Background of the Invention: Conventional demagnetising coils for ferromagnetic steel components which have become magnetised during their manufacture or inspection are run continuously from the mains at mains frequency, the decaying field required for demagnetisation being achieved by withdrawing the component to be demagnetised from the centre of the coil along a line axi-symmetric with the centre of the coil for a prescribed distance, typically 1-2 metres. The ferromagnetic steel components may have been magnetised by having been held in magnetic chucks or by having had magnetic particle inspection performed upon them.

For certain grades of hardened ferromagnetic steel, or for ferromagnetic steel (hardened or otherwise) components with parts with sharp comers or discontinuities, the abovementioned AC continuous coils produce an insufficient peak field to enable any reversal of the polarity of magnetisation in the magnetised component, thus making it impossible to demagnetise the component.

Objects and Summarv of the Invention: It is thus the principal object of the present invention to provide a solution to or amelioration of the abovementioned problem.

The invention resides in the provision of demagnetisation systems which produce a higher peak field in a given coil, thus enabling demagnetisation of components which cannot be demagnetised using conventional systems.

According to a first aspect of the invention, the decaying AC field required to demagnetise the components is derived by imposing a decay envelope, for example a near-exponential decay envelope, on the initial AC field produced by connecting to the mains. The sample may thus be placed directly in the coil, and does not need to be withdrawn from the coil to effect the decaying field. This system can thus be run at a higher initial power and hence field than the conventional continuous AC system.

According to a second aspect of the invention, an even higher initial field may be achieved by running the coil at a frequency below mains frequency, typically 3 to 9 Hertz or lower, the higher field being possible due to the lower inductance of the coil allowing a higher current for a given applied voltage. This aspect of the invention may be used alone, or in combination with the abovementioned first aspect of the invention, or with currently available systems.

A further advantage of the invention over continuously operated conventional demagnetisation coils, stems from the fact that demagnetisation is achieved with the sample static in the demagnetisation coil whereas in the conventional system the decaying field required for demagnetisation is achieved by withdrawing the component to be demagnetised from the centre

of the coil along a line axi-symmetric with the centre of the coil for a prescribed distance, typically 1-2 metres. This almost inevitably exposes the operator to repeated doses of low frequency electromagnetic radiation. To eradicate the possibility of exposure of the operator to electromagnetic radiation in the conventional system would require a large ferromagnetic cage to be built around the system, to allow the withdrawal of the component (s) to be demagnetised the prescribed distance from the centre of the coil. The withdrawn component would also need to be remotely activated and automated, increasing cost and reducing efficiency. In the system of the invention, the components would be demagnetised while static in the coil, thus enabling much more compact electromagnetic shielding to be used relative to the conventional system. Operation would also be much easier as there would be no need for withdrawal of the components from the coil during demagnetisation.

The invention will be further described in the following by reference to the accompanying drawings which illustrate an exemplary embodiment.

Description of the Drawings: Figure 1 shows schematically a power circuit of the embodiment; Figure 2 shows schematically a phase control circuit of the embodiment; Figure 3 shows schematically an envelope control circuit of the embodiment; and

Figures 4A and 4B show two housings for accommodating and shielding the coil arrangement of the embodiment.

Description of the Embodiment: 1. The Power Circuit As shown in Figure 1, the power circuit connects the mains input to the demagnetisation coil. A 30 microFarad capacitor is placed across the input to reduce high frequency interference. The capacitor is used rather than a proprietary filter due to the high currents involved.

A 3,500 microFarad capacitor is placed in series with the demagnetisation coil to filter out any DC component of voltage which would unbalance the symmetry of the demagnetisation signal. This capacitor is crucial in ensuring good demagnetisation.

Switching thyristors are controlled by the phase control circuit, which is, in turn, controlled by the envelope control circuit, giving a near exponentially enveloped AC decaying signal in the demagnetisation coil.

2. The Phase Control Circuit

The phase control circuit is as may be used in magnetising/demagnetising systems as described in W098/29883 the disclosure whereof is incorporated herein by reference.

3. Envelope Decay Circuit The envelope decay circuit controls the phase control circuit by providing an output of between 1.7V and 4.8V. An output of 1.7V causes the phase control circuit to allow full mains power to be applied through the switching thyristors, and an increase from 1.7V in the output of the envelope decay circuit causes an exponential reduction in the amplitude of the mains power across the coil, until the thyristors are switched off when the output of the exponential decay circuit reaches 3.8V, ceasing conduction in the power circuit. This gives the required exponential decay of AC voltage in the coil. A full exponential decay of the AC voltage is not required, as it would take too long, and is not necessary.

Each of Figures 4A and 4B shows a respective housing 10 and 12 for accommodating and magnetically shielding a coil (not shown) and the component (s) (not shown) placed within the coil. The illustrated housings each comprise a box having a closable access opening through which the coil and component (s) can be loaded into the box.

The box in Figure 4A has a side door 14 for closing its access opening

16 and the box in Figure 4B has two top doors 18 for closing its access opening 20. Figure 4B also shows pneumatic cylindrical push rods 22 for opening and closing the doors 18.

Advantageously, this enables the coil arrangement to be electromagnetically shielded in use.

Having described the invention in the foregoing by reference to a specific embodiment, it is to be well understood that the invention is not limited to the embodiment described and that modifications and variations to the embodiment could be made without departure from the invention.