TECHNICAL FIELD 
     This invention relates generally to an adjustment control system for a grain cleaning sieve or sieves of an agricultural combine, and more particularly, to a method of automatic operation of a sieve control system for adjusting the sieve opening size or position while mitigating error and inaccuracy resulting from tolerance stack-up of mechanical components in the system. 
     BACKGROUND ART 
     It is well known to provide an automatic system for adjusting the sieve or sieves of the grain cleaning system of an agricultural combine. Typically, the cleaning system will consist of two sieves, an upper or coarser sieve or chaffer located below the threshing mechanism or separator of the combine and having larger sieve openings for the passage of grain and smaller pieces of plant material therethrough but relatively little of the larger chaff, and a lower finer sieve located below the chaffer for receiving the grain and smaller pieces of plant material therefrom and having smaller sieve openings for passage of the grain therethrough but relatively little of the plant material. The collected and cleaned grain, once through the sieves is then typically transported by conveyor or other means to a clean grain bin on the combine, or to an accompanying grain receiving vehicle. The opening sizes of the chaffer and sieve are an important parameters for controlling the amount or yield of grain that is recovered by the combine as opposed to discharged therefrom with the chaff and other unwanted plant material and crop residue. Accordingly, the chaffer and sieve opening sizes are typically set at the commencement of the harvesting operation, and may be reset at times during the harvesting operation, to achieve a desired crop yield rate. 
     A typical sieve construction includes a plurality of elongate parallel, pivotally mounted slats, each slat including a plurality of longitudinally spaced upwardly extending inclined fingers, the slats being pivotable through a range of open positions angularly oriented to horizontal for providing a corresponding range of openings or spaces between the fingers of adjacent ones of the slats. A typical automatic sieve adjusting system includes an adjusting member which contacts each of the slats, and a linkage and/or cable arrangement connected between the adjusting member and one or more actuators driven by an electrical, fluid, or other controller for moving the linkage or cable arrangement and member and thus changing the angular orientation of the slats and as a result, the opening size. The typical controller includes at least one processor operated by stored commands and/or inputs for controlling an electrical drive motor or the like for moving the actuator. An input device such as a push button or keypad and a display device are typically located in the operator cab of the combine for changing and showing the chaffer and sieve settings. 
     Typical sieve control systems are disclosed in Rowland-Hill et al. U.S. Pat. No. 4,466,231, issued Aug. 21, 1984 to Sperry Corporation; and Diekhans U.S. Pat. No. 6,205,384, issued Mar. 20, 2001 to Claas Selbstfahrende Erntemaschinen GmbH. U.S. Pat. No. 4,466,231 in particular discloses a method for automatic sieve and chaffer adjustment which ensures that the approach to the final position or setting is always made in the direction for opening the sieve, thereby allowing for compensation for play or backlash in the mechanical linkages of the system. However, to reduce the probability of the sieve or chaffer being damaged by crop material or foreign objects as it is moved to the setting, the sieve or chaffer is brought to a fully open position so as to pass any large and potentially damaging objects therethrough, then is moved in a closing direction to a more closed position past the desired setting by an amount corresponding to an anticipated amount of backlash or play in mechanical components of the system. Then, the sieve is opened by a corresponding amount to the desired setting. Possible shortcomings of this method of operation, however, include in the instance of a chaffer, the possible passage of larger pieces of plant material, such as stalk and stem fragments, into the openings of the chaffer so as to be caught or trapped therein or suspended therefrom, so as to decrease the capacity thereof as well as possibly also interfere with the operation of the sieve below, so as to reduce the capacity or efficiency of the cleaning system. In the instance of the lower finer sieve, if fully opened during the operation thereof when larger crop material is present thereon, the undesirable crop material can pass with the grain through the sieve so as to increase the percentage of impurities in the clean grain. This may be acceptable on an occasional basis. However, if it is desired to more frequently adjust the opening size of the finer sieve, for instance, such as for automatically maintaining a selected sieve opening size, more frequently fully opening the sieve may significantly increase the amount of unwanted crop material in the clean grain. 
     Accordingly what is sought is a system for automatically adjusting a sieve of an agricultural combine which overcomes many of the problems and shortcomings set forth above. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a method of automatically controlling an opening size of a sieve of an agricultural combine is disclosed, which overcomes many of the problems and shortcomings identified above. The present method includes the steps of: 
     (a) storing a value for the opening size; 
     (b) determining an actual value for the opening size; 
     (c) comparing the actual value with the stored value, and, 
     (i) if the actual value is at least a predetermined amount greater than the stored value, then automatically closing the sieve until the actual value equals the stored value; 
     (ii) if the actual value is greater than the stored value by less than the predetermined amount, then automatically opening the sieve until the actual value is a predetermined amount greater than the stored value, then automatically closing the sieve until the actual value equals the stored value; and 
     (iii) if the actual value is less than the stored value, then automatically opening the sieve until the actual value is a predetermined amount greater than the stored value and then automatically closing the sieve until the actual value equals the stored value. 
     Preferably, the predetermined amounts are each an amount which correspond to or is only slightly or marginally greater than the amount of the anticipated tolerance stack-up for, or play in, the mechanical components of the system. The typical range of opening sizes for a particular sieve will equal several times the cumulative tolerance stack-up or play for the mechanical components of the system, which provides the advantage when the selected opening size is relatively small or in the lower portion of the range, that the sieve is only further opened by a relatively small amount, thereby limiting the number of larger pieces of plant material or contaminants that may be passed through or could become jammed or lodged in the sieve or chaffer, and the amount of smaller crop residue that would pass through the finer sieve with the clean grain. Also, because the sieve is only open a relatively small amount greater than the desired opening size, the adjustment can be completed in a time period shorter than required for opening the sieve from a smaller opening size to the fully opened position, closing the sieve to an opening size smaller than the desired size, then opening the sieve to the desired opening size, such as disclosed in U.S. Pat. No. 4,466,231. 
     As a preferred optional step, the sieve can be opened or closed to a commanded opening size, then a value for the commanded opening size stored for use as the stored value. Also preferably, during the prior step, the threshing mechanism or separator of the combine is not operating, such that any substantial flow of material therefrom to the sieve is absent. This allows the sieve to be moved, for instance, using an operator input device such as a push button or keypad, in an opening direction, a closing direction, or alternatively in both, for setting the sieve to a desired opening size without risk of becoming jammed with material therein or too large of material passing therethrough. Then, steps (a), (b) and (c) can be performed during the operation of the sieve when a flow of material from the separator is present, for making minor size adjustments for maintaining the sieve opening size at the desired value or setting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified side elevational view of an agricultural combine including a sieve adjustment control system operable according to the method of the present invention; 
     FIG. 2 is a simplified schematic diagram of the control system; 
     FIG. 3A is a high level flow diagram showing steps of the method of the invention. 
     FIG. 3B is a continuation of the flow diagram of FIG.  3 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, in FIG. 1 a conventional agricultural combine  10  is shown including a sieve adjustment control system  12  operable according to the teachings of the present method. Agricultural combine  10  generally includes a threshing mechanism or separator  14  operable for receiving crop material from a harvesting mechanism  16  and separating smaller grains, seeds, pods and related plant material from larger stalks, stems, leaves, husks and other elements of the crop material, as well as vines, weeds and the like which may be present in the harvested crop material. The smaller material falls from separator  14  into one or more augers  18  disposed below separator  14 , which collect the smaller material and convey it to a cleaning system  20 . Cleaning system  20  includes an upper, coarser sieve or chaffer  22  positioned for receiving the material from auger or augers  18 , as denoted by arrow A, and a lower, finer sieve  24  which receives grain or seeds which pass through chaffer  22 . Sieve  24 , in turn, separates or cleans the grain or seed from the remaining other material, such as pod fragments and the like, for collection by a lower auger  26  which conveys the clean grain or seeds to an elevator  28  which conveys the clean grain or seeds upwardly to a clean grain bin  30 . Chaffer  22  and sieve  24  are vibrated or reciprocated during operation by suitable means (not shown) for facilitating sifting of the crop material therethrough. Material which does not pass through is moved rearwardly on chaffer  22  or sieve  24  by the vibration or reciprocal action and is either blown or drops from the rear end thereof through an opening in the rear of combine  10  or into a return auger  32  and be carried to a return elevator  34  for conveyance back to separator  14 . 
     Referring also to FIG. 2, chaffer  22  and sieve  24 , represented by sieve  24 , each include a plurality of elongate, horizontally extending slats  36 , each slat being pivotable about a generally horizontal pivotal axis  38 . Each slat  36  is composed of a plurality of parallel, longitudinally spaced upward and downwardly inclined fingers  40 , the fingers  40  of adjacent ones of slats  36  defining sieve openings therebetween, represented by distance X in FIG. 2, for the passage of only plant material of a desired maximum size therethrough. Slats  36  are simultaneously pivotable about respective pivotal axes  38  through a range of pivotable positions for varying the opening size X of the sieve in the well known conventional manner, as denoted by arrows B. In this regard, slats  36  would be pivoted in a counterclockwise direction to increase the opening size X, and pivoted in the clockwise direction for decreasing the opening size X. 
     Sieve adjustment control system  12  is operable for automatically and simultaneously moving slats  36  through a predetermined range of pivotal positions for varying the opening size of sieve  24 , and for holding slats  36  at a position corresponding to a desired or selected opening size. To accomplish this, system  12  preferably includes at least one actuator  42 , such as, but not limited to, a conventional electric linear actuator (ELA). Actuator  42  can be connected by a suitable mechanical connecting element such as one or more cables and/or linkages, to an adjusting member  46  of sieve  24 . Adjusting member  46  will typically include a plurality of upwardly extending portions  48  which contact and support slats  36  for pivotal movement about pivotal axis  38  thereof, respectively, adjusting member  46  being linearly movable by connecting element  44  in the direction denoted by the arrow C for moving slats  36  in an opening direction, and in the direction denoted by the arrow D for moving slats  36  in a closing direction. Accordingly, actuator  42  is precisely controllably movable in a corresponding linear manner in the direction C for effecting movement of connecting element  44  and adjusting member  46  and thus slats  36  in the opening direction, and in direction D for effecting movement of connecting element  44 , adjusting member  46  and slats  36  in the closing direction. 
     Actuator  42  can be controlled using any suitable conventional controller, such as, but not limited to, controller  50  shown including at least one microprocessor and related circuitry drivingly connected to a motor connected to actuator  42  and operable for moving actuator  42  in the direction C and D, or holding actuator  42  at a selected position. Controller  50  is conventionally operable for moving actuator  42  based upon commands received from a suitable input device  52  which can be, for instance, a conventional push button or keypad device, or another input device, or a stored command or value contained in a suitable memory or register, such as a register of a display device  54 , input device  52  and display device  54  being connected to controller  50  by a suitable conductive path  56  such as a conventional wiring harness or the like. Input device  52  and display device  54  will typically be located in an operator cab of combine  10 . Information representative of a position of actuator  42 , adjusting member  46 , fingers  40  and/or slats  36 , which in turn, is representative of the sieve position or opening size, is determined by a suitable position sensor  58  and is accessible or readable by controller  50  via conductive path  56 . Controller  50  is then operable to compare information representative of a commanded or stored position or opening size from device  52  or  54 , with information representative of an actual or sensed position or opening size as determined by position sensor  58 , and controllably operate the motor of controller  50  for holding or moving actuator  42  and thus connecting element  44 , adjusting member  46  and slats  36 . 
     Here, it should be noted that structurally and operationally, chaffer  22 , is an analogous device to sieve  24 , that is, it is also a sieve, and can be automatically controlled by control system  12 , utilizing a second actuator  42 , mechanical connecting element  44 , controller  50  and position sensor  58  operable in the above-described manner, or by a second control system  12 , or other control system, as desired. In the former instance, a suitable switch or other means can be provided on the operator cab or at another desired location to allow switching devices  52  and  54  between chaffer  22  and sieve  24 , as desired or required. 
     In a sieve such as chaffer  22  or sieve  24 , the various mechanical apparatus thereof, represented by slats  36 , fingers  40 , connecting elements  44 , adjusting members  46  and upwardly extending portions  48 , are all manufactured to dimensions within certain tolerance ranges. There may also be play designed into the components of these elements and connections therebetween. Actuator  42  may also be operable for holding or moving to a position within a known range. The cumulative value of all, or selected ones, of these tolerances, play and range, can comprise what is referred to herein as a tolerance stack-up. 
     In operation, at the commencement of a harvesting operation, or at a desired time during the operation, the operator may view the information displayed by display device  54  representative of the opening size or position of slats  36  of chaffer  22  or sieve  24 . Based on the displayed information, the operator may elect or determine to change the opening size by changing the position of slats  36 , and initiate such change using input device  52 . 
     Referring also to FIGS. 3A and 3B, a high level flow diagram  60  of steps of a preferred method of the present invention are shown. According to the preferred method, after initiation of operation as denoted at start block  62 , controller  50  determines whether an open or close command from input device  52  is present, as shown by decision block  64 . If an open or close command is present, controller  50  will determine whether separator  14  is operating or not, as shown at decision block  66 . As long as separator  14  is operating, controller  50  will loop back to start block  62  and decision block  64 . If separator  14  is not operating, or ceases operating, and an open or closed command is present, controller  50  will then determine whether the command is an open or close command, as denoted by decision block  68 . If the command is an open command, controller  50  will determine whether the actual or current position is less than a predetermined value, preferably corresponding to a maximum opening size or position as denoted at decision block  70 . If the actual position is not less than the predetermined value corresponding to the maximum opening size or position (predetermined maximum value), controller  50  will loop back to start block  62 . If the actual position is less than the predetermined maximum value, controller  50  will operate actuator  42  to move in an opening direction, as denoted at block  72 , to effect a corresponding movement of connecting element  44 , adjusting member  46  and slats  36 , until a desired opening size or position of slats  36  is reached, which will be indicated by display device  54 . Here, it should be noted that the open command can be the result of an inputted value, that is, a desired value inputted via input device  52  if a keypad, or if a push button device, by holding the push button in a particular position until a desired value is displayed by display device  54 . Alternatively, the input command could be recalled from the register or display device  54  or automatically determined by controller  50 , as desired. When actuator  42  has opened the sieve sufficiently to achieve the desired or commanded opening size or position as determined or sensed by sensor  58  and displayed by device  54 , that size or position can be held, and information or a value representative of the actual opening size or position stored in the register of device  54 , as denoted at block  74 , and controller  50  will loop back to start block  62 . 
     Referring again to block  68 , if it is determined that the command is a close command, controller  50  will determine if the actual opening size or position of slats  36  is greater than a predetermined value, preferably a minimum value representative of a minimum open position or a closed position, as denoted at decision block  76 . If the actual opening size or position is not greater than the predetermined value representative of the minimum open position or closed position (predetermined minimum value), then controller  50  will loop back to start block  62 . If the actual opening size or position is greater than the predetermined minimum value, then controller  50  will controllably operate actuator  42  to move the sieve in the closing direction, as denoted at block  78 , until the commanded or desired opening size or position is reached. The actual position value determined or sensed by sensor  58  will then be stored in the register of display device  54  as denoted at block  74  and controller  50  will return to block  62 . 
     In the absence of an open or close command, as determined at block  64 , controller  50  will proceed to compare the then existing stored position value with the actual position value, as denoted at block  80 . If those values are equal, controller  50  will return to block  62 . If those values are not equal, controller  50  will determine whether the stored value is less than or greater than the actual value, as denoted at decision block  82  in FIG.  3 B. If the stored position value is greater than the actual value, controller  50  will next determine whether the actual value equals a predetermined value, as denoted at decision block  84 , which is preferably a predetermined high value. If the actual value equals this predetermined high value, then controller  50  will operate actuator  42  in an open direction until the stored value and the actual value are equal, as denoted at block  86 , and then will return to block  62 . If the actual value does not equal the predetermined high value, then controller  50  will operate actuator  42  in the opening direction until the actual value is a predetermined amount greater than the stored value, as shown at block  88 . This predetermined amount greater than the stored value preferably corresponds to or is marginally or slightly greater than the tolerance stack-up, or may be some other desired value. Then, controller  50  will operate the actuator in the closing direction until the stored value and the actual value are equal, as denoted at block  90 , and return to block  62 . 
     Referring again to block  82 , if controller  50  determines that the stored position value is less than the actual value, it will next determine if the difference between the actual position value and the stored position value equals another predetermined value, as denoted at decision block  92 . If yes, controller  50  will operate actuator  42  in the opening direction until the actual value is the predetermined amount greater than the stored value, as shown at block  88 , then will operate actuator  42  in the closing direction until the stored position value equals the actual value, as shown at block  90  and then return to start block  62 . If, at block  92 , the difference between the actual value and the stored position value does not equal the predetermined value, then controller  50  will operate actuator  42  in the closing direction until the stored value and the actual value are equal, as shown at block  90 , and then return to block  62 . 
     Thus, according to the method of the present invention two routines are utilized for controlling sieve opening size, one for opening or closing the sieve to reach a commanded position or opening size, and a second routine for monitoring a value representative of an actual sieve position or opening size and if greater than or less than a stored value, then effecting a change in the sieve position or opening size such that the actuator value will equal the stored value. In this latter routine, if the stored value is less than the actual value by at least a predetermined amount, which amount, again, preferably corresponds to or is slightly greater than the tolerance stack-up, the sieve will be actuated to move in a closing direction to reach the position or opening size corresponding to the stored value; if the stored value is not at least the predetermined amount less than the actual value, the sieve will be actuated to open until the actual value is the predetermined amount greater than the stored value, then closed until the stored value and actual value are equal. On the other hand, if the stored value is greater than the actual value but not equal to a predetermined value, preferably a maximum value, the sieve will be opened until the actual value is a predetermined amount greater than the stored value then closed until the actual value and the stored value are equal. In this way, when adjusting the sieve position or opening size to correspond to the stored value, the sieve is always moved in a closing direction by at least the predetermined amount which preferably corresponds to at least the amount of the tolerance stack-up for the mechanical components of the system. 
     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. 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.
An adjustment control system for a grain cleaning sieve or sieves of an agricultural combine, a method of automatic operation of a sieve control system for adjusting the sieve opening size or position while mitigating error and inaccuracy resulting from tolerance stack-up of mechanical components in the system.