Abstract:
A descent control device for controlling rotational motion of an object includes: at least one drive assembly for rotationally engaging the rotating object; a first substantially planar ferromagnetic moving element in rotationally locked engagement with at least one drive assembly and comprising at least one recess formed in a surface thereof having at least one magnet fixedly disposed therein; a second substantially planar ferromagnetic moving element in rotationally locked engagement with at least one drive assembly, and comprising at least one recess formed in a surface thereof having at least one magnet fixedly disposed therein; and at least one conducting plate disposed proximate the first and second moving elements. A magnetic field may be induced by rotational movement of the first or second moving element relative to at least one conducting plate in a direction to oppose acceleration of at least one drive assembly as it rotationally engages the object.

Description:
RELATED APPLICATIONS 
     The present application is a continuation in part of application Ser. No. 12/331,105, entitled “Descent Control Device”, filed on Dec. 9, 2008, and claiming priority from Canadian Patent Application No. 2,613,855 entitled “Egress Descent Control Device” which was filed on Dec. 10, 2007. Both of the aforementioned applications are incorporated by reference herein in their entirety. 
    
    
     FIELD 
     The present application relates to apparatus for controlling the descent rate of a structure along a cable, and more particularly to a descent control device for controlling the speed of descent of a cable-suspended apparatus for emergency escape from a platform on a rig. 
     BACKGROUND 
     Often it is necessary to have someone working on a rig platform (such as a derrick tubing board, for example). Sometimes, however, rig workers on such platforms are faced with a blowout or fire or some other kind of accident and need to escape quickly from the platform in order to avoid being seriously or fatally injured. Various t-bar or chair-based systems exist for providing a means for escaping from such platforms; however a difficulty encountered with known escape systems is that functionally impaired workers (e.g. workers who are in a state of shock as a result of the accident, or workers who have been burned, or disoriented by gases as a result of the accident) can have difficulties in accessing and operating them. 
     In Canadian Patent Application no. 2,539,883 filed Mar. 16, 2006 by Boscher et al and entitled APPARATUS FOR ESCAPING AREA OF ACCIDENT, which is incorporated by reference in its entirety herein, an apparatus is provided for emergency escape from a drilling rig platform along a path defined by at least one cable extending between the platform and a remote, terminal location. The apparatus includes a frame in which a top of the frame is located above a bottom of the frame when the frame is erect. The frame defines an interior space large enough to accommodate a worker. A locking system includes a locking mechanism and also a foot-actuated disengager that is located at least proximate to the bottom of the frame. The locking mechanism is adapted to interlock with a mating portion on the platform to prevent the frame from traveling away from the platform when the locking mechanism engages the mating portion. The disengager is connected to the locking mechanism and has a foot receiving surface region upon which force can be applied to displace the disengager between a first, engaged position and a second position to disengage the locking mechanism from the mating portion. The frame will travel away from the platform to the terminal location under gravity when the locking mechanism is disengaged. 
     The Boscher et al. device uses an automatic braking system attached at the bottom of the frame, beneath the disengager to permit quick descent along the path defined by the cable, but still sufficiently slowed down to prevent an excessively forceful impact when the frame arrives at the terminal location. 
     Conventional systems include braking systems that employ hand actuated levers (including overriding automatic brake settings). A preferred system disclosed in the Boscher et al device is the Rollgliss® Rescue Emergency Descent Device friction brake, model no. 3303001 manufactured by DBI/SALA &amp; Protecta. This system employs a series of brake pads that expand into a brake drum during descent to slow descent to a rate of about 15 feet/second. 
     Because of the intangible factors that will affect braking power with such systems, the rate at which enclosures equipped with such systems will fail will invariably have considerable variability, which makes it difficult to ensure compliance with applicable safety standards, such as those mandated by the Canadian Association of Oilwell Drilling Contractors (CAODC). Such standards mandate, amongst other things, that the enclosure land with a speed no greater than 12 ft/s. 
     The release of the pod pulls on the spooled cable, causing the pads to be placed in frictional contact with a drum to slow the descent of the pod. The physical contact between the pads and the drum creates a potentially hazardous risk of overheating of the components and cable and causes wear on both items which must be taken into account in maintenance operations, especially given that the device is hopefully only sporadically used. Additionally, both the pads and the drum should be subjected to regular maintenance and/or inspections to ensure that corrosion does not build up on either surface which may deleteriously impact the gripping performance of the braking system. Indeed, such braking systems face re-certification inspections after use and on a semi-annual or annual basis. 
     Moreover, such braking systems have a latch mechanism that call for manual engagement of a spooled cable clasp when the enclosure was brought to the side of the derrick or structure. Such manual engagement necessarily incurs a risk of human error, which, in the frenetic occasions when the pod is to be used, could have catastrophic consequences. 
     SUMMARY 
     In accordance with an embodiment of the invention, a descent control device for controlling rotational motion of an object is disclosed. The device comprises at least one drive assembly configured for rotationally engaging the rotating object to be controlled, a first substantially planar ferromagnetic moving element in rotationally locked engagement with the at least one drive assembly, the first moving element comprising at least one recess formed in a surface thereof having at least one magnet fixedly disposed therein, a second substantially planar ferromagnetic moving element in rotationally locked engagement with the at least one drive assembly, the second moving element comprising at least one recess formed in a surface thereof having at least one magnet fixedly disposed therein, and at least one conducting plate disposed proximate to the first and second moving elements. A magnetic field may be induced by rotational movement of the first moving element or the second moving element relative to the at least one conducting plate in a direction to oppose acceleration of the at least one drive assembly as it rotationally engages the object to be controlled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present application will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
         FIG. 1  is a perspective view of a platform with an enclosure attached by means of a cable between the platform and a terminal location; 
         FIG. 2  is a detailed side view inside of a braking assembly of an enclosure of  FIG. 1  wherein the braking assembly includes a descent control device partially behind and partially extending through the mounting plate of the braking assembly; 
         FIG. 3  is a detailed cross-sectional view of the braking assembly and the descent control device of  FIG. 2  taken along line A-A; 
         FIG. 4  is a plan view of a rotor for use in an example embodiment of the descent control device of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of the rotor of  FIG. 4  along the line B-B; and 
         FIG. 6  is a detailed cross-sectional view of an alternative embodiment if the descent control device. 
     
    
    
     DETAILED DESCRIPTION 
     The present application will now be described for the purposes of illustration only, in conjunction with certain embodiments shown in the enclosed drawings. While preferred embodiments are disclosed, this is not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present application and it is to be further understood that numerous changes covering alternatives, modifications and equivalents may be made without straying from the scope of the present application, as defined by the appended claims. 
     In particular, all dimensions described herein are intended solely to be exemplary for purposes of illustrating certain embodiments and are not intended to limit the scope of the invention to any embodiments that may depart from such dimensions as may be specified. 
     Referring first to  FIG. 1 , there is shown, an existing embodiment of an enclosure shown generally at  100  that is secured in position next to a platform  102  by a cable  104  and a locking mechanism  106 . The enclosure  100  is accessible from the platform  102  by an exit  108 . The enclosure  100  includes a braking assembly  110  affixed to a metal frame  112  of the enclosure  100 . The cable  104  is affixed, at one end, to the platform  102  by a platform anchor  114 , and at the other end, to a terminal anchor (not shown) at the terminal location (not shown). The cable  104  runs through the braking assembly  110  such that the cable  104  defines the path of descent of the enclosure  100  when the enclosure  100  is released from the platform  102 . As will be described in greater detail in  FIG. 2  and  FIG. 3 , the cable  104  is acted upon by the braking assembly  110  to slow the descent of the enclosure  100 . 
     Preferably, the cables  104  used are ½-inch diameter steel cables. 
     Preferably, the platform anchor  114 , connected to one end of cable  104 , is several feet above the exit  108 . The other end of cable  104  is connected to a terminal anchor (not shown) at the terminal location (not shown). The terminal location is located at a lower elevation and safely distant from the platform  102 . Preferably, the terminal location is horizontally distanced about 80 to 100 feet from the platform  102 . Preferably, each cable  104  is connected between the platform  102  and the terminal location by screwed in anchors that have been pull tested. 
     The platform  102  is generally of such design that an exit  108  is formed at the location where the enclosure  100  is releasably secured to the platform  102  such that a rig worker  116  can depart the platform  102  through exit  108  and enter enclosure  100 . 
     Preferably, the platform  102  is at an elevated location on a rig of the type used for drilling or servicing of wells, for example. Those skilled in the art will appreciate that at least some example embodiments of the enclosure with descent control device disclosed herein are suitable for use in conjunction with other types of platforms such as a racking board or monkey board, for example. 
     The enclosure  100  is a rigid structure designed for providing a vehicle for a rig worker  116  to depart the platform  102  in the case of an accident such as a blowout or the like. Preferably, the enclosure  100  comprises a metal frame  112  composed of a plurality of metal members that define an interior space of the enclosure  100  at least sufficiently large to accommodate one rig worker  116 . A rig worker  116  on the platform  102  can enter the enclosure  100  by departing the platform  102  through the exit  108  to enter the interior space of the enclosure  100 . 
     It will be understood that the enclosure  100  could be brought up from the terminal location to a position adjacent to the platform  102  by one of a number of different methods. In some embodiments, the weight of the enclosure  100 , including contents such as passengers or cargo may be, but is not limited to, approximately 700 lbs. 
     To releasably secure the enclosure  100  to the platform  102 , a locking mechanism  106  is employed between the enclosure  100  and platform  102 . The enclosure  100  remains secured to the platform  102  adjacent to the exit  108  when the locking mechanism  106  is engaged. The locking mechanism  106  is disengaged by a rig worker  116  entering the enclosure  100 . 
     Disengaging the locking mechanism  106  triggers the release of the enclosure  100  from the platform  102  commencing descent of the enclosure  100  from its location adjacent to platform  102  along the path defined by cable  104  to a terminal location (not shown). 
     To protect a rig worker  116  from an accidental fall off of a rig platform, it is typical for a number of safety lines  118  to be employed. Each safety line  118  connects the rig worker  116  to either the platform  102  or the enclosure  100 . Lanyards  120  connect each end of a safety line  118  to one of the rig worker  116 , platform  102 , or enclosure  100 . 
     Referring for instance to the example embodiment illustrated in  FIG. 1 , the two safety lines  118  connect the rig worker  116  to the enclosure  100  rather than the platform  102 . The impact of this setup is that a rig worker  116  in an accident situation can save the time and effort of disconnecting lanyards  120  from the platform  102  and reconnecting them to the enclosure  100  before exiting the platform  102 . 
     When a rig worker  116  enters the enclosure  100  releasing the locking mechanism  106 , the enclosure  100  will automatically commence its descent from its initial location proximate to platform  102  to a terminal location at a decreased elevation and increased horizontal displacement from the platform  102 . The path of descent of the enclosure  100  is defined by the cable  104  which runs through the braking assembly  110 . The cable  104  is anchored to the platform  102  at one end by platform anchor  114 , and anchored at the other end to the terminal location (not shown) by a terminal anchor (not shown). 
     In  FIG. 1 , a single cable  104  and a single braking assembly  110  are shown, however embodiments of an enclosure  100  with multiple braking assemblies  110  each operating upon a different cable  104  are also envisioned within the scope of the present invention. Preferably, two braking assemblies  110  are affixed on opposite sides of the enclosure  100 . Each braking assembly  110  operates upon one of two cables  104 , with those cables  104  appropriately anchored to define a path of descent of the enclosure  100  from a position adjacent to the platform  102  to a terminal location (not shown). 
       FIG. 2  shows a detailed side view inside of a braking assembly  110  of an enclosure  100  of the example embodiment in  FIG. 1 . The braking assembly  110  includes at least one descent control device  200  partially visible in front of mounting plate  202  in  FIG. 2  and extending behind the mounting plate  202  to which the braking assembly  110  is attached. Preferably, one or more descent control devices  200  are used to ensure that the enclosure  100  descends at a controlled rate and manner. 
     In the example embodiment, the braking assembly  110  includes two driven sheaves  204  between two idler sheaves  206 . All four sheaves are attached to the mounting plate  202 . Each sheave has a peripheral surface in contact with the cable  104 . In at least one example, the cable  104  runs through the braking assembly  110  passing under or over each of the four sheaves in such a manner that the cable  104  is in contact with a maximum of about ¼ of the diameter of any single sheave. At least one of the driven sheaves  204  is attached to, rotationally drives, and receives a rotational braking force from a descent control device  200  and thus acts as a drive assembly. 
     In the example embodiment, a drive gear  208  is attached to each descent control device  200 . The teeth of the drive gears  208  are mutually interlocked so as to synchronize the rate of rotation of each driven sheave  204  preventing them from slipping against the cable  104  and losing synchronization. This configuration also allows the two driven sheaves  204  to cooperatively grip the cable  104  without slipping during descent. It will be understood that alternative examples wherein a different number of driven sheaves and idler sheaves are employed may be possible. Furthermore, although in the illustrated example embodiment the cable  104  passes directly over the top of the sheave  204  and directly under the bottom of the sheave  206 , those skilled in the art will appreciate that other cable engagement configurations are possible. 
     During descent of the enclosure  100  from the platform  102 , the sheaves  204 ,  206  rotate as the enclosure  100  travels down the path defined by cable  104  from a position proximate to the platform  102  to the terminal location. During rotation, the idler sheaves  206  bear no load and offer minimal resistance to the descent of the enclosure  100 . They primarily aid in maintaining the position of the cable  104  as it passes along the driven sheaves  204 . However, each driven sheave  204  drives a descent control device  200  which generates rotational braking force slowing the descent of the enclosure  100 . How this braking force is generated is best explained with reference to  FIG. 3 . 
       FIG. 3  is an enlarged cross-sectional view along line A-A of a descent control device  200  mounted through the mounting plate  202  of  FIG. 2 . Descent control device  200  includes an input shaft  300  affixed to a rotor  302 , a flange plate  304  connected to a back plate  306  which together form a conductor surrounding the rotor  302 , a spacer  308  affixed to the input shaft  300  and holding the rotor  302  in place between the back plate  306  and the flange plate  304 , a flange bushing or bearing  310  rotatably connecting the input shaft  300  to the flange plate  304  and a back bushing or bearing  312  rotatably connecting the input shaft  300  to the back plate  306 . The rotor  302  includes a plurality of recesses  314  which receive magnets  316  in such a configuration that forms several distinct regions of polarity on the rotor  302 . In the embodiment shown in  FIG. 3 , the descent control device  200  is attached to the mounting plate  304  by fasteners  319 . 
     The input shaft  300  is preferably an elongate cylindrical member having a first end and a second end, through which the axis of rotation is defined, and a shoulder  301  about which the diameter of the input shaft  300  changes. The first end of the input shaft  300  is affixed to a driven sheave  204 . The second end of the input shaft rotationally engages the flange bearing  310  and back bearing  312 . The rotor  302  is mounted on the input shaft  300  at the shoulder  301 . The spacer  308  is mounted on the input shaft  300  on the other side of the rotor  302  such that the position of the rotor  302  relative to the input shaft  300  is fixed both rotationally and longitudinally. The input shaft  300  drives rotation of the rotor  302  when it is rotated with the rotation of a driven sheave  204 . 
     The rotor  302  is preferably a substantially planar ferromagnetic steel cylindrical disc which is centrally affixed to the input shaft  300  and held in place on the input shaft  300  between the shoulder  301  and the spacer  308 . A side view of an example rotor  302  is displayed in  FIG. 4  and in cross-sectional view in  FIG. 5 . The rotor  302  includes a plurality of recesses  314 . In one embodiment, recesses  314  are present on both sides of the rotor  302 . In an alternative embodiment (not shown) recesses  314  could pass completely through the rotor  302 . Each recess  314  receives a magnet  316  having axial magnetization. All magnets  316  are mounted in the same magnetic pole orientation such that the main flux exiting the rotor  302  is of the same polarity. The return flux goes back to the rotor  302  in the area adjacent to each magnet  316 . 
     In one embodiment, each recess  314  may be ¾″ in diameter and ⅛″ deep to receive a Neodymium rare earth or other fixed magnet  316 . In some example embodiments, two rings of recesses  314  contain 48 magnets  316 , on each side of the rotor  302 . In an alternative embodiment, the rotor  302  may itself be a magnet  316 , having a corresponding magnetic pole orientation, and obviating any use of recesses  314 . 
     The flange plate  304  is formed of a conductive metal in such a shape as to encircle the input shaft  300  and enclose one side of the rotor  302 . The flange plate  304  is connected to the flange bushing  310  which secures the axial position of the flange plate  304  relative to the input shaft  300  but permits rotational movement of the input shaft  300  relative to the flange plate  304 . In one embodiment, the flange plate  304  is attached to the mounting plate  202  by fasteners  319  to secure the descent control device  200  to the enclosure  100  and to prevent axial rotation of the flange plate  304  during rotation of the input shaft  300 . The flange plate  304  is connected to the back plate  306  at points radially distal from the rotor  302  such that the connected flange plate  304  and back plate  306  form a cavity inside of which the rotor  302  is proximate to both the flange plate  304  and the back plate  306  but may freely rotate relative thereto. The flange plate  304  forms part of the conductor in which eddy currents are induced. 
     The back plate  306  is formed of a conductive metal in a shape to enclose the second end of the input shaft  300  and the side of the rotor  302  not otherwise enclosed by the flange plate  304 . The back plate  306  is connected to the back bushing  312 , which secures the axial position of the back plate  306  relative to the input shaft  300  but permits rotational movement of the input shaft  300  relative to the back plate  306 . The back plate  306  is connected to the flange plate  304  at points radially distal from the rotor  302  by means of a plate fastener  318  such that the connected back plate  306  and flange plate  304  form a cavity inside of which the rotor  302  is proximate to both the back plate  306  and flange plate  304  but may freely rotate. The back plate  306  forms another part of the conductor in which eddy currents are induced. 
     Thus, the rotation of the rotor  302  creates a traveling wave of magnetic field relative to the conductor which induces eddy currents between the conductor and the rotor. As such, the descent control device  200  operates passively in that there is no applied power or control to operate it. As long as the magnets  316  remain magnetized and relative motion is developed between the magnets  316  and the conductor, a braking force is generated. During rotation, a traveling wave magnetic field is in motion relative to a conducting medium. The relative motion of this wave induces eddy currents in the conductive medium in a pattern which mirrors that of the driving field. The induced eddy currents interact with the field of the magnets  316  to develop a braking force. The braking force is a function of the relative strengths of the magnets  316  and induced currents and their relative phase offsets. The magnitude and phase offset of the induced current varies as a function of the relative wave velocity, magnetic field strengths, wavelength of the field and conductor resistivity. 
     The shoulder  301  surrounds and forms a portion of the input shaft  300  around which the diameter of the input shaft  300  changes. One side of the rotor  302  abuts the shoulder  301  so as to maintain a minimum spacing between the rotor  302  and the flange plate  304 . 
     The spacer  308  surrounds and abuts the input shaft  300  and abuts the other side of the rotor  302  opposite the shoulder  301  by contacting the inner race of back bearing  312 . The spacer  308  holds the rotor  302  securely in place against the shoulder  301  of the input shaft  300  to maintain a spacing between the backing plate  306  and the rotor  302 . The spacing between the rotor  302  and flange plate  306  and the spacing between the rotor  302  and the backing plate  306  prevent frictional contact between the rotor  302  and the flange plate  304  or back plate  306  and yet maintain a desired braking force of the descent control device  200 . 
     The flange bearing  310  surrounds the input shaft  300  to hold the flange plate  304  in place axially while permitting rotation of the input shaft  300 . The flange bearing  310  may be a ball bearing, bushing, spacer, sleeve, coupling or other such instrument which holds the flange plate  304  in place axially while permitting rotation of the input shaft  300 . 
     The back bearing  312  surrounds the input shaft  300  to hold the back plate  306  in place axially while permitting rotation of the input shaft  300 . The back bearing  312  may be a ball bearing, bushing, spacer, sleeve, coupling or other such instrument which holds the back plate  306  in place axially while permitting rotation of the input shaft  300 . 
     The strength of the braking force is also proportional to the distance between the rotor  302  and the conductors and thickness of the conductors. Those having ordinary skill in this art will appreciate that the braking force may be controlled by adding or removing magnets  316 ; changing the displacement between the flange plate  304  and rotor  302 ; changing the displacement between the back plate  306  and rotor  302 ; changing the diameter of back plate  306 , flange plate  304  or the rotor  302 ; changing the type or strength of the magnets  316 ; and changing the material from which the back plate  306 , flange plate  304  and the rotor  302  are composed. For example, the back plate  306  and flange plate  304  could be composed of steel, while the rotor  302  could be composed of aluminum, especially if the magnets  316  were housed in the conductor plates  304  and  306 . Alternatively, the rotor  302  could be composed of copper or laminated steel and copper or plastic. 
     Also visible in  FIG. 3 , a sheave channel  320  preferably semicircular in shape is carved into the circumferential end surface of each driven sheave  204  and idler sheave  206 . The sheave channel  320  guides and increases traction of the cable  104 . In an example embodiment, each sheave channel  320  is slotted to a specific size and spacing to accept the cable  104  there around in a traction fit and also acts to displace any debris that may have built up on the cable  104  such as snow, ice, grease, dirt, wax or the like. 
     To further assist in the removal of snow, ice, grease, dirt, wax, or the like, and to increase heat dissipation when the cable moves through the sheave channel  320 , each driven sheave  204  or idler sheave  206 , may include a series of channel bores  322  bored parallel to the axis of rotation near the circumferential end surface of each sheave and partially through the sheave channel  320 . 
     Turning now to  FIG. 4  and  FIG. 5 , an example rotor  302  is further described. In  FIG. 4  a rotor  302  shaped as a cylindrical disc includes three rings of recesses  314  each recess  314  adapted to accept a magnet  316 . At the center of the rotor  302  a key  400  is cut out of the rotor  302  to receive the shoulder  301  of the input shaft  300  in such a manner to affix the rotor  302  to the input shaft  300  for rotation together. 
     In  FIG. 5 , a cross section of the rotor of  FIG. 4  along the line B-B illustrates one embodiment where a plurality of recesses  314  exist on both sides of rotor  302  for receiving magnets  316 . 
     In a preferred example embodiment, the descent control device  200  consists of a steel rotor  302 , aluminum (6061-T6) flange plate  304  and aluminum (6061-T6) back plate  306 . The surface of the rotor  302  is spaced 0.040 inches from the flange plate  304  on one side and the same distance from the back plate  306  on the other. The flange bearing  310  and back bearing  312  are both ball bearings. The magnets  316  are NdFeB N42 0.750″ diameter, 0.125″ thick and 13,200 Gauss/3,240 surface field Gauss. 
       FIG. 6  is a cross-sectional view of an alternative embodiment of a descent control device  500  according to the present disclosure. The descent control device  500  includes an input shaft  600  on which a pair of rotors  602  are mounted. A flange plate  604  is rotatably mounted to the input shaft  600  via a flange bearing  601 , and a middle plate  605  is mounted to the flange plate  604 . Together, the flange plate  604  and middle plate  605  form a conductor substantially surrounding the first rotor  602 . A back plate  606  is rotatably mounted to the input shaft  600  via a back bearing  612 , and is affixed to the middle plate  605  at a point distant from the input shaft  600 . Together, the back plate  606  and the middle plate  605  form a conductor substantially surrounding the second rotor  602 . 
     The input shaft  600  is similar to the input shaft  300 , with the exception that is has two shoulders  601   a  and  601   b  about which the diameter of the input shaft  600  changes. The diameter of the input shaft  600  increases at a first shoulder  601   a , and decreases at a second shoulder  601   b , creating a raised portion of the input shaft  600  between the shoulders  601   a  and  601   b.    
     A first end  620  of the input shaft  600  is affixed to a driven sheave  204  and a second end  622  of the input shaft  600  rotationally engages the flange plate  604  and the back plate  606  via a flange bearing  610  and a back bearing  612 , in substantially the same manner as the embodiment shown in  FIG. 3 . Each of the flange plate  604 , the back plate  606 , the flange bearing  610  and the back bearing  612  are substantially similar to the flange plate  304 , back plate  306 , flange bearing  310  and back bearing  312 , respectively. 
     Each of the rotors  602  is similar to the rotor  302 . The rotors  602  have recesses  614  (similar to recesses  314 ) for receiving magnets  616  (similar to magnets  316 ). The rotors  602  are mounted on the input shaft  600  at the shoulders  601   a  and  601   b . Spacers  608  (that are similar to spacers  308 ) are inserted between the rotors  602  and the flange plate  604  and back plate  606  in substantially the same manner as the embodiment shown in  FIG. 3 , to fix the rotors  602  in position against the shoulders  601   a  and  601   b , both rotationally and longitudinally, relative to the input shaft  600 . 
     The middle plate  605  is provided between the flange plate  604  and the back plate  606 . The middle plate  605  can be made from the same materials as the flange plate  604  and the back plate  606 , or from any other material that the flange plate  604  and back plate  606  could be made from. The middle plate  605  engages the flange plate  604  at a point away from the input shaft  600 . An “O”-ring  618  is provided at the interface between the flange plate  604  and the middle plate  605  to help prevent contaminants such as dirt from reaching the rotors  602 . The middle plate  605  extends in an “L” shape from the flange plate  604  along the axis of the input shaft  600  and then toward the input shaft  600  between the rotors  602 . The middle plate  605  does not contact either of the rotors  602  or the input shaft  600 . In this configuration, the middle plate  605 , in combination with the flange plate  604  forms a cavity in which the first rotor  602  can rotate freely proximate to the flange plate  604  and the middle plate  605 . 
     The back plate  606  engages the middle plate  605  at the “bend” in its “L” shape. An additional “O”-ring  618  is provided at the interface between the middle plate  605  and back plate  606  to help prevent contaminants such as dirt from reaching the rotors  602 . In this configuration, the middle plate  605 , in combination with the back plate  606  forms a cavity in which the second rotor  602  can rotate freely proximate to the middle plate  605  and the back plate  606 . 
     The flange plate  604 , middle plate  605  and back plate  606  are held together by bolts (not shown) extending through channels formed by aligned bolt holes (not shown) in each of the flange plate  604 , middle plate  605  and back plate  606 . The bolts extend from a proximal end proximate the back plate  606  to a distal end proximate the flange plate  604 . The distal ends of the bolts are threaded, and the threads engage corresponding threading provided on inner surfaces of the bolt holes in the flange plate  604 . It will be apparent to those of skill in the art that other suitable means of fastening the flange plate  604 , middle plate  605  and back plate  606  together are possible. 
     Rotation of the rotors  602  creates a traveling wave of magnetic field relative to the conductors surrounding the rotors  602  and induces eddy currents between the conductors and the rotors  602 , in substantially the same manner as the embodiment shown in  FIG. 3 . It will be appreciated by those skilled in the art that descent control device  500  will create approximately twice the amount of braking force as descent control device  200 , since twice as many rotors and magnets are provided. Actual test results show that descent control device  500  generates a braking force of approximately 80 lbs when rotating at 800 rpm. This is approximately twice the braking force generated by descent control device  200  when rotating at 800 rpm, which testing has shown to be approximately 38-40 lbs. 
     In some implementations, the descent control device  500  can be interchangeable with the descent control device  200 . By connecting input shaft  600  to the driven sheave  204  in substantially the same manner that input shaft  300  can be connected to the driven sheave  204 , and mounting the descent control device  500  to the mounting plate  202  (again, in substantially the same manner in which descent control device  200  is mounted to the mounting plate  202 ), the descent control device  500  can be used to slow the descent of the enclosure  100  along its path of descent in the same manner as descent control device  200 . It should therefore be understood that references in this description to the descent control device  200  and its components shown in  FIG. 3  can apply equally to the descent control device  500  and its components shown in  FIG. 6 . 
     In operation, the enclosure  100  descends along the path defined by at least one cable  104 . Descent of the enclosure  100  along cable  104  causes rotation of at least one driven sheave  204  which causes rotation of the input shaft  300  of the descent control device  200 . Rotation of the input shaft  300  causes rotation of the rotor  302  and the magnets  316  contained in the recesses  314  of the rotor  302 . Rotation of the magnets  316  causes the magnetic field created by the axial polarity of the magnets  316  to rotate. The rotational movement of the magnetic field relative to the conductor (formed in one embodiment by the flange plate  304  and back plate  306 ) induces eddy currents in the conductor in a pattern which mirrors that of the magnetic field created by the magnets  316 . Because the eddy currents and the magnetic field mirror each other, they interact to oppose the rotation of the magnetic field. This opposition to rotation of the magnetic field translates to a braking force against the rotation of the magnets  316  in the rotor  302 , against the rotation of the input shaft  300 , against the rotation of the driven sheave  204  and against the enclosure  100  descending along the cable  104 . Consequently, the enclosure  100  descends along the path defined by the cable  104  at a rate controlled by the braking force of the descent control device  200 . 
     Because the strength of the eddy currents is proportional to the velocity of the rotor  302  relative to the stationary conductor, as the rate of descent of the enclosure  100  increases, the braking force increases. Similarly, decreasing the rate of descent of the enclosure  100  decreases the braking force. This proportionality produces a smoother deceleration and allows the enclosure  100  to descend in a controlled manner towards the terminal location (not shown), resulting in a gentle landing. Rates of descent of about 14 ft/s (peak at around 22 ft/s) have been experienced for descents from high elevations, while more moderate descent elevations result in rates of descent of about 7-8 ft/s and landing speeds as low as 2 ft/s. 
     In simulation testing, a first-order analysis assumed a single pure sinusoid traveling magnetic wave due to field rotation. The simulation assumed a pole gap field amplitude of 3240 Gauss, conductor resistivity of 4×10 −8  ohm-m (aluminum), approximate gap field wavelength of 25 mm, drive sheave diameter of 4.0″, effective rotor drag area (both sides) of 61 square inches, total weight of enclosure and contents of 600 lbs and descent by gravity at a 45 degree descent angle. This simulation of the magnetic drag (braking force) indicates that a pair of descent control devices applied to an enclosure is capable of producing a maximum of approximately 420 lbs of drag at a maximum descent speed of 28 ft/s, which is approximately equal to the gravity force of a 600 pound load descending at a 45° angle. The braking force decreases when descent speed exceeds 28 ft/s. At a descent speed of 12 ft/s, the drag force on the enclosure is approximately 180 lbs. However, this date should be treated as an estimate and approximate only. 
     In fact, the magnetic field is significantly more complex than a single pure sinusoid traveling magnetic wave, with higher order terms that will result in multiple traveling waves of different amplitudes and lengths. Each traveling wave will produce its own characteristic drag/speed curve. The total drag is the Fourier sum of the force contributed by each of these traveling waves. The higher order wave components due to edge effects tend to substantially increase the total braking force. As such, the total braking force may be in the range of 50% to 100% greater than that predicted by the single-wave analysis. 
     It will be apparent to those having ordinary skill in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present application, without departing from the spirit and scope of the present application. 
     For example, the magnets could be mounted in the back plate  306  and flange plate  304  and the conductor could be formed from the rotor  302 . 
     Similarly, a descent control device  200  can be mounted in a braking assembly  110  by rotationally connecting the input shaft  300  to the mounting plate  202  or by affixing the flange plate  304  to the mounting plate  202 . 
     In a further embodiment, the enclosure  100  includes a means for attachment to a trailer or fork lift to facilitate transportation when detached from cables  104  and/or the removable attachment of wheels to facilitate repositioning below the initial point. 
     In some implementations, the descent control device  200  (or the descent control device  500 ) can be used in many different applications to provide braking force, by rotationally connecting the input shaft  300  (or  600 ) to the moving component on which braking force is to be applied. By way of example, an enclosure such as enclosure  100  can be fixedly mounted to a loop of cable at a single point. The loop of cable can extend from a receiver sheave mounted to a raised platform from which a worker may need to escape to a terminal sheave located on the ground some distance away from the platform, defining a path of descent of the enclosure. The cable loop can move in a circular manner around the receiver sheave and terminal sheave when the enclosure descends along the path of descent. The terminal sheave can be rotationally connected to the input shaft  300  (or  600 ) of a descent control device  200  (or  500 ). In such a configuration, the rotation of the terminal sheave would drive the rotation of the rotor  302  (or  602 ), and the braking force generated by such rotation would in turn slow the rotation of the terminal sheave, thus slowing the descent of the enclosure. 
     Other embodiments consistent with the present application will become apparent from consideration of the specification and the practice of the application disclosed herein. 
     Accordingly, the specification and the embodiments disclosed therein are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the following claims.