Our resident expert on all things
robotic is merely an email away.
roboto@servomagazine.com
Tap into the sum of allhumanknowledge and get your questions answered here!
From software algorithms to material selection, Mr. Roboto strives to meet you
where you are — and what more would you expect from a complex service droid?
by
Dennis Clark
Welcome to 2011! It’s a new year with even more
amazing advances in robotics which — for some reason —
no one but us roboteers ever seem to know about! No
matter. I guess that means there are just that many more
people out there to amaze with our hobby! As I write this,
there is a lot of buzz about the Microsoft Xbox 360 add-on
“Kinect” sensor which was hacked within a week of its
release. It can be used to do 3D mapping of its
environment, as well as all the cool stuff that an Xbox 360
does with it. Neat! Another sophisticated sensor to add to
the toolbox! Keep watching those hacker blogs!
Q. I want to know that when my robot is falling down, all I need is an accelerometer, right? My robot is just a basic walker that uses servos to
move. Any help you can offer would be great!
— Kevin
A. Kevin, well, not really. It is true that an accelerometer will allow you to detect your robot ilting over. Unfortunately, as the name
“accelerometer” implies, the device measures acceleration.
It can’t distinguish between a change in attitude measured
by the change in the effects of gravity, in your robot, or the
moment-to-moment accelerations caused by your robot
simply moving. You will need one more device to help you
with your “tilt” sensor: a gyroscope, or “gyro” for short.
The accelerometer can measure a change in the effects
of gravity which can indicate a tilt, but to make sure that
you are tilting and the measurement isn’t just your robot
shambling, you can use the output of your gyro to confirm
the deduction. A gyro will measure a change in rotation
around a given axis. So, if you orient your gyro so that its
axis is perpendicular to the axis of the tilt you are interested
in (this tends to be the front-to-back vector), then when
your accelerometer sees a change that the gyro confirms
you can measure the two of them to determine how much
change there is and attempt an adequate amount of
14 SERVO 01.2011
Figure 1. Gyro breakout board.
correction to stabilize your walker.
Since I’ve been in a “code writing” kind of mood these
last few months, I’ve put together an example piece of
code that will read the angle reported by an RC aircraft
hobby gyro. I know that you can get gyros that read out in
just plain analog signals like the Pololu
LISY300AL (Figure 1) these days, but I
was convinced that
the RC gyros have
better stability and
drift less, so I
wanted to try them
out. Unlike the
Pololu gyro though,
the output of the
GWS PG-03 wasn’t
an analog voltage,
Figure 2. GWS PG-03 RC gyro.
