Siemens Simatic S7-200 System Manual page 251

PID Program Example
In this example, a water tank is used to maintain a constant water pressure. Water is
continuously being taken from the water tank at a varying rate. A variable speed pump is
used to add water to the tank at a rate that will maintain adequate water pressure and also
keep the tank from being emptied.
The setpoint for this system is a water level setting that is equivalent to the tank being 75%
full. The process variable is supplied by a float gauge that provides an equivalent reading of
how full the tank is and which can vary from 0% or empty to 100% or completely full. The
output is a value of pump speed that allows the pump to run from 0% to 100% of maximum
speed.
The setpoint is predetermined and will be entered directly into the loop table. The process
variable will be supplied as a unipolar, analog value from the float gauge. The loop output will
be written to a unipolar, analog output which is used to control the pump speed. The span of
both the analog input and analog output is 32,000.
Only proportional and integral control will be employed in this example. The loop gain and
time constants have been determined from engineering calculations and may be adjusted as
required to achieve optimum control. The calculated values of the time constants are:
K is 0.25
C
T is 0.1 seconds
S
T is 30 minutes
I
The pump speed will be controlled manually until the water tank is 75% full, then the valve
will be opened to allow water to be drained from the tank. At the same time, the pump will be
switched from manual to auto control mode. A digital input will be used to switch the control
from manual to auto. This input is described below:
I0.0 is Manual/Auto control; 0 is manual, 1 is auto
While in manual control mode, the pump speed will be written by the operator to VD108 as a
real number value from 0.0 to 1.0.
Figure 10-21 shows the control program for this application.
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Instruction Set
10-63