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servos. Boats could use almost any
size, as could most R/C cars. Robot
builders had to paw through a pile of
different types in the early days to
find a servo that had enough torque
for their unique builds.
So, it was the analog servo that
we first used. With an input pulse
train of about 1.0 to 2.0 millisecond
pulses, these little plastic boxes could
interpret pulses from a receiver (or
microcontroller) and rotate the
output shaft back and forth from
about 90 to 120 degrees or more,
depending on the servo and its
programming. With 50 20-millisecond
pulse sequences per second, longer
pulses would cause the output shaft
to proportionally turn one way, and
shorter pulses the other way. The 50
pulse sequences resulted in 50
proportional voltage pulses to the
servo’s motor — either one polarity or
another at a variable voltage.
Internal potentiometers gave an
internal feedback to the servo’s circuitry
as to direction and speed of the output
shaft. Builders soon connected
electronic speed controllers (ESCs) to
the output of an R/C receiver and
could control a motor with the received
pulse trains. These ESCs were first used
in boats and cars before being utilized
by robot experimenters.
FIGURE 2. My tabletop with a wide variety of servos.
FIGURE 3.
Robotis
Dynamixel
daisy-chain.
like the digital servos as they react
faster and don’t seem to have that
Parkinson’s buzz when not moving. The
main disadvantage to digital servos is
the higher current draw, plus they’re a
bit costlier.
The Digital Servo
Digital servos look identical to their
analog brethren and receive the same
pulse trains, but they use a
microcontroller to process the pulses
into a higher frequency series of output
pulses to drive the servo’s motor. The
resulting servo output has a more
constant torque level and almost no
‘dead-band’ movement.
Users can feel the dead-band in an
older analog servo when they try to
turn the output shaft/horn with their
fingers and feel a bit of slop back and
forth. They can also feel the buzzing of
the output caused by the lower
frequency pulses, whereas the digital
servo moves very little when a signal is
applied; the higher frequency buzz of
up to 300 Hz is negligible.
Humanoid robot designers seem to
Dynamixel and
Herkulex Servos
Back in 2005, I first saw a Bioloid
robot made by the Korean company,
Robotis, and its use of unique servo
modules known as Dynamixel rotary
actuators (more on this later). Standard
servos utilize a three-wire connection, a
power line, a neutral power line, and a
signal line. Most complex robots such
as humanoid bipedal robots or hex
walkers use as many as 18 or more
servos, and each servo must have the
two power lines and a signal line.
Builders found that they could
parallel the two power lines but were
still required to have 18 signal lines
running from their controller — one for
each servo. The Dynamixel does not
require this and interconnection is
daisy-chained as shown in Figure 3.
These servos are much more than
just a regular type in that each has a
built-in microcontroller with an
individually addressable ID. Plus, they
have the ability to send information
back to the controller as to speed,
torque, shaft position, temperature,
voltage, and load factors. There are 13
different Dynamixels in the Robotis
lineup that we’ll review here.
Other Servo Factors
Other factors to take under
consideration when selecting servos are
the motor driving circuitry and electrical
characteristics, as well as the
mechanical aspects of the overall servo
design. Earlier servos used what is
called a three-pole motor armature with
three distinct sets of wirings on three
laminated metal poles.
Some servos have gone to a five-pole armature for smoother rotation
and more torque. A few servo designs
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