Dual function sensor for a pen plotter

Abstract

In a pen plotter having a plotting head carrying one half of a pen exchanging mechanism including a holding plate pivotally attached to a plotting head for pivotal movement to raise and lower a pen gripped therein and carried thereby a spring-biased gripping finger pivotally carried by the holding plate for releasably holding a pen, this is a dual-function sensing apparatus for providing a single output signal indicating pen presence and pen up/down position. There is an optical sensor comprising a light source and a light detector separated from the light source by a space and having a sensing surface upon which a beam of light impinges and wherein the level of signal output is a function of the amount of light impinging on the sensing surface. There is a light blocking blade for selectively blocking portions of the beam of light and an activation linkage connected between the gripping finger and the light blocking blade for selectively blocking portions of the beam of light with the light blocking blade so that with no pen being held the signal output from the light detector is a first possible value, with a pen being held in a lowered position the signal output is a second possible value, and with a pen being held in a raised position the signal output is a third possible value.