Active Magnetic Field Compensation

Active Magnetic Field Compensations System MACOM II®

Eliminating magnetic interference fields in research areas (electron microscopy) and in medical areas (magnetic resonance imaging)

Background

The resolution of imaging processes in the fields of electron microscopy and magnetic resonance imaging has undergone significant improvement in recent years. However, the systems have become more sensitive to ambient conditions in the process, especially when it comes to the effects of magnetic fields in addition to vibrations, temperature, and sound waves. Since research institutes are frequently located in intra-urban areas, as a rule, the effects of trams, underground trains, electrical lines, power cables, etc. cannot be avoided. The magnetic interference fields generated by these systems in rooms where these sensitive devices are installed, are often proportionally greater than the levels permitted for such devices.

In principle, it is possible to shield off the interference fields with metallic cabins or line rooms with metal panels. However, this is considerably more expensive than interference field reduction produced by an active magnetic-field compensation system.

Principle

Magnetic fields are vector fields. If an interference field vector is superimposed by a field vector of equal magnitude though in the opposite direction, this results in obliteration of the field. In practice, no complete obliteration of the interference field is achieved by application of such interference, nevertheless, the results is a significant reduction of the field. To do this, the prevailing magnetic field is measured using a probe, then a matching opposing field is generated by an arrangement of current-conducting coils in the room. A control unit ensures that the flow is always adapted to fluctuations in the interference field.

Implementation

 In order to compensate for interference fields regardless of the waveform, a field reduction with the broadest possible frequency range is required. This is achieved by using a specially designed and patented1 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 corresponding opposing field. This way, both very slow field fluctuations - such as those generated by moving metal structures, such as lifts, steel doors or vehicles in the vicinity, and very rapid field fluctuations - those 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 the scanners. In this case, a modified MACOM II MR system that enables the probe to be positioned outside the examination room is available.
  • Probe positioning near the microscope column of transmission electron microscopes is not possible since self-fields generated by the magnetic lenses are formed. Here, a version with two probes that cover various frequency ranges was developed. This enables a significant improvement in compensation to be achieved—at least for 50 Hz fields.
  • For special devices with self-fields of higher frequencies the frequency range of the compensation system can be limited so as to prevent these self fields from impairing the compensatory action.

The design of the system also permits control from an external computer, which can be connected via a serial interface. A connection via the existing 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. Müller-BBM also provides worldwide support of the system on request—without the need for the presence of on-site staff.

A mobile system for rapid verification of the compensation unit's operation is also available, without the need to undertake structural alterations to the room. It can be assembled and dismantled in a matter of hours. This system can also be used to investigate the influences of various ambient parameters. This is useful if, for example, oscillations, airborne sound and magnetic fields are affecting the equipment to be protection in a similar manner, and there is uncertainty as to which is actually causing the interference. The mobile compensation system makes it possible to eliminate the influence of the magnetic field immediately in order to ascertain whether it was the cause of the interference.

Technical specifications2

Field reduction effect

0 to 1 kHz: < 60 dB

1 to 5 kHz: 20 bis 60 dB

10 to 50 kHz: 0 bis 20 dB

Noise (0 to 100 kHz) < 1 nT
Long-term stability < 1 μT
Maximum interference field approx. 10 T
Output current 3 x 3 A
Displays (0 to 100 kHz) Flux density or output current
PC interface RS232
Power rating max. 270 W
Design 19′′-4HE 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 room dimensions of 4m x 4m x 3m (3.65 x 3.65 x 2.74 yd). back

Additional pages