This paper proposes a new nonlinear stability analysis for the acceleration-based robust position control of robot manipulators by using disturbance observer (DOb). It is shown that if the nominal inertia matrix is properly tuned in the design of a DOb, then the position error asymptotically goes to zero in regulation control and is uniformly ultimately bounded in trajectory-tracking control. As the bandwidth of a DOb and the nominal inertia matrix are increased, the bound of error shrinks, i.e., the robust stability and performance of the position control system are improved. However, neither the bandwidth of the DOb nor the nominal inertia matrix can be freely increased due to practical design constraints, e.g., the robust position controller becomes more noise-sensitive when they are increased. The proposed stability analysis provides insights into the dynamic behavior of DOb-based robust motion control systems. It is theoretically and experimentally proved that nondiagonal elements of the nominal inertia matrix are useful in improving the stability and in adjusting the tradeoff between robustness and noise sensitivity. The validity of the proposal is verified by simulation and experimental results.

This paper extends the diagonalization method on the basis of the modal space disturbance observer (MDOB) for application to a multidegree-of-freedom (DOF) system. The aim of this method is to suppress the interference between the position and force control systems and realize a bilateral control system. The utility of the proposed method is experimentally verified by using a multi-DOF manipulator. It is confirmed that the MDOB-based decoupling method has better performance than oblique coordinate control. Conventional oblique coordinate control causes oscillation in cases where the modeling error is large and the cutoff frequency of an observer is not high enough to change the system dynamics. On the other hand, the MDOB-based decoupling method becomes unstable when the difference in mass is large.

Motion-canceling bilateral control, a teleoperation method, is proposed and implemented for telesurgical robots. Telesurgical robots have difficulty in achieving control owing to a lack of haptic feedback and the beating motion of organs. Hence, this study aims to provide a surgeon with the means for feeling the tactile sensation of the remote organ, as well as synchronizing with its motion. Therefore, a surgeon can treat the target using a master robot as if the organ is not moving even though it actually moves. The proposed method basically consists of acceleration-based bilateral control to achieve haptic feedback, and visual servoing is used to compensate for organ motion. The frequency characteristics and root locus of the proposed method are analyzed to evaluate its performance and stability, respectively. The proposal is validated through experiments using telesurgical forceps robots.

This paper analyses the robust stability and performance of Disturbance Observer- (DOb-) based digital motion control systems in discrete-time domain. It is shown that the phase margin and robustness of the digital motion controller can be directly adjusted by tuning the nominal plant model and the bandwidth of the observer. However, they have upper and lower bounds due to robust stability and performance constraints as well as noise-sensitivity. The constraints on the design parameters of the DOb change when the digital motion controller is synthesised by measuring different states of a servo system. For example, the bandwidth of the DOb is limited by noise-sensitivity and waterbed effect when velocity and position measurements are employed in the digital robust motion controller synthesis. The robustness constraint due to waterbed effect is removed when the DOb is implemented by acceleration measurement. The design constraints on the nominal plant model and the bandwidth of the observer are analytically derived by employing Bode Integral Theorem in discrete-time. The proposed design constraints allow one to systematically synthesise a high-performance DOb-based digital robust motion controller. Experimental results are given to verify the proposed analysis and synthesis methods.

This paper analyses Disturbance Observer- (DOb-) based robust force control systems in the discrete-time domain. The robust force controller is implemented using velocity and acceleration measurements. A DOb is employed in an inner-loop to achieve robustness, and another DOb, viz. Reaction Force Observer (RFOb), is employed in an outer-loop to estimate interaction forces and improve the performance of force control. First, the inner-loop is analysed. It is shown that the DOb works as a phase-lead/lag compensator tuned by the nominal design parameters in the inner-loop. The phase margin of the inner-loop controller and the bandwidth of the velocity-based (i.e., conventional) DOb are constrained not only by noise-sensitivity but also by the waterbed effect. This explains why we observe unstable responses as the bandwidth of the conventional DOb increases in practice. To eliminate the design constraint due to the waterbed effect, this paper proposes an acceleration-based DOb. Then, the robust force controller is analysed. It is shown that the design parameters of the RFOb have a notable effect on the stability of the robust force control system. For example, the robust force controller has a non-minimum phase zero (zeros) when the RFOb is not properly tuned. This may cause severe stability and performance problems when conducting force control applications. By using the stability and robustness analyses, this paper proposes new design tools which enable one to synthesize a high-performance robust force control system. Simulations and experiments are presented to validate the proposed analysis and synthesis methods.

This study aims to reveal the concealed features of human motions. For this purpose, a method of elemental separation (ES method) of human motions is proposed. First, human motions are extracted as haptic information through a bilateral control system. Then, by using the proposed method, the haptic information is divided into basic elements: 1) transformation matrices; 2) hybrid angles; 3) pure position commands; and 4) pure force commands. The proposed method is validated through simulations as well as experiments with robotic and human operators. The obtained results clearly reveal the features of human motions, although it is difficult to recognize the differences from the original responses. Nonetheless, this study provides a motion expression that should contribute toward a greater understanding of human skill.