is connected via an amplifier not shown and a line 59 to the switching magnet of a 4 2 way valve 60 the multi way valve 60 is connected via two hose lines 61 62 to the pneumatic cylinder 47 the compressed air source is marked 63 on lever 45 a switching lug 64 is fastened which cooperates with a slit initiator 65 the initiator 65 is arranged on an angle plate 66 fastened on the support 34 and is connected via a line 67 to an input of microcomputer 30 at the support 34 a switching lug 68 is fastened which cooperates with a slit initiator 69 the latter is arranged on an angle plate 70 fastened on plate 31 and is connected via a line 71 to another input of the microcomputer 30 on an l shaped support 72 which is arranged on a cranked angle plate 73 fastened on arm 1 a sensor 74 is fastened in spaced relation before the needle bar 4 the sensor 74 consists in known manner of a light transmitter and a light receiver it cooperates with a reflection foil 75 fastened on the base plate 6 fig2 and is connected via a line 76 to an input of microcomputer 30 spaced from the sensor 74 there is arranged on support 72 a second similar sensor 77 which is connected via line 78 to an input of microcomputer 30 and together with sensor 74 forms part of a feed measuring device.