The robot is divided in 3 parts:
- Part 1 (front): this part is attached to the top of part 2 (center of the robot) and is responsible to go over the next step on the stair.
- Part 2 (center): this is where the NXT bricks are located and where the motors that move parts 1 and 3 up and down are.
- Part 3 (back): this part is attached to the bottom of part 2 and is the last part to go over the next step while climbing.
Both motors used to move parts 1 and 3 up and down are geared down by 1.66:1. Each part has its own motor to move it forward while climbing the stairs (1 medium PF).
The video shows how it works.