27 Wireless Ad Hoc Sensor Networks

Transcription

1 27 Wieless Ad Hoc Senso Netwoks Wieless data communication fo agicultual puposes elies on: one o moe sensos of envionmental data (tempeatue, humidity etc.); a signal conditione; an analogue to digital convete; a micopocesso with an extenal memoy chip; and a adio module fo wieless communication between othe senso nodes and/o a base station (Popescu 2007). One of the fist wieless netwoks fo agicultue was built fo the Dutch Lofa Ago eseach poject (Visse 2005). It consisted of 50 senso boads (based on the Mica2 Cossbow wieless senso platfom) deployed in a field fo gatheing tempeatue and humidity data (Popescu 2007). The data eceived by the sensos was handled by a micopocesso implementing the TinyOS opeating system, and tansmitted via adio in the 868/96 MHz band to a field-gateway and fom thee via Wi-Fi to a pesonal compute fo data logging (Popescu 2007). Since then thee has been consideable inteest in using ad hoc wieless netwoks fo automatic data acquisition in agicultual entepises. Austalia s Commonwealth Scientific and Industial Reseach Oganisation (CSIRO) have developed a seies of devices fo agicultual wieless netwok systems. These devices incopoate: a Nodic adio with a ange of ove km that opeates on the 433- MHz o 95-MHz band; an integated sola battey chaging cicuit; and an extensive ange of sensos (Wak, et al. 2007). Thei system also incopoates a ealtime clock chip to educe micocontolle oveheads. Thei system was also based on the TinyOS opeating system (Wak, et al. 2007). In pecision agicultue, vaious paametes including soil moistue and soil tempeatue vay damatically. Off-the-shelf iigation contolles ae not effective in managing wate applications unde these cicumstances; howeve an iigation management system based on wieless senso netwoks can bette espond to high esolution wate application equiements (Mafuta, et al. 203). 27. Netwok Configuations Ealy compute netwoks could be divided accoding to thei undelying topology. They loosely divided into: a sta connected system; a bus connected system; o a ing connected system. The sta connection is the oldest topology fo netwoked systems (Pake 2000). The sta aangement uses point to point links fom peipheal netwok nodes to a cental machine (Pake 2000). In the bus configuation, the netwok nodes ae connected onto a long bus as a seies of tee junctions (Pake 2000). The ing system, as the name suggests, has a cental ing bus onto which the nodes connect (Pake 2000). Thee ae no wied infastuctues o cellula netwoks in ad hoc wieless netwoks (Li 2003). Diffeent configuations ae possible; howeve many systems

2 338 Wieless Ad Hoc Senso Netwoks use wieless nodes with omni-diectional antennae. Any tansmission fom a node can be eceived by any node within its vicinity. It is also possible to use diectional antennas to diectly link fixed nodes (Li 2003). Each mobile node has a tansmission ange. If the signal fom one node can not be diectly eceived fom a second node, the data message must be communicated though multi-hop wieless links by using intemediate nodes to elay the message (Li 2003). Consequently, each node in the wieless netwok also acts as a oute, fowading data packets fo othe nodes (Li 2003). Wieless ad hoc netwok have thei own special chaacteistics and some unavoidable limitations compaed with wied netwoks. Wieless nodes ae often poweed by batteies and have limited available stoage memoy (Li 2003). Data tansmission fom a wieless node can be eceived by seveal neighbouing nodes. This can cause intefeences (Li 2003). Unlike taditional static communication devices, the wieless nodes ae often moving duing the communication pocess (Li 2003). Seveal data outing options can be used in ad hoc netwoks. Most outing systems equie pe-knowledge of the destination node s physical location. Compass Routing fowads data via othe nodes such that the angles among the neighbouing nodes to the destination node ae the smallest possible (Li 2003). Geedy Routing tansfes data based on the shotest possible oveall tansfe distance athe than calculated tansfe angles (Li 2003). Many ad hoc netwoks in agicultual systems have limited powe esouces; theefoe consideation needs to be given to minimising the powe needed to tansfe data. This can be achieved in seveal ways. Fistly, the data should be as infomation dense as possible. This usually equies some pe-pocessing of the data befoe tansmission. Fo example, calculation and tansmission of means and standad deviations, athe than tansmitting aw data educes the size of data being tansmitted. Secondly, when a wieless netwok has a sufficient density of nodes, only a small numbe of them need to tun on at any given time to fowad data taffic (Chen, et al. 2002) Open Souce Platfoms Seveal commecial wieless systems have been developed; howeve fo small scale expeimental systems, a numbe of open souce platfoms have ae available. Two options include the Aduino and the Raspbey Pi.

3 Mobile Telephone Netwoks Raspbey Pi The Raspbey Pi is a cedit-cad sized compute that plugs into a TV (o monito) and a keyboad. It is a capable little compute which can be used in electonics pojects, and fo many of the things that a desktop PC does, like woking with speadsheets, wodpocessing and games. It can also play high-definition video (Anonymous 204b). The Raspbey Pi can suppot a complete opeating system such as Linux and can be pogamed in languages like Scatch and Python (Anonymous 204b). The Raspbey Pi platfom also suppots vaious add-ons, including wieless adaptos that suppot wieless connectivity Aduino Aduino is a tool fo making computes that can sense and contol othe devices. It is an open-souce physical computing platfom based on a simple micocontolle boad, and a development envionment fo witing softwae fo the boad. Aduino can be used to develop inteactive objects, taking inputs fom a vaiety of switches o sensos, and contolling a vaiety of lights, motos, and othe physical outputs (Anonymous 204a). The Aduino platfom suppots plug in devices called shields, including the Xbee, Wieless SD and Cellula mobile devices. The Xbee shield allows an Aduino boad to communicate wielessly using Zigbee (Anonymous 204a). The module can communicate up to 00 feet indoos o 300 feet outdoos (with line-of-sight). It can be used as a seial/usb eplacement o it can put into a command mode and configued fo a vaiety of boadcast and mesh netwoking options (Anonymous 204a). The Wieless SD shield allows an Aduino boad to communicate wielessly using a wieless module. It is based on the Xbee module, but can use any module with the same footpint. The module can communicate up to 00 feet indoos o 300 feet outdoos (with line-of-sight) (Anonymous 204a). The Cellula Shield allows the Aduino to access a mobile telephone netwok, poviding simple messaging systems (SMS), Global System fo Mobile Communications/ Geneal Packet Radio Sevice (GSM/GPRS), and Tansmission Contol Potocol/Intenet Potocol (TCP/IP) functionalities (Anonymous 204a). The cellula module equies a SIM cad (pe-paid o staight fom an existing mobile phone) and an antenna. Data can be sent using Seial.pint (Anonymous 204a) Mobile Telephone Netwoks The Univesal Mobile Telecommunications System (UMTS) is a thid geneation mobile cellula system fo netwoks based on the GSM standad. UMTS uses wideband

4 340 Wieless Ad Hoc Senso Netwoks code division multiple access (W-CDMA) adio to offe a boad ange of data and communications sevices. The technology descibed in UMTS is sometimes also efeed to as Feedom of Mobile Multimedia Access (FOMA) o 3GSM. If sufficient coveage is available, using the mobile telephone netwok as pat of an ad hoc wieless senso netwok povides geate ange than can be achieved using low powe wieless links alone. As an example, a senso system fo testing wood condition contained thee sensos: one audio senso; one tempeatue and humidity senso; and a tempeatue and pessue senso. All these sensos wee integated into Aduino Mega 2560 and 280 boads. A pogamme was witten to collect eadings fom the sensos on an houly basis and sends them to a emote place as an SMS text message using a mobile module with the Aduino boad (Tunbul, et al. 202) Powe Supply Most ad hoc netwoks used in agicultual systems opeate fom batteies. The capacity of these batteies depends on the numbe of sensos attached to the node, the sampling fequency, and the amount of data being tansfeed. Because netwoks can be costly and data has eal financial value, powe eliability becomes citical. This implies that battey powe needs to continually topped-up wheneve possible. Pat of the data steam should also include battey status so that powe supply issues can be identified ealy. Usually photovoltaic cells ae used to chage the batteies duing daylight hous Available Sola Enegy Photo-voltaic sola collectos convet sunlight into diect cuent electicity. The use of sola enegy has many advantages. In the field, it is a clean, quiet and eliable enegy souce. About seven million households aound the wold use sola hot wate systems. In emote aeas, without a connection to a public enegy gid, sola enegy is used fo heating wate and geneating electicity. The aveage sola powe density falling onto the uppe atmosphee, othewise known as the sola constant, is 353 ± 20 W m -2 (Howell, et al. 982). Depending on the wavelength, much of this adiant powe penetates the atmosphee (Duy 998). Analysing a sun-ay (Howell, et al. 982), based on the geomety displayed in Figue 27., eveals that the sola powe incident on a hoizontal plane at the eath s suface on a clea day is: R cos 2 ( ) e a - Re+ A 353 I = t é 2 s sin( x) sin( y) + cos( x) cos( y) cos( zt) ù ë û (27.)

5 Powe Supply 34 Whee I is the incident adiation powe at the eath s suface (W m -2 ), is the distance fom the sun to the eath (m), t s is the tansmission coefficient of the eath s atmosphee, R e is the eath s mean adius (m), A is the mean thickness of the atmosphee (m), a is the sola altitude (Radians), x is the Geogaphic latitude (Radians), y is the Sola declination (Radians); z is the Angula velocity of the eath s otation (Radians s - ), and t is time (s). Figue 27.: Geomety of a sola ay pathway though the Eath s atmosphee. Blewitt (973) and Howell, et al. (982) show that the sola altitude (a) may be detemined using: ( ) ( ) ( ) ( ) ( ) a= sin - é ë sin x sin y + cos x cos y cos zt ù û (27.2) Atmospheic tansmittance (t s ) is about 0.7 fo a clea day (McMullan, et al. 978, Studman 990, Stuman and Tappe 996) and much less on cloudy days. Spence (97) has developed a set of equations to descibe some of the impotant paametes fo calculating sola powe unavailability. The distance fom the eath to the sun, in astonomic units, can be calculated using: ì cos( q) sin( q) ü = ï ï 2 í ý ï cos( 2q) sin( 2q) î ïþ (27.3)

6 342 Wieless Ad Hoc Senso Netwoks Sola declination can be calculated accuately using: ì cos( q) sin( q) cos( 2q) ü y = ï ï í ý ï sin( 2q) cos( 3q) sin( 3q) î ïþ (27.4) 2p( n-) whee q = is the obit angle of the eath about the sun fo the n th day of the 365 yea. Ideally, the sola absobe should tack the sun in such a way that its suface is always pependicula to the sun s ays. In pactice, this is expensive to implement. Consequently, fixed non-tacking absobes ae commonly used in sola heating systems. Moe consistent yea-ound pefomance can be achieved when a fixed sola absobe is tilted towad the equato in such a way that it is in a plane paallel to the eath s axis. This implies that the tilt angle of the absobe should be equal to the local geogaphic latitude of the system. The basic aangement of a tilted absobe is shown in Figue Figue 27.2: Geomety of a tilted sola absobe. When this is done, equation (27.) becomes: R cos 2 ( ) e a - Re+ A 353 I = t é 2 s sin( x- b) sin( y) + cos( x-b) cos( y) cos( zt) ù ë û (27.5) Because x = b, equation (27.5) educes to: R cos 2 ( ) e a - Re+ A 353 I = t écos 2 s ( y) cos( zt) ù ë û (27.6)

7 Powe Supply 343 Figue 27.3 shows the effect of using a tilted collecto on yea-long sola powe absoption. Figue 27.3: Effect of tilting the absobing suface on sola pefomance thoughout the yea at 36o South latitude. In addition to this ay intensity thee is also a diffuse component of sola adiation due to atmospheic scatteing. The US Depatment of Enegy (999) suggests that the diffuse component of sola adiation can be appoximated by: I diff æ ö R cos 2 ( ) e a Re+ A = 2 - t é s sin ( x) sin ( y) + cos ( x) cos ( y) cos ( zt) ù ë û ç çè ø (27.7) Theefoe the total sola adiation intensity can be defined by: I = I + I (27.8) diff Powe supply is an ongoing issue fo data acquisition. Vaious methods of havesting enegy fom stay RF have been developed (Aguila 202). One of the ealiest examples of this stategy was the RF-ID system whee the ID chip havests its enegy needs fom the RF eade duing a scanning pocess. The next chapte will exploe RF-ID systems.

8 344 Wieless Ad Hoc Senso Netwoks Refeences Aguila, A RF Enegy Havesting. Unpublished thesis. Univesity of Cincinnati, Anonymous. 204a. Aduino. Anonymous. 204b. Raspbey Pi. Blewitt, M. 973, Celestial Navigation fo Yachtsmen, 5th edn, London: Edwad Stanfod. Chen, B., Jamieson, K., Balakishnan, H. and Mois, R Span: An Enegy-Efficient Coodination Algoithm fo Topology Maintenance in Ad Hoc Wieless Netwoks. Wieless Netwoks. 8(5): Duy, S. A. 998, Images of the Eath: A guide to Remote Sensing, 2nd edn, Oxfod Univesity Pess. Howell, J. R., Banneot, R. B. and Vliet, G. C. 982, Sola - Themal Enegy Systems, Analysis and Design, New Yok: McGaw-Hill. Li, X.-Y Topology contol in wieless ad hoc netwoks. Mobile Ad Hoc Netwoking Mafuta, M., Zennao, M., Bagula, A., Ault, G., Gombachika, H. and Chadza, T Successful Deployment of a Wieless Senso Netwok fo Pecision Agicultue in Malawi. Intenational Jounal of Distibuted Senso Netwoks. 203: -3. McMullan, J. T., Mogan, R. and Muay, R. B. 978, Enegy Resouces, London: Edwad Anold. Pake, C. 2000, Undestanding Computes: Today and Tomoow, Fot Woth: The Dyden Pess. Popescu, V Wieless Data Communication in Agicultual Engineeing. Tends and Pactical Expeiments. Poc. Reseach People and Actual Tasks on Multidisciplinay Sciences. Lozenec, Bulgaia Spence, J. W. 97. Fouie seies epesentation of position of the sun. Seach. 2(5): 72. Studman, C. 990, Agicultual and Hoticultual Engineeing, Wellington: Buttewoth s Agicultual Books. Stuman, A. P. and Tappe, N. J. 996, The Weathe and Climate of Austalia and New Zealand, Oxfod Univesity Pess. Tunbul, C., Bodie, G., Thanigasalam, D. b., Faell, P., Kealy, A., Fench, J. and Ahmed, B Final Poject Repot - Investigate contol of in-situ temite and decay potection and contol using micowave technologies. Univesity of Melboune US Depatment of Enegy WIMOVAC Macoclimate Module. Mach, gov/modelling/newpage9.htm Visse, O. W Localisation in Lage-Scale Outdoo Wieless Senso Netwoks. Unpublished thesis. Delft Univesity of Technology, Faculty of Electical Engineeing, Mathematics, and Compute Science Wak, T., Coke, P., Sikka, P., Klingbeil, L., Ying, G., Cossman, C., Valencia, P., Swain, D. and Bishop- Huley, G Tansfoming Agicultue though Pevasive Wieless Senso Netwoks. Pevasive Computing, IEEE. 6(2):