Marvin Schewe

A method is presented by which very small, slow, anharmonic signals can be extracted from measurement data overlaid with noise that is orders of magnitude larger than the signal of interest. To this end, a multi-step filtering process is applied to a time signal containing the time-dependent displacement of the surface of a sample, which is determined with a contactless measurement method, differential laser Doppler vibrometry (D-LDV), at elevated temperatures. The time signal contains the phase difference of the measurement and reference laser beams of the D-LDV, already greatly reducing noise from, e.g., length fluctuations, heat haze, and mechanical vibrations. In postprocessing of the data, anharmonic signal contributions are identified and extracted to show the accurate displacement originating from thickness changes of thin films and related sample bending. The approach is demonstrated on a Pr_0.1Ce_0.9O_2−δ (PCO) thin film deposited on a single-crystalline ZrO₂-based substrate. The displacement extracted from the data is ca. 38 % larger and the uncertainty ca. 35 % lower than those calculated directly from the D-LDV spectrum.

Stabilities of substituted BTEs in PMMA films are described which differ from those in solution.

Traffic monitoring technologies have become crucial for modern manned and autonomous vehicles. They aim to ensure the maximum level of safety and dependability. The commonly used monitoring technologies based on conventional LIDAR (laser imaging, detection, and ranging), vision and ultrasound sensors are effective if there is a clear line of sight for the surrounding traffic field. However, some potential accidents and traffic hazards beyond the line of sight cannot be detected by the aforementioned sensors. Thus, dedicated sensors require means to detect other traffic participants even if they are not visible due to obstacles. In this paper, a new traffic monitoring concept for detecting nearby traffic activities occurring beyond the line of sight is introduced. The principle is based on a portable laser Doppler vibrometer (LDV), which is mounted on the vehicle to measure the roadway’s vibration noise induced by nearby traffic activities. First proof-of-concept field tests were conducted using a commercial LDV and realistic public traffic conditions. A statistical classification method based on the frequency features of pavement vibration measurements showed quite good performance for identifying traffic conditions.

In many communication and sensor applications, receivers digitize phase-modulated carrier signals directly by analog-to-digital converters. The obtained phase-modulated digital carrier signals are numerically demodulated in order to extract the relevant information. However, the limited dynamic range of the analog-to-digital converter may reduce the carrier-to-noise ratio of carrier signals after digitization. Correspondingly, the resolution of the demodulated digital signal is degraded. We demonstrate a sampling and demodulation scheme for phase-modulated signals with a small modulation index. Our new scheme overcomes the limitation due to digital noise defined by the analog-to-digital converter. Through simulations and experiments, we provide evidence that our method can improve the resolution of the demodulated digital signal, when the carrier-to-noise ratio of phase-modulated signals is limited by digital noise. Our proposed method is applicable, for example, to solve the problem of a possible degradation of measurement resolution after digital demodulation in heterodyne interferometers measuring small vibration amplitudes.

The required displacement resolution for measurements of chemical expansion of volume crystals and thin films in high-temperature environments (up to 800 °C) cannot be achieved with available commercial vibrometers since they do not operate at frequencies below 1 Hz. In order to reduce the dominating environmental noise in the low frequency range, we employ a differential laser Doppler vibrometer. In this paper, we investigate the influence of the different disturbance phenomena and explore the quality of their compensations through differential measurements. We prove that the concept of differential measurements with laser Doppler vibrometry can eliminate disturbances by optical turbulence and parasitic mechanical vibrations in the setup and, thus, provides better resolution in harsh environments at frequencies as low as 1 mHz.

Zusammenfassung: Die Vibrationsmessung großer und schwer zugänglicher Konstruktionen ist oftmals mit enormen Aufwand verbunden. Durch die Positionierung des Messstrahls eines Laser-Doppler-Vibrometers mit Hilfe an Drohnen befindlicher Spiegel, sind Schwingungsmessungen an schwer zugänglichen Messpunkten möglich. In diesem Beitrag stellen wir mögliche Messmethoden vor, betrachten auftretende Störeinflüsse und zeigen anhand von Messergebnissen inwieweit diese unterdrückt werden können.

The intensity of the reflected measuring beam is greatly reduced for laser-Doppler vibrometer (LDV) measurements on rough surfaces since a considerable part of the light is scattered and cannot reach the photodetector (laser speckle effect). The low intensity of the reflected laser beam leads to a so-called signal dropout, which manifests as noise peaks in the demodulated velocity signal. In such cases, no light reaches the detector at a specific time and, therefore, no signal can be detected. Consequently, the overall quality of the signal decreases significantly. In the literature, first attempts and a practical implementation to reduce this effect by signal diversity can be found. In this article, a practical implementation with four measuring heads of a Multipoint Vibrometer (MPV) and an evaluation and optimization of an algorithm from the literature is presented. The limitations of the algorithm, which combines velocity signals, are shown by evaluating our measurements. We present a modified algorithm, which generates a combined detector signal from the raw signals of the individual channels, reducing the mean noise level in our measurement by more than 10 dB. By comparing the results of our new algorithm with the algorithms of the state-of-the-art, we can show an improvement of the noise reduction with our approach.

Zusammenfassung: Bei der kontaktlosen Schwingungsmessung im Hochtemperaturbereich mit einem heterodynen Laser-Doppler-Vibrometer begrenzen insbesondere im Frequenzbreich unter 50 Hz mehrere Faktoren die minimal auflösbaren Schwingungsamplituden. In diesem Beitrag werden der Einfluss des Hitzeflimmerns auf den Rauschpegel analysiert und Methoden vorgestellt, wie dieser Einfluss minimiert werden kann. Darüber hinaus werden weitere begrenzende Faktoren wie der Einfluss der Laufzeitunterschiede der Laserstrahlen sowie Lösungen zu deren Minimierung vorgestellt.