# How to measure pull force on a (kite) rope?

I'm building a kite flying robot, in which I've already got some sensors built in. I now also want to measure the pulling force on the rope which is attached to the kite. So I thought of attaching a hanging digital scale to the rope which can then measure the pulling force in kilograms. I of course need to read out the data from the scale using GPIO pins or USB.

Furthermore, I also have a raspberry pi installed within the kite and I want to measure some pull on ropes in the air. So if possible, I also need very small/light scales from which I can read out the data. It would be great if the scales can measure up to about 50kg, but with some pulleys about 20kg would be fine as well.

Unfortunately I can't find any digital hanging scale with a possibility to read out the data and which is reasonably light. There are some simple USB powered hanging scales, but I think they just use USB for charging, not for reading out data. And I also found this one, but that's a bit overkill (too heavy and too expensive).

So question #1: Does anybody know where I can get a simple existing hanging scale from which I can read out data?

If needed I can also build something, but I just wouldn't know where to start. I did find this page on Alibaba, where they show the contents of the scale they offer:

So as far as I understand I need the component which I highlighted. But I have no idea how that component is called (what do I search for?) and whether it is actually doable to read it out from a Raspberry Pi.

Question #2: does anybody know what the highlighted component is called, where I could possibly get it and if I can read it out from a Raspberry Pi?

In conclusion; can anybody point me in the right direction?

The component you highlighted is called a tensile load cell. You could buy one from a supplier, but it probably would be cheaper to buy a cheap digital hanging scale and taking the sensor out of it - at least I couldn't find one less than $100, ten times the price of a 50kg digital hanging scale. You will need to do some analog conversion and amplification of the signal and then convert to digital with an ADC; I haven't done so myself but Sparkfun's load cell tutorial seems a good start. The other means is to create your own scale, with a stiff spring and some sensor detecting its movement, such as a digital depth gauge - which costs$3 and has a serial output.