analog sensor part-9

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TECHVILLA


Analog sensors


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


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.


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


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


Just what does an A/D converter DO?

• Converts analog signals into binary words


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


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

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


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

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.


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.


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.


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.


Overall Better Accuracy

• Increasing both the sampling rate and the resolution you can obtain better accuracy in your AD signals.


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


Positional Sensors: potentiometer

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Processing circuit

Can be Linear or Rotational


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.


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


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


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


Light level sensitive switch

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Photojunction devices

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photodiodephototransistor


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


Photomultiplier tubes (PMT)

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• Most sensitive of light sensors (can detect individual photons)

• Acts as a current source

electrons


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


Sound transducers

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microphone speaker

• Note: voice coil can also be used to generate fast motion www.techvilla.org.in

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


Types of Temperature Sensors

ThermocouplesResistance Temperature

Detectors (RTDs)ThermistorsInfrared SensorsSemiconductors


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.


Thermocouple Applications

Plastic injection molding machinery

Food processing equipment Deicing Semiconductor processing Heat treating Medical equipment Industrial heat treating Packaging equipment


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


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

RTD Applications

Air conditioning and refrigeration servicing

Furnace servicing Foodservice processing Medical research Textile production


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



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.


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.


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



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.


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


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.



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.


Semiconductor Applications

• Hard Disk Drives

• Personal Computers

• Electronic Test Equipment

• Office Equipment

• Domestic Appliances

• Process Control

• Cellular Phones


Thermal Sensor Vendors

MincoPyrotekOmegaWatlowTexas InstrumentNational SemiconductorMaxim


Determining Factors

Low PowerSerial InterfaceSmallAccurateWide temperature

range

ExtrasI2C InterfaceTemperature Alarms


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


Lm35 temperature sensor


Interfacing


analog sensor part-9

Engineering

analog signals analog

analog digital conversion

analog sensors

analog value

binary states

digital tachometer

digital word

digital computers