Abstract:
A non-linear control (NLC) system for controlling performance of a device. The NLC system comprises a controller system and a first feedback compensator connected to the controller. The first feedback compensator compensates for linear error performance of the device; the second feedback compensator compensates for non-linear error performance of the device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to document processing machines and, more particularly, to document processing machines incorporating control systems.  
           [0003]    2. Prior Art  
           [0004]    Various document systems require that an automatic document handling apparatus within the system operate within precision tolerances so as to align a particular edge of a document with a given coordinate. For example, in an x-y coordinate system this could be the coordinates 0,0, on a copier platen where the scanner or copier optics have also been aligned to begin imaging at the 0,0 coordinates on the copier platen. This process of aligning the document relative to a given coordinate is known as document registration. In a manual copying operation, an original document is registered by the user with ruler markings along the sides of the platen. In systems having automatic document handling systems document registration is automatic and is generally accomplished by the use of various belts, rollers and sensors situated along the document path. The precision control of the various belts and rollers along the document path is generally accomplished using classical feedback control systems. If the original document is not properly registered then undesirable dark borders and shadow images may appear on the copy. In addition to document registration other document positioning functions requiring feedback control includes photo receptive (PR) drum motion control, and intermediate belt transfer (IBT) motion control, and developer motion control. In general, the desired positioning accuracy for document handling is on the order of one millimeter. As is readily appreciated an automatic document handling system must rapidly and accurately position each document. Conventional feedback control systems correct for the undesired linear motion of the document handling systems. However, the non-linear motion due to motor torque ripple, gear run-out, and roll run-out can not always be corrected due to bandwidth limitations in the classical controller. Currently, the non-linear motion due to these errors is minimized by demanding that tolerances associated with manufactured parts be as small as possible. Achieving such tolerances are difficult and expensive to maintain.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with one embodiment of the invention, a non-linear control (NLC) system for controlling performance of a device is provided. The NLC system comprises a controller system with a first and second feedback compensator connected to the controller. The first feedback compensator compensates for linear error performance of the device while the second feedback compensator compensates for non-linear error performance of the device.  
           [0006]    In accordance with another embodiment, the invention includes a method for controlling non-linear motion of a system. The method comprises the steps of sampling nonlinear performance of the system and determining at least one frequency component of the sampled non-linear performance. Next, at least one sinusoid substantially equal in magnitude to the at least one frequency component and substantially 180 degrees phase inverted with respect to the at least one frequency component is applied to the system controller.  
           [0007]    Another embodiment of the invention is directed towards a hybrid learning feedback controller (HLC) system for controlling operation of a xerographic imaging device; the xerographic imaging device having an input port and an output port. The HLC feedback controller system comprises a feed forward controller system operative to receive an actuating signal and in response to the actuating signal output a manipulated variable. The feed forward controller comprises a feed forward controller, a first input summing node, a second input summing node, a first proportionality device, and a second feedback compensator having a learning controller system for correcting non-linear motion of the device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein:  
         [0009]    [0009]FIG. 1 is a perspective view of a conventional document processing apparatus;  
         [0010]    [0010]FIG. 2 is a schematic view of paper paths and belts within the document processing apparatus shown in FIG. 1;  
         [0011]    [0011]FIG. 3 is a block diagram of a hybrid learning control system incorporating features of the present invention for controlling belts shown in FIG. 2;  
         [0012]    [0012]FIG. 4 is a flowchart of one method for learning the phase and amplitude of the sinusoidal signals to be applied to correct for non-linear motion using the hybrid system shown in FIG. 3;  
         [0013]    [0013]FIG. 5 is a detailed flowchart of one method for finding the phase of each error component frequency shown in FIG. 4;  
         [0014]    [0014]FIG. 6 a  is a frequency domain graph showing belt velocity variation before learning and applying the phase and amplitude of the sinusoidal signals as shown in FIG. 4;  
         [0015]    [0015]FIG. 6 b  is a frequency domain graph showing belt velocity variation after learning and applying the phase and amplitude of the sinusoidal signals as shown in FIG. 4;  
         [0016]    [0016]FIG. 7 is a progression graph showing the velocity variation while the algorithm shown in FIG. 4 is finding the minimum velocity variation; and  
         [0017]    [0017]FIG. 8 is a comparison graph showing belt speed variation before and after learning the phase and amplitude of the sinusoidal signals as shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Although the present invention will be described with reference to the single embodiment shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. Referring to FIG. 1, there is shown a perspective view of a document processing apparatus  10 . The apparatus  10  could be any suitable type of document processing apparatus, such as a copier, a facsimile machine, a scanner, a computer printer, or a multifunction device having two or more functions. Referring also to FIG. 2, in this embodiment the apparatus  10  is a copier which includes an original document feed system  11  and a copy document feed system  13 . A scanner or image obtainer  12  is provided under a transparent glass platen  15 . The scanned information from an original document fed through the original document feed system  13  is imaged onto paper selected from paper trays  14  or  16 . Paper selected from either of the paper trays  14 , 16  is moved by the copy document feed system  13  through the apparatus  10  by means of various intermediate belt transfers  18  and rollers  19  schematically depicted in FIG. 2. The original document feed system  11  also comprises suitable belts and rollers for automatically moving original documents through the document system. Referring now to FIG. 3 there is shown a block diagram of a hybrid learning control system, incorporating features of the present invention, generally intended to be used for controlling document transport systems in a document processing apparatus such as described above with reference to FIGS. 1 and 2 (e.g.: a copier, a facsimile machine, a computer printer, a scanner, or a multifunction device). The hybrid controller consists of a controller  340 , a feedback path consisting of feedback gain devices  334 , another feedback gain device  341  and feedback transfer element  335 ; a learning feedback system consisting of a learning feedback element  332  and learning feedback signal generator  331 . In addition, the hybrid controller consists of summing nodes  339 , 338  and amplifier  336 .  
         [0019]    The controller  340  controls the velocity of IBT (intermediate belt transfer) belts, photoreceptor drums, etc., while the learning controller  332  corrects for the non-linear motion. The combination of the controller  340  with the learning controller  332  provides precise registration between the original image and the surface receiving the reproduced image. Precise registration prevents the appearance of defects (e.g., a border between two colors not present in the original image) caused by slight misalignment of the marking element with respect to the corresponding area of the image receiving surface (e.g., a photoreceptor or a marking medium) at the time of the reproduced image is transferred. The learning controller  332  corrects for the non-linear motion by setting the signal generator  331  to add a sinusoidal signal of equal magnitude and substantially opposite phase for every error frequency to be corrected to summing node  338 .  
         [0020]    Referring now to FIG. 4 there is shown a flowchart of one method for learning the phase and amplitude of the sinusoidal signals to be applied to correct for nonlinear motion of the controlled plant (FIG. 3, item  333 ). The learning device (FIG. 3, item  332 ) digitizes  51  the output signal of the controlled plant (FIG. 3, item  333 ) and determines  52  each of the error component frequencies. The voltage magnitude of the correcting signal is found by dividing the velocity variation of each error component frequency by the transfer function of the velocity/voltage. The transfer function of velocity/voltage vs. frequency can be obtained analytically or experimentally and are determined  53  through well known techniques and is not discussed here.  
         [0021]    The phase of each error component frequency is determined  54  and is explained in more detail below. Once the phase and amplitude is known the signal generator (FIG. 3, item  331 ) is set  55  to apply a sinusoidal signal equal in magnitude and substantially opposite in phase for each error component frequency.  
         [0022]    Referring now to FIG. 5 there is shown a detailed flowchart of one method for learning (FIG. 4, item  54 ) the phase of the sinusoidal signals to correct for nonlinear motion using the hybrid system shown in FIG. 3. The plant (FIG. 3, item  333 ) output performance (i.e., belt speed) is discreetly sampled  412  at a constant interval, digitizing and transforming the sample signal from the time domain to the frequency domain. Generally, the sampled error signal is sampled  412  at a rate compatible with a Fast Fourier Transform (FFT) implementation. The phase of the error signal is synchronous to the position of the part that generates the error, this means that the phase of each error signal is referenced from the start of each revolution. Accordingly, to determine the phase of the error signal the start-phase variable is initially set to zero degrees  414  while the limit-phase variable is set to 360 degrees  414 . Next, a decision operation determines  415  the start position of the part being controlled and the signal generator (FIG. 3, item  331 ) applies  416  a sinusoid equal in magnitude at a phase equal to the following:  
         Phase=start-phase+{fraction (1/4)}((limit-phase)−(start-phase)).  
         [0023]    The plant (FIG. 3, item  333 ) output performance (i.e., belt speed) is again discreetly sampled  417  at a constant interval, digitizing and transforming the sample signal from the time domain to the frequency domain. The velocity variation amplitude of the frequency of interest is determined  418  and a decision operation  421  compares the velocity variation amplitude of the frequency of interest to a specified error tolerance. If the result of the decision operation  421  is less than the specified error tolerance the phase for the error frequency is set  419  to:  
         Phase=start-phase+{fraction (1/4)}((limit-phase)−(start-phase)),  
         [0024]    and learning stops  420 . If the result of the decision operation  421  is not less than the specified error tolerance a decision operation again determines  422  the start position of the part being controlled; the signal generator (FIG. 3, item  331 ) applies  425  the sinusoid equal in magnitude at a phase equal to the following:  
         Phase=start-phase+{fraction (3/4)}((limit-phase)−(start-phase)).  
         [0025]    The plant (FIG. 3, item  333 ) output performance (i.e., belt speed) is again discreetly sampled  426  at a constant interval, digitizing and transforming the sample signal from the time domain to the frequency domain. The velocity variation amplitude of the frequency of interest is determined  428  and a decision operation  429  compares the velocity variation amplitude of the frequency of interest to a specified error tolerance. If the result of the decision operation  429  is less than the specified error tolerance the phase for the error frequency is set  430  to:  
         Phase=start-phase+{fraction (3/4)}((limit-phase)−(start-phase)),  
         [0026]    and learning stops  420 . If the result of the decision operation  421  is not less than the specified error tolerance a decision operation determines  432  if the velocity variation amplitude determined by step  418  is less than the velocity variation amplitude determined by step  428 . An affirmative determination results in setting  432  the start-phase variable to the last known value of the start phase variable and setting  432  the limit-phase variable to:  
         Limit-phase=((limit-phase)−(start-phase)).  
         [0027]    A negative determination by decision operation  432  results in setting  436  the limit-phase variable to the last known value of the limit-phase variable and setting  436  the start-phase variable to:  
         Start-phase={fraction (1/2)}((limit-phase)−(start-phase)).  
         [0028]    The steps are then repeated until a decision operation,  421  or  429 , determines the velocity variation amplitude of the frequency of interest is less than a specified error tolerance and learning stops,  420  or  431 .  
         [0029]    In test, mathematical models simulated gears and roll run-out. In one test the center was set to 0.1 mm off-center, resulting in velocity changes of about 1.814 mm/sec (zero to peak) or 0.565% of AV/V. Typical motion quality for applications affecting the image formation in printers requires that %AV/V velocity be less than 0.1%. After learning the %AV/V was reduced to 0.00291%.  
         [0030]    Referring to FIG. 8 there is shown a comparison graph showing the belt speed variation before and after learning the phase and amplitude of the sinusoidal signals according to the steps shown in FIGS. 3 and 4. Referring to FIG. 6 a  there is shown a frequency graph showing belt velocity variation before learning and applying the phase and amplitude of the sinusoidal signals as shown in FIG. 4. Referring to FIG. 6 b  there is shown a frequency graph showing belt velocity variation after learning and applying the phase and amplitude of the sinusoidal signals as shown in FIG. 4.  
         [0031]    Referring now to FIG. 7 there is shown a progression graph showing the belt speed and velocity variation while the algorithm shown in FIG. 4 is finding the minimum velocity variation. It is readily appreciated from FIG. 7 that learning in a particular system is done once, unless one of the parts is replaced. For example, if a motor is replaced, the magnitude and phase of the correction signal should be learned again. It is also readily appreciated from the descriptions above that the disadvantages of the prior art are overcome by the present invention. Specifically, the invention allows manufacturing tolerance requirements such as pinion runout tolerances to be relaxed by dynamically compensating non-linear motion induced by finite tolerance.  
         [0032]    It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.