The basic function of ultrasonic flaw detector is

① The ultrasonic transducer (probe) is excited to generate ultrasonic pulses.

② Amplify the weak electrical signal generated by the ultrasonic transducer.

The ultrasonic pulse is displayed to the user for observation in an appropriate form.

In the traditional ultrasonic flaw detector, the synchronization circuit generates synchronous pulses at equal time intervals, so that all parts of the ultrasonic flaw detector work according to the specified beat (this is one of the important parameters of the ultrasonic flaw detector -- pulse repetition frequency).

The synchronous pulse triggers the transmitting circuit to produce a high-voltage electrical pulse, which is added to the ultrasonic transducer to excite the ultrasonic pulse. In the single probe state these high-voltage electrical and ultrasonic pulses turn the circuit to let the small-amplitude pulse, but not let the high-voltage pulse through, to protect the amplifier from being burned by the high-voltage electrical pulse. After passing through the amplifier, detector and video amplifier, the signal amplitude can reach tens of volts and is added to the vertical deflection plate of the oscilloscope tube.

On the other hand, the synchronization pulse emitted by the synchronization circuit triggers the horizontal scanning circuit, forming the well-known time baseline on the oscilloscope tube. The two work together to form A common A-shaped display.

Familiar transmitting circuits, limit-coupled circuits, and amplifiers, such as synchronous circuits, horizontal scanning circuits, oscilloscopes, detectors, and video amplifiers in Figure 1 are gone, replaced by microprocessors or digital signal processors (DSPS), displays, and analog-to-digital converters. Here, the microprocessor or DSP controls the coordination of the entire flaw detector.

The transmitting circuit, the limiting coupling circuit and the amplifier have the same functions as the analog flaw detector. The pulse output of the amplifier is handed over to the analog-to-digital converter, which converts it into digital signals, and the microprocessor processes these digital signals (such as digital detection, low-pass filtering, etc.) before handing them over to the display. It can be seen that the transmitting circuit, the limiting coupling circuit and the amplifier part are the most similar places between the digital flaw detector and the analog flaw detector. The difference is that in the analog flaw detector, the operator directly toggle the switch to adjust the instrument (such as damping, emission voltage, gain or attenuation, etc.), while in the digital flaw detector, it is by talking to the microprocessor, and then the microprocessor controls these parameters or according to a pre-set program, the microprocessor controls.

From the point of use, these differences are not important for manual inspection, but are very important for mechanical or automated inspection, and automatic control is very necessary. The biggest difference between the digital flaw detector and the analog flaw detector is the control and display of the instrument. The control part of the analog flaw detector is very simple, which only gives a fixed frequency pulse to make all parts of the instrument work synchronously. The display part is similar to the analog oscilloscope, the circuit generates sawtooths to drive the baseline when the horizontal deflection plate of the oscilloscope tube is formed, and the video amplifier amplifies the ultrasonic signal and adds it to the vertical deflection plate of the oscilloscope tube. The ultrasonic signal waveform is displayed on the fluorescent screen of the oscilloscope tube by the combined action of the two.

The digital flaw detector is much more complex, it has to go through an analog/digital conversion and subsequent software processing process. The control and display of the analog flaw detector are mainly different in the aspects of horizontal linearity, vertical linearity, display brightness and appearance. However, the influence of control and display in the digital flaw detector is more complex, which may lead to serious problems such as missed detection and false detection. That's what I'm talking about here. The following is divided into two parts, the * part involves the difference between the two types of flaw detectors in the less transmitting and receiving part, which is the basis of the digital flaw detector; The second part is the analog-to-digital conversion, signal processing and display of the digital flaw detector, which is the core of understanding the digital flaw detector.