Abstract:
A sensor module ( 100 ) for a pipeline vehicle ( 110 ) is disclosed. The sensor module ( 100 ) includes an outwardly biased sensor arm ( 120 ) pivotally connected at a hinge ( 129 ) mounted on the vehicle ( 110 ), whereby the angle between the sensor arm ( 120 ) and pipeline vehicle ( 110 ) is representative of a pipeline dimension. A magnet ( 240 ) and magnetic flux sensor ( 252 ) are mounted in the sensor module ( 100 ) to move relative to one another as the sensor arm ( 120 ) pivots relative to the vehicle ( 110 ). Measurement of change in magnetic flux can permit determination of the angle between the sensor arm and the vehicle. The sensor module ( 100 ) may be mounted on an upstanding flange ( 111 ) via a compliant (deformable) layer ( 202 ) which permits lateral deflection of the module ( 100 ) relative to the vehicle ( 110 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to pipeline vehicles, e.g. vehicles adapted to travel within a pipeline for cleaning or inspection purposes. For example, the invention may relate to inspection sensor modules for pipeline inspection vehicles (known as pipeline pigs) which can determine the internal geometry of a pipeline. 
         [0003]    2. Summary of the Prior Art 
         [0004]    It is known to inspect the inside of a pipeline using a pipeline pig which may comprise one or more interconnected vehicles which pass down the pipe. 
         [0005]    Pipeline inspection vehicles typically comprise a main central body to which sensors or other components are mounted. The vehicles may be equipped with cleaning tools for removing debris and contamination from the wall of the pipeline, and sensors for determining the pipeline integrity. 
         [0006]    The pig may be towed along the pipeline, or be fitted with pressure plates which enable propulsion by a difference in pressure across the pressure plate. Knowledge of pipeline defects is critical in preventing future pipeline failure. Defects of particular importance include cracks, regions of metal loss (due to corrosion, for example), and distortions such as dents. Metal loss and cracking are typically identified using sensors such as magnetic flux sensors and/or ultrasound sensors. These sensors are usually mounted on the outer end of sensor arms that are themselves hingedly connected to the pipeline inspection vehicle. There will be a plurality of such sensor arms, usually arranged circumferentially around the pig. Individual sensor arms can be resiliently biased against the pipeline wall using a variety of spring mechanisms so as to provide compliance over portions of the inner wall of varying diameter. 
         [0007]    By monitoring the orientation of the sensor arms relative to the pipeline inspection vehicle, the internal geometry of the pipeline may also be determined. The orientation of the sensor arms may be measured by rotary potentiometers or shaft encoders, which are fitted to the axle at the base of the sensor arm, about which the sensor arm is pivoted. 
         [0008]    However, rotary potentiometers and shaft encoders are bulky components and can prevent sensor arms being placed close together. Thus, when using these components, there is a limit to the number of sensor arms that can be provided on an outer surface of the pipeline inspection vehicle. Hence the resolution with which the interior geometry of the pipeline can be determined is also limited. Furthermore, rotary potentiometers and shaft encoders may be unsuitable for use in high pressure and dirty environments where they could be susceptible to damage due to the effects of pressure, or ingress of product or debris. 
         [0009]    WO 2006/003392 describes an inspection sensor module for a pipeline inspection vehicle in which a sensor arm is biased outwardly from the inspection vehicle by a leaf spring and movement of the sensor arm relative to the body of the inspection vehicle is measured by strain gauges attached to the leaf spring. 
       SUMMARY OF THE INVENTION 
       [0010]    At its most general, the present invention proposes monitoring the movement of a sensor arm by detecting relative movement between a magnetic flux sensor and a magnetic field. Relative movement between the magnetic flux sensor (e.g. a Hall effect sensor) and a magnet can cause the detector to detect a change in magnetic field. The detected change can permit the extent of corresponding movement of the sensor arm to be determined. 
         [0011]    In one arrangement, the magnetic flux sensor may be mounted on a sensor arm that is mounted on and movable relative to a pipeline vehicle e.g. to detect changes in pipe structure. The fixed magnetic field may be achieved by fixedly attaching one or more magnets to the pipeline vehicle. 
         [0012]    The inventors have discovered that by fixing the magnet to the pipeline vehicle (i.e. so that the position of the magnet is unaffected by the position of the sensor arm), any interaction between magnets of adjacent sensor modules remains fixed (i.e. does not change) and can readily be accounted for when evaluating the data recorded by a magnetic flux sensor positioned on the sensor arm. Thus, by moving the sensor relative to the vehicle rather than moving a magnetic field relative to the vehicle an even more accurate measurement system can be achieved. 
         [0013]    Thus, the present invention may provide a sensor module for a pipeline vehicle, the module having a support structure for mounting the module on the vehicle; a sensor arm that is movable relative to the support structure; a magnet mounted on one of the support structure and sensor arm; and a magnetic flux sensor mounted on the other one of the support structure and sensor arm, wherein the magnet and magnetic flux sensor are movable relative to one another to detect the position of the sensor arm relative to the support structure. Such a sensor module may be more robust than those which use e.g. strain gauges to detect relative movement e.g. because there are fewer moving or otherwise delicate parts in the detection structure. 
         [0014]    The magnet may be mounted on the support structure, e.g. so that it is fixed relative to the vehicle, and the magnetic flux sensor may be mounted on the sensor arm to move therewith relative to the support structure (and fixed magnet). 
         [0015]    Preferably, the sensor arm is connected at one end to the support structure. The sensor arm may be pivotally connected to the support structure. The sensor arm may be hingedly connected to the support structure. In one embodiment with a hinged connection between the sensor arm and the support structure, the support structure may house the axle for the sensor arm. 
         [0016]    Thus, the magnetic flux sensor mounted on the sensor arm may detect the orientation, i.e. angular position, of the sensor arm relative to the support structure. 
         [0017]    The sensor module may include biasing means for biasing the sensor arm towards a deployed position relative to the support structure. The sensor arm may be resiliently biased towards the deployed position, for example, by leaf springs, torsion springs, a resilient bushing or the like. Thus, when the sensor module is mounted on the pipeline vehicle, the end of the sensor arm remote from the pipeline vehicle may abut the inner wall of a pipe. If a deformation in the pipe wall is encountered, the end of the sensor arm remote from the pipeline vehicle will move radially to conform to the inner wall of the pipe. This movement will cause relative movement between the magnetic flux sensor (e.g. mounted on the sensor arm) and the magnet mounted (e.g. mounted on the support structure), whereby the magnetic flux sensor registers a change in magnetic field. The change in magnetic field may permit the position of the sensor arm, and hence the geometry of the pipeline, to be determined. By positioning the magnet on the support structure for the sensor arm, the position of the magnet remains fixed relative to the body of the pipeline vehicle when the sensor module is mounted on the vehicle. Thus, the position of the magnet may be fixed relative to other magnets provided by other sensor modules, and the level of interaction between magnets on different sensor modules is unchanging (and can be calculated or measured). The effect of this interaction on the reading obtained from the magnetic flux sensor mounted on the sensor arm can therefore be compensated or corrected. 
         [0018]    The magnetic flux sensor may be arranged on the sensor arm so that it is embedded within the arm. Similarly, the magnet may be mounted on the support structure so that it is at least partly embedded in this structure. Hence, a compact sensor module may be provided. Such compact sensors modules may be mounted on the surface of the pipeline vehicle in a closely-spaced arrangement, thus providing a high density of sensor arms on the pipeline vehicle. This arrangement allows the internal geometry of the pipeline to be determined with a high degree of resolution. 
         [0019]    It is preferable that the motion of the sensor arm relative to the support structure is such that the magnetic flux sensor traverses a region of significant change in magnetic flux density around the magnet. The magnet may be configured to present such a region at the interface between the magnet and the magnetic flux sensor. Thus, the position of the sensor arm relative to the support structure may be measured with a high degree of precision. The support structure is preferably of ferrous material to act as a magnetic yoke to facilitate configuration of the magnetic field in use. The sensor arm may also comprise an inspection sensor (e.g. a sensor block) for detecting e.g. metal loss or cracking in the pipeline wall. This inspection sensor may be located at the end of the sensor arm remote from the support structure. The inspection sensor may itself be a magnetic flux sensor or it may be an ultrasonic transducer, an electro-magnetic acoustic transducer, or a pulsed eddy-current sensor. 
         [0020]    By providing a sensor module that is adapted to determine pipeline geometry as well as detecting defects such as thinning or cracking of the pipeline wall, the spatial relationship between these defects and features of the pipeline geometry may be established. 
         [0021]    The magnetic flux sensor is preferably encased within a protective covering. This allows the sensor module to be used in high pressure environments. The magnetic flux sensor may be a Hall-effect sensor. 
         [0022]    The magnet may be a rare earth magnet, such as samarium-cobalt or neodymium-iron-boron. 
         [0023]    The above discussion has illustrated the present invention in terms of a sensor module. A second aspect of the invention may provide a pipeline vehicle having at least one, preferably a plurality, of such sensor modules mounted on its surface. 
         [0024]    The pipeline vehicle according to the second aspect of the invention may have a plurality of sensor modules provided circumferentially around a body of the vehicle, so that the movement of each sensor arm towards or away from the body of the vehicle is a radial movement in the pipe. 
         [0025]    In one embodiment, the support structure of the sensor module may be mounted on an upstanding (e.g. radially extending) flange on the outer surface of the body of the pipeline vehicle. The flange may be integral with the body or part of a separate collar mounted thereon. The support structure may include a layer of deformable, e.g. compliant, material between the support structure and flange to permit sideways deflection of the sensor module relative to the vehicle in an axial direction. This can enable the sensor module to react more robustly to sideways forces that can be exerted when the vehicle travels through curves in the pipe. The layer of deformable (preferably resilient) material may give the module enough Λplay′ with respect to the body to enable a pivotal connection between a sensor arm and support structure to be rigid e.g. to reduce or eliminate variations in a travel path of the sensor arm relative to the support structure. The deformable layer may be an independent aspect of the invention. According to that aspect there may be provided a pipeline vehicle having a sensor module mounted thereon, the sensor module including a sensor arm pivotally connected to a support structure, the support structure being mounted on vehicle to permit relative movement between the sensor arm and vehicle, wherein a deformable layer is mounted between the support structure and the vehicle to permit lateral deflection of the sensor module relative to the vehicle. The sensor arm may be constrained to pivot in a flat plane relative to the support structure, and the permitted deflection may enable relative movement between that plane and the vehicle. 
         [0026]    A further aspect of the present invention may provide a method of monitoring the characteristics of a pipe using a sensor module according to the first or second aspect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    An embodiment of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: 
           [0028]      FIG. 1  is an oblique view of a sensor module that is an embodiment of the present invention, mounted on a pipeline vehicle; 
           [0029]      FIG. 2  is an oblique view of the sensor module of  FIG. 1  which is cut away to show its internal structure; 
           [0030]      FIG. 3  is a side view of the cut away sensor module shown in  FIG. 2 ; 
           [0031]      FIG. 4  is a schematic representation of the magnet and P-shaped support bracket of the sensor module; 
           [0032]      FIG. 5  is an oblique view of a sensor module that is another embodiment of the present invention, mounted on a pipeline vehicle; 
           [0033]      FIG. 6  is an illustration of a distribution of magnetic flux lines for a section through the P-shaped support block shown in  FIG. 4 ; and 
           [0034]      FIG. 7  is a graph showing typical variation of a tangential component of magnetic field with sensor angle for a sensor module that is an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]      FIGS. 1 ,  2  and  3  illustrate a sensor module  100  according to an embodiment of the invention. The sensor module  100  is mounted on an upstanding flange  111  on an outer surface of a pipeline vehicle  110 . Although not shown in  FIGS. 1-3 , a plurality of such inspection sensor modules  100  may be provided circumferentially around the pipeline vehicle  110 , with each sensor module  100  extending laterally from the pipeline vehicle  110 . When the pipeline vehicle  110  is being used for inspecting a pipeline, the inspection sensor modules  100  extend radially from the pipeline vehicle  110 , and each inspection sensor module  100  abuts a portion of the inner wall of the pipeline. The sensor module  100  comprises a sensor arm  120  having a proximal end  122  that is connected to the pipeline vehicle  110  by a first hinge  129 . The distal end  130  of the sensor arm  120  is connected to a sensor sledge  152  by a second hinge  150 . The sensor sledge  152  has an inspection surface  154  for contacting (sliding relative to) the inner wall of a pipeline (not shown) during inspection. A sensor block  156  is mounted on the sensor sledge  152 . The hinges  129  and  150  are oriented so as to permit lateral (radial) deployment of the sensor sledge  152  relative to the pipeline vehicle  110 . 
         [0036]      FIGS. 1 to 3  show the inspection sensor module  100  in the deployed condition such that the sensor arm  120  extends laterally, e.g. radially, from the pipeline vehicle  110  and the inspection surface  154  of the sensor sledge  152  is pressed against the inner wall of the pipeline (not shown). The inspection sensor module  100  is held in the deployed position by first and second leaf springs  170 ,  172  that abut, e.g. are mounted on, a platform  114  e.g. an outward facing surface on the pipeline vehicle  110 . The second hinge  150  is resiliently biased by a third spring  174  towards a position in which the sensor sledge  152  extends along the axis of the sensor arm  120 . 
         [0037]    The first leaf spring  170  contacts a back face  132  of the sensor arm  120  so as to cause the sensor arm  120  to assume a deployed position. The front face  131  of the sensor arm  120  has a raised portion which forms a cavity  133  when the back face  132  is mounted thereon. The purpose of the cavity  133  is discussed below. 
         [0038]    The second leaf spring  172  passes behind a sensor block  156  mounted on the underside of the sledge  152  and through a spring aperture  158  formed in a downward flange  157  at the back end of the sledge  152 . The spring aperture  158  allows the leaf spring to slide freely therein so as to allow the sledge  152  to move between deployed and retracted positions without experiencing excessive torque. The action of the first leaf spring  170  causes the sensor arm  120  to be biased to a radially deployed position such that the hinge  150  lies remote to the pipeline vehicle  110  and adjacent to the inner wall of the pipeline (not shown). The action of the second leaf spring  172  and the third spring  174  is such as to press the trailing edge  160  of the sledge  152  against the inner wall of the pipeline (not shown). Thus, the action of the three springs  170 ,  172  and  174  is to maintain the sledge  152  aligned with the inner wall of the pipeline (not shown). According to this embodiment the tip of the second end  130  of the sensor arm  120  is flared in a direction radially outward so as to form a lip  135 . The purpose of the lip  135  is to prevent snag of the leading edge  151  of said sledge  152  against imperfections (e.g. projection, cracks or the like) in the surface of the pipeline inner wall. Such snag might result in damage to the inspection sensor module  100 . The structure of the first hinge  129  will now be described in more detail. The first hinge  129  comprises a P-shaped support bracket  200 , a Clevis block  210 , and a pin  230 . 
         [0039]    The P-shaped support bracket  200  acts as a support structure for mounting the inspection sensor module  100  on the pipeline vehicle  110 . The P-shaped support bracket  200  has a mounting portion  202  that is affixed to the upstanding flange  111  of the pipeline vehicle  110  by screwed fixings. The mounting portion  202  comprises a compliant layer bonded to a mounting face of the P-shaped support bracket  200 . The compliant layer may be made from polyurethane and in this particular embodiment is 2 mm thick. The compliant layer permits lateral deflection of the sensor module  100  relative to the pipeline vehicle  110 . This may permit a rigid hinge  129  to be used, which aids the accuracy of the magnetic flux sensor arrangement discussed below. The thickness of the layer, the degree of Shore hardness of the material and surface area are variable, and are determined by testing and calculation, so that the required degree of deflection is achieved for a given side load at the uppermost point of the sensor sledge  152 . The required degree of deflection will depend on a number of factors, including, but not limited to, bend radius of pipe, diameter of pipe, wall thickness of pipe, valve bore, local restrictions, sensor location and vehicle geometry. 
         [0040]    The P-shaped support bracket  200  has a head  204  that is distal from the mounting portion  202 . 
         [0041]    The Clevis block  210  has a body  212  that is affixed to the sensor arm  120  by screwed fixings  214 . Two legs extending from the body  212  to straddle the head  204  of the P-shaped support block  200  and are secured to it by the pin  230  passing through each leg and the head  204 . 
         [0042]    Thus the body  212  of the Clevis block  210  is able to move along an arc centred on the axis defined by the pin  230 . The head  204  of the P-shaped support bracket  200  is partly bounded by a curved surface  206  that is also centred on the axis defined by the pin  230 . Hence the body  212  of the Clevis block  210  is able to move along an arc that is concentric with the curved surface  206  of the head  204  of the P-shaped support bracket  200 . 
         [0043]    A magnet  240  is partly embedded in the curved surface  206  of the head  204  of the P-shaped support bracket  200  and is protected by a cover  242 . 
         [0044]    A housing  250  is encompassed by the body  212  of the Clevis block  210  and encloses a magnetic flux sensor (Hall-effect sensor)  252 . The Hall-effect sensor  252  is positioned in the region of the housing proximal to the head  204  of the P-shaped support bracket. 
         [0045]    When the pipeline vehicle  110  travels along a pipeline, the sensor arm  120  rotates in response to the varying geometry of the pipeline. This rotation causes the Hall-effect sensor  252  that is affixed to the base of the sensor arm  120  to rotate about the pin  230  and thus move relative to the magnet  240 . The Hall voltage generated by the Hall-effect sensor  252  varies in response to the changing magnetic field experienced by the sensor  252 . Thus, the position of the sensor arm  120  and the geometry of the inner wall of the pipeline may be determined. 
         [0046]    Effectively, the Hall effect sensor  252  traces an arc around the magnet  240  as the sensor arm  120  rotates. The change in field shape and amplitude of the magnetic field detected by the Hall effect sensor  252  may then be translated into an angular measurement from which the geometry of the pipeline may be deduced. 
         [0047]    The Hall effect sensor  252  of this embodiment measures the change in the tangential component of the magnetic field with angle. By positioning the magnet  240  on a component of the hinge that is fixed relative to the pipeline vehicle  110  (i.e. the P-shaped support bracket  200 ), the level of interaction between this magnet and the magnets of adjacent inspection sensor modules  100  on the surface of the pipeline vehicle  110  does not change even when adjacent sensor arms move in different ways. Thus only a simple correction to the signal obtained from the Hall-effect sensor  252  is required in order to account for this interaction. In this embodiment of the invention, housing for the Hall effect sensor  252  abuts the back face  132  of the sensor arm  120 . The output harness (not shown) of the Hall effect sensor is connected to the sensor block  156 . Where wired connections (also not shown) are used, they pass through the cavity  133  in the sensor arm  120  so as not to interfere with the springs or hinges. The Hall effect sensor  252  does not protrude from the sensor arm  120  along the axial direction of the hinge  129 . 
         [0048]    The arrangement of each Hall effect sensor  252  and the magnet  240  allows a compact sensor inspection module  100  to be provided. As a result, a large number of these modules may be mounted on a pipeline vehicle  110 . Hence, the pipeline geometry may be determined with a high degree of spatial resolution. At the same time the interaction between the magnets of the different sensor inspection modules  100  remains readily quantifiable. 
         [0049]    Because the magnet  240  and the Hall effect sensor  252  are protected by a protective cover  242  and a housing  250  respectively, the sensor inspection module  100  may be used in high pressure or corrosive environments. When covered the magnet is better protected from loose debris in the pipe. Without the cover, such debris can become trapped in hinges may affect the magnetic field produced by the magnet  240 . Neither the protective cover  242  nor the housing  250  interferes with the magnetic field from the magnet  240  and hence the performance of the sensor inspection module  100  is not compromised. The magnet  240  may be a dipole magnet that is magnetized in the through thickness direction, so that one pole is located at the exposed surface of the magnet  240 . Thus, the magnetic field exits the magnet in a radial direction relative to the exposed surface of the magnet  240  and the curved surface  206  of the P-shaped support bracket  200 . 
         [0050]    The P-shaped support bracket  200  is preferably of a ferrous material so that it can act as a magnetic yoke, thus providing a return path for the magnetic flux. Most preferably, the P-shaped support bracket  200  is made of mild steel. 
         [0051]    Thus, the magnetic flux lines exit the surface of the magnet  240  in a radial direction relative to the curved surface  206  of the head  204  of the P-shaped support bracket  200 , and then curve outwards so that they are directed to the P-shaped support bracket  200 . 
         [0052]      FIG. 6  is a schematic diagram showing typical magnetic flux lines from a magnet  240  mounted on a P-shaped support block  200  as described above. The magnetic field generated by this arrangement varies in both the radial and tangential direction relative to the curved surface  206  of the head  204  of the P-shaped support bracket  200 . Thus, the Hall effect sensor can be used to measure either the radial or tangential component of the magnetic field. In  FIG. 6  an arc  260  is drawn which represents the movement of the Hall effect sensor with changing position of the sledge in an embodiment where the tangential component is measured. 
         [0053]    Preferably, the Hall effect sensor  252  and the magnet  240  are arranged so that there is a proportional relationship between the Hall effect sensor reading and orientation angle over at least a 40° range. Preferably, this proportional relationship is achieved over at least a 50° range, most preferably a 60° range. Such an arrangement results in a nearly linear relationship between the orientation of the inspection sensor module  100  and the Hall effect sensor reading over the range of angles generally of interest. 
         [0054]      FIG. 7  is a graph showing an approximately linear relationship between tangential magnetic field and angular position of sensor relative to magnet over a range of more than 40° (e.g. between 10° and 50°). The magnet  240  may be partly embedded in the curved surface  206  of the head  204  of the P-shaped support bracket  200 . The magnet  240  may be a brick or cuboid-shaped magnet. In the case that the magnet is a brick-shaped magnet, it is preferable that the surface  244  of the magnet  240  distal from the head  204  of the P-shaped support bracket  200  be shaped so that it follows an arc concentric with the arc of the curved surface  206  of the head  204 . However, the surface  244  of the magnet  240  need not be curved. It may instead comprise a number of facets  240   a ,  240   b ,  240   c  positioned so as to approximate the shape of an arc, as shown in  FIG. 4 . Such facets may be formed through machining or forming. The use of a magnet with a facetted surface has the advantage that such magnets are easier and cheaper to shape than those with a curved surface. 
         [0055]    Preferably, the brick-shaped magnet is positioned on the P-shaped support bracket  200  with its longitudinal axis at an angle of between 25° and 60° to the longitudinal axis of the pipeline vehicle  110 . Most preferably, the magnet lies at a steep angle to the longitudinal axis of the pipeline vehicle, that is at around 60°. The inventors have found that at this orientation, the response of the Hall effect sensor  252  to the magnetic field around the magnet  240  is maximised. 
         [0056]      FIG. 4  shows a chamfered magnet  240  mounted on a P-shaped support bracket  200 . The magnet is magnetized in the through-thickness direction i.e. the magnetic field exits the magnet in a radial direction relative to the exposed surface of the magnet  240  and the curved surface  206  of the P-shaped support bracket  200 . 
         [0057]      FIG. 4  shows a single magnet  240  mounted on the P-shaped support bracket  200 . However, a plurality of magnets may be used, which may be mounted in a series extending around the curved surface  206  of the P-shaped support bracket  200 . 
         [0058]    The magnet  240  may be a rare earth permanent magnet For example, it may be a samarium-cobalt magnet. This class of magnets has a high saturation magnetization and a low temperature coefficient (i.e. change of magnetization with temperature) of −0.045%/° C. The low temperature coefficient reduces the errors introduced by temperature variations within the pipeline. 
         [0059]    In one embodiment the magnet  240  is S1TI2C017. Sintered or resin/plastic bonded SmCos or NdFeB magnets may also be suitable. 
         [0060]    The inventors have found that using the inspection sensor module  100  of the present embodiment together with a chamfered magnet, it is possible to obtain a Hall effect sensor reading that varies linearly with angle over at least a 60° range. As a result, there exists a nearly linear relationship between the orientation of the inspection sensor module  100  and the Hall effect sensor reading over the range of angles generally of interest. In the present embodiment, the second (i.e. distal) end  130  of the sensor arm  120  is connected to a sledge  152  by a second hinge  150 , and a sensor block  156  is mounted on the sledge  152 . The sensor block  156  may be a conventional metal loss sensor that detects metal loss through magnetic flux measurements or another sensor used for the detection of cracks or metal loss defects. The sensor block  156  and the sensor sledge  152  on which it is mounted are both optional components. Instead, the second end  130  of the sensor arm  120  may simply have a tip that contacts and slides along the inner wall of the pipeline. Preferably, this tip is made from a wear-resistant material such as tungsten carbide. 
         [0061]      FIG. 5  shows an alternative embodiment, in which the distal end  130  of the sensor arm  120  is modified to comprise a wheel  280  that contacts and runs along the inner wall of the pipeline.