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Question for the Oscilloscope Experts

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rsfoto, Oct 10, 2015.

  1. rsfoto

    rsfoto Member

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    Hi audioguru,

    Thanks a lot for that. At the moment I have to take a timeout as my brain is totally overloaded ... :confused:

    When I have clear mind again I will follow your steps trying to setup some circuits via searching and looking into the Internet

    ;)
     
  2. rsfoto

    rsfoto Member

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    Hi Everybody,

    Well yesterday I saw that I reached my goal with the vibration sensor. OK, I am using a not very elegant and correctly designed cirduit but I saw that it detects very minimal vibrations.

    I had the circuit on my bench running connected to my Oscilloscope. The Oscilloscope is not capable of reading the input but I saw the output while I was walking by 12 feet away and saw a swing ot the output.

    I cranked up the gain to about 150x. Today afternoon I will even go higher with the gain to see how it reacts. The amplified output at its maximum was at about 180mV so the input was around 1.2mV at the maximum and the curve I saw shoed a nice wind up and down (sinusoidal curve) I will try to catch a screen shot of how it looks and then post it.

    Well I will now proceed and solder everything onto a phenolic plate with soldering points, the one with a whole every 1/10" and test it again. I will aslo get some audio cables with a 3.5mm jack and take the piezo 6 feet away and see what happens.

    Now another story is that with my idea I was reinventing the wheel. I found in the page of the Piezo producer a PCB specially made for this sensors and after opening the datasheet PDF I saw they depicted the full circuit in schematic diagramm :angelic:

    Look here http://www.meas-spec.com/downloads/MiniSense_100_Analog_PCB.pdf I will take a look at it and try to understand it. What wonders me is that they apply a voltage onto the sensor ... Left upper corned of the circuit, marked with LDTC1, is I guess the input of the sensor.

    Nontheless it was very interesting to develop this and thank you all for your extensive and great help and especially having learned a bit more about operational amplifiers :happy:

    BTW this project is going to be used to help other amateur astronomers to find out how much their piers or columns vibrate as many of them have the observatory build on the roofs of their houses.
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    That first bit of stuff after the sensor is a unity gain buffer, then it's capicitively coupled to a resistor to remove the DC offset.
    The diodes are for OP amp protection.

    The 500K really doesn't "apply a voltage" to the sensor. It brovides a "bias return path". A "bias return path" is a DC place for the input bias (Ib) current ot go AND a predictable output when the sensor is disconnected. You could use ground or Vcc. It's like open thermocouple detection where you typically want the sesnor to register a high temperature when the sensor is not connected when heating.

    Ib is typically in the pA or nA range, but it needs a DC path to return to.
     
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  4. dave

    Dave New Member

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  5. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    I once made a bunch of portable seismic recorders at UofM in '75 designed by a grad student in the Dept of Physics, when I was an EE student working part-time for Dept of Earth Sciences in my 5th yr.

    After I debugged all his logic race conditions for the programmable timers, the analog part worked fine with 60dB gain , enough sensitivity with a large geophone , moving magnet, suspended in coil,. I could place it on the basement concrete floor and pick up all the HVAC vibration and then I could peg the needle standing away and flexing my knees just standing onto the concrete floor.
    ..... ...... .....
    You can filter out all the noise above 100 Hz as displacement drops 12dB per octave with frequency due to x being the 2nd integral of g.

    The only thing you have to measure is the low frequency noise. If you get stray hum add a notch filter, which is common as the last link showed a 100Meg load resistor try twisted pair shield with ferrite core around sensor cable or a CM choke from a telephone modem card. Raising the CM impedance improves the CMRR significantly on high impedance sources with longish cables acting as antenna at uV to mV signal levels.
     
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  6. rsfoto

    rsfoto Member

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    Thank Tony but honestly you are talking chinese to me :nailbiting: as I am just a copy and paster in this electronic stuff :(

    Will investigate what you just explained and hopefully I will understand it :wideyed:

    I just found this https://en.wikipedia.org/wiki/Band-stop_filter and if I read it correctly I need a wire coil = L of size ¿?

    As said will keep looking ...
     
  7. rsfoto

    rsfoto Member

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    OK, let me see if this is what I need

    MS100PCB.JPG
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I cannot read the fuzzy part number of the opamps but they are LMCxxx which is Cmos (VERY high input impedance). The opamps might have a maximum supply of 6V so the voltage converter IC efficiently converts the 9V battery to a regulated 5V.
    Vref biases the inputs of the opamps at half the +5V supply voltage so that their outputs can swing equally up and down, the piezo sensor is not affected by the DC.
    The voltage comparator is very sensitive so if you want a linear output then you need more gain and a half-wave detector.
     
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  9. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    if only your requirements could be defined as a spec. Gain, frequency response. input levels , output...., indicators.
     
  10. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    You've made some remarkable progress in a week, rsfoto! I am impressed.

    And I also think you've picked up another hobby... :cool:.
     
  11. rsfoto

    rsfoto Member

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    Hi Tony,

    Good question but in what I am interested is only vibration. No interest in frequency as each column or pier is different it will have a different resonance frequency. If I may say so a vibration is more or less an acceleration = g force due to soil movements so what I am measuring is g forces product of the soil movement.

    Out put as sensitive as possible. At the moment I am driving with the output of the amplifier a LM3914 and with an output signal of about 230mV i can see the second LED flickering very lightly. The bar goes from 0V to 1.25V, so if my calculation is correct and the piezo is really linear I would sense at an input of 4.2mV a g force of about 0.0042g. At the moment I do not know if that is little or much. I will have to inform myself about this value.

    So far I think I have achieved what I wanted. I will put them on my columns to get an idea. See image of my columns. Each column has a weight of about 6,850kg ~ 15,100 pounds and the telescopes are at a height of about 6.2m ~ 20 feet 4 inches

    Pano-99.jpg

    View of the inside of the observatory

    OReal14-05.jpg

    At the moment, as the circuit per attached image is, I am working with a gain of ~300.

    ActualVibrar.JPG

    Now the piezo is glued to a big window and I am waiting for cars to pass by. BTW I do live in a 4th floor about 12.5m above street level. So far I have a change in the curve when a car passes by and so I guess it is working, but as written before I can not judge it until I know what signal I do get from my columns and then I can compare more or less the columns of my friends observatories.

    vibrarsetup.jpg

    So far I am a happy

    :cool:
     
  12. rsfoto

    rsfoto Member

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  13. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    drawing error, corrected below
    • at least this would work.
    • Telescopes move images with displacement not acceleration.
    • So the signal does not represent motion error. Those that need stable images use a servo feedback mechanism to stabilize the imager not just the ground.
    • In DVD's they use a servo pulse between tracks to aligned the optical image and stay on track with motion.
    • you do care about freq. but not measuring it. The foundation resonant frequency affects displacement of imager and structure amplification, such that the lower the lower the frequency the worse the problem> i.e 1/2 of the frequency is 4x worse, 1/4 the f is 16x worse. We call this in log ratios 12 db/octave. , velocity is 6 db/octave and acceleration is what piezo sensors detect.

    WIth no DC offset the negative signal out gets clipped so you get a quasi rectified DC positive output from an AC vibration input.
    So it works but couldn't be used as a servo error correction, even if you wanted.
    But it is good for hearing cars go by and maybe a distant truck and a minor quake.... but hardly an accurate representation of true position error of the scope.
    upload_2015-10-16_19-21-36.png
     
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  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The soil is moving? I thought the pier is vibrating because the telescope is turning with the turning of our planet and the vibration will smear a long time lapse photo. If you detect vibration then delete that photo.

    Your circuit has a serious error as shown in my attachment.
    It has another error that it amplifies the DC input offset voltage of the opamp. An LM358 has a maximum input offset voltage of 7mV which produces an output as high as (7mV x 300=) 2.1V with no input signal.
    The DC gain can be reduced to 1 by adding a capacitor in series with R2.

    EDIT: The input offset voltage might be negative then the output will not show anything until the input signal is very high.
     

    Attached Files:

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  15. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    dont worry about it audioguru, he needs the offset to bias his LED.
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The LEDs are driven by an LM3914 bar graph driver with a full scale input of 1.25V.
     
  17. rsfoto

    rsfoto Member

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    Hi Tony,

    Thanks for the correction and yes my bread board is wired like that.

    The device was never meant to correct the tracking or the position for a telescope. It is just meant to be a comparing device between a known stable column and other not so stable columns.

    I know what you are telling me and that is what we call Auto guiding, but Auto guiding is not capable of guiding out the vibration of a pier because the frequency is too high ad the mounts do not react that fast. I have an example of a friend who build his column with just a 4" steel pipe and he could not do any imaging even having an Auto guider . After telling him to add concrete around his steel pipe changed and here the problem was the vibration of the 4" steel pipe.

    For Auto guiding we use either an OAG (Off Axis Guider) or a piggyback configuration (One telescope aligned with the imaging telescope) and the one piggybacked, is the one carrying the auto guider camera which feeds a software and the software takes care of the corrections of RA and DEC axis speed due to slight misalignment of the mount. Of course a badly aligned mount will introduce in my imaging the so called field rotation.

    Another device used in the Astronomy imaging is the so called Adaptive optics which apart from being able to guide can take out what we call seeing. Seeing is influenced by the atmospheric turbulence which causes the stars to get bigger then they really are.

    https://www.sbig.com/products/adaptive-optics/

    http://www.skyandtelescope.com/astronomy-news/next-gen-adaptive-optics-09092014/


    IMG_20140807_150954_vibrar.jpg IMG_20140820_200135_vibrar.jpg
     
  18. rsfoto

    rsfoto Member

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    Hi audioguru,

    Yes it moves. I mentioned in this case soil but as I wrote somewhere before som astronomers do build their observatories on the roof of their houses. Here in México all houses except the designer houses have a flat roof.

    If you ever had a telescope on the roof of your house and your kids running below it, I bet with you you can see the telescope moving while visually observing. Another source of vibration is when big trucks, like the garbage collecting trucks, etc., pass near your house you can also detect the vibrations of the pier on your house roof, etc.

    Look at message #96 . That is an example of an observatory on the roof of a house. Even worse when he does not control the telescope from a room below or he did not build the pier over a cross made by walls which normally are less prone to vibration.

    Yes the error was in the diagram but the wiring is correct. Tony already corrected it before.

    audioguru mentioned
    and yes that is another source of possible vibration on cheap mounts using stepper motors. Normally when using servo motors the problem is not there.
     
  19. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    I have seen a lot science going into eliminating resonances in many past fields of aerospace and HDD design/manufacturing

    The best foundation is deep connected to bedrock, next is 3m deep where soil is compressed at least 4x density. I saw a new house garage design with every room in the basement with 8" thick concrete walls including the garage which had 2'x3'x8' concrete monolith over the concrete garage basement pad then filled with crushed rock then a ground level pad over this.

    Buildings in Japan are built on low Q <<5) rubberized dampening material to float during an earthquake.

    A large architectural struts will of course have high Q resonance perhaps at a few hundred Hz but then floating on plastic clay dampens the coupling from surface waves but not so much from vertical seismic waves.

    If you wanted to suppress vibration, it starts by accurate instrumentation then defining the amount of attenuation, the define the structure, then consider solutions.

    In my early career I had to design an OCXO to withstand 15g upto 5kHz , 100g drop test and 50g acceleration for the harsh ride in solid fuel rocket that reaches speeds of Mach7 in a couple minutes. So I learned a few tricks about instrumentation, vibration attenuation and structural resonance.
     
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  20. rsfoto

    rsfoto Member

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    Below you can see a few curves of the behavior of the circuit (taken with a cheap Hantek 6022BE oscilloscope) :oops:

    Sensor on the wall

    wallsignal.JPG

    Sensor on the window

    ventana.JPG

    Sensor on the window and my wife knocking slightly onto it

    knocking signal.JPG

    2 sensors compared.

    Yellow line = Amplified sensor signal hanging in the air.

    Green Line = direct signal (non amplified) from the sensor also hanging in the air.

    comparación.JPG
     
  21. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    lots of EMI on source from supply perhaps, lots of DC offset from mismatch R and high gain and lots of resonance around 50Hz and higher from window resonance.
    -generally poor signals, but good enough for your purpose.

    I would use a small piezo accelerometer mounted with bees wax on telescope, semi-rigid coax to pre-=amp , dc coupled with a precision balanced ultra-low input current Charge amplifier.

    But no time to design it for you.

    Good luck
     
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