Information recording/reproducing apparatus

Abstract: An information recording/reproducing apparatus is provided with a velocity control device for moving a recording and/or reproducing head from a current track to a target track under velocity control. In the apparatus, a target velocity for the head is determined and the moving velocity of the head is detected by a detector. In the apparatus, an instruction value for an actuator for moving the head is generated at every predetermined calculating interval on the basis of the target velocity and the detected velocity so that the head follows the target velocity. In the generation of the instruction value for the actuator for the head, the calculating interval can be switched into a plural values by a switching device until the head reaches the target track.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an information recording/reproducing apparatus employing for example a magnetic disk or a magnetooptical disk, and more particularly to a velocity control device for an information recording and/or reproducing head for access to a target track, adapted for use in such apparatus. Related Background Art For velocity control of such head (or a light beam spot in case of an optical disk), there has generally been employed a closed loop control in which, as shown in Fig. 1, generated is a reference velocity proportional to the square root of the residual distance to the target position and the head is made to follow said reference velocity. In such control method, the actual head velocity is detected by a velocity detecting circuit, and the control for following said reference velocity is achieved by thus detected speed signal. Fig. 2 shows an example of said velocity detecting circuit, wherein provided are a shaping circuit 100 for shaping a tracking error signal into a binary signal; a digital differentiating circuit 101 composed of a delay circuit 102 and an exclusive OR circuit 103; and a monostable multivibrator 104. Said velocity detecting circuit functions in the following manner. At first, as shown in Fig. 3, an automatic tracking (AT) error signal is binary digitized by the shaping circuit 100 into a pulse signal with a duration corresponding to a half cycle of said error signal. Then the digital differentiating circuit 101 generates trigger pulses, for supply to the monostable multivibrator 104, at the upshift and downshift of said binary signal. In response, the monostable multivibrator 104 generates a pulsed velocity signal with a duration τ , which varies the cycle time thereof according to the change in cycle time of the AT error signal. Since the velocity is proportional to the frequency of the AT error signal, the velocity signal can be generated by the processing of said AT error signal AT. Also said digital velocity signal can be converted, by averaging, into an analog signal. However the output of said velocity detecting circuit cannot be practically used as the velocity signal when the velocity approaches to zero, as the interval of output pulses of the monostable multivibrator is extended. Consequently an exact closed loop control is difficult in the speed control method explained in Fig. 1. In addition, in the above-explained control method, the band region of the closed loop is limited by the resonance, usually in 2 to 3 kHz region, of the actuator mechanism for head driving. For this reason the response is deteriorated in a rapid deceleration, whereby the velocity at the end of a seeking operation cannot be brought to zero, and the overrun of the target position results. In order to avoid this drawback, there is employed a method of applying a large current to the actuator achieving acceleration within a short time, and effecting the deceleration in gradual manner. However, such control method results in an increased seek time because the deceleration is only gradually achieved without fully utilizing the accelerating ability. In order to avoid such drawback there is proposed a control method shown in Fig. 4, in which an open-loop bang-bang control is used for a major portion of the access distance, and the closed-loop control is adopted from immediately before the target position. This control method can reduce the seek time, because rapid deceleration is possible with a large current. However this control method may result in an overrun or leave a residual distance to the target position if the acceleration or deceleration is not conducted at appropriate points due to the influence of friction, external force etc. since the bang-bang control is conducted in an open loop. On the other hand, the aforementioned closed-loop control method has to sacrifice the seek time in order to prevent the overrun. Thus it is difficult to achieve the high speed and the accuracy at the same time in either method. The preamble of claim 1 acknowledges US-A-4719610 disclosing an apparatus using the time per track from servo position outputs to compute the velocity of the head. US-A-4 524 398 discloses an apparatus using the distance travelled by the head between servo pulses to compute the velocity of the head, whereby the number of calculations between successive tracks increases during deceleration. EP-A-0 111 654 discloses an apparatus for generating a velocity profile providing an increased number of samples at segments closer to the target. SUMMARY OF THE INVENTION In consideration of the foregoing, the object of the present invention is to provide an information recording/reproducing apparatus equipped with a velocity control device for a recording and/or reproducing head, capable of exact control even at a high speed, without influence of friction or external force. The above-mentioned object can be attained, according to the present invention, by an information recording/reproducing apparatus as defined in claim 1 or a method as defined in claim 4. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a chart showing the relationship between the reference velocity and the actuator current in a conventional velocity control method for a recording and/or reproducing head; Fig. 2 is a block diagram of a velocity detecting circuit to be used in the control method shown in Fig. 1; Fig. 3 is a timing chart showing the function of said velocity detecting circuit; Fig. 4 is a chart showing the relationship between the velocity and the current in a bang-bang control method constituting another conventional example; Fig. 5 is a block diagram of an embodiment of the information recording/reproducing apparatus of the present invention; Fig. 6 is a chart showing the relationship between the target position and the target velocity in a multiple jump operation in said embodiment; Fig. 7 is a timing chart showing the relationship among the tracking error signal, the actuator instruction value and the velocity; Fig. 8 is a flow chart of the control sequence of the multiple jump operation; Fig. 9 is a block diagram of a second embodiment; Fig. 10 is a flow chart of the control sequence of the embodiment shown in Fig. 9; Fig. 11 is a flow chart of another control sequence; Figs. 12(a) to (e) are timing charts showing the detection of zero-cross points and peak points in the velocity/position detecting circuit in the 2nd embodiment shown in Fig. 9; Fig. 13 is a chart showing the relationship among the target velocity and actual velocity of the head, the tracking error signal and the actuator instruction value in a 3rd embodiment; and Fig. 14 is a flow chart of the control sequence of said 3rd embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. Fig. 5 is a block diagram of an embodiment of the information recording/reproducing apparatus of the present invention, said embodiment applied to an optical disk recording/reproducing apparatus. Referring to Fig. 5, there are shown an optical disk 1; an optical system 2; a tracking error detector 3 for generating a tracking error signal based on the output from the optical system 2; a focus error detector 4 for generating a focus error signal based on the output from said optical system 2; A/D converters 5 for converting the error signals from said error detectors into digital signals; a digital signal processing unit 6 composed of an I/O control unit 7, a digital signal processor (DSP) 8, and memories 9, 10; D/A converters 11; a tracking actuator 12; a focusing actuator 13; and an external data input device 14. In the following there will be explained the basic function of the present embodiment. Fig. 6 is a chart showing the relationship between the target velocity of the light beam spot (or optical head) and the track position in a multijump operation. Fig. 6 also shows an ideal target velocity, but actually used are discrete target velocities. In the illustrated example, the final target track is track 14. An acceleration region extends to a track 7, and a deceleration region starts thereafter and extends to the track 14. The deceleration region is divided into a deceleration region I and a deceleration region II, with different precisions of velocity control. The division of said deceleration regions is suitably determined in consideration of the decelerating ability. In the present example, the velocity control in the decelerating region I is conducted every 1/2 track as in the acceleration region, but that in the region II is conducted every 1/4 track for finer control. More specifically, the velocity control is conducted by determining the target velocity of the light beam spot (or optical head), current velocity thereof and instruction value for the tracking actuator (or head actuator) at every 1/2 track in the acceleration region or in the deceleration region I, and at every 1/4 track in the deceleration region II. The velocity control may also be achieved by storing a velocity profile corresponding to the moving distance in a memory, and reading these data according to the moving position at every 1/2 or 1/4 track. Fig. 7 is a chart showing the relationship among the tracking error signal, the instruction value for the actuator and the velocity thereof in four tracks before reaching the target track. In this example, since the tracks 10 to 12 belong to the deceleration region I, the velocity is calculated at every 1/2 track, and the difference from the target velocity is added to the reference acceleration for driving the actuator. The velocity calculation at every 1/2 track is in fact conducted at the detected zero-cross points of the tracking error signal. For example, at a point A shown in Fig. 7, the time Ta needed from an immediately preceding zero-cross point Z to said point A is measured, and the current velocity Va is calculated from the following equation: Va = 1/2λ x 1/Ta wherein λ is the track pitch. The target velocity is V ref at the point A in Fig. 6, and the control amount act of the actuator is calculated by the following equation: act = -α + K(V ref - V a ) wherein -α is the reference deceleration, and K is the feedback gain for velocity control. Then the tracks 12 to 14 belong to the deceleration region II, the velocity is calculated every 1/4 track, and the difference from the target velocity is added to the reference acceleration for driving the actuator. In this case the velocity calculation at every 1/4 track is conducted by detecting the zero-cross points and peak points of the tracking error signal alternately and utilizing the measured time between these points. In Fig. 7, the zero-cross points and peak points are respectively indicated by "." and "*". For example, at a point B in Fig. 7, the current velocity V b is obtained from the following equation, based on the time T b measured from Y to B: V b = 1/4λ x 1/T b The target velocity in this state is V ref at the point B in Fig. 6, so that the instruction act for the actuator is calculated as follows: act = -α + K(V ref - V b ) Control based on thus calculated values provided a velocity curve as shown in Fig. 7. It will be understood that the actual velocity V t follows the ideal velocity V ref satisfactorily. In the following there will be explained equations for obtaining the target velocity V ref in Figs. 6 and 7. In general, at a residual distance S to the target position and with an acceleration ability α, the target velocity V ref for reaching V = 0 after t seconds is obtained by solving the following simultaneous equations: V ref ·t - 1/2α t 2 = S (for distance) V ref - αt = 0 (for velocity) Following equation on the target velocity is obtained by erasing t from these simultaneous equations: V ref = 2αS The residual distance S can be obtained from the following equations: S = (S A - λ/2 x N) S = (S A - {λ/2 x N + λ/4 }) wherein S A is a target moving distance (distance from the starting track to the target track), λ is the track pitch, and N is the zero-cross count. The equation (8) is used in the calculation at the detection of a zero-cross point, while the equation (9) is used in the calculation at the detection of a peak point. These equations indicate that the target velocity is proportional to the square root of the residual distance. The above-explained method allows to easily detect the current velocity from the tracking error signal. Thus, such velocity detecting method not only dispenses with a complicated velocity detecting circuit but also resolves the drawback of the conventional method of inaccurate detection at the velocity close to zero. Consequently such velocity detecting method allows to simplify the structure of the apparatus and to achieve accurate velocity control of the head. In the following the function of the present first embodiment will be explained in more details. Fig. 8 is a flow chart of the control sequence, as an example, in a multiple jump operation to a direction (inward movement). Fig. 8 shows the control sequence in case of multiple jumps from the track 0 to 14 shown in Fig. 6, with predetermined precisions in the acceleration region, deceleration regions I and II as explained before. At first a step S1 enters the tracking error signal into the digital signal processor (DSP) 8. In more detail, in Fig. 5, the light reflected from the optical disk 1 enters the optical system 2, and the tracking error detector 3 detects the tracking error signal S n , which is converted in the A/D converter 5 into a digital signal and supplied to the DSP 8 through the I/O control unit 7. In a step S2, the DSP 8 detects the zero-cross point of the tracking error signal. As an example, an estimated next tracking error signal S' n+1 is determined as follows from the entered tracking error signal S n and a preceding tracking error signal S n-1 stored in the memory 9: S' n+1 = 2S n - S n-1 Then the obtained estimated value and the current value are multiplied, and, if the product is zero or has a negative sign, the tracking error signal is identified to be at the zero-cross point. A product equal to zero or having a negative sign indicates that the estimated value is on the zero-cross point of the tracking error signal or assumes a positive or negative value beyond said zero-cross point, so that such estimated point is regarded as the zero-cross point. The step S2 effects such zero-cross point detection at each sampling of the tracking error signal, and, upon detection of said zero-cross point in a step S3, the sequence proceeds to a step S4. Except in the detection of the zero-cross point in the step S3, the DSP 8 is released until the next sampling and can therefore be used for other tasks. When the zero-cross point is detected, the content of an unrepresented counter for counting the number of appearances of the zero-cross point is increased by one. The step S4 determines the current velocity V t and the target velocity V ref as explained in relation to Figs. 6 and 7. As explained in Fig. 7, the current velocity V t can be calculated from the following equation, from the half cycle time T of the tracking error signal in this state and 1/2λ : V t = 1/2λ x 1/T Also the target velocity can be obtained as follows: V ref = 2αS wherein the residual distance S can be obtained from the foregoing equation (8) and (9). In a step S5, the DSP 8 determines the instruction value act for the actuator from thus obtained current velocity and target velocity. As the control is now in the acceleration region, said instruction value is calculated as follows: act = α + K(V ref - V t ) The instruction value is supplied through the I/O control unit 7 to the D/A converter 11 for conversion into an analog signal, which is used for driving the tracking actuator 12 (or the head actuator (linear motor) in case the velocity control is conducted on an optical head). Then a step S6 discriminates whether a half distance to the target track has been reached, namely whether the acceleration region shown in Fig. 6 has been terminated, by counting the number of crossed tracks with an unrepresented counter, and, if the count indicates that said half distance has been reached, the sequence proceeds to a step S7. Thus, in the acceleration region representing a half of the distance to the target track, the actuator is driven with the instruction value determined at every 1/2 track by repeating the steps S1 to S6. When said half distance is reached, the sequence proceeds to the step S7 to initiate the control in the deceleration region I as already explained in relation to Fig. 6. At first the step S7 enters the tracking error signal, then a step S8 effects the detection of the zero-cross point as explained before, and a step S9 discriminates whether the zero-cross point has been reached. Upon detection of the zero-cross point, a step S10 determines the current velocity and the target velocity in the identical manner as in the step S4. Then, in a step S11, the DSP 8 determines the instruction value act for the actuator as follows, based on said current velocity and target velocity: act = -α + K(V ref - V t ) The actuator is similarly driven according to said instruction value. A step S12 discriminates whether a second track in front of the target track has been reached, namely whether the deceleration region I has been terminated, similarly by means of the zero-cross counter. When the step S12 identifies the end of the deceleration region I, the sequence proceeds to a step S13 for effecting the control in the deceleration region II. Thus, in the deceleration region I, the actuator is driven with the instruction value obtained every 1/2 track, as in the acceleration region, by repeating the steps S7 - S12. In said deceleration region II, a step S13 enters the tracking error signal, then a step S14 effects the detection of the zero-cross point, and, if a step S15 identifies the zero-cross point, a step S18 determines the current velocity and the target velocity as explained before. On the other hand, if the step S15 does not identify the zero-cross point, a step S16 detects the peak value of the tracking error signal. Then, if a step S17 identifies the peak value, the step S18 determines the current velocity and the target velocity. Thus the zero-cross points and the peak points of the tracking error signal are alternately detected, and the current velocity and the target velocity are determined at each detection. Stated differently, a precise control at every 1/4 track is made possible by alternate detection of the zero-cross points and the peak points. In the step S18, the current velocity V t is determined as follows: V t = 1/4λ x 1/T wherein T is the time from the zero-cross point to the peak point. In a step S19, the DSP 8 determines the instruction value act for the actuator, based on thus obtained current velocity and target velocity according to the following equation: act = - α+K(V ref - V t ) A step S20 discriminates whether the target track has been reached, by means of the count of the zero-cross counter. Thus, in the deceleration region II, the actuator is driven with the instruction value obtained every 1/4 track by repeating the steps S13 - S20. The multiple jump operation is terminated when the step S20 identifies that the target track has been reached. Now there will be explained the detection, in the step S16, of peak point of the tracking error signal. In the present embodiment, the difference ΔS n between the entered tracking error signal S n and the previously sampled tracking error signal. S n-1 as follows: ΔS n = S n - S n-1 Then said difference ΔS n is multiplied by an immediately preceding similar difference ΔS n-1 , which is calculated as follows in the preceding sampling and stored in the memory: ΔS n-1 = S n-1 - S n-2 The peak point of the tracking error signal is identified when the product of ΔS n and ΔS n-1 is negative. As explained in the foregoing, in the present first embodiment, the instruction value for the actuator is calculated every 1/2 or 1/4 track, and the velocity is so controlled as to follow the target velocity curve, so that the light beam spot (or optical head) can be brought to the target track in ideal manner without unnecessary movement. Thus a high-speed seeking operation is rendered possible without any wasted movement. Also since the actuator is driven with the instruction value calculated every 1/2 or 1/4 track, a precise control to the target track is rendered possible without overrun. Consequently a precise control is possible even at a high speed, so that high speed and precision can be realized, unlike the conventional methods, at the same time. Fig. 9 is a block diagram of a second embodiment of the present invention, which is identical with that shown in Fig. 5, except the presence of a velocity-position detecting circuit 15. The velocity-position detecting circuit 15 serves to detect the actual velocity of the light beam spot (or optical head) and to generate a digital velocity signal. In the embodiment shown in Fig. 5, the velocity is calculated from the tracking error signal, but, in the present embodiment, the velocity is detected by a velocity detecting circuit as shown in Fig. 2. Also the velocity detection may be conducted by counting clock signals within a predetermined track interval. In the following there will be explained the function of the present second embodiment, with reference to a flow chart in Fig. 10, showing the function in a multiple jump operation. Referring to Fig. 10, at first in a step S1, the velocity-position detecting circuit 15 detects the velocity at a predetermined position, and sends a velocity signal to the DSP 8 through the I/O control unit 7. In the present embodiment, the velocity detection is conducted at each of the zero-cross points and the peak points of the tracking error signal. The method of detection of said zero-cross points and peak points for said velocity detection will be explained later in more details. Then the DSP 8 calculates the target velocity V ref at the timing of the zero-cross point or peak point, by determining the residual distance S from the current position to the target track and using the following equation: V ref = 2αS wherein S being determined from the equations (8) and (9). A step S3 calculates the instruction value act for the actuator as follows, from thus determined target velocity and the current velocity V t detected by the velocity-position detecting circuit 15: act = K(V ref - V t ) The obtained instruction value is supplied, in a step S4, through the I/O control unit 7 to the D/A converter 11 for conversion into an analog signal, which is used for driving the tracking actuator 12 (or head actuator (linear motor) in case the velocity control is conducted on the optical head). Then a step S5 discriminates whether the target track has been reached, and, if not, the sequence returns to the step S1 to repeat the sequence of the steps S1 to S5. In this manner the actuator is driven with the instruction value obtained at the zero-cross points and the peak points of the track error signal, namely every 1/4 of the track pitch, and the multiple jump operation is terminated when the target track is reached. Said multiple jump operation may also be terminated when the current velocity becomes zero as shown in Fig. 11. More specifically, a step S5 in Fig. 11 discriminates whether the current velocity has reached zero, and, if affirmative, the multiple jump operation may be terminated. It is naturally possible also to terminate the multiple jump operation when either of the conditions shown in Figs. 6 and 7 is satisfied. In the following there will be explained the detection of the zero-cross point and the peak point of the tracking error signal, by the velocity-position detecting circuit 15, with reference to Fig. 12. In Fig. 12, (a) indicates the tracking error (TE) signal, (b) indicates a binary signal obtained from said tracking error signal, (c) indicates an RF (regenerated) signal with a phase advanced by 90° from that of the TE signal, and (d) indicates a binary signal obtained from said RF signal. In general the TE signal is obtained as the difference of signals from divided sensors, whereas the RF signal is obtained as the sum, so that the TE and RF signals are mutually different by 90°. In the present embodiment, the zero-cross points and the peak points of the TE signal are determined by exclusive logic summing of the binary signals respectively obtained from the TE and RF signals. In Fig. 12, (e) indicates the output of said exclusive logic summing, with upshift coinciding with the zero-cross point of the TE signal and downshift coinciding with the peak point thereof. Such detection of the zero-cross points and the peak points is rendered possible since the TE and RF signals are mutually different by 90° in phase. The velocity-position detecting circuit 15 detects the velocity and generates a velocity signal at each of thus detected zero-cross points and peak points. This second embodiment also provides an advantage of precisely controlling the light beam spot (or optical head) even at a high speed, in identical manner as in the preceding embodiment shown in Fig. 5. Through the foregoing embodiments have been limited to an information recording/reproducing apparatus utilizing an optical disk, but the present invention is not limited to such embodiments and is naturally applicable advantageously to other recording/reproducing apparatus employing for example magnetic disks. In the following there will be explained a third embodiment of the present invention. The following description will be based on Fig. 5, since the function of said third embodiment can be achieved with an apparatus similar to that shown in Fig. 5. At first there will be explained the basic function of the present third embodiment. Fig. 13 is a timing chart showing the relationship among the target velocity curve of the light beam spot (or optical head) in a multiple jump operation, the actual velocity thereof, the tracking error signal and the instruction value for the actuator There is illustrated an example of jumping ten tracks, wherein a cycle of the tracking error signal corresponds to a track. In the present embodiment, up to the 9th track which is immediately preceding the target track, the zero-cross points of the tracking error signal are detected, and the time T 2 from a zero-cross point to a next zero-cross point is measured. The distance between the zero-cross point is measured. The distance between the zero-cross points is 1/2 of the track pitch λ, so that the velocity V t is given by: V t = λ/2 x 1/T 2 Also the target velocity V ref is given by the zero-cross count as follows: V ref = 2α(S-λ/2xN) wherein S is the target moving distance, λ is the track pitch and α is the deceleration. Then the instruction value act for the actuator is determined as follows, from the current velocity and the target velocity obtained from the equations (21) and (22): act = ±α + K(V ref - V t ) wherein ±α is the reference acceleration, and K is the feedback gain of the velocity control system. Thus the actuator is driven by the instruction value determined at every 1/2 of the track pitch up to a track immediately preceding the target track. As a result, as shown in Fig. 13, the actuator is accelerated with full power to a point Z and the velocity increases linearly. When the velocity reaches the point Z and catches up the target velocity, the deceleration is started according to the deceleration α shown in the equation (22). Then, when the light beam spot (or optical head) moves to a point B, as the velocity has considerably lowered by then, the velocity calculated at every 1/2 of the track pitch is considerably delayed from the actual velocity. Consequently the instruction value for the actuator calculated from such calculated velocity leads to an inaccurate control with significant error. For this reason the velocity is directly obtained from the tracking error signal. More specifically the velocity V t is determined as follows, from the tracking error signal S B sampled at a point B and that S A sampled at a next point A: V t = λ/4 (S A - S B )/T S wherein T S is the sampling cycle time. When the tracking error signal is normalized to a range of ± 1.0, the moving distance is 1/4λ for (S A - S B ) = 1.0 . Stated differently, a sloped portion of the sinusoidal tracking error signal is approximated by a straight line, and the velocity in a sampling section is determined from two values sampled at two points on said straight line and the sampling cycle time. Also the target velocity V ref in this state can be determined by: V ref = 2α (S - S a ) wherein S a is the moving distance up to this point and can be obtained by calculating the cumulative moving distances. Thus the instruction value for the actuator is calculated according to the equation (23), based on the current velocity and the target velocity obtained from the equations (24) and (25). In the above-explained velocity detecting method utilizing linear approximation, it is difficult to obtain the exact velocity in an area where the tracking error signal is inverted, such as X - Y in Fig. 13. Therefore, in such area where the linear approximation is not possible, the instruction value at the point X is retained, and the velocity detection by linear approximation is conducted again at the point Y. In the following explained is the actual function of the present third embodiment with reference to Fig. 14 which is a flow chart of the control sequence in a multiple jump operation in one direction (inwards) of the optical disk. It is also assumed that the control sequence is conducted according to the timing chart shown in Fig. 13. At first a step S1 enters the tracking error signal into the DSP 8. More detailedly, the light reflected from the optical disk 1 enters the optical system 2, and the tracking error detector 3 detects the tracking error signal S n , which is converted into a digital signal by the A/D converter 5 and supplied through the I/O control unit 7 to the DSP 8. In a step S2, the DSP 8 detects the zero-cross point of the tracking error signal. As an example, an estimated next tracking error signal S' n+1 is determined as follows from the entered tracking error signal S n and a preceding tracking error signal S n-1 stored in the memory 9: S' n+1 = 2S n - S n-1 Then the obtained estimated value and the current value are multiplied, and, if the product is zero or negative, the tracking error signal is identified to be at the zero-cross point. A product which is zero or negative indicates that the estimated value is on the zero-cross point of the tracking error signal or assumes a positive or negative value beyond said zero-cross point, so that such estimated point is regarded as the zero-cross point. The step S2 effects such detection of zero-cross point at each sampling of the tracking error signal, and, upon detection of said zero-cross point in a step S3, the sequence proceeds to a step S4. Except in the detection of the zero-cross point in the step S3, the DSP 8 is released until the next sampling and can therefore be used for other tasks. The step S4 determines the current velocity V t and the target velocity V ref shown in Fig. 13. The current velocity is obtained according to the equation (21) from the time between the zero-cross points and 1/2 of the track pitch, and the target velocity is obtained from the equation (22). A step S5 calculates the instruction value for the actuator, by the equation (23), based on the current velocity and the target velocity determined by the DSP 8. Said instruction value is supplied through the I/O control unit 7 to the D/A converter 11 for conversion into an analog signal, which is supplied in a step S6 to the tracking actuator 12 (or head actuator (linear motor) in case the velocity control is conducted on the optical head). A step S7 discriminates whether a track immediately preceding the target track, namely approximately the point B in Fig. 13, has been reached, by calculating the number of tracks with an unrepresented zero-cross counter, and, if said track immediately preceding the target track has been reached, the sequence proceeds to a step S8. Thus, up to the track immediately preceding the target track, a control following the target velocity curve shown in Fig. 13 is conducted by repeating the steps S1 - S7. Then the step S8 enters the tracking error signal into the DSP 8, and a step S9 discriminates whether the value of said signal is within 80% of the maximum amplitude of the tracking error signal. If the absolute values of two samples of the tracking error signal are both within 80% of the maximum amplitude, the calculation of the current velocity is identified possible by the aforementioned linear approximation of the sloped portion of the tracking error signal. If the step S9 identifies that the tracking error signals are within 80% of the maximum amplitude, a step S10 determines the current velocity and the target velocity from the tracking error signal. The current velocity V t is calculated by the equation (24), from the difference of the tracking error signal sampled in two points in an area approximated by a straight line and the sampling cycle time. Also the target velocity V ref at this point is calculated from the equation (25). A step S11 then calculates the instruction value for the actuator based on thus obtained results, according to the equation (23), and a step S13 drives the actuator by thus obtained instruction value. On the other hand, if the step S9 identifies that the tracking error signal, entered into the DSP 8, exceeds 80% of the maximum amplitude, the sequence proceeds to a step S12. Since the velocity calculation by linear approximation is difficult, the step S12 retains the preceding instruction value, and a step S13 sends said value to the actuator. Then a step S14 discriminates whether the target track has been reached, and, if not, the steps S8 - S14 are repeated. The multiple jump operation is terminated when the target track is reached. The operation may be terminated when the current velocity becomes zero. In the foregoing explanation, the preceding instruction value for the actuator is retained in an area where the tracking error signal exceeds 80% of the maximum amplitude, but such example is not limitative. For example an instruction value of zero may be given to the actuator, which shows a zero acceleration and maintains a constant speed. As explained in the foregoing, in the present embodiment, the instruction value for the actuator is calculated at every 1/2 track pitch by detecting the zero-cross points of the tracking error signal, up to a track immediately preceding the target track. Consequently, in such region, the light beam spot (or optical head) can be controlled according to the target velocity curve, thereby achieving a high-speed seeking operation without unnecessary movement. As the light beam spot (or optical head) approaches to the target track, the velocity thereof generally approaches gradually to zero, so that the interval between the zero-cross points of the tracking error signal becomes longer. Since the current velocity determined for every 1/2 track pitch is an average in such track pitch, its error from the actual velocity becomes larger as the position becomes closer to the target track. Therefore, in the last track gap immediately in front of the target track, the current velocity is exactly calculated by approximating the sloped portion of the tracking error signal with a straight line and employing signals sampled in such straight line area. Since the tracking error signal is sinusoidal, straight line approximation is possible only in the sloped portion. Consequently, in a section where such straight line approximation is not possible, the instruction value for the actuator is, for example, retained at the preceding signal level. In this manner the seeking operation of the light beam spot (or optical head) can be exactly controlled as the velocity thereof can be exactly calculated even when it is close to the target track. The foregoing embodiment describes an information recording/reproducing apparatus utilizing an optical disk, but the present invention is not limited to such embodiment and is likewise applicable advantageously for example to a recording apparatus utilizing a magnetic disk or the like. Also the present invention is subjected to various modifications within the scope and spirit of the appended claims. It is furthermore possible to combine the first and third embodiments. More specifically, it is possible to further divide the deceleration region II of the first embodiment, thereby effecting a velocity control at every 1/4 track pitch immediately after the shift from the deceleration region I, and adopting a velocity control according to the equation (24) when the position comes close to the target track.

Claims: An apparatus for performing at least one of the functions of recording information on and reproducing information from a recording medium, the apparatus being provided with a velocity control device for controlling the movement from a current track to a target track of a head for performing at least one of the functions of recording and reproducing information, the apparatus comprising: moving means (12) for moving the head in a direction so as to cross successive tracks on a recording medium; generating means (6,15) for generating a target velocity for the head; detecting means (3,5,8) for detecting the velocity of the head; digital calculation means (8) for effecting digital calculation of a control value for controlling the velocity of said moving means (12) comprising means for calculating the difference between the detected velocity and the target velocity and calculating the control value on the basis of the calculated difference at calculation intervals determined by the times taken for the head to cross successive tracks; and control means (6) for controlling said moving means (12) on the basis of the control values; said apparatus being characterized in further comprising: varying means (6) for varying the calculation intervals of said digital calculation means (8) by dividing the region between the current track and the target track into at least two sections, the varying means being arranged to increase the number of calculations between successive tracks in the section closer to the target track in relation to the number of calculations between successive tracks in the section remote from the target track An apparatus according to Claim 1, wherein said detecting means (3,5,8) is arranged to detect said velocity on the basis of a tracking error signal detected during said head movement. An apparatus according to Claim 1 or 2, wherein said varying means (6) is arranged to vary the calculation interval such that the calculation interval is set to half the period of the detected tracking error signal when said head is positioned in a first section away from the target track, and such that the calculation interval is set to one quarter the period of the detected tracking error signal when said head is positioned in a second section near the target track. A head movement control method for controlling the movement of a head for performing at least one of the functions of recording and reproducing from a current track to a target track at a controlled velocity, said method comprising the steps of: generating a target velocity for the head; detecting the velocity of the head; performing digital calculation of a control value for controlling the velocity of the head comprising calculating the difference between the detected velocity and the target velocity at calculation intervals determined by the times taken for the head to cross successive tracks; and controlling the movement of the head on the basis of the control value, said method being characterized in further comprising the step of: varying the calculation intervals used in said digital calculation step by dividing the region between the current track and the target track into at least two sections, and increasing the number of said calculations between successive tracks in the section closer to the target track in relation to the number of calculations between successive tracks in the section remote from the target track. A method according to Claim 4 wherein the step of detecting said velocity comprises the step of: detecting when the head crosses a track to produce a tracking error signal; and measuring said velocity on the basis of the tracking error signal detected during the head movement. A method according to Claim 4 including the step of: detecting a tracking error signal; and wherein the calculation interval is set to half the period of the detected tracking error signal when the head is positioned in a first section away from the target track and the calculation interval is set to quarter the period of the detected tracking error signal when said head is positioned in a second section near the target track.

Inventors: TSUKASA OGINO, TSUKASA, OGINO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G11B21
IPC codes: G11B21, G11B5, G11B7