Thanks, I was actually looking for a scale earlier but could not find it after asking a couple of people. One of the scales reads 298 g, while the other has a limit of 200g. Looks like it is indeed aluminium.
How about measuring the actual weight with a scale? There are a couple of scales on Yuta's desk.
Checked out the AUX PDH locking system at the XARM. Started by locking the AUX laser using the uPDH servo box and adjusting the test masses to maximize transmission (~0.6 achieved). There were some issues where the fundamental mode would be briefly visible and then lose lock. Higher order modes were also seen which could be removed by adjusting test masses. We also noticed the laser spot moving around a lot, as if the test masses were swaying. Finally after repeated tries we managed to lock and hold the laser to the cavity long enough to measure the open loop transfer function using the Moku:Go frequency response analyzer tool. Got an idea of the finicky and temperamental nature of the locking process.
Taking the transfer function data from the Moku:Go and using a Python script, found the UGF to be around 25.6 kHz and phase margin to be around 25.5 deg. My current goal is to keep reading up on control systems and related theory (I still feel like I lack understanding of the important principles needed), and parallelly making a small script that can take the transfer functions data and calculate some useful information (halfway done).
One issue I found with the script was that the Python control library was giving me a wrong value of Gain Margin (~0.26 where ~-5 was expected) while using the control.margin function. The other parameters phase margin and crossover frequencies agree with the data visually.
Vertex SUS DAC-0 card is kaput
Continuing our investigation of how a couple of coils seemed to be malfunctioning and the local damping was affected, we did the following:
- We calibrated ITMX actuation strength with ALS beat at 211.11 Hz and found 5.105 nm / count / f^2 or just 5% off from the previous values. So nothing seemed strange (e.g. 25% difference from single coil failure)
- We tried calibrating BS actuation strength and found it very hard to lock MICH. Furthermore the result was off by a factor of ~ 2 with respect to previous values!! Highly sus...
- We tried calibrating ITMX/ITMY actuation strengths and found they were also weaker by ~ 2 with respect to previous values. Extremely sus.. Attachment #1 summarizes these results.
- We decided to repeat our DW board transfer function estimates using single coil to Oplev transfer function measurements, but this also seemed harder in comparison with our last measurements. (couldn't even replicate using our dtt saved template results)
Finally we checked the analog electronics, breaking the BS AI to DW board connection of ULCOIL. We then used awggui to drive the single coil and the AI test mon output to read the commanded signal. We repeated this for a couple more coils, and also for other optics (PRM, SRM). We found that PRM had the same issue as BS that the DAC output couldn't drive positive voltages! In contrast, SRM worked fine. We then swapped the DAC adapter (see Attachment #2) units by swapping cables between SRM /MC3 and BS/PRM (D080303 -- not on DCC) and ruled out a failure from the buffered Test MON outputs on the AI board. So the problem was narrowed down to the DAC_0 card on c1sus chassis.
To try and fix this, we stopped the c1sus model, shutdown its frontend computer, and power cycled the IO chassis including the DC power at the front. This time, we tested the DAC outputs by driving the face coils separate from the side coils (running on DAC_2) using a 0.1 Hz sine excitation on ITMX, BS, ITMY and PRM from awggui and looking at the coil Vmon channels from the acromag on all suspensions. See Attachment #3-4 for a summary of all the tests (BS, PRM, ITMX, ITMY are bad, SRM, MC1-3 are good).
After verifying that this didn't fix the issue, we concluded that c1sus - DAC_0 fails to drive positive voltages.
- Replace DAC_0 with spare (does not seem to exist in our cds supplies)
- Swap the Side coil DAC (DAC_2) with the failing DAC (DAC_0) and drive the side coils with an offset since SIDE coils are not critical.
- Adapt the AI boards to be compatible with existing spare 18-bit DAC units.