Abstract:
In an exemplary embodiment, a transmission outputs power from two high-speed, high-powered motor/generators into two coaxial output shafts for left and right wheels of a vehicle. Embodiments of the invention provide for a transmission that is coaxially mounted with the output shafts and the two motor/generators, which allows for a space-saving design. The transmission provides at least two degrees of freedom such that torque to each of the left any right wheels of driveshaft can be separately controlled by controlling the input/output of each of the motor/generators. The transmission may include two differential units to allow a significant gear reduction such that motor/generators that require high output speeds may be used.

Description:
This application claims priority from Japanese Patent Application No. 2005-119480, filed Apr. 18, 2005, the entire content of which is incorporated herein by reference. 
   TECHNICAL FIELD 
   The invention relates to a transmission and more particularly, but without limitation to a transmission for an electric or hybrid vehicle. 
   BACKGROUND 
   A hybrid four-wheel-drive vehicle may have either the front or rear wheels driven by an engine, and the other wheels driven by a motor. Some hybrid vehicles provide a motor/generator connected to the front or rear wheels to drive these wheels by turning on the motor when necessary, or conversely, capturing rotational energy of these wheels by converting the rotational energy to electric energy to be stored, e.g., during braking. However, general practice requires that the power distribution to the wheels from the motor/generator is performed after reducing the rotational velocity of the motor/generator using a two-step parallel gear reducer. However, a transmission with a two-step parallel gear reducer causes the motor to be offset radially from an axis of a driveshaft. This results in a radially large drive system including the motor, which limits the practical application of such a design. 
   It has been proposed to provide an electrical motor in a coaxial arrangement with a rotational axis of wheels in order to conserve space by creating a hollow space at the center of the electrical motor through which a driveshaft can be located. 
   SUMMARY 
   With a transmission in which one planetary gear group is used, the problem of a radially enlarging wheel-drive system, including the motor, may be avoided by coaxially. locating the planetary gear group and the motor, but there is a structural restriction in increasing the reduction ratio, making it difficult for use in cases in which a high-power motor that is rotatable at high speed is required. 
   Additionally, with a differential unit comprising a planetary gear group when the motor power is distributed into a dual-output system, the torque distribution to the dual-output system may be fixed based on the design of the differential unit, so it is not adoptable when the torque distribution to a dual-output system needs to be freely controlled. 
   Embodiments of the invention may provide a transmission capable of outputting the rotation from a motor at a reduced rotational velocity, even with a high-power motor must rotate at high speed. Such a transmission may also be capable of freely controlling the torque distribution to a dual-output system. 
   In an embodiment of the invention, a transmission comprises a first differential unit comprising a first set of rotary elements, wherein the first set of rotary elements includes at least three rotary elements, a second differential unit comprising a second set of rotary elements, wherein the second set of rotary elements includes at least three rotary elements, wherein at least one rotary element in the first set of rotary elements is connected to at least one rotary element in the second set of rotary elements, wherein the first differential unit combines with the second differential unit to provide at least two degrees of freedom for the transmission, a first motor/generator connected to a first rotary element, wherein the first rotary element is one of the first set of rotary elements, a first output shaft connected to a second rotary element, wherein the second rotary element is one of the first set of rotary elements, a second motor/generator connected to a third rotary element, wherein the third rotary element is one of the second set of rotary elements, and a second output shaft connected to a fourth rotary element, wherein the fourth rotary element is one of the second set of rotary elements or one of the first set of rotary elements. 
   In one embodiment, a transmission comprises a first differential unit comprising a first set of rotary elements, wherein the first set of rotary elements includes at least three rotary elements, a second differential unit comprising a second set of rotary elements, wherein the second set of rotary elements includes at least three rotary elements, wherein at least one rotary element in the first set of rotary elements is connected to at least one rotary element in the second set of rotary elements, wherein the first differential unit combines with the second differential unit to provide at least two degrees of freedom for the transmission, a first motor/generator connected to a first rotary element, wherein the first rotary element is one of the first set of rotary elements, a gear reduction mechanism, a first output shaft connected to a second rotary element through the gear reduction mechanism, wherein the second rotary element is one of the first set of rotary elements, a second motor/generator connected to a third rotary element, and a second output shaft connected to a fourth rotary element. 
   An embodiment of the invention is directed to a transmission comprising a first differential unit comprising a first planetary gear group and a second planetary gear group, wherein the first and second planetary gear groups have a first common carrier and together include five rotary elements, a second differential unit comprising a third planetary gear group and a fourth planetary gear group, wherein the third and fourth planetary gear groups have a second common carrier and together include five rotary elements, wherein at least one rotary element in the first differential unit is connected to at least one rotary element in the second differential unit, wherein the first differential unit combines with the second differential unit to provide at least two degrees of freedom for the transmission, a first motor/generator, a first output shaft, a second motor/generator; and a second output shaft, wherein, in a first alignment chart, which represents the first differential unit, a first rotary element of the first differential unit, which is connected to the first motor/generator, is on a first end of the alignment chart in the direction of the order of the rotational speeds, a second rotary element of the first differential unit, which is connected to a second rotary element of the second differential unit, is on the end of the first alignment chart opposite to the first end of the first alignment chart, and the first common carrier of the first differential unit is located between the first rotary element of the first differential unit and the second rotary element of the first differential unit on the first alignment chart, wherein the first output shaft is connected to the first common carrier or to a rotary element of the first differential unit immediately adjacent to the first common carrier on the alignment chart, wherein, in a second alignment chart, which represents the second differential unit, a first rotary element of the second differential unit, which is connected to the second motor/generator, is on a first end of the second alignment chart in the direction of the order of the rotational speeds, the second rotary element of the second differential unit is on the end of the second alignment chart opposite to the first end of the second alignment chart, the second common carrier, is located between the first rotary element of the second differential unit and the second rotary element of the second differential unit on the second alignment chart, a third rotary element of the second differential unit, which is connected to the second output shaft, is located adjacent to the second common carrier on the second alignment chart, and a fourth rotary element of the second differential unit, which is fixed, is located between the second common carrier and the second rotary element of the second differential unit. 
   In an embodiment, a transmission comprises a first differential unit comprising at least a first rotary element, a second rotary element and a third rotary element, a second differential unit comprising at least a fourth rotary element, a fifth rotary element and a sixth rotary element, wherein the sixth rotary element of the second differential unit is fixed, wherein the second differential unit is located coaxially with the first differential unit, wherein the first rotary element of the first differential unit is connected to the fourth rotary element of the second differential unit, a first motor/generator, wherein the first motor/generator is connected to the second rotary element of the first differential unit, a first output shaft, wherein the first output shaft is connected to the third rotary element of the first differential unit, a second output shaft, wherein the second output shaft is connected to the fifth rotary element of the second differential unit, wherein the second output shaft is located coaxially with the first output shaft, and a second motor/generator, wherein the second motor/generator is connected to a rotary element of the second differential unit other than the fifth rotary element of the second differential unit or the sixth rotary element of the second differential unit. 
   According to the transmission associated with the present invention, the connections among the rotary elements of these differential units, and the connections of both motor/generators as well as both output shafts to these rotary elements are the connections described above, so that the rotation of both motor/generators is distributed and output to both output shafts under the deceleration by both differential units, and both motor/generators are able to be placed coaxially to both differential units that have been coaxially located in parallel. This avoids the problem of radially enlarging the transmission including the motor. 
   Because the rotation of both motor/generators is distributed and output to both output shafts while the rotation is being reduced by both differential units, a large reduction ratio may be set according to the combination of both differential units, so even when a high-power motor that is rotatable only at high speed is used, the rotation from said motors is output, assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
   Because the rotation of both motor/generators is distributed and output to both output shafts while the rotation is being reduced by both differential units, the torque distribution to both output shafts becomes freely controllable according to the output combination of both motor/generators, so it is also applicable to cases in which the torque distribution to a dual-output system needs to be freely controlled without any restriction in use. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is an outline drawing showing a concept of a transmission in one embodiment of the present invention. 
       FIG. 2  is an alignment chart of a motor transmission in the same embodiment. 
       FIG. 3  is an alignment chart wherein vectors are added to the alignment chart in  FIG. 2  for the explanation of behaviors of the transmission. 
       FIG. 4  is an explanatory figure showing the coefficients between the input/output of the transmission in the same embodiment. 
       FIG. 5  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 6  is an alignment chart of a transmission in the same embodiment. 
       FIG. 7  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 8  is an alignment chart of a transmission in the same embodiment. 
       FIG. 9  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 10  is an alignment chart of a transmission in the same embodiment. 
       FIG. 11  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 12  is an alignment chart of a transmission in the same embodiment. 
       FIG. 13  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 14  is an alignment chart of a transmission in the same embodiment. 
       FIG. 15  is an outline drawing showing a concept of a transmission in other embodiment of the present invention. 
       FIG. 16  is an alignment chart of a transmission in the same embodiment. 
       FIG. 17  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 18  is an alignment chart of a transmission in the same embodiment. 
       FIG. 19  is an outline drawing showing a concept of a transmission in another embodiment of the present invention. 
       FIG. 20  is an alignment chart of a transmission in the same embodiment. 
       FIG. 21  is an outline drawing showing a concept of a transmission in yet another embodiment of the present invention. 
       FIG. 22  is an alignment chart of a transmission in the same embodiment. 
       FIG. 23  is an alignment chart wherein vectors are added to the alignment chart of  FIG. 22  for the explanation of the behaviors of the transmission. 
       FIG. 24  is an explanatory figure showing the coefficients between the input/output of the transmission in the same embodiment. 
       FIG. 25  is an outline drawing showing a concept of a transmission in yet another embodiment of the present invention. 
       FIG. 26  is an alignment chart of a transmission in the same embodiment. 
       FIG. 27  is an outline drawing showing a concept of a transmission in yet another embodiment of the present invention. 
       FIG. 28  is an alignment chart of a transmission in the same embodiment. 
       FIG. 29  is an outline drawing showing a concept of a transmission in yet another embodiment of the present invention. 
       FIG. 30  is an alignment chart of a transmission in the same embodiment. 
       FIG. 31  is an alignment chart wherein vectors are added to the alignment chart of  FIG. 30  for the explanation of the actions of the transmission. 
       FIG. 32  is an explanatory figure showing the coefficients between the input/output of the transmission in the same embodiment. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is an outline drawing illustrating a transmission as an embodiment of the present invention. In  FIG. 1 ,  1  represents the casing, within the same casing  1  axially on the right side (left/right direction in the figure), two—i.e. a first and a second—planetary gear groups G 1  and G 2  are coaxially accommodated in parallel, and within the casing  1  axially on the left side (left/right direction in the figure), a third planetary gear group G 3  is accommodated so as to be coaxially located by the planetary gear groups G 1  and G 2 . 
   First and second planetary gear groups G 1  and G 2  are located so as to position the first planetary gear group G 1  on the left side in the figure, and one motor/generator MG 1  and the other motor/generator MG 2  are interposed coaxially between the first/second planetary gear groups G 1  and G 2  and the third planetary gear group G 3 . The motor/generators MG 1  and MG 2  have a common stator  3   s  fixed onto the casing  1 , and are equipped with a rotor  3   ro  of one motor/generator MG 1  located on the outer circumference and a rotor  3   ri  of the other motor/generator MG 2  located on the inner circumference. The motor/generator MG 1  is comprised of a stator  3   s  and an outside rotor  3   ro,  the motor/generator MG 2  is comprised of a stator  3   s  and an inside rotor  3   ri,  and the motor/generator MG 1  and MG 2  serve as a composite current dual-layer motor. 
   In this case, by applying a composite current combining the control current of both motor/generators MG 1  and MG 2 , the motor/generators MG 1  and MG 2  may be subject to individual motor drive control, or these motor/generators MG 1  and MG 2  may be functional as individual generators, respectively. Instead of comprising the motor/generators MG 1  and MG 2  as a single unit to serve as a composite current dual-layer motor, it may also be comprised as an independent unit having an individual stator. However, in any case, the motor/generators MG 1  and MG 2  are to be coaxially located together, and at the same time, be located coaxially with the planetary gear groups G 1 , G 2 , G 3 . 
   A first planetary gear group G 1  and a second planetary gear group G 2  constitute one differential unit  5 , a third planetary gear group G 3  constitute the other differential unit  6  in the present invention, and these first planetary gear group G 1 , second planetary gear group G 2 , and the third planetary gear group G 3 , respectively, are regarded as a single-pinion planetary gear group comprising a sun gear S 1 , S 2 , S 3 , a ring gear R 1  and R 2 , R 3 , and a carrier C 1  and C 2 , C 3  rotatably supporting a pinion P 1  and P 2 , P 3  that engages with the sun gear and the ring gear. Herein, to construct one differential unit  5  from the first planetary gear group G 1  and the second planetary gear group G 2 , the carrier C 1  of the first planetary gear group G 1  is connected to the ring gear R 2  of the second planetary gear group G 2 , and at the same time, the ring gear R 1  of the first planetary gear group G 1  is connected to the carrier C 2  of the second planetary gear group G 2 . 
   Then the ring gear R 2  of the second planetary gear group G 2  is connected to one output shaft Out 1 , and the carrier C 2  of the second planetary gear group G 2  is connected to the sun gear S 3  of the third planetary gear group G 3 . One motor/generator MG 1  is connected to the sun gear S 1  of the first planetary gear group G 1 , and the other motor/generator MG 2  is connected to the sun gear S 2  of the second planetary gear group G 2 . Moreover, the carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . In addition, a shaft  7  connecting the sun gear S 2  to the motor/generator MG 2  is a hollow shaft, a shaft  8  connecting the carrier C 2  to the sun gear S 3  extends from the location of the second planetary gear group G 2  to the location of the third planetary gear group G 3 , penetrating through the hollow shaft  7 , and a shaft  9  connecting the sun gear S 1  to the motor/generator MG 1  is a hollow shaft rotatably coupled to the outer circumference of the hollow shaft  7 . 
   One output shaft Out 1  protruding coaxially and rotatably from the end (right end in  FIG. 1 ) of the casing  1 , where second planetary gear group G 2  is located, is to be connected to a differential gear unit for rear left/right wheels or to a differential gear unit for front left/right wheels not illustrated herein. The other output shaft Out 2  protruding coaxially and rotatably from the end (left end in  FIG. 1 ) of the casing  1  opposite to the protruding side of the above one output shaft Out 1  is connected, for example, to a differential gear unit for rear left/right wheels or a differential gear unit for front left/right wheels not illustrated herein. 
   The above transmission unit comprised as in  FIG. 1  can be represented as in  FIG. 2  in an alignment chart; the vertical axis in the figure indicates the rotational speeds (0 is the reference, the upward direction in the figure is the forwarding rotational speed, and the downward direction is the reversing rotational speed) of the rotary elements constituting the planetary gear groups G 1  and G 2  (one differential unit  5 ) and the planetary gear group G 3  (the other differential unit  6 ), the horizontal axis indicates the distance ratio between the rotary elements constituting the planetary gear groups G 1  and G 2  (one differential unit  5 ) and the planetary gear group G 3  (the other differential unit  6 ). As described previously, the ring gear R 1  and the carrier C 2  are connected together, and the carrier C 1  and the ring gear R 2  are connected together, so one differential unit  5  comprising the first planetary gear group G 1  and the second planetary gear group G 2  is illustrated as a single rod of combined levers indicated by the same numerical mark G 1  and G 2  in  FIG. 2 , and the order of the rotational speeds (in ascending order or descending order, depending on the speed change status) of the rotary elements constituting the same differential unit  5  is in this order of sun gear S 1 , carrier C 1  (ring gear R 2 ), ring gear R 1  (carrier C 2 ), and sun gear S 2 . 
   The other differential unit  6  comprising the third planetary gear group G 3  is represented as a lever indicated by the same numerical mark G 3  in  FIG. 2 , as described because the sun gear S 3  is connected to the ring gear R 1  (carrier C 2 ), and at the same time, the carrier C 3  is fixed, and the order of the rotational speeds (in the ascending order or descending order, depending on the speed change status) of the rotary elements constituting the same differential unit  6  is in the order of sun gear S 3 , carrier C 3 , and ring gear R 3 . 
   In the alignment chart of  FIG. 2 , two rotary elements of one differential unit  5  (G 1  and G 2 ) located approximately in the middle in the direction of the order of the rotational speeds, or in other words, among the carrier C 1  (ring gear R 2 ) and the ring gear R 1  (carrier C 2 ), one output shaft Out 1  is connected to the former carrier C 1  (ring gear R 2 ), and the later ring gear R 1  (carrier C 2 ) and the sun gear S 3  of the other differential unit  6  (G 3 ) are connected together. Two rotary elements of one differential unit  5  (G 1  and G 2 ) located at each end in the direction of the order of the rotational speeds in the alignment chart of  FIG. 2 , or in other words, among the sun gear S 1  and the sun gear S 2 , one output shaft Out 1  is connected to the sun gear S 1 , as a rotary element close to the carrier C 1  (ring gear R 2 ), and the sun gear S 3  of the other differential unit  6  (G 3 ) is connected to the sun gear S 2 , as a rotary element close to the ring gear R 1  (carrier C 2 ), one motor/generator MG 1  and the other motor/generator MG 2  are connected, respectively. 
   In the alignment chart of  FIG. 2 , the other output shaft Out 2  is connected to the ring gear R 3  of the other differential unit  6  (G 3 ) located farthest from the sun gear S 3  of the other differential unit  6  (G 3 ) connected to the ring gear R 1  (carrier C 2 ) of one differential unit  5  (G 1  and G 2 ), and also in the same alignment chart of  FIG. 2 , the carrier R 3  interposed between both ends of the other differential unit  6  (G 3 ) is fixed onto the casing  1 . 
   As for the transmission unit shown in  FIG. 1 , by changing the rotational speed of the motor/generators MG 1  and MG 2  within a range indicated by the bold arrows in the alignment chart of  FIG. 2 , a mutual connection point of one differential unit  5  (G 1  and G 2 ) and the other differential unit  6  (G 3 ), and a connection point (carrier C 1  and ring gear R 2 ) of one output shaft Out 1 , a connection point of the other output shaft Out 2  (ring gear R 3 ) changes their rotational speed within the range indicated by the bold arrows, so the rotation of the motor/generators MG 1  and MG 2  may be distributed and output for both output shafts. Out 1  and Out 2 . 
   Hereafter, the manner in which the torque from the motor/generators MG 1  and MG is transmitted to the output shafts Out 1  and Out 2  is explained based on the vector indicated in  FIG. 3 , which is the same alignment chart as  FIG. 2 . As shown in the alignment charts of  FIG. 2  and  FIG. 3 , wherein L 1  is the distance between the sun gear S 1  connected to one motor/generator MG 1  and the carrier C 1  connected to one output shaft Out 1 , L 2  is the distance between the carrier C 1  to which one output shaft Out 1  is connected and the ring gear R 1  (carrier C 2 ) that is a rotary element of the differential unit  5  connected to the differential unit  6 , L 3  is the distance between the ring gear R 1  (carrier C 2 ) and the sun gear S 2  connected to the other motor/generator MG 2 , with regard to the other differential unit  6 , L 4  is the distance between the carrier C 3  and the sun gear S 3 , L 5  is the distance between the carrier C 3  and the ring gear R 3 , and given α=L 1 /L 2 , β=L 3 /L 2 , γ=L 4 /L 5 , when one motor/generator MG 1  outputs positive torque Tmg 1  as shown in  FIG. 3 , the positive torque (1+α)Tmg 1  is generated on one output shaft Out 1 , and positive torque α·γ·Tmg 1  is generated on the other output shaft Out 2 , when the other motor/generator MG 2  outputs negative torque −Tmg 2  as shown in  FIG. 3 , the positive torque β·Tmg 2  is generated on one output shaft Out 1 , and positive torque (1+β)γ·Tmg 2  is generated on the other output shaft Out 2 . Therefore, due to the torque Tmg 1  and −Tmg 2  of the motor/generators MG 1  and MG 2 , positive torque (1+α) Tmg 1 +β·Tmg 2  is applied to one output shaft Out 1 , and positive torque α·γ·Tmg 1 +(1+β)γ·Tmg 2  is applied to the other output shaft Out 2 , so the same directional torque may be output from both output shafts Out 1  and Out 2 . 
     FIG. 4  is an explanatory figure showing the relational coefficients during the above power transmission between the torque Tmg 1  as well as the number of revolutions Nmg 1  of the motor/generator MG 1 , and the torque Tout 1  and Tout 2  as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 , and between the torque Tmg 2  as well as the number of revolutions Nmg 2  of the motor/generator MG 2 , and the torque Tout 1  and Tout 2  as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 , and, in  FIG. 4 , the torque Tout 1  and Tout 2  generated from the output shafts Out 1  and Out 2  by the motor/generator torque Tmg 1  and Tmg 2 , along with the number of revolutions Nmg 1  and Nmg 2  of the motor/generators MG 1  and MG 2  determined by the number of revolutions of the output shafts Nout 1 , Nout 2  at that moment have also been stated. 
   Herein, when an equal value is given to the torque Tmg 1  and Tmg 2  of both motor/generators MG 1  and MG 2 , for the purpose of explaining the conditions so that the torque Tout 1  and Tout 2  of both output shafts Out 1  and Out 2  are equalized, it is necessary to establish, (1+α)Tmg 1 +β·Tmg 2 =α·γ·Tmg 1 +(1+β)γ·Tmg 2 , but currently, Tmg 1 =Tmg 2 , so it may be acknowledged that the distance ratio α, β, and γ, respectively, in said one differential unit  5  (G 1  and G 2 ) and in the other differential unit  6  (G 3 ) must be determined so as to obtain αγ−(1+α)≅β−(1+β)γ. 
   By determining the output of the motor/generators MG 1  and MG 2  so as to establish a relation of Tmg 1 ·Nmg 1 +Tmg 2 ·Nmg 2 +Tout 1 ·Nout 1 +Tout 2 ·Nout 2 =0 between the torque Tmg 1  and Tmg 2  as well as the number of revolutions Nmg 1  and Nmg 2  of the motor/generator MG 1  and MG 2 , and the torque Tout 1  and Tout 2  as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 , the drive power distribution to both output shafts Out 1  and Out 2  becomes arbitrarily controllable, and thus may be used as the drive power distribution control of the front/rear wheels. 
   As for the drive power distribution control, as seen in  FIG. 4 , by setting one of the number of revolutions of both output shafts Out 1  and Out 2  to zero, the output from the same output shaft may also become zero, so it then becomes possible to output the total output of both motor/generators MG 1  and MG 2  to the other output shaft. Therefore, by determining the output of the motor/generators MG 1  and MG 2  so as to establish a relation of Tmg 1 ·Nmg 1 +Tmg 2 ·Nmg 2 =Tout 1 ·Nout 1 +Tout 2 ·Nout 2 , the drive power distribution of the output shafts Out 1  and Out 2  may be changed arbitrarily from 0% to 100%, permitting an active drive power distribution control of the front/rear wheels of four-wheel-drive vehicles to enhance maneuverability, and is thus extremely useful in enhancing the running stability of vehicles due to the drive power distribution control of the left/right wheels. 
   In addition, according to the transmission of the present invention described above, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 1 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are to be the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located as in the present embodiment, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As seen from said explanation of the action and from  FIG. 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output, assuring a reduction in the required number of revolutions, and is thus also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
   Finally, as described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so without any restriction in use, it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled. 
     FIG. 5  represents a transmission of the other embodiment of the present invention, and  FIG. 6  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the double-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the double-pinion planetary gear group, the ring gear R 1  of the first planetary gear group G 1  is connected to the ring gear R 2  of the second planetary gear group G 2 , and at the same time, the carrier C 1  of the first planetary gear group G 1  is connected to the carrier C 2  of the second planetary gear group G 2 . 
   A mutually connected body of the ring gears R 1  and R 2  is connected to one output shaft Out 1 , a mutually connected body of the carriers C 1  and C 2  is connected to the sun gear S 3  of the third planetary gear group G 3  by a center shaft  8 , the other motor/generator MG 2  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  9  to the sun gear S 1  of the first planetary gear group G 1 , and one motor/generator MG 1  is connected by the hollow shaft  7  to the sun gear S 2  of the second planetary gear group G 2 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 6 , represented by an alignment chart, but the rotary members to be assigned differ from the case in  FIG. 2 , which is the same shape of alignment chart as in this figure, and similar to said embodiment, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 5 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6 , which are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located as in the present embodiment, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from said motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 7  represents a transmission of the other embodiment of the present invention, and  FIG. 8  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the double-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the double-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the ring gear R 1  of the first planetary gear group G 1  is connected to the ring gear R 2  of the second planetary gear group G 2 , and at the same time, the carrier Cl of the first planetary gear group G 1  is connected to the carrier C 2  of the second planetary gear group G 2 . 
   A mutually connected body of the carriers C 1  and C 2  is connected to one output shaft Out 1 , a mutually connected body of the ring gears R 1  and R 2  is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the inside motor/generator is regarded as MG 2 , and the outside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  7  to the sun gear S 1  of the first planetary gear group G 1 , and one motor/generator MG 1  is connected by the hollow shaft  9  to the sun gear S 2  of the second planetary gear group G 2 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 8 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 7 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located as in the present embodiment, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output, assuring a reduction in the required number of revolutions, and is thus also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 9  represents a transmission of the other embodiment of the present invention, and  FIG. 10  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is to be engaged also to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and the second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   The common carrier C 1  (C 2 ) is connected to one output shaft Out 1 , the ring gear R 2  of the second planetary gear group G 2  is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  9  to the sun gear S 1  of the first planetary gear group G 1 , and one motor/generator MG 1  is connected by the hollow shaft  7  to the sun gear S 2  of the second planetary gear group G 2 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 10 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 9 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located as in the present embodiment, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 11  represents a transmission of the other embodiment of the present invention, and  FIG. 12  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is to be engaged also to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and the second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   The ring gear R 1  of the first planetary gear group G 1  is connected to one output shaft Out 1 , the ring gear R 2  of the second planetary gear group G 2  is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  9  to the sun gear S 1  of the first planetary gear group G 1 , and one motor/generator MG 1  is connected by the hollow shaft  7  to the sun gear S 2  of the second planetary gear group G 2 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 12 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, and one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 11 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located as in the present embodiment, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 13  represents a transmission of the other embodiment of the present invention, and  FIG. 14  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is to be engaged also to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and the second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   The common carrier Cl (C 2 ) is connected to one output shaft Out 1 , the ring gear R 1  of the first planetary gear group G 1  is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  9  to the sun gear S 2  of the second planetary gear group G 2 , and one motor/generator MG 1  is connected by the hollow shaft  7  to the sun gear S 1  of the first planetary gear group G 1 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 12 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 13 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located as in the present embodiment, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 15  represents a transmission of the other embodiment of the present invention, and  FIG. 16  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is to be engaged also to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and the second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   The ring gear R 2  of the second planetary gear group G 2  is connected to one output shaft Out 1 , the ring gear R 1  of the first planetary gear group G 1  is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  9  to the sun gear S 2  of the second planetary gear group G 2 , and one motor/generator MG 1  is connected by the hollow shaft  7  to the sun gear S 1  of the first planetary gear group G 1 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 16 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 15 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 17  represents a transmission of the other embodiment of the present invention, and  FIG. 18  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is to be engaged also to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and the second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   The ring gear R 2  of the second planetary gear group G 2  is connected to one output shaft Out 1 , the common carrier C 1  (C 2 ) is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  9  to the sun gear S 2  of the second planetary gear group G 2 , and one motor/generator MG 1  is connected by the hollow shaft  7  to the sun gear S 1  of the first planetary gear group G 1 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 18 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 17 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
     FIG. 19  represents a transmission of the other embodiment of the present invention, and  FIG. 20  is the alignment chart. In the present embodiment, one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, while the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and the second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is to be engaged also to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and the second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   The ring gear R 1  of the first planetary gear group G 1  is connected to one output shaft Out 1 , the common carrier C 1  (C 2 ) is connected by the center shaft  8  to the sun gear S 3  of the third planetary gear group G 3 , the other motor/generator MG 2  (in the present embodiment, the inside motor/generator is regarded as MG 2 , and the outside motor/generator is regarded as MG 1 ) is connected by the hollow shaft  7  to the sun gear S 1  of the first planetary gear group G 1 , and one motor/generator MG 1  is connected by the hollow shaft  9  to the sun gear S 2  of the second planetary gear group G 2 . The carrier C 3  of the third planetary gear group G 3  is fixed, and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 20 , represented by an alignment chart, although the rotary members to be assigned differ from a case in  FIG. 2 , which is the same shape of alignment chart as in this figure, similar to the embodiment in  FIGS. 1 through 4 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 , so it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled with no restriction in use. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) are coaxially located in parallel as shown in  FIG. 19 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2  as well as both output shafts Out 1  and Out 2  to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and both motor/generators MG 1  and MG 2  are able to be placed coaxially to both differential units  5  and  6  that are coaxially located in parallel, thereby avoiding the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2 . 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together to be coaxially located, axially on one side, one differential unit  5  (planetary gear group G 1  and G 2 ) is coaxially located, and at the same time, from one side of which one output shaft Out 1  is coaxially extended, and axially on the other side of the motor/generators MG 1  and MG 2  that are placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other side of which the other output shaft Out 2  is coaxially extended, so the action effect related to said small sizing radially becomes more significant. 
   As described above, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is being reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so even when a high-power motor rotatable at high speed is required, the rotation from the same motors is output assuring a reduction in the required number of revolutions, so it is also applicable to a system in which such a high-power motor is used without any problem or restriction in use. 
   In each embodiment described above, although one output shaft Out 1  is directly connected to a corresponding rotary member of one differential unit  5 , the reduction ratio of the transmission may be further increased by connecting through a gear reduction mechanism, due to the reduction ratio fraction. Hence, three embodiments—in which one output shaft Out 1  is connected to a corresponding rotary member of one differential unit  5  through a gear reduction mechanism—are explained sequentially as follows, based on  FIG. 21  through  FIG. 24 ,  FIG. 25  and  FIG. 26 , and  FIG. 27  and  FIG. 28 . 
   In the present embodiment shown in  FIG. 21  though  24 , as illustrated in the outline drawing of  FIG. 21 , axially on the right side (left/right direction in the figure) within the casing  1 , one differential unit  5  comprising two coaxially located—i.e. a first and a second—planetary gear groups G 1  and G 2 , and the third differential unit  10  comprising a fourth planetary gear group G 4  are coaxially accommodated in parallel, axially on the left side (left/right direction in the figure) within the casing  1 , the other differential unit  6  comprising a third planetary gear group G 3  is accommodated so as to be located coaxially with the planetary gear groups G 1 , G 2 , and G 3 . 
   The first and the second planetary gear groups G 1  and G 2  are located so as to position the first planetary gear group G 1  on the right side in the figure, and on the right side of which, the fourth planetary gear group G 4  is located. One motor/generator MG 1  and the other motor/generator MG 2  are interposed coaxially between the first as well as second planetary gear group G 1  and G 2 , and the third planetary gear group G 3 , and as in said each embodiment, these motor/generators MG 1  and MG 2  serve as a composite current dual-layer motor comprising a common stator  3   s,  a rotor  3   ri  of the motor/generator MG 1  located on the inner circumference, and a rotor  3   ro  of the motor/generator MG 2  located on the outer circumference. However, in the present embodiment, the outside motor/generator is regarded as MG 2  and the inside motor/generator is regarded as MG 1 . Instead of comprising the motor/generator MG 1  and MG 2  as one unit to serve as a composite current dual-layer motor, it may also be comprised as an individual unit having an individual stator. However, in any case, the motor/generators MG 1  and MG 2  are to be placed coaxially together, and at the same time, be placed coaxially with the planetary gear groups G 1 , G 2 , G 3 , and G 4 . 
   As described previously, first planetary gear group G 1  and second planetary gear group G 2  construct one differential unit  5  in the present invention, third planetary gear group G 3  constructs the other differential unit  6  in the present invention, and fourth planetary gear group G 4  constructs third differential unit  10  in the present invention. Where these first planetary gear group G 1 , second planetary gear group G 2 , third planetary gear group G 3 , and fourth planetary gear group G 4 , respectively, are regarded as a single-pinion planetary gear group comprising a sun gear S 1 , S 2 , S 3 , S 4 , a ring gear R 1 , R 2 , R 3 , R 4  and a carrier C 1 , C 2 , C 3 , C 4  rotatably supporting a pinion P 1 , P 2 , P 3 , P 4  that engages with the sun gear and the ring gear. Herein, to construct one differential unit  5  comprising a first planetary gear group G 1  and a second planetary gear group G 2 , the carrier C 1  of the first planetary gear group G 1  is connected to the ring gear R 2  of the second planetary gear group G 2 , and at the same time the ring gear R 1  of the first planetary gear group G 1  is connected to the carrier C 2  of the second planetary gear group G 2 . 
   Then, although the ring gear R 1  of the first planetary gear group G 1  is connected to one output shaft Out 1 , the connection is not to be a direct connection, but rather a connection through fourth planetary gear group G 4  (third differential unit  10 ). That is, the ring gear R 1  of the first planetary gear group G 1  is connected to the sun gear S 4  of a fourth planetary gear group G 4  (third differential unit  10 ), the ring gear R 4  of a fourth planetary gear group G 4  (third differential unit  10 ) is fixed onto the casing  1 , and the carrier C 4  of a fourth planetary gear group G 4  (third differential unit  10 ) is connected to one output shaft Out 1 . 
   The carrier C 1  of the first planetary gear group G 1  is connected to the sun gear S 3  of the third planetary gear group G 3  by the center shaft  8  to the sun gear S 1  of the first planetary gear group G 1  through the hollow shaft  7  to which one motor/generator MG 1  is connected, and to the sun gear S 2  of the second planetary gear group G 2  through the hollow shaft  9  to which the other motor/generator MG 2  is connected. The carrier C 3  of the third planetary gear group G 3  is fixed and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   One output shaft Out 1  protruding coaxially and rotatably from the end (right end in  FIG. 1 ) of the casing  1 , where fourth planetary gear group G 4  is located, is to be connected to a differential gear unit for the rear left/right wheels or to a differential gear unit for the front left/right wheels not illustrated herein. The other output shaft Out 2  protruding coaxially and rotatably from the end (left end in  FIG. 1 ) of the casing  1  opposite to the protruding side of the above one output shaft Out 1  is connected, for example, to a differential gear unit for the rear left/right wheels or a differential gear unit for the front left/right wheels not illustrated herein. 
   The above transmission unit comprised as in  FIG. 21  can be represented as in  FIG. 22  by an alignment chart, the vertical axis in the figure indicates the rotational speeds ( 0  is the reference, the upward direction in the figure is the forwarding rotational speed and the downward direction is the reversing rotational speed) of the rotary elements constituting the planetary gear groups G 1  and G 2  (one differential unit  5 ) and the planetary gear group G 3  (the other differential unit  6 ) as well as the planetary gear group G 4  (third differential unit  10 ), where the horizontal axis indicates the distance ratio between the rotary elements constituting the planetary gear groups G 1  and G 2  (one differential unit  5 ) and the planetary gear group G 3  (the other differential unit  6 ) as well as the planetary gear group G 4  (third differential unit  10 ). As described previously, the ring gear R 1  and the carrier C 2  are connected together, and the carrier C 1  and the ring gear R 2  are connected together, so that one differential unit  5  comprising the first planetary gear group G 1  and second planetary gear group G 2  is illustrated as one rod of combined levers indicated by the same numerical mark G 1  and G 2  in  FIG. 22 , and the order of the rotational speeds (whether it is in the ascending order or descending order depends on the speed change status) of the rotary elements constituting the same differential unit  5  is: sun gear S 1 , carrier C 1  (ring gear R 2 ), ring gear R 1  (carrier C 2 ), and sun gear S 2 , in that order. 
   The other differential unit  6  comprising the third planetary gear group G 3  is represented as a lever indicated by the same numerical mark G 3  in  FIG. 22 , as described, because the sun gear S 3  is connected to the ring gear R 1  (carrier C 2 ) and, at the same time, the carrier C 3  is fixed, and the order of the rotational speeds (whether it is in the ascending order or descending order depends on the speed change status) of the rotary elements constituting the same differential unit  6  is: sun gear S 3 , carrier C 3 , and ring gear R 3 , in that order. 
   In the alignment chart of  FIG. 22 , two rotary elements of one differential unit  5  (G 1  and G 2 ) located approximately in the middle in the direction of the order of the rotational speeds, or in other words, between the carrier Cl (ring gear R 2 ) and the ring gear R 1  (carrier C 2 ), to the former carrier C 1  (ring gear R 2 ) to which one output shaft Out 1  is connected through a third differential unit  10  comprising the fourth planetary gear group G 4 , and the later ring gear R 1  (carrier C 2 ) and the sun gear S 3  of the other differential unit  6  (G 3 ) are connected together. In the event of connecting one output shaft Out 1  to the carrier C 1  (ring gear R 2 ) through third differential unit  10  (fourth planetary gear group G 4 ), among the ring gear R 4  and the sun gear S 4  of the third differential unit  10  (fourth planetary gear group G 4 ) located at each end, in the direction of the order of the rotational speeds in the alignment chart, the former ring gear R 4  is fixed onto the casing  1 , the latter sun gear S 4  is connected to the carrier C 1  (ring gear R 2 ) of one differential unit  5  (G 1  and G 2 ), and one output shaft Out  1  is connected to the carrier C 4  of the third differential unit  10  (fourth planetary gear group G 4 ) located approximately in the middle, in the direction of the order of the rotational speeds in the alignment chart. 
   Two rotary elements of one differential unit  5  (G 1  and G 2 ) located at each end in the direction of the order of the rotational speeds in the alignment chart of  FIG. 22 , or in other words, between the sun gear S 1  and the sun gear S 2 , to the sun gear S 1  that is a rotary element close to the carrier C 1  (ring gear R 2 ) to which one output shaft Out 1  is connected, and to the sun gear S 2  that is a rotary element close to the ring gear R 1  (carrier C 2 ) to which the sun gear S 3  of the other differential unit  6  (G 3 ) is connected, and one motor/generator MG 1  and the other motor/generator MG 2  are connected respectively. 
   In the alignment chart of  FIG. 22 , the other output shaft Out 2  is connected to the ring gear R 3  of the other differential unit  6  (G 3 ) located on the far end from the sun gear S 3  of the other differential unit  6  (G 3 ) that is connected to the ring gear R 1  (carrier C 2 ) of one differential unit  5  (G 1  and G 2 ), and also in the same alignment chart of  FIG. 22 , the carrier R 3  interposed between both ends of the other differential unit  6  (G 3 ) is fixed onto the casing  1 . 
   As for the transmission unit shown in  FIG. 21 , by changing the rotational speed of the motor/generators MG 1  and MG 2  within a range indicated by the bold arrows in the alignment chart in  FIG. 22 , a mutual connection point for one differential unit  5  (G 1  and G 2 ) and the other differential unit  6  (G 3 ), a mutual connection point for one differential unit  5  (G 1  and G 2 ) and third differential unit  10  (G 4 ), a connection point (carrier C 4 ) for one output shaft Out 1 , as well as a connection point (ring gear R 3 ) for the other output shaft Out 2 , change their rotational speed within the range indicated by the bold arrows, and thus the rotation of the motor/generators MG 1  and MG 2  may be distributed and output to both output shafts Out 1  and Out 2 . 
   In the meanwhile, based on the vector indicated in  FIG. 23 , the same as in the alignment chart in  FIG. 22 , the manner by which torque from the motor/generators MG 1  and MG is transmitted to the output shafts Out 1  and Out 2  is explained below. As shown by the alignment charts in  FIG. 22  and  FIG. 23 , wherein L 1  is the distance between the sun gear S 1  to which one motor/generator MG 1  is connected, and a mutual connection point for one differential unit  5  (G 1  and G 2 ) and the third differential unit  10  (G 4 ), L 2 , is the distance between this mutual connection point and a mutual connection point for one differential unit  5  (G 1  and G 2 ) and the other differential unit  6  (G 3 ), L 3  is the distance between this mutual connection point and the sun gear S 2  to which the other motor/generator MG 2  is connected, and with regard to the other differential unit  6 , L 4  is the distance between the carrier C 3  and the sun gear S 3 , L 5  is the distance between the carrier C 3  and the ring gear R 3 , with regard to the third differential unit  10 , L 6  is the distance between the carrier C 4  and the sun gear S 4 , L 7  is the distance between the carrier C 4  and the ring gear R 4 ; given α=L 1 /L 2 , β=L 3 /L 2 , γ=L 4 /L 5 , ε=L 6 /L 7  when one motor/generator MG 1  outputs positive torque TMg 1  as shown in  FIG. 23 , positive torque (1+α)(1+ε) Tmg 1 is generated on one output shaft Out 1  and positive torque α·γ·Tmg 1  is generated on the other output shaft Out 2 , and when the other motor/generators MG 2  outputs negative torque −Tmg 2  as shown in  FIG. 23 , then positive torque β(1+ε)Tmg is generated on one output shaft Out 1 , and positive torque (1+β)γ·Tmg 2  is generated on the other output shaft Out 2  . Therefore, due to the Tmg 1  and −Tmg 2  torque of the motor/generators MG 1  and MG 2 , respectively, on one output shaft Out 1 , a positive torque (1+α)(1+ε)Tmg 1 +β(1+Ε)Tmg 2  larger than those of each embodiment shown in  FIG. 1  through  FIG. 20  is applied, and on the other output shaft Out 2 , the same positive torque α·γ·Tmg 1 +(1+β)γ·Tmg 2  as those in each embodiment shown in  FIG. 1  through  FIG. 20  is applied, and thus, same directional torque may be output from both output shafts Out 1  and Out 2 . 
     FIG. 24  is an explanatory figure showing the relational coefficients during the above power transmission between the torque Tmg 1 , as well as the number of revolutions Nmg 1  of the motor/generator MG 1  and the torque Tout 1  and Tout 2 , as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 , and also showing the transmission between the torque Tmg 2 , as well as the number of revolutions Nmg 2  of the motor/generator MG 2  and the torque Tout 1  and Tout 2 , as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 ; furthermore  FIG. 24  also states the torque Tout 1  and Tout 2  generated from the output shafts Out 1  and Out 2  by the motor/generator torque Tmg 1  and Tmg 2 , along with the number of revolutions Nmg 1  and Nmg 2  of the motor/generators MG 1  and MG 2  determined by the number of revolutions of the output shafts Nout 1  and Nout 2  at that moment. 
   Herein, when an equal value is given to the torque Tmg 1  and Tmg 2  of both motor/generators MG 1  and MG 2 , for the purpose of explaining the conditions whereby the torque Tout 1  and Tout  2  of both output shafts Out 1  and Out 2  become equal, it is necessary to establish (1+α)(1+ε)Tmg 1 +β(1+ε)Tmg 2 =α·γ·Tmg 1 +(1+β)γ·Tmg 2 ; however, currently Tmg 1 =Tmg 2 , so it is acknowledged that the distance ratio α, β, and γ, respectively, in said one differential unit  5  (G 1  and G 2 ) and in the other differential unit  6  (G 3 ) as well as in the third differential unit  10  (G 4 ) have to be determined so as to obtain, αγ−(1+α)(1+ε)≈β(1+ε)−(1+β)γ. 
   Then, as shown in  FIG. 24 , wherein Nmg 1  and Nmg 2  are the number of revolutions of the motor/generators MG 1  and MG 2 , and Nout 1  and Nout 2  are the number of revolutions of both output shafts Out 1  and Out 2 , yielding, Nout 1 ={1/(1+β)}·Nmg 2  and Nout 2 =(1/αγ)·{(1+ε)·Nmg 1 −(1+α+ε)·Nout 1 . Thus, the revolution Nout 1  of one output shaft Out 1  can only be set according to the number of revolutions Nmg 2  of the other motor/generator MG 2 , whereas the number of revolutions Nout 2  of the other output shaft Out 2  can be set not only according to the number of revolutions Nout 1  of one output shaft Out 1 , but also according to the number of revolutions Nmg 1  of one motor/generator MG 1 . Therefore, the number of revolutions Nout 1  and Nout 2  of both output shafts Out 1  and Out 2 , respectively, may be controlled independently. 
   Additionally, when the number of revolutions Nout 2  of the other output shaft Out 2  is set at zero, automatically the number of revolutions Ngm 2  of the motor/generator MG 2  also yields zero, and in this case, as judged from  FIG. 24 , Tout 2 ={αγ/(1+ε)}·Tmg 1 +(1+β)·Tmg 2  is obtained, and therefore, even if the output of the motor/generator MG 2  is zero, it is still capable of contributing to increasing the output of output shaft Out 2 , and moreover, Tout 1 ={(1+α+ε)/(1+ε)}·Tmg 1  is obtained, and therefore, the torque Tout 1  of the output shaft Out 1  becomes controllable only by the motor/generator MG 1 , and the output shaft torque Tout 2  can be adjusted by the motor/generator torque Tmg 2 , and thus the torque Tout 1  and Tout 2  of both output shafts Out 1  and Out 2  may be controlled arbitrarily. 
   As a result, the drive power distribution of both output shafts Out 1  and Out 2  may be changed arbitrarily from 0% to 100%, permitting active drive power distribution control of the front/rear wheels of four-wheel-drive vehicles to enhance road handling abilities, thus being extremely useful for enhancing the running stability of vehicles, due to the drive power distribution control of the left/right wheels. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ), as well as third differential unit  10  (planetary gear group G 4 ) are coaxially located in parallel as shown in  FIG. 21 , and the connections among the rotary elements of these differential units, and the connections for both motor/generators MG 1  and MG 2 , as well as for both output shafts Out 1  and Out 2  to these rotary elements, are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 ,  10 , so that both motor/generators MG 1  and MG 2  are able to be coaxially placed to differential units  5 ,  6 ,  10  that are coaxially located in parallel, and thus the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2  may be avoided. 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together so as to be coaxially located, and axially on one end, one differential unit  5  (planetary gear group G 1  and G 2 ) and the third differential unit  10  (planetary gear group G 4 ) are coaxially located, at the same time, from the one end, one output shaft Out 1  is extended coaxially, and further axially on the other end of the motor/generators MG 1  and MG 2  placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other end, the other output shaft Out 2  is coaxially extended; thus, the effect of the action related to said radial downsizing becomes more significant. 
   As shown in  FIG. 24 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is reduced by both differential units  5  and  6 ,  10 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 ,  10 , so that even if a high-power motor that is rotatable at high speed is required, the rotation from the same motors is output, assuring a reduction in the required number of revolutions, and thus, also applicable to a system in which such high-power motor is used, without having any problems or restrictions in use. 
   Moreover, in the present embodiment, in the event of connecting one output shaft Out 1  to the rotary elements (carrier C 1  and ring gear R 2 ) of one differential unit  5 , without connecting them directly because the connection was made through the third differential unit  10  (fourth planetary gear group G 4 ) being a gear reduction mechanism, as previously stated, the torque Tout 1  to one output shaft Out 1  may be larger that those in each embodiment shown in  FIG. 1 through 20 . 
   In the embodiment shown in  FIG. 25  and  FIG. 26 , as represented in the outline drawing in  FIG. 25 , the outside motor/generator is regarded as one motor/generator MG 1 , and the inside motor/generator is regarded as the other motor/generator MG 2 , and one differential unit  5  is comprised by correlating a first and a second single-pinion planetary gear group G 1  and G 2  as in  FIG. 21 , and in addition, the other differential unit  6  is comprised of a third single-pinion planetary gear group G 3  as in  FIG. 21 . However, in the event of connecting one differential unit  5  to one output shaft Out 1  through the third differential unit  10  comprising a fourth planetary gear group G 4 , the same connection is made as described below. 
   That is, the ring gear R 1  of the first planetary gear group G 1  is connected to the ring gear R 4  of a fourth planetary gear group G 4 , the sun gear S 4  of the fourth planetary gear group G 4  is fixed onto the casing  1 , and by connecting one output shaft Out 1  to the carrier C 4  of the fourth planetary gear group G 4 , the connection for the output shaft Out 1  is made to the ring gear R 1  (carrier C 2 ) of one differential unit  5  through the third differential unit  10  (fourth planetary gear group G 4 ). 
   A transmission of the present embodiment having such a structure is found in  FIG. 26  represented by an alignment chart, and although the rotary members to be assigned differ slightly from the case in  FIG. 22 , the chart in  FIG. 26  is similar to the alignment chart of  FIG. 22 , and therefore, similar to the embodiments in  FIG. 21  through  FIG. 24 , the number of revolutions Nout 1  and Nout 2  of both output shafts Out 1  and Out 2 , respectively, may be controlled independently, and at the same time, the torque Tout 1  and Tout 2  of both output shafts Out 1  and Out 2  are arbitrarily controllable, and the drive power distribution of both output shafts Out 1  and Out 2  may be changed arbitrarily from 0% to 100%, permitting an active drive power distribution control of the front/rear wheels of four-wheel-drive vehicles to enhance road handling abilities, thus being extremely useful for enhancing the running stability of vehicles, due to the drive power distribution control of the left/right wheels. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) as well as third differential unit  10  (planetary gear group G 4 ) are coaxially located in parallel as shown in  FIG. 25 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2 , as well as both output shafts Out 1  and Out 2 , to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 ,  10 , so that both motor/generators MG 1  and MG 2  are able to be coaxially placed to differential units  5 ,  6 ,  10  that are coaxially located in parallel, and thus the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2  may be avoided. 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together so as to be coaxially located, and axially on one end, one differential unit  5  (planetary gear group G 1  and G 2 ) and the third differential unit  10  (planetary gear group G 4 ) are coaxially located, and at the same time, from said one end, one output shaft Out 1  is extended coaxially, and further axially on the other end of the motor/generators MG 1  and MG 2  placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other end, the other output shaft Out 2  is coaxially extended, and thus, the effect of the action related to said radial downsizing becomes more significant. 
   Because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is reduced by both differential units  5  and  6 ,  10 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 ,  10 , so that even if a high-power motor that is rotatable at high speed is required, the rotation from the motors is output, assuring a reduction in the required number of revolutions, and thus it is also applicable to a system in which such high-power motor is used, without having any problems or restrictions in use. 
   Moreover, in the present embodiment, in the event of connecting one output shaft Out 1  to the rotary elements (ring gear R 1  and carrier C 2 ) of one differential unit  5 , without connecting them directly, because the connection was made through the third differential unit  10  (fourth planetary gear group G 4 ) that is a gear reduction mechanism, the torque Tout 1  to one output shaft Out 1  may be larger than those in each embodiment shown in  FIG. 1 through 20 . 
   In the embodiment shown in  FIG. 27  and  FIG. 28 , as represented in the outline drawing of  FIG. 27 , one differential unit  5  is comprised of a first planetary gear group G 1  of the single-pinion planetary gear group and a second planetary gear group G 2  of the single-pinion planetary gear group, the other differential unit  6  is comprised of a third planetary gear group G 3  of the single-pinion planetary gear group, and the third differential unit  10  is comprised of a fourth planetary gear group of the single-pinion planetary gear group. In order to construct one differential unit  5  comprising the first planetary gear group G 1  of the single-pinion planetary gear group and second planetary gear group G 2  of the single-pinion planetary gear group, the pinion P 1  of the first planetary gear group G 1  is also to be engaged to the sun gear S 2  of the second planetary gear group G 2 , and the pinions P 1  and P 2  of the first planetary gear group G 1  and second planetary gear group G 2 , respectively, are rotatably supported by the common carrier C 1  (C 2 ). 
   Then, instead of directly connecting to one output shaft Out 1 , the common carrier C 1  (C 2 ) is connected to the output shaft Out 1 , as described below, through the third differential unit  10  comprising the fourth planetary gear group G 4 . That is, the common carrier C 1  (C 2 ) is connected to the sun gear S 4  of a fourth planetary gear group G 4 , and the ring gear R 4  of a fourth planetary gear group G 4  is fixed onto the casing  1 , and one output shaft Out 1  is connected to the carrier C 4  of a fourth planetary gear group G 4 . The ring gear R 2  of the second planetary gear group G 2  is connected to the sun gear S 3  of the third planetary gear group G 3  by the center shaft  8 , and the other motor/generator MG 2  is connected to the sun gear S 2  of the second planetary gear group G 2  through the hollow shaft  9  (in the present embodiment, the outside motor/generator is regarded as MG 2 , and the inside motor/generator is regarded as MG 1 ), and one motor/generator MG 1  is connected to the sun gear S 1  of the first planetary gear group G 1  through the hollow shaft  7 . The carrier C 3  of the third planetary gear group G 3  is fixed and the ring gear R 3  of the third planetary gear group G 3  is connected to the other output shaft Out 2 . 
   The transmission of the present embodiment with such a structure is found in  FIG. 28  represented by an alignment chart, although the rotary members to be assigned differ slightly from the case in  FIG. 22 , the chart in  FIG. 28  is similar to the alignment chart of  FIG. 22 , and therefore, similar to the embodiments in  FIG. 21  through  FIG. 28 , the number of revolutions Nout 1  and Nout 2  of both output shafts Out 1  and Out 2 , respectively, may be controlled independently, and at the same time, the torque Tout 1  and Tout 2  of both output shafts Out 1  and Out 2  are arbitrarily controllable, and the drive power distribution of both output shafts Out 1  and Out 2  may be changed arbitrarily from 0% to 100%, permitting an active drive power distribution control of the front/rear wheels of four-wheel-drive vehicles to enhance the road abilities, thus being extremely useful for enhancing the running stability of vehicles, due to the drive power distribution control of the left/right wheels. 
   According to the transmission of the present invention, one differential unit  5  (planetary gear group G 1  and G 2 ) and the other differential unit  6  (planetary gear group G 3 ) as well as third differential unit  10  (planetary gear group G 4 ) are coaxially located in parallel as shown in  FIG. 27 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2 , as well as both output shafts Out 1  and Out 2 , to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 ,  10 , and so that both motor/generators MG 1  and MG 2  are able to be coaxially placed to differential units  5 ,  6 ,  10  that are coaxially located in parallel, and thus the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2  may be avoided. 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together so as to be coaxially located, axially on one end, one differential unit  5  (planetary gear group G 1  and G 2 ) and third differential unit  10  (planetary gear group G 4 ) are coaxially located, and at the same time, from one end, one output shaft Out 1  is extended coaxially, and further axially on the other end of the motor/generators MG 1  and MG 2  placed together, the other differential unit  6  (planetary gear group G 3 ) is coaxially located, and at the same time, from the other end, the other output shaft Out 2  is coaxially extended, and thus, the effect of the action related to said radial downsizing becomes more significant. 
   Because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is reduced by both differential units  5  and  6 ,  10 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 ,  10 , so that even when a high-power motor that is rotatable at high speed is required, the rotation from the same motors is output, assuring a reduction in the required number of revolutions, and thus, it is also applicable to a system in which such high-power motor is used, without having any problems or restrictions in use. Moreover, in the present embodiment, in the event of connecting one output shaft Out 1  to the rotary elements (carrier C 1  and carrier C 2 ) of one differential unit  5 , without connecting them directly, because the connection was made through third differential unit  10  (fourth planetary gear group G 4 ) that is a gear reduction mechanism, the torque Tout 1  to one output shaft Out 1  may be larger than those in each embodiment shown in  FIG. 1 through 20 . 
     FIG. 29  through  FIG. 32  further show a transmission according to the other embodiment of the present invention,  FIG. 29  is the outline drawing,  FIG. 30  and  FIG. 31  are alignment charts, and  FIG. 32  is an explanatory figure for the correlation coefficients between the inputs and outputs. To explain the structure based on the outline drawing of  FIG. 29 , axially on the right side (left/right in the drawing) within the casing  1 , two (i.e. a second and a fourth) planetary gear groups, G 2  andG 4 , are coaxially accommodated in parallel, axially on the left side (left/right in the drawing) within the casing  1 , and a first planetary gear group G 1  and a third planetary gear group G 3  are accommodated so as to coaxially locate all of these planetary gear groups G 1  and G 2 ,G 3 ,G 4 . 
   The second and fourth planetary gear groups G 2  and G 4  are located so as to position the second planetary gear group G 2  on the left side in the figure, the first and third planetary gear groups G 1  and G 3  are located so as to position the first planetary gear group G 1  on the right side in the figure, and between the second and third planetary gear group G 2  and G 3 , one motor/generator MG 1  and the other motor/generator MG 2  are interposed coaxially. The motor/generators MG 1  and MG 2  have a common stator  3   s  fixed onto the casing  1 , and serve as a composite current dual-layer motor equipped with a rotor  3   ro  of one motor/generator MG 1  at the outer circumference and a rotor  3   ri  of the other motor/generator MG 2  at the inner circumference. Instead of comprising the motor/generators MG 1  and MG 2  as one unit to serve as a composite current dual-layer motor, it may also be comprised as an independent unit having an individual stator. However, in any case, the motor/generators MG 1  and MG 2  are to be coaxially located together, and at the same time, located coaxially with the planetary gear groups G 1  and G 2 , G 3 , G 4 . 
   The second planetary gear group G 2  and the fourth planetary gear group G 4  construct one differential unit  5  in the present invention, the first planetary gear group G 1  and the third planetary gear group G 3  construct the other differential unit  6  in the present invention, and these first planetary gear group G 1 , second planetary gear group G 2 , third planetary gear group G 3 , and fourth planetary gear group G 4 , respectively, are regarded as a single-pinion planetary gear group comprising a sun gear S 1 , S 2 , S 3 , S 4 , a ring gear R 1 , R 2 , R 3 , R 4 , a pinion P 1 , P 2 , P 3 , P 4  and a carrier C 1 , C 2 , C 3 , C 4  rotatably supporting their pinions. Herein, to construct one differential unit  5  comprising the second planetary gear group G 2  and the fourth planetary gear group G 4 , pinions P 2  and P 4  of these planetary gear groups G 2  and G 4  are to be rotatably supported by the common carrier C 2  (C 4 ), and in order to construct the other differential unit  6  comprising the first planetary gear group G 1  and the third planetary gear group G 3 , the pinions P 1  and P 3  of these planetary gear groups G 1  an dG 3  are to be rotatably supported by the common carrier C 1 (C 3 ). 
   Then, the ring gear R 2  of the second planetary gear group G 2  is connected to one output shaft Out 1 , and the common carrier C 1  (C 3 ) of the first planetary gear group G 1  and the third planetary gear group G 3  is connected to the other output shaft Out 2 . In addition, one motor/generator MG 1  is connected to the sun gear S 2  of the second planetary gear group G 2  through a hollow shaft  11 , and the other motor/generator MG 2  is connected to the sun gear S 1  of the first planetary gear group G 1  through a hollow shaft  12 . A center shaft  13  is inserted into the hollow shafts  11  and  12  and, the sun gear S 4  of the fourth planetary gear group G 2  and the sun gear S 3  of the third planetary gear group G 3  are connected together through the center shaft  13 . The ring gear R 1  of the first planetary gear group G 1  is fixed onto the casing  1 . 
   One output shaft Out 1  protruding coaxially and rotatably from the end (right end in  FIG. 1 ) of the casing  1 , where the second planetary gear group and the fourth planetary gear groups G 2  and G 4  are located, is to be connected to a differential gear unit for rear left/right wheels or to a differential gear unit for front left/right wheels, not illustrated herein. The other output shaft Out 2  protruding coaxially and rotatably from the end (left end in  FIG. 1 ) of the casing  1  opposite to the protruding side of the above one output shaft Out 1  is connected, for example, to a differential gear unit for rear left/right wheels or to a differential gear unit for front left/right wheels, not illustrated herein. 
   The above transmission unit comprised as shown in  FIG. 29  can be represented in an alignment chart, as in  FIG. 30 , where the vertical axis in the figure indicates the rotational speeds ( 0  is the reference, the upward direction in the figure is the forwarding rotational speed and the downward direction is the reversing rotational speed) of the rotary elements constituting the planetary gear groups G 2  and G 4  (one differential unit  5 ) and the planetary gear groups G 1  and G 3  (the other differential unit  6 ), while the horizontal axis indicates the distance ratio among the rotary elements constituting the planetary gear groups G 2  and G 4  (one differential unit  5 ) and the planetary gear groups G 1  and G 3  (the other differential unit  6 ). As previously described, the carriers C 2  and C 4  are mutually integrated and the ring gear R 2  is connected to one output shaft Out 1 , so that one differential unit  5  comprising the second planetary gear group G 2  and the fourth planetary gear group G 4  is illustrated as one rod of combined levers indicated by the same numerical mark as G 2  and G 4  in  FIG. 30 , and the order of the rotational speeds (whether it is in the ascending order or descending order depends on the speed change status) of the rotary elements constituting the same differential unit  5  is: sun gear S 2 , ring gear R 4  (a hypothetical ring gear of a fourth planetary gear group G 4  that does not exist in reality), common carrier C 2 (C 4 ), ring gear R 2 , and sun gear S 4 , in that order. 
   With regard to the other differential unit  6  comprising the first planetary gear group G 1  and the third planetary gear group G 3 , as previously described, the carriers C 1  and C 3  are mutually integrated and connected to the other output shaft Out 2  while the ring gear R 1  is fixed, and thus, illustrated as one rod of combined levers indicated by the same numerical mark as G 1  and G 3  in  FIG. 30 , and the order of the rotational speeds (whether it is in the ascending order or descending order depends on the speed change status) of the rotary elements constituting the said differential unit  6  is: sun gear S 3 , ring gear R 1  (a hypothetical ring gear of the first planetary gear group G 1  that does not exist in reality), common carrier C 1 (C 3 ), ring gear R 3 , and sun gear S 1 , in that order. 
   In the alignment chart in  FIG. 30 , among the sun gear S 2  and the sun gear S 4  of one differential unit  5  (G 2 ,G 4 ) located at each end, in the direction of the order of the rotational speeds, the former sun gear S 2  is connected to one motor/generator MG 1 , the latter sun gear S 4  is connected to the sun gear S 3  of the other differential unit  6  (G 1 ,G 3 ), and one output shaft Out 1  is connected to the ring gear R 2  between the common carrier C 2 (C 4 ) of one differential unit  5  (G 2 , G 4 ) and the mutual connection point of both differential units  5  and  6 . Moreover, in the same alignment chart in  FIG. 30 , among the sun gear S 1  and the sun gear S 4  of the other differential unit  6  (G 1 , G 3 ) located at each end, in the direction of the order of the rotational speeds, the former sun gear S 1  is connected to the other motor/generator MG 2 , and the latter sun gear S 4  is connected to the sun gear S 4  of one differential unit  5  (G 2 , G 4 ). Then, the other output shaft Out 2  is connected to the common carrier C 1  (C 3 ) of the other differential unit  6  (G 1 ,G 3 ), and the ring gear R 1  located between the common carrier C 1 (C 3 ), and the mutual coupling point of both differential units  5 and  6  in the alignment chart is fixed. 
   As for the transmission unit shown in  FIG. 29 , by changing the rotational speed of the motor/generators MG 1  and MG 2  within a range indicated by the bold arrows in the alignment chart in  FIG. 30 , a mutual connection point (sun gear S 3 ,S 4 ) of one differential unit  5  (G 2 ,G 4 ) and the other differential unit  6  (G 1 ,G 3 ), a connection point (ring gear R 2 ) of one output shaft Out 1  and a common carrier (C 1 ,C 3 ) of the other output shaft Out 2 , respectively, change their rotational speed within the range indicated by the bold arrows, and thus the rotation of the motor/generators MG 1  and MG 2  may be distributed and output for both output shafts Out 1  and Out 2 . 
   Based on the vector indicated in  FIG. 31  that is the same as in the alignment chart in  FIG. 30 , the manner by which torque from the motor/generators MG 1  and MG is transmitted to the output shafts Out 1  and Out 2  is explained below. As shown in the alignment charts in  FIG. 30  and  FIG. 31 , wherein L 1  is the distance between the sun gear S 2  to which one motor/generator MG 1  of one differential unit  5  (G 2 ,G 4 ) is connected and the common carrier C 2 (C 4 ); L 2  is the distance between the common carrier C 2 (C 4 ) and the ring gear R 2  to which one output shaft Out 1  is connected; L 3  is the distance between the ring gear R 2  to which the same one output shaft Out 1  is connected and a mutual connection point of both differential units  5  and  6  (sun gears S 3 ,S 4 ), and with regard to the other differential unit  6  (G 1 , G 3 ), L 4  is the distance between the fixed ring gear R 1  and the mutual connection point (sun gear S 3 ,S 4 ) of both differential units  5  and  6 ; L 5  is the distance between the same fixed ring gear R 1  and the common carrier C 1 (C 3 ) to which the other output shaft Out 2  is connected; L 6  is the distance between the common carrier C 1 (C 3 ) to which the same the other output shaft Out 2  is connected and the sun gear S 1  to which the other motor/generator MG 2  is connected; given α=L 1 /L 2 , β=L 6 /L 5 , γ=L 5 /L 4 , ε=L 3 /L 2  when one motor/generator MG 1  outputs positive torque Tmg 1  as shown in  FIG. 31 , then positive torque (1+α+ε)Tmg 1  is generated on one output shaft Out 1 , and on the other output shaft Out 2 , positive torque α·γ(1+ε)Tmg 1  is generated, and when the other motor/generator MG 2  outputs positive torque Tmg 2  as shown in  FIG. 31 , then positive torque (1+β)Tmg 2  is generated on the other output shafts Out 2 , while the torque to one output shaft Out 1  becomes 0·Tmg 2 =0, hence the torque is not transmitted. 
   Therefore, due to the torque Tmg 1  and Tmg 2  of the motor/generators MG 1  and MG 2  respectively, positive torque (1+α+ε)/(1+ε)}Tmg 1 +0·Tmg 2  is applied on one output shaft Out 1 , and positive torque {αγ/(1+ε)}Tmg 1 +(1+β)Tmg 2  is applied on the other output shaft Out 2 , and thus, same directional torque may be output from both output shafts Out 1  and Out 2 . 
     FIG. 32  is an explanatory figure showing the relational coefficients during the above power transmission between the torque Tmg 1 , as well as the number of revolutions Nmg 1  of the motor/generator MG 1  and the torque Tout 1  and Tout 2 , as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 , and the transmission between the torque Tmg 2 , as well as the number of revolutions Nmg 2  of the motor/generator MG 2 , and the torque Tout 1  and Tout 2 , as well as the number of revolutions Nout 1  and Nout 2  of the output shafts Out 1  and Out 2 ;  FIG. 32  also states the torque Tout 1  and Tout 2  generated from the output shafts Out 1  and Out 2  by the motor/generator torque Tmg 1  and Tmg 2 , along with the number of revolutions Nmg 1  and Nmg 2  of the motor/generators MG 1  and MG 2  determined by the number of revolutions of the output shafts Nout 1  and Nout 2  at that moment. 
   Herein, when an equal value is given to the torque Tmg 1  and Tmg 2  of both motor/generators MG 1  and MG 2 , for the purpose of explaining the conditions whereby the torque Tout 1  and Tout  2  of both output shafts Out 1  and Out 2  become equal, it is necessary to establish {(1+α+ε)/(1+ε)}Tmg 1 +0·Tmg 2 ={αγ(1+ε)}Tmg 1 +(1+β)Tmg 2 ; however, currently Tmg 1 =Tmg 2 , so it is acknowledged that the distance ratio α, β, γ, ε, respectively, in said one differential unit  5  (G 2 ,G 4 ) and in the other differential unit  6  (G 1 ,G 3 ) have to be determined so as to obtain αγ/(1+ε)−(1+α+ε)/(1+ε)≈(1+β) 
   And by determining the output of the motor/generators MG 1  and MG 2  so as to establish a relation of Tmg 1 ·Nmg 1 +Tmg 2 ·Nmg 2 +Tout 1 ·Nout 1 +Tout 2 ·Nout 2 =0 between the torque Tmg 1  and Tmg 2  as well as the number of revolutions Nmg 1  and Nmg 2  of the motor/generators MG 1  and MG 2 , and the torque Tout 1  and Tout 2  as well as the number of revolutions Nout 1  and Nout 2  of both output shafts Out 1  and Out 2 , the drive power distribution to both output shafts Out 1  and Out 2  become arbitrarily controllable, and thus may be used for a drive power distribution control of the front/rear wheels. 
   As for the drive power distribution control, as seen in  FIG. 32 , by setting one of the number of revolutions of both of output shafts Out 1  and Out 2  at zero, the output from the same output shaft may also becomes zero, and then it becomes possible to output the total output of both motor/generators MG 1  and MG 2  to the other output shaft. Therefore, by determining the output of the motor/generators MG 1  and MG 2  so as to establish a relation of Tmg 1 ·Nmg 1 +Tmg 2 ·Nmg 2 =Tout 1 ·Nout 1 +Tout 2 ·Nout 2 , the drive power distribution of the output shafts Out 1  and Out 2  may be changed arbitrarily from 0% to 100%, permitting an active drive power distribution control of the front/rear wheels of four-wheel-drive vehicles to enhance the road abilities, and thus being extremely useful for enhancing the running stability of vehicles, due to the drive power distribution control of the left/right wheels. 
   Moreover, according to the transmission of the present invention, one differential unit  5  (planetary gear group G 2 , G 4 ) and the other differential unit  6  (planetary gear group G 1 ,G 3 ) are coaxially located in parallel as shown in  FIG. 29 , and the connections among the rotary elements of these differential units, and the connections of both motor/generators MG 1  and MG 2 , as well as both output shafts Out 1  and Out 2 , to these rotary elements are the connections described above, so that the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  under deceleration by both differential units  5  and  6 , and so that both motor/generators MG 1  and MG 2  are able to be coaxially placed to both differential units  5  and  6  that are coaxially located in parallel, and thus the problem of radially enlarging a transmission including both motor/generators MG 1  and MG 2  may be avoided. 
   By adopting a structure wherein motor/generators MG 1  and MG 2  are placed together so as to be coaxially-located as in the present embodiment, axially on one end, one differential unit  5  (planetary gear group G 2 , G 4 ) is coaxially located, and at the same time, from one end, one output shaft Out 1  is coaxially extended, and further axially on the other end of the motor/generators MG 1  and MG 2  placed together, the other differential unit  6  (planetary gear group G 1 ,G 3 ) is coaxially located, and at the same time, from the other end, the other output shaft Out 2  is coaxially extended, and thus, the effect of the action related to said radial downsizing radially more significant. 
   As seen from said explanation of the action and from  FIG. 32 , because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is reduced by both differential units  5  and  6 , a large reduction ratio may be set according to the combination of both differential units  5  and  6 , so that even when a high-power motor that is rotatable at high speed is required, the rotation from the same motors is output, assuring a reduction in the required number of revolutions, and thus it is also applicable to a system in which such high-power motor is used, without having any problems or restrictions in use. 
   Finally, as previously described, because the rotation of both motor/generators MG 1  and MG 2  is distributed and output to both output shafts Out 1  and Out 2  while the rotation is reduced by both differential units  5  and  6 , the torque distribution to both output shafts Out 1  and Out 2  becomes freely controllable according to the output combination of both motor/generators MG 1  and MG 2 ; therefore, without having any restrictions in use, it is also applicable to cases in which the torque distribution to a dual output system needs to be freely controlled. 
   Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.