WO2008134249A1

WO2008134249A1 - Torque regeneration in a magnetic clutch

Info

Publication number: WO2008134249A1
Application number: PCT/US2008/060555
Authority: WO
Inventors: Peter Vandervaart; Wen-Ruey Hwang
Original assignee: The Timken Company
Priority date: 2007-04-25
Filing date: 2008-04-17
Publication date: 2008-11-06

Abstract

A method is provided for controlling a clutch (18) having a first magnetic system component (38), a second magnetic system component (46), the first and second magnetic system components defining therebetween a gap (50) containing condutive particles, and a coil assembly (62) operable in combination with the conductive particles to selectively magnetically couple the first and second magnetic system components together. The method inludes controlling the clutch in a first, operational mode to achieve a desired torque transfer condition between the first and second magnetic system components, and controlling the clutch in a second, torque regeneration mode to cause a magnetically-induced agitation of the conductive particles in the gap

Description

TORQUE REGENERATION IN A MAGNETIC CLUTCH

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/913,965, filed April 25, 2007, the entire content of which is hereby incorporated by reference.

BACKGROUND

[0002] The present invention relates to magnetic clutches and particularly to magnetorheological fluid type and magnetic powder type clutches.

[0003] Magnetic clutches that utilize a medium, such as fluid or powder, that contain conductive or magnetizable particles are known.

SUMMARY

[0004] A magnetic clutch assembly having any number of input or output rotors allows variable speed and torque to be transmitted from the input member(s) to the output member(s). When the magnetic clutch is operated at high slip speeds (i.e., when the magnetic clutch is not engaged to transmit torque) for prolonged periods of time, the conductive or metallic particles (e.g., iron particles) in the MR fluid can separate from the base fluid (e.g., a hydrocarbon based oil). The centrifugal field present in the absence of a magnetic field centrifuges or separates the conductive particles out of an otherwise generally homogenous distribution, and causes them to redistribute or become "packed" along one of the rotating races (e.g., the radial outer race) of the clutch. High heat can be generated at the higher slip speeds, reducing the viscosity of the base fluid of the MR fluid, further exacerbating the separation.

[0005] Likewise, with magnetic powder clutches, the high slip speed operation of the clutch can cause the powder to become "packed" or to compact along one of the rotating races (e.g., the radial outer race) of the clutch.

[0006] When the conductive particles of an MR fluid medium or the magnetic powder of a powder medium become "packed" in this manner, the ability of the clutch to magnetically engage and transmit torque upon energization can be greatly reduced. In some instances, the clutch will not be operable at all to magnetically engage, rendering the clutch "dead."

[0007] The present invention provides a clutch assembly and method of controlling a clutch assembly. The controller associated with the clutch assembly is operable to control the clutch in a first, operational mode, and in a second, torque regeneration mode that agitates or mixes the MR fluid or the magnetic powder to reduce or eliminate this "packing" phenomenon. In the torque regeneration mode, the invention contemplates controlling the operational characteristics of the clutch (e.g., input current and input member speed), either directly or indirectly (e.g., via initiation of the torque regeneration mode at a desirable time based on the application in which the clutch is used) to achieve magnetic agitation of the medium. The agitation of the medium obtains or maintains better distribution of the conductive particles in the MR fluid (e.g., a generally homogenous distribution of the conductive particles within the base fluid) or better distribution or placement of the magnetic powder within the working area or medium gap of the clutch, depending on whether the medium is an MR fluid or a magnetic powder. Unlike with prior art clutches, no mechanical mixing features or secondary magnetic coils need be added to the clutch assembly to achieve the desired agitation.

[0008] With the control capabilities of the invention, the agitation can be achieved in a number of different ways. In one embodiment, applying input power (e.g., current) to the clutch after prolonged, high slip speed operation, even when not required for torque transmission, can magnetically agitate or draw the conductive or metallic particles of the MR fluid back into suspension with the base fluid. In some embodiments, the power input can be coupled with a subsequent input member speed increase to then help mix the particles in the base fluid. This method restores the fluid back to its "unpacked" state providing the correct concentration of the conductive or metallic particles in the working area of the clutch, thereby restoring or regenerating the desired torque output capabilities. Similarly, applying combinations of input power and input member speed increases to the clutch can also "unpack" the packed or compacted magnetic powder, restoring the powder to a better distribution and placement within the working area of the clutch.

[0009] Different techniques and methodologies for controlling input power and input member speed are contemplated and can be used to achieve the desired magnetic agitation. For example, oscillations of current input, a build and decline of current input, and other systematic variations of current input can be applied. Furthermore, and likely in combination with the variations in current input, changes in the input member speed, either via direct control or by coordinating the regeneration sequence with a predicted speed increase in the input member, can help mix the particles. Additionally, the polarity of the magnetic field can be reversed to help cause the desired agitation. Other pulsing or steady state current variations, speed and load variations, and magnetic field variations can also be used to create the desired agitation, using any level of amperage up to the maximum design limit of the coil assembly of the clutch.

[0010] The agitation can be applied in a preventative maintenance mode, wherein the agitation regularly occurs after a desired number of hours of clutch operation, and/or in a corrective or reactive mode, wherein the agitation occurs after medium "packing" is detected. The controller can be programmed to trigger the torque regeneration sequence as desired. The controller can take the form of a micro-controller or digital signal processor (DSP) and can be integrated with an existing controller already present for the particular application. When used in an automotive application, the controller can be integrated with the automobile's engine control unit (ECU), or can be a stand-alone controller.

[0011] Specifically, the invention provides a method of controlling a clutch having a first magnetic system component, a second magnetic system component, the first and second magnetic system components defining therebetween a gap containing conductive particles, and a coil assembly operable in combination with the conductive particles to selectively magnetically couple the first and second magnetic, system components together. The method includes controlling the clutch in a first, operational mode to achieve a desired torque transfer condition between the first and second magnetic system components, and controlling the clutch in a second, torque regeneration mode to cause a magnetically-induced agitation of the conductive particles in the gap.

[0012] The invention further provides a system for controlling a clutch assembly. The system includes a clutch assembly having a first magnetic system component, a second magnetic system component, the first and second magnetic system components defining therebetween a gap containing conductive particles, and a coil assembly operable in combination with the conductive particles to selectively magnetically couple the first and second magnetic system components together. The system further includes a controller operable to control the clutch assembly in a first, operational mode to achieve a desired torque transfer condition between the first and second magnetic system components, and in a second, torque regeneration mode to cause a magnetically-induced agitation of the conductive particles in the gap.

[0013] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Fig. 1 is a perspective view, partially in section, illustrating a clutch embodying the invention and being used in a power steering pump application.

[0015] Fig. 2 is a section view of the clutch of Fig. 1.

[0016] Fig. 3 is a magnetic flux diagram for the clutch of Fig. 1.

[0017] Fig. 4 is a perspective view illustrating a clutch that is an alternative embodiment of the invention.

[0018] Fig. 5 is a section view of the clutch of Fig. 4.

[0019] Fig. 6 is a magnetic flux diagram for the clutch of Fig. 4.

[0020] Fig. 7 is a flow diagram of a control method embodying the invention.

[0021] Fig. 8 is a circuit diagram of one possible circuit of the invention.

[0022] Fig. 9 is a circuit diagram of another possible circuit of the invention.

[0023] Fig. 10 is a depiction of a torque regeneration sequence embodying the invention.

DETAILED DESCRIPTION

[0024] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings.

[0025] Fig. 1 illustrates a power steering pump assembly 10 including a power steering pump 14 and a clutch 18 embodying the invention. The clutch 18 enables the power steering to be deactivated when desirable to reduce horsepower loss. While the clutch 18 is shown and described for use in a power steering application, it should be understood that the clutch 18 can also be used in other applications, including but not limited to, transmission applications, engine cooling fan applications, and other applications requiring a clutch for torque responsive control.

[0026] The power steering pump 14 includes a housing 22 that contains the pump components (generally designated by the reference numeral 26). Additionally, the housing 22 includes an elongated storage chamber portion 30 that stores the power steering fluid. An input shaft 34 extends from the power steering pump 14 to receive a pulley 38 that is driven by the vehicle's engine via a belt 42 (see Fig. 2) coupled to the pulley 38. The clutch 18 is disposed on the input shaft 34 such that the pulley 38 forms a first component of the magnetic system, and in this particular application is the input member of the clutch 18. While the pulley 38 is illustrated schematically as being one piece, it would likely be formed as two or more pieces both for assembly purposes, and in order to have selective portions of the pulley 38 made of a magnetically conductive material (e.g. low carbon steel) and other portions of the pulley 38 made of non-magnetically conductive materials (e.g., aluminum or stainless steel).

[0027] While the pulley 38 forms a first component of the magnetic system, the clutch 18 includes a rotor 46 that forms a second component of the magnetic system, and in this particular application is the output member of the clutchlδ. The rotor 46 is also made of a magnetically conductive material and is coupled to the shaft 34 for rotation therewith (e.g., by press-fit, keyed connection, etc.). The illustrated rotor 46 is housed completely within the envelope defined by the outer dimensions of the pulley 38. As best shown in Fig. 2, a gap 50 is defined between an outer surface 54 of the rotor 46 and an inner surface 58 of the pulley 38, and contains conductive particles, as is understood by those skilled in the art. In the illustrated construction, the conductive particles are contained within a fluid, known as a magnetorheological fluid, and the clutch 18 is a magnetorheological fluid clutch ("MR clutch"). In other constructions, the particles can form a powder and the clutch can be a magnetic powder clutch. In the illustrated construction of the clutch 18, seals 60 keep the magnetorheological fluid in the gap 50. While the rotor 46 is illustrated schematically as being one piece, it would likely be formed as two or more pieces for assembly purposes.

[0028] Housed within the rotor 46 is a stationary coil assembly 62 including a stationary coil holder 64 made of a magnetically conductive material. The stationary coil holder 64 supports a stationary coil 66 (shown in Fig. 2, but removed in Fig. 1). Referring to Fig. 2, together the stationary coil holder 64 and the stationary coil 66 define a surface 70 spaced from an inner surface 74 of the rotor 46. The space between the surfaces 70 and 74 defines an air gap 78 between the stationary coil assembly 62 and the rotor 46.

[0029] The stationery coil holder 64 includes an extension portion 82 that extends toward the pump housing 22. The extension portion 82 is connected to the pump housing 22 such that the stationary coil assembly 62 remains stationary during rotation of the shaft 34 and the rotor 46. By keeping the coil 66 and coil holder 64 stationary, it is easy to run power (e.g., by routing wires) to the coil 66 from the pump housing 22 through or along the stationary coil holder 64. Because of the ability to have a stationary wire attachment point, no slip rings or other electrical connections are required as is the case when a rotating coil design is used. While the stationary coil holder 64 is illustrated schematically as being one piece, it would likely be formed as two or more pieces for assembly purposes.

[0030] Except for the end of the extension portion 82 that connects to the pump housing 22, the entire stationary coil assembly 62 is housed completely within the envelope defined by the outer dimensions of the pulley 38, and is also housed completely within the envelope defined by the outer dimensions of the rotor 46. Integration of the stationary coil assembly 62 with and within the pulley 38 and the rotor 46 in this manner facilitates a more compact, efficient, and cost-effective design for a clutch 18 than was previously possible in designs that mounted the stationary coil separately from and independently of the components of the magnetic system. [0031] Accurate control of the gap 50 and the air gap 78 is important to the operation of the clutch 18, and requires precise positioning of the pulley 38, the rotor 46, and the stationary coil assembly 62 relative to one another. To maintain the precise positioning of these components, the clutch 18 includes a plurality of bearings arranged between the components. A first bearing 86 is positioned between the pulley 38 and the rotor 46 and helps accurately control the spacing of the gap 50 by limiting the relative positioning between the pulley 38 and the rotor 46. The illustrated bearing 86 is a ball bearing, however, other rolling elements could be substituted for balls should space permit.

[0032] A second bearing 90 is positioned between the stationary coil assembly 62 and the rotor 46 and helps accurately control the spacing of the air gap 78 by limiting the relative positioning between the stationary coil assembly 62 and the rotor 46. The illustrated bearing 90 is a needle bearing, however, other types of roller bearings can also be substituted.

[0033] A third bearing 94 is positioned between the stationary coil assembly 62 and the pulley 38 and helps to accurately, at least partly, control the spacing of the gap 50 by limiting the relative positioning between the stationary coil assembly 62 and the pulley 38. The illustrated bearing 94 is a ball bearing, however, other rolling elements could be substituted for balls should space permit.

[0034] It can therefore be seen that each of the three main components, i.e., the pulley 38, the rotor 46, and the stationary coil assembly 62 are integrated together, and are accurately positioned relative to one another via the three bearings 86, 90, and 94, as described above. This maintains the tight control needed for the gap 50 containing the conductive particles and the air gap 78. Additionally, the three bearings 86, 90, 94 provide a direct load path for loads applied to the pulley 38 to be transmitted through the stationary coil assembly 62 and the rotor 46 to the shaft 34. At least two of the bearings 86, 90, 94, and as illustrated all three of the bearings 86, 90, 94, are located at least partially within the axial extents of the gap 50, and radially between the gap 50 and the shaft 34. Prior art stationary coil clutch designs typically required the load path to travel around the stationary coil assembly, which was separate from and independent of the components of the magnetic system, before being transmitted to the shaft. This typically resulted in overhanging loads that could lead to premature bearing failure. [0035] With reference to Figs. 1 and 2, the illustrated clutch 18 further includes a target wheel 98 coupled to the rotor 46 for rotation therewith. As best shown in Fig. 2, a sensor 102 is mounted on the stationary coil holder 64 opposite the target wheel 98 so that the speed of the pump shaft 34 can be sensed and monitored. In the illustrated embodiment, a Hall Effect sensor can be used for the sensor 102. In some embodiments, multiple Hall Effect sensors may be used to increase resolution. Other types of sensors that can also be used are magnetic speed pick-up sensors, optical sensors, and laser sensors.

[0036] Fig. 3 illustrates a model of the magnetic flux for the clutch 18 discussed above. The model of Fig. 3 illustrates that when the current is applied to the coil 66, a magnetic flux is created. In the illustrated construction, the conductive particles in the gap 50 are magnetized such that the conductive particles align across the gap 50 (as represented by the generally horizontal contour lines within the gap 50) and magnetically couple or lock the pulley 38 and the rotor 46 together for co-rotation to drive the input shaft 34 of the power steering pump 14.

[0037] Figs. 4-6 illustrate a second embodiment of a clutch 118 of the invention. As with the clutch 18, the clutch 118 can be used with a power steering pump assembly 10 as well as with other applications. While the illustrated clutch 118 utilizes a magnetorheological fluid, as with the clutch 18, in other constructions the clutch 118 can utilize a magnetic powder. The clutch 118 is a double-gap design, which can be more manufacturable than the single gap design illustrated in Figs. 1-3. Additionally, as those skilled in the art will understand, a double-gap design clutch can reduce the axial envelope of the clutch while maintaining a sufficient surface area (i.e., the surface area for two gaps instead of just one) for the magnetic coupling.

[0038] A multi-piece pulley 138 forms a first component of the magnetic system, and in this particular application is the input member of the clutch 118. The multi-piece pulley includes a first portion 139 that is coupled to a drive belt (not shown). The first portion 139 can be an aluminum casting or other non-magnetically conductive material. A second portion 140 of the pulley 138, which is also an aluminum die-cast part in the illustrated embodiment, is rotationally fixed to the first portion 139 via toothed engagement 141. The second portion 140 includes first and second magnetically-conductive material portions 142, 143 (e.g., steel) that are adjacent the two gaps, as will be discussed further below. In the illustrated embodiment, the first and second magnetically-conductive material portions 142, 143 are cast around/within the second portion 140 of the pulley 138, and can be formed by drawing or other suitable methods prior to being cast within the second portion 140 of the pulley 138. A non-magnetically conductive insert 144 (e.g., aluminum or stainless steel) is positioned within the first magnetically-conductive material portions 142 to facilitate the appropriate flux path. Finally, the pulley 138 includes an end cap 145 secured (e.g., by screws) to the second portion 140 of the pulley 138. The end cap 145 provides access to fill the clutch 118 with magnetorheological fluid or magnetic powder.

[0039] A rotor 146 forms a second component of the magnetic system of the clutch 118, and in this particular application is the output member of the clutch 118. The rotor 146 is made of a magnetically conductive material and includes a rotor portion 147 coupled for rotation with a hub portion 148 configured to receive the input shaft 34 of the pump assembly 10. The rotor portion 147 extends between the first and second magnetically-conductive material portions 142, 143 of the pulley 138 to define first and second gaps 150a, 150b. In the illustrated construction the clutch 118 utilizes magnetorheological fluid, and seals 160 keep the magnetorheological fluid in the gaps 150a, 150b. Again, magnetic powder can also be substituted for the magnetorheological fluid.

[0040] A stationary coil assembly 162 includes a stationary coil holder 164 made of a magnetically conductive material. The coil holder 164 supports a stationary coil 166. Together, the stationary coil holder 164 and the coil 166 define a surface 170 spaced from an inner surface 174 of the pulley 138. The space between the surfaces 170 and 174 defines an air gap 178 between the stationary coil assembly 162 and the pulley 138.

[0041] The stationary coil assembly 162 further includes a stationary collar 180 fixed with the coil holder 164, and a mounting bracket 182 fixed to the stationary collar 180. The mounting bracket 182 secures the clutch 118 to the pump housing 22. Note that in the embodiment of Fig. 1, the mounting bracket is not shown, but would be fixed to the extension portion 82.

[0042] Just as with the clutch 18, accurate control of the gaps 150a, 150b, containing the conductive particles and the air gap 178 is important to the operation of the clutch 118. To maintain precise positioning of the pulley 138, the rotor 146, and the stationary coil assembly 162 relative to one another, three bearings are provided. A first bearing 186 is positioned between the pulley 138 and the rotor 146. Specifically, the first bearing 186 is positioned between part of the second portion 140 of the pulley 138 and the hub portion 148 of the rotor 146. This bearing helps accurately control the spacing of the gaps 150a, 150b by limiting the relative positioning between the pulley 138 and the rotor 146. The illustrated bearing 186 is a ball bearing, however other rolling elements (e.g., needles) could be substituted for balls should space permit.

[0043] A second bearing 190 is positioned between the stationary coil assembly 162 and the rotor 146, and more specifically between the mounting bracket 182 and the hub portion 148 of the rotor 146. The second bearing 190 helps accurately control the spacing of the gaps 150a, 150b by limiting the relative positioning between the stationary coil assembly 162 and the rotor 146. The illustrated bearing 190 is a ball bearing, however, other types of roller bearings can also be substituted.

[0044] A third bearing 194 is positioned between the stationary coil assembly 162 and the pulley 138, and more specifically between the mounting bracket 182 and the first portion 139 of the pulley 140. The third bearing 194 helps to accurately, at least partly, control the spacing of the gaps 150a, 150b and the air gap 178 by limiting the relative positioning between the stationary coil assembly 162 and the pulley 138. The illustrated bearing 194 is a ball bearing, however, other rolling elements could be substituted for balls should space permit.

[0045] As with the clutch 18 as described above, it can therefore be seen that each of the three main components of the clutch 118, i.e., the pulley 138, the rotor 146, and the stationary coil assembly 162 are integrated together, and are accurately positioned relative to one another via the three bearings 186, 190, and 194. This maintains the tight control needed for the gaps 150a, 150b that contain the conductive particles and the air gap 178. Additionally, the three bearings 186, 190, 194 provide a direct load path for loads applied to the pulley 138 to be transmitted through the stationary coil assembly 162 and the rotor 146 to the shaft 34. At least two of the bearings 186, 190, 194, and as illustrated all three of the bearings 186, 190, 194, are located at least partially within the axial extents of the gaps 150a, 150b, and radially between the gaps 150a, 150b and the shaft 34. Prior art stationary coil clutch designs typically required the load path to travel around the stationary coil assembly, which was separate from and independent of the components of the magnetic system, before being transmitted to the shaft. This typically resulted in overhanging loads that could lead to premature bearing failure. [0046] With reference to Fig. 5, the illustrated clutch 118 further includes a target wheel 198 coupled to the rotor 146 at the hub portion 148 for rotation therewith. A sensor 202 is mounted on the stationary collar 180 opposite the target wheel 198 so that the speed of the pump shaft 34 can be sensed and monitored. In the illustrated embodiment, a Hall Effect sensor can be used for the sensor 202. In some embodiments, multiple Hall Effect sensors may be used to increase resolution. Other types of sensors that can also be used are magnetic speed pick-up sensors, optical sensors, and laser sensors.

[0047] Fig. 6 illustrates a model of the magnetic flux for the clutch 118 discussed above. The model of Fig. 6 illustrates that when the current is applied to the coil 166, a magnetic flux is created. In the illustrated construction, conductive particles in the gaps 150a, 150b are magnetized such that the conductive particles align across the gaps 150a, 150b (as represented by the generally horizontal contour lines within the gaps 150a, 150b) and magnetically couple or lock the pulley 138 and the rotor 146 together for co-rotation to drive the input shaft 34 of the power steering pump 14.

[0048] The above-described clutches can be controlled to incorporate the mixing or agitation capabilities described above in the Summary section. Additionally, the mixing or agitation capabilities described above in the Summary section can be applied to other magnetic clutches and to magnetic clutches for use in other applications (e.g., non- automotive applications). The controllers described below can take the form of a microcontroller or a digital signal processor (DSP) and can be integrated with an existing controller already present for the particular application. When used in an automotive application, the controller can be integrated with the automobile's engine control unit (ECU), or can be a stand-alone controller.

[0049] Fig. 7 is a flow diagram describing a method and system for control of a clutch, which can be configured in accordance with the clutches 18 and 118 described above, or differently depending upon the specific application. Block 200 depicts the control of the clutch by the controller in a first, normal operational mode. Such control can range from simple selective application of input current (e.g., on or off) to engage (lock) or disengage (unlock) the clutch, to more complicated control methodologies, for example wherein the controller controls the output speed of the output member by controlling one or both of the input current and the input member speed. This type of more complicated control can achieve complete locking engagement of the clutch resulting in 100 percent torque transfer, or less than complete clutch engagement resulting a desired torque transfer that is less than 100 percent of the possible torque transfer. In either case, when in the first, normal operational mode, the controller is operable to achieve a desired torque transfer condition between the first and second magnetic system components (i.e., the input member and the output member).

[0050] Block 204 represents a triggering event that triggers or initiates the departure from the first, normal operational mode, to a second, torque regeneration mode (see block 208). In the illustrated embodiment, the first and second modes are mutually exclusive, or in other words, cannot occur or run simultaneously. Some examples of triggering events can include predetermined amounts of time the clutch has been operating, the detection of a "packed" condition of the magnetic particles in either the MR fluid or the magnetic powder, or other application-specific events that may be used in conjunction with the torque regeneration mode. One such example of an application-specific event, in relation to an automotive application, will be discussed in more detail below.

[0051] After the triggering event is detected by the controller in block 204, the controller operates the clutch in the second, torque regeneration mode depicted by block 208 to effect the magnetically-induced agitation of the medium. The agitation of the medium obtains or maintains better distribution of the conductive particles in the MR fluid (e.g., a generally homogenous distribution of the conductive particles within the base fluid) or better distribution or placement of the magnetic powder within the working area or medium gap of the clutch, depending on whether the medium is an MR fluid or a magnetic powder. The torque regeneration mode operates without the need for any added mechanical mixing feature or any secondary magnetic coil commonly used in prior art clutch systems. After the torque regeneration mode is completed, the controller reverts to block 200 to operate the clutch in the first, normal operational mode once again.

[0052] Fig. 8 illustrates a circuit diagram of one possible embodiment of the invention. A MR clutch, and specifically the single coil of the clutch, is depicted as reference number 218 and can be configured in accordance with the clutches 18 and 118 described above, or differently depending upon the specific application. The controller 222 operates a variable current device 226 to control the input current to the clutch 218. [0053] Fig. 9 illustrates a circuit diagram of another possible embodiment of the invention. This embodiment includes a DC power supply 230, such as a 12V battery, and can be well-suited for use in automotive applications. A capacitor 234 is included in parallel to stabilize the DC power supply. A diode 238, which is also in parallel, stabilizes the current in order to minimize a ripple effect seen by the coil of the clutch 218 during the oscillation of a pulse-width-modulator (PWM) 242. The PWM 242 is used to adjust the current using the signal from the controller 222, and uses a varied duty cycle to change and control the current value. For example, if the duty cycle is 100%, which means that the circuit is closed all the time, the current is at full scale. If the duty cycle is 0%, which means the circuit is open, the current is at 0 amps. The frequency of the PWM 242 can vary depending on the application. In the illustrated embodiment, 5 kHz is used as the PWM frequency.

[0054] Different control algorithms can be programmed into the controller 222 depending on the application. Sensors, such as speed sensors at the input and output members, can provide data to the controller 222 to drive adjustments to the input current. Sensors can also communicate with the controller 222 to provide information on triggering events.

[0055] Applying power input to the clutch after prolonged, high slip speed operation, even when not required for torque transmission, can magnetically agitate or draw ("unpack") the conductive or metallic particles of the MR fluid back into suspension with the base fluid. This method restores the fluid back to its original state providing the correct concentration of the conductive or metallic particles in the working area of the clutch, thereby restoring desired torque output. Similarly, applying input power to the clutch can also "unpack" the packed or compacted magnetic powder, restoring the powder to a better distribution and placement within the working area of the clutch.

[0056] Different techniques and methodologies for controlling input power and input member speed are contemplated and can be used to achieve the desired magnetic agitation. For example, oscillations of current input, a build and decline of current input, and other systematic variations of current input can be applied. Furthermore, and likely in combination with the variations in current input, changes in the input member speed, either via direct control or by coordinating the regeneration sequence with a predicted speed increase in the input member, can help mix the particles. Additionally, the polarity of the magnetic field can be reversed to help cause the desired agitation. Other pulsing or steady state current variations, speed and load variations, and magnetic field variations can also be used to create the desired agitation, using any level of amperage up to the maximum design limit of the coil assembly of the clutch.

[0057] The agitation can be applied in a preventative maintenance mode, wherein the agitation regularly occurs after a desired number of hours of clutch operation or some other triggering event, and/or in a corrective or reactive mode, wherein the agitation occurs after medium "packing" is detected. Appropriate control systems and sensors can be incorporated to trigger the agitation as desired.

[0058] One example of a torque regeneration mode or sequence is illustrated in Fig. 10. The sequence shown in Fig. 10 is an example of a torque regeneration sequence that can be utilized in an application having a control system such as that shown in Fig. 9, and in an application in which the speed of the input member can be controlled, or at least predicted. As seen in Fig. 10, a torque regeneration sequence lasting about four minutes is used to agitate or "unpack" packed particles in the clutch. After the sequence has been triggered by a triggering event, and at time zero, a large current is input to clutch. While the illustrated current input is about seven amps, those skilled in the art will understand that the specific current values are scalable and will vary depending on the clutch and the application. In the illustrated system, the clutch will lock with about 3.5 to 4 amps of current input, such that the seven amp input is deemed a relatively high current input.

[0059] As expected, the high current input from time zero to one minute causes the clutch to lock so that the input member and the output member rotate at the same RPM. The high current input draws the conductive particles out of their packed state. From time one minute to time 1.5 minutes, the input current is dropped to 0 amps. The clutch unlocks and the output member RPM drops nearly to 0 RPM. The input member continues to run at about 1000 RPM. Next, from time 1.5 minutes to time 2 minutes, the input member speed is increased to about 3500 RPM and the current is increased to about 1.25 amps to begin a "mixing" portion of the torque regeneration sequence. The output member speed increases to about 800 RPM due to the partial engagement created by the application of current to the clutch. The high input member speed causes the now-unpacked conductive particles to "mix", either within the base fluid for fluid type clutches, or in and around the working gap area for powder type clutches. [0060] The mixing phase of the sequence continues from time 1.5 minutes to about time 3.5 minutes, where the input member speed is reduced back to 1000 RPM to end the sequence at time 4 minutes. Likewise, the current is reduced to zero amps between time 3.5 and 4 minutes. As mentioned above, the triggering event for the illustrated sequence can be a predetermined amount of time the clutch has been operating, the detection of a "packed" condition of the conductive particles in either the MR fluid or the magnetic powder, or other application-specific events that may be used in conjunction with the torque regeneration mode. For example, in an automotive application in which the input member speed is based on the engine speed so that the input member speed is not independently controllable, a triggering event may be a vehicle startup. It is predictable that shortly after engine startup, the engine speed will increase from idling to a higher operating speed and can therefore achieve the high input member speed mixing phase of the sequence. The sequence could be triggered with every engine startup, or after a predetermined number of startups (e.g., every third or fourth engine startup).

[0061] Comparison testing was performed using the torque regeneration sequence shown in Fig. 10. First, a clutch was run for a sixty -hour test duration without any torque regeneration mode, and then the clutch was tested for another sixty-hour duration but with the torque regeneration sequence run every four hours of the sixty-hour test run. The clutch performance for the first run without any torque regeneration sequence deteriorated, while the clutch performance did not deteriorate when the torque regeneration sequence was used.

[0062] Various features of the invention are set forth in the following claims.

Claims

1. A method of controlling a clutch having a first magnetic system component, a second magnetic system component, the first and second magnetic system components defining therebetween a gap containing conductive particles, and a coil assembly operable in combination with the conductive particles to selectively magnetically couple the first and second magnetic system components together, the method comprising: controlling the clutch in a first, operational mode to achieve a desired torque transfer condition between the first and second magnetic system components, and controlling the clutch in a second, torque regeneration mode to cause a magnetically-induced agitation of the conductive particles in the gap.

2. The method of claim 1, wherein the coil assembly includes a single coil, and wherein controlling the clutch in the first and second modes includes varying an input current to the single coil.

3. The method of claim 1, wherein a triggering event triggers a change in controlling the clutch from the first mode to the second mode.

4. The method of claim 1 , wherein the triggering event is one of a predetermined amount of time the clutch has been operating, and detection of a packed condition of the conductive particles.

5. The method of claim 1, wherein controlling the clutch in the second mode includes varying an input current to the coil assembly.

6. The method of claim 5, wherein controlling the clutch in the second mode further includes varying a rotational speed of at least one of the first and second magnetic system components.

7. The method of claim 5, wherein varying the input current to the coil assembly includes providing a first, non-zero input current followed by providing a second input current of about zero amps.

8. The method of claim 1 , wherein controlling the clutch in the first and second modes includes using a controller.

9. The method of claim 1, wherein controlling the clutch in the first and second modes includes using a pulse-width-modulator.

10. The method of claim 9, further comprising using a diode to stabilize a current provided to the coil assembly in response to oscillation of the pulse-width-modulator.

11. The method of claim 1 , wherein controlling the clutch in the first and second modes includes using a variable frequency device.

12. The method of claim 1 , wherein the clutch is powered by a DC power supply, and wherein controlling the clutch in the first and second modes includes using a capacitor to stabilize the DC power supply.

13. The method of claim 1, wherein the first and second modes cannot occur simultaneously.

14. A system for controlling a clutch assembly, the system comprising; a clutch assembly having a first magnetic system component, a second magnetic system component, the first and second magnetic system components defining therebetween a gap containing conductive particles, and a coil assembly operable in combination with the conductive particles to selectively magnetically couple the first and second magnetic system components together, and; a controller operable to control the clutch assembly in a first, operational mode to achieve a desired torque transfer condition between the first and second magnetic system components, and in a second, torque regeneration mode to cause a magnetically-induced agitation of the conductive particles in the gap.

15. The system of claim 14, wherein the coil assembly includes only a single coil, and wherein the controller controls an input current to the single coil in both of the first and second modes.

16. The system of claim 14, wherein the controller is operable to detect a triggering event to switch control of the clutch assembly from the first mode to the second mode.

17. The system of claim 16, wherein the triggering event is one of a predetermined amount of time the clutch has been operating, and detection of a packed condition of the conductive particles.

18. The system of claim 14, wherein the controller varies an input current to the coil assembly when controlling the clutch assembly in the second mode.

19. The system of claim 18, wherein the controller further varies a rotational speed of at least one of the first and second magnetic system components.

20. The system of claim 14, where the controller controls a variable frequency device to control operation of the clutch assembly in the first and second modes.

21. The system of claim 14, wherein the first and second modes cannot occur simultaneously.