Transportation and Logistics Modules Development

Real-time simulations are performed in order to investigate the influence of network induced time delays and disturbances on the DO process and the performance of the developed Smith predictor compensation scheme. To achieve this, network induced time delays in the form of transport delays are introduced in to the closed loop DO

process. The transfer function and the nonlinear linearising approach of the Smith predictor scheme design are investigated. The arrangements for the two approaches as described in section 5.3 are used. The random time delays are represented with transport delays and placed between the DO process and the (PI + nonlinear linearising) controllers as shown in Figure 7.9 for the nonlinear linearising case and for the transfer function approach, the delays are placed between the PI controller and the linearised DO process. This is shown in Figure 7.10

Figure 7.9: LabVIEW block diagram of real-time simulation of the Smith predictor-based compensation scheme for the closed loop DO process using

the nonlinear linearising approach

For each of the block diagrams in these Figures, the delays are increased from a given minimum value to a mximum value until a sustained oscillation (critical delay is reached). In Figures 7.9 and 7.10 the first cases investigated are those of the closed loop DO process without network delays. In the second cases, the closed loop DO process behaviour is subjected to time delays and disturbances incorporated into the closed loop DO process using saturation block in LabVIEW, while in the third cases, the Smith predictor compensation schemes are applied in order to provide robustness for the network induced closed loop DO process.

Figure 7.10: LabVIEW block diagram of real-time simulation of the Smith predictor-based compensation scheme for the closed loop DO process using

the nonlinear linearising approach

7.4.1 Description of the developed real-time simulation environment

Figure 7.11 is an example of the developed user-friendly interface that display in real-time a graphical (visual) form the changes experienced in the behaviour of the DO process as a result of the network time delays and disturbances. The interface affords the user the opportunity to evaluate in real-time the capability and performance of the developed Smith predictor approaches in chapters five and six in terms of their robustness. From the Figure 7.11, three different graphs are used to separately display the real-time simulation results of the DO process without time delays, with time delays and with time delays and the Smith predictor. The fourth graph combines the graphical results of all the three scenarios investigated. Three cylindrical tanks could be seen in the Figure 7.11 with colours blue, red and green.

The first cylinder with blue colour represents the dissolved oxygen (DO) concentration level of the closed loop DO process when there is no network induced time delay or other forms of disturbance. It could be seen that the DO concentration level in the blue tank is 2.4mg/l. The red tank represents the DO concentration level of a disturbed closed loop DO process under time delays. The green tank represents

the Smith predictor compensated DO concentration level of the DO process under the influence of network induced time delays and other forms of disturbances. The purpose of the Smith predictor scheme is to eliminate the communication network time delays and disturbances in the control system. It is meant to compensate for the network induced time delays and other forms of disturbances in the red tank and to make the green tank revert to the level of the blue tank which is the desired DO process trajectory.

During the simulation, the DO concentration level in the blue tank remained constant but the red coloured tank due to communication time delays and disturbances is seen to be going up and down throughout the process of simulation as the value of the delays are increased. It could also be observed that the green cylinder might experience some disturbances earlier in the real-time simulation, but it gradually reached steady state due to the presence of the Smith predictor compensation scheme.

7.4.2 Real-time simulation of the Smith predictor-based DO process using the nonlinear linearising approach and the transfer function approaches

The real-time simulations performed in this Chapter are previously done in Chapters 5 section 5.10 and Chapter six section for the transfer function approach and the nonlinear linearising approach respectively in the MATLAB/Simulink environment.

The reason for performing the investigations in a real-time environment using the NI-LabVIEW is to validate the results previously obtained. The real-time environment also offers a more user-friendly interface where the changes taking place in the DO process dynamics can be viewed in a more interactive way. It should be noted that simulations performed in the MATLAB/Simulink environment have very close similarities with the ones performed in the LabVIEW environment. However, there could be some situations where very few differences exist. The reason for this might be due to the fact that LabVIEW is designed for a real-time simulation which is not the case with MATLAB/Simulink