Mechanical Overview

Our mechanical objective was to provide a platform that facilitated stereo beacon detecting, tape sensing, encoder reading, drive motor velocities between 50 and 100% duty cycle, ball detection, sorting, dispensing for both goals 3 and 2, and impact sensing for goal 3 and the ball dispenser.

Stereo Beacon Detecting We found that beacon following was optimal when we used stereo beacon detecting. This consisted of two IR detectors side by side separated by a vertical panel (see Fig 1). We would drive forward when both detectors detected a particular beacon. If only one saw the beacon, the robot would turn in the corresponding direction until both detectors detected the beacon before driving forward. These were placed at an elevation of precisely 10”. There were two stereo beacon detecting sets. The first sits centered about the goal 3 ball dispenser. The second sits centered about the goal 2 ball dispenser. This way the robot can align each dispenser accurately with the corresponding goal’s beacon. Tape Sensing Tape sensors were placed as far forward as possible to maximize stability (see Figure 2). They were also aligned with the top ball hopper as we used the tape sensors to align with the ball dispenser. Two tape sensors were placed in the center for tape following. The outermost tape sensors were there for tape crossing detection. Goal 3 Dispenser Goal 3 dispenser was a rectangular hopper with a release bar (see Figure 2). The objective was to set a strip of balls into goal 3 to maximize ball distribution density. A door lock motor pulled the bar to prevent balls from falling out and pushed it to release them. Transmission The transmission consisted of a belt drive (see Figure 2). The maxon motors drove a drive pulley through a spider coupling which in turn drove the wheels through the belt. The transmission choice was selected to allow flexibility in the design should a different mechanical advantage be desired in the future. The specific mechanical advantage we chose enabled us to operate at a drive pulse of 50-100% duty cycle for almost all tasks. Encoder Reading We found early on that our performance was adversely affected by varying battery voltages. To reduce our sensitivity to batter voltages we implemented encoders on each of our drive wheels. We cut 180 slots in MDF and used a coin sensor to detect the slots. These 1mm wide slots were large enough to get a good signal from the coin sensor. Ball Detection and Sorting The balls would fall from the dispenser onto a sloped ramp which would take the balls into the corner of the robot to the sorter (see figure 4). The sorter consisted of a sloped circular surface with a slot cut in it and mounted to a motor. An IR emitter and detector sit above the sorter and another detector sit below the sorter. The detector below the sorter detects when a ball is present. The detector above the emitter then detects of the ball is black or yellow. Turning the sorter pushes the ball into either the goal 2 or goal 3 ramp to await ball release. Goal 2 Dispenser Getting balls into goal 2 required that we shoot the balls (see Figure 5). Another door lock mechanism was used to release the balls from the goal 2 ramp onto the shooter ramp. The intent of the shooter ramp was to provide the balls with a velocity vector whose direction was exactly forward and horizontal. To do this, the ramp had to rails to prevent the ball from rolling side to side and the final trajectory of the ramp was horizontal. The accuracy of the shooter was found to be high in our experiments. Goal 3 Bump Sensor The bump sensor consisted of a single momentary switch with a large metal place across the front of it (see Figure 6). This way, force contact with any part of the metal surface would trigger the sensor. Ball Dispenser Bump Sensor The ball dispenser sensor was a homemade contact sensor sitting on the end of 2” of foam (see Figure 6). The objective was to reduce the severity of impact with the ball dispenser. When the copper bar impacted the aluminum plate, a signal was pulled high and impact detected. The sensor was wide to ensure contact even in instances of poor alignment with the dispenser. Ball Dispenser Bump Sensor Goal 3 Bump Sensor