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.
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.
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.