P16229: Robofish 3.1 - Navigation
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Detailed Design

Table of Contents

Team Vision for Detailed Design Phase

During this final phase the team hoped to finalize as many design decisions as possible. In addition to this the team hoped to readdress the requirements, risks, and considerations discovered in the earlier phases of this project.

During this phase the electrical team was successful in developing and prototyping the necessary electrical components of the Robofish. All boards and such have been, at this point, laid out, purchased, and tested. The mechanical team has been working with several models in or to elucidate dimensions for the tail. A final design for the tail has not yet been settled on however a new design has been proposed which the team believes will solve the identified problems associated with the previously mentioned designs.

Overall, the team has yet to develop a complete and final design for the Robofish but is very close to completing said design. The important tasks remaining include finalizing the new tail design, finalizing testing for the proposed designs, and developing a unified concept drawing which combines the elements of both teams.

Prototyping, Engineering Analysis, Simulation

Geometric Analysis of Tail - Old Design

In order to assess the range of motion of the tail, given the previously proposed design, a geometric analysis was performed in MatLab. It was determined that the relationship between the side of the triangle made by the muscle, the length spanning the hinge to the muscle attachment point, and the length spanning the hinge to the muscle origin, could be used in the Law of Cosines to determine the angle change as a function of the muscle length change.

The analysis revealed that as the initial angle of the muscle approached 30 degrees (deemed to be the minimum feasible angle for attachment based on sizing constraints) the maximum deflection achievable increased. In addition to this for a given initial angle of the muscle, as the length of the cross bar increased the amount of deflection went down. In summary, a less steep initial angle with a shorter crossbar was determined to provide the largest range for angle of deflection.

Geometric Analysis of Tail - New Design

In light of how new the new design is for the tail the beginnings of analysis was able to be completed at this time. The first part of the analysis performed consisted of assessing the displacement angle as a function of the length change of the muscle. The arc length formula was used to convert an arc length change, related to the muscle length change, to a angle change. It was determined, as expected from the relationship between radius and angle, that as the radius of the pulley increases the angular displacement decreases.

In light of this result the torque required to move the tail through the water was partially considered. To achieve a required torque if the radius of the pulley decreases the force required increases. This fact, in conjunction with the aforementioned, results in a conundrum. As the radius of the pulley decreases the angular displacement increases but simultaneously the force required to achieve the necessary torque goes up.

Displacement Analysis
Preliminary Toque Analysis

This analysis of torque considers a 3 cm radius pulley which would a total angular deflection window of approximately 22 degrees. The torque required was derived from the expression derived by MSD team 14029 but assumes that the component of the torque required which is a function of the velocity of the object is zero. This assumption was made because the fish is not moving altogether that fast. The torque required formula was solved for a range of angular velocities and it was determined that the muscles would be able to generate the necessary force. The estimate of the required torque is very conservative and the team will continue this analysis.

See the MatLab file titles NewGeometricAnalysis.M for reference.

Battery Life Reconsidered

Initially, it was determined that 56 batteries would be needed to achieve the 10 hours runtime which was the original customer requirement. This requirement was amended after conversations with the customer to 5 hours which was determined to be a reasonable run time. The battery analysis was revisited in light of this change and in light of the battery testing performed in the subsystem design phase. It was determined that 30 to 36 batteries would be needed to achieve this new requirement.

This number is not exactly half of the batteries needed for 10 hours, as determined in the system design phase, because said number was calculated using ideal batteries. The batteries being purchased are not ideal and therefore more are needed in order to reach the desired 5 hours. Analysis indicates that these batteries could in theory achieve 7.25 hours however the as mentioned the batteries purchased will not perform as well as those used in the analysis.

Drawings, Schematics, Flow Charts, Simulations

Purpose

The remaining drawings and schematics were created to satisfy a full and complete Robofish design.

New Tail Design Schematic

A new tail design was proposed that the mechanical team believes will be able to account for the necessary length change. The roadblock with previous designs, as mentioned in the preliminary detailed design review, was that the length change associated with one muscle contracting would need to be compensated for on by the antagonistic side of the system. A new system was proposed which uses a pulley system.

This system works as follows: Affixed to a pulley will be the two muscles which are mutually tangent to the pulley wheel. When one muscle contracts the pulley wheel turns. The muscle on the other side will have its slack brought to tension by the pulley wheel. The fish tail itself will be attached to the pulley wheel which is acting as a pseudo-gear. This system can be used to increase the angle change for a given muscle length change governed by arc length-theta relationship as well as to automatically tension the antagonistic side.

Total Robofish Electronic Schematic

A total, encompassing electronic system schematic was created. The diagram will be used as a reference during the build phase to ensure the proper electrical connections. The diagram includes: micro-controllers, gate drive circuits, interface circuits, battery pack, boost converter, camera, pressure sensor, hall sensor, water detection, valves, and pump.


Robofish Interface Circuitry Board

The board design for the interface board was derived in a similar method to the gate drive board. It will be ordered from OSH Park, assembled, and tested for functionality.


Software Block Flow Diagram

A full software schematic was created to give more information about the total software process of the fish. The Raspberry Pi and Arduino microcontrollers are the focal points of the software architecture.
The Raspberry Pi, owned by P16029 Robofish team, will facilitate the interface between the camera(s) and the Arduino as well as the pulsed ballast flow valve. The Raspberry Pi will sent 4 digital signals to the Arduino to indicate whether a right or left swimming motion is desired, as well as sinking or floating. The Arduino, owned by our team will carry out the swimming procedure based on the Raspberry Pi signals including ballast control. The Arduino will also monitor the water detection circuit and pressure sensor reading and send the appropriate signals.


A sample camera view was created to demonstrate the supposed functionality of the system object-tracking. Note that the pulse width of the signal created by the Raspberry Pi will be controlled by the height of the object. When the object is farther away from the horizontal middle, the pulse width will proportionally increase.



Total System CAD

In order to work toward generating the big-picture view of the total design and to develop a platform for working on subsystem integration the team, in conjunction with team 16029, created a total system CAD. This CAD includes the pump, ballast tank, batteries, valves, etc, and most importantly, the jaw system developed by 16029 and the tail system developed by 16229.

Input and Source

  1. Selected Concepts
  2. Feasibility Models
  3. System design and interface definitions
  4. Test plans

Output and Destination

  1. Complete hierarchy of design files from system level down to components
  2. Parts list
  3. Software design that specifies coding requirements
  4. Test plans, including expected performance vs. requirement

Bill of Material (BOM)

Purpose

Confirm that all expenses and contingencies are afforded by the project financial allocation

A link to the live document for the BOM as of the DDR phase can be found here.

Assessment of Funds

In light of the purchase of the batteries, which are the most expensive individual component of the system, the team needs to address the fact that large portion of the budget has now expended. This poses significant constraints going into MSDII which necessitates a well thought out big-picture view of the design so as to prevent unnecessary expenditure. This is discussed further in the sections entitled "Plans for the next phase."

Input and Source

  1. PRP.
  2. Design Files.

Output and Destination

Completed BoM and Budget

Test Plans

Purpose

In order to demonstrate objectively the degree to which the Engineering Requirments are met as well as the overall feasibility of the design, a series of testing plans were devised for the Robofish system.

Mechanical Testing - Muscle Function

In order to determine the relationship between the mesh parameters and initial length of the muscle two tests will be performed. The first of these tests will be using two different meshes. Mesh 1 has a resting diameter equal to the outer diameter of the tubing that will be used. Mesh 2 has a minimum diameter equal to that of the tubing being used, and therefore a larger maximum diameter than mesh 1. This test will show whether or not the maximum and minimum diameters impact the extensibility/contractility of the muscle. The second of these tests will be 2 muscles with the same mesh but the initial length of the muscles will be different. This is motivated by the fact that it became clear to us that there may limit the amount of contraction possible and therefore the relative contraction decrease as a function of the increasing initial length.

What we hope to gain from these tests is as follows: We would like to determine if using a specific mesh can improve the contraction of the muscle and if using a shorter muscle is possible. Beyond simply looking at the length changes we will in the future need to characterize the force production as a function of the pressure.

Torque Analysis

Given the aforementioned analysis of the new tail system additional engineering analysis needs to be performed to assess the torque required to move the tail and the muscles ability to generate said torque.

Considerations for the New Tail

With the creation of a CAD model of the new tail design the team will be able to better characterize the physical manifestation of the proposed system. The CAD had brought into light new considerations related to the manufacturing of this system that will also play a role in selection of the pulley radius. The team is discussing the new considerations in so far as generating and assessing new risk, developing plans for manufacture, and factoring into the engineering analysis the constraints on the pulley wheel radius among others.

Gate Drive Board

The Gate Drive Board introduced in the previous review was constructed with the proper components. Full testing of the board for functionality will be performed. Also displayed, the planned stacking method of the boards.




The board was tested on one channel which gave the following results. The Yellow trace gives the BJT input control signal while the Blue trace give the solenoid output signal. Note that since this signal is connected to the negative terminal of the solenoid and the positive end of the solenoid is fixed to 24V, the actual solenoid voltage will be switched to match with the BJT input. Capturing the actual solenoid voltage cannot be done with low-voltage, non- differential probes.


Inputs and Source

  1. Engineering Requirements.
  2. Feasibility Models.

Outputs and Destination

  1. Report that summarized the degree to which Eng Reqs are satisfied.
  2. Assessment of accuracy of feasibility models.

Risk Assessment

Purpose

In order to continue to address the risks associated with the design, new risks were added to the old risk table and old risks were considered.

Screenshots

A link to the live document can be found here.

Design Review Materials

Include links to:

Plans for next phase

As a team, we need to accomplish further development of the new tail design and a more developed conceptualization of the overall subsystem integration. The new body CAD model has helped the team to meet the latter of these points and will be developed further. Development of this model is critical for future systems integration in that it will provide us with key parameters such as overall weight, center of mass, and center of buoyancy. As mentioned in the subsystem level design phase this CAD model can be modified in order to assess different ways in which the system integration can be achieved. Insofar as the former is concerned, the mechanical team will be working to address this issue prior to the beginning of MSDII in order to stay on schedule. Engineering analysis which combines physical parameters estimated in SolidWorks and analysis performed in MatLab is to be completed prior to MSDII for the new tail design.

In order to prepare for MSDII a big picture approach needs to be taken so that the team moves in the right direction, which is extremely important given the proportion of the budget that has already been exhausted. Unnecessary or extraneous expenditure could leave the team without the funds to complete the project. The remainder of this semester, the winter break, and the beginning of MSDII (if necessary) will be used to prepare this big picture conceptualization of the design before substantial building begins.


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