Magnetic sensors

Citation

Monkman, G. (1998), "Magnetic sensors", Sensor Review, Vol. 18 No. 4. https://doi.org/10.1108/sr.1998.08718daa.002

Magnetic sensors

Magnetic sensors

Gareth Monkman

Professor Dr Gareth Monkman is at Fachhochschule Regensburg, Fachbereich Elektrotechnik, Prüfeninger Str. 58, 93049 Regensburg, Germany. Tel: +49 941 943 1108/1285/114; Fax: +49 941 943 1424; E-mail: gareth.monkman@e-technik.fh-regensburg.de

Keywords Magnetic, Sensors

The ability of a freely hung loadstone to align itself in a particular and consistent direction has been known since the earliest Chinese dynasties. This property was readily exploited in the production of simple compasses, though in the early times it was not clear at what the needle was actually pointing. In fact, it was not until 1600, when William Gilbert suggested that the earth was a giant spherical magnet, that the concept of dipolar magnetism became known.

The first steps toward the measurement of magnetic fields were taken in 1750 when John Michell described the inverse square law associated with magnetic induction and its implications in the conservation of energy ­ now known as Coulomb's law (though actually rediscovered as such some 30 years later). The following year, Benjamin Franklin also noticed a connection between electricity and magnetism while observing the ability of a current-carrying coil to attract iron filings. However, it was another 70 years before this phenomenon was properly explained by Oersted and Ampere.

Parallel research on both sides of the Atlantic during the 1830s led to the invention of the dynamo, following Josef Henry's observation that coiled wires produce greater magnetic fields than straight wires. Henry's discovery of self induction, the theory behind the transformer, also opened the way to many of the electromagnetic position sensors still used to this day. These were the heydays of early electromagnetism and it was during this same period of the industrial revolution that Michael Faraday's concept of the magnetic field, as later described by Maxwell's equations, became the basic accepted theory behind virtually all forms of electromagnetic transduction, and remains the basic principle to this day.

With the advent of mechanisation, simple electromagnetic sensors entered the scene. Most were very primitive and lacked much in the way of sensitivity ­ inductive proximity sensors, tachometers, synchro-servo systems etc. Two world wars gave a boost to developments in rotary encoders such as the inductosyn, needed for the accurate positioning of ship and tank gun turrets. Post-war automation led to improvements in the resolution and sensitivity of conventional electromagnetic sensors, while a number of perhaps less well known magnetic sensory devices were also developed ­ the fact that oxygen exhibits very slight paramagnetic susceptibility gave rise to some of the first gas analysis systems; differing atomic resonance frequencies resulted in the widespread use of nuclear magnetic resonance (NMR) spectroscopy.

Though the basic underlying physics remains the same, today all these basic measurements are carried out with ever increasing finesse. Super conducting quantum interference devices (SQUIDs) have taken over from simple compasses for very accurate measurements of the earth's magnetic field. The micromachining of modern materials makes it possible to build structures so fine that simple mechanisms can be produced with all the advantages of mechanical simplicity but without the drawbacks of insensitivity normally associated with similar constructions at larger scales.

Ettinghausen, Nernst and Hall effects have all been known since the last century. However, their exploitation in the manufacture of magnetic sensors has been realised only with the availability of modern materials. The semiconductor industry has had a lot to offer in this respect. The use of specially controlled doping and the integration with CMOS technology has resulted in the MAGFET, a device which overcomes many of the insensitivities of conventional Hall effect elements.

Even the humble inductive proximity sensor, now encapsulated to IP65 standards complete with the electronic drive circuitry, variable sense range, temperature compensation and fold-back current limiting, bears little relation to the original iron cored coil through which a measurement current would be passed ­ but the basic physical principle of its operation and the measurand with which it interacts remains unchanged.

As for the future ­ present trends in microtechnology are bound to continue, but what is needed is a fresh impetus for new innovation. Medical technology and environmental concerns have taken over from the military as the driving force behind most new sensor technology. However, this applies mainly to areas where magnetic sensors are little used, such as biological and chemical. Electromagnetic compatibility and the increasing awareness of the possible influence on health from electromagnetic fields may be the next focal point. Of course the inevitable question as to what we should really be measuring remains.