design water level controller using PID control using siemens S7 200 PLC


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i am a PG student doing a project in PID control using siemens s7200 PLC. i want to know how the PID control function can be used with the same PLC.. but in the help menu of the software i didn't get much information regarding the same... also i have to develop a physical model also i.e some application... any suggestions or relevant notes..

i planned to develop a water level controller using PID ...i need some help in designing the mathematical model also.
Level is an integrating loop and can be controlled nicely with a PID controller. However, you may want to consider if this tank were in a plant, that the tank may have been put there as a buffer between process units. If this were the case, then the tank is there as a flow filter and the best PID tuning parameters are VERY simple to calculate. This from the ExperTune Inc help system (© 1993-2001 ExperTune Inc., All rights reserved):

Averaging Level Control
If your tank's purpose is to provide a more constant flow to the next stage or process operation then you may not want optimal or tight level control. In this situation you have control of the outlet flow and the inlet flow varies. The purpose of this "surge" tank is to absorb inlet flow changes and deliver as smooth as possible outlet flow without emptying or overflowing the tank.
The level in this tank system will be high when flow is high since the next upset is likely to be towards low flow. The level will be low when the flow is low since the next upset in this case is likely to be higher flow.

To tune this controller:

1) Use a P only controller. Do not use integral or derivative action.
2) Put the setpoint at the minimum allowable level - label it for operation people as "low limit". The level will drift depending on flow so a label of setpoint can be confusing when it actually is the low limit.
3) If your controller uses proportional band, Set the P action to:

(maximum allowable level in %) - (minimum allowable level in %)

If your controller uses gain, set the gain to:

100/[(maximum allowable level in %) - (minimum allowable level in %)]

4) Set Manual Reset to zero. Some controllers call this the bias.

The above equations for proportional band and gain assume the controller output is in %. With a gain of 1 (or PB of 100), a 10% change in level would give a 10% change in controller output.

See also:
john gerry
ExperTune Inc.