Abstract:
An electric drive system includes a motor output shaft rotating on a motor axis and a first electric motor. The system includes an epicyclical gear that includes a sun gear, a ring gear, a plurality of planet gears and a carrier. The sun gear, the ring gear and the carrier gear of the epicyclical gear all rotate on the motor axis, and the carrier gear is connected to the motor output shaft via a first flange. The system also includes a second electric motor interposed between the first electric motor and the epicyclical gear. The second motor shaft has a hollow center along the motor axis and the first motor shaft extends through the hollow center of the second motor shaft and is connected to the sun gear. The system also includes a second flange. The second flange connects the second motor shaft to the carrier. The first flange and the second flange are located at opposite sides of the epicyclical gear.

Description:
FIELD 
     The present invention relates to electric propulsion system, and more particularly to the electric propulsion system with high efficiency for propulsion of electric vehicles. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Electric vehicle (EV) relies on an electric motor, or multiple electric motors, for its propulsion. One of the key issues with respect to electric vehicle operation is achieving high system efficiency. Optimization of the EV system efficiency under all operating conditions has been among the top, if not the very top, endeavor of all engineering efforts devoted to the development of electric vehicle technologies. Inevitably, improvement of the electric motor efficiency is also a focus point of EV technology development. 
     Electric motors may be designed in various forms; however, each of the motors possesses operating points with higher efficiency and those with lower efficiency as well. When an electric motor is used for operation of various machine tools and equipment with fairly constant speed and torque, the system can be designed so the motor is operated at its highest efficiency for most of the time. However, when electric motors are used for electric vehicle propulsion, the situation is quite different from machine tools. 
     An electric vehicle needs to operate in a wide range of torque and speed to meet all performance requirements. For example, an electric vehicle needs to cruise in constant speed on leveled ground of city roads, in which the speed may range from low speed to high speed. The electric vehicle may also need to climb up slopes of various ramps and bridges, or even roads over to a hill. The electric vehicle may need to provide certain level of acceleration so it would not interrupt the normal traffic flow. The electric vehicle may even need to accommodate a fairly wide range of load variation, for example, in delivering vehicles or in buses. Therefore, it can be appreciated that electric motor operation in electric vehicles is quite different from that in machine tools. 
     While the electric motor operation needs to cover a wide range of torque and speed in electric vehicle propulsion and there is no way to fix the operation on the motor operating point with the highest motor efficiency, the motor design for electric vehicles faces a dilemma of providing the electric vehicle with desired high performance or providing the electric vehicle with an overall higher efficiency. When the electric motor needs to provide high performance for the electric vehicle, higher power and higher toque are needed to provide vehicle operation of high acceleration, high capability of slope climbing with desired, that is, higher, speeds while carrying higher loads. In this case, the motor may be operated in the high-efficiency point with these high-demand operations. However, when the electric motor is driven on leveled ground of city roads for most of the time, the efficiency becomes relatively low. 
     On the other hand, if the electric motor operation is to provide a higher overall efficiency over the electric vehicle operation, the motor needs to be designed with limitation where lower power and lower torque are provided. In this case, during the most operation of the electric vehicle over the leveled ground of city roads, the efficiency becomes higher. Yet, in such system, the electric vehicle can only have a lower level of acceleration, can only climb up slopes with a relatively low speed, and can only has a lower capability of starting over an inclined road of significant slope. 
     SUMMARY 
     Advantageously, an electric motor drive for electric vehicle operation is devised according to the principle of this invention where the electric vehicle may operate with higher efficiency in a lower-power and lower-torque region such as in city-road operation. On the other hand, the electric vehicle may also operate with high efficiency in a higher-power and higher-torque region such as in hard acceleration, hill climbing with decent speeds while carrying heavier loads. 
     In one feature, an electric propulsion system for a vehicle is described. The system includes a motor output shaft rotating on a motor axis and a first electric motor which includes a first motor shaft rotating on the motor axis. The first electric motor includes a first rotor which is axially connected with the first motor shaft. The first electric motor also includes a first armature which is located radially outward of the first rotor, and the first armature receives a first electric current and interacts electromagnetically with the first rotor to generate a first motor torque. The system includes an epicyclical gear with a sun gear, a ring gear, a plurality of planet gears interposed between the sun gear and the ring gear, a carrier gear of the planet gears. The sun gear, the ring gear and the carrier gear of the epicyclical gear all rotate on the motor axis, and the carrier gear is connected to the motor output shaft via a first flange. The system also includes a second electric motor interposed between the first electric motor and the epicyclical gear. The second electric motor includes a second motor shaft rotating on the motor axis, a second rotor axially connected with the second motor shaft, and a second armature which is located radially outward of the second rotor. The second armature receives a second electric current and interacts electromagnetically with the second rotor to generate a second motor torque. The second motor shaft has a hollow center along the motor axis and the first motor shaft extends through the hollow center of the second motor shaft and is connected to the sun gear. The system also includes a second flange. The second flange connects the second motor shaft to the carrier gear. The first flange and the second flange are located at opposite sides of the epicyclical gear. 
     In other features, a method of operating the electric propulsion system is described. The method includes steps of computing a system power demand based on a system torque signal and a system speed signal. The method also compares the system torque signal and the second torque rating of the second electric motor, generates the second electric current for the second electric motor based on the system torque signal and generating a zero first electric current for the first electric motor when the system torque signal is less than or equal to the second torque rating and the system power demand is less than or equal to the second power rating; and generates the first electric current for the first electric motor and generating the second electric current for the second electric motor when the system torque signal is greater than the second torque rating or the system power demand is greater than the second power rating. Under this situation, the method determines a first initial torque command for the first electric motor based on the system torque signal and the second torque rating, determines a second initial torque command for the second electric motor based on the second torque rating, sets a first initial speed command for the first electric motor equal to the system speed signal, sets a second initial speed command for the second electric motor equal to the system speed signal, determines a first update torque command for the first electric motor, wherein the first update torque command is different than the first initial torque, determines a second update torque command for the second electric motor, wherein the second update torque command is different than the second initial torque, and wherein the sum of the first update torque command and the second torque update command equals to the system torque signal, determines a first update speed command (N1) for the first electric motor, wherein the first update speed command is different than the first initial speed command, and determines a second update speed command (N2) for the second electric motor, wherein the second update speed command is different than the second initial speed command, and wherein the first update speed command, the second update speed command and the system speed signal (N) are related with each other according to a mechanical constraint imposed by the gear ratio relationship among the sun gear, ring gear and the carrier. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a plan view of an electric motor drive according to the principle of the present invention; 
         FIG. 2  is a graphical illustration of an example mode of operation of the electric motor drive according to the principle of the present invention; 
         FIG. 3  is graphical illustration of another example mode of operation of the electric motor drive according to the principle of the present invention; 
         FIG. 4  is graphical illustration of yet another example mode of operation of the electric motor drive according to the principle of the present invention; 
         FIG. 5  is a plan view of an electric propulsion system according to the principle of the present invention; 
         FIG. 6  is a flow diagram illustrating a method for operating the electric propulsion system according to the principle of the present invention; and 
         FIG. 7  is a plan view of another electric motor drive according to the principle of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     As used herein, the term module refers to an Application Specific Integrated Circuit, an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable electrical or electronic components or devices that provide the described functionality. 
     Referring now to  FIG. 1 , a plan view of an electric motor drive  10  is shown. The electric motor drive  10  may include a motor set  100 . The motor set  100  may include a primary motor  102 , a secondary motor  104  and a primary gear set  106 . The primary gear set  106  may be an epicyclical gear, or more specifically a commonly used planetary gear. The primary gear set  106  may include a ring gear  110 , a plurality of planet gears  112  meshing with the ring gear  110 , and a carrier  114  for the planet gears  112  connected with the planet gears  112  at respective hubs of the planet gears  112 . The carrier  114  may be mechanically connected to a drive shaft  116  of the motor set  100  via an output flange  118 . The drive shaft  116  may rotate on an axis AA′. The primary gear set  106  may have a sun gear  120  meshing with the plant gears  112 . 
     The primary motor  102  may have a rotor  130 . The rotor  130  may rotate on the axis AA′. The primary motor  102  may have an armature  132  that receives electric current to generate magnetic field to interact with a magnetic field generated in the rotor  130 . The rotor  130  may be connected to an output shaft  134  that also rotates on the axis AA′. The output shaft  134  of the primary motor  102  may be mechanically connected to the ring gear  110  via a primary flange  136 . The output shaft  134  may have a hollow center  138 . 
     The primary motor  102  may include an internal gear  142  interposed between and connected to the rotor  130  and the output shaft  134 . In one embodiment, the internal gear  142  may cause the speed of the rotor  130  to be faster than the speed of the output shaft  134 . In another embodiment, the internal gear  142  may cause the speed of the rotor  130  to be slower than the speed of the output shaft  134 . 
     The secondary motor  104  may include a rotor  150 . The rotor  150  may rotate on the axis AA′. The secondary motor  104  may have an armature  152  that receives electric current to generate magnetic field to interact with a magnetic field generated in the rotor  150 . The rotor  150  may be connected to an output shaft  154  that also rotates on the axis AA′. The output shaft  154  of the secondary motor  104  may be mechanically connected to the sun gear  120  through the hollow center  108  of the output shaft  134  of the primary motor  102 . 
     The motor set  100  may include a clutch  162  interposed between the output shaft  154  of the secondary motor  104  and a stationary part  164  of the motor set  100 . In one mode of operation the clutch  162  may be engaged so the output shaft  154  of the secondary motor  104  may be connected to the stationary part  164  causing the output shaft  154  to stop rotating. In another mode of operation the clutch  162  may be disengaged so the output shaft  154  of the secondary motor  104  may be free to rotate. In one embodiment, the clutch  162  may be a mechanically operated one-way clutch so it is disengaged when the output shaft  154  rotates in a certain angular direction; and the one-way clutch is engaged when the output shaft  154  is driven to the opposite angular direction. In another embodiment, the clutch  162  may be an electronically controlled clutch, of which the state of engagement and disengagement is determined by an electronic controller (not shown). 
     Referring now also to  FIG. 2 , a graphical illustration of an example mode of operation of the electric motor drive  10  is shown. For illustrative purpose only, the primary gear set  106  is assumed to be a planetary gear set having a gear ratio of 4:1 between its sun gear and ring gear. Also for illustrative purpose only, a reference motor (not shown) with rated power at 20 kW and peak power at 35 kW is assumed to meet certain desired performance specifications of an electric vehicle application. A motor efficiency map  210  of the reference motor is shown. On the efficiency map  210  an example operating point  212  is shown where the reference motor delivers a torque of 25 Nm and rotates at a speed of about 4100 rpm. The motor efficiency in such operation is little over 86% as indicated in the efficiency map  210 . 
     Instead of using the reference motor with the efficiency map  210 , an example motor set  100  is used. This example motor set  100  has a primary motor  102  smaller than the reference motor, and also has a secondary motor  104  smaller than the reference motor. For illustrative purpose only, the primary motor  102  and the secondary motor  104  are both designed and constructed identically as half-scale of the reference motor. In this example, the primary motor  102  does not have an internal gear between its rotor  130  and output shaft  134 . A motor efficiency map  220  describes the characteristics of the primary motor  102 , and a motor efficiency map  230  describes the characteristics of the secondary motor  104 . Those skilled in the art of electric machines may appreciate the difference between the efficiency maps  220 ,  230  and the efficiency map  210 . The difference lies on the half magnitude of motor torque in the half-scaled primary and secondary motors compared with that of the reference motor, however, the distribution of the motor efficiency is the same relative to the top operating speed and maximum torque. 
     In this example, operating the primary motor  102  at an operating point  222  and operating the secondary motor  104  at an operating point  232 , a higher efficiency in motor operation is attained as compared with the operation using the reference motor; yet the motor set  100  delivers the same power and torque at the drive shaft  116  as compared to the reference motor. Those skilled in electric machines may also appreciate an apparent ratio between the primary motor torque at operating point  222  and the reference motor torque at operating point  212 ; as well as the inverse of the ratio between the primary motor speed at operating point  222  and the reference motor speed at operating point  212 . The ratio arises out of the assumption of the planetary gear ratio between the sun gear and the ring gear while the primary motor does not have an internal gear. 
     Gear speeds of a planetary gear set has its operation governed by the following relationship in Equation (1): 
     
       
         
           
             
               
                 
                   
                     N 
                     c 
                   
                   = 
                   
                     
                       
                         
                           R 
                           s 
                         
                         
                           
                             R 
                             s 
                           
                           + 
                           
                             R 
                             r 
                           
                         
                       
                       ⁢ 
                       
                         N 
                         s 
                       
                     
                     + 
                     
                       
                         
                           R 
                           r 
                         
                         
                           
                             R 
                             s 
                           
                           + 
                           
                             R 
                             r 
                           
                         
                       
                       ⁢ 
                       
                         N 
                         r 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where N represents speed of respective gears, R represents radius of respective gears, subscripts c, s and r represents carrier, sun gear and ring gear, respectively. 
     Equation (1) may be written in another form as Equation (2) below:
 
 N   c   =C   g   N   s +(1− C   g ) N   r   (2)
 
     where Cg is a constant determined by the gear radii of sun gear and ring gear. 
     In this example, the secondary motor  104  is operated at zero speed and delivers zero torque. While the secondary motor  104  delivers zero torque, a reaction torque is generated by the clutch  162  that makes up the difference between the torque generated by the primary motor  102  at operating point  222  and the desired torque generated by the reference motor at operating point  212 . Without the clutch  162  while no torque is generated by the secondary motor  104 , the output shaft  154  of the secondary motor  104  would have been driven backward. A one-way clutch can allow the secondary motor  104  to rotate forward with the same direction as the primary motor  102 , and it also prevents the secondary motor  104  from rotating to the opposite angular direction against the rotation of the primary motor  102 . 
     Referring now also to  FIG. 3 , a graphical illustration of another example mode of operation of the electric motor drive  10  is shown. In this example, if the reference motor is used, an operating point  214  is desired to have the motor deliver a torque of about 60 Nm and operating at a speed of 4000 rpm. This magnitude of power delivery exceeds the capacity of any of the primary motor  102  and the secondary motor  104 . Therefore, both the primary motor  102  and the secondary motor  104  need to provide part of the power and torque for the desired electric vehicle operation. 
     For illustrative purpose only, in this example, the primary motor  102  operates at an operating point  224  that delivers half of the desired torque of 30 Nm while running at the same speed of 4000 rpm. The secondary motor  104  operates at an operating point  234  that delivers half of the desired torque of 30 Nm while running at the same speed of 4000 rpm. In combination, the motor set  100  delivers a 60 Nm at the drive shaft  116  operating at the desired speed of 4000 rpm. 
     In this example, if the reference motor is used, the motor efficiency is 90%. It also noticed that both the primary motor  102  and the secondary motor  104  operate with the same motor efficiency of 90%. In combination, the motor set  100  operates at the same motor efficiency as the reference motor. 
     Referring now also to  FIG. 4 , a graphical illustration of yet an example mode of operation of the electric motor drive  10  is shown. In this example, compared with the operation illustrated in  FIG. 3 , the desired motor performance out of the reference motor is the same. The reference motor needs to operate at the same operating point  214 . However, the primary motor  102  may operate at a different operating point  224 ′, and the secondary motor  104  may operate at a different operating point  234 ′. 
     In this example, the primary motor  102  operating point is changed from  224  to  224 ′ by reducing its operating torque and speed, and the secondary motor  104  operating point is changed from  234  to  234 ′ by increasing its operating torque and speed. The adjustments of these operating points may be determined so that the sum of the motor torques of the primary motor  102  and the secondary motor  104  results in the same as the desired reference motor torque. Further, the adjustment on the speeds of the primary motor  102  and the secondary motor  104  may be governed by Equation (2) where the carrier speed corresponds to the reference motor speed, the ring gear speed corresponds to the primary motor speed and the sun gear speed corresponds to the secondary motor speed. 
     Referring now to  FIG. 5 , a plan view of an electric propulsion system  30  is shown. The electric propulsion system  30  includes the electric motor drive  10  and a motor drive control unit  310 . The motor drive control unit  310  is electrically connected with the motor set  100  of the electric motor drive  10  to provide desired electric currents to operate the primary motor  102  and the secondary motor  104 . 
     The motor drive control unit  310  receives a system torque signal (T)  320  and a system speed signal (N)  322  from a vehicle system controller (not shown), and generates primary motor current (Ip)  350  and secondary motor current (Is)  352  for the primary motor  102  and the secondary motor  104 , respectively. The primary motor current  350  may be delivered to the armature  132  of the primary motor  102 , and the secondary motor current  352  may be delivered to the armature  152  of the secondary motor  104  via electrical connections between the motor drive control unit  310  and the motor set  100 . 
     The motor drive control unit  310  may include a system optimization unit  312 , a primary inverter  340  for the primary motor  102 , and a secondary inverter  342  for the secondary motor  104 . The system optimization unit  312  inputs the system torque signal  320  and the system speed signal  322  received by the motor control unit  310 , and generates a primary motor torque command (Tp)  324 , a primary motor speed command (Np)  326 , a secondary motor torque command (Ts)  328 , and a secondary motor speed command (Ns)  330  based on the system torque signal  320  and the system speed signal  322 . 
     The system optimization unit  312  may include a data memory module  314  and a computation module  316 . Motor efficiency data of the primary motor  102  and the secondary motor  104  may be stored in the data memory module  314 . Parameters of the primary gear set  106  may also be stored in the data memory module  314 . The data stored in the data memory module  314  may be retrieved and utilized by the computation module  316  for determining the primary motor torque command  324 , the primary motor speed command  326 , the secondary motor torque command  328  and the secondary motor speed command  330 . The computation module  316  may perform system optimization processes to determine operating points for the primary motor  102  and the secondary motor  104  that result in an optimal efficiency in operation. The system optimization process may be performed based on the motor efficiency data stored in the data memory module  314 . 
     The primary inverter  340  receives the primary motor torque command  324  and the primary motor speed command  326 , and generates the primary motor current  350  based on the primary motor torque command  324  and the primary motor speed command  326  by regulating an electric current flowing out of an energy storage device (not shown) to the armature  132  of the primary motor  102 . The energy storage device may be a battery pack. The secondary inverter  342  receives the secondary motor torque command  328  and the secondary motor speed command  330 , and generates the secondary motor current  352  based on the secondary motor torque command  328  and the secondary motor speed command  330  by regulating another electric current flowing out of the energy storage device to the armature  152  of the secondary motor  104 . The electric current generated by the inverters to the respective motor armature may be three-phase current. 
     Referring now also to  FIG. 6 , a flow diagram illustrating a method  40  for operating the electric propulsion system  30  is shown. The method  40  may be performed by the motor drive control unit  310 . The system optimization unit  312  of the motor drive control unit  310  may perform the method  40  to generate the primary motor torque command  324 , the primary motor speed command  326 , the secondary motor torque command  328 , and the secondary motor speed command  330 . 
     At each control period determined by a vehicle system controller (not shown), the method  40  may start at step  402 , and proceed to step  404  for signal input. In step  404 , the system optimization unit  312  may input the system torque signal (T)  320  and the system speed signal (N)  322  from the vehicle system controller. In step  406 , the computation module  316  may read data of a primary motor torque rating (Tmp) and data of a primary motor power rating (Pmp). The data of the primary motor torque rating and the primary motor power rating may be stored in the data memory module  314 . 
     In step  408 , the computation module  316  may compare the magnitude of the system torque signal with the primary torque rating. The computation module  316  may also compare a product of the magnitude of the system torque signal and the system speed signal with the primary power rating. The method may proceed to step  420  when the computation module  316  determines that the magnitude of the system torque signal is greater than the primary torque rating, or the product of the magnitude of the system torque signal and the system speed signal is greater than the primary power rating. The method may proceed to step  410  when the computation module  316  determines that the magnitude of the system torque signal is not greater than the primary torque rating, and the product of the magnitude of the system torque signal and the system speed signal is not greater than the primary power rating. 
     In step  410  the computation module  316  determines the primary motor torque command (Tp) and the primary motor speed command (Np) based on the system torque signal (T) and the system speed signal (N). In one embodiment, the computation module  316  may set the primary motor torque command (Tp) equal to the magnitude of the system torque signal (T), and set the primary motor speed command (Np) equal to the magnitude of the system speed signal (N). In another embodiment, the computation module  316  may set the primary motor torque command (Tp) equal to the magnitude of the system torque signal (T) multiplied by a gear ratio, and set the primary motor speed command (Np) equal to the magnitude of the system speed signal (N) multiplied by an inverse of the gear ratio. 
     In step  412  the computation module  316  determines the secondary motor torque command (Ts) and the secondary motor speed command (Ns). In one embodiment, the computation module  316  may set the secondary motor torque command equal to zero, and set the secondary motor speed command to zero. In yet another embodiment, the computation module  316  may set the secondary motor speed command to zero, but set the secondary motor torque command equal to the difference between the magnitude of the system torque signal and the primary motor torque command. 
     In step  414 , the primary inverter  340  may generate the primary motor current (Ip) based on the primary motor torque command (Tp). In step  416 , the secondary inverter  342  may generate the secondary motor current (Is) based on the secondary motor torque command (Ts). In one embodiment, when the secondary motor torque command (Ts) is set equal to zero, the secondary motor current (Is) is also set to zero. The process of method  40  may end at step  418 . 
     In step  420  following step  408  when the test result in step  408  is affirmative, the system optimization unit  312  determines an initial primary torque command (Tp0), an initial secondary motor command (Ts0), an initial primary motor speed command (Np0), and an initial secondary motor speed command (Ns0) based on the system torque signal (T) and the system speed signal (N). Each of the initial primary motor torque command and the initial secondary motor torque command may be set at half of the magnitude of the system torque signal. Each of the initial primary motor speed command and the initial secondary motor speed command may be set at the magnitude of the system speed signal. 
     In step  422 , the system optimization unit  312  may perform search for an updated operating point that results in a higher motor efficiency than any of the initial operating points defined by the initial primary motor torque command and the initial primary motor speed command, or defined by the initial secondary motor torque command and the initial secondary motor speed command. The search may be based on motor efficiency data stored in the data memory module  314 . In one embodiment, the search may be performed within an area around the initial operating point not exceeding a predefined distance away from the initial operating point. In another embodiment, a sequence of predefined distances may be used for multiple searches to determine an optimal operating point among those found with higher efficiency than the initial operating points. 
     In step  424 , the computation module  316  determines whether the search performed in step  422  is successful in finding an updated operating point with higher motor efficiency. Step  424  proceeds to step  426  when the result is positive, and step  424  proceeds to step  428  when the result is negative. 
     In step  426 , the computation module  316  sets the motor operating points according to the updated higher efficiency operating points found in step  422 . In step  428 , the computation module  316  sets the motor operating points according to the initial operating points determined in step  420 . 
     In step  430 , the primary inverter  340  generates the primary motor current  350  based on the primary motor torque command  324 , and the secondary inverter  342  generates the secondary motor current  352  based on the secondary motor torque command  328 . After performing step  430 , the process may end at step  432  for the control period. 
     Referring now to  FIG. 7 , a plan view of another electric motor drive  50  is shown. The electric motor drive  50  includes a motor set  400  which includes motors and gear set similar to those contained in the motor set  100  in  FIG. 1 . The motor set  400  may include a primary motor  102 ′, a secondary motor  104 ′ and a primary gear set  106 ′. The motor set  400  further includes a forward-stage motor  108  and a forward-stage gear set  106 ″. The forward-stage motor  108  may be similar to the primary motor  102 ′ and the forward-stage gear set  106 ″ may be similar to the primary gear set  106 ′. The primary gear set  106 ′ is connected to a drive shaft  116 ″ that rotates on an axis AA′. 
     The forward-stage motor  108  includes an output shaft  134 ″. The output shaft  134 ″ has a hollow center  138 ″. The drive shaft  116 ″ is connected to the sun gear  120 ″ of the forward-stage gear  106 ″ through the hollow center  138 ″ of the output shaft  134 ″ of the forward-stage motor  108 . The forward-stage gear set  106 ″ is connected to a drive shaft  116 ′ of the motor set  400 . The drive shaft  116 ′ rotates on the axis AA′. 
     The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.