Page 124 - Anatomy of a Robot

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COMPUTER HARDWARE 109
Disks
Hard disk (HD) drives are familiar to most people. They are in most personal comput-
ers and are occasionally a source of frustration if they misbehave. When they break,
they can cause significant headaches and losses of expensive data. The environment
they operate in is important to consider. An office environment is quite stable in most
respects. If an HD is put into a robot, it must be treated properly. Readers should extrap-
olate the following discussion to CD-ROMS and other rotating media since the fol-
lowing discussion will only address magnetic HD disks.
An HD is basically a spinning disk of magnetic material that can contain bits on its
surface. A read/write head glides over the surface and provides access to these bits
for the computer. When designing an HD into a robot, consider the following HD
characteristics:
Vibration The HD, while it’s running, holds its read/write head over the disk
surface. The distance between the two is very small, on the order of millionths of
an inch. Vibrations caused by motors, wheels, actuators, and other motions or the
robot jiggling the disk head will ruin the data. In the worst cases, the disk head
might touch the surface of the disk and scrape off the magnetic coating, ruining
the HD completely. Read the vibration specifications of the HD very carefully
before using it in a robot design. Consider replacing the HD with a more expen-
sive alternative like flash memory cards that have no moving parts. In the design
of a robot, it’s wise to restrict the amount of data that will have to be stored
onboard. If an HD must go into the robot, consider using an HD specifically
designed for laptops. Laptop HD drives are more robust than most.
Shock If the robot hits a pothole, falls over, or simply burps, the HD may expe-
rience a sudden shock. It’s not unusual for shock forces to hit 50 or 100 times the
force of gravity for a very short time period. Read the HD specifications very
carefully. There may be different shock specifications for operation and storage.
When trying to match the HD specifications to the robot’s specifications, don’t
forget to include the period when the robot is being shipped but is not operating.
If the HD cannot take the shock specified in the robot’s requirements, consider
another technology like flash cards. Another option would be to consider derating
the robot’s specifications so it will be treated more gently in operation.
Temperature Like any component within the robot, a HD will have tempera-
ture ratings. The only extra thing to consider is that temperature might cause the
HD to shrink or expand temporarily and thus make errors for a while.
Gyroscopic torque HDs contain rotating masses. As such, they will behave like
a gyroscope. Remember those spinning toys that could balance on your finger?
Gyroscopic action inside any HD will exert the same forces.