Detailed Design

Team Vision for Detailed Design Phase

Summarize:

Planned for this phase:

• Address loose ends in CAD Modeling

• Create Electrical Box Model

• Finalize Scissor Module and Solar Module

• Conduct Further Analysis on our design

• Ansys Vibration Analysis

• NIOSH Lifting Equation Calculations

• Create Test Plans that address all Engineering Requirements

• Update Documentation

• Plan for MSD II

Achieved during this phase:

• Electrical Box Model has been created

• Scissor and Solar module models have been refined

• Test Plans have been created

• Analysis has been completed

• Alternative renewable energy sources were picked

Feasibility Analysis & Simulation

Renewable Energy Subsystem Feasibility

During this phase, the finalized the two alternative energy sources to be placed onto the cart. One of the sources is a hand crank turbine that we found during the renewable energy systems benchmark. This crank can be seen below:

This energy source will allow the users to obtain up to 20 extra watts an hour. This could be used during cloudy days when the solar panel energy production will be intermittent or at the end of the day to extend the lift of the cart just a little longer after the sun goes down. This alternative source will be wired to the battery-electronics subsystem. and to be utilized, the solar module will be unluggedand this source plugged in. 

The second alternative power source was found while searching Youtube for DIY wind turbines for students. The video link is the following: 

This Youtuber is creating a very small scale vortex wind turbine. This type of wind turbine works essentially like the following. As wind flows around an object, vortex's are shed  downflow which makes an object oscillate back and forth. When a magnet and inductor are attached to the wobbling structure, the electromagnetic field is disturbed which creates energy, as seen below.

Where in the figure to the right, there is a fixed inductor (the shaded region), and a magnet, b, which moves in and out of the inductor region generating energy.  A small scale version of this created by the youtuber can be seen below.

These structures are extremely cheap, easy to make, and lightweight which is great for our cart. However, this structure will not generate a lot of energy and therefore, this power source will be used as an educational tool. Instead of being wired to the battery, this power source could have direct application to a series of LEDs which can act as a visual for how much power the sources are creating based on how many LEDs light up. An example of this could be the following:

Additionally, for the STEM students, because the power sources are so easy and cheap to make, a design challenge could be created similar to Project Lead The Way where students design these structures themselves. For background, a PLTW design challenge included building structures out of random materials around the classroom in order to fulfill an object.  So for our system, the following design challenge could be issued to the students: 

Build a Wobble Turbine System

• There are two design objectives:
  • Build the system so it is as light as possible
  • Build the system so it produces the most energy
• The system with the best energy to weight ratio wins
• Create three designs and choose the best using a pugh chart
• At the end of the class, have the system tested by the teacher using a box fan and multimeter
• Materials that can be used are the following:
  • • a magnet(s)
    • an inductor(s)
    • 5 popsicle sticks
    • 5 thick straws
    • 5”x10” thin cardboard
    • scissors
    • hot glue
    • 6 inches of duct tape
    • 5 rubber bands
    • 5 paper clips
    • A tiny spring(s)

Part of the design challenge is that the system needs to be able to withstand the force of the box fan. The students will have to design the wind turbine blades as well as the structure that they will be attached to. Students will be told the theme of the problem at the end of the previous class and given the link to this video to learn how to build the basic systems: https://www.youtube.com/watch?v=Nj3pH95aMOQ&t=266s.

Challenges like these are great teaching experiences because they make the students think objectively, efficiently, and in an engineers mindset.

Analysis

It should be noted that this analysis is an estimate of what might occur. Due to the system not being built yet, we  can not verify the accuracy of the model. However, these models can be used as indicators for where our systems are the weakest structurally. These simulations are tools that once/if verified can be used by any team in the future to verify their designs. Model based development is a  powerful field that can be very helpful for people designing systems; however, out system is too far in its infancy to be used appropriately. Instead, these simulations will be refined until they become that tool through comparing test data to simulation data once the cart is built.

A brief description of what a vibratory analysis. Every object oscillates (vibrates) in two manners. For one, an object oscillates internally at its natural frequencies which are innate to the object/structure. For two, objects can oscillate externally is a force is exerted onto them. For example, our solar module will have an internal oscillation and will be oscillated externally by the vibrations caused by the plane or car in transit. When the external oscillations match the same frequency as the object's internal oscillations than the object will deform greatly. The shape that the object makes when it is deformed in this manner is called the modal shape of the system. A famous example is Tacoma Bridge where the wind caused the bridge to oscillate at its natural frequency which lead to the bridge severely breaking. To avoid instances like this, our systems can be modeled and simulated in Ansys to find their natural frequencies and to determine the effect of external vibrations on the system. The goal is to build a system that will not break while in flight or or in the car.

The geometry for the solar module system can be seen below:

Worth noting is that the solar module system does have a metal box that surrounds the system that was ignored for the analysis. The Ansys account that I was using did not perform well with all the extra tiny parts. So to simplify the analysis, I assumed that the metal box would be structurally sound. This assumption is not a bad assumption because metal boxes ,such as the one surrounding the solar module system, are shipped often with no deformation.

The first step in running the vibratory analysis was to apply the force of gravity to the box. which is likely to be the only force acting on the box during transit. The systems static structurally response can be seen below for two different orientations. to find the orientation that limits the deformation due to vibrations the most.

For the purpose of this review,  only the modal responses for the  orientation on the right will be shown; however, the modal response for both orientations can be found in the supplementary materials tab. The solar module system had 6 mode shapes which it solved for over the course of 65-6 hours. To run the analysis, the school computers needed to be used in order to have enough computational power to solve the problem. The following natural frequencies were determined from analysis: 78 hertz, 110 hertz, 124 hertz, 128 hertz, 151 hertz , and 153 hertz. The resulting mode shapes occurred:

It should be noted that mode shape deformations in the upside down orientation were roughly half the magnitude of the mode shape deformation experienced from the other solar module orientation. The deformation shapes themselves are sensical; the object buckles up and down, and side to side which one would expect. The system also twists at the ends and concaves up and down on the individual panels. Due to the mode shapes and structural analysis making sense, one should be able to make takeaways from the analysis. 

The system was then subjected to frequencies in the modal range in order to determine maximum deformation, seen below:

The units for the system are in meters however this deformation makes sense. One would expect the system to have massive deformation is a part fails. When looking closely at the deformation plot, the highest deformation occurs at the rubber stoppers between the panels. This indicates that we may want to look into sturdier rubber. Another area of high deformation is the aluminum siding on the panels. This would be a simple fix as a strap could be used to provide more structural support.  

The geometry for the model can be seen below:

Note, the wheels were removed from the analysis because their geometry would not mesh. This is fine as the wheels are not the focus area for this analysis. Additionally, some pins and nuts were removed in order to reduce the number of contact faces. This will not affect the structural stability of the system in terms of this analysis.

The first step in running the vibratory analysis was to apply the force of gravity to the box. which is likely to be the only force acting on the box during transit. The systems static structurally response was only testing in the shown orientation due to the structural support beams obstructing the system on its top side. The deformation plot can be seen below:

Like the solar system above, the structural analysis for the scissor cart system makes sense with the local maximum deformations occurring where there is minimal support. 6 natural frequencies were found for the system: 134.85 hertz, 137.35 hertz, 139.25 hertz, 142.43 hertz, 172.96 hertz, and 183.54 hertz. Their corresponding mode shapes can be seen below:

The mode shapes seen above are sensical. Upon inspection, buckling of the scissor lift arms, rails system, and unsupported aluminum sheet endings can be seen. I believe that with more time, a twisting most shape would have been seen at higher frequencies. 

For this system, a life cycle analysis could be performed after the random vibrations. Therefore, that plot will be used with the total deformation plot above., and can be seen below:

The life cycle plot indicates that our part will break at the unsupported aluminum  ending almost instantly when the plans vibrates at the parts' natural frequency. This doesn't necessarily mean that this part of the cart will break but instead that we should take extra care of this part and watch it closely during the actual vibrations table testing. The total deformation plot shows that the maximum deformation will be seen by the scissor legs buckling. To alleviate this problem, the legs can be tied together or latched down to provide extra stability. for the system.

Sensor Feasibility

A design challenge that we came across this phase was to have additional analog ports to our system to account for all of the incoming information. The practices of benchmarking were revisited to make an informed design change. 

From the Analog Port Benchmarking we can rule out the use of an ADC as it requires a software interface to support it and uses a lot of digital ports in the process. The selection falls between the Arduino Mega and the Multiplexer.

When deciding to use a multiplexer, the MCU driving it must be considered. 

Here we see that the current MCU (MKR ZERO) has a higher clock speed than the Mega, but when multiplexing the sample rate gets divided by the amount of ports you are sampling from, reducing your overall sample rate. If the MKR ZERO were too multiplex 3 analog ports, it would have the same sample rate as the MEGA however it would need 3 digital pins to drive the multiplexing. This increases the complexity of the design. In addition, the arduino Mega is a cheaper MCU than the MKR ZERO which will help reduce costs for future iterations. For this reason the Mega can be selected as the updated Micro Control Unit. 

NIOSH Lifting Equation for Cart Expansion

Used knowledge of ergonomics and human factors to conduct a lifting analysis.

Result: 

The RWL is what the weight recommendation is for bending down and extending the scissor module from a down position to an up position.

Lifting Index = Load Weight / RWL

48 / 30.57968232 = 1.5697

Conclusion: Possible Increased Risk

This Lifting Index of 1.5697 shows that there is a slight risk of sprain or strain in doing this motion.  We will mitigate this risk by recommending two people raise the cart, or one person is careful in doing so.  

Drawings, Schematics, Flow Charts, Simulations, and Models

Mechanical System Drawings

Scissor Module

Updates

• Added reinforcement underneath

• Added connection points for electrical systems

• Added swivel handle

• Added thumb screws for height locking and wheel connection

• Added connection points for electrical systems

• Added swivel handle

• Added thumb screws for height locking and wheel connection

• Added reinforcement underneath

Solar Module

Updates

• Holes added to panels and hinges

• Hardware added

• Pads attached for cushion between frames

• Bumpers attached to top panel for stability in transit
  

Updates

• Holes added to enclosure and hinges for hardware

• Back panel of enclosure now hinges on opposite edge

• Latches added to hold position in transit

• Position locking for main workspace added via sliding tubing into place

• Feet and thumb screws added to line up with features in scissor module

Overall Model

Updates

• Independent workspace support

• Battery is separate from the rest of the electrical system

Bill of Material (BOM)

Mechanical Systems BOM

This BOM reflects a less than ideal but acceptable case for the final version of the BOM. We are hoping to be able change sources for some of the materials to cut some of our costs. With this version of the BOM, along with the electrical BOM, we anticipate having unused funding to cover future part failures

Electrical Systems BOM

This BOM has been updated from the previous iteration to include more detail regarding the physical components that hold the electrical submodule together. The design of the electrical module in CAD greatly helped to narrow down the fine detail parts and components. This is not a final iteration, similar to the mechanical BOM, we hope to reduce costs by choosing different vendors for our parts. Similarly, as more detail is added before the start of MSD II, the parts list is expected to change. 

Renewable Energy System BOM

The renewable BOM may be cheaper than anticipated if the blinds and rubber mats can be found around the house.  The lead times on shipping  are less than a few days for all products.

Test Plans

Test Plans Solar Powered Cart.xlsx

Results: 

Test setup for the switching test. The laptop was chosen as the AC load and a python script was used to monitor the status of charging.

Power seen coming from the inverter.

Power seen from the grid.

When the grid goes out, the inverter signal comes online very quickly after.

The laptop reported no changes in power while being plugged in and the sources switched.

the output of the code can be viewed here.

The data collected shows an interruption time of approximately 0.25ms which is far less than the maximum value. Therefore this test has passed.

Other Plans to Address Engineering Requirements

Risk Assessment

Live Risk Management Document

The red coloring denotes an increase in value, or an increase in importance, and the green denotes a decrease in value, or a decrease in importance. No risks were added during this phase. The likelihoods and severities that have changed in this phase have mostly been because of mitigated risks through testing and simulation performed during this phase. The only risk that became more likely was the risk that is furthest from our control. 

Design Review Materials

Links to:

• Pre-read
• Presentation
• Notes from review and action Items

Plans for next phase

Before next phase and during the winter break we plan on:

• Conducting our Gate Review on December 1st
• Meet weekly during break to address loose ends and do further research
• Order Parts

This is a Tentative Schedule for MSD II: