Communications: electrical – Audible indication – Piezoelectric
Reexamination Certificate
2001-07-05
2003-05-06
Lieu, Julie (Department: 2632)
Communications: electrical
Audible indication
Piezoelectric
C340S384100, C340S384700, C340S384720, C386S349000, C386S349000
Reexamination Certificate
active
06559758
ABSTRACT:
BACKGROUND OF THE INVENTION AND PRIOR ART
This invention relates generally to audible warning devices such as automobile or boat horns, and other acoustic signaling devices. Usually these sounder devices are designed to be as small as possible and use as little electrical power as possible, but nevertheless must be capable of producing a high intensity acoustic output. There are many designs in prior art which have proven useful with some desirable features. These include the generation of perceived intense sound of more than one frequency occurring simultaneously, as discussed in U.S. Pat. No. 4,303,908 (Enemark), with the frequency differences being non harmonically related, as in U.S. Pat. Nos. 4,204,200 (Beyl) and 4,689,609 (Ko), or using combinations of arbitrary frequencies in order to make more alarming or raucous sounds. However, sounders with the above capabilities, and which also can produce very high acoustic peak pressure levels with very high efficiencies have not been optimally addressed by prior art.
In order to obtain high efficiencies many such devices use transducer elements of a piezoelectric nature as in U.S. Pat. Nos. 3,912,952 (Kumon) and 5,990,797 (Zlotchenko). These are usually resonant at some relatively high frequency, typically 2 to 4 kHz. However at these frequencies, the sound does not have the desired warning urgency characteristic that lower frequency sounders can produce.
A major problem of prior art is obtaining alarming lower frequency sounds while using highly efficient resonant transducers. This goal has been addressed in prior art with some success by using resonant transducers and frequency modulating their output to produce an effect similar to an emergency vehicle siren as in U.S. Pat. Nos. 4,088,995 (Paladino) and 4,195,284 (Hampshire), or by amplitude modulating their higher resonant frequency with a lower frequency as disclosed in U.S. Pat. No. 4,486,742 (Kudo). This produces a perceived sound of the lower frequency and the human ear seems to be mostly unaware of the higher carrier frequency. However, in prior art there seemed to be no highly efficient solution using this approach. This invention addresses this problem by optimizing the electrical drive going to the transducer to simultaneously best take advantage of the resonant nature of the transducer and yet deliver an audible and discernable lower set of frequencies for warning or signaling purposes.
A second problem is getting the sound to propagate primarily in a preferred direction and yet not require very large radiating surface dimensions, so that the directed acoustic energy of the source is concentrated in a defined angular range. It is well known that the angular radiation pattern of a sound source is controlled by the transverse dimensions of the source relative to the wavelength of the sound. In order to confine the radiation to a specific angle the transducer dimensions transverse to the propagation direction must be at least as large as about half the wave length of the sound being generated. So, for example, a directed sound of 300 Hz of wave length about 1 meter would require a radiator of about a half meter or more across. However a frequency of 3 kHz would only require a transducer of about 5 cm diameter. This invention attains the goal of lower frequency perception, but within the angular range set by the higher resonant frequency of a small, very efficient resonant transducer, or as will be described, by an array of such transducers.
SUMMARY OF THE INVENTION
A principal object of the invention is to provide a method for producing a very high intensity warning sound from an efficient and compact device which uses higher frequency resonant transducers.
Another object of the invention is to provide a warning device which has two or more frequencies which resemble conventional automobile horns or similar devices, but which utilize the high efficiencies of resonant transducers which have resonances at much higher frequencies.
A further object of the invention is to provide a method for controlling the direction of the radiated sound from a warning device consistent with the shorter wavelength of the higher resonant frequencies, while still producing sounds at the lower frequencies with longer wavelengths.
The first and second objectives are accomplished by the following approaches. As with prior or related art an electrical signal from an oscillator with a relatively low frequency, typically about 100 to 500 Hz, is amplified by a transistor means and then the voltage of this signal is increased by a step up transformer, with its output then applied to a piezo-electric sound transducer element. Typically the oscillator in prior art has been a simple “square wave” digital logic level source, i.e. one which is on for the same duration as it is off. This then produces current flow through the transistor or other amplifier means to the transformer primary winding for 50% of the period of the oscillator. The transformer secondary winding then sends current to the piezoelectric element which is primarily a capacitive load element C. Because the transformer secondary is also an inductor with inductance L, it forms, with the capacitive transducer, a simple tank circuit with a natural frequency f
RES
given by:
f
RES
=(2*&pgr;*(
L*C
))
−1
(1)
which in this invention is preferably best tuned to the about the same frequency as the primary natural mechanical resonant frequency of the transducer, which in many cases is from about 2 to 4 kHz. In any case, whether or not so tuned, when the square wave signal from the transistor is switched from the off to the conducting or on state, the secondary winding and the transducer will electrically start to “ring” or oscillate at the effective fundamental resonant frequency. However, at the moment when the transistor turns off, the transformer secondary voltage applied to the piezoelectric transducer abruptly reverses and the force on the transducer is reversed relative to when the voltage was first applied. The resulting effects on the amplitude of the motion of the transducer's radiating element can vary widely, for if it comes at the wrong time, it can slow, stop, reverse, or decrease the amplitude of the motion and hence impair the acoustic output. If it occurs at the ideal time it will significantly enhance the output.
It is an important part of this invention to replace the “square wave” oscillator with a rectangular pulse oscillator, i.e. one with a pulse with an independently adjustable “on” duration, which is independent of the “off” duration and hence is independent of the oscillator frequency. The time that the pulse is “on”, or the “on width”, is adjusted such that the end of the conducting period occurs at the time when the motion can best be increased in the direction it is already moving. This is analogous to first pushing a person in a swing, and then as the swing reverses direction pushing the swing back in the opposite direction to increase the range or amplitude of motion. So it is with the reverse voltage applied to the transducer. It must be timed optimally, or in other words, have the correct phase relationship to the already occurring oscillating transducer motion. The pulse generating oscillator is adjusted to have the optimum pulse “on width” which maximizes the motion of the transducer element. The benefit of this is to increase the peak acoustic amplitude output of the transducer.
An additional benefit is that in general the duration of the pulse is shorter than in the square wave case, so that less power is consumed. For example if the lower frequency is 300 Hz, with a period of 3.33 msec, and the resonant frequency is 2000 Hz, with a period of 0.50 msec, then the square wave pulse would have been on for one half of 3.33 msec or 1.67 msec, while in the improved case the “on width” of the pulse is best set on the order of about half of 0.50, or 0.25 msec. In such circuits which use a coupling transformer, the power supply current flow continues to increase during the on time of the
Lieu Julie
Pyle & Piontek
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