Method and apparatus for detecting the presence or absence of a pen in

Abstract

The present invention comprises an apparatus and method for sensing whether a pen is in the grasp of a pen claw of a drawing arm or a plotter or other graphics mechanism. More specifically, the present invention comprises a pen claw movable between an upward position and an downward writing position, the pen claw being connected to a detector which detects both upward and the downward movements of the pen claw. The pen is provided with an annular ring extending therefrom and the plotter mechanism has a stop provided thereon. The method for sensing the pen's presence comprises the steps of: first, positioning the pen claw in a position proximate to the stop; second, energizing the pen claw until either the pen is substantially impeded by the stop or until the pen claw moves substantially beyond the distance at which it would have stopped if the pen ring had contacted the stop; third, simultaneously with the second step measuring the distance the pen claw has moved subsequent to energization; and fourth, depending upon the distance of movement, providing a signal indicating either that a pen is in the grasp of the pen claw or that a pen is not in the grasp of the pen claw. Thus the present invention provides a system and method for sensing a pen that uses a device for measuring the vertical movement of the pen claw which is already installed on a number of drawing arms.