499,753. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd. July 26,1937, No. 20638. Convention date, July 24, 1936. [Class 38 (iv)] [Also in Group XL] Alignment.-A device for detecting a bowed or skewed condition of the weft members of a strip of woven material comprises means for directing a pair of light beams through the strip on to photo-electric cells, means for moving the strip longitudinally whereby the beams are intercepted by the weft members, means for moving each of the light beams transversely of the strip, the lines of movement of the beams on the material being spaced apart laterally and inclined to the edges of the strip, and apparatus responsive to a difference in frequency of output of the cells. The responsive apparatus may serve automatically to control the feed of the strip so as to eliminate the bowed or skewed condition of the weft. The arrangement for detecting and correcting a bowed condition differs from that for correcting skew, in that for the latter purpose the lines of movement of the light beam are oppositely inclined to the edges of the strip, whereas for correcting bow the lines are similarly inclined. One of the photo-electric arrangements is preferably common to the systems for correct. ing both bow and skew, and such corrections may be effected simultaneously. In the apparatus shown, a strip 1 of woven material is drawn along by a tenter comprising grippers 6 on chains driven by sprockets 4, 5, which are on shafts geared together and driven by a motor 10. On its way to the grippers, the web passes over fixedly mounted idler rollers 11, 12 and two adjustably mounted rollers 13, 14. The supports for each of the latter are connected by links 17, 19 to nuts 18 threaded on a screw shaft 20 which is geared to a motor 22. When this motor is rotated in one direction or the other, under automatic control as subsequently described, the inclination of the rollers 13, 14 to the rollers 11, 12 is simultaneously and oppositely varied, so that skew in the material may be corrected before it reaches the grippers. To correct for bow, co-operating rollers 25, 29 are provided to bear on the web, the lower roller 25 being driven by gearing from the main shaft 8 so that its surface moves at a somewhat greater speed than that of the tenter chains. The pressure of the idler rollers 29 may be varied so that there is a variable amount of slip between the material and the driven roller. When the presence of bow in the weft is detected, a motor 34 is automatically caused to run in one direction or the other, and the pressure of the roller 29 on the central portion of the web is increased or decreased until the bowed condition ceases. The main drive shaft 8 is geared to a polyphase generator 49 which feeds three separate synchronous motors A<1>, B<1>, C<1>, respectively driving slotted discs A<2>, B<2>, C2 arranged beneath the web. The slots in these discs intercept linear light beams projected by suitable optical systems through the web on to photo-electric cells A3, B<3>, C<3>. The arrangement is such that, due to the relative movement of the web and the successive slots of each disc, narrow lines of light represented by the rectangles 42 in Figs. 4 and 5 periodically sweep across the web, in paths shown at A<43>, B<43>, C<43> in those Figures. The discs A<2>, C<2> rotate in the same direction, but oppositely to the disc B<2>, so that the paths of movement of the beams which fall on the cells A<3>, B3 are oppositely inclined, Fig. 4, while those of the beams falling on the cells A<3>, C<3> are similarly inclined, Fig. 5. If the web is neither skewed nor bowed, the frequency of interruption of all the light beams by the weft members during the simultaneous travels of the beams along their paths will be equal. If the web is skewed, as shown for example in Fig. 4, the frequency of interception of the beam travelling along the path A<43> (and falling on the cell A<3>) will be greater than that in the path B<43> (affecting B<3>). Similarly, a bowed condition as shown in Fig. 5 produces inequality in the frequencies of interception of the beams falling on the cells A<3> and C<3> respectively. The variations in the outputs of the cells in response to inequalities in the frequencies of interception of the light beams is utilized by means of the valve relay system described in Specification 499,601 to control the direction of rotation of the motor 22, and/or of the motor 34 as and when required, in order to effect the necessary adjustments of the web-guiding and feeding devices, whereby corrections are made in the manner described above. If desired, separate pairs of photo-electric devices may be used to detect skew and bow respectively, instead of having one unit common to both systems as in the arrangement described.