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Wireless Communication

  1. Oct 16, 2009 #1
    Hello, I am very new to this and don't know where to start. I am interested in establishing communication between two devices. The primary goal will be to frequently track the distance between these two objects, and if the distance takes on a certain value, execute some action.

    There is more to it, but that is the basics of it. Lets say a max distance of 20-30m. The devices are portable. Communication will be in open space (essentially no obstructions). Higher the frequency, the smaller the antenna?

    Any help is appreciated.
     
  2. jcsd
  3. Oct 16, 2009 #2

    chroot

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    What's your budget? More specifically, describe your needs in each of the following categories:

    1) How easy must it be to build and test?
    2) How much can it cost?
    3) How big or heavy may it be?

    - Warren
     
  4. Oct 16, 2009 #3

    berkeman

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    As chroot implies, many more details would be of help. You could just run a spring-loaded string between the objects and monitor the play in and out, eh? Why not use a string? Why does it have to be wireless?

    How about laser rangefinding? Can you track it from some base station via robotic video tracking, and hit a cube corner on the target with a laser range finder? If your accuracy requirements are not too tight, just put GPS on the base station and the rover.

    More details please...
     
  5. Oct 16, 2009 #4
    Let us know more! On the basis of the problem you've described I could recommend a piece of string tied to a switch! What's it for? (string plus switch won't work reliably for a roaming dog)
     
  6. Oct 16, 2009 #5
    1) How easy must it be to build and test?
    Must be able to be built and tested by a college student with access to labs, etc.

    2) How much can it cost?
    Cost is not an issue.

    3) How big or heavy may it be?
    Must fit in someones pocket or on their belt.

    One unit ("control unit") will remain somewhat stationary, and will define the range the second unit ("roaming unit") can travel (some circle of some radius around unit 1). It has to be wireless because the second unit may travel a considerable distance out from the first unit (30 meters).

    When the roaming unit exits this radius, it will alert the control unit and an alarm located on the roaming unit. Upon reentry to the zone both alarm and alert will stop. The control unit will check the distance of the roaming unit say once or twice a second. Is this something I need a microcontroller for?
     
    Last edited: Oct 16, 2009
  7. Oct 16, 2009 #6
    There are dog-control devices on the market, at least there were 20 years ago, which alerted the owner when the dog went out the gate. From vague and distant memory I think that these worked on a signal/no signal basis... when the dog got too far away there was no longer a received signal and the alarm went off. Perhaps you could use one of those?
     
  8. Oct 16, 2009 #7
    If the control unit had a small low power transmitter and the remote unit had a receiver that could measure its signal strength, you could compare the signal strength level in a comparator in which you could set the threshold at which the alert is triggered. If your transmitter frequency were below 490 kHz, you're allowed a decent signal strength and you'd be operating in the near field of the transmitter which means the signal strength drops faster than the inverse square law and gives you a more accurate distance measurement. If you operate between 160 and 190 kHz you're allowed up to 1 watt of power which is far more than you need.

    Furthermore operating at these frequencies it is easier to build transmitters and receivers. The antennas would be ferrite rods and you can even use higher speed opamps in the receivers. I would have the control unit transmit continuously and the remote unit would report back to the control unit once or twice a second whether it is inside or outside the radius, transmitting back to the control unit on a higher frequency. You don't need a microcontroller, a timer IC would be sufficient.
     
  9. Oct 16, 2009 #8

    f95toli

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    But that signal strength will depend on many things, not only the distance. I don't think it is possible to come up with a simple scheme for measuring distances that only use the intensity of a single radio signal and one receiver. The only way to do it would be to triangulate, although which might actually be feasible if the distance is only 30m, the two receivers could placed fairly close to each other.
     
  10. Oct 16, 2009 #9

    DaveC426913

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    I was going to say the same thing as f95toli. Signal strength will be a very unreliable measure of distance. You would get a high ratio of false alarms.
     
  11. Oct 16, 2009 #10

    vk6kro

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    If you had the control transmitting on frequency A and the roving module receiving on this frequency, the module could then receive a pulse and retransmit it on frequency B which the control is receiving.

    At 30 M distance, the time for a round trip (60 M ) would be 0.2 microseconds.

    So, the control sends a pulse and starts an onboard counter counting 100 MHz clock pulses.
    When the pulse gets back from its round trip, it stops the counter.
    These pulses are 10 nS or 0.01 uS apart.
    If the count gets to 20 or above, the roving module is too far away and this rings alarm bells.

    Counters that could count at 100 MHz are readily available.
     
  12. Oct 16, 2009 #11

    berkeman

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    Seems like the repeater delay would be too variable to make this accurate. There might be a way to make it more constant, but especially with varying signal strengths, the AGC delay alone would be pretty variable.

    Triangulation or GPS seem like the best bets...
     
  13. Oct 16, 2009 #12
    I worked many years designing and testing on location units that worked on this principal for use on overhead cranes in steel mills, auto plants, etc. While what you say about the signal strength may be true at higher frequencies, at these frequencies the wavelength is much longer than the distance being measured. Multipath or other effects we're accustomed to worrying about that affect signal strength have virtually no effect at these frequencies. The near field effect also enhances the accuracy because the signal strength drops off more rapidly with distance than it does at higher frequencies. Once calibrated, the method produces remarkably repeatable results. [Note: The buildings in which these units were installed were made of steel and filled with machinery made of steel, yet we still achieved very reliable operation.]

    The most important factor in maintaining accuracy is keeping both both transmit and receive antennas vertical.
     
  14. Oct 16, 2009 #13

    vk6kro

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    Seems like the repeater delay would be too variable to make this accurate. There might be a way to make it more constant, but especially with varying signal strengths, the AGC delay alone would be pretty variable.

    Triangulation or GPS seem like the best bets...



    The receiver could be just an RF amplifier followed by a mixer to the other frequency. No need for AGC and no appreciable delay across a small circuit board compared with the delay of an RF signal travelling 30 metres.
    This is an open area and money is no object, so signal levels could be very large. You would have a receiver that does not receive band noise.
    Accuracy would be about 1.5 metres with a 100 MHz clock. (30 M / 20 counts =1.5 M per count)

    GPS would be a nightmare because you would have to keep calculating which coordinates were within the designated circle and GPS is pretty inaccurate for precision work like this.
     
    Last edited: Oct 16, 2009
  15. Oct 17, 2009 #14

    berkeman

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    Agreed about the GPS accuracy. Interesting idea about the straight-thru shot. So basically it would just be a squelch-qualified repeat, with the input signal strength enabling the output amplifier. Might work.

    As for the power and frequency, are there any open bands in the range of where skeptic2 was talking about, or are they all licensed?

    The open ISM/microwave oven band definitely has multipath and body absorption issues for amplitude...
     
    Last edited: Oct 17, 2009
  16. Oct 17, 2009 #15
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    Last edited by a moderator: May 4, 2017
  17. Oct 17, 2009 #16

    chroot

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    Forgive me for this silly question, but why do this yourself when nearly any modern cell phone can do it already?

    - Warren
     
  18. Oct 17, 2009 #17

    vk6kro

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    How does a cell phone measure distance?
     
  19. Oct 17, 2009 #18

    f95toli

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    I guess he is refering to A-GPS.
     
  20. Oct 19, 2009 #19

    chroot

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    It's very easy to write a program which will execute some action when two internet-enabled smart phones are within some distance of each other. There are even services which allow friends to share their location data with each other.

    - Warren
     
  21. Oct 19, 2009 #20
    No-one has suggested sound. Would that be accurate enough?
     
    Last edited: Oct 19, 2009
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