The actual working ranges of industrial equipment are increasing constantly. This applies also to web and coil processing equipment being capable of handling very thin and narrow webs but also heavier and broader webs. This results in extremely high requirements regarding the accuracy of web tension measurement and control.

The customary approach to these issues is the use of dual or multiple range force sensors. In such a case the total measuring range is divided in smaller ranges in order to increase the measurement accuracy. In general the solutions require demanding mechanics and are therefore quite expensive. This sensor for the low force range has to be protected from the forces acting on the upper sensor force range. Or the sensor for the smaller forces has to be mechanically removed when working with larger forces.

The selection of the measuring ranges is typically made via the PLC. Often this results in switching and hysteresis problems in the overlapping region between the two sensors. In closed loop control this can lead to considerable difficulties.

HAEHNE has selected a different approach to achieve high resolution and accuracy resulting in a substantially lower cost solution. Typically web tension sensors have an overall measurement error of 1.0 or .5%. If such a sensor is used in lower measuring range, the total error becomes unacceptably large and not suitable for measurement and closed loop control. For this purpose web tension sensors with an overall measuring error of 0,1 % have been designed which improve the measurement results by a factor of 5. This means that now it is possible to measure and control the forces in the lower range precisely. Important is also, that such a sensor is not only protected in the large force range by mechanical stops (tenfold overload of nominal range) but also in the lower force range by the intrinsic nature of the design.

When using HAEHNE fieldbus interfaces such as Profibus or DeviceNet the measurement accuracy for small forces can be improved further by a factor of 2. Here the high resolution measurement signal is linearized in the equipment software by using correction factors in the force range under consideration.

Successful installations are proof that this approach is not only cost-effective but also very a robust under both mechanical and electrical considerations.