Measuring Motor Torque under load conditions - Page 1
Forum section: electronics / projects

author: JoeyG
author_rank: Regular Contributor
timestamp: December 21, 2023, 04:54:15 am
content: Between a motor shaft and the associated load is there a "strain gauge" style of sensor that can measure torque say between 0.05Nm and 7Nm?Something like the top level system picture attached?

author: Andy Chee
author_rank: Super Contributor
timestamp: December 21, 2023, 05:40:51 am
content: The normal way to measure torque is with the load cell(s) installed in the motor mounts, not in the load shaft.When a load is applied, the entire motor body twists. This makes the load cells see a "heavier" weight on one side and "lighter" weight on the other, the difference in weight is the load on the shaft. If you can't get the required load cell sensitivity, you can also use a torque arm.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2023, 08:28:44 am
content: The better way is to install load cells at the motor mount like Andy Chee suggests, but there are methods to measure the torque of a shaft more directly.A strain gauge circuit could be mounted to the rotating shaft and some type of wireless used to get the data. There are also non-contact magnetostriction/magnetoelastic torque sensors.

author: zapta
author_rank: Super Contributor
timestamp: December 26, 2023, 06:36:24 am
content: Here is the opposite approach, he measures the twist load to characterize the motor.

author: jbb
author_rank: Super Contributor
timestamp: December 26, 2023, 08:21:24 am
content: You definitely can get strain-gauge-on-spinning-shaft but they tend to be expensive, f###ing expensive or (my personal favourite) you can’t source the right readout electronics.Some of them use slip rings, some use some kind of coaxial transformer (1 for excitation + 1 for readout) and others appear to run on the anguished groans of postgraduate students.

author: Smokey
author_rank: Super Contributor
timestamp: December 26, 2023, 10:55:39 am
content: Obviously this is the high end of things, but it's an example of a way to measure torque in a motor system:https://www.himmelstein.comhttps://build-its-inprogress.blogspot.com/2016/04/torque-ripple-characterization-and.htmlDIY strain gauge system:Code:https://www.youtube.com/watch?v=nmabhQ0ZhnU

author: eTobey
author_rank: Super Contributor
timestamp: December 26, 2023, 09:29:21 pm
content: You can take a thin tube with a small diameter. And then place some markers on the same angle. Then take some sensors and compare how far (angle) they are apart. If you had taken a torque measurement with known values before, you can now calculate the torque.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2023, 02:22:38 pm
content: How accurate does it need to be? If it's not critical, then measuring the motor current can give a fairly good measure of torque.How about two encoders, a known distance apart on the shaft? The shaft will twist during operation, causing a phase shift in the pulses outputted by the encoders. The one nearer the shaft will lag the one closer to the motor. IF you know the Young's modulus of the shaft, then you'll be able to figure out the torque by how much it's twisted.

author: langwadt
author_rank: Super Contributor
timestamp: December 27, 2023, 02:38:11 pm
content: Quote from: Zero999 on December 27, 2023, 02:22:38 pmthat is how one type of torque meter used on big ships work, measuring the twist of the propeller shaft using two encoders

author: JoeyG
author_rank: Regular Contributor
timestamp: January 19, 2024, 07:14:48 am
content: Super helpful ALL! Many thanks

author: IDEngineer
author_rank: Super Contributor
timestamp: January 19, 2024, 03:28:44 pm
content: Seems like you'd need load cells on the motor mounts AND the driven load to capture all of the torque. Differences in mounting system compliance, torsional resistance in shafts, etc. could all conspire to make either end of the system not "equal and opposite".

author: mjkuwp
author_rank: Unknown
timestamp: January 19, 2024, 05:19:27 pm
content: I use HBM or Futek live shaft torque sensors to do this and they cost a few thousand USD. I have also built a system to measure reaction torque because this is a cheaper sensor. The cheapest would be to mount your motor on bearings that are coaxial to the load and use a lever arm to constrain the rotation of the motor. Use a compression load cell to measure the force and then obviously multiply by distance to get the torque. You will have a tiny bit of uncertainty in the measurement due to seal drag in the bearings on which your motor frame is mounted. I don't know if that would be a problem for you or not. This can be minimized by choosing shielded bearings (metal) instead of sealed bearings (rubber). the shields have lower drag than the rubber seals.There are many ways to do this and the method will depend on accuracy requirements, cost and of course if you intend to do this just one time or if this is to be designed into a product.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 19, 2024, 05:38:12 pm
content: Quote from: Andy Chee on December 21, 2023, 05:40:51 amAnother "crude" option is to mount the rearend of motor to a plate, center of plate had a hole or cylinder sticking out so the thing can be centered/held into a frame/jig. Then on that same plate you mount an arm perpendicular to shaft axis, that arm sticks out some distance and at some point it contacts a load cell.Lets say distance from shaft axis center to lever contact point is 1 meter, and when the motor torqs up it applies 1N of force onto a load cell. You just measured 1Nm of torque.