A CONCEPTUAL WHEELED ROBOT FOR IN-PIPE INSPECTIONIoan Doroftei, Mihaita Horodinca, Emmanuel Mignon

Université Libre de BruxellesActive Structures Laboratory

50, Av. F.D.Roosevelt, B-1050, Brussels, Belgium.Email: iodoroft@ulb.ac.be

Abstract : Th i s p ap e r d e s c r i b e s a n ew c on c e p t o f mob i l e r o b o t c u r r e n t l y b e i n g d e v e l o p e d a tULB a s a c o n c e p t ua l v e h i c l e f o r i n - p i p e i n s p e c t i o n . A s i n g l e DC mo t o r l o c a t e d o n t h ec e n t r a l ax i s a c t u a t e s t h e r o b o t , wh i c h c o n s i s t s o f tw o b o d i e s . Th e r o t a t i n g b o d y ha s t h r e ed oub l e wh e e l s wh i c h make an an g l e o f 10° t o a c h i e v e an h e l i c a l t r a j e c t o r y wh en t h e mo t o rr o t a t e s ; t h e b od y f i x e d t o t h e s t a t o r a l s o p o s s e s s e s t h r e e d oub l e wh e e l s o r i e n t e d pa r a l l e l t ot h e t u b e ax i s , t o a l l ow on l y ax i a l d i s p l a c em en t s . A l l t h e wh e e l s a r e moun t e d o n s p r i n g s i no r d e r t o a dap t t o c h an g i n g d i ame t e r s a l l ow t h e mo t i o n i n t o c u r v e p i p e s a nd c omp en s a t e f o ri r r e g u l a r i t i e s o n t h e i n n e r s u r f a c e o f t h e t u b e . Th e r o b o t c an mov e i n h o r i z on t a l , v e r t i c a l a sw e l l a s c u r v e d p i p e g e ome t r i e s .

Keywords: wheeled robot, in-pipe inspection.

1 Introduction

An important application for robotic systems is thearea of pipe inspection (in the oil, chemical andnuclear industry, the public water systems, andpossibly future space systems) [1-5]. In this contextand on the basis of its experience in mobile robots theActive Structures Laboratory of Université Libre deBruxelles, has developed a new concept of wheeledrobot for in-pipe inspection, called HELI-PIPE.

The robot has a number of advantages:• The vehicle has a very simple kinematics and

uses a single motor.• Low energy consumption, thanks to the

simple kinematics.• It can move in horizontal, vertical as well as

curved pipe geometries.• The robot can adapt to changing diameters

and to small obstacles on the inner surface ofthe tube.

• The robot can easily be protected againsthumid and dirty environments.

• It can be used for weld inspection, faultdetection, cleaning and repairing of internalpipe surfaces, etc.

In this paper, a structural synthesis, overallarchitecture and kinematics aspects of the HELI-PIPErobot are presented.

2 Structural Synthesis

We will consider first a plan mechanism with linearjoints, shown in figure 1. The joint 2C is a passive one

Figure 1. Plan mechanism with linear joints

and it was introduced just to increase the rigidity ofthe mechanism. It is easy to demonstrate that themechanism has a single degree of freedom (onedriving link) (6). If we assume that 1 is the driver, thedriven link 2 will have a vertical translation along theframe 0, considered fixed.

Figure 2. An equivalent plan mechanism of the robot

The relative movements of all the joints, movements

described by the vectors CBA S,S,Srrr

, are dependentby the relation:

( ) ( ) ( )2B2

A2

C SSSrrr

=+ (1)

The value of the angle α should satisfy the conditionof the auto-blocking phenomenon.

In order to obtain an equivalent plan mechanism forour robot, we will develop the mechanism from figure1. So, if we introduce the three new passive joints

432 B,B,B our mechanism will look as the oneshown in figure 2. All the guide bars 2 of the joints iBmake the same angle α with the horizontal guide barof the joint A . In order to prepare the next step of thesynthesis, we assume that the distance between the

centre of two neighbour joints iB is R32

⋅⋅π , where

R is the internal radius of the pipe. Also, the distancebetween the centre of the joints A and iB , along anaxis perpendicular on the plane of the joints iB , is R(as shown in figure 2.b).

If the plan of the joints iB is rolled on a cylinder ofradius R , around the axis AA − , as well as the joints

41 B,B respectively the axes 21 , ∆∆ are superposed,will result a spatial mechanism (see figure 3).

Figure 3. Spatial mechanism with linear joints

The guide bars of the equidistant joints iB are somehelices, disposed (with the same angle α ) on theexternal surface of a cylinder with a radius R . Thelinear joints A and C are transformed in a spatial(rotary – linear) joint CA− , disposed on the centralaxis of the mechanism.

Figure 4. Spatial mechanism with rotary joints

(a)

(b)

Figure 5. Final mechanism of the robot, withoutuniversal joint: (a) in straight pipe; (b) incurved pipe

In order to reduce the friction of the joints iB , threewheels will replace these linear joints (as shown infigure 4).

Also, the linear movement of the joint CA− can bereplaced by using three equidistant wheels, whichmake contact with the internal surface of the pipe (seefigure 5). In fact, this is the mechanism of the HELI–PIPE robot.

(a)

(b)

Figure 6. Final mechanism, with universal joint: (a) straight, (b) curved.

Many pipes or duct systems have junctions, corners,steps and big changes in their cross section. The robot,which was built on the basis of mechanism shown infigure 4, is not able to move in complex pipe shapesbut it can move in horizontal, vertical as well as curvedpipe geometries with a relatively wide radius ofcurvature (see figure 5.b). Because all the wheels are

mounted on springs, the mechanism can adapt tochanging diameters allow the motion into curve pipesand compensate for irregularities on the inner surfaceof the tube.

The wheels of the rotating body make an angle α toachieve a helical trajectory when the link 1 rotates (seefigures 3-4). In fact, the three helical trajectories ofthese wheels look like a screw with three beginnings.In this case, the movement of the mechanism intocurved pipes geometries is possible only if exist a smallaxial slippage of the driving wheels.

In order to decrease the radius of curvature of thepipe geometries and the slippage of the wheels, anuniversal joint can connect the two bodies of therobot (figure 6). In this case, in order to avoid theturning over of the bodies, it is necessary to usedouble wheels (figure 6.b).

3 Overall Architecture,Kinematics

HELI-PIPE (figure 7) is a 160 mm long wheeledrobot, with a diameter of 180 mm, for in-pipeinspection applications. Its kinematics is based on themechanism shown in figure 5. The robot is actuatedby a single DC motor located on the central axis and itconsists of two bodies, one mounted on the shaft ofthe motor, one fixed to its stator.

The rotating body has three double wheels whichmake an angle of 10° to achieve an helical trajectorywhen the motor rotates; the body fixed to the statoralso possesses three double wheels oriented parallel tothe tube axis, to allow only axial displacements. All thewheels are mounted on springs in order to adapt tochanging diameters allow the motion into curved pipesand compensate for irregularities on the inner surfaceof the tube. The robot can move in horizontal, verticalas well as curved pipe geometries with a relatively wideradius of curvature.

The present design is made for tubes of diameters inthe range of 160-180 mm, but it can be easily adaptedfor any size of pipes above 50 mm.For a complete rotation of the motor‘s shaft, we canwrite (see figures 1-4):

απ tanR2pd ⋅⋅⋅== (2)

where: d is the axial displacement of the robot; p isthe step of the helical trajectory of the drivingwheels; α is the angle of these wheels; R is theinternal radius of the tube.

(a)

(b)

Figure 7. HELI-PIPE robot: (a) final design; (b)general view

For a rotation with an angle ϕ of the rotating body,the axial displacement of the robot is:

t

tanRd

⋅=

⋅⋅=

ωϕ

αϕϕ (3)

where: n2 ⋅⋅= πω is the angular speed of therotating body; t is the time; n is the speed of themotor. In these conditions, the relation (3) becomes:

απϕ tantnR2d ⋅⋅⋅⋅⋅= (4)

The axial speed of the robot will be:nptannR2SC ⋅=⋅⋅⋅⋅= απ (5)

4 Conclusions

A new concept of mobile robot has been developed atULB, as a vehicle for in-pipe inspection. The vehiclehas a very simple kinematics thanks to a single DCmotor (located on the central axis), that actuates therobot. It consists of two bodies; one mounted on theshaft of the motor, one fixed to its stator.

Each body has three double wheels mounted onsprings in order to adapt to changing diameters allowthe motion into curve pipes and compensate forirregularities on the inner surface of the tube.

The wheels of the rotating body make an angle of 10°to achieve a helical trajectory whenthe motor rotates.

The robot can move in horizontal, vertical as well ascurved pipe geometries with a relatively wide radius ofcurvature.

References

[1] K. Taguchi, N. Kawarazaki, Development ofIn-Pipe Locomotion Robot, Proceedings of the1991 IEEE, pp. 297-302;

[2] Th. Robfman, F. Pfeiffer, Control an Designof a Pipe Crawling Robot, IFAC’96, Proceedingsof the 13th World Congres, June 30 – July 5,1996, San Francisco, USA, pp. 465-470;

[3] K. Suzumori, K. Hori, T. Miyagawa, ADirect-Drive Pneumatic Stepping Motor forRobots: Designs for Pipe-InspectionMicrorobots and for Human-Care Robots,Proceedings of the 1998 IEEE, May 1998,Leuven, Belgium, pp. 3047-3052;

[4] T. Miyagawa, K. Suzumori, M. Kimura, Y.Hasegawa, Development of Micro InspectionRobot for Small Piping, JRSJ, Vol. 17, No. 3, pp.79-85;

[5] W. Neubauer, A Spider-Like Robot thatClimbs Vertically in Ducts or Pipes, IROS’94;

[6] H. H. Mabie, Ch. F. Reinholtz, Mechanismesand Dynamics of Machinery , John Wiley & Sons,Inc., New York, 1987.

SPIDY - A MOTORLESS MICRO WALKING ROBOTIoan Doroftei, Jean-Marie Cloquet

Université Libre de BruxellesActive Structures Laboratory

50, Av. F.D.Roosevelt, B-1050, Brussels, Belgium .Email: iodoroft@ulb.ac.be

Abstract : Th i s pap e r d e s c r i b e s a l i g h tw e i g h t , s ix - l e g g e d m i c r o -wa lk in g r ob o t c u r r e n t l yb e i n g d e v e l o p e d a t ULB. Th e r o b o t h a s s ix l e g s w i t h tw o a c t i v e d e g r e e s o f f r e e d om p e r l e gand r e qu i r e s n o mo t o r s f o r i t s mo v emen t . Th e mov emen t i s a c h i e v e d b y h e a t i n g sma l l m emo r ya l l o y w i r e s (mu s c l e w i r e s ) o f 50 µm d i am e t e r , a c t i n g a s t e n d o n s f o r e a c h l e g o f t h e r o b o t .E l a s t i c r u bb e r w i r e s a r e u s e d t o r e t u r n t h e l i nk s o f t h e l e g t o t h e i n i t i a l p o s i t i o n , wh en t h emu s c l e w i r e s a r e n o mo r e p ow e r e d . A l l t h e l e g s a r e moun t e d d i r e c t l y o n t h e e l e c t r o n i c b o a r dw i t h ou t an y o t h e r f r ame s . Th e c o n t r o l b o a r d o f t h e m i c r o wa lk i n g v e h i c l e i s v e r y s imp l e a ndi t i s b a s e d o n an 8 - b i t m i c r o - c o n t r o l l e r (P IC16F84 ) . Thanks t o t h i s , t h e r o b o t c an wa lkf o rwa r d/ba ckwa r d and t u r n l e f t / r i g h t .

Keywords: walking robot, muscle wires, SMA.

1 Introduction

Superior terrain adaptability has made legged robotscandidate for exploration and inspection semi-autonomous vehicle [1], [2]. The legged vehicles offerattractive capabilities in terms of agility and obstacleavoidance. Also, the use of legs is convenient forlocomotion on soft ground where the performance ofwheels and tracks are considerably reduced,particularly in low gravity; indeed the net thrustcapability of a leg is increased by the groundcompaction while that of a wheel is reduced.

As we can see from the technical literature, walkingrobots with one, two, three, four, six or eight legs havebeen built. The number of legs affects somecharacteristics of the walking robots, such as: thestability, the efficiency, the possibility of walking withfewer legs when some of these are out of order(redundancy), the quality of the robot control, theprice, the weight, the gait, etc. (3). A wide variety ofprototypes have been constructed with various sizesand architectures [4], [5].

Shape Memory Alloys can exhibit large changes inshape when heated and cooled (capable of liftingthousands of times their own weight) and can replacemotors and solenoids for creating motion in manydevices, even robots [6], [7], [8]. They can be heateddirectly with electricity and cab be used to create awide range of motions, operating quickly and withprecise controllability.

In this context and on the basis of its experience inwalking machines, the Active Structures Laboratory ofUniversité Libre de Bruxelles has developed a

concept of lightweight six-legged micro walkingvehicle, called SPIDY.

In this paper, some structural and kinematics aspectsas well as types of gaits implemented on SPIDY robotare presented.

2 Overall Architecture,Kinematics

Six legs offer a good compromise between weight andelectromechanical complexity, on one hand, andstability, velocity and the variety of gaits, on the other

Fig. 1. Structural scheme of GENGHIS robot

hand. The kinematic architecture of the presentvehicle was used originally on a micro walkingmachine called GENGHIS (figure 1), developed atMIT [8]; a similar architecture was also used for one ofour previous prototypes [9].

SPIDY is a 55 g, 10.5 cm long, 14 cm wide and 6 cmhigh walking robot. It has six legs with two activedegrees of freedom per leg (see figure 2) in order tominimize its complexity and requires no motors forits movement.

(a)

(b)

Fig. 2. Kinematics of a leg: (a) structure; (b) design

The movement of the robot (figure 3) is achieved byheating small shape memory alloy wires (muscle wires)of 50 µm diameter, which are attached to each leg ofthe robot. Elastic rubber wires are used to return thelinks of the leg to the initial position, when the musclewires are no more powered.

(a)

(b)

Fig. 3. SPIDY robot: (a) design; (b) general view

Although the trajectory can never be a straight linebecause of the simple kinematics, the slippage doesnot cause any particular mechanical problem becauseof the small weight of the vehicle. Thanks to the smalldiameter of the SMA wires, a cycle time of about 1 seccan be achieved.

Each leg has two muscle wires, one for each d.o.f., andtwo elastic rubber wires. As we can see in figure 2.a,one muscle wire rotates the joint α in one direction(indicated by the letter a) and one rubber wire in theopposite direction, indicated by the letter a’ (when theSMA is not powered any more). The same thing willalso happen for the joint β. All the legs are mounteddirectly on the electronic board without any otherframes (figure 3), in order to simplify the architectureof the robot.

The rotating angles of the links depend of the legstructure but also of the value of the wiresdeformations. So, for a given leg structure, the valueof the rotating angle depends directly of the length of

the muscle wire.In order to increase the lengths of thewires (to increase the strokes of the legs) and to keepsmall overall dimensions for the robot, some pulleywheels are integrated in the structure of the leg (seefigure 2).

3 Control

The control board of the micro walking vehicle (seefigure 4) is very simple and it is based on an 8-bitmicro-controller (PIC16F84). This kind of micro-controller is very used for small applications becauseof a very good performance/cost ratio. More, it hasFlash program memory (this is very useful forapplications which need to change the program manytimes),

Fig. 4. Control architecture

reduces the number of external components andoperates over the standard voltage range.

For a SMA with 50 µm diameter and a linearresistance of 510 Ω/m, the recommended current is50 mA. As a function of the type of gait, which willgive us the total length of the muscle wire powered atone moment, we can compute the necessary voltageusing the basic equation of electricity (Ohm’s Law).

The control board will be connected to a PC througha serial radio link (operating range ≈ 10m); the PC actsas Man-Machine Interface and controls the walking.At this time, it is connected directly (by an umbilicalcable) to a power supply source.

Thanks to this control board, the robot can walkforward/backward and turn left/right. Because the bit

number of the PIC is limited to 8, only the tripod gaitwas implemented.

4 Conclusions

A micro-walking robot driven by SMA has beendeveloped. The movement of the robot is achieved byheating SMA of 50 µm diameter, which are attachedto each leg of the robot.

Elastic rubber wires are used to return the links of theleg to the initial position, when the muscle wires areno more powered. All the legs are mounted directly onthe electronic board without any other frames.

The control board of the micro walking vehicle isbased on an 8-bit micro-controller (PIC16F84).

References

[1] MARSNET, Report on the phase A study,ESA Publication SCI(93)2, April 1993;

[2] ROSETTA report, ESA PublicationSCI(93)7, September 1993;

[3] D. J. Todd, Walking Machines: an Introduction toLegged Robots, Kogan page, London, 1985;

[4] Special Issue on Legged Locomotion,International Journal Robotics Research 9(2), 1990;

[5] Robotics in Nuclear Facilities, Special Issue,SMIRT - 11, Tokyo, 1991;

[6] R. G. Gilbertson, Muscle Wires – Project Book,3rd edition, Mondo-tronics, Inc. San Anselmo,USA 1996;

[7] J. M. Conrad, J. W. Mills, Stiquito - AdvancedExperiments with a Simple and Inexpensive Robot,USA, 1997;

[8] J. M. Conrad, J. W. Mills, Stiquito for Beginners -An Introduction to Robotics, USA, 1999;[9] R. A. Brooks, A Robot that Walks; EmergentBehaviors from a Carefully Evolved Network,IEEE International Conference on Roboticsand Automation, 1989.