ELECTRONIC CONTROL FOR WELDING OPTICAL FIBERS

Abstract: In splicing optical fibres (3) by means of an electric arc generated between electrodes (1), during the welding the electric power is measured which is generated in the arc. This electric power is maintained constant at varying ambient conditions such as varying atmospheric pressure. A signal representing the welding voltage is tapped at the middle terminal of a voltage divider circuit (R1, R2) and a signal representing the welding current is produced over a resistor (R3). The signals are inputs to a multiplying circuit (23) and the output signal thereof is fed to one input of a comparator (29). To the other input of the comparator a reference voltage (URef.) is fed. The error voltage from the comparator (29) is an input signal to a control circuit (31) providing a control signal to circuits for generation of the voltage between the electrodes (1).

Description: ELECTRONIC CONTROL FOR WELDING OPTICAL FIBERS TECHNICAL FIELD The present invention relates to electronic control in welding and/or heating optical fibers by means of an electric arc, in particular a control of the electric quantities of the electric arc. BACKGROUND When optical fibers are welded by means of the intense heat in an electric arc variations in the arc occur when the surrounding air pressure varies. Thus the electric voltage over the electric arc is reduced at a decrease of the air pressure, what in turn results in a reduction of the electric current through the electric arc and thus a lower temperature in the electric arc and in the region of the fibers heated thereby. It results in turn in inferior welded splices generally having a larger attenuation. PRIOR ART Different ways for compensating variations in the exterior air pressure have been proposed: The European patent application EP-A1 0 504 519 for Fujikura Ltd. discloses an optical fiber welding device comprising a discharge unit 2 for obtaining an electric arc for melt-fusioning of optical fibers, a pressure sensor 6 for providing a signal corresponding to the ambient atmospheric pressure and a "discharge control unit 3" for control of the discharge in relation to the pressure signals so that to the discharge unit 2 an optimal current is delivered during the discharge. By means of feedback through a line from a resistor 251 in the welding electrode circuit the welding current can be maintained constant. The Japanese patent application JP A 63-106706, which is available to the public, also for Fujikura Ltd. relates to an optical fiber welding device of a similar kind. The German patent application DE-A1 37 24 914 made available to the public, for kabelmetal electro GmbH, describes a procedure for connecting two optical fibers by means of the energy from an electric arc. The procedure comprises among other things a control of the (gas) pressure which exists around the electric arc and of the current through the electric arc. US patent US-A 5,002,351 for Szanto et al. relates to a melt welding device for optical fibers where it, according to the abstract, is arranged that "Once an arc is established between opposed electrodes, constant power is provided during the splicing." From the specification of this patent it does not appear what is meant by "power" or how "constant power" can be achieved. SUMMARY It is an object of the invention to provide means whereby a good quality of welded splices and thus splices having a low attenuation and a good strength can be obtained when welding optical fibers to each other in an electric arc. It is another object of the invention to provide a control of the electric quantities in an electric arc which is used for welding optical fibers together, so that essentially the same quality of a finished splice is obtained at a varying surrounding air pressure. The mentioned objects are achieved by a method and a device according to the invention, the detailed characteristics of which appear from the appended claims. Thus, in welding optical fibers the electric power in the actual electric arc circuit is fed back. The electric power is measured for instance as a product of a voltage which corresponds to an average value of the rectified, for instance full wave rectified, voltage, between the welding electrodes and a current which in the same way corresponds to an average value of the rectified, for instance full wave rectified, current flowing in the arc from one electrode to the other. A first voltage can then be produced which is proportional to .the instantaneous electric power developed in the electric arc in the welding. It is fed back to a control circuit for controlling circuits for generating the high voltage over the electrodes, so that the power is maintained essentially constant during the welding. Then, a second voltage can be produced which is representative of or proportional to the current through the welding electrodes, and a third voltage which is representative of or proportional to the voltage over the welding electrodes. The second and third voltages are multiplied by each other in a multiplying circuit for generating the first voltage. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to non-limiting embodiments and with reference to the accompanying drawings in which: - Fig. 1 shows generally and schematically, as seen in the transverse direction of the fibers, welding of optical fibers in an electric arc, - Figs. 2a and 2b show voltage pulses fed to a high voltage transformer and the behaviour of the electrode voltage as a function of time in welding optical fibers, respectively, - Fig. 3 shows a diagram of an electrical important quantity in the electrode voltage as a function of the ambient air pressure, - Fig. 4 shows a diagram of the attenuation in splices between optical fibers produced at a constant electrode voltage and varying ambient air pressures, - Fig. 5 shows a circuit diagram, partly in block shape, of an electric circuit for generating an electric arc for welding optical fibers and - Fig. 6 shows an alternative embodiment of a portion of the circuit diagram of Fig. 5. DETAILED DESCRIPTION In Fig. 1 it is shown schematically, as seen from the side, electrodes 1 connected to a voltage U 8ek , where the electrodes are used for welding or in certain cases only heating one or several optical fibers 3 made of quartz, by means of an electric arc 5 generated between the points of the electrodes 1. Generally, in the production of the electric arc 5 an alternating voltage is used, so that the voltage U sek typically varies periodically. The voltage U Bβk is normally obtained from the high voltage, secondary winding of a transformer, to which a pulse shaped voltage U prim is delivered, to the primary winding thereof, see Fig. 5 and the more detailed description with reference thereto. In Fig. 2a a diagram is shown of the activating voltage pulses pr i m as a function of time and in Fig. 2b a diagram of the secondary voltage U Bβk obtained therein over the secondary winding of the transformer and between the electrodes l. The primary voltage U rim comprises in the typical case periodically recurrent, similarly shaped positive and negative square voltage pulses, the width of which (pulse width modulation) is changed in order to drive a current of varying magnitude between the electrodes 1 in the electric arc 5 or equivalently thereto change the generation of electric power in the electric arc 5. It can also be considered to use a change of the pulse heights instead (amplitude modulation) for driving a variable electric current (also for generating a varying power in the electric arc) between the welding electrodes 1. The frequency of the pulses are typically in the range of 20 - 30 kHz. Owing to the special physical processes in the electric arc 5 the secondary voltage U βek , i.e. the voltage over the electrodes 1, has a rather complicated behaviour, which is illustrated as a function of time in Fig. 2b. It can be observed, that in the ignition of the electric arc first a large short voltage pulse occurs over the electrodes, having the magnitude V τ , called the ignition voltage, after which a time period is obtained having an approximately constant, weakly decreasing voltage having an average voltage of the magnitude V B and is called the burn voltage. It is maintained as long as the voltage between the electrodes 1 still has the same polarity. For changes in the ambient air pressure around the electric arc variations occur in the electric characteristic properties of the electric arc. An example thereof is illustrated in the diagram of Fig. 3, where the burn voltage V B is plotted as a function of .the ambient air pressure for a constant electric current through the electrodes and thus in the electric arc 5. As is illustrated by the curve of this diagram, the burn voltage V B is essentially linearly dependent on the ambient pressure. The different variations in the electrical conditions for varying air pressure influence the physical conditions in the electric arc 5. If thus an optimal welding current is set to an initial value for some ambient air pressure, so that the attenuation in a splice made by the electric arc, between two optical fibers 3 for this welding current and for this air pressure is given a minimum attenuation, the attenuation will increase in a welded splice made with this same welding current, when the air pressure is changed. It is illustrated by the diagram of Fig. 4, where the loss or the attenuation in a fiber splice is plotted as a function of the ambient air pressure for a constant welding current, which is set to give a low attenuation for an air pressure of 950 mbar. In order to compensate the effects of varying pressures, previously the driver circuits for the electric arc have been arranged so that an essentially constant welding current is obtained. However, it does not always give a correct desired result, that is the splices made have not always a maintained low attenuation for varying pressures. It has appeared, that if instead the electric power generated in the electric arc is maintained constant in the production of fiber splices for varying air pressures, a better stability will be obtained and then approximately the same, continued low attenuation will be obtained in splices made for different ambient air pressures, provided that, in the corresponding manner as above, the electric power has been set to give a low attenuation in fiber splices for an initial value of the ambient air pressure. A circuit solution for maintaining the electric power in the welding arc constant is shown in Fig. 5 as included in an electric circuit which is partly shown in block shape and is designed for generating the electric arc 5 for welding optical fibers 3. Between the connection terminals 7 and 9 a direct voltage is supplied. This voltage is received by a pulse width modulating circuit 11, which to the primary winding 13 of a transformer 15 outputs a voltage U prim comprising pulses, see Fig. 2a, the width of which is controlled, i.e. pulse width modulation is used. As has been mentioned above, alternatively amplitude modulation can be used, wherein the height of the pulses in the primary voltage U prim are controlled. In the transformer 15 the voltage U prim over the primary winding 13 is transformed to a high voltage U sek over the secondary winding 17. Over the secondary winding principally a square wave voltage is induced having the same general behaviour as the primary voltage, see Fig. 2a, which by the electric arc is changed into the secondary voltage U aek according to Fig. 2b. The two terminals of the secondary winding 17 are connected to the two welding electrodes 1, between which the electric arc 5 is formed for heating and/or welding an optical fiber 3 located between the points of the electrodes 13. The secondary voltage U sek is rather high, typically in the range of 500 - 1000 volts, and is not suitable for a direct evaluation. Therefor a small voltage is extracted, which is proportional to the voltage U sek over the secondary winding 17, at the middle connection of a voltage divider circuit comprising two resistors Ri and R 2 , which have their other terminals coupled to the electrodes l or the connection terminals of the secondary winding 17. The second of these resistors, R 2 , has its other terminal coupled to ground. The extracted voltage, i.e. the voltage over R 2 , is rectified by an active, full wave rectifier or measurement rectifier 19, which is designed to rectify also small voltage signals and in the conventional way can be constructed as for instance comprising an operational amplifier having a diode in the feed back loop, and is filtered in a low pass filter 21 for eliminating too rapid voltage variations, such as primarily ripple from the electronic circuits but also the pulses obtained at the ignition of the electric arc between the electrodes, i.e. the pulses having heights equal to the ignition voltage V τ , see Fig. 2b. The rectified and filtered voltage is delivered to one terminal of a multiplier circuit 23 in the shape of a smoothed direct voltage, which reflects the average level of the voltage pulses in the secondary voltage U sec , i.e. essentially the burn voltage V B , which depends on the ambient air pressure. A voltage proportional to the electric current through the secondary winding 17 and through the welding electrodes 1, i.e. proportional to the welding current, is extracted in the conventional way by the method that a resistor R 3 is arranged serially in the circuit comprising the secondary winding 17 and the electrodes 1 and that this resistor has one terminal connected to ground. From the other terminal of this resistor R 3 , which is also connected to one connection terminal of the secondary winding 17, the voltage is extracted and is processed in the same way as the extracted voltage described above, which represents the secondary voltage. Thus the voltage is first rectified by an active full wave rectifying circuit or measurement rectifier 25 and is filtered by a low pass filtering circuit 27. The rectified and filtered voltage thus obtained is proportional to the welding current and is delivered to a second input terminal of the multiplier circuit 23. The multiplier circuit 23 is designed for multiplying the two voltages which are fed to its inputs and provides a result on a third connection line, its output, so that the voltage on this output line is proportional to the product of the two delivered voltages and thus generally is equal to the product of these two voltages multiplied by a factor K. The product voltage is then fed to one connection of a comparator 29, to the other input of which a reference voltage or control voltage U ref is provided in some suitable way, not shown, such as in the simplest conceivable case from some manually adjustable potentiometer circuit (not shown) . A comparator 29 provides on its output a voltage which is proportional to the difference of the voltage over the two inputs thereof. The signal from the comparator 29 is provided to a control circuit 31, which is arranged to control the pulse width modulating circuit 11. To the latter circuit such control signals are provided, that the pulse width is adjusted, so that the error signal on the input to the control circuit will be as small as possible. In this way the product of voltage and current in the secondary circuit comprising the electric arc is fed back, so that this product is maintained constant. The product is proportional to the electric power generated in the secondary circuit of the transformer 15, i.e. substantially the electric power generated in the electric arc 5. By the feedback and the maintaining of a voltage, which is proportional to the power, at a constant value the circuit is, as has been mentioned above, less sensitive to variations in the ambient pressure and a low attenuation is maintained in fiber splices made at varying air pressures. In Fig. 6 an alternative, more detailed circuit solution is illustrated for a portion of the circuit according to Fig. 5. The output from the multiplier circuit 23 is here connected to a switch 33, which in its normal position forwards the output signal from the multiplier circuit 23 through a resistor R 4 to the inverting input of an operational amplifier 35, which is connected as an amplifier having a gain ^ as determined by the ratio R 5 /R . R 5 is a resistor which is connected between the same inverting input of the operational amplifier 35 and the output thereof. To the not inverting input of the operational amplifier 35 the reference voltage U ref is provided. It is obtained from a processor 37, which is used for control of the total welding procedure by means of also other devices not shown here. The voltage variations on the output of the operational amplifier 35 are proportional, as given by the mentioned constant of K j of proportionality, to the variations of the difference between the voltages on the inputs of the operational amplifier 35. The processor 37 contains a D/A converter 39 for generating the analogue signal U ref and a memory 41 for a table, where for each welding type, as given by a suitable welding current I, the corresponding reference value U ref is stored. The reference value U ref determines by means of the circuit as illustrated the power generation in the electric arc. The output of the operational amplifier 35 is further connected to the inverting input connection of a comparator 43, which is comprised in the control circuit 31 of Fig. 5. To the other input terminal of the comparator 43, the not inverting input thereof, a second reference voltage U' Ref is fed. The output signal from the comparator 43 is proportional to the difference of the voltage existing over its inputs and is provided to the pulse width modulating circuit 11. When changing the state of the switch 33 the multiplier circuit 23 is disconnected and instead the voltage from the low pass filter 23, which represents the current in the electric arc, is directly delivered to the inverting input of the comparator 43. It gives a control of only the welding current, so that it will be maintained constant during the welding. Such a control can be necessary to arrange, when power control does not work, such as for instance when the points of the electrodes 1 are too bad.

Claims: CLAIMS 1. A method for welding two ends of optical fibers to each other by means of an electric arc generated between two electrodes, characterized in that the electric power in the welding is maintained constant. 2. A method according to claim 1, characterized in that a voltage is produced which is proportional to the instantaneous electric power in the welding and is fed back to a control circuit for control of an electric driver device for the electrodes, so that the power is maintained essentially constant during the welding process. 3. A method according to claim 1, characterized in that a voltage is produced, which is representative of or proportional to the current through the welding electrodes, and the voltage, which is representative of or proportional to the voltage over the welding electrodes, whereafter these two proportional voltages are multiplied by each other. 4. A device for splicing two ends of optical fibers by welding, by means of an electric arc generated between two electrodes, characterized by means for maintaining the electric power in the arc essentially constant during the welding. 5. A device according to claim 4, characterized by - a control circuit for control of an electric driver device for the electrodes, - means for producing a voltage which is proportional to the instantaneous electric power in the welding and - a line from these means for feeding the voltage back to the control circuit, so that the control circuit maintains the electric power essentially constant during the welding. 6. A device according to claim 4, characterized by - means for producing a voltage which is representative of or proportional to the electric current through the welding electrodes, and - means for producing a voltage, which is representative of or proportional to the electric voltage over the welding electrodes, and - means for multiplying these two proportional or representative voltages by each other. 7. A device according to claim 6, characterized in that the multiplying means comprise a multiplier circuit having two inputs and an output, the inputs of which are connected to the voltage producing means and on the output of which a voltage signal is provided, which is proportional to the product of the voltages, which are present on the inputs thereof. 8. A device according to claim 7, characterized in that the output terminal of the multiplier circuit is connected to an input of a comparator, that to another input of the comparator a line is arranged from a means for generating a constant reference voltage and that the output from the comparator is connected to a control circuit for control of an electric driver device for the electrodes. 9. A device according to claim 8, characterized in that the means for generating a constant reference voltage are arranged to generate different constant reference voltages depending on the type of optical fibers which are to be spliced.

Inventors: DUMITRIU ION, HULTEN OLA, DUMITRIU, ION, HULTEN, OLA
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1995
Main classification: G02B6
IPC codes: G02B6