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Spherical motors capable of omnidirectional rotations in a ball joint provide a highly dexterous actuator system yet also present great challenges in developing a high-performance regulation and tracking controller due to the complex rotor dynamics. This article presents a complementary H2-H∞(C-H2-H∞) control method for precisely controlling the multidegree of freedom (DOF) orientation of spherical motors in presence of external disturbances as well as model uncertainties and inaccuracies. In order to deal with the tradeoff between the robustness and control performance resulted from conventional control methods, the proposed controller featured with a 2-DOF structure presents an H2 controller for a nominal plant and then complements it with an additional regulator designed in H∞sense to assure robustness. Unlike traditional H∞ or mixed H2and H∞ controller, the H∞regulation is conducted online in accordance with the monitored modeling mismatch in a way that does not adversely degrade the performance delivered by H2 control, hence, improving the conservativeness of the total control performance and leading to significant improvements of tracking accuracy and response time at same time. Both numerical simulations and experiments on a spherical motor testbed are conducted to validate the superior performance of the proposed controller versus conventional control methods. [ABSTRACT FROM AUTHOR]

Theoretical modeling in any engineering design is of paramount importance as it establishes the interrelationship between variables being analyzed in a given condition. With regard to the design of a permanent-magnet spherical actuator, electromechanical modeling is crucial as it correlates the input parameters such as current to the output mechanical torque. In this paper, a new approach in electromechanical-torque formulation for this class of spherical actuator employing the magnetic-dipole-moment principle is being discussed. Derivation from first principle and the extension of this novel method in the acquisition of the 3-D resultant torque induced on the rotor are presented. The proposed approach circumvents the need for electromagnetic-energy analysis within the air gap between the rotor and stator poles and, henceforth, providing a direct computation of the resultant torque. The validity of the proposed analytical torque model was verified against numerical and empirical data. Comparisons between the 3-D torque results demonstrate the correctness and soundness of the proposed electromechanical torque model. Index Terms--Electromechanical torque modeling, magnetic-dipole-moment principle, permanent-magnet (PM) spherical actuator.

We present new results from modeling the magnetoimpedance (MI) effect, which considers explicitly the experimentally observed stripe domain structure in MI ribbon elements. Specifically, we solve the Maxwell and the Landau--Lifshitz--Gilbert equations formulated in such a way that includes an a priori known equilibrium magnetization. The equations are solved numerically for the real and imaginary parts of the magnetic field and magnetization simultaneously using a meshless method formulated in a point collocation scheme. Contrary to other models that have treated domain walls in a lumped parameter approach, we investigate the effects of the experimentally observed 180[degrees] Neel walls directly. Additionally, resulting MIR values are computed and compared to published experimental data and the case ignoring domain structure for the amorphous ribbon. It is shown that the presence of the observed domain structure leads to greatly reduced MI voltages, and contributing mechanisms are discussed. Moreover, the results have a broader impact applying to other harmonic magnetic structures with and without 180[degrees] Neel walls. Index Terms--Coupled harmonic micromagnetics, domain wall, magnetoimpedance (MI), meshless methods, MI sensor, micromagnetics, Neel wall.

Motivated by the increased needs of home-based rehabilitation for stroke patients, more and more interest has been drawn towards developing body-fixed sensors for monitoring affected joint motions. Although relatively accurate bone geometries can be obtained by scanning technologies, most human joints are approximated by simple circles and spheres to reduce the highly nonlinear kinematics to a tractable form for motion studies; many human joint-motion sensing challenges remain open. This article presents a novel magnetic tensor sensor (MTS) for noncontact tracing a human joint trajectory and a physics-based measurement model implemented on an artificial neural network (ANN) to account for un-modeled factors. The effects of input configurations and datatypes on measurement accuracy of an MTS/ANN have been numerically investigated with published data and experimentally evaluated on a prototype pantographic exoskeleton worn on a human shank/foot. As demonstrated experimentally, the MTS/ANN system calibrates the sensor intrinsic parameters, accounts for the environmental magnetic effects on the measurements, and can be trained with both offline and user-specific data. [ABSTRACT FROM AUTHOR]

We present a general harmonic formulation that takes into account, explicitly, the effects of micromagnetics for modeling the magnetic fields in a magnetoimpedance (MI) sensor element. We first relax assumptions commonly made to derive closed form solutions from a decoupled set of linear equations. We then solve numerically (using a meshless method formulated in point-collocation) the Maxwell and the Landau-Lifshitz-Gilbert equations simuitaneously for the real and imaginary parts of the magnetic fields and magnetization in the context of a cylindrical amorphous MI sensor element. Comparing our results of the effects of coupling the equations of motion against published experimental data, we found striking differences both quantitatively and qualitatively between the coupled nonlinear and decoupled linear models. The coupled nonlinear harmonic formulation presented here results in improved accuracy and more consistent qualitative behavior in accordance to reported experimental observations, particularly in the weak field regime. Also presented here are spin wave amplitude distributions showing spatial dispersion within MI elements structures, which represents information lost in decoupled formulations. Index Terms--Magnetization, magnetoimpedance, meshless methods, MI sensor, micromagnetic.

This paper presents a general method for deriving a closed-form solution for precise calculation of the magnetic field around a permanent magnet (PM) or an electromagnet (EM). The method, referred here as distributed multipole (DMP) modeling, inherits many advantages of the dipole model originally conceptualized in the context of physics, but provides an effective means to account for the shape and magnetization of the physical magnet. Three examples are given to illustrate the procedure of developing a DMP model, which derives an appropriate set of distributed dipoles from a limited set of known field points, for a general cylindrical PM, a customized PM, and a multilayer coil. The DMP modeling method has been validated by comparing simulated fields and calculated forces against data obtained experimentally and numerically; the comparisons show excellent agreement. Finally, we illustrate how the closed-form DMP models can offer an inexpensive means to visualize the effect of the EM fields on the leakage and unexpected flux paths, which have significant influences on the magnetic torque of a spherical motor. Index Terms--Actuator design, closed-form solution, dipole, Lorentz force, magnetic field, Maxwell stress tensor, pole shape.

Meshless methods have some advantages over their counterparts such as the finite-element method (FEM). However, existing meshless methods for computational electromagnetic fields are still not as efficient as FEM. In this paper, we compare two meshless methods of discretizing the computational domain of Poisson-like problems; namely, the point collocation and Galerkin methods (which use the strong and weak forms of the governing equation respectively), and their effects on the computational accuracy and efficiency of the magnetic fields. We also discuss methods of handling discontinuities at the material interface. We present several examples, which also provide a means to validate and evaluate both meshless methods. Exact solutions and/or FEM are used as a basis for comparison. In addition, we also verify the results by comparing computed magnetic forces against those measured experimentally. Index Terms--Element free, finite element, Galerkin methods, magnetic field, meshless method, strong form, weak form formulation.

Design of electromagnetic (EM) actuators often involves solving a magnetic field problem. This paper presents an adaptive meshless method (MLM) that inherits many advantages of the finite-element method (FEM) but needs no explicit mesh structure for design of EM actuators. Specifically, the paper offers a technique to estimate the distribution of numerical errors and a scheme that automatically inserts additional nodes to improve computational accuracy and efficiency. It gives several examples. The first three numerical examples, where exact solutions are available, provide a means to validate the adaptive MLM and evaluate its effectiveness against a regular MLM with a uniform node distribution. The other examples, where magnetic forces are computed from Lorenz's law, illustrate the use of adaptive MLM for practical design of an EM actuator. The paper compares the computed forces against published experimental results. Index Terms--Actuator design, adaptive meshless method, electromagnetic, finite element, magnetic field.

We consider here a class of compliant mechanisms consisting of one or more flexible beams, the manipulation of which relies on the deflection of the flexible beams. As compared with traditional rigid-body mechanisms, compliant mechanisms have the advantages of no relative moving parts and thus involve no wear, backlash, noises, and lubrication. This paper presents a formulation based on shooting method (SM) and two numerical solvers for analyzing compliant mechanisms consisting of multiple flexible members that may be initially straight or curved. Five compliant mechanisms, which are chosen to illustrate both initially straight and curved members and different types of joint/contact conditions, are formulated to exemplify analyses using the generalized shooting method for a wide spectrum of applications. The advantages of the generalized SM over the finite difference FD and finite element FE methods are demonstrated numerically. Unlike FD or FE methods that rely on fine discretization of beam members to improve its accuracy, the generalized SM that treats the boundary value problem (BVP) as an initial value problem can achieve higher-order accuracy relatively easily, and hence is more efficient computationally. In addition, the computed results were validated experimentally. It is expected that the generalized SM presented here will offer designers a useful analysis tool and will effectively facilitate the process of design and optimization of compliant mechanisms. [DOI: 10.1115/1.2202139] Keywords: shooting method, compliant mechanism, Gauss-Newton method, flexible beam

Optical fibers are drawn from preforms (fused silica glass rods) typically made up of two concentric cylinders (the core rod and the clad tube), which are usually joined in a separate fusion process. The setup time and hence manufacturing cost can be significantly reduced if the two cylinders can be joined in the same furnace in which the fiber is drawn. A good understanding of the transient temperature distribution is needed for controlling the feed rate to avoid thermally induced cracks. Since direct measurement of the temperature fields is often impossible, the geometrical design of the preform and the control of the feed rate have largely been accomplished by trials-and-errors. The ability to predict the transient temperature distribution and the thermally induced stresses will provide a rational basis to design optimization and feed rate control of the process. In this paper, we present an analytical model to predict the transient conductive-radiative transfer as two partially joined, concentric glass cylinders with specular surfaces are fed into the furnace. Finite volume method (FVM) is used to solve the radiative transfer equation (RTE). The specular surface reflectivity is obtained by the Fresnel's law and the Snell's law. The boundary intensities are obtained through the coupling of the interior glass radiative transfer and the exterior furnace enclosure analysis. The model has been used to numerically study the transient conductive-radiative transfer in the advanced melting zone (AMZ) of an optic fiber drawing process. This problem is of both theoretical and practical interest in the manufacture of optical fibers. The computational method for the radiation transfer developed in this paper can also be applied to the simulation of the fiber drawing process and other glass-related manufacturing processes.

This paper presents a distributed soft contact sensing method based on a sensitivity mapping function, which relates the change in measured voltages to that in the elastomer conductivity due to contact force acting on its surface. The sensitivity-image-based sensing system uses a small number of boundary electrodes with a multiplexer to create different electric-field patterns to generate a series of sensitivity images for machine learning, significantly reducing the number of training data typically obtained with single-point indentation measurements. The mapping function, which does not rely on the knowledge of the electric and conductivity fields during online sensing, can be trained with only a small amount of measured data in the order of the square of electrode number. The proposed method has been experimentally evaluated on two 16-electrode prototypes trained with 129 and 80 data for two typical applications, which are tactile perception on a flat surface and contact force measurement in a model knee joint, respectively. The former verifies the measurement accuracy of the contact position and force magnitude while the latter demonstrates the application of this method for measuring the internal joint forces between two curved surfaces. [ABSTRACT FROM AUTHOR]

This article presents a back-drivable spherical wrist with smart compliance for rendering human wrist-like capabilities in robotic applications. In order to synthetically impose both motion accuracy and active compliance for dedicated wrist manipulations, a robust hybrid control method is proposed that can switch between motion control (at free motion) and impedance control (at interactions with the environment). A sliding manifold is formulated so that when the control law drives the sliding manifold to zero, the motion/impedance errors will automatically converge to bounded regions despite system uncertainties/inaccuracies. Due to the direct-drive configuration of the proposed spherical wrist, the contact torque can be precisely estimated with an observer, providing an intuitive criterion of contact based on which the control modes can switch autonomously without external force/torque sensors and predefined geometric boundaries that are usually required in conventional hybrid force/position control. The concept is validated on a spherical wrist test-bed in the context of a throat swabbing process. The performance of the hybrid motion-impedance control evaluated against conventional impedance and admittance control methods demonstrate that the proposed spherical wrist enables a robotic system with both fine manipulations and smooth compliant interactions in wrist space. [ABSTRACT FROM AUTHOR]

This article presents a model-based eddy current (EC) testing method for the reconstruction of geometrical features, that may be hidden, through, or blind cavity in a large electrically conductive plate. Based on a distributed current source (DCS) modeling method, a perturbation model is formulated to convert the feature reconstruction problem into finding a possibility vector using a finite set of measured magnetic flux density (MFD) data and solved with the Tikhonov regularization method. The proposed method is numerically and experimentally validated and demonstrated by using the specimens with compound conductive/nonconductive geometrical features and comparing them with two published feature detection methods. Parametric studies are conducted to analyze the effects of feature shape, sensor configuration, and multi-coil excitation on reconstruction accuracy. [ABSTRACT FROM AUTHOR]

Undesired oscillations are often encountered in controlled systems at steady state due to high-frequency (HF) periodic disturbance and/or feedback noises. This article presents a computationally efficient alternative to derive an analytical harmonic model (AHM) that expresses the oscillatory variable as a series of harmonic kernel vectors and a position-independent harmonic amplitude vector for identifying the dominant harmonic components of the undesired effects on the manipulated variables. Illustrated in the context of an aircraft pressure-controlled piston pump (PC-PP) where the pressure feedback pulsations and the torque disturbances from the piston/slipper assemblies lead to HF swash-plate (SP) oscillation at steady state, the AHM and its significance were investigated experimentally on a PC-PP test-rig capable of simultaneously measuring the HF feedback pressure and SP-angle. Good agreements between the experimental and numerical results validate the AHM, and reveal that the HF SP-oscillation is dominated by its fundamental harmonic component, and primarily contributed by the disturbance torque. [ABSTRACT FROM AUTHOR]

This article presents an analytical method that offers both spectral and spatial information to predict local electric fields capable of driving neural activities for neuromuscular activation, and the findings of an experimental investigation on a common strategy utilizing multiple high-frequency (HF) electric fields to create an interference to recruit neural firing at depth. By introducing a cut-off frequency ${f} _{\text {cut}}$ too high to recruit neural firing in a frequency-based field descriptor, the analytical method offers an effective means to position a focused temporal interference (TI) without mechanically moving the electrodes. The experiment, which was conducted on both forearms of five healthy volunteers, validates the feasibility of the method for selective neuromuscular stimulation, where three nerve/muscles that control human fingers were independently stimulated with two current channels. The numerical and experimental findings demonstrate that the frequency-based method overcomes several limitations associated with surface-based electrical stimulation. [ABSTRACT FROM AUTHOR]

Loaded walking with a rucksack results in both gravitational and inertial forces of the load that must be borne by human carriers. The inertial force may be the source of metabolic burden and musculoskeletal injuries. This paper presents a lightweight backpack with a disturbance observer-based acceleration control to minimize the inertial force. The backpack was evaluated by seven participants walking on a treadmill at 5 km h−1 with a 19.4 kg load. Three experimental conditions were involved, including walking with a locked load (LOCKED), with an acceleration-controlled load (ACTIVE) using the designed backpack and walking with the same load using a rucksack (RUCKSACK). Our results showed that the ACTIVE condition reduces the load acceleration by 98.5% on average, and reduce the gross metabolic power by 8.0% and 11.0% as compared to LOCKED and RUCKSACK conditions respectively. The results demonstrate that the proposed active backpack can improve the loaded walking economy compared with a conventional rucksack in level-ground walking. [ABSTRACT FROM AUTHOR]

Motivated by the need to incorporate human spine mechanics into the design loop of wearable spine assistive devices, this article presents a spine-equivalent beam (SEB) model for analyzing kinematics and mechanics of a human spine in sagittal standing flexion. The model is capable of handling loads resulted from the head, arms, external payload, torso weight, and back muscle effects, and it also handles pelvis rotation. The derivation of the physiologically equivalent parameters is discussed, including initial curvature, cross-sectional area, area moment of inertia, force/moment of the back muscles, and the equivalent Young's modulus. The SEB model and its equivalent parameters are validated with published in vivo measurement collected by telemeterized implant embedded on subjects who have undergone vertebra body replacement surgery. Results show that the model is capable of predicting flexion angles and compression force experienced by the intervertebral discs throughout the flexion process, which provides a practical yet effective approach to incorporate the spine mechanics into the design of wearable spine assistive devices. [ABSTRACT FROM AUTHOR]

Motivated by the need to develop safe exoskeletons for collocated joint rehabilitation during the early stage of stroke recovery, this article presents a pantographic exoskeleton (PGE) capable of multi-degree-of-freedom motions in a single-joint collocated with the impaired joint. With minimum internal joint reactions in the presence of misalignment, the PGE “traces” the natural joint motion “like a pantograph” in the sagittal plane while performing in-bed rehabilitation, from which the joint parameters can be estimated for subsequent patient-specific motion training. An analytical model is presented to provide a rational basis for designing a collocated human-ankle/PGE mechanism, which has been numerically illustrated. The findings demonstrate that because the PGE joint do not restrict human ankle nature motion within the motion range of a typical ankle, it is less sensitive to joint misalignment as compared to conventional multijoint serial mechanisms that form a closed chain with the human limbs/joint. A prototype PGE has been designed; and its effectiveness for collocated ankle-joint manipulation has been experimentally investigated. Unlike conventional imaging methods that are prone to skin-marker errors and dependent on lumped-parameter approximations, the PGE directly measures the talus trajectory of the ankle-joint for characterizing its roll/slide motion and internal parameters. [ABSTRACT FROM AUTHOR]

In metal cutting, a good understanding of both the elastic-plastic deformation and the mechanical behavior of the workpiece material in the primary shear band (PSB) is essential to optimize machining process and ensure product quality, particularly when machining hard-to-cut materials (such as Titanium alloys) with a low thermal conductivity that gives rise to thermal stress localization near the tool-chip interface. This article presents an extended digital image correlation (DIC) method to obtain the highly intensive and localized elastic-plastic strain and stress fields in the PSB during cutting, the reconstruction of which requires the deformation history of each material point and an appropriate material constitutive model. Unlike existing DIC methods that infer elastic strains from two sequential images, the extended DIC not only reconstructs the displacement field but more importantly also captures the elastoplastic evolution of the material during the dynamic orthogonal cutting process, from which the plastic strain/stress fields in the PSB are determined experimentally. The effectiveness of the extended DIC method was evaluated numerically and experimentally for orthogonal cutting of Ti-6Al-4V alloys. [ABSTRACT FROM AUTHOR]