OK, in the meantime I have partly redesigned this robot, although the general design has not changed. See here the images of "Version 2" enhanced robot
Probably the most important change: The battery (a 11.1V (12.6V peak!) LiPo-accu went to the undercarriage, which allowed for a generally lower design and enhanced the movement characteristics, as the CoG went far down, reducing the danger of toppling.
The robot is now equipped with a gyro sensor (the one in the middle, on two stents), which allows it to keep it's course straight. I also added a short micro-USB to USB cable, which makes it easier to connect to the IDE.
For the next step I plan to make the robot a bit longer, to allow the mounting of some serious payload. This would then also mean to have 4 or 5 pulleys either side, instead of the three, I have now. Then I want to integrate a damper system, but did not yet find a good way to do so. Ideas are appreciated here :-).
At this moment, the robot is driven by a self-made Bluetooth control sketch, which connects to a BT app on Android. This is described in a separate thread. Thus, the sensors are not used at the moment, but my next step on the software side will be to add a "auto-drive" mode to the sketch and allow easy toggling between both modes "on-the-fly".