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Remote Distance Sensing

Discussion in 'General Electronics Discussion' started by sirch, Dec 6, 2012.

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  1. sirch

    sirch

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    Dec 6, 2012
    I'm considering options for knowing how far I am from a starting point (to within say 10cm over max 30m) even if I haven't got line-of-sight on the starting point. One idea I have for doing this is to put an Ultrasonic transmitter at the start point and modulate a timestamp on to its signal, then receive the signal, read the time from the signal and compare it with the actual receipt time. Assuming the clocks on the transmitter and receiver are synchronised, the time difference would then be proportional to the distance.

    What I would like input on is how do I modulate the timestamp on to the signal?

    Or is there a better way of doing this altogether?
     
  2. GreenGiant

    GreenGiant

    842
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    Feb 9, 2012
    rather than a time stamp it may be easier to set up a call/respond system, you hold a transmitter/receiver that starts a timer and sends out a signal for the set point to send out a burst and as soon as the transmitter/receiver gets the burst from the starting point it stops the timer and does a simple calculation to figure out distance.

    Now 10cm at a max of 30m is going to be tough, to get that accurate you would probably need to have monitors for air temperature, humidity, and pressure as sound travels at different speeds, even ultrasonic sounds, using sound you would definitely be able to get within a foot or two, but I wouldn't rely on it to get to the 10cm range.

    for example I took a class where we were using a micro to make a small ultrasonic sensor that would light an LED once something was less than a foot away (had a speaker and mic to read the echo) depending on the room temperature it would read from 10 inches before it tripped all the way up to about 15 inches, so over a foot you had +/- 25%
     
  3. sirch

    sirch

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    Dec 6, 2012
    Thanks GreenGiant, I had thought about a similar approach but how do I separate the response from the echoes? I guess I could use two frequencies for call and response but would it still be susceptible to harmonics? (and 40kHz transducers are commonly available and cheap).

    On the accuracy question, I take the point about temperature and pressure but I'm not so bothered about absolute accuracy, I am more interested in the delta between points and for a short period of time it would be possible to use a correction factor by initially calibrating over a known distance
     
  4. GreenGiant

    GreenGiant

    842
    6
    Feb 9, 2012
    My thought was use an RF trigger, that way you wont have to worry about halving the time then calculating, that should eliminate the problem of call/response echos

    fair enough, over short times so long as its calibrated you can definitely be fairly accurate.
     
  5. duke37

    duke37

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    Jan 9, 2011
    If you havn't got line of sight, then any response will have to have gone round an extra, unknown distance.

    With a 40kHz signal, you may need to detect the first or second cycle depending on the accuracy needed.
     
  6. sirch

    sirch

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    Dec 6, 2012
    GreenGiant - RF - (doh!) good idea. I'd got too locked in to the ultrasonic thing, I think I'll give that a go.

    duke37 - imagine you are walking down a corridor and round a corner taking readings say every second, then by summing the deltas of each reading you get a path length.
     
  7. duke37

    duke37

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    Jan 9, 2011
    If you measure the increments and sum them, you will get the right answer unless once you turn a corner you get a reflection from a wall when the increment could jump.

    To do this you will need to move the sender to a new location for each increment.
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Is this indoors? If not, any wind will screw things up big time.

    I think it's appropriate, at this point in the thread, to ask the OP WHAT DO YOU WANT TO DO? You've said you want to "know how far you are from a starting point". This doesn't tell us very much. Would you care to explain your project with a decent level of detail, or do we have to fill in a request form under the Official Secrets Act?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    2,506
    Nov 17, 2011
    Are we thinking tooooo electronic?

    You could wind 30m of cord onto a spool. Fix one end to the starting point. While unrolling the cord from the spool, have the cord turn a small wheel of known size and count the revolutions. Multiply the revolutions by the circumference of the small wheel . Voilá, there's your distance. Around corners, with heavy wind, in rain or shine.
    If you count +1 for every revolution of the small wheel, resolution is limited to 1*circumference.
    If you count +1 for every 1/n revolutions of the small wheel, your resolution is circumference/n.

    :D

    Harald
     
  10. sirch

    sirch

    109
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    Dec 6, 2012
    Ok, some fair points. I can see several applications for such a technology (e.g. search robots in collapsed buildings) but my particular usage is in cave surveying, or more particularly getting a profile of a cave passage. Use of laser distance measures and electronic compasses is fairly well established for centre-line surveys but a lot of time is spent measuring and sketching the cave profile. What I have in mind is to scan a laser around the cave passage and video the result, then process the video in software to get the profile. The issue is in determining where the profile scanner is from a known point.

    When I say "cave" the passage can be anything from a body sized tube to something the size of a cathedral (although initially I'm thinking of something like 600mm to 5m range).

    Harald Kapp - Measuring string is not really an option (it's been tried in the past) mainly due to mud on the string, getting tangled, etc.

    Duke37 - I don't think echoes are a problem because the base station sends out a signal and the first signal to arrive will have taken the shortest path to the receiver, all other reflected signals will arrive later
     
  11. sirch

    sirch

    109
    1
    Dec 6, 2012
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