4,804 research outputs found
Dynamics and Stability of Low-Reynolds-Number Swimming Near a Wall
The locomotion of microorganisms and tiny artificial swimmers is governed by low-Reynolds-number
hydrodynamics, where viscous effects dominate and inertial effects are negligible. While the theory
of low-Reynolds-number locomotion is well studied for unbounded fluid domains, the presence of a
boundary has a significant influence on the swimmer’s trajectories and poses problems of dynamic
stability of its motion. In this paper we consider a simple theoretical model of a microswimmer near
a wall, study its dynamics, and analyze the stability of its motion. We highlight the underlying
geometric structure of the dynamics, and establish a relation between the reversing symmetry of
the system and existence and stability of periodic and steady solutions of motion near the wall.
The results are demonstrated by numerical simulations and validated by motion experiments with
macroscale robotic swimmer prototypes
A new scanning method for fast atomic force microscopy
In recent years, the atomic force microscope (AFM) has become an important tool in nanotechnology research. It was first conceived to generate 3-D images of conducting as well as nonconducting surfaces with a high degree of accuracy. Presently, it is also being used in applications that involve manipulation of material surfaces at a nanoscale. In this paper, we describe a new scanning method for fast atomic force microscopy. In this technique, the sample is scanned in a spiral pattern instead of the well-established raster pattern. A constant angular velocity spiral scan can be produced by applying single frequency cosine and sine signals with slowly varying amplitudes to the x-axis and y -axis of AFM nanopositioner, respectively. The use of single-frequency input signals allows the scanner to move at high speeds without exciting the mechanical resonance of the device. Alternatively, the frequency of the sinusoidal set points can be varied to maintain a constant linear velocity (CLV) while a spiral trajectory is being traced. Thus, producing a CLV spiral. These scan methods can be incorporated into most modern AFMs with minimal effort since they can be implemented in software using the existing hardware. Experimental results obtained by implementing the method on a commercial AFM indicate that high-quality images can be generated at scan frequencies well beyond the raster scans
Brachiating power line inspection robot: controller design and implementation
The prevalence of electrical transmission networks has led to an increase in productivity and prosperity. In 2014, estimates showed that the global electric power transmission network consisted of 5.5 million circuit kilometres (Ckm) of high-voltage transmission lines with a combined capacity of 17 million mega-volt ampere. The vastness of the global transmission grid presents a significant problem for infrastructure maintenance. The high maintenance costs, coupled with challenging terrain, provide an opportunity for autonomous inspection robots. The Brachiating Power Line Inspection Robot (BPLIR) with wheels [73] is a transmission line inspection robot. The BPLIR is the focus of this research and this dissertation tackles the problem of state estimation, adaptive trajectory generation and robust control for the BPLIR. A kinematics-based Kalman Filter state estimator was designed and implemented to determine the full system state. Instrumentation used for measurement consisted of 2 Inertial Measurement Units (IMUs). The advantages of utilising IMUs is that they are less susceptible to drift, have no moving parts and are not prone to misalignment errors. The use of IMU's in the design meant that absolute angles (link angles measured with respect to earth) could be estimated, enabling the BPLIR to navigate inclined slopes. Quantitative Feedback Control theory was employed to address the issue of parameter uncertainty during operation. The operating environment of the BPLIR requires it to be robust to environmental factors such as wind disturbance and uncertainty in joint friction over time. The resulting robust control system was able to compensate for uncertain system parameters and reject disturbances in simulation. An online trajectory generator (OTG), inspired by Raibert-style reverse-time symmetry[10], fed into the control system to drive the end effector to the power line by employing brachiation. The OTG produced two trajectories; one of which was reverse time symmetrical and; another which minimised the perpendicular distance between the end gripper and the power line. Linear interpolation between the two trajectories ensured a smooth bump-less trajectory for the BPLIR to follow
Does the motor system need intermittent control?
Explanation of motor control is dominated by continuous neurophysiological pathways (e.g. trans-cortical, spinal) and the continuous control paradigm. Using new theoretical development, methodology and evidence, we propose intermittent control, which incorporates a serial ballistic process within the main feedback loop, provides a more general and more accurate paradigm necessary to explain attributes highly advantageous for competitive survival and performance
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
Inertial Load Compensation by a Model Spinal Circuit During Single Joint Movement
Office of Naval Research (N00014-92-J-1309); CONACYT (Mexico) (63462
Optimal Formation Flight Control Using Coupled Inter-Spacecraft Dynamics
Projecte fet en col.laboración amb Space Systems Laboratory (Massachusetts Institute ofTechnology, Cambridge, USAThe increasing number of formation ight space missions proposed by the scienti c
community for the near future has led many researchers to the study, development and
implementation of optimal control systems applied to a multi-spacecraft system. The
approaches taken may vary among authors, but it is generally agreed upon that having
independent controllers at each spacecraft leads to a non-optimal solution in a global
or formation-wide sense, even when independent controllers are implemented using
any of the locally optimal techniques known from the theory of control. Most of the
future formation ight missions have been designed with an interferometric purpose,
such as performing a space-based distributed telescope structure that would y into
deep space with an observational objective. In that case, where global positioning
systems such as GPS are no longer available, relative positioning not only becomes
necessary to achieve control of the multi-spacecraft system, but it also becomes a
crucial factor that would determine the performance of the system with regards to
the optical science output. In fact, if we rede ne the state vector of the plant and use
the relative states that need to be tracked instead of independent global positions,
we get to a de nition with coupled dynamics of the whole multi-agent system.
This research focuses on the control performance obtained when the controller
is designed using coupled inter-spacecraft dynamics and how this approach can lead
to an optimal solution in a global sense, both in optical performance and overall
fuel usage. The rst part of the thesis will address the theoretical advantages that
may arise within the coupled dynamics architecture and the second part analyses
the performance of the results obtained when testing the real implementation of the
controller on hardware. This study, concerning implementation and performance of
formation ight controllers in a real case scenario such as deep space interferometer
missions, will lead towards increasing mission lifetime, performance improvement and
a step forward in the eld
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