Sensors and Transducers

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

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

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


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


• Thermocouple• Operation

• Open the pair of wires in between the two ends a voltage develops

» Called Seebeck Voltage» Proportional to temperature difference

TsV

nEndsenceBetweeTempDifferT

ntpCoefficieSeebeckTems

tVoltsOpenCircuiV

Where


• 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

» Where’s the cold junction

• It’s at room temp• Voltage will be wrong• Need a 00C ref


• 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 thatthe measurement end isn’t at OOC


• 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 it’s 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


• 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


• 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 it’s thermal

characteristics» Chenicals» molten metals – new alloys

Sample Commercial thermocouple assemblies


• 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 100Ω


• 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


• 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

mWCTC /50.0


• 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


• Thermistor• Construction

• Also come manufacturer provided resistance vs temperature curves• Sample for thermistor with nominal value of 5kΩ at 00C


• 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

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


• 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.


• 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.


• 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


• 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.


• 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

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


• 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-kΩ 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


• 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

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 = ΔL/L0 , where ΔL = 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


• Strain and Stress• Strain = ΔL/L0 , where ΔL = 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 Young’s Modulus and has been tabulated for many material

• Most metals won’t stretch beyond 0.5% without deforming


• Strain and Stress• Resistor conductance can be determined from: R=ρL/A

• Where R= resistance in ohms, ρ (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% - it’s 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 Ω• Most common sizes 120 Ω and 350 Ω


• Strain and Stress• Calculations

• Where R =resistance of the gage under stress, R0 = Original resistance of the gage, ΔL = change in length of the gage, L = original length of the gage, GF = gage factor

• Typical Bridge configurations

)1(0 GFL

LRR


• 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 20’s – 115+


• Typical

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

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.

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.

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 pressurecaused by vibration. The rate of these variations determines the pitch of the tone. Higherfrequency tones result in higher pitch sounds and lower frequency tones result in lower pitchtones.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 soundwith frequencies above 20 kHz. Since the Ping))) sensor's chirps are at 40 kHz, they aredefinitely ultrasonic, and not audible to people.

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

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

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.

RFID• A significant thrust in RFID use is:

• In enterprise supply chain managementimproving 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

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 isn’t a match – the speaker groans

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

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)

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

Tactile Sensors• Can be used to detect a wide range of stimuli:

• Measure contact forces• Many of these sensors are on input devices that don’t 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

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Sensors and Transducers