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Is there any cheap way to measure how hard a servo is turning?

I am looking to measure changes of about 0.1 oz-in.

What are the common ways to measure servo torque, how much do they cost, and how precise are they?

Edit for clarification: I am looking to turn the servo at a constant speed, and measure how much the object it is turning is "fighting back."

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  • $\begingroup$ In engineering International System Units ares preferred. You should give more details about the servo rating and the object inertia $\endgroup$
    – N. Staub
    May 28 '18 at 8:04
  • $\begingroup$ there may be servos available that report the torque .... otherwise you would have to measure the servo current ...each servo would have to be calibrated with a known load $\endgroup$
    – jsotola
    May 28 '18 at 17:54
  • $\begingroup$ this kind of a thing might work ... attach it between the servo and the "object" with some kind of an adapter so that you can see the dial .... set to max torque so that it does not reach its set torque .... read the dial that shows the actual torque (use a camera to take a picture of the dial) .... would not work if the servo is spinning fast unless you have a high speed camera $\endgroup$
    – jsotola
    May 28 '18 at 23:03
  • $\begingroup$ Please include the max torque expected in your question: otherwise the 0.1 oz-in can be %1 or %80 of the total capacity. $\endgroup$
    – Gürkan Çetin
    May 29 '18 at 9:49
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If you only want to measure torque for a motor when it's not in a system, then you can do it the old-fashioned way. You'll need an extra servo horn, a light stick (about 25 cm), string, a small bucket that can be tied onto the string, and some small precision weights (you can make these out of fishing weights if you're careful).

Fasten a stick to the servo horn so that the center of the stick is at the center of the horn where the motor axle will be. Then exactly 10 cm out from the center of the horn, drill a small hole to tie a string to.

Weigh the small bucket and the string together. This is your first weight. Tie the string to the bucket, then tie it to the stick.

Set the motor so the stick is horizontal. Use a servo controller to move the motor. If it moves, then you need more weight. Add one of your weights, record how much weight is in the bucket, and then try to move the motor again. I would measure the weights in grams.

One the motor won't move, then multiply the total weights by 10, and you have the stall-torque in gram-cm.

You can do the same thing with fractional oz weights and put the hole in the stick at 5 inches from the center. You'll have to multiply by five to get the result in oz-inches.

This method has its own problems, but most of the necessary items are probably in your house or easily gotten. You'll need a precision scale to measure the weights.

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One thing you could experiment with is a load cell. Mount the servo so it can rotate about its axle but is stopped from rotating by the force on the load cell, so that the torque the servo is transferring to its axle will be proportional to the force on the cell. Calibrate with a known torque, or calibrate the cell before fitting with known weights and measure the distance. A cheap kitchen scale has a cell that will give 10kg by 1g increments, so it should be mountable to read your range of torques.

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