Research on calibration method and calibration dev

Development of calibration method and calibration device of non-metallic ultrasonic detector

non metallic ultrasonic detector (hereinafter referred to as ultrasonic detector,) is widely used in concrete construction engineering, rock mass detection, engineering geology, etc., and it is an instrument that uses ultrasonic technology to carry out non-destructive testing of internal defects of concrete. The nondestructive testing of the test piece is completed by transmitting the acoustic pulse signal to the test piece to make it pass through the test piece, and then receiving the acoustic pulse signal after passing through the test piece, and measuring the time, amplitude, frequency change, etc. of the acoustic pulse signal passing through the test piece. The transducers equipped with non-metallic ultrasonic detectors are divided into two types: planar testing transducers (hereinafter referred to as planar transducers) and circular tube radial radiation transducers (hereinafter referred to as radial transducers), which have their own different detection methods and characteristics. In view of the current problem of appropriate calibration methods for radial transducers, an in-depth study and discussion are carried out. Through the analysis and study of the working principle and technical indicators of the ultrasonic detector, the appropriate calibration methods and standard equipment are selected to calibrate the metrological performance of the ultrasonic detector. This paper introduces the calibration method in the local calibration specification of non-metallic ultrasonic detector studied and compiled, as well as the design and development of the acoustic signal method acoustic time (sound wave propagation time in the medium) calibration device

1 metrological performance requirements

according to the working principle and technical characteristics of non-metallic ultrasonic detector, five calibration items are determined: relative error of acoustic time measurement, linearity of amplitude measurement level, emission resonance frequency of radial transducer, stability of emission voltage amplitude, and channel isolation

1,) relative error of sound time measurement. Plane transducer: the measurement error of electrical signal shall not be greater than ± 0.5%, and the measurement error of acoustic signal shall not be greater than ± 1.0%; Radial transducer: the measurement error of electrical signal shall not be greater than ± 0.5%, and the measurement error of acoustic signal shall not be greater than ± 2.0%

2) amplitude measurement level linearity. The error per 10 dB shall not be greater than ± 1.5 dB

3) radial transducer emits resonant frequency. The deviation from the nominal frequency shall be within ± 10%

4) emission voltage amplitude stability. It shall not be greater than ± 5% per hour

5,) channel isolation. For multi-channel ultrasonic detectors, the nominal frequency of the transducer shall be ± 40dB

2 calibration method

2.1 relative error of sound time measurement (electrical signal method)

the transmitting end and receiving end of the ultrasonic detector are respectively connected with the burst end and output end of the signal generator. The transmitting end of the ultrasonic detector and the following are some details we teach customers. Carefully get the burst end of the signal generator. Add an attenuator during the tightness experiment, adjust the signal generator to adjust the frequency to the nominal frequency of the transducer, and the wave number is l0. Adjust the burst delay, every 100 delays μ S ultrasonic detector makes an acoustic time measurement until the upper limit of the ultrasonic detector. With the increase of delay time, the wave number can be appropriately reduced so that the time in the screen can be completely read out. Read the sound time of the ultrasonic detector and the delay time of the signal generator respectively, and calculate the relative error of the sound time of the electric signal method. The relative error calculation formula is as follows:

, where: T1 - the sound time read from the ultrasonic detector

t2 - delay time of signal generator

l -- relative error of electric signal method sound time

2.2 relative error of acoustic time measurement of plane transducer state (acoustic signal method)

the acoustic time of ultrasonic wave in air is relatively stable, but it will change with the change of temperature. Consider detecting the acoustic time of plane transducer state in air. First, measure and eliminate the zero sound, and then align the radiation of the two transducers with each other, so that the axes of the two transducers are in the same straight line, and measure the sound time with a spacing of 100 mm, 200 mm, 300 mm, 400 mm and 500 mm respectively. During the measurement, the overall waveform should be kept at 2/3 of the full screen display. The relative error of sound time measurement is calculated from the measured value of sound time and the theoretical calculated value. The theoretical calculation formula of sound time is as follows: (the sound velocity of air at 0 ° C is 331.4 m/s)

where: T - air temperature

d -- distance between two transducers

the calculation formula of the relative error of sound time is as follows:

where: T1 - the measured value of sound time; T2 -- theoretical value of sound time

l - relative error of plane transducer state sound time

2.3 relative error of acoustic time measurement of radial transducer state (acoustic signal method,)

keep the axes of the two radial transducers parallel to each other and placed at the same horizontal height of the silencing water tank. The spacing between the inner edges of the two transducers is adjusted at L1 and L2 successively (L1, L2 and their differences are not less than 100 mm), and read the corresponding acoustic time values T1 and T2 respectively. The zero sound time to generated by the ultrasonic detector, transducer and its high-frequency cable is calculated by the following formula:

place the radial transducer in the silencing water tank, and the two transducers should be on the same horizontal plane. After the zero sound time is eliminated, measure the sound time with the transducer spacing of 60 mm, 120 mm, 180 mm, 240 mm and 300 mm respectively, and calculate the relative error of sound time measurement from the measured value of sound time and the theoretical calculated value. The theoretical calculation formula of sound time is as follows:

where: D - the distance between the inner edges of two transducers

v -- sound speed in water

formula for calculating the relative error of sound time (3)

2.4 amplitude measurement level linearity

the transmitting end of the ultrasonic detector is connected with the burst end of the signal generator through an attenuator, and the receiving end of the ultrasonic detector is connected with the output end of the signal generator through a precision variable attenuator. Adjust the frequency of the signal generator to the nominal frequency of the transducer, and adjust the output amplitude of the signal generator and the precision variable attenuator to the appropriate position. Increase or decrease the output value of the attenuator by 10 dB in turn. For each measurement, adjust the received signal of the ultrasonic detector to 80% of the full screen for no less than 4 times. The difference between the amplitude displayed by the amplitude measurement function of the ultrasonic detector and the change value of the attenuator is the linear error of the amplitude measurement level

2.5 emission resonant frequency of radial transducer

set the emission voltage, sampling frequency and other parameters of the ultrasonic detector to a certain gear and keep them unchanged. Place the radial transducer and hydrophone in the anechoic water tank, and the hydrophone is connected with the oscilloscope. The oscilloscope reads the frequency corresponding to the hydrophone receiving the most human acoustic signal, that is, the emission resonant frequency of the radial transducer. The relative error of the emission resonant frequency of the radial transducer is:

, where: F1 - the measured frequency, kHz

f2 - nominal frequency, kHz

2.6 emission voltage amplitude stability

connect the transmitting end of the ultrasonic detector with the oscilloscope through the load, adjust the voltage of the ultrasonic detector, and record the amplitude of each voltage with the oscilloscope. Measure every 15 minutes for a total of 5 times. The range of amplitude change of the difference between the maximum value and the minimum value of each gear voltage relative to the average value shall meet the requirements of not more than ± 5% per hour. Voltage amplitude stability:

, where: Vmax - maximum voltage; Vmin - minimum voltage; A - voltage amplitude stability

2.7 channel isolation of multi-channel ultrasonic detector

adjust the frequency of the signal generator to the nominal frequency of the transducer, and set the output of the signal generator to the maximum. Adjust the ultrasonic detector to maximize the gain of channel L, and the received signal is 80% of the full screen. Channel 2 is short circuited to the ground. Record the amplitudes A1 and A2 of channel 1 and channel 2 of the ultrasonic detector. The channel isolation a=a1-a2 should meet the requirements of 40dB

3 calibration device design

3.1 main standards and device technical indicators

1) signal generator. Within the frequency range of the test, continuous wave signal and burst sound signal can be generated. The amplitude accuracy is better than 0.1dB, the harmonic distortion is less than 0.5%, and the amplitude stability during the test is better than 0.1 dB

2) digital oscilloscope. The sampling frequency shall be 8 times higher than the measurement frequency, and the maximum allowable error shall not be greater than ± 0.1%

3) hydrophone. Within the test frequency range, there is basically no directivity. Spherical hydrophone or small cylindrical hydrophone (the free field voltage sensitivity below 70 kHz should be greater than -l 90 dB,) is used to ensure that the signal-to-noise ratio is greater than 30 dB. During the test, the stability of the free field sensitivity of the hydrophone is better than 0.02dB

4) attenuator. The frequency range is required to be DC ~ 30 MHz, and the error per 10 dB shall not exceed 0.2 dB

5) motion control device. Stroke 600 mm, positioning accuracy ± 5 μ m. The load (including the transducer and its mounting clamp and moving slider) is 5 kg

6,) silencing water tank. The silencing water tank can meet the far-field condition of the distance between the two transducers. The maximum allowable error of thermometer shall not be greater than ± 0 2 ℃.

3.2 composition and working principle of calibration device

the calibration device of ultrasonic detector is composed of purchased standard equipment and developed acoustic time (acoustic signal method) calibration device. The standard equipment purchased includes: signal generator, digital oscilloscope, attenuator and hydrophone. The developed acoustic time calibration device is composed of silencing water tank, motion control device, control software, microcomputer and so on. Technical indicators such as acoustic time (electric signal method), amplitude measurement level linearity, emission voltage amplitude stability, channel isolation can be calibrated with standard devices such as signal generator, digital oscilloscope, attenuator, etc. the measurement method is relatively simple, and it is OK to operate according to the requirements of the calibration method, which is not introduced here. You can operate according to the requirements in the calibration method, which is not introduced here. Because the ultrasonic detector mainly depends on the propagation time of ultrasonic wave in concrete to judge the internal defects, acoustic time is a very important parameter. The following describes in detail the developed acoustic time (acoustic signal method) calibration device, which has the characteristics of high accuracy of acoustic time calibration. It is equipped with hydrophone and digital oscilloscope to calibrate the emission resonant frequency of radial transducer

3.2.1 sound time calibration device

the structure of the silencing water tank and motion control platform in the sound time calibration device is shown in Figure 1. The microcomputer controlled sliding block platform is supported above the anechoic water tank by the gantry. One transducer is fixed on the fixture of the gantry fixed block platform, and the other transducer is fixed on the fixture of the sliding block platform. The plane transducer is connected with the plane transducer fixture above the linear motion platform, and the radial transducer is connected with the radial transducer fixture in the anechoic water tank. The distance between the transducers is adjusted by moving the sliding block platform, Realize sound time calibration

in order to better simulate the field conditions of the radial transducer, an anechoic water tank applied in the ultrasonic frequency range is designed and processed. The purpose of the anechoic water tank is to establish a free sound field for the measurement of underwater acoustic time and resonant frequency of the radial transducer. By pasting an anechoic wedge specially used in water in the four walls, this wedge is made of rubber material, which can effectively absorb ultrasonic waves and reduce the impact of reflected waves, Improve measurement accuracy. In acoustic time calibration, a computer-controlled servo motor is used to make the sliding block platform move in a single axis straight line, and the displacement of the grating displacement ruler is fed back to realize the function of linear automatic positioning. The distance accuracy between transducers is the key to the accuracy of acoustic time calibration. The positioning accuracy of the calibration device is +5um, which improves the accuracy of acoustic time calibration. By changing the distance between the plane transducer and the radial transducer of the ultrasonic detector, according to the sound velocity of the transducer in the air and the radial transducer in the water, calculate the standard propagation sound, and then read out the display propagation of the ultrasonic detector