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Asics for MEMS BRILLANT Grégory 2 th of October 2006

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  • Asics for MEMSBRILLANT Grgory2th of October 2006

  • Overview

    Smart Sensor Interface ElectronicsEquivalent circuit representation of electromachanical transducers

  • Smart Sensor Interface Electronics

  • OverviewObject Oriented DesignParasitic EffectsAnalog to Digital ConversionHigh accuracy over a wide Dynamic rangePresentation of two systems

  • IntroductionInformation processing systems need sensors to acquire physical, mechanical and chemical informationSensors are inescapable in applications such as smart cars or smart homesBut: They areSolution: Smart Sensor Systems combine :SensorsSignal conditioningADCBus interfacingAnd self testing, auto-calibration, data evaluation and identification,

    EXPENSIVE

  • Object oriented Design of sensor systemWhen designing sensor systems, traditional Top/Down and Bottom/Up approach are limitedInterdisciplinary and open characters of sensor subsystemsLong design time and inflexible designsSolution: Object oriented Design. The result of the object-oriented design is a detail description how the system can be built, using objects Save costs and speed up the designIf its possible to implement the sensor element and its interface on a same chip, we speak about Smart Sensor

  • Parasitic effects in sensing elementsExcitation signals for sensing elements are usually square-wave (and not sinusoidal). Care has to be taken to avoid undesired electro-physical interactionElectrical excitation of a resistive temperature sensor causes self heating measurement errorsIn conductivity sensors the excitation can cause electrolysis corrosion

  • Parasitic effects in sensing elementsSensing elements deliver an electrical output representing the measurementBut: there are parasitic electrical effectsCapacitive humidity sensors are often shunted by a parasitic resistive componentResistive sensors are often shunted by parasitic capacitorsThe various components are founded by analyzing impedance measurement at various frequencies. Small-sized, low power integrated circuit must be used to achieve this measurements The use of additional sensor elements improve the reliability

  • Parasitic effects in sensing elementsConnecting wires and cables can affect the measurementSolution: two-port measurements 4-wire technique is applied to measure a low-ohmic sensor. The interface chip delivers an excitation current and the voltage over the sensor is measured using a high impedance input amplifierThe dual case is applied to measure a high-ohmic sensor admittance

  • Analog to digital conversionThe sensor signal is often converted to a voltage signal Standard ADC can be usedCapacitive sensing element: A/D converter requires an analog input voltage Problem: Complication of the front end ADC design because of the introduction of:Many additional transfer parametersBiasing quantitiesConversion steps

  • Analog to digital conversionSolution: sample and hold, quantization, digital filtering and conversion can be implemented in the DSP microcontrollerDSP or microcontroller are well equipped to measure frequency or time interval using of period-modulated signal

  • High accuracy over a wide Dynamic range: errorsTwo kinds of errors:Systematic errors: inaccuracy of the system parameters calibratingRandom errors: interferences, noise and instability filtering, separating common mode and differential mode signal,Calibration: sensor-under-test is compared to another one of superior qualityTrimming: sensor behavior is altered permanently to make its characteristics match the nominal as close as possible

  • High accuracy over a wide Dynamic range: choppingBut: calibration and trimming have to performed under certain conditions with respect to the temperature, supply voltage and time conditions during sensor operationSolution: Chopping techniques. Reduce random errors, noise, low frequency interferences and offsetImplementation: switches interchange the wires of a signal source at a high frequency

    Common chopper: +,-,+,-,Improved chopper: +,-,-,+,+,This sampling sequence result in a filter operation applied to the interferences

  • High accuracy over a wide Dynamic range: auto calibrationTwo signal approach: measure of a reference signal S1 in exactly the same way as the input signal SxRatio Sx/S1 or difference Sx-S1 is used to eliminates errorsThree signal approach: more accurate. Measure of two reference signals. (Sx-S1)/(S2-S1) is used.

  • High accuracy over a wide Dynamic range: amplificationDuring auto-calibration, the signals are processed in an identical wayThe system should be linear or well characterized over the full signal range this poses a problem when the signal are not in the same range of magnitudeTo achieve a high accuracy, signals should have a high dynamic range, but that is not often the caseAmplification or division by a scaling factor A

  • High accuracy over a wide Dynamic range: amplificationProblem: realize A without loosing precisionDynamic feedback instrumentation amplifier can solve this problemResistive load K=u+v+w+z Dynamic feed back is made by rotation of the resistor chainMismatches between resistors are critical 6*K switches

  • High accuracy over a wide Dynamic range: amplificationDEM amplifier can also be a solutionPossible implementation: switched-capacitorsThe rotation of the capacitors at each clock cycle can almost eliminates the effect of capacitor mistmatching

  • High accuracy over a wide Dynamic range: divisionInstead amplify the smallest signals, division of the strongest signals can also be appliedOne possible realizationA resistive voltage divider (Nr resistors) combined with a capacitive voltage divider (Nc capacitors)Division ratio: NcNr

  • Universal sensor interfaceThe Universal Sensor Interface is a complete front-end for sensor systemsThe output is based on a period modulator oscillatorThe USI converts the signals of sensing elements into period-modulated signals microcontroller and DSP compatibleSignal processing in the USIThe input signal is selected by the multiplexerChopped signal conversionPeriod length conversion

  • A wide range voltage processorExample: measurement system for thermocouple voltagesTwo measured signals: thermocouple voltage Vx and reference junction temperature TjVoff is measured to allow offset compensation All algorithmic signal processing is performed by the microcontroller. The voltages are firstly converted to the time domain

  • ConclusionIn 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

  • OverviewLumped-parameter electromechanical systemsElementary Lumped-parameter transducersEquivalent circuit representationCoupling of the transducers to the outside worldSome examples of transducers

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

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

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

  • Elementary Lumped-parametertransducers: configurationsLinear transducers are mathematically more easiest to studyLinear behavior is achieved for small signal variations around equilibrium levelsFour basics electromechanical lumped-parameter transducers:Transverse electrostatic transducerIn-plane electrostatic transducerElectromagnetic transducerElectrodynamics transducer

  • Elementary Lumped-parametertransducers: equationsCharacteristic equations: linear relations between small-signal variations of the port variable around the bias pointMatrix representationsMatrix B: effort variable as a function of state variableMatrix T: relates the effort-flow variables at the electrical port directly to those at the mechanical port

  • Elementary Lumped-parametertransducers: equationsThe coupling factor k represents the electromechanical energy conversion in lossless transducersA coupling factor of 0 means no interactions A state of equilibrium exists for 0
  • Equivalent circuit representationThere is an analogy in the mathematical descriptions between electric and mathematical phenomenaA series connection in the mechanical circuit becomes parallel in the equivalent electrical circuit

  • Equivalent circuit representationThe construction of the equivalent networks starts wit

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