Chapter 10 Built-in PID Functions
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10.7.4 How to start up using AT (Auto-tuning)
This section explains the correct AT setting method through operating the system described in 10.7.3
using the AT function. The basic AT function properly operates when the system is not started yet or when
the system has PV at least smaller (larger in case of reverse operation) than the _PIDn_AT_SV value.
Basically, AT performs different operations in different steps. The step increases from 0 to 7 and the step
of the current loop can be known by _PIDn_AT_step. In PIDSTOP mode, the AT step is 0 and increases
(automatically) as AT starts. When it reaches 7, AT is completed. The user’s random manipulation of the
step may cause malfunction.
To avoid redundant descriptions, the procedures from 10.7.3 (1) to (4) are first implemented and then the
present settings are applied. First, _PIDn_AT_SV is set. Though the _PIDn_SV value has been already
set above, the system is vibrated during Auto-tuning so that PV is more than the _PIDn_SV value. To
prevent any harm to the system in the course, an appropriate SV value should be set in _PIDn_AT_SV.
For other cases, _PIDn_AT_SV should be set the same with _PIDn_SV. The _PIDn_AT_SV value is
used only during AT and, upon completion of AT, the system is automatically started based on _PIDn_SV.
Next, _PIDn_MV_min and _PIDn_MV_max are set. During AT, the _PIDn_MV_min and _PIDn_MV_max
values are respectively considered as the minimum/maximum output of the system. During AT, the two
values differ each other in 3 cycles depending on the system speed (how fast PV reaches around SV). For
example, with _PIDn_MV_min = 0, _PIDn_MV_max = 10000, the system driving signal (MV) transferred to
the motor or heater repeats the “0 →10000 → 0” output 3 times. If there is a possibility that such a radical
change may put a heavy load on the system, _PIDn_dMV should be set.
Next, the PIDn_HYS_val value is set. _PIDn_HYS_val is used only during AT. This is a deadband that
occurs when PV reaches around SV. When PV increases, it occurs above the baseline and, when PV
decreases, it occurs below the baseline. If SV is 5000 and _PIDn_HYS_val is 100, AT increases PV until
5100 ( SV + _PIDn_HYS_val ) while maintaining MV at _PIDn_MV_max. Afterward, it decreases PV until
4900 ( SV - _PIDn_HYS_val ) while maintaining MV at _PIDn_MV_min.
The above graph shows a water level waveform obtained by setting an appropriate _PIDn_HYS_val value
(50 in the figure). A rectangular waveform should appear in MV 3 times.
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