Servo - August 2014

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