Machine Design

Sensor Sense: Finding sheets with vision sensors

Edited by Robert Repas

Vision sensors can be as simple as an inductive proximity device or can include multiple circuit boards connected to a PC. A vision system combines the two technologies to reduce an intricate decision using mathematical evaluations and complex algorithms to simple pass or fail logic.

In simplest terms, a sheet-verification sensor uses a vision system taught to recognize the contents of a page in a book. The camera takes pictures of every page as the book is assembled and compares those pages to the one it was taught. If the page is the same, the system generates a pass signal; if the pages are different, it sends a fail output, stopping the machine until an operator fixes the problem.

A sheet-verification sensor doesn’t check print quality. It only determines if the right sheet is in the proper sequence in the machine. It records only a small 2-in.2 image of the page holding enough detail to guarantee uniqueness compared to all other sheets in the book. Typically, during the teach-in process, the system takes multiple pictures along a sheet. The system then determines which image is the best one to use for sheet verification. Many times the camera images the same area twice while adjusting illumination between takes for best picture quality.

Inputs and outputs connect directly to the sheet-verification sensor so it can operate as a stand-alone unit. A photoelectric trigger activates the sensor when the sheet is about to pass in front of the camera and turns it off when the sheet leaves. An encoder that connects directly to the sensor gives a pulse-train output as the machine moves the paper forward. These pulses let the sensor determine sheet width and exactly where to photograph the page.

Much of the intelligence that was previously required from PC workstations, custom configuration software, or expensive middleware, has now been transferred to the vision sensor itself. This reduces the installation time, system complexity, and overall cost of machine ownership.

Pepperl+Fuchs supplied information for this column.

© 2011 Penton Media, Inc.

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