1. Control Programming
By Peter Woolf University of Michigan Michigan Chemical Process Dynamics and Controls Open Textbook
version 1.0
Creative commons

2. Specify with a control program!
Process from previous class..
What more do we need?
• How do these controllers influence the valves and motors?
• What is the sequence of actions?
• What happens in an emergency?
Specify with a control program!
TC1: V5
PC1: V6, V7, V8
LC3: V1, V2, V3, SV1, M3, M4
FC3: V3, M3
FC4: SV1
FC1: V1, V2, M1
FC2: V1, V2, M2
LC1: V1, V8, V2, M1
LC2: V1, V2, M2
TC2: V7

3. What is a control program?
A detailed procedure identifying what every valve should be doing under each possible condition.
A computing language for expressing control relationships
Can be discrete (on/off or high/medium/low) or continuous (open % from 0% to 100%)

4. Control Program Languages?
Control specific languages: Ladder logic, Structured text, + many proprietary versions
General computing languages: Basic, C, Pascal, and less commonly C++, Java, and Python
Here we will use a composite pseudo code that has most of the features without the syntax issues.

5. Control Programming Syntax
IF.. THEN .. ELSE
CASE
WHILE
GOTO
ALARM

6. Control Programming Syntax
IF.. THEN .. ELSE
IF T>Tset+2:
open v1
ELSE:
close v1
IF T>Tset+2:
open v1
ELSE IF T<Tset-2:
close v1
: starts a sub block
indent defines a sub block
ELSE IF allows a series of conditionals
open
closed
Tset
Tset-2
Tset+2
time

8. Problem:
Equals implies exactly equal to a controller.
Thus temp=90°C is interpreted as temp= °C at one of the sampled times which won’t happen!
Solution:
Use >, <, ≥, or ≤ instead of =

9. CASE: alternate syntax that can be cleaner than many IF
CASE: alternate syntax that can be cleaner than many IF.. THEN statements
CASE:
T>Tset+2:
v2=v2+0.1
T>Tset +1:
v2=v2+0.05
T<Tset-1:
v2=v2-0.05
T<Tset-2:
v2=v2-0.1
IF T>Tset+1:
IF T>Tset+2:
v2=v2+0.1
ELSE:
v2=v2+0.05
ELSE IF T<Tset-1:
IF T<Tset-2:
v2=v2-0.1
v2=v2-0.05
indent note nested indents
Tset
Tset-2
Tset+2
time

10. WHILE: Creates loops for things that take an unknown number of iterations.
WHILE LC1<LC1set:
open v1
close v1
WHILE TC1<TC1set:
heater on
heater off

11. GOTO: Breaks out of current run to go to a different configuration.
WHILE time<RxnTime:
IF T>Trxn:
close v1
ELSE:
open v1
IF OR(T>Talarm, P>Palarm):
GOTO FAILSAFE
FUNCTION: defines a function
FUNCTION FAILSAFE:
# an emergency condition to shut down the process
close v1, open v2
#: defines a comment

12. ALARM: Alerts the operators of a problem
ALARM: Alerts the operators of a problem. Alarms may not be sufficient danger to shut down the process, but requires outside attention.
Examples:
If storage tank of a reactant is low, then ALARM
If a reactor does not heat up in a normal time window, then ALARM
If no flow is detected even when the valve is open, then ALARM
(might be cause for FAILSAFE if the valve is critical)
If redundant sensors disagree, then ALARM

13. Common functions accessed using GOTO
FUNCTION INITIALIZE:
# runs at the start of a process to make sure all
# valves and motors are in correct position.
FUNCTION PROGRAM:
# main run
FUNCTION FAILSAFE:
# emergency mode
FUNCTION SHUTDOWN:
# normal shutdown procedure
FUNCTION IDLE:
# power down process

14. Reset counters and timers Turn off all motors Turn off heaters
Common functions accessed using GOTO
FUNCTION INITIALIZE:
# runs at the start of a process to make sure all valves
# and motors are in correct position.
Example operations:
Close all valves
Reset counters and timers
Turn off all motors
Turn off heaters

15. Open or close valves to stop the system
Common functions accessed using GOTO
FUNCTION FAILSAFE:
# emergency mode
Example operations:
Open or close valves to stop the system
Quench reactions via cooling, dilution, mixing, or other method.

16. What is the procedure for this process?
TC1: V5
PC1: V6, V7, V8
LC3: V1, V2, V3, SV1, M3, M4
FC3: V3, M3
FC4: SV1
FC1: V1, V2, M1
FC2: V1, V2, M2
LC1: V1, V8, V2, M1
LC2: V1, V2, M2
TC2: V7

17. Procedures to write: FUNCTION INITALIZE FUNCTION FAILSAFE
A measured amount, QS, of solvent is added to the reactor
A measured amount of solid B, QB, is added to the S and blended in for 5 minutes.
A total quantity of A, QA is slowly added to the reactor. The A and B react quickly producing the product polymer in an endothermic reaction. The temperature must be maintained below TR+5 degrees to prevent forming a hard polymer. The pressure of the reaction is somewhat elevated due to the partial vaporization of S. However, the tank pressure must not exceed 3 atm.
The product is held in the reactor for 30 minutes after the reaction has stopped to ensure complete conversion.
5. The S is then boiled off and recovered in an overhead condenser. This recycled S is then sent to storage for reuse.
6. The product is then cooled to ambient temperature and pumped to blending.
Tamb<TR<TS<TA<Tprod<TB
Procedures to write:
FUNCTION INITALIZE
FUNCTION FAILSAFE
FUNCTION PROGRAM

18. set all totalizers to zero
FUNCTION INITALIZE:
turn off M1, M2, M3, M4
close v1,v2,v3,v5,v6,v7,v8, SV1
set all timers to zero
set all totalizers to zero
Note v4 is manual, so not
controlled by program
Counters used for measurement

19. FUNCTION FAILSAFE: turn off M1, M2, M3 IF LC3>LC3min: turn on M4
ELSE:
turn off M4
close v1, v2, v3, v5, SV1
open v6, v7, v8
Keep agitator going if sufficient level in tank to avoid hot spots, otherwise turn off agitator
Prevent pressure increase in reactor

20. FUNCTION PROGRAM # step 1 turn on M1 open v6, v7,v8
A measured amount, QS, of solvent is added to the reactor
FUNCTION PROGRAM
# step 1
turn on M1
open v6, v7,v8
WHILE FC1tot<Qs:
adjust v1 to FC1set
IF LC1<LC1min:
ALARM
close v1
turn off M1
Open v1 to some
control set point
until charged

21. FUNCTION PROGRAM (continued) # step 2 WHILE FC4tot<QB:
A measured amount, QS, of solvent is added to the reactor
A measured amount of solid B, QB, is added to the S and blended in for 5 minutes.
FUNCTION PROGRAM (continued)
# step 2
WHILE FC4tot<QB:
adjust SV1 to FC4set
IF FC4<FC4min:
ALARM
IF LC3>LC3min:
Turn on M4
close SV1
WAIT 5 minutes

22. FUNCTION PROGRAM (continued) # step 3 WHILE TC1<TR: adjust v5 to TR
3. A total quantity of A, QA is slowly added to the reactor. The A and B react quickly producing the product polymer in an endothermic reaction. The temperature must be maintained below TR+5 degrees to prevent forming a hard polymer. The pressure of the reaction is somewhat elevated due to the partial vaporization of S. However, the tank pressure must not exceed 3 atm.
FUNCTION PROGRAM (continued)
# step 3
WHILE TC1<TR:
adjust v5 to TR
turn on M2
WHILE FC2tot<QA:
adjust v2 to FC2set
IF LC2<LC2min:
ALARM
IF OR(TC1>TR+5, PC1>3 atm):
GOTO FAILSAFE

23. FUNCTION PROGRAM (continued)
# step 3
WHILE TC1<TR:
adjust v5 to TR
turn on M2
WHILE FC2tot<QA:
adjust v2 to FC2set
IF LC2<LC2min:
ALARM
IF OR(TC1>TR+5, PC1>3 atm):
GOTO FAILSAFE
IF PC1<PC1min:
close v6, v7
ELSE:
adjust v6 to PC1set
adjust v7 to TC2set
IF LC3>LC3max:
close v2
FC2tot=QA
close v2, turn off M2

24. FUNCTION PROGRAM (continued) # step 4 WHILE timer<30 minutes:
4. The product is held in the reactor for 30 minutes after the reaction has stopped to ensure complete conversion.
FUNCTION PROGRAM (continued)
# step 4
WHILE timer<30 minutes:
adjust v5 to TR
IF OR(TC1>TR+5, PC1>3 atm):
GOTO FAILSAFE
IF PC1<PC1min:
close v6, v7
ELSE:
adjust v6 to PC1set
adjust v7 to TC2set

25. FUNCTION PROGRAM (continued) # step 5 open v5 WHILE TC1<TS+1:
5. The S is then boiled off and recovered in an overhead condenser. This recycled S is then sent to storage for reuse.
FUNCTION PROGRAM (continued)
# step 5
open v5
WHILE TC1<TS+1:
IF PC1>3 atm:
GOTO FAILSAFE
IF PC1<PC1min:
close v6, v7
ELSE:
adjust v6 to PC1set
adjust v7 to TC2set

26. FUNCTION PROGRAM (continued) # step 6 close v5 WHILE TC1>Tamb+1:
IF PC1>3 atm:
GOTO FAILSAFE
IF PC1<PC1min:
close v6, v7
ELSE:
adjust v6 to PC1set
adjust v7 to TC2set
6. The product is then cooled to ambient temperature and pumped to blending.

27. FUNCTION PROGRAM (continued)
# step 6
close v5
WHILE TC1>Tamb+1:
IF PC1>3 atm:
GOTO FAILSAFE
IF PC1<PC1min:
close v6, v7
ELSE:
adjust v6 to PC1set
adjust v7 to TC2set
close v7
open v6
turn M3 on
WHILE LC3>0:
adjust v3 to FC3set
IF LC3<LC3min:
turn M4 off
turn off M3
close v3, v7, v8

28. Programming Tips
Programs can quickly get very complicated, thus it helps to write out the program in words first, then syntax later
Remember that valves and motors stay in the state you left them
Beware of conflicts between objectives and infinite loops

29. Take Home Messages
Control programs provide a detailed description of how a process behaves
Writing control programs requires a deep understanding of the process
Write out the program in words first, syntax second.
Break up the program into smaller steps, modules, and objects
Comment your code!