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SC100/200 Series Users Manual EM-6337-B Rev.15
9. AUTO-TUNING
The PID controller provides accurate control characteristics only by adequately combining proportional band (P), integral
time (I) and derivative time (D) and tuning each parameter to most appropriate values.
The P, I and D parameters can be set on the tuning view of the Controller. In addition, it has the auto-tuning mode enabling
the user to determine approximate parameters automatically by simple panel operations.
9.1. GENERAL DESCRIPTIONS
The SC Controller uses the limit cycle oscillation for auto-tuning. The control output (MV) is given with a cyclic oscillation
(two cycles) at the reference tuning point (CV) where the PV behavior is observed. The most appropriate PID parameters
are then computed from measured values of the amplitude and frequency of the established output limit cycle waveform.
During this procedure, MV must be given with sudden changes, therefore this method is not recommended to a system that
does not tolerate such disturbance for valve operations. Furthermore, it is not suitable for use in a control system with fast
response time or that with an extremely long cycle time.
In general, the limit cycle method is recommended when the ratio L/T of dead time (L) divided by time constant (T) ranges
between 0.15 and 0.6, while the control characteristics worsen with values greater than 1, reach its controllable limit with 2,
when it is no longer suitable for PID control.
Be aware that the process may react in an unexpected way during auto-tuning procedure. Naturally the parameters obtained
in this method may not be the most appropriate. Manual adjustments are recommended to tune them nely.
Refer to Section 6.3.3. TUNING (AUTO-TUNING) VIEW to operate the Controller for the following auto-tuning procedure.
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