Proper probe positioning improves magnetic measurement accuracy

Article By : Ryan Oliver

Precise and repeatable magnetic field sensing is critical to the success of many advanced applications.

Now more than ever, permanent magnets and electromagnets play a critical role in today’s advanced applications. From high-performance electric motors to magnetic nano-machines and high-reliability actuators and sensors, precise and repeatable magnetic field sensing is critical to the success of these designs.

Hall-effect sensor systems, such as teslameters, are highly-accurate and can provide a broad measurable field ranging from below 1 μT to well beyond the strength of the world’s highest-power superconducting magnets. As with any precision measurement instrument, accuracy depends on proper use to ensure that it provides a consistently high level of performance.

Properly positioning the probe within the magnetic field will greatly improve the accuracy of magnetic measurements. Factors include:

  • Angular alignment, which includes an analysis of the angle of the field vector to the plane of the sensor
  • Sensor placement within the magnetic field, requiring knowledge of the sensor placement within the probe and the likely field distribution that is to be measured
  • Probe types, including the role of 3-axis probes
  • Consistency gained through probe alignment fixtures

Angular alignment

Hall sensor output varies with the angle of the field vector to the plane of the sensor. The output is maximum when the magnetic field is perpendicular to the device. Figure 1 displays errors encountered when the sensor is not perpendicular to the magnetic field.

illustration of measurement changes due to angular misalignmentFigure 1 Measurement reduction can be caused by angular misalignment.

Probe placement within the magnetic field

Field strengths can vary greatly over a small area, particularly when measuring very close to the source of the magnetic field. It is important to recognize that the sensor is somewhere inside the probe (Figure 2) and that the field at this location will be reported, rather than at the surface of the probe.

illustration of sensor location compared to probe tipFigure 2 The location of the sensor (indicated in red) is not at the tip of the probe.

Different probes will have the sensor positioned in different locations, meaning two different probes placed in the exact same position will actually have sensors that measure different locations. This may only be the difference of a few millimeters; however, the plot in Figure 3 shows that close to a magnet, the field can change significantly over the space of just 1 mm.

graph of flux density dropoffFigure 3 The flux density is presented as the percentage of field compared to the tip of the magnet.

A plot of the measured flux density drop-off as a function of distance (mm) from the outer surface of the magnet moving along the red line is shown in the Figure 4.

cylindrical magnet cross sectionFigure 4 This cross-section of a cylindrical magnet shows the flux density it produces.

The 3-axis probes

The 3-axis probes—whose sensors simultaneously measure the field strength across the x-axis, y-axis, and z-axis—allow the flexibility to measure magnetic fields without knowing the precise direction of the magnetic field. While a 3-axis probe may help solve one of the positioning errors discussed when using single-axis probes, it does not eliminate the effect of moving away from or toward a specific active area of measurement. Knowledge of the position of the active area and volume of the probe will be required to ensure engineers know what they are measuring.

diagram of probe internalsFigure 5 This diagram of the internals of a Lake Shore Cryotronics probe show the z-axis in yellow, the y-axis in green, and the x-axis in red.

The 3-axis sensors also introduce the potential issue of having the three different sensors occupying different positions in space. Most applications don’t require sub-millimeter co-location of these individual sensor elements. However, in applications like mapping fields with large field gradients, monolithic 3-axis sensor technology provides the ability to perform 3-axis field measurements over a very small physical area.

Measurement consistency

Stringent product specifications demand consistent measurements at carefully selected positions within the magnetic field. Probe alignment fixtures can help obtain consistent and repeatable measurements. A wide variety of probe mounting accessories and/or features are available.

illustration of probe mounting featuresFigure 6 A probe with mounting features allows for predictable and repeatable sensor placement.

While probe positioning is the most common error made in magnetic measurement, it’s important to consider other factors affecting your measurement data such as temperature, offset error, and calibration. By employing probe placement best practices, your measurements will be more accurate, precise, and dependable.

This article was originally published on EDN.

Ryan Oliver is senior product manager for instruments and sensors at Lake Shore Cryotronics.

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