Valve Positioning and Control Systems

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary function of a Square-Root Extractor in a process control system?

To linearize the output signal from a differential-pressure flowmeter. (correct)

To control the speed of an AC motor.

To amplify the electrical signals from sensors.

To convert analog signals to digital signals.

What does a larger gain (K) indicate about a controller's sensitivity to input changes?

It results in a slower response to input changes.

It decreases the sensitivity of the controller.

It increases the sensitivity of the controller. (correct)

It has no effect on the controller's sensitivity.

Which type of actuator directly influences the process variable?

Pressure transmitter.

Analog-to-Digital converter.

Final Control Element (FCE). (correct)

Square-Root Extractor.

What is the role of a Digital-to-Analog (D/A) converter in a process control system?

To convert digital signals from the controller to analog signals for actuators. Signup and view all the answers

What is the role of Integral (I) mode in a control system?

To eliminate steady-state error or offset that may persist. Signup and view all the answers

Which type of motor is primarily used in high torque applications?

DC Motor. Signup and view all the answers

How is the proportional band (PB) calculated?

PB = Controlled variable % change / Final control element % change. Signup and view all the answers

Which statement about the behavior of a controller with a narrow proportional band is correct?

It is more sensitive and responds more quickly to changes. Signup and view all the answers

Which characterizes a solenoid valve's operation?

It operates by converting an on-off electrical signal to control flow. Signup and view all the answers

What occurs if a load change persists for one minute in the context of control outputs?

The output may rise but will still have residual error after one minute. Signup and view all the answers

What is the main goal of a closed-loop control system in process control?

To ensure the controlled variable equals the set point Signup and view all the answers

What role does the feedback signal play in a closed-loop control system?

It represents the difference between the set point and the output Signup and view all the answers

Which of the following elements is NOT part of a closed-loop control system?

Load cell Signup and view all the answers

In the context of a first-order control system, what does 'k' represent?

Gain of the system Signup and view all the answers

What is a characteristic of response time in a closed-loop control system?

It has an inherent delay after a step change Signup and view all the answers

What does the term 'set point' refer to in process control?

The desired value for the controlled variable Signup and view all the answers

Which type of control utilizes a simple on-off mechanism rather than continuous adjustment?

On-off control Signup and view all the answers

What aspect of closed-loop control systems improves their response to disturbances?

Feedback implementation Signup and view all the answers

What percentage of the step input does the system approach after one time constant (𝜏)?

63.2% Signup and view all the answers

How long does it typically take to approach steady state in a control loop?

5𝜏 Signup and view all the answers

What is dead time in a control loop?

The time it takes for corrective action to start after a step input. Signup and view all the answers

Which component converts a current signal (I) to a pneumatic signal (P)?

I/P transducer Signup and view all the answers

In a P/I transducer, what does the back pressure from the bubbler sensor indicate?

Level of flammable liquid Signup and view all the answers

What signal does the I/E transducer produce from a current signal of 4-20 mA?

Proportional voltage signal of 0-10 V Signup and view all the answers

Which method does NOT help to reduce response time in a control loop?

Increasing the control loop size Signup and view all the answers

What kind of device connects in parallel with a transmitter in an I/E transducer system?

Indicator and controller Signup and view all the answers

What is the primary function of a valve positioner?

To amplify pneumatic force for valve positioning Signup and view all the answers

Which valve characteristic is most suitable for on-off control?

Quick-open Signup and view all the answers

What does the valve coefficient C represent in the equation $q=C \cdot \Delta P/SG$?

The flow capacity of the valve Signup and view all the answers

In PID control, what does the term 'Set point (SP)' refer to?

The desired value of the controlled variable Signup and view all the answers

Which control mode allows the controlled variable to follow the set point continuously?

Integral control Signup and view all the answers

What effect does a valve positioner have on actuator dead-band?

It reduces dead-band and hysteresis effects Signup and view all the answers

What does the derivative term (D) in a PID controller primarily address?

The change rate of the error Signup and view all the answers

What happens in a closed-loop control system when a feedback signal is received?

The system compares the feedback to the set point Signup and view all the answers

Study Notes

Valve Positioner

Amplifiers pneumatic signals to provide enough force for positioning the valve
Benefits: Overcome friction in valve, reduce actuator dead-band, provide linear and precise positioning, improve closed-loop frequency response

Valve Characteristics

Flow through valve: q = C (ΔP/SG)
Valve coefficient C is related to changes in the valve flow coefficient
Quick-open valve: small changes in valve position lead to large changes in flow (suitable for on-off control)
Linear valve: flow changes linearly with valve position
Equal percentage valve: flow change is proportional to the existing flow rate

Controller

Comparator: Compares the set point (SP) and feedback signal (FB) to produce an error (e(t))
PID or On-Off Controller: Uses the error signal to calculate the control output (CO) that dictates the manipulated variable (MV)
Actuator: Operates the controlled variable (CV) based on the manipulated variable
Closed loop - feedback control system - sensor provides the feedback signal

Common Control Modes

On-Off: Uses a binary control output (on or off)
Proportional: Provides a continuous control output that is proportional to the error signal
Integral: Corrects for steady-state errors by adding an output that increases linearly with the size and duration of the error
Derivative: Anticipates future changes in the error by adding an output that is proportional to the rate of change of the error

Closed-loop Control System

Objective: Maintaining the controlled Variable at the setpoint
Disturbances (load changes, environmental factors) can affect the controlled variable
Feedback loop - provides continuous adjustment to maintain the desired operating condition

Response Time of Closed-Loop

System with time delays: The control loop does not immediately respond to changes, a time constant (τ) governs the response speed
1st Order System: Input and output are related by a transfer function (k = gain, τ = time constant)
Time Constant: The time taken for the output to reach 63.2% of the step input; steady-state is approached after 5 time constants

Response Time and Dead Time

Dead time: The time it takes for the system to start responding to a change
Transient time: The time for the system to stabilize following the initial response
Two ways to reduce time delay: Proper controller selection, tuning the controller

I/P Transducer

Converts a current signal to a pneumatic signal
Pneumatic signal operates a control valve, controlling fluid flow through a pipe

P/I Transducer

Converts a pneumatic signal to a current signal
Used in devices like bubbler sensors that measure liquid levels
High levels = high pressure = high output current
Low levels = low pressure = low output current

I/E Transducer

Converts a current signal (4-20mA) to a proportional voltage signal (0-10V)
Output is sent to an indicator, recorder, or controller

Other Transducers

Square-Root Extractor: Linearizes the output of a differential-pressure flow meter
Analog-to-Digital (A/D): Converts analog signals to digital signals (used in PLCs and PCs)
Digital-to-Analog (D/A): Converts digital signals to analog signals (used in PLCs and PCs)

Final Control Element (FCE) - Actuator

Manages the process variable directly
Examples include solenoids, motors, and valves

Solenoid Valve

Electromagnet controlled
Electric signal (on/off) controls the flow

AC Motor

Used in many industrial applications (pumps, blowers)
Speed control: Varying the frequency

DC Motor

High torque applications (augers, hoists)
Can be used in control systems that require precise speed control

Sensitivity of a Controller:

Percentage input change divided by the controlled variable percentage change
Proportional Band: The percentage change in the input signal required to produce a 100% change in the output
Gain (K): The sensitivity of the controller; Higher gain means more sensitivity to error changes

Integral Mode

Eliminates offset errors by adjusting the output based on the duration of the error
When an error exists, the integral mode continuously increases the control output until the error is corrected

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz delves into the principles of valve positioners, characteristics, and control systems. Explore the benefits of amplifying pneumatic signals, different types of valves, and the functioning of PID controllers in closed-loop systems. Test your understanding of flow dynamics and control mechanisms.