GB2200270A - Apparatus for providing two-way communication in underground facilities - Google Patents

Description

51 1.4 APPARATUS FOR PROVIDING TWO-WAY COMMUNICATION 2Z_ IN'UNDERGROUND

FACILITIES /^ 0 0 2,7 0 The present invention relates to an apparatus. e.g. a radiophone. to be employed in underground facilities such as mines. for providing two-way communication wherein the utilized mode of propagation of the radio waves is based, particularly with long communication distances, on the connecting of the electromagnetic waves to the metal structures. pipeworks and other conducting bodies installed in the mine galleries and shafts.

The basic problem in the underground employment of radiophones is the strong attenuation of electromagnetic waves under these conditions. This means that such radiophones that function perfectly well on the ground do not function in a satisfactory way or in any way without special measures while they are used underground. other environmental circumstances may also set additional requirements to the equipment employed.

A radiophone connection to a mine can be established in three different ways: 1. By employing the waveguide mode of propagation with frequencies of over 200 MHz; 2. By employing leaky feeders with frequencies 2-200 MHz; and 3. By employing propagation directly through the bedrock and based on the Conducting bodies installed in the mines, in which case the frequency band 0.1-2 MHz may be possible.

_The waveguide mode is a mode of propagation for electromagnetic waves, wherein the wave motion proceeds as if within a guide. The mine gallery can be considered as a kind of a waveguide. However, the attenuation of the signal Is remarkable, and the obstacles - such as vehicles - located in the gallery cause additional attenuation. The method Is not suited for the complex network of galleries in existing mines where the communicdtion distances are long.

In the leaky feeder 'method, a long pair of cables within the .2 same shield, or a loosely braided coaxial transmission line, is coupled to the radio transmitter. The field emitted by the transmission line can be captured by a portable receiver from a distance.of a few tens of meters at the most with respect to the line. The antenna cable is installed in the mine galleries, and the use of portable equipment is possible in the vicinity of this antenna.

With a frequency below 2 MHz, such radiophone systems can be employed where the signal proceeds directly through the bedrock, or where existing conducting bodies such as power cables, pipework or hoist ropes are made use of. Direct propagation through the bedrock is dependent on the conductivity of the rock and on the employed frequency. If existing conducting bodies are utilized, the communication distances are rendered remarkably longer. The same is achieved if the base station is directly coupled to the cable which serves as the antenna.

The purpose of the present invention is to realize an apparatus, suc h as a radiophone, for providing twoway communication in underground facilities, such as a radiophone, which apparatus functions within the frequency band 0,1-2 MHz and utilizes the conducting bodies already existing in the mines'for the propagation of the sinal, the same signal being partly propagated in the bedrock as well. The essential novel features of the invention are apparent from the appended patent claims.

In accordance with the invention, advantageously a loop antenna is coupled to a transmitting and receiving apparatus providing a two-way communication connection, for instance talking connection, which apparatus utilizes digital transmission technique so that the coding of the speech into digital form is carried out by means of delta modulation. when a signal is modulated into radio frequency, frequency shift keying (FSK)'is employed.

By means of the digital modulation technique, the sensitivity of the apparatus to interference and noise existing In the mines,-owing for instance to the electric machinery employed, can be essentially reduced, because in digital transmission it is possible to transmit samples -- of for example an analogous signal - picked at sufficiently short intervals, and the original signal can be reconstructed on the basis of these samples. The said samples are advantageously coded so hat at each particular moment of time there are only two alternative signals to be transmitted. Such signals that are distorted during the transmission and contain noise can in the reception be interpreted as one of the two alternatives, in which case the distorted signals can be regenerated in the intermediate amplifiers or in the repeaters included in the apparatus.

In digital transmission, the standard of the voice quality can also be maintained as long as the received bits are correctly interpreted, evenif the signal to noise ratio would vary in the transmission. When the signal to noise ratio in digital modulation is deteriorated to a certain point, the fault probability of the received bits increases rapidly, and the original speech signal cannot be correctly decoded anymore. Therefore in the method of the present invention, the coding of the speech into digital form is carried out by means-of delta modulation, in which case the transmitted samples-are coded by one bit.. This differential coding is based on the fact that the successive samples picked of the signal correlate with each other, and on the base of the earlier samples picked of the same signal it is thus possible to-try and predict the next sample. Consequently, the difference between the real sample and the predicted sample is thesample to be transmitted. Thus the decoding of the speech cab be realized with a relatively small number of bits.

1 Owing to the good interference and noise tolerance of the digital modulation-techniques applied in accordance with the present invention, the achieved communication distanceAs also extended. In case the modulation method is not interference and noise tolerant, the Interference may cause much more limitations in the connection than the attenuation of the t+ signal on the propagation path does. Therefore the method of the present invention uses, even when a signal is being modulated into radio frequency, an angle modulation method wherein the signal to noise ratio can be adjusted with respect to the bandwidth, and better results achieved thereby.

Furthermore, the digital transmission enables a complex processing of the signal,, whereby propagation attenuation and interference can be compensated. In that case. in order to improve the mutual correlation of the successive samples picked of the signal, the number of transmitted bits is larger than should be necessary for the information in question at the transmission end. In reception. part of these redundant bits may be incorrectly interpreted without any resulting errors in the decoding of the useful signal. Moreover, owing to the digital transmission, the apparatus of the invention may employ highly i ntergrated circuits, such as delta modulation circuits and phase-lock circuits in radio frequency modulation.

The method of communication used in the apparatus -of the present invention may advantageously be for instance the socalled semiduplex. In that case a two-way talking connection works so that it is not possible to listen simultaneously when transmitting. Such methods of communication may also be utilized wherein for instance all receivers can hear the transmission from the base station or from a single radiophone. Moreover, the apparatus of the invention can be applied to the transmission of information other than speech. By taking the bandwidth limitations into account, the bandwith required by a speech channel allows, when desired, the transmission of for instance measuring, control and production Information, as well as a stationary monitoring image.

In the following the invention is explained in more detail with reference to the appended drawings, wherein Figure 1 is a scherdatical illustration of the devices In a preferred embodiment of the apparatus of the present Invention, 1 T_ Figure 2 illustrates the antenna circuit diagram advantageously suited for the embodiment of figure 1.

According to figure 1, while using the apparatus of the_7 invention in transmission, the audiosignal received from the microphone 1 Is amplified by the amplifier 2 and conducted through the limiter 3 and the filter 4 in order to make-the signal suitable in amplitude for the analog to digital converter 5. The filter 4 removes from the speech signal such frequency components that are unnecessary from the understandabilitypoint of view. The analog to digital. converter 5 converts the thus preprocessed signal into digital form, i.e. the signal is represented by means of the samples picked thereof. The length of the sample intervals can-be varied by the aid.of the timing generator 6.

The analog to digital converter 5 is operated according to the delta modulation principle so that while the value of a speech signal is growing, the converter 5 gives a bit designating logic 1, and while the signal value is decreasing, the converter gives a bit designating logic zero. These speechdescribing bits in turn control the FSK modulator 7 operated by means of frequency shift keying. With the bit zero, the modulator 7 produces a given frequency, and with the bit 1 it produces another given frequency. The thus received radiofrequency signal is amplified by means of the control stage 8 and the power stage 9 of the power amplifier. The obtained power is transmitted, via the antenna matching network 10 and the antenna switches 11, to the antenna 12.

While employing the apparatus of the invention in the reception of a signal, the weak signal received by means of the antenna 12 is conducted, via the antenna switches 11 and the matching network 13 to the receiver unit 14. This receiver unit 14 also comprises the frequency tuning 15 and the display 16, as well as the signal strength meter 17. The intermediate frequeincy signal created in the receiver 14 is conducted Into the FSK detector 18, which is advantageously operated according tS the phase lock principle. The signal G_ received from the detector 18 is further amplified and filtered in the separate amplifying and filtering stages 19, and thus a string of bits corresponding to the transmitted string of bits is reconstructed. On the basis of this obtained string of bits, the digital to analog converter 20 forms an analogous signal, which is further filterd in the filter 21 in order to eliminate the effects of the sampling. Thereafter the signal amplified in the amplifier 22 can be heard in speech, corresponding to the transmitted speech, from the loudspeaker 23 or from the headphones 24.

In-order to make the apparatus of the invention work both in the transmission and in the reception, in connection with the microphone 1 there is provided the tangent 25 - as is seen in figure 1 - whereby a transmission/reception control is achieved. This control member 25 is needed because the apparatus of the- invention includes components which are common both to the transmission and the reception, and which therefore need information as to which mode of operation is in question at each given time. Moreover, the receiver must be attenuated for the duration of the transmission, whereas the voltages of the transmission circuits must be switched off during reception.

One of the components common for both the transmission and the reception in the apparatus of the invention is the antenna 12, the operation-whereof"is described in figure 2. During transmission, the antenna 12 is tuned to series resonance in order to obtain maximaf power from the power stage 9 into the antenna 12. In the case of reception in which position the switches 26 seen in figure 2 are set there is employed parallel resonance in order to obtain maximal voltage from the antenna 12. The operation of the switches 26 is controlled by means of the control member 25. By employing the tuning of the antenna 12 illustrated In figure 2, there is achieved not only maximal power transmission but also band limiting of Interference signalt. The antenna 12 may be for example an essentially circular loop antenna. but the size of the said antenna 12 can be re'duced for instance. by advantageously 1 employing a ferrite core in the loop antenna.

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Claims (5)

1. C L A I M S

   An apparatus for providing a two-way communication system in underground facilities which operates within a frequency band 0.1-2 MHz, in order to make electromagnetic waves propagate by making use of the metal structures installed in the underground facilities, and which apparatus includes a transmitter and receiver unit and an antenna; said transmitter and receiver unit comprising an analogue to digital conve, - ter for transmission and a digital to analogue converter for reception in order to transmit the intermediate digital signal between apparatus units.
2. 2. The apparatus of claim 2, wherein the analogue to digital converter for'converting the analogue, transmittable signal into digital form is a delta modulator.
3. 3. The apparatus of claim 1 or 2, wherein the received digital signal is subjected to a detector operated on the basis of frequency shift keying in a stage prior to digital to analogue conversion.
4. 4. The apparatus of claim 1, 2 or 3, further comprising a transmission/reception control member in order to set the circuitry for transmission or reception.
5. 5. - An apparatus as hereinbefore described with reference to and as illustrated in the accompanying drawings.

   4 Published 19.118 ax The Patent Office, State I-louse. 66"71 Mgh Holborn, London WCIR 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St MaxT Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.