Michael, Krishna
This morning, we started the pump-down of BRS-2 by temporarily removing the safety interlock valve, which exists to prevent back-filling of the vacuum in case of a power outage. We expect to pump down with the cart in a day or two and switch to the ion pump, so this arrangement is okay.
Michael and I then placed 3 sets of Piezo stacks under the BRS-2 platform. We then drove the piezo stacks using the Beckhoff DAC module and an amplifier (0-100 V).
The tilt-transfer function measurement consists of driving the platform in pure rotation and measuring the response signal in the autocollimator, at a few frequencies. As the BRS-2 is connected to the pump and is not thermally shielded yet, the low-frequency noise is substantial, hence the measurement takes several hours. The attached pdf shows the measurement and a model assuming d = 10 microns (d is the distance between the center of mass (COM) and the pivot, with positive indicating COM below the pivot). The transfer function is shown relative to the drive which was roughly 1.7 microradian amplitude.
Plan for tomorrow:
1. Open the vacuum can and shift the COM up by ~10 microns.
2. Close up and begin pump down again. Remeasure the tilt-transfer function.
I've added in a new measurement point and added in curves showing error bars on the d measurement.
Thanks to Betsy's efforts, we have a portable weighing scale which we will use to add ~0.56 grams of weight (a small custom washer) to the top of the beam-balance shifting the COM up by 9.5 microns.