ABSTRACT
Materials (paper, plastic films, magnetic tape, aluminum foil, sheet metal, etc.) that are manufactured or processed as a continuous web often require that the tension of the moving web be controlled within certain limits. This may be required to prevent breakage, provide uniformly wound rolls of material, allow more rapid production, or increase product quality. Engineers have invented a variety of schemes for tension control of moving webs. 18 Figure 9-45 shows one possibility. Here a part of the processing machinery not shown pulls the web to the right at a speed vi(t). This speed may vary somewhat in an unpredictable fashion, causing the tension T to also vary. A weighted "dancer roll" creates the desired tension and also is used to detect when tension deviates from this value. Ideally, the input velocity vi and the velocity v0 provided by the supply roll being unwound would be exactly equal, the dancer roll would have no vertical motion y, and the tension would be constant at W /2, where W is the effective weight of the dancer-roll mechanism at they location. If, say, vi momentarily increased, the dancer roll would rise, which motion is measured by the displacement sensor. The sensor tells the motor driving the supply roll to increase its velocity so as to match the new vi and thus return the dancer roll to its null position. This action will tend to keep the tension constant, but one must analyze the system to see how well it can be made to work.
