so my team doesn’t have easy access to a scale and we were wondering if any other teams had this problem and had any clever physics solutions. I was thinking of measuring the vertical displacement of a couple of 2x4s resting on some books off the ground but i’m not sure that would work. any thoughts?
That would only work if you know the spring constant of the books to be able to translate displacement into force. And I doubt it would be very accurate.
You can get a cheap bathroom scale for like $10 that will be accurate within a pound. Much easier and more accurate
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Some teams weigh their robots at the post office.
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Note that those cheap scales generally don’t have remote readouts. If going that route, get a couple of them - you can stick some 2x4’s between them, get the weight of the 2x4’s, then balance the robot on those and read the weight on both scales. Subtract out the 2x4’s, and you’ve got your weight! Even better, this can help you identify your center of gravity!
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Luggage scale is a common route. They often top out at 110lbs, but its easy enough to pop off an intake or something, or weigh before its installed.
There are also fish scales in a similar form factor that’ll handle well beyond the weight limit.
We usually had somebody just pick up the robot from the scale handle, but if you’ve already built the climbing field element, hanging from that might be smart.
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At some point, we were using a scale hung from the guardrails on the second floor and climbing (I think this may have been 2017?)
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We just took our robot to a pet store with a big scale.
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This. A veterinarian or animal hospital type place usually also has a large scale. Ask nicely and they’ll probably let you bring your robot in and weigh it.
Get two cheap bathroom scales. Have two people weigh themselves and call their combined weight X. Have those same two people weigh themselves while holding the robot between them. Call their new combined weight Y. Your robot should weigh Y-X, within whatever the accuracy of your scales is (probably about 5 pounds)
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Solid strategy.
And to the original poster, the unspoken part here is that you should aim to be at least that much underweight (like the 5# mentioned) as a margin of error to avoid painful weight reduction efforts at your competition.
The only scales that matter are at the event so leave yourself a safety margin.
As spoken by me more than once:
“Robot weight - it’s a limit, not a goal”
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We bought this hanging scale this year and hang it off our mockup of the hanger field element, works pretty well https://www.amazon.com/dp/B07P7Y6XF1/ref=cm_sw_r_apanp_3Q18uLCLIw6ri
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I purchased this scale with remote display for under $30 a few months ago for weighing checked baggage. It is smaller than your robot so one would have to put a sheet of plywood on top of the scale so the robot can fit on it. The previous, hand-held baggage scale was way off.
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I purchased four inexpensive fish scales when we did mecanum in 2014 (bad choice, especially that year! Live and learn.). We hung the scales from hooks made from the wire racks used to fry turkeys, and hung the robot by its axles. This not only gave a total weight, but helped us trim the CoG towards the center of the robot.
Highly recommend you try to get a scale. There really isn’t a way get a measurement.
For $37 and available Wednesday, I’d highly recommend talking to a teacher/mentor/parent to consider making a donation.
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Gandagorn, the deflection will vary with moisture content of the 2x4. Interesting idea though.
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The price for an 8 foot long 2x4 is almost $8 now in my location. For perspective, that is about 1/3 the price of the scale I linked an between 1/5 and 1/4 the one @BrendanB linked.
Hanging scales have gotten so cheap that the only excuse not to use one is if you don’t have a place to hang it or you already have a really nice platform scale. With a hanging scale there’s no variability with the measurement as the position on a platform changes. Here’s one example.
Some outside the box ideas (some of which are clever physics solutions as requested):
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Balance a previous year’s robot (with a known weight) or number of batteries with known individual weights on a balance beam with a fulcrum
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Borrow a bathroom scale from your school’s weightlifting room/gym/locker room etc
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Built a boat and place it in a pool. Put the robot in the boat, and measure the new waterline outside the boat to see the additional water displacement, so you can calculate the weight from net displacement
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Tie the robot to a string on the ceiling near the wall, and get a spring, place it against the wall, and determine the total deflection of the spring vs the deflection angle of the string
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Accurately determine the center of gravity of the robot. Hang it from the ceiling. Swing it as a pendulum. Measure the period. Do a little math. Bob is your uncle.
Support the robot with pneumatic cylinders. Plumb the extend ports together with a pressure gauge. Using the pressure and the area of the pistons and do a little math. Bob is your aunt.