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Abstract In traditional hydraulic robotics, actuators must be sized for the highest possible load, resulting in significant energy losses when operating in lower force regimes. Variable recruitment fluidic artificial muscle (FAM) bundles offer a novel bio-inspired solution to this problem. Divided into individual MUs, each with its own control valve, a variable recruitment FAM bundle uses a switching control scheme to selectively bring MUs online according to load demand. To date, every dynamic variable recruitment study in the literature has considered homogeneous bundles containing MUs of equal size. However, natural mammalian muscle MUs are heterogeneous and primarily operate based on Henneman’s size principle, which states that MUs are recruited from smallest to largest for a given task. Is it better for a FAM variable recruitment bundle to operate according to this principle, or are there other recruitment orders that result in better performance? What are the appropriate criteria for switching between recruitment states for these different recruitment orders? This paper seeks to answer these questions by performing two case studies exploring different bundle MU size distributions, analyzing the tradeoffs between tracking performance and energetics, and determining how these tradeoffs are affected by different MU recruitment order and recruitment state transition thresholds. The only difference between the two test cases is the overall force capacity (i.e. total size) of the bundle. For each test case, a Pareto frontier for different MU size distributions, recruitment orders, and recruitment state transition thresholds is constructed. The results show that there is a complex relationship between overall bundle size, MU size distributions, recruitment orders, and recruitment state transition thresholds corresponding to the best tradeoffs change along the Pareto frontier. Overall, these two case studies validate the use of Henneman’s Size Principle as a variable recruitment strategy, but also demonstrate that it should not be the only variable recruitment method considered. They also motivate the need for a more complex variable recruitment scheme that dynamically changes the recruitment state transition threshold and recruitment order based on loading conditions and known system states, along with a co-design problem that optimizes total bundle size and MU size distribution. 

This paper describes the numerical study of oscillating circular cylinders with rigid splitter plates of different lengths. These geometries may be used as disturbance generators for the study of unsteady airfoils and wings operating in highly vortical flowfields. It has been shown that cylinders undergoing forced rotational oscillations at their natural shedding frequency can produce wakes with minimal deviation in cycle-to-cycle vortex strength and position. Adding a splitter plate allows these deviations to be reduced even further. We present cases for oscillating cylinders having splitter-plate lengths up to [Formula: see text] at a Reynolds number of 7600. Frequencies are maintained at the natural shedding frequency, and a rotational amplitude of 45 deg is used. Numerical simulations are performed using a two-dimensional unsteady Reynolds-averaged Navier–Stokes (RANS) code. Results are presented in the form of vorticity contours and cycle-averaged velocity profiles, as well as the dominant frequencies of cylinder lift force and downstream velocity angles. The results show that splitter-plate lengths shorter than [Formula: see text] adversely affect the ability to generate a coherent vortex wake due to shear layer roll-up near the trailing edge of the plate. Splitter plates longer than [Formula: see text] produced a reverse von Kármán wake with consistent cycle-to-cycle vortex shedding. 

Abstract The Multi-terrain Amphibious ARCtic explOrer (MAARCO) rover is an amphibious arctic rover designed to traverse arctic terrains and propel through water. The MAARCO rover consists of an ellipsoid chassis with links connecting to the propulsion system. The propulsion system consists of two helical drives made up of hollow cylinder ballasts wrapped in auger or screw shaped blades in opposing helical directions parallel to each other. In this paper, a 6 degree of freedom dynamic model of the MAARCO rover is created using Kane’s method dynamic modeling to demonstrate the dynamic model capabilities for an underwater vehicle’s performance. The hydrodynamic forces considered on the underwater rover include drag, buoyancy, flow acceleration, and added mass. In addition to the hydrodynamic forces the rover will experience gravity forces, control forces, net thrust from the helical drive blades, and net buoyancy from the helical drive ballast system. The equations of motion are developed from Kane’s method to reduce computational cost and simulated in MATLAB for different cases to gain further understanding and provide a visual representation of the system underwater and the dynamic models capabilities. The results of the simulations show the MAARCO rover behavior in the hydrodynamic environment. The results reveal that the Kane’s method dynamic modeling successfully develops equations of motion of a complicated system that can be implemented into a control system. 

Abstract The Multi-terrain Amphibious ARCtic explOrer (MAARCO) rover is an amphibious arctic rover designed to explore arctic regions in otherwise unsafe or restricted environments. The MAARCO rover consists of a propulsion system with two helical drives made up of hollow cylinder ballasts wrapped in auger or screw shaped blades that provide thrust to propel the vehicle as the drives rotate. Computational fluid dynamic methods provide a better understanding of the helical drives properties effects on hydrodynamic forces. In this paper, the computational fluid dynamic simulations are performed in ANSYS Fluent to observe the hydrodynamic properties of a helical drive. The drag and thrust on the helical drives are simulated for various helical drives with different blade heights and pitch lengths to determine general trends and characteristics of helical drives in water to optimize the vehicle’s abilities to navigate underwater. The helical drive drag is simulated using bluff body drag simulations with a prescribed velocity. The helical drive thrust is simulated using a multi-reference frame (MRF) mesh model with a frame motion replicate flow rotating around a stationary helical drive at a prescribed angular velocity. A convergence study was conducted to test different meshes and turbulence models to determine the most accurate drag and thrust simulation methods. The results demonstrate the effects the blade height and pitch length have on the helical drive thrust and drag properties, while maintaining a constant ballast diameter. From these results a helical drive design can be determined to optimize the net force and therefore the overall vehicle performance. 

Abstract Rovers designed for polar exploration primarily operate in flat and arid central plateau of the Antarctic and are unsuitable for multi-terrain operation such as slushy slow, wet soil, open ice and open ocean. The Multi-terrain Amphibious ARCtic explOrer or MAARCO rover is a design concept that employs a pair of helical drives in order to traverse the diverse landscape of the Arctic for autonomous research missions. A small-scale prototype (3:10 scale) of the MAARCO rover was developed to test terrestrial motion. The prototype is a screw-propelled vehicle that has two helical drives (similar to Archimedes’ screw) actuated by two independent motors to employ a differential steering system. The prototype performed several different maneuvers on a dry and wet sand — moving in a straight line, turning left, and turning right, and moving sideways by controlling the rotational speeds of the two drives. The sensors on the prototype collect data on angular orientation, linear acceleration and motor rotations. Through the tests the rover provided critical insight into the dynamics of the MAARCO rover, particularly, the effect of substrate failure or non-failure on the linear displacement of the rover. Further testing on different substrates and of the rover design on inclines surfaces will provide the MAARCO team valuable insight into designing a full-scale rover design. 

Abstract The diverse and heterogeneous terrains in the Arctic, consisting of snow, melting ice, permafrost, ice-covered lakes, sea ice and open ocean, pose serious challenges to locomotion and autonomous navigation capabilities of rovers deployed in the region for data collection and experimentation. The Multi-terrain Amphibious ARCtic explOrer or MAARCO rover is a proposed screw-propelled vehicle that uses helical drives (similar to Archimedes’ screws) to move seamlessly across the diverse terrains in the Arctic. The motion of a pair of helical drives operating in soft or fluid terrain is dictated by the response of the surrounding substrate to the stresses exerted by the rotating helical drives. If the substrate under the rover does not fail when it is moving in a straight line, the linear displacement of the rover (x) and the number of rotations of the helical drives (n) are related through x = P · n, where P is the pitch length of the helical drives. However, when the substrate fails, the linear displacement of the rover is less than P · n, i.e., x < P · n. Thus, “x = P · n” motion represents the optimal mode of operation for the rover when moving in a straight line. This paper represents the first ever attempt, to the best of author’s knowledge, to derive the conditions necessary for the application of the holonomic constraint x = P · n to the dynamics of a helical drives-based rover. 

Hardware-in-the-loop (HIL) testing is a popular control system testing method because it bridges the gap between modeling/simulation and experiments. Instead of designing a full hardware-based experiment to validate the results of a simulation, the plant hardware can be replaced with an emulator device that responds to exogenous inputs and effectively emulates the dynamic behavior of a system. This approach can be more cost-effective and modular, since the emulated plant system can be modeled in a simulation environment, implemented on a simplified piece of hardware and changed quickly without having to fabricate new parts. This paper develops the hardware and control scheme for a certain type of HIL device called a dynamic load emulator that consists of a 1-DOF linear hydraulic dynamometer equipped with in-line sensing to measure both its own position and the force exerted on it by a device-under-test. This measured force is passed to a real-time model of the emulated dynamic system. The model outputs the emulated system position, and a closed-loop controller is used to emulate this position. The emulator controller incorporates both model-based feedforward and standard feedback PI control. This paper characterizes the dynamometer-based dynamic load emulator and its controller, determining its hardware limitations and validating its capabilities when experiencing a force input from a linear spring with known parameters. Additionally, this paper demonstrates the ability of the emulator to represent the dynamics of a 1-DOF robotic joint when actuated by a pair of fluidic artificial muscles (FAMs). The primary contribution of this work is to allow for more comprehensive testing of FAM configurations, topologies, and controllers for a wide range of parameters, because the same hardware can be used to emulate multiple systems. As a result, this work will lead to more cost-effective, time-efficient, and energy-efficient designs of robotic systems and the FAMs used to actuate them.