The yarn tensioning system can be used to produce carved carpets composed of high and low rings. The colors in the low rings can be buried under the high rings of different colors to produce a two-color design. Yarns at higher tension tend to produce lower coils than when tension is reduced.
Many alternative mechanisms can be used to control and change yarn tension, thereby changing pile height. Figure 13.9 shows a relatively simple yarn tension system in which the different tension applied by the eccentric disc to the yarn as the yarn rotates will lead to the formation of alternating high and short coils. The angular position of the disc can be changed to produce a series of high / low pattern effects.
More complex pattern systems use groups of computer-controlled rollers running at different speeds to change the tension of a single yarn.

Figure 13.9 Eccentric cam yarn tensioning system for producing variable height loops.
Source: Cobble Blackburn Ltd.
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