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.
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.
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 xe2x80x9cfilter-likexe2x80x9d 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 xe2x80x9430 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, xe2x80x9cComputer-aided design of ultrasonic transducer broadband matching networksxe2x80x9d, 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 applied to piezo-transducers with low quality factor Q (Yurchenko A. V. Broadband matching of piezo-transducers of acousto-optic devices. Izvestiya VUZ., Radioelektronika, Vol 23, No. 3, pp. 98-101, (1980); Tsurochka B. N., Yurchenko A. V., An electroacoustic device, USSR Author certificate No. 1753586 Int. Cl.5 H03 07/38 (1992)) enabled optimal matching of the transducers within an arbitrary frequency band using parallel/series inductors and capacitors. It is also disclosed (T. L. Rhyne, Method for designing ultrasonic transducers using constraints on feasibility and transitional Butterworth-Thompson spectrum, U.S. Pat. No. 5,706,564) how to design an ultrasonic half-wavelength transducer with a desirable shape of the bandpass characteristic.
None of disclosed techniques suggests what a characteristic shape or bandwidth is desirable to minimize ringing. This is a many parameters task that could be solved in alternative ways depending on what factor is most important for concrete applications. Therefore, to get reduced ringing, one can consider the Murata transducer as a two-port transducer with known input impedance, apply the Fano method to get a bandwidth with acceptable transfer factor and/or an acceptable inductor value, and then smooth the phase characteristic to get acceptable transducer ringing at a given input electrical signal. Such a procedure has been used in this invention to synthesize a linear electrical circuit for ringing reduction. The circuit synthesized has been simulated and then examined experimentally. All of the above references are incorporated herein by reference.
The non-linear circuit has been simulated and the influence of its parameters on ringing reduction was investigated. In both simulations, a conditional Spice model of the Murata transducer MA40S4R/S was built on the basis of the heuristic approach. The measured transducer impedance was used as initial data.
The operation of the transducer in dual-function (i.e., transmitter-receiver) mode is fundamentally different from its transmitter mode. To see the difference, a transducer operating in dual-function mode will be considered in greater detail. In view of the interest in detecting small signals reflected back from a target, a possibility to shorten the ringing zone (dead zone as it is frequently called) will depend on what ringing is present at the electrical input to the transducer. It does not matter much what ringing will be at the transducer acoustic output. The dead zone length will be determined substantially exclusively by the relation of the received signal level to a ringing floor at the transducer electrical side. Although transient processes at the transducer electrical input and its acoustic output are connected due to electromechanical coupling, they are not identical because of the non-symmetry of the electromechanical two-port and different boundary conditions at its electrical and acoustic sides. Thus, the transient electrical process at the input of the transducer should be considered and its level compared with a level of delayed burst detected at the same points of electrical circuit. Such an analysis has been performed using the MicroSim(copyright) DesignLab 8.0 (evaluation version) Spice modeling software. Its results are presented below.
Fundamentally, the invention involves the placement of electrical possibly reactive components, inductance or inductors and/or capacitors of appropriate values in parallel/series with the ultrasonic transducer in one case and in series and parallel in the other case. Although these components have been used in the past with ultrasonic transducers they have not been of the proper value to cause a substantial reduction in transducer ringing.
Accordingly, one exemplifying embodiment of a method for reducing ringing of dual-function ultrasonic transducers in accordance with the invention comprises the step of applying at least one inductance in series and/or in parallel to the transducer electrical terminals to obtain a decreased dead zone of the transducer. At least one passive electrical circuit may be applied in series and/or parallel to. the inductance. Also, different electrical passive circuits can be applied to the transducer when the transducer is in a transmission mode than when the transducer is in a reception mode.
Although an xe2x80x9cinductancexe2x80x9d is applied, it is noted that an xe2x80x9cinductorxe2x80x9d could also be applied. In the electronics field, xe2x80x9cinductancexe2x80x9d can be realized with active circuits without any inductors which usually are simply coils. At a large value of inductance, the active circuit could often happen to be cheaper than the coil.
Each passive circuit may be a linear or non-linear circuit. For a linear circuit, the linear circuit can be synthesized using known input impedance/admittance of the transducer. It can also be optimized on the basis of a broadband matching theory. That is, the generator output impedance may be optimized to obtain acceptable ringing at a given input signal. Parametric synthesis of the circuit is also envisioned as an option. Non-linear components may be added to the linear circuit if so desired and/or necessary. The linear circuit could also be constructed with a higher order transfer function and including at least one capacitor and at least one inductor. Thus, the invention contemplates the use of, for example, a second order circuit, or two component circuit, or any other circuit with predefined number of components. Generally, passive electrical circuit can comprise any number of components by definition.
An arrangement in accordance with the invention for reducing ringing of dual-function ultrasonic transducers comprises an electrical passive circuit adapted to be coupled to the transducer, said circuit including at least one inductance adapted to be in series and/or in parallel to the transducer to obtain a decreased dead zone of the transducer.
An additional electrical passive circuit may be adapted to be coupled to the transducer and a switching device provided for switching between the circuit such that one circuit is coupled to the transducer when the transducer is in a transmission mode and the other circuit is coupled to the transducer when the transducer is in a reception mode. Instead of switching between circuits made of different components, a switching device can be built into the circuit to modify the circuit such that a first construction of the circuit is coupled to the transducer when the transducer is in a transmission mode and a second construction of the circuit, different from the first construction, is coupled to the transducer when the transducer is in a reception mode. A similar switching system is described in U.S. Pat. No. 5,267,219 (Steven J. Woodward, Acoustic range-finding system, 1993). In this system, the ringdown time of the transducer is reduced by damping that is provided by switching the transducer on the transistor and/or on an appropriate resistive circuits. No reactive elements, inductors and/or capacitors, are used in the system to shorten ringing time, therefore the net effect in such a resistive system should be worse than in a system with frequency response optimized to get acceptable ringing at a given signal shape.