Abstract:
A servo control apparatus of an optical pickup in which an offset and a gain can be adjustable for each servo control apparatus, and influences caused by changes in environmental conditions and changes with time are reduced without a decrease in operational speed, and increase in size and an increase in cost is disclosed. The servo control apparatus includes a nonvolatile memory for storing compensatory values which correspond to differences of servo characteristics of each servo apparatus and which are measured with the assistance of external measurement instruments, and a compensation portion for compensating an optical pick-up servo signal according to the compensatory values stored in the nonvolatile memory.

Description:
This is a continuation of application Ser. No. 08/022,081, filed Feb. 24, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup servo control apparatus for forcing an optical pickup to track a medium, on which information is recorded, in an optical disk unit, an optical card reader, or the like while retaining a specified positional relationship. More particularly, the present invention relates to a servo control apparatus capable of adjusting, for each optical pickup, the individual differences in characteristics of a servo control apparatus resulting from individual errors in optical pickups caused during manufacturing. 
     2. Description of the Related Art 
     An apparatus using an optical pickup includes an optical disk unit and an optical card reader. Hereafter, the present invention will be described using the optical disk unit as an example. 
     In an optical disk unit, an optical disk is rotated about a rotation axis by a spindle motor. This causes an optical pickup to move in the radial direction of the optical disk and align with the optical disk. Then, the optical pickup reads or writes information from or on the optical disk. 
     The optical pickup routes a light beam generated by a semiconductor laser serving as a light source to an objective lens via a known optical system. Then, the objective lens shrinks the light beam to provide a spotlight of a very small diameter, and irradiates the spotlight onto the optical disk. Then, the light reflected from the optical disk is routed to the optical system via the objective lens. Consequently, a light receiver in the optical system provides a light received signal associated with a change in the reflected light. 
     In this kind of optical disk unit, numerous tracks and pits are formed at intervals of several microns in the radial direction of an optical disk. To record or read information on or from the optical disk, a light beam must be forced to follow a track or pits while the state of focusing providing a beam spot of 1 micron or less in diameter is being retained. However, an optical disk may become eccentric or swell. Slight eccentricity of an optical disk displaces a focusing position of a light beam, and swelling thereof deviates a focal point of a beam spot. Therefore, the beam spot cannot be irradiated onto a track of the optical disks as it is. To solve this problem, a servo control apparatus is used to perform servo control so that an optical pickup will move on a track or pits while maintaining the focal point providing a beam spot of 1 μm or less. Specifically, a servo control apparatus for an optical pickup performs two kinds of servo control; that is, focus servo control for controlling a focal point of a light beam and tracking control for moving a beam spot to follow a track or pits. 
     In efforts to achieve the foregoing servo control, a focus actuator (focus coil) for moving an objective lens of an optical pickup perpendicularly to an optical disk in order to vary a focal point, and a tracking actuator (track coil) for moving the objective lens in the radial direction of the optical disk in order to vary an irradiation point in the tracking direction are provided. An optical system includes, for example, four elements making up a four-division light receiver, and is designed to provide a focus error signal FES associated with a deviation of the focal point of a beam spot and a tracking error signal TES associated with a displacement of the beam spot from a tracing position by processing the outputs of these elements. A focus servo control or a component of a servo control apparatus for an optical pickup processes a light received signal output by the light receiver, generates a focus error signal, and feeds back the focus error signal to the focus actuator so as to control the focus actuator. A tracking servo control processes the light received signal, generates a tracking error signal, and feeds back the tracking error signal to the tracking actuator so as to control the tracking actuator. 
     In the foregoing focus servo control or tracking servo control, an offset occurs due to individual variations in characteristics of each part of the optical pickup or to an error in mounting each part. Therefore, the focus does not always coincide with a recording surface of an optical disk when a focus error signal has a zero level. For a similar reason, the gain of the servo control system varies. Therefore, the focus servo control and tracking servo control are required to control their offsets and gains for each optical pickup. 
     In an optical pickup servo control apparatus, differential amplifiers are employed as means that use a light received signal to generate a focus error signal and a tracking error signal respectively. In the past, a bias resistor and a feedback resistor connected to these differential amplifiers have been realized with variable resistors, or a variable resistor has been connected to an offset control terminal. Thus, offsets and gains have been controlled. Furthermore, the variable resistors are adjusted in a process of manufacturing an optical pickup so that offsets will be zero and gains will be specified values. 
     However, a resistance of a variable resistor having a sliding portion tends to vary due to an environmental change or over time. Therefore, an offset and a gain, which have already been controlled, may vary as environments change or time passes. This results in unsatisfactory servo control. 
     In efforts to solve the aforesaid problems, the present applicant has disclosed an apparatus in Japanese Unexamined Patent Publication (Kokai) No. 62-222438 and Japanese Unexamined Patent Publication (Kokai) No. 62-141644. In the disclosed apparatus, a detector for detecting an offset value and a compensating means that compensates for the detected offset value are included in the focus servo control and in the tracking servo control respectively in an optical pickup, whereby an offset is automatically controlled. Japanese Unexamined Patent Publications (Kokai) Nos. 1-125733, 2-294940, 3-152722 and 4-19833 also disclose similar apparatuses each having a detector for detecting an offset value and a compensating means that compensates for the detected offset value. 
     The aforesaid kind of servo control apparatus is unsusceptible to an environmental change or a time-sequential change because no variable resistor is used. However, the servo control apparatus poses the following problems: 
     (1) Since a focus servo system or a tracking servo system is adjusted during normal operation of an optical disk unit, the normal operation is slowed down by time required for adjustment. 
     (2) For automatic adjustment, a peak detector (envelope detector), an A/D converter, and other extra circuits are needed. This leads to an increase in the scale of circuitry. Eventually, the optical disk unit becomes larger and costs increase. 
     When it comes to gain control, in Japanese Unexamined Patent Publication (Kokai) No. 63-224034, the present applicant has disclosed a technology allowing the tracking servo control to perform gain control in such a manner that permits stable servo control irrelevant of a change in the shape of a track groove. This technology can control only a gain variation resulting from the shape of a groove on an optical disk. The technology, therefore, does not cope with variations in servo gain resulting from other factors, such as, performance of an actuator, and assembly precision of an optical system. 
     The aforesaid prior art has not dealt with control of a focus servo gain. 
     For the reasons mentioned above, gain control is currently achieved using a variable resistor. 
     When a variable resistor is used to control an offset and a gain occurring in a servo control apparatus for an optical pickup, a controlled state is subject to change. This disables stable servo control. When an offset value detector is installed in the apparatus to compensate for an offset, the operating speed decreases, the size expands, and the cost increases. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to realize a servo control apparatus of an optical pickup in which an offset and a gain can be adjustable for each servo control apparatus, and influences from changes in environmental conditions and changes with the passage of time are reduced without a drop in operation speed, an increase in size and an increase in cost. 
     A servo control apparatus of an optical pickup according to the present invention includes a nonvolatile memory and a compensation portion. The nonvolatile memory stores compensatory values corresponding to differences of servo characteristics of each servo control apparatus, and these compensatory values of servo characteristics are measured with the assistance of external measurement instruments when this servo control apparatus is manufactured at a factory and so forth. The compensation portion compensates an optical pickup servo signal according to the compensatory values stored in the nonvolatile memory. 
     A servo control apparatus of an optical pickup according to the present invention is a focus servo control apparatus or a tracking servo control apparatus, or includes these two focus and tracking servo apparatuses. 
     Compensatory values of offset and gain of servo characteristics are stored in the nonvolatile memory. If a compensatory value of the gain is stored in the nonvolatile memory, the compensation portion includes a gain adjustable amplifier. If a compensatory value of the offset is stored in the nonvolatile memory, the compensation portion includes a summing circuit. 
     A compensatory value setting method of a servo control apparatus according to the present invention includes a step for measuring a signal of a portion of said servo control apparatus by external measurement means, a step for changing a compensatory values until said signal of said portion reaches a predetermined condition, and a step for storing said compensatory values into said nonvolatile memory. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings, wherein; 
     FIG. 1 is a diagram showing an appearance of a common optical disk apparatus; 
     FIG. 2 is a diagram showing a construction of a common optical disk apparatus; 
     FIG. 3 is a diagram showing a construction of a common optical pickup and a servo control portion; 
     FIG. 4 is a diagram showing a construction of a servo control portion and a control portion according to first to third embodiments of the present invention; 
     FIG. 5 is a circuit diagram of a duty measurement circuit used in the second embodiment; 
     FIGS. 6A to  6 E are diagrams illustrating a focus servo control mechanism; 
     FIGS. 7A to  7 E are diagrams illustrating a tracking servo control mechanism; 
     FIGS. 8 and 9 are flowcharts showing an operation for setting offsets of a focus servo control portion and a tracking servo control portion when this servo control apparatus is adjusted in a factory in the first embodiment; 
     FIG. 10 is a flowchart showing an operation for setting offset of a tracking servo control portion when this servo control apparatus is adjusted in a factory in the second embodiment; 
     FIGS. 11 and 12 are flowcharts showing an operation for setting offsets of a focus servo control portion and a tracking servo control portion when this servo control apparatus is adjusted in a factory in a third embodiment; 
     FIG. 13 is a diagram illustrating a offset measuring method by detecting a duty ratio of signal; 
     FIG. 14 is a diagram illustrating a offset measuring method by detecting a ratio of a positive area and a negative area in relation to a reference level; 
     FIG. 15 is a flowchart showing an operation for setting the offset values stored in an E 2 PROM in the servo control portion when this apparatus is powered ON in the first to third embodiments; 
     FIG. 16 is a diagram showing a construction of a servo control portion and a control portion of a fourth embodiment; 
     FIG. 17 is a circuit diagram showing an example of a variable gain amplifier of FIG. 16; 
     FIG. 18 is a flowchart showing an operation for setting gains of a focus servo control portion and a tracking servo control portion when this apparatus is adjusted in a factory in the fourth embodiment; 
     FIG. 19 is a flowchart showing an operation for setting the gain values stored in an E 2 PROM in the servo control portion when this apparatus is powered ON in the fourth embodiment; 
     FIG. 20 is a diagram showing a construction of a servo control portion and a control portion of a fifth embodiment; 
     FIG. 21 is a flowchart showing an operation for setting the gain value stored in an E 2 PROM and a gain value calculated from the gain value stored in the E 2 PROM in the servo control portion when this apparatus is powered ON in the fifth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As described previously, the servo control for an optical pickup requires two kinds of control; focus servo control and tracking servo control. Each control is accompanied by offset and gain control. The present invention can be implemented not only in either one of the above controls, for example, in offset control as part of focus servo control, but also in all the above controls simultaneously. 
     First of all, embodiments in which the present invention is implemented in offset control as part of focus servo control and tracking servo control for an optical pickup in an optical disk unit will be described as the first to third embodiments. 
     FIG. 1 shows an appearance of an optical disk unit of these embodiments. An optical disk  1  is inserted or ejected in directions shown by a double arrow. In an optical disk, which is used as an external memory for information equipment, a storage capacity and an operating speed have significant meanings. This makes it necessary to adjust an optical pickup servo control apparatus with high precision. 
     FIGS. 2 to  15  show the first to third embodiments. FIG. 2 shows an example of a configuration of an optical disk unit. FIG. 3 shows a configuration of an optical pickup and a servo control unit. FIG. 4 shows examples of configurations of a servo control unit and a control unit. FIG. 5 shows an example of a duty measurement circuit. FIGS. 6A to  6 E are explanatory diagrams of focus servo control. FIGS. 7A to  7 E are explanatory diagrams of tracking servo control. FIGS. 8 to  12  are flowcharts of the process for adjustment at a factory. FIG. 13 is an explanatory diagram of an offset detection method by detecting a duty rate. FIG. 14 is an explanatory diagram of an offset detection method by averaging positive and negative portions. FIG. 15 is a flowchart of the process for normal operation. 
     In the drawings,  1  denotes an optical disk.  2  denotes an optical pickup,  3  denotes a-tracking servo control circuit,  4  denotes a focus servo control circuit,  5  denotes a spindle motor,  6  denotes a tracking actuator,  7  denotes an objective lens,  8  denotes a focus actuator,  10  denotes a four-division light receiver,  11  denotes a light receiver,  12  denotes a light source (semiconductor laser),  14  and  17  denote differential amplifiers,  15  denotes an addition amplifier,  16  and  18  denote power amplifiers, R 1  to R 10 , and r 1  to r 4  denote resistors,  19  denotes a control unit,  22  denotes a digital-analog converter (hereafter, D/A converter),  23  denotes a microprocessor (MPU),  25  denotes an E 2 PROM (electrically rewritable ROM),  27  denotes a controller,  28  denotes a host,  29  denotes a magnetic field generator,  30  denotes a servo control,  31  denotes a light value control,  32  denotes a spindle motor control,  33  denotes a read/write circuit,  35  and  36  denote lenses,  37  denotes a beam splitter,  38  denotes a mirror,  39  denotes a quarter-wave plate,  40  denotes a half mirror,  41  denotes a critical angle prism,  42  denotes a moving mechanism,  43  denotes a D/A converter,  44  denotes an addition amplifier,  50  denotes a comparator (voltage comparator),  51  denotes an inverter,  52  and  53  denote AND gates,  54  and  55  denote counters, and  57  denotes a track. 
     Referring to FIGS. 2 to  5 , an optical disk unit will be described. 
     FIG. 2 shows a configuration of an optical disk unit employed in this embodiment. The optical disk unit comprises a controller  27 , a read/write circuit  33 , a control unit  19 , a servo control unit  30 , an E 2 PROM  25 , an optical pickup  2 , an optical disk  1 , a spindle motor  5 , a light value control  31 , a spindle motor control  32 , and a magnetic field generator  29 . 
     The optical disk unit is used in connection with a host  28 . 
     The optical pickup  2  and servo control unit  30  are configured, for example, as shown in FIG.  3 . 
     As illustrated, in the optical disk unit of the present invention, a moving mechanism  42  in which the optical pickup  2  is incorporated aligns the optical pickup  2  with an intended track in the radial direction of the optical disk  1  rotated by the spindle motor  5 . 
     In the optical pickup  2 , light generated by a semiconductor laser  12  serving as a light source is reduced in diameter by a lens  35 , a beam splitter  37 , a quarter-wave plate  39 , a mirror  38 , and an objective lens  7 . Then, by irradiating the reduced light onto the optical disk  1 , recording or regeneration is carried out. Light reflected from the optical disk  1  is received by the objective lens  7  and mirror  38  via the quarter-wave plate  39  and beam splitter  37 , then routed from a half mirror  40  through a lens  36  to a light receiver  11 . Then, a regenerative signal RFS is produced. Meanwhile, the reflected light is routed from the half mirror  40  through critical angle prism  41  to a light receiver  10 . Then, a tracking error signal TES and a focus error signal FES are produced. 
     As described previously, in the optical disk unit, numerous tracks or pits are formed at intervals of several microns in the radial direction of the optical disk  1 . Even slight eccentricity results in a displacement of a track. Moreover, swelling of the optical disk  1  causes a deviation of a focal point of irradiated light. Nevertheless, the irradiated light of 1 micron or less in diameter must be forced to achieve tracking. 
     For the tracking, a focus actuator  8  that moves the objective lens  7  of the optical pickup  2  vertically to modify a focal point, and a tracking actuator  6  that moves the objective lens  7  laterally in FIG. 3 to modify an irradiation point in the tracking direction are included. In addition, a focus servo control  4  for receiving a light received signal from the light receiver, generating a focus error signal FES, and driving the focus actuator  8 , and a tracking servo control  3  for receiving a light received signal from the light receiver  10 , generating a tracking error signal TES, and driving the tracking actuator  6  are included. 
     FIG. 4 shows examples of configurations of a servo control unit  30  and a control unit  19 . 
     As illustrated, a focus servo control  4  comprises a differential amplifier  14  for generating a focus error signal FES, an offset addition amplifier  15 , and a power amplifier  16  for amplifying an output of the offset addition amplifier  15 , and driving a focus actuator  8  (See FIG.  3 ). An offset addition circuit  400  is composed of the offset addition amplifier  15  and the resistors R 9 , R 10  and r 5 . 
     The offset addition circuit  400  adds an offset FOS provided by a microprocessor  23 , which will be described later, to an output (FES) of the differential amplifier  14 . 
     Outputs a and b of a light receiver  10  are applied to a negative terminal of the differential amplifier  14  via input resistors R 3  and R 4 , while outputs c and d of the light receiver  10  are applied to a positive terminal of the differential amplifier  14  via input resistors R 1  and R 2 . The differential amplifier  14  outputs (−FES) derived from (c+d)−(a+b). r 1  denotes a bias resistor, and r 2  denotes a feedback resistor. 
     A tracking servo control  3  has substantially the same configuration as the aforesaid focus servo control  4 . 
     As illustrated, the tracking servo control  3  comprises a differential amplifier  17  for generating a tracking error signal TES, an offset addition amplifier  44 , and a power amplifier  18  for amplifying an output of the offset addition amplifier  44  and driving a tracking actuator  6  (See FIG.  3 ). An addition circuit  300  is composed of the offset addition amplifier  44  and the resistors R 11 , R 12  and r 6 . 
     Outputs a and d of the light receiver  10  are applied to a negative terminal of the differential amplifier  17  via input resistors R 5  and R 6 , while outputs b and c of the light receiver  10  are applied to a positive terminal of the differential amplifier  17  via input resistors R 7  and R 8 . The differential amplifier  17  outputs (−TES) derived from (b+c)−(a+d). r 3  denotes a bias resistor and r 4  denotes a feedback resistor. 
     A control unit  19  comprises a microprocessor  23 , and D/A converters  22  and  43 , and is connected to an E 2 PROM  25  which is a nonvolatile memory. 
     In the E 2 PROM  25 , as described later, specified data (FOS and TOS) are stored during adjustment (assembly) at a factory. During normal operation, the microprocessor  23  reads data from the E 2 PROM  25 . 
     An output of the microprocessor  23  is fed to a terminal T 1  of the focus servo control  4  via the D/A converter  22  and to a terminal T 2  of the tracking servo control  3  via the D/A converter  43 . The microprocessor  23  can set a focus offset FOS or a tracking offset TOS. 
     FIG. 5 shows an example of a duty measurement circuit for use in adjusting an optical disk at a factory. 
     The duty measurement circuit comprises a comparator (voltage comparator)  50 , a resistor R 50 , an inverter  51 , AND gates  52  and  53 , and counters  54  and  55 . 
     A negative input terminal of the comparator  50  is connected to a reference power supply having a voltage Vg, and a positive input terminal thereof is connected to a point A in the tracking servo control  3  shown in FIG.  4 . 
     The comparator  50  compares the voltage at the point A with the reference voltage Vg, and outputs the result of the comparison. 
     The AND gate  52  inputs an output of the comparator  50 , and the other AND gate  53  inputs an output of the comparator  50  via the inverter  51 . 
     In this case, each of the two AND gates  52  and  53  inputs a sample clock from an external unit, produces an output signal representing the AND between an output of the comparator  50  and the sample clock, and feeds the output to the counters  54  and  55 . 
     The counter  54  counts up an output of the AND gate  52 , and outputs the count. The counter  55  counts up an output of the AND gate  53 , and outputs the count. These outputs are used to calculate a duty ratio. In this case, as described later, when N equals to M, the duty ratio is 50%. 
     Next, referring to FIGS. 6A to  6 E, and  7 A to  7 E, the operation of an optical disk unit will be described. 
     First, the basic operations of focus servo control and tracking servo control will be described in conjunction with FIGS. 6A to  6 E. 
     For focus servo control, when a four-division light receiver  10  made up of four elements a, b, c and d is employed, as shown in FIG. 6A, a state in which the focus of irradiated light coincides with the recording surface of an optical disk  1  is regarded as f. States in which the focus deviates back and forth from the recording surface are regarded as f 1  and f 2  respectively. The distribution of amounts of reflected light entering the light receiver  10  via a critical angle prism  41  becomes as those shown in FIGS. 6B to  6 D in these states. 
     To be more specific, when the focus is in the f 1  state, the distribution is as shown in FIG.  6 B. When the focus is in the f (matches) state, the distribution is as shown in FIG.  6 C. When the focus is in the f 2  state, the distribution is as shown in FIG. 6D. A focus servo control  4  receives an output of (a+b)−(c+d) from the light receiver  10 , and provides a focus error signal FES. This method is well-known as a critical angle method using the critical angle prism  41 . 
     Therefore, when a focus actuator  8  is driven according to the focus error signal FES and moves an objective lens  7  vertically, despite swelling of the optical disk  1 , the focus of irradiated light can be forced to track the recording surface of the optical disk on the submicron order. 
     For tracking servo control, as shown in FIG. 7A, the distribution of amounts of reflected light in the light receiver  10  varies according to the interference of light by a track  57  which depends on the position of irradiated light in the track  57 . 
     To be more specific, when irradiated light is in the P 1  state on the track  57 , the distribution of amounts of reflected light in the light receiver  10  becomes as shown in FIG.  7 B. When irradiated light is in the P state (exactly on the track) on the track  57 , the distribution becomes as shown in FIG.  7 C. When irradiated light is in the P 2  state on the track  57 , the distribution becomes as shown in FIG.  7 D. 
     A tracking servo control  3  receives an output of (a+d)−(b+c) from the light receiver  10 , and provides a tracking error signal TES shown in FIG.  7 E. When a tracking actuator  6  is driven to move the objective lens  7  laterally according to the tracking error signal TES, despite eccentricity of the optical disk  1 , irradiated light can be forced to follow the track  57  on the optical disk  1 . 
     Next, the process for adjustment at a factory will be described. 
     When an optical disk unit is adjusted at a factory (during assembly of a product), a focus offset FOS causing the amplitude of a signal at the point A to be maximum is detected by, for example, incrementing or decrementing a focus offset while observing a point A in a tricking servo control  3 . Then, the detected FOS values are written in an area in an E 2 PROM  25 . 
     A tracking offset TOS causing the signal at the point A to swing with OV as a center is detected by incrementing or decrementing a tracking offset while observing the point A. Then, the detected TOS value is written in an area in the E 2 PROM  25 . 
     The processing for the above detection of the first embodiment will be described with reference to FIGS. 8 and 9. 
     First, at a step  201 , the power supply is turned on to supply power to an optical disk unit. At a step  202 , a spindle motor  5  is rotated. 
     A semiconductor laser (laser diode LD) serving as a light source  12  is lit at a step  203 . At a step  204 , an optical pickup is focused by actuating a focus servo. 
     Thereafter, at step  205 , an oscilloscope is connected to a point A in a tracking servo control shown in FIG.  4 . Thereby, the voltage wave at the point A is observed. 
     Then, at steps  206  and  207 , a command indicating that a focus offset FOS should be incremented (+1) or decremented (−1) is issued to a microprocessor  23  at an external unit,. 
     In response to the command, the microprocessor  23  increments or decrements the focus offset FOS and updates a set value in a D/A converter  22 . 
     Specifically, the microprocessor  23  varies the focus offset FOS until the wave at the point A has a maximum amplitude. 
     Then, when a focus offset FOS causing the amplitude of the wave to be maximum is detected, since the FOS value allows the focus to coincide with the recording surface of an optical disk  1 , a command indicating that the focus offset value should be written in the E 2 PROM  25  is issued to the microprocessor  23  at the external unit. 
     In response to the command, the microprocessor  23  writes the FOS focus offset value (value entered at the external unit) in the E 2 PROM  25  at a step  208 . 
     At steps  209  to  213 , a command indicating that a tracking offset TOS should be incremented or decremented is issued to the microprocessor  23  at the external unit, so that the wave at the point A will swing vertically symmetrically with respect to a reference voltage Vg. 
     In response to the command, the microprocessor  23  increments or decrements the tracking offset TOS and updates a value set in the D/A converter  43 . 
     When a tracking offset TOS causing the wave at the point A to swing vertically symmetrically with respect to the reference voltage Vg is detected, a command indicating that the TOS value should be written is issued to the microprocessor  23  at the external unit. 
     In response to the command, the microprocessor  23  writes the TOS tracking offset value in the E 2 PROM  25  at a step  211 . 
     When the tracking offset is zero, the tracking error signal becomes symmetrical opposite to a level corresponding to a center position of the track. Therefore, in the aforesaid procedure of detecting a tracking offset of the first embodiment, a tracking offset value causing a tracking error signal to swing symmetrically with respect to the reference voltage Vg is detected. FIG. 10 shows the processing for detecting the tracking offset in the second embodiment. 
     In this process, a duty measurement circuit, for example, shown in FIG. 5 is used. In this case, a tracking offset TOS causing a duty ratio of a signal resulting from the comparison between the signal at the point A and the reference voltage Vg to go to 50% is detected. 
     After the operation of the step  208  in FIG. 8, an operation for connecting a duty measurement circuit to the point A is performed at a step  221 . 
     Then, a duty ratio D is checked at a step  222 . If the duty ratio D is larger than 50%, the tracking offset TOS is decremented at a step  224 . If D is smaller than 50%, the tracking offset TOS is incremented to be 50% at a step  225 . 
     When D becomes equal to 50%, the offset value is detected and written in the E 2 PROM  25  at a step  223 . 
     The aforesaid processing is achieved by, for example, connecting one of the input terminals of a comparator (voltage comparator)  50  shown in FIG. 5 to the point A and the other one of the input terminals to the reference voltage Vg. 
     If the voltage at the point A is higher than the reference voltage Vg, an output of the comparator  50  is a high-level signal. If the voltage is lower, the output signal is low. 
     Assuming that the period in which the output of comparator  50  is high is N and the period in which the output thereof is low is M, when N equals M, the duty ratio is 50%. When N is smaller than M, the duty ratio is less than 50%. 
     Next, the third embodiment in which the focus offset FOS and tracking offset TOS are detected by an other method will be described with reference to FIGS. 11 and 12. 
     In this embodiment, a focus offset FOS or a tracking offset TOS causing an information regenerative signal RFS to have a maximum level is detected. 
     First, the power supply is turned on at a step  231 . A spindle motor  5  is rotated at a step  232 . A light source (LD)  12  is lit at a step  233 . 
     Thereafter, a servo focus and a tracking servo, are actuated at a step  234 . At a step  235 , an oscilloscope is used to observe an information regenerative signal RFS produced by an optical pickup. 
     Then, at steps  236  and  237 , a focus offset FOS is varied until the information regenerative signal RFS has a maximum level. When the information regenerative signal RFS has a maximum level, the FOS focus offset value is detected and written in an E 2 PROM  25  at a step  238 . 
     Next, a tracking offset TOS is varied at steps  239  to  241 . While observing the oscilloscope, the level of the information regenerative signal RFS is increased to a maximum. 
     When the information regenerative signal RFS has a maximum level, the TOS tracking offset value is detected and written in the E 2 PROM  25  at a step  242 . 
     As described above, a focus offset FOS and a tracking offset TOS are written in the E 2 PROM  25 . The process for detecting the tracking offset TOS shown in FIGS. 9,  10 , and  12  can be combined with either of the processes for detecting the focus offset FOS shown in FIGS. 8 and 11. 
     For instance, the process shown in FIG. 9 or  10  may be preceded by the process shown in FIG.  11 . The process shown in FIG. 12 may be preceded by the process shown in FIG.  8 . 
     Of these processes, the process for detecting a tracking offset TOS will be described in more detail with reference to FIGS. 13 and 14. 
     For example, in the process shown in FIG. 10, a duty measurement circuit shown in FIG. 5 is used to set a duty ratio of 50%. FIG. 13 shows a procedure of detecting an offset using the duty ratio. 
     In this procedure, a point A in a tracking servo control circuit  3  shown in FIG. 4 is connected to an input terminal of a comparator (voltage comparator)  50  shown in FIG.  5 . Then, the voltage at the point A is compared with a reference voltage Vg. Vth in FIG. 13 denotes a mean value of levels of a tracking error signal. 
     Output pulses resulting from the comparison of the wave at the point A with the reference voltage Vg are as shown in FIG.  13 . Assuming that the pulse width is t 1  and the time interval from a trailing edge of a pulse to a leading edge of the next pulse is t 2 , the duty ratio varies depending on a change in an offset (TOS). Consequently, the pulse width t 1  and time interval t 2  (corresponding to N and M in FIG. 5) varies. 
     When the duty ratio is 50% (no offset), t 1 =t 2 . When the duty ratio is less than 50%, t 1 &lt;t 2 . Based on these relationships, an offset (TOS) providing a duty ratio of 50% can be detected. 
     The process for detecting a tracking offset while observing a wave on an oscilloscope connected to a point A (for example, the process shown in FIG. 9) is based on the technique shown in FIG.  14 . 
     Specifically, the wave at the point A is compared with a reference voltage Vg. When the wave swings above from Vg, an area S 1  results. When the wave swings below from Vg, an area S 2  ensues. When S 1  equals to S 2  (the wave becomes vertically symmetric), an offset (TOS) is calculated. 
     Normal operation of an optical disk unit performed when a user uses the optical disk unit in which a focus offset FOS and a tracking offset TOS are placed in an E 2 PROM  25  as described above will be described with reference to FIG.  15 . 
     The subsequent description will be based on the flowchart of FIG.  15 . 
     First, the power supply is turned on at a step  251 . Then, a microprocessor  23  reads a focus offset FOS from an E 2 PROM  25  at a step  252 , and sets the value in a D/A converter  22  at a step  253 . 
     Next, the microprocessor  23  reads a tracking offset TOS from the E 2 PROM  25  at a step  254 , and sets the value in a D/A converter  43  at a step  255 . 
     Then, a spindle motor  5  is rotated at a step  256 , and a light source (laser diode LD)  12  is lit at a step  257 . A focus servo is actuated at a step  258 , and a tracking servo is actuated at a step  259 . Then, the optical disk unit gets ready to operate. 
     Next, the fourth embodiment in which the present invention is implemented in the gain control performed by a focus servo control  4  and a tracking servo control  3  will be described. 
     The fourth embodiment is also an optical disk unit, having the same construction as shown in FIGS. 1 to  3 . In the drawings referenced hereinafter, components identical to those in FIGS. 1 to  3  will bear the same numerals, and the description thereof will be omitted. 
     FIG. 16 shows examples of configurations of a servo control unit and a control unit in the fourth embodiment. FIG. 17 shows an example of a configuration of a variable gain amplifier. FIG. 18 is a flowchart of the process for adjustment at a factory. FIG. 19 is a flowchart of the process for normal operation. 
     In the drawings,  15 A and  44 A denote variable gain amplifiers, R 60 , R 61 , R 62 , R 64 , and R 68  denote resistors, S 1  to S 8  denote analog switches, and Rf denotes a feedback resistor. 
     FIG. 16 shows examples of configurations of a servo control unit  30  and a control unit  19  in the fourth embodiment. Differences from the configurations shown in FIG. 4 are that variable gain amplifiers  15 A and  44 A substitute for an addition circuit made up of amplifiers  15  and  44 , and their gains can be controlled by a control unit  19 , and that in an E 2 PROM  25  are placed the gain data FSG and TSG. 
     FIG. 17 shows an example of a configuration of a variable gain amplifier  15 A for a focus servo control  4  or a variable gain amplifier  44 A for a tracking servo control  3  shown in FIG.  16 . 
     The variable gain amplifier  15 A or  44 A comprises an operational amplifier OP 1 , a feedback resistor Rf, analog switches S 1  to S 8 , and resistors R 60 , R 61 , R 62 , R 64 , and R 68 . 
     The analog switches S 1  to S 8  are turned on or off with a signal sent from a microprocessor  23  (See FIG.  4 ). When the analog switches S 1  to S 8  are all off, only the resistor R 60  is connected to a negative input terminal of the operational amplifier. If any of the analog switches is turned on, a resistor connected in series with the analog switch which has been turned on is connected in parallel with the resistor R 60 . 
     Therefore, when the analog switches S 1  to S 8  are turned on or off with a signal from the microprocessor  23 , an input resistance of the operational amplifier Op 1  varies. Thus, the gain of the variable gain amplifier can be varied. 
     Next, the process for setting a gain of a servo control in the process of adjustment of an optical disk unit at a factory will be described. 
     When an optical disk unit is adjusted at a factory (during assembly of a product), an open-loop transfer function for a focus servo and a tracking servo is calculated to provide a total servo gain. Then, a focus servo gain (FSG) and a tracking servo gain (TSG), which cause the open-loop transfer function to provide a specified value of a total servo gain, are detected and written in an area of an E 2 PROM  25 . 
     The above process will be described in conjunction with FIG.  18 . 
     In this process, the power supply is turned on at a step  301  to supply power to an optical disk unit. A spindle motor  5  is rotated at a step  302 . A semiconductor laser (laser diode LD) serving as a light source  12  is lit at a step  303 . 
     Thereafter, a focus servo is actuated under the control of a focus servo control  4  at a step  304 . 
     Next, a command indicating that FSG should be incremented or decremented is issued to a microprocessor  23  shown in FIG. 16 at an external unit, so that the open-loop transfer function for the focus servo will provide a specified value. In response to the command, the microprocessor  23  increments or decrements FSG to modify the gain for a variable gain amplifier  15 A. 
     Thus, FSG is varied at steps  305  to  307 . The value provided by the open-loop transfer function is observed using a signal analyzer or any other measuring instrument. The above procedure is repeated until the open-loop transfer function provides a specified value. 
     When FSG causing the open-loop transfer function to provide a specified value is determined, a command indicating that the FSG value should be written in the E 2 PROM  25  will be issued to the microprocessor  23  at the external unit. 
     In response to the command, the microprocessor  23  writes the FSG value in the E 2 PROM  25  at a step  308 . 
     Next, a tracking servo is actuated under the control of a tracking servo control  3  at a step  309 , and a command indicating that TSG should be incremented or decremented is issued to the microprocessor  23  at the external unit, so that the open-loop transfer function defining the tracking servo under the control of a tracking servo control  3  will provide a specified value. 
     In response to the command, the microprocessor  23  increments or decrements TSG to vary the gain for a variable gain amplifier  44 A. 
     Thus, TSG is varied at steps  310  to  312 . The value provided by the open-loop transfer function is observed using a measuring instruction. The above procedure is repeated until the open-loop transfer function provides a specified value. 
     When a TSG value causing the open-loop transfer function to provide a specified value is determined, a command indicating that the TSG value should be written in the E 2 PROM  25  is issued to the microprocessor  23  at the external unit. 
     In response to the command, the microprocessor  23  writes the TSG data entered at the external unit in the E 2 PROM  25  at step  313 . 
     Normally (when an optical disk unit is operated at a user&#39;s location after being delivered from a factory), for example, when the power supply of the optical disk unit is turned on to set up the optical disk unit, the microprocessor  23  reads data from the E 2 PROM  25 , and sets gains for the gain amplifiers  15 A and  44 A in the focus servo control  4  and tracking servo control  3 , respectively. 
     The aforesaid process will be described in conjunction with the flowchart of FIG.  19 . 
     First, the power supply is turned on at step  321  to supply power to an optical disk unit. Thereafter, a microprocessor  23  in a control unit  19  reads FSG from an E 2 PROM  25  at step  322 , and sets the read FSG value in a variable gain amplifier  15 A in a focus servo control  4  at step  323 . 
     The microprocessor  23  reads the TSG value from the E 2 PROM  25  at step  324 , and sets the TSG value for a variable gain amplifier  44 A in a tracking servo control  3 . 
     Then, a spindle motor  5  is rotated at step  326 , and a semiconductor laser (laser diode) serving as a light source  12  is lit at step  327 . 
     Next, a focus servo is actuated at a step  328 , and a tracking servo is actuated at step  329 . Thus, the optical disk unit is prepared to operate. 
     Thereafter, data read or data write is executed normally during focus servo control and tracking servo control. 
     Next, other procedure for setting gains will be presented in the fifth embodiment. 
     FIGS. 20 and 21 show the fifth embodiment of the present invention. Numerals  45  denotes an A/D converter, and  46  denotes an amplitude detector. 
     FIG. 20 shows configurations of a servo control unit  30  and a control unit  19  in the fifth embodiment. 
     In this embodiment, the configurations of a focus servo control  4  and tracking servo control  3 , which are installed in the servo control unit  30 , are identical to those in the fourth embodiment shown in FIG.  16 . The configuration of the control unit  19  differs from that in the fourth embodiment. 
     Specifically, the control unit  19  includes not only a microprocessor  23  but also an A/D converter  45  and an amplitude detector  46 . 
     The amplitude detector  46  is connected to a point A linking up with an output terminal of a variable gain amplifier  44 A in the tracking servo control  3 . 
     Then, the amplitude (voltage value) of a tracking error signal TES at the output terminal of the variable gain amplifier  44 A is detected. The detected amplitude value is converted into a digital signal by the A/D converter  45 , and then applied to the microprocessor  23 . 
     The microprocessor  23  uses the amplitude data and a gain read from an E 2 PROM  25  to set a gain (TSG) in the variable gain amplifier  44 A. 
     The process for operating the optical disk unit in which gains are set as described above will be described with reference to FIG.  21 . 
     In the fifth embodiment, the procedure until FSG and TSG are written in the E 2 PROM  25  is identical to that in the fourth embodiment. 
     With FSG and TSG written in the E 2 PROM  25 , normal operation is carried out. Referring to the flowchart of FIG. 21, the process for normal operation in the fifth embodiment will be described. 
     First, the power supply is turned on to supply power to an optical disk unit at a step  331 . Thereafter, a microprocessor  23  in a control unit  19  reads FSG from an E 2 PROM  25  at a step  332 . Then, the microprocessor  23  determines a gain and sets the determined gain for a variable gain amplifier  15 A in a focus servo control  4 . 
     Next, a spindle motor  5  is rotated. A light source (laser diode LD)  12  is lit at a step  335 . A focus servo is actuated under the control of the focus servo control  4  at a step  336 . 
     Thereafter, the microprocessor  23  in the control unit  19  reads TSG from the E 2 PROM  25  at step  337 . Then, the microprocessor  23  multiplies a standard value of a tracking error signal by a deviation of the TSG value from a standard value at a step  338 , and recognizes the product as a target value. 
     Then, the microprocessor  23  varies the gain value set for a variable gain amplifier  44 A in a tracking servo control  3  at steps  339  and  340 , so that the amplitude of a signal at the point A linking up with the output terminal of the variable gain amplifier  44 A will be equal to the target value. 
     For example, the standard value of TSG is 1, the standard amplitude value of the tracking error signal TES (output signal of a differential amplifier  17 ) is 500 mV. 
     If a TSG value read from the E 2 PROM  25  is 1.3, the target value will be 500×1.3=650 (mV). 
     The variable gain amplifier  44 A is set up so that the amplitude of the signal at the point A will be 650 mV. 
     Thereafter, a tracking servo is actuated at a step  341 . Thus, the optical disk unit is prepared to operate. 
     The “standard value of TSG” and “standard amplitude value of a tracking error signal” are values that have been calculated in the process of designing the optical disk unit and are programmed in the control unit  19 . 
     The “deviation from the standard value of TSG” is calculated by computing a value read from the E 2 PROM and the “standard value of TSG.” In the above example (of numerical values), since the standard value of TSG is set to 1, the “deviation from the standard value of TSG” is also 1 or equal to the standard value of TSG. 
     Embodiments have been described so far. The present invention can be implemented in the following modes: 
     (1) A controller  27  in FIG. 2 may be independent of an optical disk unit and placed between a host  28  and an optical disk unit. 
     (2) An E 2 PROM  25  may be replaced with any other nonvolatile memory. 
     (3) The present invention can apply to an optical disk unit having a construction different from those of the aforesaid embodiments. 
     As described above, the present invention provides the following advantages: 
     (1) Variable resistors or other sliding parts need not be included in a circuit such as a focus servo control or a tracking servo control. 
     This results in stable focus servo control or tracking servo control irrelevant of an environmental change or a lapse of time. 
     (2) When an optical system is replaced, all that should be done is to write set values of gains specific to a new optical system in a nonvolatile memory. Thereafter, focus servo control or tracking servo control can be performed according to a specified one-loop transfer function all the time. 
     (3) Circuits for servo control (for example, an envelope detector or an A/D converter) are unnecessary. Therefore, the number of parts decreases by the number of the circuits. Eventually, downsizing and cost saving can be accomplished. 
     (4) During normal operation, servo control need not be done. This results in short operation wait time.