feed forward and cascade control experiment

Chemical Engineering Process Laboratory III

Feed Forward and Cascade Control Experiment

Summary

In this experiment, the effects of the feedforward and cascade control were investigated. The CE2000 Control software was used to set the parameters of the two controllers and to monitor the process vessel level while the experiment was run on a CE117 Process Trainer.

In the first part of the experiment, the effects of process disturbance on feedforward control and its limitation were studied. The system was set up for proportional-integral (PI) control of the process vessel using the valve position as the controlled variable. Once the system had stabilized, the pump voltage was changed to introduce a disturbance to the process. The action of the feedforward control on the process as it corrected the disturbance was noted. This was repeated several times with feedforward gains of 0.2, 0.4, 0.6 and 0.8 respectively. From the results, it was found that a feedforward control with a gain of 0.2 exhibited the most optimum performance.

In the second part of the experiment, the actions of the cascade control on the process system when a disturbance is introduced, were studied. The system was again set up for PI control for both the master and slave controllers. Different types of disturbances were introduced separately throughout the experiment which include disturbances to the set point, changes to inflow rate and changes to outflow rate. The effects of the cascade control for each of the disturbances were noted.

Contents1. Introduction12. Theoretical Background22.1 Feedforward Control22.2 Cascade Control33. Experimental Setup and Procedures43.1 Experimental Setup43.2 Experimental Procedures53.2.1 Part 1: Feedforward Control53.2.2 Part 2: Cascade Control64. Results and Discussion84.1 Feedforward Control Experiment84.1.1 Feedforward Gain = 084.1.2 Feedforward Gain = 0.294.1.3 Feedforward Gain = 0.4104.1.4 Feedforward Gain = 0.6114.1.5 Feedforward Gain = 0.8124.2 Cascade Control Experiment144.2.1 Disturbances to Set Point Change144.2.2 Disturbances to Inflow Rate164.2.3 Disturbances to Outflow Rate194.3 Discussion214.3.1 Feedforward Control214.3.2 Cascade Control225. Error Analysis236. Lab Safety Analysis247. Conclusion248. References25

1. IntroductionProcess control is a key component in operating an industrial process plant. In a typical plant, many complicated processes could be running at the same time with an even larger number of process variables that must be measured and controlled. It is extremely difficult to continuously monitor and prevent disturbances to the processes manually. A computer-based process control system is thus needed to operate the plant safely while ensuring the quality of the product. The role of process control is to therefore maintain the process variable near the desired values and to automatically correct disturbances introduced to the processes. One classic and simple process control is the feedback control. In feedback control, the controlled variable is measured and this is used to adjust the manipulated variable. It is thus a reactive control system as it requires information on deviation from set point to take action. Although a feedback control is simple to implement and requires minimum information to operate, it has several inherent disadvantages. Firstly, the feedback control is unable to prevent deviation of the controlled variable from the set point during a disturbance. This is because it can only take corrective action only after a disturbance has occurred. Secondly, the feedback control may not be suitable for processes with large time constants and/or long time delays. Such processes may not be able to achieve steady-state if large disturbances occur frequently. With these important drawbacks of the feedback control, other types of process control strategy would need to be looked into to overcome these disadvantages. Two process controls, feedforward control and cascade control, both address the shortcomings of the feedback control. These two process control employ different strategies to correct disturbances while overcoming the disadvantages of the feedback control. For this study, the feedforward control and the cascade control were investigated to understand their actions on the processes when disturbances are introduced. The objectives of the experiment are to study effects of process disturbance on feedforward control and its limitation and to study how cascade control can improve the control.2. Theoretical Background2.1 Feedforward ControlIn feedforward control, the disturbance variable is measured instead of the controlled variable. This allows the feedforward control to take corrective action, based on its measurements of the disturbance variable, before the disturbance causes any deviation from the set point in the controlled variable. This would ideally mean that the feedforward control makes adjustments to the manipulated variable such that it cancels the effect of the disturbance to ensure that the controlled variable remains unaffected. The feedforward control is thus a proactive control system which is in contrast with the feedback control as it is a reactive control system. A schematic diagram of a feedforward control is as illustrated in Figure 1.

Figure 1. Schematic diagram of feedforward control. The feedfoward controller measures the disturbance variable and make adjustments in the manipulated variable to correct the disturbance before deviations from the set point occurDespite being able to overcome the feedback control main drawback, the feedforward control has several disadvantages as well. One of them is that the disturbance variable must be measured on-line which is not feasible in many situations. Another disadvantage is that it needs to have information on the process model in order to work effectively and its performance is thus dependent on the accuracy of the process model. The process model may not be known all the time and at times, only an approximate process model can be obtained. The feedforward control would thus be unable to correct for disturbances that are not accounted for and could only correct for the disturbances that it is measuring. These disadvantages led to the use of feedforward control in combination with feedback control in practical applications. Feedforward control would measure the disturbance variables and make corrective actions to decrease the effect of these measured disturbances while the feedback control help to correct for disturbances that are not measured by the feedforward control and compensates for the inaccuracies in the process model. A typical feedforward-feedback setup is shown in Figure 2.

Figure 2. Block diagram of a feedforward-feedback control system. The feedback trim is added to compensate for inaccuracies in the process model and unmeasured disturbances.2.2 Cascade ControlThe cascade control is another process control strategy which also addresses the shortcoming of the feedback control. In a cascade control system, two feedback controllers are used. One acts as the master controller which measures the primary controlled variable and uses the primary measurements to establish the set point for the secondary controller, known as the slave controller. The slave controller measures the secondary variable and the output of the slave controller adjusts the manipulated variable. It is located in the secondary loop that is nested within the primary loop. This is illustrated in Figure 3 below where the master controller in the primary loop measures the temperature of the liquid in the reactor and determines the set point for the slave controller in the secondary loop which is measuring the temperature of the jacket. The slave controller then adjusts the cooling water valve accordingly.

Figure 3. Cascade control system implemented for a chemical reactor. The primary measurement is the temperature of the chemical in the reactor while the secondary measurement is the temperature of the cooling jacket.The main advantage of the cascade control is that the second measured variable is located close to a potential disturbance. This allows the secondary controller to detect the disturbance sooner than the primary controller and corrects it immediately. The cascade control thus responds to disturbances faster than a feedback control. This also makes the cascade control less prone to errors in the process model.

3. Experimental Setup and Procedures3.1 Experimental SetupThe experimental setup includes a CE117 Process Trainer (shown in figure 4), a CE117 Mimic Panel (shown as figure 5) and the CE2000 software. Figure 4. CE117 Process TrainerPumpProcess drain valveWater tankAir ventWater reservoir

Process vesselLevel TransmitterFlow TransmitterProportional ValvePump

Figure 5. CE117 Mimic Panel3.2 Experimental Procedures3.2.1 Part 1: Feedforward ControlConnections for the mimic panel are the same as lab manual (shown in Figure 6).

Figure 6. Connections for Feedforward ControlProcedures for feedforward control process:Starting CE2000 software and loading the file C3 Feedforward Control.ict.Do connections for the mimic panel the same as figure 6; set heater control to Manual and at minimum power; set pump 2 to external; turn off cooler fan and stirrer; open drain valve and close bypass valve.Set feedforward gain to 0, and PID controller is set to 5, 0.2 and 0 respectively.Set level set point and pump voltage set to 7V.Start software by pressing the record button. System is allowed to settle within 10% of the level set point.Change pump voltage from 7V to 10V, and wait the system to reach steady state.Change pump voltage from 10V to 7V, and wait the system to reach steady state.

Press the stop button and export the data.The process is repeated for another 4 times by changing only the feedforward gain to 0.2, 0.4, 0.6 and 0.8.

3.2.2 Part 2: Cascade ControlSimilarly, follow the lab manual to do connections for the mimic panel (shown as Figure 7). Figure 7. Connections for Cascade ControlStart CE2000 software and load the file C3 Cascade Control.ict.Close process loop bypass valve, and both air vent and process vessel drain are opened fully.Set voltage to the proportional valve to 10V and the reference level to 7V.Master PID controller settings are set to 20, 1 and 0 respectively.Slave PID controller settings are set to 1, 1 and 0 respectively.Open process vessel drain valve; set pump 2 to external and run the software by pressing record button.Change level set point from 7V to 8V, and wait system to reach steady state.When the system is stabilized, change the level set point from 8V to 7V.When it reaches steady state, stop it and export the data.Change voltage to the proportional valve from 10V to 7.5, wait for stabilized.1/3 Close the process drain valve manually and wait for steady state. Change voltage from 7.5V to 10V and wait for stabilized. Stop the system and export the data.Repeat by changing voltage to 5V and 2.5V.Fully open the valve and wait for steady state. Stop the system and export the data.Repeat by changing the valve to 1/2 closed and 2/3 closed.

4. Results and Discussion4.1 Feedforward Control Experiment 4.1.1 Feedforward Gain = 0

DescriptionInitial Steady State PhaseDisturbance: 7V to 10VDisturbance: 10V to 7V

Period0.0 368.1s368.2 488.6s488.6 717.5s

Figure 8. Plot of Responses when Feedforward Gain = 0There is no feedforward controller action in this system. In the experiment, the tank was initially filled from 0 to the desired setpoint gradually with an overshoot to at 59.4s. The action of PI controller then slowly brought the system level back to the setpoint slowly during 59.4s to 123.90s.At 368.2s, a step increase of disturbance from 7V to 10V was introduced and it resulted in an offset of 0.12V which lasted until 346.2s before the PI controller takes action and eliminates the offset. During this period, the control voltage decreased gradually to 2.45V and increased back to 2.88V and oscillated around that steady state.At 488.6s, a step decrease of disturbance from 10V to 7V was introduced and it resulted in an offset of -0.12V which lasted until 506.4s before the PI controller takes action and eliminates the offset. During this period, the control voltage increased gradually to 3.9V and decreased back to 3.6V and oscillated around that steady state.

Table 1: Summary of Feedforward Control with 0 GainFeedforward Gain = 0

Setpoint (V)Level (V)Controller (V)Time to reach steady state (s)

Initial Steady State7.0007.0973.441123.90

7V 10V7.000+/- 0.0022.78854.60

10V 7V7.000+/- 0.0023.507111.00

4.1.2 Feedforward Gain = 0.2


Period0.0 374.8s374.8 507.7s507.8 589.1s

Figure 9. Plot of Responses when Feedforward Gain = 0.2In this system, there is a P controller acting as a feedforward controller with a gain of 0.2. The experiment was repeated. At the initial phase, there was a slight overshoot to at 58.1s before the system level was brought back to the setpoint during 58.1s to 110.1s.At 374.8s, a step increase of disturbance from 7V to 10V was introduced and it resulted in a negligible offset of less than 0.02V from the setpoint. During this period, there was a sharp drop in control voltage from 3.55V to 2.8 V. This is because the feedback controller paired with the feedforward control action is able to eliminate the offset. The level value was then brought back to the setpoint value through small and gradual oscillations.At 507.8s, a step decrease of disturbance from 10V to 7V was introduced. Once again, there was a very negligible offset of less than -0.02V from the set point due to the pairing of the feedback and feedforward controller. There was a sharp increase of control voltage from 2.8V to 3.6V. The level value was then brought back to the setpoint value through small and gradual oscillations.

Table 2: Summary of Feedforward Control with 0.2 GainFeedforward Gain = 0.2



7V 10V7.000+/- 0.0022.794negligible

10V 7V7.000+/- 0.0023.612negligible



Period0.0 268.6s268.7 448.6s448.6 648.5s

Figure 10. Plot of Responses when Feedforward Gain = 0.4The feedforward controller gain was increased from 0.2 to 0.4 as compared to the previous set up. At the initial phase, there was an overshoot to at 67.0s before the level was brought back to the setpoint during 67.0s to 122.5s. It can also be observed that there is a larger oscillation of the control voltage and a longer time is needed to attain steady state as compared to the 0.2 gain.At 268.7s, a step increase of the disturbance from 7V to 10 V resulted in an offset of less than -0.05V from the setpoint. During this period, there is a sudden drop in the control voltage from 4.4V to 3.2V. At 714.3s, a step decrease of the disturbance from 10V to 7V resulted in an offset of less than 0.03V from the set point. During this period, there is a sudden rise in the control voltage from 3.46V to 4.66V. In both cases, the offset is eliminated by the pairing of the feedforward controller action to the feedback controller. The controller then brought the level back to the setpoint of 7V via small and gradual oscillations.




7V 10V7.000+/- 0.0023.55663.60

10V 7V7.000+/- 0.0024.27739.80

4.1.4 Feedforward Gain = 0.6DescriptionInitial Steady State PhaseDisturbance: 7V to 10VDisturbance: 10V to 7V

Period0.0 234.2s234.2 370.3s370.3 488.3s

Figure 11. Plot of Responses when Feedforward Gain = 0.6The feedforward controller gain was increased from 0.4 to 0.6 as compared to the previous set up. At the initial phase, there was an overshoot at 63.1s before the level was brought back to the setpoint during 63.1s to 123.5s. It can also be observed that there is a larger oscillation of the control voltage and a longer time is needed to attain steady state as compared to the 0.2 gain.At 234.2s, a step increase of the disturbance from 7V to 10 V resulted in an offset of less than -0.15V from the setpoint. During this period, there is a sudden drop in the control voltage from 4.44V to 2.66V. At 370..s, a step decrease of the disturbance from 10V to 7V resulted in an offset of less than 0.15V from the set point. During this period, there is a sudden rise in the control voltage from 3.55V to 5.35V. In both cases, the offset is eliminated by the pairing of the feedforward controller action to the feedback controller. The controller then brought the level back to the setpoint of 7V via small and gradual oscillations.




7V 10V7.000+/- 0.0023.55245.00

10V 7V7.000+/- 0.0024.34158.10



Period0.0 274.3s274.4 374.4s374.5 524.8s

Figure 12. Plot of Responses when Feedforward Gain = 0.8The feedforward controller gain was increased from 0.6 to 0.8 as compared to the previous set up. At the initial phase, there was an overshoot at 63.3s before the level was brought back to the setpoint during 63.3s to 124.6s. It can also be observed that there is a larger oscillation of the control voltage and a longer time is needed to attain steady state as compared to the 0.2 gain.At 274.3s, a step increase of the disturbance from 7V to 10 V resulted in an offset of less than -0.25V from the setpoint. During this period, there is a sudden drop in the control voltage from 4.4V to 2.0V. At 374.5s, a step decrease of the disturbance from 10V to 7V resulted in an offset of less than 0.24V from the set point. During this period, there is a sudden rise in the control voltage from 3.6V to 6.0V. In both cases, the offset is eliminated by the pairing of the feedforward controller action to the feedback controller. The controller then brought the level back to the setpoint of 7V via small and gradual oscillations.




7V 10V7.000+/- 0.0023.59558.40

10V 7V7.000+/- 0.0024.38386.60

From the data collected, it can be observed and concluded that a feedforward controller gain of 0.2 is the most optimum for the system. A large oscillation in the control voltage and huge offset in the level voltage are the outcomes of the corrective action taken by the feedback controller without the feedforward controller, i.e. gain = 0. In addition, a large amount of time is needed for the level voltage to approach back to setpoint. With the implementation of 0.2 feedforward controller gain, the corrective action becomes gentler and resulted in a negligible offset in the level voltage. Also, the control voltage experienced a smaller and more gradual oscillation. However, when the feedforward controller gain is increased beyond 0.2, i.e. 0.4, 0.6 and 0.8, the corrective action becomes significantly more vigorous and there is a larger offset in the level voltage. The time taken for the level to approach back to setpoint after the disturbance also increased when the feedforward controller gain increased. Interests were put in to investigate the controller behaviour of different feedforward controller conditions. When there is a disturbance introduced, the controller voltage increased or decreased sharply in the opposite direction before corrective action taken to stabilize the system. For example, when there is a step increase in voltage (disturbance), the controller voltage decreased sharply, and vice versa. Comparing all 5 conditions, i.e. when feedforward controller gain is 0, 0.2, 0.4, 0.6 and 0.8, it showed the system with when the feedforward controller gain (= 0.2) will bring a smallest change in control voltage when disturbance was introduced. This could also explain why the level offset for 0.2 feedforward controller gain is negligible.Hence, it could be concluded that enhancement in the performance of the system is ensured for a well-tuned feedforward and feedback controllers pairing system. In this case, the optimized feedforward controller gain for this system is around 0.2. To obtain an exact value for the optimized feedforward controller gain, further fine tuning and more experiment could be conducted.

4.2 Cascade Control ExperimentWhile feedforward control is able to compensate for the disadvantages of using conventional feedback control, especially for processes with large time delays or time constants, using feedforward control requires explicit knowledge and quantification of the disturbances, which is impossible in several real life cases. In this section, we will investigate the effectiveness of using cascade control as an alternative to feedforward control. Cascade control is a control configuration that consists of a secondary controller, also known as a slave controller, nested within a larger loop. The larger loop contains the primary controller, also known the master controller. The unique characteristic of this control configuration eliminates the need for disturbances to be explicitly measured, unlike the feedforward control. There are various disturbances to the setup: disturbance to the set point change, disturbance to the inflow rate, and disturbance to the outflow rate. Graphical results of the disturbances are presented.

4.2.1 Disturbances to Set Point ChangeThe reference level, or the setpoint, was increased from 7V to 8V within seconds of starting the system. This is as seen from Figure 14 where there was a slight overshoot of the FT2 curve which adjusted the flow of the system. After the system has reached steady state, the setpoint is decreased back to 7V and allowed to attain steady state. An undershoot was observed but it was quickly adjusted back to the setpoint. The observations made were in accordance to theory which stated that offsets in a cascade control system will be eliminated.

Figure 13. Plot of Responses when Set Point ChangeIt is noted that with the inclusion of F2-Ref, which is the input to the slave controller, the accuracy of the graph is greatly reduced. Therefore, for the subsequent graphs, F2-Ref will be removed.

Figure 14. Plot of Responses when Set Point Change (Magnified)Change in set-pointTime of Change (s)Time at steady State (s)Time taken to reach steady state (s)Maximum deviation in level (V)

7V to 8V93.80122.7028.90+0.110

8V to 7V138.60164.3025.70-0.882

4.2.2 Disturbances to Inflow Rate 4.2.2.1 Inflow disturbance of 2.5VThe inflow of the system at steady state is decreased from 10V to 7.5V a set point deviation by -2.5V. When the system has reached a new steady state at 7.5V, the inflow is increased from 7.5V to 10V a set point deviation of +2.5V.10V to 7.5V7.5V to 10V

Inflow disturbanceTime of Change (s)Time at steady State (s)Time taken to reach steady state (s)Maximum deviation in level (V)

10V 7.5V42.6052.9010.30-0.026

7.5V 10V105.00118.8013.80-0.010

Figure 15. Plot of Responses with Inflow Disturbance of 2.5VThe inflow is perturbed at t=42.6s, where the inflow voltage is decreased from 10V to 7.5V. The system took 10.6s to stabilize and reach a new steady state at t=52.9s. The inflow is then perturbed again at t=105s, where the inflow voltage is now increased from 7.5V to 10V. It took roughly same time for the controller to control the perturbation and bring the system to a steady state, taking 13.8s for the system to stabilize. As seen from the graph, there are minimal oscillations when the perturbations are introduced, hence the deviations of -0.026V and -0.010V are rather small. F2-Ref graph also showed minimal deviation. It is to be noted that cascade control is able to bring the system to a steady state within seconds, while feedforward control took minutes to do so. This rapid response as compared to feedforward control is one hallmark of cascade control, as the slave controller first minimized effects of the disturbances before the effects of the disturbances reach other components in the control loop. The slave controller can hence respond quickly to the disturbance in the inflow by altering the power of the pump, hence altering the water level to reach steady state.

4.2.2.2 Inflow disturbance of 5VThe inflow of the system at steady state is decreased from 10V to 5V a set point deviation by -5V. When the system has reached a new steady state at 5V, the inflow is increased from 5V to 10V a set point deviation of +5V.5V to 10V10V to 5V


10V 5V43.7063.2020.00-0.017

5V 10V87.50107.5030.00-0.028

Figure 16. Plot of Responses with Inflow Disturbance of 5V

The inflow is perturbed at t=43.7s, where the inflow voltage is decreased from 10V to 5V. The system took 29.5s to stabilize and reach a new steady state at t=63.2s. The inflow is then perturbed again at t=87.5s, where the inflow voltage is now increased from 5V to 10V. It took 20s for the system to stabilize. From the graph above, it can be seen that the deviations are now greater in magnitude as compared to that when the inflow disturbance of the system is smaller at 2.5V instead of 5V. The maximum deviations are -0.017V and -0.028V. As compared to the previous situation where the inflow disturbance to the system is 2.5V, inflow disturbance of 5V results in a poorer controller action, as seen from an increase in the magnitude of maximum deviations and the time taken to reach steady state.

4.2.2.3 Inflow disturbance of 7.5VThe inflow of the system at steady state is decreased from 10V to 2.5V a set point deviation by -7.5V. When the system has reached a new steady state at 2.5V, the inflow is increased from 2.5V to 10V a set point deviation of +7.5V. 2.5V to 10V10V to 2.5V


10V 2.5V116.00403.90287.90-2.770

2.5V 10V567.60609.6042.00+0.189

Figure 17. Plot of Responses with Inflow Disturbance of 7.5V

The inflow is perturbed at t=116.0s, where the inflow voltage is decreased from 10V to 2.5V. The system took 287.9s to attain a steady state, where the maximum deviation in level is -2.77V. The inflow is then perturbed again at t=567.6s, where the inflow voltage is increased from 2.5V to 10V. The system took a much lesser time (t=42s) to attain steady state this time round, which the maximum deviation from set point is at a much smaller value at +0.189V. The failure of the cascade controller to handle the perturbation could be due to the controller reaching its saturation point, which will prevent the large magnitude of the perturbation from being corrected. Similarly, F2-Ref plot showed large deviations in voltage with respect to time, showing how the cascade controller is unable to handle such large perturbation to the system.

4.2.3 Disturbances to Outflow Rate 4.2.3.1 Outlet Valve: Half-closed

Figure 18. Plot of Responses with Outflow Valve Half-closedDisturbanceTime of Change (s)Time at steady state (s)Time to reach steady state (s)Maximum deviation in level (V)

Fully-opened to half-closed153.50185.6032.10+0.030

Half-closed to fully-opened252.90320.0067.10-0.090

Disturbance to the outflow rate was made by adjusting the outlet valve to half-closed. At 153.5s, the outlet valve was adjusted from fully-opened to half-closed. This change led to an accumulation of water in the vessel as the outlet flow rate was decreased. In order to maintain the water level in the vessel at its set-point, adjustments to the system was made by decreasing the pump voltage as seen from the decrease of voltage of FT2. After approaching steady state for a moment, the outlet valve was adjusted back to be fully-opened at t=252.9s. This resulted in the increase of pump voltage as seen from the rise of the FT2 curve before achieving steady state. However, the final voltage of FT2 at steady state was higher than the initial voltage before disturbance was present.

4.2.3.2 Outlet Valve: One-third-closed

Figure 19. Plot of Responses with Outflow Valve One-third-closedDisturbanceTime of Change (s)Time at steady state (s)Time to reach steady state (s)Maximum deviation in level (V)

Fully-opened to one-third-closed118.00139.0021.00+0.015

One-third-closed to fully-opened167.60185.7018.10-0.029

Another disturbance was done to the outlet valve by closing it by one-third. As this adjustment was smaller than before, the drop in voltage was also lower than before as seen from the Figure above. The explanation for the change in voltage is similar to the explanation as the previous change in outlet flow.4.2.3.3 Outlet Valve: Two-third-closed

Figure 20. Plot of Responses with Outflow Valve Two-third-closedDisturbanceTime of Change (s)Time at steady state (s)Time to reach steady state (s)Maximum deviation in level (V)

Fully-opened to two-third-closed118.80214.5095.70+0.057

Two-third-closed to fully-opened279.00325.8046.80-0.116

Lastly, the outlet valve was closed two-third, the largest adjustment made to the outlet flow rate. In this case, the drop in the pump voltage was the largest as seen from the drop in the FT2 curve. From all the responses obtained with the changes in the outflow conditions, it can be seen that the cascade system is able to provide a fast and effective response when sudden changes were made to the outflow rate.

4.3 Discussion4.3.1 Feedforward Control1. Draw a block diagram of the system.Figure 21. Block Diagram of Feedforward System

1. In what circumstances would you recommend a feedforward control system rather than a cascade or single loop feedback control system? Point out any limitations of the feedforward control system.A feedforward control system is highly recommended to be used when the model of process (process dynamic) is well investigated and known. For instances, the process dynamics should be well understood which the variables present in the system can be measured and no other unknown disturbances presented in the system. The feedforward controller will take corrective action to the known disturbances before the process is upset. However, for both the cascade and single loop feedback control system will only take corrective actions to the system after the presented disturbance has affected the system. This results in a time lag in the disturbance correction. Thus, the cascade and single loop feedback control system is recommended when the disturbances present in the system is significant and unknown.The limitation of the feedforward control system can be considered as the disturbances have to be understood and measured, so that the corrective action can be taken before the process is upset. Thus, the model of the system (process dynamics) has to be studied and known to ensure accuracy in controlling the process. Besides, feedforward control system will only correct the measured disturbance and it will not accounted for those unknown or not measured disturbances in the system. To achieve a better performance, feedforward control should be used together with a feedback control in order to account for all the disturbances present in the system.

4.3.2 Cascade Control1. Draw a block diagram of the system.

Figure 22. Block Diagram of Cascade System1. What is the advantage of cascade control compared with single loop feedback control? Point out limitations of cascade control system.The primary difference between a cascade control and a single loop feedback loop is the configuration in which cascade control has a feedback loop inside another larger feedback loop. The advantage of a cascade control is only beneficial and feasible when the secondary loop reacts at a much faster speed than the primary loop. In addition, the secondary loop has to detect the greater disturbances present in the system. Hence, only in this configuration will the cascade control be able to provide better control performance. The faster response of the secondary loop will allow any disturbances present to be quickly adjusted and corrected before it is experienced by the primary loop. Resultantly, the faster response will lead to a reduction in lag time of the primary loop, therefore reducing the sensitivity to the upsets present in the primary loop.The main limitations of a cascade control system are high capital cost due to additional instruments and equipment required. In addition, the cascade control system has a complex control structure, which requires manual tuning of the controllers in the loop where adjustments are made.

1. Look carefully at a single loop feedback control system and explain why, when the valve is considered, it can be considered as a cascade control?For a system to be considered a cascade control system there must be a presence of 2 control loops: where the secondary (slave) control loop is nested within the primary (master) control loop. The error signals from the master loop will then become the reference input to the secondary loop.

Figure 23. Diagram of cascade control inside a feedback control systemFrom Figure 23, it depicts a primary level controller acting as a master control loop, analysing the error signal and providing the reference set point for the second PID controller within the valve to control the pump speed. The addition of the valve coupled with the second PID controller actually forms a secondary loop within the bigger master loop, creating a cascade control system, allowing a rapid response to the disturbances.5. Error Analysis1) When the drain valve for cascade control was adjusted manually, the size of the step change was unable to be measured accurately. Therefore a computerized flow indication control can be implemented instead of manual opening. This will provide a much more accurate step change.

2) There are always tiny fluctuations in the graphs observed. This is probably due to the noise affecting the system. A possible noise here is the splashing of the water on the surface of the liquid when the water drips from the top of the tank, which might affect the level indicators reading of the level in the tank. Therefore to mitigate this noise, we can allow the water to flow from the bottom instead of the top of the tank so that the surface of the liquid will not have splashing and will have a smooth rise or fall of the level.

3) Time is a factor affecting the accuracy of the experiment. Since the experiment takes a significant amount of time for every try, we were unable to conduct each part of the experiment more than once. Ideally we would be able to conduct the experiment repeatedly so that the average response time can be taken. This is avoid random errors caused during the experiment.

6. Lab Safety Analysis1) Since there is filling of the tank in the experiment, we need to make sure that the level does not increase to the brim and overflow. The possible hazard of overflowing might lead to wet electrical appliances such as pump which will lead to electrical hazards. It can also create a wet environment which might be susceptible to slipping.

2) In case of any anomaly, all the electrical appliances should be turned off such as the pump and the computer.

3) All electrical appliances should be handed with dry hands

4) Before conducting the experiment, it should be ensured that all appliances are working individually so as to not have any electrical hazards when the experiment is conducted.Generally the experiment is safe chemically because the only liquid used if water.7. ConclusionFeedforward ControlCascade Control

Advantages Immediate action to prevent the measured disturbance from affecting the system Works well for significant disturbances when used in tandem with feedback control Theoretically perfect control( prevents the measured disturbance fully) When manipulated variable is affected by unknown disturbances. Corrective action in response to any unmeasured disturbance within the slave loop which prevents the controlled variable from deviating from the setpoint Useful when there is a secondary process to which its manipulated variable exists which can be adjusted by the primary loop Faster corrective response to disturbance (affecting the manipulated variable) involving the slave loop before the master loop control variable deviates from set point- efficient for high occurrence disturbance

Limitations Accurate process model required Only applicable for measurable disturbances Cannot be used if the disturbance is not measurable Tuning is challenging and time-consuming The speed of the slave loop must be ensured to be faster than the Master loop to have effective cascade control

8. ReferencesPonton J.W. (2007) Variations on Basic Feedback Control. Retrieved from http://www.see.ed.ac.uk/~jwp/control06/controlcourse/restricted/course/second/course/lecture6.html

Seaborg D.E., Edgar T. F., Mellichamp D.A., Doyle F.J. (2010). Process Dynamics and Control (3rd Edition). Wiley.

Woolf P. (2007). University of Michigan Chemical Engineering Process Dynamics and Controls Open Textbook. Retrieved from https://controls.engin.umich.edu/wiki/index.php/Main_Page

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feed forward and cascade control experiment

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