This paper details the creation of a hybrid variable recruitment control scheme for fluidic artificial muscle (FAM) actuators with an emphasis on maximizing system efficiency and switching control performance. Variable recruitment is the process of altering a system's active number of actuators, allowing operation in distinct force regimes. Previously, FAM variable recruitment was only quantified with offline, manual valve switching; this study addresses the creation and characterization of novel, on-line FAM switching control algorithms. The bioinspired algorithms are implemented in conjunction with a PID and model-based controller, and applied to a simulated plant model. Variable recruitment transition effects and chatter rejection are explored via a sensitivity analysis, allowing a system designer to weigh tradeoffs in actuator modeling, algorithm choice, and necessary hardware. Variable recruitment is further developed through simulation of a robotic arm tracking a variety of spline position inputs, requiring several levels of actuator recruitment. Switching controller performance is quantified and compared with baseline systems lacking variable recruitment. The work extends current variable recruitment knowledge by creating novel online variable recruitment control schemes, and exploring how online actuator recruitment affects system efficiency and control performance. Key topics associated with implementing a variable recruitment scheme, including the effects of modeling inaccuracies, hardware considerations, and switching transition concerns are also addressed.
Hierarchical actuators are comprised of multiple individual actuator elements arranged into a system, resulting in improved and expanded performance. Natural muscle tissue is a complex and multi-level example of hierarchical actuation, with its hierarchy spanning from the micrometer to the centimeter scale. In addition to a hierarchical configuration, muscle tissue exists in varying geometric arrangements. Pennate muscle tissue, denoted by its characteristic fibers extending obliquely away from the muscle tissue line of action, leverages geometric complexity to transform the relationship between fiber inputs and muscle tissue outputs. In this paper, a bioinspired hierarchical pennate actuator is detailed. This work expands on previous pennate actuator studies by deriving constitutive force, contraction, and stiffness models for a general pennate actuator, where the constituent fibers can be constructed from any linear actuator. These models are experimentally validated by studying a pennate actuator with McKibben artificial muscles constituting the actuator fibers. McKibben artificial muscles are used because they have a high force-to-weight ratio and are inexpensive to construct, making them an attractive candidate for hierarchical actuators and mobile robotics. Using the derived constitutive models, general pennate actuator performance is better understood by analyzing the transmission ratio, blocked force, and free contraction. Loaded contractions and stiffness during isotonic and isobaric contractions are also explored. The results allow for informed design decisions and an understanding of the associated tradeoffs when recreating the remarkable properties of pennate musculature. Future work will leverage the results of this paper to create an adaptive pennate actuator that is capable of changing configuration in response to force, contraction and stiffness demands.
This paper investigates the energetics and performance of an electrohydraulic power system with variable recruitment fluidic artificial muscle (FAM) actuators. A coupled dynamic model of the system is developed and applied to study the implications of hydraulic power system architecture for both variable recruitment actuator bundles and equivalent single-muscle actuators. This analysis extends previous FAM literature by considering both actuator recruitment methodology as well as the complete electromechanical circuit and the interactions of these two subsystems. Specifically, for both single-muscle actuators and variable recruitment muscle bundles, hydraulic architectures with a continuously-operating motor and pump are compared with a system in which the motor is intermittently shut down and restarted based on accumulator pressure. The results reveal that variable recruitment offers bandwidth advantages over the single equivalent actuator regardless of the hydraulic power architecture that is selected. However, use of the intermittently-operating motor and pump system allowed the variable recruitment system to achieve efficiency advantages over the other configurations considered. A steady-state analytic solution for the operating envelopes of the variable recruitment and single-muscle systems, including force limits and flow rate limits, was also developed and used to investigate effects of pump displacement on system bandwidth and stroke. The results of these analyses provide tools for the selection of actuator configuration, system architecture, and component design in FAM-actuated electrohydraulic robots.
Nomenclature list Kinematics-related Variables x Distance of muscle contraction y Position of point 'A' relative to point 'W' i Horizontal displacement direction j Vertical displacement direction r n Length of link n φ n Angle from the horizontal of link n Pressure in accumulator V acc Volume of gas in gas-charged accumulator V m Volume of operating fluid sent from accumulator to FAM during contraction V p Volume of operating fluid sent from pump to accumulator during system operation h l Flow head loss from accumulator to FAM Motor-related Variables θ Motor operating speed B m Motor damping coefficient
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