Abstract:
An apparatus comprising a center error creation circuit and a center error offset injection circuit. The center error creation circuit may be configured to generate a center error signal in response to light from a main laser reflected from a surface of an optical disc. The center error offset injection circuit may be configured to (i) determine a value of the center error signal when a lens in a sled housing is at a mechanical center and (ii) generate an offset signal based upon the value. The center error offset injection circuit generally measures an average value of the center error signal over a predetermined amount of time when a lens suspension which holds the lens in place is in a mechanical equilibrium state.

Description:
This is a continuation of U.S. Ser. No. 11/257,606, filed Oct. 25, 2005 now U.S. Pat. No. 7,580,331, which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to optical storage generally and, more particularly, to a method and/or apparatus for implementing a center error mechanical center adjustment. 
     BACKGROUND OF THE INVENTION 
     In a conventional optical disc system, to sense the position of the laser beam in relation to the track on the disc, the main laser beam creates a reflection from the disc. The reflection is typically picked up by 4 photo-diode sensors (or a photo detector array).  FIG. 1  is a conceptual diagram illustrating how such a photo-diode configuration is laid out in relation to the track direction. The outputs of the 4 photo-diodes (when the laser beam is focused on the disc) are shown as signals A, B, C and D, respectively. 
     A track position (e.g., the location of a laser spot relative to a track center on an optical disc) is detected by imaging the laser spot on the photo detector array. Diffraction causes a slight change in intensity on the two different sides of the photo detector array when the relative position of the laser spot and center of the track changes. The difference in intensity on the two different sides of the photo detector array is called a push-pull signal. The push-pull signal is proportional to the tracking error signal or signal TE. 
     If the laser does not shine directly through a center of the lens, an image is moved to one side and the push-pull signal changes. Such an effect is defined as the center error (CE). The center error cannot be distinguished from the push-pull effect by examining one laser spot alone. To obtain an accurate track position, a second measurement is taken one-half track away from where the first measurement was taken. With the second measurement, the center error is common to the first measurement, but the push-pull effect is reversed. By combining the first and the second measurements, an accurate track position can be determined. For DVD ROMs, a phase detection method is used to detect the track position. The phase detection method is mostly immune to the effect of center error. 
     In an optical pick-up unit (OPU), the lens is held in position by springs in a sled housing. The optical center of the lens in the OPU is defined as the position of the lens where the center error is zero (i.e., where the laser is shining through the center of the lens). However, the position of the lens where the center error is zero, may not be the natural position of the lens when both springs are in a mechanical equilibrium state (e.g., when no control force is being applied). The natural position of the lens is defined as a mechanical center. Therefore, the center error is not necessarily zero when (i) the lens is at the mechanical center or (ii) the output of a lens controller is zero. 
     The motion of the laser spot is a superposition of the motion of the sled plus the motion of the lens inside the sled housing. A track seek initiated by the motion of the lens is called a fine seek (or fine seek mode for the system). The fine seek mode is slow because the laser spot remains locked to the disc even while the laser spot is crossing the tracks. The lens may move over several hundred tracks under the fine seek mode. However, if a target track is displaced at a large distance from where the laser spot is currently positioned, the lens cannot move fast enough under the fine seek mode. Therefore, the sled motor is used to reposition the lens under such a condition. A rough seek mode includes moving the lens and the sled housing with a sled motor to move the lens to the target track. While in the rough seek mode, the laser spot is unlocked from the disc. The signal CE is used to control or position the lens to the center of the housing. Positioning the lens to the center of the housing prevents the lens from inadvertently hitting the housing when the sled motor accelerates or decelerates in the rough seek mode. Such an impact can cause the lens to loose focus. 
     When it is necessary to lock the laser spot back on the tracks, a CE controller will switch to a tracking controller (or tracking error (TE) controller) which servos on a tracking error signal. When it is necessary to lock the laser spot back on the tracks, the center error signal will no longer be used to control the lens. Any previous control output to keep the lens at the optical center and not on the mechanical center will be lost when the control of the lens is switched from CE controller to the TE controller. The change in control from the CE controller to the TE controller will introduce a transient effect that affects the lock-on-track performance at the end of the rough seek mode. Conventional methods are characterized by an increase in seek time. The seek time includes the time for the lens-to-disc motion (which may be induced by switching from the center error signal to the mechanical center) to dissipate. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an apparatus comprising a center error creation circuit and a center error offset injection circuit. The center error creation circuit may be configured to generate a center error signal in response to light from a main laser reflected from a surface of an optical disc. The center error offset injection circuit may be configured to (i) determine a value of the center error signal when a lens in a sled housing is at a mechanical center and (ii) generate an offset signal based upon the value. The center error offset injection circuit generally measures an average value of the center error signal over a predetermined amount of time when a lens suspension which holds the lens in place is in a mechanical equilibrium state. 
     The objects, features and advantages of the present invention include providing a method and/or apparatus for implementing a CE mechanical center adjustment that may (i) minimize the change in the output of a lens control output when the CE controller is switched to the track controller at the end of the rough seek mode (ii) allow the laser to lock to a track reliably, and/or (iii) improve the seek performance of the optical drive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
         FIG. 1  illustrates a photo-diode sensor distribution system; 
         FIG. 2  is a diagram illustrating a present embodiment of the present invention; 
         FIG. 3  is a detailed diagram illustrating the present embodiment of the present invention; 
         FIG. 4  is a diagram illustrating tracking modes associated with the type of controller used; 
         FIG. 5  is a diagram illustrating the present invention in a rough seek mode; 
         FIG. 6  is a flow diagram illustrating a process of the present embodiment; 
         FIG. 7  is a diagram illustrating a CE waveform which implements the present invention; and 
         FIG. 8  is a diagram illustrating a CE waveform without the CE mechanical offset injection. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2  a diagram of a system  100  in accordance with the present invention is shown. The system  100  generally comprises a block (or circuit)  102 , a block (or circuit)  104 , a block (or circuit)  105 , a block (or circuit)  108 , a block (or circuit)  114 , a block (or circuit)  116 , a block (or circuit)  118 , a block (or circuit)  120 , a block (or circuit)  122 , a block (or circuit)  124 , a block (or circuit)  126  and a block (or circuit)  128 . The circuit  102  may be implemented as a CE offset injection circuit. The circuit  104  may be implemented as a track counter. The circuit  105  may be implemented as a sled circuit. The circuit  108  may be implemented as a lens controller. The circuit  114  may be implemented as a multiplexer. The circuit  116  may be implemented as a multiplexer. The circuit  118  may be implemented as a step motor control circuit. The circuit  120  may be implemented as a digital-to-analog converter. The circuit  122  may be implemented as a digital-to-analog converter. The circuit  124  may be implemented as a digital to analog converter. The circuit  128  may be implemented as a step sled system  128 . 
     The CE offset injection circuit  102  may have an input  130  that may receive the signal CE and an output  135  that may present a signal (e.g., CEMECH_OFF). The track counter  104  may have an input  131  that may receive the signal TE and an output  133  that may present a signal (e.g., NUMTRACKS). The sled circuit  105  may have an input  140  that may receive the signal CE, an input  142  that may receive the signal (e.g., NUMTRACKS), and an input  146  that may receive a signal (e.g., TARGETSTEPSTOJUMP). The sled circuit  105  may have an output  115  that may present a signal (e.g., STEPDIRECTION), an output  117  that may present a signal (e.g., STEPTIMERCONTROL), an output  156  that may present a signal (e.g., TRACKFOLLOW_B), an output  158  that may present a signal (e.g., FINESEEK_B), and an output  160  that may present a signal (e.g., ROUGHSEEK_B). The lens controller  108  may have an input  132  that may receive the signal TE, an input  134  that may receive a signal (e.g., TARGETTRACK), an input  136  that may receive the signal NUMTRACKS, and an input  138  that may receive the signal CEMECH_OFF. The lens controller  108  may have an output  150  that may present a signal (e.g., TRACKFOLLOW_A), an output  152  that may present a signal (e.g., FINESEEK_A) and an output  154  that may present a signal (e.g., ROUGHSEEK_A). 
     The multiplexer  114  may have an input  162  that may receive the signal TRACKFOLLOW_A, an input  164  that may receive the signal FINESEEK_A, an input  166  that may receive the signal ROUGHSEEK_A and an input  174  that may receive a signal (e.g., TRACKINGMODE). The multiplexer  114  may have an output  176  that may present any one of the signals TRACKFOLLOW_A, FINESEEK_A or ROUGHSEEK_A on a signal (e.g., INT). 
     The multiplexer  116  may have an input  168  that may receive the signal TRACKFOLLOW_B, an input  170  that may receive the signal FINESEEK_B, and an input  172  that may receive the signal ROUGHSEEK_B. The multiplexer  116  may have an output  178  that may present a signal (e.g., STEPSIZE). The step motor control circuit  118  may have an input  180  that may receive the signal STEPSIZE, an input  181  that may receive a signal (e.g., STEPDIRECTION), and an input  183  that may receive a signal (e.g., STEPTIMERCONTROL). The step motor control circuit  118  may have an output  182  that may present a signal (e.g., INTA) and an output  184  that may present a signal (e.g., INTB). 
     The digital-to-analog controller  120  may have an input  185  that may receive the signal INT and an output  186  that may present a signal (e.g., CTRL). The lens system  126  may have an input  188  that receives the signal CTRL. The digital-to-analog converter  122  may have an input  190  that may receive the signal INTA and an output  194  that may present a signal (e.g., CTRLA). The digital-to-analog converter  124  may have an input  192  that may receive the signal INTB and an output  196  that may present a signal (e.g., CTRLB). The step sled system  128  may have an input  198  that may receive the signal CTRLA and an input  200  that may receive the signal CTRLB. The lens controller  108  may control a lens (not shown) in the lens system  126  based on the mode of the system  100 . The lens controller  108  may control the lens in response to the signal TE or the signal CEMECH_OFF based on the mode of the system  100 . The sled circuit  105  may control a sled motor (not shown) in the step sled system  128  in response to the signals CE, TE or STEPSIZE based on the mode of the system  100 . 
     Referring to  FIGS. 3-4 , a detailed diagram of the system  100  is shown. The lens controller  108  generally comprises a block (or circuit)  204 , a block (or circuit)  206 , and a block (or circuit)  208 . The circuit  204  may be implemented as a track follow estimator. The circuit  206  may be implemented as a fine seek estimator. The circuit  208  may be implemented as a CE controller. The lens controller  108  may control the lens system  126  based on whether the system  100  is in the track follow mode, the fine seek mode, or the rough seek mode. The track follow estimator  204  and the fine seek estimator  206  may be implemented as a track controller. The sled circuit  105  generally comprises a block (or circuit)  106  and a block (or circuit)  110 . The circuit  106  may be implemented as a rough seek step control circuit. The circuit  110  may be implemented as a sled controller. The sled controller  110  generally comprises a block (or circuit)  210 , a block (or circuit)  212  and a block (or circuit)  214 . The circuit  210  may be implemented as a CE tracking monitor. The circuit  212  may be implemented as a feed forward circuit. The circuit  214  may be implemented as a rough seek speed profile circuit. The sled controller  110  may have an input  144  that may receive a signal (e.g., STEPSTOGO). The sled controller  110  may control the step motor control circuit  118  based on whether the system  100  is in the track follow mode, the fine seek mode or the rough seek mode. The step motor control circuit  118  may drive a step motor (not shown) in the step sled system  128 . The step motor may drive the sled housing (not shown) in the step sled system  128 . 
     The signal STEPDIRECTION may provide the direction of travel for a step motor. The signal STEPTIMERCONTROL may provide the time and/or frequency between two adjacent step motor steppings. The signal TARGETSTEPSTOJUMP may provide the number of steppings needed for the step direction of the sled motor. The signal STEPTIMERCONTROLER may provide the stepping frequency of the step motor. When the system  100  is in the tracking mode. the CE tracking monitor  210  may generate the control signals STEPSIZE, STEPDIRECTION, and STEPTIMERCONTROL in response to the signal CE. When the system  100  is in the fine seek mode, the feed forward control circuit  212  may generate the control signals STEPSIZE, STEPDIRECTION, and STEPTIMERCONTROL by monitoring the number of tracks crossed on the signal NUMTRACKS. When the system  100  is in the rough seek mode, the speed profile circuit  214  may generate the control signals STEPSIZE, STEPDIRECTION, and STEPTIMERCONTROL based on the number of tracks left to go on the signal STEPSTOGO. 
     The track follow estimator  204  may position the lens on a center of the track when the system  100  is in a track follow mode. The track counter  104  may (i) count the zero crossings of the signal TE and (ii) provide the number of tracks that the lens has crossed on the signal NUMTRACKS. The fine seek estimator  206  may move the lens a predetermined number of tracks under a specified direction when the system  100  is in the fine seek mode. The CE offset injection circuit  102  generally provides the amount of offset in the signal CE needed to keep the lens centered around the mechanical center. The CE controller  208  may position the lens in the center of a sled housing during sled motion when the system  100  is in the rough seek mode in response to the signal CEMECH_OFF. The CE controller  208  may position the lens to the mechanical center with the signal CEMECH_OFF at any time when the system  100  is in the rough seek mode. The signals TRACKFOLLOW_A, FINESEEK_A, AND ROUGHSEEK_A may be digital control signals presented by the multiplexer  114 . The digital-to-analog converter  120  may convert the any one of the signals TRACKFOLLOW_A, FINESEEK_A, or ROUGHSEEK_A to an analog control signal on the signal CTRL to drive the lens system  126 . 
     The CE tracking monitor  210  may keep the lens at the center of the housing by monitoring the signal CE when the system  100  is in the track follow mode. The feed forward circuit  212  may calculate the corresponding step motor steps (or steppings) needed to move the sled motor to ensure that the lens will stay at the center of the sled housing in the fine seek mode. The feed forward circuit  212  may use the number of tracks that the lens has crossed to calculate the corresponding step motor steps when the system  100  is in the fine seek mode. The rough seek step control  106  generates a rough seek speed profile on the signal STEPSTOGO when the system  100  is in a rough seek mode. The rough seek speed profile  214  may determine each individual steppings the step motor needs to perform based on a certain step size, direction and frequency. The step motor control circuit  118  may receive any one of the signals TRACKFOLLOW_B, FINESEEK_B or ROUGHSEEK_B depending on the mode of the system  100 . The signals TRACKFOLLOW_B, FINESEEK_B AND ROUGHSEEK_B may be digital data presented by the multiplexer  116 . The signal presented by the multiplexer  116  may be determined by the signal TRACKING_MODE which indicates the mode of the system  100 . The step motor controller circuit  118  may present digital data on the signals INTA and INTB. The digital-to-analog converter  122  may convert the digital data on the signal INTA to the analog control signal CTRLA. The digital-to-analog converter  124  may convert the digital data on the signal INT_B to the analog control signal INT_B. The control signals CTRLA and CTRLB may be used to control the step motor of the step sled system  128 . 
     Referring to  FIG. 5 , a detailed diagram of a system  250  in a rough seek mode is shown. The system  250  generally comprises the CE offset injection circuit  102 , the lens controller  108 , the digital-to-analog converter  120 , a photo-diode distribution portion  252 , a CE creation circuit  254 , and a lens housing assembly  251 . The lens housing assembly  251  generally comprises an optical disc  260 , a laser  262 , a sled housing  264 , a lens  266 , and an optical disc  270 . 
     The photo-diode system  252  generally has output signals (or photo-diode signals) A, B, C and D that are generally presented to a number of inputs  253   a - 253   n  of the CE creation circuit  254 . The CE creation circuit  254  may generate the signal CE. When the system  250  is in the rough seek mode, the signal CE may be used instead of the signal TE to control the lens  266 . The CE offset injection circuit  102  may offset the signal CE to position the lens  266  at the mechanical center in the sled housing  264  while the system  250  is in the rough seek mode. The CE offset injection circuit  102  may determine the amount of offset needed to position the lens  266  at the mechanical center during a tracking open-loop calibration. When no control is exerted on the lens  266 , the CE offset injection circuit  102  measures the average value of the signal CE of the lens  266  over a period of time. The average value of the signal CE value of the lens  262  at an equilibrium position (e.g., when both springs are in a mechanical equilibrium state) may be defined as the mechanical center of the lens  266 . The optical center of the lens  266  be defined as the position of the lens  266  when the signal CE is zero (e.g., when the laser  262  shines through the center of the lens  266  the signal CE may be zero). However, when the lens  266  is at the mechanical center, the signal CE may not necessarily be at zero. 
     During the rough seek mode, the lens  266  and the laser beam  262  may not be locked to any one of the particular disc tracks  272   a - 272   n  and the sled housing  264  is repositioned by the sled motor. As noted in connection with  FIG. 5 , the rough seek speed profile circuit  214  and the step motor control circuit  118  may reposition the step sled system  128  when in the rough seek mode. The CE controller  208  may position the lens  266  near the center of the sled housing  264  to prevent the lens  266  from hitting the sled housing  264  when the sled housing accelerates or decelerates. The CE controller  208  may also keep the lens  266  at the mechanical center. At the end of the rough seek mode, the track controller (e.g., or the track follow estimator  204  and the fine seek estimator  206  depending on the mode of the system  100 ) may lock the laser beam  262  to any one of the particular disc tracks  272   a - 272   n  with the signal TE. The lens  266  may be naturally biased in the mechanical center prior to switching control from the signal CE (e.g., via the CE controller  208 ) to the signal TE (e.g., to the track controller). The lens  266  may be positioned at the mechanical center at any time of the rough seek mode. Both approaches minimize the sudden jump in control while locking the laser beam  262  through the lens  266  onto any one of the particular tracks  272   a - 272   n.    
     Referring to  FIG. 6 , a method  300  for the servo rough seek command showing when the CE mechanical offset is injected during a rough seek is shown. The method  300  generally comprises a state (or step)  302 , a decision state (or step)  304 , a state (or step)  306 , a state (or step)  308 , a state (or step)  310 , a decision state (or step)  312 , a state (or step)  314 , a decision state (or step)  316 , and a state (or step)  318 . The state  302  issues a rough seek command. The system  100  may exit out of the track follow mode or the fine seek mode and enter into the rough seek mode when it is necessary to reposition the sled housing  264  and the lens  266  with the sled motion. The decision state  304  may determine whether the sled motor has completed stepping in response to the system  100  entering into the rough seek mode. If the decision state  304  determines that the stepping of the sled motor has not completed, or that the sled housing  264  has not reached the target sled position, the method  300  moves to step  306 . The state  306  may continue to present a step motor control signal to move the sled housing  264 . The state  308  may generate the CE mechanical offset necessary to keep the lens  266  at a mechanical center while the sled housing  264  is being repositioned to the target sled position. The state  310  may center the lens  266  around the mechanical center with the CE controller  208 , then move to the decision state  304 . 
     If the decision state  304  determines that the sled stepping is complete and that the sled housing  264  has reached the target position, the method  300  moves to the decision state  312 . The decision state  312  may determine whether the track crossing is slow enough. If the decision state  312  determines that the track crossing is not slow enough, the method  300  moves to the state  308 . In the decision state  312 , the lens  266  and the laser  262  may not be locked to the disc  270 . The decision state  312  may determine when the movement of the lens  266  relative to the disc  270  is slow enough. If the decision state  312  determines that the track crossing is slow enough (or that the movement of the lens  266  relative to the disc  270  is slow enough), the method  300  moves to the state  314 . The state  314  attempts to lock the lens to the target track with the track controller. The decision state  316  determines if the target track is locked. If the target track is not locked, the method  300  moves to the decision state  312 . If the target track is locked, the method  300  moves to the state  318  and the rough seek mode is complete. 
     Referring to  FIG. 7 , a diagram illustrating the CE with a mechanical offset adjustment is shown.  FIG. 7  illustrates a rough seek CE waveform (or the signal CE with the mechanical offset) with the lens  266  kept around the mechanical center during the sled motion while in the rough seek mode. At the end of the rough seek mode, the track controller is initialized to lock on the tracks  272   a - 272   n  using the signal TE. The CE waveform fails to show a transient when the control of the lens  266  is switched from the CE controller  208  to the track controller. 
     Referring to  FIG. 8 , a diagram illustrating a CE waveform without a mechanical offset adjustment is shown. At the end of the rough seek mode, the track controller may be initialized to lock on a particular one of the tracks  272   a - 272   n  tracks using the tracking error while the lens  266  is naturally biased around the mechanical center of the lens  266 . Such a transient effect as illustrated in  FIG. 8  is due to the difference between the optical and mechanical centers of the lens  266 . The difference between the optical and mechanical centers of the lens  266  increases the difficulty and the lock time needed to lock the laser beam  262  on a particular one of the tracks  272   a - 272   n . The difference between the optical and mechanical centers of the lens  266  may reduce reliability when the system  100  is in the rough seek mode. 
     The present invention may inject a CE mechanical offset during the entire course of a rough seek to reduce the transient effect while locking the laser  262  to any one of the particular number of tracks  272   a - 272   n . The present invention may minimize the change in the signal CE control output between the CE controller  208  and the track controller. The present invention may (i) allow the laser  120  to lock on any one of the particular number of tracks  272   a - 272   n  reliably and (ii) improve the seek performance of the optical drive. The present invention may be applied to any application that involves the switching of control input signals between different modes to minimize transient effect. 
     The function performed by the flow diagram of  FIG. 6  may be implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). 
     The present invention may also be implemented by the preparation of ASICs, FPGAs, or by interconnecting an appropriate network of conventional component circuits, as is described herein, modifications of which will be readily apparent to those skilled in the art(s). 
     The present invention thus may also include a computer product which may be a storage medium including instructions which can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disk, optical disk, CD-ROM, magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, Flash memory, magnetic or optical cards, or any type of media suitable for storing information. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.