CA2484772A1 - High efficiency ultrasonic transducer driver - Google Patents

Abstract

The subject of the invention is an innovative design of an ultrasonic transducer driver.
The design utilizes two quarter-wavelength layers made out of light rigid polymer foam materials; one operating in the eddy-like wave mode, the other one in he longitudinal wave mode. The first layer transforms radial vibrations of piezoelectric ring into "eddy mode" (toroidal-like) vibration pattern, the second layer transforms the eddy mode vibrations of the first layer into pure longitudinal vibration of the front face,. Such arrangement maximizes efficiency of he overall transfer of acoustic energy from a piezo-electric element into gasous medium. At the same time, such a design achieves optimal (intrinsic) damping effect exerted onto the piezo-electric element resulting in very short ring-down, without having to apply additional damping materials or potting compound that would increase losses and reduce efficiency.

Description

High Efficiency Ultrasonic Transducer Driver DESCRIPTION
Introduction Ultrasonic transducers are devices that transform electrical oscillations into acoustic vibrations of frequencies above 16kHz; that are emitted into air, gaseous medium, into liquids or into solid materials. Most transducers are capable of reversible operation; that is acting as microphones - they can also receive ultrasonic vibrations from the medium and transform them back into electrical oscillations, albeif of much lower intensity.
Industrial applications Ultrasonic transducers are utilized as the front end devices for sonar type of sensors. For example, a typical ultrasonic transducer would emit a short acoustic wave burst in form of a'narrow beam 10-20degrees wide (full angle), which when reflected off a target placed at some distance in front of a transducer, may get received back by the transducer acting as a microphone.
The time-of-flight of such a burst is then converted to the distance to the target.
In most industrial applications, the level or distance sensing sonar operate in air.
Typical transducer design.
Since most industrial application require certain degree of robustness, the usual technique of generating ultrasonic waves in air such as electrostatic speakers with very thin metalized Mylar membranes etc are not applicable. Instead the most popular approach is the so-called Langevin design which (in many variations) is based on constructing a sandwich stack (Fig.1 ) of several disk-shaped layers of progressively lighter materials attached to the driving Piezo-electric Element, typically a sintered Lead-Zirconate-Titanate Oxide ceramic (PZT). Acoustic wave progressing from the Piezo-electric Element (called further in the text "PE") propagates from a high acoustic impedance medium (e.g. PZT crystal) into progressively lower impedance disks made of lighter materials until it reaches the front face of he ransducer that is bordering the medium (air). The biggest loss of efficiency takes place on the front face boundary where the acoustic impedance discontinuity is the greatest. Utilizing very light materials that are matched better to air are not always possible in that design because it increases the mismatch closer to the Piezo-electric element, therefore a compromise is typically made by making the first matching disk out of aluminum (rho=2.7g/cc); the second disk out of light camposite (0.5-0.5glcc) and the outer layer out of plastic sheet or light polymer foam. Additional problem 45 with the traditional design is the necessity to use damping layer (wrap), see FIG.1 in order to reduce the mechanical resonance quality factor, otherwise a transducer may exhibit very long ring-down after excitation, reducing a capability of receiving echoes from close distance. In contrast the presented here novel design addresses the issue of large acoustic impedance (Z) mismatch between 50 the heavy PE (high Z) and air (very low Z) by making use of the very special inherently low-impedance eddy-like toroidal oscillation pattern in the first matching layer (see FIG.2). This allows using rigid polymer materials or foams of very low density: Additional benefit of utilization of eddy-like oscillatory modes is that the coupling factor to a thin PE in shape of a ring, insufficiently high to 55 dispense with any damping layer altogether, greatly simplifying assembly and reducing costs in addition to lower losses (higher efficiency). The full design is represented diagrarnmatica(ly (not to scale) on FIG.3.
Explanations of terms:
PE - Piezo-electric Element, typically PZT ceramic material (Lead-Zirconate-Titanate Oxide) is used.
Eddy Mode - mode of acoustic oscillation in solid disks or bars where particles of materials perform vibratory motion along the quasi-circular lines as illustrated on FIG.2 A and B. The actual velocity field forms toroidal pattern or multiple (stacked-up radically) toroidal patterns depending on frequency and dimensions. In this description, we are focusing on the standing wave of the eddy-like mode (FIG:2A) although such vibrations may also propagate along the length of circular bars (FIG.2B). Their group and phase speeds of propagation are similar (but not identical) to the sound speed of shear wave in infinite {unbound) solid. It is typically comparable to one-half of the longitudinal sound speed in bars. In addition to slower speed; the Eddy Mode has typically much lower acoustic impedance therefore is naturally suited for matching between thin-walled lightweight PE shapes and the bulk material of the first matching layer (e.g. Layer A, FIG.3). Eddy Mode is very sensitive to the geometry and requires cylindrical symmetry.
Longitudinal Mode - mode of acoustic oscillation in solid disks; bars, other shapes, bulk solids, liquids and gases where particles of materials perform vibratory motion with velocity vectors directed predominantly along the direction of wave propagation.
Longitudinal mode is the only acoustic mode possible in gases and liquids. Longitudinal mode in solids has the highest speed of propagation and the highest'acoustic coupling impedance.

2 Operating Frequency - center frequency of the band at which an ultrasonic transducer is operated.
Acoustic Impedance - ratio of the excitation force amplitude applied to a solid, fluid or gas particle divided by the resulting particle velocity amplitude. In solids, acoustic impedance depends on the modal type of wave (e.g. shear versus longitudinal) and on the geometry of the object's boundaries. Impedace value is typically close to the product of sound speed and density. Generally, lighter solids have often lower impedance that more dense solids. As a general rule, in order to facilitate passage of acoustic energy with a minimum of reflections, through a layer of different acoustic impedance from 100 that of a source and destination, the optimal thickness of the layer should be an odd multiple of the quarter-wavelength (e.g. 1,3,5 etc). Such a layer is often called "quarter-wavelength impedance transformer".

FIG.3 illustrates the embodiment of the invention (not to scale). Thin-walled lightweight Piezo-electric Element P in shape of a ring is attached to the first matching Layer A where it excites the standing Eddy Mode wave pattern. If 110 Layer A were in shape of a full disk then for a given dimensions used in the prototype embodiment, there would have been finro toroidal eddies stacked up in the radial dimension. One would call this mode to be of the second order (N=2).
As illustrated on FIG.2 such vibration pattern would results in the counterphase movements of the right-hand side surface of the layer 2 (opposite to P). This 115 would result in undesirable partial cancellation of the longitudinal vibrations in the axial direction (i.e. along the cylindrical axis of symmetry- left-to-right on FIG.2 and 3). in order to prevent this cancellation of axial vibrations, a hole was cut in the center of layer to completely cut out the innermost eddy (see FIG.3 -HOLE).
120 As it stands, the structure of Layer A with the hole supports the outermost Eddy pattern only, acting as an impedance and vibration pattern transformer, transforming the predominantly radial vibrations of Piezo-electric Element P
into almost purely axial vibratory movements of the rightmost surface of Layer A
(on the opposite side of P) and further into the Longitudinal Mode in the bulk of Layer 125 B.
In order to achieve optimal coupling and damping of P it is important that the mass of P to mass of Layer A ratio be properly controlled and be about 10:1 (as a rule of thumb, between 5 and 20).

3 Due to a quarter wavelength thickness of Layer A, Eddy Mode vibrations retain mostly axial velocity component at its rightmost surface. However certain small radial vector componenfis may be present at that interface. Since Eddy Mode is very sensitive fo damping of its radial component, it is important that any layer 135 attached on that surface has a much lower acoustic impedance. Attempts at gluing a non-elestic film or a sheet there (e.g. metal foil etc) will most typically result in the almost complete suppression of the entire Eddy Mode pattern in Layer A. However, in the current embodiment the vital function of matching the ultra-sensitive Eddy Mode in Layer A to the further extended propagation 140 medium is played by the Layer B disk. Layer B is made out of a much lighter material than A and forms also a quarter-wavelength -impedance transformer for the longitudinal vibration: That is, Layer A forms a quarter-wavelength acoustic impedance matching transformer for the Eddy Mode vibration whereas B forms the same impedance transforming function for the Longitudinal Mode (axial 145 direction).
Acoustic wave that emerges on the rightmost surtace of Layer B is very robust and can tolerate addition of front face sheets or films (see FIG.3 item "SHEET
C") without much disturbance, as long as the following conditions are met:

- mass-loading of the front face sheet is not too high: surtace mass of sheet C
should not exceed Layer B mass by more than a factor of 2 ( in per unit surface area units, as rule ofthumb).
155 - quarter-wavelength tuning of layer2 must include the effect of, and be adjusted correspondingly for the presence of additional mass of sheet C.

RESULTS OF PROTOTYPE TESTING
Result, in form of oscilloscope screen dumps are presented on FiG.4 and 5.
A prototype of the High Efi'rciency Ultrasonic Transducer was tested by connecting to a commercial ultrasonic transducer controller for industrial storage tank level sensing. Oscilloscope was used to view the envelope signal of echo 165 profile, taken with the target that was situated at a distance of 4m from transducer (a wall of a room, perpendicular to the line of sight). Controller was programmed to drive transmitter with a single pulse. Signal from one of two original commercial ultrasonic transducers for air (of the same diameter and operating frequency as the prototype device) is shown for comparison (FiG.4 170 upper trace). FIG.5 shows the test results (upper trace) of the High Efficiency Ultrasonic Transducer Driver in :its fuN embodiment; the echo amplitude as shown is 9 times higher (19dB) with slightly better (shorter: 1.2ms versus 1.5ms) ring-down than the one produced by a standard commercial transducer. FIG.6 shows a photograph of the actual prototype driver (top view).

4 Conclusions:
This example implementation of a bare driver of the High Efficiency Ultrasonic Transducer exhibits much higher sensitivity (by 19dB) and better (shorter) 180 ringdown without any additional damping or potting; as compared with a standard commercial product. This Driver shows promising feafiures of being a superior and most likely a less labor-intensive (in manufacturing), replacement of the existing standard transducer driver design:

List of Figures FIG.1 - Typical standard transducer driver design.
FIG.2 - Illustration of eddy-like pattern of acoustic velocity field in cylindrical 190 solids.
FIG.3 - High efficiency ultrasonic transducer driver design.
FIG.4 - Oscilloscope traces ofi a standard transducer and intermediate device under test.
FIGS - Oscilloscope traces of an intermediate device under test and the fully 195 assembled high efficiency ultrasonic transducer driver.
FIG.6 - Photograph of the high efficiency ultrasonic transducer driver prototype.

Claims (8)

High Efficiency Ultrasonic Transducer Driver I Claim:

1 An ultrasonic transducer driver composed of piezo-electric ring of the fundamental radial vibrating resonant frequency that is within +/- 30% of the Operating Frequency, attached to a first disk with hole (see Layer A, FIG.3), made out of a solid material that is lighter than the piezo-electric ring itself.

2 A device as in claim 1 with a second disk (Layer B, FIG.3) made of solid material attached to the first disk (Layer A, FIG.3) to the opposite side than the piezo-electric ring.

3 A device as in claim 1 and 2 with an optional front face sheet (see SHEET C as per FIG.3).

4 A device as in claim 1 where the mass ratio of the piezo-electric ring to the Layer A (FIG:3) is between 5 and 20.

A device as claimed in 1,2, and 3 where the unit surface mass ratio of SHEET C to Layer B (FIG.3) is less or equal 2.

6 A device as claimed in,1 and 2 where the Layer A has higher density than Layer B (FIG.3).

7 A device as in claims 1,2 and 3 where the thickness of Layer A is adjusted such as to be within +/-30% of the quarter-wavelength of the Eddy Mode vibration pattern (see FIG.2) at the Operating Frequency (see Description document).

8. A device as claimed in 1,2 and 3 where the thickness of Layer B (FIG.3) is adjusted such that Layer B together with the mass of the front face sheet C (FIG.3) form a quarter wavelength structure (within +/-30%) for the Longitudinal Mode at the Operating Frequency.