Emerson Exchange 365

In reply to Jimmy Lee:

Jimmy,

First tune the secondary loop (aka lower loop) to the desired Lambda.  Now put the primary (aka upper loop) in manual and the secondary loop in Cascade.  Now you can perform step tests of the primary loop by stepping its output (which steps the SP of the secondary loop) and analyzing the response of the PV.  For example, if it is a first order self-regulating response you will determine Kp, Deadtime and Tau1. If it is an integrating response you will determine the integrating process loop gain, Kpi and the dead time.  Then you tune the primary loop with a Lambda greater then at least 5 times the Lambda of the secondary loop, as desired for loop coordination or variability regulations, both subject to minimum Lambda for robustness requirements.  For example, you might find a minimum Lambda for the primary loop from a robustness standpoint to be 50 times the Lambda of the secondary loop!  That is ok, it meets the cascade response speed separation requirements.  The integral of the primary loop controller is set by the Lambda tuning rules that you choose.  For example, if the primary loop's response is a first order self-regulating response, the integral time is set equal to the Tau of the open loop response.  If it's response is an integrator, the integral time is set equal to [2*Lambda + Dead time].

See my ISA articles on Lambda tuning!  

Loop tuning basics: Integrating processes -  https://www.isa.org/intech/201604basics/ 

Loop tuning basics: Self-regulating processes - https://www.isa.org/intech/201606basics/

 Loop tuning basics: Complex process responses  https://www.isa.org/intech/201610web/