PI Control Tuning 1

[ohman10]

I have a master and slave temperature controller that I am having trouble with. I have the setpoint at 218 on the master controller and it sends a remote SP to the slave controller for temperature regulation. The master is dropping in temperature and once it passes the SP say 217.5 it should tell the slave to start heating up except that there is a large overshoot where the master keeps on telling the slave to cool down. I have larger than desirable overshoot but the loop is still stable when locked into remote. I am only using PI control, is this a type of behavior that is indicative of having too much integral control?

 

[Compositepro]

Your description does not completely make sense to me. The master should not be telling the slave to heat or cool, it should be telling the slave what the temperature should be. The slave then determines if it need to heat or cool. In any case the overshoot is often a result of too much integral action or system time lag. Time lag is often a reason to use cascade control.

 

[danw2]

I agree with Compositepro. The master controller should feed a setpoint signal from an 'auxiliary' output to the slave's remote setpoint input and let the slave do its own PI calculation.

Many controllers have an assignable variable for an auxiliary output. Are you sure the output signal from the master is a setpoint value (in degrees) and not the master's PI output (in percent?)