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Thread: Proto Weighing Station (scale)

  1. #21
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    I don't think you would need a turntable. Just a car-length of track. Powered, aligned, yet not connected to the rest of the track. Maybe just a box tube with the track on a plunger sitting on the sensor. Much like the mechanical scales.

    You could program the system to reject the weight of locomotives and double cars, so, you wouldn't get any false alerts.
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    Quote Originally Posted by ChicagoNW View Post
    I don't think you would need a turntable. Just a car-length of track. Powered, aligned, yet not connected to the rest of the track. Maybe just a box tube with the track on a plunger sitting on the sensor. Much like the mechanical scales.

    You could program the system to reject the weight of locomotives and double cars, so, you wouldn't get any false alerts.
    Sorry, wasn't clear. I didn't mean you'd need a turntable, just that the track alignment challenge was similar (though not the same, I expect the turntable is much harder).

    FTR, I've already ordered a 500g load cell
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    Quote Originally Posted by TwinDad View Post
    FTR, I've already ordered a 500g load cell
    Awesome -- this is a really cool project. I can definitely see trying to create a working scale on my layout. Please post updates!!!

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    I'll start a thread in one of the modeling forums...
    Never mistake a guy who talks a lot for a guy who has something to say...

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    The NMRA weight recommended practice was determined by an experimental study.
    Can very well be, but doing the math, a 120 ton car shrunk to N:
    /160
    /160
    /160
    X 2000 lbs per ton
    X 16 oz per lb
    = .9375 oz
    NMRA standard for a 3-inch car = .5 + .15 + .15 + .15 = .95 oz.
    Same ballpark.

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    Quote Originally Posted by NtheBasement View Post
    Can very well be, but doing the math, a 120 ton car shrunk to N:
    /160
    /160
    /160
    X 2000 lbs per ton
    X 16 oz per lb
    = .9375 oz
    NMRA standard for a 3-inch car = .5 + .15 + .15 + .15 = .95 oz.
    Same ballpark.
    Is that car loaded or empty?

    A car riding on 70 ton trucks has a max loaded weight of about 150,000lbs. ( that is weight of the car, plus the load).

    The heaviest loaded cars on 4 axles today are allowed to weigh 286,000lbs.

    If these cars are the same length, the models should not be the same weight, but the NMRA standards say they should be.

    It's not hard to get models that fall into this category. A 50 foot boxcar from the 1970s and a modern coal gondola are about the same length after all.

    Paul

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    I realized after seeing this that I used the wrong numbers for the NMRA weights... even an 85 foot car should be well under 100g, so I also ordered a 100g load cell...

    I've actually got the code already written (obviously not tested), including a decent user interface with a 16x2 LCD.

    I'll start a thread soon... I want to get a rough idea what the structure should look like.
    Never mistake a guy who talks a lot for a guy who has something to say...

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    Is that car loaded or empty?
    These are bathtub gons. LT WT is 44100. LD LMT is 218900.

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    Quote Originally Posted by NtheBasement View Post
    These are bathtub gons. LT WT is 44100. LD LMT is 218900.
    So it weighs 22 tons empty and 131.5 tons loaded.

    The empty weight is much less than your previous calculation. The loaded weight is heavier, but not significantly.

    Of course my point is still valid. The NMRA weight only takes into account car length, and no other factors, so it is not a scale weight.

    Paul

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    No need, nobody challenging your point. Coincidence noted in my first post is that NMRA is not far off from real weight of coal.

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    Quote Originally Posted by TwinDad View Post
    So... one of these... http://www.robotshop.com/en/micro-load-cell.html

    ... and one of these ... http://www.robotshop.com/en/strain-g...hield-2ch.html

    ... and one of these ... http://www.robotshop.com/en/arduino-...ontroller.html

    ... and one of these ... http://www.robotshop.com/en/16x2-lcd...t-arduino.html

    Plus a piece of track solidly mounted on a platform in a frame that would allow the platform to flex down ever-so-slightly while also maintaining alignment with the approach tracks (think "turntable bridge").

    You'd have to measure somewhere between 1-5oz for rolling stock, much more for locomotives. There are larger load cells, but 500mg is over a pound, so I imagine it will work fine for N scale.

    And of course you could get clever and optimize down all the electronics quite a bit. The above would be just for a prototype. You could use an Arduino Pro Mini, a LCD-only module, and your load cell amplifier could be a module, not a shield.

    OR... there's this: http://www.robotshop.com/en/sparkfun-openscale.html
    Thanks, TD, for these links. I hadn't heard of RobotShop before. Now I have another place to spend money!

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    Quote Originally Posted by mhampton View Post
    Thanks, TD, for these links. I hadn't heard of RobotShop before. Now I have another place to spend money!
    FTR it's the first I've heard of them, and I've never used them myself. They just popped up when I started shopping for load cells. I actually ended up ordering mine off that online auction site direct from China. We'll see whether that was a good idea or not. Robot Shop does look like an interesting place, though...
    Never mistake a guy who talks a lot for a guy who has something to say...

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    I've ordered from Robot Shop before for work. They were fine, but we had to deal with purchase orders and all of that, which was a pain internally.

    I tend to buy from Sparkfun and Adafruit for personal projects.

    Paul

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    Looks like the amplifier is the expensive part -- @TwinDad did you find a cheaper source for those as well?

    EDIT: Looks to be several in the $2 to $5 range on ebay.

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    Quote Originally Posted by McNamee View Post
    Looks like the amplifier is the expensive part -- @TwinDad did you find a cheaper source for those as well?

    EDIT: Looks to be several in the $2 to $5 range on ebay.
    Yes, I found a pair of them for under $5, the same HX711 chip with a slightly different breakout. Assuming they are made well enough, should be a suitable substitute. I won't really be able to test it until the load cell(s) arrive, which won't be until later in February ("expected by Feb 23"). All told I have a little under $20 invested (I already have an Arduino to use) for essentially two sets of hardware.
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
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    Well, lookie here what showed up in my mail this evening!


    Load Cell Amplifier by Mark, on Flickr

    The green board on the proto board is the load cell amplifier... the cheap Chinese knock-off of the Sparkfun board. Since I don't have a load cell yet, I can't *really* test it, but it does look like I'm able to communicate with the chip and get some random values back... which is promising.

    And it's not getting hot... which is also promising. Cool chips are a GOOD thing in electronics.

    Moving forward, though, to avoid confusion, let's keep the model discussion in the other thread, and keep this one focused on the prototype...
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
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    Hey @TwinDad, did you ever get a load cell working? I've still got this link in my Favorites: https://learn.sparkfun.com/tutorials...load-cells/all and it would be cool to weigh my coal loads.

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    Nah. I got distracted. Haven’t done much of anything train related lately. My daughter is graduating HS and we have been college searching. I should probably pick this back up. I have the load cells and the electronics but never got around to building the bridge deck around it.
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
    Appalachian Railroad Technology: http://www.apprailtech.com


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