EE 4900: Fundamentals of Sensor Design
1
Lecture 4
Pressure Sensing
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Pressure Sensing
2
Q: What are we measuring?
A: Relative Pressure or Gauge Pressure.
Pressure is Force (F) per Unit Area (A); P=F/A
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Units of Pressure
SI units: Pascal (Pa), Torr
1 Pa=1 N/m2=9.869x10-6 atm=7.5x10-3 mmHg=7.5x10-3 Torr
1 atm. = 760 Torr = 101.325 kPa
1 psi (pound per square inch) = 6.89x103 Pa = 0.0703 atm
EE 4900 Fundamentals of Sensor Design
Suketu Naik
4
Pressure Sensing: Basics
Pressure sensors convert input pressures into electrical outputs
(usually voltage)
 Pressure sensors measure pressure, force, and airflow

Application
 Water level in the washer
 Car exhaust system
Car Exhaust Gas Sensor
Washer Water Level Sensor
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Pressure Sensing: Basics
Application
 Monitor blood pressure and intravenous infusion
 Control HVAC system
 Altimeter in the airplane
Blood Pressure Monitor
HVAC System Pressure Sensor
Altimeter
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Pressure Sensor Types

Mercury Pressure Sensor





U-shaped wire is suspended in mercury
The resistance of wire is balanced at two ends
As pressure applies to the left tube, mercury will be pushed into the
right tube
More resistance in the left tube and less resistance in the right tube
As a result there will be a disbalance in the bridge circuit which is
related to the change in pressure
Dynisco Melt Pressure Sensor
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Pressure Sensor Types
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Vaccum Sensors: Pirani Gauge
 Measure pressure in vacuum systems
 Based on thermal conductivity
 Platinum RTD measures amount of heat
loss which depends on the gas pressure

Pirani Gauge
Optoelectronic Pressure Sensors
 Optical cavity with Fabry-Perot interferometer
 Measure deflection of the diaphragm

EE 4900 Fundamentals of Sensor Design
Suketu Naik
Pressure Sensor Types
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Capacitive Pressure Sensors
 Si diaphragm: displacement changes capacitance
 Good for sensing low pressures
 Planar diaphragms are more sensitive

EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Pressure Sensor Types
Piezoresistive Pressure Sensors
 Deformable membrane or plate deflects (moves) due to the
pressure
 This deflection is measured by Piezoresistors

Piezoresistors
MPM283: Liquid Pressure
Sensor by MicroSensor
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Piezoresistive Pressure Sensing
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Piezoresistors are formed by a) epitaxial growth (layer is
deposited) or b) diffusion or c) ion implantation of a certain
material on Si
 Si is etched with special etchant to create the diaphragm
 The resistors are connected in half-bridge or full-bridge
configuration to measure the pressure differential

Ref:
[1] Demystifying Piezoresistors, http://www.maximintegrated.com/en/app-notes/index.mvp/id/871
[2] Piezoresistive Pressure and Temperature Sensor Cluster: http://www.microsystems.metu.edu.tr/piezops/piezops.html
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Piezoresistive Sensing: Strain Gauges
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Strain Gauges can measure
Strain: Piezoresistive Effect
Force: Strain gauge in a load cell
Pressure: Diaphragm to Force to Strain
Flow Rate: Differential Pressure
Load Cell Force Sensor
Load Cell Pressure Sensor
EE 4900 Fundamentals of Sensor Design
Load Cell Flow Sensor
Suketu Naik
Piezoresistive Sensing: Strain Gauges
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Strain Gauge
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Piezoresistive Effect
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Q: What is a Piezoresistive effect?
A: When mechanical strain (due to pressure, force etc) is applied to
a material, it deforms and its electrical resistance changes
l where, ρ=resistivity [Ωm],
Resistance of a
R
conductor
a=cross sectional area [m2],
a
2
2
l=length [m],
v=volume [m3]
l
l
l
R 
 ;
a
a l
v Constant
dR

Deformation
2 l
dl
v

dl
   dl
dR  2 l  dl  2 l   2 R
v
l
a  l
dR
dl
dR
dl
2
 Se  GF  
R
l
R
l
EE 4900 Fundamentals of Sensor Design
Suketu Naik
14
Piezoresistive Effect (Continued)
F
dl
   E  E 
a
l
dR
dl
 Se  GF  
R
l
where, σ=stress or pressure [Pa],
E=Young's modulus [Pa]
ε=Strain
Piezoresistive Effect
Se=GF=gauge factor or
strain sensitivity
Applied Stress=Pressure
R GF

   GF  
R
E
Relative Change in Resistance
R GF
R E

    p 
R
E
R GF
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Piezoresistive Pressure Sensing: Wheatstone Bridge
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Q: What does the Wheatstone Bridge measure?
A: Relative Change in Resistance
Quarter Bridge Circuit
Half Bridge Circuit
Full Bridge Circuit
Half Bridge Circuit
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Quarter Bridge Circuit
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Quarter Bridge Circuit
R1
R2
 R2
R4 
Vin
Vout  

 R1  R2 R3  R4 
R3
Unknown

Resistor Vout  0  
 R4 
R2 
Vin
Vin  
 R1  R2 
 R3  R4 
R4

where, Vout=V+OUT - V-OUT
Vin=V+IN - V-IN
R1 R3

R2 R4
R3
R4 
R2
R1
Make R3=R1
Tune R2 till Vout = 0 (Iout = 0)
Then R4=R2
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Quarter Bridge Circuit and Strain Gauge
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Quarter Bridge Circuit
R
R
Strain
Gauge
R
R+∆R
R 
 R  R
Vout  

Vin
 2 R  R 2 R 
where, Vout=V+OUT - V-OUT
Vin=V+IN - V-IN

1  GF  
Vout  
4  1  GF  

2




1 R / R 
Vin
Vout  
4  1  R / R 


2 



Vin



Prone to Temperature Variation
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Half Bridge Circuit and Strain Gauge
Half Bridge Circuit
R
R
R-∆R (compression)
Strain
Gauges
R+∆R (tension)
tension
where, Vout=V+OUT - V-OUT
Vin=V+IN - V-IN
Resistance increases under tension
Resistance decreases under compression
Compression
Common-mode effect= temperature variation is eliminated
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Half Bridge Circuit and Strain Gauge
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Half Bridge Circuit
R-∆R (compression)
Strain
Gauges
R+∆R (tension)
where, Vout=V+OUT - V-OUT
Vin=V+IN - V-IN
 R  R R 
 R 
Vout  

V

 in 
Vin
2R 
 2R
 2R 
1
Vout  GF   Vin
2
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Full Bridge Circuit and Strain Gauge
Vout=V+OUT - V-OUT
Vin=V+IN - V-IN
Full Bridge Circuit
R+∆R (tension)
Strain
Gauges
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R-∆R
(compression)
R-∆R (compression)
Strain
Gauges
R+∆R (tension)
 R  R R  R 
 R 
Vout  

V

 in 
Vin
2R 
 2R
 R 
Vout  GF   Vin
R E Vout E
 GF 
Vout  
  Vin   



R GF Vin GF
 E

σ=stress
="pressure" [N/m2]
More Sensitive than Half Bridge Circuit (why?)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Example: Pressure Sensor
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Columbia Research Labs 100P
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Assignment (Due 9/24)
Simulation
 Simulate Pressure Sensor System (Strain Gauge)
 Quarter Bridge
 Full Bridge
 Use Multisim (or Simscape or Cppsim)
Input
Pressure
[Pa]
Pressure
Sensor
Change
in resistance
Change
in voltage
Signal
Conditioning
(Amplifier
if necessary)
DAQ
(NI myDAQ)
(Wheatstone Bridge)
Display
(Labview)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Assignment (Due 9/24)
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Experiment
 Build Strain Gauge System
 Use Ni DAQ with Multisim
 Display Unknown Stress (Pressure)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Gauge Factor for Different Materials
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Metals: typically between 2 and 4
Ref: Electrical Resistance Strain Gauge Circuits, Georgia Tech
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Gauge Factors for Various Strain Gauge Grids
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Ref: Electrical Resistance Strain Gauge Circuits, Georgia Tech
EE 4900 Fundamentals of Sensor Design
Suketu Naik