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1. Steering Magnetic Robots in Two Axes with One Pair of Maxwell Coils

   Emma Benjaminson1, Matthew Travers2 (October 2020, IROS)

   Abstract—This work demonstrates a novel approach to steering a magnetic swimming robot in two dimensions with a single pair of Maxwell coils. By leveraging the curvature of the magnetic field gradient, we achieve motion along two axes. This method allows us to control medical magnetic robots using only existing MRI technology, without requiring additional hard- ware or posing any additional risk to the patient. We implement a switching time optimization algorithm which generates a schedule of control inputs that direct the swimming robot to a goal location in the workspace. By alternating the direction of the magnetic field gradient produced by the single pair of coils per this schedule, we are able to move the swimmer to desired points in two dimensions. Finally, we demonstrate the feasibility of our approach with an experimental implementation on the millimeter scale and discuss future opportunities to expand this work to the microscale, as well as other control problems and real-world applications. 

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2. Direct computation of magnetic surfaces in Boozer coordinates and coil optimization for quasisymmetry

   https://doi.org/10.1017/S0022377822000563  

   Giuliani, Andrew; Wechsung, Florian; Stadler, Georg; Cerfon, Antoine; Landreman, Matt (August 2022, Journal of Plasma Physics)

   We propose a new method to compute magnetic surfaces that are parametrized in Boozer coordinates for vacuum magnetic fields. We also propose a measure for quasisymmetry on the computed surfaces and use it to design coils that generate a magnetic field that is quasisymmetric on those surfaces. The rotational transform of the field and complexity measures for the coils are also controlled in the design problem. Using an adjoint approach, we are able to obtain analytic derivatives for this optimization problem, yielding an efficient gradient-based algorithm. Starting from an initial coil set that presents nested magnetic surfaces for a large fraction of the volume, our method converges rapidly to coil systems generating fields with excellent quasisymmetry and low particle losses. In particular for low complexity coils, we are able to significantly improve the performance compared with coils obtained from the standard two-stage approach, e.g. reduce losses of fusion-produced alpha particles born at half-radius from $$17.7\,\%$$ to $$6.6\,\%$$ . We also demonstrate 16-coil configurations with alpha loss $${<}1\,\%$$ and neoclassical transport magnitude $$\epsilon _{\text {eff}}^{3/2}$$ less than approximately $$5\times 10^{-9}$$ . 

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3. Amoeba-inspired swimming through isoperimetric modulation of body shape

   https://doi.org/10.1109/IROS47612.2022.9982120  

   Sparks, Curtis; Justus, Nathan; Hatton, Ross; Gravish, Nick (October 2022, 2022 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS))

   In this work we present the design of a swimming robot that is inspired by the body shape modulation of small microorganisms. Amoebas are small single celled organisms that locomote through deformation and shape change of their body. To achieve similar shape modulation for swimming propulsion in a robot we developed a novel flexible appendage using tape springs. A tape spring is an elongated strip of metal with a curved cross-section that can act as a stiff structure when loaded against the curvature, while it can easily buckle when loaded with the curvature. We develop a tape spring appendage that is capable of freely deforming its perimeter through two actuation inputs. In the first portion of this paper we develop the kinematics of the appendage mechanisms and compare with experiment. Next we present the design of a surface locomoting robot that uses two appendages for propulsion. From the appendage kinematics we derive the local connection vector field for locomotion kinematics and study the optimal gait for forward swimming. Lastly, we demonstrate robot swimming performance in open water conditions. The novel appendage design in this robot is advantageous because it enables omnidirectional movement, the appendages will not tangle in debris, and they are robust to collisions and contact with structures. 

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4. Dynamic Control of Probabilistic Simple Temporal Networks

   https://doi.org/10.1609/aaai.v34i06.6538  

   Gao, Michael; Popowski, Lindsay; Boerkoel, Jim (June 2020, Proceedings of the AAAI Conference on Artificial Intelligence)

   null (Ed.)

   The controllability of a temporal network is defined as an agent's ability to navigate around the uncertainty in its schedule and is well-studied for certain networks of temporal constraints. However, many interesting real-world problems can be better represented as Probabilistic Simple Temporal Networks (PSTNs) in which the uncertain durations are represented using potentially-unbounded probability density functions. This can make it inherently impossible to control for all eventualities. In this paper, we propose two new dynamic controllability algorithms that attempt to maximize the likelihood of successfully executing a schedule within a PSTN. The first approach, which we call Min-Loss DC, finds a dynamic scheduling strategy that minimizes loss of control by using a conflict-directed search to decide where to sacrifice the control in a way that optimizes overall success. The second approach, which we call Max-Gain DC, works in the other direction: it finds a dynamically controllable schedule and then attempts to progressively strengthen it by capturing additional uncertainty. Our approaches are the first known that work by finding maximally dynamically controllable schedules. We empirically compare our approaches against two existing PSTN offline dispatch approaches and one online approach and show that our Min-Loss DC algorithm outperforms the others in terms of maximizing execution success while maintaining competitive runtimes. 

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5. Reconfigurable paramagnetic microswimmers: Brownian motion affects non-reciprocal actuation

   https://doi.org/10.1039/c8sm00069g  

   Du, Di; Hilou, Elaa; Biswal, Sibani Lisa (January 2018, Soft Matter)

   Swimming at low Reynolds number is typically dominated by a large viscous drag, therefore microscale swimmers require non-reciprocal body deformation to generate locomotion. Purcell described a simple mechanical swimmer at the microscale consisting of three rigid components connected together with two hinges. Here we present a simple microswimmer consisting of two rigid paramagnetic particles with different sizes. When placed in an eccentric magnetic field, this simple microswimmer exhibits non-reciprocal body motion and its swimming locomotion can be directed in a controllable manner. Additional components can be added to create a multibody microswimmer, whereby the particles act cooperatively and translate in a given direction. For some multibody swimmers, the stochastic thermal forces fragment the arm, which therefore modifies the swimming strokes and changes the locomotive speed. This work offers insight into directing the motion of active systems with novel time-varying magnetic fields. It also reveals that Brownian motion not only affects the locomotion of reciprocal swimmers that are subject to the Scallop theorem, but also affects that of non-reciprocal swimmers. 

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