Re: Three joint arm
 

Two joints is a very good compromise of simplicity (although it isn't that simple), usefulness, and still providing a good programming challenge.

Re: Three joint arm
 

The good thing about 2 joints over 3 is that with 2 you can get away with default code and a pair of joysticks if you need to. In my experience, it gets a lot worse when you add the third joint (especially if it's a third motor-powered joint with complete freedom) and you need to start thinking about more sophisticated controls. Of course, if you can do it with one joint, your controls are even simpler still...

Re: Three joint arm
 

Team 1538's arm in 2011 was really neat, using a single joint powered by a motor and then gas shocks on the roller claw that moved the claw into the correct position without the need to power the 2nd joint directly.

Re: Three joint arm
 

Credit goes to 67's 2007 robot for the idea. :)

Re: Three joint arm
 

1731's arm in 2007 was nothing short of epic, both in how it was built and in how it was controlled. It had a z-rotating (aka yaw) turret at the base, a x-axis rotation shoulder (aka pitch), a x-axis rotating elbow (pitch), a y-axis rotating wrist (aka roll), and some sort of x-axis rotating grasping mechanism.

It was all controlled by a near-identical arm made of potentiometers on the drivers station.

Was it 100% necessary? No. But it DID work well, which was benefit 1. Benefit 2 was the incredible amount of unique materials used compared to the competition at the time. For a tech company, Benefit 1 pays the short-term bills, and Benefit 2 is what brings in licensing royalties long after other companies "innovate" in the tech space (Xerox is a great example).

Re: Three joint arm
 

While you can't tune the gas shock itself, you can certainly tune the system by adjusting the mounting points.

Re: Three joint arm
 

Although more expensive, you can also purchase adjustable gas shocks. Just be careful to do the force calculations correctly and over compensate, then release gas as needed.

http://www.mcmaster.com/#gas-shocks/=hmecz4

edit: go to reducible force springs

Re: Three joint arm
 

You change the mounting points, but this requires extra machining, along with extra calculations(surgical tubing requires none of this).

Of course, you can buy adjustable gas shocks as Akash said, this is more expensive. The nice thing about surgical tubing is that it is very cheap. But if you are willing and have the budget to spend the money, then it is worth it.

Re: Three joint arm
 

It should be noted that you get what you pay for (you may find yourself replacing sugical tubing, but the odds of having to replace gas shocks are very low).

Changing the mounting points isn't that hard. If you know what you need/what you have, take a drill and make more holes (your robot probably needs them anyway). The calculations shouldn't be that onerous, and I would guess that you would actually want to do the calculations for surgical tubing as well.