This document provides information on analog and digital sensors. It defines transducers as devices that convert one form of energy to another. Sensors are input transducers that can be passive (change resistance) or active (output voltage or current). Analog sensors output continuously variable signals, while digital sensors have only two states. Examples of analog sensors given include thermocouples and examples of digital include digital tachometers. The document then discusses analog and digital signals in more detail. It provides examples of applications for analog to digital conversion like microphones and strain gages. Finally, it covers the process of analog to digital conversion including quantizing the analog signal into discrete levels and encoding the levels with binary numbers.
3. 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)
3
Sensor Actuator
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4. 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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5. 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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6. 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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7. Just what does an
A/D converter DO?
• Converts analog signals into binary words
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8. 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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9. 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
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10. 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
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11. 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
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12. 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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13. 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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14. 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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15. 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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16. Overall Better Accuracy
• Increasing both the sampling rate and the resolution you
can obtain better accuracy in your AD signals.
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21. 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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22. 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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23. 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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24. 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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29. 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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30. 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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31. 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
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33. 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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35. 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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37. 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
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38. 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
39. RTD Applications
Air conditioning and
refrigeration servicing
Furnace servicing
Foodservice processing
Medical research
Textile production
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40. 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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41. 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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42. 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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43. 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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44. 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.
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45. 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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46. 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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47. 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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48. Semiconductor Applications
• Hard Disk Drives
• Personal Computers
• ElectronicTest Equipment
• Office Equipment
• Domestic Appliances
• Process Control
• Cellular Phones
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50. Determining Factors
Low Power
Serial Interface
Small
Accurate
Wide temperature range
Extras
I2
C Interface
Temperature Alarms
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51. 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
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Energy form conversion: electrical signals, movement, radiant energy, thermal, or magnetic energy, etc.
Sensors/Actuators – both are transducer types
Analog-sensor-produced signals can be digitized
Can be Linear or Rotational
Used in traffic lights (inductive loop buried under the road). Sense objects in dirty environment.
Does not work for non-metallic objects. Omni-directional.
Creep-action: coil or spiral that unwinds or coils with changing temperature
Solar power is 1.4kW/m^2
Brushed motor – permanent magnets on armature, rotor acts as electromagnet
Brushless motor – permanent magnet on the rotor, electromagnets on armature are switched