Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2003-03-19
2004-09-14
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C600S459000, C029S025350
Reexamination Certificate
active
06791240
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ultrasonic transducers made from piezoelectric ceramic polymer composite materials, and, more particularly, to ultrasonic transducers made from a multi-frequency composite structure that broadens the transducer bandwidth, and to methods for making such transducers.
2. Background
In general, ultrasonic transducers are constructed by incorporating one or more piezoelectric vibrators which are electrically connected to pulsing-receiving system. Conventionally, the piezoelectric member is made up of a PZT ceramic, a single crystal, a piezo-polymer composite or piezoelectric polymer. The transducers are shaped in plate form (a single element transducer) or in bars (a slotted array transducer) and the parallel opposite major surfaces thereof (which extend perpendicularly to the propagation direction) have electrodes plated thereon to complete the construction. When the piezoelectric is subjected to mechanical vibration and electrically excited, acoustic waves are then transmitted to the propagation medium with a wavelength according to the thickness of the piezoelectric. Thus, the nominal frequency of an ultrasonic transducer is obtained by determining the dimension of piezoelectric in the direction of propagation. Based on these considerations, ultrasonic transducers exhibit a unique nominal frequency that corresponds to the thickness resonance mode and thus the bandwidth of such transducers is inherently limited or bounded. A common task facing transducer designers is the optimization of the efficiency of, or otherwise improving, the electromechanical coefficient of the transducer which determines the quality of the transducer device. The most common technique of producing piezo-ceramic based ultrasonic transducers involves the provision of a backwardly damping member or backing member and/or an impedance matching layer at the transducer front face. In the first case, the sensitivity of the transducer decreases proportionally to the increase in the backing impedance, and, therefore, according to the bandwidth provided, while an improvement in both sensitivity and bandwidth can be provided by the use of a matching layer.
In practice, ultrasonic transducers are based on a judicious compromise with respect to the ratio of gain-bandwidth, and thus commonly use a medium impedance backing associated with a single or a double matching layer to achieve satisfactory performance. The set of double matching layers is composed of a first layer attached to the front surface of the piezoelectric and having an acoustic impedance between that of piezoelectric and the second matching layer, a second layer attached to the external face of the first layer and having impedance lower than that of the propagation medium. In this way, a gradient of acoustic impedances is obtained between the piezoelectric and the propagation medium, and the impedance value of each component is calculated based on a polynomial function to minimize reflection at the various interfaces.
Although the optimization techniques described above will enable transducer to provide a fractional bandwidth up to 70-80%, because of the compromise that must be accepted, the transducer sensitivity may decrease dramatically (with a heavy backing) or the fabrication of transducer may be complicated (e.g., with more than two matching layers). During the past decade, such bandwidth (i.e., a bandwidth on the order of 70%) provides acceptable performance when using standard medical diagnostic equipment or systems equipped with low dynamic range image processors. However, with the introduction of harmonic imaging techniques and full digital imaging mainframes, modern systems can now accept, and even require, an extended bandwidth scan-head to take advantage of the potential of these new technologies.
To provide the market with improved transducer products, manufacturers have made a number of new developments. One of these concerns the use of high mechanical loss piezoelectric material such as a polymer or ceramic-polymer composite. The particular structure of these materials allow increased damping of the transducer so that the impulse response is enhanced. The gain in bandwidth is about 5 to 10% with a composite and more with piezoelectric polymer but in the latter case, this increase in bandwidth is associated with a dramatic decrease in sensitivity.
Another direction which this recent research has taken focuses on multi-layer transducer structures wherein the piezoelectric device is produced by superposition of a plurality of reversed polarity single layers. The objective is to reduce the electrical mismatch between the piezoelectric impedance and those of the cable so as to minimize reflections at interface. Ringing is therefore shorter and sensitivity is improved. Unfortunately, the construction of such devices is highly difficult and requires large quantity production in order to be cost effective.
Still other techniques for broadening transducer bandwidth concern the use of a ceramic of non-uniform thickness. These techniques involve the provision of piezoelectric devices shaped to provide gradient thickness along the elevation dimension thereof so as to afford frequency and bandwidth control of the elevation aperture size and position, as well as the elevation focal depth. Transducers employing these techniques are described, for example, in the following U.S. Pat. No. 3,833,825 to Haan; U.S. Pat. Nos. 3,470,394 and 3,939,467 both to Cook; U.S. Pat. No. 4,478,085 to Sasaki; U.S. Pat. No. 6,057,632 to Ustuner; U.S. Pat. No. 5,025,790 to Dias; and U.S. Pat. No. 5,743,855 to Hanafy.
Briefly considering these patents, in the Haan patent, a thickness-mode transducer is provided which comprises an active body having non-parallel major surfaces for transmitting or receiving energy. The major surfaces of transducer are planar so that the transducer device provides a continuous variation in the resonance frequency from one edge thereof to the other.
The transducers as described in the Cook patents are of a serrated or even double serrated construction and have major opposite surfaces formed at an angle (the '467 patent). Further, the transducer front face may be of convex or concave shape.
The Sasaki patent describes transducers having an element thickness which increases from the central portion toward both edges in elevation direction. However, the variation in thickness described herein is only of two types: continuous and stepwise. The purpose of the thickness variation described in this patent is to control the acoustic radiating pattern of transducer, and neither the manufacturing method used nor the actual transducer construction are fully addressed.
Similarly, the Dias patent discloses a variable frequency transducer wherein the piezoelectric member has a gradient thickness between the center thereof and the outermost ends. Each portion has a particular thickness corresponding to a desired frequency. As a consequence, the transducer provides discrete frequencies and the frequency characteristics are not compatible with the smooth bandwidth shape required by imaging transducers.
In the transducers disclosed in the Ustuner patent, the spacing of elements increases from the first end to the second end so that the dimensions of the overall transducer array tend to be those of a trapezoidal, thereby inherently limiting the number of elements in the array.
In the Hanafy patent, a gradient transducer is produced by grinding a thicker ceramic plate to provide the desired curvature, using a numerically controlled machine. However, machining a curved surface, and especially a cylindrical surface with perfect alignment relative to the ceramic edges has been found to be a particularly delicate operation which requires superior precision with respect to the tooling used and the process employed. Thus, fabrication method described in the Hanafy patent is difficult to carry out in practice. Moreover, if the machined surface profile must be mounted on or another
Auclair Philippe
Flesch Aime
Mauchamp Pascal
Hunt, Jr. Ross F.
Stites & Harbison PLLC
Vermon
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