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NPL’s Existing FacilityA low magnetic noise environment and traceable measurement systems have been established at NPL using a triaxial ambient � eld cancellation system based on � uxgate magnetometer technology [2]. Figure 1 shows the 3 m triaxial Helmholtz coil where the ambient � eld is reduced to less than 0.5 nT at its centre. Using this approach, a noise � oor of 20 pT/√Hz at 1 Hz is achieved at the centre of the triaxial Helmholtz coil system. Also shown is the magnetic � eld variation in the laboratory before (black line) and after (red line) cancellation.

Temperature SystemTo produce the temperature conditions sensors experience during use, a commercial air temperature forcing system and thermally isolated enclosure were integrated into this existing low � eld environment. The system provides high volume, high velocity, temperature controlled air-� ow to an enclosure over the temperature range -80 to +225 °C. Integration of the temperature system required evaluation of the temperature stability and uniformity within the enclosure, and its impact on the performance of the cancelled environment. The e� ect of the air-forcing system on the cancelled environment was determined by measuring its magnetic signature as it was brought into position. It was found that the e� ect of the temperature system on the cancelled environment is of the order of 0.3 – 0.4 µT which, if not accounted for, would lead to a signi� cant increase in the uncertainties quoted. This o� set can be calibrated out by determining the sensors characteristics at ambient temperature, then by repeating the ambient measurement with the temperature system in position.

Results of Sensor Characterization StudyFor this study, two � uxgate and one anisotropic magneto-resistance (AMR) device were evaluated over the temperature range stated by the manufacturers. The sensitivity (the ratio of magnetic � ux density to output voltage) expressed in µT/V and the scaling temperature coe� cients expressed in ppm/°C were determined.

For the � uxgate sensors, an initial measurement of the sensitivity for each device was performed with the ambient � eld cancellation system at a room temperature of 20 °C. The sensitivity measurement was repeated at the same temperature with the sensor located within the enclosure of the temperature system. Any di� erence in the calculated sensitivity is the result of the DC signature of the temperature system and can be used to correct the sensitivity values obtained at other temperatures. Due to the variation in the zero o� set it is di� cult to record the sensor output to create one plot of sensitivity against temperature with complete certainty in the values obtained, this variation is shown in Figure 3.

The sensitivity values shown in Figures 4 and 5 are measured individually at each temperature along with a measurement of the sensors zero o� set with no � eld applied. The sensitivity values plotted are corrected for the o� set due to the temperature system and also for any change observed for the sensor zero o� set. The calculated sensitivity together with the associated measurement uncertainty of ± 0.05% is shown as the red marker with error bars. The expanded uncertainty evaluated for the determination of the sensitivity values, at a con� dence level of k = 2, allowing for the additional contributions due to the temperature system increased from ± 0.05% to ± 0.15%.

The characterization of the AMR sensor for scaling temperature coe� cients was made as a relative measurement with respect to an ambient temperature of 20 °C. The zero o� sets for this sensor are less susceptible to temperature due to the operating principle. The plot of the variation in the AMR’s sensitivity is shown in Figure 6.

Once the sensitivity for each device had been determined over the required temperature ranges the scaling temperature coe� cients, in ppm/°C, were derived and the results are given in the table.

Characterization of Magnetic Sensors at the Operational Temperatures

of Industrial Applications S.A.C. Harmon, M.J. Hall, S. Turner and N. Hillier

National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK

Existing measurement facilities do not allow the characterization of magnetic sensors at the operational temperatures encountered in a range of industrial applications such as down-hole drilling to space applications where the temperature is considerably di� erent to that used during their development and calibration.

Routine calibration of such sensors can be carried out by various facilities, but are limited to ambient temperatures. To enable their use in harsh temperature environments, NPL has developed measurement facilities for the characterization of these sensors over a temperature range of – 55 to + 125 °C re� ecting the actual temperatures experienced during their operation.

A temperature calibration system to enable this characterization, developed as part of the European Metrology Research Programme (EMRP) Project titled MetMags – Metrology for Advanced Industrial Magnetics [1], has been integrated into NPL’s existing low magnetic � eld facility to determine the temperature coe� cients of the most sensitive magnetic sensors.

 

Figure 2: Temperature uniformity within enclosure. PRT’s are located at each corner of the uniformity plots and cover an area of 160 mm x 100 mm.

Figure 3: Fluxgate No. 2 zero o� set vs temperature.

Figures 4 & 5: Sensitivity plots for � uxgate type sensors

Figure 6: AMR sensitivity.

Speci� ed and calculated values for scaling temperature coe� cient

Speci� cation Measured

Operating temperature

range(°C)

Scaling temperature

coe� cient(ppm/°C)

Temperature range

(°C)

Scaling temperature coe� cient(ppm/°C)

Fluxgate No. 1 – 40 to + 70 ± 20 to ± 200– 20 to + 20+ 20 to + 42+ 42 to + 66

+ 32+ 4– 22

Fluxgate No. 2 0 to + 175 Not speci� ed+ 20 to + 77

+ 77 to + 115+ 115 to + 131

+ 66+ 46+ 52

AMR Sensor – 40 to + 150 – 3600 ± 600

– 25 to 00 to + 25

+ 25 to + 50+50 to + 75

– 3736– 3663– 3427– 3155

Summary• A calibration system for the characterization of magnetic sensors over the temperature

range – 55 to +125 °C has been developed and integrated into NPL’s existing ambient � eld cancellation system.

• The e� ect due to the temperature systems proximity to the cancelled environment has been established and a calibration procedure to correct for the magnetic signature, seen as an o� set in the output of the device under test when no � eld is applied, developed.

• The results of the characterization study show a good agreement with the stated speci� cations giving con� dence that the system provides a robust and traceable measurement facility for the testing of magnetic sensors. This represents a step change in the ability to characterize sensors for automotive, geological exploration and space applications.

Acknowledgements The authors would like to thank the National Measurement System of the Department for Business, Innovation and Skills that provides funding to develop and maintain the measurement systems at NPL. They would also like to thank the EMRP MetMags project partners and stakeholders who provided sensors. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

References[1] http://www.ptb.de/emrp/metmags-home.html

[2] M. J. Hall, et al., Magnetic environment and magnetic � eld standards at NPL for the calibration of low noise magnetometers and gradiometers for cleanliness studies, Aerospace EMC, 2012 Proceedings ESA Workshop on, ISBN: 978-1-4673-0302-6, p. 1 – 6 (2012).

Figure 1: 3 m diameter triaxial Helmholtz coil used to reduce the ambient magnetic � eld at NPL, and the achieved cancellation.