20-01-2015 14:27

cadandcookies

Re: pic: 4242 Preliminary CAD 1

I really like the ramp idea. How much have you prototyped that/ how well does it work?

20-01-2015 14:40

faust1706

Re: pic: 4242 Preliminary CAD 1

Have you done a stress analysis of this yet? I don't know the material used, but looking at the design, the thought that when you try to lift a stack of totes, your elevator at the top might begin to bend depending on a few factors. I'm a programmer, though. Take this with a grain of salt.

20-01-2015 14:49

Ty Tremblay

Re: pic: 4242 Preliminary CAD 1

I'd suggest making your elevator car a little taller to space the carrier wheels out further. This will decrease the chances of having any sag in your manipulator when you have a full stack.

20-01-2015 14:51

K-Dawg157

Re: pic: 4242 Preliminary CAD 1

You have a valid point, programmer.

It would all depend on the material being used, and the orientation it was being used in. Also, how high the support from the back is up the side rails would be important.

I did some calculations with an engineer, and we figured it out that the maximum load the robot (assuming 6 tote or 5 tote bucket and noodle capabilities) would be holding, cantilevered at 6' in the air, would be ~400 foot pounds...

Make sure your material is all good and the supports will hold it and you should be set.

20-01-2015 15:00

eddie12390

Re: pic: 4242 Preliminary CAD 1

Given the configuration of this robot, what does the CoG look like at that point? It seems like you'd have to have a fairly hefty counterbalance to keep it from faceplanting.

20-01-2015 15:12

faust1706

Re: pic: 4242 Preliminary CAD 1

That was really my only fear for robots this year as far as damage; could they handle the torque of picking up a stack of n totes + the bin and not get deformed over the course of the season?

Side note: I think demos of robots this year will actually be pretty cool if they can either lift a lot of weight, or stack autonomously, or both.

There is also the concern of center of gravity, as eddie12390 has pointed out. I haven't been around frc long enough to see robots fall over because of game piece weight, if it has even ever happened.

20-01-2015 15:23

Christopher149

Re: pic: 4242 Preliminary CAD 1

My freshman year (2008), at least one robot fell over at the Wisconsin Regional (and IIRC, almost tried to fall out of the field). This was a combination of height and trackball weight.

20-01-2015 16:19

asid61

Re: pic: 4242 Preliminary CAD 1

400ft lbs?
For totes cantalievered like 14" out from the front of the robot, I only get arounf 70ft-lbs for a 60lb load. I though you were only supposed to consider the X-distance, not the Y height for torque caused by gravity.
Could you please share your calculations?

20-01-2015 16:56

apm4242

Re: pic: 4242 Preliminary CAD 1

Thanks. To answer your question - not at all. It's on the to-do list. We have some .25" clear acrylic laying around and will try this soon. We may have to put a single driven wheel at the bottom corner of the ramp to prevent totes from leaning against us rather than sliding off at the end. The downside of the ramp idea is that if we're feeding totes to the field, we're not stacking...

The plan is to put hefty weights in the back. Probably around 25lbs, maybe more depending on testing. The material is aluminum t-slot. It should be fine but we'll probably move the support a bit higher. Either way, we don't plan to lift more than 3 totes or 2 totes/1 bin if hooked from the long edge OR 2 totes or 1 tote/1 bin if hooked from the short edge.

Thanks for all the replies!

21-01-2015 09:36

Orion.DeYoe

Re: pic: 4242 Preliminary CAD 1

Um... You need to check those numbers...
The load is not cantilevered the height of the elevator. The number you should be using for your moment arm is the horizontal distance from the center of the carriage to the COG of the load (something like 12-16 inches). Multiply that distance by the weight of the load (60 lbs we'll say) and you get something like 720 - 960 in*lbs (60 - 80 ft*lbs).
It appears everything is made from 1x1 8020 extrusion. It's pretty beefy stuff and the elevator appears well-supported. I wouldn't anticipate anything horrible happening to it.
I probably would make the carriage taller though (as someone else mentioned).

21-01-2015 11:39

K-Dawg157

Re: pic: 4242 Preliminary CAD 1

EDIT:::

I have realized my mistake... I was assuming the wire would be pulling from the back of the robot, and the bottom tote would be at the top of the "pillars". I was calculating the wrong Moment of Inertia and wrong placement... I'll redo that.

Sorry for the confusion.

21-01-2015 12:12

RonnieS

Re: pic: 4242 Preliminary CAD 1

It looks like you will be running chain? Make sure you have a proper way to tension those giant runs. My vote would be to look at putting a tensioning device in the middle of your chain run. You will already be anchoring your carriage so doesn't seem to be too big of a deal, would even look at having it build into how it attaches to your carriage.

Second, I would put a cross support. Doesn't have to be directly on uppers but maybe on the angled back support braces.

Lastly, I love the ramp idea. I would look into seeing how you can stack them quick from the human loader station. Maybe by having a ramp with pistons on bottom that let one slide to floor then it raises up enough to clear first tote and slides the second tote right on top. I know our team will be looking for those robots that can make quick pre stacks.
-Ronnie

21-01-2015 14:52

apm4242

Re: pic: 4242 Preliminary CAD 1

Ronnie - Thanks for the input! We have limited experience with chain driven devices. Given the rollers and the carriage track, I'm not sure where we could put a tensioner. However, we might be able to use something like McMaster's Extended-Life Adjustable
Roller Chain Tensioners on our carriage to tenison it. One or both of the lift axles are going to be on adjustable flange bearings on the tslot. Do you think this will be sufficient to tension the chains?

RE the ramp - Right now, this ramp would only be able to make 1-stacks oriented in a consistent way for others to pick up (basically removing the inconsistencies of the chute). Although, one of our original designs was an articulating conveyor that would make quick 2 and 3-stacks from the tote chute but we nixed it because we didn't have a strong way to drive the cantilevered conveyor and grip was an issue.

21-01-2015 15:13

apm4242

Re: pic: 4242 Preliminary CAD 1

Prototyped it today. It works fairly well but does not slide without the help of the speed gained in the chute. That is to say, the tote did not slide when placed on the acrylic ramp at 16 degrees.

21-01-2015 15:16

who716

Re: pic: 4242 Preliminary CAD 1

is that a two or three stage lift, and what the goal for stacking, due you really need to go up past the secound level with two totes?

21-01-2015 15:16

mrnoble

Re: pic: 4242 Preliminary CAD 1

Please don't use acrylic.

21-01-2015 15:52

asid61

Re: pic: 4242 Preliminary CAD 1

Out of curiosity, why?

21-01-2015 15:53

Christopher149

Re: pic: 4242 Preliminary CAD 1

Prone to shattering, and cracking around bolt holes, even when all it does is support a sponsor decal.

21-01-2015 23:16

RonnieS

Re: pic: 4242 Preliminary CAD 1

That should do the trick. If not, you can always post again and we can give further instructions.
-Ronnie

22-01-2015 08:35

apm4242

Re: pic: 4242 Preliminary CAD 1

I suppose it's a 1 stage lift? It's chain driven and one side of the chain is anchored to the carriage. As far as height - that's something we're deciding. I'd rather be shorter for stability but it seems silly when all that's limiting how high you can lift is length of tslot and chain (and of course the rules). We don't plan on lifting more than 2-3 totes at a time but we'd like to be able to cap a 3-stack on the platform with a bin. ~38" (3-stacked totes) + ~27"(to the lip of the bin) = 65" off the ground with a bin held by the lip.

We've never had a problem with acrylic but this will be the first year we're considering drilling it. Maybe we'll just slide it into the tslot like last year. The .25" thickness works well with 1010.

22-01-2015 08:41

mrnoble

Re: pic: 4242 Preliminary CAD 1

Is polycarbonate (Lexan) not available for you? It will not shatter or crack.