1 Introduction
BuildingAutomationSystem (BAS) is an indispensable part of intelligent building. Its task is to monitor and control the energy use, environment, traffic and safety facilities in the building, so as to provide a safe, reliable, energy-saving and comfortable working or living environment.
2 composition and basic functions of building automation system
Building equipment automation system usually includes HVAC, water supply and drainage, power supply and distribution, lighting, elevator, fire protection, security and other subsystems. According to China's industry standards, BAS can be divided into equipment operation management and monitoring subsystem and fire protection and safety prevention subsystem, as shown in the figure. Generally speaking, BAS should consider these two subsystems at the same time. If the fire safety subsystem is set up independently, it should also establish communication with BAS monitoring center, so as to transfer the operation right according to the agreement and carry out comprehensive coordinated control in case of disaster.
The basic functions of building equipment automation system can be summarized as follows:
(1) automatically monitors and controls the start and stop of various mechanical and electrical equipment, and displays or prints the current running status.
(2) Automatically detect, display and print the operating parameters of various mechanical and electrical equipment and their changing trends or historical data.
(3) Automatically adjust all kinds of equipment according to external conditions, environmental factors and load changes, so that it always runs in the best state.
(4) Monitor and handle all kinds of accidents and emergencies in time.
(5) To realize the unified management and coordinated control of various mechanical and electrical equipment in the building.
(6) Energy management: water, electricity and gas are metered and charged to realize energy management automation.
(7) Equipment management: including equipment files, equipment operation reports and equipment maintenance management.
3 the principle of building automation control system
Building control system is a distributed computer control system based on modern control theory, also known as distributed control system (DCS). Its characteristic is "centralized management and decentralized control", that is, the microcomputer control device (DDC) distributed on the site of the controlled equipment completes the real-time detection and control tasks of the controlled equipment, which overcomes the shortage of high concentration of danger caused by centralized control of computers and the limitation of single control function of conventional instruments. The central management computer installed in the central control room has CRT display, printout, rich software management and powerful digital communication functions, which can complete tasks such as centralized operation, display, alarm, printing and optimal control, avoid the shortcomings of difficult man-machine contact and unified management after the conventional instrument control is decentralized, and ensure the equipment to run in the best state.
Here are some concepts related to distributed control system.
3.l Direct Digital Control System (DDC)
The direct digital control system (DDC) is shown in Figure 2. The computer collects real-time data through analog input channel (AI) and switch input channel (DI), then calculates according to certain rules, and finally sends out control signals to directly control the production process through analog output channel (AO) and switch output channel (DO). Therefore, DDC system is a closed-loop control system and the most common application mode of computer in industrial production.
The computer in DDC system directly undertakes the control task, so it requires good real-time, high reliability and strong adaptability.
3. 1. 1 composition of direct digital control system
Direct digital control system is mainly composed of three parts: process input channel, process control computer and process output channel.
The process input channel consists of analog input and digital input. Analog input channel consists of transmitter, sampling switch, amplifier, A/D converter and interface circuit. Among them, the function of the transmitter is to convert non-electrical signals into standard electrical signals, and the temperature, pressure and flow can be converted into 0- 10mA or 4-20mA DC signals, which can be realized by an A/D converter. The digital input channel consists of switch contacts, photoelectric couplers and interface circuits. Contact signals reflecting the on/off state of the production process are converted into digital signals through photoelectric couplers and interface circuits and sent to the computer.
The process control computer directly undertakes the operation and control tasks. Firstly, it collects various parameter signals of the controlled object through the process input channel, then performs operation according to a predetermined control law (such as PID), then sends control signals to the controlled object, and then directly controls the actuator such as the regulating valve through the output channel.
The process output channel consists of analog output and digital output. The former converts the digital control signal output by the computer into an analog voltage or current signal, and then drives the actuator such as the regulating valve through the amplifier to control the production process. This part consists of interface circuit, D/A converter, amplifier and actuator. In the latter, the computer drives the solenoid valve and relay actuator through the switching signal output by the amplifier, and the amplifier consists of interface appliances, photoelectric couplers, amplifiers and actuators.
3. 1.2 Basic algorithm of direct digital control system
According to the proportion (P), integral (I) and differential (D) of deviation, it is the basic law with mature technology and the widest application in continuous systems. Discretizing PID control law and realizing it on computer can conveniently use the accumulated mature technology, and can achieve satisfactory results through online tuning when the mathematical model or parameters of the controlled object are not clear. Therefore, the digital PID algorithm which discretizes the analog regulation law has been widely used in industrial process computer control system and has become the basic algorithm of DDC system.
Digital PID control algorithm, the ideal PID formula of analog regulator is
Where e(t)- deviation (the difference between the set value and the actual output value)
U(t)- control quantity
KP- proportional amplification factor
Ti integral time constant
TD- differential time constant
Write in the form of transfer function
In order to realize it on computer, the differential equation in continuous form must be transformed into the difference equation in discrete form. Let it be the sampling period (t is small enough compared with the system time constant), and k is the sampling sequence number (k = 0, 1, 2, ...), and the product can be calculated by the rectangular method, and differential is used instead of differential.
Where e (k) is the deviation value obtained from the kth sampling.
E(k- 1)- the deviation value obtained from the (k- 1) th sampling.
U(k)- the control quantity at the kth moment.
The smaller the sampling period t in the above formula (compared with the system time constant), the closer the controlled process is to the continuous control process, which is also called "quasi-continuous control".
3.2 Architecture of Distributed Control System
Distributed control system (DCS) appeared in the mid-1970s and developed rapidly. It integrates computer technology, control technology, graphic display technology and communication technology, and can control scattered equipment on site, which is convenient for centralized management and operation. Compared with the previous control system, it not only avoids the shortage of centralized control by a single computer, but also overcomes the shortcoming of difficult man-machine interaction of conventional instruments.
Many microcomputers in the decentralized control system have replaced a single computer in the centralized control system, which has dispersed the danger structurally and improved the reliability. Its basic structure and function are shown in Figure 3, in which field control station, data acquisition station, engineer station, operator station, monitoring computer and management computer are organically combined through data communication network to form a layered distributed control system.
3.2. 1 Distributed Control System Data Communication Network
Data communication network is the backbone of distributed control system. The structure of the whole distributed control system is essentially a network structure. Field control station, data acquisition station, engineer station, operator station and monitoring computer are all "nodes" on this network, which all contain CPU and network interface. They all have their own specific network address (node number) and can send and receive data through the network. All nodes in the network are in an equal position, and they can enjoy resources without interdependence. It also makes the functional structure and network architecture area with scattered dangers have great flexibility and expansibility, which can meet the needs of expansion and upgrading of distributed control systems and is very flexible and convenient.
(1) The characteristics of control network The communication network of decentralized control system is different from the general computer network. Compared with the general communication network, it has the following special requirements: ① high reliability and security, and the transmitted information is absolutely accurate and reliable. Therefore, redundancy technology, backup measures and self-diagnosis function are usually adopted. For example, the control station uses dual CPU boards and dual I/O boards. ② Good real-time performance. ③ Strong adaptability to the environment.
(2) Network topology: Bus network and ring network are commonly used in construction machinery automation system. In these two structures, any two nodes can communicate directly through the network, and all nodes are in an equal position.
(3) The network communication protocol constitutes the construction equipment automation system, and there must be a working language that everyone can accept and abide by to realize the dialogue between them. This is a data communication protocol standard.
BACnet protocol of building automation control network consists of physical layer, data link layer, network layer and application layer, or equivalent to the first, second, third and seventh layer protocols of OSI.
ARCnet is a token bus network with a data transmission rate of 2.5-20 bit/s, which has good real-time performance. Ms/TP is a master/slave token passing data link layer technology, which allows the use of EIA-485 hardware. BACnet realizes the communication technology between automatic control systems of different manufacturers, that is, the interconnection technology from one "island" to another.
3.2.2 Application of Fieldbus Technology-Further Decentralization of Distributed Control System
Overview of Fieldbus (1) Fieldbus is a digital bidirectional transmission and multi-branch communication network connecting intelligent field devices and automation systems. Different field buses follow different protocols and interface standards and have their own characteristics. Fieldbus technology has the following characteristics: ① Using digital signals instead of 4-20mA analog signals greatly improves the accuracy and reliability of signal conversion, so Fieldbus has a high cost performance. ② Fieldbus connects the intelligent instruments (intelligent sensors and intelligent actuators) on the equipment site into a network, and disperses the functions of control, alarm and trend analysis to the field instruments, which further disperses the control structure and leads to the change of the control system architecture. (3) Instruments and equipment from different manufacturers that meet the same fieldbus standard can be networked to achieve interoperability, and different standards can also be interconnected through gateways or routers. Fieldbus control system is an open system.
(2)LonWorks technology
LonWorks is a completely distributed local operating network (LON) technology. LonWorks network node consists of neuron chip, transceiver, firmware and I/O interface circuit. The intelligent node takes Neuronchip as the core and consists of media access control processor, network processor and application processor, which makes the node not only manage network communication, but also have control function. Block diagram of neuron chip.
Firmware is attached to the chip, which realizes LonTalk communication protocol and all task scheduling. LonTalk protocol follows the open interconnection reference model OSI proposed by ISO, and has a complete seven-layer protocol, which manages the communication of network nodes, assigns node addresses, runs embedded collision/detection avoidance algorithms, and controls physical/electrical connections.
Neuron chip not only has control function, but also has media access control processor and network processor. LonTalk protocol is solidified in the ROM of the chip, so that the micro-nodes of LonWorks do not need the fully distributed control mode of the central structure, and the control function is dispersed to the field-level instruments.
LonWorks network can use twisted pair, power line, coaxial cable, optical cable, radio, infrared and other communication media, and provide transceivers suitable for the above media, so that signals in the same network can be transmitted between different media, so it can be networked as needed, and different media can be connected through routers.
LonMark is an industry standard to prevent many manufacturers from interpreting LonWorks technology with different meanings, and to ensure that different products can be easily integrated to form a truly open system.
(3) Further decentralization of decentralized control system
The traditional distributed control system is still a centralized structure at the site control station level, but now the distributed control system is a new system developed on the basis of the original distributed control system and the construction equipment automation system using LonWorks fieldbus. Standard LAN is the original distributed control system, which uses BACnet protocol to facilitate the exchange of communication information between different subsystems of multiple suppliers and connect the isolated islands with control functions into a whole. The newly added LonWorks field bus adopts LonTalk protocol, which further disperses the control function to field-level instruments, and the standard LAN is connected with the router between the field buses. In this way, BACnet and LonMark complement each other and rely on each other to form a completely decentralized and truly open construction equipment automation system.
4. The development history of building automation system equipment and the introduction of related products
Building equipment automation system has experienced four generations of products so far:
First generation: CCMS central monitoring system (product of 1970s)
BAS has developed from an instrument system to a computer system. The central station consists of computer keyboard and CRT, and the recording instrument is replaced by printer. The information collection stations DGP scattered around the building (connected with sensors and actuators) are connected with the central station through the bus to form a central monitoring automation system. The function of DGP substation is only to upload field equipment information and issue control commands of central station. A central computer controls the whole system. The central station collects the information of each substation, makes decisions and controls all the equipment. The central station completes energy-saving control and adjustment according to the collected information and energy metering data.
The second generation: DCS distributed control system (product of 1980s)
With the development of microprocessor technology and cost reduction, DGP substation has installed CPU and developed into direct digital controller DDC. DDC substation is equipped with microprocessor chip, which can complete all control work independently, with perfect control and display functions, energy-saving management, connection with printer and installation of man-machine interface. BAS consists of four levels, namely, site, substation, central station and management system. The main feature of the distributed system is that there are only two contact modes: the central station completes monitoring, the sub-station completes control, and the sub-station is completely autonomous, which has nothing to do with the central station, thus ensuring the reliability of the system.
Third Generation: Open Distributed System (product of 1990s)
With the development of field bus technology, DDC substation is connected with the input and output modules of sensors and actuators, and LON field bus is adopted to form a distributed input and output field network layer from the inside to the equipment site, which makes the system configuration more flexible. Because of the openness of LonWorks technology, the substation has certain openness. BAS control network has formed a three-layer structure, namely management layer (central station), automation layer (DDC substation) and field network layer (LON).
The fourth generation: network integration system (2 1 century product)
With the establishment of Intranet, the construction equipment automation system will inevitably adopt Web technology and strive to occupy an important position in the enterprise network. BAS central station embeds Web server, integrates Web functions, and takes the form of web pages as the working mode, making BAS and Intranet an integrated system.
Network integration system (EDI) is a building equipment automation system using Web technology. It has a set of management software, including security system, electromechanical equipment system and fire fighting system.
EBI system provides a variety of perfect open technologies to meet the needs of different levels, and realizes the integration of all levels from field layer, automation layer to management layer. EBI system completes the integration of management system and control system. The structural diagram of the network integration system is shown in Figure 7.
At present, the large-scale and influential construction equipment supply companies include Honeywell, Johnson, KMC and Siemens of Germany.
5 concluding remarks
Building automation control technology is still a new technical field in China. With the emergence of more intelligent buildings, more advanced technologies will be added to this field, making this technology more mature and perfect.
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