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Introduction
Published in Andrew Greasley, Simulation Modelling, 2023
SD is an approach that attempts to understand the world as a system. The method was originally developed by Professor Jay Forrester when it was known as industrial dynamics. SD attempts to describe systems in terms of feedback and delays. Negative feedback loops provide a control mechanism that compares the output of a system against a target and adjusts the input to eliminate the difference. Instead of reducing this variance between actual output and target output, positive feedback adds the variance to the output value and thus increases the overall variance. Most systems consist of a number of positive and negative feedback cycles, which make them difficult to understand. Adding to this dynamic complexity is the time delay that will occur between the identification of the variation and action taken to eliminate it and the taking of that action and its effect on output. What often occurs is a cycle of overshooting and undershooting the target value until the variance is eliminated.
Operational Amplifiers and Negative Feedback
Published in D V Bugg, Electronics:, 2021
For negative feedback, there is an important simplification if BG ≫ 1. Then the 1 in the denominator can be neglected and () A≃1/B. (This result assumes the output does not saturate.) The great virtue of this arrangement is that amplification is independent of G, which can vary from one batch of amplifiers to another. If G = 105 and B = 0.01, then A ≃ 100. The exact solution (7.3a) gives A = 99.90 if G = 105 and 99.95 if G = 2 × 105, showing that a change of G by a factor 2 alters the circuit amplification A by only 0.05 %. Since B is derived from a pair of resistors, it is very stable and independent of frequency. Achieving this degree of stability is otherwise virtually impossible. The use of negative feedback makes circuit performance insensitive to the amplifier. It is a general characteristic of negative feedback that it enhances stability. We shall see later that it has other virtues too.
Control of Movement and Posture
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The negative feedback system works very well as long as the loop gain is large and delays around the loop are negligibly small. Under certain conditions, however, the negative feedback system could show instability. Suppose, for example, that the loop gain is high, but there is appreciable delay in the feedback path. If the desired movement suddenly increases to a new steady value, for example, the change in the sensed movement will be delayed. In the meantime, the error will be relatively large and positive, causing an overcorrection in the actual movement. This overcorrection will, after a certain delay, cause the error to go negative, so the actual movement will swing in the opposite direction, which, after a delay, will give rise to a large positive error, and so on. Theoretically, there will be oscillations of increasing amplitude. In practice, the amplitude of oscillations will be limited by system nonlinearities, or, if there is sufficient damping in the system, the actual movement will oscillate with decreasing amplitude about the new steady value before settling to this value. This is reminiscent of the intention tremor observed in some cerebellar disorders (Section 12.2.4.5). The oscillatory behavior could be eliminated by reducing the loop gain, at the expense of less accurate tracking, or by executing very slow movements, which severely limits the type of movement that can be executed.
Abrupt change of process behavior: The Anderson-Darling detection tool
Published in Quality Engineering, 2018
In order to produce a quality product a manufacturing process must be stable, i.e., its response parameters must have a constant distribution over time. Stability of any technological process is ensured by the dominance of stabilizing factors over factors tending to destabilize it. Stabilizing devices, such as thermostats, for example, usually use negative feedback in order to achieve sustainable balance. Process control, inter alia, also aims to keep destabilizing, destructive factors under control by not allowing them to exceed a certain threshold. A classic example of abrupt loss of stability in a process control scenario is buckling, caused by compressive stress exceeding the ultimate compressive stress that the material is capable of withstanding. Imagine, for instance a rod, initially straight, with a 50/50 chance of yielding. When too much stress is applied, this rod will bend to either the left or right (Figure 1).
Identifying leverage points to transition dysfunctional irrigation schemes towards complex adaptive systems
Published in International Journal of Water Resources Development, 2020
André F. van Rooyen, Martin Moyo, Henning Bjornlund, Thabani Dube, Karen Parry, Richard Stirzaker
Negative feedback acts as a control to balance the growth resulting from positive feedback and helps keep system’s states within safe limits. These could be good places to leverage change, but they are often ignored because they are difficult to implement. In many small-scale irrigation schemes there are no negative feedback mechanisms controlling the amount of water going to a crop. The assumption is that the more you irrigate, the better the crop will grow. Negative feedback develops when farmers reduce irrigation frequency knowing that excess water leaches expensive nutrients.