Piezoelectric Sensor Elements Research Articles

Structural Aspects and Adhesion of Polyurethane Composite Coatings for Surface Acoustic Wave Sensors

Surface acoustic wave-based (SAW) sensors are of great interest due to their high sensibility and fast and stable responses. They can be obtained at an overall low cost and with an intuitive and easy-to-use method. The chemical sensitization of a piezoelectric transducer plays a key role in defining the properties of SAW sensors. In this study, we investigate the structural and adhesion properties of a new class of coating material based on polyurethane polymeric composites. We used dark-field microscopy (DFM) and scanning electron microscopy (SEM) to observe the microstructure of polyurethane composite coatings on piezoelectric sensor elements and to analyze the effects of the chemical resistance and adhesion test (CAT) on the coating layers obtained with the polyurethane polymeric composites. The results of the microscopy showed that all polyurethane composite coatings exhibited excellent uniformity and stability after chemical adherence testing (CAT). All of the observations were correlated with the results of the ultrasonic analysis, which demonstrated the role of polyurethane as a binder to form the stable structure of the composites and, at the same time, as an adhesion promoter, increasing the chemical resistance and the adherence of the coating layer to the complex surface of the piezoelectric sensor element.

Sensitive, small, broadband and scalable optomechanical ultrasound sensor in silicon photonics

Ultrasonography1 and photoacoustic2,3 (optoacoustic) tomography have recently seen great advances in hardware and algorithms. However, current high-end systems still use a matrix of piezoelectric sensor elements, and new applications require sensors with high sensitivity, broadband detection, small size and scalability to a fine-pitch matrix. This work demonstrates an ultrasound sensor in silicon photonic technology with extreme sensitivity owing to an innovative optomechanical waveguide. This waveguide has a tiny 15 nm air gap between two movable parts, which we fabricated using new CMOS-compatible processing. The 20 μm small sensor has a noise equivalent pressure below 1.3 mPa Hz−1/2 in the measured range of 3–30 MHz, dominated by acoustomechanical noise. This is two orders of magnitude better than for piezoelectric elements of an identical size4. The demonstrated sensor matrix with on-chip photonic multiplexing5–7 offers the prospect of miniaturized catheters that have sensor matrices interrogated using just a few optical fibres, unlike piezoelectric sensors that typically use an electrical connection for each element. An optical ultrasound sensor based on a CMOS-compatible split-rib waveguide is demonstrated, offering high sensitivity, broadband detection (measured 3–30 MHz), small size (20 μm) and scalability to a fine-pitch matrix.

Glass Fiber‐Reinforced Polyurethane Composite Structures with Integrated Piezoelectric Sensor Elements and Corresponding Electronics

In this contribution, process‐integrated fabrication and embedding of piezoelectric sensors based on free‐flowing starting components into glass fiber‐reinforced polyurethane composite structures and their characterization is presented. Main focus is investigating the influence of the polarization temperature on the resulting sensor properties. For this purpose, glass fiber‐reinforced polyurethane composite structures with integrated sensors are manufactured, polarized at different temperatures and then deformed by means of cyclic 3‐point bending tests. The charge signals generated by the integrated sensors are recorded by means of a specially developed measurement setup. The results show that the charges generated by the sensors can be significantly increased by polarization at elevated temperature. Furthermore, the development of associated sensor electronics is discussed and concepts as well as an implemented prototype of the integrated protection circuit for high voltage application during polarization are presented.

Studies on the Characterization of Novel Piezoelectric Sensor Elements, Integrated in Glass Fibre-reinforced Polyurethane Composites

Within this contribution, studies on the characterization of piezoelectric sensor elements embedded in composite structures are presented. Using piezoceramic components like fibres and pearls as well as appropriate electrode structures, novel piezoelectric transducers are created and locally embedded into the composite structure, mainly to realize sensory functions. After embedding, the piezoelectric functional layer is polarized to activate the sensor. Various specifications of these sensors with different piezoceramic components and electrodes have been characterized using cyclic three-point bending and compression tests. Hereby, bending respectively compression of the composite structure leads to deformation of the embedded piezoceramic components and generation of electric charges due to the piezoelectric effect. Even if the values of the determined electric charges are relatively small, the functional capability of the novel sensors could be proven.

Development of a novel pulse wave velocity measurement system: Using dual piezoelectric elements

The aim of this study is to develop a painless system of measuring the brachial-ankle arterial pulse wave velocity (baPWV) without compression cuffs. The PWV reflects the compliance of the artery and is measured for the early diagnosis of arteriosclerotic vascular diseases. However, the conventional baPWV system, which measures four cuff pressures simultaneously, easily causes circulation block and tightening pain at the extremities. In addition, approximately 15min are required to stabilise the blood pressure for re-examination. Therefore, we developed a novel baPWV measurement system using dual piezoelectric sensor elements. The principle of this high-sensitivity pressure pulse detection system is based on adding the two in-phase outputs from the coaxially arranged dual piezoelectric sensor. As our system facilitates the measurement of the baPWV by detecting the pulsation of an artery using sensors fixed on the skin where the pulse is palpable, it does not cause pain and reduces examination time. The coefficients of correlation between the baPWV values obtained from the conventional and present methods were 0.93 (right side) and 0.90 (left side). The results suggest that our system can be used to measure the baPWV without pressure cuffs as accurately as the conventional method.

Low-Temperature Solution Processable Electrodes for Piezoelectric Sensors Applications

Piezoelectric thin-film sensors are suitable for a wide range of applications from physiological measurements to industrial monitoring systems. The use of flexible materials in combination with high-throughput printing technologies enables cost-effective manufacturing of custom-designed, highly integratable piezoelectric sensors. This type of sensor can, for instance, improve industrial process control or enable the embedding of ubiquitous sensors in our living environment to improve quality of life. Here, we discuss the benefits, challenges and potential applications of piezoelectric thin-film sensors. The piezoelectric sensor elements are fabricated by printing electrodes on both sides of unmetallized poly(vinylidene fluoride) film. We show that materials which are solution processable in low temperatures, biocompatible and environmental friendly are suitable for use as electrode materials in piezoelectric sensors.

Solution-processible electrode materials for a heat-sensitive piezoelectric thin-film sensor

Piezoelectric sensors are needed in a wide range of applications from physiological measurement applications to industrial monitoring systems. Custom-designed, highly integratable and cost-effective sensor elements can be manufactured by using flexible materials in combination with high-throughput printing for fabrication. This would also enable the embedding of ubiquitous sensors in our living environment to improve the common welfare. Here, we have fabricated flexible piezoelectric sensor elements using printing methods. We demonstrated that alternative, printable electrode materials are compatible with temperature-sensitive functional substrates. Low-temperature curable electrodes were printed on both sides of unmetallized, polyvinylidene fluoride film. Various solution-processible materials – for example, carbon nanotube–cellulose composite, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), carbon ink and silver flake ink – were used. DC sheet resistances of the electrodes and sensitivities of the sensor elements were measured. Sheet resistance varied from 10−1 to 106Ω/□ and sensitivities varied from 23 to 29pC/N. Evaporated metal electrodes as well as commercially available sensor elements were used as reference sensors. The sensing capability of printed sensors was quite insensitive to the sheet resistance of the electrodes. Further, the electrode conformation was studied by microscopy; reliability of the sensors was studied by vibration tests. In addition, sensors were tested for measuring physiological signals.

Applying tactile sensing with piezoelectric materials for minimally invasive surgery and magnetic-resonance-guided interventions

Medical technologies have undergone significant development to overcome the problems inherent in minimally invasive surgery such as inhibited manual dexterity, reduced visual information, and lack of direct touch feedback to make it easier for surgeons to operate. A minimally invasive tool incorporating haptic feedback is being developed to increase the effectiveness of diagnostic procedures by providing force feedback. Magnetic resonance imaging guidance is possible to allow tool localization; however, this engenders the requirement of magnetic resonance compatibility on the device. This paper describes the work done towards developing a sensing device using piezoelectric sensor elements to locate subsurface inclusions in soft substrates, with its magnetic resonance compatibility tested in a 1.5 T scanner. Results show that the position of a hard inclusion can be determined.

Development of Metallic Digital Strain Gauges

A joint Brunel-Southampton Universities’ research team has developed digital strain gauges based on a metallic triple-beam resonator structure with thick-film piezoelectric sensor elements. The resonator, an oscillating structure vibrating at resonance, is designed such that its resonant frequency is a function of the measurand. The resonator substrate was fabricated by a double-sided photochemical etching technique and the thick-film piezoelectric elements were deposited by a standard screen-printing process. The new metallic digital strain gauges can be used on stiff structures, have high overload capacities, low power consumption, frequency output for digital processing, and offer prospects for wireless-batteryless operation. The device can be easily mass-produced at low cost for use in a wide range of measuring systems, e.g. load cells, weighing machines, torque transducers and pressure sensors.

Development of Metallic Digital Strain Gauges

A joint Brunel-Southampton Universities’ research team has developed digital strain gauges based on a metallic triple-beam resonator structure with thick-film piezoelectric sensor elements. The resonator, an oscillating structure vibrating at resonance, is designed such that its resonant frequency is a function of the measurand. The resonator substrate was fabricated by a double-sided photochemical etching technique and the thick-film piezoelectric elements were deposited by a standard screen-printing process. The new metallic digital strain gauges can be used on stiff structures, have high overload capacities, low power consumption, frequency output for digital processing, and offer prospects for wireless-batteryless operation. The device can be easily mass-produced at low cost for use in a wide range of measuring systems, e.g. load cells, weighing machines, torque transducers and pressure sensors.

Vibration Control of Plates Using Discretely Distributed Piezoelectric Quasi-Modal Actuators/Sensors

A novel approach is presented for vibration control of smart plates using discretely distributed piezoelectric actuators and sensors. The new method consists of techniques for designing quasi-modal sensors and quasi-modal actuators. The modal coordinates and the modal velocities are obtained approximately from the outputs of the discretely distributed piezoelectric sensor elements, whereas the modal actuators are implemented by applying proper voltages on each actuator element. The observation error of the modal sensor is analyzed, and an upper bound for the observation error is determined. The control spillover of the modal actuator is also estimated, and an upper bound of the control spillover is also found. The criteria are developed for finding the optimal locations and sizes of both piezoelectric sensor and actuator elements. In the optimality criteria the optimal locations and sizes of the sensor elements can be found by minimizing the observation error of the modal sensor, and those of the actuator elements can be obtained by minimizing both the control energy and the control spillover. The results obtained using the present optimal criteria show that they do not depend on the initial condition of vibration of the structures, nor do they depend on the control gains.

Multi-mode accelerometer

An accelerometer (10) having a piezoelectric transducer (22) which produces electrical signals in response to its deflection and comprises an elongated, continuous beam of generally flat configuration and narrow cross section to provide a bending axis which facilitates deflection of the free ends in response to shock in one or more of the linear and torsional modes. The beam is supported intermediate its ends by a mount (29), with the free ends of the beam preferably extending equidistantly in opposite directions from the mount. By laminating a single piece of piezoelectric material to a rigid substrate and forming cantilevers in each deflection direction, an accelerometer with six degrees of freedom may be formed. Also, a compressive type angular accelerometer (122) may be formed from two piezoelectric sensor elements incorporated onto a single piece of piezoelectric polymer (126).