Patent Publication Number: US-2013231837-A1

Title: Electronic control of a limited slip differential

Description:
TECHNICAL FIELD 
     The invention relates to a system and a method for electronic control of a limited slip differential in a motor vehicle. 
     BACKGROUND 
     A typical motor vehicle employs a differential to transmit torque and rotation from a power source such as an internal combustion engine, an electric motor, or a combination thereof to the vehicle&#39;s road wheels via individual output or axle shafts. A differential is a device that allows each of the driving road wheels to rotate at different speeds mainly when negotiating a turn. 
     During vehicle cornering, the vehicle&#39;s wheels that are on the inside relative to the turn generally travel a shorter distance than the wheels that are on the outside of the turn. Accordingly, during cornering without a differential a vehicle&#39;s inside wheel may end up spinning, while its outside wheel may end up dragging. Such a condition may result in difficult and unpredictable handling of the vehicle, damage to vehicle tires, and strain on and possible damage to the vehicle&#39;s drivetrain. 
     A standard or “open” differential tends to transmit a largely equivalent amount of torque to both drive wheels. However, in certain driving conditions, an open differential may transfer a majority of drive torque to a wheel that has been unloaded or experiences reduced frictional contact with the road. In such a situation, the unloaded or reduced frictional contact wheel may rotate freely, thus converting a substantial amount of drive torque into tire slip and not into powering the vehicle. 
     To counteract such a loss of effective drive torque, certain higher performance vehicles employ limited slip differentials (LSDs) that allow for some difference in angular velocity of the output shafts, but impose a mechanical restriction on such a disparity. Typically the mechanical restriction is provided via a frictional interface, for example with specially configured gears or clutching elements. By limiting the difference in angular velocity between the driven wheels, useful torque can be transmitted to the road surface, as long as some traction is generated by at least one of the driven wheels. In modern vehicles, electronically controlled LSDs are sometimes used for more precise apportionment of drive torque between the drive wheels. 
     SUMMARY 
     A method is disclosed for regulating in a motor vehicle an electronic limited slip differential (eLSD) to apportion drive torque from a power source between first and second drive wheels and transmit the drive torque to a road surface. The method also includes determining maximum torque capability of each of the first and second drive wheels and identifying the wheel that is capable of transmitting a greater portion and the wheel that is capable of transmitting a lesser portion of the drive torque to the road surface. The method also includes determining a remaining portion of the drive torque by subtracting the determined maximum torque capability of the wheel capable of transmitting the lesser portion of the drive torque from the generated drive torque. 
     The method additionally includes regulating the eLSD to transfer to the wheel that is capable of transmitting the greater portion of the drive torque a portion of the drive torque that is equal to the maximum torque capability of the more capable wheel if the remaining portion of the drive torque is greater than the determined maximum torque capability of the more capable wheel. Furthermore, the method includes regulating the eLSD to transfer to the wheel that is capable of transmitting the greater portion of the drive torque the determined remaining portion of the drive torque if the remaining portion of the drive torque is equal to or less than the determined maximum torque capability of the more capable wheel. 
     The method may additionally include detecting, in real-time, changes in orientation of the vehicle relative to the road surface via at least one vehicle sensor to determine the maximum torque capability of each of the first and second drive wheels. According to the method, the at least one vehicle sensor may include a lateral acceleration sensor, a longitudinal acceleration sensor, and a yaw sensor. In such a case, the method may additionally include determining weight transfer between the first and second drive wheels in response to the received signals from the lateral acceleration, longitudinal acceleration, and yaw sensors to determine in real-time the maximum torque capability of each of the first and second drive wheels. 
     Each of the first and second drive wheels may include a pneumatic tire that establishes tractive effort with respect to the road surface. In such a case, the method may additionally include determining loading on each respective tire to determine in real-time a maximum tractive effort thereof in response to the determined weight transfer between the first and second drive wheels. According to the method, the determination of the tractive effort of each respective tire is determined via the “friction circle” concept as described herein according to physical properties of and a vertical load on the subject tire. 
     Each of the acts of determining the maximum torque capability of each of the first and second drive wheels, determining the remaining portion of the drive torque, regulating the eLSD, detecting in real-time changes in orientation of the vehicle, determining weight transfer between the first and second drive wheels, and determining loading on each respective tire may be accomplished via a controller. 
     The vehicle may additionally include a first wheel speed sensor configured to detect in, real-time, the rotational speed of the first drive wheel, and a second wheel speed sensor configured to detect, also in real-time, the rotational speed of the second drive wheel. In such a case, the method may additionally include receiving via the controller the detected rotational speeds from the respective first and second wheel speed sensors and generating feed-back control of the eLSD by comparing a desired difference in speeds of the first and second drive wheels with actual difference thereof via the controller. 
     The eLSD may include a friction plate clutch, while the controller may be additionally configured to regulate engagement of the clutch to apportion the drive torque between the first and second drive wheels. 
     Also disclosed is a vehicle that includes the described controller to perform the above method. 
     The above features and advantages, and other features and advantages of the present disclosure, will be readily apparent from the following detailed description of the embodiment(s) and best mode(s) for carrying out the described invention when taken in connection with the accompanying drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a motor vehicle equipped with an electronic limited slip differential (eLSD) for apportioning drive torque between the vehicle&#39;s driven road wheels. 
         FIG. 2  is a diagram of a friction circle describing tractive effort of a tire mounted on a road wheel such as for the vehicle shown in  FIG. 1 . 
         FIG. 3  is a diagram of friction circles for each of the driven wheels and a change in the wheels&#39; respective torque capabilities when the wheels are subject to dynamic weight transfer. 
         FIG. 4  is a flow chart illustrating a method of regulating the eLSD shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  shows a schematic view of a motor vehicle  10  which includes a vehicle body  12 . The vehicle  10  also includes a power source  14  configured to generate drive torque  15  for propelling the vehicle. As shown in  FIG. 1 , the power source  14  is an engine  16  operatively connected to a transmission  18 . The power source  14  may also include one or more motor/generators as well as a fuel cell, neither of which are shown, but a vehicle configuration employing such devices is appreciated by those skilled in the art. 
     The vehicle  10  also includes a plurality of wheels that include front wheels  20 - 1 ,  20 - 2  and rear wheels  22 - 1 ,  22 - 2 . Although four wheels,  20 - 1 ,  20 - 2 ,  22 - 1 , and  22 - 2  are shown in  FIG. 1 , a vehicle with fewer or greater number of wheels is also envisioned. As shown, the rear wheel  22 - 1  is a first drive wheel and the rear wheel  22 - 2  is a second drive wheel of the vehicle  10 . The first and second drive wheels  22 - 1 ,  22 - 2  are rotated or driven by the power source  14  for transmitting the drive torque  15  generated by the power source  14  to a road surface  19  to motivate the vehicle  10  along the road surface. Although in the particular embodiment shown and described with respect to  FIG. 1 , the wheels  22 - 1 ,  22 - 2  are depicted as the vehicle drive wheels, in a different embodiment the front wheels  20 - 1 ,  20 - 2  may similarly be configured as the vehicle drive wheels. In yet another embodiment, all four wheels  20 - 1 ,  20 - 2 ,  22 - 1 , and  22 - 2  may be configured to drive the vehicle  10  along the road surface  19 . Additionally, each of the wheels  20 - 1 ,  20 - 2 ,  22 - 1 , and  22 - 2  includes a respective pneumatic tire  21 - 1 ,  21 - 2 ,  23 - 1 , and  23 - 2  mounted thereon. 
     As shown in  FIG. 1 , a vehicle suspension system  24  operatively connects the body  12  to the front and rear wheels  20 ,  22  for maintaining contact between the wheels  20 - 1 ,  20 - 2 ,  22 - 1 ,  22 - 2  and the road surface  19 , and to maintain handling of the vehicle  10 . The suspension system  24  may include an upper control arm  26 , a lower control arm  28 , and a strut  30  connected to each of the front wheels  20 - 1  and  20 - 2 . The suspension system  24  may also include a trailing arm  32  and a spring  34  connected to each of the rear wheels  22 - 1  and  22 - 2 . Although a specific configuration of the suspension system  24  is shown in  FIG. 1 , other vehicle suspension designs are similarly envisioned. The tires  21 - 1 ,  21 - 2 ,  23 - 1 , and  23 - 2  establish a tractive effort with respect to the road surface  19  in response to the loading on each tire transmitted through the suspension system  24  during operation of the vehicle  10 , as well as being affected by the friction coefficient between the tires and the particular road surface. The tractive effort of a tire is defined herein as the maximum grip available between the tire and the road surface  19 , wherein such grip is dependent on the friction coefficient “μ” at the subject tire/road surface interface. 
     With continued reference to  FIG. 1 , a vehicle steering system  36  is operatively connected to the front wheels  20  for steering the vehicle  10 . The steering system  36  includes a steering wheel  38  that is operatively connected to the wheels  20  via a steering rack  40 . The steering wheel  38  is arranged inside the passenger compartment of the vehicle  10 , such that an operator of the vehicle may command the vehicle to follow a particular path or assume a desired orientation with respect to the road surface  19 . Additionally, an accelerator pedal  42  is positioned inside the passenger compartment of the vehicle  10 , wherein the accelerator pedal is operatively connected to the power source  14  for commanding propulsion of the vehicle  10 . 
     As shown in  FIG. 1 , a vehicle braking system is operatively connected to the wheels  20 ,  22  for decelerating the vehicle  10 . The braking system includes a friction braking mechanism  46  at each of the wheels  20 - 1 ,  20 - 2 ,  22 - 1 , and  22 - 2 . Although not shown in detail, it will be appreciated that each braking mechanism  46  may include a rotor, brake pads, and calipers. The calipers may be configured to hold the brake pads relative to the rotors, and to apply a force to the brake pads in order to squeeze the rotors for decelerating the vehicle  10 . The force applied by the braking system is controlled via a brake pedal  48 . The brake pedal  48  is positioned inside the passenger compartment of the vehicle  10 , and is adapted to be controlled by the operator of the vehicle  10 . 
     As additionally shown in  FIG. 1 , the vehicle  10  also includes an electronic, i.e., electronically controlled, limited slip differential (eLSD)  50 . The eLSD  50  is operatively connected to the power source  14  via a drive shaft  52 , and is configured to apportion the drive torque  15  generated by the power source between the first and second drive wheels  22 - 1  and  22 - 2 . The eLSD  50  is configured to limit the difference in angular velocity between the drive wheels  22 - 1  and  22 - 2  whenever one of the drive wheels becomes unloaded or otherwise loses traction. Accordingly, useful drive torque  15  can be transmitted to the road surface  19 , as long as some traction is generated by at least one of the drive wheels  22 - 1 ,  22 - 2 . The eLSD  50  may include a friction plate clutch  54  that is configured to apportion the drive torque  15  between the first and second drive wheels  22 - 1  and  22 - 2  in response to tractive effort and relative speeds of the tires  23 - 1 ,  23 - 2 . 
     The clutch  54  may include friction plates  56  and drive plates  58  configured to be selectively engaged with each other for variable apportionment of the drive torque  15  between the drive wheels  22 - 1 ,  22 - 2 . The friction plates  56  and drive plates  58  may be engaged with a selectable amount of force which may be applied either hydraulically or mechanically, such as via an electrically actuated hydraulic pump  60  or an electric motor (not shown), respectively. Accordingly, the selectable amount of force applied to engage friction plates  56  with drive plates  58  may be used to transfer a desired portion of the drive torque  15  from one of the drive wheels  22 - 1 ,  22 - 2  to the other. 
     As shown in  FIG. 1 , the vehicle  10  also includes a programmable controller  62  having a readily accessible long-term non-transient memory. The controller  62  is configured or programmed to regulate operation of the eLSD  50  to apportion the drive torque  15  between the first and second drive wheels  22 - 1 ,  22 - 2 . To that end, the controller  62  may be configured to regulate the eLSD  50  such that initially the first and second drive wheels  22 - 1 ,  22 - 2  receive predetermined baseline portions  64  and  66 , respectively, of the drive torque  15 . The baseline portions  64  and  66  of the drive torque  15  to be transferred by the eLSD  50  will typically be preset at 50% for each drive wheel  22 - 1 ,  22 - 2 . The controller  62  is also configured to determine maximum torque capability  68  of each of the first and second drive wheels  22 - 1 ,  22 - 2 . The maximum torque capability of a wheel is herein defined as the maximum amount of engine-generated drive torque  15  that the subject wheel can transfer to the road surface  19  during a particular situation. Additionally, the controller  62  is programmed to identify the wheel that is capable of transmitting a greater portion, i.e., the more capable wheel, and the wheel that is capable of transmitting a lesser portion, i.e., the less capable wheel, of the drive torque  15  to the road surface  19 . 
     The controller  62  is also configured to determine a remaining portion  70  of the drive torque  15  to be transferred to the specific drive wheel  22 - 1  or  22 - 2  that is capable of transmitting the greater portion of the drive torque. The determination of the remaining portion  70  of the drive torque  15  to be transferred to the more capable drive wheel  22 - 1  or  22 - 2  is accomplished by subtracting the determined maximum torque capability  68  of the wheel capable of transmitting the lesser portion of the drive torque from the generated drive torque  15 . Additionally, the controller  62  will regulate the engagement of the friction plates  56  and drive plates  58  in the eLSD clutch  54  to transfer a portion of the drive torque  15  that is equal to the determined maximum torque capability of the more capable wheel  22 - 1  or  22 - 2  if the remaining portion  70  is greater than the determined maximum torque capability  68  of the more capable wheel. On the other hand, if the remaining portion  70  of the drive torque  15  is equal to or less than the determined maximum torque capability  68  of the more capable wheel  22 - 1  or  22 - 2 , the controller  62  is programmed to regulate the eLSD  50  to transfer to the more capable wheel the determined remaining portion  70  of the drive torque  15 . Additionally, the controller  62  may be configured as a central processing unit that is programmed to regulate operation of the power source  14  and the amount of drive torque  15  generated thereby. 
     As shown in  FIG. 1 , the vehicle  10  additionally includes vehicle sensors mounted on the vehicle body  12  and configured to detect in real-time g-forces and changes in orientation of the vehicle relative to the road surface  19 . Generally, the g-forces sensed by such sensors may act on the vehicle  10  as a result of, and, therefore, be indicative of cornering, forward acceleration, and/or braking of the vehicle and the forces generated during such maneuvers. The vehicle  10  may employ a stability control system (not shown) and the subject sensors may be part of that system. The controller  62  is configured to receive signals from the vehicle sensors to determine the torque capability of each of the first and second drive wheels  22 - 1 ,  22 - 2 . Such vehicle sensors may include a lateral acceleration sensor  72  configured to detect as the vehicle  10  moves laterally with respect to the road surface  19 , a longitudinal acceleration sensor  74  that is configured to detect acceleration or deceleration of the vehicle along the centerline of the vehicle labeled as X, and a yaw sensor  76  configured to detect a yaw rate of the vehicle body  12 . 
     In response to the received signals from the sensors  72 ,  74 , and  76 , and as the vehicle  10  performs various maneuvers, the controller determines dynamic weight transfer between the first and second drive wheels  22 - 1 ,  22 - 2 . Such determination of the weight transfer between the first and second drive wheels  22 - 1 ,  22 - 2  in turn permits the controller  62  to determine in real-time the maximum torque capability of each of the first and second drive wheels. Additionally, in response to the determined weight transfer between the first and second drive wheels  22 - 1 ,  22 - 2  the controller is configured to determine loading on each respective tire  23 - 1 ,  23 - 2 , and, in conjunction with the friction coefficient between the subject tires and the road surface  19 , to determine in real-time the maximum tractive effort of the tires  23 - 1 ,  23 - 2 . 
     The determination of the tractive effort of each respective tire  23 - 1 ,  23 - 2  may be determined according to the “friction circle” concept illustrated in  FIG. 2 . The friction circle, a circle of forces, or a traction circle is a concept that is frequently used to analyze and describe the dynamic interaction between a vehicle&#39;s tire and the road surface. Typically, a diagram, such as shown in  FIG. 2 , is generated where a tire is viewed from above so that the road surface lies in the “x-y plane”. In such a diagram, the vehicle that the tire is attached to is generally depicted as moving in the positive “y” direction. In the diagram of  FIG. 2 , the vehicle  10  is shown as cornering to the right, i.e., in the positive “x” direction which points to the center of a corner being negotiated by the vehicle. The tire is rotating in a plane  78  that is at an angle  80  to a direction  82  that the tire is actually moving in. The angle  80  is termed the “slip angle” and accounts for how much the tire slides off the given course that is actually selected by the vehicle&#39;s steering  36  system. 
     A tire can generate a force by the mechanism of slip, which force is represented by a vector  84  in  FIG. 2 . The vector  84  lies in a horizontal plane where the subject tire meets the road surface. When the subject tire rolls freely, with no torque applied thereto by the vehicle&#39;s brakes or power source, the direction of vector  84  is perpendicular to the plane  78 . On the other hand, when torque is being applied to the tire either by the brakes or the power source, the vector  84  will be either at an acute or at an obtuse angle with respect to the plane  78 , respectively. The magnitude of vector  84  is limited by the boundary of a dashed friction circle  85 , but the vector  84  may be any combination or sum of the vector&#39;s component along the x-axis and its component along the y-axis that does not exceed the boundary of the dashed circle  85 . As an additional note, the diagram depicted in  FIG. 2  is an idealized theoretical representation of the friction circle, for a real-world tire, the circle is likely to be closer to an ellipse, with the y-axis being slightly longer than the x-axis. 
     In  FIG. 2 , the tire is shown as generating a force component  86  along the x-axis of the force represented by a vector  84 , which, when transferred by the vehicle&#39;s suspension system in combination with similar forces from the other tires, will cause the vehicle to turn to the right. Additionally, there is also a small component  88  of force in the negative y direction. This represents frictional drag between the tire and the road surface that will, if not countered by some other force, cause the vehicle to decelerate. Frictional drag of this kind is an unavoidable consequence of the mechanism of slip, by which the tire generates lateral force. The diameter of the friction circle  85 , and therefore the maximum horizontal force that the tire can generate, is affected by multiple factors. Such factors may include the design properties of the tire tread and the tire&#39;s inner structure, the tire&#39;s rubber compound, the tire&#39;s condition, for example its age and temperature, quality of the road surface, and the vertical load imposed by the vehicle body on the tire through the suspension system. Accordingly, the tractive effort of a particular tire as determined by the friction circle  84  may change in real-time depending on such factors, and thereby affect the ability of the respective wheel to put the particular portion of the drive torque down to the road surface. 
     During operation of the vehicle  10 , as the vehicle negotiates a turn or a curve, dynamic weight transfer will tend to unload the inside tire  23 - 1  or  23 - 2 , i.e., the tire mounted on the wheel  22 - 1 ,  22 - 2  that is inside or closest to the center of the curve. In response to the inside tire being unloaded and thus experiencing reduced traction capability, the eLSD  50  will be directed to transfer a portion of the drive torque  15  to the outside drive wheel, i.e., the other of the two drive wheels  22 - 1 ,  22 - 2 . Such transfer of a portion of the drive torque  15  to the outside wheel, will permit more of the drive torque to be transmitted to the road surface  19  through the tires  23 - 1  and  23 - 2 , and thus more effectively power the vehicle  10  through the given turn. 
       FIG. 3  represents an example of change in tractive effort of each of the tires  23 - 1 ,  23 - 2  due to dynamic weight transfer, such as during vehicle cornering. As shown in  FIG. 3 , tractive effort of the unloaded tire, in this situation tire  23 - 1 , is decreased, while that of the tire that receives additional load from weight transfer, in this situation tire  23 - 2 , is increased. Such a situation will typically occur when the vehicle  10  is turning to the left and the tire  23 - 1  mounted on the drive wheel  22 - 1  becomes the inside tire with respect to the center of the turn. In  FIG. 3  the dashed circle  85  represents the friction circle of the particular tire in a baseline or statically loaded condition, while the solid circle  89  represents the torque capability of the same tire subject to weight transfer. 
     With resumed reference to  FIG. 1 , the vehicle  10  may additionally include a first wheel speed sensor  90  configured to detect in real-time rotational speed of the first drive wheel  22 - 1  and a second wheel speed sensor  92  configured to detect in real-time rotational speed of the second drive wheel  22 - 2 . The controller  62  may then also be configured to receive the detected rotational speeds from the respective first and second wheel speed sensors  90 ,  92  to generate feed-back control of the eLSD  50  by comparing a desired or preprogrammed difference in speeds of the first and second drive wheels  22 - 1 ,  22 - 2  with actual difference thereof. The desired difference in speeds of the first and second drive wheels  22 - 1 ,  22 - 2  is typically zero when the vehicle  10  is traveling in a straight line, and has an appropriate magnitude for a specific turn such that there is a minimum of tire slip. However, it may also be desirable to employ a specific predetermined speed difference between the drive wheels  22 - 1 ,  22 - 2  to assist with controlling handling of the vehicle  10 , such as via collaboration with the vehicle&#39;s stability control system (not shown). 
       FIG. 4  depicts a method  100  of regulating the eLSD  50  in the vehicle  10  to apportion drive torque  15  from the power source  14  between first and second drive wheels  22 - 1 ,  22 - 2  and transmit the drive torque to the road surface  19 , as described above with respect to  FIGS. 1-3 . The method commences in frame  102  with the vehicle  10  being operated relative to the road surface  19 , and then proceeds to frame  104 . In frame  104 , the method may include identifying via the controller  62  predetermined baseline portions  64  and  66  of the drive torque  15  generated by the power source  14  to be transferred by the eLSD  50  to each of the first and second drive wheels  22 - 1 ,  22 - 2 . From frame  104 , the method advances to frame  106 , where the method includes determining via the controller  62  maximum torque capability of each of the first and second drive wheels  22 - 1 ,  22 - 2  and identifying the wheel that is capable of transmitting a greater portion and the wheel that is capable of transmitting a lesser portion of the drive torque  15  to the road surface  19 . 
     During frame  106 , the method may additionally include detecting in real-time changes in orientation of the vehicle  10  relative to the road surface  19  via at least one of the vehicle sensors  72 ,  74 , and  76  to determine via the controller  62  the weight transfer between the first and second drive wheels  22 - 1 ,  22 - 2 . Accordingly, based on thus determined weight transfer between the first and second drive wheels  22 - 1 ,  22 - 2 , the controller  62  may then determine in real-time the maximum torque capability of each of the first and second drive wheels. As noted above, the vehicle&#39;s orientation may change relative to the road surface  19  in response to variation in drive torque  15  generated by the power source  14 . Accordingly, the loading on each drive wheel  22 - 1 ,  22 - 2  and the resultant tractive effort of each tire  23 - 1 ,  23 - 2  may be determined as a function of change in drive torque  15  during various maneuvers of the vehicle  10 , such as negotiating a turn under power. 
     After frame  106 , the method moves on to frame  108 . In frame  108 , the method includes determining via the controller  62  the remaining portion of the drive torque  15  to be transferred to the drive wheel  22 - 1  or  22 - 2  that is capable of transmitting the greater portion of the drive torque by subtracting the determined maximum torque capability of the wheel capable of transmitting the lesser portion of the drive torque from the generated drive torque. Following frame  108  the method will advance to frame  110 , where the method determines whether the remaining portion of the drive torque  15  is greater than the determined maximum torque capability of the more capable wheel. 
     If in frame  110  it is determined that the remaining portion of the drive torque  15  is greater than the determined maximum torque capability of the more capable wheel, the method proceeds to frame  112 . In frame  112  the method includes regulating the eLSD  50  via the controller  62  to transfer the portion of the drive torque  15  that is equal to the maximum torque capability of the drive wheel  22 - 1  or  22 - 2  that is capable of transmitting the greater portion of the drive torque. On the other hand, if in frame  110  it is determined that the remaining portion of the drive torque  15  is not greater, i.e., is equal to or less, than the determined maximum torque capability of the more capable wheel, the method will proceed from frame  108  to frame  114 . In frame  114 , the method includes regulating the eLSD  50  via the controller  62  to transfer the determined remaining portion  70  of the drive torque  15  to the drive wheel  22 - 1  or  22 - 2  that is capable of transmitting the greater portion of the drive torque. 
     Additionally, following either frame  112  or  114  the method may advance to frame  116 . In frame  116  the method includes receiving via the controller  62  rotational speeds from the respective first and second wheel speed sensors  90 ,  92 . Furthermore, from frame  116  the method may proceed to frame  118 . In frame  118  the method includes generating feed-back control of the eLSD  50  via the controller  62  by determining an actual difference in speeds of the first and second drive wheels  22 - 1 ,  22 - 2  and then comparing a desired speed difference between the drive wheels with the actual speed difference there between. 
     The detailed description and the drawings or figures are supportive and descriptive of the invention, but the scope of the invention is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed invention have been described in detail, various alternative designs and embodiments exist for practicing the invention defined in the appended claims.