Page 170 - Mechatronic Systems Modelling and Simulation with HDLs

7.5 DEMONSTRATOR 4: DISK DRIVE 159

by an analogue/power circuit, i.e. motion-control. The voice coil motor transforms
the current into motion of the head assembly which may or may not contain res-
onances. The mechanical motion results in a track position which is fed into the
firmware controller. The track content including track-id — as well as the logic to
detect it — is reduced to a more or less trivial digital model, since it does not add
to the overall function of the servo control.
The simulation of the servo control is performed on the basis of the mixed-mode
simulator Saber. The analogue part of the system — motion-control and mechan-
ics — are modelled in the analogue hardware description language MAST. Some
basic digital modelling including the synchronisation between the regulator C-
routine and the rest of the system is carried out using the digital capabilities
of MAST.

7.5.6 Simulation and results

The high-level model of the servo control as described in the previous section can
be used for concept engineering. For example, it is trivial to change the number of
servo fields in a track, which at a fixed rotational speed determines the frequency
of position measurements and thus the frequency of the digital controller. In the
same manner, most of the other variables determining the function and performance
of a drive may be varied. This can even be carried out systematically, e.g. by a
parameter sweep or — if more than one parameter is involved — on a Monte-Carlo
basis. All these variations can be simulated efficiently. For instance, the simulation
of a long seek over 20 000 tracks takes about 80 CPU seconds for 20 ms real-time
on a SUN Ultra 60 workstation, see Figure 7.17. One can easily spot the ‘bang-
bang’ strategy of maximum acceleration, constant speed at the speed-limit and
maximum deceleration to lock into the new track position. Incidentally, the speed
limit is due to the fact that the read/write-head rides on an air cushion. This requires
an air-stream in the direction of the tracks, which is created by the rotations of the
disk(s). The movement for track seeking is perpendicular to that and could seriously
disturb the mechanism described above beyond a certain speed. In the next step, the
motion-control part of the servo control model was replaced by its implementation,
represented by about 300 CMOS transistors and some DMOS power transistors.
The real-life circuitry was thereby verified in The Virtual Disk Drive, which is
far more meaningful than the results of classical analogue test benches. With the
same configuration as the previous long-seek analysis, the simulation takes about
9 CPU hours. Note that most seek operations are performed much quicker and that
track-following can be reasonably assessed in an even shorter period. In addition
to the standard tasks of seeking and track following, it is now easily possible to
review the implementation of special features, e.g. a request to park the heads in
the landing zone.
The virtual testing of analogue circuitry is currently under evaluation. The devel-
opment of a high-level system model is absolutely indispensable to this. This will