Fluidic artificial muscles (FAMs) have emerged as a viable and popular robotic actuation technique due to their low cost, compliant nature, and high force-to-weight-ratio. In recent years, the concept of variable recruitment has emerged as a way to improve the efficiency of conventional hydraulic robotic systems. In variable recruitment, groups of FAMs are bundled together and divided into individual motor units. Each motor unit can be activated independently, which is similar to the sequential activation pattern observed in mammalian muscle. Previous researchers have performed quasistatic characterizations of variable recruitment bundles and some simple dynamic analyses and experiments with a simple 1- DOF robot arm. We have developed a linear hydraulic characterization testing platform that will allow for the testing of different types of variable recruitment bundle configurations under different loading conditions. The platform consists of a hydraulic drive cylinder that acts as a cyber-physical hardware-in-the-loop dynamic loading emulator and interfaces with the variable recruitment bundle. The desired inertial, damping and stiffness properties of the emulator can be prescribed and achieved through an admittance controller. In this paper, we test the ability of this admittance controller to emulate different inertial, stiffness, and damping properties in simulation and demonstrate that it can be used in hardware through a proof-of-concept experiment. The primary goal of this work is to develop a unique testing setup that will allow for the testing of different FAM configurations, controllers, or subsystems and their responses to different dynamic loads before they are implemented on more complex robotic systems.
This paper investigates the effect of resistive forces that arise in compressed fluidic artificial muscles (FAMs) within a variable recruitment bundle. Much like our skeletal muscle organs that selectively recruit different number of motor fibers depending on the load demand, a variable recruitment FAM bundle adaptively activates the minimum number of motor units (MUs) to increase its overall efficiency. A variable recruitment bundle may operate in different recruitment states (RSs) during which only a subset of the FAMs within a bundle are activated. In such cases, a difference in strain occurs between active FAMs and inactive/low-pressure FAMs. This strain difference results in the compression of inactive/lowpressure FAMs causing them to exert a resistive force opposing the force output of active FAMs. This paper presents experimental measurements for a FAM for both tensile and compressive regions. The data is used to simulate the overall force-strain space of a variable recruitment bundle for when resistive force effects are neglected and when they are included. Counterintuitively, an initial decrease in bundle free strain is observed when a transition to a higher RS is made due to the presence of resistive forces. We call this phenomenon the free strain gradient reversal of a variable recruitment bundle. The paper is concluded with a discussion of the implications of this phenomenon.
The use of soft, compliant actuators has recently gained research attention as a potential approach to improve human-robot interaction compatibility. Fluidic artificial muscles, or McKibben actuators, are a popular class of soft actuator due to their low cost and high force-to-weight ratio. However, traditional McKibben actuators face efficiency problems, as in most actuation schemes, the actuator is sized for the largest possible load, resulting in energy loss when operating at lower force regimes. To address this issue, our group has developed a bio-inspired actuation strategy called variable recruitment. In variable recruitment, actuators are placed within a bundle and can be sequentially activated depending on the required load. This strategy mimics the hierarchical architecture of mammalian muscle tissue and improves system efficiency and bandwidth while allowing for variable stiffness properties. Previous variable recruitment models and controllers assume that the force output of each actuator is independent and that these forces sum to provide the total bundle force. However, our recent work has shown that there is significant interaction between actuators within a bundle, particularly at lower recruitment states. This is because at these states, inactive or partially activated actuators resist bundle motion and reduce total force production. In this paper, we study these resistive effects at low recruitment states by considering two different variable recruitment configurations: a fixed-end configuration (with resistive forces) and a tendon configuration (designed with tendons to eliminate resistive forces). We then assess the tradeoffs between the two configurations. We found that while using the tendon configuration eliminates resistive forces, if we consider both configurations with the same overall system length, the tendon configuration has less overall system free strain because its FAMs have to be shorter than those of the fixed-end configuration. However, despite this difference in free strain, our results still show that the tendon configuration can have higher maximum load capacity and efficiency than the fixed-end configuration and that the specific application and system requirements will dictate the proper configuration choice.
KEYWORDS: Smart materials, Solar energy, Wind energy, Energy harvesting, Wind measurement, Thin film solar cells, Intelligence systems, Aerodynamics, Thin films, Electromagnetism, Sun
This paper considers aerodynamic interactions among an array of tensioned ribbon energy harvesters capable of harvesting both wind and solar energy. Each harvester consists of a thin-film solar cell ribbon supported in tension by a pair of piezoelectric bimorph beams in an inverted-U configuration. These ribbons experience aeroelastic flutter when subjected to crossflow, and the energy from these vibrations can be harvested through the piezoelectric beams. The effect of wind speed on the interaction between two fluttering inverted U-shaped aeroelastic energy harvesters configured in a tandem array was investigated, as previous work suggests that synergistic wake interactions can occur between multiple fluttering energy harvesters. An experimental apparatus was constructed and two thin-film solar ribbons were placed in tandem at a fixed separation distance. Each ribbon was given an applied pre-tension, and wind tunnel testing was performed for a range of wind speeds between 7.5 m/s and 12.5 m/s for each ribbon when fluttering in isolation and when fluttering in tandem. Tandem array efficiency was calculated from the experimental data, and it was determined that there is a wind speed at which peak tandem array efficiency (significantly greater than unity) occurs. It was found that this peak corresponds to the wind speed at which constructive interference due to frequency lock between the two fluttering ribbons begins. Results also show tandem efficiency benefits in both the downstream and upstream harvester, as opposed to previous results that show benefits primarily in the downstream harvester. It is hypothesized that these upstream benefits are due to possible base excitations in the apparatus that have been transmitted by the downstream harvester.
KEYWORDS: Energy harvesting, Wind energy, Solar energy, Data centers, Data modeling, Wind measurement, Instrument modeling, Data acquisition, Intelligence systems, Statistical modeling
Researchers have performed theoretical investigations of flow induced limit cycle oscillations (LCOs) of tensioned ribbons. Furthermore, attempts have been made to tap into the energy harvesting capability of such ribbons, owing to its structural simplicity, low weight and ease of fabrication. However, in order to tune the ribbon to perform optimally at a given location, a robust, reliable model of the ribbon is essential to predict the limit cycle behavior. The model needs validation across a broad spectrum of its operating envelope based on experimentally obtained results. This paper seeks to provide experimental data for a sample tensioned ribbon in cross flow to serve as basis for validation of an aeroelastic model. This paper experimentally characterizes a PTFE (polytetrafluoroethylene) ribbon of aspect ratio 18 across a range of applied axial preload tension and wind speeds.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.