Servo - 201306

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ROS laptop over Wi-Fi. The PS3-eye camera and Primesense RGBD camera were cabled to the laptop.

We chose to use ROS mainly because of the vision processing packages available, but it brought additional benefits such as easy communication between different software packages, sophisticated logging, and the ability to replay video from a run to debug software.

Mechanical and Electrical Design

We were inspired by the Spirit rover which (at the time) was about to land on Mars. As a showpiece at DPRG outreach events, it would be instantly recognizable to children and adults alike. We also thought the rover mechanical design would provide great handling on rough terrain. It used six-wheel drive to maximize drive power from small light motors. All six wheels were steered which enabled Ackerman-style cornering, as well as crabbing. To steer, we turned the front and back wheels in opposite directions, which afforded a near zero radius turning ability. We built several iterations of the robot as we found and addressed various challenges before arriving at the final design.

The first challenge was the suspension. So that the Mars rover can climb over rocks as big as twice its wheels’ diameter — all the while keeping six wheels on the ground — its rocker-bogies have stub axles for each wheel and an independent suspension that drives a self-leveling platform. We were unsuccessful in our attempt to duplicate that aspect of the design; the weight of the laptop on the platform stripped the plastic gears that were supposed to keep the platform level. We ended up tying both front wheels and the platform together into a single rigid assembly which resulted in sometimes only having five wheels on the ground.

Another challenge was the weight of the laptop which caused flexing in the lower chassis structures. That problem was compounded by the wheels which had straight sides and

FIGURE 2. Paul and Jason debate jRover chassis design.

didn’t slip sideways on grass or carpet (as they must to avoid deforming the chassis). Also, they were steered with motors which had considerable backlash compared to servos. We had decided to use motors for steering because they’re stronger, but the backlash meant the wheels were never perfectly aligned.

Adding to that problem, the integrated motor encoders made backlash-induced misalignment undetectable. Also, the wheels were mounted on beams extending from the bogie pivot point, and the beams would twist. The result of all this was the wheels would run in or out from their intended track, twisting the beams as they did. We never solved these problems satisfactorily.

Motor speed and steering motor position were determined from encoders integrated into the VEX motors. Determining “straight” for the wheel-steering assembly was a challenge since there was no indication of absolute position. We installed a mechanical stop on the steering head, and at startup, we drove the steering head against the stop, then rotated the head back a fixed amount that was calculated to center it.