The Robot Base
While we could have
made everything from scratch,
we decided to base this robot
on the Parallax Stingray robot
chassis. I had purchased a
used Stingray (in pieces) from
a friend and had just finished
putting it back together. That
saved a lot of time creating a
motorized base and enabled
us to focus on other aspects
of the robot. The Stingray kit
was introduced in October
2009 and was available for
several years until it was
discontinued in 2013. If you
don’t have or can’t find a
Stingray, you can build a
similar platform. A new
version called the Manta Ray
was just covered in the
June/July issues of SERVO
which could be modified as
the platform for this robot.
So, we started out with the
original Stingray kit, the PING))) kit of
three sensors, and the infrared line
follower kit. The controller on the
Stingray is a Parallax MSR1 controller
(based on the Propeller processor)
which uses an L6205 dual H-bridge
driver for controlling the two main
drive motors. It also provides dual
5V), a USB interface for
programming, and voltage translators
for the I/O.
The TXB0108 translator chips
convert the 3.3V I/O on the Propeller
for 5V devices. Those translator chips
seem to work fine for this
application, but I usually prefer to
handle the I/O interfacing myself.
The base controller handles all the
motion of the robot and controls the
two main drive motors. The original
motors on the Stingray run
fast ( 7.2V, 310 RPM) and
don’t have a lot of torque.
Although they could move
the lightweight frame of the
Stingray okay, once you start
adding a lot of options those
motors are underpowered.
We upgraded them to new
6V-15V gearmotors with
encoder.
These motors are much
more powerful and run at a
slower speed that is more
appropriate for this robot.
With the larger motors, an
extra heatsink was installed
on the L6205 H-bridge to
ensure it stays cool. Another
possible motor option is to
use some of the Pololu 37D
motors w/encoder. Those are
also bolt-on replacements for
the original Stingray motors.
Instead of altering the
wiring on the new motors, I
made a small adapter board that
would allow the original connectors
to be used. It handles the wiring for
the motor power and the quadrature
encoder signals. The addition of
quadrature encoders on each of the
motors will help with navigation code
later on. The board also has room for
any extra circuits that I may want to
interface with the base controller.
The power source started out as
Building the
MD-1 Robot
SERVO 08.2014 59
Initial Stringray base before upgrades.