Progress Report - Mechanical Design (02/12/2012)

Since my latest blog post, the AUV has continued to progress significantly. The compressed air tank, tank/high-pressure regulator, and secondary/low-pressure regulator were successfully installed their simple, elegant Delrin mount. The tank was filled to capacity (i.e. 3,000 psi) at a local scuba diving shop named Coral Reef, and successfully integrated with the network of nylon gas distribution tubes. Furthermore, the single-acting air cylinder for the grasp/release mechanism was successfully installed on its recently designed and manufactured mount. The simple, efficient angle-bracket mount was intelligently designed for its efficient use of material, relative ease of manufacture, and with dimensions that enable the jaws of the grasp/release mechanism to successfully pick up the rescue object (i.e. laurel wreath) without the bottom camera enclosure interfering. The single-acting air cylinder, as well as the torpedo launchers (with the redesigned (embedded neodymium magnets in place of the appended low-pull bar magnets), low friction, proper density, properly balanced torpedoes) were successfully tested in air.

An aluminum heat dissipation platform and the electronics rack were also successfully redesigned and manufactured. This redesign was prompted by the recent decision to use the Zotac mini-itx board in place of the Beagleboard-xM as the main control unit due to the need for greater processing power (particularly for the computer vision algorithms). The new heat dissipation platform was designed and manufactured to slide snugly into the hull end cap slots, and also to provide a wider region in the center (between the two lithium-ion batteries) on which to mount the new 6.7" x 6.7" Zotac PC. The revised electronics rack features one level which spans the majority of the interior hull, and essentially forms a bridge over the batteries and Zotac PC. This electronics rack not only features an efficient use of material and an aesthetic appeal, but also provides much easier access to the circuit boards than in the previous multi-level design. Furthermore, the redesigned electronics rack uses the same amount of material as the original design, and also is very cleanly welded at the joints (in place of cumbersome, interfering nuts, bolts, and angle brackets) to provide a strong, durable, elegant result.

The SEACON connectors also arrived, so I carefully drilled and tapped (using a mill tool plate attachment that operates in polar coordinates) each of the aluminum end caps to enable the installation of the female bulk head connectors. In addition, I cut the wires of each of the peripheral subsystems to about 3", and then measured and cut each of the corresponding male SEACON cables to length in order to ensure the reduction of excess cable (and thus weight). Each of the cables was then stripped to prepare them for soldering, heat shrink, and Plasti-Dip on Monday. The male cables for the SBT150 thrusters were also soldered to the leads of the thrusters inside the back end caps, and the cable was then potted using a waterproof epoxy. Plast-Dip will also be brushed over the external epoxy in order to provide a slightly cleaner look, as well as another layer of protection against water leakage.

The new acrylic camera enclosure boxes also arrived and were carefully mounted to their redesigned supporting platforms. I conducted exact measurements of these enclosures in order to ensure that their respective fixed end caps will fit firmly and securely, reducing the potential for leakage following the addition of the 100% silicone caulking around the four outer edges. The redesigned mounts for the cameras were also successfully installed (in the center of each of the enclosures (and the vehicle)) recently. I discovered a product called Duco Cement which was proved to be a fantastic find. It is a completely clear, very strong adhesive that dries within about 15 minutes. It has been used to adhere the camera enclosures to their respective aluminum base plates, as well as to adhere both of the camera enclosure mounts to the inside surface of the acrylic enclosures, and to adhere the polycarbonate pressure transducer supports to the center line of the top of the acrylic hull.

By the end of this upcoming week, I expect to have the redesigned fixed end caps for the camera enclosures manufactured and installed, all the male cables successfully connected to their respective peripheral subsystems (including the pressure transducer, which should arrive by Friday, but excluding the kill switch and the servo motor of the marker dropper (both will be ordered Monday)), the electronics rack successfully installed inside the hull with holes drilled in line with the circuit board mounting holes, the thrusters re-installed on the vehicle, the files required to make the solenoid valve PCBs obtained and submitted, the grasp/release mechanism jaws water cut, and the pre-amplifier circuit for the hydrophones tested on a breadboard. The kill switch (which will now be mounted to the frame on the back of the vehicle behind the compressed air tank), new servo motor, and new SEACON connectors (for the kill switch and to enable code to be uploaded to the main control unit without having to repeatedly open the end caps) will also be ordered on Monday, and should arrive the following week.

The vehicle is still on pace for mechanical and electrical completion by spring break (i.e. March 5th), but there is considerable work to do until that point in order to reach this goal. Currently my main concern is interference in the signal from the hydrophones, and the successful design and integration of the custom pre-amplifiers for the hydrophones. I will be drawing on my Mechatronics II knowledge, experience, and resources in order to make this a success as well. Also, once the vehicle is successfully completed and refined in preparation for underwater mission testing in March and April, I will direct my attention to helping Hang Zhang develop control algorithms for the thrusters using information from the IMU and pressure transducer as input sensory information.

Lastly, the vehicle was recently weighed, and is on pace for a projected 87 lb. Due to the 84 lb threshold, we might lose a few points (110 lb is the max weight in order to qualify to compete). This is not of major concern as this was both anticipated, and has a relatively small impact in reference to the entire mission. Furthermore, there is a potential that either (a) the weight limit will be increased slightly, or (b) this projected weight ends up being a slight overestimate.

I will provide another update on the status of the AUV in the coming weeks.

                      -Eric Sloan (ME Project Manager, 15th Annual AUVSI Robosub Competition)