Need help on writing PID loop in RSLogix5000

[bupesh]

Hi All,

I am looking for some help, on writing a logic to control the Power output from a VFD using PID... I will appreciate your help.

Regards,
BK

 

[TConnolly]

Question #1: Are you wanting to use the ladder PID instruction or the function block PIDE instruction? The PIDE is a superior instruction but it will require that you have a version of Logix5K that supports FB and ST language, if you have a ladder only version of Logix5K then you will have to use the Ladder PID instruction.

Suggestion #1: The quality and quantity of the help you will receive is directly proportional to the quality and quantity of the information you provide about what you are trying to do. So far its pretty vague.

 

[bupesh]

Thank you Alaric,

I will try to, we are using Ladder Logix 5k. This is regarding the Ventilation Fans rite now.. the Fans run with VFD and the VFD get the command through logix to run at X speed.

we are thinking to run the VFD with power control. like running the vfd by changing the desired amount of AMPS.

is that information ok to help me out.

Regards,
BK

 

[TConnolly]

If the application is simple ventilation then I don't really see the need to use a PID control. Open loop control will probably be more than sufficient. If you want 50% fan speed send a command to the drive to run at 30hz. You can do this using a simple scaling instruction or CPT instruction to write to the output word controlling the drive, or if you are using 1756 IO, then just write your speed percentage to the card and configure the card to to do the scaling.

If you are trying to control something else, temperature, humidity, etc. by varying the fan speed then we really do need more information.