Page 125 - Anatomy of a Robot

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110 CHAPTER THREE
Any HD will resist being turned. If the robot will be spinning or moving in such
a way that the HD is thrown around some, better do some worst-case calculations
on the rotational velocity of the HD. The specifications of the HD may not be very
clear about the torque that the operating drive can withstand without making
errors. If possible, arrange for any robotic motions affecting the HD to be copla-
nar with the rotating mass of the HD. Picture an HD placed on a flat surface. Most
HDs are shaped like a brick, with the disk surface spinning like a record rotating
about a vertical axis. The operating HD can be moved sideways just fine without
engendering gyroscopic forces. But if the HD is twisted sideways, it will resist
with gyroscopic forces. These forces could ruin data or burn out the motor bear-
ings over time. Read the specifications for the HD very carefully. If no specifica-
tion for rotational acceleration exists, beware. All disks do have a limit for this, so
find out what it is.
Spin-up time HDs take a couple of seconds to spin up to operating speeds. If
the information on the HD must be instantly accessible, don’t allow the HD to spin
down automatically with disuse. Some computer systems will enable HDs to spin
down to save power. If this is an issue, make sure the computer is not allowed to
turn off the disk motor.
Longevity HDs only have finite lifetimes. There should be Mean Time Between
Failure (MTBF) information (discussed later) for the HD, which takes certain fail-
ure mechanisms into account. Barring electrical failure, either the spindle bearing
will wear out or the HD magnetic surface will wear out. In addition, some issues
may arise regarding data degenerating over long time periods and becoming prob-
lematic. CDs have this problem, and HDs probably do too.
Error rates HDs do make errors. Generally, the signals that are recorded are
more than sufficient to allow a proper read of the data.
Bad disk surface HDs also have a mechanism to avoid bad spots on the disk
surface. A separate place exists in the HD surface to record the location of bad
sectors on the surface so they can be shipped from the factory without having
perfect media. The HD can then avoid those bad spots altogether. However, the
disk surface can also develop new bad spots. If the operating system can detect
such an occurrence, it can compensate for it.
Bad write or read Occasionally, the disk just makes a mistake. Errors can
occur because of media problems, vibration, and probably phases of the moon!
Usually, the operating system software is programmed to detect the problem
and compensate for it. If the design of the robot is such that data must never be
lost, then multiple disk images should be written. It is also possible to put in
multiple disks to accomplish this. Read up on Redundant Array of Inexpensive
Disks (RAID) systems if this is a requirement for the design of the robot. Check