Introduction: Movable Bridge

We are SUMMIT, coming from University of Michigan-Shanghai Jiao Tong University Joint Institute (JI). The Joint Institute, located in Minhang District, Shanghai, was established in 2006 by two universities, University of Michigan and Shanghai Jiao Tong University. JI features both American and Chinese teaching styles, aiming to cultivate the qualities of innovation and leadership for engineers with a global view.

VG100 Introduction to Engineering is a unique course of JI which requires the spirit of teamwork and the ability to design and analyze. Project 1&Competition is about making a movable bridge which is controlled by Arduino and can be deployed and retracted from one side. We started the project from Sep.24th 2019 and ended passing several tests on the game day, Oct.16th 2019.

On the game day, we get scores 32.25 of 100: passing the first size test, achieving the bonus (supporting all the weights) but failing the deployment, retraction and load test. Our bridge loaded 1.05kg on the 75% position and 0.6kg on the 25% position with the deflection of 2 mm.

There's a brief introduction to our rules and regulations.

A. Size Test

a. The bridge should span 700mm to 750mm, with the width of 160mm to 200mm.

b. A car (90mm in width, 20mm in height) can go over it steadily.

c. The bridge could be stuffed into a cuboid box (350mm * 350mm * 250 mm).

B. Function Test

a. The bridge, attached to the large abutment, could be automatically deployed and retracted, both within 1 minute.

C. Load Test

a. The bridge could bear the weight of more than 1000g or 3.33 times of its own weight.

b. The load recorded is the weight causing a deflection of 2mm.

D. Regulations

a. Balsa wood and wood glue (Elmers’) are the only materials we can use to build the structural part of the bridge.

b. Other materials (especially metal) can only be used in electronic devices.

Video links: Click here and you can see our bridge!

Step 1: Concept Diagram

Step 2: Key Components Concept Diagram

Here are the concept diagrams of our key components.

The first picture is the truss of our bridge.

The second picture shows the joggled joint.

The last picture is the pedestal of our bridge.

Step 3: Analysis

Step 4: Material List

Step 5: Circuit Diagram and Programming

Step 6: Deck Manufacture

1. Cut the balsam wood board(1000mm*100mm*5mm), glue them into 3 pieces (2×320mm*155mm*5mm, 150mm*155mm*5mm).

2. Construct isosceles right triangles in the in Solidworks (Every four triangles[16.8mm*16.8mm*24mm] form a square with a distance of 4mm).

3. Hollow out the two longer pieces with laser cutting (Following model made previously in Solidworks).

Step 7: Support Manufacture

1. Cut the wooden sticks (6mm*6mm) with a length of (each for 2):

A. 144mm

B. 199mm

C. 72mm

D. 102mm

E. 51mm

F. 111mm

G. 160mm

2. Polish both ends of all the sticks with abrasive blocks. (30,45-degree right triangles off, respectively according to the diagram)

3. Glue them together with wood glue. (Repeat the above steps for the other side)

Step 8: Deck Reinforcement

1. Prepare special sticks (for the longer boards)

A. 200mm*10mm*10mm for three, with two symmetrical “holes” (10mm*10mm*5mm) and one 57mm*10mm*5mm “hole” in the middle

B. two for 260mm*10mm*10mm, with three 6mm*6mm*4mm “holes” (from left to right at 55mm, 120mm, 185mm) 2. Build enforcement structure

A. Splice all the prepared supports together.

B. Fix a link-board (240mm*100mm*5mm) into the stick A (glued). (Repeat the above steps for the other side)

3. Prepare special sticks (for the middle board)

150mm*1mm*1mm for three, with two 5mm*4mm*1mm “holes" on the top left, right sides.

4. Build enforcement structure

A. Fix two 60mm*10mm*4mm link-boards into the “holes” of the three sticks.

B. Glue the middle board onto the sticks symmetrically.

Step 9: Pedestal Manufacture

1. Prepare the wooden sticks (6mm*6mm) with a length of:

A.180mm for four

B.130mm for two

C.310mm for four

2. Glue them together according to the diagram.

3. Cut a 100mm*200mm*3mm baseboard.

4. Glue the four longest sticks to the baseboard.

Step 10: Assembly and Perfection

1. Glue the support (Built in Step 5) on sticks B.

2. Glue the combined structure on the longer boards (40mm from the edges of deck).

3. Place the three boards in a line.

4. Stick the hinges (two for each joint) onto the link-board and the baseboards.

Step 11: Circuit Assembly

1. Motor section

A. Hold the motor in a four-stick cubic (you can adjust the size to your motor).

B. Prepare larger circle plates (r=15mm for two, r=7mm for one).

C. Glue the plate with the string attached to the middle.

D. Stab the axis into the plate.

2. Servo motor section

Glue the servo motor on both the supporting plate and the hinge (separately).

3. Circuit connection: Follow the circuit diagram.

Step 12: Final System View

Our bridge has good load-bearing capacity, and has the ability of retraction and deployment.

Step 13: Trouble-shooting Section

1. Fail to meet the size test

Solution: Change the two-part bridge to three-part bridge.

2. Fail to complete the retraction test

Solution: a. Change the motor to gear motor to increase the power.

b. Ensure an angle between rope and deck at the node and put the node a little farther outwards.

c. Use plastic materials like straw between the contact surface of wood and rope to reduce the friction.

d. Program to use rope to lift the second part together with the first part and when the angle between the deck and horizontal becomes 60°, program to fold the third part which is the farthest from the motor.

3. Fail to fold or unfold the third part

Solution: a. Fix two motor servos at the joint between the second and the third part (Remark: Make sure the center shafts of the hinge and the motor servo are in a straight line.).

b. Design to reduce the weight of the third part by removing useless structure or reconstructing with a new structure that is easy to be pulled up.

4. Have difficulty in decreasing the weight

Solution: a. Use less materials and change iron hinges to plastic ones.

b. Hollow out the deck.

c. Make the bridge according to the minimum width standard.

5. Have difficulty in decreasing the deflection

Solution: Make sure the joints are connected tightly when loading.

6. Fail to run the program when the program itself is correct

Solution: a. Check whether the program has been uploaded to the Arduino board.

b. Check whether the board has been provided electricity.

c. Check whether the circuit is connected correctly.

d. Check whether the leads are in good contact.

Thank you for reading our manual! Hope it can help you build your bridge!