The basic mechanical concept of the robot can be seen in Figure 2. The propulsion is generated both with two horizontal cylinders (2.1) that have their normal force generated by gravity, and by two pairs of vertical cylinders (2.2) that are pressed against each other by a spring system (1). To increase the stability, the payload is mounted at a very low position (4). When riding on the cable, the arms for the vertical cylinders (3.2) remain at a constant angle towards the central part of the chassis (3.1). As soon as an obstacle is reached, the front arms open and the front vertical propulsion units embrace the obstacle. While opening, the forces generated by the springs (1) will increase and try to manoeuvre the system back into a centre position on the obstacle. The front vertical cylinders may lose direct contact with the obstacle or the ground cable. Since the two side cylinders are in a tapered shape, the cylinder pair always falls back into a centre position. When the rear pair of vertical cylinders reaches the obstacle, the front pair has already passed. The obstacle passing sequence is then similar to the one described for the front pair. More details of the obstacle-passing-sequence can be seen in Figure 5.