Air-Coupled Ultrasonic Probe Integrity Test Using a Focused Transducer with Similar Frequency and Limited Aperture for Contrast Enhancement

Abstract

Air-coupled ultrasonic probes require a special design approach and handling due to the significant mismatch to the air. Outer matching layers have to be soft so can be easily damaged and excitation voltages might cause the degradation of electrodes or bonding between the layers. Integrity inspection is desired during design, manufacturing, and exploitation. Spatial distribution of a transduction efficiency over piezoelement surface is proposed as a convenient means for the air-coupled probe integrity inspection. Focused transducer of similar center frequency is used to scan the surface of the inspected probe. However, such approach creates a challenge, i.e., area of the scanning beam is much smaller than the total receiving area of the inspected probe, therefore, contrast and imaging resolution are significantly degraded. Masking aperture made from cardboard and felt, placed at the focal point was proposed as solution. Far-range sidelobes were suppressed down to the noise floor (-50 dB) and the near-range sidelobes were reduced down to -17 dB. The proposed modification allows to use a similar frequency focused transducer. Probe integrity inspection can be carried out at significantly enhanced contrast and lateral resolution. Natural and artificial defects can be detected by the use of the proposed method.

Keywords: air-coupled ultrasound; beam sidelobes; focused transducer; probe characterization; ultrasound transducer.

Figure 1

Probe integrity inspection setup.

Figure 2

Focused 0.9 MHz 15 mm diameter dual piezoelement transducer used for transmission.

Figure 3

Reflection mode directivity profile of the unmodified focused transducer: (a) along x-axis at y = 0 mm, (b) along y-axis at x = 0 mm, and (c) 2D gray-scale profile in dB.

Figure 4

Transmission mode directivity profile of the unmodified focused transducer: (a) along x-axis at y = 0 mm, (b) along y-axis at x = 0 mm, and (c) 2D gray-scale profile in dB.

Figure 5

Transducer mask used for sidelobes’ cancelation: (a) construction drawing and (b) photo of the mask mounted on the focused transducer.

Figure 6

Aperture influence on transmission mode directivity profile: (a) along x-axis at y = 0 mm, (b) along y-axis at x = 0 mm, and (c) 2D gray-scale profile in dB.

Figure 7

Air-coupled probes used in experiments for integrity inspection: (a) round wideband 0.65 MHz probe and (b) rectangular single element polypropylene-based 0.32 MHz probe.

Figure 8

Round probe (no artificial defects) integrity test results using unmodified focused transducer: (a) along x-axis at y = 3 mm, (b) along y-axis x = −4.75 mm, and (c) C-scan profile in dB.

Figure 9

Steel spacer as artificial defect placed on probe’s surface.

Figure 10

Round probe (steel spacer as artificial defect) integrity test results when using unmodified focused transducer: (a) along x-axis at y = 2 mm, (b) along y-axis at x = 3.5 mm, and (c) C-scan profile in dB.

Figure 11

Copper tape triangle as artificial defect placed on probe’s surface.

Figure 12

Round probe (copper tape triangle as artificial defect) test results with unmodified focused transducer: (a) along x-axis at y = 2 mm, (b) along y-axis at x = 4.75 mm, and (c) C-scan profile in dB.

Figure 13

Electrode defect image of the rectangular polypropylene-based probe: (a) front view of the whole probe, (b) microscope zoom-in over damaged area under 6× magnification, and (c) microscope zoom-in over damaged area under 24× magnification.

Figure 14

Rectangular probe (no artificial defects) integrity test results when using unmodified focused transducer: (a) along x-axis at y = 5.25 mm, (b) along y-axis at x = −8.25 mm, (c) 2D gray-scale profile in dB.

Figure 15

Round probe (no artificial defects) integrity test results using sidelobes-masking aperture: (a) along x-axis at y = 2.25 mm, (b) along y-axis x = −4.25 mm, and (c) 2D gray-scale profile in dB.

Figure 16

Spectral distribution plots at soldering point location when using sidelobes-masking aperture: (a) along x-axis at y = 2.25 mm and (b) along y-axis at x = −4 mm.

Figure 17

Round probe (steel spacer as artificial defect) integrity test results when sidelobes-masking aperture was used: (a) along x-axis at y = 2 mm, (b) along y-axis at x = 2.5 mm, and (c) C-scan profile in dB.

Figure 18

Spectral scans (steel spacer as artificial defect) when using sidelobes-masking aperture: (a) along x-axis at y = 2 mm and (b) along y-axis at x = 2.5 mm.

Figure 19

Round probe’s (copper tape triangle as artificial defect) integrity test results when using masking aperture: (a) along x-axis at y = 1.25 mm, (b) along y-axis at x = 4 mm, and (c) C-scan profile in dB.

Figure 20

Spectral scans (copper tape triangle as artificial defect) when using masking aperture: (a) along x-axis at y = 1.25 mm and (b) along y-axis at x = 4 mm.

Figure 21

Rectangular probe integrity test results using masking aperture: (a) along x-axis at y = 5.25 mm, (b) along y-axis at x = −8.25 mm, and (c) C-scan profile in dB.

Figure 22

Accumulation of the sidelobe’s signals creates a halo effect if no sidelobe-masking aperture is used (a). Application of the aperture at focal spot location can reduce the halo effect (b).