Patent Publication Number: US-9884614-B2

Title: Control system for vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national phase application based on the PCT International Patent Application No. PCT/IB2015/000550 filed Apr. 22, 2015, claiming priority to Japanese Patent Application No. 2014-090866 filed Apr. 25, 2014, the entire contents of both of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system for controlling a mechanism that transmits power for a travel of a vehicle. The invention particularly relates to a control system for a power transmission mechanism that includes a selectable one-way clutch. 
     2. Description of Related Art 
     The invention related to a vehicular transmission that includes a selectable one-way clutch (hereinafter described as an SOWC) is described in US 2009/0084653 A. The SOWC described in this US 2009/0084653 A includes: a first ring and a second ring that are arranged to face each other; and a strut that is arranged between these first ring and second ring. The first ring is provided with a pocket which a tip of the strut enters and is engaged with. The second ring is provided with a through opening. The strut is housed in the through opening. The strut is configured to be pushed out to the pocket side by a projecting tooth of an apply ring via a spring, the projecting tooth of the apply ring being inserted in the through opening from a back surface side of the second ring (an opposite side of a surface that faces the first ring). A combination of the strut and the pocket is provided in two types that are a first type and a second type. In the first type, the strut and the pocket are engaged when the SOWC transmits torque in a forward travel direction. In the second type, the strut and the pocket are engaged when the SOWC transmits the torque in a reverse travel direction. A blocking device is arranged between the first ring and the second ring in a manner to rotate within a specified angle range. The blocking device is a ring-shaped plate like the first ring and the second ring. This blocking device is provided with a window through which the strut passes. In addition, a return mechanism that presses the blocking device in a direction to separate the blocking device from the first ring is provided between the blocking device and the first ring. Then, the above apply ring is configured to be moved by an actuator in a rotational axis direction. Accordingly, it is configured that the second ring and the blocking device are pressed to the first ring side via the apply ring and the projecting tooth by pressing the apply ring to the first ring side by thrust of the actuator. 
     In a disengaged state in which the above SOWC is not engaged in any of rotational directions of the forward travel direction and the reverse travel direction, the strut is pressed in the through opening of the second ring by the blocking device. In other words, the first ring is completely separated from the blocking device and the second ring. The actuator is actuated in such a disengaged state to press the second ring and the blocking device to the first ring side. As a result, the blocking device is brought into contact with an opposing surface of the first ring. The blocking device that is in contact with the first ring rotates in a rotational direction of the first ring by a friction force that is generated at this time. Then, when a position of the window of the blocking device matches a position of the through opening of the second ring (that is, the strut) in the rotational direction, the strut is pushed out from the window to the first ring side. As a result, the tip of the strut is engaged with the pocket that is formed in the first ring. In other words, the SOWC is switched to an engaged state. 
     Noted that the invention related to an SOWC is described in US 2013/0062151 A. The SOWC is constructed of: a strut; a notch plate that is formed with a notch, the notch being engaged with the strut; a pocket plate that is provided with a recess section for housing the strut; a control plate that is formed with an opening through which the strut passes and that controls an engaged state between the strut and the notch. 
     In the SOWC described in US 2009/0084653 A above, in the case where the SOWC is switched from a disengaged state (MODE 1) to an engaged state (MODE 2), the SOWC is controlled such that a negative difference (differential rotation) is once produced between a rotational speed of the first plate and a rotational speed of the second plate. Thereafter, the SOWC is controlled such that a differential rotation speed becomes positive. The SOWC is switched from the disengaged state to the engaged state in a state that the differential rotation speed has actually become positive. In other words, the blocking device is brought into contact with the first ring for rotation. The tip of the strut, which is projected from the window of the blocking device, is engaged with the pocket of the first ring. In this way, the SOWC is switched from the disengaged state to the engaged state. At this time, a time lag is generated between an operation of the blocking device and an operation of the strut. Accordingly, the SOWC is switched to the engaged state after a specified time period has elapsed since the differential rotation speed thereof becomes positive. Noted that, in this SOWC, a state of a positive differential rotation corresponds to a direction in which the strut and the pocket are engaged. The state of the positive differential rotation corresponds to a rotational state in which the torque can be transmitted between the first ring and the second ring. A state of the negative differential rotation corresponds to the direction in which the strut and the pocket are not engaged. The state of the negative differential rotation corresponds to the rotational state in which the torque is not transmitted between the first ring and the second ring. 
     Just as described, in the SOWC described in US 2009/0084653 A, the strut and the pocket are engaged when the differential rotation speed is positive. In addition, as described above, the SOWC can transmit the torque when the differential rotation speed is positive. Accordingly, there is a case where a load is applied to the strut, which is pressed to the pocket side, immediately after the strut enters the pocket. Regarding this problem, in this SOWC described in US 2009/0084653 A, the blocking device for controlling an operation of the strut is indirectly operated, that is, operated by following an operation of any other component of the SOWC. In other words, the SOWC is not configured that the operation of the blocking device is actively controlled. Accordingly, there is a case where the strut and the pocket are engaged at an improper position as described in this US 2009/0084653 A. The improper position refers to a position that is located in a middle of a path for the strut to be engaged at a specified position in the pocket. If the strut is engaged at such an improper position, an area of a contact portion between the strut and the pocket is reduced. Thus, surface pressure of the contact portion is increased. As a result, durability of the SOWC is possibly degraded. 
     SUMMARY OF THE INVENTION 
     The invention provides a control system for a vehicle with which a selectable one-way clutch can appropriately be engaged. 
     A first aspect of the invention is a control system for a vehicle. The control system includes a selectable one-way clutch, a motor, and an electronic control unit. The selectable one-way clutch includes a first clutch member, a second clutch member, a strut, and a switching mechanism. The first clutch member and the second clutch member are configured to rotate relatively to each other. At least a part of the strut is configured to be operated such that the part of the strut is projected from the first clutch member side to the second clutch member side. The switching mechanism is configured to selectively set a first state and a second state. The first state is a state that the switching mechanism permits projection of the strut from the first clutch member side to the second clutch member side. The second state is a state that the switching mechanism inhibits the projection of the strut. The selectable one-way clutch is configured to be switched between an engaged state and a disengaged state. The engaged state is a state that restricts the relative rotation in either a positive rotational direction or a reverse rotational direction in the first state that the part of the strut is projected from the first clutch member side to the second clutch member side and is engaged with a part of the second clutch member. The disengaged state is a state that permits the relative rotation in the positive rotational direction and the relative rotation in the reverse rotational direction in the second state that the strut is not projected to the second clutch member side. The motor is configured to control a rotational speed of either the first clutch member or the second clutch member. The electronic control unit is configured to produce differential rotation by controlling the rotational speed by the motor. The differential rotation includes positive differential rotation and negative differential rotation. The positive differential rotation is the relative rotation in which the relative rotation is restricted in the engaged state. The negative differential rotation is the relative rotation in which the relative rotation is permitted in the engaged state. The electronic control unit is configured to execute the following processes in an order of (1) to (4) in the case where the electronic control unit switches the selectable one-way clutch from the disengaged state to the engaged state: (1) controlling the motor such that the differential rotation becomes the negative differential rotation; (2) switching from the second state to the first state such that the part of the strut is projected from the first clutch member side to the second clutch member side in a state that the differential rotation is the negative differential rotation; (3) controlling the motor such that the differential rotation becomes the positive differential rotation; and (4) engaging the part of the strut with the part of the second clutch member in a state that the differential rotation is the positive rotational direction. 
     According to the above aspect, in the case where the selectable one-way clutch is switched from the disengaged state to the engaged state, rotation of the motor is controlled such that the differential rotation speed of the selectable one-way clutch becomes negative. Then, the switching mechanism is actuated such that the strut can be projected to the position which is between the two clutch members and at which the strut can be engaged (the first state) in the state that the differential rotation speed is negative. When the differential rotation speed of the selectable one-way clutch is positive, the torque can be transmitted between the two clutch members. Accordingly, a load is possibly applied to the strut. On the contrary, when the differential rotation speed is negative, the torque is not transmitted between the two clutch members. Accordingly, no load is applied to the strut, and the strut can easily be operated. Thus, by setting the first state as described above in the state that the differential rotation speed is negative, the strut can easily be operated and reliably be engaged at a specified position. Therefore, the selectable one-way clutch in the disengaged state can reliably and appropriately be switched to the engaged state. 
     In the above aspect, first target differential rotation speed may be set as a target value of the differential rotation speed that is used when the electronic control unit controls the motor such that the differential rotation becomes the negative differential rotation. The electronic control unit may be configured to control the motor such that the differential rotation speed is maintained at the first target differential rotation speed until the part of the strut is projected to the second clutch member side. 
     According to the above aspect, in the case where the differential rotation speed of the selectable one-way clutch is brought into the negative state as described above, the rotation of the motor is controlled such that the differential rotation speed is maintained at the first target differential rotation speed that is set as a value on the negative side. When the differential rotation speed is controlled, control disturbance or a fluctuation in control occurs due to a fluctuation in torque of the engine, input of disturbance torque, or the like, for example. For this reason, the first target differential rotation speed as described above is set in consideration of such control disturbance or such a fluctuation in the control. Thus, the strut can be operated in a state that the differential rotation speed is reliably negative. In other words, the strut can be operated in a state that the load is never applied to the strut. Thus, the strut can easily be operated and reliably be engaged at the specified position. 
     In the above aspect, the switching mechanism may include an actuator that is configured to operate the strut. After the electronic control unit controls the motor such that the differential rotation becomes the negative differential rotation, the electronic control unit sets the first state in the state that the differential rotation is the negative differential rotation. In such a case, the electronic control unit may be configured to initiate actuation of the actuator before the differential rotation speed reaches the first target differential rotation speed. 
     According to the above aspect, in the case where the differential rotation speed of the selectable one-way clutch is maintained at the first target differential rotation speed that is set as the value on the negative side, the actuator starts being actuated to operate the strut before the differential rotation, which is controlled to become the first target differential rotation speed, reaches the first target differential rotation speed. For this reason, time required for switching of the selectable one-way clutch to the engaged state can be shortened. 
     In the above aspect, the switching mechanism may include an actuator that is configured to operate the strut. After the electronic control unit controls the motor such that the differential rotation becomes the negative differential rotation, the electronic control unit sets the first state in the state that the differential rotation is the negative differential rotation. In such a case, the electronic control unit may be configured to initiate the actuation of the actuator when the differential rotation speed is zero or the positive value that is close to zero. 
     According to the above aspect, in the case where the differential rotation speed of the selectable one-way clutch is maintained at the first target differential rotation speed that is set as the value on the negative side, the actuator starts being actuated to operate the strut when the differential rotation speed, which is controlled to become the first target differential rotation speed, is zero or a positive value that is close to zero. Thus, the strut can be operated and engaged in a state that the differential rotation speed is close to zero, at which the strut is less likely to be influenced by inertia torque. Therefore, shock received by the strut during the engagement can be suppressed. In addition, since the actuator starts being actuated before the differential rotation speed reaches the first target differential rotation speed, the time required for switching of the selectable one-way clutch to the engaged state can be shortened. 
     In the above aspect, the switching mechanism may include the actuator that is configured to operate the strut. The first target differential rotation speed may be set as the target value of the differential rotation speed that is used when the electronic control unit controls the motor such that the differential rotation becomes the negative differential rotation. Second target differential rotation speed may be set as a target value of the differential rotation speed that is used when the electronic control unit controls the motor such that the differential rotation becomes the positive differential rotation. The electronic control unit may be configured to execute the following processes in an order of (i) to (v): (i) actuating the actuator to set the first state such that the part of the strut is projected to the second clutch member side in the state that the differential rotation speed is maintained at the first target differential rotation speed; (ii) then, controlling the motor by the electronic control unit such that the differential rotation becomes the positive differential rotation; (iii) engaging the part of the strut with the part of the second clutch member in the state that the differential rotation is the positive differential rotation; (iv) controlling the motor after the actuation of the actuator is completed and an operation of the part of the strut to be projected to the second clutch member side is completed, such that the differential rotation speed is increased to the second target differential rotation speed; and (v) controlling the motor such that the differential rotation speed is maintained at the second target differential rotation speed until engagement of the part of the strut and the part of the second clutch member is completed. 
     According to the above aspect, in the case where the differential rotation speed of the selectable one-way clutch is maintained at the first target differential rotation speed, which is set as the value on the negative side, timing for starting the actuator operation or for increasing the differential rotation speed to the positive side so as to engage the selectable one-way clutch and thus to transmit the torque, is set in consideration of time at which the actuation of the actuator or the operation of the strut is completed. Then, also in the case where the differential rotation speed of the selectable one-way clutch that is brought into the negative state as described above becomes positive, the time at which the actuator starts being actuated or the time for increasing the differential rotation speed to the positive side, so as to engage the selectable one-way clutch and thus to transmit the torque, is set. Therefore, the selectable one-way clutch in the disengaged state can reliably and appropriately be switched to the engaged state. 
     In the above aspect, an absolute value of the first target differential rotation speed may be set higher as a magnitude of predicted control disturbance is increased. 
     According to the above aspect, as described above, when the differential rotation speed is controlled, the control disturbance or the fluctuation in control occurs due to the fluctuation in torque of the engine, the input of disturbance torque, or the like, for example. In this case, the first target differential rotation speed is set such that the absolute value of the first target differential rotation speed becomes higher as a magnitude of such control disturbance is increased. Accordingly, the strut can be operated in a state that the load is never applied to the strut. 
     In the above aspect, an internal combustion engine, a drive wheel, and a power transmission may further be provided. The power transmission mechanism may include a fixed section and a first differential mechanism. Either the first clutch member or the second clutch member may be coupled to the fixed section. The fixed section may be configured not to rotate or move. The first differential mechanism may include a first rotary element, a second rotary element, and a third rotary element. The first rotary element, the second rotary element, and the third rotary element may be configured to perform a differential action with respect to each other. The internal combustion engine may be coupled to the first rotary element. The motor and the other of the first clutch member or the second clutch member may be coupled to the second rotary element. The first differential mechanism may be configured to output torque from the third rotary element to the drive wheel. 
     In the above aspect, the internal combustion engine, the drive wheel, and the power transmission may further be provided. The power transmission mechanism may include the fixed section, the first differential mechanism, and a second differential mechanism. Either the first clutch member or the second clutch member may be coupled to the fixed section. The fixed section may be configured not to rotate or move. The first differential mechanism may include the first rotary element, the second rotary element, and the third rotary element. The first rotary element, the second rotary element, and the third rotary element may be configured to perform a differential action with respect to each other. The internal combustion engine may be coupled to the first rotary element. The motor may be coupled to the second rotary element. The first differential mechanism may be configured to output torque from the third rotary element to the drive wheel. The second differential mechanism may include a fourth rotary element, a fifth rotary element, and a sixth rotary element. The fourth rotary element, the fifth rotary element, and the sixth rotary element may be configured to perform the differential action with respect to each other. The first rotary element may be coupled to the fourth rotary element. The second rotary element may be coupled to the fifth rotary element. The other of the first clutch member or the second clutch member may be coupled to the sixth rotary element. In the second differential mechanism, the fifth rotary element may be configured to rotate in an opposite direction from a rotational direction of the fourth rotary element by stopping a rotation of the sixth rotary element. 
     The above aspect can be applied to the control device for the power transmission mechanism in which a speed of the internal combustion engine can be controlled by the motor via the differential mechanism. In this case, the selectable one-way clutch can selectively restrict rotation of any of the rotary elements of the differential mechanism in a specified direction. Then, the selectable one-way clutch can reliably and appropriately be switched from the disengaged state to the engaged state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a view of an example of a configuration of a power transmission mechanism in a hybrid vehicle to which the invention is applied; 
         FIG. 2  includes collinear diagrams on a planetary gear mechanism that constitutes the power transmission mechanism in the hybrid vehicle shown in  FIG. 1 ; 
         FIG. 3  is a view of another example of the configuration of the power transmission mechanism in the hybrid vehicle to which the invention is applied; 
         FIG. 4  includes collinear diagrams on a compound planetary gear mechanism that constitutes a power split mechanism and an overdrive mechanism in the hybrid vehicle shown in  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of a configuration of a selectable one-way clutch that can be a subject of the invention; 
         FIG. 6  is a view of a housing section and a pocket, the housing section being formed in a first clutch plate and the pocket being formed in a second clutch plate of the selectable one-way clutch shown in  FIG. 5 ; 
         FIG. 7  is a flowchart for illustrating an example of control that is executed by a control device of the invention; 
         FIG. 8A  is a diagram for illustrating an example of the map that is applied when the control shown in the flowchart in  FIG. 7  is executed; 
         FIG. 8B  is a diagram for illustrating an example of the map that is applied when the control shown in the flowchart in  FIG. 7  is executed; 
         FIG. 9  is a time chart that shows an example of a change in differential rotation speed of the selectable one-way clutch when the control shown in the flowchart of  FIG. 7  is executed; 
         FIG. 10  is a view of yet another example of the configuration of the power transmission mechanism in the hybrid vehicle to which the invention is applied; and 
         FIG. 11  includes collinear diagrams on the planetary gear mechanism that constitutes the power split mechanism in the hybrid vehicle shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Next, a specific description will be made on the invention with reference to the drawings. The invention can be applied to a control system of which a power transmission mechanism in a hybrid vehicle is a subject. First, an example of a configuration of the power transmission mechanism will be described.  FIG. 1  schematically shows the power transmission mechanism in a hybrid vehicle Ve of double-spindle two-motor type. The hybrid vehicle Ve includes: an engine (Eng)  1  as an example of an internal combustion engine of the invention; a first motor (MG 1 )  2  an example of a motor of the invention; and a second motor (MG 2 )  3  as drive power sources. 
     The first motor  2  is configured to mainly execute control of a speed of the engine  1  and cranking of the engine  1 . This first motor  2  is also configured to function as one of the drive power sources in a two-motor travel mode (a two-motor EV mode) in which a vehicle travels by the two motors. Together with the engine  1 , the first motor  2  is coupled to a power split mechanism  4  that is an example of the first differential mechanism of the invention. 
     In this example shown in  FIG. 1 , the power split mechanism  4  is constructed of a planetary gear mechanism of single pinion type that includes a sun gear  5 , a carrier  6 , and a ring gear  7  as rotary elements. A rotor of the first motor  2  is coupled to the sun gear  5  that is an example of a second rotary element of the invention among the rotary elements. In addition, an output shaft (a crankshaft) of the engine  1  is coupled to the carrier  6  that is an example of a first rotary element of the invention. The ring gear  7  that is an example of a third rotary element of the invention is an output element. An output gear  8  as an output member is attached to the ring gear  7 . The output gear  8  meshes with a counter driven gear  9 . The counter driven gear  9  is attached to a counter shaft  10 . A counter drive gear  11  that has a smaller diameter than the counter driven gear  9  is attached to the counter shaft  10 . The counter drive gear  11  meshes with a ring gear  13  in a differential gear  12 . Then, the differential gear  12  outputs drive torque to right and left drive wheels  14 . 
     The second motor  3  is configured to mainly function as a drive power source for a travel. A drive gear  15  is attached to a rotor shaft of the second motor  3 . The drive gear  15  meshes with the counter driven gear  9 . This drive gear  15  has a smaller diameter than the counter driven gear  9 . In this manner, the drive gear  15  and the counter driven gear  9  constitute a speed reduction mechanism. 
     A selectable one-way clutch (hereinafter described as an SOWC)  17  is provided between the sun gear  5 , to which the first motor  2  is coupled, and a casing  16  that is an example of a fixed section of the invention. This SOWC  17  is a clutch that is configured to enable relative rotation in both directions of positive rotation and reverse rotation so as to prevent torque transmission in a disengaged state. This SOWC  17  is a clutch that is configured to restrict the relative rotation in only one direction of the positive rotation and the reverse rotation so as to transmit the torque in the direction of the relative rotation and to enable the relative rotation in an opposite direction therefrom, so as to prevent the torque transmission in an engaged state. Here, the positive rotation refers to rotation in the same direction as a rotational direction of the engine  1 . This rotational direction refers to a positive rotational direction. The reverse rotation (or negative rotation) refers to rotation in an opposite direction from the rotational direction of the engine  1 . This rotational direction refers to a reverse rotational direction. In addition, similar to the SOWC described in above-described US 2009/0084653 A, this SOWC  17  is configured to be able to transmit the torque when a difference in the rotational speed (differential rotation) between the two rotary members that are involved in torque transmission, that is, between a first clutch plate  24  and a second clutch plate  25 , which will be described below, is positive. This SOWC  17  is configured not to transmit the torque when the differential rotation speed is negative. Noted that the specific configuration of this SOWC  17  will be described below. 
     The first motor  2  and the second motor  3  are connected to an electrical storage device and a controller unit such as an inverter, which are not shown. In addition, the motors  2 ,  3  are electrically connected to each other to enable electrical power transfer therebetween. Furthermore, an electronic control unit (ECU)  18  is provided to control these electrical storage device and controller unit, the SOWC  17 , or the like. This electronic control unit  18  is constructed of a microcomputer as a main body. This electronic control unit  18  is configured to receive detection signals indicative of a vehicle speed, an accelerator operation amount, an engine speed, estimated output torque, a rotational speed and torque of each of the motors  2 ,  3 , an operating state of the SOWC  17 , and the like as data. In addition, this electronic control unit  18  is configured to output command signals for controlling each of the motors  2 ,  3  and the SOWC  17 , the command signals being obtained by performing computation based on the input data. 
       FIG. 2  includes collinear diagrams on the planetary gear mechanism that constitutes the above power split mechanism  4 . A top diagram in  FIG. 2  indicates a forward traveling state in a hybrid mode (an HV mode or a power split mode). In this state shown in the top diagram in  FIG. 2 , the engine  1  is driven, and thus the carrier  6  rotates in the positive rotational direction. In addition, due to the forward travel of the vehicle Ve, the ring gear  7  rotates in the positive rotational direction. At this time, the SOWC  17  is disengaged, and thus the sun gear  5  and the first motor  2 , which is coupled to the sun gear  5 , can rotate in either direction of the positive rotation or the reverse rotation. In this state shown in the top diagram in  FIG. 2 , the first motor  2  functions as an electrical power generator while making the positive rotation. In other words, the first motor  2  outputs torque in a negative direction (a downward direction in the top diagram in  FIG. 2 ) and thereby controls the speed of the engine  1  to a speed at which excellent fuel efficiency can be realized. In this case, the electrical power generated in the first motor  2  is supplied to the second motor  3 . Then, the second motor  3  functions as the motor and outputs drive power for the travel. 
     A second diagram from the top in  FIG. 2  indicates a transient state (a transition state) in which the SOWC  17  is switched between the state shown in the top diagram in  FIG. 2  and a state shown in the third diagram from the top in  FIG. 2 , which will be described below. In other words, the second diagram from the top in  FIG. 2  indicates a transient state when the SOWC  17  is switched from the disengaged state (free) to the engaged state (locked) in which the positive rotation of the sun gear  5  is stopped. In this state shown in the second diagram from the top in  FIG. 2 , the first motor  2  functions as the motor and rotates the sun gear  5  in a reverse rotational direction. As a result, the negative differential rotation is produced in the SOWC  17 . In other words, the SOWC  17  does not transmit the torque. Thus, when the engagement control of the SOWC  17  is executed in this state, the torque is not applied to the strut  26  of the SOWC  17 , which will be described below. 
     A third diagram from the top in  FIG. 2  indicates a state in which the positive rotation of the sun gear  5  is stopped by the SOWC  17  and in which the vehicle travels forward by the drive power of the engine  1  or by the drive power of the engine  1  and the drive power of the second motor  3  (in a so-called parallel mode). In this state shown in the third diagram from the top in  FIG. 2 , a rotational speed of the ring gear  7  is higher than the engine speed (a rotational speed of the carrier  6 ), and thus the torque is output from the ring gear  7 . When the second motor  3  is operated as the motor in this state, the drive power thereof is added to the drive power that is output from the ring gear  7  and is transmitted to the drive wheels  14  via the differential gear  12 . Also in this case, the first motor  2  and the sun gear  5  are fixed, and energization thereof is stopped (in an OFF state). Thus, the excellent fuel efficiency can be realized when the vehicle travels at a high speed. 
       FIG. 3  shows another example of the configuration of the power transmission mechanism that can be the subject of the invention. In this configuration shown in  FIG. 3 , an overdrive (O/D) mechanism  19  is added to the above-described configuration shown in  FIG. 1 . This configuration shown in  FIG. 3  is also an example of the configuration in which the overdrive mechanism  19  is selectively locked by the SOWC  17 . The overdrive mechanism  19  is an example of a second differential mechanism of the invention. In this example shown in  FIG. 3 , the overdrive mechanism  19  is constructed of a planetary gear mechanism of double pinion type that includes a sun gear  20 , a carrier  21 , and a ring gear  22  as rotary elements. The carrier  6  in the above-described power split mechanism  4  is coupled to the carrier  21  that is an example of a fourth rotary element of the invention. Accordingly, it is configured that the output torque of the engine  1  is transmitted to these carrier  6  and carrier  21 . In addition, the sun gear  5  in the power split mechanism  4  is coupled to the sun gear  20  that is an example of a fifth rotary element of the invention. Accordingly, it is configured that the torque of the first motor  2  is transmitted to these sun gear  5  and sun gear  20 . Furthermore, the above-described SOWC  17  is arranged between the casing  16  and the ring gear  22  that is an example of a sixth rotary element of the invention. It is configured that the SOWC  17  restricts (inhibits) rotation of the ring gear  22  in a specified direction, so as to set an overdrive state. The rotary elements of the planetary gear mechanism of the single pinion type that constitutes the power split mechanism  4  and the rotary elements of the planetary gear mechanism of the double pinion type that constitutes the overdrive mechanism  19  are coupled as described above. In this way, a so-called compound planetary gear mechanism having the four elements is constructed. Since the rest of the configuration of the power transmission mechanism is the same as the configuration shown in  FIG. 1 , components shown in this  FIG. 3  are denoted by the same reference numerals as those used in  FIG. 1 , and the description thereof will not be made. 
       FIG. 4  includes collinear diagrams on the above compound planetary gear mechanism, and a top diagram in  FIG. 4  indicates the forward traveling state in the hybrid mode (the HV mode or the power split mode). In this state shown in the top diagram in  FIG. 4 , the engine  1  is driven, and thus the carrier  6  rotates in the positive rotational direction. In addition, due to the forward travel of the vehicle Ve, the ring gear  7  rotates in the positive rotational direction. At this time, the SOWC  17  is disengaged, and thus the sun gear  5  or the ring gear  22 , and the first motor  2 , which can rotate the sun gear  5  and the ring gear  22 , can rotate in either direction of the positive rotation or the reverse rotation. In this state shown in the top diagram in  FIG. 4 , the first motor  2  functions as an electrical power generator while making the positive rotation. In other words, the first motor  2  outputs the torque in a negative direction (a downward direction in the top diagram in  FIG. 4 ) and thereby controls the speed of the engine  1  to the speed at which the excellent fuel efficiency can be realized. In this case, the electrical power generated in the first motor  2  is supplied to the second motor  3 . Then, the second motor  3  functions as the motor and outputs the drive power for the travel. 
     A second diagram from the top in  FIG. 4  indicates a transient state (a transition state) in which the SOWC  17  is switched between the state shown in the top diagram in  FIG. 4  and a state shown in the third diagram from the top in  FIG. 4 , which will be described below. In other words, the second diagram from the top in  FIG. 4  indicates a transient state when the SOWC  17  is switched from the disengaged state (free) to the engaged state (locked) in which the SOWC  17  stops the positive rotation of the ring gear  22 . In this state shown in the second diagram from the top in  FIG. 4 , the first motor  2  functions as the motor and rotates the sun gear  5  or the ring gear  22  in the reverse rotational direction. The rotational speed at this time is the rotational speed at which the ring gear  22  rotates in the reverse rotational direction, and the negative differential rotation is produced in the SOWC  17 . In other words, the SOWC  17  does not transmit the torque. Thus, when the engagement control of the SOWC  17  is executed in this state, the torque is not applied to the strut  26  of the SOWC  17 , which will be described below. 
     A third diagram from the top in  FIG. 4  indicates a state in which the positive rotation of the ring gear  22  is stopped by the SOWC  17  and in which the vehicle travels forward by the drive power of the engine  1  or by the drive power of the engine  1  and the drive power of the second motor  3 . In this state shown in the third diagram from the top in this  FIG. 4 , the ring gear  22  in the overdrive mechanism  19  is fixed so as not to rotate in the positive rotational direction. Then, torque in the positive rotational direction is applied to the carrier  21 . Accordingly, the sun gear  20  rotates in the reverse rotational direction. In the power split mechanism  4 , the sun gear  5  is integrated with the sun gear  20  in the overdrive mechanism  19  and rotates in the reverse rotational direction. Accordingly, in the power split mechanism  4 , since the torque of the engine  1  is applied to the carrier  6  in the state that the sun gear  5  rotates in the reverse rotational direction, the ring gear  7  as the output element rotates at the higher rotational speed than the carrier  6  (that is, the engine  1 ). In other words, the overdrive state is generated. When the second motor  3  is operated as the motor in this state, the drive power thereof is added to the drive power that is output from the ring gear  7  and is transmitted to the drive wheels  14  via the differential  12 . Noted that, in this overdrive state, the first motor  2  is fixed with the ring gear  22  and controlled to be in the OFF state. Thus, the excellent fuel efficiency can be realized when the vehicle travels at a high speed. 
     Here, the configuration of the SOWC  17  will be described. In the power transmission mechanism that is the subject of the invention, for example, the SOWC that is described in above-described US 2009/0084653 A, the SOWC that is described in above-described US 2013/0062151 A, an SOWC that is described in US 2010/0252384 A, or the like can be adopted. Furthermore, the SOWC  17  that is configured as shown in  FIG. 5  and  FIG. 6  can be adopted. These  FIG. 5  and  FIG. 6  show an engagement mechanism  23  in the SOWC  17 . This engagement mechanism  23  is mainly constructed of the first clutch plate  24 , the second clutch plate  25 , a strut  26 , and an actuation mechanism  27 . 
     The first clutch plate  24  is formed in a disc shape as a whole. The second clutch plate  25  that is also formed in the disc shape as the first clutch plate  24  is arranged to face this first clutch plate  24 . Of these clutch plates  24 ,  25 , the first clutch plate  24  is an example of a first clutch member of the invention. Of these clutch plates  24 ,  25 , the second clutch plate  25  is an example of a second clutch member of the invention. These clutch plates  24 ,  25  are retained to enable relative rotation to each other. For example, the one clutch plate  24  ( 25 ) is attached to the above-described casing  16 . The other clutch plate  25  ( 24 ) is coupled to the sun gear  5  in the example shown in  FIG. 1 . Alternatively, in the example shown in  FIG. 3 , the other clutch plate  25  ( 24 ) is coupled to the ring gear  22 . 
     The first clutch plate  24  is provided with a recessed section that is elongated in a rotational direction at a position that is in a front surface of the first clutch plate  24  and that is shifted to a radially outer side from the center of rotation of the first clutch plate  24 , that is, at a specified position on an outer peripheral side. This recessed section is a housing section  28  for housing the strut  26 . The second clutch plate  25  is provided with a pocket  29  that is a recessed section in the substantially same shape as the housing section  28  at a radial position that is in a surface of the second clutch plate  25  facing the first clutch plate  24  and that corresponds to the housing section  28 . A plate-shaped engagement piece whose cross section is substantially the same as that of the housing section  28 , that is, the strut  26  is housed in the housing section  28 . The strut  26  is arranged in the housing section  28  in a manner to swing with a support pin  30  being the center, the support pin  30  being provided at the center in a longitudinal direction of the strut  26  and facing the radial direction of the first clutch plate  24 . A depth of the recessed section of the housing section  28  is changed at the support pin  30 . More specifically, an upper half of the housing section  28  in  FIG. 5  has a thickness that is substantially equal to a thickness of the strut  26  or that is slightly larger than the thickness of the strut  26 . Then, a lower half of the housing section  28  in  FIG. 5  has a thickness that is larger than the thickness of the strut  26 . In this way, the strut  26  is configured to be able to swing with the support pin  30  being the center. 
     A spring  31  that causes an elastic force to act in a direction to push out one end side of the strut  26  from the housing section  28  is arranged in a shallow portion of the housing section  28 . In addition, an actuator  32  that presses another end side of the strut  26  in the direction to push out from the housing section  28  is arranged in a deep portion of the housing section  28 . This actuator  32  only needs to be able to apply a pressing force to the other end side of the strut  26 . For example, a hydraulic actuator such as a hydraulic piston or an electromagnetic actuator such as a solenoid that generates thrust by using an electromagnetic force can be adopted. Accordingly, in a state that the actuator  32  does not press the other end of the strut  26 , it is configured that the one end of the strut  26  is pressed by the spring  31  and is projected from the housing section  28  toward the pocket  29  on the second clutch plate  25  side. In addition, in a state that the actuator  32  presses the other end of the strut  26 , it is configured that the strut  26  rotates about the support pin  30  in a direction to compress the spring  31  and that the entire strut  26  is housed in the housing section  28 . In other words, it is configured to inhibit the strut  26  from being projected to the second clutch plate  25  side. 
     As described above, the spring  31  and the actuator  32  constitute the actuation mechanism  27  for operating the strut  26 . Then, as described above, a state that the actuator  32  does not press the other end of the strut  26  and that the one end of the strut  26  is pressed by the spring  31  and projected from the housing section  28  toward the pocket  29  on the second clutch plate  25  side corresponds to a state that the projection of the strut  26  to the second clutch plate  25  is permitted. That is, such a state is an example of the first state of the invention. In addition, a state that the actuator  32  presses the other end of the strut  26 , that the strut  26  rotates about the support pin  30  in a direction to compress the spring  31 , and that the entire strut  26  is housed in the housing section  28  corresponds to a state that the projection of the strut  26  to the second clutch plate  25  side is inhibited. That is, such a state is an example of the second state of the invention. Accordingly, the actuation mechanism  27  for operating the strut  26  by the above spring  31  and actuator  32  is an example of a switching mechanism of the invention. 
     Noted that, in the above engagement mechanism  23 , an appropriate elastic member such as a spring may be interposed between the actuator  32  and the other end of the strut  26  in order to relax the pressing force generated by the actuator  32  or to permit swinging of the strut  26  in the state that the actuator  32  presses the other end of the strut  26 . In addition, the following description will be made on an example in which it is configured that, when the actuator  32  is controlled to be OFF, the actuator  32  presses the other end of the strut  26  so as to bring the engagement mechanism  23  into a disengaged state. The following description will further be made on an example in which it is also configured that, when the actuator  32  is controlled to be ON, the actuator  32  cancels pressing of the other end of the strut  26  so as to bring the engagement mechanism  23  into an engaged state. 
     As described above, the pocket  29  that is provided in the second clutch plate  25  is a portion which the one end of the strut  26  projected from the housing section  28  enters and is engaged with. Accordingly, in the engagement mechanism  23 , in a state that the one end of the strut  26  is projected to the second clutch plate  25  side, in the case where torque in the positive rotational direction acts on either one of the clutch plates  24 ,  25 , the strut  26  is meshed between the housing section  28  and the pocket  29 . That torque in the positive rotational direction acts on either one of the clutch plates  24 ,  25  means that torque in an upward direction of  FIG. 5  acts on the first clutch plate  24  or that torque in a downward direction of  FIG. 5  acts on the second clutch plate  25 . As a result, the clutch plates  24 ,  25  are integrally coupled in the rotational direction. That is, the relative rotation of the first clutch plate  24  in the upward direction of  FIG. 5  to the second clutch plate  25  is restricted. In other words, the relative rotation of the second clutch plate  25  in the downward direction of  FIG. 5  to the first clutch plate  24  is restricted. The restricted rotational direction in this case is the positive rotational direction in each of the power transmission mechanisms shown in above-described  FIG. 1  and  FIG. 3 . A state that the positive rotation of the above-described sun gear  5  or ring gear  22  is restricted (or inhibited), just as described, is the engaged state of the engagement mechanism  23  or the SOWC  17 . 
     In the engaged state of the SOWC  17  as described above, in the case where the torque in the reverse rotational direction (the negative rotational direction) acts on either one of the clutch plates  24 ,  25 , the surface of the strut  26  is pressed by an edge portion of an opening end of the pocket  29  in the second clutch plate  25 . That the torque in the reverse rotational direction (the negative rotational direction) acts on either one of the clutch plates  24 ,  25  means that the torque in the downward direction of  FIG. 5  acts on the first clutch plate  24  or that the torque in the upward direction of  FIG. 5  acts on the second clutch plate  25 . As a result, the strut  26  acts against the elastic force of the spring  31  and is pushed in the housing section  28 . In other words, the engagement by the strut  26  is canceled, and the clutch plates  24 ,  25  can rotate relatively. Then, when the actuator  32  presses the other end of the strut  26 , the strut  26  rotates in such a direction that the one end thereof enters the housing section  28  while compressing the spring  31 . As a result, the strut  26  is housed in the housing section  28 . Accordingly, the member that connects the clutch plates  24 ,  25  no longer exist, and thus the clutch plates  24 ,  25  can rotate relatively in either direction of the positive rotation or the negative rotation. This state is the disengaged state of the engagement mechanism  23 , that is, the SOWC  17 . 
     As described above, the engaged state and the disengaged state of the SOWC  17  are switched by the operation of the actuator  32 . Accordingly, it is possible by detecting the operating state or an operation amount of the actuator  32  to determine whether the SOWC  17  is in the engaged state or the disengaged state on the basis of the detection result. For this reason, the engagement mechanism  23  is provided with a stroke sensor  33  for performing the detection as described above. An appropriate sensor that has conventionally been known can be adopted for this stroke sensor  33 . For example, the stroke sensor  33  may be a type of sensor that detects a stroke of the actuator  32  by capacitance or electrical resistance that varies by the operation amount of the actuator  32 , a type of sensor that detects the stroke of the actuator  32  optically, or the like. In addition, instead of detecting the stroke of the actuator  32 , a so-called ON/OFF sensor may output a signal at an advanced end and a retracted end of the actuator  32 . 
     As described above, the SOWC  17  can be engaged when the differential rotation between the first clutch plate  24  and the second clutch plate  25  is positive. In other words, in the state of the positive differential rotation of the SOWC  17 , the strut  26  is fitted to and engaged with the pocket  29  at the position between the first clutch plate  24  and the second clutch plate  25 . Accordingly, the torque can be transmitted between the first clutch plate  24  and the second clutch plate  25  via the strut  26 . For this reason, if the strut  26  is operated to be engaged with the pocket  29  in the state of the positive differential rotation of the SOWC  17 , there is a case where the torque is applied to the strut  26  that is in the middle of a path for the strut  26  to be completely engaged at a specified position in the pocket  29 , and thus the strut  26  is engaged with the pocket  29  at an improper position. In such an improper engaged state, surface pressure that acts on a contact portion between the strut  26  and the pocket  29  is increased. As a result, the load applied to the strut  26  is possibly increased. 
     In view of this, the control device according to the invention that has the above power transmission mechanism as the subject is configured to execute control, which will be described below, such that the strut  26  is not engaged with the pocket  29  at the improper position during the engagement of the SOWC  17 .  FIG. 7  is a flowchart for illustrating an example of such control. A routine shown in this flowchart of  FIG. 7  is repeatedly executed at specified short time intervals. In addition, in the routine shown in this flowchart of  FIG. 7 , the control is executed with an assumption that the vehicle Ve travels in the disengaged state of the SOWC  17  and that the differential rotation speed of the SOWC  17  is positive. 
     In the flowchart of  FIG. 7 , it is first determined whether the SOWC  17  will be engaged (step S 1 ). If a negative determination is made in this step S 1  due to the lack of a request for engaging the SOWC  17 , this routine is once terminated without executing control in the following steps. 
     On the contrary, if a positive determination is made in step S 1  due to the presence of the request for engaging the SOWC  17 , a process proceeds to step S 2 . For example, in the example of the configuration shown in  FIG. 1 , when the vehicle Ve travels forward by the output of the engine  1  and the output of the second motor  3 , the rotation of the first motor  2  and the rotation of the sun gear  5  are locked. In such a case, the SOWC  17  is engaged. Alternatively, in the example of the configuration shown in  FIG. 3 , when the overdrive state is set, the rotation of the ring gear  22  in the overdrive mechanism  19  is locked. In such a case, the SOWC  17  is engaged. 
     In step S 2 , synchronous control is initiated. Here, the synchronous control refers to a series of control of the rotational speed that is executed when the SOWC  17  is engaged. The synchronous control also refers to control in which the differential rotation speed of the SOWC  17  is once maintained to be negative for the engagement of the SOWC  17  and then is gradually increased to the positive side, so as to bring the SOWC  17  into an engageable state. More specifically, the differential rotation speed of the SOWC  17  is synchronized with unlock-side target differential rotation speed, and the rotation of the first motor  2  is controlled such that the differential rotation speed of the SOWC  17  is maintained at the unlock-side target differential rotation speed for a specified time period. The unlock-side target differential rotation speed corresponds to a target value that is used when the differential rotation speed of the SOWC  17  becomes negative (that is, on the unlock side). This unlock-side target differential rotation speed is set in advance on the basis of a result of an experiment, a simulation, or the like in consideration of a fluctuation in the rotational speed that is caused by control disturbance, such as a fluctuation in torque of the engine  1  or input of disturbance torque, such that the differential rotation speed of the SOWC  17  becomes negative even with occurrence of such a fluctuation. 
     Furthermore, the above unlock-side target differential rotation speed can be set by changing the value in accordance with a magnitude of the predicted control disturbance. More specifically, the unlock-side target differential rotation speed can be set such that the value thereof is changed to be increased as the magnitude of the predicted control disturbance is increased. For example, a map shown in  FIG. 8A  or  FIG. 8B  can be used to set the unlock-side target differential rotation speed. In an example shown in  FIG. 8A , the unlock-side target differential rotation speed is set such that an absolute value of the unlock-side target differential rotation speed is increased as a throttle opening degree of the engine  1  is increased. In the engine  1 , the engine torque is increased as the throttle opening degree is increased. Then, when the engine torque is increased, the fluctuation in torque is also increased. Accordingly, the fluctuation in the differential rotation speed of the SOWC  17  is also increased by the occurrence of the significant fluctuation in torque, just as described. In such a case, the absolute value of the unlock-side target differential rotation speed is increased (that is, the value of the unlock-side target differential rotation speed is reduced on the negative side). Thus, even with the occurrence of the fluctuation, the differential rotation speed of the SOWC  17  can be maintained to be negative. 
     In addition, in an example shown in  FIG. 8B , the unlock-side target differential rotation speed is set such that the absolute value of the unlock-side target differential rotation speed is increased as an amount of increase in the throttle opening degree of the engine  1 , that is, an amount of change of the throttle opening degree in a direction to be opened wider is increased. It is difficult to estimate the engine torque when the engine  1  is in a rapid transient state that the amount of increase in the throttle opening degree is large. Accordingly, a discrepancy between the estimated engine torque and the actual engine torque is possibly increased as the amount of increase in the throttle opening degree is increased. With such an increased discrepancy, the fluctuation in the differential rotation speed of the SOWC  17  is possibly increased. Also in such a case, the absolute value of the unlock-side target differential rotation speed is increased (that is, the value of the unlock-side target differential rotation speed is reduced on the negative side). Thus, even with the occurrence of the fluctuation, the differential rotation speed of the SOWC  17  can be maintained to be negative. 
     Once the synchronous control is initiated as described above, it is determined whether the differential rotation speed of the SOWC  17  has reached actuator ON differential rotation speed (step S 3 ). The actuator ON differential rotation speed is differential rotation speed at which ON control of the actuator  32  is initiated for the engagement of the SOWC  17  in the synchronous control of the SOWC  17 . This actuator ON differential rotation speed is zero or a value close to zero. Furthermore, this actuator ON differential rotation speed is set as a value that is lower than the lock-side differential rotation speed, which will be described below. This actuator ON differential rotation speed is also set in advance on the basis of a result of an experiment, a simulation, or the like, for example, in consideration of responsiveness of the actuator  32 . Noted that, in this step S 3  of the control, instead of the above actuator ON differential rotation speed, it can be determined whether the differential rotation speed of the SOWC  17  has reached the unlock-side target differential rotation speed. 
     A negative determination is made in this step S 3  if the differential rotation speed of the SOWC  17  has not reached the actuator ON differential rotation speed. In this case, the process returns to step S 2 , and the above synchronous control is continued. On the contrary, a positive determination is made in step S 3  if the differential rotation speed of the SOWC  17  has reached the actuator ON differential rotation speed. In this case, the process proceeds to step S 4 . Then, the above synchronous control is continued, and the ON control of the actuator  32  is initiated. 
     If the ON control of the actuator  32  is initiated, it is determined whether a stroke (an operation) of the actuator  32  is completed (step S 5 ). Once the ON control is initiated, the actuator  32  is operated to cancel pressing of the strut  26 , so as to project the strut  26  to the second clutch plate  25  side. Accordingly, in this step S 5 , the operating state of the actuator  32  can be determined on the basis of the detection signal of the stroke sensor  33  or the ON/OFF sensor. 
     A negative determination is made in this step S 5  if the operation of the actuator  32  for engagement of the engagement mechanism  23  has not been completed. In this case, the process returns to step S 4 , and the above synchronous control and the ON control of the actuator  32  are continued. 
     A positive determination is made in step S 5  if the operation of the actuator  32  for the engagement of the engagement mechanism  23  has been completed. In this case, the process proceeds to step S 6 . Then, it is determined whether a specified time period has elapsed since the initiation of the ON control of the actuator  32 . This specified time period is estimated and set in advance as time required for the strut  26  to move to a specified position for engagement by the operation of the actuator  32  in consideration of the responsiveness of the actuator  32  and the operation of the strut  26 . This specified time period is also set in advance on the basis of a result of an experiment, a simulation, or the like, for example. 
     A negative determination is made in this step S 6  if the specified time period has not elapsed. In this case, the process returns to step S 4 , and the above synchronous control and the ON control of the actuator  32  are continued. 
     If a positive determination is made in step S 6  due to a lapse of the specified time period, a process proceeds to step S 7 . Then, lock rotation control is initiated. This lock rotation control is control in which the differential rotation speed of the SOWC  17 , which is maintained at the unlock-side differential rotation speed on the negative side (that is, the unlock side), is increased to be the differential rotation speed on the positive side (that is, the lock side), so as to bring the SOWC  17  into the engageable state. More specifically, the rotation of the first motor  2  is controlled such that the differential rotation speed of the SOWC  17  is a value on the positive side. 
     Once the lock rotation control is initiated in step S 7  as described above, the differential rotation speed of the SOWC  17  is then increased to the lock-side target differential rotation speed and maintained at the lock-side target differential rotation speed (step S 8 ). The lock-side target differential rotation speed is the differential rotation speed at which the differential rotation of the SOWC  17  is the positive differential rotation and at which SOWC  17  can be engaged. In addition, as shown in  FIG. 9 , this lock-side target differential rotation speed is set as a value that is lower than a lower limit of ratchet minimum differential rotation speed, which will be described below. 
     As described above, the engagement mechanism  23  of this SOWC  17  is brought into the engageable state when the differential rotation of the SOWC  17  is the positive differential rotation. In addition to the above, this SOWC  17  is provided with a ratchet function that inhibits the engagement between the strut  26  and the pocket  29  when the differential rotation speed is excessively high. More specifically, this SOWC  17  is configured that the strut  26  is flicked by an opening portion of the pocket  29  and thus cannot be engaged with the pocket  29  (that is, an ratchet action is exerted) when the differential rotation speed of the SOWC  17  is higher than specified differential rotation speed. The specified differential rotation speed in this case is the ratchet minimum differential rotation speed. Accordingly, the SOWC  17  is configured that it can be engaged (locked) when the differential rotation speed thereof is lower than the ratchet minimum differential rotation speed and that it is ratcheted and thus cannot be engaged when the differential rotation speed thereof is higher than the ratchet minimum differential rotation speed. If the SOWC  17  is engaged at the high differential rotation speed, the impact caused by the engagement of the strut  26  and the pocket  29  becomes substantial. However, due to the provision of the ratchet function as described above, the impact or shock during the engagement can be suppressed. 
     Furthermore, a fluctuation in the ratchet minimum differential rotation speed is unavoidable due to a structure of the engagement mechanism  23 . In other words, when the differential rotation speed of the SOWC  17  is close to the ratchet minimum differential rotation speed, there is a case where the SOWC  17  can be either locked or ratcheted. For this reason, as shown in  FIG. 9 , an upper limit of the ratchet minimum differential rotation speed and a lower limit of the ratchet minimum differential rotation speed are set in this lock rotation control. When the differential rotation speed of the SOWC  17  is higher than the upper limit of the ratchet minimum differential rotation speed, the SOWC  17  is always ratcheted to prevent the engagement thereof. On the contrary, when the differential rotation speed of the SOWC  17  is lower than the lower limit of the ratchet minimum differential rotation speed, the SOWC  17  is always engaged. 
     If the lock rotation control is executed in step S 7  and step S 8 , it is determined whether the differential rotation speed of the SOWC  17  is lower than engagement stop differential rotation speed (step S 9 ). As shown in  FIG. 9 , this engagement stop differential rotation speed is set as a value that is higher than the upper limit of the above ratchet minimum differential rotation speed. Accordingly, a negative determination is made in this step S 9  if the differential rotation speed of the SOWC  17  is equal to or higher than the engagement stop differential rotation speed. In this case, the process proceeds to step S 10 , and engagement stop control for stopping a series of the synchronous control, which includes the above lock rotation control, is executed. Then, this routine is terminated once. 
     On the contrary, a positive determination is made in step S 9  if the differential rotation speed of the SOWC  17  is lower than the engagement stop differential rotation speed. In this case, the process proceeds to step S 11 . Then, it is determined whether the differential rotation speed of the SOWC  17  is lower than retry differential rotation speed. As shown in  FIG. 9 , this retry differential rotation speed is set as a value that is lower than the upper limit of the above ratchet minimum differential rotation speed and is also higher than the lower limit of the ratchet minimum differential rotation speed. Accordingly, when the differential rotation speed of the SOWC  17  is higher than this retry differential rotation speed, a possibility that the SOWC  17  is ratcheted and thus cannot be engaged is increased. Thus, a negative determination is made in this step S 11  if the differential rotation speed of the SOWC  17  is equal to or higher than this retry differential rotation speed. In this case, the process returns to step S 2 , and the above-described synchronous control is executed again. In other words, the differential rotation speed of the SOWC  17  is returned to the unlock-side target differential rotation speed that is set on the negative side, and the series of the control from the synchronous control in step S 2  onward is executed again. 
     On the contrary, a positive determination is made in this step S 11  if the differential rotation speed of the SOWC  17  is lower than this retry differential rotation speed. In this case, the process proceeds to step S 12 . Then, it is determined whether the differential rotation speed of the SOWC  17  becomes substantially zero. In other words, it is determined in this step S 12  whether the SOWC  17  has become engaged. It can be determined that the SOWC  17  has become engaged, in the case where the differential rotation speed of the SOWC  17  that is maintained at the lock-side target differential rotation speed becomes substantially zero. Accordingly, a negative determination is made in this step S 12  if the differential rotation speed of the SOWC  17  has not become substantially zero. In this case, the process returns to step S 8 , and the lock rotation control, in which the differential rotation speed of the SOWC  17  becomes and is maintained at the lock-side target differential rotation speed, is continued. 
     A positive determination is made in step S 12  if the differential rotation speed of the SOWC  17  has become substantially zero. In this case, the process proceeds to step S 13 . That is, if it is determined that the SOWC  17  has become engaged, the process proceeds to step S 13 . Then, transition of torque from the first motor  2  to the SOWC  17  is initiated. More specifically, the first motor  2  is controlled such that the output torque thereof becomes zero. For example, in the example of the configuration shown in  FIG. 1 , the output torque of the first motor  2  becomes zero in a state that the rotation of the first motor  2  and the rotation of the sun gear  5  are locked by the SOWC  17 . Alternatively, in the example of the configuration shown in  FIG. 3 , the output torque of the first motor  2  becomes zero in a state that the rotation of the ring gear  22  in the overdrive mechanism  19  is locked by the SOWC  17 . Then, this routine is terminated once. 
     A time chart in  FIG. 9  shows an example of the change in the differential rotation speed of the SOWC  17  when the above control shown in the flowchart in  FIG. 7  is executed. Once a determination for switching the SOWC  17  to the engaged state is established in a state that the vehicle Ve travels in the disengaged state of the SOWC  17  (time t 1 ), the above-described synchronous control is initiated. Then, the rotation of the first motor  2  is controlled such that the differential rotation speed of the SOWC  17  is reduced to the negative side (time t 2 ). 
     The differential rotation speed of the SOWC  17  is gradually reduced to the negative side and eventually reaches the actuator ON differential rotation speed, which is set as a specified value close to zero. At this time, the actuator  32  is controlled to be ON (time t 3 ). Thereafter, once it is detected by the detection signal of the stroke sensor  33  or the like that the stroke of the actuator  32  is completed (time t 4 ), a timer is actuated. Then, the lock rotation control is initiated at a time point (time t 5 ) when a specified time period has elapsed since the completion of the stroke of the actuator  32  is detected. The specified time period set here is a time period that is estimated as time required for the operation of the actuator  32  to be completed and for the strut  26  to move to the specified position for engagement, as described above. 
     Meanwhile, while the operation of the actuator  32  is controlled as described above, the differential rotation speed of the SOWC  17  is gradually reduced to the unlock-side target differential rotation speed that is set on the negative side. Then, once the differential rotation speed of the SOWC  17  becomes the unlock-side target differential rotation speed, the rotation of the first motor  2  is subject to feedback control such that the differential rotation speed is maintained at the unlock-side target differential rotation speed. This control for maintaining the differential rotation speed of the SOWC  17  at the unlock-side target differential rotation speed is continued until the above time t 5 , that is, until a time point at which the completion of the stroke of the actuator  32  is estimated. Accordingly, the operation of the strut  26  for the engagement of the SOWC  17  is always performed in the state that the differential rotation speed of the SOWC  17  is negative. Thus, the strut  26  can be operated from the second clutch plate  25  of the SOWC  17  without being applied with the torque. As a result, such a situation that the strut  26  and the pocket  29  are engaged at an improper position that is located in a middle of a path for the strut  26  to be engaged at a specified position in the pocket  29  can be avoided. 
     Once the lock rotation control is initiated at the time t 5 , the rotation of the first motor  2  is controlled such that the differential rotation speed of the SOWC  17  is increased to the lock-side target differential rotation speed that is set on the positive side. Then, once the differential rotation speed of the SOWC  17  becomes the lock-side target differential rotation speed, the rotation of the first motor  2  is subject to feedback control such that the differential rotation speed is maintained at the lock-side target differential rotation speed. As described above, the lock-side target differential rotation speed is set as the value on the positive side that is lower than the lower limit of the ratchet minimum differential rotation speed at which the SOWC  17  is always engaged. For this reason, the SOWC  17  can be smoothly shifted to the engaged state in the state that the differential rotation speed thereof is maintained at the lock-side target differential rotation speed. 
     Then, the engagement of the SOWC  17  is completed, and thus the differential rotation speed of the SOWC  17  becomes zero (time t 6 ). Accordingly, it can be determined that the engagement of the SOWC  17  is completed by monitoring the change of the differential rotation speed of the SOWC  17 . Once it is determined that the engagement of the SOWC  17  is completed, just as described, the torque transition from the first motor  2  to the SOWC  17  is initiated (time t 7 ). More specifically, the first motor  2  is controlled such that the output torque thereof becomes zero. 
     As it has been described specifically so far, according to the control device of the invention, in the case where the SOWC  17 , the differential rotation speed of which is positive, is switched from the disengaged state to the engaged state, the rotation of the first motor  2  is controlled such that the differential rotation once becomes the negative differential rotation. Then, the actuation mechanism  27  of the SOWC  17  is actuated such that the projection of the strut  26  to the second clutch plate  25  at the position between the first clutch plate  24  and the second clutch plate  25  is permitted in the state that the differential rotation is the negative differential rotation. When the differential rotation of the SOWC  17  is the negative differential rotation, the torque is not transmitted between the first clutch plate  24  and the second clutch plate  25 . Thus, no load is applied to the strut  26 . In other words, the strut  26  can easily be operated. Thus, the strut  26  can easily be operated and reliably be engaged at a specified position by setting such a state that the projection of the strut  26  to the second clutch plate  25  is permitted as described above in the state that the differential rotation is the negative differential rotation. Therefore, the SOWC  17  in the disengaged state can appropriately and reliably be switched to the engaged state. 
     In the above-described specific example, the configuration in which the SOWC  17  is used as a brake for selectively stopping the rotation of the sun gear  5  or the ring gear  22  is described. Meanwhile, in this embodiment, the SOWC  17  can also be configured such that it is used as a clutch for selectively transmitting the torque between the two rotary members. Such an example is shown in  FIG. 10 . A part of the configuration shown in  FIG. 1 , which is described above, is modified for this example of the configuration shown in  FIG. 10 . More specifically, instead of the engine  1 , the output gear  8  is coupled to the carrier  6  in the planetary gear mechanism, which constitutes the power split mechanism  4 . In addition, instead of the output gear  8 , the engine  1  is coupled to the ring gear  7 . Furthermore, the SOWC  17  shown in this  FIG. 10  is configured to selectively couple the ring gear  7  (the engine  1 ) and the sun gear  5 . A direction of engagement of the SOWC  17  in this case is a direction in which the torque is transmitted from the engine  1  to the sun gear  5  in the positive rotational direction. The rest of the configuration is the same as the configuration shown in  FIG. 1 . Accordingly, components shown in this  FIG. 10  are denoted by the same reference numerals as those used in  FIG. 1 , and the description thereof will not be made. 
     In the power transmission mechanism that is configured as shown in  FIG. 10 , the HV mode (or the power split mode) and a direct connection mode (or the parallel mode) can be set. In the HV mode, the power output by the engine  1  is divided by the output gear  8  and the first motor  2 . In the direct connection mode, a differential action of the power split mechanism  4  is stopped, and the entire power split mechanism  4  is integrated for rotation. The SOWC  17  shown in this  FIG. 10  is engaged when the vehicle Ve travels forward in the above direct connection mode. 
       FIG. 11  includes collinear diagrams on the planetary gear mechanism that constitutes the power split mechanism  4  shown in  FIG. 10 . A top diagram in  FIG. 11  indicates a state that the vehicle Ve travels forward in the HV mode with the SOWC  17  being disengaged. In this state shown in the top diagram in  FIG. 11 , the torque in the positive rotational direction of the engine  1  is transmitted to the ring gear  7 . Meanwhile, reaction torque in the reverse rotational direction that is generated in conjunction with the travel of the vehicle Ve acts on the carrier  6 . Accordingly, the torque in the reverse rotational direction acts on the sun gear  5 . This corresponds to the torque in the positive rotational direction of the engine  1  with respect to the sun gear  5 . However, since the SOWC  17  is disengaged, the sun gear  5  rotates reversely as shown in the top diagram in  FIG. 11 , for example. In this case, the first motor  2  that is coupled to this sun gear  5  functions as the electrical power generator and applies the torque in the positive rotational direction (the upward direction in the top diagram in  FIG. 11 ) as the reaction force to the sun gear  5 . As a result, the torque of the engine  1  is amplified and transmitted to the output gear  8 , which is coupled to the carrier  6 . In addition, the first motor  2  controls the speed of the engine  1  to the speed at which the excellent fuel efficiency can be realized. Furthermore, the electrical power generated in the first motor  2  is supplied to the second motor  3 . Then, the second motor  3  functions as the motor. In other words, some of the power of the engine  1  that has been converted to the electrical power is converted to the mechanical power again, and is transmitted to the drive wheels  14 . 
     A second diagram from the top in  FIG. 11  indicates a transient state (a transition state) in which the SOWC  17  is switched between the state shown in the above top diagram in  FIG. 11  and a state shown in the third diagram from the top in  FIG. 11 , which will be described below. In other words, the second diagram from the top in  FIG. 11  indicates the transient state when the SOWC  17  is switched from the disengaged state (free) to the engaged state (locked) in which the SOWC  17  restricts the relative rotation between the sun gear  5  and the engine  1 . In this state shown in the second diagram from the top in  FIG. 11 , the first motor  2  functions as the motor and rotates the sun gear  5  in the positive rotational direction. At this time, the rotational speed of the sun gear  5  exceeds the speed of the engine  1 . In other words, the first motor  2  is controlled such that the engine  1  rotates in the reverse rotational direction (the negative rotation) relative to the sun gear  5 . Such relative rotation is an example of the differential rotation in the invention, and in this state shown in the second diagram from the top in  FIG. 11 , the negative differential rotation is produced. In other words, the SOWC  17  does not transmit the torque. Thus, when the engagement control of the SOWC  17  is executed in this state, the torque is not applied to the strut  26  of the SOWC  17 . Noted that, in this state shown in the second diagram from the top in  FIG. 11 , the direction of the differential rotation, the direction of the torque of the first motor  2 , and the rotational direction of the first motor  2  are opposite from those in the examples shown in the collinear diagrams of above-described  FIG. 2  and  FIG. 4 . However, as described above, the SOWC  17  shown in  FIG. 10  is configured such that the differential rotation produced in this state shown in the second diagram from the top in  FIG. 11  becomes the negative differential rotation. Accordingly, the same control as the control example shown in the flowchart of  FIG. 7  is executed so that the SOWC  17  can reliably and appropriately be engaged. 
     A third diagram from the top in  FIG. 11  indicates a state that the vehicle Ve travels forward in the direct connection mode. In the forward travel state, as described above, the torque in the direction to cause the reverse rotation of the sun gear  5  acts thereon, and the engine  1  attempts to rotate in the positive rotational direction relative to the sun gear  5 . Accordingly, when the SOWC  17  is controlled to be in the engaged state, the above-described strut  26  is interposed (meshed) between the housing section  28  of the first clutch plate  24  and the pocket  29  of the second clutch plate  25 , and thus the sun gear  5  and the engine  1  are coupled to rotate integrally in the positive rotational direction. As a result, since the two rotary elements are integrated, the entire power split mechanism  4  rotates integrally. In other words, the engine  1  is directly connected to the output gear  8 . 
     Noted that, in the configuration shown in  FIG. 10 , the first motor  2  is coupled to the first clutch plate  24  of the SOWC  17 , and the rotational speed of the first clutch plate  24  is controlled by the first motor  2 . Meanwhile, in the above-described configuration shown in  FIG. 1 , the first motor  2  is coupled to the second clutch plate  25  of the SOWC  17 , and the rotational speed of the second clutch plate  25  is controlled by the first motor  2 . In addition, in the above-described configuration shown in  FIG. 3 , the first motor  2  is coupled to the second clutch plate  25  of the SOWC  17  via the overdrive mechanism  19 , and the rotational speed of the second clutch plate  25  is controlled by the first motor  2 . As described above, the first motor  2  is configured that it can control the rotational speed of either clutch member of the first clutch plate  24  or the second clutch plate  25  of the SOWC  17 , and is an example of the motor of the invention. 
     Furthermore, the invention can be applied to a control device for a power transmission mechanism that includes a stepped transmission or a continuously variable transmission other than the power transmission mechanism that is installed in the hybrid vehicle Ve as described above. Thus, the motor in the invention may be a motor for controlling the SOWC only.