Modern InstrumentationPHYS 533/CHEM 620

Lecture 11Light, Force, Strain, and Pressure Sensors

Amin JazaeriFall 2007

Classification of Sensors• Proprioceptive (Internal state) v.s.

Exteroceptive (external state) – measure values internally to the system (robot), e.g.

battery level, wheel position, joint angle, etc,– observation of environments, objects

• Active v.s. Passive – emitting energy into the environment, e.g., radar,

sonar– passively receive energy to make observation, e.g.,

camera• Contact v.s. non-contact• Visual v.s. non-visual

– vision-based sensing, image processing, video camera

Light energy• For a sensor, we’re interested in the light power

that falls on a unit area, and how well the sensor converts that into a signal.

• A common unit is the lux which measures apparent brightness (power multiplied by the human eye’s sensitivity).

• 1 lux of yellow light is about 0.0015 W/m2.• 1 lux of green light (50% eff.) is 0.0029 W/m2.• Sunlight corresponds to about 50,000 lux • Artificial light typically 500-1000 lux

Electromagnetic SpectrumElectromagnetic SpectrumVisible Spectrum

700 nm 400 nm

Light sensors• Simplest light sensor is an LDR (Light-

Dependent Resistor).• Optical characteristics close to human eye. • Can be used to feed an A/D directly without

amplification (one resistor in a voltage divider).• Common material is CdS

Sensitivity: dark 1 M,10 lux 40 k,1000 lux 400 .

(a) General block diagram of an optical instrument. (b) Highest efficiency is obtained by using an intense lamp, lenses to gather and focus the light on the sample in the cuvette, and a sensitive detector. (c) Solid-state lamps and detectors may simplify the system.

Light sources and detectors

Sources

• Incandescent bulb

• Light emitting diode (LED)

• Gas and solid state lasers

• Arc lamp

• Fluorescent source

Detectors

• Thermal detector (pyroelectric)

• Photodiode

• Phototransistor

• Charge-coupled device (CCD)

• Photoconductive cell

• Photomultiplier tube

• Block diagram of a single beam spectrophotometer. The prism serves as the dispersing device while the monochromator refers to the dispersing device (prism), entrance slit, and exit slit. The exit slit is moveable in the vertical direction so that those portions of the power spectrum produced by the power source (light source) that are to be used can be selected.

Readoutdevice

Detector

Cuvette

Exit slit

Red

VioletPrism

Monochromator

Entrance slit

Light source

I0

I

Photoemissive Sensors

Photomultiplier An incoming photon strikes the photocathode and liberates an electron. This electron is accelerated toward the first dynode, which is 100 V more positive than the cathode. The impact liberates several electrons by secondary emission. They are accelerated toward the second dynode, which is 100 V more positive than the first dynode, This electron multiplication continues until it reaches the anode, where currents of about 1 A flow through RL. Time response < 10 nsec

Phototube: have photocathode coated with alkali metals. A radiation photon with energy cause electron to jump from cathode to anode.Photon energies below 1 eV are not large enough to overcome the work functions, so wavelength over 1200nm cannot be detected.

Photoconductive CellsPhotoresistors: a photosensitive crystalline materials such as cadmium Sulfide (CdS) or lead sulfide (PbS) is deposited on a ceramic substance.

The resistance decrease of the ceramic material with input radiation. This is true if photons have enough energy to cause electron to move from the valence band to the conduction band.

light-dependent resistors (LDRs) are slow, but respond like the human eye

Photojunction SensorsPhotojunction sensors are formed from p-n junctions and are usually made of silicon. If a photon has enough energy to jump the band gap, hole-electron pairs are produced that modify the junction characteristics.

Voltage-current characteristics of irradiated silicon p-n junction. For 0 irradiance, both forward and reverse characteristics are normal. For 1 mW/cm2, open-circuit voltage is 500 mV and short-circuit current is 8 A.

Photodiode: With reverse biasing, the reverse photocurrent increases linearly with an increase in radiation.

Phototransistor: radiation generate base current which result in the generation of a large current flow from collector to emitter.Response time = 10 microsecond

Photovoltaic SensorsPhotovoltaic sensors is a p-n junction where the voltage increases as the radiation increases.

Spectral characteristics of detectors, (c) Detectors. The S4 response is a typical phototube response. The eye has a relatively narrow response, with colors indicated by VBGYOR. CdS plus a filter has a response that closely matches that of the eye. Si p-n junctions are widely used. PbS is a sensitive infrared detector. InSb is useful in far infrared. Note: These are only relative responses. Peak responses of different detectors differ by 107.

Photovoltaic Sensors

• Photovoltaic– light falling on a pn-junction

can be used to generate electricity from light energy (as in a solar cell)

– small devices used as sensors are called photodiodes

– fast acting, but the voltage produced is not linearly related to light intensity A typical photodiode

Photodiode vs. Photoresistor

• Photoresistor: simple but slow

• Photodiode/phototransistor: complex but fast– Phototransistor vs. Photodiode:

• Higher current• Slower (100KHz)• Higher dark current

Light sensors – high end• At the cutting edge of light sensor sensitivity are

Avalanche photodiodes. • Large voltages applied to these diodes

accelerate electrons to “collide” with the semiconductor lattice, creating more charges.

• These devices have quantum efficienciesaround 90% and extremely low noise.

• They are now made withlarge collection areas andknown as LAAPDs (Large-Area Avalanche Photo-Diode)

Light sensors – cameras• Two solid-state camera types: CCD and CMOS.• CCD is the more mature technology, and has

the widest performance range. – 8 Mpixel size for cameras– Low noise/ high efficiency for astronomy etc. – Good sensitivity (low as 0.0003 lux, starlight)

• CCDs require several chips,but are still cheap ($50 +)

• Most CCDs work in near infraredand can be used for night visionif an IR light source is used.

Light sensors – cameras• CMOS cameras are very compact and

inexpensive, but haven’t matched CCDs in most performance dimensions.

• Start from $20(!)

• Custom CMOS camerasintegrate image processingright on the camera.

• Allow special functions likemotion detection, recognition.

Polarized Light

• Normal light: light wave travels at all orientation (w.r.t. horizon)

• Polarized light: all the light traveling in a given orientation.

• Two normal filters => no passing light

• The amount of light can be controlled

Applications

• object presence detection

• object distance detection

• surface feature detection (finding/following markers/tape)

• wall/boundary tracking

• rotational shaft encoding

• bar code decoding

Sensor limitations

• Light reflectivity:– Surface color

• Black: does not reflect• White: reflects

– Texture• Ambient light:

– Measure with and w/o emitter– Subtract from each other

• Sensor calibration– Calibration to be done repeatedly. Why?

• => Partially observable

Force Sensing

AF Strain Sensing:

APF Pressure Sensing:

amF Acceleration Sensing:

xkF Elastic Sensing:

Sensing Element in Force Sensing Element in Force SensorsSensors

There are many types of sensors can

be used to measure force (or torque)!

Resistive type force sensors, such as

strain gages ands load cells, are very

commonly used in force

measurements.

Load Cell Load Cell (Example of Spec. Items)

PerformancePerformance Load range: 5 to 250 lbs Non-Linearity: 0.05% F.S. Hysteresis: 0.03% F.S. Non-Repeatability: 0.03% F.S. Output: 3 mV/V Resolution: InfiniteEnvironmentalEnvironmental Temp. operating: 0 to 130 °F Temp. compensated: 30 to 130 °FMechanicalMechanical Static overload: 50% over capacity

FullScale

Force sensors - Strain Gauges

• Strain gauge - The main tool in sensing force.• Strain gauges, measure strain• Strain can be related to stress, force, torque and

a host of other stimuli including displacement, acceleration or position.

• At the heart of all strain gauges is the change in resistance of materials due to change in their length due to strain.

Strain GagesStrain Gages

Characteristics:

1) able to measure strains of m/m2) small in size and light in weight3) able to response to high frequency signals4) wide range of linear response5) has stable calibration constant (gage factor)6) flexible in use and wide range applications7) low in cost8) easy compensation to various factors

Fundamentals of Strain Fundamentals of Strain GagesGages

(strain) e

(stress) E

eL

eT

FF

llA

Elastic Modulus:

LT ee

The resistance of a strain gage:Al

R

Material resistivity

Element length

Cross section area

When a strain gage is strained, the change in resistance is:

R

AAR

llR

R

Poisson’sratio

Axialstrain

Transversestrain

Strain GagesStrain Gages

Relative change in resistance:

Because:

Define a Gage factor G:

Gage factor of a strain gage:

LeR

RG

)e(2e2D

D2

A

ALT

Then:

;el

lL

Poisson’sratio

Example Specs of Strain Example Specs of Strain GagesGages

Temperature RangeTemperature RangeNormal: -100 to +350 FShort-Term: -320 to +400 F

Strain RangeStrain Range+3% for gage lengths under 1/8 in+5% for 1/8 in and over

Fatigue LifeFatigue Life105 cycles at +1500 microstrain 106 cycles at +1500 microstrain

with low modulus solder.

Strain Gauge

• For any given strain gauge the gauge factor is a constant

• Ranges between 2 to 6 for most metallic strain gauges

• From 40-200 for semiconductor strain gauges. • The strain gauge relation gives a simple linear

relation between the change in resistance of the sensor and the strain applied to it.

Stress and Strain

Two-axis strain gauge

120 degree rosette

45 degree rosette

45 degree stacked rosette

membrane rosette

Semiconductor strain gauges

• Operate like resistive strain gauges • Construction and properties are different. • The gauge factor for semiconductors is much

higher than for metals. • The change in conductivity due to strain is much

larger than in metals. • Are typically smaller than metal types • Often more sensitive to temperature variations

(require temperature compensation).

Semiconductor strain gauges

• All semiconductor materials exhibit changes in resistance due to strain

• The most common material is silicon because of its inert properties and ease of production.

• The base material is doped, by diffusion of doping materials (usually boron or arsenide for p or n type) to obtain a base resistance as needed.

• The substrate provides the means of straining the silicon chip and connections are provided by deposition of metal at the ends of the device.

Amplification for Strain Amplification for Strain GagesGages

Sensitive instrumentation is required to measure the small changes in resistance produced by strain gauges.

Wheatstone bridgeWheatstone bridge is typically used to measure resistances accurately and dynamically over a very large range (1 to 1,000,000

Application of Strain GagesApplication of Strain Gages

+e

x l

-eF

Strain gages are used in cantilever type load cells

w

t

-

R4 R3

R1 R2

V0

Vs

+

-

R1 R3

R2

R4

4

4

2

2

1

1

3

32

32

32s0 R

R

R

R

R

R

R

R

)RR(

RRVV

Application of Strain GagesApplication of Strain Gages

Strain gages are used in pillar type load cells

LT

L

eEA

Fe

EEA

Fe

F

eT

eL

R3

, R4

, R1

R2

F Poisson’sratio

Application of Strain GagesApplication of Strain Gages

Strain gages are used in torque cells :

3142 eeee

31 rG

Te

3

gage1gage

0

rG

TFeF

V

V

ulusmodShear)1(2

EG

factorGageF

TorqueT

gage

Strainedcompressed

Common Strain Gage Common Strain Gage ArrangementsArrangements

+ -

R4 R3

R1 R2

VS + -

R4 R3

R1 R2

VS

+ -

R4 R3

R1 R2

VS + -

R4 R3

R1 R2

VS

Practical Implementation of Practical Implementation of Strain GagesStrain Gages

Strain gages Bridge Amplifier / filter ComputerForce Input

Preparation:• clean with sandpaper• degreaser solvent • glue = adhesive + curing agent• clams for curing cycle (24 h)

Parallel

Most critical step !

Practical Implementation of Practical Implementation of Strain GagesStrain Gages

Wheatstone bridge and amplification circuit

RF

-

+

V1

Vout

R1

A

B

R3

V2

R2

-

R4 R3

R1 R2

V0

Specifications:• Chip 741• R1 = R2 • R3 = RF • Gain: RF / R1

Tactile sensors

• Tactile sensors are force sensors but: • Definition of “tactile” action is broader, the

sensors are also more diverse. • One view is that tactile action as simply sensing

the presence of force. Then:– A simple switch is a tactile sensor– This approach is commonly used in keyboards – Membrane or resistive pads are used – The force is applied against the membrane or a

silicon rubber layer.

Tactile sensors

• In many tactile sensing applications it is often important to sense a force distribution over a specified area (such as the “hand” of a robot).

• Either an array of force sensors or • A distributed sensor may be used. • These are usually made from piezoelectric films

which respond with an electrical signal in response to deformation (passive sensors).

A tactile sensor

Tactile sensors

• Operation:• The polyvinylidene fluoride (PVDF) film is

sensitive to deformation. • The lower film is driven with an ac signal • It contracts and expands mechanically and

periodically. • When the upper film is deformed, its signal

changes from normal and the amplitude and or phase of the output signal is now a measure of deformation (force).

Pressure Sensors

What is pressure?

Pressure = force per area in fluids

1 N/m2 = 1 Pa Pascal

Engineers use the bar1 bar = 105 Pa = 1 athmosphere ~ 10 m of water column

Ranges of pressure measurement:Athmosphere: 1 barHydraulics, pneumatics: 6 -10 barCar industry 1 - 5 bar (tyre), 20 bar (air conditioning)Medicine: Blood: 100 mbar; in the human body 10 to 100 mbarDeep sea level: 4.000m ~ 400 barThin film processes: 1 to 1000 Pa (0,01 to 10 mbar)Rough vacuum: 10 Pa (changing pumps) High vacuum: down to 10-8 Pa

Pressure sensors

• Pressure sensors come in four basic types :• Absolute pressure sensors (PSIA): pressure sensed

relative to absolute vacuum.• Differential pressure sensors (PSID): the difference

between two pressures on two ports of the sensor is sensed.

• Gage pressure sensors (PSIG): the pressure relative to ambient pressure is sensed. (Most common)

• Sealed gage pressure sensor (PSIS): the pressure relative to a sealed pressure chamber (usually 1 atm at sea level or 14.7 psi) is sensed.

Piezoresistive pressure sensors

• Piezoresistor is a semiconductor strain gauge• Most modern pressure sensors use it rather than

the conductor type strain gauge. • Resistive (metal) strain gauges are used only at

higher temperature or for specialized applications

• May be fabricated of silicon– simplifies construction – allows on board temperature compensation,

amplifiers and conditioning circuitry.

Piezoresistive pressure sensors

• Basic structure: – two gauges are parallel to one dimension of

the diaphragm– The two gauges can be in other directions

Piezoresistive pressure sensors

• The change in resistance of the two piezoresistos is:

R1R1

= R2R2

= 12 y x

is an average sensitivity (gauge) coefficient andx and y are the stresses in the transverse directions

Piezoresistive pressure sensors

• Piezoresistors and the diaphragm are fabricated of silicon.

• A vent is provided, making this a gage sensor. • If the cavity under the diaphragm is

hermetically closed and the pressure in it is P0, the sensor becomes a sealed gage pressure sensor sensing the pressure P-P0.

• A differential sensor is produced by placing the diaphragm between two chambers, each vented through a port (figure).

Differential pressure sensor

Piezoresistive pressure sensors

• A different approach is to use a single strain gauge

• A current passing through the strain gauge

• Pressure applied perpendicular to the current.

• The voltage across the element is measured as an indication of the stress and therefore pressure.

Construction

• Many variations

• Body of sensor is particularly important

• Silicon, steel, stainless steel and titanium are most commonly used

• Ports are made with various fittings

• The contact material is specified (gas, fluid, corrosivity, etc.)

Various pressure sensors

Miniature pressure sensors

Pitran pressure sensors (absolute)

150 psi differential pressure sensor

100 psi absolute pressure sensor (TO5 can)

15 and 30 psi differential pressure sensors

Capacitive pressure sensors

• The deflection of the diaphragm constitutes a capacitor in which the distance between the plates is pressure sensitive.

• The basic structure (not shown) consists of two metalic plates

• These sensors are very simple and are particularly useful for sensing of very low pressure.

• At low pressure, the deflection of the diaphragm may be insufficient to cause large strain but can be relatively large in terms of capacitance.

Capacitive pressure sensors

• The capacitance may be part of an oscillator,• The change in its frequency may be quite large making

for a very sensitive sensor. • Other advantages

– less temperature dependent – stops on motion of the plate may be incorporated, - not sensitive

to overpressure.

• Overpressures of 2-3 orders of magnitude larger than rated pressure may be easily tolerated without ill effects.

• The sensors are linear for small displacement but at larger pressures the diaphragm tends to bow causing nonlinear output

Magnetic( Inductive) pressure sensors

• A number of methods are used • In large deflection sensors an inductive position

sensor may be used or an LVDT attached to the diaphragm.

• For low pressures, variable reluctance pressure sensor is more practical.

• The diaphragm is made of a ferromagnetic material and is part of the magnetic circuit shown in Figure 6.32.

Variable reluctance pressure sensor

Magnetic pressure sensors

• The reluctance is directly proportional to the length of the air gap between the diaphragm and the E-core.

• Gap changes with pressure and the inductance of the two coils changes and sensed directly.

• A very small deflection can cause a very large change in inductance of the circuit making this a very sensitive device.

• Magnetic sensors are almost devoid of temperature sensitivity allowing these sensors to operate at elevated temperatures.

Other pressure sensors

• Optoelectronic pressure sensors - Fabri-Perot optical resonator to measure small displacements. – light reflected from a resonant optical cavity is measured

by a photodiode to produce a measure of pressure sensed.

• A very old method of sensing low pressures (often called vacuum sensors) is the Pirani gauge. – based on measuring the heat loss from gases which is

dependent on pressure. The temperature is sensed and correlated to pressure, usually in an absolute pressure sensor arrangement.

Pressure sensors - properties

• Semiconductor based sensors can only operate at low temperatures (50 to +150C).

• Temperature dependent errors can be high unless properly compensated (externally or internally).

• The range of sensors can exceed 50,000 psi and can be as small as a fraction of psi.

• Impedance is anywhere between a few hundred Ohms to about 100 k, depending on device.

• Linearity is between 0.1 to 2% typically

Pressure sensors - properties

• Other speciffications include:– Maximum pressure, burst pressure and proof pressure

(overpressure) – electrical output - either direct (no internal circuitry) or after

conditioning and amplification. – Digital outputs are also available. – Materials used (silicon, stainless steel, etc.) and compatibility

with gases and liquids are specified – port sizes and shapes, connectors, venting ports – cycling of the pressure sensors is also specified – hysteresis (usually below 0.1% of full scale) – repeatability (typically less than 0.1% of full scale).

How to construct a pressure sensor

Pressure is transformed into deflection of a membrane

Tasks to do:

• Understand the elastic deformation of a membrane

• Construct a membrane

• Sense the deflection

• Construct a sensor housing

• Build an electronic circuit

Metal membrane pressure sensor

Figure from: Hesse, Schnell: Sensoren für die Prozess- und Fabrikautomation

Capacitive pressure sensor

Capacity of two plates

d

AC r0

Advantages:

• Sensitive

• Stable, small T-drift (ceramic technology)

• Small, E.g. 2mm Si chip size for eye pressure sensor

Disadvantages:

• Nonlinear (1/d)

• Electronics complicated

• Capacitive bridge circuit

• C - Frequency conversion

A: Area; d: distance

Inductive pressure sensor

Inductive bridge circuit

Very T-stable

Large devices,

large membranes for

small differential pressure

Si micromachined pressure sensor

Why Si technology?

• Batch process: >1000 chips on a wafer

• Precise control of technology

• k-factor in Si is ~ 100 (>>2!)

• Monolithic integration with electronics

• Very advanced and available technology

• Housing processes well known

Piezoresistive effect in Si

Semiconductors: deformation changes band structure

large change in conductivity

k positive or negative

k sensitive on temperature, doping and crystal orientation

Longitudinal: Current parallel to strain: i

Transversal: Current vertical to strain: i

Rule of thumb for p-doped Si:

Longitudinal: k~ +100

Transversal: k~ -100

Principle of Si p-sensor

A: longitudinal

B: transversal

Silicon piezoresistive pressure sensor

Figure from: Bonfig, Sensoren

Si

Si

Pyrex glass

Metal socket

NitrideMetal

Oxide Piezoresistor

Low pressure: vacuum gauges

Ranges:

Rough vacuum at 1 Pa to 100 Pa. In this range, mechanical pumping is switched to turbo pumping for high vacuum. A sensor is needed to trigger the valves.

High vacuum below 0,1 Pa. This is measured for process control.

Usually, a vacuum system has one sensor for control of the pumping down and pump control (pirani type) and one sensor for the control of the final pressure (ionisation type).

Quartz Pressure Sensor

• A typical Quartz crystal sensor with inbuilt micro-electric circuitry and a diaphragm.

• These sensors measure dynamic pressures, and are not generally used for static pressure sensing.

• Proper and accurate alignment of the sensor is very important for higher sensitivity.

• Sensors used in high temperature conditions(e.g. combustion chamber of an engine) use either recess mounting, baffled diaphragm or thermal protection coatings to reduce negative signal effects.

Pros and Cons

• Have a high Stiffness value and produce a high output with very little strain.

• Ideal for rugged use.• Excellent linearity over a

wide amplitude.• Ideal for continuous

online condition monitoring smart systems.

• Can be used only for dynamic pressure sensing as in case of static sensing the signals will decay away.

• Operation over long cables may affect frequency response and introduce noise and distortion, the cables need to be protected.

Typical Application-Combustion Monitoring• Pressures developed during

the combustion process is continuously measured by sensors mounted on the cylinder heads.

• Continuous Pressure monitor(CPM) systems are the basic data acquisition and data reduction software and hardware units.