Feedback loops feed-down

The behaviour of clock-gene products in the SCN provides strong support to the current model of the circadian clockwork as an autonomous intracellular mechanism based upon interlocked feedback and feed-forward loops. Intercellular signalling, more specifically convergent mechanisms for up- and down-regulation of Per expression, is critical to circadian entrainment, which can now be explained in mechanistic terms rather than described by formalisms. 

The furnace is heated by low voltage (usually 10 V) and high current (up to 500 A) from a well stabilized step-down transformer. For optimum precision, the voltage should be well stabilized, often by a feedback loop which may be temperature feed-back based (see Section 3.6.1). A rapid rise-time of the temperature is also preferable, because of theoretical considerations of peak shapes. This has implications for power supply design and furnace design, as will be shown below. Currently, furnaces are available that reach temperatures of up to 3000°C, and temperatures of 2500°C should be reached in less than 2 s in a well designed furnace. 

Rippin and Lamb point out that if we provide a perfect feedforward control system to adjust Lr so that feed changes in flow and composition do not change jVr, then the transmission Xr s) is broken and there is no feedback of Xr(s) down the column. It is possible to accomplish the same thing by making l/xp sufficiently smaller than the resonant frequency of the closed-loop composition-control system. This has been shown experimentally by Aikmam to be true of a plant column. The mathematical explanation is simple. 

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