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Key Practical Considerations for Valves and Controllers in Control Systems

This overview covers critical issues related to valves and controllers, specifically focusing on proportional band, controller action, failure modes, and reset windup. The proportional band is defined as the percentage change in controller input required for a full output change. Understanding the implications of controller actions and the importance of appropriately selecting failure modes for valves is crucial for safety and efficiency. Additionally, the document discusses integral action, resetting mechanisms, and techniques for addressing derivative problems in control systems.

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Key Practical Considerations for Valves and Controllers in Control Systems

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  1. BAE 3023 Important Practical Valve and Controller Issues

  2. Proportional band BAE 3023 Proportional Band Definition: The % of controller input change required to cause a 100% controller output change Note: If you wish to turn the proportional gain on a controller down you must turn PB UP !

  3. BAE 3023 • Control equations http://www.manufacturing.net/ctl/article/CA186117 • Conventional PID equation: • Other forms may be used ie: • Check the manufacturers literature on the controller you use!

  4. Controller Action BAE 3023 Controller action Definition: The sign on Kcor PB. If the action on a controller is backwards it will attempt to drive the process in the wrong direction. The process will be at either its maximum or minimum levels

  5. Failure Modes BAE 3023 Valve Failure modes: Actuators in general and valves in particular may be selected or designed to fail to maximum, minimum, or current settings. The selection of failure mode is a part of the safety design considerations in a system. Discuss practical examples: Students be prepared to answer a question on failure mode for valves on an exam:

  6. Reset windup BAE 3023 Reset windup: Definition: The integral action that is accumulated when the manipulated variable is at its maximum and the controlled variable is not forced to the setpoint. Discuss and example: Correction for reset windup: Most controllers set the integral action to zero when the manipulated variable is at its maximum and the controlled variable is not forced to the setpoint.

  7. Reset time vs. Reset rate BAE 3023 Reset (integral) time (tI) vs. Reset rate (tRI) : From before a PID controller equation is: It can also be written: Note: if a controller uses tRI, the effect of adjustment of the integral setting will be opposite of controllers using tI It can also be written:

  8. Fixing the setpoint derivative problem BAE 3023 For the control system we developed earlier: Notice that a step in qset will cause a step in e and the derivative of e will become infinite

  9. The fix BAE 3023 We can eliminate the derivative of the setpoint and still get the effect we desire from the derivative action. So let: This equation along with filtering of qout_measured is used in commercial controllers with derivative action.

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