## 3 Results and Discussion  
The automatic control of an articulated bus, presents, at a high level, some features similar to other automatic vehicles, mainly regarding instrumentation and communications [66; 67; 68; 69]. However, from the point of view of practical implementation and low-level control, there are some challenging differences. One major difference is inertia, which means greater power is involved and, so, more demanding control and safety measures must be applied, those being of special relevance when coping with down slopes. As a consequence, braking strategies and accelerations (longitudinal, lateral) are now of greater relevance with respect to those of cars or small buses as is the case of the European project CityMobil2 [70]. A second key difference with automatic cars or small buses [66] is the very long length of an articulated bus (about 18 m), which, adding the position of the steering wheel ahead of the front wheels (the bus driver is seated ahead of the front wheels), makes turning maneuvers very cumbersome (in fact, during our experiments, one of the most \"striking\" sensations was when approaching a curve and the steering wheel did not turn for a while, then it started to turn when the front wheels where required to follow the curve). Furthermore, because of the bus length and the presence of its central articulation, the trajectories of the front, middle and rear wheels present more differences than in the case of a shorter vehicle [66], where practically the rear wheels are able to track  
Figure 11: The architecture of the experimental platform. Figure 12: On-board instrumentation and control system of the experimental platform. the front wheels' trajectory [27]. This intrinsic articulated bus characteristic requires special attention when calculating the required steering angles for the desired bus trajectory. The longitudinal control of the articulated bus is carried out so that the vehicle will be able to move ahead of the test lane at a desired speed. The speed used in most experiments was relatively slow due to safety reasons and to the intricacy of the trajectory (please refer to Figure 1) intended more for checking control performance than to go very fast. The average speed in the experiments was ranging from 10 km/h to 25 km/h. However, in the straight section of the path, in some controlled experiments, the speed went up to 60 km/h. Figure 13 shows the longitudinal and lateral control scheme implemented in the bus Volvo B10M ART-RA-IN. Regarding longitudinal control, the system acts on the PotBox of the bus, which has an action range of 0 to 5 DC volts, rather than acting on the throttle, because the control over acceleration/speed of the bus is better in this case and more controllable. The acceleration is limited (saturated) in order to protect the PotBox and not to exceed the desired speed over limits that cannot be controlled, according to the experimental tests. When the bus exceeds the desired speed, then the acceleration controller stops and selects the brake controller. The brake controller has minimum and maximum limits, which determine the workspace for this controller. Within these limits, the brake controller generates a saw-tooth signal to act on the brake actuator, which consists of a DC servomotor (see Figure 3). This makes the brake subsystem perform stopping of the bus as a gentle deceleration. When the speed decreases to a pre-set threshold, the brake subsystem stops acting and starts the accelerator subsystem. The bus speed is measured by a speed sensor whose resolution is greater than that offered by the original bus tachometer. Regarding lateral control, an electric actuator composed of a DC motor, gearbox and encoder has been installed on the steering column (see Figure 5). The actuator receives commands from the lateral control system, so that the vehicle will track the desired trajectory. It takes into account the steering limit, which in this case is \(\pm\)900\({}^{\circ}\).