Team Update 5

[writchie]

This appears to be a major change to the game. It changes the apparent design intent of the hitch from a rigid tight fit along the pitch axis to a wobling loose fit. The nominal pitch planes of the robot and trailer can now differ by more than 10 degrees instead of less than 1 degree, even more with the inevitable enlargement of the hitch pin holes in the aluminum c-channel that will now occur.

This pretty much invalidates the strategy of transferring the bulk of the trailer weight to the wheels of the robot (through CG), increasing the normal force on driven wheels by as much as 30 lbf.

This kind of rule change, half way through the build season, would seem to at least warrant an explanation as to why the change was necessary and what it is intended to accomplish rather than just slipping it in as a drawing change.

What is the lesson to be learned here? Our team happens to have submitted a Q&A (still unanswered) to confirm the alignment angles implied by the original drawings (and pointing out that the hitch pin is actually too tight of a fit under worse case tolerances). Instead we get what amount's to a new spec for the robot. The trailer is an integral part of the robot this year and the hitch and its attachment are the most critical part of this "system".

What is the engineering lesson to be conveyed to our students by this change? 1) Don't point out to the customer that her specs have a minor problem because instead of adding a bit of extra tolerance she might change the whole design intent and force you into re-design at your expense? 2) Don't assume that specs that are supposed to be frozen are actually frozen? 3) Be careful, a seeming innocuous drawing change can invalidate an entire design approach? 4) S#%*$t happens - get used to it?

I'm sure I've missed a few more.

[Akash Rastogi]

Originally Posted by writchie

This kind of rule change

Rule change? Well, guess I missed something.

[Woody1458]

Originally Posted by Akash Rastogi

Rule change? Well, guess I missed something.

Look at Section 6, drawing changes I think is what writchie meant.

[Don Wright]

Originally Posted by writchie

This appears to be a major change to the game... (trimmed by me to save space...)

I'm sorry but I would have to respectively disagree here. I believe that the intent of the trailer all along was to be towed behind the robots as a typical trailer (boat, rv, etc) would be behind a car or truck... This is a pivoting, freely rotating connection.

I believe that designing strategies around small oversights in things like tolerances in drawings in order to gain an advantage is at the risk of the team and should they risk doing such, face the facts that their strategy is null and void when the inconsistency in the drawing is fixed.

That is the lesson that should be learned here for your team, IMHO.

[MikeDubreuil]

Originally Posted by Don Wright

I'm sorry but I would have to respectively disagree here. I believe that the intent of the trailer all along was to be towed behind the robots as a typical trailer (boat, rv, etc) would be behind a car or truck... This is a pivoting, freely rotating connection.

I can sympathize with Wally as early on I had a similar idea. The intent of the trailer is not properly defined by name itself; nor is intent defined in The Arena Rev-A. The intent of the trailer is clearly defined by the drawings. When you change drawings you change intent.

With that said, I do believe the GDC reserves the right to change the game at any time. We could have a new game piece in the next team update

[writchie]

Originally Posted by MikeDubreuil

With that said, I do believe the GDC reserves the right to change the game at any time. We could have a new game piece in the next team update

Ah!. I missed that. (It's not in the spec - it's in the contract - I knew we needed a lawyer on the team). Now we have a good engineering lesson. Look for the fine print. When the customer has some fine print that says something like "the government reserves the right at any time to change the specifications ....." beware. You may have to anticipate such changes and their possible consequences (and build that possibility into your price ).

Fortunately, we had a Plan B contingency for the case where the idea didn't work. The idea worked but the specs have changed so Plan B will now come in handy.

[Taylor]

Originally Posted by writchie

Fortunately, we had a Plan B contingency for the case where the idea didn't work. The idea worked but the specs have changed so Plan B will now come in handy.

Remember to put a diaper on your helper monkey.

[JesseK]

Imagine what happens to the trailer, practically speaking, when it starts whipping around the back of the robot. The trailer will want to tip left/right (aka roll) when it changes lateral directions. This puts extreme forces on the contact point of the trailer tongue and the robot hitch. Enlarging the hole at the contact point allows for the trailer to roll and tip slightly without damaging the trailer tongue or the hitch. I for one am very glad of this change as it reduces the torsional reinforcement our hitch mount needs since our drive train is designed to strafe and drift. Part of engineering is to at least have considered the realm of implications and possibilities of a particular design before going forward. It's impossible to figure them all out, which is why engineers usually create room for error in their designs.

Why do people consistently post that they think the GDC is out to get them when they clarify the rules?

[writchie]

Originally Posted by JesseK

Imagine what happens to the trailer, practically speaking, when it starts whipping around the back of the robot. The trailer will want to tip left/right (aka roll) when it changes lateral directions. This puts extreme forces on the contact point of the trailer tongue and the robot hitch. Enlarging the hole at the contact point allows for the trailer to roll and tip slightly without damaging the trailer tongue or the hitch. I for one am very glad of this change as it reduces the torsional reinforcement our hitch mount needs since our drive train is designed to strafe and drift. Part of engineering is to at least have considered the realm of implications and possibilities of a particular design before going forward. It's impossible to figure them all out, which is why engineers usually create room for error in their designs.

1. The original hitch design (Prior to REV A) allowed the trailer to roll freely. REV A (before the kickoff) added a close fit pin which eliminated this roll.

2. If the GDC wanted to allow limited roll, it could have modified the hole added by REV A. This would not have affected pitch.

3. REV B now allows about plus or minus 10 degrees of roll and pitch. Under some pitch loads, it also allows the 1018 steel cylinder to be levered against the aluminum C channel.

4. I'm glad that you think this change benefits your design. When the design of a component changes there are usually some that benefit, some that are harmed, some for which the change is irrelevant, and some who remain in ignorant bliss.

Originally Posted by JesseK

Why do people consistently post that they think the GDC is out to get them when they clarify the rules?

I don't know. But what does that have to do with this discussion? We certainly don't think the GDC is out to get us. And this certainly isn't a clarification! But the fact is that the design of a major component has been changed in a significant way and we still think that such spec changes should at least be accompanied by an explanation.

It may be that this is new territory for the GDC with a major component of the robot supplied as part of the field. This introduces a new element to the engineering - mating with an externally specified and supplied component. I think that this is a good thing that opens up new opportunities. But I think that specs for a component of the robot system and a the specs for game piece are of a different character. If the intent is for the piece to be an integral part of the robot on the floor, then the specs should be frozen at kickoff and not changed without both good reason and cogent explanation.

Just my $0.02

[writchie]

Picture of side by side Hitch, Swivel Rev A and Rev B

[Rick Wagner]

Originally Posted by writchie

Picture of side by side Hitch, Swivel Rev A and Rev B

http://www.chiefdelphi.com/forums/at...5&d=1232678195

The link is not working for me.

[rwood359]

Where do you find the updated GE drawings?
All I can find on the FIRST site is 9036A and 9040.

[writchie]

Originally Posted by rwood359

Where do you find the updated GE drawings?
All I can find on the FIRST site is 9036A and 9040.

They are in the latest 2009 Game Specific Drawings.zip file.

Should be here http://www.usfirst.org/uploadedFiles...20Drawings.zip

[rwood359]

Originally Posted by writchie

They are in the latest 2009 Game Specific Drawings.zip file.

Should be here http://www.usfirst.org/uploadedFiles...20Drawings.zip

That zip file has 9036a and 9040. Not 9036B and 9040A, that's why I'm confused. The updated drawings have the old revs.

[JesseK]

Originally Posted by writchie

... not changed without both good reason and cogent explanation.

Heh, you're right, we win some, we lose some. I truly believe this update was to prevent trailer damage, which is part of the field as you stated. Damage prevention techniques are stated and implied in several spots of the manual (since Day 1) and on the Q&A.

Don't get me wrong though; I'm all for milling a 1/4" slot to allow for roll, which would prohibit pitch. I suppose we can call the GDC lazy for going with the simple method of drilling a bigger hole.