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The conventional force control has proposed load-side torque control based on the resonance ratio control (RRC) enhanced by an instantaneous state observer using the acceleration sensor; however, environmental stiffness variation ( $K_e$ variation) is not considered in such systems. This article evaluates the robust stability of the conventional method based on open-loop characteristics and the stability analysis results indicate that the conventional method becomes unstable during $K_e$ variation. To realize superior high-robust stability, this article proposes a high-robust force control based on the duality of the two-inertia system. This article constructs an equivalent RRC (ERRC) combining the motor-side velocity control and the load-side velocity observer and demonstrates a torque-velocity duality. The results confirm that the load-side torque control based on ERRC is robust against $K_e$ variation through the analysis of open-loop characteristics, and illustrate that the ERRC has the same as the conventional RRC proposed in position or velocity controls. The effectiveness of the proposed control method is validated through numerical simulations and experiments.

This paper focuses on single inertialization on the load-side parameter of two-inertia system and employs the equivalent disturbance compensator (EDC). Conventional EDC is designed in case of the plant system without friction; thus, the back-forward drivability control using the EDC is affected by friction. In this paper, we propose the back-forward drivability control using EDC and DOB for single inertialization of two-inertia system. DOB is used to compensate the friction and the EDC is applied to not only improve the back-forward drivability, but also suppress the vibration on load-side output. This proposal consists of basic actuator and force sensor and it is available to general robot actuator that uses current sensor, motor encoder, and force sensor. The effectiveness of proposed back-forward drivability control is verified through the numerical simulation.

In an industrial machine modeled with a two-inertia system, the amplitudes of the input and torsion torques should be matched. However, owing to the effect of resonance frequency, the amplitude of the torsion torque may increase with the input torque. In this study, the amplitudes of the input and torsion torques are matched by designing the electric inertia such that the gain of the torsion torque with respect to the input torque is one. Because the parameters of the two-inertial system are used in the design of the electric inertia, the correct parameters must be determined. However, it is difficult to identify the friction of the torsion element in a two-inertial system. Hence, the electric inertia is corrected using a system that identifies the friction of the torsion element. The effectiveness of the proposed method is confirmed experimentally.

In a two-inertia system, load-side control is relatively simple to design using a cascade controller with a torsion torque controller in the minor-loop. However, the control bandwidth of the outer controller, such as the load-side reaction force controller, is affected by the bandwidth of the minor-loop. Thus, this paper proposes load-side reaction force control based on model-matching control. To realize a simple model-matching control, an equivalent disturbance compensator (EDC) is applied to torsion torque controller. The EDC is able to be used to suppress disturbance, such as the reaction force to the torsion torque. Moreover, the torsion torque and environmental vibration mode are separated. The effectiveness of the proposed control is verified through numerical simulations and experiments.

Industrial machines are modeled with a two-inertia system. A resonance frequency may not be able to set higher than the frequency band used in operation. When the frequency of the input torque for operation matches a resonance frequency, the vibration of the torsion torque increases. Increased torsion torque vibrations adversely affect machine operation. A method of suppressing an increase in torsion torque vibration includes adding a mechanical damper. The mechanical damper attenuates even at frequencies other than the resonance frequency. Hence, the mechanical damper has an energy loss. Vibration suppression by control is performed without mechanical damper. Generally, in the vibration suppression control to the multi-inertia system, the control bandwidth is set lower than the resonance frequency. If the resonance frequency is lower than the frequency band used for driving, the high-frequency torque used for driving is suppressed. In this paper, the mechanical parameters are changed electrically using electric inertia and friction. When the proposed method is applied to a two-inertia system, the bandwidth is expanded to a frequency higher than the resonance frequency, and an increase in torsion torque vibration is suppressed. The effectiveness of the proposed method is confirmed using simulations and experiments.

To realize vibration suppression for a three-inertia system, this paper proposes a current control system using resonant frequency damping disturbance observer based robot servo system. The resonance frequency components for achieving vibration suppression is estimated by resonant frequency damping disturbance observer. Through the proposed current control system, vibration suppression is achieved against a three-inertia system. The effectiveness of the proposed method is confirmed through experiments

Controlling torsion torque in geared motors is essential for precise control of load-side dynamics. An I-P torsion torque controller (TTC) structure combined with a friction observer based on the disturbance observer has been previously proposed for this means. However, in motors with strong nonlinear friction, this method is not able to quickly compensate nonlinear friction when rotation direction changes, severely affecting the torsion torque control performance. This paper proposes a variable-order friction observer with a switching law based on the motor-side velocity (state) and torque command (reference) that is able to compensate for the effects of friction in the torsion torque response, while also preventing limit cycles due to the friction compensation itself. First, a review of the conventional I-P TTC with zero-order friction observer method is given. Followed by this we explain the decomposition process of a high-order friction observer into a variable-order form. Next, a stability analysis of the variable-order friction observer is done. Then, appropriate switching functions are designed to effectively compensate friction and prevent limit cycles. Finally, the effectiveness of the proposed method is proven by numerical simulation results and experimental results with a 1 DOF robotic arm.

Good forward-drivability characteristics are essential for applications such as position, velocity, and force control. Geared motors, usually employed in industrial robots, require lubrication in their gears, and consequently, some form of sealing is needed to avoid leaking. While oil seals prevent leaking, they increase nonlinear frictional effects, which result in stick-slip during velocity zero-crossings, thereby deteriorating the forward-drivability. This paper proposes a stick-slip suppression method based on a velocity-driven N-order stick compensator (VDNSC), which works as an order-variable friction compensator formed by the velocity-driven switching of the internal components of a high order Q-filter. Experiments with torque and velocity control confirm that the proposed method improves the forward-drivability, allowing for more precise motion even under large friction.

Load-side acceleration sensing based on load-side acceleration control is proposed for a two-inertia resonance system. The proposed method is divided into two stages. In the first stage, the feedback component from the load-side is canceled using the estimated value of the instantaneous state observer and the load-side acceleration. The two-inertia resonance system is converted into a simple second-order system using this method. The proposed method configures a control system of cascaded loops for the second-order system. Therefore, the load-side acceleration control is achieved by two specified bandwidths. The proposed method is easy to implement for conventional robots and yields superior cost-savings compared with other types of load-side acceleration control. The effectiveness of the proposed load-side acceleration controller is verified by theoretical analysis, numerical simulations, and experiments.

Conventionally, the two-inertia system with environment has two resonant frequencies and one anti-resonant frequency. In the case of using resonance ratio control (RRC) for a two-inertia system with environment, it is difficult to determine the resonance ratio. Thus, this paper focuses on the duality between torque and velocity, and a two-spring resonant system is constructed based on motor-side velocity control. In addition, RRC based on motor-side velocity control and load-side velocity observer (LVOB) is proposed, and the duality between the two-inertia resonant system and two-spring resonant system is demonstrated. Finally, load-side torque control based on RRC is confirmed, and that the RRC is the same as conventional RRC. Compared with the RRC of two-inertia system and RRC of two-spring system, a duality is demonstrated, and the effectiveness of the proposed control method is verified through numerical simulation and experiment.

This paper proposes force sensorless force control using a notch-type dual disturbance observer. The force control is structured by acceleration and speed controllers. The reaction force estimation system reduces the effect of the static friction using a dither signal. The proposed notch-type dual disturbance observer combines notch- type zero-order friction-free disturbance observer (FF - DOB) and notch-type first-order FFDOB. The proposed disturbance observer has the disturbance suppression characteristics of a notch-type zero-order FFDOB on the high-frequency side and a notch-type first-order FFDOB on the low-frequency side. It has a wide estimation band and allows the improvement of the suppression characteristics against low frequency disturbances. The validity of the proposed notch-type dual disturbance observer is verified through experimental results using an actual industrial robot.

In recent years, many kinds of renewable-energy solutions, such as wind, power, and photovoltaic energy, have attracted much attention as well as the distributed power grids into which they are integrate. These systems have a highly reliable power supply. However, to connect DC buses to other devices, many isolated converters are require to ensure isolation from the DC bus. Isolation is essential for safety purposes. However, the use of many isolated converters is not preferable because of problems with the increasing system size. Triple action bridge (TAB) converters have the features of bidirectional power flow and small size, which is suitable for integration into distributed power grids. TAB converters that are based on dual active bridge (DAB) converters have useful characteristics of DABs. However, there remains a problem with respect to the interaction between two current loops with TABs, and there are few solutions to this problem when an AC supply is connected to the system. In order to enable the connection from a TAB converter to an AC system, this paper proposes a method to solve the interference problems in the circuit and control. The suitability of the propose solution is shown using simulations.

Many gears and reducers are built in the joints of industrial machinery and robots. The harmonic drive is used for many robots. However, harmonic drive has nonlinear characteris- tics such as lost motion and hysteresis. The vibration suppression performance deteriorates when a parameter fluctuation occurs during vibration suppression control. In the proposed control method, a state feedback is constructed, and the control gain is sequentially changed according to the spring constant during operation, based on the torsion angle $\theta_{s}$ and previously measured spring characteristics. The effectiveness of the proposed method is confirmed by simulation and experimental results.

This paper proposes a back-forward drivable control based on torsion torque control (TTC) that achieves both back-forward drivability and vibration suppression for a two-inertia system. The proposed system combines the TTC that consists of I-P controller and velocity deviation feedback and a motor-side normalization compensator (MNC) to facilitate human-robot interactions. For the velocity deviation estimation, load-side torque observer (LTOB) and load-side velocity observer (LVOB) are evaluated on estimation performance. The better observer is applied to the TTC from the analysis results. The effectiveness of the proposed control system is verified by simulation and experimental results, and the vibration of the back-forward drivable response is considerably suppressed.

This paper proposes force sensorless force control by using a notch-type friction free disturbance observer. The force control is structured by using acceleration and speed controllers. The reaction force estimation system reduces the effect of the static friction using a dither signal. A conventional friction free disturbance observer becomes unstable when a low-frequency dither signal is applied. In order to solve this problem, a notch-type friction free disturbance observer is employed, which allows the use of low-frequency dither signals. This paper explains the difference between the conventional design method and the proposed design method. The validity of the proposed notch-type friction free disturbance observer is verified through the experimental results by using an actual industrial robot.

To operate two-inertia systems with high speed and precision, a control method to suppress mechanical resonance is required. A conventional current control system consists of a proportional integral controller and decoupling controller, and the conventional current control system has high motor torque control performance. However, the conventional current control system tends to excite mechanical resonance. This paper proposes a new current control system to suppress the resonance of a two-inertia system by using the cross-coupling term of a surface permanent magnet synchronous motor. The proposed control system consists of a proportional controller and voltage disturbance observer to estimate the voltage disturbance without including the cross-coupling term that is used as a damping control. The effectiveness of the proposed control system is validated by numerical simulations and experiments.

In this paper, the aim of precise motion control, considering thrust vector selection method that allows Surge, Sway and Yaw motion of the 4-axis driven electric ship. As a thrust vector selection method, we propose a method of actively changing the position of the center of thrust vector. The proposed method controls Yaw angular velocity and Sway velocity. Compared to the conventional method of Yaw angular velocity control, confirm the improvement of the motion performance of the proposed method. By experiments, the usefulness of the proposed method is confirmed.

This paper proposes force sensorless force control using the notch-type friction free disturbance observer. The force control is structured by acceleration and speed controllers. The reaction force estimation system reduces the effect of the static friction using a dither signal. A conventional friction free disturbance observer becomes unstable when a low-frequency dither signal is applied. To solve this problem, the notch-type friction free disturbance observer is employed, which allows the use of low-frequency dither signals. This paper explains the difference between the conventional design method and the proposed design method. The validity of the proposed notch- type friction free disturbance observer is verified through the experimental results using an actual industrial robot.

This paper considers the contact phase in a sensorless torque control system for a two-mass system. A plant system is a two mass system with an environmental switch. The environmental switch is used for considering unstable contact behavior. The state observer (SOB) and zero-order disturbance observer (ZODOB) are used to estimate the state variable of a plant system. In sensor-less torque control system, the ZODOB has better performance than the SOB. When plant system is not in contact with the environment the estimated value using the SOB is not good for control system. It makes control system becomes instability in some cases. When the ZODOB is fed back to torque control system, the stability of control system is preserved. Experiments are performed to confirm the effectiveness of using the ZODOB in sensor-less torque control system.

SUMMARY A method for estimating the sway angle using an observer has already been proposed. The state observer estimates the sway angle accurately and must use the detected sway angle value. However, the estimated sway angle has an error owing to rope length error, friction force, and wind. Moreover, the container mass cannot be determined, and therefore the observer parameter is not suitable. We already proposed robust antisway control for overcoming rope length error without adding a new sensor. Further, we designed a friction disturbance observer to cancel out the influence of the friction force. In this paper, we first propose a container mass estimation method when a crane system performs rolling up control. The observer parameter can be selected using the estimated mass value. Second, in crane parallel shift control, we propose a robust antisway control even when there is a wind disturbance. We design a wind disturbance observer and propose a wind disturbance estimator to separate the friction observer output from the wind disturbance observer output. We confirm through experiments that the proposed method can reduce vibration.

SUMMARY This study proposes a variable dynamic threshold for a jerk signal for achieving contact detection in industrial robots. Sensorless force control is useful in applications in advanced industrial robots. The external torque of each joint is estimated accurately by using a disturbance observer and by inverse dynamics calculations. However, the phase lag of the disturbance observer and the dynamic parameter error create force estimation errors. These errors hamper quick and reliable contact detection in industrial robots. Therefore, this study proposes a useful approach to contact detection that uses a variable dynamic threshold for a jerk signal. The proposed variable dynamic threshold reduces contact detection failures caused by force estimation errors during the free motion of a robot. The proposed threshold is suitable for robot motion and improves the speed of contact detection. Numerical simulation results show that the proposed method is valid for the parameter variations in an actual industrial robot. In addition, experimental results show that the dynamic threshold is a useful technique for application to general robot systems as well. The dynamic threshold is useful in applications for any industrial sensorless force control system.