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Background
Background
The oceans comprise over seventy percent of Earth’s surface and consequently play a critical role in our climate system. As funding for conventional research tightens, organizations such as the National Science Foundation seek cost-effective methods to understand environments that play a large role in our global climate. The Autonomous Underwater Explorer (AUE) is sponsored by the Jaffe Laboratory for Underwater Imaging located at Scripps Institution of Oceanography. The collaboration with MAE was sought to improve the design of an existing linear actuator that controls the buoyancy of the AUE through changes in volume. In the original system, distressing mechanical drive components compromised the critical time at depth to take measurements from the hydrophone.
The oceans comprise over seventy percent of Earth’s surface and consequently play a critical role in our climate system. As funding for conventional research tightens, organizations such as the National Science Foundation seek cost-effective methods to understand environments that play a large role in our global climate. The Autonomous Underwater Explorer (AUE) is sponsored by the Jaffe Laboratory for Underwater Imaging located at Scripps Institution of Oceanography. The collaboration with MAE was sought to improve the design of an existing linear actuator that controls the buoyancy of the AUE through changes in volume. In the original system, distressing mechanical drive components compromised the critical time at depth to take measurements from the hydrophone.
Objectives / Requirements
The objectives of this system all aim to increase the amount of time the hydrophone is able to record while increasing the reliability of the system. Specifically:
1. Decrease the noise of the system heard by the hydrophone by a factor of two
2. Increase the reliability and consistency of system
3. All changes must fit within existing volume constraints
4. The energy budget must permit normal field operations
Original System
Original System
As shown in Figure 1, the original system's components in question were as follows:
1. Two inline Pololu motors
2. A worm gear connection
3. A lead screw
Figure 1: Original AUE Prototype
Final Design
Final Design
Figure 2 shows the finalized design of the AUE. The original two Pololu motors were replaced with a flat geared motor from Micromo. In order to meet the volume constraints of the prototype, a battery was removed from the pack. Although this new motor requires more time to fully extend the piston, it has significantly reduced noise. The worm gear system was replaced with a simpler and quieter design of two inline spur gears. Additionally, the lead screw was replaced with a much more efficient ball screw, decreasing energy lost to friction.
Figure 2: Finalized AUE Design with Flat Geared Motor
Process
We took two major approaches to our design solution:
1. Improve the speed and efficiency of the piston system through new motors, gears, and linear actuator, effectively making it quieter by changing the motor.
2. Reduce the noise actually generated by the piston system and picked up by the hydrophone through the use of damping.
Speed Improvement
Several motors were considered and tested as replacements for the old motors and worm gear. The gear ratios were selected based on the torque needed to turn the ball screw at the maximum depth of 100m, the torque available from the motor, and the volume constraints of the prototype.
Initial Noise Reductions
The original AUE had only metal-on-metal connections, and thus the motors and gears were rigidly attached to the hydrophone mounted on the exterior of the unit. This caused significant noise to be picked up through the mechanical vibrations the frame holding the hydrophone experienced. At the suggestion of our adviser Dr. Tustaniwskyj, we sought to physically isolate the motors and gears from the frame holding the hydrophone.
Several options were used to pursue this:.
vibration damping grommets were placed in the frame
damping mounts were used to hold the motors
gear system changed from worm gear to two spur gears
Testing
Multiple rounds of testing were completed to quantitatively deduct the ultimate solution. First, three different types of vibration damping mounts were attached to a metal frame with a hydrophone and motor to determine which mount damped the most mechanical vibrations. Using audio recording and MATLAB analysis we analyzed our first test data. Then, all three prototype motors were attached to the AUE's internal frame to test audio and the time required for full extension. Using the lever system show in Figure 3, we applied various loads to recreate the pressure inherent at up to 100 meters of depth.
Figure 3: Testing Apparatus to Simulate Hydrostatic Pressure
The following video is a demonstration of the prototype's capabilities. It demonstrates the AUE's ability to extend the piston with a load applied.
Conclusion
Once the team decided on the flat geared motor for the final design, the motor was tested with both the lead screw and ball screw to determine the optimal design based on efficiency and noise. Due to deterioration of the sandwich mounts and grommets after a short period of testing, it was decided to remove these items and allow metal to metal contact. Noise was significantly reduced from simply using the flat geared motor and the ball screw. Finally, cycle testing was performed to ensure reliability. After numerous tests, the AUE proved to be reliable. The only task remaining is to test the device for true reliability to ensure the prototype has waterproof integrity.
UCSD Scripps Floats
UCSD Scripps Floats
Spring 2013 MAE156B Sponsored Project
Spring 2013 MAE156B Sponsored Project
University of California, San Diego
University of California, San Diego
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