This paper focuses on voltage-integral-based reference tracking modulation (VIRTM) and direct torque control (DTC), proposing the control strategy that minimizes switching frequency while achieving excellent dynamic performance. In the proposed scheme, the optimal voltage integral trajectory calculated by exhaustively searching all switching patterns in an offline process is applied to the DTC. One of the advantages of the proposed scheme is its superior versatility and robustness, as it does not rely on motor parameters. Secondly, it significantly reduces the switching frequency at all operating points while preserving the dynamic characteristics of DTC. The effectiveness of the proposed scheme has been validated through experiments, demonstrating up to a 28% reduction in switching frequency compared to conventional standard DTC.

This paper presents a modulation method that minimizes the impact on common-mode noise and switching loss. The proposed method introduces a priority factor into the evaluation function to satisfy both requirements and adjust these characteristics. The effectiveness of the proposed method is tested through simulations and experiments, which confirm the switching and common-mode voltage frequencies.

Advancements in automation have pushed forward the development of collaborative robots, which are designed to work together with humans. The majority of actual commercial collaborative robots use geared motors with some kind of torque sensing, and their performance is directly related to how well torque is controlled. However, imperfections in the gear fabrication causes transmission error in the geared motor, producing undesired vibrations which affect the torque control performance. In this study, an integratorless 2DOF (two degrees-of-freedom) torque control scheme is proposed to improve the torque response in such case. The proposed control scheme uses first-order FPIDO (force-position-integrated disturbance observer) which nominalizes and achieves motor side acceleration control, allowing to use the natural mechanical dynamics of the motor side as an integrator instead of implementing it on the control system. This is combined with a derivative-type torsional disturbance observer (DTDOb), which uses torque sensor information to compensate all torsional disturbances and nominalize the torsion torque dynamics, achieving 2DOF (Two degrees of freedom) torque control. The effectiveness of the proposed methodology is verified by simulation and experimental results.

This paper proposes a load-side acceleration control scheme for geared motors affected by unknown backlash and friction. A backlash-deactivated disturbance observer (BDDOb) is proposed to achieve load-side acceleration control while ensuring quick traversing of backlash. The BDDOb works by executing conditional estimation-compensation of load disturbances, and is driven by a backlash detection scheme based on torsion torque sensing which does not require any information on the backlash gap width. In addition, a variable-order force-position-integrated disturbance observer (VOFPIDO) is proposed to quickly compensate nonlinear friction without requiring information of the nonlinear friction magnitude by applying velocity-dependent switching. Experimental results prove the effectiveness of the proposed method.

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.

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.

This paper proposes a new sensorless force sensing approach to realize the advanced wideband force control system. The force observation is based on the harmonic estimation integrated singular spectrum analysis based disturbance observer. The singular spectrum analysis is constructed to improve the force sensing performance and achieve the remarkably wide bandwidth by effectively separating the fundamental response and noise when the disturbance observer pole is notably raised. Additionally, the harmonic estimation based on the least squares method is configured to identify and cancel the periodic component caused by dither signal in the force estimation. The proposed method achieves the high performance force sensation with considerably widened force sensing bandwidth. The effectiveness of the proposed method is verified by numerical simulation results.

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.

In a single-phase to three-phase electrolytic capacitor-less inverter, the inverter must control not only the motor but also the source current because a power factor correction circuit or energy buffer does not exist. To obtain a sinusoidal source current, a control method that modifies the output voltage directly is proposed. However, the motor current control occasionally becomes unstable because the control method modifies the output voltage to control the source current. To control the source current while considering the motor current response, this paper proposes a motor current regulation based direct DC-link current control. The proposed method enhances the stability of the motor current control system by realizing the sinusoidal source current with the motor current reference modification. The effectiveness of the proposed method is experimentally verified.

In the electrolytic capacitorless inverter system, resonance occurs between the inductance on the grid side and the DC-link capacitor. Because of this resonance, the regulation of the input current harmonic may not be satisfied in some cases. In order to satisfy harmonic regulation, a method for controlling this input current by direct DC-link current control has been proposed using PWM modulation. In PWM modulation, the switching frequency depends on the carrier frequency. This paper proposes a control method that reduces the switching frequency and satisfies the harmonic regulation via model predictive direct current control (MPDCC). This paper is confirmed that the proposed method reduces harmonics of input current and switching frequency. The effectiveness of the proposed method is verified by numerical simulation.

In this paper, we present a method to rapidly discretize a continuous time-variant system in real time. Three discretization methods are compared. The proposed method does not calculate for discretization, and therefore, the calculation cost is 1/100 of that of the conventional zero-order hold model. The presented method has high stability in higher order systems and the position and size of the internal parameters are determined for each sampling time. The effectiveness of the Finite-Difference Time-Domain (FDTD) method is demonstrated by comparing responses with a continuous system by using simulations. By experiments, we reproduce the haptic sensations based on the time-variant environmental model expressed by Tustin approximation and FDTD method.

In order to realize a fine force sensorless force control, this paper proposes a new notch-type friction-free disturbance observer(DOB) and a new notch-type friction-free reaction force observer(FFRFO). Generally, a force sensorless force control system always has influence on static friction phenomenon. To overcome this problem, this paper proposes a new force sensorless control system using the notch-type friction free reaction force observer, whose inner system is the proposed acceleration control based on the notch-type friction free disturbance observer. The effectiveness of the proposed system is confirmed by experiments using an actual industrial robot.

In a single-phase to three-phase inverter that does not have an inductor or electrolytic capacitor, the inverter must control not only the motor but also the input power factor because there is no power factor correction circuit or energy buffer. To improve the input power factor, several control methods are proposed. However, if the motor has large spatial harmonics caused by the rotor structure, the bandwidth limitations of the control system adversely affect the source current response and thereby cause source current harmonics. This paper proposes a harmonic current reduction controller that makes it possible to obtain a sinusoidal source current. The proposed controller focuses on the relationship between the source current and the average DC-link current and controls the source current using a feedforward control method. The effectiveness of the proposed method is experimentally verified.

This paper proposes a robust position control based on state feedback using double disturbance observer for two mass system. In load-side position control based on state feedback using disturbance observer, performance of control system is depend on bandwidth of disturbance observer. To solve problem, the load-side position control using I2-PI controller based on state feedback is proposed. The proposed system consists of PD position controller, motor-side acceleration controller, I2- PI controller equivalent of states feedback, velocity deviation feedback, and a zero-order load-side disturbance observer (ZLDOB) with variable noise-covariance (VNC) Kalman filter. The simulation and experimental results confirm that the load-side position control using I2-PI controller based on state feedback reduces the vibration in response load-side position. Additionally, the proposed method reduces the vibration in response load-side acceleration.

For the high performance of force/torque control, an approach using a disturbance observer is known to be effective. However, a conventional disturbance observer cannot perfectly compensate for the disturbance torque when the disturbance torque (for example, impact torque) is instantaneous. This paper proposes a force and position sensors integrated disturbance observer (FPIDO), and analyzes its disturbance-suppression performance. Moreover, this paper focuses on high-performance torque control using a torsion torque sensor for a two-inertia system. The disturbance-suppression performance of the conventional disturbance observer and the FPIDO are compared using experimental results for motor-side acceleration control and load-side torque control. Finally, the experimental results verify the effectiveness of the proposed disturbance observer.

This paper proposes a new sensorless force control system based on reaction force estimation system considering static friction and kinetic friction for industrial robot. As industrial robot uses the harmonic gear and the ball screw, the proposed force control system is structured by the resonance ratio control based speed controller. Moreover, as harmonic gear has a static friction and kinetic friction, the reaction force estimation system needs to reduce the influence on static friction and kinetic friction force. The proposed friction-free force estimation system has the two technics. The first one is the dither signal based static friction free force estimation, and the second one is the Stribeck model based kinetic friction free estimation. Using these technics, the friction-free reaction force estimation system is constructed. In this paper, the validity of proposed system is verified by the experimental results using the actual industrial robot.

This paper focuses on storage and reproduction systems for environmental haptic sensations. A large force vibration is caused by the differential of viscosity momentum in the reproduction system based on force control. To solve this problem, the reproduction system based on momentum control has been proposed in previous our research. The improved reproduction system has a configuration that the momentum control loop is added to the force control loop. The high-stiffness environment in which the actuator does not move cannot be reproduced because the system generates the force command that correspond to the position of the actuator. Hence, this paper newly proposes storage and reproduction systems based on position control considering the environmental mechanical admittance. The proposed system is able to reproduce the environmental information without additional control loop. The effectiveness of the proposed method is confirmed by experiments.

Recently, a soft robot considering the contact of the human has attracted attention, and the zero stiffness torque control is required. Because a soft robot causes vibration at the load side, vibration suppression control of a soft robot is required. In this paper, the authors discuss position control and torque control of a soft robot by using a torsion torque sensor based on a resonance ratio control. Position control can be used to design parameters. However, torque control cannot be used to design parameters because the proportional gain Kp and resonance ratio gain Kr are not independent with regard to integral-proportional differential(I-PD) torque control. Therefore, in the case of torque control, resonance ratio control is equivalent to I-PD control.

In the field of high-precision position control, the position response requires that there is no vibration. However, a permanent magnet synchronous motor (PMSM) has a wide variety of torque-ripple components, including cogging torque and flux harmonics, which degrade the control performance. Typically, these components are defined as a disturbance torque, and compensation is accomplished by using a disturbance observer. However, a disturbance observer uses a low-pass filter for noise reduction, and it causes phase and amplitude shifts in an estimated value. As a result, it is not possible to perform the appropriate compensation. This paper considers a new structure of disturbance observer based on a zero-order disturbance observer and torque ripple table. It is possible to accurately estimate the instantaneous disturbance torque by using the proposed disturbance observer. The effectiveness of the proposed disturbance observer is confirmed by numerical and experimental results.

This paper proposes a sensorless vector control method based on Primary flux linkage and sinusoidal current tracking control for Interior permanent magnet synchronous motor. The proposed sensorless vector control can control torque at different speeds. Furthermore, the proposed sensorless vector control method estimates the primary flux linkage vector. At various speeds, it is possible to control the torque without relying on position or speed information.

This paper proposes a stable force control system to achieve the force control of industrial robots. The proposed stable force control system is designed by using an I-P force control system and a resonance ratio control system considering the required interaction with the environment. To design the resonance ratio control system considering the contact with the environment, this paper analyzes the stability of the proposed resonance ratio control and the bandwidth of a disturbance observer. This paper designs the resonance ratio control system using an instantaneous state observer considering the results of the stability analysis. In addition, a stable force control system is designed based on the resonance ratio control system using an instantaneous state observer. In this paper, the effectiveness of the proposed force control system is confirmed by performing numerical simulations and experiments using an industrial robot arm.

The approach used to implement mechanical impedance can be generally separated into the following two types: an impedance control system and an admittance control system. When an impedance control system reproduces a hard environment, large vibrations are generated. In contrast, an admittance control system is excellent at reproducing a high-stiffness environment for utilizing the inverse of the environmental impedance. However, stability analyses have not been conducted on admittance control systems. Therefore, in this study, a stability analysis for an implementation system of mechanical impedance is conducted using a pole placement. The analysis results indicate that the pole placement of the impedance control system is moved toward the direction of an imaginary axis based on the influence of the environmental stiffness. Hence, large vibrations occur when the environmental stiffness is determined to be a large value. On the other hand, for the admittance control, when the environmental stiffness is dominant, pole placement is not affected by the environmental stiffness. The superiority of the admittance control system in a high-stiffness environment was shown through an experiment.

This paper proposes a method of dynamic parameter tuning for a sensorless force control system with I-P(Integral-Proportional) current controllers. All free parameters of the whole system are determined theoretically by pole assignment method. This paper describes that gain of current controllers and estimation bandwidth of a disturbance observer have to be set according to stiffness of target environment, although gain of a force deviation and a quality factor of the disturbance observer are derived as constant values. In order to adapt to the stiffness variation, the proposed system changes dynamically the gain of I-P current controllers and the bandwidth of the disturbance observer. Furthermore, comparative experiments confirm that the force and I-P current control systems with dynamic parameter tuning is effective.

The force control system of a typical injection molding machine obtains force information from the machine's environment via a force sensor. However, because of Force sensors often have disadvantages such as signal noise, high cost, and a narrow bandwidth, a reaction force observer was developed for electric injection molding machines. This method has a steady-state error owing to the influence of nonlinear friction. We realized sensorless force feedback control based on a friction compensation element, dither signal inserting method, and high order reaction force observer (HORFO). The HORFO does not influence either the inserted dither signal or the nonlinear friction phenomenon. However, as the desired parameters of the HORFO cannot be fixed at each driving point, the sensorless force feedback control method cannot always be used for fine pressure control. To overcome this problem, we propose new sensorless force control system based on HORFO in this paper. Experimental results confirmed the robustness of the proposed sensorless force control system.

Force sensing is an important part of a motion-copying system. To achieve the high performance of the control system, high quality of force sensing is required. This paper focuses on suppressing harmonic disturbances in the force estimation to improve the performance of force sensing. A friction-free disturbance observer with a dither signal is applied for both motion-saving and motion-loading systems. The dither signal reduces the friction effect on force estimation and the friction-free disturbance observer eliminates the oscillatory signal on force estimation. Moreover, the friction-free disturbance observer achieves effective harmonics suppression in force estimation. All control algorithms are implemented in FPGA to achieve short sampling period, and enable a wider force sensing bandwidth. The effectiveness of the proposed method is verified by experimental results.

In this work, we present a bilateral control system with different mechanisms of master and slave sides. A linear shaft motor and a ball screw perform the roles of the master and slave, respectively. To reduce the friction effect on force estimation due to the ball screw mechanism, a periodic signal is inserted into the control signal of the slave side. The addition of periodic signal causes oscillatory force responses on both master and slave sides. Therefore, a high-order disturbance observer is designed for force sensing operation on the slave side to reduce the effect of oscillatory disturbance on force information. The control algorithm consists of a conventional disturbance observer and a high-order disturbance observer for master and slave sides, respectively. The numerical simulation results verify the effectiveness of the proposed method.

This paper studies motion reproducibility of a motion-copying system, which is capable of storing and reproducing motion of human operators. In the paper, the motion reproducibility is expressed as transfer function from a force response of the storage phase to a force response of the reproduction phase. According to analysis of the derived transfer function, this paper confirms that the motion reproducibility is varied depending on variations of force gain, position gain, bandwidth of disturbance observer, environmental stiffness, and viscosity frictions. By the experiment, validity of the analysis method of the motion reproducibility is verified.