERBUS bus uses full-duplex data transmission, providing extremely high real-time data transmission performance.
The field modules use Phoenix Contact's INLINE products. The BK module is the bus coupler for each substation; each BK module can support 63 input/output modules, and the BK modules are connected via bus cables. Inline provides all the modules needed for information acquisition and data transmission for control purposes; this system uses digital input, digital output, analog input, and analog output modules.
3. Variable air volume air conditioning system based on cascade control
The air conditioning unit system is the key part of the control section of the entire building automation system. The following is the operation screen of the No. 1 air conditioning unit of the PHOENIX factory building automation system.
We chose a variable air volume (VAV) air conditioning system. Compared to a constant air volume (CAV) system, the VAV system offers significant energy savings, more flexible control, and higher air conditioning quality. However, it requires a higher initial investment, is more complex to control, and demands a higher level of management expertise.
For air conditioning systems in large spaces like the PHOENIX plant, which require high temperature control accuracy, we propose a VAV terminal control scheme based on cascade control, drawing on process control theory. The control process is as follows:
The average temperature at various points in the room is used as the primary control parameter, and the supply air temperature is used as the secondary control parameter, forming a cascade loop. Both the primary and secondary controllers are PID controllers.
y1: Main control parameter (average indoor temperature) y2: Secondary control parameter (supply air temperature)
The output of the main PID controller is the set value of the air supply temperature, and the output of the secondary PID controller is the valve opening degree. Both are output to the air valve actuator through the PLC's AO port to control the valve opening degree.
4. Conclusion
Since its launch in January 2006, this system has demonstrated excellent stability with no network failures. It successfully passed the German acceptance test and received unanimous praise.
The program development for this system was conducted using PHOENIX's PC WorX 3.03 compiler. This compiler is very powerful, capable of compiling mixed statements, function blocks, and ladder diagrams, and supports online debugging. For the host computer configuration software, we chose Genesis-32. This software is not only very easy to configure, but its open VBA interface environment is also conducive to secondary development of the system. We used VBA to develop the SMS alarm module and the overall system runtime schedule.
References
[1] Song Jing et al. Terminal regulation and control method for variable air volume air conditioning system. Intelligent Building. 2006.8:20-23
[2] Wang Yuan. Interbus-S bus and its applications. Instrumentation User Magazine. 1998.5:19-22
[3] Huang Shaogang, Jiang Hao, Yan Xiaoping. Application of Interbus fieldbus technology in automotive production logistics. Journal of Southeast University. 2003.9: Vol.33
