Last year I summarized an Emerson Exchange presentation by James Beall, Advanced PID Functions for Improved Control Performance. At this year’s Emerson Exchange, he did a reprise version, Interesting and Useful Features of the DeltaV PID Controller. I’ll highlight some additional nuggets of wisdom that he imparted.
When it comes to limits being placed on setpoints and for a loop that is in automatic mode, it’s usually best to let the operators have total control on the setpoint and not apply setpoint limits to the PID control when it is in the cascade mode. He did add that when it comes to cascaded loops like a level control master PID loop and a flow slave loop, it’s a good idea to set limits on the output of the master loop rather than the sepoint of the slave loop, and that there is a lot of flexibility in the DeltaV PID control algorithm to do this.
With cascade control, James noted that mode tracking and bumpless transfer is provided and that limited conditions in the slave loop are taken care of through the BKCAL feature. DeltaV Books Online describes the input and output:
BKCAL_IN is the analog input value and status from a downstream block’s BKCAL_OUT output that is used by a block for bumpless transfer. This connection is necessary if the PID is a master to another controller in a cascade. Without the connection, the slave controller will not make the transition to CAS and the master PID will never be active.
BKCAL_OUT is the value and status sent to an upstream block to prevent reset windup and provide bumpless transfer to closed loop control.
In conditions where the slave loop is limited, you can enable PID external reset. You would most often use it in the primary loop of a cascade and have it compensate for unexpected slow secondary-loop response. This is done by selecting the FRSIPIDOPTS “Dynamic Reset Limiting” on the master loop and the CONTROL_OPTS “Use PV for BKCAL_OUT” on the slave loop.
I summarized James’ discussion of gain scheduling, which provides up to three regions of different PID tuning with a smooth transition between regions. This year, he described a parameter FRSIPID_OPTS that modifies the proportional gain as a function of the error (process variable, PV minus setpoint, SP). You can use this non-linear gain function to make the tuning more aggressive as the separation between PV and SP increases. It also can be used to create the “error squared” PID function.
James cautioned that using this non-linear gain function on an integrating process, like levels, can cause oscillations at the reduced gain. For these applications, the reset time should be based on the product of gain and the minimum gain modifier (NL_MINMOD), which will result in a larger reset time to prevent oscillations. He suggests using the gain scheduler to provide non-linear tuning on integrating processes.
There’s more wisdom shared on valve output characterization, anti-reset windup limits, adaptive control, and loop simulations, which I’ll leave with you should you choose look at the embedded presentation.
