Untethered, controllable, mobile microrobots have been proposed for numerous applications, ranging from micro-manipulation, in vitro tasks (e.g., operation of microscale biological substances) to in vivo applications (e.g., targeted drug delivery; brachytherapy; hyperthermia, etc.), due to their small-scale dimensions and accessibility to tiny and complex environments. Researchers have used different magnetic actuation systems allowing custom-designed workspace and multiple degrees of freedom (DoF) to actuate microrobots with various motion control methods from open-loop pre-programmed control to closed-loop path-following control. This article provides an overview of the magnetic actuation systems and the magnetic actuation-based control methods for microrobots. An overall benchmark on the magnetic actuation system and control method is also discussed according to the applications of microrobots.

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Magnetic Actuation Based Motion Control for Microrobots: An Overview

We have designed a Halbach magnet array by using a numerical optimization method based on finite-element analysis. The magnetization direction of each element is defined as the design variable. The optimal magnet arrays composed of two and three linear magnet layers can then be investigated to increase the attractive, repulsive, and tangential magnetic forces between magnet layers. We have applied a magnet array maximizing the tangential force to a torsional spring composed of two- and three-magnet rings. The two-dimensional finite-element analysis incorporates optimization techniques such as the sequential linear programming and the adjoint variable method.

Design of a Halbach Magnet Array Based on Optimization Techniques

On the dry and wet sliding performance of potentially new frictional brake pad materials for automotive industry

Third body formation on brake pads and rotors

Challenges of continuum robots in clinical context: a review

Tribological study of gray cast iron with automotive brake linings: The effect of rotor microstructure

Operating characteristics of the bump foil journal bearings with top foil bending phenomenon and correlation among bump foils

This paper proposes a Gaussian process-based method for trajectory learning and generation of individualized gait motions at arbitrary user-designated walking speeds, intended to be used in generating reference motions for robotic gait rehabilitation systems. We utilize a nonlinear dimension reduction technique based on Gaussian process dynamical models (GPDMs), in which the internal dynamics is modeled as a second-order Markov process evolving in a lower-dimensional latent space. After the GPDM parameters are identified with training data obtained from gait motions of healthy subjects walking at different speeds, our method then employs Gaussian process regression (GPR) to predict the initial two states of the latent space dynamics from any arbitrary desired walking speed and the anthropometric parameters of the test subject. Motions are then generated by directly mapping the latent space dynamics to joint trajectories. Experimental studies involving more than 100 subjects indicate that our method generates gait patterns with 30% less mean square prediction errors compared to recent state-of-the-art methods, while also allowing for arbitrary user-specified walking speeds.

Gaussian Process Trajectory Learning and Synthesis of Individualized Gait Motions

Safety and performance are often required for robots interacting with humans. A variable stiffness joint (VSJ) is a useful device to reduce the shock against a human being by decreasing the joint stiffness quickly. In this study, a novel concept of VSJ using permanent magnets is suggested. It can have stronger stiffness compared with previous systems to satisfy payload conditions. However, this system requires a positioning mechanism to move the rotor to the axial direction for the variable stiffness. This study focuses on the optimal design of a permanent-magnet-type VSJ. The analysis is based on the finite-element simulation. The virtual work method is used to calculate the force and torque of VSJ. The modification of the initial concept model is preceded and a prototype is implemented and tested to compare analysis results. Also, the Halbach array concept is added to increase the torque of VSJ. The parameter optimization scheme is applied based on the design of experiments and a final design is suggested

Optimal Design of a Variable Stiffness Joint Using Permanent Magnets

This paper proposes novel motion modes for a microrobot which enable several driving options during its 2-D locomotion. The robot motion based on conventional methods shows stick-slip phenomena which produce imprecise jerking movement. We solved the imprecise control problem by using the concept of kinetic friction force. Furthermore, we introduced several motion modes that can generate smooth motion under different environments and presented the pros and cons of each motion mode. A motion mode is properly selected depending on the environment or application purpose, and it can exhibit high performance in the execution of the task. The proposed method using a suitable mode of operation of the microrobot has the potential to be applied in various medical fields.

Seung Jong Kim

Papers

Tribological study of gray cast iron with automotive brake linings: The effect of rotor microstructure

Operating characteristics of the bump foil journal bearings with top foil bending phenomenon and correlation among bump foils

Gaussian Process Trajectory Learning and Synthesis of Individualized Gait Motions

Optimal Design of a Variable Stiffness Joint Using Permanent Magnets

Novel Motion Modes for 2-D Locomotion of a Microrobot