Patent Publication Number: US-8527125-B2

Title: System and method for controlling traction

Description:
TECHNICAL FIELD 
     This patent disclosure generally relates to a traction control system and, more particular, to systems and methods for controlling traction during retarding for electric drive machines. 
     BACKGROUND 
     Vehicles having mechanical drive systems typically transmit torque to their drive wheels via gear arrangements, which are commonly known as differentials. A differential typically transfers rotational motion from an input shaft to each of two wheels disposed on both ends of a drive axle. Differentials are typically able to allow two wheels that are connected to a single axle to rotate at different speeds. Conditions requiring such differential motion may occur when the vehicle is turning or when the two wheels are experiencing different traction conditions. A loss of traction may result in a wheel sliding. Electric drive vehicles with rear-wheel drive are susceptible to such loss of traction that may be increased due to electric retarding applied to the rear wheels, which requires relatively higher ground friction than with all-wheel braking systems. 
     Even though differentials are effective in preventing wheel slipping or sliding for vehicles or machines, they are typically absent from vehicles having systems driving each wheel independently from the others, such as, vehicles having electrical or hydrostatic drive systems. Such vehicles typically lack a direct mechanical linkage between drive wheels because each drive wheel is independently powered by a motor that is associated with that wheel. 
     The disclosed systems and methods are directed to overcoming one or more of the problems set forth above. 
     SUMMARY 
     The disclosure describes, in one aspect, a traction control system for a machine having an electric drive configuration. The traction control system includes an electric motor associated with at least one wheel and adapted to provide braking torque to the wheel. The control system further includes a controller configured to determine a rotational speed of the at least one wheel, compare the rotational speed to an allowable slide threshold, and adjust the braking torque to the at least one wheel during retarding if the speed is less than the allowable slide threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an electric drive machine having a control system in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 2  is a flow chart illustrating one embodiment of a method of controlling traction in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 3  is a flow chart illustrating one embodiment of a method of controlling traction during retarding in accordance with an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to systems and methods for controlling traction of an electric drive machine. An exemplary embodiment of a machine  100  is shown schematically in  FIG. 1 . The machine  100  may be an off-highway truck, as shown, or any other vehicle that has an electric drive system, including passenger vehicles, trains, earthmoving machines, and mining vehicles. In an illustrated embodiment, the machine  100  includes an electric drive system  102  operatively coupled to travel mechanisms  104  to propel movement of the machine  100 . 
     The travel mechanism  104  may include wheels and axles on each side of the machine  100 . In the illustrated embodiment, the travel mechanisms  104  include a set of front wheels  105  on each side of the machine  100  and a set of rear dual wheels  106  on each side of the machine  100 . The travel mechanisms  104  allow the machine  100  to travel on the surface of a type of terrain, such as earth surface terrain. The travel mechanisms  104  are shown as wheels, but it is contemplated that the travel mechanisms  104  may be any type of tractive or traction mechanism known, such as, for example, tracks and belts. 
     The electric drive system  102  includes an engine  107 , alternator  108 , rectifier  110 , inverters  112 ,  114 , and motors  116 ,  118 . The engine  107  may provide power for the machine  100  and other machine components. Suitable engines may include gasoline powered and diesel powered engines. In some embodiments, the engine  107  may be a diesel engine that generates and transfers power to other components of the machine  100  through a power transfer mechanism, for example, a shaft (not shown). In the illustrated embodiment, the engine  107  provides power to the alternator  108 . The alternator  108  generates a three-phase alternating current, which produces electrical power. 
     In some embodiments, the rectifier of the electric drive system  102  may convert the three-phase alternating current to a direct current. One or more of the inverters  112 ,  114  convert the direct current to alternating current to power one or more of the electric motors  116 ,  118 . The electric motors  116 ,  118  represent motors that transfer the electric power received from the alternator  108  into power that drives one or more of the travel mechanisms  104 . For example, in some embodiments, the motors  116 ,  118  may be wheel motors used to drive a wheel or wheels to propel the machine  100 . In some embodiments, the rear dual wheels  106  may be independently or directly driven such that each of the motors  116 ,  118  may correspondingly drive each of the driven rear dual wheels  106 . A speed of the motors  116 ,  118  may be controlled by controlling the frequency of the alternating current produced by the inverters  112 ,  114 . 
     In some embodiments, a single motor drives all of the travel mechanisms  104 , while in some embodiments, a plurality of motors drives the travel mechanisms  104 . In the illustrated embodiment, for example, an electric motor  116 ,  118  is associated with each travel mechanism  104  embodied as the rear dual wheels  106 , including a right motor  116  and a left motor  118 . In some embodiments, the engine  107  may be used to power some of the plurality of motors, while a separate electric power source or power storage unit such as a battery (not shown) may be used to power the remaining of the plurality of motors. In some embodiments, the motors  116 ,  118  may be driven directly from the separate electric power source. 
     The engine  107 , alternator  108 , rectifier  110 , inverters  112 ,  114 , and motors  116 ,  118  may be operatively coupled to provide power sufficient to propel the machine  100  in a forward or a reverse driving direction during a driving phase or propel phase of operation. When operating the machine  100  in the driving phase, the motors  116 ,  118  provide a propel torque sufficient to propel the machine  100  in the forward or the reverse driving directions. In some embodiments, the electric drive system  102  may include a final drive (not shown), which includes a planetary gear set connected between the motors  116 ,  118  and the travel mechanisms  104 , to convert the speed of the motors  116 ,  118  into an appropriate magnitude of the propel torque to propel the machine  100  in the forward or reverse driving directions. 
     Further, the electric drive system  102  may dissipate power sufficiently to retard or provide braking to the machine  100  during a retarding phase of operation. During the retarding phase of operation, the inverters  112 ,  114 , motors  116 ,  118 , and a braking chopper  120 , collectively define an electric retarding system  122 . When operating the machine  100  in the retarding phase, the motors  116 ,  118  may provide a braking torque sufficient to cause the machine  100  to slow down and/or come to a complete stop. In some embodiments, the motors  116 ,  118  during retarding may generate alternating current that is converted to direct current by the inverters  112 ,  114  and that flows through the brake chopper  120 , which provides direct current to direct current conversion, and into a braking grid or resistor grid  124 . In the illustrated embodiment, the power that is generated by the motors  116 ,  118  during retarding may be used to power a fan  126  or other appropriate cooling system to reduce a temperature resulting from the heat energy radiating from the braking grid  124 . 
     In some embodiments, the machine  100  may also include a braking system  128  that includes the electric retarding system  122  and one or more service brakes  130 ,  132  for retarding or braking the movement of the machine  100 . In some embodiments, the braking system  128  and the one or more service brakes  130 ,  132  may be associated with corresponding travel mechanisms  104 . In some embodiments, the braking system  128  and the one or more service brakes  130 ,  132  may be associated with the front wheels  105  and/or the rear wheels  106 . In the illustrated embodiment, the braking system  128  includes the electric retarding system  122  and the one or more service brakes  130 ,  132  embodied as a right service brake  130  and a left service brake  132 . The service brakes  130 ,  132  may be hydraulic friction, hydro-mechanical, or mechanical brakes. 
     In some embodiments, all of the braking required to reduce a speed of the machine  100  may be provided by the electric retarding system  122 . In some embodiments, all of the braking required to reduce the speed of the machine  100  may be provided by the service brakes  130 ,  132 . In the illustrated embodiment, if the electric retarding system  122  is not capable of providing all of the braking required, a portion of the braking required to reduce the speed of the machine  100  is provided by the electric retarding system  122  and a portion of the braking required to reduce the speed of the machine  100  is provided by the service brakes  130 ,  132 . 
     The service brakes  130 ,  132  may be manually actuated by an operator, which also allows the operator to manually control the speed of the machine  100 . In some embodiments, the service brakes  130 ,  132  may be mechanically, electro-mechanically, hydraulically, pneumatically, or actuated by other known methods. In the illustrated embodiment, the service brakes  130 ,  132  may be automatically actuated by a control system  134 . In some embodiments, the control system  134  may determine an appropriate ratio of retarding torque splits between, for example, the left and right set of dual wheels  106 , or between the rear wheels  106  and the front wheels  105 . In other words, the portion of braking provided by the electric retarding system  122  may be split between the left and right travel mechanisms  104  and/or between the rear dual wheels  106  and the front wheels  105 . 
     In the illustrated embodiment, the control system  134  may be in communication with the electric drive system  102  through a data link interface  136 . Additionally, or alternatively, the control system  134  may be in communication with the electric drive system  102  and other machine components wirelessly or remotely. In some embodiments, the control system  134  may send a command to the one or more components in response to signals collected and transmitted from one or more sensors. The control system  134  may receive sensor signals directly from the one or more sensors or indirectly such as, for example, from the data link interface  136 . In the illustrated embodiment, the one or more sensors include one or more speed sensors  138  that may measure, collect, and transmit signals to the control system  134  indicative of the speed of the machine  100 . 
     The speed sensors  138  may send speed signals to the control system  134  in response to requests, or the speed sensors  138  may be configured to send speed signals periodically, or in response to a machine event, such as an increase in speed, or a deceleration, and other such events. In some embodiments, the speed sensors  138  may measure a rotational speed of an axle used in the travel mechanisms  104  or other drive train components that are associated with a ground speed (or linear tire speed) of the machine  100 . In some embodiments, the speed sensors  138  may be capable of measuring an actual ground speed or travel speed of the machine  100 . In some embodiments, the speed sensors  138  may be configured or arranged to measure a rotational speed of idling wheels. For example, in the illustrated embodiment, the idling wheels are the front wheels  105 . In some embodiments, the ground/travel speed may be determined by measuring the rotational speed of each idling wheel  105  and calculating the average of the measured speeds. 
     In some embodiments, the speed sensors  138  may be configured or arranged to measure a rotational speed of the powered or driven wheels. For example, in the illustrated embodiment, the powered or driven wheels are the rear dual wheels  106 . The rotational speed may also be representative of a rotating machine RPM. In some embodiments, the speed sensors  138  may be capable of sensing the direction of rotating components associated with the motors  116 ,  118 . For example, the speed sensors  138  may include one or more hall effect sensors (not shown). In some embodiments, the one or more hall effect sensors are associated with each of the right motor  116  and the left motor  118 . 
     In the illustrated embodiment, the control system  134 , which may be configured to perform certain control functions, is operatively connected to the electric drive system  102  through the data link interface  136 . The data link interface  136  may represent one or more interface devices that interconnect one or more data links with the control system  134 . It is contemplated that the data link interface  136  may include other standard data links and may be configured in a manner different from the illustrated embodiment without departing from the teachings of this disclosure. 
     The control system  134  is operatively connected to an operator interface  140  that may include a plurality of operator input devices such as, for example, a steering device  142 , an accelerator pedal or throttle  144 , a shift lever  146 , a retarder lever  148 , and a display  150  for communicating information and commands between the operator and the control system  134 . The steering device  142  may be configured or adapted to control the direction of travel of the machine  100  by controlling, for example, a steering angle of the travel mechanisms  104 . In some embodiments, the steering device  142  may be actuated by electrical, mechanical, or hydraulic power. 
     In the illustrated embodiment, the steering device  142  is hydraulically actuated and may include known hydraulic and/or electrical components that may cause one or more linkages to pivotally move to change a steering angle of the machine  100 . The operator interface  140  may include a steering angle sensor  152  associated with the steering device  142  and adapted or configured to measure the steering angle of the travel mechanisms  104 , and thus, the steering angle of the machine  100 . 
     In some embodiments, the operator interface  140  may include an accelerator pedal position sensor  154  that is associated with the accelerator pedal  144 , which is used to determine a requested engine speed that corresponds to a desired motor power. In some embodiments, the desired motor power may correspond with a depression of the accelerator pedal  144 . The accelerator pedal  144  may be configured to control an acceleration and/or deceleration of the machine  100 . The accelerator pedal position signal may be transmitted from the accelerator pedal position sensor  154  to the other components of the control system  134  to indicate an amount of torque requested by the operator. 
     The control system  134  may control the electric drive system  102  to produce a desired propulsion of the machine  100  in the forward or the reverse driving directions. The control system  134  may manage torque commands for the motors  116 ,  118  by taking into account a number of factors, such as operator requests, current machine speed, engine power availability, machine speed limits, and environment factors, including drivetrain and component temperatures. In some embodiments, the control system  134  may determine a desired torque to transmit to the motors  116 ,  118  based on one or more of the accelerator pedal position signal, a requested gear command signal from the shift lever  146 , a retarder lever position signal, a payload status, and/or speed limits. 
     For example, the operator interface  140  may include a shift lever position sensor  156  associated with the shift lever  146  to detect an operator&#39;s intention to change from one position of the shift lever  146  to another position of the shift lever  146 . The requested gear command signal may represent such gear selections as park, reverse, neutral, drive, or low. The operator may engage the shift lever  146  to control the driving direction of the machine  100 . For example, the shift lever  146  may include at least a drive and a reverse position associated respectively with the forward and reverse driving directions of the machine  100 . 
     The control system  134  may operatively interact with the operator interface  140  and other components to determine the ground speed of the machine  100 . For example, the control system  134  may determine the ground speed of a centerline of the machine  100  based at least in part on the rotational speed of at least one of the idling wheels  105  and the steering angle of the machine  100 . Nevertheless, it is contemplated that any suitable method may be used to determine the travel or ground speed of the machine  100 . 
     In the illustrated embodiment, the control system  134  includes one or more data structure, such as, for example, one or more maps, which may include two dimensional arrays or lookup tables, in memory. The maps may contain data in the form of equations, tables, or graphs. The control system  134  may contain a map that correlates a steering angle value to a slip or slide ratio. The control system  134  may be configured or adapted to calculate a specific slip/slide ratio that corresponds to a specific steering angle, and may further perform this calculation continuously as the steering angle changes during operation. 
     The slip and slide ratios are non-dimensional values indicative of relative speeds between two wheels that are connected to the same axle or that are connected to the machine  100  at opposing sides. For example, the slip ratio may be a ratio of rotational speeds between right and left rear wheels  106  (i.e., rotational speed of the right rear wheel  106  divided by the rotational speed of left rear wheel  106 ), which should be about equal to 1 when no slip/slide is present and the machine  100  is travelling in a straight line. 
     The control system  134  may use the slip/slide ratio map in an algorithm that is adapted to adjust the torque commanded to each individual driven wheel  106 ,  106 . A flowchart for a method of controlling traction by adjusting the torque commanded to each individual driven wheel  106 ,  106  is generally shown at  200  in  FIG. 2 . The control system  134  is arranged for simultaneous control of two driven wheels, each of which is driven by a respective motor  116 ,  118 . The wheels are designated as “right” or “left” to indicate that they are arranged on either side of the machine  100  along a single axle. One can appreciate that the methods disclosed herein are equally applicable for machines having more or fewer than two driven wheels. 
     As shown in  FIG. 2 , the control system  134  receives inputs from various systems of the machine  100 , for example, an input from the steering angle sensor  152 . The control system  134  may also receive inputs from at least one of the speed sensors  138 . Based on these inputs, the control system  134  is adapted to calculate and apply torques to the motors  116  and  118  to control traction of the machine  100 . The control system  134  may use signals from the speed sensors  138  that are associated with each of the driven wheels  106 ,  106  to determine the rotational speeds of each driven wheel  106  individually. A wheel speed of the left driven wheel  106  is determined at  202  based on input from the at least one speed sensor  138 . 
     The control system  134  may use a signal from the steering sensor  152  to account for machine turns in controlling the traction of the machine  100 . In the illustrated embodiment, the control system  134  is adapted or configured to determine a steering angle of the machine at  204  based on a signal from the steering angle sensor  152 . The control system  134  may use signals from the speed sensors  138  that are associated with the non-driven wheels  105 ,  105  to determine the ground speed of the machine  100 . The travel speed or ground speed of the machine is measured at  206 , and the wheel speed of the right driven wheel is determined at  208  based on input from the at least one speed sensor  138 . 
     The control system  134  may determine a normalized or corrected speed or speed ratio for each one of the driven wheels  106 ,  106 . For example, a speed ratio for the left driven wheel, V L,TS , may be calculated at  210  by dividing the wheel speed for the left driven wheel  106 , which was calculated at  202 , by the ground speed of the machine  100 , which was calculated at  206 . In addition, the control system  134  may calculate a speed ratio V R,TS  for the right driven wheel  106  at  212  by dividing the wheel speed for the right driven wheel  106 , which was calculated at  208 , by the ground speed of the machine, which was calculated at  206 . 
     These normalizations or corrections of the drive wheels&#39; speeds should be equal to 1 when the machine speed or ground speed matches the speed of each wheel  106 , that is, when there is no slipping or sliding, and should change to a value above or below 1 when there is slipping or sliding. As can be appreciated, each speed ratio V R,TS  and V L,TS  will increase above 1 when the ground speed of the corresponding wheel  106 ,  106  is greater than the speed of the machine  100 , such as, for example, when that wheel  106 ,  106  is slipping for lack of grip with the ground, and will be less than 1 when the machine is travelling faster than the speed of the corresponding wheel  106 ,  106 , such as, for example, when the wheel  106 ,  106  is becoming stuck or when the wheel  106 ,  106  is sliding during retarding. 
     The control system  134  also receives information indicative of the angle of the steering device  142  via a signal from the steering angle sensor  152 . The steering angle information is input to a table at  214  to determine the expected slip or slide ratio, SR E , or the expected slip or slide that results when the machine  100  is turning and the wheels  106 ,  106  arranged along a single “axle” line are following circular paths that are at different distances from a center point of the turning radius of the machine  100 . In other words, the expected slip ratio SR E  accounts for differences in rotational speed for the wheels  106 ,  106  that are not mechanically linked to each other. 
     When the machine is turning, the steering angle determined at  204  is used to calculate an expected slip or slide ratio SR E  at  214 . The calculation of the expected slip or slide ratio SR E  at  214  may include a lookup table of slip/slide ratio versus steering angle or may be any other type of calculation, such as, for example, a function having the steering angle and slip/slide ratio values as variables. Nevertheless, the expected slip or slide ratio SR E  as well as the speed ratios V L,TS  and V R,TS  are non-dimensional or normalized parameters. Specifically, the expected slip or slide ratio SR E  represents the expected slip or slide or difference in wheel speed that will occur when the machine  100  is turning. 
     The expected slip or slide ratio SR E  is considered as the ratio between the speed of a wheel following an inner path of the turn and the speed of the corresponding wheel following an outer path of the turn. For example, when the machine is turning left, the left driven wheel  106  will follow an inner path that may be curved or circular about a turn center (not shown), while the right driven right wheel  106  will follow an outer path that is disposed at a greater radial distance from the turn center relative to the radial distance of the left driven wheel  106 . 
     The control system  134  uses the expected slip or slide ratio SR E  to perform a second normalization or correction of the speed ratios V R,TS  and V L,TS  to account for steering. For example, when the machine  100  is turning, one or both speed ratios V R,TS  and V L,TS  may change from the base value of 1, even though there may be no slippage due to loss of traction. This change may be the result of the different trajectories followed by the driven wheels  106 ,  106  during the turn. In this situation, the expected slip or slide ratio SR E  can be used to account for the differences in wheel speed that are attributed to the turn, such that the respective speed ratio V R,TS  and V L,TS  for each driven wheel can be adjusted to the base value of 1 during the turn. 
     For example, the speed ratio of the wheel travelling on the inside track during a sharp turn may assume a speed ratio of one-half (½), indicating that the wheel is travelling at half the speed of the machine  100 . The expected slip or slide ratio SR E  that corresponds to the specific turn angle may also be set to one-half (½), such that the ratio between the speed ratio and the expected speed ratio is equal to 1. Hence, the result of each of these normalizations is a corrected speed ratio, which is calculated for each driven wheel  106 . 
     In the illustrated embodiment, a left wheel corrected speed ratio V L,TS,SR  is calculated at  216  by dividing the speed ratio V L,TS  ( 210 ) for the left wheel  106  by the expected slip or slide ratio SR E  ( 214 ). Similarly, a right wheel corrected speed ratio V R,TS,SR  is calculated at  218  by dividing the speed ratio V R,TS  ( 212 ) for the right wheel  106  by the expected slip or slide ratio SR E  ( 214 ). Both the left wheel corrected speed ratio V L,TS,SR  and the right wheel corrected speed ratio V R,TS,SR  represent non-dimensional values that are indicative of slipping or sliding of the machine&#39;s driven wheels  106 ,  106  during either straight line or turning motion of the machine  100 . 
     The corrected slip ratios V R,TS,SR  and V L,TS,SR  are not values of actual slip or slide. Instead, the corrected slip ratios V R,TS,SR  and V L,TS,SR  are non-dimensional slip or slide parameters or ratios that qualify and quantify a slip or slide condition for the driven wheels  106 ,  106  disposed along the same drive axle of the machine  100 . The corrected speed ratios V R,TS,SR  and V L,TS,SR  are inclusive or account for any straight-line motion slip or slide, which may be due to uneven traction, as well as speed differentials in the driven wheels  106 ,  106  that can result from turning. 
     Having determined the corrected speed ratios V R,TS,SR  and V L,TS,SR , the control system  134  compares each to a speed ratio threshold value, T SR . The speed ratio threshold value T SR  may be considered as a threshold slip or slide condition that the machine  100  may tolerate during operation in propulsion or retarding modes of operation. Each corrected speed ratio V L,TS,SR  and V R,TS,SR  is compared to the threshold value T SR  individually such that the slip or slide of each driven wheel  106 ,  106  can be determined separately. The threshold value T SR  can be a constant, non-dimensional parameter, for example, 10 percent (%), which represents the extent of slipping or sliding that can be present in the operation of the machine  100  without requiring intervention by the control system  134  to the torques commanded to each of the driven wheels  106 ,  106 . The threshold T SR  may alternatively be a variable that depends on an operating parameter of the machine, for example, the ground speed of the machine  100 . 
     In the illustrated embodiment, the threshold T SR  is determined at  220  based on the ground speed of the machine  100  ( 206 ) using, for example, a lookup table. The threshold T SR  is compared to each corrected speed ratio V L,TS,SR  and V R,TS,SR  at, respectively,  222  and  224 . Based on the comparisons at  222  and  224 , the control system  134  makes two independent determinations of whether one or both corrected speed ratios V L,TS,SR  and V R,TS,SR  exceed the threshold T SR . When the control system  134  determines that at least one corrected speed ratio V L,TS,SR  and/or V R,TS,SR  has exceeded the threshold T SR , the control system  134  intervenes to adjust the torque being commanded to the wheel that is slipping or sliding, by adjusting the torque being commanded at  226  and/or  228  to the corresponding motor  116  and/or  118 . 
     The control system  134  may operate at a preset frequency or cycle time, for example, at 125 Hz. At each cycle, the control system  134  may compare each of the corrected speed ratios V R,TS,SR  and V L,TS,SR  with the threshold value T SR  to determine whether a slip or slide condition is present and whether the slip or slide condition exceeds the allowable slip or slide threshold for the ground speed of the machine  100 . When one or both of the corrected speed ratios V R,TS,SR  and V L,TS,SR  are determined to be higher than the calculated threshold value T SR , the control system  134  may adjust the torque commanded to the corresponding wheel, for example, by decreasing the torque being commanded to that wheel  106 . 
     This adjustment to the speed of rotation of a corresponding wheel  106 ,  106  purports to bring each corresponding corrected speed ratio to a value that is within the threshold value T SR . In this embodiment, the control system  134  may assume a more active role in reducing slip or slide of the driven wheels  106 ,  106  during operation. The control system  134  continuously calculates a slip or slide ratio error or, alternatively, a difference between each corrected slip or slide ratio, V R,TS,SR  and V L,TS,SR  and the threshold value T SR . Stated differently, the continuously calculated corrected slip or slide ratios V R,TS,SR  and V L,TS,SR  may be considered as “actual” slip or slide ratios that are reflective of a slip or slide condition for each of the driven wheels  106 ,  106 . 
     These actual slip or slide ratios V R,TS,SR  and V L,TS,SR  should always be within an acceptable range, which depends on the threshold value T SR . Here, the control system  134  calculates a difference between each corrected speed ratio V L,TS,SR  and V R,TS,SR  and the threshold T SR  to generate an error. The error may be used to drive a PI controller (not shown) but that is included within, respectively,  226  and  228 . The control system  134  may further include various other sub-routines or power circuits that command a torque to each motor at  230  and  232 . 
     In some embodiments, the control system  134  may include one or more controllers. In some embodiments, the one or more controllers may include one or more control modules (e.g. ECMs, ECUs, etc.). The one or more control modules may include processing units, memory, sensor interfaces, and/or control signal interfaces (for receiving and transmitting signals). The processing unit may represent one or more logic and/or processing components used by the control system  134  to perform certain communications, control, and/or diagnostic functions. For example, the processing unit may be configured to execute routing information among devices within and/or external to the control system  134 . 
     Further, the processing unit may be configured to execute instructions from a storage device, such as memory. The one or more control modules may include a plurality of processing units, such as one or more general purpose processing units and or special purpose units (for example, ASICS, FPGAs, etc.). In some embodiments, functionality of the processing unit may be embodied within an integrated microprocessor or microcontroller, including integrated CPU, memory, and one or more peripherals or in multiple microprocessors or microcontrollers. The memory may represent one or more known systems capable of storing information, including, but not limited to, a random access memory (RAM), a read-only memory (ROM), magnetic and optical storage devices, disks, programmable, erasable components such as erasable programmable read-only memory (EPROM, EEPROM, etc.), and nonvolatile memory such as flash memory. 
     Industrial Applicability 
     The industrial applicability of the systems and methods for controlling traction in an electric drive machine described herein will be readily appreciated from the foregoing discussion. In accordance with certain embodiments, the disclosed control system may be applicable to any machine that has wheels driven independently from each other, for example, a machine having an electric or hydrostatic drive system that uses a motor connected to each wheel. Each of the motors may be operated independently and without mechanical connections with other motors. The disclosed control system may be helpful in situations where one or both of the driven wheels of the machine are slipping or sliding due to, for example, poor traction when the machine is travelling in a straight line, in a forward or reverse driving direction, when the machine is turning, when the machine is operating in a retarding mode, such as, for example, when braking, or any other conditions that cause differential speeds to occur in the driven wheels. 
       FIG. 3  illustrates an exemplary embodiment of the control system  134  and the process ( 300 ) of adjusting an electric motor adapted to provide braking torque to at least one wheel until a speed ratio based in part on the speed of the at least one wheel is equal to an allowable slide threshold during retarding. The control system  134  may determine that machine is in a retarding mode of operation (Step  302 ), such as, for example, when an operator engages the retarder lever  148 . The control system  134  is adapted to determine a ground speed (Step  304 ). In some embodiments, the ground speed is an estimate of the travel speed of the machine  100  made based on speed measurements of traction motors, or on one or more independent ground speed measurements. In the illustrated embodiment, at least one of the speed sensors  138  is associated with the non-driven wheels  105 , and the ground speed measurements are based on the measured speeds of the non-driven wheels  105  combined with the driven wheels  106 , as discussed in detail above. For convenience, all speeds are expressed in relation to traction motor RPM, although it is contemplated that the speeds can be expressed in wheel RPM or machine speed in km/hr. 
     The control system  134  further determines an amount of slide that will be allowable (Step  306 ) before an attempt to correct or adjust is initiated. In the illustrated embodiment, the allowable wheel slide threshold is established for use during retarding and is set approximately below the ground speed of the machine  100 . For example, in some embodiments, the allowable slide threshold may be established as a percentage of ground speed. If, for example, a ten percent (10%) slide may be allowed, and the measure ground speed corresponds to 1000 RPM on the traction motors, than any motor speed down to 900 RPM would be allowed without correction. Thus, any motor speed less than 900 RPM will be below the threshold. 
     The control system  134  determines the motor speed of at least one of the driven wheels  106  (Step  308 ). In some embodiments, the control system  134  may determine the rotational speeds of each of the driven wheels  106 ,  106  to compare to the slide threshold. The actual motor speed (from Step  308 ) is compared (Step  310 ) to the allowable slide threshold (from Step  306 ). If the motor speed is less than the slide threshold (Step  310 ; Yes), the control system  134  is adapted to adjust the torque provided to the motor associated with the wheel until the motor speed is within to the slide threshold. 
     In the illustrated embodiment, control system  134  controls each motor independently so that each wheel speed is separately controlled and each wheel speed is essentially maintained substantially equal to the other. In some embodiments, if the motor speed is less than the slide threshold, the control system  134  is adapted to hold mechanical brake torque constant while modulating the electric motor torque until the wheel speed is within the slide threshold. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.