• ### Project Log 5 : Final assembly and code

With the help of an arduino UNO, a servo SG90 and a micro interuptor, we have assembled the holding mechanism for the crossbow. When the conveyor belt pulls the elastic back, the servo will activate causing its arm to rotate 90 degrees downwards and hold the elastic. It will hold it for 3 seconds until it moves back to its original position releasing the arrow. Here is the code below. We also added support to the axes for the conveyor belt to make sure it stays stable and straight.

#include <Servo.h>

Servo interupteur;

void setup() {
interupteur.attach(9);

void loop() {
interupteur.write(90);
delay(3000);
interupteur.write(180);     delay(3000);

}

• ### Project Log 4 : Motor and track assembly

Built the main structure of the crossbow using the laser printer found in the lab where I made holes to hold the 2 axis of the track. The first hole was 2.5cm in diameter to hold the DC motor and the second hole was 1.5cm in diameter to hold a ball bearing where a 1cm axis would be fit inside it. Also cut using the laser printer, a 25 by 4cm wood that would be just above the track with a 0.5 cm by 20cm slit located in the middle which will allow the hook to pull back on the string. This will also have the arrow sitting on top of it ready to be launched.

• ### Project Log 3 : Motor calculations and study

luis.poujol05/06/2024 at 14:30 1 comment

Went to the lab to choose a suitable motor for my project. I first calculated the amount of torque needed to shoot my arrow. With a mass m = 10g and a diameter d = 1cm for the motor. Using F = mg = 0.01x10 = 0.1N which would mean a torque T = 0.1Ncm. We wanted to accelerate this arrow by about a factor of 10 which would mean a T = 1Ncm. I then looked for motors with a rating above 1Ncm or 0.1kgcm

Both options can provide more torque than needed at T = 4kgcm which I found was a good safety net accounting for the friction and weight of the system (axle and conveyor belt). The first stepper motor operates with 12V and would require a driver whilst the DC geared motor only requires 6V and can operate without a driver which would allow better testing. I therefore chose the 2nd option.

I will also need a servo that will rise up when the string is pulled back to lock the string in place and will release once the arrow is ready to be launched.

• ### Project Log 2 : First prototype and components

First prototype using cardboard and sticks. Refined the idea to use tracks as well as wheels on the axis to have more torque and friction. Use of a rubber band.

Components we want to order

 Arduino UNO 23.90€ 1 29.8 https://www.gotronic.fr/art-arduino-uno-a000066-12420.htm Stepper Motor for conveyor belt 12.99€ 2 25.98 https://www.amazon.com/Stepper-Motor-Printer-Milling-Machine/dp/B071P2H1F9/ref=sr_1_20?sr=8-20 Push Buttons 8.68€ 1 8.68 https://www.amazon.com/dp/B01E38OS7K/ Tracks and wheel provided by school Stepper Motor driver 15.99€ 1 15.99 https://www.amazon.com/dp/B0CGRMQB9P/ Coupler and 8mm axis 9.99€ 2 19.98 https://www.amazon.com/dp/B07S1LKMQ6/

• ### Project Log 1 : Docs and Sources

We want to replicate this "chu ko nu" crossbow and enhance it with the automatic reloading system we will design.

-A 2x4 in any type of wood, prefer few defects of course (at least 27" long)
-1/2" Sch 40 PVC pipe (at least 22" long)
-Red oak 1x2 (53" or more, other hardwoods are OK)
-A 1/2" x 5.5" x 48" poplar board
-3/8" poplar dowels (recommend three 48" dowels)
-5/16" dowel (at least 10")
-Larger dowel, 3/4" to 1" diameter (at least 6" long)
-Glue
-white glue
-epoxy
-hot glue stick
-String (I used 9/16" venetian blind chord, it is mixed polymers)
-Two 1/4-20 bolts, 3" long, with nuts.
-Wood screws (at least 6, ones that are 2"-3" long would be great)

OPTIONAL:
-Wood stain
-Spray on clear coat
-Brown spray paint
-1.5" wide hinge
-1.5" wide by 1/4" thick oak or poplar board (10")

https://maker.pro/custom/tutorial/what-type-of-motor-is-best-for-my-project

We want to use 1-4 brushless DC or stepper motor to pull on a drive wheel which will spin and pull a conveyor belt around the main shaft of the crossbow. There will be a hook attached to the conveyor belt which will hook on the crossbow string and pull it as it moves along the shaft. as it goes back, it will let go of the string as it goes around the shaft again.