FEATURE Web Tension Control & Guiding The importance of load cells
The trend towards faster, wider web width machinery and the processing of a broader range of substrates has resulted in a greater emphasis being placed on tension control equipment, says Montalvo
Montalvo is one of the industry leaders in web tension control, and for over 65 years has
specialised in manufacturing web control products and offering complete services and support in major industrial markets worldwide. Its products lines range from analogue and digital tension controllers, load cells, range expanders and sensors. Henning Pedersen is a Montalvo applications specialist and electro- mechanical engineer; he is also responsible for sales, service and technical support for Montalvo Europe. Here, he explains how important the measurement of web tension is in effective production.
Continual management Without an accurate method of measurement and control, product quality and productivity will be affected, and web breaks are costly affairs. In order to control web tension, there must be continual and precise measurement in the web in a format that is meaningful to the process line. Electronic load cells are at the beginning of any sequential measurement process; they convert tension to electrical signals, and these determine the effectiveness of stable tension. If the signals are imprecise or fail because of poor operating conditions, the fault becomes apparent very quickly. Converters are finding that production
requirements are becoming more demanding and the need for greater material flexibility and faster line speeds is increasing; this necessitates wide measurement ranges, signal stability and long-term reliability.
Montalvo load cells (transducers) are mounted on a sensing or measuring roll in continuous contact with the web material. System output signals are relayed to tension controllers or indicators. They are designed to operate over wide temperature ranges without electronic temperature compensation. Overall system resolution with a load cell system can be as good as one to two per cent of load rating.
The load cell is the most vulnerable component of a measurement system. They are exposed to the rigors of the operating environment and consequently must be able to accommodate high-speed resonance changes without experiencing signal distortions. Failure in the quality of the signals cannot be compensated for by any other tension component. The importance of load cells is
disproportionate to their size. Manufactured to tight tolerances, they need to be sturdy, durable and able to withstand jolts, overloads, EMC disturbances and occasional operator mishandling. In addition, they must be sensitive and quick in determining tension fluctuation. Montalvo load cells use materials such as stainless steel, which can resist even aggressive environments.
Load cell control methods Operation of a Montalvo load cell begins with an excitation voltage applied to the load cells wired in the Wheatstone Bridge configuration. Force due to web tension deflects the load cell beams, causing compression and tensile forces on the beams. The end result is an output voltage
A Montalvo tension control package
directly proportional to web tension. Excitation voltage can be supplied by a wide variety of electronic packages largely dependent upon the task at hand. For example, if a drive requires a tension trim signal, inexpensive amplifier modules, such as a Montalvo M-3200 or Montalvo A-4 load cell amplifier module, provide excitation and calibration circuits. This type of device also provides current voltage and meter output proportional to web tension.
Load cell system operation ‘controlling an unwind brake’ Excitation voltage is sent to a set of load cells; web tension forces create output changes that are returned to the controller. This output is compared to a tension set point established by the operator. The controller compares actual web tension to the desired set point and sends corrective output to the I/P Converter for brake air pressure regulation, or to a voltage converter for an electric brake. Since the web itself is part of the feedback loop, the control can compensate for speed and roll diameter changes.
The same principles apply to drive applications for unwinds and rewinds. Intermediate systems differ slightly. These applications require a speed reference from a tachometer on the ‘master’ drive; together with load cell input a corrective trim signal is sent to the slave for tension control.
Standard Mount Dead Shaft-Idler Load Cells (left) and a T/er-Series Live Roller Load Cell 20
May 2015
Montalvo T: +1 1800 226 8710
www.montalvo.com
www.convertermag.co.uk
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