PIDE cv output at last value at switch on

[pauly]

I am using the PIDE function in Logix 5000 for a presure control application and if the power is cycled to the PLC the customer wants the control output to the valve on power up to be the same as it was previously. When the PLC is switched off and back on and the SP and PV are equal, the output from the PIDE CV is zero. Does anybody have an idea how I can maybe initialise the output or prevent the PIDE function resetting to zero on power up?

 

[Lancie1]

Save your variables, especially the CV, to non-volitile memory. Add a new rung with the First Scan bit S:1/15. Use the First Pass bit to:
1. Upon power-up and the first scan of the PLC, set the PID to Manual mode,
2. Restore the saved variables to the PID function.
3. Make sure to copy the old CV value to the PID Setpoint.
4. At some later timer after pressure has stabilized, do a "bumpless transfer" and switch the PID back to Auto mode, if needed.

 

[pauly]

Non-volatile memory?

How do I do that? Which memory areas are non volatile?

 

[kamenges]

Basically everything is non-volatile. The memory locations are reset based on their usage, that is which instructions they are used in and whether AB has decided that these usages should be initialized. For example, if TestBit is declared as a BOOL and use in an OTE instruction it will be reset. But if TestBit is used as an OTL it will not be reset. The CV of a PIDE is obviously one of the values AB thought needed to be initialized .

So, as as Lancie1 says, create a tag of the appropriate data type and keep it updated with the CV of the PIDE. Then do as he indicates on a first scan of the plc. You will get your old value back.

Keith

 

[pauly]

First scan on Controllogix

Is there a bit you can access in Controllogix for the first scan?

 

[kamenges]

The Logix platform only has a few system bits. The firstscan is one of them, fortunately. It is 'S:FS'.


Keith

 

[Lancie1]

I am sure Keith is correct about S:FS. I did not think about it being different in Logix 5000. If the PID works the same as in RSLogix 500, when you switch a PID to Manual Mode, the Setpoint (SP) becomes the CV output. So you need to make sure that the Setpoint is set to be equivalent to the Old CV, otherwise in Manual mode the PID will change the CV to match the Setpoint.

If you don't switch to Manual, the Auto mode will try to adjust the output based on setpoint and the error signal. If power has been off, this usually results in a change to the Control Variable, the condition you are trying to avoid.