Rotating Torque Sensing

[victorfrankl]

So the human powered submarine project is in its early stages, we have almost a year to play. Right now we're setting up to play with propeller designs. Available power is about 1/2 horsepower (max, in their dreams). Shaft speed tbd. We're trying to build a test cell to test various propellers for thrust at various speeds. We have the mechanics of it figured out. A battery drill delivers about the right power, is variable speed. Harbor Freight 4.5" RAgrinders ($12!) for right angle ~3:1 gearboxes, etc. Once we get this in the water it's going to be obvious (these are ~middleschoolers w/o a lot of experience yet) that we need to measure shaft speed and torque, i.e. have a dynamometer. I haven't looked into measuring shaft speed yet but imagine a bike computer setup will work fine. I don't mind them doing the math (power = torque x speed) by hand until we have a lot of experience to imprint it a bit better in their minds. At that point we can probably use the same magnet/sensors, maybe w/o even any signal conditioning to feed the Teensy. But torque has me wrapped around the axle :- ) . One idea is to measure the static torque on the drill as it's driving the whole mess. A simple load cell in that case, but the controlling hand might accidentally apply some torque as it squeezes the trigger. I'd actually love to have an actual rotating torque measurement. A purely/largely mechanical solution would also be fine. High accuracy is not needed, good repeatability is a good thing, tho. Shaft speed is limited to that of the drill (less than 600 rpm, 10 cps). I could have a whole rotating Teensy torque sensor with wireless talking to another wireless Teensy. That seems awkward. Any ideas? Thanks in advance, btw.

 

[PaulStoffregen]

Wish I could help, but I've never used load cells.

Teensy 3.2 does have a programmable gain amplifier (PGA) with differential input, so maybe that can help? It's been discussed elsewhere on this forum, so maybe those terms can help you search for those old threads?

 

[GremlinWrangler]

The Boring mechanical way to measure torque may be a mechanical scale either directly measuring the pushing force from the drill powered propellor and/or on leverarm of known radius measuring the 'weight' of the twisting motion. Or you can do it really old school and have a torsion member supporting the drive train and measure the amount of weight it takes to pull the torsion member back to the zero point once it is under load (so torsion bar needs no calibration, just be repeatable) . Bonus points for using a 5 meter arm so your adjustment weights are tiny.

All these can be teensiesed but for seeing and feeling things having something you can touch may be worth the the effort in fabrication as a first step, since this requires engineering desired forces to load through a single point for measurement, while isolating other forces.

If you do go to electronic strain or torsion measurement you can do the same thing, use static weight on a known lever to deform the instrumented torsion member to produce an electronic strain reading, and in theory the same reading measured later while twisted by thrust load will be equal to that weight.

 

[jwatte]

Note that stall torque is different from operating torque. Most motors will burn out if you keep stalling them, and are only safe to operate at significantly less than maximum stall torque.

Dynamic torque sensors can be bought, and they mount inline with the shaft, but they are somewhat bulky usually. They're also several thousand dollars.
https://www.digikey.com/products/en?keywords=1604-500-ND

Strain gages could be used to measure torque, by measuring elastic deformation (twist) in a shaft, but you'd have to machine the shaft to properly insert the sensor, which is likely a machining project beyond a typical m