CAD
After the team agreed on the goals of the bot and the hardware needed to implement our strategy, the chassis was designed to hold all the mechanical and electrical subsystems. The basic structure of it was separated out into three platforms: a bottom platform that held the drivetrain and tape sensors, a top layer which had a servo attached to a dunking arm, and a middle layer that held all the circuits and microcontroller, in addition to contact switches. All parts were laser cut out for the ease, speed, and precision in this process.
The bot was designed to maximize it's use of the 12" x 12" x 12" initial requirements we were restricted to. Because the hoop was approximately 18" off the ground, we needed to consider both the initial height of the bot and the length of the arm. We selected the width of each platform to be 11 inches. This allowed us not only to accommodate for the large motors we used and the shaft couplers, bearings, and wheels they were attached to. This served to allow us to place a large arm on the top platform. The arm, from the pivot point of the servo, was about 9.5 inches. In addition, the chassis used standoffs between each platform to separate each platform. The bottom and middle platform were separated by 1 inch, and the middle and top were separated by 8 (?) inches. This placed the top of the servo (the highest point of the bot in the off position) at just shy of 12 inches from the ground. Overall, this gave us a 21.5 inch maximum height for our dunking bucket, enough to clear the basket.
The bot was designed to maximize it's use of the 12" x 12" x 12" initial requirements we were restricted to. Because the hoop was approximately 18" off the ground, we needed to consider both the initial height of the bot and the length of the arm. We selected the width of each platform to be 11 inches. This allowed us not only to accommodate for the large motors we used and the shaft couplers, bearings, and wheels they were attached to. This served to allow us to place a large arm on the top platform. The arm, from the pivot point of the servo, was about 9.5 inches. In addition, the chassis used standoffs between each platform to separate each platform. The bottom and middle platform were separated by 1 inch, and the middle and top were separated by 8 (?) inches. This placed the top of the servo (the highest point of the bot in the off position) at just shy of 12 inches from the ground. Overall, this gave us a 21.5 inch maximum height for our dunking bucket, enough to clear the basket.
Overview of Bot Chassis Bottom Platform with Standoffs and Mounting Plates
Bottom Platform: Tape Sensors and Motors
The bottom platform was designed to hold both the tape sensors and the motor assemblies. The areas not occupied by these devices also served to hold onto our batteries. To accommodate these devices, special plates were created to hold them. These plates had tabs that fit into corresponding holes in the bottom platform. For the tape sensors, these plates were secured using hot glue. Two of these plates were placed at the center of the bot. A vertical slot in this plate allowed for a bolt to be placed through the tape sensor and tape sensor plate; in addition this slot allowed us to adjust the height of the tape sensor. Corresponding holes in the bottom platform allowed the tape sensors to hang through the bottom of the plate at the edges of the tape. A third tape sensor mount can be seen at the front of the bot; this one is slightly different and was in fact a late modification to the original design. We faced problems making precise 90 degree turns on to the black tape, which was a requirement for our design. This third tape sensor at the front of the vehicle allowed for that to occur. It was attached to the same bolts that secured our front caster. Since we did not have exact dimensions of the caster when we ordered it, we created three sufficiently large slots in the same geometric shape required to secure the caster.
Each motor had two plates supporting it: a bearing plate that protected the motor and shaft from radial and axial loads and a 'face plate' that attached the motor in place. The motor was secured via screws that fit into the front end of the motor through the face plate. Since the motor produces torques and vibrations, we wanted to make sure the plates securing the drivetrain did not deform from these force and reduced the odds of vibrations impacting our performance. To control for this, two bolts on either side of the plate were placed through the bottom platform into these plates. These exerted a downward force on nuts placed in the wider section of the slots cut out for the bolts. The location of these plates were determined by determining a reasonable placement of the wheels and determining the clearance needed for a shaft coupler between them.
There turned out to be a slight problem with this setup. The bottom platform was rather crowded due to the size of the motors and the fact that we also placed our batteries here. We weren't planning on needing to adjust the tape sensors after placing them on and testing them, but they required being moved at some point. Due to their location and the fact that the tape sensor mounts were glued in, it was difficult to perform this adjustment - we essentially had to tear apart our bot and place it back together.
Middle Platform: Electronics and Contact Switches
The middle platform attached to the bottom platform using holes on the periphery of both platforms designed to allow standoffs to screw into them. Slots in the middle platform also connected onto the motor face and bearing mounts, providing additional stability to the motor assemblies. The main design challenge with this board turned out to be creating an array of holes which we could use to screw our perfboards into. We used what appeared to be a size that accommodated a variety of perfboards, using a grid with holes every half inch. Unfortunately, the L293 board had a different hole arrangement, making it's mounting awkward but not impossible. This revelation was used in the future to guide design for the top platform. Breadboards that were initially used for testing circuits were left due the time requirements for soldering and the risk of introducing an inadvertent error in this switch over.
After determining the dimensions on our contact switches, a fixed hole and three holes used for angle adjustment were placed to allow the testing of the contact switch in different mountings and ultimately secure one orientation. A slot was not used due to concerns that hitting the wall would end up loosening and shifting back the contact switch. To compensate for this lack of fine-tuning ability from mounting orientation, contact switches with long handles were used. The handles were then bent at a point where their performance was optimal. The contact switch mountings caused us to change the pattern of standoff holes on this and the top platform relative to the bottom; however this was easy to design around without needing to recreate the base.
Top Platform: Dunking Subsystem and Control Switches
The top platform was designed to primarily hold the dunking system, consisting of a servo, an arm, and a bottle for holding balls. Additional features were added on here, including a switch which would easily toggle both batteries into the on state simultaneously and fuse holders in order to prevent either circuit from drawing dangerous levels of current. Slots for potential perfboards and holes for mounting the Arduino on the underside of the platform were also included. The important factor in the design of this top platform was determining the width of the servo. This allowed us to place two slots the servo could mount to and adjust the position of the servo accordingly. It was also important to ensure that there was a slot cut into the platform; this cut allowed for the bottle used for holding the balls to be within the maximum size range of the bot. Each hole for the switch and fuse holder was designed to be slightly larger than their screw diameter, so that they fit well without falling through the platform.
The connecting arm for the dunking system had to be designed so that it did not collide with the top platform in it's zero position and so that it did not reach out past the maximum 12" range. To ensure this, the servo was placed as far forward on the bot as possible without sacrificing significant adjustment range. We also ensured that the servo and it's horn did not collide with the platform. After this, an arm was designed to be screwed into the servo horn. The arm length was determined from the center of the servo horn to its end, and the assumption was made that the bottle would not extend far past the end of the arm. Measuring the distance of the center of the servo horn to the front point of the robot, it was determined that 9.5" would be ensure a good reach while remaining within the 12" x 12" x 12" volume requirement.