WO2008022358A1

WO2008022358A1 - Magnetic field detector

Info

Publication number: WO2008022358A1
Application number: PCT/ZA2007/000050
Authority: WO
Inventors: Leon Le Roux De Beer
Original assignee: Teqnovo (Pty) Ltd
Priority date: 2006-08-13
Filing date: 2007-08-10
Publication date: 2008-02-21

Abstract

The invention provides a magnetic field detector, which detector includes reed switch means and alternating magnetic field pre-tensioning. The reed switch means is a reed switch. A source of the alternating magnetic field is a coil connected to an alternating voltage source. In use, the reed switch is placed next to or within a coil which is connected to an alternating voltage source which produces an alternating magnet field over the reed switch so that the field over the reed switch cycles between N/S and S/N.

Description

Magnetic Field Detector

Field of the Invention

The invention relates to magnetic field detectors.

Background to the Invention

Instruments that are sensitive to foreign magnetic fields require protection against willful or accidental magnetic interference. Screening is normally only partially effective. An effective way is to provide a warning or take the instrument out of service if it is subjected to a magnetic field strong enough to cause malfunction. This can normally easily be achieved by fitting devices such as Hall effect sensors. These devices, however, normally have a relatively large power consumption and are unsuitable if the instrument itself is powered by a small battery that must operate for up to say ten years without a need to change the battery. The choice of magnetic detector therefore usually revolves around cost, power consumption and sensitivity.

An interferential water meter, fitted with an electronic read out, shut off valve and an automatic on-board power supply, provides an example of a difficult problem. It requires a low cost, highly sensitive, accurate and low power consumption magnetic field detector.

It is well known that the sensitivity of a reed switch can be improved upon by placing a permanent magnet near the read switch, as shown in figure 1.

The magnet is placed far enough from the reed switch so that the magnetic field of Me small magnet is not sufficient to close the reed switch. Additional magnetic flux from an external source, if in the same North / South direction, can provide the shortfall in flux density needed to close the reed switch. By moving the small magnet further away or closer to the reed switch the pre- tensioning of the reed switch can be set to the external field strength the system must respond to. There are, however, two main disadvantages:

a) It is difficult to accurately position the pre-tensioning magnet; and

b) The device is magnetic pole sensitive, i.e. if the external field is N/S relative to the N/S field of the pre-tensioning magnet, the external field would strengthen the pre-tensioning field. If the external field is, however, S/N, the external field strength required to close the reed switch would have to equal the pre-tension magnet field plus the field required to close the non-pre-tensioned reed switch. This system is furthermore insensitive due to hystereses.

To detect external fields from any direction at least three devices are required.

This device is neither cost-effective nor consistently accurate. Power consumption is, however, low when correctly integrated.

The S/N, N/S insensitivity of a single pre-tensioned reed switch may be at least partially overcome by using two pre-tensioned reed switches as shown in figure 2.

Summary of the Invention

The invention provides a magnetic field detector, which detector includes reed switch means and alternating magnetic field pre-tensioning. The reed switch means may be a reed switch.

The alternating magnetic field source may be a coil connected to an alternating voltage source.

The reed switch may be placed next to or within a coil which is connected to an alternating voltage source.

This produces an alternating magnet field over the reed switch i.e. the field over the reed switch cycles between N/S and S/N.

The amplitude and/or frequency of the alternating voltage source may be selectable.

The selection may be made in response to the magnetic field caused by an external stimulus, such as an interfering external magnetic field, that must be detected.

The coil thus may replace the pre-tension magnet of figure 1.

The pre-tensioning may be set very accurately by adjusting the applied voltage to the coil.

By using an alternating voltage the device can check for an external field of either pole direction every full cycle.

The invention extends to an interferential flow meter including a magnetic field detector of the invention.

The flow meter may be of the interferential type and may include an electronic read out and an on-board power supply The flow meter may be a water meter.

The water meter may be a pre-paid water meter.

Specific Description of an Example of the Invention

Without being limited in scope of the invention either by the example or by the theory, this can be presented as follows:

Figure 1 shows, in schematic representation, a reed switch of the prior art;

Figure 2 shows, in schematic representation, a pair of reed switches which attempt to address the shortcomings of Figure 1 ;

Figure 3 shows, in schematic representation, a magnetic field detector of the invention;

Figure 4 shows an alternating magnetic field; and

Figure 5 shows, in schematic representation, a proposed solution to the problem posed by the problem to the invention and as addressed by the detector of Figure 3.

With reference to Figures 3 to 5, the N/S or S/N field to close the reed switch is not a fixed value for a batch of reed switches but is a constant for a given reed switch if hysteresis effects are ignored.

Cycling reduces the hysteresis effects and the device can quickly be set to the desired level of external field detection. If the field strength to be detected is in the order of magnitude of the earth's magnetic field at that location, the instrument must be set in a fixed position before final calibration.

For this type of sensitive sensing a zero offset of the alternating coil voltage is required in order to obtain a magnetic field that cycles around zero when the cycling field is combined with the static field of the earth at that location.

Only a single coil voltage cycle is required to detect an external field which is larger than the maximum allowed i.e. the set value.

The minimum power consumption is the power required per cycle. The average consumption is determined by how often detection of an external field is required i.e. how often the coils are pulsed.

The above explanation makes use of a sinusoidal wave form. A square wave saw tooth wave or the like, may also be used.

Claims

1. A magnetic field detector, which detector includes reed switch means and alternating magnetic field pre-tensioning.

2. A detector as claimed in claim 1 , wherein the reed switch means is a reed switch.

3. A detector as claimed in claim 1 or claim 2, wherein a source of the alternating magnetic field is a coil connected to an alternating voltage source.

4. A detector as claimed in any one of claims 2 and 3, wherein the reed switch is placed next to or within a coil which is connected to an alternating voltage source which produces an alternating magnet field over the reed switch so that the field over the reed switch cycles between N/S and S/N.

5. A detector as claimed in any one of the preceding claims, wherein the amplitude and/or frequency of the alternating voltage source is selectable.

6. A detector as claimed in claim 5, wherein the selection is made in response to the magnetic field caused by an external stimulus, such as an interfering external magnetic field, that must be detected.

7. A detector as claimed in any one of claims 3 to 6, wherein the coil replaces a pre-tension magnet.

8. A detector as claimed in any one of claims 3 to 7, wherein the pre- tensioning is set very accurately by adjusting the applied voltage to the coil.

9. A detector as claimed in any one of the preceding claims, wherein by using an alternating voltage an external field of either pole direction is checked every full cycle.

10.An interferential flow meter including a magnetic field detector as claimed in any one of the preceding claims.

11.A flow meter as claimed in claim 10, which is of the interferential type and includes an electronic read out and an on-board power supply

12. A flow meter as claimed in claim 10 or claim 11 , which is a water meter.

13. A flow meter as claimed in claim 12, which is a pre-paid water meter.

14. A magnetic field detector as claimed in claim 1 , substantially as herein described and illustrated.

15.An interferential flow meter as claimed in claim 10, substantially as herein described and illustrated.

16. A new magnetic field detector or a new interferential flow meter substantially as herein described.