SENSORS and Transducers

[project report maker]

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¢ SENSORS
¢

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PRESENTED BY
PANKAJ KUMAR BHARTI
ROLLNO-10602048
NATIONAL INSTITUTE OF TECHNOLOGY,ROURKELA

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¢ Transducers

¢ 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 stimulus
¢ Sensor Systems
“ 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 System
“ Example 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 internet
¢ Primary Transducers
¢ Conventional Transducers
large, but generally reliable, based on older technology
“ thermocouple: temperature difference
“ compass (magnetic): direction
“
¢ Microelectronic Sensors
millimeter sized, highly sensitive, less robust
“ photodiode/phototransistor: photon energy (light)
¢ infrared detectors, proximity/intrusion alarms
“ piezoresisitve pressure sensor: air/fluid pressure
“ microaccelerometers: vibration, ˆ†-velocity (car crash)
“ chemical senors: O2, CO2, Cl, Nitrates (explosives)
“ DNA arrays: match DNA sequences
¢ Example Primary Transducers


¢ Light Sensor
“ photoconductor
¢ light à DR
“ photodiode
¢ light à DI
“ membrane pressure sensor
¢ resistive (pressure à D R)
¢ capacitive (pressure à DC)
“
¢ Displacement 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

“ Amplifiers
¢ Strain Gage: Gage Factor
¢ Remember: for a strained thin wire
“ DR/R = DL/L “ DA/A + Dr/r
¢ A = p (D/2)2, for circular wire
¢ Poisson™s ratio, m: relates change in diameter D to change in length L
“ DD/D = - m DL/L
¢ Thus
“ DR/R = (1+2m) DL/L + Dr/r
“ Gage Factor, G, used to compare strain-gate materials
“ G = DR/R = (1+2m) + Dr/r
DL/L DL/L
¢ Temperature 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. à signal
¢ Fiber-optic Temperature Sensor
¢ Sensor operation
“ small prism-shaped sample of single-crystal undoped GaAs attached 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 implantation
¢ Example MEMS Transducers
¢ MEMS = micro-electro-mechanical system
“ miniature transducers created using IC fabrication processes
¢
¢ Microaccelerometer
“ cantilever beam
“ suspended mass
¢ Rotation
“ gyroscope
¢ Pressure
¢ Passive 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
“ Vout varies as R3 changes
¢ Operational Amplifiers


¢ Properties
“ open-loop gain: ideally infinite: practical values 20k-200k
¢ high open-loop gain à virtual short between + and - inputs
“ input impedance: ideally infinite: CMOS opamps are close to ideal
“ output impedance: ideally zero: practical values 20-100W
“ 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 amplifier

¢ Basic Opamp Configuration
¢ Voltage Comparator
“ digitize input
“ Voltage Follower
“ buffer
¢ Non-Inverting Amp
¢ More Opamp Configurations
¢ Summing Amp
¢ Differential Amp
¢ Integrating Amp
¢ Differentiating Amp
¢ Converting Configuration
¢ Current-to-Voltage
¢ Voltage-to-Current
¢ Instrumentation Amplifier
¢ Robust differential

gain amplifier
¢ Input stage
“ high input impedance
¢ buffers gain stage
“ no common mode gain
“ can have differential gain

¢ Gain stage
“ differential gain, low input impedance
¢ Overall amplifier
“ amplifies only the differential component
¢ high common mode rejection ratio
“ high input impedance suitable for biopotential electrodes with high output impedance
¢ Instrumentation Amplifier w/ BP Filter
¢ Connecting Sensors to Microcontrollers


¢ Analog
“ many microcontrollers have a built-in A/D
¢ 8-bit to 12-bit common
¢ many have multi-channel A/D inputs
¢
¢ Digital
“ serial I/O
¢ use serial I/O port, store in memory to analyze
¢ synchronous (with clock)
“ must match byte format, stop/start bits, parity check, etc.
¢ asynchronous (no clock): more common for comm. than data
“ must match baud rate and bit width, transmission protocol, etc.
“ frequency encoded
¢ use timing port, measure pulse width or pulse frequency
¢ Connecting Smart Sensors to PC/Network
¢ Smart sensor = sensor with built-in signal processing & communication
“ e.g., combining a dumb sensor and a microcontroller

¢ Data Acquisition Cards (DAQ)
“ PC card with analog and digital I/O
“ interface through LabVIEW or user-generated code

¢ Communication Links Common for Sensors
“ asynchronous serial comm.
¢ universal asynchronous receive and transmit (UART)
“ 1 receive line + 1 transmit line. nodes must match baud rate & protocol
¢ RS232 Serial Port on PCs uses UART format (but at +/- 12V)
“ can buy a chip to convert from UART to RS232
“ synchronous serial comm.
¢ serial peripheral interface (SPI)
“ 1 clock + 1 bidirectional data + 1 chip select/enable
“ I2C = Inter Integrated Circuit bus
¢ designed by Philips for comm. inside TVs, used in several commercial sensor systems
“ IEEE P1451: Sensor Comm. Standard
¢ several different sensor comm. protocols for different applications
¢ Sensor Calibration


¢ Sensors can exhibit non-ideal effects
“ offset: nominal output ‰  nominal parameter value
“ nonlinearity: output not linear with parameter changes
“ cross parameter sensitivity: secondary output variation with, e.g., temperature
¢ Calibration = adjusting output to match parameter
“ analog signal conditioning
“ look-up table
“ digital calibration
¢ T = a + bV +cV2,
“ T= temperature; V=sensor voltage;
“ a,b,c = calibration coefficients
¢
¢ Compensation
“ remove secondary sensitivities
“ must have sensitivities characterized
“ can remove with polynomial evaluation
¢ P = a + bV + cT + dVT + e V2, where P=pressure, T=temperature