Abstract:
A dozer blade control system controls the position of a bulldozer blade, maintaining the blade at a constant position as the dozer travels through a worksite. The control system monitors the angle of the dozer blade with respect to the earth and when it senses that the dozer blade is tilting, it corrects the dozer blade&#39;s position by extending or retracting hydraulic cylinders that couple the dozer blade to the chassis of the crawler-tractor. When the dozer chassis pitches forwards, the blade begins to tilt forward and to drop closer to the ground. The control system senses this forward rotation of the blade and retracts the hydraulic cylinders that couple the blade to the chassis, causing the blade to return to and maintain its original position. Conversely, when the dozer chassis pitches backwards and the blade begins to tilt backward and rise higher above the ground, the control system extends the hydraulic cylinders coupling the blade to the chassis and lowers the blade, causing the blade to return to and maintain its original position with respect to the earth.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to bulldozers. More particularly, it relates to systems for keeping the blade of a bulldozer at a selected position as the bulldozer is operated.  
       BACKGROUND OF THE INVENTION  
       [0002]     “Bulldozers” or “dozers”, as those terms are used herein, refer to crawler-tractors that are equipped with a blade for scraping the ground or pushing material along the ground. The blade is pivotally connected to the crawler-tractor chassis such that they can pivot up and down. Blade controls are provided to the operator in the cab of the vehicle. These controls permit the operator raise and lower the blade with respect to the chassis of the crawler-tractor. One of the most common uses for blades on bulldozers is to level or otherwise contour ground for construction of houses, buildings, parking lots, and roads. Often the terrain that the bulldozer starts working is quite uneven and rough. As it passes over this rough terrain, the bulldozer chassis often begins to pitch.  
         [0003]     When the chassis pitches up and down, the blade pitches as well. As the blade pitches up the blade digs the earth shallower. As the blade pitches down, it digs into the earth deeper, duplicating in the earth the fluctuations of the dozer chassis as it pitches over the rough terrain. Instead of evenly leveling the terrain, a bulldozer tends to reproduce the very rough terrain over which it drives.  
         [0004]     A skilled operator can reduce the pitching of the blade by anticipating the pitching of the chassis and moving the blade in the opposite direction. By manually pitching the blade in a direction opposite to the direction the chassis pitches and at exactly the same time, the operator can grade the terrain more level than if the blade merely pitches with the chassis. This ability to anticipate the motion of the chassis and pitch the blade in the opposite direction takes a good deal of skill, and that skill can only be acquired through experience.  
         [0005]     Even a talented driver, however, cannot travel at full speed over rough terrain, but must reduce his speed to accommodate the pitching of the dozer blade as the dozer chassis pitches up and down as it travels over the ground.  
         [0006]     The process of leveling the ground using a bulldozer blade is called “grading”. Systems for automatically grading the ground have been devised that use sensors mounted on a bulldozer blade and laser light sources located at the corners of a field to be graded. These light sources transmit light to the sensors attached to the bulldozer blade.  
         [0007]     As a bulldozer equipped with these systems pitches backward or forward, the blade begins to pitch up or down, causing the light falling on the sensor to fall or rise, respectively. A controller coupled to the sensor controls blade pitching by raising and lowering the blade to keep it in the same position with respect to the ground.  
         [0008]     This system, however, requires the careful placement and adjustment of light sources and an unobstructed view of the bulldozer blade.  
         [0009]     What the inventors have discovered is that for many applications this laser-guided whole-field system is overkill. Many operators, especially novice operators, would be significantly benefited by a system that merely monitors bulldozer pitching as it goes over rough terrain and keeps the blade in a relatively constant position and at a relatively constant height as the chassis of the bulldozer pitches forward and backward.  
         [0010]     What is needed, therefore, is a system for reducing dozer blade pitching as the dozer chassis pitches. What is also needed is a dozer that has a system for reducing dozer blade pitching. What is also needed is a system for keeping the dozer blade at a relatively constant height and at a relatively constant position as the chassis of the tractor-crawler pitches. What is also needed is a system that at least partially relieves the operator of the burden of manually raising and lowering the blade as the vehicle pitches while traveling over the ground. What is also needed is a system that permits the operator to grade faster by automatically controlling blade pitching. What is also needed is a system that automatically controls blade pitching faster than an operator can manually control blade pitching. It is an object of this invention to provide such system and bulldozer.  
       SUMMARY OF THE INVENTION  
       [0011]     In accordance with a first aspect of the invention, a bulldozer is provided, comprising a crawler-tractor; a ground-engaging blade coupled to the crawler to raise and lower with respect to the crawler-tractor; at least one hydraulic lift cylinder configured to position the blade; a blade position sensor to provide a signal indicative of a position of the blade; and an electronic controller coupled to the blade and to the at least one lift cylinder to automatically raise the blade with respect to the crawler-tractor when the crawler-tractor pitches forward, and lower the blade when the crawler-tractor pitches backward, in response to position signals received from the blade position sensor.  
         [0012]     The bulldozer may include two horizontally disposed arms pivotally coupled to left and right sides of the crawler-tractor that support the blade, an operator input device coupled to the controller, the input device including a member manually operable to transmit a signal indicative of a target blade position to the controller; wherein the controller includes a feedback control loop configured to drive the blade to a target position. The blade position sensor may be coupled to the controller to transmit the blade position signal to the controller. The controller may be configured to raise and lower the blade in response to the signal indicative of blade position. The signal indicative of blade position may indicate a rate of change of the blade angle that is perpendicular to a generally horizontal axis and laterally extending axis. The operator input device may be configured to generate the signal indicative of the target blade position in a first mode of operation and may be configured to generate a signal indicative of a desired rate of blade lifting in a second mode of operation. The blade position sensor may be fixed to the blade. The controller may include a CPU, RAM, and ROM.  
         [0013]     In accordance with a second aspect of the invention, a pitch control system for controlling the pitch of the bulldozer blade is provided, including a blade position sensor configured to be fixed to the blade of the bulldozer to provide a signal indicating an actual position of the blade, a manually operable operator input device configured to be coupled to the controller to provide the controller with a signal indicative of a target position; and an electronic controller configured to be coupled to the blade position sensor and to the input device to receive the target position signal and the actual position signal, to determine the difference between the target position and the actual position and to calculate a valve signal for a hydraulic valve coupled to a blade lift cylinder that will drive the blade to the target position when the bulldozer pitches.  
         [0014]     The signal indicating the actual position of the blade may also indicate the angular turning rate of the blade. The controller may include a CPU, a RAM, and a ROM, and the ROM may contain digital instructions that (a) determine the difference between the actual and the target positions and (b) may calculate the valve signal that will drive the blade to the target position. The angular turning rate may be the rate of change of the blade angle that is perpendicular to a generally horizontal axis that extends perpendicular to the length of the crawler-tractor. The blade position sensor may be an angular turning rate sensor. The input device may include a manually operable member that generates a signal that lowers the target position when moved in a first direction, and that raises the target position when moved in a second direction opposite the first direction.  
         [0015]     In accordance with a third aspect of the invention, a computer-implemented method of controlling the pitching of a bulldozer blade during movement over the ground is provided, the method including the steps of (1) reading a blade actual position that indicates an actual position of the bulldozer blade; (2) comparing the blade actual position signal with a blade target position signal; (3) determining a hydraulic valve signal that is calculated to drive the blade from the actual position to the target position; and (4) driving the blade to the target position. The step of comparing may include the step of calculating an error signal indicating the difference between the blade&#39;s actual position and the blade&#39;s target position. The step of determining may include the step of calculating a control signal from the error signal, the control signal having a derivative component, a proportional component, and an integral component. The step of reading a blade actual position may include the step of reading the blade actual position from an angular turning rate sensor and integrating the turning rate to determine the blade actual position. The turning rate may be the rate of turning about a generally horizontal and laterally extending axis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a side view of a bulldozer in accordance with the present invention.  
         [0017]      FIG. 2  is a hydraulic and schematic diagram of a blade pitch control system in accordance with the present invention as shown on the bulldozer of  FIG. 1 .  
         [0018]      FIG. 3  is a flowchart of the functions performed by the controller of  FIG. 2  when it executes its stored program and controls blade pitching.  
         [0019]      FIG. 4  is a control diagram illustrating the control operations performed by the electronic controller of  FIG. 3  that regulate the pitch of the blade. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     “Dozer” or “bulldozer” as used herein refers to a crawler-tractor coupled to a blade. “Crawler-tractor” refers to any of the class of work vehicles having a chassis, with an engine and ground-engaging endless-loop tracks that are disposed on either side of the chassis, that are driven by the engine, and that move the chassis over the ground.  
         [0021]     “Blade position” and “blade height” are used in the discussion below to refer to the position or height of the blade with respect to the ground on which the bulldozer is supported and the angle of the blade with respect to the chassis and with respect to the horizon. If the crawler-tractor chassis pitches forward, lowering the front of the chassis closer to the ground, the automatic pitch control system disclosed herein raises the blade with respect to the dropping front of the dozer to maintain a relatively constant blade height with respect to the ground. If the chassis pitches backward, raising the front of the chassis, the system lowers the blade to maintain a relatively constant blade height with respect to the ground.  
         [0022]     Referring to  FIG. 1 , a dozer  100  is illustrated. The dozer includes a chassis  102  and an engine  104  fixed to the chassis  102 . Dozer  100  also includes left side and right side drive systems  106 , each of which further includes a drive wheel  108  that is driven by engine  104  and an endless track  110  that is coupled to and driven by the drive wheel. Dozer  100  also includes a laterally extending blade  112  that is mounted to a left arm  114  and a right arm  116 . The arms are pivotally coupled to the chassis at their rear ends and are supported at their front ends by left and right hydraulic lift cylinders  118 , 120 .  
         [0023]     The left and right cylinder portions  122  of the hydraulic lift cylinders are coupled to the chassis and the left and right rod ends  124  are coupled to the left and right arms. When the operator retracts cylinders  118 , 120 , they shorten in length and lift blade  112 . Dozer  100  has an operator&#39;s compartment or cab  126  from which the operator operates dozer  100 . Among other controls, the cab includes an operator input device  128  that the operator manipulates to raise and lower blade  112 .  
         [0024]     Device  128  preferably includes a lever  130  having a neutral central position. The operator can move the lever in one direction from neutral to raise the blade and can move the lever in the other direction to lower the blade.  
         [0025]      FIG. 2  shows blade pitch control system  132  in detail. System  132  includes a blade position sensor  134 , an electronic controller  136  that is coupled to device  128 , a speed sensor  138 , a pilot hydraulic valve  140 , a main hydraulic valve  142 . System  132  also includes an operator switch  144  that is coupled to controller  136 .  
         [0026]     Electronic controller  136  is a digital microprocessor-based controller, having a RAM, ROM, CPU, sensor input and signal conditioning circuits, valve driver circuits, and serial communications circuits. The sensors and switches are coupled to the sensor input and signal conditioning circuits, the pilot valve is coupled to the valve driver circuits and other digital controllers are coupled to the serial communications circuit. The ROM stores the CPU instructions that constitute the program, the RAM provides working space for the CPU to store values that change during operation, and the CPU executes the program instructions stored in ROM. All these components are coupled together by a data, address and control bus in a conventional manner.  
         [0027]     Device  128  preferably includes a variable resistor or shaft encoder coupled to lever  130  to provide a signal proportional to (and indicative of) lever position. This signal is provided to controller  136  on the signal line coupling the two. Lever  130  is mounted to pivot about a pivotal axis when grasped and deflected by the operator.  
         [0028]     Lever  130  is preferably spring loaded such that it returns to a central neutral position when released by the operator. In this way, movement in one direction away from the neutral position is identified by controller  136  as a request to raise blade  112  and movement in the other direction is identified by controller  136  as a request to lower blade  112 .  
         [0029]     Speed sensor  138  is coupled to wheel  108  to provide a signal indicative of wheel speed and vehicle speed. Sensor  138  may be a Hall Effect device, shaft encoder, or other device configured to indicate the rotational speed and direction (velocity) of wheel  108  or the speed of the vehicle.  
         [0030]     Pilot hydraulic valve  140  includes a coil  146  that is coupled to the valve driver circuit of controller  136 . Valve  140  is a proportional control valve that regulates flow in both directions through valve  140 . The output of pilot valve  140  is applied to both ends of main hydraulic valve  142 . The output of valve  140  opens valve  142  proportional to the magnitude and direction of the signal applied to coil  146  of valve  140 . Thus, the greater the signal applied to coil  146 , the faster the movement of blade  112 . In a preferred arrangement, a bulldozer can be retrofitted with a blade pitch control system such as that described herein, by coupling pilot hydraulic valve  140  to an existing bulldozer blade control system to that bulldozer&#39;s existing main hydraulic valve  142 . In this manner, the operator can use the bulldozer&#39;s existing blade control input devices to drive the bulldozer&#39;s existing valve  142  and control the bulldozer blade position, or the operator can release those controls (which may be electrical, mechanical, fluidic or a combination of any of the three) and control the blade using the blade control system described herein.  
         [0031]     The movement of the valves (and hence the blade) is a function not just of the magnitude of the applied signal but also the direction of the signal. If the signal is applied in one direction, the blade moves upward. If the signal is applied in the opposite direction, the blade moves downward.  
         [0032]     Blade movement is therefore proportional to, and in the direction indicated by, the electrical signal which controller  136  applies to coil  146 .  
         [0033]     Blade position sensor  134  provides a signal indicative of the position of the blade—preferably, the angle of the blade or the rate of change of the blade angle as it pitches forward and backward. Blade position sensor  134  preferably includes an angular or rotational turning rate sensor, a sensor that senses the rate of rotation about an axis. Such sensors include, for example, pitch, yaw, or roll rate sensors. In the preferred embodiment, the sensor is fixed to the side-to-side center of the blade and is responsive to the pitching of the bulldozer blade about a lateral (side-to-side) axis.  
         [0034]     Whenever the blade is either raised or lowered with respect to the chassis, the entire blade rotates about a lateral axis defined by the trailing ends of the two arms that support the blade. The trailing ends of these arms are rotationally coupled to the chassis of the crawler-tractor. Whenever the hydraulic lift cylinders are extended or retracted, the blade, in effect, rotates about a generally horizontal and lateral axis defined by the pivot points where the bulldozer arms are coupled to the chassis.  
         [0035]     Similarly, whenever the chassis itself pitches forward or backward going over a stump or rock, for example, the blade also rotates about the generally horizontal and lateral axis.  
         [0036]     In both of these cases, the position sensor is mounted to the blade to sense the blade&#39;s angular rotation about a lateral axis and transmits a signal indicative of this movement to controller  136 . Whether the blade tilts (i.e. rotates) forward and moves downward toward the ground or tilts (i.e. rotates) backward and moves away from the ground due to (1) extending or retracting the hydraulic lift cylinders or (2) because the chassis of the vehicle pitches, makes no difference: the effect is the same. The angular rotation of the blade with respect to a lateral axis is proportional to the blade&#39;s height. Thus, the height of the blade can be maintained in a generally constant position by maintaining the blade at a constant angle of tilt or pitch.  
         [0037]     If the blade position sensor  134  is a rate sensor, its rate of rotation signal may be integrated by (or at) the sensor itself to provide an absolute position signal. Alternatively, the signal may be transmitted to controller  136  as a rate of rotation signal and integrated by (or at) controller  136  to provide a signal that indicates absolute blade position (angle).  
         [0038]     Operator switch  144  has an “off” and an “on” position. When the switch is in the “on” position, the switch signals controller  136  to automatically reduce blade pitching. When switch  144  is in the “off” position, the switch signals to controller  136  that the controller should not automatically reduce blade pitching. Alternatively, the system can include a gyro rate control. The gyro rate control can be used to adjust sensitivity.  
         [0039]     When the operator switch is in the “off” position, however, direct control of blade  112  position is possible by operator manipulation of device  128  by lever  130 . Controller  136  applies a signal to coil  146  proportional to and in the direction indicated by the movement of lever  130  of input device  128 . When the controller senses that the operator has moved lever  130  in the “R” direction, controller  136  signals coil  146  to raise the blade at a speed proportional to the deflection of lever  130  in the R” direction. When the controller senses that the operator has moved lever  130  in the “L” direction, controller  136  signals coil  146  to lower the blade at a speed proportional to the degree of deflection of lever  130  in the “L” direction. Thus, when switch  144  is “off”, controller  136  is configured to move the blade up-and-down at a rate that corresponds to the degree of deflection of lever  130 . In this mode, lever  130  signals the rate at which blade  112  rises and falls  
         [0040]     When the operator switch is in the “on” position, controller  136  controls blade pitching by monitoring the blade&#39;s angular position with sensor  134  and driving the blade up or down with valve  140 , to keep it at a generally constant angle with respect to the earth. This automatic pitch control is described below in conjunction with  FIGS. 3 and 4 .  
         [0041]     When the switch is in the “on” position, controller  136  operates, generally, by (1) receiving signals indicative of blade position from blade position sensor  134 , (2) receiving signals indicative of a preferred or target blade position from device  130  and (3) combining the two signals to keep blade  112  at the preferred or target position.  
         [0042]     Controller  136  determines the operator&#39;s preferred blade position from the signals that are provided by input device  128 . It compares that position with the actual blade position and, based upon the difference between the two, drives the blade to the target position. It does this by controlling pilot valve  140 , which in turn controls main valve  142 , which in turn controls hydraulic lift cylinders  118 , 120 , which in turn raise and lower blade  112  with respect to the crawler-tractor.  
         [0043]     Control system  132  is coupled to a source of hydraulic fluid  148 . This source includes a hydraulic pump that is driven by the bulldozer&#39;s engine. The system is also coupled to a hydraulic fluid reservoir  150  to which fluid is returned. The source  148  and reservoir  150  are coupled to the valves to provide the hydraulic fluid used to operate the valves and the hydraulic cylinders.  
         [0044]     The components described above, including the blade position sensor  134 , the electronic controller  136 , operator input device  128 , speed sensor  138 , pilot hydraulic valve  140 , main hydraulic valve  142 , and operator switch  144 , collectively constitute pitch control system  132 .  
         [0000]     Primary Pitch Control Mode  
         [0045]      FIG. 3  is a flow chart illustrating the programming of controller  136  and the operation of the blade pitch control system. Controller  136  is configured to execute the steps shown in  FIG. 3  whenever the operator switch  144  is in the “on” position. The steps shown in  FIG. 3  are repeated by controller  136  on a preferred interval of 10 to 100 milliseconds.  
         [0046]     In the first step  152  of  FIG. 3 , controller  136  reads the signal from the input device  128 , which indicates whether the operator is requesting that the blade be raised, lowered, or held in the same position. In step  154 , controller  136  checks the signal from input device  128  to see if lever  130  is in neutral. Due to its spring loading, lever  130  remains in its neutral position until the operator moves it to another position and returns to neutral when released.  
         [0047]     If the lever is in neutral, the process continues to step  156 , if is not in neutral, controller  136  continues to step  158 . In step  158 , controller increments or decrements “TARGET”, a digital value stored in the memory of controller  136  (and identified herein for convenience as “TARGET”) that indicates the operator&#39;s preferred or target position for blade  112 .  
         [0048]     Controller  136  increments TARGET if the operator has moved lever  130  in the “raise” (“R” in  FIG. 2 ) direction from the neutral position (“N” in  FIG. 2 ). Controller  136  increments TARGET an amount proportional to the distance that lever  130  is deflected in the “raise” direction. Controller  136  decrements TARGET if the operator has moved lever  130  in the “lower” (“L” in  FIG. 2 ) direction from the neutral position. Controller  136  decrements TARGET an amount proportional to the distance that lever  130  is deflected in the “lower” direction.  
         [0049]     Once controller  136  has changed TARGET, processing continues with step  156 . In step  156 , controller  136  reads the signal from the blade position sensor  134 . This signal generally indicates the actual position of the blade with respect to the ground. Controller  136  then stores the value of this signal in a memory location in controller  136  identified for convenience herein as “ACTUAL” herein. Whenever the vehicle pitches forward, the blade both moves downward and tilts forward at the same time. Whenever the blade is lowered using hydraulic lift cylinders  118 , 120 , the blade is not only lowered but also tilted forward. In both cases, the angle of the blade indicates the blade&#39;s position.  
         [0050]     Having read the signal from sensor  134  in step  156 , controller  136  proceeds to step  160 , which represents the feedback control loop executed by controller  136  for controlling the position of blade  112 . In step  160 , controller compares the actual position of the blade derived from the signal of sensor  134  to the target position of the blade provided by the value TARGET stored in the controller&#39;s memory circuits. If the blade is not at the target position (TARGET), controller  136  is programmed to open pilot valve  140  an amount appropriate to incrementally move the blade back to its target position.  
         [0051]     As the blade moves back toward its target position with each iteration of the steps of  FIG. 3 , the actual blade position (ACTUAL) that controller  136  reads in step  156  gets closer and closer to the target position (TARGET). This process is called “feedback control” and the repeated iterations through steps  156  and  160  are called a “feedback control loop” or “feedback control algorithm”. They are called this since (1) the controller repeatedly loops through the steps and (2) the process relies upon feedback from the physical system being controlled (i.e. the position of the blade and hence the signal from sensor  134 ) to determine the appropriate control actions to be taken. In this example, the control action taken by controller  136  is closing or opening valve  140 .  
         [0052]      FIG. 4  is a control diagram of the PID (proportional-integral-derivative) feedback control loop executed by controller  136 .  
         [0053]     While this particular control loop is representative of a typical feedback control algorithm, it should be understood that it is just one of many automatic feedback control algorithms that may be used to control blade position. The selection of an appropriate feedback control algorithm depends upon many factors, including the particular size, shape, and mass of the structures being controlled (e.g. the blade  112  and the arms); the configuration and capacity of the devices controlling them (e.g. the hydraulic valves  140 , 142  and cylinders  118 , 120 ); and the speed, resolution, and accuracy of the sensors and instrumentation (e.g. blade position sensor  134 ).  
         [0054]     The control loop of  FIG. 4  is preferably implemented in software, in which the control loop items shown in  FIG. 4  are programming constructs. In the control diagram, the target blade position, “TARGET”, ( 162 ) is summed at junction  164  with the actual blade position, “ACTUAL”, ( 166 ) provided by sensor  134  to provide an error signal on line  168 . This error signal is provided to a proportional gain block  170 , a differential block  172  and an integral block  174 .  
         [0055]     The blade position can be expressed in absolute terms or in relative terms as an angle or a distance. The units used by controller  136  are immaterial. What is important is that whatever units are used, the blade position (height or angular rotation) be kept generally constant in the vicinity of TARGET.  
         [0056]     The proportional block generates an output on line  176  that is proportional to the error signal. The derivative block generates an output on line  178  that is proportional to the derivative of the error signal (the time rate of change of the error signal), and the integral block generates an output on line  180  that is proportional to the integral of the error signal (the sum of the errors over time). Summing junction  181  combines the proportional, the integral, and the derivative components of the signal and provides that combined signal on line  182 . The combined signal is then applied to pilot valve  140  (block  184 ). When pilot valve  140  changes its position, it changes the position of main hydraulic valve  142  (block  186 ), which changes the position of blade  112  (block  188 ), by moving it up or down. When blade  112  changes position, it moves position sensor  134  (block  190 ) which is coupled to the blade. Sensor  134  responds accordingly by generating a signal indicating the new actual position (ACTUAL) of the blade.  
         [0057]     To summarize the operation of the automatic pitch control system shown in  FIGS. 3 and 4 , the system has a preferred or target blade position (TARGET) to which it constantly drives the blade.  
         [0058]     Whenever the chassis of the vehicle pitches forward, the blade position sensor  134  senses the forward rotation (or pitching) of the blade about a lateral axis as the blade drops towards the ground. Controller  136  executes a feedback control loop to correct the blade&#39;s position using the hydraulic valves and the hydraulic lift cylinders to retract the hydraulic cylinders and to lift the blade upward. This has a double effect of maintaining the blade at a generally constant angle of tilt and maintaining the blade at a generally constant height relative to the earth.  
         [0059]     Similarly, when the vehicle&#39;s chassis pitches backwards, it causes the blade to tilt backwards and the blade to lift higher above the ground. Blade position sensor  134  senses this backwards rotation of the blade of and signals controller  136 . Controller  136 , in turn, executes the feedback control loop to correct the blade&#39;s position by extending the hydraulic cylinders and lowering the blade downward toward the ground. This also has the double effect of maintaining the blade at a generally constant angle of tilt with respect to the earth and maintaining the blade at a generally constant height with respect to the earth.  
         [0060]     The operator can change the target blade position by moving lever  130 . Whenever the target blade position changes, the control loop executed by controller  136  responds by automatically controlling the position of the blade.  
         [0061]     When the operator switch is in the “off” position, controller  136  commands the valves to open (and hence the blade to move) in a direction proportional to the degree of deflection of lever  130  from its neutral position. If the bulldozer chassis pitches when the operator switch is in the “off” position, the hydraulic cylinders do not move with respect to the chassis. The blade pitches just as the chassis pitches, either digging deeper into the ground when the chassis pitches forward or rising up out of the ground when the chassis pitches backward.  
         [0000]     Automatic TARGET Determination  
         [0062]     In the example illustrated above, the target position of the blade is selected manually by the operator who can change the target blade position at any time by operating the input device. However, in an alternative mode of operation, controller  136  is configured to automatically determine the initial blade position in automatic pitch control mode based upon the average blade position during manual operation.  
         [0063]     In this alternative mode of operation, controller  136  is configured to periodically read the actual position of the blade from sensor  134  during manual operation of the bulldozer (i.e. when switch  144  is turned off). During this manual operation, the operator gradually adjusts the blade position over time with input device  128  until he finds the optimum blade position.  
         [0064]     As the operator adjusts the blade during manual operation, controller  136  is configured to automatically read successive blade positions (i.e. the blade position signal) over a period of time. Controller  136  is configured to average these successive signals to determine an average actual blade position. Controller  136  is therefore aware of the operator&#39;s desired blade position even before the operator turns the blade pitch control system “on”. Once the operator engages the blade pitch control system by turning switch  144  “on”., controller  136  already knows the current height of the blade and can immediately take over and keep the blade at that height.  
         [0065]     Once the pitch control system is turned on, controller  136  uses the position it calculated during manual mode as its initial target blade position (TARGET). Thus, from the moment the operator turns the automatic pitch control system “on”, controller  136  starts controlling the blade position to keep the blade at the same position that the operator was manually keeping it.  
         [0000]     Alternative Pitch Control Mode  
         [0066]     In the example illustrated in FIGS.  34  above, the target position of the blade is changed by the operator whenever the operator manipulates input device  128 . In the automatic pitch control mode, whenever the operator moves lever  130  up or down, the target blade position (TARGET) changes up or down, responsively.  
         [0067]     In an alternative configuration, however, controller  136  is configured to change the target position (TARGET) when the input device is moved in one direction, but not to change the target position when the operator moves the lever in the other direction. In this mode, whenever the operator moves lever  130  in the “L” (or “lower”) direction, for example, controller  136  responds by lowering the target position (TARGET) of the blade. Controller  136  continues controlling blade pitching, but does so with a new and lower target blade position.  
         [0068]     When the operator moves lever  130  of input device  128  in the “R” or “raise” direction, however, controller  136  is configured to not change (i.e. to not raise) the target blade position. Instead, controller  136  raises the blade as though the switch  144  was “off”, and temporarily ceases to automatically control blade position. Controller  136  remembers the target blade position, however, and does not change it. Controller  136  just ceases to drive the blade to the target position until the operator again signals his desire for automatic blade pitch control.  
         [0069]     In this alternative configuration, controller  136  interprets the operator&#39;s upward movement of lever  130  not as a request to raise or increase the target blade position, but as a request to (1) temporarily raise the blade to avoid obstructions, and (2) temporarily disable automatic pitch control until the obstruction is passed.  
         [0070]     The operator can continue through the field for any distance with the blade held by controller  136  in this raised position. Controller  136  will not begin automatically controlling blade pitch again until the operator signals controller  136  to restart automatic control using lever  130  in a special manner.  
         [0071]     In this way the operator does not have to turn the automatic pitch control “off” with switch  144 , then raise the blade, then wait for the obstruction to pass, then turn the automatic blade pitch control “on” again with switch  144  when he wishes to return to automatic pitch control at the original height.  
         [0072]     The operator signals his desire to restart automatic blade pitch control in a manner opposite the way he signaled his desire to leave automatic blade pitch control.  
         [0073]     When the operator wishes to turn automatic blade pitch control back on in this alternative configuration, he moves lever  130  in the “L” (lower) direction (again, without manipulating switch  144 ). Controller  136  responds to this lever movement in the “L” direction by lowering the blade just as it does when switch  144  is “off” and without changing the target blade position.  
         [0074]     With the operator holding lever  130  in the “L” position, the controller begins to lower the blade toward its target blade position. The blade eventually drops to the target blade position. Controller  136  is aware of this approach toward the target blade position since controller  136  is configured to continuously monitor the actual blade position during this blade descent. As the blade descends, controller  136  is configured to compare the actual blade position with the (lower) target blade position.  
         [0075]     Eventually controller  136  determines that the blade is within a small and predetermined distance of the target blade position. At this point, controller  136  stops functioning as though switch  144  is “off”, and restarts its automatic control of blade position.  
         [0076]     Once controller  136  has restarted its automatic control of blade position, if the operator releases lever  130  of input device  128  the controller merely continues automatically controlling the blade position.  
         [0077]     Alternatively, if the operator does not release lever  130  but keeps holding it in the “L” position once automatic control has been reengaged, controller  136  does not keep lowering the blade, but controls the blade height at the target blade position.  
         [0078]     To summarize the operation in this alternative automatic pitch control mode, the operator can lower the target blade position with lever  130 , but cannot raise the target blade position by moving lever  130  in the “raise” direction. Instead, when the operator raises the lever that signals controller  136  to (1) raise blade  112  above the target blade position and (2) immediately stop moving the blade just as though the blade pitch control is disabled when the operator releases lever  130 . The purpose of this operating mode is to permit the operator to briefly raise the blade above stumps, rocks, or other protrusions, to temporarily disable automatic blade positioning without changing the target blade height and to permit restarting of automatic blade pitch control without having to manipulate switch  144 .  
         [0079]     Assuming the operator wishes to lower the blade further (i.e. below the current target blade position the controller  136  has locked onto), the operator must first release lever  130 , permitting it to return to the neutral position. This return to neutral is immediately sensed by controller  136 .  
         [0080]     Once controller  136  senses the blade had returned to neutral, it keeps controlling the blade position automatically, but permits the operator to (1) lower the blade and the target blade position by moving the lever in the “L” direction, or (2) raise the blade above the target blade position by moving the lever in the “R” direction. If the operator wishes to keep lowering the blade in the automatic mode once the controller has “locked on”, he must first release the lever. In another alternative embodiment, when the operator lowers the blade to within the small and predetermined distance of the target blade position, the system does not automatically begin controlling blade position at the target blade height before the operator releases the lever, but waits until the operator releases the lever, permitting it to return to its neutral position.  
         [0081]     In another alternative embodiment, the speed sensor senses the vehicle&#39;s speed through the field and provides it to controller  136 . Controller  136 , in turn, changes the response rate of its PID control loop to respond faster when the vehicle is moving faster through the field, and to respond slower when the vehicle is moving slower through the field.  
         [0082]     In yet another alternative embodiment, a GPS can be coupled to controller  136  to provide location information to controller  136  that could provide better information on the vehicle&#39;s path through the field.  
         [0083]     It will be understood that changes in the details, materials, steps, and arrangements of parts which have been described and illustrated to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure within the principles and scope of the invention.  
         [0084]     For example, rather than providing a blade position sensor that is responsive to the blade being raised and lowered and responsive to the blade pitching about a substantially lateral axis, a blade position sensor can be provided that is responsive to rotating about a longitudinal axis, or “rolling”. In this case, the blade position sensor would sense the rolling of the blade about a longitudinal axis such as occurs when one side of the bulldozer is raised above the other side. This happens, for example, when the track on one side of the bulldozer runs over our rock or a stump. Many bulldozers are configured with hydraulic cylinders coupled to the blade that control the blade&#39;s roll angle. These hydraulic cylinders cause one side of the blade to raise and the other side of the blade to lower. In this arrangement, if the chassis begins to roll to the right, with the left side tracks being lifted higher above the ground than the right side tracks, controller  136  will sense the corresponding rolling of the blade to the right by monitoring this alternative or additional blade position sensor that senses rotation about the blade&#39;s longitudinal axis. Using the same control algorithm described above with regard to bulldozer pitching, controller  136  is configured to maintain the roll angle of the blade constant.  
         [0085]     In a second alternative arrangement, a blade control system can be provided with a blade position sensor responsive to rotation about a longitudinal axis and rotation about a lateral axis (such as illustrated herein). In this arrangement, electronic controller  136  could be coupled to both of these blade position sensors, and could be configured to execute two PID control loops, one controlling rotation around the longitudinal axis and the other controlling rotation around the lateral axis. In effect, controller  136  would control the height of the blade as well as its left to right tilt angle.  
         [0086]     The position sensor is shown as a single physical device coupled to the center of the blade of the bulldozer. In an alternative embodiment, the position sensor  134  may be located elsewhere, such as a long the dozer arms, or it may comprise two physical devices, one device to provide chassis position and one disposed to provide a position signal indicative of the difference between the chassis position and the blade&#39;s position. These two devices may include a position sensor, such as item  134 , coupled to the dozer chassis and an angle sensor coupled between the blade and the chassis to provide a chassis-to-blade angle signal. By combining signals from these two physical devices, one showing or indicating the dozer chassis position with respect to the ground, and the other showing or indicating the position of the blade with respect to the chassis, the system described herein would function just as well.  
         [0087]     The foregoing description illustrates the preferred embodiment of the invention; however, concepts, as based upon the description, may be employed in other embodiments without departing from the scope of the invention. Accordingly, the following claims are intended to protect the invention broadly as well as in the specific form shown.