This paper investigates the behavior of piezoelectric elements as strain sensors. Strain is measured in terms of the charge generated by the element as a result of the direct piezoelectric ef- fect. Strain measurements from piezoceramic (PZT) and piezofilm (PVDF) sensors are compared with strains from a conventional foil strain gage and the advantages of each type of sensor are dis- cussed, along with their limitations. The sensors are surface bonded to a beam and are calibrated over a frequency range of 5-500 Hz. Correction factors to account for transverse strain and shear lag ef- fects due to the bond layer are analytically derived and experimentally validated. The effect of tem- perature on the output of PZT strain sensors is investigated. Additionally, design of signal condition- ing electronics to collect the signals from the piezoelectric sensors is addressed. The superior performance of piezoelectric sensors compared to conventional strain gages in terms of sensitivity and signal to noise ratio is demonstrated.

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Fundamental Understanding of Piezoelectric Strain Sensors

HIS article presents an overview and assessment of the technology leading to the development of intelligent structures. Intelligent structures are those which incorporate actuators and sensors that are highly integrated into the structure and have structural functionality, as well as highly integrated control logic, signal conditioning, and power amplification electronics. Such actuating, sensing, and signal processing elements are incorporated into a structure for the purpose of influencing its states or characteristics, be they mechanical, thermal, optical, chemical, electrical, or magnetic. For example, a mechanically intelligent structure is capable of altering both its mechanical states (its position or velocity) or its mechanical characteristics (its stiffness or damping). An optically intelligent structure could, for example, change color to match its background.17 Definition of Intelligent Structures Intelligent structures are a subset of a much larger field of research, as shown in Fig. I.123 Those structures which have actuators distributed throughout are defined as adaptive or, alternatively, actuated. Classical examples of such mechanically adaptive structures are conventional aircraft wings with articulated leading- and trailing-edge control surfaces and robotic systems with articulated manipulators and end effectors. More advanced examples currently in research include highly articulated adaptive space cranes. Structures which have sensors distributed throughout are a subset referred to as sensory. These structures have sensors which might detect displacements, strains or other mechanical states or properties, electromagnetic states or properties, temperature or heat flow, or the presence or accumulation of damage. Applications of this technology might include damage detection in long life structures, or embedded or conformal RF antennas within a structure. The overlap structures which contain both actuators and sensors (implicitly linked by closed-loop control) are referred to as controlled structures. Any structure whose properties or states can be influenced by the presence of a closed-loop control system is included in this category. A subset of controlled structures are active structures, distinguished from controlled structures by highly distributed actuators which have structural functionality and are part of the load bearing system.

Intelligent structures for aerospace - A technology overview and assessment

A finite element formulation for vibration control of a laminated plate with piezoelectric sensors/actuators is presented. Classical laminate theory with the induced strain actuation and Hamilton's principle are used to formulate the equations of motion. The total charge developed on the sensor layer is calculated from the direct piezoelectric equation. The equations of motion and the total charge are discretized with four-node, 12-degreeof-freedom quadrilateral plate bending elements with one electrical degree of freedom. The piezoelectric sensor is distributed, but is also integrated since the output voltage is dependent on the integrated strain rates over the sensor area. Also, the piezoelectric actuator induces the control moments at the ends of the actuator. Therefore, the number, size, and locations of the sensors/actuators are very important in the control system design. By selective assembling of the element matrices for each electrode, responses with various sensor/actuator geometries can be investigated. The static responses of a piezoelectric bimorph beam are calculated. For a laminated plate under the negative velocity feedback control, the direct time responses are calculated by the Newmark-/? method, and the damped frequencies and modal damping ratios are derived by modal state space analysis.

Finite Element Modeling of Piezoelectric Sensors and Actuators

Issues related to modeling and implementation of a self-sensing piezoelectric actuator are investigated The necessary formulation for modeling the simultaneous sensing and actuation phenomenon is provided Open and closed loop experiments performed on a cantilevered beam test specimen are described The sensitivity of the results to representative errors introduced in the implementation of the transducer is demonstrated analytically and experimentally The self-sensing actuator is also implemented using an active piezoelectric strut in a truss structure

Self-sensing piezoelectric actuation - Analysis and application to controlled structures

The twenty-first century could be called the 'Multifunctional Materials Age' The inspiration for multifunctional materials comes from nature, and therefore these are often referred to as bio-inspired materials Bio-inspired materials encompass smart materials and structures, multifunctional materials and nano-structured materials This is a dawn of revolutionary materials that may provide a 'quantum jump' in performance and multi-capability This book focuses on smart materials, structures and systems, which are also referred to as intelligent, adaptive, active, sensory and metamorphic The purpose of these materials from the perspective of smart systems is their ability to minimize life-cycle cost and/or expand the performance envelope The ultimate goal is to develop biologically inspired multifunctional materials with the capability to adapt their structural characteristics (such as stiffness, damping and viscosity) as required, monitor their health condition, perform self-diagnosis and self-repair, morph their shape and undergo significant controlled motion over a wide range of operating conditions

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Smart Structures Theory

A series of piezoelectric sensors which measure the strain rate of a structyre directly arc dcvclopcd hy recognizing the fact that'due to the high output irnpedancc nature oS piczoclectric scnsors the measured signal i.; strongly influenced by the impctlancc matching circuit. tlxperimentnl data which verifies thc performance of a local strain ratc sensor is presented. A uniaxial strain ratc sensor which cornplctely eliminates the cross-axis sensitivity is developed, as well as a pure shcar strain rate scnsor which measurcs the in-planc shcar rate. T o achieve rnorc clTcctivc distributed control, a singlc-input singlc-output multi-modc scnsor/actuator dcsign approach is prcscntcd for controlling N > I modes of a bcam type stlucturc which docs not exhibit any o r the spillover phcnomcnon into tlic uncontrolled modes typically fourid in othcr flcxihlc structure vibration control approaches. It is shown that tlic two-mode scnsor/actuator case is capahlc of achieving critical damping for both rnodcs undcr control. 'I'hc pcrformance of this sensor/actuator approach [or controlling rnorc than two modes are also prcscnted. Even though negative vclocity fccdback has traditionally been the primary fccdhack variablc uscd in flexible structure vihration control 11-31, in many cases velocity feedback is closely related to strain ratc control [4-61. Strain gagcs havc bccn used successf~illy Tor strain measurement during the last half century, but scnsors or gagcs that measure the local strain ratc of a structurr directly havc not been avaiiablc. Tratlitionallv,

T. O'sullivan

Papers

Piezoelectric strain rate sensor and actuator designs for active vibration control