Response Electronics CL6011B Especificações descarregar pdf (Página 271)

5Ć9Configuration Tips

Original Ć June 1990 CE4.2:CL6211

5.5 Zero Dropout on Analog Inputs

Due to the accuracy limitations of certain field devices it is often desirable

to force an analog input value to zero when it drops below a certain

point. In the case of flow transmitter inputs where square root is used, a

small offset in the calibration of the field device may cause a significant

flow indication when there is actually zero flow. These offset indications

may cause inaccuracies in accumulations, and distraction for the system

operators.

In the computing and interactive controllers, this situation is may be

remedied by using the following FST code segment:

AINSQR(1, FI–1 ), {Read Input; use proper tag name

for E.U. conversion}

LSM( DROPOUT ), {Detect lower value than trip point}

TFR( ZERO ,ACCUM), {Switch in Zero if flow below

trip point}

After these instructions, the SVA will either contain 0.0 if the input is

below the trip point, or the actual flow. The register DROPOUT should

be configured as a reference register with initial value enabled. The initial

analog value of DROPOUT should be set to the desired trip point. The

analog portion of register DROPOUT may be included in a register DDP

list if it desired to make the trip point tunable. Note that any input failure

monitoring FST instructions should be placed between the AINSQR and

LSM instructions.

5.6 Loop Mode FST Functions

The loop mode functions perform the operating mode changes and

mode status checking for the controller.

There is an interaction between the IF TRUE SET functions and

commands from an operator station or the data highway. Commands for

mode changes from an operator station or data highway are entered into

the proper direct control unit at the end of the FST execution. This

ensures that all loops will have the same data base during the FST

execution. The IF TRUE SET functions will set the mode of the loop at

the time of execution of that function block. For example, if the IFTMAN

function has a logic high SVD input, it will immediately change the mode

of the currently executing loop. If this function has a logic high input and

is placed before the CNTRL function, then that loop is put in the manual

mode even if commands sent by the operator station or data highway

are to the contrary. This type of setup can lock out changes in the

control mode by an operator station or data highway.

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Response Electronics CL6011B Especificações Página 271