Rieter

Classification

Index

These operate in accordance with either the open-loop or the closed-loop principle. In addition to the advantages and disadvantages of these systems listed in  Technology of Short-staple Spinning, the following should be mentioned here:

  • open-loop control can also compensate variations of short (to medium) wavelength, but
  • closed-loop control can compensate only medium and long-term variations.

This implies that piecings arising from the combing operation can be partly eliminated with the aid of the open-loop system but not with the closed-loop device. That is why the closed-loop control system is unsuitable for application in short staple spinning. With closed-loop control the autoleveler drawframe can be used, if at all, only as the first drawframe passage, because a doubling operation is always needed after this process stage on a succeeding drawframe. However, the faults or the quality deterioration are not leveled out in this second drawframe passage either; they pass into the yarn. The autoleveler drawframe can only be installed as the last passage in the line with an open-loop control device. A further major influencing factor is the leveling speed. Leveling has to be performed so fast that any change in sliver weight will be corrected while still maintaining a safety reserve. This means that the correction speed of the system has to be far faster than the fastest possible change in the sliver cross section.

On the other hand, long-term stability can be improved with closed-loop systems. For this reason, and also because of the lack of self-monitoring in open-loop control, drawframes that operate with this principle can usefully be fitted with a monitoring device having a display. Leveling drawframes of this kind (open-loop control) are mostly used for the second passage, because the piecings have then already been drawn farther apart and faults arising from the first passage can also be compensated. Leveling drawframes with closed-loop control can therefore be used only in the first passage. Since both open-loop and closedloop systems exhibit noticeable advantages and disadvantages, some time ago several manufacturers equipped their leveling drawframes with both systems in combination. Compensation is usually effected in a range of ±25%.