The basic theory of a switching control system was proposed in the Soviet Union in 1950's, and thereafter the term "variable structure system (VSS)" and "sliding mode control (SMC)" have been used for such systems. In the period of 1957--1962, the plant was a linear second-order systems with feedback based on an error signal and/or its derivative and it was assumed that the feedback factor could take one of two possible values. For such systems, the behavior of such systems was studied in the phase plane by Filippov, and the invariance condition of the overall system with respect to the disturbances and uncertainties was investigated by Drazenovic.
In the 1970's, the VSS has been also studied outside the Soviet Union by translating some of works written in Russian.
Recently, the VSS has been developed as a general design scheme for a wide spectrum of system types including multi-input/multi-output (MIMO) systems, nonlinear systems, discrete models, robot manipulators, and so on. In addition, the objective of the VSS has been greatly extended to variety control problems such as stabilization, regulation, tracking, model following, and so forth.
This kind of wide application is caused by several good features of the VSS. The most attractive feature of the VSS is an invariance property against system uncertainties and external disturbances. When the VSS is used as a control method for a plant, the overall system shows the same performance under the existence of disturbances and uncertainties if the system is in the sliding mode. Therefore, the system performance is insensitive to parameter variations. Although this invariance/robustness feature is very attractive, the VSS has some week points.
Even though the VSS has an attractive property, it has two major disadvantages. The first disadvantage is that, as mentioned above, the VSS has invariance property. This property, however, can be guaranteed if the system is in the sliding mode. In other words, the output response can be affected by uncertainties and disturbances before the system goes into the sliding mode. This is called "reaching phase problem".
The other problem is "chattering phenomena". Since the VSS uses switching functions to get an invariance property under the existence of system uncertainties and disturbances, it is inevitable for the control input to show the chattering phenomena.
In order to alleviate these problems, the variable structure control design scheme for robot manipulator is proposed in this thesis. The proposed method uses virtual plant/controller concept to avoid the chattering problem, and tracking error trajectory planning is also proposed to overcome the reaching phase problem. To relax the assumption that the acceleration has to be measurable, a sliding mode observer is also proposed. For the overall system, the stability is proved and the domain of attraction is derived.
가변구조제어기는 1950년대에 구 소련에서 제안되었다. 1970년대 중반까지는 계속 소련에서 연구가 이루어져오다가 1970년대 후반에 들어서면서 서구에도 이 이론이 알려지기 시작하여 급격히 많은 연구가 이루어져왔으며 적용 분야도 다변수 시스템, 비선형 시스템, 이산 모델, 로봇 매니퓰레이터 등 굉장히 다양하고 넓다.
이러한 가변구조제어기는 매개변수 변화 및 불확실성이나 외란 등에 강인한 특성을 가지므로 많은 연구자들을 매료시켜왔으나 다음과 같은 큰 두 가지 단점을 내포하고 있었다.
첫째는 리칭 페이즈(reaching phase)이다. 이는 시스템이 슬라이딩 모드(sliding mode)에 도달하기 이전의 기간을 지칭하는 말로써, 이 구간에서는 강인한 특성을 기대하기 힘들게된다.
다른 한 가지는 채터링 현상(chattering phenomena)이다. 가변구조제어기는 그 특성상 매우 빠른 속도로 스위칭(switching)을 해야한다. 따라서, 이로 인하여 모델링 되지 않았던 동특성을 유발시킬 수도 있으며, 실제 기구부는 무한한 샘플링 주파수를 갖지 못하므로 전체 시스템이 불안정해질 수도 있는 단점이 있다.
따라서, 본 논문에서는 가변구조제어기의 큰 두 가지 단점인 '리칭 페이즈'와 '채터링 현상'을 극복하는 제어기를 제안하였다. 제안된 제어기는 리칭 페이즈를 효과적으로 제거하였으며 채터링 현상 또한 없앴다. 그러나 4장에서 제안된 제어기는 가속도 정보를 필요로 하기 때문에, 5장에서는 가변구조관측기를 이용하여 가속도 정보를 추정하므로써 전체 시스템의 안정도를 증명하였다.
또한, 제안된 제어기의 유효성을 보이기 위하여 컴퓨터 시뮬레이션 및 실험을 수행하였다.
