Transducer questions

[bob]

Sture has been _extremely_ helpful in explaining some important parameters in designing transducers. At this point, I am ready to try some experimenting, but I still have some remaining questions, which I wanted to post here. Perhaps Sture or some others can help out:


What effect does changing the crystal thickness have? I now understand the effect of changing length and width, but what about thickness? Can any value be used, or are we constrained?


How do we cut the piezoelectric crystals to size? One site with fairly expensive crystals offered to cut them to custom sizes, because cutting them in a shop is not supposed to be very easy. Any ideas??


In the transducer design with a steel bar substrate, how are the top electrodes attached? I was thinking that if they are soldered on, the solder lump might affect the sonic output. Perhaps there are ways to minimize this problem, by attaching the electrode in a certain place, or using a different material besides solder.


Are there limits on the substrate bar width? Should it be 1/4 wave, or the same width as the elements? Can it be wider or will we start to get standing waves?


Thanks!


-Bob

[Sture Hultqvist]

Re: Transducer questions

Bob


The thickness of the crystal gives the resonant frequency. However you can run the transducer on a lower frequency with proportional loss in sensitivity. With the normal material Pz 5 you will have - around - 600 KHz for 3 mm, 500 KHz for 4 mm, 250 KHz for


6 mm.


Crystals up to a thickness of -around- 8 mm is made like cookies. Thicker crystals is molded (correct english ?) and have lower quality. The cookies are made in sheets, round or square. A normal sheet size is - around - 50 x 50 mm. After sintering a silver surface is screen printed and then the sheets are polarised with a high electrical field and a temperature over the Curie point. After this the sheets are cutted into the ordered size with diamond cutter.


At the design with the steel bar backing the top electrode is simply soldered with a wire. The steel bar is in the same size as the total array with cutted slices between each crystal to minimize longitudinal ringing. The thickness is - as I understand - choosen to minimize vertical ringing or standing wawes. The thickness is a odd number of 1/4 wawelenghts. With a sound speed of 6000m/s in the steel backing the first odd number is 3 mm for 500KHz.


Backing can be made with other material as a composite with tungsten and epoxy.


Sture

[Tod]

Re: Transducer questions

Sture,


Was it you how said they used cork around the ceramic. I've seen that on fishfinder transducers. If you don't have an insulator between the ceramic and the steel is the steel the other electrode?


Tod

[Sture Hultqvist]

Re: Transducer questions

Let see.. I have the limitations that english is not


my native langue


The transducer I investigated did have the crystal


glued directly on the steel bar. This was surrounded


of PUR-foam as isolator, except of the top. In this


case the steel bar bar was one electrode.


The reason that I opened this transducer was it was dead.


And the reason for this was that the crystals have loosen from the bar. It might be tricky to glue


crystals on the bar.


I have myself tested this for some years ago but


I didn't slice the bar and this transducer become


very much "ringing". However the gluing worked well.


I didn't see any improvment compare to another backingfree transducer I builded.


The idea putting cork between the crystals and the bar


might be very good. Then you use the bar only for aligment and not as backing.


Sture