# Patent

## Title
Position feedback device with prediction

## Abstract

An apparatus for use with an encoder feedback device includes a comparator, a counter, and a prediction unit. The encoder feedback device is coupled to rotate with a rotating load and operable to generate at least one feedback position signal indicative of movement of the rotating load. The comparator is operable to receive the feedback position signal and generate a first position signal including a plurality of edges based on the feedback position signal. The counter is operable to receive the first position signal and count the edges to periodically generate position values at a predetermined update interval. The prediction unit is operable to receive a position data request at a first time and predict a position of the rotating load at the first time as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates generally to motor control and, more particularly, to a position feedback device that predicts position for incoming position data requests that occur between position updates.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Rotating motors are typically controlled by a motor drive that receives a reference motor velocity signal and, based on the motor velocity signal, produces and outputs a torque signal that is applied to the motor. Adjustment of the torque signal based on changes to the reference velocity signal relative to a feedback velocity signal ensures that the motor rotates at the reference velocity.
Some applications require precise motor control across multiple, synchronized motors. For example, an electronic line shaft may be employed in a printing application to move the paper or other material over rollers and through various stages of the printing process. Typical printing processes employ multiple colors, each applied at different locations along the line. Hence, to ensure print quality, the various stages are synchronized. A lack of synchronicity between the stations results in misregistration between the colors, leading to unacceptable product that may need to be scrapped.
Previous generations of printing technology employed a mechanical line shaft mechanically linked to the various printing stations. Rotation of the line shaft by an electric motor activated rollers and other printing station tools along the line to conduct the printing process. In a mechanical line shaft system, factors such as play in the mechanical linkages, stretching of the paper web, and torsional flexing of the line shaft itself make it difficult to achieve and maintain synchronicity between the printing stations, especially during periods of acceleration and deceleration of the printing system. It has been observed that when synchronicity is not maintained, product generated includes excessive flaws and is often unacceptable for intended use. Mechanical line shafts also have reduced flexibility in addressing print changes. Hence, where changes are required, down time may be excessive.
More modern printing systems, commonly referred to shaftless printing systems or electronic line shaft systems, employ a plurality of motors and associated rollers that are electrically synchronized, as opposed to mechanically synchronized. Lack of synchronicity in an electronic line shaft results in similar problems, such as color misregistration, evident in a mechanical line shaft system.
When operating a plurality motors synchronously in an automated system, several factors exist that may cause the position of the motors to deviate from each other even though they are all operating pursuant to a single reference velocity signal. For instance, motor inertia between motors at different stations is often non-uniform and can cause one motor to drift from the other motors.
Position errors in a drive system are controlled by a position regulator that acts on the difference between a reference position and a feedback position determined using a position feedback device such as, for instance, an optical encoder. That difference is commonly referred to in the motor control industry using terms such as “following error”, “tracking error”, and “position error”. The resolution of the position feedback device determines the number of discrete position references by which the position of the drive may be controlled.
One known position feedback device, commonly referred to as a Heidenhain encoder scans optical markings disposed about the periphery of a disk that rotates with the load. The encoder generates a two-channel output, one being a sine wave and the other a cosine wave. Typically, these channel signals are passed through filtering circuitry to convert them to square wave or edge signals. The edges are counted and used as position references for determining the rotational position of the drive. The edge counts are stored in a counter, such that forward motion increments the counter while reverse motion decrements the counter. The speed of the motor is typically determined by comparing the counter values over a predetermined time interval and dividing the number of counts by the time interval. The value stored in the position counter may be referred to as course position.
In some applications controlling the drive based on course position provides sufficient precision. However, in other applications, a more precise position control is desired. A technique for increasing the resolution of the optical encoder involves sampling the sine and cosine signals generated by the encoder prior to converting into square waves for edge counting. The sine/cosine data provides information concerning the incremental position of the drive (i.e., position between the edges used to generate the course position). An incremental position value is determined by computing the arctangent of the sine/cosine signals to yield a fractional angular position. Thus, the position of the drive is represented by a composite value in which the most significant bits are generated by the course position stored in the counter and the least significant bits are generated using the incremental position resulting from the arctangent function. The incremental position technique can be employed to increase the resolution of the encoder by up to several orders of magnitude.
Even with the increased resolution made available using the arctangent technique, some position error still remains in the feedback signal due to the discrete nature of the hardware used to generate the position data. The course and incremental position are updated at predetermined intervals, however, typical feedback units operate asynchronously with respect to the position and velocity regulators used to control the drive. Hence, the drive unit may request position data between updates by the feedback unit. In such an instance, the feedback unit provides the position data as of the last update (i.e., sample and hold). In applications with stringent precision requirements, this error in the position feedback signal is unacceptable.
Thus, it would be desirable to provide more accurate position measurements when the feedback unit is operated asynchronously with respect to the drive regulators. In a printing application, it would be advantageous to increase the accuracy of the position feedback to ensure the quality of printed product, thereby reducing waste.
BRIEF SUMMARY OF THE INVENTION
The present inventors have recognized that a position feedback device may be implemented with prediction to increase the accuracy of the position measurements when the feedback unit is operated asynchronously with respect to the drive regulators.
One aspect of the present invention is seen in an apparatus for use with an encoder feedback device. The apparatus includes a comparator, a counter, and a prediction unit. The encoder feedback device is coupled to rotate with a rotating load and operable to generate at least one feedback position signal indicative of movement of the rotating load. The comparator is operable to receive the feedback position signal and generate a first position signal including a plurality of edges based on the feedback position signal. The counter is operable to receive the first position signal and count the edges to periodically generate position values at a predetermined update interval. The prediction unit is operable to receive a position data request at a first time and predict a position of the rotating load at the first time as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.
Another aspect of the present invention is seen in a method for determining position of a rotating load. At least one feedback position signal indicative of movement of the rotating load is received. A first position signal including a plurality of edges is generated based on the feedback position signal. The edges are counted to periodically generate position values at a predetermined update interval. A position data request is received at a first time. A position of the rotating load at the first time is predicted as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.
These and other objects, advantages and aspects of the invention will become apparent from the following description. The particular objects and advantages described herein may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:
FIG. 1 is a simplified diagram of an electronic line shaft in accordance with one embodiment of the present invention;
FIG. 2 is a simplified block diagram of the electronic line shaft of FIG. 1 from a control perspective;
FIG. 3 is a block diagram of a motor control system in accordance with the present invention;
FIG. 4 is a block diagram of a feedback unit in the motor control system of FIG. 3;
FIG. 5 is a block diagram of a velocity compensation unit in the motor control system of FIG. 3; and
FIG. 6 is a graph of velocity versus time during an acceleration event illustrating lost velocity-seconds and velocity-seconds restored in accordance with the present invention.



While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.”
Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to FIG. 1, the present invention shall be described in the context of an electronic line shaft 10. In the illustrated embodiment, the electronic line shaft 10 is employed to control a printing process, however, the application of the present invention is not limited to any particular process or application. The phrase “electronic line shaft” is intended to apply to any system in which two or more motors are controlled in a synchronized fashion to facilitate a process.
A plurality of print stations 15, 20, perform printing operations on a moving web, 25 (e.g., paper). The printing operations performed by the print stations 15, 20 may vary. For example, some printing systems print using 4 color print processes. Each print station 15, 20 prints a different color. Other print stations 15, 20 perform operations such as cutting, binding, folding, etc. Motor-driven rollers 30, 35, 40 move the web 25 through the print stations 15, 20. Although the rollers 30, 35, 40 are shown as being separate from the print stations 15, 20, in some embodiments, they may be integrated. Each motor-driven roller 30, 35, 40 has an associated controller 45, 50, 55, respectively. The controller 45 operates as a master controller and the controllers 50, 55 operate as slave controllers. The master controller 45 generates reference position and velocity data for the slave controllers 50, 55 so that synchronization may be achieved. Synchronization of the rollers 30, 35, 40 allows synchronization of the print stations 15, 20 to effectively perform the printing process.
Turning now to FIG. 2, a block diagram of the electronic line shaft 10 from a control perspective is provided. The electronic line shaft 10 includes a master drive 100 and one or more slave drives 110, only one of which is illustrated, a synchronization unit 165, a signal source 130, and a ramp generator 135. Drive 100 includes a motor 102, a motor control system 104, first and second delay elements 150 and 160, respectively, a virtual encoder 145, and an encoder 106. Slave drive 110 includes a motor 112, a motor control system 114 and an encoder 116. Motors 102, 112, motor control systems 104, 114, and encoders 106, 116 operate in similar fashions and therefore, to simplify this explanation, only motor 102, encoder 106, and motor control system 104 will be described here in any detail. Motor control system 104 generates appropriate voltages and control signals for controlling motor 102. Encoder 106 generates position information as motor 102 rotates. A plurality of radially displaced optical markings (not shown) are disposed about the periphery of a disk that rotates with the load (e.g., the rollers 30, 35, 40 in FIG. 1) associated with motor 102. Encoder 106 includes a scanner that identifies the passage of each marking to enable the determination of load position as described in more detail below.
Motor 102 receives a torque input signal 120 from the motor control system 104 and rotates the load at a reference velocity in response to the torque input signal 120. In general, the master drive 100 receives a command velocity signal 125 from signal source 130, and converts the command velocity signal into torque signals to drive motor 102. The torque signals are adjusted during operation based on factors such as a deviation between the feedback load position and reference load position, a deviation between the feedback velocity and reference velocity, and motor inertia that prevents the motor 102 from immediately reacting fully to a change in the torque input signal 120.
Referring still to FIG. 2, the signal source 130 may comprise any conventional device capable of receiving an input related to a reference velocity of motor rotation. The input can either be manually entered (e.g., via a man machine interface) or can be automatically provided to the signal source 130 via an automated control system. Ramp generator 135 receives the command velocity signal 125 from the signal source 130 and produces a reference velocity signal 140 that transitions or ramps up or down to the input command velocity signal 125. In this regard, the ramp generator 135 prevents abrupt changes in the speed command and, therefore, the torque command that is input to the motor 102 to reduce stress that would be experienced by the motor components if the torque signal were to abruptly change. The signal source 130 and ramp generator 135 may be collectively referred to as a signal generator.
During operation, when the command velocity signal 125 is applied to ramp generator 135, the ramp generator 135 determines the difference between the current command velocity signal 125 and the previous command velocity signal 125. The ramp generator 135 then determines a period of time necessary to transition the reference velocity signal 140 to a level corresponding to the command velocity signal 125. For example, the reference velocity signal 140 may be ramped linearly by the ramp generator 135, or may be ramped hyperbolically or in any other suitable manner that smoothly transitions the motor 102 to the command velocity signal 125.
The ramp generator 135 outputs the reference velocity signal 140 to virtual encoder 145 and to delay element 150. The virtual encoder 145 is virtual in that it is programmed in firmware of drive 100. The construction and operation of the virtual encoder 145 is described in greater detail in U.S. Pat. No. 6,850,021, issued Feb. 1, 2005, entitled, “PRECISION VIRTUAL ENCODER,” commonly assigned to the assignee of the present application, and incorporated herein by reference in its entirety. In general, the virtual encoder 145 receives the reference velocity signal 140 from the ramp generator 135 and, based on a constant scale factor of the pulses per revolution of the motor 102 (e.g., 4096 pulses per revolution), integrates the input reference velocity signal 140. The virtual encoder 145 thus produces and outputs a reference position signal 155 that is a virtual equivalent of an integer position marking count that would originate from a real, physical encoder. The reference position signal 155 is provided to delay element 160. The reference velocity signal 140 and reference position signal 155 are also provided to synchronization unit 165 for communication to the slave drive 110 and any other slave drives in the electronic line shaft 10.
The outputs of delay elements 150 and 160 are provided as delayed velocity and position signals to motor control system 104. The reference velocity signal 140 and reference position signal 155 are delayed by the delay elements 150, 160 to provide sufficient time for the synchronization unit 165 to propagate the values to the slave drives 110 and their associated motor control systems 114 so that the master drive 100 and slave drives 110 may act on the control information in a synchronous fashion. The construction and operation of the synchronization unit 165 is described in greater detail in U.S. patent application Ser. No. 09/862,941, filed May 22, 2001, entitled, “APPARATUS FOR MULTI-CHASSIS CONFIGURABLE TIME SYNCHRONIZATION”, U.S. patent application Ser. No. 09/862,256, filed May 22, 2001, entitled, “PROTOCOL AND METHOD FOR MULTI-CHASSIS CONFIGURABLE TIME SYNCHRONIZATION,” and U.S. patent application Ser. No. 09/862,249, filed May 22, 2001, entitled, “SYSTEM AND METHOD FOR MULTI-CHASSIS CONFIGURABLE TIME SYNCHRONIZATION”, each commonly assigned to the assignee of the present application and incorporated herein by reference in its entirety.
In general, the synchronization unit 165 generates a timing signal in conjunction with the reference position and velocity and provides them to the slave drives 110. Responsive to the timing signal, the motor control systems 104, 114 act on the data to compare the feedback velocity and position to the reference values and make control adjustments synchronously and accordingly. Thus, the delay elements 150, 160 in the motor control system 104 provide a functional time equivalent of the delay in the position and speed commands that are delivered by the synchronization unit 165 to the slave motor control systems 114.
Turning now to FIG. 3, a simplified block diagram illustrating an exemplary motor control system 104 is provided. The operation of the motor control system 114 (see also FIG. 2) is similar, and is not described here in the interest of simplifying this explanation. The motor control system 104 includes a feedback unit 200 for generating position feedback data based on the outputs of the encoder 106, a position regulator 202 for controlling position errors, a velocity regulator 225 for controlling velocity errors, a velocity noise filter 230 for filtering position data to determine the velocity of the motor 102, an inertia compensation unit 250 for adjusting the control based on the expected inertial response of the motor 102, a velocity compensation unit 285 that affects the velocity control during periods of acceleration/deceleration, an inertia adaption unit 290 for generating acceleration feedback, first and second summers 215, 245 (i.e., adjustors), and a motor controller 295 that adjusts the torque input signal 120 applied to motor 102.
The reference position signal 155 is provided to position regulator 202. The position regulator 202 also receives a feedback position signal 205 from feedback unit 200 that reflects a measurement of the position count determined by encoder 106. Position regulator 202 subtracts the feedback position signal 205 from the reference position signal 155 to generate a position error signal 210 corresponding to the error between the feedback position and reference position. The position error signal 210 is one component used to eventually determine the torque input signal 120 applied to the motor 102. In general, the velocity of the motor 102 is adjusted to correct the position error by adding a component to the velocity if the feedback position count trails the reference position and subtracting a component from the velocity if the feedback position count is greater than the reference position. In generating the position error signal, the position regulator 202 converts the count error to a per unit velocity consistent with the reference velocity signal 140 by multiplying the count by a factor relating the seconds per edge of the encoder 106 at the base speed of the motor 102. The summer 215 receives the position error signal 210 and the reference velocity signal 140.
Returning to FIG. 3, inertia compensation unit 250 includes a derivative module 255 and a multiplier 265. Derivative module 255 receives the reference velocity signal 140 and, as the label implies, determines the derivative of the reference velocity signal 140 output by the ramp generator 135 (i.e., ramp rate where the derivative is an acceleration signal 260). The acceleration signal 260 is provided to multiplier 265. Multiplier 265 also receives an inertia coefficient signal 270 related to the inertia of motor 102. Multiplier 265 multiplies the inertia coefficient signal 270 and the acceleration signal 260 to provide an inertia compensation signal 275 that is provided to summer 245.
Summer 245 adds the inertia compensation signal 275 to the velocity regulator output signal 240 to generate a net output signal 280 for adjusting the torque input signal 120 applied to the motor 102.
The inertia compensation unit 250 is provided because changes in torque input signals 120 to motor 102 are resisted by the inertia of the motor 102, whether spinning or at rest. The inertia compensation signal 275 thus provides an additional signal that counteracts the inherent resistance of motor 102 to changes in velocity. It should be appreciated that when reference velocity signals 140 is decreasing, the derivative calculated by derivative module 255 is negative, thereby reducing the torque input signal 120 applied to motor 102. The inertia coefficient 270 is determined during the commissioning of the system and represents the time required to accelerate the inertia of the motor/load to base speed at rated torque. In embodiments, where the inertia adaption unit 290 is enabled, the inertia coefficient 270 is increased by approximately 50% from the system inertia, because the inertia adaption unit 290 electronically adds 50% to the effective system inertia of the system.
Referring still to FIG. 3, the velocity compensation unit 285 receives the acceleration signal 260 and generates a velocity compensation signal 287 which is provided to summer 215. The operation of the velocity compensation unit 285 is discussed in greater detail below with reference to FIGS. 6 and 7.
Summer 215 adds signals 140, 210 and 287 and provides its output 220 to a summer 226 in the velocity regulator 225. The other input to the summer 226 in the velocity regulator 225 is provided by the velocity noise filter 230. Velocity noise filter 230 receives various input values, N and Tv, during a commissioning procedure and uses those values along with a feedback position signal 205 from encoder 106 to generate a feedback velocity signal 235. Operation of velocity noise filter 230 is described in greater detail below. The summer 226 in the velocity regulator 225 subtracts the feedback velocity signal 235 from the sum 220 output by summer 215 to generate an error signal. The error signal is filtered by a velocity error filter 227, and the filtered error signal is provided to a proportional-integral (PI) controller 228. The output of the PI controller 228 is a velocity regulator output signal 240 that corresponds to the difference between the sum 220 and the feedback velocity signal 235. The velocity regulator output signal 240 is provided to summer 245. As described in greater detail below, the velocity error filter 227 is coordinated to cooperate with the velocity noise filter 230 to attenuate the sideband components introduced by the velocity noise filter 230. The operation of the PI controller 228 for controlling the velocity error is well known to those of ordinary skill in the art, and in the interests of simplifying this description, is not detailed herein.
Still referring to FIG. 3, inertia adaption unit 290 generates an acceleration feedback component for inclusion by the summer 245 for adjusting the net output signal 280 provided to the motor controller 295. Inertia adaption unit 290 creates an electronic inertia of precise magnitude to minimize velocity regulator gain change when a mechanical inertia becomes disconnected from the motor. For instance, when using a gear-box or spring coupling at high frequencies. System stability is increased in such systems, especially when load inertia is much greater than motor inertia. The inertia adaption unit 290 may not be used in some embodiments. Typically, the inertia adaption unit 290 is not used if the system inertia is <3 times the motor inertia. The inertia adaption unit 290 may be used if there is a gear-box and/or spring coupling with a resonant frequency in the range of 30 to 200 Hz, or if the desired velocity bandwidth exceeds two thirds of the maximum bandwidth divided by the inertia ratio. The construction and operation of the inertia adaption unit 290 is described in greater detail in U.S. patent application Ser. No. 10/662,556, filed Sep. 15, 2003, entitled, “METHOD AND APPARATUS FOR PROVIDING OPTIMAL ACCELERATION FEEDBACK,” commonly assigned to the assignee of the present application, and incorporated herein by reference in its entirety.
Referring to FIG. 3, the inertia adaption unit 290 may be configured to receive the feedback velocity signal 235 from the velocity noise filter 230 for determining the acceleration feedback, or alternatively, the inertia adaption unit 290 may receive the unfiltered position data from the feedback unit 200 (i.e., as indicated by the dashed line) and calculate an instantaneous velocity using the last two position values and the time interval between the values.
The motor controller 295 adjusts the torque input signal 120 based on variations between feedback and reference position, feedback and reference velocity, and inertia effects, as described above. The construction and operation of the motor controller 295 are known and not described in greater detail herein.
Turning now to FIG. 4, a simplified block diagram of the feedback unit 200 interfacing with encoder 106 is provided. The encoder 106 detects the passage of optical markings present on a disk that rotates along with the rotating load during operation and employs a two-channel system that outputs a sine component (i.e., channel A) and a cosine component (i.e., channel B) corresponding to detections of the optical markings. Phase differences between the pulse trains from each channel may be used to determine motor direction. The feedback unit 200 includes a comparator 300, a counter 305, a sampling unit 310, an incremental position estimator 315, a prediction unit 320, and a position register 325.
Comparator 300 converts the sine/cosine signals generated by the encoder 106 into square waves. Counter 305 counts rising and falling edges of the square wave signals, where four successive counts represent a rising edge of the A channel, a rising edge of the B channel, a falling edge of the A channel, and a falling edge of the B channel. The value stored in the counter 305 represents the coarse position of the load (e.g., motor 102 in this example).
Still referring to FIG. 4, the sampling unit 310 samples the sine/cosine signals at a predetermined update interval. The sampling unit 310 includes sample and hold circuitry and analog-to-digital conversion circuitry to generate the sine/cosine samples. Incremental position estimator 315 determines an incremental position of the drive 100 between edge counts in the counter 305 based on the sine/cosine values. The ratio of the A signal component to the B signal component represents the tangent of the rotational angle of the shaft. To determine the incremental position, incremental position estimator 315 computes the arctangent of the A/B ratio. In one embodiment, incremental position estimator 315 accesses an arctangent look-up table indexed by the value of the A/B ratio to determine the arctangent. Other techniques for calculating the arctangent may be used, such as a polynomial approximation. To synchronize the course and incremental position values, the incremental position estimator 315 generates a latch signal 330 that locks in the values of the counter 305 and sampling unit 310 when the incremental position is determined.
Position register 325 stores the position value 335 including a course component 340 and a fine component 345. The course component 340 corresponds to the value stored in the counter 305. Incremental position estimator 315 stores the computed incremental component in the position register 325 as the fine component 345.
Referring briefly to FIG. 3, the feedback unit 200 operates asynchronously with respect to the position regulator 202 and velocity noise filter 230. Hence, when a position interrupt is received its timing is typically misaligned with respect to the position updates generated by the counter 305 (i.e., course) and incremental position estimator 315 (i.e., fine) shown in FIG. 4.
Returning to FIG. 4, prediction unit 320 estimates the position of drive 100 when the position interrupt is received and updates the value stored in the position register 325 accordingly. Prediction unit 320 determines an instantaneous velocity for the drive 100 based on the previous two values stored in the position register 325 and the update time interval, in at least some embodiments, using the following equation:




  
    
      
        
          
            v
            ⁡
            
              (
              
                t
                k
              
              )
            
          
          =
          
            
              
                x
                ⁡
                
                  (
                  
                    t
                    k
                  
                  )
                
              
              -
              
                x
                ⁡
                
                  (
                  
                    t
                    
                      k
                      -
                      1
                    
                  
                  )
                
              
            
            
              T
              k
            
          
        
        ,
      
    
    
      
        (
        1
        )
      
    
  




where Tk represents the update interval of the feedback unit 200.

Prediction unit 320 predicts the position at the time of the position interrupt, ts, using the equation:

x(ts)=x(tk)+v(tk)·Δt  (2)

where Δt=(ts−tk) representing the temporal misalignment between the update interval of the feedback unit and the position interrupt request. Prediction unit 320 updates the value stored in the position register 325 based on the predicted position. Hence, the position values provided by the feedback unit 200 are aligned with the position interrupt request based on the predicted position offset, thereby increasing the accuracy of the position data.
With continued reference to FIG. 3, the operation of the velocity noise filter 230 and velocity error filter 227 are now described in greater detail. From a noise perspective the velocity noise filter 230 and velocity error filter 227 are in series. In general, the velocity noise filter 230 is a finite impulse response (FIR) filter performing a moving average function using N=2n data points to determine a velocity value. The value of n may represent a noise index and may be configured in the drive firmware to provide differing filter responses. The velocity error filter 227 is an infinite impulse response (IIR) that attenuates sidebands of the FIR velocity noise filter 230.
The velocity noise filter 230 operates on accumulated position provided by the feedback unit 200 and outputs a near ideal velocity value that is band-limited. An exemplary transfer function for the velocity noise filter 230, independent of the position feedback device type, can be expressed as:




  
    
      
        
          
            G
            ⁡
            
              (
              Z
              )
            
          
          =
          
            
              1
              -
              
                Z
                
                  -
                  N
                
              
            
            
              
                T
                v
              
              ⁢
              N
            
          
        
        ,
      
    
    
      
        (
        3
        )
      
    
  




where: N=number of taps, typically ranging from 1 to 256 in powers of two,

    
    
        Tv=sample time of the filter, and
        Z=exp(sTv)
    
    


By configuring the number of taps, N, in the velocity noise filter 230, the bandwidth and anticipated noise level is controllable. In general, the bandwidth decreases as the number of taps increases and lower bandwidth reduces noise level. Noise is thus reduced by increasing N.
The velocity error filter 227 is implemented using a second order IIR filter. An exemplary transfer function for the filter 227 can be expressed as:




  
    
      
        
          
            G
            ⁡
            
              (
              s
              )
            
          
          =
          
            1
            
              
                (
                
                  1
                  +
                  
                    
                      T
                      f
                    
                    ⁢
                    s
                  
                
                )
              
              2
            
          
        
        ,
      
    
    
      
        (
        4
        )
      
    
  




where Tf=filter time constant in seconds. A higher order filter is contemplated and may be employed in some embodiments. The velocity error filter 227 attenuates high frequency sidebands of the FIR velocity noise filter 230. The bandwidth of the velocity error filter 227 is typically set at a multiple of the bandwidth of the velocity noise filter 230. For instance, the bandwidth of the velocity error filter 227 may be set at 6 times that of the velocity noise filter 230. Other multiples, such as between about 3 and 9, or other values may be used. For example, in embodiments where the inertia adaption unit 290 is enabled, the bandwidth of the velocity error filter 227 may be set at 3 times the selected velocity bandwidth.

Turning now to FIG. 5, a simplified block diagram of the velocity compensation unit 285 of FIG. 3 is provided. The velocity compensation unit 285 includes a velocity compensation gain calculator 500 and two multipliers 510, 520 cooperating to generate the feed forward velocity compensation signal 287.
Referring again to FIG. 3, in the illustrated embodiment, position regulator 202 and velocity regulator 225 operate at different update intervals. Tx represents the interrupt interval of the position regulator 202, and Tv represents the interrupt interval of the velocity regulator 225. In the illustrated embodiment, the reference velocity signal 140 is oversampled at a rate 4 times that of the reference position signal 155, so Tx=4Tv. Other sampling rate arrangements are contemplated, including no oversampling, a higher level of oversampling, or a lower level of oversampling.
Velocity compensation unit 285 receives the sample time of velocity regulator 225 (Tv), the sample time of the position regulator 202 (Tx) and the time delay of the velocity noise filter 230 (i.e., based on N), during a commissioning procedure. In addition, compensation unit 285 receives the acceleration signal 260 (DV/DT) from the derivative module 255. However, in an embodiment without inertia compensation, the derivative module 255 may be incorporated into compensation unit 285.
Multiplier 510 multiplies the acceleration signal 260 by the position regulator sample time Tx. Multiplier 520 then multiplies the output of multiplier 510 by a velocity compensation gain factor, Vcomp_gain, generated by the velocity compensation gain calculator 500 to generate the velocity compensation signal 287 that is, in turn, provided as an input to summer 215 shown in FIG. 3.
To illustrate operation of velocity compensation unit 285, a simple example is described in which the velocity noise filter has one tap (i.e., n=0, N=20=1). The value of Vcomp_gain is normalized to unity when n=0. The output 260 of the derivative module 255 and Tx (sec) are multiplied by multiplier 510 to generate an intermediate velocity compensation signal 530. Note that in steady state, the value of intermediate velocity compensation signal 530 is zero because the value output by derivative module 255 is zero (i.e., no acceleration). When accelerating, the velocity compensation signal 287 restores an increment of velocity-seconds lost to the sample and hold process, as illustrated in FIG. 6. The velocity compensation unit 285 uses a feed forward compensation technique to anticipate the velocity seconds that are lost due to the discrete position samples and restore the lost velocity-seconds. Restoring lost velocity-seconds of the proper level secures an ideal correction and a near zero position error at the time of interrupt. The compensation provided by the velocity compensation signal 287 results in a reduced position error, thus reducing the observable performance difference between steady state and acceleration/deceleration periods of operation.
The velocity noise filter 230 imparts a delay that varies depending on the number of taps, N. It is known that delays through an FIR filter can be made precisely linear by design. Because the velocity noise filter 230 is linear in the illustrated embodiment, the filter delay is precisely known and can be factored into the compensation calculation of the velocity compensation gain calculator 500. The velocity noise filter 230 is run at the same sampling rate as the velocity regulator 225, Tv, at a task frequency that is an exact multiple of the position regulator 202. The velocity noise filter 230 could also be run at the same rate. In terms of timing, the velocity tasks could be performed after the position regulator 202 (i.e., Tv after Tx) or before the position regulator 202 (i.e., Tv before Tx). The timing relationships are predetermined. In either case, a precise formula can be applied via velocity compensation gain calculator 500 to restore lost velocity-seconds. The formula for Vcomp_gain where the velocity task is performed prior to the position task is:




  
    
      
        Vcomp_gain
        =
        
          1
          -
          
            
              [
              
                
                  
                    T
                    v
                  
                  
                    T
                    x
                  
                
                ·
                
                  
                    (
                    
                      N
                      -
                      1
                    
                    )
                  
                  2
                
              
              ]
            
            .
          
        
      
    
    
      
        (
        5
        )
      
    
  




The formula for Vcomp_gain where the position task is performed prior to the velocity task is:




  
    
      
        Vcomp_gain
        =
        
          1
          -
          
            
              [
              
                
                  
                    T
                    v
                  
                  
                    T
                    x
                  
                
                ·
                
                  
                    (
                    
                      N
                      +
                      1
                    
                    )
                  
                  2
                
              
              ]
            
            .
          
        
      
    
    
      
        (
        6
        )
      
    
  




Returning to FIG. 2, the reference velocity signal 140 and reference position signal 155 are sent to other the slave drives 110 controlling motors 112 that are to be operated synchronously with the motor 102. It should be appreciated in this regard that the master drive 100 sends signals to a plurality of slave drives 110. The cooperation between the master drive 100 and the slave drives 110 ensure that all motors operate at the same velocity and at the same position, and that adjustments are made to correct position errors when a feedback position of a given motor does not equal the reference position of the motor. Accordingly, only one virtual encoder is necessary for a system operating a plurality of synchronously controlled motors.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

## Claims

1. An apparatus for use with an encoder feedback device coupled to rotate with a rotating load and operable to generate at least one feedback position signal indicative of movement of the rotating load, comprising:
a comparator operable to receive the feedback position signal and generate a first position signal including a plurality of edges based on the feedback position signal;
a counter operable to receive the first position signal and count the edges to periodically generate position values at a predetermined update interval; and
a prediction unit operable to receive a position data request at a first time and predict a position of the rotating load at the first time as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.

2. The apparatus of claim 1, wherein the counter is operable to generate a course component of the position values based on the counted edges, and the apparatus further comprises an incremental position estimator operable determine a fine component of the position values based on the feedback position signal generated by the encoder.

3. The apparatus of claim 2, wherein the feedback position signal comprises a first channel signal and a second channel signal, and the incremental position estimator is operable to calculate a ratio between the first channel signal and the second channel signal, and determine the fine component based on the arctangent of the ratio.

4. The apparatus of claim 1, wherein the subset includes the two most recent position values, and the prediction unit is operable to determine a velocity of the motor as a function of the two most recent position values and the predetermined update interval and predict the position of the motor based on the most recent position value and the determined velocity.

5. The apparatus of claim 4, wherein the prediction unit is operable to multiply the determined velocity by the difference between the first time and the predetermined update interval to determine a position offset and add the position offset to the most recent position value to generate the predicted position.

6. The apparatus of claim 1, further comprising a position regulator operable to receive the aligned position signal and a reference position signal and control the motor based on the difference between the reference position signal and the predicted position signal.

7. The apparatus of claim 1, further comprising:
a velocity filter and operable to receive the aligned position signal and generate a feedback velocity signal based on the aligned position signal; and
a velocity regulator operable to receive the feedback velocity signal and a reference velocity signal and control the motor based on the difference between the reference velocity signal and the feedback velocity signal.

8. An electronic line shaft, comprising:
a signal source operable to generate a reference velocity signal;
a virtual encoder coupled to the signal source and operable to generate a reference position signal based on the reference velocity signal; and
a master drive unit operable to receive the feedback position signal, the reference velocity signal, and the reference position signal, comprising:
a motor;
a first encoder associated with the motor to generate at least one feedback position signal indicative of rotation of the motor;
a motor controller operable to provide a torque input signal to the motor;
a feedback unit, comprising:
a comparator operable to generate a first position signal including a plurality of edges based on the position feedback signal;
a counter operable to count the edges to periodically generate position values at a predetermined update interval; and
a prediction unit operable to receive a position data request at a first time and predict a position of the drive at the first time as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.

9. The electronic line shaft of claim 8, further comprising at least one slave drive unit operable to receive the reference velocity and reference position signals in a synchronous manner with respect to the master drive unit.

10. The electronic line shaft of claim 8, wherein the counter is operable to generate a course component of the position values based on the counted edges, and the feedback unit further comprises an incremental position estimator operable determine a fine component of the position values based on the feedback position signal generated by the first encoder.

11. The electronic line shaft of claim 10, wherein the feedback position signal comprises a first channel signal and a second channel signal, and the incremental position estimator is operable to calculate a ratio between the first channel signal and the second channel signal, and determine the fine component based on the arctangent of the ratio.

12. The electronic line shaft of claim 8, wherein the subset includes the two most recent position values, and the prediction unit is operable to determine a velocity of the motor as a function of the two most recent position values and the predetermined update interval and predict the position of the motor based on the most recent position value and the determined velocity.

13. The electronic line shaft of claim 12, wherein the prediction unit is operable to multiply the determined velocity by the difference between the first time and the predetermined update interval to determine a position offset and add the position offset to the most recent position value to generate the predicted position.

14. The electronic line shaft of claim 8, wherein the master drive further comprises a position regulator coupled to the prediction unit and operable to receive the aligned position signal and the reference position signal and provide a control signal to the motor controller for adjusting the torque input signal based on the difference between the reference position signal and the predicted position signal.

15. The electronic line shaft of claim 8, wherein the master drive unit further comprises:
a velocity filter operable to receive the aligned position signal and generate a feedback velocity signal based on the aligned position signal; and
a velocity regulator operable to receive the feedback velocity signal and the reference velocity signal and provide a control signal to the motor controller for adjusting the torque input signal based on the difference between the reference velocity signal and the feedback velocity signal.

16. A method for determining position of a rotating load, comprising:
receiving at least one feedback position signal indicative of movement of the rotating load;
generating a first position signal including a plurality of edges based on the feedback position signal;
counting the edges to periodically generate position values at a predetermined update interval;
receiving a position data request at a first time; and
predicting a position of the rotating load at the first time as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.

17. The method of claim 16, wherein counting the edges further comprises counting the edges to generate a course component of the position values, and the method further comprises determining a fine component of the position values based on the feedback position signal.

18. The method of claim 17, wherein the feedback position signal comprises a first channel signal and a second channel signal, and determining the fine component further comprises:
calculating a ratio between the first channel signal and the second channel signal; and
calculating the arctangent of the ratio to determine the fine component.

19. The method of claim 16, wherein the subset includes the two most recent position values, and the method further comprises:
determining a velocity of the motor as a function of the two most recent position values and the predetermined update interval; and
predicting the position of the motor based on the most recent position value and the determined velocity.

20. The method of claim 19, wherein predicting the position further comprises:
multiplying the determined velocity by the difference between the first time and the predetermined update interval to determine a position offset; and
adding the position offset to the most recent position value to generate the predicted position.

21. A printing system including a plurality of printing stations for processing a web, comprising:
a signal source operable to generate a reference velocity signal;
a virtual encoder operable to generate a reference position signal based on the reference velocity signal; and
a plurality of drive units for moving the web through the printing stations in accordance with the reference position signal and the reference velocity signal, each of the drive units comprising:
a motor;
a first encoder operable to generate at least one feedback position signal indicative of rotation of the motor;
a motor controller operable to provide a torque input signal to the motor;
a feedback unit, comprising:
a comparator operable to generate a first position signal including a plurality of edges based on the position feedback signal;
a counter operable to count the edges to periodically generate position values at a predetermined update interval; and
a prediction unit operable to receive a position data request at a first time and predict a position of the drive at the first time as a function of at least a subset of the position values generated prior to the first time and a misalignment between the first time and the predetermined update interval to generate an aligned position signal.

22. The printing system of claim 21, wherein one of the plurality of drive units is designated as a master drive unit, and the remaining drive units are designated as slave drive units, further comprising a synchronization unit operable to receive the reference velocity signal and the reference velocity signal and distribute the reference velocity signal and the reference velocity signal to at least the slave drive units, the slave drive units being operable to process the reference velocity signal and the reference position signal synchronously with respect to the master drive unit.