Advanced process control schemes and its design

1. ,
Advanced Control
Schemes

2. •In general, not all processes can be neatly characterized by
first-order lags. In some cases, a process will produce a
response curve like that shown in Figure 9.1.
In this process, the maximum rate of change for the output
does not occur at time zero (to) but at some later time (t1).
This is called dead time in process control: the period of time
(td) that elapses between the moment a change is introduced
into a process and the moment the output of the process begins
to change.
Dead time is shown in Figure 9.1 as the time between t1 and
to , or td = t1 - to.
Dead Time Lag:

3.  Dead time is the most difficult condition to
overcome in
process control.
 During dead time, there is no process response
and therefore
no information available to initiate corrective action.
 To illustrate the concept of dead time, consider
the temperature feedback control system shown
in Figure 9.2.

4. Effect of Dead-Time(Time Delay)
 Process with large dead time (relative to the time
constant of the process) are difficult to control by
pure feedback alone:
o Effect of disturbances is not seen by controller for
a while.
o Effect of control action is not seen at the output
for a while.
This causes controller to take additional
compensation unnecessary.
o This results in a loop that has inherently built in
limitations to
control.

5. Causes of Dead-Time:
 Transportation lag (long pipelines)
 Sampling downstream of the process
 Slow measuring device: GC
 Large number of first-order time constants in
series (e.g. distillation column)
 Sampling delays introduced by computer control

6. Multivariable Loops:
 Multivariable loops are control loops in which a primary
controller
controls one process variable by sending signals to a
controller of
a different loop that impacts the process variable of the
primary
loop.
 For example, the primary process variable may be the
temperature of the fluid in a tank that is heated by a
steam jacket (a pressurized steam chamber surrounding
the tank).
 To control the primary variable (temperature), the
primary (master) controller signals the secondary (slave)
controller that is controlling steam pressure.
 The primary controller will manipulate the setpoint of the
secondary controller to maintain the setpoint
temperature of the primary process variable.

7. Advanced control loops
 These are:
 Feedforward
 Ratio
 Cascade
 Override
 Selective control
 Batch control
 Split range control etc.,

8. The Smith Predictor:

9. Control with Smith Predictor:
(Kc=1,Ki=1,Kd=1):

10. Selective Control:
 For every controlled variable, there must be at
least one
manipulated variable. In some applications

11. Selective Control - Example: