Coupler for electrical waveguides and mechanical waveguides

A system for coupling energy between electrical and mechanical waves includes a mechanical waveguide for propagating a mechanical wave having a mechanical wavelength at a given frequency, and an electromechanical energy converter for coupling energy between electrical and mechanical waves attached to a portion of the waveguide and capable of propagating an electrical wave having an electrical wavelength substantially equal to the mechanical wavelength at the given frequency. The portion has a length, measured in units of coupled wavelength, which is selected on the basis of the reciprocal of the coupling strength of the electromechanical converter and a selected amount of wave energy to be coupled. The function is based primarily on desired efficiency and may also be an odd integer multiple of the coupling strength reciprocal, preferably one. Piezoelectric elements are the preferred electromechanical energy conversion elements. This system is applicable to damping of structural waves, transferring structural waves from one mechanical waveguide- to another, and for creating a linear motor.

FIELD OF THE INVENTION 
The present invention relates to systems for converting energy between 
mechanical waves and electrical waves. More particularly, the invention 
relates to systems for coupling wave energy between mechanical and 
electrical waveguides and to applications of coupling systems for damping, 
isolating, and creating resonance of mechanical waves. 
BACKGROUND OF THE INVENTION 
Systems for coupling wave energy are commonly available for optical and 
microwave waveguides. Such systems couple energy of a wave of one type 
propagating in one waveguide into wave energy of the same type propagating 
in a second waveguide. The present work relates to such coupling between 
electrical and mechanical waves in electrical and mechanical waveguides. 
The closest similar work known to the Applicants is that of Baer and Kino 
("A Travelling Wave Ultrasonic Transducer," in Proc. 1982 Ultrasonics 
Symposium, pp. 498-501, San Diego, Oct. 27-29, 1982) who considered 
coupling an electrical delay line to a piezoelectric stack to generate 
longitudinal waves in the stack, thus creating an ultrasonic transducer. 
Coupling into and from a mechanical waveguide was not performed. Hagood 
and von Flotow ("Damping Of Structural Vibrations With Piezoelectric 
Materials And Passive Electrical Networks," J. Sound And Vibration, Vol. 
146, No. 2, pp. 243-268, 1991) considered tuned L-R-C coupling to 
vibrating structures. 
Known components which are capable of converting mechanical energy to 
electrical energy include piezoelectric, electrostrictive, 
magnetostrictive and electromagnetic devices. Such components have been 
used, for example, for vibration damping, sensing, motors, and 
transducers. Such systems have not been used for coupling wave energy 
between electrical and mechanical waves. 
Accordingly, it is an object of the present invention to provide a system 
for coupling energy between electrical and mechanical waves. 
SUMMARY OF THE INVENTION 
To accomplish the foregoing and other objects, there is provided a system 
for coupling energy between electrical and mechanical waves. The system 
includes a mechanical waveguide for propagating a mechanical wave having a 
mechanical wavelength at a given frequency and an electromechanical energy 
converter, for coupling energy between electrical and mechanical waves. 
The electromechanical energy converter, or waveguide coupler, which has a 
specific coupling strength with the waveguide is attached to a portion of 
the mechanical waveguide, and is capable of propagating an electrical wave 
having an electrical wavelength substantially equal to the mechanical 
wavelength at the given frequency. The coupled portion of the waveguide 
has a length, measured in units of the coupled wavelength, which is 
selected on the basis of the reciprocal of the coupling strength and a 
selected amount of energy to be coupled. This coupling length is 
preferably substantially equal to an odd integer multiple (preferably one) 
of the reciprocal of the coupling strength of the electromechanical 
converter. Normally, this length is greater than several wavelengths. 
In a preferred embodiment of the present invention, the electromechanical 
converter includes piezoelectric elements distributed over the coupling 
portion of the mechanical waveguide. The number of piezoelectric elements 
used is preferably greater than two per coupled wavelength. In a 
particular embodiment, the mechanical waveguide is an aluminum beam and 
the electrical wave is propagated by an L-C ladder circuit. 
Another embodiment of the present invention includes an electrical 
impedance which dissipates electrical energy obtained by coupling the 
energy of a mechanical wave. This embodiment is particularly useful for 
damping mechanical waves in a structure. 
In another embodiment of the present invention, the energy received in an 
electrical waveguide is converted back into mechanical energy in a second 
waveguide. Such an embodiment is useful, for example, for isolating 
mechanical structures from waves propagating in other surrounding 
structures. 
In yet another embodiment of the present invention, the electrical energy 
converted from mechanical energy at one end of the mechanical waveguide, 
e.g. a beam, is coupled back into the opposite end of the same waveguide, 
thus forming a loop. When the loop through which the electrical and 
mechanical waves flow has a length equal to an integer multiple of the 
wavelength, resonance may be obtained. This structure is useful, for 
example, for creating a linear motor when the mechanical wave in the beam 
is a bending wave.

DETAILED DESCRIPTION 
In the following detailed description of illustrative embodiments of the 
present invention, similar reference numbers are utilized to indicate 
similar structures. It should be understood that the embodiments shown in 
these figures are merely examples of the present invention shown for 
illustrative purposes and that numerous modifications to these examples 
should be apparent to those of ordinary skill in the art from the detailed 
description below. 
Referring now to FIG. 1, a mechanical waveguide 11 is coupled to an 
electrical waveguide 12 via an energy converter or waveguide coupler 13. 
Although the mechanical waveguide 11 as shown in FIG. 1 is a solid beam, 
the invention may also be applicable to other mechanical waveguides in 
which kinetic and potential wave energy may be propagated, including both 
fluid and solid, discrete and continuous systems. The electrical waveguide 
12 shown in FIG. 1 is an L-C ladder circuit including inductors L and 
capacitors C. Other types of electrical waveguides may also be used to 
receive an electrical wave propagated in the coupler 13 in response to a 
wave in the mechanical waveguide 12. The coupler 13 may also receive an 
electrical wave, from either an electrical wave generator, for instance, 
or electrical waveguide 12, for coupling it to the mechanical waveguide 
11. Other electrical circuits which receive or provide an electrical wave 
may also be used in place of an electrical waveguide 12 provided that the 
electrical and mechanical wave speeds remain matched. 
Coupler 13 includes electromechanical energy conversion elements. A variety 
of such elements are well known, and include piezoelectric, 
electrostrictive, magnetostrictive, and electromagnetic elements. These 
elements have both mechanical and electrical properties which provide a 
coupling strength with a waveguide which represents the ratio of the input 
energy of one type of wave to the output energy of a second type of wave. 
That is, the coupling strength of the elements in response to a mechanical 
wave represents how much mechanical energy of the wave is transferred by 
the elements and how much of the transferred energy is converted to 
electrical energy by the elements. Conversely, the coupling strength of 
the elements in response to an electrical wave represents how much 
electrical energy of the wave is transferred by the elements and how much 
of the transferred energy is converted to mechanical energy by the 
elements. The amount of wave energy transferred by the elements depends on 
how well the elements are coupled to receive the wave and is known as the 
efficiency factor. The amount of transferred energy which is converted by 
the elements is a known constant for given elements and is known as the 
coupling coefficient. For example, the electromechanical energy conversion 
ratio for piezoceramics may be obtained from suppliers or from the IEEE 
Standard 176--1978 on Piezoelectricity. In general, the coupling strength 
is the product of the coupling coefficient and the efficiency factor. 
Given the coupling strength of the electromechanical elements used in 
coupler 13, an optimal coupling length (the portion of the mechanical 
waveguide to which the coupler 13 is connected) may be determined. This 
determination will be described in more detail below. 
The construction of the mechanical waveguide 11, electrical waveguide 12 
and coupler 13, in accordance with the present invention, is dependent on 
at least the desired operating conditions of the waveguides. That is, both 
waveguides and the coupler are tuned to propagate a signal having 
substantially the same wavelength at a given frequency. The mechanical 
properties of the mechanical waveguide 11 and coupler 13, particularly- 
stiffness and mass, and the electrical properties of the electrical 
waveguide 12 (if used) and coupler 13, particularly inductance and 
capacitance, are selected so that the wavelength of a wave propagated in 
the mechanical waveguide at a selected frequency is substantially equal to 
the wavelength of a wave propagated in the electrical waveguide at the 
selected frequency. In many applications, the mechanical structure and the 
frequency of the mechanical wave are both predetermined. In such cases, 
the inductance and capacitance of the coupler 13 and the electrical 
waveguide 11 are adjusted to obtain matching of the wavelength and 
frequency of the electrical and mechanical waves. In other applications, 
the frequency may be adjustable which provides more degrees of freedom for 
tuning of the waveguides (11, 12) and the coupler 13. 
Given the operating conditions, including the desired wavelength and 
frequency of the wave to be coupled, an optimal coupling length (as 
mentioned above) may be determined. This length is the distance between 
the distant edges of the electromechanical energy conversion elements of 
the coupler 13. The optimal coupling length, measured in units of the 
wavelength of the coupled wave, is substantially equal to an odd integer 
multiple (preferably one) of the reciprocal of the coupling strength. The 
most efficient coupling may be obtained when the coupling length is 
exactly equal to the reciprocal of the coupling strength. At optimal 
coupling, all of the signal of one type of wave is converted to the other 
type of wave. Efficiency may be reduced according to the following 
cosinusoidal function of the deviation from the optimal coupling length, 
assuming matched phase speeds: 
EQU 1/2(cos(deviation.pi./optimal length)+1) (1) 
Efficiency is reduced because the wave is either incompletely coupled, 
possibly resulting in reflections of the wave, or over-coupled, resulting 
in the conversion of the propagating wave back into the original type of 
wave in the waveguide of origin. Both types of non-optimal coupling result 
in the formation of a standing wave in the original waveguide if the 
waveguide has an end off which reflections occur. Deviations from the 
optimal coupling length may also result from deviations in the operating 
conditions and in the materials used in the waveguides or coupler. Such 
deviations may make exact tuning difficult to obtain; however, substantial 
efficiency may still be obtained. For most applications, an efficiency of 
90% or better is preferable. In general, the selection of the coupling 
length should be based on the optimal coupling length. It should be 
understood that a fraction of the optimal coupling length may be used if 
only a part of a wave is to be coupled. This fraction may be determined 
according to the error function (1) above. 
The coupling used in the system of the present invention is weak, because 
only a small fraction of a wave is coupled per wavelength. As illustrated 
in FIG. 1 (though not to scale), the amount of wave transferred between 
waveguides gradually increases from one end of the coupler (21) to the 
other end (22). Assuming that the coupler 13 has the optimal coupling 
length, all of the wave is transferred by the end of the coupler. If the 
coupler 13 were longer, a wave propagating in the coupler would begin to 
be converted back into the other type of wave. Weak coupling results in 
reduced reflections due to sudden change in the characteristics of the 
waveguide. 
The embodiment shown in FIG. 1 is a particular application of the present 
invention for coupling of bending waves in a continuous mechanical 
waveguide to an electrical wave in a discrete electrical waveguide. The 
electromechanical elements are piezoelectric elements 14, which are 
oriented to respond more strongly to bending waves than to other types of 
waves. The manner of orientation of these elements to obtain efficient 
coupling of an arbitrary wave is known to those of ordinary skill in the 
art. As mentioned above, other types of electromechanical elements may 
also be used. The maximum spacing of multiple piezoelectric elements 14, 
in this embodiment, though not shown to scale in FIG. 1, depends on the 
Nyquist criterion. That is, more than two piezoelectric elements per 
wavelength are required, because these elements may be considered as 
samplers of the mechanical wave. Several elements per wavelength are 
preferable because the sampling resolution is increased. Also, coherent 
scattering may occur when the number of elements per wavelength is an 
integer, resulting in the generation of a standing wave. This problem may 
be reduced by providing a non-integer number of elements per wavelength. 
In the embodiment shown in FIG. 1, adjacent piezoelectric elements 14 are 
interconnected via inductors 15 in a manner similar to the construction of 
the LC ladder of the electrical waveguide 12. For tuning purposes, the 
inductance of inductors 15 and the capacitance of the piezoelectric 
elements 14 are considered as part of the electrical waveguide 12. Also 
for tuning purposes, the mechanical properties of at least the 
piezoelectric elements 14 are also considered in determining the 
properties of the mechanical waveguide 11. In fact, any non negligible 
mechanical effects of the coupler 13 and electrical waveguide 12 on the 
mechanical waveguide 11 need to be considered when determining the wave 
propagation properties of the mechanical waveguide 11. It is possible to 
substantially minimize the mechanical effects of the electrical waveguide 
12 on the mechanical waveguide 11 by providing a connection which is soft 
and adds negligible stiffness. For instance, copper wires 16 having a 
small diameter, for example 100 microns, may be used to connect the 
piezoelectric elements to the inductors 15. 
For an implementation of the embodiment shown in FIG. 1, an aluminum beam 
with a single-sided lamination of piezoelectric material was used. With 
this construction, the mechanical effects of the piezoelectric material on 
the mechanical waveguide are easily understood. The beam had a length of 
670 mm, a width of 12.7 mm and a thickness of 2.191 mm (2 mm of aluminum, 
191 .mu.g of piezoelectric material). The piezoelectric material was 
etched to form a number of electrodes, and thus piezoelectric elements. 
Each element had a length of about 6.35 mm; the coupling length was about 
370 mm. Adjacent piezoelectric segments were interconnected by inductors 
having an inductance of 50 mH and a resistance of 70 ohms. It was 
determined that maximum coupling could be obtained at a frequency of 7.8 
kHz and a wavelength of 46 mm, resulting in a phase speed of 360 meters 
per second. 
One application of the system of the present invention is for damping waves 
traveling through structures. An illustrative embodiment is shown in FIG. 
2, utilizing the coupler 13 shown in FIG. 1. The mechanical waveguide 11 
may be part of a structure in which a wave travels. For example, an 
airplane engine may generate waves in the skin of the airplane wing. The 
coupler 13 converts the energy of the mechanical wave into electrical 
energy which is output at the end of coupler 13 into an impedance 17, such 
as a resistor. The impedance should provide a matched termination to 
eliminate reflections of the electrical wave. 
The energy conversion system of the present invention may also be used for 
transferring mechanical energy in one structure, or mechanical waveguide, 
to another structure, or mechanical waveguide. An illustrative embodiment 
of this use is shown in FIG. 3. Similar to the embodiment shown in FIG. 1, 
a mechanical waveguide 11 and an electrical waveguide 12 are 
interconnected by a coupler 13. In a similar fashion, a second mechanical 
waveguide 11' is connected to the electrical waveguide 12 via a second 
coupler 13'. It should be understood that the electrical waveguide 12 in 
this instance may also be a wire. Using such a system, a wave propagating 
through one mechanical waveguide may be transferred into a second 
mechanical waveguide. A particular use of this application involves 
isolation of mechanical structures from mechanical waves. For instance, a 
wave may be transferred around a structure which may cause noise or 
reflection, such as a point of increased stiffness, e.g. due to an 
attachment point, such as a bolt 23, on the structure. If a mechanical 
wave in a structure is completely absorbed by a coupler 13, the wave may 
be transferred to another part of the same structure as an electrical 
wave, and recoupled into the structure by another coupler 13. Portions of 
the structure intermediate the two couplers 13 are thus isolated from 
mechanical waves in the structure. 
Yet another application of the present invention involves using a coupler 
13 to implement a linear motor, as shown in FIG. 4. Prior art linear 
motors normally consist of a circular or oval mechanical waveguide, and a 
drive mechanism which generates a mechanical wave in the waveguide. The 
mechanical wave is not converted into an electrical wave. The circular or 
oval shape provides a loop through which the mechanical wave propagates, 
whereby resonance is obtained when the length of the loop is an integer 
multiple of the wavelength of the mechanical wave. 
A coupler according to the present invention may be used in combination 
with singular mechanical d 11 such as a beam which has two ends, and a 
drive 20 to create a linear motor. The drive 20 may be connected in a 
manner similar to the endless loop type of linear motors of the prior art. 
The o mechanical waveguide 11 is provided with couplers 13 and 13' at its 
respective ends which are tuned, along with the drive 20, to a wavelength 
at a selected frequency. The implementation shown in FIG. 4 is 
particularly suitable for bending wave lingar motors. 
Assuming, in FIG. 4, that the wave in the mechanical waveguide is traveling 
from left to right, the coupler 13' converts the energy of the mechanical 
wave into electrical energy. The output of coupler 13' is coupled to the 
input of the second coupler 13 which is connected to the opposite end of 
the mechanical waveguide 11. The coupling of the two couplers may be 
completed simply by a wire 18, provided that resonance may be obtained. 
That is, the electrical wave fed back to coupler 13 and converted into a 
mechanical wave should dynamically reinforce the wave in the mechanical 
waveguide 11 produced by drive 20 (i.e., the drive signal and feedback 
signal are in phase). In order to obtain resonance, the effective length 
of the loop through which the mechanical and electrical waves propagate 
should be equal to an integer multiple of the wavelength of the wave being 
propagated. If the coupling length is optimal and the coupler is 
appropriately matched to the mechanical wave, the length of the loop is 
determined by the length of the mechanical waveguide from the beginning of 
the first coupler 13 to the end of the second coupler 13' as shown by, 
respectively, points A and B in FIG. 4, along with the effective length of 
any electrical waveguide between the output of the second coupler 13' and 
the input of the first coupler 13. The length of the electrical waveguide 
is equal to the ratio of the phase shift to 360.degree. provided by the 
waveguide at the operating frequency. The criterion for resonance may also 
be understood as requiring the total phase shift through the loop to be 
equal to 2k.pi., where k is an integer. A wire, such as shown in FIG. 4, 
has effectively a negligible length for calculating the length of this 
loop. If the length of the loop is not an integer multiple of the 
wavelength, inductors and capacitors may be added as part of the coupling 
18 between the couplers 13 and 13'. For this purpose, it may be preferable 
to provide a tunable LC circuit which allows for compensation of the 
length of this loop due to variations in the operating frequency. Other 
options for tuning the system to achieve resonance include varying the 
drive frequency, and thus the wavelength, of the propagating wave, and 
varying the length of the mechanical waveguide by adjusting the relative 
positions of couplers 13 and or 13' or by other suitable means. 
Modifications of and adaptations to the present invention may also include 
providing a tapered coupling of strength gradually increasing with length 
along the coupler. That is, the coupling strength of the electromechanical 
energy converter could be varied as a function of the position within the 
length of the coupler. Such a modification may be made, for example, by 
providing different electromechanical energy converters with different 
coupling strengths. By gradually increasing the coupling strength, 
possible negative effects of coupling, such as reflection due to the 
sudden change in the characteristics of the waveguide, may be reduced. 
Having now described a few embodiments of the invention, it should be 
apparent to those skilled in the art that the foregoing is illustrative 
only and not limiting, having been presented by way of example only. 
Numerous other embodiments and modifications thereof are contemplated as 
falling within the scope of the present invention as defined by the 
appended claims and equivalents thereto.