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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."
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.
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.
If you want to directly measure the output torque of the servo you can add a strain gauge to the output shaft or other part of the output linkage.
Strain gauges are very low cost, but they require some design and calibration as well as electronics to read the analog signals. But you can design and deploy them to measure the exact signal that you want.
You can read more about strain gauges on Wikipedia. There's a lot of companies who make them and tutorials on how to use them. They are generally the technology inside of loadcells. Doing it yourself trades off the cost against others having calibrated and developed roboust electronic circuits. And the products themselves cannot be as well customized or integrated into the infrastructure in the same way.
I like using reaction torque sensor arrangements to measure the torque into "real" loads:
See, for example, this diagram from Futek's site. The commercial solutions like this tend to be pretty pricey, hundreds or thousands of dollars depending on the torque you need to measure. I've only priced out and purchased big ones, so I don't know about "servo" scale cost-wise.
For a cheap apparatus, you can mount the servo body on excellent low-friction bearings on-axis with the servo axle and restrain it from turning through a known lever arm and electronic scale (or a weight and string). I've built reaction-lever systems of this type around Flintec planar beam cells sold for weighing applications. These are relatively inexpensive and have good absolute calibrations. Of course, the bearings in this kind of setup contribute some error to your measurement that may be hard to characterize.
Your sensitivity requirements are quite stringent so you may have to experiment with measurement protocol and calibration no matter what you do. For a commercial torque cell, you can probably do a lot better than the nominal percentage accuracy (often something like 0.1% of full scale) but you'll have to do some work to trust the results.
You might build your own special apparatus that's "softer" than a normal torsion load cell or other reaction setup, so deflections at small torques are lower, but the performance of that will depend on the dynamics of your load. If the load varies, it could set up oscillations of a "soft" reaction torque device and the servo body.
Most reaction torque approaches won't lend themselves that well to integration into a tight space in an existing device, though.
Letting people know more constraints on your application would also be helpful. Can you freely modify the servo body mounting or not? Do you have ample space to install things, or are you trying to add this internally to a compact existing design?
