Piezoelectric transducer

Details Piezoelectric transducer Piezoelectric transducer

2003-03-28

2004-03-02

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Piezoelectric polymers

C310S334000, C310S319000, C310S338000, C310S339000

Reexamination Certificate

active

06700314

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to piezoelectric sensors and actuators and, more particularly, to piezoelectric laminates, segmented piezoelectric film sensors, and the application thereof to modal analysis.
DESCRIPTION OF THE PRIOR ART
During the past decade, distributed piezoelectric sensors and actuators (collectively, transducers) have been used increasingly in the field of active vibration sensing and control. The basic idea is to bond a piezoelectric lamina onto a large surface of a thin vibrating structure. By collecting the piezoelectric charge induced by surface strains, one can estimate the vibrational state of the structure. Reversely or conversely, by applying an appropriate control voltage to the transducer, it is possible to excite (actuate) the structure and thus, for example, to generate an artificial damping effect or monitor structural behavior. Among the advantages of this approach are the light weight of the piezoelectric transducer and the possibility to embed the device in thin, flexible structures that are prone to vibrate. Use of this technology can be found from the aerospace industry to manufacturers of high-end consumer goods such as “smart skis” (i.e. skis that incorporate such transducer technology).
For this technology to be efficient, it is necessary for the polarization of the piezoelectric lamina to vary in a well-defined manner over the surface of the transducer. Since the piezoelectric sensitivity of a laminate is normally uniform, a weighting function must be applied to the transducer with the help of some controlled physical process.
It has been demonstrated that by measuring or inducing strains in a thin bending structure (such as a beam or plate) by means of a shaped piezoelectric lamina bonded on its surface, it is possible to sense or actuate directly a given modal contribution of a transverse motion, provided that the boundary conditions satisfy certain requirements that result in the orthogonality of these modes.
In this method, it can be shown that the local piezoelectric sensitivity of the transducer must follow an appropriate spatial sensing distribution (or weighting) function &Lgr;(&agr;
1
, &agr;
2
) in order to interact with a unique mode. It should be noted that the use of the two parameters &agr;
1
and &agr;
2
do not limit the domain of &Lgr; to a plane. Instead, &agr;
1
and &agr;
2
are bi-dimensional curvilinear coordinates that can define any point on the neutral surface of a three-dimensional shell. Since the piezoelectric sensitivity of a laminate is usually uniform, a weighting function must be implemented with the help of a controlled physical process. Sometimes, the operation of imposing a given polarization profile on a planar piezoelectric transducer is also called “shading”. There are several available procedures for shading.
In one procedure, the longitudinal piezoelectric constants of the piezoelectric material can be adjusted to a value proportional to a weight function &Lgr;(&agr;
1
, &agr;
2
), by means of a manufacturing method such as repoling, doping or the dosage of a two-phase composite. It should be noted however that these techniques are costly and difficult to implement in practice.
In another procedure, a given area of a piezoelectric transducer is made effective by the presence of a pair of electrodes which can either collect the charges generated piezoelectrically (sensor mode) or impose an electric potential between them (actuator mode). Thus, an elementary type of spatial weighting is obtained by limiting the areas provided with electrodes. In this case, the weighting function takes only two values: &Lgr;=1 inside the shape boundaries, or &Lgr;=0. In practice, a photolithographic process can implement this profile.
For a beam, it is sufficient to implement a one-dimensional weight function. In this case, the appropriate variation of piezoelectric sensitivity can be obtained by varying the width of the electrode down the length of the beam since there is no deflection along this direction. To impose the correct sign of the weight (spatial sensing distribution) function, areas are defined where the polarization of the piezoelectric material should be accordingly positive or negative, either by bonding the laminate “face up” or “face down”, or by inverting its contacting electrodes.
In the more general case of a structure whose deflection varies along two directions, a bi-dimensional spatial sensing distribution function can be approximated by a lattice of small electrodes that are turned either “on” or “off”. However the task remains to connect electrically the individual active electrodes and to impose the correct function sign. In practice, this is only feasible if the “on” electrodes happen to be grouped in a few contiguous domains.
In spite of the elegance of their concept, modal sensors and actuators have an important limitation. A manufacturing process allowing control the weight function by repoling, doping, or dosing a two-phase composite is usually not available at the level of the application engineer. Furthermore, these steps would be very costly to implement. As a consequence modal sensors have not been applied to structural elements, such as plates and shells, unless their weight function could be reduced to a one-dimensional function or a product of such functions by separation of variables.
This difficulty and other factors have promoted the use of an approximated version of modal sensors, called segmented piezoelectric sensors. In this design the distributed effect of a piezoelectric laminate is replaced by an array of size-limited, discrete piezoelectric sensors, each of which are measured separately, and the outputs of which are being sampled, multiplied by discrete weight factors (calculated by the method of modal filtering), and then added. The main advantage of this method is that it shifts the operation of fixing the weight factors from the manufacturing process to an electronic operation, making it much more flexible. Thus, except for the number of channels, it is not more difficult to build such a system for a variety of shells and plates rather than a beam.
However, segmented piezoelectric sensors also have their shortcomings. For one, segmented piezoelectric sensors are only able to model a finite number of modes (the more transducers in the array, the higher this number). If unmodelled modal contributions (residual modes) are present, they constitute a source of noise. For another, segmented piezoelectric sensors are much less compact than modal sensors. Each channel requires a full measurement chain including coaxial cable, low current or charge sensitive amplifier and analog to digital converter. Furthermore, a digital signal processing board is required to carry out the computations to estimate the modal coordinates. By contrast, a modal sensor simply needs a unique coaxial cable and a low current or charge sensitive amplifier. For this reason it is more difficult to embed segmented piezoelectric sensors as elements of an intelligent structure. Finally, because of the analog to digital conversions involved with segmented piezoelectric sensors, it is difficult to include segmented transducers in sensing-actuating applications, such as the frequency-stabilizing element in a resonator. On the other hand, the use of a modal sensor in a resonating circuit is straightforward and may open the door to applications where the frequency of such a system could be used to monitor physical parameters, like the temperature of the structure or a variation of pressure exercised on it.
Because of the limitations imposed by these technologies, it is very difficult to obtain a polarization profile whose shape varies in function of two geometric dimensions. Thus, in order to apply this active control scheme to structures whose deflections vary along two independent coordinates, such as vibrating plates and shells, one of the following simplifying methods is normally used.
One such method is that the behavior of a bending plate may be approxim

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