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SupaVillain
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Apparently in order to learn something I have to be stumped and then post that i cant find it, then immediately after i find it!.
https://www.youtube.com/watch?v=3hlp401JN0M
- this video is a jackpot of info because it actually tears down an FTIR (from 1989) and explains a lot about its parts. I had no idea that the laser
was used for tracking the movement of the sliding mirror and that therefore an expensive air bearing with expensive programmable control isnt
necessary. That makes the electronics part way easier. Setup of a nichrome wire thingy may be simpler because i dont know anything about setting up a
globar. Back to electronics, they are more complicated than the current times and reality that we live in today because in this video the computer
hardware of this old machine wasnt as advanced back then and im hooking this thing up to my computer anyways instead of having it use a computer of
its own. Nowadays, 1 or 2 chips of RAM could replace all that mess of RAM chips they have... Also this video sheds light on the requirements of the
resolution for the ADC chip (analog to digital for detector to computer connection), being 16 bit, but the guy in the video said this was an NaCl and
ZnSe FTIR instead of KBr and the detector quality didnt seem to impress me so Im going to say that the ADC chip needs AT LEAST 16 bits of
resolution... more resolution cant hurt but looking at these chips, more of that means more things gotta be hooked up to all kinds of crap.
Another video i saw on youtube introduced the OMNIC software which i believe is open source (free) and the guy in this video explained that the
interferogram is more of a recorded thing over a short span of time like a few seconds. He stated that the moving mirror moves slowly. Ill be looking
into more of the details on the moving mirror (how far it travels and how its movement affects the interferogram output so that i find out the tech
specs of it and therefore see how i need to design it
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SupaVillain
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Smaerd i didnt see your post until after i posted again, i will look into those things thanks, another thing i have realized might gimme info on how
the moving mirror affects the interferogram is to actually download the OMNIC software and tinker with it, also looking into OMNIC software tutorials
might show users tweaking the instructions of the parts and the displying an outcome that will tell me something.
I also keep forgetting to address the component of the sample compartment in which I found the attenuated total refraction (ATR) is the best for this
because it totally takes away sample preparation but I don't know how expensive this is compared to traditional sample compartments. This link below
is valuable in that it shows a diagram of how the beam will pass though it and lead back into direction of the detector.
http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumenta...
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smaerd
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ATR is wonderful but expensive. Home-made ATR is definitely possible, still probably expensive, http://link.springer.com/article/10.1007%2Fs00897970140-X /
"‘Do-It-Yourself’ Attenuated Total Reflectance Cell Designed and Constructed in a Laboratory Course: A Versatile and Economical Alternative to
Commercial Designs". The Chemical Educator
October 1997, Volume 2, Issue 4, pp 1-16.
If I had access to springer I would have gotten the article for you.
For testing the instrument prior to sample cell development thin sheets of polystyrene are often used as sample and are of course very cheap and easy
to find.
I think the mirror's speed depends on what the operation is. For scans it's probably pretty slow(but a nice constant rate to achieve a uniform
dataset), but when I fixed a few FT-IRs they were doing a quick jerking motion during the start-up routine. Maybe it was a calibration routine to find
where the mirror was located.
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SupaVillain
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Honestly i think that i feel pretty confident about the sliding mirror from its movement to the final product of the interferogram, from the links you
provided. Thank you! Seems like the ultimate output interferogram is the averaged or high resolution result made from thousands of little lower
resolution scans from across the whole movement of the mirror.
all that really maters.... is that the final interferogram is made of a large amount of detector scans, for example, a scan at every micron increment
passed by the mirror if your actuator has micron resolution. If you really think about it, theres no reason why making this slower or faster would
make a difference in the final product. The highest resolution is found by taking a detector scan at every smallest unit amount of distance that the
mirror can take steps by. However, the mirror's movement doesnt have to be a bunch of little steps, it could be a continuous movement in which each
detector scan is paired with the recorded distance it has traveled at the time of the scan. This may be more difficult due to lagging of the
processing made by the electronics.
As far as tracking the movement though, somehow this supposed to be done with the HeNe laser and a "fringe" thingy. I will look more into that next. I
really hope to make a cheap sliding track and have all the technical tracking figured out by the hopefully strong and high resolution capabilities of
this laser fringe combo thing.
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aga
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The ADC sampling time will be the bottleneck in a low-cost rig.
If the mirror moves faster than that, you'll just miss samples, lowering your overall resolution.
E.g. for a PIC16F1823 that i happen to be using at the moment, a reliable ADC reading takes 1us.
From experience, i would take at least 16 readings, bubble sort, and use the median value, so an absolute minimum of 1uS, more like 100us per reading,
so the mirror has to move slower than than between positions.
Is the 'IR Source' really just a heated wire ?
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SupaVillain
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Yes nichrome wire, i mean its a heating element, IR radiation is just heat basically.
By 1us you mean 1 microsecond right?
Yea honestly though a lot of the price of the cost of an FTIR is in the wage hours taken to design it, make it, and test it. But since I love doing
this crap i dont count that in on my own self. Im counting the cost of materials though, and thats the honest cost of em, with even mostly highest
quality components, for example a Ge thin film on KBr substrate beamsplitter
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SupaVillain
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It's hard to find a way to count the fringes since they dont really sell something for that commonly.
Edit - lmao and then i immediately find out that what im labeling the as is incorrect for rhe industry terminology. Ebay and google searches produce a
wealth of info for "laser distance sensors".
[Edited on 7-12-2014 by SupaVillain]
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m1tanker78
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SupaVillain: What do you plan to do for optics? The expense has always put me off. I planned to attach the moving mirror to a speaker
cone (instead of a proper linear actuator) with enough dynamic range, calculate latency from momentum and electrical path(s), plug that into the
software, and 'track' the position of the mirror by the phase of the sine wave applied to the speaker. Naturally, the zero crossing point would be the
center of travel. With some tweaking, one could find an ideal frequency or range of frequencies to drive the speaker. Amplitude (deflection) would
also be a controllable parameter.
With a 16 bit DAC, I figure the resolution would be more than adequate once the dynamic quirks (jitter) of the speaker and driver are worked out.
Tank
Chemical CURIOSITY KILLED THE CATalyst.
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SupaVillain
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Not sure which part of the optics you mean lol im new to this stuff, i plan to use highly reflective mirrors that i can buy or make myself with PVD or
CVD, a Ge on KBr beamsplitter and i think they call them "parabolic windows" which take the IR radiation and focus it into a skinnier beam so that it
fits the size of the detector. Ill be ordering my KBr crystal lenses instead of trying to cut my own but ill do my own PVD magnetron sputtering to
deposit the thin film of Ge on them. I mean theres a site you can order KBr crystals like almost 1 inch by almost 2 inches for like 40 bucks and i was
gonna cut that in half. Not sure what the cost of magnetron sputtering is but ive seen some pretty ghetto DIY setups on youtube
So far ive planned that the detector will be pyroelectric lithium tantalate crystal from here http://www.silverlight.ch/laser.php
as far as the tracking of the moveable mirror... that is what i have just found out a great deal about...
the HeNe laser i used entirely to generate the whole "interference" part of the interferometer. The thing about interference is that it's basically
the two split beams giving off these waves called "fringes". As you move the sliding mirror in its linear path you will find these fringes to keep
coming out from the center of the beam.... and counting these fringes helps you track the movement of the mirror. In order to digitalize this, a
phototube, more specifically a PHOTOMULTIPLIER is used to count the fringes somehow. Im currently looking at how these photomultipliers are hooked up
and used for this purpose. Now thats tracking the movement, and as far as actually MAKING the movement, im going to try to go as cheap as possible
with something like an arduino step motor or maybe do a periscope type thing like in the FTIR teardown video i posted. I personally think the quality
of movement doesnt matter its how high resolution you can track it that matters, because that simply will see all that was done in the movement and
pair that data with what was scanned by the detector
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smaerd
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Jeese I don't know supavillian. I would really reccomend using more things off the shelf, especially when it comes to optics. I'm sure that you're
aware that KBr will degrade in moist environments (IE air) over time.
I would also recommend avoiding a photomultiplier tube at almost all costs. I don't recall seeing one in the FT-IR's I've peaked in too. I'm sure
there's another way to handle what you're trying to do.
Can you find any spectral data on the detectors you listed? I didn't see any on the website.
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aga
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ISTR that digital vernier calipers use a fringe-counting technique, and they're really cheap these days.
Obviously they achieve pretty fine and repeatable resolution.
http://www.ebay.co.uk/itm/Digital-Vernier-Caliper-Gauge-Micr...
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SupaVillain
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Hmmmm. Yes i mean the whole point of the laser is to make the fringes and make them countable upon the moving mirror but it really isnt required. Ive
just been curious as to see if thats how this could really be done.
Yes but smaerd why wouldnt a photomultiplier work? I think the cheapest movement tracking is to get one of those analog knobs that displays how far
its been turned, as it turns, upon an arduino, very simple cheap setup. Unless i can figure out how to get the "caliper tong teeth things" from the
caliper thing that aga has mentioned, to be moveable along with the mirror, and i guess a hookup could be made from its electronics to plug the
distance values into the FTIR software instead of sending it to its little LCD display
i think im gonna just give up on the photomultiplier because i not only would have to figure out from a scarce info resources how to hook it up and
put it in an environment in which it could actually read the fringes.
Might not even have a point for the HeNe laser at all anymore!!! Is the laser used for the analysis as well or just for the mirror movement tracking?
It is valuable to remove things from the recipe list that you wouldve had to buy before!
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SupaVillain
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So yes the vernier calipers do provide measurements as you move the prongs continuously as you would need, but i may look into higher definition
alternatives for this.
[Edited on 8-12-2014 by SupaVillain]
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aga
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higher definition = more gears maybe ?
i suppose there's a limit, but increase the gearing ratio by 100:1 should mean increasing the resolution by a factor of 100.
[Edited on 8-12-2014 by aga]
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m1tanker78
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SupaVillain, it seems you're a visual learner. Here is a random video I pulled from yt (though probably a bit extreme). Apparently, the audio guys
call it 'excursion'. I call it deflection but whatever. Imagine the moving mirror attached to the center of the speaker cone. You'd have to google
terms like 'sine wave', 'phase', etc. You could theoretically do away with optical or mechanical tracking of the moving mirror by simply keeping track
of the angular phase (google) of the signal that drives the speaker. Any laser light that operates in the visual range of the spectrum could be used
to align the optics. Even those $2 cheapies from the corner store will work.
The fringe thing as I understand is simply a standing wave but could be used to confirm that the incident beam does in fact interfere with the
phase-shifted beam.
https://www.youtube.com/watch?v=ZI6C_4KHtnM
Chemical CURIOSITY KILLED THE CATalyst.
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SupaVillain
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Yes either optical encoder or gears like that, sometimes i worry that these gears may not be exact or precise enough but then when I think about a
manual watch or clock, i guess there's nothing to worry about
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SupaVillain
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Tanker i didnt see your post until i posted, what youre saying sounds very interesting but yes i am a visual learner so much googling is about to
happen of the terms you just brought forth
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SupaVillain
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Yo tanker i think i found what youre talkin about with the mirror at the center of the speaker cone and the laser projecting off the excursion
measurement, in this yt video here, skip to like 3:30 in the video to see and yea i had no idea what you were talking about until i paired it up with
this visual representation lol
https://www.youtube.com/watch?v=EjReQE9B_7E
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SupaVillain
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Okay what I need to know is can the excursion of the speaker be done slowly and make only one movement? Like from point a to point b once instead of
this repetitive motion?
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aga
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Hmm.
It *can* be, but to get smooth operation, you would really want to drive it with a pure sine wave.
That would make it move in a repeatable way, without stuttering.
Avoiding saturation at the ends would be necessary, or it'll smoothly bang into end-limit, giving 'strangeness' at that point.
Google for 'pure sine wave oscillator' and there's loads of example circuits.
If you go for a Big speaker, google again for Class A audio amplifiers (class A gives the least distortion).
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SupaVillain
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Yea i dont know this sounds more complicated and not as precise as gears with a ratio and an optical encoder
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m1tanker78
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Quote: Originally posted by SupaVillain  | | Okay what I need to know is can the excursion of the speaker be done slowly and make only one movement? Like from point a to point b once instead of
this repetitive motion? |
Definitely. The resolution is going to depend on your drive circuit. If you use a microcontroller with 16 bit digital to analog converter, you can
increment the deflection or 'excursion' of the speaker cone by more than 32,000 increments forward and more than 32,000 increments back. Your AVR or
whatever you use would cycle through and create one [almost] smooth motion. You can't drive a speaker directly. You'd need to drive an amplifier of
some sort and account for dynamic irregularities of the speaker. Pretty much all this can be done in software.
As you saw in the video, you can measure the excursion directly but if you're going to be measuring very small changes, you could angle the backboard
to give higher resolution.
By how much does a typical commercial FTIR move the mirror?
Tank
Chemical CURIOSITY KILLED THE CATalyst.
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aga
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It's just a bit of electronics !
Probably more reliable than a mechanical system too.
Having said that, you could try the mechanical system, and then try the electronic one later, and compare results.
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SupaVillain
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I have massive respect for what methods ya'll are talkin about but i dont even know how to research that stuff lol. I kept diving into dense
photomulitiplier research and eventually it led away and towards other more modern technologies. I came up with info about using photo diode arrays or
CCD's and such to take a picture of the fringes once theyre projected off by an optical flat or some similar setup. Once you get these fringes as a
picture at certain points you can basically use OPENFRINGE software to analyze them and turn it into some kind of displacement data eventually. Theres
other software like metropro & mx, labview, and any other "static fringe analysis" or "phase measuring interferogram" software. I eventually
basically came to a point where once i got to the fringes analysis, everything turned into software names, and sometimes people talked of the
interferogram being made of this, where actually the interferogram and spectrum are made of a combo of this fringe analysis program and the data found
from the FTIR's detector. Detecting these fringes and measuring with them can apparently provide measurement at a "fraction of a micrometer" i guess
is the best way to say.
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SupaVillain
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Well i think im gonna go with traditional sample compartment since its easy and simple and ATR cant even do gas analysis. However they use a small
diamond with the popular "brilliant" cut and do a "single bounce" refection thru it, meaning laser pointed at the bottom right corner and reflects to
bounce against the top of it from inside and then back out the bottom left corner, if looking at a horizontal diagram... pretty simple but you may
need to focus the radiation down into a point to fit in the tiny diamond (top of diamond where sample sits is usually 1.5 mm diameter) and then the
top has some simple thing that screws down onto the sample and applies a little pressure....
but yea im probably going to just do the traditional sample compartment cause i guess i could change it at any time
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