Control Systems Unit 3

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.