Chapter 10 Built-in PID Functions
10 - 27
(2) Determination of SV
In order to set SV, the PV value of the system desired by the user should be determined. To put it easily, in
order to maintain the water level at 250mm, the PV value for 250mm should be determined. This value can
be determined through numerical analysis of the system but it will be more exactly determined by
experimenting with the reaction of the control object. Analysis with the current system suggests that, with
the water level of 250mm, PV outputs the value of 8333, but an actual operation showed that, with the
water level of 250mm, the sensor output value was 3250. The reasons for such an error are the inaccurary
of the sensor, the error of the measuring base point, etc. So, the actually measured value of 8250 should
be used as the state value when the water level is 250mm. This value will be used as the SV value for
control of 250mm.
(3) Control Setting
After the previously developed program is downloaded to the PLC, then monitoring begins. The next step
is to set the variables registered in the Variable Monitor window. The following figure shows the screen of
settings in the Variable Monitor window of the example program.
Settings were made for SV, K_p, and MV_max.
The actually measured value of 8250 was set for SV and 5 was randomly selected for Kp.
MV_max, an item to limit the maximum value of MV, was set to 10000 according to the analog module.
