Analog sensor Part-9

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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)
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Sensor 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 8.75-10.0

Quantizing
The 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
7 111

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 ConverterTypesBy 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 Level
Light 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/Pressur
e
Strain Gauge, Pressure Switch, Load
Cells
Lifts & Jacks,
Electromagnetic, Vibration
Position
Potentiometer, Encoders,
Reflective/Slotted Opto-switch, LVDT
Motor, Solenoid, Panel
Meters
Speed
Tacho-generator, Reflective/Slotted
Opto-coupler, Doppler Effect Sensors
AC and DC Motors, Stepper
Motor, Brake
Carbon Microphone, Piezo-electric

Positional Sensors: potentiometer
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Processing circuit
Can be Linear or Rotational

Positional Sensors: LVDT
Linear 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

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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photodiode
phototransistor

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 electromagnet
Brushless 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

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 ofTemperature Sensors
Thermocouples
ResistanceTemperature
Detectors (RTDs)
Thermistors
Infrared Sensors
Semiconductors

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, Rugged
High temperature operation
Low cost
No resistance lead wire problems
Point temperature sensing
Fastest response to temperature
changes
Least stable, least repeatable
Low sensitivity to small temperature
changes
Extension wire must be of the same
thermocouple type
Wire may pick up radiated electrical
noise if not shielded
Lowest accuracy
AdvantagesAdvantages DisadvantagesDisadvantages

ResistanceTemperature 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, EarThermometers, Fiber opticThermometers,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
• ElectronicTest Equipment
• Office Equipment
• Domestic Appliances
• Process Control
• Cellular Phones

Thermal SensorVendors
Minco
Pyrotek
Omega
Watlow
Texas Instrument
National Semiconductor
Maxim

Determining Factors
Low Power
Serial Interface
Small
Accurate
Wide temperature range
Extras
I2
C Interface
Temperature Alarms

National Semiconductor
LM75/LM76
I2
C Interface
-55º to 125ºC range
±2/ ±1º accuracy
9 bits/ 12 bits or ±0.0625ºC resolution
3/3.3 to 5.5 operating voltage
0.25 to 0.5 µA operating current, 4/5µA shutdown
current
100ms/400ms conversion rate(9/12 bit)
Online sample request
8 pin SOP package
Needs 400kHz clock for I2
C Interface

Lm35 temperature sensor

Interfacing

Analog sensor Part-9

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