Preliminary Detailed Design

Team Vision for Preliminary Detailed Design Phase

In this phase, we planned to prototype our scissor lift system and solar panel folding module, create detailed preliminary designs including CAD models and an updated electrical schematic, and create a preliminary Bill of Materials.  The table below further explains these goals and addresses completion status.

Prototyping, Engineering Analysis, Simulation

Scissor Lift Prototype:

BOM:

Result:

Hot glue and 3 inch nails. was used to create structure for the prototype base and top piece.  Paper Clips were used to secure scissor arms together.

Solar Module Prototype: 

BOM:

Result: 

3 Possible Folding Designs

Sequential Folding of Solar Module 

Larger Scale Solar Module Prototyping 

Test Plans

This test resulted in the observation of the scissor legs working as expected.  Insights gained are that the scissor lift will be able to be lifted by one person from one side of the lift and gave us ideas to further prototype. 

This test resulted in an optimal folding solar panel concept.  The insights gained are that we will want to distribute the weight of the module on all four sides of the cart, and this also gave us the opportunity to start making detailed CAD models.

The results of this test were the successful use of the feet on the solar module fitting into the cut outs of the scissor lift module.  This gave us confidence that this method is applicable for our cart. 

The results of this test were that the panel provides ample power (34.43W)

The results of this test were that data was successfully plotted on a PC and displayed on an I2C device simultaneously. Potentiometers were used to simulate analog data inputted to the system. 

Design and Flowcharts

Collapsibility Module Flowchart

This flowchart explains the interactions between the components of the collapsible system.  There are hardware interactions and also applied force.

Solar Module Flowchart

This flowchart explains the interactions between the components of the solar module system.  There are wire connections along with hardware.

Electrical Module Flowchart

This flowchart explains interactions between the components of the electrical system. Power and data transfer are shown, as well as user interaction.

Feasibility Analysis & Simulation

The prototype solar panels image below has the solar panel naming convention written on it that will be used throughout this section.

The first panel analyzed were the side panels which can be seen below in the free body diagram.

As shown above, the only forces acting on the panel are the force of gravity and the moment and shear force that the hinge creates on the side of the panel. The weight of each solar panel is about 8 lbs. The panel was assumed to be fixed at the face where the solar panel glass meets (brown in the image below) the aluminum frame (silver below) on the end with the hinge. The resulting free body diagram in Ansys can be seen below.

Of note, the front panel will have the same free body diagram except the shear and moment would be acting on the longer edge of the solar panel. Due to this, the structural support of the solar panel will deform less than the structural support would for the side panel. Therefore, if the deformation is acceptable for the side panel, then the deformation will be acceptable for the front panel.

Below is the free body diagram for the middle panel. The only forces acting on the panel are the moment and shear forces due to the hinges. 

Seen below is the middle panel in Ansys. The fixed point is assumed to be the face where the aluminum support and glass meet at the end with no hinge. This point was chosen for consistency between the side panel analysis and this analysis. This also allows for buckling along the fourth side with is not a ruled out case.

  Three support structures, seen below, were tested to find the deflection of the solar panels.  The supports were created with 0.06" aluminum alloy strips.

The side panel did not need to be tested with cross structural support because the panel  did not have forces which would cause bending in that direction. Below an example of an Ansys output can be seen for total deformation.  The example is of the middle panel with a cross structural support.

The table below summarized the deflection amounts seen in the analysis. In all cases, the solar panel structural support deflects less than 1 mm indicating that the solar panels do not require legs and can rather be hinged to the middle panel and hang freely. This assumes that no one pushes these panels down and that while the solar panels are being unfolded, the user is gentle with the panels. For all panels, no structural support is required under the panels although it can be added to create a higher factor of safety.

It should be noted that deflection less than 5 mm would have been allowable based on what we want.

Due to the complex shapes that the top and bottom scissor tables have, no preliminary free body diagram was created and instead the dynamics will be explained using the Ansys free body diagram., such as for the bottom scissor table shown below.

The forces acting on the bottom table are the weight of the table and the weight of the electronics subsystem which is spread across the entire top face of the table. The structure is fixed at the cylinder holes which the scissor leg pin fits into and the contact face between the rollers and track. This position is assumed to be at the fully extended  orientation of the cart, which would result in the most deformation. The system is also fixed on the bottom where the wheels would be attached. The weight of the electronics subsystem was assumed to be 60lbs which is roughly 1.5 times heavier than expected.  The Ansys free body diagram for the bottom table can be seen below.

The structure is slightly tilted to show that the forces are acting into the screen. The top scissor table is the same as the bottom scissor table  but flipped. The forces acting on the table are the weight of the table and the weight of the solar panel system which will sit on top of the table. The force is distributed over the area of the structural support of the solar panel, which is the contact interface between the two systems. The fixed points are the same as the bottom table, described above. 

The following structural supports were tested, seen below, as well as a few other combinations which varied the number of supports and having a slab or not. The supports were created from 0.06" aluminum alloy. The longitudinal supports consisted of 3 strips of the aluminum alloy side by side.

The slab is simply an aluminum sheet places horizontally over the longitudinal supports, between the rail structures. An example of the deformation can be seen below, and is the bottom table with 3 longitudal supports and a slab.

The resulting deformation values for each tested orientation can be found below in the table. In order to obtain deflection values of less than 5mm, the top table requires a slab but the bottom table can be left bare. The factor of safety applied to the top table was 2 and 1.5 for the bottom table.  

It should be noted that this analysis was not performed yet but rather was just set up. This test will look at the subsystems when they are collapsed and folded to see if the systems fail due to vibrations during transit for both airplane and car traveling. An image of the setup can be seen below. 

In order to complete this analysis, a version of Ansys which is stronger than the student version will need to be obtained.

The following schematic represents the forces that the pins and roller shafts will experience in the system. The bottom cart has 8 total, with 4 rollers and 4 pins for the scissor legs to rotate about.

Pin Schematic

It is assumed that the weight of the scissor legs, table top system, and solar panel system will be evenly distributed over the 8 shafts.  These systems have a combined weight of roughly 60 lbs. Using a factor of safety of two, the weight that each shaft experiences is 15 lbs.  

The equation for determining the shear on the shaft is: τave= F/A  where F is 483 pound force, A is the area of the circular shaft, or pi*R^2, and τave is the average shear stress.

The maximum allowable shear stress is dependent upon the material used. We chose to use carbon steel for the rollers and zinc coated steel for the pins, which have τmax values of 40,000psi and 50,000psi, respectfully. The resultant minimum shaft diameters were 0.02355in and 0.0211in for the rollers and pins, respectfully.

Because the rollers and pins that we are researching have such larger diameters, it may be possible to use them as the stopping mechanism for the scissor lift. Simply, the scissor lift would be raised to the desired height, and a pin (or in this case a screw) could be screwed into the rail system in front of the roller to prevent it from rolling downwards. Assuming one screw is used, hand calculations were performed and can be seen below. The resulting minimum thickness is 0.0138in, or 13 times smaller than the pins we are looking to buy.

The reason why the roller exerts one-fourth the weight on the pin is because the top table has four rollers all of which equally shoulder the weight. This force is pushing the rollers to the sides in an attempt to collapse the system. The equation was utilized the in the same manner for calculating the minimum diameters of the pin shafts and rollers.

Electrical Schematic

The electrical schematic from last year was updated with some simplifications.

Power combination has been deemed out of the scope of this project. The input power will be determined by a SPDT switch controlled by the user and the currently connected renewable source. With removal of the schottkey diodes and additonal wiring, the cart becomes more efficient and easy for the user to understand.  

Deep Discharge of batteries

Not all batteries are created for the same purpose and therefore our system operation depends on the type of battery used. Deep discharge batteries are the most desired type of battery for this project as they can be safely discharged up to 50% capacity. However the use of a deep discharge battery in our system may not be feasible as deep discharge batteries may not be locally sourced in Columbia. In addition to this, lead acid batteries classify as a hazardous material, shipping may not a feasible option either. Flooded batteries are a type of lead acid battery that are readily available in our customer location of Columbia. Despite only being capable of safe discharges up to 20%, this type of battery can prove to be a viable option in our system.

The average capacity of a flooded type lead acid car battery is about 70 amp-hours (Ah). These batteries have a voltage of 12V so their equivalent watt-hours are 840 Wh. Discharging the battery 20% would yield a power of 168 Wh.

If a sample load is assumed to be 110W, the system could operate for roughly 1.5 hours. This estimation does not factor in the renewable power generation or inverter inefficiencies.

This should be a sufficient run time whiteout grid power. If needed, more batteries of the same type can be connected in parallel to increase run time.  

As can be seen in the electrical schematic, a relay has been added to the output of the inverter to prevent over discharge of the battery. This will prevent the load from over discharging the lead acid battery. Preferably a deep discharge battery would be used, so more of the battery capacity would be used before the low voltage mark has been reached.

Figure from Renogy's Adventurer Manual

The table of battery charging parameters shows for all battery types the low voltage mark occurs at the same electrical potential. With the MCU sensing the voltage across the battery, it can prevent over discharge irrespective of battery type.

Bill of Material (BOM)

Mechanical System BOM

This BOM reflects a less than ideal case for the final version of the BOM. We are hoping to be able change some of the materials to be lighter and cheaper if we can be confident that they will be strong enough. There is also some uncertainty in the manufacturing processes we will be using to fabricate some of the parts, which could lead to some changes as well. With this version of the BOM, along with the electrical BOM, we anticipate having unused funding to cover future part failures.

Electrical System BOM

This electrical BOM shows the overlap between the previous years purchases and the current needs of the team. Most of the big ticket electrical items have already been purchased; connectors, and displays represent the majority of the expenses. The total cost to us very low compared to our budget. The mechanical BOM has far more new parts to purchase, so a majority of the budget can be allocated to it. 

CAD Models

Scissor module

CAD model for scissor system

• 7.5 inch tall wheels

• 5 inch tall collapsed scissor section

• 30 inch tall extended scissor section

• More to be added to design for height locking

Solar panel module

CAD model for solar panels and hinge subassembly

• Different length hinges allow for stacking panels

• Panels are folded out individually for ease of use

• Clearance between panels when folded for pads to be attached to frames

CAD model of solar panels with the enclosure

• Enclosure folds out into table

• Middle solar panel is the mechanical hub of the module

• Cutout in enclosure base for access to back of center panel

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. The four bottom grey rows are risks added during this phase. Covid shutdowns was raised to a likelihood of two due to St. John Fischer closing down.  Previously purchased equipment not functioning properly in assembly was raised to a two due to the previous teams design not working as anticipated. The severity however was lowered to a one because we have designed around their cart whenever needed. Poor analysis was raised to a likelihood of two due to us finding more incorrect analysis from the previous team. Insufficient power likelihood was lowered to a 1 after the research that Matthew and Christian have done showing that the battery has plenty of power for the desired use. cases. The likelihood of the cart requiring two people for set up was raised to a two because the modules are heavier than anticipated. The severity was lowered to a 0.5 however because the this case is very dependent upon the person using the cart. The severity of not being able to make the design modular was changed to a 3 because of how heavy the cart would be if it wasn't in modules. The likelihood of the battery being overcharged and deep discharged was lowered to a one because we directly set this parameter. Additionally identified risks are the roller and track system and hinge systems not working properly., the cart not educating the user effectively, and the customers having conflicting ideas. 

Design Review Materials

Include links to:

• Pre-read
• Phase Review Presentation
• Review Notes
• Action Items

Pre-Read

Plans for next phase

Matt's 3 Week Plan

Garrett's 3 Week Plan

Christian's 3 week plan

Jake's 3 week plan