Patent Publication Number: US-8527158-B2

Title: Control system for a machine

Description:
TECHNICAL FIELD 
     This disclosure relates generally to a control system and, more particularly, to a control system for controlling movement of a work implement near a desired end of travel position of a work tool. 
     BACKGROUND 
     Control of machines, such as excavators and material handlers, can be a complex task requiring a significant amount of skill on the part of an operator and typically requires manipulation of multiple input devices such as joysticks. An example may be the movement of a work tool, such as a bucket, along a desired path in a consistent, controlled manner from a first location, such as a dig location, to a second location, such as a dump location. Upon reaching the dump location, the operator will typically operate the input devices to slow down the movement of the work tool in order to accurately position the work tool and, in the case of a bucket, minimize any spillage from the bucket until it is in its desired dump location. 
     One example of a machine that includes automated control over a portion of movement of a work tool is disclosed in U.S. Pat. No. 5,968,104 (the &#39;104 patent) issued to Egawa, et al. on Oct. 19, 1999. In particular, the &#39;104 patent discloses a hydraulic excavator having an area limiting excavation control system. The area limiting excavation control system has a setting device permitting an operator to set an excavation area at which an end of a bucket is allowed to move. The area limiting excavation control system also includes angle sensors disposed at pivot points of a boom, an arm, and a bucket for detecting respective rotational angles and velocities thereof, a tilt angle sensor for detecting a tilt angle of the excavator&#39;s body in a back-and-forth direction (fore/aft direction), and a pressure sensor for detecting a load pressure of the boom as it is moved upward in response to signals generated by a control lever. 
     The excavation control system limits the speed of the bucket based on changing machine parameters. Specifically, as the bucket nears a boundary of the operator set excavation area during a fore/aft or up/down movement operation (i.e., during a digging operation), the speed of the bucket is slowed in the direction of the boundary such that the bucket stops at the boundary of the excavation area without exiting the desired excavation area. Stopping of the bucket is controlled by adapting flow rate characteristics of control valves associated with movement of the bucket based upon changing machine parameters such as speed, load, position, posture, and temperature. Although the area limiting control system of the &#39;104 patent may improve operator control and machine performance of a hydraulic excavator under some conditions, its system does not provide a full and efficient solution to certain challenges facing the machine operator. 
     The foregoing background discussion is intended solely to aid the reader. It is not intended to limit the innovations described herein nor to limit or expand the prior art discussed. Thus the foregoing discussion should not be taken to indicate that any particular element of a prior system is unsuitable for use within the innovations described herein, nor is it intended to indicate any element, including solving the motivating problem, to be essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims. 
     SUMMARY 
     In one aspect, a control system for use with a machine is provided. The control system may include a controller configured to store target signals indicative of end of travel positions of a boom, a stick and a frame and receive an operator input device actuation signal indicating a desired movement of the work tool. The controller may also determine positions and velocities of the boom, the stick and the frame based at least in part upon signals received from sensors on the machine and generate command signals to control movement of the boom, the stick and the frame based upon the operator input device actuation signal, proximity of the boom, the stick and the frame to their respective end of travel positions and the velocities of the boom, the stick and the frame. The controller may also transmit the command signals to control movement of the boom, the stick and the frame near the desired end of travel position of the work tool. 
     Additional and alternative features and aspects of the disclosed control system including a method and a machine will also be appreciated from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an excavator with an adjacent target vehicle in accordance with the disclosure; 
         FIG. 2  is a simplified schematic of control components of a control system within the excavator of  FIG. 1 ; 
         FIG. 3  is a flowchart illustrating a process for controlling components of the excavator near an end of travel position; 
         FIG. 4  is a flowchart illustrating a process for generating target signals indicative of an end of travel position; 
         FIG. 5  is a flowchart illustrating an alternate process for generating target signals indicative of an end of travel position; 
         FIG. 6  is a flowchart illustrating still another process for generating target signals indicative of an end of travel position; and 
         FIG. 7  is a flowchart illustrating a process for sequentially controlling components of the excavator between two end of travel positions. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10  having multiple systems and components that cooperate to excavate and load earthen material onto a nearby target machine such as a haul vehicle  12 . In one example, machine  10  may embody a hydraulic excavator. It is contemplated, however, that machine  10  may embody other types of excavation machines such as a backhoe, a front shovel, a wheel loader, or another similar machine as well as a material handler. Machine  10  may include, among other things, an implement system  14  configured to move a work tool  16  between a first position such as a dig location  18  (e.g., within a trench) and a second position such as a dump location  20  (e.g., over a target machine such as a haul vehicle  12 ), and an operator station  22  for manual control of implement system  14 . 
     Implement system  14  may include a linkage structure utilizing fluid actuators to move work tool  16 . More specifically, implement system  14  may include a boom member  24  vertically pivotal relative to frame  42  and propelled by a pair of adjacent, double-acting, boom hydraulic cylinders  28  (only one being shown in  FIG. 1 ). Implement system  14  may also include a stick member  30  vertically pivotal about a horizontal axis  32  between boom members  24  and stick member  30  and propelled by a single, double-acting, stick hydraulic cylinder  36 . Implement system  14  may further include a single, double-acting, work tool hydraulic cylinder  38  operatively connected to work tool  16  to pivot work tool  16  vertically about a horizontal axis  40  through stick member  30  and work tool  16 . Accordingly, stick member  30  pivotally connects work tool  16  to boom member  24  by way of axes  32  and  40 . Frame  42  may be horizontally pivotally connected relative to an undercarriage member  44 , and moved about vertical axis  46  by a swing motor  49 . It is contemplated that a greater or lesser number of fluid actuators may be included within implement system  14  and connected in a manner other than described above, if desired. 
     Each of hydraulic cylinders  28 ,  36 ,  38  may embody linear actuators having a tube and a piston assembly (not shown) arranged to form two distinct pressure chambers. The pressure chambers may be selectively supplied with pressurized fluid and drained of the pressurized fluid to cause the piston assembly to displace within the tube, thereby changing the effective length of hydraulic cylinders  28 ,  36 ,  38 . The flow rate of fluid into and out of the pressure chambers may relate to the speed of extension or retraction of hydraulic cylinders  28 ,  36 ,  38 , while a pressure differential between the two pressure chambers may relate to the force imparted by hydraulic cylinders  28 ,  36 ,  38  on the associated linkage members. The extension and retraction of hydraulic cylinders  28 ,  36 ,  38  results in the movement of work tool  16 . 
     Similar to hydraulic cylinders  28 ,  36 ,  38 , swing motor  49  may also be driven by differential fluid pressure. Specifically, swing motor  49  may be a rotary actuator including first and second chambers (not shown) located on opposite sides of an impeller (not shown). Upon filling the first chamber with pressurized fluid and draining the second chamber of fluid, the impeller is urged to rotate in a first direction. Conversely, when the first chamber is drained of fluid and the second chamber is filled with pressurized fluid, the impeller is urged to rotate in an opposite direction. The flow rate of fluid into and out of the first and second chambers impacts the rotational speed of swing motor  49 , while a pressure differential across the impeller impacts the output torque thereof. 
     Numerous different work tools  16  may be attachable to machine  10  and controllable via operator station  22 . In addition to the bucket depicted in  FIG. 1 , work tool  16  may include any device used to perform a particular task such as, for example, 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 pivot and swing relative to machine  10 , work tool  16  may alternatively or additionally rotate, slide or move in any other manner known in the art. 
     Operator station  22  may include one or more operator input devices  48  embodied as single or multi-axis joysticks ( FIG. 2 ) located proximate to an operator seat (not shown). Operator input devices  48  may be proportional-type controllers configured to position and/or orient work tool  16  by producing a work tool position signal that is indicative of a desired or commanded work tool speed and/or force in a particular direction. It is contemplated that different operator input devices may alternatively or additionally be included within operator station  22  such as, for example, wheels, knobs, push-pull devices, switches, pedals, and other operator input devices known in the art. 
     As illustrated in  FIG. 2 , machine  10  may include a control system  50  including the hydraulic cylinders  28 ,  36 ,  38  and swing motor  49  together with other fluid components that cooperate to move work tool  16  in response to input received from operator input device  48 . In particular, control system  50  may include one or more fluid circuits (not shown) configured to produce and distribute streams of pressurized fluid. A boom control valve  52 , a stick control valve  54 , a bucket control valve  56  and a swing control valve  58  may be situated to receive the streams of pressurized fluid and selectively meter the fluid to and from hydraulic cylinders  28 ,  36 ,  38  and swing motor  49 , respectively, to regulate the motions thereof. Specifically, boom control valve  52  may have elements movable in response to input from the operator to control the motion of boom hydraulic cylinders  28  associated with boom member  24 , bucket control valve  56  may have elements movable to control the motion of work tool hydraulic cylinder  38  associated with work tool  16 , stick control valve  54  may have elements movable to control the motion of stick hydraulic cylinder  36  associated with stick member  30 , and swing control valve  58  may have elements movable to control the swinging motion of frame  42  imparted by swing motor  49 . 
     Since the elements associated with boom, stick, bucket and swing control valves  52 ,  54 ,  56 ,  58  are similar and function in a similar manner, only the operation of boom control valve  52  will be discussed herein. In one example, boom control valve  52  may include a first chamber supply element (not shown), a first chamber drain element (not shown), a second chamber supply element (not shown), and a second chamber drain element (not shown). To extend boom hydraulic cylinders  28 , the first chamber supply element is moved to allow the pressurized fluid to fill the first chambers of boom hydraulic cylinders  28  with pressurized fluid while the second chamber drain element is moved to drain fluid from the second chambers of boom hydraulic cylinders  28 . To move boom hydraulic cylinders  28  in the opposite direction, the second chamber supply element is moved to fill the second chambers of boom hydraulic cylinders  28  with pressurized fluid while the first chamber drain element is moved to drain fluid from the first chambers of boom hydraulic cylinders  28 . It is contemplated that both the supply and drain functions may alternatively be performed by a single element associated with the first chamber and a single element associated with the second chamber, or by a single valve that controls all filling and draining functions, if desired. 
     The supply and drain elements may be movable by solenoids (not shown) in response to a command. More specifically, hydraulic cylinders  28 ,  36 ,  38  may extend and swing motor  49  may rotate at a speed that substantially corresponds to the flow rate of fluid into and out of the first and second chambers, and with a force that substantially corresponds to the pressure of the fluid. To achieve an operator-desired or commanded speed and/or force indicated via the operator input device, a command based on an assumed or measured pressure may be sent to the solenoids (not shown) of the supply and drain elements that causes them to open an amount corresponding to the necessary flow rate. As such, a larger opening of the supply and drain elements will generally result in faster movement of cylinders  28 ,  36 ,  38  and swing motor  49  while a smaller opening will generally result in slower movement. When the supply and drain elements are completely closed, movement will be generally inhibited. The command may be in the form of a flow rate command or a valve element position command. 
     Control system  50  may also include a controller  60  in communication with operator input device  48  and boom, stick, bucket and swing control valves  52 ,  54 ,  56 ,  58  to coordinate the movements described above. Controller  60  may embody a single microprocessor or multiple microprocessors that include a means for controlling the operation of control system  50 . Numerous commercially available microprocessors can be configured to perform the functions of controller  60 . It should be appreciated that controller  60  could readily be embodied in a general machine microprocessor capable of controlling numerous machine functions. Controller  60  may include memory, secondary storage devices, processors, and any other components for running an application. Various other circuits may be associated with controller  60  such as power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other types of circuitry. 
     One or more maps relating the input device position signal, desired actuator speed or force, associated flow rates and pressures, and/or valve element positions associated with movement of hydraulic cylinders  28 ,  36 ,  38  and swing motor  49  may be stored in the memory of controller  60 . Each of these maps may include a collection of data in the form of tables, graphs, and/or equations. In one example, desired speed and commanded flow rate may form the coordinate axis of a 2-D table for controlling the first and second chamber supply and drain elements described above. The commanded flow rate required to move the fluid actuators at the desired speed and the corresponding valve element position of the appropriate supply and drain elements may be related in another separate 2-D map or together with desired speed in a single 3-D map. It is also contemplated that desired actuator speed may be directly related to the valve element position in a single 2-D map. Controller  60  may be configured to allow the operator of machine  10  to directly modify these maps and/or to select specific maps from available relationship maps stored in the memory of controller  60  to affect fluid actuator motion. It is contemplated that the maps may additionally or alternatively be automatically selectable based on modes of machine operation, if desired. 
     Controller  60  may be configured to receive input from operator input device  48  and to command operation of control valves  52 ,  54 ,  56 ,  58  in response to the input and based on the relationship maps described above. Specifically, controller  60  may receive the input device position signal indicative of a desired speed and/or force of work tool  16  in a particular direction, and reference the selected and/or modified relationship maps stored in the memory of controller  60  to determine flow rate values and/or associated positions for each of the supply and drain elements within control valves  52 ,  54 ,  56 ,  58 . The flow rates or positions may then be commanded of the appropriate supply and drain elements to cause filling and/or draining of the first or second chambers at rates that result in the desired work tool movement. 
     Control system  50  may be equipped with one or more sensors  64  for monitoring the position and velocity of the various components of machine  10 . In one example, the sensors  64  may be position sensors associated with each of hydraulic cylinders  28 ,  38 ,  36  to determine the amount of displacement of each of the boom hydraulic cylinders  28 , the stick hydraulic cylinder  36  and the work tool hydraulic cylinder  38 , respectively. In another example, the sensors  64  may be angle sensors associated with the pivot joints of implement system  14 . In another example, the sensors  64  could be inclinometers and/or gyroscopes. The sensors  64  may also include tilt sensors configured to detect a pitch and a roll of frame  42  and may include sensors for determining the position and orientation of the machine relative to a local or global references such as by using, for example, a local laser system or one or more global position system (“GPS”) sensors. In some instances, a pair of GPS sensors may be used. In other instances, a single GPS sensor may be used to determine global position together with a compass to determine orientation. In addition, the sensors  64  may include elements capable of determining velocity or angular velocity as well as load sensors configured to detect a payload of work tool  16  (i.e., a mass of material contained within and transported by work tool  16 ). The combination of sensors  64  may be chosen as desired for the particular needs and applications of machine  10 . Based on signals generated from sensors  64  and based on known kinematics of machine  10 , controller  60  is configured to command control valves  52 ,  54 ,  56 ,  58  to position work tool  16  relative to the dig and dump locations at the speeds and/or forces desired by the operator. In addition, based on the signals generated by sensors  64 , controller  60  may be able to derive and record some or all of the positions, velocities, accelerations, orientations, masses, and/or inertias of implement system  14 . 
     In some situations, controller  60  may be configured to selectively limit movement of implement system  14 . More specifically, when performing repetitive tasks such as digging and loading a haul vehicle  12 , an operator may desire to use controller  60  to control or limit movement of the work tool as it approaches one or more locations. For example, if work tool  16  is a bucket and the operator is performing a sequence of digging operations at dig location  18  and then moving the bucket to a dump location  20  over haul vehicle  12 , the operator may want to utilize automated or semi-automated controls to assist in positioning work tool  16  in conjunction with the dumping operation. Referring to flowchart  100  in  FIG. 3 , the operator initially defines and stores a target or end of travel position for dumping the bucket. The definition and storage can be accomplished in numerous manners but in each instance, as indicated at stage  102 , desired target signals for sensors  64  indicative of the end of travel position of each of boom member  24 , stick member  30  and frame  42  may be identified. In addition, sensors  64  indicating pitch and roll of frame  42  may also be used to identify desired pitch and roll target signals. At stage  104 , the generated target signals are stored within controller  60 . 
     As an operator manipulates one or more operator input devices  48  to perform any of a variety of tasks with implement system  14 , signals generated by the operator input devices  48  are transmitted to and received by controller  60  at stage  106 . Data signals from the various sensors  64  of machine  10  are received by controller  60  at stage  108 . If desired, the data signals may be conditions such as by filtering, amplification and format or protocol conversion at stage  110 . Controller  60  utilizes the data from sensors  64  to determine the positions and velocities of work tool  16 , boom member  24 , stick member  30  and frame  42  at stage  112 . Pitch and roll data may also be utilized by controller  60  to further define the positions of work tool  16 , boom member  24 , stick member  30  and frame  42 . At stage  114 , based upon data received from sensors  64 , controller  60  determines whether work tool  16  is moving towards or away from the end of travel position. If the work tool is moving away from the end of travel position (and thus stage  114  is not satisfied), controller  60  will not modify the operator input signals from the operator input devices  48  (stage  120 ) and control system  50  will be controlled by the operator input signals at stage  122 . However, if the work tool is moving towards the end of travel location (i.e., the dump location), the signals from the operator input devices are compared to values in the data map based upon the positions and velocities of work tool  16 , boom member  24 , stick member  30  and frame  42  and potentially the pitch and roll of frame  42  at stage  116 . 
     The data map may be generally configured to assist in slowing movement of each of boom member  24 , stick member  30  and frame  42  and thus work tool  16  as the work tool approaches the end of travel position to simplify the operator&#39;s efforts in positioning the work tool at the end of travel position. Accordingly, if, and to the extent that, the operator input signals from the operator input devices  48  are less than the values in the data map, controller  60  will not modify the operator input signals from the operator input devices  48  at stage  120  and the operator input signals will be used as command signals by controller  60  to control the control system  50  at  122 . However, for those aspects of the operator input signals that meet the conditions of stage  116  (i.e., any operator input signals that are greater than the data map values), command signals for controlling control system  50  are generated using data from the data map at stage  118  and subsequently transmitted to the control system at stage  122 . With such a control system, as the work tool  16 , such as a bucket, approaches the end of travel position such as dump location  20 , movement of work tool  16  generated by movement of each of hydraulic cylinders  28  and  36  and swinging movement generated by swing motor  49  is slowed down or damped to the lesser of the speed directed by the operator through operator input devices  48  or that specified by the data map of controller  60 . In other words, if the operator is moving boom member  24  and/or stick member  30  vertically and/or swinging frame  42  more slowly than the speed that would result from a command based on the data map, the operator&#39;s input signals will control the operation of each component that is moving more slowly than the data map value while those components for which the operator input device commands movement that is faster than the data map value will be moved at the data map value. It should be noted that since controller  60  may assume control based on speed, the location at which controller  60  assumes control may vary. For example, the faster the movement of boom member  24 , the more likely it is to be controlled at a greater distance from the end of a travel position in order to reduce the likelihood of an abrupt change in speed or deceleration of boom member  24 . 
     As stated above, the generation of target signals indicative of the end of travel position at stage  102  may be performed in a variety of manners. Flowchart  130  in  FIG. 4  depicts one example in which, at stage  132 , the operator utilizes operator input devices  48  to position work tool  16  in a desired end of travel position such as dump location  20 . Upon actuating a target operator interface such as switch  62  ( FIG. 2 ) at stage  134 , data from each sensor is read at stage  136  to determine target data values with the work tool  16  at the desired end of travel position. More specifically, the data signals from each of the sensors  64  associated with boom member  24 , stick member  30  and frame  42  are read and utilized as target signals to indicate the desired end of travel positions for each of boom member  24 , stick member  30  and frame  42  at stage  102  of  FIG. 3 . If machine  10  also includes other sensors  64 , such as those for indicating pitch and roll, as well as those for indicating local or global position and orientation, the data signals from those sensors may also be read and utilized as additional target signals indicative of the desired end of travel positions. 
     In another example of generating target signals at end of travel positions, flowchart  140  in  FIG. 5  depicts the generation of target signals beginning with machine  10  entering into a learning mode such as by actuating an operator interface (e.g., switch  62 ) at stage  142 . Other manners of entering a learning mode may be used including configuring controller  60  to enter a learning mode upon turning the machine on. Once in a learning mode, the operator utilizes operator input devices  48  to position work tool  16  in a desired end of travel position at stage  144  such as dump location  20  for dumping the bucket. Upon moving work tool  16  to perform a predetermined operation stage  146 , such as by dumping the bucket, each sensor is read at stage  148  to determine data values with the work tool  16  at the desired end of travel position. More specifically, the data signals from each of the sensors  64  associated with boom member  24 , stick member  30  and frame  42  are read and utilized as target signals to indicate the desired end of travel positions for each of boom member  24 , stick member  30  and frame  42  at stage  102  of  FIG. 3 . If machine  10  also includes other sensors  64 , such as those for indicating pitch and roll, as well as those for indicating local or global position and orientation, the signals from those sensors may also be read and utilized as additional target signals indicative of the desired end of travel position. 
     In still another example, target signals may be generated or calculated by controller  60  as depicted in flowchart  150  in  FIG. 6 . At stage  152 , dimensions of machine  10  are entered into controller  60 . These dimensions include detailed dimensions of work tool  16  together with each component that affects the movement of work tool  16  such as boom member  24 , stick member  30 , frame  42  and undercarriage member  44 . The dimensions of each component together with data from the sensors  64  associated with work tool  16 , boom member  24 , stick member  30  and frame  42  permit controller  60  to calculate the specific position of the boundary of work tool  16  (as well as the boundaries of the other components of machine  10 ) and to control the movement of machine  10  based on the calculated positions. At stage  154 , the global position and orientation of machine  10  as well as the pitch and roll of frame  42  are entered into controller  60 . The entry of such data may be manual, automated or a semi-automatic combination. A pair of spaced apart GPS sensors  64  may be mounted on machine  10  and used to determine the machine&#39;s global position, orientation and pitch and roll. In an alternate structure, a single GPS sensor  64  may be utilized to determine the global position of machine  10  together with a compass to indicate the orientation of machine  10  and other sensors to determine pitch and roll. 
     At stage  156 , dimensions of a target machine, such as haul vehicle  12 , are entered into controller  60 . The dimensions of the target machine may include, in the haul vehicle example, the length and width of the body or dump box  13  ( FIG. 1 ) together with the height of the top of the dump box above a reference ground surface. At stage  158 , the global position and orientation of the target machine together with its pitch and roll are entered into controller  60 . A pair of spaced apart GPS sensors  64  may be mounted on the target machine and used to determine the target machine&#39;s global position, orientation and pitch and roll. In one alternative structure, a single GPS sensor  64  may be utilized to determine the global position of the target machine together with a compass to indicate the orientation of the target machine and additional sensors utilized to determine the pitch and roll of the target machine. As a result, the exact location of the perimeter of the top of dump box  13  is known. 
     At stage  160 , a desired end of travel position such as a dump location relative to the top of dump box  13  may be set. An operator may set a specific location such as one relative to the length and width of the dump box together with the height relative to the top of the dump box or may utilize a default setting that may set the dump location as a predefined location relative to the dimensions of the dump box such as, for example, at its center and a predetermined height above the top of the dump box. Based upon the input dimensions, location, orientation and pitch and roll of the target vehicle, a desired end of travel position of work tool  16  such as a bucket is calculated at stage  162 . At stage  164 , controller  60  utilizes the calculated end of travel position of work tool  16  together with the input dimensions, location, orientation and pitch and roll of machine  10  to generate target signals indicative of end of travel positions of boom member  24 , stick member  30  and frame  42 . 
     If desired, controller  60  may be configured to limit movement of work tool  16  at a series of two or more spaced apart positions in a predetermined sequence. For example, depending on the size of the bucket at the end of the implement system  14 , the size of haul vehicle  12  and the material being moved, it may be desirable to dump the material at alternating positions in the haul vehicle. Such a process is set forth in flowchart  170  in  FIG. 7  and operates in a manner similar to that set forth in flowchart  100  in  FIG. 3  but with a target or end of travel position changing after each dumping operation. As with the process of  FIG. 3 , first target signals are generated at stage  172  corresponding to the first end of travel position. The first target signals may be generated in a manner as set forth in  FIG. 4  in which the work tool is moved to the desired first end of travel position at stage  132 . A target operator interface such as switch  62  is actuated at stage  134  and the sensors  64  are read at stage  136  with the data from the sensors being used as first target signals indicative of the first end of travel position. These first target signals are then stored within controller  60  at stage  174  of  FIG. 7 . The process is then repeated to generate second target signals at stage  176  by defining a second end of travel position such that the work tool is moved to the desired second end of travel position in a manner similar to that indicated at stage  132 . A target operator interface such as switch  62  is actuated at stage  134  and the sensors  64  are read at stage  136  with the data from the sensors being used as second target signals indicative of the second end of travel position. These second target signals are then stored within controller  60  at stage  178 . 
     Controller  60  sets the target position to correspond to the first end of travel position at stage  180 . Once the end of travel position has been set, the operator may operate machine  10  as desired until the work tool reaches the first end of travel position at stage  182 . While moving to the first end of travel position, the process of stages  106 - 122  of  FIG. 3  is followed with the first target signals corresponding to the first end of travel positions being used as the target signals of  FIG. 3 . Once a predetermined operation, such as dumping the bucket at the first end of travel position has occurred (stage  184 ), controller  60  changes the target signals identified in  FIG. 3  to correspond to the second target signals corresponding to the second end of travel positions at stage  186 . If, for any reason, the operator moves the bucket to the first end of travel position but does not perform the predetermined operation such as dumping the bucket (stage  184 ), controller  60  will continue to limit movement of work tool  16  near the first end of travel position until the predetermined operation has occurred. After the target signals are changed to the second target signals, at stage  186 , the process of stages  116 - 132  of  FIG. 3  is followed with the second target signals corresponding to the second end of travel positions being used as the target signals of  FIG. 3 . The operator may operate machine  10  as desired until work tool  16  reaches the second end of travel position at stage  188 . Once the predetermined operation, such as dumping the bucket at the second end of travel position, has occurred (stage  190 ), controller  60  changes the target signals identified in  FIG. 3  to correspond back to the first target signals corresponding to the first end of travel positions at stage  180 . This sequence of alternating between first and second end of travel positions may be repeated until a signal is sent to the controller to terminate the alternating sequence of operation. 
     Other manners of defining or establishing multiple end of travel positions may be used such as using the process set forth in  FIG. 5  and instructing controller  60  to read the data from the sensors at stage  148  the first time work tool  16  performs a predetermined operation, such as dumping the bucket, and storing the data as the target signals corresponding to the first end of travel positions at stage  174  of  FIG. 7 . An operator may continue operation of machine  10  and the controller may read the sensor data at stage  158  a second time once work tool  16  performs the predetermined operation, such as dumping the bucket, at a second location and storing the data as the target signals corresponding to the second end of travel positions at stage  178  of  FIG. 7 . In another alternate manner of defining multiple end of travel positions, an operator may select a mode in which the controller  60  includes a set pattern of end of travel positions, such as based upon the type and size of the target vehicle. In still another alternate manner of defining multiple end of travel positions, the process set forth in  FIG. 6  may be used to generate the target end of travel signals by calculating their values based on the dimensions of machine  10  and haul vehicle  12  as well as the global position, orientation and pitch and roll of each. During stage  160  of  FIG. 6  at which the operator may set a specific location relative to the length and width of the dump box  13  and height relative to the top of the dump box, the operator may specify desired locations as the first and second end of travel positions or instruct the controller to calculate the first and second end of travel positions based upon the dimension of the dump box. Although the process set forth in  FIG. 7  is depicted with two end of travel positions, more than two end of travel positions may be utilized by defining additional end of travel positions and a desired sequence of alternating between end of travel positions. 
     It should be noted that in each example of generating or calculating target signals corresponding to the end of travel position, such signals may include the pitch and roll of frame  42 . If, after storing the target signals, the current pitch or roll of frame  42  were to change, controller  60  may function to recalculate the target signals for each of the boom member  24 , the stick member  30 , and the frame  42  based upon the kinematics of the machine  10  and store such recalculated signals as the target signals. Similarly, if the pitch and roll of the target vehicle were monitored and utilized as part of the generation or calculation of the target signals of the boom member  24 , the stick member  30 , and the frame  42  as well the target pitch and roll of frame  42 , and the current pitch and roll of the target vehicle were to change, controller  60  may function to recalculate the target signals for each of the boom member  24 , the stick member  30 , and the frame  42  based upon the kinematics of the machine  10  and store such recalculated signals as the target signals. 
     INDUSTRIAL APPLICABILITY 
     The industrial applicability of the control system described herein will be readily appreciated from the foregoing discussion. The present disclosure is applicable to many machines and many tasks accomplished by machines. One exemplary machine for which the control system is suited is an excavator. However, the control system may be applicable to any excavation machine or material handler that benefits from control of a work tool near an end of travel position. 
     The disclosed control system may modify command signals from an operator of a machine when a work tool reaches a position a predetermined distance from the end of travel position in order to slow or control movement of the work tool. If the work tool is a spaced from the end of travel position a distance greater than the predetermined distance, the work tool is moving away from the end of travel position or the movement is slower than a modified command that would be generated by the controller, the machine is controlled by commands from the operator rather than by a controller of the control system. In an example, the control system is configured to slow down or damp movement of aspects of an implement system in order to slow the movement of the work tool near an end of travel position. It is generally desirable to avoid abrupt changes in movement of a work tool in order to reduce any spillage of material from the work tool. The control system simplifies the operation of a machine by assisting the operator to avoid rapid deceleration and assist in precisely positioning a bucket near an end of travel position prior to a dumping operation. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.