Team Update 5
 

It's been posted at http://www.usfirst.org/uploadedFiles...Update%205.pdf and includes a link to a .pdf on how to restore used Rover Wheels (http://www.usfirst.org/uploadedFiles...d%20Wheels.pdf).

Re: Team Update 5
 

Corrected second link:

http://www.usfirst.org/uploadedFiles...d%20Wheels.pdf

I like this update--it makes the standard for rover wheel condition clear, while forcing nobody to stockpile wheels. (Don't get me wrong, a spare set would be nice, but it's not essential.)

Re: Team Update 5
 

Not a bad Update. Nothing earthshaking, but I do like the little note about zapping the driver's station....

Btw, the Rover Wheel restore is an excellent trick. It's so simple and easy to do that I highly doubt any teams will have to replace their wheels during competition unless the wheels break.

Re: Team Update 5
 

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. ;)

Re: Team Update 5
 

Rule change? Well, guess I missed something.

Re: Team Update 5
 

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

Re: Team Update 5
 

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.

Re: Team Update 5
 

Any change in nominal force designed to give a traction advantage is illegal, and is inferred here in the Q&A.

No change in the rules, just a clairification.

Re: Team Update 5
 

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 ;)

Re: Team Update 5
 

A typical trailer hitch would have 3 degrees of freedom (like a standard ball hitch). The original hitch swivel (prior to RevA) had 2 (yaw and roll). The 0.27 hole added on Rev A removed the roll leaving only yaw. If you examine the REV A drawing, you will see that .27 dia through hole was added in Rev A (eliminating roll) and the 0.26 dia hole was unchanged. If you closely examine rev b you will see the change from 0.27 dia to .375 dia. Note that it previously was 0.26 dia not 0.27 implying that the change from 0.26 to 0.27 was some unreleased change between Rev A and Rev B. This by the way would be a proper change given that the plus or minus 0.01 tolerance left a 0.25 nominal pin an a 0.25 worst case hole, a bit tight for a clearance fit.

The Rev B change is not a tolerance issue anymore than changing a shaft hole from 0.26 to 0.375 would be a tolerance issue. This change provides a nominal 0.0625 annular ring around the pin allowing it to slosh around 1/8th of an inch or rotate plus or minus 10+ degrees in pitch and roll and bang against the sides with every change in robot direction. Like using too small of a ball on a typical trailer hitch, it can be expected to de-stabilize the attachment. Note also that the hole is now so big that it encroaches well into the ball radius.
The idea of transferring weight from the trailer, i.e. changing the CG of the robot/trailer system occurred during our conceptual design review. It was a natural side effect of our two wheel drive concept. The ability to transfer pitch load through the hitch was being reviewed and if it was good enough for stability then it was also good enough for shifting CG. The design intent of the trailer is clear from the the drawing. Changing from .26 to .27 or even .270 would be correcting an inconsistency. Changing from .26 to .375 is a change in design intent. This change adds two degrees of freedom (both yaw and pitch).

I fully agree with you regarding designing strategies around small oversights or technicalities. We don't believe in that either. But we are trying to demonstrate engineering to our students which in part means looking closely at the rules (requirements) and specifications and understanding them enough to exploit any competitive advantages that are available. In fact, we were fully embracing the idea that the robot and trailer are a system and were paying particular attention to the specifications for the interface of the two systems. This interface is now different in very substantive ways and this fact may have other unforeseen side affects not sufficient considered or tested.

Thanks for your input.

Re: Team Update 5
 

I believe the GDC intends to deliver a stable and well-defined game at kickoff, but the GDC members being human, each year the game will generally have a few areas that need clarification and refinement. I don't believe they will ever intentionally make major changes to the play of the game during the build season, so I would not look for new game pieces or major rule changes.

Re: Team Update 5
 

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.

Re: Team Update 5
 

Remember to put a diaper on your helper monkey.

Re: Team Update 5
 

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? :confused:

Re: Team Update 5
 

The Q&A answer was:

"Any vacuum/suction/fan system that alters the traction characteristics of the ROBOT would be considered a violation of Rule <R06>".

Other Q&A answer regarding dynamic wheel height expressly confirms that wheels that can be raised or lowered to change which wheel(s) bear the weight of the Robot do not violate <R06>.

So IMHO transferring mass from the trailer to Robot wheels via dynamic wheels would not be precluded by Rule 06. This, however, is academic as the design change you are calling a "clarification" would seem to precludes this approach.