Electronic Troubleshooting Chapter 12 Sensors and Transducers.

Electronic Troubleshooting

Chapter 12Sensors 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• Temperature Sensors

• Types of Temperature Sensors• Thermocouple• Resistance temperature device(RTD)• Thermistor• Monolithic IC SensorsNOTE: See Chart on next slide or page 344

• 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

Chart on page 344

Example 12-1 page 345to 12-4 on page 365


• 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

Commercial thermocouple assemblies – see page 348


• 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

» Example Problems on page s 349 & 350

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


• 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

• Typical application• See slide on the next page or the bottom of page 355• Monitors droplets falling through an IV administration set

» Drip rate is set by nurses with a small valve not shown• IR LED is the Light beam sourse• The drops block enough light to turn off the phototransistor

» Positive spikes on it’s collector feed an inverter that squares off and amplifies the spikes

» Sent to a counter, alarm, or monitoring equip


• 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

• Example Problems 12-4 and 12-5 on page 359

• 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

Electronic Troubleshooting Chapter 12 Sensors and Transducers.

Documents

sound sensors

temperature block

room temperature

absolute temperature

cool end temperature

temperature difference

characteristic transducers

wires junction