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Questions and Answers
What is the value of CO when the error is zero?
What is the effect of a higher proportional band?
What is the action of a reverse-acting controller?
What factors determine the correct sign of the controller?
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What happens when the valve fails in the example provided?
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What is the outcome of a correct controller action selection?
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What is the response to a load disturbance in a closed loop system called?
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What type of process has variables that converge to a steady state when disturbed?
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What is the purpose of a feedback controller in a closed loop system?
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What is the name of the controller that uses a bias signal and a proportional term?
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What is the purpose of Integral action in PI control?
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What happens if the derivative time (τD) is too large?
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What is the response to a setpoint change in a closed loop system called?
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What type of process has variables that go to infinity mathematically?
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What is the purpose of Derivative action in PID control?
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What is the main disadvantage of using PI control?
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What is the principle behind the 1st Law of Process Control?
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What is the advantage of using different languages in Process Control?
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Study Notes
Feedback Controllers
- CO is the value when the error is zero, and Kc is the proportional gain or controller gain.
- Some manufacturers use an alternative term, proportional band (PB), where a higher PB means a lower gain and vice versa.
Controller Action
- The sign of the controller depends on three factors: the action of the transmitter, the action of the valve, and the effect of the manipulated variable (MV) on the controlled variable (CV).
- There are two types of controllers: reverse-acting (increases-decrease) and direct-acting (increases-increase).
Controller Action Selection
- To select the correct controller action, examine the valve, transmitter, and the effect of MV on CV.
- Example: if the valve allows steam into the process and fails shut, the controller should decrease the steam flow when there is a problem.
Feedback Control
- Feedback control is also known as closed-loop control.
- The closed-loop response to a load disturbance is called the regulatory response or closed-loop load response.
- The closed-loop response to a set point change is called the servo response.
Stability
- A process is unstable if its variables go to infinity.
- A stable or self-regulatory process has variables that converge to a steady state when disturbed.
Types of Controllers
- There are three types of controllers discussed in this course: proportional controller (P-only), proportional-integral controller (PI), and proportional-integral-derivative controller (PID).
Proportional Control
- Proportional control action is based on the error signal.
- A bias signal is a constant and is known as the reset rate.
- If there is no error, the controller output does not move.
Proportional-Integral (PI) Control
- Integral action eliminates the steady-state error after a set point or disturbance change.
- However, it degrades the dynamic response of the control loop and can make it more oscillatory and unstable.
Derivative Action
- Derivative action uses the trend of the PV to make necessary adjustments.
- In theory, this action should always improve the dynamic response.
- If the derivative time (τD) is too large, there will be significant oscillations in the CV.
Proportional-Integral-Derivative (PID) Control
- PID control combines proportional, integral, and derivative actions.
- The ideal PID control is rarely implemented in practice, and instead, "derivative on PV only" is used.
Process Control Laws
- The first law states that the best control system is the simplest one that will do the job.
- The second law states that you must understand the process before you can control it.
- The third law states that liquid levels must always be controlled.
Process Control Languages
- Different approaches or languages are used to solve problems in process control.
- The choice of language depends on the complexity of the problem.
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Description
Test your knowledge of feedback control systems, including closed loop response, regulatory response, and servo response. Learn about basic concepts, stability, and more.