Introduction to seismic sensors (subject 3.2) Peter Novotny PACMAN meeting, CERN, 7 October 2014 What is seismic sensor? Instrument which measures ground motion. (Not only earthquakes!) Ground Motion in frequency domain Power Spectral Density β PSD of ground motion Cultural noise Micro seismic peak RMS of ground motion How does seismic sensor work? Relative motion (mass to ground) π¦ π π π Ground acceleration π€ β Absolute motion (ground) Dynamic described by 2nd Newtonβs law => Forced damped harmonic oscillator. ππ + ππ + ππ = βππ How does seismic sensor work? Transfer functions: ππ€π¦ = π πππππ‘ππ£π πππ πππππππππ‘ = π πππ πππ’π‘π πππ πππππππππ‘ Laplace transform ππ + ππ + ππ = βππ ππ€π¦ π πππππ‘ππ£π π£ππππππ‘π¦ = = π πππ πππ’π‘π π£ππππππ‘π¦ ππ€π¦ π πππππ‘ππ£π πππ πππππππππ‘ = = π πππ πππ’π‘π πππππππππ‘πππ Natural frequency ππ = π π !! Information about absolute motion is in relative motion!! π/ππ How we measure relative motion? Using different types of transducers: β’ Pen (mechanical seismometers - obsolete) β’ Resistive β’ Capacitive β’ Piezoelectric β’ Optical (Interferometers, encoders) β’ Inductive β coil + magnet β’ Eddy current β’ LVDT - Linear variable differential transformer β’β¦ Seismic sensor parameters β’ Bandwidth β fmin - fmax β’ Sensitivity S β conversion factor from D,V,A to Volts β’ Self noise N β determine resolution β’ Resolution R = S/N β smallest measureable value β’ Dynamic range β Max/Min measureable value β’ Active/passive β need of power supply β’β¦ requirements on sensor β’ Bandwidth = (0.1 β 100) Hz β’ Resolution = β Sensitivity and β Self-noise β’ for R = 0.1um β’ for NANO STABILISATION R = 0.1nm β’ Stray magnetic fields resistance β’ Weight < few kg & compact size β’ Temperature stability β’ βRadiation resistanceβ => only for stabilisation β’ Price β 4000 sensors for CLIC β’β¦ Geophones β’ Output voltage proportional to ground velocity above natural frequency. β’ Simple and compact solution requirements οΌ Low noise οΌ Weight and size οΌ Price × Natural freq. > 10 Hz × Magnetic resistance Geophones with feedback => seismometers β’ Utilise the feedback control of mass position to extend bandwidth to low frequencies. Seismic mass Leaf spring sensing & force coils Geophones with feedback => seismometers β’ Output voltage is proportional to ground velocity in certain frequency bandwidth. requirements οΌ οΌ οΌ β’ × × Very low noise Resolution Broad bandwidth Weight and size Magnetic resistance Price Accelerometers β’ Output voltage proportional to ground acceleration below natural frequency. β’ Different types: β’ Piezoelectric, piezoresistive, capacitive, Force Balanced (FBA) β’ Possibility of utilizing MEMS Piezoelectric MEMS capacitive acc. Principle + size and price reduction - high self noise requirements οΌ οΌ β’ × × Bandwidth Magnetic resistance Price Higher noise at lower freq. Resolution MEMS realisation Final product Noise in seismic sensors Interferometer β’ Sources of noise: β’ Thermomechanical (Brownian) noise Model Noise [um*s-2/Hz1/2] 87 0.9 @1Hz 0.25 @10Hz 0.1 @100Hz bb rms <4um*s-2 Wilcoxon 731A 0.3 @2Hz 0.1 @10Hz 0.04 @100Hz Brüel & Kjær 8340 14.7 @0.1Hz 14.7 @55Hz 393B31 0.6 @1Hz 0.1 @10Hz 0.04 @100Hz SF1600S.A 3 in 0.1-100Hz BW TSA-100S rms 0.22@1Hz rms 0.2@10Hz rms 0.7@10Hz Producer Endevco β’ Molecular collisions with mass β’ Thermoelectrical (Johnson) noise β’ Random thermal excitation of charge carriers β’ Schottky noise Capacitive sensor β’ Random movement of charge across potential barriers β’ Flicker 1/f noise β’ Impurities in semiconductors β’ Discretisation noise β’ ADC converters PCB COLIBRYS Metrozet Classification of seismic sensors Output voltage proportional to: β’ Displacement β’ Velocity: β’ geophones β’ feedback geophones = seismometers β’ Acceleration: β’ Accelerometers β’ Force Balanced Accelerometers (FBA) By type of sensing transducer: β’ β’ β’ β’ β’ Capacitive Inductive Piezoelectric Optical β¦ Other classifications: β’ Bandwidth β’ Short period, Long period, Broad band, Very BB β’ Signal intensity β’ Strong motion, High sensitivity Few sensors and their producers Producer GURALP ion (Sensor Nederland B.V.) Sercel (Mark Products) Kinemetrics (Streckeisen) Nanometrics REFTEK Lennartz electronic Metrozet Eentec SINUS GeoSIG Endevco Wilcoxon DYTRAN DJB Instruments MMF Brüel & Kjær PCB COLIBRYS R-sensors METTECH GEOTECH INSTRUMENTS type of sensors Seismometers geophones geophones Seismometers + FBA Seismometers Seismometers Seismometers Seismometers + FBA FDBCK MET geophones + FBA Piezo-velocity sensor geophones + FBA accelerometers accelerometers accelerometers accelerometers accelerometers accelerometers accelerometers accelerometers MET geophones + acc MET Seismometers Secondments β’ Development of the seismic sensor for stabilisation of the Final Focus (of CLIC) β’ Equivalent of 7 months β’ High precision mechanic manufacturer (already started) β’ Their measuring and control system β’ 3 months starting in March 2015 β’ Project: β’ Measurement and adjustment techniques for precision applications Thank you for your attention!
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