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ID Date Author Type Categorydown Subject
  129   Tue Nov 24 23:17:02 2009 ZachMiscSUSAD587KN voltage noise

I got a few AD587KN (high-precision 10V reference) samples today from AD. I hooked them up to see how much quieter my DC supply would be. The results are pretty good, with the voltage noise reduced by a factor of 5-10 throughout. The first two attachments below are comparisons of the noise in

1. The +12V regulator (MC78M12) alone

2. The AD785KN reference with V_in = +12 V provided by the regulator

3. The same as in 2, only now with an additional "noise reduction" capacitor (a 1-uF capacitor from pin 8 to ground forms a LPF with an internal 4-k resistor, giving a corner frequency of 40 Hz to reduce high-frequency noise),

plotted with the same frequency ranges and settings as those in the previous post.

The reference comes very close to its noise spec of 100 nV/rt(Hz) @ 100 Hz. The only issue is that it seems to have much more line pickup than the regulator (which seems almost completely insensitive to line noise), and this is worsened by the extra capacitor. Attachment 3 is a close-up of the low-frequency spectrum around 60 Hz. I suspect that this will be alleviated somewhat when I move away from the breadboard phase.

I want to rig this up so that I can stabilize the supply voltage to the transimpedance amp and LED, but in order to do so I will need to build a higher-current source using a power transistor, like either of those shown in attachment 4 (the AD587LN is only able to provide <10mA). 

  130   Thu Nov 26 02:59:35 2009 ranaElectronicsSUSLED Driver circuit

We want to have a simple low noise circuit to drive the LED. Our plan is to use the AD587 followed by a filter/buffer.


from 0.1-10 Hz, produce less RIN in the LED light than shot noise by a factor of 3.

With 35 mA of LED drive, we get ~35 uA of photocurrent (no magnet/flag). The shot noise of 35 uA is ~3.5 pA/rHz.

So the RIN from shot noise is 1e-7. So we shoot for a RIN of 3e-8 from the LED.


The AD587 voltage reference has a relative noise of 1e-7 at 0.1 under very good conditions (perhaps our vacuum system will be so kind). So we have to get a factor of 3 filtration at 0.1 Hz.

The following circuit should it for us: its a 2nd order Butterworth implemented in a Sallen-Key configuration. The noise is reasonable and the cutoff frequency is so low (0.03 Hz) because of the latest in capacitor technology.

We can buy metal poly caps which are as large as 47uF and have a reasonable physical size and tolerance and noise.

On page 2 of the plot you can see that the noise performance of this filter is limited by the input voltage noise of the FET opamp (op1) (AD743 - soon to be obsolete). The noise of the BUF634 (op2) is insignificant in this configuration. What we really need to make this good is a part with just as good of an input current noise spec as the AD743 but 3x less voltage noise at 0.1 Hz. I offer one cookie to whomever can find an opamp that fits those parameters.

These images show the circuit diagram (left) and the proto setup (right):


update: added a 230 Ohm series resistor between the BUF634 output and the LED to step the voltage down to the 1.7V that the LED wants.

  131   Thu Nov 26 15:17:01 2009 KojiElectronicsSUSLED Driver circuit

I found a Quad Opamp OP497 (neither dual nor single!), but this is not enough to expel AD743.
Dis-continuation of OP497 is also close except for one SMD package.

AD743 (reference):
LF Voltage Noise: 0.38 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 2.9 nV/√Hz @ 10 kHz
Current Noise: 6.9 fA/√Hz @ 1 kHz

LF Voltage Noise: 0.3 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 15 nV/√Hz @ 1kHz
Current Noise: 5 fA/√Hz @ 1kHz


What we really need to make this good is a part with just as good of an input current noise spec as the AD743 but 3x less voltage noise at 0.1 Hz. I offer one cookie to whomever can find an opamp that fits those parameters.

  132   Mon Nov 30 18:53:28 2009 ZachElectronicsSUSLED Driver circuit

Not having much luck. I found the LT1028, which has 10x better low-frequency voltage noise (35 nVpp, 0.1 Hz to 10 Hz), but its current noise is worse by a ridiculous factor of 1000:

Screen_shot_2009-11-30_at_6.51.02_PM.png           Screen_shot_2009-11-30_at_6.51.16_PM.png




I found a Quad Opamp OP497 (neither dual nor single!), but this is not enough to expel AD743.
Dis-continuation of OP497 is also close except for one SMD package.

AD743 (reference):
LF Voltage Noise: 0.38 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 2.9 nV/√Hz @ 10 kHz
Current Noise: 6.9 fA/√Hz @ 1 kHz

LF Voltage Noise: 0.3 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 15 nV/√Hz @ 1kHz
Current Noise: 5 fA/√Hz @ 1kHz


What we really need to make this good is a part with just as good of an input current noise spec as the AD743 but 3x less voltage noise at 0.1 Hz. I offer one cookie to whomever can find an opamp that fits those parameters.


  133   Tue Dec 1 00:49:19 2009 KojiElectronicsSUSLED Driver circuit

The situation is well illustrated in the following application note of Analog Devices:

Low Noise Amplifier Selection Guide for Optimal Noise Performance

Even though the graph is created for 1kHz, it is very clear that AD743
has superb performance for high source impedance purposes:
combination of low current noise and low voltage noise.

If the source impedance of Rana's circuit (400kOhm@DC) are reduced to 20K or so,
OP-27 type OPamp can come into the scope. However this means we need to use 10 times
larger capacitors. This is almost impossible for now, though innovation on the caps
can change the situation.

LT1028 is an AD797 type opamp. This works greatly with the smaller source impedance.

  134   Wed Dec 2 18:29:07 2009 ZachMiscSUSLow-noise LED driver

 Yesterday, I rebuilt Rana's low-noise LED driver in the Bridge elab. It is based on a 2nd-order active lowpass filter (using the Sallen-Key topology). The schematic is shown below. The circuit is essentially the same as the one Rana posted a few days ago, only the R and C values are all around twice what they are in his schematic. This results in the same corner frequency of fc = 0.03 Hz.




I hooked it up and measured Vsk,out = 9.76 V. I then used it to drive a 50-ohm resistor, and measured VLED = 1.71 V, then measured the current to be ILED = 33.5 mA.

After ensuring that it was supplying the correct voltage, I hooked it up to the LED and took a spectrum of the voltage noise across it over the two frequency bands I have been using in previous posts. The following are comparison plots of the noise here and the noise with the simple RC filter used before, calibrated to displacement noise.



RA: although these plots have Displacement in the y-axis, they are NOT measurements of actual displacement noise. They are estimates for the contribution to the displacement noise made by the LED RIN based on measurements of the voltage noise across the LED.



Something is clearly wrong: not only is the new configuration worse at lower frequencies, but the rolloff seems to go as 1/f and not 1/f2. Investigating after dinner...


  135   Thu Dec 3 02:35:39 2009 ranaMiscSUSLow-noise LED driver


 OP27 current noise is too high - use AD743.

  136   Mon Dec 7 02:59:38 2009 ZachMiscSUSLED Driver noise (with AD743)

 I retook the measurement from my last post, this time using an AD743 in place of the OP27 (per Rana's comment). The results are below.



RA: Although it says m/rHz, this is not measured displacement noise, but rather estimated displacement noise due to the LED noise. The previously measured conversion from the LED RIN to apparent displacement is used to convert from the voltage noise of the LED driver to the contribution to the OSEM's displacement readout.



Seems better than before, but not quite what expected. I observed that the transfer function of the S-K filter was what it should be up until a decade or two above the corner frequency, after which it appeared to spring zeroes out of nowhere and level off at high frequency. I tried to see what would happen if I changed the resistor values, and the following plot is what I got.


This plot seems familiar from my electronics courses, but I haven't put a finger on what is causing this behavior yet. I'm sure that the answer is somewhere in H&H (or in the brain of a kind soul who happens to be reading this--wink wink).

  137   Mon Dec 7 12:01:14 2009 ranaMiscSUSAOSEM LED Driver noise (with AD743)

The low frequency noise looks pretty good now. The funny shape is most likely a thermal transient due to having not enough insulation. You need to droop some Kleenex over the circuit to stop the thermal air currents and then put a second box over the first box. Then its probably best to sit outside of the room when taking the measurement to reduce the human noise.

  138   Tue Dec 8 10:07:39 2009 ZachElectronicsSUSSallen-Key filter attenuation limit

 Last night, I was going to retake the noise measurement with the added elements that Rana suggested, but instead I spent a ridiculous amount of time trying to figure out what is going on with my Sallen-Key filter. I now know a lot more about their limitations, but am still at a loss as to what is happening in this case. The problem is that no one seems to be using a corner frequency anywhere near this low (at least not anyone trying to explain these filters). The following is a comparison of an ideal 2nd-order Butterworth filter and a real one using an LT1464.


The filter behaves as expected a ways past the corner frequency, rolling off at 40 dB/decade. As the signal increases in frequency, the capacitors' impedances decrease, until (at point 'b'), they fall below the output impedance of the amp, causing the response to climb at 40 dB/decade. This happens for a short while, until the unity-gain bandwidth frequency (~ 1-10 MHz, 4 MHz for the AD743) of the op amp is reached, and the filter can attenuate no more ('c'), so the response flattens out to 0 dB/decade.

Different component values affect the high-frequency behavior of the filter, as shown below.



This makes sense, since with smaller capacitors it takes a higher frequency to fall below the output impedance of the amp. In any case, though, the final flatline always happens near the UGB frequency. The following is a plot of the transfer functions I measured (also in last post). I did not change the C values--only the R's--so the corner frequency is different in each case. What I observe is some R-dependent maximum attenuation, which sets on well before the 4-MHz UGB frequency of the AD743. The strangest part is that for small enough R this maximum "attenuation" is actually a positive gain.


I suppose it is not a huge deal, as I can increase the stopband attenuation as R while only increasing the Johnson noise as 2 * sqrt(R), but it would be nice to get some insight into what 's going on.

NOTE: While it appears that the high-frequency flatline in the other plots occurs for a different reason, it still seems to be R-dependent. I could not find any explanation for what determines this asymptotic behavior.

  139   Tue Dec 15 01:48:22 2009 ZachMiscSUSAOSEM noise measurement

 I tracked down some more AD743s at Wilson House 2.0 today (thanks to Rich). I was then able to simultaneously use 743s to both drive the LED and amplify the readout. Below is an 800-line DC - 25 Hz noise spectrum of

- The voltage across the LED (DC level: VLED = 1.59 V) -- AC coupled

- The output of the amp with a fully lit PD (DC level: Vout,full = -2.95 V) -- DC coupled, through bandpass filter

- The output of the amp with the LED out -- DC coupled, through bandpass filter

- The output of the amp with an open input -- DC coupled, through bandpass filter

all calibrated to equivalent displacement noise. For the LED plot, this was done by using the measured current ratio between the LED and the PD when fully lit along with the measured OSEM response of 0.05 A/m. The other three were converted using this response along with the transimpedance gain of the amp (100,000 V/A). For all measurements, the OSEM was covered by a box, and the circuit was draped with a cloth and put under a box within another box (to reduce air currents).


The funny low-frequency junk from the previous driver spectrum is gone--thanks to the isolation from air currents--but the line seems a bit higher overall (trying to figure out why). There also still seems to be the funny effect of noise added by the LED above the level of the voltage noise across it, and I think it's somewhat strange that the "Dark" noise is lower than the amp noise with no input. We can probably still do better..

  140   Tue Dec 15 09:28:39 2009 ranaMiscSUSPD front end noise

This is the LISO estimate of the PD front end noise. It could be improved somewhat by using a higher value resistor, but there's no point.

The shot noise level for the 20-40 uA of current we have is more than 1 pA/rHz so we should be OK above 30 mHz.

So even the level of the dark noise below seems too high and also the 10,000 V/A statement. The feedback resistor we had used to be 100k...

  141   Tue Dec 15 10:58:56 2009 ZachMiscSUSPD front end noise

10k was a typo--fixed.


This is the LISO estimate of the PD front end noise. It could be improved somewhat by using a higher value resistor, but there's no point.

The shot noise level for the 20-40 uA of current we have is more than 1 pA/rHz so we should be OK above 30 mHz.

So even the level of the dark noise below seems too high and also the 10,000 V/A statement. The feedback resistor we had used to be 100k...


  142   Thu Dec 17 10:50:16 2009 ZachMiscSUSAOSEM noise measurement

 I retook the measurement from the previous post, since the noise in the amp appeared much higher than it should. It looks much better now, but still not great. Above around 3 Hz, the amp noise is at the LISO-predicted level of ~8e-12 m/rHz equivalent displacement noise. Below this, it seems to show some 1/f-ish noise back to DC. LISO predicts some low-frequency noise as well, due to the increase in the AD743's current and voltage noise there, but it doesn't begin until below 1 Hz, and it doesn't seem quite as steep. I'm trying to figure out what is causing this, but the best solution might be to move to a more solid, soldered arrangement.
LED: Voltage across the LED (DC value 1.61 V), AC coupled
Bright: Noise at output of amp, LED on and PD connected, DC coupled with bandpass
Dark: Noise at output of amp, LED off and PD connected, DC coupled with bandpass
Amp: Noise at output of amp, PD disconnected, AC coupled (I checked this DC coupled and through the bandpass, as well, but there is no significant difference)
Amp (theoretical): LISO-predicted noise at the output of the amp with no input


  143   Fri Dec 18 11:56:12 2009 ZachMiscSUSNew AOSEM LED problem

 I picked up the other AOSEM from the 40m today, so that I could compare it with the one I've measured in an attempt to get to the bottom of the noisy LED problem. It began uneventfully: I measured the impedance of the LED by connecting an ammeter in series and slowly increasing the voltage. I got ILED = 36 mA at about VLED = 1.7, giving ZLED ~ 47 ohms.

Then, I powered up the LED driver, and tested it with a 50-ohm resistor (as usual), measuring V ~ 1.7 across it. Having confirmed that everything was working properly, I hooked the LED up, and measured NO current. I hooked directly back up to the DC supply and found the same result. The thing appears to be blown, but I have no idea how. I went through every precaution I have been taking with the other OSEM, which worked fine when I switched it back in. Crap.

One thing I noticed before I hooked anything up was that the small white pieces attached to the LED and PD on either side of the OSEM opening were very loose when compared to the other OSEM. When I first measured no current, I tried applying some pressure to the LED side, and some current flowed across, but only about 1/10 of what it should have been.

  146   Wed Jan 13 14:13:03 2010 ZachMiscSUSASOSEM comparison

EDIT: I have calibrated the y axis of the plot to meters

Last night, I got around to testing some of the other AOSEM samples, to see how the noise varied between them. What I found was rather strange: the noise in all the new ones (#s 2-6) was about the same, but they were all quite a bit noisier than the previous one I have been testing (#1). The only difference between them, as far as I can tell, is that the first specimen has a coil wound around it already, while the others just have a rubber band. Also, the newer ones all have an impedance of ~ 44-45 ohms, while I measured 47 ohms for the first (though, among the new ones, the slight variation in Z seems to have no correlation with the small differences in noise level). For those wondering, YES, I did remeasure the noise in the 1st one; I am not using old data.

Either my meddling with the old one has somehow made it quieter or something is amiss.


  147   Tue Jan 26 22:22:17 2010 ZachMiscSUSAOSEM LED, PD current comparison

Norna and Rich: I am sorry for taking so long to get you this measurement. I plan to do the noise measurements on the standalone LEDs this week.

The following table gives the current through each OSEM's LED (measured using the voltage drop across the 238-ohm resistor in series), as well as the measured photocurrent (the DC output of the amplifier divided by its transimpedance gain of 100,000 V/A), and the ratio of the two. The plot from the previous post is reproduced below for analysis--I realized that I did still have this plot saved in units of V/rHz. In some cases (e.g. #3, #4), the noise level seems to be correlated to the photocurrent, but not all of them follow this pattern. The issue of #1 being significantly quieter than the other set remains, as well.

AOSEM LED and PD Current Comparison
1 35.1 31.4 1120
2 35.5 38.5 923
3 35.5 57.6 616
4 35.4 30.5 1160
5 35.4 42.9 826
6 35.5 48.2 737
7 35.5 39.2 906
8 35.4 39.8


9 35.2 34.0 1040
10 35.5 43.8 810




  148   Fri Feb 12 11:33:04 2010 MottMiscSUSState of the Shaker

The Shaker project is coming along nicely.  I am currently looking into using the built-in ability to download a waveform to the front end to do the sweeps, but we are running into memory problems, and I get the sense from Tony that it was not really designed to do this.  Currently we are able to download a waveform to the frontend, run the generator according to it, and make a measurement over a full run of the waveform.  If we can crack the limited time constraint and figure out the averaging, this is going to be the most straightforward solution.

I am working, in parallel, with Gert (who is out of the office at the moment) on using pure script to do this, although I am worried about starting and stopping the generator so frequently.  Apart from anything else, there is a slight hang in the frontend when the generator start method is called; it is not noticeable when the button is pushed in the app, but I think it adds quite a bit of latency to the program.  I am still waiting to hear from Gert about how to acquire a time series; hopefully we can figure that out by early next week, since it is critical.  I am also not entirely sure how we force the program to do all the analysis on the time series after it is acquired.  Ideally we would want the analysis to run in parallel and update the frontend continuously, but I am not sure this is possible with VBA (I don't think you can do multithreaded programming) and I am not sure I would know how to do so even if it is!



  149   Fri Feb 12 11:35:19 2010 MottMiscSUSPiezos

Engineering was very helpful showing me how to make the leads we need for the piezos; I will go crimp some more at the beginning of next week. 

The new structures should be coming in soon, so we will have a dedicated structure for the piezo damping, at which point we can really get cracking.

  150   Thu Feb 18 17:43:15 2010 MottMiscSUSPiezos


I finished crimping all the connectors we will need for the piezos.  We are now just waiting for the new structures to arrive so we can start gluing the piezos on.

  151   Sat Jun 26 13:59:57 2010 Vladimir DergachevMiscSUSovernight tiltmeter plots
And here are the plots from overnight run:



* I picked a nice drift segment out of the whole run which showed some
junks in the beginning and a few near the end, possibly caused by external

* The drift with oscillations is still there. It is likely they are


* The best spectrum comes from lvdt2. It is likely that LVDT1 receives
extra noise from clamping zeners in the fine channel of its preamplifier.


* The full spectrum reaches the limit of the coarse channel only at
high frequencies:


The fine channel of LVDT2 dips a little bit lower, which is easier to
see on linear X scale:


The electronics operates around 6.6 kHz and the conversion to DC is
done digitally. Thus we should see flat noise floor from it, except for
the effect of voltage references which are used both in ADCs and in the
triangle driver and any other noise source that affects the amplitude
(such as a current setting resistor in the triangle driver).

Riccardo - I think it would help to isolate the effects of mechanics
noise from driving electronics if we had a test fixture for LVDTs.
Something like a U bracket for the excitation part of the LVDT and a
screw-on cover for the pickup coil. It would be nice to have a choice
between a plain cover, a cover with a slot and a PEEK cover.

This essentially follows the suggestion Eric made at the last meeting,
except I would avoid usage of all-analog readout as I am not confident I
can debug it easily. We can still do it as a confirmation once we know
what the baseline curves are from our current system.
  152   Wed Jul 7 00:14:55 2010 Vladimir DergachevNoise HuntingSUS2e-9 rad/sqrt(Hz)
Here is a fresh plot:


Changes: reduced noise in preamps, reduced gain in fine channels so that
they just reach 1e-10 rad/sqrt(Hz) - this produces large overlap without
having to trim LVDTs.

We are now under 2e-9 rad/sqrt(Hz), but there is still work to be done as
can be clearly seen from the correllation plot:


Or from overlay of coarse and fine channels:


The negative correllation values are clearly electronic (likely excitation
coil driver).

Some of it subtracts:


but there is a lot of room until we reach the pink line (which is the
limit of readout electronics for fine channels).
  156   Sat Jul 17 22:35:16 2010 Vladimir DergachevNoise HuntingSUSStandalone LVDT measurements
I have finally assembled the bracket to hold a standalone LVDT.
All plots are at:


From the attached plot you can see the usual 1/f noise at low frequency, which is most likely caused by current setting resistor which is cooled by air currents.
Voltage references and the output driver are the secondary suspects.

The 1/f rise is much smaller than what we see from the tiltmeter, so that slope must be due to mechanical noise as we expected from correlation plots. Once the magnets arrive we can reduce the number of wires to test the size of the effect.

Also, the standalone LVDT is a fairly good proximity detector - putting a hand close to it produces a very large change in the reading. It would be interesting to see how differential LVDT coils perform in this regard.
  155   Sat Jul 17 22:35:16 2010 Vladimir DergachevNoise HuntingSUSStandalone LVDT measurements
  159   Wed Sep 1 22:49:36 2010 Vladimir DergachevMiscSUSLatest tiltmeter spectrum
Latest tiltmeter sensitivity plot.

Explanation of the legend: lvdt1_fine_cal, lvdt2_fine_cal are left and right sensors they are right on top of each other and general tilt spectrum.
The red curve shows common mode. It has some noise of its own mostly due to imperfect cancellation between left and right sensors, but mostly it shows what electronics is definitely capable of.
Bracket refers to standalone LVDT mounted on a bracket and shows what a single LVDT can do - it is calibrated the same was as the other two. The pale pink curve on the bottom is hard limit from
amplifier and ADC sensitivity.

This did not use any feedback.

The large peak in the middle is the tiltmeter proper frequency. We tuned it higher so it is easier to compare performance between open loop and close loop cases.
  160   Thu Sep 2 13:27:43 2010 Vladimir DergachevMiscSUShysteresis measurements

There is no evidence of hysteresis in the latest measurements. The plot below zooms in to the few cycles in the middle of the upper plot.

The thick black areas are created by the proper mode of the tiltmeter and air currents.

  252   Sat Jul 23 17:22:01 2011 haixingDailyProgressSUSmatching the magnets

Yi Xie and Haixing,

We used the Gauss meter to measure the strength distribution of bought magnets, which follows a nice Gaussian distribution.
We pick out four pairs--four fixed magnets and four for the levitated plate that are matched in strength. The force difference is
anticipated to be within 0.2%, and we are going to measure the force as a function of distance to further confirm this.

In the coming week, we will measure various transfer functions in the path from the sensors to the coils (the actuator). The obtained
parameters will be put into our model to determine the control scheme. The model is currently written in mathematica which can
analyze the stability from open-loop transfer function.

  256   Tue Jul 26 01:25:17 2011 haixingDailyProgressSUShistogram of magnets

Just to add a little bit more details to the previous elog:

To obtain matched magnets, we measured the magnetic field strength of the magnets. We have two type of magnets: the
first one (for fixed magnets) is 1 inch in diameter and 1/32" inch thickness; the other one (for the levitated plate) is 1/2 inch
in diameter and 1/8" inch thickness [refer to the schematic for illustration]:


In total, we bought 12 1'' ones and 12 1/2" ones [we want to get the distribution before ordering more]. We used a Gauss meter
to measure the strength [in the axial direction]. We used a plastic block to fixed the distance between the Gauss meter and
the magnets.

For the 1" ones, the measured values are {94.9, 126.3,84.6, 109.8, 117.1, 94.2, 104.8,96.5,116.3 108.5,98.0,122.6}. The histogram
is the following [normalized with respect to the total number and the horizontal axis is Gauss]:

We fitted it with a Gaussian distribution with mean of 106.1 Gauss and variance of 12.8 Gauss.

For the 1/2" ones, the measured values are {126.7,131.9,127.9,129.3,125.8,133.1,132.4,124.8,130.7,125.0,136.2,135.0}. We
fitted it with a Gaussian distribution with mean of 130.0 Gauss and variance of 3.9 Gauss.

The 1/2" ones have a much smaller variance.

Even though the quantity is small, we were able to find 4 pairs of matched ones that are differed by 5%. Interestingly, since
the force between two magnets depends on the product of their strength, we can choose the magnets in such a way that
if the fixed magnets is 5% weaker, we can compensate it by choosing the levitated magnets is 5% stronger. This needs to
be confirmed by the force measurement. Just in case, we have ordered more 1" magnets.

  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.


  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.


  260   Tue Jul 26 14:13:27 2011 haixingHowToSUSnumber of magnets need to achieve 1% imbalance

In Elog 256, we showed that the 1" magnets have a mean of 106 Gauss with a variance of 12.8 Gauss.
The question would be if we want to have an imbalance of 1% how many magnets we need to buy.Here
Here we will make an estimate by assuming that the distribution of strength is Gaussian---a reasonable assumption
given what we have measured. The distribution would simply be

with and . Through numerical integration, one can find out the probability content for the
magnet strength falling into [105, 107] (within the 1% error around the mean)  is 0.062. Therefore, if we want
to have 4 matched magnets that have 1% error around the mean, the number of magnets we need to order is
approximately 4/0.062 = 64. Since we have already got 12, extra 50 would be enough (the quantity that we order
today), unless we are not lucky.

Steve: I asked  K&J  Magnetics to select matched pairs of 4, but they declined.

  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.


  286   Tue Aug 9 10:25:25 2011 HaixingDailyProgressSUSmaglev circuit board

In order to use Labview for maglev, we need to have an analog interface for the input (OSEM) and output (coil).
I have designed a new board based upon the old circuit design we had previously for the maglev. Here we only
keep the LED drive and coil drive part. The LED drive is a second-order low pass filter with Sallen-Key topology
with a corner frequency around 4.5Hz, designed by Rana.The coil drive is a voltage follower with Gain of 2 where
we use BUF634 to boost the current of quad opamp L1125.

The schematics for the LED drive is given by the figure below:


The schematics for the coil drive is given by the figure below:


The final board is


The Altium file for this board is in the attachment titled: analogy_circuit.zip


  287   Tue Aug 9 15:57:10 2011 YiDailyProgressSUSCurves of magnets and coil forces
  • coil one

Measurement I with coil DC power off

d={18.2683, 21.7983, 26.4683, 29.8133, 32.9883, 35.995, 40.3383, 43.8483, 48.5483, 52.7983}  (units: mm)

Fz={0.137984, 0.093835, 0.051989, 0.036064, 0.026264, 0.020139, 0.013671, 0.010339, 0.007252, 0.005439}  (units: N)



Through Mathematics fitting,



Measurement II with coil DC power on

 d= {1.58683, 1.93983, 2.40683, 2.74133, 3.05883, 3.3595, 3.79383, 4.14483, 4.61483, 5.03983} (units: cm)

Fz={0.15464399999999998`, 0.108388`, 0.061789`, 0.043953`, 0.032732`, 0.025529`, 0.017934`, 0.0137689} (units: N)



a=0.024358   b=-0.0253739



  • coil two

Measurement I with coil DC power off

d={1.81683, 2.16983, 2.63683, 2.97133, 3.28883, 3.5895, 4.02383, 4.37483, 4.84483, 5.26983} (units: cm)

Fz={0.13426, 0.092218, 0.052773, 0.036995, 0.027293, 0.020384, 0.014357, 0.010731, 0.007399, 0.00539} (units: N)




Measurement II with coil DC power on

d={1.58683, 1.93983, 2.40683, 2.74133, 3.05883, 3.3595, 3.79383, 4.14483, 4.61483, 5.03983} (cm)

Fz={0.150626, 0.105693, 0.062475, 0.044884, 0.03381, 0.025725, 0.018522, 0.014161, 0.010094, 0.007546}  (N)



a=0.0237513, b=-0.0259415



  • coil three

Measurement I with coil DC power off

d= {1.81683, 2.16983, 2.63683, 2.97133, 3.28883, 3.5895, 4.02383, 4.37483, 4.84483, 5.26983} (cm)

Fz={0.127204, 0.085848, 0.049588, 0.034839, 0.026019, 0.019698, 0.013867, 0.010388, 0.00735, 0.005439}  (N)




Measurement II with coil DC power on

d= {1.58683, 1.93983, 2.40683, 2.74133, 3.05883, 3.3595, 3.79383, 4.14483, 4.61483, 5.03983}  (cm)

Fz={0.143178, 0.098833, 0.058751, 0.042287, 0.032193, 0.024843, 0.017885, 0.013671, 0.009849, 0.007546} (N)



a=0.0228077, b=-0.0253862



  • coil four

Measurement I with coil DC power off

d= {1.81283, 2.16583, 2.63283, 2.96733, 3.28483, 3.5855, 4.01983, 4.37083, 4.84083, 5.26583}  (cm)

Fz={0.127204, 0.085848, 0.049588, 0.034839, 0.026019, 0.019698, 0.013867, 0.010388, 0.00735, 0.005439} (N)





Measurement II with coil DC power on

d={1.58683, 1.93983, 2.40683, 2.74133, 3.05883, 3.3595, 3.79383,  4.14483, 4.61483, 5.03983}  (cm)

Fz={0.15092, 0.104468, 0.06174, 0.044198, 0.03332, 0.025774, 0.018326, 0.014014, 0.009996, 0.00735} (N) 


a=0.02419, b=-0.0252273



  •  summary

The amount DC bias needs to be counteracted is around 0.06N. The working point is: around 0.6 cm.

With power off, there are:

                First pair       Second pair      Third pair     Forth pair
constant:   0.152813        0.148525         0.140065     0.146585

  301   Wed Aug 10 23:28:05 2011 Yi and HaixingDailyProgressSUSupdate on maglev

Today we tried to use the national instrument card together with Labview to acquire data from the OSEM for measuring cross coupling.
The levitated plate is supported by four springs around the equilibrium point [below is a view from the side]:


One issue we encountered was that the flag can easily touch the edges of the OSEMs due to imperfection
in the design and the slight horizontal drift of the equilibrium point.


To fix this, we find a possible solution: we can use 1''inch diameter Polycarbonate Round Tube to replace the case of OSEM.

 For this purpose, I have ordered the following components today:
1. 1''inch diameter Polycarbonate Round Tube with length of 1 feet. Mcmaster Carr Part number: 8585K14
2.  IR Emitters with wavelength 890nm and also 935nm. Newark Part number:
08F2922 and 08F2957
3.  the corresponding IR photodiodes. Newark Part number: 91F1840 and 32C9152

Concerning the circuit board for LED and coil drive, I found that we might not need to order extra new board from PCBexpress.
We have an extra one left from the previous ordering. I will look into it more carefully to see whether we can use it or not.

Concerning the feedback control part, Yi has worked out the control matrix by assuming certain values for the sensing matrix elements [we are trying
to measure them]. This afternoon, Koji gave us very precious suggestions and materials for implementing the feedback control in Labview.

  304   Fri Aug 12 01:37:04 2011 Yi and HaixingDailyProgressSUSupdate

On the analog part, we modified the old circuit such that we can use it directly in the digital control.
The schematics of the original circuit goes as follows:


Basically, we only need the LED and coil drive part. We removed the resistors R17, R30, R31, R41 such that the
coil drive part is now disconnected from the derivative and integral control part [we will replace it by digital control].
We also replace the potential meter with fixed resistors. To make it more clear, let us look at only one signal path
of the coil drive
[the first one]. The modifications are shown by the following two figures [left one is the original coil
drive and the right one is the modified one].

zoom_1.png >>>>>>>>>>>zoom_2.png

Now we have four analog channels for both LED and coils. By combining them with the DAC and ADC in 
the two National Instruments cards, we can move to phase of implementing the digital control.

On the digital part, we find an example on the Internet about using Labview for proportional-derivative-integral (PID)
control. The virtual instrument file is attached: pid_control_labview_example.vi

In this example, they used a simulated plant to demonstrate how to realize PID control in Labview, which is very useful to us.
The modifications that we need to make
are the following:
1. Replace the simulated plant with our DAC and ADC vi module (Koji showed us yesterday)
2. Extend it to four input and four output channels (from SISO to MIMO);
3. Include the control matrix.

In the next few days, we will try the PID control first with a single input and single output to make ourselves get familiar with the
Labview interface and also demonstrate the control principle. We later can proceed to multiple channels.

  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



  315   Tue Aug 16 13:25:31 2011 Yi and HaixingMiscSUSnew sensor design for maglev

As we mentioned in the earlier that OSEM constrained the position of working point
of the flag in our design, due to a slight drift of the equilibrium point in the horizontal
direction [as indicated by the figure below]:


In order to solve this issue, we have the following design for the sensing. This allows a flexible tuning of the
LED and PD in the horizontal direction and get the right position for sensing the flag motion.

The above scheme is not difficult to fabricate. We do not need to go to the mechanical shop and we can make them by
ourselves. Right now, we got the required components (LED, PD, Polycarbonate tube, and screws, and we need to find
something for the movable plate

If you have any better ideas, please let us know by commenting on this log.

  317   Tue Aug 16 23:45:38 2011 Yi and HaixingDailyProgressSUSnew sensors for maglev

We tried to make our new sensors as what we designed [as shown by the figure below]:

[The reason for this new design was posted on Elog 315]

We glued the LED and PD on aluminum plates and soldered wires on them [shown by the figure below]. As it turns out,
if we simple make the gap [for holding the plate] slightly smaller than the aluminum plate, we do not need extra screws to fix
the plate, which makes the scheme a lot simpler.


  321   Thu Aug 18 15:48:46 2011 Yi and HaixingDailyProgressSUSissues in digital control of single DOF

To better understand the digital control system, we first tried to control a single degree of freedom (DOF) with Labview
and NI DAQ system yesterday. We relaxed the constraint on the angular motion of the levitated plate [it was constrained
by mechanical springs originally]. This allows us effectively to have a single DOF system to work with [as shown by the
figure below]:

The experimental setup and its schematic goes as follows:


We used SR560 as a low-pass filter for anti-aliasing. The corner frequency for the low-pass filter is 100Hz.

After adjusting the working point, we get the error signal. In the figures below, we showed the error signal (orange curve) from the OSEM and the control
signal (cyan curve) from the analog output of the national instrument (NI) card, before (left figure) and after (right figure) the low-pass filter is turned off


From the signal we can see that the system is oscillatory. It does not decrease when we apply the derivative control. From the control signal, we can
see that the sampling rate is very low, and the control signal is clearly discrete with a rate around 50ms.
Probably this is why we can not have a stable
control. Can someone give us some suggestions on how to proceed? Thanks.

  322   Thu Aug 18 20:27:21 2011 KojiDailyProgressSUSissues in digital control of single DOF

 - The previous entry showed that the sampling rate is 1ms.

If the loop is really running at 50ms, you should see an error output from the "timed-loop structure".

If the timeout error is found, the servo does not make sense anymore.


- Why don't you take the transfer function of the digital servo filter separately from the closed loop?


  323   Thu Aug 18 21:38:41 2011 HaixingDailyProgressSUSissues in digital control of single DOF

Thank you very much for your reply.

>> - The previous entry showed that the sampling rate is 1ms.

Yes, indeed it was. Actually, even in the current setup, the sampling rate for the channel is set to be 1kHz.
Jan told me the highest sampling frequency that we can get is of the order of 100kHz.

>> - If the loop is really running at 50ms, you should see an error output from the "timed-loop structure".

When we changed the loop time constant from 1ms to 100ms, it seems that there is no change at all. The control signal
still behaves like that. Maybe we do not know how to set it up correctly. However, there is no error output from the "timed-loop structure".
We will look into this more carefully.  Right now, we really have very poor knowledge of the digital system. I will come up to 40m
to bother you with few more questions tomorrow, if you will be around.

  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.

  342   Tue Sep 6 00:08:07 2011 HaixingDailyProgressSUSLevitation of two degrees of freedom

After fine tuning the working point, we successfully levitate two degrees of freedom of the
plate---the second stage before we can levitate the entire plate which has three degrees
of freedom that need to be controlled (other three are stable). The configuration is shown
schematically by the figure below. The two degrees freedom of the levitated plate are the
tilt motion [differential motion of magnet 2 and 3] and the pitch motion [the motion of magnet 4].
The plate is held by a spring at where the magnet 1 is located. The control force is applied
onto the magnet 2 and magnet 4. The reason why we choose these two degrees of freedom
is that they are weakly coupled to each other---we do not need to worry about the cross coupling
of these two at the moment.

The photo for the levitated plate and the control signal on the oscilloscope are:

levitation.png            oscilloscope.png
(The green and blue are the control signals (green without low-pass filter); yellow and magenta curves are
the sensor outputs for the OSEMs)

The experimental setup is shown by figure below. We a national instrument card PCI-6289 for ADC and DAC.
The feedback controller is a hybrid of digital [for tuning the gain and the integral controller] and analog
[for lead compensation].


We replaced the old computer with a new one. Now we can simultaneously run four channels with a time
loop of nearly 2ms [we had 5ms with the old computer]. For the feedback, we used a simple proportional
and integral (PI) control in Labview [the derivative control is realized by using analog circuit instead] the
with a proportional gain equal to 1 and the integration time of 0.1 min.

Before we try to levitate the third degree of freedom, we will firs measure the open-loop transfer for this
scheme to get a better understanding of the transfer function of the plant. In addition, we will measure
cross coupling among different sensors.

  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 ;-)

  381   Mon Jan 23 18:06:55 2012 Giordon StarkDailyProgressSUSCoating Thermal Noise - Setup

 Spoke with Rana today. Meeting up with Zach on Wednesday to scrounge parts together and make some rough calculations of the SNR.

The attached PDF needs a lock-in amplifier and extra electronics and we need to fill in missing details about the equipment such as the range of intensity for the laser beam, and so on.

Calculating the SNR will be relatively tough since we'll need to know power spectral densities of some things such as the photodiodes, and how the birefringence stress in the mirror affects our signal (by looking at some mode and making order-of-magnitude calculations).

We're going to run our first iteration without a vacuum. We'll use some thin disk (lower eigenfrequencies = easier measurement) which has a mode around 80 kHz-ish. We might wanna try the old 40m mirrors with modes at 28kHz first. Alastair is going to find out if Glasgow has any coated disks that they don't want anymore. Also talked to Rana more about the support which is described as a "cats' cradle" for holding the mirror. It sounds like it will be rather awesome as long as we're good about placing the threads along the nodal points of the mirror when it's excited. Finally - we should find Phil or Alastair's "old rig" for finding the electrostatic drive (ESD) and see if it's usable. If it isn't, we'll use it as inspiration for making a new one which we send off to a company later in the week.


  566   Tue Aug 28 16:16:00 2012 Norna RobertsonMiscSUSProposed quad pendulm with 143 kg silicon test mass for LIGO-III

Our high school summer student Madeleine Waller has produced a set of slides describing work she has been doing over the summer working with Calum and  Norna.

She has been using Solidworks to look at how a proposed quad suspension with 143kg silicon mass might fit into an existing BSC chamber with a  BSC-ISI, a Transmon and an arm cavity baffle.

See G1200828 on the DCC at https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=95782


  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.



  579   Fri Oct 5 15:17:47 2012 HaixingNoise HuntingSUSEstimate of gas damping and associated noise in maglev

Here I make an estimate of the gas damping for the maglev.

The damping rate:

I use the formula given by Cavalleri et al. presented in the article titled Gas damping force noise on a macroscopic test body in an infinite gas reservoir [PLA 374, 3365–3369 (2010)]. According to Eq. (18) in their paper, the viscous damping coefficient for a rigid body is given by


Here p≈10^5Pa is the air pressure, S is the surface area of the rigid body (0.05m^2 the floating plate in our case), m_0≈28  is the air molecular mass, T≈273K is the environmental temperature. Plug in the number, we get


Given mass of the floating plate to be around 0.56 kg, we get mechanical damping rate to be:


which is very large. This means the floating plate is an strongly over-damped oscillator if the resonant frequency is around the design value of 0.1Hz. To have a quality factor of even order of unity, we need to pump down by one hundredth of the air pressure.

The displacement noise:

We can use the fluctuation-dissipation theorem to estimate the displacement noise. The force spectrum is given by


The displacement noise around the mechanical resonant frequency reads:


Given a resonant frequency to be around 0.1 Hz, we have


This is smaller than the seismic noise which is approximately three orders of magnitude higher.


  580   Mon Oct 8 09:58:27 2012 ranaNoise HuntingSUSEstimate of gas damping and associated noise in maglev

If I hold up a piece of paper in the lab, I can see it move because of the air currents. Since the maglev and a piece of paper have roughly the same resonant frequency, I think your estimate is not covering the whole picture.

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