Patent Publication Number: US-2010120578-A1

Title: Drive and steering control system for an endless track vehicle

Description:
FIELD OF THE INVENTION 
     The present invention relates to endless track vehicles and more specifically to drive and steering control systems for such vehicles. 
     BACKGROUND OF THE INVENTION 
     The endless track vehicle has been in existence for over a century and has been applied to a wide variety of work machine situations. One such application is found in the harvesting of sugar cane. An endless track vehicle traverses the field and has a section severing the sugar cane and a section processing it for delivery to a hopper or second vehicle for carrying away for further processing. Traditionally, such harvesters have been controlled in the manner of endless track vehicles with separate levers, each controlling one of the tracks in terms of direction and rotational speed to propel the vehicle in a controlled manner at a selected speed. Another such control implementation is the use of a T-handle which has a pivoting movement to control speed and a rotational movement to control differential track RPM so as to change direction. A further implementation is the use of a so called “joy stick” that responds to forward movement for vehicle velocity control and side-to-side movement for turning movements. 
     While such control systems generally provide a way for an endless track vehicle to be controlled, they increase operator fatigue, particularly with respect to hand movements of the operator since the operator must compensate for variations in component manufacturing variations. Furthermore, there is a required learning curve for vehicle operators making a transition from traditional wheeled vehicles to endless track vehicles. 
     What is needed in the art therefore is an effective way to control endless track vehicles with a system that reduces operator fatigue and learning requirements. 
     SUMMARY OF THE INVENTION 
     In one form, the invention is a drive and steering control system for an endless track vehicle. The system includes a pair of variable volume hydrostatic pumps respectively connected to a pair of hydraulic motors, each driving one of a pair of endless tracks for the vehicle. An operator controlled speed control mechanism is included for controlling forward and reverse velocity of the vehicle. A steering wheel is provided for manipulation by an operator to control direction of the vehicle and a controller receives inputs from the steering wheel and the speed control mechanism for controlling the RPM of the hydraulic motors to control the velocity and turning of the vehicle. 
     In another form, the invention is an endless track vehicle having a frame, a prime mover and a pair of endless tracks for ground movement. The prime mover drives a pair of variable volume hydrostatic pumps, respectively connected to a pair of hydraulic motors, each of which drives one of the endless tracks. A speed control mechanism is provided for controlling the forward and reverse velocity of the vehicle in response to an operator input. A steering wheel is mounted on the frame for manipulation by an operator to control turning of the vehicle. Finally, a controller receives inputs from the steering wheel and the speed control mechanism for controlling the RPM of the hydraulic motors to control the turning and velocity of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an endless track vehicle with a drive and steering control system embodying the present invention; 
         FIG. 2  is schematic view of the drive and steering control system for the endless track vehicle of  FIG. 1 ; and 
         FIG. 3  is a graph showing the relationship between the steering wheel inputs and differential track speed for different operating conditions of the vehicle of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 and 2 , there is shown an endless track vehicle  10  having a frame  12  for mounting a pair of endless track assemblies  14  and  16  on opposite sides of the frame  12 . A prime mover  18 , usually in the form of a compression ignition or diesel engine, is used to provide propulsion for the vehicle  10  and various agricultural processing functions. The prime mover  18  is connected to a variable volume dual path hydrostatic pump assembly comprising first pump  20  and second pump  22 . The variable volume hydrostatic pumps  20  and  22  are respectively connected to hydraulic motors  26  and  28  respectively driving endless track assemblies  14  and  16 . The variable volume hydrostatic pump  20  is connected to motor  26  for bidirectional flow of fluid through a pair of conduits  30 . Thus, pump  20  can operate hydraulic motor  26  in a forward, neutral and reverse direction with variable RPM. Pump  22  is connected to motor  28  by a pair of conduits  32  and also permits variable volume bidirectional flow to control the RPM of motor  28 . 
     A harvesting mechanism  36  is provided at the forward end of vehicle  10 . This mechanism may take many forms depending upon the function required. For example, for sugar cane the mechanism  36  would be a mechanism that gathers and severs the sugar cane for further processing, not shown to enable a clearer understanding of the invention. A further mechanism  38  may be provided for delivering the harvested material either to a storage container or to an additional vehicle used to carry the sugar cane away. 
     An operator control cab  40  is provided at the forward end of vehicle  10  and includes an operator seat  42  and vehicle speed control mechanism  44 . The speed control mechanism may take the form of a number of control inputs such as a dual output propulsion potentiometer controlled by an operator lever  45 . A steering wheel mechanism  46  is provided for operator manipulation. Steering wheel mechanism  46  may be in many forms but in the usual form it provides a steering wheel  47  and a control mechanism to provide control inputs as described below. The steering wheel mechanism  46  is mechanically centered in a straight ahead direction usually using some form of bidirectional yieldable urging so that without an input the steering wheel is urged to a center, or neutral position. 
     The speed control mechanism  44  and steering wheel mechanism  46  provide signal inputs via lines  48  and  50  to a microprocessor  51 . Microprocessor  51  receives control inputs from the speed control mechanism  44  and steering wheel mechanism  46  to send signals via lines  53  and  55  to pumps  20  and  22  to vary the volume and thus the speed and direction of the endless tracks  14  and  16 . In a usual fashion, the control input provided by connections  53  and  55  is to manipulate a variable swash plate, although many other inputs may be employed. The RPM of the motors  26  and  28 , and thus the endless track speed is sensed by sensors  52  and  56  and fed back to microprocessor  51  by lines  54  and  58  respectively. Thus, the actual RPM of motors  26  and  28  is fed back to microprocessor  51  for manipulation in the manner described below. 
     The propulsion and steering control system includes the three sensor steering input device  46  mechanically centered and coupled to the standard steering wheel  47 , a dual output propulsion potentiometer as the main forward/reverse ground drive speed input mechanism  44 , the microprocessor  51  and individual RPM sensors  52  and  56  for feedback from the final drive motors. The disclosed steering system permits use of a standard steering column/wheel mechanical input for steering and a separate propulsion input for propelling forward or reverse. 
     The forward/reverse speed control system is a closed loop control. The desired speed command is generated from the speed control mechanism  44 . Mechanically linked to the input handle  45  is a propulsion potentiometer sensor. A dual output potentiometer with a main signal and redundant secondary signal is integrated into the system. 
     The feedback signal is the average ground speed measured from both motors  26  and  28 . The microprocessor  51  uses redundant input sensors and output drivers for improved reliability and operator safety. In the event a speed sensor feedback signal is lost, the microcontroller will alert the operator, via a diagnostic trouble code and immediately switch to open loop control of the propulsion and steering system. The speed sensors  52  and  56  provide the control system with speed and direction inputs. Any residual error between the left and right tracks ground speed is used to further close the loop on each side to match speeds and achieve straight tracking. 
     The steering input mechanism  46  measures the rotational position of steering wheel  47 . The device has fixed end-stops with 560 degrees of rotational lock to lock (280 degrees each direction from the spring centered position). The steering input mechanism  46  includes a self-centering mechanical spring, has a positive feel at the center, and requires low effort to steer. Three identical steering wheel position sensors provide redundancy from the steering wheel  47  to the steering control mechanism  46 . 
     A closed loop control strategy is used to control the speed and steering of the vehicle  10  and consists of an inner loop to individually control the speed of each side of the tracks  14  and  16 . An outer loop, speed tracking will monitor the differential speed between both sides of the tracks, compares the error with the steering command and feedbacks the result proportionally to each side in such a way that one side of the tracks speeds up and the other slows down. 
     In manual mode, steering of the vehicle to a desired turn radius is achieved by generating a variable ratio steering command based on the steering wheel position and vehicle speed. The steering command is added to the speed command so that one track is sped up and the other slowed down so that forward/reverse ground speed is maintained. For vehicles equipped with Global Position Systems (GPS), the control system uses position and course information of the vehicle to calculate a track course error and a lateral error and generate the proper steering command to guide the vehicle along predefined parallel tracks. 
       FIG. 3  illustrates the correlation between steering wheel postion and the relative speed of the motors  26  and  28 . In general, the vehicle  10  turn radius and direction of turn is a function of steering wheel  47  angular position, wheel RPM, engine RPM, and direction of travel. Changing direction of travel maintains the original turn radius. Steering action is reduced but not eliminated when the vehicle stops and a park brake is not engaged. A transitional counter rotation mode is built into the steering algorithm to prevent sudden steering from occurring when a park brake is disengaged while the steering wheel is not at the spring centered and the vehicle is stationary. 
     Another mode of steering referred to as the “endstop ramp” is also built into the steering algorithm in order to allow the operator maximum turning rate while maintaining fine steering around the spring centered position, the gain curve is extended by utilizing a ramping function when the operator is at the steering wheel endstops. The ramp rate is proportional to vehicle speed/engine RPM. In stationary counter rotation mode it is proportional to engine speed. The graph illustrates the maximum and minimum steering curves due to changes in engine speed and vehicle speed. 
     The feedback signal from the sensors  52  and  56  is fed back into the microprocessor  51  to provide a closed loop system in which the motors actually rotate at the commanded signal. This compensates for manufacturing variations between the pumps. The residual error between the left and right tracks is thus corrected to achieve a straight tracking which is particularly important for minimizing operator fatigue. 
     The turning of the vehicle is a function of the position of steering wheel  47  and that signal via line  50  overlays the speed signal from line  48  to produce differential controls at pumps  20  and  22  as a function of the wheel angular position, the engine speed, track speed and direction of travel. By using a standard steering wheel  47 , the need for special operator adjustment to the standard drive for a tracked vehicle is avoided. 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.