Positional feedback

Figure Bl.19.12. Basic principles of SECM. (a) With ultramicroelectrode (UME) far from substrate, diflfiision leads to a steady-state current, ij, (b) UME near an insulating substrate. Flindered diflhision leads to < ij, 3D. (c) UME near a conductive substrate. Positive feedback leads to go. (Taken from [62],...

To make an oscillator from an amplifier requires, in the language of electronics, positive feedback. In lasers this is provided by the active medium being between two mirrors, both of them highly reflecting but one rather less so in order to allow some of the stimulated radiation to leak out and form the laser beam. The region bounded by the mirrors is called the laser cavity. Various mirror systems are used but that shown in Figure 9.1, consisting of... 

At still higher fields carriers can acquke enough energy from motion in an electric field to create electron—hole paks by impact ionization. Eor siUcon the electron ioniza tion rate, which is the number of paks generated per cm of electron travel, depends exponentially on electric field. It is about 2 X 10 cm for a 50 kV/cm field at 300 K. The electric field causes electrons and holes so created to travel in opposite dkections. They may create other electron—hole paks causing positive feedback, which leads to avalanche breakdown at sufficiently high fields. 

Feedback Control In a feedback control loop, the controlled variable is compared to the set point R, with the difference, deviation, or error e acted upon by the controller to move m in such a way as to minimize the error. This ac tion is specifically negative feedback, in that an increase in deviation moves m so as to decrease the deviation. (Positive feedback would cause the deviation to expand rather than diminish and therefore does not regulate.) The action of the controller is selectable to allow use on process gains of both signs. 

The key parts of the positioner/actuator system, shown in Fig. 8-74 7, are (1) an input-conversion network, (2) a stem-position feedback network, (3) a summing junction, (4) an amplifier network, and (5) an actuator. 

The stem-position feedback network converts stem travel to a useful form for the summer. This block includes the feedback linkage. 

On springless actuators where the actuator is not usable for throttling control without position feedback,... 

A number of current coupled ocean-atmosphere climate models predict that the overturning of the North Atlantic may decrease somewhat under a future warmer climate.While this is not a feature that coupled models deal with well, its direct impact on the ocean s sequestration of carbon would be to cause a significant decline in the carbon that is stored in the deep water. This is a positive feedback, as oceanic carbon uptake would decline. Flowever, the expansion of area populated by the productive cool water plankton, and the associated decline... 

Note that in Figure 4.12 there is a positive feedback loop. Flence the closed-loop transfer function relating and C (.v) is... 

Positive feedback from customers shall be recorded, filed with client data, and posted on the company noticeboard... 

As more fuel is converted into combustion products per unit of volume and time, expansion flow becomes stronger. Higher flow velocities go hand in hand with more intense turbulence. This process feeds on itself that is, a positive feedback coupling comes into action. In the turbulent stage of flame propagation, a gas explosion may be described as a process of combustion-driven expansion flow with the turbulent expansion-flow structure acting as an uncontrolled positive feedback (Figure 3.2). 

Figure 3.2. Positive feedback, the basic mechanism of a gas explosion.

Most investigators used tubes open only at the end opposite the point of ignition. For tubes with very large aspect ratios (length/diameter), the positive feedback mechanism resulted in a transition to detonation for many fuels, even when the tubes were unobstructed. Introduction of obstacles into tubes reduced considerably the distance required for transition to detonation. 

The channel experiments produced results similar to those from tubes. Introduction of venting (decrease of the degree of confinement) greatly reduces effectiveness of the positive-feedback mechanism. Obstacles appear to enhance the combustion rate considerably. 

Experimental research has shown that a vapor cloud explosion can be described as a process of combustion-driven expansion flow with the turbulent structure of the flow acting as a positive feedback mechanism. Combustion, turbulence, and gas dynamics in this complicated process are closely interrelated. Computational research has explored the theoretical relations among burning speed, flame speed, combustion rates, geometry, and gas dynamics in gas explosions. 

Coordinated by the positive feedback these simple rules induce between robots and their environment, the result, over time, is a seemingly intelligent, coordinated sorting activity. Clusters of randomly distributed objects spontaneously and quite naturally emerge out of a simple set of autonomous local actions having nothing at all to do with clustering per se. 

Local and transient Ca2+ increases that propagate throughout the cytosol of individual cells in the form of waves. Ca2+ waves are generated by a positive feedback activation of Ca2+ release from the intracellular Ca2+ stores through ryanodine receptors or inositol IP3 receptors. 

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