Paper feed sensing mechanism for printer

Abstract

Both the operative position of a movable platen and the presence of paper to be advanced past the platen in a line at a time printer are affirmatively sensed by a single mechanism. A cantilevered spring arm extending from a fixed mount includes a terminal switch sensor at its free end in proximity to the paper. At an intermediate region along its length the spring arm traverses adjacent to but spaced apart from the platen, and an adjustable set screw on the arm is disposed for contact with the platen. When the platen is in the operative position, behind the print hammers, and the printer is loaded with paper, the spring arm biases the actuator into contact with the paper, transferring the switch contact to a given position and thereby indicating that both needed prerequisites for printer operation are satisfied. If the platen is disengaged from its normal position, as for paper loading, the spring arm is concurrently moved away from the paper and the switch actuator freed to transfer the switch to its opposite contact. In the absence of paper the switch actuator is also freed regardless of platen position. The length of travel of the spring arm is substantially greater than that of the switch actuator and the spring arm has a greater spring force than the switch actuator arm, insuring both forceful operation of the switch despite variations in paper thickness, and easy vernier adjustment of the trip point of the switch.