- February 01, 1996, Sarnelli, Robert
A new type of load cell uses an electromagnetic transducer to achieve more reliable tension measurement, reportedly resulting in improved machine performance, application range, and productivity.
Tension control has become an essential part of web processing in the converting industry to meet modem productivity and quality requirements. And the trend toward faster, wider machinery with greater application versatility is making this function even more important. But the trend also poses new technical challenges, which are accentuated by the increased demand for application flexibility in existing web processing lines. One such challenge is in the area of measuring the web tension.
If tension is not controlled, product quality and plant productivity almost inevitably will be affected. Distortions in a material that has been stretched as a result of excessive tension can be just as damaging as folds or creases resulting from tension that has been too low. Web breaks - and consequent shutdowns - caused by inadequate tension control obviously will drag down productivity. Slowing down the line may help, but it also will be at the expense of throughput.
In order to control web tension, you must have continual, precise measurement of actual web tension in a format that is meaningful to the line's tension control system. That system, in turn, regulates tension through the drives or the use of brakes at strategic points in the line to match a preset, ideal web tension level.
The Importance of Load Cells
This vital sequence of measurement and control originates with the load cells. Their transducers convert tension to electrical signals, and the quality of these signals as a measurement of web tension is a key variable in determining the effectiveness of the tension-stabilizing effort. If the signals are imprecise - or if load cells fail frequently due to exposure to operating conditions - the results will soon show up on the winder and in operating inefficiency.
Today's demanding operating requirements are changing the expectations of these vital components and increasing their significance in line operations. Greater flexibility in processed materials and line speeds accentuates the need for wider measurement ranges of individual units, better signal stability, and long-term performance reliability. The ability to achieve adequate measurement accuracy and sensitivity levels is no longer a major concern in specifying load cells; the key questions now are "over how wide a range?" and "for how long?"
Load cells with measurement transducers that operate on the basis of electromagnetic signal generation have been shown to excel in demanding applications in other industries with operating characteristics similar to those found in converting. A new type of load cell based on that technology and ideal for converting web processing lines promises enhanced functionality and performance reliability over what is typically seen in many light- to medium-tension applications today.
Web tension fluctuations result from many events that occur in the course of normal line operation. Such events include switches in processed materials, starts and stops, splices, fluctuations in temperature and humidity, adaptation to roll-diameter changes, and drive variations.
These transient variations can readily be smoothed out, and ideal tension levels maintained, by regulating the speed of one or several AC, DC, or geared stepping motors that typically constitute the drive system. In alternative arrangements, pneumatic or electric brakes might be activated at strategic points in the machinery. In some cases, the web tension control function is integrated with the overall controls of the machine or process.
The Basic System
The measurement system that provides the essential input on web tension deviations basically consists of the in-line load cells that generate the signals in the first place; electronics for a variety of signal enhancements and an overall control function; and, finally, instrumentation.
Load cells are mounted on a deflection (measurement) roll in continuous direct contact with the web material. Depending on design and application criteria, they may be mounted beneath the pillow block bearings at one or both ends of the deflection roll, or directly on the roll shaft end.
The signals from the transducers are enhanced in various ways by the central processing electronics, which, in turn, provide system output signals to tension controllers and operator instruments. To allow for versatility of measurement, these electronics units also provide measurement control functions, and they should be readily expandable with optional functions that can be added after installation of the system.
Another important performance consideration is that signals throughout the system should reach their destinations unimpeded and undistorted by potential electrical interference from plant surroundings.
The load cells themselves are the most vulnerable components of the measurement system, because they are directly exposed to the rigors of the operating environment. They must be able to absorb the consequences of product changes on the line - such as E-stops, resonance at critical speeds, and interference from power cables-without distortion of their signals. And, they should tolerate being subjected to inadvertent abuse from line operation. It's not unheard of, for example, for a machine attendant to use a roll equipped with load cells as a stepladder to reach a certain point of the machinery.
Lapses in the quality of the signals the load cells produce cannot be compensated for by other components of the system. The quality of the signals from each individual transducer has a direct effect on machine operating efficiency and product quality. Failures and unreliable signals inevitably lead to unexpected line stoppages to recalibrate or replace the faulty sensors.
Sensitive and Strong
So, their presence in the manufacturing line calls for the load cells that ideally are sturdy and durable components, able to withstand overloads, jolts, electronic emissions, contamination, vibration, and occasional rugged handling without distorting signals. But load cells must also be sensitive and "nimble," quickly detecting the slightest variances in tension and continuously generating signals that are accurate and stable.
The need for sensitivity is increased, because the material being processed wraps the deflection roll at a certain angle, which affects how much force the transducer actually senses. If the wrap angle is shallow, the force exerted on the load cell will be only a small fraction of total web tension. For this reason, sensitivity to slight variations in force is critical. In addition, changes in the ambient temperature may affect load cell performance and must be compensated for in order to maintain overall system sensitivity and accuracy.
Operating Principle Is Key
In all currently used transducers, such as strain gauge and LVDT devices, the signal is induced through some type of physical movement, such as bending, stretching, or compressing. In one type of strain gauge transducer, for example, a mechanical force applied to the load cell stretches small resistive strain gauges glued to a measurement area in the unit. The stretching alters the electrical resistance of the gauges, which, in turn, gives rise to the electrical measurement signals.
Inducing a signal in this way results in a necessary design compromise. While pliability (ease of deflection) produces enhanced accuracy as well as a range of measurement, it must be balanced against the need for sturdiness, or rigidity, which is required to maintain signal quality when the load cell is exposed to mechanical interference. So the accuracy of this type of load cell cannot be maximized without making it prone to breakdowns, and its durability cannot be maximized without jeopardizing its accuracy. But it is also possible to generate a transducer signal in another way-by measuring magnetic change. And this leads to performance and design attributes that are strikingly different from what converting line operators are used to. Perhaps most importantly, this technique makes it possible to combine the attributes of measurement sensitivity, range, and mechanical rigidity in the transducer design.
In order to produce a signal, this technology relies upon what is known as the "magnetoelastic effect" of steel. This simply means that the permeability of a material for magnetic flux is changed by mechanical force.
Basically, the transducer consists of two perpendicular electromagnetic coils of wire embedded in a block of steel. A high-frequency AC current is run through one of the coils to create an alternating magnetic field whose flux will not intersect with the second coil, as shown in Figure 1a.
When a mechanical force is applied to the block, and its ability for magnetic transfer is diminished, the electromagnetic field becomes distorted, starts to overlap with the second coil, and produces an electrical signal directly proportional to the mechanical force, as shown in Figure 1b. This technique is known as the "Pressductor Principle."
The consequent independence from physical movement in generating a transducer signal produces highly desirable load cell characteristics. The transducer registers a load at a lower stress level and produces a stronger signal; it is more sensitive. Typical output ranges in the hundreds of millivolts AC, whereas conventional devices produce outputs of tens of millivolts DC.
The load cell can be bulkier and stiffer without detracting from its sensitivity or accuracy. In other words, it can be made more tolerant to overloads, shockloads, and vibration, and not at all susceptible to mechanical fatigue. Overloads of 500% or more of nominal ratings (without mechanical stops) will not cause failures or necessitate recalibration. Fatigue failures experienced in deflection load cells as the measurement membranes weaken and fail due to exposure to vibration, especially at machine resonance points, are, by definition, eliminated. Resonance frequencies are kept exceptionally high.
Six to Ten Times Higher
Assuming an infinitely stiff base for an 8-in.-diameter roll yields a resonant frequency at about 60,000 fpm, perhaps six to ten times higher than conventional load cells. The resonant frequency of the measurement system can readily be made high enough to ensure stable performance even at very high application speeds.
In addition, the electromagnetic operating principle yields AC signals - and by design at a high frequency - with exceptionally low electrical impedance, as well as high strength. Combined, these factors yield clearer and more distinct system input that is virtually invulnerable to electrical interference from plant surroundings, so the integrity of the measurement is preserved all the way to the system's signal processing unit. The relatively few turns of wire in the transducer coils keep impedance low. Transducer output at an odd AC frequency (not a multiple of 60 Hz), typically 330 Hz, makes it possible to filter out any interference at higher or lower frequencies before they corrupt the signals being sent to the control units and operator instruments.
And, because magnetic changes within the steel of the transducer are inducing the signals, it's easy to see why load cell functionality is totally unaffected by fluids, dust, etc., that can seep into other types of load cells.
To sum up, the use of electromagnetic technology makes it possible to design load cells with greater physical tolerance, more reliable performance, and wider measurement range.
* Shaft mount for use on dead shaft, live shaft, and driven rolls
* Electromagnetic signal generation technology
* Key system characteristics:
Application range 10 to 20,000 lbs web tension Measurement range 40:1 Stiffness 10x most physically resistive technologies Overload tolerance 500%, without mechanical stops Size 105 to 225 mm in diameter. Temperature stability 0.007%/deg F (50% of physically resistive technologies)
Robert Sarnelli is product manager for sensors at the Weighing & Force Measurement Div. of ABB Industrial Systems, Brewster, NY. He has almost 20 years of experience in the design and application of products and systems for measurement functions in the converting and many other industries. He has published papers on web tension measurement Sarnelli holds BSEE and MBA degrees.