This experiment is carried out by mixing an acidic solution of pH Essay

This experiment is carried out by mixing an acidic solution of pH 4 (diluted HCl solution) and alkali solution of pH 9 (diluted NaOH solution) in a reaction vessel in which the basic solution is manually controlled while the acidic solution is added automatically via the control valve to decrease the pH when the current measured value has a higher pH value compared to the set point (SP) pH value. Otherwise, the control valve remain closed (0% valve opening) to permit only the addition of pH 9 solution to increase the pH to the SP value.

Three different types of controller are used in this experiment, namely Proportional (P) controller, Proportional-Integral (PI) controller and Proportional-Integral-Derivative (PID) controller, in order to compare and contrast each of three types of controller. The result generated from the data logger is presented in a plotted graph in order to study the characteristic, strength and drawbacks of each of the three type of controllers.

4.1 Proportional (P) controller

A Proportional (P) controller applies the correction action proportionally based on the error signal which is the difference between the measured value (PV) and the set point (SP).

From the graph in Figure 1, it is observed that at the beginning of the reaction, the PV is lower than the SP, thus the valve is fully closed (0% valve opening) to allow the input of pH 9 solution to increase the pH to the SP. At this point, the response of controller to increase the PV to the SP is considerably fast because a P controller the output is directly proportional to the error and the proportion coefficient can be altered to control the speed of the process. After the PV begin to exceed the SP (PV=60.1% and SP=60%), the valve automatically opens to allow pH 4 solution to bring down the

pH back to the SP. However, after the process stabilised, the PV stayed constant at PV=68% while the SP=60%. This happens due to the fact that a P controller cannot eliminate an offset error that occur within the system. The offset error is the difference between the set point (SP) and the actual measured value (PV), SP-PV, which is always resulted when a disturbance occur in a proportional control system. Thus, it can be deduced that a P controller have a short rise time which is suitable for fast response systems, however, the major drawback is that it cannot eliminate the offset error that occur. Besides that, overshoot problem is noticeable due to the fast response of a P controller without a mechanism to slow down the control signal as the PV approach the SP.

Figure 1 The correlation between the measured value (PV) and valve opening when the set point (SP) is set to SP=60% in a P controller

100

pH (%)

Response of P controller (SP=60%)

Valve Opening (%)

Measured Value

Set Pont (SP)

Figure 2 The offset error shown in the pH process control software interface for the control system using a P controller

4.2 Proportional-Integral (PI) controller

Previously, the P controller depends solely on the proportional corrective action to respond to error signal generated due to the deviation of the measured value (PV) from the set point (SP). However, P controller is unable to eliminate the offset error in the pH control system. This problem can be solved by introducing the integral action on top of the proportional action in the P controller. This type of controller is known as a Proportional-Integral (PI) controller. The integral term in the PI controller allows the control signal to be proportional to the integral of the error signal and the integral gain, which eliminates the offset error by putting an effort to move the proportional band closer to the set point until the PV error is nullified and the SP is achieved. From the graph in Figure 3, at the beginning of the process, the measured pH value is below the SP, thus, the control valve remain closed (valve opening=0%) to allow the pH 9 solution to increase the pH to the set point SP=60%. When the PV begin to exceed the SP (when PV=60.1% and SP=60%), the control valve opens to allow the inflow of pH 4 solution to decrease the PV to the SP. However, we can notice a high overshoot and slow settling time in the PI controller due the integral term that causes a slow response. The advantage for a PI controller is that the offset problem can be eliminated. From Figure 4, large disturbance and noise is observed in the operation of a PI controller highlights another drawback of using a PI controller.

Figure 3 The correlation between the measured value (PV) and valve opening when the set point (SP) is set to SP=60% in a PI controller

Figure 4 Large disturbance and noise that be observed in a control system using a PI controller

4.3 Proportional-Integral-Derivative (PID) controller

The previous application of a PI controller successfully eliminated the offset error that occurs in P controllers, but resulted in high overshoot and slow settling time. The solution to this would be introducing the derivative action into the existing

100

pH (%)

Response of PI controller (SP=60%)

Valve Opening (%)

Measured Value

Set Pont (SP)

proportional-integral action of the PI controller, creating a proportional-integral-derivative corrective mechanism of the error signal as in PID controllers. The derivative term will predict the upcoming trend of the measured value (PV) by taking into consideration the rate of change of error with time. As the PV move rapidly approaching the SP, the control signal is gradually slowed down to allow the PV to reach the SP with minimal overshoot. When the PV moves away from the SP, the control signal is increased to put more effort to bring the PV back to the SP. From Figure 5, at the beginning of the reaction, the PV is lower than the SP, thus, the control valve is fully closed (valve opening=0%) to allow the inflow of pH 9 solution to increase the measured pH value to the SP. At this point, the response is fast due to the advantage of proportional action. Unlike in the P and PI controller, the derivative term in a PID controller slows down the control signal when it approaches the SP, which greatly reduced overshooting and settling time. Figure 6 shows the advantages of using a PID controller which is short rise time, minimal overshoot, short settling time and no offset error.

Figure 5 The correlation between the measured value (PV) and valve opening when the set point (SP) is set to SP=60% in a PID controller

100

pH (%)

Response of PID controller (SP=60%)

Valve Opening (%)

Measured Value

Set Pont (SP)

Figure 6 The output control signal of PID controller has a short rise time, gradually decreases approaching the SP due to derivative action, minimal overshoot and short settling time

5.0 CONCLUSION AND RECOMMENDATIONS

In a nutshell, it can be concluded that all the three types of the controllers, namely P, PI and PID controller play its main function to apply the corrective mechanism based on the error signal generated by comparing the measured value (PV) and the set point (SP) either by proportional action alone, or with the combined action of the integral term and/or derivative term. P controller has the advantage of fast response and controllable speed due to its proportional action on the gain and error signal but the drawback is that the offset error cannot be eliminated. By introducing the integral term, a PI controller managed to compensate the offset error that occur in P controllers, however, the integral term does not predict the future trend of the error signal, thus, it slows down the overall response of the process control system. On the other hand, adding a derivative mode as in the PID controller enables the system to predict the upcoming measure value (PV) based on the rate of change of error. Therefore, a PID controller takes the advantage of proportional action to provide a fast response when the PV is far from the set point (SP) and gradually slowing the rate then the derivative term gets smaller as it approaches the SP to minimise overshoot and reduce settling time. A PID controller also take the advantage of the integral term to eliminate the offset error.

Hence, it can be deduced that P controller has a fast response but the major drawback is the offset error. P controllers are applicable in certain process that requires the control signal to be proportional to the error signal. PI controllers managed to eliminate the offset error but gives a slow response, making it widely used for the system that do not require a fast response since the setup is easy while the cost is relatively cheap. PID controllers take the advantage of fast response from proportional action, error elimination from integral action and the rate of change in error from the derivative action to achieve a short rise time, minimal overshoot, and short settling time. From the experiment, PID is the most recommended type of controller since its advantages outstand the other types of controller but the selection of appropriate controller lies greatly upon the type of industrial processes and its application.

REFERENCES

Alwan, G. M. (2008). pH-control problems of wastewater treatment plants. Al-Khwarizmi Engineering Journal, 4(2), 37-45

Chandra, S., & Nidhi, Y. (February 2017). Comparative analysis of P/PI/PID controllers for pH neutralization process. International Journal of Advanced Technology in Engineering and Science, 5(2).

Kambale, S. D., George, S., & Zope, R. G. (2015). Controllers used in pH neutralization process: A review. International Research Journal of Engineering and Technology (IRJET), 2(3), 354-361

Sharmila, B., & Vidhyanandhan, L. (2016). Modelling and designing of controllers for pH process. Journal of Advances in Chemistry, 12(15), 4872-4883

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