Background
Systems for research and manufacturing of very small structures in the semiconductor industry, biochemistry and with electro medical equipment like pattern generators, electron microscopes and magnetic resonance tomographs (MRT) make high demands on magnetic interference fields. Typical sources of interference like trams, trains, power cables but also the electric current of electric installations of the building itself as well as electric machinery and equipment cause magnetic interference fields with flux densities of several 100 nT up to several 1000 nT, whereas many sensitive systems are already considerably influenced by magnetic interference fields of about 10 nT. As passive screening of magnetic fields by metallic alloys is difficult and expensive, active magnetic compensation systems are a practical alternative.

Principle
Magnetic fields are vector fields. If an interference field vector is superimposed by another vector of the same size but with the opposite direction, the field is cancelled. In practice, the use of this interference mechanism considerably reduces the magnetic interference field. For this purpose, the prevailing magnetic field is measured on location and by means of an arrangement of current-carrying coils a suitable opposing field is created. An automatic control unit ensures that the current is set according to the changes of the interference field.

Realisation
In order to compensate any interference field, a field reduction in the largest possible frequency range is required. This is achieved by a specifically developed and patented¹  sensor which measures the magnetic field from 0 Hz (constant fields) up to frequencies of several 100 kHz. This sensor’s signal is fed to an analogue automatic control unit. A broad band power amplifier generates the current for the corresponding opposing field. Distribution of the magnetic flux density without (left side) and with compensation (right side). Thus, both very slow changes of the magnetic field - which are created for example by moving metal constructions like elevators, steel doors or motor vehicles in the surroundings - but also very fast changes of the magnetic field - caused for example by switching actions in the electric installation - are reduced effectively.

With the concept of the system, control by an external computer is also possible using a serial interface. The operation of the system by remote control is also possible. If the external computer is connected to a company-internal intranet or the Internet, all parameters can be remotely adjusted and data collected. By request, the system can thus be supported by Müller–BBM worldwide without local presence of technicians.