Patent Publication Number: US-9428884-B2

Title: Articulation covering complete range of steering angles in automatic articulation feature

Description:
TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to articulated machines such as motor graders and, more particularly, relates to a system and method for automatically controlling articulation of an articulated machine. 
     BACKGROUND OF THE DISCLOSURE 
     An articulated machine, such as a motor grader, is a versatile apparatus for road work, ditch work, site preparation and other surface contouring and finishing tasks. The versatility of a motor grader is provided in large part by its multiple course setting and course change options. In particular, a motor grader typically includes a steering function implemented via steerable ground engaging wheels while also allowing some degree of course correction or steering via lateral arching or articulation of the machine frame. In this manner, for example, a motor grader may be steered and articulated to follow a curve without driving the rear wheels across the area inside the curve and disturbing the just graded area. 
     As should be recognized from the above, motor graders, and other articulated machines, are complex pieces of heavy machinery and are operatively complex. Controlling a motor grader includes numerous hand-operated controls to steer the front wheels, position the blade, control articulation, control auxiliary devices such as rippers and plows, and various displays for monitoring machine conditions and/or functions. Control of a motor grader requires highly skilled and focused operators to position the blade while controlling steering. 
     In order to track front wheels to back wheels, some systems may limit steering angle to the maximum corresponding articulation angle. For example, U.S. Pat. No. 8,548,680 tracks articulation angle to steering angle up to the maximum articulation angle and then limits further changes to steering angle. 
     However, there may be cases when an operator would like additional control of the articulation angle that does not necessarily involve having the rear wheels follow in the front wheel tracks. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the present disclosure, in a motor grader, or other articulated machine, that has steerable front wheels attached to a front frame with the front frame and a rear frame pivotally connected at an articulation joint a method of managing articulation includes determining, at a controller, a steering angle of the front wheels. The method also includes adjusting, via the controller, an articulation angle at the articulation joint at a first ratio of more than one degree of the steering angle to one degree of articulation angle. 
     In another aspect of the disclosure, a method of managing articulation in a motor grader, or other articulated machine includes receiving, via a steering angle sensor, a steering angle of front wheels of the motor grader and setting, via a controller, an articulation angle between the front frame and the rear frame corresponding to a first ratio that is greater than 1:1 ratio steering angle to articulation angle when the steering angle is in a first range. The method may also include setting, via the controller, the articulation angle corresponding to a second ratio that is less than the first ratio and greater than zero when the steering angle is in a second range. 
     In yet another aspect of the disclosure, a system for adjusting an articulation angle in a motor grader or other articulated machine, includes a front frame and a rear frame pivotally connected at an articulation joint with front wheels attached to the front frame that are steerable over a steering angle range. The system may include a steering angle sensor that reports a steering angle of the front wheels and an articulation actuator that adjusts the articulation angle between the front frame and the rear frame over an articulation angle range. The steering angle range being greater than the articulation angle range. The system may also include a controller coupled to the steering sensor and the articulation actuator. The controller may use a transfer function to determine mapping of the first range of steering angle to the second range of articulation angle. The controller may define a first sub-range of the steering angle range and a second sub-range of the steering angle range, the first and second sub-ranges containing no common steering angle values. A first portion of the transfer function with a first non-zero slope maps the first sub-range to a first sub-range of articulation angle and a second portion of the transfer function with a second non-zero slope maps a second sub-range of steering angle to a second sub-range of articulation angle with a second non-zero slope. The first and second non-zero slopes having different slope values. 
     In yet another aspect of the disclosure, a method of managing a relationship between a steering angle of front steerable wheels of an articulated machine and an articulation angle between a front frame and a rear frame in the articulated machine includes receiving an operator-initiated signal indicating a desired steering path. Following receipt of the signal, a relationship between the steering angle of the steerable front wheels and the articulation angle may be adjusted according to a ratio of more than one degree of steering angle to one degree of articulation angle. 
     Other features and advantages of the disclosed systems and principles will become apparent from reading the following detailed disclosure in conjunction with the included drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a motor grader constructed in accordance with the present disclosure; 
         FIG. 2  is a top view of the motor grader of  FIG. 1 ; 
         FIG. 3  is a schematic top view of a motor grader during an automatic articulation mode of operation in accordance with the present disclosure; 
         FIG. 4  is a block diagram of an exemplary steering control system in accordance with the present disclosure; and 
         FIG. 5  is a simplified illustration of a cab control for an automatic articulation control function; 
         FIG. 6  is a flow chart depicting a process of implementing an automatic articulation control function; 
         FIGS. 7-10  are exemplary transfer functions related to full range and non-linear articulation to steering angle control; and 
         FIGS. 11-12  are prior art transfer functions related to partial range steering control angle. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The present disclosure provides a system and method that uses an automatic articulation system for enabling an operator of a motor grader or other articulated machine to optimize use of the articulation capabilities of the articulated machine. In particular, the system and method may employ a controller that prevents automatic articulation control functions from starting or disables automatic articulation control when any of a number of conditions are detected. In addition, some automatic articulation control features may allow limited operation to correct an undesired articulation angle prior to disabling or suspending operation. As disclosed below, the automatic articulation control functions improve both safety and ease of use as well as providing additional articulation tracking modes. 
       FIG. 1  is a schematic side view of an articulated machine  10 , specifically, a motor grader in accordance with an embodiment of the present disclosure. The articulated machine  10  includes a front frame  12 , rear frame  14 , and a work implement  16 , e.g., a blade assembly  18 , also referred to as a drawbar-circle-moldboard assembly (DCM). The rear frame  14  includes a power source (not shown), contained within a rear compartment  20 , that is operatively coupled through a transmission (not shown) to rear traction devices or wheels  22  for primary machine propulsion. 
     As shown, the rear wheels  22  are operatively supported on tandem axels  24  which are pivotally connected to the machine between the rear wheels  22  on each side of the articulated machine  10 . The power source may be, for example, a diesel engine, a gasoline engine, a natural gas engine, or any other engine known in the art. The power source may also be an electric motor linked to a fuel cell, capacitive storage device, battery, or another source of power known in the art. The transmission may be a mechanical transmission, hydraulic transmission, or any other transmission type known in the art. The transmission may be operable to produce multiple output speed ratios (or a continuously variable speed ratio) between the power source and driven traction devices. 
     The front frame  12  typically supports an operator station  26  that contains operator controls  106 , along with a variety of displays or indicators used to convey information to the operator, for primary operation of the articulated machine  10 . The front frame  12  may also include a beam  28  that supports the blade assembly  18  and which is employed to move the blade  30  to a wide range of positions relative to the articulated machine  10 . The blade assembly  18  includes a drawbar  32  pivotally mounted to a first end  34  of the beam  28  via a ball joint (not shown) or the like. The position of the drawbar  32  is typically controlled by hydraulic cylinders: a right lift cylinder  36  and left lift cylinder  38  ( FIG. 2 ) that control vertical movement, and a center shift cylinder  40  that controls horizontal movement. The right and left lift cylinders  36 ,  38  are connected to a coupling  70  that includes lift arms  72  pivotally connected to the beam  28  for rotation about axis C. A bottom portion of the coupling  70  may have an adjustable length horizontal member  74  that is connected to the center shift cylinder  40 . 
     The drawbar  32  may include a large, flat plate, commonly referred to as a yoke plate  42 . Beneath the yoke plate  42  is a circular gear arrangement and mount, commonly referred to as the circle  44 . The circle  44  is rotated by, for example, a hydraulic motor referred to as the circle drive  46 . Rotation of the circle  44  by the circle drive  46  rotates the attached blade  30  about an axis ‘A’ perpendicular to a plane of the drawbar yoke plate  42 . The blade cutting angle is defined as the angle of the work implement  16  relative to a longitudinal axis  48  of the front frame  12 . For example, at a zero degree blade cutting angle, the blade  30  is aligned at a right angle to the longitudinal axis  48  of the front frame  12  and beam  28  ( FIG. 2 ). 
     The blade  30  is also mounted to the circle  44  via a pivot assembly  50  that allows for tilting of the blade  30  relative to the circle  44 . A blade tip cylinder  52  is used to tilt the blade  30  forward or rearward. In other words, the blade tip cylinder  52  is used to tip or tilt a top edge  54  relative to the bottom cutting edge  56  of the blade  30 , which is commonly referred to as a blade tip. The blade  30  is also mounted to a sliding joint associated with the circle  44  that allows the blade  30  to be slid or shifted from side-to-side relative to the circle  44 . The side-to-side shift is commonly referred to as blade side shift. A side shift cylinder (not shown) or the like is used to control the blade side shift. 
     Motor grader steering is accomplished through a combination of both front wheel steering and machine articulation. As shown in  FIG. 2 , steerable traction devices, such as right and left wheels  58 ,  60 , are associated with the first end  34  of the beam  28 . Wheels  58 ,  60  may be both rotatable and tiltable for use during steering and leveling of a work surface  86  ( FIG. 1 ). Front wheels  58 ,  60  are connected via a steering apparatus  88  that may include a linkage  90  and a hydraulic cylinder (not shown) for rotation at front wheel pivot points  80 ,  FIG. 3 , and tilt cylinders  92  for front wheel tilt. Front steerable wheels  58 ,  60  and/or rear driven traction devices  22 , may include tracks, belts, or other traction devices as an alternative to wheels as is known in the art. The front wheels  58 ,  60  may also be driven, as is the case in motor graders provided with all wheel drive. For example, the power source may be operatively connected to a hydraulic pump (not shown) fluidly coupled to one or more hydraulic motors (not shown) associated with the front wheels  58 ,  60 . 
     Referring to  FIGS. 1 and 3 , the articulated machine  10  includes an articulation joint  62  that pivotally connects front frame  12  and rear frame  14 . Both a right articulation cylinder  64  and left articulation cylinder  66  ( FIG. 3 ) are connected between the front frame  12  and rear frame  14  on opposing sides of the articulated machine  10 . The right and left articulation cylinders  64 ,  66  are used to pivot the front frame  12  relative to the rear frame  14  about an articulation axis B ( FIG. 1 ). In  FIG. 2 , the articulated machine  10  is positioned in the neutral or zero articulation angle position with the longitudinal axis  48  of the front frame  12  aligned with a longitudinal axis  68  of the rear frame  14 . 
       FIG. 3  is a schematic top view of a articulated machine  10  with the front frame  12  rotated at an articulation angle a defined by the intersection of longitudinal axis  48  of front frame  12  and longitudinal axis  68  of the rear frame  14 , the intersection corresponding with the position of articulation joint  62 . In this illustration a positive α is indicative of a left articulation from the perspective of an operator facing forward, while a negative α (not shown) would be indicative of a right articulation. A front wheel steering angle θ is defined between a longitudinal axis  76  parallel to the longitudinal axis  48  of front frame  12 , and a longitudinal axis  78  of the front wheels  58 ,  60 , the angle θ having an origin at a pivot point  80  of the front wheels  58 ,  60 . This is demonstrated in connection with right front wheel  58 , but equally applies to left front wheel  60 . As with articulation angle, a positive θ is defined as the front wheels  58 , 60  being to the left of longitudinal axis  76  and a negative θ defined as the front wheels  58 ,  60  being to the right of longitudinal axis  76 . 
     In in the interest of brevity and minimization of any risk of obscuring the principles and concepts in accordance to the present disclosure, unless other specified, the description of steering angles and articulation angles are assumed to be magnitudes with absolute values. That is, a range of 0 degrees to 20 degrees of articulation refers to 0 degrees to +20 degrees and 0 degrees to −20 degrees. Similarly, a range of steering angle of 0 degrees to 12 degrees refers to a range including both 0 degrees to +12 degrees and 0 degrees to −12 degrees. Another steering angle range of +12 degrees to 50 degrees includes steering angles of +12 degrees to +50 degrees and −12 degrees to −50 degrees. Lastly, steering angle ranges and articulation angle ranges are assumed to have non-overlapping values, so that when one range is, for example, a steering angle of 0 degrees to +12 degrees and another range is +12 degrees to +50 degrees, one value is either slightly higher or lower than exactly 12 degrees. 
     With reference now to  FIG. 4 , a block diagram of an exemplary steering control system  100  in accordance with an embodiment of the disclosure is provided. The control system  100  generally includes an electronic controller  102  configured, for example, via a control algorithm, to receive a plurality of instructions from various sensors and/or operator commands, and to responsively provide instructions to control various machine actuators and/or communicate with the machine operator. Controller  102  may include various components for executing software instructions designed to regulate various subsystems of the articulated machine  10 . For example, the controller  102  may include a processor  103 , a memory  105 , that may include a random access memory (RAM) and a read-only memory (ROM). The memory  105  may also include a mass storage device, data memory, and/or on a removable storage medium such as a CD, DVD, and/or flash memory device, but does not include propagated media such as a carrier wave. The controller  102  may execute machine readable instructions stored in the memory  105 . The controller  102  may also include input/output hardware  107  coupled to various sensors and output devices described below. 
     The control system  100  may be configured to control machine articulation for machine articulation based upon operator control of the front wheel steering. Accordingly, the controller  102  may be configured to receive an indication of the front wheel steering angle θ. In some examples, the articulated machine  10  includes one or more steering angle sensors  104  that may be associated with one or both of the right and left front wheels  58 ,  60 . In some such examples, the steering angle sensor  104  is configured to monitor the wheel steering angle θ by monitoring angles of rotation of steering linkages and/or pivot points at the front wheels. 
     The steering angle sensors  104  may be configured to monitor the wheel steering angle by measuring the extension amount of an actuator (not shown), such as a hydraulic actuator, that controls the steering of front wheels  58 ,  60 . Other sensor configurations are well known in the art. The steering angle sensors  104  may provide data “indicative of” the steering angle, which should be understood to mean direct measurements of the quantity or characteristic of interest, as well as indirect measurements, for example of a different quantity or characteristic having known relationships with the quantity or characteristic of interest. 
     The controller  102  may be configured to receive a signal from one or more operator steering controls  106  that may be employed to provide an indication of steering angle θ. These controls  106  may be, for example, a steering wheel  106  as shown in  FIGS. 1-2 , or any other type of operator input device, such as a dial, joystick, keyboard, pedal or other devices known in the art. In one embodiment, for example, a steering wheel sensor may be provided that senses the rotation or position of the steering wheel  106  to provide an indication of steering angle θ. Whether received via the steering angle sensors  104  or operator steering controls  106 , a steering signal may be generated that is used in the controller to determine a steering angle of the front wheels  58 ,  60 . 
     One or more articulation sensors  108  may be employed to provide an indication of the articulation angle a at the axis B between the rear frame  14  and front frame  12 . In some examples, the articulation sensor  108  is a pivot sensor disposed at articulation joint  62  to sense rotation at articulation axis B. Additionally or alternatively, the articulation sensor  108  may be configured to monitor the extension of right and/or left articulation cylinders  64 ,  66 . Steering angle sensors  104  and articulation sensors  108  could be any type of sensor known in the art, including, for example, potentiometers, extension sensors, proximity sensors, angle sensors and the like. 
     Other inputs that may be associated with the control system  100  may include instructions provided from a mode selector  110  disposed, for example, in operator station  26 . The mode selector  110  may include a slider  214  for selecting an operating mode and a dial  216  for selecting a steering angle to articulation angle mode, discussed more below. The operating mode may be employed to select among various modes of operation including, for example, a manual mode, or one or more automatic modes. Other input mechanisms and selections may also be used. 
     Addition inputs may include machine speed sensors  112  and transmission sensors  114  located, for example, in rear compartment  20 . Machine speed sensors  112  may be any sensor configured to monitor machine travel speed, for example, sensors associated with any of the front wheels, rear wheels, axle shafts, motors, or other components of the drive train. A transmission sensor  114  may be associated with the transmission to provide an indication of a current gear or output ratio. Alternatively, an indication of current gear or output ratio may be provided by data associated with operator controls for the transmission (not shown). 
     The control system  100  may also include outputs that affect operation of the articulated machine  10 . Power steering instructions  118  may be provided to control steering actuators  120 . Articulation actuators  64 ,  66  may be controlled by articulation control instructions  122  that may result, depending on operating mode, from either operator input via articulation controls  116  or developed automatically at controller  102 . A state or mode of operation of the automated articulation function may be transmitted to an operator via communication instructions  124  and display panel  126 . 
     Referring to  FIG. 5 , a cab control  125  for the automatic articulation control function may include a mode selector  110 , discussed above, and a display panel  126 . The display panel  126  may include an “On” indicator  202  and an “Inactive” indicator  204 . The Inactive indicator  204  may generally indicate that one or more conditions is preventing activation of the automatic articulation control function. In the illustrated embodiment, more than one indicator may be used to represent some of the different operating states separately, such as “Armed”  206 , “Center-Only”  208  and “Error Condition”  210 . Under some conditions, more than one display may be active at a time, such as ‘Error Condition’  210  and ‘Inactive’  204 . In some embodiments, a message area  212  may provide more detailed information to an operator or may provide instructions for the operator on steps to take should the automatic articulation control function be in one of the indicated states. In different embodiments, other indicators and combinations of display technology can used to receive mode selections and to convey information about the state of the automatic articulation control function, related messages, and help information. These display technologies may include touch screens, voice recognition, etc. 
     INDUSTRIAL APPLICABILITY 
       FIG. 6  illustrates an exemplary control process  300  for managing automatic articulation behavior. The control process may be executed by a processor  103  of the controller  102  using computer-executable instructions stored in the memory  105 . 
     At a block  302 , the position of the mode selector  110  may be determined through known mechanisms. If the mode selector is in one of the automatic mode selection positions execution may continue to block  304 . If the speed of the articulated machine  10  is below a threshold speed, for example, less than one or two miles per hour including stopped, the ‘yes’ branch may be followed to a block  306 . 
     Generally, at block  306 , when the mode selector  110  is set to an automatic operation setting, a determination is made if the articulated machine  10  is in a state where the operator may not be aware of the automatic articulation mode setting. Without a safety override, automatic articulation may occur without the operator expecting it. Changes in articulation angle alter the steering pattern of the vehicle, so if such changes occur when an operator does not expect it, the articulated machine  10  may turn differently than anticipated and could result in an accident. 
     So, at block  306 , conditions are checked that indicate an operator may not be aware that the mode selector  110  is set to on. These conditions may include a machine key cycle or machine power cycle. That is, that when the engine was last started, the mode selector  110  was set to an “ON” position. Another exemplary condition may be when a seat or cab sensor indicates that no operator is present when the mode selector  110  is set to an on position. In these cases, an operator may not be aware of a pre-existing automatic articulation mode selection. 
     If any of these exemplary conditions, or other similar conditions are identified, the ‘yes’ branch from block  306  may be followed to block  308 . At block  308 , the automatic articulation control function may be disabled or inhibited. This does not represent, necessarily, an error condition, and may simply require that the operator toggle the mode selector  110  to “OFF” and back to “ON”, as indicated at block  310 . An appropriate indicator may be activated at the display panel  126 . After toggling the mode selector  110 , execution may return to block  304 . In an embodiment, these conditions can disable the automatic articulation control function independently of groundspeed. That is, alternate control processes may evaluate these factors independently from groundspeed checking or in parallel with groundspeed checking 
     If, at block  304 , the groundspeed of the articulated machine  10  is below the threshold speed, the ‘yes’ branch may be taken, as before, to block  306 . At block  306 , if the conditions associated with inhibiting operation of the automatic articulation control function are clear, the ‘no’ branch from block  306  may be taken to block  312 . 
     At block  312 , a check may be made to determine if the transmission is in neutral, for example using transmission sensor  114 . Because the transmission is in neutral and groundspeed is at or near zero, an assumption may be made that the vehicle is parked or stopped. However, even while stopped an operator or maintenance person may turn the front wheels  58 ,  60  to check for tire condition or to access a mechanical component for inspection or maintenance. Similar to the conditions above, should an operator turn the front wheels  58 ,  60  while the vehicle is not moving, and if there were no check for this condition, the articulated machine  10  may change its articulation angle causing the front frame  12 , rear frame  14 , or both to move. Such unexpected movement could injure personnel or damage nearby equipment or buildings. Therefore, if the transmission is in neutral, the ‘yes’ branch from block  312  may be taken to block  326  and the automatic articulation control function may be disabled or inhibited. Once disabled, the automatic articulation control function may be re-activated according to the steps beginning at block  302 . An appropriate indicator may be activated at the display panel  126 . 
     Returning to block  312 , if the speed of the articulated machine  10  is below the threshold speed and none of the inhibit conditions are present, and if the transmission is not in neutral an assumption may be made that the articulated machine  10  is operating normally and execution may continue at block  314  or, in an embodiment block  318  (not specifically depicted). For example, the articulated machine  10  may be operating on a steep uphill grade or may be scraping a heavy or difficult work surface  86  that causes the machine  10  to come to a momentary halt or at least to slow below the threshold speed. In this situation, if the automatic articulation control function were automatically disabled it could create an inconvenience at the least and at the most cause a safety hazard should the operator be expecting automatic articulation when unbeknownst to him or her it had been disabled. The decision point at block  312  allows automatic articulation control to continue functioning in this situation for both safety and convenience reasons. Note that this condition is normally only be reached when the automatic articulation control function is already active and operating. 
     Returning to block  304 , if the groundspeed of the articulated machine  10  is greater than the threshold speed the ‘no’ branch may be taken from block  304  and execution may continue at block  314 . At block  314 , an actual articulation angle and a desired articulation angle may be compared. A value of actual articulation angle may be determined directly or indirectly from data received via articulation sensors  108 . Desired articulation angle may be calculated based on inputs from steering controls  106  and/or steering angle sensors  104 , that is, by evaluating a current steering angle to determine the desired articulation angle. In some embodiments, the current steering angle to desired articulation angle may be a function of a nonlinear transfer function or some other mapping algorithm, as described in more detail below. 
     Any of several conditions may contribute to an actual articulation angle not being equal to a desired articulation angle. In one case, an operator may engage the automatic articulation control function, e.g. at block  302 , when the current steering angle dictates a desired articulation angle that is simply not equal to the current articulation angle. For example, the articulated machine may be in alignment with zero articulation angle and the steering wheels oriented in a 35 degree left turn. In another example discussed more below, an error condition that has caused the automatic articulation control function to temporarily be disabled may clear but the actual and desired articulation angles may have diverged during the time when the automatic articulation control function was disabled. Activation of the automated articulation in these cases could cause a sudden and dramatic change in articulation angle and could cause changes to steering that may be difficult or impossible for an operator to control. In a worst-case scenario, if articulation were at −20 degrees and steering were at +45 degrees, activation of the automatic articulation control function would cause the rear frame  14  to rapidly move a full 40 degrees. 
     In some prior embodiments of automatic articulation control an operator might be required to manually observe the steering angle of the front wheels  58 ,  60  as well as the current articulation angle and attempt to activate the automatic articulation control function at an exact time when the two angles appear to be in alignment. This was found to be both difficult and a significant distraction to an operator. 
     At block  314 , when the actual and desired articulation angles are not equal, or within a threshold angle range such as 0.2 degrees to −0.2 degrees the ‘no’ branch may be taken to block  320 . 
     At block  320 , the automatic articulation control function may be armed, that is, in a standby state so that the electronic controller  102  can monitor the actual and desired articulation angles and when they are within the threshold angle range engage the automatic articulation control function. This relieves the operator of the need to manually observe and time activation of the function but preserves the desirable characteristic of avoiding a rapid and significant change in articulation angle of the articulated machine  10  when engaging the automatic articulation control function. 
     When at block  314  the actual and desired articulation angles are at zero or are within the threshold angle range, the ‘yes’ branch may be taken to block  316 . At block  316 , the controller  102  may screen for any of several error conditions including but not limited to a groundspeed of the articulated machine  10  being over a limit, an invalid signal or input signal such as no groundspeed signal being available at the controller  102 , or other errors such as steering sensor errors. In an embodiment, the maximum limit for groundspeed or threshold groundspeed may be about 20 mph, but may vary for different kinds of articulated machines  10  or even for different operation conditions. When no error conditions are found, execution may follow the ‘no’ branch to block  318 . 
     At block  318 , the automatic articulation control function may be activated and control articulation of the articulated machine  10  according to whatever control strategy is active. More details on different control strategies are discussed below with respect to  FIGS. 7 through 10 . 
     In an exemplary embodiment, the articulated machine  10  may be operated at an articulation angle α with a magnitude greater than zero between the front frame  12  and the rear frame  14  responsive to instructions from the electronic controller  102 . For example, from a fully aligned position when the front wheels  58 ,  60  turn to the left (+θ) the electronic controller  102  may cause the articulated machine  10  to articulate to the left, designated as a positive articulation angle or +α. Similarly, from a fully aligned position when the front wheels  58 ,  60  turn to the right (−θ) the electronic controller  102  may cause the articulated machine  10  to articulate to the right designated as a negative articulation angle or a −α. Whether articulated to the left or to the right, a magnitude of the angle α is non-zero whenever the front frame  12  and rear frame  14  are not aligned. 
     Returning to block  316 , when an error condition is present the ‘yes’ branch from block  316  may be taken to block  322 . At block  322 , a determination is made as to whether the articulated machine  10  is in an articulated state with a non-zero articulation angle, that is, with a magnitude outside a threshold angle range discussed above. 
     If at block  322 , the articulated machine  10  is not articulated execution may take the ‘no’ branch to block  326  and the automatic articulation control function may be disabled. An appropriate indicator may be activated at the display panel  126 . 
     If, at block  322 , the articulated machine  10  has some nonzero angle of articulation, were the automatic articulation control function simply disabled, the articulated machine  10  may be fixed at some angle of articulation that is counterproductive to future steering angle settings. For example, if an articulated machine  10  is in a left-hand turn with a steering angle of +20 degrees and a corresponding articulation angle of +20 degrees at which time the ground speed increases above a groundspeed limit, simply disabling the automatic articulation control function would cause the articulation angle to remain at a +20 degree angle of articulation even though a steering angle may change to the right through zero or beyond. This would create an awkward situation where the machine is articulated to the left and the steering is articulated to the right, causing the articulated machine  10  to “crab” along the work surface. 
     To avoid this situation, execution may continue at block  324 . At block  324 , the automatic articulation control function may be configured to operate in a return-to-zero mode so that it responds only to steering commands that would cause the articulation angle to return to zero. That is, any detected steering angle that would increase the desired articulation angle is ignored and any detected steering angle that causes the desired articulation angle to decrease is processed. When the articulation angle decreases to zero, or is within the minimum threshold angle range, execution continues at block  326  and the automatic articulation control function is disabled. 
     When, at block  316 , the error condition has cleared and the actual and desired articulation angles are approximately equal at block  314  the automatic articulation control function may be reactivated and normal operation continued at block  318 . 
     The exemplary control process  300  of  FIG. 6  is but one representation of steps that may be followed to implement the controls and features disclosed. A person of ordinary skill in the art would recognize that other implementations could be developed that implement the safety and control functions discussed above. For example, various inhibit and error conditions could drive interrupts so that automatic articulation control could be performed via a state change paradigm. 
       FIGS. 7-10  illustrate exemplary transfer functions related to full range and non-linear steering angle to articulation angle control. These figures illustrate an exemplary embodiment where steering angle can range from about −50 degrees to +50 degrees and articulation angle can range from about −20 degrees to +20 degrees. Other articulation machines may have different steering and articulation angle ranges. The principles disclosed here apply to those different ranges as well. 
     The articulation angle transfer functions described below allow an operator a rich selection of operating modes for automatic articulation control. In various embodiments, the dial  216  portion of the mode selector  110  may be modified to allow individual selection of these examples or other similar transfer functions for use in various operating environments. 
       FIG. 7  illustrates a non-linear transfer function  400  of steering angle on the x-axis to articulation angle on the y-axis. The transfer function  400  shows a first range  402  with a first slope of about 1 or steering angle to articulation angle ratio of approximately 1:1 from about—10 degrees to about +10 degrees. The transfer function  400  shows a second range  406  below about −10 degrees and above  404  about +10 degrees that has a second slope of about ¼ or about 1 degree of articulation angle to 4 degrees of steering angle. 
     In an embodiment, rather than using a fixed steering angle degree to transition from a first range  402  to a second range  404 ,  406 , the controller  102  may use a threshold percentage of steering angle, such as a range of about 45% to 55% of maximum steering angle. The first and second ranges  402 ,  404 - 406  constituting sub-ranges of the full steering angle range. Each sub-range has steering angle values that are unique, that is not in common with other sub-ranges. 
     In contrast to the prior art transfer functions  500  of  FIG. 11 or 510  of  FIG. 12 , that have a 1:1 correspondence of steering angle to articulation angle and then cap either articulation or steering, the transfer function  400  provides at least some change in articulation over the full range of steering angles. The approximately 1:1 ratio in the first range of  FIG. 4  may allow the rear wheels to track the front wheels of the articulated machine  10 , for example when grading or scraping around a curve, such as a cul-de-sac. The second range  404 ,  406  above and below about 10 degrees of steering angle allows continuous increases in articulation angle over the full remaining steering angle, allowing the operator to significantly improve turning radius, when desired. 
       FIG. 8  illustrates an alternate embodiment of  FIG. 7 , showing a transfer function  408  with a first range  410  having a first ratio and a second range  412 ,  414  having a second, lower ratio of steering angle to articulation angle. The embodiment of  FIG. 8  shows ‘porch’ regions  416 ,  418  that illustrate an embodiment where the final few degrees of steering angle do not change the articulation angle. The principal of  FIG. 7  is maintained in that the separate transfer function slopes of the first and second ranges allow a front-to-back wheel tracking region (range  410 ) and a greater than 1:1 region (range  412 ,  414 ). 
       FIG. 9  illustrates an alternate embodiment from  FIGS. 7 and 8  and illustrates a transfer function  420  with one region  422  having a constant slope with a rate of more than one degree of steering angle to one degree of articulation angle. The transfer function  420  offers consistent steering to articulation changes over the full range of steering angles and offers an operator a predicable rate of change of articulation, but does not necessarily provide front wheel to back wheel tracking. 
       FIG. 10  is an alternate embodiment of the transfer function  420  of  FIG. 9  and illustrates a transfer function  430  with a constant ratio region  432  that is greater than 1:1 steering angle to articulation angle but with porches  434  and  436  so that articulation does not necessarily track to the full extent of the steering range. 
     The exemplary embodiments illustrated in  FIGS. 7-10  may be modified with additional regions having various linear or non-linear slopes. The above illustrations are not limiting with respect to additional variations of transfer curve implementations. 
     The exemplary embodiments illustrated in  FIGS. 7-10  may also have applicability to special cases when operating in reverse. For example, an operator-initiated signal may indicate a desired steering path, or steering path change. When traveling in the forward direction this may most naturally take place by sending a signal that controls front wheel steering angle. However, in some cases, it may be advantageous to first adjust articulation angle and have the steerable wheels track the adjustment to articulation angle. This may be particularly true when operating in reverse. 
     Another mode of automatic articulation control may be supported that allows such directional control to be accomplished when operating in reverse through adjustments to the articulation angle operator control, generally via a joystick (not depicted), which then drives changes to the steering angle. The changes to the steering angle based on articulation angle may use the same variations of transfer functions discussed above that incorporate a more than 1:1 ratio of steering angle to articulation angle to map at least a portion of the articulation angle range to steering angle range. 
       FIGS. 11-12  depict prior art transfer functions related to partial range steering control angle. As discussed above,  FIG. 11  shows a prior art transfer function  500  with a constant 1:1 region  502  and zero slope regions  504  and  506 . That is, for any steering angle greater than +20 degrees or less than −20 degrees, the articulation angle is fixed at a corresponding +20 degrees or minus 20 degrees. 
       FIG. 12  illustrates a transfer function  510  with a constant 1:1 region  512  and constrained steering regions  514  and  516  where the steering is limited to +20 degrees and −20 degrees respectively when the maximum articulation angles of +20 and −20 degrees are reached. 
     The present disclosure relates generally to a method of improving steering control for an articulated machine having front wheel steering. In general, the disclosed systems receive steering commands from the operator, and, based upon the steering command or signals indicative of front wheel steering angle, automatically command articulation according to a predetermined formula. Automatic control of articulation angle can reduce operator distractions during operation, improve turning radius, cause the rear wheels to track in the path of the front wheels, etc. 
     The automatic articulation mode is instantiated and executed via the computerized execution of instructions stored on a physically-embodied computer-readable medium or memory, e.g., a disc drive, flash drive, optical memory, ROM, etc. The controller  102  may be physically embodied in one or more controllers and may be separate from or part of one or more existing controllers such as one or more engine controllers and/or transmission controllers. 
     It will be appreciated that the present disclosure provides a system and method for facilitating an automatic articulation mode with selectable modes and enhanced safety and performance features. While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.