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Thread: Proper N scale dynamometer car.

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    Default Proper N scale dynamometer car.

    I'm looking for all our fabricators and electronics tinkerers.

    There are plenty of miniature "strain gauges" available in popular electronic distributors catalogues. These are solid state devices that can measure the strain, or pull, and by extension weight of something. They get incorporated into a product such as this: https://www.amazon.com/Smart-Weigh-P.../dp/B00ISDG4O2

    What I have wanted for a long time is a working n scale dynamometer car. That is not something that looks prototypical, but rather something that actually measures the train forces.

    Conceptually this is simple. A simple mounting arrangement so you can attach a MT truck to each end of one of these strain measuring devices along with a very small "computer" that can read the output of the strain gauge and print the current and perhaps max value on a display. Basically the exact same stuff that's in the suitcase weighing handle above, simply attached to two N scale trucks.

    Installing this "car" right behind the locomotive it would be possible to get precise measurements on tractive effort (this locomotive has 4.75 lbs of pull before slipping), to measure the resistant of different cars (metal v plastic v wheels with wipers), and to measure the pull of entire trains. It would be possible to see the effects of grades and curves, helping folks tune their layouts.

    Problem is, I don't know how to make all the parts work. I'm handy in this area, but not handy enough. I'm happy to collaborate with folks on it though. Is there anyone else interested in working on such a project?
    --
    Leo Bicknell

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    I think that I'd approach this using the force transducer on the coupler drawbar rather than splitting the whole car in two. Use a section of rectangular brass tubing to mount the coupler and support that in a "sleeve" made from acetal. Brand names for acetal include Celcon and Delrin. This is the slippery engineering plastic typically used for truck frames, hand rails, gears, and the like.

    Take the brass tubing and remove the sides and bottom where the coupler mounts, so there is just a flat plate to screw the coupler to. The coupler will be in line with the center of the rectangular tubing, more or less. The acetal sleeve can be made by taking a small sheet of acetal the same thickness as the brass tubing and making two bars on either side of the tubing, with a plate above and below it. This will need to be assembled with screws and nuts. You may need to shim it slightly to make sure the brass tubing slides freely. regular paper should work for that, just make sure it doesn't hit the brass tubing.

    You can calibrate it by attaching your dynamometer car, sitting on the track, to a force instrument and pulling on it. Apply a force (like a small weight hanging off the end of the track) and adjust your dynamometer so they read the same.
    Tim Rumph
    Modeling the Southern Railway in N-Scale

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    Consider dissecting something like this and see if you can get the pieces to fit on a flat car. https://www.amazon.com/Etekcity-Mult...en+scale&psc=1

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    Most of the scales are "push" (e.g. compression) based, so I'm not sure how to put the pieces together in a useful way for this application.

    The hanging luggage scales are one of the few small "pull" applications I've seen off the shelf. The problem with them are most are in lbs up to 80lbs, and while I don't know the exact range I figure N scale pull is going to be more in the perhaps 0-2lbs range.

    Hanging scales are the right form factor...but for the most part they assume the thing you're hanging is heavy, 100lbs+, and thus are a bit large for N scale. For O or maybe even HO, one could be used outright: https://www.amazon.com/JTHKX-Digital...ng+scale+small
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    Leo Bicknell

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    This gentleman made an HO one. I dont know if it can be shrunk down to N scale. It is way over my head. https://youtu.be/Kg0BMU5j-8g

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    Quote Originally Posted by bicknell View Post
    Most of the scales are "push" (e.g. compression) based, so I'm not sure how to put the pieces together in a useful way for this application.

    The hanging luggage scales are one of the few small "pull" applications I've seen off the shelf. The problem with them are most are in lbs up to 80lbs, and while I don't know the exact range I figure N scale pull is going to be more in the perhaps 0-2lbs range.

    Hanging scales are the right form factor...but for the most part they assume the thing you're hanging is heavy, 100lbs+, and thus are a bit large for N scale. For O or maybe even HO, one could be used outright: https://www.amazon.com/JTHKX-Digital...ng+scale+small
    I suspect compression vs tension is just a matter of mechanics - levers or something - that you can solve. The actual teeninsy elements bend.

    It might we worth the ten bucks just to see what is inside. Plus you get on/off with automatic zeroing, LCD readout, etc. The zeroing in particular is gonna be a must have. What might be a big issue is these probably have 4 separate sensors, one in each corner of the platen. Not sure what the circuit does when 3 get zero bend and the other gets all of it; it might do some averaging or something.

    I have this particular scale - handles weight up to a pound and gives measurements down to 0.1 gram. Accurate to that level from what I can tell by weighing balance scale weights.

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    The push and pull can be gotten around. The problem with these is that they work mostly with beam bending, and that beam need to be perpendicular to the force. Forgive me if I misunderstood, but I thought that you'd be using something like this: https://www.digikey.com/products/en/...=1&pageSize=25 and just were looking mechanical stuff. At $10 it's reasonably priced, and .22 pounds (3.5 oz) it seems like the right range. You just need something to convert the output to a number.

    Don't get me started on kgf. It's not a unit.
    Tim Rumph
    Modeling the Southern Railway in N-Scale

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    Quote Originally Posted by SteveSteven View Post
    This gentleman made an HO one. I dont know if it can be shrunk down to N scale. It is way over my head. https://youtu.be/Kg0BMU5j-8g
    The link to the article about it has a ton of good information!

    http://model-railroad-hobbyist.com/node/26134

    To which I say, yes, let's build that, in N scale!
    --
    Leo Bicknell

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    Quote Originally Posted by Tim R View Post
    The push and pull can be gotten around. The problem with these is that they work mostly with beam bending, and that beam need to be perpendicular to the force. Forgive me if I misunderstood, but I thought that you'd be using something like this: https://www.digikey.com/products/en/...=1&pageSize=25 and just were looking mechanical stuff. At $10 it's reasonably priced, and .22 pounds (3.5 oz) it seems like the right range. You just need something to convert the output to a number.

    Don't get me started on kgf. It's not a unit.
    Yes, you're exactly down the path I've already lightly explored. SparkFun has a good article on using them: https://learn.sparkfun.com/tutorials...ith-load-cells

    The problem, if you look at that bar style, is that the force is perpendicular to the long side, making them hard to hide in a N scale car (or at least, pass tunnels etc).

    I think we need one a bit smaller. An "s" shape one would have a better direction of pull, but I don't see them made smaller. There are membrane sensors that look like they would be smaller, but they seem to work in compression only. That might be something that can be worked around by making the pull compress a space, I'm not sure.

    So yes, right path, but on the gauge I think there's still a need for a smaller/easier to package component. Once we have that the other part is much more straight forward (although still complex), some sort of circuit board/computer to read the sensor and convert to a display.
    --
    Leo Bicknell

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    I found this, and I'm thinking something like it is what will be required to fit this into N.

    https://www.robotshop.com/media/file...act_manual.pdf

    Basically I'm thinking an L shaped mounting plate could be made that holds the coupler on the bottom, and slides front to back. When there is a pull on the coupler the vertical part would squeeze the sensor against the front bulkhead. The interface board and some sort of small "computer" to translate into numbers would then be stuffed in the car.
    --
    Leo Bicknell

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    I've been researching more hardware for this application. It's hard to find hardware small enough for N scale! However, I think I have some options. I'm also going to tag @TwinDad as he helped some in the past with this sort of thing.

    A "Teensy LC" (https://www.sparkfun.com/products/13305) is a small single board computer that I think we could use. The board is about 35mm x 18mm, so it wouldn't quite fit inside an N scale car flat (too wide), but isn't any wider than a car for prototyping. They offer a small OLED (https://www.sparkfun.com/products/14048) add on for a display that stacks on top of it.

    My thinking for version 1.0 is to make a flat plastic frame similar to http://www.aztectrains.com/accessories.html but a bit longer. The Teensy would then get stuck on top with double sided tape, and the display on top of it (facing up, for now).

    Then, to detect the force I would use a https://www.robotshop.com/en/capacit...-n-22-lbs.html with a https://www.robotshop.com/en/capacit...2c-output.html I2C interface board wired into the I2C on the Teensy. This would somehow be connected to the front coupler.

    What's missing is a power supply, probably will just use a battery on a following car for prototyping.

    I think if we can make this "science fair" project work with off the shelf parts this is ripe to integrate into N scale. It should be straightforward to design a board with dimensions to go inside an RPO style car that contains the Teensy + Display + I2C interface for the sensor all on one smaller board with no external pins. Then have someone do a shapeways design for the car frame/coupler mount that will take some existing RPO shell.

    Seems like all the parts are there, and should be relatively straightforward to incorporate. Might need some deeper pockets (e.g. a Kickstarter, or a company who wants to bankroll and sell) to make the final, integrated version.
    --
    Leo Bicknell

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