Page 294 - Anatomy of a Robot

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MOTORS AND ACTUATORS 279
deliberately. They are also not well geared for changing loads; they can lose track
of their position if the load varies in a sudden manner.
Cooling Steppers can be fairly open and easy to cool. If they remain stationary
for some time, the current in the coil can be reduced. A good controller will do
that automatically.
Controls Steppers have relatively complex controllers. They are generally com-
puterized since the computer must keep track of the position and momentum of
the motor. More complex controllers have more than just on-off control of the coil
voltage and current.
Portability Steppers tend to be lightweight and fairly sturdy. They are not par-
ticularly good with large or varying loads, but they function reliably in most appli-
cations.

Exotic Motors

PIEZO-ELECTRIC MOTORS

Piezo-electric materials are ceramics that change shape when an electric field is applied
across them. Watch alarms and phone ringers are the most common applications of such
materials. They don’t move much, but they can move often. They are used for small
motions like creeping and fine adjustments. If the robot must have very fine, accurate
positioning, piezo-electrics can provide the movements. They can move large loads,
albeit slowly. More info on these motors can be found at the following web sites:

www.piezo.com/intro.html
http://web.umr.edu/~piezo/

ORGANICS
Some organic crystals expand and contract when a current is passed through them. No
simpler motor exists. Unfortunately, these tend to be very fragile.
Here are some more web sites with information about motors:
www.instantweb.com/o/oddparts/acsi/motortut.htm
www.slewin.clara.net/elec/tmotor.htm
www.cs.uiowa.edu/ jones/step/
www.motionnet.com/cgi-bin/search.exe?a=cat&no=1708