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Machine Design

Sensor Sense: Position alignment using vision sensors

Just-in-time manufacturing and a demand for reduced inventory have pushed the adoption of automated storage-and-retrieval systems (ASRSs). These complex systems contain a series of bays for product storage that can stack up to 20-units high.

Tracks located in the aisle between bays steer an automated car as it moves to a desired bin location. To monitor drive position, an encoder or other measurement device guides the vehicle to the destination. Because of belt wear, wheel slippage, or chain expansion over time, the retrieval unit may not end up at the exact location. It may be off by only a fraction of an inch, but that’s enough for the system to malfunction.

A new line of vision sensors has been designed to solve this type of problem. A hole drilled in the horizontal or vertical support member of the storage rack at each bin location acts as a position register. A camera system installed on the ASRS detects the hole position within the camera’s field of view. If it is off center, four digital outputs, +X, -X, +Y, and -Y, tell the system how far and in what direction to travel to center the camera precisely over the hole. Once centered, the bin can be pulled without damage to the rack structure. The ASRS can also use the corrected position to recalibrate itself.

Of course, applications using vision positioning aren’t limited to high-bay warehouses. For example, low-temperature cameras designed for cold storage can track perishable products as they move through a chiller. More-specialized applications can include determining the roundness, diameter, or location of holes in an object without the expense of a full vision system. MD

Pepperl+Fuchs supplied information for this column.

Edited by Robert Repas

© 2011 Penton Media, Inc.

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