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H48C Data logger on the propeller - how fast can the H48C be sampled? — Parallax Forums

H48C Data logger on the propeller - how fast can the H48C be sampled?

OakGraphicsOakGraphics Posts: 202
edited 2007-09-25 15:23 in Propeller 1
Hello - I am looking at making an accelerometer data logger with the prop - something that could show when I exceeted say 2 g on any direction, and flag an alarm, or show me graphically, or send out the real-time values out via usb· / rs282 so I can monitor it on a pc.· But my question is how fast could the h48C be sampled accurately from the propeller?· I have read posts of the bs2 doing maybe 4 samples a second - which is way too slow for the application I was looking for.· I would hope for· 100 x that, and would be happy if it could do 1000x faster then that.· Any hope of these type of samples per second?·· If not with this device (H48C) - any suggestions on what I could use?

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2007-09-24 06:26
    OakGraphics,

    According to the datasheet,· the H48C is limited to 200 samples per second. Reference: http://www.parallax.com/dl/docs/prod/acc/HitachiH48C3AxisAccelerometer.pdf

    There is no reason preventing the Propeller from being able to meet that requirement.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • OakGraphicsOakGraphics Posts: 202
    edited 2007-09-25 14:36
    Beau Schwabe (Parallax) said...
    OakGraphics,

    According to the datasheet,· the H48C is limited to 200 samples per second. Reference: http://www.parallax.com/dl/docs/prod/acc/HitachiH48C3AxisAccelerometer.pdf

    There is no reason preventing the Propeller from being able to meet that requirement.

    I am wondering if this is too slow for instant g-force (shock) detection.·· How many samples per second should·I be looking for to record shock-type of events?

    I was doing some searching, and noticed sparkfun had a 5 dof sensor that might do the trick for my application:
    http://www.sparkfun.com/commerce/product_info.php?products_id=741

    It also does the 3g of detection, but could be sampled faster (500/second), as well as giving 2 rotation axis.·
    I am also wondering if I need to detect higher levels of g-force as well.·· I guess the question is what can a human withstand in shock in g-force?·· I would like to make a sensor that detects when possible harmful levels of g-force shock is done.

    ·
  • David BDavid B Posts: 592
    edited 2007-09-25 15:19
    Here's a site that documents the origin of Murphy's law, which arose during tests of what G-forces a body could withstand. This may not be exactly what you're asking, in that I'd assume shock might be defined as a rapid change of acceleration, where I think this rocket sled simply measured the level of acceleration, not its derivative.

    They had a guy withstand 35 Gs when strapped in securely to this rocket sled! So I guess you need to redesign your sensor!

    http://www.improbable.com/airchives/paperair/volume9/v9i5/murphy/murphy1.html
  • mcstarmcstar Posts: 144
    edited 2007-09-25 15:23
    As far as humans go, instantaneous g-force is not nearly as harmful as long term sustained g-force. As it turns out we are pretty robust.· Pilots can pull forces approaching 9gs (of course they have special suits to help them out there).· At about 7-8g's you'll start to pass-out due to the blood flow not getting to the head under normal conditions, but we are talking tens of seconds , not 1000ths of a second.· I have to wonder how the 35G guy was even able to breath·with his chest weighing in at 35x's it's normal weight!··If his chest and ribs normally weighed just 10lbs, that means he was breathing with a 350lb weight on his chest!· Maybe they used forced air or something.

    Anyway,·when a very quick impulse hits your body (less than a second or so), it pretty much passes through the body unaffected.· This is because according to F=MA or Force = Mass x Acceleration, the time needed for a Force to cause a change in speed is inversely proportional to and multipled by the Mass that force is affecting.· This is the property called inertia or an object's tendency to stay at the speed an directly of travel it is currently at.·· Or stated another way, the impulse of a very quick change in acceleration is more of a shock wave then an accelerating force for objects with appreciable mass. Bascially, if the force lasts for 1/1000 of a second it has very little appreciable effect on most things.· It's more of a sub-sonic vibration·with such a small impulse. ·Only things with very very tiny mass are affected by such accelerations as far as can be measured by most devices.

    For what it's worth, at the sensor's maximum acceleration of 3g's, you are· going to have a hard time even standing up.· I don't think you'll need the sensor to tell you something's wrong at that point.·

    Post Edited (mcstar) : 9/25/2007 3:32:47 PM GMT
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