Scavenger was constructed in this order
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Scavenger back then |
The newer structures were largely built over the old ones, giving a relatively tall robot. The main body was also heavily braced, resulting in a relatively heavy robot. While the robot required a lot of motor torque for propulsion, it was stable at high speeds due to its weight and required less space for rotation since the parts were packed upwards. In hindsight, we could have spent more time planning the chassis before building it, given that we had a lot of trouble fitting the encoder wheel (freewheel) into the limited space between the two gearboxes. The placement of the caster wheel was also highly dependent on whether putting it at the front or at the back would provide better balance for the robot, something which probably could have been avoided with a better design.
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Scavenger runs using gearboxes with a 125:1 gear ratio and a tank drive mechanism. It has two wheels on each side (sharing the same axle) and is powered by one motor on each side. The high gear ratio is enough to provide a large torque to power the heavy robot. |
The caster wheel of Scavenger is located in the back of the chassis. (Though since Scavenger drove backwards most of the time, technically speaking, the back is the front.) It went through many modifications after various problems occured in testing - the tire alone was not enough to hold the weight of the robot, and a weak joint between axles had it entirely pop off in one occasion. |
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The wheels used to spin the gearboxes are linked to a motor using a 25:1 gear ratio. The gear ratio provides a balance between speed and sufficient torque to overcome the resistance in the gearbox due to the meshing of gears. Using tires rather than gears also proved to be a good idea, as it allowed more space for error and was less likely to get stuck.
A servo motor was oriented such that it rotates in the plane of the lever, and a contraption comprising primarily axles was attached to the servo motor to go behind the lever and drag it backwards towards the motor. The contraption was aligned such that it was high enough to avoid the lever as the robot moved in and was able to rotate low enough to activate the breakbeam sensor in the lever. As the contraption pulls the lever multiple times, we configured the contraption so that it does not rotate up to its original height between balls so as to minimize the time taken to collect the balls. This required careful testing as we needed a sufficient angle range to activate the breakbeam sensor and also ensure the servo motor moves at a reasonable speed.
Balls were fed onto a platform that was angled. Collection of balls was thus primarily driven by gravitational forces. The balls were held in using a "fence" that would only open when the robot approached the dropzone. The platform was initially able to accommodate only 15 balls, but it was later expanded to take about 25 balls. As the expanded platform was no longer rectangular in shape, there were areas where the balls could remain caught in, and the robot was programmed to jiggle at the dropzone to shake the balls out.
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Tires to protect Scavenger from crashes. These were added after Scavenger got severely injured during testing. |
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Free wheel to measure distance. We originally implemented a very quick reaction timeout for driving straight using the encoder(300 milliseconds as opposed to 4 seconds), but it somehow failed on the day of impound. |