Control device for electromagnetic clutch

A control device for an electromagnetic clutch includes: a first switch that is connected to one end of an electromagnetic coil forming the electromagnetic clutch; a second switch that is connected to the other end of the electromagnetic coil; and a microcomputer that controls an on/off state of both switches. The control device further includes: a first diode having its anode terminal connected between the electromagnetic coil and the first switch and its cathode terminal connected between the second switch and an in-vehicle power source; and a second diode having its anode terminal connected to the ground and its cathode terminal connected between the electromagnetic coil and the second switch. The microcomputer stops power supply to the electromagnetic coil by turning off both switches.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-048810 filed on Mar. 12, 2014 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to control devices for electromagnetic clutches.

2. Description of the Related Art

Conventionally, electromagnetic clutches are widely known in the art that are capable of permitting and cutting off torque transmission between a first rotary member and a second rotary member by controlling current application to an electromagnetic coil. See, e.g., Japanese Patent Application Publication No. 2009-138798 (JP 2009-138798 A). Even if power supply to the electromagnetic coil is stopped in order to cut off torque transmission between the first and second rotary members, a current flowing in the electromagnetic coil does not immediately become zero due to a back electromotive force that is generated due to electromagnetic induction so as to oppose a change in the amount of current flowing in the electromagnetic coil. This causes delay in response of the electromagnetic clutch.

As a solution to this problem, Japanese Patent Application Publication No. 2004-92691 (JP 2004-92691 A), for example, proposes a control device for an electromagnetic clutch in which a load such as a resistor is connected in parallel to an electromagnetic coil so that a current due to a back electromotive force flows in the load when power supply to the electromagnetic coil is stopped. The back electromotive force that is generated in the electromagnetic coil when power supply to the electromagnetic coil is stopped is consumed by conversion to heat in the load. Accordingly, a current flowing in the electromagnetic coil can quickly become zero, and responsiveness of the electromagnetic clutch can be improved.

In recent years, higher responsiveness has been required for the electromagnetic clutches, and the required level of responsiveness has not been achieved even with such a configuration as described in JP 2004-92691 A. Accordingly, there is a demand for a new technique capable of implementing higher responsiveness.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a control device for an electromagnetic clutch that is capable of implementing high responsiveness.

According to an aspect of the invention, a control device for an electromagnetic clutch that permits and cuts off torque transmission between a first rotary member and a second rotary member includes: a first switch that is connected to one end of an electromagnetic coil forming the electromagnetic clutch; a second switch that is connected to the other end of the electromagnetic coil; a first diode that allows a current to flow from the one end of the electromagnetic coil to a power supply line connecting the second switch and a power source; a second diode that allows a current to flow from a reference potential point to the other end of the electromagnetic coil; and a control circuit that controls an on/off state of the first and second switches. The control circuit stops power supply to the electromagnetic coil by turning off both the first and second switches.

According to the above configuration, a back electromotive force that is generated when power supply to the electromagnetic coil is stopped is consumed as a current flows from the second diode to the power source via the electromagnetic coil and the first diode. That is, the back electromotive force of the electromagnetic coil is consumed by regeneration by the power source. Accordingly, the current flowing in the electromagnetic coil can quickly become zero, and responsiveness of the electromagnetic clutch can be improved as compared to the case where the back electromotive force of the electromagnetic coil is consumed by conversion to heat using a load such as a resistor.

DETAILED DESCRIPTION OF EMBODIMENTS

A first embodiment of the invention will be described below with reference to the accompanying drawings.

As shown inFIG. 1, a vehicle1is a four-wheel drive vehicle based on a front-wheel-drive vehicle. An engine2serving as a driving source is mounted on the front part (the left side inFIG. 1) of the vehicle1. A transaxle3is attached to the engine2. The transaxle3includes a clutch mechanism4in addition to a transmission, a front differential, a transfer case, etc. (which are not shown). Front wheels FR, FL are always coupled to the transaxle3via a pair of right and left front axles5R,5L. A propeller shaft6is coupled to the transaxle3via the clutch mechanism4.

A pinion shaft (drive pinion shaft)8is coupled to the propeller shaft6via a torque coupling7, and a rear differential9is coupled to the pinion shaft8. Rear wheels RR, RL are coupled to the rear differential9via a pair of right and left rear axles10R,10L. A well-known bevel gear differential is used as the rear differential9of the present embodiment. An electromagnetic clutch11is disposed between the rear differential9and the right rear axle10R.

The clutch mechanism4is capable of permitting and cutting off torque transmission between an output shaft (not shown) of the transaxle3and the propeller shaft6. The torque coupling7is configured so that a friction engagement force between clutch plates (not shown) provided on the propeller shaft6side and on the pinion shaft8side changes according to the amount of current that is applied to an electromagnetic coil (not shown) of the torque coupling7. The torque coupling7transmits torque based on the friction engagement force between the clutch plates from the propeller shaft6on the input side to the pinion shaft8on the output side. That is, the torque coupling7is capable of changing torque (torque transmission capacity) that can be transmitted to the pinion shaft8(the rear wheels RR, RL). The electromagnetic clutch11is capable of permitting and cutting off torque transmission between a side gear (not shown) of the rear differential9and the rear axle10R.

A control device12that controls operation of the clutch mechanism4, the torque coupling7, and the electromagnetic clutch11is connected to the clutch mechanism4, the torque coupling7, and the electromagnetic clutch11. The clutch mechanism4of the present embodiment is connected to the control device12via a host electronic control unit (ECU). An accelerator operation amount sensor13and wheel speed sensors14ato14dare connected to the control device12. The control device12calculates a vehicle speed V and a wheel speed difference ΔW between the front wheels FR, FL and the rear wheels RR, RL based on wheel speeds Vfr, Vfl, Vrr, Vrl that are detected by the wheel speed sensors14ato14d. The control device12controls operation of the clutch mechanism4, the torque coupling7, and the electromagnetic clutch11based on the vehicle speed V, the wheel speed difference ΔW, and an accelerator operation amount signal S_a from the accelerator operation amount sensor13.

Specifically, the control device12computes a control target value of the torque transmission capacity (target torque) of the torque coupling7based on the vehicle speed V, the wheel speed difference ΔW, and the accelerator operation amount signal Sa. If the target torque is zero, the control device12operates so as to switch the vehicle1into a two-wheel drive mode. That is, the control device12brings the clutch mechanism4into a disengaged state, namely the state where the clutch mechanism4cannot transmit torque, to cut off torque transmission from the engine2to the propeller shaft6, and also brings the electromagnetic clutch11into a disengaged state to cut off torque transmission from the side gear to the rear axle10R. At this time, the control device12also stops power supply to the torque coupling7.

If the target torque is larger than zero, the control device12operates so as to switch the vehicle1into a four-wheel drive mode. That is, the control device12brings the clutch mechanism4into an engaged state, namely the state where the clutch mechanism4can transmit torque, to permit torque transmission from the engine2to the propeller shaft6, and also brings the electromagnetic clutch11into an engaged state to permit torque transmission from the side gear to the rear axle10R. At this time, the control device12controls operation of the torque coupling7by power supply to the electromagnetic coil so that the torque transmission capacity becomes equal to the target torque.

Accordingly, in the two-wheel drive mode, the torque of the engine2is transmitted only to the front wheels FR, FL through the transaxle3and the front axles5R,5L. In contrast, in the four-wheel drive mode, the torque of the engine2is transmitted to the front wheels FR, FL through the transaxle3and the front axles5R,5L, and is also transmitted to the rear wheels RR, RL through the propeller shaft6, the pinion shaft8, the rear differential9, and the rear axles10R,10L as necessary. That is, in the present embodiment, the front wheels FR, FL function as main drive wheels to which torque is always transmitted, and the rear wheels RR, RL function as auxiliary drive wheels to which torque is transmitted as necessary.

The configuration of the electromagnetic clutch will be described in detail below.

As shown inFIG. 2, the electromagnetic clutch11is capable of permitting and cutting off torque transmission between a rod-shaped first rotary member21and a cylindrical second rotary member22. The first rotary member21is coupled to the side gear of the rear differential9so as to be rotatable together therewith. The second rotary member22is coupled to the rear axle10R so as to be rotatable together therewith. The first rotary member21and the second rotary member22are supported on the same axis L in a housing23so as to be rotatable relative to each other. In the following description, the “first direction side” refers to the rear axle10R side (the right side inFIG. 2) on the axis L, and the “second direction side” refers to the rear differential9side (the left side inFIG. 2) on the axis L.

The housing23includes first and second housings31,32having the shape of a bottomed cylinder. A bottom33of the first housing31has a through hole34extending therethrough in the axial direction. A bottom35of the second housing32has a through hole36extending therethrough in the axial direction, and an extended cylindrical portion37is extended from the outer bottom surface of the bottom35toward the first direction side. The housing23is formed by coupling the opening ends of the first and second housings31,32.

The first rotary member21is supported so as to be rotatable relative to the first housing31via a rolling bearing41provided in the through hole34of the first housing31. The first rotary member21has a round rod-shaped shaft portion42, an extended portion43, and a cylindrical portion44. The extended portion43is extended radially outward from the first direction side (the second rotary member22side) end of the shaft portion42. The cylindrical portion44is extended in the axial direction from the radially outer end of the extended portion43toward the first direction side. A first spline fitting portion45as a first meshing portion is formed on the opening end of the inner peripheral surface of the cylindrical portion44.

The second rotary member22is supported so as to be rotatable relative to the first rotary member21and the second housing32via rolling bearings46,47. The rolling bearing46is provided on the inner periphery of the cylindrical portion44of the first rotary member21, and the rolling bearing47is provided on the inner periphery of the extended cylindrical portion37of the second housing32. An annular fixed member48is fixed to the outer periphery of the second rotary member22so as to adjoin the first direction side of the rolling bearing46. The second direction side end of the rear axle10R is spline-fitted in the second rotary member22. The second rotary member22is thus coupled to the rear axle10R so as to be rotatable therewith. A snap ring49is interposed between the second rotary member22and the rear axle10R to restrict relative axial movement between the second rotary member22and the rear axle10R.

The electromagnetic clutch11includes a meshing member51, a biasing member52, and a cam mechanism53. The meshing member51is coupled to the second rotary member22so as to be movable in the axial direction relative to the second rotary member22and so as not to be rotatable relative to the second rotary member22. The biasing member52biases the meshing member51in the axial direction. The cam mechanism53together with the meshing member51synchronizes (matches) rotation of the first rotary member21with rotation of the second rotary member22. The electromagnetic clutch11further includes an electromagnetic coil54, an armature55, and a pressing mechanism56. The electromagnetic coil54generates a magnetic force when a current is applied thereto. The armature55is moved in the axial direction by the magnetic force of the electromagnetic coil54. With the axial movement of the armature55, the pressing mechanism56presses the meshing member51against the biasing force of the biasing member52to move the meshing member51in the axial direction.

The meshing member51has a cylindrical portion61and a friction engagement portion62having the shape of a bottomed cylinder. The friction engagement portion62is extended radially outward from the first direction side end of the cylindrical portion61and then extended toward the second direction side. The cylindrical part of the friction engagement portion62is tapered toward the second direction side. Namely, the outer peripheral surface of the cylindrical part of the friction engagement portion62is tilted radially inward so that the thickness (length in the radial direction) of the friction engagement portion62gradually decreases toward the second direction side. The cylindrical portion61of the meshing member51is spline-fitted on the second rotary member22. The meshing member51is thus coupled to the second rotary member22so as to be movable in the axial direction relative to the second rotary member22and so as not to be rotatable relative to the second rotary member22. An accommodating hole63extending in the axial direction is formed in the second direction side end face of the cylindrical portion61. A second spline fitting portion64serving as a second meshing portion capable of meshing with the first spline fitting portion45is formed on the outer periphery of the second direction side end of the cylindrical portion61.

The second spline fitting portion64is formed in such a range in the axial direction that the second spline fitting portion64meshes with the first spline fitting portion45when the meshing member51is located on the first direction side and that the second spline fitting portion64is disengaged from the first spline fitting portion45as the meshing member51moves toward the second direction side. When the first and second spline fitting portions45,64mesh with each other, the electromagnetic clutch11is engaged so that torque can be transmitted between the first and second rotary members21,22. When the first and second spline fitting portions45,64are disengaged from each other, the electromagnetic clutch11is disengaged so that torque cannot be transmitted between the first and second rotary members21,22.

A spring member such as a coil spring is used as the biasing member52. The first direction side end of the biasing member52is coupled to the bottom surface of the accommodating hole63of the meshing member51, and the second direction side end of the biasing member52is coupled to a side surface of the fixed member48. The biasing member52is thus disposed in an axially compressed state between the fixed member48and the meshing member51. The biasing member52therefore biases the meshing member51toward the first direction side, namely in such a direction that the second spline fitting portion64meshes with the first spline fitting portion45.

The cam mechanism53includes annular first and second cam members71,72, a plurality of balls (cam followers)73interposed between the first and second cam members71,72, and an annular stopper74that restricts axial movement of the second cam member72.

The first cam member71is fitted on the cylindrical portion44of the first rotary member21so as to be rotatable relative to the cylindrical portion44of the first rotary member21. The first cam member71has a cylindrical friction engagement portion75extended from the radially outer end of the first cam member71toward the first direction side. The friction engagement portion75is tapered toward the first direction side. Namely, the inner peripheral surface of the friction engagement portion75is tilted radially outward so that the thickness of the friction engagement portion75gradually decreases toward the first direction side. The friction engagement portion62of the meshing member51thus strongly frictionally engages with the friction engagement portion75of the first cam member71as the meshing member51moves toward the second direction side. The second cam member72is spline-fitted on the cylindrical portion44. The second cam member72is thus coupled to the first rotary member21so as to be movable in the axial direction relative to the first rotary member21and so as not to be rotatable relative to the first rotary member21. The stopper74is fixed to the second direction side end of the outer periphery of the cylindrical portion44.

A plurality of cam grooves76,77tilted with respect to the circumferential direction are formed at regular angular intervals on the opposing surfaces of the first and second cam members71,72. Each of the cam grooves76,77of the present embodiment has a U-shaped section, and is formed so that its depth increases from the center in the circumferential direction toward both ends in the circumferential direction. The balls73are disposed in the opposing cam grooves76,77and held between the first and second cam members71,72.

When the friction engagement portions62,75are not in friction engagement with each other, the balls73are located in the centers in the circumferential direction of the cam grooves76,77in this cam mechanism53, and no axial pressing force is generated in the first and second cam members71,72. If the meshing member51moves toward the second direction side so that the friction engagement portions62,75frictionally engage with each other and the first and second cam members71,72rotate relative to each other, the balls73move in the circumferential direction within the cam grooves76,77, and the second cam member72is separated from the first cam member71and pressed against the stopper74. An axial pressing force is thus generated between the first and second cam members71,72, which restricts rotation of the first cam member71relative to the first rotary member21. Torque is thus transmitted from the first cam member71to the meshing member51, whereby the rotational speed of the first rotary member21becomes closer to that of the second rotary member22. That is, rotation of the first rotary member21is synchronized with that of the second rotary member22.

The electromagnetic coil54is formed by winding around a resin bobbin81a wire82in which a current supplied from the control device12flows. The electromagnetic coil54is held by an annular yoke83made of a ferromagnetic material such as iron. The yoke83is supported by the inner periphery of the second housing32. Fitting holes84opening toward the first direction side are formed in the yoke83at intervals in the circumferential direction. A plurality of fitting holes85opening toward the second direction side are formed in the second housing32at positions facing the respective fitting holes84in the axial direction. The ends of each columnar pin86that is disposed parallel to the axis L are fitted in the fitting holes84,85.

As shown inFIGS. 2 and 3, the armature55is formed in the shape of a disc. The armature55has a through hole91formed in its center, and insertion holes92formed at intervals in the circumferential direction. The second rotary member22is inserted through the through hole91, and the pins86are inserted through the respective insertion holes92. The armature55further has a plurality of (six in the present embodiment) pressing protrusions93protruding radially inward from the inner peripheral surface of the through hole91. Facing surfaces93aof the pressing protrusions93which face the axial end faces of engaged portions of a piston102described below are tilted surfaces tilted with respect to the thickness direction of the armature55(the direction parallel to the axis L).

The armature55is biased in such a direction that the armature55is separated from the yoke83by a coned disc spring94placed between the armature55and the yoke83. When no current is applied to the electromagnetic coil54, the armature55contacts the bottom35of the second housing32by the biasing force of the coned disc spring94. When a current is applied to the electromagnetic coil54, the armature55contacts the yoke83against the biasing force of the coned disc spring94due to the magnetic force that is generated by the electromagnetic coil54. Rotation of the armature55relative to the second housing32and the yoke83is restricted by the pins86inserted through the insertion holes92. The armature55is thus guided by the pins86to move between a first position where the armature55contacts the bottom35of the second housing32and a second position where the armature55contacts the yoke83. The second housing32is provided with a stroke sensor95that detects the position of the armature55. The stroke sensor95is connected to the control device12.

The pressing mechanism56has a plurality of engagement portions101that are not movable in the axial direction relative to the second housing32(housing23) and not rotatable relative to the second housing32(housing23), and the cylindrical piston102having a plurality of levels of engaged portions that are engaged by the engagement portions101at different positions in the axial direction. As the armature55moves in the axial direction, the engagement portions101of the pressing mechanism56engage with the engaged portions of a different level located at a different position in the axial direction out of the plurality of levels.

As shown inFIGS. 2 and 4, the engagement portions101are formed at regular angular intervals in the circumferential direction. Each of the engagement portions101has an L-shape protruding radially inward from the inner peripheral surface of the through hole36of the second housing32and then protruding toward the second direction side along the axis L. Distal end faces101aof the engagement portions101face the axial end faces of the engaged portions of the piston102described below. Like the facing surfaces93aof the pressing protrusions93of the armature55, the distal end faces101aare tilted surfaces tilted with respect to the direction parallel to the axis L.

As shown inFIGS. 2 and 5, the piston102has a cylindrical shape, and is disposed on the first direction side of the meshing member51and fitted on the outer periphery of the second rotary member22. The piston102is clearance-fitted on the second rotary member22so as to be movable in the axial direction relative to the second rotary member22and to be rotatable relative to the second rotary member22. A rolling bearing103such as a needle roller thrust bearing is disposed between the piston102and the meshing member51. The piston102is thus biased toward the first direction side, namely toward the engagement portion101side, by the biasing member52via the meshing member51and the rolling bearing103. A base end face102aof the piston102on the second direction side, which contacts the rolling bearing103, is in the form of a planar surface. When the armature55is pulled in by the electromagnetic coil54and moves in the axial direction, the piston102presses the meshing member51toward the second direction side, namely in such a direction that the first spline fitting portion45of the first rotary member21is disengaged from the second spline fitting portion64of the meshing member51.

The plurality of levels of engaged portions that are engaged by the engagement portions101at different positions in the axial direction are formed on the first direction side end of the piston102so as to be adjoin each other in the circumferential direction. In the present embodiment, the piston102has four levels of engaged portions. Specifically, the plurality of levels of engaged portions are first to fourth engaged portions104to107, and six sets of these engaged portions are formed along the circumferential direction.

The first to fourth engaged portions104to107are formed as follows. The second engaged portion105is formed on the right side of the first engaged portion104so as to adjoin the first engaged portion104, the third engaged portion106is formed on the right side of the second engaged portion105so as to adjoin the second engaged portion105, and the fourth engaged portion107is formed on the right side of the third engaged portion106so as to adjoin the third engaged portion106, when the piston102is viewed from the first direction side. A wall108protruding in the axial direction is formed at the opposite circumferential end of the fourth engaged portion107from the third engaged portion106.

The first to fourth engaged portions104to107are formed at different positions in the axial direction in the piston102. Specifically, the second engaged portion105is located farther away from the base end face102athan the first engaged portion104is. The third engaged portion106is located farther away from the base end face102athan the second engaged portion105is. The fourth engaged portion107is located farther away from the base end face102athan the third engaged portion106is.

Axial end faces104ato107aof the first to fourth engaged portions104to107are tilted with respect to the circumferential direction of the piston102. Specifically, the axial end face104aof the first engaged portion104is tilted so that its circumferential end which is located on the second engaged portion105side is located closer to the base end face102athan the other circumferential end is. Similarly, the axial end face105aof the second engaged portion105is tilted so that its circumferential end which is located on the third engaged portion106side is located closer to the base end face102athan the other circumferential end is. The axial end face106aof the third engaged portion106is tilted so that its circumferential end which is located on the fourth engaged portion107side is located closer to the base end face102athan the other circumferential end is. The axial end face107aof the fourth engaged portion107is tilted so that its circumferential end which is located on the wall108side is located closer to the base end face102athan the other circumferential end is. An axial end face108aof the wall108is tilted in the same direction as the axial end faces104ato107aof the first to fourth engaged portions104to107. A circumferential side surface108bof the wall108faces the fourth engaged portion107.

The facing surfaces93aof the pressing protrusions93of the armature55and the distal end faces101aof the engagement portions101contact the axial end faces104ato107aof the first to fourth engaged portions104to107. Specifically, the facing surfaces93aof the pressing protrusions93of the armature55contact the radially outer parts of the axial end faces104ato107a, and the distal end faces101aof the engagement portions101contact the radially inner parts of the axial end faces104ato107a.

The interval between the distal end face101aof the engagement portion101and the base end face102aof the piston102is the shortest when the engagement portion101engages with the first engaged portion104. The meshing member51is thus moved toward the first direction side by the biasing force of the biasing member52, and the first spline fitting portion45of the first rotary member21meshes with the second spline fitting portion64of the meshing member51. Namely, the electromagnetic clutch11is engaged. The interval between the distal end face101aof the engagement portion101and the base end face102aof the piston102is the longest when the engagement portion101engages with the fourth engaged portion107. The meshing member51is thus pressed by the piston102to move toward the second direction side, and the first spline fitting portion45is disengaged from the second spline fitting portion64. Namely, the electromagnetic clutch11is disengaged. If the armature55is located at the second position where the armature55contacts the yoke83when the engagement portion101engages with the second engages with portion105or the third engaged portion106, the first spline fitting portion45is disengaged from the second spline fitting portion64. Namely, the electromagnetic clutch11is disengaged.

Operation of the pressing mechanism will be described.

FIGS. 6A to 6Dare perspective views showing the armature55and the piston102. Regarding the second housing32, only the portion near the engagement portions101is shown inFIGS. 6A to 6D.FIGS. 7A to 7Dare schematic views showing the piston102together with the pressing protrusion93of the armature55and the engagement portion101as viewed from outside in the radial direction.

FIGS. 6A and 7Ashow a first state where the engagement portion101engages with the first engaged portion104and the armature55is located at the first position. In the first state, the axial end face104aof the first engaged portion104is pressed against the distal end face101aof the engagement portion101by the biasing force of the biasing member52and faces the facing surface93aof the pressing protrusion93of the armature55. The engagement portion101contacts a circumferential side surface104bof the first engaged portion104, and the pressing protrusion93of the armature55faces the axial end face104aat a position separated in the circumferential direction of the piston102from the circumferential side surface104b. The circumferential side surface104bis a surface forming the difference in level between the first engaged portion104and the second engaged portion105, and is a flat surface parallel to the axis L. In the first engaged portion104, the angle between the axial end face104aand the circumferential side surface104bis an acute angle.

FIGS. 6B and 7Bshow a second state where the armature55has been moved from the first state shown inFIGS. 6A and 7Ato the second position by power supply to the electromagnetic coil54. During the transition from the first state to the second state, the facing surface93aof the pressing protrusion93of the armature55contacts the axial end face104a, and the pressing protrusion93presses the piston102toward the second direction side (the meshing member51side). In the second state, the engagement portion101is separated from the circumferential side surface104bof the first engaged portion104, and the piston102rotates in the direction of arrow A by a first predetermined angle due to sliding between the axial end face104aof the first engaged portion104and the facing surface93aof the pressing protrusion93of the armature55. The circumferential side surface104bof the first engaged portion104contacts a circumferential side surface93bof the pressing protrusion93of the armature55due to the rotation of the piston102.

That is, as the armature55moves from the first position to the second position, the armature55moves the piston102toward the second direction side (the meshing member51side) and rotates the piston102by the first predetermined angle. The first predetermined angle is an angle corresponding to the distance d1between the pressing protrusion93of the armature55and the circumferential side surface104bof the first engaged portion104as shown inFIG. 7A.

When the armature55is located at the second position, the distal end face101aof the engagement portion101is located at a distance from the second engagement portion105and faces the axial end face105a. That is, when the armature55moves to the second position, the piston102rotates by the first predetermined angle, so that the pressing protrusion93contacts the circumferential side surface104b, and the distal end face101aof the engagement portion101faces the axial end face105aof the second engaged portion105adjoining the first engaged portion104.

FIGS. 6C and 7Cshow a third state where power supply to the electromagnetic coil54has been stopped, and the armature55is returning from the second position to the first position. In the third state, the distal end face101aof the engagement portion101contacts the axial end face105aof the second engaged portion105. Due to this contact between the distal end face101aof the engagement portion101and the axial end face105aof the second engaged portion105, the piston102is subjected to a rotational force in the direction of arrow A. However, this rotation in the direction of arrow A is restricted by contact between the circumferential side surface93bof the pressing protrusion93of the armature55and the circumferential side surface104bof the first engaged portion104.

FIGS. 6D and 7Dshow a fourth state where the armature55has returned to the first position, and the piston102has rotated in the direction of arrow A until a circumferential side surface105bof the second engaged portion105contacts a circumferential side surface101bof the engagement portion101. In the fourth state, the axial end face105aof the second engaged portion105of the piston102subjected to the biasing force of the biasing member52slides on the distal end face101aof the engagement portion101, whereby the piston102rotates relative to the engagement portion101by a second predetermined angle. The engagement portion101thus engages with the second engaged portion105. The second predetermined angle is an angle corresponding to the distance d2between the circumferential side surface105bof the second engaged portion105and the engagement portion101in the third state shown inFIG. 7C. That is, as the armature55moves from the second position to the first position, the piston102is further rotated by the second predetermined angle, and the engagement portion101engages with the second engaged portion105adjoining the first engaged portion104.

As described above, the pressing mechanism56moves the meshing member51in the axial direction against the biasing force of the biasing member52as the armature55reciprocates between the first position and the second position. In the present embodiment, the piston102has the first to fourth engaged portions104to107that are formed as four steps. Accordingly, power supply to the electromagnetic coil54is conducted and stopped three times, and the armature55reciprocates three times between the first position and the second position, whereby the piston102rotates from the position where the engagement portion101engages with the first engaged portion104to the position where the engagement portion101engages with the fourth engaged portion107. As shown inFIG. 7A, the distance d4from the base end face102aof the piston102to the axial end face107aof the fourth engaged portion107is longer than the distance d3from the base end face102aof the piston102to the axial end face104aof the first engaged portion104. The piston102advances and withdraws in the axial direction in a range corresponding to the difference between the distance d4and the distance d3.

FIGS. 8A to 8Dare schematic views illustrating operation at the time the engagement portion101switches from the state where it engages with the fourth engaged portion107to the state where it engages with the first engaged portion104, and the electromagnetic clutch11switches from the disengaged state to the engaged state.

FIG. 8Ashows a state where the engagement portion101engages with the fourth engaged portion107, and the armature55is located at the first position. In this state, the engagement portion101contacts the axial end face107aof the fourth engaged portion107and the circumferential side surface108bof the wall108.

FIG. 8Bshows a state where the armature55has moved to the second position. During the transition from the first state to the second state, the pressing protrusion93presses the piston102toward the meshing member51side, and the engagement portion101is separated from the circumferential side surface108bof the wall108. The piston102therefore rotates in the direction of arrow A by the first predetermined angle.

FIG. 8Cshows a state where the armature55is returning from the second position to the first position. In this state, the distal end face101aof the engagement portion101contacts the axial end face108aof the wall108, and the piston102is subjected to a rotational force in the direction of arrow A.

FIG. 8Dshows a state where the armature55has returned to the first position, and the engagement portion101has rotated in the direction of arrow A until it engages with the first engaged portion104. During the transition from the state ofFIG. 8Cto the state ofFIG. 8D, the piston102is shifted to a large extent in the axial direction in the entire range corresponding to the difference between the distance d3and the distance d4, and the second spline fitting portion64of the meshing member51meshes with the first spline fitting portion45of the first rotary member21.

When the piston102thus moves toward the first direction side (the opposite side from the meshing member51) according to the axial movement of the armature55, the second spline fitting portion64meshes with the first spline fitting portion45by the biasing force of the biasing member52. More specifically, when the engagement portion101is separated from the fourth engaged portion107formed at the farthest position from the meshing member51out of the first to fourth engaged portions104to107and engages with the first engaged portion104formed at the closest position to the meshing member51, the second spline fitting portion64of the meshing member51meshes with the first spline fitting portion45by the biasing force of the biasing member52, and the first rotary member21is coupled to the second rotary member22so that torque can be transmitted therebetween. Namely, the electromagnetic clutch11is engaged.

That is, the armature55needs to reciprocate three times between the first position and the second position in order for the electromagnetic clutch11to switch from the engaged state to the disengaged state. However, the armature55needs only to reciprocate once between the first position and the second position in order for the electromagnetic clutch11to switch from the disengaged state to the engaged state.

Transition of the state of the electromagnetic clutch that is caused by the operation of the pressing mechanism will be described below.

As shown inFIG. 9A, in the state where the engagement portion101engages with the first engaged portion104, the first and second spline fitting portions45,64mesh with each other. The electromagnetic clutch11is thus in the engaged state. In order to switch the electromagnetic clutch11from the engaged state to the disengaged state, current application to the electromagnetic coil54is controlled to reciprocate the armature55a plurality of times between the first position and the second position so that the engagement portion101engages with the fourth engaged portion107. The meshing member51thus moves toward the second direction side in the axial direction against the biasing force of the biasing member52, and the first and second spline fitting portions45,64are disengaged from each other. The electromagnetic clutch11is thus switched to the disengaged state. As shown inFIG. 9B, in the state where the engagement portion101engages with the fourth engaged portion107, the first and second spline fitting portions45,64do not mesh with each other and the friction engagement portions62,75do not frictionally engage with each other even if the armature55is moved to the first position by stopping power supply to the electromagnetic coil54. The disengaged state of the electromagnetic clutch11is thus maintained.

As shown inFIG. 9C, in order to switch the electromagnetic clutch11from the disengaged state to the engaged state, power is supplied to the electromagnetic coil54to move the armature55to the second position and to move the meshing member51toward the second direction side, thereby frictionally engaging the friction engagement portions62,75. After the first and second rotary members21,22are synchronized with each other, power supply to the electromagnetic coil54is stopped to move the armature55to the first position so that the engagement portion101engages with the first engaged portion104. The first and second spline fitting portions45,64thus mesh with each other, and the electromagnetic clutch11is switched to the engaged state (seeFIG. 9A).

The electrical configuration of the control device will be described below. Description of the configuration other than that associated with control of the electromagnetic clutch will be omitted for convenience.

As shown inFIG. 10, the control device12includes first and second switches201,202and a microcomputer203. The first and second switches201,202control power supply from an in-vehicle power source B to the electromagnetic coil54of the electromagnetic clutch11. The microcomputer203is a control circuit that outputs control signals S1, S2for controlling the on/off state of the first and second switches201,202. For example, semiconductor switching elements that are turned on/off in response to a gate voltage such as metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs) are used as the first and second switches201,202of the present embodiment. The control signals S1, S2that are output from the microcomputer203are voltage signals that are applied to gate terminals of the first and second switches201,202.

The first switch201has its drain terminal connected to one end of the electromagnetic coil54and its source terminal connected to the ground. A temperature sensor204that detects the temperature (detected temperature T) of the first switch201is provided near the first switch201.

The second switch202has its drain terminal connected to the in-vehicle power source B via a power supply line Lp and its source terminal connected to the other end of the electromagnetic coil54. A current sensor205is provided on the power supply line Lp. The current sensor205detects a value of an actual current flowing in the electromagnetic coil54(actual current value I). A capacitor206and a coil207are connected to the power supply line Lp in order to smooth a current that is applied to the power supply line Lp. A drive relay208that is formed by a mechanical relay, a field effect transistor (FET), etc. is provided on the in-vehicle power source B side of the coil207. The power supply line Lp is conductive when the drive relay208is on, and is not conductive when the drive relay208is off. When the power supply line Lp is conductive, power according to the voltage of the in-vehicle power source B can be supplied to the electromagnetic coil54

The control device12further includes a first diode211and a second diode212. The first diode211allows a current to flow from the one end of the electromagnetic coil54to the in-vehicle power source B side of the current sensor205in the power supply line Lp. The second diode212allows a current to flow from the ground as a reference potential point to the other end of the electromagnetic coil54. Specifically, the first diode211has its anode terminal connected to the one end of the electromagnetic coil54and its cathode terminal connected between the current sensor205and the coil207in the power supply line Lp. The second diode212has its anode terminal connected to the ground and its cathode terminal connected to the other end of the electromagnetic coil54.

An IG signal S_ig indicating the on/off state of an ignition switch (hereinafter referred to as the “IG”) of the vehicle is input to the microcomputer203. When the microcomputer203receives an IG signal S_ig indicating that the IG is on, the microcomputer203outputs a relay control signal S_rl that turns on the drive relay208. When the microcomputer203receives an IG signal S_ig indicating that the IG is off, the microcomputer203outputs a relay control signal S_rl that turns off the drive relay208. If the actual current value I is larger than a threshold current Ith, the microcomputer203outputs a relay control signal S_rl that turns off the drive relay208. For example, the threshold current Ith is a current value indicating that a ground fault etc. of a wire between the control device12and the electromagnetic clutch11has occurred. The threshold current Ith is obtained in advance by experiments etc.

The stroke sensor95is connected to the microcomputer203. The stroke sensor95detects the position of the armature55of the electromagnetic clutch11, and the microcomputer203detects the position of the armature55based on a position signal S_po that is output from the stroke sensor95. An initial position of the piston102is stored in advance in the microcomputer203of the present embodiment, and the microcomputer203determines if the electromagnetic clutch11is in the engaged state or in the disengaged state based on the initial position of the piston102and the number of times the armature55reciprocates between the first and second positions.

In addition to the temperature sensor204and the current sensor205, the accelerator operation amount sensor13and the wheel speed sensors14ato14dare connected to the microcomputer203. The detected temperature T of the first switch201, the actual current value I flowing in the electromagnetic coil54, the accelerator operation amount signal S_a, and the wheel speeds Vfr, Vfl, Vrr, Vrl are input to the microcomputer203. Based on these received state quantities, the microcomputer203outputs the control signals S1, S2to the first and second switches201,202to control power supply to the electromagnetic coil54. At this time, in the case of switching the vehicle1into the two-wheel drive mode, the microcomputer203controls power supply to the electromagnetic coil54so as to bring the electromagnetic clutch11into the disengaged state. In the case of switching the vehicle1into the four-wheel drive mode, the microcomputer203controls power supply to the electromagnetic coil54so as to bring the electromagnetic clutch11into the engaged state.

The microcomputer203repeatedly conducts and stops power supply to the electromagnetic coil54based on the position signal S_po. The microcomputer203thus operates the pressing mechanism56to bring the electromagnetic clutch11into the engaged state or the disengaged state. Specifically, the microcomputer203supplies power to the electromagnetic coil54until the armature55moves from the first position where the armature55contacts the bottom35of the second housing32to the second position where the armature55contacts the yoke83. The microcomputer203stops power supply to the electromagnetic coil54when the armature55reaches the second position, and resumes power supply to the electromagnetic coil54if the armature55reaches the first position. The microcomputer203repeats this operation to operate the pressing mechanism56. By operating the pressing mechanism56, the microcomputer203continues to stop power supply with the engagement portion101engaging with the first engaged portion104in the case of bringing the electromagnetic clutch11into the engaged state, and continues to stop power supply with the engagement portion101engaging with the fourth engaged portion107in the case of bringing the electromagnetic clutch11into the disengaged state.

If the detected temperature T is equal to or lower than a preset threshold temperature Tth when power is supplied to the electromagnetic coil54, the microcomputer203of the present embodiment applies a pulse-width modulation (PWM)-controlled control signal S1to the first switch201and applies a control signal S2held at Hi level to the second switch202to supply power to the electromagnetic coil54. That is, if the detected temperature T is equal to or lower than the threshold temperature Tth, the microcomputer203drives the first switch201by PWM and holds the second switch202in the on state to supply power to the electromagnetic coil54.

If the detected temperature T is higher than the threshold temperature Tth when power is supplied to the electromagnetic coil54, the microcomputer203applies a control signal S1held at Hi level to the first switch201and applies a PWM-controlled control signal S2to the second switch202to supply power to the electromagnetic coil54. That is, if the detected temperature T is higher than the threshold temperature Tth, the microcomputer203holds the first switch201in the on state and drives the second switch202by PWM to supply power to the electromagnetic coil54. The duty ratios of the control signals S1, S2are computed by performing current feedback control so that the actual current value I follows a target current value.

When stopping power supply to the electromagnetic coil54, the microcomputer203holds the control signals S1, S2at Lo level (stops outputting the control signals S1, S2) and turns off both the first and second switches201,202to stop power supply to the electromagnetic coil54.

Control procedures for the electromagnetic clutch by the microcomputer will be described below.

As shown in the flowchart ofFIG. 11, the microcomputer203performs an initial process such as turning on the drive relay in response to an IG signal S_ig indicating that the IG is on (step S100). The microcomputer203then obtains state quantities (step S101) and determines whether to conduct or stop power supply to the electromagnetic coil54based on the state quantities (step S102).

If it is determined that power should be supplied to the electromagnetic coil54(step S102: YES), the microcomputer203determines if the detected temperature T of the first switch201is higher than the threshold temperature Tth (step S103). If the detected temperature T is equal to or lower than the threshold temperature Tth (step S103: NO), the microcomputer203drives the first switch201by PWM and holds the second switch202in the on state to supply power to the electromagnetic coil54(step S104), and the routine proceeds to step S107described below. If the detected temperature T is higher than the threshold temperature Tth (step S103: YES), the microcomputer203holds the first switch201in the on state and drives the second switch202by PWM to supply power to the electromagnetic coil54(step S105), and the routine proceeds to step S107.

In the case of stopping power supply to the electromagnetic coil54based on the state quantities obtained in step S101(step S102: NO), the microcomputer203turns off both the first and second switches201,202(step S106), and the routine proceeds to step S101.

In step S107, the microcomputer203determines if the actual current value I is larger than the threshold current Ith. If the actual current value I is equal to or smaller than the threshold current Ith (step S107: NO), the microcomputer203continues to supply power to the electromagnetic coil54, and the routine proceeds to step S101. If the actual current value I is larger than the threshold current Ith (step S107: YES), the microcomputer203outputs a relay control signal S_rl that turns off the drive relay208(step S108), and thus stops the electromagnetic clutch11until the IG is turned on again after being turned off.

Functions of the present embodiment will be described.

In the case where the first switch201is driven by PWM and the second switch202is held in the on state when power is supplied to the electromagnetic coil54, the first switch201is repeatedly turned on and off in a very short time. The first switch201therefore tends to overheat due to large switching loss. A major factor in heat generation of the second switch202is saturation loss according to the on-resistance of the second switch202. Accordingly, the second switch202generates less heat than in the case where the second switch202is driven by PWM, and the temperature of the second switch202is less likely to increase to a predetermined temperature according to the ambient temperature, or higher. Similarly, in the case where the first switch201is held in the on state and the second switch202is driven by PWM when power is supplied to the electromagnetic coil54, the temperature of the first switch201is less likely to increase to the predetermined temperature or higher, and the second switch202tends to overheat. According to the present embodiment, if the detected temperature T of the first switch201becomes higher than the threshold temperature Tth, the second switch202instead of the first switch201is driven by PWM, and the first switch201is held in the on state. The first switch201is therefore cooled by natural heat dissipation etc., and overheating of the first switch201is suppressed.

The moment power supply to the electromagnetic coil54is stopped, a back electromotive force is generated in the electromagnetic coil54due to electromagnetic induction. At this time, in the present embodiment, the first and second switches201,202are off. The back electromotive force generated in the electromagnetic coil54is therefore consumed as a current flows from the second diode212to the in-vehicle power source B through the electromagnetic coil54, the first diode211, the coil207, and the drive relay208. That is, the back electromotive force of the electromagnetic coil54is consumed by regeneration by the in-vehicle power source B. Accordingly, a current flowing in the electromagnetic coil54quickly becomes zero as compared to the case where the back electromotive force of the electromagnetic coil54is consumed by, e.g., conversion to heat using a load such as a resistor.

If the actual current value I is larger than the threshold current Ith and it is estimated that a ground fault has occurred, the drive relay208is turned off. This suppresses an overcurrent in the second switch202etc.

Advantageous effects of the present embodiment will be described below.

(1) The back electromotive force that is generated at the moment when power supply to the electromagnetic coil54is stopped is consumed by regeneration by the in-vehicle power source B. Accordingly, a current flowing in the electromagnetic coil54can quickly become zero, and responsiveness of the electromagnetic clutch11can be improved. Moreover, heat generation of the control device12can be suppressed as compared to the case where the back electromotive force is consumed by conversion to heat using a resistor.

(2) The electromagnetic clutch11is switched between the engaged state and the disengaged state by reciprocating the armature55a plurality of times in the axial direction by the presence and absence of the pull-in force that is generated by the electromagnetic coil54. This configuration is very effective in quickly consuming the back electromotive force generated in the electromagnetic coil54by regeneration by the in-vehicle power source B.

(3) If the detected temperature T of the first switch201becomes higher than the threshold temperature Tth, the second switch202is driven by PWM instead of the first switch201. This can suppress overheating of the first switch201.

A second embodiment will be described below with reference to the drawings. For convenience of description, the same configurations as those of the first embodiment will be denoted with the same reference characters, and description thereof will be omitted.

As shown inFIG. 12, the control device12does not include the drive relay208, and the coil207is connected to a high potential-side terminal of the in-vehicle power source B without interposing a switch such as a relay therebetween. If the actual current value I is larger than the threshold current Ith, the microcomputer203holds the control signals S1, S2at Lo level (stops outputting the control signals S1, S2) to hold the first and second switches201,202in the off state.

Control procedures of the microcomputer after power supply to the electromagnetic coil will be described below.

As shown in the flowchart ofFIG. 13, if the actual current value I is equal to or smaller than the threshold current Ith (step S107: NO), the microcomputer203continues to supply power to the electromagnetic coil54, and the routine proceeds to step S101, as in the first embodiment. If the actual current value I is larger than the threshold current Ith (step S107: YES), the microcomputer203holds both the first and second switches201,202in the off state (step S201) to stop the electromagnetic clutch11until the IG is turned on again after being turned off.

In the control device12with the above configuration, the first and second switches201,202are held in the off state if the actual current value I is larger than the threshold current Ith and it is estimated that a ground fault has occurred. This suppresses an overcurrent in the second switch202etc.

Advantageous effects of the present embodiment will be described below. The present embodiment has the following advantageous effect in addition to the above advantageous effects (1) to (3) of the first embodiment.

(4) If the actual current value I is larger than the threshold current Ith, the microcomputer203holds the first and second switches201,202in the off state, which suppresses an overcurrent in the second switch202etc. That is, the second switch202functions also as a relay that cuts off power supply to the electromagnetic coil54. Accordingly, the drive relay208can be eliminated, and the control device12can be reduced in size.

The above embodiments can be modified as appropriate and carried out in the following forms.

In the second embodiment, if the actual current value I is larger than the threshold current Ith, both the first and second switches201,202are held in the off state. However, only the second switch202may be held in the off state.

In the above embodiments, in the case of supplying power to the electromagnetic coil54, an overcurrent is suppressed based on comparison between the actual current value I and the threshold current Ith regardless of which of the first and second switches201,202is driven by PWM. However, the invention is not limited to this. For example, the comparison between the actual current value I and the threshold current Ith may be made only in the case of supplying power to the electromagnetic coil54by driving the first switch201by PWM and holding the second switch202in the on state.

In the above embodiments, if the actual current value I is larger than the threshold current Ith, the electromagnetic clutch11is stopped until the IG is turned on after being turned off. However, the invention is not limited to this. For example, whether a ground fault of a wire between the control device12and the electromagnetic clutch11has been continuing or not may be detected, and the electromagnetic clutch11may be driven again if the ground fault of the wire is no longer present.

In the above embodiments, the microcomputer203may not compare the actual current value I with the threshold current Ith.

In the above embodiments, the first and second spline fitting portions45,64are formed as the first and second meshing portions in the first and second rotary members21,22. However, the invention is not limited to this. For example, crown gear-shaped teeth portions in which radially protruding teeth are arranged side by side in the circumferential direction may be formed as the first and second meshing members in the first and second rotary members21,22.

In the above embodiments, the engagement portions101are integral with the second housing32. However, the invention is not limited to this. The engagement portions101may be separate members from the second housing32.

In the above embodiments, in the state where the armature55is located at the second position (the state where a current is being applied to the electromagnetic coil54), the armature55may not contact the yoke83, and clearance may be present between the armature55and the yoke83.

In the above embodiments, the first cam member71forming the cam mechanism53has the friction engagement portion75. However, the invention is not limited to this. The electromagnetic clutch11may not be provided with the cam mechanism53. For example, an engagement member having the same shape as the first cam member may be fixed to the first rotary member21.

In the above embodiments, the temperature of the first switch201may not be detected. A temperature sensor that detects the temperature of the second switch202may be provided, and the switch to be driven by PWM and the switch to be held in the on state may be switched based on comparison between the detected temperature that is detected by the temperature sensor and the threshold temperature Tth.

The temperature sensor204may not be provided. In this case, when supplying power to the electromagnetic coil54, one of the first and second switches201,202is always driven by PWM, and the other switch is always held in the on state.

In the above embodiments, when operating the pressing mechanism56, power supply to the electromagnetic coil54is conducted and stopped based on the position of the armature55which is detected by the stroke sensor95. However, the invention is not limited to this. For example, power supply to the electromagnetic coil54may be conducted and stopped by estimating a first predetermined time after power supply to the electromagnetic coil54is started that the armature55has moved from the first position to the second position, and estimating a second predetermined time after power supply to the electromagnetic coil54is stopped that the armature55has moved from the second position to the first position.

In the above embodiments, the stroke sensor95detects the position of the armature55, and the microcomputer203estimates the position of the piston102based on the initial position of the piston102and the number of times the armature55has reciprocated between the first and second positions, thereby determining whether the electromagnetic clutch11is in the engaged state or in the disengaged state. However, the invention is not limited to this. For example, a stroke sensor that detects the position of the piston102may be provided, and the position of the piston102may be detected based on a position signal from the stroke sensor to determine whether the electromagnetic clutch11is in the engaged state or in the disengaged state.

In the above embodiments, the anode terminal of the second diode212is connected to the ground as the reference potential point. However, the invention is not limited to this. For example, the reference potential point may be a low potential-side terminal of the in-vehicle power supply B, and the anode terminal of the second diode212may be connected to the low potential-side terminal.

In the above embodiments, semiconductor switching elements are used as the first and second switches201,202. However, the invention is not limited to this, and mechanical switches etc. may be used as the first and second switches201,202.

In the above embodiments, the control device12may conduct and stop power supply to an electromagnetic coil of any other clutch such as the torque coupling7in a control manner similar to that for the electromagnetic coil54of the electromagnetic clutch11.