I5 Flashcards
Process variable
The variable that is controlled
Sensor
Measures and senses the process variable, provides feedback to controller
Actuator / Controlled device
Alters the process variable
Set Point
Provides information to the controller about desired value of process variable
Error (signal)
The difference between the set point signal and the feedback signal
Command signal
Controlling device / controller
Controls the actuator, compares feedback signal to set point signal to generate an error signal
Open Loop control system
Controlled manually
Open-loop system operator
A person that monitors the process variable and operates the controlling device as needed.
The process variable has no effect on the operation of the controlling device.
This means the system (is/is not) self correcting, is (open/closed)-loop.
Not self correcting, Open-loop.
Closed-loop control system
Controlled automatically
Closed-loop system is also called
Closed-loop feedback control system
Why is a closed loop system called a closed-loop feedback control system?
Because an automatic system requires feedback from the process variable
Discontinuous control system
full power is applied or removed to modify the process variable
Continuous control system
power supplied to the process variable is controlled from very low to full power
3 examples of process variables
Motor speed
Fluid pressure
Liquid level
Room temperature
Liquid flow
What is the function of a sensor?
To measure the process variable and provide feedback information to the controller
What is a set point?
The desired value of the process variable
What is the difference between an open-loop and a closed-loop control system?
An open-loop system is a manual system, it is not self correcting, and the controlling device operates independently of the process variable
A closed-loop system is automatic, self correcting, and the controller relies on feedback from the process variable to modify it.
5 system elements of a closed-loop control system
Process variable
Set point
Sensor
Controller
Actuator
Difference between continuous and discontinuous control system
Continuous system supplies variable power
Discontinuous supplies full power or no power
Transducer
Generally a sensor used for automatic control
Converts energy in to a form suitable for the controller
Analog sensor signal
Supplies continuous variable output that varies smoothly
Digital sensor signal
Is displayed in fixed, discrete steps
Common type of sensors
Motion
Force
Fluids
Temperature
Light
3 types of motion
Linear: motion in a straight line
Angular: The rotary movement of a shaft in degrees clockwise or counter-clockwise from a reference position
Rotary: Typically the speed of rotation of a shaft and its direction
Sensor of force
Bonded wire strain gauge
Wire bonded to a plastic sheet in grid form
How does a bonded wire strain gauge work?
When force is applied to the column it’s overall length reduces
This change is transferred to the wire of the gauge
When the wire is compressed it’s effective length reduces
Cross sectional area increases
This translates into an overall reduction in resistance
Types of fluid pressure sensors
Bourdon tube
Bellows
Diaphragm
Piezoelectric pressure transducer
Fluid flow sensors
Orifice plates and manometers
Magnetic flow meter
Liquid level sensors
Float liquid level sensor
Capacitive liquid level sensor
Ultrasonic wave
Temperature sensors
Resistive temp. sensor
Resistance temp. detector
Thermistors
Thermocouple
Bimetallic sensor
Light sensors generally work on one of the following principles…
Photoemission: electrons liberated from material when light energy falls on it
Photoconduction: resistance of material is reduced with light
Photovoltaic: voltage is generated in a cell due to light falling on it
What is a transducer?
A device that converts one form of energy into another
List 4 basic principles used to sense linear motion
Resistance change
Inductance change
Capacitance change
Transformer coupling change
What resistance parameters are changed when a strain gauge is subjected to a compression force?
Decrease in length and increase in cross sectional area
How can a piezoelectric crystal work as a sensor?
It can separate charges (create voltage) according to the amount of mechanical pressure applied.
3 basic principles used in light sensors
photoemission
photoconduction
photovoltaic
Function of a transmitter
Accept sensor information and provide standard types of output signals for transmission over long distances
How does a bourdon tube work?
It is a semicircle of slightly flattened steel. As pressure in the tube increases, it straightens.
How do bellows work?
Fluid pressure can be applied from either end, the bellow expands ans pressure increases.
What does a controller typically consist of?
A comparator and an output section
What does the comparator do?
Generates an error signal by measuring how far away the feedbacksignal is from the set-point signal.
Negative feedback is?
Desired.
The error signal is the difference between the set point and the feedback signal.
Positive feedback is?
Undesired.
Connection error or wrong selection of system components.
sum of set point and feedback signals.
Four basic modes of control?
On/Off
Proportional
Proportional plus integral
Proportional plus integral plus derivative (PID)
What is the difference between positive and negative feedback?
Positive is the sum of set-point and feedback signal
Negative is the difference of set-point and feedback signal
What are the general classifications of actuators?
Electrical
Hydraulic
Pneumatic
Function of actuator?
Receive signal from controller, modify process variable accordingly
