National Instruments PID Control Toolkit 371685C-01 - Manual (Page 20)

Chapter 3 Using the PID Control Toolkit
© National Instruments Corporation 3-3 LabWindows/CVI PID Control Toolkit User Manual
Open-Loop (Step Test) Tuning Procedure
The open-loop (step test) tuning procedure assumes that you can model any process as a
first-order lag and a pure deadtime. This method requires more analysis than the closed-loop
tuning procedure, but the process does not need to reach sustained oscillation. Therefore, the
open-loop tuning procedure might be quicker and more reliable for many processes. Observe
the output and the PV on a strip chart that shows time on the x-axis. Complete the following
steps to perform the open-loop tuning procedure.
1. Put the controller in manual mode, set the output to a nominal operating value, and allow
the PV to settle completely. Record the PV and output values.
2. Make a step change in the output. Record the new output value.
3. Wait for the PV to settle. From the chart, determine the values as derived from the sample
displayed in Figure 3-1. The variables represent the following values:
• Td—Deadtime, in minutes
• T—Time constant, in minutes
• K—Process gain = change in PV/change in output
Figure 3-1. Output and Process Variable Strip Chart
4. Multiply the measured values by the factors shown in Table 3-2 and enter the new tuning
parameters into your controller. Table 3-2 provides the proper values for a quarter-decay
ratio. If you want less overshoot, reduce the gain (K
c
).
Table 3-2. Factors for Determining Tuning Parameter Values (Open Loop)
Controller PB (Percent) Reset (Minutes) Rate (Minutes)
P 100 (KT
d
/T) — —
PI 110 (KT
d
/T) 3.33 T
d
—
PID 80 (KT
d
/T) 2.00 T
d
0.50 T
d