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Product Name: Tension Control 101

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A simple overview from DFE project engineer (and staff lecturer), John Volpe What is web tension? In the materials processing industry, words like “stress” and “strain” take on a different meaning from our typical, everyday usage of these terms. I suppose there is a similarity in that they both are associated with the effects produced by extending up to and beyond limits (strain). Stress is the resulting condition of a solid structure (or person) being placed under pressure or strain; and tension is a state that can result from strain and stress. This article attempts to explain in simple terms some of the mechanics of tension control in the industrial sense. “Tension control” as applied to materials processing, refers to the dynamic control of the tension produced by a web (or filament) being pulled, usually through a machine’s processing zone, at a precise magnitude. This is accomplished by either pulling at one end (in a rewind or intermediate zone) and/or increasing the drag at the other end (the unwind zone); the tension changing in response to a comparison between a measured and a desired setting. To investigate the basics of a tension control system, let’s look at a primitive case where we have two ordinary spring scales supporting a roller in a processing zone of interest. The web rides over the roller in such a manner that the tension applied to the web is transferred to the roller. In the case of measuring tension in an unwind zone, a person could be stationed at this zone, reading the scales (a scaleman) and a second person (a brakeman), at the unwind stand, could control a brake to increase or decrease the drag on the web. The scaleman would determine the desired reading on the scales (i.e. 20 lb., 51kg, or 12 stones) in his head, based on his experience with this web material, and as the readings went up and down, he would holler to the brakeman to either increase or decrease the braking force to keep the scale readings at the set point. The scaleman and scale would be acting as a tension measurement system and the brakeman and brake would be the tensioning device. If Benjamin Franklin needed web control on his press, this might have been the method that he would have chosen. However this is not the 18th century, but almost the 21st. Our choices and methods have changed quite a bit since then. In our present world, we have replaced the scaleman and the brakeman with an electronic control system. Tension Transducers Today the scales have been replaced with what we call tension transducers (often, incorrectly, referred to as “load cells”). Transducers are basically tension sensors composed of precisely designed beams that support tiny strain gages. Strain gages are electrical resistors that change their resistance as they are stretched. They act much like a piece of tubing carrying water through it. If you stretched the tubing, the water has a longer and narrower path to travel and therefore a greater resistance. The further the gages are stretched, the higher the resistance. Of course, we are talking about minute stretching in the vicinity of thousandths or millionths of an inch for each inch of strain gage length. Residing inside the transducer housing, the transducer beam compresses and stretches as weight is applied, and the more weight the greater the compressing and stretching. If the strain gages are bonded (glued) to the beam in these compression and stretching areas, then a resistance change is measured that is in relation to the amount of beam compression and stretching. The magnitude of the resistance change is in direct proportion to the magnitude of the weight applied to the beam. If we place transducers at each end of an idler roll and suspend the roll (in a machine frame), we have a replacement for the spring scale in the scaleman/brakeman example. If we subtract out the initial weight of the roller, the transducers can measure the added force placed upon the roller from the web. Now that we have a method of determining the amount of force exerted by the web upon the roller, we have to do something with it. This leads us to a device mysteriously called a PID controller. The Measurement Part Of A Modern Tension Controller In our primitive system, where we had a scaleman who watched the scales and directed the brakeman to adjust torque on the unwind brake, we have an analogy to today’s electronic systems. Electronic control systems, however, can provide far greater performance than a manual system could. An electronic controller contains a circuit that looks at both the desired tension value and the actual tension value. The desired value is set with a potentiometer (appropriately enough called a setpot) by the user. Then as the web machine runs, tension on the roller of interest is measured by the transducers. But before the measured tension is compared to the setpoint value, it must be electronically adjusted to match the reference scale of the setpoint. After all, we should only compare apples with apples. Before this scaling adjustment is made, the measured tension has the roll weight subtracted from it, so as only to count the tension contribution from the web, not the roll. The output signal corresponding to actual tension is then amplified to coincide with the magnitude of the desired tension setpoint. We call the first step of subtracting the roll weight “zeroing”, while the scaling procedure is called “calibrating”. These are two very important aspects of any electronic measurement system. P + I + D: The Corrective Actions of a Tension Controller Proportional Action: the P of PID Once the roll weight and transducers have been zeroed and calibrated, we can compare the electrical signal of the actual web tension to our desired tension setpoint. The result of this comparison is an error signal. Because web processes are dynamic systems, with actual tension changing from moment to moment, except for the moments when actual tension is exactly equal to setp