Web tension sensors use a voltage or current source to measure web tension parameters. They are used on continuous processing machinery for applications such as printing, slitting, cutting, embossing, punching and forming. Web tension sensors consist of transducers and are used with strain gages, devices that measure changes in electrical resistance. As weight is applied, the transducer beam stretches or compresses and the strain gage measures the corresponding change in electrical resistance. Typically, transducers are placed at each end of an idler roll that is suspended in a machine frame. The initial weight is subtracted from the roller, and the transducers measure the added force placed upon the roller from the web. The output signal that corresponds to the actual tension is then amplified to coincide with the magnitude of the desired tension setpoint.
Selecting Web Tension Sensors
Selecting web tension sensors requires an analysis of dimensional parameters, tensiometer configurations, shaft applications, and mounting options. Roll width and roll diameter are the most important dimensional parameters to consider. Tensiometer configurations use a single roller, multiple rollers, a pair or sensors, or a roller with an integrated sensor. Web tension sensors that are designed for “live” shaft applications sense tension on actuated or powered rolls. By contrast, devices that are designed for “dead” shaft applications sense tension on non-powered or idle rolls. There are several mounting options for web tensions sensors. Some sensors mount between pillow blocks or use cantilevered brackets. Others secure to rollers via a flange or clamp. Web tension sensors that are mounted via studs or threaded into through-holes are also available.
Specifications for web tension sensors include display type, controls, port type, and features. Analog meters, digital readouts, and video displays are common display types. Controls can be adjusted manually or programmed with a computer. Web tension sensors with analog front panels include potentiometers, dials, and switches. Digital devices include a numeric keypad or menus. There are several common port types. Serial communication devices use RS232, RS422, RS485 or universal serial bus (USB) ports to transmit sensor signals over relatively long distances. Devices that connect to a computer through a parallel port are also available. Some web tension sensors are compatible with programmable logic controllers (PLC). Others can be placed on a network or are compatible with transistor-transistor logic (TTL). In terms of features, web tension sensors often include controller functions, pushbutton system calibration, or temperature compensation. Devices that include an integral chart recorder can plot data on a strip chart, in a circular pattern, or on a video display.