PID Cascade Help

Hi,

I am trying to automatically regulate a process ( liquid ) in a tube using an ultrasonic sensor, differential level meter, control valve, and VFD with motor & pump. My question is, can I cascade two PID’s in RSLogix 5000 to control this process and would it work ? For example, if I had the master PID using the ultrasonic sensor to control the valves and the slave PID using the differential level meter to control speed of the motor. Will this setup work ? I am new to using PID’s in RSLogix 5000 so I apologize in advance if I am making any simple mistakes.

Kind regards,
24V

http://www.plctalk.net/qanda/showthread.php?t=43386

http://www.plctalk.net/qanda/showthread.php?t=113117

The above threads may help you out.
Regards,

@24V go to:
https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf

Pages 15 - 19 (pdf) will help you a lot.

I'm not real clear on the application. I'm guessing liquid level in a tube, the tube being oriented vertically instead of horizontally. I'm further guessing that the motor and pump would be used for filling and the valve for emptying. It seems to me the ultrasonic and the differential pressure sensor are two ways to get the same information, namely the height of the fluid in the tube.

Assuming I am right this isn't a good process candidate for cascade control. Cascade control occurs when the output of one control loop (the "outer" loop) becomes the setpoint for another loop (the "inner" loop). Generally speaking the two loops will have different feedback values that are affected by different disturbances.

One of the reasons to use cascade control is if the driver affecting the process does not respond in a stable or linear way to the output of the loop controlling the process. If this is a case a second control loop (the inner loop) can be designed to control the driver so it will more closely follow the commands from the process loop (the outer loop).

What is the specific issue you are running into?

Keith