Servo circuit for magnetic disk apparatus

Abstract: A servo circuit for a magnetic disk apparatus having a target speed generating circuit (13a) for generating a target speed in accordance with an amount of movement; a speed signal producing circuit (14) for producing a real speed from a position signal from a servo object (11); and a speed error generating circuit (15) for controlling the speed of the servo object (1) based on an error between the target speed and the real speed; in accordance with the amount of movement, acceleration and then deceleration control being performed for positioning at the target position, wherein provision is made of an acceleration detection circuit (19) for detecting acceleration in progress from an output of the speed error generating circuit (15), and an acceleration-in-progress detection signal of the acceleration detection generating detection circuit (12) is used for changing a cutoff frequency of the speed error generating circuit (15) between acceleration in progress and deceleration in progress. Also, a servo circuit for a magnetic disk apparatus provided with a servo loop which supplies a seek current based on an error signal (DE) between a target speed (E1) and real speed (E2) to a voice coil motor (210) and performs speed control on a head carriage (212) toward a target track position, provided in the servo loop with an addition means for adding a predetermined addition value (Ecc) to the error signal (DE) at the stage of acceleration after a start of the speed control or provided, at an amplifier circuit (316) positioned at a latter stage of an amplifier circuit (314) for determining the frequency characteristics of the servo loop, with a gain switching means (318) which switches a preset gain (G1) to a larger gain (G2) between the start of a seek operation and the completion of acceleration. Further, a servo circuit for a magnetic disk apparatus which performs speed control over a servo object based on a speed error between a target speed and a real speed, provided with a speed signal generating circuit (414A) which generates the real speed of the servo object based on a position signal of the servo object, the speed signal generating circuit provided with a band changing circuit (4140) for changing the band of a speed signal producing circuit (414B) in accordance with a seek difference.

Description: The present invention relates to a servo circuit for a magnetic disk apparatus which performs speed control over a servo object based on an error between a target speed and a real speed, more particularly relates to ones which perform speed control by passing a seek current corresponding to the error between the target speed and the real speed to a voice coil motor and to one having a speed signal generating circuit for finding the real speed. Circuits of this general type are known from our earlier application EP-A-0,272,076. Servo circuits are used in magnetic disk apparatuses for positioning magnetic heads on specific tracks. In such servo circuits, it is desired to reduce the time required for the speed control and to shorten the access time and to obtain stable positioning even with higher speeds. In particular, in recent years, higher speeds have been achieved in the seek access operations, which move and stop a head in a disk apparatus at a target track position. Along with the higher speeds of seek access operations, much higher speeds have been sought for so-called "one difference" seek access operations wherein the heads are moved to the adjoining track position. A demand has therefore arisen for a servo circuit with a high response commensurate with the higher speeds of "one difference" seek operations. Explaining the related art in more detail, in a servo circuit, speed control is used up to near the target position, then position control is switched to so as to control the positioning to the target position. However, the frequency characteristic of the servo object has a resonance point, i.e., a resonance point of the servo arm such as the carriage of the servo object. If the frequency of a speed control signal, explained later, appears in the band which covers this resonance point, mechanical oscillation appears during the seek operation and therefore there is no hope for improvement of the floating stability of the magnetic head and the stability after switching to fine control. The frequency band of an amplifier of a speed error detection circuit, explained later, is determined by the time constant of a resistor and capacitor of a filter. The cut-off frequency must be made smaller than the resonance point. In recent years, however, higher speed access has become desirable. With a "one difference" seek operation, the acceleration/deceleration current cycle of the speed control signal becomes close to the cut-off frequency. Therefore, the current waveform, in particular, the current waveform during acceleration, becomes corrupted and no further current can be passed. Since it is difficult to raise the resonance point, the problem arises that it is difficult to shorten the time for speed control. In another related art, provision is made of a servo loop for speed control which generates a target speed in accordance with a difference of a current track position from a target track and supplies to a voice coil motor a seek current corresponding to the difference between the target speed and the real speed. Here, the target speed has the characteristic of maintaining a predetermined maximum speed level when the difference is above a predetermined amount and of falling in proportion to a mean value of the difference, for example, when the difference is under a predetermined amount. The target speed becomes the minimum speed level in the case of a difference of one track. However, with such a servo circuit, since acceleration is performed by passing a seek current corresponding to the difference between a target speed and real speed, determined by the difference at the start of the seek operation, when the target speed level is extremely small, such as with a "one difference" seek operation, the rise of the acceleration current flowing to the voice coil motor becomes slow. That is, in a "one difference" seek operation, when the seek current begins to flow at the start of the seek operation and the real speed appears, the target speed level is extremely small, so the difference between the real speed and target speed disappears and the seek current is suppressed. As a result, the seek current at the time of acceleration becomes triangular in form, the rise of the acceleration current deteriorates, and the access time of a "one difference" seek operation becomes longer. In still another servo circuit, provision is made of an additional speed error preparing circuit. This circuit, however, sometimes suffers from loop resonance, which prevents a normal seek operation. Such loop resonance occurs more easily the larger the seek difference, but if this is prevented by limiting the bands of component circuits so as to try to suppress the oscillation component, the time for the speed control becomes longer and a high speed seek operation, in particular, a high speed seek operation for a small amount of difference such as a difference of one track becomes difficult. Another of our earlier applications, EP-A-0260138 discloses an apparatus for improved servo control. This is aimed at reducing the overshoot which occurs at the end of speed control when switching to position control. This reduction in overshoot is obtained by including a differential control means in the position control circuit. The differential control means has a variable gain which is initially high to overcome the overshoot, but is subsequently reduced to give accurate positioning. Yet another of our earlier applications, EP-A-0342130 discloses a servo circuit having a forward or reverse switching control. The gain characteristics of the amplifier are identical in the forward or reverse direction since the amplifier amplifies the absolute difference between actual and target position, and the amplified signal is inverted if the head must move in the reverse direction. According to the present invention a servo circuit for a magnetic disk apparatus for performing speed control of a servo object to a target position in accordance with an amount of movement of the servo object under the acceleration and then deceleration control, comprising: a target speed generating means for generating a target speed signal in accordance with the amount of movement of the servo object; a speed signal generating means for generating a real speed from a position signal obtained from the servo object; a speed error preparing means for controlling the speed of the servo object based on an error between the target speed and the real speed; and an acceleration detection means for detecting acceleration in progress based on an output of the speed error preparing means; characterised in that a cut-off frequency of the speed error preparing means is changed between acceleration in progress and deceleration in progress based on an acceleration-in- progress signal output from the acceleration detection means through a switch, such that the cutoff frequency during deceleration control is lower than the cutoff frequency during acceleration control. A further object of the present invention is to effectively prevent the occurrence of loop resonance during seek control irregardless of the magnitude of the seek difference and to enable a stable, high speed seek operation even in the case of a small difference. This is achieved by a servo circuit for a magnetic disk apparatus which performs speed control over a servo object based on a speed error between a target speed and a real speed, provided with a speed signal generating circuit which generates the real speed of the servo object based on a position signal of the servo object, the speed signal generating circuit provided with a band changing circuit for changing the band of a speed signal producing circuit in accordance with a seek difference. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 and Figs. 2A and 2B are views explaining the prior art; Fig. 3 is a view explaining problems in the prior art; Fig. 4 is a view explaining a conventional "one difference" seek operation; Fig. 5 is a view explaining the constitution a servo circuit according to the present invention; Figs. 6A and 68 are views explaining the operational characteristics of the servo circuit of Figure 5; Fig. 7 is a view explaining the constitution of a speed signal generating circuit of the servo circuit of Figure 5; Figs. 8A to 8H are views explaining operational waveforms of the speed signal generating circuit of the servo circuit of Figure 5; Figs. 9A to 9C are views of the principle of an example of the present invention; Fig. 10 is a view of the constitution of the example of Figure 9; Fig. 11 is a view of the constitution of main parts of the constitution of Fig. 10; Fig. 12 is a waveform diagram of important parts of the example of Figures 9 to 11; Fig. 13 is a view explaining a further example of the present invention; Fig. 14 is a view explaining the basic constitution of another example of the present invention; Figs. 15A to 15D are views explaining the operational characteristics of the speed signal generating circuit of the another example of the present invention; Fig. 16 is a view explaining the constitution of an embodiment of the another example of the present invention; Fig. 17 is a graph for explaining the characteristic of a final example of the present invention; and Fig. 18 and 19 are filter circuits for the final example of the present invention. Before describing the preferred embodiments, a more detailed explanation will be given of the related art for background purposes. Figure 1 is a view explaining a prior art. In the figure, 11 is a servo object, i.e., a magnetic disk mechanism, which has a voice coil motor 11a, a carriage 11b for carrying a magnetic head 110, and a magnetic disk 11c. Reference numeral 12 is a position signal demodulation circuit which demodulates a position signal Ps from servo information read from the servo surface of a magnetic disk 11c, 13a is a target speed generating circuit which generates a target speed Vc for positioning to a target position in accordance with an amount of movement from a main processing unit (mentioned later), and 13b is a forward (FWD) /reverse (RVS) switching circuit which switches the target speed of the forward direction or target speed of the reverse direction from the target speed generating circuit 13a in accordance with the forward/ reverse switching signal of the main processing unit 18. Reference numeral 14 is a speed signal preparing circuit which prepares a real speed Vr from a position signal Ps and a current signal ic, mentioned later. Reference numeral 15 is a speed error preparing circuit which generates a speed control current (signal) in accordance with a speed error ΔV between a target speed Vc and a real speed Vr and has an amplifier 150 for generating the speed error AV and a filter 151. Reference numeral 16 is a position error preparing circuit which produces a position error from the position signal Ps and current signal ic and generates a position control signal; and 17a is a coarse/fine switching unit which switches from speed control of the speed error preparing circuit 15 to position control of the position error preparing circuit 16 by a coarse (speed control)/fine (position control) switching signal from the main processing unit. Reference numeral 17b is a power amplifier which amplifies the output of the switching unit 17a and drives the voice coil motor lla of the magnetic disk mechanism 11; and 17c is a current detection circuit which detects the drive current of the power amplifier 17b and generates a current signal ic. Reference numeral 18 is a main processing unit which is constituted by a microprocessor, detects the position in accordance with the position signal Ps, outputs a remaining movement to a target speed generating circuit 13a, generates a forward/reverse switching signal in accordance with the forward/reverse direction, and generates a coarse/fine switching signal near the target position. In this servo circuit, speed control is used up to near the target position, then position control is switched to so as to control the positioning to the target position. However, the frequency characteristic of the servo object 11 has the resonance point f2 as shown in Fig. 2A. This resonance point f2 is a torsional resonance point of the servo arm such as the carriage llb of the servo object 11. If the frequency of the speed control signal appears in the band which covers this resonance point, mechanical vibration appears during the seek operation (movement) and therefore there is no hope for improvement of the floating stability of the magnetic head 110 and the stability after switching to fine control. Therefore, the frequency band of the amplifier 150 of the speed error detection circuit 15 is determined by the time constants of the resistor and capacitor of the filter 151, and the cut-off frequency fl of the frequency band shown in Fig. 2B must be made smaller than f2, for example, f2 = 2fl to 1.5 fl. In recent years, however, higher speed access times have become desirable. With a "one difference" seek operation, as shown in Fig. 3, the acceleration/ deceleration current IC cycle of the speed control signal has become close to the cut-off frequency fl. Therefore, as shown by the solid line of Fig. 3, the current waveform of the IC, in particular, the current waveform during acceleration, becomes corrupted and no further current can be passed, as shown by the dotted line. Since it is difficult to raise the resonance point f2 of the mechanical characteristics, the problem arises that it is difficult to shorten the time for speed control. In another related art, provision is made of a servo loop for speed control which generates a target speed in accordance with a difference of a current track position with a target track and supplies to the voice coil motor a seek current corresponding to the difference between the target speed and the real speed. Here, the target speed has the characteristic of maintaining the predetermined maximum speed level when the difference is above a predetermined value and of falling in proportion to a mean value of the difference, for example, when the difference is below a predetermined value. The target speed becomes the minimum speed level when there is a difference of one track. However, with such a servo circuit, since acceleration is performed by passing a seek current corresponding to a difference between the target speed and real speed determined by the difference at the start of the seek operation, when the target speed level is extremely small, such as with a "one difference" seek operation, the rise of the acceleration current flowing to the voice coil motor becomes slow. That is, as shown in Fig. 4, in a "one difference" seek operation, when the seek current begins to flow at the start of the seek operation at the time tl and the real speed appears, the target speed level is extremely small, so the difference between the real speed and target speed disappears and the seek current is suppressed. As a result, the seek current at the time of acceleration becomes triangular in form, the rise of the acceleration current OA deteriorates, and the access time of a "one difference" seek operation becomes longer. Figure 5 is a block diagram showing the basic constitution of a servo circuit according to the present invention. In Fig. 5, reference numeral 411 is a magnetic disk mechanism used as the servo object. This is provided with a voice coil motor VCM 4111, a carriage 4113 mounting a magnetic head 4112, and a magnetic disk 4114. Reference numeral 412 is a position signal demodulation circuit which demodulates a position signal Ps showing the position of movement from servo information read by the servo head 4112 from the servo surface of the magnetic disk 4114. Reference numeral 413 is a target speed generating circuit which generates a target speed Vc for positioning the magnetic head 4112 of the servo object 411 at a target position in accordance with an amount of movement commanded by a main processing unit. Reference numeral 414 is a speed signal generating circuit which prepares a real speed Vr of the magnetic head 4112 in the servo object 411 from the position signal Ps and the current signal ic. The current signal ic is a compensating signal which is added for the smooth production of the real speed Vr. Reference numeral 415 is a speed error preparing circuit which is provided internally with a filter 4150 and generates a speed error signal AV for controlling the servo object 411 based on a speed error between the target speed Vc and the real speed Vr. Reference numeral 416 is a position error preparing circuit which produces a position error of the servo object 411 with respect to a target position from the position signal Ps and the current signal ic and generates the position control signal ΔP. Reference numeral 417 is a servo object drive circuit which performs processing for switching from speed control by the speed error preparing circuit 415 to the position control of the position error preparing circuit 416 by a coarse (speed control)/fine (position control) switching signal from the main processing unit, amplifies the power of the error signal ΔV or ΔP, performs processing for outputting the voice coil motor current Ic for driving the voice coil motor 4111 of the servo object 411, detects the voice coil motor current Ic, and performs processing for generating the current signal ic. Reference numeral 418 is a main processing unit which is constituted by a microprocessor, detects the position of the magnetic head 4112 based on the position signal Ps, outputs the remaining amount of movement, i.e., the remaining difference, to the target speed generating circuit 413, and generates a coarse/fine switching signal near the target speed. Reference numeral 419 is an acceleration detecting circuit, which detects the speed error signal AV generated by the speed error preparing circuit 415 and generates an acceleration-in-progress signal SKA which changes the band of the speed error preparing circuit 415 in accordance with the amount of seek difference during acceleration and deceleration. The servo circuit constructed as above performs speed control up to near the target position and then switches to position control and performs control for positioning to the target position. This control operation is basically the same as the servo control operations of conventional servo circuits, so the remaining explanation will primarily concentrate on the operation of the speed error preparing circuit 415 referring to Figs. 6A and 6B. When the speed control starts due to a command from the main processing unit 418, the target speed generating circuit 413 generates a target speed Vc during acceleration control from the amount of movement, that is, amount of difference, commanded by the main processing unit 418. Further, the speed signal generating circuit 414 receives the position signal Ps from the position signal demodulation circuit 412 and the current signal ic from the servo object drive circuit 417 and prepares a real speed Vr of the servo object 411 (magnetic head). The speed error preparing circuit 415 generates a speed error signal ΔV corresponding to the speed error between the target speed Vc generated by the target speed generating circuit 413 and the real speed Vr generated by the speed signal generating circuit 414. On the other hand, the acceleration detecting circuit 419 detects from the speed error signal AV generated by the speed error preparing circuit 415 that the speed error preparing circuit 415 is in the middle of acceleration control and thereupon adds the acceleration-in-progress signal SKA to the speed error preparing circuit 415. When the filter 4150 of the speed error preparing circuit 415 receives the acceleration-in-progress signal SKA from the acceleration detecting circuit 419, the cut-off frequency f10 becomes infinitely large or higher than even the resonance point f2 of the servo object 411. Due to this, the cut-off frequency fl0 of the speed error preparing circuit 415 during acceleration control becomes infinitely large or higher than even the resonance point f2 of the servo object 411 as shown in Fig. 6A. The finitely large cut-off frequency fl0 in this case is set to a high frequency in a range where no oscillation will occur in the acceleration current. The servo object drive circuit 417 amplifies the power of the speed error signal ΔV received from the speed error preparing circuit and outputs the voice coil motor current Ic for driving by acceleration the voice coil motor 4111 of the servo object 411 and also generates a current signal ic, i.e., the detected current of the voice coil motor current Ic. By this, the curve of the voice coil motor current Ic corresponding to the speed error signal ΔV generated by the speed error preparing circuit 415 during acceleration control becomes a very clean, sharply inclined one, as shown by Fig. 6B, so it is possible to end the acceleration control in a short time. When the acceleration control is ended, the main processing unit 418 switches to deceleration control. When deceleration control is switched to upon the command of the main processing unit 418, the target speed generating circuit 413 generates a target speed Vc for deceleration control. The speed error preparing circuit 415 generates a speed error signal ΔV (negative) corresponding to the speed error between a target speed Vc generated by the target speed generating circuit 413 and the real speed Vr generated by the speed signal generating circuit 414. On the other hand, the acceleration detecting circuit 419 stops the generation of the acceleration-in-progress signal SKA upon completion of the acceleration control of the speed error preparing circuit 415. When the speed error preparing circuit 415 enters deceleration control, the filter 4150 of the speed error preparing circuit 415 lowers the cut-off frequency to a frequency below the resonance point f2 of the servo object 411. At this time, the larger the difference, the lower the cut-off frequency is made. By this, the cut-off frequency of the speed error preparing circuit 415 during deceleration control is reduced to fll, f12, ... fln the larger the amount of difference, as shown in Fig. 6A. The servo object drive circuit 417 amplifies the power of the speed error signal ΔV received from the speed error preparing circuit 415 and outputs a voice coil motor current Ic corresponding to the seek current for controlling under deceleration the voice coil motor 4111 and also generates a current signal ic, i.e., the detected current of the voice coil motor current Ic. By this, the curve of the deceleration current, i.e., the voice coil motor current Ic corresponding to the speed error signal ΔV generated by the speed error preparing circuit 415 during deceleration control, becomes as shown in Fig. 6B. That is, there is less corruption of the deceleration curve the smaller the amount of difference, and the deceleration control time, that is, the speed control time, falls such as tn, ... t2, tl. In the above way, the cut-off frequency of the speed error preparing circuit is made to change between the times of acceleration control and deceleration control and also the cut-off frequency during deceleration control is made to change in accordance with the amount of difference, so during acceleration control, the corruption of the acceleration current supplied to the servo object can be reduced and the degree of the corruption of the deceleration current during deceleration control can be reduced the less the amount of difference, so even when the amount of difference is small, it is possible to stably shorten the speed control time. By this, it is possible to perform speed control stably and at a high speed even with an extremely small difference such as a difference of one track and thus stable, high speed seek operations become possible. Next, an explanation will be made of the constitution and operation of the speed signal generating circuit 414 referring to Fig. 7 and Figs. 8A to 8H. In Fig. 7, reference numeral 4141 is a differential circuit which differentiates the position signal Ps and generates a differential speed Vps. Reference numeral 4143 is an amplifier which amplifies the current signal ic up to a predetermined level. Reference numeral 4144 is an offset adjusting circuit which generates an offset compensating signal for adjusting the offset of the real speed Vr generated from the speed signal generating circuit 414. Reference numerals 4145a to 4145c are variable resistors, the variable resistor 4145a being used to adjust the output gain of the amplifier 4143a, the variable resistor 4145b being used to adjust the output gain of the differential circuit 4141, and the variable resistor 4145c being used to adjust the output gain of the offset adjusting circuit 4144. Reference numeral 4142 is a real speed generating circuit, which amplifies and integrates the added signal of the level adjusted differential signal Vps, current speed ic, and the offset signal and prepares the real speed Vr. In this construction, the position signal Ps shown in Fig. 8A is applied to the differential circuit 4141 from the position signal demodulation circuit 412. The position signal Ps has a waveform which crosses zero each time the magnetic head passes through the track. The differential circuit 4141 differentiates the position signal Ps and generates a differential speed Vps shown in Fig. 8B. This differential speed Vps becomes zero at each of the largest points of the position signal Ps and peaks at the each point where it crosses zero. The envelope connecting the peaks gives the real speed Vr of the servo object 411 (magnetic head). If just the differential signal Vps generated from the differential circuit 4141 is given to the real speed generating circuit 4142, differentiation is performed and the envelope of the differential speed Vps is obtained. In this case, at places where the peak interval of the differential speed Vps is small, that is, at places of fast speed of the Vr, a good envelope characteristic is obtained, but at places with large peak intervals, that is, places of slow speed (starting and ending times of seek operations), good envelope characteristics cannot be obtained. To correct the deviations in the envelope characteristics, the current signal ic may be added. The current signal ic shown in Fig. 8C from the servo object drive circuit 417 is amplified by the amplifier 4143 and added to the differential speed Vps. The current signal ic resembles the voice coil motor current Ic supplied to the servo object, so can effectively correct the deviations in the envelope mentioned earlier. The real speed generating circuit 4142 integrates and amplifies the added signal of the differential speed Vps, the current signal ic, and the offset correction signal and generates a real speed Vr having a smooth characteristic shown in Fig. 8D. Note that since an accurate real speed Vr is produced, the output gains of the current signal ic, the differential speed Vps, and the offset correction signal are initialized by the variable resistors 4145a to 4145c. If the speed signal generating circuit 414 in the servo circuit has a fixed frequency band regardless of the seek difference oscillation may cause a problem. The band of the speed signal generating circuit 414 is defined by the band of the real speed generating circuit 4142, that is, the time constants of the integration resistor 4142b and capacitor 4142c. Therefore, when oscillation is generated during seek acceleration control due to the mechanical resonance of the servo object 411, as shown in Fig. 8E, the position signal Ps takes on a waveform with the oscillation superposed. When the amount of movement of the servo object 411 (magnetic head), that is, the amount of seek difference, is large, the acceleration time and the acceleration voice coil motor current Ic become large, so oscillation readily occurs. When oscillation is generated in the position signal Ps in this way, oscillation is also caused in the differential speed Vps as shown in Fig. 8F, and oscillation is caused in the real speed Vr generated by the real speed generating circuit 4142 (Fig. 8G). Since this oscillating real speed Vr is received and oscillation occurs in the speed error signal ΔV generated by the speed error preparing circuit 415, oscillation also occurs in the voice coil motor current Ic generated by the servo object drive circuit 417 (Fig. 8H). As a result, the amplitude of the servo object 411 is amplified, loop resonance occurs, and a normal seek operation is no longer possible. By limiting the bands of one or both of the speed signal generating circuit 414 and the speed error preparing circuit 415 so as to try to suppress the oscillation component, the time for the speed control becomes longer and a high speed seek operation, in particular, a high speed seek operation with a small amount of difference such as a difference of one track, becomes difficult. However, by changing the cut-off frequency, the problem of speed, and of resonance may be overcome. As shown in Figure 9A, there is provided a servo circuit for a magnetic disk apparatus having a target speed generating circuit 13a for generating a target speed in accordance with an amount of movement; a speed signal producing circuit 14 for producing a real speed from a position signal from a servo object 11; and a speed error generating circuit 15 for controlling the speed of the generating circuit 15 for controlling the speed of the servo object 11 based on an error between the target speed and the real speed; in accordance with the amount of movement, acceleration and then deceleration control being performed for positioning at the target position, wherein provision is made of an acceleration detection circuit 19 for detecting acceleration in progress from an output of the speed error generating circuit 15 and an acceleration-in-progress detection signal of the acceleration detection generating detection circuit 12 is used for changing a cut-off frequency of the speed error generating circuit 15 during acceleration and deceleration. Explaining the mode of operation in more detail, as shown by Figs. 9B and 9C, the cut-off frequency is made infinitely large or a large f2 during acceleration and is changed to a small fl during deceleration, thereby reducing the corruption of the acceleration current during acceleration. During acceleration, the power amplifier does not function as a current source and a voltage large enough that a power source voltage is applied to the motor is input, so merely the acceleration current increases. This is equivalent to no servo control being exercised. Therefore, even if the cut-off frequency is made infinitely large or large, it is difficult for mechanical (servo object) oscillation to occur. Accordingly, the cut-off frequency may be made large, the corruption of the acceleration current reduced, and the acceleration time shortened. Due to this, as shown by the solid lines of Figs. 9B and 9C, the acceleration time is shortened compared with the prior art shown by the dotted line and therefore the speed control time can be shortened. Further, during deceleration, the cut-off frequency is returned to fl, so mechanical oscillation does not occur overall. Figure 10 is a view of an example of the present invention, and Fig. 11 is a view of the constitution of key parts of Fig. 10. In the figure, portions the same as those shown in Figs. 9A to 9C are given the same reference numerals. In the figure, the filter 151 is constituted by a parallel circuit of a resistance R with a switch SW and a capacitor C. Reference numeral 190 is a comparator which performs zero volt slicing of the output VCMCRT (speed error current) of the amplifier 150 and outputs a speed slice signal VSL, and reference numeral 191 is an inversion circuit which inverts the speed slice signal VSL. Reference numerals 192 and 193 are each three-input AND gates which generate a high level output only when all of the three inputs are high level. The AND gate 192 obtains the high level AND of the forward movement signal *MvF from the main processing unit (MPU) 18, the capture velocity signal *CVL generated when the real speed from the main processing unit 18 is less than a predetermined level, and the output of the comparator 190. The AND gate 193 obtains the high level AND of the reverse movement signal *MvR from the main processing unit 18, the capture velocity signal *CVL from the main processing unit 16, and the output of the comparator 191. Reference numeral 194 is an OR gate which obtains the OR of the outputs of the AND gates 192 and 193, and reference numeral 195 is an inversion circuit which inverts the output of the OR gate 194 and generates an end acceleration signal EAC. Reference numeral 196 is a flip-flop which is cleared by an access start *ASP from the main processing unit 18, inverts using as a clock the end acceleration signal EAC, and generates an acceleration-in-progress signal SKA from the Q output. Figure 12 is a waveform diagram of key parts of an example of the present invention. In forward movement, the main processing unit 18 generates a target speed FwV c in the forward direction from the target speed generating circuit 13a. The main processing unit 18 generates a forward movement signal *MvF. Further, in the case of a small difference and small speed, it generates a capture velocity signal *CVL from the real speed Vr. In the speed error preparing circuit 15, the speed error between the target speed FwVc and the real speed Vr is obtained at the amplifier 150 and a speed error signal AV is generated. At this time, the flip-flop 196 of the acceleration detection circuit 19 is cleared by the access start signal *ASP generated along with the start of the seek operation (access), so a high level acceleration-in progress signal SKA is generated from the Q output. By this, the switch SW of the filter 151 turns off, the cut-off frequency of the amplifier 150 becomes infinitely large, and the speed error signal AV rises sharply. The speed error signal AV is zero volt sliced by the comparator 190, so when the speed error signal AV is positive, a high level speed slice signal VSL is generated. Therefore, a high level output is generated from the AND gate 192 and a low level end acceleration signal EAC is generated through the OR gate 194 and the inversion circuit 195. When the speed error signal AV becomes zero, the speed slice signal VSL of the comparator 190 becomes the low level, the output of the AND gate 192 becomes the low level, the end acceleration signal EAC through the OR gate 194 and inversion circuit 195 becomes the high level, the flip-flop 196 inverts using this point of change as a clock, and the acceleration-in-progress signal SKA of the Q output becomes the low level. Therefore, the switch SW of the filter 151 turns on and the cut-off frequency of the amplifier 150 becomes fl. In the succeeding deceleration period, the speed error signal ΔV is restricted in band by the cut-off frequency fl. Due to this, the current VCMCRT flowing through the power amplifier 17b to the voice coil motor lla rises substantially linearly during the acceleration period as shown in Fig. 12. On the other hand, in reverse movement, when starting the access for a difference of one track, the main processing unit 18 generates an access start signal *ASP, generates a reverse direction target speed RvVc from the target speed generating circuit 13a, generates a reverse movement signal *MvR, and generates a capture velocity signal *CVL after slicing the speed signal and detecting the speed-down. In the speed error preparing circuit 15, the speed error between the target speed RvVc and the real speed Vr is obtained and the speed error signal ΔV generated. At this time, due to the access start signal *ASP, the flip-flop 196 of the acceleration detection circuit 19 is cleared, so a high level acceleration-in-progress signal SKA is generated from the Q output. By this, the switch SW of the filter 151 is turned off, the cut-off frequency of the amplifier 150 is made infinitely large, and the speed error signal ΔV rises sharply. The speed error signal ΔV is zero volt sliced by the comparator 190 and, when the speed error signal ΔV is negative, a low level speed slice signal VSL is generated. Therefore, a high level output is generated from the AND gate 193, and a low level end acceleration signal EAC is generated through the OR gate 194 and inversion circuit 195. When the speed error signal ΔV becomes zero, the speed slice signal VSL of the comparator 190 becomes high level, the output of the AND gate 193 becomes low level, the end acceleration signal EAC through the OR gate 194 and inversion circuit 195 becomes high level, the flip-flop 196 inverts using this point of change as a clock, and the acceleration-in-progress signal SKA of the Q output becomes low level. Therefore, the switch SW of the filter 151 turns on, and the cut-off frequency of the amplifier 150 becomes fl. As a result, during the deceleration period, the speed error signal ΔV is limited in band by the cut-off frequency fl. By this, the current VCMCRT flowing through the power amplifier 17b to the voice coil motor 11a rises substantially linearly during the acceleration period as shown in Fig. 12. In this way, in a "one difference" seek operation, the switch SW of the filter 151 is turned off by the access start signal, the cut-off frequency is made infinitely large, switch SW is turned on by the end of the acceleration, and the cut-off frequency becomes fl, which is determined by the capacitor C and the resistor R. Figure 13 is a view explaining another example of the present invention. In this example, the filter 151 is constituted by a parallel circuit of a resistor R, a capacitor C12, and a switch SW and capacitor C2. This filter 151 can be modified to the band changing circuit 4140 shown in Fig. 15A. Therefore, when the switch SW is turned ON, as shown in Fig. 9C, the cut-off frequency changes to fl, and when the switch SW is turned OFF, the cut-off frequency changes to f2. By the same operation as in Fig. 10 and Fig. 11, in a "one difference" seek operation, during acceleration, the switch SW is turned OFF by the acceleration-in-progress signal SKA and the cut-off frequency becomes f2, so the corruption of the acceleration signal is reduced, while during deceleration, the switch SW is turned ON and the cut-off frequency becomes fl. In this example, the band was restricted even during acceleration, so there was absolutely no worry of mechanical oscillation, but the corruption of the acceleration current is greater than in the example of Fig. 10 and the acceleration time is longer. In the above examples of the present invention, the explanation was made taking as an example a magnetic disk apparatus, but the invention may be applied to other apparatuses as well and it is possible to change the cut-off frequency with respect to the desired distance of movement. Figure 14 is a block diagram of the basic constitution of a further example of the present invention. As shown in Fig. 14, there is provided a servo circuit for a magnetic disk apparatus which performs speed control over a servo object based on a speed error between a target speed and a real speed, wherein provision is made of a speed signal generating circuit 414A which generates the real speed Vr of the servo object based on a position signal Ps of the servo object 411 from a position signal demodulation circuit, the speed signal generating circuit provided with a band changing circuit 4140 for changing the band of a speed signal producing circuit 414B in accordance to a seek difference. As shown in Fig. 15A, when the seek difference is small, a frequency f31 close to the resonance point f2 of the servo object is set to. The larger the seek difference, the lower the frequency f32, ... f3n than the resonance point f2 is set to. Preferably, the speed signal generating circuit is provided with a differentiating circuit 4141 which differentiates the position signal to generate a speed, a real speed generating circuit 4142 which generates the real speed based on the differential signal, and a band changing circuit 4140 which changes the band of the real speed generating circuit 4142 according to the seek difference. The operation of this example will now be explained referring to the graphs of Figs. 15A to 15D. Although Fig. 15B to 15C are the same waveforms as that of Figs. 8D and 8F, these figures are used for explaining the example of the present invention. Before the start of the seek operation, the speed signal generating circuit 414A is changed in band by the band changing circuit 4140 in accordance with the seek difference. As shown in Fig. 15A, the larger the seek difference, the small the band set to. The speed signal generating circuit 414A generates a real speed Vr of the servo object based on the position signal Ps. The real speed Vr is prepared, for example, by differentiating the position signal Ps and producing a differential speed Vps, then integrating (smoothening) the differential signal Vps. In the case of a small difference such as in a "one difference" seek operation, as explained in Fig. 5, the band of the speed error preparing circuit is set wide, but the band of the speed signal generating circuit 4141 is also set correspondingly wide, so the acceleration control time is shortened and a high speed seek operation is possible. In the case of a small difference, the oscillation frequency generated also becomes high, so even if the bands of the speed signal generating circuit 414A and speed error preparing circuit are made wide, it is possible to effectively suppress the oscillation component and to prevent the occurrence of loop resonance. When the seek difference is large, the band of the speed signal generating circuit 414A is narrow, so as shown in Fig. 15B, even if oscillation occurs in the position signal Ps due to the mechanical oscillation generated in the servo object, at the real speed Vr output, the oscillation component is effectively suppressed and does not appear (Fig. 15D). Therefore, the occurrence of loop resonance is prevented and a stable seek operation is made possible. If the band of the speed signal generating circuit 414A is made narrow, the time constant of the speed signal generating circuit 414A becomes large, so the time for speed control increases, but since the overall speed control time is long, the degree of that increase is small and the increase in the speed control time in the case of such a seek difference does not pose a problem in practice. As mentioned above, the band of the speed signal preparing circuit is changed in accordance with the seek difference, so irregardless of the magnitude of the seek difference, it is possible to effectively prevent the occurrence of loop resonance in seek control and it is possible to achieve stable, high speed seek operations even in the case of a small seek difference such as a difference of one track. Figures 15A to 15D are waveform diagrams used for explaining the operation of the example. Figure 16 is a block diagram of the constitution of an embodiment of the speed signal generating circuit. The speed signal generating circuit 414A and the band changing circuit 4140 are as explained in Fig. 14. Other portions corresponding in constitution to those in the related servo circuit are given the same reference numerals. That is, reference numeral 411 is a servo object, and 412 is a position signal demodulation circuit for generating a position signal Ps. Reference numeral 413 is a target speed generating circuit for generating a target speed Vc, and 415 is a speed error preparing circuit which generates a speed error signal ΔV from the speed error between the target speed vc and the real speed Vr generated by the speed signal generating circuit 414A. Reference numeral 416 is a position error preparing circuit which produces a position error ΔP based on the position signal Ps and the current signal ic. Reference numeral 417 is a servo object drive circuit which amplifies the power of the speed error signal ΔV during speed (coarse) control and the position error signal ΔP during position (fine) control, generates a voice coil motor current Ic for driving the servo object 411, supplies it to the servo object 411, detects the voice coil motor current Ic, and generates the current signal ic. Reference numeral 418 is a main processing unit which supplies a band changing signal BCX1 to BCXn to the band changing circuit 4140 of the speed signal generating circuit 414A, sends the necessary control signals to the circuits, and controls the overall seek operation of the servo circuit. Reference numeral 419 is an acceleration detecting circuit, which detects the speed error signal AV generated by the speed error preparing circuit 415 and generates an acceleration-in-progress signal SKA which changes the band of the speed error preparing circuit 415 in accordance with the amount of seek difference during acceleration and deceleration. In the speed signal generating circuit 414A, 4142 is a real speed generating circuit. In this real speed generating circuit 4142, 4142a is a amplifier such as an operational amplifier, 4142b is an integration resistance, and 4142c is an integration capacitor. Reference numeral 4142d is an offset compensation resistor of the amplifier 4142a. Reference numeral 4144 is an offset adjustment circuit which adjusts the offset of the real speed Vr. Reference numerals 4145a, 4145b, and 4145c are variable resistors for adjusting the gains of the differential circuit 4141, the amplifier 4143, and the offset adjustment circuit 4144. The capacitance of the integration capacitor 4142c is a small capacitance required in the case of the minimum seek difference (difference of one track). In the band changing circuit 4140, reference numerals 4146₁ to 4146 n are capacitors for band adjustment. Reference numerals 4147₁ to 4147 n are switches provided in series with the corresponding capacitors 4146₁ to 4146 n and are turned on and off by the band changing signals BCX1 to BCXn from the main processing unit 418. In the servo circuit of Fig. 16, the operations of the circuits other than the real speed preparing operation of the speed signal generating circuit 414A and the overall seek operation are the same as the operations of the circuits and the overall seek operation of the related servo circuit, so the following explanation will concentrate on the operation of the speed signal generating circuit 414A. Before the start of the seek operation, the main processing unit 418 turns on the band changing signal BCXi corresponding to the seek difference. The band changing circuit 4140 of the speed signal generating circuit 414A, when receiving this band changing signal BCXi, turns on the corresponding switch 4147i. By this, the capacitor 4146i becomes parallel with the resistor 4142b along with the capacitor 4142c, so the band of the real speed generating circuit 4142, that is, the speed signal generating circuit 414A, is set to be narrower the larger the seek difference, as shown in Fig. 15A. Further, the offset adjustment circuit 4144 generates an offset compensation signal which performs adjustment so that the offset of the real speed Vr generated by the speed signal generating circuit 414A becomes zero. Further, the values of the variable resistors 4146a to 4146c are initialized corresponding to the output gains of the differential circuit 4141, the amplifier 4143, and the off-set adjustment circuit 4144 so that the correct real speed Vr is produced. When the seek operation is started, the differential circuit 4141 differentiates the position signal Ps input from the position signal demodulation circuit 412 and generates the differential signal Vps (Figs. 15B and 15C). Note that Figs. 15B and 15C shows the case where there is oscillation in the position signal Ps and the differential signal Vps. On the other hand, the amplifier 4143 amplifies the current signal ic generated from the servo object 411. The real speed generating circuit 4142 amplifies and integrates the differential signal Vps, current signal ic, and offset adjustment signal adjusted in level by the variable resistors 4145a to 4145c and prepares the real speed Vr. The real speed generating circuit 4142 is band restricted by the band changing circuit 4140, so even if there is oscillation in the differential speed Vps, this oscillation component is suppressed and does not appear in the output real speed Vr (Figs. 15C and 15D). Therefore, the oscillation is suppressed and does not appear either in the speed error signal ΔV generated by the speed error preparing circuit 415 based on the speed error between the real speed Vr and the target speed Vc from the target speed generating circuit 413 and the voice coil motor current Ic of the servo object drive circuit 417. By this, even if mechanical oscillation occurs, even temporarily, in the servo object 411 at the time of the start of the seek operation, loop resonance can be prevented and a high speed seek operation can be stably performed. The band changing circuit 4140 may be made a filter in construction and inserted at the output side or input side of the real speed generating circuit 4142, and that band changed and controlled in accordance with the amount of seek difference. Further, a plurality of capacitors can be combined in the band changing circuit 4140 of Fig. 16 so as to obtain the desired capacitance. By this, it is possible to produce a desired capacitance corresponding to various seek differences by a small number of capacitors. The speed signal generating circuit can of course be applied to servo circuits other than the related servo circuit. As explained above, the invention changes the band of the speed signal generating circuit in accordance with the amount of seek difference, so it is possible to effectively prevent the occurrence of loop resonance during the seek control irregardless of the magnitude of the seek difference. Due to this, it is possible to achieve stable, high speed seek operations even with small seek differences such as a difference of one track. A further example of the present invention will now be explained with respect to Figs. 17 to 19. Fig. 17 is a graph for explaining the characteristic of the example. When the filter of the speed error preparing circuit 15 receives the acceleration-in-progress signal SKA from the acceleration detection circuit 19, the cut-off frequency f₁₀ of the filter is set to the infinite frequency or the higher frequency than the resonance point frequency f₂. In this case, the cut-off frequency f₁₀ is set to the extent without occurring the vibration of the acceleration current. When the acceleration is completed, the main processing unit 18 changes from the acceleration control to the deceleration control, and the acceleration-in-progress signal SRA is stopped. The filter of the speed error preparing circuit 15 is set the lower frequency than the resonance point frequency f2 when the speed error preparing circuit 15 starts to perform the deceleration control. In this case, the cut-off frequency becomes lower in accordance with the larger difference amount. Accordingly, in the deceleration control, the cut-off frequency of the filter of the speed error preparing circuit lowers as shown by f₁₁, f₁₂, --- f 1n in accordance with the larger difference amount. Figures 18 and 19 are filter circuits for the example. The filter shown in Fig. 19 is the same structure as the filter shown in Fig. 18 except that the capacitor 5152a having smaller capacitance is added in parallel to the resistor 5151. In Fig. 18, the resistors and capacitors are connected each other as shown in the figure. The switch 5153 is switched by the acceleration-in-progress signal SKA from the acceleration detection circuit 19. The switches 5153a to 5153n are switched by the filter switching signals FCS₁ to FCS n . These switching operation are the same as the preceding example of the present invention. The capacitor 5152a shown in Fig. 19 is set to the smaller value in the extent without occurring the vibration of the servo object.

Claims: A servo circuit for a magnetic disk apparatus for performing speed control of an servo object (411) to a target position in accordance with an amount of movement of the servo object under the acceleration and then deceleration control, comprising: a target speed generating means (413) for generating a target speed signal in accordance with the amount of movement of the servo object (411); a speed signal generating means (414) for generating a real speed from a position signal obtained from the servo object (411); a speed error preparing means (415) for controlling the speed of the servo object (411) based on an error between the target speed and the real speed; and an acceleration detection means (419) for detecting acceleration in progress based on an output of the speed error preparing means (415); characterised in that a cut-off frequency of the speed error preparing means (415) is changed between acceleration in progress and deceleration in progress based on an acceleration-in- progress signal output from the acceleration detection means (419) through a switch, such that the cutoff frequency during deceleration control is lower than the cutoff frequency during acceleration control. A servo circuit as claimed in claim 1, wherein said speed error preparing means comprises a CR filter (151) for changing the cut-off frequency, the change of the cut-off frequency being performed by connection or disconnection of a capacitor of the CR filter based on the acceleration-in-progress signal. A servo circuit as claimed in claims 1 or 2, wherein said cut-off frequency is set to a predetermined frequency by connecting the capacitor in the deceleration in progress, and set to a higher frequency than the predetermined frequency by disconnecting the capacitor in the acceleration in progress. A servo circuit for a magnetic disk apparatus according to any of the preceding claims, in which a seek current based on the error signal is supplied to a voice coil motor (11a) for performing speed control on a servo object toward a target track position, wherein said speed signal generating means comprises a band changing circuit (4140) for changing the band of the real speed signal in accordance with the seek difference. A servo circuit as claimed in claim 4, wherein said speed signal generating means further comprises a differentiating circuit (4141) for differentiating the position signal to generate the speed signal, and a real speed generating circuit for generating the real speed based on the differential signal. A servo circuit as claimed in claim 4, wherein said band changing circuit comprises a plurality of switches (4147) and capacitors (4146), each switch and capacitor being connected in series, and each of pairs of series connected switch and capacitor being connected in parallel. A servo circuit as claimed in claim 6, wherein each of the switches is switched by a band changing signal generated from the main processing unit (418). A servo circuit for a magnetic apparatus according to any of the preceding claims, further comprising: a main processing means (418) for setting an amount of the movement of the servo object and positioning the servo object to the target position by the deceleration after acceleration; wherein the speed error preparing means includes a filter (151) for changing a cut-off frequency between acceleration in progress and deceleration in progress, and changing the cut-off frequency at the deceleration in correspondence with an amount of the movement based on an acceleration-in-progress signal output from the acceleration detection means. A servo circuit as claimed in claim 8, wherein said filter adjusts the cut-off frequency so as to become an infinity frequency in the acceleration, and to lower the cut-off frequency in the deceleration when an amount of the movement becomes larger. A servo circuit as claimed in claim 8, wherein said filter adjusts the cut-off frequency so as to become a finite frequency higher than the resonance point frequency of the servo object in the acceleration, and to lower in the deceleration when an amount of the movement is larger.

Inventors: HASHIMOTO SHUICHI, SHINOHARA TOHRU SAYOSOU, HASHIMOTO, SHUICHI, SHINOHARA, TOHRU SAYOSOU 202
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G11B5
IPC codes: G11B5, G11B21