Distance sensor measures sag of web material to control take-up speed
February 3, 2010
A global manufacturer of fibreglass-based asphalt roofing shingles requested a demonstration of the AR1000 laser distance sensor to improve the performance of their production lines. The roofing shingle product is a laminated material produced in a continuous process where long stretches of material are draped between rollers while the material cures and is advanced towards automatic cutting stations. Near the end of the process, before the material is cut into individual sheets, there is a large sag created by the mismatch of speeds between two rollers controlled by variable frequency drives. It is important to properly control this "slack". If there is no sag, the material is held in tension and this could alter the product thickness or at worst, tear it. If there is too much sag in the material loop, the product collects on the floor and could ruin the product prior to packaging.
The speed of the take-up roller is currently controlled using a series of two photo eyes at different heights. If the top photo eye does not detect the presence of the sag loop, it speeds up the delivery of material. If one photo eye detects the material, the delivery and take-up speeds are equal. If the bottom photo eye detects the loop, it means that the loop is too long (low) and the take-up needs to be sped up. The production process creates considerable dust and airborne debris which collected on the photo eye lenses and caused detection problems.
Process improvement using a non-contact distance meter
This customer wants better control over this process and needs a more precise measurement of the depth of the sag in material. The AR1000 was installed above the web process and aimed downward so that the laser spot measures to the bottom of the loop. A 4-20 mA analog signal transmits to a PLC which controls the speeds of the variable frequency drives. Although these laser rangefinders have been successfully implemented in similar applications, the integration engineers wanted to verify that the optical sensor could accurately measure to the dark asphalt material. The asphalt laminate is impregnated with colored granules and their presence offered an ideal measurement surface for excellent reflection of the laser. The device accurately measured the length of the sag loop to within a few millimeters at 5 Hz sampling speed. The orientation of the laser sensor was downward so little dust or debris could find its way up the dust protection tube of the AR1000 and obstruct the sensor lens. To learn more about this application, please contact Acuity laser measurement or the local integrator for the job, Integrity Automation of Telford, PA.
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