DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO...

DAM-Île de FranceDép

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10/04/23 DIF/DASE/B.ALCOVERRO 1

INFRASOUND SENSOR CALIBRATION DEVICE 13/11/2001

Infrasound Sensor Infrasound Sensor Dynamic CalibratorDynamic Calibrator

ALCOVERRO Benoît

DASE/TMG


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Dynamic CalibrationDynamic Calibration

Dynamic calibration requires an infrasonic source with:– precise pressure signal applied to the sensor (some percent).

– no phase distortion introduced by the calibrator (or measured error).

– [0.001 Hz - 100 Hz] linear frequency range needed to verify the CTBT band [0.02 Hz - 4 Hz] (flat in amplitude and smooth phase).

– Low sensitivity to noisy signals.

Solution to generate a pressure:– Variation of a sealed volume at constant

temperature.– If V/V0 small, at first order,

P = P0 . P = P0 . VV P is static pressure dependant.

Volume variation V

Sealed volume V0

Static pressure

P0

Pressure generated P0+ P


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The sensor is connected to a sealed cavity where the pressure is generated.

The use of small volumes (< 50 l) & light piston (< 100 g ) could increase the linearity up to 100 Hz (linear amplitude & smooth phase responses up to 10 Hz).

Low energy required to generate the pressure ( < 5 W for 10 Pa generated by Electro-mechanical driver)

Small size portable solution.

Basic Acoustical PrincipleBasic Acoustical Principle

V

Small sealed volume (30 l)

External pressure

P0

P0+ P

Displacementof piston

Sensor to be calibrated

Connection pipe P0+ P

pg

Driver

FrontCavity

Connexion Pipe

RearCavity

SensorP Cal


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P0+ P


pg

Driver

FrontCavity

RearCavity

SensorP Cal

Actual solutionActual solution

Cavities without leaks : low frequencies generation.

Sensor inlet inside the front cavity & small volumes : extend the high frequencies response. (resonant frequency could be around 100 Hz : flat amplitude & phase 30 Hz)


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Leaks VerificationLeaks Verification

Verification by the mean of a Static pressure calibrator. Measurement over 1 hour must be stable.(P < 0.2 hPa for 5 hPa over pressure) Could be made before calibration


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Accurate pressure adjustmentAccurate pressure adjustment

Pressure generated is temperature & static pressure dependant:

– the displacement of the piston must be adjusted before each calibration.

The solution: use a calibrated microphone:– measure the pressure at high frequency

(> 10 Hz)– attach to an absolute pressure reference

The frequency used for adjustment must be in the overlap band between microphone & calibrator:

– < 30 Hz for calibrator– > 10 Hz for current capacitance

microphones


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Microphone CalibrationMicrophone Calibration The microphone used is flat & currently calibrated from 20 Hz to 2 kHz within (+/- 2.3 %). A 1kHz calibrator allows the measurement chain calibration at 10 Pa (+/- 1%) 16 bits digitizers used in control device allows 1 LSB error ( < 0.01 %)

This is the link with absolute reference Pressure (Quality procedures)


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Complete systemComplete system Precise pressure generation requires adjustments before calibration: The calibrator accuracy depends on the microphone calibration:

– currently < 3 % but < 1% possible by specific calibration

SIGNALGENERATOR

16 bits

PowerAmplifier

PressureMeasurement

16 bitsCALCULATOR

Sync GPSPCLogicalcommands

Pref

Calibrated Microphone


FEEDBACK LOOP & COMMAND CALIBRATORMLS + / - Vref

Sinus + / - Vref

Response of calibrated sensor

Gain Narrow band filter mean value

calculation


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Calibration BenchCalibration Bench


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Calibration FeaturesCalibration Features Signals generated:

– Sine Waves at finite frequencies [0.025 - 50] Hz

– Positives Pulses from [1 - 200] ms width

– Maximal length sequences with order [10 - 14] & [1 - 100] Hz sample rate for Transfer functions measurements.

Amplitude from [1 - 50] Pa (with on board power amplifier & 12 V DC supply)

Stimulus could be synchronized to an external trigger (1PPS for ex.)

Self calibration at 20 Hz is used to reach less than 3 % in absolute amplitude error (if no specific microphone calibration used) but could be < 1 %.

High frequency calibrator response could be measured by microphone & M.L.S. to precise H.F. measurements.

Frequency response : flat in Amplitude & Time over [DC - 30 Hz]. Temperature stability: < 3 °C for < 1% error in amplitude.


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For calibration, pseudo-random sequences are used. The pseudo-random sequences characteristics:

– Binary sequences between +/- maxi.

– Random like distribution but identical sequences (depending on order)

– Spectral density = 1 over bandwidth

– maximal energy applied to the sensor

– Ref VANDERKOOY JAES Vol 42, N° 4 April 1994 Transfer function measurements:

Full range Calibration Full range Calibration using Pseudo-random sequences using Pseudo-random sequences

SENSOR TO BE TESTED

DIGITISER(one channel)

Pseudo-randomsequence(order n)Generated by calibrator

Correlation

Pseudo-randomsequence(order n)locally generated

Sensorresponse(Amplitude & Phase)

Possibility to be distant


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Self Calibration & Sine Wave Self Calibration & Sine Wave


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M.L.S. M.L.S.


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Ex: Sensor sensitivity response Ex: Sensor sensitivity response

1 measurement 16384 pts MLS S.R. = 25 Hz duration = 2614

s 10 Pa excitation

Sensor Nominal sensitivity: 20 mV/Pa


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Ex: Sensor response + tolerances Ex: Sensor response + tolerances

- 1.0 % / static calibration

20 mV/Pa nominal sensitivity

19.75 mV/Pa in static calibration

19.55 mV/Pa in dynamic calibration


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Ex: MB2000 DC output responseEx: MB2000 DC output response

The calibrator is flat from 0.005 up to 10 Hz

(1.005 mV/Pa in static calibration)


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Conclusion Conclusion Effective broadband infrasound calibrator Internationally patented On board MLS allows amplitude & phase measurements with

1 channel digitizer Self calibration requires no adjustments

Laboratory calibrator

On the field calibrator

remarks

Size (h x ) 0.55 x 0.40 0.30 x 0.2 (m)

Freq. response [0.001 – 30] Hz [0.01 – 10] Hz With flat amplitude & linear phase

Pressure range [1 – 50] Pa [1 – 10] Pa 12 V powered (< 5 W)

Absolute pressure Accuracy:

3 % < 1 %

3 % < 1 %

If currently mic. calibration If specific mic. calibration

State Industrialization Under dev.

DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO...

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Transcript of DAM-Île de France Département Analyse, Surveillance, Environnement 06/06/2014 DIF/DASE/B.ALCOVERRO...