Advanced PID Functions for Improved Control Performance

I saw Emerson’s James Beall the other week and asked him for a copy of his Emerson Exchange presentation, Interesting and Useful Features of the DeltaV PID Controller.

Every year, James presents to standing-room-only crowds and his presentation (given twice) this year was no exception. PID or proportional-integral-derivative control is definitely not a new concept. I did some Googling around and found references to it dating back to 1922 when N. Minorsky published an article on its use for automatic ship steering control.

While PID control has been around for a long time, technologists keep adding innovations, like degrees of freedom to the proportional action and the derivative action.

James began by describing three common PID forms: parallel, standard (a.k.a. ISA form), and series (a.k.a. classical form.) The standard form is the default choice in the DeltaV PID function block and the series form is an option. James counseled that the choice is based on your prior experience and personal preference. The series and standard forms are identical if the derivative action is not used. Also, your choice of forms can impact the conversion of tuning constants from a previous control system.

The PID function block also has a STRUCTURE parameter that provides two degrees of freedom for the proportional and derivative actions. On a change of setpoint (SP), you can scale these actions (BETA = proportional action scaling, GAMMA = derivative action scaling) between 0 and 100%.

The PID function block has an integral dead band (IDEADBAND) for when the error (SP minus PV) gets within this dead band. At this point, the integral action stops. James described a level controller application that feeds a downstream unit in order to reduce the movement of the controller output when the error enters the dead band.

James discussed three setpoint filters based on rate of change. One filter provides a time constant in seconds of the first order SP filter (SP_FTIME). Another provides a ramp rate at which downward setpoint changes (SP_RATE_DN) or upward setpoint changes (SP_RATE_UP) are acted on when the loop is in automatic mode.

Limits can also be placed on highest and lowest setpoints allowed, whether or not these limits are obeyed when the loop is in cascade or remote cascade mode, or whether output limits of the master loop in a cascade pair are used to limit the setpoint to the slave loop in cascade and remote cascade mode.

On the subject of cascade-control loops, James shared how mode tracking, bumpless transfers, and other loop interactions are automatically handled by the PID block’s BKCAL interblock communications.

Gain scheduling is another PID control innovation for loops with nonlinearities where different regions of the PID controller can have different PID tuning parameters. The DeltaV PID function block can have up to three regions with different tuning parameters, based on a selected state variable (output, process variable, error, production rate, etc.) The algorithm provides a smooth transition between the regions.

James also provides guidance on valve output characterization and anti-reset windup limits in the presentation. Although these advanced PID functions can appear quite technical, they can significantly improve the performance of PID control and provide ways to handle difficult process dynamics. The bottom line to getting this control right is better control performance and a more efficient process.

You can read about the full capabilities of the PID function block in the 9.3 version of DeltaV Books On-line.

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