GB2191291A

GB2191291A - Magnetic pick-offs

Info

Publication number: GB2191291A
Application number: GB08612046A
Authority: GB
Inventors: Alfred William Piercey
Original assignee: GABRIEL MICROWAVE Ltd
Current assignee: GABRIEL MICROWAVE Ltd
Priority date: 1986-05-17
Filing date: 1986-05-17
Publication date: 1987-12-09
Anticipated expiration: 2006-05-17
Also published as: GB2191291B; GB8612046D0

Abstract

In order to avoid the use of wires connecting the pick-off to an external power supply or to a remote controller or indicator, the pick-off comprises a casing, a sensing coil 2, within the casing, and a light source 3 disposed within the casing in close proximity to the coil 2 and electrically coupled to the coil 2 so as to be powered by the sensing current from the coil 2. A light signal proportional to the sensing current may thereby be transmitted to the remote controller 5 or indicator by way of an optical communication link 4 without requiring the supply of power to the pick-off from an external power supply. This arrangement substantially eliminates the problems of the pick up of external electromagnetic interference by the wires. <IMAGE>

Description

SPECIFICATION Magnetic pick-offs This invention relates to magnetic pick-offs, and is concerned more particularly, but not exclusively, with such pick-offs for measuring one or more parameters of a controlled machine or system and for supplying these measurements to an electronic controller for controlling the machine or system.

It is well known that the wires carrying measurement signals to such electronic controllers must be arranged or shielded so as to minimise the effects of external electromagnetic interference on such signals. However it is not always practicable to completely protect such a system from external interference, particularly where the measurement signals are of very low power. Prior proposals for overcoming this problem have included use of a preamplifier located within the pick-off, and/or the use of fibre-optic coupling between the pick-off and the controller. However neither of these methods has proved entirely satisfactory since the preamplifier and the light modulation circuitry are powered from an external power source and may themselves be affected by interference picked up by the power supply or the supply leads.For example, where such an arrangement is used to control the turntable ladder of a fire engine, transmitted radio signals may be picked up by the supply leads, which may act as fractional wavelength aerials, and this may cause a malfunction of the controller.

It is an object of the invention to provide a novel form of magnetic pick-off which substantially avoids such difficulties.

According to the present invention there is provided a magnetic pick-off comprising a casing, a sensing coil within the casing, and a light source disposed within the casing in close proximity to the coil and electrically coupled to the coil so as to be powered by the sensing current from the coil, whereby a light signal proportional to the sensing current may be transmitted to a remote controller or indicator by way of an optical communication link without requiring the supply of power to the pick-off from an external power source.

It will be appreciated that such a pick-off does not require the use of wires connecting the pick-off to the controller or indicator or of leads connecting an external power supply to the pick-off. Instead a light source is powered by the sensing current without the application of external power, and the resulting light signal is transmitted to the controller by way of the optical communication link. Such an arrangement substantially eliminates the problems of interference by radio signals and other electromagnetic field effects.

In certain applications and under certain conditions the sensing current may be inadequate to directly power the light source, in which case the pick-off may comprise power storage means for storing the sensing current over a period of time and for discharging a current pulse at the end of the storage period of sufficient power to cause the light source to flash. The power storage means preferably includes a capacitor.

In an alternative embodiment for use in applications where the sensing current may be inadequate to directly power the light source, the pick-off includes a regenerative magnetic circuit, for example of the Weigan wire type, which serves to produce, in response to sensing of a relatively slow flux variation by the pick-off, a relatively fast flux variation for a short period sufficient to generate instantaneous power to cause the light source to flash.

Where circumstances permit, the pick-off may include a frequency multiplier for enabling multiple light pulses to be generated by the light source in response to a single actuating signal.

The light source may comprise a light-emitting diode, and optionally also a light modulation circuit for modulating the light-emitting diode.

The invention also provides a control system comprising such a pick-off in combination with a controller adapted to receive a light control signal.

In order that the invention may be more fully understood, various embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 shows a magnetic pick-off unit connected to an electronic controller by an optical fibre link, together with a corresponding schematic circuit diagram of a first embodiment of the invention; Figure 2 is a schematic circuit diagram of a second embodiment; Figure 3 is a schematic diagram of a variable reluctance magnetic sensor; and Figures 4 and 5 are schematic circuit diagrams of third and fourth embodiments.

Referring to Figure 1, a tacho-alternator 1 incorporates an alternator winding 2 and a light-emitting diode 3 connected directly to the alternator winding 2. The output from the winding 2 modulates the light-emitting diode 3, and the resulting light signal is passed along an optical fibre link 4 to a controller 5 which incorporates an opto-electronic stage 6 and a control stage 7.

Referring to Figure 2, the alternator 1 may generate enough power to drive additional electronic circuits for processing the basic signal to generate multiple pulses and thus achieve frequency multiplication, before modulating the light-emitting diode 3. These additional circuits may comprise a power supply stage 8 and an electronic modulator or frequency multiplier 9.

The alternator 1 may be a variable reluctance magnetic sensor, which is essentially a low powered, inefficient alternator. As shown in Figure 3, such a sensor 10 may be formed by a small coil 11 wound around a soft-iron polepiece 72 which is loosely coupled to a permanent magnet 13. Rotation of a ferrous toothed wheel 14 in front of the polepiece 12 causes modulation of the magnetic flux through the coil 11, thereby generating a sensing current at the output of the coil 11.

The sensor 10 may directly modulate the light-emitting diode 3, as shown in Figure 1.

However the output from such a sensor 10 will be of fixed mean power and voltage for a given frequency and air gap. Furthermore, in order to enable appropriate communication with the controller 5 along the optical fibre link 4, it is necessary to generate a ievel of illumination at the controller input which is adequate for the input interface. This requires a threshold current through the light-emitting diode 3 at a minimum, but approximately constant, voltage. In certain applications and under certain conditions, the sensor 10 may not be able to generate the required power continuously on demand, in which case it may become necessary to convert the sensor low mean power to pulses of short duration but of high instantaneous power.

Figure 4 shows an embodiment for use in such applications. In this embodiment the high voltage, low current output from the coil 11 is supplied to a power storage circuit 15 which charges a capacitor 16 and subsequently discharges the capacitor 16 through the lightemitting diode 3 under the control of a trigger signal and control circuit 17, thereby generating short synchronised pulse discharges through the light-emitting diode 3.

In an alternative embodiment, shown in Figure 5, a regenerative magnetic circuit of the Weigan wire type, comprising a polepiece 18 and a coil 19, is connected between the coil 11 and the light-emitting diode 3. The magnetic circuit exhibits hysteresis having a snap action. The Weigan wire, by means of a coaxial magnetostrictive effect, converts a low flux-change-rate to a fast flux-change-rate for a short period. This flux-change envelope can then generate the voltage-current required to drive the light-emitting diode 3. This magnetostrictive effect can be achieved either directly by means of a suitably designed sensor, or by allowing the normal sensor to generate the current which modulates the Weigan wire, as shown in Figure 5.

The above described arrangements are particularly suitable for inclusion in magnetic pickoffs of the type described in Magnetic Perception Heads: Principles and Practice, Electronic En gineering, June 1978 issue, the contents of which are incorporated herein by reference.

Claims

1. A magnetic pick-off comprising a casing, a sensing coil within the casing, and a light source disposed within the casing in close proximity to the coil and electrically coupled to the coil so as to be powered by the sensing current from the coil, whereby a light signal proportional to the sensing current may be transmitted to a remote controller or indicator by way of an optical communication link without requiring the supply of power to the pickoff from an external power source.

2. A magnetic pick-off according to claim 1, further comprising power storage means for storing the sensing current over a period of time and for discharging a current pulse at the end of the storage period of sufficient power to cause the light source to flash.

3. A magnetic pick-off according to claim 2, wherein the power storage means includes a capacitor.

4. A magnetic pick-off according to claim 1, further comprising a regenerative magnetic circuit for producing, in response to sensing of relatively slow flux variation by the pick-off, a relatively fast flux variation for a short period sufficient to generate instantaneous power to cause the light source to flash.

5. A magnetic pick-off according to claim 4, wherein the regenerative magnetic circuit is of the Weigan wire type.

6. A magnetic pick-off according to any preceding claim, which includes a frequency multiplier, for enabling multiple light pulses to be generated by the light source in response to a single actuating signal.

7. A magnetic pick-off according to any preceding claim, wherein the light source includes a light-emitting diode.

8. A magnetic pick-off according to any preceding claim, wherein the light source includes a light modulation circuit.

9. A magnetic pick-off substantially as hereinbefore described with reference to the accompanying drawing.

10. A control system comprising a magnetic pick-off according to any preceding claim in combination with a controller adapted to receive a light control signal.