Answer / sam

There is only two modes of control if you really think about it.
On/Off Control and PID Control.

Where some confusion can come in, is when you look at he
various CONTROLLER modes available on a DCS.
PID controllers are also sometimes linked together like in
cascade or feed forward control configuration, but the
control modes is still PID control.

Look at On/Off control:
You can have a open/close control valve in the field that is
controlled by the ESD or DCS system that will either open or
close this valve.
The ESD system do not do any control but it can open or
close a valve during a emergency as a step in it's blow down
or shut down sequence. This is in general not consider as
control just a action.
You can also have another open/close control valve in the
field that is controlling the level in a tank. In this case
it is connected to a Decap controller in the DCS. a Dcap
controller have a set-point that can be set to control the
level of the tank, but if a on/off valve is used as is, this
valve will open and close all the time to try and keep the
level on the set-point. So to prevent this the controller
have a small dead band around the set-point that can be
adjusted. This will then prevent the valve from opening and
closing until it reaches the dead-band threshold position.
Normally about 5% over and under the set-point. It is still
on/off control.

Looking at PID control:
Any mode in conjunction with proportional control is
possible on a PID controller it just depends on the
application. For example PID, PI, PD, P.
You can see them each as individual control modes if you
want, but I prefer to just stick to PID or on/off control
modes since there is a definite and distinct control mode
difference between the two. I see Dcap, PI, PD and P modes
as sub divisions on the two main control modes.
P = Proportional
I = Integral
D = Derivative
Good Luck

Answer / suraj

There are Two basic mode ON/OFF mode and PID control.
but in PID mode there are also three mode
ONE MODE
P=proportional
D=derivative
I=intrgral
TWO MODE
PI=proportional+derivative
THREE MODE
PID=proportional derivative intrgral

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