GB1579601A

GB1579601A - Keyhole welding or cutting

Info

Publication number: GB1579601A
Application number: GB4757577A
Authority: GB
Original assignee: Central Electricity Generating Board
Current assignee: Central Electricity Generating Board
Priority date: 1978-05-25
Filing date: 1978-05-25
Publication date: 1980-11-19

Description

(54) IMPROVEMENTS IN OR RELATING TO KEYHOLE WELDING OR CUTTING (71) We, CENTRAL ELECTRICITY GENERATING BOARD, a British Body Corporate, of Sudbury House, 15 Newgate Street, London, EC1A 7AU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to keyhole welding and is concerned more particularly with automatic control of the welding operation.

As will be explained hereinafter, the invention may also be applied to the automatic control of a cutting process.

Keyhole welding may be effected using a plasma-arc or a laser beam or an electron beam and enables deep seam welds to be produced.

Considering for example, plasma-arc welding, this Is a welding process in which fusion is produced by the heat of a constricted arc. Compared with TIG (tungsten inert gas) welding, a much higher energy density is employed in the arc and a much higher gas velocity and momentum are produced by constraining the arc to flow through a nozzle. Plasma-arc welding enables deep seam welds to be produced by forming a "keyhole" in the workpiece. This keyhole welding mode differs from conventional fusion welding by the TIG process, which is essentially a surface melting process as the arc cannot penetrate to the depth of the weld pool. In plasma-arc welding, the arc depresses the surface of the liquid metal by pressure of the gas flow and the plasmaarc thus produces a keyhole in the weld pool. When the process is operated correctly, with the arc being traversed along the line of the weld, the metal, which is melted in front of the advancing keyhole, flows around to the rear where it forms the weld bead.

The keyholing process is essentially a compromise between a conventional fusion welding process and a plasma cutting operation. In the former, there is no penetration to the underside of the workpiece to form a keyhole and no more than a surface depression is achieved in the weld pool. In plasma cutting, on the other hand, the molten metal is ejected from the welding area by the force of the plasma jet. For keyhole welding, it is necessary to ensure that the molten metal flows from the front of the advancing keyhole around to the rear to solidify into a weld.

Although keyhole welding has been described more particularly with reference to the use of a plasma-arc, a similar keyholing process is employed in laser and electron beam welding.

It has previously been proposed to control a keyhold welding operation by monitoring the light emission from the efflux plasma which issues from the exit side of the keyhole. In a paper "Operational envelopes for plasma keyhole welding" by W.McLean and B.E. Pinfold at the 3rd International Conference on "Advances in Welding" at Harrogate May 1974, it was proposed that a photo-sensor could be used to detect thislight for monitoring when breakthrough occurs. In the discussion at the Conference on that paper, J.C. Metcalfe pointed out that, by observation of the quantity of light emitted and of the form of the efflux plasma, it is possible to control the parameters of the welding operation to maintain a predetermined size of efflux plasma during the welding process. Such a control system is further described by J.C. Metcalfe and M.B.C. Quigley in Central Electricity Generating Board Technical Disclosure Bulletin No. 261 (April 1976) where the light from the efflux plasma is monitored to detect breakthrough and to then initiate relative movement between the workpiece and welding head. Instabilities can be detected and controlled by monitoring for repeated signals from the breakthrough detector.

It is an object of the present invention to provide an improved form of automatic feedback control for a keyhole welding process.

According to one aspect of the present invention, a method of controlling a keyhole welding process using a welding head with means for effecting relative movement between the welding head and a workpiece comprises the steps of monitoring the amplitude of the light emitted from the efflux plasma and simultaneously varying, in accordance with the monitored light amplitude, both the power input to the welding head and the rate of relative movement between the welding head and the workpiece. The power input to the welding head or the rate of traverse will be referred to hereinafter as the controlling variables. One or more other variables, e.g. the gas-flow rate in a plasma-arc welding process may also be simultaneous controlled in accordance with the monitored light output.

For plasma-arc welding and electron beam welding, the welding current is a measure of the power input to the welding head. For a laser beam welding device, the power in the laser beam is the equivalent controlling variable. In explaining the invention, it is convenient to assume a plasmaarc welding operation and to refer to the welding current. In carrying out the method of this invention, both the welding current and rate of traverse are simultaneously varied so as to form a one variable system.

Any change in the amplitude of the monitored light away from a datum level will cause a change in both the welding current and the speed of traverse. The relationship between these controlling variables is predetermined for a given operation, e.g. a particular plate thickness and a particular gas flow setting but it has to be appropriately chosen for that operation.

Considered in another way, one can plot a series of curves representing lines of constant efflux plasma light emission using welding current as the ordinate and speed of traverse as the abscissa. Each plate thickness, gas flow setting etc. will correspond to a different curve and there is an optimum operating point on each curve. The line through these points (which is analogous to a load line in, for example, valve character istics), represents the required operating conditions and thus determines how both the welding current and the rate of traverse should change for a given change in the amplitude of the light from the efflux plasma. This line may be straight or may be curved. It will be seen therefore that once the optimum relationship between the controlling variables has been determined over the range of monitored light amplitude for the required welding conditions, then the "load-line" is determined and the controlling variables may be both altered in an appropriate manner in accordance with changes in the monitored light. The control system may be a simple proportional control in accordance with light amplitude or may include an integral term or may use three-term control.

The "load-line" may thus be determined for steady state conditions and the effects of transients can be dealt with in the controller. The load-line can be determined empirically for any given welding operation.

Although reference has been made above to the control of two controlling variables, e.g.

welding current and rate of traverse, it is possible to use more than two controlling variables in this way. For example, in plasma-arc welding, the gas flow might also be used. It will be seen that this is, in effect, operating on a load-line drawn through a set of surfaces in three dimensions, representing constant efflux plasma, the three controlling variables defining the three coordinate axes. Torch height may also be used as a further additional controlling variable. In practice however, two controlling variables may be adequate.

The invention furthermore includes within its scope keyhole welding apparatus having a welding head and traverse means for effecting relative movement between the welding head and a workpiece, wherein the means are provided for monitoring the amplitude of the light emitted from the efflux plasma to give a control signal and wherein means are provided responsive to the control signal and operative to control in a predetermined relationship both the power to the welding head and the rate of relative movement between the welding head and workpiece.

Preferably means are provided for automatically stopping the welding operation when the control system calls for a value of a controlling variable outside its working range, e.g. a welding current, in a plasmaarc welding operation, beyond predetermined maximum and minimum limits. Preferably also for starting a weld operation, means are provided for regulating the controlling variables according to some predetermined open-loop manner until penetra tion is detected, whereupon automatic control of traverse of speed and welding power may be used. A similar technique would be used at the end of the weld to close the keyhole.

The means responsive to the control signal may comprise power supply control means and a variable speed drive each operatively responsive to said control signal.

Separate controllers may be provided for the variable speed drive and for the power supply control and for any other variables to be controlled, e.g. gas flow rate. Each may be a proportional controller or a proportional and integral controller or a three-term controller. Provision may be niade for separately adjusting each of the controllers to preset the proportionality factors and/or time constants. The control signal may be an error signal and the control of the rate of movement and of the power to the welding head may be control of the deviation from a pre-set value.

Limit means may be provided to stop the welding operation if the control system calls for power exceeding a predetermined maximum or less than a predetermined minimum.

For starting a welding operation, means may be provided for varying the power and speed in a predetermined manner until breakthrough is detected by the light monitoring means.

The above described techniques and apparatus can equally well be used for the controlling of a cutting operation and thus the invention includes within its scope a method of controlling a plasma-arc or laser beam or electron beam cutting process using a cutting head with means for effecting relative movement between the cutting head and a workpiece which method comprises the steps of monitoring the light emitted form the efflux plasma and simultaneously controlling, in accordance with the monitored light amplitude, both the power input to the cutting head and the rate of relative movement between the cutting head and the workpiece.

The invention further includes within its scope, apparatus having a plasma-arc or laser beam or electron beam cutting head for cutting a workpiece wherein means are provided for monitoring the amplitude of the light emitted from the efflux plasma to give a control signal and wherein means are provided responsive to the control signal and operative to control in a predetermined relationship both the power to the cutting head and the rate of relative movement between the cutting head and the workpiece.

The following is a description of one form of the invention, reference being made to the accompanying drawings in which: Figure 1 illustrates diagrammatically a control system for a plasma-arc welding apparatus for deep-seam welding; and Figure 2 is a graphical diagram for explaining the operation of the control system.

In Figure 1, there is shown a plasma-arc torch 10 producing a plasma flux 11 which torch is gradually traversed along a workpiece 12 in a direction indicated by arrow 13. In this particular embodiment the relative movement is effected by means of a variable speed drive 14 moving the workpiece. The flux has to penetrate the workpiece 12 melting the metal and producing the keyhole at 15. The light emitted by the efflux plasma 27 is detected by a photosensor 16. The photo-sensor 16 is shown diagrammatically; it may include optical transmission means, e.g. a rod or rods of glass or transparent plastics, (preferably coated), for collecting and transmitting the light to a photo-transducer. It is desirable to keep the distance of the photo-sensor from the position of welding constant and conveniently the light collecting device is incorporated in a supply pipe and nozzle assembly which provides an inert backing gas atmosphere at the rear of the weld.

The output of the photo-sensor 16 is dependent on the quantity of light emitted by the efflux plasma 27 and this output is compared with a reference signal from a reference signal source 17 in a comparator 18 to provide an error signal on a lead 19.

The signal from the photo-sensor may be processed, e.g. amplifed and/or clipped to produce a signal for comparison with the reference signal which deviates substantially symmetrically with respect to the reference level. The error signal is applied to first and second controllers 20, 21 for controlling respectively the variable speed drive 14 and a power supply control unit 22 regulating the plasma current in the welding arc. In this particular embodiment, each of the controllers 20, 21 provides both proportional and integral outputs dependent on the magnitude of the error signal on lead 19. The relationships between the proportionality factors and the time constants may be set by adjustment of the resistors and/or capacitors of the operational amplifiers 23 in each of the controllers. The proportionality factors for the proportional control components relate to the steady state conditions and are predetermined, for any given workpiece and gas flow rate, by finding the optimum relation between welding current and traverse speed for each of a number of magnitudes of the efflux plasma. The time constants of the integral components are adjusted in accordance with overall transient response of the system.

The control signals in the above arrangement are essentially error signals and are used to control the deviation of the controlled parameters from some predetermined value. If gas flow is also to be controlled, a third controller would be provided in addition to the controllers 20, 21.

Figure 2 is a graphical diagram showing a number of curves A, B, C each of which is a line of constant efflux plasma plotted with the welding current as ordinate and speed of traverse as abscissa. A given size of efflux plasma will have different lines (A, B, C etc.) if the welding conditions are changed.

For example line B could represent the combinating of current and welding speed for a workpiece of a particular thickness, line C could refer to a thinner workpiece and line A to a thicker one. However the weld quality varies along each of the lines and only the segments A,A2, B1B2, C,C2, correspond to good welds. The line XY, representing the relationship between cur rcnt and speed determined by the gains within the control system, is chose to intersect each of the curves A, B, C etc. at the best place. XY may be a straight line or a curve. Thus, for any given efflux plasma amplitude, the line XY defines both the required welding current and the speed of traverse. The welding current and speed of traverse are the two controlling parameters in the above-described system.

It will be readily apparent that there may be more than two controlling parameters. In particular, gas flow rate may also be used as a controlling variable. In this case the diagram of Figure 2 becomes a threedimensional diagram with the curves A, B, C replaced by two-dimensional surfaces.

The optimum conditions are still represented by a line XY. For practical pruposes, however, control of two parameters with possible a simplified control of one or more further parameters may generally be adequate.

A limit sensor 24 senses when the current supply to the arc called for by the power supply control 22 reaches maximum or minimum values and operates to stop welding by switching off the current, reducing the gas supply at 25 and also stopping the traverse of the workpiece by the variable speed drive 14, in some predetermined cycle.

For starting a welding operation, the unit 22 is set to give a predetermined start current and movement of the workpiece is inhibited until a breakthrough detcctor 26, responsive to the output of sensor 16, determincs that penetration has been achieved. On detection of such initial breakthrough, the power supply unit 22 and the drive 14 are released from the starting conditions so that they can then operate under the control of controllers 2(), 21 as described above. Provision may be made to change the gas flow on initiation and/or completion of traverse.

Although a simple form of analogue control has been described, it will be readily apparent that a digital control sytem may be employed.

In the above, there has been described more particularly a method of an apparatus for controlling a keyhole welding operation.

It will be immediately apparent that a similar technique and apparatus may be used for controlling a cutting process using for example a laser beam or an electron beam or a plasma-arc and in which an efflux plasma is produced on the opposite face of the material to that over which the cutting head is moved. The light amplitude of the efflux plasma is monitored and used to control the power input to the cutting head and the rate of relative movement between the cutting head and the workpiece.

WHAT WE CLAIM IS: 1. A method of controlling a keyhole welding process using a welding head with means for effecting relative movement between the welding head and a workpiece which method comprises the steps of monitoring the amplitude of the light emitted from the efflux plasma and simultaneously controlling, in accordance with the monitored light amplitude, both the power input to the welding head and the rate of relative movement between the welding head and the workpiece.

2. A method as claimed in claim 1 and for controlling a plasma arc welding or an electron beam welding process wherein the welding current is controlled to control the power input to the welding head.

3. A method as claimed in claim 1 and for controlling a laser beam welding process wherein the power in the laser beam is controlled to control the power input to the head.

4. A method as claimed in claim 1 and for controlling a plasma arc welding process wherein the gas flow to the welding head is also controlled in accordance with the monitored light amplitude.

5. A method as claimed in any of the preceding claims wherein the distance of the welding head from the workpiece is also controlled in accordance with the monitored light amplitude.

6. Keyhole welding apparatus having a welding head and traverse means for effecting relative movement between the welding head and a workpiece, wherein means are provided for monitoring the amplitude of the light emitted from the efflux plasma to give a control signal and wherein means are provided responsive to the control signal and operative to control in a predetermined relationship both the power to the welding head and the rate of relative movement

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. used to control the deviation of the controlled parameters from some predetermined value. If gas flow is also to be controlled, a third controller would be provided in addition to the controllers 20, 21. Figure 2 is a graphical diagram showing a number of curves A, B, C each of which is a line of constant efflux plasma plotted with the welding current as ordinate and speed of traverse as abscissa. A given size of efflux plasma will have different lines (A, B, C etc.) if the welding conditions are changed. For example line B could represent the combinating of current and welding speed for a workpiece of a particular thickness, line C could refer to a thinner workpiece and line A to a thicker one. However the weld quality varies along each of the lines and only the segments A,A2, B1B2, C,C2, correspond to good welds. The line XY, representing the relationship between cur rcnt and speed determined by the gains within the control system, is chose to intersect each of the curves A, B, C etc. at the best place. XY may be a straight line or a curve. Thus, for any given efflux plasma amplitude, the line XY defines both the required welding current and the speed of traverse. The welding current and speed of traverse are the two controlling parameters in the above-described system. It will be readily apparent that there may be more than two controlling parameters. In particular, gas flow rate may also be used as a controlling variable. In this case the diagram of Figure 2 becomes a threedimensional diagram with the curves A, B, C replaced by two-dimensional surfaces. The optimum conditions are still represented by a line XY. For practical pruposes, however, control of two parameters with possible a simplified control of one or more further parameters may generally be adequate. A limit sensor 24 senses when the current supply to the arc called for by the power supply control 22 reaches maximum or minimum values and operates to stop welding by switching off the current, reducing the gas supply at 25 and also stopping the traverse of the workpiece by the variable speed drive 14, in some predetermined cycle. For starting a welding operation, the unit 22 is set to give a predetermined start current and movement of the workpiece is inhibited until a breakthrough detcctor 26, responsive to the output of sensor 16, determincs that penetration has been achieved. On detection of such initial breakthrough, the power supply unit 22 and the drive 14 are released from the starting conditions so that they can then operate under the control of controllers 2(), 21 as described above. Provision may be made to change the gas flow on initiation and/or completion of traverse. Although a simple form of analogue control has been described, it will be readily apparent that a digital control sytem may be employed. In the above, there has been described more particularly a method of an apparatus for controlling a keyhole welding operation. It will be immediately apparent that a similar technique and apparatus may be used for controlling a cutting process using for example a laser beam or an electron beam or a plasma-arc and in which an efflux plasma is produced on the opposite face of the material to that over which the cutting head is moved. The light amplitude of the efflux plasma is monitored and used to control the power input to the cutting head and the rate of relative movement between the cutting head and the workpiece. WHAT WE CLAIM IS:

1. A method of controlling a keyhole welding process using a welding head with means for effecting relative movement between the welding head and a workpiece which method comprises the steps of monitoring the amplitude of the light emitted from the efflux plasma and simultaneously controlling, in accordance with the monitored light amplitude, both the power input to the welding head and the rate of relative movement between the welding head and the workpiece.

between the welding head and workpiece.

7. Apparatus as claimed in claim 6 wherein means are provided for automatically stopping the welding operation when the control system calls for a value of a power input and/or rate of traverse outside the working range.

8. Apparatus as claimed in either claim 6 or claim 7 wherein, for starting a weld operation, means are provided for regulating the power input and/or rate of traverse according to some predetermined open-loop manner until penetration is detected.

9. Apparatus as claimed in any of claims 6 to 8 wherein the means responsive to the control signal comprise power supply control means and a variable speed drive each operatively responsive to said control signal.

10. Apparatus as claimed in claim 9 wherein separate controllers are provided for the variable speed drive and for the power supply control.

11. Apparatus as claimed in claim 10 wherein each of the controllers is adjustable to preset proportionality factors and/or time constants.

12. Apparatus as claimed in any of claims 6 to 11 and having a reference signal source for comparison with a signal from a sensor responsive to the amplitude of the light from the efflux plasma to provide an error signal as the control signal.

13. A method of controlling a plasmaarc or laser beam or electron beam cutting process using a cutting head with means for effecting relative movement between the cutting head and a workpiece which method comprises the steps of monitoring the light emitted from the efflux plasma and simultaneously controlling, in accordance with the monitored light amplitude, both the power input to the cutting head and the rate of relative movement between the cutting head and the workpiece.

14. Apparatus having a plasma-arc or laser beam or electron beam cutting head for cutting a workpiece wherein means are provided for monitoring the amplitude of the light emitted from the efflux plasma to give a control signal and wherein means are provided responsive to the control signal and operative to control in a predetermined relationship both the power to the cutting head and the rate of relative movement between the cutting head and the workpiece.

15. A method of controlling a keyhole welding process or a cutting process substantially as hereinbefore described with reference to the accompanying drawings.

16. Apparatus for controlling a keyhole welding process or a cutting process substantially as hereinbefore described with reference to the accompanying drawings.