"external" torque sensor
"external" torque sensor
(OP)
Hi,
here is my problem: I'd like to measure the torque produced by an electrical motor very accurately. (accurate<O.OO5 N.m)
The point is: I'd like to use an "external" sensor and not a classic axial torque sensor.
I've tried to use piezo electric sensors, in order to measure the reaction force between the housing of the motor and the ground.
But I have, with this solution, some vibration issues (1st natural frequency occurs at 260Hz..) because the pz sensors aren't stiff enough to support the motor.
Any idea/suggestion about external/indirect torque measurement ?
Any help would be appreciated!
Tom (french! sorry for my poor english, I did my best..)
here is my problem: I'd like to measure the torque produced by an electrical motor very accurately. (accurate<O.OO5 N.m)
The point is: I'd like to use an "external" sensor and not a classic axial torque sensor.
I've tried to use piezo electric sensors, in order to measure the reaction force between the housing of the motor and the ground.
But I have, with this solution, some vibration issues (1st natural frequency occurs at 260Hz..) because the pz sensors aren't stiff enough to support the motor.
Any idea/suggestion about external/indirect torque measurement ?
Any help would be appreciated!
Tom (french! sorry for my poor english, I did my best..)





RE: "external" torque sensor
I'm tempted to say mount it in a pair of bearings and then usa a small weight on an arm to counteract the torque
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: "external" torque sensor
Or else you could lean the leaver on a load cell instead of the weight.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: "external" torque sensor
RE: "external" torque sensor
the solution with the arm/calibrated weight or load cell looks interesting but I am not sure it would be accurate enough regarding the accuracy I need !( <0.005 Nm)
I'm looking for 'external' sensor because I'm afraid the shaft can't support any strain gage sensor..
Basically I had this idea: mouting the motor body in a pair of bearing and use piezo sensors (the most accurate I could find) to counteract the torque. But I've still some natural frequency issues and I'm not sure the bearing are stiff enough to allow such precise measurememt !
Anyway,BobM3 would you like to tell me more about the "optical methods" you described ? Do you know any company which could sell such equipment ?
RE: "external" torque sensor
http://vibrac.com/products/transducers.aspx
Sounds like you can't use an inline unit but maybe you can use the idea and make something that would work for your setup. I've also heard of using gear teeth and prox switches to measure the shaft deflection.
There's also magnetoelastic:
http://www.mdi-sensor.com/
RE: "external" torque sensor
" I've also heard of using gear teeth and prox switches to measure the shaft deflection." would you like to explain more precisely ?
Thanks for your contribution to this very helpful and professionnal forum !
Thomas
RE: "external" torque sensor
This method definitely requires some software processing of the two recovered signals. As the torsion bar twists under load, the load end becomes time delayed with respect to the drive end.
It is a case of measuring the load end delay time, and dividing this into the tooth to tooth time interval. These time delays are both rpm dependent, but the ratio stays the same for a given transmitted torque. Problems with vibration and time jitter require some heavy software averaging of many successive readings to get a smooth high resolution torque reading.
I have a rolling road chassis dynamometer at home that uses this method of torque measurement, and I wrote the software for it, so am acutely aware of the problems and limitations.
I much prefer Greg Locock's idea of a couple of support bearings and a torque arm. Very simple, very easy to calibrate, and highly accurate. This can rest on a weighing scale, or use a load transducer cell. Physical weights mounted on a torque arm can be a problem, because they may tend to thrash around between the mechanical limit stops.
RE: "external" torque sensor
RE: "external" torque sensor
RE: "external" torque sensor
Thanks for your contribution to this very helpful and professionnal forum !
Thomas