Patent Publication Number: US-9850639-B2

Title: Excavation system having velocity based work tool shake

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to an excavation system and, more particularly, to an excavation system having velocity based work tool shake. 
     BACKGROUND 
     Excavation, mining, or other earth removal activities often employ machines, such as load-haul-dump machines (LHDs), wheel loaders, carry dozers, etc. to remove (i.e. scoop up) material from a pile at a first location (e.g., within a mine tunnel), to haul the material to a second location (e.g., to a crusher), and to dump the material at the second location. Productivity of the material removal process depends on the efficiency of a machine during each excavation cycle. For example, the efficiency increases when the machine can sufficiently load a machine tool (e.g., a bucket) with material at the pile within a short amount of time, haul the material via a direct path to the second location, and dump the material at the second location as quickly as possible. 
     As the machine travels from the first location to the second location, some of the material in the tool may spill from the tool and fall on the machine or along the path travelled by the machine. In some applications, for example, underground mining operations, spillage can create hazardous conditions by creating obstructions in the path of the machine. Because the amount of space available in underground operations is relatively small, cleanup of the spilled material is difficult and may also cause reduction in productivity of the machines. 
     U.S. Pat. No. 8,160,783 of Shull that issued on Apr. 17, 2012 (“the &#39;783 patent”) discloses a digging control system for loading a work implement of a machine with material from a pile. In particular, the &#39;783 patent discloses a controller configured to initiate tilting of the work implement when the controller determines that the loading of the work implement exceeds a threshold loading. The &#39;783 patent also discloses that the controller monitors a tilt angle of the work implement and ceases tilting of the work implement when the tilt angle of the work implement equals a threshold tilt angle. By controlling tilting of the work implement in this manner, the controller of the &#39;783 patent aims to reduce the average loading of the work implement during lifting and tilting of the work implement, reducing the energy expended by the machine. Further, the controller of the &#39;783 patent aims to prevent needless pushing of the material forward into the pile. 
     Although the digging control system disclosed in the &#39;783 patent discloses controlling tilting of the work implement to reduce the energy consumption of the machine, the disclosed system may not prevent spillage of material from the work implement. In particular, although the control system of the &#39;783 patent may help ensure that the material is loaded into the work implement instead of being pushed forward into the pile by the work implement, material may still fall out of the work implement as the machine moves from the loading location to a dumping location. 
     The excavation system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art. 
     SUMMARY 
     In one aspect, the present disclosure is directed to an excavation system for a machine having a work tool. The excavation system may include at least one sensor configured to generate a signal indicative of a load exerted on the work tool. The excavation system may also include a lift actuator configured to lift the work tool above a ground surface. The excavation system may further include a tilt actuator configured to tilt the work tool. The excavation system may include a controller in communication with the sensor, the lift actuator, and the tilt actuator. The controller may be configured to detect engagement of the work tool with a material pile based on the signal. The controller may also be configured to operate the work tool to load the work tool with an amount of material. Further, the controller may be configured to determine whether loading of the work tool has been completed. The controller may be configured to operate the lift actuator to lift the work tool when the loading has been completed. The controller may also be configured to operate the tilt actuator to shake the work tool. In addition, the controller may be configured to cause the machine to withdraw from the material pile after shaking the work tool. 
     In another aspect, the present disclosure is directed to a method of controlling a machine having a work tool. The method may include sensing a parameter indicative of a load exerted on the work tool. The method may also include detecting engagement of the work tool with a material pile based on the parameter. Further, the method may include operating the work tool to load the work tool with an amount of material. The method may also include determining whether loading of the work tool has been completed. The method may include lifting the work tool above a ground surface, using a lift actuator of the machine, when the loading has been completed. The method may also include shaking the work tool using a tilt actuator of the machine. In addition, the method may include causing the machine to withdraw from the material pile after shaking the work tool. 
     In yet another aspect, the present disclosure is direct to a machine. The machine may include a frame. The machine may also include a plurality of wheels rotatably connected to the frame and configured to support the frame. The machine may further include a power source mounted to the frame and configured to drive the plurality of wheels. The machine may also include a work tool operatively connected to the frame, driven by the power source, and having a tip configured to engage a material pile. The machine may include a lift actuator configured to lift the work tool above a ground surface. The machine may also include a tilt actuator configured to tilt the work tool. Further, the machine may include a speed sensor associated with the plurality of wheels and configured to generate a first signal indicative of a travel speed of the machine. The machine may also include a torque sensor associated with the powertrain and configured to generate a second signal indicative of a torque output of the powertrain. In addition, the machine may include an acceleration sensor configured to generate a third signal indicative of an acceleration of the mobile machine. The machine may also include a controller in communication with the speed sensor, the torque sensor, and the acceleration sensor. The controller may be configured to detect engagement of the work tool with the material pile based on at least one of the first, second, and third signals. The controller may also be configured to operate the work tool to load the work tool with an amount of material. Further, the controller may be configured to determine whether loading of the work tool has been completed. The controller may also be configured to lift the work tool, using the lift actuator, when the loading has been completed. The controller may be configured to perform a first rack of the work tool. The controller may also be configured to monitor a tilt cylinder velocity. Further, the controller may be configured to perform a first unrack of the work tool, when the tilt cylinder velocity is less than a threshold velocity. The controller may also be configured to monitor a tip angle of the work tool while performing the first unrack of the work tool. In addition, the controller may be configured to perform a second rack of the work tool when the tip angle is about equal to a target tip angle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side-view illustration of an exemplary disclosed machine; 
         FIG. 2  is a side-view illustration of the machine of  FIG. 1  operating at an exemplary disclosed worksite; 
         FIG. 3  is a diagrammatic illustration of an exemplary disclosed excavation system that may be used in conjunction with the machine of  FIG. 1 ; 
         FIG. 4  is a flowchart illustrating an exemplary disclosed method of excavation performed by the excavation system of  FIG. 3 ; and 
         FIG. 5  is a flowchart illustrating an exemplary disclosed method of work tool shake performed by the excavation system of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary embodiment of a machine  10 . In the disclosed example, machine  10  is a load-haul-dump machine (LHD). It is contemplated, however, that machine  10  could embody another type of excavation machine (e.g., a wheel loader or a carry dozer). Machine  10  may include, among other things, a power source  12 , one or more traction devices  14  (e.g. wheels), a work tool  16 , one or more lift actuators  18 , and one or more tilt actuators  20 . Lift actuators  18  and tilt actuators  20  may connect work tool  16  to frame  22  of machine  10 . In one exemplary embodiment as illustrated in  FIG. 1 , lift actuators  18  may have one end connected to frame  22  and an opposite end connected to a structural member  24 , which may be connected to work tool  16 . Work tool  16  may be connected to structural member  24  via pivot pin  26 . Lift actuators  18  may be configured to lift or raise work tool  16  to a desired height above ground surface  28 . In one exemplary embodiment as illustrated in  FIG. 1 , tilt actuators  20  may have one end connected to frame  22  and an opposite end connected to linkage member  30 , which may be connected to work tool  16 . Tilt actuators  20  may be configured to alter an inclination of a lower surface  32  of work tool  16  relative to ground surface  28 . 
     Power source  12  may be supported by a frame  22  of machine  10 , and may include an engine (not shown) configured to produce a rotational power output and a transmission (not shown) that converts the power output to a desired ratio of speed and torque. The rotational power output may be used to drive a pump (not shown) that supplies pressurized fluid to lift actuators  18 , tilt actuators  20 , and/or to one or more motors (not shown) associated with wheels  14 . The engine of power source  12  may be a combustion engine configured to burn a mixture of fuel and air, the amount and/or composition of which directly corresponding to the rotational power output. The transmission of power source  12  may take any form known in the art, for example a power shift configuration that provides multiple discrete operating ranges, a continuously variable configuration, or a hybrid configuration. Power source  12 , in addition to driving work tool  16 , may also function to propel machine  10 , for example via one or more traction devices (e.g., wheels)  14 . 
     Numerous different work tools  16  may be operatively attachable to a single machine  10  and driven by power source  12 . Work tool  16  may include any device used to perform a particular task such as, for example, a bucket, a fork arrangement, a blade, a shovel, or any other task-performing device known in the art. Although connected in the embodiment of  FIG. 1  to lift and tilt relative to machine  10 , work tool  16  may alternatively or additionally rotate, slide, swing open/close, or move in any other manner known in the art. Lift and tilt actuators  18 ,  20  may be extended or retracted to repetitively move work tool  16  during an excavation cycle. 
     In one exemplary embodiment as illustrated in  FIG. 2 , the excavation cycle may be associated with removing a material pile  34  from inside of a mine tunnel  36 . Material pile  34  may constitute a variety of different types of materials. For example, material pile  34  may consist of loose sand, dirt, gravel etc. In other exemplary embodiments, material pile  34  may consist of mining materials, or other tough material such as clay, rocks, mineral formations, etc. In one exemplary embodiment as illustrated in  FIG. 2 , work tool  16  may be a bucket having a tip  38  configured to penetrate the material pile  34 . Machine  10  may also include one or more externally mounted sensors  40  configured to determine a distance of the sensor from pile face  42 . Each sensor  40  may be a device, for example a LIDAR (light detection and ranging) device, a RADAR (radio detection and ranging) device, a SONAR (sound navigation and ranging) device, a camera device, or another device known in the art for determining a distance. Sensor  40  may generate a signal corresponding to the distance, direction, size, and/or shape of the object at the height of sensor  40 , and communicate the signal to an on-board controller  44  (shown only in  FIG. 3 ) for subsequent conditioning. 
     Alternatively or additionally, machine  10  may be outfitted with a communication device  46  that allows communication of the sensed information to an off-board entity. For example, excavation machine  10  may communicate with a remote control operator and/or a central facility (not shown) via communication device  46 . This communication may include, among other things, the location of material pile  34 , properties (e.g., shape) of material pile  34 , operational parameters of machine  10 , and/or control instructions or feedback. 
       FIG. 3  illustrates an excavation system  48  configured to automatically determine various operational parameters of machine  10  to improve efficiency of machine  10  in an excavation cycle. Excavation system  48  may include, among other things, sensor  40 , controller  44 , communication device  46 , speed sensor  50 , at least one load sensor  52 , lift sensor  56 , tilt sensor  58 , lift pressure sensor  60 , and tilt pressure sensor  62 . Controller  44  may be in communication with each of these sensors and numerous other components of excavation system  48  and, as will be explained in more detail below, configured to detect engagement of work tool  16  (referring to  FIG. 2 ) with material pile  34 , to determine a repose angle α of material pile  34 , to determine a tip angle β of tip  38 , to determine one or more tilt control parameters for work tool  16 , etc. This information may be used for remotely or autonomously controlling machine  10 , including, among other things, to control operation of work tool  16 . 
     Controller  44  may embody a single microprocessor or multiple microprocessors that include a means for monitoring operations of excavation machine  10 , communicating with an off-board entity, and detecting properties of material pile  34 . For example, controller  44  may include a memory, a secondary storage device, a clock, and a processor, such as a central processing unit or any other means for accomplishing a task consistent with the present disclosure. The memory or secondary storage device associated with controller  44  may store data and/or routines that may assist controller  44  to perform its functions. Further the memory or storage device associated with controller  44  may also store data received from the various sensors associated with machine  10 . Numerous commercially available microprocessors can be configured to perform the functions of controller  44 . It should be appreciated that controller  44  could readily embody a general machine controller capable of controlling numerous other machine functions. Various other known circuits may be associated with controller  44 , including signal-conditioning circuitry, communication circuitry, hydraulic or other actuation circuitry, and other appropriate circuitry. 
     Communication device  46  may include hardware and/or software that enable the sending and/or receiving of data messages through a communications link. The communications link may include satellite, cellular, infrared, radio, and/or any other type of wireless communications. Alternatively, the communications link may include electrical, optical, or any other type of wired communications. In one embodiment, on-board controller  44  may be omitted, and an off-board controller (not shown) may communicate directly with sensor  40 , speed sensor  50 , one or more load sensors  52 , lift sensor  56 , tilt sensor  58 , lift pressure sensor  60 , tilt pressure sensor  62 , and/or other components of machine  10  via communication device  46 . 
     Speed sensor  50  may embody a conventional rotational speed detector having a stationary element rigidly connected to frame  22  (referring to  FIG. 1 ) that is configured to sense a relative rotational movement of wheel  14  (e.g., of a rotating portion of power source  12  that is operatively connected to wheel  14 , such as an axle, a gear, a cam, a hub, a final drive, etc.). The stationary element may be a magnetic or optical element mounted to an axle housing (e.g., to an internal surface of the housing) and configured to detect the rotation of an indexing element (e.g., a toothed tone wheel, an embedded magnet, a calibration stripe, teeth of a timing gear, a cam lobe, etc.) connected to rotate with one or more of wheels  14 . The indexing element may be connected to, embedded within, or otherwise form a portion of the front axle assembly that is driven to rotate by power source  12 . Speed sensor  50  may be located adjacent the indexing element and configured to generate a signal each time the indexing element (or a portion thereof, for example a tooth) passes near the stationary element. This signal may be directed to controller  44 , which may use this signal to determine a distance travelled by machine  10  between signal generation times (i.e., to determine a travel speed of machine  10 ). Controller  44  may record the traveled distances and/or speed values associated with the signal in a memory or other secondary storage device associated with controller  44 . Alternatively or additionally, controller  44  may record a number of wheel rotations, occurring within fixed time intervals, and use this information along with known kinematics of wheel  14  to determine the distance and speed values. Other types of sensors and/or strategies may also or alternatively be employed to determine a travel speed of machine  10 . 
     Load sensor  52  may be any type of sensor known in the art that is capable of generating a load signal indicative of an amount of load exerted on work tool  16 , for example by material pile  34  when work tool  16  comes into contact with material pile  34 . Load sensor  52  may, for example, be a torque sensor associated with power source  12 , or an accelerometer. When load sensor  52  is embodied as a torque sensor, the load signal may correspond with a change in torque output experienced by power source  12  during travel of machine  10 . In one exemplary embodiment, the torque sensor may be physically associated with the transmission or final drive of power source  12 . In another exemplary embodiment, the torque sensor may be physically associated with the engine of power source  12 . In yet another exemplary embodiment, the torque sensor may be a virtual sensor used to calculate the torque output of power source  12  based on one or more other sensed parameters (e.g., fueling of the engine, speed of the engine, and/or the drive ratio of the transmission or final drive). When load sensor  52  is embodied as an accelerometer, the accelerometer may embody a conventional acceleration detector rigidly connected to frame  22  or other components of machine  10  in an orientation that allows sensing of changes in acceleration in the forward and rearward directions for machine  10 . It is contemplated that excavation system  48  may include any number and types of load sensors  52 . 
     Lift sensor  56  may embody a magnetic pickup-type sensor associated with a magnet (not shown) embedded within lift actuators  18 . In this configuration, lift sensor  56  may be configured to detect an extension position or a length of extension of lift actuator  18  by monitoring the relative location of the magnet, and generate corresponding position and/or lift velocity signals directed to controller  44  for further processing. It is also contemplated that lift sensor  56  may alternatively embody other types of sensors such as, for example, magnetostrictive-type sensors associated with a wave guide (not shown) internal to lift actuator  18 , cable type sensors associated with cables (not shown) externally mounted to lift actuator  18 , internally- or externally-mounted optical sensors, LIDAR, RADAR, SONAR, or camera type sensors or any other type of height-detection sensors known in the art. From the position and/or velocity signals generated by lift sensor  56  and based on known geometry and/or kinematics of frame  22 , lift actuators  18  and tilt actuators  20 , and other connecting components of machine  10 , controller  44  may be configured to calculate a height of work tool  16  above ground surface  28 . In one exemplary embodiment, controller  44  may be configured to calculate a height of lower surface  32  of work tool  16  above ground surface  28 . In another exemplary embodiment, controller  44  may be configured to calculate a height of tip  38  of work tool  16  above ground surface  28 . In yet another exemplary embodiment, controller  44  may be configured to calculate a height of pivot pin  26  (shown in  FIGS. 1 and 2 ) of work tool  16  above ground surface  28 . 
     Tilt sensor  58  may also embody a magnetic pickup-type sensor associated with a magnet (not shown) embedded within tilt actuator  20 . In this configuration, tilt sensor  58  may be configured to detect an extension position or a length of extension of tilt actuator  20  by monitoring the relative location of the magnet, and generate corresponding position and/or tilt velocity signals directed to controller  44  for further processing. From the position and/or tilt velocity signals generated by tilt sensor  58  and based on known geometry and/or kinematics of frame  22 , lift actuators  18  and tilt actuators  20 , and other connecting components of machine  10 , controller  44  may be configured to calculate tip angle “β,” representing an angle of inclination of lower surface  32  of work tool  16  relative to ground surface  28 . It is also contemplated that controller  44  may be able to use signals generated by one or more tilt sensors  58  to determine a rack angle “β rack ” and/or an unrack angle “β unrack ” of work tool  16 . As used in this disclosure, β rack  refers to a change in the angular position of work tool  16  from its current position as work tool  16  is tilted away from ground surface  28 . Likewise, as used in this disclosure, β unrack  refers to a change in the angular position of work tool  16  from its current position as work tool  16  is tilted towards ground surface  28 . It is also contemplated that tilt sensor  58  may alternatively embody other types of sensors such as, for example, magnetostrictive-type sensors associated with a wave guide (not shown) internal to tilt actuator  20 , cable type sensors associated with cables (not shown) externally mounted to tilt actuator  20 , internally- or externally-mounted optical sensors, rotary style sensors associated with joints pivotable by tilt actuators  20 , or any other type of angle-detection sensors known in the art. 
     One or more lift pressure sensors  60  may be strategically located within the one or more lift actuators  18  to sense a pressure of the fluid within lift actuators  18 . Lift pressure sensor  60  may generate a corresponding signal indicative of the pressure within lift actuator  18  and direct the signal to controller  44 . Likewise, one or more tilt pressure sensors  62  may be strategically located within the one or more tilt actuators  20  to sense a pressure of the fluid within tilt actuators  20 . Tilt pressure sensor  62  may generate a corresponding signal indicative of the pressure within tilt actuator  20  and direct the signal to controller  44 . Controller  44  may use the information received from the one or more sensors and components of machine  10  to control operations of machine  10 , as will be described in more detail below. 
       FIGS. 4 and 5  illustrate exemplary methods that may be performed by excavation system  48 .  FIGS. 4 and 5  will be discussed in more detail in the following section to further illustrate the disclosed concepts. 
     INDUSTRIAL APPLICABILITY 
     The disclosed excavation system may be used in any machine at a worksite where it is desirable to remotely or autonomously control the machine while ensuring that a work tool of the machine is sufficiently loaded with material. For example, the disclosed excavation system may be used in a LHD, wheel loader, or carry dozer that operates under hazardous conditions. The excavation system may assist control of the machine by automatically loading the work tool with material from a material pile and shaking the work tool to ensure loose material falls out of the work tool on the material pile before the machine withdraws from the material pile to travel to a dump location. Operation of excavation system  48  will now be described in detail with reference to  FIGS. 4 and 5 . 
       FIG. 4  illustrates an exemplary disclosed method of excavation  400  performed by excavation system  48 . Method  400  may include a step of engaging auto-load digging (Step  402 ) for machine  10  at any time during forward travel of machine  10 . The auto-load digging functionality may help ensure that sufficient amount of material is loaded in work tool  16  during each excavation cycle. In step  402 , controller  44  may initiate the auto-load digging functionality in response to a variety of inputs. For example, controller  44  may automatically initiate auto-load digging in response to a detection of forward travel (e.g., in response to a signal from speed sensor  50 ). In another example, controller  44  may initiate auto-load digging in response to a proximity to material pile  34  (e.g., in response to a signal from sensor  40 ). In yet another example, auto-loading may be initiated manually by a local or remote operator. Any combination of these inputs (and others) may be utilized to initiate auto-load digging functionality. 
     Method  400  may include a step of detecting pile impact, for example, detecting contact of work tool  16  with material pile  34  (Step  404 ). In one exemplary embodiment, controller  44  may orient work tool  16  so that lower surface  32  of work tool  16  is disposed generally parallel to ground surface  28 . As machine  10  travels towards material pile  34  with work tool  16  disposed generally parallel to ground surface  28 , controller may receive signals from various components of machine  10 . Controller  44  may detect contact of work tool  16  with material pile  34  based on a sharp change in acceleration of machine  10 . Alternatively or additionally, controller  44  may detect a slowing down of machine  10  by detecting a sharp change in torque output of power source  12  (i.e., by an increase in torque output). Accordingly, controller  44  may continuously compare monitored values of torque output and acceleration to respective threshold values to detect engagement of work tool  16  with material pile  34 . 
     Method  400  may include a step of operating the work tool (Step  406 ). To operate the work tool in step  406 , controller  44  may issue commands to one or more lift actuators  18  and tilt actuators  20  to lift work tool  16  and rack and unrack work tool  16  as work tool  16  penetrates material pile  34 . By actuating the lift actuators  18  and tilt actuators  20  in this manner, controller  44  may help ensure that material from material pile  34  may be removed and loaded into work tool  16 . 
     Method  400  may include a step of determining whether loading of work tool  16  with material is complete (step  408 ). Controller  44  may determine whether loading of work tool  16  is complete based on one or more of a plurality of conditions. For example, controller  44  may determine that loading of work tool  16  is complete when a height of pivot pin  26  above ground surface  28  exceeds a target height. Alternatively, controller  44  may determine that loading of work tool  16  is complete when an amount of material in work tool  16  exceeds a target amount. Controller  44  may also determine that loading of work tool  16  is complete when tip  38  has penetrated material pile  34  by a distance that exceeds a target penetration distance. In another exemplary embodiment, controller  44  may determine that loading of work tool  16  is complete when a tip angle β of tip  38  exceeds a tip angle target. Controller  44  may determine that loading of work tool  16  is complete when controller  44  detects that tip  38  of work tool  16  has been extracted from material pile  34 . When controller  44  determines that loading of work tool  16  is not complete (Step  408 : No) based on any of the above conditions, controller  44  may return to step  406  to continue operating work tool  16  to load work tool  16  with material. Thus, controller  44  may cycle through steps  406  and  408  to continuously monitor whether loading of work tool  16  is complete as work tool  16  is loaded with material. When controller  44  determines, however, that loading of work tool  16  is complete (Step  408 : Yes), controller may proceed to step  410 . 
     In step  410 , controller  44  may shake work tool  16  to cause any loose material in work tool  16  to spill out on material pile  34 . Controller  44  may shake work tool  16  by racking and unracking work tool  16  multiple times in quick succession. In one exemplary embodiment, controller  44  may rack and unrack work tool  16  at least 2 times in step  410 . Further details regarding the process of shaking work tool  16  will be discussed below with respect to  FIG. 5 . 
     Method  400  may include a step of causing machine  10  to withdraw from material pile  34  after shaking work tool  16  (Step  412 ). After withdrawing from material pile  34 , machine  10  may proceed along a designated path to a dump location to dump the contents of work tool  16  at the dump location. By shaking work tool  16  before withdrawing machine  10  from material pile  34 , method  400  may help ensure that loose material from work tool  16  may be spilled on material pile  34  for pickup by machine  10  during a subsequent excavation cycle. Further, by helping ensure that loose material from work tool  16  is spilled on material pile  34 , method  400  may help ensure that loose material does not spill along the path from material pile  34  to the dump location. This in turn may help to keep the path clear of debris and reduce and/or eliminate the need to clean the path of any spillage from work tool  16  as machine  10  travels over the path. 
       FIG. 5  illustrates an exemplary disclosed method  500  of shaking work tool  16  performed by excavation system  48 . Method  500  may include a step of lifting work tool  16  above ground surface  28  (Step  502 ). In step  502 , controller  44  may issue commands to cause lift actuators  18  to lift or raise work tool  16  above ground surface  28 . In one exemplary embodiment, controller  44  may do so by issuing commands to operate pumps or other components to pump hydraulic fluid into lift actuators  18  causing lift actuators  18  to extend and raise work tool  16  above ground surface  28 . 
     Method  500  may include a step of determining whether a target extension (i.e. target length) has been reached by lift actuator  18  (Step  504 ). When controller  44  determines that lift actuator  18  has reached a target extension (Step  504 : Yes), controller  44  may proceed to step  508 . When controller  44  determines, however, that lift actuator  18  has not reached the target extension (Step  504 : No), controller  44  may proceed to step  506  of determining whether lifting has timed out. In one exemplary embodiment, the target length ranges from about 15% to 20% of a maximum length of extension of lift actuator  18 . 
     Controller  44  may initialize a timer (i.e. set the timer to 0) when executing step  502  to lift work tool  16 . Controller  44  may monitor an elapsed time as lift actuators  18  lift work tool  16  above ground surface  28 . Controller may periodically compare the elapsed time with a target lift time, which may represent a maximum amount of time for lifting work tool  16  to the target height. Controller  44  may determine that lifting has timed out (Step  506 ), when the elapsed time exceeds the target lift time and lift actuator  18  has not reached the target extension. When controller  44  determines that lifting has timed out (Step  506 : Yes), controller  44  may proceed to step  508 . When controller  44  determines, however, that lifting has not timed out (Step  506 : No), controller  44  may return to step  502  to continue lifting work tool  16 . Controller  44  may cycle through one or more of steps  502 - 506  to lift work tool  16  and help ensure that work tool  16  is free from material pile  34  before shaking work tool  16  to remove loose material from work tool  16 . 
     Method  500  may include a step of performing a first rack of work tool  16  (Step  508 ). In step  508 , controller  44  may issue commands to cause tilt actuators  20  to rack (i.e. tilt) work tool  16  away from ground surface  28 . In one exemplary embodiment, controller  44  may do so by issuing commands to operate pumps or other components to pump hydraulic fluid into tilt actuators  20  causing lift actuators  18  to extend and tilt work tool  16  away from ground surface  28 . 
     Method  500  may include a step of determining whether a tilt velocity V tilt  is less than a threshold tilt velocity of work tool  16 . Controller  44  may use signals from, among other things, tilt sensor  58  to determine a tilt velocity of work tool  16  at periodic intervals as work tool  16  tilts away from ground surface  28 . When controller  44  determines that tilt velocity V tilt  of work tool  16  is less than a threshold velocity (Step  510 : Yes), controller  44  may proceed to step  514 . When controller  44  determines, however, that tilt velocity V tilt  of work tool  16  is greater than the threshold velocity (Step  510 : No), controller  44  may proceed to step  512  of determining whether first rack has timed out. 
     Controller  44  may initialize a timer (i.e. set the timer to 0) when executing step  508  of racking work tool  16 . Controller  44  may monitor an elapsed time as tilt actuators  20  tilt work tool  16  away from ground surface  28 . Controller may periodically compare the elapsed time with a target rack time, which may represent a maximum amount of time permitted for racking work tool  16 . Controller  44  may determine that first rack has timed out (Step  512 ), when the elapsed time exceeds the target first rack time and tilt velocity V tilt  of work tool  16  remains higher than the threshold velocity. When controller  44  determines that first rack has timed out (Step  512 : Yes), controller  44  may proceed to step  514 . When controller  44  determines, however, that first rack has not timed out (Step  512 : No), controller  44  may return to step  508  to continue racking work tool  16 . Controller  44  may cycle through one or more of steps  508 - 510  to rack work tool  16 . 
     Method  500  may include a step of performing a first unrack of work tool  16  (Step  514 ). In step  514 , controller  44  may issue commands to cause tilt actuators  20  to unrack (i.e. tilt) work tool  16  towards ground surface  28 . In one exemplary embodiment, controller  44  may do so by issuing commands to operate pumps or other components to pump hydraulic fluid out of tilt actuators  20  causing tilt actuators  20  to contract and tilt work tool  16  towards ground surface  28 . 
     Method  500  may include a step of determining whether a tip angle β exceeds β target , a target tip angle (Step  516 ). Controller  44  may use signals from, among other things, tilt sensor  58  to determine the tip angle β of work tool  16  at periodic intervals as work tool  16  tilts towards ground surface  28 . When controller  44  determines that tip angle β of work tool  16  exceeds the target tip angle β target  (Step  516 : Yes), controller  44  may proceed to step  520 . When controller  44  determines, however, that tip angle β of work tool  16  is less than the target tip angle β target  (Step  516 : No), controller  44  may proceed to step  518  of determining whether first unrack has timed out. 
     Controller  44  may initialize a timer (i.e. set the timer to 0) when executing step  514  of unracking work tool  16 . Controller  44  may monitor an elapsed time as tilt actuators  20  tilt work tool  16  toward ground surface  28 . Controller may periodically compare the elapsed time with a target unrack time, which may represent a maximum amount of time permitted for unracking work tool  16 . Controller  44  may determine that first unrack has timed out (Step  518 ), when the elapsed time exceeds the target first unrack time and tip angle β of work tool  16  remains higher than the target tip angle β target . When controller  44  determines that first unrack has timed out (Step  518 : Yes), controller  44  may proceed to step  520 . When controller  44  determines, however, that first unrack has not timed out (Step  518 : No), controller  44  may return to step  514  to continue unracking work tool  16 . Controller  44  may cycle through one or more of steps  514 - 518  to unrack work tool  16 . 
     Method  500  may include a step of performing a second rack of work tool  16  (Step  520 ). Controller  44  may perform processes similar to those described above for step  508  to perform the second rack of work tool  16 . Method  500  may include a step of determining whether a tilt velocity V tilt  is less than a threshold velocity of work tool  16  (Step  522 ). Controller  44  may perform processes similar to those described above for step  510  to determine whether a tilt velocity V tilt  is less than a threshold velocity of work tool  16 . When controller  44  determines that tilt velocity V tilt  of work tool  16  is less than the threshold velocity (Step  522 : Yes), controller  44  may end method  500 . When controller  44  determines, however, that tilt velocity V tilt  of work tool  16  is greater than the threshold velocity (Step  522 : No), controller  44  may proceed to step  524  of determining whether second rack has timed out. 
     Controller  44  may perform processes similar to those described above for step  512  to determine whether second rack has timed out. When controller  44  determines that second rack has timed out (Step  524 : Yes), controller  44  may proceed to step  526 . When controller  44  determines, however, that second rack has not timed out (Step  526 : No), controller  44  may return to step  520  to continue racking work tool  16 . Controller  44  may cycle through one or more of steps  520 - 524  to rack work tool  16 . 
     Method  500  may include a step of performing a second unrack of work tool ( 526 ), determining whether the tip angle β exceeds β target  (Step  528 ), and determining whether the second rack has timed out (Step  530 ). Controller  44  may perform processes similar to those described above for steps  514 ,  516 , and  518  when performing steps  526 ,  528 , and  530 , respectively. In step  528 , when controller  44  determines that tip angle β of work tool  16  exceeds the target tip angle β target  (Step  528 : Yes), controller  44  may proceed to step  532 . When controller  44  determines, however, that tip angle β of work tool  16  is less than the target tip angle β target  (Step  528 : No), controller  44  may proceed to step  530  of determining whether second unrack has timed out. In step  530 , when controller  44  determines that second unrack has timed out (Step  530 : Yes), controller  44  may proceed to step  532 . When controller  44  determines, however, that second unrack has not timed out (Step  530 : No), controller  44  may return to step  526  to continue unracking work tool  16 . Controller  44  may cycle through one or more of steps  526 - 528  to unrack work tool  16 . 
     Method  500  may include a step of performing a third rack of work tool  16  (Step  532 ). Controller  44  may perform processes similar to those described above for steps  508  or  520  to perform the third rack of work tool  16 . Method  500  may also include a step of determining whether the third rack has timed out (Step  534 ). Controller  44  may perform processes similar to those described above for steps  512  or  524  to determine whether third rack has timed out. When controller  44  determines that third rack has timed out (Step  534 : Yes), controller  44  may end method  500 . When controller  44  determines, however, that third rack has not timed out (Step  526 : No), controller  44  may return to step  532  to continue racking work tool  16 . Controller  44  may cycle through one or more of steps  532 - 534  to rack work tool  16 . 
     As illustrated in  FIG. 5  and described above, controller  44  may perform a first rack of work tool  16  (Step  508 ), followed by a first unrack of work tool  16  (Step  514 ), and a second rack of work tool  16  (Step  520 ) to shake work tool  16  to allow loose material to spill from work tool  16  onto material pile  34 . While performing second rack of work tool  16  (Step  520 ), if controller  44  determines that the tilt velocity Vtilt of work tool  16  is higher than the threshold velocity and if the third rack times out (i.e. the third rack cannot be completed in the allocated time), then controller  44  proceeds to perform a second unrack of work tool  16  (Step  526 ), followed by a third rack of work tool  16  (Step  534 ). The additional second unrack (Step  526 ) and third rack (Step  534 ) may allow controller  44  to help ensure work tool  16  is not stalled or stuck and can move freely before allowing machine  10  to withdraw from material pile  34 . By performing the process of repeatedly racking and unracking work tool  16  according to method  500 , controller  44  may help ensure that loose material from work tool  16  can be dislodged at material pile  34 , which may prevent debris from falling from work tool  16  onto the path travelled on by machine  10 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed excavation system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed excavation system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.