Load-lifting mast especially adapted for use with automatically-guided

Abstract

A load-lifting mast especially adapted for an automatically-guided, driverless vehicle has automatic features for ensuring accuracy and reliability of operation despite the absence of a driver. For load-lowering purposes, a slack chain sensor senses whether or not the load-supporting carriage is supported by the mast, and the carriage is withdrawn from the load when no support by the mast is indicated. The slack chain sensor also cooperates with a carriage height control system by overriding it to cause lowering past a target height until the carriage is supported independently of the mast. A carriage height sensor self-calibration system continually recalibrates the height-sensor readings automatically while the mast is in use to compensate for height sensor slip, chain stretching, and other mechanical variables. The stack chain sensor cooperates with the self-calibration system to enable it to reference to the ground or other surface upon which the vehicle travels to compensate for such other variables as tire wear. The mast is preferably powered by an electric motor-driven screw member having a wear-preventing, universal-joint-type connection to the carriage-lifting mechanism to prevent the imposition of unsymmetrical loading on the screw member. The electric motor has field effect transistor controls operable over a wide range of source voltages.