The present invention relates to suppression of magnetic fields and, more particularly, to an active feedback system for suppressing extremely low frequency (e.l.f.) alternating magnetic fields in a large volume of space. For purposes of the application, it should be understood that the word "alternating" is used to represent any time-varying magnetic field.
Time-varying magnetic fields are generated by a.c. mains wiring and associated distribution transformers. Occasionally, switching high current d.c. circuits or moving d.c. powered equipment and vehicles or even movement of large ferromagnetic bodies in the earth's geomagnetic field can generate extremely low frequency a.c. magnetic fields.
Such magnetic fields may be troublesome. For example, prolonged exposure to even milligauss (mG) magnetic disturbances may result in danger to the health of humans and animals. A recent Swedish study, for example, was reported at a conference sponsored by the U.S. Department of Energy and the Electric Power Research Institute. The study indicated that children exposed to relatively weak magnetic fields (0.1-0.3 .mu.Teslas) from power lines near their homes develop leukemia at almost four times the expected rate. Another found that men exposed to similar magnetic field levels in the workplace had three times the expected rate of one form of leukemia.
Moreover, uncontrolled magnetic fields may adversely affect sensitive equipment. For example, both scanning (SEM) and transmission (TEM) electron microscope resolution is known to deteriorate when spurious magnetic fields above approximately 0.1 .mu.Tesla are present.
While certain passive shielding mechanisms, notably those fabricated from "mu-metal" materials, can isolate sensitive equipment or personnel from magnetic fields, such mechanisms are generally intended only to protect limited areas. As a practical matter, it has proven to be impossible to compensate for magnetic disturbances, at a reasonable cost, for volumes of space as great as a room, much less a building.
U.S. Pat. No. 4,380,716 issued to Romeo et al on Apr. 19, 1983, discloses an external d.c. (static) magnetic field compensator for color aberration due to axial fields traversing a CRT. Color purity signals from monochromatic sensors located at the CRT corners are used in a zero frequency response, closed loop, servo feedback system. Current is applied to a compensation coil set providing first order cancellation of color distortions.
U.S. Pat. No. 4,963,789 issued to Buhler on Oct. 16, 1990, discloses a technique for dynamically neutralizing a magnetic field within a specified volume by generating compensating magnetic fields at a number of locations around the volume, sensing the field in the volume via fluxgate sensors, and controlling the value of the compensating fields in a closed loop, microprocessor-controlled servo system to null the field. An external mu-metal shield and degaussing coil are provided as part of the servo magnetic compensation system. Low level e.l.f. magnetic fields are not discussed in Buhler, since the application of the technique is for neutralizing relatively high level magnetic fields to correct CRT color and distortion.
A magnetic field cancelling system Model No. SC07, manufactured by Spicer Consulting, Bedford, England, is described in a brochure dated Jun. 3rd, 1991. The system is intended to reduce alternating ambient magnetic fields and can be used to improve the performance of electronic instruments which are sensitive to magnetic fields, such as electron microscopes and electron beam metrology tools. The system contains three power amplifiers which drive currents through cables to generate a cancelling field. X, Y and Z magnetic fields are measured; corresponding signals are amplified and fed back in anti-phase to the power amplifiers. The negative feedback reduces the field at the sensor location.
It would be advantageous to provide a cost-effective magnetic field compensation system.
It would also be advantageous to ensure that such a compensation system were reconfigurable and/or portable.
It would also be advantageous to provide a compensation system responsive to a wide range of frequencies and/or amplitudes of time-varying magnetic fields.
It would also be advantageous to provide a magnetic field compensation system that is both easily installable and easily operable.