WO2008035257A1

WO2008035257A1 - A system and method for determining the position of a plurality of laser spots on an optical disc

Info

Publication number: WO2008035257A1
Application number: PCT/IB2007/053705
Authority: WO
Inventors: Dayu Chen; Nicolas Baumgartner
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-09-20
Filing date: 2007-09-14
Publication date: 2008-03-27
Also published as: TW200832388A

Abstract

A system for determining an angular measure (AM) of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc, said system comprising: -a detecting unit for detecting light signals derived from said plurality of spots, said detecting unit comprising a set of detectors aligned according to a same direction D, each of said detectors being intended to receive one light signal derived from one spot of said plurality of spots, each of said detectors having a first light-sensitive area and a second light-sensitive area positioned on both sides of said direction D; a first output signal (OS1A; OS1B) being derived from said first light-sensitive area and a second output signal (OS2A; OS2B) being derived from said second light- sensitive area and -a generating unit (320) comprising: a) a first module (320) for generating, for each of said detectors, a set of first offset signals by calculating, the difference between said first output signal (OS1A;OS1B) and said second output signal (OS2A;OS2B); b) a second module for generating a set of second offset signals by calculating the difference of each pair of first offset signals; and c) a third module for determining said angular measure (AM) according to said set of second offset signals.

Description

A SYSTEM AND METHOD FOR DETERMINING THE POSITION OF A PLURALITY OF LASER SPOTS ON AN OPTICAL DISC

FIELD OF THE INVENTION

The present invention relates to a system and a method for determining the position of a plurality of laser spots on an optical disc, and more particularly, to a system and a method for determining an angle measure of a plurality of laser spots on an optical disc.

BACKGROUND OF THE INVENTION

In an optical storage drive the position of the spots on the disc has to be controlled accurately. For a single spot drive, the radial actuator of the objective lens is in charge of this control. In case of multi-spots, both radial and rotational movements must be controlled. Therefore, an additional so-called angular actuator is needed, for rotating the laser-spots on the optical storage disc. The angular actuator is needed not only to guarantee that all spots are on track but also to have the possibility for advanced jump strategies (i.e. various displacements of the spots on the optical disc).

There is a method where angle error signal is generated from the difference of the radial Push Pull error signal from two spots (Push Pull method). The control scheme includes at least a module that converts a plurality of radial tracking signals into a radial error signal and a rotation error signal, and a controller module that converts the radial error signal and the rotation error signal into a radial actuator input and a rotational actuator input. The rotational error signal can also be called angle error signal. So a radial actuator will take the radial actuator input and control the radial movement of the two spots through a servo loop, and an angle actuator will take the angle error signal to control the angle movement of the two spots through a servo loop. As a result, the two spots can be kept on track. The Push Pull method can also be applied to the situation where more than two spots have been involved.

It may happen that a plurality of spots have to be displaced along a relative long distance on an optical disc, such as a jump movement when looking for a data storied on the disc. During this kind of movement, spots travel over multiple tracks, the real angle position of the spots cannot be indicated by using the Push Pull method. Indeed, the Push Pull method only allows to generate the angle error when the spots deviate a little to the track, it can not normally work due to its periodical characteristic after a relatively long distance jump. For example, in case the spots are happened on track (such as the situation of A, B or C in fig 1, showing a set of 3 spots successively positioned at different angles compared to tracks of the optical disc), the output signals by using the Push Pull method are same, meaning that they cannot indicate the angel difference of the 3 spots with respect to the tangent of the disc track. Therefore, the angel information is lost after a jump.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system and method for determining the angle measure of a plurality of spots with respect to an optical disc.

To this end, it is proposed a system for determining an angular measure of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc, said system comprising:

-a detecting unit for detecting light signals derived from said plurality of spots, said detecting unit comprising a set of detectors aligned according to a same direction D, each of said detectors being intended to receive one light signal derived from one spot of said plurality of spots, each of said detectors having a first light-sensitive area and a second light-sensitive area positioned on both sides of said direction D; a first output signal being derived from said first light-sensitive area and a second output signal being derived from said second light-sensitive area and

-a generating unit comprising:

a) a first module for generating, for each of said detectors, a set of first offset signals by calculating, the difference between said first output signal and said second output signal;

b) a second module for generating a set of second offset signals by calculating the difference of each pair of first offset signals; and c) a third module for determining said angular measure according to said set of second offset signals.

It is also proposed a method comprising steps having functionalities defined by features of the system according to the invention.

By using the fixed relation between the second offset signal and the angular measure, the angular measure is determined.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l shows a set of three spots successively positioned at different angles compared to tracks of the optical disc.

Fig.2 illustrates a diagram of three spots on the different plane and the output signals obtained thereto according to one embodiment of the invention.

Fig.3 illustrates a system according to one embodiment of the invention for determining an angular measure of a plurality of spots.

Fig.4 illustrates the relation between tangent error signals and angles of a plurality of spots.

Fig.5 illustrates diagrams of multiple spots on the detector plane.

In the above drawings, the same reference numeral indicates the same, similar or corresponding element or function.

DETAILED DESCRIPTION OF THE INVENTION

The technical measures of the present invention will be described in detail hereinafter by way of embodiments with reference to the drawings. Fig.2 illustrates a diagram of three spots on the different plane and the output signals obtained thereto according to one embodiment of the invention.

The first column shows three spots (A, B, C) on the disc with different angle to the tracks.

It is indicated in the first column the laser spots are landed on the corresponding tracks on the disc information plane. After the laser get reflected from the disc surface each beam is guided by the light path to their corresponding photo detector which will transform the intensity signal from the laser beam to an electrical signal.

The second column shows these three spots (A, B, C) in the detector plan with the same angle as on the disc. Each detector is divided further into two quadrants. The laser beam landed on the detector is shown in column two.

The electrical signal from the detector quadrant is proportional to the beam intensity and the area of the beam landed on such a quadrant. Therefore the signal changes with the position of the laser beam landing position.

The different angle of the beams will thus have different angle measure signal as indicated in column three.

The angle of the spots with respect to the tangential direction of the tracks can be obtained by subtracting the tangential signals of different spots. The arrows show the centre of rotation of the spot array. The third column shows the corresponding offset signal of spot - 1 subtracted from the offset signal of spot +1. One method to obtain the offset signal of each spot is to subtract the signal of the left part detector from the signal of the right part detector.

The angular measure AM, for example, may be measured between a line L interconnecting said plurality spots and the tangent T of the track of said optical disc, as shown in Fig.2 (α;β). Fig.3 illustrates a system 300 according to one embodiment of the invention for determining an angular measure AM of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc.

For sake of understanding, the invention is described with 3 spots only, but similar explanations would apply to a higher number of spots or only 2 spots.

The invention provides a system which comprises a detecting unit 310 for detecting (also corresponding to a step of detecting) light signals derived from said plurality of spots, said detecting unit 310 comprising a set of detectors A, B, C aligned according to a same direction D, each of said detectors A, B, C being intended to receive one light signal derived from one spot of said plurality of spots, each of said detectors having a first light- sensitive area (left part of each detector as shown by Fig.3) and a second light-sensitive area (right part of each detector as shown by Fig.3) positioned on both sides of said direction D; a first output signal such as OS IA and OS IB being derived from said first light-sensitive area and a second output signal such as OS2A and OS2B being derived from said second light-sensitive area.

Said set of detectors A, B, C may also be conventional four-quadrant detectors. In this situation, each said first and second light-sensitive area contains two quadrants. The signals from 2 quadrants are added to form said first output signal or said second output signal.

Said set of detectors A, B, C are aligned with a same direction D, and said direction D corresponds to the direction of the tangent of the tracks on the optical disc. Said direction does not necessarily cross the central point of every detector, but it preferably crosses the central of gravity of said set of detectors.

Said system 300 also comprises a generating unit 320, said generating unit 320 comprises a first module 321 for generating (also corresponding to a step of generating), for each of said detectors, a set of first offset signals by calculating, the difference between said first output signal and said second output signal. Said first module 321 may, for example, comprises a set of subtracters SA and SB. SA takes the input OS IA and OS2A from detector A and calculates the difference (OS2A-OS 1A) as the first offset signal; similarly, SB takes the input OS IB and OS2B from detector B and calculates the difference (0S2B-0S1B ) as the first offset signal.

Said generating unit 320 also comprises a second module 322 for generating (also corresponding to a step of generating) a set of second offset signals by calculating the difference of each pair of first offset signals. Said second module 322 may, for example, comprise a set of subtracters and switches. In Fig.3, only one subtracter S and two switches around S are shown. Subtracter S may take a pair of first signal such as the first offset signal from SA and the first offset signal from SB, and calculate the difference (OS2A- OSIA)- (OS2B-OS 1B ) as the second offset signal. In this case, the two switches SWA and SWB switch to take inputs from SA and SB Similarly, other subtracters in said second module might generate different second offset signals.

The second offset signal is also called tangent error signals in this invention. Fig.4 illustrates the relation between tangent error signals TE and angles AO of a plurality of spots.

The angle is between a line interconnecting said plurality spots and the tangent of the track of said optical disc. Around angle zero the curve is considered as being linear. When the angle increased there will be a saturation area. When the angle further increased the spot is landed out of the detector therefore the signal goes back to zero. The working area to have the right tangential error signal is the linear area.

Therefore, the second offset signal corresponds to the tangent displacement of a spot, because the distance between spots is fixed, so the angular measure may be determined.

Said generating unit 320 also comprises a third module 323 for determining (also corresponding to a step of determining) said angular measure AM according to said set of second offset signals. By using the fixed relation between said second offset signal and said angular measure AM, said angular measure AM is determined. One method of determining said angular measure (AM) is by calculating (also corresponding to a step of calculating) the average of said set of second offset signals. It is possible that there are more than one second offset signal generated, in this case the third module 323 is intend to calculate the average of said set of second offset signal in order to get more accurate AM.

If one spot is writing while the other spot is reading, or when one spot is landed on a written part of the disc while the other spot is landed on the blank part of the disc (different reflectivity), the large difference of the reflected light requires a normalization of the error signal. Considering this factor, said generating unit 320 further comprising a normalizing module 324 for normalizing (also corresponding to a step of calculating) said set of first offset signals so as to overcome the different reflectivity of said set of first offset signals.

For example, as illustrated by Fig.3, the normalizing module 324 may comprises a set of adder such as AA and AB and a set of divider such as DA and DB. OS IA and OS2A are fed into AA which then output the sum: OS 1A+OS2A. Divider DA takes OS 1A+OS2A and OS2A -OS IA as the inputs and calculates the output (OS2A -OSIA)/ (OS 1A+OS2A). Because signal OS 1A+OS2A reflects the integral light intensity of detector A, the difference in the light reflectivity is compensated. In this case, SWA switches to take inputs from DA and SWB switches to take input from DB

There is a situation that the angular measure (AM) may be determined only by using two detectors: a first detector A and a second detector B. In this case, said generating unit generates said angular measure (AM) according to the formula:

AM , K representing a parameter, OSl₁

representing a first output signal generated by the first light-sensitive area of said first detector (A), OS 2 _A representing a second output signal generated by the second light- sensitive area of said first detector (A), OS\_B representing a first output signal generated by the first light-sensitive area of said second detector (B), OS2_B representing a second output signal generated by the second- sensitive area of said second detector (B).

Based on the formula above of AM, the first module 321 comprises a first subtracter SA and a second subtracter SB. SA and SB generate first offset signals (OS2A-OS1A) and (OS2B- OSIB ) respectively.

It should be noticed that the first offset signals (OS2A-OS1A) and (0S2B-0S 1B ) could be directly input to the second module 322, which comprises a subtracter S. Subtracter S then generates a second offset signal (OS2A-OS 1A) -(0S2B-0S 1B ), which is proportional to AM. In this case, SWA switches to take inputs from DA and SWB switches to take input from DB

The processing module 324 is used to normalize the first offset signals (0S2_A - OSl _A ) and (0S2B-0S 1B ). The processing module 324 may comprise a first divider DA, a first adder AA, a second divider DB and a second adder AB. AA generates the output signal ( 0S2_A + OSl _A ), which is fed into DA AB generates the output signal (0S2B+0S 1B), which is fed into DB DA and DB generate output signals ( 0S2_A - 0Sl_A )/( 0S2_A + OSl_A ) and (0S2B-0S 1B )/ (0S2B+0S 1B) respectively.

The second module 322 may comprise a subtracter S and two switches SWA and SWB. Switches SWA and SWB switch to take the inputs from DA and DB respectively. Then

subtracter S generates a second offset signal which

is proportional to said angular measure (AM). The second offset signal is sent to the third module

323, finally an angular measure (AM) is generated according to the following formula:

Fig.5 illustrates diagrams of multiple spots on the detector plane.

This invention also applies to the situation where the number of spots used for angle measurement is more than 2. Suppose there are totally n+m+1 detectors (n, m being integrator numbers greater than 0). There is a random selected base detector and there are n detectors in the one side of the base detector (n being an integrate number greater than 0) and there are m detectors on the other side of the base detector (m being an integrate number greater than 0). The formula giving the angle measurement is as follows:

In this formula, d_t and d_} represent the distance from i detector and j detector to the base detector respectively; C₁ and c_} are parameters; L₁ represents an output signal generated by the first light-sensitive area of the i detector and R₁ represents an output signal generated by the second light-sensitive area of the j detector, as shown by Fig.5. In Fig.5, X₁ reflects the distance between the central of a spot on the detector and the central line of

R - T the detector. X₁ = C₁ • — '-, ΔXi = X₁ - x₀ .

R + L

This invention also provides a system where said first detector A and said second detector B are positioned to receive light signals from spots situated at the extremity of said plurality of spots. In this situation, the obtained angular measure is more accurate, compared to the situation that said detector A and said detector B is relatively closer. Because around angle zero the curve is considered as linear, and the working area to have the right tangential error signal is the linear area, the system may comprises a unit (not shown) for limiting (also corresponding to a step of limiting) said angular measure in a predetermined range. The predetermined range is preferably between -10° and 10°.

A player/recorder for playing/recording an optical disc may comprise a system as described above. For example said player/ recorder may be a blue disk player/recorder.

A computer may also comprise a system as described above.

The present invention can also be implemented by means of a suitably programmed computer program product for determining an angle measure (AM) of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc, said computer program product comprising instruction codes for carrying out the steps as described above.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" or "comprises" does not exclude other elements or steps, and the indefinite article "a' or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A system (300) for determining an angular measure (AM) of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc, said system comprising:

-a detecting unit (310) for detecting light signals derived from said plurality of spots, said detecting unit (310) comprising a set of detectors aligned according to a same direction (D), each of said detectors being intended to receive one light signal derived from one spot of said plurality of spots, each of said detectors having a first light-sensitive area and a second light-sensitive area positioned on both sides of said direction (D); a first output signal (OS IA; OS IB) being derived from said first light-sensitive area and a second output signal (OS2A; OS2B) being derived from said second light-sensitive area and

-a generating unit (320) comprising: a) a first module (321) for generating, for each of said detectors, a set of first offset signals by calculating, the difference between said first output signal (OS1A;OS 1B) and said second output signal (OS2A;OS2B); b) a second module (322) for generating a set of second offset signals by calculating the difference of each pair of first offset signals; and c) a third module (323) for determining said angular measure (AM) according to said set of second offset signals.

2. A system (300) as claimed in claiml, said generating unit (320) further comprising a processing module (324) for normalizing said set of first offset signals.

3. A system (300) as claimed in claiml or claim 2, wherein said third module (323) is also in charge of calculating the average of said set of second offset signals for determining said angular measure (AM)".

4. A system (300) as claimed in claiml, wherein said set of detectors comprises a first detector (A) and a second detector (B), wherein said first detector (A) and said second detector (B) are positioned to receive light signals from spots situated at the extremity of said plurality of spots.

5. A system (300) as claimed in claim 1, further comprising a unit for limiting said angular measure (AM) in a predetermined range.

6. A system (300) as claimed in claim 5, wherein said predetermined range is between [-10°; 10°].

7. A method for determining an angle measure (AM) of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc, said method comprising the steps of:

-detecting light signals derived from said plurality of spots by a detecting unit, said detecting unit comprising a set of detectors aligned with a same direction, each of said set of detectors being intended to receive one light signal derived from one spot of said plurality of spots, each of said set of detectors having a first light- sensitive area and a second light-sensitive area positioned on both sides of said direction (D); a first output signal (OS1A;OS1B) being derived from said first light-sensitive area and a second output signal (OS2A;OS2B) being derived from said second light-sensitive area; and

-generating a set of first offset signals by calculating, to each of said detectors, the difference between said first output signal (OSl) and said second output signal (OS2);

-generating a set of second offset signals by calculating the difference of each pair of first signals; and

- determining said angular measure (AM) according to said set of second offset signals.

8. A method as claimed in claim7, said method further comprising a step for normalizing said set of first offset signals.

9. A method as claimed in claim7 or claim 8, said step of determining is done by calculating the average of said set of second offset signals.

10. A method as claimed in claim 7, further comprising a step of limiting said angular measure in a predetermined range.

11. A method as claimed in claim 10, wherein said predetermined range is between [- 10°; 10°].

12. A player/recorder for playing/recording an optical disc, said player/recorder comprising a system as claimed in claim 1, 2, 3, 4, 5 or 6.

13. A computer comprising a system as claimed in claim 1, 2, 3, 4, 5 or 6.

14. A computer program product for determining an angle measure (AM) of a plurality of spots intended to be applied to an optical disc, with respect to said optical disc, said computer program product comprising instruction codes for carrying out the steps of the method as claimed in claim 7, 8, 9,10 or 11.