Abstract:
A non-feedback proportional system is provided to achieve desired operating characteristics which will allow the control of a hydrostatic transmission to closely approximate the control performance of a displacement feedback control system by establishing certain parameters relating to (1) the servo spring rate; (2) the valve plate index; (3) valve plate crossport/trap; (4) valve plate porting geometry; (5) swashplate offset; and (6) piston bore volume.

Description:
BACKGROUND OF THE INVENTION 
     Displacement feedback control systems (DFCs) and non-feedback proportional (NFPs) have previously been used for axial piston hydrostatic pumps. The Control Block Diagrams A and B shown in FIGS. 16 and 17 the basic differences between the Feedback (DFC) and non-feedback (NFP) control systems. 
     The control of the pump necessarily involves controlling the position of the swashplate of the pump. With a DFC mechanism, the swashplate position is primarily a function of only an input signal, manual, electrical or hydraulic. The DFC controls position of a servo piston and swashplate system proportional to the input signal utilizing a mechanical feedback linkage. With an NFP control, the swashplate position is a function of the input signal and the moments imposed on the swashplate which are dependent on the input speed to the pump, the operating pressure for the pump, and the swashplate angle. 
     The DFC system has several functional advantages, but is expensive to manufacture. The NFP is less expensive but also has certain beneficial characteristics including a softer ride for the vehicle driven by the transmission, and inherent characteristics similar to a low performance anti-stall system. However, on certain types of vehicles, these characteristics can be drawbacks. This is especially true of vehicles requiring constant speed (i.e., constant swashplate position), aggressive performance, responsiveness independent of vehicle load, and applications which use a micro-processor based anti-stall system sensing engine speed. In addition, the NFP control is typically less stable than a DFC due to the lack of compensation provided by the feedback mechanism. 
     Therefore, it is a principal object of this invention to provide a NFP system to allow improved control for a hydrostatic pump. 
     More specifically, it is an object of this invention to provide a NFP system to achieve desired operating characteristics which will allow the control of a hydrostatic transmission to closely approximate the control performance provided of a DFC system. 
     These and other objects will be apparent to those skilled in the art. 
     SUMMARY OF THE INVENTION 
     A method is provided for allowing a hydraulic pump with a non-feedback proportional control to closely approximate the performance of a displacement feedback control by taking a hydraulic pump including a rotatable piston group engaging a pivotal swashplate, with the pump having associated therewith an input power shaft and a servo piston mechanically connected to the swashplate to influence a torque imposed on the swashplate when rotational power is imposed on the pump, and a neutral return spring connected to the swashplate and a pump housing; providing a valve plate to control fluid flow between pistons in the group and pressure and return fluid conduits; providing in the valve plate a plurality of elongated arcuate slots extending therethrough concentrically located at a constant radius with respect to a center of the plate and an axis of rotation of the piston group, with each slot having opposite ends, an elongated notch at one end of some of the slots and extending away from the end to form a bottom with sidewalls extending upwardly with respect to the bottom; providing a valve plate index for the valve plate to approximately a −1.5° to −0.5° wherein the valve plate index is defined as the location of a pressure transition zone relative to top or bottom dead center positions of a piston in the rotatable piston group being in either a fully retracted or fully extended position in its operational movement; providing a cylinder block with a piston port for the piston group with fluid inlet and outlet ports in communication with the pistons of the piston group; providing valve plate crossport of approximately 3° to 9° wherein the valve plate crossport is defined as the amount of angle of rotation during which the piston port in the cylinder block is connected to both the inlet and outlet ports at the same time; maintaining the swashplate at a first swashplate offset of −0.015 in. to +0.015 in. in a first direction parallel to axes of rotation of the piston group; maintaining the swashplate at a second swashplate offset of −0.060 in. to +0.060 in. in a second direction perpendicular to a longitudinal axis of the pistons; maintaining the ratio of the volumes of the fluid in the piston bores at a top dead center position of movement of the piston in the bore to the volume at a bottom dead center position of 0.53 to 0.73 and adjusting the spring rate of the return spring to a range of approximately 470-670 pounds/inch; whereby the dependency of the angular position of the swashplate is influenced by less than 50% on operating conditions of the transmission and is increasingly influenced by more than 50% by an input signal acting thereon to ensure stability throughout the transmission&#39;s operating range for speed, pressure and swashplate angle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a hydrostatic transmission adaptable for use with this invention; 
     FIG. 2 is a schematic cross sectional view of the transmission of FIG. 1 in forward, neutral and reverse modes; 
     FIG. 3 is a schematic diagram showing the control forces acting on the swashplate; 
     FIG. 4 is a schematic view of the progressive positions of the servo spring; 
     FIG. 5 is a plan view of the valve plate; 
     FIG. 6 is a partial plan view of the valve plate taken on line  6 — 6  of FIG. 5; 
     FIG. 7 is a partial plan view of the fluid inlet into one of the grooves in the valve plate; 
     FIG. 8 is a sectional view taken on line  8 — 8  of FIG. 7; 
     FIG. 9 is a sectional view taken on line  9 — 9  of FIG. 8; 
     FIG. 10 is a diagram of valve plate indexing; 
     FIG. 11 is a bottom plan view showing the piston locations associated with inlet and outlet fluid pressures; 
     FIG. 12 is a perspective view of the bottom of the rotating group showing inlet and outlet ports; 
     FIG. 13 is a diagram showing the input signal effect on the swashplate with the pivot point of the swashplate being offset with respect to the pivotal axes of the swashplate trunnion; 
     FIG. 14 is an enlarged scale cross section through a piston in a rotating group at its top dead center position; and 
     FIG. 15 is a view like FIG. 14 showing the piston in a bottom dead center position. 
     FIG. 16 is a control block diagram for a feedback (DFC) control system. 
     FIG. 17 is a control block diagram for a non-feedback (NFP) control system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The overall goal of this invention is to control the slope and separation of control torque curves as a function of speed, system pressure, and swashplate angle. This includes: (1) reducing the dependency of the swashplate angle on non-input signal parameters (i.e., system pressure and speed; (2) increasing the dependency of the angle of the swashplate on the input signal; (3) ensuring the stability throughout the operating range of the vehicle for speed, pressure and swashplate angle; and (4) reducing the swashplate vibration and noise in conjunction with steps (1)-(3). 
     The foregoing goal is achieved by bringing together a plurality of known control concepts, fine tuning some or all of these concepts within certain parameters, as discussed hereafter, wherein a control system results which will allow the control of a hydrostatic transmission to approximate the control performance of a DFC system. 
     With reference to FIG. 1, a hydrostatic transmission  10  for use in a vehicle such as a compact utility tractor has a housing  12  having a Z-shaped configuration with an input shaft  14  parallel to but offset from an output shaft  16 . As seen in FIG. 2, the housing  12  has a center section  18  with internal fluid conduits  20  and  22  which fluidly connect hydrostatic pump  24  with hydrostatic motor  26 . Input shaft  14  is conventionally connected to pump  24 , and output shaft  16  is operatively connected to motor  26 . 
     The pump  24  has a conventional rotating group  28  mounted on a valve plate  30  and include a plurality of reciprocal pistons  32  with “outer” ends which conventionally engage the flat side of swashplate  34  which is pivotably mounted on a trunnion (not shown). Fluid from conduits  20  and  22  flow through conventional ports in the valve plate  30  to fluidly connect the pump  24  and the motor  26 . The input shaft  14  freely extends through a conventional aperture (not shown) in the swashplate. The angle of the swashplate determines the mode of output shaft  16 . In the neutral position, there is no fluid flow in conduits  20  and  22  between the pump  24  and motor  26 . For positions other than neutral the rotation of output shaft  16  is shown by the arrows F and R in FIG. 2 reflecting forward and rearward directions of travel. The corresponding angular position of the swashplate corresponding to the arrows F and R are also shown in FIG.  2 . The numeral  36  designates an input signal actuated servo piston which is conventionally connected to the swashplate  34 . The movement of the piston in valve  36  is designated by the arrows in FIG. 2 for the forward and reverse directions of rotation of the output shaft  16 . This invention deals essentially with the control of the position of swashplate  34 . 
     The known control concepts referenced above are (1) the servo spring rate; (2) the valve plate index; (3) valve plate crossport/trap; (4) valve plate porting geometry; (5) swashplate offset; and (6) piston bore volume. Each of these concepts will be discussed below. 
     Servo Spring Rate 
     FIG. 3 is a schematic drawing showing the control forces acting on the swashplate  34 . The numeral  38  designates a servo spring which extends between the swashplate  34  and the housing  12 . Every spring has a spring rate which is a measure of its physical makeup. In conjunction with this invention, the spring rate of servo spring  38  should be in the range of 470 to 670 pounds/inch. 
     The servo spring rate provides a differential load/torque to the swashplate  34  as the servo piston (not shown) of valve  36  and servo spring  38  are moved a given distance. The effect of piston bore pressure (discussed below) to average control torque required can be reduced, by increasing the spring rate so that pressure dependency is reduced. The spring rate overshadows differences due to pressure. The spring rate is also important for obtaining desired resolution of control torque between low and high swashplate angles. 
     As shown in FIG. 3, spring  38  acts in the direction of the arrow  39 . A more detailed view of spring  38  appears in FIG.  4 . As indicated in FIG. 3, spring  38  acts on swashplate  34  to return it to the neutral position of FIG.  2 . As such, the spring is a part of the input signals acting on the swashplate. With the other criteria set forth herein, it is important to maintain the spring rate of the spring  38  within the range recited above to achieve the optimum utility of the invention. The arrows T 1 -T 4  in FIG. 3 designate the following: 
     
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 T1 = Total Torque About Trunnion 
               
               
                   
                 T2 = Pressure Moment About Trunnion 
               
               
                   
                 T3 = Inertia Moment About Trunnion 
               
               
                   
                 T4 = Neutral Return Spring Moment About Trunnion 
               
               
                   
                   
               
             
          
         
       
     
     Valve Plate 
     The valve plate  30  is germane to the concepts of valve plate index, valve plate crossport/trap, and valve plate porting geometry. Each of these features are discussed below. 
     Valve Plate Index 
     As previously indicated, valve plates are located adjacent a center section and are in communication with center section conduits to receive or deliver fluid under pressure to the cylinder bores of reciprocating pistons located within the rotating group rotatably mounted on the side of the plate opposite to the center section. 
     The valve plate  30  is shown in FIGS. 5-10. It is circular in shape and has a center aperture  40  to permit the input shaft  14  to loosely extend therethrough in spaced condition to the aperture. A plurality of arcuate ports  43  appear in spaced relation on a circular axis that is concentrically located with respect to aperture  40 . The ports are separated by lands  47 . FIGS. 7,  8  and  9  show the details of the fluid inlet end  54  of ports  43 . A tapered ramp  56  (FIGS. 7 and 8) having a V-shaped cross section (FIG. 9) is located adjacent certain of the ports  43 . The lower ends  58  of the ramps  56  communicate with an end  54  of the ports  43  (FIG. 8) The ramp  56  increases in width as its elevation decreases towards the port  43  (FIG.  7 ). In lieu of a ramp, a rectangular notch having a bottom and sidewalls can be used. The ends of ports  43  opposite to end  54  has an outlet similar to end  54 . 
     With reference to FIG. 10 the valve plate index controls the timing of precompression and decompression relative to TDC (top dead center) and BDC (bottom dead center) positions. Valve plate index is conventionally defined as:        Index   =         a   Lead     -     a   Trial       2                            
     Valve plate index is the location of the pressure transition zone of a piston  32  in the rotating group  28  relative to the rotational position where the piston is either fully retracted (bottom dead center) or fully engaged (top dead center). Index is defined as positive in direction of cylinder block rotation. A positive index tends to increase neutral-seeking torque while a negative index tends to decrease neutral seeking-torque. Valve plate index affects the dependency of control torque relative to input speed. The valve plate index of this invention is preferably at about a negative 0.870°, and within the range of negative 1.5° to a negative 0.5°, because it relies additionally on crossport, porting geometry, and the X-offset (to be discussed hereafter) to contribute to stroke reducing torque. 
     Valve Plate Crossport/Trap 
     Valve plate crossport is defined as the amount of angle of rotation during which the cylinder block piston ports  60  and  62  (FIG. 11) are connected to both the inlet and outlet ports  64  and  66  (FIG. 12) at the same time. Trap occurs when the cylinder block piston ports are blocked and are not connected to either the inlet or outlet ports  64  and  66 . 
     Ports  60  and  62  (FIG.  11 ), ports  64  and  66  (FIG.  12 ); and ports  43  (FIG. 5) all register with each other at times during the operation of the invention. 
     Valve plate crossport/trap affects the dependency of control torque relative to input speed and system pressure and affects noise and vibration due to swashplate oscillation. Specifically, it affects the piston bore pressure rise rate which affects the average control torque. A 6° angle crossport works in conjunction with the porting geometry to provide a steeper bore pressure profile and reduce the effect of pressure on the average control torque. The angular range for crossport should be within the range of 3-9°. 
     Valve Plate Porting Geometry 
     This geometry was discussed above in relation to FIGS. 5-10 with regard to Valve Plate Index, and particularly in regard to FIGS. 7,  8  and  9 . 
     Valve plate porting geometry affects the cross sectional area of inlet (FIG. 9) and outlet ports ( 64  and  66 —FIG. 12) and the rise rate of piston bore pressure (discussed below) from the inlet port to the outlet port which has an effect on noise. Porting geometry interacts with valve plate index and crossport/trap to affect the dependency of control torque relative to system pressure, input speed, and swashplate angle. The goal of the porting geometry was to make the inlet area large. This increases the rise rate of the bore pressure profile and reduces the effect of pressure on the average control torque. 
     Swashplate Offset 
     Swashplate offset in an “X” direction (described below) is the distance between the trunnion rotation center and point of resolution of torque producing forces (“sweetspot”). See FIG.  13 . Swashplate offset affects the control torque separation between low angle and high angle and is dependent on swashplate angle, system pressure, and speed. This offset should be in the range of −0.015 to 0.015 inches. 
     Offset in a “Y” direction (described below) should be in the range −0.060 to +0.060 inches. (FIG.  13 ). 
     As shown in FIG. 13, the “X” offset is in a direction parallel to the axes of the pistons  32 . The “Y” offset is in a direction perpendicular to the axes of the pistons. 
     Piston Bore Volume 
     Piston/bore volume of the pumping device is the amount of oil in the piston bore which must be compressed or decompressed during the pumping cycle. Piston/bore volume of the cylinder block bore and piston affects the rise rate of piston bore pressure from the inlet port to the outlet port thus, affecting the separation of the control torques relative to speed, pressure, and angle. FIG. 14 shows a piston  32  in its TDC position showing a piston bore  68  in the block  70  of rotating group  28 . When in the TDC position, the piston  32  creates a volume  72  in the bore  68 . FIG. 15 shows the piston  32  in its BDC position creating a volume  72 A in bore  68 . As can be seen, the volume  72  of FIG. 14 is smaller than the volume  72 A in FIG.  15 . Thus, as indicated above, the ratio of piston bore volumes at TDC to BDC (i.e., volumes  72  to  72 A) should be 0.53 to 0.73. Dotted line  76  shows the position of the end  78  of piston  32  when the piston (and swashplate) are in a neutral position denoting a “neutral” volume within the piston. 
     While each of the general concepts discussed above have been previously utilized in control systems, they have not been combined within the parameters outlined heretofore. The foregoing concepts used according to these parameters do achieve the stated goals of the invention, and result in the dependency of the angular position of the swashplate being influenced less than 50% on operating conditions of the transmission, and is influenced by more than 50% by the input signal acting thereon to ensure the stability of the transitions operating range for speed, pressure and swashplate angle.