Dynamic analysis of capacitive micromachined ultrasonic transducers

Electrostatic transducers are usually operated under a DC bias below their collapse voltage. The same scheme has been adopted for capacitive micromachined ultrasonic transducers (cMUTs). DC bias deflects the cMUT membranes toward the substrate, so that their centers are free to move during both receive and transmit operations. In this paper, we present time-domain, finite element calculations for cMUTs using LS-DYNA, a commercially available finite element package. In addition to this DC bias mode, other new cMUT operations (collapse and collapse-snapback) have recently been demonstrated. Because cMUT membranes make contact with the substrate in these new operations, modeling of these cMUTs should include contact analysis. Our model was a cMUT transducer consisting of many hexagonal membranes; because it was symmetrical, we modeled only one-sixth of a hexagonal cell loaded with a fluid medium. The finite element results for both conventional and collapse modes were compared to measurements made by an optical interferometer; a good match was observed. Thus, the model is useful for designing cMUTs that operate in regimes where membranes make contact with the substrate.     

电容式微加工超声传感器(cMUT)的结构设计及仿真

与传统的压电传感器相比,电容式微加工超声传感器(cMUT)性能优良,有着广泛的应用前景．为此．介绍了cMUT的基本结构和工作原理,并提出了一种新型结构．利用有限元分析软件ANSYS对这种结构进行模态分析．得到其共振频率和一阶振动图;利用ANSYS的耦合场分析模块和参数化设计语言(APDL),对结构进行力电耦合仿真,获得其直流偏压和作为发射器时所需的交流电压值．分析结果表明,所设计的结构符合设计要求．     

Finite element analysis of underwater capacitor micromachined ultrasonic transducers

A simple electromechanical equivalent circuit model is used to predict the behavior of capacitive micromachined ultrasonic transducers (cMUT). The equivalent circuit model of the cMUT lacks important features such as coupling to the substrate and the ability to predict crosstalk between elements of an array of transducers. To overcome these deficiencies, a finite element model of the cMUT is constructed using the commercial code ANSYS(R). Calculation results of the complex load impedance seen by single capacitor cells are presented, then followed by a calculation of the plane wave real load impedance seen by a parallel combination of many cells that are used to make a transducer. Crosstalk between 1-D array elements is found to be due to two main sources: coupling through a Stoneley wave propagating at the transducer-water interface and coupling through Lamb waves propagating in the substrate. To reduce the crosstalk level, the effect of structural variations of the substrate are investigated, which includes a change of its thickness and etched trenches or polymer walls between array elements     

A new regime for operating capacitive micromachined ultrasonic transducers

We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k/sub T//sup 2/) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes.     

Directional scholte wave generation and detection using interdigital capacitive micromachined ultrasonic transducers

Directional generation and detection of Scholte waves and other guided modes in liquids and microfluidic channels by capacitive micromachined ultrasonic transducers (cMUTs) is reported. An interdigital transducer structure along with a phased-excitation scheme is used to enhance the directionality of Scholte interface waves in microfluidic environments. Finite element models are developed to predict the performance of the devices in both fluid half-spaces and microchannels. Experiments on the interdigital cMUTs show that a five-finger-pair device in a water half-space has 12 dB of directionality in generating Scholte waves at the design frequency of 10 MHz. A 10-finger device operating at 10 MHz in a water-filled microchannel has 13.4 dB of directionality. These directionality figures agree well with the modeling results. Using the results of the finite element model of a cMUT in a fluid half-space, it was determined that 41% of the acoustic power radiated into the fluid is contained in the Scholte wave propagating in the desired lateral direction. Transducers are demonstrated to perform bidirectional pumping in fluid channels with input power levels in the milliwatt range. Interdigital cMUTs fabricated using low temperature processes can be used as compact ultrasonic transducers with integrated electronics for sensing and actuation in fluidic environments.     

Fabrication of gap-optimized CMUT

A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer. All previous approaches either require an additional metallic electrode or do not possess a structured sacrificial layer and, consequently, may make exact adjustment of the membrane dimensions difficult. The present set ups are especially suited for the fabrication of cMUT with gap heights ranging between 50 nm and 2 /spl mu/m between the electrodes. Large gaps are a prerequisite to enabling sufficient deflections of the membrane and, therewith, to generating high pressure gradients. On the other hand, small gap sizes are desirable for detecting weak ultrasonic sources. This paper focuses on the fabrication process of cMUT to realize electrode separation above 500 nm and, in addition, on the manufacturing of cMUT with gaps below 500 nm. The successful realization has been proven by some basic experimental investigations. Finally, the fundamental equations of a frequently chosen simulation model are documented, as a number of ambiguities exist in the common literature.     

Capacitive micromachined ultrasonic transducer design for high power transmission

Capacitive micromachined ultrasonic transducers (cMUTs) were developed to meet the demands of the ultrasonic industry. To achieve maximum efficiency, the conventional operation of the cMUT requires a bias voltage close to the collapse voltage. Total acoustic output pressure is limited by the efficiency of the cMUT and the maximum-allowed pulse voltage on the membrane. In this paper, we propose the collapse-snapback operation of the cMUT: the membrane is collapsed onto the substrate in the collapsing cycle, and released in the snapback cycle. The collapse-snapback operation overcomes the above-mentioned limitations of the conventional operation. The collapse-snapback operation utilizes a larger range of membrane deflection profiles (both collapsed and released profiles) and generates higher acoustic output pressures. The static finite element calculations were performed to design cMUTs with specific collapse and snapback voltages by changing the electrode parameters (radius (re), position (de), and thickness (te)). These designs were refined for optimum average displacement per cycle. An electrode radius greater than 60% of the membrane radius significantly improved the displacement per volt. Moderately thick membranes (te approximately 0.2 microm) were preferred, as thicker membranes reduced the displacement per volt. Under proper bias conditions, the collapse-snapback operation, designed for high-power transmission, allowed the application of pulse voltages larger than the difference of collapse and snapback voltages. Dynamic finite element calculations of an infinite cMUT array on the substrate loaded with acoustic fluid medium were performed to determine the dynamic response of the cMUT. Commercially available FEM packages ANSYS and LS-DYNA were used for static and dynamic calculations, respectively. The cMUTs were fabricated for optimal performance in the collapse-snapback operation. The transmit experiments were performed on a 2-D cMUT array using a calibrated hydrophone. Taking into account the attenuation and diffraction losses, the pressure on the cMUT surface was extracted. The cMUT generated 0.47 MPa (6 kPa/V) and 1.04 MPa (11 kPa/V) in the conventional and collapse-snapback operations, respectively. Therefore, collapse-snapback operation of the cMUTs was superior for high-power transmission.     

Extending the frequency range of a Wien-bridge oscillator usingcomposite operational amplifiers

The author describes the use of composite operational amplifiers to extend the useful operating frequency range of an operational amplifier-based Wien-bridge oscillator. The effect of the finite gain-bandwidth product (GB) of the operational amplifier (OA) is analyzed. This analysis shows that for a certain GB the frequency of oscillation is less than the theoretical value and that the error increases with frequency. A composite OA is then used to decrease the errors due to GB. This is verified theoretically and experimentally. Although the composite OA improves the frequency performance of the oscillator, the amplitude of the output decreases with frequency. An automatic-gain control circuit is incorporated to keep an output amplitude constant over the useful frequency range. Distortion measurements are presented     

Characterization of micromachined ultrasonic transducers using light diffraction tomography

This paper demonstrates that light diffraction tomography can be used to measure the acoustic field of micromachined ultrasonic transducers (MUT) in cases in which standard methods like hydrophone and microphone measurements fail. Two types of MUTs have been characterized with the method, one air-coupled capacitive MUT (cMUT) and one waterloaded continuous wave (CW) miniature multilayer lead zirconate titanate (PZT) transducer. Light diffraction tomography is an ultrasound measurement method with some special characteristics. Based on the interaction of light and ultrasound, it combines light intensity measurements with tomography algorithms to produce a measurement system. The method offers nonperturbing pressure measurements with high spatial resolution. It has been shown that, under certain circumstances, light diffraction tomography can be used as an absolute pressure measurement method with accuracy in the order of 10% in water and 13% in air. The results show that air-coupled cMUTs in the frequency range of about 1 MHz as well as the extreme near field of a miniaturized CW 10 MHz water-loaded transducer were successfully characterized with light diffraction tomography.     

Piezoceramics for high-frequency (20 to 100 MHz) single-elementimaging transducers

The performance of transducers operating at high frequencies is greatly influenced by the properties of the piezoelectric materials used in their fabrication. Selection of an appropriate material for a transducer is based on many factors, including material properties, transducer area, and operating frequency. The properties of a number of piezoceramic materials have been experimentally determined by measuring the electrical impedance of air-loaded resonators whose thickness corresponds to resonance frequencies from 10 to 100 MHz. Materials measured include commercially available compositions of lead zirconate titanate (PZT) with relatively high dielectric constants and a modified lead titanate (PT) composition with a much lower dielectric constant. In addition, materials which have been designed or modified to result in improved properties at high frequencies are studied. Conclusions concerning the influence of the microstructure and composition on the frequency dependence of the material properties are made from the calculated properties and microstructural analysis of each material. Issues which affect transducer performance are discussed in relation to the properties. For transducers larger than about 1 mm in diameter, the use of a lower dielectric constant material is shown to result in a better electrical match between the transducer and a standard 50 Ω termination. For transducers whose impedance is close to that of the connecting cables and electrical termination, equivalent circuit model simulations show improved performance without the need for electrical matching networks. Measurements of fabricated transducers show close agreement with the simulations, validating the measurements and showing the performance benefits of electrically matched transducers     

Miniature multimode monolithic flextensional transducers

Traditional flextensional transducers classified in seven groups based on their designs have been used extensively in 1-100 kHz range for mine hunting, fish finding, oil explorations, and biomedical applications. In this study, a new family of small, low cost underwater, and biomedical transducers has been developed. After the fabrication of transducers, finite-elements analysis (FEA) was used extensively in order to optimize these miniature versions of high-power, low-frequency flextensional transducer designs to achieve broad bandwidth for both transmitting and receiving, engineered vibration modes, and optimized acoustic directivity patterns. Transducer topologies with various shapes, cross sections, and symmetries can be fabricated through high-volume, low-cost ceramic and metal extrusion processes. Miniaturized transducers posses resonance frequencies in the range of above 1 MHz to below 10 kHz. Symmetry and design of the transducer, polling patterns, driving and receiving electrode geometries, and driving conditions have a strong effect on the vibration modes, resonance frequencies, and radiation patterns. This paper is devoted to small, multimode flextensional transducers with active shells, which combine the advantages of small size and low-cost manufacturing with control of the shape of the acoustic radiation/receive pattern. The performance of the transducers is emphasized.     

A new detection method for capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (cMUT) have become an alternative to piezoelectric transducers in the past few years. They consist of many small circular membranes that are connected in parallel. In this work, we report a new detection method for cMUTs. We model the membranes as capacitors and the interconnections between the membranes as inductors. This kind of LC network is called an artificial transmission line. The vibrations of the membranes modulate the electrical length of the transmission line, which is proportional to the frequency of the signal through it. By measuring the electrical length of the artificial line at a high RF frequency (in the gigahertz range), the vibrations of the membranes can be detected in a very sensitive manner. For the devices we measured, we calculated the minimum detectable displacement to be in the order of 10^-5 Å/√Hz with a possible improvement to 10^-7 Å/√Hz     

Real-Time Assessment and Diagnosis of Process Operating Performance

Over time, the performance of processes may deviate from the initial design due to process variations and uncertainties, making it necessary to develop systematic methods for online optimality assessment based on routine operating process data. Some processes have multiple operating modes caused by the set point change of the critical process variables to achieve different product specifications. On the other hand, the operating region in each operating mode can alter, due to uncertainties. In this paper, we will establish an optimality assessment framework for processes that typically have multi-mode, multi-region operations, as well as transitions between different modes. The kernel density approach for mode detection is adopted and improved for operating mode detection. For online mode detection, the model-based clustering discriminant analysis (MclustDA) approach is incorporated with some a priori knowledge of the system. In addition, multi-modal behavior of steady-state modes is tackled utilizing the mixture probabilistic principal component regression (MPPCR) method, and dynamic principal component regression (DPCR) is used to investigate transitions between different modes. Moreover, a probabilistic causality detection method based on the sequential forward floating search (SFFS) method is introduced for diagnosing poor or non-optimum behavior. Finally, the proposed method is tested on the Tennessee Eastman (TE) benchmark simulation process in order to evaluate its performance.     

Parametric linear modeling of circular cMUT membranes in vacuum

We present a lumped element parametric model for the clamped circular membrane of a capacitive micromachined ultrasonic transducer (cMUT). The model incorporates an electrical port and two sets of acoustic ports, through which the cMUT couples to the medium. The modeling approach is based on matching a lumped element model and the mechanical impedance of the cMUT membrane at the resonance frequencies in vacuum. Very good agreement between finite element simulation results and model impedance is obtained. Equivalent circuit model parameters can be found from material properties and membrane dimensions without a need for finite element simulation.     

A High Quality Controlled Current Source

A technique for realizing a high-quality controlled current source is described. A monolithic operational amplifier is used as the main building block, and a modified current mirror is used to recover the signal current from the power supply leads of the amplifier. The proposed mirror exhibits low input resistance, which reduces errors due to the noninfinite power-supply rejection characteristics of the amplifier, and a current-splitting network allows the realization of output impedances in the range of tens of megohms using standard components. The entire circuit operates in a class-AB mode for high drive capability with low values of quiescent current. A simple application is included to show that a voltage amplifier can readily be constructed which exhibits a 3-dB gain-bandwidth product forty times larger than the gain-bandwidth product of the operational amplifier used. Detailed experimental results are included to verify predicted frequency response and gain characteristics.     

Apodization of multilayer bulk-wave transducers

Recent experiments have demonstrated the use of superlattice transducers for bulk acoustic waves in the gigahertz frequency range. The transducers consisted of multilayers of ZnO or LiNbO(3) with alternating crystal orientations or polarizations. A procedure for calculating the electromechanical characteristics of general multilayer transducers in which the individual layers are anisotropic and piezoelectric and have arbitrary crystal orientation is described. The algorithm used is numerically stable and easily implemented for use on a personal computer using commercial software. A network model is also derived to provide both an approximate analysis of multilayer transducer performance and an insight into synthesis procedures. Examples are used to compare the two approaches and illustrate an initial design procedure for broadband transducers.     

Characterization of air-coupled transducers

This paper describes a theoretical and experimental study for determination of the through-air system impulse response and insertion loss with different air-coupled ultrasonic transducers. Wide-band piezopolymer transducers (PVDF) are employed in both transmission and reception modes and their behavior assessed by means of mathematical modeling and experiment. Specifically, a linear systems approach, modified to include the influence of attenuation in the propagation medium, was used to design suitable PVDF transducers for wide-band operation in air. Suitable devices were then manufactured for determination of the transmission and reception response characteristics of piezocomposite and electrostatic transducers when operating in the air environment. A range of transducers was evaluated, including 1-3 connectivity composites of different ceramic volume fraction and mechanical matching conditions, in addition to electrostatic devices of varying design. To complement the investigation, relative performances for narrow-band operation are also presented under transmission and transmit-receive conditions. Despite the obvious measurement difficulties, good agreement between theory and experiment was observed and the methodology is shown to provide a convenient and robust procedure for comparison of through-air transducers operating in the frequency range 50 KHz to 2 MHz. Although highly resonant, the most effective composite transducers under consideration demonstrate an improvement in two-way insertion loss of 22.4 dB and 11.5 dB over a corresponding electrostatic pair, under narrow-band and wide-band operation, respectively     

Evaluation of on-line flue gas measurements by MISiCFET and metal-oxide sensors in boilers

Metal insulator silicon carbide field-effect transistor sensors, metal-oxide sensors, and a linear Lambda sensor in an electronic nose was used to measure on-line in hot flue gases from a boiler. Flue gas from a 100-MW pellets-fuelled boiler has been used to feed the experimental setup. Several reference instruments, which measure the flue gases in parallel to the sensor array, are connected to the electronic nose. Data was collected during six weeks and then evaluated. Using principal component analysis as the data evaluation method, different operating modes for the boiler have been identified in the data set. The different modes could be described in terms of high or low O/sub 2/ and CO concentration. Furthermore, we have shown that it seems possible to use a sensor array to determine the operating mode of the boiler and, by partial least-squares models, measure the CO concentration when the boiler operates in its optimum mode.     

Dynamic characteristics of 2-2 piezoelectric composite transducers

Lateral resonant modes associated with specific lateral structures in piezoceramic-polymer composites are the main drawback that interferes with performance of ultrasonic transducers utilizing the composites as driving elements. This paper presents a field solution to characterize dynamic behavior of lateral modes in composite transducers with 2-2 connectivity. The solution is derived based on a concept of Lamb wave propagation along piezoceramic and polymer layers in the thickness direction of the composite transducers. The behavior of 2-2 composite is investigated over a broad spectrum ranging from quasi-static stage up to mode transition zones. Dispersion curves for various modes were calculated and compared with experimental results. A safe zone can be identified from the dispersion curves where the interference of lateral modes to thickness oscillation is negligible. Strong interactions between thickness modes and lateral modes are observed in the transition zones where two modes merge together in the frequency domain. Electromechanical thickness coupling coefficient, k_t, is calculated for the first time to include the effect of aspect ratio of the composite. Results show that the coupling coefficient, k_t, varies dramatically as a function of both ceramic volume fraction and the aspect ratio. As k_t reaches its maximum value, the vibration of the ceramic phase is completely decoupled from that of the polymer phase     

Quantitative model for evaluating the quality of an automotive business process

One of the key issues to business process control is the identification of measurable process attributes. For manufacturing processes these are typically physical parameters of the process (e.g. temperature, set points) or physical attributes of the manufactured product (e.g. dimension, functional performance). However, for business processes the metrics are more abstract. The challenge has been to develop metrics that capture the contributing subtle and hard to measure factors for business process control. This paper presents an analytical model that uses the weights-of-evidence concept to convert answers to audit or self-assessment questions into a single numerical process quality index. This index is used to forecast process success or failure and monitor its performance from start to end. The application of the approach is illustrated with an automotive industry product development sub-process where the process performance metric is the field warranty data, i.e. incidents per thousand vehicles (IPTV). The analytical model converts process self-assessment (failure mode and effect analysis) questions into a single numeric process quality index. The validity of the model is reflected in the strength of the correlation between the index and the IPTV results. Also, in this paper a measure is developed for identifying critical process quality assessment questions. This measure quantifies the deviation in the automotive business process that should have more focus. The significance of the analytical model proposed in this research is that the project managers or quality assurance auditors may be able to use the metric to predict product quality at any point in the product development process.