What is process control?
A process control is changing the process variable to the desired quantity according to the measured feedback variable from the process. A Process control system always aim to achieve a process variable at the desired quantity, so that the system is said to be in controlled
The selection of what process control mode to use in a process is the function of the characteristics of the process.
1. Process equation:
A process control loop regulates some dynamic variable in a process. This controlled variable a process parameter may depend on many other parameters and thus suffer changes from many different sources. Select that parameter as the process parameter. If the measurement of the controlled variable shows a deviation from the setpoint, then the controlling parameter is changed, which in turn changes the controlled variable.
The entire process should be described by the process equation, for each process the equation changes considering the control parameters. For example, a temperature system and its process equation is shown below:
Here the liquid temperature TL is the functions that are listed below the equation. To bring the parameter back to the setpoint, we change the steam flow rate, but it is important to include all the parameters that effect the liquid temperature.
2. Process load:
In a process, it is possible to identify a set value for the process parameter that results in a controlled variable having the setpoint value. This set of parameters is called nominal set. The required controlling variable value under this condition is the nominal value of the parameter.
If the setpoint is changed, the control parameter is altered to cause the variable to adopt this new operating point. The load is still nominal, however, because the other parameters are to assumed unchanged.
Suppose one of the parameters change from nominal, causing a corresponding shift in the controlled variable. Then there is process load change is occurred. The controlling variable is adjusted to compensate for this load change and its effect on the dynamic variable to bring it back to the setpoint.
Another type of change involves a temporary variation of one of the load parameters. After this excursion, the parameter return to the nominal value. This variable is called transient. A transient is not a load change because it is not permanent.
3. Process lag:
Process control operation is essentially a time-variation problem. At some point in time, a process load change or transient causes a change in the controlled variable. The process-control loop responds to ensure that, some finite time later, the variable return to the setpoint value. Part of this time is consumed by the process itself and is called the process lag.
A significant characteristic of some processes is the tendency to adopt a specific value of the controlled variable for a nominal load for no controlled operations. The controlled operation may be significantly affected by such self-regulation.