Tracking device using multiple piezoelectric elements for track centering
BACKGROUND OF THE INVENTION 
     This invention relates to a tracking device for a helical scanning VTR in which recording tracks are formed at an angle to the longitudinal direction of a magnetic tape or a magnetic recording and reproducing system in which concentric or spiral recording tracks are formed on a magnetic disc and, more particularly, to a tracking device for displacing a magnetic head in the direction of the track width according to tracking information. 
     In the usual tracking device of this type, a magnetic head is mounted on the tip of a bimorph element (which is comprised of a piezoelectric element), and the center position of the magnetic head gap is held coincident with the center position of the track at all times by applying a predetermined voltage to the bimorph element according to tracking information. 
     In this tracking device, however, the mechanical strength of the bimorph element itself is comparatively low. Therefore, the magnetic head is liable to be greatly deviated from its regular position by an external force such as a force produced when the tip of the magnetic head is brought into contact with the recording surface of a magnetic tape or a magnetic disc. In such a case, it is necessary for restoring the magnetic head to the optimum operating position to apply a considerably large correcting voltage to the bimorph element. This means that the dynamic range of an amplifier for supplying the correcting voltage must be considerably increased, which is disadvantageous from the standpoint of cost. The deviation mentioned above also gives an off-set to a tracking servo system. In order to absorb this off-set, it is necessary to extremely increase the servo gain of the tracking servo system. To do so, there are problems in view of ensuring the stability of the servo system. The bimorph element is formed by cementing together two thin ceramics, or it comprises a piezoelectric element and a non-piezoelectric film such as a metal film or a polymer film cemented to the non-piezoelectric element. In order to cause a great displacement of the magnetic head, therefore, the bimorph element must have a considerable length. To increase the length of the bimorph element, the length of the ceramics must also be increased. For this reason, the ceramics must also be quite thick in order to ensure its mechanical strength. In this case, a considerably high drive voltage must be applied to the bimorph element in order to obtain sufficient operation. 
     Because of the relationship of the thickness, length, drive voltage and deformation of the bimorph element as mentioned above, it is difficult to design a bimorph element which is well-suited for tracking. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a tracking device, which permits stable tracking without being influenced by the external force produced when the magnetic head is brought into contact with the recording surface. 
     According to the invention, there is provided a tracking device, which controls the tracking of the magnetic head by making use of a thickness extensional mode of piezoelectric elements. 
     According to the invention, there is also provided a tracking device, with which high mechanical strength of the element for driving the magnetic head can be ensured without the need for increasing the length of the element and which can also be readily assembled and adjusted. 
     According to the invention, there is further provided a tracking device, in which the magnetic head is sandwiched between first and second piezoelectric members, and opposite polarity voltages are applied to the first and second piezoelectric members according to tracking information so that the magnetic head is moved according to the deformation of the first and second piezoelectric members. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing a disc apparatus using an embodiment of the tracking device according to the invention; 
     FIG. 2 is a perspective view showing the tracking device of FIG. 1; 
     FIG. 3 is a circuit diagram showing an equivalent circuit of the tracking device of FIG. 2; 
     FIG. 4 is a circuit diagram showing a tracking detecting section for detecting tracking information; and 
     FIG. 5 is a perspective view showing a tracking device using a helical scan system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a rotary magnetic disc 11 having recording tracks as typically shown by 12. A step motor 13 for moving a magnetic head has its shaft integrally coupled to a lead screw 14. A head actuator or a head holder 15 is screwed on the lead screw 14. The lead screw 14 and head holder 15 are coupled to each other with a ball bearing thread structure where balls are interposed between the screw 14 and holder 15, and the head holder 15 is moved in the radial direction of the disc 11 as the lead screw 14 is rotated by the step motor 13. 
     FIG. 2 is a perspective view showing a specific construction of the head holder 15 shown in FIG. 1. As is shown, the head holder 15 includes an H-shaped frame 20 having a sufficient mechanical strength. A magnetic head 22 having a gap G is mounted, in a state sandwiched between first and second piezoelectric members 23a and 23b formed of a plurality of superposed piezoelectric elements e.g. ceramics (barium titanate) or Rochelle salt, between opposed legs 21a and 21b of the frame 20. A positive voltage and a negative voltage provided from voltage amplifiers 24 and 25 are applied as control voltages to the first and second piezoelectric members 23a and 23b. The one ends of the the first and second piezoelectric members 23a and 23b are bonded by means of adhesive to the respective legs 21a and 21b. The other ends of first and second piezoelectric members 23a and 23b are also bonded by means of adhesive to the opposite sides of the magnetic head 22. 
     FIG. 3 is a view showing the detailed construction of the first and second piezoelectric members 23a and 23b. In FIG. 3, only the first piezoelectric member 23a is shown. The member 23a is formed of insulators 31 to 36 and piezoelectric elements 41 to 45 which are alternately superposed. Electrode layers 41a and 41b to 45a and 45b are provided on the opposite sides of the respective piezoelectric elements 41 and 45. The electrode layers 41a to 45a on the left side of piezoelectric elements 41 to 45 in the Figure are grounded, while the electrode layers 41b to 45b on the right side of the piezoelectric members are connected to the output terminal of the voltage amplifier 24 noted above. In accordance with tracking information, a control voltage is produced from the voltage amplifier 24, and is applied to the piezoelectric elements 41 to 45 in parallel. If the control voltage is a positive voltage, a biasing force in the direction of arrow A in FIG. 3 is produced. If the control voltage is a negative voltage, a biasing force in the direction of arrow B is produced. The biasing force noted above is applied to the head 22 in the same direction as the first and second piezoelectric members 23a and 23b. More particularly, when the piezoelectric elements of the piezoelectric member 23a are expanded in the direction of its thickness, as shown in FIG. 2, according to the applied voltage, the voltage applied from the inverted voltage amplifier 25 to the other piezoelectric member 23b tends to contract the member 23b. The simultaneous expansion of the piezoelectric elements of the member 23a and contraction of the piezoelectric elements of the number 23a have an effect of displacing the magnetic head 23 in the direction of arrow A. 
     FIG. 4 shows a tracking information detector. A pair of reproducing heads 51 and 52 are provided for detecting tracking information. These heads 51 and 52 are positioned such that their respective gap centers coincide with the opposite edges of the relevant track 12 when the magnetic head 22 is in the optimum tracking state. They can be moved in unison with the magnetic head 22 in the direction of the track width. The outputs of the tracking information detection heads 51 and 52 are enveloped by respective enveloping circuits (envelope detectors) 53 and 54, and the outputs thereof are fed to a differential amplifier 55. The output of the differential amplifier 55 is supplied as tracking information to the voltage amplifier 24. 
     With the tracking device of the above construction, when a tracking error of the head 22 occurs, a difference is produced between the outputs of the tracking information detection heads 51 and 52. As a result, a positive or negative output is produced at tracking information from the differential amplifier 55 according to the difference. The tracking information is amplified by the voltage amplifier 24 and also by the inverted voltage amplifier 25. The opposite phase voltages which are produced from the respective amplifiers 24 and 25 are applied as control voltages to the superposed piezoelectric elements of the members 23a and 23b. The piezoelectric members 23a and 23b thus exert driving forces to the head 22 in the direction of arrow A or B in FIG. 2, whereby the tracking control of the head 22 is performed such that its center position is brought back into register with the center position of the track 12. Since the head 22 is sandwiched between the opposite side piezoelectric members 23a and 23b, which are in turn firmly clamped between the opposed legs 21a and 21b of the frame 20, it will not be greatly deviated even if an external force is exerted to it. Thus, unlike the prior art, there is not need of particularly increasing the dynamic range of the amplifier 24 for applying large voltages to the piezoelectric members 23a and 23b for the correction of the deviation. Further, there is no need of ensuring a servo system gain for absorbing an off-set, so that the stability of the servo system can be obtained. Further, since the head 22 is uniformly urged at its opposite sides by the pair of piezoelectric members 23a and 23b between the parallel legs 21a and 12b of the head 22, as shown in FIG. 2, the vertical and parallel status of the head 22 can be readily obtained without provision of any particular adjusting means. 
     Further, when applying the invention to a helical scan type VTR, the head holder as shown in FIG. 5 may be mounted on a rotary cylinder, on which a magnetic tape is wound. In this case, the lead screw 14 is unnecessary, and it is only necessary to mount the frame 20 such that its axis X--X&#39; is parallel with the axis of rotation of the rotary cylinder. 
     Further, while the above embodiment has used the superposed piezoelectric elements as the piezoelectric members to sandwich the head 22 therebetween, it is also possible to use a single piezoelectric element depending upon the size of the head 22. 
     As has been described in the foregoing, according to the invention, the magnetic head is firmly sandwiched between the opposite side piezoelectric members which are in turn firmly clamped between parallel and opposite legs of a frame having high mechanical strength. Thus, it is possible to provide a tracking device with which the magnetic head can be stably head and which is not adversely affected irrespective of any external force applied to it so that stable tracking control can be obtained. 
     Further, with the tracking device according to the invention the head actuator can be reduced in size and be made more rigid compared to the prior art head actuator using a bimorph element. Further, the head actuator of the tracking device according to the invention can be used not only as a magnetic head actuator for a video tape recorder or a magnetic disc recorder but also as a tracking actuator for an optical head of an optical disc recorder or as a focusing actuator. The invention is particularly suitable for a head actuator of a small-size portable magnetic recorder.