Progress Report - Mechanical/Electrical Design (02/22/2012)

Since my most recent update, I have continued sedulous work on the AUV. I properly and securely installed all of the SEACON connectors to both the hull and camera enclosure end caps (adding silicone grease to each of the o-rings as well as the male plug connectors, and applying Loctite thread sealant to the middle threads of the female socket connectors), completed the construction of the camera enclosures (and successfully installed the forward facing camera in the top enclosure), designed and manufactured the grasp/release mechanism, waterproofed the "waterproof" (hobby servos like the one carried over from last year's team are not rated for sustained submersion of up to 16 ft--not even close) servo motor (Traxxas 2056), designed, manufactured, and installed the kill switch mount and kill switch (features a military grade rubber boot (IP86* rating--sustained submersion)), installed the newly arrived pressure transducer, developed a design for the amplification/filtering PCB for the hydrophones, and designed and manufactured the solenoid valve interface PCB using  a program called Dip Trace which I downloaded and then learned via a tutorial and introductory documentation in a day.

The grasp release mechanism design consists essentially of mirrored 3-bar (or 4-bar in a liberally general sense) slider crank mechanism. I briefly consulted Dr. Hollis (Mechanical Systems II and Modeling and Simulation Professor) in regards to a couple of the design details (e.g. the use of bushings over bearings, lubrication suggestions, etc.) as well as an approach to deriving the motion profile of the proposed linkage system. I ended up reviewing my Mechanical Systems I text book (I keep all my engineering and advanced mathematics books) and found a set of equations that could be directly applied to this model. It was during these calculations when I realized that the current design wouldn't output the intended range of motion of the jaws, and in fact would have induced significant stress build-up due to not physically being able to move in such a way in order to allow the single-acting air cylinder to translate the full 2" during actuation. I then set up a sketch on Pro/E, allowing me to make immediate design modifications by providing an efficient means of adjusting various design parameters (link lengths (i.e. distance between pivots) and claw shapes). Ultimately, I concluded that it was best to get a new ($18) single-acting air cylinder that has 0.5" of travel (it looks the same as the other single-acting air cylinder, but is about 2" shorter as well). This modification, along with the redesigned jaws provided the desired motion of the grasp/release mechanism, and without hindering the ability of the single-acting air cylinder to extend to its full capacity. The design features 8.5" of grasping range   (closing to a very slight out-of-plane overlap), a custom piston adapter, and a fixed pivot (achieved via minor additions to the frame, as well as another custom adapter piece). Furthermore, it uses bushings, 1/4" - stainless steel dowel pins, e-clips, and shaft shims.

I successfully tested the newly acquired Traxxas 2056 servo motor a few days ago as well, serving as final confirming evidence that the other two servo motors were, in fact, burned out as suspected. After extensive research on servo motor waterproofing methods, materials were ordered, and test were performed on these dead servos. The conclusion was to apply silicone grease to the output shaft of the gear train, submerge and assemble the servo in a high-viscosity mineral oil bath, ensure the screws were tightly fastened (the screws already had o-rings on them), clean the shell using alcohol, apply Duco Cement along the two interfaces of the servo shell (as insurance), add an lubricated (silicone grease) o-ring around the output shaft (externally), and then tightly screw the servo horn onto the output shaft. Finally, Plasti-Dip was used where the wires protrude from the back of the servo motor in order to ensure a definitive water tight seal.

The kill switch issue was the other waterproof task that was on my mind. The original source provided an astronomical quote that even earned complaints from their most recent customers--the Bellagio and the Navy. It was decided that it was far more economical (and necessary) to attempt to custom waterproof the toggle switch that was inherited from last year's team. Rough ideas were thrown around, including the implementation of a balloon around the toggle switch to prevent water while not restricting actuation. The general concept was decent, but I wasn't too fond of the sophomoric technique that was being implied. Thus, I conducted more extensive research and discovered an established company that manufactures silicone boots that are directly compatible with a vast array of toggle, push button, etc. switches. An appropriate boot for the toggle switch was obtained and this, combined with the addition of Plasti-Dip to the leads of the switch, should provide a reliable water tight seal. Furthermore, it has been decided to use this comfortable-actuation switch not only as our kill switch, but also as our mission start switch--preventing the need for an additional switch, and thus additional threaded hole in the rear end cap for the complementary set of SEACON connectors. The switch is conveniently mounted on the top of the back face of the AUV--directly behind the compressed air tank, and out of the way of thrusters or any mechanical subsystem.

I also discovered, installed, and briefly learned a PCB design program called Dip Trace. I was able to derive the solenoid valve interface PCB (containing four identical circuits in a compact arrangement) which has dimensions of 3.35" x 1.45", and also has built-in mounting holes. The transistors, resistors, solenoid valve wires, and protection/snubber/flyback diodes will simply need to be soldered to this board. Lastly, I sat down with Antony on Friday and confirmed my initial determination that we in fact need 25 dB of amplification in the pre-amplifier for the hydrophones. I have derived circuit design with the aid of National Semiconductor's Webench, which will consist of of four identical circuits containing a third-order active band-pass filter connected to two gain stages (i.e. two-stage amplifier using op-amps). The active filter should sufficiently filter out noise from the delicate hydrophones, and the amplifier should provide the necessary gain in signal so that as the AUV enters into its pinger detection state (following completion of the Kill Caesar mission), the signal amplitude should be about 0.4 V, and as the AUV approaches about 1.5 m, the signal strength should be about 4.5 V. Since these signals will be sent to A/D converters for eventual digital signal processing, it was critical to design the circuit so that the signal would not saturate the A/D converters within the desired range from the target pinger. I will construct this complex and robust circuit using Dip Trace as well. However, due to the severe magnitude of this particular PCB, I will likely end up sending the file off to a company (possibly National Semiconductor) to construct the custom PCB using more advanced milling equipment than is currently available at the FAMU - FSU College of Engineering.

*We are also currently pursuing further funding from the college, and also are in the process of submitting/have submitted the competition entry fee, and requested the lending of a transponder (pinger) from the competition staff (they have items that teams can borrow for up to four weeks, including IMUs, PCs, pingers, hydrophones, etc.).

I will update my blog again at the start of spring break. Until then, I will be continuing the progress of the AUV until the task is done, we go to competition, and we win the competition.

      -Eric Sloan (Mechanical Engineering Project Manager - 15th Annual AUVSI Robosub Competition)