16 SERVO 01.2014
I have a few interesting questions
this month to answer, and some new
boards to ponder the usefulness of ...
Q. I am trying to decide between analog and digital servos for my robot arm
project. Is the extra cost of the digital
servos worth it?
— Tony
A. To some extent, this answer will involve opinion. Since you are talking “analog vs. digital”
servos, I assume you mean hobby
servos like those used in RC planes
and cars. My answer, therefore, is “it
depends.” The digital servos respond
faster, hold better, and are more
precise than analog servos. So, if
precision and speed as well as
constant torque (regardless of the
setting) matter, then digital servos are
what you want to use.
If this is a garage project for a
dynamic art exhibit, I’d go for the
cheaper analog servos since accuracy
and repeatability are typically not
what is required.
Another consideration is power. If
you have lots of power reserves, the
digitals are usable. If you have power
limitations, then you should consider
the analog servos. Digital servos
typically are more power hungry than
their analog brethren.
If you really want a good robot
arm, you should consider the digital
smart Dynamixel or Herkulex
networkable servos. These servos will
allow you to get positional feedback
as well as current, load, and
temperature. They can be addressed
individually and talk back to their
controller.
Q. My quadcopter goes through lithium-ion batteries like crazy, and my charger
can’t keep up. What should I look for
in a fast lithium-ion charger that is
affordable, say, less than $100?
— Marcus
A. Again, this question relies omewhat upon opinion. I have used a few really cheap
chargers (ones that are under US $40)
and they have either let me down or
left me “underwhelmed.” There are
two multi-chargers that I have been
very pleased with, however: the
Duratrax Intellipeak ICE and the Hitec
X4 DC multicharger.
The Hitec X4 is well above your
$100 limit, but the Hitec AC/DC X1
charger costs about $62US at a
number of shops. I trust that charger.
The Duratrax ICE DC charger is under
$90US.
Both of these chargers will handle
just about every battery type there is,
and will charge, discharge, and
discharge/charge cycle. (The latter of
which is used to find out how robust
your battery is.)
by Dennis Clark
Our resident expert on all things robotic is merely an email away.
roboto@servomagazine.com
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january2014_MrRoboto.
Ask Mr. Roboto
As I write this article, the 2013 FIRST LEGO League (FLL) season is winding down for my team. The kids did well and scored in the Accomplished and Exemplary parts of
the rubrics, as well as landing in the upper half of the
robot scores at the competition tables. They suffered many setbacks
and confusing moments when their robot stubbornly refused to do
what it had been doing on our table back at the school. In this,
they experienced what we robot hobbyists — and to some extent,
robo-embedded engineers — experience all the time. That is, they
discovered that sometimes our assumptions about our robot's
environment are incorrect and we need to account for even more
variables than we thought! It is a sobering discovery which always
leads to a more robust design. The team got lucky on their last
competition run when, magically, the robot started doing what they
expected! Then, on the last task they ran, Dragon Bytes decided to
solve the puzzle its own way and scored better than the kids had
designed. There were many howls of laughter, knuckle-bumps, and
high fives at that table! Call it an apocryphal tale if you wish, but
sometimes you just get lucky!
