Sehoon Oh

Automated Impedance Matching System for Robust Wireless Power Transfer via Magnetic Resonance Coupling

Recently, a highly efficient midrange wireless transfer technology using electromagnetic resonance coupling has been proposed and has received much attention due to its practical range and efficiency. The resonance frequency of the resonators changes as the gap between the resonators changes. However, when this technology is applied in the megahertz range, the usable frequency is bounded by the industrial, scientific, and medical (ISM) band. Therefore, to achieve maximum power transmission efficiency, the resonance frequency has to be fixed within the ISM band. In this paper, an automated impedance matching (IM) system is proposed to increase the efficiency by matching the resonance frequency of the resonator pair to that of the power source. The simulations and experiments verify that the IM circuits can change the resonance frequency to 13.56 MHz (in the ISM band) for different air gaps, improving the power transfer efficiency. Experiments also verified that automated IM can be easily achieved just by observing and minimizing the reflected wave at the transmitting side of the system.

Design and Analysis of Force-Sensor-Less Power-Assist Control

Due to the recent trend of the application of robots and other mechatronic devices to human life support, the force control draws more attention than ever. However, to use force sensors in all the required cases makes the system not only expensive but also bulky. Force-sensor-less power-assist control (FSPAC) which uses only encoders to obtain the external force information and provides force control performance can address this problem. Due to its simplicity and wide application, FSPAC is an essential technology to control a motor in a human-friendly way, but the design of FSPAC is different from the conventional feedback controllers so a new design methodology needs to be established. In order to attack this problem, this paper generalizes and analyzes the structure and characteristics of FSPAC. The generalized structure reveals how FSPAC can achieve the assistance, and the transfer function analysis based on the structure addresses the robustness and assistance performance evaluation problems. The robustness of FSPAC is analyzed in terms of the gain margin and robust stability, and the limitation of the power assistance to guarantee the robust stability is derived. Then, the evaluation way of feedback control design in FSPAC is provided. All the discussion in this paper provides the readers with understanding and appropriate design methodology of FSPAC.

A High-Precision Motion Control Based on a Periodic Adaptive Disturbance Observer in a PMLSM

This paper presents a novel disturbance compensation scheme to attenuate periodic disturbances on repetitive motion using permanent magnet linear synchronous motors (PMLSMs), and this scheme is called the periodical adaptive disturbance observer. The scheme is based on assumptions that all measured states and disturbances are periodic and repetitive when the tasks executed by PMLSM motion systems have periodic and repetitive characteristics. In the proposed control scheme, a lumped disturbance is estimated by the classical linear disturbance observer (DOB) for the initial time period and stored in memory storages. It consists of parametric errors multiplied by states, friction force, and force ripple, and then, it is updated for each time period by the periodic adaptation law. This scheme requires no mathematical models of disturbances and adaptation laws of model parameters such as the mass of the mover and viscous friction coefficient. Also, it is possible to compensate for disturbances above as well as below the bandwidth of the Q-filter (LPF) of DOB. The effectiveness of the proposed control scheme is verified by various experiments that take into account varying frequency components of disturbances along the operating speed of a mover of PMLSM such as force ripple and friction force.

Comparing Approaches for Actuator Redundancy Resolution in Biarticularly-Actuated Robot Arms

Biarticular actuators-actuators spanning two joints-play a fundamental role in robot arm designs based on the human musculoskeletal actuation structure. Unlike kinematic redundancy, actuator redundancy resulting from biarticular actuation brings advantages such as increased stability, reduced link inertia, and decreased nonlinearity of the end-effector force with respect to the force direction. The way the actuator redundancy is resolved is a fundamental problem, as it strongly characterizes robot arms performance. In this study, the six most significant actuation redundancy resolution approaches in the literature-1-norm, 2-norm, infinity-norm, phase different control (PDC), nonlinear hase different control (NLPDC), and linear programming (LP)-are analyzed with respect to their design, and experimentally compared with each other using BiWi, a biarticularly actuated and wire-driven robot arm. In addition, an integrated control framework to resolve actuator redundancy maximizing end-effector force and simultaneously minimizing the necessary input torques is proposed.

Frequency-Shaped Impedance Control for Safe Human–Robot Interaction in Reference Tracking Application

In the control of industrial robots, both safety and reference tracking performance are required. For safe human-robot interaction, robots should exhibit low mechanical (or controlled) impedance so that they react to the interaction forces in a compliant manner. On the other hand, the reference tracking requires for the robots to reject exogenous disturbances, which results in an increased impedance. In order to achieve these two conflicting objectives, a frequency-shaped impedance control (FSIC) method is proposed in this paper. The proposed method utilizes the two different functionalities of the disturbance observer (DOB): a disturbance estimation function as an observer and a disturbance rejection function as a feedback controller. Namely, the DOB is utilized as an observer at the frequencies where the robots interact with humans, while it is used as a feedback controller (i.e., disturbance rejection controller) at the frequencies where the reference tracking is required. The proposed approach is realized by shaping a filter of the DOB in the frequency domain so that the impedance is manipulated to achieve both the compliant interaction and reference tracking. The compromised reference tracking performance in the frequency range, where the impedance is set low, can also be supplemented by feedforward control. A typical feedback controller and a feedforward controller are designed in addition to the DOB-controlled system as the whole control system to enhance reference tracking performance and the betterment of stability robustness. The proposed method is verified by experimental results in this paper.

A generalized control framework of assistive controllers and its application to lower-limb-exoskeletons

Various control methods have been studied for the natural assistance of human motions by exoskeletal robots, i.e., wearable robots for assisting the human motions. For example, impedance control and compliance control are widely used for controlling interaction forces between a human and a robot. When an accurate measurement of the human muscular force is available (e.g., electromyography), a direct use of the estimated human joint torque is possible in the control of an assistive robot. The human motions in a daily living, however, are so complex that they are constituted by multiple phases, such as walking, sitting, and standing, where the walking can be further categorized into multiple sub-phases. Therefore, a single control method cannot be the best option for all the motion phases; a switch in the control algorithms may be necessary for assisting human movements in multiple motion phases. In this paper, a generalized control framework is proposed to incorporate the various assistive control methods in one general controller structure, which consists of Feedforward Disturbance Compensation Control, Reference Tracking Feedback Control, Reference Tracking Feedforward Control, Model-based Torque Control. The proposed control framework is designed taking into consideration of the linearity of each control algorithm, and thus it enables the continuous and smooth switching of assistive control algorithms, and makes it possible to analyze the stability of the overall control loop. The proposed method is implemented into a lower-limb exoskeleton robot and is verified by experimental results. A generalized control framework is proposed to incorporate the various assistive control methods in one general controller structure.The proposed control framework enables the continuous and smooth switching of assistive control algorithms.The proposed control framework makes it possible to analyze the stability of the overall control loop.The proposed method is implemented into a lower-limb exoskeleton robot and is verified by experimental results.

High-Precision Robust Force Control of a Series Elastic Actuator

A series elastic actuator (SEA) is a promising actuation method in force control applications that intelligently interacts with environments. The SEA is characterized by a spring placed between a load and an actuator, which is an electric motor in most cases. Since the spring plays the role of a transducer between position (i.e., the spring deflection) and force, it is able to control the output force (torque) precisely by utilizing typical position control methods. However, the force control performance of the SEA is considered to have limitations due to its elasticity, and thus, to be inferior to rigid actuators in terms of bandwidth. This paper proposes that the force control performance of the SEA can be improved by exploiting the dynamic model of the SEA. To this end, the SEA is modeled and analyzed utilizing the two-mass dynamic model, which is a well-known and widely accepted model of the flexible system. The disturbance observer and feedforward controller are introduced as the model-based control algorithms for the SEA to achieve the high-precision force control. In addition to high-bandwidth force control, the proposed controller can address the robust stability and performance against model parameter variance and exogenous disturbances. For the analytic and quantitative assessment of the proposed force control system, the dynamic characteristics of an SEA under various control algorithms are analyzed, and the experimental results are provided for an actual SEA system in this paper.

Disturbance Attenuation Control for Power-Assist Wheelchair Operation on Slopes

This paper proposes a practical and effective disturbance attenuation control algorithm to support the safe and comfortable operation of power-assist wheelchairs. The power-assist wheelchair is an option for mobility that can be propelled by the users arm while also providing additional assisting torque based on the measured user torque to relieve the users propulsion effort. The control algorithm proposed in this paper detects external forces other than the applied human torque and attenuates the effect of the external force to improve safety and ease of use. The proposed 2-D disturbance attenuation control can reduce the effect of external force in the longitudinal direction and the rotational direction respectively. Gravity on slopes is considered the major disturbance, and the effectiveness of the proposed controller on slopes is verified by experiments.

Two-Degree-of-Freedom Control of a Two-Link Manipulator in the Rotating Coordinate System

As applications and tasks of robotic manipulators become more diverse and complicated, the desired motions of the robots also become more sophisticated and complicated. In spite of this diversity of tasks, the coordinate system to describe the tasks has not been changed much; the conventional Cartesian coordinate system is still the most widely used coordinate system. It is found in this paper that the rotating coordinate system significantly simplifies the kinematics of a two-link robotic manipulator with the biarticular actuation coordination, which is inspired from human muscles that can generate torques at adjoining two joints simultaneously. Taking the advantage of this simple kinematic relationship by the rotating coordinate system and the biarticular actuator coordination, the dynamics of the two-link manipulator is analyzed, and a disturbance observer (DOB) is designed based on the derived dynamics to nominalize the actual dynamics and to reject undesired disturbances. The proposed DOB-based control algorithm can achieve better control performance in the rotating coordinate system, and comparative experiments verify the effectiveness of the proposed coordinate system and control methods.

A Mobile Motion Capture System Based on Inertial Sensors and Smart Shoes

Motion capture systems play an important role in health-care and sport-training systems. In particular, there exists a great demand on a mobile motion capture system that enables people to monitor their health condition and to practice sport postures anywhere at any time. The motion capture systems with infrared or vision cameras, however, require a special setting, which hinders their application to a mobile system. In this paper, a mobile three-dimensional motion capture system is developed based on inertial sensors and smart shoes. Sensor signals are measured and processed by a mobile computer; thus, the proposed system enables the analysis and diagnosis of postures during outdoor sports, as well as indoor activities. The measured signals are transformed into quaternion to avoid the Gimbal lock effect. In order to improve the precision of the proposed motion capture system in an open and outdoor space, a frequency-adaptive sensor fusion method and a kinematic model are utilized to construct the whole body motion in real-time. The reference point is continuously updated by smart shoes that measure the ground reaction forces.