In this work we present construction details of a precision, standalone, and compact digital synthetic impedance for application in the field of vibration damping. The presented device is based on an embedded ARM microcontroller with external AD and DA converters and a special analog front-end. The performance of the device is tested by comparing the actually synthesized impedance with several prescribed impedances and shows very good match. Fine-tuning ability of the device, which is crucial for the considered application, is also demonstrated and reaches as small step as 0.1% for the most complicated impedance structure and drops below the level of direct measurability with less complex structures. The real application in vibration damping is demonstrated on a simple and well understood case of a one-dimensional vibrating spring-mass system with piezoelectric actuator embedded as the interface between source of vibrations and vibrating mass.
Active acoustic metasurfaces (AAMSs) have been recently recognized as very efficient sound shielding structures, which can have large lateral dimensions perpendicular to the direction of the sound wave propagation but very short lateral dimension along the direction of the sound wavevector. The sound shielding principle of AAMSs is based on control of the specific acoustic impedance (SAI). This is achieved by means of an active tuning of elastic properties of piezoelectric transducers, which, therefore, represent the core element of the AAMSs. Using this approach, it is possible to actively control the acoustic coefficients of transmission and reflection of AAMSs. An important point, which has been recently discovered, is the fact that the great suppression of the transmission coefficient can be achieved in the regime, when the SAI of the AAMS is negative. The function of the AAMS in varying operational conditions or in a wide frequency range, however, put delicate stability conditions on the negative values of SAI. In order to keep the AAMS in the stable operation, a concept of adaptive acoustic metasurfaces (AdAMSs) is introduced in this paper. Methods for the real-time estimation and the active control of the SAI values of the AdAMSs are presented. It is shown that the accurate control of the distribution of the SAI on the surface of the AdAMS enables to control the transmitted sound field not only in the magnitude but also in the direction of the transmitted sound wave.
This paper presents a new method for radius of curvature measurement by interferometers. The radius measurement is carried out directly in the interferometer confocal position without the need for a specific hardware and thus allows us to measure a much more diverse range of optical surfaces than standard methods. The method is based on measuring a number of phase maps and displacements at several steps through the confocal null position. Radius of curvature is then computed as the tangent slope of the measured defocus–displacement pair values in the confocal position. A relative accuracy of the method is approximately 0.05%, which makes the method suitable for a vast number of applications. Results of the method are verified using standard confocal cat’s eye technique.
A novel radius of the curvature measurement method for optical spherical surfaces using absolute interferometry is proposed. A measurement setup is designed and built around a common-path Fizeau interferometer. The cavity length (volume of air between reference and tested surfaces) can be measured by the absolute wavelength tuning interferometry. An interconnection of data from three different tunable laser diodes (central wavelengths 780, 785 and 852 nm) allows us to measure the cavity length with uncertainty from tens to hundreds of nanometres. Once the reference radius of curvature is known/measured/calibrated, the radius of surface under test can be computed applying the value of the cavity length. The radius of curvature is measured directly in confocal position of the interferometer with relative precision of about 10 ppm. Moreover, unlike standard radius measurement by interferometry, the uncertainty of the introduced method can be optimized by selecting a suitable transmission sphere. In the paper, the method is described, tested, and verified by measuring several specimens featuring different radii of curvature. The results are analysed and furthermore compared to other measurement device.
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