Jens Rautenberg

Continuous sonar sensing for mobile mini-robots

Ultrasonic sensors enable mobile autonomous systems to obtain information about obstacles in large environments. In the presented work, a 5 cm broad array of three piezo-ceramic ultrasonic transducers is employed for getting two-dimensional impressions of the surroundings. Deviating from the pulse echo measurement techniques used so far the time-continuous transmitting and receiving from modulated pseudo-random sequences are considered. Thus the narrow bandwidth of a piezo-ceramic transducer can be compensated by an increased measuring period. An advanced analysis of the correlated signals allows the rejection of phantoms caused by multiple reflections. Furthermore, a classification of objects such as wall, corner, log or cylinder is possible.

Computer-assisted design of transducers for ultrasonic sensor systems

In this contribution, possibilities and methods for computer-assisted design of ultrasound transducers are described. These transducers are essential for an ultrasonic sensor design, e.g. for continuous non-invasive determination of quantities that are important in process technology. To achieve technical reliability and robustness, the precise determination of all acoustic properties of the used sensor materials is of great importance. Problem-oriented modeling, numerical simulation, special optimization algorithms and improved methods for the visualization of propagating waves offer new and promising possibilities for developing ultrasonic transducers with enhanced properties.

Multi Reflection of Lamb Wave Emission in an Acoustic Waveguide Sensor

Recently, an acoustic waveguide sensor based on multiple mode conversion of surface acoustic waves at the solid—liquid interfaces has been introduced for the concentration measurement of binary and ternary mixtures, liquid level sensing, investigation of spatial inhomogenities or bubble detection. In this contribution the sound wave propagation within this acoustic waveguide sensor is visualized by Schlieren imaging for continuous and burst operation the first time. In the acoustic waveguide the antisymmetrical zero order Lamb wave mode is excited by a single phase transducer of 1 MHz on thin glass plates of 1 mm thickness. By contact to the investigated liquid Lamb waves propagating on the first plate emit pressure waves into the adjacent liquid, which excites Lamb waves on the second plate, what again causes pressure waves traveling inside the liquid back to the first plate and so on. The Schlieren images prove this multi reflection within the acoustic waveguide, which confirms former considerations and calculations based on the receiver signal. With this knowledge the sensor concepts with the acoustic waveguide sensor can be interpreted in a better manner.

Transient modeling of ultrasonic guided waves in circular viscoelastic waveguides for inverse material characterization

In this contribution, we present an efficient approach for the transient and time-causal modeling of guided waves in viscoelastic cylindrical waveguides in the context of ultrasonic material characterization. We use the scaled boundary finite element method (SBFEM) for efficient computation of the phase velocity dispersion. Regarding the viscoelastic behavior of the materials under consideration, we propose a decomposition approach that considers the real-valued frequency dependence of the (visco-)elastic moduli and, separately, of their attenuation. The modal expansion approach is utilized to take the transmitting and receiving transducers into account and to propagate the excited waveguide modes through a waveguide of finite length. The effectiveness of the proposed simulation model is shown by comparison with a standard transient FEM simulation as well as simulation results based on the exact solution of the complex-valued viscoelastic guided wave problem. Two material models are discussed, namely the fractional Zener model and the anti-Zener model; we re-interpret the latter in terms of the Rayleigh damping model. Measurements are taken on a polypropylene sample and the proposed transient simulation model is used for inverse material characterization. The extracted material properties may then be used in computer-aided design of ultrasonic systems.

Sensitivitätssteigerung bei der inversenMaterialparameterbestimmung für Piezokeramiken

Zusammenfassung Für eine realitätsnahe Finite-Elemente-Simulation von Piezokeramiken ist es notwendig, dass deren Materialparameter präzise bekannt sind. Zur Bestimmung werden zunehmend numerische Verfahren auf Basis inverser Methoden eingesetzt, die einen Einfluss aller Materialparameter auf den als Messgröße genutzten Impedanzverlauf voraussetzen. Dieser kann durch die Nutzung einer segmentierten Elektrode in Kombination mit einem elektrischen Vornetzwerk vergrößert werden, wodurch die Sensitivität erhöht und die Bestimmung aller relevanter Materialparameter an einer einzelnen Probe ermöglicht wird.

Messsystem zur Bestimmung akustischer Kenngrößen stark absorbierender, transversal isotroper Kunststoffe

Zusammenfassung Es wird ein wellenleiterbasierter Ansatz zur Bestimmung von Modellparametern für die numerische Simulation der Schallausbreitung in stark absorbierenden, transversal isotropen Kunststoffen vorgestellt. Aufbauend auf einer ersten strahlentheoretischen Schätzung wird innerhalb eines inversen Ansatzes eine multimodale wellentheoretische Simulation der Schallausbreitung im Wellenleiter realisiert. Abstract In this contribution we present a waveguide-based approach for the determination of highly absorbing, transversely isotropic polymers for numerical simulation purposes. Based on a multimodal simulation of wave propagation the directional as well as frequency dependent acoustic properties are computed within an inverse problem.

An improved mode-tracing algorithm to compute dispersion curves of acoustic waveguides

Semi-analytical approaches for the description of dispersion in acoustic waveguides are based on root finding in a characteristic function. A mode-tracing algorithm helps to find all solutions of this problem by consecutively bracketing single roots. In this contribution we present a multistage and adaptive mode-tracing algorithm, which uses an adaptive step size for the prediction of consecutive solutions as well as an adaptive estimation of the search space around these predictions. Furthermore we present a validation stage to ensure the correlation between consecutive solutions alias waveguide modes.

Time-causal material modeling in the simulation of guided waves in circular viscoelastic waveguides

For the description of linear viscoelasticity, the fractional Zener model may be used. Based on the spectral decomposition of the elasticity matrix as proposed by Theocaris, we generalize the one-dimensional analysis of the material model into three dimensions and discuss appropriate simplifications to reduce the amount of unknowns for the material description. Then, a decomposition approach that considers the real valued frequency dependence of the viscoelastic moduli and the real valued frequency dependence of their attenuation separately is proposed. The Scaled Boundary Finite Element Method is used for the efficient computation of the phase velocity dispersion and the modal wave fields given a frequency dependent but real valued viscoelasticity matrix. Utilizing the modal expansion approach, the transmitting and receiving transducer are taken into account to compute the modal amplitudes. Combining these modal amplitudes, the phase velocity dispersion and re-introducing the viscoelastic attenuation results in a transfer function of the viscoelastic waveguide including excitation and receiving conditions. The performance of the proposed simulation model is shown by comparison to measurements taken on a polypropylene sample.

Reliable computation of roots in analytical waveguide modeling using an interval-newton approach and algorithmic differentiation

For the modeling and simulation of wave propagation in geometrically simple waveguides such as plates or rods, one may employ the analytical global matrix method. That is, a certain (global) matrix depending on the two parameters wavenumber and frequency is built. Subsequently, one must calculate all parameter pairs within the domain of interest where the global matrix becomes singular. For this purpose, one could compute all roots of the determinant of the global matrix when the two parameters vary in the given intervals. This requirement to calculate all roots is actually the methods most concerning restriction. Previous approaches are based on so-called mode-tracers, which use the physical phenomenon that solutions, i.e., roots of the determinant of the global matrix, appear in a certain pattern, the waveguide modes, to limit the root-finding algorithms search space with respect to consecutive solutions. In some cases, these reductions of the search space yield only an incomplete set of solutions, because some roots may be missed as a result of uncertain predictions. Therefore, we propose replacement of the mode-tracer approach with a suitable version of an interval- Newton method. To apply this interval-based method, we extended the interval and derivative computation provided by a numerical computing environment such that corresponding information is also available for Bessel functions used in circular models of acoustic waveguides. We present numerical results for two different scenarios. First, a polymeric cylindrical waveguide is simulated, and second, we show simulation results of a one-sided fluid-loaded plate. For both scenarios, we compare results obtained with the proposed interval-Newton algorithm and commercial software.

Sensitivity study of signal characteristics for an inverse waveguide based approach of material characterization

A very promising approach for the ultrasonic identification of highly damping viscoelastic polymers has been proposed by Rautenberg [1]. He uses transmission measurements through hollow cylindrical shaped test samples which are modeled as waveguides. The measurement effect relies on multiple mode-conversions at the waveguides outer and inner boundaries and is evaluated based on an inverse approach. In this contribution we discuss different signal characteristics of the obtained dispersed signals with respect to their sensitivity to certain material parameters. Based on these sensitivities we propose a multiphase optimization scheme to increase the optimizations convergence.