Sensors and Transducers

1
Sensors and Transducers

• Characteristic
• Transducers converts the form of energy
• A microphone coverts sound energy into electrical
  energy
• A speaker converts electrical energy into sound
• Sensors are transducers that used to detect
  and/or measure something
• Used to convert mechanical, thermal, magnetic,
  chemical, or etc variations into electrical
  voltages and currents

2
Sensors and Transducers

• This course only focuses on a limited number of
  sensor types. Specifically
• Some Temperature sensors
• Some light sensors
• Some strain gages
• Many other types exist
• Smart Sensors
• Infrared
• Switches
• RFID
• etc

3
Sensors and Transducers

• Temperature Sensors
• Types of Temperature Sensors
• Thermocouple
• Resistance temperature device(RTD)
• Thermistor
• Monolithic IC Sensors
• Thermocouple
• Characteristics
• Most common sensor
• A pair of dissimilar wires welded together at the
  sensing location
• A temperature difference from the welded end and
  the other end causes a DC voltage at the non
  welded end
• Can be used under extreme conditions
• Ovens, Furnaces, Nuclear tests

4
Sensors and Transducers

• Temperature Sensors

5
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Operation
• When wires made of dissimilar metals are
• Welded together at both ends
• With different temperatures at both ends
• Current flows

6
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Operation
• Open the pair of wires in
• between the two ends a
• voltage develops
• Called Seebeck Voltage
• Proportional to
• temperature difference

7
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Operation
• Equation is linear over only a small range of
  temperatures
• Tables of corrected voltages in 10 increments is
  available from the NBS for each type
• Reference Junction
• Voltage developed is dependent upon the
  temperature difference between ends NOT
  Absolute Temperature of the welded end
• Wheres the
• cold junction
• Its at room temp
• Voltage will be wrong
• Need a 00C ref

8
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Reference Junction
• Lab Set up
• Not practical for most situations
• Practical Reference Junction
• solutions
• Electronic Ice points
• Available for All types
• of thermocouples
• Encased electronic device
• that balances an internal bridge
• circuit which generates a voltage to cancel out
  effect that
• the measurement end isnt at OOC

9
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Practical Reference Junction
• solutions
• Isothermal block
• Usually used with
• computerized (also microcontrollers) data
  collection systems
• The isothermal block is a good conductor of heat
  not electrical current
• However its resistance is effected measurably by
  changes in temperature
• Block is always near the point were the voltages
  are measured
• Computerized measuring system calculates cool end
  temperature based on the block resistance and
  corrects the voltage reading

10
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Typical Problems
• A short of the two wires
• Junction then will be at the point of the short
• Temperatures readings will be incorrect
• No Reference Junction Compensation
• Temperatures readings will be incorrect
• Test Short the inputs to the compensator and
  room temperature should be the new reading
• If extensions of the thermal couple wires are
  used they should be of a larger size and material
• Different materials create Incorrect readings
  since the connection of dissimilar materials
  creates a new junction
• Larger size is needed for IR drops

11
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Typical Problems
• Noise pick-up
• Long leads form an antenna uses shielding.
  e.g., grounded over braiding of copper
• Extreme Temperature Gradient
• Can damage the thermocouple should have
  protection
• Environment can change the metal and its
  thermal characteristics
• Chenicals
• molten metals
• new alloys

Sample Commercial thermocouple assemblies
12
Sensors and Transducers

• Temperature Sensors
• Resistance Temperature Device
• Key principle
• As the temperature of a resistor increases so
  does its resistance
• Measure the change in the resistance of a known
  resistor calculate the temperature change
• Linear relation ship for smaller changes more
  linear than thermocouples NBS has correction
  tables for the typical types of measurement
  resistors
• Typical construction
• Wire wound resistor in on a ceramic core using
  platinum wire
• Stable (linear) over a wide range of temperatures
• Temperature coefficient 0.00385/0C
• Typical Values 10 several kilo-ohms
• Most common value 100?

13
Sensors and Transducers

• Temperature Sensors
• Resistance Temperature Device
• Measuring Circuit Types
• RTD Bridge circuit
• Constant Current Source
• RTD Bridge circuit
• Platinum resistor is remote from the bridge
  circuit which is isolated from the sensing point
• Bridge is balanced at a known temperature
• Eliminates consideration of the connecting leads
• Voltage developed is proportional to the
  temperature change

14
Sensors and Transducers

• Temperature Sensors
• Resistance Temperature Device
• Constant Current through RTD
• Voltage across the RTD rises and the resistance
  increase with the rise in temperature
• The constant current also increases the
  temperature of the resistance and effects the
  temperature reading
• The correction factor for common platinum RTDs
  has been determined

15
Sensors and Transducers

• Temperature Sensors
• Thermistor
• Resistors with high negative temperature
  coefficients
• Resistance decreases with an increase in
  temperature
• High temperature coefficients means that there is
  a significant change in resistance for a small
  temperature change
• Construction
• Semiconductor material
• In either tube or bead shapes
• Can be used as a plain resistor in circuits such
  as a bridge or voltage divider
• Come in a Wide range of values
• Also come with manufacturer provided resistance
  vs temperature curves

16
Sensors and Transducers

• Temperature Sensors
• Thermistor
• Construction
• Also come manufacturer provided resistance vs
  temperature curves
• Sample for thermistor with nominal value of 5k?
  at 00C

17
Sensors and Transducers

• Temperature Sensors
• Monolithic IC Sensors
• Current or voltage types are available
• They have linear output voltages or currents with
  temperature changes
• Typical values 1µA/0K 10mV/0K
• 1 0K 1 0C

18
Sensors and Transducers

• Light Sensors
• Typical uses of the sensors
• Measure intensity of the light
• Detect the presence or absence of light
• Types of Light Sensors
• Photovoltaic Cells
• Photoconductive Cells
• Photo Diodes
• Phototransistors
• Photovoltaic Cells
• aka, Solar Cells
• Semiconductor material that generates a voltage
  when light shines on it
• 2.5 by 5 cm cell can produce 0.4 V with 180mA of
  current

19
Sensors and Transducers

• Light Sensors
• Photovoltaic Cells
• Sometimes used to detect the presence of light
• Photoconductive Cells
• aka, photoresistors
• Characteristics of Photoresisters
• Uses bulk resistivity which decreases with
  increasing illumination, allowing more
  photocurrent to flow.
• Signal current from the detector can be varied
  over a wide range by adjusting the applied
  voltage.
• Thin film devices made by depositing a
• layer of a photoconductive material
• on a ceramic substrate.

20
Sensors and Transducers

• Light Sensors
• Photoconductive Cells
• Characteristics of Photoresisters
• Metal contacts with external connection. These
  thin films have a high sheet resistance.
  Therefore, the space between the two contacts is
  made narrow and long for low cell resistance at
  moderate light levels.

21
Sensors and Transducers

• Light Sensors
• Photoconductive Cells
• Light Intensity Application
• With little or no light the
• voltage at point X is low
• As the intensity of the
• light on the sensor increases
• the voltage at X will increase
• By adjusting Rf,a usable output range of voltages
  that the is proportional to the light intensity
  can be obtained
• Presence or Absence of
• Light application
• Activates a electromechanical
• counter when the light is blocked

22
Sensors and Transducers

• Light Sensors
• Photoconductive Cells With a Microcontroller
• Critical aspect of this application a BASIC
  command for measuring the RC decay time on a
  connected circuit
• RCTIME command is designed to measure RC decay
  time on a circuit like the one below. The lower
  the count recorded the brighter the light
  measured
• RCTIME Pin, State, Duration
• Pin argument is the number of
• the I/O pin that you want to measure
• State argument - 1 if the voltage across the
  capacitor starts above 1.4 V and decays downward.
  0 if the voltage across the capacitor starts
  below 1.4 V and grows upward
• Duration argument has to be a variable that
  stores the time measurement, which is in 2 µs
  units
• Very simple circuit range of measured light is
  limited only by the size of the variable used to
  store the count.

23
Sensors and Transducers

• Light Sensors
• Photodiodes
• A diode that is forward biased by light
• Very fast reactions to changing light levels
• Same physical size as LEDs
• Have small windows through which light is
• sensed
• Testing is simple
• When the window is blocked
• High resistance is read
• Shine a bright light (several footcandles) on it
  while still connected to an ohmmeter
• The resistance will drop significantly
• Phototransistors
• Usually used instead of photodiodes when low
  light levels are measured

24
Sensors and Transducers

• Light Sensors
• Phototransistors
• Usually used instead of photo resistors when low
  light levels are broken at high rates
• Typical ratings
• Like low power transistors
• 30-50V maximum collector to emitter voltages
• Max collector currents of 25mA

25
Sensors and Transducers

• Light Sensors
• Replacement Considerations
• Best option is an exact replacement
• If not possible match the following
  characteristics
• Voltage, current, power ratings physical size
• Light sensitivity
• Can be specified nm (human sight 400 -700) nm
  (700nm red light)
• Called spectral response
• Can also be specified in angstroms Å. 10 Å 1nm
• Light Insensitivity
• For photoresistors X-k? at Y-footcandles
• 1 Foot candle light falling on 1 square foot
  one foot from a standardcandle
• For phototransistors Collector current at a
  specified light level

26
Sensors and Transducers

• Light Sensors
• Other Problems with light sensing systems
• Burned out, weak, or obstructed light sources
• Can be a simple problem of dirty light filters or
  lens
• Light shields may have been misaligned by a bump
• Mechanical Sensors
• Characteristics
• Used to measure
• Force
• motion
• position
• The chapter covers Strain gages
• They measure Forces
• Weight is a common force

27
Sensors and Transducers

• Strain gages
• Characteristics
• Sensors used to measure change in the dimensions
  of solid objects caused by forces
• Information is critical to designs of mechanical
  systems
• Used in load cells which are used to measure
  weights of objects
• Measurements can range from a few pounds to the
  weight of a fully loaded tractor trailer rig
• Strain and Stress
• Strain ?L/L0 , where ?L change in length
  due to a force
• and L0 the
  original length before the force was applied
• Can be caused by tension or compression forces

28
Sensors and Transducers

• Strain gages
• Strain and Stress
• Strain ?L/L0 , where ?L change in length
  due to a force
• and the
  original length before the force was
• applied
• Can be caused by tension or compression forces
• Stress is a measure of the force applied that has
  been normalized to a unit area
• Stress F/A , where F the total force
  applied and A cross-
• sectional area
• The ratio of Stress/Strain is a constant value
  for each material
• Called Youngs Modulus and has been tabulated for
  many material
• Most metals wont stretch beyond 0.5 without
  deforming

29
Sensors and Transducers

• Strain gages
• Strain and Stress
• Resistor conductance can be determined from
  R?L/A
• Where R resistance in ohms, ? (rho) is the
  resistivity of the material, L length of the
  material, A is the cross-sectional area of the
  material
• If the gage material under stress increases it
  length by0.4 - its resistance will increase by
  0.4
• Some commercial gages have been designed to yield
  multiples of the change in length in change of
  resistance A Gage Factor
• Construction
• Metal or semiconductor foil woven back and forth
  to increase the length
• Range of common values 30 -3000 ?
• Most common sizes 120 ? and 350 ?

30
Sensors and Transducers

• Strain gages
• Strain and Stress
• Calculations
• Where R resistance of the gage under stress, R0
  Original resistance of the gage, ?L change in
  length of the gage, L original length of the
  gage, GF gage factor
• Typical Bridge configurations

31
Sensors and Transducers

• Strain gages
• Typical Bridge configurations
• The 1/4 bridge has a gain factor of 1
• Change of resistance causes the bridge to
  unbalance
• The ½ bridge has two strain gages
• One in tension mode and one in compression mode,
  like in the metal beam drawing bottom right of
  previous slide
• rg1 is stretched and rg2 is compressed
• Changes double the resistance change
• GF 2
• The full bridge has four gages and a GF of four
• Problems with Strain Gages
• Temperature changes
• If outside the circuitry must have temperature
  compensation
• e.g., Las Vegas temperatures range from the 20s
  115

32
Sensors and Transducers

• Strain gages
• Typical

33
Some Smart Sensors

• Smart Sensor Characteristics
• Inside every smart sensor
• One or more primitive sensors
• Primitive sensors are devices or materials that
  have some electrical property that changes with
  some physical phenomenon
• Additional, built-in electronics makes a smart
  sensor "smart
• Smart sensors able to do one or more of the
  following
• Pre-process their measured values into meaningful
  quantities
• Communicate their measurements Orchestrate the
  actions of primitive circuits and sensors to
  "take" measurements
• Make decisions and initiate action based on
  sensed conditions, independent of a
  microcontroller
• Remember calibration or configuration settings
• Three samples of smart sensors follow

34
Ultrasonic Distance Detector

• Example Parallax Ping)))
• The Ping))) sensor interfaced
• with a Controller measures
• distances to objects
• Range of 3 centimeters to 3.3 meters
• Accurate to the centimeter
• How it works
• The controller starts by sending the Ping)))
  sensor a pulse to start the measurement.
• Then, the Ping))) sensor waits a short period of
  time, enough for the controller to start a
  elapsed time counter.
• Then, at the same time the Ping))) sensor chirps
  its 40 kHz tone, it sends a high signal to the
  controller.

35
Ultrasonic Distance Detector

• Characteristics of the Parallax part
• How it works
• When the Ping))) sensor detects the echo with its
  ultrasonic microphone, it changes that high
  signal back to low.
• The controller uses a variable to store how long
  the high signal from the Ping))) sensor lasted.

36
Ultrasonic Distance Detector

• Characteristics of the Parallax part
• How it works
• This time measurement is how long it took sound
  to travel to the object and back.
• Using this measurement and the speed of sound in
  air, you can make your program calculate the
  object's distance in centimeters, inches, feet,
  etc.

The Ping))) sensor's chirps are not audible
because 40 kHz is ultrasonic. What we consider
sound is our inner ear's ability to detect the
variations in air pressure caused by vibration.
The rate of these variations determines the pitch
of the tone. Higher frequency tones result in
higher pitch sounds and lower frequency tones
result in lower pitch tones. Most very young
people can hear tones that range from 20 Hz,
which is very low pitch, to 20 kHz, which is very
high pitch. Subsonic is sound with frequencies
below 20 Hz, and ultrasonic is sound with
frequencies above 20 kHz. Since the Ping)))
sensor's chirps are at 40 kHz, they
are definitely ultrasonic, and not audible to
people.
37
Accelerometer

• Acceleration
• A measure of how quickly speed changes.
• Just as a speedometer is a meter that measures
  speed
• An accelerometer is a meter that measures
  acceleration.
• Some of the measurements that an accelerometer
  can take
• Acceleration
• Tilt and tilt angle
• Incline
• Rotation
• Vibration
• Collision
• Gravity

38
Accelerometer

• Accelerometers are already used in many different
  devices, including personal electronics,
  specialized equipment and machines. Here are just
  a few examples
• Self-balancing robots
• Tilt-mode game controllers
• Model airplane autopilots
• Car alarm systems
• Crash detection/airbag deployment systems
• Human motion monitoring systems
• Leveling tools

39
RFID

• Definition
• Radio-frequency identification (RFID) is an
  automatic identification method, relying on
  storing and remotely retrieving data using
  devices called RFID tags or transponders.
• An RFID tag is an object that can be applied to
  or incorporated into a product, animal, or person
  for the purpose of identification using
  radiowaves. Some tags can be read from several
  meters away and beyond the line of sight of the
  reader.
• Most RFID tags contain at least two parts. One is
  an integrated circuit for storing and processing
  information, modulating and demodulating a (RF)
  signal, and other specialized functions. The
  second is an antenna for receiving and
  transmitting the signal.

40
RFID

• A significant thrust in RFID use is
• In enterprise supply chain management
• improving the efficiency of inventory
• tracking and management.
• Types of Tags
• RFID tags come in three general
• varieties- passive, active, or semi-passive
  (also known as battery-assisted). Passive tags
  require no internal power source, thus being pure
  passive devices (they are only active when a
  reader is nearby to power them), whereas
  semi-passive and active tags require a power
  source, usually a small battery.

Walmart RFID Tag
41
RFID

• Simple Security System based on RFID
• The system enables the reader
• Waits for a Tag to be read
• Compares the read identification to a list of
  known IDs
• It there is a match the an electronic door lock
  is opened
• We will use a Red LED
• In addition the name associated with that Tag
  will be printed in the debug window
• And the Piezo electric speaker beeps
• If there isnt a match the speaker groans

42
Contact and Touch Sensors

• Characteristics
• As complex as a touch screen on a CRT or LCD
  monitor
• Use in such varied applications as Customer
  service Kiosks in stores to video poker EGMs in
  casinos
• To a tactile sensors
• Used as input devices to video games, to the joy
  stick in the cockpit of a F-16 fighter .
• Contact sensors can be as simple as a switch
• Used in numerous systems to detect a changed
  condition

43
Touchscreens

• Overview
• Touchscreens systems come in three technologies
  each having three components  The technologies
  are
• Surface Acoustic Wave (SAW)
• Resistive and,
• Capacitive
• Information provided to the system
• X, Y coordinates of the point touched
• Used as substitute for button activation
• Surface Acoustic Wave (SAW)
• It is based on sending acoustic waves across a
  clear glass panel with a series of transducers
  and reflectors.
• When a finger touches the screen, the waves are
  absorbed, causing a touch event to be detected at
  that point (X, Y coordinates)

44
Tactile Sensors

• Can be used to detect a wide range of stimuli
• Presence or absence of a grasped object
• Successfully pick up object, or failed to grasp
  object
• To discovering a complete tactile image.
• Texture, impact, slip and other contact
  conditions generate specific force and position
  patterns
• This information again can be used to identify
  the state of manipulation.
• Objects size, shape and mass can be used to test
  whether the object has been rotated ninety
  degrees and flipped , etc
• Measure contact forces
• Critical for Joy Stick implementations

45
Tactile Sensors

• Can be used to detect a wide range of stimuli
• Measure contact forces
• Many of these sensors are on input devices that
  dont move
• i.e., a joy stick
• The greater the force in a direction the greater
  of the output
• The output verses input levels can be either
  linear or exponential
• Control stick on an F-16
• For easy and accurate
• control of the aircraft during
• high G-force combat
• maneuvers, a side stick
• controller is used