Web tension indicators measure and report tension parameters in web or other flat-sheet production processes. Some web tension indicators also provide rate control, basis weight control, and thickness control. Devices with totalizer, web alignment, and web breakage detection capabilities are also available. Web alignment refers to the placement of a sheet or web on the rollers. Sometimes, one edge is monitored and kept in the same relative place throughout the cycle. Products with breakage detection can sense a zero-tension (breakage) event and send signals or sound alarms to shut down the process. Web tension indicators with data logger and chart recorder capabilities are also available. Units that are sold in Europe comply with the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronics Equipment (WEEE) directives from the European Union (EU).

Control Methods

Web tension indicators use several different control methods. Limit control (off-on control, bang-bang control) establishes set points or limits that, when reached, causes the web tension indicator to send a signal to stop or start a process variable. Linear control matches a variable input signal with a correspondingly variable control output. Signal conditioning, filtering, and amplification are used to produce the proper output control signal. Proportional, integral, and derivative (PID) control requires real-time system feedback. Feedforward control provides direct-control compensation from the reference signal. This control technique is either open-loop or used in conjunction with more advanced PID controls. Fuzzy logic is a type of control in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Advanced or nonlinear controls for web tension indicators use algorithms for adaptive gain and neural networking.

Performance Specifications

Performance specifications for web tension indicators include the number of inputs, the number of outputs, input types, output types, and the number of zones (if applicable). The number of inputs is the total number of signals sent to the web tension indicator. The number of outputs is the sum of all outputs used to control, compensate or correct the production process. Input types for web tension indicators include direct current (DC) voltages, current loops, analog signals from resistors or potentiometers, frequency inputs, and switch or relay inputs. Output types include analog voltages, current loops, switch or relay outputs, and pulses or frequencies. Some web tension indicators can send inputs or receive outputs in serial, parallel, Ethernet or other digital formats which indicate a process variable. Other products can send inputs and receive outputs from information that is converted to or from an industrial fieldbus protocol such as SERCOS or PROFIBUS^®, a registered trademark of PROFIBUS International.

Selecting Web Tension Indicators

Selecting web tension indicators requires an analysis of form factors and user interface options. Some products are designed to mount on a wall, chassis, or enclosure. Others fit in a standard 19-in. telecommunications racks. Web tension indicators that are designed for personal computers (PC) often have a printed circuit board (PCB) form factor or plug directly into a computer backplane. Standalone, benchtop and free-standing units are commonly available. In terms of user interface options, web tension indicators can feature a digital front panel with a keypad and displays, or an analog front panel with knobs, switches and meters. Suppliers of web tension indicators also offer computer-programmable, Ethernet-ready, and networkable products.