I would recommend using 2 PID controllers instead the split range valves/controller you are asking about.

The PID Controller for the small value would control level.

The second PID Controller would control the small valve (which is controlled by the 1st PID controller)position, ideally to 50%.

The rational for using 2 PID Controllers is as follows:

The large control valve would typically be used for Gross level control where-as the small control valve should be used for fine control. In other words, the large value should get you to level setpoint +/- some gross accuracy (say +/- 10%). Where-as the small control valve should get one to level setpoint +/- some tighter accuracy (say +/- 1%). By keeping the small valve at 50% (by the second PID controller) it leaves +/- 50% valve adjustment in order to maintain the tight level control.

With this 2 PID controller algorithm you must make sure the controller time responses are apart to avoid the controllers from interfering which each other. If they interfere with each other the two controllers would end up chasing each other resulting in the valves swinging. Ideally the time constants between the 2 controllers should be a factor of 10 (but at a minimum should be at least a factor of 5. In this algorithm the small valve would be tuned as the fast controller and the large valve would be de-tuned to make it a slow responding controller. This way the small valve controller can respond to disturbances in the system in order to maintain tight level control. However, the large valve controller would slowing respond such that the small control valve is maintained around 50%.

An example response scenario with the small valve at 50% and the large valve at 20% with level at setpoint. If a disturbance occurs, say the tank output flow increases, the level would start to decrease. The fast/small control valve would open in order to increase the make-up flow (ideally to match the increase in output flow). On the slower responding large valve PID controller with the small valve open above 50% (starting point/setpoint) the large controller (properly tuned) would start to open more which increases more make up flow to more than the output flow. This causes the small valve to respond by closing (towards and ideally to 50%) in order to maintain level by again matching the make-up flow with the output flow and maintain the desired level setpoint.

Hope this is understandable and helps you resolve your issue

cabreytj

Hi.

Does anyone have an example of split-range PID control for S7-300 (FB41) ?

regards.

CBo

hi Pratapks,
not sure to understood if you are talking about split-range function or something different but similar. Lets me do an example:
- PID output range is between 0-100%
- 2 analog valve 0-100% range (0=completely closed, 100=comletelly open)
- 40% split-range point (just for example)

PID output = 0% : 1st valve =100% , 2nd valve 0%;
PID output = 40% : 1st valve = 0% , 2nd valve 0%;
PID output =100% : 1st valve = 0% , 2nd valve 100%;

This is split-range function Sydney&Mario Bragança said (standard split-range function) already present in PCS7 library where it's possible change the split-range points and invert output range to make workiing the valvs in opposite way.

If you need a different function can you explain with more detail or numeric example?

Regards

Hi Berg,

I have created my own logic using FB41, if you wish. the control comparision you can change as per your requirments.