How Can I Measure the torque applied by a servo motor?

Question

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

Can anyone suggest a tool which 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.

Answer 1

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.

Answer 2

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.

Answer 3

I'm sure you know that torque is rotational force about an axis. It is a combination of distance from the center of the shaft and the force. It seems that you are being suggested all kinds of fancy tools (no disrespect to anyone), how about keeping it simple and cheap (and a bit inaccurate)? put tape around the shaft to grow the diameter (make it tight so it doesn't compress under the next step). Then put a string and a weight (pennies in a plastic bag). When your motor stalls, measure the weight of the pennies, and multiply it by the radius of the shaft. Since tape doesn't weigh much, and it's offset by the counterweight on the other side, it shouldn't skew your readings too much. It's one way to what you are looking for.

You didn't specify this in questions, but I am assuming you want starting torque, which is the highest anyway, and goes down with speed.

Good luck.

Answer 4

Measuring the force applied to a measured moment arm will give you what is needed to compute the torque.

I think the simplest way is to use a servo horn and a kitchen scale (if you already have one).

On the servo horn, measure the distance from the axis to the mounting hole you'll use. When that arm of the servo horn is horizontal, your measured distance will be the moment arm for the torque.

To measure the force, to use an electronic kitchen scale to measure how much lighter a weight gets when the servo tries to lift it.

Choose a weight a little too heavy for the servo to lift. With the weight on the scale, use string or something and rig the servo to try to lift it at "arm's length" -- that is, with the servo horn arm horizontal. With the weight on the scale, but servo not lifting, zero (tare) the scale. As the servo struggles to lift the weight, whatever force it exerts will reduce the scale's measured weight, directly reading the force you want, albeit negative.

If you are measuring your only servo, consider that some servo designs cannot tolerate stall. Overheating and broken gears are the symptoms of engineering to minimize weight and manufacturing cost.