Published papers

Bilateral teleoperation is a network control system that connects distant locations under force sensation and contributes to task execution. The motivation of this study is to focus on the importance of symmetry in bilateral teleoperation systems and to design controller configurations with symmetry. This study employs the acceleration control-based four-channel bilateral teleoperation (AC4BT). AC4BTs have the regulator to zero in resultant force and the controller in positional difference, which can precisely achieve bidirectional force transmission and position synchronization while maintaining its symmetric structure. However, the coupled design of the force regulator and position controller remains challenging. The mutual impedance-based motion control approach derives and discusses the intrinsic wave dynamics in AC4BT under communication delay. The mutual impedance determines the transmission characteristics of force and position in bilateral teleoperation since it corresponds to the characteristic impedance in distributed-parameter systems. This study proposes the force-proportional-integral (force PI) and position-proportional-derivative (position PD) controllers, with the gain settings canceling the frequency-dependent terms. Without interfering with the realization of the control objectives, this brings the phase relationship between the force and velocity in manipulation closer to that performed in a no-delay situation. Experimental results verify the proposed approach.

This paper presents a vibration suppression approach for systems with elastic couplings, which combines standard control and observation structures with a novel wave-space transformation. By formulating the system dynamics as a wave system and designing a traveling wave controller to reject reflected waves, the proposed method achieves robust vibration suppression against variations in load-side inertia. Unlike conventional approaches that rely on online identification or adaptive mechanisms, which inherently require persistent excitation and may compromise suppression performance, the proposed method eliminates the need for such estimation. The paper details the modeling, controller design, and state-estimation scheme. Experimental results validate the effectiveness of the proposed method, showing superior immunity compared with conventional ones under varying load-side conditions and discussing the criteria for parameter selection for the observers and controller.

Vibration or ripple reduction is a common topic in motion control, electrical systems, and motor drives. Although wave model-based vibration suppression is novel, the control structures must be systematized. Thus, this study addresses the comparative analysis of control structures derived from the wave model. The primary contribution is to discuss the similarities and differences between an integral-proportional controller and a proportional controller with a disturbance observer through analyses of their transfer function and experimental verification.

This paper presents issues related to the damping of torsional vibrations in a system with elastic coupling. A novel PI-based control structure with additional feedback from torsional torque and its derivative is proposed. For the estimation of the required variables, the integral observer is proposed. Analytical expressions are presented to enable the selection of parameters of the control system. The relationship between the considered system and popular structures with a PI controller and one additional feedback from torsional torque and the derivative of torsional torque is pointed out. The proposed control structure is tested under simulation and experimental studies.

The article presents issues related to vibration damping in a dual-mass system. A control structure with a PI controller and with additional feedback from the torsional torque and its derivative is used for the study. A higher-order integral estimator is proposed to obtain information on these variables. Results showing a significant improvement in the quality of estimation of these quantities compared to the classical observer are presented. Theoretical considerations are confirmed by simulation and experimental studies.

Robust force control systems guarantee robustness to disturbance, including external force. Few previous studies, however, clarify transitions between contact and non-contact. This paper considers an admittance control with the desired relation between force and velocity to achieve safety transitions. Also, the proposed admittance control keeps robust by disturbance observer (DOB). The DOB-based admittance control intrinsically contains robust force control, and a controller during non-contact and one during contact differ only in damping parameter settings. It allows simple contact regulation on transitions. Experiments adopt and validate the proposed method for workspace control of two-degree-of-freedom (2-DOF) manipulators. This paper indicates that the control-based approach, as well as the mechanism-based approach, is one of the effective strategies in human-robot coexisting situations.

Vibration suppression has attracted interest in achieving fast, stable motion control in industrial fields. This study uses the wave partial differential equation (PDE) to model resonant systems and connects a lumped load and the PDE model in series. The lumped load is governed by the ordinary differential equation (ODE). This integrated model replaces the conventional two-mass resonant model, referred to as the PDE-ODE model in this study. A matrix representation of the PDE model is eigenvalue decomposed. The decomposition provides the basis transformation of the variables on the actual space into those on a wave space. Based on this transformation, the proposed controller equips the decoupling of the traveling/reflected waves for vibration suppression. Impedance matching explains the conditions necessary for the decoupling and control of vibration. The suppression method requires only the PDE part's parameter, making the control system resistant to inertial variations.

This study introduces a methodology for vibration damping based on hybrid control of velocity (or angular velocity) and force (or torque). The hybrid velocity/force integral-proportional controller is developed using the wave model, which accounts for interferences between traveling and reflected waves that induce vibration. Central to the proposed approach is treating the traveling wave as a state variable and the reflected waves as disturbances, thereby enabling velocity control of the load in a backstepping manner. Furthermore, this study addresses the issue of steady-state deviation under load disturbances by incorporating a high-pass filter into the force integral feedback. Experimental results validate the effectiveness of the proposed controller and filter.

Bilateral control realizes position synchronization and the “law of action and reaction” between two points. To advance applications of bilateral control, it is necessary to emphasize environmental adaptability, safety, and task execution. Accordingly, the bilateral system is required to consider contact with unknown environments and realize stable contact to avoid harm to the operator or the object. Thus, a bilateral control system using elastic manipulators is proposed in this paper. Moreover, the integration of mechanical and control designs for stable interaction with the operator and the environment is discussed. The proposed bilateral system with elasticity is modeled as a pair of two-mass resonant systems. Accordingly, a modal space control (MSC) is introduced with the aim of realizing the control goals, vibration suppression, and good operationality. Modal transformation independently configures the force and position controllers of each other in virtual common and differential spaces. MSC comprises reaction torque feedback loops in the modal space and modal space load-side disturbance observer (MLOB). MLOB compensates for load-side disturbance, including the interference between the common and the differential modes. It realizes disturbance suppression, decoupling control, and nominalization of parameters in the form suitable for each force and position control system.

Tackling communication delays attracts interest in improving the stability and transparency of bilateral teleoperation. This study presents a novel concept; mutual impedance. Under the delays, this impedance is intrinsic in four-channel acceleration-based bilateral control (4ch ABC). In 4ch ABC, wave propagations can characterize the transmissions of force and velocity, which are described in distributed-parameter systems. The delays induce interference between applied force and velocity response and decrease transparency. The mutual relationship between force and position controllers can explain the interference. The proposed mutual impedance is the indicator of the impact of the relationship. This study notes that the impedance comprises a ratio of force and position controllers. Thus, a force proportional-integral (PI) controller with a gain constraint between the position controller is introduced. One-degree-of-freedom (1-DOF) motion experiments compare the proposed method with conventional methods in superiority. The applicability of the proposed approach to delay fluctuations and multi-DOF motion is verified through 3-DOF manipulators.

Because of their high efficiency, electric drives have become a staple solution in numerous applications, such as robotic manipulators and exoskeletons. However, such constructions need to be designed with lighter materials, which are also more prone to twisting and bending. Because of torsion affecting the mechanical structure, precision of motion is deteriorated. In this study, a classic IP controller is coupled with a resonant filter to improve the performance of the drive affected by the torsional torque. The results include both numerical analysis and experimental verification.

This study introduces quasi-real-time (QRT) motion control, explicitly incorporating measured sampling periods into the digital control loop. Digital control in mechatronic systems typically relies on the assumption of fixed sampling periods. However, practical systems often experience sampling fluctuations due to hardware limitations, sensor-related interrupt processing, and communication delays. To address this issue, a systematic discretization method for continuous-time transfer functions is proposed to enable controller, observer, and filter design under sampling fluctuations. Experimental results demonstrate that the proposed approach performs more robustly than conventional fixed-sampling implementations. QRT motion control relaxes stringent real-time (RT) constraints and is a foundation for user-friendly, general-purpose operating system (GPOS)-based RT execution frameworks in advanced mechatronic applications.

Recently, the rational motion of robots is required for further applications. Human motion gives many clues to studies of robotics. This study focuses on the fact that a human adjusts grip force when he/she performs an impact motion with a tool. The tools often have elasticity, and the human adjustment enables an effective impact motion. For reproduction of the impact motion with a flexible manipulator, this study models the elasticity as a wave system. The flow of the reflected wave is controlled during the impact motion based on the human adjustment of grip force. Since this study is based on the research of vibration suppression in elastic structures, it provides not only rational but also stable motion control. The experimental results show the validity of the proposed method.

Electric drives are responsible for torque generation in most industrial processes and devices, including electromobility. The operational performance of industrial drives is often limited by torsional vibrations and time-varying load inertia. To address these challenges, this paper proposes and compares three robust control structures based on PI controllers augmented with additional feedback from the first and second derivatives of torsional torque. A higher-order integral disturbance observer (IDO) is employed to estimate the unmeasurable derivatives of torsional torque, enabling accurate compensation. Furthermore, a bio-inspired optimization algorithm (Britch-inspired Optimization Algorithm, BiOA) is utilized to determine the optimal gain coefficients for the PI controller and the additional feedback loops. The proposed methodologies are validated through simulations and experimental testing on a physical setup, demonstrating improved reference-trajectory tracking and enhanced torsional vibration suppression under varying load conditions.

In this paper, the structure of a model to estimate human motion from surface-electromyography (sEMG) was defined mathematically and physically. Especially, an infinite impulse response (IIR) filter and an element description method were focused to generate the model in this paper. Human motion can be estimated from sEMG. However, the relationship between sEMG and human motion has a complex characteristics such as a non-linear relationship. The cause of the above complex relationship can be thought the muscle contraction velocity. Then, the IIR filter was applied to consider about the previous sEMG value in the estimation. In addition, some conventional models have been a black-box model and it is not easy to interpret the model mathematically and physically. From the above, an element description method (EDM) was applied to generate the model in this paper. EDM is one of the system identification method and it is easy to interpret the model mathematically and physically because EDM can generate the block diagram of the model. In this way, the proposed method could solve two conventional methods about the muscle contraction velocity and the black box model. Then, it was possible to define the structure of the model by the above in this paper. In this paper, the models about a wrist and a fingertip motion were generated and considered about the structure. As the results, the structure of models was defined with a hyperbolic tangent function. It will be possible to apply the model to various motions by just identifying the parameters about the relationship between each joint and muscle because the amount of muscles and range of sEMG are deferent from each muscle.