What is a RadBot? It was the name of my team's awesome final project in RPI's Introduction to Engineering Design class! I want to make it clear up front that this project was a collaboration between myself and a number of other students in my class group, not a personal project. While I would not usually document other people's work, I think that it would be a shame not to share some of the interesting parts of this project.
Specs:
Purpose: A robotic radiation detection and mapping platform
Chassis: Abrasive waterjet cut and TIG welded 6061 and 5052 aluminum alloys. Tab and slot fastening with a space frame design.
Drivetrain: 4WD Pololu 30:1 metal gearmotors
Suspension: 4 corner trailing arm system
Electronics: SaberTooth 2x12 RC motor controller, Arduino Nano with custom breakout board, Xbee transmitter and receiver, photoresistors, EM-406a GPS module, and HobbyKing radio control
Top speed: ~9 mph
Software: Processing program for wireless reception, Arduino language for on board sensing and transmission, Java parsing program, and Matlab for data analysis
For those who are unfamiliar with RPI's intro to engineering course (known as IED, but hopefully not in the explosive way), it is a semester long class tailored to teaching the engineering research, design, and manufacturing processes found in industry. For many students, it is the first "real world" engineering design work they participate in. It is commonly dreaded for its tendency to consume time, force students into dysfunctional groups, and inexplicably break working projects before final presentations. Fortunately, I would say that I went into the class better prepared than most due to the experience gained from personal projects and RPI Formula Hybrid.
I also had the advantage of getting a great group to work with. Together we decided upon an ambitious final project. This was partly to stubbornly ignore the common knowledge rule of IED (keep everything simple, and never rely on complicated electronics) and prove that it is possible to do a more technically challenging project. Additionally, we just found the concept very cool.
The concept behind our project was to create an inexpensive, yet rugged remotely operated vehicle that could enter environments with dangerous levels of ionizing radiation and map out the intensity of those localized readings. By using a GPS and wireless transmitters, the intensity across an entire landscape could be plotted. While the idea was conceived to assist in the rescue efforts surrounding reactor meltdowns or natural disasters, the concept could easily be applied to any contaminant or toxin. A lengthy paper was produced going into much more of the business justification and details behind this, but I won't elaborate on it more here. What this meant was that we needed a robust prototype RC vehicle that could operate in adverse environments and produce reliable data.
I contributed to multiple areas of the project, but my first task was chassis design. As seen above, the chassis made use of the abrasive waterjet cutting process to "jigsaw puzzle" the vehicle frame together. In the spirit of racecar design, I loosely emulated a space frame design when considering the load paths moving through the structure. Essentially, the goal was to ensure that every structural member was loaded in pure tension or compression, creating a network of 2 force members intersecting in nodes. This ensured that no material was loaded in bending, which would have created a potential failure point.
Once the chassis was in place, the suspension could be installed. This consisted of relatively simple trailing arms with motor mounts direct driving the wheels. Lever arms projected outward from the suspension axles, to which compression springs were mounted. Despite its lack of damping, the suspension worked surprisingly well for such a rudimentary and low cost setup.
Electronics were installed in the fore and aft compartments, with the NiMH battery pack in the center. If you are wondering what the "tower" at the front is, it is intended to be a remote camera mount. It also gives more room to mount a Geiger counter.
Once the electronics were installed and tested, RadBot got taken outside to prove its capabilities.
But roaming around the RPI '86 field was only half of RadBot's job. The other half was Science! As described above, RadBot was initially designed to collect ionizing radiation readings. While the Geiger counter model we had found would have been easy to integrate into the robot, we made a last minute switch to reduce the cost of the project. So RadBot ended up using a slightly less exciting photo-resistor as its "radiation" sensor. Still sensing radiation, only now the less deadly kind (if you remember to wear sunscreen). In order to test the system, we stayed up until 3am testing in the dead of night to get readings from streetlights so the sun wasn't a variable we had to worry about. After a few days of tweaking, the system worked great:
Note the red areas on the map above. Those were all light poles on the field. The squiggling path visible in this image was actually intentional, so as to cover as much area near the lights as we could. GPS accuracy was actually surprisingly good, typically coming within 5 feet of the robot's true position.
RadBot did struggle with a few problems. First and foremost, we made the rookie mistake of having nearly equal track and wheelbase lengths between the wheels. This meant that the skid steering system we intended to use was initially highly ineffective, as all the motor torque went into scrubbing the wheels. Shortening the rear suspension trailing arm allowed us to reduce the wheelbase, effectively solving the issue. The Pololu motors were also troublesome. They typically worked great for the first day or two of driving, then developed difficulties starting. Once spun, they could get up to speed, but never had the same torque that they started out with. My theory is that because we had the two motors wired per channel of the motor controller, the stalling of one could have damaged the other. I may test this in the future to get to the bottom of the problem. In all likelihood, we were just using these motors in a much harsher application than they were ever intended for. While the gearboxes held up excellently, opening the failed motors revealed a coating of carbon dust on the inside of the casing.
Overall, RadBot was a great example of an ambitious project that succeeded in the face of limited time and resources. In addition, my team kindly allowed me to keep the robot at the end of the semester. Maybe RadBot will get a revisit sometime in the future....
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