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I know there must be a "tool" that can measure oz-in of torque. I do not want to trust what the servo manufacturers state on their site for torque values so I want to test them for myself.

Anyone know what tool I can use to do this? I have used fishing scales before, but I need something more sensitive than that and my units are pretty small such as around 20 oz-in.

Thanks.

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  • $\begingroup$ 20 oz-inch is equal to 1.44 kg-cm. Why are you considering that "small"? Can't you just place a weight on a scale and use the servo to lift it and then see how weight it subtracted? Just be careful of course not to leave the servo like that for too long. $\endgroup$ – Damjan Dakic Mar 21 '14 at 20:17
  • $\begingroup$ I don't have a bunch of different weights to check different size servos. And I really want to see how much torque the servo is pulling along its travel. $\endgroup$ – RC Hobbyist Mar 22 '14 at 0:10
  • $\begingroup$ A torque wrench? $\endgroup$ – Guy Sirton Mar 22 '14 at 3:27
  • $\begingroup$ You forgot to add a smiley face. I was hoping to get a viable solution to this problem. I really need to be able to check to see if the servos I am using are up to the task. I would prefer to test my servos rather than relying on manuf. data. But then again, doesn't everyone want to verify their equip? $\endgroup$ – RC Hobbyist Mar 22 '14 at 5:38
  • $\begingroup$ @RCHobbyist, you don't need different weights, you just place one that is certainly heavier than the servo can lift, it will pull it up with the maximal force it can produce which will in turn reduce the weight information on the scale. $\endgroup$ – Damjan Dakic Mar 22 '14 at 11:12
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One option for you if you are willing to spend ~ 100 dollars is a particle brake. I just saw a 15 in-lb particle brake on ebay for 99 dollars. If you search, you may be able to find a better deal. You can couple a particle brake to your shaft and the torque that the brake applies is proportional to voltage. Their torque is usually pretty linear with respect to voltage.

If you can't spend that much, then I don't have a lot of options for you. But there are some checks you can do. Start with checking winding resistance. Measure the resistance between any 2 of the 3 motor leads and make sure it matches the manufacturer's spec. Stall torque is proportional to resistance, so this is an important value. I would suggest using a strobe or motor tachometer (~ 30 dollars). Measure no-load speed and current. Attach your motor to another motor and back drive the motor. Take an oscilloscope and look at the waveform between any 2 of the 3 motor leads. Measure the peak voltage and record the speed at the time of the measurement. The speed (in RPM) divided the peak voltage should give you the motor's Kv value. The peak voltage divided by the speed (in RPM) should give you the motor's back-emf constant. Even more important is if you take the peak voltage divided by the speed (in rads/sec), then you get a back-emf constant in Volts/(rad/sec) which is equivalent to your torque constant in Nm/A. From your no-load current and your torque constant, you should be able to estimate what your torque is based on the current draw of your motor.

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  • $\begingroup$ What's the tolerance on those brakes? It's hard to imagine you can make an accurate measurement. If you back-drive another motor you're relying on the second motor? Surely you can't determine torque from resistance, as you say it depends on the motors' torque constant which in this question is not trusted. $\endgroup$ – Guy Sirton Mar 26 '14 at 3:39
  • $\begingroup$ Measureing resistance just to make sure it matches what the manufacturer says. Just a check. As long as you can get an accurate speed measurement with a strobe or tach, that's all you need. I'm not sure what the accuracy of a particle brake is. It probably depends on the size and manufacturer. For a cheap alternative to a dyno its not a bad choice in my opinion. $\endgroup$ – Eric Mar 26 '14 at 3:59
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There are lots of ways of measuring torque from first principles. Obviously the first problem is that you need a sensor, you have lots of options:

  • You can use a torque sensor. ( e.g. imada.com/torque-testers/torque-gauge ) couple it directly to your shaft. Drive your motor. Read the sensor. Your error would be the sensor's accuracy plus any errors introduced due to inaccurate coupling.
  • You can use some sort of force sensor. Your scale is one example. You mount an arm on the motor and you read the force at a given distance. This gives you torque. With a scale you want to: idle your motor, zero the scale with the weight of your arm on it, and then energize the motor (pushing on the scale) and read the scale. Any zeroing error plus measurement error on the arm would eat into your accuracy. Variations on this is putting a large weight on and driving up. It's all the same principle.
  • You can use a position sensor. Mount a large mass of known moment of inertia onto the motor and start driving it. Take position measurements and differentiate them to find velocity and then acceleration. From acceleration you can work back to torque given the moment of inertia inertia. This has the advantage of being able to measure torque at speed. This is basically an inertia dynamometer. You accuracy will depend on how accurately you know the moment of inertia of the system (including the shaft), you timing resolution, your position sensor resolution, friction potentially introduced by the sensor (e.g. an encoder) etc.
  • velocity sensor. same princple as above.
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