Abstract:
A braking system for a movable unit which travels along a cable includes a plate of conductive material extending from the cable to define a braking zone having a start and an end along at least a portion of the cable. There is a brake unit movable along the cable and positionable at the start of the braking zone. The brake unit has magnets positionable on opposite sides of the conductive material. The brake unit is engagable by the movable unit when the movable unit reaches the start of the braking zone to couple the two units together. The movable unit acts to push the brake unit through the braking zone such that movement of the magnets of the brake unit relative to the conductive material induces eddy currents in the conductive material to create a braking force between the brake unit and the plate of conductive material to brake the brake unit and the movable unit. In an alternative arrangement, the magnets are installed directly in the movable unit to eliminate the separate brake unit. The braking system provides for reliable, low ‘g’ force, high energy absorption operation in all weather conditions with minimal maintenance.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a braking system for a vehicle travelling on a cable, and more particularly, to a braking system for a recreational cable line ride.  
       BACKGROUND OF THE INVENTION  
       [0002]     Recreational cable line rides are becoming popular in high profile resort areas such as Whistler, British Columbia, Canada. Cable line rides generally involve riders traveling on a carriage or trolley that moves along a cable run suspended between two end points. Often, the cable run extends between two sides of a valley, and the carriage and rider move from a first, higher end point to a second, lower end by gravity. When the carriage and rider reach the lower end of the cable run, it is necessary to brake and stop the carriage so that the rider can safely disembark from the ride.  
         [0003]     Current braking systems for cable line rides tend to rely on friction braking or a buffer system incorporating energy absorbing springs to slow and stop the carriage. Such systems are prone to wear and require rigorous maintenance to ensure safe and reliable operation. Their effectiveness also tends to be adversely affected by weather conditions. Operation in wet or icy conditions renders friction brakes significantly less effective.  
         [0004]     Linear magnetic brake technology is well developed and is currently applied to roller coaster, trolley on fixed tracks, and larger water slide rides to provide deceleration from high speeds. These braking systems are substantially maintenance free. There are no moving parts, and no electrical source required to run the system since the technology relies on permanent magnets and aluminum conductors with no wearing surfaces.  
         [0005]     Linear magnetic brake technology works according to the principle that moving a metal plate such as an aluminum or copper conductor plate in the air gap of a magnet induces current in the metal plate. The current will flow back through the zero-field areas of the metal plate and thus create a closed current eddy loop. A flow of current always means there is a magnetic field as well. Due to Lenz&#39;s law, the magnetic field created by the eddy current reacts against the direction of movement. Instead of mechanical friction, ‘magnetic friction’ is created.  
         [0006]     This technology is also referred to as linear eddy-current brakes in reference to the eddy currents set up in a conductor plate. Linear eddy-current brakes are always the best choice when demands for reliability and safety are highest. These brakes provide a smooth braking action as the braking force builds up continuously when the conductor plate moves relative to the permanent magnets. Braking with permanent magnets works independently of any other system and is free of wear and tear even in severe weather conditions, including lightening strikes, ice, snow, rain and high wind. Typically, these brakes are also corrosion and UV resistant. Governing authorities readily accept magnetic brakes as “fail safe” since the technology has been thoroughly tested and certified in the specific applications in which it has been used commercially to date.  
         [0007]     To date, the technology involved in linear magnetic brakes has not been applied to the braking environment of a cable line system. This represents a major challenge. Current linear magnetic braking applications are typically built into a solid structural framework over which a heavy car on a track carries a conductor plate or fin through the magnets arranged in several sections in a deceleration zone. Alignment of the conductor plates and the magnets is ensured. In the case of suspended cables, any linear magnetic braking system has to accommodate movements in the cable, the slope of the cable and movements due to temperature fluctuations both in the cable and in the conductor plates. This represents a significant problem in ensuring consistent alignment between the permanent magnet associated with one of the carriage to be braked and the cable, and the conductor plate associated with the other of the carriage and the cable to ensure that the magnet and the conductor plate are able to move past each other to generate the desired magnetic braking force.  
       SUMMARY OF THE INVENTION  
       [0008]     The braking system of the present invention has been developed to address the foregoing problems and to adapt the linear magnetic braking system to the new environment of a cable system.  
         [0009]     The present invention provides a reliable, ‘fail safe’ linear magnetic braking system that is adapted for use with a suspended cable system. The present invention provides a smooth, low ‘g’ braking effect in all weather conditions with minimal maintenance.  
         [0010]     Accordingly, the present invention provides a braking system for a movable unit which travels along a cable comprising:  
         [0011]     a plate of conductive material extending from the cable to define a braking zone having a start and an end along at least a portion of the cable;  
         [0012]     a brake unit movable along the cable and positionable at the start of the braking zone, the brake unit having magnets positionable on opposite sides of the conductive material, and the brake unit being engagable by the movable unit when the movable unit reaches the start of the braking zone;  
         [0013]     whereby the movable unit acts to push the brake unit through the braking zone such that movement of the magnets of the brake unit relative to the conductive material induces eddy currents in the conductive material to create a braking force between the brake unit and the plate of conductive material to brake the brake unit and the movable unit.  
         [0014]     In a further aspect, the present invention provides a method for braking a movable unit which travels along a cable comprising:  
         [0015]     providing a plate of conductive material extending from the cable to define a braking zone having a start and an end along at least a portion of the cable;  
         [0016]     positioning a brake unit movable along the cable at the start of the braking zone, the brake unit having magnets positionable on opposite sides of the conductive material;  
         [0017]     engaging the brake unit with the movable unit when the movable unit reaches the start of the braking zone to cause the movable unit to push the brake unit through the braking zone whereupon movement of the magnets of the brake unit relative to the conductive material induces eddy currents in the conductive material to create a braking force between the brake unit and the plate of conductive material to brake the brake unit and the movable unit.  
         [0018]     The present invention relies on a conductor plate mounted underneath the cable to define a braking zone. The conductor plate is formed from a plurality of interconnected segments to accommodate the curvature of the cable. An incoming carriage or trolley carrying a rider contacts and engages a travelling brake unit housing permanent magnets that is positioned at the start of the braking zone. Both the carriage and the brake unit then travel through the braking zone where magnetic braking occurs.  
         [0019]     During magnetic braking, the kinetic energy of the moving carriage coupled with the moving brake unit is converted into thermal energy which is rapidly dissipated from the conductor plate. The carriage and brake unit decelerate while the conductor plate heats up due to induced eddy currents. The braking force is dependent on the entry velocity of the carriage into the braking zone and the material of the conductor plate (i.e. the plate&#39;s specific resistance). Braking force will build up with speed until deceleration reaches a maximum and will then drop off, leaving a residual velocity after the braking zone. A secondary buffer zone at the end of the cable may be provided to bring the carriage to a complete stop The secondary buffer zone may be composed of an array of elastomer damping units in series and co-axial with the cable.  
         [0020]     The braking zone may be as long as 20 metres for higher velocity rides  (15 -18 m/s) and as short as 10 metres for slower rides (8-10 m/s). At the end of the braking zone the velocity of the carriage will be slowed down to 3 m/s. The frequency of incoming carriages is such that the conductor plate would have sufficient time to cool from induced heat. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Aspects of the present invention are illustrated, merely by way of example, in the accompanying drawings in which:  
         [0022]      FIG. 1  is an elevation view of a cable line system incorporating a preferred embodiment of the linear magnetic braking system of the present invention;  
         [0023]      FIG. 1   a  is a detail view of the braking zone of cable system of  FIG. 1 ;  
         [0024]      FIG. 2  is a detail side elevation view of a preferred embodiment of a movable unit or carriage supporting a rider;  
         [0025]      FIG. 2   a  is a front view of the carriage and rider;  
         [0026]      FIG. 3  is a detailed perspective view of the carriage and brake unit according to a preferred embodiment of the invention;  
         [0027]      FIG. 3   a  is a detail side view of the conductive plates that define the braking zone showing the manner in which they are attached to the cable;  
         [0028]      FIG. 3   b  is a section view through the cable and conductor plate taken along line  3   b - 3   b  of  FIG. 3   a;    
         [0029]      FIGS. 4, 4   a  and  4   b  are side elevation, end and plan views, respectively, of a preferred carriage;  
         [0030]      FIGS. 5, 5   a  and  5   b  are side elevation, end and plan views, respectively, of a preferred brake unit;  
         [0031]      FIGS. 6 and 7  are detail perspective view showing the sequence of events as the carriage engages and couples with the brake unit by operating of the coupling device;  
         [0032]      FIGS. 8 and 8   a  are views of a preferred secondary buffer system with a recoil control damper for bringing the carriage and brake unit to a full stop; and  
         [0033]      FIGS. 9 and 9   a  are views of an alternative embodiment of the present invention in which the carriage and brake unit are combined into a single unit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     Referring to  FIGS. 1 and 1   a,  there is shown an exemplary cable line ride system  2  incorporating a preferred embodiment of the magnetic braking system of the present invention. Cable line ride  2  comprises a cable  4  suspended between a first, higher end point  6  and a second, lower end point  8  on opposite sides of a valley  10 . In the illustrated embodiment, first, upper end point  6  is created using applicant&#39;s SYSTEM FOR SUSPENDING STRUCTURES FROM TREES as described in co-pending U.S. patent application No. 10/859,699 filed on Jun. 4, 2004, the disclosure of which is incorporated herein by reference. First upped end point  6  defines a launch platform for the cable line ride. Second lower end point  8  includes a raised structure  12  defining a landing platform  14 . It will be appreciated that alternative structures for anchoring the ends cable  2  are possible in order to suspend the cable in place.  
         [0035]     Movable units in the form of carriages  20  support riders  21  for travel along cable  2  from upper end point  6  to lower endpoint  8  by gravity.  FIGS. 2 and 2   a  show a preferred embodiment of carriage  20  mounted on cable  2 . Rider  21  is suspended below carriage  20  and cable  2  by a harness system  24 . In the illustrated embodiment, harness system  24  supports the rider in substantially a sitting position. Support cables  26  extend downwardly from carriage  20  to define a support structure formed from straps  28  and a spreader bar  30  for gripping by the rider. Alternative arrangements for supporting the rider below cable  2  are possible and will be readily apparent to a person skilled in the art. Carriage  20  illustrated and described in more detail below is only one example of a movable unit suitable for travel along cable  20  that will work with the braking system of the present invention.  
         [0036]     When carriage  20  and rider  21  reach the lower end of cable  2 , it is necessary to brake and slow the carriage so that the rider can safely disembark from the ride at landing platform  14 . This is achieved using the braking system  30  of the present invention.  FIG. 1   a  provides a more detailed view of the lower end  8  of cable  2  including braking system  30  for slowing carriages  20  travelling along cable  2 . Preferably, lower end point  8  of cable  2  includes an upwardly sloping section to assist in slowing of the carriage, but this is not necessary with the present braking system.  
         [0037]     Braking system  30  includes a plate  32  of conductive material extending from cable  2  to define a braking zone  34  at the lower end of the cable having a start  36  and an end  38 . The plate of conductive material defining the braking zone may be as long as 20 metres for higher velocity rides (15-18 m/s) and as short as 10 metres for slower rides (8-10 m/s). Preferably, plate  32  of conductive material is formed from aluminum which has good cooling characteristics and is flexible to accommodate movement of the cable, however, it is understood that other conductive material may be used.  
         [0038]     A brake unit  40  movable along the cable, and positionable at start  36  of the braking zone is also provided. As will be discussed in more detail below, brake unit  40  includes magnets positionable on opposite sides of plate  32 . The brake unit is engagable by a carriage  20  as the carriage descends along cable  2  and reaches start  36  of braking zone  34 . Carriage  20  acts to push brake unit  40  through the braking zone  34  such that movement of the magnets of the brake unit relative to the stationary conductive material of plate  32  induces eddy currents in the conductive material with the result that a braking force acting on brake unit  40  is created. As brake unit  40  slows down due to braking, following carriage  20  is also slowed down.  
         [0039]     FIGS.  3  to  3   b  provide detail views of preferred embodiments of the conductive plate  32 , carriage  20 , and brake unit  40  of the braking system.  
         [0040]     Turning first to  FIGS. 3   a  and  3   b,  conductive plate  32  is preferably formed from a continuous plate mounted to the cable at a plurality of spaced, connection points  42  along the length of the braking zone to accommodate flexing of the cable. The continuous plate  32  includes a channel member  44  along an upper edge  46  to receive cable  2 . Each of the plurality of connection points  42  comprises an opening  48  through plate  32  adjacent channel member  44 , and a band  50  looped over the cable, under the channel member and through the opening to connect plate  32  to the cable as best shown in section view  3   b.  Plate  32  is formed with a slit  52  extending from a lower edge  54  upwardly to each opening  48  of the plurality of connection points  42  to define interconnected plate segments  32   a  joined along upper edge  46  of the continuous plate. Plate segments  32   a  are free to separate from each other along each slit  52  to permit flexing of the continuous plate with the cable. Preferably, three is a clip  56  overlapping each slit  52  at lower edge  54  of the continuous plate to maintain alignment of the interconnected plate segments in the plane of the cable.  
         [0041]      FIG. 3  shows carriage  20  and brake unit  40  on cable  2  just prior to carriage  20  engaging brake unit  40  at the start of the braking zone. For clarity of the drawings, note that  FIG. 3  does not show conductive plate  32  attached to cable  2 .  
         [0042]     Brake unit  40  comprises a generally cylindrical body  60  which rotatably supports at least one roller  62 . In the illustrated embodiment, a pair of spaced rollers  62  are shown. Each roller is a conventional unit with an internal hub fitted onto axle  64  extending transversely to the body of the brake unit. The tread surface  66  of each roller  62  is preferably formed from a hard elastomer such as urethane of 90 durometer hardness, however, it will be understood that other suitable materials of different hardness can be used. Tread surface  66  is concave and dimensioned to receive and run along the upper surface of cable  2  in order to movably support body  60  on the cable.  
         [0043]      FIGS. 5-5   b  provide additional views of brake unit  40 . As best shown in  FIG. 5   a,  which is an end view of the brake unit, cylindrical body  60  includes a downwardly opening central channel  68  aligned with rollers  62  to permit body  60  to straddle the cable and attached conductive plate  32 . Cylindrical body  60  also includes a lower magnet housing  70  for mounting of permanent magnets  72  on opposite sides of central channel  68  to position the magnets on opposite sides of plate  32 . Preferably, the magnets are rare earth magnets which offer good magnetic strength for their size and are resistant to demagnetization.  
         [0044]     Within central channel  68 , pairs of alignment rollers  74  extend inwardly from opposite sides to engage plate  32 .. Alignment rollers  74  maintain the central channel  68  substantially centred about cable  2  and plate  32 .  
         [0045]     Referring to  FIGS. 3, 4 ,  4   a  and  4   b,  carriage  20  is also preferably formed as a generally cylindrical body  80  rotatably supporting at least one roller  82 . In the illustrated embodiment, a pair of spaced rollers  82  are employed. Each roller  82  is a conventional unit with an internal hub fitted onto axle  84  extending transversely to the body of the carriage and with a concave tread surface  86  of hard urethane to movably support body  80  on the cable. As best shown in the end view of  FIG. 4   a,  cylindrical body  80  is formed with a downwardly opening central channel  88  aligned with rollers  82  to permit body  80  to straddle and travel along cable  2 . As best shown in  FIG. 4 , lines  24   a  of harness system  24  for supporting a rider are preferably looped over and through cylindrical body  80  to extend downwardly on opposite sides to anchor the harness system to the body of carriage  20 . At each end of body  80 , pairs of flared housing plates  90  extend downwardly and diverge. Plates  90  provide mounting points for cable clamps  88  to secure lines  24   a,  and act to keep the lines away from cable  2 . Cable guide blocks  92  are mounted to the internal surface of plates  90  and act to centre body  80  on cable  2 . Access windows  94  are formed through cylindrical body  80  to permit adjustment of guide blocks  92 .  
         [0046]     When carriage  20  approaches the braking zone after descending along cable  2  and initially contacts brake unit  40  to begin the braking process, it is preferable that the carriage and the brake unit are releasably coupled together to prevent carriage  20  from repeatedly striking and rebounding from brake unit  40  as they travel through the braking zone. To achieve this, brake unit  40  preferably includes a coupling device  100  to permit releasable coupling of carriage  20  to the brake unit on initial contact between the two.  
         [0047]      FIGS. 3, 6  and  7  show various aspects of a preferred coupling device  100  and its operation. In  FIGS. 6 and 7 , portions of the cylindrical body of the brake unit  40  have been removed for clarity.  
         [0048]     Initially,  FIG. 3  shows carriage  20  approaching brake unit  40  at the start of the braking zone. Coupling device  100  is positioned at the end of brake unit  40  facing carriage  20 . The coupling device comprises a docking cavity  102  to receive an end of carriage  20 , and at least one coupling hook  104  to engage and hold the end of the carriage within the docking cavity. At the same time, carriage  20  is formed with at least one end having at least one latching site for coupling hook  104 . In the illustrated embodiment, there are a pair of coupling hooks  104  to engage a pair of latching sites  106  which include openings  106   a  through flared housing plates  90 .  
         [0049]      FIG. 6  shows carriage  20  just as it makes contact with brake unit  40 . The cylindrical outer body of the brake unit is not show to provide an unobstructed view of docking cavity  102  and coupling hooks  104 . Docking cavity  102  is a depression shaped to receive the protruding end of carriage  20 . Cavity  102  is formed in a movable block  108  which is slidably mounted within the body of the brake unit. Block  108  is shown partially sectioned in  FIG. 6  and includes a lower slot  110  to accommodate cable  2 . On opposite sides of block  108 , coupling hooks  104  are positioned for movement between a default engaged position to hold and retain the end of carriage  20 , and a released position to permit disengagement of the hooks from latching sites  106 . Spring  112  extending between anchor posts  114  on each hook and through passage  118  in block  108  to bias the hooks toward each other and into the default latched position. Hooks  104  pivot about elongate pins  120 . Pins  120  are slidably retained at their upper and lower ends in slots  122  formed in the cylindrical body of the brake unit to guide movement of block  108  (see  FIG. 3 ). As carriage  20  initially contacts brake unit  40 , latching sites  106  on plates  90  force hooks  104  apart to the released position against the biasing force of spring  118  to allow the end of carriage  20  to move into docking cavity  102 .  FIG. 7  shows latching sites  106  fully engaged in docking cavity  102 . After latching sites  106  are fully engaged in the docking cavity, spring  118  is able to bias hooks  104  back into the default latched position such that the hooks  104  are engaged in latching sites openings  106   a  to couple the carriage and brake unit together. Block  108  is adapted to absorb the initial impact of the engagement of the latching sites  106  with docking cavity  102  by moving in the direction indicated by arrow  124  in  FIG. 7 . This slidable movement is guided and accommodated by pins  120  moving in slots  122  in the outer cylindrical body. At the end of the travel of block  108 , impact absorbing elements  126  associated with the brake unit act to further absorb the impact of the carriage engaging with the brake unit. Preferably, impact absorbing elements  126  comprise at least one deformable ring member which resiliently deforms when contacted by block  108  as the block slides in the direction of arrow  124 . Elements  126  are mounted to an internal wall  127  of brake unit  40 . Spring  118  extending through block  108  between posts  112  tends to bias block  108  forwardly in the opposite direction to arrow  124  to ensure that block  108  and docking cavity  102  are properly positioned to receive the end of a carriage  20 . Note that the coupling device  100  is arranged such that latching hooks  104  are engaged with latching openings  106   a  prior to block  108  sliding into contact with impact absorbing elements  126  to ensure that the hooks hold and retain the end of the carriage during any impact of block  108  with elements  126 .  
         [0050]     Latching hooks  104  are formed with tabs  130  that protrude through slots  132  formed in the cylindrical body of brake unit  40  as best shown in  FIG. 3 . Tabs  130  are manipulated by a rider or operator to pivot hooks  104  out of latching site openings  106   a  to permit disengagement of carriage  20  from brake unit  40  after the brake unit has performed its braking function. The brake unit  40  is then returned to its starting position at the start  36  of braking zone  34  ( FIG. 1   a ) to engage and stop the next carriage  20  and rider  21  travelling down cable  2 . Movement of brake unit  40  to the start  36  of braking zone  34  can be accomplished in several ways. For example, as shown in  FIG. 5 , brake unit  40  can be equipped with a motor  150  to drive rollers  62  along cable  2  to the start of the braking zone. Motor  150  is preferably battery powered and under radio control to allow an operator to readily control the position of the brake unit. Alternatively, a close line type system for manually moving brake unit  40  to the beginning of the braking zone can be used. If the braking zone is readily accessible to the operator based on the geometry of the cable at the braking zone, it may simply be a matter of the operator moving the brake unit manually pushing it to the start of the braking zone.  
         [0051]     The nature of the braking forces generated in the linear magnetic braking system of the present invention mean that the carriage and rider are not brought to a complete stop at the end of braking zone  34 . The braking system does substantially reduce the speed of the carriage along the cable, for example, from a speed of 18 m/s at the beginning of the braking zone to a speed of 3 m/s at the end of the zone. Depending on the configuration and dimensions of the cable and landing platform  14 , this lower speed may permit a rider to slow themselves to a complete stop by standing up in the harness and putting their feet on the landing platform (see  FIG. 1 ). To further assist in stopping carriage  20  and the rider  21 , a buffer section  160  may be installed on cable  2  after braking zone  34 . Preferably, buffer section  160  comprises an array of elastomer damping units in series and co-axial with the cable adapted to absorb and cushion movement of the brake unit and carriage and bring them to a complete stop.  FIGS. 8 and 8   a  show buffer section  160  in detail. Elastomer damping units  161  comprises a pair of generally disc shaped bodies  162  having a central hole  164  to permit passage of cable  2  through the bodies. An associated axially aligned spring  163  extends between the pair of disc bodies  162  which are formed with an annular flange to receive and retain the ends of the spring. Damping units  161  are strung together end to end in series to define buffer section  160 . Typically, up to  15  damping units would be positioned on a cable to define a buffer section. Each disc body  162  is preferably formed from an ultrahigh molecular weight (UHMW) plastic and serves to align and centre the springs over cable  2 . When a carriage and brake unit impact the exposed end  166  of the buffer section, springs  163  are compressed and this deformation of the springs absorbs the momentum of the carriage and brake unit. To avoid the springs recoiling and sending the carriage and brake unit back along the cable, a recoil control device can also be incorporated into buffer section  160 . In a preferred arrangement, recoil control device  170  comprises a pair of control cables  172  extending through each disc body  162  parallel to and on opposite sides of cable  2  as best shown in  FIG. 8   a,  which is a section view through the buffer section taken along line  8   a - 8   a  of  FIG. 8 . Control cables  172  are wound onto the drum  174  of a hydraulic damper unit  176  with a sprague clutch. The drum  174  of hydraulic damper unit  176  operates to take up slack in the control cables on compression of the springs to prevent recoil of the springs. Control cables  172  may be connected to counter-weights to maintain the control cables taut.  
         [0052]      FIGS. 9 and 9   a  shows a further embodiment of the present invention in which the carriage  20  and brake unit  40  are combined into a single unit  200 . In this alternative arrangement, a plate of conductive material  32  suspended below cable  2  still defines the braking zone  34 , however, there is no longer a separate braking unit  40 . Instead, as best shown in  FIG. 9   a,  carriage  200  is fitted with permanent magnets  72  for positioning on opposite sides of the plate of conductive material  32  when the carriage reaches the start of the braking zone. As in the previous embodiment, movement of the magnets of the carriage relative to the conductive material induces eddy currents in the conductive material to create a braking force between the carriage and the plate of conductive material to brake the movable unit.  
         [0053]     This arrangement eliminates the need to circulate carriages from the end of the ride to the beginning for the next rider as each cable has it&#39;s own captive carriage running back and forth on the cable. Referring to  FIG. 9   a,  the return of carriage  200  along cable  2  to the start of the run is preferably performed by a DC electric drive  202  incorporating a gear box and clutch powered by a solar charged battery system. The battery system is charged through solar panel  204 . The drive  202  only operates to move the carriage uphill along the cable. As in the previous embodiment, movement of the carriage down the cable is by gravity. In other words, carriage  200  moves down the cable carrying a rider under gravity and returns up the cable empty using drive  202 . A radio control unit may be incorporated in drive  202  to allow the operator to move carriage  200  under remote control back to the start of the cable.  
         [0054]     The alternative arrangement shown in  FIGS. 9 and 9   a  will tend to yield higher speed rides because the carriage is heavier with its additional magnet and drive components. Such a carriage would be advantageous for lighter riders. Another advantage of this alternative system is that it eliminates the impact of the carriage on the brake unit. This system is less expensive overall because there will tend to be fewer carriages in the system, and a separate brake unit is not required.  
         [0055]     Although the present invention has been described in some detail by way of example for purposes of clarity and understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims.