WO1997031438A1

WO1997031438A1 - Method and device for telemetry on a metal structure protected by current tapping

Info

Publication number: WO1997031438A1
Application number: PCT/FR1997/000317
Authority: WO
Inventors: Jean-Marc Vernet
Original assignee: Ipsi
Priority date: 1996-02-20
Filing date: 1997-02-20
Publication date: 1997-08-28
Also published as: FR2745133A1; AU2099097A; FR2745133B1

Abstract

A telemetry method for a metal structure (19) protected from oxidation by current tapping (1, 2). The method uses the structure (19) itself as the medium for data transmission from at least one transmitter connected to the structure, said data being recognised by a receiver (4, 5, 6, 7) connected to another part of the structure (19). Ground connections (16) are used to achieve a positive potential difference with the protected structure (19). The power of the current tapping station (1, 2) and said positive potential difference between at least one of said ground connections (16) and the metal structure (19) are used to inject a variable current (I) modulated by said transmitter depending on the data to be transmitted through the structure (19). The resulting current variations are measured by the receiver and used to determine the data picked up and transmitted by the transmitter. As said transmitter is self-contained, it may be placed anywhere on the structure (19).

Description

METHOD AND DEVICE FOR TELEMETRY ON A METAL WORK PROTECTED BY DRAWING

The present invention relates to a method and a device for telemetry on metal work protected by racking.

The technical field of the invention is that of the electrical transmission of information by conductor.

The main application of the invention is the production of a telemetry device operating on a metal structure, such as a pipe, protected against oxidation by drawing off, which uses the structure itself as a support for transmission. information and can ensure and transmit, for example, the control of cathodic protection at any given point in the structure.

Currently, there are different systems for transmitting information concerning metallic structures, such as that described in patent application FR2097265: this document teaches a system for transmitting information using an underwater pipe, between a transmitter and a receiver. , each connected at a given distance to the said pipe, according to a known general basic principle which is also that of the present invention: the emission systems described in this document use the earth as return, as in all known systems even before the filing of this patent application; the sacrificial anodes of the cathodic protection system of the pipe, which have an oxidation potential lower than that of the pipe, and draw current therefrom, are presented there either as problems to be overcome or as a means for power repeaters; such an emission system requires a fairly significant specific energy supply.

We also know the patent AT356982 which describes a system fairly close to the previous one, which sends an alternating current thanks to a current source of its own, which of course limits the possibilities of implantation.

Thus all these systems and others require too much energy to remain autonomous for a long time, and, if necessary, the autonomy of their repeaters is limited to the very short life of a sacrificial anode; in addition, they pose maintenance problems, are sensitive to changes in the electrical characteristics of the metal structure over time, and use technologies that are both more expensive than installing transmission lines independent of the protected structure, and less economical to use than a periodic statement by specialized personnel. Moreover, many large companies are reduced to adopting this second solution, which is however not a panacea since a defect arising between two readings can cause major inconvenience (poor bias voltage causing the drilling of a pipe , or drop in gas pressure in a pipe quickly causing a lack of gas at the end of a wired, etc.).

The problem is therefore to be able to produce a telemetry device on a metal structure, protected against oxidation by drawing off electric current, which device using a transmitter placed anywhere on the structure and not only at one end , thus making it possible to transmit all the information found anywhere in the structure, such as the cathodic protection control, which can be autonomous, and of manufacture, installation and simple use.

A solution to the problem posed is a telemetry method as well as any device allowing the implementation of such a method, operating on a metal structure protected against oxidation by racking, which use the structure itself as a support for the transmission of information, from at least one transmitter connected to said work and recognized by a receiver also linked further to this same work; according to the invention,

- earth connections are used which offer, if possible, an oxidation potential greater than the oxidation potential of the metal structure, or otherwise, allowing at least a positive potential difference with the protected structure, by choosing to preferably earth connections made of copper, brass, bronze, cast iron with a high silicon, coal, coke or graphite content. - earth plugs are used to have a positive potential difference with the protected structure,

- using the power of the withdrawal station and said positive potential difference existing between at least one of the said earth connections and the metal structure, to inject a variable current modulated according to the information to be transmitted in the structure ,

- Measuring said current variation by the receiver and deducing therefrom the information noted and transmitted by the transmitter, said transmitter being autonomous can be installed at any point of the structure. In preferred embodiments:

- said variable current is injected by causing the opening, controlled by said transmitter, of an electric bridge between one of said earth connections and the structure, the variable current then always flowing from the earth connection to the work ; - Earth plugs are used offering an oxidation potential greater than the oxidation potential of the metal structure, said earth electrodes behaving with respect to the structure like natural cathodes, which make it possible to increase the current injected, and therefore provide additional power to the emission; - the potential difference between the protected structure and one of the so-called earth connections is used to generate the electrical energy which the transmitters need,

- if necessary, this voltage is increased to a level compatible with the operation of the electronic cards of said transmitters, - the fact is used that the variation of said current injected into said metal structure by the electrical bridge can be identified at the station racking or placing the receiver whatever the shape and size of the protected structure, as long as the cathodic polarization voltage with respect to the earth is correct at all points of the structure,

- We define for this spectral components of said current to be transmitted low enough not to be much more attenuated than the DC component (frequency components in the bandwidth of the structure). In the case where there are several transmitters on the said structure, it generates from each transmitter placed on the structure its own signals and allow the receiver to identify the transmitter of these signals and to know the or the measurements it transmits.

The result is new telemetry methods and devices operating on a metal structure protected against oxidation by racking.

A method according to the invention is broken down into a method for transmitting signals according to the invention, a method for supplying transmitters according to the invention, a method for coding information according to the invention, and a method for reception of the signals transmitted on the structure according to the invention.

The method according to the invention makes it possible to produce the device according to this same invention, which solves the problems mentioned above.

Indeed, it is autonomous for a long time, poses very few maintenance problems given its simplicity, is independent of the geometry of the protected structure and of its aging, and is inexpensive.

In preferred embodiments, a device according to the invention comprises:

- transmitters, each consisting of a specific device according to the invention for transmitting signals, of a specific device according to the invention for measuring the bias voltage, and of a specific device according to the invention for powering electronics,

- a special receiver for the reception of signals transmitted on the structure,

- earth connections preferably made of a material offering a high oxidation potential such as copper, brass, bronze, cast iron with a high silicon content, coal, coke and graphite.

The transmission method according to the invention has its origin in two valid observations when the polarization is correct at all points of the structure: - all the current injected into a structure sufficiently polarized by a polarization station passes through this station since the potential which it imposes at any point of the structure is lower than its oxidation potential, - it is always possible, at any point , to inject current into the structure, by making a point electrical bridge between the polarized structure and its medium, if it is conductive. To do this, it suffices to use an earth connection whose oxidation potential is greater than or equal to the potential of the structure protected by racking. If the oxidation potential of the earth connection is less than or equal to the oxidation potential of the metal structure (for example, if the earth connection is in a less noble metal than the metal in which the pipe is made), it is the devices used to polarize the metal structure which provide the power necessary for this current injection. If, on the other hand, the oxidation potential of the earth electrode is greater than the oxidation potential of the metallic structure, the earth electrode forms, compared to the metallic structure, a natural cathode, which contributes to supplying the power necessary for current injection (without cathodic protection applied to the structure, the earth electrode could already inject current into it).

A first variant of the method according to the invention for transmitting information on a correctly polarized structure is therefore, in a first characteristic, the following: each transmitter controls an electrical bridge between an earth connection and the protected structure, and takes advantage of the positive potential difference to allow a variable current to flow between the two - therefore entering the structure -, according to a law known by the receiver, placed at the level of the racking station. The law of variation of the current is specific to each transmitter, and can be modulated according to the (or) value (s) measured (s) by the transmitter.

Any current modulation will be seen by the receiver, provided that its frequency components are sufficiently low. Since we can often be satisfied with a measurement per minute, even by the hour (measurement of the bias voltage for example), there is theoretically no low limit to the base frequency of the modulation (we could go down to a few tenths of Hertz). We note that the identification of very low frequencies presents no difficulty in digital signal processing (Fourier transform, correlation or autocorrelation). It is also noted that, if the emission method according to the invention is functionally a current emission method, an emitted signal will be especially seen at the level of the voltage draw-off station if this draw-off station is assimilated to a source. current (resistance of the substation and of the weir large compared to the resistance equivalent to the earth of the structure). This same signal will be more easily measured by current if the racking station approaches a voltage source (especially the case of new structures in the ground).

The receiver may not be able to detect variations in the injected current if the structure injects it into its environment (leaks). But this case corresponds to faulty cathodic protection, and the receiver will be able to deduce therefrom the existence of a problem deserving of the intervention of a team.

It is noted that the current injected into the polarized structure protects against oxidation, by drawing off, the earth electrode used to inject this current. This goes in the direction of the realization of practically wear-free devices, therefore autonomous for a long time. In addition, if the earth connection belongs to a third party, where possible (use of third-party works), the transmission process cannot cause its oxidation, but, on the contrary, limit it.

We also note that according to this transmission method, according to the invention, the inherent power of the transmitter is not used, but rather that of the withdrawal station, possibly supplemented by that of the earth connection, natural cathode by relation to the structure. In addition, placing the receiver at a forced passage of the injected signal, one can in theory and in practice play with the shape and size of any metallic structure, provided that it is protected by a single racking, and that one is ready to lower the base frequency of the injected signals, until it is compatible with the passband (frequencies hardly more attenuated than the zero frequency). A possible variant of this transmission method according to the invention is to place the receiver at a location other than the racking station. However, depending on the topology of the protected structure, this process will be less efficient and will require greater sensitivity of the receiver (in particular if the receiver is at the end of the structure). Other variants of this process are to place several transmitters and one or more receivers on a structure protected by several rackings.

The attenuation of the signals will be stronger, but compatible with good reception in most cases, especially if the signals do not find in their path a racking station of low resistance (source of tension for the structure), which could stop their transmission.

The transmission method according to the invention can use multiple low frequency waveforms: periodic square signal, pulses, sinusoidal signal ... (the list is not exhaustive).

However, the one that does not undergo any deformation during transmission is the sinusoid (even a well-insulated pipe is a deforming transmission line, except for sinusoids), and it simplifies the detection logic, especially since it will be issued for a long time. On the other hand, the easiest waveform to generate is probably the square wave.

The transmission method according to the invention includes a method of coding the information transmitted (generally signature of the transmitter and measurement (s) transmitted). This coding method can use a form of frequency modulation, the frequency band allocated to the transmissions being divided into absolute frequencies each dedicated to a particular value (or combination of values) of the measurement (s) made. ) by a particular transmitter. If N is the maximum number of different values (or combinations of values) that a transmitter can transmit, and M the maximum number of transmitters that we want to be able to put on the same protected structure, then the coding process must use MxN different frequencies in the allocated transmission band. This coding method implies that M signals can be transmitted simultaneously. Note that the coding precision is limited only by that of the oscillators (quartz or piezoelectric) used for transmission and reception. A close variant is to use analog coding, only the frequency band per transmitter being fixed, the latter transmitting a frequency which is a continuous function of the measurement or measurements to be transmitted (returns to N "infinite").

A possible variant of the coding method is to use a different frequency per transmitter, the data to be transmitted, coded in binary, being transmitted by a succession of transmissions and rests.

We note that these principles can be applied without too many problems to any periodic waveform, if we are only interested in the first harmonic, or fundamental frequency, and that the frequency band used does not exceed the octave.

In general, the coding method can be, or resemble, any known coding / modulation method, determined as a function of the objectives of transmission reliability, transmission speed, the number and nature of the information to be transmitted, frequency of transmission cycles, etc.

In particular embodiments of the emission device according to the invention, a switch is used which can make an electrical bridge between the protected structure and its environment, therefore injecting current into the structure, via two connections: one to the protected structure, the other to an "earth connection", the best possible. The signal transmitted is, in this case, a succession of "all or nothing" slots of variable durations, which will generate, for example, a particular "spectral signature". In one embodiment of this transmission device according to the invention, the switch is controlled by simple electronics which can chop the current injected at high frequency.

The variant of the emission process implemented on such a device according to the invention can be as follows: the current injected at high frequency is chopped to give it the desired low frequency form (throughout the text, the term high frequencies s 'applies to the spectral components higher than the cutoff frequency of the metal work concerned, and the term low frequencies applies to the frequency components close to the zero frequency, which are not much attenuated more than this last. For example, if we want to transmit information over several tens of kilometers, along a large diameter pipe, we can consider 1 kHz as a high frequency, absorbed, and 2Hz as a low frequency, slightly attenuated). It can be a so-called pulse width modulation, pulse frequency, or a combination of the two. More generally, according to the invention, the technique of modulation of the injected signal may be any technique that, during each time interval (small compared to the rise or fall time of the desired low frequency signal during this precise time interval), the ratio of the cumulative duration of the "on" states to the cumulative duration of the "open" states fixes the average current injected into the structure. This type of pulse control is also very widespread for the control of DC motors. Simple wired logic or a small microcontroller can easily control the switch, making it easily generate the most unusual forms of low frequency current, but it is the sinusoid which will be the least deformed, and will allow the simplest detection. (to be precise, it will then be a sinusoid superimposed on a continuous component, because one cannot generate alternations "taking current" to the protected structure. In this case, the continuous component constitutes the "lost" current by the transmission device, a logical consequence of the use of the transmission method according to the invention). It is noted that it is possible to chop the current at frequencies close to the base frequency of the signal to be generated, or even at the extreme of generating a square signal in place of a desired sinusoid at the level of the receiver. The disadvantage is that we generate unwanted harmonics at the start, imposing limits in the choice of frequencies to use if we use a frequency band wider than the octave, and generally adding noise in a band frequencies that don't necessarily need them.

However, chopping a current at low frequency disturbs the electromagnetic environment less in general. In addition, the deliberate choice of a square wave can have two advantages:

- it has a sinusoidal component at its natural frequency of amplitude higher than that of the sinusoid which one can generate without harmonics under the same conditions (ratio 4 / π, demonstrable thanks to the series of Fourier), which increases the range of the emission if we only try to detect the fundamental frequency,

- if you only open and close the switch at low frequency, you can add a large filtering capacitor in parallel (or an inductive coil in series), which will prevent noise at industrial frequencies (50 / 60Hz ), often exceeding in amplitude the bias voltage, and therefore causing the conduction of the protective and / or freewheeling diodes of a possible MOS transistor, does not oxidize or bias the earth electrode (depending only on these parasitic currents are injected or removed from the earth). Indeed, the RC circuit (respectively RL) thus constituted with the resistance of the earth electrode can sufficiently filter the industrial frequencies to allow the earth electrode to be crossed outside the periods when it is used only by a current of zero mean value (respectively very low). An undeniable advantage of this transmission device according to the invention is its simplicity. In practice, the switch may be a relay, a bipolar, MOS, or IGBT switching transistor, or even a thyristor or a triac, the list not being exhaustive. The relay, a real switch, but greedy in current control, is poorly suited to an autonomous system. The transistors, without being real switches (because they are not symmetrical), behave in the environment of the invention like switches, the voltage present at their terminals normally having always the same polarity. And if industrial noises, outside the spectral band used for transmission, and not filtered, or not sufficiently, by a possible capacitor or a filter coil placed for them, instantly reverse this polarity, consecutive aberrations will also be in outside this band. The MOS and IGBT transistors are the most suitable for an autonomous system, since they require only a very low control power. The thyristor or the triac are not reasonably suitable, although their use is possible.

A variant of the transmission method according to the invention uses, as an electrical bridge, a transistor in unsaturated operating mode (generally linear). A current of the desired shape is then injected directly into the structure. The devices implementing this variant of the emission process naturally dissipate more energy in the transistor and are more difficult to produce, especially in the context of a stand-alone transmitter. Another variant of the transmission method according to the invention uses, as an electrical bridge, a network of resistors connectable in parallel or in series via switches. A device implementing this method can consist of a few resistors which can be put in parallel by a set of switching transistors.

A complement to the emission method according to the invention consists, whatever the electric bridge used, of using an energy reservoir - battery or large capacitor - periodically recharged by the supply device according to the invention, and which will serve, during emissions, increase the current injected into the protected structure. In practice, by a set of electronic switches (transistors, relays, etc.), the battery or the capacitor will be inserted into the earthed chain, electric bridge, metal structure to increase the flow of current. However, a device implementing this variant process has technical reason to exist only if it is impossible to decrease the value of the earth resistance (which is almost always possible).

Depending on the method for receiving the signals transmitted, it is possible to use both a current and a voltage sensor. If the racking station is mainly a current source compared to the structure (see above), it will be preferable to use a voltage sensor. In the event that the station behaves as a voltage source (see above again), a current sensor will be preferable.

In one embodiment, the receiving device according to the invention therefore comprises a current probe, measuring the current flowing from the protected structure to the racking station, or even in the protected structure, wherever possible. In a second embodiment, the receiving device uses a tension sensor, placed at any point of the structure, but preferably at the level of the racking station, if there is only one, and between an earth connection if possible without noise and the said structure.

According to a variant of the reception method, an analog low-pass filter (or band-pass) is used, to ensure anti-aliasing spectrum filtering, and to reduce the useful dynamic range. The high cut-off frequency of this filter depends of course on the transmission frequencies used, but a cut-off frequency of 10Hz leaves both a reasonable frequency band for transmissions, and is sufficiently far from the harmonics and sub-harmonics coming from the sector so that a second order filter attenuates the components at 100 / 120Hz, coming from the rectifier bridge of the racking station (respectively at 50 / 60Hz, industrial frequencies) by a factor of 100 (respectively 25), which makes it possible to use behind components with a dynamic range of around 1000 on many structures. The filtered signal is then sampled and transformed into digital data, then processed by a digital signal processing means, which, for example, rejects the 50 and / or 60Hz, its harmonics and main sub-harmonics (sufficient cardinal sine filter ) then which performs as many autocorrelations in parallel as there are possibilities of transmitted signals. The purpose of rejecting frequencies from the sector is to avoid "saturating" the processor with values that are too disparate during the detection processing proper. In addition, if the sampling frequency is well chosen, a cardinal sine filter only results in a succession of additions, which pose no risk of loss of information during the calculation.

However, in general, the signal processing used in this variant of the reception method according to the invention, perhaps any digital method making it possible to detect and demodulate a useful signal embedded in noise. According to particular embodiments, the reception device therefore comprises an analog low-pass or band-pass filter (for example 0.1 Hz to 10 Hz), followed by an analog-digital converter, and, to carry out the signal processing, and depending on the computing power required, an electronic card, a specific logic circuit, a signal processing processor, a microprocessor or a microcontroller, or a desktop microcomputer.

The method of feeding, according to the invention, the transmitter card follows from two observations:

- The transmitter, content to transmit to control the electrical bridge between the earth electrode and the structure, only needs to operate the electrical power necessary for the operation of a low-consumption logic circuit controlling the control of a MOS transistor, and possibly some low consumption sensors and converters (in practice, an intelligent transmitter based on a microcontroller can consume less than 100 μA).

- A correct bias voltage of the metal structure with respect to the earth being always less than -0.8V (measurement with Cu-CuSθ4 electrode), there is therefore normally a potential difference greater than 0.9V between an earth connection made of copper or carbon (potential of 0.1V in the earth) and the protected structure. This figure is increased in seawater. This voltage can be used by a DC / DC voltage converter to generate 5V: there are also integrated circuits manufactured to implement this function on a printed circuit. Based on the first observation, a first variant of the power supply method consists in using a fully autonomous source of electrical energy, such as a battery, a possible power supply device according to the invention: the capacity of such existing devices allows a theoretical autonomy of at least 10 years.

Based on the two previous observations, a second variant of the supply process consists in using the potential power available between the earth connection and the protected structure (approximately equal to U ² /4.Rt, if U is the potential difference and Rt the "resistance" of earth), via a voltage converter device, to directly supply the electronics of the transmitter or to charge a device accumulating electrical energy, which will feed in return the electronics of the transmitter. In the case where the electronics are supplied directly with the converter, it may stop if the bias voltage of the structure becomes greater than -0.8V, but the receiver will be aware of the problem immediately, especially if the transmitter does not start again as soon as the tension is correct again.

The devices according to the invention for supplying the transmitters can therefore be:

- A simple long-life battery, for example Lithium-Thionyle, which has the advantage of simplicity and the fact of not polarizing the earth connection.

- A voltage converter, for example a switching power supply or a charge pump.

- A voltage converter associated with a battery or a large capacitor. In this case, the converter can be very rudimentary, and controlled by a microcontroller.

Note that the transmitter is in all cases autonomous, and can therefore be installed anywhere on the protected metal structure.

Grounding at the transmitters requires special care. Indeed, for the injected current to be maximum, it must offer the highest possible oxidation potential, the lowest possible earth resistance, and good resistance to polarization.

According to a first embodiment, a good performance / price compromise, the earth connection according to the invention is a simple copper stake planted directly in the earth or an electrolyte surrounded by earth (earth resistance for example of the order of 30 Ohms). The very high oxidation potential of copper makes it possible to have a maximum potential difference between the stake and the protected structure, which allows the emitter to inject more current than with an ordinary ground stake of the same dimensions. , and authorizes operation of the switching power supply at pipe bias voltages just below -0.8V.

According to a second embodiment, the earth connection according to the invention is a carbon, coke or graphite electrode, optionally surrounded by a regulating mixture based on graphite powder. Its potential is comparable to that of a copper stake, but requires a larger buried volume (with the advantage of slower aging). According to other embodiments, and depending on the resistivity of the ground, the earth connection can consist of a stake, a plate or any metal wire buried in the earth itself or an electrolyte, itself in contact with the soil, preference being given to metals with a high oxidation potential such as copper, brass, bronze and cast iron with a high silicon content.

According to another embodiment, an existing metal structure will be used as an earth connection. In this case, care must be taken to guarantee the direction of current flow (from the external structure to the protected structure) by using a component or a rectifier assembly (for example a diode), as well as its maximum intensity, by using a current limiter (a simple resistance can sometimes be enough), so as not to harm these external works: the direction of the current guarantees that the external work is not oxidized, and the maximum intensity limited that the '' there is no risk of disturbing equipment electric or electronic connected moreover to this external structure, or even to disturb the operation of another racking. In terms of transmitters, care should also be taken to use frequencies which do not disturb the transmission equipment possibly using the external structure.

A first variant of the method for measuring the cathodic polarization voltage according to the invention consists in measuring the voltage between the ground rod (used by the electric bridge and possibly the voltage converter), and the structure to be protected. The measurement must be carried out when no current comes from the earth rod, therefore when the emission device and the voltage converter are stopped.

According to a second variant of the method for measuring the bias voltage according to the invention, a potential tap separate from the ground socket is used for the measurement. This process is by far the most accurate, since the potential of the earth electrode used for transmission and supply changes over time.

The device used to measure the voltage can for example be a simple analog digital voltage sampler / converter (in the extreme a comparator), the input of which is filtered by a conventional anti-aliasing filter (cutoff frequency at least twice lower than the sampling frequency, to avoid that alternative components, of frequencies multiple of the sampling frequency are seen as continuous components). The telemetry method according to the invention can easily be used as a basis for a remote control method according to the invention which, at the level of the receiver, uses the measurements of the quantities carried out remotely to regulate these quantities, by acting on actuators having an effect on these quantities, and controlled directly or not by the receiver. We could cite other advantages of the present invention but those mentioned above already show enough to prove its novelty and interest. The description and the figures below represent, on the one hand, simplified diagrams of the operating method of a device according to the invention, and on the other hand exemplary embodiments of such a device, but have no limiting character: other embodiments are possible within the scope of the scope of this invention, such that a person skilled in the art knowing electronics would be able to realize them. FIG. 1 schematically represents an environment and devices for implementing the transmission method according to the invention.

FIG. 2 represents a simplified installation diagram of a device according to the invention on a metal pipe protected by a conventional cathodic protection system.

FIG. 3 represents a diagram of an embodiment of a transmitter according to the invention.

In FIGS. 1 and 2, the various constituent elements of a cathodic protection system by conventional withdrawal are first identified:

- weir 1,

- the polarization station proper 2, - here a rectifier station, but 1 and 2 could represent a sacrificial anode which can also ensure current draw - - the metal structure 19 protected against oxidation by draw 1 and 2 - here, a buried gas pipe, which serves as a support for the transmission of information,

- "land" 21 - here, truly the land, not the sea -,

- the current injected into the metal structure by default of insulation or induction 9.

FIG. 1 also shows the fundamental and essential devices for implementing a variant of the transmission method according to the invention, which provides for the use of an all-or-nothing electric bridge. At the transmitter, placed anywhere on the structure:

- an earth electrode 16, offering an oxidation potential allowing to have a positive potential difference with the protected structure 19, of "apparent earth resistance" Rt, including in this context the resistance due to the conduct between the transmitter and station racking, the resistance of the racking station, and its resistance compared to the earth,

- forming part of any transmitter according to the invention, an electrical bridge 23: here, according to the variant cited above, a switch controlled by a command C (switch open or closed when C respectively equal to 0 or 1), which can electrically connect the earth electrode 16 to the pipe 19.

The potential difference existing between the earth connection 16 and the pipe 19 when the switch 23 is open is positive, equal to (ΔV).

If Vo_t and Vo_c are the oxidation potentials of the earth connection and the pipe, and Vc is the potential of the pipe protected by the withdrawal, then we can write:

ΔV - (Vo_t - Vc) or also:

ΔV - (Vo_t - Vo_c) + (Vo_c - Vc)

It is clear here that the potential difference between the earth electrode 16 and the structure 19 is the sum of the no-load voltage of the battery constituted by the earth electrode and the pipe (Vo_t - Vo_c), and the lowering of voltage caused by withdrawal from the pipe, independently of the earth connection (Vo c - Vc).

When C - l, the current injected I into the pipe is quickly equal to lm - ΔV / Rt. When C - 0, the injected current is zero.

It is noted in passing that the injected current I, positive or zero, always flows from the earth connection 16 to the structure 19, and that the withdrawal station 1,2 always supplies at least part of the power required, the other part being possibly supplied by the natural cathode with respect to the work which is, if necessary (if Vo_t> Vo_c), the earth electrode 16. The latter provides all the more power than its potential oxidation is high.

A simple variant of the transmission method according to the invention consists in this environment of imposing on C to follow a law C (t), which is a square signal (succession of 0 and 1) of frequency fo as high as possible but in the pipe bandwidth. Of this way, a current is injected mainly comprising a continuous part roughly equal to Im / 2 (average of the injected current), a sinusoidal part at the frequency fo, of amplitude substantially equal (because perhaps already slightly attenuated) to 2.1 m / π, and sinusoids with frequencies higher than 3fo, perhaps already having trouble entering the structure (selfic effect of the pipe attenuating the amplitude of the injected current by a coefficient a (f) / | a (f) | <1), and whose amplitudes are anyway less than or equal to 2Im / 3π (therefore at least three times lower than that of the fundamental). ωo = 2πfo being the pulsation of the first harmonic of C (t), at the level of the racking station 2, the racking current Is follows a law:

Is (t) = IsO (t) + KO.Im/2 + 2Im / π.Kl .sin (ωot + fl) + 2Im / 3π.K3. | a (3fo) | .sin (3ωot + f3)) + ..., where IsO (t) is the law followed by the draw-off current in the absence of transmitter, KO, Kl and K3 the attenuation coefficients of the conduct respectively continuous, at pulsation ωo and 3ωo. ωo being by choice at the limit of the passband of the pipe, Kl is substantially equal to KO, but K3 <(or even <<) K0.

Is (t) therefore mainly consists of a "continuous noisy" component (IsO (t) + KO.Im/2) and a sinusoid of pulsation ωo, due to the transmitter, of amplitude K0.2. Im / π. The higher frequency harmonics generated by the transmitter generate non-random low power noise which, if it does not interfere with the receiver according to the invention, may possibly disturb other equipment. In the latter case, we may choose to generate signals closer to the sinusoid.

According to a variant of the reception method according to the invention, the current Is (t) is measured, and, by autocorrelation, the apparent amplitude of the spectral component of frequency fo is calculated. If at some point it is close to K 1.2.Im/π, we deduce the presence of an emitter.

This variant of the method will be all the more reliable when lm is large (good bias voltage and low earth resistance), when there is little noise (in particular in the frequency band surrounding fo), and when the we can autocorrelate over a long period (uncertainty on the frequency inversely proportional to the observation time, and therefore noise rejection increasing function of the observation time).

Then, in FIG. 2, the elements specific to a particular embodiment of the transmission device according to the invention are identified:

- the DC-DC converter (10A or 10B), (switching power supply or charge pump),

- the low consumption control logic (11A or HB) - here a microcontroller card - supplied respectively between the output of the converter (10A or 10B) and the structure 19.

- The pulse generator 12A (or 12B), controlled by 1 1A (respectively 11B) according to the data of 15 (respectively 18).

- The transistor serving as a controlled electrical bridge 13 or 20, controlled by 12A (respectively 12B) which can be open (1 = 0) or closed. In the latter case I ≈ Im after a long time in front of the system time constants

- a surge arrester 14, which may consist of a low voltage transil diode (the bias voltage must not be less than - 10V). The resistance of the transistor 13 or 20 passing must be large enough so that the transistor does not break in the event of overcurrent (source-drain or emitter-collector voltage then equal to the maximum voltage across the transil diode). If necessary, a series resistance may be added to the transistor.

Next come in FIG. 2 the elements specific to the measurement of bias voltage and of another quantity (here the pressure at the end of the wireframe for example):

- an analog-digital converter 15 measuring the voltage of the earth electrode 16, after low-pass anti-aliasing filtering by 15A, relative to the structure 19. To obtain the most reliable measurement possible, it is better, depending on the invention, perform 13 open and 10A temporarily stopped (the duration of a conversion is short enough for the power supply filter capacitors of the electronic card to keep the power supply at a correct level during the conversion). The output of the converter is operated by 1 1 A, for control 12A and consequently 13 so that a signal characteristic of the measurement made is injected into 19.

a pressure sensor, or pressure-voltage converter 17, the voltage output of which is optionally filtered by the low-pass filter 18A,

- an analog-digital converter 18, transforming the voltage from 18B into values used by 11B, to control 12B and therefore 20 so that a signal characteristic of the measured pressure is injected into 19. We can then always discern in FIG. 2, the elements specific to the reception of the measurements made:

- a current-voltage converter 7 placed between the protected pipe and the source of the draw-off current. In another embodiment according to the invention, more suitable in the case of poorly insulated structures, a simple voltage sensor placed between the structure and an earth connection will be preferable.

- a low-pass analog filter 6. A band-pass filter, removing the DC component, makes it possible to increase the dynamic range useful for the transmission, but causes the absolute value of the bias voltage to be lost at the start of the wire, which can be very useful for driving 8 in a safer way.

A variant according to the invention consists in using two analog channels: a channel for transmission, which uses a bandpass filter (0.1 to 10 Hz for example), and a channel for measuring the bias voltage at the start, which uses a simple low pass.

- a low speed precision 8 to 24 bit analog-digital converter 5 according to the initial filtering and the maximum attenuation in the structure.

- a digital processing card 4, determining the frequencies present, and deducting the transmitters present and the measurement that they transmit. The signal processing carried out in the reception device according to the invention can be a rejection of the harmonics and of some sub-harmonics of the local frequency of the sector (a simple filter in cardinal sinus being sufficient), followed by a rapid Fourier transformation, or a series of correlations, or autocorrelations.

- an indicator light 3, which could also be an audible or modem alarm, which indicates a problem on the protected structure (no protection or lack of pressure for example).

A few additions transform the telemetry device into a complete remote control device, according to the invention:

- an electronic chopper 8, controlled by the processing card 4, and inserted between the original racking station (roto-transformer in most cases) or any voltage source 2 on the one hand, and the structure to protect 19 on the other hand. The latter allows the digital processing card to adjust the initial bias voltage so that the cathode bias voltage is satisfactory at all points in the structure where it is measured. - If necessary, a gas compressor 22 electronically controllable, which will be controlled so that the pressure at the end of the wireframe is satisfactory.

FIG. 3 shows a transmission device according to the invention, and according to a particular embodiment: The electronics are supplied between 42 and 44 by a long-lasting 2V lead battery (8 to 10 years).

Earth taken from earth 16 is connected at 40, and the structure 19 to be protected, at a negative potential with respect to earth, is connected at 41. The microcontroller 30, operating from 2 to 6 volts, (here, a

PIC 16LC84 from the American manufacturer Microchip) controls all of the card's functions: the transmission function 13, 43 and 30, the switching power supply function and battery recharge 30 to 38, 38A and 45, and the voltage measurement function polarization 37, 39, 39A and 45. We note that 30 is involved in these three main functions, 37 and 45 in the last two.

The emission function is provided by a MOS transistor 13 with a threshold voltage lower than 2 volts (here, an SI6426DQ from the manufacturer Ternie-Siliconix) in series with a resistor 43, controlled by the microcontroller 30. The resistance above all has a protective purpose in the event of an overvoltage, which should above all be absorbed by the transil diode 14. By controlling the duty cycle of its command, the microcontroller very precisely controls the current which it injects into the structure . Capacitor 14A can be added to absorb strong industrial alternative components (higher than the bias voltage of the structure), and prevent 14 (or even 13 in reverse) from clipping them unbalanced, which could cause oxidation or a fast polarization of 16. It is noted that the assembly according to the invention could very well directly use an output of the microcontroller (ie integrated transistors) to inject current into the structure (passage of the output of high impedance in the state " zero "and vice versa), but reliability would no longer be guaranteed (the output of this microcontroller only supports 150 mA at peak, current which can be exceeded in the event of overcurrent, while the MOS transistor used supports 30A at peak. In this last case, there is a lot of margin). The battery supply and recharge function can be analyzed as follows: - the diode 34 allows the emission function to function correctly when the capacitor 32 is charged. The latter serves as an energy reservoir, allowing the current in the coil 3 1 not to be limited by the earth resistance and the inductance of the structure; the coil 31 is charged with an energy proportional to the square of the intensity of current which crosses it when the transistor 33 is made passing through 30, and discharges a good part of this energy in the main supply of the card , when 33 is suddenly "opened" by the microcontroller 30. The capacitor 36 makes it possible to limit the fluctuations of the supply during this operation. The battery (between 42 and 43) is charged when the switching power supply is actuated by 30, or when the bias voltage is higher than 2.6V, and limits the supply voltage of the transmitter to approximately 3 volts maximum , when the bias voltage of the structure is not too high.

The polarization voltage measurement function is performed by the double comparator 37 associated with the voltage reference 45, to the set of resistors 39 and capacitor 39A, which make it possible to detect in this case whether the average voltage of the structure is above or below the threshold set by the operator. The microcontroller 30 uses the corresponding output of 37 to determine which form of command C it should use, and when it should use it.

The voltage reference 45, the set of resistors 38, the capacitor 38A and the comparator 37, make it possible to determine whether the supply of the card is not beyond the safety margins, in which case the microcontroller protects the card by closing transistor 13 and / or 33. This latter case occurs when the structure is over-polarized.

Claims

1. A telemetry method operating on a metal work (19) protected against oxidation by racking (1) and (2), which uses the work itself (19) as a medium for the transmission of information, from '' at least one transmitter connected to the structure, and recognized by a receiver (4), (5), (6), (7) also linked further to this same structure (19), characterized in that:

- earth plugs (16) are used to have a positive potential difference with the protected structure (19), - the power of the withdrawal station (1,2) and said existing positive potential difference are used between at least one of said earth connections (16) and the metal structure (19), to inject a variable current (I) modulated by said transmitter as a function of the information to be transmitted in the structure (19), - the said variation in current is measured by the receiver and the information read and transmitted by the transmitter is deduced therefrom, the said transmitter being autonomous can be installed at any point in the structure (19).

2. Method according to claim 1, characterized in that the injection of said variable current (I) is caused by the opening controlled by said transmitter of an electric bridge (13,20) between one of said earth connections (16 ) and the structure (19), the variable current (I) then always flowing from the earth connection to the structure (19)

3. Method according to any one of claims 1 and 2 characterized in that earth connections are used (16) offering an oxidation potential greater than the oxidation potential of the metal structure (19), said sockets earth behaving with respect to the structure (19) like natural cathodes, which make it possible to increase the injected current (I), and therefore provide additional power to the emission.

4. Method according to any one of claims 1 to 3 characterized in that: - the potential difference is used between the protected structure (19) and one of said earth connections (16) to generate the electrical energy which the transmitters need,

- if necessary, this voltage is increased to a level compatible with the operation of the electronic cards of said transmitters.

5. Method according to any one of claims 1 to 4, characterized in:

- use is made of the fact that the variation of said current injected (I) into said metal structure (19) by the electric bridge (13,20), is identifiable at the level of the withdrawal station (1, 2) where the receiver (4,5,6,7) whatever the shape and the size of the protected structure (19), as long as the cathodic polarization voltage with respect to the earth (21) is correct at all points of the work (19),

- We define for this spectral components of said current (I) to be transmitted low enough not to be much more attenuated than the DC component (frequency components in the bandwidth of the structure).

6. Method according to any one of claims 1 to 5 characterized in that it generates from each transmitter placed on the work (19) its own signals allow the receiver to identify the transmitter of these signals and to know the measure (s) it transmits.

7. Method according to any one of claims 1 to 5, characterized in that one uses the measurements of the quantities carried out remotely and recovered by the receiver (7), (6), (5), and (4) to regulate these quantities, by acting on actuators (for example (8) or (10)) having an effect on these quantities, and controlled directly or not by the receiver.

8. Device for implementing the method according to any one of claims 1 to 7 characterized in that transmitters use, as electric bridge, one or more transistors, MOS (13), bipolar (20), or IGBT, discrete (13) or (20) or not, to inject a variable current into the metal structure (19).

9. Device for implementing the method according to any one of claims 1 to 8, characterized in that earth connections are made of a material offering a high oxidation potential, such as copper, brass, bronze, high silicon cast iron, coal, coke or graphite.

10. Device for implementing the method according to any one of claims 1 to 9, characterized in that transmitters comprise at least one capacitor (14A) in parallel, or an inductive coil in series, with the device clipping (14) to prevent any strong alternating components of the voltage present between the earth electrode (16) and the structure (19) from causing unnecessary oxidation or polarization of the earth electrode (16), for example repetitive and unbalanced conduction of (14).