fringe lock system
Ann
control my interference fringe. My experiment setup is placed on the
vibration-free table. But the fringe shift is visible to eyes. So I
decided to try fringe control circuit.
I have 2 questions in mind.
1. The displacement of the piezo is propotional to the applied voltage in
the range of 0-100V. The max. displacement is 15um. The wavelength of the
laser is 633nm, if the angle is 0, to shift one fringe, I only need
105.5mv to get the path change (double the movement of the mirror) of
316.5nm. The output from the photodector already gave me the voltage shift
of about 20mv. I only need to amplify it by about 5?
2. It is so easy to pass one or more fringes if the applied voltage to the
piezo is too high. I tried to use a simple FET with RC circuit to control
it. Still it was no good. And I believe it's the key point. But I have no
idea how I can solve it.
Any suggestions and information will be greatly appreciated.
And useful papers, books......
Thanks.
Ann
Phil Hobbs
>
> I am trying to use a photodetector, a amplifier circuit, and a piezo to
> control my interference fringe. My experiment setup is placed on the
> vibration-free table. But the fringe shift is visible to eyes. So I
> decided to try fringe control circuit.
>
> I have 2 questions in mind.
>
> 1. The displacement of the piezo is propotional to the applied voltage in
> the range of 0-100V. The max. displacement is 15um. The wavelength of the
> laser is 633nm, if the angle is 0, to shift one fringe, I only need
> 105.5mv to get the path change (double the movement of the mirror) of
> 316.5nm. The output from the photodector already gave me the voltage shift
> of about 20mv. I only need to amplify it by about 5?
>
> 2. It is so easy to pass one or more fringes if the applied voltage to the
> piezo is too high. I tried to use a simple FET with RC circuit to control
> it. Still it was no good. And I believe it's the key point. But I have no
> idea how I can solve it.
Fringe surfing is very hard to do well. There are many sources of
fringe wiggles, e.g. expansion and warping due to temperature swings,
small amounts of wavelength instability, noise, bumping the table,
microphonics. Many of these can exceed the range of any piezo whatever,
so that once in awhile your system will run out of range and lose lock.
Getting it to lock stably in a decent bandwidth requires a properly
designed control system. You didn't tell us things like the resonant
frequency of the piezo stage plus its load, how high the Q of the
resonance was, the RC time constant you were using, the amplifier gain,
and so on. Actually, within reason, you'll get better performance by
using a whole lot of amplifier gain--at least 100 times more than you
calculate. This excess gain is what allows a very small error (say 0.01
fringe) to generate a full-scale control voltage, which is what you need
to get a wide locking range with small errors. The problem is that you
have to have the right time constant, or the whole thing will oscillate.
If you can append a few more details, we could be of more help.
Cheers,
Phil Hobbs
Bret Cannon
As Phil Hobbs mentioned in his reply, to control a fringe position with a
piezo you want a servo system with lots of negative feedback just as is used
in op-amps. For some background you might want to read an introduction to
op-amps, say in Art of Electronics by Horowitz and Hill. If you are only
needing to hold a fringe stable for a few minutes or less and since the
fringe movement is slow enough to see, you might be able to do what you want
by using split photodiode and amplifying the difference in the photocurrent
between the two halves. Position it so that when the fringe is in the
correct position there are equal photo-currents from both halves. Use the
amplified difference photocurrent to drive the piezo where the sign of the
amplification is chosen to drive the fringe to the middle of the split
photo-diode. Start with an output time constant of about 1 second and
increase the amplifier gain until it the system becomes unstable and then
reduce the gain by a factor of 3.
This is very crude and a long ways from optimum, but it might meet your
needs. Servo-control systems can get quite involved if you need high
bandwidth and you are working mechanical devices, which generally have
resonances where they are most inconvenient and turn a stabilization system
into an oscillator.
Bret Cannon
Ann <my...@hbar.wustl.edu> wrote in message
news:54fb9b24.02102...@posting.google.com...
Doug Dulmage
I think you've gone a bit overkill on this.. I built two different fringe
stablizers for when I was doing production holography, and I didn't
need to come near as close as the quality and speed of a piezo
actuator. I got by just fine using some modified speaker voice coils
running off d.c. coupled amplifiers hooked up to the photodetector with
a simple gain control. I ended up with two gain controls, with an offset
adjustment, one large gain and then about a 10 turn fine gain adjust,
but I found that most of the time that I could set the detector into one
arm of the interferometer and get it to lock and hold on a fringe using
either just the offset or the gain. Actually, as I used a continous power
level on my exposures, the amplitude available at the detector was pretty
constant over the are of the table that I could place it, so I could usually
just get things to settle down by moving the detector mount around by
hand and then gently tightening it down with a couple of 1/4-20's.
I figure my bandwidth was fairly flat up to about 1khz, and that was
more than enough to cancel out any disturbance that could throw off the
hologram.
Their is a holography company in Utah, I'm sorry that I'm at a
complete loss as to the name of the company, it's named after the
owner. Oh, it's Ralcon Development. If you go to his site, he has quite
a long section their on active fringe stabilizers, as well as a link to the
person that builds his. I believe that the commercial units that he uses
sells for about $900, but it's almost identical to mine, and as I machined
my own mounts and stuff, I built the whole thing for less than $100..
Check out Ralcon, he's been using them with great success for
a loooong time, and has a lot of great info. And I based mine on what he
had suggested and it worked really well. I could pretty much pound a hammer
on the table stop while making a photopolymer hologram with a 10 second
exposure and the fringes whole hold, and my coherence length was about
3 feet or so total on the arms of my setup.
Good luck!
Doug
Ann
no idea about the frequency about the piezo stage. The piezo has unloaded resonance
frequency f0=18khz, the push/pull force capacity is 1000/50N. The load I have added
is just a pole with a mirror. The weight is small conpared to its capacity.
I have tried:
1. Output from the photodetector and a reference voltage goto a differential
opamp, then after several step of amplification to the piezo. I tried
to adjust the amplication of the circuit to get the right phase shift.
By looking at the oscilloscope, sometimes the vibration can be reduced,
sometimes it was even worse( the amplitude), sometimes, I just saw identical
sin wave sweep by. I thought that's because of the voltage I added to the piezo
( the amplification) was too high. The amplification I used from about 1
to 100.
2.As I stated here, using just simple comparator, compare the signal from the
photodetector and the reference voltage, then connected it to the FET with RC
circuit. I thought that the switch here would solve the problem. I used the time
constant as 10ms. Really in this design, i didn't use amplifier.
I hope you have a better idea what I had done.
>Actually, within reason, you'll get better performance by
> using a whole lot of amplifier gain--at least 100 times more than you
> calculate.
--How is this possible? I am confused.
Ann
AES
dul...@visi.com (Doug Dulmage) wrote:
> I think you've gone a bit overkill on this.. I built two different fringe
> stablizers for when I was doing production holography, and I didn't
> need to come near as close as the quality and speed of a piezo
> actuator. I got by just fine using some modified speaker voice coils
> running off d.c. coupled amplifiers hooked up to the photodetector with
> a simple gain control.
Way back when -- mid 1960s maybe? -- there was a small company in
Berkeley -- University Labs? -- that built early low-cost scanning
Fabry-Perot interferometers for laser research using ordinary speaker
voice coils as the mirror scanning elements. Very low cost -- the
primary structure was made from plastic -- but they worked just fine.
Loudspeaker coils (when did "loudspeakers" just become "speakers"?) may
lack many of the features of piezo elements for this task in terms of
stability, speed of response, and so on, but they're much better matched
to solid state driving circuitry, and seem to do the job in many
situations.
Bob May
of worms when you approach the limits of the system. In other words, slow
systems (where you aren't trying to drive any part of the system near its
resonant frequency or speed limit) are easy to develop while fast systems
can be a real pain in the posterior.
I don't know how fast the fringes are moving but if they are easily visible
with the eye as moving about, you should be able to do the design without
any real troubles.
The first thing to worry about is the loop gain. If the gain gets high
enough, you have an oscillator. This means that the thing will just start
flopping about and not really follow the target. As you reduce gain, the
system will eventually start trying to track the target until you get the
gain down far enough and then you can then see the target being tracked but
the whole system is very nervous and subject to sudden wanderings, often
going back into oscillatory motions for moments.
For a start, reduce the amount of motion that the piezo does to the mirror
or whatever you are adjusting until you need to do a lot of voltage change
to get a fringe motion. At this point, you should notice that it will be
tracking the slower motions of the fringe. You may also want to do some
other setup that will allow for the easy control of the fringes that the
electronics is tracking and move those fringes to some speed to see that the
electronics can track the fringes. When you do a step motion to the fringe,
the signal to the piezo mover should be a nice step with no overshooting of
the voltage. Once you get this done, do the proper setup and check the
voltage of the output of the amp again, looking for smooth changes in the
voltage without ringing of the output.
Servoloops can be fun and there are engineers out there that do nothing but
this as it is a very complex job when things get moving fast relative to the
ability of the parts of the loop to respond.
--
Bob May
Global WARMING???
What I want to know is when I can start growing wheat in Greenland again!
Phil Hobbs
> Ha, you brought up some questions I have never thought before. Honestly, I have
> no idea about the frequency about the piezo stage. The piezo has unloaded resonance
> frequency f0=18khz, the push/pull force capacity is 1000/50N. The load I have added
> is just a pole with a mirror. The weight is small conpared to its capacity.
The usual problem with putting piezo actuators in closed-loop systems is
that they have very pronounced resonances, often with Q values of 30 or
more, that force us to slow the control loop *way* down (a factor of 20
to 30) to avoid oscillation. The best method for fringe surfing is to
arrange two photodiodes so that they see opposite phases of the fringe
pattern--e.g. when PD1 is on a bright fringe, PD2 is on a dark fringe.
Wire the photodiodes in back-to-back parallel (anode to cathode), and
connect this combination between the inverting input of an op amp and
ground. Run the op amp's output into your piezo driver (or possibly
just into the piezo, via a 1k resistor). Ground the noninverting input
of the op amp, and connect a large capacitor (at least 1 uF to start
with) between the op amp's output and its inverting input. Run the op
amp from +-15V supplies, and use one with good output drive, such as an
LF156 or TL081.
Watch the op amp's output on an oscilloscope, and see what it does. If
it sits at some level between +-12V, and jiggles around when you breathe
on your optical system, it's in lock. Try reducing the feedback
capacitance, which will increase the speed of the loop, improve the
fringe locking, and (if you go too far) make the loop itself unstable.
As to why a high-gain feedback loop works better, I second the
suggestion to read Horowitz and Hill's *The Art of Electronics*.
Keeping a copy in the bathroom for a year and reading it at every
opportunity is a painless way to learn a lot about elementary circuits.
Cheers,
Phil Hobbs
IBM T. J. Watson Research Center
Doug Dulmage
You probably nailed the vintage of the actuator or voice coils on the head.
They came from a laser shop that made dichro type holograms back in the
60's and 70's and then the owner went out of biz, the stuff was moved into
a moving company storage area, and we picked it all up for the cost of the
back rent owed, so they had moved 3 semi loads of gear from this shop,
including one special truck just to move a 2ft thick by 12 by 9 foot granite
slab that came in at about 11,000 lbs. I had to take what I wanted that would
fit into a 28ft Ryder truck and junked the rest, which as far as I know is
still sitting out in the werehouse in Fremont. I got a nice twin tube lexel 95
out of it with temp controlled etalon, but the controller was shot but a nice
Red Lion PID temp controller that would learn the heating and cooling curves
of the laser came in really handy for getting rid of or slowing down mode
hops, and then I found the guts of these two stabalizers in one of the 50 or
so boxes of stuff that I had collected, the amps and stuff were shot, but the
mirrors on the coils worked fine, came out to about a 4 ohm load, so it was
easy to find a d.c. coupled amp with offset (I basically hacked up a general
scanning type galvo closed loop amp) and used the feedback portion for
the galvo position detector to hook up my photodetector. It was kind of a
no-brainer to put together, and the frequency response was much higher
than I needed, and my new lab was sitting under the northwest approach
to Minneapolis airport, planes going overhead weren't the problem, it was
when they would put their engines into thrust reverse, we were about 5
miles away from the runway and these huge standing waves would get
setup, and of course there was a doppler shift as the plane moved down
the runway and I could turn off the driver and watch the fringes move in
proportion to the shift of the standing waves. With the driver on, I could
make an exposure while a 747 or C130 from the Natl. Guard was landing
and the fringes wouldn't budge, and I had quite a bit of coherence length
from the etalon. Actually, I have a picture of the table with the fringe
stabilizer on it at my ugly web pages, maybe you'd recognize the mirror
mount/voice coil, here's the URL, but you better have a fast cable or DSL
connection..
Doug
http://www.visi.com/~dulmage/page4.html for the holography setup
http://www.visi.com/~dulmage/ for a bunch of laser marking
stuff
BS
If you need high frequency locking, I suggest you actually lock your fringes
around a fixed frequency, not around a fixed position. For instance,
introduce a 100Hz movement into one of the mirrors of your setup via a piezo
and correct this movement with a second mirror of the setup (also on a
piezo). The advantage here is that because your mirrors are already
vibrating at a 'high' frequency (but out of phase so as to cancel their
movements), you get a much higher bandwith in the correction of random phase
shift errors. In addition, you don't actually need to use two photodetectors
(which are not so handy) but only one. A shift of the fringes in one
direction then results in a decrease (or respectively increase) of the
'carrier' frequency and hence direction of the shift is known and can be
corrected for.
BS.
"Ann" <my...@hbar.wustl.edu> a écrit dans le message de news:
54fb9b24.02102...@posting.google.com...
HoloLab
> If you need high frequency locking, I suggest you actually lock your fringes
> around a fixed frequency, not around a fixed position. For instance,
> introduce a 100Hz movement into one of the mirrors of your setup via a piezo
> and correct this movement with a second mirror of the setup (also on a
> piezo). The advantage here is that because your mirrors are already
> vibrating at a 'high' frequency (but out of phase so as to cancel their
> movements), you get a much higher bandwith in the correction of random phase
> shift errors. In addition, you don't actually need to use two photodetectors
> (which are not so handy) but only one. A shift of the fringes in one
> direction then results in a decrease (or respectively increase) of the
> 'carrier' frequency and hence direction of the shift is known and can be
> corrected for.
> BS.
I have always believed the approach as described above with a carrier frequency should be way better. I remember reading a paper about this approach back in 1995 or so but have been frustrated unsuccessfully trying to find it.
Anyhow, my question is about how the control system would look like with the carrier frequency approach? I am guessing it is not the same design as with two photodiodes but rather a PLL approach? If anyone has more info on a possible design I am interested!
HoloLab
HoloLab
>The best method for fringe surfing is to
> arrange two photodiodes so that they see opposite phases of the fringe
> pattern--e.g. when PD1 is on a bright fringe, PD2 is on a dark fringe.
> Wire the photodiodes in back-to-back parallel (anode to cathode), and
> connect this combination between the inverting input of an op amp and
> ground.
Could you detail how the "Wire the photodiodes in back-to-back parallel (anode to cathode)" works because as I understand it, that would add the charges measured by the two photodiodes while I would normally want to subtract one from the other, no?
Many thanks
Loic
Phil Hobbs
occasionally, and glad to see visitors with actual optics to discuss. ;)
Fringe surfing is usually done naïvely by using one photodiode,
dithering around a dark fringe, detecting the fundamental component of
the resulting photocurrent with a lock-in, and feeding that back to some
actuator to zero it out. (The actuator can be lots of things, e.g. a
piezo mirror in an interferometer or a current-tuned diode laser.)
The problem with that approach is that you're servoing around a point
where your SNR is zero. (Not 0 dB, _zero_, i.e. all noise and no
signal.) The error signal builds up only quadratically with mistuning,
so the null is very poorly determined, making the lock very noisy. You
can use some huge dither to get round this, but that usually screws up
the measurement in other ways.
The two photodiode approach requires a spatial fringe pattern, e.g. a
slightly misaligned interferometer. If the two PDs straddle a bright
fringe, subtracting the photocurrents (by wiring the PDs in parallel,
usually) [*] gives a signal that goes linearly through zero when the
fringe is exactly centered. If you pick the separation right (roughly
the 70% points), you get a nice sharp null signal with lots of SNR.
Feeding back on that gives you a good locking signal without needing dither.
AC approaches are possible but much more complicated--making a moving
fringe pattern needs an acousto-optic modulator or the equivalent, and
you need a fair bit of RF signal processing to get that right.
(It's far from impossible--I did something vaguely like that for my
thesis long ago, but it wasn't a quick or easy job.)
Cheers
Phil Hobbs
[*] The best way of wiring PDs in parallel is to wire them in series
between opposite-polarity bias supplies. ;) Either way, the currents
subtract, which is the key point.
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
Phil Hobbs
two output beams are conveniently separate, you don't need a spatial
fringe--you can just detect the two beams separately and subtract as
above.
That's the basis for a very good but little-known laser locking technique.
Cheers
Phil Hobbs
HoloLab
I will experiment with wiring the detectors in series between opposite-polarity bias supplies, seems like a simple and sound setup, just what I needed :)
My application is about stabilizing long exposures of holography setups; nothing new but I like to revisit things by myself.
From what I understand now:
- the single detector approach has the advantages of not requiring a specific fringe spacing and enabling simple phase stepping the interferometer (by inserting a bias voltage in the loop) but has the disadvantage of requiring fringe contrast calibration and is prone to error if the laser output power varies.
Here is a nice article on that setup: https://www.researchgate.net/publication/263582518_A_Laser_Interferometer_for_the_Undergraduate_Teaching_Laboratory_Demonstrating_Picometer_Sesitivity
- the two detector approach is immune to laser output power fluctuations but requires proper fringe spacing
In both cases I don't quite see why it is much more complicated to introduce a (low frequency) carrier frequency in the optical setup by means of an additional mirror on a transducer in order to improve the "reactivity" of the loop to some external vibration as suggested above but I might be missing something.
Best regards
Loic
PS: if anyone is interested in holography here is a link to my channel on what I do in holography (purely amateur, nothing to sell):
https://www.youtube.com/@hololab7368/videos
Phil Hobbs
> Greetings Phil and many thanks for your reply.
>
> I will experiment with wiring the detectors in series between
> opposite-polarity bias supplies, seems like a simple and sound setup,
> just what I needed :)
>
> My application is about stabilizing long exposures of holography
> setups; nothing new but I like to revisit things by myself.
>
> From what I understand now:
>
> - the single detector approach has the advantages of not requiring a
> specific fringe spacing and enabling simple phase stepping the
> interferometer (by inserting a bias voltage in the loop) but has the
> disadvantage of requiring fringe contrast calibration and is prone to
> error if the laser output power varies. Here is a nice article on
> that setup:
> https://www.researchgate.net/publication/263582518_A_Laser_Interferometer_for_the_Undergraduate_Teaching_Laboratory_Demonstrating_Picometer_Sesitivity
>
> - the two detector approach is immune to laser output power
> fluctuations but requires proper fringe spacing
spacing to your actual fringes--if the diodes are dx apart, twisting the
mount effectively gives you dx cos theta, which can be significantly
smaller.
There's also nothing that says the diodes have to be looking at the same
fringe, just that they be on opposite slopes, and that you not have a
full fringe across the diameter of the diode.
>
> In both cases I don't quite see why it is much more complicated to
> introduce a (low frequency) carrier frequency in the optical setup by
> means of an additional mirror on a transducer in order to improve the
> "reactivity" of the loop to some external vibration as suggested
> above but I might be missing something.
You could do it with a photoelastic modulator and a lock-in, and look
for the second harmonic to go to zero. That puts you halfway up a
bright fringe, so you don't have the SNR problem of surfing dark fringes.
Cheers
Phil Hobbs
HoloLab
> > fluctuations but requires proper fringe spacing
> Or else a rotation mount, so that you can match a fixed photodiode
> spacing to your actual fringes--if the diodes are dx apart, twisting the
> mount effectively gives you dx cos theta, which can be significantly
> smaller.
> There's also nothing that says the diodes have to be looking at the same
> fringe, just that they be on opposite slopes, and that you not have a
> full fringe across the diameter of the diode.
I have started to design a circuit, partly based on the article previously mentioned and your parallel wiring proposal of the diodes, if you may confirm this is what you meant:
https://flic.kr/p/2og5WDR
Thank you
Loic
Phil Hobbs
the summing junction, and the offset pot doesn't add anything useful, I
don't think.
HoloLab
> Pretty much. You probably don't need the resistor between the PDs and
> the summing junction, and the offset pot doesn't add anything useful, I
> don't think.
> Cheers
Here were my reasonings (which are likely flawed!):
- Resistor R5 between the PDs and the summing junction is there to yield a gain equal to R4/R5
- The offset (Bias) pot is necessary because when the PDs are at equilibrium the output of the loop is 0volts, which is not the best working position for a piezo, since negative voltages may occur
Again, thank you for your help. Once I get a working setup I will share it (schematics, PCB, etc)
Phil Hobbs
>
>> Pretty much. You probably don't need the resistor between the PDs
>> and the summing junction, and the offset pot doesn't add anything
>> useful, I don't think. Cheers
>
> Many thanks. Here were my reasonings (which are likely flawed!):
>
> - Resistor R5 between the PDs and the summing junction is there to
> yield a gain equal to R4/R5
> - The offset (Bias) pot is necessary because when the PDs are at
> equilibrium the output of the loop is 0volts, which is not the best
> working position for a piezo, since negative voltages may occur
>
> Again, thank you for your help. Once I get a working setup I will
> share it (schematics, PCB, etc)
>
HoloLab
So I dug a bit on the topic of transimpedance amplifiers and found this good article: https://www.analog.com/en/technical-articles/optimizing-precision-photodiode-sensor-circuit-design.html
So I revised my circuit (new version heer https://flic.kr/p/2oguPQL) with the following changes:
1/ removed the useless resistor after the PDs and added selectable pre-scaling on the first amplifier stage for best noise performance
2/ added a second op amp to act as an adder in order to add the piezo bias AFTER the various gain stages. This has the advantage of not amplifying the bias as gains are adjusted
Thank you for any feedback!
Loic
Henry Nebrensky
> > On Thursday, October 24, 2002 at 11:01:10 AM UTC+2, BS wrote:
> > If you need high frequency locking, I suggest you actually lock your fringes
> > around a fixed frequency, not around a fixed position. For instance,
> > introduce a 100Hz movement into one of the mirrors of your setup via a piezo
> > and correct this movement with a second mirror of the setup (also on a
> > piezo). The advantage here is that because your mirrors are already
> > vibrating at a 'high' frequency (but out of phase so as to cancel their
> > movements), you get a much higher bandwith in the correction of random phase
I'm possibly being thick here, and my hands on holography experience is decades out of date, but I'm struggling to find an intuitive understanding for why vibrating the mirror is better vis-a-vis random phase shifts: surely half the time the mirror is travelling in the wrong direction, so first you have to stop it, and then move it to the right place but you have now less time to do so (owing to time spent halting it)...
Maybe if the vibration was mains-electricity related (motors, etc.) it would make more sense?
Thanks
Henry
Henry Nebrensky
> In both cases I don't quite see why it is much more complicated to introduce
> a (low frequency) carrier frequency in the optical setup by means of an additional
> mirror on a transducer in order to improve the "reactivity" of the loop to some
> external vibration as suggested above but I might be missing something.
complicated" than not doing so; the issue is whether the extra complication is worth it.
Are you actually limited by vibration problems?
Thanks
Henry
HoloLab
HoloLab
HoloLab
https://flic.kr/p/2ogEQci
Henry Nebrensky
> Not really but I am currently working with photopolymer films which are very slow, potentially yielding long exposure times, which increase the chance of a random phase shift and slow thermal drifts. So I am putting a bit of energy on the fringe-locking topic :)
Thanks
Henry
HoloLab
> I've never worked with photopolymer... can it work in the Denisyuk geometry? That can be more tolerant of noise and air movement around the laser.
Although the Denisyuk setup is simple, robust and yields realistic holograms, it is not suitable for all applications. See an example here (https://youtu.be/qrpcj7Es1_c) of a transmission interferogram of an entire room that I also recorded on PP. If you look at the way the interference fringes move, you can guess that the main source of vibrations is a rigid body movement of the optical table with respect to the room, which severely degrades the hologram quality at recording time, and which should be compensable by a fringe-locker :)
Henry Nebrensky
Unfortunately this old thing struggles with youtube - I 'll try to have a
look at those soon.
On Tuesday, 14 February 2023 at 09:39:52 UTC, HoloLab wrote:
> ... If you look at the way the interference fringes move, you can guess
> optical table with respect to the room, which severely degrades the
> hologram quality at recording time, and which should be compensable by a
> fringe-locker :)
Taking some off the ref. beam and bouncing off the wall?
Thanks
Henry
HoloLab
> I'm probably missing something obvious, but what then are you locking to?
> Taking some off the ref. beam and bouncing off the wall?
I have actually just completed the prototype fringe-locker circuit and transducer this evening. As youo can see the PCB is mounted on a (3D printed) holder which can be tilted to best align with the fringes.
Stay tuned for some real fringe locking tests!
https://flic.kr/p/2ohYP1Y
https://flic.kr/p/2oi2uvp
https://flic.kr/p/2oi4Cqx
HoloLab
HoloLab
And the project page: https://github.com/Loic74650/FringeLocker
This first version works ok but could certainly work better I think. In particular a more powerful output stage with a stiffer piezo would likely improve the frequency response of the control loop.
BOM total is just under USD26, so I called it the "Poor Man's Fringe Locker" :))
Many thanks for the inputs, in particular @Phil Hobbs
Phil Hobbs
You need more loop gain, though--probably a good 20 dB more. It should
be able to really lock them suckas. If the loop wants to oscillate at
higher gain, you can use more feedback capacitance.
Cheers
Phil Hobbs
HoloLab
>
> You need more loop gain, though--probably a good 20 dB more. It should
> be able to really lock them suckas. If the loop wants to oscillate at
> higher gain, you can use more feedback capacitance.
>
> Cheers
>
> Phil Hobbs
Phil Hobbs
worth measuring that, because R_out * C_piezo is probably the dominant
pole in the loop right now.
If you need any help with frequency-compensating the loop, ask.
HoloLab
> worth measuring that, because R_out * C_piezo is probably the dominant
> pole in the loop right now.
>
> If you need any help with frequency-compensating the loop, ask.
> Cheers
What do you mean by "probably the dominant pole in the loop right now"?
I have increased the capacitance on the output low pass and I can now increase the gain without oscillating.
Now I have increased the output gain by "a lot" but somehow it does not seem to improve much; I might be missing something
Phil Hobbs
that it responds gracefully but quickly to small changes in both control
inputs and external forcing.
That is, if you make a small but abrupt change in either the setpoint or
the output loading, you want the controlled variable (fringe position
here) to recover quickly and smoothly, without much overshoot and
(ideally) with zero DC error.(*)
The loop properties depend almost entirely on the *loop gain*. You
notionally break the feedback loop someplace convenient, while keeping
all the DC levels the same, introduce a perturbation (e.g. a small step
from an LTspice voltage source), and calculate the response of the rest
of the loop up to the other side of the break. (It's the same anywhere
in the circuit.)
If you post the details of the forcing function (that 0.6 Hz sine wave)
and how it's connected, the capacitance of the piezo, and the
peak-to-peak output from the photoreceiver, we can work out a sensible
frequency compensation scheme that'll give you better fringe stability.
Cheers
Phil Hobbs
(*) The restriction to small perturbations avoids getting mixed up with
the other main problem in control systems, namely *windup*.
HoloLab
OK I think we are more or less on the same page but I am coming more from the mechanics angle rather than electronics, so the semantics could be a little different. From what I recall from my Univeristy course in automation, a pragmatic approach to getting coarse tuning parameters for a PID feedback system is to measure the time constant of the system, its resonance frequency (when possible), and the "Ziegler-Nichols" method lets you estimate reasonable PID parameters. However in this case we are only dealing with a P loop so I was
thinking of just playing with the low pass filter and gain setting to tune it. But I am likely oversimplifying the problem. And the more I think about it the more I am thinking a pure Integrator type of loop could be more accurate (in removing the last DC offset error but does it really matter in my case).
Anyhow, believe it or not I don't have any instrument to measure capacitance (please advise a good HP, Fluke or alike instrument and will look it up on eBay) but the spec sheet of the Murata 7BB-20-6L0 piezo specifies a 10nF capacitance, a 6.3KHz resonance freuquency and 1KOhms Resonant Impendance. On the bench I measured a resonant frequency of the whole system as closer to 10KHz which is surprizing as I would have expected it to be less since we have a mass (the mirror) glued on the piezo.
Then I tried to measure the time constant of the system by inputing a step function into the piezo and measure the output of the PDs on a scope but alas the signal generator is completely messing up the signal when switched on. Analog electronics fun for you :)
I will retry by plugging/unplugging by hand a battery output into the piezo input and see if I can measure anything on the scope.
HoloLab
- In purple the input voltage to the piezo
- In yellow the PDs output voltage
- Time/div setting is 5ms on the scope
So I would say the Time Constant of the system is in the order of 10-20ms, agree?
HoloLab
I have tried to increase the gains at various places in my schematics but when doing so it seems the closed loop is actually doing more or less the same, maybe worse, so I have been wondering why and my guess is that it has to do with the Gain Bandwidth Product of the op amp?
The spec sheet of the TL074 I am using states a 3MHz Gain Bandwidth Product. If I understand this correctly it means that if I set a gain of 1million, Gain will start decreasing by 3dB at 3Hz, correct? Which could explain why I get the feeling the closed loop is not doing better at high gain. And so my only option would be to cascade more op amps in my circuit?
Phil Hobbs
> Many thanks Phil,
>
> OK I think we are more or less on the same page but I am coming more
> from the mechanics angle rather than electronics, so the semantics
> could be a little different. From what I recall from my Univeristy
> course in automation, a pragmatic approach to getting coarse tuning
> parameters for a PID feedback system is to measure the time constant
> of the system, its resonance frequency (when possible), and the
> "Ziegler-Nichols" method lets you estimate reasonable PID parameters.
> However in this case we are only dealing with a P loop so I was
> thinking of just playing with the low pass filter and gain setting to
> tune it. But I am likely oversimplifying the problem. And the more I
> think about it the more I am thinking a pure Integrator type of loop
> could be more accurate (in removing the last DC offset error but does
> it really matter in my case).
>
> Anyhow, believe it or not I don't have any instrument to measure
> capacitance (please advise a good HP, Fluke or alike instrument and
> will look it up on eBay) but the spec sheet of the Murata 7BB-20-6L0
> piezo specifies a 10nF capacitance, a 6.3KHz resonance freuquency
> and 1KOhms Resonant Impendance.
nanofarads just fine.) .
> On the bench I measured a resonant frequency of the whole system as
> closer to 10KHz which is surprizing as I would have expected it to
> be less since we have a mass (the mirror) glued on the piezo.
Okay, it'll look roughly like a series RLC: 1k ohm, 10 nF, 25
millihenries. That's a Q of only 1.6, which is fairly surprising.
Typical Qs are around 30.
> Then I tried to measure the time constant of the system by inputing a
> step function into the piezo and measure the output of the PDs on a
> scope but alas the signal generator is completely messing up the
> signal when switched on.
> Analog electronics fun for you :) I will retry by plugging/unplugging
> by hand a battery output into the piezo input and see if I can
> measure anything on the scope.
piezo voltage corresponds to a full cycle? Once we have all the gains
and bandwidths, we can calculate the frequency compensation easily.
Cheers
Phil Hobbs
HoloLab
Peak to peak photocurrent is a bit tricky to measure with my current setup, that will take some time as I am busy in the coming weeks.
Piezo voltage range is +/- 18V max
Meanwhile I have played a bit with the gains and low pass filter and result is already better, see a video at the link below:
https://youtu.be/GYKY7e883Bo
HoloLab
Current is 150nA (nano-amps). In practice the amount of light will vary from setup to setup so we could assume this peak current will vary from say, 50nA to 1000nA. If you need to pick a value, take 150nA