I am working on upgrading a research project on wind energy. They are trying to measure the performance of wind blade designs in a small wind tunnel (i.e. their models are relatively small).
What they were doing was measure the torque generated by the wind by mounting on the axis of the wind blade a bicycle discbrake and engaging it. (They have gone through many mechanical iterations)
Currently they are using a servo motor to pull a bike cable that activates the clamps. What they do is: blade is accelerated, and then the brakes are applied and bring the blade to a halt. While the blase is slowing down, they are measuring the torque and rpm. In order to maintain a constant force on the clamp they are using a spring to maintain a constant tension of the bike cable.
I am upgrading the measurement software side, and two of the wishlist feature requests were :
- to control the system to apply enough clamping force that will maintain the number or rpms. (apparently they had tried that in the past with a PID but it did not work out).
- to maintain a constant torque on the design (i.e. clamping force on the discbrake).
When I saw the system I could see the following issues:
- given the discbrake design, the available travel between engaging and free running is very small (a few hundred microns). That makes the system the system very non linear and very difficult to control (in a few microns you go from 0 to max braking power - even with a lever).
- there is a lot of play on the discbrake (its wobbly) - due to the mounting of the shaft- , however because the shaft is spinning at 3000 to 10000 rpm it tends to self align.
Overall, I wasn't hopeful that I could achieve the level of control with their current system.
So my question (inspired while I was reading this question) is what mechanisms can I look into to apply a controllable clamping force (or equivalenly clamping torque) on a 3[mm] spinning shaft which is rotating at 3000-10000 rpm?