Taming Extruder Temperature Control

I caught up with Emerson’s Mark Coughran, a senior process variability consultant whom you may recall from earlier process tuning and optimization posts.

Mark shared a story of a plastics manufacturer that was challenged to bring a new product to market with a new extruder. This manufacturer needed to run trials with varying polymer formulations at various temperatures and speeds while trying to perfect the production process.

The plant control engineer was struggling with control strategy and necessary tuning to hold the required temperature. The temperature loop wasn’t responding to setpoint step changes and was oscillating even when no disturbances were present.

When Mark arrived to lend his experience to this challenge, many anxious folks greeted him. The project engineer was glad to see him. The project manager asked if he could stay the weekend. The plant manager assured Mark that the situation had visibility at the highest levels of the organization. A corporate engineer added pressure by saying single loop control worked just fine at a similar plant. I imagine that Mark didn’t enjoy all this attention.

He and the project engineer began by measuring the process dynamics–both the linear and non-linear components. For the linear process responses, Mark applied Lambda tuning.

For the non-linear portions of the overall process dynamics, the approach was to mitigate these nonlinearities as much as possible. They performed four actions to accomplish this. The first step was to improve the control strategy by changing the master loop configuration to prevent interaction with the slave loops. Next Mark helped identify and have an unnecessary interlock removed that disturbed the control loop.

The process dynamic measurements uncovered an extremely high process gain, which was reduced by establishing pressure control upstream from the extruder. Finally, the output pulsing was adjusted to better match the control strategy with the control valve dynamics.

After applying these changes, the temperature process variables tracked the setpoint changes over the operating range of the trials. Mark typically likes to work with the process manufacturer to financially quantify the results to prove the value of his services. It also helps the people he works with look good to their upper management. Unfortunately, until this new product gets to market its value is not yet determined and the control engineer didn’t want to speculate. In this case, robust control was established and the level of anxiety dropped considerably.

2 comments

  1. This post brings back many PID tuning memories. It also emphasizes the critical importance of understanding the “process,” and its relationship to the loop.

  2. Jack, Thanks for your comment!
    With continual advancements in technology, the toolset for process engineers keeps growing. Learning the dynamics of the process before you apply any of these is definitely the best first step.
    Take it easy, Jim

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