Abstract:
A differential steering control system and method for a machine includes a motor for steering the machine in a leftwards or a rightwards direction, a continuously variable transmission for moving the machine in a forwards or a backwards direction, and a control unit connected to the motor and the continuously variable transmission. The control unit determines the speed of the continuously variable transmission and based upon the determined speed, modifies a speed of the motor in order to achieve/maintain a desired turn radius for the machine.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional patent application No. 60/876813, filed Dec. 22, 2006. 
     
    
     TECHNICAL FIELD  
       [0002]    This invention relates generally to a steering control system, and more particularly, to a differential steering control for a machine having a continuously variable transmission. 
       BACKGROUND  
       [0003]    Hydro-mechanical steering differential systems are employed in large track-type tractors or machines to provide smooth, equal, and uninterrupted power and torque flow to both the left and the right tracks at all times when the tractor is moving. It is also important to provide power to the tracks whenever the tractor is turning in either the left or right directions. A hydro-mechanical steering differential system is typically powered by a mechanical input and a hydraulic input. The mechanical input is provided from the transmission and the hydraulic input is received from an engine driven variable displacement pump. 
         [0004]    In conventional machines, such as a track-laying type of machine, turning control is performed by an operator steering lever or mechanism. However, one problem with the use of such operator steering levers or mechanisms is that it is difficult to have a desired constant turning radius of the machine. For example, it is sometimes difficult to control the machine when attempting to turn with a constant turning radius in a region of low machine speeds or high loads. 
         [0005]    U.S. Pat. No. 5,611,405, entitled “Turning Speed Control System for Use in a Track-Laying Vehicle” teaches a turning speed control system. The &#39;405 patent provides a system that reduces vehicle speed in order to achieve a commanded turn radius when the steering motor is rotating at maximum speed. 
         [0006]    The &#39;405 patent however, reduces vehicle speed based on turning such that when a turning motor reaches a maximum displacement, and additional turning is demanded but not available, the transmission ratio is reduced. This effect may cause the engine to lug, or increase emissions. 
         [0007]    The present disclosure is directed to overcoming one or more of the problems as set forth above. 
       SUMMARY OF THE INVENTION  
       [0008]    In one embodiment, a steering control system for a machine is provided. The steering control system includes a continuously variable transmission, a motor connected to the continuously variable transmission, and a control unit connected to the motor and the continuously variable transmission. The control unit modifies a speed of the motor in response to a change in at least one of a speed of the continuously variable transmission and a load on the engine in order to maintain a desired turn radius. 
         [0009]    In another embodiment, a differential steering control system is provided. The differential steering control system includes a motor for steering the machine in a leftwards or a rightwards direction, a continuously variable transmission for moving the machine in a forwards or a backwards direction, a steering device for providing an indication of a desired turn radius, and a control unit connected to the motor, the continuously variable transmission, and the steering device. The control unit determines a speed of the continuously variable transmission and the desired turn radius, and based upon a change in the determined speed and the desired turn radius, the control unit controls a speed of the motor in order to achieve and maintain the desired turn radius. 
         [0010]    In yet another embodiment, a method for controlling a differential steering device for a machine having a continuously variable transmission is provided. The differential steering device includes a motor for turning the machine in a leftward or rightward direction. The method includes the steps of determining at least one of a speed of the continuously variable transmission and a load on the engine, determining a desired turn radius, and modifying a speed of the motor based upon the at least one of the determined speed and load to maintain the desired turn radius. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    The accompanying drawings, which are incorporated and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
           [0013]      FIG. 1  illustrates a schematic view of a machine-level steering differential control system according to one embodiment of the present invention; 
           [0014]      FIG. 2  illustrates a schematic view of a machine-level steering differential control system according to another embodiment of the present invention; and 
           [0015]      FIG. 3  illustrates a schematic view of a steering differential control system according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0016]    Reference will now be made in detail to embodiments or features of the invention. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. Whenever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0017]      FIG. 1  illustrates a steering control system  10  incorporated within a machine  12  for providing differential steering to a left traction device  14  such as a wheel, tire, or track, and a right traction device  16 , such as a wheel, tire, or track, of a drive train  18 . The drive train  18  includes an engine  22  and a continuously variable transmission (“CVT”)  24  connected to the engine  22  through a drive shaft  26 . The continuously variable transmission  24  may be hydromechanical, electromechanical, or purely electric, including a generator and motor. In the embodiment of  FIG. 1 , the continuously variable transmission  24  is illustrated as a hydromechanical transmission. Except as otherwise indicated, the CVT  24  will be referred to as a hydromechanical transmission. 
         [0018]    The hydromechanical transmission  24  is connected to a steering differential  28  through a shaft  30  to provide a mechanical input thereto. A pump  32  provides hydraulic input to a motor  34  through fluid lines  38  to drive the steering differential  28 . The pump  32  may be a reversible, variable displacement type and driven by the engine  22  through a suitable drive connection  36 . 
         [0019]    Referring to  FIGS. 1 and 2  jointly, the steering differential  28  drives the left traction device  14  through a left axle shaft  48 , which is coupled to a left final drive  44 . The steering differential  28  drives the right traction device  16  through a right axle shaft  50 , which is coupled to a right final drive  52 . Movement of the machine  12  may be in either the forward or reverse direction and the machine  12  may also be placed in neutral. The machine  12  may be maneuvered to move or turn in either the left or right directions when a relative velocity exists between the left traction device  14  and the right traction device  16 . 
         [0020]    A control unit  54  monitors and controls the direction and speed of the machine  12  through the hydromechanical transmission  24  and the motor  34 . The control unit  54  may be electrically connected to the engine  22 , the hydromechanical transmission  24 , the pump  32 , and the motor  34 , as well as additional sensors  56  (see  FIG. 3 ) placed on the left or right traction devices  14 ,  16 , or on the left or right axle  48 ,  50  to determine speed, direction, errors, temperature, and the like. 
         [0021]      FIG. 2  illustrates a CVT  24  embodied as an electric drive train. The electric drive train includes a generator  42 , power electronics  43 , and an electric motor  46 . The electric motor  46  provides propulsion to move the machine  12 . In this particular embodiment, the electric motor  46  receives power from the generator  42  or the power electronics  43 . The steering motor  34  is also electric. 
         [0022]    Referring now to  FIG. 3 , the control unit  54  includes a microprocessor  58  for executing a specified program, which controls and monitors various functions associated with the machine  12 . The microprocessor  58  includes a memory  60 , such as ROM (read only memory)  62 , for storing a program, and a RAM (random access memory)  63  which serves as a working memory area for use in executing the program stored in the memory  62 . Although the microprocessor  58  is shown, it is also possible and contemplated to use other electronic components such as a microcontroller, an ASIC (application specific integrated circuit) chip, or any other integrated circuit device. 
         [0023]    The control unit  54  electrically connects to the engine  22 , a steering device  64 , a direction selector  66 , an output speed sensor  68 , a differential speed sensor, or sensors,  70 , the pump  32 , the motor  34 , and other speed and temperature type sensors  56 . The steering device  64  may be a wheel, joystick, pedals, or the like. The direction selector  66  may be a Forward-Neutral-Reverse (“FNR”) device, or the like, and provides desired directional information to the control unit  54 . The output speed sensor  68  provides information indicative of an output speed of the hydromechanical transmission  24 , and the differential speed sensor  70 . 
       INDUSTRIAL APPLICABILITY  
       [0024]    In operation, the hydromechanical transmission  24  provides propulsive power to the driven elements,  14 ,  16 . The steering device  64  provides a signal to the control unit  54  indicative of an operator&#39;s desire to turn to the left or to the right at a desired turn radius. The control unit  54  commands an up-stroking or a de-stroking of the pump  32  to increase or decrease the speed of the motor  34 . Alternatively, the control unit  54  may command additional power supply to increase the speed of the motor  34  if it is an electric motor  46 . 
         [0025]    The motor  34  either increases or decreases a rotational speed of either of the left or right axle shafts  48 ,  50  to create a relative speed there between. In effect, the left or right final drives  51 ,  52  increase or decrease to create a relative speed between the two to effect a turning action. 
         [0026]    At high or low machine speeds, the turn radius of the machine  12  is achieved by one or both of a reduction in the speed of the hydromechanical transmission  24  and increasing the speed of the motor  34 . At higher machine speeds, the speed of the motor  34  is changed to achieve a commanded turn radius. However, at slower machine speeds, the speeds of the motor  34  and the hydromechanical transmission  24  may be changed to achieve the commanded turn radius. If the speed of the hydromechanical transmission  24  is reduced because of external loads, then control unit  54  may reduce the speed of the motor  34  to maintain the commanded turn radius. 
         [0027]    As the machine  12  begins turning, the control unit  54  receives information from the steering device  64 , the engine  22 , the pump  32 , the motor  34 , the hydromechanical transmission  24 , and other speed sensors. The hydromechanical transmission  24  includes logic controls for controlling the input/output ratio and automatically adjusts according to a desired engine running speed. As a result, when a load causes the engine to lug below the desired engine running speed, the hydromechanical transmission input/output ratio adjusts to modify the input/output ratio such that the engine speed increases to a desirable level. However, as the hydromechanical transmission  24  changes ratio to maintain engine speed, the steering system commands less steering speed to maintain the desired turn radius. Accordingly, the motor  34  decreases speed a corresponding amount to maintain the desired turn radius. If the motor  34  is already operating at a maximum speed, the control unit  54  commands the continuously variable transmission to decrease transmission ratio. In effect, the vehicle  12  slows, which results in a decreased turn radius. 
         [0028]    It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed system and method without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only.