Series 65
INSTALLATION
980029EB
Edition 2019-10-11
63/103
4.9.4.2
Tuning of control performance with fixed PI pressure controller
Optimizing P gain and I gain
This valve may be used for downstream or upstream pressure control depending on configuration. The PI
parameters of the pressure controller require correct adjustment. These parameters must be set once
during system setup and are stored in the device memory which is power fail save. Based on the PI
controller configuration, the valve is able to run fast and accurate pressure control cycles. The PI
parameters can be evaluated using below instruction.
•
In downstream control mode valve will move towards open when current
pressure is higher than set point.
•
In upstream control mode valve will move towards close when current pressure is
higher than set point.
Introduction
PI controller mode is used if for any reason (e.g. too long system time constant) the adaptive control
mode does not provide satisfying control performance.
In PI controller mode the parameters P gain and I gain have to be set according to the systems
characteristics. The best set of parameters can be found by using the empiric method below.
1.
Optimizing P gain and I gain
1.1
Pressure and gas flow for optimization
A PI controller delivers the best results for a certain working point (pressure/gas flow). If there is only one
working point, this pressure and gas flow has to be used for optimizing P and I gain. If there are several
working points that have to be covered, the pressure for optimizing is the medium pressure between
highest and lowest pressure to be controlled, the gas flow for optimizing is the highest flow out of all
working points.
Two different pressure set points are necessary for optimization. Set point 1 (SP1) is the pressure for
optimizing as determined above. Set point 2 (SP2) is about 10 - 20% lower than SP1.
Example: pressure range:
4 – 10 Torr
Flow range:
2 – 4 slm
Pressure set points and gas flow for optimization:
SP1
=
7 Torr
SP2
=
6 Torr
Gas flow
=
4 slm
1.2
Optimizing P gain
While optimizing P gain, the gas flow determined above has to be constant all the time.
Start optimization with P gain set to 1.0 and I gain set to 0.0.
Set chamber pressure to SP2, wait until the pressure is stable. Set pressure to SP1. If the transition from
SP2 to SP1 results in a significant pressure over shoot or even does not stabilize at all, the P gain is too
high. If there is no over shoot and the pressure reaches SP1 asymptotically and very slow, P gain is too
low.
The optimal P gain value is found if the transition from SP2 to SP1 results in a slight pressure over shoot.
It does not matter if there is still a deviation between SP1 and actual pressure.
