Patent Publication Number: US-7712559-B2

Title: Front-wheel drive steering compensation method and system

Description:
This application is a continuation of application Ser. No. 10/928,167, filed on Aug. 30, 2004 now U.S. Pat. No. 7,325,636, the content of which is incorporated herein by reference in its entirety. 

   TECHNICAL FIELD 
   This disclosure is directed to a steering compensation method and, more particularly, to a front-wheel drive steering compensation method for a work machine, such as a motor grader. 
   BACKGROUND 
   Typical motor graders have all-wheel drive capability with one or more front motors for driving the front wheels and a separate rear transmission for driving the rear wheels. The transmission and the clutch for the front motors typically include a free-wheel capability, meaning that the front wheels are allowed to rotate at a speed faster than they are being driven by the front motors. Therefore, when the rear motor drives faster than the front motors, the front wheels roll freely and will not drag. This is important because the front wheels provide steering capability to the motor grader, and if the front wheels drag, the motor grader&#39;s ability to turn is limited. 
   Although the front and rear wheels typically rotate at the same speed during straight ahead travel, when turning, one or both of the front wheels are required to rotate faster than the rear wheels. In fact, a full turning motor grader may have front wheel speeds that are up to 50% faster than the rear wheel speeds. When this occurs, the front wheels may rotate faster than the driving front motors, thereby causing free-wheeling. Whenever the front wheels are free-wheeling, they are not providing traction into the turn, which can reduce the steerability of the motor grader, and increase the turning radius. Motor graders having a tandem arrangement of rear wheel sets resist turning more than those with a single rear wheel set. Accordingly, the problems associated with free-wheeling on a motor grader with tandem rear wheel sets may be even greater. 
   One system for driving front wheels on a motor grader is disclosed in U.S. Patent Application Publication No. US 2002/0027025 (the &#39;025 publication) to Kobayashi et al. The &#39;025 publication discloses a system for rotating the front wheels faster than the rear wheels based on the turning radius and the revolution number of the rear wheels. The system measures a front wheel steering angle and an articulation angle, and controls the speed of the front wheels based on the measured factors. However, the &#39;025 publication discloses that a single pump drives both front wheels. Further, the &#39;025 publication discloses an open loop system that cannot compensate one wheel independent of the other to increase traction in a turn. Such a system may result in a loss of traction from at least one front wheel during a turn. 
   This disclosure is directed to a system and method for independently controlling the front wheel speeds of a work machine by independently adjusting the desired front wheel speeds based on a steering angle or a combination of steering and articulation angles, for example. 
   SUMMARY OF THE INVENTION 
   One aspect of the present disclosure is directed to a method of controlling the speed of a right wheel and a left wheel on a work machine. The method includes receiving a speed command based at least partially on an operator input and monitoring at least one wheel steering angle of at least one wheel. A first wheel speed command may be determined based at least partially on the at least one wheel steering angle. In addition, a second wheel speed command may be determined based at least partially on the at least one wheel steering angle. The first wheel speed command and the second wheel speed command may be output to independently control the speed of the right and the left wheels. 
   In another aspect, the present disclosure is directed to a system for controlling the speed of a right wheel and a left wheel on a work machine. The system includes an operator input device and a speed module configured to generate a speed command based at least partially on a signal from the input device. At least one wheel angle sensor is associated with at least one of the right and the left wheels. The wheel angle sensor may be configured to monitor at least one wheel steering angle of at least one of the right and left wheels. A steering compensation module may be configured to determine a first wheel speed command based at least partially on the at least one wheel steering angle and to determine a second wheel speed command based at least partially on the at least one wheel steering angle. The steering compensation module may be configured to output the first wheel speed command and the second wheel speed command to independently control the speed of the right and the left wheels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a pictorial representation of a side view of an exemplary motor grader. 
       FIG. 2  is a pictorial representation of a top view of the exemplary motor grader of  FIG. 1 . 
       FIG. 3  is a block diagram of a control system for controlling the speed of the front wheels of a motor grader to provide traction during a turn. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to exemplary embodiments that are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   An exemplary embodiment of a motor grader  100  is illustrated in  FIGS. 1 and 2 . The motor grader  100  includes a rear frame section  102  and a front frame section  104 . The rear frame section  102  includes a rear frame  106  and an engine in an engine compartment  108 . The engine in the engine compartment  108  is mounted on the rear frame  106  and drives or powers rear wheels  110  on the motor grader  100 . 
   The front frame section  104  includes a front frame  112 , a blade assembly  114 , and an operator cab  116 . The front frame  112  extends from front wheels  118  toward the rear wheels  110 , and supports the operator cab  116 . The operator cab  116  contains the many controls necessary to operate the motor grader  100 . 
   The blade assembly  114  includes a blade  120  and a linkage assembly  122  that allows the blade  120  to be moved to a variety of different positions relative to the motor grader  100 . The linkage assembly  122  includes a drawbar  124 , a right lift cylinder  126 , a left lift cylinder  128  ( FIG. 2 ), a center shift cylinder  130 , and a coupling  132 . 
   The drawbar  124  is mounted to the front frame  112 , and its position is controlled by the right lift cylinder  126 , the left lift cylinder  128 , and the center shift cylinder  130 . The coupling  132  connects the three cylinders  126 ,  128 , and  130  to the front frame  112 . The coupling  132  can be moved during blade repositioning, but is fixed stationary during earthmoving operations. The height of the blade  120  is controlled primarily with the right and left lift cylinders  126 ,  128 . The right and left lift cylinders  126 ,  128  may be controlled independently and, thus, may be used to angle the blade  120  relative to the ground. The center shift cylinder  130  is used primarily to sideshift the drawbar  124  and all the components mounted to the end of the drawbar  124 , relative to the front frame  112 . 
   The drawbar  124  includes a large, flat plate, commonly referred to as a yoke plate  134 , shown in  FIG. 2 . Beneath the yoke plate  134  is a large gear or circle  136  ( FIG. 1 ). The circle  136  may be rotated using methods known in the art to pivot the blade  120  about a blade axis A to establish a blade cutting angle. 
   As best seen in  FIG. 2 , a right articulation cylinder  138  and a left articulation cylinder  140  are respectively mounted to the right and left side of the rear frame  106 . The right and left articulation cylinders  138 ,  140  are used to rotate the rear frame section  102  relative to the front frame section  104  about an articulation axis B shown in  FIG. 1 . The angle of the rear frame  106  relative to the front frame  112  is referred to herein as an articulation angle. In  FIG. 2 , the motor grader  100  is positioned at a zero articulation angle. 
     FIG. 2  also shows that the front wheels  118  include a right front wheel  142  and a left front wheel  144 . The front wheels  118  are configured to turn relative to the front frame  112  to steer the motor grader  100  using known methods. The angle formed between the direction of the front wheels  118  and the front frame  112  is referred to herein as a wheel steering angle. For example, when the wheels are facing directly forward, and the work machine is not articulated, the wheel steering angle is zero. Any pivoting of the wheels  118  relative to the front frame  112  increases the wheel steering angle by the amount that the front wheels  118  are pivoted. 
     FIG. 3  shows a control system  300  for controlling the speed of the front wheels  118  of the motor grader  100  to ensure front wheel traction during a turn. The control system  300  includes a speed control  302 , a steering compensation control  304 , and a wheel driving system  306 . The control system  300  is configured to generate a desired speed ratio and then generate an adjusted speed ratio, based on the desired speed ratio and the steering angle and/or articulation angle. The desired speed ratio is a ratio that is equal to the rear transmission speed ratio multiplied by an aggression factor controlled by an operator. The rear transmission speed ratio is a ratio between the transmission output speed and the transmission input speed. The aggression factor is adjustable factor that controls the desired speed of both front wheels relative to the rear wheel speed. The adjusted speed ratio is a ratio calculated to increase or decrease the front wheel speeds to provide traction during a turn. 
   The speed control  302  is configured to determine a desired speed ratio by multiplying the rear transmission speed ratio by the aggression factor as controlled by an operator. Thus, the operator may control the front wheels to operate at a faster or slower speed than the rear wheels. The speed control  302  includes a transmission  308 , one or more operator input device(s)  310 , and a speed module  312 . The transmission  308  may be any standard transmission and may be associated with the speed module  312  in a manner to send signals to the speed module  312  representative of a transmission speed ratio from the transmission  308 . The transmission speed ratio may be derived from a calculation of the transmission speed output divided by the motor speed input. Accordingly, the transmission speed ratio may differ from gear to gear. 
   The operator input device  310  may be any standard analog or digital input device including a dial, a joystick, a keyboard, a pedal, and/or other input device known in the art, used alone or in combination. In one exemplary embodiment, the operator input device  310  is associated with the speed module  312  to control the front wheel speed relative to the rear wheel speed by adjusting the aggression factor. For example, the operator input device  310  may be configured adjust the aggression factor to allow an operator to control the front wheel speed relative to the rear wheel speed within an allowable range. In one exemplary embodiment, the allowable range allows the operator to select a front wheel speed that is between 90% and 120% of the rear wheel speed. Accordingly, in operation, an operator may use the input device  310  adjust the aggression factor to set the front wheel speed at a percentage of the rear wheel speed, such as, for example, 110%. In this exemplary embodiment, the front wheel speed would therefore be 10% higher than the rear wheel speed. 
   The speed module  312  is associated with transmission  308  and the operator input device  310 , and may be any module capable of performing a calculation and determining the desired speed ratio based on the transmission speed ratio from the transmission  308  and the adjustment factor from the operator input device  310 . In addition, the speed module  312  may consider the current operating gear of the transmission  308  when determining the desired speed ratio. 
   The steering compensation control  304  is configured to adjust the desired speed ratio and output an adjusted speed ratio to control each front wheel to provide sufficient traction at all times during a turn. The steering compensation control  304  may include a wheel angle sensor  314 , a frame sensor  315 , an optional sensor processing  318 , an optional sensor processing  319 , and a steering compensation module  320 . 
   The wheel angle sensor  314  may be one or more sensors associated with one or both of the right and left front wheels  142 ,  144  and may be configured to monitor the right and/or left front wheels  142 ,  144  to determine a wheel steering angle based on signals from the wheel angle sensor  314 . As explained above, the wheel steering angle is the turning angle of the wheel, relative to the front frame  112 , and may be controlled using any standard steering system on the motor grader, such as a steering wheel in the operator cab  116 . In one exemplary embodiment, the wheel angle sensor  314  is configured to monitor the wheel steering angle by monitoring the angles of steering linkages at the front wheels  118 . In another exemplary embodiment, the wheel angle sensor  314  is configured to monitor the wheel steering angle by measuring the extension amount of an actuator, such as a hydraulic actuator, that controls the steering of the front wheels  118 . The wheel angle sensor  314  may be located at any number of different positions where it can monitor the amount of turn of a front wheel, and may be a system to sense movement of the controls, such as movement of the steering wheel in the operators cab  116 , movement of a steering joystick, or at other locations. It should be noted that the wheel angle sensor  314  may be any system configured to determine the steering angle, and may include one or more additional controllers that may process or filter a signal indicative of a wheel steering angle. 
   The frame sensor  315  is one or more sensors configured to monitor the articulation angle at the axis B between the rear frame section  102  and the front frame section  104  of the motor grader  100 . In one exemplary embodiment, the frame sensor  315  is a pivot sensor disposed at the articulation axis B and configured to measure the pivot angle at the articulation axis B. In another exemplary embodiment, the frame sensor  315  is configured to monitor the extension amount of an actuator, such as right and/or left articulation cylinders  138 ,  140 , that may be used to control the amount of articulation between the rear frame section  102  and the front frame section  104  of the motor grader  100 . The wheel angle sensor  314  and the frame sensor  315  could be any type of sensor known in the art, including a potentiometer, an extension sensor, a proximity sensor, and an angle sensor, among others. It should be noted that in some exemplary embodiments, the wheel angle sensor  314  and the frame sensor  315  are directly connected to and monitored by the steering compensation module. In other exemplary embodiments, signals representative of the values determined by the wheel angle sensor  314  and the frame sensor  315  are received from another control module on the work machine and may be transmitted over wire or wireless data links. 
   The steering compensation module  320  may include a processor and a memory device configured to store one or more control routines, which could be software programs, for determining the adjusted speed ratio for each front wheel. The steering compensation module  320  may be associated with the wheel angle sensor  314 , the frame sensor  315 , and the speed module  312 . Based on the wheel steering angle, the frame articulation angle, and/or the desired speed ratio from the speed module  312 , the steering compensation module  320  is configured to calculate an adjusted speed ratio for each front wheel so that the wheels provide traction at all times during a turn. The adjusted speed ratio is a ratio calculated to increase or decrease the front wheel speeds to provide traction during the turn. The adjusted speed ratios for the right and left front wheels  142 ,  144  are referred to herein as a right adjusted speed ratio and a left adjusted speed ratio, respectively. 
   In one exemplary embodiment, the steering compensation module  320  determines the right and left adjusted speed ratios based on a stored look-up table, such as a mapping function. The mapping function for the right front wheel  142  is different than the mapping function for the left front wheel  144 , and may factor in the different turning radius of an inside and outside wheel during a turn. In one exemplary embodiment, the mapping function includes either a linear or a non-linear curve on a two-axis map that may include, for example, a wheel steering angle on one axis and a steering compensation factor on the other axis. The map may use a separate curve for each measurable articulation angle. Then, based on the wheel steering angle and the articulation angle, the steering compensation module  320  uses the map to determine the right and left adjusted speed ratios based on the desired speed ratio received from the speed module  312 . 
   In another exemplary embodiment, the steering compensation module  320  is configured to calculate the right and left adjusted speed ratios using trigonometric equations. In this exemplary embodiment, the steering compensation module  320  determines the adjusted speed ratio necessary to maintain traction during a turn based upon the configuration and/or size of components of the motor grader  100 . For example, the adjusted speed ratio for each wheel may be dependent on the distance between the right and left front wheels  142 ,  144 , the distance between the front and rear wheels  118 ,  110 , and/or any number of other parameters. 
   When a wheel angle sensor  314  is associated with each front wheel, the optional sensor processing  318  may be configured to filter out one sensor signal, while allowing the other to pass to the steering compensation module  304  for processing. Accordingly, in this embodiment, only one signal is considered by the steering compensation module  320  to calculate both the right and left adjusted speed ratios. In one exemplary embodiment, the sensor processing  318  may be configured to filter out the signal from the wheel angle sensor  314  associated with the outside wheel in a turn. Accordingly, in this embodiment, only the signal from the wheel angle sensor  314  associated with the inside wheel is actually considered by the steering compensation module  320 . Therefore, the steering compensation module may include two different mapping functions that process the same signal; one that determines the left adjusted speed ratio and one that determines the right adjusted speed ratio. 
   Optional sensor processing  319  may be associated with the frame sensor  315  and may be configured to filter out one sensor signal, while allowing one or more other signals to pass to the steering compensation module  304  for processing. Accordingly, in the embodiment using the sensor processing  319 , only one signal representative of the articulation angle is considered by the steering compensation module  320  to calculate both the right and left adjusted speed ratios. The sensor signals may be associated with frame sensors  315  associated with each of the right and left articulation cylinders  138 ,  140 . 
   In another exemplary embodiment, the steering compensation module  320  may be configured to calculate the left adjusted speed ratio and the right adjusted speed ratio, based upon signals from the respective wheel angle sensor  314  associated with both of the left and right front wheels. The left adjusted speed ratio and the right adjusted speed ratio may be output from the steering compensation module  320  and provided to the wheel driving system  306 . 
   The wheel driving system  306  is configured to power the front wheels  118  and may include a left hydrostatic transmission (HST) controller  322 , a right HST controller  324 , a left HST  326 , a right HST  328 , and left and right speed sensors  316 ,  317  associated with the left and right front wheels  144 ,  142 . The left and right HST controllers  322 ,  324  may be configured to receive the respective left and right adjusted speed ratios from the steering compensation module  320  and receive signals indicative of the actual speeds of the left and right front wheels  144 ,  142  from the respective left and right speed sensors  316 ,  317 . Further, the left and right HST controllers  322 ,  324  may be configured to process the adjusted speed ratios and measured wheel speeds and output control signals to the left and right HSTs  326 ,  328 , respectively. The left and right HSTs  326 ,  328  each include a pump and a motor, with the pumps and motors being controlled independently by the HST controllers  322 ,  324 . The motors may each be respectively disposed within a hub of the left and right front wheels  142 ,  144 . 
   The left and right speed sensors  316 ,  317  may be associated with the left and right front wheels  144 ,  142 , respectively, and may be configured to measure parameters indicative of the actual wheel speeds or any other rotational speed directly proportional to wheel speed that may be associated with a gear reduction. Signals representative of the wheels speeds may be sent to the left HST controller  322  and a right HST controller  324 , respectively. During straight line travel, the speeds of the front wheels  142 ,  144  will be substantially equal. However, during turning, the speeds of the front wheels  142 ,  144  vary. The left and right speed sensors  316 ,  317  are configured to measure the speed of each respective front wheel independent of the speed of the other front wheel. The sensors  316 ,  317  may be any sensor capable of measuring such information, including a wheel rpm sensor, a wheel speed sensor, or other sensor. In one exemplary embodiment, the actual wheel speed may be measured by measuring the motor speed of the left and right HSTs  326 ,  328 , which is proportional to wheel speed by a front final gear ratio. 
   Further, because the speed of each front wheel is constantly monitored by the left and right speed sensors  316 ,  317 , the left and right HST controllers  322 ,  324  may compare the actual speed of each front wheel to the speed desired by the adjusted speed ratio and make corrections for any differences. Thus, if the actual speed varies from the desired speed, the left and right HST controllers  322 ,  324  may send a signal to speed up or slow down the wheel so that the actual speed corresponds to the speed desired by the adjusted speed ratio. This input from the left and right speed sensors  316 ,  317  provides a closed loop-system for controlling the front wheel speeds. 
   In one exemplary embodiment, the control system  300  may be configured to operate using only front-wheel drive, where the rear wheels roll freely. In this embodiment, the operator input device or devices  310  is configured to operate as a continuously variable transmission control, controlling the hydrostatic transmission drive ratio (HST ratio) to the front wheels of the motor grader  100 . The speed module  312  may be configured to output the desired HST ratio to the steering compensation module  320 , which may determine an adjusted left and right HST ratio so that the right and left front wheels both maintain the same amount of traction and the machine maintains the same ground speed as the rear wheels during a turn. Accordingly, the front wheels may be sped up during a turn to maintain a constant speed at the rear wheels. Alternatively, the front wheels may be controlled during a turn to maintain a constant speed at the front wheels. The steering compensation module may use the mapping functions or trigonometric equations to calculate the adjusted left and right speed ratios based on the wheel steering angle and the frame articulation angle. 
   INDUSTRIAL APPLICABILITY 
   An exemplary method for controlling the speed of the front wheels of a motor grader to provide continuous traction in a turn will now be described. During driving, the speed module  312  monitors and receives a transmission speed ratio signal from the transmission  308 . In addition, the speed module  312  monitors the gear of the transmission. An operator of the motor grader  100  may select the gear and, in addition, may control the operator input device  310  to control the front wheel speed relative to the rear wheel speed. The speed module  312  calculates a desired speed ratio based upon a signal representative of the aggression factor from operator input device  310 , the speed ratio from the transmission  308 , and in some exemplary embodiments, the transmission operating gear. The speed module  312  then outputs the desired speed ratio to the steering compensation control  304 . 
   The steering compensation control  304  determines a front wheel speed that provides sufficient traction at all times during a turn using the desired speed ratio from the speed module  312 , a signal from the wheel angle sensor  314 , and/or a signal from the frame sensor  315 . The signal from the wheel angle sensor  314  is representative of the steering angle of the front wheels  118  of the motor grader  100 . In one exemplary embodiment, a wheel angle sensor  314  is associated with each front wheel. In another exemplary embodiment, a wheel angle sensor  314  is associated with only one front wheel. 
   In the exemplary method described, the wheel angle sensor  314  is associated with each front wheel. A sensor processing  318  monitors the signal from each wheel angle sensor  314  and selects which signal to send to the steering compensation module  320 . The sensor processing  318  filters unselected signals so that they are not considered by the steering compensation module  320 . For example, the sensor processing  318  may allow only the signal of the wheel angle sensor  314  associated with the wheel on the inside of a turn to be received at the steering compensation module  320 . In another exemplary embodiment, the sensor processing allows signals from wheel angle sensors  314  associated with both the left and right front wheels to be received at the steering compensation module  320  for processing. 
   The frame sensor  315  monitors the articulation angle between the rear and front frame sections  102 ,  104  of the motor grader  100 . The frame sensor  315  may do this, for example, by monitoring pivoting at the articulation axis B or may monitor the extension amount of actuators, such as articulation cylinders  138 ,  140 . A sensor processing  319  monitors the signal from each frame sensor  315  and selects which signal to send to the steering compensation module  320 . The steering compensation module  320  receives the selected signal representative of the articulation angle from the frame sensor  315 . 
   The steering compensation module  320  processes the signals from the wheel angle sensor  314  and the frame sensor  315 , along with the desired speed ratio from the speed module  312 , to determine left and right adjusted speed ratios for the left and right front wheels  142 ,  144 . The processing may include using a look-up table, such as a mapping function, to determine the left and right adjusted speed ratios. The mapping function may be based upon the articulation angle with each possible articulation angle having a separate map. Using the wheel steering angle, the articulation angle, and/or the desired speed ratio, the steering compensation module  320  determines and outputs right and left adjusted speed ratios representing front wheel speeds necessary to provide traction in a turn. These may also be calculated using trigonometric equations. 
   Once calculated, the left and right adjusted speed ratios are sent respectively to the left HST controller  322  and the right HST controller  324 . The left and right HST controllers  322 ,  324  convert the adjusted speed ratio signals to control signals to control the respective left and right HSTs  326 ,  328 . The left and right HSTs  326 ,  328  control the speed of the left and right front wheels independent of one another to provide traction during the turn. 
   Left and right speed sensors  316 ,  317  are associated with the left and right front wheels  142 ,  144  and measure parameters indicative of the actual wheel speeds. The left and right speed sensors  316 ,  317  independently electronically communicate a signal indicative of the wheel speeds to the left and right HST controllers  322 ,  324 . The left and right HST controllers  322 ,  324  compare the actual speed of each front wheel  142 ,  144  to the speed desired by the adjusted speed ratio of each front wheel. If the actual speed varies from the desired speed, the left and right HST controllers  322 ,  324  compensate the adjusted speed ratios by controlling the left or right HST so that the actual measured speed corresponds to the desired speed of the adjusted speed ratio. Therefore, the system is a closed loop system, and the left and right HST controllers  322 ,  324  receive signals indicative of the actual speed to ensure the wheel speeds correspond to the desired speed of the adjusted speed ratio for each wheel. 
   In one exemplary embodiment, the adjusted speed ratio for the inside wheel in a turn is less than the adjusted speed ratio for the outside wheel. This compensates for the difference in turning radius between the two front wheels. Therefore, the inside wheel drives slower than the outside wheel so that both wheels provide a similar amount of traction to properly turn the motor grader. In addition, it should be noted that the steering angles may differ between the left and right front wheels during a turn by angling the inside wheel more than the outside wheel, as known in the art. 
   The exemplary work machine  100  in  FIG. 1  is an articulated motor grader. In some instances, the operator may articulate the work machine  100  at a desired angle, and turn the front wheels  118  to a desired steering angle so that the articulated work machine travels in a straight direction. By considering the articulation angle, the steering compensation module  320  is able to recognize that both the right and left front wheels  142 ,  144  should be driven at the same speed. However, in another exemplary embodiment, the motor grader is not articulated. In this embodiment, the steering compensation control  304  calculates left and right adjusted speed ratios based only on the desired speed ratio, and the wheel steering angle. This effectively occurs on an articulated work machine when the articulation angle is zero. 
   Although this disclosure describes providing traction during a turn on a motor grader, the disclosed system may be used on any articulated truck, tractor-scraper, and compactor, for example, that relies upon separately powered front wheels. In addition, it may be used on any wheeled non-articulated truck, including an off-highway truck, a wheel loader, and others using separately powered front wheels. Thus, various work machines having a need for front wheel traction in a turn can be benefited. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed embodiments without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.