Voices: Magazine Exclusives

Software that Reduces Maintenance of Multivariable Predictive Controllers

The Software Is Easy to Use. It Takes About 30 Minutes to Create

By Charles and James Cutler

The degradation of the models used by Multivariable Predictive Controllers (MPC) is primarily due to changes in the PID controller's tuning or configuration.  The physical plants vessels, piping, and inventories which set the basic dynamics do not change significantly with time.  As the models degrade, the performance of the controller eventually reaches such a low point that the controller is either turned off or the process unit is retested to upgrade the model.  Turning the controller off is not desirable, since the full value of the controller is lost.   Retesting is usually a high cost event because it requires engineering support from a contractor or some other part of the organization.  Many times the poor performance of the controller is due to equipment failure such as a valve becoming stuck or losing its plug.  The loss of one key valve that requires a shutdown to fix, forces the controller to be put on manual which can destroy the entire MPC's model and cause the controller to be turned off.

Also Read: Model Predictive Control - Past, Present and Future, Part 1

Cutler Technology's Universal Process Identification (UPID) software will be used to demonstrate how the problems with PID configuration and tuning can be reduced to a minimum.  A depropanizer model will be used as the process example.  The independent manipulated variables in the model are the set points for the feed rate, the top pressure on the column, the tray 5 temperature (5 trays from top), and the heat medium flow to the reboiler.  The tray 5 temperature is controlled by manipulating the reflux to the column.  The top pressure controller by passes the overhead condensing system and sends the hot vapors directly to the overhead accumulator to increase the pressure.  The MPC on the column controls the top temperature and bottom temperature.  The differential pressure across the column and the valve position on the pressure controller are the main constraints.  These controlled variables are dependent variables in the controller's model.  The other dependent variables in the controller model in Display 1 are there to facilitate the rearrangement of the control matrix when it becomes prudent to put one or more of the PID controllers in manual in the MPC model.

In Display 1, it should be noted that the PV for a specific manipulated variables comes to 1.00, i.e. the PV comes to its setpoint.  The response of this PV for the other manipulated variable is zero or comes back to zero at steady state for the other manipulated variables.  This response is true because the manipulated variables are independent from one another.  The valve positions of the manipulated variables change to make this independence possible.

In Display 2 the Top Pressure PID controller has been switched to manual in the model.  The Top Pressure valve position becomes the manipulated variable.  The setpoint for the pressure replaces the valve position as a dependent variable and tracks the pressure PV.   During the summer when the cooling water temperature goes up, it becomes desirable to switch the pressure controller to manual, to tell the MPC that it needs to use the other manipulated variables to control the pressure on the column.  As can be seen in Display 2, all of the other manipulated variables have a significant effect on the pressure.  The MPC controller with the pressure PID controller in manual can completely close the valve on the bypass around the condensers.

Closing the valve permits the maximum cooling for the column.

The Tray 5 Temperature PID controller is switched to manual in Display 3   The PID controller on tray 5 temperature is a cascaded loop that moves the setpoint on the reflux to the column.  The MPC controller model in Display 3 has the reflux valve position as one of its independent manipulated variables.  
In Display 4, both the temperature PID controller and pressure PID controller are in manual.   The MPC model is significantly changed from the original model when all the setpoints were functioning.  There are many reasons PID controllers are switched to manual during the course of a run.  A run may last a number of years, so every effort should be made to keep MPC on control.  A controller may be switched to manual when a valve is sticking so frequently that it is putting cycles into the entire process unit.  The process piping may not have adequate by passes to permit the valves to be blocked in and removed for repaired.

In Display 5, the models for the key constraint CVs are compared for the four set point case and the two set point case.  The valve position responses are obviously different from the set point responses.  Major differences in the gains also exist for the setpoint versus setpoint responses.  When the configurations of the PID controllers within a multivariable controller are changed, it affects the entire controller model.

Changes in the process mode of operation may cause one or more controllers to become unstable.   The PID controllers must be tuned to gain stability.   The new tuning changes the MPC model.  In Display 6 the PID controllers have become unstable.  The software allows the PIDs controllers in the MPC model to be tuned to match the plants tuning as illustrated in Display 7.  The ability to change the PID controller's tuning and configuration in the MPC model to match the current conditions on the process unit can significantly increase the on stream factor, the quality of the MPC control, and the profitability of the controller.

The software is easy to use.  The six models that follow, took about 30 minutes to create.  The controller has fourteen dependent variables.  Seven of the dependent variables were dropped out of the displays to make them readable.

More from this voice

Title

Working with IS and non-Signals on the Plant Floor

Galvanic Isolators Can Solve Some of Your Intrinsic Safety Problems

06/29/2009

WirelessHART Successfully Handles Control

WirelessHART Successfully Handles Control -- Tests on Stripper and Absorber Show Results Rival Those With Wired Devices

12/07/2010

Wireless Wisdom: What’s Your Strategy? Where’s Your Committee?

A Forum of Wireless Experts at ISA Expo 2008 Turns into a Cautionary Consensus About the Dangers of Falling in Love with Technologies and Applications That May Run Counter to Your Long-Term Wireless Strategy and Corporate Goals

01/12/2009

Why Manufacturing Is Leading Economic Recovery

The Following Opinion Piece Is by Dr. Chris Kuehl, Economic Analyst, Fabricators & Manufacturers Association, International

02/04/2011

Which Way Wireless?

ISA100.11a and WirelessHART – Your Future Wireless

03/28/2008

When Is It Time to Retire Your PAT System?

The Answer’s Not as Simple as It Might Appear

07/14/2008

What's the Best Defense Against Stuxnet?

A Comparison of Which Tools Are the Best for Finding Stuxnet in a System

05/28/2012

Web Exclusive: Fixed-Price Projects

Looking at Fixed-Price Work

07/14/2009

Virtually Simulating the Next Generation of Clean Energy Technologies

NETL's AVESTAR Center Is Dedicated to the Safe, Reliable and Efficient Operation of Advanced Energy Plants With Carbon Capture

01/05/2012

Using Modular Procedural Automation to Improve Operations

Standard Automation Methodology Improves Operations and Prevents Incidents by Enabling the Sharing of Best Practices Among Operators

11/29/2012

Upgrading a Wastewater SCADA System

The City of Yuma installed a SCADA system to control and monitor its water and wastewater distribution system remotely. But as the city expanded, the existing SCADA system lacked the needed support, performance and diagnostic capabilities.

08/30/2007

Training and Keeping Your Employees

How to Get and Keep the Best Techies

03/26/2008

To Hell and Back

How to Rebuild a Control Room in Five Days. Lessons in Disaster Recovery and Emergency Response

08/25/2010

Think Things Are Small Now?

What the One-Molecule Sensor Might Mean for Process Control

08/05/2008

The World's Largest Economy

In China, Manufacturing is Still an Honorable Profession

07/14/2010

The Status Quo Isnt

Web Applications May Be the Next Big Thing in Computing. Are You Ready?

02/05/2008

The Project Staff at Celulosa Arauco

Environment, Safety, and Quality Goals Realized in Unusually Successful Start-Up

04/27/2009

The One Machine

What Is the Value of a Telecommunications Network

12/22/2008

The Nuclear Mess

Just When We Were All Getting Convinced to Let the Genie Out of the Bottle Again, We're Back Where We Were

05/13/2011

The Future of Process Management

To Thrive in the Current Environment, We Need to Change the Way We Think and the Systems We Use to Manage Our Companies

03/29/2011