motors 21 23 25 are operated to position analog photocell 27 directly over the selected line 37 and with the required orientation about the axis of shaft 24 at this point blank 2 is fed through station 6 to scan the selected line 37 and obtain a detection curve 28 which is memorized by processor 35 together with associated data comprising data relative to the position of head 26 during detection and data supplied by encoder 33 relative to the position assumed at each instant by blank 2 during detection which curve 28 is then used as reference signal s2 during actual control in the event blank 2 to be controlled is of a type already known to processor 35 i e in the event the reference signal relative to the blank has already been memorized by the processor together with the relative associated data the above automatic learning procedure is naturally skipped and the blank type is simply entered on keyboard 36 to accurately instruct emitting device 30 and machine 18 concerning the type of reference signal to be emitted and the position of head 26 respectively as shown in fig2 during actual control analog photocell 27 emits a signal s1 consisting of a given detection curve 28 while emitting device 30 supplies in time with photocell 27 a reference signal s2 consisting of a given reference curve 31 as shown in the fig2 example only particularly indicative portions in this case portions a.