A Tri Star wheel consists of a three spoked wheel, with 3 leaf wheels on the end of each spoke, all powered. This means that at rest, each Tristar wheel is likely to have two leaf wheels in contact with the ground. On flat terrain, the leaf wheels will simply turn, and give simple, and relatively efficient traction. The whole assembly, leaf wheels and star is referred to as a "meta-wheel".
However, once rough terrain is hit, and the leaf wheels cannot turn, then the whole meta-wheel turns, pushing one set of leaf wheels up and over the obstacle. This ability to climb obstacles make TriStar wheels very, very adept at dealing with rough, rocky and uneven terrain. More so than Caterpillar tracks, or simple individually powered wheels with a very flexible suspension (used in the Spirit Mars rovers).
This means also being able to build a robot capable of handling stairs - a classic problem that plagued both real and fictional robots, even The Daleks.
They even offer an advantage for slow going amphibious vehicles and the meta wheel can double up as paddles.
The complexity for Tristar wheels is handled in the mechanical linkage of the leaf wheels to the main axle for the hub of each Tristar. This means that the drive mechanism can simply be Skid Steering or similar, although these could be used with a Adder Subtractor Drive for some very serious ability to deal with obstacles and hold course on relatively simple programming.
The mechanical linkage is a set of gears, including slipgears which when powered will drive the leaf wheels until they cannot move and then power will go to turning the metawheel instead - an additional differential may be required to distribute the rotation between the leaf wheels and meta-wheel. The image above (courtesy of Doug Carlson) demonstrates one possible Lego implementation. Note that the differential gear is used instead of slip gears.
This image shows tristar wheels being used in a complete vehicle. It was also created by Doug Carlson.
|The Robot Builder's Bonanza: 99 Inexpensive Robotics Projects||Gordon McComb, Myke Predko||Not only does this cover Tristar wheels and Lego, but it is a very handy robot building book anyway. A must for a robot hobbyists bookshelf. This is the Third Edition|
|Damnation Alley||Roger Zelazny||This is a superb SciFi read by a top SciFi writer. Set in a post-holocaust future, an Anti-Hero must cross a great expanse of some of the roughest areas to bring aid Boston, with an alternative of a life in jail. The Landmaster is the vehicle of choice.|
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.