Published papers

© 2018 IEEE. Disturbance observer based compensation is a popular technique in motion control to compensate for the difference between an actual plant and its nominal model. Conventional implementation with pseudo derivative performs poorly when the measurement noise is large and the bandwidth has to be decreased. Low pass derivative FIR filters provide a frequency response closer to the ideal response than comparable pseudo derivative filters. This paper proposes a disturbance observer with low pass derivative FIR filter for DC motor with position measurement. The proposal is validated with numerical simulations.

© 2019 IEEE. This paper proposes the bilateral control method for a pair of manipulators which have different structures considering the fluctuation of an equivalent mass matrix. Bilateral control is one of the methods that controls position and force bidirectionally between two actuators. By using this method, operators can get haptic sensation from the remote environment during teleoperation. When considering the practical situation, the desired structure of robots would be different between the operator and remote sides depending on a task or environment. Therefore, bilateral control between two manipulators with different structure needs to be considered. In the method using Workspace Observer (WOB), conventionally, the nominal value is used for the equivalent mass matrix. However, an error occurs between the nominal and true values of equivalent mass matrix, and this error deteriorates the stability or noise-sensitivity of the system. In addition, it deteriorates the accuracy of the force estimation since Workspace Reaction Force Observer (WRFOB) estimates external force including this error. To overcome this problem, the control system needs to be designed based on the equivalent mass, which fluctuates depending on the posture of manipulators. By using the estimated equivalent mass matrix for WOB, the mass fluctuation can be considered, however, the interference between position control and force control occurs as the equivalent mass is different between the master and slave. Therefore, in the proposed method, Modal space Disturbance Observer (MDOB) with estimated task mass matrix is used to reduce the interference. Experiments were conducted to validate the proposed method.

© 2018 IEEE. Technologies for the skill succession and machine automation are strongly demanded in some developed countries under the circumstance of declining working population due to the lower birth rate and the aged society. Motion copying system (MCS) is one of the methods for the skill succession and machine automation. MCS is able to play back the saved motion in the case that the environmental conditions between the saving and the loading phases are the same. Conventionally, the pure position controller, the pure force controller, or Hybrid Position and Force Control (HPFC) is applied to MCS. The reproduction performance is, however, degraded when the environmental variations happen. In such case, impedance control is applicable to MCS instead of HPFC. The purpose of this paper is to conduct the comparative study of some control strategies: position control, force control, HPFC, the Position-Based Impedance Control (PBIC), and the Force-Based Impedance Control (FBIC), and clarify which control strategy is appropriate for the tasks. Simulations were conducted for supporting the argument.

© 2018 IEEE. Some developed countries will encounter the shortage of working population, and machine automation is considered as one of the promising methods to overcome its issue. This paper considers Motion Reproduction System (MRS) as a method of the machine automation. MRS can be easily repeated the executed motion; however, this system is sensitive to environmental variations. One of simple countermeasures is introducing compensation values that are designed based on the environmental variations, but these values contain errors and degrade the motion reproduction. This paper proposes the modification of control impedance on the basis of the type of environmental variations, and the trustworthy information is much more weighted. The effectiveness of the proposal is verified through simulation and experiments.

© 2019 The Institute of Electrical Engineers of Japan. The 4ch bilateral control is a teleoperation method that transmits the position and force information between the master and slave, and the reproducibility and operationality are often used to evaluate it. Conventionally, the reproducibility and operationality have been utilized only for performance evaluation, although many controllers based on the 4ch bilateral control scheme have been proposed. Therefore, the aim of this paper is to determine the controller structure based on the target reproducibility and operationality in advance. The contribution of this paper is to develop an appropriate controller from the reproducibility and operationality through numerical computations and analyses. The structure of the proposed controller is verified through simulations and experiments.

© 2019 IEEE. Haptic teleoperation has been studied as an attractive technology for a long time. Although the haptic teleoperation has been already realized by conventional methods, their physical meanings are remained vague and unclear. This paper explains bilateral control for the haptic teleoperation using an equivalent circuit model. Owing to the circuit model, which takes into account characteristics of electrical actuators, a control strategy and its physical meanings are clearly illustrated. Torque (force) control and position (velocity) control for the haptic teleoperation can be designed independently because of mode decomposition. Feedback controller design for each control is based on analyses of sensitivity derived from the equivalent circuit. The control strategy derived from the equivalent circuit have been confirmed by experiments and resulted in the haptic teleoperation.

© 2018 IEEE. In this paper, a new approach for high-precise force control in repeated tasks is proposed. In the general force control, the feedback control tends to be used for a matching the output force to the desired force safely. However, when the feedback control is used, the input of the plant does not change until the error is observed. The characteristic makes reactivity bad. In the position control, the model-based feedforward control is often used to solve this problem. However, the model-based feedforward control cannot be used in force control due to the difficulty of making the accurate model. The main reason of this problem is the uncertainty of the environmental impedance. Owing to the uncertainty, the model-based feedforward control cannot work properly. To solve this problem, Iterative Learning Control (ILC) is used in this paper. ILC does not require the accurate model, and it is able to match the output force to the desired one through repeated trials. However, the convergence condition in force control has not been analyzed until now. In this paper, by taking account of the environmental impedance of the object, a new convergence restriction is proposed. Under the proposed condition, the force control in repeated tasks becomes more precise. The procedure was demonstrated on simulations.

© 2018 IEEE. Lower paralysis interrupts the mobility and depresses quality of life. Some incomplete paraplegia have an ability to walk in rehabilitation. However, incomplete paraplegia consumes larger energy for the gait motion than able-bodied people. Therefore, it is difficult for them to walk a long time. To solve this problem, many walking assist exoskeletons have been developed. In the case of walking assist, assist timing is one of the essential factors. The mismatch of assist timing interferes user's motion, thus it is required to adjust the assist timing. To control the assist timing, gait phase detection methods are used. Gait motion is divided into several phases. Many researchers introduce the gait phase detection method to control the assist timing. However, there is a mismatch between assist torque and ideal torque which is required by the user. There are no researches to evaluate the gait phase detection methods based on the analysis of the transition of assist torque required by the user. The purposes of this research are to propose the novel FSBM and evaluate the gait phase detection methods. In the experiment, a subject walks on the treadmill with wearing knee exoskeleton. The motor of the exoskeleton is controlled by the bilateral controller. Therefore, the assist torque can be adjusted to be suited to the user by manipulating master side motor. After the experiment, the assist torque and detected gait phases of proposed FSBM are compared to the conventional methods.

© 2018 IEEJ Industry Application Society. In recent years, with the declining birthrate and the aging of the population, the demand for nursing care support equipment is increasing. Among them, wheelchairs are already common as nursing care support equipment. Therefore, demand for wheelchair research is high, and electric wheelchair is required to have higher functionality. In this paper, prevention of falling of the wheelchair as a part of enhancing the functionality of the electric wheelchair is described. Several studies on prevention of rollover of electric wheelchairs have already been reported, but since rider motion state is not taken into account in the calculation of rollover conditions, there is a problem that the rollover conditions does not conform to the actual situation. Therefore, the purpose of this study is to estimate the state of the rider by IMU sensor and to calculate accurate rollover condition. Several simulation results are shown to confirm the proposed approach.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. The parameters of walking have been studied from the viewpoints of joint rotation and translation of body. The balance and symmetry of walking are indispensable features to understand for healthy walking, while also being a personal characteristic. However, quantification has not been easy to carry out in the case of the conventional gait parameters COG (center of gravity) and ZMP (zero moment point). In this approach, the CP (crossing point) is proposed to quantify the concept of symmetry and balance by comparing it to the COG and ZMP. The CP is estimated based on the intersection between the hip line and the ankle line. While the hip line is fixed on the upper body where the COG is, the ankle line is altered depending on the each footfall, where the ZMP is. Therefore, the values of COG, ZMP, and CP have similar or different tendencies in terms of whether balanced walking results in symmetry or not. The validity of this is verified by carrying out a simulation with robot walking, and an experiment using human walking. Through additional experiments, it was noticed that the CP was able to improve the role of COG and ZMP in terms of not only stability, but also its relationship with the movement range of the lower limbs.

© 2020 IEEE. The number of robots has been increasing and they would widely conduct activities in a variety of situations. The amounts of motion data distribution would also become larger and larger: therefore, the amounts of each motion data are expected to be compressed. The compression of motion data has been widely discussed, but this study focuses on the success-based approach and considering a position/force task. Since the purpose of machine automation is to conduct some tasks, the data can be compressed in the extent that the task is safely and successfully completed. The sampling time of command loading is changed for reducing the amount of motion data. The algorithm for investigating the required sampling times in each motion period and each degrees-of-freedom is developed and applied, and the appropriate values were experimentally obtained. The data compression of motion commands was confirmed as well as the task success based on its compressed motion command.

© Science Press 2020. In recent years, there has been concern over labor shortages. Robots are being introduced for transportation and other tasks. When robots are used in the environment with humans, it is desirable that the size is small and the output is relatively small from a viewpoint of safety. However, in the case of using one robot, there is a limit to the transport capacity. To address this issue, this paper describes the cooperative control of two mobile robots to realize the transportation ability that cannot be achieved by one robot. In the cooperative control, external force detection from the environment is important to enhance the adaptability of the robot. In the previous studies, force sensor was used for each robot, and it was possible to transport even when an external force is applied to the robot. However, the cost increases as the number of robots used increases. Therefore, this paper proposes compliance control that enables transport with reduced force fluctuation without external force sensor. Position control and sensorless force control are assigned to each robot, and when an external force is applied to sensorless force control robot, the control method is switched between the robots to realize the fluctuation suppression of the grasping force.

© 2019 IEEE. In robotic systems, reduction drives are utilized to increase actuators' force. Actuators with a constant reduction ratio can not achieve characteristics of both high velocity driving and high force driving. In order to perform various tasks, high force drive and high velocity drive are required for robotic actuators since required driving characteristics are different among tasks. Therefore, a mechanism which can change a reduction ratio continuously is needed. This paper proposes a mechanism that can change a reduction ratio with a linear electro-hydrostatic actuator (EHA). The proposed mechanism switches between two discrete reduction ratios by switching two oil paths with solenoid valves. Furthermore, this paper realizes a continuously variable reduction ratio by switching the solenoid valves quickly. This paper confirms that the reduction ratio changes continuously and widely by simulations.

© Science Press 2020. In recent years, the practical use of personal mobility of wheeled inverted pendulum mobile robot such as Segway has been advanced. In this paper, this paper focuses on a two-wheeled wheelchair, which is a sitting-type wheeled inverted pendulum personal mobility. In order to improve the traveling performance, the purpose of this paper is to efficiently climb over the steps of the two-wheeled wheelchair. In a situation where the wheelchair climbs over a step, there is a drawback that is a trade-off between stabilizing the posture and applying a torque large enough to climb over the step. In order to solve this drawback, modeling is carried out including the model of the passenger in the situation where the wheelchair climbs over a step. This paper shows a control system to feedback the gravity term applied to the wheel when the wheelchair climbs over a step. By these proposals, the improvement of the traveling performance at the time of climbing over a step is realized. The usefulness of the proposed method is confirmed via the simulation result.

© 2019 IEEE. In IMUs, three sensors, acceleration, gyroscope, and magnetic, are implemented. Data get from these sensors are used to acquire posture and position of each sensor. Theoretically, from a simple integration of an acceleration and an angular velocity, position and posture could be calculated. The IMU is applied to many fields due to its user friendly structure. Human motion detection is one of interesting applications. However, small error in the measured data are also integrated, hence position error and posture error, which is also called drift, occur. The purpose of this paper is to estimate position and posture using IMUs without human model. Two IMUs are used to estimate posture and position. For posture estimation, complementation of posture angle are done from an 'angular velocity' and a 'gravity acceleration'. For position estimation, complementation by relative relationship between 2 sensors has been proposed. This is the novel part of this paper. Two ways of position complementation algorithm are proposed. One method is complementation by Kalman Filter, and the other is complementation by error estimation. Error estimation is based on a theory of mode transfer between relative space and world space. The validation of proposed method is done by experiment.

© 2019 IEEE. Fall risk is a serious problem especially for the elderly. Fall accident causes fracture, and it leads to bedridden. Early detection of balance inability and rehabilitation training is important to decrease the risk. Quiet standing test is one of the physical tests to assess the balance inability of human. When the human is standing, he/she controls the plantar force to keep the balance. Center of pressure (COP) is the representative parameter to explain the plantar force movement. During the quiet standing test, COP fluctuates unconsciously. There are many researches that analyze the relationship between the COP fluctuation and balance inability. Many statistical indicators have been developed to assess the fluctuation specification.In contrast, several researchers have been tried to reveal the COP fluctuation mechanism by introducing stochastic mathematical model. Ornstein-Uhlenbeck process and diffusion equation are often introduced to the model. The mathematical models have been developed, and analyzed how visual input is related to the model parameters. If the mathematical model can explain the COP fluctuation completely, it becomes easier to assess the balance ability by comparing the model parameters.However, statistical indicators are still used in the clinical cases, thus there is room to discuss which statistical indicators to be used for assessment. The purpose of this research is to explore the relationship between statistical indicators and numerical model by evaluating whether the numerical model reproduces the same values with real data. The reproduced indicators and original indicators are compared, and the magnitude of the errors are evaluated.

© 2020 IEEE. In these days, multilevel inverters are focused thanks to their advantages, such as small voltage stress, low electromagnetic interferences, and harmonics reduction. Generally, renewable energy sources output a low DC voltage. From the viewpoint of efficiency, a single-stage conversion system which can directly change the lower DC-link voltage to higher AC voltage is required. Some single-stage conversion systems have been proposed before. However, these systems have large energy storage such as an inductor and a capacitor. This paper proposes a multilevel boost inverter stepping up the AC output peak voltage at any boost ratio by changing the number of switched capacitors. To reduce the capacitance, the proposed inverter has multiple conduction patterns when the output voltage is boosted. This characteristic distributes the discharging time to multiple capacitors. This inverter can set any maximum boost ratio and apply to the multiphase applications. The operation principle and the boost ability with multiple boost ratios were confirmed by simulations and experiments. The capacitance reduction was also confirmed by simulations.

© 2019 IEEE. Recently, for renewable energy applications, multilevel inverters are an attractive option due to their advantages, such as harmonics reduction, low electromagnetic interferences, and small voltage stress. Additionally, lower DC-link voltage should be converted to higher AC voltage, which needs an inverter and a DC-DC boost converter or a transformer. However, the two-stage conversion reduces efficiency and reliability in renewable energy applications. Hence, a single-stage conversion system which can directly change the lower DC-link voltage to higher AC voltage is required. Some single-stage conversion systems have been proposed before. However, the maximum boost ratio and the level of these systems are constant, which limits applications. This paper proposes a multilevel boost inverter stepping up the AC output peak voltage at any boost ratio by changing the number of switched capacitors. The operation principle and the boost ability with multiple boost ratios are confirmed by simulations.

© 2020 IEEE. This paper aims to lower the operational force for bilateral control under time delay while guaranteeing stability. Bilateral teleoperation system enables humans to work in remote places. However, time delay on network impairs the stability and the performance. The conventional method has trade-off problem between stability and operationality. This paper introduces the concept of equilibrium acceleration, which includes both position and force information, and proposes a method to detect vibration based on the difference of equilibrium acceleration. As a result, the algorithm can be considered simply, and it is possible to stabilize only during vibration and improve the operationality in free motion. Since this paper does not focus on frequency, but vibration, damping control works only during vibration and the operational force can be reduced compared to the conventional method. The effectiveness of the proposed method is verified by simulations and experiments.

The aim of this paper is to propose a novel robotic hand exoskeleton which can assist disabled people with grasping objects. Existing exoskeletons cannot measure force during contact motion, and this makes it difficult to achieve accurate grasp assistance. In the proposed system the exoskeletal finger is attached to the index and middle fingers, and the robotic finger is place on the palm side, which functions as a substitute of a thumb. The upper exoskeletal finger exerts assistive force on operator's fingers, and the lower robotic finger works as a force sensor by the use of Reaction Torque Observer. Thus, human grasping force is measured during grasping, and the environmental and human forces are estimated from measured torque. The proposed system realizes power assist control without using additive sensors like force sensors or EMG sensors. The effectiveness of the proposed method is demonstrated through simulation and experiment.