Patent Publication Number: US-2009222232-A1

Title: Reel motor torque calibration during tape motion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present applications claims priority to U.S. Provisional Appl. Ser. No. 60/980,004 filed Oct. 15, 2007. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to tape drives and more specifically to torque calibration of tape drive reel motors. 
     BACKGROUND 
     Tape drives typically employ a cartridge reel motor and a drive reel motor to wind tape from a cartridge reel, in a tape cartridge, to a drive reel, in the tape drive, and back. During winding of tape from either of the cartridge reel or the drive reel, the supply reel, that is, the reel from which tape is winding off from, ought to maintain an appropriate amount of tension in the tape such that the winding process can be performed without the tape getting tangled in tape drive parts. In addition, various tape format specifications call out tension values to be applied to the tape during read and write operations. 
     The tension applied to the tape via the supply reel is typically controlled by a corresponding supply reel motor applying torque. Application of the torque is generally done via torque control functions that can be implemented in tape drive control algorithms. The torque control functions are typically optimized based on the reel motor&#39;s torque constant which is sometimes referred to as “K T .” K T  may additionally be utilized in the operations of loading a tape cartridge, reel motor velocity control loops and reel motor position control loops. 
     For a given manufacturer&#39;s line of reel motors, however, K T  may vary from an ideal value from one reel motor to the next. This situation disadvantageously can result in a non-ideal amount of torque being applied by a reel motor which in turn translates to non-optimal tape drive performance. 
     SUMMARY 
     The present invention, in particular embodiments, is directed to methods, apparatuses and systems directed to calculation of a reel motor torque constant (“K T ”). In one implementation, calibration logic energizes supply and take-up reel motors to wind tape from a supply reel to a take-up reel. The calibration logic, during winding, selectively disengages the supply reel motor and measures one or more attributes of the supply reel motor, such as the resulting voltage observed at the winding terminals of the supply reel motor and the angular velocity of the supply reel motor during the period of disengagement. The calibration logic then calculates K T  based on the observed attributes. 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, apparatuses and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated. In addition to the aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting. 
         FIG. 1  is a perspective view of an LTO-type magnetic tape cartridge as viewed from an upper side thereof; 
         FIG. 2  is another perspective view of the magnetic tape cartridge, showing the lower side thereof; 
         FIG. 3  is a perspective view of a magnetic tape drive, showing the external appearance thereof; 
         FIG. 4  is a perspective view schematically showing the internal configuration of the magnetic tape drive; 
         FIG. 5  is another perspective view of the internal configuration shown in  FIG. 4  as viewed from the lower side thereof; 
         FIG. 6  is a diagram illustrating a tape threading mechanism; 
         FIG. 7  is a flowchart diagram illustrating a method for measuring supply reel motor parameters used to calculate a reel motor torque constant, in accordance with an example embodiment; 
         FIG. 8  is a flowchart diagram further illustrating certain operations of the method of  FIG. 7 , in accordance with an example embodiment; 
         FIG. 9  is a flowchart diagram further illustrating a supply reel motor coast operation of the method of  FIG. 7 , in accordance with an example embodiment; 
         FIG. 10  is a flowchart diagram further illustrating a supply reel motor spin to end of tape operation of the method of  FIG. 7 , in accordance with an example embodiment; 
         FIG. 11  is a diagram illustrating calculation of a reel motor torque constant based on the measured parameters from the method of  FIG. 8 , in accordance with an example embodiment; and 
         FIG. 12  is a schematic diagram illustrating an example computing system architecture that may be used to implement portions of the claimed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, apparatuses and methods which are meant to be illustrative, not limiting in scope. 
     Aspects of the claimed embodiments are directed to calculation of a reel motor torque constant (“K T ”). In one implementation, while a cartridge is in a tape drive, calibration logic energizes supply and take-up reel motors to wind tape from a supply reel to a take-up reel. The calibration logic then selectively disengages the supply reel motor and measures parameters of the supply reel motor. The calibration logic then calculates K T  based on the measured parameters. The resulting torque constant may be stored in the tape drive logic for use by logic and processes of the tape drive, such as tension control algorithms and the like. 
     For the K T  measurement while a supply reel motor is not actively powered-up but still being rotated by pull of tape from a take-up reel and associated motor, rotation of the supply reel motor acts as a generator resulting in a sinusoidal voltage at the supply reel motor winding terminals. The magnitude of this sinusoidal voltage is proportional to the K T  of the supply reel motor. This is due to the relationship of the magnets inside the supply reel motor moving by the winding in the motor. The magnetic field from the magnets is seen by the winding, changes in magnitude and polarity as the winding moves past the series of magnets. When the winding “sees” a change in the magnetic field, a current is induced in the winding. When the supply reel motor is “off,” the motor winding terminal voltages are not being driven by an external circuit and voltage on the terminals can be observed and that voltage is generated by the magnetically induced current in the winding. Since the supply reel motor is off but rotating, the current generated in the windings raises the voltage at the winding terminal. As the winding rotates by the series of magnets the voltage seen at the winding terminal will sinusoidally change from a positive voltage to a negative voltage and so on, until the supply reel motor stops rotating. 
     It should be understood that the claimed embodiments can be applied to both reel motors (cartridge reel and drive reel motors) of a typical tape drive in that measurements are recorded for both of the reel motors. Due to this, two K T  values can be produced—one for each reel motor and a particular K T  value will be applied to the applicable reel motor that was utilized to generate that particular K T  value. 
     Calibration logic may be implemented as tape drive firmware utilizing example architecture  461  of  FIG. 13 . Additionally, the tape drive firmware may be implemented as a portion of a tape drive controller such as controller  413  of  FIG. 6  which controls, amongst other operations, operation of various drive motors such as the reel motors. Controller  413  may also be implemented in architecture  461  of  FIG. 13 . 
     In another implementation, the calibration logic may be embodied as a client system that utilizes architecture  451 , the calibration logic of the client system in turn operable to perform the claimed embodiments on a tape drive. 
     The parameters measured by the calibration logic, in one implementation, are back electromotive force (“BEMF”) voltage across windings of the supply reel motor and angular velocity of the reel motor during a measurement period which can also be referred to as revolutions-per-minute (“RPM”) and, as previously mentioned, are used in the K T  calculation. K T  may then be utilized as an adjustment factor for various torque control functions utilized by reel motor and tape control algorithms. 
     In one implementation, calibration logic energizes reel motors of a tape drive to wind tape, from a supply reel to a take-up reel, with low tape tension in order to minimize perturbations to the tape tension and take-up reel loop. This involves the calibration logic energizing the reel motors to achieve a desired tape speed and provide a tape speed stabilization period once a desired tape speed is reached. Next, the calibration logic lowers the supply reel tension, via application of torque by the supply reel motor, at a first rate to a target level. Once the supply reel tension reaches the target level, the calibration logic further lowers supply reel tension at a second rate until the supply reel tension is approximately zero (0) Newtons. 
     At approximately zero Newtons, the supply reel motor is effectively disengaged and is coasting. Restated, the supply reel and supply reel motor continue to revolve due to tension in the tape from the take-up reel and the take-up reel motor. Once the supply reel motor is disengaged, the calibration logic provides a stabilization period and then measures the supply-reel parameters. 
     In one implementation, the supply reel motor is disengaged for one revolution and the reel motor parameters are measured during pre-defined time intervals spanning the one revolution. 
     In another implementation, measurements are taken for a portion of a revolution and the calibration logic sets the tape speed to the desired level when the first portion of measurements have been completed, provides a stabilization period, lowers the supply reel tension to the target level at the first rate and further lowers the supply reel tension to approximately zero Newtons at the second rate. In turn, the calibration logic measures the reel motor parameters for another portion of the revolution. The calibration logic repeats the cycle until a full revolution of measurements have been made. Advantageously, this implementation reduces variation in the measurements due to angular positioning of the reel motor. The reduction in variation is due to repeatable variations in magnet strength and spacing variations over a full revolution of a motor. 
     Before the claimed embodiments are explained in detail,  FIGS. 1-6  will first be presented which generally describe a tape cartridge ( FIGS. 1-2 ), a tape drive enclosure ( FIG. 3 ) and reel motors situated in the tape drive enclosure and how they interact with a cartridge reel and a drive reel to wind tape to and from the cartridge ( FIGS. 4-5 ). Additionally, an example mechanism for threading the tape from the cartridge reel to the drive reel will also be presented via  FIG. 6 . 
     Beginning with  FIGS. 1-2 ,  FIG. 1  is a perspective view of an LTO-type magnetic tape cartridge  2  and  FIG. 2  is another perspective view of the magnetic tape cartridge  2  that shows the lower side. The magnetic tape cartridge  2  has a cartridge casing  4  accommodating a magnetic tape wound around a cartridge reel. The magnetic tape cartridge  2  has one side surface formed at its front end with a shutter (lid)  6  normally biased in its closing direction. This one side surface of the magnetic tape cartridge  2  is further formed with two notches  8  and  10  exposed to the lower surface of the magnetic tape cartridge  2 . As shown in  FIG. 2 , the magnetic tape cartridge  2  has a chucking mechanism  12  composed of a magnetic member  13  such as an iron member and an annular gear  15 . The chucking mechanism  12  is connected to the cartridge reel accommodated in the cartridge casing  4 . 
       FIG. 3  is an external perspective view of a magnetic tape drive  14 , and  FIG. 4  is an internal perspective view schematically showing the internal configuration of the magnetic tape drive  14 .  FIG. 5  is another perspective view of the internal configuration shown in  FIG. 4  as viewed from the lower side. As shown in  FIG. 3 , the magnetic tape drive  14  has a housing  16  whose front end surface is formed with a cartridge loading slot (insertion slot)  18 . Referring to  FIGS. 4 and 5 , a cartridge reel motor  31 , a drive reel motor  33 , and a magnetic head  27  for recording and reproducing data are mounted on a base  20  provided in the tape drive  14 . 
     As shown in  FIGS. 4 and 5 , the magnetic tape cartridge  2  is adapted to be inserted into a carrier  22  movably provided in the tape drive  14 . A magnetic tape  25  is adapted to be supplied from a cartridge reel  24  provided in the magnetic tape cartridge  2 , next moving past the magnetic head  27 , and then being taken up by a drive reel  30  provided in the tape drive  14 . The condition shown in  FIGS. 4 and 5  is a condition where the carrier  22  holding the magnetic tape cartridge  2  has been moved to a cartridge mounting position and the chucking mechanism  12  connected to the cartridge reel  24  in the magnetic tape cartridge  2  is chucked (engaged) to a chucking mechanism of the cartridge reel motor  31  in the tape drive  14 . The drive reel  30  is rotated by the drive reel motor  33 . 
       FIG. 6  is a diagram illustrating a tape threading mechanism. A hub filler  402  is shown riding along the guide rail  408 , with tape  25  attached. The end of the tape  25  is fixedly attached to a leader in pin  404 , which is releasably attached to the hub filler  402 . The other end of the tape  25  is wound around the cartridge reel  24  of cartridge  4 . The cartridge reel  24  is mechanically coupled to the cartridge reel motor  412 . The cartridge reel motor  412  rotates during a tape unloading operation to retract the tape  25  into the tape cartridge  2 . 
     During a tape loading operation, the hub filler  402  attaches to the leader pin  404  in the tape cartridge  2 . The hub filler  402  is then driven to the drive reel  30  by a guide arm  416  and a guide arm motor  414  along a guide rail  408 . As the hub filler  402  is transported to the take-up reel  410 , tape  25  is dragged out of the cartridge  2 . The hub filler  402  then attaches to the drive reel  30 , attaching the tape  25  to the drive reel  30 . The hub filler  402  then attaches to the drive reel  30  at a drive reel opening  407 . The drive reel  30  and the hub filler  402  are designed such that when the tape  25  is attached to the drive reel  30 , the drive reel  30  can be rotated by a drive reel motor  33  to wrap or unwrap tape  25  around the drive reel  30  during a read/write operation. 
     During the tape read/write operation, the hub filler  402 , leader pin  404  and tape  25  are attached to the drive reel  30 . The drive reel  30  and the cartridge reel  24  are rotated to run the tape across a read/write head (not shown) for exchange of data between the tape drive mechanism and the tape  25 . During the tape unloading operation, the hub filler  402 , leader pin  404 , are transported from the drive reel  30  along the guide rail  408  to the cartridge  2 . Upon the hub filler  402  and the leader pin  404  being retracted into the cartridge  2 , the leader pin  404  is detached from the hub filler  402 . 
     The cartridge reel motor  31 , guide arm motor  414  and the drive reel motor  33  are typically electrical motors controlled by a controller  413  during the loading, read/write and unloading operations. The controller  413  provides electrical power and/or control signals to these motors ( 31 ,  33 ,  414 ) to control the magnitude and direction of the motor movements. Different combinations of motor movements are used during the different operations. For instance, during a loading operation, the guide arm motor  414  may be induced to cause the guide arm  416  to drive the hub filler  402  to the drive reel  30 . 
     As mentioned in the background section, a reel from which tape is unwinding can be referred to as a supply reel while the reel to which the tape is being fed can be referred to as a take-up reel. With that in mind, it should be understood in view of the claimed embodiments, that the cartridge reel  24  can be a supply reel when tape is wound from the cartridge reel  24  to the drive reel  30 . Or, when tape is being wound from the drive reel  30  to the cartridge reel  24 , then the drive reel  30  can be termed as the supply reel and the cartridge reel  24  can be referred to as the take-up reel. 
     In a similar manner, cartridge and drive reel motors ( 31 ,  33 ) can also be alternately-named depending on the direction of tape travel. For example, when the cartridge reel  24  is functioning as a supply reel, the cartridge reel motor  31  can then be labeled as a supply reel motor and the drive reel motor  33  can be named a take-up reel motor. As in the above-stated reel examples, the reel motor naming can also be reversed when the drive reel motor  33  is driving the drive reel  30  to supply tape to the cartridge reel  24  which is being turned by the cartridge reel motor  31 . 
     The claimed embodiments will now be further detailed via the flowcharts of  FIGS. 7-10 . Additionally, calculation of K T  will be described via  FIG. 11 . 
       FIG. 7  is a flowchart diagram illustrating a method  700  for measuring supply reel motor parameters used to calculate a reel motor torque constant, in accordance with an example embodiment. Method  700  describes a particular implementation wherein a tape cartridge  2  is inserted into a drive  14 , calibration logic measures cartridge reel motor parameters, of cartridge reel motor  31 , as tape  25  is wound from cartridge reel  24  to drive reel  30 , when the cartridge reel motor  31  is selectively disengaged. Once measurements are completed for the cartridge reel motor  31 , the calibration logic spins a balance of the tape  25  onto the drive reel rotor  35  and the process is repeated in the reverse direction—calibration logic measures drive reel parameters (BEMF voltage and RPM) while the tape  25  is spun to the cartridge reel  24  during the intervals when the drive reel motor  31  is disengaged. 
     Upon loading ( 702 ) of a tape  2  into a tape drive  14 , the control logic initiates spinning ( 704 ) of the tape  2  from the cartridge reel  24  to the drive reel  30 . The calibration logic then disengages the cartridge reel motor  31  thus allowing it to coast ( 706 ) as it is being rotated by the cartridge reel  24  due to the tape being pulled by the drive rotor  30 , via the drive rotor  33 . The calibration logic then disengages ( 706 ) the cartridge reel motor  31  and measures ( 708 ) voltage and RPM of the cartridge reel motor  31  while it is disengaged. Some methods of measuring RPM include using hall and optical sensors in a reel motor and measuring a radius of tape on a reel. If additional cartridge reel motor  31  measurements ( 710 ) are not required, the calibration logic spins ( 712 ) a remaining portion of tape  25  from the cartridge reel  24  to the drive reel  30 . Otherwise, calibration logic repeats operations  704 ,  706  and  708 . 
     As previously mentioned, measurement of the BEMF voltage and RPM can be done at time intervals during one revolution of a reel motor, in one implementation. In another implementation, operations  704 - 708  are performed during sub-revolutions and repeated until a full revolution of reel motor measurements are completed. Restated, the cartridge reel motor  31  is energized, disengaged and measurements are recorded—all three (energize, disengage and record measurements) repeatedly until measurements for a full revolution have been completed. 
     Once the tape  25  has been spun ( 712 ) to the drive reel rotor  30 , control logic spins the tape ( 714 ) in the opposite direction from the drive reel rotor  30  to the cartridge reel rotor  24  and disengages ( 716 ) the drive reel motor  33 . Calibration logic then records BEMF voltage and RPM ( 718 ) of the drive reel motor  33 , during the intervals when the drive reel motor is disengaged. If additional measurements ( 720 ) are required, calibration logic energizes ( 722 ) the cartridge reel motor  31  and repeats operations  716  and  718  as necessary. Once measurements for the drive reel motor  33  are complete ( 720 ), the calibration logic spins ( 724 ) the tape back to the cartridge reel  24  and unloads the tape ( 726 ). 
       FIG. 8  is a flowchart diagram further illustrating operations  704  and  722  of  FIG. 7 . Operations  702  and  722  characterize, in some implementations, the process of energizing the reel motors, one of which is a supply reel motor, and then disengaging the supply reel motor. This process can be referred to as “selectively disengaging” the supply reel motor, in some implementations. 
     First, calibration logic ramps tape speed up to a target measurement speed ( 802 ) and allows the tape speed to stabilize for a period of time ( 802 ). Next, calibration logic lowers supply reel tension ( 804 ) at a first rate to a target level. The supply reel referred to in operation  804  may be the cartridge reel  24  in operation  704  or the drive reel  30  in operations  712  and  722 . The calibration logic then lowers the supply reel tension ( 706 ) at a second rate until the reel motor ( 31  or  33  as applicable) is no longer engaged and applying approximately zero Newtons of torque. 
     One reason for ramping down the tension on the supply reel motor, in the fashion described via  FIG. 8 , is to prevent harm to the tape  726  by avoiding an abrupt transition in tape tension which may occur if the supply reel motor were to be quickly disengaged. In a similar manner, after measurements have been recorded on the supply reel motor, tape speed is ramped at an appropriate rate to the desired tape speed in order to prevent tape damage. 
       FIG. 9  is a flowchart diagram further illustrating the coast operations ( 706 ,  716 ) operations of  FIG. 7 . Once operation  806 , of  FIG. 8 , is completed, calibration logic disengages the supply reel motor to allow it to coast ( 900 ) and implements a stabilization period ( 902 ). 
       FIG. 10  is a flowchart diagram further illustrating the spin to end of tape operation ( 724 ) of  FIG. 7 . Once the measurements of both reel motors ( 31 ,  33 ) have been completed, the calibration logic raises the tape tension to a desired level ( 1002 ) and ramps down the tape speed ( 1004 ) near the end of the tape  25 , as viewed by the drive rotor  30 , as the tape  25  is winding back into the cartridge  2 . 
       FIG. 11  is a diagram illustrating calculation of a reel motor torque constant based on the measured parameters from the method of  FIG. 7 , in accordance with an example embodiment. Schematically represented is a supply reel motor  1102  that is selectively disengaged by control logic  1100  to allow for measurement of BEMF voltage and RPM. Once measurements are collected for a full revolution, the calibration logic processes the measurements through a gain/level shift circuit  1104 , an analog-to-digital converter  1106  and the BEMF voltage measurements are further processed through volts conversion  1108 . The calibration logic then processes the measurements through a low pass filter  1110 , removes any DC bias in the measurement ( 1112 ) by subtracting the DC average of the average of the BEMF voltage measurements ( 1114 ) and calculates a root-mean-square of the measurements ( 1116 ). The calibration logic then uses the results  1118  of the root-mean-square  1116  and an average tape speed  1120 , calculated from the collected RPM measurements, and an ideal torque constant  1122  to calculate ( 1124 ) a torque constant correction factor. 
     In respect to specific portions of  FIG. 11 , the average BEMF voltage  1114  may be calculated via the following Equation I wherein “x” are the individual collected voltage measurements as processed up through the low pass filter  1110 : 
     
       
         
           
             
               
                 
                   
                     x 
                     rms 
                   
                   = 
                   
                     
                       
                         
                           1 
                           N 
                         
                          
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             N 
                           
                            
                           
                             x 
                             i 
                             2 
                           
                         
                       
                     
                     = 
                     
                       
                         
                           
                             x 
                             1 
                             2 
                           
                           + 
                           
                             x 
                             2 
                             2 
                           
                           + 
                           … 
                           + 
                           
                             x 
                             N 
                             2 
                           
                         
                         N 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   I 
                 
               
             
           
         
       
     
     Additionally, the K T  calculation  1118  may be calculated via Equation II: 
     
       
         
           
             
               
                 
                   Kt 
                   = 
                   
                     
                       rms 
                        
                       
                           
                       
                        
                       BackEMF 
                        
                       
                           
                       
                        
                       Voltage 
                       × 
                       
                         2 
                       
                       × 
                       0.955 
                     
                     
                       
                         ( 
                         
                           Average 
                            
                           
                               
                           
                            
                           Angular 
                            
                           
                               
                           
                            
                           Velocity 
                         
                         ) 
                       
                       × 
                       2 
                        
                       Pi 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   II 
                 
               
             
           
         
       
     
     Calibration logic may be implemented, as part of a controller  413  in one implementation, in a tape drive firmware utilizing computer architecture such as the example architecture  461  of  FIG. 12 . Architecture  461  typically includes a processor  453 , cache  454  and memory  463 . Additionally, architecture  461  will typically include an I/O bus  459 , I/O ports  490  and non-volatile storage  492  to store instructions that can be executed by processor  453 . 
     In another implementation, the measurements performed on a supply reel motor may be directed by and collected by a host computer which can then perform the associate reel motor torque constant calculation. 
     While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.