Servo circuit for magnetic disk apparatus

Abstract: A servo circuit for a magnetic disk apparatus which controls a servo object (212) based on an error between a target speed and a real speed, in which a pre-determined control voltage is added to the control signal for the servo object at the start of speed control.

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. An alternative servo circuit for a magnetic disk apparatus is shown in our co-pending application EP-A-0,378,329 having the same date of filing as the present application. According to this circuit, the cut-off frequency of the speed generating circuit is changed depending on the acceleration and deceleration of the servo object. In another co-pending application, EP-A-0, 614, 174 (Attorneys' reference: 80/4782/02) also having the same date of filing as the present application, the gain of an amplifier is made large at the start and end of the seek operation, but is reduced during the seek operation. According to the present invention a servo circuit for a magnetic disk apparatus for supplying a seek current based on an error between a target speed and a real speed to a voice coil motor and performs speed control on a servo object toward a target track position, comprising: a target speed generating means for generating a target speed depending on the initial jump distance 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 the error between the target speed and the real speed; and an acceleration means provided between the speed error preparing means and the voice coil for adding a pre-determined control voltage for acceleration of the speed of the servo object to the error during the stage of acceleration after a start of the speed control. Examples of the present invention will now be described and contrasted with the prior art in respect of the accompanying drawings, in which:- 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 of a related servo circuit; Figs. 6A and 68 are views explaining the operational characteristics of the related servo circuit; Fig. 7 is a view explaining the constitution of a speed signal generating circuit of the related servo circuit; Figs. 8A to 8H are views explaining operational waveforms of the speed signal generating circuit of the related servo circuit; Figs. 9A to 9C are views explaining the principle of a second aspect of the present invention; Fig. 10 is a view of the constitution of an embodiment of the second aspect of the present invention; Fig. 11 is a view of the constitution of an embodiment of an acceleration control circuit of the second aspect of the present invention; Fig. 12 is a timing chart of the operation of the embodiment of Figs. 10 and 11; Fig. 13 is a view of the constitution of another embodiment of the second aspect of the present invention; Fig. 14 is an operational timing chart of the embodiment of Fig. 13. 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 ΔV 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 11a 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 11b 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 another servo circuit previously proposed by the applicant to meet this demand ("related" servo circuit). 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 ΔV 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 ΔV 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 ΔV 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 f10 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 f10 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 fl1, fl2, ... 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, t1. 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. The speed signal generating circuit 414 in the related servo circuit has a fixed frequency band regardless of the seek difference. 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. Such loop resonance occurs more easily the larger the seek difference, but if this is prevented 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. This is a problem of not only the related servo circuit, but conventional servo circuits as well. In the present application, there is disclosed a servo circuit for a magnetic disk apparatus provided with a servo loop which supplies a seek current based on an error signal ΔE 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, wherein provision is made in the servo loop of an addition means 214 for adding a pre-determined addition value (voltage) Ecc to the error signal ΔE at the stage of acceleration after a start of the speed control. Preferably, as shown in Fig. 9A, the acceleration means 214 has a switching means 218 for selectively supplying the predetermined acceleration value Ecc to an addition point 216 where the error signal ΔE of the servo loop is input and turns on the switching means 218 at the start of the seek operation and turns off the switching means 218 at detection of completion of the acceleration. More preferably, as shown in Fig. 9B, the acceleration means 214 has a switching means 218 for selectively supplying the predetermined acceleration value Ecc to an addition point 216 where the error signal ΔE of the servo loop is input and is provided with a timer means 220 which turns on the switching means 218 at the start of the seek operation and turns off the switching means 218 after the elapse of a predetermined time from the start of the seek operation. In the servo circuit for a magnetic disk apparatus according to the present invention, even in a "one difference" seek operation where the target speed is extremely small, a predetermined acceleration value is added to the servo loop regardless of the error between the target speed and the real speed, so during the acceleration period, the seek current can rapidly rise to the saturation level and a sufficient acceleration current can flow, so the access time for a "one difference" seek operation can be greatly reduced and demands for a higher speed magnetic disk apparatus can be met. Figure 10 is a view of an embodiment of the present invention. In Fig. 10, reference numeral 222 is a main processing unit constituting a control logic. When an access signal is received from a host computer, target speed data for the seek operation is generated based on the current position of the head at that time and the target track position and a forward command c1 or reverse command signal c2 is output in accordance with the target track direction. Reference numeral 224 is a target speed generating circuit which converts target speed data corresponding to the forward or reverse command from the main processing unit 222 to analog signal voltage by a digital to analog converter, generates a forward target speed signal through a switch 226-1 turned on by a forward command signal cl at the time of forward control or outputs a reverse control target speed signal through a switch 226-2 turned on by a reverse command signal c2 during reverse control. Reference numeral 228 is a velocity error amplifier which receives as input from the target speed generating circuit 224 a forward or reverse target speed voltage El through a resistor R1 or R2, finds the error signal ΔE with the real speed E2 input through a resistor R3, and generates an error signal ΔE amplified by an amplifier 230 connecting a capacitor C1 and resistor R4 to a feedback circuit. The output of the velocity error amplifier 228 is input to a speed control/position control switching circuit 232. The speed control/position control switching circuit 232 is provided with a switch 234-1 which receives as input the output of a velocity error amplifier 228 and a switch 234-2 which receives as input a position signal from a not shown position signal generating circuit. The switch 234-1 is turned on by a coarse signal (speed control signal) from the main processing unit 222, while the switch 234-2 is controlled by a fine signal (position control signal) from the main processing unit 222. That is, in the interval from the start of the seek operation to when the difference with respect to the target track becomes zero, the coarse signal is valid and the switch 234-1 is turned on, so speed control is performed. When the difference becomes zero, the fine signal becomes valid and the switch 234-2 is turned on, so position control maintaining the head on the target track is switched to. The outputs of the switches 234-1 and 234-2 are connected in common through the resistors R8 and R9, and are input to an amplifier 236 provided with a feedback resistor R10 and input resistor Rll, and are supplied to a next stage power amplifier 238 through the amplifier 236. The output voltage from the amplifier 236 is converted to drive current by the power amplifier 238, and the drive current is passed to the voice coil motor 210. The voice coil motor 210 is connected mechanically to a head carriage 212 mounting a head 240 and controls the movement of the head 240 in the radial direction of a not shown disk medium. The servo signal from the disk medium obtained by the head 240 is given to the demodulation circuit 242. The real speed E2 is found by the speed detection circuit 244 from the position signal demodulated by the demodulation circuit 242 and the current signal flowing to the voice coil motor 210 detected by the power amplifier 238 and is fed back to the input stage of the amplifier 230 provided in the velocity error amplifier 228 through the resistor R3. This part of the constitution of the servo circuit is the same as in the prior art. However, in the present invention there is the addition and new provision, at the input stage of the amplifier 236 provided in the speed control/position control switching circuit 232, of an acceleration means for compulsorily adding a predetermined acceleration voltage at the acceleration stage of the start of the seek operation. That is, the input of the amplifier 236 is provided with, as the addition means, a circuit for inputting a plus side power voltage + Ecc through the resistor R6 and the switch 218-1 and a circuit for inputting a minus side power voltage - Ecc through a resistor R7 and switch 218-2. The switch 218-1 for inputting the power voltage + Ecc is controlled to turn on or off by the forward acceleration signal C3, while the switch 218-2 for inputting the power voltage - Ecc is controlled to turn on or off by the reverse acceleration signal C4. Figure 11 is a view of an example of the acceleration control circuit of the present invention for generating a forward acceleration signal C3 and reverse acceleration signal C4 to the switches 218-1 and 218-2 provided in the speed control/ position control switching circuit 232 of Fig. 10. In Fig. 11, an error signal ΔE from the velocity error amplifier 228 is input to the acceleration control circuit. Also input from the main processing unit 222 are the forward command signal C1, reverse command signal C2, seek-in-progress signal C3 showing that the seek operation is in progress, and the seek start pulse P. The error signal ΔE is given to the plus input terminal of the comparator 246. The minus input terminal of the comparator 246 is grounded. When the error signal ΔE becomes zero voltage, that is, after the start of the seek operation, the point of time when the real speed voltage E2 reaches the target speed voltage E1 and matches the same is detected as the point of the end of the acceleration and the high level output is inverted to the low level output. The output of the comparator 246 is input to the AND gate 250 through the inverter 248 and is input directly to the AND gate 252. The AND gate 250 receives as input the forward command signal C1 and the seek-in-progress signal C3. Further, the AND gate 252 receives as input the reverse command signal C2 and the seek-in-progress signal C3. The outputs of the AND gates 250 and 252 are input to the NOR gate 254 and are further input to the clear terminal CLR of the flip-flip 258 through the inverter 256. To the clock terminal CLK of the flip-flop 258 is applied the seek start pulse P. Further, the seek start pulse P is applied to the D terminal through the capacitor. The Q output of the flip-flop 258 is input to the NAND gates 260 and 262. The NAND gate 260 receives as input the forward command signal Cl through the inverter 264. Further, the NAND gate 262 receives as input the reverse command signal C2 through the inverter 266. The output of the NAND gate 260 becomes the forward acceleration signal C3 for adding the acceleration voltage + Ecc to the error signal DE of the servo loop, while the output of the NAND gate 262 becomes the reverse acceleration signal C4 for adding the predetermined acceleration value - ECC to the error signal DE of the servo loop. Next, the operation of the embodiment of Figs. 10 and 11 will be explained referring to the operation timing chart of Fig. 12. In Fig. 12, first, at the time tl when a seek start pulse P is output for a "one difference" forward seek operation, along with the seek start pulse P, the main processing unit 222 makes the forward command signal C1 and the seek-in-progress signal C3 the high level. Therefore, the flip-flip 258 receiving the seek start pulse P is set and the C6 signal, serving as the Q output, falls from the high level to the low level. Further, at the start of the seek operation, the error signal ΔE is at its maximum level, the comparator 246 produces a high level output, the AND gate 250 is inhibited by the inversion of the inverter 248, and even if both the forward command signal C1 and the seek-in-progress signal C3 become the high level, the C5 signal from the NOR gate 254 becomes the low level. In this way, the flip-flop 258 is set by receiving the seek start pulse P. When the C6 signal, serving as the Q output, falls from the high level to the low level, a low level signal is input in the NAND gate 260 from the inverter 264 based on the high level forward command signal C1, so the forward acceleration signal C3, which had been the high level up to then, falls to the low level. Due to the operation of the acceleration control circuit shown in Fig. 11, the forward acceleration signal C3 for the switch 218-1 of the speed control/position control switching circuit 232 shown in Fig. 10 becomes the low level and the switch 218-1 closes. By this, the power voltage of +Ecc is supplied through the resistor R6 to the input of the amplifier 236. On the other hand, at the same time as the start of the seek operation, target speed data corresponding to a "one difference" seek operation is given to the target speed generating circuit 224, a target speed signal E1 with an extremely small signal voltage is input through the switch 226-2 turned on by the forward command signal C1 to the velocity error amplifier 228, simultaneously the real speed signal E2 at that time is input from the speed detection circuit 44, and an error signal ΔE between the target speed E1 and real speed E2 is input to the amplifier 236 through a switch 234-1 in an on state by a coarse signal. Therefore, a power voltage +Ecc input under the control of the reverse acceleration signal C3 is added to the error signal ΔE at the input stage of the amplifier 236, a control voltage serving as the added value (ΔE + Ecc) is input by the amplifier 236 to the power amplifier 236, and, as a result, the power amplifier 238 passes a quickly rising acceleration seek current to the voice coil motor 210. Receiving this seek current, the head carriage 212 moves the head 240 to the target track. Next, at the time t2, when the error signal ΔE for the comparator 246 reaches zero voltage, the output of the comparator 246 inverts to the low level and the end of the acceleration is detected. The low level output of the comparator 246 places the AND gate 250 in an enable state through the inverter 248, the C5 signal from the NOR gate 254 becomes the high level, the flip-flop 258 is reset, and, as a result, the C6 signal of the Q output is returned to the high level. Therefore, by the forward acceleration signal C3 from the AND gate 260 returning to the high level, the switch 218-1 is turned off and the acceleration control by the addition of the power voltage + Ecc is released. Therefore, after the time t2, a deceleration current flows based on the error signal ΔE produced by the error between the target speed signal El and the real speed signal E2. At the end of the seek operation at the time t3, the coarse signal returns to the high level, the speed control ends, and simultaneously the fine signal changes from the high level to the low level. When the switch 234-2 turns on, position control based on the position signal is switched to. Further, at the time t4, if a seek start pulse P for the reverse seek operation is generated based on an access from a host computer, the reverse command signal C2 from the main processing unit 222 becomes the high level. In the same way as the forward seek operation, the reverse acceleration signal C4 from the NAND gate 262 provided in the acceleration control circuit of Fig. 11 becomes the low level, the switch 218-2 of Fig. 10 turns on, whereby the power voltage - Ecc is compulsorily applied to the servo loop, and the seek current of the "one difference" seek operation rises well at the acceleration stage. Figure 13 is a view of the constitution of another example of the present invention. In the example of Fig. 13, for a predetermined time from the start of the seek operation, a predetermined acceleration value + Ecc or -Ecc is added to the servo loop. In Fig. 13, the construction of the servo circuit from the main processing unit 222 to the speed detection circuit 244 is the same as that of the embodiment of Fig. 10. Only the construction of the acceleration control circuit shown in Fig. 11 differs. That is, in the embodiment of Fig. 13, new provision is made in the main processing unit 222 of a timer function by a timer means generating a timer signal C7 during a predetermined time from the start of the seek operation. A timer signal C7 from the main processing unit 222 is input to the NAND gates 260 and 262 provided in the acceleration control circuit 270. At other inputs of the NAND gates 260 and 262 are input a forward command signal C1 and reverse command signal C2 through the inverters 264 and 266, respectively. A switch 218-1 provided in the speed control/position control switching circuit 232 is controlled by the forward acceleration signal C3 from the NAND gate 260. Further, the switch 218-2 is controlled by the reverse acceleration signal C4 from the NAND gate 262. Next, an explanation will be made of the operation of the example of Fig. 13 referring to the operational timing chart of Fig. 14. In Fig. 14, when the forward seek start pulse P is generated at the time tl, the main processing unit 222 activates the built in timer in accordance with the start of the seek operation and generates a low level timer signal C7 over the set time T. Therefore, receiving this timer signal C7, the forward acceleration signal C3 from the NAND gate 260 provided in the acceleration control circuit 270 becomes the low level, the switch 218-1 turns on, and therefore the power voltage + Ecc is compulsorily added to the error signal ΔE of the servo loop and the seek current rises quickly in the acceleration period, maintaining a saturation level. At the time t2 after the elapse of a set time T from the start of the seek operation, the timer signal C7 from the main processing unit 222 stops, that is, returns to the high level, the forward acceleration signal C3 from the acceleration control circuit 270 returns to the high level, the switch 218-1 turns off, and the addition of the power voltage + Ecc to the servo loop is stopped. After the time t2, deceleration control by the deceleration current produced by the error signal ΔE between the target speed signal E1 and the real speed E2 is begun. At the time t3, position control is switched to. Further, at the time t4, if a seek pulse for a reverse seek operation is obtained, the main processing unit 22 generates a low level timer signal C7 for a set time T in the same way as the forward seek operation. A low level reverse acceleration signal C4 is output from the acceleration control circuit 270, and the switch 218-2 is turned on, whereby the power voltage - Ecc is added to the error signal ΔE of the servo loop, the seek current at the acceleration stage during the reverse seek operation is quickly raised and the saturation level maintained, and, after the elapse of the set time T, the addition of the power voltage - Ecc is stopped. By the compulsory addition of a predetermined acceleration value to the error signal of the servo loop at the stage of acceleration at the start of the seek operation, even in a "one difference" seek operation where the target speed signal is extremely small, the seek current can be raised well and a sufficient acceleration characteristic can be obtained. Also, higher speeds can be realized even for seek access operations for small differences such as a difference of one track.

Claims: A servo circuit for a magnetic disk apparatus for supplying a seek current based on an error between a target speed and a real speed to a voice coil motor (210) and performs speed control on a servo object (212) toward a target track position, comprising: a target speed generating means (224) for generating a target speed depending upon the initial jump distance of the servo object (212); and a speed signal generating means (244) for generating a real speed from a position signal obtained from the servo object (212), and a speed error preparing means for controlling the speed of the servo object based on the error between the target speed and the real speed; characterised in that the circuit further comprises: an acceleration means (214) provided between the speed error preparing means and the voice coil for adding a pre-determined control voltage for acceleration of the speed of the servo object (212) to the errorr during the stage of acceleration after a start of the speed control. A servo circuit as claimed in claim 1, wherein said acceleration means comprises a switching means (218) for selectively supplying the predetermined control voltage to an addition point (216) where the error is input, and a voltage source for supplying a predetermined DC voltage, said switching means (218) being turned ON at the start of the seek operation and turned OFF at detection of completion of the acceleration. A servo circuit as claimed in claim 1, wherein said acceleration means (214) comprises a timer means (220) for turning ON the switching means (218) at the start of the seek operation and turning OFF the switching means (218) after the elapse of a predetermined time from the start of the seek operation, and a voltage source for supplying a predetermined DC voltage.

Inventors: HASHIMOTO SHUICHI, SHINOHARA TOHRU, HASHIMOTO, SHUICHI, SHINOHARA, TOHRU, SHINOHARA, TOHRU, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G11B5
IPC codes: G11B5