Abstract:
A robot drive system preferably used for a vessel hull cleaning and/or inspection robot includes a first frame portion rotatably supporting a first axle with a first wheel thereon, and a second frame portion rotatably supporting a second axle with a second wheel thereon. A joint connects the first frame portion to the second frame portion and defines an expendable and contractible portion between the first frame portion and second frame portion. An actuator subsystem is configured expand and contract the expandable and contractible portion to move the first frame portion relative to the second frame portion at the joint to angle the first axle relative to the second axle to steer the robot.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part application which claims the benefit of and priority to U.S. patent application Ser. No. 12/313,643, filed Nov. 21, 2008 under 35 U.S.C. §§119, 120, 363, 365, and 37 C.F.R. §1.55 and §1.78, incorporated herein by this reference. The subject application is related to U.S. patent application Ser. No. 12/583,346, filed Aug. 19, 2009 and U.S. patent application Ser. No. 12/587,949 filed Oct. 14, 2009. 
    
    
     FIELD OF THE INVENTION 
     The subject invention relates to a drive system, typically for a hull robot configured to clean and/or inspect the hull of a vessel. 
     BACKGROUND OF THE INVENTION 
     Co-pending U.S. patent application Ser. No. 12/313,643 filed Nov. 21, 2008 discloses a new autonomous hull robot including turbines driven by water flowing past the hull while the vessel is underway. The turbines operate (e.g., power) the cleaning and the drive subsystems of the robot. 
     Most prior hull cleaning robots suffer from several shortcomings. Typically, the robots are connected to a cable and powered and controlled by an on-board power supply and control subsystem and are able to operate only on a stationary vessel. 
     BRIEF SUMMARY OF THE INVENTION 
     It is desirable to be able to turn the robot as it traverses the hull of a vessel. Typically, the drive system for the robot includes one or more wheels, rollers, or magnetic tracks, e.g., structures which roll on the hull. Complex steering systems or steering systems with numerous moveable components are not desirable. 
     If the steering angle of the drive system is very small, the resulting turning radius may be fairly large, but, since the hull of a ship is very large in area compared to the size of the robot, a large turning radius may be satisfactory. That realization enables the innovation of a drive system which can include relatively few moving parts and which is robust and simple in design. Alignment and adjustment of various components may not be required. 
     This invention features, in one aspect, a robot drive system comprising a frame including a first frame portion rotatably supporting a first axle with a first wheel thereon. A second frame portion rotatably supports a second axle with a second wheel thereon. A joint connects the first frame portion to the second frame portion and defines an expendable and contractible portion between the first frame portion and second frame portion. An actuator subsystem is configured expand and contract the expandable and contractible portion to move the first frame portion relative to the second frame portion at the joint to angle the first axle relative to the second axle to steer the robot. 
     The first and second axles may each include a pair of magnetic wheels. The first and second wheels may be drum shaped and include alternating magnetic and ferromagnetic material. The second frame portion can include a module with spaced side walls for the second axle or it may house two axles of a magnetic track subsystem. The second frame portion may include a third wheel on a third axle spaced from the second axle and a magnetic track about the second and third wheels. 
     Typically at least one of the first and second frame portions includes a propulsion subsystem for driving the robot. One propulsion subsystem includes a motor with a drive shaft. There may be a drive train between the drive shaft and a wheel. For example, the drive train may include a first pulley coupled to the wheel, a second pulley coupled to the drive shaft, and a belt about the first and second pulleys. 
     In one embodiment the joint includes a flexible member between the first frame portion and the second frame portion. In other embodiments, the joint includes a hinged portion between the first frame portion and the second frame portion. 
     In one version, the actuator subsystem includes a shaft including threads in one direction on a first end and threads in an opposite direction on a second end. The first end of the shaft extends into a threaded orifice of the first frame portion and the second end extends into a threaded orifice of the second frame portion. The threaded orifices may include barrel nuts therein. One preferred actuator subsystem also includes means for rotating the shaft. In one design the means for rotating the shaft includes a piston coupled to the shaft and extending from a cylinder connected to the pivotable joint. The actuator subsystem may further include a coupling member between the piston and the shaft. In another design, the actuator subsystem includes a motor associated with the first frame portion driving a shaft threaded into the second frame portion. 
     In some embodiments, one of the first and second frame portions includes a second joint, a second expandable and contractible portion, and a second actuator subsystem configured to expand and contract the second expandable and contractible portion and angle the first axle relative to the second axle at the second joint. The joint can be located on one side of the frame or located interior to the frame defining first and second expandable and contractible portions. In this example, there is typically an actuator subsystem on each side of the joint configured to expand and contract the first and second expandable and contractible portions. 
     The invention also features a robot drive system comprising a frame including spaced side walls, a first axle rotatably disposed between the spaced side walls, a second axle, spaced from the first axle, and rotatably disposed between the spaced side walls. An expandable and contractible portion in the frame is located between the first and second axles. An actuator subsystem is configured to expand and contract the expandable and contractible portion to angle the first axle relative to the second axle to steer the robot. In one preferred design, the expandable portion includes a gap in the frame and a joint spanning the gap and the joint includes a flexible member spanning the gap. 
     An example of a robot drive in accordance with this invention features a first frame portion housing a first magnetic rolling means, a second frame portion housing a second magnetic rolling means, and at least one expandable and contractible portion defined by a joint between the first and second frame portions. An actuator subsystem is configured to expand and contract the expandable portion and flex the joint to angle the first magnetic roller means relative to the second magnetic roller means to steer the robot. A propulsion subsystem is included for at least one of the first and second magnetic roller means to drive the robot. In one example, the first and second magnetic rolling means each include one or more magnetic wheels, drums, and/or tracks. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic three-dimensional bottom view of a hull robot in accordance with the invention showing an example where a robot drive subsystem is mounted to a rotatable turret; 
         FIG. 2  is a schematic three-dimensional bottom view of an example of a hull robot including cleaning brushes; 
         FIG. 3  is a schematic three-dimensional top view showing an example of a drive system in accordance with the invention; 
         FIG. 4  is a schematic top view showing in more detail a portion of the actuator subsystem for the robot drive shown in  FIG. 3 ; 
         FIG. 5  is a schematic three-dimensional top view showing another example of a robot drive in accordance with the subject invention; 
         FIG. 6  is a schematic three-dimensional top view showing an embodiment of a robot drive system in accordance with the invention including two roller wheels; 
         FIG. 7  is a schematic three-dimensional bottom view showing an example of a motorized propulsion subsystem in accordance with the invention; 
         FIG. 8  is a schematic three-dimensional view of a robot drive in accordance with the invention showing a hinged joint between two frame portions; 
         FIG. 9  is a schematic three-dimensional top view showing another example of an actuator subsystem in accordance with the invention; 
         FIGS. 10A and 10B  are schematic views showing an example of a robot drive incorporating a magnetic track; 
         FIG. 11  is a schematic top view showing an example of another robot drive frame configuration in accordance with the invention; 
         FIG. 12  is a schematic three-dimensional top view showing an example of a robot drive with two magnetic tracks; 
         FIG. 13  is a schematic top view showing still another example of a robot drive in accordance with the invention; 
         FIG. 14  is a block diagram showing several of the primary components associated with a typical hull robot in accordance with the invention; 
         FIG. 15  is a schematic three-dimensional partial front view showing several of the components associated with an example of a magnetic track drive module; 
         FIG. 16  is a schematic three-dimensional front view showing one example of a switchable permanent magnetic element associated with a drive module; 
         FIG. 17  is a schematic cross-sectional side view showing the permanent magnet element of  FIG. 16  in its shunted state; 
         FIG. 18  is a schematic cross-sectional side view showing the permanent magnet element of  FIG. 16  in its non-shunted state; 
         FIG. 19  is a schematic three-dimensional side view of an example of a tunnel body constraining the individual permanent magnet elements; 
         FIG. 20  is a schematic three-dimensional side view showing an example of a portion of the mechanism which drives the tunnel body relative to the permanent magnet elements; 
         FIG. 21  is a schematic three-dimensional side view of a segmented tunnel body; 
         FIG. 22  is a schematic three-dimensional front view showing spaced side plate members flexibly supporting the segmented tunnel body shown in  FIG. 21 ; 
         FIG. 23  is a schematic three-dimensional front view showing in more detail the flexure members of  FIG. 22 ; 
         FIG. 24  is a schematic three-dimensional front view of the inside of one of the panels of  FIG. 22 ; and 
         FIG. 25  is a schematic three-dimensional front view of the panels shown in  FIG. 24  depicting how a feature in the panel acts as the switch actuator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
       FIGS. 1-2  show robot  10  including robot body  12  with turbine intake vents  14   a  and  14   b . Cleaning brushes  16   a ,  16   b , and  16   c  are shown in  FIG. 2 . A magnetic drive system  22  is typically used to adhere the robot to the hull and to maneuver the robot about the hull. 
     In the examples shown, turbines  26   a  and  26   b  drive generators  28   a  and  28   b , respectively. Turbines  26   a  and  26   b  are driven by water flowing past the vessel hull when the vessel is underway. Generators  28   a  and  28   b  recharge a power source such as a battery. One or more motors are powered by the power source. An electronic controller is also powered by the power source. 
     For example,  FIG. 1  shows motor  72   c  driving turret  24 . Motor  72   c  is powered by the power source and is controlled by an electronic subsystem or controller. Motor  72   c  drives worm gear  120  engaged with peripheral gear  122  on turret  24 . Turret  24  rotates with respect to the hull via a shaft or the like. Other actuator systems for adjusting the position of turret  24  are possible. In this way, as the robot turns via drive system  22 , turbines  26   a  and  26   b  can be kept in alignment with the flow of water past the hull. 
     Typically, other subsystems are included as components of the robot, for example, a navigation subsystem, a communication subsystem, and the like. Preferably robot body  12  need not be tethered to any kind of an on-board power or control subsystem. The turbine subsystem can operate the drive subsystem (and, in one example, a cleaning subsystem) directly or via a generator charging a power subsystem (e.g., a battery pack) which supplies power to one or more motors driving the drive subsystem and/or the cleaning subsystem. The battery pack can also be used to energize the other electronic and/or electromechanical subsystems associated with the robot. It is also possible for a generator to drive one or more motors directly. 
       FIG. 3  shows an example of drive system  22  in more detail. System  22  includes frame  100  with spaced side wall  102   a  and  102   b  interconnected by end walls  104   a  and  104   b . Spaced axles  106   a  and  106   b , in this example, are rotatably disposed between side walls  102   a  and  102   b  such that frame  100  defines first frame portion or section  108   a  housing axle  106   a  and second frame portion  108   b  houses axle  106   b . Axles  106   a ,  106   b  support, in this particular example, wheels such as spaced magnetic wheels  110   a  and  110   b  on axle  106   a  and spaced wheels  110   c  and  110   d  on axle  106   b . By wheels, as disclosed herein, we mean wheels in the conventional sense, drum shaped wheels (also referred to as rollers), and even a pair of spaced wheels, drums, or sprockets used, for example, in a magnetic track, and other rolling structures. For example, as disclosed herein, axle  106   b  can support a drum type wheel or there may be two spaced axles supported by frame section  108   b  supporting spaced drums or sprockets for a magnetic track housed by frame section  108   b . The same is true with respect to frame section  108   a . One example of a magnetic track module is discussed in reference to  FIGS. 15-25 . 
     In the example shown in  FIG. 3 , frame  100  defines pivotable, bendable, and/or flexible joint  112  between frame portion  108   a  and frame portion  108   b . Stated another way, frame  100  includes expandable and contractable portion  114  between frame portions  108   a  and  108   b . In this particular example, joint  112  is a section of frame side wall  102   b  and portion  114  comprises a gap  115  in frame  100  side wall  102   a  and gap  117  in top wall  103  between metal frame portions  108   a  and  108   b . Joint  112  in this particular example is a portion of the frame side wall  102   b  which can be bent when desired. Joint  112  spans gap  117  which, in the illustrated embodiment, narrows from a wide end near side wall  102   a  to a more narrow end near side wall  102   b . Joint  112  is, in this example, a locally flexible integral portion of an otherwise stiff frame side wall  102   b  and top wall  103  and can bend a few degrees. 
     Actuator subsystem  116  is configured to move frame portion  108   a  relative to frame portion  108   b  and flex joint  112  while expanding and contracting expandable gaps  115  and  117  in the direction shown. This action, in turn, angles first axle  106   a  relative to second axle  106   b  and turns the drive and the robot it is attached to. If axles  106   a  and  106   b  are, for example, a foot apart, the turning radius can be between 5 to 40 feet, which is sufficient for operation on the hull of a ship. 
     In this particular example, actuator subsystem  116  includes shaft  120 ,  FIG. 4  with threads  122   a  extending into a threaded orifice in frame portion  108   a  and threads  122   b  on the other end extending into a threaded orifice in frame portion  108   b . Threads  122   a  and  122   b  are in different directions so turning shaft  120  in one direction expands gap  114  and turning shaft  120  in the opposite direction contracts gap  114 . To reduce any stress on shaft  120 , rotating barrel nuts  124   a ,  124   b  may be incorporated in frame portions  108   a ,  108   b , respectively, to receive the threads on the opposite ends of the shaft. 
     Piston  132  driven in and out of cylinder  130  is coupled to shaft  120  via pivoting joint  134 . In this way, actuating cylinder  130  rotates shaft  120 . Cylinder  130  is typically coupled to the frame, (see  FIG. 7 ) at or near joint  112 . The combination of cylinder  130  and piston  132  may be an electrically driven linear actuator as is known in the art or a pneumatically drive subsystem as is also known in the art. Typically, the electricity required for actuator subsystem  116  is provided by generators  28   a  and/or  28   b ,  FIGS. 1-2  via, for example, a battery subsystem. 
       FIG. 5  shows an example where frame portions  108   a  and  108   b  are interconnected at side wall  102   b  via joint  112 ′ flexure  140  which spans the narrow end of gap  119  between the two frame sections. In  FIG. 6 , the spaced axles support drum shaped magnetic wheels  150   a  and  150   b . Roller  150   b , in this example, is driven via a propulsion subsystem which includes motor  72   a  driving axle  106   b  via gears  152   a  and  152   b . Electricity for motor  72   a  also can be provided by generators  28   a  and/or  28   b ,  FIGS. 1-2 , via the battery subsystem. 
     Rollers  150   a  and  150   b  are magnetic and may include bands of alternating magnetic material  156  and ferromagnetic material  158  for directing magnetic flux into the hull.  FIG. 6  also shows how second frame portion  150   b  includes module  60  housing axle  106   b  and roller  150   b . Module  60  may also house a magnetic track as disclosed herein. 
       FIG. 7  shows an alternative arrangement where the propulsion subsystem includes motor  72   a ′ with drive shaft  170  and a drive train between drive shaft  170  and roller wheel  150   b  including pulley  172   a  on drive shaft  170 , pulley  172   b  coupled to axle  106   b , and belt  174  about pulleys  172   a  and  172   b . Again, electricity for powering motor  172   a  is typically provided by generators  28   a  and/or  28   b ,  FIG. 1 . Alternative propulsion subsystems are within the scope of the invention. 
       FIG. 8  shows an example of where joint  112 ″ includes hinge  180  with hinge pin  182  between frame sections  102   a  and  102   b  each of which include a portion of the hinged joint. 
     In another design, motor  72   d ,  FIG. 9  is associated with frame portion  102   a  and drive shaft  120 ′ is threaded into frame portion  102   b . Operating motor  72   d  in one direction expands gap  114  and operating motor  72   d  in the opposite direction contracts gap  114  angling roller  150   a  relative to roller  150   b . Motor  72   d  is typically powered by electricity supplied via generators  28   a  and/or  28   b ,  FIGS. 1-2 . 
     If a magnetic track is desired,  FIGS. 10A-10B  depict how frame portion  102   b  includes two spaced axles  106   b  and  106   c  supporting drums, sprockets, or the like for driving magnetic track  190 . 
     In the design shown in  FIG. 11 , joint  112 ′″ is a flexible frame section located interior of the frame side and end walls. There may now be two adjustable expandable and contractable sections  114   a  and  114   b  and two actuator subsystems  116   a  and  116   b . The result, in this design, is a smaller turning radius. Gap  114   a  may be contracted while gap  114   b  is expanded and vice versa. 
       FIG. 12  shows an example with two magnetic tracks  190   a  and  190   b  one each associated with frame portion  102   a ,  102   b , respectively.  FIG. 13  shows an example where frame section  102   b  includes a second joint  112   b  and actuator subsystem  116   b  for joint  112   b . Joint  112   a  is between frame portion  102   a  and  102   b  and is associated with actuator subsystem  116   a.    
     The novel drive system of the invention is thus highly versatile and can incorporate numerous different features and combinations. 
       FIG. 14  illustrates an example where turbine subsystem  26  (see turbines  26   a  and  26   b ,  FIG. 1 ) are actuatable by fluid flowing past the hull when the vessel is underway. Generators  28  recharge power source  38  which may include one or more batteries. One or more motors are powered by power source  38 . Typically, motor  72   b  (See  FIG. 2 .) is coupled to the cleaning subsystem  82  which may comprise a number of rotatable brushes as shown in  FIG. 2  via drive train  74   b . One or more motors  72   a  are associated with drive subsystem  22  mounted to turret  24 . For example, as discussed above, there is typically one motor for one of the drive wheels, rollers, or sprockets and also at least one motor associated with the actuator subsystem of the robot drive. As shown in  FIGS. 1 and 14 , motor  72   c  is typically associated with turret  24  for rotating the same. The direction of travel of the robot as well as actuation of the propulsion subsystem and the actuator subsystem to turn the robot is controlled by electronic control subsystem  46 ,  FIG. 14 . Operation of the propulsion subsystem, turret, and actuator subsystem can be based on inputs, for example, from navigation subsystem  78  and/or communications subsystem  80  and/or one sensors  90 . See U.S. patent application Ser. No. 12/313,643 incorporated herein by this reference. 
       FIG. 15  schematically depicts certain components of a version of a magnetic track module for the drive. There are typically a plurality of permanent magnet elements such as element  200 . Switch assembly  202  switches element  200  between a shunted and a non-shunted state. Actuator  204  actuates switch  202  typically between a shunted state when element  200  is not adjacent the vessel hull and a non-shunted state when element  200  is adjacent the vessel hull. Tunnel body  206  is configured to constrain the movement of element  100  which typically includes some type of carriage  208 . There are also some means to drive tunnel body  106  with respect to permanent magnet element  200  as shown by arrow  210 . 
       FIG. 16  shows a design where permanent magnet element  200  includes diametrically polarized cylindrical magnet  220  rotatably disposed in a bore of housing  222 . Housing  222  includes non-magnetic material  224  (e.g., aluminum, plastic, or the like) sandwiched between ferromagnetic material  226   a  and  226   b  (e.g., steel). Switch  202  is attached to cylindrical magnet  220  and rotates it as shown in  FIGS. 17-18 . In  FIG. 17 , magnet  220  is shunted since the magnetic field flows from the north pole, outwardly through ferromagnetic material  226   a  and  226   b , and to the south pole. The attraction of magnet  220  to vessel hull  230  is thus minimized. Activating switch  202  rotates magnet  220  as shown in  FIG. 7  so each pole is proximate ferromagnetic material  226   a  or  226   b . As shown in figure, the south pole is in contact with ferromagnetic material  226   a  and the north pole is in contact with ferromagnetic material  226   b . The magnetic field flows from the north pole of the magnet into body  226   b , to the ship&#39;s hull  230 , to body  226   a , and then back to the south pole of the magnet. In this non-shunted state, the attraction of magnet  220  to hull  230  is maximized. 
     Typically, switch  202  is activated to shunt magnet  220  as permanent magnet element  200  reaches the end of its travel on the hull and switch  202  is again activated to actuate magnet  220  as permanent magnet element  200  again comes into contact with the hull. In this way, power usage is minimized and yet there is still a very strong tractive force provided to keep the robot on the hull. Power usage is minimized because power is not wasted in removing the individual permanent magnet elements from the hull. Also, damage to the hull is minimized since the permanent magnet elements are not switched to their non-shorted states until they are actually in contact with the hull. Each permanent magnet element may include a protective covering to also reduce damage to the vessel hull. The intent is to control the holding force exerted by the magnets but at the same time use permanent magnets which consume no power unlike electromagnets. 
       FIG. 16  also shows carriage  208 ′ with spaced rotating bearings  240   a  and  240   b  and connectors  242   a - 242   d . Bearings  240   a  and  240   b  ride in side tracks in tunnel body  206 ′,  FIG. 19 . In  FIG. 19 , oval shaped side track  252  is shown. One axle is typically disposed through orifices  253   a  and  253   b  in body  206 ′ and another axle is typically disposed through orifices  255   c  and another orifice, not shown. 
       FIG. 20  shows how tunnel body  206 ′ supports a drive train such as spaced sprocket wheels including wheel  260  on axle  261  (which may be driven by motor  72   a  and drive train  74   a ,  FIG. 14 ). Chain  262  extends around the spaced sprocket wheels. Bearing  240   b  of carriage  208 ′ of permanent magnet element  200   a  is constrained in track  252  of tunnel body  206 ′ and connectors  242   c  and  242   d  extend into chain  262 . 
     Since tunnel body  206 ′ is coupled to the robot body, and since permanent magnet elements  200   b - 200   e  in their non-shunted states are strongly attracted to the vessel hull, chain  262  actually drives tunnel body  206 ′ forward (and rearward) and thus the robot body is driven with respect to the vessel hull via the rotation of chain  262  and about sprocket  260  and a similar sprocket at the other end of the tunnel body. Both sprockets are on axles typically housed by one of the drive frame portions, e.g. frame portion  102   b ,  FIG. 12 . 
       FIG. 20  also shows that permanent magnet element  200   a  is shunted via the position of switch  202 . Permanent magnet element  200   x  is either an element first coming into position to be attracted to the hull  230  or it is leaving hull  230  depending on the direction of robot travel. If permanent magnet element  200   x  is just coming into position to be attracted to hull  230 , it is switched from the shunted position shown to its non-shunted position once permanent magnet element  200   x  occupies the position of permanent magnet element  200   b . If permanent magnet element  200   x  is just leaving hull  230 , or is about to leave the hull, it is switched into a shunted state just after it occupies the position of permanent magnet element  200   b.    
       FIG. 21  shows a segmented design for tunnel body  206 ″ to allow for articulation of the tunnel body and track system to maximize the contact area for each permanent magnet element in the presence of non-uniformities  270   a  and  270   b  on hull  230 . In  FIG. 22 , spaced frame panels  280   a  and  280   b  support tunnel body  206 ″ via flexures  282   a ,  282   b , and the like,  FIGS. 22-23 . Typically, there is at least one flexure for each tunnel body segment as shown in  FIG. 23 . Side frame panels  280   a  and  280   b  are affixed to the robot body or to a turret rotatably attached to the robot body and are associated with one of the drive frame portions, e.g., frame portion  102   b ,  FIG. 12 . 
       FIG. 22  also shows an actuation feature such as closed loop groove  284   a  on the inside of panel  280   b . As shown in  FIG. 24 , these grooves in the side panels function to actuate the switches of the permanent magnet elements. At groove ends  286   a  and  286   b  there is a jog. If the direction of travel of the hull robot is as shown by arrow  288  and the vessel hull is at the bottom of the figure, jog  286   b  actuates the permanent magnet element switches to shunt the permanent magnet elements and at jog  286   a  the switches are actuated again to return the permanent magnet elements to their non-shunted configuration.  FIG. 25  shows more complete switching assemblies  202   a - 202   d  and depicts how switch  202   a  is in its shunted position but switch  202   b , via groove jog  286   a , has been actuated to its non-shunted position. Similarly, jog  286   b  turns switch  202   c  to the shunted position for the remainder of its travel about the front and top of panel  280   a  corresponding to the front and top of tunnel body  206 ′,  FIG. 20 . 
     In one preferred design, the tunnel body performs two functions: it constrains the movement of the permanent magnet elements and also serves to house the propulsion mechanism (e.g., a chain about two sprockets) connected to the carriages of the permanent magnet elements. This design also provides structural support against slack in the drive assembly. The side plates also serve two functions: they flexibly support the tunnel body and they include means for actuating the switches of the permanent magnet elements. In the preferred design, the magnetic elements are switched between their minimum tractive state and their maximum tractive state irrespective of the direction of travel of the robot. These are not limitations of the subject invention, however, as other designs are possible. 
     Other features associated with the typical hull robot are disclosed in the patents cited in the Background section hereof and incorporated herein by this reference. Also, U.S. patent application Ser. No. 12/313,643 filed Nov. 21, 2008 by the assignee hereof discloses additional features which may be associated with a hull robot. The drive system disclosed herein, however, is not limited to use in connection with such a vessel hull robot. The drive module, for example, can be used on any ferromagnetic body including but not limited to vessel hulls, underwater structures, and the like. “Hull,” as used herein, then, broadly means a structure to be traversed. 
     Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.