8,810 research outputs found
Feedback Control as a Framework for Understanding Tradeoffs in Biology
Control theory arose from a need to control synthetic systems. From
regulating steam engines to tuning radios to devices capable of autonomous
movement, it provided a formal mathematical basis for understanding the role of
feedback in the stability (or change) of dynamical systems. It provides a
framework for understanding any system with feedback regulation, including
biological ones such as regulatory gene networks, cellular metabolic systems,
sensorimotor dynamics of moving animals, and even ecological or evolutionary
dynamics of organisms and populations. Here we focus on four case studies of
the sensorimotor dynamics of animals, each of which involves the application of
principles from control theory to probe stability and feedback in an organism's
response to perturbations. We use examples from aquatic (electric fish station
keeping and jamming avoidance), terrestrial (cockroach wall following) and
aerial environments (flight control in moths) to highlight how one can use
control theory to understand how feedback mechanisms interact with the physical
dynamics of animals to determine their stability and response to sensory inputs
and perturbations. Each case study is cast as a control problem with sensory
input, neural processing, and motor dynamics, the output of which feeds back to
the sensory inputs. Collectively, the interaction of these systems in a closed
loop determines the behavior of the entire system.Comment: Submitted to Integr Comp Bio
Vibration serviceability of footbridges under human-induced excitation : a literature review
Increasing strength of new structural materials and longer spans of new footbridges, accompanied with aesthetic requirements for greater slenderness, are resulting in more lively footbridge structures. In the past few years this issue attracted great public attention. The excessive lateral sway motion caused by crowd walking across the infamous Millennium Bridge in London is the prime example of the vibration serviceability problem of footbridges. In principle, consideration of footbridge vibration serviceability requires a characterisation of the vibration source, path and receiver. This paper is the most comprehensive review published to date of about 200 references which deal with these three key issues.
The literature survey identified humans as the most important source of vibration for footbridges. However, modelling of the crowd-induced dynamic force is not clearly defined yet, despite some serious attempts to tackle this issue in the last few years.
The vibration path is the mass, damping and stiffness of the footbridge. Of these, damping is the most uncertain but extremely important parameter as the resonant behaviour tends to govern vibration serviceability of footbridges.
A typical receiver of footbridge vibrations is a pedestrian who is quite often the source of vibrations as well. Many scales for rating the human perception of vibrations have been found in the published literature. However, few are applicable to footbridges because a receiver is not stationary but is actually moving across the vibrating structure.
During footbridge vibration, especially under crowd load, it seems that some form of human–structure interaction occurs. The problem of influence of walking people on footbridge vibration properties, such as the natural frequency and damping is not well understood, let alone quantified.
Finally, there is not a single national or international design guidance which covers all aspects of the problem comprehensively and some form of their combination with other published information is prudent when designing major footbridge structures. The overdue update of the current codes to reflect the recent research achievements is a great challenge for the next 5–10 years
Comprehensive structural model of the mechanochemical cycle of a mitotic motor highlights molecular adaptations in the kinesin family
Kinesins are responsible for a wide variety of microtubule-based, ATP-dependent
functions. Their motor domain drives these activities but the molecular adaptations
that specify these diverse and essential cellular activities are poorly understood. It
has been assumed that the first identified kinesin - the transport motor kinesin-1 – is
the mechanistic paradigm for the entire superfamily, but accumulating evidence
suggests that this is not the case. To address the deficits in our understanding of the
molecular basis of functional divergence within the kinesin superfamily, we studied
kinesin-5s, which are essential mitotic motors whose inhibition blocks cell division.
Using cryo-electron microscopy and subnanometer resolution structure
determination, we have visualised conformations of microtubule-bound human
kinesin-5 motor domain at successive steps in its ATPase cycle. Following ATP
hydrolysis, nucleotide-dependent conformational changes in the active site are
allosterically propagated into rotations of the motor domain and uncurling of the drugbinding
loop L5. In addition, the mechanical neck-linker element that is crucial for
motor stepping undergoes discrete, ordered displacements. We also observed large
reorientations of the motor N-terminus that indicate its importance for kinesin-5
function through control of neck-linker conformation. A kinesin-5 mutant lacking this
N-terminus is enzymatically active, and ATP-dependent neck-linker movement and
motility is defective although not ablated. All these aspects of kinesin-5
mechanochemistry are distinct from kinesin-1. Our findings directly demonstrate the
regulatory role of the kinesin-5 N-terminus in collaboration with the motor’s structured
neck-linker, and highlight the multiple adaptations within kinesin motor domains that
tune their mechanochemistries according to distinct functional requirements
A Design-Led, Materials Based Approach to Human Centered Applications Using Modified Dielectric Electroactive Polymer Sensors
This paper describes a design-led exploratory scoping study into the potential use of an industry standard dielectric electroactive polymer (DEAP) sensor for applications in assistive healthcare. The focus of this activity was to explore the physical format and integration of soft materials and sensor combinations with properties that afford an opportunity for accurate and unobtrusive real time body mapping and monitoring. The work involved a series of practical investigations into the capacitance changes in the sensor brought on by deformation through different ways of stretching. The dielectric sensors were selected as a direct mapping tool against the body based on the similarity of the stretch qualities of both the sensor and human skin and muscle resulting in a prototype vest for real time breathing monitoring through sensing thoracic movement. This involved modification of the standard sensors and handcrafting bespoke sensors to map critically relevant areas of the thorax
Swimming by spinning: spinning-top type rotations regularize sperm swimming into persistently symmetric paths in 3D
Sperm modulate their flagellar symmetry to navigate through complex
physico-chemical environments and achieve reproductive function. Yet it remains
elusive how sperm swim forwards despite the inherent asymmetry of several
components that constitutes the flagellar engine. Despite the critical
importance of symmetry, or the lack of it, on sperm navigation and its
physiological state, there is no methodology to date that can robustly detect
the symmetry state of the beat in free-swimming sperm in 3D.How does symmetric
progressive swimming emerge even for asymmetric beating, and how can beating
(a)symmetry be inferred experimentally? Here, we numerically resolve the fluid
mechanics of swimming around asymmetrically beating spermatozoa. This reveals
that sperm spinning critically regularizes swimming into persistently symmetric
paths in 3D, allowing sperm to swim forwards despite any imperfections on the
beat. The sperm orientation in three-dimensions, and not the swimming path, can
inform the symmetry state of the beat, eliminating the need of tracking the
flagellum in 3D. We report a surprising correspondence between the movement of
sperm and spinning-top experiments, indicating that the flagellum drives
''spinning-top'' type rotations during sperm swimming, and that this parallel
is not a mere analogy. These results may prove essential in future studies on
the role of (a)symmetry in spinning and swimming microorganisms and
micro-robots, as body orientation detection has been vastly overlooked in
favour of swimming path detection. Altogether, sperm rotation may provide a
foolproof mechanism for forward propulsion and navigation in nature that would
otherwise not be possible for flagella with broken symmetry
Which One is Me?: Identifying Oneself on Public Displays
While user representations are extensively used on public displays, it remains unclear how well users can recognize their own representation among those of surrounding users. We study the most widely used representations: abstract objects, skeletons, silhouettes and mirrors. In a prestudy (N=12), we identify five strategies that users follow to recognize themselves on public displays. In a second study (N=19), we quantify the users' recognition time and accuracy with respect to each representation type. Our findings suggest that there is a significant effect of (1) the representation type, (2) the strategies performed by users, and (3) the combination of both on recognition time and accuracy. We discuss the suitability of each representation for different settings and provide specific recommendations as to how user representations should be applied in multi-user scenarios. These recommendations guide practitioners and researchers in selecting the representation that optimizes the most for the deployment's requirements, and for the user strategies that are feasible in that environment
Discovery and recognition of motion primitives in human activities
We present a novel framework for the automatic discovery and recognition of
motion primitives in videos of human activities. Given the 3D pose of a human
in a video, human motion primitives are discovered by optimizing the `motion
flux', a quantity which captures the motion variation of a group of skeletal
joints. A normalization of the primitives is proposed in order to make them
invariant with respect to a subject anatomical variations and data sampling
rate. The discovered primitives are unknown and unlabeled and are
unsupervisedly collected into classes via a hierarchical non-parametric Bayes
mixture model. Once classes are determined and labeled they are further
analyzed for establishing models for recognizing discovered primitives. Each
primitive model is defined by a set of learned parameters.
Given new video data and given the estimated pose of the subject appearing on
the video, the motion is segmented into primitives, which are recognized with a
probability given according to the parameters of the learned models.
Using our framework we build a publicly available dataset of human motion
primitives, using sequences taken from well-known motion capture datasets. We
expect that our framework, by providing an objective way for discovering and
categorizing human motion, will be a useful tool in numerous research fields
including video analysis, human inspired motion generation, learning by
demonstration, intuitive human-robot interaction, and human behavior analysis
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