My robot's overall design was inspired by the Gradall Excavator. It has five degrees of freedom and a range of mobile bases. Unlike most excavators, Gradall doesn't have an elbow. Instead, it uses a straight telescopic joint. This is its main advantage since elbows tend to stick up in the air and have a nasty habit of colliding with trees, power lines, and overpasses.
The other benefit of Gradall's telescopic joint is its ability to twist its arm. The entire boom can roll 90 degrees in each direction. This is rather difficult to do with a traditional elbow arm - the human forearm uses two bones (the Ulna and the Radius) that twist together. I would dearly like to see an elbow excavator use this technique. The roll motion gives Gradall added dexterity, though in practice it is rarely used.
For my robot the telescopic motion is ideal. It allows me to counter-balance much of the arm behind the main up/down fulcrum (impossible to do with an elbow) thus giving me a great deal more lifting power.
Although the rolling motion on my robot isn't functional yet, when it does become available it will be able to rotate 180 degrees in each direction (twice as far as Gradall). This will not happen for a while, since there is no need for the roll motion until I replace the electromagnet with a gripper.
Gradall is a registered trademark for hydraulic excavators built by The Warner & Swasey Co.