4,139 research outputs found
Textural Ordination Based on Fourier Spectral Decomposition: A Method to Analyze and Compare Landscape Patterns
We propose an approach to texture characterization and comparison that directly uses the information of digital images of the earth surface without requesting a prior distinction of structural âpatches'. Digital images are partitioned into square âwindows' that define the scale of the analysis and which are submitted to the two-dimensional Fourier transform for extraction of a simplified textural characterization (in terms of coarseness) via the computation of a âradial' power spectrum. Spectra computed from many images of the same size are systematically compared by means of a principal component analysis (PCA), which provides an ordination along a limited number of coarseness vs. fineness gradients. As an illustration, we applied this approach to digitized panchromatic air photos depicting various types of land cover in a semiarid landscape of northern Cameroon. We performed âtextural ordinations' at several scales by using square windows with sides ranging from 120 m to 1 km. At all scales, we found two coarseness gradients (PCA axes) based on the relative importance in the spectrum of large (> 50 kmâ1), intermediate (30â50 kmâ1), small (10â25 kmâ1) and very small (<10 kmâ1) spatial frequencies. Textural ordination based on Fourier spectra provides a powerful and consistent framework to identifying prominent scales of landscape patterns and to compare scaling properties across landscapes
SABBAC: online Structural Alphabet-based protein BackBone reconstruction from Alpha-Carbon trace
SABBAC is an on-line service devoted to protein backbone reconstruction from alpha-carbon trace. It is based on the assembly of fragments taken from a library of reduced size, selected from the encoding of the protein trace in a hidden Markov model-derived structural alphabet. The assembly of the fragments is achieved by a greedy algorithm, using an energy-based scoring. Alpha-carbon coordinates remain unaffected. SABBAC simply positions the missing backbone atoms, no further refinement is performed. From our tests, SABBAC performs equal or better than other similar on-line approach and is robust to deviations on the alpha-carbon coordinates. It can be accessed at
Reliable ABC model choice via random forests
Approximate Bayesian computation (ABC) methods provide an elaborate approach
to Bayesian inference on complex models, including model choice. Both
theoretical arguments and simulation experiments indicate, however, that model
posterior probabilities may be poorly evaluated by standard ABC techniques. We
propose a novel approach based on a machine learning tool named random forests
to conduct selection among the highly complex models covered by ABC algorithms.
We thus modify the way Bayesian model selection is both understood and
operated, in that we rephrase the inferential goal as a classification problem,
first predicting the model that best fits the data with random forests and
postponing the approximation of the posterior probability of the predicted MAP
for a second stage also relying on random forests. Compared with earlier
implementations of ABC model choice, the ABC random forest approach offers
several potential improvements: (i) it often has a larger discriminative power
among the competing models, (ii) it is more robust against the number and
choice of statistics summarizing the data, (iii) the computing effort is
drastically reduced (with a gain in computation efficiency of at least fifty),
and (iv) it includes an approximation of the posterior probability of the
selected model. The call to random forests will undoubtedly extend the range of
size of datasets and complexity of models that ABC can handle. We illustrate
the power of this novel methodology by analyzing controlled experiments as well
as genuine population genetics datasets. The proposed methodologies are
implemented in the R package abcrf available on the CRAN.Comment: 39 pages, 15 figures, 6 table
Continental collision, gravity spreading, and kinematics of Aegea and Anatolia
International audienceWe have carried out experiments using a layered medium of sand and silicone to investigate the lateral extrusion of a material which spreads over its own weight while being compressed by the advance of a rigid indenter. Boundary conditions in the box mimic those prevailing in the Anatolian-Aegean system. Both shortening in front of the rigid piston, which models the northward motion of Arabia, and extension resulting from the gravity spreading of the sand-silicone layer are necessary to initiate the lateral extrusion. Strike-slip faults accommodate the lateral escape and link the normal faults accompanying gravity spreading with the thrust faults in front of the rigid indenter. Strike-slip faults begin to accommodate extrusion at a late stage in the experiments after the normal and thrust faults have developed. Experiments also show that the initial geometry of the boundary of the spreading layer may result in the formation of two arcs behind which material extends, in a manner analogous to the Hellenic and Cypriot arcs, without invoking a rheological change at the junction of the two arcs. The experiments also suggest that southward motion of the eastern part of the spreading region is compensated by the northward advance of the piston, which is a possible explanation for the slower movement of the Cypriot arc compared to the Aegean arc
Refined Instrumental Variable method for non-linear dynamic identification of robots
The identification of the dynamic parameters of robot is based on the use of the inverse dynamic identification model which is linear with respect to the parameters. This model is sampled while the robot is tracking âexcitingâ trajectories, in order to get an over determined linear system. The linear least squares solution of this system calculates the estimated parameters. The efficiency of this method has been proved through the experimental identification of a lot of prototypes and industrial robots. However, this method needs joint torque and position measurements and the estimation of the joint velocities and accelerations through the bandpass filtering of the joint position at high sample rate. So, the observation matrix is noisy. Moreover identification process takes place when the robot is controlled by feedback. These violations of assumption imply that the LS estimator is not consistent. This paper focuses on the Refined Instrumental Variable (RIV) approach to over-come this problem of noisy observation matrix. This technique is applied to a 2 degrees of freedom (DOF) prototype devel-oped by the IRCCyN Robotic team
Dynamic identification of a 6 dof industrial robot without joint position data
Off-line robot dynamic identification methods are mostly based on the use of the inverse dynamic model, which is linear with respect to the dynamic parameters. This model is sampled while the robot is tracking reference trajectories that excite the system dynamics. This allows using linear least-squares techniques to estimate the parameters. This method requires the joint force/torque and position measurements and the estimate of the joint velocity and acceleration, through the bandpass filtering of the joint position at high sampling rates. A new method called DIDIM has been proposed and validated on a 2 degree-of-freedom robot. DIDIM method requires only the joint force/torque measurement. It is based on a closed-loop simulation of the robot using the direct dynamic model, the same structure of the control law, and the same reference trajectory for both the actual and the simulated robot. The optimal parameters minimize the 2-norm of the error between the actual force/torque and the simulated force/torque. A validation experiment on a 6 dof Staubli TX40 robot shows that DIDIM method is very efficient on industrial robot
A new closed-loop output error method for parameter identification of robot dynamics
Off-line robot dynamic identification methods are mostly based on the use of
the inverse dynamic model, which is linear with respect to the dynamic
parameters. This model is sampled while the robot is tracking reference
trajectories that excite the system dynamics. This allows using linear
least-squares techniques to estimate the parameters. The efficiency of this
method has been proved through the experimental identification of many
prototypes and industrial robots. However, this method requires the joint
force/torque and position measurements and the estimate of the joint velocity
and acceleration, through the bandpass filtering of the joint position at high
sampling rates. The proposed new method requires only the joint force/torque
measurement. It is a closed-loop output error method where the usual joint
position output is replaced by the joint force/torque. It is based on a
closed-loop simulation of the robot using the direct dynamic model, the same
structure of the control law, and the same reference trajectory for both the
actual and the simulated robot. The optimal parameters minimize the 2-norm of
the error between the actual force/torque and the simulated force/torque. This
is a non-linear least-squares problem which is dramatically simplified using
the inverse dynamic model to obtain an analytical expression of the simulated
force/torque, linear in the parameters. A validation experiment on a 2
degree-of-freedom direct drive robot shows that the new method is efficient
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