National Instruments PID Control Toolkit 371685C-01 - Manual (Page 19)

Chapter 3 Using the PID Control Toolkit
LabWindows/CVI PID Control Toolkit User Manual 3-2 ni.com
Tuning Controllers Manually
The following controller tuning procedures are based on the work of Ziegler and Nichols, the
developers of the Quarter-Decay Ratio tuning techniques derived from a combination of
theory and empirical observations (Corripio 1990). Experiment with these techniques and the
process control simulation examples to compare them. For different processes, one method
might be easier or more accurate than another. For example, some techniques that work best
when used with online controllers cannot stand the large upsets described here. To perform
these tests, set up your control strategy with the PV, SP, and output displayed on a large strip
chart with the axes showing the values versus time. Refer to the Closed-Loop (Ultimate Gain)
Tuning Procedure and
Open-Loop (Step Test) Tuning Procedure sections for more
information about disturbing the loop and determining the response from the graph. Refer to
Tuning of Industrial Control Systems as listed in Appendix A,
References , for more
information about these procedures.
Closed-Loop (Ultimate Gain) Tuning Procedure
Although the closed-loop (ultimate gain) tuning procedure is very accurate, you must put your
process in steady-state oscillation and observe the PV on a strip chart. Complete the following
steps to perform the closed-loop tuning procedure.
1. Set both the derivative time and the integral time on your PID controller to 0.
2. With the controller in automatic mode, carefully increase the proportional gain (K
c
) in
small increments. Make a small change in the SP to disturb the loop after each increment.
As you increase K
c
, the value of the PV should begin to oscillate. Keep making changes
until the oscillation is sustained, neither growing nor decaying over time.
3. Record the controller proportional band (PB
u
) as a percent, where PB
u
= 100/K
c
.
4. Record the period of oscillation (T
u
) in minutes.
5. Multiply the measured values by the factors shown in Table 3-1 and enter the new tuning
parameters into your controller. Table 3-1 provides the proper values for a quarter-decay
ratio. If you want less overshoot, increase the gain (K
c
).
Note Proportional gain (K
c
) is related to proportional band (PB) as follows: K
c
= 100/PB.
Table 3-1. Factors for Determining Tuning Parameter Values (Closed Loop)
Controller PB (Percent) Reset (Minutes) Rate (Minutes)
P 2.00 PB
u
— —
PI 2.22 PB
u
0.83 T
u
—
PID 1.67 PB
u
0.50 T
u
0.125 T
u