Feedback Control Systems Quiz

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Questions and Answers

What is the value of CO when the error is zero?

Zero (correct)

One

What is the effect of a higher proportional band?

No effect on gain

Lower gain (correct)

Gain depends on the sign of the controller

Higher gain

What is the action of a reverse-acting controller?

Increase-decrease (correct)

Increase-increase

Decrease-decrease

Decrease-increase

What factors determine the correct sign of the controller?

Action of the transmitter, valve, and effect of MV on CV Signup and view all the answers

What happens when the valve fails in the example provided?

It shuts, preventing steam from entering the process Signup and view all the answers

What is the outcome of a correct controller action selection?

The controller gives the correct action Signup and view all the answers

What is the response to a load disturbance in a closed loop system called?

Regulatory response Signup and view all the answers

What type of process has variables that converge to a steady state when disturbed?

Stable process Signup and view all the answers

What is the purpose of a feedback controller in a closed loop system?

To reject disturbances Signup and view all the answers

What is the name of the controller that uses a bias signal and a proportional term?

Proportional controller Signup and view all the answers

What is the purpose of Integral action in PI control?

To eliminate the steady-state error after a set point or disturbance change Signup and view all the answers

What happens if the derivative time (τD) is too large?

There will be significant oscillations in the CV Signup and view all the answers

What is the response to a setpoint change in a closed loop system called?

Servo response Signup and view all the answers

What type of process has variables that go to infinity mathematically?

Unstable process Signup and view all the answers

What is the purpose of Derivative action in PID control?

To improve the dynamic response of the control loop Signup and view all the answers

What is the main disadvantage of using PI control?

It degrades the dynamic response of the control loop Signup and view all the answers

What is the principle behind the 1st Law of Process Control?

The best control system is the simplest one that will do the job Signup and view all the answers

What is the advantage of using different languages in Process Control?

Each language is used for a specific type of problem Signup and view all the answers

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.