EE 4900: Fundamentals of Sensor Design 1 Lecture 4 Pressure Sensing
Pressure Sensing Q: What are we measuring? A: elative Pressure or Gauge Pressure. Pressure is Force (F) per Unit Area (A); P=F/A 2
SI units: Pascal (Pa), Torr Units of Pressure 3 1 Pa=1 N/m 2 =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) = 6.89x10 3 Pa = 0.0703 atm
Pressure Sensing: Basics Pressure sensors convert input pressures into electrical outputs (usually voltage) Pressure sensors measure pressure, force, and airflow 4 Application Water level in the washer Car exhaust system Car Exhaust Gas Sensor Washer Water Level Sensor
Application Pressure Sensing: Basics Monitor blood pressure and intravenous infusion Control HVAC system Altimeter in the airplane 5 Blood Pressure Monitor HVAC System Pressure Sensor Altimeter
Mercury Pressure Sensor Pressure Sensor Types 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 6 Dynisco Melt Pressure Sensor
Pressure Sensor Types Vaccum Sensors: Pirani Gauge Measure pressure in vacuum systems Based on thermal conductivity Platinum TD measures amount of heat loss which depends on the gas pressure 7 Pirani Gauge Optoelectronic Pressure Sensors Optical cavity with Fabry-Perot interferometer Measure deflection of the diaphragm
Pressure Sensor Types Capacitive Pressure Sensors Si diaphragm: displacement changes capacitance Good for sensing low pressures Planar diaphragms are more sensitive 8
Pressure Sensor Types Piezoresistive Pressure Sensors Deformable membrane or plate deflects (moves) due to the pressure This deflection is measured by Piezoresistors 9 Piezoresistors MPM283: Liquid Pressure Sensor by MicroSensor
Piezoresistive Pressure Sensing 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 10 ef: [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
Piezoresistive Sensing: Strain Gauges 11 Strain Gauges can measure Strain: Piezoresistive Effect Force: Strain gauge in a load cell Pressure: Diaphragm to Force to Strain Flow ate: Differential Pressure Load Cell Force Sensor Load Cell Pressure Sensor Load Cell Flow Sensor
Piezoresistive Sensing: Strain Gauges 12 Strain Gauge
Piezoresistive Effect 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 esistance of a conductor d 2 dl l l a 2 2 l l l a a l v d 2 l dl v d 2 l dl 2 l v a d ; Deformation S e dl l dl l where, ρ=resistivity [Ωm], a=cross sectional area [m 2 ], l=length [m], v=volume [m 3 ] Constant 2 GF dl l 13
Piezoresistive Effect (Continued) 14 F dl E E a l d S e dl l GF where, σ=stress or pressure [Pa], E=Young's modulus [Pa] ε=strain S e =GF=gauge factor or strain sensitivity GF E Piezoresistive Effect GF Applied Stress=Pressure elative Change in esistance GF E p E GF
Piezoresistive Pressure Sensing: Wheatstone Bridge Q: What does the Wheatstone Bridge measure? A: elative Change in esistance 15 Quarter Bridge Circuit Half Bridge Circuit Full Bridge Circuit Half Bridge Circuit
Quarter Bridge Circuit 1 2 3 4 Quarter Bridge Circuit Unknown esistor V 2 4 out V in 1 2 3 4 Vout 0 1 3 2 4 1 2 2 V in 3 4 4 V in 16 where, V out =V +OUT - V -OUT V in =V +IN - V -IN 3 4 2 1 Make 3 = 1 Tune 2 till V out = 0 (I out = 0) Then 4 = 2
Quarter Bridge Circuit and Strain Gauge Quarter Bridge Circuit 17 + where, V out =V +OUT - V -OUT V in =V +IN - V -IN Strain Gauge V 2 2 out V in V V 1 GF 4 GF 1 2 out V in 1 / 4 / 1 2 out V in Prone to Temperature Variation
Half Bridge Circuit and Strain Gauge Half Bridge Circuit 18 - (compression) Strain Gauges + (tension) tension where, V out =V +OUT - V -OUT V in =V +IN - V -IN esistance increases under tension esistance decreases under compression Compression Common-mode effect= temperature variation is eliminated
Half Bridge Circuit and Strain Gauge Half Bridge Circuit 19 - (compression) Strain Gauges + (tension) V out 2 2 V in 2 V in V out 2 1 GF V in where, V out =V +OUT - V -OUT V in =V +IN - V -IN
Full Bridge Circuit and Strain Gauge Full Bridge Circuit Strain Gauges + (tension) - (compression) - (compression) Strain Gauges + (tension) 20 V out =V +OUT - V -OUT V in =V +IN - V -IN V out 2 2 V in V in V out GF V in V out GF E V in E GF V V E GF More Sensitive than Half Bridge Circuit (why?) out in σ=stress ="pressure" [N/m 2 ]
Example: Pressure Sensor 21 Columbia esearch Labs 100P
Simulation Assignment 2 Simulate Pressure Sensor System (Strain Gauge) Quarter Bridge Full Bridge Use Multisim (or Simscape or Cppsim) 22 Input Pressure [Pa] Pressure Sensor Change in resistance Change in voltage (Wheatstone Bridge) Signal Conditioning (Amplifier if necessary) DAQ (NI mydaq) Display (Labview)
Assignment 3 Experiment Build Strain Gauge System Use Ni DAQ with Multisim Display Unknown Stress (Pressure) 23
Gauge Factor for Different Materials Metals: typically between 2 and 4 24 ef: Electrical esistance Strain Gauge Circuits, Georgia Tech
Gauge Factors for Various Strain Gauge Grids 25 ef: Electrical esistance Strain Gauge Circuits, Georgia Tech