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Asics for MEMS BRILLANT Grégory 2th of October 2006 Overview I. Smart Sensor Interface Electronics II. Equivalent circuit representation of electromachanical transducers October 2006 Smart Sensor Interface Electronics Overview I. II. III. IV. V. October 2006 Object Oriented Design Parasitic Effects Analog to Digital Conversion High accuracy over a wide Dynamic range Presentation of two systems Introduction Information processing systems need sensors to acquire physical, mechanical and chemical information Sensors are inescapable in applications such as smart cars or smart homes But: They areEXPENSIVE Solution: Smart Sensor Systems combine : Sensors Signal conditioning ADC Bus interfacing And self testing, auto-calibration, data evaluation and identification, … October 2006 Object oriented Design of sensor system When designing sensor systems, traditional Top/Down and Bottom/Up approach are limited Interdisciplinary and open characters of sensor subsystems Long design time and inflexible designs Solution: 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 design If it’s possible to implement the sensor element and its interface on a same chip, we speak about Smart Sensor October 2006 Parasitic effects in sensing elements Excitation signals for sensing elements are usually square-wave (and not sinusoidal). Care has to be taken to avoid undesired electro-physical interaction Electrical excitation of a resistive temperature sensor causes self heating → measurement errors In conductivity sensors the excitation can cause electrolysis → corrosion October 2006 Parasitic effects in sensing elements Sensing elements deliver an electrical output representing the measurement But: there are parasitic electrical effects Capacitive humidity sensors are often shunted by a parasitic resistive component Resistive sensors are often shunted by parasitic capacitors The 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 October 2006 Parasitic effects in sensing elements Connecting wires and cables can affect the measurement Solution: 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 amplifier The dual case is applied to measure a high-ohmic sensor admittance October 2006 Analog to digital conversion The sensor signal is often converted to a voltage signal → Standard ADC can be used Capacitive 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 parameters Biasing quantities Conversion steps October 2006 Analog to digital conversion Solution: sample and hold, quantization, digital filtering and ∑Δ conversion can be implemented in the DSP microcontroller DSP or microcontroller are well equipped to measure frequency or time interval → using of period-modulated signal October 2006 High accuracy over a wide Dynamic range: errors Two kinds of errors: Systematic errors: inaccuracy of the system parameters → calibrating Random errors: interferences, noise and instability → filtering, separating common mode and differential mode signal,… Calibration: sensor-under-test is compared to another one of superior quality Trimming: sensor behavior is altered permanently to make its characteristics match the nominal as close as possible October 2006 High accuracy over a wide Dynamic range: chopping But: calibration and trimming have to performed under certain conditions with respect to the temperature, supply voltage and time conditions during sensor operation Solution: Chopping +,-,+,-,… techniques. Reduce Common chopper: random errors, noise, low frequency Improved chopper: +,-,-,+,+,… interferences and offset This sampling sequence result in a filter Implementation: switches interchange the operation to the at interferences wires of a applied signal source a high ≠ frequency October 2006 High accuracy over a wide Dynamic range: auto calibration Two signal approach: measure of a reference signal S1 in exactly the same way as the input signal Sx Ratio Sx/S1 or difference Sx-S1 is used to eliminates errors Three signal approach: more accurate. Measure of two reference signals. (Sx-S1)/(S2-S1) is used. October 2006 High accuracy over a wide Dynamic range: amplification During auto-calibration, the signals are processed in an identical way The 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 magnitude To achieve a high accuracy, signals should have a high dynamic range, but that is not often the case Amplification or division by a scaling factor A October 2006 High accuracy over a wide Dynamic range: amplification Problem: realize A without loosing precision Dynamic feedback instrumentation amplifier can solve this problem Resistive load K=u+v+w+z Dynamic feed back is made by rotation of the resistor chain Mismatches between resistors are critical 6*K switches October 2006 High accuracy over a wide Dynamic range: amplification DEM amplifier can also be a solution Possible implementation: switchedcapacitors The rotation of the capacitors at each clock cycle can almost eliminates the effect of capacitor mistmatching October 2006 High accuracy over a wide Dynamic range: division Instead amplify the smallest signals, division of the strongest signals can also be applied One possible realization A resistive voltage divider (Nr resistors) combined with a capacitive voltage divider (Nc capacitors) Division ratio: NcNr October 2006 Universal sensor interface The Universal Sensor Interface is a complete front-end for sensor systems The output is based on a period modulator oscillator The USI converts the signals of sensing elements into period-modulated signals → microcontroller and DSP compatible Signal processing in the USI The input signal is selected by the multiplexer Chopped signal conversion Period length conversion October 2006 A wide range voltage processor Example: measurement system for thermocouple voltages Two measured signals: thermocouple voltage Vx and reference junction temperature Tj Voff is measured to allow offset compensation All algorithmic signal processing is performed by the microcontroller. The voltages are firstly converted to the time domain October 2006 Conclusion In the smart sensor systems presented, measurement techniques are implemented using a limited number of low-cost, lowpower 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. October 2006 Equivalent circuit representation of electromachanical transducers Overview I. II. III. IV. V. October 2006 Lumped-parameter electromechanical systems Elementary Lumped-parameter transducers Equivalent circuit representation Coupling of the transducers to the outside world Some examples of transducers Introduction A 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 –versa Example: microphone in which a sound pressure is converted into an electrical signal Equivalent 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. October 2006 Lumped-parameter electromechanical systems Lumped parameter (or discrete) system: physical properties (mass, capacitance, inductance,…) are concentrated or lumped into single physical elements The 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 system Example: basic configuration of an electrostatic transducer October 2006 Energy exchange Exchange of energy of a transducer and the outside world is achieved trough ports: pair of conjugate dynamic variables, the effort variable and the flow October 2006 Elementary Lumped-parameter transducers: configurations Linear transducers are mathematically more easiest to study Linear behavior is achieved for small signal variations around equilibrium levels Four basics electromechanical lumpedparameter transducers: Transverse electrostatic transducer In-plane electrostatic transducer Electromagnetic transducer Electrodynamics transducer October 2006 Elementary Lumped-parameter transducers: equations Characteristic equations: linear relations between small-signal variations of the port variable around the bias point Matrix representations Matrix B: effort variable as a function of state variable Matrix T: relates the effort-flow variables at the electrical port directly to those at the mechanical port October 2006 Elementary Lumped-parameter transducers: equations The coupling factor k represents the electromechanical energy conversion in lossless transducers A coupling factor of 0 means no interactions A state of equilibrium exists for 0<k<1 Typical values for k are between 0.05 and 0.25 October 2006 Equivalent circuit representation There is an analogy in the mathematical descriptions between electric and mathematical phenomena A series connection in the mechanical circuit becomes parallel in the equivalent electrical circuit October 2006 Equivalent circuit representation The construction of the equivalent networks starts with the splinted transfer matrix of the electrostatic transducers Center matrix: transducer Left matrix: electrical admittance Right matrix: mechanical impedance Each of the constituent can be represented by an equivalent network October 2006 Equivalent circuit representation There is no one way to decompose a matrix But: each decomposition has its own network representation The choice of which circuit to use is dictated by the application October 2006 Coupling of the transducers to the outside world The exchange of energy of the transducer and the outside world is done trough ports Laws of equilibrium link the transducer via their port relation to the external elements Electrical parts: Kirchoff’s voltage and current laws Mechanical parts: Newton law (∑Fi=0) and continuity of space (∑Ui=0, U: incremental velocity) The mechanical laws are directly obtained by invoking the Kirchoff’s laws to the mechanical part in the equivalent circuit representation October 2006 Examples of lumped-parameters electromecanical sytems I. II. III. IV. October 2006 Condenser Microphone In-plane parallel microresonators Vibration sensors Electromechanical feedback Condenser microphone, force and displacement transducers Operating principles: the force to be measured is exerted on the mass. The motion of the mass is converted into an electrical signal, a current, which flows in part through a resistors → production of an output voltage. This voltage is a measure for the applied force or displacement. October 2006 Condenser microphone, force and displacement transducers October 2006 Condenser microphone, force and displacement transducers If the applied force is the result of an acoustic pressure, the transducer can be used as an electrostatic or condenser microphone October 2006 In-plane parallel microresonators In-plane parallel microresonators using electrostatic interdigitated structures for excitation and detection of the vibrational motion are used as transducing elements in a wide variety of applications October 2006 Vibration sensors Vibration sensors are employed for measurements on moving vehicles, on buildings, or on machinery or as seismic pickups The basic principle of vibration measurements is simply to measure the relative displacement of a mass connected to the vibrating body. The transducer detects the mass displacement Xm relative to the displacement Xin of the vibrating body October 2006 Electromechanical feedback Electromechanical feedback (or force balancing) is often employed for applications requiring a great accuracy The system measures the force Fm exerted directly on the mass The upper capacitor senses the induced mass displacement resulting in a change of the plate charge October 2006 Electromechanical feedback The output voltage of the charge amplifier Va is next amplified by a high-gain (servo) amplifier T The output voltage Vout is fed back to the lower capacitor. This generates an electrostatic force which is proportional to the relative mass displacement and which always opposes motion of the mass from the rest position. This way, the mass itself is kept very close to the zero-displacement position October 2006 Conclusion The majority if the circuits presents 3 different parts: An electrical part An electromechanical coupling part A mechanical part The equivalent circuits can be used to determine the frequency and the transient response of the transducer The equivalent circuit theory applied to the study of transducer characteristics is a basis for further investigations October 2006