Towards Testable Neuromechanical Control of Architectures for Running

Our objective is to provide experimentalists with neuromechanical control hypotheses that can be tested with kinematic data sets. To illustrate the approach, we select legged animals responding to perturbations during running. In the following sections, we briefly outline our dynamical systems approach, state our over-arching hypotheses, define four neuromechanical control architectures (NCAs) and conclude by proposing a series of perturbation experiments that can begin to identify the simplest architecture that best represents an animal\u27s controller

Energy Absorption During Running by Leg Muscles in a Cockroach

Biologists have traditionally focused on a muscle\u27s ability to generate power. By determining muscle length, strain and activation pattern in the cockroach Blaberus discoidalis, we discovered leg extensor muscles that operate as active dampers that only absorb energy during running. Data from running animals were compared with measurements of force and power production of isolated muscles studied over a range of stimulus conditions and muscle length changes. We studied the trochanter-femoral extensor muscles 137 and 179, homologous leg muscles of the mesothoracic and metathoracic legs, respectively. Because each of these muscles is innervated by a single excitatory motor axon, the activation pattern of the muscle could be defined precisely. Work loop studies using sinusoidal strains at 8 Hz showed these trochanter-femoral extensor muscles to be quite capable actuators, able to generate a maximum of 19-25 W kg-1 (at 25ºC). The optimal conditions for power output were four stimuli per cycle (interstimulus interval 11 ms), a strain of approximately 4%, and a stimulation phase such that the onset of the stimulus burst came approximately half-way through the lengthening phase of the cycle. High-speed video analysis indicated that the actual muscle strain during running was 12% in the mesothoracic muscles and 16% in the metathoracic ones. Myographic recordings during running showed on average 3-4 muscle action potentials per cycle, with the timing of the action potentials such that the burst usually began shortly after the onset of shortening. Imposing upon the muscle in vitro the strain, stimulus number and stimulus phase characteristic of running generated work loops in which energy was absorbed (-25 W kg-1) rather than produced. Simulations exploring a wide parameter space revealed that the dominant parameter that determines function during running is the magnitude of strain. Strains required for the maximum power output by the trochanter-femoral extensor muscles simply do not occur during constant, average-speed running. Joint angle ranges of the coxa-trochanter-femur joint during running were 3-4 times greater than the changes necessary to produce maximum power output. None of the simulated patterns of stimulation or phase resulted in power production when strain magnitude was greater than 5%. The trochanter-femoral extensor muscles 137/179 of a cockroach running at its preferred speed of 20 cm s-1 do not operate under conditions which maximize either power output or efficiency. In vitro measurements, however, demonstrate that these muscles absorb energy, probably to provide control of leg flexion and to aid in its reversal

Lift-off dynamics in a simple jumping robot

We study vertical jumping in a simple robot comprising an actuated mass-spring arrangement. The actuator frequency and phase are systematically varied to find optimal performance. Optimal jumps occur above and below (but not at) the robot's resonant frequency $f_0$. Two distinct jumping modes emerge: a simple jump which is optimal above $f_0$ is achievable with a squat maneuver, and a peculiar stutter jump which is optimal below $f_0$ is generated with a counter-movement. A simple dynamical model reveals how optimal lift-off results from non-resonant transient dynamics.Comment: 4 pages, 4 figures, Physical Review Letters, in press (2012

Comparative Design, Scaling, and Control of Appendages for Inertial Reorientation

This paper develops a comparative framework for the design of an actuated inertial appendage for planar reorientation. We define the Inertial Reorientation template, the simplest model of this behavior, and leverage its linear dynamics to reveal the design constraints linking a task with the body designs capable of completing it. As practicable inertial appendage designs lead to physical bodies that are generally more complex, we advance a notion of “anchoring” whereby a judicious choice of physical design in concert with an appropriate control policy yields a system whose closed loop dynamics are sufficiently captured by the template as to permit all further design to take place in its far simpler parameter space. This approach is effective and accurate over the diverse design spaces afforded by existing platforms, enabling performance comparison through the shared task space. We analyze examples from the literature and find advantages to each body type, but conclude that tails provide the highest potential performance for reasonable designs. Thus motivated, we build a physical example by retrofitting a tail to a RHex robot and present empirical evidence of its efficacy. For more information: Kod*la

Optically Targeted Search for Gravitational Waves Emitted by Core-Collapse Supernovae during the First and Second Observing Runs of Advanced LIGO and Advanced Virgo

We present the results from a search for gravitational-wave transients associated with core-collapse supernovae observed within a source distance of approximately 20 Mpc during the first and second observing runs of Advanced LIGO and Advanced Virgo. No significant gravitational-wave candidate was detected. We report the detection efficiencies as a function of the distance for waveforms derived from multidimensional numerical simulations and phenomenological extreme emission models. The sources with neutrino-driven explosions are detectable at the distances approaching 5 kpc, and for magnetorotationally driven explosions the distances are up to 54 kpc. However, waveforms for extreme emission models are detectable up to 28 Mpc. For the first time, the gravitational-wave data enabled us to exclude part of the parameter spaces of two extreme emission models with confidence up to 83%, limited by coincident data coverage. Besides, using ad hoc harmonic signals windowed with Gaussian envelopes, we constrained the gravitational-wave energy emitted during core collapse at the levels of 4.27 x 10-4 M⊙·c2 and 1.28 x 10-1 M⊙·c2 for emissions at 235 and 1304 Hz, respectively. These constraints are 2 orders of magnitude more stringent than previously derived in the corresponding analysis using initial LIGO, initial Virgo, and GEO 600 data

GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo during the First Half of the Third Observing Run

We report on gravitational wave discoveries from compact binary coalescences detected by Advanced LIGO and Advanced Virgo between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. By imposing a false-alarm-rate threshold of two per year in each of the four search pipelines that constitute our search, we present 39 candidate gravitational wave events. At this threshold, we expect a contamination fraction of less than 10%. Of these, 26 candidate events were reported previously in near real-time through GCN Notices and Circulars; 13 are reported here for the first time. The catalog contains events whose sources are black hole binary mergers up to a redshift of ~0.8, as well as events which could plausibly originate from binary neutron stars, neutron star-black hole binaries, or binary black holes. For the latter group, we are unable to determine the nature based on estimates of the component masses and spins from gravitational wave data alone. The range of candidate events which are unambiguously identified as binary black holes (both objects ≥ 3M⊙) is increased compared to GWTC-1, with total masses from ~14M⊙ for GW190924_021846 to ~150M⊙ for GW190521. For the first time, this catalog includes binary systems with asymmetric mass ratios, which had not been observed in data taken before April 2019. Given the increased sensitivity of Advanced LIGO and Advanced Virgo, the detection of 39 candidate events in ~26 weeks of data (~1.5 per week) is consistent with GWTC-1

GW190521: A Binary Black Hole Merger with a Total Mass of 150 M⊙

On May 21, 2019 at 03:02:29 UTC Advanced LIGO and Advanced Virgo observed a short duration gravitational-wave signal, GW190521, with a three-detector network signal-to-noise ratio of 14.7, and an estimated false-alarm rate of 1 in 4900 yr using a search sensitive to generic transients. If GW190521 is from a quasicircular binary inspiral, then the detected signal is consistent with the merger of two black holes with masses of 85+21−14  M⊙ and 66+17−18  M⊙ (90% credible intervals). We infer that the primary black hole mass lies within the gap produced by (pulsational) pair-instability supernova processes, with only a 0.32% probability of being below 65  M⊙. We calculate the mass of the remnant to be 142+28−16  M⊙, which can be considered an intermediate mass black hole (IMBH). The luminosity distance of the source is 5.3+2.4−2.6  Gpc, corresponding to a redshift of 0.82+0.28−0.34. The inferred rate of mergers similar to GW190521 is 0.13+0.30−0.11  Gpc−3 yr−1

Model Comparison from LIGO-Virgo Data on GW170817\u27s Binary Components and Consequences for the Merger Remnant

GW170817 is the very first observation of gravitational waves originating from the coalescence of two compact objects in the mass range of neutron stars, accompanied by electromagnetic counterparts, and offers an opportunity to directly probe the internal structure of neutron stars. We perform Bayesian model selection on a wide range of theoretical predictions for the neutron star equation of state. For the binary neutron star hypothesis, we find that we cannot rule out the majority of theoretical models considered. In addition, the gravitational-wave data alone does not rule out the possibility that one or both objects were low-mass black holes. We discuss the possible outcomes in the case of a binary neutron star merger, finding that all scenarios from prompt collapse to long-lived or even stable remnants are possible. For long-lived remnants, we place an upper limit of 1.9 kHz on the rotation rate. If a black hole was formed any time after merger and the coalescing stars were slowly rotating, then the maximum baryonic mass of non-rotating neutron stars is at most 3.05M⊙, and three equations of state considered here can be ruled out. We obtain a tighter limit of 2.67M⊙ for the case that the merger results in a hypermassive neutron star