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.

Description:
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&#39; 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 T v , 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 &lt;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 T k  represents the update interval of the feedback unit  200 .
 
   Prediction unit  320  predicts the position at the time of the position interrupt, t s , using the equation:
 
 x ( t   s )= x ( t   k )+ v ( t   k )·Δ t   (2)
 
   where Δt=(t s −t k ) 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=2 n  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,
         T v =sample time of the filter, and   Z=exp(sT v )       
   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 T f =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. T x  represents the interrupt interval of the position regulator  202 , and T v  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 T x =4T v . 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  (T v ), the sample time of the position regulator  202  (T x ) 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 T x . 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=2 0 =1). The value of Vcomp_gain is normalized to unity when n=0. The output  260  of the derivative module  255  and T x  (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 , T v , 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., T v  after T x ) or before the position regulator  202  (i.e., T v  before T x ). 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.