Hi,
I'm redesigning my dynamometer. I had been using a servomotor as a torque measuring load but I want to replace it with a rotating torque transducer and a frictional brake load. This is to reduce the load moment of inertia.
Torque transducers are expensive; $3,000 and up for a magnetostrictive 5Nm to 10Nm device and they are easily damaged by torque overload, typically 150% of rated value.
It occurred to me to look at making my own. Hooke's Law says there is a linear relationship between applied torque and resulting torsional angle of any elastic material. My idea is to take a long thin steel rod, apply torque to it and measure the resulting torsional angle. As long as it stays within the elastic limits, the angle will be proportional to torque. This angle would be measured by differencing the counts from encoders mounted at both ends of the torsion rod.
The drawback to using a spring is it stores mechanical energy. Rate damping must be used to prevent undesirable ringing (oscillation) in the spring. I have elected to use a viscous oil and vanes to provide 2nd-order damping. See the attached pdf.
The torsion rod (piano wire?) would have to be of sufficient diameter and length to require +/-500 to +/-1,000 in-oz (3.5Nm to 7Nm) torque to give a +/-45 degree torsion angle and still remain within elastic limits. A 2,048 line encoder at both ends would give +/-1,000 counts over that angle range.
The design of the rotor and stator vanes is such that they would mesh and prevent any rotation much beyond +/-45 degrees, protecting the torsion rod from over-torque. The torsion rod (red) is thin so it would be susceptible to 'whip' at high RPMs. That is why it is sheathed by the rotor (green) and stator (blue) assemblies. The torsion rod would be fixed (set screw?) at the ends but free to torsionally rotate within the respective sheaths. They would also add stiffess to the assembly, taking bending stresses off of the torsion rod.
I'm thinking this thing should have only an oz-in or so of bearing friction yet be able to measure 1,000 in-oz torques. Vane length and oil viscosity would have to be adjusted to give a critically damped mechanical response.
What do you think? All wet or just half-baked?
Mariss