Fine-wire sensors achieve new precision

Manufacturing Engineering, Jan 1999

A new generation of eddy current measuring systems utilizing finewire sensors helps meet the challenge of producing today's precision manufactured high-tech products. Replacing the measurement tools of the past, such as calipers, dial indicators and even micrometers, fine-wire sensors accurately measure microchips and computer parts, aid in physics research, and even monitor rocket engine noise and vibration.

The most critical component of eddy current measurement systems is a sensor made from customdesigned coiled wire. When a high frequency oscillating current is directed through the coil, a varying electromagnetic field surrounds it. When the sensor is placed close to a conductive target (the object being measured, monitored, or fabricated), the impedance of the sensor coil changes. This produces a signal that precisely defines the position of the target in relation to the sensor.

The wire is the heart of the eddy current measuring system and its quality must be absolutely consistent from sensor to sensor and batch to batch. California Fine Wire (Grover Beach, CA), has custommade such wire since 1961.

Making the wire is an exacting process. Initially, large-diameter wire is drawn though a series of progressively smaller diameter dies and rewound onto spools. Each pass through a die further reduces the wire's diameter and dramatically increases its length; a 10 0.020" diameter (3 m, 0.5 mm) wire drawn to half its diameter will create a 0.010" (0.25 mm) wire four times the original length, or 40 (12 m). In one test, shining a standard D-cell flashlight on the beam from a distance of several feet induced almost undetectable localized heating in the beam, causing it to bend 20 to 30 nanometers, a displacement that was easily noted by a sensor. Circle 210.