Patent Publication Number: US-2021187687-A1

Title: Knife edge location sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to U.S. Provisional Application Ser. No. 62/950,728, filed Dec. 19, 2019, the disclosure of which is incorporated by reference herein in its entirety for any purpose whatsoever. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to computer numeric controlled (CNC) machines that use a knife as a cutting tool for cutting shapes of flexible material. More specifically, the present invention relates to computer numeric controlled (CNC) machines that use a knife as a cutting tool and include a sensor for determining a knife offset to reduce cutting error. 
     BACKGROUND OF THE INVENTION 
     Computer numeric controlled (CNC) machines may use a knife as a cutting tool for automated cutting of shapes typically of flexible materials such as fabric. A well-known usage of such a machine is automatic cutting of garment parts from stacked layers of fabric. Typically, the knife reciprocates in a direction parallel to the knife edge, and the knife is periodically sharpened by an automatic sharpener built into the machine. Successive sharpens wear back the knife edge causing the location of the edge to have an offset from the ideal or original edge location. It is not unusual for a knife edge to wear back 2.5 millimeters or more from its leading edge. Accurate CNC cutting must compensate for this offset of the knife edge. This is analogous to a well-known method called cutter diameter compensation where the programmed path input to a CNC machine is modified to account for cutter diameter differences from the nominal diameter. However, as discussed herein, the cutting tool is a knife instead of a cylindrical cutting tool, such as an end mill. 
       FIG. 1  illustrates an outline or toolpath of an example part suitable for CNC cutting using a knife. The outline includes notch  101 , which is a common feature of garment parts, and is used by sewing machine operators as an alignment point between adjacent sewn parts. The adverse effect of an uncompensated knife edge offset is that cut features such as notches are displaced from their intended locations. For example, an actual cut notch  102  is shown displaced from the toolpath location of the notch  101 . The cutting direction for the example of  FIG. 1  is counter-clockwise, which is in the direction indicated by the arrow  100 . The magnitude of displacement principally geometrically follows the knife edge offset. 
     The most common method of controlling adverse effects of knife edge offset is to replace the knife before the wear from sharpening exceeds some threshold amount. This method does not compensate for offset, instead it limits the amount of error. Other methods known to the prior art do compensate for the knife edge offset. These methods require either an estimate or a measurement of the knife edge location. One solution estimates the knife edge offset by tracking the number of sharpens and using the count to predict the amount of wear. The accuracy of this method is dependent on the precision and knowledge of the relationship between the number of sharpens to an amount of wear. This relationship is complex because the sharpener abrasive becomes less aggressive as it is used, and consequently the rate at which the abrasive wears back the knife diminishes with use of the abrasive. The method also requires resetting the sharpen counter when a new knife is installed, and this may be a manual operation subject to human error. The operator may forget or not know to reset a counter. 
     The prior art also includes methods for measuring the knife edge offset using a non-contact sensor. For example, US Patent Publication No. 2015/0082957, the disclosure of which is incorporated herein in its entirety, teaches a non-contact proximity sensor for measuring the knife edge offset. The location of the edge is found without contacting the knife. The use of the non-contact method in the prior art is motivated by the difficulty of measuring the location of a reciprocating edge by direct contact with a probe. The invention of this disclosure overcomes this difficulty by using the sharpener grinding wheels as the contact probe. 
     In the prior art, a knife sharpen cycle has fixed parameters including contact pressure and duration of contact between the abrasive and knife. As mentioned, the sharpener abrasive media becomes less aggressive with use. Consequently, the amount of knife edge wear is dependent on the state of aggressiveness of the abrasive media. When the abrasive media is new and aggressive, a sharpen cycle tends to wear too much material from the knife. 
     Accordingly, there is a need in the art for an apparatus that accurately determines a knife edge offset for a cutter toolhead. Further, there is a need in the art for an apparatus for preventing removal of too much material from the knife edge during sharpening by adjusting contact duration, abrasive wheel speed or pressure as needed to achieve uniform material removal from the knife for each sharpen. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention is to provide a sensor that measures the size of a gap between an arm and a frame of a cutter toolhead. The sensor produces an electrical signal directly related to the size of the gap. The size of the gap is directly related to a knife edge offset for a knife of the cutter toolhead. 
     Another object of the invention provides that a first abrasive wheel and a second abrasive wheel act as a probe in contact with a knife of the cutter toolhead and mechanically govern the size of the gap between an arm and a frame of a cutter toolhead. Alternative sensors include capacitive proximity sensors, linear voltage displacement transducers, resistive potentiometers, encoders or any sensor that may produce a computer readable electrical signal related to the relative distance between two surfaces. 
     Another object of the invention provides that a knife edge offset may be determined by a computer-controller using data from a sensor that measures the size of a gap between an arm and a frame of a cutter toolhead. The sensor data is read by the computer-controller while a first abrasive wheel and a second abrasive wheel are in contact with a knife of the cutter toolhead. The sensor data may be read or sampled multiple times at a rate over the duration of a sharpen cycle to obtain a stored set of samples. The knife edge offset is obtained from the average sensor data by a linear function, table lookup calculation or other functional mapping commonly accomplished by a computer-controller. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, referred to herein and constituting a part hereof, illustrate a preferred embodiment of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  illustrates an outline or toolpath of an example part suitable for CNC cutting using a knife. 
         FIG. 2  shows a cutter toolhead with a sensor for measuring the location of the knife edge in accordance with the invention. 
         FIG. 3  shows a cutter toolhead with grinding wheels engaged with the knife. 
         FIG. 4  shows a cutter toolhead with grinding wheels disengaged from the knife. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is generally applicable to a computer-controlled machine for cutting two dimensional shapes on a planar work surface. The machine includes a gantry that positions a cutter toolhead using two or more servo motors to follow controlled tool paths within the plane parallel to the work surface. A material to be cut is placed on the work surface. U.S. Pat. No. 4,205,835, the disclosure of which is incorporated herein in its entirety, describes a bristle bed work surface suitable for supporting a material while cutting with a reciprocating knife. 
       FIG. 2  illustrates a cutter toolhead that includes a knife  10  that is powered to reciprocate normal to the work surface by a knife motor  16  and a drive means that includes a crank arm  17 . The knife motor  16  provides most of the work done to cut the material. The knife  10  and a knife drive means that includes a crank arm  17  may be positioned in the states of either tool-up or tool-down by an elevator platform  19  that is typically actuated by a pneumatic cylinder. In the tool-down state, the knife  10  pierces and cuts the material placed on the planar work surface. In the tool-up state, the knife  10  is located in a clearance plane above the material and in this state the knife may be moved to start points of cut lines. The tool-up state is also used to position the knife  10  for sharpening.  FIG. 2  shows the knife in the tool-up state. 
     As illustrated in  FIG. 3 , the knife  10  is periodically sharpened by a first abrasive wheel  11  and second abrasive wheel  12 . The first abrasive wheel  11  and second abrasive wheel  12  are rotatably coupled to an arm  13 , and are both driven by a grinder motor by means of a transmission comprised of gears or belts. In the preferred embodiment, the arm  13  rotates about pivot  22  relative to a frame  26 , and the arm  13  is rotationally actuated by a rotary pneumatic cylinder  24  to either an engaged position or disengaged position. In the engaged position shown in  FIG. 3 , the first abrasive wheel  11  and second abrasive wheel  12  are in contact with the knife  10  to sharpen the knife  10 . 
     In the disengaged position shown in  FIG. 4 , the first abrasive wheel  11  and second abrasive wheel  12  are stored away from the knife  10 , and in this state cutting and other non-sharpen operations occur. 
     A preferred embodiment of this invention includes a sensor  20  attached to the frame  26  that measures the size of a gap  21  between the arm  13  and the frame  26 . The sensor  20  produces an electrical signal readable by the computer-controlled machine directly related to the size of the gap  21 . The size of the gap is directly related to the knife edge offset. That is, the first abrasive wheel  11  and second abrasive wheel  12  act as a probe in contact with the knife  10  and mechanically govern the size of the gap  21 . Alternative sensors include capacitive proximity sensors, linear voltage displacement transducers, resistive potentiometers, encoders or any sensor that may produce a computer readable electrical signal related to the relative distance between two surfaces. Another embodiment of the invention measures the angle between the arm  13  and the frame  26 . The angle may be measured by a rotary encoder or other equivalent sensor producing a computer readable electrical signal. Yet another embodiment of the invention has the arm  13  slidably coupled to the frame  26  instead of rotating about a pivot  22 . For example, the arm  13  may be mounted to a linear bearing that would allow the first abrasive wheel  11  and second abrasive wheel  12  to slide along a line to create an engaged position where the knife  10  is sharpened and a disengaged position where the abrasive wheels are stored. Actuation of the sharpener could be achieved by a straight-line type pneumatic cylinder instead of a rotary pneumatic cylinder  24 . 
     The knife edge offset is determined by the computer-controller using data from the sensor  20 . The sensor  20  output is read by the computer-controller while the first abrasive wheel  11  and second abrasive wheel  12  are in contact with the knife  10 . The sensor  20  output may be read or sampled multiple times at a rate over the duration of the sharpen cycle to obtain a stored set of samples. Each sample may be slightly different due to vibration and electrical noise. If the rate is 100 samples per second and the sharpen duration is 0.5 seconds, then the set of samples would include 50 stored values. The set of samples may be averaged by the computer-controller to obtain an average sensor output, and is an estimate less susceptible to the effects of vibration and electrical noise. The knife edge offset is obtained from the average sensor output by a linear function, table lookup calculation or other functional mapping commonly accomplished by a computer-controller. 
     In the preferred embodiment, the knife edge offset is calculated from the average sensor output using a linear function. Preferably, the knife edge offset is nominally zero for a new knife and increases as the knife  10  wears. The slope of the linear function may be such to obtain the knife edge offset in standard dimensional units such as millimeters. In the preferred embodiment the knife edge offset is further processed by the computer-controller. Each sharpen cycle will generate a new knife edge offset value. Because of vibration and electrical noise, in practice some variation will exist in the sequence of values. Those skilled in the art will recognize a smoother estimate may be obtained by calculating a weighted average of the current and some of the previous knife edge offset values. In the preferred embodiment, this estimate will substitute for the original knife edge offset. 
     Information of the knife edge offset may be used by the computer-controller to compensate for a worn knife edge. Without compensation, for example, the toolpath location of the notch  101  in  FIG. 1  is displaced to the actual cut notch  102  location. The computer-controller executes a compensation algorithm that modifies the toolpath geometry such that the actual knife edge follows the toolpath. The solution to this problem is well-known to those skilled in the art of inverse kinematics. 
     Another use of the knife edge offset is for automatically determining when the knife  10  requires replacement. After each sharpen the computer-controller may compare the knife edge offset with a threshold value. The machine may warn the operator or stop the machine and require a knife  10  replacement should the knife edge offset exceed the threshold value. 
     Yet another use of the knife edge offset is for automatically determining when a new knife has been installed. The computer-controller may detect a new knife by looking for the knife edge offset to fall to a near zero value after previously sustaining a much larger value. Information of when a knife is new and when it needs to be replaced makes it possible for the computer-controller to count the number of sharpens that a particular knife receives over the course of its life and notify the operator of a pending necessary knife change or cease operation when the knife is worn to it&#39;s useful life. 
     Yet another use of the knife edge offset is to determine an aggressiveness estimate of the first and the second abrasive wheel, and use the aggressiveness estimate to adjust sharpener cycle parameters such as grind time to achieve consistent material removal from the knife in a single sharpen. The aggressiveness estimate may be calculated as the change in knife edge offset per grinding wheel revolution. Preferably, the aggressiveness estimate would be calculated as an average value of multiple sharpens, for example the most recent 100 sharpen cycles. It is desirable for the sharpener cycle parameters be maintained such that change of knife edge offset per sharpen nearly always equal a target value. For example, the target value may be 0.8 microns per sharpen. The aggressiveness estimate information would be used by the computer-controller in a feedback loop that adjusts sharpener cycle of grind time or abrasive wheel speed. A decreasing aggressiveness estimate may be compensated by increasing either or both the grind time or abrasive wheel speed. Either compensation increases the number of abrasive wheel revolutions per sharpen thereby increasing material removal per sharpen. 
     The aggressiveness estimate may be used by the computer-controller to detect when the first and second abrasive wheel need replacement. The aggressiveness estimate will slowly decrease as the abrasive wheels age. Eventually the aggressiveness estimate will fall to level too low where it is no longer practical to compensate for decreasing knife material removal by increasing grind time or abrasive wheel speed. The computer-controller may monitor the aggressiveness estimate, and when the estimate falls below a threshold value, the operator would be notified or forced to change the first abrasive wheel  11  and second abrasive wheel  12 . 
     It may be appreciated that abrasive wear to first abrasive wheel  11  and second abrasive wheel  12  will contribute to the readings of sensor  20 . Abrasive wear, however, is assumed to be small and negligible relative to knife wear. More specifically, abrasive wheel  11  and second abrasive wheel  12  are preferably cubic boron nitride (CBN). The grain sizes for cubic boron nitride abrasive wheels 151 microns (0.0059 inches). About 55% of these grains are encapsulated to mechanically hold them to the wheel. Consequently, the contribution of abrasive wear to the readings of sensor  20  is only 45% of the grain size or 68 microns (0.0027 inches). These values are negligible when compared to the 2500 micrometer (0.10 inches) possible knife wear. 
     The invention in its broader aspects is not limited to the specific embodiments herein shown and described and departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.