Pid Program Example - Siemens SIMATIC S7-200 System Manual

SIMATIC Instructions

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 9-28 shows the control program for this application.
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S7-200 Programmable Controller System Manual
A5E00066097-02