Transducers and Instrumentation

Published on 06/02/2015 by admin

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Last modified 06/02/2015

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Transducers and Instrumentation

Christopher J. Gallagher and John C. Sciarra

Piezoelectric Effect

Understanding the piezoelectric effect takes the mystery out of “Just what the hell is that little gizmo at the end of my probe, anyway?”

To make a sound wave, you need to wiggle something.

Bang-a-gong, the metal vibrates, and the sound waves go forth. Now let’s just tie a little creature to the end of a gastroscope, and have him bang-a-gong fast enough to create 7 million cycles/second for 20 minutes straight.

No go. We need a better way to get so much wiggling. The guy banging the gong just won’t do.

Millions of times per second? Better go to electricity, that’s the only thing that can give you that many wiggles per second. But how to get electricity to wiggle something? Electrify a gong?

Piezoelectric crystals to the rescue! These are quartz or ceramic things that have a magical property. When a current is applied to them, the polarized particles align perpendicular to the face of the crystal. When the current goes off, the particles no longer align. This alternating aligning and nonaligning results in the face of the crystal bowing out, then coming back, in effect wobbling just like the gong.

(Who the hell figures this stuff out the very first time, I want to know.)

OK, groovy, so this electrical thing makes a mechanical wave. How does a piezoelectric crystal “hear”?

Well, according to the Principle of Electromechanical Turn-It-Around-ness, when a wave comes into and hits the piezoelectric crystal, it causes a mechanical deformation that then makes a current change. So,

Electricity makes a mechanical wave.

A mechanical wave makes electricity.

Then, through some kind of voodoo known only to electrical engineers and people with pocket protectors, the TEE sorts all this out and gives you an image.

Crystal Thickness and Resonance

A thin crystal resonates at a high frequency (think of a thin wine glass that goes “TING!” when you tap it). A thick crystal (think glass beer stein; better yet, get one and fill it to the top if you’re slogging through this physics junk) resonates at a low frequency. No big shocker there.

Damping

When the signal comes back to the crystal, you don’t want the crystal to wiggle too wildly. Hence, behind the piezoelectric crystal, damping material is in place. The damping material allows a short pulse length, hence improved resolution. Go back to the concept of the ringing gong. After our hero has hit the gong, he doesn’t want it ringing and ringing. He grabs the gong; that allows it to become still, and then he can hit the gong again.

Sound Beam Formation

Electricity in a short burst (typically 1 to 6 microseconds) hits the crystal and produces the short blast of ultrasound by means of the piezoelectric effect. The damping material keeps the crystal from “wiggling” too long, as mentioned above. These short bursts allow better axial (along the direction of the beam) resolution.

(As you can see, the Content Outline of the PTEeXAM chops up the individual items you need to know. In reality, this stuff all flows together in one smooth explanation in the TEE review course syllabus and in Otto’s textbook.)

Focusing

A sound beam tends to spread apart, like ripples in a pond. (I can almost envision a “TEE Haiku” coming out of this.) TEE needs a tight beam to be able to make some split-second measurements of small places, so the transducer focuses the sound beam. A mechanical lens does this.

Axial and Lateral Resolution

(Here again, we’re chopping up stuff that should run together.)

Axial Resolution

Short bursts—that is, high frequency—give you better axial (along the line) resolution. Why? You will have a lot of information bouncing back to you, so you’ll be able to tell, “Aha! This reflected signal tells me something is just right there, and this other signal tells me that something is just a little further along the line.” If this, admittedly weak, explanation doesn’t convince you, then try this line of reasoning: Imagine very infrequent signals going out. How could you tell things are close together then?

Lateral Resolution

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