Sensor Technologies

Phase LinearityDescribe how well a system preserves the phase relationship between frequency components of the inputPhase linearity: f=kfDistortion of signal Amplitude linearity Phase linearity

Sensor Technology - TerminologyTransducer is a device which transforms energy from one type to another, even if both energy types are in the same domain.Typical energy domains are mechanical, electrical, chemical, magnetic, optical and thermal.Transducer can be further divided into Sensors, which monitors a system and Actuators, which impose an action on the system.Sensors are devices which monitor a parameter of a system, hopefully without disturbing that parameter.

Categorization of SensorClassification based on physical phenomenaMechanical: strain gage, displacement (LVDT), velocity (laser vibrometer), accelerometer, tilt meter, viscometer, pressure, etc.Thermal: thermal coupleOptical: camera, infrared sensorOthers Classification based on measuring mechanismResistance sensing, capacitance sensing, inductance sensing, piezoelectricity, etc.Materials capable of converting of one form of energy to another are at the heart of many sensors. Invention of new materials, e.g., smart materials, would permit the design of new types of sensors.

Linear Variable Differential Transformer*

Paradigm of Sensing System DesignZhang & Aktan, 2005

Instrumentation ConsiderationsSensor technology;Sensor data collection topologies;Data communication;Power supply;Data synchronization;Environmental parameters and influence;Remote data analysis.

MeasurementMeasurement output: interaction between a sensor and the environment surrounding the sensor compound response of multiple inputs

Measurement errors: System errors: imperfect design of the measurement setup and the approximation, can be corrected by calibration Random errors: variations due to uncontrolled variables. Can be reduced by averaging.

SensorsDefinition: a device for sensing a physical variable of a physical system or an environment

Classification of Sensors Mechanical quantities: displacement, Strain, rotation velocity, acceleration, pressure, force/torque, twisting, weight, flow Thermal quantities: temperature, heat. Electromagnetic/optical quantities: voltage, current, frequency phase; visual/images, light; magnetism. Chemical quantities: moisture, pH value

Specifications of SensorAccuracy: error between the result of a measurement and the true value being measured.Resolution: the smallest increment of measure that a device can make.Sensitivity: the ratio between the change in the output signal to a small change in input physical signal. Slope of the input-output fit line.Repeatability/Precision: the ability of the sensor to output the same value for the same input over a number of trials

Accuracy vs. Resolution

Accuracy vs. PrecisionPrecision without accuracyAccuracy without precisionPrecision and accuracy

Specifications of SensorDynamic Range: the ratio of maximum recordable input amplitude to minimum input amplitude, i.e. D.R. = 20 log (Max. Input Ampl./Min. Input Ampl.) dBLinearity: the deviation of the output from a best-fit straight line for a given range of the sensorTransfer Function (Frequency Response): The relationship between physical input signal and electrical output signal, which may constitute a complete description of the sensor characteristics.Bandwidth: the frequency range between the lower and upper cutoff frequencies, within which the sensor transfer function is constant gain or linear.Noise: random fluctuation in the value of input that causes random fluctuation in the output value

Attributes of SensorsOperating Principle: Embedded technologies that make sensors function, such as electro-optics, electromagnetic, piezoelectricity, active and passive ultraviolet.Dimension of Variables: The number of dimensions of physical variables.Size: The physical volume of sensors.Data Format: The measuring feature of data in time; continuous or discrete/analog or digital.Intelligence: Capabilities of on-board data processing and decision-making.Active versus Passive Sensors: Capability of generating vs. just receiving signals.Physical Contact: The way sensors observe the disturbance in environment.Environmental durability: will the sensor robust enough for its operation conditions

Strain GaugesFoil strain gaugeLeast expensiveWidely usedNot suitable for long distanceElectromagnetic InterferenceSensitive to moisture & humidity

Vibration wire strain gaugeDetermine strain from freq. of AC signalBulky

Fiber optic gaugeImmune to EM and electrostatic noiseCompact sizeHigh costFragile

Strain SensingResistive Foil Strain GageTechnology well developed; Low cost High response speed & broad frequency bandwidthA wide assortment of foil strain gages commercially availableSubject to electromagnetic (EM) noise, interference, offset drift in signal. Long-term performance of adhesives used for bonding strain gages is questionableVibrating wire strain gages can NOT be used for dynamic application because of their low response speed.Optical fiber strain sensor

Strain SensingPiezoelectric Strain SensorPiezoelectric ceramic-based or Piezoelectric polymer-based (e.g., PVDF)Very high resolution (able to measure nanostrain)Excellent performance in ultrasonic frequency range, very high frequency bandwidth; therefore very popular in ultrasonic applications, such as measuring signals due to surface wave propagationWhen used for measuring plane strain, can not distinguish the strain in X, Y directionPiezoelectric ceramic is a brittle material (can not measure large deformation)

Courtesy of PCB Piezotronics

Acceleration SensingPiezoelectric accelerometerNonzero lower cutoff frequency (0.1 1 Hz for 5%)Light, compact size (miniature accelerometer weighing 0.7 g is available)Measurement range up to +/- 500 gLess expensive than capacitive accelerometerSensitivity typically from 5 100 mv/gBroad frequency bandwidth (typically 0.2 5 kHz)Operating temperature: -70 150 C

Acceleration SensingCapacitive accelerometerGood performance over low frequency range, can measure gravity!Heavier (~ 100 g) and bigger size than piezoelectric accelerometerMeasurement range up to +/- 200 gMore expensive than piezoelectric accelerometerSensitivity typically from 10 1000 mV/gFrequency bandwidth typically from 0 to 800 HzOperating temperature: -65 120 C

Accelerometer

Force SensingMetal foil strain-gage based (load cell)Good in low frequency responseHigh load ratingResolution lower than piezoelectricity-basedRugged, typically big size, heavy weight

Courtesy of Davidson Measurement

Force SensingPiezoelectricity based (force sensor)lower cutoff frequency at 0.01 Hz can NOT be used for static load measurementGood in high frequencyHigh resolutionLimited operating temperature (can not be used for high temperature applications)Compact size, light

Courtesy of PCB Piezotronics

Displacement SensingLVDT (Linear Variable Differential Transformer):Inductance-based electromechanical sensorInfinite resolution limited by external electronicsLimited frequency bandwidth (250 Hz typical for DC-LVDT, 500 Hz for AC-LVDT)No contact between the moving core and coil structure no friction, no wear, very long operating lifetimeAccuracy limited mostly by linearity0.1%-1% typicalModels with strokes from mms to 1 m available

Photo courtesy of MSI

Displacement SensingLinear Potentiometer Resolution (infinite), depends on?High frequency bandwidth (> 10 kHz)Fast response speedVelocity (up to 2.5 m/s)Low costFinite operating life (2 million cycles) due to contact wearAccuracy: +/- 0.01 % - 3 % FSOOperating temperature: -55 ~ 125 C

Photo courtesy of Duncan Electronics

Displacement TransducerMagnetostrictive Linear Displacement TransducerExceptional performance for long stroke position measurement up to 3 mOperation is based on accurately measuring the distance from a predetermined point to a magnetic field produced by a movable permanent magnet. Repeatability up to 0.002% of the measurement range. Resolution up to 0.002% of full scale range (FSR)Relatively low frequency bandwidth (-3dB at 100 Hz)Very expensiveOperating temperature: 0 70 C

Photo courtesy of Schaevitz

Displacement SensingDifferential Variable Reluctance Transducers Relatively short strokeHigh resolutionNon-contact between the measured object and sensor

Courtesy of Microstrain, Inc.

Type of Construction Standard tubularFixing Modeby 8mm diameterTotal Measuring Range2(+/-1)mmPneumatic RetractionNoRepeatability0.1umOperating Temperature Limits-10 to +65 degrees C

Velocity SensingScanning Laser Vibrometry No physical contact with the test object; facilitate remote, mass-loading-free vibration measurements on targets measuring velocity (translational or angular)automated scanning measurements with fast scanning speed However, very expensive (> $120K)

Laser VibrometryReferencesStructural health monitoring using scanning laser vibrometry, by L. Mallet, Smart Materials & Structures, vol. 13, 2004, pg. 261the technical note entitled Principle of Vibrometry from Polytec

Shock (high-G) SensingShock Pressure SensorMeasurement range up to 69 MPa (10 ksi)High response speed (rise time < 2 sec.)High frequency bandwidth (resonant frequency up to > 500 kHz) Operating temperature: -70 to 130 CLight (typically weighs ~ 10 g)

Shock AccelerometerMeasurement range up to +/- 70,000 gFrequency bandwidth typically from 0.5 30 kHz at -3 dBOperating temperature: -40 to 80 CLight (weighs ~ 5 g)

Photo courtesy of PCB Piezotronics

Angular Motion Sensing (Tilt Meter)Inertial Gyroscope (e.g., http://www.xbow.com)used to measure angular rates and X, Y, and Z acceleration.

Tilt Sensor/Inclinometer (e.g., http://www.microstrain.com)Tilt sensors and inclinometers generate an artificial horizon and measure angular tilt with respect to this horizon.

Rotary Position Sensor (e.g., http://www.msiusa.com)includes potentiometers and a variety of magnetic and capacitive technologies. Sensors are designed for angular displacement less than one turn or for multi-turn displacement.

Photo courtesy of MSI and Crossbow

MEMS TechnologyWhat is MEMS?Acronym for Microelectromechanical SystemsMEMS is the name given to the practice of making and combining miniaturized mechanical and electrical components. K. Gabriel, SciAm, Sept 1995.Synonym to:Micromachines (in Japan)Microsystems technology (in Europe)

Leverage on existing IC-based fabrication techniques (but now extend to other non IC techniques)Potential for low cost through batch fabricationThousands of MEMS devices (scale from ~ 0.2 m to 1 mm) could be made simultaneously on a single silicon wafer

MEMS TechnologyCo-location of sensing, computing, actuating, control, communication & power on a small chip-size deviceHigh spatial functionality and fast response speedVery high precision in manufactureminiaturized components improve response speed and reduce power consumption

MEMS Fabrication TechniqueCourtesy of A.P. Pisano, DARPA

Distinctive Features of MEMS DevicesMiniaturizationmicromachines (sensors and actuators) can handle microobjects and move freely in small spacesMultiplicitycooperative work from many small micromachines may be best way to perform a large taskinexpensive to make many machines in parallelMicroelectronicsintegrate microelectronic control devices with sensors and actuators

Fujita, Proc. IEEE, Vol. 86, No 8

MEMS AccelerometerCapacitive MEMS accelerometerHigh precision dual axis accelerometer with signal conditioned voltage outputs, all on a single monolithic IC Sensitivity from 20 to 1000 mV/gHigh accuracyHigh temperature stability Low power (less than 700 uA typical) 5 mm x 5 mm x 2 mm LCC package Low cost ($5 ~ $14/pc. in Yr. 2004)

Courtesy of Analog Devices, Inc.

MEMS AccelerometerPiezoresistive MEMS accelerometerOperating Principle: a proof mass attached to a silicon housing through a short flexural element. The implantation of a piezoresistive material on the upper surface of the flexural element. The strain experienced by a piezoresistive material causes a position change of its internal atoms, resulting in the change of its electrical resistancelow-noise property at high frequencies

Courtesy of JP Lynch, U Mich.

MEMS Dust MEMS dust here has the same scale as a single dandelion seed - something so small and light that it literally floats in the air.

Source: Distributed MEMS: New Challenges for Computation, by A.A. BERLIN and K.J. GABRIEL, IEEE Comp. Sci. Eng., 1997

Sensing SystemReferenceZhang, R. and Aktan, E., Design consideration for sensing systems to ensure data quality, Sensing issues in Civil Structural Health Monitoring, Eded by Ansari, F., Springer, 2005, P281-290

Linear Variable Differential Transformer*