FIG. 8, FIG. 10 is a partial block diagram of a modified control system and sensor construction, FIG. 11 is an explanatory view of an operative area of the sensor shown in FIG. 10, FIG. 12 is a schematic plan view showing a conveyor cart equipped with the sensor shown in FIG. 10 and a running track layout, FIGS. 13A and 13B are a flowchart showing an