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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
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¢ 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

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TRANSDUCERS


A transducer is a device that converts one type of energy to another. The conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or any other form of energy. While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer.

Briefly a transducer has to perform two major functions:

(1) To measure or sense a physical quantity in a measurement system
(2) To convert the measured value to a useful output







 Basic Requirements of a Transducer

(1)Accuracy and precision: Every transducer should follow an ideal or theoretical output/measured relationship. The accuracy of a transducer is defined as the ratio of the error to the full-scale output. Precision is the closeness with which measurements are distributed about their mean value.

(2) Repeatability : The transducer should reproduce same measured value when same input is applied consecutively under the same conditions in the same direction.

(3)Linearity : The input-output characteristics of the transducer should be linear. The transducers are designed with linear output/measured relationship as this tends to facilitate data reduction.

(4) Resolution : A transducer should possess both high resolution and high accuracy at the same time.

(5) Environmental Characteristics : When the transducer operates under the conditions which are different from the room conditions ,errors should not appear in the transducer output. In addition to this external forces should not affect the performance of transducer. Hence a transducer should not be affected by temperature, vibration and other external environmental condition.

(6)Dynamic Response : In case of input varying with time, the transducer should be able to respond to the changes in the input as quickly as possible.

(7) Ruggedness : It should be able to withstand overloads for short duration and adequate safety measures should be present for overload protection.


TRANSDUCER CLASSIFICATION

There is no standard way for the transducer classification but the following types of classification are most commonly used:

 Classification Based on application
a) Sensor
b) Actuator
c) Combination


(a) A sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical and/or digital signal). For example, a pressure sensor might detect pressure(a mechanical form of energy) and convert it to electricity for display at a remote gauge.
(b) An actuator accepts energy and produces movement (action). The energy supplied to an actuator might be electrical or mechanical (pneumatic, hydraulic, etc.). An electric motor and a loudspeaker are both transducers, converting electrical energy into motion for different purposes.
© Combination transducers have both functions; they both detect and create action. For example, a typical ultrasonic transducer switches back and forth many times a second between acting as an actuator to produce ultrasonic waves, and acting as a sensor to detect ultrasonic waves.

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SENSORSa.k.a.Interfacing to the Real World:Review of Electrical Sensors and Actuators



Andrew Mason
Associtate Professor, ECE

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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

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