Analog sensor Part-9
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Transcript of Analog sensor Part-9
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TECHVILLA
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Analog sensors
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Terminology• Transducers convert one form of energy into
another• Sensors/Actuators are input/output
transducers• Sensors can be passive (e.g. change in
resistance) or active (output is a voltage or current level)
• Sensors can be analog (e.g. thermocouples) or digital (e.g. digital tachometer)
3Sensor Actuator
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Analog Signals
Analog signals – directly measurable quantities in terms of some other quantity
Examples:• Thermometer – mercury height rises as temperature rises• Car Speedometer – Needle moves farther right as you
accelerate• Stereo – Volume increases as you turn the knob.
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Digital Signals
Digital Signals – have only two states. For digital computers, we refer to binary states, 0 and 1. “1” can be on, “0” can be off.
Examples:
• Light switch can be either on or off
• Door to a room is either open or closed
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Examples of A/D Applications
• Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals
• Strain Gages - determines the amount of strain (change in dimensions) when a stress is applied
• Thermocouple – temperature measuring device converts thermal energy to electric energy
• Voltmeters
• Digital Multimeters
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Just what does an A/D converter DO?
• Converts analog signals into binary words
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Analog Digital Conversion 2-Step Process:
• Quantizing - breaking down analog value is a set of finite states
• Encoding - assigning a digital word or number to each state and matching it to the input signal
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Step 1: Quantizing
Example: You have 0-10V signals. Separate them into a set of discrete states with 1.25V increments. (How did we get 1.25V? See next slide…)
Output States
Discrete Voltage Ranges (V)
0 0.00-1.25
1 1.25-2.50
2 2.50-3.75
3 3.75-5.00
4 5.00-6.25
5 6.25-7.50
6 7.50-8.75
7 8.75-10.0www.techvilla.org.in
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QuantizingThe number of possible states that the
converter can output is:N=2n
where n is the number of bits in the AD converter
Example: For a 3 bit A/D converter, N=23=8.
Analog quantization size:Q=(Vmax-Vmin)/N = (10V – 0V)/8 = 1.25V
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Encoding
• Here we assign the digital value (binary number) to each state for the computer to read.
Output States
Output Binary Equivalent
0 000
1 001
2 010
3 011
4 100
5 101
6 110
7 111www.techvilla.org.in
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Accuracy of A/D Conversion
There are two ways to best improve accuracy of A/D conversion:
• increasing the resolution which improves the accuracy in measuring the amplitude of the analog signal.
• increasing the sampling rate which increases the maximum frequency that can be measured.
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Resolution
• Resolution (number of discrete values the converter can produce) = Analog Quantization size (Q)(Q) = Vrange / 2^n, where Vrange is the range of analog voltages which can be represented
• limited by signal-to-noise ratio (should be around 6dB)
• In our previous example: Q = 1.25V, this is a high resolution. A lower resolution would be if we used a 2-bit converter, then the resolution would be 10/2^2 = 2.50V.
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Sampling Rate
Frequency at which ADC evaluates analog signal. As we see in the second picture, evaluating the signal more often more accurately depicts the ADC signal.
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Aliasing• Occurs when the input signal is changing
much faster than the sample rate.
For example, a 2 kHz sine wave being sampled at 1.5 kHz would be reconstructed as a 500 Hz (the aliased signal) sine wave.
Nyquist Rule:• Use a sampling frequency at least twice as
high as the maximum frequency in the signal to avoid aliasing.
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Overall Better Accuracy
• Increasing both the sampling rate and the resolution you can obtain better accuracy in your AD signals.
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A/D Converter Types By Danny
Carpenter
• Converters
• Flash ADC
• Delta-Sigma ADC
• Dual Slope (integrating) ADC
• Successive Approximation ADC
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Transducer types
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Quantity being
Measured
Input Device(Sensor)
Output Device(Actuator)
Light LevelLight Dependant Resistor (LDR),
Photodiode, Phototransistor, Solar Cell
Lights & Lamps, LED's & Displays, Fiber
Optics
Temperature
Thermocouple, Thermistor, Thermostat, Resistive
temperature detectors (RTD)
Heater, Fan, Peltier Elements
Force/Pressure
Strain Gauge, Pressure Switch, Load Cells
Lifts & Jacks, Electromagnetic,
Vibration
PositionPotentiometer, Encoders,
Reflective/Slotted Opto-switch, LVDT
Motor, Solenoid, Panel Meters
SpeedTacho-generator,
Reflective/Slotted Opto-coupler, Doppler Effect Sensors
AC and DC Motors, Stepper Motor, Brake
SoundCarbon Microphone, Piezo-
electric CrystalBell, Buzzer, Loudspeaker
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Positional Sensors: potentiometer
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Processing circuit
Can be Linear or Rotational
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Positional Sensors: LVDTLinear Variable Differential Transformer
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Positional Sensors: Inductive Proximity Switch
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• Detects the presence of metallic objects (non-contact) via changing inductance
• Sensor has 4 main parts: field producing Oscillator via a Coil; Detection Circuit which detects change in the field; and Output Circuit generating a signal (NO or NC)
Used in traffic lights (inductive loop buried under the road). Sense objects in dirty environment.Does not work for non-metallic objects. Omni-directional.
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Positional Sensors: Rotary Encoders
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• Incremental and absolute types• Incremental encoder needs a counter, loses
absolute position between power glitches, must be re-homed
• Absolute encoders common in CD/DVD drives
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Temperature Sensors
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• Bimetallic switch (electro-mechanical) – used in thermostats. Can be “creep” or “snap” action.
• Thermistors (thermally sensitive resistors); Platinum Resistance Thermometer (PRT), very high accuracy.
Creep-action: coil or spiral that unwinds or coils with changing temperature
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Thermocouples
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• Two dissimilar metals induce voltage difference (few mV per 10K) – electro-thermal or Seebeck effect
• Use op-amp to process/amplify the voltage• Absolute accuracy of 1K is difficult
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Light sensors: photoconductive cells
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• Light dependent resistor (LDR) cell
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Light level sensitive switch
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Photojunction devices
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photodiodephototransistor
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Photovoltaic Solar Cells
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• Can convert about 20% of light power into electricity
• Voltage is low (diode drop, ~0.6V)
Solar power is 1.4kW/m^2
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Photomultiplier tubes (PMT)
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• Most sensitive of light sensors (can detect individual photons)
• Acts as a current source
electrons
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Motion sensors/transducers
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• Switches, solenoids, relays, motors, etc.• Motors
• DC• Brushed/brushless• Servo• Stepper motors
• AC
Stepper motor
Brushed motor – permanent magnets on armature, rotor acts as electromagnetBrushless motor – permanent magnet on the rotor, electromagnets on armature are switched
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Sound transducers
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microphone speaker
• Note: voice coil can also be used to generate fast motion www.techvilla.org.in
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Piezo transducers
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• Detect motion (high and low frequency)• Sound (lab this week), pressure, fast motion• Cheap, reliable but has a very limited range
of motion
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Types of Temperature Sensors
ThermocouplesResistance Temperature
Detectors (RTDs)ThermistorsInfrared SensorsSemiconductors
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Thermocouples
Two wires of different metal alloys.
Converts thermal energy into electrical energy.
Requires a temperature difference between measuring junction and reference junction.
Easy to use and obtain.
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Thermocouple Applications
Plastic injection molding machinery
Food processing equipment Deicing Semiconductor processing Heat treating Medical equipment Industrial heat treating Packaging equipment
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Thermocouples
Simple, RuggedHigh temperature operationLow costNo resistance lead wire
problemsPoint temperature sensingFastest response to
temperature changes
Least stable, least repeatableLow sensitivity to small
temperature changesExtension wire must be of the
same thermocouple typeWire may pick up radiated
electrical noise if not shieldedLowest accuracy
AdvantagesAdvantages DisadvantagesDisadvantages
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Resistance Temperature Detectors (RTDs)
Wire wound and thin film devices.
Nearly linear over a wide range of temperatures.
Can be made small enough to have response times of a fraction of a second.
Require an electrical current to produce a voltage drop across the sensor
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RTD Applications
Air conditioning and refrigeration servicing
Furnace servicing Foodservice processing Medical research Textile production
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RTDs
• Most stable over time
• Most accurate
• Most repeatable temperature measurement
• Very resistant to contamination/
• corrosion of the RTD element
• High cost• Slowest response time• Low sensitivity to small
temperature changes• Sensitive to vibration (strains the
platinum element wire)• Decalibration if used beyond
sensor’s temperature ratings• Somewhat fragile
AdvantagesAdvantages DisadvantagesDisadvantages
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Thermistors
• A semiconductor used as a temperature sensor. • Mixture of metal oxides pressed into a bead, wafer or other shape.• Beads can be very small, less than 1 mm in some cases.• The resistance decreases as temperature increases, negative temperature
coefficient (NTC) thermistor.
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Thermistors
• Most are seen in medical equipment markets.
• Thermistors are also used are for engine coolant, oil, and air temperature measurement in the transportation industry.
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Thermistors
• High sensitivity to small temperature changes
• Temperature measurements become more stable with use
• Copper or nickel extension wires can be used
• Limited temperature range• Fragile• Some initial accuracy
“drift”• Decalibration if used
beyond the sensor’s temperature ratings
• Lack of standards for replacement
AdvantagesAdvantages DisadvantagesDisadvantages
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Infrared Sensors
• An infrared sensor intercepts a portion of the infrared energy radiated by an object.
• Many types Optical Pyrometers, Radiation Pyrometers, Total Radiation Pyrometers, Automatic Infrared Thermometers, Ear Thermometers, Fiber optic Thermometers, Two-Color Pyrometers, Infra-Snakes, and many more.
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Infrared Applications
• Manufacturing process like metals, glass, cement, ceramics, semiconductors, plastics, paper, textiles, coatings.
• Automation and feedback control
• Improve safety in fire-fighting, rescues and detection of criminal activities.
• Used to monitor and measure human body temperatures with one second time response.
• Reliability and maintenance needs from building heating to electrical power generation and distribution
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Infrared Sensors
• No contact with the product required
• Response times as fast or faster than thermocouples
• No corrosion or oxidation to affect sensor accuracy
• Good stability over time• High repeatability
• High initial cost• More complex - support
electronics required• Emissivity variations affect
temperature measurement accuracy
• Field of view and spot size may restrict sensor application
• Measuring accuracy affected by dust, smoke, background
• radiation, etc.
AdvantagesAdvantages DisadvantagesDisadvantages
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Semiconductors
• Are small and result from the fact that semiconductor diodes have voltage-current characteristics that are temperature sensitive.
• Temperature measurement ranges that are small compared to thermocouples and RTDs, but can be quite accurate and inexpensive.
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Semiconductor Applications
• Hard Disk Drives
• Personal Computers
• Electronic Test Equipment
• Office Equipment
• Domestic Appliances
• Process Control
• Cellular Phones
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Thermal Sensor Vendors
MincoPyrotekOmegaWatlowTexas InstrumentNational SemiconductorMaxim
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Determining Factors
Low PowerSerial InterfaceSmallAccurateWide temperature
range
ExtrasI2C InterfaceTemperature Alarms
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National SemiconductorLM75/LM76
I2C Interface-55º to 125ºC range±2/ ±1º accuracy9 bits/ 12 bits or ±0.0625ºC resolution3/3.3 to 5.5 operating voltage0.25 to 0.5 µA operating current, 4/5µA
shutdown current100ms/400ms conversion rate(9/12 bit)Online sample request8 pin SOP packageNeeds 400kHz clock for I2C Interface
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Lm35 temperature sensor
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Interfacing
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