How to measure thrust/power etc?

Thread Starter

JDR04

Joined May 5, 2011
356
I would like to be able to measure the difference of thrust between 2 rc dc motors who have propellers on them.

It would be great if I could use a PIC that will drive a couple of 7 segment displays. The rest I do not know how one would go about it so all suggestions will be welcomed.

Thanks folks....
 

danadak

Joined Mar 10, 2018
4,057
DIY transformer with a movable core spring loaded. Measure the AC coming out of the transformer as position of core moves with A/D. Do peak detect or RMS in the code.

Or use transformer in oscillator circuit, and use linearization on force vs frequency to get at linear transfer function for force sensor.

Or spring loaded LVDTs. Or low power laser measuring deflection of motor case along the axial direction, again a spring load, either in tension or compression.

Or photocell and light source, attache a vane to motor case to modulate light level and measure again the axial direction, again a spring load, either in tension or compression.

Regards, Dana.
 
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djsfantasi

Joined Apr 11, 2010
8,066
Or a mechanical solution. Build a floating frame for the motors (spring suspension or let it ride on polished rails). Then mechanically connect the motor to a linear potentiometer. Or better yet, a slide potentiometer. Have the μP read the pot measurement via an analog pin and the built-in ADC.
 

drc_567

Joined Dec 29, 2008
1,156
Thrust is simply force.
A spring extends by a distance, say X, when a force, call it F, is applied. A spring constant, usually k, is a characteristic property of any particular spring denoting its strength or weakness.
Thus,
F=kX
So, choose a spring that has a reasonable extension distance for the force being measured. By measuring the distance resulting from the applied force, you have a numeric quantity that can be measured one way or another.
A test stand using a small sail and a spring that extends to a measurable distance could conceivably work.
... LEDs might require a potentiometer of some type.
 
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Thread Starter

JDR04

Joined May 5, 2011
356
To whom are you responding?

What do you mean you’re stuck with the application of them (?)?
I was replying to djsfantasi. Its the actual putting tegether of the components I'm having difficulty with.

Would a rotary motion sensor work? Do they work the same way with resistance like a potentiometer?
 

wayneh

Joined Sep 9, 2010
17,160
I'd hang the arrangement from a string and see how high the motor/propellor can pull it away from vertical. Kind of like a poor-man's method for measuring muzzle velocity.
 

oz93666

Joined Sep 7, 2010
737
This is normally done with ordinary kitchen digital scales ...a wide range of scales available for various thrust ranges .. accurate digital readout

Weigh the unit , then weigh with motor running , just make sure the down draft does not push on the scale .... you can aim the thrust up . Put unit on scale , press "Tare " to reset and when you switch on the prop, the reading is a direct measure of the thrust .
 
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dendad

Joined Feb 20, 2016
3,982
Thanks dendad.Do you think a digital readout scale would be more accurate?
Yes, I think so. But the mechanical one is very simple. You could in fact make your own with a spring, pointer and pulley. hang measured weights on it to calibrate it.
 

wayneh

Joined Sep 9, 2010
17,160
Thanks wayneh, will investigate this further.
I prefer the simplicity of the solution in #11. The pendulum trick does work well for measuring muzzle velocity but your needs are different. You can get digital kitchen scales for as little as $5 that weigh with 1g precision.
 
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