It is my understanding that Robot Inspectors are under the directive to measure frame perimeters with a flexible tape measure. This is how I’ve seen it done in the past, and the LRI’s I’ve talked to implied that the same would be true this season.
We bought ourselves a few flexible tape measures so we could ensure our frame perimeter would pass inspection, since we’ve had issues with being right on the edge in previous years. We never really learn that we shouldn’t design our frame perimeter to be exactly 120"…
Well this year we finally learned and decided to design our robot with 119.5" perimeter. We got our frame put together yesterday and measured it with our flexible tape measures to ensure we were under the limit… and to our shock we measured at 120.5". A bunch of finger pointing went on, some blamed our modeling software, others claimed that the drive rails were fabricated incorrectly, and ultimately people were pretty much freaking out.
We knew we designed this thing to be under, so it made no sense that we were oversized. We decided to use another flexible tape measure with the same results… then we decided to take a regular tape measure to measure the perimeter… and we found it to be 119.5"
So we laid all of our tape measures out as shown in the pictures below and found some pretty significant differences between the measurements. In these pictures the flexible tape measures are stretched as much as they would stretch, and still weren’t close to the measurement of the standard tape.
Obviously flexible tape measures have been used for years to measure frame perimeters in FRC. This issue would’ve been brought up many times over if it were more prevalent.
We decided to measure our 2019 robot’s frame perimeter since we had some issues passing this part of inspection at each of our events last year… we even had to bevel the corners of our robot to pass inspection at Champs. When we measured with our standard tape we found out we were 1/4" under the limit, and shouldn’t have had any issues passing inspection. I suspect somebody didn’t pull their flexible tape measure tight enough when measuring our robot at champs last year.
TL;DR: Flexible tape measures can stretch, and this introduces a variable that can make or break the legality of a team’s robot. Inspectors need to ensure they pull their flexible tape measures tight when measuring frame perimeter. Teams should take this into consideration with their designs and not design up to the maximum size, for many reasons.
Those flexible tapes have the opposite problem of stretch (which really would not be a problem for oversized teams anyway); those tapes have nonstandard inches. That much variation can maybe be blamed on qc at the factory but it’s a good reason to be extra careful.
time for a Q&A about RI tape measure tolerances and specifications, eh? Hopefully AM will get official FRC tape measures in stock a week or two before Week 1 competitions.
The difference between the design goal and the limit is less than 0.5%. What is the accuracy of the manufacturing processes your team is using? What is the tolerance of the measuring instrument? In my metrology course, I was taught that one must take into account the accuracy of the measuring instruments as well as the desired accuracy of the product from the specification when doing the design work.
As @anon91718515 stated, it looks like either the flexible tape measures have shrunken relative to the steel one or the steel one stretched significantly (improbable).
At all the events I have done inspection at (various Houston area, Houston Champs, Ont Prov Champs, CPR) all the RI’s used steel tape measures and added up the measurements of the length and width to calculate the frame perimeter. I don’t recall seeing any flexible tape measures.
I’d argue the opposite. At all the events I’ve seen (various Texas locations, Houston champs) they’ve used flexible tape measures and wrapped it around. I’ve never seen steel tape measures used.
It appears that there is significant inconsistency in how the frame perimeter is measured at events, even the same event. Often, I have had to use my own (Stanley) steel tape measure because the inspection station did not have enough (steel) tape measures for all the RI’s.
That was essentially my conclusion as well. If you’ve only seen steel and I’ve only seen flexible, there’s definitely some oddities there.
Any event I’ve RId at the measuring devices were provided by the LRI. I imagine we were seeing different divisions at Champs and it was a matter of different LRIs at the other events.
Yes. Sometimes, I used my own clipboard and pens too. I’m not complaining. It’s what I expect when participating at a largely volunteer run event. I’ve had to use my own clipboard and pens when serving as s stroke judge at swim meets, including at state championships.
I can tell you from experience (I’m a professional seamstress) that you never trust a flexible tape measure.
As long as you use the same one for everything (I don’t really have a choice, I must use a flexible one for a lot of my work) it’s not really a problem. However with this, you’re switching back and forth. Whenever I buy a new tape, I always check it against a steel one, and check those against each other too because I’ve had large variations on those as well (though less likely).
The real problem is we can’t really measure EVERYONE’S. I agree everyone should always design well within the constraints to allow for this type of thing, but if it were me and all my sides added up to the correct amount with a steel tape measure and an inspector doesn’t want to pass me I would ask for the LRI and show them the difference.
Those Starrett tape measures are steel. Would they go around the corners of a frame? Neither the Specifications section nor the Bulletin 664 gives any sort of accuracy specification.
We always design 1" under. One year we were found to be 1/8" over because steel tape measures have non-trivial bend radii. We pointed this out to the inspector and asked them to measure length + width then double it, and were indeed found to be at 119".
Measuring tape calibration shouldn’t be a practice day nightmare for kids just trying to compete.
Make it obvious to the RI’s that your robot has a frame perimeter under 120". Teams should apply this thinking to their mechanisms too. There were several local teams that the Refs kept calling out repeatedly to push their ball carrying mechanisms back inside the frame perimeter before the match started. It appeared that as the mechanisms “aged”, they got floppy and didn’t hold the intended position well.
We have built to 120" perimeter the past 3 seasons and never had an issue with this. We have used versaframe with the 1" spaced holes and we refrain from any protrusions. I have not been present to witness every inspection but the ones I did observe, they used a flexible tape to wrap the perimeter but, without asking, have also measured each side separately. I was not close enough to see whether the tape read 120" or not, because the inspector pulled the tape off quickly.
We have talked about what-if scenarios in the past if the measurement came in oversized. If we were within a 1/4 of an inch, we were going to round off the corners with a file (while making sure we did not violate the bumper rules). We never really what-if’d a situation where we were 1.5" - 2" over as the comparison of these tapes suggest might be the case.
If that had happened, I expect we would have asked for a re-measurement as the tolerance on a long bar of versaframe is pretty tight and there would be no way that an accurate measurement would produce that kind of reading. I’m not sure what recourse you have if their “official” tape measure is not accurate, but if you can show that it measures accurately using another commercially available tape measure, I think you have a case for appeal. Honestly, I would probably point them to the pictures at the beginning of this thread as it clearly shows differences between the various measuring tapes. None of the tapes are calibrated in an instrument sense, but the metal tapes you buy at the hardware store are at least certified to an NIST standard. If the flexible tape does not have an NIST or ANSI certification mark on it, I think it is acceptable to question it.
I’m not sure how districts manage inspection stations, but for regionals, FIRST sends flexible tape measures with the inspection crate, and the expectation is to use them to measure the robots. I have two of them myself, and I’ve verified them against a known perimeter (my team’s robot) each season.
When you have minor protrusions on your frame perimeter (bolt heads, weld beads, etc), depending on how you wrap the tape measure around the frame you can get different measurements. Consider a robot that’s built to exactly 120". There’s a weld bead in each corner holding the frame together, and it sticks out 1/16" on each face of the robot. When you measure with a steel tape measure, you hit the flat sides, front, and back… but a flexible tape measure is going to include those minor protrusions. That adds 1/8" front/back, and another 1/8" side to side - double those and combine them, and you’ll measure 1/2" over sized.
The key really is to build undersized. We aim for at least 1" under on a frame perimeter measurement. That small amount really isn’t going to make a difference in your robot, but it could save you a headache at competition!
Indeed. Our design goal was 118", far easier to maintain tolerances and really with no perceptible influence on robot performance. I mean, is 1/2" better than 2", really? Is there a performance gain?
Nevertheless, the OP carries a good lesson: Bring your calibrated-to-NIST measure with you to help the inspectors verify their measurement instruments.
