Hall probe, fluxmeter, NMR…
Field mapping: one size does not fit all

Many researchers, engineers and technicians dealing with magnetic field mapping are not aware of the variety of technologies available. From particle accelerators to the automobile industry, the problems vary widely depending on the type of field measured and the precision required. Fortunately, a suitable, reliable solution can be found for (almost) all situations. A brief review of the state of the art.
 

Engineers and technicians producing and installing MRI systems and scientists working on particle accelerators are well aware of the difficulties of magnetic field mapping. As the magnetic design of electric motors, sensors and actuators becomes increasingly sophisticated, industrial manufacturers are encountering similar difficulties. Tricky aspects of a single-point measurement – for example, accurately positioning the probe – become a nightmare when attempting to map a field completely by measuring hundreds or thousands of points in succession. An obvious solution is simultaneous measurement using multiple devices.

The question remains: what kind of device? "The main three technologies for the precision measurement of intense magnetic fields – Hall probe, fluxmeter and NMR – can all be used for mapping”, explains Metrolab’s Philip Keller. "Each has its field of application, limitations and advantages."

Hall probes: watch out for angular errors
Hall probes are quick and easy to use and measure the three field components with medium precision (usually no better than 1,000 ppm). The main limitation is that they must be calibrated to correct offset, non-linearity and temperature sensitivity, and, because of offset drift, recalibrated in a zero field prior to each use. Crosstalk between axes must also be taken into account, and the probes must be positioned extremely precisely: an angular error of 1° can translate into a measurement error of 17,000 ppm!

Positioning jigs perform a dual function: moving the probe and measuring its position. An example? At the Laboratório Nacional de Luz Síncrotron (LNLS) in Brazil, researchers use a Hall system to map undulator magnets, which consist of a row of permanent magnet pairs with alternating polarities. The mapper comprises three orthogonal Hall probes mounted on a three-axis positioning platform that runs the full length of the undulator (4.2 m). The position is accurate to within 5 µm, yielding better than 1,000 ppm precision even in regions with a gradient of 66 T/m!

Fluxmeter: it all depends on the coil
Practical as they may be, Hall probes are ineffective in a number of configurations – for example, measuring the flux density inside an iron yoke, a high-frequency AC field, or very low fields. The fluxmeter – a coil combined with a voltage integrator – is often a suitable alternative.

 

It provides excellent precision for the magnetic measurement (as good as 10 ppm), and special integrators that provide an on-the-fly read out of the partial integrals allow rapid field mapping with almost arbitrary spatial resolution. It all depends on the geometry of the coil and, once again, the mechanical jig. "The fluxmeter’s greatest quality, its flexibility, may also be its biggest flaw", says Philip Keller with a smile. “It requires good technical knowledge and tailor-made coils that can become very, very high-tech!"

NMR: To be more precise…
Lastly, in cases where maximum precision is required, nuclear magnetic resonance (NMR) must be used. "An NMR teslameter can measure very strong fields, from about 40 mT on up, with a precision of just a few ten ppb", stresses Philip Keller. "On the other hand, its use is strictly limited to homogenous and static fields, and it does not indicate the direction of the field."

NMR mappers are primarily used to manufacture and install MRI systems, NMR spectrometers, and certain high-precision mass spectrometers. In the old days, manufacturers used a single-probe NMR system for this task – and when flexibility or investment dollars are at a premium, this is still an option. Otherwise, multi-probe NMR systems are the clear favourite: the NMR probes are arranged ("quite simply", one might say) on a semicircular plate that itself moves around an axis. The field inside the volume swept out this way is calculated from measurements on the periphery by solving Maxwell’s equations.

MRI manufacturers such as GE and Philips use this technology at each stage of the industrial process, from R&D to installation in hospitals, where the machine must be shimmed to correct for imperfections in the magnet as well as the magnetic environment of the room. The technicians who have to carry out this tricky operation, in situ, within very short times, know how much they owe to modern mapping solutions!

 

For more information
A white paper, based on a talk at this spring's Magnetics 2007 conference, provides much more detail on these three mapping technologies.