ECE 480, Prof. A. Mason Sensors p.1SENSORSa.k.a.Interfacing to the Real World:Review of Electrical Sensors and ActuatorsAndrew MasonAssocitate Professor, ECETeach: Microelectronics (analog & digital integrated Circ., VLSI)Biomedical Engineering (instrumentation)Research: Integrated Microsystems (on-chip sensors & circuits)ECE 480, Prof. A. Mason Sensors p.2Transducers•Transducer– a device that converts a primary form of energy into a corresponding signal with a different energy form• Primary Energy Forms: mechanical, thermal, electromagnetic, optical, chemical, etc.– take form of a sensor or an actuator•Sensor(e.g., thermometer)– a device that detects/measures a signal or stimulus– acquires information from the “real world”• Actuator (e.g., heater)– a device that generates a signal or stimulusrealworldsensoractuatorintelligentfeedbacksystemECE 480, Prof. A. Mason Sensors p.3usablevaluesSensor SystemsTypically interested in electronic sensor– convert desired parameter into electrically measurable signal• General Electronic Sensor–primary transducer: changes “real world” parameter into electrical signal–secondary transducer: converts electrical signal into analog or digital values• Typical Electronic Sensor Systemrealworldanalogsignalprimarytransducersecondarytransducersensorsensorinputsignal(measurand)microcontrollersignal processingcommunicationsensor dataanalog/digitalnetworkdisplayECE 480, Prof. A. Mason Sensors p.4Example Electronic Sensor Systems• Components vary with application– digital sensor within an instrument• microcontroller– signal timing– data storage– analog sensor analyzed by a PC– multiple sensors displayed over internetµCsignal timingmemorykeypadsensorsensor displayhandheld instrumentPCcomm. cardsensor interfaceA/D, communicationsignal processingsensore.g., RS232PCcomm. cardinternetsensorprocessorcomm.sensorprocessorcomm.sensor bus sensor busECE 480, Prof. A. Mason Sensors p.5Primary Transducers• Conventional Transducerslarge, but generally reliable, based on older technology–thermocouple: temperature difference– compass (magnetic): direction• Microelectronic Sensorsmillimeter sized, highly sensitive, less robust– photodiode/phototransistor: photon energy (light)• infrared detectors, proximity/intrusion alarms– piezoresisitve pressure sensor: air/fluid pressure– microaccelerometers: vibration, ∆-velocity (car crash)–chemical senors: O2, CO2, Cl, Nitrates (explosives)– DNA arrays: match DNA sequencesECE 480, Prof. A. Mason Sensors p.6Example Primary Transducers•Light Sensor– photoconductor•light Æ ΔR–photodiode•light Æ ΔI– membrane pressure sensor• resistive (pressure Æ Δ R)• capacitive (pressure Æ ΔC)ECE 480, Prof. A. Mason Sensors p.7Displacement Measurements• Measurements of size, shape, and position utilize displacement sensors•Examples– diameter of part under stress (direct) – movement of a microphone diaphragm to quantify liquid movement through the heart (indirect) • Primary Transducer Types– Resistive Sensors (Potentiometers & Strain Gages)–Inductive Sensors– Capacitive Sensors– Piezoelectric Sensors• Secondary Transducers–Wheatstone Bridge–AmplifiersECE 480, Prof. A. Mason Sensors p.8Strain Gage: Gage Factor• Remember: for a strained thin wire– ΔR/R = ΔL/L – ΔA/A + Δρ/ρ•A = π (D/2)2, for circular wire• Poisson’s ratio, μ: relates change in diameter D to change in length L– ΔD/D = - μ ΔL/L•Thus – ΔR/R = (1+2μ) ΔL/L + Δρ/ρ•Gage Factor, G, used to compare strain-gate materials–G = ΔR/R = (1+2μ) + Δρ/ρΔL/L ΔL/LLDdimensional effect piezoresistive effectECE 480, Prof. A. Mason Sensors p.9Temperature Sensor Options• Resistance Temperature Detectors (RTDs)– Platinum, Nickel, Copper metals are typically used– positive temperature coefficients• Thermistors (“thermally sensitive resistor”)– formed from semiconductor materials, not metals• often composite of a ceramic and a metallic oxide (Mn, Co, Cu or Fe)– typically have negative temperature coefficients• Thermocouples– based on the Seebeck effect: dissimilar metals at diff. temps. Æ signalECE 480, Prof. A. Mason Sensors p.10Fiber-optic Temperature Sensor• Sensor operation– small prism-shaped sample of single-crystal undoped GaAsattached to ends of two optical fibers– light energy absorbed by the GaAs crystal depends on temperature– percentage of received vs. transmitted energy is a function of temperature• Can be made small enough for biological implantationGaAs semiconductor temperature probeECE 480, Prof. A. Mason Sensors p.11Example MEMS Transducers• MEMS = micro-electro-mechanical system– miniature transducers created using IC fabrication processes• Microaccelerometer– cantilever beam– suspended mass•Rotation– gyroscope•PressureElectrodesRingstructureDiaphragm (Upper electrode)Lower electrode5-10mmECE 480, Prof. A. Mason Sensors p.12Passive Sensor Readout Circuit• Photodiode Circuits• Thermistor Half-Bridge–voltage divider– one element varies• Wheatstone Bridge– R3 = resistive sensor– R4 is matched to nominal value of R3–If R1= R2, Vout-nominal= 0–Voutvaries as R3changesVCCR1+R4ECE 480, Prof. A. Mason Sensors p.13Operational Amplifiers• Properties– open-loop gain: ideally infinite: practical values 20k-200k•high open-loop gain Æ virtual short between + and - inputs–input impedance: ideally infinite: CMOS opamps are close to ideal–output impedance: ideally zero: practical values 20-100Ω– zero output offset: ideally zero: practical value <1mV– gain-bandwidth product (GB): practical values ~MHz•frequency where open-loop gain drops to 1 V/V• Commercial opamps provide many different properties–low noise– low input current–low power–high bandwidth– low/high supply voltage– special purpose: comparator, instrumentation amplifierECE 480, Prof. A. Mason Sensors p.14Basic Opamp Configuration• Voltage Comparator– digitize input• Voltage Follower– buffer• Non-Inverting Amp•Inverting AmpECE 480, Prof. A. Mason Sensors p.15More Opamp Configurations• Summing Amp• Differential Amp•Integrating Amp• Differentiating AmpECE 480, Prof. A. Mason Sensors p.16Converting Configuration• Current-to-Voltage• Voltage-to-CurrentECE 480, Prof. A. Mason Sensors p.17Instrumentation Amplifier•Robust differential gain amplifier•Input
View Full Document
Unlocking...