1. Field of the Invention
The present invention relates to an improved information recording and reproducing apparatus. More particularly, the present invention relates to an information recording and reproducing apparatus, in which a drive device using an electromechanical transducer allows a recording and reproducing head to track an information recording track formed on a recording medium.
2. Description of the Related Art
There has been increased a demand for a hard disk device (hereinafter simply referred to as a disk device) of a small size and a large capacity for use in a personal computer. In order to increase the storage capacity of the disk device, it is required to narrow a mutual interval between information recording tracks (hereinafter simply referred to as a track) formed on a disk (i.e., a track density) and increase a recording density on each of the tracks. For the purpose of fulfilling such a demand for a high recording density of the disk device, it is necessary to provide a tracking mechanism for positioning a recording and reproducing head with respect to a target track formed on the disk with high accuracy. Incidentally, the recording and reproducing head signifies a head for performing both or either one of writing and reading information in and from the disk.
As the tracking mechanism for the recording and reproducing head in the disk device, there has been known a tracking mechanism consisting of an actuator using a voice coil motor and an actuator using a piezoelectric element. However, it has been pointed out that such a tracking mechanism has inconveniences such as a complicated structure, fabrication at a high cost and a low reliability.
Thus, the present applicants have proposed a tracking mechanism including a drive mechanism (an actuator) using an electromechanical transducer, as disclosed in the gazette of Japanese Unexamined Patent Application No. 11-251426.
This tracking mechanism uses an actuator actuated in two kinds of drive modes: in a rough movement mode, in which a drive pulse is supplied to a piezoelectric element as one type of electromechanical transducers, to generate expansion and contraction displacement vibrations at different speeds, that is, an expansion speed and a contraction speed, so that a driving member and a moving member frictionally coupled to the driving member are moved relatively to each other, thereby allowing the moving member to be moved in a wide range; and in a fine movement mode, in which a DC voltage is applied to the piezoelectric element, so that the moving member can be moved with high accuracy by an expansion displacement or a contraction displacement generated in the piezoelectric element, wherein a recording and reproducing head is fixed to the moving member.
At the time of seeking, i.e., when a desired track is sought and the recording and reproducing head is moved to the track, the drive pulse is supplied to the piezoelectric element in the rough movement mode, so that the recording and reproducing head is speedily moved to the desired track; in contrast, at the time of following, i.e., when the recording and reproducing head is allowed to precisely follow the desired track with high accuracy, the drive mode is switched to the fine movement mode, and then, the DC voltage is applied to the piezoelectric element, thereby finely moving the moving member.
With the configuration disclosed in the above-described gazette, the drive pulse of a rectangular wave form (hereinafter referred to as a rectangular wave) is supplied to the piezoelectric element in the rough movement mode, the moving speed of the moving member, i.e., the recording and reproducing head is adjusted by varying a duty ratio D (a ratio ON/(ON+OFF)) of the rectangular wave. This produces a merit that a drive circuit for supplying the drive pulse can be achieved by a simple circuit using a transistor device capable of an ON/OFF control.
Also in the fine movement mode, it was convenient that the moving speed of the recording and reproducing head was adjusted by varying the duty ratio D of the rectangular wave. However, the recording and reproducing head cannot be finely moved in the fine movement mode by varying the duty ratio D of the rectangular wave for the reasons described below. Thus, in the prior art, as described above, the moving member is moved with high accuracy with an expansion displacement or a contraction displacement generated with the application of the DC voltage to the piezoelectric element in the fine movement mode.
FIG. 7 is a graph illustrating the interrelationship among the frequency f and duty ratio D of a drive pulse (a rectangular wave) to be supplied to a piezoelectric element and a moving speed V of a recording and reproducing head in an actuator using the piezoelectric element, wherein it is shown that even if the duty ratio D is constant, the moving speed V can be varied according to the frequency f of the rectangular wave.
In other words, a curve A shows the relationship between the duty ratio D and the moving speed V of the recording and reproducing head when the frequency f of the rectangular wave is low (f=ft, wherein fs>ft); in contrast, a curve B shows the relationship between the duty ratio D and the moving speed V of the recording and reproducing head when the frequency f of the rectangular wave is high (f=fs, wherein fs>ft).
When the duty ratio D falls in the vicinity of 0.25 (or 0.75), the moving speed V of the recording and reproducing head becomes maximum both when the frequency f is low and when the frequency f is high. However, since the number of pulses per unit time is greater when the frequency f is higher, the maximum of the moving speed V is greater than that when the frequency f is lower.
In contrast, when the duty ratio D falls in a low speed region in the vicinity of 0. 5 or 0.0 (also at 1.0), the recording and reproducing head slides in a moving direction, resulting in the generation of a neutral zone N in which the moving speed V is never varied in response to the variations in the duty ratio D, as illustrated in FIG. 7. As is obvious from FIG. 7, the width of the neutral zone N is greater as the frequency f is higher; on the contrary, it is smaller as the frequency f is lower. As a consequence, the characteristics of the neutral zone have made the low speed region unsuitable for low-speed driving.
Tracking operations in the information recording and reproducing apparatus include a seeking operation by which the recording and reproducing head is speedily moved to a target track and a following operation by which the recording and reproducing head is allowed to follow on the target track with high accuracy.
High speed performance is required for the seeking operation, that is, the moving speed of the recording and reproducing head need be high. In the meantime, it is important that the following operation has performances of the positioning accuracy of a high resolution and the linearity in which the moving speed is linearly varied according to the variations in the duty ratio D in the low-speed region.
In the case where a speed control method for varying the moving speed of the recording and reproducing head according to only the variations in the duty ratio D is adopted in the tracking mechanism, the drive circuit can be configured more simply than that in the speed control method in which the rough movement mode and the fine movement mode are switched, as disclosed in the above-descried gazette. However, there has arisen a serious problem that both of the characteristics of the neutral band and the high speed must be achieved in the low-speed region in the vicinity of a duty ratio D of 0.5.
In the following operation by which the recording and reproducing head is allowed to precisely follow the track, the recording and reproducing head detects a positional error between the center of the recording and reproducing head and the center of the track, and then, performs a feedback control so as to set the positional error to zero. At this time, it is necessary to move the recording and reproducing head at a fine speed in both of positive and negative directions.
However, when the recording and reproducing head is to be moved at a fine speed in both of positive and negative directions only by varying the duty ratio D, the above-described characteristics of the neutral band in the low-speed region hinder the generation of the fine speed, and therefore, the positioning accuracy in the following operation is degraded, thereby raising a problem that the track interval cannot become small, that is, a high recording density cannot be achieved.
As described above, if the frequency of the drive pulse is decreased, the width of the neutral zone becomes small, and thus, the fine speed can be achieved. However, the maximum of the moving speed becomes small, thereby raising a problem that a time required for the seeking operation is prolonged.
Furthermore, the moving speed of the recording and reproducing head during the seeking operation is decreased from the maximum speed when the recording and reproducing head is moved to a desired track down to about zero when the recording and reproducing head approaches the desired track. When the recording and reproducing head approaches the desired track, the recording and reproducing head cannot be moved at the fine speed due to the above-described characteristics of the neutral band, thereby raising a problem that the desired track cannot be sought finally.