How ultrasonic flaw detector constructed?

Answer 1

ANSWER I'm assuming you are referring to a machine used to conduct an ultrasonic non-destructive test. Generally speaking, the machine works like this sonar: an ultrasonic transceiver creates a mechanical pulse and measures the time it takes for the echo to return. The echo delay time indicates how far into the material the sound wave went before it was reflected. This reflection happens because the material stops -- usually it's either the far side of the material, or a flaw. For instance, inspecting a solid metal plate of constant thickness is pretty straightforward. Visualize, or mark, an inspection grid on the plate being inspected. Place the transceiver at the first grid location and fire a pulse. Measure the echo time and store it. Repeat for all the grid locations, and then plot the times. The plot could be a three dimensional plot, in which case it would look very much like a scaled drawing of the plate. Or it could be two dimensional and use color to indicate the echo times. Either way, a flaw is indicated where the time changes from the expected value. Naturally, you would typically take multiple readings and average them. You might also calculate the variation in readings to determine how good a measurement you have. Also, there is usually another medium required to help couple the ultrasonic signal into the material to be inspected, called a couplant. It usually is a clear gel. The test could be run on a machine that positions the transceiver (or moves the plate and holds the transceiver still) using an X-Y table. It could also use a line of multiple transceivers and move the plate past them in one direction. Or an entire array of transceivers could be placed over the plate and the entire echo time map could be measured all at the same time. The transceiver is usually driven with a single strong pulse. You usually get better results if the pulse looks more like a sine than a square wave, because square waves include other frequencies besides the one you are driving and these other frequencies use some of the power. The reason to prefer a single frequency is that the received signal usually is passed through a very tight bandpass filter to avoid recording false echoes. A single pulse reduces uncertainty in the time measurement, where multiple pulses are easier to detect. The receiver is usually not enabled until the transmitter has finished driving the output signal, and this establishes a minimum echo time that can be measured. Some receivers use adjustable gain amplifiers with the gain increasing with time because the longer it takes for the echo to return, the more material the signal has to pass through, and the weaker the echo will be. [note that I've left out any discussion of propagation velocity, signal spread, etc.]