维基百科里是这样定义系统(System)的:
System (from Latin systēma, in turn from Greek σύστημα systēma) is a set of entities, real or abstract, comprising a whole where each component interacts with or is related to at least one other component and they all serve a common objective. Any object which has no relation with any other element of the system is not part of that system but rather of the system environment. A subsystem then is a set of elements, which is a system itself, and a part of the whole system.
翻成中文大体意思就是: 系统,是一系列的真实的或者抽象的对象,以及对象之间的交互影响构成的.
维基百科中是这样定义动力学系统(Dynamical System)的:
The dynamical system concept is a mathematical formalization for any fixed "rule" which describes the time dependence of a point's position in its ambient space. The mathematical models used to describe the swinging of a clock pendulum, the flow of water in a pipe, or the number of fish each spring in a lake are examples of dynamical systems.
A dynamical system has a state determined by a collection of real numbers, or more generally by a set of points in an appropriate state space. Small changes in the state of the system correspond to small changes in the numbers. The numbers are also the coordinates of a geometrical space—a manifold. The evolution rule of the dynamical system is a fixed rule that describes what future states follow from the current state. The rule is deterministic: for a given time interval only one future state follows from the current state.
而系统动力学(System Dynamics)是这样定义的:
System Dynamics is an approach to understanding the behaviour of complex systems over time. It deals with internal feedback loops and time delays that affect the behaviour of the entire system.
系统动力学是过程导向的研究方法， 擅长于大量变量、高阶非线性系统的研究，系统中的因、果回馈关系环环相扣，例如研究世界人口、生产活动、污染、自然资源等问题的“世界动力学模式”(Forrester, 1973)、研究都市发展动态的“都市动力学模式”(Forrester, 1969)等。系统动力学应用的领域非常广泛，包含生态、经济、社会、组织、管理、环境保护等。系统动力学研究的主要贡献是对于动态系统反直觉行为的深入了解，透过行为背后的结构性原因(互动机制)来解释为何行为产生如此的变化形态；其次透过电脑的模拟提供了政策设计与学习的练习场。
系统动力学对问题的理解，是基于系统行为与内在机制间的相互紧密的依赖关系，并且透过数学模型的建立与操弄的过程而获得的，逐步发掘出产生变化形态的因、果关系，系统动力学称之为结构。所谓结构是指一组环环相扣的行动或决策规则所构成的网络，例如指导组织成员每日行动与决策的一组相互关连的准则、惯例或政策，这一组结构决定了组织行为的特性。构成系统动力学模式结构的主要元件包含下列几项，“流”(flow)、“积量”(level)、“率量” (rate)、“辅助变量”(auxiliary) (Forrester, 1961)。
控制理论(Control Theory)是这样定义的:
In engineering and mathematics, control theory deals with the behavior of dynamical systems. The desired output of a system is called the reference. When one or more output variables of a system need to follow a certain reference over time, a controller manipulates the inputs to a system to obtain the desired effect on the output of the system.
基于以上的东西,我们基本上可以知道,系统是一个由相互存在各种联系的实体构成的集合.动力学系统则是泛指系统成员之间的规则以及变化依赖于时间的变化的系统.系统动力学是用数学的办法来研究系统行为,导向的一种办法.控制理论则是用数学的办法建立规则来引导系统向我们需要的形式转化.
一个动力学系统基本上可以将它当前的输出信号看成是依赖于它当前输入信号以及历史输入信号的一个函数.这样的一个系统的控制器基本上应该是一个反馈控制器.下图是一个简单的闭环反馈控制系统:
图中P是目标系统.C是控制器.r 是参考信号(Reference), y 是输出信号(output), r 与 y 相减,就得到系统误差(Error) e. 控制器C根据e来调整输出控制信号(Control Signal) u 作为系统P的输入信号(Input Signal). 这个控制系统的目的就是让系统P的输出信号 y 尽量的和参考信号 r 一致. 这个是一个简单的单入单出(Single-In-Single-Out, SISO)系统.
http://www.xbstudio.net/blog/?tid=18
