Feedforward Control Methods, Benefits and Resources

by , | Mar 27, 2024 | Operational Excellence, Production

Jim Cahill

Jim Cahill

Chief Blogger, Social Marketing Leader

Process Automation Hall of Fame member, Greg McMillan

Greg McMillan receives best paper award for his 2024 AIChE Spring Meeting presentation, Feedforward Control Methods and Benefits

This week at the AIChE Spring Meeting, Process Automation Hall of Fame member Greg McMillan was awarded the best paper for his paper and presentation, Feedforward Control Methods and Benefits. Here is the paper’s abstract:

Guidelines and test results are provided on feedforward control dynamic compensation and configuration to improve load and setpoint responses. The prevalent confusion on feedforward multiplier and summer use is explained and resolved. The methods and benefits of upgrading to ratio control including online adaptation are detailed and the extension to plantwide control is noted. The strategies for preemptive action needed in procedure automation and in the open loop backups critical for preventing compressor surge and Resource Conservation and Recovery Act (RCRA) pH violations are addressed.

He opened the presentation by sharing realities and opportunities for feedforward control:

  • Nearly all inputs that affect production are process streams and utility streams.
  • The Process Flow Diagram (PFD) reveals the many effects of streams on plant performance, but unfortunately, PFDs are often outdated.
  • Changes in stream flow are usually fast, and the degree of disruption is determined by the lack of automation (e.g., manual actions), Safety Instrumented System (SIS) and sequential actions, interactions, measurement 5Rs (resolution, repeatability, rangeability, response time, and reliability), and PID algorithm and tuning.
  • Energy or component flow depends upon stream temperature and composition.
  • Flow feedforward can address these issues and:
    • Enable unit operations to respond in unison to production rate changes via plantwide use
    • Facilitate smarter procedure automation for startups and transitions
    • Provide smarter open loop backups for compressor surge protection and Resource Conservation and Recovery Act (RCRA) violation prevention
    • Provide smarter state-based control to minimize impact of equipment failures

Greg shared these thoughts when comparing a feedforward multiplier strategy versus a summer strategy:

  • When there is no secondary flow or speed controller, the feedforward summer in the primary controller is used to directly manipulate a valve position or power input signal. A secondary controller is too slow for pressure control.
  • When there is a flow or speed controller, Ratio control is predominantly used where a secondary flow or speed controller setpoint is manipulated to follow a leader flow or speed that is multiplied by a desired ratio. Ratio control is used to assist primary PIDs for composition, level, pH, temperature, & quality control.
  • In Ratio control the leader and follower flow first go to a Ratio block whose output is the input to a Bias/Gain block whose output is the cascade setpoint for a flow or speed controller. The setpoint (SP) of each block (ratio or bias) can be set by operator (auto mode) or automatically corrected by a primary PID (cascade mode). The manipulation of the primary PID of the Bias SP and Ratio SP is effectively a feedforward summer and feedforward multiplier, respectively. The use of the ratio and bias/gain blocks provides the operator visibility and accessibility in ratio control particularly important for understanding and procedural automation during startup, changes in products and abnormal operations.
  • The Bias SP is manipulated for volumes with back mixing due to agitation, turbulence or boiling (e.g., vessels and columns). The Ratio SP is manipulated for volumes with plug flow (conveyors, extruders, inline systems (static mixers), kilns, and film, fiber, sheet, and spin lines).
  • The actual and desired Ratio SP are displayed. During startup, until the process is at its normal operating point, the primary controller is often in manual. In this case, the operator runs with a manually set bias and ratio without correction.

Some applications for ratio control include blend composition, column temperature, combustion temperature, drum level, extruder quality, heat exchanger temperature,  neutralizer pH, reactor reaction rate, reactor composition, sheet/web/film line machine direction (MD) gauge, spin line fiber diameter gauge, and header pressure.

The presentation addressed cascade ratio or cascade bias correction, loop and load disturbance dynamics, feedforward dynamic compensation and gain computation, and effects of delay mismatches on various process dynamics. Greg closed the presentation with examples including steam header feedforward and decoupler dynamic compensation, ratio control dynamic compensation & adaptive feedback correction of ratio & bias, liquid reactor ratio control of reactants with feed maximization, and compressor surge protection and efficiency maximization.

If you’re interested in more on this topic and application, reach out to Greg in LinkedIn, or visit some of his suggested resources for this presentation:

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