GB2509069A

GB2509069A - A Method of Positioning an Electron Beam

Info

Publication number: GB2509069A
Application number: GB1222897.9A
Authority: GB
Inventors: Alex O'farrell
Original assignee: Aquasium Technology Ltd
Current assignee: Aquasium Technology Ltd
Priority date: 2012-12-19
Filing date: 2012-12-19
Publication date: 2014-06-25
Anticipated expiration: 2032-12-19
Also published as: GB201222897D0; GB2509069B

Abstract

There is provided. a method. of positioning an electron beam comprising: i) impinging an electron beam (16) on a region to be modified; ii) measuring intensity of electromagnetic radiation produced from the region; and iii) modifying the electron beam characteristics in response to the measured intensity of the electromagnetic radiation. An initial scan is undertaken to identify a 10 position where a minimum or maximum intensity occurs to identify the location of a joint or to focus the electron beam (16). There is also provided apparatus for positioning an electron beam (16) comprising a sensing means (26) to measure intensity of electromagnetic radiation emitted from a weld region, and a controller (30) in communication with the sensing means and. in communication with an actuation means (32) capable of adjusting characteristics of an electron is beam. The controller (30) is configured to alter the actuation means (32) to adjust the electron beam characteristics in response to measured intensity. The electron beam may be used in welding, melting, drilling or machining applications

Description

Title: A Method of Positioning an Electron Beam

Field of the invention

This invention relates to a method of positioning an electron beam and to apparatus used in such a method.

Background to the invention

When using an electron beam in a vacuum to modify metals and ceramics, for example by welding, melting, drilling or machining, it is necessary to ensure that the a electron beam is targeted accurately on items to be modified. This ensures that the modification, such as a weld, is performed at the correct position and that cncrgy from the beam is used efficiently. Location of the beam with respect to a region to be modified is generally done by visual inspection. Whilst the initial position of the beam may be correct, the beam can drift off position as a weld progresses. With is developments in electron beam technology, it has become important to be able to identif' joints more accurately and to track along joint seams more accurately than has been possible before.

Summary of the invention

In accordance with a first aspect of the present invention, there is provided a method of positioning an electron beam comprising: (i) impinging an electron beam on a region to be modified; (ii) measuring intensity of electromagnetic radiation, such as visible light, generated from the region in response to the electron beam; and (iii) modifying the electron beam characteristics in response to the intensity of the electromagnetic radiation.

By measuring the intensity of electromagnetic radiation, it is possible to accurately measure joint position and surface focus before modification of an object by the electron beam. Adjusting the beam characteristics in response to the measured intensity improves beam accuracy and energy efficiency. Whilst the electron beam may be used in welding of two items to form a single object, the electron beam may also melt, drill or machine objects, for example to introduce patterns into an object.

I

The intensity may be measured using sensing means, such as a photodiode, where appropriatc used in combination with lenses and deflecting mirrors to ensure the radiation is incident on the sensing means. Other sensing devices may be used as appropriate, for example photo-muhiplier tubes, cameras, light-dependent resistors and the like.

The method may further comprise undertaking an initial scan prior to welding to identil at least one position where a desired intensity occurs, such as a maximum or io minimum intensity. This creates a reference position relative to which the electron beam characteristics can be modified. Desired intensities for a plurality of positions may be recorded, for example in a look-up table, so as to create a welding profile used to control electron beam characteristics over a weld region during welding. This is particularly useful where the dimensions of the region to be welded vary, typically is with respect to height or depth.

The initial scan may identify a position where intensity of the electromagnetic radiation is at a minimum, thereby to identify location of a joint, or a position where intensity of the electromagnetic radiation is at a maximum, thereby to ensure accurate focussing of an electron beam. Where joint location is required, typically the beam current will be in the range O.lmA to O.5mA. Using such a very low energy scan ensures that the joint site is not damaged by the scan prior to welding taking place.

Scans to assess focus of the electron beam and looking to identify maximum intensity will typically be undertaken using beam currents in the range O.lmA to 5mA. Such high energy beam currents can, depending on the material concerned, cause damage to the material being scanned. Where this is likely to happen, the scan may involve repositioning the beam for each focus current setting of the focus scan.

Electron beam characteristics that may be modified include the position of the beam, so as to ensure that the electron beam accurately tracks along a joint, and separately or in combination modifying the focus of the electron beam.

In accordance with another aspect of the invention, there is provided apparatus for positioning an electron beam, the apparatus comprising a sensing means or device to measure intensity of electromagnetic radiation emitted from a weld region, a controller in communication with the sensing means and in communication with an actuation means capable of adjusting characteristics of an electron beam, wherein the controller is configured to alter the actuation means to adjust the electron beam characteristics in response to measured intensity.

a The apparatus may further comprise focussing means, such as a lens, to focus the clcctromagnctic radiation onto thc sensing means.

Where the sensing means produces an analogue output signal, the controller preferably comprises an analogue to digital converter to convert analogue signals is from a sensing means into digital signals for case of comparison.

The invention will now be described, by way of example, and with reference to the following drawings in which: Figure 1 shows a schematic diagram of vacuum welding apparatus using the present invention; Figure 2 shows a schematic diagram comparing beam intensities when scanning a joint position; Figure 3 shows an explanatory diagram of electron beam focussing; Figure 4 shows a schematic diagram comparing beam focus on a workpiece; and Figures 5 to 7 show graphs of intensity against coil focus current for increasing sensitivity.

Description

Figure 1 shows electron beam vacuum apparatus 10 where items 12, 12' to be welded together are held within vacuum chamber 14. Electron beam 16 is generated from cathode 18 and deflected to weld along joint 20 between items 12 and 12' by adjusting the magnetic field of electromagnetic coils 18. Leaded window 22 is integrated into the wall of the vacuum apparatus to allow the inside of the chamber to be viewed. Photons associated with electromagnetic radiation emitted from the items in response to the electron beam, such as infra-red, visible, or ultraviolet radiation, are unaffected by the magnetic field generated by coils 18 and mirror 23 positioned within chamber 14 deflects the electromagnetic radiation onto leaded window 22.

The electromagnetic radiation transmitted through leaded window 22 is focussed by lens 24 external to chamber 14 onto photodiode 26 which produces electrical signals dependent on the received intensity of the electromagnetic radiation. Photodiode 26 is connected to analogue to digital processor, or controller, 30 which in turn is connected to beam adjuster 32 which adjusts the configuration of the beam and in particular the a ramp deflection coil current applied to coils 18.

If desired, photodiode 26 can be positioned within chamber 14 and intensity readings taken in situ. Devices equivalent to photodiode 26 can be used, such as light-dependent resistors, photo-multiplier tubes, cameras and the like.

Whilst discussion of the invention will be in relation to detecting visible light, the invention is equally applicable to other ranges of the electromagnetic spectrum such as infra-red and X-ray.

When objects are to be welded together and joint 20 needs to be accurately tracked for a high quality weld, the present invention allows the beam position relative to welding joint 20 to be identified accurately, tracked and adjusted as the weld progresses.

Where electron beam 16 impinges on one of the items 12, 12' in a region located away from joint 20, the electromagnetic radiation emitted from the weld region, i.e. the region where the electron beam encounters the workpiece, will have a high light intensity because the entire electron beam is incident on the weld region. This is illustrated in Figure 2(a) by large diameter beam circle 34 representing the intensity of the electromagnetic radiation emitted from the weld region. When the beam is incident on the joint between items 12, 12', the electromagnetic radiation from the weld site will have reduced intensity as some of the energy from electron beam 16 passes into the joint gap between the two items. Thus the lowest intensity beam, see small diameter beam circle 36, is detected when the beam is incident on joint 20, see Figure 2(b). The term weld region' encompasses any region where the electron beam 16 is incident on an item to modify its configuration in any way, for example by welding, melting, drilling, machining or similar.

When a joint position is to be detected, electron beam 16 is placed on low power, typically having a beam current of 0.1 mA to 0.5 mA, so as not to undertake a weld, and scanned vertically across horizontal joint 20. Photodiode 26 receives the electromagnetic radiation emitted during the scan and generates electrical signals dependent on the measured intensity which are passed to controller 30 which includes a a current to voltage amplifier and an analogue to digital convertor. By undertaking a single vertical scan across horizontal joint 20 in thc direction of arrow 40, controller identifies the position where the lowest light intensity occurs and logs this reading in a look-up table. This position corresponds to the position at which electron beam 16 is on joint. Once the scan is finished, controller 30 instructs beam controller 32 to is adjust the ramp deflection coil current applied to coils 18 to position beam 16 precisely on the region where the lowest intensity was generated, i.e. on the joint. By using such a joint finding technique, joint 20 can be found consistently within 30 microns.

Tracking of a joint that changes in height or focus is done using a teach and replay system at multiple positions around the workpiece. If the focus or joint height needs to be altered during a weld, for example due to an irregular joint shape or weld piece shape, the part has its joint heights and focus levels logged at different positions around or along the part to create a welding profile before the weld begins in a teaching' process. The system then operates according to the welding profile, recalling the logged values and ramping between them when undertaking welding in a replay process.

To set up for a new type of workpieee, a rough visual alignment of the joint is sufficient initially. Then a vertical sweep is run across the joint, controller 30 logging intensity with position to identi' the position at which lowest intensity is identified.

Controller 30 then instructs beam controller 32 to adjust the current supplied to deflector coils 18 to position beam 16 on joint 20.

The aforementioned technique creates a welding profile from an initial low energy non-welding scan to adjust the position of the electron beam during welding to ensure welding continues accurately along a joint. It is also possible to analyse the light intensity created by the beam interacting with the target to adjust the focus of the beam. Figure 3 shows an example of exemplary electron beams 42 when their focus on workpiece 44 is adjusted using coils 46. When the beam is in focus on workpiece 44, Figure 4(b), the electron beam is at its narrowest diameter and maximum intensity, so transferring maximum energy to the workpiece. When coils 46 arc adjusted to a make beam 42 under-focussed, Figure 3(a), or over-focussed, Figure 3(c), then beam 42 is broader and more diffiisc and energy is less concentrated. When welding, such a beam will not weld as effectively and lower intensity electromagnetic radiation will be generated from the welded site.

is By measuring intensity with photodiode 26 at differing focus values, controller 30 can assess which focus value produces the maximum light intensity and so which is the correct focus value for the distance to the target or workpieee. This allows extremely accurate focussing and ensures that all beam energy can be efficiently used on the sample to be welded. Again a welding profile can be created for focus values if the dimensions of the weld region vary.

For assessing focus values, a higher energy beam is needed, typically up to SmA beam current, which can damage certain materials. Thus high speed raster scan patterns can be used to deflect the electron beam to enable the light level reading for each focus value to be taken without the beam penetrating and damaging the surface.

This method of focus scanning enables the beam to be repositioned to an undamaged piece of metal for every focus current setting of a focus scan. This ensures the focus reading is consistently always of the surface of the material which is not always achievable by a manual operator.

Graphs showing how to identi' the surface focus value are shown in Figures 5 to 7.

Typically an initial focus scan will be run at coarse intervals to roughly identify the focus value. This can be seen in Figure 5 where a scan is run from 350mA to 450mA focus current at ImA increments and variation in the intensity readings shows that the focus value is in the high 39OmA area.

Once having identified that the focus value is in this area, another scan is run from 393mA to 4O3niA focus current at imA increments, see Figure 6 which shows the focus to be at 396mA. If necessary a further scan can be run, see Figure 7 in which a scan has been run from 395mA to 398mA focus current at O.2mA increments showing the focus to be at precisely at 396.2mA. The focus can thus be found with much higher accuracy than can be done manually by an operator. Auto-focussing in this io way can be performed with beam currents from 0.1 mA up to SmA.