Abstract:
The invention relates to a load transporting mono-robot ( 10 ), comprising (i) a gantry ( 19 ) having two lateral uprights ( 11 ) that are connected at their upper ends by a cross beam ( 12 ), each of the lower ends being equipped with propulsion means linked to the upright ( 11 ) by a motorized pivot ( 18 ), and (ii) means for gripping a load that are positioned between the lateral uprights ( 11 ) linked to the cross beam ( 12 ) by a kinematic chain for positioning and orientation that is configured to allow the means for gripping a load to rotate about an axis substantially normal to the cross beam ( 12 ) and is located substantially in the plane defined by the gantry ( 19 ), and to allow the means for gripping a load to rotate about an axis substantially normal to the plane defined by the gantry ( 19 ). The invention also relates to a method for transporting a load that uses a plurality of mono-robots ( 10 ) and also to two methods for crossing obstacles, ensuring the stability of a poly-robot and its load.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a National Phase Application of Patent Application PCT/FR2015/050483 filed on Feb. 27, 2015, which claims the benefit of and priority to French Patent Application No. 14/51661 filed Feb. 28, 2014, the contents each of which are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention concerns a mono-robot for transporting long loads and a method for transporting long loads using this mono-robot. 
       BACKGROUND 
       [0003]    The transport of a long load such as, for example, a pipeline segment, a wind turbine blade, a stretcher or a beam or construction reinforcements, may prove to be difficult due to the length of the load itself. 
         [0004]    Traditionally, the mechanized transport of a long load is performed by a vehicle having a chassis on which the load is positioned as is the case of vehicles presented in the patents EP 1465789 and EP 2328795. 
         [0005]    However, the positioning of the load on this type of vehicle requires the use of an external machine such as a lifting truck or a jib crane. 
         [0006]    Furthermore, the vehicles of the prior art are often standardized and may not be adapted to the load to be transported. Furthermore, due to the presence of a long chassis and the length of the load to be transported, the vehicles of the prior art may only move forward with difficulty over rough terrain. 
       BRIEF SUMMARY 
       [0007]    In this technical context, an object of the present invention is to provide a transport solution of long loads easy to be loaded, adaptable to the type of load to be transported and may cross obstacles. 
         [0008]    In the present document, mono-robot, unitary robot, and poly-robot are defined as a combination of several mono-robots working together. 
         [0009]    According to a general definition, the invention relates to a load transporting mono-robot which comprises a gantry crane with lateral uprights connected at their upper ends by a transverse beam, each of the lower ends being equipped with propulsion means connected to the upright by a driven pivot. The mono-robot further comprises, gripping means of a load positioned between the lateral uprights, and connected to the transverse beam by a positioning and orientation kinematic chain. The positioning and orientation kinematic chain is configured to allow the rotation of the gripping means of a load about an axis substantially normal to the transverse beam and substantially belonging to the plane defined by the gantry crane, and the rotation of the gripping means of a load about an axis substantially normal to the plane defined by the gantry crane. 
         [0010]    The invention then provides a mono-robot, allowing a ventral seizing of an object to be transported. It about an important point of the invention because the ventral transport of a load allows the assembly comprising of a mono-robot and a load to keep a high stability by presenting a center of gravity close to the ground. 
         [0011]    The mono-robot according to the invention is, moreover, easily configurable in order to perform the transport of any type of load. 
         [0012]    The invention can thus be adapted to a wide variety of geometries and masses of the loads to be transported as the mono-robot may be fastened at any point of the load. It is possible to combine several mono-robots on a same load for distributing the mechanical forces. 
         [0013]    Furthermore, each mono-robot can perform complex movements which allow it to cross obstacles when implemented with other mono-robot for transporting a load. The mono-robot has thus a great agility which distinguishes it from the long loads transporting vehicles of the prior art. 
         [0014]    Furthermore, the positioning and orientation kinematic chain connecting the gripping means to the transverse beam may be configured to allow the translation of the gripping means of a load along a direction substantially normal to the plane defined by the gantry crane. 
         [0015]    In this manner, the mono-robot may be displaced along a load in order to cross an obstacle or to optimize the position of the center of gravity of the load with respect to the bearings of the mono-robot. 
         [0016]    Preferably, the positioning and orientation kinematic chain may be configured to allow the translation of the gripping means of a load in the plane defined by the gantry crane along a direction normal to the transverse beam. 
         [0017]    Thus the mono-robot according to the invention presents a fast loading and easy implementation mode. Indeed, the positioning and orientation kinematic chain allows the gripping means to seize the load on the ground and to lift it for transport. The mono-robot can then seize a load placed on the ground by standing directly over the concerned load, without recourse to annex lifting equipment. 
         [0018]    Furthermore, the gripping means of a load are connected to the transverse beam by the positioning and orientation kinematic chain comprising kinematic connections of the cylindrical, rotoid, prismatic or universal group. Furthermore, the finger-spherical connection has the same degrees of freedom as a universal type connection and may be substituted therefor. It is specified that the positioning and orientation chain may have a serial or parallel architecture (open or closed) with one or more contours. 
         [0019]    Thus, the gripping means have all the degrees of freedom and all the movements required for seizing the load. Furthermore, the positioning and orientation kinematic chain allows displacements of the mono-robot relative to the transported load to better adjust the position of its center of gravity. The positioning and orientation kinematic chain also allows the mono-robot to displace one of the propulsion means in the three dimensions of the space by bearing on the other propulsion means. 
         [0020]    According to a preferred embodiment, the propulsion means belong to the group comprising: a wheel, a caterpillar and an omnidirectional wheel. 
         [0021]    According to one embodiment, each lower end of the gantry crane is equipped with a single wheel connected to the upright by a driven pivot. Other embodiments are possible by equipping each lower end of the gantry crane of an omnidirectional wheel or a caterpillar. 
         [0022]    Furthermore, a gripping means of a load comprises a clamp having one or more jaws configured to seize and retain a load, each jaw being equipped with a movable end roller in rotation relative to the jaws and allowing the translation of a load with respect to the jaw, and at least one latch adapted to immobilize in rotation one or more rollers relative to the jaw. 
         [0023]    This technical disposition allows displacing the mono-robot with respect to the load seized by the gripping means. Furthermore, the latches are used to allow accurately adjusting the position of a mono-robot along the seized load and locking said position. 
         [0024]    The present invention also concerns a method for transporting a load by a load transporting poly-robot which comprises the following steps:
       supply of number M of mono-robots with M greater than or equal to 2;   distribution of the mono-robots along a load;   gripping by the gripping means of each mono-robot of said load or an intermediate chassis connected to a load;   lifting of the load;   actuation of the propulsion means of each mono-robot.       
 
         [0030]    The invention thus allows the transport of a long load by several mono-robots whose displacements are coordinated. According to this aspect of the invention, the load fulfils the function of chassis which connects at least two mono-robots. The invention thus becomes a poly-robot without chassis as the chassis function is carried out by the load to be transported itself. This disposition of the invention is quite advantageous in that it allows the economy of a chassis which is costly and cumbersome. 
         [0031]    Furthermore, the invention provides a method for transporting a load by a load transporting poly-robot which comprises the following phases of crossing an obstacle:
       positioning the poly-robot against an obstacle;   for each mono-robot m (m=1 . . . M) of the poly-robot:
           reconfiguration phase of the assembly of the poly-robot to maximize its stability in anticipation of the raising of a propulsion means of the mono-robot m,   raising of a first propulsion means of the mono-robot m at an altitude greater than the altitude of the obstacle;   crossing phase of the obstacle by the first propulsion means of the mono-robot m,   landing phase on the obstacle of the first propulsion means of the mono-robot m,   reconfiguration phase of the assembly of the poly-robot to maximize its stability in anticipation of the raising of the second propulsion means of the mono-robot m,   raising of the second propulsion means of the mono-robot m at an altitude greater than the altitude of the obstacle;   crossing phase of the obstacle by the second propulsion means of the mono-robot m,   landing phase on the obstacle of the second propulsion means of the mono-robot m.   
               
 
         [0042]    Advantageously, the reconfiguration of the poly-robot before crossing the obstacle by a wheel allows the invention to remain stable for all the duration of crossing the obstacle. Thus, the invention allows crossing an obstacle by at least two mono-robots transporting a load. The combination of ventral gripping and mono-robots endowed with a complex connection kinematic chain allows the crossing of significant obstacles. 
         [0043]    Furthermore, the reconfiguration phase includes one or more following steps and intended for stabilization:
       translation of substantially longitudinal axis of a mono-robot m relative to the load so as to approach said mono-robot to the center of gravity of the load;   rotation of substantially vertical axis of a mono-robot m relative to the load so as to approach a propulsion means bearing on the ground of the mono-robot m to the position of the propulsion means which will be subsequently lifted by a mono-robot m+1.       
 
         [0046]    By being thus positioned, the mono-robots—whose number is at least two—allow the poly-robot to increase its stability during the lifting of a wheel. 
         [0047]    Advantageously, the crossing phase of an obstacle according to the invention allows a mono-robot to cross obstacles having a significant height by bearing both on its first wheel and the rest of the poly-robot in order to lift its second wheel. 
         [0048]    According to another embodiment, the invention provides a method for transporting a load by a load transporting poly-robot comprising two mono-robots. Said method comprises the following steps of front crossing of an obstacle:
       rotation of substantially longitudinal axis of a mono-robot allowing the positioning, at an altitude greater than the altitude of the obstacle, of the propulsion means which crosses the obstacle;   rotation of substantially vertical axis of the mono-robot, allowing the positioning of the propulsion means lifted above the obstacle;   rotation of substantially longitudinal axis of the mono-robot allowing the propulsion means to be placed on the obstacle.       
 
         [0052]    In another embodiment, the invention concerns a method for transporting a load by a load transporting poly-robot comprising at least three mono-robots presenting the phases of front crossing of an obstacle comprising the steps of:
       positioning the load transporting poly-robot against an obstacle;   for each of the successive mono-robots of the poly-robot, a front crossing phase in three steps:
           translation of substantially vertical axis of the mono-robot considered at an altitude greater than the altitude of the obstacle;   advance of the poly-robot and the load over the obstacle until bringing the next mono-robot against the obstacle;   translation of substantially vertical axis of the mono-robot considered to allow it to place its propulsion means on the obstacle.   
               
 
         [0058]    The front crossing phase of an obstacle according to the invention allows a poly-robot including at least three mono-robots to cross obstacles having a significant height by bearing on at least two mono-robots bearing on the ground. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0059]    Other features and advantages of the invention will become clear from the following description with reference to the appended drawings which show several embodiments of the invention. 
           [0060]      FIG. 1  is a schematic perspective view of a mono-robot according to the invention; 
           [0061]      FIG. 2  is a perspective view of a long load transporting poly-robot according to the invention in an implementation of the invention with two mono-robots; 
           [0062]      FIG. 3  is a schematic perspective view of another embodiment of a long load transporting poly-robot using a longitudinal translation means of the load by a specific gripper with rolling rollers, in an implementation with two mono-robots; 
           [0063]      FIG. 4  is a schematic perspective view of a poly-robot transporting a flexible load stiffened by an intermediate chassis; 
           [0064]      FIGS. 5 to 55  show in top, perspective and side views, a crossing mode of an obstacle by a poly-robot comprising two mono-robots. 
           [0065]      FIGS. 56 to 79  illustrate in top and side views, a crossing mode of an obstacle by a poly-robot comprising at least three mono-robots. 
       
    
    
     DETAILED DESCRIPTION 
       [0066]    In the present document, the following axes are conventionally defined:
       longitudinal axis, an axis substantially normal to the plane defined by the gantry crane;   vertical axis, an axis substantially comprised in the plane defined by the gantry crane and perpendicular to the transverse beam;   transverse axis, an axis substantially belonging to the plane defined by the gantry crane and parallel to the transverse beam.       
 
         [0070]    As shown in  FIG. 1 , the transport mono-robot  10  has a generally reverse U-shaped structure and comprises two lateral uprights  11  and a transverse beam  12 , forming a gantry crane  19 . 
         [0071]    The end of each lateral upright  11  comprises a propulsion means, for example a wheel  17  connected to the upright  11  by a driven pivot  18 . 
         [0072]    It might be also envisaged to replace the wheels by omnidirectional wheels, caterpillars or any other propulsion means. 
         [0073]    The mono-robot  10  is herein, schematically represented. The gantry crane may be composed of mechanically welded metal members or appropriately assembled composite members. 
         [0074]    Furthermore, the mono-robot  10  comprises gripping means positioned in the gantry crane  19  between the lateral uprights  11 , so as to enable seizing a load. 
         [0075]    According to the embodiment shown herein, the gripping means are connected to the beam  12  by a positioning and orientation kinematic chain comprising a prismatic connection (or slide) P of substantially longitudinal axis, a rotoid connection R 1  (or pivot) of substantially longitudinal axis and a cylindrical connection C (or sliding pivot) of a substantially vertical axis. 
         [0076]    These orientations are specified in the neutral position shown by  FIG. 1 . 
         [0077]    It is specified that some or all of the connections P, R 1  and C may be driven. 
         [0078]    It must be specified that the connections P, R 1  or C are described by way of example and that other chains with serial or parallel structures may be considered. 
         [0079]    Furthermore, the stabilization of the mono-robot  10  during its displacement may be ensured by sensors controlling the acceleration, the rotations and the translations of the mono-robot  10 . According to other embodiments, the stabilization of the mono-robot  10  may be performed by an additional rolling member connected to the gripping means  15  or connected to a pole fastened to the gantry crane  19 . 
         [0080]    As it may be seen in  FIG. 3 , the gripping means may comprise a clamp  15  comprising two jaws  15 A and  15 B connected for example by a pivot R 2  to enable seizing a long load  300 . In other non-illustrated embodiments of the invention, the clamp  15  may have a jaw which exerts a holding on a fixed surface; it is also conceivable to provide the clamps with more than two jaws ( 3 ,  4  or more). 
         [0081]    Furthermore, as it is visible in  FIG. 3 , each jaw  15 A- 15 B of the clamp  15  may be connected to a rotatably movable roller  16 . When the clamp  15  supports a load  300 , the rotation of the rollers  16  allows the translation of the load  300  and may then ensure the function of the prismatic connection P. 
         [0082]    Furthermore, the rollers  16  may be locked in rotation to block the position of a mono-robot  10 A- 10 B relative to the load  300 . 
         [0083]    When seizing a load  300 , the clamp  15  descends by performing a vertical translation due to the cylindrical connection C. Then when the clamp  15  clasps the load  300 , the load  300  is lifted by a vertical translation of the clamp  15  due to the cylindrical connection C. 
         [0084]    The poly-robot  100  as described in  FIGS. 2 and 3  can indifferently handle two types of loads: on the one hand, the load alone if it is sufficiently stiff; on the other hand, an assembly comprising of an intermediate chassis  200  on which a load  300  is fastened in the case where the latter proves to be too soft to ensure the mechanical connection between the mono-robots  10  of the poly-robot  100  ( FIG. 4 ). 
         [0085]    As shown in  FIG. 2 , a poly-robot  100  for transporting long load  300  may be carried out by using at least two mono-robots  10 A and  10 B. 
         [0086]    Two mono-robots  10 A and  10 B are positioned along the load  300 . It is thus seen that the load  300  ensure the function of intermediate chassis of the poly-robot  100  being blocked in the gripping means  15  of each mono-robot  10 A and  10 B. 
         [0087]    In this embodiment, it may be appreciated that the load  300  fulfils the function of connection member between the mono-robots  10 . Thus, this implementation avoids the use of a chassis which is commonly found in the devices of the prior art, which is an important advantage of the invention. This embodiment allows a weight gain and allows the poly-robot to transport a long load on rough terrain hardly accessible to the devices of the prior art. 
         [0088]    In another embodiment shown in  FIG. 4 , when the long load  300  does not have sufficient mechanical strength to ensure the function of the intermediate chassis between the two mono-robots  10 A- 10 B, it may be expected to add to the load an intermediate stiffening chassis. In the example shown in the figures, the intermediate chassis is formed by a profile  200 . The profile  200  may comprise a series of clips  210  which allow the connection of the long load  300  to the profile  200 . 
         [0089]    In this case, the clips  210  are mechanical, but it is possible to consider, for example, electromagnetic or pneumatic clips  210  to be adapted for any type of load  300 . 
         [0090]    The driven pivots  18  allow the poly-robot  100  to roll in a straight line, and to perform a turn by acting on the rotation speeds of each of the wheels, for example by differentiating the rotation speed of the two wheels  17  of a same mono-robot  10  selected depending on the desired trajectory. 
         [0091]    The control and coordination of the positioning and orientation kinematic chain of the wheels  17  may be carried out by a monitoring electronic such as, for example, a microcontroller. It is possible to provide an on-board control console, or it is also possible to provide a wireless remote control system. 
         [0092]    Furthermore the positioning and orientation kinematic chain of each mono-robot  10 A and  10 B allows the poly-robot  100  to cross an obstacle. 
         [0093]    The invention may be implemented by a poly-robot  100  which comprises at least two mono-robots  10 . 
         [0094]    The crossing of an obstacle may be performed by adjusting the position of the center of gravity of the poly-robot  100  to optimize the balance so as to allow successively lifting each of the wheels  17  while guaranteeing the permanent quasi-static balance of the system. 
         [0095]    For its best understanding, the crossing method of an obstacle by a poly-robot  100  comprising at least two mono-robots  10  is detailed hereinafter. 
         [0096]    During the rolling, the poly-robot  100  may meet an obstacle as illustrated in  FIGS. 5-6-7 . 
         [0097]    The crossing of an obstacle is made according to a succession of sequences comprising the phases of: reconfiguration, crossing, reconfiguration, crossing, rolling, and this, as many times as required for each of the mono-robots M of the poly-robot. 
         [0098]    For the sake of simplicity, the following description is made in relationship relative to a poly-robot  100  comprising two mono-robots  10 . It is understood that the invention is applied to a poly-robot  100  which may include M (with M greater than or equal to 2) mono-robots according to the load to be transported. 
         [0099]    In the example of a poly-robot with two mono-robots  10 A and  10 B, so that the poly-robot  100  is stable when lifting the wheel  17   a,  the poly-robot  100  initiates a reconfiguration phase ( FIGS. 8-9-10 ). The mono-robot  10 B is oriented to position the projection of the center of gravity of the poly-robot  100  in the sustenance triangle formed by the wheels  17   b,    17   c  and  17   d,  as far as possible from the edges of said sustenance triangle. 
         [0100]    The mono-robot  10 B performs a substantially longitudinal axis translation along the load  300  by means of the prismatic connection Pb, and a rotation about the substantially vertical axis due to the cylindrical connection Cb. The poly-robot  100  is then in the position illustrated in  FIGS. 8-9-10 . 
         [0101]    As shown in  FIGS. 11-12-13 , the crossing of the obstacle is initiated by the raising of the wheel  17   a.  The wheel  17   a  is raised by a rotation of substantially longitudinal axis of the mono-robot  10 A around the load  300  due to the rotoid connection R 1   a  or R 1   b.    
         [0102]    The thrust of the mono-robot  10   b  and the wheels  17   c - 17   d  then causes a rotation of substantially vertical axis of the cylindrical connection Ca of the mono-robot  10 A and the positioning of the wheel  17   a  above the obstacle as it is visible in  FIGS. 14-15-16 . 
         [0103]    Then a rotation of substantially longitudinal axis of the mono-robot  10 A allows the bearing of the wheel  17   a  on the obstacle, as shown in the  FIGS. 17-18-19 . 
         [0104]    As it may be seen in  FIGS. 20-21-22 , the mono-robot  10 B is oriented to position the projection of the center of gravity of the poly-robot  100  within the sustenance triangle formed by the wheels  17   a,    17   c  and  17   d,  as far as possible from the edges of said sustenance triangle. The orientation of the mono-robot  10 B is carried out as described hereinabove. 
         [0105]    Analogously to what the wheel  17   a,    17   b  has undergone, the wheel is raised as shown in  FIGS. 23-24-25 . 
         [0106]    The wheel  17   b  is then positioned above the obstacle, as seen in  FIGS. 26-27-28 , then placed on the obstacle as illustrated in  FIGS. 29-30-31 . Thus the wheel  17   b  can cross the obstacle. The poly-robot  100  then performs a rolling phase. 
         [0107]    As observable in  FIGS. 32-33-34 , the mono-robots  10 A and  10 B, each performs a rotation of substantially vertical axis in order to be positioned in rolling position in a straight line. The poly-robot  100  then moves forward so as to position the mono-robot  10 B against the obstacle. 
         [0108]    As illustrated in  FIGS. 35-36-37 , before lifting the wheel  17   c  of the poly-robot  100 , the poly-robot performs a reconfiguration phase. The mono-robot  10 A is oriented so as to position the projection of the center of gravity of the poly-robot  100  within the sustenance triangle formed by the wheels  17   a,    17   b  and  17   d,  as far as possible from the edges of said sustenance triangle. 
         [0109]    The wheel  17   c  can thus initiate the crossing of the obstacle. For this, the wheel  17   c  is lifted as visible in  FIGS. 38-39-40 . 
         [0110]    The wheel  17   c  is positioned above the obstacle, and then is placed on the obstacle as visible in  FIGS. 41-42-43 . 
         [0111]    As shown in  FIGS. 44-45-46 , in order to lift the wheel  17   d,  the poly-robot  100  carries out a reconfiguration phase. 
         [0112]    As seen in  FIGS. 44-45-46 , the mono-robot  10 A is displaced so as to position the projection of the center of gravity of the poly-robot  100  within the sustenance triangle formed by the wheels  17   a,    17   b  and  17   c,  as far as possible from the edges of said sustenance triangle. 
         [0113]    The wheel  17   d  is then ready to cross the obstacle. 
         [0114]    As visible in  FIGS. 47-48-49 , the mono-robot  10 B lifts the wheel  17   d.  Then, the wheel  17   d  is positioned above the obstacle and placed on the obstacle as shown in  FIGS. 50-51-52 . 
         [0115]    The poly-robot  100  having then crossed the obstacle, the mono-robots  10 A and  10 B are oriented in the rolling position in a straight line as visible in  FIGS. 53-54-55 . 
         [0116]    The invention can also be implemented by a poly-robot  100  which comprises at least three mono-robots  10 , the crossing of an obstacle may be performed by successively lifting each of the three mono-robots  10 . 
         [0117]    It must be specified that the invention is not limited to the poly-robot with three mono-robots illustrated in  FIGS. 56 to 79 . The invention may be implemented with more than three mono-robots. 
         [0118]    During the raising of one of the mono-robots  10 , the poly-robot  100  bears on the other mono-robot  10  in contact with the ground or the obstacle. 
         [0119]    For its good understanding, the crossing method of an obstacle by a poly-robot  100  comprising at least three mono-robots  10  is described hereinafter. 
         [0120]    During the rolling, the poly-robot  100  may meet an obstacle as illustrated in  FIGS. 56-57 . 
         [0121]    As seen in  FIGS. 58-59 , the mono-robot  10 D, by means of the prismatic connection Pd, is displaced along the load  300  to reconfigure the balance of the poly-robot  100  for lifting the mono-robot  10 C. 
         [0122]    As seen in  FIGS. 60-61 , the mono-robot  100  then performs a translation of substantially vertical axis, due to the cylindrical connection Cc, so as to be lifted at an altitude greater than the altitude of the obstacle. 
         [0123]    As seen in  FIGS. 62-63 , the two mono-robots  10 D- 10 E which serve as bearing for the poly-robot  100  move forward to position the mono-robot  100  over the obstacle. 
         [0124]    As seen in  FIGS. 64-65 , the mono-robot  100  performs a translation of substantially vertical axis to be placed on the obstacle. 
         [0125]    The poly-robot  100  moves forward to position the mono-robot  10 D against the obstacle, as observable in  FIGS. 64-65 . 
         [0126]    In the same way as the mono-robot  100  the mono-robot  10 D is raised then placed on the obstacle, as seen in  FIGS. 66 to 71 . 
         [0127]    The poly-robot  100  moves forward to position the mono-robot  10 E against the obstacle. 
         [0128]    In order to lift the mono-robot  10 E, the mono-robot  10 D performs a translation along the load  300  to ensure the stability of the poly-robot  100 , as it may be seen in  FIGS. 72-73 . 
         [0129]    In the same manner as the mono-robots  100  and  10 D, the mono-robot  10 E is raised then placed on the obstacle, as observable in  FIGS. 74 to 79 . 
         [0130]    Of course, the invention is not limited to the embodiments shown hereinabove, but it encompasses, on the contrary, all the variants, in particular the case where the poly-robot includes a number M of mono-robots greater than three and alternative propulsion means, such as omnidirectional wheels or caterpillars as an alternative of the represented wheels.