Ultrasonic multilayer paint thickness measurement
AssigneeGeneral Motors Corporation
InventorsFrederick J. Trulson, August F. Scarpelli, George L. Sarosy
Life of this patent
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Abstract
Ultrasonic thickness measurement of layers of paint on a substrate produces pulse echoes indicative of each layer interface. Several ultrasonic waveforms are averaged in a digital oscilloscope and the resultant waveform is analyzed by a computer programmed to recognize wave forms resembling pulse echoes, compare such waveforms to stored reference waveform characteristics, and selecting the waveform having the best fit to the reference. Each selected pulse echo is graded based on amplitude, peak separation, peak symmetry, and spacing from a previous echo to determine the quality of the waveform. Layer thickness is calculated from the spacing between pulse echoes and the velocity of sound in the layer material.
Claims22
A method for measuring layer thicknesses of multiple thin layers separated by layer interfaces comprising the steps:
- transmitting ultrasonic pulses through the layers,
- receiving echo pulses reflected from the layer interfaces,
- deriving from the echo pulses a signal waveform revealing characteristic M-shaped and W-shaped echo pulse shapes and storing the signal waveform as digital data,
- defining characteristic reference shapes,
- determining zones in the signal waveform corresponding to predicted positions of the layer interfaces,
- locating candidate echo pulse shapes in one of the zones,
- grading the candidate echo pulse shapes by comparing the candidate echo pulse shapes to one of the reference shapes and assigning a score to each of the candidate echo pulse shapes based on its fit to the one of the reference shapes,
- selecting one of the candidate echo pulse shapes having the best score and storing a position thereof in the signal waveform corresponding to the location of one of the layer interfaces,
- repeating the locating, grading and selecting steps for at least another zone to select another candidate echo pulse shape and store a position thereof in the signal waveform corresponding to the location of another of the layer interfaces, and
- determining layer thickness form the stored positions of the selected candidate echo pulse shapes in adjacent zones.
A method for measuring layer thicknesses of paint layers between layer interfaces in multilayer paint on a substrate comprising the steps:
- transmitting ultrasonic pulses through the paint layers,
- receiving echo pulses reflected from the layer interfaces,
- deriving from the echo pulses a signal waveform revealing characteristic M-shaped and W-shaped echo pulse shapes and storing the signal waveform as digital data,
- defining characteristic reference shapes,
- determining zones in the signal waveform corresponding to predicted positions of the layer interfaces,
- locating candidate echo pulse shapes in the one of the zones corresponding to the first of the layer interfaces,
- grading the candidate echo pulse shapes by comparing the candidate echo pulse shapes to one of the reference shapes and assigning a score to each of the candidate echo pulse shapes based on its fit to the one of the reference shapes,
- selecting one of the candidate echo pulse shapes having the best score and storing a position thereof in the signal waveform corresponding to the location of the first of the layer interfaces,
- repeating the locating, grading and selecting steps for at least another zone to select another candidate echo pulse shape and store a position thereof in the signal waveform corresponding to the location of another of the layer interfaces, and
- determining layer thickness from the positions of the selected echo pulse shapes in adjacent zones.
In apparatus for measuring layer thicknesses of multiple thin layers separated by layer interfaces, the apparatus having means for transmitting ultrasonic pulses through the layers and receiving echo pulses reflected from the layer interfaces, the improvement comprising, in combination:
- means for deriving from the echo pulses a signal waveform revealing characteristic M-shaped and W-shaped pulse echo pulse shapes, and storing the signal waveform as digital data,
- means for defining characteristic reference shapes,
- means for determining zones in the signal waveform corresponding to predicted positions of the layer interfaces,
- means for locating candidate echo pulse shapes in the zones,
- means for grading the candidate echo pulse shapes by comparing the candidate echo pulse shapes to one of the reference shapes and assigning a score to each of the candidate echo pulse shapes based on its fit to the one of the reference shapes,
- means for selecting one of the candidate echo pulse shapes having the best score and storing a position thereof in the signal waveform corresponding to the location of one of the layer interfaces, and
- means for repeating the locating, grading and selecting steps for at least another zone to select another candidate echo pulse shape and store a position thereof in the signal waveform corresponding to the location of another of the layer interfaces, whereby the difference in the stored positions of the selected echo pulse shapes in adjacent zones indicates layer thickness.
In apparatus for measuring layer thicknesses between layer interfaces of multilayer paint on a substrate by transmitting ultrasonic pulses through the paint layers and receiving echo pulses reflected from the layer interfaces, the improvement comprising, in combination:
- means for deriving from the echo pulses a signal waveform revealing characteristic M-shaped and W-shaped echo pulse shapes and storing the signal waveform as digital data,
- means for defining characteristic reference shapes,
- means for determining zones in the signal waveform corresponding to the positions of the layer interfaces,
- means for locating candidate echo pulse shapes in the one of the zones corresponding to the first of the layer interfaces,
- means for grading the candidate echo pulse shapes by comparing the candidate echo pulse shapes to one of the reference shapes and assigning a score to each of the candidate echo pulse shapes based on its fit to the one of the reference shapes,
- means for selecting one of the candidate echo pulse shapes having the best score and storing a position thereof in the signal waveform corresponding to the location of one of the layer interfaces,
- means for repeating the locating, grading and selecting steps for at least another zone to select another candidate echo pulse shape and store a position thereof in the signal waveform corresponding to the location of another of the layer interfaces, whereby the difference in the stored positions of the selected echo pulse shapes in adjacent zones indicates layer thickness.
Dependencies are inferred from the claim text.