Design, analysis, and validation of an orderly recruitment valve for bio-inspired fluidic artificial muscles

Abstract

Biological musculature employs variable recruitment of muscle fibers from smaller to larger units as the load increases. This orderly recruitment strategy has certain physiological advantages like minimizing fatigue and providing finer motor control. Recently fluidic artificial muscles (FAM) are gaining popularity as actuators due to their increased efficiency by employing these bio-inspired recruitment strategies such as active variable recruitment (AVR). AVR systems use a multi-valve system (MVS) configuration to selectively recruit individual FAMs depending on the load. However, when using an MVS configuration, an increase in the number of motor units in a bundle corresponds to an increase in the number of valves in the system. This introduces greater complexity and weight. The objective of this paper is to propose, analyze, and demonstrate an orderly recruitment valve (ORV) concept that enables orderly recruitment of multiple FAMs in the system using a single valve. A mathematical model of an ORV-controlled FAM bundle is presented and validated by experiments performed on an ORV prototype. The modeling is extended to explore a case study of a 1-DOF robot arm system consisting of an electrohydraulic pressurization system, ORV, and a FAM-actuated rotating arm plant and its dynamics are simulated to further demonstrate the capabilities of an ORV-controlled closed-loop system. An orderly recruitment strategy was implemented through a model-based feed forward controller. To benchmark the performance of the ORV, a conventional MVS with equivalent dynamics and controller was also implemented. Trajectory tracking simulations on both the systems revealed lower tracking error for the ORV controlled system compared to the MVS controlled system due to the unique cross-flow effects present in the ORV. However, the MVS, due to its independent and multiple valve setup, proved to be more adaptable for performance. For example, modifications to the recruitment thresholds of the MVS demonstrated improvement in tracking error, albeit with a sacrifice in efficiency. In the ORV tracking performance remained insensitive to any variation in recruitment threshold. The results show that compared to the MVS, the ORV offers a simpler and more compact valving architecture at the expense of moderate losses in control flexibility and performance.

Investigation of the tradeoffs between tracking performance and energetics in heterogeneous variable recruitment fluidic artificial muscle bundles

“…The FAMs within the bundle can be sequentially based on required load, allowing them to operate much closer to source pressure, increasing overall efficiency. This was first shown to be true for a bundle in quasistatic conditions [5], and was later shown in both dynamic simulation and experimental studies [6][7][8][9][10]. Variable recruitment studies have been performed for both pneumatic and hydraulic artificial muscles bundles, both of which offer certain advantages.…”

“…These thresholds are implemented in Meller et al and are chosen ad hoc based on system performance [7]. In Vemula et al, recruitment thresholds are used, but only to eliminate slack from MUs before they are recruited, not to provide additional force capabilities before a previously recruited MU reaches saturation [10].…”

“…Due to the pressure-dependent free contraction behavior of the MU, it will not begin to generate force until its internal pressure becomes sufficiently large for its contraction to equal that of the bundle. This is known as 'de-slacking' [10,19]. During de-slacking, it is assumed that the force on the MU is zero, and thus the pressure in the MU is equal to the free contraction pressure corresponding to the MU's instantaneous length.…”