AUTO-TUNE FOR PID CONTROL
Auto-Tune is a user initiated function in which the controller automatically
determines the PID settings based upon the process characteristics. During
Auto-Tune, the controller temporarily causes the system to oscillate by
cycling the output power from 0 to 100%. The nature of these oscillations
determines the settings of the controller’s parameters.
Note: If the induced oscillations caused by Auto-Tune can cause system
problems or are otherwise unacceptable, the Manual Tuning Procedure can
be used as a tuning alternative.
Prior to initiating Auto-Tune, it is essential that the controller be
configured to the application. In particular, Control Hysteresis (CHYS) and
Auto-Tune code (tcod) must be set in the Output Parameters section.
Auto-Tune requires the hysteresis setting to be above the process noise floor;
generally, 2 - 5 degrees or 1-2% of range is adequate. For most applications,
the control hysteresis should be set at 2 (factory setting). The Auto-Tune code
may be set to yield the response characteristics shown in Figure 28,
Auto-Tune Code. A code setting of zero gives the fastest response with
possible overshoot, and a code of two gives the slowest response with
minimum overshoot.
The following controller parameters are automatically adjusted by
Auto-Tune according to the characteristics of the process:
Proportional Band (ProP)
Integral Time (Intt)
DerivativeTime(dErt)
Input Filter (Fltr)
Output Power Dampening (OPdP)
As shown in Auto-Tune Operation, Figure 29, Auto-Tune Operation cycles
the process at a control point 3/4 of the distance between the current process
value (at the instant Auto-Tune is started) and the setpoint. The 3/4 control
-51-
Figure 28, Auto-Tune Code
Figure 29, Auto-Tune Operation
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