The Logic Behind The Magic. “Control Function Block "

Process quantities (level, flow, pressure, ..etc) in plant need to be maintained at target value and a difference from these values could result in a dangerous condition arising or loss production.

To maintain these process quantities condition over an extended period of time without any operator involvement. Automatic control system is usually installed and used.

The element of these control system (loop) are:

1. Transmitter: Process variable is measured by the primary element of the transmitter and the measured signal is converted to electric 4 to 20 mA signal.
2. Controller: Controller receive a signal from the transmitter and compares this value with the set point and computes the output signal to achieved the set point.
3. Final Control Element: Correction device in the control loop. Usually a valve

General Control Loop Algorithm.for Automatic Control System

1. Manual loader (open loop system).

This Control function merely allows the operator to manually operater the valve or actuator instrument that is receiving the signal from it using Analog output point (AO).

OPDIR + Defines the direct /  reserve action of the Anaolg output

     2. Simple PID Control Loop

PID Control loop consist of:

-          An analog input tag that serves as receiving input su\ignal from the field instrument

-          An analog output tag that serves as sending output signal tp valve or actuator

-          An PID regulatory control tag that performs the control based on the configuration paramenters (PID)

 

CRLACTN = Define the direct / reserve action of this algorithm’s output.

Range : 0 – Direct (PV increase, output increase)

             1 – Reserve (PV increase, output decrease)

PVEUHI = PV high range in engineering unit

PVEULO = PV low range in engineering unit

      3. Simple loop with selector

 

Switch over is performed manually by operator with selector

      4. Gap Control Loop

Gap Control works in the principle of two-user definenable control regions.

The first region is far from set point (outside the gap) and requires a strong control action to turn the process around and bring it back to set point.  The normal controller settings are used outside the gap. The second region is close to the set point (inside the gap) within which controller detunes itself based on a configurable gain multiplier M (between 0.0 to 0.1) and devide the integral time by M for integral processes.

       5. Split Output Control Loop

Split control loop output consist of :

1. Several regulatory control tags (PID, AutoMan)
2. Several analog output tags to send output signals to valves

AutoMan function block serve the purpose of:

1. an intermediate tag to transfer control signal from the main controller to analog output
2. ensuring bumpless transfer of output signal from the main controller to the analog output point when recovery of failed analog output signal occurred.
3. Allowing the operator to manually operate th evalue individually when necessary.

        6. Ratio Control Loop

This is an algorithm that typically used in the control of the flow/ pressure of a gas or liquid as a ration of another flow/pressure.

1.      Calculator PV algorithm is used to calculate the actual ratio achieved, for display and printing.

2.      The operator is allowed to change the SP

 7. Cascade Control Loop

Cascade Control Loop consist of:

a.       Several analog input tags that serves as receiving input signal from field transmitter

b.      Several analog output tags that serves as sending output signal to actuator such as control valve

c.       Several PID reguratory Control Tags that performs the control based on the configured parameters (PID etc)