Control Systems Unit 3

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Questions and Answers

In control valve terminology, what does AO stand for?

Air to Open (correct)

Automatic Operation

Adjustable Output

Actuator Opening

What does AC stand for in control valve terminology?

Adjustable Capacity

Automatic Control

Actuator Control

Air to Close (correct)

What is a 'fail-safe action' in control valves?

The speed at which the valve closes

The ability to operate at low temperatures

The default action a valve takes upon loss of control signal (correct)

The maximum pressure the valve can withstand

What does the control valve sizing coefficient Cv represent?

The flow rate through the valve at a specified pressure drop (A) Signup and view all the answers

Cv is typically defined as the flow rate in gallons per minute (GPM) that a valve can pass for a pressure drop of:

1 PSI (B) Signup and view all the answers

Which of the following factors affect the Cv of a control valve?

All of the above (D) Signup and view all the answers

What is the primary need for a positioner in a control valve system?

To ensure the valve reaches the desired position accurately (D) Signup and view all the answers

What is cavitation in control valves?

Formation and collapse of vapor bubbles in the fluid (C) Signup and view all the answers

What effect does noise have on control valve systems?

Reduces the life span of valve components (B) Signup and view all the answers

Which method is effective in reducing noise in control valves?

Using noise attenuating trims or silencers (A) Signup and view all the answers

What is the main purpose of a control system?

To maintain a desired output (B) Signup and view all the answers

What does the term 'error' refer to in control systems?

The difference between the desired and actual output (B) Signup and view all the answers

What is a variable range in the context of control systems?

The range of possible inputs (D) Signup and view all the answers

What does 'cycling' in a control system imply?

Periodic oscillation of output (C) Signup and view all the answers

What is 'direct action' in a control system?

Increase in controller output with increase in error (D) Signup and view all the answers

Which of the following is a characteristic of a two-position control mode?

On/Off control (A) Signup and view all the answers

What is a multi-position control mode?

It has multiple discrete positions (A) Signup and view all the answers

In floating control mode, the controller output:

Varies continuously within a range (A) Signup and view all the answers

Proportional control mode adjusts the controller output:

Proportional to the error (D) Signup and view all the answers

Integral control mode is primarily used to:

Eliminate steady-state error (C) Signup and view all the answers

Study Notes

Unit 3: Control Systems

Control System Fundamentals

The main purpose of a control system is to maintain a desired output.
Error in a control system refers to the difference between the desired and actual output.
A variable range in control systems refers to the range of possible outputs.
Cycling in a control system implies periodic oscillation of the output.

Control Modes

Direct action in a control system means the controller output increases with an increase in error.
Two-position control mode is a type of discontinuous control where the output has only two positions: on and off.
Multi-position control mode has multiple discrete positions.
Floating control mode is a type of discontinuous control where the output varies continuously within a range.
Proportional control mode adjusts the controller output proportional to the error.
Integral control mode is used to eliminate steady-state error.
Derivative control mode is used to predict future error and respond to the rate of change of error.

Controller Types

A Proportional-Integral (PI) controller is used to eliminate steady-state error and dampen oscillations.
A Proportional-Derivative (PD) controller is used to improve transient response.
A PID controller combines proportional, integral, and derivative actions.
Reset windup in a PID controller occurs when the integral action accumulates excessively.
Rate before reset in a PID controller refers to the derivative action being applied before the integral action.

Tuning Control Systems

The main goal of tuning a PID controller is to optimize the controller parameters for desired performance.
Performance criteria for tuning PID controllers include quarter amplitude decay ratio, integral time performance indices, and minimum settling time.

Unit 4: Tuning PID Controllers

Tuning Methods

The Ziegler-Nichols method is a heuristic method for tuning PID controllers.
The Process Reaction Curve method is an open-loop tuning method that involves observing the system's transient response to a step input.
The quarter amplitude decay ratio is a performance criterion that aims to achieve rapidly decaying oscillations.
Integral of the absolute error (IAE), integral of the squared error (ISE), and integral of the time-weighted absolute error (ITAE) are performance indices that consider the cumulative error over time.

Ziegler-Nichols Method

The Ziegler-Nichols method involves finding the ultimate gain (Ku) and ultimate period (Pu) of the system.
The ultimate gain is the gain at which the system starts to oscillate.
The ultimate period is the period of oscillation at the ultimate gain.
PID parameters are set using pre-determined formulas based on Ku and Pu.

Limitations of Ziegler-Nichols Method

The Ziegler-Nichols method can result in excessive oscillations in some systems.
It is often used to provide good starting values for further tuning.

Unit 5: Final Control Elements

Signal Conversion

Current-to-pressure (I/P) converters convert electrical signals to pneumatic signals.
Pressure-to-current (P/I) converters convert pressure signals to electrical signals.

Control Valves

Final control elements are necessary to directly regulate the flow of energy or material.
Control valves are classified by their method of operation.
Rangeability refers to the ratio of maximum to minimum controllable flow rate.
Turndown is the ratio of maximum capacity to minimum controllable flow.
Valve capacity is usually measured in liters per second (L/s) or gallons per minute (GPM).

Control Valve Characteristics

Inherent control valve characteristics refer to the valve's behavior when operating without external influences.
Installed control valve characteristics refer to the valve's behavior in the actual process system.
Cv is the valve sizing coefficient that represents the flow rate through the valve at a specified pressure drop.

Unit 6: Control Valve Types

Globe Valves

Globe valves have a spherical body with a disc.
They are suitable for throttling and have excellent throttling capability.
Single-seated globe valves are typically used in applications requiring tight shutoff.
Double-seated globe valves are preferred where balanced forces on the valve stem are needed.

Other Control Valve Types

Gate valves are suitable for on/off applications with minimal pressure drop.
Angle valves are used for 90-degree flow direction changes.
Butterfly valves are compact and lightweight, but have limited throttling capability.
Ball valves are used in on/off applications with minimal pressure drop.

Control Valve Performance

Positioners are used to ensure the valve reaches the desired position accurately.
Cavitation occurs when vapor bubbles form and collapse in the fluid.
Flashing occurs when the fluid pressure drops below the vapor pressure.
Noise in control valves can reduce the life span of valve components.

Note: These notes are a summary of the key points and concepts covered in the provided text. They are intended to be a concise and organized study aid.

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Description

Test your understanding of control systems with these questions on their purpose, error, and variable ranges. Covers the basics of control systems and their applications.