Proceedings of the 2006 IEEE/RSJInternational Conference on Intelligent Robots and Systems

October 9 - 15, 2006, Beijing, China

Ultra Low Frequency Electromagnetic WaveLocalization and Application to Pipeline Robot

Hongjun ChenDepartment of Electrical Engineering

Harbin Institute of TechnologyHarbin, China 150001

Email: hongjun@hit.edu.cn

Xiaohua ZhangDepartment of Electrical Engineering

Harbin Institute of TechnologyHarbin, China 150001

Email: zxhhitee@hit.edu.cn

Junyuan LiDepartment of Electrical Engineering

Harbin Institute of TechnologyHarbin, China 150001

Email: lijunyuan@hit.edu.cn

Abstract- The paper analyzes the magnetic field distribu-tion of emitter, and studies the localization technique basedon ultra low frequency electromagnetic wave sensors. As theelectromagnetic detection is sensitive to transform medium, whichis related to its dielectric coefficient and magnetic inductivecapacity, the paper presents a magnetic dipole model to simplifythis problem and studies a new localization method for pipelinerobot application. In order to improve the position precision,multi sensors are installed in the system. Based on constraintsatisfaction problem method, an electromagnetic wave localiza-tion algorithm is presented, and the localization error is analyzed.Experiments are designed to verify the analysis and model. Theexperimental results show that the localization method basedon ULF electromagnetic wave has a satisfied precision for thepipeline robot application.

I. INTRODUCTION

Pipelines have found many applications in industries, suchas chemistry, oil transportation, and municipal administration.The pipelines should be inspected and maintained after anoperation period in order to ensure safe operation. Pipe robotor pipe pig provides the special services for maintenanceand repair of the pipeline systems and pressure vessels fortransmission and distribution of gases or fluids. Unlike otherkind mobile robots, these applications require robot to workin a special environment where less detailed information isavailable. Localization is an important technique in pipelineinspection robot applications[ 1] [2]. Although some researchershave used vision technology to get information of innerpipeline, it is still a serious problem in the pipeline application,especially when we need to know the robot's precise positionin pipeline. The nuclear ray can be used to achieved this aim,but it is not a better solution considering workers' health safe.As the current techniques cannot provide a satisfied localiza-tion precision in the long pipeline inspecting procedure, weneed consider this serious problem in other way. Now theproblem is how to make the signal penetrate the steel pipeline,water, and mud during the pipeline inspecting operation.

Ultra low-frequency(ULF) provides a reasonable solutionfor pipeline robot localization. Although normal wireless com-munication has wider applications in industries and mobilephone, it cannot be utilized in pipeline inspection robotbecause of the high electrical conductivity of steel pipeline

and water, and signals are attenuated rapidly as they propagatedownward through it. The ULF electromagnetic wave canpenetrates not only sealed steel vessel but also water andmud even though its power is attenuated. Static electric andmagnetic fields arise from both natural and man-made sources,whereas electric and magnetic fields in the ULF range (3-3000Hz) are mostly associated with man-made sources. The degreeto which a signal is attenuated depends on its frequency. Thelower the frequency, the far a signal can be received. TheULF electromagnetic wave can be received considerably far inapplication, and broadcasts using this mode provide a primarylink under special working conditions. This is an importantcharacteristics and helps us to solve the above informationtransmission task in the pipeline robot application.The ULF technique has found many applications in indus-

tries and military. In 1970s and 1980s, the US military depart-ments studied the extremely low frequency electromagnetictechnology and proved that such system is the only system ca-pable of providing continuous communications to submarinesoperating at designed depths and speeds. A. Brandolini,G. D'Antona and M. Faifer etc. studied the low frequencymagnetic flux density measurements[3]. D. L. Atherton studiedthe magnetic inspection technique for far-side anomalies insteel pipe[4]. J. A. Da Silva developed an intelligent locationsystem for a pipeline project[5]. He studied application ofthe extra low frequency transmission and reception technologyin the pipeline. By recording the magnetic sensor signals inthe inspection process, the system can achieve more accurateposition information and prevent dangerous work when theoperation risks are high. The magnetic sensor provides centi-metric precision, overcomes blindness of pipeline robot, andpresents a solution for robot location in pipeline inspection.Robert Torbin studied an inspection system for unpiggablepipelines[6]. And Xianhe Ding studied magnetic distancesensing technique[7]. E. Prigge and J. How developed a signalarchitecture for magnetic local position[8].The seabed oil pipeline robot achieves internal inspection

and evaluation of pipeline conditions using the on-line in-spection tools such as magnetic and ultrasonic. Althoughsome researchers studied the localization using ULF technique,the recognitions are usually manual as the sensor signalis complicated, this paper studies the automatic recognition

1-4244-0259-X/06/$20.00 C)2006 IEEE1201

and localization problem of pipeline robot. The maintenanceship and the inner inspection robot is shown in Fig.l. Therobot inspects various defects of pipeline and robot's positionshould be known while moving along pipeline in order toprocess the maintenance and rescue for emergency. And themaintenance ship outside pipeline should detect the movementof robot during the normal inspecting operation. For thispipeline inspecting system, a set ULF electromagnetic deviceis utilized to localize the inner mobile robot. The receiverson the maintenance ship sense the magnetic field signal andoutput voltage signals to computer to process analyze, andrecognize.

In order to illustrate the principle of localization, a modelof ULF electromagnetic dipole is presented in Section II. Alocalization algorithm based on multi sensors is studied inSection III. And Section IV gives the experiment results andanalysis.

where I is the current of the circular line, n is the number ofthe circular loop wire, R is the radius of the circular line, and,uo is the magnetic conductive rate in free space[9].By using vector calculation, the magnetic filed dB can be

written as Eq.(2).

dB,uo IndzdL x r-4w r2,uo Indz(-R sin OdOl + R cos 0d0j)47 [(x0- Rcos 0)2 + (-Rsin 0)2 + (zo Z)2]3/2

x [(xo-R cos 0)i + (-R sin Oj) + (zo -z)k](2)

where 0 is the angular between x axis and radius at point C.Then the components dBx and dB, along axis x and z canbe written

dBx ,uolndz (zo- z)R cos047 (R2 -2R cos 0xo + x +(zo z)2)3

dB _ uolndz R(R- xo cos 0)47 (R2 + X2-2R cos 0xo + (zo z)2)3

Integer Eq.(3), then the component Bx along x axisI 7T

Bx = 847r J dz I[R2

= oInrR f(O)dO4w

R(zo -z) cos0d02R cos Oxo + xo + (zo

-dO/2(3)

/2d(4)

- Z)2]3/2

(5)where

Fig. 1. The pipeline inspecting and maintenance system f(0)II. MODEL OF MAGNETIC DIPOLE

The coil generating ULF electromagnetic wave can beregarded as a magnetic dipole when its length is far longerthan its diameter[9]. Consider a circular loop of line shown inFig.2 in unbounded space. According to the Biot-Savart law,

A

y

C

g(0)Fig. 2. Magnetic field of a circular loop

the magnetic field intensity dB, which is generated from theunit current line IdL at point C(R cos 0, R sin 0, z), can beexpressed in Eq.(l).

dB =uo IndzdL x 4r (1)47 r3

In the sameexpressed.

cos 0(R2 2R cos Ox0 +x 2)1/2 x

[/R2 2R cos 0xo +X2 + (zo + 1)2

1/R2 -2R cos 0xo+ X2 + (zo 1)2way, the component Bz along axis z can be

1 7T

B_9oI / dz f R(R- xocos0)dO2R cos 0xo + (zo= 7F

=,u0InR g (0)d04w97

where

Z)2]3/

(6)

-(R- xo cos 0)( -2 2R cos Ox0 + xo)F ~~(Zo-1)WR~~ 2Rc(os0o+ +zo 1)Y\R2 -2R cos Oxo + X2 + (Zo -1)2

(zo +1)1R2 2R cos0xo+xo + (zo + 1)2

In the engineering implementation, Eq.(5) and Eq.(6) shouldbe simplified. Considering the case I >» R, a small loop can

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be referred as a magnetic dipole. The magnetic field of thisdipole is shown in Fig.3.

xFEv

*a)

a)

Fig. 3. Magnetic field of a magnetic dipole

If the length of circular loop is 21, then the momentof magnetic dipole is expressed as Eq.(7). The capacity ofmagnetic charge is Eq.(8).

pipeline, water, and mud. The emitter is installed along move-ment direction on pipeline robot, and the receivers are installedvertical to the movement direction. Then the relationshipbetween lB, and z is shown in Fig.5. The characteristicof the receiver is a hump. The output of receiver increaseswhile move to emitter and equals to zero when the positionof receiver is same as that of emitter.

1.4xlO-'1.2x10-71.Oxio-78.OxlO-6.OxlO--4.OxlO--2.OxlO-8

0.0-2.OxlO- -

0 1 2Distance(m)

Fig. 5. Relationship between IB1I and z

m = 2nlIS (7)m2qm = -1 = nrIwR2 (8)21

Then the field at point D is generated by magnetic chargeqn and -q and can be expressed as the components alongx axis and z axis.

B_n-IR2/loxIR (2x[ +( Z)2)3/2 (2+ ( + Z)2)3/2

(9)B tnIR2,uOz z - z+ I

Bz =

4 [ + (Z 1)2)3/2 (2+ (Z + 1)2)3/2](10)

It is shown from Fig.4 that when z is near the origin,

FEv

p

._,

-9._,

1.5xlO-'1 .OxiO-

5.OxlO-0.0

-5.OxlO

-l1OxlO

-1.5xlO-10 -5 0 5 10

Distance (m)

Fig. 4. Relationship between B. and z

B. Installation of receivers

In order to improve the detecting precision and speed, threereceivers are installed along the movement direction in thelocalization system. The structure of this installation is shownin Fig.6. S0, Sl, and S2 are electromagnetic receivers and areplaced with equal space.

FH

S2(- y"0,)F

So(O, °) S (Yo,O)Receiver on vehicle

(Y,Z)

Steel pipeline (D Emitter

--p2121

Fig. 6. The installation of sensors

As the emitter moves along the pipeline, receivers installedon the maintenance ship sense the magnetic field from emitterand output voltage signals. The key problem is how to fusethe sensor outputs and calculate the robot motion parameters.

the component B, along x axis increases when z increases.This relationship is the basic principle for the pipeline robotlocalization.

III. LOCALIZATION WITH ULF ELECTROMAGNETIC WAVE

A. Electromagnetic Emitter

In the application of pipeline inspection, the frequency ofemitter is designed 23Hz considering the penetration steel

C. Constraint satisfaction problem algorithmConstraint satisfaction problem(CSP) is an effective method

to solve uncertain parameters calculation problem[10] [11]. Inthe robot localization system, the receiver's output provide thedistance information between emitter and receiver individuallyand rises a CSP. According to the CSP constrain relationship,the effect of electromagnetic transmission from media conduc-tivity and parameter uncertainty can be released by selecting

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k ki _,I-

i

Z

r

The factor tmn in matrix Tmn satisfies in the constrainrelationship Eq.(1 1). Then the factor aij of its unitary matrixAmn can be calculate in Eq.(15)

a_= O tij v (m-n)[d-d,d+d]1 tij C (m -n)[d -d,d+d]

The position y of robot can be calculated.

Y (yo + y +y)/3IV. EXPERIMENT AND ANALYSIS

(15)

(16)

Fig. 7. Virtual structure of ULF localization system

the sensor's output. It is shown from Fig.drawla that everysensor's output has four position. And the calculated positionmust meet the distance constrain of sensors installation. It isa typical CSP.

In the practical application, it is difficult to obtain magneticfield by mathematical analysis as the transmission media isvery complex. The normal method in application is to buildthe relationship between sensor's output x and the distance y.Assume the outputs of sensors So,S1,S2 are xo, x1, and

X2 respectively, and the relative distances between sensor andemitter are yo, Yi, and Y2. The space between sensors is d.Then yo, Yl, and Y2 satisfy Eq.(l1) constrain.

Yi -yo= dY2-Yl= dY2- Yo = 2d

In order to verify the above localization algorithm, threereceivers are used to sense the changed magnetic field, an USBinterface data acquisition with 14 bits resolution is connectedto a notebook. The computer operation system is Windows XPand the programmed is developed with Borland C++ Builderlanguage. In order to improve the detection system signal-noise rate, an adaptive gain is designed to change the adaptamplifier's output range. A digital Tektronix oscillograph isutilized to record the experiment results. Fig.8 is the the sealedULF localization unit used for the project. The receiver'soutput is shown in Fig.9.

(1 1)

Considering the sensor noise Ax in the system measure-ment, then the above constrain relationship becomes an inter-regional constrain. Assume the range of distance error is Ad,then we have the new constrain relationship.

Fig. 8. The sealed ULF emitter unit

-yo C [d-d,d+d]yi C [d-d,d+d]Yyo C [2d -2d, 2d + 2d]

It is shown from Fig.5 that there are four positions for every

sensor output. And consider the following assumptions.Assume yi, y2, y3, and y4 are matches for the outputxo.

Assume y, yl2 y3, and y4 are matches for the output

1 -Assume y2, y2, y3, and y4 are ma

X2.

Define the characteristic matrix of Sm

Ym Ym Ym Ym2 2 2 2

Tm - Ym Ym Ym Ym

Ym Ym Ym Ym

Then define the constrain matrix of senso

Tmn Tm Tn m,n =0,1,2

Fig. 9. Measurement of ULF receiver signal

tches for the output Firstly, an experiment is designed and implemented using

only one receiver in order to verify the localization algorithmis Tm. based on the magnetic dipole model. The receiver and note-

book are placed on a cart. The emitter is powered and is put inthe pipeline. The cart moves along the experimental pipeline.

m = 0,1, 2 (13) The output of receiver output at different distance is recordedand shown in Fig. 10. We can see that the curve of outputmatches the above theory curve and the magnetic dipole model

r. is true.Secondly, the repetition experiments were done with the

m > n (14) same emitter and receiver. We did this localization process

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Yi{ Y2

Y2(12)

1.4 -

0.8°0.61

' 0.4

0.0-150-100 -50 0 50 100 150

Distance (cm)

I--,

cti)*i)aDn

Fig. 10. Experiment of ULF localization

1.4-

1.2-

1.0-

0.8-

0.6-

0.4-

0.2-An.r

50 100 150 200Distance (cm)

Fig. 12. Localization experiment using three receivers

and recorded the output of receiver. With the same localizationalgorithm, we can calculate the distance between emitter andreceiver. Three repetition experiment results are shown inFig. 1. We can see that these curves have a good match.

C),a::

1.6-1.4-1.2-1.0 -

0.8-0.6-0.4-0.2-0.0-

1* 2

3* 4

-150 -100 -50 0 50

Distance (cm)100 150

Fig. 11. Repeat localization experiment

Then considering the redundance of sensor information andestimation reliability, we achieved a triple sensors experimentand recorded the receivers output. The algorithm based on CSPis used to calculate the position of robot. The interval of thereceivers is 80cm, and the emitter is placed at 150cm. Thelength of pipe is 300cm, and the pipe thickness is 9mm. Thesampling interval is 5cm, and 5 groups of data are collectedas reference. The experimental result is shown in Fig.12. It isshown from this figure that the output of receiver is differentas the distance between emitter and three receivers is not thesame.

The localization based on ULF electromagnetic wave hasa good precision. The experiments show that the dynamiclocalization error is within ±10cm when the relative speedbetween robot and maintenance ship is within 25cm/s. Al-though this method have a satisfied precision, the algorithmneed to be improved in order to release the relative speedconstrain. And the localization at the curve pipeline is still an

interesting problem to study in the future.

V. CONCLUSION

By using the special penetrating characteristic of ULFelectromagnetic wave, this paper presents a magnetic dipolemodel, studies a new automatic localization method, and

develops an algorithm is developed for pipeline robot appli-cation. Multi sensors are designed to improve the localizationprecision. Experiments are designed to verify the model. Theexperimental results show that the localization based on ULFelectromagnetic wave has a satisfied precision. The dynamicallocalization error is within ±10cm while the relative speed iswithin 25cm/s. However, the presented algorithm should beimproved in order to release the relative speed constrains andadapt different pipeline conditions, such as curve pipeline.

REFERENCES

[1] J. Moraleda, A. Ollero and M. Orte, "Arobotic system for internalinspection of water pipelines," IEEE Robotics & Automation Magazine,1999, 3 (6), pp. 30-41.

[2] Roh Se-Gon and Choi Hyouk Ryeol, "Differential-drive in-pipe robotfor moving inside urban gas pipelines," IEEE Transactions on Robotics,2005, 21(1), pp. 1-17.

[3] A. Brandolini, G. D'Antona, M. Faifer, M. Lazzaroni, and R. Otto-boni, "Low frequency magnetic flux density measurements based on

navigation agents," Proceedings of the ISA/IEEE Sensors for IndustryConference, 2004, pp. 86-90.

[4] D. L. Atherton, "Stress-shadow magnetic inspection technique for far-side anomalies in steel pipe," NDT International, 1983, 16(3), pp. 145-149.

[5] J. A. Da Silva, 0. De Jesus Ribeiro Afonso, J. A. C. Dos Santos,M. Matsuura, R. De Castro Antunes, etc., "A smart location system,"Proceedings of the Biennial International Pipeline Conference, 2004,vol.2, pp. 975-981.

[6] R. Torbin, W. Leary, and G. Vradis, "Robotic inspection system for un-

piggable pipelines," Proceedings of the Biennial International PipelineConference, IPC, vol. 2, 2004, pp. 1077-1086.

[7] Xianhe Ding, Katsutoshi Kuribayashi and Takao Hashida, "Developmentof a planar type micro electro magnetic distance sensor using high aspectratio photoresist and its application," Proceedings of the InternationalSymposium on Micro Machine and Human Science, 1999, pp. 227-233.

[8] Eric A. Prigge and Jonathan P. How, "Signal architecture for a distributedmagnetic local positioning system," IEEE Sensors Journal, 2004, 4(6),pp. 864-873.

[9] Edward J. Rothwell and Michael J. Cloud, "Electromagnetics", CRCPress 2001

[10] R. Dechter and D. Frost, "Back jump-based back tracking for ConstraintSatisfaction Problems", Artificial Intelligence, 2002, vol.136, pp. 147-188.

[11] Schmidt Linda, Shi Hai and Kerkar Sameer, "A constraint satisfactionproblem approach linking function and grammar-based design genera-tion to assembly" Journal of Mechanical Design, Transactions of theASME, 2005, 127(2), pp. 196-20.

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* . g. * *gg .~~

250 300() ()v.v-r

0