Effects of time response on the point spread function of a scanning radiometer

An analytical expression has been obtained that describes the time variation of time-resolved signals transmitted through or reflected by a homogeneous scattering slab as measured with a detection system having a square-impulse response. This expression can be used to improve the match between theoretical and experimental time-resolved signals measured with a system having a finite response time. It can also be used to assess the effect of a finite detection response time on the time-domain characterization of a turbid medium. The expression can be adapted to detection systems that are not time invariant.

A full-wave equation that describes nonlinear propagation in a heterogeneous attenuating medium is solved numerically with finite differences in the time domain (FDTD). This numerical method is used to simulate propagation of a diagnostic ultrasound pulse through a measured representation of the human abdomen with heterogeneities in speed of sound, attenuation, density, and nonlinearity. Conventional delay-andsum beamforming is used to generate point spread functions (PSF) that display the effects of these heterogeneities. For the particular imaging configuration that is modeled, these PSFs reveal that the primary source of degradation in fundamental imaging is reverberation from near-field structures. Reverberation clutter in the harmonic PSF is 26 dB higher than the fundamental PSF. An artificial medium with uniform velocity but unchanged impedance characteristics indicates that for the fundamental PSF, the primary source of degradation is phase aberration. An ultrasound image is created in silico using the same physical and algorithmic process used in an ultrasound scanner: a series of pulses are transmitted through heterogeneous scattering tissue and the received echoes are used in a delay-and-sum beamforming algorithm to generate images. These beamformed images are compared with images obtained from convolution of the PSF with a scatterer field to demonstrate that a very large portion of the PSF must be used to accurately represent the clutter observed in conventional imaging.

In multi- and hyperspectral imaging, spatial coregistration of the point spread functions (PSFs) of all bands within each pixel is critical for the integrity of measured spectra. There is a need to define a standardized method for characterizing coregistration error. We propose a method that estimates PSFs from the product of line spread functions (LSFs) recorded in two orthogonal directions. Coregistration is then evaluated according to the PSF difference metric [T. Skauli, Opt. Expr. vol. 20, p. 918 (2012)]. Experimental results on two pushbroom hyperspectral cameras show good correspondence with measurements based on the full PSF, provided that LSF scan directions are aligned with axes of symmetry in the optics. Even a maximally unfavourable choice of scan directions gives meaningful estimates of coregistration error. The proposed method may have potential as a standard for coregistration characterization.

Imaging of pressure-sensitive paint (PSP) for pressure measurement on moving surfaces is problematic due to the movement of the object within the finite exposure time of the imager, resulting in the blurring of the blade edges. The blurring problem is particularly challenging when high-sensitivity PSP with a long lifetime is used, where the long luminescence time constant of exponential light decay following a burst of excitation light energy results in blurred images. One method to ameliorate this effect is image deconvolution using a point spread function (PSF) based on an estimation of the luminescent time constant. Prior implementations of image deconvolution for PSP deblurring have relied upon a spatially invariant time constant in order to reduce computational time. However, the use of an assumed value of time constant leads to errors in the point spread function, particularly when strong pressure gradients (which cause strong spatial gradients in the decay time constant) are involved. This work introduces an iterative method of image deconvolution, where a spatially variant PSF is used. The point-by-point PSF values are found in an iterative manner, since the time constant depends on the local pressure value, which can only be found from the reduced PSP data. The scheme estimates a super-resolved spatially varying blur kernel with sub-pixel resolution without filtering the blurred image, and then restores the image using classical iterative regularization tools. A kernel-free forward model has been used to generate test images with known pressure surface maps and a varying amount of noise to evaluate the applicability of this scheme in different experimental conditions. A spinning disk setup with a grazing nitrogen jet for producing strong pressure gradients has also been used to evaluate the scheme on a real-world problem. Results including the convergence history and the effect of a regularization-iteration count are shown, along with a comparison with the previous PSP deblurring method.

Aeroelastic analysis for flap of airfoil in transonic flow

Light that would typically be discarded at a confocal microscope's detector pinhole will be collected and processed to allow a reduced spatial sampling rate and thus an improved image acquisition time. It is shown that collecting and appropriately processing the out-of-focus light will allow an axial sampling rate below that specified by the Nyquist criterion. To achieve this, a central detector pinhole and a number of out-of-focus regions are collected concurrently. This corresponds to imaging through several different channels, with differing point spread functions, in parallel. Since the spatial sampling rate is below the Nyquist frequency, aliasing occurs in the data from each of the channels. However, since the point spread functions are different, the aliasing effects are different in each channel. This allows the ensemble of aliased images to be processed into a single dealiased and deconvolved image. This potential utility of out-of-focus light is demonstrated through simulated examples for differing collection schemes and scanning rates. Results are shown for under-sampling by up to a factor of four. Collecting the out-of-focus light also improves instrument collection efficiency.

Distributed control systems are crucial for regulating manufacturing processes by controlling field devices. Delays in device operation can significantly impact the time response of the process control system, affecting overall stability. To enhance network performance and process time response, it is essential to study the inherent delays of field and network devices. This paper examines process control loops involving a 3-layered network based on the OSI model’s physical, network, and application layers. The proposed networked control system assesses time delays within and between field devices configured through these layers, which are essential for control loops, including open, closed, and cascade loops. Analyzing these time delays enables effective control loop configuration, improving the process control time response and overall stability.

This research introduces a filtering circuit design integrated with the DC-link of a medium power variable speed drive (VSD) system. The designed filtering circuit employs modern power electronic circuits to improve time response characteristics in both grid and dc-link sides. Depending on the widely used multi-level converters, a conventional three-level H-bridge four-quadrant chopper scheme was developed into a cascade five-level H-bridge four-quadrant chopper scheme. The time response of the proposed system was performed with both choppers, and results showed that voltage drop on cascade chopper transistors was reduced to half in comparison with voltage drop on conventional four-quadrant chopper transistors. Moreover, the sharp fluctuations in system's waves were mitigated; consequently, time response characteristics in both steady and transient states were remarkably improved. The total harmonic distortion factor THD% of the input current and voltage was reduced to 26.2% and 2.43% respectively and the ripple factor RF for DC-link current was reduced to 0.196.

Perfluorocarbons such as perfluoroctylbromide (PFOB) can be used as contrast agents in the vascular system for fluorine-19 magnetic resonance imaging or as synthetic oxygen carriers. F-19 imaging has been proposed for studying the vascular system, capillary flow, tissue perfusion, and tumor oxygenation. A major difficulty is that F-19 compounds often have complex multipeak spectra. These peaks result in chemical shift artifacts, lower signal-to-noise ratios, and blurred images. Each peak also excites a different section when a section-select gradient is applied. Direct inverse filtering is the simplest deconvolution method for correcting such artifacts; however, two major difficulties present themselves: functional singularity and noise amplification at high frequencies. The use of a new reblurred deconvolution (RED) method appears to overcome these problems. Although this method is based on iterative deconvolution in the spatial domain, the computational overhead is negligible. Since the point spread function and object data are already available in the time domain as FID data, RED appears to be useful for eliminating chemical shift artifacts and suppressing noise amplification while restoring the original image without loss of resolution.

Aberration, clutter, and reverberation degrade the quality of ultrasonic images. When an acoustic pulse propagates through tissue these effects occur simultaneously and it is difficult to obtain independent estimates for the precise source of the point spread function broadening. The purpose of this paper is to characterize the sources of clutter and reverberation with a simulation of ultrasonic propagation through the abdomen. A full-wave equation that describes nonlinear propagation in a heterogeneous attenuating medium is solved numerically with finite differences in the time domain (FDTD). Three dimensional solutions of the equation are verified with water tank measurements of a commercial diagnostic ultrasound transducer and are shown to be in excellent agreement in terms of the fundamental and harmonic acoustic fields, and the power spectrum at the focus. The linear and nonlinear components of the algorithm are also verified independently. In the linear non-attenuating regime solutions match results from Field II, a well established software package used in transducer modeling, to within 0.3 dB. In addition to thermoviscous attenuation we present a numerical solution of the relaxation attenuation laws that allows modeling of arbitrary frequency dependent attenuation, such as that observed in tissue. A perfectly matched layer (PML) is implemented at the boundaries with a novel numerical implementation that allows the PML to be used with high order discretizations. A -78 dB reduction in the reflected amplitude is demonstrated. The numerical algorithm is used to simulate a focused ultrasonic pulse propagating through a histologically determined representation of the human abdomen. An ultrasound image is created in silicon using the same physical and algorithmic process used in an ultrasound scanner: a series of pulses are transmitted through heterogeneous scattering tissue and the received echoes are used in a delay-and-sum beamforming algorithm to generate a images. The resulting harmonic image exhibits characteristic improvement in lesion boundary definition and contrast when compared to the fundamental image. We demonstrate a mechanism of harmonic image quality improvement by showing that the harmonic point spread function is less sensitive to reverberation clutter.

A sliding mode observer is applied towards the operation of a switched reluctance motor drive. The sliding mode observer estimates rotor position and velocity to control the conduction angles of the machine. The particular case of an automotive hydraulic brake system motor is considered in detail where the conduction angles are modified with velocity feedback to provide optimum time response to brake system commands. Nonlinear modeling of a switched reluctance motor is described and a computer simulation is developed based on data from an experimental switched reluctance motor system. The sliding mode observer is implemented with a fixed-point digital signal processor and the discrete-time implementation is first examined under locked rotor conditions. After confirming the accuracy of the computer simulation with experimental data, the design considerations in selecting observer coefficients with regard to sampling time, convergence rate and transient stability are discussed. In conclusion, the effects of flux estimation errors on the system time response during a start-up transient are examined.

Chapter 9 - Fast terminal sliding mode controller for high speed and complex maneuvering of unmanned aerial vehicles

In this paper an Optimization based linear control design method is used to achieve a coordinated design of Power System Stabilizer (PSS) and Thyristor Controlled Series Capacitor (TCSC) for power system damping improvement with pre specified control design requirements. The settling time is considered as main performance criteria in time domain specification and gain and phase margins for frequency domain specifications. The main objective of the coordination of controllers is to improve the global stability through damping of oscillations by improving the damping ratio of the system and minimizing the settling time of the system response. Thus coordination increases the reliability of the system and maintains the required damping even one or more controllers are out of service and hence reduces the need for additional controller. Optimization based linear control design is used to determine the optimal gain and time constant setting of lead-lag controller parameters for PSS and TCSC. The coordinated PSS and TCSC controller is tested in the Single Machine Infinite Bus Power System (SMIB). Simulation is carried out using Matlab2011b. From the time domain simulation results it is observed that oscillations are damped out, settling time is reduced and damping ratio is improved. From bode plot it is inferred that gain and phase margin of the system is further improved to satisfy the system requirements. In the overall analysis, optimization based linear control design implemented for the coordination of PSS and TCSC controller satisfies the practical requirements of the power system oscillation damping problem, also shift the eigen values to the left half of the s-plane as far as possible and hence enhances the overall system stability

Integrated transmission vibration reduction technology based on the “isolating-reducing-optimizing” method

The Scanning Polarimetric Imaging RAdiometer at 91 GHz with an angular resolution of 0.5° was used to investigate the dynamics of the atmosphere. We introduced a new imaging method by continuously scanning the sky over a range of elevation angles in a fixed azimuth direction. The measurements were realized during three different situations: clear sky, sky with water clouds, and sky with cirrus clouds. In most situations, the scan direction was nearly parallel to the mean atmospheric flow. Particularly interesting structures were found in the images with water clouds. In contrast, cirrus clouds are highly transparent. Simulations of the applied imaging method helped to interpret the cloud images, particularly concerning the cloud movement. Characteristic shapes were identified as signatures of motions along the scan line, which were used to estimate the horizontal velocity of water clouds. It was also possible to estimate the integrated water vapor from the clear sky images. They allow a visualization of water vapor parcels. Some of these images contain similar signatures as the clouds, indicating advection of water vapor along the scan line. In the future, we plan to extend these measurements and to combine them with multifrequency observations.