Recent content by GrahamJ

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  1. G

    PID Tuning Without a Plant Model on a Slow System

    Tuning cascade controllers starts with finding the transfer function and tuning for setpoint response. Then simply keep moving up the food chain, remembering that the tuning of the inner loop will affect the dynamics of its outer loop but not visa versa. Choosing Fast, Slow or Medium response...
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    PID Tuning Without a Plant Model on a Slow System

    This is the data I get from a standard step test that usually takes between 2 to 3 minutes or a matter of seconds for fast loops. We perform a Digital Laplace Transform of the response to determine an accurate model of the process in the frequency domain. From the transfer function we cancel...
  3. G

    PID Tuning Without a Plant Model on a Slow System

    Certainly without further input from the OP we can keep guessing but from the original sketch it shows a typical Pasteuriser arrangement where the criteria is a time/temperature relationship of the product and not the product temperature in the bottles. Indeed it would not be uncommon for the...
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    PID Tuning Without a Plant Model on a Slow System

    Perhaps the OP could clarify but my view is the bottles are NOT part of the load for the heater so therefore would not be a load index to any feedforward. The primary load indices for the heater will be the incoming temperature from the holding and/or the product flow. I believe we are violent...
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    PID Tuning Without a Plant Model on a Slow System

    Hey pnachtwey, I was reluctant to use the term ITAE formula as I agree with you that the OP has misapplied it. The formula he/she quoted was something akin to one proposed by Lopez many years ago and, as I said, rarely gives any robustness in the tuning. In my attached file I gave an example...
  6. G

    PID Tuning Without a Plant Model on a Slow System

    OK, the ITAE formula you used is based on giving minimum response time and ALWAYS yields proportional gains that are too aggressive. In my experience when applying this formula I would reduce the theoretical gain by as much as a half. However, I never use this formula as it does not provide the...
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    PID Tuning Without a Plant Model on a Slow System

    Let's return to tuning later but in all probability this loop can be helped immensely by the addition of a cascade temperature loop on the hot water set. Your Master controller is the final product temperature which provides a setpoint to a new controller that controls the hot water temperature...
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