Patent Publication Number: US-2012026849-A1

Title: Optical disc device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of PCT International Application PCT/JP2010/002549 filed on Apr. 7, 2010, which claims priority to Japanese Patent Application No. 2009-099022 filed on Apr. 15, 2009. The disclosures of these applications including the specifications, the drawings, and the claims are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to an optical disc device, and more particularly to focus control performed by the optical disc device. 
     In recent years, recording media such as compact discs (CDs) and digital versatile discs (DVDs) are being used for car-mounted disc drives, video recorders, camcorders, etc. Optical disc devices using such recording media may fail to continue its focus control (focus servo) when having undergone an impact from outside. Therefore, techniques of improving the trackability of focus control and, even if defocus occurs, avoiding collision between an objective lens and an optical disc are in high demand. 
     In optical disc devices, a coil for focusing in an optical pickup is driven in accordance with a drive signal, to perform focus control in which the position of the optical pickup is controlled so that a light beam is focused on an information-recording layer of an optical disc. The drive signal is generated from a focus error signal obtained based on light reflected from the optical disc. The focus error signal indicates the distance between the focal point of the light beam and the information-recording layer of the optical disc. When defocus occurs due to an impact from outside, etc., the amount of light reflected from the optical disc decreases. When the reflected light amount falls, and continues to be, below a threshold, it is determined that defocus has been detected. Once this is determined, an avoidance pulse is output as the drive signal to increase the distance between the objective lens of the optical pickup and the optical disc, thereby controlling to avoid collision between the objective lens and the optical disc. An example of such an optical disc device is disclosed in Japanese Patent Publication No. H11-185259 and Japanese Patent Publication No. 2008-210489. 
     SUMMARY 
     The focus error signal does not become completely zero even when focusing completely fails. Since the loop for focus control remains closed until defocus is detected, the drive signal gradually increases by a low-frequency compensation filter, whereby the objective lens becomes closer and closer to the optical disc. If the objective lens is excessively close to the optical disc, it may collide with the optical disc. To avoid this, the time taken until it is determined that defocus has been detected may just be shortened. However, this has a demerit that, since the frequency of detection of defocus increases, the focus servo tends to fail. 
     It is an objective of the present disclosure to provide an optical disc device capable of keeping an objective lens from colliding with an optical disc. 
     The optical disc device of an embodiment of the present disclosure includes: a focus error signal generation section configured to generate a focus error signal based on reflected light from an optical disc; and a focus control section configured to generate a drive signal for focus control of an optical pickup from the focus error signal. The focus control section includes a first filter adjustment portion configured to activate a first filter adjustment signal when the absolute value of the focus error signal is equal to or more than a first error threshold, a proportional term computing unit configured to multiply the focus error signal by a predetermined value, an integral term computing unit configured to multiply the focus error signal by an integral gain and integrate the multiplied result of the integral term computing unit, a differential term computing unit configured to differentiate the focus error signal, an adder configured to add up the computed results of the proportional term computing unit, the integral term computing unit, and the differential term computing unit and output the added result as the drive signal, and an acceleration measurement section configured to measure the acceleration of an objective lens of the optical pickup in a direction from the objective lens toward the optical disc. The integral term computing unit decreases the integral gain when the first filter adjustment signal is active. The first filter adjustment portion decreases the first error threshold when the acceleration measured by the acceleration measurement section is equal to or more than an acceleration threshold. 
     With the above configuration, in which the integral gain is decreased when the absolute value of the focus error signal is equal to or more than the first error threshold, an increase in the value of the integral term can be reduced. Therefore, the objective lens can be kept from getting too close to the optical disc, permitting avoidance of collision between the objective lens and the optical disc. 
     According to the embodiment of the present disclosure, since the increase in the value of the integral term is reduced at the time of defocus, the objective lens can be kept from getting too close to the optical disc. Thus, collision between the objective lens and the optical disc can be avoided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example configuration of an optical disc device of an embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing an example configuration of a focus control section in  FIG. 1 . 
         FIG. 3  is a block diagram showing an example configuration of a filter adjustment portion in  FIG. 2 . 
         FIG. 4  is a flowchart showing a flow of processing by the optical disc device of  FIG. 1 . 
         FIGS. 5A-5F  are charts respectively illustrating the distance between an objective lens and an optical disc ( 5 A), a focus error signal ( 5 B), a reflected light amount ( 5 C), a drive signal ( 5 D), a detection signal ( 5 E), and a first filter adjustment signal ( 5 F). 
         FIG. 6  is a block diagram showing a variation of the focus control section in  FIG. 1 . 
         FIG. 7  is a block diagram showing an example configuration of a filter adjustment portion in  FIG. 6 . 
         FIG. 8  is a flowchart showing a flow of processing by the optical disc device having the focus control section of  FIG. 6 . 
         FIGS. 9A-9G  are charts respectively illustrating the distance between an objective lens and an optical disc ( 9 A), a focus error signal ( 9 B), a reflected light amount ( 9 C), a drive signal ( 9 D), a detection signal ( 9 E), a first filter adjustment signal ( 9 F), and a second filter adjustment signal ( 9 G). 
         FIG. 10  is a block diagram showing a variation of the filter adjustment portion of  FIG. 3 . 
         FIG. 11  is a block diagram showing a variation of the filter adjustment portion of  FIG. 7 . 
         FIG. 12  is a flowchart showing a flow of processing by the optical disc device having the filter adjustment portions of  FIGS. 10 and 11 . 
         FIG. 13  is a block diagram showing a configuration of a variation of the optical disc device of  FIG. 1 . 
         FIG. 14  is a block diagram showing an example configuration of a focus control section in  FIG. 13 . 
         FIG. 15  is a block diagram showing an example configuration of a filter adjustment portion in  FIG. 14 . 
         FIG. 16  is a block diagram showing an example configuration of another filter adjustment portion in  FIG. 14 . 
         FIG. 17  is a flowchart showing a flow of processing related to setting of an error threshold by the optical disc device of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described hereinafter with reference to the accompanying drawings. Note that, throughout the drawings, components denoted by reference numerals same in the last two digits correspond with each other: they are the same or similar components. 
     The function blocks as defined herein can be implemented typically by hardware. For example, each of the function blocks can be formed on a semiconductor substrate as part of an integrated circuit (IC). As used herein, the IC includes a large-scale integrated circuit (LSI), an application-specific integrated circuit (ASIC), a gate array, a field programmable gate array (FPGA), etc. Alternatively, part or the entire of each of the function blocks can be implemented by software. For example, such a function block can be implemented by a program executed on a processor. In other words, the function blocks to be described herein may be implemented by hardware, by software, or by an arbitrary combination of hardware and software. 
       FIG. 1  is a block diagram showing an example configuration of an optical disc device of an embodiment of the present disclosure. The optical disc device of  FIG. 1  includes a disc motor  102  that rotates an optical disc  101  mounted, an optical pickup  104 , a detector  106 , a drive signal generation block  108 , and a drive section  110 . The drive signal generation block  108  includes preamplifiers  112 , a focus error signal generation section  114 , a focus control section  116 , a switch  118 , a reflected light amount detection section  122 , a defocus detection section  124 , and an avoidance pulse generation section  126 . 
     The optical pickup  104  in  FIG. 1  directs focused laser light onto the optical disc  101 . The detector  106  receives light reflected from the optical disc  101  via the optical pickup  104 . The detector  106  has four light receiving elements, which individually generate electric signals corresponding to the received light amounts and output the signals to the preamplifiers  112 . The preamplifiers  112  amplify the input signals and output the results to the focus error signal generation section  114  and the reflected light amount detection section  122 . 
     The focus error signal generation section  114  generates a focus error signal FE indicating the distance between the focal point of the light beam and the information-recording surface of the optical disc  101  based on the signals amplified by the preamplifiers  112 , and outputs the signal to the focus control section  116 . The focus control section  116  performs automatic gain control (AGC), phase compensation, and low-frequency compensation for the focus error signal FE, and outputs the resultant drive signal DR to the switch  118 . 
     The switch  118  normally selects the drive signal DR and outputs the signal to the drive section  110  as a drive signal DS for focus control of the optical pickup  104 . The control section  110  outputs the drive signal DS after amplifying its current, to drive the coil for focusing in the optical pickup  104 , thereby performing focus control in which the position of the optical pickup  104  is controlled so that the light beam is focused on the information-recording layer of the optical disc. 
     The reflected light amount detection section  122  generates a reflected light amount signal corresponding to the amount of the reflected light based on the signals amplified by the preamplifiers  112 , and outputs the signal to the defocus detection section  124 . The defocus detection section  124  detects defocus that may occur due to an impact from outside. In other words, the defocus detection section  124  compares the reflected light amount signal with a predetermined detection level, and, if the time period during which the reflected light amount signal is smaller than the predetermined detection level (defocus detection period) reaches a predetermined time length, determines that defocus has been detected. Once determining the detection of defocus, the defocus detection section  124  outputs a signal indicating the defocus to the avoidance pulse generation section  126  and the switch  118 . 
     Having received the signal indicating the defocus, the avoidance pulse generation section  126  generates an avoidance pulse AP. The switch  118  selects the avoidance pulse AP and outputs the signal to the drive section  110  as the drive signal DS. The avoidance pulse AP is a square wave pulse having a voltage with which the objective lens in the optical pickup  104  is forced away from the optical disc  101 . 
     Thus, having received the signal indicating the defocus, the avoidance pulse generation section  126  outputs the avoidance pulse AP to the drive section  110 . With this signal, the objective lens moves in the direction away from the optical disc  101 . In this way, the objective lens can be avoided from colliding with the optical disc  101 . 
       FIG. 2  is a block diagram showing an example configuration of the focus control section  116  in  FIG. 1 . The focus control section  116  includes a first filter adjustment portion  131  and a computation portion  134 . The computation portion  134  includes an integral term computing unit  136 , a proportional term computing unit  137 , a differential term computing unit  138 , and an adder  139 . 
     The integral term computing unit  136  integrates the focus error signal FE, to increase the gain in the low-frequency region of the focus error signal FE. More specifically, the integral term computing unit  136  multiplies the focus error signal FE by an integral gain and adds up the multiplied results at predetermined intervals, to obtain the integrated result. The proportional term computing unit  137  multiplies the focus error signal by a predetermined value: i.e., the proportional term computing unit  137  determines the degree of amplification of the focus error signal FE. The differential term computing unit  138  differentiates the focus error signal FE to perform phase compensation of the focus error signal FE. 
       FIG. 3  is a block diagram showing an example configuration of the filter adjustment portion  131  in  FIG. 2 . The filter adjustment portion  131  includes an error detector  142  and a signal adjuster  144 .  FIG. 4  is a flowchart showing a flow of processing by the optical disc device of  FIG. 1 .  FIGS. 5A-5F  are charts respectively illustrating the distance between the objective lens and the optical disc ( 5 A), the focus error signal FE ( 5 B), the reflected light amount ( 5 C), the drive signal DS ( 5 D), a detection signal FEdet 1  ( 5 E), and a first filter adjustment signal FA 1  ( 5 F). The defocus detection period shown in  FIG. 5C  refers to, for example, the time period from the time at which the reflected light amount falls below a predetermined value through the time for which the reflected light amount remains below the predetermined value until the time at which the defocus detection section  124  determines that defocus has been detected. In  FIG. 5D , the avoidance pulse AP generated by the avoidance pulse generation section  126  after the detection of defocus is shown. 
     The operation of the optical disc device of  FIG. 1  will be described hereinafter with reference to  FIGS. 1-5E . The focus error signal generation section  114  in  FIG. 1  generates the focus error signal FE based on the signals amplified by the preamplifiers  112 , and outputs the signal FE to the focus control section  116  (S 102 ). 
     The error detector  142  in  FIG. 3  determines whether or not the absolute value of the focus error signal FE is equal to or more than an error threshold FEth 1  (S 110 ). The processing proceeds to S 112  if the error detector  142  determines that the absolute value of the focus error signal FE is equal to or more than the error threshold FEth 1  (i.e., if an error is detected), or proceeds to S 122  if the error detector  142  determines that the former is less than the latter. The error threshold FEth 1  may be a value set previously by a controller (not shown) outside the focus control section  116  or may be a fixed value. 
     In S 112 , the error detector  142  sets the detection signal FEdet 1  at “1” and outputs the signal. In S 122 , the error detector  142  sets the detection signal FEdet 1  at “0” and outputs the signal. In S 114 , with the detection signal FEdet 1 =1, the signal adjuster  144  activates a first filter adjustment signal FA 1  (sets the signal FA 1  at “1”) and outputs the signal to the integral term computing unit  136 . In S 116 , with the first filter adjustment signal FA 1  being active, the integral term computing unit  136  decreases the integral gain from its initial value, for example, and sets the gain at the resultant decreased value. 
     In S 124 , the signal adjuster  144  detects a falling edge of the detection signal FEdet 1 . The processing proceeds to S 126  if a falling edge has been detected, or proceeds to S 128  if no falling edge has been detected. The signal adjuster  144  has a down counter. In S 126 , the down counter is set at a fixed value, to start a first extension for the defocus detection period. Assume herein that the value set for the down counter is a fixed value larger than the value corresponding to the defocus detection period, for example. The down counter counts down in accordance with a clock. 
     In S 128 , the signal adjuster  144  determines whether the first extension has finished or not, i.e., whether the value of the down counter is zero or not. The processing proceeds to S 114  if the value of the down counter is not zero. At this time, the down counter decrements the value. The processing proceeds to S 130  if the value of the down counter is zero. 
     In S 130 , the signal adjuster  144  inactivates the first filter adjustment signal FA 1  and outputs the signal to the integral term computing unit  136 . In S 132 , with the first filter adjustment signal FA 1  being inactive, the integral term computing unit  136  sets the integral gain at the previous value before the decrease in S 116  (e.g., the initial value). 
     In S 134 , the integral term computing unit  136  integrates the focus error signal FE using the set integral gain and outputs the integrated result to the adder  139 . In S 184 , the proportional term computing unit  137  multiplies the focus error signal FE by a predetermined value and outputs the multiplied result to the adder  139 . Also, the differential term computing unit  138  differentiates the focus error signal FE and outputs the differentiated result to the adder  139 . In  5136 , the adder  139  adds up the computed results of the integral term computing unit  136 , the proportional term computing unit  137 , and the differential term computing unit  138 . In S 138 , the adder  139  outputs the added result as the drive signal DR. 
     When the first filter adjustment signal FA 1  is activated as shown in  FIG. 5F , the integral gain of the integral term computing unit  136  decreases. This reduces increase in the drive signal DS of  FIG. 5D , and thus reduces decrease in the distance between the objective lens and the optical disc of  FIG. 5A . In other words, the objective lens can be kept from getting too close to the optical disc. Thus, collision between the objective lens and the optical disc during the defocus detection period can be avoided. 
     The drive signal generation block  108  may be formed on a single semiconductor substrate, or only part of the drive signal generation block  108  including the focus control section  116  may be formed on a single semiconductor substrate. Otherwise, part of the focus control section  116  may be formed on another semiconductor substrate. 
       FIG. 6  is a block diagram showing a variation of the focus control section  116  in  FIG. 1 . A focus control section  216  of  FIG. 6  is different from the focus control section  116  in  FIG. 1  in that a second filter adjustment portion  232  is additionally provided, and is used in the optical disc device of  FIG. 1  in place of the focus control section  116 .  FIG. 7  is a block diagram showing an example configuration of the filter adjustment portion  232  in  FIG. 6 . The filter adjustment portion  232  includes an error detector  246  and a signal adjuster  248 . 
       FIG. 8  is a flowchart showing a flow of processing by the optical disc device having the focus control section  216  of  FIG. 6 .  FIGS. 9A-9G  are charts respectively illustrating the distance between the objective lens and the optical disc ( 9 A), the focus error signal FE ( 9 B), the reflected light amount ( 9 C), the drive signal DS ( 9 D), the detection signal FEdet 1  or FEdet 2  ( 9 E), the first filter adjustment signal ( 9 F), and a second filter adjustment signal ( 9 G). 
     The operation of the optical disc device having the focus control section  216  of  FIG. 6  will be described hereinafter with reference to  FIGS. 6-9G . The steps S 102  through S 134  are similar to those in  FIG. 4 . Description of these steps is therefore omitted here. 
     The error detector  246  in  FIG. 7  determines whether or not the absolute value of the focus error signal FE is equal to or more than an error threshold FEth 2  (S 160 ). The processing proceeds to S 162  if the error detector  246  determines that the absolute value of the focus error signal FE is equal to or more than the error threshold FEth 2 , or proceeds to S 172  if the error detector  246  determines that the former is less than the latter. The error threshold FEth 2  may be a value set previously by a controller outside the focus control section  216  or may be a fixed value. 
     In S 162 , the error detector  246  sets the detection signal FEdet 2  at “1” and outputs the signal. In S 172 , the error detector  246  sets the detection signal FEdet 2  at “0” and outputs the signal. In S 164 , with the detection signal FEdet 2 =1, the signal adjuster  248  activates the second filter adjustment signal FA 2  and outputs the signal to the proportional term computing unit  237  and the differential term computing unit  238 . In S 166 , with the second filter adjustment signal FA 2  being active, the proportional term computing unit  237  and the differential term computing unit  238  set at least one of the proportional gain or the differential gain at a value increased from its initial value, for example. 
     In S 174 , the signal adjuster  248  detects a falling edge of the detection signal FEdet 2 . The processing proceeds to S 176  if a falling edge has been detected, or proceeds to S 176  if no falling edge has been detected. The signal adjuster  248  has a down counter. In S 176 , the down counter is set at a fixed value, to start a second extension for the defocus detection period. Assume herein that the value set for the down counter is a fixed value larger than the value corresponding to the defocus detection period, for example. The down counter counts down in accordance with a clock. 
     In S 178 , the signal adjuster  248  determines whether the second extension has finished or not, i.e., whether the value of the down counter is zero or not. The processing proceeds to S 164  if the value of the down counter is not zero. At this time, the down counter decrements the value. The processing proceeds to S 180  if the value of the down counter is zero. 
     In S 180 , the signal adjuster  248  inactivates the second filter adjustment signal FA 2  and outputs the signal to the proportional term computing unit  237  and the differential term computing unit  238 . In S 182 , with the second filter adjustment signal FA 2  being inactive, the proportional term computing unit  237  and the differential term computing unit  238  set the proportional gain and the differential gain at the previous values before the increase in S 166  (e.g., the initial values). 
     In S 184 , the proportional term computing unit  237  multiplies the focus error signal FE by a predetermined value, further multiplies the multiplied result by the proportional gain, and outputs the result to the adder  239 . The differential term computing unit  238  differentiates the focus error signal FE, multiplies the differentiated result by the differential gain, and outputs the result to the adder  239 . In S 136 , the adder  239  adds up the computed results of the integral term computing unit  236 , the proportional term computing unit  237 , and the differential term computing unit  238 . In S 138 , the adder  239  outputs the added result as the drive signal DR. 
     When the first filter adjustment signal FA 1  goes active to decrease the integral gain, the trackability of steady-state fluctuations of the focus control system degrades, but instead the stability margin increases. This makes it possible to increase at least one of the proportional gain or the differential gain. Since more importance is placed on the trackability of steep fluctuations than on the trackability of steady-state fluctuations right after occurrence of a disturbance, it is useful, for stability of the focus control system, to decrease the integral gain and increase at least one of the proportional gain or the differential gain. 
     In the focus control section  216  of  FIG. 6 , when the second filter adjustment signal FA 2  goes active as in  FIG. 9G , at least one of the proportional gain of the proportional term computing unit  237  or the differential gain of the differential term computing unit  238  is set at an increased value. Therefore, right after occurrence of a disturbance, the trackability of steep fluctuations of the focus control system can be improved. 
       FIG. 10  is a block diagram showing a configuration of a variation of the filter adjustment portion  131  of  FIG. 3 . A filter adjustment portion  331  of  FIG. 10  is different from the filter adjustment portion  131  of  FIG. 3  in having a signal adjuster  344  in place of the signal adjuster  144 .  FIG. 11  is a block diagram showing a configuration of a variation of the filter adjustment portion  232  of  FIG. 7 . A filter adjustment portion  332  of  FIG. 11  is different from the filter adjustment portion  232  of  FIG. 7  in having a signal adjuster  348  in place of the signal adjuster  248 . The filter adjustment portions  331  and  332  are used in the focus control section of  FIG. 6  in place of the filter adjustment portions  231  and  232 , respectively.  FIG. 12  is a flowchart showing a flow of processing by an optical disc device having the filter adjustment portions  331  and  332  of  FIGS. 10 and 11 . 
     The signal adjusters  344  and  348  have their down counters. In S 125  in  FIG. 12 , the value to be set for the down counter of the signal adjuster  344  (an extension amount EX 1 ) is given to the signal adjuster  344  from a controller outside the focus control section  216 , for example. In S 175 , similarly, the value to be set in the down counter of the signal adjuster  348  (an extension amount EX 2 ) is given to the signal adjuster  348  from an external controller, for example. The extension amounts EX 1  and EX 2  can be changed with the system states such as the type of the optical disc, the readout rate from the optical disc, and the rotation control scheme, for example. Note that the extension amount EX 1  should be a value at least larger than the defocus detection period, but the extension amount EX 2  does not need to be a value larger than the defocus detection period. 
     In S 126 , with the setting of the extension amount EX 1  for the down counter of the signal adjuster  344 , the first extension for the defocus detection period is started. In S 176 , with the setting of the extension amount EX 2  for the down counter of the signal adjuster  348 , the second extension or the defocus detection period is started. The other steps of the processing in  FIG. 12  are similar to those described above with reference to  FIGS. 4 and 8 . 
     The defocus detection period may be changed with the system states such as the type of the optical disc, the readout rate from the optical disc, and the rotation control scheme and the use environments of the optical disc device (e.g., car-mounted, etc.). In this case, also, the extension time of the first filter adjustment signal can be changed in association with the change of the defocus detection period. 
     The extension time of the second filter adjustment signal can also be changed with the system states and the use environments of the optical disc device described above. Therefore, the trackability can be improved only for steep fluctuations of the focus control system right after occurrence of a disturbance, and thus the focus control system can be stabilized during the defocus detection period. 
       FIG. 13  is a block diagram showing a configuration of a variation of the optical disc device of  FIG. 1 . An optical disc device of  FIG. 13  is different from the optical disc device of  FIG. 1  in further having an acceleration measurement section  407  and having a drive signal generation block  408  in place of the drive signal generation block  108 . The drive signal generation block  408  is different from the drive signal generation block  108  in  FIG. 1  in having a focus control section  416  in place of the focus control section  116 . 
       FIG. 14  is a block diagram showing an example configuration of the focus control section  416  in  FIG. 13 . The focus control section  416  is different from the focus control section  216  of  FIG. 6  in having filter adjustment portions  431  and  432  in place of the filter adjustment portions  231  and  232 .  FIG. 15  is a block diagram showing an example configuration of the filter adjustment portion  431  in  FIG. 14 . The filter adjustment portion  431  includes an error detector  442 , a signal adjuster  444 , an acceleration determiner  452 , and a threshold adjuster  454 .  FIG. 16  is a block diagram showing an example configuration of the filter adjustment portion  432  in  FIG. 14 . The filter adjustment portion  432  includes an error detector  446 , a signal adjuster  448 , an acceleration determiner  456 , and a threshold adjuster  458 . 
       FIG. 17  is a flowchart showing a flow of processing on setting of the error thresholds FEth 1  and FEth 2  by the optical disc device of  FIG. 13 . Note that, in addition to the processing in  FIG. 17 , processing similar to that in  FIG. 4 ,  8 , or  12  is performed, for which the error thresholds FEth 1  and FEth 2  set in the processing in  FIG. 17  are used. The operation of the optical disc device of  FIG. 13  will be described with reference to  FIGS. 13-17 . 
     In S 402 , the acceleration measurement section  407  measures the acceleration of the objective lens of the optical pickup  104  with respect to the optical disc  101 , and outputs the measured acceleration AS to the filter adjustment portions  431  and  432  of the focus control section  416 . Assume herein that the direction from the objective lens toward the optical disc  101  is normal. 
     In S 404 -S 408 , the filter adjustment portion  431  performs the following processing: in S 404 , the acceleration determiner  452  determines whether or not the acceleration AS is equal to or more than an acceleration threshold ASth 1 , and outputs the determination result to the threshold adjuster  454 . The processing proceeds to S 406  if the acceleration AS is equal to or more than the acceleration threshold ASth 1 , or otherwise proceeds to S 408 . In S 406 , the threshold adjuster  454  decreases the input error threshold FEth 1  and outputs the result to the error detector  442 . In S 408 , the threshold adjuster  454  outputs the input error threshold FEth 1  to the error detector  442  as it is. 
     In S 404 -S 408 , also, the filter adjustment portion  432  may perform the following processing: in S 404 , the acceleration determiner  456  determines whether or not the acceleration AS is equal to or more than an acceleration threshold ASth 2 , and outputs the determination result to the threshold adjuster  458 . The processing proceeds to S 406  if the acceleration AS is equal to or more than the acceleration threshold ASth 2 , or otherwise proceeds to S 408 . In S 406 , the threshold adjuster  458  decreases the input error threshold FEth 2  and outputs the result to the error detector  446 . In S 408 , the threshold adjuster  458  outputs the input error threshold FEth 2  to the error detector  446  as it is. The acceleration thresholds ASth 1  and ASth 2  may be values set previously by a controller outside the focus control section  416  or may be fixed values. 
     When the error threshold FEth 1  decreases, the error detector  442  is more likely to detect an error (FEdet 1  is more likely to become 1). When the error threshold FEth 2  decreases, the error detector  446  is more likely to detect an error (FEdet 2  is more likely to become 1). Therefore, by allowing at least one of the filter adjustment portions  431  or  432  to perform the processing of S 404 -S 408  to decrease at least one of the error thresholds FEth 1  or FEth 2 , the stability of the focus control system can be maintained even if the acceleration in the direction from the objective lens toward the optical disc  101  is large. 
     As described above, in the embodiment of the present disclosure, collision between the objective lens and the optical disc can be avoided. Thus, the present disclosure is useful for optical disc devices, etc., in particular, optical disc devices used in portable apparatuses and car-mounted apparatuses susceptible to vibrations and impacts, for example. 
     Many features and advantages of the present disclosure are apparent from the written description, and thus, it is intended by the appended claims to cover all such features and advantages of the disclosure. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the disclosure to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.