Progress Report - Mechanical/Electrical Design (03/03/2012)

        The grasp/release mechanism, waterproofing of the marker dropper servo motor, implementation of a waterproof mission/kill switch, solenoid valve interface PCB, installation of several circuit boards in the electronics rack, and initiation of the replacement of our pressure transducer were successfully completed since my latest update.

The grasp/release mechanism was designed and mechanically simulated on Pro/Engineer - Mechansim. The design required the purchase of a shorter air cylinder with only 0.5" of actuated displacement (compared to the 2" displacement of the original single-acting air cylinder) in order to allow for the proper mechanical motion, grasping range, and size of the device. The device is essentially a mirrored 3-bar slider-crank mechanism, which uses press-fit bushings, as well as precision cut stainless steel dowel pins and e-clips in order to provide the necessary pin joints and motion constraints.

The servo motor for the marker dropper was waterproofed following extensive research and experiments on the two burned-out motors (one of which was inherited). Ultimately, a combination of Molykote silicone grease, high viscosity mineral oil, an o-ring, and DUCO cement were used, Silicone grease was liberally, yet cleanly added inside the gear shaft chamber where the output shaft exits the chamber, and where the input shafts enter the chamber. This chamber was then closed, and the back plate was removed to expose the electronics. I then poured the mineral oil into a container and rested the container at an angle in order to allow the oil ot accumulate toward the bottom, providing the necessary isolated depth of oil required for the next step. At this point, I completely submerged the servo motor (again, with the back plate removed and electronics exposed) in the mineral oil, and after allowing all the bubbles to escape, tightly closed up the back plate without ever allowing any part of the servo motor to surface. Once the servo motor had been successfully reassembled securely while submerged, I removed the motor from the mineral oil, carefully cleaned of the surfaces with soap and water, followed by some alcohol, and then applied DUCO cement to where the front plate connects to the body plate, and where the back plate connects to the body plate. This was to ensure no progressive leakage of water into the gear train chamber, or leakage of mineral oil out of the electronics chamber.Lastly, an o-ring was tightly pressed around the exposed tip of the output shaft, and the servo horn was torqued down tightly over this o-ring. The silicone grease around the o-ring was cleanly redistributed to form a clean, symmetrical barrier, and the electrical leads on the back of the servo motor were sealed at the base (i.e. where they exit the electronics chamber) using Plasti-Dip; this completed the process.

I completed the design of the solenoid valve PCB using DipTrace, making sure that no connection lines in the circuit intersected (thus required a multi-layer construction, of which the in-house milling facility was not capable) and submitted the Gerber files for in-house milling with specific requests to remove unnecessary copper on the top of the board, as well as around the holes on the bottom of the board. The result was a clean, compact, organized PCB to which I thereafter successfully soldered the components (i.e. resistors, NPN transistors, and wires that route to the leads from the SEACON connectors that route to the solenoid valves, ground, the output of the 9V regulator, and the corresponding I/O pins on the Arduino Uno board nearest this PCB on the electronics rack. Simple actuation of any of the four I/O (designated as output) pins opens the corresponding solenoid valve, and provided the air cylinder is set to output 100 psi, will launch one of the two torpedoes, close the grasp/release mechanism jaws, or open the grasp/release mechanism jaws (once closed).

I also assembled all the circuit boards (i.e. Zotac mini-itx, both Arduino boards, Phidget Spatial IMU, solenoid valve PCB, and the L298 Dual H-Bridge motor drivers) and organized them on the electronics rack so as to minimize unused space, provide sufficient room between boards to allow room for protruding wires, and ensure that the IMU would be located directly at the center/heart of the AUV. Furthermore, the goal was to designate as much space as possible for the final circuit board-- the amplification/filtering circuit for the hydrophones. Once this layout was established, I made final dimensions of the boards and hole locations, and recreated this layout on Pro/Engineer. Using the bottom left hole location as a local origin, and creating corresponding x- and y- axis through this point, I was able to chart the relative coordinates of all the other mounting hole locations. These coordinates were placed in Excel, and after securing the electronics rack on the mill and zeroing the drill to this local origin, I simply had to move the tooling plate to each of these coordinates and drill (the clearance holes were all the same). This provided an efficient, effective way to properly prepare the circuit boards for installation (using standoffs and screws).

I realized about a week ago the need for either an absolute or sealed-gauge pressure transducer since the reference vent cable was not anticipated and wouldn't have been able to be accomodated, so following a few phone calls and urgent emails, I was able to return ours and the company was willing to replace it with a sealed-gauge version. In addition, the company (Impress-Sensors Technology Ltd) was willing to send the version that directly output 0-5VDC, and which also had a marine bronze housing (even more corrosion resistant than the 316 stainless steel housing)--all for no additional cost. Sealed gauge was chosen over absolute because with absolute, the device outputs 0V in a vacuum. Thus, at atmospheric pressure, the device would have already output 2.5V, thereby significantly reducing the accuracy of the device for our application. The sealed gauge transducer, however, will output 0V at atmospheric pressure, and 5V at around 30 feet, making it almost ideal for our application. The sealed gauge uses 1atm as a reference, though, so if the atmospheric pressure at the testing site or competition site is slightly different, there would be very slight error introduced. This offset error, however, can be accommodated for by running a calibration test at each of these locations.

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