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AUTO SELECT

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WHAT IS AUTO SELECT?

Auto selector sometimes also known

as override control. One controller canoverride a manipulated variable from

another controller when operatingconditions exceed the limit or

constraint specified.

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WHY DO WE USE AUTO

SELECT?

It is used to choose appropriatecontrolled variables for a specific

reason. Automatic selector can choose

the highest or the lowest or the median

from two or more controlled variables.

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WHEN DO WE USE AUTO

SELECT?

Selectors facilitate on-line modification ofcontrol strategies as a function of changing

operating conditions without disturbing theprocess. The typical used are:

Protection of equipment

Auctioneering

Redundant Instrumentation Variable structuring or median select.

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PROTECTION OF

EQUIPMENT

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As shown in Figure 9.7, there are twocontrollers (i.e TIC1 and TIC2) producing

two controller output signals (MV1 andMV2) connecting to the auto selector. Oneof the output controller signals is selectedto the final control element (TCY) based onwhich one is the lowest. Under normalcondition, it is designed that the selector

selects MV 1 based on measurement doneby TE1. However, if there is a disturbance(air leak) the heater become too hot due tothe deviation measured at TE1. TIC 1 willproduces high signal of MV1 and theselector will switch to MV2. At thismoment TIC2 will overrides TIC1 incontrolling the heater. Hence it protectsheater from burn out due to large signalprovided by MV1.

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AUCTIONEERING

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Auctioneering describes theapplication of an auto-selector

where multiple inputs of a similarnature are polled and one is

selected as the highest or lowest.Figure 9.8 shows a reactor where

the process is to controlled basedon the highest temperature.

Coolant will be supplied to theprocess to modify the reaction

based on the highest temperature. Ifone controlled variable is used, itmay not necessary indicate thehottest section of the reactor.

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REDUNDANT

INSTRUMENTATION

Redundant instrumentation is a

common application of auto-selector where a high or lowselector is used to choose a

functioning measurement in the

event of a measurement failure. Asshown in Figure 9.8, if one of thetransmitter fails, the auto select will

refers to the highest transmitterreading

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VARIABLE STRUCTURING

OR MEDIAN SELECT

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A median selector is an arrangementwhere high and low selectors are used to

detect the median/middle of a series ofsignals. This is very useful system whenthe measurement output to a controller, ifdriven to the highest or lowest extreme,

could cause disruption of the process. Theplant/equipment is then protected againstan individual instrument extremely driven

in either direction.

Fig. 9.9 is a diagram of a typical chemicalprocess application of median-select. Theconfiguration below allows only median

temperature signal pass through to thetemperature controller.

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ADVANTAGES Multiple signals from transmitter can bemanipulated to control a loop with highersensitivity (high-low select) or by taken

the average/median of the signalsmeasured (median select).

In the event of instrument failure,redundant instrumentation play importantrole to avoid disruption in the process

In the case of override control, it canprotects against exceeding equipmentoperating condition.

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