Introduction: Movable Bridge

Our team is MaJIc6 with four members, from University of Michigan-Shanghai Jiao Tong University Joint Institute (UM-SJTU JI), which locates in the campus of Shanghai Jiao Tong University in Minhang District, Shanghai, China. Joint Institute is a prestigious engineering college, which provides students with great educational resources and combines the advantages of western education and eastern education.

-Course & Project information

VG100 Introduction to Engineering is a special curriculum of Joint Institute which emphasizes on cultivating students’ technical and communication skills on engineering. The Movable Bridge is our project one for 2019 fall semester which requires us to design and construct a bridge made of balsa wood, which can be deployed and retracted over two abutments automatically and is able to hold a load of a certain weight.

Game Rules & Regulations:

1. The length of the bridge should be within 700mm and 750mm and the width should be within 160mm and 200mm.

2. A car with the size of 90mm in width and 20mm in height can be fully placed on all parts of the bridge.

3. The bridge should be able to fit in a box with the size 350 * 350 * 250mm before the deployment process and after the retraction process.

4. The deployment and retraction process should be both within 1 minute.

5. The bridge should be able to hold a load of more than 1kg or 3.33times of its own weight.

Step 1: Concept Diagram

Concept diagrams are shown in the picture.

Step 2: Material List

Step 3: Building Part 1

1. Place four upright wood sticks(60*8*8mm) on the four corners of the board(330*16*5mm).

2. Place another two sticks with the same size in the middle of the long sides of the board.

3. Stick two beams(330*8*8mm) on the top of the upright sticks.

4. Connect the beams with three sticks(144*8*8mm), two on the edge of the beams and one in the middle.

5. Select two slant sticks between each two upright sticks.

6. Stick a small stick (40*8*8mm) at the end of Part 1. The function of the stick is to prevent Part 1 from sliding out. Then Part 1 is finished.

Step 4: Building Part 2

1. Get a stick(10*10*25mm), a plastic bottle gap(d=3mm), and a pushpin.

2. Drill on the center of the gap.

3. Drill on the stick, 2mm from bottom on the central line.

4. Connect them.

5. Repeat the 4 steps above 5 times. Then paste 6 rollers on the bottom board.

6. Place six rollers on a board (325*20*5mm), three on each side. The use of the rollers is to reduce the friction between the two wood boards.

7. Place four upright sticks (147*8*8mm) on the four corners of the wood board.

8. Place another two sticks with the same size in the middle of each long side of the board.

9. Stick two beams (325*8*8mm) on the top of the upright sticks to stabilize them.

10. Connect the beams with four sticks (184*8*8mm), two on the edge of the beams and two on the middle.

11. Stick two slant sticks between each corner stick and middle stick.

12. Stick one stick (317*10*10mm) on each inner side of the frame.

13. Stick one stick (45*8*8mm) on each front end of the stick in step 7.

14. Stick two sticks (184*10*10mm) stuck together on bottom of the sticks in step 8.

15. Stick two sticks (184*8*8mm) stuck together on the front opening of the frame, 117mm from the bottom.

16. Stick one stick (184*10*10mm) between two beams, 63mm from the front opening.

17. Stick one stick (43*8*8mm) on each side of the stick in step 11, 10mm from the edge.

18. Fix an axle between two sticks (55*10*10mm). Then fix them between the stick in step 11 and the front opening, on the central line.

19. Stick the wood board (180*78*2mm) on the top of the frame, with one side stuck on the top of the sticks in step 3, and the other side stuck on the bottom of the stick on the after end of the frame.

20. Stick four sticks (24*10*10mm) on the corners of a wood board (70*70*5mm).

21. Stick the sticks in step 15 on the outer bottom of the frame, 61mm from the front opening.

22. Stick servo1 on the back of stick in step 10. Then Part 2 is finished.

Step 5: Building Part 3

1. Get a wood board (305*200*5mm).

2. Stick two pins on two front corners of the board.

3. Fix a pulley on the top of a stick (335*10*10mm).

4. Stick the pole in step 3 on the board, 80mm from the afterbody end, on the central line.

5. Fix two sticks to stabilize the pole.

6. Fix two sticks (40*10*10mm) stuck together on the board, lying against the back of the pole.

7. Fix the motor on the sticks in step 6, with one side stuck on the pole.

8. Fix a stick(70*8*8mm) to servo2.

9. Fix servo2 on the board, 11mm from the after-end edge, 15mm form the right edge.

10. Symmetrically do step 8,9 with servo3. Then Part 3 is finished.

Step 6: Assembling All Parts

1. Connect a nylon string (d=1mm) with the back of Part 1. The string should be long enough. (about 2m)

2. Pull the string over the axle in Part 2, then over the pulley in Part 3.

3. Insert Part 1 into Part 2 from the front opening.

4. Wind the line to the motor.

5. Stick the rear end of Part 2 with the pins in Part 3.

6. Connect wires as Circuit Diagram. (See Circuit Diagram)

7. The assembling process of the whole system is finished.

Step 7: Realization of Deployment and Retraction

A. Deployment

1. Servo2 and servo3 rotate 0.1°/s simultaneously for 80s. Motor rotates anticlockwise to loosen the string in the meanwhile.

2. Motor keeps rotating for 13s. The bridge is then laid flat by gravity.

3. Motor rotates in opposite direction for 15s to fasten the string, pulling out Part1.

4. Motor rotates anticlockwise for 2s to loosen the string, ensuring Part 1 is laid on the abutment.

5. The deployment process ends.

B. Retraction

1. Motor rotates clockwise for 25s, pulling the bridge upright.

2. Part 1 is now slant and blocked.

3. Motor begins rotating to loosen the string.

4. Servo1 rotates 30°/s clockwise for 2.5s to pull Part 1 upright.

5. Servo1 rotates 90°/s anticlockwise to its origin position.

6. Motor continues rotating anticlockwise to loosen the string for 15s. Part 1 slowly declines.

7. The retraction process ends.

Step 8: Circuit Diagram

The circuit diagram is shown in the picture.

Step 9: Final View of the Movable Bridge

The final views of the bridge are shown in the pictures.

Step 10: Arduino Codes


Servo servo1, servo2, servo3;

char screen_in;

int pos_of_servo1, pos_of_servo2, pos_of_servo3;

int motor[3][3];

void setup() {




motor[1][0] = 7;

motor[1][1] = 8;

for (int i = 1; i <= 2; i++) {

pinMode(motor[i][0], OUTPUT);

pinMode(motor[i][1], OUTPUT);




int m_move(int i) {

digitalWrite(motor[i][0], LOW);

digitalWrite(motor[i][1], HIGH);


int m_antimove(int i) {

digitalWrite(motor[i][0], HIGH);

digitalWrite(motor[i][1], LOW);


int m_stop(int i) {

digitalWrite(motor[i][0], LOW);

digitalWrite(motor[i][1], LOW);


int deploy() {

servo1.write(pos_of_servo1 = 18);

servo2.write(pos_of_servo2 = 96);


for (int i = 1; i <= 80; i++) {

















return 0;


int retract() {

servo1.write(pos_of_servo1 = 98);

servo2.write(pos_of_servo2 = 16);





for (pos_of_servo3 = 105; pos_of_servo3 >= 30; pos_of_servo3--) {




for (pos_of_servo3 = 30; pos_of_servo3 <= 105; pos_of_servo3++) {







void loop() {

char OPT;

if (pos_of_servo1 < 90) {

servo1.write(pos_of_servo1 = 18);

servo2.write(pos_of_servo2 = 96);

servo3.write(pos_of_servo3 = 105);


OPT =;

if (OPT == 'D') {



while (1) {

OPT =;

if (OPT == 'R') {





while (1);



Step 11: Manual for the Project

Manual for the Project.

Step 12: Video Link