GB1574032A

GB1574032A - Apparatus for spectographic identification of metals

Info

Publication number: GB1574032A
Application number: GB1603277A
Authority: GB
Original assignee: Central Electricity Generating Board
Current assignee: Central Electricity Generating Board
Priority date: 1976-08-17
Filing date: 1978-05-24
Publication date: 1980-09-03
Also published as: NL165274B; DE2717130C2; NL165274C; NL7609109A; DE2717130A1

Description

(54) IMPROVEMENTS IN OR RELATING TO APPARATUS FOR SPECTROGRAPHIC IDENTIF1CATION OF METALS (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 a method of and apparatus for the spectrographic identification of metals.

There are many circumstances in which it is required to check the identity of a metal alloy to ensure that a correct material has been used. The problem arises very particularly with steel where many different alloys are in wide commercial use. A typical example of such a problem is in a large boiler for a power station which might have 500,000 steel tubes; it may be required to check that each of these tubes is of a correct high chromium steel and not of a mild steel with a low chromium content.

For this purpose optical spectroscopes have been used. Visual spectographic displays are complex to analyse and much effort and thought has been given to the production of apparatus for the automatic analysis of spectrographic information. Such apparatus however has been very complex and consequentially far too large and cumbersome to be utilised for many of the simple operations where it would be desirable to have a hand-held instrument with automatic readout which can be utilised to distinguish between different alloys of a metal having a known predominant material, for example between different steel alloys.

According to this invention, there is provided a method of identification of a metal alloy in which there is a known predominant material, which method comprises the steps of forming an arc between an electrode and the metal to be identified, photoelectrically observing the arc in a repetitive sequence to produce electrical signals representative of the amplitudes of at least two different spectral components, and separately summing the signals representative of the different components to determine, when the sum of the signals corresponding to one component has reached a predetermined value, the magnitude-of the sum of the signals corresponding to another component or each of the other components.

The invention furthermore includes within its scope an apparatus for the identification of a metal alloy in which there is a known predominant material comprising means for forming an arc between an electrode, and the metal to be identified, an optional spectrometer dispersing the light from the arc into spectral components, photoelectric means arranged to produce repetitive electrical signals corresponding to the amplitudes of at least two spectral different components, separate summing means for each component arranged to sum the magnitudes of the successive signals representing th respective components and means for indicating the magnitude of the summed value of the signals corresponding to one component when the summed value of signals corresponding to another component has reached a pre determined magnitude.

This apparatus would normally be used by summing the signals representing the spectral component corresponding to the predominant metal in the alloy, e.g. iron in a steel alloy, this summing being continued until the sum reaches a predetermined value. The summation of the electrical signals representing the other component or components, which has been proceeding simultaneously, is then stopped and the magnitude of the summed signals for the or for each of the other components is then a measure of the proportion of alloying constituents giving the chosen spectral component or components. In a simple case, to determine the amount of chromium in steel, one would have to select a spectral component representative of iron (e.g. a narrow wavelength band at about 460 nm) and another spectral component (e.g.

the 520 nm line) representative of chromium.

When the summed signals from the iron component reach a predetermined value, the magnitude of the summed signals of the chromium component would be representative of the percentage of chromium; the instrument may be calibrated using alloys of known composition. It has been found readily possible with such an instrument to provide a direct meter indication of the percentage of chromium in a steel alloy containing up to 10% by weight of chromium, the indication being accurate to about 0.1% by weight of chromium. Conveniently two or more components are measured at the same time in this way. For many purposes it is sufficient to indicate the proportions of one or two components but it is readily possible to provide indication of up to say ten components in a portable instrument.

For producing the electrical signals, a linear array of photo-transducers may be provided onto which optical signals from the spectrometer are directed so that the various transducers are responsive to different spectral components; in this case, the electrical signals from the various phototransducers may be electrically scanned, e.g.

by gating signals from selected transducers into respective integrators for providing the required summed outputs.

Very conveniently however the scanning of the optical signals is effected by a rotating mirror directing the various optical components in succession onto a single photo-transducer, the output of which is gated in synchronism with the rotation of the mirror to select outputs for feeding to a plurality of separate integrators.

A reference level detector may be provided for indicating when the output of one of the integrators has reached a predetermined value, means being provided controlled by said reference level detector for stopping further integration of the other signal or signals when the output of said one of the integrators has reached said predetermined value. For this purpose the outputs from the further integrator or integrators may be fed through gates to analogue signal indicators, said gates being controlled by the reference level detector.

Preferably means are provided for delaying the start of integration of signals until a short time after the arc has been struck. This is to give time for the arc to penetrate any surface layer on the object to be identified before the spectrographic identification is initiated.

Means may be provided for preventing an output indication being obtained unless the reference level detector indicates that a signal has been integrated to the predetermined value within predetermined time limits. If the reference signal corresponds to the predominant material in the metal to be identified, the reference signal integrator will reach the predetermined value within some short time. If it does not do so, the instrument may not have been set correctly to select appropriate signals or there is some malfunction or the object being treated is of some very different material.

It will be seen that essentially the above- described apparatus indicates the proportion of the selected component or components to the predominant constituent in the material. By proper selection of spectral components which are unique to certain elements, it is readily possible to find the approximate proportions of these components relative to the major constituent of the alloy and irrespective of the presence of other components in the alloy. The apparatus enables direct numerical indication to be obtained by calibration. This indication can be given in analogue or digital form.

Conveniently the apparatus comprises a probe unit connected by a flexible cable to an indicator unit, the probe unit containing the aforesaid electrode for striking the arc. The probe unit may also contain the optical spectrometer together with the photo-transducer or transducers for providing the electrical signal output. Alternaitvely the optical spectrometer together with the photo-transducer or transducers may be arranged in said indicator unit, a flexible optical light guide being provided between the probe unit and the indicator unit for the transmission of light from the arc to the spectrometer.

Preferably said electrode is arranged within a surrounding guard member and means, e.g. a solenoid, may be provided responsive to the current through the electrode for effecting relative movement between the electrode and the guard member, e.g. withdrawal of the electrode within the guard member, after the arc has been struck. With such a construction the operator has merely to touch the electrode onto the object to be identified; the current through the electrode then causes the electrode to be withdrawn so producing the arc within said guard member.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagram illustrating the construction of a probe unit forming part of a metal identification apparatus; Figure 2 is a block diagram illustrating the apparatus for processing the electrical signals from the probe unit; and Figure 3 illustrates a modified form of probe unit.

Referring to Figure 1 there is shown diagrammatically a probe unit comprising an outer casing 10 within which is an electrode 11 for striking an arc, the electrode being arranged within a cylindrical housing 12 having an aperture 13 for light to be received from the arc. In one convenient construction the housing 12 is springloaded so that, when the probe unit is pushed towards a metal object to be identified, indicated at 15, the housing 12 is pushed backwards in the probe unit against springpressure until the electrode 11 touches the object 15. A solenoid 16 around the electrode carries the current through the electrode. As soon as current begins to flow through the electrode, the solenoid 16 is energised and causes the electrode 11 to be withdrawn inwardly against springpressure into the housing 12 for a short distance.The electrode 11 is energised as a suitable voltage such that an arc extinguishes as the electrode is fully withdrawn; i.e. the available voltage is less than the arc sustaining voltage. A pulsed D.C. supply is used so that the electrode 11 returns to contact the object 15 before the next pulse to repeat the cycle. The operator thus merely holds the probe up and pushes it towards the object 15 to be tested. With the above-described construction giving a predetermined arc length, with the operator merely having to hold the housing 12 in contact with the object to be tested, it is possible to use a low voltage arc,. and typically a 60 volt pulsed D.C. supply is provided. The pulsing of the supply is controlled in a manner to be described later.

The light from the arc is reflected by a mirror 20 and focussed by a lens 21 on a slit 22 from whence the light passes through a collimating lens 231 and prism 24 forming the dispersive element of the spectrometer.

The dispersed light passes through a focussing lens 25 onto a rotatable mirror 26 which reflects the light via a fixed mirror 27 through a slit 28 onto a photomultiplier tube 29. The lens 25 serves to focus the light onto the slit 28. The optical system in the known way operates to split up the light from the arc into the spectral components; the mirror 26 is rotated at a uniform speed by an electric motor (not shown) so that the various spectral components in turn are scanned past the slit 28 and hence sensed by the photomultiplier tube 29. The output of this photomultiplier tube 29 thugs comprises an analogue signal cyclically varying in time. In practice it comprises a series of pulse signals of different magnitudes arising from the spectral line component in the light from the arc due to the various materials present.The probe unit also contains a further light source 30 and the photo-sensor unit 31, typically a photoemitting diode and solid-state photo-sensor, which will sense when the rotating mirror 26 is in a particular angular direction (normal to the bisector of the angle between the lines from the centre of the mirror 26 and to the further light source 30 and to the sensor 31). Thus for each revolution of the mirror, a pulse will be obtained from this further sensor 31 giving a timing signal indicating when the mirror is in a particular angular position.Provided the mirror 26 is rotated at a known, uniform speed, any selected spectral component of the light source will be sensed by the photomultiplier tube at some definite time interval after this timing signal, which thus provides a reference for identifying the various spectral components and enabling required components to be selected.

Referring to Figure 2, the probe unit shown at 40 has a lead 41 on which the electrical output from the photomultiplier is fed to a strobing unit 42. The reference signal from the further sensor 31 is fed on a further lead 43 to the strobe unit and to a timing generator 43. The timing generator 43, inter alia, provides timing signals for controlling gates in the strobing unit 42, these timing signals being synchronised by the signal from the further sensor 31.

The timing generator 431 effects switching of a time delay circuit 48 controlling the pulsing of a Dl.C. supply circuit 49 which provides the pulsed D.C. for the arc. This pulsing is timed to ensure that the arc is conducting only when the rotatable mirror 26 is in the particular angular -position to reflect light onto the slit 28. This pulsing of the D.C. supply is used to extend the electrode life and to reduce heat transfer into the spectrometer coupared with the use of a continuous D.C. supply.

The refractive index of the prism 24 will depend on the temperature and hence the angular dispersion of the prism 24 will change with change of temperature. If no compensation were made for this change in angular dis-- persion with temperature, the -analogue output of the photomultiplier tube 29 will change in time position relative to the output of the timing generator. A resistor 33, the resistance of which varies with temperature, is attached tothe prism 24, as shown in Figure 1, and is electrically connected in an initial timing delay unit 34 within the timing generator 43.This initial timing delay unit is thus controlled by the temperature of the prism 24 and is arranged to adlust the timing signals for certain controlling gates (to be described later) so as to keep the selected spectral component of the analogue output of the photomultiplier tube 29 within its controlling gate as the temperature of the spectrometer varies.

The strobing unit 42 receives the photomultiplier output on leads 41, 44 and is operative to pass selected spectral components to output lines 45, 46, 47. The gates in the strobing unit 42 are each open for a very short time period (e.g. 4 microseconds), controlled by the timing generator 43, such that the output signal of each gate is representative of the magnitude - of a single spectral line component. The signal on lead 45 is a spectral component of the major constituent in the metal to be identified, that is to say iron in the case of a steel alloy. The signals on leads 46, 47 are spectral components of two further elements for example chromium and manganese.The timing generator 431 essentially selects the delay from the reference pulse from the further sensor 31 to the start of the required spectral component and also selects the duration for which this component is to be observed. The strobing unit 42 thus provides, for each cycle of rotation of the mirror, three output signals on leads 45, 46, 47 respectively, which outputs are fed to three separate integrators in an integrating unit 50 to give respective integrated outputs on leads 51, 52 and 53. The integrated output on lead 51 is fed to a reference level detector 54 operative, when the integrated output on lead 51 reaches a predetermined value, to close two further gates 55, 56.These gates 55, 56 are opened, as described later, shortly before the start of the integration period and the integrated outputs on leads 52, 53 are fed respectively through these gates 55, 56 to indicators 57 and 58 displaying the amplitude of the integrated signal. These may be analogue indicators for example having a pointer movable across a scale or may be digital indicators arranged to indicate the analogue magnitude of the respective integrated outputs. Preferably the indicators have holding means for holding their indications until instrument is re-set.

The input to the reference level detector is set to zero by a push-button 60 by a signal on lead 61. This push-button 60 serves also to start the operation of the timing generator 43 via lead 62. This timing generator 43 also contains a much longer period timing system. Firstly it provides a delay signal, typically of 3 seconds. for acti vating the integrators in an integrating unit 50 as shown at 63. The purpose of this delay is to avoid utilising light from the arc immediately after the arc is struck when it may contain spectral components due to surface contamination of the article under test or which are otherwise not properly representative of the constituents of the material below the surface layer or coating.

The timing generator 43 also controls a timing unit 64 which inhibits any output from the reference level detector 54 after an adjustable time period, which might typically be set in the range of 10 to 20 seconds. Thus if the integrated reference signal component on lead 51 is of low amplitude for any reason, e.g. if the instrument has not been set to select the correct main constituent of the alloy, then no readings will be given on the indicators 57, 58. A lamp 66 is operated by the reference level indicator in these circumstances to indicate that no valid indication is being given. A further lamp 67 indicates that a signal has been fed to the gates 55, 56 and therefore that a valid indication is being given.

This instrument would be used for identifying selected components in an alloy having a known major component. It may be calibrated using specimens of known materials and, in this case, provided spectral lines can be selected which are unique to the particular components, it enables the magnitudes of these various components to be indicated. The indicators may be calibrated to indicate the percentage proportions of the selected components. Although only two such indicators are shown in Figure 2, more than two may readily be employed.

It will be seen that the probe unit may readily be constructed as a lightweight hand-held unit. It is connected by a flexible cable to the indicator unit containing the electronic circuitry which may be made as a separate portable unit. Although, in the embodiment described. a prism is used as the dispersive system, other optical dispersive systems may be employed. The dispersive system and the optical components may be arranged in a vacuum.

Figure 3 illustrates a modification of the probe unit of Figure 1. The same reference numerals are used to indicate corresponding components and mention will be made only of the distinctive features of the construction of Figure 3. The probe unit of Figure 3 is made lighter and smaller than that of Figure 1 by putting the spectrometer prism together with the photo-sensor and other associated components in the indicating unit instead of in the probe. To do this, a fibre optic flexible light guide 70 is provided for transmitting the light from the probe unit to the rspectrometer. The light from the arc is reflected by the mirror 20 and focussed by the lens 21 onto a termination of this light guide 70. The fibre termination may form the entrance slit of the spectrometer, so avoiding the necessity for a separate scribed slit.

With the arrangement of Figure 3, the spectrometer is situated in an environment having better temperature stability compared with the arrangement of Figure 1.

The reduced size and weight of the probe unit facilities the examination of the objects in less easily accessible locations.

WHAT WE CLAIM IS: 1. A method of identification of a metal alloy in which there is a known predominant material. which method comprises the steps of forming an arc between an electrode and the metal to be identified, photoelectrically observing the arc in a repetitive sequence to produce electrical signals representative of the amplitudes of at least two different spectral components, and separately summing the signals representative of the different components to determine, when the sum of the signals corresponding to one component has reached a predetermined value, the magnitude of the sum of the signals corresponding to another component or each of the other components.

2. A method as claimed in claim 1 wherein the signals corresponding to said one component are signals of a distinctive spectral component of said predominant mater vial.

3. Apparatus for the identification of a metal alloy in which there is a known predominant material which apparatus comprises means for forming an arc between an electrode and the metal to be identified, an optical spectrometer dispersing the light from the arc into spectral components, photoelectric means arranged to produce repetitive electrical signals corresponding to the amplitudes of at least two different spectral components, separate summing means for each component arranged to sum the magnitudes of the successive signals represesting the respective components and means for indicating the magnitude of the summed value of the signals corresponding to one component when the summed value of the signals corresponding to another component has reached a predetermined magnitude.

4. Apparatus as claimed in claim 3 wherein, for producing the electrical signals, a linear array of phototransducers is provided onto which optical signals from the spectrometer are directed so that the various transducers are responsive to different spectral components.

5. Apparatus as claimed in claim 4 wherein the electrical signals from the various photo-transducers are electrically scanned by gating signals from selected transducers into respective integrators for providing the required summed outputs.

6. Apparatus as claimed in claim 3 wherein a rotatable mirror is provided for scanning of the optical signals-by rotation of the mirror to direct the various optical components in succession onto a single photo-transducer, and wherein-means are provided for gating the output of the phototransducer in synchronism with the rotation of the mirror to select outputs for feeding to a plurality of separate signal integrators.

7. Apparatus as claimed in either claim 5 or claim 6 wherein a reference level detector is provided for indicating when the output of one of the integrators has reached a predetermined value and wherein means are provided controlled by said reference level detector for stopping further integration of the other signal or signals when the output of said one of the integrators has reached said predetermined value.

8. Apparatus as claimed in claim 7 wherein, for each further integrator, there is provided an analogue signal indicator with a gate through which the output of the respective integrator is fed to the analogue signal indicator, the or each of said gates being controlled by the reference level Idetector.

9. Apparatus as claimed in either claim 7 or claim 8 wherein means are provided for preventing an output indilation being obtained unless the reference level detector indicates that a signal has been integrated to the predetermined value within predetermined time limits.

10. Apparatus as claimed in any of claims 3 to 8 wherein means are provided for delaying the start of summation of signals for a period after the arc has been struck.

11. Apparatus as claimed in any of claims 3 to 10 and comprising a probe unit connected by a flexible cable to an indicator unit, the probe unit containing said electrode for striking the arc.

12. Apparatus as claimed in claim 11 wherein the probe unit also contains the optical spectrometer together with the photo-transducer or transducers for providing the electrical signal output.

13z. Apparatus as claimed in claim 11 wherein the optical spectrometer together with the photo-transducer or transducers are arranged in said indicator unit, a flexible optical light guide being provided between the probe unit and the indicator unit for the transmission of light from the arc to the spectrometer.

14. Apparatus as claimed in any of claims 3 to 13 wherein said electrode is arranged within a surrounding guard member and wherein means are provided responsive to the current through the electrode for effecting relative movement between the electrode and the guard member to withdraw

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. termination may form the entrance slit of the spectrometer, so avoiding the necessity for a separate scribed slit. With the arrangement of Figure 3, the spectrometer is situated in an environment having better temperature stability compared with the arrangement of Figure 1. The reduced size and weight of the probe unit facilities the examination of the objects in less easily accessible locations. WHAT WE CLAIM IS:

1. A method of identification of a metal alloy in which there is a known predominant material. which method comprises the steps of forming an arc between an electrode and the metal to be identified, photoelectrically observing the arc in a repetitive sequence to produce electrical signals representative of the amplitudes of at least two different spectral components, and separately summing the signals representative of the different components to determine, when the sum of the signals corresponding to one component has reached a predetermined value, the magnitude of the sum of the signals corresponding to another component or each of the other components.

the electrode after the arc has been struck.

15. A method of identification of a metal alloy substantially as hereinbefore described with reference to the accompanying drawings.

16. Apparatus for the identification of a metal alloy substantially as described with reference to Figures 1 and 2 or with reference to Figure 3 of the accompanying draw- ings.