Re: arm design
 

I agree with you generally. But it also depends greatly on a the complexity of your movements. In most cases using pneumatics that stop midstroke are similiar to dead reckoning programming. With adjustments and fine tuning you can get good results for simple, broad movements. But for complex, precise movement you are generally out of luck.

An example of this was my teams 2003 robot. We had two cyclinder lifting our stacker. As part of our autonomous we would lift our stacker part way up. It came to the same position almost everytime. But it wouldn't work well it if had to be precise.

This is where some design comes in. IF you design a robot that is able to pick up tetras with broad movements, you would likely be able to tune a program to do it in automounous (thinking about things like air pressure, piston speed, what the tetra weight will do). If it must be precise in it's movements (move two inches, then one inche, and so on) it will likely be hard for autonomous and equally so for the driver's and any programming in user mode.

Re: arm design
 

an interesting thing that i designed and tested was a joystick controlled regulator with psi relief valve. the more forward i moved the joystick the more pressure was allowed in, and if i back out the joystick pressure would be relieved. this made it so that with a load on the piston you can precisely position it where you wish, but it takes some practise.

Re: arm design
 

Ok, Here's My $0.02:
Alright, First off, Our Team is Using Pneumatics All the way. ONE Piston for our uber-cool arm. :p
To give you an idea of the range we can accomplish:
-The Arm Fits within starting size (duh)
-The Arm will utilize Mid-Stroke Positioning (This is "Stopping the piston in between fully out and in") The Mid-Stroke Positioning allows us to stop our arm at any point between retracted and extended. We developed a system in 2003 which also allowed us to do this with our staged forklift, and for those who shun mid-stroke positioning to be "inaccurate", I will try to have some pics or video for you to see how "inaccurate" it is ;)
But anyway, for those interested, the "basic" way to stop the piston mid-way is to block the Exhaust output from the solenoid controlling your piston.
Do a search and you'll quickly find more info on the subject.
Back to our Arm!
-It's capable of "grabbing" a tetra on the ground and capping it onto the center goal, up to 2-3 tetras on Center
-It stacks one at a time (Which I know many of you out there are stacking before hand ;) )
-It can stack 5-6 Tetras on the "Standard" goals
-Our arm will weigh approx. < 20lbs, Including the Four ~4.5 - 5' Counter-Support beams spreading across the chassis.
-The "grabber" section of the arm extends from 8" to 10'4" from ground level.

Now, to be fair, here are a few issues we may see developing with our arm:
-Lack of Air (I'm not too Worried about this one, by the looks of our Pneumatics Board, but always something to look out for! Especially with our Pistons...)
-Tipping (Once again, we've put alot of work into maintaining our CG in our Design, plus its simple and small (surprisingly, yes), so I'm not at worried about this yet either. We also do not extend our arm out very far outside of our robots x,z axis size (38x28)

I hope to have more Info up soon, but for those interested, here's a GIF of a (much) older model of the arm in motion:
http://www.raptar.net/ArmAnim.gif
(This was made a while back, much changes since then ;) )

Hope the info helps!