Dirt insensitive light beam based limit switch - has variable light beam intensity to give reliable performance by increasing intensity when dirt is present

摘要

A light beam based limit switch for application typically to machine tools, in which a movement of some element must be limited, is relatively insensitive to dirt. The degeneration of the performance of the switches in the presence of dirt is reduced by the use of a variable intensity light source, which can be set either by hand or by an automatic control circuit so that the associated light sensitive receiver always receives an adequate signal. When an upper light intensity limit is reached, an alarm of some kind is actuated, indicating the need for cleaning of the system.