Patent Publication Number: US-2010126786-A1

Title: Electric drive inertia ratio for ttt

Description:
TECHNICAL FIELD  
     The present disclosure generally relates to electric powertrains, and more particularly relates to an apparatus and method for optimizing the load acceptance and maneuverability of a vehicle employing an electric powertrain. 
     BACKGROUND  
     Many vehicles used in construction, agriculture and industry employ a track-type form of locomotion. More specifically, the engine of the vehicle is coupled to a transmission that drives a rotating sprocket. An endless loop of track is trained around the sprocket and a plurality of idlers in the undercarriage of the vehicle, such that rotation of the sprocket causes rotation of the track. The track is formed from a plurality of shoes hingedly pinned together, with each shoe having a radially outwardly extending grouser for engaging the ground and providing traction. Accordingly, upon rotation of the sprocket, the track rotates around the undercarriage and the vehicle is propelled. 
     The power for such locomotion is typically provided by an internal combustion engine, more specifically a diesel-type internal combustion engine. Such engines provide extremely high torque at low ground speeds and thus are able to accept loads of substantial size. As the load increases, for example if the track-type tractor is grading soil, pushing gravel, or the like, the engine speed is simply increased to accommodate the load and meet the torque demand. 
     While effective, such engines run on diesel fuel and thus increased engine speed increases fuel consumption, adding to the cost of operation while at the same time increasing emissions. With the ever rising cost of fuel, and increasing environmental demands of the public and of governmental regulations regarding emissions, such reliance on diesel engines for use in tractors is becoming increasingly unacceptable. 
     In an effort to abate these concerns, track-type tractors have been developed which use electric motors and powertrains in combination with internal combustion engines. US Patent Publication No. 2007/0080236 is one example of such technology. In the &#39;236 application, an internal combustion engine is coupled to a generator which in turn is coupled to one or more electric motors. The motors are potentially able to provide added torque to the powertrain, but do so without consuming additional fuel or releasing additional emissions. A control circuit is also provided to store the energy provided by the generator when the motor or motors are not being employed, or when the electric motors are being employed in a regenerative braking capacity and thus producing energy. 
     While such an electric powertrain and tractor are significant advancements in the field, such powertrains continue to be improved, with one current focus being directed toward optimizing the balance between the ability of the tractor to accept and handle a given load, while at the same time providing the tractor with acceptable maneuverability. More specifically, while generally large mass motors can be used to provide the desired level of torque for load acceptance, as the mass and therefore inertia of the motor increases, the ability of that motor to start and stop quickly decreases. This relationship necessarily means that larger mass motors, and track-type tractors employing such motors, have a lessened ability to start, stop, and reverse direction quickly. 
     One potential solution has been thought to provide low mass/low inertia motors on such vehicles. Such motors can more quickly start, stop and reverse direction. As such vehicles are often called upon to repeatedly push matter forward, reverse direction, and then push forward again and again, this is potentially advantageous. However, the assignee has learned that while low mass or inertia motors do allow for such improved maneuverability, they come at the cost of decreased load acceptance. In other words, such low mass motors are often unable to provide sufficient torque to meet the demands of the load. This can directly translate to track slippage and thus reduced productivity. 
     SUMMARY OF THE DISCLOSURE  
     In accordance with one aspect of the disclosure, an electric powertrain is provided which may include an electric motor having an inertia, and a driven member operatively coupled to the electric motor. The driven member has an inertia and the ratio of the motor inertia to the driven member inertia may be between about 1 and about 2.5. 
     In accordance with another aspect of the disclosure, a track-type tractor is disclosed which may include a main frame, an electric motor, a gear reduction assembly, and a track. The electric motor is mounted to the main frame and includes a rotor having a mass. The gear reduction assembly is operatively coupled to the electric motor and includes an overall gear reduction ratio. The mass of the rotor multiplied by the square of the overall gear reduction ratio of the gear reduction assembly produces the inertia of the motor. The track is operatively coupled to the gear reduction assembly and has a rolling radius. The track-type tractor includes a total mass. The total mass of the track-type tractor multiplied by the square of the rolling radius of the track produces an inertia of the tractor. The ratio of the motor inertia to the tractor inertia may be within the range of about 1 and about 2.5. 
     In accordance with a still further aspect of the disclosure, a method of manufacturing a track-type tractor is provided which may include operatively coupling an electric motor to a gear reduction assembly wherein the electric motor and gear reduction assembly have a motor inertia, operatively coupling the gear reduction assembly to a track of the track-type tractor wherein the track-type tractor has a tractor inertia, and sizing the electric motor relative to the track-type tractor such that the track-type tractor may have a Stemler Factor less than the ratio of the motor inertia to the tractor inertia. 
     These and other aspects and features of the present disclosure will become more apparent upon reading the following description when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a side elevational view of an exemplary vehicle employing the teachings of the present disclosure; 
         FIG. 2  is a schematic block diagram of the vehicle of  FIG. 1 ; 
         FIG. 3  is a schematic block diagram of the electric powertrain of the vehicle of  FIG. 1 ; and 
         FIG. 4  is a flowchart depicting a sample sequence of steps which may be practiced in accordance with an exemplary method employing the teachings of the present disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the present disclosure to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the present disclosure. 
     DETAILED DESCRIPTION  
     Referring now to the drawings, and with specific reference to  FIG. 1 , a vehicle constructed in accordance with the present disclosure is generally referred to by reference numeral  100 . While the vehicle  100  depicted is a track-type tractor, it is to be understood that the teachings of the present disclosure can be used on any number of different vehicles used in construction, agriculture and industry, including but not limited to bulldozers, farm tractors, graders, skid-steer loaders, front-end loaders, excavators, and the like. 
     As shown, the vehicle  100  includes a main frame  102  to which an internal combustion engine, or other prime mover or main power source  104  is mounted. The engine  104  may be any known type of engine such as a diesel-type internal combustion engine, a gasoline-type internal combustion engine, natural gas engine, a gas turbine engine, or the like. An operator cab  106  is also provided atop the main frame  102 . 
     Below the main frame  102 , an undercarriage  108  is positioned for propelling the vehicle  100 . The undercarriage may be operatively coupled to the engine  104  by a mechanical link  110  (see  FIG. 2 ), such as a transmission, a gear assembly, a differential steering unit or the like. As for the undercarriage  108  itself, it may include a drive sprocket  112 , a pair of idler wheels  114 , and a plurality of mid-rollers  116 , around all of which is trained an endless loop or track  118 . The ground-engaging track  118  may include a plurality of shoes  120  hinged together by pins  122 . Each shoe  120  may include a grouser  124  for direct engagement into the underlying ground (not shown). Also, as shown in  FIG. 2 , a second track  126  may be provided on the vehicle  100  in laterally flanking position relative to the first track  118 . 
     Depending on the type of tractor being constructed, the vehicle  100  can then be configured with a number of different implements to perform a given function. For example, with the bulldozer depicted in  FIG. 1 , the vehicle  100  includes a pair of push arms  128  extending from a roller frame  130  of the vehicle and coupled to a blade  132 . In order to lift, tilt, and lower the blade  132 , one or more hydraulic cylinders  134  may be connected to the blade  132  and the vehicle  100 . The hydraulic cylinders  134  may in turn be connected to a hydraulic system  136  of the vehicle  100  as shown in  FIG. 2 . Among other things, the hydraulic system  136  may include a pump  138  powered by the engine  104 . 
     Turning now to  FIGS. 2 and 3 , the electric powertrain of the present disclosure is generally referred to by reference numeral  140 . As shown schematically therein, the electric powertrain  140  may include a generator  142  operatively coupled to the engine  104  to turn at least a portion of the mechanical energy generated by the engine  104  into electrical energy. The generator  142  is then in turn operatively coupled to one or more electric motors  144 . The electric motors  144  may be operatively coupled to a gear reduction assembly  146  which is in turn connected to the drive sprocket  110 . The gear reduction assembly  146  may include any number of different gearing arrangements and components to step the speed of the motor  144  down to a more useable speed and torque for use by the drive sprocket  112 . Such arrangements may include, but not be limited to, planetary gear systems. The gear reduction assembly will also have an overall gear reduction ratio as more fully described below. 
     As the electric motors  144  may be employed by the vehicle  100  in dynamic fashion, an energy storage device  148  may be used to store the electric energy created by the generator  142  for subsequent use. The energy storage device  148  may be a battery, a capacitor, a flywheel, or any other known type. In addition, the energy created by the motor  144  when run in reverse, such as by regenerative braking, can also be stored in the energy storage device  148 . 
     The type of electric motor  144  employed has no bearing on the relationships and equations referenced herein and may be any known type including, but not limited to, AC, DC, permanent magnet, induction, switched-reluctance, hybrid combination, sealed, brushless, and/or liquid cooled. Similarly, the generator may be any known type including, but not limited to AC, DC, permanent magnet, induction, switched-reluctance, hybrid combination, sealed, brushless, and/or liquid cooled. The motor  144  and generator  142  may be controlled by power electronics or motor drives  150  with one or more speed, torque or other sensors  152  being operatively associated with the motor  144  or generator  142  to provide a closed loop feedback control. In additional, while not germane to the focus of this disclosure, for the purpose of completeness, it is helpful to understand that the power electronics  150  may include a power converter, an inverter controller, and/or motor software, and may be configured to convert and control electricity, for example, provided to the electric motor  144 , thereby providing control of speed and torque for the propulsion of the vehicle  100 . The power electronics  150  may be housed in a compartment (not shown), which may be sealed and liquid cooled. 
     In order to provide the vehicle  100  with the optimum balance between load acceptance and maneuverability, the inventors have devised a ratio that enables an electric powertrain to be sized relative to the vehicle. The ratio concerns the inertia of the motor or motors to the inertia of the vehicle. More specifically, the ratio is summarized as follows:
         If       

         I   ms   =I   m *( GR   t ) 2 , and 
         I   v   =m   v *( rr   t ) 2 ),         then       
       1 ≦I   ms   /I   v ≦2.5,         where,
           I ms  is the inertia of the motor at the sprocket,   I m  is the inertia of the motor,   GR r  is the overall gear reduction ratio,   I v  is the inertia of the vehicle,   m v  is the total mass of the vehicle, and   rr t  is the rolling radius of the track.   
               
     While a target inertia ratio of about 1.8 has been identified as the optimum balance enabling the vehicle to handle a load while at the same time providing exceptional maneuverability, the inventors have identified ranges of ratios which have heretofore been unknown and which provide an advantageous balance of such concerns. For example, a ratio within the range of about unity (1) to about 2.5 affords the vehicle with such a balance. Within that range, subsets have also been identified which, depending on the particular application of the vehicle may provide optimal performance. In other words, depending on whether the vehicle is a front-end loader, a skid-steer loader, an excavator, a grader, a back-hoe, a farm tractor, or the like, ranges of about 1.3 to about 2.3, about 1.5 to about 2.1, or about 1.7 to about 1.9, among others, may be advantageously employed. As used herein with respect to a numerical value, “about” means within plus or minus ten percent of the stated value. 
     By way of example, if the vehicle  100  is a bulldozer, load acceptance may be more desirable than maneuverability. In such a case, the manufacturer may size the motor(s)  144  to have a target inertia ratio toward the high end of 2.5. Conversely, if the vehicle  100  is a skid-steer loader or forklift, the maneuverability to start, stop and reverse directions with relatively smaller loads may be more desirable than load acceptance, in which case the manufacturer can size the motor(s)  144  such that the resulting target inertia ratio is more toward the low end of 1.0. Of course, target inertia ratios below 1.0 and above 2.5 are possible as well, and encompassed within this disclosure, but such a range has been identified by the inventors as optimal to avoid track slippage, improve productivity, decrease fuel consumption, enhance load acceptance, and optimize maneuverability. 
     In arriving at these uniquely identified ratios, the inventors have also derived a relationship between motor torque and inertia ratio that has been previously unknown and which enables manufacturers to size an electric motor and gear reduction assembly to a given vehicle to ensure the proper balance between the load acceptance and maneuverability concerns mentioned above. This relationship, referred to as the Stemler Factor (S f ), is represented by the following formulae: 
         S   f   =F   m   *rr   t *(1 /e   g )*(1 /T   m ),         where:
           F m =the stall pull of the motor,   rr t =the rolling radius of the track,   e g =the mechanical efficiency of the gear reduction assembly, and   T m =the motor torque.   
               
     To calculate the stall pull (F), the following calculation is made: 
         F=T   m   *N*GR   r *(1 /rr   t ),         where:
           N=the number of motors in the vehicle powertrain, and the remaining variables are defined as set forth above.   
               
     Using the above calculations, the inventors have identified that the electric motor and gear reduction assembly should be sized relative to the vehicle employing the motor such that the Stemler Factor is maintained at a level less than or equal to the target inertia ratio mentioned above. If the Stemler Factor is so managed, the optimal balance between load acceptance and vehicular maneuverability can be attained. 
     INDUSTRIAL APPLICABILITY  
     The disclosed powertrain or powertrain may be applicable to any machine used in construction, farming, or industry such as, for example, vehicles having ground engaging tracks. Such track-type vehicles may be, but are not limited to, bulldozers, front-end loaders, graders, skid-steer loaders, and the like. By virtue of using an electric powertrain, exhaust emissions are reduced and fuel efficiency may be increased. In addition, but virtue of the unique optimization disclosed herein, a proper balance between load acceptance and maneuverability in such vehicles is attained leading to less track slippage and improved productivity. The operation of such a vehicle and optimization method will now be explained. 
     One exemplary application in which the teachings of this disclosure may be employed is in the manufacture of a track-type tractor. In order to reduce fuel consumption and limit emissions that may result from such combustion, it may be desirable to manufacture a track-type tractor with a powertrain which at least in part utilizes electric motors. However, sizing such motors relative to the vehicle to provide both load acceptance and maneuverability has proven, until now, to be challenging to the industry. 
     The vehicle of the present disclosure is able to do so. Such a vehicle  100  is shown in  FIGS. 1-3 . As shown, the vehicle  100  may include a main frame  102  to which a prime mover or power source  104  is provided such as a diesel-type internal combustion engine. The engine  104  may be used for a number of different things, including but not limited to driving a mechanically driven transmission  110  operatively coupled to the drive sprocket  112 , which in turn is operatively coupled to the track  118  identified above. The engine  104  may also be used to power the pump  138  of the hydraulic system  136  to drive the various hydraulic cylinders  134  on the vehicle  100  used to lift, lower, tilt, or otherwise move the work tools, e.g. the blade  132 , provided on the vehicle  100 . 
     One other function the engine  104  may perform is to power the generator  142 . As discussed above the generator  142  converts at least a portion of the mechanical energy created by the engine  104  into electrical energy. This electrical energy may then be used to operate the one or more electric motors  144  on the vehicle  100  provided as part of the electric powertrain  140 . Each motor  144  may include a stator  154  and rotor  156  as is conventional, with a drive shaft  158  extending from the center of the rotor  156 . The drive shaft  158  may then be operatively coupled to the gear reduction assembly  146  to step the relative rotational speed of the motor  144  down to a speed usable by the drive sprocket  112 . In so doing, the locomotion of the vehicle  100  can be aided by the electric motors  144  when desired. Moreover, when the electric motors  144  are not in use, the electric energy created by the generator  142  can be stored in an electric energy storage device  148  such as a battery, capacitor, flywheel, or the like, so that when the motor  144  is to be employed, this reserved energy can be utilized, thereby allowing the engine  104  to run slower, use less fuel, and release fewer emissions. 
     However, without the contributions of this disclosure, the proper sizing of the electric motor or motors in such a vehicle would not be possible. The inventors have derived a series of calculations and relationships between the size and torque of the motor to the size and application of the vehicle which enable manufacturers to optimize the ability of the vehicle to both accept the load confronted in the application, while at the same time providing the vehicle with exceptional maneuverability. The following is one exemplary set of such calculations and decisions which can be made in accordance with the method of this disclosure. 
     Referring now to  FIG. 4 , a first calculation may be to calculate the inertia of the vehicle  100  in question. This is represented by calculation  160  in  FIG. 4 , and is arrived at by multiplying the entire mass of the vehicle  100  by the square of the rolling radius of the track  118 . Once the inertia of the vehicle  100  in question is known, the manufacturer will know what the inertia of the motor  144  to be selected should be because the optimum ratio of motor inertia to vehicle inertia has been identified as about 1.8. Using the above equation for motor inertia, the manufacturer can then select a motor  144  and gear reduction assembly  146  (see step  162  in  FIG. 4 ) that ensure that ratio is about 1.8. More specifically, as shown by calculation  164  in  FIG. 4 , the mass of the motor rotor  156  may be multiplied by the overall gear reduction ratio of the gear reduction assembly  146  to arrive at the motor inertia. That motor inertia can then be divided by the vehicle inertia to arrive at the target inertia ratio. As shown by decision  168 , if the target inertia ratio is about 1.8, the selected motor  144  and gear reduction assembly  146  may be used with the vehicle  100  in question to provide the optimum balance between load acceptance and maneuverability as shown by step  170 . 
     If not, the manufacturer can decide if the target inertia ratio is within an acceptable range for the particular type of vehicle being sized as shown by step  172 . As identified above, that range may be between about 1 and about 2.5, and depending on the particular application for the vehicle (e.g., bulldozer vs. skid steer loader), the calculated target inertia ratio may be acceptable. If it is, the motor  144  can be used as shown by step  170 . If it is above or below that range however, the manufacturer may choose to revert back to step  162  and select a different motor size and/or gear reduction assembly as shown by step  174 . Of course, the foregoing is but one sequence of calculations which may be made in accordance with this disclosure. For example, the inertia of the vehicle  100  need not be calculated first, but rather the inertia of the motor  144  could be calculated first with the inertia of the vehicle  100  then being selected to achieve the targeted inertia ratio. 
     As a further step to ensure the optimum balance is achieved, the manufacturer may also employ the Stemler Factor. As shown in  FIG. 4 , either in addition to calculating the target inertia ratio, or independent of the target inertia ratio, the Stemler Factor for a particular motor  144  and vehicle  100  combination can be calculated as shown in step  176 . If done concurrent with the target inertia ratio, once the target inertia ratio is known, the Stemler Factor dictates that it should be less than or equal to that target inertia ratio. In other words, if the target inertia ratio is calculated to be 1.8, the Stemler Factor of the combination should he less than or equal to 1.8 as well. 
     Referring in part to the above equations, that Stemler Factor is calculated by multiplying the stall pull of the motor  144  by the rolling radius of the track  118  by the inverse of the gear reduction assembly efficiency by the inverse of the motor torque. The stall pull used in that calculation may be arrived at by multiplying the motor torque by the number of motors  144  in the electric powertrain  140  by the inverse of the rolling radius. If the Stemler Factor is less than the target inertia ratio as determined by decision  178 , the manufacturer can use the selected motor  144  (see step  170 ) and be even more assured that motor  144  and gear reduction assembly  146  have been properly selected for the given vehicle to ensure the optimum balance between load acceptance and maneuverability is reached. If not, the motor can be reselected and the process reverted back to step  162 . 
     Since the optimum target inertia ratio is within the range of about 1 to about 2.5, and the Stemler Factor should be less than or equal to the target inertia ratio, the Stemler Factor should be within the range of about 0.9 to about 2.5. The inventors have found a Stemler Factor of about 1.7 for a motor and vehicle combination having a target inertia ratio of about 1.8 to be optimal. Again, however, similar to the target inertia ratio, depending on the intended environment and application for the vehicle, other values may be acceptable, with exemplary Stemler Factors being within ranges such as about 1.5 to about 1.9, about 1.3 to about 2.1, and about 1.1 to about 2.3. 
     Based on the foregoing, it can be seen that the present disclosure sets forth an electric powertrain, a vehicle such as a track-type tractor, and a method of manufacturing a track-type tractor with an optimized balance between load acceptance and maneuverability. By properly sizing the motor and gear reduction assembly relative to a given vehicle these competing interests can be balanced, and by employing the unique, previously unknown relationships identified by the inventors and quantified herein, that balance and improved productivity for the vehicle can be attained.