User:Pranav Rathi/Notebook/OT/2011/06/21/Noise issue with the optical tweezers: Difference between revisions

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The test completes with the following summary:  
The test completes with the following summary:  
* Vibration transmissivity wise structure is most sensitive to 100 to 150Hz and more sensitive to the frequencies less than 400Hz.
* Power coupling from frequencies higher than 150Hz wise 295Hz is the strongest candidate.
* Airborne noise wise structure is most sensitive to frequencies less than 140Hz.
* 120 Hz is the mechanical resonant frequency of the structure.
* 180Hz and 295Hz are the secondary resonance.




[[Category:Construction of Optical Tweezers and devices]]
[[Category:Construction of Optical Tweezers and devices]]

Revision as of 17:09, 11 October 2011

This page is dedicated to the different noise issues we experience during the data acquisition. I might decorate and elaborate this page very well but for now I am concentrating on the more important issue being faced right now. I am experiencing a weird oscillation in the data plane while acquiring the data. I mean by data plane is the plane in which the data is acquired. Some preliminary information about the oscillations:

  • Oscillations are more prominent during the DOG scan over the DOG profile.
  • Oscillations in X-signal are not continuous, they come and go.
  • Oscillations in y-signal are also like that.
  • But oscillations in the sum signal look continuous.
  • According to my observations two or more oscillations exists; 60~70 Hz and 120 Hz.
  • I do not know the amplitude yet.
  • The prime suspects are x-Piezo, QPD(x and y channel interference); environmental acoustic noise/unidentified noise object (UNO, DAQ cross talk. This list might increase.

June/21/2011

The investigation is under progress.[1]

June/22/2011

  • There are two different oscillations exist. One is around 60Hz, and these are due to on-track (amplifier and controller of QPD). This oscillation is on-track gain dependent and never goes away even when the QPD is not connected to on-track. These oscillations can be controlled by manipulating the ground of the on-track power supply. I designed a weird circuit to do that and it is successfully controlling the noise.
  • 120 Hz is the oscillation exists. This oscillation is not continuous like 60Hz. It comes and goes. I am not surprised that this oscillation is piezo damping dependent. I compared sets of data, between piezo is damped and not damped. I found some difference and looking more into it.........

June/23/2011

The 60Hz periodic oscillations are due to the power supply came with on-track. The power supply does not have ground enforced polarity and maybe it is a half-wave rectifier (I am not positive on that). I tried different power supplies with the same result. The ground manipulation on on-track power supply worked but I was not satisfied. So I used a different power supply model: TR2V1000N00 with regulated +12V DC at 300mA. This power supply gives a regulated voltage with ground. If I use this power supply without the cancellation circuit, than it is no different. So I used a cancellation circuit in which I have a 300uF 200V capacitor in parallel to the output of the power supply and a ground which is directly connected to the common. The capacitor absorbs any fluctuation in the current and keeps the modulation at minimum, and any unwanted noise is grounded. Another possible noise source might be AC power-lines and shutter-powersupply situated close to the signal carrying wires. A comparison is shown below.

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June/24/2011

I have also tested 12 V battery option. This option is little better overall noise wise. On-track needs 12V DC at 300mAmps. Any rechargeable-battery over 5 Ah can give 15 hours of constant power which is more than enough. Another exercise done today is isolation of the data acquisition hardware from various AC power lines. Data acquisition hardware includes, on-track, BNC wires, DAQ and QPD. I revised the whole electrical setup for tweezers, so now all the AC lines are far away from the acquisition hardware. This will definitely help in reducing the overall noise. On Monday I will verify it. Next Task on Monday: investigation of 120 Hz oscillations with nano drive piezo-alternate.

June/27/2011

60Hz noise issue is fully resolved now. The were two causes behind this noise. First x,y and sum signal BNC-wires were running close to the AC line and shutter-Powersupply. Power-supplies with transforms have 60Hz electromagnetic filed which might induce current into a conductor running close to it through electromagnetic induction. AC lines can do the same. Second the power supply for on-track was missing the ground.

I rearranged everything so the signal wires are isolated from any power-lines and power-supplies in vicinity. I changed the power-source for on-track to battery ( am using RYOBI cordless power drill's battery, which can supply power for hours to on-track, and we have two of them. We can use one when the spare is on charge). This solves the problem.

Result

The results are presented in slide 2 to 7. Slide 2 and 3 shows the noise through power-spectrum. I wrote a lab view program in lab view v7.1 for noise detection see slide 29. This program can take the data produced by the feedback program as an input and find-out any existing frequencies. The most prominent frequencies can be clearly identified on x-axis which is frequency axis. Y-axis is power-axis in DB. We uses quadrant photo diode as a detector, which gives three separate responses; x-signal, y-signal and sum signal of both. Sum signal is most appropriate to identify the noise frequencies.

Stiffness Vs time presents the sum signal Vs time so it is more appropriate to find the frequencies in comparison to x-signal OR y-signal Vs time. The data in slide 2 was taken on may/18/2011. 60Hz noise with its 3rd and 5th order multiples (180 and 300) can be clearly identified in power-spectrum graph.

Data in slide 4 was taken on June/02/2011. Similar result can be seen here too. The 60Hz noise oscillations in a form of beating can be seen by zooming into the data as shown in the satellite figure.

Slide 5 shows the solution with ontrack and its power-supply. Slide 6 and 7 data was taken on june/30 and july/14. There are no prominent oscillations, the relative strength of the noise is in the order of other background noise which is great!!

In case of 120Hz, i found that piezo might be the problem. I compared few sets of data between piezo dumped and not dumped. I found that when piezo is dumber the oscillation are weaker in magnitude. I am looking more into it, but if this is the case than i doubt it that this can be solved without changing the piezo stage.

Aug/16/2011

Designing a new microscope stage.

Aug/20/2011: Investigation of 120Hz oscillation problem

In the beginning there were two major noise-traces in the data. One was 60 Hz and other one was 120Hz. 60 Hz was due to two main causes; AC power line running close to the signal-wires and ontrack-power-source was not being grounded. This problem was solved by rearranging all the wires and using DC power-source (battery) for ontrack QPD-driver.

120 Hz noise was not easy to investigate; It is an airborne noise, the bandwidth of this noise is roughly 10 to 15 Hz, which means it goes from 110 to 125 Hz, and this might not the only frequency. So I had to device a way to investigate this noise in the optical tweezers: The entire device I made including the techniques and results are presented in the slide-presentation. So I will refer to the corresponding slide-number while discussing the relative.

In slides 9 and 10 a data comparison is shown: Ideal Vs noisy. The 120 Hz noise oscillations can be clearly seen. An optical tweezers can resolve the DNA-unzipping to the DNA base-pairs which are .338nm apart. This information is vital when DNA is unzipped in the presence of site-specific proteins. This noise definitely interferes with that.

To solve this problem the first step is to investigate the structure (setup) dependence (OR find the resonance of the setup) on the airborne noise frequency and vibration-frequency. The optical-tweezers setup consists of variety of materials like Aluminum, Steel, Glass, plastic and rubber. Each material has its unique response to the acoustics called its mechanical-response to its resonant frequency. This response is also 3-D design dependent; for an example a plane thick square sheet (12"X12") of aluminum can propagate a particular frequency with different attenuation in comparison to the same sheet with a grid of wholes (1/4" of hole diameter at every square inch). But mechanical-resonance will somewhat stay the same. This affects both vibration-frequency and airborne noise frequency. So first I did an experiment to test for the resonant frequency (most sensitive frequency band) for the whole setup all together which includes aluminum stages and breadboard, glass slide and steel posts.


Test for frequency response of the setup

I am not going into the whole chapter of transmissibility Vs frequency and airborne acoustic noise Vs material frequency response to that noise by the materials. I have chosen an easier way. I wrote a lab view V9 program to produce an acoustic wave from 0Hz to 22 KHz. It can generate sin, triangle, square and sawtooth waves of any amplitude with any offset with a power-spectrum display DB Vs frequency see slide 12. I used computer built-in sound-card and an external low power sound amplifier of unknown power-output. Since the computer sound-card, external amplifier and speakers all have their own range of frequency response, so no matter what frequency I generate at the program, it will only show-up at the speakers if it falls in that range, which is 100Hz to 20KHz. But this range is enough for the test; I am more interested in the range of 100 to 1000Hz, because I think this is what I am seeing in my data. The frequencies above 1.5KHz are useless for direct observation because we use a low-pass filter at 1.5kHz in data acquisition.

The test setup is relay simple see slide 11. I have a speaker connected to the output of the external amplifier which is connected to the sound-card output at the ear-phone jack. First I placed the speaker on the optical table about 1 meter away from the tweezers and second placed the speaker on a chair so no physical contact between the tweezers and the speaker. The sound and vibration produced by the speaker can affect the tweezers in two ways; by direct acoustic airborne noise and vibration transmissibility through the structure in first and only through acoustic airborne noise in second. I will start at 100Hz and take data (DOG; I made a stuck bead sample with water) every 5Hz increment.

There are 3 things to consider in advance in this test; first it is not necessary that the vibration frequency is same as the sound frequency produced by the speaker at all frequencies, second it is possible that the structure (setup) can absorb the energy from one frequency and resonate at another. But this behavior should not affect the results much. In other words we can still find the resonant frequency or frequencies of the structure by producing different frequencies at the speaker. Third, (because of the first and second) in the data we can see a result due to beating between structure vibration frequency, speaker generated frequency and other frequencies present. So counting oscillations at the time axis does not always work to find the frequencies present, because the oscillations do not have to belong to a single frequency. So the right way is to do a FFT-power-spectrum of the data, so all prominent frequencies can be found simultaneously.

My expectation from this test is to find: Which frequency or frequencies are more transmissive through structure, which frequency or frequencies are more intervening through airborne noise, which is more dominating between vibration transmissibility and airbornen and at what frequency range? This will help me with redesigning the setup with particular choice of the materials.

The frequencies are found out of a huge background of airborne noise, it is like finding the noise out of noise in the data. The data I receive through our Feedback program is a 2D array with number of options at x and y axis. So I wrote a noise detection program in lab view V7.1 see slide 30, which takes this and identify any prominent frequencies. We use a quadrant photo diode as a detector, which gives three separate responses; x-signal, y-signal and sum signal of both. Sum signal is most appropriate to identify the week noise frequencies. And x-signal is more appropriate in finding the noise in relative magnitude in DB. The program finds the frequencies through FFT.

Result

The test results are presented in the slide 13 to 24 and power-spectrum from 25 to 29. The results are generated through a DOG scan right across a 1μm stuck bead with water as buffer. Each time a scan is done with different frequency generated at the speaker starting from 0Hz when no frequency is generated (OFF). Similar test is also done when the speaker was not in physical contact with the optical table but was at the same distance from the trap.

First the slides 13 and 25: The data in these slides is taken when the speaker was off (0Hz). A DOG profile in slide 13 is given with a lower section zoomed in the inset: The oscillations can be counted to 12 over .1 second periods (this is 120Hz noise), but the oscillation profile looks of beating. Why is that (It means there might be more than 1 frequency)? In slide 25 the power-spectrum shows two prominent bands; around 120Hz, 180Hz and 295Hz. 120 Hz band is wider and much stronger than other two, maybe this is why it is more prominent in the beating. So it is sure now that there are more than 1 frequency bands exist in the data (similar is shown by the data collected when speaker was on the chair slide 25 right portion).

From slide 13 to 16 the DOG-profiles are presented over a frequency span of 150Hz. As we reach 120 Hz the profile gets thicker and gets thickest at 120 Hz (I tried to keep the amplitude same as I changed the frequency every time). I remember that I saw the bead vibrating in the camera at this frequency, which suggests that the present setup is most sensitive (vibration transmissibility and noise absorption wise) to this frequency. In slide 14 the 120Hz DOG is presented; the oscillation looks perfect (sine wave) no beating in that section. In slide 26 the power spectrum is presented; 120 Hz band can be clear seen with its higher-order bands of 240, 360, 480 and 600 Hz bands from left to right. The multiple bands are due to the constructive and destructive interference between the original and reflected waves. The waves are reflected at the ends of the optical table and interfere like the water waves in a pound are if generated at the center travel to the ends reflected back at the ends and interfere with the original. Since 120Hz is the only frequency band around, so all the generated bands are integer multiple of it with power descending with ascending order. The same thing also happens with other frequencies close to 120 Hz like 115 and 125Hz but it is strongest at 120Hz, so 120Hz is definitely more favorable, than other frequencies.

For next few frequencies the profile keeps going thinner until for 180 Hz in slide 17. This is another frequency we see in the 0Hz profile. In slide 27 the power-spectrum is presented in comparison with when speaker was not in physical contact. Profile on left (physical contact) is much stronger then on right (no-contact), which shows that this frequency is also definitely vibration transmissibility favorable. On right the 120 Hz frequency is dominating on 180Hz when the speaker is not in contact, but on left 180Hz is dominating when it is in contact.

After 180Hz 300Hz is next on to hit. In slide 28 power-spectrum shows that in-contact profile is much stronger than non-contact profile; suggests that this frequency is vibration transmissibility favorable. After 300Hz, no single frequency is dominating so there is effective interference between the frequencies. As can be seen in the slide 29; 600Hz is as strong as 120 Hz in both left and right (in-contact and no-contact).

The data is briefly analyzed in the slides 31 to 35. In the slide 32 a plot is presented with three strongest frequency Vs scan frequency. The blue line shows the strongest frequency band received. At 0Hz the first strongest frequency is 120Hz after that the received strongest frequency is the scan frequency up to 400Hz, after that the strongest frequency discontinued to be scan frequency and becomes 120Hz for all the higher scan frequencies. This suggests that the structure (setup) supports (less attenuation comparable to other higher frequencies) more to the vibration frequencies less than 400Hz. In slide 32 the plot shows the relative magnitude (DB) of 120Hz Vs scan frequency. At 0Hz the blue line starts with 15DB reaches 44DB at scan frequency of 120Hz, after that it remains below 15DB. This plot also shows the strongest frequency (120Hz) magnitude after 400Hz which still remains below 15DB; this suggests that there was not any power coupling to this frequency, otherwise it must be higher than 15DB. This suggests three things: The structure is more sensitive to the vibration-frequencies below 400Hz (the transmissibility is higher for the frequencies below 400Hz), power-coupling to 120Hz is void and there is not much airborne noise intervening above 400Hz. In slide 32 second and third strongest frequencies behave non-monotonically. Second strongest starts at 180Hz, between 105 and 150Hz it is the second harmonics of the scan frequency after 150 it drops down to 120Hz. After 400Hz it stays at 295Hz for more than once. Third strongest shows same behavior, it starts at 295hz becomes 3rd harmonics for frequencies from 100 to 130 Hz and drops down to 295 stays there for more than once. This suggests that the structure is definitely very-very sensitive to vibration-frequency band of 100-150Hz and sensitive to frequencies less than 400Hz. On average the 295Hz stay over the 0Hz magnitude for scan frequencies over 150Hz, which might be due to power coupling or noise absorption to this band (I am not sure).

Slides 33 and 34 presents when the speaker was not in physical contact with tweezers but placed at the same place and distance to avoid any transmissive-vibrations generated at the speaker. Slide 33 shows the first strongest frequency of 120Hz at 0Hz. From 105 to 140 the strongest is similar to the scan and then it drops to 120 and stays there for all the higher scan frequencies. Slide 34 presents the relative magnitude of 120Hz Vs scan frequency. At 0Hz it starts at 18.7DB and goes to 28DB at 120Hz scan frequency. After 140 Hz this plot presents the values for the first strongest frequency which still remain less than 18.7DB. This again suggests two things: First structure is more sensitive to the airborne noise frequencies less than 140Hz and there is no sufficient airborne noise power coupling (out of higher frequencies) to 120Hz. Second strongest frequency starts at 180Hz (-1.3DB) between 145 and 180Hz it is same as scan frequency, from 200Hz it stays on 295Hz which is on average of 1.5DB. This is above 0Hz DB which suggests that for the scan frequencies above 200Hz there might be power coupling to this frequency via airborne noise. Slide 35 compares the in-contact and no-contact data. All three profiles of no-contact (120,180 and 295Hz) are higher than the in-contact. This is due to the vibration dominance and interference created by the strong vibration scan frequency at any scan frequency. The resonance for all three frequencies can be clearly seen in the data.

The test completes with the following summary:

  • Vibration transmissivity wise structure is most sensitive to 100 to 150Hz and more sensitive to the frequencies less than 400Hz.
  • Power coupling from frequencies higher than 150Hz wise 295Hz is the strongest candidate.
  • Airborne noise wise structure is most sensitive to frequencies less than 140Hz.
  • 120 Hz is the mechanical resonant frequency of the structure.
  • 180Hz and 295Hz are the secondary resonance.
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