New Applications of Location System in Zigbee WSNs for the Fire System

DENG Hu-bin，ZHANG Lei (Department of Information Engineering,East China Jiaotong University,Nanchang,China,330013)

ray1985@foxmail.com

Abstract:Currently, fire often came suddenly and unconsciously. When the firemen saved the people and their properties in the scene of fire with seldom useful system to monitor the fire and strive for time, the need of efficient instrument became intense. In this paper our design of location system based on the location algorithm of received signal strength indication (RSSI) in Zigbee WSNs, was a good example system for the fire system. The system provided the commanders outside with the condition of the fire site and the definite location of firefighters inside。Finally, our location system is tested in real indoor environment and the precision improves. Keywords: WSN; Zigbee; Location; RSSI; Fire system 1 INTRODUCTION

With the maturity and development of wireless sensor network technology, the location services have drawn increasing attention, the fixed small-scaled location service demands are on the increase. Location service system in Zigbee WSNs becomes more commercioganic and more useful, location system based on RSSI in Zigbee WSNs has a good solution to cut the cost, reduce the energy consumption, be small and be used indoor environment. Receive signal strength indication(RSSI) location technology is based on the node received signal strength to calculate the signal propagation loss, using the theoretical and empirical models, then turn the transmission loss into distance and calculate the location of the node by the location engine which is integrated into CC2431.

Now the danger of Fire has a serious impact on the safety of people's properties and lives, it is extremely dangerous that the firemen go directly into the unknown scene of fire without any equipment to instruct them of the circumstances of the fire scene. The micro-WSNs nodes with especial sensing, computing and communicating capabilities can help the firemen monitoring the scene of fire's condition, people could know the details of the fire situation without entering the field, and make the most effective measure to reduce the lost to the least. 2 SYSTEM STRUCTURE

The WSNs node is the basic unit of the network. The stable operation of the nodes is an important guarantee for the reliability of the entire network. In different applications, the sensor type is different, but WSNs node typically includes sensor module, CPU module, wireless RF module and power supply module.

The sensor module is mainly responsible for collecting the physical information or analog signals and converting it into electrical signals or digital signals, then

sending the useful signals to the microcontroller; CPU module handles the signals and information;

Wireless RF module works for sending or receiving data; power supply module provides the system with stable and reliable source of energy. The location system structure schematic is Fig.1.

In our system, only the coordinator has the serial port. One WSN just has one coordinator particularly; coordinator node is responsible for monitoring the host and WSNs communication, it sends queries to the lower level node, and receives the data from the lower level node and sends the received data to the host via serial port. The blind node has a location engine which is used to calculate the coordinates. The reference node includes sensors.

Fig. 1 System structure

3 NODE DESIGN There are 3 node types: blind nodes 、 reference

nodes and coordinator nodes. The blind nodes are the one to be localized; the reference nodes send its reference information, the blind nodes calculate the distances then send it to the coordinator node; the coordinator node is a node which connects to the host with one serial port, sending and receiving useful information.

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Blind nodes and reference nodes have identical hardware structure, while the interface circuit of coordinator node is replaced by the RS232 communication interface; only the blind nodes include CC2431, the reference nodes and the coordinator node

include CC2430. The function of the nodes is burning into store memory by the emulator. The nodes are powered by a couple of battery, and in the life cycle, there is almost no other energy supply. The sensor node schematic diagram is Fig.2.

Fig.2 Sensor node schematic diagram

The processor is CC2431 or CC2430, they are both

ultra low power consumption Soc (System-On-Chip).

They include a predominant RF transceiver, and an

enhanced 8051. With 128KB flash ROM and 8KB RAM,

they combine the Zigbee protocol stack with many other

powerful parts.CC2431 and CC2430 for the wireless

communication equipments, transfer data at the most

speed of 250kbps, and rapid multipoint network is

supported.CC2431 has a location engine. The other parts

are the same as CC2430

Our design uses the unbalanced whip antenna, so a

balun is used in order to optimize performance. The balun will match the RF input/output to 50Ωwith low-

cost discrete inductors and capacitors. Almost all the I/O

pins are pulled out. The power plate powered by a couple

of battery includes one MAX3232 transceivers

connecting to RS-232. 4 LOCATION SYSTEM

The location engine is implemented to estimate the position of blind nodes. The reference nodes send the known coordinates helping the blinds nodes calculate

their positions. The blind nodes are often mobile and linked with other objects which are to be tracked. The location engine needs at least a set of three references coordinates with a set of measured parameters.

The location system estimates with resolution of 0.5 meters less than 40 µs. The Location ranges are 64 x 64 meters. The location error can be less than 3 meters depending on the factors of the using environment and the hardware circuit. There are about 22 parameters to be fixed. These parameters consist of two radio parameters 、 sixteen RSSI values and four Searching border coordinates.

In our system, we use eight reference nodes to exact the result. Two blinds nodes are tested in the system. The Location engine implements a distributed computation algorithm, which uses the received signal strength indicator (RSSI) values from known reference nodes? In the node-level the implementation of the position calculation reduces network traffic and communication delays, otherwise present in a centralized method. We use Chipcon's software: Z-Location Engine, to track the locus of the nodes.

The theoretical RSSI value can be from the formula：RSSI = - (10n·lgd + A) (1)

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Assuming that the radiation pattern is an omni-

directional, the radio parameter “A” is defined as the absolute value of the average power in dBm received at a close-in reference distance of one meter from the transmitter. Generally speaking, the range of A is supposed to be [30.0, 50.0] with precision 0.5. A typical value for A is 40.0.

The parameter “n” is defined as the path loss exponent that describes the rate at which the signal power decays with increasing distance from the transmitter. According to different environment, we choose different n values. The actual parameter n value written to the Location Engine is an integer index value selected from a lookup table shown in Table 1.

Table 1: n parameter lookup table

n n index n n index

0 1.000 16 3.375

1 1.250 17 3.500

2 1.500 18 3.625

3 1.750 19 3.750

4 1.875 20 3.875

5 2.000 21 4.000

6 2.125 22 4.125

7 2.250 23 4.250

8 2.375 24 4.375

9 2.500 25 4.500

10 2.625 26 4.625

11 2.750 27 5.000

12 2.875 28 5.500

13 3.000 29 6.000

14 3.125 30 7.000

15 3.250 31 8.000

With the two radio parameters and 16 RSSI values, we can probably estimate the distance between the sending and the receiving equipment. Collecting the measured data with the arithmetic mean we can get an approximate value which could reflect the current environment. All the tested parameters are saved in the RF register MEASPARM. Before the reference coordinates, [x0, y0, x1, y1... x8, y8], are loaded into the RF register REFCOORD, "1" must be written to the register bit LOCENG.REFLD. When all reference coordinates have been loaded, write "0" to LOCENG.REFLD. The same is to the register LOCENG.PARLD. All the tested parameters must be written sequently [A, n, xmin, xmax, ymin, ymax, rssi0, rssi1 ... rssi15, rssi16], then write "0" into LOCENG.PARLD. Write "1" to LOCENF.RUN to run the location estimate. The estimated coordinates can be read from the LOCX and LOCY registers when

LOCENG.DONE is set to "1". Location Engine does not produce any interrupt requests. So it hardly uses the CPU. The estimated X and Y from the LOCX and LOCY will show the relative position. The flowchart of the location engine operation is Fig.3

Fig. 3: Location Engine Operation

According to the average distance estimated by (1), the error of estimation reduced. At last we tested our design in indoor environment; According to the environment we choose “A” equals 45 and “n” equals 16. With the eight reference nodes, the two blind nodes were Located and displayed by the Z-Location Engine. The result showed that the location precision improves. The location display picture is Fig. 4.

Fig. 4 Display by Z-Location Engine (M)

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5 Conclusions According to our experiment, the location accuracy is improved. The location technology based on the location algorithm of RSSI in Zigbee WSNs can be used in various indoor occasions. Zigbee provides a strong expansion of capacity for the stability of the entire system, at the same time reducing the cost. Therefore our location system shows the fire system an example, and the effective collecting, monitoring and locating technology can achieve a very good and high value in the fire system.

6 ACKNOLEGEMENTS This work were sponsored by East China Jiaotong University Graduate Innovation Foundation and the technology proliferation plan item of Nanchang city (Caiqi [2008], NO.68), science and research foundation of East China Jiaotong University (09XX05).

REFERENCES

[1] Texa instruments CC2431 Datasheet [2] Texa instruments CC2430 Datasheet [3] ZigBee Alliance.ZigBee Specification 2006[S]. [4] IEEE Std 802.15.4-2003 Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks[S]. [5] Zhou Z, Cui J H, Zhou S. Localization for large-scale underwater sensor networks [J]. Lecture Notes in Computer Science, 2007, 4479: 108-119. [6] Zhou Y, He J H, Chen K. An area localization scheme for large scale underwater wireless sensor networks [C], International Conference on Commu- nications and Mobile Computing. Kunming, China: IEEE, 2009: 543-547. [7] Whitehouse, K., Karlof, C., & Culler, D. (2007). A practical eval-uation of radio signal strength for ranging-based localization. SIG-MOBILE Mobile Computing Communication Review, 11(1), 41–52.

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