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 转帖: 扫盲,关于系统稳定和负反馈常识的简单介绍! |
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作者：Anonymous 在 罕见奇谈 发贴, 来自 http://www.hjclub.org
为了让没学过系统控制论或是学过系统控制论但没学好的人扫盲关于系统控制中一些最基本的常识,比如: 什末叫稳定系统,什末叫负反馈,系统稳定的必要条件是什末,... 我特从网上摘下一篇最浅显的文章,供各位短时间恶补一下这方面的知识. 其中重要部分我做了点翻译,欢迎讨论!
由于本坛是个公共场所,按照美国法律是禁止赌博的,凡有赌博这种不良嗜好的人请去拉斯维加斯去赌.任何对本题目的争议,都仅以对控制理论的理解和知识为辩论的胜负!无知还无理狡赖的人请自重!
下面是原文:
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Stability and Instability (稳定和不稳定)
Dynamic systems are said to be stable if their variables return to, or towards, their original states following disturbances(一个动态系统被认为是稳定的,如果在扰动下的变量趋于回复到它们初始的状态). For example, the temperature of a room is stable because, when a door is opened (= exogenous disturbance), the thermostatic control system returns the room to its previous temperature. Similarly, the sycamore aphid population also appears to be stable because, although it fluctuates considerably, it always returns towards its mean density following a disturbance. Hence, stable systems tend to persist in a state of balance in variable environments. They are said to be homeostatic or regulated.
Negative feedbacks(负反馈) generally cause variables to return towards their original values and, therefore, they tend to act as stabilizing forces in dynamic systems(负反馈总体来说是系统中趋于将变量回复它们的初始状态,所以它们在动态系统中是一个稳定力). Hence, negative feedbacks tend to lead, with time, to equilibrium or balance in mechanical and ecological systems; i.e., they regulate or control the variables. However, although negative feedback is a necessary condition for stability(尽管,负反馈是系统稳定的必要条件), it is not sufficient to ensure stability(但它并不充分). To guarantee stability, the negative feedback must act rapidly and gently, otherwise the variables may oscillate to varying degrees around their equilibrium points.
Delays in the action of negative feedback processes are usually caused by the order of the feedback loop (how many dynamic variables are involved in the loop). Hence, it generally takes more time for the signal to pass through long -feedback loops involving many variables and, for this reason, systems with many variables are usually less stable than ones with few variables. Instabilities due to time delays in the feedback structure are usually manifested as cycles of increasing period and amplitude. Hence, the extreme 9-10 year cycles of the larch budmoth are probably due to feedback involving the budmoth and one or more other species in the community, species that are affected by budmoth numbers and which also affect budmoth numbers directly or indirectly.
Dynamic systems are said to be unstable if their variables continue to move away from their original states following a disturbance. The human population, for instance, is currently exhibiting unstable dynamics because it is increasing continuously, as may be the collapsing blue whale population. One of the main causes of instability in dynamics is positive feedback. In contrast to stabilizing negative feedback, +feedback accentuates or amplifies changes in state and is the force behind population explosions, inflation spirals, arms races, and organic evolution. Positive feedback can also create unstable breakpoints or thresholds in dynamic systems.
The feedback structure of dynamic systems determines its stability properties, which, in turn, have a dominant influence on the dynamic patterns and regularities we observe in the system. Because of this, it is important that we understand how feedback loops are created in ecological systems, and how to detect and manipulate those feedback loops to produce stable, self-sustaining systems.
The concepts of stability and instability can be illustrated by reference to a ball resting on different landscapes (see Figure)
A = Stable equilibrium. The ball moves back towards its equilibrium position when disturbed. If the valley is infinitely deep the system is said to be globally stable.
B = Unstable equilibrium. The ball moves away from its equilibrium when disturbed.
C = Neutrally stable equilibrium. The ball stays wherever it is placed.
D = Metastable equilibrium. The ball returns towards equilibrium as long as disturbances are not too large. The equilibrium is not globally stable because once the ball has crossed the unstable equilibrium (the peak), it is unlikely to return to the stable point.
E = Metastable equilibria with multiple stable states. The ball returns towards one or the other equilibrium depending on which side of the unstable equilibrium it is. The system is globally stable.
作者：Anonymous 在 罕见奇谈 发贴, 来自 http://www.hjclub.org |
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