What factors determine the sensitivity of ultrasonic Flaw Detector?

　1. The main reason

　　(1) Transmitting circuit part

　　1. The power supply voltage E is too low

　　2. When the thyristor or thyristor conducts electricity, the internal resistance is too large.

　　3. The repetition frequency of the trigger wave is too high or the load resistance R of the thyristor is too large, so that the capacitor C in the transmitting circuit cannot be charged to the power supply voltage E.

　　4. Capacitor C leakage 5. In the resonant transmitting circuit, the resonant frequency deviates too much from the natural frequency of the probe.

　　(2) Receiving circuit part

　　1. Electronic tube or transistor aging.

　　2. The tube or transistor operating point (bias current or bias voltage) is not at a good point.

　　3. Low power supply voltage (filter capacitor leakage)

　　4. The tuning circuit is disordered, and the amplification frequency is inconsistent with the transmission frequency.

　　(3) Probe part of ultrasonic flaw Detector

　　1. The sensitivity of the chip is low (the product of the sensitivity of transmission and reception is low).

　　2. The natural frequency of the chip is inconsistent with the emission frequency or amplification frequency.

　　3. The silver layer of the wafer falls off

　　4. The protective diaphragm is worn.

　　5. The oil layer between the protective film and the chip is lost (the protective film of some probes is directly bonded to the chip).

　　6. The damping of the absorbing block is too large.

　　7. The impedance of the chip does not match the impedance of the probe lead (such as connecting a high-impedance quartz chip with a low-impedance cable).

　　8. The probe has low detection efficiency on high-temperature working objects exceeding its Curie temperature.

　　9. Poor contact between probe wire and socket head.

　　(4) Oscilloscope tube part

　　1. Low deflection sensitivity.

　　2. Accelerator voltage is too high.

　　2. Inspection method

　　(1) Check the transmitter circuit

　　1. Use a multimeter to measure whether the power supply voltage of the ultrasonic Thickness Gauge is too low

　　2. Use a multimeter to measure whether the average value of the screen (anode) voltage of the thyristor (SCR) is high or low

　　3. Check whether the capacitor C is leaking

　　4. Use a pulse oscilloscope (or use a heterodyne wavelength meter) to measure whether the frequency of the emitted wave is consistent. When checking with a test block, put the probe on the test block, it is the reflected wave on the fluorescent screen, adjust the iron core of the coil L in the transmitting circuit, the reflected wave is the highest

　　(2) Check the receiving circuit

　　1. Measure whether the voltage at all levels is normal.

　　2. Check the tuning frequency and magnification, there are several methods:

　　(1) Connect the output terminal of the high-frequency signal generator to the input terminal of the receiving circuit, connect the voltmeter to the output terminal of the detection circuit, change the frequency of the high-frequency signal generator (the output voltage remains unchanged), and write down the voltmeter value. The frequency at which the voltage is at its maximum should match the transmission frequency, and record the magnification. If the frequency deviation is too large, it is necessary to adjust the coils at all levels of the flaw Detector.

　　(2) Put the probe on the test block so that there are reflected waves on the fluorescent screen, and use an oscilloscope to observe whether the waveforms of the screen electrodes (or collectors) at each level are larger than the first level (from the front level to the back level), if there is a level The waveform is not large enough, indicating that there is a problem with this stage.

　　(3) If the amplifying circuit is single-tuned, place the probe on the test block and adjust the iron cores of the coils at all levels to make the reflected wave the highest.

　　3. Check whether the electronics or transistors are aging.

　　4. Adjust the working point (the cathode resistance of the electron tube, the emitter resistance of the transistor and the base voltage divider resistance).

　　5. Check whether the tuning coil is moldy or broken.

　　(3) Check the probe part

　　1. First use a good probe to detect. If the sensitivity of the ultrasonic flaw Detector is improved, it means that there is a problem with the original probe and needs to be replaced.

　　2. If the chip is degraded, it can be re-polarized. Some instruments have a polarized socket. Insert the probe into the socket and turn on the power for ten minutes. If there is no polarized socket, an appropriate voltage can be drawn out in the high-voltage circuit of the oscilloscope tube inside the flaw Detector. The polarization voltage of barium titanate is 2000 volts per millimeter and that of lead zirconate titanate is 4000 volts per millimeter. When the wafer is polarized for the first time, the wafer is clamped in the fixture, and invaded into a container containing transformer oil, and the container is placed on an electric furnace, heated to the Curie temperature (barium titanate 120°C, lead zirconate titanate 300°C ), turn on the voltage, cut off the power supply of the electric furnace after 20 minutes, let the transformer oil cool down to room temperature naturally, and then turn off the high voltage.

　　3. The piezoelectric performance of the chip can be checked by a simple method, that is, use a high-sensitivity tube voltmeter to check, press one end of the ground wire on one end of the chip, press the gauge stick slightly on the other end of the chip, and watch the deflection of the pointer of the voltmeter Whether it is large, if there is no deflection or the deflection is very small, the piezoelectric performance is not good.