WO2020248516A1 - 空气耦合超声干涉法 - Google Patents

空气耦合超声干涉法 Download PDF

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WO2020248516A1
WO2020248516A1 PCT/CN2019/119709 CN2019119709W WO2020248516A1 WO 2020248516 A1 WO2020248516 A1 WO 2020248516A1 CN 2019119709 W CN2019119709 W CN 2019119709W WO 2020248516 A1 WO2020248516 A1 WO 2020248516A1
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frequency
air
ultrasonic
coupled ultrasonic
difference
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PCT/CN2019/119709
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English (en)
French (fr)
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沈宇平
谢明明
朱绪祥
周新宗
赵军辉
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苏州博昇科技有限公司
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Priority to JP2021573800A priority Critical patent/JP7285029B2/ja
Priority to EP19932344.5A priority patent/EP3982080A4/en
Priority to US17/614,538 priority patent/US11892541B2/en
Publication of WO2020248516A1 publication Critical patent/WO2020248516A1/zh

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B17/00—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations
    • G01B17/06—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations for measuring contours or curvatures
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
    • G01S7/534—Details of non-pulse systems
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
    • G01S15/02—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems using reflection of acoustic waves
    • G01S15/06—Systems determining the position data of a target
    • G01S15/08—Systems for measuring distance only
    • G01S15/32—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
    • G01S15/34—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
    • G01S15/02—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems using reflection of acoustic waves
    • G01S15/06—Systems determining the position data of a target
    • G01S15/08—Systems for measuring distance only
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
    • G01S15/02—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems using reflection of acoustic waves
    • G01S15/06—Systems determining the position data of a target
    • G01S15/08—Systems for measuring distance only
    • G01S15/32—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
    • G01S15/36—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated with phase comparison between the received signal and the contemporaneously transmitted signal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
    • G01S15/88—Sonar systems specially adapted for specific applications
    • G01S15/93—Sonar systems specially adapted for specific applications for anti-collision purposes
    • G01S15/931—Sonar systems specially adapted for specific applications for anti-collision purposes of land vehicles

Definitions

  • the invention relates to the technical field of air-coupled ultrasonic ranging, in particular to the air-coupled ultrasonic interference method.
  • the high-precision measurement method in the free-form surface measurement method of the nano-machined surface of the complex structure generally uses the three-axis coordinate machine technology platform in the industry.
  • This technology platform includes high-precision ruler systems such as linear rulers, precision screw rods, induction synchronizers, grating rulers, magnetic rulers and light wave wavelengths, etc.; including guide rail systems such as sliding guides, rolling bearing guides and air bearing guides; including driving and balancing , Turntable and other technical module systems, plus a three-dimensional probe that is technically independent of other parts of the three-axis coordinate machine.
  • Probes are divided into contact type and non-contact type.
  • the contact probe type probe is the main type of probe currently used, and the measurement accuracy of the probe type is generally not better than the micron level.
  • optical probes are mainly divided into optical, ultrasonic and electromagnetic categories. Among them, optical probes are currently a hot research topic at home and abroad. Optical probes are divided into line structured light method, laser triangulation method, laser semi-focus spot imaging method and so on. At present, the best domestic laboratory indicators seen in the public literature are realized by the semi-spot imaging laser probe. Its repeat aiming uncertainty is the best in the order of microns, and the measurement sensitivity can reach 30mV/ ⁇ m. The inclination angle of the measured curved surface can be up to 25°. However, this method is greatly affected by the type of material. The color of the material and the light absorption properties of the material surface will affect the sensitivity of the measurement, so different standard samples are required for different materials. Sometimes standard samples of materials are difficult to obtain.
  • Laser interferometric probes are mainly most sensitive to small relative changes and insensitive to absolute quantities.
  • the absolute measurement accuracy is restricted by the three-dimensional coordinate aiming of the console.
  • the laser interference probe has very high requirements for the laser reflection characteristics of the surface of the workpiece to be measured. Too high or low reflectivity is not good. Some special absorbing materials need to be replaced with different wavelength lasers. Therefore, laser interferometry has not been widely promoted and applied.
  • the air ultrasonic method mainly uses the time difference method.
  • the higher the frequency the higher the accuracy of ranging.
  • the attenuation of 1MHz high-frequency ultrasonic waves in the air is so great that it cannot be used for common distance measurement.
  • the air-ultrasonic jet-lag method is greatly affected by external interference, so the ranging accuracy of the jet-lag method in practical applications is greatly compromised.
  • the air-coupled ultrasonic ranging and reversing radar widely used in the automotive industry generally uses frequencies in the order of tens of KHz, and its ranging accuracy is usually on the order of one emission wavelength.
  • the ranging accuracy of a 40KHz ultrasonic reversing radar is about 10mm.
  • the technical scheme of the present invention is: air-coupled ultrasonic interferometry. If the air-coupled ultrasonic probe is directly facing the surface of the workpiece, the ultrasonic waves emitted by the ultrasonic transducer will be emitted back and forth on the surface of the workpiece.
  • the wavelength ⁇ of ultrasonic waves propagating in the air is
  • c is the air velocity and f is the ultrasonic frequency.
  • phase difference ⁇ of the first time the echo from the surface of the workpiece reaches the air-coupled ultrasonic transducer is:
  • L is the sound path of the ultrasonic wave from the transducer to the surface of the workpiece and then reflected back to the transducer.
  • the distance from the air-coupled ultrasonic transducer to the workpiece is L/2.
  • f 1 and f 2 are the frequencies corresponding to two adjacent phase periods.
  • formula (6) can also be derived in another method. Assuming that the total sound path at a certain frequency f 1 is an integer n times the wavelength ⁇ 1, then
  • the total sound path at this frequency is an integer (n+1) times the wavelength ⁇ 2
  • the sound path L at this frequency is an integer multiple of the wavelength N 1 ; define the highest frequency F 2 in the bandwidth f B of the air-coupled ultrasonic transducer, The sound path L at this frequency is also an integer multiple of the wavelength N 2 .
  • N 1 and N 2 can be accurately obtained by the acoustic transit method or the interferometric method.
  • usually the difference between the transducer bandwidth and the two frequencies is very small, namely
  • ⁇ L/2 L/2 ⁇ (
  • Formulas (12) and (14) provide us with calculation formulas for estimating the relative accuracy and absolute accuracy of the measurement.
  • Formula (14) shows that in order to improve the absolute accuracy of the measurement, it is necessary to increase the bandwidth f B (or F 2 -F 1 ), reduce the measured distance L/2, and reduce the measurement uncertainty of F 1 and F 2 .
  • the bandwidth can be appropriately relaxed in the air ultrasonic ranging application, for example, -20dB bandwidth can be selected .
  • formula (14) can be organized as:
  • the distance between the transducer and the workpiece is 45mm
  • the frequency step is 10Hz
  • the frequency measurement error is 10Hz, according to the formula (15 ) It can be seen that the distance measurement error is
  • the two-dimensional graph is a curve similar to the change of a sine wave, and a regression algorithm can be used to fit the curve with a sine wave function.
  • the wider the frequency band the more accurate the simulation result.
  • the period of the simulated sine wave function can be regarded as ⁇ when ⁇ is 2 ⁇ in claim 2.
  • the time-domain signal selection range for interference is within the time interval from the time of the second echo to the continuous excitation time of the ultrasonic wave plus the time of the first echo.
  • the ultrasonic technology is based on long-term continuous emission exceeding one echo time and a wide-span frequency scan within a relative bandwidth. It has the advantages of high measurement accuracy and strong anti-interference ability, and its corresponding technology in laser interference is relatively difficult. achieve.
  • the relative ranging accuracy of this technical theory is proportional to the ratio of the frequency measurement error to the bandwidth of the air-coupled ultrasonic transducer.
  • the short-distance ranging accuracy can reach the sub-micron level, and the long-distance ranging accuracy can be better than 1% of the wavelength.
  • the distance measurement technology can be applied to high-precision scanning of free-form surface contours at a short distance, and can be applied to anti-jamming distance measurement by ultrasonic radar during auto-driving.
  • Figure 1 is a schematic diagram of air-coupled ultrasonic probe ranging
  • Figure 2 is a physical picture of an air-coupled ultrasonic probe
  • Figure 3 is a frequency diagram of 399.8KHz, 50Vpp, 120 cycles of continuous excitation to produce coherent phase length;
  • Figure 4 is a frequency diagram of coherent cancellation caused by continuous excitation at 402KHz, 50Vpp, and 120 cycles;
  • Figure 5 is a frequency diagram of 399.8KHz, 50Vpp, 3 period continuous excitation for time difference measurement;
  • Fig. 6 is the variation of the integrated average value of the amplitude of the interference part and the frequency when the 550KHz transducer is placed 25mm above the workpiece.
  • an air-coupled ultrasonic transducer to vertically align the free-form surface of the tested workpiece, as shown in Figure 1. Choosing a focused transducer can reduce the focal spot and improve the lateral resolution; choosing an air-coupled ultrasonic planar transducer with a small diameter and high frequency can also achieve a similar effect.
  • the outer diameter of the air-coupled ultrasonic transducer is generally between 10mm and 50mm, as shown in the actual photo in Figure 2, and can be directly installed as a probe on a three-axis coordinate measuring machine after adding appropriate fixtures.
  • the focal spot of the focusing transducer is generally between 1mm and 5mm, and the focal length is between 5mm and 50mm; the diameter of the planar transducer wafer is generally between 10mm and 50mm, and the near field area is generally between 10mm and 100mm.
  • Air-coupled ultrasonic transducers generally use planar types in high frequency bands (above 1MHz).
  • the air-coupled ultrasonic automatic scanning system is selected as the test platform, and a piezoelectric air-coupled ultrasonic transducer with a center frequency of 400KHz is selected and fixed vertically above the surface of the workpiece about 42mm, and a piezoelectric air-coupled ultrasonic transducer with a center frequency of 550KHz is selected.
  • the transducer is vertically fixed about 25mm above the surface of the workpiece.
  • a 400KHz transducer spontaneously sent and received the acoustic piezoelectric signal with time curve, as shown in Figure 3 to 5.
  • the three-period excitation data in Figure 5 can be used as a rough time difference method for distance measurement, and the accuracy of multiple repeated measurements is better than 1mm.
  • Figures 3 and 4 are continuous wave excitation.
  • the time of continuous excitation exceeds the time of the first echo and is less than the time of the second echo, so the part of the continuous excitation that exceeds the time of the first echo and the first echo occur Interference effect.
  • Some frequencies are coherent and coherent, and some frequencies are coherent and destructive, and are reflected in the second echo (marked by the box in the figure).
  • Fig. 6 shows the variation of the absolute value of the amplitude of the interference part and the frequency change obtained when a higher frequency 550KHz transducer is placed about 25mm above the workpiece.
  • the curve can be band-pass filtered once to become a sine wave curve that is smoother and easier to process.
  • Calculations with an accuracy of the order of microns can be obtained by extracting more accurate frequency data from Figure 4.
  • an accurate fitted sine function period is obtained.
  • the period multiplied by 2 ⁇ is the ⁇ when ⁇ is 2 ⁇ in formula (5), so the sound path L can be calculated by formula (5).
  • the absolute error that the author can quickly obtain in practical experiments is usually better than 1% of a wavelength.
  • the frequencies within the bandwidth can be quickly sampled and swept according to the Nyquist law of minimum sampling to determine N 1 and N 2 in equation (10).
  • the rough distance L C can be obtained by low-period pulse emission according to the figure below in Figure 3.
  • the continuous maximum frequency sweep step F S in the bandwidth can be obtained as
  • the frequency step sweep in the middle of the bandwidth can also be omitted.
  • the period in Figure 4 can be roughly estimated as c/L C , so that the initial values of N 1 and N 2 in equation (11) can be obtained. In this way, a fine scan of one frequency cycle at each end of the bandwidth is sufficient to accurately determine F 1 and F 2 in equation (10).
  • the relative accuracy of the measurement is equal to the ratio of the frequency measurement error of the electronic device to the specific application frequency band of the air-coupled ultrasonic.
  • the accuracy of frequency measurement is very high, and the application of air-coupled ultrasonic transducers in the air has a relatively high frequency band, so this technology can give extremely high distance measurement accuracy.
  • the short-distance ranging accuracy can reach sub-micron level, and the long-distance ranging accuracy can be better than 1% of the wavelength.
  • the short distance can be used for high-precision scanning of free-form surface contours, and the long distance can be used for ultrasonic radar anti-jamming ranging during auto driving.

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  • Engineering & Computer Science (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Acoustics & Sound (AREA)
  • Length Measuring Devices Characterised By Use Of Acoustic Means (AREA)
  • Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)
  • Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)

Abstract

一种空气耦合超声干涉法，包括将空气耦合超声换能器作为测头正对工件表面，超声波在工件表面发生来回反射，测得第一次工件表面反射回波达到空气耦合超声换能器的相位差；基于超声波频率及波长的变化，则将距离的测量转变成声波相位相对于声波角频率的变化率；声波角频率的变化为空气耦合超声换能器带宽f B内最高频率F 2与最低频率F 1之差乘以2π；声波相位的变化为F 2频率下与F 1频率下声程除以波长所得整倍数的差值乘以2π。该超声技术基于长时间的持续发射和大跨度的频率扫描，具有测量精度高和抗干扰能力强的优点，其在激光干涉中的对应技术比较难以实现。

Description

空气耦合超声干涉法 技术领域
本发明涉及空气耦合超声测距技术领域,特别是空气耦合超声干涉法。
背景技术
复杂结构高精度纳米加工以后，需要对表面的自由曲面进行高精度测量。但是静态的自由曲面的高精度测量方法和技术一直是一个难题。超精密加工后表面轮廓精度超精密测量难、测量成本高、表面表征理论和测量方法未有应用性突破。
目前对复杂结构纳米加工表面的自由曲面测量的方法中高精度的测量方法行业内普遍采用三轴坐标机技术平台。这个技术平台包括高精度的标尺系统如线纹尺、精密丝杆、感应同步器、光栅尺、磁尺及光波波长等；包括导轨系统如滑动导轨、滚动轴承导轨和气浮导轨等；包括驱动、平衡、转台等技术模块系统，另外再加上技术上与三轴坐标机其它部分相对独立的三维测头。总体来说，对于三坐标机系统，位置和尺寸测量精度达到微米量级的已经是高端产品，关键要看测头测量精确度的技术水平。测头分为接触式与非接触式。接触式的探针类测头是目前主要使用的测头类型，探针类的测量精度一般不会优于微米量级。
非接触式测头的种类主要分为光学、超声和电磁几类。其中光学类测头是目前国内外研究的热点。光学类测头里面又分为线结构光法、激光三角法、激光半焦斑成像法等。目前公开文献见到的国内最好的实验室指标是采用的半光斑成像式激光测头实现的，其重复瞄准不确定度最好的效果达到微米量级，测量灵敏度可达30mV/μm，激光跟踪瞄准被测曲面倾角可达25°。但是该方法受材料类别影响非常大，材料的颜色以及材料表面的吸光性质都会影响测量的灵敏度，所以对不同的材料需要不同的标准试样。有时候材料的标准试样是很难获取的。
激光干涉式测头主要最对微小的相对变化量敏感，对绝对量不敏感，绝对测量精度受制于控制台的三维坐标瞄准。而且激光干涉测头对被测工件表面的激光反射特性要求很高，反射率太高太低都不好，有些特殊的吸波材料还需要要更换不同波长的激光。所以激光干涉法目前没有受到大规模推广应用。
空气超声法测距主要用时差法。一般来说，频率越高，测距精度越高。但是1MHz量级的高频超声波在空气中的衰减很大，以至于无法用于常见距离的测量。另外空气 超声时差法受外界干扰较大，所以实际应用中时差法的测距精度大打折扣。举例来说汽车行业广泛应用的空气耦合超声测距倒车雷达采用的频率一般在几十KHz量级，其测距精度通常在一个发射波长的量级。比如一个40KHz的超声波倒车雷达的测距精度大约在10mm左右。
发明内容
本发明的技术方案是：空气耦合超声干涉法，如果空气耦合超声测头正对着工件表面，超声波换能器发射出的超声波在工件表面就会发生来回发射。超声波在空气中传播的波长λ为
λ＝c/f    (1)
式中c是空气声速，f是超声波频率。
如果频率变化，空气超声波测头声程不变，但是相位会改变。第一次工件表面反射回波到达空耦超声换能器的相位差Φ为：
Φ＝2πL/λ    (2)
式中L是超声波从换能器到工件表面再反射回换能器的声程，空气耦合超声换能器到工件的距离则为L/2。(1)式代入(2)式中有
Φ＝2πLf/c   (3)
如果超声波频率发生微小变化，相位差也发生微小变化。(3)式对微小频率变化△f展开得到微小相位差变化△Φ：
△Φ＝2πL△f/c＝L△ω/c     (4)
其中△ω为微小角频率变化。继续整理后有
L＝c·△Φ/△ω    (5)
从公式(5)可知，我们把距离的测量转变成了相位相对角频率ω的变化率，这个变化率是一个与被测量距离线性相关的常数。令式(5)中的△Φ为一个周期2π，则式(5)变为
L＝c·/△f＝c·/(f 2-f 1)    (6)
式中f 1和f 2是相邻的两个相位周期对应的频率。
优选的是，公式(6)还可以以另一种方法推出。假设在某一频率f 1下总声程为波长λ1的整数n倍，则
L＝nλ 1＝nc/f 1   (7)
如果连续增加频率，可以得到一个频率f 2，此频率下总声程为波长λ 2的整数(n+1)倍
L＝(n+1)λ 2＝(n+1)c/f 2   (8)
联合(7)和(8)式消去n求解L也可以得到公式(6)。
定义空气耦合超声换能器带宽f B内最低的一个频率F 1，该频率下声程L为波长的整数倍N 1；定义空气耦合超声换能器带宽f B内最高的一个频率F 2，该频率下声程L也为波长的整数倍N 2。我们后面将会说明N 1和N 2可以用声时差法或者干涉法准确地求出。我们后面也将说明，通常换能器带宽与这两个频率的差值相差极小，即
F 2-F 1≈f B   (9)
为了表述方便，我们以后会用频带宽度f B来代替公式(9)中的频率差来说明该频率差的物理意义。把频率差和相位差代入到公式(5)中有，
L＝c·(N 2-N 1)/(F 2-F 1)   (10)
因为c，N 2,N 1都为非常容易确定的常数，由误差分析可知式(10)的总声程相对误差为
δL/L＝δ(F 2-F 1)/|F 2-F 1|＝(|δF 2|+|δF 1|)/|F 2-F 1|    (11)
或者空气耦合超声换能器测头到工件的距离相对测量误差为：
(δL/2)/(L/2)＝(|δF 2|+|δF 1|)/|F 2-F 1|   (12)
总声程L的测量绝对误差为
δL＝L·(|δF 2|+|δF 1|)/|F 2-F 1|   (13)
或者空气耦合超声换能器测头到工件的距离测量绝对误差为
δL/2＝L/2·(|δF 2|+|δF 1|)/|F 2-F 1|≈L/2·(|δF 2|+|δF 1|)/f B    (14)
公式(12)和(14)为我们提供了估算测量相对精度和绝对精度的计算公式。公式(14)说明，要想提高测量的绝对精度，需要增大带宽f B(或者F 2-F 1)，减小被测距离L/2,减小F 1和F 2的测量不确定度。对于空气耦合超声换能器，由于单探头在被测工件表面的反射率几乎为100％，信号非常强，所以在空气超声测距应用中可以适当放宽应用带宽的范围，比如可以选用-20dB带宽。通常商业应用级别的压电型空气耦合超声换能器在空气中的插入损耗-6dB带宽在20％～50％左右，-20dB带宽能够达到100％。因此，公式(14)中的带宽f B可以近似用换能器的中心频率f W代替。不失一般性，公式(14)可以整理为：
δL/2≈L/2·(|δF 2|+|δF 1|)/f w   (15)
举例说明，如果我们选用一个非常常见的300KHz的空气耦合超声压电换能器，换能器到工件的距离选为45mm，频率步进选为10Hz或者说频率测量误差为10Hz，根据公式(15)可知距离测量误差为
δL/2≈45mm·(10+10)/(300k)＝3×10 -3mm＝3μm    (16)
这个测量精度已经远远高于脉冲反射方法。
我们再回头论证一下公式(9)的合理性。由以上所选取的实验参数可知相邻两个周 期的频率差为
(f 2-f 1)＝c/L＝340/(90×10 -3)＝3800Hz＝3.8KHz     (17)
公式(9)中f B与(F 2-F 1)的最大差值百分比为：
2×3.8KHz/300KHz＝2.5％  (18)
所以公式(9)是有根据近似成立的，在说明物理意义时我们可以用带宽f B来代替F 2-F 1的频率差。这进一步说明带宽越宽，干涉测量方法越精确。
我们采用一种特殊的超声波激励方式来实现上述干涉法测量。我们用正负变化的正弦波或者方波一个固定频率持续激发空气耦合超声换能器，激发的时间大于一次回波的时间，小于第二次回波的时间。第一次回波时间和第二次回波时间可以通过一个短周期的脉冲反射方法快速获得。然后我们变换激发频率按照上面的激励方法进行频率扫描检测。以扫查角频率为横坐标，以发生干涉的时域信号的绝对值积分为纵坐标作二维图。则该二维图为一个类似正弦波变化的曲线，可以用一个正弦波函数使用回归算法来拟合该曲线。频带越宽，模拟的结果就越准确。模拟出来的正弦波函数周期可以认为是权利要求2中在△Φ为2π时的△ω。发生干涉的时域信号选取范围为从第二次回波时间始至超声波持续激发时间加上第一次回波时间为至的时间区间以内。
本发明的优点是：
第一、该超声技术基于超过一个回波时间的长时间的持续发射和相对带宽内大跨度的频率扫描，具有测量精度高和抗干扰能力强的优点，其在激光干涉中的对应技术比较难以实现。
第二、该技术理论相对测距精确度正比于频率的测量误差值与空气耦合超声换能器的带宽比值，其短距离测距精度可以达到亚微米量级，长距离测距精度可以优于1％的波长。
第三、该测距技术短距离可以应用于自由曲面轮廓的高精度扫描，长距离可以应用于汽车自动驾驶时超声波雷达抗干扰测距。
附图说明
下面结合附图及实施例对本发明作进一步描述：
图1是空气耦合超声测头测距示意图；
图2是空气耦合超声测头实物图；
图3是399.8KHz、50Vpp、120周期连续激发产生相干相长的频率图；
图4是402KHz、50Vpp、120周期连续激发产生相干相消的频率图；
图5是399.8KHz、50Vpp、3周期连续激发用于时差法测量的频率图；
图6是550KHz换能器置于工件上方25mm时的干涉部分幅值绝对值积分平均值与频率的变化。
具体实施方式
实施例：
选取一个空气耦合超声换能器垂直对准被测工件自由曲面，如图1所示。选用聚焦式换能器可以缩小焦斑，提高横向分辨率；选用直径小的频率高的空气耦合超声平面换能器也能达到类似效果。空气耦合超声换能器的外径一般在10mm到50mm之间，如图二的实物照片所示，加上适当的夹具后可以直接作为测头安装在三轴坐标测量机上。聚焦换能器焦斑一般在1mm到5mm之间，焦距在5mm到50mm之间；平面换能器晶片直径一般在10mm～50mm，近场区一般在10mm～100mm之间。空耦超声换能器在高频段(1MHz以上)一般都选用平面型。
选用空气耦合超声自动扫描系统作为测试平台，分别选择一个中心频率为400KHz的压电空气耦合超声换能器垂直固定在离工件表面大约42mm的上方，以及一个中心频率为550KHz的压电空气耦合超声换能器垂直固定在离工件表面大约25mm的上方。
一个400KHz换能器自发自收的声压电信号随时间变化的曲线，如图3至5所示。其中最图5中3个周期激发的数据可以用作比较粗略的时差法测距，多次重复测量精度优于1mm。图3和图4是连续波激发，连续激发的时间超过了第一次回波的时间，小于第二次回波时间，于是超过第一次回波时间的连续激发部分与第一次回波发生干涉效应。有的频率相干相长，有的频率相干相消，并且在第二次回波时反应出来(如图中方框所标示处)。将干涉部分幅值取绝对值积分平均，就会得到随频率类似正弦波变化的曲线。作为实例，图6为一个更高频率550KHz换能器放置于工件上方约25mm时得到的干涉部分幅值绝对值积分平均值与频率的变化。实际应用中，该曲线可以进行一次带通滤波变成比较光滑和易于数据处理的正弦波曲线。
从图6可以用肉眼估算大约从530KHz到570kHz干涉相位变化了6个周期，将该数据代入到公式(10)有：
Figure PCTCN2019119709-appb-000001
也就是说换能器与工件上被测点的距离为51mm/2＝25.5mm，这与实验设置时放置的大致位置25mm比较接近。精确度达到微米量级的计算可以从图四中提取更加精确的频率数据得到。比如对图6进行滤波后用一个正弦波函数进行回归拟合，得到的精确 的拟合的正弦函数周期。该周期乘以2π即为公式(5)中△Φ为2π时的△ω，从而声程L可以通过公式(5)计算出。
当换能器降低频率到40KHz与100KHz之间工作在与工件距离1米到20米的范围时，作者在实际应用实验中能够快速得到的绝对误差通常优于一个波长的1％量级。
在频率扫描时，为了提高测距速度，除了带宽两端以外，带宽以内的频率可以按照奈奎斯特最低采样定律快速采样扫频以确定方程式(10)中的N 1和N 2。按照前述的持续发射得到相干信号之前，可以先按照图三中的下图以低周期脉冲发射得到粗略的距离L C，根据下式可得带宽内连续最大频率扫描步进F S为
Figure PCTCN2019119709-appb-000002
为了更进一步提高测距速度，该带宽中间的频率步进扫描也可以省略。图四中的周期可以大致估算为c/L C，这样可以得到式(11)中N 1和N 2的初值。这样在带宽的两端各精细地扫描一个频率周期足以精确地确定式(10)中的F 1和F 2。
本发明所提出的空气耦合超声高精度测距干涉技术中，测量的相对精度等于电子设备对频率测量的误差与空气耦合超声具体应用频带的比值。实际工程应用中，对频率的测量精度非常高，加上空气耦合超声换能器在空气中的应用相对频带非常高，所以该技术可以给出极高的距离测量精度。其短距离测距精度可以达到亚微米量级，长距离测距精度可以优于1％的波长。其中，短距离可以应用于自由曲面轮廓的高精度扫描，长距离可以应用于汽车自动驾驶时超声波雷达抗干扰测距等。
上述实施例仅例示性说明本发明的原理及其功效，而非用于限制本发明的。任何熟悉此技术的人士皆可在不违背本发明的精神及范畴下，对上述实施例进行修饰或改变。因此，举凡所属技术领域中具有通常知识者在未脱离本发明的所揭示的精神与技术思想下所完成的一切等效修饰或改变，仍应由本发明的权利要求所涵盖。

Claims (11)

  1. 空气耦合超声干涉法，将空气耦合超声换能器作为测头正对工件表面，其特征在于：超声波在工件表面发生来回反射，测得第一次工件表面反射回波达到空气耦合超声换能器的相位差；基于超声波频率及波长的变化，则将距离的测量转变成声波相位相对于声波角频率的变化率；所述声波角频率的变化为空气耦合超声换能器带宽f B乘以2π；所述声波相位的变化为带宽上边界频率时与带宽下边界频率时声程除以波长所得数值的差值乘以2π。
  2. 根据权利要求1所述的空气耦合超声干涉法，其特征在于：声波相位相对于声波角频率的变化率，即声波相位差相对于声波角频率差的比值，其与声程的公式关系为：
    L＝λ·(△Φ/2π)/(△f/f)＝c·△Φ/△ω，
    其中△Φ为声波相位差，△ω为声波角频率差。
  3. 根据权利要求2所述的空气耦合超声干涉法，其特征在于：声波相位相对于声波角频率的变化率是一个与被测距离线性相关的常数，其也可以转化为相邻的两个相位周期对应的频率之差，其与声程的公式关系为：
    L＝c·/△f＝c·/(f 2-f 1)。
  4. 根据权利要求2所述的空气耦合超声干涉法，其特征在于：将声波角频率差和声波相位差用空气耦合超声换能器带宽f B内声程为波长整数倍的最高频率F 2与声程为波长整数倍的最低频率F 1之间的关系来替换，则声程的公式转化为：
    L＝c·(N 2-N 1)/(F 2-F 1)。
  5. 根据权利要求4所述的空气耦合超声干涉法，其特征在于：超声波在一个固定频率检测时持续激发的时间大于一次回波的时间，小于第二次回波的时间。
  6. 根据权利要求5所述的空气耦合超声干涉法，其特征在于：超声波进行扫频检测，以扫查角频率为横坐标，以发生干涉的时域信号绝对值积分为纵坐标作二维图，对该二维图所呈现的正弦波变化曲线用正弦波函数回归算法来拟合；扫频跨度的大小与拟合数据的误差大小反相关；模拟出来的正弦波函数周期为相邻的两个相位周期对应的角频率之差2π(f 2-f 1)，且此时声波相位差△Φ为2π。
  7. 根据权利要求6所述的空气耦合超声干涉法，其特征在于：发生干涉的时域信号范围为：从第二次回波时间开始，至超声波持续激发时间加上第一次回波的时间为结束的时间区间。
  8. 根据权利要求7所述的空气耦合超声干涉法，其特征在于：根据误差分析法，声程的相对 误差公式为：δL/L＝δ(F 2-F 1)/|F 2-F 1|＝(|δF 2|+|δF 1|)/|F 2-F 1|。
  9. 根据权利要求8所述的空气耦合超声干涉法，其特征在于：声程的绝对误差公式为：
    δL＝L·(|δF 2|+|δF 1|)/|F 2-F 1|。
  10. 根据权利要求9所述的空气耦合超声干涉法，其特征在于：第一次工件表面反射回波达到空气耦合超声换能器的相位差Φ＝2πL/λ；其中λ为波长；L是超声波从换能器到工件表面再反射回换能器的声程，声程等于两倍的被测距离。
  11. 根据权利要求10所述的空气耦合超声干涉法，其特征在于：超声波频率发生变化时，波长发生变化，相位也发生变化；相位变化的差值对频率变化的差值的关系为：
    Figure PCTCN2019119709-appb-100001
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