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Fall Presentation
Winter Presentation
Results
Table 1 - Test Results
The table shows the results from the 3 tests done to evaluate the performance of the device. The device was able to meet all the requirements that were tested. Test 1 looked at the midpoint height of the bridge when it is opened. Test 2 evaluated the cycle time of the bridge. This included opening, staying open for 10 seconds, and closing. Test 3 evaluated the deflection of the bridge when a 190 N load was applied. Testing the device and having it be successful in the tests shows that the processes took to analyze, decide and construct from those decisions was a success. The analyses created for the tests were close to the tested values. The tests themselves were based off the requirements of the engineering specifications.
Introduction
This project establishes design and building techniques of a bridge through engineering analyses. Building with balsa wood is an accessible way to show an accurate representation of what an actual bridge may experience. This includes the loads on the members from external forces resulting in stresses on the system. Critical points where failure is expected can be utilized to design the bridge that effectively accommodates these restrictions. All these factors can then be used to design each piece of the bridge and build accordingly. This website will show the process to complete this.
Figure 1 - Sketch Design
Analysis Summary
By using various engineering analysis techniques, the design elements of the bridge can be found. Methods such as Statics can be used to calculate forces in the bridge and reactions at the points where the bridge will make contact with the abutments. Mechanics of Materials can also be used to calculate stresses and moments in the bridge which will show dimension parameters used for design.
Minimum Open Angle
Based from the project specifications, the midpoint of the bridge must raise 140 mm when it is open. By using trigonometric ratios the angle can be found since the length of the hypotenuse and opposite side are known. The angle calculates to 38.48°. The bridge will open at this angle.
Figure 2 - Analysis 1
Figure 3 - Analysis 4
Figure 3 - Analysis 4 Cont.
Areas of Truss Members
By using Statics and method of joints, the forces in the truss members can be found. All members except the vertical members will experience compression or tension. By using the stress equation, the area of each member can found. By referencing the stress from matweb.com, the tensile strength of tropical balsa wood is 1 MPa. Since the force and stress are known, the area can be calculated. After rounding up to a standard size, the members will have an area of 1/2" x 1/2".
Weight of Bridge
By following the requirement that the bridge must weigh less than 85 g, the total weight of the bridge is found. After defining the lengths of the members at one end of the bridge, the volume of the members can be found. The members can include the truss members, road deck, and cross members. By using the density of balsa wood, the weight of the components can be calculated. After all the calculations, the total projected weight of the bridge is 73.632 g, staying under the requirement.
Figure 4 - Analysis 3
Schedule
Fall quarter consisted of analyzing portions of the bridge using engineering techniques. Then using the analyses, dimensions were defined to be used to create drawings of the bridge. This required careful planning as some analyses were needed to be completed first to be able to complete drawings. Following the Gantt Chart below, most tasks could be completed within the estimated time. Some being completed much faster than expected. In Winter, construction of the bridge will begin. It is important to make sure that all components that are needed to assemble the bridge are the correct size and amount. In Spring, testing of the bridge and completing its deliverables will be done. In this quarter, its important to fulfill all the needed criteria in a successful bridge and ensure all complementing information is complete and presentable.
Figure 5 - Fall Gantt Chart
Figure 6 - Winter Gantt Chart
Figure 7 - Spring Gantt Chart
Budget
The budget consists of parts, labor, estimated total project cost, funding source and project updates. Parts such as balsa wood sticks and sheets and Arduino boards will be acquired online from stores like Amazon and specializedbalsa.com. Shipping times from Amazon can be expected to be at least 3 days. Other sites will plan for longer shipping times. Any 3D printed parts will be covered at CWU. These parts can be seen in Table 1 - Parts List and Cost. Using standard rates for labor, the projected labor costs $2,407.50. The total project cost will be $2543.72. This includes part, labor, and a buffer for additional expenses. A breakdown of these costs can be seen in Table 2 - Budget. The source of funding will be covered by the student and CWU. Subsequent updates on the budget will be noted during the Winter and Spring quarters.
Table 1 - Parts List and Cost
Table 2 - Budget
Construction
The construction of the bridge will include all operations done on a part to get it from a stock piece to be able to use in assembly. Since almost all pieces were designed to be the same cross section, the length will only be affected. Before sticks are to be used for manufacturing, they are visually inspected for any knots and chipped edges. After, they are marked to the desired length and cut 2-3mm more to account for lost material due to cutting. They are cut with a hack saw or table saw. Any excess material will be sanded down by hand or using a belt sander. Since some parts require a chamfer at the edges, they are marked to the desired angle and sanded down. These processes are used for all balsa wood parts. Additional parts that require extra manufacturing such as drilling a through hole are done using a drill press. Below further explains the manufacturing steps taken to create several parts for the bridge.
Figure 8 - Drawing Tree
After inspecting this piece for knots or chipped edges, it is measured and marked to the desired length. The piece shown is ready to cut.
Figure 9 - Stick marked to cut
The part is cut using a hack saw/table saw. Any excess material is sanded down with sand paper or a belt sander. The piece shown is ready to use for assembly
Figure 10 - Manufactured Stick
This part is cut to length using a hack saw/table saw. Any excess material is sanded down with sand paper or a belt sander. Since this piece is thin, more caution is taken when cutting and sanding to prevent splitting the wood. An 8mm diameter hole is placed in the center for testing. This is done using a drill press.
Figure 11 - Manufactured Road Deck
This part is cut to length using a hack saw/table saw. Any excess material is sanded down with sand paper or a belt sander. A 1/4" diameter hole is drilled through the center.
Figure 12 - Vertical Articulation Member
This part is cut to length using a hack saw/table saw. 45° chamfers are created using a belt sander or sand paper. This is done using a paper template to accurately draw out the chamfers. This will be used to add stability to the articulation tower.
Figure 13 - Inner Diagonal Articulation Member
This part is created in SolidWorks to the designed specifications. It is then printed using MakerBot. A wire that connects to the bridge will thread through the hole. The spool will then connect to the motor allowing the bridge to open and close.
Figure 14 - Spool
The bridge will be attached together using wood glue. The video shows the process of how the trusses were made. Originally, the truss would be assembled with the bottom horizontal member. However, as shown in the video, the upper truss was made separately to accommodate the road deck being in the middle of the bottom horizontal member and the rest of the truss above it.
Video 1 - Building Truss
The video shows the completed upper trusses and the bottom section containing the road deck and bottom horizontal members. The trusses will be glued to the bottom section one at a time and crossbars will be added in between to add stability to the bridge.
Video 2 - Trusses and Road Deck
The video shows the final construction of the circuitry used in the articulation of the bridge. An Arduino Motor Shield is mounted on top of an Arduino Rev3 Uno. This will allow fine control of the motor. Red and yellow switch buttons are wired to a breadboard and connects the the Arduino. The yellow button spins the motor clockwise while the red button spins counterclockwise. The components are powered by a 9V battery.
Video 3 - Circuitry
The video demonstrates the general motions of how the articulating bridge will perform. The motor is connected to the spool which connects to the bridge with a wire. For demonstration purposes, the motor was placed at the top but will be placed lower and towards the back in the final build.
Video 4 - Demo
Testing
Evaluation of the bridge in Spring consisted of meeting certain test criteria. This includes meeting requirements for dimensions, weight, strength and stability. Testing the device included finding the articulation height where the midpoint of the bridge needed to be above 140 mm. The cycle time was tested requiring the time to open, stay open for 10 seconds, and then close all within 60 seconds. The load and deflection of the bridge was also tested requiring a 190 N center load on the bridge with a deflection of less than 25 mm.
Looking at the testing procedure as a whole, an important factor was getting data that was consistent throughout all the tests. This was especially important during the first and second test that required the articulation of the bridge. After several trials for the first test, the motor that controls the articulation was noticeably running at a slower speed than past trials. It was realized that the battery had little to no charge which affected the performance of the motor. After replacing the battery, the motor spun at a consistent rate. This consideration was used in the second test that also used the articulation to make sure that the test were consistent.
Test 1: Articulation Height
Test 1 looks at the height of the bridge's midpoint when it is open. The requirement is the midpoint needs to be at least 140 mm from the resting plane. The equipment needed are the device, ruler, and a flat surface.
Figure 15 - Test 1 Setup
Controlling the motor to stop at the desired height requires programming the Arduino to spin the motor some amount of milliseconds. To calculate this, a reference is taken by programming the motor to spin 1000 ms which raised the bridge 31.5 mm. Using this relation, it can be applied to the 140 mm height by dividing 140 by 31.5 to find a ratio of 4.4. This can be applied to the 1000 ms to find a time of 4400 ms.
Figure 16 - Test 1 Measuring
The procedure includes setting up the bridge for the test. This requires the string to be tensioned to the right amount and making sure to use a battery with sufficient battery. Using a battery with little charge affects the performance of the motor resulting in inconsistent data. After the bridge is set up, the test is performed by raising the bridge. When the motor stops, the height is measured and the test is repeated for 2 more trials.
Video 5 - Test 1 Procedure
Test 2: Cycle Time
Test 2 evaluated the performance of the motor and its ability to raise the bridge within the required time limit of 60 seconds. The time is predicted through calculations and then compared to the actual measured time. The bridge is timed using a stopwatch and measures how long it takes for the bridge to go from fully closed to fully open.
Figure 17 - Test 2 Setup
The bridge is considered fully open when its midpoint is above 140 mm. The calculated prediction came out to be 5.83 seconds with the measured value of 6.64 seconds. The possible reason for the difference was the initial calculations did not consider the resistances of the bridge, wire, and spool acting on the motor.
Figure 18 - Test 2 Results
The cycle time of the bridge includes raising, holding at fully open for 10 seconds, and lowering. The final cycle time comes out to 23.28 seconds. This is found by taking the measured value of 6.64 seconds and multiplying by 2 to account for opening and closing the bridge. Then 10 seconds is added to account for an 'object' travelling under.
Video 6 - Test 2 Procedure
Test 3: Deflection
Test 3 looks at the deflection of the bridge when a 190 N load is applied to the center of the bridge. The bridge is fixed onto the Instrom using a threaded rod that slots through the road deck and is fastened using a flat plate, nut, and washer, shown in Figure 18. The Instrom pulls the rod upwards which applies the 190 N load on the center of the bridge. The deflection can then be analyzed from the data.
Figure 19 - Instrom with bridge
The test is performed by starting the Instrom. A 190 N load will be steadily applied. The Instrom will stop once 190 N is reached or the bridge fails resulting in a sudden drop in load. The bridge did support a load of 190 N with a deflection of 3.55 mm.
Video 7 - Performing Test 3
Shown to the right is a before and after of the bridge. Looking closely at the middle area where the load is applied, the bridge is noticeably deformed.
Video 8 - Before and After
The graph to the right shows the data from the test. The graph dips down in two places with the possible reason that the road deck shifted as the load increased. The road deck is supported by several cross members underneath. The area where the load is applied has a relatively large gap where the are no cross members which could be the main reason for the dips in load.
Figure 20 - Deflection vs. Load Data
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