I'm doing a pet project in which I want to fly a kite from my computer. For this to happen my plan is to build a little box hanging under the kite which includes a raspberry pi (connected to my laptop by wifi), a battery, a motor controller and an electromotor to pull the cords. I am now in the quest of finding a suitable motor.
In reading around and asking previous questions here on stackexchange I found I would need a basic DC motor including a transmission because they have a high torque-to-weight ratio and are easy to control. Furthermore, I will attach a little wheel (diameter == 3cm) to the motor so that the rope can be hung around it and the motor its force can be translated to rope motion.
== Minimum Revs/Minute ==
I want to be able to change the rope length about 40cm in 2 seconds. Since the wheel has a circumference of ≈ 9.4cm (π * 3), it has to be able to have at least 4.24 revs per 2s or 2.12 revs per second, or 127 RPM.
== Minimum Torque in Kg/cm ==
I haven't got the kite as of yet, but I want to fly a "very small kite-surf kite" in "moderate wind". This means that at full force it will be able to pull me off my position, but not lift me in the air. So I estimate the maximum rope pull at about 50Kilograms, divided by 4 ropes equals 12.5Kilogram/rope. Since I line the two steering ropes in a loop, pulling one will release the other. This means that the rope being released "helps" pulling the other rope in. Seeing that the rope pulling force will vary between the two ropes and I also have to overcome inertia I guestimate the power needed to adjust the rope lengths at 40% of the total force, or 5 kilograms. Since we're using a wheel with a radius of 1.5cm (diameter == 3cm) we take 5 kilograms times 1.5 which results in a minimum needed torque of 7.5Kg/cm.
So I searched around on websites and found the motors below. All sites are in Dutch, so I just translated the info below.
- Free spinning: 200 RPM, 300mA, 12 kg-cm
- Average force: 1A
- Full force: 5A
- RPM on force 104 RPM
- Max. torque 9 kg/cm · permanent 3 kg/cm
- Free spinning at 1,5 V 120 RPM (why 1.5 volt if its a 12V motor?!?)
- Max. Current 2.1 A
- Max. torque 0.6 Nm (doesn't this equal 0.06 Kg/cm?)
- free spinning at 1,5 V 110 RPM
- Continuous RPM: 85 RPM
- free spinning current: 0,10 A
The problem I have now is that the information is so different (RPM in free spinning vs. with force attached to the motor) and varies so much (12 vs 0.06kg/cm?!?) that I'm kind of unsure of my whole train of thought.
So my question: does it make any sense what I'm writing here, or do I make mistake upon mistake upon mistake? What advice would you give me here? Is the first motor clearly the best, or am I overlooking things?
All tips are welcome!