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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.

This paper focuses on controlling the load-side acceleration (LSA) of a two-inertia system with the LSA. Here, an LSA control-based system on integral-proportional (I-P) drive-side velocity control is proposed for an industrial robot with mechanical resonance. The LSA control based on I-P drive-side velocity control with a two-inertia system adopts a wideband acceleration sensor capable of acquiring direct current (DC). The proposed structure is similar to that of the control system that adopts the disturbance observer (DOb)-based acceleration control of a single-inertia system as a minor loop. In addition, the proposed LSA control system comprises a drive-side velocity controller and load-side velocity disturbance observer, which are employed to simplify the plant system and, compensate for the velocity disturbance on the load-side, respectively. Furthermore, the limitations of the conventional acceleration controller and the effectiveness of the proposed LSA controller are verified via theoretical analysis, numerical simulations, and practical experiments.

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.

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

This paper proposes an approach and contact controls based on the force impulse regulator. The force impulse regulator is constructed using two-spring resonant system using the motor-side velocity control and the force and position sensors integrated disturbance disturbance observer (FPIDO). Control gains are determined using pole placement method and the force impulse command is determined according to desired approach speed. The proposed approach is verified through the numerical simulation and experimental results.

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.

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.

In previous research, the back-forward drivability of load-side torque control system based on a motor-side acceler­ation control has proven to be high, but contact motion has not yet been realized in human-robot interaction such as powered exoskeleton and direct teaching. To address this issue, this study proposes a load-side torque control system that achieves both bouncing suppression and human-interaction. This paper focuses on load-side acceleration because the bouncing motion at environmental contact and the human-robot interaction motion are able to be evaluated to its performances intuitively. Human-robot interaction is applied in low-frequency, and bouncing suppression is applied in high-frequency. The experimental results verify the effectiveness of the proposed method.

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.

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.

Industrial robots are required to perform high-speed and high-precision motion. Industrial robots are controlled using a conventional PI velocity controller with state feedback system for a two-inertia system. In general, the axis is modeled as a two-inertia system. However, a three-inertia system is required for modeling the axis because the industrial robot is required to have more high-performances motion. This paper proposes a vibration-suppression control system for a three-inertia system based on a PI velocity control system with state feedback system using the two-inertia approximation model. The new two-inertia system for the state observer is designed as an approximation model based on the three-inertia system. The effectiveness of the proposed system is confirmed by numerical simulations and experiments.

Recently, the scaling up of the storage capacity and data transfer rate of digital storage media has been required. However, increasing in the storage capacity and transfer rate makes optical head control more difficult. Thus, a tracking control system for optical discs must exhibit a high degree of precision control. Consequently, a new two-stage feedforward control (TSFFC) system for high-precision control is proposed in this paper. The proposed system is constructed using two zero phase error tracking (ZPET) control systems based on error prediction and an error-based disturbance observer (EDOB) that uses a notch filter to suppress non periodic disturbances. The proposed control system is designed for DDU-1000 for digital versatile discs (DVDs). The experimental results demonstrate that the proposed system effectively suppresses tracking errors.

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.

Currently, a high-precision tracking control system is needed for large-capacity optical disc drive system. This paper proposes a new full field-programmable gate array (FPGA) tracking control system based on an Error-Based Communication Disturbance Observer (ECDOB) with consideration of a time delay of the optical disc drive system. The FPGA control system sometime has a large influence on both the integer time delay and the non-integer time delay of the control system. In order to consider the time delay, this paper reconstructs the feedback control system by using the limited pole placement method. Moreover, this paper proposes a new error based communication disturbance observer. The experimental and numerical simulation results confirm that the tracking performance of proposed system is better than that of conventional feedback control system. In full FPGA tracking control of optical disc systems, the proposed error-based communication disturbance observer based tracking controller exhibits good performance.

This paper proposes a force control system including contact process using acceleration-sensor-based instantaneous state observer for high-stiffness gear drive. The proposed system is structured by PI velocity control system and IP force control system based on resonance ratio control system. An instantaneous state observer is used to a resonance ratio control system for a high-stiffness gear drive. In proposed system, the robot arm of a high-stiffness gear drive is achieved to move closer to the environment by using PI velocity control system. Moreover, the robot arm is achieved to come in contact with the environment by using IP force control system. Additionally, the force control system including contact process is needed a smooth switching of these control system, and smooth force control. To achieve the smooth switching of control system, an initial value of an integrator of force control system is calculated by the condition that a motor-side torque reference is constant when switching the control system. In addition, the algorithm is proposed to achieve the smooth force control system. The proposed method is achieved the smooth switching of control system and smooth force control system. The effectiveness of proposed method is confirmed by numerical simulation and experiments.

This paper compares performance of a double feedforward control system that considers for time delay. Recently, digital content size continues to increase. In order to cope, manufacturers of digital storage media try to increase capacity by narrowing the track pitch of these digital storage media. This digital controller designs and implements in a Field-Programmable Gate Array (FPGA). However, the control system includes non-integer the time delay. The time delay element exists in its characteristic polynomial. Therefore, the system is possibly destabilized by time delay. In the feedforward system, the feedforward compensation value is delayed by time delay. Considering these factors, a high-precision pickup control system that considering time delay is needed for optical discs. This paper proposes a new double feedforward control system considering time delay. In this paper, the tracking performance of the control system is compared to the conventional feedback control system and the new error based communication disturbance observer. In optical disc systems, the proposed double feedforward control system considering time delay has a good performance.

To achieve force control of an industrial robot, this paper proposes an I-PD force control system based on an instantaneous state observer. The structure of the proposed system is based on a resonance ratio control system and a feedback signal of the reaction force response. In this paper, the gain of a pseudo derivation is designed as the feedback gain. From the results of the designed gains, the feedback gain Kf is a negative value. Hence, the resonance ratio control system becomes equivalent to a state feedback. For the stability of the proposed I-PD force control system based on the acceleration control system and a state feedback, a new analysis method is required. This paper analyzes an open-loop of the proposed system considering the bandwidth of the observer. The results of an analysis show that an observer with a wide bandwidth is required. Therefore, the parameters of proposed system using the instantaneous state observer are designed based on the coefficient diagram method. The effectiveness of the proposed method is confirmed by performing numerical simulations based on the model of an industrial robot arm. The results show that the proposed method is effective for the stable force control of an industrial robot arm.

It is desirable for a container crane to operate smoothly and quickly. For this purpose, the control system of a container crane should be capable of antisway control for suppressing vibrations. A vision sensor system is often used to detect the sway angle. However, since a control system with a vision sensor has a delay time when determining the angle, it sometimes leads to deterioration of control performance owing to the delay time. In order to overcome this problem, this paper proposes a new antisway crane control system based on a dual-state observer with sensor-delay correction. However, because of nonlinear friction in the crane, the estimation accuracy achieved by using the observer is poor. To overcome this problem, this paper proposes a disturbance observer considering friction disturbance. The control performance and effectiveness of the proposed robust control system based on the estimated information are shown to be satisfactory by experimental results. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(3): 36–46, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22412

This paper proposes a new fine-motion-control method for realizing high-accuracy and high-speed contact motion of industrial robots by employing sensorless force control. Today, although industrial robots have become considerably important in the modern industrial society, their functions are limited. A typical limited function is the positioning motion control of robots used in the manufacturing industry. Contact motion is necessary for almost all new applications. In this study, by employing the proposed motion control, smooth and quick contact motion of industrial robots is realized by using a sensorless I-P (Integral-Proportional) force feedback controller. The proposed method is simple and effective, takes into account both the inertia of a robot and the behavior of the I-P force controller. In the experiments, a three-degree-of-freedom robot is brought into contact with an object (a concrete block or a rubber board) by the I-P force control using the proposed method. Further, in the experiment, the motion of the robot's end-effector was considered. The validity of the proposed method is confirmed by using a six-axis force sensor and an acceleration sensor in the contact motion experiments.

An optical disk drive system must realize high precision tracking control. For this purpose, the tracking control system that combined a robust feedback controller and a zero phase error feedforward tracking controllertracking control system has been proposed. This proposed feedforward tracking control system effectively suppresses the tracking error caused by track eccentricity. However, it is sometimes difficult for a conventional feedforward tracking control system to realize high precision tracking control. Hence, this paper proposes a new perfect tracking control (PTC) system that uses a prediction state observer. Numerical simulation results and experimental results confirm that the proposed system effectively suppresses the tracking error for a disk rotation speed of 8000[rpm] and operates stably. Therefore, the proposed system realizes high precision tracking control.

This paper proposes the new design method of error-prediction control systems combining the ZPET control and the robust feedback control. The error-prediction control system based on ZPET control can reduce the tracking error caused by periodic disturbances more efficiently than the conventional repetitive control technique. However, the proposed tracking servo system does not reduce the residual tracking error caused by the low-pass filter of the feedforward compensation and by the long sampling time. In order to overcome this problem, this paper proposes the new structure of the feedforward tracking control system. The modified ZPET control tracking servo system for optical disk recording system does not include the low-pass filter and its sampling time becomes faster. On the other hand, the sudden disturbance observer reduces the influence of nonperiodic disturbances. The experimental results point out that the proposed tracking servo system has a precise tracking response against both the periodic disturbances and the nonperiodic disturbances. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(4): 51–59, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20800

This paper proposes a new robust feedforward tracking control system suppressing tracking error including harmonics disturbance and DC offset for optical disk recording system. The proposed tracking control system consists of triple compensation loops of tracking error which are the robust feedback control loop, a new harmonics disturbance suppression loop based on Zero Phase Error Tracking(ZPET) feedforward control and a new additional DC offset compensation loop. The feedback controller is the most inner loop which is based on the high gain servo system. The harmonics disturbance suppression loop based on ZPET control is the middle loop which suppresses the influence of many periodic disturbances. The final outer loop is a new additional DC offset compensation loop which suppresses the DC offset caused by ZPET feedforward control. The experimental results point out that the proposed system well realizes the fine tracking performance for optical disk recording system.