EP1671320A2

EP1671320A2 - Disc drive apparatus

Info

Publication number: EP1671320A2
Application number: EP04770107A
Authority: EP
Inventors: Marcel F. Heertjes; Franciscus L. M. Cremers; Frank B. Sperling; George A. L. Leenknegt; Horst Rumpf
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2003-10-02
Filing date: 2004-09-28
Publication date: 2006-06-21
Also published as: JP2007507822A; KR20060088890A; WO2005034103A3; WO2005034103A2; TW200516572A; CN1860533A

Abstract

A method is described for controlling an optical disc drive apparatus (1) of a type comprising: scanning means (30) for scanning a record track of an optical disc (2) and for generating a read signal (SR); actuator means (50) for controlling the positioning of at least one read/write element (34) of said scanning means (30) with respect to the disc (2); a control circuit (90) for receiving said read signal (SR) and generating at least one actuator control signal (SCR) on the basis of at least one signal component of said read signal (SR), the control circuit (90) having at least one variable control parameter. The method comprises the steps of: deriving from said read signal (SR) at least two signals (REn; MIRn) of mutually different type; and varying the setting of said at least one variable control parameter as a function of said at least two signals. An other method is described for discriminating two kinds of disturbances originated by a) disc defects, b) schocks.

Description

Disc drive apparatus

FIELD OF THE INVENTION The present invention relates in general to an optical disc drive apparatus for writing/reading information into/from an optical storage disc.

BACKGROUND OF THE INVENTION As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can only be read by a user. The optical storage disc may also be a writeable type, where infonnation may be stored by a user. For writing information in the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand optical means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam. Since the technology of optical discs in general, the way in which information can be stored in an optical disc, and the way in which optical data can be read from an optical disc, is commonly known, it is not necessary here to describe this technology in more detail. For rotating the optical disc, an optical disc drive typically comprises a motor, which drives a hub engaging a central portion of the optical disc. Usually, the motor is implemented as a spindle motor, and the motor-driven hub may be arranged directly on the spindle axle of the motor. For optically scanning the rotating disc, an optical disc drive comprises a light beam generator device (typically a laser diode), an objective lens for focussing the light beam in a focal spot on the disc, and an optical detector for receiving the reflected light reflected from the disc and for generating an electrical detector output signal. The optical detector comprises multiple detector segments, each segment providing an individual segment output signal. During operation, the light beam should remain focussed on the disc. To this end, the objective lens is arranged axially displaceable, and the optical disc drive comprises focal actuator means for controlling the axial position of the objective lens. Further, the focal spot should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, at least the objective lens is mounted radially displaceable, and the optical disc drive comprises radial actuator means for controlling the radial position of the objective lens. In many disc drives, the objective lens is arranged tiltably, and such optical disc drive comprises tilt actuator means for controlling the tilt angle of the objective lens. For controlling these actuators, the optical disc drive comprises a controller, which receives an output signal from the optical detector. From this signal, hereinafter also referred to as read signal, the controller derives one or more error signals, such as for instance a focus error signal, a radial error signal, and, on the basis of these error signals, the controller generates actuator control signals for controlling the actuators such as to reduce or eliminate position errors. In the process of generating actuator control signals, the controller shows a certain control characteristic. Such control characteristic is a feature of the controller, which may be described as the way in which the controller behaves as reaction to detecting position errors. Position errors may, in practice, be caused by different types of disturbances.

The two most important classes of disturbances are:

1) disc defects

2) external shocks and (periodic) vibration The first category comprises internal disc defects like black dots, pollution like fingerprints, damage like scratches, etc. The second category comprises shocks caused by an object colliding to the disc drive, but shocks and vibrations are mainly to be expected in portable disc drives and automobile applications. Apart from the difference in origin, an important distinction between disc defects on the one hand and shocks and vibration on the other hand is the frequency range of signal disturbances: signal disturbances caused by disc defects are typically high-frequency, while shocks and vibrations are typically low- frequency. A problem in this respect is that adequately handling disturbances of the first category requires a different control characteristic than adequately handling disturbances of the second category. Conventionally, the controller of a disc drive has a fixed control characteristic, which is either specifically adapted for adequately handling disturbances of the first category (in which case error control is not optimal in the case of disturbances of the second category) or specifically adapted for adequately handling disturbances of the second category (in which case error control is not optimal in the case of disturbances of the first category), or the control characteristic is a compromise (in which case error control is not optimal in the case of disturbances of the first category as well as in the case of disturbances of the second category). As long as a controller applies linear control technique, there is always a compromise between low-frequency disturbance rejection and high-frequency sensitivity to measuring noise. In the state of the art, it has already been proposed to change the gain of the controller, depending on the type of disturbance experienced. For instance, reference is made to US patent 4.722.079. In order to be able to implement a controller having variable gain, it is necessary to determine which class of disturbance is at hand. Said US patent 4.722.079 describes a system where an optical read signal is processed to determine disturbance class, but this system requires a 3 -beam optical system. US patent 5.867.461 also describes a system where an optical read signal is processed to determine disturbance class. In this known system, the envelope is determined ' of the high frequency signal contents. One disadvantage of this method is that it relies on data written on the disc; it is not applicable in the case of blank discs. Another disadvantage is that this method requires complicated circuitry, inter alia for detecting upper peaks and lower peaks, for filtering in order to detect upper envelope and lower envelope, for analysing these envelopes, and for storing signals in a memory. A general problem relates to adapting the control characteristics in a disc drive which should be capable of handling mechanical shocks as well as disc defects. Changing the control characteristics such that one type of disturbance is handled better may seriously affect the controller's capability to handle a disturbance of another type. Further, the present inventors have found that a single detection signal is not always a reliable source of information for discriminating between mechanical shocks and disc defects. In other words, a certain signal may be generated in the case of a mechanical shock as well as in the case of a disc defect, so that an event may erroneously be determined as a disc defect or as a mechanical shock. A general objective of the present invention is to provide a method for determining more reliably whether an event corresponds to the occurrence of a mechanical shock or a disc defect. Further, it is an objective of the present invention to provide a method for determining disturbance class which can be implemented relatively easily. Further, it is an objective of the present invention to provide a method for changing control characteristics of the controller on the basis of the results of the above- mentioned determinations.

SUMMARY OF THE INVENTION According to a first important aspect of the present invention, a method for discriminating different types of disturbances in a disc drive apparatus is based on receiving and processing at least two types of signals, each derived from the detector signal. A signal of first type is a signal which is based on summation of two or more individual segment output signals; a preferred signal of first type is generated by generating a summation, possibly a weighed summation, of the individual segment output signals of all detector segments. Such signal of type 1 is a measure for the reflectivity of the disc (i.e. "disc quality"), and will mainly be sensitive to disc defects. A signal of second type is a signal which is based on summation of two or more individual segment output signals, wherein at least one of those signals is multiplied by -1. Such signal of type 2 will merely indicate the occurrence of a position error. Signals of first type may give a certain response, indicating a disc defect or a shock; likewise, signals of second type may give a certain response in the case of a disc defect or a shock. According to a second important aspect of the present invention, a disc defect or a shock is only determined if both signals of first and second type are in agreement with each other, so the chance on erroneous judgement is reduced. It is noted that a distinction between shocks in vertical direction (i.e. parallel to the disc rotation axis) and shocks in horizontal direction (i.e. perpendicular to the disc rotation axis) can be made by taking into account the focus error signal or the radial error signal, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: Figure 1A schematically illustrates relevant components of an optical disc drive apparatus; Figure IB schematically illustrates an embodiment of an optical detector in more detail; Figure 2 is a block diagram schematically illustrating an actuator control circuit; Figure 3 is a graph schematically illustrating a function for varying a parameter of the actuator control circuit; Figure 4 is a timing diagram of signals for illustrating the operation of the present invention.

DESCRIPTION OF THE INVENTION Figure 1A schematically illustrates an optical disc drive apparatus 1, suitable for storing information on or reading information from an optical disc 2, typically a DVD or a CD. For rotating the disc 2, the disc drive apparatus 1 comprises a motor 4 fixed to a frame (not shown for sake of simplicity), defining a rotation axis 5. The disc drive apparatus 1 further comprises an optical system 30 for scanning tracks (not shown) of the disc 2 by an optical beam. More specifically, in the exemplary arrangement illustrated in figure 1A, the optical system 30 comprises a light beam generating means 31, typically a laser such as a laser diode, arranged to generate a light beam 32. In the following, different sections of the light beam 32, following an optical path 39, will be indicated by a character a, b, c, etc added to the reference numeral 32. The light beam 32 passes a beam splitter 33, a collimator lens 37 and an objective lens 34 to reach (beam 32b) the disc 2. The objective lens 34 is designed to focus the light beam 32b in a focal spot F on a recording layer (not shown for sake of simplicity) of the disc. The light beam 32b reflects from the disc 2 (reflected light beam 32c) and passes the objective lens 34, the collimator lens 37, and the beam splitter 33, to reach (beam 32d) an optical detector 35. In the case illustrated, an optical element 38 such as for instance a prism is interposed between the beam splitter 33 and the optical detector 35. The disc drive apparatus 1 further comprises an actuator system 50, which comprises a radial actuator 51 for radially displacing the objective lens 34 with respect to the disc 2. Since radial actuators are known per se, while the present invention does not relate to the design and functioning of such radial actuator, it is not necessary here to discuss the design and functioning of a radial actuator in great detail. For achieving and maintaining a correct focusing, exactly on the desired location of the disc 2, said objective lens 34 is mounted axially displaceable, while further the actuator system 50 also comprises a focal actuator 52 arranged for axially displacing the objective lens 34 with respect to the disc 2. Since focal actuators are known per se, while further the design and operation of such focal actuator is no subject of the present invention, it is not necessary here to discuss the design and operation of such focal actuator in great detail. For achieving and maintaining a correct tilt position of the objective lens 34, the objective lens 34 may be mounted pivotably; in such case, as shown, the actuator system 50 also comprises a tilt actuator 53 arranged for pivoting the objective lens 34 with respect to the disc 2. Since tilt actuators are known per se, while further the design and operation of such tilt actuator is no subject of the present invention, it is not necessary here to discuss the design and operation of such tilt actuator in great detail. It is further noted that means for supporting the objective lens with respect to an apparatus frame, and means for axially and radially displacing the objective lens, as well as means for pivoting the objective lens, are generally known per se. Since the design and operation of such supporting and displacing means are no subject of the present invention, it is not necessary here to discuss their design and operation in great detail. It is further noted that the radial actuator 51, the focal actuator 52 and the tilt actuator 53 may be implemented as one integrated actuator. The disc drive apparatus 1 further comprises a control circuit 90 having a first output 92 connected to a control input of the motor 4, having a second output 93 coupled to a control input of the radial actuator 51, having a third output 94 coupled to a control input of the focal actuator 52, and having a fourth output 95 coupled to a control input of the tilt actuator 53. The control circuit 90 is designed to generate at its first output 92 a control signal SCM f°^r controlling the motor 4, to generate at its second control output 93 a control signal SCR for controlling the radial actuator 51, to generate at its third output 94 a control signal SCF f°^r controlling the focal actuator 52, and to generate at its fourth output 95 a control signal SCT for controlling the tilt actuator 53. The control circuit 90 further has a read signal input 91 for receiving a read signal SR from the optical detector 35. Figure IB illustrates that the optical detector 35 may comprise a plurality of detector segments. In the case illustrated in figure IB, the optical detector 35 comprises six detector segments 35a, 35b, 35c, 35d, 35e, 35f, capable of providing individual detector signals A, B, C, D, SI, S2, respectively, indicating the amount of light incident on each of the six detector segments, respectively. Four detector segments 35a, 35b, 35c, 35d, also indicated as central aperture detector segments, are arranged in a four-quadrant configuration. A centre line 36, separating the first and fourth segments 35a and 35d from the second and third segments 35b and 35c, has a direction corresponding to the track direction. Two detector segments 35e, 35f, also indicated as satellite detector segments, and which may themselves be subdivided into subsegments, are arranged symmetrically besides the central detector quadrant, on opposite sides of said centre line 36. Since such six-segment detector is commonly known per se, it is not necessary here to give a more detailed description of its design and functioning. It is noted that different designs for the optical detector 35 are also possible. For instance, the satellite segments may be omitted, as known per se. Figure IB also illustrates that the read signal input 91 of the control circuit 90 actually comprises a plurality of inputs for receiving all individual detector signals. Thus, in the illustrated case of a six-quadrant detector, the read signal input 91 of the control circuit 90 actually comprises six inputs 91a, 91b, 91c, 9 Id, 91e, 91f for receiving said individual detector signals A, B, C, D, SI, S2, respectively. As will be clear to a person skilled in the art, the control circuit 90 is designed to process said individual detector signals A, B, C, D, SI, S2, in order to derive data signals and one or more error signals. A radial error signal, designated hereinafter simply as RE, indicates the radial distance between a track and the focal spot F. A focus error signal, designated hereinafter simply as FE, indicates the axial distance between a storage layer and the focal spot F. It is noted that, depending on the design of the optical detector, different formulas for error signal calculation may be used. Generally speaking, such error signals each are a measure for a certain kind of asymmetry of the central optical spot on the detector 35, and hence are sensitive to displacement of the optical scanning spot with respect to the disc. A special signal which can be derived by processing said individual detector signals is the mirror signal MIRn, obtained by a weighed summation of all individual detector signals A, B, C, D, SI, S2 according to Rn = A + B + C + D + W(Sl + S2) (1) wherein W indicates a weighing factor, typically in the order of about 15. This signal is a measure for the reflectivity of the disc. In contrast, the usual error signals, such as REn, are derived by suitably adding some of the (or all of the) individual detector signals A, B, C, D, SI, S2, at least one (but not all) of the signals being inverted (i.e. multiplied by -1), as will be known to persons skilled in the art. By way of example, a radial error signal REn can be defined according to (A ₊ D) - (B ₊ Q - WjSl - S2) A + B + C + D + SI + S2

W being a weighing factor. The control circuit 90 is designed to generate its control signals as a function of the error signals, to reduce the corresponding error, as will be clear to a person skilled in the art. In this case, the control circuit 90 has a variable control characteristic which depends on the type of error. In the case of errors due to disc defects, the control characteristic of the control circuit 90 is adapted according to a first strategy in order to adequately handle disc defects. In the case of errors due to external shocks, the control characteristic of the control circuit 90 is adapted according to a second strategy in order to adequately handle external shocks, which second strategy differs from the first strategy. The exact nature of these control characteristics which are adapted is a matter of design. For instance, it is possible that a gain in the control circuit is adapted. It is also possible that for instance a cut-off frequency of a filter of the control circuit is adapted. The choice of which parameter of the control circuit to adapt is no subject of the present invention. Further, control circuits with variable gain are known per se, so it is not necessary here to explain in more detail how a circuit parameter is actually changed in order to adapt the control characteristics. Thus, stated generally: the control characteristic of the control circuit 90 contains a number of variable control parameters. The setting of at least one of these variable control parameters is varied as a function of at least two detection signals, namely at least the two signals of first and second type described above, this function being such that said first strategy is reliably implemented in the case of disc defects whereas said second strategy is reliably implemented in the case of external shocks or vibrations. In the following, this aspect of the invention will be explained in more detail for an exemplary case of a control circuit having variable gain. Figure 2 is a block diagram schematically illustrating a part of an examplary control circuit 90 in more detail. For the sake of discussion, this part of control circuit 90 may relate to the control of the radial actuator 51. It is assumed that the control circuit 90 comprises a PID controller 60 having an input 61 and an output 69. A radial error signal is received at input 61, and subjected to a first common amplifier 71 with variable gain kl. The PID controller 60 comprises three parallel branches, i.e. a P-branch 62, an I-branch 63, and a D-branch 64, each receiving the amplified input signal, and each comprising amplifiers 72, 73, 74, respectively, with a variable gain k2, k3, k4, respectively. The P-branch 62 comprises a P-processor 65, providing a P-signal Sp to a summation point 68. The I-branch 63 comprises an I-processor 66, providing an I-signal S to the summation point 68. The D- branch 64 comprises a D-processor 67, providing a D-signal SD to the summation point 68. The summation point output signal S+ is subjected to a second common amplifier 75 with variable gain k5. At least one of said gains k,, i indicating 1-5, and preferably all of said gains, are varied depending on at least two types of signals, each derived from the optical detector signal SR. Thus, each gain kj can be described as a function f;(xl, x2) of two signals xl and x2, xl and x2 indicating the two types of signals, respectively. In the preferred embodiment, the mirror signal MIRn is used as signal xl of first type, and the normalized radial error signal REn is used as signal x2 of second type. ' The mirror signal MIRn is a measure for the reflectivity of the disc. Normally, this signal has a substantially constant value, indicated hereinafter as M, which is substantially independent from shocks or vibrations, but which rapidly decreases in a case of disc defects. Thus, MIRn allows to make a distinction between disc defects and shocks; for instance, it is simply possible to decide that a disc defect is occurring if MIRn < α-M, wherein α is a predetermined threshold factor, for instance α = 0.9. Alternatively, instead of the mirror signal MIRn, another signal which is based on summation of two or more individual segment output signals might be used as a signal of first type. An example of such alternative signal, suitable for use as first type signal in the context of the present invention, is CA = A+B+C+D. The normalized radial error signal REn is a measure for optical spot displacement in the radial direction. Alternatively, instead of the normalized radial error signal REn, another signal which is based on summation of two or more individual segment output signals, at least one (but not all) of those signals being multiplied by -1, might be used as a signal of second type. Examples of such alternative signal, suitable for use as second type signal in the context of the present invention, are: S1-S2; A-B; FEn. By way of example, figure 3 illustrates a possible first function fl for varying the first gain kl. It is noted that functions for varying the other gains k2-k5 may be identical to fl, but it is also possible that individual functions for varying the other gains k2-k5 are mutually different, although the general shape of these functions will at least be similar. Figure 3 is a graph containing a curve 80 showing gain kl (vertical axis) as a function of radial error REn (horizontal axis) for different values of MIRn. It should be clear that it is also possible to show kl as a function of MIRn (horizontal axis) for different values of REn, or to show kl as a three-dimensional graph. In the embodiment illustrated, the first gain kl has a nominal value kl,0. As long as the radial error is relatively small, i.e. below a threshold Rp, the first gain kl is kept constant at its nominal value kl,0 (horizontal stem 83 of curve 80). If the radial error is above said threshold Rj, the first gain kl is increased (first branch 81 of curve 80) or decreased (second branch 82 of curve 80) depending on the second type input signal MIRn. If MIRn has a value indicating disc defects, the first gain kl is decreased. Otherwise, if MIRn has a value indicating shocks or vibrations, the first gain kl is increased. The amount of decrease or increase, respectively, depends on the magnitude of REn. Thus, the value of the first gain kl is selected in dependency of two signals. In principle, the present invention is already embodied if the value of only one of the said gains depends on at least two signals as described above. Preferably, however, more than one of the said gains are varied on the basis of at least two signals, more preferably all of the said gains. Each gain may be associated with a corresponding threshold, although preferably the threshold Rj is the same for all gains. Further, each of said gains may be set in accordance with individual functions of two (or more) signals, although preferably such function (curve 80) is the same for all gains. Figure 4 is a graph illustrating the effect of the present invention in relation to the above-mentioned gain kl. The lefthand part of figure 4 illustrates the case of a (periodic) vibration, for instance at 40 Hz, while the righthand part of figure 4 illustrates the case of a disc defect (a black dot). Dashed curves 101 and 102, respectively, illustrate the behaviour of the radial error signal REn if all gains would be kept constant at their respective nominal values, so that the control circuit 90 would perform a linear control. Curve 101 shows a sine-shaped behaviour of REn in the case of a (periodic) vibration. In the case of a black dot, curve 102 shows large peaks corresponding with the optical spot entering and leaving the black dot. Solid line 103 illustrates the behaviour of the radial error signal REn when the present invention is applied in the case of a (periodic) vibration. In an initial time interval before time tl, the radial error signal REn is below threshold R_τ, so that gain kl is at its nominal value kl,0 (this corresponds to stem 83 of curve 80 in figure 3). At time tl, the radial error signal REn rises above threshold RT, while MIRn has its nominal value M, indicated by curve 111. Corresponding to branch 81 of curve 80 in figure 3, gain kl is increased from its nominal value kl,0, in this case to reach a maximum value kljyj_AX, as illustrated by curve 121. It is true that REn still rises above threshold R_T, but the maximum value reached by REn is lower than in the case of linear control (curve 101). When REn decreases, gain kl also decreases, and at time t2, when REn drops below threshold R , gain kl reaches its nominal value kl,0 again. This process repeats itself, each time when the absolute value of REn rises above threshold R . Solid line 104 illustrates the behaviour of the radial error signal REn when the present invention is applied in the case of a disc defect. In an initial time interval before time t3, the radial error signal REn is below threshold RT, SO that gain kl is at its nominal value kl,0 (this corresponds to stem 83 of curve 80 in figure 3). At time t3, the radial error signal REn rises above threshold RT, while MIRn has dropped below the switching threshold αM, indicated by curve 112. Corresponding to branch 82 of curve 80 in figure 3, gain kl is decreased from its nominal value kl,0, in this case to reach a minimum value kl_M^, as illustrated by curve 122. It is true that REn still rises above threshold RT, but the maximum value reached by REn is lower than in the case of linear control (curve 102). When REn decreases, gain kl increases, and at time t4, when REn drops below threshold RT, gain kl reaches its nominal value kl,0 again. The optical spot has now left the black dot, and the mirror signal MIRn has regained its nominal value M. It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. In the above, the present invention has been explained with reference to an embodiment where one or more gains are set in dependency of two optical signals. It is also possible to adapt other parameters of the actuator control, such as for instance cut-off frequencies of high-pass or low-pass filters, or central frequencies of band-pass or band- reject filters. It should be clear to a person skilled in the art that any control parameter which has different optimal settings for the cases of shock and disc defects may be adapted in the manner described above. Further, it is possible that any such control parameter is set on the basis of three or more signals. In the exemplary embodiment described above, it is for example possible to use the focal error signal as a third signal, indicating shocks in the Z-direction. In the above, the present invention has been explained with reference to an embodiment with a six-segment optical detector. It should be clear to a person skilled in the art that detectors having different design are also possible, in which case the formulas for error signals may be different. In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is perfonned by one or - more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, etc.

Claims

CLAIMS:

1. Method for discriminating different types of disturbances in an optical disc drive apparatus (1), the disc drive apparatus (1) comprising: scanning means (30) for scanning a record track of an optical disc (2) and for generating a read signal (SR); the method comprising the steps of: deriving from said read signal (SR) at least two signals (REn; MIRn) of mutually different type; deciding that a disturbance corresponds to a shock or vibration or the like if one of said signals is within a first range of values (REn > RT) while another of said signals is within a second range of values (MIRn > α-M), and deciding that a disturbance corresponds to a disc defect if said one of said signals is within said first range of values (REn > RT) while said another of said signals is within a third range of values (MIRn < α-M).

2. Method according to claim 1, wherein said one of said signals (REn; FEn) is a signal which is a measure for optical spot displacement.

3. Method according to claim 1, wherein said one of said signals (REn; FEn) is a signal which is a measure for a certain kind of asymmetry of the central optical spot on the detector (35).

4. Method according to claim 1, wherein said one of said signals (REn) is a radial error signal.

5. Method according to claim 1, wherein said one of said signals (FEn) is a focus error signal.

6. Method according to claim 1, wherein said another of said signals is a signal (MIRn) which is a measure for the reflectivity of the disc.

7. Method according to claim 1, wherein said scanning means (30) comprise an optical detector (35) comprising multiple detector segments (35a, 35b, 35c, 35d, 35e, 35f) for receiving a reflected light beam (32d) and for generating individual detector signals (A, B, C, D, SI, S2), respectively; and wherein said one of said signals (REn; FEn) is a signal which is based on summation of two or more individual segment output signals, wherein at least one but not all of those individual segment output signals is multiplied by -1.

8. Method according to claim 1, wherein said scanning means (30) comprise an optical detector (35) comprising multiple detector segments (35a, 35b, 35c, 35d, 35e, 35f) for receiving a reflected light beam (32d) and for generating individual detector signals (A, B, C, D, SI, S2), respectively; and wherein said another of said signals (MIRn) is a signal which is based on summation of two or more individual segment output signals.

9. Method according to claim 8, wherein said another of said signals is the normalised mirror signal (MIRn) which is the summation of all individual detector signals (A, B, C, D, SI, S2) from all detector segments (35a, 35b, 35c, 35d, 35e, 35f).

10. Method according to claim 8, wherein said another of said signals is the central aperture signal (CA) which is the summation of all individual detector signals (A, B, C, D) from four central aperture detector segments (35a, 35b, 35c, 35d) arranged in a four- quadrant configuration.

11. Method for controlling an optical disc drive apparatus (1), the disc drive apparatus (1) comprising: scanning means (30) for scanning a record track of an optical disc (2) and for generating a read signal (SR); actuator means (50) for controlling the positioning of at least one read/write element (34) of said scanning means (30) with respect to the disc (2); a control circuit (90) for receiving said read signal (SR) and generating at least one actuator control signal (SCR) on the basis of at least one signal component of said read signal (SR), the control circuit (90) having at least one variable control parameter; the method comprising the steps of: discriminating different types of disturbances, using the method of claim 1; and varying the setting of said at least one variable control parameter in accordance with a first strategy (81) in the case of a disc defect, and varying the setting of said at least one variable control parameter in accordance with a second strategy (82) in the case of a shock or vibration.

12. Method for controlling an optical disc drive apparatus (1), the disc drive apparatus (1) comprising: scanning means (30) for scanning a record track of an optical disc (2) and for generating a read signal (SR); actuator means (50) for controlling the positioning of at least one read/write element (34) of said scanning means (30) with respect to the disc (2); a control circuit (90) for receiving said read signal (SR) and generating at least one actuator control signal (S_CR) on the basis of at least one signal component of said read signal (SR), the control circuit (90) having at least one variable control parameter; the method comprising the steps of: deriving from said read signal (SR) at least two signals (REn; MIRn) of mutually different type; and varying the setting of said at least one variable control parameter as a function of said at least two signals (REn; MIRn).

13. Method according to claim 12, wherein said at least one variable control parameter is a gain of a signal processing component of said control circuit (90).

14. Method according to claim 12, wherein a first type signal of said at least two signals is a signal (MIRn) which is a measure for the reflectivity of the disc.

15. Method according to claim 12, wherein said scanning means (30) comprise an optical detector (35) comprising multiple detector segments (35a, 35b, 35c, 35d, 35e, 35f) for receiving a reflected light beam (32d) and for generating individual detector signals (A, B, C, D, SI, S2), respectively; and wherein a first type signal of said at least two signals is a signal which is based on summation of two or more individual segment output signals.

16. Method according to claim 15, wherein said first type signal is the normalised mirror signal (MIRn) which is the summation of all individual detector signals (A, B, C, D,

SI, S2) from all detector segments (35a, 35b, 35c, 35d, 35e, 35f).

17. Method according to claim 15, wherein said first type signal is the central aperture signal (CA) which is the summation of all individual detector signals (A, B, C, D) from four central aperture detector segments (35a, 35b, 35c, 35d) arranged in a four-quadrant configuration.

18. Method according to claim 12, wherein a second type signal of said at least two signals is a signal (REn; FEn) which is a measure for optical spot displacement.

19. Method according to claim 12, wherein a second type signal of said at least two signals is a signal (REn; FEn) which is a measure for a certain kind of asymmetry of the central optical spot on the detector (35).

20. Method according to claim 12, wherein said scanning means (30) comprise an optical detector (35) comprising multiple detector segments (35a, 35b, 35c, 35d, 35e, 35f) for receiving a reflected light beam (32d) and for generating individual detector signals (A, B, C, D, SI, S2), respectively; and wherein a second type signal of said at least two signals is a signal which is based on summation of two or more individual segment output signals, wherein at least one of those signals is multiplied by -1.

21. Method according to claim 18, wherein said second type signal of said at least two signals is a radial error signal (REn).

22. Method according to claim 18, wherein said second type signal of said at least two signals is a focus error signal (FEn).

23. Method according to claim 12, wherein said at least one variable control parameter (kl) has a nominal value (kl,0) as long as said second type signal (REn) is below a predefined threshold (RT); wherein the value of said at least one variable control parameter (kl) is changed in a first direction (increased; 121) if said second type signal (REn) is above said predefined threshold (R_τ) while said first type signal (MIRn) fulfils a first condition (MIRn > αM) indicating a disturbance originating from mechanical shocks or vibrations; and is wherein the value of said at least one variable control parameter (kl) is changed in a second direction (decreased; 122) differing from said first direction if said second type signal (REn) is above said predefined threshold (RT) while said first type signal (MIRn) fulfils a second condition (MIRn < αM) indicating a disturbance originating from disc defects.

24. Method according to claim 12, wherein said at least one variable control parameter (kl) has a nominal value (kl,0) as long as said second type signal (REn) is below a predefined threshold (RT); wherein the value of said at least one variable control parameter (kl) is changed in a first direction (increased; 121) if said second type signal (REn) is above said predefined threshold (R_τ) while said first type signal (MIRn) fulfils a first condition (MIRn > αM) indicating normal reflectivity of the disc; and is wherein the value of said at least one variable control parameter (kl) is changed in a second direction (decreased; 122) differing from said first direction if said second type signal (REn) is above said predefined threshold (RT) while said first type signal (MIRn) fulfils a second condition (MIRn < αM) indicating reduced reflectivity of the disc.

25. Disc drive apparatus (1) comprising: scanning means (30) for scanning a record track of an optical disc (2) and for generating a read signal (SR); actuator means (50) for controlling the positioning of at least one read/write element (34) of said scanning means (30) with respect to the disc (2); a control circuit (90) for receiving said read signal (SR) and generating at least one actuator control signal (SQR) on the basis of at least one signal component of said read signal (SR), the control circuit (90) having at least one variable control parameter; the control circuit (90) being adapted to perform the method of claim 1.

26. Disc drive apparatus (1) comprising: scanning means (30) for scanning a record track of an optical disc (2) and for generating a read signal (SR); actuator means (50) for controlling the positioning of at least one read/write element (34) of said scanning means (30) with respect to the disc (2); a control circuit (90) for receiving said read signal (SR) and generating at least one actuator control signal (SCR) on the basis of at least one signal component of said read signal (SR), the control circuit (90) having at least one variable control parameter; the control circuit (90) being adapted to perform the method of claim 12.