Browse Prior Art Database

Double Threshold Reflective Item Sensor

IP.com Disclosure Number: IPCOM000045298D
Original Publication Date: 1983-Mar-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 3 page(s) / 36K

Publishing Venue

IBM

Related People

Cato, RT: AUTHOR

Abstract

This article describes a double threshold reflective item sensor for a checkout scanner where an infrared light source and detector are on one side of the item path and a reflector is on the opposite side. The sensor responds to below-normal voltages produced when an item interrupts the beam path and to above-normal voltages caused by reflections from a shiny item.

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Double Threshold Reflective Item Sensor

This article describes a double threshold reflective item sensor for a checkout scanner where an infrared light source and detector are on one side of the item path and a reflector is on the opposite side. The sensor responds to below-normal voltages produced when an item interrupts the beam path and to above-normal voltages caused by reflections from a shiny item.

Referring to the drawing, supermarket checkout scanners may have a reflective item sensor including an infrared light source 10 and a photodetector/amplifier 12 on one side of an item path indicated by arrow 14. Light emitted by source 10 is reflected from mirrors 16 and 18 unless the beam path is interrupted by an item being moved along path 14. For ease of illustration, the mirrors 16 and 18 are shown as being widely separated. In fact, they would be much closer so that an item would normally interrupt both transverse light paths simultaneously.

Conventionally, an item sensor responds to a below-normal voltage produced by the photodetector/amplifier 12 when the beam paths are blocked. Reflections from shiny items may, however, cause a greater than normal amount of light to become incident on the photodetector/ amplifier 12. Since a conventional item sensor responds only to below-normal voltages, such shiny items are never "seen".

The item sensor to be described responds to voltages falling both above and below a dead band centered on a normal voltage Vn. The light source/detector are digitally calibrated. The output of the photodetector/ amplifier circuit 12 is applied to the negative input of a comparator amplifier 20, the positive input of which is connected to a Vn reference node in a voltage divider circuit. When the system is first powered up, the state of open collector driv...