30,368 research outputs found
Robot Impedance Control and Passivity Analysis with Inner Torque and Velocity Feedback Loops
Impedance control is a well-established technique to control interaction
forces in robotics. However, real implementations of impedance control with an
inner loop may suffer from several limitations. Although common practice in
designing nested control systems is to maximize the bandwidth of the inner loop
to improve tracking performance, it may not be the most suitable approach when
a certain range of impedance parameters has to be rendered. In particular, it
turns out that the viable range of stable stiffness and damping values can be
strongly affected by the bandwidth of the inner control loops (e.g. a torque
loop) as well as by the filtering and sampling frequency. This paper provides
an extensive analysis on how these aspects influence the stability region of
impedance parameters as well as the passivity of the system. This will be
supported by both simulations and experimental data. Moreover, a methodology
for designing joint impedance controllers based on an inner torque loop and a
positive velocity feedback loop will be presented. The goal of the velocity
feedback is to increase (given the constraints to preserve stability) the
bandwidth of the torque loop without the need of a complex controller.Comment: 14 pages in Control Theory and Technology (2016
Geometric Scattering in Robotic Telemanipulation
In this paper, we study the interconnection of two robots, which are modeled as port-controlled Hamiltonian systems through a transmission line with time delay. There will be no analysis of the time delay, but its presence justifies the use of scattering variables to preserve passivity. The contributions of the paper are twofold: first, a geometrical, multidimensional, power-consistent exposition of telemanipulation of intrinsically passive controlled physical systems, with a clarification on impedance matching, and second, a system theoretic condition for the adaptation of a general port-controlled Hamiltonian system with dissipation (port-Hamiltonian system) to a transmission line
Stability evaluation of rotor/bearing system
A stability study of rotor/bearing systems is presented. Even though it was limited to study of a fully lubricated bearing subject to oil whirl, and further limited to low eccentricity region for linearity and with only one type of lubricant, it can be seen that the perturbation methodology, together with the sorting of the impedance terms into direct and quadrature with respect to input force can be very useful to the general study of stability. Further, the concept of active feedback should assist to increase knowledge in rotor system stability. While there remains a large amount of study to be accomplished, perhaps some more tools are available to assist this field of analysis
Arbitrary beam control using passive lossless metasurfaces enabled by orthogonally-polarized custom surface waves
For passive, lossless impenetrable metasurfaces, a design technique for
arbitrary beam control of receiving, guiding, and launching is presented.
Arbitrary control is enabled by a custom surface wave in an orthogonal
polarization such that its addition to the incident (input) and the desired
scattered (output) fields is supported by a reactive surface impedance
everywhere on the reflecting surface. Such a custom surface wave (SW) takes the
form of an evanescent wave propagating along the surface with a spatially
varying envelope. A growing SW appears when an illuminating beam is received.
The SW amplitude stays constant when power is guided along the surface. The
amplitude diminishes as a propagating wave (PW) is launched from the surface as
a leaky wave. The resulting reactive tensor impedance profile may be realized
as an array of anisotropic metallic resonators printed on a grounded dielectric
substrate. Illustrative design examples of a Gaussian beam
translator-reflector, a probe-fed beam launcher, and a near-field focusing lens
are provided
Time-varying Huygens' meta-devices for parametric waves
Huygens' metasurfaces have demonstrated almost arbitrary control over the
shape of a scattered beam, however, its spatial profile is typically fixed at
fabrication time. Dynamic reconfiguration of this beam profile with tunable
elements remains challenging, due to the need to maintain the Huygens'
condition across the tuning range. In this work, we experimentally demonstrate
that a time-varying metadevice which performs frequency conversion can steer
transmitted or reflected beams in an almost arbitrary manner, with fully
dynamic control. Our time-varying Huygens' metadevice is made of both electric
and magnetic meta-atoms with independently controlled modulation, and the phase
of this modulation is imprinted on the scattered parametric waves, controlling
their shapes and directions. We develop a theory which shows how the scattering
directionality, phase and conversion efficiency of sidebands can be manipulated
almost arbitrarily. We demonstrate novel effects including all-angle beam
steering and frequency-multiplexed functionalities at microwave frequencies
around 4 GHz, using varactor diodes as tunable elements. We believe that the
concept can be extended to other frequency bands, enabling metasurfaces with
arbitrary phase pattern that can be dynamically tuned over the complete 2\pi
range
Generalized Parity-Time Symmetry Condition for Enhanced Sensor Telemetry
Wireless sensors based on micro-machined tunable resonators are important in
a variety of applications, ranging from medical diagnosis to industrial and
environmental monitoring.The sensitivity of these devices is, however, often
limited by their low quality (Q) factor.Here, we introduce the concept of
isospectral party time reciprocal scaling (PTX) symmetry and show that it can
be used to build a new family of radiofrequency wireless microsensors
exhibiting ultrasensitive responses and ultrahigh resolution, which are well
beyond the limitations of conventional passive sensors. We show theoretically,
and demonstrate experimentally using microelectromechanical based wireless
pressure sensors, that PTXsymmetric electronic systems share the same
eigenfrequencies as their parity time (PT)-symmetric counterparts, but
crucially have different circuit profiles and eigenmodes. This simplifies the
electronic circuit design and enables further enhancements to the extrinsic Q
factor of the sensors
Planar immersion lens with metasurfaces
The solid immersion lens is a powerful optical tool that allows light
entering material from air or vacuum to focus to a spot much smaller than the
free-space wavelength. Conventionally, however, they rely on semispherical
topographies and are non-planar and bulky, which limits their integration in
many applications. Recently, there has been considerable interest in using
planar structures, referred to as metasurfaces, to construct flat optical
components for manipulating light in unusual ways. Here, we propose and
demonstrate the concept of a planar immersion lens based on metasurfaces. The
resulting planar device, when placed near an interface between air and
dielectric material, can focus electromagnetic radiation incident from air to a
spot in material smaller than the free-space wavelength. As an experimental
demonstration, we fabricate an ultrathin and flexible microwave lens and
further show that it achieves wireless energy transfer in material mimicking
biological tissue
Horn-like space-coiling metamaterials toward simultaneous phase and amplitude modulation
Acoustic metasurfaces represent a family of planar wavefront-shaping devices garnering increasing attention due to their capacity for novel acoustic wave manipulation. By precisely tailoring the geometry of these engineered surfaces, the effective refractive index may be modulated and, consequently, acoustic phase delays tuned. Despite the successful demonstration of phase engineering using metasurfaces, amplitude modulation remains overlooked. Herein, we present a class of metasurfaces featuring a horn-like space-coiling structure, enabling acoustic control with simultaneous phase and amplitude modulation. The functionality of this class of metasurfaces, featuring a gradient in channel spacing, has been investigated theoretically and numerically and an equivalent model simplifying the structural behavior is presented. A metasurface featuring this geometry has been designed and its functionality in modifying acoustic radiation patterns experimentally validated. This class of acoustic metasurface provides an efficient design methodology enabling complete acoustic wave manipulation, which may find utility in applications including biomedical imaging, acoustic communication, and non-destructive testing.We thank Boston University Materials Innovation Grant and Dean's Catalyst Award. We also thank the Boston University Photonics Center for technical support. (Boston University Materials Innovation Grant; Dean's Catalyst Award
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