3LP: a linear 3D-walking model including torso and swing dynamics

In this paper, we present a new model of biped locomotion which is composed of three linear pendulums (one per leg and one for the whole upper body) to describe stance, swing and torso dynamics. In addition to double support, this model has different actuation possibilities in the swing hip and stance ankle which could be widely used to produce different walking gaits. Without the need for numerical time-integration, closed-form solutions help finding periodic gaits which could be simply scaled in certain dimensions to modulate the motion online. Thanks to linearity properties, the proposed model can provide a computationally fast platform for model predictive controllers to predict the future and consider meaningful inequality constraints to ensure feasibility of the motion. Such property is coming from describing dynamics with joint torques directly and therefore, reflecting hardware limitations more precisely, even in the very abstract high level template space. The proposed model produces human-like torque and ground reaction force profiles and thus, compared to point-mass models, it is more promising for precise control of humanoid robots. Despite being linear and lacking many other features of human walking like CoM excursion, knee flexion and ground clearance, we show that the proposed model can predict one of the main optimality trends in human walking, i.e. nonlinear speed-frequency relationship. In this paper, we mainly focus on describing the model and its capabilities, comparing it with human data and calculating optimal human gait variables. Setting up control problems and advanced biomechanical analysis still remain for future works.Comment: Journal paper under revie

Push recovery with stepping strategy based on time-projection control

In this paper, we present a simple control framework for on-line push recovery with dynamic stepping properties. Due to relatively heavy legs in our robot, we need to take swing dynamics into account and thus use a linear model called 3LP which is composed of three pendulums to simulate swing and torso dynamics. Based on 3LP equations, we formulate discrete LQR controllers and use a particular time-projection method to adjust the next footstep location on-line during the motion continuously. This adjustment, which is found based on both pelvis and swing foot tracking errors, naturally takes the swing dynamics into account. Suggested adjustments are added to the Cartesian 3LP gaits and converted to joint-space trajectories through inverse kinematics. Fixed and adaptive foot lift strategies also ensure enough ground clearance in perturbed walking conditions. The proposed structure is robust, yet uses very simple state estimation and basic position tracking. We rely on the physical series elastic actuators to absorb impacts while introducing simple laws to compensate their tracking bias. Extensive experiments demonstrate the functionality of different control blocks and prove the effectiveness of time-projection in extreme push recovery scenarios. We also show self-produced and emergent walking gaits when the robot is subject to continuous dragging forces. These gaits feature dynamic walking robustness due to relatively soft springs in the ankles and avoiding any Zero Moment Point (ZMP) control in our proposed architecture.Comment: 20 pages journal pape

Imprecise dynamic walking with time-projection control

We present a new walking foot-placement controller based on 3LP, a 3D model of bipedal walking that is composed of three pendulums to simulate falling, swing and torso dynamics. Taking advantage of linear equations and closed-form solutions of the 3LP model, our proposed controller projects intermediate states of the biped back to the beginning of the phase for which a discrete LQR controller is designed. After the projection, a proper control policy is generated by this LQR controller and used at the intermediate time. This control paradigm reacts to disturbances immediately and includes rules to account for swing dynamics and leg-retraction. We apply it to a simulated Atlas robot in position-control, always commanded to perform in-place walking. The stance hip joint in our robot keeps the torso upright to let the robot naturally fall, and the swing hip joint tracks the desired footstep location. Combined with simple Center of Pressure (CoP) damping rules in the low-level controller, our foot-placement enables the robot to recover from strong pushes and produce periodic walking gaits when subject to persistent sources of disturbance, externally or internally. These gaits are imprecise, i.e., emergent from asymmetry sources rather than precisely imposing a desired velocity to the robot. Also in extreme conditions, restricting linearity assumptions of the 3LP model are often violated, but the system remains robust in our simulations. An extensive analysis of closed-loop eigenvalues, viable regions and sensitivity to push timings further demonstrate the strengths of our simple controller

Concurrent silent strokes impair motor function by limiting behavioral compensation

Silent strokes occur more frequently than classic strokes; however, symptoms may go unreported in spite of lasting tissue damage. A silent stroke may indicate elevated susceptibility to recurrent stroke, which may eventually result in apparent and lasting impairments. Here we investigated if multiple silent strokes to the motor system challenge the compensatory capacity of the brain to cumulatively result in permanent functional deficits. Adult male rats with focal ischemia received single focal ischemic mini-lesions in the sensorimotor cortex (SMC) or the dorsolateral striatum (DLS), or multiple lesions affecting both SMC and DLS. The time course and outcome of motor compensation and recovery were determined by quantitative and qualitative assessment of skilled reaching and skilled walking. Rats with SMC or DLS lesion alone did not show behavioral deficits in either task. However, the combination of focal ischemic lesions in SMC and DLS perturbed skilled reaching accuracy and disrupted forelimb placement in the ladder rung walking task. These observations suggest that multiple focal infarcts, each resembling a silent stroke, gradually compromise the plastic capacity of the motor system to cause permanent motor deficits. Moreover, these findings support the notion that cortical and subcortical motor systems cooperate when adopting beneficial compensatory movement strategies. © 2012 Elsevier Inc.

Stress precipitates functional deficits following striatal silent stroke: A synergistic effect

Stress has been linked to structural and functional outcomes after stroke. Moreover, the striatum, both dorsal and ventral, is a vital regulator of stress perception and associated physiological responses. This study investigates potential synergistic effects of focal stroke in the ventrolateral striatum and restraint stress on motor and spatial performance. Adult male Long-Evans rats were pre-trained in a skilled reaching task and randomly assigned to sham, stroke-only, stress-only and stroke. +. stress conditions. Ventrolateral striatal focal ischemia was induced by endothelin-1 (ET-1) infusion. Rats in stress-only and stroke. +. stress groups received 21. days of mild restraint stress after stroke. All rats were tested in the skilled reaching task and the ziggurat task (ZT) for post-stroke motor and spatial performance. There was no effect of ventrolateral striatal ischemia or stress alone on motor and spatial performance. Notably, stroke and stress interacted synergistically to reduce reaching success and to disrupt qualitative aspects of movement performance in the absence of histological differences in lesion size. Thus, stress can precipitate behavioural deficits after focal ischemia even in the absence of significant functional deficits on its own. These results emphasize the importance of prevention programmes to control post-stroke levels of stress in clinical populations. © 2011 Elsevier Inc

Rats with hippocampal lesion show impaired learning and memory in the ziggurat task: A new task to evaluate spatial behavior

Spatial tasks are widely used to determine the function of limbic system structures in rats. The present study used a new task designed to evaluate spatial behavior, the ziggurat task (ZT), to examine the performance of rats with widespread hippocampal damage induced by N-methyl-d-aspartic acid (NMDA). The task consisted of an open field containing 16 identical ziggurats (pyramid shaped towers) arranged at equal distances. One of the ziggurats was baited with a food reward. The task required rats to navigate through the open field by using a combination of distal and/or proximal cues in order to locate the food reward. The ability to acquire and recall the location of the goal (baited) ziggurat was tested in consecutive training sessions of eight trials per day for 10 days. The location of the goal ziggurat was changed every second day, requiring the rats to learn a total of five different locations. Several parameters, including latency to find the target, distance traveled, the number of visits to non-baited ziggurats (errors), and the number of returns were used as indices of learning and memory. Control rats showed a significant decrease in distance traveled and reduced latency in locating the goal ziggurat across trials and days, suggesting that they learned and remembered the location of the goal ziggurat. Interestingly, the hippocampal-damaged group moved significantly faster, and traveled longer distances compared to the control group. Significant differences were observed between these groups with respect to the number of errors and returns on test days. Day 11 served as probe day, in which no food reward was given. The controls spent more time searching for the food in the previous training quadrant compared to the hippocampal group. The findings demonstrate that the ZT is a sensitive and efficient dry task for measuring hippocampus-dependent spatial performance in rats requiring little training and not associated with some of the disadvantages of water tasks. © 2007 Elsevier B.V. All rights reserved

Effect of stacking interactions on charge transfer states in photoswitches interacting with ion channels

The activity of ion channels can be reversibly photo-controlled via the binding of molecular photoswitches, often based on an azobenzene scaffold. Those azobenzene derivatives interact with aromatic residues of the protein via stacking interactions. In the present work, the effect of face-to-face and t-shaped stacking interactions on the excited state electronic structure of azobenzene and p-diaminoazobenzene integrated into the Na V1.4 channel is computationally investigated. The formation of a charge transfer state, caused by electron transfer from the protein to the photoswitches, is observed. This state is strongly red shifted when the interaction takes place in a face-to-face orientation and electron donating groups are present on the aromatic ring of the amino acids. The low-energy charge transfer state can interfere with the photoisomerization process after excitation to the bright state by leading to the formation of radical species. </p