Focus draw-in method in an optical disc device

Abstract: A focus draw-in system for an optical disc device, which includes the steps of detecting a peak (11,12) of a focus error signal, and after detecting the peak, applying a driving force to an optical device (7) in a direction of the detected peak until the relative speed between a disc face and the optical device (7) becomes small, and effecting focus control at a time point when the relative speed between the disc face and the optical device has become small. By the above system of the present invention, the focus draw-in function with a high performance capable of effecting an efficent draw-in even when the relative speed between the disc face and the optical device is large, may be advantageously realized.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to an optical disc device, and more particularly, to a focus draw-in method for an optical disc device. It has been a recent trend that a large member of products related to optical discs such as a compact disc player and the like, are introduced into the market. Commonly, data of an optical disc are arranged in a spiral shape from an inner periphery to an outer periphery of the optical disc in the form of pits, and the data are settled thereon by the presence or absence of such pits. The role of an optical servo is to direct a focal point of light emitted form the optical device onto a predetermined row of pits during reproduction of data on the optical disc, thereby to establish a state where the data are readily read out. The optical servo includes a focus control which functions to bring the optical device into focus on the disc face, and a tracking control for focusing onto the pit row on the disc face. For the control procedure, it is so arranged that, after rotating the disc by a motor, the optical device is driven in a direction perpendicular to the disc surface to effect the focus draw-in function. For the explanation of the focus draw-in function, formation of a focus error signal will be described hereinafter. As shown in Fig. 8, the focus error signal becomes zero when the focal point is located on a disc face, while an absolute value of the focus error signal is increased as the focal point is spaced from the disc face in the perpendicular direction, and is reduced after reaching a peak value. In the focus control, it is so arranged that, when the focus error signal is present within an S-shape peaks (i.e. within a range between peaks for an S-shape curve: referred to as S-shape peaks hereinafter), the absolute value of the focus error signal is reduced to zero for control so as to focus on the disc face, and the "focus draw-in" refers to the function at a stage prior to execution of the focus control. Subsequently, conventional focus draw-in systems as follows will be explained. In a focus servo device known from JP-B-6 313 261, it is so arranged that there is provided a reference voltage with the same polarity as a focus error signal to be produced when the optical device is spaced from the disc beyond a "just focus" point, and having an absolute value smaller than that of the polarity value of said focus error signal, whereby when the focus error signal crosses the reference voltage at the side of the "just focus" point beyond the polarity value, the processing is transferred into the focus control. Meanwhile, in a focus lens servo draw-in device according to JP-B-6 334 528, there is provided a reference voltage of the same polarity as a focus error signal to be produced when the optical device approaches the disc beyond a "just focus" point, and having an absolute value smaller than that of the polarity value of said focus error signal, whereby when the focus error signal crosses the reference voltage at the side of the "just focus" point beyond the polarity value, a counter force gradually attenuated from a predetermined level is applied to the optical device, and when it crosses the reference voltage again, the processing is transferred into the focus control. However, in the first prior art arrangement as described above, in the case where the relative speed between the disc face and the optical device is large, it is impossible to stabilize within the S-shape region of the focus error even upon transfer into the focus control, thus resulting in undesirable deviation from the S-shape region. Meanwhile, in the second prior art as referred to above, when the relative speed between the disc face and the optical device is large, it is also impossible to stabilize within the S-shape region of the focus error, even if a counter force is applied, similarly resulting in deviation from the S-shape region. SUMMARY OF THE INVENTION Accordingly, an essential obiect of the present invention is to provide a focus draw-in system for an optical disc device which presents a high focus draw-in performance. Another object of the present invention is to provide a focus draw-in system as described above which may be readily applied to optical disc devices through simple construction. In accomplishing these and other objects, according to the present invention, there is provided a focus draw-in method in an optical disc device, which method comprises the steps of detecting a value of a focus error signal, and, after detecting said value, applying a driving force to an optical device, and effecting focus control, characterized in that said value is a peak of the focus error signal, and the driving force is applied to the optical device in the direction of said detected peak until a time point when a changing point of said focus error signal indicative of a temporarily stopped condition of said optical device is detected, and the focus control is effected at said time point. Further advantageous embodiments of the invention are defined in the subclaims. By the steps of the present invention as described above, even in the case where the relative speed between the disk face and the optical device is high, and stabilization within the S-shape region of the focus error can not be achieved upon closing of the servo-loop, a strong braking force may be continuously exerted from the peak of the focus error by applying a driving force to the optical device in the direction of the peak as detected after the peak detection, and the focus is drawn-in at the time point when the relative speed between the disc face and the optical device has become small or low, and therefore, focus draw-in at a high capacity may be realized even when the relative speed between the disc face and the optical device is large. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which; Fig. 1 is a schematic block diagram showing a general construction of a focus draw-in circuit to which a focus draw-in system of the present invention may be applied, Fig. 2 is also a block diagram showing an internal construction of a micro-computer employed in the focus draw-in circuit related to the present invention, Fig. 3 is a flow-chart for a focus draw-in system according to a first embodiment of the present invention, Figs. 4(A) and 4(B) are graphical diagrams for explaining a general concept of the focus draw-in system of the present invention, Figs. 5(A) and 5(B) are flow-charts for the focus draw-in system according to a second embodiment of the present invention, Fig. 6 is a flow-chart for the focus draw-in system according to a third embodiment of the present invention, Fig. 7 is also a flow-chart for the focus draw-in system according to a fourth embodiment of the present invention, and Fig. 8 is a graphical diagram showing the relation between the focus position of the optical device and focus error signal. DETAILED DESCRIPTION OF THE INVENTION Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings, a focus draw-in system according to the present invention will be explained in detail hereinafter. Figs. 1 to 4(B) related to a first embodiment according to the present invention, and represent the case where it is so arranged that a focus error signal is sampled by an A/D converter, and a control by a micro-computer is effected based on the error signal as sampled. Fig. 1 is a block diagram showing a general construction of the focus draw-in circuit according to the present invention, which includes a motor 8 for rotating a disc 1 as an information recording medium, an A/D converter 16 inserted between an optical device 7 and a micro-computer 17 and adapted to convert a focus error signal in the analog form received as an input, into a digital data of 8 bits for input to the micro-computer 17, which effects calculation, judgement, etc. based on the 8 bit data so as to apply PWM (Pulse Width Modulation) data to a PWM converter 18 coupled therewith. The PWM converter 18 connected to a driver 6 receives the PWM data which is the output of the micro-computer 17 and outputs a pulse width modulation signal to said driver 6. The driver 6 outputs an electric current proportional to the input voltage value to an actuator 24 connected thereto. This actuator 24 having the output current of the driver 6 as the input drives the optical device 7 by said input current in a direction perpendicular to the disc face. The optical device 7 arranged to be thus driven by the actuator 24, also produces the focus error signal. Fig. 2 shows an internal construction of the micro-computer 17 referred to above, which includes an input port 19 which receives an output of the A/D converter 16, and outputs to a CPU (central processing unit) 21. The CPU 21 outputs an address to a ROM (Read Only Memory) 20 which is a memory containing a program for the micro-computer 17, and receives the program for the address from the ROM 20, and based on the program thus inputted, functions to read data of the input port 19, to read or write data with respect to a RAM (Random Access Memory) 22, to effect calculation or judgement, or to output data to an output port 23 connected to the CPU 21. Referring also to Fig. 3 showing a flow chart for the focus draw-in system according to the first embodiment of the present invention, an S-shape region intruding direction detecting step 10 detects from which of positive or negative direction the focus error signal begins to appear, and in the case where it intrudes into the S-shape region in a positive direction, the step is shifted to a positive peak detecting step 11, while it is transferred to a negative peak detecting step 12 upon intrusion thereof into the S-shape region from a negative direction. The positive peak detecting step 11 detects a positive peak of the error signal, and after detection of the positive peak, the step is shifted to a positive direction braking step 13. This positive direction braking step 13 continues to drive the optical device 7 in the positive peak direction until an inclination of the error signal becomes zero or positive after completion of the positive peak detecting step 11, and when the inclination of the error signal becomes zero or positive, the step is transferred into a focus control step 15. The negative peak detecting step 12 detects the negative peak of the error signal, and after detection of the negative peak, the step is transferred to a negative direction braking step 14. This negative direction braking step 14 continues to drive the optical device 7 in the negative peak direction until an inclination of the error signal becomes zero or negative after completion of the negative peak detecting step 12, and when the inclination of the error signal becomes zero or negative, the step is transferred into the focus control step 15. This focus control step 15 effects a focus control adapted to reduce an absolute value of the focus error signal to zero after completion of braking. Figs. 4(A) and 4(B) show waveform diagrams representing the focus draw-in functions, and Fig. 4(A) relates to the case where the focus error signal intrudes into the S-shape region in a positive direction, while Fig. 4(B) shows the case where the intrusion of the focus error signal is effected in a negative direction. It is to be noted here that numerals in Figs. 4(A) and 4(B) correspond to the numerals representing the respective steps in the flow-chart of Fig. 3. Still referring to Figs. 1 to 4(B) as described so far, functioning according to the first embodiment of the present invention will be described hereinafter. It is to be noted here that in the embodiment to be described, the focus error signal is sampled by the A/D converter 16, and the sampled data is of 8 bits, with a sampling frequency set at 44.1 KHz. In the S-shape region intrusion detecting step 10 in Fig. 3, the error signal is sampled after search driving to see if the focus error signal exceeds a threshold value or not. For the search driving, driving data is applied from the micro-computer 17 to the PWM converter 18 so as to move the optical device 7. The search driving is effected by triangular waves, and appearance of the S-shape curve for the focus error signal is monitored. When the threshold signal exceeds the threshold value, it is regarded as intrusion into the focus error signal S-shape region, and the step is shifted to a subsequent positive/negative peak detecting step. In the micro-computer 17 in Fig. 2, the data of the focus error signal is applied to the CPU 21 from the input port 19, while the threshold value data in the program of the ROM 20 is also inputted to the CPU 21, whereby the two data are compared, and based on the result of said comparison, the address of the ROM 20 is selected. The negative threshold value is represented by a constant B, while the positive threshold value is denoted by a constant A. The reason for setting the positive and negative threshold values in the above arrangement is such that there are two days in which the S-shape for the focus error appears, since the polarity of the focus error signal during the peak time differs depending on directions in which the focus of the optical device 7 approaches the surface of the disc 1 as will be seen from Fig. 8. It is to be noted here that for the constants A and B, figures equal to each other in the absolute value are to be employed. Subsequently, with respect to the peak detection, there will be considered a case where a positive threshold value has been exceeded. In the flow-chart of Fig. 3, when the focus error signal exceeds the constant A which is the positive constant, the flow is transferred to that of the positive peak detecting step 11. In the positive peak detecting step 11, the focus error signal is sampled by the A/D converter 16 after the search driving, and is applied to the CPU 21 through the input port 19, whereby comparison between the sampled error signal and the size of the maximum value is effected. The initial value of the maximum value is set to zero and is stored in the RAM 22. During execution of the positive peak detecting step 11, since the focus error signal is simply increasing until the positive peak is exceeded, the maximum value is continuously renewed and stored in the RAM unless the error signal smaller then the maximum value is received. This positive peak detecting step 11 is represented by the numeral 11 in Fig. 4(A), which detects the maximum value. Upon passing the peak for the focus error signal, the error signal becomes smaller than the maximum value, and enters the flow for the next positive direction braking step 13 through the positive peak detecting step 11. The positive direction braking step 13 functions in the manner as described hereinafter. Firstly, the focus error signal is sampled to be applied to the CPU 21, and is also stored in the RAM 22. Subsequently, data of the focus error signal before one sample and that of the present sample are compared with each other for size. At the time point when the function is transferred into the positive braking function, since the error signal is reduced after passing the S-shaped peak, the error of the present sample has become smaller than that before one sample, and thus, the flow is shifted into the subsequent positive peak direction driving. In the positive peak direction driving, the optical device 7 is driven to the maximum extent in the direction in which the error signal is increased within the focus error S-shape peaks, i.e. in the positive peak direction, whereby the speed in the direction for decreasing the error signal is reduced. While the error signal is decreasing, the braking function is continued, and the maximum driving force is continuously applied in the direction to stop the relative motion between the surface of the disc 1 and the optical device 7. Subsequently, when the focus error of the present sample becomes equal to or larger than that before one sample, it is judged that the relative speed between the surface of the disc 1 and the optical device 7 is in the vicinity of zero, and the focus draw-in is completed through the positive direction braking step 13, and the flow is shifted into the ordinary focus control step 15. Although the ordinary focus control step 15 intends to reduce the absolute value of the focus error signal to zero, since the focus control is started when the absolute value of the focus error signal is changed from the reduction to increase in this case, the relative speed between the surface of the disc 1 and the optical device 7 is in the vicinity of zero, and thus, the draw-in may be readily effected. Hereinafter, the draw-in function when the focus error signal exceeds the negative threshold value will be explained. In the S-shape region intruding direction detecting step 10 of Fig. 3, when the sampled focus error signal falls below the constant B which is the negative constant, the flow is shifted to the negative peak detecting step 12. At the negative peak detecting step 12, the focus error signal is sampled after the search driving, and the sampled error signal and the minimum value are compared for size. If the initial value of the minimum value is set to zero, the minimum value is continuously renewed unless the error signal larger than the minimum value is received, since the error signal is negative in this case. The above negative peak detecting step 12 is represented by the numeral 12 in Fig. 4(B), which detects the minimum value. When the negative peak of the focus error signal is exceeded, the error signal becomes larger than the minimum value, and the flow enters the next negative direction braking step 14 through the negative peak detecting step 12. Subsequently, the function of the negative direction braking step 14 will be explained hereinbelow. In the first place, the focus error signal is sampled to be stored in the RAM 22, then, data of the focus error signal before one sample and that of the present sample are compared with each other for size. At the time point when the function is transferred into the negative braking function, since the error signal is increased over the S-shape peak, the error of the present sample has become larger than that before one sample, and thus, the flow is shifted into the subsequent negative peak direction driving. In the negative peak direction driving, the optical device 7 is driven to the maximum extent in the direction in which the error signal is decreased within the focus error S-shape peaks, i.e. in the negative peak direction, whereby the speed in the direction for increasing the error signal is reduced. While the error signal is increasing, the braking function is continued, and the maximum driving force is continuously applied in the direction to stop the relative motion between the surface of the disc 1 and the optical device 7. Subsequently, when the focus error of the present sample becomes equal to or smaller than that before one sample, it is judged that the relative speed between the surface of the disc 1 and the optical device 7 is in the vicinity of zero, and the focus draw-in is completed through the negative direction braking step 14, and the flow is shifted into the ordinary focus control step 15. Although the ordinary focus control step 15 intends to reduce the absolute value of the focus error signal to zero, since the focus control is started when the absolute value of the focus error signal is changed from the increase to reduction in this case, the relative speed between the surface of the disc 1 and the optical device 7 is in the vicinity of zero, and thus, the draw-in may be readily effected. As is seen from the foregoing description, according to the first embodiment of the present invention, the focus draw-in system for the optical disc device is constituted by the steps of detecting the peak of the focus error signal, and after detecting the peak, applying the maximum driving force to the optical device in the direction of the detected peak until the relative speed between the disk face and the optical device becomes small, and effecting the focus control at the time point when said relative speed therebetween has become small, whereby the draw-in may be easily effected even in the case where the relative speed between the disk and the optical device is large. Referring to Fig. 5, there is shown a flow-chart for the focus draw-in system according to a second embodiment of the present invention. It should be noted here that the construction of the focus draw-in circuit for Fig. 5 is generally the same as that of the first embodiment of Fig. 3, and the essential difference thereof from the circuit of Fig. 3 is such that an out-of S-shape region detecting circuit 26 is added before the S-shape region intruding direction detecting step 10, immediately after starting of the focus draw-in. The function of the focus draw-in system according to the second embodiment of the present invention arranged as described above will be explained hereinafter. With respect to the out-of S-shape region detecting step 26, the error signal is first sampled after the search driving. Since the search driving is effected in the similar manner as in the first embodiment, detailed description thereof is abbreviated here for brevity of explanation. Then, it is judged whether or not the absolute value of the sampled error signal is smaller than the constant C. If it is smaller, 1 is added to a counter, while, if not, the counter is cleared to 0. The counter is constituted by the micro-computer 17, and the value added by the CPU 21 is stored in the RAM 22, and the counting function is effected by applying the data of the RAM 22 again to the CPU 21 for effecting addition. At the starting, the counter has been cleared to 0. The sampled data is of 8 bits, with the sampling frequency of 44.1 KHz. When the value of the counter reached a constant D, it is regarded to be in the out of the focus error S-shape region, and the flow enters the subsequent S-shaped region intruding direction detecting step 10. It is to be noted that the out-of focus error S-shape region is represented by the portion out of the range for the focus error S-shape region in Fig. 8. When the focus draw-in is to be effected, there is no guarantee that the initial state is out of the S-shape region, and thus, erroneous function for the draw-in is prevented by moving into a series of focus draw-in function after confirmation that the state is out of the S-shape region. For example, upon supposition that the focus draw-in happens 7 is located in the vicinity of the face of the disc 1 (i.e. in the "just focus" state), the focus error signal is increased in the absolute value so as to be spaced from the just focus point. Thus, the absolute value of the focus error signal is reduced by passing over the peak, but in this case, it is judged to have entered the focus error S-shape peaks, irrespective of its deviation therefrom, and the optical device is driven towards the outside of the focus error S-shape region, and thus, the absolute value of the focus error signal is only subjected to the simple decrease so as to be out of the focus error S-shape region. In the second embodiment as described so far, the erroneous function in the case where the focus draw-in is started from inside of such S-shape region is advantageously prevented. Since the construction and function of the focus draw-in system according to the second embodiment of the present invention after the out-of S-shape region detecting step 26 are the same as those of the first embodiment of Fig. 3, detailed description thereof is abbreviated here for brevity, with like steps being designated by like reference numerals. As described so far, according to the second embodiment of present invention, through addition of the out-of S-shape region detecting step 26 to the steps of the first embodiment, the mal-functions in the draw-in when the focus draw-in function is started from inside the focus error S-shape region, can be advantageously eliminated. Referring to Fig. 6, there is shown a flow-chart for the focus draw-in system according to a third embodiment of the present invention. It should be noted here that the construction of the focus draw-in circuit of Fig. 6 is generally the same as that of the first embodiment of Fig. 3, and the essential difference thereof from the circuit of Fig. 3 resides in that there is added a counter direction driving step 27 which drives in a direction opposite to that of the braking during transfer to the focus control step 15 after completion of the positive or negative direction braking function. Since the construction and function of the focus draw-in system according to the third embodiment of he present invention before the counter direction driving step 27 are the same as those of the first embodiment of Fig. 3, detailed description thereof is abbreviated here for brevity, with like steps being designated by like reference numerals. In the above arrangement of Fig. 6, the reason for driving in the counter direction is such that, when there are many cases where the polarity of inclination of the error signal varies or becomes zero before the focus error signal is subjected to zero-crossing, with a strong braking force being applied in a constant direction by the braking function, the draw-in function may be effected more smoothly when there is given a slight speed in the direction in which the focus error signal becomes zero, than when there is a tendency to over-shooting. As described above, in the third embodiment of Fig. 6, the stability for the focus draw-in may be improved through addition of the counter direction driving step 27 to the first embodiment of Fig. 3. Referring further to Fig. 7, there is shown a flow-chart for the focus draw-in system according to a fourth embodiment of the present invention. It should be noted here that the construction of the focus draw-in circuit for Fig. 7 is generally the same as that of the first embodiment of Fig. 3, and the essential difference thereof from the circuit of Fig. 3 resides in that a noise discriminating step 28 is inserted between the positive and negative peak detecting steps 11 and 12 and the positive and negative direction braking steps 13 and 14 so that the braking function may not be erroneously started by the noise of the focus error signal. As the conditions for moving into the braking through the peak detection, it is so arranged to shift into the subsequent braking when the value becomes smaller than that obtained by subtracting a positive constant E from the absolute value of the error signal by taking into account, the case in which noise is contained in the focus error signal. By the above arrangement, the peak detection accurately detects exceeding of the peak even when noise is present in the error signal. The constant E is set to be smaller than the absolute value of the peak value so as to cover the noise component. Since the construction and function of the focus draw-in system according to the fourth embodiment of the present invention after the nose discriminating step 28 are the same as those of the first embodiment of Fig. 3, detailed description thereof is abbreviated for brevity, with like steps being designated by like reference numerals. As described so far, according to the fourth embodiment of present invention, through addition of the noise discriminating step 28 to the steps of the first embodiment, it becomes possible to prevent erroneous shifting to the braking function by the error of the focus error signal. It should be noted here that in the foregoing embodiments, although the focus error signal is subjected to the sampling for the control by the micro-computer, the arrangement may be replaced by an analog circuit depending on necessity.

Claims: A focus draw-in method in an optical disc device, which method comprises the steps of detecting a value of a focus error signal, and, after detecting said value, applying a driving force to an optical device (7), and effecting focus control, characterized in that said value is a peak (11,12) of the focus error signal, and the driving force is applied to the optical device (7) in the direction of said detected peak until a time point when a changing point of said focus error signal indicative of a temporarily stopped condition of said optical device (7) is detected (13, 14), and the focus control is effected at said time point. The method as claimed in claim 1, wherein the driving force to be applied is of the maximum force. The method as claimed in claim 1, comprising an S-shape region intruding direction detecting step (10) of a focus error signal wherein said peak detecting steps comprise a positive peak detecting step (11) having a first output of said S-shape region intruding direction detecting step (10) as an input, and a negative peak detecting step (12) having a second output of said S-shape region intruding direction detecting step (10) as an input, said force applying steps comprise a positive direction braking step (13) having an output of said positive peak detecting step (11) as an input, and a negative direction braking step (14) having an output of said negative peak detecting step (12) as an input, and said focus control step (15) has outputs of said positive direction braking step (13) and of said negative direction braking step (14) as inputs. The method as claimed in claim 1, wherein the steps of detecting a peak of a focus error signal are carried out after detecting that an absolute value of the focus error signal is smaller than a certain predetermined value. The method as claimed in claim 3, further comprising an out-of S-shape region detecting step (26) of a focus error signal, wherein said S-shape region intruding direction detecting step (10) has an output of said out-of S-shape region detecting step (26) as an input. The method as claimed in claim 1, wherein the step of effecting focus control is carried out after driving the optical device (7) in a counter direction. The method as claimed in claim 3, further comprising a counter direction driving step (27) having outputs of said positive direction braking step (13) and of said negative direction braking step (14) as inputs. The method as claimed in claim 1, wherein the steps of applying a driving force to the optical device (7) are carried out upon detection that an absolute value of the focus error signal becomes smaller than a value obtained by subtracting a predetermined value from an absolute value of the peak value of said focus error signal. The method as claimed in claim 8, wherein said predetermined value is above zero and below the absolute value of the peak value of the focus error signal. The method as claimed in claim 3, further comprising a noise discriminating step (28) having an output of said positive peak detecting step (11) as a first input, with its first output serving as an input of said positive peak detecting step (11), and having an output of said negative peak detecting step (12) as a second input, with its second output serving as an input of said negative peak detecting step (12), wherein the positive direction braking step (13) has a third output of said noise discriminating step (28) as an input, and the negative direction braking step (14) has a fourth output of said noise discriminating step (28) as an input.

Inventors: IIDA MAKOTO, IIDA, MAKOTO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G11B7
IPC codes: G11B7