Servo circuit control system

Abstract: A control system of a servo circuit having a speed control unit (13) for controlling the speed of a servo object (11); a position control unit (14) for controlling the position of the servo object (11); a switching unit (15) for switching the connections of the servo object (11) between the speed control unit (13) and the position control unit (14); a main control unit (16); and optionally a speed zero detection circuit (39, Figure 12A) which compares the real speed (Vr) and a slice level and detects if the real speed (Vr) is less than a fixed value. A method for speed control of such a system includes a step of producing a target speed using an amount of movement X in an amount of required movement d as a remaining amount of movement and a step of producing, after producing a target speed for movement by the amount of movement X, a target speed of equal speed for the remaining amount of movement Y (Fig 3B). A method for controlling the switching of the position control of the servo circuit in such a system includes a step wherein the main processing unit (16) judges if the conditions for switching to the position control exist during the speed control; a step of monitoring the conditions for switching for a fixed time X after detection of the existence of the switching conditions; a step of switching to the position control in accordance with the existence of the switching conditions for the fixed time X; and a step of returning the to the step of judgement of the switching conditions if they collapse within the fixed time. Another method of control in such a system comprises producing and outputting a target speed using an amount of movement (X) in a total amount of movement (d) as a remaining amount of movement, producing and outputting a target speed of an equal speed of the remaining amount of movement Y, and switching the value of the slice (SLl) level in the speed zero detector circuit (39) movement (X) and after the movement (Y).

Description: The present invention relates to a servo circuit which performs speed control and then position control over an object under servo control (hereinafter referred to as a "servo object"). The applicant's earlier European Patent Application, EP-A-0295015 discloses a servo circuit of this general type, in which a speed control means controls the speed of the servo object based on an error between the actual speed and target speed. The actual speed is obtained from a position signal generated by an encoder of the servo object. A position control means is also provided and control of the servo object is by either the position control or speed control means, as controlled by a main processing unit. A similar system is shown in US-A-4480279. Wide use is being made of servo circuits for the positioning of magnetic heads on tracks of magnetic disk devices and other precision positioning. In such servo circuits, there is a desire for a technique enabling stable positioning even at high movement speeds. Also, there is a demand for a method for stable switching control even in the face of external disturbances and other influences when switching from speed control to position control. In recent years, magnetic disk apparatuses etc. have tended to become higher in the speed of positioning in seek (movement) operations. Due to these higher speeds, there is a need for increasing the degree of acceleration during deceleration. If the degree of acceleration during deceleration is increased, the entry speed at the final stage of the speed control will not be constant, but will fluctuate in accordance with the amount of the seek operation (movement). When the degree of acceleration in the deceleration is small and a relatively long time is taken for the deceleration (for example, when over two times the acceleration time is taken for the deceleration), the entry speed substantially remains the same, but if the degree of acceleration during the deceleration is made large and the positioning time shortened, the entry speed fluctuates in accordance with the amount of the seek operation. These fluctuations in the entry speed create fluctuations in the amount of overshoot, invite fluctuations and prolongation of the convergence time in the fine control (position control), and end up halving the effects of the reduction of time. Also, once the switching conditions are reached, the position control is switched to unconditionally after a fixed time, so position control is switched to even if the switching conditions only appear to have been reached due to disturbances in the position signal of a servo object due to external disturbances or if the position signal is disturbed by external disturbances after that. In such cases, overshooting or undershooting occur, position errors easily occur, too much time is taken for positioning, the reliability of the position control (seek) falls, and an increase in the positioning (seek) time is invited. The present invention has as its first object the reduction of the fluctuations in the entry speed and the reduction of the positioning time even with a large degree of acceleration during deceleration. The present invention has as a second object the improvement of the reliability of position control and the reduction of the positioning time. The present invention has a third object, in addition to the first object, the prevention of erroneous operation at the time of switching from speed control to position control. According to the present invention, a servo circuit for performing speed control and then position control determined by a difference representing the required amount of movement of a servo object under servo control comprises a speed control means for controlling the speed of the servo object based on an error between a target speed (V c ) and real speed obtained from a position signal of the servo object; a position control means for controlling the position of the servo object to the target position; a switching means for switching the connection of the servo object between the speed control means and the position control means; and a main processing means characterised in that the servo circuit is adapted so that the main processing means calculates a first difference (X) which the servo object has to be moved first and a second differences (Y) which the servo object has to be moved subseqently so that the second difference (Y) has a predetermined value, and the sum of the first difference (X) and the second difference (Y) equals the difference (d) representing the required amount of movement, and generates a first target speed profile for moving the servo object for the first difference (X) and a second target speed profile for moving the servo object for the second difference (Y) in which the second target speed is substantially constant. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A to 1C are explanatory views of the prior art; Figures 2A and 2B are explanatory views of another prior art; Figures 3A and 3B are views of the principle of the first aspect of the present invention; Fig. 4 is a view of the constitution of an embodiment of the first aspect of the present invention; Fig. 5 is a flow chart of the seek processing of the embodiment of the first aspect of the present invention; Figs. 6A and 6B are explanatory views of the operation of the embodiment of the first aspect of the present invention; Figs. 7A and 7B are views of the principle of the second aspect of the present invention; Fig. 8 is a view of the constitution of an embodiment of the second aspect of the present invention; Fig. 9 is a flow chart of the processing of the embodiment of the second aspect of the present invention; Fig. 10 is a view of the constitution of another embodiment of the second aspect of the present invention; Fig. 11 is a flow chart of the processing of the other embodiment of the second aspect of the present invention; Figs. 12A, 12B, and 12C are views of the principle of the third aspect of the present invention; Fig. 13 is a view of the constitution of an embodiment of the third aspect of the present invention; Fig. 14 is a view of the constitution of key portions in the constitution of Fig. 13; Fig. 15 is a flow chart of the seek processing of the embodiment of the third aspect of the present invention; Fig. 16 is explanatory views of the operation of the embodiment of the third aspect of the present invention; and Figs. 17 and 18 are explanatory views of the gain control operation of the embodiment of the third aspect of the present invention. Before describing the preferred embodiments, a more detailed explanation will be given of the prior art. Figures 1A to 1C are explanatory views of a prior art. As shown in Fig. 1A, the servo circuit has a speed control unit 3 for controlling the speed of a servo object 1; a position control unit 4 for controlling the position of the servo object 1; a switching unit 5 for switching the connections of the servo object 1 between the speed control unit 3 and the position control unit 4; and a main processing unit 6 which controls the switching of the switching unit 5 from the state of the servo object 1. The main processing unit 6 has a difference counter 6a which stores a difference (amount of movement) d and changes the target speed Vc given to the speed control unit 3 in accordance with the remaining difference d of the difference counter 6a as shown in Fig. 1B. That is, the difference counter 6a is decremented in response to a position signal (track crossing pulse) of the servo object and produces a step-like target speed in accordance with its content, i.e., the remaining difference d. The step-like target speed becomes zero for 0.5 track before the target track in a magnetic disk apparatus. It is smoothing circuit by a smoother of the speed control unit 3 and becomes a continuous curve as shown by Fig. 1B. In this way, in the prior art, the target speed was produced in accordance with the difference d for speed control. In recent years, however, magnetic disk apparatuses etc. have tended to become higher in the speed of positioning in seek (movement) operations. Due to these higher speeds, there is a need for increasing the degree of acceleration during deceleration. If the degree of acceleration during deceleration is increased, the entry speed at the final stage of the speed control will not be constant, as shown in Fig. 1C, but will fluctuate in accordance with the amount of the seek operation (movement). When the degree of acceleration in the deceleration is small and a relatively long time is taken for the deceleration (for example, when over two times the acceleration time is taken for the deceleration), the entry speed substantially remains the same, but if the degree of acceleration during the deceleration is made large and the positioning time shortened, the entry speed fluctuates in accordance with the amount of the seek operation. These fluctuations in the entry speed create fluctuations in the amount of overshoot, as shown in Fig. 1C, invite fluctuations and prolongation of the convergence time in the fine control (position control), and end up halving the effects of the reduction of time. Figures 2A and 2B are explanatory views of another prior art. As shown in Fig. 2A, like the first prior art shown, the servo circuit has a speed control unit 3 for controlling the speed of a servo object 1; a position control unit 4 for controlling the position of the servo object 1; a switching unit 5 for switching the connections of the servo object 1 between the speed control unit 3 and the position control unit 4; and a main processing unit 6 which controls the switching of the switching unit 5 from the state of the servo object 1. In such a servo circuit, in the prior art, when conditions were reached for switching from speed (coarse) control to position (fine) control, the main processing unit 6 operated the switching unit 5 unconditionally by the switching signal to change from the speed control to position control. For example, in a magnetic disk apparatus, as shown in Fig. 2B, the conditions for switching are considered to stand when, among a capture velocity signal indicating that the speed of the servo head has fallen below the target speed, an equal signal indicating that the servo head is above the same cylinder as the target cylinder, and an on-track signal indicating that the servo head is within a fixed distance of the target position, two of them, i.e., the capture velocity signal and equal signal, or three of them, the capture velocity signal, equal signal, or on-track signal, stand. A fixed time T (for example, 500 µsec) after the switching conditions stand, position (fine) control is switched to. In this prior art, however, once the switching conditions are reached, the position control is switched to unconditionally after a fixed time, so position control is switched to even if the switching conditions only appear to have been reached due to disturbances in the position signal Ps of the servo object 1 due to external disturbances or if the position signal Ps is disturbed by external disturbances after that. In such cases, as shown by the dotted line of Fig. 2B, overshooting or undershooting occur, position errors easily occur, too much time is taken for positioning, the reliability of the position control (seek) falls, and an increase in the positioning (seek) time is invited. The aspects of the present invention will now be explained. According to a first aspect of the present invention, there is provided a control system of a servo circuit having a speed control unit 13 for controlling the speed of a servo object 11 based on an error of a given target speed and real speed; a position control unit 14 for controlling the position of the servo object 11; a switching unit 15 for switching the connections of the servo object 11 between the speed control unit 13 and the position control unit 14; and a main processing unit 16 which revises the remaining amount of movement based on a position signal Ps from the servo object 11 and generates a target speed Vc in accordance with the remaining amount of movement; the control system featuring a method for speed control of the servo circuit including a step of producing a target speed using an amount of movement X in an amount of movement d as a remaining amount of movement and a step of producing, after producing a target speed for movement by the amount of movement X, a target speed of equal speed for the remaining amount of movement Y (X + Y = d). In Fig. 3, explaining the mode of operation of the first aspect of the present invention, a target speed is produced using an amount of movement X in an amount of movement d as the remaining amount of movement, a target speed of an equal speed is produced for the remaining amount of movement Y, the equal speed target speed is used to absorb fluctuations in the entry speed, which fluctuates according to the amount of movement, and adjustment made to substantially the same entry speed. Therefore, even if the degree of acceleration during deceleration is made large and a higher speed is attempted, no fluctuations occur in the entry speed, the amount of overshoot is kept constant, and stable position is made possible even at a high speed. Figure 4 is a view of the constitution of an embodiment of the first aspect of the present invention. In the figure, portions the same as those shown in Figs. 1A to 1C and Figs. 3A and 3B are given the same reference numerals. Reference numeral 11 is a servo object, which has a voice coil motor 11a and a servo head 11b moved by the voice coil motor 11a, while reference numeral 12 is a position signal preparing circuit, which prepares a position signal from a signal read by the servo head 11b. Reference numeral 13a is a speed detection circuit, which detects a real speed Vr from the position signal Ps and the later mentioned detection current ic, and reference numeral 13b is a speed error detection circuit, which generates a speed error ΔV between the later mentioned target speed Vc and the real speed Vr and performs speed control. Reference numeral 14 is a position error detection circuit, which generates a position error signal ΔP from the position signal Ps and the detection current ic and performs position control, and reference numeral 15 is a power amplifier and switching unit, which has a changeover switch and power amplifier and switches connections of the speed error detection circuit 13b or position error detection circuit 14 to the servo object 11 by a coarse (speed control)/fine (position control) switching signal. Reference numeral 16 is a main processing unit, which is constituted by a microprocessor and which generates a target speed curve Vc in accordance with the amount of movement and also monitors the position of the servo object 11 by the later mentioned track crossing pulses to generate a signal for switching from coarse control (speed control) to fine control (position control) near the target position. The reference number d is a counter for storing the difference amount. Reference numeral 17 is a control current detection circuit, which detects the control current Is of the power amplifier 15 and generates a detection current signal ic, and reference numeral 18 is a track crossing pulse generating circuit, which generates track crossing pulses from the position signal Ps and outputs them to the main processing unit 16. Figure 5 is a flow chart of the seek processing of the embodiment of the first aspect of the present invention. Figures 6A and 6B are explanatory views of the operation of the embodiment of the first aspect of the present invention. Referring to the figure, [1] The main processing unit (hereinafter referred to as the "MPU") 16 calculates an amount of movement (difference) d from the target track position and the current track position and given by an upper unit sets it in the difference counter 16a. Next, the difference d of the difference counter 16a is decremented by "1". That is, the difference d is reduced by "1" in advance. Then the MPU 16 generates a target speed according to the difference d to start the speed control [2] The MPU 16 monitors the track crossing pulses of the track crossing pulse generating circuit 18 and when detecting a track crossing pulse, revises the content of the difference counter 16a to (d-1) and issues a target speed according to that difference d. [3] The MPU 16 judges if the difference d is zero and if not zero returns to step [2]. On the other hand, if the MPU 16 judges that the difference is zero, it judges that the seek operation of the difference (d-1) is completed, sets "1" in the difference counter 16a so as to perform the seek operation for the remaining "1", and generates a target speed according to this difference d. [4] Next, the MPU 16 monitors the track crossing pulses from the track crossing pulse generating circuit 18 and if it detects a track crossing pulse, decrements the difference d of the difference counter 16a by "1". By this, the difference d of the difference counter 16a becomes zero, so the MPU 16 sets the difference zero. That is, it judges that the seek operation of the difference of 1 is completed. [5] Next, the MPU 16 judges if the real speed of the speed detection circuit 3a is zero and waits until it is zero. When the real speed becomes zero, the MPU 16 waits a fixed time, then switches from the speed control to position control by a coarse/fine switching signal. Then, the MPU 16 monitors the on-track signal of the position error detection circuit 14 (signal showing that servo head is within fixed distance of target track) and if it detects an on-track state for a fixed period, ends the operation judging that the positioning is completed. Explaining this operation referring to Figs. 6A and 6B, first the amount of movement d (=5) is decremented by "1" and a seek operation for the position (d-1) (=4) is executed. After this, a seek operation for the position of the remaining "1" is executed. This seek operation of the position of the remaining "1" means movement of a position of "1", so an equal speed target speed is given and an equal speed seek operation performed. That is, under the conditions of X = d-1 and Y = 1, the seek operation of the difference d is split into two seek operations. Showing this by tracks of a magnetic disk, the seek operation of the position (d-1) is performed until 1.5 tracks before the target track, then a seek operation of a position of 1 of an equal speed is performed from the 1.5 tracks before to 0.5 track before. The seek current becomes as shown in Fig. 6B. The target speed Vc from MPU 16 takes a step-like form, as shown by Fig. 6A, of "4", "3", "2", "1", and "0", then "1" and "0". By running this through a smoother circuit, not shown, of the speed error detection circuit 13b, the continuous speed Vcs of Fig. 6A is obtained, which draws a curve which decreases from d=4 to d=0, rising by the d=1 after d=0 and then decreases again. Therefore, even when the degree of acceleration during deceleration is large and the entry speed fluctuates, the fluctuations are absorbed by the fixed speed, equal speed seek zones and the speed control can be ended at substantially the same entry speed. As explained above, according to the first aspect of the present invention, since the target speed is produced using the amount of movement X in the amount of movement d as the remaining amount of movement and an equal speed target speed is produced for the remaining amount of movement Y, the fluctuations in the entry speed, which fluctuates according to the amount of movement, are absorbed by the equal speed seek zones and substantially the same entry speed can be adjusted to. Even with a large degree of acceleration during deceleration, it is possible to keep the overshoot constant and achieve high speed, stable positioning. According to a second aspect of the present invention as shown in Figs. 7A, 7B and 8, there is provided a control system of a servo circuit having a speed control unit 23 for controlling the speed of a servo object 21; a position control unit 24 for controlling the position of the servo object 21; a switching unit 25 for switching the connections of the servo object 21 between the speed control unit 23 and the position control unit 24; and a main processing unit 26 which controls the switching of the switching unit 25; featuring a method for controlling the switching of the position control of the servo circuit including a step wherein the main processing unit 26 judges if the conditions for switching to the position control stand during the speed control; a step of monitoring the conditions for switching for a fixed time T after detection of the standing of the switching conditions; a step of switching to the position control in accordance with the standing of the switching conditions for the fixed time T; and a step of returning to the step of judgement of the switching conditions in accordance with the collapse of the switching conditions within the fixed time. Optionally, there may be included a further step for switching to the position control irregardless of the standing or not standing of the switching conditions after the elapse of a second fixed time S after the detection of the standing of the switching conditions. Explaining the mode of operation of the second aspect of the present invention, position control is switched to if the switching conditions do not collapse within a fixed time T after the switching conditions once are reached, while the establishment of the switching conditions is waited for if the conditions collapse. Therefore, there is no collapse of the switching conditions after the position control is switched to and it is possible to reduce error and shorten the positioning time. Further, in the second aspect of the present invention, if the position signal Ps is seriously disturbed, position control would not be switched to no matter how long in the above mentioned control, so position control is compulsatorily switched to after the elapse of a second fixed time S. By this method, while there is a possibility of position error, this is better than indefinitely delaying the switching. Further, there is a possibility of positioning under position control. Next, an embodiment of the second aspect of the present invention will be explained. Figure 8 is a view of the constitution of the embodiment of the second aspect of the present invention. In Fig. 8, reference numeral 21a is a voice coil motor, which performs a seek operation for the magnetic head; and reference numeral 21b is a servo head (magnetic head), which reads servo information on the servo surface of the magnetic disk. Reference numeral 22 is a position signal preparing circuit, which prepares a position signal from a signal read by the servo head 21b. Reference numeral 23a is a speed detection circuit, which detects a real speed Vr from the position signal Ps and the later mentioned detection current ic, and reference numeral 23b is a speed error detection circuit, which generates a speed error ΔV between the later mentioned target speed Vc and the real speed Vr and performs speed control. Reference numeral 23c is a capture velocity generating circuit, which detects that the real speed Vr is below a fixed target speed and generates a capture velocity signal CAV. Reference numeral 24 is a position error detection circuit, which generates a position error signal ΔP from the position signal Ps and the detection current ic and performs position control, and reference numeral 25 is a power amplifier and switching unit, which has a changeover switch and power amplifier and switches connections of the speed error detection circuit 23 or position error detection circuit 24 to the servo object 21 by a coarse (speed control)/fine (position control) switching signal. Reference numeral 26 is a main processing unit, which is constituted by a microprocessor and which generates a target speed curve Vc in accordance with the amount of movement and also monitors the position of the servo object 21 by the later mentioned track crossing pulses to generate a signal for switching from coarse control (speed control) to fine control (position control). Reference numeral 27a is a control current detection circuit, which detects the control current Is of the power amplifier 25 and generates a detection current signal ic, and reference numeral 27b is a track crossing pulse generating circuit, which generates track crossing pulses from the position signal Ps and outputs them to the main processing unit 26. Reference numeral 28a is an equal signal generating circuit, which is set with a number of tracks for movement L from the main processing unit (MPU) 26, performs subtraction by the position signal Ps, and generates an equal signal indicating that the servo head 21b has reached the target cylinder when the number of tracks becomes zero. Reference numeral 28b is an on-track signal generating circuit, which generates an on-track signal from the position signal Ps showing that the servo head 21b has come within a fixed distance from the target position. Reference numeral 28c is an AND gate, which takes the logical AND of the capture velocity signal CAV, the equal signal, and the on-track signal and generates a switching condition standing signal. Reference numeral 29a is a timer, which is set by the MPU 26, times the fixed time T, and generates the coarse/fine switching signal. Figure 9 is a flow chart of the seek processing of the embodiment of the third aspect of the present invention. Referring to the figure, [1] The MPU 26 sets a difference d corresponding to the distance of movement in its built-in difference counter. Then, the MPU 26 generates the target speed Vc and starts the speed control. [2] The MPU 26 monitors the track crossing pulses of the track crossing pulse generating circuit 27b and when detecting a track crossing pulse, revises the content of the difference counter to (d-1). [3] The MPU 26 judges if the content of the difference counter is zero and if not zero returns to step [2]. On the other hand, if the content of the difference counter is zero, the MPU 26 sets the difference "0". [4] The MPU 26 then monitors the switching condition standing signal of the AND gate 28c and judges if the switching conditions stand. [5] If the switching conditions stand, then the MPU 26 starts the timer 29a. Further, during the time T of the timer 29a (about 500 µsec), the MPU 26 monitors the switching condition standing signal of the AND gate 28c and judges if the said signal turns off in the time T. If the switching condition standing signal goes off within the time T, the timer 29a is reset and the process returns to the judging of the standing of the switching conditions of step [4]. [6] On the other hand, if the switching condition standing signal does not go off within the time T and the switching conditions still stand, the timer 29a is not reset. Therefore, after the timer 29a counts the time T, it generates a coarse/fine switching signal to the switching unit 25 and switches to position control by the position error detection circuit 24. Then, the MPU 26 monitors the on-track signal of the on-track signal generating circuit 28b and if the on-track signal goes on for a fixed time, judges that the seek operation is completed and ends the processing. In this way, after the establishment of the switching conditions, the switching signal standing signal is monitored for a fixed time T and if the switching conditions do not collapse, the position control is switched to and if they collapse the process is repeated from the judgement of the standing of the switching conditions. Therefore, even if the switching conditions are reached, if they are not sustained, the position control is not switched to, so it is possible to eliminate the switching to position control caused by disturbed seek waveforms where switching conditions coincidentally stand and it is possible to achieve stable, high speed positioning. Figure 10 is a view of the constitution of another embodiment of the second aspect of the present invention. In the figure, portions the same as those shown in Fig. 8 are given the same reference numerals. Reference numeral 29b is a second timer, which is started by the MPU 26, counts a time S (for example, 3 msec), then generates a coarse/fine switching signal; reference numeral 29c is an OR gate, which takes the OR of the outputs of the first timer 29a and second timer 29b and outputs a coarse/fine switching signal to the switching unit 25. Figure 11 is a flow chart of the processing of the other embodiment of the second aspect of the present invention. In this embodiment, the flow of Fig. 9 is augmented with a step [7] between steps [3] and [4] and a step [8] between steps [4] and [5]. Referring to the figure, [7] After step [3], the MPU 26 judges if the timer 29b has counted a fixed time Y. If it has not counted it, it proceeds to step [4]. If it has counted it, it generates a coarse/fine switching signal through the OR gate by a count over output of the timer 29b and switches to position control. [8] If the switching conditions stand at step [4], the MPU 26 starts the timer 29b and proceeds to step [5]. Therefore, in this embodiment, when the switching conditions fail to be sustained for the fixed time T no matter how long it takes, once a fixed time S (S≧T) elapses, the MPU 26 switches compulsatorily to position control irregardless of the standing or not standing of the switching conditions. If the state where the switching conditions fail to be sustained for the fixed time T continues forever, position control would never be entered, so the limit of the time S is established and if the switching conditions fail to be sustained for the fixed time T even after the elapse of the time S, the MPU 26 recognizes this as a seek error and switches compulsatorily to the position control. In the above embodiments, the timer 29a, AND gate 28c, and equal signal generating circuit 28a are constituted by hardware separate from the MPU 26, but the same functions may be achieved by firmware of the MPU 26. Further, the timer 29b and OR gate 29c may be realized by firmware of the MPU 26 in a similar manner. In the above embodiments, further, an explanation was made of the example of a magnetic disk apparatus, but the aspect of the invention may be applied to other apparatuses as well. As explained above, according to the second aspect of the present invention, after the switching conditions have been once reached, position control is switched to if the switching conditions do not collapse within a fixed time T, while if they collapse, the establishment of the switching conditions is waited for, so it is possible to prevent execution of position control in a state where the switching conditions have collapsed and to reduce error and reduce the positioning time. Further, according to the second aspect of the present invention, position control is compulsatorily switched to after the elapse of the second fixed time S, so it is possible to prevent the state where the switching conditions collapse and position control is never switched to. According to a third aspect of the present invention, as shown in Figs. 12A, 12B and 13, there is provided a control system of a servo circuit having a speed control unit 33 for controlling the speed of a servo object 31 based on an error of a given target speed and real speed; a position control unit 34 for controlling the position of the servo object 31 based on a position signal from the servo object 31; a switching unit 35 for switching the connections of the servo object 31 between the speed control unit 33 and the position control unit 34; a main processing unit 36 which revises the remaining amount of movement based on the position signal and generates a target speed in accordance with the remaining amount of movement; and a speed zero detection circuit which compares the real speed and a slice level and detects if the real speed is less than a fixed value; based on an output of the speed zero detection circuit 39, the main processing unit 36 switching the switching unit 35 from speed control to position control, the control system characterized by producing and outputting a target speed using an amount of movement X in an amount of movement d as a remaining amount of movement, producing and outputting a target speed of an equal speed of the remaining amount of movement Y (X + Y = d), and switching the value of the slice level at the time of the start of movement and after the movement. Explaining the mode of operation of the third aspect of the present invention, first, as in the first aspect, a target speed is produced using an amount of movement X in an amount of movement d as the remaining amount of movement, a target speed of an equal speed is produced for the remaining amount of movement Y, the equal speed target speed is used to absorb fluctuations in the entry speed, which fluctuates according to the amount of movement, and adjustment made to substantially the same entry speed. Therefore, even if the degree of acceleration during deceleration is made large and a higher speed is attempted, no fluctuations occur in the entry speed, the amount of overshoot is kept constant, and stable position is made possible even at a high speed. Second, with such speed control, the real speed Vr follows the target speed Vc and, as shown in Fig. 12C, when the amount of movement X is completed, the speed curve Vr is crossed to the slice level SL₁ for the detection of the switching conditions for the speed control/position control, and position control is switched to during the equal speed zone of the amount of movement Y, resulting in erroneous operation. Therefore, the slice level SL is switched to the low level SL₂ upon the completion of the amount of movement X after the start of the movement so as to prevent erroneous operation and guarantee the speed control of an equal speed zone of the amount of movement Y and also it is made possible to switch to position control at a suitable timing. Figure 13 is a view of the constitution of an embodiment of the third aspect of the present invention. In Fig. 13, reference numeral 31 is a servo object, which has a voice coil motor 31a and a servo head 31b moved by the voice coil motor 31a, while reference numeral 32 is a position signal preparing circuit, which prepares a position signal from a signal read by the servo head 31b. Reference numeral 33a is a speed detection circuit, which detects a real speed Vr from the position signal Ps and the later mentioned detection current ic, and reference numeral 33b is a speed error detection circuit, which generates a speed error ΔV between the later mentioned target speed Vc and the real speed Vr and performs speed control. Reference numeral 34 is a position error detection circuit, which generates a position error signal ΔP from the position signal Ps and the detection current ic and performs position control, and reference numeral 35 is a power amplifier and switching unit, which has a changeover switch and power amplifier and switches connections of the speed error detection circuit 33b or position error detection circuit 34 to the servo object 31 by a coarse (speed control)/fine (position control) switching signal. Reference numeral 36 is a main processing unit, which is constituted by a microprocessor and which generates a target speed curve Vc in accordance with the amount of movement and also monitors the position of the servo object 31 by the later mentioned track crossing pulses to generate a signal for switching from coarse control (speed control) to fine control (position control) near the target position. Reference numeral 37 is a control current detection circuit, which detects the control current Is of the power amplifier 35 and generates a detection current signal ic, and reference numeral 38 is a track crossing pulse generating circuit, which generates track crossing pulses from the position signal Ps and outputs them to the main processing unit 36. Reference numeral 39 is a speed zero detection circuit, which slices a real speed Vr by a predetermined slice SL to generate a speed zero signal CAPVL and an end deceleration signal ENDDC and which changes the slice level SL between a high and low stage by the command of the main processing unit (MPU) 36. Reference numeral 310 is a gain control circuit, which detects equal speed zones from the speed error signal ΔV an the speed zero signal CAPVL and controls the gains of the amplifier of the power amplifier and switching unit 35. Figure 14 is a view of the constitution of essential portions of the constitution of Fig. 13. The speed zero detection circuit 39 has a slice switching circuit 390 which switches values of the fixed value slice voltage SL (Va, Vb) by operation by a switch SW by slice switching signals GS from the MPU 36; an inversion amplifier 391 which inverts the real speed Vr; a first comparator (CMP) 392 which compares the slice voltage Va (SL) with the real speed Vr; a second comparator (CMP) 393 which compares the inverted real speed Vr with the slice voltage Vb (SL); an EX-OR circuit 394 which generates a "high" level output when the outputs of the two comparators 392 and 393 differ; an AND circuit 395 which takes the logical AND of the speed zero signal *CAPVL of the EX-OR circuit 394 and the equal signal EQUAL output from the MPU 36 when the difference is zero; and an inversion circuit 396 which inverts the speed zero signal *CAPVL. The gain control circuit 310 has a comparator (CMP) 3100 which performs a zero volt slice operation on the speed error signal ΔV and outputs an acceleration/deceleration signal VERSL; an inversion circuit 3101 which inverts the output of the comparator 3100; an AND gate 3102 which takes the negative logical AND (positive logical OR) of the speed zero signal CAPVL, the inversion acceleration/deceleration signal *VERSL, and the forward signal *MvFWD from the MPU 36; and AND gate 3103 which takes the negative logical AND of the speed zero signal CAPVL, the acceleration/deceleration signal VERSL, and the reverse signal *MvRVS from the MPU 36; an OR gate 3104 which takes the positive logical OR of the outputs of the two AND gates 3102 and 3103; and a flip-flop 3105 which is set by the end acceleration signal *ENDAC of the OR gate 3104, is reset by the deceleration over signal DFOVER, which is a high level signal while the target speed from the MPU 36 is not changed (while there is no deceleration), and genrates the gain switching signal *BIBIN. The power amplifier and switching unit 35 has a coarse switch 350 which turns on by a coarse command signal from the MPU 36 and outputs a speed error signal ΔV; a fine switch 351 which turns on by a fine command signal from the MPU 36 and outputs a position error signal ΔP; an amplifier 35 which amplifies the signals of the two switches 350 and 351 and switches the gain by operation of the switch SW by the gain switching signal *BIBIN; and a power amplifier 35b which passes a current CSENS to the coil of the voice coil motor 31a in accordance with a power amplifier drive output PADRV of the amplifier 352. Figure 15 is a flow chart of the seek processing of the embodiment of the third aspect of the present invention. Figure 16 is explanatory view of the operation of the embodiment of the third aspect of the present invention. Referring to the figures, [1] The main processing unit (hereinafter referred to as the "MPU") 36 calculates an amount of movement (difference) d from the target track position given by a higher unit and the current track position and sets it in the difference counter 36a. Next, the difference d of the difference counter 36a is decremented by "1". That is, the difference d is reduced by "1" in advance. Then the MPU 36 generates a target speed according to the difference d to start the speed control [2] The MPU 36 monitors the track crossing pulses of the track crossing pulse generating circuit 38 and when detecting a track crossing pulse, revises the difference counter 36a to (d-1) and issues a target speed according to that difference d. [3] The MPU 36 judges if the difference d is zero and if not zero returns to step [2]. On the other hand, if the MPU 36 judges that the difference is zero, it judges that the seek operation of the difference (d-1) is completed, sets "1" in the difference counter 36a so as to perform the seek operation for the remaining "1", and generates a target speed according to this difference d. Along with this, the MPU 36 sets "1" in the speed detection slice switching latch (not shown), makes the slice switching signal GS "1", turns the switch SW of the slice switching circuit 390 of Fig. 14 off, and makes the slice voltage SL the low level. That is, in performing the seek operation of the remaining "1", as shown in Fig. 16, the slice level SL is made the low level. [4] Next, the MPU 36 monitors the track crossing pulses from the track crossing pulse generating circuit 38 and if it detects a track crossing pulse, decrements the difference d of the difference counter 36a by "1". By this, the difference d of the difference counter 36a becomes zero, so the MPU 36 sets the difference zero and outputs an equal signal EQUAL to the speed zero detection circuit 39. That is, it judges that the seek operation of the difference of 1 is completed. [5] Next, the MPU 36 judges if the speed is zero by the end deceleration signal *ENDDC of the speed zero detection circuit 39. If the end deceleration signal *ENDDC rises, it is considered that the real speed has become zero. The MPU 36 waits a fixed time, then turns off the switch 350 by a coarse/fine switching signal, turns on the switch 351, and thus switches from speed control to position control. Next, the MPU 36 sets the speed detection slice switching latch to "0", turns on the switch SW of the circuit 390, and makes the slice voltage the high level. Then, the MPU 36 monitors the on-track signal of the position error detection circuit 34 (signal showing that servo head is within fixed distance of target track) and if it detects an on-track state for a fixed period, ends the operation judging that the positioning is completed. Explaining this operation referring to Figs. 6A and 6B as explained above, first the amount of movement d (=5) is decremented by "1" and a seek operation for the position (d-1) (=4) is executed. After this, a seek operation for the position of the remaining "1" is executed. This seek operation of the position of the remaining "1" means movement of a position of "1", so an equal speed target speed is given and an equal speed seek operation performed. That is, under the conditions of X = d-1 and Y = 1, the seek operation of the difference d is split into two seek operations. Showing this by tracks of a magnetic disk, the seek operation of the position (d-1) is performed until 1.5 tracks before the target track, then a seek operation of a position of 1 of an equal speed is performed from the 1.5 tracks before to 0.5 track before. The seek current becomes as shown in Fig. 6B. The target speed Vc from MPU 36 takes a step-like form, as shown by Fig. 6A, of "4", "3", "2", "1", and "0", then "1" and "0". By running this through a smoother circuit, not shown, of the speed error preparing circuit 33b, the continuous Vcs of Fig. 6A is obtained, which draws a curve which decreases from d=4 to d=0, rising by the d=1 after d=0 and then decreases again. Therefore, even when the degree of acceleration during deceleration is large and the entry speed fluctuates, the fluctuations are absorbed by the fixed speed, equal speed seek zones and the speed control can be ended at substantially the same entry speed. Next, the reason why the slice level SL is switched from the high level to the low level is that, as shown by Fig. 16 the target speed Vc rises due to the seek operation for the position "1" after the seek operation of the position (d-1) and the real speed Vr follows this, forming a peak after once falling, as shown in the figure. In such a case, if slice level SL₁ is to be prevented from changing as shown by the dotted line, the real speed Vr crosses the slice level SL₁ at t1 immediately after the seek operation of the position (d-1), a speed zero signal such as CAPVL′ is caused from time t₁ , an end deceleration signal such as ENDDC′ is generated at the time t2 of the difference zero, and, after the elapse of the time t, fine control is switched to. This means a shift to fine control at the point of the peak of the real speed Vr, making it impossible to enjoy the full effects of the equal speed seek operation. Further, fine control is performed at the time when the real speed Vr is large, so overshoot is caused and the limit time becomes longer. Therefore, at the point of time t₃ when the slice level SL₁ is switched to the low level SL₂ , the peak of the real speed Vr is passed, and the real speed Vr sufficiently falls, the speed zero signal CAPVL is generated, the end deceleration signal ENDDC is generated, and then the switching to the fine control is performed. By this, the effects of an equal speed seek operation can be fully enjoyed and fine control is switched to at the point of time when the real speed Vr sufficiently falls, so the overshoot can be reduced and the limit time made shorter. If the slice level SL is fixed to the low level SL₂ from the start, troubles will occur as explained with reference to Fig. 17 and Fig. 18. The gain control circuit 310 explained in Fig. 13 and Fig. 14 detects the equal speed zones from the speed zero signal CAPVL, turns on the switch SW of the amplifier 352 of Fig. 14, reduces the amplifier gain, and prevents vibration of the type easily occurring with equal speed control. Further, in Fig. 17 and Fig. 18, for simplification of the explanation, no explanation is made using the two stage seek operation of Fig. 6. Figure 17 shows the ordinary operation. If the slice level SL is a high level with respect to the real speed Vr, irregardless of the offset of the real speed Vr, the speed zero signal CAPVL becomes the low level during the acceleration, equal speed, and deceleration and otherwise becomes the high level. On the other hand, the speed error signal ΔV is detected as the acceleration/deceleration signal VERSL by the comparator 3100 of the gain control circuit 310. Therefore, the end acceleration signal *ENDAC showing the end of the acceleration is output from the AND gate 3104 through the AND gate 3102 and 3103, and the equal speed zone low level gain switching signal *BIBIN is output from the flip-flop 3105. By this, the switch SW of the amplifier 352 is turned off, the equal speed zone gain is reduced, and the VCM current CSENS of the power amplifier 35b no longer causes vibration in the equal speed zone. As opposed to this, if the slice level SL is fixed to the low level, as shown in Fig. 18, the offset of the real speed Vr at the start of the seek operation will exceed the slice level SL (Vb) and the speed zero signal CAPVL will be considered the low level from before the start of the seek operation. Therefore, the end acceleration signal *ENDAC will be output from the OR gate 3104 from before the start of the seek operation, the flip-flop 3105 will be reset, the gain signal will be at a low level from before the seek operation, and the gain of the amplifier 352 will end up reduced. Due to this, since the amplifier 352 will have a low gain even during the acceleration period after the start of the seek operation, as shown in Fig. 18, the current during the acceleration (drive current PADRV, VCM current CSENS) will become small and the necessary acceleration will not be possible. In this way, there is a problem with making the slice level a low level in advance. At the very least, the slice level needs to be made high at the start of the seek operation. From the above, at the first, the slice level is made high (SL₁) and, as explained with reference to Fig. 15 and Fig. 16, at the second stage of positioning of "1", the slice level is switched to the low level (SL₂). After the switching to the position control, the slice level is returned to the high level. In the above embodiment, the slice level was made the low level from the start of the seek operation of the position "1" to the switching to the position control, but as mentioned earlier the reason why trouble is caused if the slice level is kept the low level is the offset of the real speed Vr, so it is possible to maintain the slice level up until the start of the seek operation and then switch to the low level. As explained above, according to the third aspect of the present invention, since the target speed is produced using the amount of movement X in the amount of movement d as the remaining amount of movement and an equal speed target speed is produced for the remaining amount of movement Y, the fluctuations in the entry speed, which fluctuates according to the amount of movement, are absorbed by the equal speed seek zones and substantially the same entry speed can be adjusted to. Even with a large degree of acceleration during deceleration, it is possible to keep the overshoot constant and achieve high speed, stable positioning. Further, even with such a speed control, since the slice level is switched, the effects of the speed control by the equal speed target speed can be fully enjoyed and it is possible to switch to position control at a suitable timing. In the above embodiments, an explanation was made of the example of a magnetic disk apparatus, but the invention may be applied to other apparatuses as well. Also, an explanation was made using a position of Y=1, but other values may be used as well. Above, an explanation was made using embodiments, but the present invention may be modified in many ways within the scope of the appended claims. These modifications are not excluded from the present invention. The different aspects of this invention can also be used together in combination.

Claims: A servo circuit for performing speed control and then position control determined by a difference (d) representing the required amount of movement of a servo object (11,21,31) under servo control, comprising: a speed control means (13,23,33) for controlling the speed of the servo object (11,21,31) based on an error between a target speed (V c ) and real speed obtained from a position signal of the servo object (13,23,33); a position control means (14,24,34) for controlling the position of the servo object (13,23,33) to the target position; a switching means (15,25,35) for switching the connection of the servo object (11,21,31) between the speed control means (13,23,33) and the position control means (14,24,34); and a main processing means (16,26,36) characterised in that the servo circuit is adapted so that the main processing means (16,26,36) calculates a first difference (X) which the servo object has to be moved first and a second difference (Y) which the servo object has to be moved subsequently so that the second difference (Y) has a predetermined value, and the sum of the first difference (X) and the second difference (Y) equals the difference (d) representing the required amount of movement, and generates a first target speed profile for moving the servo object for the first difference (X) and a second target speed profile for moving the servo object for the second difference (Y) in which the second target speed is substantially constant. A servo circuit as claimed in claim 1, wherein said main processing means (16,26,36) comprises a difference counter (16a,36a) for setting the difference (d) calculated from a target track position and a current track position. A servo circuit as claimed in claim 2, wherein the difference (d) of the difference counter (16a,36a) is decremented by "1" in advance, that is, the difference is reduced by "1" in advance. A servo circuit as claimed in claim 1, 2, or 3, wherein the first difference (X) corresponds to the difference "d-1" and the second difference (Y) corresponds to the difference "1", respectively. A servo circuit as claimed in claim 1 or 4, wherein said second target speed having equal speed is applied to the second difference (Y) as a one position seek operation having constant entry speed. A servo circuit according to any of the preceding claims, further characterised by a speed zero detection means (39) for comparing the real speed with a slice level (SL) and detecting if the real seed is less than the slice level (SL), switching the switching means (35) from speed control to position control based on an output of the speed zero detection means (39), and switching the slice level (SL) from the high level (SL₁) to the low level (SL₂) at the time of the start of movement and after the movement. A servo circuit according to any of the preceding claims, in which the main processing means (26) judges if the conditions for switching to the position control stands during the speed control, monitors the conditions for switching for a fixed time after detection of the standing of the said switching conditions, switches to the position control in accordance with the standing of the switching conditions for the said fixed time, and returns to the step judgement of the switching conditions when the switching conditions is collapsed within the said fixed time. A servo circuit as claimed in claim 7, wherein said main processing means (26) further controls the switching of the switching means (25) in such a way that the main processing means (26) switches to the position control irregardless of the standing or not standing of the switching conditions after the elapse of a second fixed time after the detection of the standing of the said switching conditions. A servo circuit as claimed in claim 7, wherein said servo circuit further comprises a timer means (29a) for counting the fixed time and generating a coarse/fine switching signal. A servo circuit as claimed in claimed 8, wherein said servo circuit further comprises a second timer means (29b) for counting the second fixed time and generating the coarse/fine switching signal.

Inventors: HASHIMOTO SHUICHI, SHINOHARA TOHRU SAYO-SO, HASHIMOTO, SHUICHI, SHINOHARA, TOHRU 202 SAYO-SO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G11B21
IPC codes: G11B7, G11B5, G11B21