GB2081941A

GB2081941A - Closed loop current control for electron beam gun

Info

Publication number: GB2081941A
Application number: GB8026034A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1980-08-09
Filing date: 1980-08-09
Publication date: 1982-02-24
Also published as: DE3130383A1; FR2488442A1

Abstract

An electron beam gun 10, eg. for welding, is connected to a supply 18 by way of a resistor chain 30. The current is controlled by a triode 36 responsive to a control voltage 54 and a feedback signal derived from a filament current monitor 42 coupled through an LED and optical link arrangement. The grid of the gun is kept at a constant potential derived from line 14. <IMAGE>

Description

SPECIFICATION Closed loop current control for EBW The present invention relates to a control means for an electron beam gun. The invention is particularly aimed at means by which an electron beam welding apparatus may be controlled.

It is the present practice, to control the current delivered to the cathode of an electron beam gun, via a series of resistors, resistance value being selected by operation of a rotary switch which results in a stepped change in current. Such an arrangement lacks sensitivity and smoothness in the change from one power setting to another.

It is also known, to achieve relatively smooth power variation by connecting a thermionic triode between the gun power output and a reference value and use any differences between them, to cause the triode output to change the relative potential of the gun grid. However, this does not take account of secondary effects such as gun grid emission and power sur ply current leakage, which affect the actual beam current.

The present invention seeks to provide an improved electron beam gun control means.

According to the present invention, electron beam gun control means comprises input means for connecting between a high voltage power supply and the cathode filament of an electron beam gun, return means for connecting between said filament and said high voltage power supply, a bias resistor chain connected between said input and return means, a thermionic triode having its cathode connected via a tap to the bias chain and its anode connected to said input means, a current monitor connected between said anode and gun cathode filament, means for detecting any current flow in said monitor and deriving a voltage signal corresponding thereto, a control potential, summing means for summing the derived voltage and control potential and means for deriving from the sum a further signal, said means for deriving said further signal being connected to the triode grid so as to vary its potential relative to its cathode potential as dictated by the bias resistor chain and so enabling variation in power input to the gun filament.

Preferably the current monitor comprises a light emitting diode.

The means for detecting current flow in said monitor and devising a voltage signal corresponding thereto may comprise a light pipe and a photo detector, wherein said light pipe transmits light emitted by the light emitting diode as a result of current flowing therethrough, to said photo detector which converts said light to a voltage, the magnitude of which is proportional to the light intensity.

Preferably the photo detector output is amplified by a temperature controlled gain amplifier.

The means for deriving a further signal with which to vary the grid potential of the triode, comprises a further current source adapted for the receipt of the summing means output so as to bring about current flow, a further current monitor, light pipe and photo detector with amplified output connected in series with the grid.

The invention will now be described, by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic electron beam gun control circuit in accordance with the invention, Figure 2 is a pictorial view of an electron beam gun control pack in accordance with the invention.

In Fig. 1, an electron beam gun 10 has a cathode filament 12 which is heated by electrical means (not shown) connected via line 16 to a power supply 18. Power supply 18 is immersed in an oil bath 20 so as to reduce the incidence of flash over of high voltage electricity.

In operation the gun grid (not shown) has a fixed electrical potential of about - 1 50 kv applied thereto via line 14 and the electron stream 22 emitted by filament 12 is accelerated by anode 24 towards a workpiece 26, in the normal manner, and the working circuit is completed via line 28 to the positive side of power supply 18.

Lines 14 and 40 are bridged by a number of bias resistors 30 which in turn, are tapped by connection 32 to the cathode 34 of a thermionic triode 36. The anode 38 of triode 36 is connected at 40, to input line 16 of filament 12.

A current monitor 42, which in the present example is a light emitting diode, is inserted in line 16 so as to indicate the current flowing therein to filament 12. A light pipe 44 has one end adjacent monitor 42 and the other end protrudes from oil bath 20 and is positioned adjacent a photo detector 46.

On diode 42 emitting light in accordance with current flowing through it, photo detector 46 senses the light and generates a voltage dependent on light intensity and passes it via an amplifier 48 and a resistor 50, to a summing junction 52.

Diode 42 and photo detector 46 are affected by temperature changes. Therefore diode 42 must include a compensating network (not shown) and the amplifier 48 must be of the kind known as a temperature controlled, gain set amplifier.

The remaining side of summing junction 52 receives a voltage from an operator controlled console 54, which includes a signal mode select switch 56, the function of which, is to select the primary mode of operation of the apparatus. The example shown herein has four modes which are: a) External input 58. This transfers control to an external input voltage which can be derived from say, a wave form generator or any other source of voltage suitable to provide current control.

b) A pulser 60. The pulse frequency is adjustable from 1 to 100 cycles/second in 1 cycle increment with pulse width control from 10% to 90% of steady state, in 1 % increments. This input could also include a slope control input 62 i.e. an input which varies the mark/space ratio of each pulse so that e.g.

where the apparatus is used to weld a ring, the beam on/off ratio is varied at start and finish of the operation so as to maintain temperature change at the workpiece at a minimum. This technique avoids cracking of the workpiece. An apparatus with which to achieve variations of mark/space ratio, is disclosed and claimed in British patent speicfica- tion 1,331,113.

c) Steady beam. In this mode, a control potentiometer 64 is utilised to control the beam current.

d) Filament calibration, wherein the closed loop is broken and the resulting open circuit prevents the potentiometer settings from corresponding with the current meter.

Whatever mode of input is selected at switch 54, that input is passed via control potentiometer 64, an amplifier 66 and a resistor 68, to summing junction 52.

Resistor 50 is biased relative to resistor 68 so as to give higher gain to the feed back signal from photo detector 46 than from amplifier 66. In the present example the gain is in the order of 800:1.

The two inputs to junction 52 are passed to summing amplifier 70 and the sum signal is then inverted by the inverter 74. The inverted signal is then passed as a voltage to the current flow control 76 of a further current source, the circuit for which includes a further light emitting diode 78. It will be seen that fluctuations in current flow through monitor 42, will result in fluctuations in current flow through light emitting diode 78, and will result in varying degrees of light intensity in diode 78. The light emitted is passed via a further light pipe 80, to a further photo detector 82 and converted to a voltage, then passed via amplifiers 84, 85 and buffer transistor 86, to the grid 88 of thermionic triode 36.The arrangement is such that, on an undesirable drop in current being sensed by monitor 42, the end result will be triode grid 88 being made more positive with respect to cathode 34, thus increasing the current throughput of triode 36, to filament input line 16. An undesirable increase in current flow through monitor 42, will result in grid 88 being made more negative with respect to cathode 34, so as to reduce current flow through triode 36 and therefore, through line 16 to filament 12.

The control means described hereinbefore, is designed to fit as a pack, into existing electron beam units units such as those used for welding and such a pack will now be described with reference to Fig. 2.

Transformer 118 has sockets 90, 92 fitted to its top, prior to immersion in oil bath 20. A subframe 94 is then placed in the bath.

Subframe 94 has a flat top 96 with an aperture 98 cut in it, so that the lower portion of a unit 100 can be passed through.

Unit 100 has a lower black box 102 which contains triode 36, monitor 42, the receiving end of light pipe 44, the exit end of light pipe 80, photo detector amplifiers 82, 84 85 and buffer transistor 86. Unit 100 also has an upper black box 104 which contains the exit end of light pipe 44, photo detector 46, amplifier 48, resistors 50, 68, summing junctin 52, summer 72, inverter 74, further current source 76, light emitting diode 78 and the receiving end of light pipe 80.

Black boxes 102, 104 are spaced apart by rigid insulators 106, 108, which terminate at their lower ends with the appropriate connectors (not shown) for plugging into the connectors 90, 92 on transformer 118. The upper black box 104 has a connecting point 110 for receiving a plug 11 2 which via a cable 114 bridges the console 54 of Fig. 1 and unit 100.

Fitting unit 100 into the transformer consists of simply lowering the unit through aperture 98 and pu hing the lower ends of insulators 106, 108 into respective sockets 90, 92.

This connects the high voltage (as much as 150 kv) D.C. supply to the electron beam gun circuit. It also ensures that a plate 116, under black box 104, fills the aperture 98, thus closing the oil bath 80. Lower black box 102 is thus immersed in the oil but, upper black box 104 remains outside. The only connection between them for the purpose of operation, are light pipes 44 and 80. It follows that although electrical current flow in the lower black box 102 initiates electrical current flow in upper black box 104, there are no electrical connections bridging them. This is achieved by light signals. The apparatus is therefore safe to use.

Claims

1. Electron beam gun control means comprising input means for connecting between a high voltage power supply and the cathode filament of an electron beam gun, return means for connecting between said filament and said high voltage power supply, a bias resistor chain connected between said input and return means, a thermionic triode having its cathode connected via a tap to the bias chain and its anode connected to said input means, a current monitor connected between said anode and gun cathode filament, means for detecting any current flow in said monitor and deriving a voltage signal corresponding thereto, a control potential, summing means for summing the derived voltage and control potential and means for deriving from the sum a further signal, said means for deriving said further signal being connected to the triode grid so as to vary its potential relative to its cathode potential as dictated by the bias resistor chain and so enabling variation in power input to the gun filament.

2. Electron beam gun control means as claimed in claim 1 wherein the current monitor comprises a light emitting diode.

3. Electron beam gun control means as claimed in claim 2 wherein said means for detecting said current flow and deriving a voltage signal therefrom comprises a light pipe and photo detector, wherein said light pipe transmits light emitted by the light emitting diode as a result of current flow therethrough, to said photo detector which converts said light to a voltage, the magnitude of which is proportional to the light intensity.

4. Electron beam gun control means as claimed in clam 3 wherein said photo detector output is amplified by a temperature controlled gain amplifier.

5. Electron beam gun control means wherein said means for deriving a further signal with which to vary the grid potential of said triode, comprises a further current source adapted for the receipt of the summing means output so as to bring about current flow, a further current monitor, light pipe, and photo detector with amplified output connected in series with said grid.

6. Electron beam gun control means sç stantially as described in this specification with reference to the accompanying drawings.

6. Electron beam gun control means substantially as described in this specification with reference to the accompanying drawings.

CLAIMS (Filed 28 Jul 1981)

1. Electron beam gun control means comprising electrical input means for connecting between a high voltage power supply and the cathode filament of an electron beam gun, means which in operation of the gun provides an electrical return path to the high voltage power supply, a bias resistor chain connected between said input and return means, a thermionic triode having its cathode connected via a tap to the bias chain and its anoe connected to said input means, a current monitor connected between said anode and gun cathode filament, means for detecting any current flow in said monitor and deriving a voltage signal corresponding thereto, a control potential, summing means for summing the derived voltage and control potential and means for deriving from the sum a further signal, said means for deriving said further signal being connected to the triode grid so as to vary its potential relative to its cathode potential as dictated by the bias resistor chain and so enabling variation in current input to the gun filament.

4. Electron beam gun control means as claimed in claim 3 wherein said photo detector output is amplified by a temperature controlled gain amplifier.

5. Electron beam gun control means wherein said means for deriving a further signal with which to vary the grid potential of said triode comprises a further current source adapted for the receipt of the summing means output so as to bring about current flow, a further current monitor, light pipe, and nhoto detector with amplified output connected in series with said grid.