EMC

Active fields cancel interference

A new technique greatly improves the performance of computer monitors and other equipment subject to stray, varying magnetic fields. Tom Shelley reports

It is now possible to negate the effects of magnetic interference on computer monitors by sensing fields in three dimensions and then applying current to field coils to counter them.

The technique has been developed to improve the performance of colour monitors subject to interference or varying terrestrial magnetic fields.

It further has the potential to be applied to any sensitive piece of electronic equipment or even whole rooms, greatly improving the ease of making sensitive measurements. And just as it can be used to stop unwanted stray fields getting in, there also exists the possibility of using variations of the technique to stop interference getting out.

Microvitec of Bradford has developed the technique in order to reduce the cost of making shadow mask colour monitors more immune to changes in the earth’s magnetic fields and stray field interference. Changes in magnetic field strength manifest themselves in movement of the electron beam relative to the holes in the shadow mask, leading to flickering images and loss of brightness. Magnetisation of the shadow mask tends to result in wrong colours.

Conventional monitors are normally supplied roughly set up for their intended location and may then be finely adjusted to produce a good image.

Monitors in ships and aircraft, on the other hand, move from place to place, and encounter fields whose direction and magnitude may vary.

And while their locations remain fixed, monitors in power stations, some factories, and underground stations suffer varying magnetic field interference from conditions around them.

Microvitec now have systems which both cancel the effects of external magnetic fields and demagnetise (degauss) shadow masks in between frames.

Magnetic field is sensed using fluxgate sensors made by SCL, formerly Speake & Co. These sensors, which are extremely low cost, were the subject of the cover feature article in Eureka’s August 1995 cover feature story.

The output from the sensors is fed to the inputs of amplifiers. The outputs drive rectangular coils round the working parts of the equipment. Sufficient coil current is supplied to reduce detected fields to zero.

With the system demonstrated to Eureka, it is possible to suppress the effects of non-varying fields of up to 10 Gauss, 20 times greater than that of the earth’s field; and 50 Hz AC mains generated fields of up to 1 Gauss.

Should interference from other sources be encountered, it is thought likely to be able to suppress fields varying at frequencies up to 1 to 10 kHz. Frequency and field strength suppression is limited only by amplifier response and power output.

Degaussing of the shadow mask is by supplying a decreasing AC current to a coil running round the periphery of the screen.

Most large colour monitors on sale today have some degaussing ability, usually implemented when the monitor is turned on. If picture quality deteriorates during use, degaussing can also be implemented at the touch of a button. The degaussing action manifests itself as a violent rippling and flickering of the screen, after which screen colours are normally restored to what they should be. Failure to degauss can lead to colours becoming sickly, or in extreme cases, complete colour inversion. If warning red is changed by mask magnetisation to comforting green, such colour inversion could in process control situations have unfortunate consequences.

Microvitec technology can perform degaussing once every 20 seconds between frames. It could, with the existing technology, be increased to once every 6s. If any customer should ever need it, it could, in theory be applied between every frame.

If an existing monitor needs to be protected against stray fields, Microvitec have also developed a prototype unit which will fit any existing monitor. This could in theory be made any size, including large enough to protect an entire room. The technique could then be applied to improve the efficacy of sensitive scientific research work, or medical scanning.

By using SQUIDS (Superconducting Quantum Interference Detectors), for example, it is possible to detect magnetic activity in different parts of the human brain, but field strengths are minute, and presently require working in carefully shielded rooms.

And just as cancelling fields can be used to prevent stray magnetic fields getting in, it should also be possible to use the technique to prevent fields getting out, as part of a strategy to reduce interference, and to prevent eavesdropping on computer monitors.

In the latter case, the offending signals are at frequencies much too high to respond to in a servo loop. It should however, be possible to add a cancelling field to signal fields generated by the monitor.

Microvitec

SCL

Design Pointers

* Using the active field cancellation system, it is possible to cancel steady magnetic fields of up to 10 Gauss, and fields varying at 50Hz at up to 1 Gauss.

* Frequency response is up to 1 to 10 KHz, but with suitable servo amplifiers, there is no limit to either field strength or frequency

* The technique can be applied to any piece of equipment of any size, and may, in theory, also be used to suppress stray fields getting out, as well as interfering fields getting in.

Expensive measures offer present solutions

Liquid crystal display monitors are totally impervious to the effects of magnetic fields. Large display areas are so far unavailable and screens of even moderate size are expensive.

The more common approach is to surround conventional cathode ray tube monitors with mu metal, an alloy which short circuits magnetic fields in the same way that conductive metal shielding short circuits the electric components of radio waves.

Mu metal is a fairly expensive material and difficult to work with. Work hardening, for example, can severely affect its shielding properties. The result is that mu metal shielding tends to add from 400 to 800 to the cost of a monitor, in addition to increasing weight. It also fails to cover the front of a screen. Some protection can be given by forming a tunnel round the sides of the screen, but such an approach allows screens to only be viewed from directly in front.

Other types of monitor, such as full colour electroluminescent, are still undergoing development.

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