Descent control device

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.

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 & 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.

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 toFIG. 1, there is shown, an existing embodiment of an enclosure shown generally at100that is secured in position next to a platform102by a cable104and a locking mechanism106. The enclosure100is accessible from the platform102by an exit108. The enclosure100includes a braking assembly110affixed to a metal frame112of the enclosure100. The cable104is affixed, at one end, to the platform102by a platform anchor114, and at the other end, to a terminal anchor (not shown) at the terminal location (not shown). The cable104runs through the braking assembly110such that the cable104defines the path of descent of the enclosure100when the enclosure100is released from the platform102. As will be described in greater detail inFIG. 2andFIG. 3, the cable104is acted upon by the braking assembly110to slow the descent of the enclosure100.

Preferably, the platform anchor114, connected to one end of cable104, is several feet above the exit108. The other end of cable104is 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 platform102. Preferably, the terminal location is horizontally distanced about 80 to 100 feet from the platform102. Preferably, each cable104is connected between the platform102and the terminal location by screwed in anchors that have been pull tested.

The platform102is generally of such design that an exit108is formed at the location where the enclosure100is releasably secured to the platform102such that a rig worker116can depart the platform102through exit108and enter enclosure100.

Preferably, the platform102is 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 enclosure100is a rigid structure designed for providing a vehicle for a rig worker116to depart the platform102in the case of an accident such as a blowout or the like. Preferably, the enclosure100comprises a metal frame112composed of a plurality of metal members that define an interior space of the enclosure100at least sufficiently large to accommodate one rig worker116. A rig worker116on the platform102can enter the enclosure100by departing the platform102through the exit108to enter the interior space of the enclosure100.

It will be understood that the enclosure100could be brought up from the terminal location to a position adjacent to the platform102by one of a number of different methods. In some embodiments, the weight of the enclosure100, including contents such as passengers or cargo may be, but is not limited to, approximately 700 lbs.

To releasably secure the enclosure100to the platform102, a locking mechanism106is employed between the enclosure100and platform102. The enclosure100remains secured to the platform102adjacent to the exit108when the locking mechanism106is engaged. The locking mechanism106is disengaged by a rig worker116entering the enclosure100.

Disengaging the locking mechanism106triggers the release of the enclosure100from the platform102commencing descent of the enclosure100from its location adjacent to platform102along the path defined by cable104to a terminal location (not shown).

To protect a rig worker116from an accidental fall off of a rig platform, it is typical for a number of safety lines118to be employed. Each safety line118connects the rig worker116to either the platform102or the enclosure100. Lanyards120connect each end of a safety line118to one of the rig worker116, platform102, or enclosure100.

Referring for instance to the example embodiment illustrated inFIG. 1, the two safety lines118connect the rig worker116to the enclosure100rather than the platform102. The impact of this setup is that a rig worker116in an accident situation can save the time and effort of disconnecting lanyards120from the platform102and reconnecting them to the enclosure100before exiting the platform102.

When a rig worker116enters the enclosure100releasing the locking mechanism106, the enclosure100will automatically commence its descent from its initial location proximate to platform102to a terminal location at a decreased elevation and increased horizontal displacement from the platform102. The path of descent of the enclosure100is defined by the cable104which runs through the braking assembly110. The cable104is anchored to the platform102at one end by platform anchor114, and anchored at the other end to the terminal location (not shown) by a terminal anchor (not shown).

InFIG. 1, a single cable104and a single braking assembly110are shown, however embodiments of an enclosure100with multiple braking assemblies110each operating upon a different cable104are also envisioned within the scope of the present invention. Preferably, two braking assemblies110are affixed on opposite sides of the enclosure100. Each braking assembly110operates upon one of two cables104, with those cables104appropriately anchored to define a path of descent of the enclosure100from a position adjacent to the platform102to a terminal location (not shown).

FIG. 2shows a detailed side view inside of a braking assembly110of an enclosure100of the example embodiment inFIG. 1. The braking assembly110includes at least one descent control device200partially visible in front of mounting plate202inFIG. 2and extending behind the mounting plate202to which the braking assembly110is attached. Preferably, one or more descent control devices200are used to ensure that the enclosure100descends at a controlled rate and manner.

In the example embodiment, the braking assembly110includes two driven sheaves204between two idler sheaves206. All four sheaves are attached to the mounting plate202. Each sheave has a peripheral surface in contact with the cable104. In at least one example, the cable104runs through the braking assembly110passing under or over each of the four sheaves in such a manner that the cable104is in contact with a maximum of about ¼ of the diameter of any single sheave. At least one of the driven sheaves204is attached to, rotationally drives, and receives a rotational braking force from a descent control device200and thus acts as a drive assembly.

In the example embodiment, a drive gear208is attached to each descent control device200. The teeth of the drive gears208are mutually interlocked so as to synchronize the rate of rotation of each driven sheave204preventing them from slipping against the cable104and losing synchronization. This configuration also allows the two driven sheaves204to cooperatively grip the cable104without 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 cable104passes directly over the top of the sheave204and directly under the bottom of the sheave206, those skilled in the art will appreciate that other cable engagement configurations are possible.

During descent of the enclosure100from the platform102, the sheaves204,206rotate as the enclosure100travels down the path defined by cable104from a position proximate to the platform102to the terminal location. During rotation, the idler sheaves206bear no load and offer minimal resistance to the descent of the enclosure100. They primarily aid in maintaining the position of the cable104as it passes along the driven sheaves204. However, each driven sheave204drives a descent control device200which generates rotational braking force slowing the descent of the enclosure100. How this braking force is generated is best explained with reference toFIG. 3.

FIG. 3is an enlarged cross-sectional view along line A-A of a descent control device200mounted through the mounting plate202ofFIG. 2. Descent control device200includes an input shaft300affixed to a rotor302, a flange plate304connected to a back plate306which together form a conductor surrounding the rotor302, a spacer308affixed to the input shaft300and holding the rotor302in place between the back plate306and the flange plate304, a flange bushing or bearing310rotatably connecting the input shaft300to the flange plate304and a back bushing or bearing312rotatably connecting the input shaft300to the back plate306. The rotor302includes a plurality of recesses314which receive magnets316in such a configuration that forms several distinct regions of polarity on the rotor302. In the embodiment shown inFIG. 3, the descent control device200is attached to the mounting plate304by fasteners319.

The input shaft300is preferably an elongate cylindrical member having a first end and a second end, through which the axis of rotation is defined, and a shoulder301about which the diameter of the input shaft300changes. The first end of the input shaft300is affixed to a driven sheave204. The second end of the input shaft rotationally engages the flange bearing310and back bearing312. The rotor302is mounted on the input shaft300at the shoulder301. The spacer308is mounted on the input shaft300on the other side of the rotor302such that the position of the rotor302relative to the input shaft300is fixed both rotationally and longitudinally. The input shaft300drives rotation of the rotor302when it is rotated with the rotation of a driven sheave204.

The rotor302is preferably a substantially planar ferromagnetic steel cylindrical disc which is centrally affixed to the input shaft300and held in place on the input shaft300between the shoulder301and the spacer308. A side view of an example rotor302is displayed inFIG. 4and in cross-sectional view inFIG. 5. The rotor302includes a plurality of recesses314. In one embodiment, recesses314are present on both sides of the rotor302. In an alternative embodiment (not shown) recesses314could pass completely through the rotor302. Each recess314receives a magnet316having axial magnetization. All magnets316are mounted in the same magnetic pole orientation such that the main flux exiting the rotor302is of the same polarity. The return flux goes back to the rotor302in the area adjacent to each magnet316.

In one embodiment, each recess314may be ¾″ in diameter and ⅛″ deep to receive a Neodymium rare earth or other fixed magnet316. In some example embodiments, two rings of recesses314contain 48 magnets316, on each side of the rotor302. In an alternative embodiment, the rotor302may itself be a magnet316, having a corresponding magnetic pole orientation, and obviating any use of recesses314.

The flange plate304is formed of a conductive metal in such a shape as to encircle the input shaft300and enclose one side of the rotor302. The flange plate304is connected to the flange bushing310which secures the axial position of the flange plate304relative to the input shaft300but permits rotational movement of the input shaft300relative to the flange plate304. In one embodiment, the flange plate304is attached to the mounting plate202by fasteners319to secure the descent control device200to the enclosure100and to prevent axial rotation of the flange plate304during rotation of the input shaft300. The flange plate304is connected to the back plate306at points radially distal from the rotor302such that the connected flange plate304and back plate306form a cavity inside of which the rotor302is proximate to both the flange plate304and the back plate306but may freely rotate relative thereto. The flange plate304forms part of the conductor in which eddy currents are induced.

The back plate306is formed of a conductive metal in a shape to enclose the second end of the input shaft300and the side of the rotor302not otherwise enclosed by the flange plate304. The back plate306is connected to the back bushing312, which secures the axial position of the back plate306relative to the input shaft300but permits rotational movement of the input shaft300relative to the back plate306. The back plate306is connected to the flange plate304at points radially distal from the rotor302by means of a plate fastener318such that the connected back plate306and flange plate304form a cavity inside of which the rotor302is proximate to both the back plate306and flange plate304but may freely rotate. The back plate306forms another part of the conductor in which eddy currents are induced.

Thus, the rotation of the rotor302creates 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 device200operates passively in that there is no applied power or control to operate it. As long as the magnets316remain magnetized and relative motion is developed between the magnets316and 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 magnets316to develop a braking force. The braking force is a function of the relative strengths of the magnets316and 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 shoulder301surrounds and forms a portion of the input shaft300around which the diameter of the input shaft300changes. One side of the rotor302abuts the shoulder301so as to maintain a minimum spacing between the rotor302and the flange plate304.

The spacer308surrounds and abuts the input shaft300and abuts the other side of the rotor302opposite the shoulder301by contacting the inner race of back bearing312. The spacer308holds the rotor302securely in place against the shoulder301of the input shaft300to maintain a spacing between the backing plate306and the rotor302. The spacing between the rotor302and flange plate306and the spacing between the rotor302and the backing plate306prevent frictional contact between the rotor302and the flange plate304or back plate306and yet maintain a desired braking force of the descent control device200.

The flange bearing310surrounds the input shaft300to hold the flange plate304in place axially while permitting rotation of the input shaft300. The flange bearing310may be a ball bearing, bushing, spacer, sleeve, coupling or other such instrument which holds the flange plate304in place axially while permitting rotation of the input shaft300.

The back bearing312surrounds the input shaft300to hold the back plate306in place axially while permitting rotation of the input shaft300. The back bearing312may be a ball bearing, bushing, spacer, sleeve, coupling or other such instrument which holds the back plate306in place axially while permitting rotation of the input shaft300.

The strength of the braking force is also proportional to the distance between the rotor302and 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 magnets316; changing the displacement between the flange plate304and rotor302; changing the displacement between the back plate306and rotor302; changing the diameter of back plate306, flange plate304or the rotor302; changing the type or strength of the magnets316; and changing the material from which the back plate306, flange plate304and the rotor302are composed. For example, the back plate306and flange plate304could be composed of steel, while the rotor302could be composed of aluminum, especially if the magnets316were housed in the conductor plates304and306. Alternatively, the rotor302could be composed of copper or laminated steel and copper or plastic.

Also visible inFIG. 3, a sheave channel320preferably semicircular in shape is carved into the circumferential end surface of each driven sheave204and idler sheave206. The sheave channel320guides and increases traction of the cable104. In an example embodiment, each sheave channel320is slotted to a specific size and spacing to accept the cable104there around in a traction fit and also acts to displace any debris that may have built up on the cable104such 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 channel320, each driven sheave204or idler sheave206, may include a series of channel bores322bored parallel to the axis of rotation near the circumferential end surface of each sheave and partially through the sheave channel320.

Turning now toFIG. 4andFIG. 5, an example rotor302is further described. InFIG. 4a rotor302shaped as a cylindrical disc includes three rings of recesses314each recess314adapted to accept a magnet316. At the center of the rotor302a key400is cut out of the rotor302to receive the shoulder301of the input shaft300in such a manner to affix the rotor302to the input shaft300for rotation together.

InFIG. 5, a cross section of the rotor ofFIG. 4along the line B-B illustrates one embodiment where a plurality of recesses314exist on both sides of rotor302for receiving magnets316.

In a preferred example embodiment, the descent control device200consists of a steel rotor302, aluminum (6061-T6) flange plate304and aluminum (6061-T6) back plate306. The surface of the rotor302is spaced 0.040 inches from the flange plate304on one side and the same distance from the back plate306on the other. The flange bearing310and back bearing312are both ball bearings. The magnets316are NdFeB N42 0.750″ diameter, 0.125″ thick and 13,200 Gauss/3,240 surface field Gauss.

FIG. 6is a cross-sectional view of an alternative embodiment of a descent control device500according to the present disclosure. The descent control device500includes an input shaft600on which a pair of rotors602are mounted. A flange plate604is rotatably mounted to the input shaft600via a flange bearing601, and a middle plate605is mounted to the flange plate604. Together, the flange plate604and middle plate605form a conductor substantially surrounding the first rotor602. A back plate606is rotatably mounted to the input shaft600via a back bearing612, and is affixed to the middle plate605at a point distant from the input shaft600. Together, the back plate606and the middle plate605form a conductor substantially surrounding the second rotor602.

The input shaft600is similar to the input shaft300, with the exception that is has two shoulders601aand601babout which the diameter of the input shaft600changes. The diameter of the input shaft600increases at a first shoulder601a, and decreases at a second shoulder601b, creating a raised portion of the input shaft600between the shoulders601aand601b.

A first end620of the input shaft600is affixed to a driven sheave204and a second end622of the input shaft600rotationally engages the flange plate604and the back plate606via a flange bearing610and a back bearing612, in substantially the same manner as the embodiment shown inFIG. 3. Each of the flange plate604, the back plate606, the flange bearing610and the back bearing612are substantially similar to the flange plate304, back plate306, flange bearing310and back bearing312, respectively.

Each of the rotors602is similar to the rotor302. The rotors602have recesses614(similar to recesses314) for receiving magnets616(similar to magnets316). The rotors602are mounted on the input shaft600at the shoulders601aand601b. Spacers608(that are similar to spacers308) are inserted between the rotors602and the flange plate604and back plate606in substantially the same manner as the embodiment shown inFIG. 3, to fix the rotors602in position against the shoulders601aand601b, both rotationally and longitudinally, relative to the input shaft600.

The middle plate605is provided between the flange plate604and the back plate606. The middle plate605can be made from the same materials as the flange plate604and the back plate606, or from any other material that the flange plate604and back plate606could be made from. The middle plate605engages the flange plate604at a point away from the input shaft600. An “O”-ring618is provided at the interface between the flange plate604and the middle plate605to help prevent contaminants such as dirt from reaching the rotors602. The middle plate605extends in an “L” shape from the flange plate604along the axis of the input shaft600and then toward the input shaft600between the rotors602. The middle plate605does not contact either of the rotors602or the input shaft600. In this configuration, the middle plate605, in combination with the flange plate604forms a cavity in which the first rotor602can rotate freely proximate to the flange plate604and the middle plate605.

The back plate606engages the middle plate605at the “bend” in its “L” shape. An additional “O”-ring618is provided at the interface between the middle plate605and back plate606to help prevent contaminants such as dirt from reaching the rotors602. In this configuration, the middle plate605, in combination with the back plate606forms a cavity in which the second rotor602can rotate freely proximate to the middle plate605and the back plate606.

The flange plate604, middle plate605and back plate606are held together by bolts (not shown) extending through channels formed by aligned bolt holes (not shown) in each of the flange plate604, middle plate605and back plate606. The bolts extend from a proximal end proximate the back plate606to a distal end proximate the flange plate604. 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 plate604. It will be apparent to those of skill in the art that other suitable means of fastening the flange plate604, middle plate605and back plate606together are possible.

Rotation of the rotors602creates a traveling wave of magnetic field relative to the conductors surrounding the rotors602and induces eddy currents between the conductors and the rotors602, in substantially the same manner as the embodiment shown inFIG. 3. It will be appreciated by those skilled in the art that descent control device500will create approximately twice the amount of braking force as descent control device200, since twice as many rotors and magnets are provided. Actual test results show that descent control device500generates a braking force of approximately 80 lbs when rotating at 800 rpm. This is approximately twice the braking force generated by descent control device200when rotating at 800 rpm, which testing has shown to be approximately 38-40 lbs.

In some implementations, the descent control device500can be interchangeable with the descent control device200. By connecting input shaft600to the driven sheave204in substantially the same manner that input shaft300can be connected to the driven sheave204, and mounting the descent control device500to the mounting plate202(again, in substantially the same manner in which descent control device200is mounted to the mounting plate202), the descent control device500can be used to slow the descent of the enclosure100along its path of descent in the same manner as descent control device200. It should therefore be understood that references in this description to the descent control device200and its components shown inFIG. 3can apply equally to the descent control device500and its components shown inFIG. 6.

In operation, the enclosure100descends along the path defined by at least one cable104. Descent of the enclosure100along cable104causes rotation of at least one driven sheave204which causes rotation of the input shaft300of the descent control device200. Rotation of the input shaft300causes rotation of the rotor302and the magnets316contained in the recesses314of the rotor302. Rotation of the magnets316causes the magnetic field created by the axial polarity of the magnets316to rotate. The rotational movement of the magnetic field relative to the conductor (formed in one embodiment by the flange plate304and back plate306) induces eddy currents in the conductor in a pattern which mirrors that of the magnetic field created by the magnets316. 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 magnets316in the rotor302, against the rotation of the input shaft300, against the rotation of the driven sheave204and against the enclosure100descending along the cable104. Consequently, the enclosure100descends along the path defined by the cable104at a rate controlled by the braking force of the descent control device200.

Because the strength of the eddy currents is proportional to the velocity of the rotor302relative to the stationary conductor, as the rate of descent of the enclosure100increases, the braking force increases. Similarly, decreasing the rate of descent of the enclosure100decreases the braking force. This proportionality produces a smoother deceleration and allows the enclosure100to 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 4×10−8ohm-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 plate306and flange plate304and the conductor could be formed from the rotor302.

Similarly, a descent control device200can be mounted in a braking assembly110by rotationally connecting the input shaft300to the mounting plate202or by affixing the flange plate304to the mounting plate202.

In a further embodiment, the enclosure100includes a means for attachment to a trailer or fork lift to facilitate transportation when detached from cables104and/or the removable attachment of wheels to facilitate repositioning below the initial point.

In some implementations, the descent control device200(or the descent control device500) can be used in many different applications to provide braking force, by rotationally connecting the input shaft300(or600) to the moving component on which braking force is to be applied. By way of example, an enclosure such as enclosure100can 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 shaft300(or600) of a descent control device200(or500). In such a configuration, the rotation of the terminal sheave would drive the rotation of the rotor302(or602), 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.