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  SUS Lab eLog, Page 1 of 37  Not logged in ELOG logo
ID Date Author Typedown Category Subject
  60   Sun Nov 4 23:22:50 2007 waldmanUpdateOMCOMC PZT and driver response functions
I wrote a big long elog and then my browser hung up, so you get a less detailed entry. I used Pinkesh's calibration of the PZT (0.9 V/nm) to calibrate the PDH error signal, then took the following data on the PZT and PZT driver response functions.:

  • FIgure 1: PZT dither path. Most of the features in this plot are understood: There is a 2kHz high pass filter in the PZT drive which is otherwise flat. The resonance features above 5 kHz are believed to be the tombstones. I don't understand the extra motion from 1-2 kHz.
  • Figure 2: PZT dither path zoom in. Since I want to dither the PZT to get an error signal, it helps to know where to dither. The ADC Anti-aliasing filter is a 3rd order butterworth at 10 kHz, so I looked for nice flat places below 10 KHz and settled on 8 kHz as relatively harmless.
  • Figure 3: PZT LSC path. This path has got a 1^2:10^2 de-whitening stage in the hardware which hasn't been digitally compensated for. You can see its effect between 10 and 40 Hz. The LSC path also has a 160 Hz low path which is visible causing a 1/f between 200 and 500 Hz. I have no idea what the 1 kHz resonant feature is, though I am inclined to point to the PDH loop since that is pretty close to the UGF and there is much gain peaking at that frequency.
Attachment 1: 071103DitherShape.png
Attachment 2: 071103DitherZoom.png
Attachment 3: 071103LSCShape.png
Attachment 4: 071103DitherShape.pdf
Attachment 5: 071103DitherZoom.pdf
Attachment 6: 071103LSCShape.pdf
Attachment 7: 071103LoopShape.pdf
  63   Mon Nov 5 14:44:39 2007 waldmanUpdateOMCPZT response functions and De-whitening
The PZT has two control paths: a DC coupled path with gain of 20, range of 0 to 300 V, and a pair of 1:10 whitening filters, and an AC path capacitively coupled to the PZT via a 0.1 uF cap through a 2nd order, 2 kHz high pass filter. There are two monitors for the PZT, a DC monitor which sniffs the DC directly with a gain of 0.02 and one which sniffs the dither input with a gain of 10.

There are two plots included below. The first measures the transfer function of the AC monitor / AC drive. It shows the expected 2 kHz 2d order filter and an AC gain of 100 dB, which seems a bit high but may be because of a filter I am forgetting. The high frequency rolloff is the AA and AI filters kicking in which are 3rd order butters at 10 kHz.

The second plot is the DC path. The two traces show the transfer function of DC monitor / DC drive with and with an Anti-dewhitening filter engaged in the DC drive. I fit the antidewhite using a least squares routine in matlab constrained to match 2 poles, 2 zeros, and a delay to the measured complex filter response. The resulting filter is (1.21, 0.72) : (12.61, 8.67) and the delay was f_pi = 912 Hz. The delay is a bit lower than expected for the f_pi = 3 kHz delay of the AA, AI, decimate combination, but not totally unreasonable. Without the delay, the filter is (1.3, 0.7) : (8.2, 13.2) - basically the same - so I use the results of the fit with delay. As you can see, the response of the combined digital AntiDW, analog DW path is flat to +/- 0.3 dB and +/- 3 degrees of phase.

Note the -44 dB of DC mon / DC drive is because the DC mon is calibrated in PZT Volts so the TF is PZT Volts / DAC cts. To calculate this value: there are (20 DAC V / 65536 DAC cts)* ( 20 PZT V / 1 DAC V) = -44.2 dB. Perfect!

I measured the high frequency response of the loop DC monitor / DC drive to be flat.
Attachment 1: 07110_DithertoVmonAC_sweep2-0.png
Attachment 2: 071105_LSCtoVmonDC_sweep4-0.png
Attachment 3: 07110_DithertoVmonAC_sweep2.pdf
07110_DithertoVmonAC_sweep2.pdf 07110_DithertoVmonAC_sweep2.pdf
Attachment 4: 071105_LSCtoVmonDC_sweep4.pdf
071105_LSCtoVmonDC_sweep4.pdf 071105_LSCtoVmonDC_sweep4.pdf
  82   Thu Nov 8 00:55:44 2007 pkpUpdateOMCSuspension tests
[Sam , Pinkesh]

We tried to measure the transfer functions of the 6 degrees of freedom in the OMS SUS. To our chagrin, we found that it was very hard to get the OSEMs to center and get a mean value of around 6000 counts. Somehow the left and top OSEMs were coupled and we tried to see if any of the OSEMs/suspension parts were touching each other. But there is still a significant coupling between the various OSEMs. In theory, the only OSEMS that are supposed to couple are [SIDE] , [LEFT, RIGHT] , [TOP1, TOP2 , TOP3] , since the motion along these 3 sets is orthogonal to the other sets. Thus an excitation along any one OSEM in a set should only couple with another OSEM in the same same set and not with the others. The graphs below were obtained by driving all the OSEMS one by one at 7 Hz and at 500 counts ( I still have to figure out how much that is in units of length). These graphs show that there is some sort of contact somewhere. I cant locate any physical contact at this point, although TOP2 is suspicious and we moved it a bit, but it seems to be hanging free now. This can also be caused by the stiff wire with the peek on it. This wire is very stiff and it can transmit motion from one degree of freedom to another quite easily. I also have a graph showing the transfer function of the longitudnal degree of freedom. I decided to do this first because it was simple and I had to only deal with SIDE, which seems to be decoupled from the other DOFs. This graph is similar to one Norna has for the longitudnal DOF transfer function, with the addition of a peak around 1.8 Hz. This I reckon could very be due to the wire, although it is hard to claim for certain. I am going to stop the measurement at this time and start a fresh high resolution spectrum and leave it running over night.

There is an extra peak in the high res spectrum that is disturbing.
Attachment 1: shakeleft.pdf
Attachment 2: shakeright.pdf
Attachment 3: shakeside.pdf
Attachment 4: shaketop1.pdf
Attachment 5: shaketop2.pdf
Attachment 6: shaketop3.pdf
Attachment 7: LongTransfer.pdf
LongTransfer.pdf LongTransfer.pdf LongTransfer.pdf
Attachment 8: Shakeleft7Nov2007_2.pdf
Attachment 9: Shakeleft7Nov2007_2.png
  87   Fri Nov 9 00:23:12 2007 pkpUpdateOMCX and Z resonances
I got a couple of resonance plots going for now. I am still having trouble getting the Y measurement going for some reason. I will investigate that tommorow. But for tonight and tommorow morning, here is some food for thought. I have attached the X and Z transfer functions below. I compared them to Norna's plots - so just writing out what I was thinking -

Keep in mind that these arent high res scans and have been inconviniently stopped at 0.5 Hz Frown.

Z case --

I see two small resonances and two large ones - the large ones are at 5.5 Hz and 0.55 Hz and the small ones at 9 Hz and 2 Hz respectively. In Norna's resonances, these features arent present. Secondly, the two large peaks in Norna's measurement are at 4.5 Hz and just above 1 Hz. Which was kind of expected, since we shortened the wires a bit, so one of the resonances moved up and I suppose that the other one moved down for the same reason.

X case --

Only one broad peak at about 3 Hz is seen here, whereas in Norna's measurement, there were two large peaks and one dip at 0.75 Hz and 2.5 Hz. I suspect that the lower peak has shifted lower than what I scanned to here and a high res scan going upto 0.2 Hz is taking place overnight. So we will have to wait and watch.

Pitch Roll and Yaw can wait for the morning.
Attachment 1: Xtransferfunc.pdf
Xtransferfunc.pdf Xtransferfunc.pdf Xtransferfunc.pdf
Attachment 2: Ztransferfunc.pdf
Ztransferfunc.pdf Ztransferfunc.pdf Ztransferfunc.pdf
  93   Mon Nov 12 10:53:58 2007 pkpUpdateOMCVertical Transfer functions
[Norna Sam Pinkesh]

These plots were created by injected white noise into the OSEMs and reading out the response of the shadow sensors ( taking the power spectrum). We suspect that some of the additional structure is due to the wires.
Attachment 1: VerticalTrans.pdf
VerticalTrans.pdf VerticalTrans.pdf VerticalTrans.pdf VerticalTrans.pdf
  102   Wed Nov 14 16:54:54 2007 pkpUpdateOMCMuch better looking vertical transfer functions
[Norna Pinkesh]

So after Chub did his wonderful mini-surgery and removed the peek from the cables and after Norna and I aligned the whole apparatus, the following are the peaks that we see.
It almost exactly matches Norna's simulations and some of the extra peaks are possibly due to us exciting the Roll/longitudnal/yaw and pitch motions. The roll resonance is esp prominent.

We also took another plot with one of the wires removed and will wait on Chub before we remove another wire.
Attachment 1: VerticalTransPreampwireremovedNov142007.pdf
VerticalTransPreampwireremovedNov142007.pdf VerticalTransPreampwireremovedNov142007.pdf VerticalTransPreampwireremovedNov142007.pdf VerticalTransPreampwireremovedNov142007.pdf
Attachment 2: VerticalTranswiresclampedNov142007.pdf
VerticalTranswiresclampedNov142007.pdf VerticalTranswiresclampedNov142007.pdf VerticalTranswiresclampedNov142007.pdf VerticalTranswiresclampedNov142007.pdf
  105   Thu Nov 15 17:09:37 2007 pkpUpdateOMCVertical Transfer functions with no cables attached.
[Norna Pinkesh]

The cables connecting all the electronics ( DCPDs, QPDs etc) have been removed to test for the vertical transfer function. Now the cables are sitting on the OMC bench and it was realigned.
Attachment 1: VerticaltransferfuncnocablesattachedNov152007.pdf
VerticaltransferfuncnocablesattachedNov152007.pdf VerticaltransferfuncnocablesattachedNov152007.pdf VerticaltransferfuncnocablesattachedNov152007.pdf VerticaltransferfuncnocablesattachedNov152007.pdf
  158   Mon Aug 23 22:07:39 2010 JenneThings to BuySeismometryBoxes for Seismometer Breakout Boxes

In an effort to (1) train Jan and Sanjit to use the elog and (2) actually write down some useful info, I'm going to put some highly useful info into the elog.  We'll see what happens after that....

The deal:  we have a Trillium, an STS-2, a GS-13 and the Ranger Seismometers, and we want to make nifty breakout boxes for each of them.  These aren't meant to be sophisticated, they'll just be converter boxes from the many-pin milspec connectors that each of the seismometers has to several BNCs so that we can read out the signals.  These will also give us the potential to add active control for things like the mass positioning at some later time.  For now however, the basics only.

I suggest buying several boxes which are like Pomona Boxes, but cheaper.  Digi-Key has them.  I don't know how to link to my search results, so I'll list off the filters I applied / searched for in the Digi-Key catalog:

Hammond Manufacturing, Boxes, Series=1550 (we don't have to go for this series of boxes, but it seems sensible and middle-of-the-line), unpainted, watertight.

Then we have a handy-dandy list of possible sizes of nice little boxes. 

The final criteria, which Sanjit is working on, is how big the boxes need to be.  Sanjit is taking a look at the pinouts for each seismometer and determining how many BNC connectors we could possibly need for each breakout box.  Jan's guess is 8, plus power.  So we need a box big enough to comfortably fit that many connectors. 

  163   Tue Dec 21 06:58:09 2010 ranaThings to BuySeismometryTrillium Noise Plot

Nanometrics has a couple of seismometers which are cheaper than the T240 which may be of interest to us: better than the Guralp CMG-40T, but cheaper and easier to use than the STS-2.


  193   Thu May 5 21:39:07 2011 tara,ryanThings to BuyBladesblade holding block

We made a drawing for a structure hat will hold the maraging blade. The details aren't complete yet. The holes for the clamping will be  identified,  but the sketch shows the rough idea.

     We want to clamp the blade to a structure. The drawing for the clamp will be provided by Ryan (he found it in the dcc.) The structure is consisted of the base and the pillar. Although a monolithic structure is better, it might be to expensive to carve out a big piece of Al block, so Koji suggested that we do it like this. The base will be mounted on the table, and the pillar will be mounted on the base by 4 screws. The height of the pillar is not decided yet. It depends on how big the Al mass block we need to pull down the blade by its weight, and how the mirror for reflecting the beam up will be mounted, but it should be around 6 - 8 inches.

    The mass block will be used for mounting the end mirror of the interferometer + a translational stage. This way we can steer the beam with 2 mirrors and adjust the arm length. We will determine the weight, so we can estimate the size of the mass block, assuming we will use Al.


Attachment 1: base.PDF
Attachment 2: pillar.PDF
  196   Mon May 9 22:25:26 2011 tara,ryan, mingyuanThings to BuyBladesblade holding block

 We made a sketch for the weight clamp that will carry the mass block on the end of the blades. This will be done in Solidwork tomorrow.


   We plan  to load a block of mass under the tip of the blade by using a pair of knife edge pieces so that the rubbing between the mass block and the blade is minimized.

 The edge of the blade cannot be too large, or it will be noisy when the blade is driven. On the other hands, if the blade angle is too small (sharper blade), the stress on the blade due to the weight will be too large and cause plastic deformation on the blade, which we don't want. We plan to make it flat ~ 1mm wide, with 120degree open angle.

 The yield tensile strength of maraging steel is ~ 1 -2 GPa. With the contact area at the knife edge we can calculate the maximum clamping force.

The width of the edge is ~ 5cm

The thickness of the edge ~ 1mm.

so the maximum force should not exceed ~ 1 GPa x 0.05 m x 0.001 m ~10^4 newton.

We will use spring washers to make sure that we do not tighten the clamps together with too much force and cause plastic deformation on the blade.



Attachment 1: IMG_1554.JPG
  197   Tue May 10 16:59:36 2011 tara,ryan, mingyuanThings to BuyBladesblade holding block

We finalized the drawing for blade clamping system. The drawings are posted here and in Crackle ATF Wiki. We will submit the drawings to the machine shop tomorrow.

         For each blade, the clamping system will consist of: 1)Steel base, 2)Steel pillar, 3) Steel top clamp, 4) Al knife edge top piece,5)Al knife edge bottom piece,and 6) Al end piece.

1) Steel base x1:   The steel base is 3"x3"x0.5" . It has 4 counter sunk holes that allow us to mount the steel pillar on it. It has 3" rails on both sides, so we can mount it on the table. Extra clamps can be used to hold the base on the table.

2) Steel pillar x1:   It is 5.5" height with 2"x2" square cross section.  There are 4 tapped 1/4-20 holes , 1" in depth, on the bottom for mounting it on the base. There are 2 tapped 3/8 , 1" in depth, on top for clamping two clamps along with the blade.

3) Steel top clamping piece x1, This will clamp the blade on the pillar.

4) Aluminum knife edge, top piece x1,

5) Aluminum knife edge, bottom piece x1: (4&5) The two knife edge pieces will be used for loading the mass block on the maraging blade tip. The explanation is written in this entry.

6) Aluminum end piece that holds the mirror mount on the blade tip x1: We want to have a steerable mirror for the IFO. So we need a mirror mount. The block will hold the mount and the blade tip together through screws. This piece is uploaded in the above entry.



 The assembly (without the blade and the mirror mount) is shown below.


Attachment 1: base.PDF
Attachment 2: pillar.PDF
Attachment 3: edge_bottom.PDF
Attachment 4: top_edge.PDF
  200   Wed May 11 22:42:28 2011 tara,ryan, mingyuanThings to BuyBladesblade holding block

We submitted the drawing to the machine shop today. The works should be done before May 23rd.


   The base/ pillar/ blade clamp will be made from stainless steel. The knife edge pieces and mirror mount at the blade tip will be made from aluminum.


  209   Wed May 25 20:04:28 2011 taraThings to BuyCracklepurchases

I ordered opto mechanical mounts for turning the beam vertically. See the details in psl log.

I also orderedspring lock washers and wave washers. There will be used when we clamp the guillotine things for putting the load on the tip of the blade.

The pressure from the clamp should not exceed the yield strength of the maraging steel blade. So the spring lock washer should give us some limits of pressure on the blade. There is no specification about how much pressure it would be, so I ordered two kinds of washer for testing.

  626   Tue Mar 12 15:08:49 2013 ericqThings to BuyCrackleFiber thoughts

It is clear that the fiber situation needs to be improved. Rana says that the way to reduce the fiber's intensity noise is to use a long (5m?) length of fiber inside of the chamber, tied solidly to the stack; as the beam jitter induced intensity noise is a result of the jittering beam coupling into higher order modes. I am working on calculating the attenuation of higher order modes in a single mode fiber as a function of length, as this should indicate how much fiber we would need. 

I have also been in email contact with Dan Clark at Stanford, who did some fiber feedthrough work for a seismic platform interferometer. He sought to achieve a design that would be fully compatible with aLIGO standards, and thus considerably more stringent than the requirements for our situation. He got ahold of a metalized polarization maintaining fiber, and soldered it directly to a flange blank, with pigtails on each end. This has the advantage of only having metal-to-metal vacuum seals, so UHV can be achieved. However, he told me that this all lives out of the way, and doesn't need to be touched often.

Our situation differs somewhat, because we occasionally need to work near the flange, and thus have a greater risk of breaking a fiber that is semipermanently mounted to the flange. Additionally, our vacuum requirements are much looser. I think a feedthrough that has female fiber connectors on each side along with patch cables such as the one we're using would be a robust solution. This way, the fiber can be detached from the flange while leaving the coupling intact, if things need to be moved around. 

My current thought is that we can make a feedthrough out of something like this Thorlabs mating sleeve, since an assembled feedthrough (like from CeramTech, which may not even be polarization maintaining) is quite expensive.

Thus, the solution could be made of the following things:


  826   Mon Aug 11 15:55:46 2014 ranaThings to BuyCrackleFast PZT mirror for high BW locking

 In order to bypass the mechanical resonance problems that people have been having with the blades (i.e. they're not good for high BW locking), today we discussed using a stiff PZT mirror in one of the Michelson arms.

In principle, we would be concerned that we get crackle from this PZT element, but the LLO people have done some crackle measurements on the OMC PZTs so that we should have a good upper limit on that component and its good enough ??

Stiff, high BW, PZT actuated mirror mounts have been used for laser locking:

  1. http://holofringe.com/PiezoMirrors.html
  2. http://www.repairfaq.org/sam/lasersam.htm
  3. Nergis's thesis
  4. JILA idea

High Voltage Piezo drivers:

  1. http://www.falco-systems.com/products.html (~1500$)
  2. http://www.thorlabs.us/thorproduct.cfm?partnumber=MDT694B (~1000$)
  1152   Tue Aug 18 15:50:55 2015 GabrieleThings to BuyCrackleDesign of new magnet clamps and test blades wire clamps

We want to avoid gluing magnets at all. So I designed some small plates that will host the magnet in the center, clamped with set screws. Those small plates can then be attached to the breadboard and to the blocks using screws. The plates have some slots where the screws will go in, so that we can adjust thei vertical position.

There are two different versions, one for the breadboard magnets (6 pieces) and one for the block later magnets (4 pieces). I also redesigned the triangular stands that we are using for the vertical magnets: in the bottom side you can see the four tapped holes where the magnet plate will be attached. When we want to install the magnets using the plates, we'll have to remove the two lateral stiffening beams and have the holes machined. Hopefully we can do that quickly at the machine shop.

I also redesigned the wire clamps for the test blades: the hole positons have been corrected to preperly align to the holes in the blades. Here too I added a clamp for the magnet, so no glue anymore.

Attachment 1: MagBlock.PDF
Attachment 2: MagClamp.PDF
Attachment 3: TestBladeWireClampBlockNew.PDF
Attachment 4: TestBladeWireClampNew.PDF
Attachment 5: VertMagStand.PDF
  1335   Thu Oct 29 08:38:12 2015 GabrieleThings to BuyCrackleElectronics V2

Orders have been placed for the improved version of the electronics. The basic ideas are described in T1500539 and detailed schematics are available in D1500402.

  1337   Thu Oct 29 12:29:35 2015 GabrieleThings to BuyCrackleMechanical upgrades

The drawings of the Crackle2.1 mechanical upgrade are available from LIGO-E1500420-x0. Thanks Eddie!

  58   Fri Nov 2 12:18:47 2007 waldmanSummaryOMCLocked OMC with DCPD
[Rich, Sam]

We locked the OMC and look at the signal on the DCPD. Plots included.
Attachment 1: 071102_OMC_LockedDCPD.gif
Attachment 2: 071102_OMC_LockedDCPD.pdf
  59   Sat Nov 3 16:20:43 2007 waldmanSummaryOMCA good day's work

I followed up yesterday's test of the PZT with a whole mess of characterizations of the PZT control and finished the day by locking the OMC with a PZT dither lock and a 600 Hz loop. I haven't analyzed any of the data yet, so its not calibrated in physical units and etc. etc. etc. Since a lot of the sweeps below are of a "drive the PZT, look at the PDH signal" nature, a proper analysis will require taking out the loop and calibrating the signals, which alas, I haven't done. Nonetheless, I include all the plots because they are pretty. The files included below are:

  • DitherLock_sweep: Sweep of the IN2/IN1 for the dither lock error point showing 600 Hz UGF
  • HiResPZTDither_sweep: Sweep of the PZT dither input compared to the PDH error signal. I restarted the front end before the sweep was finished accounting for the blip.
  • HiResPZTDither_sweep2: Finish of the PZT dither sweep

More will be posted later.
Attachment 1: 071103_DitherLock_sweep.png
Attachment 2: 071103_DitherLock_sweep.pdf
Attachment 3: 071103_HiResPZTDither_sweep.png
Attachment 4: 071103_HiResPZTDither_sweep.pdf
Attachment 5: 071103_HiResPZTDither_sweep2.png
Attachment 6: 071103_HiResPZTDither_sweep2.pdf
  178   Wed Jan 26 10:34:53 2011 JanSummarySeismometryFIR filters and linear estimation

I wanted to write down what I learned from our filter discussion yesterday. There seem to be two different approaches, but the subject is sufficiently complex to be wrong about details. Anyway, I currently believe that one can distinguish between real filters that operate during run time, and estimation algorithms that cannot be implemented in this way since they are acausal. For simplicity, let's focus on FIR filter and linear estimation to represent the two cases.

A) FIR filters


A FIR filter has M tap coefficients per channel. If the data is sampled, then you would take the past M samples (including sample at present time t) of each channel, run them through the FIR and subtract the FIR output from the test-mass sample at time t. This can also be implemented in a feed-forward system so that the test-mass data is not sampled. Test-mass data is only used initially to calclulate the FIR coefficients, unless the FIR is part of an adaptive algorithm. For adaptive filters, you would factor out anything from the FIR that you know already (e.g. your best estimates of transfer functions) and only let it do the optimization around this starting value.

The FIR filter can only work if transfer functions do not change much over time. This is not the case though for Newtonian noise. Imagine the following case:


where you have two seismometers around a test mass along a line, one of them can be closer to the test mass than the other. We need to monitor the vertical displacement to estimate NN parallel to the line (at least when surface fields are dominant). If a plane wave propagates upwards, perpendicular to the line, then there will be no NN parallel to this line (because of symmetry). The seismic signals at S1 and S2 are identical. Now a plane wave propagating parallel to the line will produce NN. If the distance between the seismometers happens to be the length of the plane wave, then again, the seismometers will show identical seismic signals, but this time there is NN. An FIR filter would give the same NN prediction in these two cases, but NN is actually different (being absent in the first case). So it is pretty obvious that FIR alone cannot handle this situation.

What is the purpose of the FIR anyway? In the case of noise subtraction, it is a clever time-domain representation of transfer functions. Clever means optimal if the FIR is a Wiener filter. So it contains information of the channels between sensors and test mass, but it does not care at all about information content in the sensor data. This information is (intentionally if you want) averaged out when you calculate the FIR filter coefficients.

B) Linear estimation


So how to deal with information content in sensor data from multiple input channels? We will assume that an FIR can be applied to factor out the transfer functions from this problem. In the surface NN case, this would be the 1/f^2 from NN acceleration to test-mass displacement, and the exp(-2*pi*f*h/c) - h being the height of the test mass above ground - which accounts for the frequency-dependent exponential suppression of NN. Since the information content of the seismic field changes continuously, we cannot train a filter that would be able to represent this information for all times. So it is obvious, that this information needs to be updated continuously.

The problem is very similar to GW data analysis. What we are going to do is to construct a NN template that depends on a few template parameters. We estimate these parameters (maximum likelihood) and then we subtract our best-estimate of the NN signal from the data. This cannot be implemented as feed forward and relies on chopping the data into stretches of M samples (not necessarily the same value for M as in the FIR case). Now what are the template parameters? These are the coefficients used to combine the data stretches of the N sensors. This is great since the templates depend linearly on these parameters. And it is trivial to calculate the maximum-liklihood estimates of the template parameters. The formula is in fact analogous to calculating the Wiener-filter coefficients (optimal linear estimates). If we only use one parameter per channel (as discussed yesterday) or if one should rather chop the sensor data into even smaller stretches and introduce additional template coefficients will depend on the sensor data and how nature links them to the test mass. Results of my current simulation suggest that only one parameter per channel is required.

When I realized that the NN subtraction is a linear estimation problem with templates etc, I immediately realized that one could do higher-order noise subtraction so that we will never be limited by other contributions to the test mass displacement (and here I essentially mean GWs since you don't need to subtract NN below other GWD noise, but maybe below the GW spectrum if other instrumental noise is also weaker). Something to look at in the future (if this scenario is likely or not, i.e. NN > GW > other noise).

  183   Fri Jan 28 21:09:56 2011 JanSummarySeismometryNN subtraction diagram

This is how Newtonian-noise subtraction works:


  257   Tue Jul 26 02:16:54 2011 haixingSummarySUSforce measurement

Yi and Haixing,

In the afternoon, we made a force measurement between the 1" diameter magnet and 1/2" diameter magnet.
The experimental setup goes as follows:

By adjusting the distance between the two magnets, we can obtain force as a function of distance. We measured
the repelling force instead of attracting force, which avoids two magnets getting stuck to each other. The measurement
data are listed below:

weight (g)         distance (mm)
   0.05                    95.53
   0.06                    90.92
   0.07                    88.00
   0.08                    84.60
   0.09                    82.29
   0.10                    79.39
   0.11                    77.27
   0.15                    75.27
   0.17                    71.61
   0.23                    68.29
   0.27                    64.86
   0.33                    61.59
   0.40                    57.15
   0.48                    56.33
   0.60                    53.21
   0.75                    50.22
   1.01                    47.11
   1.20                    43.40
   1.38                    40.90
   1.59                    39.50
   1.90                    38.00
   2.32                    35.89
   2.78                    33.61

We made a fit with the analytical expression for the force between two current loops, which is a
good approximation for the force between two thin disk magnets (separation larger than their thickness).


The fitted curve is shown by the figure below [the right one is the zoom-in version of the left one]:

We will make a similar measurement for other three pairs of magnets tomorrow morning, which allows us to calibrate the mismatch
and calculate how much DC biased current in the control coil is needed to counteract the mismatch.


  258   Tue Jul 26 03:04:33 2011 ranaSummaryCrackleAC Biased Resistor Bridge setup

As we discussed in the meeting today, I set up a Wheatstone Bridge with an AC bias to see if we could make a test of the Crackle demod setup.

I used 5.5k wirewound resistors. Since they're wirewound, I expect almost no excess noise (c.f. Frank's thesis). I used clip probes to take the differential bridge into the A and B inputs of an SR560 set to (A-B) mode, with a gain=1000, and a 10kHz low pass.

I started with a pile of 1% resistors and quickly found a pair that gave a differential bridge voltage of ~0.8 mVpp with a AC bias of 8 Vpp. So the balance is very good (1 part in 10000).

The attached plot shows the differential bridge voltage (I took the SR785 data and divided out by the G=1000 of the SR560). Also plotted is my calculation for the Johnson noise of the bridge.

The drive frequency is ~9.1 Hz. Clearly there is a lot of harmonic distortion at this bridge measurement point.


Q1)   How will we measure the excess resistor noise in the presence of this distortion?

Q2)   Will there be a similar distortion issue in the blade Crackle measurement?

Attachment 1: bnoise.png
  259   Tue Jul 26 13:48:11 2011 haixingSummarySUSorder list for maglev

Today, Steve helped me to order more magnets and other mechanical parts for the maglev.
The detailed items go as follows:
1.  1" diameter and 1/32" thickness magnets (Grade N42). Quantity: 50.  The Supplier: K & J magnetics
     [The reasoning for the quantity is due to its large variance in the magnet strength, as shown in the ELOG 256]
2.  1/2" diameter and 1/8" thickness magnets (Grade N42). Quantity: 20.  The Supplier: K & J magnetics
3.  1 pack of Brass fully threaded 1/2" rods [They are used as flags in the position sensing]
4.  4 packs of 5 precision stainless spring (0.18" outer diameter and 0.018 wire)  The size of the spring is choosen
     in such a way that it can fit into a 8-32 screw. [This is for the cross coupling measurement. With spring, we can
     first create a stable setup and measure the cross coupling by driving the levitated plate with coil (see the schematics below) ]
5.  4 packs of 5 precision stainless spring (0.18" outer diameter and 0.026 wire). This is another size for the same
     purpose of cross coupling.

In addition, I used techmart to order another BNC terminal block [with 18 analog inputs and 2 analog outputs. The
type is 2090A, and the link is given by http://sine.ni.com/nips/cds/view/p/lang/en/nid/203462] for the national instruments
DAC card. We had already got one in the basement lab. This new ordered one gives us additional two analog outputs.
In total, we will get four analog outputs which would be enough for the first-step digital control before Cymac
will be available in one month.


  270   Thu Jul 28 23:36:35 2011 ranaSummaryCrackleAC Biased Resistor Bridge setup

I upgraded the Resistor Bridge setup and now have pretty good results:

  1. Most of the distortion was coming from the SR560 which I was using as the inst. amp. I put a AD620 (in the gain=1000 config) onto the proto-board and the distortion went way down.
  2. Next was to replace the DS345 with the signal source from the SR785. This is a very low distortion source and removed the presence of much harmonics (and sub-harmonics).
  3. Then I trimmed the bridge by hand: the main resistors are 5.5k. I needed to adjust the bridge legs by ~0.1-0.2 Ohms. I got some 11.3 and 11.5 Ohm 1% resistors from the kit and the appropriate combo gave me a fractional balance of 120 uV / 5 V = 2e-5.

Using a freq bin width of 31 mHz, I find that the residual 2f and 3f peaks are pretty small. Now, I should use the AD734 multiplier chip to square the signal and try out the 2f demod.

Attachment 1: a.png
  277   Thu Aug 4 19:49:17 2011 Yi and HaixingSummarySUSupdate on the current status of maglev

An update on the current status of maglev:

(1) We installed the springs in our setup. Right now, the levitated plate is held stably by those springs.
     We are going to measure the cross coupling, once the Labview is working. In addition, we need to have
     four current buffers to drive the coils.

(2) The second BNC terminal block for the National Instrument card has arrived. Now, we have enough
     input channels (36 in total) and output channels (4 in total) to implement the feedback control. I am current
     learning labview and Jan Harms is helping me use it to take data.

(3) The new magnets have arrived, and we will make similar measurements of the strength distribution, as
      we did earlier. We try to find better matched magnets, possibly down to maybe 1%.

(4)  We measured the current force on the levitated magnets. The measurement setup is the same as
      the magnetic force measurement, except for that we now connect the coil to a DC power supplier to
      deliver a constant current flow to the coil. This measurement allows us to determine the DC biased current
      that needs to counteract the imbalance in the DC magnetic force. The data is under analyzing, and the result
      will be posted soon.


  314   Tue Aug 16 09:23:19 2011 Yi and HaixingSummarySUSupdate on maglev

Yesterday, we test the ADC and DAC for the maglev. Specifically, we measured the time delay in the
digital path by using the method suggested by Koji. This is useful for us to model the stability of the system.
Basically, we connect the analog input (AI) to a function generator, and make a direct connection in the
Labview from the ADC and DAC.We then compare the time delay between the signals from the function
generator and from the analog output  (AO). The setup is shown by the figure below [we have rescaled
the two outputs for a clear display]:

Labview virtual-instrument file for the ADC and ADC:

We used function generator to produce square waves at different frequencies: 50 Hz, 100 Hz, and 200 Hz
to avoid the possibility of delaying by an integral number of periods if we only use one frequency. These three
frequencies measurement all give the same measurement result---the delay is around 2.8 ms.

We are now trying to put together all four AI and AO channels. We need to make corresponding change to the
SISO PID controller to MIMO. We are still working on it. A flash show of the unfinished PID controller.vi
is shown by the figure below [We are making very simple modifications to the  example given in



  318   Thu Aug 18 01:19:21 2011 Larisa ThorneSummaryCrackleVanessa and Larisa's LIGO SURF presentation

 Hopefully y'all logged on to EVO and listened to our talk.

Attachment 1: Crackling_Noise_in_Blade_Springs.pptx
  334   Mon Aug 29 11:25:24 2011 Yi and HaixingSummarySUSlevitation of one degree of freedom

After modifying the digital and analog part, we demonstrated levitation of a single degree of freedom [one corner of the
the levitation plate (as explained in the previous Elog 321)]. This time, we again use the trick of eddy-current damping
by placing an aluminum plate above the levitated magnet to obtain enough phase margin. Because we have a poor modelling
of our plant, the circuit we design [the details will be shown below] cannot provide enough phase margin. We are now
trying to measure the plant transfer function [only possible if it is levitated] and modify our circuit. In the next stage, we will try
to levitate two corners of the plate, which has two degrees of freedom, before we jump to levitate the entire plate (three degrees
of freedom that need to be controlled in the current scheme).

We took a photo of the plate corner and also the control and error signals from the oscilloscope.

levitation.png control_signal.png

(the yellow curve shows the error signal from the sensor and the blue curve shows the control signal).

Yesterday, we try to measure the entire open-loop transfer function [especially to get the TF for the plant part] by
injecting signal after the sensor with preamplifier SR560 as a summing amplifier (as shown by the figure below). Due to
the transient signal (before reaching the steady state) from the OSEM always saturate the SR560, we cannot get the right
control signal to achieve a stable levitation. We then try to use Labview to measure the transfer function by using the
build-in vi: "response function measurement. vi", but the resulting curve is very bumpy and we cannot make any sense out
of it. The possible solution is to make our own summing amplifier which allows a large voltage input and output.


During the last few days, we have modified both the digital and analog parts of our system. The detailed modifications and
related issues are shown as follows:

>> Digital part:

[TF measurement] We change the time-loop structure, and now the sampling rate becomes higher than what has been shown
in Elog 321. To tell the new sampling rate and the time delay of the digital path, we use SR785 to make a direct frequency
measurement, instead of using oscilloscope. We can tell the time delay from the phase. The bode plot of the TF for the digital path
[a direct path with 2-order low pass filter around 170 Hz] is shown by the figure below:

From this curve, we learn that the sampling rate is around 300Hz (from the dip of the spectrum?), and the time delay is 4.6ms
---not a very decent DAQ, but sufficient at this preliminary stage.

[issue in computational power] We found a very critical problem in our digital system---the computer does not work properly (the computer is
a quite old one) and screws up the gain if we run other programs simultaneously (even open IE) or other graphic processes. Below we show the
difference in TF of the digital part between turning on and off the waveform chart in Labview for showing real-time control signal.

As we can see that the gains at high frequencies (above 5Hz) go down significantly.

[issue in PID controller] Initially, we used the build-in "PID.vi" in the Labivew to try the digital control. As it turns out that the derivative part of the
PID does not work properly
. We can clearly see many spikes in the control signal if we set the gain of the derivative control to be nonzero. This is
partially because the discreteness of the signal from the ADC, and the simply derivative control in the "PID.vi" is not band-limited. The high frequency
part of the signal screws up the derivative controller. Instead, we realize lead compensation by using an analog circuit. In the mean time, we will
try to add lead compensation by using a digital filter which is band-limited.

>> Analog part:


We have made many small changes to the analog circuit [as shown by the figure above]. Initially, channel two and three are coupled, as we want their signal difference.
Now we decouple them. We then have four parallel channels for the feedback control. We replaced many components to realize the following transfer function
[left panel shows the amplitude and the right panel shows the phase in degree (red cure is the calculated one and blue curve is measured one)]:

Initially, we thought that we have a reasonable good understanding of the plant, and the above circuit can provide a stable levitation
by using the Nyquist stability criterion, which turns out to be not the case. The design open-loop transfer function has a unit gain
frequency at 8Hz with a phase margin of 20 degree, as shown by the figure below:

The transfer function that we assumed for the plant goes as follows [based on our measurement]:

* coil to levitated magnet: 7.7 x 10^{-3} N/V
* magnet itself is modelled by a negative spring---the negative spring constant is -50N/m with mass equation to 240g
* the flag to the sensor (i.e., the displacement to the sensor output): 71 V/m

Now by using the stably-levitated system, we will be able to have more accurate measurements of the plant TF, and we can then
figure out what is the right filter for the lead compensation.

  339   Thu Sep 1 21:10:58 2011 dan, valeraSummaryCrackleComments on Michelson ifo noise for crackle experiment

Here are some comments:

- The noise from Michelson ifo, that Dan posted yesterday, appeared to be just above the SR785 noise. But now Dan knows how to do the whitening to beat this noise down. The Michelson spectrum was not corrected for the loop gain. The voltage noise from Michelson was ~30nV/rtHz refered to the PD output at 100 Hz.  Today we measured the Thorlab PD100A dark noise to be around 15 nV/rtHz at 100 Hz (not bad for a cheap PD with ~10 V full range). We also tried to measure the laser intensity noise and found that we would expect it to be several times higher than the Michelson in-lock spectrum we got yesterday(?). The laser noise measurement was done by blocking one of the arms with a black glass dump. So the laser noise needs more investigation.

- Also for the reference the free swinging Michelson ifo p-p value was 60 mV. The DC value of the mid-fringe was also ~60 mV. So the contrast defect (Pmin/Pmax) was not great ~30%.

- Frank turned off the computer in the SUSlab for us as this computer was the largest audible source of noise. We expected to see the reduction of acoustic peaks in the spectrum around few hundred Hz but we are not able to lock the Michelson today for unknown reason. There were virtually no changes from yesterday configuration apart from some minor alignment due to replacement of the laser post. The p-p value is the same as yesterday. Anyway the chamber will be pumped eventually so the acoustic noise will not be a problem.

- We borrowed a SR650 from the CDS group to do the power demodulation measurement. We found that if the SR650 is setup to bandpass 80(hp)-100(lp) Hz then a 2 V p-p sine wave at 1 Hz is attenuated by the filter to 70 uV p-p value.

- Another option for the power demodulation is to record the time serieses of the Michelson output and the excitation signal using SR785 - it holds up 9 hours of data at 256 Hz - and then do the digital bandpassing, squaring, frequency doubling, and demodulation all in Matlab. 

- The up shot is that with the noise of 5e-14 m/rtHz above 100 Hz one can start doing the power demodulation to go below this noise by another factor of ~10 or more.

Attachment 1: pdnoise.png
Attachment 2: lasernoise.png
  341   Sat Sep 3 12:58:24 2011 valeraSummaryCrackleComments on Michelson ifo noise for crackle experiment

The laser noise measurement could have been compromised by clipping or scattering since we added the weight to the stack between the time when the Michelson noise was taken and the laser noise was measured. After that Dan found that the  stack was touching the chamber. So I suggest that the laser noise measurement should be repeated right after the low noise Michelson spectrum is achieved.


Here are some comments:

- The noise from Michelson ifo, that Dan posted yesterday, appeared to be just above the SR785 noise. But now Dan knows how to do the whitening to beat this noise down. The Michelson spectrum was not corrected for the loop gain. The voltage noise from Michelson was ~30nV/rtHz refered to the PD output at 100 Hz.  Today we measured the Thorlab PD100A dark noise to be around 15 nV/rtHz at 100 Hz (not bad for a cheap PD with ~10 V full range). We also tried to measure the laser intensity noise and found that we would expect it to be several times higher than the Michelson in-lock spectrum we got yesterday(?). The laser noise measurement was done by blocking one of the arms with a black glass dump. So the laser noise needs more investigation.

  348   Thu Sep 8 21:55:46 2011 haixingSummarySUSfull levitation

The levitated plate is finally fully levitated without any physical touch of the tuning screws, but
with Eddy-current damping
for gaining enough phase margin [we need to modify the circuit to
remove the aluminum plates].


The error signal of the OSEM and the control signal for the coils are indicated in the following
figure of the Labview front panel (the fourth channel is not used, as we only have three degrees
of freedom to control right now):


The procedure for this levitation goes as follows: we first lock the magnet 1, and then magnet 3.
After the steady state is reached, we slowly increase the proportional gain of magnet 4 up to 0.5.
When the error signal from the OSEM approaches to zero, we gradually detach the tuning screw.
The changes have to be made very slow such that the control has time to response,
as our control bandwidth is quite small.  


Somehow, we are lucky in the sense that the three degrees of freedom [pith, yaw and vertical]
are weakly coupled to each other
. We realize the levitation, by simply using three independent

To realize this, I made few small improvements of the maglev device:

1. The bottom fixed plate is adjusted such that it gives less constraint on the position of the OSEM to
    avoid the flags to touch the edge of OSEM (my newly designed ones do not work due to crappy
    hand-making by myself).

2. The tuning screws are wrapped with Teflon tapes to make them firm. Previously, the tapped holes
    are slightly larger than the screw size, and we cannot use them for a very fine tuning. Especially,
    they drift around during the transient times, as they are hit by the vibrating levitated plate.

3. We reattached those aluminum plates for Eddy-current damping.

We now need to fully characterize the system.

PS: The video for this levitation does not look awesome, so I did not post it ;-)

  353   Sat Sep 10 01:57:46 2011 DanSummaryCrackleSummary in August

Summary in August


Attachment 1: summary.PDF
summary.PDF summary.PDF summary.PDF summary.PDF summary.PDF summary.PDF summary.PDF
  360   Thu Sep 22 15:45:08 2011 Vanessa AconSummaryCrackleLIGO Summer project summary and report

 I attached the pdf of my summer project paper for anyone's perusal.

Attachment 1: Progress6.pdf
Progress6.pdf Progress6.pdf Progress6.pdf Progress6.pdf Progress6.pdf Progress6.pdf Progress6.pdf Progress6.pdf
  449   Tue Apr 10 21:16:29 2012 RanaSummaryCoating QDisk was stuck: started new sweep


 The sweeps looked strange to me, as if there was something broken. Zach assured me that the solder/paste is sound. He tested the voltage continuity of the ESD just before pumping down.

I used the HV knob to change the ESD's DC voltage from -2 kV to +2 kV to look for arcing, etc. There is indeed some arcing (the next version should have less pointy edges).

However, the big discovery for me is that any voltage beyond +/- 500 V is enough to stick the optic to the ESD by the electrostatic force. All of our runs for the past few days have been at +1-2 kVdc, so the optic was stuck the whole time.


I have now set the DC voltage to +200 V and confirmed by eye that the optic is swinging (its obvious with a flashlight): the red laser beam swings around in the chamber with a several second period.

I have set up a sweep with a 0.1 Vpp with a 200 Hz span around 5030 Hz and with a 5 sec settling time and a 55 s integration time per point. Also the 'auto resolution' feature of the sweep is on. Let's see what happens.


Next time around, we should set up active damping or make the yaw frequency higher by a factor of 5-10.

  450   Tue Apr 10 21:35:27 2012 ranaSummaryCoating QFEM w/ COMSOL

 I took the Giordon FEM file and changed the material from 'silica glass' to 'Corning 7940' since I think the material parameters more closely resemble the thing we have in the can.

Then I changed the mesh size parameter to 'Extremely Fine'. The eigenfrequencies changed with every resolution, which indicates that we're not yet meshing fine enough...

I've uploaded a new version to the SVN: its in COMSOL/CoatingQ/  where we can put all of our coating Q matlab codes, documents, models, etc.

Attachment 1: modes.gif
modes.gif modes.gif modes.gif modes.gif modes.gif modes.gif modes.gif modes.gif
  454   Fri Apr 13 16:37:52 2012 ranaSummaryCoating QFEM w/ COMSOL

  Since we care about STRESS induced bi-refringence, I've plotted here the radial and azimuthal stress components for the modes instead of displacement.


Attachment 1: mode_stress.gif
mode_stress.gif mode_stress.gif mode_stress.gif mode_stress.gif mode_stress.gif
Attachment 2: mode_stress_phi.gif
mode_stress_phi.gif mode_stress_phi.gif mode_stress_phi.gif mode_stress_phi.gif mode_stress_phi.gif
  577   Thu Oct 4 11:34:03 2012 HaixingSummarySUSUpdate on Maglev

Here I give an update on the current status of the maglev project.


The setup:

The solidworks schematics (false-colored) for the setup is shown by the figure below, from top to bottom:


  1. Top fixed plate (in gray): It is used to mount the coil bobbins and also three linear DC motors (not shown in this schematics) for pushing the floating plate to the working position.
  2. Six coil bobbin (white cylinder): Three of coils (in red-orange) are for counteracting the DC mismatch of the magnets; the other are for controlling the vertical motion of the floating plate. On the bottom of each coil bobbin, there is a magnet which attracts the magnet mounted on the floating plate [mentioned later]. In the hollow center of the bobbin, there is the hall-effect sensor which has a large linear dynamical range and is for acquiring first-stage locking of the floating plate before switching to the short-range optical-lever sensing.
  3. Floating plate (in green): This is the central part of the setup and there are six magnets mounted on the plate (push-fit). We want to levitate it and lock it around the local extrema of the magnetic force between the magnets on the bobbin and on the floating plate, which ideally would have a very low rigidity and achieve a low resonant frequency levitation (for seismic isolation).
  4. Corner reflector (in gold): There are three corner reflectors, and together with the laser form the optical lever to sense the six degrees of freedom of the floating plate. The sensing of different DOFs are coupled to each other, and we need to diagonalize the sensing matrix. This is also where the long-range hall-effect sensing comes into play and it allows us to first lock the floating plate, and we can then diagnose the coupling for the optical-lever sensing.
  5. Small-coil bobbins (small white cylinder): These are for the first-stage sensing and controlling the horizontal motion of the floating plate before switching to the optical-lever sensing. In each of them, there is also a hall-effect sensor.
  6. Collimator  (on the gray mirror mount): This is to fix the optical fiber for the 635nm laser light, which is part of the optical-lever sensing mentioned earlier.
  7. Mirror (on the green mirror mount): In the next-stage experiment, this will be one of the mirror for the Fabry-Perot cavity. On the side of the mirror, we have mounted two small magnets, which are for sensing and controlling the angular degree of freedom of the floating plate.
  8. Middle fixed plate (in between the poles): This plate is to mount the two small-coil bobbins (for sensing and controling the angular DOF). In addition, there are three linear DC motors (not shown in this schematics) mounted on this plate, together with the three DC motors mounted on the top fixed plate, we can place the floating place to be near the working position and also prevent the floating plate stuck to the magnets (very strong) on the coil bobbins.
  9. Quadrant photo-diode (QPD) box (in blue on the bottom fixed plate): In each of box, there is a QPD which is to sense the reflected laser light from the corner reflector on the floating plate. We are using Hamamatsu 4-element photodiode S4349 . Together with the corner reflectors, they form the short-range optical-lever sensing.

Current status:


  1. Mechanical parts: There are two parts not yet ready: the small bobbins and auxiliary components attached on the linear DC motors, due to later modifications to the earlier design; all the other parts are in place. The coil bobbins are now winded with coils.
  2. Optical parts: Apart from the mirror mounts, the main components Laser diode: LPS-635-FC - 635 nm, 2.5 mW, A Pin Code, SM Fiber-Pigtailed Laser Diode, FC/PC; Diode mount: TCLDM9 - TE-Cooled Mount For 5.6 & 9 mm Lasers;  Driver: LDC201CU - Benchtop LD Current Controller, ±100 mA; Coupler (for splitting): FCQ632-FC - 1x4 SM Coupler, 632 nm, 25:25:25:25 Split, FC/PC; Collimator: F280SMA-B - 633 nm, f = 18.24 mm, NA = 0.15 SMA Fiber Collimation Pkg; Collimator adapter: AD11NT - Ø1" (Ø25.4 mm) Unthreaded Adapter for Ø11 mm Collimators are now ready.
  3. Analogy Electronic parts: The pcb boards for the hall-effect sensors and QPD box have been fabricated, and now need to be stuffed. The coil drivers are not yet ready.
  4. Digital parts: The binary input-output box has not yet powered up. The pcb board for the chassis power just arrived, and needs to be stuffed. Rana and I worked out the schematics for AA/AI but not yet the pcb layout.

Plan (Assembly stage):


  1. Mechanical parts: The small bobbins and auxiliary components for the linear DC motors need to be fabricated. The lead time is around three to four weeks. During this period, I will mostly work on the the electronics and also try to get the digital part ready (for this I need helps from Rana and Jamie). In addition, I will design a closure for covering the setup to reduce some noise from the air and acoustics.
  2. Optical parts: I plan to work on them once I finish the electronics. The tasks are: (i) designing the optical layout; (ii) test and diagnose different components, especially the laser diode.
  3. Electronics: I will mostly focus on this part in the near term: (i) stuffing the pcb board for the hall-effect sensors and QPD box; (ii) modifying the old coil driver circuits to accommodate this new setup with more input and outputs; (iii) powering up the Binary input-output box and test it for prototyping; (iv) working together with Rana and Jamie on the AA/AI.
  4. Digital part: This would heavily rely on the help of Jamie and Rana.


Plan (Testing stage):


  1. Acquiring lock of the floating plate by using the hall effect sensors. This is relatively easy compared with the optical-lever sensing and control, as different degrees of freedom are not coupled strongly.
  2. Characterizing the cross coupling among different degrees of freedom in the optical-lever sensing scheme.
  3. Measuring the resonant frequency of the levitation, and testing the tunability of this resonant frequency by locking the plate at different locations to see how low we can achieve.



  604   Wed Nov 7 15:27:04 2012 Matt A.SummaryCoating QCoating Q analysis

 I've been working on the COMSOL model for analyzing Coating Q measurements. 


First, I built the COMSOL model (attached, TwoPartDisc_Base.mph) out of two connected cylinders, one for the coating and one for the substrate, using typical thicknesses of a single layer on a 3" substrate. The substrate was made of silica, and the coating was made of tantala (nominally, I took the material params for silica and changed the Young's modulus to 140 GPa and the Poisson's ratio to 0.23). In a real sample, the silica substrate is beveled and there is a thumbnail section of the coating missing (for welding to the suspension fiber). It is generally assumed that these differences are negligible.


First, I calculated the energy ratio per coating thickness for a thin film on a thick substrate, i.e. dU_film/U_total. This is the typical value reported in the literature for extracting the coating loss from the measured loss of a coated substrate. These values can be compared to the literature values to make sure my model isn't completely wrong. Unfortunately, not many papers actually list what values they use, and even fewer list the Young's modulus of the coating, although I'll assume it's some calculation for a multilayer coating made of silica and tantala, so it'll be somewhere between 70 and 140 GPa. 

Energy Ratio Comparison
Mode R (Harry 07) R (Matt)
Bf 1584 2269
Dh 1659 2491
Bf2 1581 2276
Dh2 1624 2408

These are the first four modes of the substrate, at frequencies Bf: 2700, Dh: 4000, Bf2: 6000, Dh2: 9000 Hz, and usually the only four that are measured in the literature. The difference in energy ratios come from the difference in Young's modulus, and possibly Poisson's ratio. I've done a sweep of the Young's modulus of the coating from 70-280 GPa and you can see that these numbers agree around 100 GPa, which seems reasonable for a multilayer coating. The plot below shows the four model results below for the first 32 modes (many are double modes, and have the same frequency and energy ratio). The red points are equivalent to a pure silica coating (ignoring the effects of Poisson's ratio), the Green points would be for pure tantala, the blue would be for pure hafnia, and I don't know why I did the black, we're not likely to use a material that stiff.


Varying the Poisson's ratio also has an effect on the energy ratio, but reasonable values of Poisson's ratio are pretty much all in the range of 0.2-0.3 (although silica is at 0.17). The result of varying within this range is shown below.


In most cases, this variation isn't a big deal.

Finally, I calculated the film bulk and shear energy ratios, U_bulk-film/U_film (R_bulk) and U_shear-film/U_film (R_shear), using the equations in Hong 12. (n.b. I had to use the equations in Hong to calculate the total energies as well, as the sum of shear and bulk seemed to add exactly a factor of two to the total energy calculated by COMSOL. I still don't know why this is true, but it doesn't matter to the ratios because this factor is removed in the ratio.)

For completeness, the equations used in comsol are:

U_bulk = 3D integral of (0.5 * solid.K * (solid.eel11+solid.eel22+solid.eel33)^2)

     where solid.K is the bulk modulus and solid.eel## are the diagonal elements of the strain tensor at each point.

U_shear = 3D integral of (2/3*solid.G*(solid.eX^2+3*solid.eXY^2+3*solid.eXZ^2+solid.eY^2+3*solid.eYZ^2+solid.eZ^2-solid.eY*solid.eZ-solid.eX*solid.eY-solid.eX*solid.eZ))

     where solid.G is the shear modulus and solid.e## are various components of the strain tensor at each point.

Below are two plots from the two parametric sweeps done above. The first shows the bulk energy ratio as Young's modulus is changed, and the second shows the bulk energy ratio as the Poisson's ratio is changed. Of course, the plots for shear ratios would just be 1-R_bulk.



As you can see, varying the Young's modulus has no effect on the bulk and shear energy ratios, which makes sense as the total, bulk, and shear energies are all linear with Y:

solid.K = Y/(3*(1-2*sig))       [sig = Poisson's ratio, Y = Young's modulus]

solid.G = Y/(2*(1+sig))

Changing the Poisson's ratio does change the energy ratios because the bulk and shear energies are not linear with sig, it it obviously makes a big difference, even with a small variation in sig (0.2-0.3).


It seems that it'll be important to get the Young's modulus and Poisson's ratio right when we measure different coatings directly.


Finally, there are two modes that are considered 'shear modes'. These are seen at ~37 kHz and ~42 kHz in the plots above. They are especially obvious in the plots of film to total energy ratios, where they are much lower than the other modes. This makes sense as most of the strain is in the direction perpendicular to the coating, where the cross-sectional area is very small. So while these modes might be interesting in terms of their bulk and shear film ratios (at least the 37 kHz mode, the other seems to approach the various butterfly modes), it would be very difficult to extract a film loss with them, as the effect of the film on the total loss would be very small. 

Instead, I think it's reasonable to focus on the numerous butterfly and drumhead modes, as there seems to be a clear divide in terms of their film shear and bulk ratios, and they all have larger total ratios. In fact, as a number of papers have already measured the film ratios at at least one butterfly and one drumhead mode, we can go back and look at their results (assuming our own Poisson's ratio (sig = 0.23).

The uncertainties on these will be large, since in most cases, only one drumhead mode is measured only once, and the butterfly modes will have multiple measurements. Also, the drumhead mode will most likely have higher suspension loss, and so its loss can be over-estimated. Finally, while the lowest measured loss is usually the one reported in the literature, this is usually just an upper limit, and the true loss might be much lower. Needless to say, the uncertainties on these are not really Gaussian. That being said, the errors I quote below are from the lscov function in Matlab, which does a least-squares fit to the data and assumes gaussian uncertainties. Take it as a zero-order measure if anything. 

Also, reading form the Hong paper, the ratio of phi_bulk to phi_shear is the figure of merit when determining how the difference between the two loss angles affects the coating brownian noise in the detector. Higher values are generally worse. In current calculations, it is assumed that this ratio is one.

From Penn et al. 03:

Samples Measured
Sample X-lambda silica layers Y-lambda tantala layers # layers
B 1/4 1/4 2
C 1/4 1/4 30
D 1/8 1/8 60


1/8 3/8 30
F 3/8 1/8 30


Sample Modes Measured phi_bulk (1e-4) phi_shear (1e-4) phi_bulk/phi_shear
B Bf+, Bfx, Dh 3.4 pm 2.0 3.1 pm 0.4 1.1
C Bf+, Bfx, Dh 3.9pm0.2 2.6 pm 0.1 1.5
D Bf+, Bfx, Dh 3.7 pm 0.4 2.7 pm 0.1 1.4
E Bf+, Bfx, Dh 7.2 pm 0.1 3.7 pm 0.1 1.9
F Bf+, Bfx, Dh 2.7 pm 0.8 1.5 pm 0.2 1.8

Comparing B and C, it seems like the effect is not solely due to the coating/substrate interface. Comparing C and D, we see that the number of layers doesn't seem to make much difference. Comparing C, E, and F is confusing, as if only one of the coatings was causing the increased shear loss, C would fall between E and F. I haven't decided what to make of this, other than that these could be influenced by the uncertainties discussed above. The drumhead mode in E was higher than one might expect, so it could have been an errant measurement, and the modes in F were all about a factor of 2 lower loss, so they might have greater environmental influence.


From Crooks et al. 2004 (loss values estimated from plot)

Sample X-lambda silica layer Y-lambda tantala layer # layers
A 1/4 1/4 30
B 3/8 1/8 30
C 1/8 3/8 30



Sample Modes Measured phi_bulk (1e-4) phi_shear (1e-4) phi_bulk/phi_shear
A Bfx, Bf+, Dh 3.6 pm 0.2 2.3 pm 0.1 1.6
B Bfx, Bf+, Dh, 2Bfx, 2Bf+ 2.6 pm 0.3 1.4 pm 0.1 1.9
C Bfx, Dh 6.8 3.1 2.2

 Comparing A, B, and C is the same as comparing C, E, and F above, and we see roughly the same trend, but with slightly higher ratios. Perhaps the two materials cancel each other somehow, perhaps it has something to do with layer thickness, or it could be due to offsets from our assumed Poisson's ratio.













Attachment 1: TwoPartDisc_Base.mph
Attachment 2: Yf_Bulk.png
Attachment 3: Yf_EnergyRatio.png
Attachment 4: Pf_Bulk.png
Attachment 5: Pf_EnergyRatio.png
  605   Wed Nov 7 15:55:35 2012 Matt A.SummaryCoating QCoating Q analysis

I also analyzed data from Harry 07 on titania-doped tantala, but the elog wouldn't let me write anymore. Here it is:



Sample titania concentration (cation %)
0 0
1 6-8.5
2 13-20.8
3 24-22.5
4 54.5-54

All samples were multilayer 1/4-lambda stacks of silica and Ti:tantala, at 30 layers.

Sample Modes Measured phi_bulk (1e-4) phi_shear (1e-4) phi_bulk/phi_shear
0 Bfx, Bf+, Dh 3.3 2.4 1.4
1 Bfx, Bf+, Dh, 2Bfx, 2Bf+ 3.4 pm 7.6 3.2 pm 1.8 1.1
2 Bfx, Bf+, Dh, 2Bfx, 2Bf+, 2Dhx, 2Dh+ 2.6 pm 0.3 1.5 pm 0.1 1.7
3 Bf, Dh, 2Bf, 2Dhx, 2Dh+ 2.8 pm 0.3 1.4 pm 0.1 2.0
4 Bfx, Bf+, Dh, 2Bf, 2Dh 3.1 pm 0.3 1.4 pm 0.1 2.2

It looks like adding titania actually increases the bulk/shear ratio. The only point that does not fit this trend is sample 1, which has huge errorbars, due to some unusually large losses on the Bfx and Dh modes. If we remove just the Bfx mode, the ratio shoots up to 4.8, and if we remove the Dh and the Bfx, we get down to 1.8. Unfortunately, in removing the Dh, we lose out on a mode with very different energy ratios than the Bf modes.  


I should also note that the first coatings in the last post were from REO, the second were from LMA, and those above are also from LMA.

Also, if I didn't list any uncertainties in the tables, it's because there are effectively only two points and two unknowns, so the fit is perfect. For example, even though sample 0 above has three modes measured, both Bf modes have the same loss, and the same energy ratios for the two butterflies, so you're reduced to two points: Bf and Dh.

  616   Wed Jan 16 12:40:39 2013 haixing, kojiSummarySUSGeneral signal conditioning circuit for maglev

Here is a general signal conditioning circuit for whitening and dewhitening of the signal from the sensor and actuator (multiple channels) in maglev. Previously, I was designing different circuits for whitening and dewhitening. Koji pointed out that by manipulating the zero and poles, we can realize them with the same circuit by choosing the proper values for the resistors and capacitors. In addition, by bypassing certain stages, we can use one type of PCB board for the sensor (the hall-effect sensor and quadrant photodiode), and the actuator (the coil).

The pcb board and the associated alitum file (altium_file.zip) are attached.


This circuit contains five stages, and each can be bypassed by using a three-terminal header and jumper:

The first one is to set the DC offset.


The next three are generic filters, each with one zero and two poles.


(different footprints for the capacitor for a generic purpose)

I will explain the detail in the additional information part appended to this elog.

The last stage is a current boost for coil drive.


-------Additional information----------

In the below, I will briefly explain the idea of the filter part:


The transfer function for this circuit is given by


s0: 1/C1(R1+R2)

s1: 1/(C1 R2)  

s2: 1/(C2 R3)

At very low frequency, the gain is determined by -(R3/R1), which is chosen to be -1 in our case. Given different values for s0, s1, and s2, we can have high-pass filter (s2>s1>s0) [cut-off above s2], or a low-pass filter (s1>s0>s2) [cut-off above s1].

For example, in our case, we have chosen

  1. A high-pass filter (more precisely, a bandpass): s0/2π = 0.5 Hz, s1/2π = 5 Hz, and s2/2π = 200 Hz [cut-off].
    The parameters for the components are: R1 = R3 = 28.6 KΩ, R2 = 3.2 KΩ, C1 = 10μF, C2 = 27.8nF
    The amplitude is shown by the figure below:

  2. A low-pass filter: s2/2π = 0.5 Hz, s0/2π = 5 Hz, and s1/2π = 200 Hz [cut off].
    The parameters for the components are: R1 = R3 = 32 KΩ, R2 = 820 Ω, C1 = 0.97 μF, C2 = 10 μF.
    The amplitude is shown by the figure below:

  3. To balance the whitening and dewhitening in the entire loop, we have chosen the zero and poles such that the product of the above two filters is close to unity at low frequencies, as shown by the figure below:



Attachment 1: altium_file.zip
  617   Wed Jan 16 16:19:25 2013 haixing, kojiSummarySUSGeneral signal conditioning circuit for maglev

- I forgot why you don't have a voltage reference (cf. AD587) for the offset subtraction.

- Don't you want to put an output impedance at the output of the current driver?

  618   Wed Jan 16 19:41:15 2013 haixingSummarySUSGeneral signal conditioning circuit for maglev


- I forgot why you don't have a voltage reference (cf. AD587) for the offset subtraction.

- Don't you want to put an output impedance at the output of the current driver?

 > For your first comment, you are right. I am diverting some voltage from the power, which has a huge noise. This is rather bad.

> For your second comment, it turns out that my coils have quite high impedance, e.g., of the order of hundred Ohm. The maximal current, given the maximal voltage 15V, is smaller than what the buffer can provide. But it is good to add an output impedance for flexibility and also limits the current.

Thank you. I will implement these comments in the next revision of the PCB.

  619   Thu Jan 17 23:40:12 2013 haixingSummarySUSGeneral signal conditioning circuit for maglev


- I forgot why you don't have a voltage reference (cf. AD587) for the offset subtraction.

- Don't you want to put an output impedance at the output of the current driver?


I implemented both of your comments in this revision. The altium file is also attached. Thanks.

Attachment 2: general_signal_conditioning_board_20130117.zip
  620   Mon Jan 21 22:33:26 2013 haixingSummarySUSfront and rear panels for signal conditioning boxes

Here are the front and rear panels for the signal conditioning boxes. The front-panel files are attached.

For the coils:



For the QPD:



For the Hall-effect sensors:



For the linear motors [using simple DC control]:
Small panel-mounted voltage meter reads out the force gauge signal that indicates the weight of the floating plate, from which we know roughly how far we are away from the working point where the gravity is balanced by the DC magnetic force. We use two on-mom switches to control the linear motor (up and down).



Attachment 9: front_and_rear_panel_files.zip
  662   Tue Jul 9 01:24:56 2013 GiorgosSummarySUSHE sensors test, arrangement, and offset & Strain Gauge arrangement

Today, we first talked about the connection of the HE conditioning boards to the HE sensors and the arrangement of the wires on the connector. There are 7 HE sensors named after their position (e.g. W=West). Starting from the right, the first four pins denote the sensors that lie above the plate.The bottom row is the bottom part of the connector to the HE signal. X denotes the pins not used and the last three pin places on the left are for the sensors imbedded in the coils, which are though--for the time being--not used.




We tested the transfer functions (TF) of our 7 HE sensor conditioning boards. Six of them had identical TF, same as the ones we expected and one of them (S1) had a similar TF, but a totally different phase. We extracted them to a floppy disk and inserted them to a computer, where we created files that contain the data of the TF plots. Tomorrow, we need to plot the data in Mathematica. We also measured the offset for our Hall-effect sensors on the oscilloscope. We used Vin=0 to measure the actual offset and then adjust R2 to null it. Here are the recorded offsets:

AC1:2.37V, AC2: not working, AC3:2.34V, S1:2.5V, S2:2.5V, N:2.5V, W:2.46V.

We also looked at the connector for the Strain Gauge (S.G.) and DC motors (M). We have six connections for each. We named our S.G. boards, depending on the location of the corresponding--in our setup--strain gauge. IMoving from the right to the left, the strain gauge sensors correspond to: TS (top south), TW, TE, BN (bottom north), BS, BE. We found a problem with the BE op-amp; it must be broken. We tested the output signal of some boards and we did not find a steady DC amplified voltage we expected; we thought of introducing a low-pass filter (since DC signals have ideally a 0Hz frequency)before the signal reaches the strain gauge op-amp.


Tomorrow, we will measured the TF of the Strain Gauge boards to see what is wrong. We will also insert a low-pass filter with a cut-off frequency around 10Hz.

Attachment 2: Strain_Gauge_&_Motors_Connections.png
  663   Tue Jul 9 18:13:29 2013 GiorgosSummarySUSStrain Gauge Voltage Offset

Strain Gauge Boards

Our conditioning boards did not have a low-pass filter. That is a problem, since these circuits were designed to amplify a DC voltage offset, but the op-amp cannot provide that gain at very high frequencies. We introduced a capacitor to create a low-pass filter and made sure the cut-off frequency of our setup was lower than the one of the op-amp: f= 1/(RC*2pi). For our R=24kΩ, we chose C=0.1μF. So, we built a low-pass filter for our 6 strain gauge boards and then measured the DC voltage offset. Our digital voltage meter can read up to 200mV, so we adjusted the one adjustable resistor to get the offset voltage as low as a few mV. As we slightly pussed on the strain gauge sensors, the voltage increased indicating that our circuits work fine.

Panels for Coil Actuators & Hall-Effect sensors' power boards

At the end of the day, I marked the holes for 5 power boards on the panels of our coil (3 power boards) and Hall-effect sensors' box (2 power boards).

  665   Thu Jul 11 01:01:30 2013 GiorgosSummarySUSLED connections, Power Boards, and DC magnetic field boards

LED connections

We use LEDs to indicate whether our power boards work. For one power board, we need two LEDs, one for the positive and one for the negative voltage. For boxes that contain more than one power board, we will still use onlyy one pair of LEDs, since we only care to test whether our power supply works. We have six LEDs and we will use 2 for the HE sensors board, 2 for the coil-actuation box and 2 for the strain gauge box. Today, we made the connections for our LEDs; on our power boards, there are two letters: A and K. A is for the signal wires (positive/red or negative/black) and K is for the ground wires.

Power Boards

The missing components for our power boards have arrived, so we finished our power board circuits, drilled the holes on the box panels and screwed the power boards. We also created more power lines, such that we have enough for the 7 coil-actuator and 3 DC offset boards, as well as the HE sensor boards. We prepared power lines for all HE sensor boards and 6 coil-actuator ones, but ran out of components; we still need to create power lines for one coil-actuator board and 3 DC offset ones.

DC magnetic field offset

For the boards that will provide tuning of the magnetic field, we only use a current booster circuit (configuration with a current buffer in a feedback loop with a low-noise op-amp). We built all three boards.






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