for thread preparation mechanism 2 the other thread preparation mechanisms 3 to 5 can also be equipped with thread break detectors 43 to 45 and connected to the monitoring device 29 fig3 shows a practical embodiment corresponding to the block diagram in fig2 more particularly for two thread preparation mechanisms 2 and 3 in this configuration the weft threads 15 and 16 pass successively and respectively through the detectors 46 and 47 the prewinders 10 and 11 the detectors 48 and 49 the thread brakes 50 51 and 52 53 thread compensators 54 and 55 a weft detector 56 and the thread guide elements 34 and 35 of the thread presentation mechanism 14 the prewinders 10 and 11 each consist of a prewinder drum 57 and a rotating winding tube 58 through which the turns 59 are wound onto the prewinder drum the prewinders each have one or more turn detectors 60 which monitor the quantity of thread on the prewinder drums 57 the detectors 48 and 49 are preferably eye shaped motion detectors which also form a guide for the threads as these leave the prewinder drums 57 the weft detector 56 consists of a series of motion detectors of which two 61 and 62 respectively are used by the weft threads.