Cascade PID Control in Micrologix 1100

Hey guys,

After spending most of a day Googling looking for the answer, I finally decided to make a thread and ask the experts.

I'm trying to implement a cascade PID control loop (level of a tank controlling flow out of the tank) in RSLogix Micro for a Micrologix 1100, but I have very little idea how the instructions work. I mainly work in the Studio 5000 environment and the PID instruction is a little more straight-forward to me there.

I've read up on how the analog values need to be integers in the 0-16383 range, so I've scaled the raw data coming from the level and flow transmitters (6241-31207) to 0-16383 and stored them in separate integer files, N10:0 and N11:0 respectively. I then created two PID instructions, giving them the PID files PD9:0 and PD9:1. I have the PV of the master PID set to the scaled level transmitter value (N10:0), and I tried setting the CV to PD9:1.SPS, but that gives me an error. I'm kind of operating under the assumption that CV is the output, and I thought that the .SPS bit would take that output, but it didn't. Long story short, I'm not sure my syntax/approach to the PID instruction is correct, and I can't figure out how to send the output of the master to the setpoint of the slave. Any help or direction would be great.

I think I solved it using a MOV instruction to put the CV value from the master instruction into the setpoint of the slave instruction. I'm a bit too used to indirect addressing using Studio/Logix 5000. If anyone has anything to add, I'd love a bit more insight.

mylespetro said:

I think I solved it using a MOV instruction to put the CV value from the master instruction into the setpoint of the slave instruction. I'm a bit too used to indirect addressing using Studio/Logix 5000. If anyone has anything to add, I'd love a bit more insight.

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In general, yes, the CV of the outer loop goes into the SP of the inner loop. However, I wonder why a cascaded loop at all? Couldn't the tank level serve as the PV to the PID that controls the output flow device (pump or valve)? I guess what was missing from your first post is what is the second PV in this cascaded setup?

robertmee said:

In general, yes, the CV of the outer loop goes into the SP of the inner loop. However, I wonder why a cascaded loop at all? Couldn't the tank level serve as the PV to the PID that controls the output flow device (pump or valve)? I guess what was missing from your first post is what is the second PV in this cascaded setup?

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Hey Robert,

In this case, it is a common condensate collection tank from two separate steam turbines that is pumped out by one of two pumps (selectable by selector switch). The second PV is a flowmeter measuring the discharge flow of the pumps. I'm not exactly sure what the reasoning behind cascade control is, but that was what the powers that be decided. I believe it is to be a pretty tightly controlled level in the tank, so that may be it.

Couldn't the tank level serve as the PV to the PID that controls the output flow device (pump or valve)?

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Good question but that assumes there is a PID that controls the output flow device. If also assumes the outflow of the device is linear with the speed or opening.

I'm not exactly sure what the reasoning behind cascade control is, but that was what the powers that be decided. I believe it is to be a pretty tightly controlled level in the tank, so that may be it.

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The outer loop is the level control. The output or CV should be a value proportional to the desired flow rate out of the tank. The inner loop tries to achieve that flow rate.
Both loops only need PI control.

Peter Nachtwey said:

The outer loop is the level control. The output or CV should be a value proportional to the desired flow rate out of the tank. The inner loop tries to achieve that flow rate.
Both loops only need PI control.

Click to expand...

Yes, that's the way it operates. I was having a logistical problem with sending the output of the master PID instruction to the setpoint of the slave PID instruction, but I figured it out with a MOV instruction.

Thanks again guys