Patent Publication Number: US-2007121439-A1

Title: Optical disc drive and method for controlling the position of a lens

Description:
This invention relates to an optical disc drive comprising:  
      a lens for focusing and positioning a radiation beam on an optical disc,  
      wherein the radiation beam is reflected by the optical disc;  
      means for causing the optical disc to rotate with a disc rotational frequency, and  
      detection means for receiving the reflected radiation beam and generating a radial error signal indicating a position of the lens relative to the optical disc,  
      lens position motor for moving the lens,  
      a servo control circuit having a tracking mode for controlling the position of the lens in response to the radial error signal, comprising a first motor control circuit for controlling the lens position motor.  
      The invention also relates to a method for controlling the position of a lens in an optical disc drive, the method comprising the steps of:  
      causing an optical disc to rotate with a disc rotational frequency;  
      controlling the position of the lens with a lens position motor.  
      It is typical in an optical disc drive to provide an optical head for recording and reading information in the track of a rotating optical disc, with a lens actuator provided on the optical head for displacing a light spot in a direction traversing the track of the optical disc. Such an arrangement is described in U.S. Pat. No. 6,163,513. Such an optical disc drive typically consists of a radial lens position motor and an axial lens position motor for controlling the lens that positions the laser spot in radial and axial directions. These motors are positioned on an unit that is positioned on a positioner (or “sledge”) that can be moved by a linear motor, or by a rotating motor and a transmission. In order to control the spot in a radial sense, control loops are required.  
      There is a need for short response times in the control loops that control the laser spot. For example, before the read-out of a disc, initialization has to be done once with the disc. Initialization determines a tracking offset value used in a track control loop. Also a radial lens position error may be determined during initialization to be used later during a rough search.  
      At initialization, there is a need to achieve tracking as quickly as possible. The time to achievement of tracking is determined in part by the bandwidth of the control loop that controls the offset of the radial error signal. The optical disc is typically not perfectly centered and eccentricity of the disc gives rise to frequency modulated signals in this control loop. These are typically filtered out using a low pass filter, but this in turn reduces the bandwidth of the control loop and, therefore, its responsiveness.  
      Furthermore, during non-tracking situations, the sledge can be controlled to perform a rough search. During a rough search the sledge causes the lens to jump radially over the disc while the actuator is controlled to maintain a central position in respect to the sledge. Although the number of tracks passed during such a jump can be counted in order to evaluated more precisely the radial position, typically this kind of jump is effectuated without counting of the tracks. The jump is therefore approximate (rough) and needs a correction jump after the position is evaluated by data reading. Again, the bandwidth of the control loop that controls the sledge limits the time to completion of a rough search.  
      There is a need to improve the response time of control circuits for optical disc drives.  
      According to a first aspect of the present invention, the optical disc drive control circuit is provided with means for applying an alternating signal to the lens position motor.  
      The method according to the invention further comprises a step of applying an alternating signal to the lens position motor.  
      By applying the alternating signal to the lens position motor the control of the lens position motor is modulated. Consequently, the control loop that controls the lens position motor can have higher bandwidth and therefore greater responsiveness. For example, where the first motor control circuit has a low-pass filter with a cut-off frequency, this cut-off frequency can be selected relative to the frequency of the alternating signal.  
      The alternating signal preferably has a frequency higher than the disc rotational frequency, and an amplitude sufficient to cause the lens to shake with an amplitude of at least about 0.8 to 1.0 times the track pitch, for instance 0.88 times. By employing a radial offset control feedback loop with a time constant that is low in relation to the rotational frequency of the disc, faster offset determination is achieved with a correspondingly shorter start-up time.  
      In an embodiment of the optical disc drive the optical disc drive further comprises  
      a sledge for moving the lens position motor and the lens in radial direction relative to the optical disc, and  
      a second motor for control of the sledge,  
      wherein the servo control circuit comprises a second motor control circuit for controlling the second motor.  
      The first motor control circuit preferably has means for detecting the position of the lens relative to a sledge, and providing a lens position feedback signal which is combined with the alternating signal to give a modulated signal to the lens position motor.  
      In a favorable embodiment of the optical disc drive according to the invention the lens position signal is fed to a low-pass filter with a cut-off frequency less than the frequency of the alternating signal and an output of the low-pass filter is fed to the lens position controller.  
      The low pass filter can have a higher cut-off frequency allowing higher control bandwidth because of the raised frequency contents of the position signal due to the alternating signal.  
      In a further embodiment of the optical disc drive according to the invention the servo control circuit comprises a radial offset control feedback loop. The radial offset control feedback loop can be implemented by either measuring a lens position offset in the case where a lens position signal is available or measuring a radial offset in the radial error signal itself.  
      In an embodiment of the invention the radial offset control feedback loop is able to operate in a first mode and in a second mode, wherein in the first mode the lens is moved in a neutral position and a lens position offset in the lens position signal is measured and in the second mode the lens position signal is corrected with the measured lens position offset.  
      In an other embodiment of the invention a radial offset of the radial error signal is measured and subtracted from the radial error signal.  
      By applying the alternating signal to the radial to the lens position motor during initialization of the radial offset feedback loop, the initialization process can be performed more quickly.  
    
    
      Preferred embodiments of the invention are now described, by way of example only, with reference to the drawings, in which  
       FIG. 1  shows an optical disc reader in accordance with an aspect of the present invention, incorporating a control circuit also in accordance with the present invention,  
       FIG. 2  shows an embodiment of a control circuit in accordance with the present invention,  
       FIG. 3  shows optional details of the control circuit of  FIG. 2 ,  
       FIG. 4  shows a plot of the radial error signal, and  
       FIG. 5  is a continuation of  FIG. 4  showing the radial offset during initializing. 
    
    
      Referring to  FIG. 1 , an optical disc drive of an optical disc reader is shown for reading an optical disc. An optical disc  10  is shown side-on, mounted on a spindle  11  of a disc motor  12 . Associated with the disc motor  12  is a disc rotational speed controller  13 . Beneath the disc  10  is a lens  20  that controls a beam of a laser (not shown). The lens  20  is mounted on a sledge  22 , driven by a sledge motor  25 . A voice coil motor (VCM)  24  controls the position of the lens  20  relative to the sledge  22 . A control circuit  30  controls the motor  12 , the sledge motor of the sledge  22  and the VCM  24 . The control circuit  30  also receives feedback signals from these respective elements.  
      In operation, the motor  12  causes the disc  10  to rotate at a predetermined rotational frequency. The motor control circuit  13  controls the steady rotation of the disc  10 . The lens  20  focuses the laser onto a track on the underside of the disc  10 . The VCM  24  controls the position of the lens  22  relative to the track in the direction of arrow A. Sledge  22  moves the lens  20  and its associated VCM  24  radially in relation to the disc  10  along the direction of the arrow B.  
      Referring to  FIG. 2 , the control circuit  30  is shown in dotted outline and is shown connected to the VCM  24  and the sledge motor  25 . To the right of these motors are illustrated an actuator  40 , a sledge motor transmission element  41  and a sledge  42 . These are not physical elements, but represent the control response functions of the VCM  24 , the sledge motor  25  and the sledge  22 , respectively. Combining function  43  is shown connected to elements  40  and  42 , illustrating the combined response of those elements. The combined response from summer  43  represents the performance of the disc  10 , which is fed back to a pre-processor  50  of the control circuit  30 .  
      Within the control circuit  30 , there are two control loops, a first for controlling the VCM comprises the pre-processor  50 , a radial controller  52 , a mixer element  54  receiving a signal from a signal injector  56  and a first gain element  58 . The second control loop for controlling the sledge comprises the pre-processor  50 , an optional radial offset control loop  60 , the radial controller  52  and a second gain element  62 . The radial offset control loop  60  can also be implemented in the first control loop. When the radial offset control loop  60  is implemented in the second control loop the radial offset of the radial error signal  55  is measured, and subsequently subtracted from the radial error signal. When the radial offset control loop  60  is implemented in the first control loop the lens position offset is measured, and subsequently subtracted from the lens position error signal  53 .  
      In operation, the pre-processor  50  receives a signal  51  from an optical detector (not shown) associated with the disc  10  and its disc drive. The pre-processor  50  creates a lens position error signal  53  and a radial error signal  55 . These signals are passed to the radial controller  52 . The radial controller  52  has an actuator control output  57  passing an actuator control signal to the mixer  54  and a sledge control output  59  passing a sledge control signal to the sledge driver  62 . As will be described in greater detail, a periodic signal is injected by signal injector  56  into mixer  54 . In operation, the output of the sledge driver  62  drives the sledge motor  25  and the output of the actuator driver  58  drives the VCM  24 . The resulting movement of these motors, as represented by elements  40  to  43 , results in a change to the signal  51  read by the optical detector and, accordingly, the control loop is closed.  
      The control loops are disturbed by track cross modulation when the radial control is not tracking (while focused). This track cross modulation signal finds its origin in the radial error signal  55 . This is a periodic signal. When there is no tracking, the laser beam crosses tracks, depending on the eccentricity of the disc  10 . This results in a frequency modulated sinusoidal signal. The number of sine waves passed per disc rotation depends on the eccentricity, and the rate of modulation depends on the disc speed.  
      The VCM  24  moves the lens  20  that controls the laser beam position on the optical disc  10 . The sledge  22 , driven by motor  25  positions the VCM  24  and its lens in such a way that the lens is in its middle position. In order to jump to another track on the disc, the lens might have to make a large excursion. In that case, the sledge  22  moves the VCM  24  to another position along arrow B in  FIG. 1 . The lens  20  must remain in a middle position during such movement and must resist against the acceleration force exerted by the sledge. The lens position error signal  53  indicates the relative position of the lens with respect to the sledge. This is derived from the optical detector (not shown). The lens position feedback loop keeps the lens in the middle position. The lens position error signal  53  has track cross modulation which disturbs the lens position control. This track cross modulation component is reduced by a low pass filter  65  in or associated with the radial controller  52 .  
      The cut off frequency of the low pass filter  65  in or associated with the radial controller  52  has to be low in order to have sufficient reduction of the track cross modulation. There is a relationship between maximum control bandwidth of the position control and the filter cut off frequency, because of the stability of the control. A low cut off frequency of the low pass filter of the radial controller  52  gives the control loop a low control bandwidth and, accordingly, a poor reduction of disturbances that arise from the moving sledge.  
      A periodic signal is generated in signal generator  56  and applied to the actuator control signal  57 , such that when amplified by actuator driver  58 , its effect on the VCM gives an amplitude of movement of about 0.88 times the track pitch. The frequency of the signal is higher, and preferably substantially higher, than the disc revolution frequency. The preferred frequency of the alternating signal is 2 kHz. This is suitable for disc speeds from 3 to 160 rotations per second. In this way, the frequency modulation in the lens position detector signal becomes high frequency. A cut-off frequency close to the alternating signal frequency, but below this frequency, can be chosen for the filter  65 . The preferred cut-off frequency of the low pass filter in the lens (actuator) position loop is about 1 kHz. This is a higher cut-off frequency than has previously been possible and, because of this, the controlled bandwidth of the lens position loop can be higher. This results in better tracking performance of the lens with respect to the sledge.  
      Referring to  FIG. 3 , the radial controller  52  is shown in phantom outline for the purposes of illustrating detail thereof. Connected between the radial error signal  55  from the pre-processor  50  and the radial controller  52  is the radial offset controller  60  (which has been described as optional in relation to  FIG. 2 ).  
      The radial controller  52  comprises a lens position controller  101  coupled to the input to the lens position error signal  53 . A track controller  102  is provided coupled to the input of the radial error signal  55  via a difference element  103 . Connected between the lens position controller  101  and the lens position control signal output  57  are a first multiplexer (switch)  110  and a second multiplexer (switch)  111 . The first multiplexer  110  receives the lens position control signal from the lens position controller  101  at its upper (negative) input and receives a tracking control signal  104  from the track controller  102  on its lower input. It also has an switch input  112 , which causes the multiplexer to pass its upper input to its output when high and its lower input to its output when low. The second multiplexer  111  receives the output of the first multiplexer  110  at its upper input and has its lower input grounded. The second multiplexer  111  has a switch input  113  also causing it to pass its upper input to its output when high and its lower input to its output when low.  
      A supervisor micro-controller  115  is provided with a track control output  116  connected to the switch input  112  of the first multiplexer  110  and an initialize output  117  connected to the switch input  113  of the second multiplexer  111 . The supervisor micro-controller  115  has a communication channel  118  for receiving control commands from a user.  
      Connected between the track controller  102  and the sledge control signal output  59  is a sledge controller  120 , the input of which is a radial control input and the output of which is a sledge control output.  
      The track controller  102  is effective when the laser spot has to read out data on the disc. The track controller  102  receives an input signal (via pre-processor  50 ) from a photo diode detector (not shown) that detects the tracking error between the laser spot and the disc track to be read. A tracking offset value, determined in an initialization controller (in micro-controller  115 ), is subtracted from the tracking error signal. (Initialization is described in greater detail below.)  
      The offset reduced tracking error signal (output from difference element  103 ) is the input for the lens track controller  102  that controls the radial position of the lens. The task of the this controller is to reduce the tracking error to an acceptable limit. The track controller  102  provides control to the positioner (sledge  42 ) under control of a positioner controller in the micro-processor  115 , so that the track control signal  104  is passed to the radial lens position motor (VCM  24 ). The task of the positioner controller is to keep the positioner (sledge  42 ) in a neutral position in respect to the lens, which is realized by keeping the control signal  57  of the radial lens position motor (VCM  24 ) within predefined limits using feedback control. The measure by which the controller reacts to an error at its input depends on its gain. Higher gain and higher control bandwidth results in faster reaction. The controller gain (together with the characteristics of the motors) limits the error. The controller gain is not constant over the frequency band but has a frequency compensator, as is known in the art, to keep the system stable.  
      Referring now to the radial offset controller  60 , this comprises a gain element  130  having a third gain value (an offset learning gain) k 3 , connected to the upper input of a third multiplexer (switch)  132 . The third multiplexer  132  has its lower input grounded. The third multiplexer  132  has a switch input  134  causing it to pass its upper input to its output when high and its lower input to its output when low. The output of the third comparator  132  is connected via a summer  135  to a delay element  136  having a delay 1/z. At an output of the delay element  136 , there is a feedback loop  138  feeding back to the summer  135 . Also at the output of the delay element  136  is a feedback loop  140  feeding back to a negative input of the difference element  103 .  
      The initialize output of the supervisor micro-controller  115  is a logical signal which, when true, causes the radial error offset control to switch on by causing the third multiplexer  132  to pass its upper input to its output, thereby closing the radial offset control loop. It also ceases control of the VCM (and hence the actuator) by causing the second multiplexer  111  to switch its grounded lower input to its output. This signal is temporarily true when a disc is started up.  
      When a disc is started up, initialization needs to be carried out. For this purpose an initialization controller within micro-controller  115  is provided. This initialization controller is in action during some time before the first reading of a disc in order to determine the tracking offset value used in the track control loop. Also a radial lens position error is determined which is used in the lens position controller  101  during a rough search.  
      The initialization loop contains the same parts as mentioned above, but the radial lens position motor (VCM  24 ) is not controlled. The control loop reduces the mean value of the radial error by subtracting an offset signal. This tracking offset signal is kept in a register in microcontroller  115  as the tracking offset value and is available in the track controller  102 . In the same way a control loop may reduce the mean value of the lens position error signal  53 . The radial lens position motor is not controlled whilst the axial lens position motor keeps the lens in focus. In this situation the tracking error signal ( FIG. 4 , described below) is a frequency modulated signal. Each time when a track passes under the laser spot a wave form indicating the tracking error is detected and the frequency of wave forms that passes depends on the number of tracks per second that pass under the laser spot. The initialization loop is perturbated by this frequency modulated component of the tracking error and this limits the speed at which this loop can find an acceptable tracking offset value.  
      One aspect of the invention concerns shaking the lens in radial sense in this loop by applying a sine signal to the radial lens position motor (VCM  24 ). In this way the spot will always pass the tracks with a relatively high frequency. Therefore offset iterations can be done quicker and the drive can proceed by tracking the disc earlier.  
      The initialize output of the supervisor micro-controller  115  may also be true at the entrance of a new zone on the disc. The controller may divide the disc into several zones for this purpose.  
      After an initialization period, the initialize signal becomes false causing the offset of the radial error signal to be removed, and the lens position control begins.  
      After initialization, when a rough search is required and the sledge is required to move from one position to another, the micro-controller  115  provides a “false” signal on its track control output  116 . This causes the actuator to be positioned in its neutral position. During rough searching, the lens controller  101  (rather than controller  102 ) controls the VCM. The sledge controller  120  performs a rough search upon receipt of a command  119  from the micro-controller  115 . During the rough search, the sledge is controlled independently from other aspects of the radial controller  52 . In this situation the VCM (the actuator) is switched to the lens position controller  101 .  
      The rough search loop is effective when the laser spot has to jump to another radial position on the disc that is not sufficiently near that the jump can be effectuated by the lens alone. In this case the positioner (actuator  40 ) is displaced by the positioner controller (lens position controller  101 ).  
      A rough search controller is provided (not explicitly shown, but embodied in the micro-controller  115  and the sledge controller  120 ) which receives a signal (via pre-processor  50 ) from a photo diode detector (not shown) that detects the lens position in respect to the positioner (the sledge). This is the lens position error signal  53 . The rough search controller also uses lens position controller  101  to reduce this lens position error signal to an acceptable level.  
      A lens position error offset value, determined in the initialization loop (described above), is subtracted from the lens position error signal  53 . The lens position error signal  53  can have a relatively high cross talk from the radial error. Therefore the same frequency modulated signal can perturbate this lens position control loop. To reduce this perturbation the lens position loop contains a low pass filter  65 . This low pass filter limits the bandwidth of the loop and therefore the responsiveness of the loop.  
      Another aspect of the invention concerns shaking the lens in a radial sense in this loop by adding a sine signal from signal generator  56  to the radial lens position motor. In this way the spot will always pass the tracks with a relatively high frequency. Therefore the cut-off frequency of the low pass filter  65  can be increased and by that also the band width of the control. This results in better responsiveness and reduces the lens lag during fast positioner displacements.  
      During tracking operation the actuator is controlled in respect to the track of the disc and the sledge is controlled to stay in a neutral position in respect to the lens (using the lens control signal  104 ), but during rough searching the actuator is controlled (by lens position controller  101 ) to maintain a central position in respect to the sledge. During tracking the lens is master, while during rough search the sledge is master.  
      The task of the controller  115  is to configure the controllers by putting them in right operation modes. The sledge controller  120  is able to control the sledge during a rough search. To do this it receives a command from micro-controller  115 . In this situation, shaking of the actuator control signal by means of signal generator  56  causes the low frequency components of the lens position error signal  53  to be reduced and allows the control bandwidth to be increased. This in turn reduces the time taken to perform a rough search.  
      When the laser beam is required to follow a track, the micro-controller  115  provides a “true” signal on output  116 , whereupon the first multiplexer  110  causes the lens position control signal from lens position controller  101  to be passed to the second multiplexer  111 , which is now switched to receive its upper input, thereby passing it to the VCM  24  (and the actuator  40 ).  
      The radial offset control  60  is used during tracking only, and the offset itself is evaluated during the initialization operation.  
      Thus, the lens  20  is controlled in its radial position by feedback control using the radial error signal  55  generated in the photodetector. When radial control is switched off, offset is removed from the signal by offset feedback control using the radial offset controller  60  and the radial error signal is now a frequency modulated signal, depending on the eccentricity of the disc  10 .  
      When the eccentricity is low and the speed is low, the frequency of the track crossing signal is low and the offset controller tends to follow the slow signal trend. The bandwidth of this control needs to be low in order to determine the offset with sufficient accuracy. By applying a periodic signal from signal generator  56  to the VCM  26  with an amplitude of about 0.8 to 1.0 times (and preferably about 0.88 times) the track pitch and a frequency significantly or substantially higher than the disc revolution frequency, the modulation in the lens position error signal  53  becomes high frequency. Accordingly, the time constant of delay element  136  can be chosen at a lower value than would otherwise be possible. A preferred value for this time constant is about 25 ms. This results in a faster offset determination and shorter start-up time for the optical disc reader.  
      Referring now to  FIGS. 4 and 5 , the improvement provided by the features of the invention is illustrated by showing the radial error and the radial offset for an optical disc drive during initialization, with and without application of the alternating signal to the VCM that moves the lens. The figures show the response of the system at different times following start-up. Curve  400  of  FIG. 5  represents the lens focus without radial shaking of the lens. As can be seen, there is a significant departure from focus shortly after initialization and the focus settles down only after about 0.03 seconds. By contrast, curve  401  shows the focus offset using the periodic control signal from the signal generator  56  and here it is seen that there is no significant loss of focus from the very start of initialization.  FIG. 4  shows the corresponding radial error signals  55 , ranging from a maximum of 1 to a minimum of −1 and it can be seen that the radial error signal has high frequency below about 0.015 seconds and the frequency of the error drops at around about 0.02 seconds, rising again after about 0.025 seconds.  
      Accordingly, a control circuit for an optical disc reader, and an optical disc reader having such a control circuit, have been described in which the lens radial actuator, for example a VCM, is modulated in a radial direction using a alternating signal while it is not tracking. This increases the minimal track crossing frequency. By increasing the minimal track crossing frequency, track cross modulation components, particularly in the lens position control loop, can be decreased using low pass filtering at an increased cut off frequency. This reduces the startup time and increases control accuracy in control of the lens position.  
      Further modifications of the invention can be made by one of ordinary skill in the art within the scope of the invention and further advantages of the invention will be apparent. A single processor or unit may fulfill the functions of several means recited in the claims. A single means recited may be fulfilled by several independent means. Where an element or step is described as comprising one or more elements or steps, the term “comprising” does not exclude other elements or steps. The indefinite article “a” or “an” does not exclude a plurality.