Video tape recorder heads

Abstract: In a video tape recorder, in order to prevent clogging of a head gap without affecting head tracking and tape-to-head fit, a head tip 14 is cantilevered on a rotary drum 1, and the tip is vibrated ultrasonically in a direction A transverse to the direction of the track 19a of the tape.

Description: This invention relates to a head drum apparatus for a video tape machine and is particularly concerned with reducing clogging of the head. In accordance with the preamble of claim 1, Japanese Utility Model application JP-A-23224/1984 discloses a head unit of a video tape recorder which can be vibrated in order to reduce clogging of the head unit. Referring to Figures 6 and 7 of the accompanying drawings, the known head unit 12 is mounted on a rotary drum 4 wound with a magnetic recording medium in the form of a tape 19. The head unit 12 is provided with a mounting element 13 mounted on the drum 4 by a screw. An exciting element 15 is mounted on the mounting 13, and in turn a head tip 14 is mounted on the exciting element 15. The tip end of the head tip 14 projects slightly from the rotary drum 4 so as to contact the tape 19. The head tip 14 has a head gap, and signals can be read from and written to the tape 19 by the head tip 14. The exciting element 15 is electrically connected to an ultrasonic signal generator and is arranged so that the head tip 14 is vibrated in a direction radially of the drum, i.e. towards and away from the tape 19 as shown by arrow B in Figure 7. The ultrasonically vibrating head tip 14 shakes off foreign matter (mainly, bonding material and magnetic powder separated from the tape) attached thereto, and seeks to prevent read and write failure, or dropout, due to clogging of the head gap. The resonant frequency of the exciting element 15 is determined by the Young's modulus thereof and the mass of the head tip 14 as a load. In a typical example, the head tip 14 has a frequency as low as 15 to 20 kHz, and accordingly, a problem has arisen in that the head tip 14 cannot sufficiently shake off the foreign matter attached thereto. As a result, dropout can still occur in reading or writing of the signals. Moreover, the head tip 14 vibrates in the direction towards and away from the tape 19. Therefore, disadvantages occur because of poor tape-contact between the head tip 14 and the tape 19, or a speed error in the rotation of the head. Also the head tip 14 is secured on the exciting element 15, and a piezoelectric element which is used as the exciting element 15 has an unstable property in the sense that it tends to shift the position of the head tip 14 with variation in temperature. This can cause failure of tracking of the head. The present invention is concerned with preventing or reducing clogging of the head gap and eliminating or reducing signal dropout in a way which does not affect, or has a reduced effect on, tape-to-head contact, drum rotation, and/or head tracking. In accordance with the present invention, the vibrating device acts on a head mounting element so as to vibrate the mounting element and the head at an ultrasonic frequency in a direction which is transverse to the track which the head traces on the magnetic tape and such that the ultrasonic vibrations have an insignificant effect on tracking. Because the head is vibrated transversely of the track, rather than towards and away from the tape, the problems with tape-to-head contact and drum rotation are obviated. Preferably, the head mounting element is arranged as a cantilever to mount the head on the drum. With such an arrangement, the resonant frequency is typically considerably greater than that for the known vibration arrangement, which results in better prevention of clogging. Furthermore, with this arrangement, the vibrator device can be arranged so that temperature variations do not cause tracking failure. It is known from patent specification US-A-4,882,635 to use an electro-mechanical transducer to adjust the position of a head so as to adjust tracking, and it is known from patent specification AT-B-370 253 to use a piezoelectric device. With the present invention, preferably the vibrator device comprises a piezoelectric device and means to energise the piezoelectric device at an ultrasonic frequency, and preferably the piezoelectric device has a first end acting on the mounting element and a second end mounted stationarily with respect to the drum, the energising means further comprising means for applying a DC bias signal to the piezoelectric device, the DC bias signal being adjustable thereby to adjust the head in a direction transverse to the track. Specific embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figs. 1 to 3 show a first embodiment according to the present invention, in which: Fig. 1 is an exploded oblique view showing mounting of a head unit to a rotary drum, viewed from below, Fig. 2 is a sectional view of Fig. 1, and Fig. 3 is a side view, partially sectioned, of the rotary drum; Figs. 4 and 5 show a second embodiment, in which: Fig. 4 is an exploded oblique view showing mounting of a head unit to a rotary drum, viewed from below, and Fig. 5 is a sectional view of Fig. 4; Figs. 6 and 7 show the prior art, in which: Fig. 6 is a plan view of a rotary drum, and Fig. 7 is a sectional view of Fig. 6; and Fig. 8 is an end view of a modified head tip. Referring now to the first embodiment of the present invention, Fig. 3 shows a sectional view of a rotary drum 1. A drum support 2 is fixed on a chassis (not shown). A drum 3 is supported on the drum support 2 and composed of an upper rotary drum 4 and a lower fixed drum 5. The rotary drum 4 is composed of a drum body 6 of a short cylindrical shape, a flange 7 projecting inwardly from the lower end of the drum body 6, a spindle support 8 fixed on the flange 7, and a cover plate 9 covering an upper surface of the drum body 6. All of the above are secured by screws which are not shown in the drawing. The spindle support 8 is formed on its center with a spindle hole 8a in which a shaft 10 is a force fit. The lower portion of the shaft 10 passes through the fixed drum 5 and the drum support 2. Ball bearings are arranged between the shaft 10 and the drum body 6 and the drum support 1. The rotary drum 4 is supported rotatably by the fixed drum 5 and the drum support 2. Also a plurality of head units 12 (in this embodiment, two) are provided on the lower and outer circumference of the flange 7 of the drum body 6. As shown in Figs. 1 and 2, each head unit 12 is provided with a head base 13 fixed on the flange 7, head tips 14 provided on the tip end of the head base 13, and exciting or vibrator elements 15 for vibrating the head base 13. A shallow recess 7a for receiving the head tip 14 is formed in part of the upper surface of the outer circumference of the flange 7 of the rotary drum 4. Also, a deeper recess 7b is formed below the shallow recess 7a for receiving the exciting elements 15. Inwardly of the recesses 7a,7b of the flange 7 is formed a base mounting screw hole 7c. The head base 13 is formed from a brass plate and is provided in its base end with a mount hole 13a and on its tip end with a pair of projections 13b. The upper surfaces of the projections 13b are tapered. A slit 13c is formed in the head base 13, extending from between the projections 13b to the vicinity of the mount hole 13a. The head tips 14 are bonded by a bonding agent on the tip-end bottom surfaces of the projections 13b to either side of the slit 13c. The head tips 14 are wound with coils 20 whose ends are soldered on a contact plate 21 bonded by a bonding agent on the upper surface of the head base 13. The pair of head tips 14 form a head gap 18 therebetween, which permits reading and writing to a magnetic recording tape 19 with a predetermined track-width. The exciting elements 15 are layered or laminated piezoelectric elements which are bonded by a bonding agent to the bottom surface of the head base 13 to either side of the slit 13b. The layered piezoelectric elements 15 contract and expand with voltage applied so that the head base 13 is vibrated in the width or transverse direction of the track 19a of the tape 19. The layered piezoelectric elements 15 are accommodated in the recess 7b. The head base 13 thus constituted abuts on the flange 7 of the rotary drum 4 through a washer 16. The head base 13 is secured on the rotary drum 4 by a base mounting screw 17 engaging with the base mounting screw hole 7c, and passing through the through hole 13a of the head base 13 and the through hole 16a of the washer 16. The ends of the head tips 14 protrude slightly from the circumference of the rotary drum 4. Due to a larger size of the through hole 13a of the head base 13 than that of the base mounting screw 17, the amount by which the head tip 14 projects relative to the rotary drum 4 can be adjusted by displacement of the head base 13. An explanation will now be made of the operation of the first embodiment referring to Figs. 1 to 3 as follows. In Fig. 2, with voltage applied, the piezoelectric element 15 expands and contracts ultrasonically in the vertical direction, and the head base 13 vibrates ultrasonically in the width direction (arrow A in Fig. 2) of the track 19a of the tape 19. It is to be noted that the head base 13 is fixed on its one end by the base mounting screw 17 to the flange 7 of the rotary drum 4 and is constructed as a cantilever having the head tip 14 at its free end. The resonant frequency of the head base 12 in a cantilever bending mode is determined by its effective length, thickness, Young's modulus, the piezoelectric element 15 as a load and mass of the head tip 14, etc. For typical values of these parameters a resonant frequency is produced which is extensively larger (about 200 kHz in this embodiment) than the resonant frequency (about 1 kHz) of the prior art arrangement of Figs. 6 and 7, and the amplitude of vibration at the resonant frequency can be reduced to a small degree such as 0.5µm. Therefore, the head tip 14 vibrates ultrasonically and thus shakes off the magnetic powder or bonding dust which has adhered from the tape 19. Thus, the head gap is prevented from clogging. It is to be noted that, if dropout (failure to record or write) arises due to the clogging phenomenon during overwriting (writing again, on the already recorded tape 19), the previously recorded signal remains as it was on the portion of the tape where dropout occurred and both intended and unwanted signals are recorded together on the same tape. When reproducing such a tape, and if the unwanted signals are reproduced normally, it is not possible to discriminate the dropout, and a compensating process therefore cannot be performed during reproduction. Then, failure such as suddenly changing the reproduced picture occurs. According to the present embodiment described above, because the clogging phenomenon can be reliably eliminated, such failure can be prevented. Moreover, although the head tip 14 vibrates in the width direction of the track 19a of the tape 19, the width of the track 19a is as large as 40µm to 50µm, and the vibration amplitude is as small as about 0.5µm. Therefore the effect of the head tip 14 vibration on the tracking is insignificant. The layered piezoelectric element 15 may vary its characteristic or positioning due to external conditions such as temperature. However, the head tip 14 is fixed directly on the head base 13, and thus it is not affected by displacement of the piezoelectric element 15, and no deterioration arises in tracking. The head tip 14 vibrates in the width direction of the track 19a relative to the tape 19. This means the tape 19 always contacts the head tip 14, and there is no adverse effect on tape-to-head contact or fit. The second embodiment of the present invention shown in Figs. 4 and 5 has the same construction as the first embodiment in many respects. However, in the second embodiment, a step adjuster 22 is provided for adjusting the heights of the head tips 14 of the rotary drum 3, and this is of use when two or more head units are employed. The step adjuster 22 abuts the bottom surface of the layered piezoelectric element 15, and adjusts the height of the respective head tip 14 by pushing up or even pulling down the layered piezoelectric element 15. By means of varying slightly the height of the head tips of the two head units, two tracks can be formed at the same time using only one rotating drum. In Figs. 4 and 5, the flange 7 of the rotating drum 4 is provided on the bottom of each deeper recess 7b with two screw holes 7d vertically passing down through the flange 7 to its bottom surface. Height adjusting screws 23 engage with the screw holes 7d from the bottom surface of the flange 7. The upper surfaces of the height adjusting screws 23 abut on the bottom surfaces of the respective layered piezoelectric elements 15. With this arrangement, the heights of the head tips of the head units can be varied by removing the cover plate 9 (as shown in Fig. 3) of the rotary drum 4 and by turning a head of a desired height adjusting screw 23 using a screw-driver from above the rotating drum. The step adjustment can thus be performed easily. In the second embodiment described above, the height adjusting screws are used for step adjustment. However, adjustment can be realized without using such height adjusting screws. That is, the construction may be modified such that height adjustment is provided by making the bottom surfaces of the layered piezoelectric elements 15 abut on the bottoms of the respective deep recesses 7b, and making the layered piezoelectric elements 15 expand or contract by applying a desired d-c voltage thereto. With such a constitution, the manufacturing process becomes much easier and the number of parts and assembling man-hours are reduced. The modification shown in Fig. 8 will now be described. This modification may be applied to either of the two previously described embodiments. As shown, each head tip 14 comprises a pair of portions 14a,14b, around each of which a respective portion of each coil 20 is wound. Each pair of head tip portions defines a gap therebetween, and, as is known, the gap may extend in directions G which are inclined at angles of 75° and 105° relative to the direction T of the track on the tape. With such a configuration the exciting elements 15 are mounted on the head base 13 such that, and the cantilevering of the head tips is such that the directions of vibration A are inclined at approximately the same angle, 75° or 105°, relative to the track direction T as the respective head tip gap directions G. By virtue of this configuration, timebase errors (which could otherwise occur if the head tips are vibrated in a direction other than the gap direction) can be eliminated. In the case where the modification of Fig. 8 is applied to the second embodiment of Figs. 4 and 5, the adjusting screws 23 may be arranged to be inclined similarly, so that the screws 23 are coaxial with the respective exciting elements 15.

Claims: A head drum apparatus for a video tape apparatus, comprising: a rotary drum (1) around which a magnetic tape (19) can be wound; a head mounting element (13) mounted on the drum; at least one transducing head (14) mounted on the mounting element so that the transducing head can trace a track (19a) on the magnetic tape during rotation of the drum; and a vibrator device (15) for vibrating the head at an ultrasonic frequency; characterised in that: the vibrator device acts on the mounting element so as to vibrate the mounting element and the transducing head at the ultrasonic frequency in a direction (A) transverse to the track such that the ultrasonic vibrations have an insignificant effect on tracking. An apparatus according to claim 1, wherein the mounting element has a first portion which is mounted on the drum and a second portion (13b) on which the transducing head is mounted so that the mounting element forms a cantilever, the vibrator device acting on the mounting element part-way between the first and second portions thereof. An apparatus according to claim 1 or 2, further comprising an adjustment mechanism (22), and wherein the vibrator device has a first end acting on the mounting element and a second end acted on by the adjustment mechanism, whereby the adjustment mechanism can adjust the head in a direction (A) transverse to the track. An apparatus according to any preceding claim, wherein the vibrator device comprises a piezoelectric device (15) and means to energise the piezoelectric device at an ultrasonic frequency. An apparatus according to claim 4, wherein the piezoelectric device has a first end acting on the mounting element and a second end mounted stationarily with respect to the drum, the energising means further comprising means for applying a DC bias signal to the piezoelectric device, the DC bias signal being adjustable thereby to adjust the head in a direction transverse to the track. An apparatus according to any preceding claim, wherein the head comprises a pair of head portions (14a,b) defining therebetween a head gap which extends in an inclined direction (G) relative to the track, and wherein the vibrator device is arranged so as to vibrate the head in the inclined direction of the head gap.

Inventors: SAITO ETSURO C O SONY CORPORAT, SAITO, ETSURO, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G11B5
IPC codes: G11B5