I have been struggling, for over a month now, to find a way to measure instantaneous torque from a servo motor. Here's a servo that look similar to the one I'm using: https://www.servocity.com/html/spg805a-bm_standard_rotation.html
I need to find a way to measure the torque in an accurate, but inexpensive manner-which means a torque transducer wouldn't work for me. In my project, there will be times where a nonconstant force will be opposing the motion of the servo (just imagine that a rod, mounted horizontally, is attached to the shaft of the servo, and someone applies a force against the motion of the rod) i.e., the torque of the servo/shaft isn't always constant. Moreover, the speed of the rotation will be different at times as well. There has to be a way to do this, right? I am reading data in through an arduino, connected to the servo, and am free to use sensors. At first I wanted to measure the current uptake but don't think it's going to be accurate. Furthermore, even if I did go with using current, I'd like to validate my results through other/more direct means. Is it possible to somehow incorporate a strain gauge or something, and attach it to the shaft somehow to measure the torque directly (I'm not really sure how strain gauges work)? I asked my friend about this and he talked about using compressible "blocks", with a spring attached in the middle, and have the ends of the springs attached to two strain gauges. The spring might compress/relax differently according to differing levels of torque apparently. I couldn't really understand his idea, nor could I understand how such a thing could attach to the shaft, but he said it's something of a classic/old technique used for measuring rotary torque. I also pondered of measuring the acceleration (to calculate torque) using an accelerometer would work, but I guess I wouldn't be measuring the electrical torque of the actual servo at that point? I don't even know..
I'm really new to all this stuff (and still bad at it) so I apologize if I'm not being clear about anything. I'm really just looking for a clear/logical, sensible, and most direct way/idea I can implement to measure torque-even if it's not easy. I'm willing to learn and put in the effort.