Set point sensing system for numerically controlled machine tools

Abstract

An automatic system for sensing and establishing the set point for numerically controlled machine tools. Before operating a numerical control (N/C) machine tool, such as a milling machine, it is necessary to precisely locate a basic reference point on the workpiece or work holding fixture. The disclosed system comprises a capacitive bridge sensing device positioned at the set or reference point. When a machine tool is positioned precisely at the set point, a null reading is observed on suitable readout means. When the tool is moved from null, a reading proportional to displacement is observed. The sensor basically consists of four individual capacitor plates arranged in a circle around a center point, in 'X' and 'Y' directions. The plates are excited by an audio oscillator and coupled into a bridge instrumentation oscillator. As a grounded machine tool is moved adjacent to these plates the capacitance between the individual plates and the grounded tool varies. In each of the 'X' and 'Y' directions the plus and minus capacitance are differentially measured using the amplifier to observe the phase difference in the bridge. The output of the differential amplifier is a sinewave whose phase is proportional to the grounded tool's displacement from center, which can be displayed on a digital readout device. An operator may then precisely locate the center position by manipulating the N/C machine and observing the readout.