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A Design of Greenhouse Monitoring & Control

System Based on ZigBee Wireless Sensor Network

Zhou Yiming, Yang Xianglong, Guo Xishan, Zhou Mingang, Wang Liren

Department of Bio-System Engineering, Zhejiang UniversityHangzhou 310029, Chinabenton.zym@tom.com

AbstractThe wireless sensor network (WSN) is one of the most

significant technologies in the 21st century. As an open andglobal standard for WSN, ZigBee shows advantages on low-cost,

low power consumption and self-forming. The current researches

of ZigBee wireless sensor network on industrial automation,electronic products, smart buildings and medical care were

presented and, as an explorative application of ZigBee wirelesssensor network in protected agriculture overcoming the limits of

wire connection, its applied design for greenhouse management

was proposed by introducing both the hardware and software

architectures. The node power consumption was also discussed.Finally, the architecture of ZigBee-based mesh networkgreenhouse management was indicated.

Keywords-wireless sensor network; ZigBee; greenhouse;

monitoring and control system; application

I. INTRODUCTION

Wireless sensor network, which integrates sensortechnology, MEMS technology, wireless communicationtechnology, embedded computing technology and distributedinformation management technology, has been under rapiddevelopment during recent years. Because of the wideapplication prospect, it interests the world. Business Week

ever predicted in 1999 that WSN technologies would be one ofthe most important technologies in 21st [1].

ZigBee is an open and global standard for WSN aiming at alow rate, low cost, low power consumption and self-formingwireless communication. The major applications of ZigBeefocus on sensor and automatic control, such as militaryapplication, industrial control, smart buildings and environmentmonitoring. Agriculture automation is also an applicable fieldrecommended by ZigBee alliance. This paper presents thecurrent researches of ZigBee wireless sensor network onindustrial automation, electronic products, smart buildings andmedical care. The example of greenhouse managementapplication is also given. The architectures including thehardware and software design is proposed.

II. PRESENT RESEARCHES ON ZIGBEE TECHNOLOGIES

Applications on industrial control, smart buildings,electronic products, house automation and medical care wererecommended by ZigBee alliance. Literatures also show thatmost studies focused on these fields.

A. Industrial control

Jin et al. (2006) [2] reported to apply the ZigBeetechnology to mine safe production. The ZigBee devicewirelessly transmitted the data collected by the sensors oncolliers body to the gateway which transferred them to thecomputer finally. The computer analyzed the data and insuredthe safety. Cao et al. (2006) [3] presented his application ofZigBee on wireless natural gas meter record and transmission.The implementation of ZigBee wireless network into Modbusfieldbus control system was developed by Zhou et al. (2006)[4] which not only ensured the security and real time of theconnection but also reduced the cost of deployment andredeployment. Srkimki et al. (2006) [5] presented theapplicability of the ZigBee technology to electric motor rotormeasurements. Requirements for data transmission, electricalstructure and powering of a sensor were also discussed and a

prototype wireless ZigBee-based torque sensor was built andtested.

B. Electronic products and house automation

Huang et al. (2006) [6] discussed the problems of ZigBee-based WSN in supporting multimedia services, and proposed

their reference design that can overcome these problemsincluding the aspects of sensor nodes hardware and softwarearchitecture, network configuration and data transmission. Animage sensor network platform was developed by Pekhteryevet al. (2006) [7] for testing transmission of images over ZigBeenetworks that support multi-hopping. These technologies can

be adopted by electronic products like mobile phones and soon.

Zhang et al. (2005) [8] discussed some key issues in homenetworking with ZigBee. Feng et al. (2005) [9] proposed asolution for home electric appliances network based on ZigBeetechnology and the ZigBee device types, hardware design,routing protocol and network topology were introduced indetail. Yu (2006) [10] presented the ways to achieve the

automatic and long-distance remote control center in familiesby connecting all types of intelligent electrical appliances,monitoring and alarming equipments through ZigBee network

based on ARM embedded systems.

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C. Smart Buildings

Egan (2005) [11] indicated the application prospect ofZigBee technology on smart buildings. While Yang et al.(2005) [12] conducted an experiment in a building to monitorthe fire hazard using a ZigBee-based Ad Hoc network and theresult was of satisfaction.

D. Medical care

Wang et al. (2006) [13] presented an architecture as well as

circuit implementation of ZigBee receiver for personal medicalassistance using 868/915 MHz band, which was compliant tothe physical layer of IEEE standard 802.15.4. An embeddedremote health care system based on wireless sensor networktechnology was developed by Zhao et al. (2005) [14]. Thedesigns of several sensor nodes and the care base-station andthe wireless communication based on IEEE 802.15.4 / ZigBeestandard between them were introduced. By connecting the

base-station with a networked home PC, the doctor could checkthe data through another networked computer to see if the

patient was fine.

III. DESIGN

However, wireless technologies have been widely used in

agriculture nowadays [15][16], but most of them wererestricted to simple communication between control computerand end devices like sensors instead of wire connection, whichcouldnt be strictly defined as wireless sensor network. As itsincreasing applications on agriculture, researchers summarizedthe following five aspects of environmental monitoring,

precision agriculture, machine and process control, buildingand facility automation, traceability systems [17].

Greenhouse monitoring and control system has been greatlydeveloped during the past decades. The sensors were used tomonitor the parameters, and a computer with expert strategieson greenhouse management was applied to analyze the data toimplement automated control. However, these systems havemostly been wire connected and this limits the use because of

the high expenses in installation and maintenance. Althoughreferences show that wireless sensors based on severalcommunication techniques have already been used ingreenhouse, WSN is believed to eliminate the big costs of justwiring [18][19]. According to the greenhouse actualities,ZigBee WSN technologies are thought the most suitable forgreenhouse application because of its low cost, low powerconsumption, self-forming and high network capacity. Theapplicable design dedicated for greenhouse management isintroduced as follows.

A. Star network in greenhouse

ZigBee defines the network, security, and applicationframework profile layers for an IEEE 802.15.4-based system.ZigBees network layer supports three networking topologies;star, mesh, and cluster tree. Star networks are common and

provide for very long battery life operation. The ZigBee logicaldevice types are ZigBee Coordinators, ZigBee Routers, andZigBee End Devices. The coordinator initializes a network,manages network nodes, and stores network node information.The router participates in the network by routing messages

between paired nodes. The end device acts as a leaf node in thenetwork.

ZigBee-based wireless monitoring and control system inone greenhouse is composed of a coordinator and several enddevices including sensor nodes and actuator modules organizedas a star network shown in Fig. 1. By running greenhousemanagement software, the coordinator periodically receives thedata from the wireless sensor nodes and displays them on itsLCD. Meanwhile, it sends orders to actuator modules in the

network to control the electrical machines automatically ormanually.

Figure 1. The star network applied in one greenhouse management

B. Node hardware

The design of wireless transceiver and its energyconsumption was a difficulty before JN5121 was developed.The JN5121 is the first in a series of low power, low costIEEE802.15.4 compliant wireless microcontrollers. Combiningan on chip 32-bit RISC core, a fully compliant 2.4GHzIEEE802.15.4 transceiver, 64Kb of ROM and 96Kb of RAM,

provides a versatile low cost solution for wireless sensornetworking applications. The high level of integration helps toreduce the overall system cost. In particular, the ROM enablesintegration of point-to-point and mesh network stack protocols,and the RAM allows support of router and controller functions

without the need for additional external memory. The JN5121uses hardware MAC and highly secure AES encryptionaccelerators for low power and minimum processor overhead.Integrated sleep oscillator and power saving facilities are

provided, giving low system power consumption. The devicealso incorporates a wide range of digital and analogue

peripherals for the user to connect to their application [20].

As an example of a sensor end device integratedtemperature, humidity and light, the design is shown in Fig. 2.

Figure 2. The hardware design of a sensor node

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The SHT1x is a single chip relative humidity andtemperature multi sensor module comprising a calibrateddigital output. The TSL2550 is a digital light sensor with atwo-wire, SMBus serial interface. SHT1x and TSL2550 are

both digital sensors with small size and low powerconsumption. The extra flash is used to store the programs.Antenna is connected to the built-in transceiver through a

balun. Other sensor nodes can be obtained by changing thesensors. The actuator module comprises JN5121, relays and

some anti-jamming circuits serving as a driver for electricalmachines.

Compared with end devices, coordinator also contains aLCD and four buttons for displaying, settings and manualoperations.

The sensor nodes are powered from onboard batteries andthe coordinator also allows to be powered from an external

power supply determined by a jumper.

C. Node software

The application system consists of a coordinator andseveral end devices. The general structure of the code in each isthe same, with an initialization followed by a main loop. In themain loop, interrupts are used extensively to synchronize

operation, which allows the device to put the CPU to sleep forlong periods whilst nothing is happening [21].

The Fig. 3 shows the software flow of coordinator. Uponthe coordinator being started, the first action of the applicationis the initialization of the hardware, stack and applicationvariables. Then it sends out regular beacons containing a

beacon payload of 8 bytes. The first byte of the beacon payloadcontains a specific value so that the end devices can use this toverify that the coordinator is running the demonstration. Aseach end device associates with the coordinator it is given ashort address with which to identify it. Also the keys are onlychecked 20 times per second. This avoids the need for any keyde-bounce software algorithm without giving a perceivedoperating delay. Any MLME or MCPS events are processed as

they occur, handling association requests and received framesfrom end devices. The application puts the CPU itself into dozemode whenever possible.

The buttons have different functions under different LCDscreen. The Network Screen shows the current values for allsensor nodes and states for all actuators. To select a NodeDisplay screen, the NODE button should be pressed. Thisscreen displays the value for one sensor node and the linkquality. Pressing the SETINGS button causes the NodeSetting screen which allows the high and low limit values to beset. If it is an actuator node, the three buttons act as manualoperations of OPEN, CLOSE and STOP. To selectanother sensor node, the NODE button should be pressedorderly until the Network Screen shows again.

Figure 3. The software flow diagram of coordinator

The Fig.4 shows the software flow of a sensor node. Aseach sensor node is switched on, it scans all channels and, after

seeing any beacons, checks that the coordinator is the one thatit is looking for. It then performs a synchronization andassociation. Once association is complete, the sensor nodeenters a regular loop of reading its sensors and putting out aframe containing the sensor data. As the beacons from thecoordinator contain a payload, the sensor node receives anMLME indication from the stack whenever a beacon arrives.This is used to trigger the next read of the sensors.

Figure 4. The software flow diagram of end device

D. Node Power Consumption

The node power consumption mainly includes JN5121current consumption and sensors on board power consumption[22]. Because the coordinator in our application can be

powered from an external power supply besides batteries, sosensor node power consumption is discussed below.

1) JN5121 current consumption.: The sensor node in the

enabled star network that continuously performs the following

actions in one minute:

Wakes from sleep.

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Reads data from a sensor connected to the two-wireserial interface.

Performs a Clear Channel Assessment (CCA).

Transmits a data frame containing a payload of 64bytes.

Sleeps (without holding the memory contents) beforerepeating Step 1.

Table 1 shows the current consumption of JN5121 whileperforming various operations. All quoted currents are typical

values at 3V and 25.

TABLE I. THE CURRENT CONSUMPTION OF JN5121

Operation Current(mA) Notes

Sleep

(RAM contents not held)0.0035

Sleep

(RAM contents held)0.025

CPU active

(wireless transceiver off)9.00

CPU active

(wireless transceiver on)12.80

Radio transmitting 44.00 CPU in doze mode

Radio receiving 49.00 CPU in doze mode

The timings of all 802.15.4 operations are based on onefundamental piece of information, the symbol rate. Whenoperating in the 2.4-GHz band, the symbol rate is defined as62500 symbols per second. Each symbol comprises four bits ofinformation and, therefore, the over-air data rate is 250 kbps.So, the time mentioned below taken to perform common802.15.4 network operations is all based on this data rate.

The time taken by the JN5121 to perform operations that

are not related to the 802.15.4 protocol is shown in Table.

TABLE II. THE TIME TAKEN BY JN5121 DISRELATED TO THE 802.15.4PROTOCOL

Operation Time (ms) Notes

Oscillator start time 2.5

Application load time 0.84*n n is application size in Kbytes

Now, we can calculate the power consumption of eachaction.

a) Wake from sleep: Since the contents of RAM have

not been held, the time taken to wake from sleep mode can becalculated by adding the oscillator start time to the application

load time. Therefore, the time to wake from sleep equals13.084 ms (0.25+0.84*12.6, see Table ). When loading the

application, the wireless transceiver will not be operating and

therefore the current drawn is 9mA.

b) Read data from sensors: It is assumed that it takes5ms to read data from the sensors attached to the two-wire

serial interface. During this time, the current drawn by the

JN5121 is 9mA.

c) Perform CCA: Before a data frame can betransmitted, the CSMA/CA algorithm is used to check that the

channel is not being used. The CCA takes 8 symbol periods(0.128ms) to complete. If the channel is found to be busy, the

back-off periods should be added. One back-off period is

equal to 20 symbol periods. Assume that the channel is found

to be clear after the CCA and that the random back-off period

is 2, the time taken to execute the CSMA/CA algorithm can be

calculated as follows.

Back-off Period = (22-1)*20/62.5=0.96 ms

CCA Period =0.128 ms

During the back-off period, the application is running andthe transceiver is on although it is not transmitting andreceiving. The current drawn during this period is 12.8mA.During a CCA, the radio receiver is on and therefore thecurrent drawn is 49mA.

d) Transmit data: The data frame includes 6 bytes ofPhysical layer header and 13 bytes of the MAC layer header.

The payload size of sensors data is 6 bytes. The data

transmission period can be calculated as follow.Data Transmission Period= (6+13+6)*8/250=0.704ms

The current drawn during this period is 44mA, the radiotransmitting current.

e) While Sleep: The current drawn during the

approximate one minute sleep period is 3.5A.

2) Sensors power consumption: The supply current of the

humidity and temperature Sensor SHT1x and the ambient light

sensor TSL2550 is 0.55mA and 0.35mA while measuring,

0.3A and 10A while sleep respectively. The periods related

these two modes are 5ms and 1 minute.Using the above time and current data, it is possible to

calculate the average current required by the application is

17.4A approximately. So the node should therefore becapable for about five years when powered by two 750mAh

batteries.

E. ZigBee-based mesh network for greenhouses management

The star network is limited to one greenhouse management,so its not available for many greenhouses around in one area.However, ZigBees self-forming and self-healing meshnetwork architecture permits data and control messages to be

passed from one node to other node via multiple paths. Thisfeature extends the range of the network and can be competentfor many greenhouses application.

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Figure 5. The architecture of ZigBee-based mesh network for greenhouses

management

Fig. 5 shows the architecture of ZigBee-based meshnetwork for greenhouses management. The embeddedcontroller, served as ZigBee coordinator in the network, canreceive the real-time sensor data from routers in allgreenhouses and control the electronic machines by sendingthem orders. The data from each greenhouse are displayed onLCD. The star network introduced in section 3.1 is applied in a

certain greenhouse. The difference is that the coordinator instar network acts as a router in mesh network which

participates in the network by routing messages between pairednodes. The router in a remote greenhouse can talk with thecoordinator through its neighbor routers. The multiple pathsensure the reliability of communication.

Using JN5121 as a transceiver, the embedded controlleradopts the ARM embedded system to achieve a complicatedgreenhouse management system by planting Linux operationsystem and expert strategies [22]. It also can be connected byremote computer via GPRS or Ethernet which allows peopleworking at home. The design of a router refers to section 3.2.

IV. CONCLUSION

Greenhouse prevents the plant from the effects of climate,inspect and so on, which makes great sense for agricultural

production. The automation and high efficiency on greenhouseenvironment monitoring and control are crucial. ApplyingZigBee-based WSN technologies to greenhouses is arevolution for protected agriculture which overcomes the limitsof wire connection systems. Such a system can be easilyinstalled and maintained. After several moths test, the systemhas been working normally and shows its competent abilitieson precision irrigation and animal facilities.

ACKNOWLEDGMENT

The work is supported by the foundation of ZhejiangProvincial Science and Technology Department (No.

2005C22060).

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