RSlogix5000 PID with discrete outputs

Hello,

I have already searched for that in the forum but did not get any satisfying answer, so here it goes:

I have a 1769-L36ERM that controls an Auma regulating valve. The valve expexts from the PLC two outputs, one for close and one for open. It also provides to the PLC a 4-20mA signal of the position of the valve. The use of the valve is to reduce the temperature of a hot water line, by injecting cooler water into it.
I intend to program a PID loop where my PV will be the temp of the water line and the SP will come from a SCADA.
From past experience I know that The PID block in Logix5000 provides its output in percent as an analog value. The question is how can this be converted to two discrete signals for closing and opening the valve. I have read that it be programmed as a time proportional conversion.
What I am looking for is to avoid programming that myself. I would prefer a well proven built in instruction that does that like it happens with Siemens.

Another question is what to do with the position feedback I get from the valve, apart from using it for monitoring and trending purposes.

Regards

Please download this file:
http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf
Please go to pages 22-24 there is a good explanation of Split-Range Time Proportioned Loops.

I know what grnick50 is talking about since I have discussed these "step controllers" with Pandiani, remember him?
This application isn't exactly a split range application like in the Rockwell pdf.

Since grnick50 has the valve position as feed back this will be a cascaded loop system.
The outer temperature loop would be fairly standard. The output will be percent of valve opening like grnick50 says. The inner loop will be very simple. If the control output % is greater than the spool position the turn the open output on. If the control output % is less than the spool position % then turn the close output on. There will probably need to be a dead band so the valve doesn't open and close constantly when close to the temperature set point.

The outer loop can be slow but the inner loop should be fast so the valve doesn't go past its set point.

I don't think grnick50 will find a ready made block for this but it should be simple to implement.

As usual, Peter nailed this one.
The inner loop doesn't need to be a PID, just two compares, and yes, you will probably need to put at least 1% of deadband before taking a control action in the inner loop.

When using a setpoint that way, from a PID you can do something like:

CMP Valve_Actual < (Command_Pos - 1) OTE OPEN_Valve

CMP Valve_Actual > (Command_Pos + 1) OTE CLOSE_Valve

I also think it is a good solution. Thank you Peter.