Sensor Types Flashcards
physical quantities measured using electrical resistance
displacement/length
strain
temperature
physical quantities measured using capactiance
displacement
sound pressure (microphone)
physical quantities measured using inductance
displacement
Ohm’s law
V = IR
Voltage = current x resistance
often resistors make use of this
resistor and variable resistor symbols
check slide 7
Voltage Divider formula
-check diagram on slides 7
V = I(R₁ + R₂) Vₐ = (R₂/R₁+R₂)xV
voltage divider
-used to split voltage drop across two resistors so that there is a certain voltage drop across the first resistor and then across the second resistor
Equivalent resistance/Effective resistance of resistors placed in series
𝑅ₑᵩ/Rₑ𝒻𝒻 = R₁ + R₂ + … + 𝑅ₙ = ∑𝑅ₙ
Equivalent resistance/Effective resistance of resistors placed in parallel
1/𝑅ₑ𝒻𝒻 = 1/𝑅₁ + 1/𝑅₂ + … + 1/𝑅ₙ =∑1/Rₙ
wheatstone bridge
a common circuit in resistance based measurement devices
-consists of two voltage dividers in prallel
V₀ = Vₐ -Vᵦ = [R₂/(R₁+R₂) - R₄/(R₃+R₄)]V
wheatstone bridge balanced
-Voltage V₀ is zero if bridge is balanced
ie.
V₀ if R₂/(R₁+R₂) = R₄/(R₃+R₄)
R₂/R₁ = R₄/R₃
Wheatstone bridge uses
- measure unknown resistance
- operated in balanced or unbalanced mode
- bridge output proportional to unknown resistance
devices that use Wheatstone bridges
- hot wire anemometer (flow measurement)
- strain gauges
Heat, resistance, and current
𝐻∝𝐼²𝑅
wheatstone bridge/hot wire anemometer? advantages
- relative cheap electrical systems
- can measure from 0-50kHz (flat freq response)
- small size
- can measure temp w/ a modification to set up
- less than 1% error in practical setups
- custom probes available to suit application
wheatstone bridge/hot wire anemometer? disadvantages
- physically intrusive (probe may change flow trying to measure)
- requires access for cables, probe holder, etc
- delicate + easily broken
- probe repair reqs specialist
- reqs non-linear calibration
strain
physical distortion of an object in response to external stimuli. These external stimuli include linear forces, pressure, torsion, thermal expansion.
result of these forces
the dimensions of the body change.
strain can be elongation (+) or compression (-)
strain formula
Strain = change in length/unit length
ε=Δ𝐿/𝐿
strain unit
microstrain με
1 microstrain = 1 part per million (ppm
why measure strain
- to determine stresses in structuers
- stress data used to asses structural reliability, safety, service live, changes in behaviour of material
- strain measurements can be used to calculate weight, pressure, etc. Can design sensors/transducers for other quantities using strain gauges
small gauge length
better resolution
large gauge length
increased sensitivity
for uniform conductor of length L, area A, and resistivity ρ formula
𝑅 = ρ𝐿/𝐴
manufacturing a strain gauge
manufactured by etching a wire or foil pattern onto a substrate called a carrier matrix.
advantage of this approach
can design many different shapes or form factors.
Including:
- Single gauges
- Orthogonal pairs
- Rossettes
mounting strain gauges
- using adhesive
- welding
Gauge Factor
determines relative change in resistance with strain.
high GF gives high output per unit strain
source of error in strain gauges
gauges respond to strain perpendicular to the primary sensing axis
transverse sensitivity in foil gauges due to
0End loop effects (often orientated in the transverse direction)
-Width to thickness ratio of the foil gridlines
reduce transverse sensitivity
by increasing the cross-sectional area of the end loops. This lowers their resistance, hence lowering the net change in resistance from these elements of the gauge.
gauge resistance and temp
- Gauge resistance changes with temperature, in fact it is also a temperature sensor.
- With temperature changes there is differential expansion between the gauge and substrate
- temperature sensitivities give rise to temperature induced apparent strain.
removing apparent strain
-measure the temperature, calculate the apparent strain and remove it
or
-design a circuit to compensate for the temperature change.
alternative approach to removing apparent strain
- install identical strain gauge in unstrained location at same temp as measurement point.
- strain gauge output should be due to the temperature effect only.
- can subtract these signals ‘electrically’ using a Wheatstone Bridge.
drawback to alternative approach
0Needs an extra gauge (cost)
-Difficult to achieve unstressed, identical temperature condition
advantages of resistance foil strain gauges
- Behaviour well understood
- Reasonable cost
- Small, flexible – can be installed on curved surfaces
- Weldable vers available
- Manufactured using photographic techniques – configuration possible
- variety of signal conditioning equipment available
disadvantages of resistance foil strain gauges
- Sensitive to temperature – may need compensation
- Low output from Wheatstone Bridge – reqs amplification
- Needs well regulated power supply
- Tricky installation process
- Wiring problem in some situations
- Can’t be reused
single element bridge
- Uses one strain gauge per bridge
- Non-linear output
- Lowest signal level output of possible configurations
- No temp compensation
- Often used with Self Temperature Compensating Gauges. Have relatively flat responses across range of temperatures (special alloys used)
- Also used in temperature measurements with RTDs
two element bridge
- LHS – both gauges change in same direction
- RHS – gauges vary in opposite direction
- Double the output level of single gauge bridge design
four element bridge
- all bridge arms have active strain gauges
- Linear output
- Highest signal level output
- Each arm of bridge acts like a potentiometer or rheostat
- Industry standard design for load cells used in force measurement
constant current operation
- Amplifier acts to equalise + and – inputs
- stable reference voltage, Vᵣₑ𝒻 , sets the current needed to establish req bridge voltage
- Sense resistor Rₛₑₙₛₑ provides feedback
- Resistance of lead wires doesn’t matter
RTDs
Resistive Temperature Detectors
two main microphone types
- condenser (high quality + expensive)
- electret (varying quality, cheap)
capacitor
-prevents dc, bu allows transmission of alternating current
how capacitors made
- using two parallel metal plates separated by gap which contains non-conductive material.
- called the dielectric + could be air (condenser microphones) or a polymer (electret microphones).
Amount of charge stored in plates
measured by amount of charge per volt (Farads)
how condenser microphone works
- one plate of capacitor is made from flexible metal diaphragm.
- other plate is rigid metal with a small air gap between plates.
- When sound waves hit front plate, cause it to vibrate.
- vibration changes distance between two plates + hence the capacitance.
- change in capacitance produces a change in voltage output of device.
how to make microphone unaffected by changes in atmospheric presure
- include small hole which equalises pressure between air gap and atmospheric pressure.
- microphone is placed in circuit with a DC supply voltage (known as the polarising voltage).
natural freq of a system formula
𝜔ₙ=√(𝑘/𝑚)
Where 𝑘 is the stiffness and 𝑚 is the mass.
how to make natural freq of microphone diaphragm far above an of freqs which it will be exposed to when measuring sound
- done so system doesnt shake itself apart
- need high stiffness and low mass
high stiffness effect on microphone
- reduce mic’s sensitivity
- increase range of operating frequencies
increasing sensitivity of mic
-make area of diaphragm larger (more sound energy hitting sensor + larger displacements)
importance considerations in the design of the condenser microphone
Supply voltage
Output impedance
Diaphragm stiffness and mass
Diameter of the diaphragm
source of error for condenser mics
sensitivity to changes in humidity
- The air hole which balances pressure also allows humid air into sensor. If very humid, a spark can jump between the polarising plates.
- Sparks appears as spikes in output data, can damage sensor’s diaphragm.
electret microphones
modified version of the condenser where the airgap between the metal plates has been replaced by a polymer.
polymer in electret
designed so that it is permanently polarised
- electret is permanently polarised without need for a polarising supply voltage
- not vulnerable to humidity problems
3 most commonly used temperature sensors
- Thermocouples
- Resistance Temperature Detector (RTD)
- Thermistors
temp sensors differ in
- Operating range
- Sensitivity
- Linearity
- Accuracy
- Cost
- Application Domains
thermocouple
- Most commonly used
- Voltage output proportional to (absolute) temperature
thermocouple operating principle
- Seebeck Effect
- Metals may be joined by soldering welding, or twisting together
- Commercial varieties use welded or soldered joints
seebeck effect
the junction of two dissimilar metals (alloys) generates a voltage proportional to temperature
seebeck voltage formula
eₐᵦ = αT
α = seebeck coefficient T = temp
8 common letter designated thermocouple types
- C, R, S types
- E, J types
- K, T types
C, R and S types
high temp
low sensitivity
E and J types
highest sensitivity
K and T types
sub-zero temps
reference unction
- make use of a a second thermocouple junction to sense a ‘known’ temperature, our reference.
- place reference junction (J2) in an ice bath (0°C)
- Hence reference junction is called a cold junction
reference junction formula
V = α(Tᵤₙₖₙₒ𝓌ₙ - Tᵣₑ𝒻)
isothermal block
- to make sure junctions J3 and J4 are at same temperature, mount them on an isothermal block.
- isothermal block has sufficient thermal inertia to damp out fluctuations in ambient temperature, maintaining both at same temperature.
- isothermal block is an electrical insulator, but good conductor of heat
advantages of thermocouples
- Mechanically durable – easy to package and transport
- Wide operating range
- Easy to construct
- Relatively inexpensive
- Accuracy of about ±1-2°C without any calibration!
disadvantages of thermocouples
- Low output voltage
- Slow measurements times
- Output is non-linear – requires linearisation somehow
used in RTDs
-platinum, very accurate + stable
RTDs main features
- Platinum, nickel or nickel alloys most common metals used
- resistance of 100Ω most common.
- better linearity than thermocouple.
- Platinum has best performance.
RTDs industry standard
Pt100 - 100 Ω at 0 °C, 0.385 Ω/°C
The RTD makes use of a direct resistance measurement. There are some significant disadvantages to this approach:
- slope of 0.385Ω/°C is v small change
- resistance value of sensor is small, only 100 Ω
- resistance of lead wires can cause v large errors
We can approximate the RTD curve with the Callendar-Van Dusen equation. Which has two versions:
(used for calibration)
𝑅(𝑡)=𝑅(0)[1+𝐴𝑡+𝐵𝑡² ]
𝑅(𝑡)=𝑅(0)[1+𝐴𝑡+𝐵𝑡²+(𝑡−100)𝐶𝑡³ ]
sources of error in RTD sensors
- self heating: current in RTD may cause temp to rise
- thermal shunting: attaching RTD may change measurement - mass absorbs some heat
- thermal EMF: platinum-copper junction causes thermocouple effect
these errors depends on size of RTD sensor
small RTD
- fast response time
- low thermal shunting
- high self-heating error
Large RTD
- slow response time
- poor thermal shunting
- low self-heating error
thermistors
- special type of RTD known as a Thermal Resistor
- Material used is ceramic or polymer
- operating range is -90ºC to +130ºC
- more sensitive than RTD but smaller range
- non-linear resistance-temperature curve
two types of thermistor
- Positive Temperature Coefficient (PTC)
- Negative Temp Coefficient (NTC)
NTC thermistor resistance
decreases with increasing temperature
Steinhart-Hart Equation
used to convert resistance value to temp
1/𝑇 = 𝐴 + 𝐵×ln𝑅 + 𝐶×(ln𝑅 )³
T in Kevil
Constants A, B C are specified for each device
Thermocouple adv and disadv
- widest range: -184°C to +2300°C
- high accuracy + repeat-ability
- needs cold junction compensation
- low voltage output
RTD adv and disadv
- Range: -200°C to +850°C
- fair linearity
- requires excitation
- low cost
Thermistor adv and disadv
- range: 0°C to +100°C
- poor linearity
- requires excitation
- high sensitivity
