souped up ultrasonic range finder

Discussion in 'The Projects Forum' started by NewStudent, May 8, 2008.

  1. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    I'm working on a souped up ultrasonic range finder. I successfully amplified the signal driving the 40kHz piezo transmitter. The transmitter and receiver are located 0.75 inch apart. The problem is crosstalk directly between the transmitter and receiver, see scope trace 1 (no target). The crosstalk signal hides the target detection, see scope trace 2 ( target at 1 inch) and scope trace 3 ( target at 2 inches). I would like to use a PIC 16F877. Does anyone have a software or hardware suggestion of how to remove the unwanted crosstalk signal?
     
  2. Caveman

    Active Member

    Apr 15, 2008
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    Are you talking about ultrasonic crosstalk, or electrical? If it is ultrasonic (which it probably is) you need to direct the ultrasonic waves more.
     
  3. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    Thanks Caveman

    The crosstalk is ultrasonic. Do you have a suggestion of how to "direct the ultrasonic waves more"?
    I tried:
    felt pads under the transducers
    cotton balls between the transducers
    paper dividers and cones between the transducers
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Can you post you schematic so that we can see what the circuitry looks like?

    hgmjr
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    How about some lead foil as a dampener? John
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Given the distance, could it be that the receiver is simply continuing to vibrate after all that sound energy falls on it? The damping over time suggests something like that.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Would a resonant tube (cardboard tube cut to length, or multiple thereof) make the the signal more directional? I remember something similar using a bundle of straws glued together for a directional mike in an old Radio Shack article.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Try reducing the amount of amplification until you no longer receive the crosstalk.

    Either that, or re-orient your receiver further away from the transmitter.
     
  9. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    Thanks hgmjr
    Here is the schematic you asked for.
     
  10. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    Sounds like a good idea. I will give it a try.
     
  11. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    I think you are correct. The transmitter is driven at 40kHz for 8 cycles.

    In trace1 you can see the crosstalk at the receiver with no target.

    In trace3 you can see the crosstalk in the first 400us. You can also see the crosstalk + the signal reflected from the target in the next 700ns. The receiver continues to ring for another 500us.

    I trying to develop code and/or a circuit to ignore the crosstalk and detect the signal reflected from the target.
     
  12. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    Yes, I think a tube might make the signal more directional but unfortunately I need the full beam width.
     
  13. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    Yes, reducing the strength of the driving signal and/or reducing the amplification does reduce the crosstalk. Unfortunately, it also reduces the ability of the sensors to detect both small targets and targets at a distance.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    There may be another issue going on here. It looks like there is a "beat frequency" going on. You're not allowing the piezo to vibrate at it's own natural frequency; you're artificially forcing it to resonate at a frequency that's close. So, you're getting both the frequency you're stimulating it with, and the natural frequency of the piezo transducer.

    But it also could be that the beat frequency is the difference between the transmitted signal frequency and the natural resonant frequency of the receiver transducer.

    Rather than having your uC generate a frequency to blast your piezo transducer with, why not set up a driver using an inverter, with a couple of variable caps on either side of the transducer in the 10-50pF range to "tweak" it in?
     
  15. Caveman

    Active Member

    Apr 15, 2008
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    There are a couple of things here.

    As far as acoustic crosstalk, any sort of rigid structure will likely direct the beam. I used to work on ultrasonic motion sensors. That's what the grills on them do. And they will place plastic between the transducers to keep the direct path down.

    The second issue is with your drive circuit. You should have a capacitor in series with the transmitting transducer. This is to remove any DC component which could damage the device. Otherwise it looks good.
     
  16. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    Wow, I hadn't noticed the "beat frequency" but I see it now that you point it out. The transducers are a matched pair rated at 40kHz and I think I'm driving them at that frequency.

    As to your suggested solution, I'm afraid I have no idea what you are talking about. Can you be more explicit or better yet provide a schematic.
     
  17. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    I had no acoustic crosstalk when driving the transmitter at ±5V but when I raised the voltage to ±10V the problem appeared. I have tried a wide variety of materials placed between the transducers and varied the space between the transducers from 0.75” to 2.25” all with little success.

    Can you suggest size and type for the capacitor?
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    Just as a suggestion, what if you place a short across the receiver temporarily. If you continue to see a signal on the oscope, then you have to conclude that at least some of the output is electrical noise rather than acoustical in origin.

    hgmjr
     
  19. Caveman

    Active Member

    Apr 15, 2008
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    Something with a small impedance at 40kHz, like 0.1uF to 1uF range. Ceramic works well.
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    The lousy old LM358 is low power so it is slow. It can barely have a gain of 10 at 40kHz then its output is triangle-waves instead of square-waves.
     
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