Automated Controls Flashcards
Define control systems
Set of mechanical or electronic devices that regulate other devices or systems by way of control loops
Define open loop
Controlled only by set point commands, without feedback measurement signals. Uses one control like on/off or a timer
Define closed loop
Uses feedback signals from the system to provide automatic control and maintain specific settings. Uses two or more controls.
Define negative feedback
reduce the change or output. Dampen or buffer changes.
Which feedback provides system stability/equilibrium?
Negative feedback
Define positive feedback
increases the change or output. Enhances or amplifies changes. Moves system away from equilibrium.
What is another term for negative feedback?
Self-regulated
What are some examples of a negative feedback system?
House temp closed loop control system, flyball governor, water level control, cruise control
What is an example of positive feedback?
Schmitt Trigger
Define direct action (non-inverting)
Controller output signal will respond to any controller input signal change in the same direction
Define reverse acting (inverting)
Controller output signal will respond to any controller input signal change in the opposite direction
What is the key function/component of an open loop system?
Timers
What is the fundamental process for a flyball governor?
Centripetal force
What is the purpose of a solvent tank?
Provide a constant level of solvent regardless of various load behaviours.
proportional control in proportion to demand
What type of loop and feedback system is a solvent tank?
Proportional closed loop negative feedback system
What are the four steps in a control system/control block?
Process, measurement, controller/error detector, final control
Define a transfer function?
Represents the relationship between output signal of a control system and the input signal
Each transfer function can be described using what two terms?
Transfer gain K (static) and speed of response D (dynamic)
Digital signal is what type of control mode?
Discontinuous - On/Off
Analog signal is what type of control?
Continuous - proportional, integral, derivative
What is the first block of the block diagram?
Process - what needs to be controlled
Ex) water level
What is the second block of the block diagram?
Measurement Sensor - senses the controlled variable and produces a corresponding signal which is sent to the controller.
Can be called controlled variable (CV), measured variable (MV), or process variable (PV)
What is the third block in the block diagram?
Controller/Error Detector. Block receives measured variable from sensor, controller decides on what size and direction of output signal to send.
What is the fourth block in the block diagram?
Final Control - received output signal from controller, adjusts energy or material flow to or from a process.
Also called manipulated variable (MV).
What are some advantages of closed loop systems?
Faster, more consistent, and more dependable than humans. Can use more variables than humans can.
What are some disadvantages of closed loop systems?
Equipment is more complex and more expensive, installation is more difficult, equipment maintenance is challenging and requires a lot of knowledge. Possibility of ‘run away’ control system.
What is a run away control system?
The result of a faulty control system when the error signal remains at non-zero constantly.
How is a run away prevented?
Safety override control systems should be installed in critical control applications.
What is a proportional closed loop control system?
the output of one block is the input of the next block - a series of action and reaction responses makes it proportional.
What are some other terms for a proportional control system?
self-correcting system, negative feedback control system
What does PFD stand for?
Process Flow Drawings
What does P&ID stand for?
Piping and instrumentation diagram or Process and instrumentation diagram
What is the purpose of a P&ID?
Shows the ‘key components’ of equipment and the mechanical interconnections of pipes and ducts. (instruments, controllers, final controls, computers, software etc)
How will a primary location (normally accessible) symbol be shown?
A horizontal line through the shape
How will a field mounted symbol be shown?
Just the symbol, no lines through centre
How will an auxiliary location (normally accessible) symbol be shown?
Two lines horizontally through the centre
What is the symbol for discrete instruments?
Circle
What is the symbol for shared display, shared control?
Circle inside of a square
What is the symbol for computer function?
Hexagon
What is the symbol for a PLC?
Diamond inside of a square
How are normally inaccessible or behind the panel symbols shown?
The same as it’s regular symbol, but a dashed line instead of a solid line
What does ISA stand for?
International Society of Automation
What does ANSI stand for?
American National Standards Institute
What is the difference between ISA and ANSI?
ISA is the organization, ANSI is the standard
On a P&ID, what do the first letters of a symbol represent?
The measured or initiating variable, and then the modifier
On a P&ID, what do the succeeding letters of a symbol represent?
Readout or passive function, then output function, then modifier
What does the first letter D mean on a P&ID symbol?
Differential
What does the first letter F mean on a P&ID symbol?
Flow rate
What does the first letter I mean on a P&ID symbol?
Electrical current
What does the first letter L mean on a P&ID symbol?
Level
What does the first letter P mean on a P&ID symbol?
Pressure
What does the first letter T mean on a P&ID symbol?
Temperature
What does the second letter A mean on a P&ID symbol?
Alarm
What does the second letter C mean on a P&ID symbol?
Control
What does the second letter H mean on a P&ID symbol?
High
What does the second letter L mean on a P&ID symbol?
Low
What does the second letter I mean on a P&ID symbol?
Indicator
What does the second letter T mean on a P&ID symbol?
Transmit
What does the second letter V mean on a P&ID symbol?
Valve
What does the second letter Y mean on a P&ID symbol?
Convert
The transfer function of a control system component primarily relates to the ______ and the ______ relationship of the input and the output?
Size, and timing
Define and electrical transducer
A device which receives information in one form and converts it into another form
What are some inputs of transducers?
Temperature, pressure, speed, oxygen, carbon dioxide
What is the output of a temperature transducer?
Voltage/resistance signals
What is the output of a pressure transducer?
resistance signals
What is the output of a chemical transducer?
Electrical signals
What is the output of microphones?
Mechanichal vibration - Electrical signals
What is the output of speakers?
Electrical signals - mechanical vibration
What are the two basic types of input transducers?
Active, Passive
What is an active transducer?
Converts ambient physical variable monitored directly to a corresponding electrical signal.
Generates energy output caused Ryan energy force rather than an external power source to operate them.
ex) thermocouple, solar panel
What is a passive transducer?
Change their characteristic when exposed to energy but cannot generate energy on their own.
ex) strain gauge - when a weight is placed on scale the strain gauge resistance changed and the passive transducer measured the change in resistance
Are sensors input or output transducers?
Input
What is an example of an output transducer?
Speakers, actuators, LED’s. Converting electrical signals into physical changes
Define an electrical sensor element
A device that detects a quantity or energy and directly converts it into an electrical form
The form of an electrical sensor output can be AC, or DC, True or False?
True, can be both AC or DC depending on the chemical makeup and/or electromechanical makeup of the sensor itself.
True or False, the output of an electrical sensor element itself is too low of a level to be directly useful in control applications.
True
Define an electrical transducer
Device which receives information in one form and converts it into another form.
Is an electrical sensor element a type of electrical transducer?
Yes
Define an transmitter
An elaborate transducer that includes a sensor and additional components to modify sensors output to produce a sStandardized transmission signal
What are the standard current output signals of a transmitter?
4-20mA
What are the standard voltage output signals of a transmitter?
0-10V, 0-5V, 105V, -10 - 10V
What are the standard pressure output signals of a transmitter?
3 - 15 psi, or 20 - 100 kPa
How is am amplifier used in a transmitter?
The primary input measured in millivolts uses an amplifier to convert that very small current into a standard current signal
What is the purpose of the filter in a transmitter?
stripping off or removing any electrical noise that might be present on the modified signal coming from the amplifier
What is the main application that transducers and transmitters are usually found in?
Wherever measurements of physical or chemical quantities are required. Usually industrial setting for controlling diverse processes like mining, manufacturing, anything with the process of level and pressure, liquid or gas temps. etc.
What other applications are transducers and transmitters found in other than process control?
Consumer products, continuous monitoring and recording, remote data acquisition, hazardous environments, biomedical
What is the purpose of standardization of signals?
So that transmitters can easily interface with a wide range of industrial instruments. ex) all instruments have signal inputs of 4-20mA so a data logger can be used to log PI’s, FI’s, FT’s, etc etc
What are the main advantages of using a current loop signal?
- Less susceptible than voltage signals to electrical interface (Noise) because of low impedance input of current receivers
- Can be transmitted over long distances without losses, opposed to voltage which will suffer voltage drop
- Power supply is external to the transmitter and can be connected with a two wire
- Line breakage can be detected unmistakably
What are the main disadvantages of a current loop signal?
-If there is a break in any one of the series connections, the entire system will malfunction
- Length of transmission lines and number of instruments that can be driven is limited
- Two - wire current transmitters require a minimum voltage (typically 12V) to operate
For voltage of 1-5V, what is the standard current?
4-20mA
For voltage of 1-10V, what is the standard current?
10-50mA
How are 4-20mA current signals connected?
In series
What is the input/output relationship of a pressure transmitter?
Proportional, constant, ideal, linear.
Signal current to voltage transducer has a linear 4:1 conversation factor (signal scale factor)
Define range (input and output) of a transducer
the stated minimum and maximum measurement values
ex) 4-20mA or 0-10V
What is the minimum value of range in the transducer?
Zero value
What is the maximum value of range in the transducer?
Full scale value
What is the difference between range and span?
Span is the algebraic difference between the min and max value range.
ex) Range: 4-20mA, span is 16mA
Define static transfer function
Input to output relationship of a device in equation form, ignoring the (dynamic) time response
ex) temperature measurements
What is the equation of the basic device static transfer function?
y = mx + b
transmitter output = (output span/input span) (input meas. - input zero value) + output zero value
y = output
m = gain (slope)
x = input
b = bias (y intercept)
Define span accuracy
The limit within which the output may deviate. Usually specified as a percent.
ex) +- 0.5%
What is the formula to find span accuracy?
% accuracy = max deviation from actual value/theoretical span x 100%
What are the four most common types of errors that may be found in transducer systems?
Bias (zero) error, Span (gain) error, linearity (gain variation), and hysteresis/dead band
What is Bias (zero) Error?
Actual output value - theoretical output value. Measured at the minimum range (zero) value.
ex) 8mA - 4mA = 4mA of bias error
What is span error (gain error)?
the difference in the actual output signal span compared to the theoretical span. Caused bydeviation from the ideal information transfer gain, expressed as a percent.
ex) actual span: 24mA - 4mA = 20mA
Theoretical span: 20mA - 4mA = 16mA
Error in % = 20mA - 16mA/16mA x 100% = +25%
What is linearity error (variable gain error)
maximum deviation from the theoretical output after bias and span error have been removed. Expressed as a percentage of the theoretical span, also known as non-linearity.
Non linearity % = max deviation/theortetical span x 100%
ex) assume maximum deviation from theoretical is 0.5mA.
non linearity in % = 0.5mA/16mA x 100% = 3.125%
What is hysteresis error?
present when the device under test gives different results when readings are taken in both the upscale and downscale directions. directional error associated with friction and backlash in mechanical devices and systems.
What are the seven most common types of object detection sensors?
Photoelectric, electro-mechanical, pneumatic, magnetic, inductive, capacitive, ultrasonic
What are the three types of photoelectric sensors?
Through-beam, retroreflective, and diffused
What is an example of an electro-mechanical switch?
Limit switch
What are the two types of magnetic sensors?
Reed switch, hall effect switch
What is a reed switch?
Operates when exposed to magnetic field - there are two contacts inside that close when there is a magnetic field.
Cannot carry high currents, must be protected by fast acting fuse
What is the hall effect?
Production of voltage difference across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current
What is a linear hall effect sensor?
Provides an analog output that’s proportional to the magnetic flux density which enables precise position measurements.
ex) used widely in automotive to measure angles of mirrors
What is a digital hall effect sensor?
activated by an external magnetic field. The output signal from the hall effect sensor is the function of magnetic field density around the device.
What is an inductive sensor?
Device that detects metal objects without physical contact.
What are the advantages of an inductive sensor?
withstand harsh environments, has a long life, easy to install, high switching rate
What are the disadvantages of an inductive sensor?
Sensing rate is short, can only detect metallic targets
What is a capacitive sensor?
device that detects presence of objects without making physical contact
What is the main advantage of a capacitive sensor?
It can detect nearly any object/material, not just metal like an inductive sensor
What is an ultrasonic sensor?
A device that measures distance based on rate and time of ultrasonic waves
What are the three main characteristics that will affect ultrasonic sensors?
Density of air, temperature, humidity
What are the six types of Linear Position Sensors?
linear motion potentiometer, LM variable inductor, LV variable capacitor, Linear variable differential transformer (LVDT), ultrasonic sensor, RF types of position sensors
What is a linear motion potentiometer?
Device that measures an objects linear position by converting mechanical motion into electrical signal. Uses a sliding contact or wiper that moves along a resistive element changing the resistive value.
What is a linear motion variable inductor?
conductive tube surrounds an inductive probe, the position of the tube or rod varies the inductance oft he probe. The resonant frequency of the oscillator is measured and the change in frequency is converted into an electrical signal.
What is a linear motion variable capacitor?
uses linear motion to change the capacitor value. Set of plates or a cylinder that can move with a probe/plunger, changing the area of overlap between plates or materials.
What is a linear variable differential transformer (LVDT)?
a device that converts linear motion into electrical signal. Used in aerospace, power generation, and industrial automation.
Primary winding and two secondary windings, primary winding is connected to an AC source producing flux in the air gap. The flux induces voltages in the secondary windings. Moveable iron core is placed inside the LVDT, the displacement being measured is connected to the iron core. The difference between voltages of the two secondary windings is the output voltage.
What is an ultrasonic sensor?
Measures fluid levels in tanks, distances, and detects objects. A pulse of high frequency is transmitted and the time it takes for the echo to return is measured.
What are the five types of angular position/motion sensors?
Potentiometer sensors, rotary motion differential transformer (RMDT), capacitive sensors, angular resolvers, encoders
What is a potentiometer?
variable resistor that controls resistance in a circuit. Has a resistive element with a moving contact called a wiper that can be adjusted.
What is the differential output formula for a RMDT?
Vzero = V1 - V2
What is an optical encoder?
Electromechanical device, monitors the direction of rotation, position, or velocity of a rotary or linear operating mechanism.
A Schmitt Trigger is what type of sensor?
Optical Encoder
What are the four major elements of an optical encoder?
a light source, light sensor, optical disk, signal conditioning circuitry
What is an absolute encoder?
- Device that reads a unique code on a disc that rotates with the shaft.
- Code is derived from light sensors on each track, a parallel output from these detectors produce highs and lows to form a 4-bit pure binary whole word that indicates the position.
- Binary count of 0000 to 1111 is possible
- Disk is divided into sixteen 22.5 degree sections
What is a load cell?
Converts force such as tension, compression, pressure, or torque into an electrical signal.
Define a border resistor
A resistor connected to the secondary side of a CT in parallel to the load, used to lower voltage on the secondary.
What is the formula for pressure?
P = F/A
P - Pressure (psi)
F - Force (pounds)
A - Area (square inches)
What are the four basic scales?
Gauge, absolute, differential, and vacuum
What is a gauge pressure scale?
Instruments use atmospheric pressure as the reference point, can be positive or negative
What is absolute pressure scale?
Instruments with absolute pressure scale are referenced to absolute zero or the complete absence of pressure. Positive values only.
What is a differential pressure scale?
Used to express the difference in pressure between two measured pressures. It is determined by subtracting the lower reading for the higher reading.
What is vacuum pressure?
Measured below atmospheric pressure, the zero point for vacuum pressure is referenced at the atmospheric pressure
Which type of pressure measurement is usually more accurate?
Absolute pressure.
Absolute pressure = gauge pressure + atmospheric pressure
What are three examples of primary deflection type pressure sensors?
Bourdon element, bellows element, diaphragm element
What three components do deflection type pressure sensors consist of?
primary element, secondary element, signal conditioner
What is the purpose of the primary element of a deflection type pressure sensor?
Converts the measured pressure into a proportional mechanical displacement
What is the purpose of the secondary element of a deflection type pressure sensor?
Converts mechanical displacement into an electrical quantitty
What is the purpose of the signal conditioner in a deflection type pressure sensor?
Converts the change of the electrical quantity into a corresponding proportional signal (for use by a controller, PLC, etc)
What are some types of secondary elements (signals)?
Resistance, capacitance, inductance
Which method of sensing is used for Open Tank Level Measurement?
Pressure-based liquid level sensor using a differential pressure cell. High pressure liquid is applied to only one side of the DP cell, and atmospheric pressure is applied to the other side.
Which method of sensing is used for Closed Tank Level Measurement?
Pressure based liquid level sensing using a differential pressure cell. On a closed tank there is a small holding tank at the top of the tank, which is connected to the other side of the DP cell instead of atmospheric pressure. The DP cell is reading high pressure from the tank side and differential pressure from the small tank on the other side (low pressure side).
What is the displacement float measuring method?
The buoyant force on a stationary float is proportional to the liquid level around the float
What are the three other types of liquid level measuring methods?
Displacement float, capacitance probe, ultrasonic system
What are the two types of ultrasonic flowmeter technologies?
Doppler type, transit time type
What is a doppler flowmeter?
Sends an ultrasonic signal across a pipe, calcualtes the flow velocity by measuring the doppler frequency shift. pipe must be full and fluid must contain particles
What is a transit time flowmeter?
Measures the difference in travel time between alternating ultrasonic beam pulses, transmitted in both the direction of flow, and the opposite direction of the flow. Time difference is used to calculate the flow rate.
What are the three most common temperature transducers?
Thermocouple, resistance temperature detector, thermistor
What is the seebeck effect?
The EMF that develops across two points of an electrically conducting material when there is a temperature difference between them
What is the hot and cold junction of a thermocouple?
Hot - measuring point on a thermocouple
Cold - the reference point on the thermocouple
What is a thermopile?
Multiple thermocouples connected in series
What is a thermo-well?
Cylindrical tube that protects temperature sensors from harsh industrial conditions
What is a resistance temperature detector (RTD)?
Sensor whose resistance changes as its temperature changes. REsistance increases and temp of sensor increases. RTD has a positive temperature coefficient.
Which sensor has a negative temperature coefficient?
Thermistor
Which sensor has a positive temperature coefficient?
RTD
What are some advantages of the thermocouple?
Self powered, simple, strong, wide temp range
What are some advantages of the RTD?
Most stable, most accurate, more linear than thermocouple
What are some advantages of the thermistor?
High output, fast, two wire ohms
What are some advantages of the IC sensor?
Most linear, highest output, inexpensive
What is an infrared pyrometer?
device used to detect infrared energy emitted from an object
How does an infrared pyrometer work?
Uses infrared radiation.
Total amount of energy received by infrared pyrometer is the sum of radiated energy (E), reflected energy (R), and transmitted energy (T).
Only radiated or emitted energy reflects the temperature
Define time lag
From the time that the system gets the command to the time the control valve opens
Define dead time
from the time the control valve opens to the time the process variable starts to change
Define pure lag time
dynamic response
What are the effects of time lags?
May make a feedback control system going into oscillation and being unstable. Gain of the control loop must be adjusted, reduced in order that the system maintains at the desired stability
What are the blocks for the process level control diagram?
Process (tank level), Level transmitter (sensor/measurement), level controller, final control element (valve)
What are the two main components of the controller block in the process level control?
Summing point, calculation unit
What is the purpose of the controller in the process level control?
Detect error (set point - process value), produce an output correction in response to the error, provide the user with an adjustment for the desired value input
What is feedback in the process level control?
Provides difference between the process variable and its desired value, which can take action in the appropriate direction to oppose the deviation
What is set point?
The desired value of the process variable
ex) desired level of the tank
What are the four basic modes of control
- On/Off (two position) signal fully on or fully off
- Proportional: magnitude of the output signal is directly proportional to the size of the error detected
- Integral (reset) the output signal is in proportion to both the time duration and the size of the error detected
- Derivative (rate) the output signal is in proportion to the rate of change of either the error or the measured process variable
What is dead band?
Upper setpoint - lower set point
Also called differential gap
What are the three continuous control modes?
Proportional, integral, derivative
What is the purpose of proportional control?
Provide smoother control than on/off control and to reach steady state of the process
What is an error based proportional feedback control?
Proportional control will always have permanent error. The permanent error or offset is necessary to keep the inflow amount of process supplies. The value of permanent error or offset depends on the outflow demand.
What is integral control (reset)?
- used in most applications to remove offset errors.
- controller output signal will continuously change via the integral mode until the error signal again returns to zero. After every disturbance the process variable is always returned to the set point
What is derivative control (rate)?
help compensate for dynamic errors in the control loop. It can reduce both the magnitude of error deviation and increase the speed of the control loop recovery.
What is PID?
Proportional integral and derivative control
What are some final control elements?
Solenoids, electrical motors, stepper motors, servo motor, valves
What is the main practical use of secondary cells?
Used in emergency power systems because they are rechargeable
I am a temperature detector which operates on the principle that when one junction of joined dissimilar metals is heated, an EMF proportional to the temperature difference of the junctions is produced.
Whom am I?
A thermocouple
When force is applied to a strain gauge and causes it to bend, what is produced?
Resistance
What is being measured at the bottom of an open tank when a pressure transducer is connected?
Hydrostatic head pressure
What type of electrical output does a transducer have?
Voltage
What type of electrical output does a transducer have?
Voltage
Automatic control of process systems can be both open or closed loop, true or false
True
The “Bias” of a PID controller output signal is normally established by the operator using the controller Manual Mode adjustment. (True/False)
True
The “Bias” of a PID controller output signal is normally established by the operator using the controller Manual Mode adjustment. (True/False)
True
In a Doppler ultrasonic flow meter, the difference between transmitter and receiver frequencies become __________ as the velocity of the fluid increases.
Greater
Which electrical component does not have discrete characteristics
Thermistor
What circuit block is designed to provide precision voltages to the other circuit blocks?
The reference supply
Which component of a control system compares the desired output to the actual output?
The comparator
