AIR-COUPLED ULTRASONIC INTERFEROMETRY METHOD

Abstract

An air-coupled ultrasonic interferometric method is disclosed. An air-coupled ultrasonic transducer, as a probe, is placed directly facing the surface of a workpiece, and an ultrasonic wave is reflected back and forth between the ultrasonic transducer and the surface of the workpiece; the phase difference of the first echo reflected from the surface of the workpiece and reaching the air-coupled ultrasonic transducer is measured; based on the change of the ultrasonic frequency and wavelength, the measured distance is transformed into the rate of change of the acoustic phase with respect to the acoustic angular frequency, wherein the change in the acoustic angular frequency is a product obtained by multiplying 2π by the difference between the highest frequency F2 and the lowest frequency F1 within the bandwidth fB of the air-coupled ultrasonic transducer.

Claims

1. An air-coupled ultrasonic interferometric method, using an air-coupled ultrasonic transducer as a probe to directly face the surface of a workpiece, characterized in that: an ultrasonic wave is reflected back and forth between the ultrasonic transducer and the surface of the workpiece, and the phase difference of a first echo reflected from the surface of the workpiece and reaching the air-coupled ultrasonic transducer is measured; based on the change of the ultrasonic frequency and wavelength, the measured distance is transformed into the rate of change of the acoustic phase with respect to the acoustic angular frequency, wherein the change in the acoustic angular frequency is a product obtained by multiplying 2π by the bandwidth f.sub.B of the air-coupled ultrasonic transducer, and the change in the acoustic phase is a product obtained by multiplying 2π by the difference between the sound paths at the upper and lower boundary frequencies of the bandwidth respectively divided by the wavelength, wherein the relationship between the rate of change of the acoustic phase with respect to the acoustic angular frequency (i.e. the ratio of the acoustic phase difference to the acoustic angular frequency difference) and the sound path is as follows:
L=λ●(ΔΦ/2π)/(Δf/f)=c●ΔΦ/Δω, where ΔΦ is the acoustic phase difference, and Δω is the acoustic angular frequency difference; wherein the rate of change of the acoustic phase with respect to the acoustic angular frequency is a constant linearly related to the measured distance, and can also be transformed into the difference between the frequencies corresponding to two adjacent phase periods, having the relationship with the sound path as follows:
L=c●/Δf=c●/(f.sub.2f.sub.1); and wherein if the acoustic angular frequency difference and the acoustic phase difference are replaced by the relationship between the highest frequency F.sub.2 (whose corresponding sound path is an integer multiple of the wavelength) and the lowest frequency F.sub.1 (whose corresponding sound path is an integer multiple of the wavelength) within the bandwidth fs of the air-coupled ultrasonic transducer, the formula of the sound path will be converted as follows:
L=c●(N.sub.2N.sub.1)/(F.sub.2=F.sub.1).

2. (canceled)

3. (canceled)

4. (canceled)

5. The air-coupled ultrasonic interferometric method according to claim 1, characterized in that: the ultrasonic continuous excitation time at a fixed frequency of detection is greater than the time of the first echo and less than the time of the second echo.

6. The air-coupled ultrasonic interferometric method according to claim 5, characterized in that: the ultrasonic wave is used for frequency sweep detection, then a two-dimensional diagram is made with a sweeping angular frequency as the abscissa and the integral of the absolute value of a time-domain signal allowing interference to occur as the ordinate, and then a sine wave change curve presented in the two-dimensional diagram is fitted with the sine wave function regression algorithm; the span of the frequency sweep is inversely related to the error of the fitting data; the period of the fitted sine wave function is the difference 2π(f.sub.2−f.sub.1) between the angular frequencies corresponding to two adjacent phase periods, and the acoustic phase difference ΔΦ is 2π at this time.

7. The air-coupled ultrasonic interferometric method according to claim 6, characterized in that: the range of the time-domain signal allowing interference to occur is the time interval from the time of the second echo to the ultrasonic continuous excitation time plus the time of the first echo.

8. The air-coupled ultrasonic interferometric method according to claim 7, characterized in that: according to the error analysis method, the relative error formula of the sound path is δL/L=δ(F.sub.2−F.sub.1)/|F.sub.2−F.sub.1|=(|δF.sub.2|+|δF.sub.1|)/|F.sub.2−F.sub.1|.

9. The air-coupled ultrasonic interferometric method according to claim 8, characterized in that: the absolute error formula of the sound path is δL=L●(|δF.sub.2|+|δF.sub.1|)/|F.sub.2−F.sub.1|.

10. The air-coupled ultrasonic interferometric method according to claim 9, characterized in that: the phase difference of the first echo reflected from the surface of the workpiece and reaching the air-coupled ultrasonic transducer is Φ=2πL/λ, where λ the wavelength, and L is the sound path of the ultrasonic wave emitted from the transducer to the surface of the workpiece and then reflected back to the transducer and is equal to twice the measured distance.

11. The air-coupled ultrasonic interferometric method according to claim 10, characterized in that: when the ultrasonic frequency changes, the wavelength and phase will change; the relationship between the difference between the phases before and after the change and the difference between the frequencies before and after the change is $\Delta \Phi =\frac{2\pi L}{c}\Delta f.$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below with reference to drawings and examples:

[0032] FIG. 1 is a schematic diagram of ranging of the air-coupled ultrasonic probe;

[0033] FIG. 2 is a physical image of the air-coupled ultrasonic probe;

[0034] FIG. 3 shows the frequency of coherent construction generated by continuous excitation of 120 cycles at 399.8 KHz and 50 Vpp;

[0035] FIG. 4 shows the frequency of coherent cancellation generated by continuous excitation of 120 cycles at 402 KHz and 50 Vpp;

[0036] FIG. 5 shows the frequency for measurement by the time difference method generated by continuous excitation of 3 cycles at 399.8 KHz and 50 Vpp; and

[0037] FIG. 6 shows the variation of the average value of the integral of the absolute value of the amplitude in the coherent region obtained when the 550 KHz transducer is located 25 mm above the workpiece with frequency.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] Example:

[0039] An air-coupled ultrasonic transducer is placed vertically facing the free-form surface of a measured workpiece, as shown in FIG. 1. A focusing transducer can reduce the focal spot and improve the lateral resolution, and an air-coupled ultrasonic planar transducer with small diameter and high frequency can also achieve similar effects. The air-coupled ultrasonic transducer has its outer diameter generally in the range of 10-50 mm (as shown in the physical photo in FIG. 2), and it can be directly installed as a probe on a three-axis coordinate measuring machine through a suitable fixture. The focusing transducer has its focal spot generally in the range of 1-5 mm, and its focal length in the range of 5-50 mm; the planar transducer has its wafer diameter generally in the range of 10-50 mm, and its near-field area generally in the range of 10-100 mm. The air-coupled ultrasonic transducer generally uses the planar type in a high frequency band (above 1 MHz).

[0040] An air-coupled ultrasonic automatic sweeping system is selected as the test platform; a piezoelectric air-coupled ultrasonic transducer with the center frequency of 400 KHz is fixed vertically about 42 mm above the surface of the workpiece, and a piezoelectric air-coupled ultrasonic transducer with the center frequency of 550 KHz is fixed vertically about 25 mm above the surface of the workpiece.

[0041] The curves of the acoustic piezoelectric signal emitted and received by the 400 KHz transducer over time is shown in FIGS. 3-5. The data obtained from 3 cycles of excitation in FIG. 5 can be used for rough ranging by the time difference method, with the accuracy of multiple repeated measurements superior to 1 mm. FIGS. 3 and 4 are obtained from continuous wave excitation, wherein the time of continuous excitation is greater than the time of the first echo and less than the time of the second echo, and thus the portion of the continuous excitation exceeding the time of the first echo produces an interference effect with the first echo. Some frequencies are of coherent construction and some frequencies are of coherent cancellation, which are shown in the second echo (as marked by the box in the figures). By obtaining the average value of the integral of the absolute value of the amplitude in the coherent region, a curve that varies with frequency in a manner similar to a sine wave will be obtained. FIG. 6 shows the variation of the average value of the integral of the absolute value of the amplitude in the coherent region obtained when a transducer with a higher frequency of 550 KHz is placed about 25 mm above the workpiece with frequency. In practical application, the curve can be band-pass filtered once into a sine wave curve that is smoother and easier to process.

[0042] From FIG. 6 it can be estimated with the naked eye that the interference phase in the range of about 530-570 KHz has changed for 6 cycles. By substituting the data into formula (10), the following formula can be obtained:

[00001]$\begin{array}{cc}L=c.Math.\left(N_2-N_1\right)/\left(F_2-F_1\right)=340m/s\times \frac{6}{\left(570-530\right)\mathrm{KHz}}=51\mathrm{mm}& \left(19\right)\end{array}$

[0043] That is to say, the distance between the transducer and the measured point on the workpiece is (51/2) mm=25.5 mm, which is close to the approximate position of 25 mm where the transducer is placed during the experiment. Calculations with an accuracy of the order of micron can be achieved by extracting more accurate frequency data from FIG. 4. For example, by filtering the curve in FIG. 6 and then regression-fitting it with a sine wave function, an accurate fitted sine function period can be obtained. The period is multiplied by 2π to get Δω when ΔΦ is equal to 2π in formula (5), so the sound path L can be calculated by formula (5).

[0044] When the transducer reduces the frequency to the range of 40-100 KHz and works in a position 1-20 m from the workpiece, the author can quickly obtain an absolute error that is usually superior to the order of 1% of one wavelength in the actual experiment.

[0045] During frequency sweeping, in order to increase the ranging speed, the frequencies within the bandwidth except the two ends of the bandwidth can be used to quickly sample and sweep frequency according to the Nyquist's minimum sampling theorem, so as to determine N.sub.1 and N.sub.2 in formula (10). Before the coherent signal is obtained by the aforementioned continuous emission, the rough distance L.sub.C can be obtained by low-period pulse emission according to FIG. 3, and then the continuous maximum frequency sweeping step F.sub.S in the bandwidth can be obtained according to the following formula:

[00002]$\begin{array}{cc}\mathrm{Fs}>2\times \frac{c}{L_C}& \left(20\right)\end{array}$

[0046] In order to further increase the ranging speed, the frequency step sweep in the middle of the bandwidth can also be omitted. The period in FIG. 4 can be roughly estimated as c/L.sub.C, from which the initial values of N.sub.1 and N.sub.2 in formula (11) can be obtained. In this way, finely sweeping in one period at a frequency at both ends of the bandwidth is sufficient to accurately determine F.sub.1 and F.sub.2 in formula (10).

[0047] In the high-precision air-coupled ultrasonic ranging interferometric technology proposed by the present invention, the relative accuracy of the measurement is equal to the ratio of the error of frequency measurement by electronic equipment to the specific applied frequency band of the air-coupled ultrasound. In practical engineering applications, the accuracy of frequency measurement is very high, and the relative frequency band of the air-coupled ultrasonic transducer used in the air is very high, so this technology can provide extremely high ranging accuracy. This technology enables its short-distance ranging accuracy to reach the order of submicron and its long-distance ranging accuracy to be superior to 1% of the wavelength. This ranging technology can be used at a short distance for high-precision sweeping of free-form surface contours, and at a long distance for ultrasonic radar anti-interference ranging for cars in auto-driving, etc..

[0048] The above examples only exemplarily illustrate the principles and effects of the present invention, but are not used to limit the present invention. Those skilled in the art can make modifications and variations on the above examples without departing from the spirit and scope of the present invention. Therefore, it is intended that the appended claims of the present invention cover all the equivalent modifications and variations made by those of ordinary skill in the art to the present invention without departing from the spirit and technical idea of the present invention.