Patent Description:
Rotational coupling devices such as brakes and clutches may use a variety of force transmitting mechanisms to cause movement of components of the coupling device to engage or disengage the device. In some conventional devices, an electromagnet is used to draw an armature into engagement with a stationary or rotating member of the device in order to, respectively, inhibit rotation or cause rotation of the armature and a corresponding structure to which the armature is connected (e.g., a shaft, pulley, gear, etc.). When it is desired to disengage the armature from the stationary or rotating member, another force transmitting mechanism such a spring moves the armature away from the member. These conventional devices typically work well for their intended purpose. The devices do have a significant drawback, however. When the electromagnet is deenergized, residual magnetism exists in the armature and the stationary or rotating member with which the armature is engaged. This residual magnetism delays release of the armature and causes undesirable friction/rubbing between the armature and the stationary or rotating member.

<CIT> discloses an armature hub <NUM>, a sintered hub <NUM> and a stopper plate <NUM> of synthetic resin material, which are integrally secured by insert molding. A spline groove 8a of an armature <NUM> is set in a spline groove 5a of the stopper plate <NUM>. The armature <NUM> is held to the armature hub <NUM> by the attracting force of a magnet <NUM>. <CIT> discloses a heat removing apparatus used on an electromagnetic fan clutch which comprises a driven rotor and a driving rotor. A coil winding for outputting an induced current is disposed on the driven rotor or the driving rotor. <CIT> discloses an early design of electromagnetic torque device.

The inventors herein have recognized a need for a rotational coupling device that will minimize and/or eliminate one or more of the above-identified deficiencies.

A rotational coupling device is provided according to claim <NUM>. In particular, a rotational coupling device is provided including a shaft mounted collar with permanent magnets that is used to release an armature from engagement with another member of the device.

A rotational coupling device in accordance with one embodiment of the invention includes an armature configured for coupling to a shaft for rotation therewith about a rotational axis. The armature is configured for movement axially relative to the shaft. The device further includes an electromagnet assembly disposed on a first axial side of the armature and fixed against rotation relative to the rotational axis. The device further includes a collar disposed on a second axial side of the armature opposite the electromagnet assembly. The collar is configured for rotation with the shaft, but fixed against axial movement relative to the shaft. The collar includes a permanent magnet. The electromagnet assembly urges the armature in a first axial direction into engagement with a member of the coupling device to apply a torque between the member and the armature when a current having a first polarity is provided to the electromagnet assembly. In one embodiment, the member may comprise the electromagnet assembly itself and the torque brakes rotation of the armature. In another embodiment, the member may comprise a rotor disposed axially between the armature and the electromagnet assembly and the torque results in rotation of both of the armature and the rotor. The permanent magnet urges the armature in a second axial direction to release the armature from the member and thereby disengage transmission of torque between the armature and the member when the current is not provided to the electromagnet assembly.

A rotational coupling device in accordance with another embodiment of the invention includes an armature configured for coupling to a shaft for rotation therewith about a rotational axis. The armature is configured for movement axially relative to the shaft. The device further includes an electromagnet assembly disposed on a first axial side of the armature and fixed against rotation relative to the rotational axis. The device further includes a collar disposed on a second axial side of the armature opposite the electromagnet assembly. The collar is configured for rotation with the shaft, but fixed against axial movement relative to the shaft. The collar includes a permanent magnet. The device further includes a controller configured to provide a current having a first polarity to the electromagnet assembly to establish an electromagnetic circuit between the armature and the electromagnet assembly and urge the armature in a first axial direction into engagement with a member of the coupling device to transmit a torque between the member and the armature. In one embodiment, the member may comprise the electromagnet assembly itself and the torque brakes rotation of the armature. In another embodiment, the member may comprise a rotor disposed axially between the armature and the electromagnet assembly and the torque results in rotation of both of the armature and the rotor. The controller is further configured to terminate the current to terminate the electromagnetic circuit between the armature and the electromagnet assembly. The permanent magnet urges the armature in a second axial direction following termination of the electromagnetic circuit to disengage the armature from the member.

A rotational coupling device in accordance with another embodiment is provided according to claim <NUM>.

In one embodiment, the member may comprise the electromagnet assembly itself and the torque brakes rotation of the armature. In another embodiment, the member may comprise a rotor disposed axially between the armature and the electromagnet assembly and the torque results in rotation of both of the armature and the rotor. The permanent magnet urges the armature in a second axial direction to disengage the armature from the member when a second current having a second polarity is provided to the electromagnet assembly.

A rotational coupling device in accordance with the present teachings is advantageous relative to conventional rotational coupling device. In particular, the inventive device releases an armature mounted on a shaft from engagement with a stationary or rotating member using a collar with one or more permanent magnets that is mounted on the same shaft. The collar and magnets cause a rapid release of the armature that overcomes the residual magnetism in the armature and stationary or rotating member to reduce the release time and friction between the armature and member. The collar also eliminates the need for springs or other axial mechanical force transmitting mechanisms thereby reducing the number of moving parts in the coupling device and improving the life of the device.

The foregoing and other aspects, features, details, utilities, and advantages of the invention will be apparent from reading the following detailed description and claims, and from reviewing the accompanying drawings illustrating features of this invention by way of example.

Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, <FIG> illustrates a rotational coupling device <NUM> in accordance with one embodiment of the invention. Device <NUM> is configured to function as a brake and provides a braking torque to a shaft <NUM> (and any structure coupled to shaft <NUM> such a gear, pulley, blade, etc.) rotating about a rotational axis <NUM> in order to slow or halt rotation of shaft <NUM>. It will be understood by those of ordinary skill in the art that device <NUM> may be used in a wide variety of industrial and other applications requiring a brake. Device <NUM> may include an armature <NUM>, means, such as electromagnet assembly <NUM>, for urging armature <NUM> in a first direction along axis <NUM>, means, such as a collar <NUM> housing one or more permanent magnets <NUM>, for urging armature <NUM> in a second axial direction along axis <NUM>, and a controller <NUM>.

Armature <NUM> is provided to transmit a braking torque to shaft <NUM>. Armature <NUM> may be made from metals or metal alloys or other materials having relatively low magnetic reluctance such as iron or steel. In accordance with various embodiments, armature <NUM> is made from metal alloys having a relatively high carbon content such that that armature <NUM> has a relatively high remanence following exposure to electromagnetic fields. In accordance with certain embodiments, armature <NUM> is made from a material having a content of at least <NUM> percent by weight of carbon or a carbon equivalent (a "carbon equivalent" comprises a combination of carbon (C), manganese (Mn), chromium (Cr), molybdenum (Mo), vanadium (V), nickel (Ni) and copper (Cu) often represented by the formula CE = C + Mn/<NUM> + (Cr + Mo + V)/<NUM> + (Ni + Cu)/<NUM>). Armature <NUM> may be annular in shape and may be disposed about, and centered about, axis <NUM>. Armature <NUM> is configured for coupling to shaft <NUM> for rotation therewith about axis <NUM>, but is configured for movement axially relative to shaft <NUM>. In particular, the radially outer surface of shaft <NUM> and the radially inner surface of armature <NUM> may have complementary, torque transmitting, shapes that fix armature <NUM> and shaft <NUM> against relative rotation, but permit relative axial movement including complementary splines, teeth or flats (e.g., a single or double D shape or hexagonal shape). Armature <NUM> includes friction surfaces on opposed sides <NUM>, <NUM> configured to engage electromagnet assembly <NUM> and collar <NUM>, respectively, during engagement and disengagement of the brake.

Electromagnet assembly <NUM> provides a means for urging armature <NUM> in one direction along axis <NUM> away from collar <NUM> and into engagement with assembly <NUM> to transmit a braking torque from assembly <NUM> to armature <NUM> and engage the brake. Assembly <NUM> is disposed on one axial side <NUM> of armature <NUM> opposite collar <NUM> and is fixed against rotation relative to axis <NUM>. Assembly <NUM> includes a housing <NUM> or field shell and a conductor <NUM>. Housing <NUM> provides structural support for, and orients, conductor <NUM>. Housing <NUM> also forms part of an electromagnet circuit with armature <NUM> when current is supplied to conductor <NUM>. Housing <NUM> may be annular in shape and disposed about, and centered about, axis <NUM>. Housing <NUM> may be made from materials having a relatively low magnetic reluctance such as ferromagnetic materials including steel. In accordance with various embodiments, housing <NUM> may be made from metal alloys having a relatively high carbon content such that that housing <NUM> has a relatively high remanence following exposure to electromagnetic fields. In accordance with certain embodiments, housing <NUM> is made from a material having a content of at least <NUM> percent by weight of carbon or a carbon equivalent. Housing <NUM> may define a radially extending end wall <NUM> and axially extending, radially aligned, inner and outer walls <NUM>, <NUM> (or poles) that extend axially from end wall <NUM> towards armature <NUM>. Conductor <NUM> may comprise a conventional wound coil or similar conductor and is configured to be received within housing <NUM> between walls <NUM>, <NUM>. Current supplied to conductor <NUM> creates or weakens an electromagnetic circuit that includes armature <NUM> and housing <NUM> depending on the strength and polarity of the current and the current state of device <NUM> as discussed in greater detail below. The electromagnetic circuit urges armature <NUM> towards electromagnet assembly <NUM> and away from collar <NUM> against the magnetic forces of magnets <NUM> to engage brake <NUM>.

Collar <NUM> and magnets <NUM> provide a means for urging armature <NUM> in the opposite direction along axis <NUM> towards collar <NUM> and away from electromagnet assembly <NUM> to disengage armature <NUM> from assembly <NUM> and release the brake. Collar <NUM> may be made from metals or metal alloys or other materials having a relatively high magnetic reluctance such as aluminum. Collar <NUM> is disposed on side <NUM> of armature <NUM> opposite electromagnet assembly <NUM>. Collar <NUM> may be annular in shape and may be disposed about, and centered about, axis <NUM>. Collar <NUM> is coupled to shaft <NUM> for rotation therewith and is also fixed against axial movement relative to shaft <NUM>. Referring to <FIG>, in one embodiment, collar <NUM> includes one or more radially extending threaded bores <NUM> configured to receive set screws used to secure collar <NUM> to shaft <NUM>. The bores <NUM> may be equally circumferentially spaced about collar <NUM> and axis <NUM>. Collar <NUM> further includes one or more axially extending bores <NUM> configured to receive magnets <NUM>. Bores <NUM> may also be equally circumferentially spaced about collar <NUM> and axis <NUM>.

Referring again to <FIG>, magnets <NUM> form part of a magnetic circuit with armature <NUM> that urges armature <NUM> away from electromagnet assembly <NUM> and towards collar <NUM> along axis <NUM> to release the brake. Magnets <NUM> comprise permanent magnets and may comprise neodymium iron boron (Nd-Fe-B) magnets or other known permanent magnets. Magnets <NUM> are arranged such that the poles of each magnet <NUM> are axially aligned. In the illustrated embodiment, collar <NUM> includes two magnets <NUM> with one magnet <NUM> having a pole having a first polarity (e.g., North) facing armature <NUM> and a pole having a second polarity (e.g. South) facing away from armature <NUM> while the other magnet <NUM> has a pole having the second polarity facing armature <NUM> and a pole having the first polarity facing away from armature <NUM>. Although the illustrated embodiment includes two magnets <NUM>, it should be understood that the number of magnets <NUM> may vary. In the illustrated embodiment, the two magnets <NUM> are located diametrically opposite one another within bores <NUM> of collar <NUM>. In general, magnets <NUM> may be equally circumferentially spaced on collar <NUM> and about axis <NUM>.

Controller <NUM> is provided to control the delivery of current to conductor <NUM> and, therefore, the operation of device <NUM>. Controller <NUM> may comprise a programmable microprocessor or microcontroller or may comprise an application specific integrated circuit (ASIC). Controller <NUM> may include a central processing unit (CPU). Controller <NUM> may also include a memory and an input/output (I/O) interface through which controller <NUM> may receive a plurality of input signals and transmit a plurality of output signals. Controller <NUM> controls the delivery of current to conductor <NUM> from a power source (not shown) such as a battery or capacitor.

Controller <NUM> is configured to control the operation of device <NUM> by controlling the delivery of current to conductor <NUM> in order to apply and release the brake. In some embodiments, current may be delivered to conductor <NUM> continuously or for relatively long durations to engage the brake and maintain the brake in an engaged state. In other embodiments, device <NUM> may operate as a bistable brake in which short duration current pulses cause device <NUM> to move between engaged and disengaged states and to remain in a given state after the pulse ends until the next current pulse is provided to conductor <NUM>. When device <NUM> is disengaged (i.e., when armature <NUM> is disengaged from housing <NUM> of electromagnet assembly <NUM> as a result of the magnetic forces generated by magnets <NUM>), controller <NUM> may engage the brake by delivering a current having a first polarity to conductor <NUM>. The current establishes an electromagnetic circuit including armature <NUM> and housing <NUM>. In particular, the current generates a magnetomotive force in an amount equal to the number of turns (N) in the conductor <NUM> multiplied by the amount of current (I). The magnetomotive force generates a magnetic flux (φ) that traverses the air gap between the armature <NUM> and housing <NUM> with the amount of flux (φ) depending on the magnetic reluctance (R) in the electromagnetic circuit. The flux (φ) in the electromagnetic circuit creates an attractive force (F) between the armature <NUM> and housing <NUM> opposing the magnetic forces of magnets <NUM> and that is a combination of forces at the inner and outer poles formed by walls <NUM>, <NUM> of housing <NUM>: F = φ<NUM>/(area of outer pole) + φ<NUM>/(area of inner pole). The amount of current (I) supplied must be sufficient to generate an attractive force (F) greater than the magnetic force of magnets <NUM> in order to urge armature <NUM> in an axial direction away from collar <NUM> and towards electromagnet assembly <NUM> to engage brake <NUM>.

As noted above, in some embodiments device <NUM> may be configured to act as a bistable brake in which device <NUM> is configured to maintain an attractive force (F) that exceeds the magnetic force of magnets <NUM> even after the current is terminated. As discussed above, at least one of armature <NUM> and housing <NUM> may be made from a material having a relatively high carbon content. As a result, the armature <NUM> and housing <NUM> have a relatively high remanence that continues to exist even after current is no longer provided to conductor <NUM>. Due to this remanence, a magnetic circuit among armature <NUM> and housing <NUM> is maintained after termination of the current and the brake remains in an engaged or applied state. In one embodiment, at least one of armature <NUM> and housing <NUM> has a content of at least <NUM> percent by weight of carbon or a carbon equivalent so as to have a relatively high remanence. The exact material composition of armature <NUM> and/or housing <NUM> may vary, however, based on other factors that influence the amount of attractive force between armature <NUM> and housing <NUM> such as the size of the air gap between armature <NUM> and housing <NUM> when the brake is disengaged and the area of the poles formed by the inner and outer walls <NUM>, <NUM> of housing <NUM>. Further, the material composition may be chosen in consideration with other factors that influence residual magnetism including annealing, mechanical stresses (e.g., coining) and heat treatment of materials. In general, the material for armature <NUM> and/or housing <NUM> is selected so as to produce a residual attractive force between armature <NUM> and housing <NUM> that exceeds the magnetic force of magnets <NUM> when considering these other factors.

When it is desired to disengage or release the brake, controller <NUM> may simply terminate delivery of the current to conductor <NUM>. Alternatively, in the case of the bi-stable brake wherein armature <NUM> and housing <NUM> have a relatively high remanence, controller <NUM> may provide current to conductor <NUM> having a polarity that is opposite the polarity of the current used to engage device <NUM>. This current again generates a magnetomotive force in an amount equal to the number of turns (N) in the conductor <NUM> multiplied by the amount of current (I), but this magnetomotive force operates in a direction opposite the force generated by the current used to engage device <NUM>. This coercive magnetomotive force reduces the residual magnetic flux (φ) traversing the air gap between the armature <NUM> and housing <NUM>. As a result, the current weakens the magnetic circuit among armature <NUM> and housing <NUM> thereby allowing magnets <NUM> to move armature <NUM> along axis <NUM> away from electromagnet assembly <NUM> and towards collar <NUM> to disengage device <NUM>. Because the magnetic force of magnets <NUM> moves armature <NUM> during disengagement of device <NUM>, the current used to disengage device <NUM> may have a magnitude that is less than the magnitude of the current used to engage device <NUM> (in which the force exerted by magnets <NUM> must be overcome). Further, the amount of current required to disengage or release device <NUM> may be minimized by considering the size of the air gap between armature <NUM> and housing <NUM>. In particular, the existence of the air gap also opposes the residual attractive force between armature <NUM> and housing <NUM>. In a graph of a conventional demagnetization curve, an air gap line can be plotted in the second quadrant of the graph from the origin and with a slope equal to the length of the air gap divided by the area of the air gap. The intersection of the line and the demagnetization curve identifies the residual magnetic flux remaining when no current is provided to conductor <NUM>. From this point, one can determine the amount of current required to generate a coercive magnetomotive force that is sufficient, when combined with the impact of the air gap, to overcome the residual attractive force between armature <NUM> and housing <NUM>.

As described above, generation of current by controller <NUM> is used to move device <NUM> between a (fully) engaged position and a (fully) disengaged position. In some embodiments, however, it may be desirable to apply a partial braking torque to control the rate of motion in a rotating body. In these embodiments, controller <NUM> may be further configured to generate current pulses of alternating polarity at a relatively high frequency to produce a smaller braking torque. Controller <NUM> may generate these pulses in response to a set of programming instructions (i.e. software) stored in a memory, in response to sensor feedback (e.g., the speed of the rotating body, or position of armature <NUM> along axis <NUM>) and/or in response to user commands entered through a conventional user interface.

Referring now to <FIG>, a rotational coupling device <NUM> in accordance with another embodiment of the invention is illustrated. Device <NUM> is configured to function as a clutch. It will be understood by those of ordinary skill in the art that device <NUM> may be used in a wide variety of industrial and other applications requiring a clutch. Device <NUM> selectively transmits torque between shaft <NUM> and a shaft <NUM>. Shaft <NUM> may be driven by a motor or another power source (it should be understood, however, that shaft <NUM> may alternatively be configured as a driven shaft with shaft <NUM> driven by a motor or other power source). In the illustrated embodiment, shaft <NUM> is configured to rotate about the same rotational axis <NUM> as shaft <NUM>. Device <NUM> may include many of the same components as device <NUM> including armature <NUM>, electromagnet assembly <NUM>, collar <NUM>, magnets <NUM> and a description of these components may be found above. Device <NUM> differs from device <NUM> in that device <NUM> may further include a rotor <NUM> coupled to shaft <NUM> and a controller <NUM> configured to control the operation or device <NUM>.

Rotor <NUM> is provided to transmit torque between shaft <NUM> and armature <NUM> and, consequently, shaft <NUM>. Rotor <NUM> may be made from metals or metal alloys or other materials having relatively low magnetic reluctance such as iron or steel. In accordance with various embodiments, rotor <NUM> is made from metal alloys having a relatively high carbon content such that that rotor <NUM> has a relatively high remanence following exposure to electromagnetic fields. In accordance with certain embodiments, rotor <NUM> is made from a material having a content of at least <NUM> percent by weight of carbon or a carbon equivalent. Rotor <NUM> may be annular in shape and may be disposed about, and centered about, axis <NUM>. Rotor <NUM> is disposed axially between armature <NUM> and electromagnet assembly <NUM>. Rotor <NUM> is coupled to shaft <NUM> for rotation therewith. In particular, the radially outer surface of shaft <NUM> and the radially inner surface of rotor <NUM> may have complementary, torque transmitting, shapes that fix rotor <NUM> to shaft <NUM> against relative rotation. Rotor <NUM> may further be fixed against axial movement relative to shaft <NUM> through the use of fasteners such as screws, welds or adhesives or through the use of snap rings or similar devices on either side of rotor <NUM>. Rotor <NUM> includes a friction surface <NUM> configured to engage friction surface <NUM> on armature <NUM> during engagement of the clutch.

Controller <NUM> is provided to control the delivery of current to conductor <NUM> and, therefore, the operation of device <NUM>. Controller <NUM> may again comprise a programmable microprocessor or microcontroller or may comprise an application specific integrated circuit (ASIC). Controller <NUM> may include a central processing unit (CPU). Controller <NUM> may also include a memory and an input/output (I/O) interface through which controller <NUM> may receive a plurality of input signals and transmit a plurality of output signals. Controller <NUM> controls the delivery of current to conductor <NUM> from a power source (not shown) such as a battery or capacitor.

Controller <NUM> is configured to control the operation of device <NUM> by controlling the delivery of current to conductor <NUM> in order to apply and release the clutch. In some embodiments, current may be delivered to conductor <NUM> continuously or for relatively long durations to engage the clutch and maintain the clutch in an engaged state. In other embodiments, device <NUM> may operate as a bistable clutch in which short duration current pulses cause device <NUM> to move between engaged and disengaged states and to remain in a given state after the pulse ends until the next current pulse is provided to conductor <NUM>. When device <NUM> is disengaged (i.e., when armature <NUM> is disengaged from rotor <NUM> as a result of the magnetic forces generated by magnets <NUM>), controller <NUM> may engage the clutch by delivering current having a first polarity to conductor <NUM>. The current establishes an electromagnetic circuit including armature <NUM>, housing <NUM>, and rotor <NUM>. In particular, the current generates a magnetomotive force in an amount equal to the number of turns (N) in the conductor <NUM> multiplied by the amount of current (I). The magnetomotive force generates a magnetic flux (φ) that traverses the air gaps between rotor <NUM> and housing <NUM> and between rotor <NUM> and armature <NUM> with the amount of flux (φ) depending on the magnetic reluctance (R) in the electromagnetic circuit. The flux (φ) in the electromagnetic circuit creates an attractive force (F) between the armature <NUM>, rotor <NUM> and housing <NUM> opposing the magnetic forces of magnets <NUM> and that is a combination of forces at the inner and outer poles formed by walls <NUM>, <NUM> of housing <NUM>: F = φ<NUM>/(area of outer pole) + φ<NUM>/(area of inner pole). The amount of current (I) supplied must be sufficient to generate an attractive force (F) greater than the magnetic force of magnets <NUM> in order to urge armature <NUM> in an axial direction away from collar <NUM> and towards rotor <NUM> to engage the clutch.

As noted above, in some embodiments device <NUM> may be configured to act as a bistable clutch in which device <NUM> is configured to maintain an attractive force (F) that exceeds the magnetic force of magnets <NUM> even after the current is terminated. As discussed above, at least one of armature <NUM> and rotor <NUM> may be made from a material having a relatively high carbon content. As a result, the armature <NUM> and rotor <NUM> have a relatively high remanence that continues to exist even after current is no longer provided to conductor <NUM>. Due to this remanence, a magnetic circuit among armature <NUM> and rotor <NUM> is maintained after termination of the current and the clutch remains in an engaged or applied state. In one embodiment, at least one of armature <NUM> and rotor <NUM> has a content of at least <NUM> percent by weight of carbon or a carbon equivalent so as to have a relatively high remanence. The exact material composition of armature <NUM> and/or rotor <NUM> may vary, however, based on other factors that influence the amount of attractive force between armature <NUM> and rotor <NUM> such as the size of the air gap between armature <NUM> and rotor <NUM> when the clutch is disengaged. Further, the material composition may be chosen in consideration with other factors that influence residual magnetism including annealing, mechanical stresses (e.g., coining) and heat treatment of materials. In general, the material for armature <NUM> and/or rotor <NUM> is selected so as to produce a residual attractive force between armature <NUM> and rotor <NUM> that exceeds the magnetic force of magnets <NUM> when considering these other factors.

When it is desired to disengage or release the clutch, controller <NUM> may simply terminate delivery of the current to conductor <NUM>. Alternatively, in the case of the bi-stable clutch wherein the armature <NUM>, housing <NUM> and/or rotor <NUM> have a relatively high remanence, controller <NUM> may provide current to conductor <NUM> having a polarity that is opposite the polarity of the current used to engage device <NUM>. The current again generates a magnetomotive force in an amount equal to the number of turns (N) in the conductor <NUM> multiplied by the amount of current (I), but this magnetomotive force operates in a direction opposite the force generated by the current used to engage device <NUM>. This coercive magnetomotive force reduces the residual magnetic flux (φ) traversing the air gap between the armature <NUM> and rotor <NUM>. As a result, the current weakens the magnetic circuit among armature <NUM> and rotor <NUM> thereby allowing magnets <NUM> to move armature <NUM> along axis <NUM> away from rotor <NUM> and electromagnet assembly <NUM> and towards collar <NUM> to disengage device <NUM>. Because the magnetic force of magnets <NUM> moves armature <NUM> during disengagement of device <NUM>, the current used to disengage device <NUM> may have a magnitude that is less than the magnitude of the current used to engage device <NUM> (in which the force exerted by magnets <NUM> must be overcome). Further, the amount of current required to disengage or release device <NUM> may be minimized by considering the size of the air gap between armature <NUM> and rotor <NUM>. In particular, the existence of the air gap also opposes the residual attractive force between armature <NUM> and rotor <NUM>. In a graph of a conventional demagnetization curve, an air gap line can be plotted in the second quadrant of the graph from the origin and with a slope equal to the length of the air gap divided by the area of the air gap. The intersection of the line and the demagnetization curve identifies the residual magnetic flux remaining when no current is provided to conductor <NUM>. From this point, one can determine the amount of current required to generate a coercive magnetomotive force that is sufficient, when combined with the impact of the air gap, to overcome the residual attractive force between armature <NUM> and rotor <NUM>.

As described above, generation of current by controller <NUM> is used to move device <NUM> between a (fully) engaged position and a (fully) disengaged position. In some embodiments, however, it may be desirable to apply a partial torque to control the rate of motion in a rotating body. In these embodiments, controller <NUM> may be further configured to generate current pulses of alternating polarity at a relatively high frequency to produce a smaller torque. Controller <NUM> may generate these pulses in response to a set of programming instructions (i.e. software) stored in a memory, in response to sensor feedback (e.g., the speed of the rotating body, or position of armature <NUM> along axis <NUM>) and/or in response to user commands entered through a conventional user interface.

A rotational coupling device <NUM> or <NUM> in accordance with the present invention is advantageous relative to conventional rotational coupling devices. In particular, the inventive device <NUM> or <NUM> releases an armature <NUM> mounted on a shaft <NUM> from engagement with a stationary or rotating member <NUM> or <NUM>, respectively, using a collar <NUM> with one or more permanent magnets <NUM> that is mounted on the same shaft <NUM>. The collar <NUM> and magnets <NUM> cause a rapid release of the armature <NUM> that overcomes the residual magnetism in the armature <NUM> and stationary or rotating member <NUM> or <NUM>, respectively, to reduce the release time and friction between the armature <NUM> and member <NUM> or <NUM>. The collar <NUM> also eliminates the need for springs or other axial mechanical force transmitting mechanisms thereby reducing the number of moving parts in the coupling device and improving the life of the device.

Claim 1:
A rotational coupling device, comprising:
an armature having a shape that is complementary to a shape of a shaft such that the armature is configured for directly coupling to the shaft for rotation therewith about a rotational axis and for movement axially relative to the shaft;
an electromagnet assembly disposed on a first axial side of the armature and fixed against rotation relative to the rotational axis; and,
a collar disposed on a second axial side of the armature opposite the electromagnet assembly, the collar configured for rotation with the shaft, but fixed against axial movement relative to the shaft, the collar housing a first permanent magnet
wherein the electromagnet assembly urges the armature in a first axial direction to release the armature from the collar and urge the armature into engagement with a member of the coupling device to apply a torque between the member and the armature when a current having a first polarity is provided to the electromagnet assembly and the first permanent magnet urges the armature in a second axial direction to release the armature from the member and thereby disengage transmission of torque between the armature and the member when the current is not provided to the electromagnet assembly
characterized in that the member of the coupling device comprises the electromagnet assembly and the torque brakes rotation of the armature.