Peter-Christian Eccardt

Linear and nonlinear equivalent circuit modeling of CMUTs

Using piston radiator and plate capacitance theory capacitive micromachined ultrasound transducers (CMUT) membrane cells can be described by one-dimensional (1-D) model parameters. This paper describes in detail a new method, which derives a 1-D model for CMUT arrays from finite-element methods (FEM) simulations. A few static and harmonic FEM analyses of a single CMUT membrane cell are sufficient to derive the mechanical and electrical parameters of an equivalent piston as the moving part of the cell area. For an array of parallel-driven cells, the acoustic parameters are derived as a complex mechanical fluid impedance, depending on the membrane shape form. As a main advantage, the nonlinear behavior of the CMUT can be investigated much easier and faster compared to FEM simulations, e.g., for a design of the maximum applicable voltage depending on the input signal. The 1-D parameter model allows an easy description of the CMUT behavior in air and fluids and simplifies the investigation of wave propagation within the connecting fluid represented by FEM or transmission line matrix (TLM) models.

Micromachined transducers for ultrasound applications

Within the last decade many interesting solutions for micromachined electroacoustic transducers have been presented, most of them microphones for the audio range. So far they could not compete significantly on the market with miniaturized conventional transducer techniques such as electret microphones. One of the main reasons for this was a limited sensitivity due to a high noise level without offering significant cost advantages. Now two facts raised new attention to micromachined transducers: For high-frequency ultrasound imaging the reduction in size of a single transducer element for 1D and even more for 2D arrays is more and more limited by fabrication and cabling technology. On the other hand, new microfabrication technologies have emerged, allowing a highly reproducible fabrication of electrostatically driven membranes with gap heights below 400 nm. One of the most interesting facts is that with a recently developed process step micromechanical membranes can be fabricated within a modified BiCMOS process. This allows the combination of transducer elements with the driving, preamplifying and multiplexing electronics on a single chip, thus reducing parasitic capacities and noise level significantly. This paper first outlines the history of micromachined transducers. Then it describes the internal structure of a micromechanical transducer element and its acoustical properties. The main differences in comparison to piezoelectric bulk transducers are the significantly lower acoustic impedance of the membranes and the nonlinear electromechanical working principle, leading to consequences in array design, which are discussed. Mathematical models and experimental results for transducer bandwidth, membrane deflection and radiation patterns of transducer arrays are presented and compared with the properties of piezoelectric transducer arrays. The influence of the membranes relative deflection, the poling voltage, the cabling capacity and the preamplifier characteristics upon transmission level and signal-to-noise ratio are discussed. Finally an outlook for potential applications of micromachined transducers, especially in array configurations, is given.

Surface micromachined ultrasonic transducer

A spacer layer (7) with a cavity (8) etched out therein and a diaphragm (2) arranged thereabove on the spacer layer are located on a silicon substrate (1) with a doped region (5) formed therein, whereby the doped region and the diaphragm are electrically connected via terminal contacts (4, 6) to electronic components (13) that are likewise integrated in the substrate (1). The electronic component are a component part of the operating circuit that can also be used for the drive of the diaphragm and for evaluating the diaphragm oscillations. The integration makes it possible to arrange the micromachined transducer components as array that can be electronically driven as phased array.

A resonant CMUT sensor for fluid applications

Fluid waves at the interface of CMUTs (capacitive micromachined ultrasound transducers) to the surrounding fluid are an often discussed and unwanted effect for medical imaging applications, as they cause ringing artifacts. A new approach for a surface wave sensor is presented which uses these dispersive surface waves for sensing fluid properties like mass density and viscosity. After a short introduction to the theory and our FEM model we will present first results showing the sensitivity of the sensor to the viscosity of different silicone oil samples and will discuss the results.

Finite element simulation of acoustic wave propagation within flowing media

A finite element method for acoustic wave propagation is established, taking into account the influence of a time-independent inhomogeneous flow on the acoustic waves. The simulation scheme is verified for homogeneous flow and validated with geometrical acoustics for inhomogeneous flow profiles. As an application, the wave propagation within ultrasound flow meters for liquids is computed for different flow profiles.

Simulation of Acoustic Wave Propagation Within Flowing Media Using Simplified Boundary Element Techniques

The development of a novel ultrasonic gas meter1 for household applications demonstrated that ultrasound technique can be a very precise and cost effective method for flow meters. Fig. 1 shows the basic principle of an ultrasound meter based on the delay time measurement principle. The time of flight between the two transducers positioned along the flow channel is influenced by the mean flow along the measurement path. The mean flow rate can be calculated as

Sensing physical fluid properties with CMUT arrays

Fluid waves at the interface of CMUTs (Capacitive Micromachined Ultrasound Transducers) to the surrounding fluid are an often discussed and unwanted effect for medical imaging applications, as they cause ringing artifacts. A new approach for a surface wave sensor is presented which uses these dispersive surface waves for sensing fluid properties like mass density and viscosity. After a short introduction to the theory and our FEM model we will present first results. A mixture of water and glycerin was used to investigate the sensitivity of the sensor to dynamic viscosity.

Weg- und Winkelsensoren

Weg- und Winkelaufnehmer werden uberall dort benotigt wo veranderliche Positionen, (Abstande, Langen oder Winkel) erfasst werden mussen, um entweder als reiner Messwert ausgegeben zu werden oder in einem Regelkreis zumeist die Regelgrose als Messwert zur Verfugung zu stellen. Grundsatzlich unterscheidet man zwischen beruhrenden (tastenden) und beruhrungslosen Aufnehmern. Zu den beruhrenden Sensoren zahlen dabei alle Aufnehmer, die ein spezielles Messobjekt als Weg- oder Winkelvermittler benotigen, das wahrend der Messung fest mit dem eigentlichen Messobjekt verbunden ist. Diese Bezeichnung gilt auch dann, wenn die eigentliche Messung gegen das vermittelnde Objekt im Endeffekt beruhrungslos erfolgt. Beruhrungslose Weg- und Winkelaufnehmer konnen ohne einen solchen Vermittler direkt gegen das zu vermessende Objekt messen.

Boundary element solutions for acoustic wave propagation in media with nonuniform flow

The interaction between acoustic wave propagation and a fluid in motion can be described by a system of coupled equations. For many applications like ultrasound flowmeters, medical ultrasound diagnostics, environmental acoustics or air conditioning systems, the influence of the acoustic waves upon the flow can be neglected. This allows a separation and therefore a successive solution of the problem. The results of finite element (FEM) simulations based on a modified wave equation with the flow profile as a boundary condition have been previously presented. In this paper, a new boundary element (BEM) technique is reported to solve this problem. One major advantage over FEM is the reduced dimension of the problem, leading to easier meshing and, especially for unbounded domains, to reduced computational effort. For the new BEM approach the Greens function had to be modified to consider flow. Based on an approximate Greens function a boundary element solution was developed. This new approach is compared with the previously implemented FEM scheme as well as with experiments measuring the sound pressure in a flow channel for various geometries and flow profiles. The results show good agreement between experiment and simulation. The limitations of this approach will be discussed.