Abstract:
A method for tuning operation of servo motors includes selecting a plurality of discrete positions in a guillotine blade cycle for which to determine tuning coefficients, determining tuning coefficients at the discrete positions, interpolating tuning coefficients for positions between the discrete positions, and applying the determined and the interpolated tuning coefficients to the servo motor. The servo motors may be used in connection with a guillotine cutter for separating individual sheets from a continuous web. The guillotine cutter blade may be driven by a servo motor to cyclically lower and raise to transversely cut the web transported below the cutter blade.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to fine tuning the operation of a high speed guillotine cutter at the input portion of a high speed inserter system. In such a system, individual sheets are cut from a continuous web of printed paper for use in mass-production of mail pieces. 
     BACKGROUND OF THE INVENTION 
     Inserter systems, such as those applicable for use with the present invention, are typically used by organizations such as banks, insurance companies and utility companies for producing a large volume of specific mailings where the contents of each mail item are directed to a particular addressee. Also, other organizations, such as direct mailers, use inserts for producing a large volume of generic mailings where the contents of each mail item are substantially identical for each addressee. Examples of such inserter systems are the 8 series, 9 series, and APS™ inserter systems available from Pitney Bowes Inc. of Stamford, Conn. 
     In many respects, the typical inserter system resembles a manufacturing assembly line. Sheets and other raw materials (other sheets, enclosures, and envelopes) enter the inserter system as inputs. Then, a plurality of different modules or workstations in the inserter system work cooperatively to process the sheets until a finished mail piece is produced. The exact configuration of each inserter system depends upon the needs of each particular customer or installation. 
     Typically, inserter systems prepare mail pieces by gathering collations of documents on a conveyor. The collations are then transported on the conveyor to an insertion station where they are automatically stuffed into envelopes. After being stuffed with the collations, the envelopes are removed from the insertion station for further processing. Such further processing may include automated closing and sealing the envelope flap, weighing the envelope, applying postage to the envelope, and finally sorting and stacking the envelopes. 
     At the input end of the inserter system, rolls or stacks of continuous printed documents, called a “web,” are fed into the inserter system by a web feeder. The continuous web must be separated into individual document pages. This separation is typically carried out by a web cutter device. In a typical web cutter, a continuous web of material with sprocket holes on both sides of the web is fed from a fanfold stack from web feeder into the web cutter. The web cutter has a tractor with pins or a pair of moving belts with sprockets to move the web toward a guillotine cutting module for cutting the web cross-wise into separate sheets. Perforations are provided on each side of the web so that the sprocket hole sections of the web can be removed from the sheets prior to moving the cut sheets to other components of the mailing inserting system. Downstream of the web cutter, documents can be transported to a right angle turn that may be used to reorient the documents, and/or to meet the inserter user&#39;s floor space requirements. 
     In a typical embodiment of a web cutter, the cutter is comprised of a guillotine blade that chops transverse sections of web into individual sheets. This guillotine arrangement requires that the web be stopped during the cutting process. As a result, the web cutter transports the web in a sharp starting and stopping fashion. 
     In a feed cycle, the paper is advanced past the blade of the guillotine cutter by a distance equal to the length of the cut sheet and is stopped. In a cut cycle, the blade lowers to shear off the sheet of paper, and then withdraws from the paper. As soon as the blade withdraws from the paper path, the next feed cycle begins. The feed and cut cycles are carried out in such an alternate fashion over the entire operation. 
     In some web cutters, it is desirable to achieve a cutting rate of 25,000 cuts per hour or more, for example. This means that the web cutter has a feed/cut cycle of 144 ms. Typically the length of the cut sheet is 11 inches (27.94 cm). If the time to complete a cut cycle is about 34 ms, then the total time in a feed cycle is 110 ms. This means that the web must be accelerated from a stop position to a predetermined velocity and then decelerated in order to stop again within 110 ms. As guillotine cutters are required to generate pages even faster (up to 36,000 cuts per hour), precise motion control coordinated over various mechanisms must be implemented in order to eliminate web breakage and to reliably cut sheets of proper length at high rates. 
     In this environment, it is important to be able to precisely control the guillotine cutter to accurately perform its cuts during the brief time window available. Since the guillotine blade servo motor is subject to varying torques throughout the up and down cycle of the guillotine blade, it has been found to be difficult to tune the driving servo motor in order to achieve the exacting performance required. 
     SUMMARY OF THE INVENTION 
     For a typical closed loop motion control system with fixed hardware gains and servo update rate, determining servomotor tuning coefficients is a function of inertial and friction loading reflected back to the servo motor. For mechanisms that have inertial and friction loads that are not constant, determination of tuning coefficients that provide satisfactory or optimized motion control performance can be difficult, if not impossible to achieve. One such mechanism that has varying friction and inertial properties reflected to the motor shaft is a crank-rocker mechanism. The crank-rocker mechanism is typically utilized as a means to provide motion to a guillotine cutter blade assembly. 
     The present invention provides a method for improved tuning of servo motors used to drive guillotine cutters. Rather than providing a single tuning coefficient to the motor, the tuning coefficient is continuously varied during the blade&#39;s cutting cycle. The novel method for selecting the varying tuning coefficients allows rapid and precise cutting and minimizes lag or overshooting. 
     In a first step of the tuning process, a plurality of discrete positions in the blade cycle are selected for analysis of the optimal tuning coefficients at those positions. For each of those discrete positions, tuning coefficients are determined. In one preferred embodiment, the motor is commanded to move through approximately three degrees (of the three hundred sixty blade cycle) at the discrete position. The actual displacement corresponding to the command is observed. The tuning coefficients for that discrete location are then determined by adjusting the coefficients up or down, and repeating the test until the desired motion is achieved. In the preferred embodiment, the step of determining tuning coefficients is done using PID (proportional, integral, derivative) control techniques with a PID controller providing control signals to the motor amplifier. 
     After the tuning coefficients have been determined for the discrete locations in the blade cycle, the coefficients for the remainder of the blade cycle are determined through interpolation. In a preferred embodiment, linear interpolation is used. The controller then applies the measured and interpolated coefficients to the amplifier that controls the motor. 
     In the preferred embodiment, the step of selecting the discrete positions includes selecting 90 degrees, 180 degrees, 270 degrees, and 360 degrees in a guillotine blade cycle. These four positions roughly correspond to peaks and valleys in the coefficients needed to work with the varying torques that are required over the blade cycle. The 180 degree position represents a bottom dead center position and 360 degrees represents a top dead center position in the blade cycle. These top and bottom positions also represent points in the cycle with low torque requirements and low tuning coefficients. The horizontal positions of 90 and 270 degrees represent high torque positions that will require peak coefficients. 
     One of skill in the art will understand that the gearing ratio of the motor to the blade cycle need not be one to one. Thus, more or less than one rotation of the motor can result in one cycle of the blade. The tuning coefficients are based on the blade position, regardless of the gearing ratio between the blade cycle and the motor. 
     By testing for the proper coefficients at those four discrete quadrant positions, the appropriate bases for linear interpolation are achieved. Interpolation may also be done based on a sinusoidal shaped curve. 
     Further details of the present invention are provided in the accompanying drawings, detailed description, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a ,  1   b , and  1   c  depict a view of a guillotine cutter blade cutting across a sheet of web in varying stages. 
         FIG. 2  is a diagrammatic representation of a preferred embodiment of rotary driven cutter blade. 
         FIG. 3  depicts a graph of preferred motion control profiles for steady state operation of an inserter input module. 
         FIG. 4  depicts a feedback control loop for controlling and tuning the guillotine blade servo motor. 
         FIGS. 5A and 5B  depict ranges of interpolated servo motor tuning coefficients over a blade cycle. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1   a - 1   c  depict the guillotine cutter  21  through a downward cutting motion, starting at a beginning position in  1   a , to a finished cut position in  1   c . Guillotine cutter blade  21  preferably has an edge that is vertically inclined at an angle above the path of web  120 . As the blade  21  is lowered ( FIG. 1   b ) the blade  21  edge comes into contact with the web and cuts across its width (from right to left in  FIGS. 1   a - c ). In  FIG. 1   c , the blade has reached its bottom position, and the whole width of the web  120  has been cut. In an alternative scenario, blade  21  can be stopped at the position shown in  FIG. 1   b , and only the right half of the web has been cut. This technique is used when the web  120  is comprised of side-by-side sets of sheets, and where only one of the sheets belongs to the mailpiece that is currently being processed. The other half of the web  120  can be cut when the system is ready to start processing the collection of sheets for the next mailpiece. 
       FIG. 2  is a diagram depicting a preferred embodiment for driving the motion of the cutter blade  21 . Cutter blade  21  is linked to a rotary motor  22  by an arm  25 . As the motor  22  makes a 360 degree rotation in the clockwise direction, the cutter blade  21  undergoes a complete down and up cutting cycle. When the arm  25  is rotated to point TDC, the blade  21  is positioned at top-dead-center above the web  120 . When the motor  22  has rotated the arm  25  to position BDC, the blade will be at bottom-dead-center of its cutting cycle. 
     In this example, TDC and BDC have small moment arms and require lower torques for those positions. Friction is also low on the blade  21  at TDC and BDC, which is a further reason for low torque requirements at those positions. Accordingly, it is expected that motor  22  will require less gain to be driven at those positions. 
     Positions A-H of the rotary motor  22  in  FIG. 2  are other key positions in the cutting cycle. Position A represents the point on the rotation where the blade  21  first comes into contact with the web. Position A in  FIG. 2  would roughly correspond to the position of the blade  21  depicted in  FIG. 1   a . Position D in  FIG. 2  represents a half-cut position that corresponds to the blade  21  position in  FIG. 1   b . Rotary position E represents the position in the rotary cycle of motor  22  where the web  120  has been completely cut ( FIG. 1   c ). The blade  21  completes its downward movement at BDC in the rotary cycle, and rises back up from BDC to TDC. At position H, while rising, the blade  21  rises above the horizontal position of the web  120 . The cutter transport resumes transport of the web after point H in the rotary cutting cycle has passed. 
     Positions C and F have large moments arms, and therefore greater torque requirements on motor  22 . At position C, paper is being cut, adding a further frictional component. At position F, the blade  21  is being raised against the force of gravity, and will thus require a larger torque output from the motor  22 . Accordingly, it is expected that larger gains will be needed at positions C and F for tuning the control of the motor  22 . 
       FIG. 3  depicts the motion control profiles for the cutter transport  90 , the web handler transport, and the rotary motor  22  of cutter  21 . This graph shows time on the x-axis and velocity on the y-axis. Cutter transport profile  61  has a triangular shape indicating constant acceleration and deceleration for its controlled motion. In steady state operation web handler profile  62  is preferably a straight line, indicating constant velocity feeding a loop that is expanded and contracted while the cutter transport undergoes the accelerations of profile  61 . Blade profile  63  represents the rotary motion of the motor  22  for driving the blade  21 . As seen in this preferred embodiment, the blade profile is triangular, indicating constant acceleration during the downward stroke to BDC, and decelerating a constant rate while returning back to TDC. 
     The blade  21  begins its motion profile  63  when the displacement of the cutter transport is such that, after the blade  21  has reached displacement A, the cutter transport will have come to rest. Blade displacement, A, is the blade position from TDC where the blade just contacts the inner sheet of web  120  minus some amount for margin (includes servo settle time). 
     The use of closed loop position control systems, as illustrated in  FIG. 4 , are well known in the motion control industry. At some periodic rate, a motion profile (PD) is injected at point  70  and provides a desired position into a summing junction  71 , also referred to herein as a comparator. Actual position is subtracted from the desired position to provide a position error. This error is injected into a digital filter (or controller)  72  that outputs a DAC (digital to analog converter) value. In the industry, a preferred digital filter  22  is commonly known as a PID (Proportional, Integral, Derivative) filter. However, any suitable algorithm that converts position error into a DAC power stage  73  (also referred to as an amplifier or drive) can be used to provide a value to a motor  74  to provide the desired quality of motion at the mechanical load  76 . 
     The DAC value is scaled accordingly to match the inputs and outputs of the power stage or amplifier  73 . Such scaling is achieved with a digital filter that contains tuning coefficients. The filter outputs a percentage of the range between maximum and minimum values that can be applied to the amplifier  73 . In addition to providing the proper gain for the system, the tuning coefficients are also selected to provide desired position accuracy, desired system response and stability. The tuning coefficients may also be referred to as the “gain” of the system. The tuning coefficients may also be characterized as a sum of a subset of parameters that contribute to system stability. In a PID system, proportional gain, derivative gain, and integral gain are the primary components for determining the overall gain. These, and other less significant tuning parameters, are well known in the art and need not be described in further detail here. 
     Many commercially available amplifiers  73  use+/−10 VDC as an acceptable analog input signal. The power stage  73  converts this input signal and outputs a winding current that is proportional to the input signal. With new components, the digital filter  72  may output a digital value whereby the power stage  73  can accept this digital value and accomplish the same as the analog version. Winding current is delivered to the motor  74  and is typically proportional to motor  74  output torque. This ultimately provides motion to the mechanism  76 . An encoder  75  or other suitable feedback device located on the motor  74  or on the mechanism  76  provides the actual position back to the summing junction  71 , completing the closed loop. In an inserter machine application, this entire process typically updates at a period of 500 microseconds (or 2 KHz), ultimately providing the desired quality of motion at the cutter mechanism  75 . 
     In the preferred embodiment, tuning operations are performed at separate positions in the cutter blade  22  cycle. Tuning is preferably performed at TDC (0 or 360 degrees), position C (90 degrees), BDC (180 degrees) and at position F (270 degrees) as depicted in  FIG. 2 . For each of these discrete positions, the blade is preferably moved through approximately three degrees of the cycle. Thus, at position  70  in  FIG. 4  a motion command PD is input requiring a corresponding small displacement. The untuned PID filter  72  multiplies the position error signal by a default gain which is then amplified to produce movement. Motor  74  performance is monitored for instability, overshoot and lag of the actual position relative the commanded position. The operator doing the tuning, can then adjust the tuning coefficient of the PID filter  72  to correct the difference between the observed performance and the desired performance of the motor  74  for driving the blade through that discrete portion of its cycle. 
     The system is then tested again using the new tuning coefficient, and the resulting operation of motor  74  is observed. One of skill in the art will be familiar with tuning processes for adjusting gains to find an optimal tuning coefficient, and further details need not be included here. 
     In the preferred embodiment, the tuning coefficients are tested and determined in this way for the four quadrant points of the blade cycle (90, 180, 270, and 360 degrees, also shown as positions C, BDC, F, and TDC in  FIG. 2 ). These four points are at, or are very close to, places where maximum or minimum torques are being required from the motor. 
     In the preferred embodiment, tuning coefficients for untested points between these tested quadrant points are determined using interpolation. Linear interpolation is appropriate, but curved interpolation algorithms may also be used. 
     For an example of linear interpolation, lets assume we know the tuning coefficient XTDC for the position TDC and the tuning coefficient XC for the 90 degree position (position C in  FIG. 2 ). The following equation provides the linear interpolation for finding the tuning coefficient, X, for a position, θ, located between θTDC (0 degrees) and θC (90 degrees).
 
 X =(( XC−XTDC )(θ−θ TDC ))/(θ C−θTDC )
 
     Linear interpolation is an algebraic process that is easily accomplished when the correct parameters are known.  FIG. 5A  depicts an exemplary graph of tuning coefficients determined for a 360 degree blade cycle, and for which tuning coefficients (K) have been determined by a testing method at the four quadrant positions. The sloped lines between the points represent the tuning coefficients (K) used by PID filter  72  as determined by linear interpolation. The slopes and equations for those lines are easily calculated and the appropriate tuning coefficient is easily determined for points on those lines.  FIG. 5B  depicts an alternative exemplary embodiment of a graph of tuning coefficients (K) for which a sinusoidal curve has been used between the tested points. The invention is not limited to any particular mathematical method of interpolation, and any shaped curve may be used to interpolate between points. 
     For interpolation to be useful, it is important that the tested data points reflect the high and low points in the range of proper tuning coefficients. For example, if only TDC and BDC were tested, interpolation would be useless, since none of the higher tuning coefficients needed for the higher torque scenarios at 90 and 270 degrees would be recognized. For the preferred embodiment, that is why the four quadrant points were selected for testing, and for the basis of the interpolation. 
     Although the invention has been described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the scope of this invention.