The Greenhouse Environment Monitoring System Based on Wireless Sensor Network Technology Ling-ling LI,Shi-feng YANG,Li-yan WANG,Xiang-ming GAOCollege of Electronic Information and AutomationTianjin University of Science & TechnologyTianjin, Chinae-mail lilinglingxjt@Abstract—ZigBee is a newly-emerging wireless network technology with short distance, low cost, low rate, low power, high capacity, high security and high reliability, and it is currently widely used in short-range wireless network area. Based on the ZigBee technology, the wireless sensor network has distinguished advantages in greenhouse environment monitoring system, and it supports self-organiz ing networks as well. This paper describes the greenhouse environment monitoring based on ZigBee, briefs the characteristics of these two systems, including the hardware and the software of the network coordinator node and the sensor node. The experiment shows that the node and the network coordinator are in normal communication condition, the network stability is fine, the datais consistent with the real environment, and thus wireless sensor networks can meet the requirements in the applications.Keywords: Wireless sensor network, ZigBee, Greenhouse, Environment MonitoringI.I NTRODUCTIONWireless sensor network (WSN) integrates the sensor network technology, information processing technology and network communication technology with the feature of small size, low cost and easy maintenance, which has a wide application in the area of environment data collection, security monitoring and target tracking. It comprises a great many wireless sensor nodes deployed in the monitoring region, and through wireless communication a multi-hop self-organizing network system is formed. Its role is to coordinate the perception, acquisition and process of the information of its perceptual objects within the area covered by the network, and returned data to the observer. At present, large amount of widely-distributed electronic detection devices and implementing facilities are greatly used in greenhouse production, resulting in intertwining cables in the greenhouse. Furthermore, the corresponding location of the installations and facilities should be adjusted when the crops experience evolution successions, and sometimes the cables connecting various devices and facilities should also need to be re-arranged, which not only causes the increase of the investment costs of the greenhouse and the difficulties of the maintenance, but also may affect plant growth. To realize scientifically and rationally automatic detection of the parameters of the greenhouse environment, this study introduced a new low-cost and short-range wireless network transmission technology into the Greenhouse Monitoring System, and created a greenhouse wireless sensor network monitoring system. The monitoring system consists of monitoring and control center, network coordinator and a number of terminal nodes. It could realize speedy and accurate test of greenhouse parameters. It has a great useful value in improving the maintainability and space utilization.II.Z IGBEE T ECHNOLOGYZigBee is new wireless communication technology with short distance, low complexity, low energy consumption, slow data rate and low cost, and it is based on IEEE 802. 15.4 Standard with the capacity of coordinating mutual communication among thousands of tiny sensors [1]. Through the radio waves, these sensors can transmit the data from one sensor to another with small energy cost and high efficiency. Compared with various existing wireless communication technology, ZigBee technology has the lowest energy consumption and cost. Because of the slow data rate and the small range of communication, ZigBee technology is extremely suitable for agricultural field which has small amount of data flows. The technical features of this technology also make it the best choice for wireless sensor networks. Therefore, it has the practical significance when applied in the crop environmental monitoring systemZigBee has the following features:ZigBee uses a variety of power-saving modes to guarantee that it could be used for at least six months to two years powered by two AA batteries.ZigBee uses the avoidance collision mechanism in CSMA-CA and pre-set a prior particular time slot for a fixed bandwidth communications service in order to avoid competition and conflict when sending data. MAC layer adopts a fully confirmed data transport mechanism, and each packet sent by the receiver must wait for confirmation.Zigbee has self-organizing features that one node can sense other ones without any human interventions, and connect with each other automatically to create a completed network. It also obtains self-recovery function that the network can repair itself when a node is added or deleted, the position of a node is changed, or a breakdown occurred. It also can adjust the topology structure to ensure that the whole system can work normally without any human interventions [2].Proceedings of the 2011 IEEE International Conference on Cyber Technology in Automation, Control, and Intelligent Systems March 20-23, 2011, Kunming, ChinaZigBee protocol is patent-free that the equipment cost is reduced effectively. ZigBee's working frequency is flexible, namely 2.4GHz license-free frequency, which means unlimited communication without using fees.One ZigBee network can accommodate up to 254 servo devices and a master device, while more than 200 ZigBee network could exist in one region.ZigBee supports data integrity and authentication functions, and the encryption algorithm adopts AES-128 function. All applications could determine their own security properties flexibly [3].III.T HE B UILDING P RINCIPLE OF W IRELESS N ETWORKZigBee has 3 topology structures which are Star, Cluster tree, and Mesh and 2 physical equipments which are FFD and RFD. FFD is always used as coordinator of network, selector of router and terminal device for ZigBee in these 3 topology structures. RFD is used in the terminal device only. The network coordinator’s work is to initialize and update the topology of the network, transmit the network symbol, manage network nodes, store the nodes’ information, provide the associated routing information between nodes and store the data of nodes, etc. In order to send, receive the routing data, the router should join in the ZigBee network. And then it can allow other router and terminal devices to connect. The router can send and receive the data when it joins in to the whole network however the router cannot access other devices join in. Furthermore, the data just only can be sent and received by the master-node not through the router.The ad hoc network’s function is realized by the startup of the ZigBee coordinator. ZigBee network is also named ZigBeePAN. When the coordinator is used for the first time, ZigBee can carry out energy scanning in multiple channels, and after that, it will scan the remaining channels. At this time the coordinator will send a broadcast that is one-hop symbol request. Any other coordinators and routers nearby will send the symbol frame to the former coordinator to respond to the symbol request. Beacon frame contains information including the PAN, the PAN identifier, and whether the equipment is allowed to join in the network or not. Once the coordinator completes the energy and PAN scanning, it will analyze all the received symbols and try to start an unused PAN ID and channel which is unique. Once the router and terminal equipment are activated, they will scan the PAN ceaselessly, receive symbols and configure to join in any ZigBeePAN, or a PAN with special PAN ID. There are two parameters for coordinator or router to decide whether accept the new device to join in: the permission of joining in and the number of nodes of terminal deceive. Terminal device use its master node to duff the received data, while coordinator and each router support the limited terminal sub-node. Once the number of terminal sub-nodes reaches the limit of coordinator or routers, no other terminal device would be allowed to join in ZigBee technology supports the device address and application-layer address. Device address provides the destination address of the packet devices. The application-layer shows a special application receiver named ZigBee terminal. There are 2 types of address in ZigBee based on 802.15.4 agreement, they are 16-bits network address and 64-bits network address. Every node should be allotted one 16-bits address when it comes into the network. Every node has its unique network address in the network, however the network address can be changed. In the other hand, the 64-bits address is the only one and not be changed any more. In the data sending, the data is always sent to the 16-bits target device address. In fact, before the sending, ZigBee device must research the whole addresses in the network. The sending data bring the 64-bits target device address, when the address of receiving is matched and then feedback a data packet, provide the address of network. The transmitter sends the date when it receives the feedback. Before sending the date, mode should guarantee the 16-bits address and the path of the target node are ensure, if there is no mode of network address to be matched, the network will give up the data packet. Fig.1 is the flow chart of Network address searching and mode oftransmission.Fig1. The flow chart of network address searching and mode of transmissionIV.T HE G ENERAL S CHEMEThe entire network includes a monitoring center and a ZigBee network. This is a hierarchical network structure, with the sensor nodes set at the bottom, ZigBee master node (coordinator) in the middle and monitoring centre in the above. As the monitoring centre, a computer is used to display the environmental monitoring data and send commands to the network. ZigBee network, which consist the primary nodes and the terminal nodes, is responsible for the collection of environmental data. In each network, it must have a ZigBeecoordinator to be responsible for initiating the network and its management and maintenance, including allocating the network address for newly-added devices, allowing a node to join in or leave, and issuing and updating the network security key etc. In order to avoid the nodes to join into any other network which may result in uneven distribution of power of network nodes, in this application, the network is divided into a number of smaller star-shaped networks, and each star-shaped network is defined as a group. The central node of the star-shaped network processes the data sent by terminal device, and then transmit data to the ZigBee primary node. There are two ways to connect ZigBee network and monitoring center, and usually the serial port is used to connect the monitoring center and coordinator directly. If it is inconvenient to use the monitoring center at the spot for a long period of time, GPRS is used to send data to a monitoring center with GPRS receiving device.The monitoring Center should monitor the working status and health condition of the sensor nodes, indicate the root addresses of all data, the data collected by the sensors and their trends for changes, and adjust the working task of the nodes accordingly. The health condition of a node includes residual energy, sensors, and the working status of communication components etc. By monitoring the sensor status, the user can adjust the duty cycle of sensor nodes in time and re-allocate the tasks in order to avoid the failure of the node appearing too early and extend the lifetime of the entire networkV.H ARDWARE OF THE SYSTEMTerminal device node should obtain the feature of small size, low power consumption and good suitability. Its main task is collecting, processing and transmitting parameters concerning environmental temperature, humidity, and carbon dioxide concentrations etc. Generally, a wireless sensor node is made up of a sensor module, a processor module, a wireless communication module and an energy supply module. Processor module and wireless communication modules use the CC2430 chip, and two AA batteries are used to supply the power. Sensor modules use a temperature and humidity sensor and a carbon dioxide sensor. The carbon dioxide sensor output analog signals, which is then inputted to the MCU after experiencing MCU12 bit ADC transformation, while the digital signal collected by temperature and humidity sensors is inputted to the MCU through the I / O port, and then through the spread spectrum signal O-QPSK they are transmitted to the carrier, and finally the signal will be sent to the master node through the antenna.CC2430 is the system-on-chip solution for the 2.4GHz IEEE802.15.4/ZigBeeTM in TI. CC2430 chip integrates the RF front of ZigBee, memory and microcontroller. It contains an 8-bit 8051MCU, it has a programmable Flash in 32/64/128k and RAM in 8K, as well as ADC, timer, AES-128 security co-processor, watchdog timer, power-on reset circuit, out power detection circuit, 32K Hz crystal sleep mode timer and 21 programmable I / O pins, P0 and P1 port are completely 8-bit port, P2 port is only 5 bits are available using the software set up a group of SFR registers bit and byte enable these pins as universal I / O port or as a peripheral parts of I / O port to be used for connecting ADC, Timers or USART [4-5]. Fig.2 is the diagram of Master node hardware design.Fig2. The hardware diagram design of master node The microprocessor is the C8051F series provided by Cygnal Company. C8051F31X series are the fully integrated mixed-signal system-on-chip which has the CIP-51 core whose instructions are all fully compatible with 8051 set, and it constitutes a microcontroller that integrates the data acquisition or control system required in almost all analog and digital peripherals and other features. These peripheral components or functions include: ADC, programmable gain amplifier, DAC, DAC, voltage comparator, voltage reference, temperature sensor, SMBus/I2C, UART, SPI, timers, PCA, internal oscillator, watchdog timer devices and power monitors. The high level of integration of peripheral components provides great convenience for the design of application system of single chip microprocessor because of their small size, low power consumption, high reliability, and high performance. Meanwhile they also help to reduce the cost of the whole system. [6-7]VI.S OFTWARE OF THE SYSTEMAfter the network is switched on by the coordinator, sensor nodes join the network and connect to the coordinator. The keyboard is used to choose which group they want to join into. By default, the data of temperature, humidity and carbon dioxide concentration are set to be collected and sent once per 30s, and the battery voltage is collected and sent once per 60s. Data should be sent to the coordinator, and then be uploaded to the monitoring center coordinator. The network monitoring center could send commands to monitor the situation closely as well, and the terminal device in each dormant always check up whether there are messages and orders after awakening. If orders come, they will be implemented immediately. For example, the monitoring center issue the command to stop collecting temperature data, and all terminal devices will receive it forwarded by the coordinator when they wake up, so they will stop collecting temperature data.Fig3. The flow chart of user dutyFor the terminal device, after the hardware is powered up, it should implement the main function which first reset all the components, start the system state detection function and transfer to MCU initialization, peripheral equipment, etc. and then go to the task loop. Command includes hardware interface, network layer, application access, and user-defined tasks. If nothing occurs, the system would be in sleep mode; when there is a case, the system is switched to working condition to respond to events and process them. Once all is done, it would return to sleep mode. The collected types of data can control the state of self-definition, and the current definitions are: temperature, humidity, carbon dioxide concentration and the battery voltage. Terminal equipment will also receive the messages (usually control commands) sent by the master node, such as start / stop collecting data, setting acquisition time. The key events are mainly for the node network and function’s configurations, such as joining the network, address binding, select the collected data, and select encryption functions, etc. Figure 3 is the flow chart of sensor network node network.VII.C ONCLUSIONThis paper implements a greenhouse environmental monitoring system based on the ZigBee wireless sensor network. By means of the oretical analysis and experimental test, it has been confirmed that the system can efficiently capture greenhouse environmental parameters, including temperature, humidity, and carbon dioxide concentration and it shows normal communication between nodes and the network coordinator, fine network stability, and conformity between data and real situations. It also obtains strong adaptability, good confidentiality and high reliability, at the same time, the system simplifies the on-site equipment installation and demolition transfer and makes it more suitable for greenhouse environmental data monitoring needs. This system has a wonderful practical value used in the complex greenhouse environmental monitoring.R EFERENCES[1]TIMMONS N F,SCANLON W G. Analysis of the performance of IEEE802. 15. 4 for medical sensor body area networking [J] . IEEE Wireless Communication, 2004, 26 (8):16224.[2]Chipcon1SmartRF CC2430 [EB /OL] 1 http: / /www1chipcon1com/ ,2005 /2006.[3]ZigBee Allianance, ZigBee Specification [Z],2004.[4]Chipcon AS, CC2430 Preliminary Data Sheet [Z].2006.[5]Texas Instruments, Z-Stack User’s Guide-CC2430DB[Z],2005.[6]LI WENZHONG, DUAN ZHAOYU. C8051F series monolithicintegrated circuit and short distance wireless data communication [M].Beijing: Beijing University of Aeronautics and Astronautics Publishing house,2007.[7]SUN LIMIN. 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