Method for restarting a weaving loom after stopping and/or changing the warp chain position

The present invention relates to a method for controlling the starting position of a weaving loom after stopping, in particular a method for restarting the weaving loom after stopping in view of an adjustment.

As known, during weaving loom operation it may be necessary to occasionally stop the loom for various reasons and then to restart it after performing specific operations. In particular circumstances, while the loom is stationary, operations are carried out that temporarily alter the position of the warp chain on the loom and/or the relative positions of warp, let-off motion cylinder and fabric-tensioning roller.

In any case, every time the warp chain is temporarily moved backwards or forwards, it is necessary to recreate the initial conditions before restarting the loom, i.e. bring all the weaving members and the yarn in the exact position they were in before the loom was stopped: if not, when restarting at a steady speed, conspicuous defects will appear in the fabric.

According to the prior art, in order to bring the loom back to the initial conditions, the rotation angle described by the main shaft of the loom during down time (i.e. during the time between two steady conditions) is tracked, for example by means of an encoder, to be later reproduced backwards by the main motor.

Ideally, this approach should achieve the desired result, especially thanks to the inherent control accuracy of modern encoders and electric motors.

However, since the warp yarns and weft threads have a certain elasticity and the fabric resting on the loom bench is subject to a remarkable friction - amongst other things, of a very different scale in dynamic or semi-static conditions, respectively - reference to the encoder only on the mechanical transmission shaft is not sufficient to reset the initial condition. As a matter of fact, although the initial "mechanical" condition of the loom is actually reset by the encoder only, the initial "textile" condition of the warp chain may be lost due to the yarn elastic and friction forces.

In such circumstances the reed beating line, upon restart after adjustment operations, may no longer fall exactly on the margin or the leading edge of the cloth as it was completed when the loom stopped.

As is understandable, the phase displacement between initial "mechanical" condition and initial "textile" condition may also be of a minimal extent (being the larger, the more elastic the yarn is), but in any case it inevitably causes defects in the fabric (commonly referred to as "overhead beating" defects and "open space in cloth") which it is desirable to eliminate.

JP04-065555 discloses a method for restarting a weaving loom, whereby after stopping the loom an adjustement on the mechanical members of the loom is carried out on the basis of a textile phase displacement measured by comparing first and second image obtained by a image sensor.

It is an object of the present invention to supply a method for restarting the loom which allows to correctly reset the conditions of the warp chain even after an adjustment operation, in particular a method allowing to reset even the initial "textile" conditions independently from reference to the initial "mechanical" conditions.

Such object is achieved by the method described in the accompanying main claim.

Other inventive features of the method are described in the dependent claims.

Further features and advantages of the method according to the invention will in any case be clearer from the following detailed description of a preferred embodiment of the same.

A warp chain runs from a let-off cylinder to a fabric-tensioning roller passing through the weaving area where the weaving machine is located. In particular, the position in which the fabric is actually originated corresponds with the reed beating line, i.e. the line along which the last weft thread inserted is beaten onto the cloth.

According to the invention, in the weaving loom, in the bench area onto which the fabric runs, is installed an image acquisition device or camera capable of taking up at least a portion of the fabric travelling in front of a window frame which is fixed with respect to the loom reference, preferably a small window frame of limited size, advantageously smaller than 10x10 cm and, for example, measuring about 2x2 cm.

The image of the fabric, preferably in a digital format, is then managed and processed, for example keeping only a thin linear scanned image thereof in the warp direction.

The image of the fabric is acquired at preset times by a digital taking up system, for example the acquisition device described in the Italian patent application simultaneously filed by the same Applicant.

The method of the invention provides that, when the loom is stopped for the required adjustments, a digital image of the framed fabric portion is acquired through the taking up device: such digital image is kept at least temporarily in a memory area, for example either in a RAM of a suitable integrated circuit, or in a RAM or a hard disk of a computer connected to the acquisition device.

Once the operator has completed his operations on the warp chain, the loom is brought back to the conditions in which it was at the time of stopping, using known methods, for example causing the loom motor to reproduce backwards the rotation angle detected by an encoder integral with the main shaft.

Afterwards, the loom control system instructs the taking up device to acquire a second image of the framed fabric portion, which is compared against the first image stored in the memory area.

The comparison between the two images occurs by means of suitable numerical algorithms so that it may be established whether there is a perfect match - except for a tolerance factor set beforehand - or whether the two images are so different that adjustment is required. In this last case, from the numerical comparison of the two digital images, the amount and direction of correction is obtained by which the warp chain must be shifted to bring the leading edge of the cloth back to exactly the same position it was in when the loom was stopped.

The numerical comparison between the two digital images can be performed in a variety of ways, some of which are immediately available to a person skilled in the field of computerised anti-counterfeiting systems.

A possible comparison method, for example, consists in identifying the position of light intensity peaks in each image compared against a fixed reference, for example the edge of the acquisition window frame of the taking up device: from the difference in the peak positions in the two images and from the enlarging rate at which the fabric image is acquired, it is possible to determine the amount and direction by which the loom must be moved forwards or backwards, with respect to the mechanical condition existing at the time when the loom was stopped, in order to reset the initial "textile" condition.

Similar results can be achieved through an analysis of virtual interference fringes obtained from the overlapping of the two acquired images, fringes which give an immediate measure of the phase shift between the two images.

The measure of the adjustment, processed by the central control system of the loom on the basis of the comparison of the two images and transmitted to the loom motors, can be slightly larger than that calculated strictly on the basis of the phase shift existing between the two images, for example to take into account the elasticity of the actual warp chain and the remarkable friction existing between the cloth and the bench in nearly static conditions. The adjustment is carried out through two or more iterative steps between which the image comparison phase is repeated.

Advantageously, the fabric portion whose image is acquired lies in a position as close as possible to the leading edge of the cloth, so as to increase accuracy when resetting the initial conditions: in fact, this avoids residual positioning errors deriving from the elasticity of the cloth portion between the acquisition window frame and the beating line.

Once the desired initial "textile" condition is reset, the control system provides to restart the loom.

Furthermore, according to the method of the invention, after resetting the initial "textile" condition through the adjustment intervention determined by the comparison of the fabric images, even the initial "mechanical" condition is reset again: in this latter step the yarn-specific elasticity and the existing friction against the loom bench ensure that the obtained initial "textile" condition is not lost again.

With the method according to the invention the set objects are thus achieved, since the method allows to reset - after loom stopping and adjustment intervention affecting a displacement of the warp chain - not only the initial "mechanical" condition, but also - most importantly - the initial "textile" condition.

It is understood, however, that the invention is not limited to the specific embodiment described above, which is a mere non-limiting example of the scope of the invention, but that a number of variants are possible, all within the reach of a skilled person in the field, without departing from the scope of the invention as claimed.