Final Design
External and Internal Overview
Design Decisions Explained
1. Waterproofing: Wetmates and O-rings
Wetmate connectors are the norm when it comes to electrical connections that are exposed to marine environments for prolonged periods of time. Their high tolerance to pressure makes it the perfect candidate for use on this project. We chose a connector designed for Ethernet connections and power transmission. 8 contacts will be used for data transmission and the additional 5 contacts can be used for power for the lighting or additional instrumentation that may be added to the housing later.
O-rings are the norm when it comes to sealing an attachment or lid. The O-ring is compressed along the sides of the groove in order to provide a pressurized seal between the two rigid mountings to prevent water from entering.
2. Clear Imaging : Window and LED's
Since housing is to undergo repeated cycles of soil penetration, it is essential the window remain as clear and scratch resistant as possible. Sapphire was chosen as the sight window material for its resistance to scratches. Another advantage of sapphire is its transmittance as seen in figures below
Through experimentation we found that the best way to set the lights are at the smallest angle possible in relation to the window. In other words as horizontal as possible. Generally this position produces a lot of glare but for our application where we are working with minimal exposure times, it produces the best images. The camera is positioned in such a fashion in which it only views the sediment window. The LED light is a high intensity 200 lumen light that produces a 30 degree viewing angle so that the light does not optically interfere with itself.
3. Vibration Dampening: Flexures
The mounting for the camera makes use of spring steel flexures. The flexures keep the vibration of the housing from affecting the image from the camera too much. The dimensions of the flexures was determined by analyzing the equation for natural frequency. The length and width of the flexures were designed to give the camera system a specific moment of inertia that corresponds to the natural frequency of oscillation.
4. Forces on Housing : Buckling and Penetration Depth
The Smallest Load Value, that is the minimum load to buckle our housing under the least ideal conditions is F=2115kN. Since the Vibrocorer applies an approximate 22kN, it does not apply enough force to buckle the Housing. The plot below shows the Critical Load on Square Tube, vs. Length of the Tube, Legend corresponds to the inner diameter of the tube
The figure above is a picture of Terzaghi's bearing capacity theory. The housing would push the soil in section I which will push sections II which will then push sections III. When the weight of the soil equals the applied force, then the housing will no longer be able to go any deeper.
A large risk was that the Vision Core could not penetrate the sediments at the needed depth of 10 ft. Using the Terzaghi’s bearing capacity equation below, the Vision Core theoretically did not have any issues with penetrating sediments at the operating depth.
D=(Q_u-(1.3×C×N_c )-(0.4×γ×B×N_γ))/(γ×N_q )
Where,
D: Penetration Depth [m] , Qu: Pressure from driving force, weight of system and cross section area of penetration. [N/m^2 ], C: Cohesion of soil [N/m^2], Y : Unit weight of soil [N/m^3 ], B: Width of footing [m], Nc, Nq, Nr: Terzaghi’s bearing capacity factors depend on soil friction angle, φ.
The figure above shows how deep the housing (without a tip) can penetrate the soil for different Vibrocoring settings
Performance Videos