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Measuring distance — Parallax Forums

Measuring distance

Zap-oZap-o Posts: 452
edited 2011-02-11 20:38 in General Discussion
I want to see if the wheel is already invented for part of this idea I have. I would like to measure distance up to 30 mm but in .18 mm increments. Is there any sensor out there that could measure to this detail?

- Approach 1. using a GP2D120 into a 16 bit ADC
http://www.hobbyengineering.com/specs/gp2d120.pdf

- Approach 2. Make a 850nm LASER diode transmitter and a 850nm receiver circuit.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=365-1145-ND

Comments

  • TtailspinTtailspin Posts: 1,326
    edited 2011-02-09 20:25
    From the data sheet, Approach 1 looks the easiest to implement...:nerd:

    But.. Approach 2 has a LASER... and every project should have a laser..:cool:
  • FranklinFranklin Posts: 4,747
    edited 2011-02-09 20:49
    - Approach 2. Make a 850nm LASER diode transmitter and a 850nm receiver circuit
    Joe Grand has been working on this and with the help of all those here has not gotten it to work yet. The speed is the killer, light is too fast for the processors here.
  • Zap-oZap-o Posts: 452
    edited 2011-02-10 07:18
    I was also thinking about using a time to digital IC.

    Pulse laser on, start a timer and wait for the reflection to be received. Its this simple right?
  • TtailspinTtailspin Posts: 1,326
    edited 2011-02-10 07:31
    Seems like, as soon as the laser is pulsed, there would not be time to start a timer...
    unless Your trying to measure the distance to the sun or planet neptune.

    At least I would like to see the IC chip that does this..:smile:
    not saying there is not an IC that does this, just haven't seen one in this small town I live in.
    (we have more barn's than classroom's)

    The ping ultrasonic works at around 700 to 800 miles per hour, depending on weather..
    The laser, well she work a little faster than that, I know You know this, I was just saying..:cool:
  • kwinnkwinn Posts: 8,697
    edited 2011-02-10 08:49
    Speed of light is 300,000,000 meters/second in vacuum and slightly slower in air.
    To measure distance with a resolution of 0.18mm (0.00018meters) would require a minimum timer resolution of ( 0.00018x2)/300 microseconds or 1.2 picoseconds
    ( 0.0000000000012 seconds ). That would require a clock frequency of 833 GHz for the counter.
  • TtailspinTtailspin Posts: 1,326
    edited 2011-02-10 19:11
    This does sound to easy to be true... 
    start the timer    ' so you allready have the timer going
send the pulse   ' laser beam sent
recieve pulse      ' using laser beam reciever
stop timer          ' ? how long does it take to stop a timer ?
do the math       'check time to recieve laser beam
    I think it would still all have to be done at 833 Ghz at least. :nerd:

    How do those "Tapeless" tape measure's work?
    I haven't bothered mashing one with a sledge hammer to see whats it's gut's looks like.:tongue:
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2011-02-11 10:38
    @Ttailspin I think distance to celestial bodies is a bit excessive, seeing as light travels about 1 feet every 1ns (one clock cycle at 100MHz on the Prop is 10ns). Using a counter on the Propeller, you could get 5ft resolution (light has to travel in both directions). I bet with the right software/hardware setup, you could measure things as close as 20 or 40ft with 5ft resolution.
    Plus, to bounce a laser off Neptune, you would have to wait for over 8 hours to see the return.
    The speed of light is fast, but there is plenty of hardware that can work with light on a human scale (of course 0.18mm is not entirely a human scale).
  • TtailspinTtailspin Posts: 1,326
    edited 2011-02-11 19:55
    @Ttailspin I think distance to celestial bodies is a bit excessive,...
    But, Bobb, I did not place them...so I can not take blame for the overall distance to one or T'other..:innocent:

    And yes, I might did exaggerate's a small part, considering Zap-o mentions 30mm in the original post.
    well, to this, I can only confess my love of the golden hammer theory...
    (if the only tool one has, is a hammer, one tends to treat everything as a nail).:cool:


    Anyways, I have to thank You guys for actually typing out the numbers, and mathmatic formulas,
    think I learned way more then I expected,(I have lots of tape measures.) but seeing the numbers in format.
    Speed of light is 300,000,000 meters/second in vacuum and slightly slower in air.
    To measure distance with a resolution of 0.18mm (0.00018meters) would require a minimum timer resolution of ( 0.00018x2)/300 microseconds or 1.2 picoseconds
    ( 0.0000000000012 seconds ). That would require a clock frequency of 833 GHz for the counter.
    Thats good stuff, thanks Kwinn.

    So is this...thanks Bobb.
    seeing as light travels about 1 feet every 1ns (one clock cycle at 100MHz on the Prop is 10ns). Using a counter on the Propeller, you could get 5ft resolution

    I am not what You would call a math wiz. so things like this help Me alot.:smile:
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-02-11 20:38
    There is a related discussion in the Robot forum. I mention my hope of using a laser with a Wii camera to measure distance (post #9)

    I think most "tapeless" tape measures use sonar. I don't think they are very accurate.
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