Comparison Analysis of Model Predictive Controller with Classical PID Controller For pH Control Process

Indonesian Journal of Electrical Engineering and Informatics (IJEEI)
Vol. 4, No. 4, December 2016, pp. 250~255
ISSN: 2089-3272, DOI: 10.11591/ijeei.v4i4.247  250
Received September 14, 2016; Revised October 21, 2016; Accepted November 9, 2016
Comparison Analysis of Model Predictive Controller
with Classical PID Controller for pH Control Process
V.Balaji*
1
, Dr. L.Rajaji
2
, Shanthini K.
3
1
Department of Electrical Engineering, Singhania University, Pacheri Bari, Rajasthan, India
2,3
ARM College of Engineering and Technology, MaraiMalai Nagar, Chennai, India
e-mail: balajieee79@gmail.com
Abstract
pH control plays a important role in any chemical plant and process industries. For the past four
decades the classical PID controller has been occupied by the industries. Due to the faster computing
technology in the industry demands a tighter advanced control strategy. To fulfill the needs and
requirements Model Predictive Control (MPC) is the best among all the advanced control algorithms
available in the present scenario. The study and analysis has been done for First Order plus Delay Time
(FOPDT) model controlled by Proportional Integral Derivative (PID) and MPC using the Matlab software.
This paper explores the capability of the MPC strategy, analyze and compare the control effects with
conventional control strategy in pH control. A comparison results between the PID and MPC is plotted
using the software. The results clearly show that MPC provide better performance than the classical
controller.
Keywords: pH control, PID, MPC, FOPDT, matlab
1. Introduction
The Process control theory is evolving and new types of controller methods are being
introduced.PID controllers arecommonly used due to its simplicity and effectiveness. In most of
the industries PID controllers are used. Still there is no generally accepted design method for
this controller. In 1970’s MPC controller has been introduced as a way of controlling a wide
range of processes. Now a day‟s control systems engineers in the industry are adopting
computer aided control systems design for modeling, system identification and estimation.
These made a path to study MATLAB software .By adopting simulations the students may
easily visualize the effect of adjustingdifferent parameters of a system and the overall
performance of the system can be viewed. In this paper itis demonstrated how to create a
model predictive controlfor a first order system with time delay in a MATLAB Simulink and also
explains the difference betweenMPC and conventional controller.pH control plays a vital role in
the process industry.The traditional method is to use classical PID method and the advanced
control strategy includes ModelPredictive Controller. In this paper the tuning has beendone
using Z-N Method and results have been compared between, PID and Model Predictive
method.
2. Model Predictive Control
Model predictive control (MPC) has become a standard technology in the high level
control of chemical processes. MPC or receding horizon control is a form of control in which the
control action is obtained by solving on-line, at each sampling instant, a finite open-loop optimal
control problem, using the current state of the plant as the initial state; the optimization yields an
optimal control sequence in which the first control move is applied to the plant.
Here the controller tries to minimize the error between predicted and the actual value
over a control horizon and the first control action is being implemented. Model predictive
controllers rely on dynamic models of the process, most often linear empirical models obtained
by system identification. MPC is also referred to as receding horizon control or moving horizon
control (Qin and Badgwell, 2003).[3]
Figure 1 shows the behavior of an MPC system can be quite complicated, because the
control action is determined as the result of the online Optimization problem. The problem is
constructed on the basis of a process model and process measurements. Process

IJEEI ISSN: 2089-3272 
Comparison Analysis of Model Predictive Controller with Classical PID … (V.Balaji)
251
measurements provide the feedback (and, optionally, feed-forward) element in the MPC
structure. Figure 1 shows the structure of a typical MPC system feed-forward) element in the
MPC structure.
Figure 1. Model Predictive control Scheme
Figure 2. Basic concept of MPC [1]
2.1. The Receding Horizon
The control calculations are based on future predictions as well as current
measurements. Future values of output variables are predicted using a dynamic model of the
process and current measurements. Fig. 2 shows the concept of prediction horizon and control
horizon.
2.2. Prediction and Control Horizons
Prediction horizon has a length equal to the number of samples in future for which the
MPC controller predicts the plant output [1]. Prediction (P) is typically as far ahead as two to
three times the dominant time constant of the system. Suppose the process is sampled at say
one twentieth of that time constant: the output prediction horizon could then be up to some 60

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252
steps ahead [2]. The length of control horizon is equal to the number of samples within the
prediction horizon where the MPC controller can affect the control action [1].
2.3. Receding Horizon Approach
(i)At the kth sampling instant, the values of the manipulated variables, u, at the next M
sampling instants, {u(k), u(k+1), …, u(k+M -1)} are calculated. (ii)This set of M ―control moves‖
is calculated so as to minimize the predicted deviations from the reference trajectory over the
next P sampling instants while satisfying the constraints. (iii) Typically, an LP or QP problem is
solved at each sampling instant. Then the first ―control move‖, u(k), is implemented. (iv)At the
next sampling instant, k+1, the M-step control policy is re-calculated for the next M sampling
instants, k+1 to k+M, and implement the first control move, u(k+1).(v) Then Steps 1 and 2 are
repeated for subsequent sampling instants.
3. Experimental Setup
The pH process is adjusted by controlling the flow rate of ammonia. This action adjusts
the flow rate of the Ammonia, thus the input to the controller is the pH reading of the mixing
vessel which is compared against the required set point. At the same time the output voltage
obtained from the controller is used to adjust the solenoid valve or motorized valve to control the
Ammonia flow rate. This output tends to maintain the mixing vessel pH to a desired value. The
Figure 3 shows the pH controlling process.
Figure 3. Process diagram of pH control
3.1. Approximating pH process to FOPDT
For the step input of (0 to 30% opening of Ammonia flow rate valve), we note the
following characteristics of its step response: to approximate into First Order Plus Delay Time
(FOPTD) model,
(i) The response attains 63.2% of its final response at time, t = τ+θ. (ii) The line
drawn tangent to the response at maximum slope (t = θ) intersects the
y/KM=1 line at (t = τ+ θ). (iii) The step response is essentially complete at
t=5τ. In other words, the settling time is ts=5t. The graphical the analysis to
determine the FOPDT model is shown in the fig 4. Therefore the FOPDT
model transfer function becomes

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Figure 4. Graphical Analysis to Obtain the Model
4. Methodology
The Design of conventional PID controller and advanced controller are done using the
Matlab tools. Figure 5 and 6 shows the diagram of PID and MPC tuning in the Matlab
environment.The setpoints needed to be adjusted are 1.8,1.9,2.8 and 4.8.The PID controller
gives good setpoint tracking when kp= 0.10, τI=0.17 and τd=0.033. The MPC is tuned prediction
horizon of 3, control horizon of 1 and control interval of 1.
Figure 5. Simulink Diagram for PID Design

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Figure 6. Simulink Diagram for MPC Design
5. Results and Analysis
The graph between time and the output signal has been obtained for PID, and MPC
controller as shown in Figure 7. The comparison between these controllers has been done and
the best controller has been obtained.
Figure 7. Output Response of PID and MPC Controller
We can observe from Figure 7 that how fast the MPC can reach the set-point. In the
response of the PID we can easily the fulucations from the beginging itself and it is time
consuming to reach the set point. Figure 8 shows the graph of input adjustment for both the
controllers.

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255
The output response of the MPC is faster than the response of the PID controller.
Figure 8. Input Adjustments by MPC and PID
6. Conclusion
The obtained transfer function is processed using classical and advanced controllers
such as PID, and MPC. The values which are obtained from the tuning methods are simulated
using MATLAB. It is seen from the response curve that MPC controller provides a better
response with minimum time when compared with PID and IMC. So it is concluded that MPC
controller is efficient for a pH process.
References
[1] Hans-Petter, Halvorsen. Model Predictive Control in labVIEW. Dept. of Electrical Engineering
Information Technology and Cybernetics, Telemark University College. 2011.
[2] Jonathan L. Process Automation Handbook - A guide to Theory and Practice.
[3] Bemporad, A. Model Predictive Control: Basic concepts, Controllo di Processo e Sistemi di
Produzione. 2009.
[4] Camacho and Bordons. Model Predictive Control. Springer. 2004.
[5] Rossiter J. Model based Predictive Control - A practical approach. CRC Press. 2003.
[6] Mathworks.com. Model Predictive Control Toolbox. 2012.
[7] J.Prakash, K.Srinivasan. Design of Non Linear PID Controller and Non Linear Model Predictive
Controller for a Continuous stirred tank Reactor. ISA Transactions. 2009; 48: 273-282.
[8] S.Abirami, H.Kala, P.B.Nevetha, B.Pradeepa, R.Kiruthiga, P.Sujithra. Performance Comparison Of
Different Controllers For Flow Process. International Journal of Computer Applications. 2014; 90(19):
17-21.
[9] E.F. Camacho, C. Bordons. Model Predictive Control in the Process. Springer-verlay. 1995.
[10] Ljung. System Identification: Theory for the User. Printice-Hall: Englwood cliffs. 1987

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