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ingly enough one that perhaps will be improved by a factor of 10 in the
next decade. Incidentally, the speed of rotation of floppy disks is about
1 ⁄ 10 that of the hard disk, or 360 rpm. In addition, the head touches the
magnetic surface of the floppy disk, limiting the storage life of the unit.
The typical magnetizing force needed to lay down the magnetic orientation
is 400 mA-turn (peak-to-peak). The result is a write current of only
40 mA for a 10-turn, thin-film inductive head.
Although the thin-film inductive head could also be used as a read
head, the magnetoresistive (MR) head has improved reading characteristics.
The MR head depends on the fact that the resistance of a soft ferromagnetic
conductor such as permolloy is sensitive to changes in
external magnetic fields. As the hard disk rotates, the changes in magnetic
flux from the induced magnetized regions of the platter change the
terminal resistance of the head. A constant current passed through the
sensor displays a terminal voltage sensitive to the magnitude of the
resistance. The result is output voltages with peak values in excess of
300 V, which exceeds that of typical inductive read heads by a factor of
2 or 3�1.
Further investigation will reveal that the best write head is of the
thin-film inductive variety and that the optimum read head is of the MR
variety. Each has particular design criteria for maximum performance,
resulting in the increasingly common dual-element head, with each
head containing separate conductive paths and different gap widths.
The Areal density of the new hard disks will essentially require the
dual-head assembly for optimum performance.
As the density of the disk increases, the width of the tracks or
cylinders will decrease accordingly. The net result will be smaller
heads for the read/write function, an arm supporting the head that
must be able to move into and out of the rotating disk in smaller
increments, and an increased sensitivity to temperature effects which
can cause the disk itself to contract or expand. At one time the
mechanical system with its gears and pulleys was sensitive enough to
perform the task. However, today’s density requires a system with less
play and with less sensitivity to outside factors such as temperature
and vibration. A number of modern drives use a voice coil and ferromagnetic
arm as shown in Fig. 11.48. The current through the coil will
determine the magnetic field strength within the coil and will cause
Read/Write
head
Voice coil
FIG. 11.48
Disk drive with voice coil and ferromagnetic arm.
Ferromagnetic
shaft
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