Study of a capacitive sensing technology for the measurement of perpendicularity of narrow-deep slot-walls in precision molds

This paper presents a novel approach to slot-wall measurement in precision molds. The method measures the perpendicularity of slot-walls in the hard to measure very narrow and deep slots in precision molds. A tabletop hybrid measurement centre combining micro-wire electrical discharge machining (w-EDM) with automatic optical inspection (AOI), and capacitive sensing (CS) is developed. Microprobes are machined using micro w-EDM on the measurement centre. AOI facilitates the designed machine vision system to acquire images from the mold's features. These images are binarized for recognition, which helps the microprobe speedy position into the contour. Capacitive sensing, by which a signal with very low voltage and high frequency is employed between the probe and slot-wall, is conducted for precise detection of perpendicularity. A four-step probe feed approach is proposed for improved measurement accuracy. The technical feasibility of capacitive sensing was experimentally confirmed.