I have a magnet coupled to a gearbox with a relationship of 13000 rpm:1, I need to measure torque in the gearbox. I was trying to measure it with strain gauges but the device is very small (1.5 cm diameter tops) and the accuracy is almost none. I know that Hall sensors would help me on this but to be honest I don't know how to couple them. (Torque is going to be small, approximately from 0.002 mNm to 1.5 Nm)
If you can measure the speed of the shaft and if you know how much power you are driving the gear box, you can calculate torque:
torque = power / shaft speed
Of course you'd have to account for small errors like losses due to motor efficiency and friction.
The easy but expensive way is to buy a torque measurement dyno that can measure torque directly.
I am assuming that you are trying to use the Hall sensors to measure speed? The Hall Sensors must be spaced 120 electrical degrees apart and you'd need to build a tachometer to measure speed--seems like a lot of work.
Usually, each hall sensor has a face plastic body and 3 leads. Two leads are for the + and - power. The output must be pulled up with a resistor.
You may want to look at ABB's Torductor-S technology