Selection of controller in Plant

In process industry generally there are three basic controller modes- proportional controller, integral or reset controller and derivative or rate controller. There are no rules written that at what time or place which controller should be used but there are some specific guidelines to look at which controller can be used to eliminate the error in the process.

Proportional control:  The proportional mode of control is  generally included in all feedback control system in process industries. Because its quick responsive nature for the error, it activates the controller as soon as it detects an error and begins its corrective action. A feedback controller with only proportional action can be used in plan, because only one proportional control setting is required to tune the controller and this controller is simplest and easy. But proportional only controller has a built in defect called offset that severely limits its use in process industries.

Consider the use of proportional controller to control the liquid level in the tank and if any change in the inlet flow change the liquid level. But the proportional only controller can not bring the liquid level to the required set-point. The sustained difference between the set-point and measurement at steady state is called “offset”.

Why does offset occur?  

 The proportional controller

manipulated variable= Mo + kc(error)

where             Mo =  Manual mode

                      Kc= Proportional constant

By the above equation it is illustrated that at zero error the only possible output from the controller is Mo ( Manual mode output). Whenever a set-point changes more or less the manipulated variable should change to meet the set-point. from the above equation, the only way to have any other controller output is for the error to be something other than zero. i.e there should be steady state error to make the controlled output in new steady state condition.

Offset error at steady state

In most industrial applications offset is a fatal error. In controlling steam boiler pressure control, it is mandatory keep the pressure at the set-point (X). A pressure of X+10 or X-10 is not at all good for the safety of boiler and plant i.e the offset cannot be tolerated in this condition. But in some cases like preventing overflow or draining of the tank, the proportional only  control can be used with the offset. For these exceptional cases called averaging control offset is of no concern.

How to eliminate the offset?

To eliminate the offset, an operator takes the controller in manual mode and adjusts the manual mode output to reset the offset error and again it can be switched back to the automatic. i second method, a facility is incorporated in latest controllers to adjust the manual output without switching back to automatic to manual mode. In these two methods the operator intervention is required where there is scope of error or misuse. To eliminate this later a wonderful solution is  incorporated in control system to automatically resets the error i.e called reset or  integral mode.

Proportional and integral controller

The integral mode, normally used in conjunction with the proportional controller which is called as proportional and integral (PI) controller. In the process industry almost 70-90 % controllers are PI controllers based on its control strategies.The integral term acts as a summer circuit that looks at error signal and adjusts the controller output until error reaches zero. As a result, there cannot be any offset error when integral mode is used. The integral mode adds to or subtracts from the manual output as the operator does it to reduce the offset error at steady state. The addition of integral term reduces the steady steady error, however it also adds a time constant to the system which provokes the oscillations in system.

Due to its dynamic characteristics  of integral mode, of course requires a time for controller output but it helps in reducing the noise in flow control applications. While the proportional mode acts immediately to the flow variations and tends to pass the noise measurement to the control valve which introduces process oscillations. The integral mode (Low pass filter) can be used to filter or smooth the noise and give the gradual output instead of sudden output value. In the tuning of such flow controllers the relative amount of proportional and integral values are adjusted to get satisfactory response and smooth output signal.

The above discussion concludes that the Proportional and integral mode of control can be used for “flow control” in the process industry. But in some process that have large time constants, the use of integral mode results in accumulation of integral component because of the dead time of the process.

Proportional and derivative controller

The derivative control is used to counteract the dead time in the systems which have large time constants. Generally temperature controllers are slow to react so that use of derivative action automatically comes in mind in the concept of temperature controller. Other slow processes, however may also benefited by the use of derivative mode. The derivative action exerts a stabilizing influence that allows larger amount of proportional and integral action. However the advantages of derivative controller for slow process it should use with proportional action which increases the dynamics of process.

Finally the derivative action is not without problems that may arise in control system. First, it is very sensitive to noise in the measurement signal so filtering (adding integral component) of input signal must be added in some applications like flow control applications.  Second, the derivative action is also sensitive to rapid change in set point and this can be handled by choosing the derivative action based on measurement signal instead of error changes. Third is tuning problem is so complicated in derivative controller to obtain a satisfactory performance because of its sensitive nature to noise.  In addition, using proportional action with derivative introduces a constant offset which has to be reset by the operator or automatically by introducing integral mode.

PID controller

The problems arising in the above controllers can be minimized by combining the three control components which is called Proportional-Integral-derivative (PID) controller. The use of PID controller is advantageous over the above controllers that the lag arisen by the integral mode can be cancelled by the derivative action. The integral mode also acts as filtering function for input noise in signal measurement. But  the selection of proportional gain, integral and derivative time constants for obtaining the optimized performance in the process control. It is not hot so hard to find one constant like proportional controller compared to PID controller. It is somewhat more difficult to find the two factors like proportional and integral constants for better performance. It can be extremely difficult to find the three settings in PID controller. Trial and error method is the best to find the best matching constants with experience. However the tuning techniques available for controller tuning techniques can reduce the trial and error method to two or three trails.  

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