Control System Strategies
The
purpose of a process control system is to maintain some process variable at the
setpoint. The configuration and complexity of process control system depend on
the characteristics of process under control.
Single Variable Process Control:
It
is the most elementary type process control. In this strategy there is only a
single variable. The variable is measured directly and maintained at the
setpoint regardless of any other process variables. It is the most simplest and
cheapest process control strategy.
In the
above example, there is process tank. Water is fed through an inlet tube. Water
is taken out through an outlet tube. There is a flow controller to maintain
constant flow through the inlet tube. The water inside the tank is heated.
There is a temperature control system to maintain the tank temperature.
Consider
the flow control system. The flow is measured directly and maintained at the
setpoint with the help of a flow controller. The flow is not affected by any
other control loops.
Independent Single Variable Control:
In certain processes, the control of one variable may be effected by the manipulation of some other variable. In this case the controlled variable is called the dependent variable and the manipulated variable is also known as the independent variable. The controlled variable depends on the independent variable.
In the
example, there is a process tank in which water is collected. Water is fed in
to the tank through an inlet tube. There is an outlet tube to carry water out
of the tank. There is a level sensor in the tank. The level controller
maintains the water level by manipulating the inlet flow. Here the water level
in the tank depends on the inlet flow rate.
Interactive variable control system:
In certain processes, there may be more than one control loops. In certain cases, the action of a control loop may affect the other loops. The loops may interact. In this case the controller design becomes complicated. The corrective action of one loop disturbs the other loop
In the
above example, there is process tank. Water is fed through an inlet tube. Water
is taken out through an outlet tube. There is a flow controller to maintain
constant flow through the inlet tube. The water inside the tank is heated.
There is a temperature control system to maintain the tank temperature.
Under nominal conditions, the
flow in to the tank is held constant and the temperature is also held constant.
If the setpoint of the flow control system changes, the flow control loop
adopts a new flow rate, which will appear as a load change to the temperature
control system.
Compound Variable Control System:
The word compound means
combination of more than one variable. In compound variable control system,
more than one process variables are measured. The variables are combined and
used to maintain a single variable under control. Ratio control system is an
example of compound control system.
In
this example the control system maintains the ratio of two reactants A and B. One of the flow rates is measured but
allowed to float (that is, not regulated), and the other is both measured and
adjusted to provide the specified constant ratio. The flow rate of reactant A
is measured and added, with appropriate scaling, to the measurement of flow rate
B. The controller reacts to the resulting input signal by adjustment of the control
valve in the reactant B input line.
Ref: Curtis Johnson- Process Control Instrumentation Technology ( pages 558 to 560)
