Controller Design

Safety Procedures

Please be aware of any potential hazards prior to operating any experiment. If you're designing a controller for the pH bath then this experiment requires 0.1 M hydrochloric acid and 0.1 M sodium hydroxide, concentrations strong enough to cause skin irritation. In the event of exposure, run water over the affected area, contact lab personnel, and if additional medical attention is required use the protocols at the lab exits to contact EH&S or call 911.

Other Safety Concerns:

• Wear a lab coat and safety glasses at all times

• Check all pump connections before starting pumps.

• Rinse the pH probe with DI water after use and return it to its container with its storage fluid. Do not replace the storage fluid!

Programs, Equipment, and Instruments

For the pH control experiment:

• Stir plate

• pH probe with storage fluid

• Peristaltic pump with speed controller

• Computer with LabVIEW

For the temperature control experiment:

• Combination hot plate / stir plate

• Temperature sensor

• Peltier cooler

• Fixed speed pump

• Computer with LabVIEW

Objectives

Assume that your system can be represented by the following simplified block diagram:

1. (lab) Perform an open loop step test to generate data of the form Y(t) as a function of time for a given actuator signal A(t)

   1. For pH control, A(t) is % power sent to pumps (-100% to +100%)

   2. For temperature control, A(t) is % power sent to Peltier cooler (0% to 100%).

2. (math) Propose a model (process) transfer function Gp = Y(s)/A(s). Fit this transfer function to the data generated in the previous step to determine any unknown gains or time constants.

3. (math) Use Direct Synthesis to determine the coefficients of a PID controller to be used as Gc.

4. (math) Use Simulink to predict the closed loop step response using the model and PID settings calculated above. Note Ysp(t), A(t), and Y(t) for comparison in the next step.

5. (lab) Use the controller VI with your calculated PID coefficients to determine the experimental response (Ysp(t), A(t), Y(t)) of the system.

6. (math) Compare the experimental to theoretical closed loop responses.

7. (lab+math) Repeat 3-6 using IAE or IMC tuning methods.

Tips

1. Plan your experiments before coming to the lab.

2. For the pH control system, make sure you are always working in the pH buffer region! To find your buffer region, perform a titration on a buffer solution from about pH 4 to pH 9. The flat region around pH 6-7 is the buffer region (sometimes it drifts).

3. For the temperature control system, always start your sample at room temperature. Don't use the heating function of the hotplate. 

References

1. Seborg, D.E.; Edgar, T.F.; Mellichamp, D.A., Process Dynamics and Control, 2nd Ed.; John Wiley and Sons: New Jersey, 2004, Ch. 12.

2. Riggs, J.B.; Karim, M.N.; Chemical and Bio-Process Control, 4th ed.; Ferret Publishers: Lubbock, Texas, 2016

3. Shimizu, K., An overview on the control system design of bioreactors. Adv. in Biochem. Eng./Biotech. 1993, 50, 65-84

4. Liden, G., Understanding the bioreactor. Bioproc. Biosyst. Eng. 2002, 24, 273-279.

5. Komives, C.; Parker, R.S., Bioreactor state estimation and control. Cur. Opin. Biotech. 2003, 14, 468-474.

6. Seborg, D.E. et al., Process Dynamics and Control, 2nd ed.; John Wiley and Sons: Hoboken, NJ, 2004, pp 317-319.

7. Dochaina, D.; Tali-Maamarb, N.; Babary, J.P., On modelling, monitoring, and control of fixed bed bioreactors. Comp. & Chem. Eng. 1997, 21, 1255-1266.

8. Parulekar, S.J.; Lim, H.C., Modeling, optimization, and control of semi-batch bioreactors. Adv. Biochem. Eng./Biotech. 1985, 32, 207-258. (link)

9. Sung, S.W.; Lee, I., pH control using a simple set point change. Ind. Eng. Chem. Res. 1995, 34, 1730-1734.

10. Wright, R.A.; Kravaris, C., Nonlinear control of pH processes using the strong acid equivalent. Ind. Eng. Chem. Res. 1991, 30, 1561-1572.

11. Anil, C.; Sree, R.P., Tuning of PID controllers for integrating systems using direct synthesis method. ISA Transactions. 2015, 57, 211-21

12. Chau, P., Process Control: a first course with MATLAB; Cambridge University Press: New York, 2002.