Patent ID: 7573018

Claim:
A monitoring system for a conveyor based transport system that features a series of spaced vertical shafts, each said shaft supporting a least one carrier, said monitoring system comprising: a first optical sensor at a first level where said shafts travel; a second optical sensor at a second level where both empty carriers and objects transported by said carriers may be registered by said second optical sensor, said second level being vertically offset from said first level, and, controls for said optical sensors, said controls monitoring said first optical sensor to determine when a shaft is present and monitoring said second optical sensor to detect objects transported by a carrier on a detected shaft; wherein, said controls distinguish between an empty carrier and a loaded carrier by comparing the ratio of actual time registered by said second optical sensor to actual time registered for a respective shaft at said first level, to a pre-measured ratio of time registered for an empty carrier to time registered for a respective shaft.