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TECHNICAL FIELD 
   This invention relates to the field of excavating work machines, and, more particularly, to a system for modulating a boom assembly for linear excavation. 
   BACKGROUND 
   Work machines that have boom assemblies serve a variety of functions, such as digging ditches, grading surfaces, and laying pipe. In order to carry out these functions, it is advantageous for the boom assembly to extend and retracted in such a manner that the work implement is kept on a linear path during the function. An operator controls the movement of the boom assembly by moving control levers or joysticks. Hydraulic actuators, connected to the boom assembly, receive the operator commands and move the boom assembly accordingly. 
   When grading a surface, the operator extends the boom assembly out and places the tip of the work implement into the material at an appropriate depth and angle. In order to create the linear surface, the operator must raise the boom and draw the stick in at a coordinated rate, such that the work implement follows a linear path. This takes high operator skill to coordinate the movement of the boom and stick and remove the appropriate amount of material. 
   Some manufacturers have tried to anticipate such a scenario and have means to coordinate the movement of the boom assembly. One known control device is found in U.S. Pat. No. 4,332,517, issued to Michiaki Igarashi et al. on Jun. 1, 1982. Igarashi discloses a control device whereupon one cylinder is manually controlled and the operation of the remaining cylinders are calculated using angle detectors provided on the boom, bucket, and arm cylinders. 
   The present invention is directed to overcoming one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   A method of modulating a boom assembly to perform in a linear manner is disclosed. The boom assembly includes a boom and a stick. The method comprising the steps of sending at least one lever signal to a control device indicative of operator desired direction and desired velocity of the boom and the stick, calibrating the lever signals to provide a boom command signal and a stick command signal, applying an algorithm to the boom command signal and the stick command signal, which the algorithm uses command signal mapping, and providing a modulating factor to the control device as a result of the algorithm. 
   A method of using a work machine to grade a surface is disclosed. The work machine having a boom, a stick, and a work implement coupled to the stick, each of the boom and stick is controllable by at least one lever. The method includes the steps of activating at least one lever to produce a command signal comprising at least one of a stick command signal and a boom command signal, communicating the command signal to a control device, and using the control device to modulate the command signal in accordance with a command signal mapping such that said work implement travels in a linear path. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a drawing of an embodiment of a work machine; 
       FIG. 2  is a diagrammatic view of an embodiment of an algorithm. 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a work machine  100  being attached with a boom assembly  102 . The boom assembly  102  comprises a boom  104  pivotally connected to a boom support bracket  106  of the work machine  100 , a stick  108  pivotally connected to the boom  104 , and a work implement  110  pivotally connected to the stick  108 . A boom actuator  112 , such as a hydraulic cylinder, having one end connected to the boom  104  and the other end connected to the boom support bracket  106 , rotates the boom  104  relative to the work machine  100  about a horizontal axis. A stick actuator  114 , such as a hydraulic cylinder, having one end connected to the boom  104  and the other end connected to the stick  108 , rotates the stick  108  relative to the boom  104  about a horizontal axis. A work implement actuator  116 , such as a hydraulic cylinder, having one end connected to the stick  108  and the other end connected to the work implement  110 , rotates the work implement  110  relative to the stick  108  about a horizontal axis. 
   An operator&#39;s cab  118 , being positioned to view the boom assembly  102 , includes a plurality of levers  120  for commanding the boom  104 , stick  108 , and work implement  110 . The plurality of levers  120  are connected to a control device  122  within the work machine  100 . The control device  122 , such as a programmable electronic control module (ECM), is capable of sending command signals to control the respective boom, stick, and work implement actuators  112 ,  114 , and  116 , upon operator commands. The plurality of levers  120  are operator controlled and capable of sending lever signals to the control device  122 , indicative of the position of the plurality of levers  120 . The control device  122  applies a pre-determined calibration factor to the lever signal and converts the lever signal to a command signal. For exemplary purposes, the calibrated command signals for the boom  104 , stick  108 , and work implement  110  would have a range of −1000 to +1000, respectively, depending on the operator desired direction and desired velocity of the rotating boom  104 , stick  108 , and work implement  110 . The −1000 would represent a full command signal to rotating in one of the clockwise or counter-clockwise direction. The +1000 would represent a full command signal to rotate in the opposing direction of the −1000. If the plurality of levers  120  is in the neutral position, 0 would represent the command signal. 
   In  FIG. 2 , the control device  122  executes an algorithm in a continual manner capable of providing a modulating factor  201  to the command signal for controlling the boom  104 , as a result of command signal mapping. For example, an operator gives full commands for the boom  104  and stick  108  represented by calibrated command signals of −1000 for the boom and −1000 for the stick. The algorithm maps the command signals for the boom  104  and stick  108  and provides the modulating factor  201  that changes the command signal to the boom to −500. The details of the algorithm and calculations are disclosed hereinafter. 
   A boom map  202  receives a boom command signal  203  from the control device  122 , indicative of the lever signal from the plurality of levers  120 . The boom map  202  provides a boom map output constant  205  that is indicative of the boom command signal  203 . For exemplary purposes, the boom map  202  includes a pre-defined map  204  on an X and Y axis. The X axis represents the boom command signal  203 , with a scale of −1000 to +1000, indicative of the maximum and minimum values of the boom command signal  203 , and the Y axis represents the boom map output constant  205  with a scale of 0 to 1, indicative of the maximum and minimum boom map output constant  205  values. The boom command signal  203  of less than 0 would provide the boom map output constant  205  of 1, and the boom command signal  203  equal to or greater than 0 would provide the boom map output constant  205  of 0. 
   A subtraction factor map  206  receives a calculated signal  208  that is indicative of calculating the boom and stick command signals  203 , 209  from the control device  122 . The subtraction factor map  206  provides a subtraction factor map output constant  211  that is indicative of the calculated signal  208 . For exemplary purposes, the calculated signal  208  is a result of adding the boom and stick command signals  203 ,  209 . The subtraction factor map  206  includes a pre-defined map  210  on an X and Y axis. The X axis represents the calculated signal  208 , with a scale of −2000 to +2000, indicative of the calculated signal maximum and minimum values, and the Y axis represents the subtraction factor map output constant  211  with a scale of 0 to 0.5, indicative of the subtraction factor map output constant  211  maximum and minimum values. The calculated signal  208  between 0 and −1000 would provide a proportional subtraction factor map output constant  211  of 0.5 to 0, respectively. The calculated signal  208  between 0 and +1000 would provide a proportional subtraction factor map output constant  211  of 0.5 to 0, respectively. The calculated signal  208  less than −1000 and greater than +1000 would provide a subtraction factor map output constant  211  of 0. 
   A stick map  212  receives the stick command signal  209  from the control device  122  that is indicative of the lever signal from the plurality of levers  120 . The stick map  212  provides a stick map output constant  213  that is indicative of the stick command signal  209 . For exemplary purposes, the stick map  212  includes a pre-defined map  214  on an X and Y axis. The X axis represents the stick command signal  209 , with a scale of −1000 to +1000, indicative of the stick command signal  209  maximum and minimum values, and the Y axis represents the stick map output constant  213  with a scale of 0 to 1, indicative of the stick map output constant  211  maximum and minimum values. The stick command signal  209  between −700 and −1000 would provide the stick map output constant  213  of 1. The stick command signal  209  between −700 and 0 would provide the proportional stick map output constant  213  of 1 to 0, respectively. The stick command signal  209  of greater than 0 would provide the stick map output constant  213  of 0. 
   Calculating the boom, subtraction factor, and stick output constants  205 ,  211 , and  213  provides a final subtraction factor  216 . For example, the boom, subtraction factor, and stick output constants  205 ,  211 , and  213  are multiplied together to produce the final subtraction factor  216 . The range of the final subtraction factor would be between 0 and 0.5, indicative of the maximum and minimum values of the multiplication of the boom, subtraction factor and stick output constant  205 ,  211 , and  213 . 
   Calculating the final subtraction factor  216  and a full boom constant  218  provides a pre-dampened modulating factor  219 . For example, the final subtraction factor  216 , with a range of 0 to 0.5 is subtracted from the full boom constant  218  of 1, indicative of a constant given to the maximum boom command signal  203 , to provide a pre-dampened modulating factor  219  of 0.5. 
   The pre-dampened modulating factor  219  then passes through a rate limit control  220 , which is provided to control the rate at which the modulating factor  201  can increase or decrease with respect to time, to produce smooth transitions. For example, the rate limit control  220  would allow a change of modulating factor (MF)  201  of the magnitude of ΔMF/1 s. 
   The modulating factor  201  is then provided to the control device  122  for modulating the boom command signal  203 . For example, the modulating factor  201  of 0.5 is multiplied by the boom command signal  203  of −1000. As a result, a percentage of the boom command signal  203  of −500 is sent to control the boom  104 . 
   INDUSTRIAL APPLICABILITY 
   When the operator is performing a linear function, the plurality of levers  120  are positioned to produce the desired direction and velocity of the boom  104 , stick  108 , and work implement  110 . The plurality of levers  120  send lever signals to the control device  122  where a calibration factor is applied to provide boom and stick command signals  203 ,  209 . The boom and stick command signals  203 ,  209  are sent by the control device  122  to the control the respective boom  104  and stick  108 , and rotate them respective of one another. 
   The control device  122  executes the algorithm continually to provide a modulating factor  201  to the boom command signal  203  that is indicative of command signal mapping. Boom and stick command signals  203 ,  209  are mapped using boom, subtraction factor, and stick pre-defined maps  204 ,  210 , and  214 , to produce the subtraction factor  216 . Subtracting the subtraction factor  216  from the full boom constant  218  provides the pre-dampened modulating factor  219 . The rate limit control  220  applied to the pre-dampened modulating factor  219  provides a smooth transition in instantaneous of the modulating factor  201 . The modulating factor  201  is provided to the control device  122  for modulating the boom command signal  203 . The modulated boom command signal controls the boom rotation and allows coordination between the boom and stick to allow for linear movement of the work implement.

Summary:
A method of modulating a boom assembly to perform in a linear manner. The boom assembly includes a boom and a stick. The method comprising the steps of sending at least one lever signal to a control device indicative of operator desired direction and desired velocity of the boom and the stick, calibrating the lever signals to provide a boom command signal and a stick command signal, applying an algorithm-to-the boom command signal and the stick command signal, which the algorithm uses command signal mapping, and providing a modulating factor to the control device as a result of the algorithm.