pulse stretcher which, in effect, is an amplifier. So, a 1%
difference in width from the comparator can generate a
50% drive to the H-bridge. The H-bridge sets the polarity of
the voltage going to the motor according to the direction
signal. The percent drive to the H-bridge decreases as the
position approaches the command point. To prevent
“hunting” (an oscillation around the final position), there
is a small dead-band. Once the difference between the
command point and position is within the dead-band, the
motor drive goes to zero.
Three types of material are used to make gears for
• Nylon: the most commonly used. Nylon gears are
lightweight and run smoothly with low wear, but are at
the low end in durability and strength.
• Metal: the strongest material. Metal gears are heavy and,
due to wear on the teeth, will develop “slop” (looseness) in
the gear train. Slop causes loss of position accuracy and
sometimes causes a mechanical instability or oscillation
under certain types of load. If you have the money,
titanium gears offer superior wear resistance.
• Karbonite: carbon reinforced plastic (not to be confused
with carbonite, which is highly explosive). Hitec’s Karbonite™
gears are stronger and more durable than plain nylon, but
just as lightweight. They wear better than metal gears, but
metal is still the strongest.
Two important specs for a servo are speed and torque.
Speed is specified as how long it takes to rotate through a
given angle, such as 0.15 seconds for 60 degrees. Torque is
given in ounce-inches (oz-in) or kilogram-centimeters (kg-cm).
Speed and torque are given for specific voltages, usually 4.8V
and 6V. One factor that affects speed and torque is the bearing
surface for the output shaft. Possibilities are plastic, metal
sleeve, or ball bearings. Servos vary in size and weight with
more powerful servos being bigger and heavier. There are
micro, mini, and standard sizes, as well as some “maxi” sizes.
motor position faster.
That means faster
starting torque, a
and more holding
torque. The trade-off
is that digital servos
can draw a lot more
current than analog
servos. They can kill
a battery quickly.
Because of the built-in H-bridge and gear train, people
sometimes modify servos to rotate continuously to become,
in effect, inexpensive gear-head motors. But servos were
Digital vs. Analog
While digital servos are more expensive than
analog models, they offer a lot of advantages.
Because they contain a microprocessor, some can
be programmed for parameters such as speed,
direction of rotation, range of rotation, and
dead-band. Or, you can skip the programming and
use them as they are right out of the box.
Because they can receive input commands faster
than analog servos, digital servos can update the
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