Active magnetic field compensation

Active magnetic field compensation system
MACOM II®

Compensating magnetic interference fields in research (electron microscopy) and in medicine (magnetic resonance imaging)

Background

The resolution of imaging processes in electron microscopy and magnetic resonance imaging has undergone significant improvement in recent years. However, the systems have become more sensitive to ambient conditions at the same time, especially when it comes to the effects of magnetic fields in addition to vibrations, temperature and sound waves. Interfering impacts e. g. from trams, underground trains, electrical lines, power cables etc. are usually unavoidable: in rooms where magnetically sensitive devices are installed, these sources often generate interfering magnetic fields far beyond the maximum permissible values. In principle, it is possible to shield off the interference fields with metallic chambers or by lining rooms with metal sheets – certainly at prices that are considerably higher than the cost of the interference field reduction by means of an active magnetic-field compensation system.

Principle

Magnetic fields are vector fields. If an interference field vector was superimposed by a field vector of equal magnitude, though in the opposite direction, this would, in theory, result in the obliteration of the interference field. In practice, the interference field will not be fully obliterated through the application of an opposite field, but, at least its field strength can be significantly reduced. To do so, the prevailing magnetic field is measured using a probe and a matching opposite field is generated by an arrangement of current-carrying coils. A control unit ensures that the flow is always adapted to fluctuations in the interference field.

Implementation

In order to compensate magnetic interference fields regardless of the waveform, a field reduction with the widest possible frequency range is required. This is achieved by using a specially designed and patented sensor that measures the magnetic interference field from 0 Hz (constant fields) up to frequencies of some 100 kHz. This sensor signal is fed into an analogue control unit; a broadband power amplifier generates the currents for the respective opposite field. Thus, very slow field fluctuations (e. g. generated by moving metal structures, such as lifts, steel doors or vehicles in the vicinity) as well as very rapid field fluctuations (e.g. produced by switching operations in the electrical installation) are effectively reduced.

Various modifications to the standard device have been developed for special applications.

  • The measuring probe cannot be installed near magnetic resonance scanners because of the strong self-field generated by these scanners. In this case, with MACOM II® MR a modified system is available whose probe can be positioned outside the examination room.
  • Probe positioning near the microscope column of transmission electron microscopes is not possible since the magnetic lenses generate self-fields. Here, a version with two probes covering different frequency ranges was developed. Thus, a significant improvement in the compensation can be achieved—at least for 50 Hz fields.

The design of the system also permits control from an external computer, which can be connected via a serial interface. A connection via Ethernet interface is also possible, thus enabling the direct connection of the compensation system to the company's Intranet or the Internet. All parameters can then be set and all data queried via any computer connected to the network. On request, Müller-BBM also provides worldwide remote support of the system.

Mobile test unit

A mobile system for rapid verification of the compensation unit's operation is also available, which can be used to investigate the influences of various ambient parameters. This allows to identify the source of interference, if several sources are equally affecting the equipment to be protected and if there is uncertainty as to which is actually causing the interference.

Assembling and dismantling of the test unit takes only a few hours, no structural alterations to the room are needed.

Technical specifications2

Field reduction effect

0 Hz to 1 kHz: 60 dB

1 kHz to 5 kHz: 60 dB to 40 dB

5 kHz to 100 kHz: 40 dB to 0 dB

Noise (0 Hz to 100 kHz) < 1 nT
Long-term stability < 1 nT
Maximum interference field approx. 10 µT
Output current 3 x 3 A
Displays (0 Hz to 100 kHz) Flux density or output current
PC interface RS232, Ethernet
Power rating max. 600 W
Design 19′′-4U insert or desktop housing

1 Patent number: 10224582 at the German Patent and Brand Office
2 The specifications are given relative to the location of the probe and for room dimensions of 4m x 4m x 3m

Additional pages