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2 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 43 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor X Wang Hongbo College of Mechancal Engneerng, Yanshan Unversty Qnhuangdao, , Chna. Introducton In the practcal applcaton of the moble robot, the frst encountered problem s postonng. The postonng s an mportant basc functon for an autonomous moble robot, whch s the premse of the robot to complete ts msson, and s also currently the closely watched research topc (Meng et al., 000; Wang et al., 008; Panzer et al., 008). Postonng technology s ntended to obtan more accurate poston and orentaton of the robot based on the nformaton of the pror envronment map and the nformaton from multple sensors. The postonng methods can be roughly categorzed as relatve poston measurements and absolute poston measurements (Zhang et al., 005; Guccone et al., 000). Relatve postonng ncludes the use of encoders, gyroscopes and accelerometer (Zunad et al., 006). Absolute postonng nvolves usng beacons (actve or passve) (Venet et al., 00), global postonng systems (GPS) (Wllgoss et al., 003), landmark recognton (Borensten et al., 997) and model matchng methods (Fang, et al., 006). GPS s wdely used for absolute postonng of the moble robot, but t cannot properly receve satellte sgnals ndoors and has lower accuracy, whch makes ts ndoor applcaton to a moble robot dffcult (Km and Park, 008). In recent years, the robot postonng based on ZgBee wreless sensor networks has become a new study hotspot (Leland et al., 006; La et al., 007). ZgBee wreless sensor network s a new wreless standard havng the followng features: low cost, low power, low speed, short tme delay, large network capacty and hgh relablty (Yang and Lu, 007). Although ZgBee has hgher postonng accuracy than GPS, t cannot meet the needs of accurate postonng for a moble robot. Therefore, the more accurate posonng method has to be employed. Many references have approached the postonng problem of moble robots by employng landmarks (Se et al., 00; Armngol et al., 00; Bas and Sablatng, 006). The key dea of the postonng method s to use specal marks that nclude a wealth of geometrc nformaton under perspectve projecton so that the camera locaton can be easly computed from the mage of the gude marks. Researchers have chosen celng lghts as landmarks snce they can be easly detected due to the hgh contrast between the lght and the celng surface. Also, the celng lghts do not need to be nstalled specally (Wang and Ishmatsu, 004; Nguyen et al., 007). Therefore, we choose celng lghts as landmarks to realze the accurate postonng of the moble robot.

3 44 Robot Localzaton and Map Buldng Ths chapter presents a postonng method for a moble robot based on the ZgBee wreless sensor networks and vson sensor. Usng the ZgBee, the absolute poston of the moble robot can be estmated. To obtan the accurate postonng, we use vson sensor (Jang, 008; Yang et al., 008), and choose celng lghts as landmarks to realze the relatve postonng of the moble robot. The method not only overcomes the shortcomngs of low postonng accuracy usng ZgBee wreless sensor networks, but also decreases the computaton complexty of usng landmark absolute postonng. To gude the moble robot, the path plannng and navgaton control method are descrbed. Fnally, a moble robot system s ntroduced and the postonng experments are conducted.. The Postonng System Based on ZgBee ZgBee s an emergng short-range, low-rate wreless network technology whch s based on the IEEE Ths s a standard of the IEEE wreless personal area network workng group that s known as IEEE techncal standards.. The Composton of the ZgBee Wreless Channel IEEE works n the ndustral, scentfc, medcal (ISM) band. It defnes two workng bands: the 868/95MHz and.4ghz frequency bands. They correspond to two physcal layer standards, and both physcal layers are based on the drect sequence spread spectrum (DSSS) technology. The 868MHz band s the addtonal ISM band n Europe and the 95MHz band s the addtonal ISM band n Amerca. They cannot be used n Chna. In the IEEE , the ISM band s dvded nto 7 channels wth three data rates. The.4GHz band has 6 channels of 50kb/s rate and the channel spacng s 5MHz; the 95MHz band has 0 channels of 40kb/s rate and the channel spacng s MHz; the 868 MHz band has only one channel of 0kb/s rate. The wreless channels used n ZgBee are shown n Fg.. Fg.. The dstrbuton of frequency and channels. Postonng System of ZgBee The ZgBee wreless sensor networks postonng system ncludes two categores of nodes: the reference node and the postonng node (blnd node, also known as moble node). A node whch has a statc locaton s called a reference node. Ths node must be confgured wth X and Y values that are correspondng to the physcal locaton. The man task for a

4 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 45 reference node s to provde an nformaton packet that contans X and Y coordnates for the blnd node, also referred to as an anchor node. A reference node can be run on ether a CC430 chp or a CC43 chp. A blnd node wll communcate wth the neghbourng reference nodes, collectng X, Y and RSSI (Receved Sgnal Strength Indcator) values from each of these nodes, and calculate ts poston X, Y based on the parameter nput usng the locaton engne hardware. Afterwards the calculated poston should be sent to a control staton. Ths control staton could be a PC or another node n the system. A blnd node must use a CC43 chp. The bggest dfference between CC43 chp and CC430 chp s that the CC43 chp ncludes a lcensed locaton engne hardware core from Motorola. Adoptng ths core, the postonng accuracy of about 0.5m that s much hgher than that of GPS can be acheved, and postonng tme s n the range of 40μs..3 Postonng Theory The locaton algorthm used n CC43 locaton engne s based on RSSI values. The RSSI value wll decrease when the dstance ncreases. The basc RSSI based rangng prncple can be descrbed as follows: The emsson sgnal strength of the transmttng node s known, and the recevng node calculates the sgnal's transmsson loss n spread accordng to the sgnal receved. Usng a theoretcal or emprcal model, the transmsson loss wll be converted nto the dstance. The bass of the dstance measurement based on theoretcal model s the wreless sgnal transmsson theory. In free space, the sgnal strength the recever receves s gven by the followng formula PG t tgr Pr ( d), () (4 ) d L where: P t s the transmtter power; Pr ( d) s the recevng power; G t and Gr are the transmttng antenna gan and recevng antenna gan, respectvely; d s the dstance between the sender and the recever; L s the system loss factor; λ s the wavelength. Based on ths prncple, IEEE gves a smplfed channel model. The receved sgnal strength s a functon of the transmtted power and the dstance between the sender and the recever. The receved sgnal strength wll decrease wth ncreased dstance and can be expressed n the followng equaton RSSI (0n log d A), () where: n s the sgnal propagaton constant that s related to the sgnal transmsson envronment; A s the receved sgnal strength at a dstance of one meter that s related to the transmtter power. Fgure shows how the curve of the receved sgnal strength value changes wth the dstance between the transmtter and the recever. 0.4 Postonng Process The CC43 locaton engne uses dstrbuted computng methods and hardware to compute the locaton. Ths consumes less CPU resources. The locaton engne collects 3 to 6 reference nodes messages. If t receves more than 6 node s messages, t wll arrange the

5 46 Robot Localzaton and Map Buldng values of d n ascendng sequence and use 6 nodes of the RSSI values n front. Network communcatons between the adjacent nodes exchange coordnates of the poston and RSSI nformaton. In the process, the blnd node obtans the RSSI values and the coordnates of adjacent reference nodes of the known address. Accordng to the RSSI values, the blnd node can calculate the dstances between tself and the reference nodes. Fnally, the blnd node uses the Maxmum Lkelhood Estmaton Method to calculate ts coordnate. Fg.. The curve of the receved sgnal strength value The RSSI-based localzaton algorthm can be dvded nto the followng steps: () The blnd node sends a request to all the reference nodes to obtan the sgnal of locaton nformaton. For the blnd node n the sensor networks, when t needs to determne ts poston, t wll broadcast to all the surroundng reference nodes, requestng ther coordnates and ID nformaton. () The reference nodes send ther nformaton of coordnates and ID after the blnd node s request s receved. (3) The blnd node receves coordnates from reference nodes wth dfferent ID, and measures RSSI values of dfferent reference nodes. (4) When the blnd node receves the coordnates and the RSSI values of m nodes, t wll convert the RSSI values to the correspondng dstances d. Then t arranges the values of d n ascendng sequence and establshes the correspondng collectons of dstances and coordnates. Dstances collecton: Dstance ={ d, d,, d m }, d < d < < d m ; Coordnates collecton: Coordnate= { ( x, y ), ( x, y ),..., ( xm, y m ) }. (5) In order to mprove the postonng accuracy and reduce the postonng error caused by dstance measured, the blnd node judges the measured dstances. The dstance d s nvald when t s greater than the lmt value. The dstance d and the coordnate must be removed from the collectons. The blnd node retans the rest of the n (n 3) group s nformaton. (6) For the n group s nformaton of n reference nodes, the Maxmum Lkelhood Estmaton Method s used to calculate the blnd node s coordnate.

6 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 47.5 The Maxmum Lkelhood Estmaton In the three-dmensonal space, f the dstances from one pont to 4 reference ponts are known, the coordnate of an unknown pont can be determned. In the sensor networks, the coordnate system s two-dmensonal. As long as the dstances from the blnd node to 3 reference nodes are known, the poston of blnd node can be calculated. The postonng algorthms based on rangng nclude Trlateraton, Trangulaton, Weghted Centrod Algorthm and Maxmum Lkelhood Estmaton. In ths chapter, we use the Maxmum Lkelhood Estmaton Algorthm. The known coordnates of n reference nodes are ( x, y ), ( x, y ), ( x3, y 3 ),..., ( xn, y n ), respectvely. The dstances between the blnd node O and the n reference nodes are d, d, d 3,..., d n. The coordnate of the blnd node s ( x, y ). From Fg. 3, we have Fg. 3. Maxmum Lkelhood Estmaton ( x x) ( y y) d, ( xn x) ( yn y) dn (3) x xn ( x xn ) x y yn ( y yn ) y d dn xn xn ( xn xn ) x yn yn ( yn yn ) y dn dn. (4) The lnear equatons n (4) can be expressed as AX b. (5) Usng the Mnmum Mean Square Error Method, we can obtan the coordnate of the blnd node T T X ( A A) A b, (6) where: ( x xn ) ( y yn ) A, (7) ( xn xn ) ( yn yn )

7 48 Robot Localzaton and Map Buldng x xn y yn dn d b, (8) xn xn yn yn dn d n X [ x y] T. (9) 3. Postonng Based on Vson Sensor In order to obtan the accurate postonng, the celng lghts are chosen as landmarks and the vson sensor s used to recognze the landmarks. 3. Landmark Recognton Usng a vson system, the central lne and contour of the celng lght can be obtaned. However, the camera may acqure some unnecessary mages such as sunlght through wndows or any other lght sources. These unnecessary mages have to be elmnated. (a) The raw mage from camera (b) Land recognton from all lght sources Fg. 4. An example of landmark recognton

8 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 49 By means of the labellng program, the geometrcal data (area, length, wdth, poston, mnmum and maxmum horzontal/vertcal coordnates) of every whte cluster and the number of whte clusters n the mages can be obtaned. From the geometrcal data and the dstance of the neghbourng whte cluster, the desred mages can be pcked out from among others. An example of landmark recognton s shown n Fg. 4. Fgure 4(a) shows a raw mage from camera. From the labellng and geometrc calculaton, the celng lghts can be selected as landmarks from all lght sources as shown n Fg. 4(b). 3. The Coordnate System of a Sngle Lght On the bass of the raster coordnates of the sngle fluorescent lght, we have the poston vectors p xu yu f ( =, ) of projecton ponts P of lght ends Q n the camera coordnate system o c x c y c z c as shown n Fg. 5. From the perspectve transformaton, the relaton between q and p can be expressed as Celng Lght Q y j z j o j Q x j Celng Plane q kp q k ' kp p q cj q jm n c P Image Plane z c P ' P Parallel Plane p p ' p y c z m Fg. 5. Relaton between the coordnate systems of the camera and a sngle lght q kp ( =, ), (0) q x y z ndcates the poston vector of lght end Q n the camera where c c c x c o c q mc x m coordnate system, k s the proportonalty coeffcent between two vectors. If the length L of the lght s known and the mage plane and the celng plane are parallel, the proportonalty coeffcent k can easly be obtaned as follows o m y m

9 430 Robot Localzaton and Map Buldng kp kp q q L k k k p p p p p p. () Once k s known, the vector q can be obtaned usng equaton (0). It s more effcent to place the camera on a plane that s not parallel to the plane of the celng lghts so that the sght of camera can be ncreased. In ths case, equaton () cannot be used. In order to calculate k, we consder a plane that passes through the pont P and s parallel to the celng plane. In the camera coordnate system, the equaton of a parallel plane can be wrtten as follows T ' n ( p p )=0. () In the above equaton, c ' p ndcates the poston vector from pont oc to pont ' ' P and the ' pont P s the crossng pont of the parallel plane and the strght lne ocq, nc s the normal unt vector through the pont p of the parallel plane n the camera coordnate system and can be obtaned usng the followng equaton n A T T c (0 0 ), (3) where A represents the rotaton transformaton matrx of the camera coordnate system relatve to the coordnate system of moble robot and s known n advance. The relaton ' between the poston vectors p and p can be expressed n the followng equaton p tp. (4) Substtute equaton (4) nto (), and the parameter t can be obtaned as follows T c t n p T n p. (5) ' From equaton (4), the poston vector p can be obtaned. When the poston vector p n ' equaton (0) and () s replaced by p, the equaton (0) and () can be used to obtan q and k. Afterq s obtaned, the lght coordnate system can be determned. The orgn o j of the lght coordnate system s at the mddle pont of ponts Q and Q. The x-axs s along the drecton from Q to Q. The unt vector of the z-axs of the lght coordnate system s parallel to the z-axs of the coordnate system o w x w y w z w fxed on the moble robot. The transformaton matrx of the lght coordnate system relatve to the camera coordnate system can be expressed as follows B e x e y e z, (6) where q q ex e 0 0, z e y ez e x. (7) q q, c

10 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor The Coordnate System of Double Lghts From the raster coordnates of the double lghts, we have the vectors x y f p ( u u =,, 3, 4) n the camera coordnate system shown n Fg. 6. From perspectve transformaton, the relaton between q and p can be expressed as Celng Lght y j o j z j x j Q Q 4 Q Q 3 q 3 k3p3 q kp Celng Plane q kp q cj q 4 k4p4 z c q jm Image Plane Fg.6. Relaton between the coordnate systems of the camera and the double lghts q kp ( =,...4), (8) where q ndcates the poston vector of lght end Q n the camera coordnate system, and k s the proportonalty coeffcent between two vectors. Snce the two lghts are P P P P 3 4 p p 3 p p x 4 c oc y c parallel and the lengths of the two lghts are equal, the followng vector equaton can be obtaned k p k p k p k p. (9) From cross product of the vector p4 and equaton (9), we have the followng equaton k p p k p p k p p. (0) By nner product of the vector p and equaton (0), the followng equaton can be obtaned k p p p p p p. () k The ratos of k3 and k can be expressed n the followng equaton p p4 p k3 c3k, c3. () p p p 4 3 Usng a smlar method, the coeffcents k and k 4 can be expressed as follows qmc x m o m z m y m

11 43 Robot Localzaton and Map Buldng k k c k, c 4 4 c k, c 4 p p p p p p p p p p p p , (3). (4) Snce the dstance between Q and Q s the length of the lghts, the followng equaton can be obtaned k p k p k p c ( p p ) c p. (5) From the above equaton, the proportonalty coeffcent k can be calculated k q q p c ( p p ) c p. (6) The k 3, k4 and k can be obtaned usng equatons (), (3) and (4). From the equaton (8), the q ( =,, 3, 4) can be calculated. After q s obtaned, the relaton between the coordnate systems of the lght and the camera can be establshed. The lght coordnate system o xyz s set as shown n Fg. 6. The orgn o of the lght coordnate system s chosen at the mdpont of ponts Q and Q 3. The poston vector of the orgn s q ( q 3 q ) /. The x-axs s along the drecton c from Q to Q. The z-axs s along the normal lne of the plane that s composed of three ponts Q, Q and Q 3. The unt vector of z-axs can be determned as follows e z q4 q3 q q3 q q q q (7) The unt vector of the y-axs can be determned usng rght hand law. The transformaton matrx B of the lght coordnate system relatve to the camera coordnate system can be obtaned usng equaton (6). p x y f ( =,, 3, 4) of the For a lght wth a rectangular lampshade, the vectors u u four vertces can be obtaned. Usng a smlar method presented above, we can determne the coordnate system of the celng lght. 3.4 Postonng of the Moble Robot The self-locaton of a moble robot s a prerequste step to determne the poston and orentaton of the moble robot relatve to the landmarks and to ts world coordnate system. Snce the rotaton matrx A s known, the homogenous transformaton matrx of the coordnate systems between the moble robot and celng lght can be expressed as A qmc Β qcj T, (8) 0 0

12 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 433 where q mc ndcates the poston vector of the camera coordnate system relatve to the moble robot coordnate system. The nverse matrx of above matrx can be wrtten n the followng form C q jm T, (9) 0 where T T C B A, (30) T T T q B q B A q. (3) jm cj mc In equaton (30), C represents the rotaton matrx of coordnate system of the moble robot relatve to celng lght. In equaton (3), q expresses the poston vector of the moble jm robot n the coordnate system of the lght. Consderng that the z-axs of the lght coordnate system and the z -axs of the moble robot coordnate system are always parallel, the rotaton angle be calculated as follows jm m of the moble robot relatve to the lght coordnate system can sn c tan tan, (3) jm cos c where c s the element of the second row and the frst column n matrx C. When only one landmark s n sght of the camera, we assume the landmark s j-th celng lght. Snce the poston vector of the j-th celng lght n the world coordnate system s known, the poston vector of the moble robot n a two-dmensonal world coordnate system o xy can be obtaned as follows r r r, (33) om oj jm where r s a two-dmensonal vector wth x and y coordnates as shown n Fg. 7. The orentaton of the moble robot n the world coordnate system can be obtaned usng the followng equaton om oj jm, (34) where om and oj ndcate the orentaton of the coordnate system fxed on the moble robot and the j-th lght coordnate system relatve to the world coordnate system respectvely. We consder a general case where k landmarks are n sght of the camera as shown n Fg. 7. It s readly understandable that the more landmarks n sght are, the hgher the accuracy s n the self-localzaton attaned usng the Method of Least Squares (Tajma and Komak 996). Snce the dstance between the moble robot and landmark has an effect on the accuracy of the measurement, the weghtng factor for every landmark should be consdered. From equaton (33), the sum of the squares of the measurement error wth a weghtng factor can be wrtten as

13 434 Robot Localzaton and Map Buldng Fg. 7. Landmarks n sght of camera o j y Moble Robot r y om y x x x y m r oj x m r jm k Landmark y j x j j where k l J rom rom roj rjm, (35) j l jm r l s the weghtng factor of every landmark, l can be taken as the length of the l r jm celng lght. A necessary condton to mnmze J rom s that the followng equaton must be satsfed J r l k j om r 0 om roj rjm. (36) r ( ) om l rjm From the above equaton, the poston vector r om of the moble robot relatve to the world coordnate system can be expressed as follows k l r oj rjm ( ) j l rjm r om. (37) k l ( l r ) j Usng a smlar method, the orentaton of the moble robot relatve to the world coordnate system can also be obtaned as k l oj jm ( ) j l rjm om. (38) k l ( l r ) j jm jm 4. Path Plannng and Navgaton Control A major problem n gudng a moble robot to ts ntended goal s the path plannng or fndpath problem. Many references on path plannng have been publshed to address the problem (Desaulners and Vlleneuve 000; Zhuang et al. 006). In ths secton, the method of path plannng based on landmarks and the control scheme for autonomous navgaton based on path plannng are presented.

14 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor Navgaton Map and Network Matrx Navgaton of a moble robot usng landmarks depends upon a navgaton map shown n Fg. 8. The navgaton map should contan the nformaton about the poston of every landmark and about possble paths. The possble paths are determned as a seres of lnes that connect neghborng nodes stuated near the landmarks. The navgaton map wth n landmarks can be expressed usng the followng matrces Matrx R mark mark o o on R r r r, (39) R r r r. (40) nn s composed of the poston vectors of all landmarks (celng lghts) n the world coordnate system. Matrx R ndcates the poston vector of every node relatve to the correspondng landmark. Ths matrx determnes the navgatonal course that drfts away the landmarks takng nto consderaton the obstacles n the path. The poston vectors of all nodes can be expressed as node n mark R r r r R R. (4) y o Fg. 8. Navgaton map Node k r k r j x d jk Node j 3 r n r o r Landmar In order to fnd the optmal path among all possble paths, we ntroduce the followng network matrx D 0 d d, (4) 0 0 dn dn n d 0 dn where d 0 j ndcates the dstance between node and j, that s, d r r. If there s an 0 j j obstacle between node and j, or node and j are not adjacent, we use d 0 j to express that the path between node and j s not feasble. Snce the dstance from node to j and the dstance from node j to are equal, the network matrx s a symmetry matrx.

15 436 Robot Localzaton and Map Buldng 4. Shortest Path 0 Based on the network matrx D, the shortest path from the startng poston to the selected node (goal poston) can be found. Here, the Floyd Shortest Path Algorthm (Mneka, 978) s used to fnd the shortest path. Before ntroducng the algorthms, some notatons need to be defned: d denotes the length of a shortest path from node to node j, where only the frst m- m j node s allowed to be the ntermedate node. If no such path exsts, then let d ths defnton of d m j m j. From, t follows that d denotes the length of the shortest path from to j that uses no ntermedate nodes. d represents the length of a shortest path form to j. N j m D denotes the m 0 j m matrx whose, j-th element s d. N D s the matrx of shortest path length whose, j-th element expresses the length of the shortest path among all nodes. p ndcates the next-to-last node n the shortest path from to j that s called the l j penultmate node of that path. V represents the path vector of the shortest path whose every element s the node path number (landmark number). Suppose the shortest path conssts of L nodes, then p l ( l,,3,... L) V v, v v of the shortest path, j where v l consttutes the path vector L l p j. The Floyd Shortest Path Algorthm starts wth D 0 and calculates D from D 0. Next, D s N N calculated from D. Ths process s repeated untl D s obtaned from D. The algorthm can be descrbed as follows: 0 () From the navgaton map, determne the network matrx D. m j path () For m =,, N, successvely determne the elements of m of D usng the followng recursve formula (3) After m m m m j m mj j L m D from the elements d mn d d, d. (43) As each element s determned, t records the path t represents. Upon termnaton, the N, j-th element of matrx D represents the length of a shortest path from node to j. m Note that d 0 for all and all m. Hence, the dagonal elements of the matrces D, N D,... D do not need to be calculated. Moreover, m m only d j (j > ) s calculated n matrx D. Let pj N D s obtaned, the penultmate node j and p L j. If p the path vector from to j s N j p 0 j l pj 0 D s a symmetry matrx, and can be found as follows:, there are only two nodes n the shortest path and V j p p. Otherwse, L path (, ) ( j, j )

16 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 437 for p l ( l,,3,... L ), suppose that the penultmate node n a shortest path from to j l j s any node k, that s, p k( k, j and s not repeated). If k s satsfed wth the followng equaton we have pj j d d d, (44) N 0 N k kj j k. Then, the second-to-last node n ths path s the penultmate node p 3 k on a shortest path from to k. Ths process can be repeated untl all the nodes n ths path from to j have been tracked back. In ths step, only D 0 and D N matrces are requred for the value of k to be determned. (4) When all the nodes n the shortest path from to j have been found, the path vector from to j can be obtaned as follows L L v, v v p, p p V. (45) path L j j j 4.3 Autonomous Navgaton Based on Path Plannng In ths secton, we wll present the control scheme for the autonomous navgaton of the moble robot based on the path vector. When the path vector s known, the poston vectors of all nodes n the shortest path can be expressed n the followng matrx v v v vl R r r r. (46) From Fg. 9, two vectors adjonng three nodes n the path can be expressed as a rv r v, v v a r r ( L-). (47) The angle between the two vectors can be obtaned usng the followng equaton tan a a k, (48) a a where k represents the normal unt vector of the plane o-xy. Usng the above equaton, we can obtan the control scheme for the moble robot from node + to node +. When < 0, the moble robot turns degree to the rght, otherwse, t turns degree to the left. In the above control scheme, the moble robot must navgate along the vector a and the movng drecton must be concdent wth the drecton of the vector a. However, when a moble robot navgates tself from node to node +, t wll unavodably encounter an obstacle. After the obstacle s avoded, the moble robot wll drft off the vector a as shown n Fg. 0. In order to enable the moble robot to move to node + after obstacle avodance, the angle between two vectors b and c should be obtaned

17 438 Robot Localzaton and Map Buldng y Node Node a r v a Node r v r v o Fg. 9. Vector between adjonng nodes x ( ) tan c b k, (49) c b where b rv r om. When the moble robot moves to node +, the control scheme for the moble robot from node + to node + can be obtaned as follows tan ( a b ) k. (50) a b When the moble robot navgates from node to +, t needs to know whether the node + has been reached. The poston vector between the moble robot and node can be obtaned as follows d r r. (5) The angle between the vectors a and From Fg. 0, the projecton of vectors follows om v d s calculated n the followng equaton ( ) tan d a k. (5) d a d and b on the vector a can be calculated as L d cos b cos( ). (53) a When the dstance between two nodes s bgger than the projecton of the vectors b and d on vector a, that s, a La, the moble robot has not yet reached the node +. When a L s obtaned, node + has been arrved at. In ths case, the moble robot can a automatcally turn to the rght or to the left at angle and navgate from node + to +.

18 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 439 y Node a b a Node Node d r om c Poston of Robot Movng Drecton o x Fg. 0. Poston and movng drecton of the moble robot after obstacle avodance 5. Moble Robot System and Postonng Experments 5. Moble Robot System Fgure shows an omn-drectonal moble robot developed by us. The moble robot has 8 wheels as shown n Fg.. The 8 wheels are dvded nto 4 groups that have the same transmsson mechansm. One wheel of each group s the drvng wheel and the other s the free wheel. Snce two belts drven by two motors make four drvng wheels move n a synchronous way, the moton of the robot platform s a translaton moton. The mechancal desgn and knematcs of the moble robot were dscussed n earler work (Wang, et al., 008). The control system of the moble robot conssts of one set of the ZgBee wreless sensor networks (C5RF-3-ZDK), a CCD camera (BASLER A30FC), a laser sensor (BANNER LT3), 8 ultrasonc sensors (BTE054: US Sensor ), a motor controller (PI-6), three all-n-one motors, a SH7044 mcrocomputer and a notebook PC as shown n Fg. 3. The ZgBee wreless sensor networks and the vson sensor are employed for the robot postonng, and the ultrasonc sensors are used to detect obstacles n the way. The laser sensor s not used n ths experment. Fg.. The omn-drectonal moble robot

19 440 Robot Localzaton and Map Buldng Fg.. The drvng system of the moble robot Fg. 3. The control system of the moble robot 5. Postonng Experments In ths paper, the postonng system of ZgBee wreless sensor networks ncludes blnd node and 8 reference nodes. The experments were carred out n the corrdor of our laboratory (Fg. ). The blnd node s fxed on the moble robot, 8 reference nodes are nstalled n the navgatonal envronment as shown n Fg. 4. Fg. 4. Expermental envronment

20 Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor 44 We let the robot navgate from the start poston to the end poston, and collected the postonng data. We obtaned the errors of postonng by comparng the expermental data wth the theoretcal calculaton results. The poston errors are shown n Fg. 5. Fg. 5. Poston errors 6. Concluson A postonng method of the moble robot based on the ZgBee wreless sensor networks and vson sensor was presented. Usng the ZgBee postonng system, the rough absolute postonng could be obtaned. The postonng system and postonng theory of ZgBee were descrbed. In order to obtan the accurate postonng, the celng lghts were chosen as landmarks and the vson sensor was used to recognze the landmarks. The postonng method usng the celng lght landmarks was proposed. The navgaton map that contans the nformaton about the poston of every landmark and possble path n the navgatonal envronment was establshed and the network matrx that expressed the navgaton map was obtaned. Then the control method of autonomous navgaton was descrbed. The postonng experments based on the ZgBee and vson sensor for the moble robot developed by our laboratory were conducted and expermental results showed the effectveness of the postonng method. Acknowledgement Ths work was supported by the Foundaton of Hebe Educaton Commttee under Grant 00849, and the Scentfc and Technologcal Foundaton for the Returned Overseas Chnese Scholars, Hebe Provnce.

21 44 Robot Localzaton and Map Buldng 7. References Armngol, J.M., Escalera, A., Moreno, L. and Salchs, M.A. (00). Moble robot localzaton usng a non-lnear evolutonary flter. Advanced Robotcs, Vol. 6, No. 7, pp ISSN: , (Onlne). Bas, A. and Sablatng, R. (006). Landmark based global self-localzaton of moble soccer robots. Lecture Notes n Computer Scence, Sprnger Berln, Hedelberg, Vol. 385, pp ISBN: ISSN: , (Onlne). Borensten, J., Everett, H.R., Feng, L. and Wehe, D. (997). Moble robot postonng: sensors and technques. Journal of Robotc Systems, Vol. 4, No. 4, pp ISSN: Desaulners, G. and Vlleneuve, D. (000). The shortest path problem wth tme wndows and lnear watng costs. Transportaton Scence, Vol. 34, No. 3. pp ISSN: Fang F., Ma, X.D. and Da, X.Z. (006). Moble robot localzaton based on mproved model matchng n hough space. 006 IEEE/RSJ Internatonal Conference on Intellgent Robots and Systems. Bejng, Chna. pp ISBN: X. Guccone, S., Muscato, G., Nunnar, G., et al. (000). Robots for volcanos: the state of the art. Proceedngs of the 3rd Internatonal Conference on Clmbng and Walkng Robots, Madrd, Span, pp ISSN: Jang, Z.H. (008). Vson-based cartesan space moton control for flexble robotc manpulators. Int. Journal of Modellng, Identfcaton and Control, Vol. 4, No. 4, pp ISSN: Km, S.Y. and Park, S. (008). Estmaton of absolute postonng of moble robot usng U- SAT. Lecture Notes n Computer Scence, Sprnger Berln, Hedelberg, Vol. 467, pp ISBN: ISSN: , (Onlne). La, X.Z., Yang, S.X., Zeng, G.X., She, J.H. and Wu, M. (007). New dstrbuted postonng algorthm based on centrod of crcular belt for wreless sensor networks. Int. Journal of Automaton and Computng, Vol. 4, No. 3, pp ISSN: Leland, E., Bradford, K. and Jenkns, O.C. (006). Robot localzaton and control. Crcut Cellar, Issue 88, pp ISSN: Meng, Q.H., Sun, Y.C. and Cao, Z.L. (000). Adaptve extended Kalman flter (AEKF) - based moble robot localzaton usng sonar. Robotca, Vol. 8, pp ISSN: Mneka, E. (978). Optmzaton algorthms for networks and graphs. Marcel Dekker Inc., New York. ISBN: Nguyen, V.T., Jeong, M.S., Ahn, S.M., et al., (007). A robust localzaton method for moble robots based on celng landmarks. Lecture Notes n Computer Scence, Sprnger-Verlag, Vol. 467, pp ISBN: ISSN: , (Onlne). Panzer, S., Pascucc, F. and Setola, R. (008). Smultaneous localzaton and mappng of a moble robot va nterlaced extended Kalman flter. Internatonal Journal of Modellng, Identfcaton & Control, Vol. 4, No., pp ISSN: Se, S., Lowe, D. and Lttle, J. (00). Moble robot localzaton and mappng wth uncertanty usng scale-nvarant vsual landmarks. Internatonal Journal of Robotcs Research, Vol., No. 8, pp ISSN:

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23 444 Robot Localzaton and Map Buldng

24 Robot Localzaton and Map Buldng Edted by Hanafah Yussof ISBN Hard cover, 578 pages Publsher InTech Publshed onlne 0, March, 00 Publshed n prnt edton March, 00 Localzaton and mappng are the essence of successful navgaton n moble platform technology. Localzaton s a fundamental task n order to acheve hgh levels of autonomy n robot navgaton and robustness n vehcle postonng. Robot localzaton and mappng s commonly related to cartography, combnng scence, technque and computaton to buld a trajectory map that realty can be modelled n ways that communcate spatal nformaton effectvely. Ths book descrbes comprehensve ntroducton, theores and applcatons related to localzaton, postonng and map buldng n moble robot and autonomous vehcle platforms. It s organzed n twenty seven chapters. Each chapter s rch wth dfferent degrees of detals and approaches, supported by unque and actual resources that make t possble for readers to explore and learn the up to date knowledge n robot navgaton technology. Understandng the theory and prncples descrbed n ths book requres a multdscplnary background of robotcs, nonlnear system, sensor network, network engneerng, computer scence, physcs, etc. How to reference In order to correctly reference ths scholarly work, feel free to copy and paste the followng: Wang Hongbo (00). Moble Robot Postonng Based on ZgBee Wreless Sensor Networks and Vson Sensor, Robot Localzaton and Map Buldng, Hanafah Yussof (Ed.), ISBN: , InTech, Avalable from: InTech Europe Unversty Campus STeP R Slavka Krautzeka 83/A 5000 Rjeka, Croata Phone: +385 (5) Fax: +385 (5) InTech Chna Unt 405, Offce Block, Hotel Equatoral Shangha No.65, Yan An Road (West), Shangha, 00040, Chna Phone: Fax: