GB1604222A - Controlling impinge ment of high-energy beam of charged particles on a workpiece - Google Patents

Description

(54) IMPROVEMENTS IN CONTROLLING IMPINGEMENT OF A HIGH-ENERGY BEAM OF CHARGED PARTICLES ON A WORKPIECE (71) We, STEIGERWALD STRAHL TECHNIK GMBH, a joint stock company organised under the laws of the Federal Republic of Germany, of Haderunstrasse la, 8000 Munich 70, Federal Republic of Germany, 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: The present invention relates to controlling impingement of high-energy beams of charged particles on workpieces.

When a workpiece is processed with a beam of charge carriers or charged particles, such as an electron or ion beam, a problem arises of directing the beam onto a selected zone to be treated or processed by the beam, e.g.

welded, hardened or machined, in spite of disturbing effects such a spurious electrostatic and/or magnetic fields deflecting the beam, and in spite of deviations of the workpiece from the desired shape and orientation and errors caused by transport and positioning devices which effect the relative position between the workpiece and the beam source, and a further problem arises of keeping auxiliary devices, such as devices for supplying an additional material such as a filler wire, aligned with the zone treated by the beam.

Both problems are closely related with each other, because when the relative position between the beam and the workpiece is changed, e.g. to enable the beam to follow a somewhat distorted gap between two workpiece parts to be joined by welding with the beam, the position of the auxiliary device likewise must be changed to keep it in the desired relative position with respect to the workpiece. The above problem is aggravated by the fact that modern beam machine tools such as electron beam welding machines operate with beam powers of 100 kilowatts and more so that very severe environmental conditions prevail in the vicinity of the zone treated by the beam.

It is an object of the present invention to provide methods and apparatus with which predetermined position relations can be reliably maintained between the beam of charge carriers, a workpiece zone to be treated by the beam, and an additional device cooperating with said zone and said beam.

A method according to the present invention is defined by claim 1 hereinafter, and apparatus according to the present invention is defined by claim 5 hereinafter.

In a preferred embodiment of the present invention, the relative position of the beam and the desired zone of treatment is determined by means of X-rays, while the relative position of the zone actually treated by the beam and the auxiliary device, such as a device for feeding an additional or filler material to said zone, is established by sensing the position of the beam in respect to sensing electrode means which have a predetermined position relative to a mechanical element for supporting said auxiliary device.

The reference position may be a point on a stationary machine element, e.g. a base plate, or a point on an element of an auxiliary device, such as a nozzle of a filler wire supply device, which cooperates in the processing with said beam at the actual zone of impingement.

The preferred field of application of the invention is electron beam machining, such as electron beam welding.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic perspective view of a workpiece during electron beam welding with supply of a filler wire, to show a typical situation in which the invention can be used with advantage; Figure 2 is a sectional view in a plane 11-Il of Figure 1, which goes midways through the gap welded; Figure 3 is a section view in a plane Ill-Ill of Figure 1 perpendicular to the gap welded; Figure 4A is a schematic view of a X-ray measuring system for determining the relative position of an electron beam with respect to a gap to be welded by the beam; Figure 4B is a more detailed view of the system zone in Figure 4A; Figure 5 is an oblique view onto the surface of the workpiece shown in the direction of the line of sight of an X-ray sensor of the system of Figure 4A; Figure 6A is a section view along a line VI-VI of Figure 5; Figure 6B shows a wave form of a reference signal used in the system of Figure 4A; Figure 6C shows a series of pulses reflecting the actual position of the beam in the system of Figure 4A; Figure 7A, B and C are similar to Figure 6A, B and C, respectively and show the situation where the beam is misaligned in respect to a gap to be welded; Figure 8 is partial view of a modification of the system shown in Figure 4A and 4B; Figure 8A is a section view in a plane VIII--VIII of Figure 8; Figure 9A and 9B are section views of portions of workpiece to be welded; Figure 10 shows a diagram which can be used in a modification of the system shown in Figure 4A; Figure 11 is a schematic view for Cx- plaining a modification of the X-ray measuring method described earlier; Figure 12 is a schematic view of a system which can be used for performing the method described in the reference to Figure 11; Figure 13 shows wave forms used in the system of Figure 12; Figure 14 is a perspective view of a part of preferred system for determining the relative position of an electron beam with respect to fixed reference coordinates; Figure 15 is a schematic view of a modification of the system shown in Figure 14; Figure 16 is a partial schematic view of a further modification of the system she'van in Figure 14; and Figure 17 is a block and schematic diagram of a system which can be used in combination with the embodiment shown in Figure 16.

Electron beam welding is well known, e.g.

from U.S. Patent No. 2,987,610, and Figure 1 shows a typical situation at the welding site in an electron beam (E.B.) welding machine not shown. An electron beam 10, which is produced by a beam gun 30, is oscillated across a gap 12 between two edges of two metal plates 14, 16 to be joined by the E.B. welding process and produces a pool 18 of molten workpiece material and additional or filler material 20 supplied to the pool 18 in form of a wire or rod through an appropriately positioned nozzle 22. A relative motion is generated between the workpiece 14--16 and the electron beam 10 so that the pool 18 moves along the gap 12 and forms, upon solidifying, a weld seam 24.

It is essential for producing a sound weld seam that a predetermined relation is maintained between the beam 10, the gap 12 and the nozzle 22 supplying the additional material to the pool of molten metal. Specifically, the zone covered by the oscillating beam should be aligned with the gap, and the filler wire should enter the surface of the pool at the zone struck by the beam.

Because of inevitable inaccuracies in the edges defining the gap 12 and/or spurious magnetic and/or electric fields deflecting the beam from its normal, straight path, it is in practice not possible to ensure a predetermined path for the beam with respect to the workpiece by prealigning or preprogramming the relative mtoion between the beam and the workpiece. Thus, the actual position of the beam with respect to the workpiece region to be worked on by the beam must be determined and any deviations from a desired relationship must be compensated for by some kind of regulation. The relative position between the beam 10 and the gap in the workpiece 14--16 is established by means of X-ray radiation and any detected errors in such relative position is compensated for by deflecting the beam. This will be explained in more detail below. Compensatory detection of the beam can be elected with two pairs of electromagnetic deflection coils 26-26 and 28-28 spaced along the beam path, and the beam can be maintained on target at the gap 12 as shown in dotted lines in Figure 1, and in Figure 3. However, the nozzle 22 supplying the filler wire 20 does not follow the beam deflection since the filler wire supply device (not shown) of which the nozzle 22 is a part, is stationary relative to part of the electron beam machine, which part may be, for example, a stationary base plate 23 of the E.B. machine. The actual position of the beam 10 close to impingement is therefore established with respect to a re ference position, such as a predetermined point of the base plate 23, and, thus, with respect to a stationary support for the wire feeding device which comprises the nozzle 22. A position sensor comprising sensing electrode means is used for this purpose, and any deviation detected is used for varying the position of the feeding nozzle 22 in a sense and direction to restore the desired position relationship of the nozzle 22 and the gap 12. The position sensor using sensing electrode means will also be described in detail below.

The combination of an X-ray sensor for establishing the relative position of the beam with respect to the workpiece region to be treated by the beam, and a "mechanical" sensor comprising sensing electrode means for establishing the actual location of the zone worked on by the beam with respect to fixed machine coordinates, provides for a very reliable system which is effectively operable also under the severe environmental conditions which prevail when a workpiece is worked on by a beam having a power in the order of 100 kilowatts.

In the following, preferred method and apparatus for establishing the actual position of the beam with respect to a gap between two workpieces to be joined by E.B.

welding are described. The method and apparatus to be described are likewise applicable to other types of working on a workpiece with a beam of charged particles, e.g.

hardening, machining, re-melting or repair welding if the zone to be worked on can be distinguished from the adjacent portions of the workpiece. This can be secured e.g. by providing the workpiece with a groove or a layer having an X-ray emissivity other than that of the adjacent material.

Figure 4A is a schematic view of a preferred example of the sub-system for maintaining the electron beam 10 aligned with the gap 12 between two plates 14, 16 to be welded together. The sensor portion of this sub-system is shown in more detail in Figure 8. The sub-system comprises an X-ray sensor 32 which includes a shielding housing 34 made of lead, and a X-ray detector 36, such as a Geiger-Muller tube, mounted within the housing 36. The housing 34 is provided with a collimator section 38 which forms an X-ray entrance passage having a slit-shaped cross section defining a likewise slit-shaped field of view 42 (Figure 5). The sensor 32 is mounted stationary with respect to the base-plate 23 or the gun 30.

The output of the X-ray detector 36 is coupled to a regulating circuit 44 which also receives on line 46 an a.c. voltage related to the voltage which is used to oscillate the beam across the gap 12 during the welding operation. The regulating circuit 44 produces on line 48 an error voltage which is supplied to the first pair of deflection coils 26 arranged to deflect the beam 10 in a direction transversely to the length direction of gap 16, and via an inverter 50 to the second pair of deflection coils 28 to provide a parallel shift of the mean beam axis, as shown in Figure 3. One of the pairs of deflection coils may be used also for producing the transverse oscillation of the beam 10.

During operation, the entrance passage 40 of the collimator section 38 is aimed preferably to a point 52 (Figure 4A) of the pool 18 of molten material which is within the gap 12, so that the wall portions of the plates 14, 16 which define the gap 12, form a lateral collimator for the X-ray radiation produced by the beam 10 striking the molten metal forming the pool 18. Alternatively, the X-ray sensor 32 may also be aimed to a point 52' (Figure 4B) at the bottom of the pool 18. A lead shield 54 (Figure 4B) may be provided to prevent X-ray radiation produced at a beam entrance surface 56 of the workpiece from entering the X-ray detector.

As a further alternative, an X-ray sensor 32' may be provided on a beam exit side of the workpiece as shown in dotted lines in Figure 4A.

The operation of the X-ray sensor subsystem will now be described with additional reference to Figures 6 and 7. During welding of the seam 24, the electron beam 10 is deflected across the gap 12 as indicated by a double-headed arrow 58 in Figure 5.

The electron beam 10 produces X-rays when it strikes the metal forming the workpiece and the pool 18. Since the field of view of the X-ray sensor 32 is limited by the passage 40 (which may have a cross-section area of 0.6 mm by 10 mm) to a small, ribbon-shaped volume, the intersection of which at the workpiece surface being shown by the dotted line 42 in Figure 5, the X-ray detector 36 receives X-rays only when the beam crosses and penetrates into ;the gap 12 and strikes point 52.

If the middle axis 60 of the beam is aligned with the gap 12, as shown in Figure 6A, the X-ray detector 36 will produce an output pulse 62 exactly when the current 64 (Figure 6B) used in the deflection coils 26 or 28 for oscillating the beam, is zero. The regulating circuit 44 may comprise a transmission gate circuit having a signal input to which a signal corresponding to the deflection current 64 is applied, a gate input, to which a signal corresponding to the output pulses 62 is applied, and a signal output which produces a voltage corresponding to those portions of the input signal which are gated through by the gating pulses. Thus, the transmission gate will produce a zero mean output voltage in case of the situation shown in Figure 6.

If, however, the beam axis 60 is misaligned with respect to the gap 12, the X-ray detector 36 produces output pulses 62' at other points of time than that when the deflection current 64 is zero. Thus, the transmission gate will produce an error output voltage of non-zero value which can be used to produce an additional current in the de flection coils for realigning the mean beam axis to the gap 12.

Alternatively, the X-ray sensor 32 may be aimed to a spot 52" (Figure 8) where the electron beam 10 enters the gap 12 and the pool 18. This is preferred in case of well matching workpiece edges and a gap 12 of very small or zero width. Generally, the X-ray emissivity of the solid workpiece material adjacent the pool 18 is larger than that of the molten metal of the pool 18 and the output signal of the X-ray detector produces in such case a level signal having a dip when the beam strikes the pool 18. This dipped level signal may be inverted and shaped and then used as described with reference to Figures 6 and 7. In addition, a shoulder of limited depth may be formed at the matching edges of the workpiece parts 14, 16 bo be joined so that a slot 66 of limited depth d is formed in the beam entrance surface of the workpiece which is detected by the X-ray sensor 32. In such case, the detection of the gap or butt is not seriously impaired by a limited misalignment of the workpiece edges in the direction of the beam axis, as shown in Figure 9B.

An X-ray detector having a dotffhaped field of view may be used, the collimator being oscillated so that the line of sight of such a detector scans the area from which the X-rays to be detected are emitted. Thus, the elongated field of view 42 is equivalent to a periodically reciprocated spot-shaped field of view.

In a practical embodiment, as shown in Figure 4B, the slit-shaped front aperture of the collimator section entrance passage 40 may be positioned at a distance D of e.g. 165 mm from the spot 52' from which the X-rays to be detected are emitted. This spot 52' is in this case deep within the gap 12 and the axis of the entrance passage 40 forms an angle of about 37.5 degrees with the surfaces of the plates 14, 16.

As shown in Figures 8 and 8A, the collimator section 38 of the X-ray sensor 32 may comprise a thick-walled tube member 68 made of lead, and a diaphragm system 70, 72 mounted in end portions of said tube member 68 as shown in Figure 8. Each diaphragm system comprises at least two bodies 74, 76 made of lead and having a D-shaped cross section for defining a slit-shaped passage for the X-ray radiation 78 to be detected. The slit-shaped passage may be filled with a plateshaped body 80 of a material, such as al;- minium, which has a low absorptie,l coefficient for the X-ray radiation involved.

Under certain conditions, the X-ray detector 36 may fail to produce a position pulse 62 (Figure 6C) or a corresponding dip, e.g. when the line of sight is obstructed by a tack weld 82 as schematically shown in Figure 4B. Such situation can be coped with by using a circuit as shown in Figure 10.

This circuit may form a portion of regulating circuit 44 (Figure 4A). The output of X-ray detector 36 is coupled to the input of an integrating circuit 84 which inegrates the position pulses 62. The output of the integrating circuit 84 is connected to the input of a threshold circuit 86 having a threshold such that an output signal is produced when the detector 36 fails to produce the position pulses 62. A switch 88 is connected in series with line 48 and opened by the output signal of threshold circuit 86. A high input impedance amplifier 90 is coupled between the switch 88 and the deflection coils 26, and a storage capacitor 92 is coupled to the input of amplifier 90. The movable contact of the switch 88 is coupled to the collector of a transistor 94 which forms the transmission gate described with reference to Figure 4A, and this connection is further coupled to an operating voltage supply B through a load resistor During normal operation, the switch 88 is dosed (as shown by a dashed line) and a varying error voltage appears across the storage capacitor 92 and controls the deflection of the beam to keep it aligned on the gap 12 (Figure 4A). The charge on the capacitor 92 quickly follows variations of the error voltage because the RC time constant formed by load resistor 96 and storage capacitor 92 is relatively small. When the detector 36 fails to produce position pulses, this situation is detected by circuit 84-86 and the output signal of threshold circuit 86 opens the switch 88. The RC-time constant which is now effective is determined by the high input impedance of the amplifier 90, and the last value of the error voltage is stored across capacitor 92, so that the amplifier 90 continues to keep the beam in the position prevailing immediately before the failing of the position pulses.

Other possible obstructions, which may cause an interruption of the position signal, are spacers inserted between the workpiece parts to be welded together or foreign matter such as scale on or in the gap 12.

The tack welds or spacers may be provided manually with a small groove, similar to the groove 66 mentioned in Figure 9, before welding.

The above described methods and devices provide for a continuous "on-line" tracking of the working site, such as the gap 12, at the location of impingement of the beam. This concept can be modified as below described with reference to Figures 11 and 12. The modification will be described with reference to electron beam welding, it is, of course, likewise applicable to other types of machining operations using a beam of charged particles.

According to the modification, the welding process is repeatedly, preferably periodically, interrupted for a short period of time, during which the relative position of the beam (more specifically its mean middle axis) is measured with respect to the workpiece.

Each interruption may have a duration of e.g.

about 1 to 2 milliseconds, and successive interruptions may be spaced by a period of time of e.g. one or a few seconds. The duration of each interruption is sufficiently short to avoid any harmful effects on the welding process.

During the short interruption, or measurement interval, the normal transverse beam deflection (oscillation) is interrupted and the beam is deflected along a triangular path 100 (Figure 11) over the workpiece surface in advance of the present welding site 18. Thus, the beam crosses the gap 12 or groove 66 some distance, e.g. 5 mm ahead of the welding site with a sufficiently high velocity to avoid any substantial melting of the workpiece material. The detection of the X-ray radiation can be effected by a detector of the type described as reference to Figure 4, and the detector system may be keyed to limit the detection to the transverse portion 100a of the triangular path 100.

Figure 12 is a block diagram of an apparatus for effecting the beam deflection described with reference to Figure 11. The apparatus comprises two pairs of magnetic deflection coils 102, 104 for deflecting the beam across and lengthwise the gap 12, respectively. The deflection coils 102 are coupled to the output of a sine wave generator 106 and to the output of a circuit 108 for producing a symmetrical sawtooth waveform 110 (Figure 13).

The deflection coils 104 are connected to the output of a circuit 112 providing a triangular waveform 114. A timer and trigger circuit 116 controls the circuit 106, 108, 112 by periodically blocking the sine wave generator 106 and simultaneously triggering the generation of the sawtooth and triangular waveforms 110 and 114, respectively. Circuits performing the functions of blocks 106, 108, 112, 116 are known and need not to be described in detail.

The positional error signal provided by the described X-ray measuring systems can be integrated over several measuring cycles to avoid undesirably fast fluctuations caused by irregularities of the gap by sputter particles and the like. This integration can be effected e.g. by an appropriate selection of the time constant of the load resistor 96 and the storage capacitor 92 (Figure 10).

The above described apparatus compensates any detected positional errors by an appropriate deflection of the beam.

Variation of the relative position of the beam gun 30 with respect to the workpiece 14-16 can be effected as described herein after.

The width of the position pulses 62 (Figure 6) can be detected and used for controlling the supply rate of the filler material.

The position of the beam with reference to a coordinate system fixed in respect to the E.B. machine or E.B. gun is measured in the vicinity of the workpiece surface by a second sensor system comprising physical electrode means. A first example of this is described below with reference to Figure 14.

The sensor system shown in Figure 14 comprises a vertical shaft 140 mounted for rotation or pivoting about an axis 142 on sup port and drive means (not shown) which has a fixed position with respect to the beam gun 30. The shaft 140 carries a radially pro truding sensor electrode 144 in form of a needle or thin rod made of a refractory mat erial such as carbon or tungsten. The shaft 140 is further provided with means for gen erating a signal related to the angular position of the shaft, e.g. an angle position encoder shown schematically by a series of equally spaced markings 146 cooperating with a senu sor 148.

During operation, the shaft 140 is rotated or pivoted such that the needle shaped sens ing electrode 144 periodically travels through the beam 10 and produces a corresponding electrical output signal.

These generally pulse-shaped output sig nals follow each other with the revolution frequency of the shaft 140. After adjustment of the beam or any misregistration of the beam with the gap, the output signal of the sensor 148-which e.g. by frequency division may be transformed e.g. into a sine wave having one period for each revolution of the shaft 140--is compared with the altered temporal sequence of the needle shaped sensor 144. This output signal has a predetermined time relationship with a "zero" marking of the series of markings 146 if the beam 10 is adjusted to the axis of the beam gun.

Thus the position of the beam centered with the gap with respect of the gun axis may then be detected by a phase shift measurement between the signal sequence of sensors 144 and 148. The error signal produced by the latter phase shift measurement is used to move the filler wire supply and other auxiliary devices which are connected with the mechanical housing of the gun into the corrected beam position or into a desired predetermined position with respect to the welding site.

Spurious magnetic fields caused by remanent magnetism of the workpiece or other forces may also cause a deflection of the beam in a direction parallel to the longitudinal direction of the gap 12. To cope l with a disturbance of this type, a second sensing system comprising a rotatable or pivotable shaft 140" carrying a further needleshaped electrode 144" may be provided in addition to the system described with reference to Figure 14. The plan view of Figure 15 shows an arrangement, which comprises a first shaft 140' carrying a needle-shaped electrode 144' for producing, as described with reference to Figure 14 an error signal for aligning e.g. the auxiliary device in a direction normal to the gap 12 or welding direction, while the second system comprising the shaft 140" and the needle-shaped electrode 144' is used in a similar manner for aligning the auxiliary device in relation to the gun in a direction parallel to the gap 12 (longitudinal direction).

According to a further modification shown in Figure 16, a shaft 140a carries at least two needle-shaped sensing electrodes 144a and 144b, spaced in the direction of the axis 60 of the undeflected beam. A double deflection system 160 comprising two pairs of deflection coils 26, 28 spaced along the axis 60 can be used to deflect the beam, as shown in Figure 16, such that it impinges with an essential normal direction on the workpiece surface in spite of any spurious magnetic field 162.

(The actual position of the shaft 140a is in front of or behind the axis 60; in Figure 16 it is shown in the plane of drawing only to make the drawing more legible).

Figure 17 shows a circuit which can be used in combination with the sensing system described with reference to Figure 16. An angular position sensor 148 produces a signal corresponding to the angular position of the rotating shaft 140a. This signal is converted by a frequency dividing and wave shaping circuit 163 into a position reference sine wave.

A first error signal generating circuit 164 receives said sine wave at a first input and the output pulses from electrode 144a at a second input. The circuit 164 may comprise a transmission gate as described with reference to Figure 4A and provides at an output 166 an error signal indicating any deviations of the beam 10 from the reference position corresponding to the undeflected beam axis 60, which may occur the plane of rotation of the electrode 144a. This error signal controls the deflection current in deflection coils 26 to minimize said error. Further the pulses from the second sensing electrode 144b and the reference sine wave are applied to a second error signal generating circuit 168 which produces a second error signal which is used to control the deflection current m the deflection coils 28. The error signal generating circuits 164 and 168 produce the respective error signal with differing time constants so that no hunting is possible and a compromise is established which provides for an essentially normal impingement of the beam on the workpiece surface.

The "needle sensor systems" described above may be modified in a similar manner as described with reference to Figures 11 and 13. Thus, the needle electrodes may be mounted stationary with respect to the E.B.

welding machine coordinates and the sensing may be effected by deflecting the beam along a triangular path with respect to the sta tionary sensing electrode or electrodes.

The position of the sensing electrodes should be as close as feasible to the workpiece surface, e.g. in a distance of no more than about 5 to 15 mm.

The needle-shaped electrodes 144a, 144b may be slightly offset in a direction normal to the axis 142 of the shaft 140a to ensure that the undeflected straight beam does not cast a shadow of the upper electrode 144a onto the lower electrode. Alternatively, the electrodes 144a and 144b may be offset in circumferential direction, e.g. to extend in opposite directions, and in such case means are provided to supply the circuits 164 and 168 with appropriately phase-shifted versions of the reference sine wave.

To prevent hunting, the regulating system for the relative position of the beam and the desired reg

Claims (18)

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

   with a disturbance of this type, a second sensing system comprising a rotatable or pivotable shaft 140" carrying a further needleshaped electrode 144" may be provided in addition to the system described with reference to Figure 14. The plan view of Figure 15 shows an arrangement, which comprises a first shaft 140' carrying a needle-shaped electrode 144' for producing, as described with reference to Figure 14 an error signal for aligning e.g. the auxiliary device in a direction normal to the gap 12 or welding direction, while the second system comprising the shaft 140" and the needle-shaped electrode 144' is used in a similar manner for aligning the auxiliary device in relation to the gun in a direction parallel to the gap 12 (longitudinal direction).

   According to a further modification shown in Figure 16, a shaft 140a carries at least two needle-shaped sensing electrodes 144a and 144b, spaced in the direction of the axis 60 of the undeflected beam. A double deflection system 160 comprising two pairs of deflection coils 26, 28 spaced along the axis 60 can be used to deflect the beam, as shown in Figure 16, such that it impinges with an essential normal direction on the workpiece surface in spite of any spurious magnetic field 162.

   (The actual position of the shaft 140a is in front of or behind the axis 60; in Figure 16 it is shown in the plane of drawing only to make the drawing more legible).

   Figure 17 shows a circuit which can be used in combination with the sensing system described with reference to Figure 16. An angular position sensor 148 produces a signal corresponding to the angular position of the rotating shaft 140a. This signal is converted by a frequency dividing and wave shaping circuit 163 into a position reference sine wave.

   A first error signal generating circuit 164 receives said sine wave at a first input and the output pulses from electrode 144a at a second input. The circuit 164 may comprise a transmission gate as described with reference to Figure 4A and provides at an output 166 an error signal indicating any deviations of the beam 10 from the reference position corresponding to the undeflected beam axis 60, which may occur the plane of rotation of the electrode 144a. This error signal controls the deflection current in deflection coils 26 to minimize said error. Further the pulses from the second sensing electrode 144b and the reference sine wave are applied to a second error signal generating circuit 168 which produces a second error signal which is used to control the deflection current m the deflection coils 28. The error signal generating circuits 164 and 168 produce the respective error signal with differing time constants so that no hunting is possible and a compromise is established which provides for an essentially normal impingement of the beam on the workpiece surface.

   The "needle sensor systems" described above may be modified in a similar manner as described with reference to Figures 11 and 13. Thus, the needle electrodes may be mounted stationary with respect to the E.B.

   welding machine coordinates and the sensing may be effected by deflecting the beam along a triangular path with respect to the sta tionary sensing electrode or electrodes.

   The position of the sensing electrodes should be as close as feasible to the workpiece surface, e.g. in a distance of no more than about 5 to 15 mm.

   The needle-shaped electrodes 144a, 144b may be slightly offset in a direction normal to the axis 142 of the shaft 140a to ensure that the undeflected straight beam does not cast a shadow of the upper electrode 144a onto the lower electrode. Alternatively, the electrodes 144a and 144b may be offset in circumferential direction, e.g. to extend in opposite directions, and in such case means are provided to supply the circuits 164 and

   168 with appropriately phase-shifted versions of the reference sine wave.

   To prevent hunting, the regulating system for the relative position of the beam and the desired region of welding preferably has a time constant which is shorter, e.g. by one magnitude, than the time constant of the system regulating the position of the auxiliary device with respect to the actual site of welding (pool 28).

   Matter described hereinbefore is described and claimed in Application No. 25355/78, Serial No. 1,604,223.

   WHAT WE CLAIM IS: 1. A method of controlling the zone of im pingement of a high-energy beam of charged particles on a workpiece, the method com prising the steps of: sensing a desired zone of impingement rela tive to the actual zone of impingement of said beam and determining a spatial relation shp of the said zones to produce a first error signal representative of deviation from a de sired spatial relationship between the actual and desired zones of impingement; sensing the beam so as to determine an actual position in the beam with respect to a reference position to produce at least one second error signal representative of de viation between the said actual position of the beam and the reference position, the reference position being related to the posi tion of an auxiliary device intended to co operate with the actual zone of impingement; using the first error signal for compensat ing deviation from the said desired relation ship between the actual and desired zones of impingement; and using said second error signal or sig

   nals to enable the auxiliary device to be in a position permitting cooperation thereof with the actual zone of impingement.
2. 2. A method according to claim 1, wherein the said first error signal is produced by detecting X-ray emission from the actual zone of impingement and is used to change the position of the beam relative to the workpiece by deflection by an amount and in a direction to minimize the deviation.
3. 3. A method according to claim 1 or 2, wherein the said actual position in the beam is adjacent the actual zone of impingement and is sensed by means of physical electrodes to establish the actual position of the beam closely adjacent of the workpiece.
4. 4. A method according to any preceding claim, wherein, during a detection period, the position of the beam is changed along a predetermined path crossing the desired zone of impingement, the relative position of said beam with respect to the desired zone is detected during said crossing and the first error signal is produced on the basis of the result of such detection, said detection period being short enough to avoid impairing the intended effect of the beam on the workpiece at the said actual zone of impingement.
5. 5. Apparatus for effecting controlled impingement of a high-energy beam of charged particles on a workpiece, the apparatus comprising first sensing means for sensing a desired zone of impingement relative to the actual zone of impingement of said beam and determining a spatial relationship of the said zones to produce a first error signal representative of deviation from a desired spatial relationship between the actual and desired zones of impingement; second sensing means for sensing the beam so as to determine an actual position in the beam with respect to a reference position to produce at least one second error signal representative of deviation between the said actual position of the beam and the reference position, the reference position being related to the position of an auxiliary device intended to cooperate with the actual zone of impingement; means adapted to use the first error signal for compensating deviation from the said desired relationship between the actual and desired zones of impingement; and means adapted to use the second error signal or signals to enable the auxiliary device to be in a position permitting cooperation thereof with the actual zone of impingement.
6. 6. Apparatus according to claim 9, wherein said desired zone is a gap between two workpiece portions to be joined by beam welding, and wherein said auxiliary device is a device for supplying additional material into a welding zone produced by said beam during the welding process.
7. 7. Apparatus according to claim 5 or 6, wherein said first sensing means includes an X-ray sensor responding to X-ray radiation received from the actual zone of impingement.
8. 8. Apparatus according to claim 7, wherein said X-ray sensor comprises a collimator limiting the field of view of the sensor to a ribbon-shaped volume, and is so positioned that the said ribbon-shaped volume intercepts the actual zone of impingement of the beam.
9. 9. Apparatus according to claim 5, wherein said second sensing means indudes electrode means for sensing the actual position of the beam with respect to the said reference position by relative movement of the electrode means with respect to the beam and producing an electrical output signal when the beam is intercepted by said electrode means.
10. 10. Apparatus according to claim 9, wherein said electrode means comprises electrodes for establishing an actual position in the beam with respect to two coordinates.
11. 11. Apparatus according to claim 9, wherein said electrode means comprises at least two electrodes spaced apart along a beam reference direction or a beam gun axis for establishing the direction of the beam with respect to said axis adjacent said actual zone of impingement.
12. 12. Apparatus according to claim 5, wherein means are provided for deflecting said beam, during a detection period, across the desired zone of impingement, said detection period being so short that the intended effect of the beam on the workpiece at the said actual zone of impingement is not impaired, and the said first sensing means sense the impingement of the beam during said detection period.
13. 13. Apparatus according to claim 12, wherein said deflection means comprises deflection coils and a waveform generator for deflecting said beam along a triangular path having a portion crossing said desired zone of impingement.
14. 14. Apparatus according to claim 12, wherein said means for sensing the position of said deflected beam comprises a stationary sensing electrode.
15. 15. Apparatus according to claim 11, wherein at least two deflection systems are provided spaced in the direction of a mean beam axis to control the angle of incidence of said beam on said desired zone of impingement in response to two second error signals produced by operation of the second sensing means.
16. 16. Apparatus according to claim 5, wherein said means for using said second error signal comprises means for varying the position of a beam generator with respect to the workpiece.
17. 17. A method according to claim 1 and substantially as described hereinbefore with reference to Figures 6, 7 and 14, or to Figures 6, 7 and 15, or to Figures 6, 7, 16 and 17, of the accompanying drawings.
18. 18. Apparatus according to claim 5 and substantially as described hereinbefore with reference to Figures 1, 8, and 14 or 15, or to Figures 1, 4B, 8, 10 and 14 or 15, or Figures 1, 4A and 8, or to Figures 16 and 17 of the accompanying drawings.