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The principle of automatic control

Steady state is rare and, in General, the design capacity of the system will exceed the load, because the load is constantly changing. State support in the conditioned space, do not remain constant, if the capacity of the plant is uncontrolled.

As an example, consider the Plenum ventilation system shown in Fig. 13.1. When the outside air temperature rises above the design winter value fabric heat losses will diminish, and the heater battery capacity is too great, with a subsequent rise in temperature. Keep it nominally constant value four things to happen. First, it should be measured. Secondly, the change of temperature should be used to send a signal to the heater battery. Third, the strength of the signal, you must make the corresponding changes in battery output. Finally, the time spent on all this to happen should not be so long that further changes loads occur at the same time, battery output becomes significantly the load phase.

Please note that the change of temperature to be measured if the battery capacity will be changed, meaning that the temperature may not be permanent.

Such a deviation is inherent in control systems but sustainable deviation (offset)can be prevented, over time, in certain sophisticated systems described next. There are several methods adopted for signal transmission, but this power must be associated with the measured deviation so that the capacity of the regulating device battery can assess their reaction to it.

The principle of using the observed deviation in the load give corrective in response capacities called " negative feedback " and management system, using this principle, called " closed loop". Another form of control is the system of 'open loop", which do not use negative reviews, but instead regulates the power in advance way. For example, as the fabric of heat loss is proportional to the difference between the inside and outside temperature, as well heater battery power depends on the inflow of hot water temperature may decrease the value of the latter as the outside temperature rises, according to the estimated schedule. Room temperature, then can be maintained at a constant level, actually having measured. This is compensated control) works as an open loop. Open loop system, as a rule, contains subsystems, which are themselves closed in order to work properly...

 
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