Is Ratio control a kind of Cascade or Feedforward control

A

Thread Starter

Anonymous

Is ratio control considered as a type of feedforward control or cascade control. I personally think that it is a kind of cascade control because the output from one controller sets the setpoint of another controller. However some technical text groups ratio control under feedforward. Can anyone tell me which of these claims are true and Why??
 
G

G.Navaneethan

It is necessary to distinguish the terms
1. Ratio Control
2. Cascade Control
3. feed forward control

If you are familiar with Boiler control system, it is simple.

Ratio Controller.
Liquid fuel is sent to the burners. This is measured by Fuelflow transmitter and controlled by
fuel control Valve. simultaneously Combustion air is sent to combustion chamber. This is measured by
airflow transmitter and controlled by suction damper. Practically you can evaluate for each fuel flow rate, the exact amount of combustion air needed and find the exact ratio. Fuel flow transmitter signal will be the input of the ratio relay(not Controller) and its output is the needed combustion air. This calculated value will be the set point of the airflow Controller.

Feedforward Cotroller
When there is a boiler load change, Steam flow senses first, (before all signals). This follwed up with falling/raising drum pressure and so on. Steam temperature change will follow last slowly. Sensing the steam flow first and anticipate other changes will follow in succession, Control the temperature in advance. (Using anticipatory Relay)

Cascade Control
Drum level is controlled by feed water valve. In high pressure boilers, while increasing the load, Drum pressure falls. Due to swelling action, momentarily the drum level rises first And then falls rapidly. (This gives a wrong signal to level controller to reduce feed, but in reality it is the other way). So drum level will not be connected dierctly to feed water valve. Instead, steam flow will be the setpoint of feed water flow controller. Drum level Changes are effected through drum level controller and its outputs are summed with Steam flow, with suitable bias or suppression. When drum level is normal, feed flow = steam flow.
 
Y

Y.K.JARIWALA

Ratio control can be only treated as feedforward
loop only if one variable is controlled with
respect to other flow, but this is called wild
feedforward loop. True ratio control is when
both streams are controlled in feedback mode, ie
closed ratio control loop , but can not be treated as feedforward loop.
Ratio is involved when stoichiometric ratio is
involved which is also availble in boilers.fuel oil ratio.

Feedforward loop is eg , tripple element drum
level is
Material in - material out = material accumulated
water flow - steam flow = Ax[DL/DT]
WATER IN = LIC + STEAM FLOW

IT IS STEAM FLOW WHICH IS FEEDFORWARD SIGNAL
to LIC [ BOILER DRUM LEVEL ] to set demand for
water flow controller.
most sensitive variable in boiler is Pressure.

In above example it is water flow controller is
in cascade mode [Slave loop ]

I hope to have clarified your doubt

Jari
[email protected]
 
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