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MECHANICAL ENGINEERING DESIGN PROJECTS

FINAL STATUS REPORT

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SUBMITTED BY

Ogbemi Ekwejunor-Etchie

Jacob Spector

Daniel Wood

Sanders Colbert

May 7, 2013


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TABLE OF CONTENTS

PROJECT OVERVIEW ................................................................................................................................................................ 3

OVERALL DESIGN ..................................................................................................................................................................... 5

TESTING/PROTOTYPING RESULTS ....................................................................................................................................... 344

PROPOSED IMPROVEMENTS/LESSONS LEARNED ............................................................................................................... 399

REQUIREMENTS COMPLIANCE.............................................................................................................................................. 45

COST .................................................................................................................................................................................... 477


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PROJECT OVERVIEW

At present, there is no available solar panel system that is roof-mountable, capable of solar tracking, able to withstand hurricane-force winds, and is not unnecessarily complex. Systems with any three of those four attributes are available (or at least patented), but all four of those qualities are needed for a system to be economically viable for installation in regions that are both low-income and prone to heavy storms.

Evidence has shown that fixed, roof-mounted solar panels can withstand hurricanes if installed to proper specifications, as long as the roof they are attached to is secure. However, fixed panels are significantly less efficient than panels that can rotate to track the sun’s movement. Current State of the Art:

Looking through patent listings, one can find several solar trackers designed for high wind environments built on the relatively simple central point of rotation design. The “high winds” referenced in the patents aren’t explicitly defined as being hurricane force. Some manufacturers, however, claim their solar tracker systems are “hurricane rated” (Sundog Solar, LED Lighting Management, MegaWatt Solar), although the ratings seem to either apply only to the tracker frame, or have no data to back up the claims. Having just the frame being hurricane rated is risky, as hurricane force winds could be strong enough to rip PV panels from the frame. A more promising design is that proposed in patent US20090101135, which can retract itself to be nearly flush with the base.

All of the above systems, however, rely on being mounted on the ground using poured concrete footings, and are thus unsuitable for being mounted on a roof. In background research, only one suitably wind-resistant roof-mounted multi-axis system could be found, described in patent US20110048406. This system, while appearing to be perfectly capable of resisting hurricane force winds by retracting flat into its frame, seems to be too mechanically complex for reliable operation; the large numbers of hinge joints would be susceptible to dirt and leaves causing them to stick, as well as corrosion from the elements. This impracticality perhaps explains why this system has yet to be manufactured.

Far simpler roof-mounted systems have been made. The system shown in patent US20100288062 is a good example, with the only moving parts besides the panel being a couple linear actuators and a brace. Our team’s overarching goal is to make a roof-mounted two-axis system with this level of simplicity, while providing the

http://sundogsolarwind.com/index.asp?q=solar-electric-solar-tracker

http://ledlightingmanagement.com/led-lighting-management/content/hurricane-rated-solar-dual-axis-tracker-large

http://www.megawattsolar.com/technology/components

http://www.google.com/patents/US20090101135?printsec=drawing&dq=solar+panel+wind+retract&ei=ozpjUIDeDKni0QHC94CABw#v=onepage&q=solar%20panel%20wind%20retract&f=false

http://www.google.com/patents/US20110048406

http://www.google.com/patents/US20100288062?printsec=drawing#v=onepage&q&f=false


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storm resistance of the complex roof-mounted system or the retractable foundation mounted system described earlier.

Design Metrics:

System should be able to withstand a 140 mph sustained wind speeds, and gusts of up to 180 mph (consistent with a Category 4 hurricane direct hit).

Power output should be at least 100W/sq. meter Should produce electricity with at least 10% efficiency if PV panels are used; 15% if thermal systems

are used. Tracking should use no more than 10% of produced power. Should be a better option economically than both existing trackers and weatherized fixed panels given

a strong hurricane frequency (strong enough to permanently damage non-ruggedized systems) of one every 10 years.

Final Solution

The final solution, as seen in the accompanying model, consists of several components that will allow for single-axis tracking and the ability to retract during non-daylight hours and dangerous weather conditions. It will also have the ability to flip over in order to protect the front side of the solar panel from debris while it is retracted.

The following is a list of important components and their functions:

Linear Actuator: Our linear actuator runs on 12V and has a maximum stroke length of 18” and a load capacity of 400 lbs. Its primary function is to raise and lower the arms which hold the panel in place.

Gear Motor: A 12 V gear motor is used for turning the panel while it is tracking the sun. The motor is mounted into a rectangular cavity in the arm.

Base Frame: The frame is made out of 80-20 aluminum due to its low cost and versatility for attaching brackets anywhere along the frame.

Locking System: magnetic locks that will together provide 260 lbs (1200 N) of locking force.

Circuitry: The final solution consists of three main circuits: o Light Sensor Controller: This circuit’s purpose is to conduct solar tracking during the panel’s

operation. o Motor Controller: There are two H-bridge motor drivers to control the linear actuator motor

and the gear motor movement. o Accelerometer Board: The 3-axis accelerometer circuit is necessary for setting the actuator and

the panel to the proper angle and position during both raising and retracting of the system, as well as adjusting the panel angle during passive tracking, or “cloudy” mode.


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Solar Panel: The solar panel itself is a monocrystalline panel which was chosen because of its relatively high energy conversion rates (15-20%) and its longevity (most manufacturers give it a 25-year warranty). The panel for our prototype is 150W.

Industry Standards: There are several industry standards on the installation, fire classification, wind resistance and materials that need to be met when designing a solar panel array. We will examine the standards from ANSI and the Solar America Board for Codes and Standards. The American National Standards Institute (ANSI) approved the 2009 Uniform Solar Energy code, which outlines the standard for the inspection, installation and maintenance of solar powered systems and components. There are standards for the installation of the PV modules and the shingles used as roof covering. This means that we will need to make our system compatible with these standards.

An important consideration will be to make sure that the fire classification rating is up to standard. As stated in the codes, the PV should not in any way diminish the minimum fire safety requirements for the roof. This is because roofs have always received fire ratings which are based on the roof’s ability to minimize the spread of a fire as well as the ability of a fire penetrating the roof. So our design will have to maintain a certain level of fire safety and should not diminish the fire safety of the roof in place.

In terms of installation, there are standards that the roof should constructed in a manner that the load imposed by the solar panel system does not cause the roof to suffer any structural damage. Although the building of the roof is not part of our task, we must make sure that the loads created by our system is small enough to not have any effect on an industry standardized roof.

OVERALL DESIGN

Design Process

Our team employed two main methods to narrow down and “flesh out” our solutions for the positioning subsystem. First, our team utilized FEA in order to determine if proposed solutions would be structurally sound in normal operating conditions (i.e., sustained winds less than 30 mph). Second, an analytical static force and moment analysis was performed on the retraction mechanism in order to determine if the proposed solutions would successfully be able to raise and lower the panel and panel arm.

FEA

Through the use of SolidWorks and other features such as the FEA and the Design Insight feature on SolidWorks the previous list of possible solutions was narrowed down through cost, weight, functionality and stress analyses to a general “outline” of a design.

This solution consisted of a linkage moved by a linear actuator to raise and retract the panel, and then a gear motor attached to the end of the linkage to rotate the panel through its range of angles. The panel was to be


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stowed away in inclement weather by retracting the actuator (once the panel has been rotated so it will be horizontal when fully retracted). To prevent the linkage, henceforth referred to as the “arms”, from blowing over, it was reinforced with a brace. This solution had the advantage of only using one linear actuator. It should also be easy to control, as the angle is set by setting the angle of the gear motor. An issue, however, was figuring out how to attach the gear motor to one of the arms.

To determine whether the system would break under normal operating weather conditions, FEA was used to simulate the stresses caused by these conditions. The FEA was performed in SolidWorks, conveniently using the existing SolidWorks models of the existing proposed solutions. For each of the proposed solutions, the forces applied were a wind force of 240 Pa (corresponding roughly to a direct incident wind speed of 45 mph, 150% of the planned retraction wind speed of 30 mph) and a panel weight of approximately 150 lbs. The panel arm, panel, and linkages were assumed all to be made out of aluminum. The FEA results (Figure 1) showed that the internal stresses in the panel arm, linkages, and joints were at most around 10 MPa, well below the minimum failure strength of aluminum, 55 MPa. Thus, it was concluded that this design would be sufficiently strong to withstand normal operating conditions.

Figure 1: Finite Element Analysis


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Figure 2: Design Insight Analysis

Figure 2 shows the design insight, which shows a continuous path between the loads and restraints. The translucent portion of the Design Insight plot shows where the load is carried less effectively than the solid parts of the plot. This means that the translucent portions can be removed if we are pushed by our weight constraints.

Modeling Actuator Loads

All of the proposed solutions used a linear actuator to raise and lower the panel (for more information on the specifics of linear actuators, see Section 2). Thus, in order to determine what force would be required of the linear actuator, the raising and lowering mechanism was simplified into a 2D linkage, consisting of the actuator, panel arm, and base, as well as two forces, panel weight, and actuator force, as shown in the drawing below:


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Figure 3: Retraction Mechanism

The main parameters are retracted actuator length Y, base mount distance D, panel arm mount point X, panel arm overhang Z, retracted panel arm angle ϴmin, and panel weight W. From this can be calculated the

maximum required actuator force F, and maximum panel arm angle ϴmax, which requires also calculating the minimum actuator angle ϴp, using the equations below:

F(min)=W(Z+X)cos(ϴmin)Xsin(ϴmin+ϴp) ϴp=Xsin(ϴmin)/Y ϴmax =arccosX2+D2-4Y22XD Using MATLAB, the ratio F/W was plotted as a function of Y, and ϴmax was also plotted as function of Y, for the maximum reasonable (without making the base too high) ϴmin of 10 degrees, and a value of X+Z of 2.5 ft, exactly ½ of the intended panel length of 5 ft. Combining these, a plot of F/W as a function of maximum attainable angle can be made.


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Figure 4: Actuator Force vs. Actuator Length

Figure 5: Actuator Length vs. Max Angle


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Figure 6: Actuator Force vs. Max Angle

The results showed that in order to minimize the force required by the actuator for a given max angle, the mount point should be placed as far up the panel arm as possible. However, the main finding here was that to raise the panel angle above 35 degrees (what may be considered a reasonable minimum to get a good range of motion along that axis), the force required by the actuator was about 1.6 times the weight of the panel. Given that the panel may weigh over 100 lbs., and we really wanted a much greater range of motion, it was clear that the 150lb force actuator we originally planned to use would be insufficient.

First General Design

The first prototype solution consisted of the design described above, but with added slits to the solar panel equal to the width of the linear actuator. These enabled the panel to rotate 180 degrees while retracting so the solar cells will be facing away from the sun while the panel is in its retracted phase. Adding this feature (seen in Figure 26 & 27) helps in preventing debris and heavy precipitation from damaging the solar cells on the panel. This addition will not affect the panel’s sun-tracking movements or any other features of the panel. The only thing that was lost was a small amount of area, which will decrease the amount of power that is produced. However, the lost power output did not outweigh the advantages of preventing damage to the solar cells, especially since this system is meant to last a long time and require minimal maintenance.


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Figure 7: First Full Solution Rendering

Figure 8: First Full Solution Rendering


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First Full Solution Specifics:

Section 1: Positioning Subsystem

The positioning subsystem performs two main functions. First, it physically rotates the panel about one axis in order to reach the desired panel angle commanded by the control system. Second, it lowers the panel in severe storms, and then raises it again afterwards. While these functions may seem separated, it turns out that the mechanical design of one function greatly affects the functionality of the other; thus, for design purposes they are considered one (very critical) subsystem.

Linear Actuator

A linear actuator is needed to raise and lower the arms, and in turn, the panel. It must have a long enough stroke length such that the panel can be fully retracted into the base frame. It must also be able to support the static and dynamic load of the panel, panel mount, and linkage without failure occurring.

We needed the actuator to be able to support the approximately 100 lb panel and be able to travel 2 ft. With the addition of torsion springs to effectively reduce the supported weight, we felt that Firgelli Automation’s FA-05-12-X” model actuator will effectively suit our needs1. It has a static and dynamic load capacity of 150 lbs., and it has a stroke length between 1” and 30”. Its speed at full load is 0.4”/sec, which is slow, but speed doesn’t matter as much load capacity. It utilizes a 12V DC motor, which is an easy voltage to provide with a standard power supply.

Single Axis Sun Tracking System

The tracking system was to be made up of several subcomponents, including solar sensors, a gear motor to rotate the panel, and a H-bridge to control the motor. The solar sensors are used to determine the position of the sun and convert light into voltage. These sensors will be partitioned by height. When the sun moves, this will cause a height difference between the solar sensors, and subsequently a voltage difference. The amplifier will measure out the voltage difference between the two sensors and will activate the motor through the driving circuit until the panel moves to a point where the voltage difference is near zero again.

For the phototransistor, we originally intended to use the infrared phototransistors available in the GM Lab, which is probably very similar to Vishay’s Silicon PIN Phototransistor2, which has a peak wavelength of 940 nm.

For a power supply we considered using Toshiba TA7267BP model3. It can supply voltage as high as 18V, and can operate under high temperatures. The gear motor we were looking to use should be 12V and relatively compact. Minebea’s PAN14 model4 would be one that fits that description.

1 http://www.firgelliauto.com/Tube%20Actuator%20Catalog%20Dec%202005.pdf

2 http://www.vishay.com/docs/83472/tefd4300f.pdf

3 http://www.toshiba.com/taec/components2/Datasheet_Sync//261/3592.pdf

4 http://www.nmbtc.com/pdf/motors/PAN14.pdf


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Single Axis vs. Dual Axis Tracking Analysis

After reading several reports on the analysis on the differences of efficiency, cost, and maintenance between the single and dual axis tracker, we decided to proceed with a single axis tracker. Within the solar industry there is an argument whether or not the added complexity of a two axis tracker makes the added cost of a two-axis tracker worthwhile. It is already known that the difference between a fixed system and a one-axis tracker of 24- 40% is a large enough increase in efficiency and power output to rationalize its implementation. But in the case of single axis vs. two axes some studies have shown to be as low as 10%. As some manufacturers claim this additional net energy increase is lost due to the additional cost, maintenance and increased risk of failure. Since we are switching to a single-axis tracker, we will only need one gear motor.

Figure 9: Efficiency Comparison

Section 3: Electricity Generation (Monocrystalline Solar Panels)

In order to determine our final solar panel type we took into consideration many factors such as space, power output, maximum efficiency and price per wattage. From the different choices of monocrystalline, polycrystalline and thin film, using these criteria we decided on the monocrystalline solar panel.

Monocrystalline solar panels are made from a single, pure silicon structure, and are considered to be the most dependable for producing electricity from the sun. One of the major benefits for this panel is its longevity as most solar panel manufacturers give it a 25-year warranty. It is also the most efficient panel as energy conversion rates are typically 15-20%. Monocrystalline panels also have an advantage over thin-film solar panels since they produce up to four times the amount of electricity using the same amount of panel space. Because space is a major consideration in our project this is a huge advantage for the project. The biggest


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limitation for us, within the scope of our project was that monocrystalline panels are the most expensive type of panels. Another limitation was that the circuitry is very sensitive to damage from dirt or snow.

Referencing the graph below, one can see the efficiencies of monocrystalline solar cells far of 27% surpass the efficiency of polycrystalline solar cells of around 20%. This consideration drove our decision to choose monocrystalline cells over polycrystalline cells. As we are confined by space on the roof, monocrystalline cells have a higher Watt of power output per panel than the polycrystalline cells. Since we are trying to create a more effective and efficient alternative to current solar tracking technologies, our panels should have the highest efficiencies possible within our cost constraints.

Figure 10: Solar Panel Type Efficiency Graph


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Section 4: Physical Storm Protection Mechanism (Mag Locks)

The evaluation method that we used to select our physical storm protection mechanism was based on comparing the power requirements, cost, and lifetime durability of the proposed mechanisms. After comparing a motor controlled mechanical latch and magnetic lock system, we decided to proceed with a magnetic lock system.

Magnetic locks use electrical currents to induce magnetic fields to provide a locking force. An armature plate would need to be implanted into the panel frame. Once the panel is fully retracted into the casing, an electric current in the casing would induce another magnetic field in the casing that would provide a very strong locking force for as long as the current is maintained. The main advantage to this proximity lock is the lack of moving parts which is fully optimized for long periods of weatherization. Since there is no physical latch, save the force field between the panel and the casing, there is no hinge that can rust. The principal drawback to electromagnetic locks is the power drained from supplying a constant current.

In order to reduce the maximum power consumption the maximum capacity was limited to 2 amps. We considered making magnetic locks from scratch by making our own electromagnets and armature plates. However, this was determined to be infeasible, and we decided to purchase magnetic locks instead. Several magnetic locks were to be used to distribute the locking force over the panel area in order to prevent the panel from warping under load.

Section 5: Base Design

The initial base design consisted of five main components: the frame, the windshield, the base plate, the panel arm bracket, and the fixtures. The frame provided the structural stiffness for the whole system; all components are attached to it directly or indirectly. The wind shield went on top of the frame and locks, and serves to protect the frame from corrosion, and most importantly, deflect wind around the panel itself when in the retracted position, in order to move stress from the mechanical components (which would be damaged due to the stresses caused by hurricane force winds) to the much more structurally sound frame. The base plate was to be attached to the bottom of the frame, and serve as a place to mount small components that don’t need to be mounted directly to the frame, such as circuit boxes, wires, and other non-force-loaded components. The panel arm bracket was to attach the panel arms onto the frame. It was to be placed either on the outside of the frame, as shown in the picture below (requiring a cut in the frame), or on top of the frame, as can be seen in some of the positioning system renderings. Finally, the fixtures are the components that attach the frame to the roof.

Shown below are two images of the proposed base solution without the fixtures, the first an “exploded” view, and the second, an assembled view. Looking at the exploded view (Figure 17), the wind shield is the curved piece on top, while the frame consists of the square tube, flat plate, and small bracket. As can be seen in the


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second, assembled view (Figure 12), the parts all bolt together to form a compact, low-profile base. Having a low profile was intentional, as this decreases the area exposed to wind gusts parallel to the roof. The large square hole in the center of the frame serves as a recess for the panel to retract into for stowage.

Figure 11: Base, Exploded View


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Figure 12: Base, Assembled View

We initially intended to make the base frame out of steel square tubing. Square steel tube is very resistant to torsion forces, and is lighter than an equivalently stiff frame made out of plate steel. Square aluminum tube was also considered, for weight reasons, but it was not available in as many sizes as steel square tube. This frame was to be painted or powder-coated to prevent rust. The frame could be made either by cutting pieces of the right length and welding them together, or bending the tube into the right shape.

The wind shield, due to its irregular curved shape and extreme exposure to the elements, was intended to be made out of some sort of high density plastic. Plastic can be formed into an aerodynamically optimum shape, and is resistant to corrosion. The optimal shape of the wind shield was to be determined later by computational fluid dynamics (CFD) analysis. We were originally planning to also test it in the wind tunnel, but wind tunnel testing abandoned since it would be impractical to test for failure in the wind tunnel without risking damage to the wind tunnel itself. CFD modeling was also to be used to determine the lift forces the panel can withstand without being driven off of the roof.

The panel arm bracket will just be a piece of angled metal bolted onto the frame having appropriate holes for inserting the bushings that will serve as a hinge for the panel arm. Depending on the final design of the positioning system, the panel arm could mount either on top of the frame, face outward from the frame, or face inward from the frame. Welding the panel arm bracket to the frame was considered, but rejected after seeing that welding was unnecessary for structural reasons, and would simply add unnecessary cost.


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The base plate would probably be also made out of some piece of high-density plastic. It could feasibly be made out of just a plastic sheet with holes cut out, removing the need for more expensive molding or casting. Large sections of the plate can be left out if they aren’t supporting any components; this also allows water to drain out of the system.

For fixtures, two main designs were considered: attaching the frame by screwing large lag bolts directly through the roof into the rafters (this is the standard technique for installing solar panels), or, to increase strength, running bolts through the roof to brackets attached to the rafters mounted inside the house, as shown in Figure 3 below. This, however, would greatly increase the amount of labor required, and increase the likelihood of roof leaks as a result of the installation. This decision was resolved by researching the rated strengths of lag bolts in wood. For a 5/16” lag bolt embedded 3” into a wooden rafter, the rated pullout strength is a minimum of 615 lbs. per screw (Reference: http://www.solarpanelstore.com/pdf/SnapNrack-corru_block.pdf); for four screws, this would be equal to a pullout force resistance of more than a ton, far more than the 875 lbs. of lift maximum that it is expected the panel would face. Given this, it is safe to assume that lag bolts will be sufficient to hold the panel place in hurricane conditions. For actual installation into a roof, of course, proper flashing and sealant would be used around the holes to prevent leaks.

Figure 13: Base Design Sketch

http://www.solarpanelstore.com/pdf/SnapNrack-corru_block.pdf

http://www.solarpanelstore.com/pdf/SnapNrack-corru_block.pdf


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Final Design Modifications

Due to financial constraints and intermediary testing, the design had been modified and completed during the the last semester. The updated design, as seen from Figure’s 14, 15, 16, and 17, retains most of the same features of the old design, but contains some minor improvements.

Figure 14: Full Design, Isometric View


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Figure 15: Full Design, Top View

Figure 16: Full Design, Side View – Panel Down


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Figure 17: Full Design, Side View – Panel Up

The following is a list of the specific changes to the design since early January:

Linear Actuator

Our initial design consisted of using Firgelli Automation’s FA-05-12-24 model actuator with 12V input voltage, a stroke length of 24” and a load capacity of 150 lbs. With that actuator, we would have had to attach torsion springs to the frame to reduce the load of the arms and the panel that the actuator would have had to endure. There are a couple of complications that surfaced while we were researching different springs. The first is that the edges of panel could possibly come into contact with the torsion springs while the panel is retracting and flipping from its panel-side-up configuration to the debris protected panel-side-down configuration. The other issue is that the torsion springs with our required radius and spring constant would have cost close to $100, which we couldn’t afford with our budget.

To avoid having to complicate our system with torsion springs, we decided to switch our actuator to Firgelli Automation’s FA-400-L-12-18 model, which still has a 12V input voltage, but also has a stroke length of 18” and a load capacity of 400 lbs. With the increased load capacity, the actuator comes really close to providing enough torque to lift up the arm in the worst-case scenario (3600 in-lbs). And this actuator ($130+shipping) is a lot more cost effective than the old actuator with the torsion spring ($220+shipping). To account for the shorter stroke length, we adjusted the length of the extruded piece of 80-20 aluminum from the base frame so that the actuator and arm angles required for movement could still be achieved.

Arm Design

In the initial design, the arms’ only function was to serve as a means to connect the linear actuator to the solar panels so they can be raised and lowered as well as supported. However, it became apparent that the arms would be the best place to attach the gear motor since attaching it anywhere else would cause large stresses on the structure, especially if it were attached to the base.


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To create the gear motor attachment, we cut a rectangular hole in the arm for the gear motor to slide into and to be held in place by brackets on both sides of the arm. This design is much more stable than a design that simply attached the motor to the side of the arm because then a counterweight would have been necessary to offset the off balance weight on the arm, and that would have added even more weight that the actuator would have had to support.

Figure 18: Arm with Motor Attachment Space

Arm-Panel Frame Attachment

After figuring out how to effectively mount the motor to the arm, the next step was to determine how to attach the motor to the panel frame. Directly attaching the motor to the frame would cause too much stress in the gearbox, so we designed a chain drive mechanism to connect the motor to frame. This design is especially advantageous because not only will we be able to reduce the stress on the motor, but we can also adjust the position of the motor on the arm thereby tuning its center of mass.


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From Figure 19 below, the lower sprocket is attached directly to the motor shaft and the upper sprocket is attached to a shaft that holds the two arms in place. The rectangular pieces attached to each end of the shaft will be attached to the angle section on the panel frame. The large shaft is held in place by two circlips (not pictured) located at the inner face of the large bushings the shaft rotates in.

Figure 19: Complete Arm Assembly Figure 20: Exploded Arm Assembly


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Panel Frame

New to the design is the panel frame that will be used to hold the panel in place and be the attachment point for the connection to the arm. The frame will be made up of three 26” long pieces of 1” x 1” x 1/8” thick aluminum square tubing that will be held in place by a 48” long section of 3” x 3” x 3/8” aluminum angle. Each of these pieces will be bolted into the hub plate. Pictures of the panel frame can be seen in Figures 30 & 31 in the “List of Components” section.

Base Design

Although our design specified welded steel tube frames for the base, for prototyping purposes we decided to use 80-20 extruded aluminum due to its ease of use (no welding required) and cheaper cost to prototype (steel tubing would be cheaper for full-scale production).

List of Components

Linear Actuator

Our 12V linear actuator, with maximum stroke length of 18” and load capacity of 400 lbs, arrived on 2/28 and it meets our required specifications. The only issue with it is that when it reaches the maximum stroke length, it remains stuck in that position and is not able to retract to a lower stroke length. During testing and operation, we will have to be careful not to let the actuator reach within ½” of its maximum length.

Gear Motor Figure 21: Linear Actuator

The gear motor we ordered is a 12V DC motor with a torque value of 5.5 N-m. The gearbox is flush with the motor, which makes it very easy to mount it to the arm. The motor was purchased from McMaster-Carr.

Figure 22: Gear Motor


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Light Sensor Controller

This circuit’s purpose is to conduct solar tracking during the panel’s operation. The tracker has three modes (sunny, cloudy, and night) to calibrate it according to the weather and the time of day. At the moment, the circuit utilizes phototransistors as the light sensor, but we will make the seamless switch to CdS photocells for our final model. CdS photocells are better than phototransistors for outdoor use because their peak wavelength sensitivity (540nm) is in the visible light range, whereas the phototransistors’ peak sensitivity (940nm) is in the infrared range. The response time is a little slower than phototransistors, but since the sun moves very slowly, sun tracker response speed is not a major concern. Each light sensor has utilizes two trim potentiometers (2MΩ and 100KΩ) for both coarse and fine sensitivity. The output from each sensor is then buffered before being fed to the microcontroller inputs, allowing the use on pull-down resistors on the microcontroller inputs to reduce noise.

Figure 23: Light Sensor Circuit Diagram


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Figure 24: Light Sensor Tuning and Buffer Circuit

Motor Controller

We have two H-bridge motor drivers to control the linear actuator motor and the gear motor. The driver for the actuator was custom-built and can handle up to 10 amps, and the driver for the gear motor was bought (eBay link here) and can handle up to 4 amps. The actuator driver circuit uses four MOSFETs to form the H-bridge, and a fifth to allow for PWM speed control. The H-bridge also uses an inverter to use one binary microcontroller output to switch directions, and two small NPN transistors to amplify the 5V logic signals to the 12V necessary to control the H-bridge MOSFETs. All MOSFETs will have heat sinks for cooling purposes. Each circuit was fabricated or procured by 3/10.

Figure 25: Linear Actuator Driver Diagram

http://www.ebay.com/itm/L298-Based-Stepper-DC-Motor-Driver-Board-Arduino-L298N-Controller-Module-USA-/150765336052?ssPageName=ADME:L:OC:US:3160


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Figure 26: Linear Actuator Driver

Accelerometer Boards

The 3-axis accelerometer circuits are necessary for setting the actuator and the panel to the proper angle and position during both raising and retracting of the system. They offer easy connection to other circuit components, along with convenient mounting holes for mounting these accelerometers to the panel frame or actuator. A LED lights up if the accelerometer has power. These circuits were completed on 3/10.

Figure 27: Accelerometer Circuit Diagram Figure 28: Accelerometer Circuit Boards


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Control Box

The control box serves as an enclosure for the microprocessor board, the two H-bridges, the light sensor sensitivity tuning board, sensor inputs, and the control switches. The enclosure itself was bought from Radio Shack, and is about 8” x 6” x 5”. The various circuit boards are attached with small screws, as is a terminal block that ties together all the high-current wires, including the power supply output wires, the H-bridge input wires, the H-bridge output wires/motor and actuator wires, and the magnetic lock output wires. BNC connectors are used to connect the light sensors, gear motor, and magnetic locks to the enclosure, allowing for quick disconnection. The six switches are all toggle switches, two of which are double-throw to switch between REV-OFF-FWD for the actuator and gear motor in manual mode. The power and three manual mode switches all have indicator LED’s above them, while the two mode selection switches do not. There are also two potentiometers to control the actuator and gear motor speeds while in manual mode.

Panel Arms

Both arms were machined out of aluminum using the ProtoTrak in Penn’s Machine Shop. The arm on the right in Figure 29 is ½” while the arm on the left is slightly thicker at 5/16” thick. This is because there are threaded holes for which to attach the motor that require an extra layer of thickness for the screws to fit. Both arms are 32” long.

As seen from Figure 29 below, we added some slots next to the rectangular area where the motor will be placed. This gives us a little flexibility for position the motor within that area.

Figure 29: Panel Arms


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Panel Frame

The panel frame is made up of aluminum square tubing attached to aluminum angle sections with 5/16” bolts put through holes drilled in both pieces. The panel frame is

Figure 30: Panel Frame Angle Sections Figure 31: Panel Frame Rendering


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Hub Plates Figure 32: Hub Plate

The hub plates are used to help connect the panel frame to the arm. Specifically, this piece will be attached to the frame L-brackets and the shaft. This piece is made of a 5”x 5.5” piece of ¼” thick aluminum. The hub plates were fabricated on 3/18.

Shaft Figure 33: Shaft

The aluminum shaft is the piece that holds the arms together and holds the upper sprocket in place. The indented area on the shaft indicates where the upper sprocket will be held in place using set screws. The ends of the shaft are where the pieces that connect the arm to the panel frame will be attached. The shaft was fabricated on 3/15.


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Chain Drive Mechanism

The chain drive mechanism consists of two finished-steel bored roller chain sprockets and a single-strand roller chain. The sprocket attached to the gear motor has a ¼” pitch and ¼” bore. The sprocket attached to the shaft has a ¼” pitch and a 1” bore. The chain is 2.5’ long. The configuration is shown in Figure 34 below.

Figure 34: Chain Drive Mechanism Setup

Base Frame

The base frame will be made up of four 62” long pieces of aluminum extruded 80-20 fastened together in a rectangle. We tapped 5/16” holes on both ends of two of the 80-20 pieces so we can attach T-slot fasteners to the ends of those 80-20 pieces. These holes were tapped on 3/12.


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Figure 35: Extruded 80-20

Figure 36: Fasteners attached to 80-20


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Electromagnetic Locks

The locking system is run on two magnetic locks that will together provide 260 lbs (1200 N) of locking force. The locks have been tested to draw a current of 0.29 amps at 12 volts with a net power draw of 7 Watts while both are engaged. The locks will be attached to a raised surface and the ends of the panel frame angle sections, as seen in Figure 39.

Figure 37: Magnetic Lock Figure 39: Magnetic Lock Attachment

Solar Panel

Our system is suited to operate with two standard-sized 150 W solar panels, which are attached to a frame made up of aluminum square tubing and a length of aluminum 3”x3” standard angle. Designing the system around standard, off-the-shelf panels helps to decrease the cost of the system.


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TESTING/PROTOTYPING RESULTS

First Prototype

The first prototype was a small-scale version of the final prototype and was made primarily out of press-fit MDF. The base was 18”x20” and the surrounding walls were 4” tall. The wall height was chosen arbitrarily and does not reflect the proportional height of the base frame we are aiming for in the full-scale prototype. The motors used were initially 30 mm circular gear motors that were mounted to the arms and held together by motor holders that were press-fit into the panel. We borrowed a linear actuator with a maximum stroke length of 6” for this prototype. The purpose of making this prototype was primarily to test geometries to see if the panel and raise and retract in the way we had designed it to. Figure 40 below displays the first version of the first prototype. Figure 41 below displays the breadboard circuit used to manually control the movement of the actuator.

Figure 40: First Version of First Prototype Figure 41: First Prototype Circuit

The first version of this prototype did not work because the central portion of the panel would interfere with the arms while the panel was retracting. We learned from this that the actuator-arm attachment point had to lower down on the arm to avoid this interference. To fix this issue, we changed the design of the arm in an attempt to avoid any interference. This change is exhibited in Figure 42 below.

One successful aspect of this version of the prototype was that the circuit for manually controlling the actuator worked really well, and that it wouldn’t be too difficult to take the next steps to control the actuator digitally.


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Figure 42: Second Version of First Prototype

From this prototype, we found that there was still interference between the arm and the panel, so we redesigned the full-scale second prototype to not have a central portion to connect the panel, and essentially have two solar panels with one connected to each arm. We were always going to use a right angle gear motor, like the one in Figure 22, instead of the motor we used in this prototype. However, from conducting tests with this prototype, we learned that we couldn’t place the motor at the top of the arm because of potential interference with the panel and uneven load distribution. This led us to redesign the arm of the full-scale prototype to include a way to attach the motor somewhere in the center.

Light Sensing Circuit

Two tests were done to see if the light sensing circuits were working correctly. The light circuits were first prototyped on a breadboard. Afterwards, in the first test, we checked to see if it switched between the sunny, cloudy, and night modes based on the light levels on one phototransistor (in the actual prototype, the phototransistors would be replaced with the LDRs). We did so by moving a light source (a diffuse battery powered night-light) near the mode-switching phototransistor, and saw that, with the calibration settings, it read "sunny" when the light was within about 2 inches, "cloudy" when within 6", and "night" when farther than about 6" away. The current mode was read from outputs from the microcontroller to a PC terminal.

In the second test, we tested controlling the motor with the two directional phototransistors by having them about 3" apart and moving the light source along the line between the two. It successfully switched the direction of the motor depending on the position of the light, and switched off the motor when within about 1/4" of the center.

Actuator and Gear Motor Drivers


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The tests done with the H-bridge motor drivers were to see if they could produce the correct motor speed and direction from the outputs of the microcontroller, at the expected motor voltage of 12V. The purchased L298-based H-bridge used to drive the gear motor worked correctly the first time. However, initial tests found that the home-built actuator H-bridge failed at voltages above 8.4V, due to the difference between the 5V logic voltage used by the inverter and the power voltage. This was fixed by adding amplifying PNP transistors to the direction logic outputs.

Retraction and Raising Test (Accelerometer)

There were significant issues with the accelerometer when initially tested in the full-scale prototype. Very often the accelerometer would stop functioning when the gear motor turned on, causing the panel to rotate past the desired position when retracting and lowering. When in manual mode, turning on the gear motor would often cause the system to “freeze” and become unresponsive. It was determined that the motor was generating noise and voltage spikes that caused the accelerometer to “freak out”. This issue was fixed by creating separate power sources for the logic (M2, accelerometer, sensors) and the electromechanical components (actuator, gear motor, magnetic lock), a 4-AA battery pack for the former, and the existing big power supply for the latter. The grounds for these two inputs were tied together far back in the power supply, not in the control box, in order to maximally separate the gear motor noise from the accelerometer. After this fix, the system correctly raised itself to the proper angle and rotated the panel into position, and then did the reverse for retraction.

Light Sensor Calibration

The light sensor sensitivities were calibrated for indoor and outdoor use. This will involve two steps. First, the sensitivity trim potentiometers were adjusted so there is a noticeable voltage drop between sunny conditions and cloudy conditions. Second, based off of the measured difference between two panel light sensor values for various angles relative to the sun, it was determined whether the difference between the two sensor values needs to be amplified with a differential op amp, in order to produce a noticeable difference in the panel sensor values for even a very small panel-sun angle. Once light sensors are calibrated, the mode and position lock threshold values are adjusted in the code accordingly.

Integrated System Testing

After testing each component individually, we performed an integrated test which tested the system’s retraction and deployment systems under tracking situations. We performed the manual mode test, where the user flips the switch and the system performs the retraction procedure enabling the locking system. Next we performed the manual deployment procedure to return the panels to its zero state location. In order to perform the fully automated tests we calibrated the light sensors were recalibrated for indoor use and deployed the system. Using a flashlight we performed successful integrated tracking tests.

Full Mechanical System-Circuit Testing


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Upon verifying that the integrated circuit system testing worked according to our specifications, we moved towards building the full mechanical assembly and testing that with the circuitry. The complete assembly can be seen in Figure 43 below:

Figure 43: Full Mechanical Assembly

While we were testing, we came across a few obstacles that almost severely hindered our progress. The first issue was that our initial gear motor broke during testing. This gear motor, which is not the same one that we used in the final assembly, can be seen in Figure 44 below:


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Figure 44: Initial Gear Motor

This gear motor is a 12V DC motor with a torque value of 4.91 N-m. While we were testing the motor driver with the full assembly, the edge of the solar panel hit the floor, and the gears in the gear motor sheared off while it was trying to rotate the panel against the ground. The reason the panel was in a position to hit the ground was because we were testing the motor driver at the incorrect linear actuator angle. The linear actuator was at about half the stroke that it would usually be at during operation. This was an experimental error on our part. This accident occurred about ten days before the MEAM Senior Design Demonstration Day, and because the motor we were using had been ordered from China, there was a very low probability that we could get the same motor delivered in time for demonstration. To solve this dilemma, we found a McMaster-Carr gear motor that had similar power and torque ratings, could fit our arm design, and could be delivered via overnight-shipping. This gear motor, seen in Figure 22, actually worked a lot better than our previous gear motor since it had a slightly higher torque value of 5.5 N-m. We had no issues after using this gear motor.

The second obstacle involved the use of the accelerometers with the full assembly, which is detailed in the “Retraction and Raising Test (Accelerometer)” subsection above.

Besides those obstacles, everything else seemed to work according to specifications. The linear actuator worked well with the actuator driver and had no trouble lifting the weight of the panel. The motor driver moved the new McMaster-Carr gear motor easily, and the chain drive mechanism was able to rotate the panel without any issues. We were able to move both the linear actuator and the gear motor seamlessly under manual control.

After fixing the noise issues with the accelerometer, we were able to complete a full retraction in a little over a minute without the panel obstructing the ground or the panel frame. We were also able to successfully conduct light-sensing tests with a fluorescent light source. The next step would be to see if the light sensors can calibrate sunlight and track the sun during the day.


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PROPOSED IMPROVEMENTS/LESSONS LEARNED

Electronics Improvements

Gear Motor & Actuator: Currently, the electromechanical components for the system are not waterproofed, as they would need to be to be used outside. The linear actuator can be taken care of easily by simply using a linear actuator manufactured with higher water resistance rating, such as IP66 or IP67 (as opposed to the current actuator, which is only rated IP43). Such fully waterproof linear actuators are not hard to find (one made by Firgelli can be found here), although they do add to the actuator price. To further protect the actuator from debris and dirt from getting inside the mechanism, the actuator should have an accordion-like boot like the one pictured in Figure 45. Any linear actuator used in the future should at least be a different model than the (probably) Chinese one we used, which suffered from manufacturing defects like misaligned mounting points and malfunctioning limit switches.

Such is not the case with the gear motor however, as IP66 or higher gear motors are not as readily available (and very expensive when they are). To address this problem, Figure 45: Waterproof Linear Actuator

it is proposed that the gear motor be entirely enclosed with a hard plastic cover, which will be attached to the arm. A rubber gasket between the plastic cover and the arm will ensure that seal remains waterproof. To prevent water from reaching the motor from the other side of the arm, the motor would be mounted to the arm with just five close-fitting holes, instead of five slots: four for screws attaching the motor, and one with a bushing or bearing for the motor output shaft. With proper fits and seals on these holes, no water should get through them to the motor. Of course, the ability to slide the motor to adjust tension will be lost, so the hole placement and chain size will have to be more carefully thought out to ensure proper chain tension. The gear motor should also be slightly more powerful, in order to be able to rotate the panel in high winds. Sensors: The sensors in the future will need better than the makeshift ones used so far. The accelerometer and the panel light sensors will need waterproof enclosures (probably plastic) that are bolted directly to the panel frame, while the ambient light sensor will need a similar enclosure sturdily mounted to the base. The light sensors’ enclosures will need to have a clear “window” for light to shine through. The cables to the control box should enter these sensor enclosures through Amphenol-type connectors (female on the enclosure, male on the cable ends), which provide quick-disconnect capability for facilitating assembly and maintenance, while maintaining the waterproof seal of the enclosure. To save on costs, the accelerometer used by our team (a mIMU) could easily be replaced with a cheaper alternative accelerometer that doesn’t have gyros or a magnetic field sensor. To increase the reliability of the panel sensor readings, it may be necessary to have four sensors instead of two: one on the top and bottom of each of the two panels. Magnetic Lock: Based on testing results showing the lock to be stronger in tension than in shear, the lock mount should be changed so that the lock faces up instead of sideways, with the armature plate mounted to

http://www.firgelliauto.com/product_info.php?cPath=93&products_id=323


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the outer face of the solar panel with some sort of machined bracket. The contacting faces of the magnetic lock and armature plate will need to be rust-proofed, probably by some sort of surface coating, as using stainless steel would affect the magnetic properties of the lock. The hole in the lock where the wires enter should be sealed to prevent moisture from entering and causing corrosion or short circuits. Like with the sensors, to assist with assembly and maintenance the cable from the magnetic lock to the control box should be able to be disconnected, by use of a waterproof plug-socket assembly. Linear Actuator Driver: To improve performance and reliability of the linear actuator, the homemade H-bridge used as the linear actuator driver should be replaced with a commercially available 10A H-bridge. A commercially available H-bridge will not only have better safety features and heat dissipation, but also may eliminate the need for an inverter on the control board (although the inverter would probably be eliminated by other improvements described in the control board section). Commercially available H-bridges would also be much smaller than the homemade one, while only adding about $10 to the price of the system. Control Board, Switches, Indicators: In a final setup, the control board (the board that contains the microprocessor, sensor inputs, control inputs etc.) would be made on a custom-printed PCB rather than a breadboard or protoboard. For compactness, the light sensor sensitivity adjustment board would be merged with the control board. The hex inverter used for direction control could probably be eliminated from the circuit, by allocating two M2 outputs each to actuator direction control and gear motor direction control, instead of the one each currently used. This would restrict the number of inputs available for manual control, so the manual speed controls will probably have to be eliminated; these were hardly used in testing anyway, so this won’t be a significant issue. For a final production model, the microcontroller itself could be incorporated into the PCB, to save on weight and costs, but for next revisions it should be left as a separate component (so it can be switched out if “fried”). Switching between the modes should be done with a knob with four detents rather than the two switches currently used, as this is more intuitive. There should also be a one-digit display incorporated into the system displaying the current mode number. For a final rooftop-mounted model, while the control box will be inside the house, the manual control switches would need to be on the roof, so long connector cables will be necessary leading from the switches to the control box; a small enclosure would thus be needed for those switches, including a cover for the switches. Control Box: The control box, currently made from cheap plastic with roughly-drilled holes, ought to be replaced with a sturdier, smaller (the smaller circuit boards will allow for this) control box, with the holes for switches and connectors drilled using a milling machine to ensure proper alignment. The main power and linear actuator wires ought to connect to the control box via a plug and socket, instead of to a terminal block inside as they do currently, to allow for easy disconnection; heavy gauge Molex connectors would work well for this. Cable Management: Clearly, the current system of taping the various cables to the arm, panel, and base needs to be improved. The magnetic lock and actuator wires should be routed through the base to keep them from interfering with anything and reduce their visibility. The light sensor wires should be routed through the inside


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of the frame along the inside edge of the panel, also reducing their visibility. All the cables running down the arm (light sensors, gear motor, accelerometer) should be grouped into one “bundle” that is attached securely to the arm with metal clips in a way that eliminates interference while preserving enough slack for the panel to rotate freely. Many of the cables themselves will need to be made thicker for durability reasons. Special care should also be paid to make sure the plastic in the outer sheath of the cables doesn’t degrade over time in sunlight. All of the cables for the system will then need to be grouped and passed through a hole in the roof to the control box in the house. Power Generation/Storage: Right now, the solar panel isn’t connected to anything. In the future, the solar panel wires should be extended into the house, where inside they could be connected to both a standard solar panel grid tie-in system as well as a bank of batteries for storing energy. These batteries should be able to power the movement of the solar tracker system if the grid power goes out (a transfer switch would be able to tell if the power went out), with the solar panel able to provide sufficient energy over the course of the day to indefinitely maintain sufficient power in the batteries (in case the power didn’t come back for weeks, or if this was deployed in an area with no grid). Gel or Absorbed Glass Mat (a good description can be found here) batteries would be the best for such a system. The transfer switch will have to be custom designed given the unusual application, but probably won’t consist of much more than a voltage sensor and a couple of relays. Chains, Sprockets, and Shafts Chain Cover: To protect the chain and sprockets from getting gummed up with dirt and debris, they should all be protected with a single, long cover made out of either sheet metal or plastic, with a hole in one end for the main shaft to pass through. Unlike the gear motor cover, this cover doesn’t need to be waterproof, just sufficient to keep large debris out. The cover will also serve to prevent heavy rain from washing the lubricant

off the sprockets and chain. The cover would probably be attached to the arm with machine screws. Figure 46: Improved Chain-Sprocket System

Shafts: To take lateral stress off of the motor shaft, it would be helpful to extend the length of the motor shaft, and then have the end of the shaft pass through a bracket (via a bushing or bearing) in turn attached to the arm. This will keep the motor shaft parallel to the main shaft even under load, in turn taking stress off of the internal gears of the gear motor. The aforementioned chain and sprocket cover would fit over this bracket. It was intended to use circlips to keep the main shaft from sliding axially for the demo prototype; they were not installed, however, due to time constraints. These ought to be installed by cutting appropriate grooves in any future revisions.

http://www.freesunpower.com/batteries.php


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Lubricants: The lubricant for the chain, the main shaft, and the arm pin joints should be changed from heavy lubricant oil to some type of grease. The actuator-arm joint and actuator-base joint, currently unlubricated, should also be greased to prevent corrosion and binding. Since it will be difficult to re-lubricate those joints once the system is on the roof, using a grease that will stay in place offers a big advantage over a oil that will need to be reapplied regularly. Greases also resist being washed out by rain much better than liquid lubricants do. The added friction of grease over oil is of little consequence, since all the components always rotate very slowly, minimizing the viscous friction. Grease from the Mobilith SHC series meant for marine applications should be a good choice. Using grease will also minimize washout from diving rain, better maintaining performance after storms. Actuator Joints and Arm-Base Joint Actuator Joints: Both actuator joints are pretty crude in the current model, and should be refined. For the actuator-arm joint, the threaded rod should be replaced with a proper turned shaft for lower friction, with shaft collars or circlips to keep the actuator from sliding axially along the shaft, which could cause binding. The shaft could be threaded for about 1 ¼” on the ends to allow it to be held in place by nuts. This joint should be greased with the same grease as that used for the main shaft and chain. If there is too much friction still in this shaft’s rotation, the threaded rod ends could be replaced with press-fit bearings to allow the actuator-arm shaft to rotate relative to the arm. For the actuator-base joint, the current pin should be replaced with one that is a tighter fit to remove the slop in that joint. Of course, both of these fixes are dependent on replacing the current actuator with misaligned mounting holes with one with properly aligned mounting holes Arm Joints: The current arm-base joints, consisting of shoulder bolts and bushings, allow too much play in that joint, allowing the arm to twist slightly. This should be fixed by replacing the shoulder bolts with a shaft that is long enough to pass through both mounting brackets, turned to a diameter that is a closer match for the bushings. The new shaft and bushing diameter should be larger than the current 5/16” in order to reduce the risk of bending the shaft (probably ½”). To hold the shaft axially, it could be press-fit into the brackets, with the bushings moved from the brackets to the arms. If both arm bushings’ flanges are pushed up against the brackets, the arm will be constrained from moving side to side without adding too much friction. The arm-base joint should, of course, be greased. Figure 47: Arm-Base Joint Figure 48: Panel- Shaft Attachement

http://www.exxonmobil.com/MarineLubes-En/products_auxiliary-machinery_greases_mobilith-shc-460.aspx


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Panel Frame and Arms Weight Distribution: With proper adjustments to panel frame length, arm length, and base dimensions, the system could be redesigned such that the panel is centered about the main shaft, rather than being off-center is the case currently, eliminating the need for counterweights at the other end of the frame. This will not only reduce material and machining costs, but also reduce the load that the actuator needs to lift. Better vertical weight distribution can also be achieved, by figuring out the panel assembly’s precise center of gravity and then attaching it to the shaft such that the panel frame’s center of gravity lines up with the shaft center. This would remove the need for the current design’s slotted plate that allows the panel frame to be moved up and down slightly so one can “guesstimate” the panel assembly’s center of gravity; the shaft collar could directly connect to the angled aluminum at the appropriate height. Panel Frame Stiffness: To improve frame stiffness and to prevent the square tubes from loosening due to vibration, the tubes should be welded to the angle aluminum piece in a production version. The angle aluminum can be reduced in thickness to ¼”, from the current 3/8”, to save weight since the extra strength isn’t necessary. Replacing the angle with stiffer C-channel aluminum would save even more weight, made possible by the fact the square tubes can be welded from their ends (whereas they can only be bolted through their sides). To protect the backside of the panel from debris when retracted, some sort of thin metal or hard plastic plate the same dimensions as the panel should be sandwiched between the backside of the panel and the square tubing when the panel is bolted to the tubing. To remedy the issue of the panels twisting relative to each other, there should be more braces between the two panel frames at the magnetic lock end. Braces should be at several heights on the angle or C-channel aluminum in order to constrain the panels in multiple axes, ensuring they remain parallel. Arm Braces: Although the current arm braces work well, they don’t look professional and wouldn’t do for a production model. The threaded rods and 8020 piece should be replaced with turned rods with necked ends that have tapped holes in the ends. The length of the larger diameter section of the rod would be the desired distance between the two arms, and the necked ends would fit snugly in holes drilled into the arm. Pressed against the inner faces of the two rods and multiple points, the arms will stay parallel. The rods will be held


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in place by bolts and large washers screwed into the ends of these rods. Figure 49: Arm Braces

Corrosion Prevention: To prevent galvanic corrosion from dissimilar metal contact between the aluminum frame and arm components and stainless steel fasteners, the screws should be anodized and/or appropriate plastic spacers or sealants should be used to prevent the fasteners’ steel from coming in direct contact with aluminum. If the system was to be used somewhere right by the coast where salt spray is a concern, a zinc sacrificial anode may need to be used. Although not necessary from an engineering standpoint, from an aesthetic point of view it would be helpful to powder-coat the arms and panel frame for a production version. Base Wind Shield: The base in its current form is missing a critical component, the wind shield, due to time and cost constraints. Our intended design for the wind shield was to make it out of fiberglass, formed over a curved wooden frame to give it the appropriate shape. Although difficult to work with, fiberglass would be an ideal material given its light weight, durability, and resistance to corrosion, as evidenced by its widespread use in boat construction. The intended shape of the wind shield can be seen in the rendered images of the ReTracktor system. The wind shield would simply bolt onto the base frame. Base Frame and Spring: For any high volume production, the base frame should be changed from expensive yet easy to work with 8020 to welded square or rectangular steel or aluminum tubing. This will not only save on material costs, but also be stronger since the frame would be one solid piece. This would make the need for the plywood base in the current prototype unnecessary, as the box tube frame would be stiff enough on its own. The actuator assist spring would then be supported by an extension of the metal frame similar to the frame part supporting the magnetic locks. This would solve the issue encountered with the current prototype where compressing the spring would deform the plywood supporting it. The spring also needs to be longer in order to provide assisting force through a greater angular range, as currently the actuator now often doesn’t have enough power to raise the arm once the arm has been raised up enough to uncompress the spring. Additional Features Anemometer: Besides the power generation, which was discussed in the electronics section, the feature our team most would have liked to have included is an anemometer connected to the microprocessor to monitor the wind speed. The design would have been based off of the instructions found here, except using a generator instead of a Hall effect sensor, in order to take advantage of the analog-to-digital converter in the M2, and save the need to write software to measure pulse frequency. The anemometer would be mounted at least 10 feet away from the panels to make sure the anemometer is never in the wake of the panels, which would give an inaccurate reading. The anemometer would be connected to the control box by the same cables and connectors used by the light sensors. Internet Weather Input: A useful feature would be to have the system receive storm warnings via the internet, and automatically retract the panel if a storm is imminent. This would add a lot of complexity to the circuitry,

http://www.hackersbench.com/Projects/anemometer/index.html


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however, requiring much higher-level programming and a much more expensive processor. Designing this would be beyond the programming and circuit design experience of our design team. Automatic Light Sensor Calibration: This would be a feature added into the software allowing the system to automatically adjust the light sensor sensitivities based off of the readings from a standard light source centered between the two light sensors (bare 100W incandescent bulb 20’ away, for example), to the correct levels needed to ensure tracking accuracy. This would entail adding some sort of calibration switch, and a lot of experimental testing to find an ideal “standard light”.

REQUIREMENTS COMPLIANCE

Structural Integrity

In order for our tracking system to be in compliance with industry best practice, our customer requirements stipulated that we fulfill all building code requirements for structural integrity and fire safety. For building and fire codes we used the 2009 Uniform Solar Energy Code, which has been approved by the American National Standards Institute (ANSI) as our benchmark. These requirements outline widely used standards for installation and maintenance of solar powered systems and components. Because our plug and play tracking system utilized photovoltaic cells that have already been approved for roof mounting, our fire-safety requirements mandated that the additional electrical components in our tracking system not increase the roof’s ability to spread fire above code requirements of the International Building Code (IBC).

Following building code requirements in section 1509 of the IBC regarding roof installations, our system cannot be installed within 20 feet of property lines in order to maintain the appropriate fire separation distance between roof-mounted mechanical components and adjacent buildings. There was some concern from a prototyping standpoint of the fire safety aspects of our circuit design, which displaced a large amount of heat. This was not regarded to be a significant risk in any production model since a fully enclosed printed circuit board would be used in any scaled manufacturing run. The use of closed printed circuits in manufacturing for any full production model would generate a significantly less heat while drawing less power and would not be expected to present additional fire-safety implications for our system.

Our weatherization metrics mandated that our system be able to withstand 140 mph winds with temporary gusts of 180 mph while retracted, which is consistent with a direct hit from a category 4 hurricane. The customer requirements for performance stipulated that our tracking system not consume more than 10% of power output with a photovoltaic efficiency of at least 10%.

In our initial schedule we had intended to perform failure testing by suspending our system upside down and hanging weights in order to simulate the pressure and force effects of large wind loads since we did not have access to a large enough wind tunnel. These tests had to be foregone due to unforeseen prototyping delays during the integration between the control drivers and motors. Despite our inability to pursue field testing, we did perform finite element analysis on our system using the lift forces determined from aerodynamic theory and CFD modeling and determined that the maximum stresses expected on our aluminum frame while


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retracted was around 15Mpa, which was an order of magnitude lower than the aluminum tensile strength of 55-240 MPa. This gives us confidence that our design is significantly strong enough to withstand the required wind loads in the customer requirements.

Economic Performance

The most important customer requirement with this product is, of course, “Will it save me money?” To figure this out, we can compare the capital cost present values of two scenarios, for a 25 year maximum panel life and 25 year study period. The first scenario is the installation of one 300W ReTracktor unit, and the second is the installation of 300W of traditional solar panels. We will assume that the production version of the system will be about $150 less than the cost to build the prototype—a reasonable assumption, given the expense of building the frame out of 8020, and the markup on a lot of parts for buying them through McMaster-Carr rather than from the manufacturers themselves. This will make the price of the system $600 + the panel cost, which at a minimum would be $420 dollars, giving a total system cost of about $1020. Assuming labor costs of $100 to install the system (a conservative estimate), the present value of the capital costs is simply the installation cost, or -$1120.

If it is assumed that the cost to install the simple panels is the panel cost ($420) + $100 for mounting hardware + installation labor costs ($100), then the cost to install the simple system is $620. If it is assumed that a very destructive hurricane passes through the area once every 7 years (not unreasonably for the Gulf Coast), then the solar panels will have to be re-installed three times, at years 7, 14, and 21. Given a discount rate of 5%, the present value of this scenario is -$1,596. Since the ReTractor has the least negative present worth, it clearly wins in terms of capital costs. These economic benefits don’t even include the 30% greater power output due to the sun tracking capability, although it’s impossible to put an accurate dollar amount on that gain, since the value of power output varies wildly depending on the rebates and incentives in the state and area for solar power.

Taking into account the reinstallation costs for the normal system at years 7, 14, and 21 while accounting for retail power prices of $.10/Kw*Hr with increases of 2% a year, a discount rate of 5%, and 26.4% net power bonus due to tracking the net present value assuming 5 hours a day of sunlight is $-113 for the ReTractor compared to $-870 for the comparable simple system before tax credits are accounted for.

Background Power Draw

One of the design metrics was that the system should consume no more than 10% of the power generated by the panel to move all the components and run the sensors and M2 controller. Based off of experimental measurements of current draws, this is clearly achieved. The gear motor and full speed draws about 0.33 A @ 12 V, or 4W. If it turns at about 0.6 RPM, and rotates about 360 degrees during the day, then the motor will have rotated for about 1.67 minutes per day. From this, the total energy consumed by the gear motor in a day


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is calculated to be about 400J. The actuator while raising and lowering draws about 3A @ 12V, or 36W. Raising and lowering takes about 2 minutes in each direction, making the daily actuator energy consumption 4320J. The M2 microprocessor, accelerometer, and sensors, running continuously, draw about 60mA at 5V, or 0.3W. This uses up 25,920J of energy during the day. Adding these together, the ReTractor uses 30.6 kJ during a typical day.

In terms of production, a solar panel in southern North American receives on average about 5.0 hours of full sunlight during the day. Since this is a tracking solar panel, we can assume that the panel can continuously produce its nominal 300W of power during these five hours, producing 1.5 kWh, or 5400kJ. Thus, the energy consumed by the ReTractor, on average, is only about 0.6% of the system’s power output, a negligible amount and well within the design metric.

Space Efficiency

Another metric was that the system’s maximum power output divided by the system’s total footprint area should be no less than 100W/m2. The expected output of the system is 300W, and the footprint of the system currently is 62 in. x 65 in., giving a total area of 4030 in2, or 2.60 m2. Thus, the power-to-space ratio is 115.4W/m2, above the minimum threshold.

Solar Cell Efficiency

The solar system is required to convert at least 10% of the solar radiation hitting the footprint into electricity. At a solar insolation of 1000W/m2, the total radiation hitting the footprint is 2600W. Thus, the system is able to convert 11.5% of that (300/2600) into electricity. The efficiency of the panels themselves is significantly higher, 14.9%, since much of the footprint is not taken up by the panels.

The plug and play aspect of our tracking system also allows the owner to replace the panels with more efficient PV designs as Photovoltaic efficiencies increases. Since the current state of the art in solar cell efficiency is rapidly improving, our system allows the customer to take advantage of these additional efficiency increases as they occur.

COST

Team 10 Budget: $1200

Team 10 Expenditures

Quant Cat/Model # Description Unit Cost Total Cost

Vendor: McMaster-Carr

25 6338K411 Bronze Flanged Bushings, ¼” ID, ¼” Length $0.74 $18.50


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1 95526A550 Lag Bolts, 3/8”, 2.5” length $11.17 $11.17

16 91259A537 Shoulder Screws, ¼” dia., ½” L $0.90 $14.40

5 91264A247 Shoulder Screws, ¼” dia., ½” L $1.57 $7.85

6 6138K54 ¼” ID SS Bearings, Double-Sealed $5.76 $34.56

Vendor: TSINY Motors,

through Ali Express

3 TS-JSX4458 DC 12V/3rpm/50kg.cm High-torque worm Gear

motor,planet geared motor,micro dc motor

Express Shipping: FEDEX Added : $18.59

$36.88 $129.23

Vendor: 80-20 Inc.

4 #1530-Lite, 62” 80-20 Extrusion, 1.5”x3”, 62” Long $38.13 $152.52

1 Part #3380 15 Series T-Slot End Fasteners, 10 Pack $16.00 $16.00

1 Part #3286 Economy T-Nut 15 Series, 25 Pack $3.68 $3.68

4 Part #4350 15 Series 4 Hole 90-Degree Joining Plate $5.60 $22.40

Vendor: Firgelli Automations

1 FA-400-L-12-18 400 lb Force 18” Stroke Linear Actuator $129.99 $129.99

Vendor: Access Controller &

RFID Reader Module

& Secu, through

AliExpress

2 FCL-60KG 60kg Force Magnetic Lock

EMS Shipping Added: $22.84

$12.63 $48.10



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1 2737T151 Steel Finished-Bore Roller Chain Sprocket for

#25 Chain, ¼” Pitch, 23 Teeth, ¼” Bore

$7.80 $7.80

1 2737T157 Steel Finished-Bore Roller Chain Sprocket for

#25 Chain, ¼” Pitch, 23 Teeth, 1” Bore

$7.80 $7.80

1 6261K171 Standard ANSI Roller Chain #25, Single Strand,

¼” Pitch, .13” Dia, 2’ Length

$7.46 $7.46

2 9677T2 Face-Mount One-Piece Shaft Collar 1” Bore, 1-

3/4” Outside Diameter, .49” Width

$8.22 $16.44

2 6338K414 3/8” Shaft Diameter, 1/2” OD, 1/4” Lg Bronze

Flanged Bushing

$0.70 $1.40

2 6338K436 1” Shaft Diameter, 1.25” OD, 3/4” Lg Bronze

Flanged Bushing

$2.76 $5.52

Vendor: Penn Machine Shop

1 N/A Aluminum Stock $70.00 $70.00


2 93985A401 Type 416 SS Precision Hex Socket Shldr Screw

3/8” Shoulder Diameter, 5/16” L Shoulder,

5/16”-18 Thrd

$6.49 $12.98

2 98408A156 Side-Mount External Retaining Ring (E-Style) SS,

for 63/64” and 1” Shaft Diameter

$2.11 $4.22

Vendor: First Solar

1 N/A 150 Watt 12V Solar Module (With expedited

shipping)

$205 $215


1 2737T151 Steel Finished-Bore Roller Chain Sprocket, for #25 Chain, 1/4" Pitch, 23 Teeth, 1/4" Bore

$10.56 $10.56


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1 6261K171 Standard ANSI Roller Chain, #25, Single Strand, 1/4" Pitch, Rollerless, .13" Diameter, 3' L

$11.19 $11.19

5 6261K108 #25 Connecting Link for, Standard ANSI Roller Chain

$1.00 $5.00

2 92620A587 Grade 8 Alloy Steel Hex Head Cap Screw, Zinc Yellow Plated, 5/16"-18 Thread, 1-1/2" L, Fully Thread, Packs of 10

$7.22 $14.22

1 6409K11

Compact DC Gearmotor, 12 VDC, 0.6 RPM

$44.96 $44.96

Final Cost____________________________________________________$1022.95

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