P18250: Smart Buoy
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Preliminary Detailed Design

Table of Contents

Team Vision for Preliminary Detailed Design Phase and Quick Summary

The goal of phase 3 is to complete as much of the detailed design work as possible. The subsystems we will focus on designing this phase are the components that are the highest risk, most complex, and ones with which other components are highly dependant. By getting these out of the way early in the design phase, it will allow for complications to spill over into the next phase if necessary, and should hopefully free up some time at the end of MSD 1 to comfortably wrap things up and prepare for MSD 2.

At the end of phase 3, both the mechanical and electrical/computer subsystems made significant progress in designing their subsystems and choosing components. Although the design of many components will be spilling over into phase 4, the team is confident that the most difficult decisions and designs are set, and moving forward go relatively smoothly.

Below is the documentation of the systems that were designed during this phase, the bill of materials, some feasibility updates, test plans, risk assessment, and lastly the team’s plan for wrapping up MSD 1.

Mechanical Subsystems

Hull

Three materials were considered for creating the catamaran hulls: reinforced foam, large diameter pvc pipe, or custom inflatables. Reinforced foam was deemed to have longevity concerns and a tendency to break. Pvc pipe was considered too expensive given our budget. The best remaining idea was to build custom inflatables using vinyl fabric.

A prototype was created during this phase to test the feasibility of creating inflatables. The concerns were if we have the facilities to manufacture the inflatables, and if it would keep the desired shape when inflated. The shape was cut out of a tarp to simulate the vinyl fabric, and the stitching was performed in the Construct. The prototype was a success in creating the desired shape when inflated, but additional testing will be required during MSD II to ensure proper air-tight seal of the stitches and valve stem using vinyl cement.

Buoy prototype

Buoy prototype

As was conducted last phase, a simple equation can be used to calculate the volume that each hull must displace in order to meet the engineering requirements for buoyancy. The FBD and equations can be seen below.

Submerged volume calculations

Submerged volume calculations

As a result, in order to support 200lb, each pontoon must displace a volume of 1.6 ft^3.

In the system level design, ⅓ of the volume was used to calculate the displaced volume of the pontunes. The engineering requirement however is that ⅓ of the height of the pontoons be submerged. These two quantities are different as can be seen in the figure below.

Difference between ⅓ height and ⅓ volume

Difference between ⅓ height and ⅓ volume

The team used two methods to solve the new hull design problem; an analytical approach and a simulated approach using 3D CAD modeling. The following are two equations used to calculate the volume as a function of the geometry of two possible cross-sections.

Circular cross section volume as a function of arc angle

Circular cross section volume as a function of arc angle

Triangular cross section volume as a function of height

Triangular cross section volume as a function of height

One of the biggest results to come from these calculations is that a triangular cross section would need to be very large in total volume in order to have the same displaced volume as a circular cross section. Due to these results, we chose to use a circular cross section.

CAD model of inflated pontoon

CAD model of inflated pontoon

Each pontoon will be a total of 72” long. They each comprise of a 15” diameter 42” long tube, which tapers up to a point at 45 degrees at both ends. The total volume of each pontoon is 9.33 ft^3, with a submerged volume of 1.57 ft^3, just enough to apply 100lb of buoyant force per pontoon.

Hull mounting tabs

Hull mounting tabs

In order to mount the pontoons to the frame, rings will be placed along the tip seam, and a side fold of each pontoon. This rings will then be bolted to the frame along rails that run the length of the cylindrical portion of the pontoons

Pontoon cut-out shapes

Pontoon cut-out shapes

The fabric comes in 60” rolls sold by the yard. Laying out the different hull sections as closely as possible while still allowing for extra material for the seams results in just under 7ft of fabric per pontoon. In order to have the fabric for three pontoons (2 for the hull and 1 as an extra/backup), we will need 21 ft or 7 yd of the 60” wide material.

In order to ensure the inflated pontoons stay watertight, vinyl cement will be used seal all of the seams. Each pontoon has about 210” of seams. Assuming each seam has 1.5in of cement applied to it, the total required area for 3 pontoons sums up to 6.5ft^2. A 4 oz can of vinyl cement can cover 8ft^3, which should be plenty.

Frame

In designing the frame for the ‘Smart Buoy’, three different materials were considered to make up the beams; XR100, and three sizes of 80/20. Below are their cross sections, and geometric properties. Each of these are made of extruded 6105-T5 Aluminum, with a yield stress of 39,900ksi

Frame material cross sections

Frame material cross sections

Frame Material Dimensions

Frame Material Dimensions

With the chosen structure for the frame that utilizes the minimum length of material possible in order to ensure the desired strength and rigidity, the total length of material that would be required was calculated which can be seen below.

Frame Material Length

Frame Material Length

Another thing to check when designing the frame and choosing materials was weather or not it would be able to support the load it would be subject to. The FBD and singularity function show the equations for the max internal moment in the frame member with the highest stresses.

Frame Max Stress Calculation

Frame Max Stress Calculation

Factor of Safety to Yielding Calculations

Factor of Safety to Yielding Calculations

Frame Material Comparison

Frame Material Comparison

Based on the above comparison, The team chose to use XR 100 for the frame due to it being the most economical option, as well as having sufficient mechanical properties for this application.

The CAD model for the Frame can be seen below:

Frame CAD Model

Frame CAD Model

Hull and Frame

Attaching the pontoons to the frame results in the following CAD model which makes up the results of the Mechanical Subsystems from this phase.

Frame CAD Model

Frame CAD Model

Electrical Subsystems

To further development, decisions were made about the specific parts to be used for all of the major systems in our project. Below are the parts selected or the thoughts being made towards an eventual choice.

High Level schematics

The high level schematics of the EE/CE subsystems is shown below:

High Level Schematics

High Level Schematics

Microcontroller

After researching different microcontrollers for the smart buoy project , the team decided to choose Arduino Due as it satisfies the microcontroller requirements below.

Arduino Due

Arduino Due

High Level Schematics

High Level Schematics

Advantages of Arduino based boards:

Winch

For our winch we had a few requirements that we wanted to satisfy:

Through further research the best winch we were able to find that satisfied the requirements was the Pontoon 35 Electric Anchor Winch from trac outdoor. Inside a manual for one of their wireless controllers that is compatible with the winch, we found the wiring from the control unit that would connect to the buttons which can be found here. We are confident that we can have our microcontroller be able to drive this winch up and down. If this does not work our backup plan is to have the up down function be driven by an H-Bridge controller.

Some of the other features of this winch is:

Trolling Motor/Thrust Motor

We already have a Minn Kota Endura C2 45 from a previous groups’ project. During this phase, additional research was done by looking through the manual and online product reviews. Through this research we found that the motor has the ability to run both forwards and backwards as well as rotate 180 degrees. Some battery and power specifications were learned (12 volt deep cycle and fuse needed). In addition, we found: electrical specifications, wiring diagram, and a wiring gauge guide.

GPS Module

The GPS module chosen was the Adafruit Ultimate GPS Breakout. The reasons for this decision:

Based on these things, and the fact that the other qualities (update rate, accuracy) are agreeable, we chose this GPS.

Adafruit Ultimate GPS Breakout

Adafruit Ultimate GPS Breakout

Heading Sensors

To calculate the heading of our buoy, we need an accelerometer and a magnetometer. The Sparkfun IMU Breakout was the module chosen. It contains an accelerometer, a magnetometer, and a gyro all in the same module. The board is similar to the GPS module in terms of ease of integration, and there are tutorials available for setting it up. These factors made us choose this module over others.
Sparkfun IMU Breakout

Sparkfun IMU Breakout

Lights

We found out that a motor powered craft our size, that we need a 360 degree overhead white light and a 120 degree red and green bidirectional light in the front. These are requirements so that at night, boats can see what direction our boats are coming from. This way they can find out who has the right of way. Diagrams can be found here. and here.

Through research, we found that the most cost efficient options that met our requirements can be found here and here.

Battery and Solar Panel

Battery and solar panel equipment will be provided from the customer. The battery chosen is to be a 110Ah lead acid AGM battery. Initial power calculations estimated that a 50Ah battery would suffice for our application, so the 110Ah will be more than enough in terms of capacity.

The solar panel will also be provided by the customer as well as an MPPT charge controller. The charge controller will interface between the battery and the solar panel and allow easy charging of the battery.

Solar Panel

Solar Panel

Solar Panel Specifications

Solar Panel Specifications

Motor Control

To control forward and reverse directionality for the trolling motor, we decided an H-Bridge driver circuit would be necessary. We have narrowed down our choice to two Pololu H-Bridge drivers that are simple to control and should be able to handle our expected current draws.

Our selections are based on data and information from an older MSD project, P16250, which used the same trolling motor. The 16250 team has already tested the motor with a similar H-Bridge driver circuit; we are confident in our selection based on their test data.

H-Bridge

H-Bridge

H-Bridge Specifications

H-Bridge Specifications

Communications

We had decided in the previous phase to use XBee RF transceivers for our communication between the buoy and the user. After considering range requirements and integration problems, we decided to go with XBee SX modules. Specifically, we decided to go with a development kit that includes two XBee SXs and one XBee Pro SX, along with boards for integration and antennas for longer range capabilities. The kit is more expensive than we have previously allocated to communication, but these additions will save us a lot of time and possibly money when we design our PCB board or decide to cut out having a PCB board altogether.
XBee SX Module

XBee SX Module

State Diagram

Below is a preliminary state diagram for the operation of the buoy. The states are focused around the changes to the buoy’s location, the state of the thrust motor, and the state of the anchor.
State Diagram Flowchart

State Diagram Flowchart

Bill of Material (BOM)

Bill of Materials

Bill of Materials

The working document of the Bill of Materials can be found here.

Updated Weight

Weight Breakdown

Weight Breakdown

Test Plans

The working document of test plans can be found here.

Frame

Hull

Frame and Hull

Battery

GPS

Microcontroller

Motor Control

Trolling Motor

Winch

Wireless Communications

Risk Assessment

Risk Assessment Sheet

Risk Assessment Sheet

The live Risk Assessment sheet can be found here.

Design Review Materials

The Design Review Presentation can be found here.

Plans for next phase

Plans for Upcoming Phase 4

Plans for Upcoming Phase 4

From the mechanical side, the biggest priorities are designing the motor steering control, and laying out the components of the frame and ensuring the center of mass is centered on the frame. There are also a few things to clean up such as designing the custom frame brackets.

On the electrical side, the focus will mainly be creating schematics for all of the circuitry that will be required. The connections between all of our chosen components needs to be determined, and any extra things between them in order for them to interact in a efficient and safe manner. In addition to schematics, the navigation algorithm will also be a high priority, along with supporting code and tests.


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