It is the detector adjusting the edge of the web in line as light source or ultra sonic emitting sensor, so detects the edge of the web without touching them.

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And so the signal on the receipt part detects the position of the web, amplified power at the receipt part provides voltage to moving coil. After that the voltage provided to moving coil is transformed to mechanical power, which moves spool, sends pumped oil pressure from hydraulic pump to cylinder, and moves the opposite wheels of un-winder and rewinder left and right, and positions the web at the center of detector sensor. If the edge of the web is located at the center of sensor, voltage is not supplied to moving coil, so spool would maintain the balance. In case of opaque materials, light emitting part is consist of LED and receiving units is consist of CDS as so that is semi-permanent and excellent detecting.

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You can call "Line Position Controller" or "Edge Follow Controller"

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Please see the full product brochure here.