Αισθητήρες και Συστήµατα Οργάνων · • Familiarisation with structures and associated models. 3 ... • I-V characteristic of a diode is nonlinear E.g

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Αισθητήρες και Συστήµατα Οργάνων

•  Εισαγωγή

•  Βασική Θεωρία Μετρήσεων

•  Αρχές των Βασικών Αισθητήρων

•  Αισθητήρες ΜΕΜΣ

•  Σήµατα και Θόρυβο

•  Ενισχυτές Σηµάτων

•  Σύνδεση και Προστασία Σηµάτων

•  Συλλογή δεδοµένων και Μετατροπείς Δεδοµένων

•  Ηλεκτρική Ασφάλεια σε Ιατρικά Συστήµατα

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Aims

•  Review some of the sensing principles in the context of Miniature Sensors / Microsensors

•  To get an overview of different types of MEMS microsensors

•  Familiarisation with structures and associated models.

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Sensors Revisited

•  A sensor may be defined as a device that converts and non-electrical input quantity to an electrical signal

•  An actuator is a device that converts into •  A processor modifies an electrical signal

(amplifiers, filters etc) but does not convert its primary form.

•  A transducer can be either a sensor or an actuator. •  Some devices can be operated as both

e.g. coil-magnet based microphone/speaker

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Primary Energy Domains

•  Electrical E •  Thermal T •  Radiation R •  Mechanical Me •  Magnetic M •  Bio(chemical) C

Microsensors Introduction

Vectorial representation (a) sensors and (b) actuators in energy domain space

Microsensor/Microactuator Classification

Classification of sensors and transducers in terms of the electrical property that is changed Classification of sensors and transducers in terms of the nature of the electrical signal

Micro Thermal Sensors

Thermal Sensors measure primarily thermal quantities, e.g. •  Temperature ç Most important!! •  Heat flow •  Thermal conductivity

Most material properties depend on temperature, e.g. stiffness, strength, conductivity….etc. Four principles for thermal measurement

a.  Resistive Temperature Microsensors b.  Microthermocouples c.  Thermodiodes and Thermotransistors d.  SAW Temperature Sensors

Microsensors Thermal Sensors

a. Resistive Temperature Microsensors •  Temperature of an object can be measured using:

•  Thermistor (platinum resistance thermometers) Electrical resistivity ρ varies with absolute T


b. Microthermocouples •  Potentiometric temperature sensor - a temperature dependant open-

circuit voltage VT appears relying on the Seebeck effect where different materials have different thermoelectric power


b. Microthermocouples


b. Microthermocouples •  Example of a temperature microsensor with good

experimental values


c. Thermotransistors •  I-V characteristic of a diode is nonlinear

E.g. with operation in constant current mode the forward diode voltage is directly proportional to the absolute temperature

Thermotransistor principle: The Diode Equations

•  If an equal current is passed through both diodes the difference in voltage is proportional to the absolute temperature

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Id1

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Id1 ≅ Is expVf1

VT

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Id 2 ≅ NIs expVf 2

VT

Id2

1 : N

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∂Vf =VT lnN

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VT =kTe

Vf1 Vf2

The Conventional BGR

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•  Voltage across resistors R1 and R2 is identical

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∂Vf =VT ln NR2R1

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⇒ I1R1 = I2R2

⇒ I2 =R1R2I1

Vf1 Vf2

dVf

I1 I2 VR1 VR2

Kuijk, K.E.; , "A precision reference voltage source," Solid-State Circuits, IEEE Journal of , vol.8, no.3, pp. 222- 226, Jun 1973doi: 10.1109/JSSC.1973.1050378

The Conventional BGR

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•  Since the same current I2 flows through R2 and R3

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Vref =Vf 1 +VR 2

Vref = Is expVf

kT e+R2R3

kTeln N R2

R1

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VR 2 = dVfR2R3

Vf1 Vf2

dVf

I1 I2 VR2 VR1

•  Hence we get

CTAT PTAT


d. Surface Acoustic Wave (SAW) Temperature Sensors •  SAW is proportional to temperature on piezoelectric materials (quartz,

lithium niobate, etc.) •  In- and output transducer are placed on piezoelectric substrate to generate

and measure SAW (transduction of el. to mech. energy and vice versa)

Wireless SAW Temperature Sensor 1.  Antenna receives el. wave and transduces it to a SAW 2.  SAW travels to reflector and back 3.  Antenna transduces SAW back to el. wave 4.  Receiver measures phase angle of outgoing

and incoming el. wave, which is linearly related to the temperature

Microsensors Radiation Sensors

Radiation sensors can be classified according to the type and energy of the measurand, but not all of them can be integrated


Radiation sensors can be distinguished by their underlying operation principle

1.  Photoconductive Radiation excites a number of e- from the valance band of a semiconductor material into its conduction band and thus creates both e- and holes

2.  Photovoltaic Radiation induces a voltage across a semiconductor junction (photovoltaic effect)

3.  Pyroelectric Radiation heats up the surface of the crystal and thereby induces the charge to flow off its surface and creating a voltage

4.  Microantenna The microantenna can detect low-energy microwave signals and transduce these to SAWs or simply sense it


Some commercial radiation microsensors

Microsensors Mechanical Sensors

Mechanical sensors seem to the most important sensors •  Large variety of different mechanical measurands (see table 8.4) •  Successful application in mass market


Most important classes of mechanical sensors

•  Acceleration / deceleration •  Displacement •  Flow rate •  Force / torque •  Position / angle •  Pressure / stress


Micromechanical Compounds and Statics •  Basic building blocks are used for a whole host of different microsensors,

-actuators and MEMS

Cantilever beam

Bridge

Diaphragm / Membrane


Assumption for material of microstructures

•  Homogeneous •  Uniform •  Elastic

⇒  Simple theory for

deformation under mechanical load applicable •  Force •  Torque •  Stress •  Pressure

Example Free end of a cantilever beam will

deflect by distance Δx when a point load Fx is applied to it (no gravity considered)

Em = Young’s modulus

Im second moment of area

Im is related to width and thickness by

Fx may be written as: Km = stiffness or spring constant


Mechanical force to displacement (or vice versa) •  The simple cantilever beam can be used to convert mechanical force into

displacement (as the example shows) •  Cantilever beam, bridge and diaphragm can be used to measure distributed

force such as stress •  A diaphragm can be used to measure a hydrostatic of barometric

pressure


Some important parameters for the STATIC deflections Some important parameters for the DYNAMIC deflections


Pressure Microsensors •  First type of silicon micromachined sensors (late 1950s)

•  Most mature silicon micromechanical device •  Widespread commercial availability today •  Automotive is the largest market


Pressure Microsensors •  Most common methods to

fabricate pressure microsensors •  Bulk micromachining •  Surface micromachining

•  Basic principle of piezoresistive (upper illustration) and capacitive pressure sensor (lower illustration)


Microaccelerometer •  Microaccelerometers are based on the cantilever

principle in which an end mass (or shuttle) displaces under an inertial force

•  Dynamics can be described in simple terms by the second-order system of a mass-spring damper


Two basic principle of microaccelerometers •  Capacitive pickup of the seismic mass movement

•  Piezoresistive pickup of the seismic mass movement


Microaccelerometer •  Mostly come with high g and low g variations •  Have sophisticated damping and overload

protections

Example applications •  Automotive – ABS: 0 to 2 g •  Automotive – suspension: 0 to 2 g •  Automotive – air bag: 0 to 50 g •  Automotive – navigation systems: 0 to 2 g


Up to date acceleration sensor: Freescale MMA6270QT Pricing: $ 5.07 (> 500) to $ 8.03 (< 10) •  ACCELERATION SENSOR, XY, 1.5G •  Acceleration Range:± 1.5g to ± 6g •  No. of Axes:2

•  Gravity Range:± 1.5g / ± 6g •  Sensitivity:1.5/2/4/6g 800/600/300/200 mV/g

•  Sensor IC Case Style:QFN •  No. of Pins:16 •  Supply Voltage Range:2.2V to 3.6V

•  Operating Temperature Range:-40°C to +105°C •  SVHC:Cobalt dichloride (18-Jun-2010) •  Case Style:QFN

•  Max Operating Temperature:85°C •  Min Temperature Operating:-20°C

•  Max Supply Voltage:3.6V •  Min Supply Voltage:2.2V •  Supply Current:500µA

•  Termination Type:SMD •  Amplifier IC Type:Accerlerometer Sensor X-Y

axis

•  Max Frequency:350Hz •  Max Supply Current:800µA

•  Max Supply Voltage DC:3.6V •  Min Supply Voltage DC:2.2V •  No. of Channels:2

•  Supply Voltage:3.3V

Source: www.farnell.com

reshapin

g the digital revolution

What is a Gyro?

ω!!!

×= va 2α=acceleration, v=velocity ω=angular velocity

v

a

Basis of a Tuning Fork Vibratory Gyro

p =m× vp=momentum, m=mass, v=velocity


Microgyrometers •  Gyroscopes measure the change in orientation of an object Basic principle •  Transfer of energy from one resonator to another due to Coriolis

force •  A mass m supported by springs in the x- and y-axes and rotated

around the z-axis at an angular velocity Ω has the following equation of motion


Basic calculation A mass m supported by springs in the x- and y-axes and rotated around the z-axis at an angular velocity Ω has the following equation of motion

where -2mΩx‘ and -2mΩy‘ describe the Coriolis forces and the resonant f are

Assume the resonators are excited and behave harmonically with the amplitudes a(t) and b(t). By fixing the amplitude of one oscillator (a0) by feedback and then for synchronous oscillators (ω0x = ω0y), the equation simply reduces to Under constant rotation, the steady-state solution to equation above is a constant amplitude b0 where


Up to date +-500 dps analog gyroscope: ST Microelectronics LPR450AL Pricing: $ 5.22 (> 3000) to $ 15.64 (< 10)

•  2.7 V to 3.6 V single-supply operation •  Wide operating temperature range (-40 °C to +85 °C) •  High stability over temperature

•  Analog absolute angular-rate outputs •  Two separate outputs for each axis (1x and 4x amplified)

•  Integrated low-pass filters •  Low power consumption ([email protected]) •  Embedded power-down

•  Embedded self-test •  Sleep mode •  High shock and vibration survivability

•  ECOPACK® RoHS and “Green” compliant

Source: www.digikey.com

Microsensors Magnetic Sensors

Introduction •  Magnetic sensors measure the magnetic flux B •  Various kinds of magnetic sensors exist (see table)

1.  Magnetogalvanic devices 2.  Magnetoresistive devices 3.  Magnetdiodes / -transistors


Magnetogalvanic Microsensors •  The sensors principle is based on the Hall-Effect •  The Hall-Coefficient in semiconductors is quite high

Definition When a current Ix is passed down a slab of material of length l and thickness d and a perpendicular magnetic flux density Bz is applied, a voltage VH appears across the slab perpendicular to Ix and Bz

n = carrier density RH = Hall Coefficient d = material thickness Bz= Magnetic field in z-axis Ix=current in x-axis VH = Hall voltage (See next slide for Fleming LHR)

Remember Fleming Left Hand Rule (motors)

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Magnetoresistive Devices •  Basic principle is to use the magentoresistivity of a

semiconductor •  The resistance of a semiconducting material is

influenced by the application of an external magnetic flux density Bz and called magnetoresistivity

•  The resistance R of a slab of material depends on the Hall angle θH and is given by

R0 = R at zero flux density, kg is dependent on the geometry of the slab.

•  Hall angle θH is the angle by which the direction of the current Ix is rotated as a reaction of the Laurentzian force that acts on the charge carriers. Sensors slab should be wider rather than longer and Hall voltage should be shorted out.


Magnetodiodes Structure (a): using a silicon-on-sapphire IC process in which carriers are injected

from both the n+ and p+ junction and drift under the action of an electric field. With a perpendicular magnetic field, the Suhl* effect occurs, where the carriers are deflected to the enclosing interfaces, where the recombination rates vary. High recombination occurs at the Si-Al2O3 interface and low at Si-SiO2

* Suhl effect: when a strong transverse magnetic field is applied to an n-type semiconducting filament, holes injected into the filament are

deflected to the surface, where they may recombine rapidly with electrons or be withdrawn by a probe.

* McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


Magnetodiodes Structure (b): standard CMOS device looking like a transistor but being operated

as diode. The reversed biased p-n junction becomes the high recombination surface, whereas the Si-SiO2 interface has a low recombination rate.

* McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

•  The poor control of the recombination rates in a magnetodiode makes them problematic (depends on surface roughness/temperature etc).


Magnetotransistors •  Two effects as result of the Lorentz force take place

1.  Generation of a Hall voltage 2.  Deflection of the injected minority carrier current (measured using a split

collector) •  Magnetotransistors can be made from standard CMOS process


Acoustic Devices •  Main problem of Hall effect ICs is the low sensitivity •  New approach: microsensors based on a delay-line SAW device How it works •  The propagation of the SAW is modified by the magnetoelastic coupling

between the magnetic spin and the strain fields. The SAW device acts like a strain sensor detecting the strain in the Garnet film. The Garnet films strain depends on the field’s magentostriction.

•  The change in the acoustic velocity ϑ of the wave results in a change in the resonant frequency ϑ0 of the SAW oscillator and hence,

•  The shift in oscillator frequency is a nonlinear function F(·) of the magnetic flux density Bz


•  By providing an dc magnetic bias the frequency sensitivity to magnetic fields can be increased to give a field resolution of micro-Tesla.

•  The high sensitivity of a SAW magnetic sensor is a significant advantage over Hall sensors


Superconducting Quantum Interference Device (SQUID) •  The most sensitive of all magnetic sensors •  Require a superconducting material coil interrupted by a extremely

thin insulation barrier to form a Josephson junction. •  Quantum tunneling enables conduction across junction •  A current is driven across the junction •  A voltage across the junction is created when the current is above the

critical current level. •  The critical current level is a function of the applied field. •  This voltage also flips polarity each time the field is increased by:

Φ0 = h/(2e) •  Hence can measure fields in two ways:

•  Keep current above critical, take field from 0 to measured value. •  Find critical current.

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Plot of current vs. voltage for a SQUID. Upper and lower curves correspond to nΦ0 and (n+1/2)Φ0 respectively

Periodic voltage response due to flux through a SQUID. The periodicity is equal to one flux quantum, Φ0


SQUIDs •  The most sensitive of all magnetic sensors is known as a

superconducting quantum interference device (SQUID)


Further Reading for SQUIDS

•  http://rich.phekda.org/squid/technical/part3.html •  http://en.wikipedia.org/wiki/SQUID •  http://cnx.org/content/m22750/1.3/

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Microsensors Bio(chemical) Sensors

Introduction •  Bio(chemical) sensors measure one or more chemical substances by

transducing its concentration to another (e.g. electrical) signal •  Bio(chemical) sensors can be classified as follows:


Basic Components •  The analyte molecules

interact with the chemically sensitive layer and produce a physical change that is detected by the transducer and are converted into an electrical output signal

•  This transduction may be reversible or irreversible


Conductimetric Devices •  Conductimetric gas sensors are based on the principle of

measuring a change in the electrical resistance of a material upon the introduction of the target gas

•  Most common type employs a solid-state material as the gas-sensitive element

How it works The device consists of a wire-wound platinum heater coil inside a ceramic former onto which a thick layer of porous tin oxide is painted manually. The film is then sintered at a high temperature so theat the appropriate nanocrystalline structure is formed. The electrical resistance of the sintered film is then measured by a pair of gold electrodes an basic potential divider circuit.


Potentiometric Devices •  There is a class of field-effect gas sensors based on metal-insulator

semiconductor structures in which the gate is made from a gas-sensitive catalytic metal

•  Structure (a): field-effect transistor •  Structure (b): gas-sensitive capacitor

References

Microsensors MEMS and Smart Devices, Chapter 8 Julian W. Gardner, Vijay K. Varadan, Osama O. Awadelkarim ISBN: 978-0-471-86109-6

Documents

Αισθητήρες και Συστήµατα Οργάνων · • Familiarisation with structures and associated models. 3 ... • I-V characteristic of a diode is nonlinear E.g