BRAKING MECHANISMS

Abstract

An eddy-current braking mechanism including a rotor, rotatable about a rotor axis; at least one electrically conductive member coupled to the rotor for rotation therewith; at least one magnet configured to apply a magnetic field extending at least partially orthogonal to the plane of rotation of the conductive member, and characterised in that upon rotation of the rotor, the conductive member is configured to move at least partially radially from the rotor axis into the applied magnetic field.

Claims

1-36. (canceled)

37. An eddy-current braking mechanism comprising: a rotor, rotatable about a rotor axis; at least one arm coupled with the rotor for rotation therewith; at least one magnet coupled with the at least one arm; and at least one electrically conductive member positioned circumferentially about the rotor; wherein the at least one arm and the at least one magnet coupled therewith move relative to the rotor axis when the rotor rotates around the rotor axis; wherein upon rotation of the rotor, the at least one magnet moves radially such that at least a portion of the at least one magnet overlaps the at least one electrically conductive member in an axial direction; and wherein a biasing device is attached to the at least one arm and to the rotor to provide a bias opposing the outward or inward radial movement of the at least one arm.

38. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one arm is coupled directly to the rotor and the at least one magnet is coupled directly to the at least one arm.

39. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one arm is pivotally coupled to the rotor and pivots about a pivot axis.

40. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one arm has a center of mass on or eccentric to the pivot and rotor axes.

41. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one arm is pivotally attached to the rotor at a point eccentric to the rotor axis.

42. The eddy-current braking mechanism as claimed in claim 37, wherein the biasing device is attached to the rotor at a point eccentric to the pivot axis.

43. The eddy-current braking mechanism as claimed in claim 37, wherein a stop is provided for limiting the range of movement of the at least one arm.

44. The eddy-current braking mechanism as claimed in claim 43, wherein the stop is positioned at a point of maximum overlap between the at least one magnet and the at least one electrically conductive member.

45. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one electrically conductive member is arranged in two generally circular shapes or arrays, one on each opposing side of the plane of rotation of the at least one magnet.

46. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one electrically conductive member shape or array is provided on one side of the rotor.

47. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one magnet does not overlap the at least one electrically conductive member in a rotor axis direction when the rotor is stationary.

48. The eddy-current braking mechanism as claimed in claim 37, wherein the at least one magnet overlaps at least a portion of the at least one electrically conductive member in a rotor axis direction when the rotor is stationary.

49. The eddy-current braking mechanism as claimed in claim 37, wherein the biasing device is attached to the at least one arm at a point distal to the rotor axis and to the rotor at a position to provide a bias opposing the at least one arm movement resulting from rotor rotation.

50. The eddy-current braking mechanism as claimed in claim 37, wherein the biasing device is attached to the at least one arm and at least one coupled magnet at a point distal to the rotor axis and the rotor at a position to bias the at least one arm against the direction of movement of the at least one arm resulting from rotor rotation.

51. The eddy-current braking mechanism as claimed in claim 37 wherein the mechanism comprises at least two arms and the arms move independently.

52. The eddy-current braking mechanism as claimed in claim 37 wherein the mechanism includes at least two arms, each arm having the at least one magnet coupled therewith and wherein the at least two arms nest together when the rotor is stationary.

53. The eddy-current braking mechanism as claimed in claim 37 wherein the mechanism includes at least two arms, each arm having the at least one magnet coupled therewith and wherein the rear of each arm abuts with the front of the next arm when the rotor is stationary.

54. The eddy-current braking mechanism as claimed in claim 37 wherein the at least one arm is T-shaped.

55. The eddy-current braking mechanism as claimed in claim 37 wherein the at least one arm has or have an at least partially arc shaped cross-section shape.

56. The eddy-current braking mechanism as claimed in claim 37 wherein the at least one arm has or have a unidirectional configuration with braking torque only applied in one rotation direction and not the opposing direction.

57. The eddy-current braking mechanism as claimed in claim 37 wherein the rotor is a wheel.

58. An eddy-current braking mechanism including: a rotor, rotatable about a rotor axis; at least two arms coupled with the rotor for rotation therewith; at least one magnet coupled with each of the at least two arms; and at least one electrically conductive member positioned circumferentially about the rotor; wherein the at least two arms and the at least one magnet coupled therewith pivot about a pivot axis or axes upon rotation of the rotor; wherein, upon rotation of the rotor, the at least two arms and the at least one magnet coupled therewith move at least partially away from the rotor axis via the pivot axis or axes such that at least a portion of the magnets overlap the at least one electrically conductive member resulting in application of a frictionless braking force to the magnets and the at least two arms as the magnets overlap an increasing portion of the at least one electrically conductive member thereby providing a controlled speed of rotation of the rotor over an operating range of applied torques by balancing an increase in an applied torque with an equal and opposite increase in a braking torque arising from an induced eddy-current; and wherein the at least two arms nest together when the rotor is stationary.

59. An eddy-current braking mechanism including: a rotor, rotatable about a rotor axis; at least two arms coupled with the rotor for rotation therewith; at least one magnet coupled with each of the at least two arms; and at least one electrically conductive member positioned circumferentially about the rotor; wherein the at least two arms and the at least one magnet coupled therewith pivot about a pivot axis or axes upon rotation of the rotor; wherein, upon rotation of the rotor, the at least two arms and the at least one magnet coupled therewith move at least partially away from the rotor axis via the pivot axis or axes such that at least a portion of the magnets overlap the at least one electrically conductive member resulting in application of a frictionless braking force to the magnets and the at least two arms as the magnets overlap an increasing portion of the at least one electrically conductive member thereby providing a controlled speed of rotation of the rotor over an operating range of applied torques by balancing an increase in an applied torque with an equal and opposite increase in a braking torque arising from an induced eddy-current; and wherein the rear of each arm abuts with the front of the next arm when the rotor is stationary.

60. An eddy-current braking mechanism including: a rotor, rotatable about a rotor axis; at least one T-shaped arm coupled with the rotor for rotation therewith; at least one magnet coupled with the at least one T-shaped arm; and at least one electrically conductive member positioned circumferentially about the rotor; wherein the at least one T-shaped arm and the at least one magnet coupled therewith pivot about a pivot axis or axes upon rotation of the rotor; wherein, upon rotation of the rotor, the at least one T-shaped arm and the at least one magnet coupled therewith move at least partially away from the rotor axis via the pivot axis or axes such that at least a portion of the at least one magnet overlaps the at least one electrically conductive member resulting in application of a frictionless braking force to the at least one magnet and the at least one T-shaped arm as the at least one magnet overlaps an increasing portion of the at least one electrically conductive member thereby providing a controlled speed of rotation of the rotor over an operating range of applied torques by balancing an increase in an applied torque with an equal and opposite increase in a braking torque arising from an induced eddy-current.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0156] Further aspects and advantages of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which:

[0157] FIG. 1 shows a plot of Torque vs. Speed for an exemplary prior art eddy-current braking mechanism;

[0158] FIG. 2A shows a schematic plan diagram of an eddy current braking mechanism according to one preferred embodiment of the present invention, the rotor being stationary;

[0159] FIG. 2B shows a schematic plan diagram of the eddy current braking mechanism of FIG. 2A with the rotor rotating under an intermediate braking torque;

[0160] FIG. 2C shows a schematic plan diagram of the eddy current braking mechanism of FIGS. 2A and 2B with the rotor rotating under a maximum braking torque;

[0161] FIG. 3 shows a schematic side elevation of part of the eddy current braking mechanism of FIG. 2;

[0162] FIG. 4 shows a schematic side elevation of part of an alternative configuration to the eddy-current braking mechanism shown in FIGS. 2 and 3;

[0163] FIG. 5A shows a force diagram of the eddy-current braking mechanism shown in FIGS. 2 and 3 when a torque is initially applied to the rotor, i.e. at a ‘start-up’;

[0164] FIG. 5B shows a force diagram of the eddy-current braking mechanism shown in FIGS. 2 and 3 when the applied torque is increasing;

[0165] FIG. 5C shows a force diagram of the eddy-current braking mechanism shown in FIGS. 2 and 3 when a constant torque applied is matched by the braking torque, i.e. at ‘steady-state’;

[0166] FIG. 5D shows a force diagram of the eddy-current braking mechanism shown in FIGS. 2 and 3 at maximum braking torque;

[0167] FIG. 5E shows a force diagram of the eddy-current braking mechanism shown in FIGS. 2 and 3 when the applied torque is decreasing;

[0168] FIG. 6 shows a plot of Torque vs. Speed of the rotor used with the braking mechanism of FIGS. 2-3 and 5;

[0169] FIG. 7 shows a plot of Speed vs. Torque of the rotor used with the braking mechanism of FIGS. 2-3 and 5;

[0170] FIG. 8 shows a plot of Speed vs. Torque of an alternative configuration of the braking mechanism of FIGS. 2-3 and 5;

[0171] FIG. 9 shows yet another plot of Speed vs. Torque of an alternative configuration of the braking mechanism of FIGS. 2-3 and 5;

[0172] FIG. 10A shows a schematic plan diagram of an eddy current braking mechanism according to a second preferred embodiment of the present invention;

[0173] FIG. 10B shows an enlarged view of part of the braking mechanism shown in FIG. 10A;

[0174] FIG. 11 shows a schematic illustration of an eddy current braking mechanism according to another embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0175] FIG. 1 shows a plot of Torque vs. Speed for an exemplary prior art eddy-current braking mechanism that utilises a conductive disc configured to rotate in a magnetic field. Eddy-currents are induced in the disc when the disc rotates and a reactive magnetic field is generated opposing the applied magnetic field. The opposing magnetic fields create a reactive force opposing movement of the disc through the magnetic field.

[0176] The magnitude of the braking torque applied to the disc is dependant on the magnetic field strength and the speed of rotation, thus as speed increases, the braking torque also increases. This system will limit the speed to a certain level depending on the applied torque. However, the braking torque and therefore equilibrium speed are only linearly proportional to the speed within a predetermined operating range (as shown in FIG. 1), until a threshold ‘characteristic speed’ (S) is reached where the braking torque becomes non-linear and peaks before beginning to reduce with further speed increases.

[0177] The prior art systems are thus only effective at regulating the speed with a linear response to the applied torque until the characteristic speed is reached. Thus, these prior art systems are unsuitable for auto-belay and other applications where it may be desirable to maintain a constant speed over a wider range of applied torques.

[0178] FIGS. 2A-2C, 3 and 5A-5E show an eddy-current braking mechanism according to one preferred embodiment of the present invention as generally indicated by arrow 1. For clarity, in FIGS. 5A-5E only one conductive member 3 is shown attached to the rotor 2.

[0179] The braking mechanism 1 is coupled to a spool of line (not shown) forming part of an auto-belay device (not shown). The spool of line is connected to a rotor 2 of the braking mechanism 1 and will thus rotate with the rotor 2. A line 23 extends from the spool to a harness of a user. The rotor 2 has a biased retracting mechanism (not shown) for opposing the extension of line 23 from the spool and for automatically retracting the line 23 when the line tension (and applied torque) is reduced, e.g. when a user is ascending while climbing.

[0180] The rate of line dispensing from the spool can thus be regulated by controlling the speed of rotation of the rotor 2 with the braking mechanism 1.

[0181] The braking mechanism 1 includes the rotor 2, rotatable about a rotor axis X and three electrically conductive members provided in the form of pivoting arms 3 coupled to the rotor 2. The arms 3 are pivotally attached to the rotor 2 at points 8 eccentric to the rotor axis X.

[0182] A plurality of magnets 4 are provided and fixed in position relative to the rotor axis X. The magnets 4 form two circular arrays 5 (only one shown in FIG. 2) on opposing sides of the plane of rotation of the arms 3 and rotor 2.

[0183] FIG. 3 shows the magnets 4 positioned either side of the plane of rotation of the arms 3.

[0184] Each magnet array 5 is arranged coaxially with the rotor 2 and applies a magnetic field 6 extending orthogonal to the plane of rotation of the arms 3.

[0185] The magnets 4 of the two magnet arrays 5 have opposite poles substantially opposing each other. Thus, a magnetic field 6 is created that extends between the opposing poles (North opposing South) of opposing magnets 4, in a direction orthogonal to the plane of rotation of the rotor 2 and arms 3.

[0186] Steel or other ferromagnetic backing 7 (shown in FIG. 3) is attached to the outer surface of each magnet array 5 on an opposing side to the arms 3. This steel backing 7 helps reinforce the magnetic field 6 as well as potentially protecting the magnets 4 from impact damage.

[0187] An alternative configuration is shown in FIG. 4 where only one magnet array 5 has steel backing 7.

[0188] As shown in the progression from FIG. 2A to FIG. 2C, upon a tangential force F.sub.App being applied to the rotor 2 (e.g. from a climber descending), the rotor 2 will rotate and the arms 3 will pivot about pivot points 8. As the applied force F.sub.App accelerates the rotor 2, the arms 3 will move into, and intersect the applied magnetic field 6. Any movement of the arms 3 through the applied magnetic field 6 (e.g. when rotating) induces eddy-currents in the arms 3 which in turn generate reactive magnetic fields opposing the applied magnetic field 6.

[0189] The arms 3 have an arc-shaped outer edge 10 matching the profile of the magnet array 5 so that the maximum area of field 6 is intersected while also minimising size and weight of the arms 3. The arms 3 are shaped to nest together when the rotor is stationary, i.e. the ‘rear’ of each conductive member 3 is shaped to abut with the ‘front’ of the next conductive member 3. It will be appreciated that reference herein to the rotor being “stationary” refers to the rotor not rotating or moving relative to the magnetic field 6.

[0190] As the arms 3 pivot about the pivot points 8, a progressively greater part of each arm 3 moves into and intersects the magnetic field 6. The arms 3 are also shaped so that in the contracted position shown in FIG. 2A, the arms 3 fit together to occupy the minimal amount of space possible, thereby minimising the size requirements of the braking mechanism 1 while maximising the potential braking torque when in the magnetic field 6 as shown in FIGS. 2B and 2C.

[0191] Biasing devices are provided in the form of springs 12 attached to the arms 3 at points 13 distal to the pivot axis 8 and to the rotor 2 at a position 14 spaced from the pivot axis 8 in the direction of rotation R to be braked, i.e. shown as clockwise in FIG. 2. The springs 12 thereby provide a bias opposing the pivoting (and thereby radial) movement of the arms 3. The strength of the springs 12 can be changed to control the movement of the arms 3 toward the magnetic field 6 and therefore the characteristics of the braking mechanism 1.

[0192] The pivoting range of the arms 3 is constrained by the springs 12 to one sector, thereby ensuring that the arms 3 will only move into the magnetic field 6 when rotating in one direction. Such a ‘unidirectional’ configuration is useful in auto-belay applications where it is undesirable to have a braking effect on the line 23 when ascending, as this may oppose the line retraction mechanism and potentially create slack in the line 23.

[0193] Safety stops (not shown) are attached to the rotor 2 and engage with the arms 3 to limit the range of arms 3 pivoting movement. The stops are formed by a sliding engagement between a protrusion (not shown) attached to the arms 3 and a rigid slot (not shown) that is fixed to the rotor 2. The protrusion is free to slide in the slot but is limited by the extent of the slot which limits the range of movement of the arms 3. The stops (not shown) thus provide a ‘safety’ feature to ensure that if the spring 12 breaks, detaches or otherwise fails, the arms 3 will still apply a braking torque (preferably maximum) to the rotor 2. The stop also assists in transferring braking torque to the rotor 2 when the protrusion reaches the extent of the slot The arms 3 are mounted eccentrically to the rotor axis X such that each arm 3 has a center of mass 9 eccentric to the pivot 8 and rotor axes X such that when the rotor 2 rotates, the arms 3 will move radially outward and pivot the arms 3 about the pivot point 8.

[0194] In an auto-belay application, tension is placed on the line 23 wrapped about the rotor 2 or connected spool by a load (e.g. a human) and thereby applies a torque (T.sub.App=F.sub.App×r) on the rotor 2 to cause rotation.

[0195] The applied magnetic field 6 induces eddy-currents in the arm 3 and a reactive magnetic field is generated that opposes the applied magnetic field 6. The repelling force between the applied and reactive magnetic fields thus provides a reactive force F.sub.EDDY opposing the movement of the arms 3 through the magnetic field 6. FIGS. 5A to 5E are partial schematic diagrams showing the forces acting on each arm 3. For clarity, only one arm 3 is shown in FIGS. 5A to 5E.

[0196] It will be appreciated that the force diagrams of FIGS. 5A-5E do not show an accurate detailed analysis of the many and varied dynamic forces acting on the arm 3 and thus the forces shown are simplistic and indicative only. The force diagrams 5A-5E, are provided to show a simplified example of the primary forces acting on the arm. Each diagram 5A-5E includes a box with the main forces added to show the approximate net force at the center of mass 9. It should be appreciated that these forces are indicative only and the force lines may not be of accurate length or direction.

[0197] FIG. 5A shows a force diagram of the eddy-current braking mechanism 1 in an initial ‘start-up’ stage where there is only a tangentially applied force F.sub.App and the spring 12 is not extended. As this force F.sub.App is applied tangentially to the rotor, a torque T.sub.App is applied to the rotor and it will accelerate from rest. Components (F.sub.App (8) and F.sub.App (13)) of this force F.sub.App are respectively applied to the arm 3 via the pivot point 8 and spring connection 13.

[0198] It should be appreciated that in another configuration, an arm(s) may be shaped and positioned such that in the start-up phase at least a portion of the arm intersects the magnetic field. An eddy-current braking effect will thus be applied as soon as the rotor starts to rotate.

[0199] Also, as the arm 3 is connected to the rotor 2, when rotating it will accelerate toward the rotor axis X under centripetal acceleration. The centripetal force is applied to the body via the connections 8, 13. F.sub.cp is the force exerted by the mass centroid 9 on the arm 3 resisting the centripetal acceleration of the arm 3.

[0200] The arm 3 also has an inertia resisting changes in movement. For the purposes of this analysis this inertia will relate to the arm mass and moment of inertia acting about the mass centroid 9.

[0201] The forces shown in FIGS. 5A-5E are detailed in the following table with approximate formulae. It will be appreciated that these formulae and forces are approximate and indicative only.

TABLE-US-00001 Force Symbol Indicative relationship formula Applied Force F.sub.App Force applied by tension on line 23. Applied Force through F.sub.APP (8) Component of F.sub.App acting through pivot point 8 pivot point 8. Approx equal to T.sub.App − F.sub.App(13)R.sub.2 R.sub.1 where R.sub.1 is the distance of the pivot point 8 from the rotor axis X, and R.sub.2 is the perpendicular distance of the spring 12 from the rotor axis X. Applied Force through F.sub.App (13) Component of F.sub.App acting through connection 13 connection 13. Approx equal to F.sub.s. Applied Torque T.sub.App Approx equal to F.sub.App x r where r is the radius of the rotor 2 to the line 23. Applied Force through F.sub.R (8) A resultant force from the pivot 8 combination of F.sub.App(8), F.sub.App(13), F.sub.cp and F.sub.EDDY acting on the rotor via pivot 8 Resultant Force F.sub.R A resultant of the force vectors F.sub.App(13), F.sub.App(8), F.sub.cp and F.sub.EDDY acting at the arm 3 mass centroid 9. Resultant Moment M.sub.R A resultant moment acting about the arm 3 mass centroid 9 due to the of the force vectors F.sub.App(13), F.sub.App(8), F.sub.cp and F.sub.EDDY and their respective lever arms. Braking force caused F.sub.EDDY Braking force caused by eddy- by eddy-current current reactive magnetic field braking effect. interacting with applied magnetic field 6. Approx proportional to area of magnetic field 6 intersected by the arms 3; strength of the magnetic field 6 intersecting the arms 3; resistivity of the arms 3; and relative velocity of the arms 3 with respect to the magnetic field 6. Braking torque T.sub.EDDY Torque applied to the conductive caused by eddy- arm 3 by the braking force F.sub.EDDY. current braking effect Approx equal to the F.sub.EDDY x l where l is the perpendicular distance from the line of action of the force F.sub.EDDY to the pivot point 8, i.e. R.sub.3 in the drawings. Spring Bias force F.sub.s Approx equal to kx + c where k is the spring constant, x is the extension from equilibrium and c is the spring pre-tension. Braking torque on T.sub.B Proportional to the components of rotor the braking force F.sub.EDDY acting through the pivot point 8 and connection 14. Centrifugal force F.sub.cp Approx equal to the mass of the acting on the arm 3 arm 3 at the mass centroid 9 mass centroid 9 multiplied by v.sup.2/R.sub.1 where v is the tangential velocity of the rotor at the pivot point X.

[0202] In the start-up state shown in FIG. 5A, the combined force F.sub.R of the forces F.sub.App (8) and F.sub.App (I3) act approximately on the mass centroid 9 of the arm 3. The offset of the forces F.sub.App (8) and F.sub.App (13) from the mass centroid 9, R.sub.FApp(8) and R.sub.FApp(13) respectively, provide a moment M.sub.R about the mass centroid 9 of the arm 3. The force F.sub.R therefore acts to accelerate the arm radially outwards and the moment M.sub.R acts to rotationally accelerate the arm in the same rotational direction as the rotor 2.

[0203] The arm 3 is constrained at the pivot point 8 but is not rigidly fixed at connection 13.

[0204] As the arm 3 is accelerated outward by the resultant force F.sub.R, the arm rotates about pivot point 8 in an anticlockwise direction with respect to the rotor 2 and the spring 12 is extended thus increasing the spring bias F.sub.s and the applied force F.sub.App(I3). With an increase in F.sub.s a larger proportion of T.sub.App is transferred to the arm 3 via the spring 12 and the force F.sub.App (8) applied through the pivot 8 is reduced. The resultant force F.sub.R acting on the mass centroid 9 changes the direction in a clockwise motion.

[0205] As the applied direction of F.sub.R moves forward of a radial line from the axis X to the mass centroid 9, the force accelerates the arm 3 in a clockwise direction.

[0206] FIG. 5B shows the applied force F F.sub.App accelerating the rotor 2 and attached arm 3. The arm 3 is pivoted at a greater angular displacement than that shown in FIG. 5A and now intersects the magnetic field 6. The rotor 2 and arm 3 have gained angular velocity about the rotor axis X and the arm 3 is accelerated towards the rotor axis X under centripetal acceleration. The mass centroid 9 applies a centrifugal force F.sub.cp to the arm 3.

[0207] In addition to the rotary forces, the eddy-current braking force F.sub.EDDY is also applied as the arm 3 is moving through the magnetic field.

[0208] The resultant force F.sub.R of the forces F.sub.App (8), F.sub.App (13), F.sub.cp1, and F.sub.EDDY act on the mass centroid 8 to accelerate the arm 3 further outward from the rotor axis X. The resulting anticlockwise rotation of the arm 3 about the pivot 8 increases the distance between connection 13 and connection 14 thereby extending the spring 12. The extension of the spring 12 increases the spring bias F.sub.s and correspondently increases F.sub.App (13) applied to connection 13.

[0209] The rotor 2 will continue to accelerate and the arm 3 will continue to pivot anticlockwise until the force F.sub.App (I3) applied by the spring 12 on the arm 3 is sufficiently large to balance the forces acting on the arm such that F.sub.R and M.sub.R reduce to zero. At this point, the braking torque T.sub.B applied to the rotor through the transfer of F.sub.EDDY via pivot 8 and connection 14 equals the applied torque T.sub.App, the angular acceleration is thus equal to zero and the rotor 2 will rotate at a constant speed. A steady-state equilibrium position is then reached as shown in FIG. 5C.

[0210] The variables that contribute to the braking torque T.sub.B applied to the rotor 2 can all be controlled by appropriate calibration of the springs 12 and mass centroid 9, and thus the braking mechanism 1 can provide substantial control over the braking torque T.sub.B response to suit the particular application.

[0211] Any changes in the applied torque T.sub.App will result in a commensurate increase in the radial displacement of the arm 3 and the braking torque T.sub.EDDY applied by the magnetic field 6 and the reactionary force F.sub.R. However, it will be appreciated that the maximum braking torque T.sub.B achievable is constrained by the physical parameters of the mechanism 1.

[0212] FIG. 5D shows the arm 3 at a point of maximum radial displacement where the maximum magnetic field is intersected by the arm 3. The braking torque T.sub.B is equal to the applied torque T.sub.App. However, any further increases in applied torque T.sub.App will not result in the arm 3 moving radially outward as the spring is extended to its maximum extent and the arm 3 is in contact with the safety stop (not shown). The braking torque T.sub.B can therefore not increase any further. Any further increases in applied torque T.sub.App will therefore accelerate the rotor 2.

[0213] FIG. 5E shows a decreasing applied torque T.sub.App on the braking mechanism 1.

[0214] As the applied torque T.sub.App is reduced, a commensurate decrease in F.sub.App (8) occurs to balance the applied torque T.sub.App while the spring bias force F.sub.s remains temporarily unchanged. The resultant force F.sub.R from the forces F.sub.App (8), F.sub.App (I3), F.sub.cp1, and F.sub.EDDY act on the mass centroid 9 such that it accelerates the arm 3 inward towards the rotor axis X. The resulting clockwise rotation of the arm 3 about the pivot 8 decreases the distance between connection 13 and connection 14. The resulting reduction in extension of the spring 12 results in a reduction in spring bias F.sub.s and F.sub.App (I3). At the same time there is a reduction in the area of the arm 3 intersected by the magnetic field 6 with a corresponding reduction in the eddy-current braking force F.sub.EDDY.

[0215] The arm 3 continues to rotate clockwise about pivot 8 until the forces acting on the arm 3 balance such that the magnitude of F.sub.R is zero with a corresponding reduction in the acceleration of the mass centroid 9 to zero and thus the system in in a state of equilibrium. At this point the braking torque T.sub.B generated by the transfer of the eddy-current braking force F.sub.EDDY through the pivot 8 and connection 14 balances the applied torque T.sub.App and the acceleration of the rotor 2 is thus zero.

[0216] The speed of rotation can therefore be limited by adjusting the spring bias F.sub.s to ensure that the braking torque T.sub.B increases proportionally to the applied torque T.sub.A and both forces are kept equal throughout an ‘operating range’ of applied torques.

[0217] As aforementioned, the magnitude of the reactive force F.sub.EDDY is dependant on the: [0218] area of magnetic field 6 intersected by the arms 3; [0219] strength of the magnetic field 6 intersecting the arms 3; [0220] resistivity of the arms 3; and [0221] relative velocity of the arms 3 with respect to the magnetic field 6.

[0222] The braking mechanism 1 shown in FIGS. 2-5 provides automatic variation in both the area A of the applied magnetic field 6 intersected and the distance R between the arms 3 and rotor axis X by variation in the radial movement of the arms 3 into the applied magnetic field 6. Thus, in the operating range, changes in the applied torque T.sub.App will result in a commensurate change in the braking torque T.sub.B applied to the rotor 2.

[0223] It will be appreciated that the maximum braking torque achievable will depend on the physical constraints of the mechanism, e.g. size and strength of magnets, size, thickness and conductivity of the arm 3. Furthermore, the rotor 2 must experience a minimum applied torque, and therefore minimum rotational acceleration and speed, before the arm 3 applies a sufficient braking torque to limit the rotation speed.

[0224] The braking mechanism 1 limits the speed in an operating range between these maximum and minimum applied torques. Speed profiles of the braking mechanism 1 showing the operating range are shown in FIGS. 6 and 7.

[0225] As can be seen from FIGS. 6 and 7, the speed initially increases with applied torque T.sub.App until the resultant force F.sub.R acting on the mass centroid 9 accelerates the arms 3 outward into the magnetic field 6 and the reactive braking force F.sub.EDDY is applied. The resultant braking torque T.sub.B will increase and then equal the applied torque T.sub.App. The speed of rotation is thereby limited to a constant value as no acceleration can occur due to the applied torque T.sub.App being continually matched by the braking torque T.sub.B. Increases in applied torque T.sub.App are matched by increases in braking torque T.sub.B until an upper limit is reached where the maximum area of magnetic field 6 is intersected and thus the magnetic field reactive force F.sub.B generated is proportional to speed only. After the upper limit, the speed profile is similar to prior art devices which vary the braking torque T.sub.B with speed only.

[0226] Different speed responses to applied torques can be achieved by varying the spring bias. Examples of alternative speed profiles are shown in FIGS. 8 and 9.

[0227] The profile shown in FIG. 8 is achievable by providing a relatively ‘weak’ spring (i.e. small restoring bias and spring constant) compared with the embodiment shown in FIGS. 6 and 7 such that the braking torque applied upon magnetic field intersection is greater than the applied torque T.sub.A throughout the operating range. Thus, the speed of rotation is reduced with increasing applied torque T.sub.App until the applied torque T.sub.App exceeds the braking torque T.sub.B.

[0228] Alternatively, as shown in FIG. 9, a relatively ‘strong’ spring (i.e. large restoring bias and spring constant) may be used such that the applied torque T.sub.A exceeds the braking torque T.sub.B over the operating range. Thus, the speed of rotation increases linearly with increasing applied torque T.sub.App until the braking torque T.sub.B exceeds the applied torque T.sub.App.

[0229] It will thus be appreciated that the present invention may be modified to accommodate any speed response required for the application simply by adjusting or changing the spring 12.

[0230] FIG. 10 shows a braking mechanism 100 according to another embodiment of the present invention with the arms provided in the form of plates 103. The plates 103 are capable of radial movement along tracks 101 provided in the rotor 102. The plates 103 are coupled via the tracks 101 to the rotor 102 so that the plates rotate with the rotor 102 and tracks 101. Springs 112 are attached to the rotor 102 and to the plates 103. The springs 112 when extended—apply a biasing force F.sub.s to bias the plates 103 toward the rotor axis X.

[0231] The torque T.sub.App applies a tangential force F.sub.App on the plates 103 and the spring applies a centripetal force F.sub.cp. The centripetal acceleration of the springs 112 toward the rotor axis X results in the plates 103 moving radially outwards into the magnetic field 106 while extending the springs 112. Thus, the braking force F.sub.B applied will vary proportional to the tangential velocity of the plates 103 and the spring bias F.sub.s.

[0232] In contrast to the braking mechanism 1 of FIGS. 2-5, it will be appreciated that this braking mechanism 100 does not provide a set limit to the speed as the movement of the plates 103 is proportional to the rotor speed, rather than also to the applied torque as in the braking mechanism 1.

[0233] The magnet array (not shown) of this ‘linear’ embodiment is provided in the same configuration as that shown in the first preferred ‘pivoting’ embodiment shown in FIGS. 2 and 3.

[0234] As the plates 103 move radially outward under any rotor rotation (i.e. primarily under centrifugal effects), this ‘linear’ embodiment provides a braking mechanism 100 that works independent of the direction of rotation of the rotor 102.

[0235] Although the braking mechanism 100 provides a braking effect independent of the rotation direction, the braking torque varies only with the speed of rotation (and therefore centripetal acceleration) and not the torque applied. The speed will only be limited when the braking torque equals the applied torque and thus a greater applied torque (e.g. a heavier person) will result in the speed being limited at a higher equilibrium speed than a correspondingly ‘lighter’ person. Thus, this braking mechanism 100 does not provide the level of control of the braking mechanism 1 shown in FIGS. 2-5.

[0236] Another embodiment of a braking mechanism is generally indicated by arrow 201 in FIG. 11. In this embodiment an array of magnets (204) is mounted on a cradle (220). A rotor (202), having pivotally mounted conductors (203), is mounted on an axle (205) for rotation about the rotor axis (X).

[0237] The cradle (220) is configured to rotate about the rotor axis (X) and is connected to it by a gear transmission (230). In the arrangement shown in FIG. 11 the gear transmission (230) is configured such that the cradle (220) (including the magnetic array (204)) rotates in an opposite direction to the rotor (202) (and conductors (203)) thus increasing the relative angular velocity of the rotor (202) and conductor members (203) relative to the magnetic array (204). Such an arrangement for the braking mechanism may achieve an increased braking effect.

[0238] Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the appended claims.