FRC 95 The Grasshoppers 2019 Build Thread


#1

EDIT: Here is link to our OnShape project.

Hello CD. Welcome to the Grasshopper’s 2019 build thread.

As usual process we started brainstorming robot archetypes after carefully reading the manual. We defined these archetypes roughly by identifying their capabilities in a chart. After downselecting the list looks like this:

We then compiled a weighted decision matrix using a sumproduct operation (that someone from 1114 or 610 (?) suggested in a prior build thread) to rank the archetypes. Unusually, many of the archetypes had roughly the same value for our weighting.

We elected to go with the MVP archetype. Our main objective will be to handle and score game pieces quickly and reliably at all positions, with a secondary objective of completing endgame tasks. For a team of our size we think this is the right scope.

CAD has started for the drive base. We’re looking at up to 10WD with 6in wheels with 6-minicim + ball shifter transmissions. Depending on our HAB climbing approach we might reduce this to 8wd or 6wd. We’re targeting speeds a little slower than what we had last year, 15.5ft/s high and 7ft/s low free speeds compared to 18/8 from last year.

We are prototyping a few different approaches to ground-loading CARGO and HATCHES as well as handling HATCHES. We are looking to iterate our elevator from last year, gear ground collector from 2017, and ball collector from 2012/2014. A real mix-and-match approach of known techniques.

Always happy to hear feedback and answer questions at any point.


Missing Build thread from FRC 95
#2

This is my favorite thread of the year, every year! As always thanks for doing this, year after year we look towards this thread for it’s numerous insights and resources :slight_smile:


#3

Always look forward to your threads every season. Very informative and mentoring a rookie team this year, it’s going to be a great resource to show the students what a successful build season looks like


#4

Looking forward to keeping track of this thread all year. Last year was awesome, and I’m sure this year will be no less. Good luck in Deep Space!


#5

I like this thread every year, it’s great to see you document the process and decisions your team makes!

Interesting lack of ground clearance on the drivetrain, I’m interested to see if it could get hung up on the metal barriers that hold the Cargo or if it gets sideways on the HAB platform.


#6

Right now we’re designed to have 1.25in of ground clearance, but may increase it on further evaluation.


#7

We’ve upped our ground clearance to ~1.9in. Reviewing 148’s CAD of Uppercut has been quite informative. 148 has a ton ofavailable resources that everyone should leverage!

Hatch ground collector prototype based on our 2017 gear ground pickup looks promising.

Edit: tally one day lost to snow already. Hooray NE weather!


#8

James,

Super glad that 95 is doing another season-long build thread. I actually looked into your 2017 elevator last night and was quite impressed. If you or your team would be up for sharing any additional videos/info/resources regarding your 2017 elevator in the coming days, I’m sure that would be greatly appreciated!


#9

Hey James, did you simply use thin polycarb underneath or is it a different material? We tried the thinnest poly we had and it was still a bit too stiff for my liking. Try thin hdpe or anything like that?


#10

Thanks! Here you (and everyone) go.

CAD Link (converted to OnShape, it might be a mess). A number of OTS parts like pulleys didn’t import for some reason, but it gives the general idea.

More details are available in our 2018 IMGUR album. https://imgur.com/a/5ut3k

Big points of learning
-Roller contacts are the best
-Joint stiffness between stages and rails is critical
-Thinner cord/rope is better
-We never used a powered ‘down’ rope and were satisfied with performance (against all conventional FRC wisdom)
-Design in clean and robust hard-stops from the beginning, same with limit switches
-Using an encoder on the spool was accurate enough for 2018

Here is a video of our gear ground collector for reference.

The ‘teeth’ in the 2017 gear collector were HDPE sheets (0.040in/1mm IIRC). They were reasonably robust, but did require replacement at regular intervals and could catch on the field’s gaffer tape.

The ‘teeth’ in our 2019 prototype are PTFE. This is likely too soft to use in competition. I suspect we’re going to use delrin or hdpe on the final version.


#11

Oh neat, I was wondering if it might be PTFE. Looking forward to seeing if it ends up being robust enough in your testing. I’m leaning towards HDPE after a little more prototyping


#12

That is certainly what we concluded in 2017.

We have a laser cutter now, and delrin cuts really nicely (apparently) so we’re going to add it to the mix.


#13

If you need something really thin, flexible, and relatively robust, try polycarbonate film from McMaster. It varies between .005”-.04”. I used some .01 on a school project, it wouldn’t shatter or tear and can be cut with heavy duty scissors. It’s very flexible though so you might need some clever bends or supports to stiffen it. It doesn’t need heat to bend because of how thin it is. Also great for sponsor panels as it’s super light and durable.


#14

Here is exactly what we used in 2017. Getting it on a roll was accidentally the best. We used the natural lay of the material to spring it down into the carpet.


#15

Do you plan on using the same ground pickup to remove the panel from the loading station?


#16

Good question, I was thinking the same.


#17

No.

:smile:


#18

Coyness aside, we’re are strongly considering a ground pickup that hands off to another mechanism. It increases our total mechanism count (which is a bummer) but allows us to optimize the main mechanism for loading station collection/scoring, which have comparable geometries. It also means that if the ground collector gets smashed on something we’re still capable of scoring.


#19

Here are two more hatch ground load prototype videos.

We had lovely success using ~1/8in thick MDS filled nylon (we just happened to have some), again using the coil set direction as a spring to ride smoothly on the carpet.

On the CAD side of things we have made a few big decisions:

The elevator concept will proceed with a 3-stage approach instead of 2-stage so that we can have a comfortable amount of stage overlap. Schematically, it might wind up looking like this:

If we use 10WD to reach HAB LEVEL 2, we will use 3x axles of 6in wheels and 2x axles of 4in wheels to improve packaging and weight.

Ground clearance changes, and a few other changes, have cleaned up the drive base a bit.

We’re continuing to prototype a hatch grabber and will start our cargo collector work this weekend. We have postponed this development because we think it’s a much lower risk development than hatch handling. Some of us (sigh) have been around since 2004, the last time these game elements were used.


#20

What’s the reasoning behind using a continuous elevator over a cascading one? We’re still debating which to use, so I’m interested what made you choose one over the other.