Process Control System characteristics

By Youssef Edward
February 24, 2019

Process control refers to controlling a certain variable in
a process plan. The set of variables in the process all contribute to the
control of the controlled variable. If one of variables affecting the controlled
variable is changed, the controlled variable will also change. The set of
variables for a given setpoint refers to the process load.

Process Equation

The process equation is the relationship between the
controlled variable and the parameters affecting that variable in the
surrounding environment. For instance, consider a tank system having inlet and
outlet for water flow. Suppose we have to maintain the temperature of the water
to a specific point. We may list the parameters affecting the temperature value
inside the tank as:

  1. Input flow rate.
  2. Output flow rate.
  3. Stem temperature.
  4. Inlet water temperature.
  5. Ambient temperature.

If any one of those parameters change, then the inside temperature
will change in turn. To regulate the temperature inside the tank, we have to
take any one of those variable as the controlling variable. The controlling
variable is the one that controls the final element in the process.

Note that to maintain control, we don’t need the
relationship exactly, we can make control without knowing the relationship.
More important we need to know whether the relationship is proportional or not.

Process Load

Regarding a certain setpoint, the controlling parameters
will be known as nominal values from the nominal relationship of the process.
Those set of controlling parameters are called the process load. If the
setpoint is changed, then the process load will change.

Process Lag

Process lag refers to the amount of time the final control
element takes to respond to environment changes and make the actual control.
This is a function of the process nature and not the controller itself. In
other words, the controller may be so fast, but the actual time for the
controlled variable response is high compared to the response time of the

For instance, in the above example, if the input flow rate
is suddenly changed, the stem valve will change. The time it takes to open or
close may be in the range of 1 or 2 seconds while the control loop response may
be in the range of 1 or 2  ms.  This difference is too high. It will not be a
big advantage to design a fast controller while there is a big delay in the
final control element.


if a sudden change occurs in the controlling parameter, a
big delay may occur before the controlled variable reaches the exact setpoint.
This is due to the process control lag. That is, the final control element
response may be large. Also this is due to the nature of the process.

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