Cascade controller set-point units 1

[sysengineer]

When a controller accepts a remote set-point in cascade mode from another (primary) controller does the output of the primary controller have to be scaled into the engineering units of the secondary measured value (MV)?

 

[xnuke]

Yes.

xnuke
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[controlnovice]

As Xnuke says: Yes.

But, I've worked on some DCS Systems that don't allow changing the Output Engineering Units of the Primary Controller - they were always 0 - 100% and could not be changed to match the engineering units of the secondary controller (like degC). It's very frustrating.....

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[danw2]

input the controller output signal into a math block, scale it, use the math block output

 

[sysengineer]

Thanks controlnovice. On the DCS systems that don't support changing of engineering units is it just assumed that 0 to 100% output of the primary controller represents 0 to 100% of the the secondary controller measured variable? I.e. if the secondary controller measured variable is 0 to 50 litres/min then the remote setpoint is also a 0 to 50 litres/min signal?

 

[controlnovice]

Yes:

0-100% of the Master controller output represents the full range of the Secondary controller PV...in your example 0 - 50 lpm.

Keep in mind that typically the Setpoint range of the secondary controller matches the PV Range...so your SP Range would also be 0 - 50 lpm.

If you limit your setpoint maximum to, for example 40 lpm, the 100% output still represents the 50 lpm but the secondary controller won't go above 40.

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[sysengineer]

Thank you this was really helpful.