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Crash Sensor — Parallax Forums

Crash Sensor

CumQuaTCumQuaT Posts: 156
edited 2009-01-24 21:33 in General Discussion
Hi all,

I've been working on a project for the last, ooh, 3 months or so to develop a padded bag (the sort used in martial arts and sports) that will accurately record the force of a blow struck to it. I'm using the Basic Stamp to process the whole thing, and so far, after several revisions of the sensor mounted in the bag, am still unsuccessful.

My first plan was to use a dual axis accelerometer, which didn't work. The impacts were much greater than the sensor could read. Same story with the Tri-axis. Then I put in the Piezo Film sensor, and put a small weight on the top of it (so the impact would make it wobble, and thus show impact) but, lo and behold, nadda.

So I thought I'd open it up on here. What ideas to people have for a sensor that could measure a hit for me? I'm not just looking to sense the hit, I want to know exactly how hard the hit was. The sensor is mounted in the center of the bag, which is in turn held vertically and very still by a person. A target has been placed on the bag to show the exact location of the sensor (for maximum accuracy of the reading) and a cable runs from the back of the bag to the controller box which is stored on the holder's belt.

So does anyone have any good ideas on how I can build this sensor? I imagine it'd be something similar to what is used in car crash test dummies... Can't seem to find those sensors being sold to the general consumer though...

Thanks in advance for your help, guys!

Comments

  • sylvie369sylvie369 Posts: 1,622
    edited 2009-01-15 02:11
    You might benefit from digging around in this thread:

    http://forums.parallax.com/showthread.php?p=771713
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-15 02:11
    Thanks! I'll have a look.
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-15 02:15
    I might also add that I am getting readings from the device, it never resets, but they're always jumbled. A small punch will register about 14,000 but then a large will register 9,000 and then a followup small punch will register as 6,000

    Just for extra info...
  • FranklinFranklin Posts: 4,747
    edited 2009-01-15 02:38
    Could you attach your code to your next post?

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    - Stephen
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-15 02:43
    Sure! Here you go.

    In the 'TakeHit' subroutine, it needs to check for a button press so that it can go back to the score recording screen. (Just so you know)
    bs2
    bs2
    Digital_Trainer.bs2
    4K
  • kwinnkwinn Posts: 8,697
    edited 2009-01-15 17:32
    It seems to me that the biggest problem you have here is transferring the force of the blow from the bag to the sensor. If the sensor is in the center of the bag and a loose weight is hitting the sensor, then what you are really measuring is the acceleration of the bag. That acceleration would be dependent on how hard the bag is hit and how well the person holding the bag still does his job (ie signal out = force of blow - force absorbed by bag holder). What you need to measure is the force in the x and y direction at the top and bottom of the bag and calculate the total force from that. The best idea I can come up with is to mount the bag on a post with sensors at the top and bottom. Perhaps a padded board (2"x8"x2') mounted on a wall or post would be better. Then you would only need one sensor at the top and one at the bottom.
  • SRLMSRLM Posts: 5,045
    edited 2009-01-15 19:09
    Is the bag going to be in one location most of the time? Force is simply mass*acceleration. If you know the mass (constant of the bag) you can measure the acceleration with a distance sensor. By finding the maximum acceleration, you can calculate the force. If the hit is expected to be linear, then you can use just one sensor. If your design has hits comming from multiple sides then you'll need more sensors and just take the maximum. Plus, it protects your delicate electronics.
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-16 00:28
    Hi all. The bag will only ever be hit from one direction, and unfortunately, due to the circumstances of the use of the bag, will always have to be held by someone. The hold is very loose and the bag is pretty much allowed to move, with very little resistance (a firmly held bag can cause injury to someone hitting it) so distance sensors may not work too well for that...

    Using acceleration sensors would be optimal in this instance, but the accelerometers provided by Parallax don't seem to be good enough for it. The G's imparted by a skilled kick upon the bag far exceed 3Gs...

    So to cope with that, I have mounted a small box inside the bag, right where it will be hit. Inside the box is a small PCB with one of the Piezo Film Flex Sensors sticking vertically out of it. At the top of the sensor is a small weight, so when the bag moves backwards suddenly, the piezo sensor is forced to bend forward (like someone's head in a car crash) and I am measuring the bend...
  • WhitWhit Posts: 4,191
    edited 2009-01-16 01:52
    CumQuaT,

    Are you talking about these? http://www.parallax.com/Store/Sensors/PressureFlexRPM/tabid/177/CategoryID/52/List/0/Level/a/ProductID/384/Default.aspx?SortField=ProductName%2cProductName

    Have you thought of something like this?
    http://www.parallax.com/Store/Sensors/PressureFlexRPM/tabid/177/CategoryID/52/List/0/Level/a/ProductID/499/Default.aspx?SortField=ProductName%2cProductName

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    Whit+


    "We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-16 01:54
    I'm talking about this one:

    http://www.parallax.com/Store/Sensors/PressureFlexRPM/tabid/177/CategoryID/52/List/0/Level/a/ProductID/89/Default.aspx?SortField=ProductName,ProductName


    The load sensor only detects up to 10lbs. A good sidekick can generate up to 800lbs of force, unfortunately...
  • WhitWhit Posts: 4,191
    edited 2009-01-16 02:02
    CumQuaT,

    The company that makes the iLoad makes one that can measure up to 200 lbs, but that is still not enough.

    There must be some way to do this though.

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    Whit+


    "We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-16 02:08
    Yeah, I have hope lol theoretically, my little impact sensor with the Vibra Mass Tab should work... I've checked the amount of bend in it during a hit, and it never maxes out... Maybe the basic stamp just isn't fast enough to register it properly?

    Grargh! I'm sure with our combined minds we can think of something.....
  • steve_bsteve_b Posts: 1,563
    edited 2009-01-16 13:29
    There are bigger accelerometers out there....do you know what kind of G's to expect? Although I think this really depends on the bag moving.

    Why not put a Smart Cinderella slipper on the kicker? Put an accelerometer in the glove or slipper....it should be able to detect acceleration/deceleration and you'd be able to determine force from that?!

    How about a couple of strain-type gauges? I would've said put one from the floor to the bottom of the bag, but the person holding the bag would defeat this.
    How rigid is the bag from it's topcenter down through to the bottom center?
    You could put a plate in the center of the bag....when the target zone is hit, another plate is pushed towards this sensing plate and would create a capacitance.
    You'd have to have power running through this and I'm not sure how much power you'd have to put through to be able to measure.
    You could also use a couple of coils....like an isolation transformer....you hit the target and it pushes one coil towards the other and the electromagnetics induce in each other.
    Or a coil and magnet setup....one induces a voltage in the other....

    Cheers

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    <FONT>Steve



    What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
    Lee Travino after the second time being hit by lightning!
  • CumQuaTCumQuaT Posts: 156
    edited 2009-01-17 06:39
    You know, I was close to doing the Cinderella slipper idea before I came on here... Any ideas as to why my current sensor design would be not working?
  • steve_bsteve_b Posts: 1,563
    edited 2009-01-23 13:43
    I'd say it has more to do with the bag material absorbing the impact.
    The sensor has a small "sense" area....you need to couple this sense area to your bag surface.

    People don't want to kick a hard bag, so the bag is cushioned and padded to absorb the hit and protect the foot.
    I'm sure you might be able to leave the accelerometer in there and do some math to see that the "little" sensor movement relates to an impact of "X".
    Of course, your only way of testing this is to swing a known weight from a known height and calculate it's force on impact. Then do a look up table.

    Would the size of the impact object change the table? A big foot hitting at X force would hit more cushion and therefore not translate in to Y force....but a small foot hitting at the same X force would impact less cushion (think force per sq inch) but would be sensed as a Y+Z or a 2Y??

    Unless you move the sensor up to the impact/contact zone, you'll constantly have this issue.

    It's probably easier to put the sensor in to a glove/slipper and you could measure for the peak force....granted, you have to play with wires....

    Keep us posted!

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    <FONT>Steve



    What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
    Lee Travino after the second time being hit by lightning!
  • john_sjohn_s Posts: 369
    edited 2009-01-24 21:33
    How about making your own load cell sensor from conductive fabric

    http://www.instructables.com/id/Knit_touchpad/

    or conductive foam

    http://forums.parallax.com/showthread.php?p=764574


    Post Edited (john_s) : 1/24/2009 11:44:05 PM GMT
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