Non-rigid Body Research Articles

Finite Element Analysis on Real Contact Area Based on Fractal Characterization

No surface in engineering is absolutely smooth. It is important to analyze and calculate the real contact area for a better understanding of friction, wear, lubrication and thermal conductance. To obtain the accurate real contact area between rough surface and smooth surface, a rough-non-rigid-smooth surface contact finite element model is proposed in which the rough surface is characterized by fracture theory. In finite element modeling and analyzing process, MATLABEXCEL and AutoCAD are used to process data, and the smooth surface is considered to be non-rigid body. Compared with the traditional modeling, this method can obtain data quickly and is closer to the actual situation.

An Image Registration Method for Colposcopic Images

A nonrigid body image registration method for spatiotemporal alignment of image sequences obtained from colposcopy examinations to detect precancerous lesions of the cervix is proposed in this paper. The approach is based on time series calculation for those pixels in the first image of the sequence and a division of such image into small windows. A search process is then carried out to find the window with the highest affinity in each image of the sequence and replace it with the window in the reference image. The affinity value is based on polynomial approximation of the time series computed and the search is bounded by a search radius which defines the neighborhood of each window. The proposed approach is tested in ten 310-frame real cases in two experiments: the first one to determine the best values for the window size and the search radius and the second one to compare the best obtained results with respect to four registration methods found in the specialized literature. The obtained results show a robust and competitive performance of the proposed approach with a significant lower time with respect to the compared methods.

Research of Hand Gesture Recognition in Multi-Touch Projector-Camera System

This paper studys the feasibility of the shadow of interactive gestures on the projection screen as the control instructions in a projection interactive system. With regard to complex background of the projection screen, we put forward to detect mobile gestures and the shadows of gestures by using the way of the Gaussian mixture model. Relative to the non-rigid body interaction gestures, The shadow of gestures has a simpler two-dimensional characteristics and greater differences from the other regions of the projection screen. Based on this characteristics, we adopt Ostu threshold segmentation algorithm to remove the interference of the gestures. Then we use the mixed way of the radial symmetry transform and curvature threshold to position the interactive fingertips as interactive instruction of the projector-camera system. Conctroling the screen in a wide range without carrying any equipment is completed. The experimental results show that this method is easy to operate and has a high accuracy rate.

The continuous non-rigid 3D body motion recovery algorithm based on the Murkowski distance

The continuous non-rigid 3D body motion recovery algorithm based on the Murkowski distance

Analysis of state-of-the-art single-thruster attitude control techniques for spinning penetrator

The attitude dynamics and manoeuvre survey in this paper is performed for a mission scenario involving a penetrator-type spacecraft: an axisymmetric prolate spacecraft spinning around its minor axis of inertia performing a 90° spin axis reorientation manoeuvre. In contrast to most existing spacecraft only one attitude control thruster is available, providing a control torque perpendicular to the spin axis. Having only one attitude thruster on a spinning spacecraft could be preferred for spacecraft simplicity (lower mass, lower power consumption etc.), or it could be imposed in the context of redundancy/contingency operations. This constraint does yield restrictions on the thruster timings, depending on the ratio of minor to major moments of inertia among other parameters. The Japanese Lunar-A penetrator spacecraft proposal is a good example of such a single-thruster spin-stabilised prolate spacecraft.The attitude dynamics of a spinning rigid body are first investigated analytically, then expanded for the specific case of a prolate and axisymmetric rigid body and finally a cursory exploration of non-rigid body dynamics is made. Next two well-known techniques for manoeuvring a spin-stabilised spacecraft, the Half-cone/Multiple Half-cone and the Rhumb line slew, are compared with two new techniques, the Sector-Arc Slew developed by Astrium Satellites and the Dual-cone developed at Surrey Space Centre. Each technique is introduced and characterised by means of simulation results and illustrations based on the penetrator mission scenario and a brief robustness analysis is performed against errors in moments of inertia and spin rate. Also, the relative benefits of each slew algorithm are discussed in terms of slew accuracy, energy (propellant) efficiency and time efficiency. For example, a sequence of half-cone manoeuvres (a Multi-half-cone manoeuvre) tends to be more energy-efficient than one half-cone for the same final slew angle, but more time-consuming. As another example, the new Sector-Arc Slew and Dual-cone techniques are designed to overcome a specific restriction on attainable slew angle that is associated with the half-cone manoeuvre, giving one additional degree of freedom for designers to fine-tune.

Nonrigid motion correction in 3D using autofocusing withlocalized linear translations

AbstractMR scans are sensitive to motion effects due to the scan duration. To properly suppress artifacts from nonrigid body motion, complex models with elements such as translation, rotation, shear, and scaling have been incorporated into the reconstruction pipeline. However, these techniques are computationally intensive and difficult to implement for online reconstruction. On a sufficiently small spatial scale, the different types of motion can be well approximated as simple linear translations. This formulation allows for a practical autofocusing algorithm that locally minimizes a given motion metric — more specifically, the proposed localized gradient‐entropy metric. To reduce the vast search space for an optimal solution, possible motion paths are limited to the motion measured from multichannel navigator data. The novel navigation strategy is based on the so‐called “Butterfly” navigators, which are modifications of the spin‐warp sequence that provides intrinsic translational motion information with negligible overhead. With a 32‐channel abdominal coil, sufficient number of motion measurements were found to approximate possible linear motion paths for every image voxel. The correction scheme was applied to free‐breathing abdominal patient studies. In these scans, a reduction in artifacts from complex, nonrigid motion was observed. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.

Towards a theoretical foundation for morphological computation with compliant bodies

The control of compliant robots is, due to their often nonlinear and complex dynamics, inherently difficult.The vision of morphological computation proposes to view these aspects not only as problems, but rather also as parts of the solution. Non-rigid body parts are not seen anymore as imperfect realizations of rigid body parts, but rather as potential computational resources. The applicability of this vision has already been demonstrated for a variety of complex robot control problems. Nevertheless, a theoretical basis for understanding the capabilities and limitations of morphological computation has been missing so far. We present a model for morphological computation with compliant bodies, where a precise mathematical characterization oft he potential computational contribution of a complex physical body is feasible. The theory suggests that complexity and nonlinearity, typically unwanted properties of robots, are desired features in order to provide computational power. We demonstrate that simple generic models of physical bodies,based on mass-spring systems, can be used to implement complex nonlinear operators. By adding a simple readout(which is static and linear) to the morphology, such devices are able to emulate complex mappings of input to output streams in continuous time. Hence, by outsourcing parts of the computation to the physical body, the difficult problem of learning to control a complex body, could be reduced to a simple and perspicuous learning task, which can not get stuck in local minima of an error function.

High temporal resolution retrospective motion correction with radial parallel imaging

A method for motion correction in multicoil imaging applications, involving both data collection and reconstruction, is presented. A bit-reversed radial acquisition scheme, in conjunction with a rapid self-calibrated parallel imaging method, Generalized auto-calibrating partial parallel acquisition (GRAPPA) operator for wider radial bands (GROWL), is used to achieve motion correction at a high temporal resolution. View-by-view in-plane motion correction is achieved in 2D imaging, while 3D motion correction is achieved for every two consecutive slice-encoding planes in 3D imaging. In the proposed technique, GROWL contributes in two aspects: First, a central k-space circle/cylinder used as the motion-free reference is generated from a small number of radial lines/planes; Second, undersampled k-space regions resulting from rotation and inconsistent (e.g. intraview and nonrigid body) motion can be filled in. When compared with navigator-based motion correction methods, the proposed method does not prolong scan time and can be applied to short-TR sequences. Magn Reson Med, 2011. © 2011 Wiley-Liss, Inc.

Efficient Registration of Nonrigid 3-D Bodies

We present a novel method to perform an accurate registration of 3-D nonrigid bodies by using phase-shift properties of the dual-tree complex wavelet transform [Formula: see text]. Since the phases of [Formula: see text] coefficients change approximately linearly with the amount of feature displacement in the spatial domain, motion can be estimated using the phase information from these coefficients. The motion estimation is performed iteratively: first by using coarser level complex coefficients to determine large motion components and then by employing finer level coefficients to refine the motion field. We use a parametric affine model to describe the motion, where the affine parameters are found locally by substituting into an optical flow model and by solving the resulting overdetermined set of equations. From the estimated affine parameters, the motion field between the sensed and the reference data sets can be generated, and the sensed data set then can be shifted and interpolated spatially to align with the reference data set.

Rotating full- and reduced-dimensional quantum chemical models of molecules

A flexible protocol, applicable to semirigid as well as floppy polyatomic systems, is developed for the variational solution of the rotational-vibrational Schrödinger equation. The kinetic energy operator is expressed in terms of curvilinear coordinates, describing the internal motion, and rotational coordinates, characterizing the orientation of the frame fixed to the nonrigid body. Although the analytic form of the kinetic energy operator might be very complex, it does not need to be known a priori within this scheme as it is constructed automatically and numerically whenever needed. The internal coordinates can be chosen to best represent the system of interest and the body-fixed frame is not restricted to an embedding defined with respect to a single reference geometry. The features of the technique mentioned make it especially well suited to treat large-amplitude nuclear motions. Reduced-dimensional rovibrational models can be defined straightforwardly by introducing constraints on the generalized coordinates. In order to demonstrate the flexibility of the protocol and the associated computer code, the inversion-tunneling of the ammonia ((14)NH(3)) molecule is studied using one, two, three, four, and six active vibrational degrees of freedom, within both vibrational and rovibrational variational computations. For example, the one-dimensional inversion-tunneling model of ammonia is considered also for nonzero rotational angular momenta. It turns out to be difficult to significantly improve upon this simple model. Rotational-vibrational energy levels are presented for rotational angular momentum quantum numbers J = 0, 1, 2, 3, and 4.

Motion Estimation of Real Object through Virtual Buoy Motion

This paper describes a method for estimating 3D motion. Some methods for detecting the 3D scene flow (=3D-Flow) of an arbitrary point on free-form surface by using video sequence have been already suggested. When the target object is a rigid body, motion parameters can be calculated analytically because all 3D-Flow vectors are based on same conversion matrix. On the other hand, when the target object is a non-rigid body, it is difficult to calculate overall motion involving local deformation. In this study, we define detected 3D-Flow vectors as “flow-field” (or “force-field”), and store the virtual buoy (float) into this field, and calculate motion of the virtual buoy by using simple physical simulation. Motion of the object in the real world is estimated through the motion of the virtual buoy.

Theoretical and experimental analysis of the electromechanical behavior of a compact spherical loudspeaker array for directivity control

Sound directivity control is made possible by a compact array of independent loudspeakers operating at the same frequency range. The drivers are usually distributed over a sphere-like frame according to a Platonic solid geometry to obtain a highly symmetrical configuration. The radiation pattern of spherical loudspeaker arrays has been predicted from the surface velocity pattern by approximating the drivers membranes as rigid vibrating spherical caps, although a rigorous assessment of this model has not been provided so far. Many aspects concerning compact array electromechanics remain unclear, such as the effects on the acoustical performance of the drivers interaction inside the array cavity, or the fact that voltages rather than velocities are controlled in practice. This work presents a detailed investigation of the electromechanical behavior of spherical loudspeaker arrays. Simulation results are shown to agree with laser vibrometer measurements and experimental sound power data obtained for a 12-driver spherical array prototype at low frequencies, whereas the non-rigid body motion and the first cavity eigenfrequency yield a discrepancy between theoretical and experimental results at high frequencies. Finally, although the internal acoustic coupling affects the drivers vibration in the low-frequency range, it does not play an important role on the radiated sound power.

Jamming and Wedging Analysis of Non-Rigid Assembly with Friction Contact

It is well known that the friction force can lead to the analysis complexity of the contact constraint model. Based on Coulomb’s frictional model and unilateral constraint model, non-rigid bodies’ compliant dynamic model was established. Considering the whole motion of the assembly system and elastic contact deformation caused by assembly forces, the nearly rigid contact analysis is proposed. Jamming and wedging must be avoided for successful robot assembly operation. Based on the compliant dynamic model the conditions for jamming and wedging are provided in the formulations of the constrained dynamic system of peg-in-hole insertion.

On the definition of the natural frequency of oscillations in nonlinear large rotation problems

Computational multibody system algorithms allow for performing eigenvalue analysis at different time points during the simulation to study the system stability. The nonlinear equations of motion are linearized at these time points, and the resulting linear equations are used to determine the eigenvalues and eigenvectors of the system. In the case of linear systems, the system eigenvalues remain the same under a constant coordinate transformation; and zero eigenvalues are always associated with rigid body modes, while nonzero eigenvalues are associated with non-rigid body motion. These results, however, cannot in general be applied to nonlinear multibody systems as demonstrated in this paper. Different sets of large rotation parameters lead to different forms of the nonlinear and linearized equations of motion, making it necessary to have a correct interpretation of the obtained eigenvalue solution. As shown in this investigation, the frequencies associated with different sets of orientation parameters can differ significantly, and rigid body motion can be associated with non-zero oscillation frequencies, depending on the coordinates used. In order to demonstrate this fact, the multibody system motion equations associated with the system degrees of freedom are presented and linearized. The resulting linear equations are used to define an eigevalue problem using the state space representation in order to account for general damping that characterizes multibody system applications. In order to demonstrate the significant differences between the eigenvalue solutions associated with two different sets of orientation parameters, a simple rotating disk example is considered in this study. The equations of motion of this simple example are formulated using Euler angles, Euler parameters and Rodriguez parameters. The results presented in this study demonstrate that the frequencies obtained using computational multibody system algorithms should not in general be interpreted as the system natural frequencies, but as the frequencies of the oscillations of the coordinates used to describe the motion of the system.

Non-rigid metric reconstruction from perspective cameras

The metric reconstruction of a non-rigid object viewed by a generic camera poses new challenges since current approaches for Structure from Motion assume the rigidity constraint of a shape as an essential condition. In this work, we focus on the estimation of the 3-D Euclidean shape and motion of a non-rigid shape observed by a perspective camera. In such case deformation and perspective effects are difficult to decouple – the parametrization of the 3-D non-rigid body may mistakenly account for the perspective distortion. Our method relies on the fact that it is often a reasonable assumption that some of the points on the object’s surface are deforming throughout the sequence while others remain rigid. Thus, relying on the rigidity constraints of a subset of rigid points, we estimate the perspective to metric upgrade transformation. First, we use an automatic segmentation algorithm to identify the set of rigid points. These are then used to estimate the internal camera calibration parameters and the overall rigid motion. Finally, we formulate the problem of non-rigid shape and motion estimation as a non-linear optimization where the objective function to be minimized is the image reprojection error. The prior information that some of the points in the object are rigid can also be added as a constraint to the non-linear minimization scheme in order to avoid ambiguous configurations. We perform experiments on different synthetic and real data sets which show that even when using a minimal set of rigid points and when varying the intrinsic camera parameters it is possible to obtain reliable metric information.

Librational response of Enceladus

Physical librations could significantly contribute to Enceladus' geophysics through their influence on tidal stress. Therefore it is important to determine their behavior and the present paper is devoted to estimating Enceladus' libration in longitude. In a rotational model of Enceladus with no global ocean, we introduce the main perturbative terms of its orbital longitude and the tidal coupling. The main librations of Enceladus are related to indirect perturbations of the orbit of Enceladus by Dione (11 years and 3.7 years periods) with amplitudes of 933.4″ (1.14 km) and 676.6″ (827 m), respectively. These amplitudes are almost independent of the body's triaxiality. The third main libration is due to the direct gravitational attraction of Saturn and its period is equal to that of the mean anomaly of Enceladus with an amplitude between 93.1″ and 113.5″ (i.e., 112 and 139 m), depending on triaxiality. These amplitudes are consistent with the upper bound of 1.5° (6.6 km) inferred from observations with the Cassini‐Huygens spacecraft. The nonrigid body libration amplitudes due to tidal coupling are negligible. Nevertheless, tidal dissipation induces a small phase shift up to 0.57° corresponding to a displacement of Enceladus' figure of 1 m along the moon's equator at the mean anomaly period.

Transport Properties of Small Spherical Particles

Recently, a theoretical framework for nanoparticle transport in the laminar flow regime has been proposed. The theory features a rigorous gas-kinetic theory analysis. It considers the effect of nonrigid body collision, and the theory is shown to reproduce the Chapman-Enskog theory of molecular transport in the small particle size limit, Epstein's model of particle drag in the rigid-body limit, and the Stokes-Cunningham equation for the drag on micrometer size particles. This theoretical framework provides the hope that bits and pieces of particle transport theories formulated over the last century can now be unified into a generalized theory. This paper discusses an unresolved fundamental issue related to this generalized theory, namely, the transition from specular scattering applicable to molecule-molecule collision to diffuse scattering governing molecule-"particle" collision.

A Model for Vibrational Effects of Nonrigid Vehicles

The wheel polygonization of state‐of‐the‐art lightrail streetcars is studied using a mechanical model for the vibrational frequency response of a vehicle with a nonrigid body, which reveals relevant novel wheel resonances.

SNR improvement in GRE-EPI first-pass myocardial perfusion images with non-rigid body registration and KLT filtering

Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) are often limited in first-pass myocardial perfusion images due to the demands for ultra-fast image acquisition and the use of parallel imaging techniques. Spatial or temporal low pass filtering can enhance SNR but may blur boundaries and generate artifacts. The Karhunen-Loeve Transform (KLT) filter takes advantage of temporal correlation to remove random noise without compromising either spatial or temporal resolution, thereby enhancing SNR in dynamic images [1, 2], but the performance of this filter has not been carefully evaluated for first-pass perfusion CMR. Image registration to compensate for respiration and variability in ECG triggering is a necessary step in quantitative evaluation of first-pass images. We hypothesize that registration can also increase the temporal correlation between images and reduce the potential for artifacts induced by KLT filtering.

Neural Network Mapping and Clustering of Elastic Behavior From Tactile and Range Imaging for Virtualized Reality Applications

To fully reach its potential, virtualized reality needs to go beyond the modeling of rigid bodies and introduce accurate representations of deformable objects. This paper explores neural networks and vision-based and tactile measurement strategies to investigate the intricate processes of acquisition and mapping of properties characterizing deformable objects. An original composite neural network framework is applied to guide the tactile probing by clustering measurements representing uniform elasticity regions and, therefore, direct sensors toward areas of elasticity transitions where higher sampling density is required. The network characterizes the relationship between surface deformation and forces that are exemplified in nonrigid bodies. Beyond serving as a planner for the acquisition of measurements, the proposed composite neural architecture allows the encoding of the complex force/deformation relationship without the need for sophisticated mathematical modeling tools. Experimental results prove the validity and the feasibility of the proposed approach.