PROCESS CONTROL SYSTEMS
LAP 10 – Control Loop Performance
1) Resolution
a) Control loop optimization and its importance
i) Process of ensuring that all loop components are
selected and configured to provide the desired system
response
ii) Ensures that the PV will be maintained at the setpoint
iii) Optimization is composed of two parts
(1) Physical
(a) Instrument resolution and accuracy are sufficient
(b) Process system components are properly sized
(2) Programming
(a) Controller parameters are properly configured –
“Tuning”
iv) An estimated 70-80% of process control loops are not
optimized
(1) Increased operating costs
(2) Decreased efficiency
(3) Increased wear on system instruments and
components
b) Instrument resolution and its importance
i) Resolution is the smallest distinguishable difference
between two measurements or values
ii) Smallest difference an instrument can sense or control
iii) Determines how closely a process can maintain a desired
setpoint
c) Calculating instrument resolution in units of the measured
parameter
i) Rmp = Sp/Res
(1) Rmp = Resolution in Units of Measurement
Parameters
NOTES:
(2) Sp = Span
(3) Res = Instrument Resolution
ii) Example: A temperature sensor has a span of 5° C and a
resolution of 5 points. What is the Rmp?
(1) Rmp = 5° C/5 = 1° C
2) Accuracy & Repeatability
a) Accuracy
i) Measure of how closely an instrument’s output matches
the actual value of the process variable
ii) The accuracy of each instrument in a process has an
effect on the overall accuracy of the process
iii) Accuracy of each instrument must be high enough to
ensure that the required overall process accuracy can be
met
iv) Sources of inaccuracy
(1) Gear backlash
(2) Hysteresis and stiction in valves
(3) Improper calibration of process instruments
(a) Most common source of inaccuracy and the
easiest to correct
b) Five methods of expressing accuracy
i) A factor of the measured variable
(1) A pressure sensor has a stated accuracy of ± 1 psi
(2) If the indicated process pressure is 100 psi, the actual
pressure may be anywhere between 99 and 101 psi
ii) Percentage of the span
(1) A level sensor has a span of 50 inches and a stated
accuracy of 1%
(2) Multiply the span by the percentage accuracy
(a) 50 X .01 = .5
Accuracy = ± .5 inch across
the span
(b) If the indicated level is 25 inches, the actual level
may be anywhere between 24.5 and 25.5 inches
iii) Percentage of the upper range value (URV)
(1) A temperature sensor is rated for the range of 100° –
120° C with a stated accuracy of 2.5%
(2) Multiply the Upper Range Value(URV) by the
percentage accuracy
(a) 120 X .025 = 3 Accuracy is ±3° across the
range
(b) If the indicated temperature is 105° C, the actual
temperature may be anywhere between 102° and
108° C
iv) Percentage of the scale length
(1) An indicating scale has a length of 20 inches and a
stated accuracy of 1%
(2) Multiply the scale length by the percentage accuracy
(a) 20 X .01 = .2
Accuracy is ± .2 inches across
the scale
(b) If the actual level is 12 inches, the measured level
may be anywhere between 11.8 and 12.2 inches
v) Percentage of the displayed reading
(1) A controller displays the level in a tank as 75 feet
with a stated accuracy of 2%
(2) Multiply the displayed reading by the percentage
accuracy
(a) 75 X .02 = 1.5
Accuracy is ± 1.5 ft across the
range
(b) If the displayed level is 75 ft, the actual level may
be anywhere between 73.5 and 76.5 ft.
c) Repeatability
i) The ability of an instrument to consistently give the same
reading or output if the same input is repeated a number
of times
ii) Repeatability is often erroneously used synonomously
with accuracy
(1) These terms are not interchangeable
iii) Results that are both accurate and repeatable are often
difficult to achieve
(1) Repeatability is sometimes preferred over inaccuracy
because it is often easier to compensate for
inaccuracy than repeatability
3) Open-Loop Tuning
a) Loop tuning and its importance
i) Process of determining the best control
settings(Proportional, Integral, and Derivative) for
optimal loop performance and entering them into the
controller
ii) Usually the last step in control loop optimization
iii) Performed on initial startup and repeated periodically as
needed
iv) Goal
(1) Control the process variable (PV) as accurately as
possible
(2) Ideal control (tight control) would feature
(a) Rapid response
(b) Minimal overshoot
(c) No offset error
(3) Ensure the quality of the final product
v) Standards for determining when a loop is tuned
(1) Quarter wave display response
(a) The second oscillation is ¼ the size of the first
oscillation
vi) Problem
(1) Determining which control mode to use and how
much of each to apply
(2) Generic guide
CONTROL LOOP
Flow
CONTROLLER MODE
PROPORTIONAL INTEGRAL
Always
Usually
DERIVATIVE
Never
Level
Temperature
Analytical
Pressure
Always
Always
Always
Always
Usually
Usually
Usually
Usually
Rarely
Usually
Sometimes
Sometimes
vii) Tuning methods
(1) Procedures used for learning the dynamics of the
process
(2) Three basic categories
(a) Open-Loop methods
(b) Closed-Loop methods
(c) Tuning software
(3) Methods focus on the two major system dynamics
(a) Time
(b) Amount
b) Open-loop tuning and an application
i) Determines the PID settings for closed-loop operation by
testing the system dynamics with the controller in the
manual(open-loop) mode
ii) Open-loop tuning methods include:
(1) Process Reaction Curve
(2) Point of Inflection
(3) Open-loop Process Gain
iii) Advantage of Open-Loop tuning
(1) Responds quickly to disturbances or changes in the
setpoint
(2) Good choice for pH control loops
iv) Disadvantage of Open-Loop tuning
(1) Requires that the system be taken out of automatic
mode
(a) May be disruptive to the process
c) Tuning a loop using the process reaction curve open-loop
method
i) Create a reaction curve for the process
ii) Use the reaction curve to determine the reaction rate and
lag time
iii) Calculate the values for the PID settings using the
Reaction Rate and Lag Time
iv) Enter the calculated values into the controller
v) Test the process for the desired response
d) Tuning software and applications
i) Computer software designed to determine the PID values
based on the process data entered
ii) Provides a way to simulate a process and predict the
optimal settings
iii) Advantage
(1) Actual process is not disturbed while determining the
settings
(2) Controller settings are updated by going “on-line”
with the software and downloading the settings to the
controller
(3) Versatile and can be used with any control loop
(4) Popular in networked settings
4) Closed-Loop Tuning
a) Process that determines the PID settings for closed-loop
operation by testing the system dynamics with the controller
in the automatic mode (closed loop).
i) Controller continues to control the process while the
settings are determined and changed
ii) Common closed-loop tuning methods include:
(1) Ultimate gain method
(2) “Short cut” method
iii) Each method produces a different process response
(1) Response curves are examined and the data gathered
is inserted into standard equations to determine the
PID settings
iv) Advantage of closed-loop tuning
(1) Includes the effects of control valve hysteresis and
process dead time
(2) Easier to find a good compromise between tight
control and fast response
v) Disadvantage of closed-loop tuning
(1) Controller maintains control of the process during
tuning
(a) Process must tolerate short term changes during
the tuning process
(b) Often used in pressure and temperature control
loops because they are less sensitive to brief
changes
(c) Would typically not be used in a flow control
loop
b) Describe how to tune a loop using the ultimate gain closedloop method
i) Developed by Zeigler and Nichols in 1942
ii) Determines the ultimate gain of the system
(1) Value of Proportional gain Kp that causes a stable
sine wave response
(a) Values for ultimate gain and the period of the sine
wave are plugged into standard formulas to
determine the needed PID settings
iii) Tuning a loop using the ultimate gain closed-loop
method
(1) Set the controller so that only proportional control is
active
(2) Bump the setpoint and create a response curve
(3) Adjust the gain until the response is a stable sine
wave
(4) Determine the ultimate gain and the ultimate period
(5) Calculate the PID settings using the ultimate gain and
ultimate period
(6) Enter the calculated values into the controller
(7) Test the process for the desired response