Asics for MEMS BRILLANT Grégory 2 th of October 2006

Asics for MEMS

BRILLANT GrégoryBRILLANT Grégory

22th th of October 2006of October 2006

October 2006October 2006

Overview

I.I. Smart Sensor Interface ElectronicsSmart Sensor Interface Electronics

II.II. Equivalent circuit representation of Equivalent circuit representation of

electromachanical transducerselectromachanical transducers

Smart Sensor Interface Electronics


Overview

I.I. Object Oriented DesignObject Oriented Design

II.II. Parasitic EffectsParasitic Effects

III.III. Analog to Digital ConversionAnalog to Digital Conversion

IV.IV. High accuracy over a wide High accuracy over a wide Dynamic rangeDynamic range

V.V. Presentation of two systemsPresentation of two systems


Introduction

Information processing systems need sensors to Information processing systems need sensors to acquire physical, mechanical and chemical acquire physical, mechanical and chemical informationinformationSensors are inescapable in applications such as Sensors are inescapable in applications such as smart cars or smart homessmart cars or smart homesBut:But: They areThey areSolution:Solution: Smart Sensor Systems Smart Sensor Systems combine :combine :

SensorsSensorsSignal conditioningSignal conditioningADCADCBus interfacingBus interfacingAnd self testing, auto-calibration, data evaluation and And self testing, auto-calibration, data evaluation and identification, …identification, …

EXPENSIVEEXPENSIVE


Object oriented Design of sensor system

When designing sensor systems, traditional When designing sensor systems, traditional Top/Down and Bottom/Up approach are limitedTop/Down and Bottom/Up approach are limited

Interdisciplinary and open characters of sensor Interdisciplinary and open characters of sensor subsystemssubsystemsLong design time and inflexible designsLong design time and inflexible designs

Solution:Solution: Object oriented DesignObject oriented Design. . The result of the object-oriented design is a detail The result of the object-oriented design is a detail description description howhow the system can be built, using objects the system can be built, using objects Save costs and speed up the designSave costs and speed up the design

If it’s possible to implement the sensor element If it’s possible to implement the sensor element and its interface on a same chip, we speak about and its interface on a same chip, we speak about Smart SensorSmart Sensor


Parasitic effects in sensing elements

Excitation signals for sensing elements are Excitation signals for sensing elements are usually square-wave (and not sinusoidal). usually square-wave (and not sinusoidal). Care Care hashas to be taken to avoid undesired to be taken to avoid undesired electro-physical interactionelectro-physical interaction

Electrical excitation of a resistive temperature Electrical excitation of a resistive temperature sensor causes self heating sensor causes self heating →→ measurement measurement errorserrors

In conductivity sensors the excitation can In conductivity sensors the excitation can cause electrolysis cause electrolysis → → corrosioncorrosion



Sensing elements deliver an electrical output Sensing elements deliver an electrical output representing the measurementrepresenting the measurementBut:But: there are there are parasitic electrical effectsparasitic electrical effects

Capacitive humidity sensors are often shunted by a Capacitive humidity sensors are often shunted by a parasitic resistive componentparasitic resistive componentResistive sensors are often shunted by parasitic Resistive sensors are often shunted by parasitic capacitorscapacitors

The various components are founded by The various components are founded by analyzing impedance measurement at various analyzing impedance measurement at various frequencies. frequencies. Small-sized, low power integrated circuit must be Small-sized, low power integrated circuit must be used to achieve this measurements used to achieve this measurements The use of additional sensor elements improve The use of additional sensor elements improve the reliabilitythe reliability



Connecting wires and cables can affect the Connecting wires and cables can affect the measurementmeasurementSolution:Solution: two-port measurements two-port measurements

4-wire technique is applied to 4-wire technique is applied to measure a low-ohmic sensor. measure a low-ohmic sensor. The interface chip delivers an The interface chip delivers an excitation current and the excitation current and the voltage over the sensor is voltage over the sensor is measured using a high measured using a high impedance input amplifierimpedance input amplifierThe dual case is applied to The dual case is applied to measure a high-ohmic sensor measure a high-ohmic sensor admittance admittance


Analog to digital conversion

The sensor signal is often converted to a The sensor signal is often converted to a voltage signal voltage signal →→ Standard ADC can be Standard ADC can be usedused

Capacitive sensing element: A/D converter Capacitive sensing element: A/D converter requires an analog input voltage requires an analog input voltage → → Problem: Problem: Complication of the front end ADC Complication of the front end ADC design because of the introduction of:design because of the introduction of:

Many additional transfer parametersMany additional transfer parameters

Biasing quantitiesBiasing quantities

Conversion stepsConversion steps


Analog to digital conversion

Solution: Solution: sample and hold, quantization, digital sample and hold, quantization, digital filtering and ∑Δ conversion can be implemented filtering and ∑Δ conversion can be implemented in the DSP microcontrollerin the DSP microcontrollerDSP or microcontroller are well equipped to DSP or microcontroller are well equipped to measure frequency or time interval measure frequency or time interval → using of → using of period-modulated signalperiod-modulated signal


High accuracy over a wide Dynamic range: errors

Two kinds of errors:Two kinds of errors:Systematic errors:Systematic errors: inaccuracy of the system inaccuracy of the system parameters parameters → → calibratingcalibratingRandom errors:Random errors: interferences, noise and interferences, noise and instability instability → → filtering, separating common mode and filtering, separating common mode and differential mode signal,…differential mode signal,…

Calibration:Calibration: sensor-under-test is compared to sensor-under-test is compared to another one of superior qualityanother one of superior qualityTrimming:Trimming: sensor behavior is altered sensor behavior is altered permanently to make its characteristics match permanently to make its characteristics match the nominal as close as possible the nominal as close as possible


High accuracy over a wide Dynamic range: chopping

But:But: calibration and trimming have to calibration and trimming have to performed under certain conditions with performed under certain conditions with respect to the temperature, supply voltage respect to the temperature, supply voltage and time and time conditions during sensor conditions during sensor operationoperationSolution:Solution: Chopping techniques.Chopping techniques. Reduce Reduce random errors, noise, low frequency random errors, noise, low frequency interferences and offsetinterferences and offsetImplementation:Implementation: switches interchange the switches interchange the wires of a signal source at a high wires of a signal source at a high frequencyfrequency

≠Common chopperCommon chopper: +,-,+,-,…: +,-,+,-,…

Improved chopper:Improved chopper: +,-,-,+,+,… +,-,-,+,+,…

This sampling sequence result in a filter This sampling sequence result in a filter operation applied to the interferencesoperation applied to the interferences


High accuracy over a wide Dynamic range: auto

calibrationTwo signal approach:Two signal approach: measure of a measure of a reference signal S1 in exactly the same reference signal S1 in exactly the same way as the input signal Sxway as the input signal Sx

Ratio Sx/S1 or difference Sx-S1 is used Ratio Sx/S1 or difference Sx-S1 is used to eliminates errorsto eliminates errors

Three signal approach:Three signal approach: more accurate. more accurate. Measure of two reference signals. Measure of two reference signals.

(Sx-S1)/(S2-S1) is used.(Sx-S1)/(S2-S1) is used.


High accuracy over a wide Dynamic range:

amplificationDuring auto-calibration, the signals are During auto-calibration, the signals are processed in an identical wayprocessed in an identical wayThe system should be linear or well The system should be linear or well characterized over the full signal range characterized over the full signal range → → this poses a problem when the signal are not in this poses a problem when the signal are not in the same range of magnitudethe same range of magnitudeTo achieve a high accuracy, signals should have To achieve a high accuracy, signals should have a high dynamic range, a high dynamic range, but that is not often the but that is not often the casecaseAmplification or division by a scaling factor AAmplification or division by a scaling factor A



amplificationProblem: Problem: realize A without loosing realize A without loosing precisionprecision

Dynamic feedback instrumentation Dynamic feedback instrumentation amplifier amplifier can solve this problemcan solve this problem

Resistive load K=u+v+w+z Resistive load K=u+v+w+z

Dynamic feed back is made by rotation of Dynamic feed back is made by rotation of the resistor chainthe resistor chain

Mismatches between resistors are critical Mismatches between resistors are critical

6*K switches6*K switches



amplificationDEM amplifier DEM amplifier can also be a solutioncan also be a solution

Possible implementation: switched-Possible implementation: switched-capacitorscapacitors

The rotation of the capacitors at each The rotation of the capacitors at each clock cycle can almost eliminates the clock cycle can almost eliminates the effect of capacitor mistmatchingeffect of capacitor mistmatching


High accuracy over a wide Dynamic range: division

Instead amplify the smallest signals, Instead amplify the smallest signals, division of the strongest signals can also division of the strongest signals can also be appliedbe applied

One possible realizationOne possible realizationA resistive voltage divider (Nr resistors) A resistive voltage divider (Nr resistors) combined with a capacitive voltage divider (Nc combined with a capacitive voltage divider (Nc capacitors)capacitors)

Division ratio: NcNrDivision ratio: NcNr


Universal sensor interface

The The Universal Sensor InterfaceUniversal Sensor Interface is a is a complete front-end for sensor systemscomplete front-end for sensor systemsThe output is based on a period modulator The output is based on a period modulator oscillatoroscillatorThe USI converts the signals of sensing The USI converts the signals of sensing elements into period-modulated signals elements into period-modulated signals → → microcontroller and DSP compatiblemicrocontroller and DSP compatibleSignal processing in the USISignal processing in the USI

The input signal is selected by the multiplexerThe input signal is selected by the multiplexerChopped signal conversionChopped signal conversionPeriod length conversionPeriod length conversion


A wide range voltage processor

Example:Example: measurement system for measurement system for thermocouple voltagesthermocouple voltagesTwo measured signals:Two measured signals: thermocouple thermocouple voltage Vx and reference junction voltage Vx and reference junction temperature Tjtemperature TjVoff is measured to allow offset Voff is measured to allow offset compensation compensation All algorithmic signal processing is All algorithmic signal processing is performed by the microcontroller. The performed by the microcontroller. The voltages are firstly converted to the time voltages are firstly converted to the time domaindomain


Conclusion

In the smart sensor systems presented, measurement techniques are implemented using a limited number of low-cost, low-power integrated circuits only.By applying synchronous detection, auto calibration and advanced chopping, high immunity is obtained for interfering signals, 1/f noise and parameter drift. The dynamic range of the signals can be extended using dynamic amplifiers and dynamic dividers.

Equivalent circuit representation of

electromachanical transducers


Overview

I.I. Lumped-parameter Lumped-parameter electromechanical systemselectromechanical systems

II.II. Elementary Lumped-parameterElementary Lumped-parametertransducerstransducers

III.III. Equivalent circuitEquivalent circuit representation representation

IV.IV. Coupling of the transducersCoupling of the transducersto the outside worldto the outside world

V.V. Some examples of transducersSome examples of transducers


Introduction

A A transducertransducer is a device that converts one type of is a device that converts one type of energy to another, or responds to a physical energy to another, or responds to a physical parameter. A transducer is in its fundamental parameter. A transducer is in its fundamental form a passive component. form a passive component. Electomechanical transducersElectomechanical transducers are used to convert are used to convert electrical energy into mechanical energy and vice electrical energy into mechanical energy and vice –versa–versaExample:Example: microphone in which a sound pressure microphone in which a sound pressure is converted into an electrical signalis converted into an electrical signalEquivalent circuit approach:Equivalent circuit approach: the electrical and the electrical and mechanical portions of the transducers are mechanical portions of the transducers are represented brepresented by electrical equivalents y electrical equivalents → single → single representation of device that operate in more than one representation of device that operate in more than one energy domain.energy domain.


Lumped-parameter electromechanical systemsLumped parameter (or discrete) system:Lumped parameter (or discrete) system: physical properties (mass, capacitance, physical properties (mass, capacitance, inductance,…) are concentrated or lumped inductance,…) are concentrated or lumped into single physical elementsinto single physical elementsThe parameters which involve ordinary The parameters which involve ordinary differential equations are called linear differential equations are called linear lumped parameters. lumped parameters. Lumped-parameter modeling is valid as Lumped-parameter modeling is valid as long as the wavelength of the signal is long as the wavelength of the signal is greater than all dimensions of the systemgreater than all dimensions of the systemExample:Example: basic configuration of an basic configuration of an electrostatic transducer electrostatic transducer


Energy exchange

Exchange of energy of a transducer and Exchange of energy of a transducer and the outside world is achieved trough the outside world is achieved trough ports: ports: pair of conjugate dynamic variables, the pair of conjugate dynamic variables, the effort variableeffort variable and the and the flow flow


Elementary Lumped-parameter

transducers: configurationsLinear transducersLinear transducers are mathematically are mathematically more easiest to studymore easiest to studyLinear behavior is achieved for small Linear behavior is achieved for small signal variations around signal variations around equilibriumequilibrium levels levelsFour basics electromechanical lumped-Four basics electromechanical lumped-parameter transducers:parameter transducers:

Transverse electrostaticTransverse electrostatic transducertransducer

In-plane electrostatic In-plane electrostatic transducertransducer

Electromagnetic transducerElectromagnetic transducerElectrodynamics transducerElectrodynamics transducer



transducers: equationsCharacteristic equations: Characteristic equations: linear relations linear relations between small-signal variations of the port between small-signal variations of the port variable around the bias pointvariable around the bias point

Matrix representationsMatrix representations

Matrix B:Matrix B: effort variable as a function of effort variable as a function of state variablestate variable

Matrix T:Matrix T: relates the effort-flow variables relates the effort-flow variables at the electrical port directly to those at at the electrical port directly to those at the mechanical portthe mechanical port



transducers: equationsThe The coupling factor kcoupling factor k represents the represents the electromechanical energy conversion in electromechanical energy conversion in lossless transducerslossless transducersA coupling factor of 0 means no A coupling factor of 0 means no interactions interactions A state of equilibrium exists for 0<k<1A state of equilibrium exists for 0<k<1Typical values for k are between 0.05 and Typical values for k are between 0.05 and 0.25 0.25


Equivalent circuit representation

There is an analogy in the mathematical There is an analogy in the mathematical descriptions between electric and mathematical descriptions between electric and mathematical phenomenaphenomenaA series connection in the mechanical circuit A series connection in the mechanical circuit becomes parallel in the equivalent electrical becomes parallel in the equivalent electrical circuitcircuit



The construction of the equivalent networks The construction of the equivalent networks starts with the splinted transfer matrix of the starts with the splinted transfer matrix of the electrostatic transducerselectrostatic transducers

Center matrix: transducerCenter matrix: transducerLeft matrix: electrical admittanceLeft matrix: electrical admittanceRight matrix: mechanical impedanceRight matrix: mechanical impedance

Each of the constituent can be represented by an Each of the constituent can be represented by an equivalent network equivalent network



There is no one way to decompose a matrixThere is no one way to decompose a matrix

But:But: each decomposition has its own network each decomposition has its own network representationrepresentation

The choice of which circuit to use is dictated by The choice of which circuit to use is dictated by the applicationthe application


Coupling of the transducersto the outside world

The exchange of energy of the transducer and The exchange of energy of the transducer and the outside world is done trough the outside world is done trough portsportsLaws of equilibrium link the transducer via their Laws of equilibrium link the transducer via their port relation to the external elementsport relation to the external elementsElectrical parts:Electrical parts: Kirchoff’s voltage and current Kirchoff’s voltage and current lawslawsMechanical parts:Mechanical parts: Newton law (∑Fi=0) and Newton law (∑Fi=0) and continuity of space (∑Ui=0, U: incremental continuity of space (∑Ui=0, U: incremental velocity) velocity) The mechanical laws are directly obtained by The mechanical laws are directly obtained by invoking the Kirchoff’s laws to the mechanical invoking the Kirchoff’s laws to the mechanical part in the equivalent circuit representationpart in the equivalent circuit representation


Examples of lumped-parameters

electromecanical sytems

I.I. Condenser MicrophoneCondenser Microphone

II.II. In-plane parallel microresonatorsIn-plane parallel microresonators

III.III. Vibration sensorsVibration sensors

IV.IV. Electromechanical feedbackElectromechanical feedback


Condenser microphone, force and displacement

transducersOperating principles:Operating principles:

the force to be measured is exerted on the mass. the force to be measured is exerted on the mass. The motion of the mass is converted into an electrical signal, a The motion of the mass is converted into an electrical signal, a current, which flows in part through a resistors current, which flows in part through a resistors →→ production production of an output voltage. of an output voltage. This voltage is a measure for the applied force or This voltage is a measure for the applied force or displacement.displacement.



transducers



transducersIf the applied force is the result of an If the applied force is the result of an acoustic pressure, the transducer can be acoustic pressure, the transducer can be used as an used as an electrostaticelectrostatic or or condenser condenser microphone microphone


In-plane parallelmicroresonators

In-plane parallel In-plane parallel microresonators using microresonators using electrostatic electrostatic interdigitated interdigitated structures for structures for excitation and excitation and detection of the detection of the vibrational motion are vibrational motion are used as transducing used as transducing elements in a wide elements in a wide variety of applications variety of applications


Vibration sensorsVibration sensors are Vibration sensors are employed for employed for measurements on moving measurements on moving vehicles, on buildings, or vehicles, on buildings, or on machinery or as seismic on machinery or as seismic pickups pickups The basic principle of The basic principle of vibration measurements is vibration measurements is simply to measure the simply to measure the relative displacement of a relative displacement of a mass connected to the mass connected to the vibrating body. vibrating body. The transducer detects the The transducer detects the mass displacement Xmass displacement Xm m relative to the relative to the displacement Xdisplacement Xin in of the of the vibrating bodyvibrating body


Electromechanical feedback

Electromechanical Electromechanical feedback (or force feedback (or force balancing) is often balancing) is often employed for employed for applications requiring applications requiring a great accuracya great accuracyThe system measures The system measures the force the force Fm Fm exerted exerted directly on the mass directly on the mass The upper capacitor The upper capacitor senses the induced senses the induced mass displacement mass displacement resulting in a change resulting in a change of the plate charge of the plate charge


Electromechanical feedback

The output voltage of the The output voltage of the charge amplifier Vcharge amplifier Va a is next is next amplified by a high-gain amplified by a high-gain (servo) amplifier T(servo) amplifier TThe output voltage VThe output voltage Voutout is is fed back to the lower fed back to the lower capacitor. capacitor. This generates an This generates an electrostatic force which is electrostatic force which is proportional to the relative proportional to the relative mass displacement and mass displacement and which always opposes which always opposes motion of the mass from motion of the mass from the rest position. the rest position. This way, the mass itself is This way, the mass itself is kept very close to the zero-kept very close to the zero-displacement positiondisplacement position


Conclusion

The majority if the circuits presents 3 The majority if the circuits presents 3 different parts:different parts:

An electrical partAn electrical partAn electromechanical coupling partAn electromechanical coupling partA mechanical partA mechanical part

The equivalent circuits can be used to The equivalent circuits can be used to determine the frequency and the transient determine the frequency and the transient response of the transducerresponse of the transducerThe equivalent circuit theory applied to The equivalent circuit theory applied to the study of transducer characteristics is a the study of transducer characteristics is a basis for further investigationsbasis for further investigations

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Asics for MEMS BRILLANT Grégory 2 th of October 2006