New Slitter Positioning System Uses Linear Sensor to Shave Setup Time

Pulp & Paper, Oct 2004 by Edeal, David, Turcotte, Jeffrey

While 0.02 in. may seem like a small value for a slitting machine, these machines can process hundreds of feet of paper per minute, so a small amount of positioning error can quickly add up to a lot of wasted material. Within the R-Series Profibus microprocessor, there is a code that enables the sensor to automatically correct for linearity errors within the sensing element. This technology allowed the Arkansas mill to purchase a single, accurate device that can replace multiple expensive rotary feedback devices, which are more susceptible to mechanical influences and/or deficiencies.

Previous and competitive designs require that the external sensor interface include a linearity "mapping" or laser test bed calibration to correct for the small linearity errors. This mapping can either be manually entered into parameter files in the machine controller or retrieved from the sensor by the controller prior to the start of normal operation. The problem with the former approach is that the mapping only applies to a particular sensor. Therefore, if the sensor needs to be changed out for any reason, the mapping will have to be replaced, requiring additional setup time that means the machine is not productive. The problem with linearity mapping within the sensor is that it requires valuable memory space, which is typically at a premium in these small-form factor sensors. Fewer measurements can be stored so that less memory is used, but this reduces the sensor output accuracy.

More measurements mean better accuracy, but the increased memory capacity is costly and uses more space. The R-Series Profibus "self-correcting" sensor utilizes a "smart" correction algorithm that, coupled with a unique calibration test procedure, produces higher accuracies within the same sensor envelope without special sensor interface or machine controller requirements or any loss of output signal update rate. This proprietary design enables customers such as slitter manufacturers to use a single, longer sensor (300-600 in.) in place of multiple sensors that have been utilized in the past. Using a single sensor reduces costs and simplifies the machine controller programming because one sensor, depending on length, is usually less than the cost of two high-end encoders.

Positioning System Demonstrates Accuracy

To better meet customer needs for quick turnaround and quality products, the Arkansas paper mill has adopted a variety of new technologies to improve its manufacturing processes. When approached by the controls integrator with the 460-in. single sensor solution, the mill's engineering manager agreed that the technology would achieve the accuracy and reliability goals for the Wartsilla winder to support customer demands. Also, sending out rolls that consistently stay within the allowed specifications was critical to the mill's success since returned rolls cost a great deal in terms of money and reputation.

The new positioning system was installed along with a total rebuild of the PLC/HMI controls on the winder. Ultimately, the positioning portion consisted of two 300-in. and two 460-in. linear position sensors. These devices were mounted in a fixed location on the frame of the machine with the feedback being provided by magnets attached to brackets on the various elements of the slitters and stations. All four sensors were connected through a Profibus network to a communications card that resides in the PLC rack.