Basics Of Calibration - Modal Shop

o What is calibration? Verifies amplitude response and system linearity over theintended range of use Provides physical meaning to the electrical output Typical Manufacturer calibration includes:––––––Reference sensitivityFrequency responseOutput bias level or supply voltageTransverse sensitivityResonant frequencyTime constant

o Why calibrate? To ensure the sensor performs according to the specificationand to provide meaning to the electrical outputo Why recalibrate? Good measurement practice that helps to ensure accurateresults and that the sensor is still “healthy”.– Reference sensitivity, frequency response, output bias level, transversesensitivity, resonant frequency and time constant Often required by ISO 9001, QS9000 or contractual reasonso “Do-it-yourself” decision Risk vs. Reward

o What are the standards? ISO 9001 Company Quality Management System ISO 17025 Calibration Laboratory Quality System– Identifies the general requirements for the competence of calibrationlaboratories– Accreditation bodies include A2LA, ACLASS, NVLAP, etc ISO 16063 Methods for the Calibration of Vibration and ShockTransducers––––16063 Part 11.16063 Part 13.16063 Part 21.16063 Part 22.Primary vibration calibration by laser interferometryPrimary shock calibration by laser interferometryVibration calibration by comparison methodShock calibration by comparison method

o Calibration data is fairly meaningless withoutknowing the measurement uncertaintyo Measurement Uncertainty often misstated May not include component from reference standard May not be compliant with relevant standard– For example, neglect shaker transverse or drift May not use coverage factor of k 2o Systemic Component Generally speaking – the measurement accuracyo Random Component Generally speaking – the measurement precision

ooooReference and Signal Conditioner UncertaintyTransverse Motion of ExciterReference Sensitivity “Drift”Measurement Channel Inaccuracies

!o Mounting and Cabling Torque Variation Cable Strain Relief Accelerometer Base Strain SensitivityooooRelative Motion Between SUT and ReferenceOperator TechniqueElectrical NoiseLab Conditions Temperature Humidity Pressure

"o Excerpt from The Modal Shop’s published ISO17025 A2LA certified uncertainty budget

o Static versusDynamicCalibration

o Absolute Methods Test where the sensor is subjected to a known,accurate and reliable standard of nature– Drop Test/Gravity Inversion Test– Gravimetric– Laser Interferometry

#o Sensor is rotated 180 Degrees in the Earth’sgravity so that it experiences a 2g (-1 g to 1 g)step function Requires long DTC or DC response for accurate results Signal Conditioning and readout device must be DC coupled

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o Accelerometer is allowed to free-fall in Earth’sgravity which varies by approximately /-0.25%around the globeEarth’s Gravity (9.80665 m/s2)0 Deg Latitude: 9.789 m/s290 Deg Latitude: 9.832 m/s2Altitude Correction: -3 mm/s2per 1000 m above sea level

o Step function output (mV) divided Earth’s gravity (1g) equals the sensitivity of the accelerometer

o Relative Methods Test where the sensor and calibrated reference aresubjected to the identical input acceleration. The ratio of theoutput signals provides the calibration factor.– Back-to-Back– Hand held shaker– Impact

o Test sensor mounts “back-to-back” with areference accelerometer that has been previouslycalibrated by primary means or by a recognizedlaboratory

o ISO 16063 Part 21 Vibration calibration by comparison to a reference transducero Most common of all calibration system types Typically used by manufacturers and metrology labso Reference transducer can be either external“piggyback” or internal to excitero Approximate uncertainty, including both systematicand random errors: 1 to 20 Hz : /-3%20 to 5000 Hz : /-1.5%5000 to 10000 Hz : /-3% 10000 Hz : /-4.5%

#!o Reference Accelerometer is typically the largestsingle component of the system uncertaintybudgeto Consider calibrating reference directly to LaserPrimary for improved uncertaintyo Quartz as piezoelectric material provides natural,stable characteristicso ICP technology provides low noise susceptibility,particularly at low frequencieso Shear mode construction eliminates base straino Hermetic sealing for humidity insensitivity

#Transverse Motion (% of axial)o Transverse bending and rocking motion ofshaker causes significant measurement erroro Calibration air-bearing shakers meeting ISO16063 recommendations eliminate this error200180160140120100806040200typical f lexurebased shakerresponse396C10/C11air bearingshakers0100020003000ISO recommendedlimit400050006000Frequency (Hz)70008000900010000

#o Uncertainty often misstated due to neglectingtransverse motion effects, often as much as 3-5%

%o Test sensor mounts “back-to-back” with areference accelerometer and anvil, struck witha pneumatically operated projectileTest SensorReference AccelAnvilImpact ShockPneuShock shock tower

%o ISO 16063 Part 22 shock calibration bycomparison to a reference transducero Measures transient (impact) acceleration levelsup to 10,000 g to calculate sensitivityo Verifies amplitude linearityand performance of sensorat intended test levels

%&'()*"Compare peaks to get sensitivityZero shift isapparent duringfree fallDecelerationby “fingers”Duration – shouldbe 5x resonanceDuration of free fall is set by height of “fingers”

!o Back-To-Back Calibration – Resonance Test Stepped sine FRF measurement of test sensor to referencestandard identifies the unit’s mounted resonant frequencyo Verifying consistencyof sensor’s mountedresonant frequencyprovides additionalassurance of sensorhealth

o Handheld Calibrator Quick, easy field check and/orcalibration of entire system fromsensor through cable to analysisequipment Provides a fixed g force via abuilt-in servo-stabilization Battery operation provides forportable use, while DC adaptersallows for extended operation A handheld shaker is relative toan internal reference in shaker%

,o Portable Vibration Calibrator Quick, easy field check and/orcalibration of entire system fromsensor through cable to analysisequipment Provides selectable amplitudeand frequency settings for in-situcal from 7 Hz to 10 kHz Integral reference accelerometerprovides traceability throughcomparison method Supports acceleration, velocityand displacement%

-. o Laboratory Qualifications OEM (Original Equipment Manufacturer)– Advantages of using an OEM OEM is capable of checking all functions of the instrument OEM has knowledge of latest changes One Stop Shopping OEM is able to upgrade instrument at customerrequest and provide calibration services Accredited Laboratory– Has been audited by A2LA, NVLAP or other approved organizationto ISO Guide 25, ANSI Z540 or ISO 17025– Per QS-9000, non -OEM Laboratories must be accredited to one ofthe above standards.

-. o Traceability The ability to show that a standard or an instrumenthas been calibrated or certified by a higher orderstandard which is then calibrated by a still higherorder, and on and on, thus the standard orinstrument is eventually related back to theestablished national standards.

-. o Customer Responsibilities Verify that the Laboratory is qualified to calibrate theinstrument being submitted Verify that the Quality System is adequate through audits onsite or written Communicate any needs or requirements when theLaboratory is being evaluated to determine if your needs willbe met

– 16063 Part 11. Primary vibration calibration by laser interferometry – 16063 Part 13. Primary shock calibration by laser interferometry – 16063 Part 21. Vibration calibration by comparison method – 16063 Part 22. Shock calibration by comparison method