User`s guide

Series D8 User’s Guide Chapter 4: Operation and Setup
Doc. 0600-3120-2000 Watlow Anafaze 91
Table 4.6 Input Readings and Calculations
Table 4.7 Scaling Values
Autotuning
Autotuning is a process by which a controller calculates the
correct PID parameters for optimum control. Only the heat
output of a loop may be autotuned.
How Does Autotuning Work?
Autotuning is performed at the maximum allowed output. If
an output limit has been set, then autotuning occurs at that val-
ue. Otherwise, the control output is set to 100 percent.
The PID constants are calculated according to process re-
sponse to the output. The loop need not reach or cross the set
point to successfully determine the PID parameters.
The controller looks at the delay between when power is ap-
plied and when the system responds and uses this information
to determine the proportional band. The controller then looks
for the slope of the rising temperature to become constant to
determine the integral term. The controller mathematically
derives the derivative term from the integral term.
When the controller finishes autotuning a loop, it switches the
loop to automatic mode. If the process reaches 80 percent of
the set point or the autotuning time exceeds 30 minutes, the
controller switches the loop to automatic mode and applies the
PID constants it has calculated up to that point.
Autotuning is started at ambient temperature or at a tempera-
ture above ambient. However, the temperature must be stable
Process
Variable
Displayed
Sensor
Input
Reading in
Percent of Full Scale
65 GPM 4.75 V (4.75 V / 5.00 V) x 100% = 95%
3 GPM 0.5 V (0.5 V / 5.00 V) x 100% = 10%
Parameter Value
Input range high 65 GPM
Input high signal 95.0%
Input range low 3 GPM
Input low signal 10.0%