Classifications

• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
  • G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B21/00—Head arrangements not specific to the method of recording or reproducing
  • G11B21/02—Driving or moving of heads
  • G11B21/10—Track finding or aligning by moving the head ; Provisions for maintaining alignment of the head relative to the track during transducing operation, i.e. track following
  • G11B21/106—Track finding or aligning by moving the head ; Provisions for maintaining alignment of the head relative to the track during transducing operation, i.e. track following on disks
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
  • G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
  • G11B7/0901—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following only
  • G11B7/0903—Multi-beam tracking systems
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
  • G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
  • G11B7/0908—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for focusing only
  • G11B7/0909—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for focusing only by astigmatic methods
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
  • G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
  • G11B7/094—Methods and circuits for servo offset compensation
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
  • G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
  • G11B7/0945—Methods for initialising servos, start-up sequences

Definitions

• the invention relates to a method for compensating offset voltages in a focus control loop, by means of which a light beam from a light source is focused on a record carrier, and / or in a track control loop, by means of which the light beam is guided on the data tracks of the record carrier, the light beam from Recording medium is reflected on a photodetector with a plurality of photodiodes, from whose output voltages the difference in focus and / or tracking error signal is generated by difference formation.
• a light beam is focused on the record carrier by means of a focus control loop and guided on the data tracks of the record carrier by means of a track control loop.
• the optical scanning device of such devices as CD players, magneto-optical devices for playback and recording, recording and playback devices of DRAW discs or video disc players is equipped with a laser diode, a plurality of lenses, a prism beam splitter, a diffraction grating and a photodetector. Structure and function of an optical scanning device, a so-called optical pick-up, are in electronic components & applications, Vol. 6, No. 4, 1984 on pages 209-215.
• the light beam emitted by the laser diode is focused on the CD disk by means of lenses and reflected from there onto a photodetector.
• the data stored on the CD disk and the actual value for the focus and the tracking control loop are obtained from the output signal of the photodetector.
• the actual value for the focus control loop is referred to as focusing error, while the expression radial tracking error is selected for the actual value of the track control loop.
• a coil serves as the actuator for the focus control loop, via the magnetic field of which an objective lens can be moved along the optical axis.
• the focus control loop now ensures that the light beam emitted by the laser diode is always focused on the CD disk.
• the track control loop which is often also referred to as a radial drive, the optical scanning device can be displaced in the radial direction with respect to the CD disk. This allows the light beam to be guided on the spiral data tracks on the CD.
• the radial drive is made up of a so-called coarse and a so-called fine drive.
• the coarse drive is designed, for example, as a spindle, by means of which the entire optical scanning device comprising the laser diode, the lenses, the prism beam splitter, the diffraction grating and the photodetector can be moved radially.
• the fine drive the light beam can be tilted in the radial direction, for example, by a predeterminable small angle, so that the light beam can travel a small distance along a radius of the CD disk solely by this tilting movement.
• FIG. 1 shows the photodetector PD, the optical scanning device of a CD player, in which three laser beams L1, L2 and L3 are focused on the CD disk. Such a scanning device is called three-beam pick-up in the literature mentioned at the beginning.
• the middle light beam L1 is the main beam
• the two light beams L2 and L3 are the beams +1. and -1. Order that are generated from the main beam L1 by means of a diffraction grating.
• photodetector PD In the photodetector PD, four square-shaped photodiodes A, B, C and D are assembled so that they in turn form a square. With regard to this square formed from the four photodiodes A, B, C and D, two further square-shaped photodiodes E and F are located diagonally opposite one another.
• the two outer light beams L2 and L3, of which the front L2 strikes the photodiode E, the rear L3 strikes the photodiode F, generate the tracking error signal TE ES-FS.
• AS, BS, CS, DS, ES and FS denote the photo voltages of the photodiodes A, B, C, D, E and F.
• the central light beam L1 is circular when precisely focused on the large square formed by the photodiodes A, B, C and D, while at Defocusing takes elliptical shape.
• the focus control loop recognizes from the negative value of the focus error signal FE that the distance between the objective lens and the CD disk is too great. Therefore, the objective lens is moved by the actuator of the focus control loop towards the CD disk until the focus error signal FE becomes zero.
• the focus control loop recognizes from the positive value of the focus error signal FE that the objective lens is too close to the CD disk. The objective lens is therefore moved away from the CD disk by the actuator until the focus error signal al FE becomes zero.
• the tracking error signal TE has the value zero.
• FIG. 1b shows the case in which the light beams L1, L2 and L3 are shifted to the right of the track.
• the actuator of the tracking control loop now moves the optical scanning device to the left until the tracking error signal TE becomes zero.
• Now the actuator of the track control loop moves the optical scanning device to the right until the track error signal TE becomes zero.
• the control amplifier of the focus control loop also has an offset voltage, the size of which depends on the one hand on the temperature and on the other hand is subject to a long-term drift.
• the drift of the offset voltage and other parameters of an amplifier over time are caused by the aging of the amplifier.
• the focus error signal FE (AS + BS) - (BS + DS) is formed in a differential amplifier. Because this differential amplifier also has an offset voltage, and because the photodiodes A, B, C and D emit different voltages or currents than the ideal photodiodes with the same luminance, there is another source of disturbing offset voltages.
• the invention solves this problem by comparing the focus error and / or tracking error signal with a predefinable reference variable when the control loop is closed, and by supplying a compensation variable to the actuator of the focus and / or tracking control loop, which is changed until the focus error and / or tracking error signal coincides with the reference variable.
• a second solution to this problem provides that in a first process step with an open or switched-off control loop and uniform illumination of the photodiodes to the focus error and / or tracking error signal, a first compensation variable is added and changed until the sum agrees with a first reference variable that then, in a second method step with a closed control loop, the focus error and / or tracking error signal is compared with a second reference variable and that the actuator of the focus and / or tracking control loop is supplied with a second compensation variable and is changed until the focus and / or tracking error signal matches the reference size.
• a third solution to this problem is that in a first process step with the control circuit open or switched off and with uniform illumination of the photodiodes, the focus and / or tracking error signal is stored as a reference variable, that in a second subsequent process step with the control circuit closed, the actuator of the control circuit Compensation variable supplied and changed until the focus and / or tracking error signal matches the stored reference variable.
• Figure 2 shows a circuit arrangement for performing the method according to claim 1
• Figure 3 shows a circuit arrangement for performing the method according to claim 3
• Figure 4 shows a circuit arrangement for performing the method according to claim 4
• FIG 5 shows a circuit arrangement for carrying out the method according to claim 5.
• FIG. 1 there is a voltage + U at the interconnected cathodes of the photodiodes A, B, C and D.
• the interconnected anodes of photodiodes A and C are connected to the addition input, while the interconnected anodes of photodiodes B and D are connected to the subtraction input of a differential amplifier DV, the output of which is connected via a resistor R1 to the input of a control amplifier RV and via a further resistor R2 is connected to the non-inverting input of a comparator VL.
• the non-inverting input of the comparator VL is at a reference potential via a capacitance C1.
• a reference voltage UR is present at the inverting input of the comparator VL, the output of which is connected to the input E1 of a microprocessor MP.
• the output A1 of the microprocessor MP is connected to the input of a digital-to-analog converter DA1, the output of which is connected to the output of the control amplifier RV and the one connection of the actuator SG is.
• the other connection of the actuator SG which is designed as a coil, is at reference potential.
• the focus control loop is made up of ideal components which do not have any offset voltages.
• the main beam L1 forms a circle on the four photodiodes A, B, C and D, as shown in FIG. 1a. Because all four photodiodes A, B, C and D receive the same light energy and convert it into an electrical current, they emit the same output voltages or currents. Therefore, the voltage at the output of the differential amplifier DV is zero. Because the control amplifier RV is also assumed to be ideal, the voltage at its output and therefore also at one connection of the actuator SG is also zero. The actuator SG, often also called the actuator, therefore moves the objective of the optical scanning device until the voltage at the output of the control amplifier RV becomes zero. Assuming ideal components, the focus is then precise because the voltage at the output of the differential amplifier DV is also zero.
• control amplifier RV has an offset voltage
• differential amplifier DV and the photodiodes A, B, C and D are still to be considered ideal.
• the actuator SG will move the lens until the voltage at the output of the control amplifier RV becomes zero.
• the control amplifier RV which is now assumed to be real, however, the input voltage and thus also the voltage at the output of the differential amplifier differ from zero.
• the light beam L1 is therefore no longer circular, but how shown in Figure 1b or 1c slightly elliptical, which indicates that is not precisely focused.
• the voltage at the output of the differential amplifier DV is compared in the comparator VL with a reference voltage UR, which is chosen to be zero in the exemplary embodiment specified.
• the microprocessor MP now changes the digital values at its output A1, which the digital-to-analog converter DA1 converts into an analog voltage and feeds the actuator SG until the comparator VL at the input E1 of the microprocessor MP indicates that the voltage at Output of the differential amplifier DV has become zero. Because the light beam L1 is now imaged circularly by the lens onto the photodiodes A, B, C and D, the focus is precise.
• the microprocessor MP retains the value at its output A1, so that an analogue is constantly being transmitted due to the digital-to-analog converter DA1 Compensation voltage is present at actuator SG.
• the device e.g. a CD player is now ready to play. It is particularly advantageous to carry out the compensation each time the CD player is switched on.
• the offset voltage of the control amplifier RV changes, for example as a result of aging or temperature fluctuations, this has the consequence that the voltage at the output of the differential amplifier DV also changes and no longer corresponds to the reference voltage UR. Because the comparator VL indicates this to the microprocessor MP when the CD player is switched on, the microprocessor is able to readjust the offset compensation voltage and thus to ensure optimal compensation.
• a major advantage of the circuit arrangement shown in FIG. 2 is that the offset voltage of the control amplifier RV is automatically compensated each time the CD player is switched on.
• the circuit arrangement from FIG. 3 differs from the circuit arrangement from FIG. 2 in that it is supplemented by a digital-to-analog converter DA2, the output of which is connected to the output of the differential amplifier DV and the input of which is connected to an output A2 of the microprocessor MP.
• DA2 digital-to-analog converter
• the differential amplifier DV also has an offset voltage.
• the photodiodes A, B, C and D are also real, i.e. not considered completely identical components that emit different voltages or currents with the same lighting Therefore, in the case of focusing, if the lens images the light beam L1 in a circle as in FIG. 1a on the four photodiodes A, B, C and D, the voltage at the output of the differential amplifier DV does not become zero as desired, but a positive or negative Value, e.g. accept + a.
• the four photodiodes A, B, C and D are uniformly illuminated when the focus control loop is open or switched off. This state can be easily achieved by switching off the light source, because then the photodiodes A, B, C and D are in the dark. When the light source is switched off, it also does not matter where the lens is located and whether it is moving because no feedback can take place. In this state - with the light source switched off - the voltage at the output of the differential amplifier DV is compared with the reference voltage UR in comparison with the VL.
• the digital values which the microprocessor MP now outputs at its output A2 are converted into an analog voltage by the digital / analog converter DA2.
• the microprocessor MP now changes the digital values at its output A2 until the comparator VL indicates that the analog voltage at the output of the digital-analog converter DA2 has compensated for the voltage at the output of the differential amplifier DV.
• the digital value at this time at the output A2 of the microprocessor MP is retained. This measure ensures that the voltage at the input of the control amplifier RV is zero when the light beam L1 is imaged circularly onto the photodiodes A, B, C and D as shown in FIG.
• optical offset another previously neglected offset size, often referred to as optical offset, is due to the optics of the optical pickup. This means the following: If the light beam is precisely focused on the record carrier, the light beam does not become, as is the case with ideal, completely error-free optical components, due to the never-to-be-avoided optical defects of the optical components - the lenses, the prism beam splitter and the diffraction grating would be circular, but slightly elliptical on the photodetector with the four photodiodes A, B, C and D. When the light beam is precisely focused on the recording medium, the voltage at the output of the differential amplifier DV is therefore not zero, as desired, despite compensation for its offset voltage. Rather, it has a positive or negative value. How this offset voltage, which is caused by the optical offset, can also be compensated, is explained with the aid of the circuit arrangement shown in FIG.
• the reference voltage UR can be changed.
• an output A3 of the microprocessor MP is connected to the control input of the reference voltage source UR, which e.g. can be designed as a digital-to-analog converter.
• a fixed value is selected for the reference voltage UR.
• a third process step to compensate for the optical offset mentioned is carried out during the production of the CD player, which proceeds as follows.
• a CD test plate is inserted into the CD player.
• the digital values determined in the first and second method steps at the outputs A1 and A2 of the microprocessor MP are retained during the third method step and are not changed.
• the light beam is now focused precisely on the recording medium, the CD test plate.
• the exact focus is determined with the help of the CD test plate, because if the focus is precise, the jitter in the RF signal is lowest. However, the exact focusing can also be checked, for example, using a microscope. It is now determined by what value the reference voltage UR is to be changed, so that the optical offset is also compensated, because the offset voltage of the differential amplifier DV was already compensated in the first and that of the control amplifier RV in the second step.
• the reference voltage UR is therefore changed until the jitter in the RF signal assumes a minimum because the light beam is then focused precisely on the inserted test plate. The one on this The value of the reference voltage UR found in this way is fixed. The CD player is now ready for use.
• the offset voltage of the differential amplifier DV or the control amplifier RV changes in later game operation, they will e.g. compensated each time the device is switched on in accordance with the first and the second method step.
• a first process step as in the circuit arrangement from FIG. 3, uniform illumination of the photodiodes A, B, C and D is ensured by switching off the light source.
• the microprocessor MP changes the reference voltage UR until the comparator VL1 indicates to it that the reference voltage UR with the voltage at the output of the differential amplifier DV, which, for example + a may be the same.
• the first method step is changed in a manner similar to the circuit arrangement from FIG. 3.
• the focus control loop open or switched off, the light beam is precisely focused on the CD disk during production of the CD player and the exact focusing is checked with the aid of a microscope or by determining the jitter minimum in the HF signal using the CD test plate.
• the microprocessor MP changes the reference voltage UR until the comparator VL1 indicates that the ref limit voltage UR agrees with the voltage at the output of the differential amplifier DV, which may be + b, for example.
• the second method step takes place with the light source switched on.
• the actuator SG would hold the lens in a position in which the voltage at the output of the control amplifier RV becomes zero.
• the voltage at the input of the control amplifier RV will generally not be + a or + b as desired, as would be the case with precise focusing, but rather a positive or negative value different from zero depending on the size of the offset voltage of the control amplifier RV.
• the microprocessor MP changes the digital value at its output A1, which is converted by the digital-to-analog converter DA1 into an analog compensation voltage supplied to the actuator SG, until the comparator VL indicates to it that the voltage at the output of the differential amplifier DV corresponds to the reference voltage UR, which has the value + a or + b in the assumed numerical example.
• the digital value then at the output A1 of the microprocessor MP is retained, so that the correct analog compensation voltage is constantly applied to the actuator SG because of the digital-analog converter DA1.
• the CD player is now ready to play.

Landscapes

• Optical Recording Or Reproduction (AREA)
• Amplifiers (AREA)
• Networks Using Active Elements (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=6358758

Family Applications (2)

Family Applications Before (1)

Country Status (14)

Families Citing this family (19)

Family Cites Families (7)

• 1988
  • 1988-07-15 DE DE3824039A patent/DE3824039A1/de not_active Withdrawn
• 1989
  • 1989-07-13 DE DE8989112884T patent/DE58902327D1/de not_active Expired - Lifetime
  • 1989-07-13 WO PCT/EP1989/000816 patent/WO1990000797A1/de not_active Application Discontinuation
  • 1989-07-13 AT AT89112884T patent/ATE80960T1/de not_active IP Right Cessation
  • 1989-07-13 KR KR1019900700555A patent/KR0145299B1/ko not_active IP Right Cessation
  • 1989-07-13 EP EP89112884A patent/EP0350939B1/de not_active Expired - Lifetime
  • 1989-07-13 HU HU894721A patent/HUT53467A/hu unknown
  • 1989-07-13 EP EP89907726A patent/EP0378650A1/de active Pending
  • 1989-07-13 JP JP1507484A patent/JP2726131B2/ja not_active Expired - Lifetime
  • 1989-07-13 ES ES198989112884T patent/ES2035459T3/es not_active Expired - Lifetime
  • 1989-07-15 MY MYPI89000996A patent/MY104135A/en unknown
  • 1989-07-15 CN CN89104824A patent/CN1020007C/zh not_active Expired - Lifetime
• 1990
  • 1990-03-15 DK DK067990A patent/DK67990A/da not_active Application Discontinuation
• 1991
  • 1991-09-30 US US07/769,331 patent/US5148423A/en not_active Expired - Lifetime
• 1992
  • 1992-10-16 GR GR920402337T patent/GR3006012T3/el unknown
• 1997
  • 1997-01-23 HK HK8997A patent/HK8997A/xx not_active IP Right Cessation

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events