Step 4: First step Metalwork and construction
The project can be divided into metal fabrication / construction of the running gear and electronics/programming. The fabrication can be as clever or straightforward as you wish. Someone has even built a "SegwayTM" style vehicle out of wood using wheelchair motors for example.
I started by mocking up the 2 steel side panels in cardboard. Although these were made as shown in the image initially, I later removed the raised sections at each end to permit a flat deck across the top.
Note regarding batteries: Lithium batteries give most power for their size and weight followed by Nickel Hydride followed by lead-acid. I used lead acid batteries because (a) cheap, (b) can give large current for small periods if needed when hitting obstacle for example, (c) complex chargers not required.