Elena Garcia

The endophytic mycobiota of Arabidopsis thaliana

Fungal endophytes are receiving increasing attention as resources to improve crop production and ecosystem management. However, the biology and ecological significance of these symbionts remains poorly understood, due to a lack of model systems for more efficient research. In this work, we have analyzed the culturable endophytic mycobiota associated, in the wild, with leaves and siliques of the model plant A. thaliana. We have studied the effect of biotic and abiotic factors in the frequency of fungal endophytes in plant specimens, and in the species composition of the endophytic community. Our results indicate that the frequency of Arabidopsis plants hosting endophytes depends on the time of the year and the phenological stage of the plant, and that the probability of endophyte colonization increases as the life cycle of the plant progresses. The diversity of the endophytic assemblages of natural A. thaliana populations was high, and precipitation and temperature were the two main factors determining the diversity and species composition of the communities. We propose A. thaliana and its endophytes as a model system for an integral approach to the principles governing the endophytic lifestyle, taking advantage of the molecular tools and the abundant knowledge accessible from the host plant.

An Adjustable Compliant Joint for Lower-Limb Exoskeletons

The field of exoskeletons and wearable devices for walking assistance and rehabilitation has advanced considerably over the past few years. Currently, commercial devices contain joints with stiff actuators that cannot adapt to unpredictable environments. These actuators consume more energy and may not be appropriate for human-machine interactions. Thus, adjustable compliant actuators are being cautiously incorporated into new exoskeletons and active orthoses. Some simulation-based studies have evaluated the benefits of incorporating compliant joints into such devices. Another reason that compliant actuators are desirable is that spasticity and spasmodic movements are common among patients with motor deficiencies; compliant actuators could efficiently absorb these perturbations and improve joint control. In this paper, we provide an overview of the requirements that must be fulfilled by these actuators while evaluating the behavior of leg joints in the locomotion cycle. A brief review of existing compliant actuators is conducted, and our proposed variable stiffness actuator prototype is presented and evaluated. The actuator prototype is implemented in an exoskeleton knee joint operated by a state machine that exploits the dynamics of the leg, resulting in a reduction in actuation energy demand and better adaptability to disturbances.

Impedance Control for Legged Robots: An Insight Into the Concepts Involved

The application of impedance control strategies to modern legged locomotion is analyzed, paying special attention to the concepts behind its implementation which is not straightforward. In order to implement a functional impedance controller for a walking mechanism, the concepts of contact, impact, friction, and impedance have to be merged together. A literature review and a comprehensive analysis are presented compiling all these concepts along with a discussion on position-based versus force-based impedance control approaches, and a theoretical model of a robotic leg in contact with its environment is introduced. A theoretical control scheme for the legs of a general legged robot is also introduced, and some simulations results are presented.

Detailed Study of Amplitude Nonlinearity in Piezoresistive Force Sensors

This article upgrades the RC linear model presented for piezoresistive force sensors. Amplitude nonlinearity is found in sensor conductance, and a characteristic equation is formulated for modeling its response under DC-driving voltages below 1 V. The feasibility of such equation is tested on four FlexiForce model A201-100 piezoresistive sensors by varying the sourcing voltage and the applied forces. Since the characteristic equation proves to be valid, a method is presented for obtaining a specific sensitivity in sensor response by calculating the appropriate sourcing voltage and feedback resistor in the driving circuit; this provides plug-and-play capabilities to the device and reduces the start-up time of new applications where piezoresistive devices are to be used. Finally, a method for bypassing the amplitude nonlinearity is presented with the aim of reading sensor capacitance.

On the biomimetic design of agile-robot legs.

The development of functional legged robots has encountered its limits in human-made actuation technology. This paper describes research on the biomimetic design of legs for agile quadrupeds. A biomimetic leg concept that extracts key principles from horse legs which are responsible for the agile and powerful locomotion of these animals is presented. The proposed biomimetic leg model defines the effective leg length, leg kinematics, limb mass distribution, actuator power, and elastic energy recovery as determinants of agile locomotion, and values for these five key elements are given. The transfer of the extracted principles to technological instantiations is analyzed in detail, considering the availability of current materials, structures and actuators. A real leg prototype has been developed following the biomimetic leg concept proposed. The actuation system is based on the hybrid use of series elasticity and magneto-rheological dampers which provides variable compliance for natural motion. From the experimental evaluation of this prototype, conclusions on the current technological barriers to achieve real functional legged robots to walk dynamically in agile locomotion are presented.

System identification applied to contact modeling: An experimental investigation

Many robotics applications require contact with the environment, from traditional pick and place task to legged locomotion. Nevertheless, to increase adaptability to different terrains it is necessary to know its contact properties. These properties can be known beforehand or extracted from contact forces. In this paper a practical evaluation using adaptive filtering techniques to extract the properties from different environmental conditions, with the intention to increase the adaptability of the robot to different environments is presented. We use the well-known linear spring-dashpot model and fit its parameters to four different materials in order to establish which of the reviewed methods (the recursive and windowed least squares algorithms) perform better to describe the material properties. The results show that the recursive least squares provide better tracking performance while the windowed least squares gives a smoother response.

Exploiting joint synergy for actuation in a lower‐limb active orthosis

– Lower‐limb exoskeletons and powered orthoses are external devices that assist patients with locomotive disorders to achieve correct limb movements. Current batteries cannot meet the long‐term power requirements for these devices, which operate for long periods of time. This issue has become a major challenge in the development of these portable robots. Conversely, legged locomotion in animals and humans is efficient; to emulate this behaviour, biomimetic actuation has been designed attempting to incorporate elements that resemble biological elements, such as tendons and muscles, in the mechanical systems. The purpose of this paper is to present a mechanism that resembles a human tendon to achieve and utilise the synergic actuation of the leg joints., – In this paper, we present a mechanism that resembles a human tendon to move the ankle joint and utilise the synergic actuation of hip and knee joints. Implementation of the proposed transmission system in the ATLAS active orthosis prototype allowed for a better ankle gait fit, which resulted in a more natural stride and, as expected, optimised energy consumption in the locomotion cycle and actuation energy requirements., – The fitted passive ankle motion provides toe‐off impulse, increases support force, and helps provide ground clearance., – A synergetic underactuated movement in the ankle joint, implemented by two cables in each leg, improves the functionality of the device without increasing the leg weight and while maintaining a reduced size. To achieve a correct and efficient motion in the ankle of an active orthosis, two steel cables were attached in the ATLAS orthosis. These cables act as a synergic biarticular linkage and transfer motion from the hip and knee joints. Synergic ankle motion provides impulse in the toe‐off, increases support force, and provides ground clearance. These goals are achieved with low energy expenditure because of synergical actuation, and high inertia is prevented in the more distal limb.

On the Necessity of Including Joint Passive Dynamics in the Impedance Control of Robotic Legs

Bioinspired quadruped robots are among the best robot designs for field missions over the complex terrain encountered in extraterrestrial landscapes and disaster scenarios caused by natural and human-made catastrophes, such as those caused by nuclear power plant accidents and radiological emergencies. For such applications, the performance characteristics of the robots should include high mobility, adaptability to the terrain, the ability to handle a large payload and good endurance. Nature can provide inspiration for quadruped designs that are well suited for traversing complex terrain. Horse legs are an example of a structure that has evolved to exhibit good performance characteristics. In this paper, a leg design exhibiting the key features of horse legs is briefly described. This leg is an underactuated mechanism because it has two actively driven degrees of freedom (DOFs) and one passively driven DOF. In this work, two control laws intended to be use in the stan ce phase are described: a control law that considers passive mechanism dynamics and a second law that neglects these dynamics. The performance of the two control laws is experimentally evaluated and compared. The results indicate that the first control law better achieves the control goal; however, the use of the second is not completely unjustified.

Parameterized inverted and double pendulum model for controlling lower-limb active orthosis

Lower-limb active orthosis have been traditionally controlled by tracking clinical gait analysis (CGA) angle patterns. This approach however is very rigid and difficult to modify. This paper proposes a method based on the parameterization of simple dynamic models that explain human walking, which is more flexible and more intuitively modified than directly applied CGA patterns. In addition, a comparison between the angle trajectories obtained by this method and CGA is presented. In order to further test the method, it is applied to a simulated biped featuring the same mass and limb length distribution as a quadriplegic girl showing good results with the simplest possible controller, even in step-to-step transitions which are not explicitly considered by the parameterization. Finally, the joint coordinates used in the simulation were also implemented in the ATLAS prototype showing a natural looking gait.