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oscillator
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frequency output transformers... self-oscillator
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oscillator circuit... high frequency
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This contribution presents the necessary conditions for the determination of the thermal properties of a gas in a channel under flowing conditions. The measurement principle relies on the modulation of the penetration depth of harmonic thermal waves by means of the oscillation frequency. Harmonic waves produced on the wall of the flow channel are confined within flow boundary layer that forms over the surface of the channel’s wall. Simulations and experiments using gases show that thermal conductivity and the volumetric heat capacity can be determined under flow conditions if v/(Dw) < 1, where v is the average flow speed in the channel, w the frequency and D the channel diameter. This verifies the results reported in the literature for liquids only.
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We present a novel approach of using a high-speed camera to combine a frequency modulated Doppler global velocimetry (FM-DGV) system and a particle image velocimetry (PIV) system in order to monitor the 3D velocity field with an increased spatial resolution of 300 µm. This approach facilitates a simultaneous measurement of all three velocity components with a high measurement rate of 10 kHz. Hence, the sound induced oscillation velocities for tone frequencies of several kHz can be resolved in addition to the mean flow velocity. Therefore, the measurement system is promising for investigating the soundflow interaction, which is demonstrated for the analysis of the sound damping mechanism at a bias flow liner used for aircraft noise reduction.
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We report the experimental investigation of the non-steady-state photoelectromotive force in aluminium nitride crystal. The sample is illuminated by an oscillating interference pattern formed by two coherent light beams and the alternating current is detected as a response of the material. The experiments are performed for two geometries, where arising photocurrent is parallel or perpendicular to the optical axis of the crystal. Dependencies of the signal amplitude versus light intensity, temporal and spatial frequencies are measured. The photoelectric parameters of the material are estimated for the light wavelength λ = 532 nm.
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Resonant sensors are used in a wide range of applications, e.g. as microbalances, chemical sensors in liquid and gaseous environments, and for physical property sensing of liquid and viscoelastic media. Our sensor electronics will be used for the readout of resonant sensors for liquid properties, which means that the Q-factor of such devices is much lower than that of resonators operating in vacuum or gaseous environments. Hence oscillator circuits are not preferable evaluation circuits for this application. Moreover, the damping may be frequency dependent, so measurement over a wide frequency range is mandatory. Besides the common method of using precise laboratory instruments for the measurement of the impedance spectra of these sensors, various approaches have been reported [3-8]. For the interpretation of the measurement results not only the amplitude but also the phase angle of the sensor impedance is of interest allowing a more accurate characterization of the resonance behavior. In this paper we introduce the system concept and some measurement results in comparison to results obtained with a commercial lock-in amplifier.
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Capacitive sensors are popular in industry because they have a simple construction and they perform well. As the name of the sensor suggests, the electrical model of the capacitive sensor is capacitance, the value of which is related to the value of the input variable. Naturally, the main function of the capacitivesensor electronic interface is to measure capacitance. This can be done in a number of ways by using different excitation signals. With harmonic excitation signals (u or i), the reactance of the unknown capacitance can be measured. By knowing the frequency, the excitation, and the measured signal values, the value of the unknown capacitance can be found. Another way to measure capacitance is to make it part of the frequencydefining circuit of an oscillator, together with another passive component. For a first-order (relaxation) oscillator this is usually a resistor, whereas for a second-order (harmonic) oscillator this can be an inductor. In this particular case the information carrier of the measured capacitance value is the frequency/period of the generated signal. Altematively, the sensor capacitance Cx can be extracted by measuring the charge Qx=Cx chef, which is stored in it. Here Uref is a reference voltage used to (re)charge the sensor capacitance. There are three main performance parameters used to evaluate the level of performance of capacitivesensor electronic interface: (1) resolution; (2) measurement time (i.e., the time from the moment we want to know the value of the measurand and the moment when the result is available); and (3) stability. Additionally, in some applications, other parameters are also important, such as power consumption, linearity, and dynamic range. The required accuracy is achieved by calibration with an accurate reference. This paper addresses the high-end industrial applications of capacitive sensors for measuring very small displacements in the nanometer and the sub-nanometer range, for which very high performance is required with respect to resolution, measurement speed, and drift. First, the general features and challenges of a typical industrial working environment are briefly discussed. Then, the basic limitations of the capacitive-sensor and the interface electronics are addressed. Lastly, methods for improving the longterm stability, resolution, and immunity to interference are presented.
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Acoustic biosensors allow the label-free detection of molecules and the analysis of binding events. In general, they are based on quartz crystal resonators, in which the mode of oscillation depends on the cut and geometry of the quartz crystal. If mass is applied on to the surface of the quartz resonator, the frequency of the oscillation decreases. By measuring the change of frequency, it is possible to determme the change in mass. Measurement of mass by using quartz crystal resonators was first examined by Sauerbrey, who showed that the frequency change of the crystal resonator is a linear function of the mass per area or absolute. The advantages of acoustic sensor Systems that exploit the piezoelectric effect to measure mass binding and molecular interactions have long been discussed, with the technology proffered as an alternative to optical biosensors. The technique has also been shown to be capable of detecting subtie changes in the solution-surface interface that can be due to density-viscosity changes m the solution, viscoelastic changes in the bound interfacial material, and changes in the surface free energy. More specifically, Signal transduction via the piezoelectric mechanism operates well in complex, often optically-intractable media. We demonstrate here that with improvements to acoustic biosensor liquid handling, thermal control, surface chemistries and microfluidics, increases in sensitivity are achieved that enable both high and low molecular weight analytes to be detected. By formation of a non-planar three-dimensional matrix to which a member of a specific binding pair can be attached, an increased amount of the other member of the pair can be captured. This not only increases receptor binding capacity and sensitivity, but also has the effect of reducing the degree of non-specific binding to the surface by effectively masking the chemical and physical properties of the metallic electrode surface. The technology can thus be applied to an extremely wide range of biological and chemical entities with a molecular weight range from less than 200 Daltons through to an entire bacterium or cell.
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The gyroscopes established one of the fundamental reference for attitude and heading in aerospace applications. The information about angular velocity gives input not only for autopilot, but also damping devices, as yaw or pitch dampers for example. The MEMS gyroscopes (Micro Electro Mechanical Systems) are much less reliable than their laser or fibre optic cousins. Nevertheless, availability and low price of MEMS components cause growing area of application in avionics for General Aviation aeroplanes. This paper presents certain results of flight data analysis registered during flight testing campaign of the new class aeroplane experimental low power single engine turbo-prop utility I-31T. The researches were focused on identification of oscillation modes, distinctive for the new aircraft, such as engine precession, whirl or shimmy. The data came from three-axis MEMS gyroscope recorder, placed near to the aircraft centre of gravity. Wavelet transformation gives better precision in time domain than Fourier transformation, especially for signals of low frequency.
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