2.996/6.971 Biomedical Devices Design Laboratory Lecture 10: SensorsSwitchesReed SwitchesPiezoresistive Strain GagesForce Sensitive Resistors (FSR)Photoresistor (Light-Dependent Resistor)Resistance Measurement CircuitsResult only dependent on the ratios of the test and reference resistorsWheatstone BridgeInstrumentation AmplifierServo’ed Split BridgeCapacitance (E-field) SensingCap Sensing – The quick and dirty wayLow-Impedance MethodHigh-Impedance MethodMagnetic Field SensingHall-effect SensorsApplications of Magnetic SensorsGiant Magnetoresistance (GMR)Spin-Dependent ConductionGMR Sensors from NVEHysteresis and Biasing2.996/6.971 Biomedical Devices Design LaboratoryLecture 10: SensorsInstructor: Dr. Hong MaOctober 22, 2007Switches• Pull-ups and pull-down• De-bouncingReed Switches• Magnetically operated• Non-contact operation• Door open/close detectionImage removed due to copyright restrictions.Piezoresistive Strain Gages• Gage factor– Common metals G~2-3– Platinum G=6– Semiconductors G~40-200• Metals have greater elongation limits– Constantan foil:• Up to 3-5% elongation•G=2– Higher strains are limited by bonding()12 (12)dR dl dlCGGRl lυυε=++−==Images removed due to copyright restrictions.Force Sensitive Resistors (FSR)• Pressure sensitive polymer– Decrease in resistance with applied pressure• Not suitable for precision measurements!– Force accurace range from ±5 to ±25%• Repeatable mechanics is key!Image removed due to copyright restrictions.Photoresistor(Light-Dependent Resistor)• Very slow (response time ~100ms)• Hysteresis behavior• CdS ~ 480nm• ZnS ~ 320nm• CdSe ~ 720nm• PbS ~ 2000nmImage removed due to copyright restrictions.Resistance Measurement Circuits• Quick-and-dirty techniques:– Trans-impedance amplifier– Resistive divider– Timer-comparatorResult only dependent on the ratios of the test and reference resistorsWheatstone Bridge• Match resistors to remove common-mode interference• Not linear• Linearize using multiple sensorsInstrumentation Amplifier• Available as a packaged amplifier, e.g. INA118•RGand VREFaccessible externallyServo’ed Split Bridge•Vout= -x*Vin/2• Require dual supplies if x>0Capacitance (E-field) Sensing• Cost effective• Infinite resolution• Require transient excitation signalImage removed due to copyright restrictions.Cap Sensing – The quick and dirty way• Exchange the resistor and capacitor• Measure capacitor discharge timeLow-Impedance MethodHigh-Impedance MethodConventional Pin-OutOPA129 Pin-OutMagnetic Field Sensing• H-field– Applied field– Units (CGS): Oe (Oersted)– Units (SI): Ampere Turns / Meter•B-field– Applied + induced field– B = µ(H + M)– Units (CGS): Gauss– Units (SI): Tesla (1 Tesla = 104Gauss)• In air (in CGS units): 1 Oe = 1 Gauss• Earth’s magnetic field: 0.3 – 0.6 Gauss• Sensitivities:– Hall-effect: >50 Oe– GMR: 0.1 to 0.5 OeHall-effect Sensors• Ubiquitous• Low cost• Non-contact• Line-of-sight not required• Poor accuracy– >50 Oe magnetic field required• Generally used in switch modeApplications of Magnetic SensorsGiant Magnetoresistance (GMR)• Nobel Prize in Physics 2007• Discovered in 1988, brought to market in 1997• Albert Fert (France) and Peter Grünberg (Germany)Image removed due to copyright restrictions.Spin-Dependent Conduction• ~5nm layers• Section 2 could be a conductor or insulator• Practically 8 - 12% conductivity changeGMR Sensors from NVEHysteresis and Biasing• All magnetic materials exhibit hysteresis