Effects of Sensor Size

Sensor size directly impacts the resolution of the measurement system. All sensors have an axial length, circumferential width, and radial height, which are collectively referred to as the sensor aperture. Therefore, they provide an average measurement of the flux passing though the sensor. The resolution of a system is defined by the circumferential width of the sensor. The use of narrow sensors improves system resolution by providing more signals for analysis from a given metal-loss region.

Sensor width exerts several effects on characterization. These effects are related to:

  1. The number of sensors that measure the leakage field at a metal-loss region
  2. Possible positions of the sensor relative to the metal loss.
Electronic circuits and packaging are enabling the implementation of smaller and smaller sensors. However, there is a practical limit on minimum sensor width. The information from ten sensors in a metal-loss region exceeds the information gathered by just one, but one hundred sensors may not provide significantly more information. Additional sensors provide little value when they are much narrower than the width of the leakage field. Note that because of circumferential spreading of flux leakage, the width of the leakage field is larger than the defect.

Sensor Position

When the sensor width is on the order of or greater than the width of a defect, flux leakage levels may not be properly measured. In this case, the impact of possible sensor positions relative to a metal-loss region can affect the characterization potential of an MFL inspection system. The amplitude of the measured flux leakage signal depends on the alignment of the centerline of the sensor with respect to the maximum depth of the metal-loss region,

When the maximum depth and the sensor center are aligned, a maximum signal is recorded. Otherwise, a smaller amplitude is measured. When depth estimates are based on amplitude, variations in sensor-to-defect alignment produce variations in depth estimates.

Effects of Sensor Position

The effect of sensor position is quantified in the above figure. These results are for a 2-inch hemispherical pit. The top two graphs represent output from a series of 0.40-inch wide sensors, and the bottom two graphs represent output from a series of 2- inch wide sensors. The left two curves illustrate situations where the centers of the pit and middle sensor are aligned. The right two graphs illustrate situations where two middle sensors straddle the center of the pit. In all graphs, the output of a series of 0.020-inch sensors is shown as a reference.

This figure demonstrates that the measured peak amplitude can be different from the actual peak amplitude. The difference between the two is a function of the defect geometry, the sensor size, and the sensor position relative to the defect. The difference is especially large for larger sensors.

A number of other factors related to sensor width affect the characterization potential of an MFL inspection system. These factors include the amount of information available for estimating metal-loss width and shape from an array of sensors.