Tom Carroll can be reached at TWCarroll@aol.com.
pots tend to be a little
noisy and not as linear
as one may desire,
especially the 3/4” turn
types that are in a lot of
circuitry. The 1/4” types
that I mentioned
previously have either
three hard leads on the
bottom that can be
soldered to a circuit
board, or one can use
the three twisted wire
types so the builder can
position the pot where
FIGURE 10. Solenoid operation
from Society of Robots site.
Before I discuss linear actuators
and servos that are designed around a
lead-screw with a traveling nut that is
attached to a gearmotor, I would like
to discuss some inexpensive
alternatives to these devices. Solenoids
are popular as a linear movement
device and are used in many
applications such as valves and
Figure 10 from the Society of
Robots site illustrates the simple
technology involved. An energized
wound coil pulls or pushes (usually
pulls or retracts) a ferrous ‘slug’ to
create linear motion.
Linear motion distance is usually
limited to less than an inch, and the
force is very non-linear in that the
beginning of the force is small and
increases almost exponentially to the
end of the stroke — just like a piece of
metal drawn to a magnet. There are
applications in robot design such as
the release of a weapon or similar, but
few robots use electromagnetic
same way as Nitinol. When an electric
current is applied to the wire and it
heats past its ‘activation temperature’
— the point at which it was ‘trained’ —
it contracts about 10% of its length.
Figure 11 shows a robot hand using
five of the wires to contract and cause
the fingers to curl.
Figure 12 shows a simple door
latch application using SMA wire. A
five meter length of the 0.006”
diameter wire with a resistance of
1.3Ω per inch is available at
RobotShop for about $22. It is a tiny
(just a wire) linear actuator but
applications require a lot of current
(500 mA or more), have slow
response times, and large hysteresis.
Read more about SMA technology
from an online search engine.
A third device is the automotive
car electric door lock shown in Figure
13. It is not a solenoid but is a
motorized actuator. They are quite
inexpensive at surplus houses at about
SMA door lock.
FIGURE 13. Inexpensive door lock actuator.
SERVO 12.2012 77