Communications – electrical: acoustic wave systems and devices – Transmitter systems – With beam forming – shaping – steering – or scanning
Reexamination Certificate
2002-07-30
2004-05-04
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Transmitter systems
With beam forming, shaping, steering, or scanning
C367S903000
Reexamination Certificate
active
06731569
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrical means and methods for reducing or suppressing ringing of ultrasonic transducers, and more particularly, to the design and construction of electrical circuits to suppress ringing of ultrasonic air-coupled resonant transducers.
BACKGROUND OF THE INVENTION
Occupant sensors are now being used on production automobiles that make use of ultrasonic transducers in a system to locate and identify the occupancy of the front passenger seat of an automobile to suppress deployment of an airbag if the seat is empty, if a rear facing child seat is present or if an occupant is out-of position, that is so close to the airbag that the deployment is likely to cause greater injury to the occupant than its non-deployment. Depending on the particular design, an occupant can get quite close to the transducers, sometimes as close as 10 cm.
Ultrasonic transducers can be used both to send and to receive ultrasonic waves. However, commercially available ultrasonic transducers, such as the Murata MA40S4R/S, due to their high quality factor Q continue to emit ultrasound even after all power to the transducer has been turned off. As a result, residual electrical oscillations at the transducer terminals deteriorate and mask weak received signals. This is known as ringing and is similar to the sound that a bell continues to emit after it has been struck.
This ringing prevents the use of such a transducer as a receiver until the ringing has subsided to the point that the received waves exceed the magnitude of the waves being emitted. Such transducers effectively cannot sense a reflection from a target closer that some particular distance from the transducer depending on the amount of ringing, which for a standard MuRata transducer may be as much as about 30 cm. Thus, when it is necessary to sense the presence of an object closer than the ringing zone, ultrasonic systems heretofore have required that the transducers be used in pairs, one for sending and another for receiving. The requirement to use pairs of transducers increases the cost of the system and occupies valuable real estate in the vehicle. Thus, there is a need of a method to reduce this ringing so as to enable a single transducer to be used both for sending and receiving from targets as close as about 10 cm.
OBJECTS AND SUMMARY OF THE INVENTION
To suppress ringing of off-the-shelf ultrasonic transducers, one can use acoustic/mechanical or electrical means. The latter is simpler and requires less effort. An objective of this invention is to provide electrical passive circuits and/or switching circuits which suppress ringing of a commercially available ultrasonic transducer such as the Murata MA40S4R/S transducer to permit reflections to be sensed from objects located as close as about 10 cm from the transducer. Although MuRata is a well-known supplier of open cone type transducers, there are many manufacturers and suppliers of this and other types of air-coupled resonant transducers, and the invention is equally applicable to them. For example, it may be applied to the APC or Massa air-coupled ultrasonic transducers.
Two types of circuits are used in practicing this invention: a linear circuit, developed on the basis of the Fano theory utilizing the principle of physical feasibility to get a “filter-like” circuit structure (Fano R. M., Theoretical limitations on the broadband matching of arbitrary impedance, Journal of the Franklin Institute, Vol. 249, pp. 57-84 and 139-154 (January-February 1950)), and a non-linear circuit, developed by Automotive Technologies International, Inc. of Rochester Hills, Mich. (ATI).
An important purpose of this invention is to obtain an acceptable ringing of the transducer at a given drive signal using passive electrical components. There is a known general rule that the broader a transducer transfer function is, the shorter is the transducer ringing. Various electrical matching circuits with inductors and capacitors were being applied to the resonant transducers to widen their transfer function (May J. E., Waveguide ultrasonic delay lines, Physical Acoustics, Edited by W. P. Mason, Vol. 1A. Academic Press, NY-London (1964); White D., A transducer with a locking layer and other transducers, Physical Acoustics, Edited by W. P. Mason, Vol. 1B. Academic Press, NY-London (1964)). However, the transfer factor decreases if the characteristic is widened arbitrarily. An example of this is Massa's commercial ultrasonic transducer of E-152 series, which being tuned with an inductor and a resistor has less sensitivity. Inductive circuits were also applied to medical ultrasonic transducers to widen their frequency response and make their impulse response shorter. (R. E. McKeighen, Influence of pulse drive shape and tuning on the broadband response of a transducer, Proc IEEE Ultrasonics Symposium, Vol 2, pp. 1637-1642, IEEE Cat. # 97CH36118, 1997; R. E. McKeighen, Design Guidelines for Medical Ultrasonic Arrays, SPIE International Symposium on Medical Imaging, Feb. 25, 1998, San Diego, Calif.). The author discloses circuits of the specific, low-pass filter structure that were build on the base of finite element simulations and experiments carried out with a concrete type of the medical transducer with lossy backing, that is, with rather low quality factor Q. The impulse shortness is observed at the level of about —30 dB that is enough for this type of transducers but not suitable for air-coupled ones with high Q. The authors also did not achieve any real ringing reduction of the transducer itself, that is, reduction of electrical oscillations at its electrical terminals (electrodes). Also, as far as there is no theory underlying the simulations, the study done is only applicable to the concrete type of the transducer investigated.
The known theories of broadband matching of arbitrary impedance, including Fano's, developed on the basis of physical feasibility approach (Wai-Kai Chen, Theory and Design of Broadband Matching Networks, Pergamon Press, Oxford N.Y. Toronto Sydney Paris Frankfurt, 1976; Matthaei G. L., Young L., Jones E. M. T., Microwave filters, impedance matching networks, and coupling structures, Vol. 1, McGraw-Hill Book Company, NY 1964)) give techniques of how to integrate a lumped model of matched impedance into a filter-like structure, and then to build an optimal matching circuit that provides, for example, a maximum transfer factor at a given bandwidth.
Similar approaches are disclosed in (G. A. Hjellen, J. Andersen, R. A. Sigelmann, “Computer-aided design of ultrasonic transducer broadband matching networks”, IEEE Trans on Sonics and Ultrasonics, Vol SU-21, No. 4, PP. 302-305, October, 1974; C. H. Chou, J. E. Bowers, A. R. Selfridge, B. T. Khuri-Yakub, and G. S. Kino. The Design of Broadband and Efficient Acoustic Wave Transducers, Preprint G.L: Report No. 3191 November 1980. Presented at 1980 Ultrasonics Symposium, Nov. 4-7, 1980, Boston, Mass.). In the first case, the authors built a three-element lumped R-L-C model of the high frequency (5.5 MHz) transducer, integrated it in the pass-band filter-like structure with series inductive and capacitive elements, and then applied a parametric synthesis procedure to those elements to get a wide Butterworth-like characteristic of the electrical power absorbed by the transducer. They did not analyze and reduce ringing of the transducer. In the second case, the authors also applied parametric synthesis to high frequency (3 MHz and 35 MHz) lossy backing transducers operating into water, and build reactive matching circuits with inductors and capacitors to get either a desirable frequency response or a compact impulse response of the transducer. They shortened the impulse response of the 35 MHz transducer from 15 full cycles to 3 full cycles. However, they do not disclose neither ringing reduction of the transducer at its electrical terminals nor the drive signal shape at which this compactness of the impulse response was achieved.
One of optimal matching techniques, namely Fano's, being
DuVall Wilbur E.
Johnson Wendell C.
Yurchenko Oleksandr V.
Automotive Technologies International Inc.
Pihulic Daniel T.
Roffe Brian
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