K. Venkateswara

The damping turn-table for BRS is now functioning as expected. The attached pdf shows an example of how it damps the BRS beam balance when it exceeds a certain threshold (which is currently roughly 200 counts amplitude). I also show the control signal which controls the turn-tables position.

The plot starts with the BRS being damped with a Q of ~ 30. At 600s, I stopped the BRS code and doubled the feedback gain to reduce the Q to ~15. At 650s, the code was restarted. There was a spike in the control signal (due to the restart) which kicked up the beam-balance slightly after which it began damping correctly. The turn-table controller is set to turn off 700s after the threshold is reached which seems to be around ~1700s. After that the natural Q of the balance (~5000) is restored. The slow oscillation is due to the high-pass filter. Most of the parameters of this scheme are easy to change and I may improve them soon.

Before getting the turn-table to work correctly, I had to sort out a couple of issues: First, Richard McCarthy added a grounding cable to the platform on Wednesday but the 'grouding issue' I saw on Tuesday was not related to that. It was due to an intermittent short between the controller board and it's housing. After fixing that, the control signal looked clean. However a different issue cropped up: It looked like the turn-table would occasionally apply a huge torque on the beam-balance. I had seen this happen before so I realized that it was due to vibrational coupling to some higher-order mode of the balance (maybe bounce). The stepper motor and turn-table make some noise/vibration which, when it hits the right frequency, imparts a large torque on the balance. To fix this, it was necessary to change something on the turn-table so that the vibration produced was different. After a couple of trial and error attempts, I finally managed it by adding o-rings under some of the bolts of the turn-table.