Page 165 - Mechatronic Systems Modelling and Simulation with HDLs

154 7 MECHATRONICS

Magnetic field
RW-Head
Pivot

Coil

Figure 7.13 Voice coil motor

The data on the disk is organized in circular tracks. The outermost track may
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be located at a radius of 1.8 inch for a typical 3.5 inch form factor drive. This
means that the maximum length of a track is 11.3 inches, which gives rise to a
linear speed of about 1880 inch/s for a 10 000 rev/min drive. This, together with a
bit density within a track of 400 Kbit/inch, for example, in turn leads to a peak data
rate of 750 Mbit/s. Interestingly enough, 1880 inch/s — in terms of the velocity of
a car — is far beyond any speed limit, while bits of the length of about 60 nm are
reliably read.
Thetrackdensityofdrivescurrentlyunderdesignisinthe rangeof30 000–100 000
tracks per inch. Together with bit densities of 300–500 Kbit/inch this leads to surface
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densities of 9–50 Gbit/inch . Given the current track densities, the track pitch is in
the range of 250 nm (100 000 tracks/inch) to 850 nm (30 000 tracks/inch). Controlling
the head position to a precision of 10% of the track width, to make sure that most
of the track width adds to the signal and to keep noise at bay, results in a required
precision of 25 nm–85 nm when controlling the track following.

7.5.3 Circuit development for disk drives

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Designing circuits for disk drives is currently an ASIC business with a low number
of customers, i.e. the disk drive companies, and high volumes. For instance, 223
million disk drives were shipped in the year 2000. This is still the case even
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though several attempts to create ASSPs have been made in the past and are
underway at the moment. The electronics of a disk drive — together with the related
firmware — can be partitioned into a few functions:

• Servo control
The servo control detects the current position of the head using so-called servo
marks which are embedded into the tracks. Several hundreds of these marks are
available throughout a full rotation of the disk. On the basis of this information
the voice coil motor is controlled to allow for track following and seeking.

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Note that none of the dimensions of a 3.5 inch drive is actually 3.5 inch. The disk’s form factor is just
the same as for a 3.5 inch floppy drive.
4 Application specific integrated circuit.
5 Application specific standard product.