Αισθητήρες και Συστήµατα Οργάνων

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 1

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. 2

Sensors Revisited A sensor may be defined as a device that converts and non-electrical input quantity E to an electrical signal E An actuator is a device that converts E into E 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 3

Primary Energy Domains Electrical E Thermal T Radiation R Mechanical Me Magnetic M Bio(chemical) C 4

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

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

Thermal Sensors b. Microthermocouples Potentiometric temperature sensor - a temperature dependant opencircuit voltage V T appears relying on the Seebeck effect where different materials have different thermoelectric power

Thermal Sensors b. Microthermocouples

Thermal Sensors b. Microthermocouples Example of a temperature microsensor with good experimental values

Thermal Sensors 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 I d1 I s exp V f 1 VT I d1 V f1 I d 2 NI s exp V f 2 VT I d2 V f2 If an equal current is passed through both diodes the difference in voltage is proportional to the absolute temperature 1 : N V f = V T lnn V T = kt e 13

The Conventional BGR Voltage across resistors R1 and R2 is identical I 1 I 2 V R1 V R2 I 1 R 1 = I 2 R 2 I 2 = R 1 R 2 I 1 dv f V f1 V f2 # V f = V T ln N R 2 % $ R 1 & ( ' 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 14

The Conventional BGR I 1 I 2 VR2 V R1 Since the same current I 2 flows through R 2 and R 3 V R 2 = dv f R 2 R 3 Hence we get dv f V ref = V f 1 +V R 2 V f1 V f2 V ref = I s exp V f kt e + R 2 kt R 3 e ln " N R 2 $ # R 1 % ' & CTAT PTAT 15

Thermal Sensors 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

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 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

Radiation Sensors Some commercial radiation 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

Mechanical Sensors Most important classes of mechanical sensors Acceleration / deceleration Displacement Flow rate Force / torque Position / angle Pressure / stress

Mechanical Sensors Micromechanical Compounds and Statics Basic building blocks are used for a whole host of different microsensors, -actuators and MEMS Cantilever beam Bridge Diaphragm / Membrane

Mechanical Sensors Assumption for material of microstructures Homogeneous Uniform Elastic Example Free end of a cantilever beam will deflect by distance Δx when a point load F x is applied to it (no gravity considered) Simple theory for deformation under mechanical load applicable Force Torque Stress Pressure E m = Young s modulus I m second moment of area I m is related to width and thickness by F x may be written as: K m = stiffness or spring constant

Mechanical Sensors 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

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

Mechanical Sensors Pressure Microsensors First type of silicon micromachined sensors (late 1950s) Most mature silicon micromechanical device Widespread commercial availability today Automotive is the largest market

Mechanical Sensors 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)

Mechanical Sensors 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

Mechanical Sensors Two basic principle of microaccelerometers Capacitive pickup of the seismic mass movement Piezoresistive pickup of the seismic mass movement

Mechanical Sensors 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

Mechanical Sensors 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

What is a Gyro? p = m v p=momentum, m=mass, v=velocity Basis of a Tuning Fork Vibratory Gyro a v a! = 2v!! ω α=acceleration, v=velocity ω=angular velocity res

Mechanical Sensors 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

Mechanical Sensors 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 (a 0 ) by feedback and then for synchronous oscillators (ω 0x = ω 0y ), the equation simply reduces to Under constant rotation, the steady-state solution to equation above is a constant amplitude b 0 where

Mechanical Sensors 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 (7mA@3.6V) Embedded power-down Embedded self-test Sleep mode High shock and vibration survivability ECOPACK RoHS and Green compliant Source: www.digikey.com

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

Magnetic Sensors Magnetogalvanic Microsensors The sensors principle is based on the Hall-Effect The Hall-Coefficient in semiconductors is quite high Definition When a current I x is passed down a slab of material of length l and thickness d and a perpendicular magnetic flux density B z is applied, a voltage V H appears across the slab perpendicular to I x and B z n = carrier density R H = Hall Coefficient d = material thickness B z = Magnetic field in z-axis I x =current in x-axis V H = Hall voltage (See next slide for Fleming LHR)

Remember Fleming Left Hand Rule (motors) 38

Magnetic Sensors 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 B z and called magnetoresistivity The resistance R of a slab of material depends on the Hall angle θ H and is given by R 0 = R at zero flux density, k g is dependent on the geometry of the slab. Hall angle θ H is the angle by which the direction of the current I x 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.

Magnetic Sensors 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-Al 2 O 3 interface and low at Si-SiO 2 * 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.

Magnetic Sensors 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-SiO 2 interface has a low recombination rate. The poor control of the recombination rates in a magnetodiode makes them problematic (depends on surface roughness/temperature etc). * McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright 2003 by The McGraw-Hill Companies, Inc.

Magnetic Sensors 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

Magnetic Sensors 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 ϑ 0 of the SAW oscillator and hence, The shift in oscillator frequency is a nonlinear function F( ) of the magnetic flux density B z

Magnetic Sensors 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

Magnetic 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: Φ 0 = h/(2e) Hence can measure fields in two ways: Keep current above critical, take field from 0 to measured value. Find critical current.

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 46

Magnetic Sensors SQUIDs The most sensitive of all magnetic sensors is known as a superconducting quantum interference device (SQUID)

Magnetic Sensors 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/ 48

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:

Bio(chemical) Sensors 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

Bio(chemical) Sensors 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 gassensitive 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.

Bio(chemical) Sensors 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 gassensitive 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