The special issue on Control Systems in Mechatronics is a significant and timely issue since many robotics and mechatronics engineers now pay attention to the research field of motion control and control theory. In Japan, advanced motion control technology is a key technics to improving the performance of robot systems and/or mechanical automation equipment. The definition of motion control in this issue is the control of mechanical systems driven by electrical actuators such as a do servo motor or an ac servo motor. The means or strategy of motion control has so far been of interest only to electrical engineers and mechanical engineers; it has not been as familiar to robotics engineers. Recently, a control system has been developed with industry applications. Advanced motor control technology in Japan is based on the robust control system, such as the disturbance observer, the H00 control system, the two-degrees-of-freedom control system and so on. The disturbance observer has a simple structure, and it is quite valid for disturbance torque rejection. The robust control system based on the disturbance observer is now widely used in robot and mechanical systems in Japan. The disturbance observer is the original Japanese technology designed by two electrical engineers, Prof. Ohnishi and myself, from the viewpoint of the electrical actuator but control theory. Ho control is linear control technics popular around the world. It can make the desired loop shaping of frequency characteristics for a plant system such as the actuator of a mechanical system. The robust control system based on the mixed sensitivity problem of H00 control theory has good frequency characteristics. Moreover, the availability of large amounts of computational power has enabled us to use complex control theory, and actuators for robotics applications are now mainly electrical ones because of the remarkable progress in power electronics. This change in the control of mechanical systems is a new and attractive one. Motion control is becoming a field of interest to control, electrical, and mechanical engineers who work in robotics. In this issue, the eight papers and the two news reports have been selected to show the current topics concerned with control systems in mechatronics. The first paper is a review paper titled ""robust motion control by the disturbance observer"". Prof. Ohnishi describes the physical meaning of motion control and the purpose of robust control. This review paper also shows the effectiveness of motion control based on the disturbance observer. Four papers in this issue deal with robot motion control systems using the disturbance observer. Mr. Oda explains the decoupling force control method of redundant robot manipulation by workspace disturbance observer which is not a joint space disturbance observer such as an ordinary disturbance observer. Dr. Komada explains the hybrid position/force control method based on second derivatives of position and force, which uses the force-based disturbance observer. Dr. Shimada explains the servo system considering a robot of low stiffness, which is based on the disturbance and velocity observer. This observer is mounted with each joint. Prof. Kuroe explains the decoupling control method of robot manipulation using a variable structure disturbance observer which is not an ordinary linear disturbance observer. The other three papers in this issue deal with robot motion control using the other advanced control system. Prof. Ohishi, myself explains the hybrid position/force control method without a force sensor, which is based on H00 acceleration controller and torque observer. This torque observer is the same observer as the ordinary disturbance observer. Mr. Fujimoto explains the three dimensional digital simulation of legged robots for advanced motion control. Mr. Kang explains the state estimation for mobile robots using a partially observable Markov decision process. This method can estimate the mobile robot state precisely and robustly. The two news reports in this issue deal with control and robot laboratory news from Japanese universities such as news generated by Prof. Hori of the University of Tokyo and Prof. Hori of Mie University. Both Prof. Horis are famous and active researchers in advanced motion control. This issue scans only one aspect of control systems, not the whole. Adaptive control, learning control, and other advanced control methods such as the LMI method are not mentioned. The subject of control systems in mechatronics is now expanding and developing. I greatly appreciate the efforts of the reviewers and authors in producing this issue, and I thank the Chief-Editor, Prof. Toshio Fukuda, for encouraging us to prepare it.
Discrete-Time Analysis and Synthesis of Disturbance Observer-based Robust Force Control Systems
