GB2279536A - Receiver/transmitter for electromagnetic communications - Google Patents

Abstract

Apparatus for electromagnetic communications underground, e.g. in deep coal mines, comprises an inductive transducer (1) (e.g. based on a ferrite toroid) arranged to be removably and inductively coupled, either with a long piece of metalwork such as a pipeline or cable (9), or with a loop antenna, according to choice by the operator of the apparatus. The apparatus can be powered by a hand-cranked generator and is free of the need for galvanic connection either to an external power source orto an external signal line. <IMAGE>

Description

Electronic Apparatus and Methods for Electromagnetic Communications Field of the Invention This invention relates to electronic apparatus and methods for electromagnetic communications. Among other things it relates to transmitters, receivers and transceivers and their use for establishing communications in underground environments such as mines, e.g. in deep coal mines, e.g. in emergency conditions that may cut off usual channels of communication.

Background of the Invention Emergency communications systems can be of use for example in underground tunnels and excavations if for example a roof fall has trapped workers on the blind side. Such an accident is possible whenever underground workers are located in a place without an alternative route of escape to the surface, and it is often not feasible to provide such alternative routes. In the UK the rate of incidents in which miners have been trapped behind a fall of ground is very small. Even so, rescue teams have not always known the severity of a fall when it has occurred, or the condition of the trapped men.

Contact, especially voice contact, with trapped workers on the other side of such a fall is highly desirable to help rescue operations, to increase the probability of saving life, and to facilitate cooperation between rescuers and any workers trapped by such a fall, as well as to reassure those trapped.

Up to the present there is no universally applicable scheme for such emergency communications that is commercially available.

There are through-the-overburden one-way message transmitting systems.

But tests have shown that these do not work in deep coal mines of the kind that exist, for example, in the UK. There are also cable-linked communications systems, e.g. telephone lines, but such systems are vulnerable to be cut off by damage in the event of a rock burst or roof fall. Hammering on pipes or other metalwork has been resorted to, but it has limited capacity for conveying useful information.

It remains an aim to provide robust and effective communications systems suitable for use in emergencies such as those that can arise underground as already mentioned.

The Present Invention According to the present invention, there is provided electronic apparatus suitable for communications underground, e.g. in deep coal mines, comprising a transmitter and/or a receiver for propagating a transmitted signal to a remote receiver or for receiving a signal propagated from a remote transmitter, without requiring an electric-current connection between the receiver and the transmitter, said apparatus comprising an inductive transducer arranged to be removably and inductively coupled, either with a long piece of metalwork such as a pipeline or cable, or with a loop antenna, according to choice by the operator of the apparatus.

An example of the invention described in further detail below has for example the following notable features: It incorporates both a receiver and a transmitter; the inductive transducer is common to the receiver and the transmitter; the inductive transducer comprises a toroidal transformer with a high-permeability, e.g. ferrite, core; the transformer has a removable section of its core arranged to be removed and replaced by an operator to allow the core to be linked inductively with the long metalwork or the loop antenna.

Apparatus such as that described herein is capable of use for signal propagation effectively in two modes, accordingly as the transceiver unit is linked inductively to long metalwork or to a loop antenna.

The electrical configuration is preferably such that the choice between the use of long metalwork or loop antenna is without significant effect on the frequency-tuning of the apparatus. The apparatus can be made to operate at an essentially fixed signal frequency, e.g. a frequency in the range 30-600 kHz. The lower end of that range has been found highly useful especially when continuous underground metalwork is available for inductive linking with the transmitter and receiver, such as power cables, rails or pipes. A preferred frequency range for such an example is about 30-70kHz. At such frequencies, useful signal propagation in the alternate mode via a loop antenna can also be achieved.

In an alternative arrangement, for use in environments where it may be necessary to link the transducers inductively with metalwork which is not continuous between transmitter and receiver, e.g. where there are multiple cables in a mine tunnel but they are not connected together, it can be useful to employ a higher signal frequency, e.g. up to about 600 kHz, especially for example in the range 250 - 600 kHz, since in this frequency range it is found that useful parasitic coupling of signals between separate cables and metalwork within a mine tunnel can be maximised. Dual-frequency or multi-frequency transmitter/receiver units can also be provided.For example, the receiver and transmitter can if desired use plural frequencies e.g. a lower frequency in the band 30-70 kHz, and a higher frequency in the band 250-600 kHz, to increase likelihood of establishing effective communications in difficult conditions.

Most useful embodiments of the invention are free of the need for any external power source: they can be housed in a casing that also contains a generator or battery, most preferably a hand-cranked generator. Most often it is desirable to provide an enclosure and a general construction for the apparatus which accords with the well-established intrinsic safety (IS) standards for operation of equipment within a flammable envoronment, and to use a generator of a configuration which is free of a commutator or brushes, e.g. a stepping motor used in generator mode.

The invention can be embodied for example, as described and illustrated below, as an emergency communications unit in a casing that contains a transmitter-receiver and a hand-cranked generator, together with a microphone, loudspeaker, a push-to-talk switch (or button), and a ferrite toroid inductive coupler, together with a separate loop antenna, for optional use, which can be simply unfolded and coupled with the toroidal coupler, as described herein.

Examples of the unit can be labelled as equipment for use in emergency and placed at strategically chosen places in the mine workings, so as to be accessible to any mine worker in emergency.

Examples of such a transmitter/receiver have provided for example two-way voice communication through more than 50 metres of ground, or along considerably longer distances of continuous underground metalwork, e.g. underground power cables or metal pipes or rails, e.g.

up to several kilometres distance in an electrically quiet mine roadway. Examples of the system described herein can promise high reliability and availability even after several years of storage or deposit in the mine workings. Such receiver/transmitters can be made so as to operate in flammable atmospheres, with a simple method of operation accessible to the general underground work-force after relatively little training.

Further Details of the Invention, and Description of the Drawings Embodiments of the invention are described in further detail below, without limitation on the scope of the invention, and with reference to the accompanying drawings, of which: Figure I is a diagrammatic perspective view of an emergency communications unit according to an embodiment of the invention, shown in one of its in-use positions in which it is inductively coupled with a power cable in a mine.

Figure 2 is a diagrammatic perspective view of the same unit as in Figure 1, shown in another of its in-use positions in which it is inductively coupled with a loop antenna laid out in a mine roadway.

Figure 3 shows some constructional detail of the unit of Figures 1 and 2, in diagrammatic perspective and in partly exploded form.

Figure 4 is a schematic diagram of system sub-components of the communications unit of Figures 1 to 3.

Figure 5 is a functional block diagram of the receiver embodied in the unit of Figures 1 to 4.

Figure 6 shows electronic circuit arrangements corresponding to the receiver functions of Figure 5.

Figure 7 is a functional block diagram of the transmitter embodied in the unit of Figures 1 to 4.

Figure 8 shows electronic circuit arrangements corresponding to the transmitter functions of Figure 7.

Figure 9 shows a form of transmitter output circuitry for use as an alternative to some of the arrangements of Figure 8.

Referring to Figure 1 of the drawings, there is shown generally an emergency underground two-way voice communications unit suitable for use e.g. in deep coal mines, having a casing 1, an instruction label 2, a microphone/loudspeaker point 3, a push-to-talk button 4, a carrying handle 5, a generator crank handle 6 of a hand-powered generator to proved electrical power for the unit, and a generally rectangular aperture 7 surrounded by a toroidal coupler which is shown in detail in later figures but is visibly represented in Figure 1 by its slidably removable and replaceable side 8 which can be removed and replaced by the operator. In Figure 1, the unit is shown in one of its in-use positions in which it is inductively coupled with a power cable 9 in a mine.In order to reach this position the operator has opened the toroidal coupler by slidingly removing coupler side 8, engaged the aperture 7 with a suitably chosen cable 9 in a mine roadway and replaced the coupler side 8 to leave the unit and its toroidal coupler linked with the cable 9. A suitable cable to choose in an emergency is a cable believed by the operator to extend to a point beyond a roof-fall or other obstruction so that it can be reached by persons with similar communications units on the other side of the obstruction. Any other long metalwork such as a rail or pipe can be used instead.

In order to operate the unit in 'receive' mode the operator simply turns the generator crank handle 6 and listens for voice communication through the loudspeaker at 3. In order to transmit speech signals the operator also pushes on button 4 while turning the crank handle 6 and speaks to the microphone at 3.

An optical indicator preferably a light-emitting diode can be provided (not shown in Figure 1) to light up and indicate to the operator that the crank handle 6 is being turned at a sufficient rate to power the device effectively.

Figure 2 shows the same unit in another of its in-use positions in which it is inductively coupled with a loop antenna ('aerial') laid out in a mine roadway. A preferred loop antenna suitable for use with the example unit described herein is a flexible loop of a size sufficient to provide a square loop of side 5 metres, comprising 18 turns of insulated wire preferably forming a continuous electrical loop. The number of turns is not critical and for example 12-20 turns have been used. It has also been found that acceptable communications can often be set up even if the loop 6 becomes open circuit.The loop is not provided with its own separate electrical tuning components, these are unnecessary in connection with the arrangements described herein, in which the main toroid coupler coil is tuned, and there is also no need nor provision here for galvanic connection between the loop antenna (or other guide metalwork) and the main communications unit.

The unit described herein can often give better performance when inductively coupled with long metalwork extending continuously from a transmitting station to (for example) a generally similar receiving station as in Figure 1, but in the absence of access to such metalwork, which of course cannot be guaranteed in emergency, the transmission arrangement of Figure 2 can be used instead. The arrangements described herein enable these dual modes of signal propagation to be used alternately at the option of the operator in response to exigencies of an emergency without any adjustment of the communications unit other than the choice of metalwork or antenna with which to link it, there is no need to perform any electrical tuning adjustment in connection with that choice.

Packages each containing an example of the communications unit, together with the (physically separate) loop for optional use with it, suitably folded and packed, are preferably packed and placed together, labelled as equipment for use in emergency, at strategically chosen places in the mine workings, so as to be accessible to any mine worker in emergency.

Figure 3 shows mechanical constructional detail in partly exploded diagrammatic view, of the emergency communications unit.

Figure 3 shows casing parts 1, forming a casing that houses electrical components and system boards (not shown here) and a ferrite toroidal coupler 2, made in two mechanically separate parts, a U-section 2a and an I-section 2b (effectively of bar shape), arranged for close juxtaposition to form a high-permeability core for an inductive coupler, forming a magnetic circuit of low magnetic reluctance particularly at the close contact surfaces between the ferrite U-section 2a and the I-section 2b.

Casing parts 1 together enclose U-section 2a of the ferrite toroidal coupler 2. A useful degree of resistance to shock is provided by moulded insert members 3, located within the casing so as to assist positive component location and especially provide shock-absorbance for the ferrite toroid 2. Insert 3 can also help to retain fragments of toroid 2 in functioning close proximity together, with a path of low magnetic reluctance, even if some shock in an accident causes the ferrite material of the coupler to fracture.

Spring clip components 4 are fixed to casing part 1 for retaining a carrier 5 to which is mounted I-section 2b of the toroidal coupler 2.

Carrier 5 together with ferrite I-section 2b is slidably removable and replaceable by the operator of the unit between (a) an assembled and operational position in which I-section 2b is in close contact with U-section 2a to complete a magnetic toroid, and carrier 5 is firmly retained under spring clip parts 4, and (b) a temporary disassembled position to enable an operator to link or unlink the toroid with metalwork such as for example the cable or the loop antenna shown in Figures 1 and 2. (In Figure 4, further described below, I-section 2b is also shown in such a disassembled position, designated 2b'.) The sliding action provided by the spring arrangement provided by spring clip components 4 can help to maintain the electromagnetic characteristics of the toroid after operation in a dirty environment by providing a 'self-cleaning' action when the operator re-closes the toroid by slidingly replacing carrier 5 with I-section 2b in close assembled association with U-section 2a.

The toroidal coupler 2 is constructed so as to leave a central aperture that can accommodate as many as possible of the types of cable and pipe and metalwork to be encountered in the mining industry in which the system is to be deployed. In the context of deep coal mines this conveniently means in practice an internal clearance of at least 105 mm, and after allowance for sleeving and coil windings this has translated for example to a rectangular, substantially square, toroid with an aperture llOmm square. Subject to these requirements of practicality, the ferrite toroid should be from the electrical point of view as small as possible in its magnetic path-length. The values quoted here represent a preferred compromise. A rectangular U and I section as shown in the drawings is presently preferred; in other embodiments an acceptable alternative would be to employ a split ring ferrite.

To provide some temperature stability of the toroid electrical characteristics, a ferrite composite has been preferred, in which each of the U-section and the I-section making up the toroid comprises linear ferrite slabs, comprising L2 ferrite slabs and Q3 ferrite slabs of complementary temperature characteristics, bonded by epoxy resin so that ferrite components of L2 and Q3 grades each make up a layer contributing 50% each to the thickness of the toroid. The arms of the toroid are approximately 25 mm x 45mm in cross-section, and this thickness is made up from four ferrite components each 25 x 1(mm in section, two each of the L2 and Q3 ferrite grades, epoxy-bonded together.This layered construction provides some temperature compensation and overall an adequately low temperature coefficient of magnetic permeability over the temperature range 10 to 50 degrees C.

In alternative examples, other ferrite grades in other ratios can be used to provide desired degrees of temerature compensation.

Also shown diagrammatically in Figure 3 is an electronic module 6 comprising microphone/loudspeaker, push-to-talk button and power-generation indicator lamp, a coupler coil 7 shown in exploded view but in actuality it is wound around one of the arms of ferrite U-section 2a of the toroidal coupler 2, and a hand-cranked generator 8. The microphone/loudspeaker unit of module 6 is of waterproof type, of medium impedance 180 ohms or more, and is effectively cushioned in a resilient mounting to minimise vibrational noise problems. A semiflexible strap 9 is also provided to connect casing 1 to carrier 5 which carries the ferrite I-section 2b of the toroidal coupler.Strap 9 helps manipulation and prevents loss of the removable I-section especially during operation to link or unlink the unit with an antenna or metalwork, and also preserves the original orientation of I-section 2b relative to U-section 2a. The unit also has carrying handles 10.

Altogether the arrangements shown are ergonomically satisfactory and facilitate use of the unit by personnel who have little or no experience of such apparatus. The siting of the push-to-talk switch and the crank handle facilitate operation in total darkness.

It has been found that placing the components in the manner shown here can result conveniently in acceptably low electromagnetic, electrical, mechanical and vibrational interference between subcomponents. The spacing and wiring layout can also contribute to ruggedness and facilitate limiting catastrophic damage in the event of bumps, shocks or partial crushing of the unit.

Figure 4 is a schematic diagram of system sub-components of the communications unit of the preceding Figures, and further shows functional relations of parts indicated and numbered similarly in Figure 3. Toroid coupler 2 comprises, as before, a U-section 2a and a removable and replaceable I-section 2b. Coil 7 is tuned by a tuning capacitor 7a and is connected in autotransformer mode to both a transmitter board and receiver board of the electronic module 6, which in turn is functionally connected to a push-to-talk switch 6a, a microphone and loudspeaker 6b, fed with power from the hand-cranked generator 8, which is provided with gearing arrangements, via a rectifier subcircuit 6c provided with a minimum-power indicator 6d.

Figure 5 is a functional block diagram of the receiver subsystem, and Figure 6 shows electronic circuit arrangements of this receiver subsystem.

The system example illustrated here uses a scheme of frequencymodulation of the voice-frequency signals upon a carrier signal in the LF/VLF range, here chosen as 36 kHz.

It has been found possible and useful, in choosing many of the electronic components for the receiver and transmitter and with the aim of maximum component integration, to use a number of components normally supplied and commercially available for cellular radio transmitter/receivers, even though these components are typically parameterised for operation with VHF/UHF carrier signal frequencies in the range 50 MHz - 1 GHz.

The receiver design is based on a double heterodyne up-conversion arrangement, with a first intermediate frequency of 105 kHz and a second intermediate frequency of 455 kHz. These frequencies are in no way critical and others can be adopted.

The receiver arrangements indicated in Figures 5 and 6 comprise input circuitry featuring an isolating and matching transformer (L1, L2) selected for good common-mode rejection, connected from the antenna coil (coil 7 in Figures 3 and 4), provided with some transient high-level overload protection (e.g. from power-line transients and the risk of damage therefrom) by back-to-back small-signal Schottky diodes to give excursion limiting, connected across the secondary L2.

In experiments so far this arrangement has turned out preferable to the possible alternative use of series-connected enhancement mode field effect transistors to provide transient overload protection, since the semiconductor flicker-noise levels from the FETs were found undesirable. PIN and avalanche diode arrangements have also been tried and found less preferable.

A first conversion stage is provided by a 'Gilbert Cell' multiplier double balanced mixer-oscillator enabling notably good noise performance (integrated-circuit ICl and associated circuitry, of which in particular L3 and C6-C8 provide an oscillator signal of 141 kHz in an L-C Colpitts configuration).

Integrated circuit IC2 provides a second conversion to 455 kHz using a mixer signal derived from an external discrete-transistor crystalcontrolled oscillator TR1 and associated components, at a frequency of 560 kHz. IC2 also provides 2nd i.f. amplification and with its associated components, including crystal filter X2 of widely available type CFG455G, provides suitable band-pass and notch filter characteristics, plus the discriminator circuitry to demodulate the f.m. signal and provide an a.f. signal. It is noted that noise performance is improved by using a relatively narrow band-pass characteristic. In practice a nominal voice-frequency bandwidth of +/- 2.5 kHz has been successfully used, though this is not critical.

The a.f. signal is gated in response to a voltage signal indicative of stable received-signal strength (RSS), provided by circuitry associated with IC2, to provide noise muting in the absence of consistent received signal, and is then passed to a linear power amplifier. The power output stage is implemented by IC4 with associated components powering a loudspeaker/microphone capsule (of about 180 ohms or alternatively greater impedance) in receive mode.

This a.f. output stage operates effectively as a tri-state output (i.e. with high impedance in the 'off' state), giving acceptably little shunting of the loudspeaker/microphone capsule in transmit mode, so that the transmitter and receiver circuits can in practice be hard-wired together as indicated in the diagrams.

Care is taken, in known manner, with the routing of earth currents to avoid unwanted couplings. Power regulation (8v) and some of the decoupling is provided by IC5 which also operates as an electronic power-switch for the receiver stages, actuated by a push-to-talk switch as mentioned above.

Figure 7 is a functional block diagram of the transmitter subsystem, and Figure 8 shows electronic circuit arrangements of this transmitter subsystem, together with detail of circuitry associated with the generator.

f A high impedance (c. 220 k-ohm) input buffer stage is provided (IC1A and associated components), this feeds a 'soft' progressive-action limiter (D1, D2 and associated components) followed by a bandwidthrestricting low-pass a.f. amplifier (IC1B and associated components).

The soft limiter has been found to permit acceptance of a wide dynamic range of acoustic input with good intelligibility and acceptably low levels of intermodulation products and other distortions.

Conversion to a frequency-modulated 36 kHz carrier is provided, first, by a voltage-controlled oscillator modulated by the processed a.f.

signal (IC2 and associated components), then by a pulse-shaping switching buffer stage (IC4A,B,C) to produce a substantially rectangular-wave signal which is subject to passive tuned low-pass filtering (L1 and associated components). The circuit illustrated implements a high-stability voltage-controlled oscillator with adequately low sensitivity to temperature and supply variation.

The filtered carrier-frequency f.m. signal is finally amplified in a linear output stage (IC3 and associated components) before coupling (via C20-21 and R27-28) to a tuned toroidal output transformer. The output stage is essentially tri-state (high impedance in the 'off' condition) to allow the toroid autotransformer to be hard-wired to the receiver input circuitry as well. With some circuit designs it may be desirable to include a precision microwave relay to provide mechanical switching of the antenna between transmitter and receiver but units embodying the circuits of Figures 5-8 have been made without requiring such a relay.

The winding configuration adopted for the coil Lt of the tuned toroidal output transformer (also shown at 7 in Figures 3 and 4) which is wound around an arm of the ferrite U-section core 2b of toroidal coupler 2 of Figures 3 and 4, is a parallel-tuned autotransformer with 18 turns, tapped at 2 turns. A suitable nominal value for the associated tuning capacitor Ct (also shown at 7a in Figure 4), for a signal frequency of 36 kHz, can be 20 nF; in alternative arrangements there can for example be a capacitor Ct with a value in the range 10-30 nF. The transmitter 'circuit illustrated in Figure 8 can apply a nominal transmit drive voltage to the coupler tuned winding Lt of about 6 v rms.

With the arrangement illustrated, it has been found possible to implement a tuned output circuit in transmit mode which also functions as a tuned input circuit in receive mode, with reasonable closeness to optimal matching for each function. The arrangement for tuning the resonant frequency of the primary winding of the coupler coil is also such that in each case the tuned frequency is adequately independent of the external metalwork of the untuned loop antenna or of the cabling, so that the operator does not need to adjust any tuning components during use.

With further reference to Figure 8, a transmit delay when the push-to-talk switch is activated is implemented by IC4D, TR1 and associated circuitry, and can enable acceptably clean transmit-receive switching without troublesome transients or receiver unmute delays.

The power-switching of complete circuit blocks is implemented in the design of Figures 5-8 and can help to avoid overloading of the hand-cranked generator.

The transmitter circuit values illustrated in this example can involve some degree of compromise in relation to the risk of detonating electro- explosive devices (EEDs), which include electric detonators commonly used in mining environments (as assessed by methods appropriate to BS 6657: 1991 and corresponding mine safety regulations). If greater safety margins are desired in relation to the risk of detonation by induced currents from the transmitter of the emergency communications unit, then the transmit amplifier gain and signal drive levels can be reduced, or current-limiting resistors placed in series with the transmit amplifier output, in each case at some cost to the effective range and noise performance of the system.

Alternatively, or in addition, operators of the devices can also be advised to exercise care in positioning the devices and associated loop antennas, so as to avoid close contiguity between the communications components'and any EEDs in the vicinity where the system is in use.

Figure 8 also shows detail of the power supply circuitry. The hand-cranked generator used in this example is a multiphase stepper motor with 1.8 degrees step angle, and phase inductance and resistance 4mH and 3.6 ohms respectively, which avoids the use of brushes or commutator, and can be chosen, as here, to provide twin phase a.c.

outputs. Use of a stepper motor as a generator in the context of hand-operated signalling equipment is believed to be novel per se. The arrangement set out here can provide high reliability and low electromagnetic interference in close proximity to the sensitive receiver circuitry. The multiphase feature also helps power supply noise suppression, and with the two-phase arrangements used here, the generator can acceptably be housed in a common housing with a receiver of submicrovolt sensitivity. Gearing is provided by a bidirectional jam-free low-noise gearbox (a 1:3 step-up, in-line epicyclic gearbox) so that the unit need be hand-cranked no faster than 1.5 revs/second by the user to provide 12-24 volt output.

The present arrangement can be operated (hand-wound) for several minutes at a time with little or no fatigue.

Figure 8 also shows twin full-wave diode rectifier circuits, one for each of the generator output phases, with the d.c. outputs of the full-wave rectifiers stacked. Each arm of each full-wave rectifier comprises a diode shunted by a 220 nF capacitor to provide 'soft recovery' rectification and minimise signal interference. The generator (stepper motor) metalwork is connected directly to the system star-point signal ground node.

In further relation to minimising interference from vibrational and other noise, the microphone/loudspeaker insert in this example is chosen to have an electrical and/or mechanical minimum response matched as closely as possible to the mechanical noise frequency spectrum of the generator.

The d.c. output line is shunted by reservoir capacitors and a voltage-limiting zener regulator. A light-emitting diode in series with a further zener functions as an indicator and lights up when the generator is being cranked fast enough to provide adequate power to operate the unit.

Figure 9 shows a simplified form of transmitter output circuitry for use as an alternative to the transmitter output stage arrangements of Figure 8. The output stage of Figure 9 is based on a tri-state complementary-pair MOSFET switching stage (Tr91-92) fed e.g. from a splitter circuit to be associated with the pulse-shaping buffer stages IC4A-C in Figure 8. No linear transmitter output stage is provided in this variant, and coupling from the switching output pair to the toroidal coupler coil is achieved via a low-pass L-C filter (L91-92, C92 with series resistors R91-R92) to provide a function similar to that of components L1 and C17-18 in Figure 8. This circuit arrangement can provide enhanced impedance matching and power transfer to the toroidal coupler.

Among the modifications and variations which can be applied to the examples of the emergency communications unit described and illustrated herein, are the following: Provision may be made, in addition to the loudspeaker/microphone unit (or, less preferred, instead of that unit) for a morse-key or tone signalling arrangement using a key and corresponding output speaker or indicator, of any of a variety of types, especially for use where communications signal propagation conditions are so poor that voice communications are unintelligible.

The frequency of the device may alternatively be in a higher range than that of about 30-70 kHz, e.g. in the range 250-600 kHz especially for use where inductive coupling has to be made to non-continuous guide metalwork, e.g. damaged cables or railing.

It is possible, although much less preferred, to use battery power in place of hand-operated generator power, using e.g. sealed gelled electrolyte lead-acid batteries, e.g. to DIN 43534. However reliability is a problem with such batteries. Hermetically welded lithium thionyl chloride cells might be used if they could be made safe enough to leave an acceptably low risk of explosion.

The present invention is susceptible of many other modifications and variations and the present disclosure extends to other combinations and subcombinations of the features mentioned above and illustrated in the drawings.

Claims (1)

1. CLAIMS:

   1: Electronic apparatus suitable for communications underground, e.g.

   in deep coal mines, comprising a transmitter and/or a receiver for propagating a transmitted signal to a remote receiver or for receiving a signal propagated from a remote transmitter, without requiring an electric-current connection between the receiver and the transmitter, said apparatus comprising an inductive transducer arranged to be removably and inductively coupled, either with a long piece of metalwork such as a pipeline or cable, or with a loop antenna, according to choice by the operator of the apparatus.

   2: Apparatus according tb claim 1, comprising both a receiver and a transmitter.

   3: Apparatus according to claim 2, wherein the receiver and transmitter use plural frequencies e.g. a lower frequency in the band 30-70 kHz, and a higher frequency in the band 250-600 kHz, to increase likelihood of establishing effective communications in difficult conditions.

   4: Apparatus according to claim 2, wherein the inductive transducer is common to the receiver and the transmitter.

   5: Apparatus according to claim 1, wherein the inductive transducer comprises a toroidal transformer with a high-permeability, e.g.

   ferrite core, the transformer having a removable section of its core arranged to be removed and replaced by an operator to allow the core to be linked inductively with the long metalwork or the loop antenna.

   6: Apparatus according to claim 1, wherein the electrical configuration of the inductive transducer is such that the choice between the use of long metalwork or loop antenna is without significant effect on the frequency-tuning of the apparatus.

   7: Apparatus according to claim 1, arranged to operate at an essentially fixed signal frequency, in the range 30-600 kHz, e.g.

   about 30-70 kHz, e.g. 36 kHz.

   8: Apparatus according to claim 7, arranged to provide voice frequency and/or tone signalling communications using a frequency-modulated carrier at said fixed signal frequency.

   9: Apparatus according to claim 1 or 2, which is of general construction in accordance with intrinsic safety (IS) standard for operation in a flammable environment, and arranged in a casing that also contains a generator or battery, e.g. a hand-cranked generator of a configuration which is free of a commutator or brushes, e.g. a stepping motor used in generator mode.

   10: An emergency communications unit comprising in a package an apparatus according to claims 2 and 9, wherein the casing houses a transmitter-receiver and a hand-cranked generator, together with a microphone, loudspeaker, a push-to-talk switch (or button), and a ferrite toroid inductive coupler, together with a separate loop antenna, for optional use.

   11: A method of establishing emergency communications underground, comprising operating apparatus according to claim 1 at each of two stations of which at least one is underground.

   12: Communications apparatus substantially as described herein with reference to any of the features of any one or more of the foregoing claims, description and accompanying drawings.