Subsystems DesignThe Subsystem Design phase consisted of identifying and proving feasibility in the critical subsystems. The analyses and their results are shown below.
Critical Subsystem Determination
Subsystems were determined to be critical for the following reasons
- Highest Technical Risk
- Most Challenging Technically
- Most Important Engineering Requirements
- Most Important System Level Behavior
The systems that met one or more of these criteria were:
- Propulsion system
- Fluid system
- Muscle system
- Control system
- Power system
- Honorable mention: Orientation control system
Power Consumption Analysis
Battery selection was done by determining what types of batteries had high power density (were lightweight), selecting the best options of each type, and choosing the best one based on cost, weight, and power.
Lithium Polymer (LiPo) batteries were found to be the best option
- High energy to weight ratio.
- Relatively inexpensive.
- Does require the use of a “smart charger” for safe charging.
- Presents additional risk item: Battery catching on fire, happens during charging if done incorrectly.
- Action to mitigate risk: Design fish such that battery can be removed for charging.
The chosen battery has a capacity of 4 Amp-Hours. This translates to an expected battery life of 1.17 hours at full operation, far exceeding the customer requirement of 15 minutes.
Kinematic Analysis Using Matlab
Kinematic Analysis is performed using Matlab. The primary goal of this analysis is to simulate fish forward and turning motion by tuning control parameters found in literature. We hope to use these parameters to aid our force analysis and CAD design.
Torque due to Drag Force CalculationsIn order to actuate the fins, the muscles must overcome the pressure drag from the water in front of it. This pressure drag is dependent upon the velocity the fins move through the water, both from forward motion and from side-to-side oscillation. The approach is to integrate the torque due to the drag force along the length of the fin, using the components of velocity that are perpendicular to the fin surface.
Though skin drag is also present, and will affect the final forward velocity of the fish, it was neglected here. The reason is that it doesn't affect the force required from the muscles, because the skin friction acts along the fin, causing no essentially no torque about the fin hinge point.
Diagrams showing and combining the apparent fluid velocities, the governing equation, and final equation are shown below. Refer here for the details, explanation of terms, and derivation.
We used the existing McKibben muscle test rig in order to obtain pressure vs. force and pressure vs. displacement (in order to calculate pressure vs. strain). The first results showed a very large "dead zone" extending to approximately 30psi before any actuation occurred. This was due to having space between the muscle tubing and the fabric mesh, as the "dead zone" was the pressure required to expand the tubing to the point where the mesh was tight. The slope of the force vs. pressure graph, one actuation is occurring, is then related to the ratio of the tubing's inner circumference over its thickness.
The lessons learned during the first round of testing were then applied when making new muscles, leading to much better results. The muscle was constructed using a mesh that was much tighter to the tubing, and the tubing had a much thinner wall section. Tests were run with two different fabric mesh types. Both were tight to the tubing, but one had to be scrunched together more than the other to slide it over the tube. In the end, both of these muscles had the same strain and force outputs, leading to the conclusion that the mesh simply has to be tight, and decreasing the mesh diameter below the tubing diameter doesn't greatly affect the muscle.
Muscle Force and Strain: Testing and Requirements Summary
With a muscle lever arm of 2cm (.787"), the force required to overcome the pressure drag on the fins was found to be 3.66 pounds. At 20psi, our muscles produced a force output of 4 pounds. The reaction forces from the other fin sections were neglected during this analysis, but it clear to say that our muscles constructed of a limited assortment of scrap materials have demonstrated feasibility.
The muscles produced a strain output of 13% at 20psi. Assuming a maximum rotation of 30 degrees in each direction, and the lever arm of 2cm, required 6.1" long muscles in order to achieve the required actuation distance.
The muscles used were very small however. Using larger muscles will allow for the muscles to be moved further in, further decreasing the required muscle length.
Buoyancy AnalysisThe weights of the main components were summed, and from there the volume of water was calculated. The buoyancy control system was further developed as well. This resulted in the concept of inflatable air bladders located at multiple points throughout the fish, able to control the forward pitch and side-to-side roll of the fish.
Component Spacing AnalysisThe sealed compartment is sized to fit the battery, Arduino controller, solenoids, and manifold comfortably while leaving room for wiring and plumbing.
Subsystem Design Review
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|Subsystem Level Prep||Subsystem Level Build & Test||Subsystem & System Level Build, Test, Integrate||Systems Level Build, Test, Integrate||Verification & Demonstration of Results|
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