GB2224568A - Fibre-optic continuity tester device - Google Patents

Abstract

A fibre-optic cable is connected between a transmitter unit (Figure 1) and a receiver unit (Figure 2). Light from a light emitting diode (8) in the transmitter unit is transmitted over the cable if it is continuous to be received by a light sensitive diode (12) in the receiver unit. This light activates the receiver unit and causes either a light emitting diode (18) or an acoustic sounder (20) to be activated to indicate continuity in the cable. The receiver unit circuitry may act as a filter to remove low frequencies, particularly those associated with a local power supply. The transmitted light may be pulsed at 3 kHz. <IMAGE>

Description

FIBRE-OPTIC CONTINUITY TESTER This invention relates to a fibre-optic continuity tester and is particularly concerned with a device for testing in a ron-destructive manner the continuity of a fibre-optic cable or conductor.

Increasingly fibre-optics are being used particularly in circumstances where intrinsic safety is paramount to conduct information fran one point to another. Fibre-optics are particularly useful where there is a substantial amount of magnetic or electrical interference in the neighbourhood of the conductor which might upset signals of small value Which were being conducted along a wire in a more conventional electronic transmission of information.

Many fibre-optic conductors are themselves very small and do not have a great breaking strain. It is therefore very easy for them to be broken and for the break not to be readily visibie by an outside examination of the conductor. In order to test the continuity of the conductor fran one end to another it is necessary to shine a light fran one end to a remote end and to monitor that the transmission of light is correct. Furthermore, it is essential that the ends of the fibre-optic oonductor at either end are properly positioned so as to receive light directly fran a source.

It is an object of the present invention to provide an improved fibre-optic continuity tester Which is able to be used in an intrinsically safe environment and which is simple in its construction but yet reliable.

According to the present invention a fibre-optic continuity tester device comprises a transmitter unit including a light emitting source and a. receiving unit including a light sensitive detector, means for connecting the- ends of a fibre-optic cable under test between the source and the detector and means for indicating when light is transmitted from the source to the detector.

The light source is preferably light emitting diode (LED), the light sensitive detector may include an alarm device of an audio nature such as an acoustic sounder. Both the light emitting source and the light sensitive detector are preferably powered in a solid state electronic circuit. The circuit of the receiver preferably includes filtering means to remove low frequencies paricularly those Which are of the frequency of any local power supply or harmonics thereof. The frequency circuit may also be designed to ensure that large levels of ambient light do not disrupt the sensitivity of the detector. The circuitry is preferably operated from an internai low voltage battery power source.

Figures 1 and 2 of the accanpanying drawings show respectively a transmitter unit and a receiver unit circuit of a fibre-optic continuity tester device in accordance with the invention. The device is for testing continuity of fibre-optic conductors used in a coal mine and because of this the design of the circuitry has to be intrinsically safe. This means that if there is a spark it has insufficient energy to cause an explosion.

Referring first to Figure 1 which shows the circuitry of the transmitter unit this shows power fran a supply source is applied to terminals 1. The power is 9 volts and the lower terminal in the drawing is held at zero-volts level. The current fran the terminals is fed into a potential divider comprised by two resistors 2 and fed to the non-inverting input terminal of a solid state amplifier 3. The inverting input terminal of this amplifier is connected to zero-volts via a capacitor 4 and there is a feedback circuit between the output of the amplifier 3 via resistors 5 and 6 respectively to the inverting and non-inverting terminals.The output is also fed fran 3 to a resistor 7 to a light emitting diode 8 having its negative side connected to the ground rail. Energisation of the 1 8 causes this LED to be activated and to emit a modulated light beam having a wave form as shown at 9 and a modulation frequency of approximately 3 kilohertz. The end of the optical fibre cable being tested is held in a fitment (not shown) adjacent to the LD 8 to ensure that light falls on its end at the correct incidence.

Referring now to Figure 2 Which shows the receiver unit it will be seen that this unit again receives voltage fran a 9 volt source fran terminals 11 the lower one of Which is at zero-volts. A photodiode 12 adjacent the remote end of the fibre-optic cable under test is connected between the terminals 11 in series with a resistor 13. Light received by the photodiode 12 causes it to became conductive and to feed current through integrating and amplifying circuitry canprising amplifiers 14, 15 and 16 and associated components to one of two alternative circuits governed by a switch 17.

In the position shown the switch 17 is connected in series with a further light emitting diode 18 and if there is continuity of the fibre-optic cable the diode 18 is activated to give an output indicating this continuity. If the switch 17 is in its alternative position, i.e.

to the right as shown Figure 2, then the voltage is imposed across the zener diode 19 and the oscillator circuit connected to an acoustic sounder 20 is activated and the sounder 20 emits an audible signal indicating continuity.

The circuitry shown includes canponents Which have values calculated in order to apply a filtering effect to the signals traversing them. For example, any ambient power source having a frequency at 50, 100 or 150 hertz Which may arise for example fran electric lights are filtered out so that no spurious activation of the Trn 18 or sounder 20 occurs.

Furthermore only relatively high frequency signals are transmitted so that if there is a large ambient or background light level marking the receiver unit then this d does not confuse the operation of the tester and only selected high frequency levels of signal are transmitted and recorded.

Because of the low current levels involved in this invention it is very good for use in hazardous atmospheres such as coalmines or petroleum refinery installations Where intrinsic safety of electrical components is essential.

The whole tester can be made into a small canpact unit operated by a simple 9 volt battery and sealed in a suitable box.

The tester is efficient, reliable and effective in operation and inexpensive to manufacture.

Claims (7)

1. A fibre-optic continuity testing device canprising a transmitter unit including a light emitting source and a receiving unit including a light sensitive detector, means for connecting one end of a fibre-optic cable to the light emitting source and the other end to the light sensitive detector of the receiving unit.

2. A device as claimed in claim 1 wherein the light emitting source canprises a light emitting diode.

3. A device as claimed in claim 1 or claim 2 wherein the light sensitive detector includes sound emitting means for indicating the continuity of a cable under test.

4. A device as claimed in any preceding claim Wherein the light sensitive detector includes light emitting means for indicating the continuity of the cable under test.

5. A device as claimed in any preceding claim and including filter circuitry to remove low frequencies.

6. A device as claimed in any preceding claim Wherein the light is transmitted fran the light emitting source at a frequency in the order of 3 kilohertz.

7. A fibre-optic continuity tester device substantially as hereinbefore described and with reference to Figures 1 and 2 of the accanpanying drawings.