Thursday, March 26, 2009
Company name
Company address
RE/ Process control loop analysis Loop-Tag
Dear Sir,
We submit this report on the loop Loop-Tag. This analysis should help you improve the performance, organize
maintenance and update your information.
Please call us with questions.
Regards,
ExperTune Inc.
81919388
Page 1 of 1
Legend
Initial Tests
Automatic Mode
Manual mode
Notes:
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81919388
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Legend
Hysteresis Check
Found
0.528 %
Hysteresis
0.822
Gain
Noise
Valve:
Acceptable values
less than 1%
ideal
from 1 % to 2%
acceptable
more than 3%
to be checked
less than 0.5
too small
from 0.5 to 2
ideal
more than 3
too high
less than 3%
acceptable
0.873 %
N/A
Note: Enter your valve recommendations here.
If your hysteresis is more than 1% for valves with positioners or 3% for valves without positioners you should
consider repairing or changing equipment to reduce hysteresis and improve control. Often, the addition of a valve
positioner will correct the problem.
With a sticky valve, the air signal to the valve will have to change by an amount equal to the hysteresis before the
valve stem will move. Once the valve has begun to move in one direction it will continue to move if the air signal
keeps moving in the same direction. When the air signal reverses direction, the valve will not move until the air
signal has changed in the new direction by an amount equal to the hysteresis.
Hysteresis is not limited to valves, it is also present in mechanical linkages, and can have other sources, but
valves are the most typical sources.
ExperTune Inc.
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Page 2 of 18
Legend
Stiction Check
Found
Stiction is less than 1%
Stiction
CO High
CO Low
Valve:
Acceptable values
less than .2%
ideal
more than .3%
to be checked
61
60
N/A
Note: Enter your valve recommendations here.
In many processes, 0.5 % of stiction is too much. Stiction guarantees cycling and variability. Stiction is much
more harmful than the other valve problems. For example, hysteresis is undesirable, but usually not really a
problem. Another example is the valve characteristic, which can be compensated by a non-linear function inserted
in the controller output or in the positioner.
ExperTune Inc.
81919388
Page 3 of 18
Legend
Linearity
Linearity
Notes
N/A
Loop requires characterization or MUST be detuned. The process gain GP varies from
0.283 to 1.96; the ratio of these values is 6.92. This ratio should be under 2 and the
process gain values should be between .5 and 3.
Many control loops are difficult to tune because they are non-linear. This means that the process gain changes as
a function of the measurement or controller output. Without any linearization, the controller will need to be tuned
for the condition when the process gain is the highest. This results in sluggish tuning everywhere else.
Linearizing these loops will improve control since the controller will be better tuned over the complete operating
range. With a characterizer, you may be able to get uniform performance across the entire range of your process so you can run your production at its optimum rather than de-tuning for oscillations.
ExperTune Inc.
81919388
Page 4 of 18
Legend
Asymmetry
Process Model With Increasing PV
Asymmetry
Notes:
Process Model With Decreasing PV
N/A
Enter Your Asymmetry Description Here
Compare PID tuning or models identified with the ExperTune Process Modeler. Does the process respond
differently in the up direction versus the down? If so, can you reduce or eliminate the discrepancy? If not, then
you must use the more conservative tuning.
To determine the tuning parameters and the appropriate filter, use the worst case: least aggressive tuning or worst
model. The worst model is one with the largest dead time and a highest process gain.
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Test Chosen To Compute Tuning Parameters tsampling : 0.5sec
Note: This part of the test must represent the worst situation to ensure the loop stability. The worst case is a
high process gain GP, a long dead time Td and a small time constant Tc.
Bode Plot
Tuning Parameters
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Tuning Parameters, Performance, And Robustness
Process model found
Old and new tuning parameters
(fastest response)
Robustness
Notes
New
P
50
220
I
35
4.6
D
0
.52
F
0
.026
RRT
17
49
Performance Indices (From current to new)
Performance Increase
73%
Response of the loop to a set-point change
Parameters
Current
Robustness Increase
294%
Valve Travel Index (based on noise simulation)
118%
Valve Reversal Index (based on noise
simulation)
-4.59%
Response of the loop to a disturbance or a load
change
N/A
Initial battery of tests
ExperTune Inc.
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Page 7 of 18
Legend
Loop Summary Table
Notes
Performance Indices
Performance Increase: The Performance Increase shows how much better your plant responds to upsets with
the new tuning. Usually the Performance Increase is directly proportional to the money you can save using the
new tuning. With poor tuning, an upset in the direction towards expensive results causes you to give away
product. Or, a load causes off-spec product. With better tuning you can give away less expensive ingredients
while staying on spec.
Robustness Increase: This is the percent that the robustness improves with the new tuning. It is based on
comparing the new and current tuning. If the loop is less robust with the new tuning, then the Robustness
Increase will be negative. You can use the Robustness Increase as a quick and rough way of assigning a
numerical value to the change in robustness from current to new settings. If either loop is unstable, the
Robustness Increase will show N/A.
Valve travel and reversal indices: Use these values to predict valve wear by comparing current to new. The less
controller output travel and direction changes, the less valve wear. The Valve Travel Index is the percent
improvement in controller output travel from current to new. If the new PID tuning parameters (and filter) result
in more travel than the current ones, the Valve Travel Index will be negative. The Valve Reversal Index is the
percent improvement in controller output reversals from current to new. If the new PID tuning parameters (and
filter) result in more reversals than the current ones, the Valve Reversal Index will be negative.
Robustness Plot
There is always a trade-off between tuning the loop tightly, and robustness or sensitivity to the process changing.
If tuned too fast, a small change in the process could cause the loop to be unstable. Use the robustness plot to
assess this tradeoff. Robustness plots graphically show the tradeoff between tight tuning and stability
The two axis of the plot are process gain and dead time. The plot has a region of stability and a region of
instability. The solid (red and blue) lines on the robustness plot are the limit of stability. To the right and above
the solid lines (higher gains and dead times) the closed loop process is unstable. To the left and below the solid
lines, the closed loop system is stable. There is a cross in the center of the blue line trapezoidal-like figure. This
cross is at the actual process gain and dead time. On the graph, moving away from the cross shows what happens
when gain and dead time changes. Process gain and controller gain changes have the same effect on closed loop
system stability.
ExperTune Inc.
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Legend
The blue line is for the current PID tuning. The red line is for the new PID parameters. Generally, a safety factor
or divisor of 2 is "reasonable" for a loop. These points are represented by the vertices of the blue line "box" in the
robustness plot. Use it as a design aid. For practical system stability keep the limit of stability line outside the
"box". The vertices are connected by lines that are straight on a log log plot.
ExperTune Inc.
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Legend
Time Line Analysis
Time Line Analysis
Notes
Time Line Items
Sample Interval
Filter Time
Derivative Time
Dead Time
Equivalent Dead Time
Integral Time
Lag Time
Relative Response Time (RRT)
Values
0.5
0.054
1.1
3
5.3
4.1
2.3
43
Evaluation Ratios
Dead Time / Sample Interval
Filter Time / Derivative
Controller Integral / Derivative
Filter / Dead Time
Values
6
0.049
3.7
0.01
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Evaluated Time Items
Sample Interval to Process
Dead Time
PV Filter to Controller
Derivative
Controller Derivative to
Controller Integral
PV Filter to Process Dead
Time
Evaluations
Caution: Better if range greater than 9
Excellent
OK
OK
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Performance Evaluation
Performance Evaluation
Notes
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Closed Loop Before and After
Before
After
Set point change
Statistical analysis
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Histogram
Note :
Power Spectral - Before
Note :
Power Spectral - After
Note :
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Auto or Cross Correlation - Before
Note :
Auto or Cross Correlation - After
Note :
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Conclusions And Recommendations
Enter your conclusions here.
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Legend
To Do
Valve
Hysteresis
N/A
0.528 %
Gain
Linearity
N/A
Enter your valve recommendations here.
Loop requires characterization or MUST be detuned..
Asymmetry
N/A
Enter Your Asymmetry Description Here .
PID parameters
N/A
Initial battery of tests
0.822
Conclusions And Recommendations
Enter your conclusions here.
N/A
ExperTune Inc.
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Legend
OK
CF
MO
VM
RL
RA
N/A
Equipment performing well
Values configured at the plant
To be modified
Should be checked and analyzed more
Should be repaired in the plant
To be repaired (during next shut-down)
Not applicable
© 1998-2009 ExperTune Inc. ExperTune gives you permission to modify, copy and print this document for your
own or customer’s Loop Analysis reporting use.
ExperTune Inc.
81919388
Page 18 of 18