Abstract:
A method and system for controlling a feed-speed to a bandsaw having a sawblade trained around a drive wheel and a spaced-apart driven wheel, and a strain adjustment mechanism for adjusting the strain on the sawblade by changing the spacing therebetween. The method includes sensing a change in the spacing and making a corresponding change in the feed-speed. The system controls an input device for feeding a workpiece to the bandsaw, and includes a sensor and a controller for performing the aforementioned functions.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to controlling the feed-speed of a workpiece to be cut by a bandsaw, and more particularly, a bandsaw of the type used in sawmills for cutting logs where it is often referred to as a bandmill. 
       BACKGROUND 
       [0002]    A bandsaw is a cutting apparatus that turns or trains an endless loop of a cutting element having a serrated or toothed edge (hereinafter sawblade) around two spaced-apart wheels. It is often used for cutting wood. 
         [0003]    More particularly with reference to  FIG. 1 , a typical bandsaw  10  has a drive wheel D 1  which drives a driven wheel D 2  by turning a sawblade SB which loops around the two wheels. The sawblade is typically formed of steel. Bandsaws are provided in sizes that are small enough to allow for use on a table-top, and range up to the very large sizes used in sawmills for cutting logs, where the distance between the rotational centers C 1  and C 2  of the wheels is typically 10 feet or more. 
         [0004]    The sawblade slides across a pair of “guides” G 1  and G 2  that push it outwardly, away from centerline CL of the wheel centers. This causes the sawblade to make a slight positive angle θ relative to the centerline CL as it leaves the driven wheel D 2  and approaches the upper guide G 2  (or alternatively, depending on the direction of rotation of the wheels, as it approaches the driven wheel after having left the upper guide), and as it approaches the driving wheel D 1  after having left the lower guide G 1  (or alternatively, depending on the direction of rotation of the wheels, as it leaves the driving wheel and approaches the lower guide). 
         [0005]    A log “L” to be cut lengthwise with the sawblade SB is fed between the guides G 1  and G 2 , in a direction perpendicular to the plane of the Figure, typically for producing “cants” and “flitches” as indicated in  FIG. 2 . 
         [0006]    It is desired to maintain a substantially constant (tensile) stress on the sawblade, which decreases when the sawblade lengthens. To compensate for this lengthening, or “strain,” the sawblade, the position of the driven wheel D 2  is adjusted along the axis indicated as “A” by a strain adjustment mechanism indicated generally as “SM.” 
         [0007]      FIG. 3  shows an example of such a mechanism employing a typical elastomeric bellows “BL.” It may be noted that, due to the fulcrum and lever arrangement shown for mounting the driven wheel D 2 , the axis “A” is actually a circular arc, but for small rotations of the wheel center C 2  about the pivot point P 1  this are is reasonably approximated by a straight line. 
         [0008]    The bellows is served by a pressurized line (air or hydraulic fluid) which pressurizes it to a degree determined by a human operator who initially sets the pressure in the line. The position of the driven wheel is a consequence of a balance between the expansion force of the bellows and the resistance of the sawblade, which provides for a position adjustment whereby the driven wheel moving farther apart from the drive wheel as the sawblade becomes increasingly strained. The expansion force provided by the bellows remains constant, independent of the strain, so long as the pressure setting remains unchanged, resulting in a constant stress on the sawblade at all times. 
         [0009]    U.S. Patent Publication No. 2015/0158097 describes a problem known in the art as instability (referred to as “an oscillation . . . called ‘snaking’ . . . ”) of the sawblade, and proposes as a solution to sense changes in the forces detected by force sensors at the guides, particularly the guide that is downstream of the log, in terms of the direction of movement of the sawblade, i.e., the guide G 1  if the sawblade circulates counterclockwise, or the guide G 2  if the sawblade circulates clockwise, in  FIG. 1 . These changes are described as being precedent to instability of the sawblade, and are used to control the speed of feeding the workpiece to the bandsaw, hereinafter “feed-speed.” 
       SUMMARY 
       [0010]    A system and method for controlling a feed-speed to a bandsaw is disclosed herein. The bandsaw has a sawblade trained around a drive wheel and a spaced-apart driven wheel, and a strain adjustment mechanism for adjusting the strain on the sawblade by changing the spacing therebetween. 
         [0011]    The method includes sensing a change in the spacing and making a corresponding change in the feed-speed. 
         [0012]    The system controls an input device for feeding a workpiece to the bandsaw, and includes a sensor and a controller for performing the aforementioned functions. 
         [0013]    It is to be understood that this summary is provided as a means of generally determining what follows in the drawings and detailed description and is not intended to limit the scope of the invention. Objects, features and advantages of the invention will be readily understood upon consideration of the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a schematic representation of a bandsaw. 
           [0015]      FIG. 2  is an end-view of a log showing cuts defining “cants” and “flitches.” 
           [0016]      FIG. 3  is an isometric view of a prior art bandsaw using an elastomeric bellows to provide for sawblade strain adjustment. 
           [0017]      FIG. 4  is a plot of strain versus time for a hypothetical sawblade, showing the effect of a relatively small impact force. 
           [0018]      FIG. 5  is a plot of strain versus time for a hypothetical sawblade, showing the effect of a relative large impact force. 
           [0019]      FIG. 6  is a plot of strain versus time for a hypothetical sawblade, showing the effect of a gradually and steadily increasing strain. 
           [0020]      FIG. 7  is a plot of feed-speed versus time for the circumstance shown in  FIG. 6  according to the present invention. 
           [0021]      FIG. 8  is a plot of strain versus time for the sawblade of  FIG. 6 , showing the effect of a sudden shock in addition to the effect of gradually and steadily increasing strain. 
           [0022]      FIG. 9  is a plot of feed-speed versus time for the circumstance shown in  FIG. 8  according to the present invention. 
           [0023]      FIG. 10  is the schematic representation of  FIG. 1 , modified for measuring strain according to the present invention. 
           [0024]      FIG. 11  is a block diagram of a feed-speed control system according to the present invention. 
           [0025]      FIG. 12  is the isometric view of  FIG. 3 , modified for measuring strain according to the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    It is an objective of the present invention to improve on the prior art methodology described above for controlling feed-speed to a bandsaw in a sawmill. The strain adjustment mechanism responds to elastic (reversible) increases in the length of the sawblade that result from the ever increasing amounts of heat generated as the sawblade wears, and plastic (permanent) increases in the length of the sawblade that occur as a result of the sawblade encountering inhomogeneities in the workpiece, in an effort to maintain constant stress on the sawblade. 
         [0027]    The present inventor has recognized that maintaining a constant stress on a sawblade that is stretching either because it is getting hotter due to wear or because it is plastically deforming as a result of normal use increases the risk of premature and catastrophic failure of the sawblade if the same feed-speed is also being maintained. 
         [0028]    It is also recognized that, when the sawblade experiences a shock, such as when encountering a knot in a log, the sawblade transmits a (transient) oscillatory force to the driven wheel, which (likewise transiently) oscillates in response due to the fact that the elastomeric bellows, which serves as a load-bearing joint between the driven wheel and the frame of the bandsaw, has compliance. This produces oscillatory changes in the strain of the sawblade independent of any strain adjustment. 
         [0029]    All of these effects can be discerned by measuring the strain on the sawblade. By contrast, the only one of these effects that can be discerned by measuring the forces at the guides is shock, and only if the shock is of a sufficient magnitude—the cumulative effects of the multitude of minor shocks that occur over the life of a sawblade will be missed. 
         [0030]    The present invention provides for measuring the strain on the sawblade, and controlling the feed-speed in response, typically by reducing it as the strain increases. 
         [0031]      FIGS. 4 and 5  show, respectively, examples of elastic and plastic changes in strain, in plots of strain versus time. In both Figures, strain is shown oscillating sinusoidally for illustrative purposes, but in reality the oscillations would typically be more randomized (i.e., they would have additional Fourier components). 
         [0032]    In  FIG. 4 , the strain oscillates around a baseline BL 1 . Upper and lower “control limits” UCL 1  and LCL 1  may be superimposed on the plot, representing desired limits on the changes in strain so that the strain does not vary more than a desired number of standard deviations from the baseline, e.g., 3 or 5. 
         [0033]    Prior to an initial time “IT 1 ” the strain on a sawblade cutting a log varies elastically and regularly about a baseline BL 1 . At an initial time IT 1 , the sawblade is shocked by a sudden but relatively low magnitude impact force, such as by encountering a small knot in the log. As a result of the impact, the strain deviates unusually from the baseline until it settles down after a “settling” time “ST 1 ” to resume its ordinary magnitude of oscillation around the baseline. In this example the impact force is not large enough to plastically deform the sawblade, so the strain settles around the original baseline BL 1 . 
         [0034]    By contrast, in  FIG. 5 , the impact force is larger and causes a more severe oscillation, resulting in a permanent strain of the sawblade that results in the strain settling, after the settling time “ST 2 ,” to a new baseline BL 2 . The new baseline may be provided with new control limits “UCL 2 ” and “LCL 2 ,” which may or may not represent the same magnitude of deviation around the baseline as the previous control limits. 
         [0035]    The control limits in both cases can be used to recognize the occurrence of an event that bears watching and/or calls for a change in the feed-speed, and to identify times when the strain has settled around a baseline, as evidenced by the strain remaining within the control limits for an extended period of time. 
         [0036]      FIG. 6  shows a strain for which the baseline is a trend line, representative of the sawblade undergoing gradual and steadily increasing strain, which can be either or both because the sawblade is steadily heating up as it wears, and is therefore undergoing steady thermal expansion (which is elastic, or reversible strain), and because the sawblade is gradually accumulating plastic deformations in response to encountering relative minor inhomogeneities in the workpiece (which is irreversible strain). The control limits, here upper control limit UCL 3  and lower control limit LCL 3 , follow the same trend, and in the example shown, there is never an event which causes the strain to fall outside the control limits. 
         [0037]      FIG. 7  shows how the feed-speed may be adjusted in response to the strain shown in  FIG. 6 . In particular, in this example the feed-speed is reduced in direct proportion to the increase in strain. It should be understood that the proportionality shown is arbitrary, which can be appreciated by recognizing that no units are being provided on the vertical axes in either Figure. The only point here is that the feed-speed is being linearly reduced as a result of the strain being linearly increased. 
         [0038]      FIG. 8  shows an example like  FIG. 6 , except that there is an event, occurring at time “t 1 ,” that suddenly shocks the sawblade and causes the strain to fall outside the control limits that have been set for the initial baseline “BL 3 .” Where the sawblade is sawing a log, such an event could be, for example, where the sawblade encounters a knot on the log. In response to the event, the sawblade undergoes a sudden increase in strain due to plastic (irreversible) deformation. 
         [0039]    After the event, the strain settles around a new (increased) baseline “BL 4 ” with essentially the same standard deviation as before, so that it is appropriate to recognize new control limits, from UCL 3  and LCL 3  to “UCL 4 ” to “LCL 4 ” respectively, that compensate for the increase in baseline. 
         [0040]      FIG. 9  shows how the feed-speed may be adjusted in response to the strain shown in  FIG. 8 . In this example the feed-speed is generally reduced in direct proportion to the increase in strain, except that it is in addition suddenly decreased by an additional amount after the event at time t 1 . This may be an amount considered to provide a predetermined safety margin after which the feed-speed may be increased again, such as shown at “t 2 ,” after the sawblade has settled from the shock of the event. The amounts of the initial additional decrease in feed-speed in response to the event, and any subsequent increase, may be determined by experience in view of the factors indicated further below. 
         [0041]      FIG. 10  shows the prior art bandsaw of  FIG. 1  modified for measuring strain according to the invention. The structure indicated as “30” illustratively represents a dial indicator, for measuring a displacement of the driven wheel caused by a strain adjustment, the displacement being representative of the strain on the sawblade. 
         [0042]      FIG. 11  shows a feed-speed control system  20  for controlling a feed-speed to a bandsaw according to the present invention. The system  20  has a sensor  22  which is an electronic version of the aforementioned dial indicator for sensing displacement. An example is the MTS Temposonics® E series position sensor, available from the MTS Systems Corporation of Cary N.C. 
         [0043]    The sensor  22  outputs an electrical signal “S 1 ” representative of an amount of displacement of a structural member associated with the strain adjustment mechanism.  FIG. 10  shows an exemplary mounting arrangement that provides for this. 
         [0044]    The signal S 1  is input to a controller  24 , which is typically but not necessarily a digital computer, which is configured or programmed to track changes in the magnitude of the strain signal, and in response to these changes, produce a time-varying output speed-control signal “S 2 ” which controls the speed of an input device  26 , typically a carriage, conveyor, or rollcase, for feeding the workpiece to the bandsaw. 
         [0045]    Examples of the control logic performed by the controller  24  have been discussed in connection with  FIGS. 6-9 , and may be as simple as to reduce the feed-speed in direct proportion to measured changes in strain, which can be either increases or decreases, or to measured or otherwise known increases in baseline, with the change in feed-speed being inversely related to the changes in strain, e.g., if the strain increases, the feed-speed is reduced. 
         [0046]    There are, however, a number of complicating factors or variables that may be and preferably are taken into account when determining whether and/or how much to adjust the feed-speed. A partial list of such variables for consideration particularly in a sawmill, and how that factor generally affects the decision about how much to reduce the feed-speed when it is being reduced according to the principles of the invention, are:
   1) density of the wood (greater density calls for more reduction in feed-speed)   2) whether the wood is frozen or not (frozen wood calls for more reduction in feed-speed)   3) whether there is a “frost tooth” on the sawblade (frost tooth calls for more reduction in feed-speed)   4) experience with or knowledge about the particular bandsaw being used (feed-speed reduction can be more or less, depending)   5) lumber quality (higher quality calls for more reduction in feed-speed)   6) gullet capacities of the saw teeth (small gullet capacity calls for more reduction in feed-speed)   7) depth of cut (large depth of cut calls for more reduction in feed-speed)   8) efficacy of the cooling system used in the bandsaw (better cooling allows for less reduction in feed-speed)   9) gauge (thickness) of the sawblade (thicker allows for less reduction in feed-speed)   10) stress on the sawblade (more stress allows for less reduction in feed-speed)   11) speed (sfpm) of the sawblade (higher sawblade speed allows for less reduction in feed-speed)   12) hardness (e.g., Rockwell) of the sawblade (higher hardness allows for less reduction in feed-speed)   13) maintenance status of the feed system (less maintenance calls for more reduction in feed-speed)   14) spacing between the guides (closer allows for less reduction in feed-speed)   15) whether one of the guides is moveable (if moveable, allows for less reduction in feed-speed)   
 
         [0062]    Adjustments to feed-speed may be made when there is a change in strain or at a subsequent time, and they may be made continuously, periodically, or aperiodically as desired. 
         [0063]    The strain adjustment mechanism SM may be modified to provide for mechanical amplification of the strain to increase the sensitivity of the strain measurement. This is indicated in the example of  FIG. 10 , where a lever arm “LA” has been attached to a link “LK” that connects the wheel center C 2  of the driven wheel D 2  to the wheel pivot point “P 1 ,” to effectively extend its length, so that a given angle of rotation of the link will produce an amplified movement along the axis A. The same modification can be made to the bandsaw of  FIG. 3 , such as shown in  FIG. 12 . 
         [0064]    The strain on the sawblade of a bandsaw is rigorously determined by the spacing between the drive and driven wheels; accordingly, any means for measuring or inferring this spacing can be used for making strain measurements according to the present invention. 
         [0065]    It is to be understood that, while a specific system and method for controlling a feed-speed to a bandsaw has been shown and described as being preferred, variations may be made, in addition to those already mentioned, without departing from the principles of the invention. 
         [0066]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by