7,564 research outputs found
Experimental comparison of parameter estimation methods in adaptive robot control
In the literature on adaptive robot control a large variety of parameter estimation methods have been proposed, ranging from tracking-error-driven gradient methods to combined tracking- and prediction-error-driven least-squares type adaptation methods. This paper presents experimental data from a comparative study between these adaptation methods, performed on a two-degrees-of-freedom robot manipulator. Our results show that the prediction error concept is sensitive to unavoidable model uncertainties. We also demonstrate empirically the fast convergence properties of least-squares adaptation relative to gradient approaches. However, in view of the noise sensitivity of the least-squares method, the marginal performance benefits, and the computational burden, we (cautiously) conclude that the tracking-error driven gradient method is preferred for parameter adaptation in robotic applications
Relaxation Tribometry: A Generic Method to Identify the Nature of Contact Forces
Recent years have witnessed the development of so-called relaxation
tribometers, the free oscillation of which is altered by the presence of
frictional stresses within the contact. So far, analysis of such oscillations
has been restricted to the shape of their decaying envelope, to identify in
particular solid or viscous friction components. Here, we present a more
general expression of the forces possibly acting within the contact , and
retain six possible, physically relevant terms. Two of them, which had never
been proposed in the context of relaxation tribometry, only affect the
oscillation frequency, not the amplitude of the signal. We demonstrate that
each of those six terms has a unique signature in the time-evolution of the
oscillation, which allows efficient identification of their respective weights
in any experimental signal. We illustrate our methodology on a PDMS
sphere/glass plate torsional contact
Brownian ratchet in a thermal bath driven by Coulomb friction
The rectification of unbiased fluctuations, also known as the ratchet effect,
is normally obtained under statistical non-equilibrium conditions. Here we
propose a new ratchet mechanism where a thermal bath solicits the random
rotation of an asymmetric wheel, which is also subject to Coulomb friction due
to solid-on-solid contacts. Numerical simulations and analytical calculations
demonstrate a net drift induced by friction. If the thermal bath is replaced by
a granular gas, the well known granular ratchet effect also intervenes,
becoming dominant at high collision rates. For our chosen wheel shape the
granular effect acts in the opposite direction with respect to the
friction-induced torque, resulting in the inversion of the ratchet direction as
the collision rate increases. We have realized a new granular ratchet
experiment where both these ratchet effects are observed, as well as the
predicted inversion at their crossover. Our discovery paves the way to the
realization of micro and sub-micrometer Brownian motors in an equilibrium
fluid, based purely upon nano-friction.Comment: main paper: 4 pages and 4 figures; supplemental material joined at
the end of the paper; a movie of the experiment can be viewed
http://www.youtube.com/watch?v=aHrdY4BC71k ; all the material has been
submitted for publication [new version with substantial changes in the order
of the presentation of the results; differences with previous works have been
put in evidence
A nonlinear disturbance observer for robotic manipulators
A new nonlinear disturbance observer (NDO) for robotic manipulators is derived in this paper. The global exponential stability of the proposed disturbance observer (DO) is guaranteed by selecting design parameters, which depend on the maximum velocity and physical parameters of robotic manipulators. This new observer overcomes the disadvantages of existing DOs, which are designed or analyzed by linear system techniques. It can be applied in robotic manipulators for various purposes such as friction compensation, independent joint control, sensorless torque control and fault diagnosis. The performance of the proposed observer is demonstrated by the friction estimation and compensation for a two-link robotic manipulator. Both simulation and experimental results show the NDO works well
Model structure detection and system identification of metal rubber devices
Metal rubber (MR) devices, a new wire mesh material, have been extensively used in recent years due to several unique properties especially in adverse environments. Although many practical studies have been completed, the related theoretical research on metal rubber is still in its infancy. In this paper, a semi-constitutive dynamic model that involves nonlinear elastic stiffness, nonlinear viscous damping and bilinear hysteresis Coulomb damping is adopted to model MR devices. After approximating the bilinear hysteresis damping using Chebyshev polynomials of the first kind, a very efficient procedure based on the orthogonal least squares (OLS) algorithm and the adjustable prediction error sum of squares (APRESS) criterion is proposed for model structure detection and parameter estimation of an MR device for the first time. The OLS algorithm provides a powerful tool to effectively select the significant model terms step by step, one at a time, by orthogonalizing the associated terms and maximizing the error reduction ratio, in a forward stepwise procedure. The APRESS statistic regularizes the OLS algorithm to facilitate the determination of the optimal number of model terms that should be included into the dynamic model. Because of the orthogonal property of the OLS algorithm, the approach leads to a parsimonious model. Numerical ill-conditioning problems confronted by the conventional least squares algorithm can also be avoided by the new approach. Finally by utilising the transient response of a MR specimen, it is shown how the model structure can be detected in a practical application. The identified model agrees with the experimental measurements very well
Energy dissipation prediction of particle dampers
This paper presents initial work on developing models for predicting particle dampers (PDs) behaviour using the Discrete Element Method (DEM). In the DEM approach, individual particles are typically represented as elements with mass and rotational inertia. Contacts between particles and with walls are represented using springs, dampers and sliding friction interfaces. In order to use DEM to predict damper behaviour adequately, it is important to identify representative models of the contact conditions. It is particularly important to get the appropriate trade-off between accuracy and computational efficiency as PDs have so many individual elements. In order to understand appropriate models, experimental work was carried out to understand interactions between the typically small (1.5–3 mm diameter) particles used. Measurements were made of coefficient of restitution and interface friction. These were used to give an indication of the level of uncertainty that the simplest (linear) models might assume. These data were used to predict energy dissipation in a PD via a DEM simulation. The results were compared with that of an experiment
Secret objectives: promoting inquiry and tackling preconceptions in teaching laboratories
In its most general form, a `secret objective' is any inconsistency between
the experimental reality and the information provided to students prior to
starting work on an experiment. Students are challenged to identify the secret
objectives and then given freedom to explore and understand the experiment,
thus encouraging and facilitating genuine inquiry elements in introductory
laboratory courses. Damping of a simple pendulum is used as a concrete example
to demonstrate how secret objectives can be included. We also discuss the
implications of the secret objectives method and how this can provide a link
between the concepts of problem based learning and inquiry style labs
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