Ge 90-70 pid

I have used the PID on furnaces with 4-20 ma valve motors and I had no problems converting the CV to an analog signal..My next furnace has 120V valve motors that needs a 120v signal to open and a different one to close. This is for controlling the heat in the furnace.

So if the PV gets below the SP, I need the PID to send a 120V signal to "Open" the valve motor. When the PV gets above the SP I need it to send another output to close the valve motor.

Can I convert a register from the CV to an output that while it is going up, the valve will open and while it is going down the valve will close?

I'm lost.

In general terms the technique is called "heat/cool". In essence the change in output is converted to a variable time delay, and the valve is moved in the proper direction on that basis. I've used this technique on other PLCs (AutomationDirect) and their manual has some good examples. Check the GE manual to see if they cover this, or consult your local representative. (In Afghanistan I realize that option might not be viable!)

Pid Time Proportional:

I think what you are looking for is a PID with Time Proportional Output. Do a search for that term, and you should find some examples. I know that Allen Bradley has some examples on their Knowledgebase.

Stu.....

sthompson said:

I think what you are looking for is a PID with Time Proportional Output.

Click to expand...

No, that will not work because the valve integrates the one and off signal and the valve will end up either fully open or closed.

The position of the valve would make things much easier.

Peter Nachtwey said:

No, that will not work because the valve integrates the one and off signal and the valve will end up either fully open or closed.

The position of the valve would make things much easier.

Click to expand...

Perhaps I am misunderstanding the use of Time Proportional in this case. If I have led the OP astray, I apologize. Thank you for the correction Peter.

What would you suggest for this type of Control Peter?

I have worked on Furnaces with a similar Valve for air control, and they had position feedback on them, as well as End of Travel Switches for Full Open and Full Closed.

The OP will have to tell us if this is the case with his Valves.

Stu.....

sthompson said:

Perhaps
I have worked on Furnaces with a similar Valve for air control, and they had position feedback on them, as well as End of Travel Switches for Full Open and Full Closed.

The OP will have to tell us if this is the case with his Valves.

Stu.....

Click to expand...

Yes they are position feedback and fully open and close LS.
The only way I was figuring it was to use the feedback and scalle it the same as the output of the PID. Once I get the scaling right I could possibly use LE_INT and GT_int to get the valve motor to open and close. I would just have to tune the PID to make it move slowly so there would not be alot of "hunt" in the valve motor.

Dean

Good you have position feed back

goldprospector said:

Yes they are position feedback and fully open and close LS.
The only way I was figuring it was to use the feedback and scalle it the same as the output of the PID. Once I get the scaling right I could possibly use LE_INT and GT_int to get the valve motor to open and close. I would just have to tune the PID to make it move slowly so there would not be alot of "hunt" in the valve motor.

Dean

Click to expand...

You need something like an inner and outer loop but your inner loop is going to be quite crude because it is only on-off. The outer loop should compute a valve position from 0-100%. The inner loop needs to move to the set point generated by the outer loop. Simple enough.

So goldprospector, what is the time constant for the oven? Take the time it takes to reach a steady state temperature, subtract the dead time and divide by 5. How often are you going to update the outer loop PID and how fast does the valve move in %per update? Also, please say this valve is relatively linear with respect to flow vs position because it will keep things simpler.

The obvious thing to do is to simply open or close the valve until it reaches the set point. The valve would always be active then and that may not be good because of wear and tear. It may be better to open or close the valve until is reduces the error by a certain amount amount like half each time period or close within 1% to 2% and stop.