NDT.net - June 2002, Vol. 7 No.06

PIEZOTRANSDUCERS CREATING NARROW WEAKLY DIVERGING ULTRASONIC BEAM

Alex Karpelson
Kinectrics
800 Kipling Avenue, Toronto,
Ontario M8Z 6C4 Canada

Corresponding Author Contact:
Email: alex.karpelson@kinectrics.com

I. Introduction

II. Far field synthesis

B. Experiment

III. Near field synthesis

IV. Lens synthesis

V. Conclusions

  1. The transducer synthesis problem for far zone directivity pattern can be solved in general form using equations (4)-(5). From these, one can calculate the pressure distribution on the surface of piston piezoelement that generates the desired directional function.
  2. This method was applied to develop piezotransducer creating a narrow weakly diverging ultrasonic beam. It was determined that piston transmitter with an oscillating pressure distribution on its surface forms acoustic field with a narrow main lobe and small side lobes in the directivity pattern.
  3. A numerical computation algorithm was developed for calculating optimum parameters of this distribution.
  4. This method of acoustic beam forming was used to design and manufacture a piezotransducer capable of creating a narrow weakly diverging axi-symmetric ultrasonic beam ("needle-like" beam). A piezo-disk with a system of annular electrodes on both surfaces was designed, manufactured and tested. The desired pressure distribution was realized by driving various electrodes by voltages with alternative polarities and different amplitudes through capacitor dividers.
  5. Experimental testing of the transducer demonstrated that piston transmitter with an oscillating pressure distribution on the surface forms acoustic field with a narrow main lobe and small side lobes in the directivity pattern. Numerical modeling results for the acoustic fields are in good agreement with the experimental data, thus confirming the validity of the general approach.
  6. The near zone synthesis problem for piston transmitterr was solved. Parameters of pressure distribution on the surface of transducer, which forms a narrow weakly diverging axi-symmetric "needle-like" ultrasonic beam in the near zone, were optimized and computed.
  7. A transducer creating such a beam was manufactured and tested. Experimental data confirm the initial assumptions, method and accuracy of calculations.
  8. Transducers with ring electrodes look exactly like ordinary probes and can be used with any commercial ultrasonic flaw-detector. They have a flat front surface and can work as both immersion and contact probes, focusing ultrasonic beam within a large axial range.
  9. Using a rectangular piezoelement with a system of linear electrodes, a transducer creating a "knife-like" ultrasonic beam (narrow and weakly diverging in one direction but wide and uniform in other direction) can be made.
  10. The lens synthesis problem was developed and solved, enabling the determination of the lens surface profile forming the desired acoustic field. This method was applied to a lens creating a narrow weakly diverging axi-symmetric ultrasonic beam in the near zone. Shapes of two lenses forming such an acoustic field were computed: logarithmic lens and axicon.
  11. Lenses with these profiles were designed, made and tested. The measurement data match the computations and thus confirm the theoretical approach and calculation method.
  12. The acoustic field created by axicon is wider and more diverging than the field formed by the "logarithmic" lens which, in turn, is wider and more diverging than field, created by piezoelements with an oscillating pressure distribution on the surface. However, the lenses are much simpler and more energetically efficient.
  13. If the "logarithmic" surface of the lens is attached to a piezoelement, then its front surface is flat; and in this case, the probe can be used as both an immersion and a contact transducer. At the same time, this transducer has a stretched focal zone, providing focusing within a large axial range.
  14. A transducer with a cylindrically focused "logarithmic" lens will create a "knife-like" ultrasonic beam: narrow and weakly diverging in one direction but wide and uniform in the other direction.

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