AU616573B2

AU616573B2 - Hardness testing machine incorporating an image analysis system

Info

Publication number: AU616573B2
Application number: AU44330/89A
Authority: AU
Inventors: Charles Thomas Austin
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-10-30
Filing date: 1989-11-02
Publication date: 1991-10-31
Anticipated expiration: 2007-10-28
Also published as: AU595953B2; GB2197463B; JPH01501114A; GB2197463A; AU4433089A; GB8625957D0; EP0292514A1; WO1988003345A1; KR880702028A; AU8105387A

Description

I J-1 7 COMPLETE SPEC EICAT1N FOR OFFICE USE Application Number: Lodged: Complete Specification Priority: Class Int. Class Lodged: Accepted: Published: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Address for Service: Address for Service: CHARLES THOMAS AUSTIN The Old Rectory, West Lydford, Somerton, Somerset, TAll 7DQ, United Kingdom.

SMITH SHELSTON BEADLE 207 Riversdale Road Box 410) Hawthorn, 3122, Victoria, Australia A i* Complete Specification for the invention entitled: HARDNESS TESTING MACHINE INCORPORATING AN IMAGE ANALYSIS SYSTIE The following statement is a full description of the best method of performing it known to me/us: this invention, including Page 1 Our Ref: #4630 PS:LG '1 2 1 This invention relates to a hardness testing machine incorporating a system for analysis of an image, such as one produced by a closed-circuit television (CCTV) camera. In preferred embodiments, the invention is applicable to a static indentation hardness testing machine, in which an image of an indentation is analysed using a CCTV camera and a processor.

In a static indentation hardness testing machine, the size of an indentation made in a metal object is used as a 4 Yfd« measure of the hardness of that object. It is important, therefore, that the size of the indentation be measured

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o oo« accurately.

SIt is well known that, in order to be able to analyse e an image accurately, it is necessary for the object to have good illumination. For many kinds of object this presents no particular difficulty and previously, in the field of hardness testing, it has been usual to measure the sizes of indentations using a microscope which uses single level of illumination for different objects. In a prior proposal ,'2Q using a CCTV camera, described in EP-A-0048346, a constant illumination for different test objects of the same or S' different type was used. However, it has been determined that it is difficult with constant illumination of metal objects, to analyse the image with constant accuracy even S254 when in some cases, the test objects are different samples from The same production run. It has been established that the variations of surface finish and reflectivity can vary significantly and produce inaccuracies of analysis.

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891102,!DssneO3.austin.soe. r 1. -3- As mentioned above, when using a static indentation hardness testing machine, there is a need to determine the size of an object (such as an indentation in a sample) which is viewed through a CCTV camera, using an image processor. In such a machine, it has been proposed to calibrate the processor by presenting to the CCTV a reference object, which forms a calibration image, and which might take the form of a calibrated scale, a representation of an object of the type under test, or an actual sample of an object of the type nOo«.o under test, the representation or the sample being of 0 0 o° known size. If data concerning the size of the reference object are input into the processor, which is <ooo 15 able to generate a measurement of the image size in 00V* 0o00 units such as pixels or chord lengths, the sizes of objects subsequently placed in the field of view of the CCTV camera may be determined. These measurements may be made by comparison between the size of the object in 6 40 0 0@ 20 units such as pixels or chord lengths compared with the known size of the calibration image in those units.

Sucih a system, however, has major practical o0 disadvantages. In general, the calibration image is ':08:4 placed by hand, and needs to be positioned extremely 00 25 accurately if subsequent measurements are to be made 4 accurately, while the data concerning the size of the calibration image must be input manually into the processor, with the risk of error by the operator. In addition, whenever re-calibration of the processor is carried out, the measuring operations of the machine must be interrupted. As a result, there is a tendency to re-calibrate only infrequently. However, images produced by CCTV cameras may drift, or distort, with time or with variations in temperature. Such changes may have a serious effect on the accuracy of a measurement system. This is particularly important 4 1 because of the time taken to re-calibrate a processor, and hence the infrequency with which re-calibration is carried out.

The present invention seeks to overcome the disadvantages associated with known calibration systems, by providing a hardness testing machine having a measurement apparatus in which the calibration image is correctly and automatically positioned, without the need for an operator, and without interruption of the sequence of measurements.

Thus, the possibility of operator error, and the effects of camera drift with time and/or temperature, are substantially 8 1 o« eliminated because re-calibration can be achieved in a short time, and hence more frequently.

0600 According to the present invention, there is provided a static indentation hardness testing machine, for determining the hardness of a sample on the basis of the size of an indentation made in the sample, the machine including a measurement apparatus for comparing the size of the indentation with that of a reference object, the apparatus '*240, comprising:a means for forming an image of the indentation whose size is to be determined, when the sample is in a viewing position; and a reference object, mounted in the apparatus, which may 6 a s be viewed, using the means for forming an image, instead of, or in addition to the sample.

In preferred embodiments of the second aspect of the invention, the sample and the reference object may be viewed via a beam splitting mirror, and are located such that an image of one is obtained by transmission of light through 4 the beam splitting mirror and an image of the other is obtained by reflection of light from the beam splitting mirror. Preferably, the sample is located at the viewing position, which is located in the focal plane of a CCTV camera lens, and illuminated, while the reference object is also located in a focal plane of the camera lens and is illuminated, and hence 8 9 1102,!sspeOl3.austin.spe.

viewed through the camera, only when there is no object in the viewing position. For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:- Figure 1 is a block schematic diagram showing an image illumination apparatus, forming part of an image analysis system in accordance with a first aspect of the present invention; o Figure 2 is a cross-section through a measurement °0"o o apparatus, forming part of an image analysis system in S0 0o oo accordance with a second aspect of the invention; and 0 ooo Figure 3 is a view from below of the measurement 0 15 apparatus shown in Figure 2.

0oo. Figure 1 shows, schematically, an image illumination apparatus forming part of a hardness testing machine according to an embodiment of the first oos aspect of the invention. A sample 1 is viewed through 0 6 20 a television camera 2. In the preferred embodiment of 0 00 o a° the invention, the sample bears an indentation, the o size of which is to be neasured. The image from the 4 ocamera 2 is transferred to a frame store 4 and then to 000*04 0 0 a processor 3, in which the image is analysed. The <o0 o 25 processor has access to the contents of an associated memory 20, and controls the voltage supplied to a lamp which illuminates the object 1.

It has been found that accurate analysis of an image, and hence accurate measurement of the size of the indentation in the sample, can be consistently achieved if the mean grey level at certain points in a fixed pattern which surrounds the stored image of the hardness indentation lies within a particular range of grey levels. The fixed pattern is of importance because of the build up of material in the immediate vicinity of a hardness indentation, caused by the

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I 6 displacement of material during the indentation itself.

If the level of illumination is uncontrolled, the mean grey level found at the same points will vary widely, depending upon the reflectivity of the sample in which the indentation is made. The reflectivity will depend upon the nature of the metal itself, and on the surface finish of the sample.

The image viewed by the camera 2 is divided, by the frame store 4, into pixels, to each of which is assigned a grey level between 0 and 63. Appropriate o0 o programming of the processor allows the size of the 0°o largest anticipated indentation to be identified, on 0oo the basis of known data about the hardness test. The 0 0 ooo processor then selects a number of pixels forming a 15 pattern approximately surrounding that notional ooo indentation and calculates the mean grey level of these pixels. This ensures that the sampled pixels do not, in fact, form part of the image of the actual oo00 indentation itself, but only of the surrounding °00 20 material. As a result of a large number of such tests, 0o 0° it has been possible to determine a range of such mean grey levels for which an accurate analysis of the image may be achieved. Therefore, the calculated mean grey S level in each case is compared with the end-points of 25 that range, which are stored in the memory 20. If the calculated mean grey level is below the desired range it is necessary to increase the illumination provided by the lamp 5, while if the calculated mean grey level is above the required range, it is necesary to decrease the level of illumination.

In an alternative embodiment, analysis of the image may be carried out in real time, if the processor 3 has sufficient memory capacity. In this case, it would not be necessary to provide the frame store as a separate device.

The lamp is of a conventional type, and is it II1 controlled by a lamp controller which is able to provide any one of 16 available discrete voltage outputs. Therefore, if it is required to increase the level of illumination, the voltage to the lamp is increased by one step, while if it is required to decrease the level of illumination, the voltage to the lamp is decreased by one step. In order to obtain a mean grey level within the desired range, it may be necessary to repeat this operation, and to alter the voltage supplied to the lamp several times.

'"QO0 Figures 2 and 3 illustrate the apparatus used for 00 .0 mounting the sample and the reference object, in order o oo to allow the reference object to be viewed without the 2 0 need for interrupting the measurement operations, and 0 000 15 to substantially eliminate measurement errors caused by camera drift.

A camera extension tube 6 is mounted horizontally to the front of the camera 2 shown in Figure 1, and is mounted inside a support tube 7. At the end of the 20 camera extension tube 6 is located a beam splitting mirror 8, behind which is provided a front silvered mirror 9. Above the front silvered mirror is positioned a reference object 10, which is back-lit by ,a light source mounted in a holder 11. The beam 25 splitting mirror is mounted in optical blocks 12, 13, which are connected to further optical blocks 14, The optical blocks are provided with polarisers 16, 17, and, adjacent the beam splitting mirror, there is provided an optical window 18, and, on the other side of the mirror, a diffuser 19.

This apparatus is intended to be used in connection with a hardness testing machine, and in particular a static indentation hardness testing machine for performing, for example, the Brinell hardness test. In this particular test, a measure of the hardness of a material is formed by measuring the I ~Y~~lrul-rrr~r~ size of an indentation formed in a sample as a result of the application of a known load. The mean diameter of the indentation formed by a spherical ball is inversely proportional, for a given load, to the Brinell Hardness Number of the material. A sample, with an indentation formed therein, is positioned below the optical wndow 18 along the axis B. An image of the indentation is then reflected by the beam splitting mirror along axis A to the camera via the extension tube 6, which contains a lens to enlarge the image sufficiently for measurement purposes. coo* 000 oo 00 0 0 a 0 0 00 00 0 A*q 0 4a04 0 44.44r 44.4 4044 I I i 41 II 11 l 1 t 4 l l i ''4444 t t IIoI 4 4 441 4 44

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00 -9rooo 0 0 o 00 00 0 0 o 0000 S00o 0 O000 0 o 0 0 0 a 0 kv The beam splitting mirror 8 is chosen such that approximately 50% of incident light is transmitted through the mirror and approximately 50% is reflected.

Thus, it is possible to illuminate the indentation from above the mirror, and this is done by positioning a light source, such as a fibre optic cable remotely supplied with light from the lamp 5, along the axis C.

As a result of the properties of the beam splitting mirror, it is also possible to view, through the camera, a second image. In this embcodiment of the invention, an image of a back-lit reference object is reflected from the front silvered mirror 9, and partially transmitted through the beam splitting mirror 8 along the extension tube 6 to the camera 2. The reference object is an accurately made representation of a hardness indentation, surface etched onto polished opal glass. This object is mounted in the focal plane of the camera, and is visible to the camera only when it is back illuminated from a light source mcunted in the holder 11 along the axis D.

As the reference object is fixed in position, it is illuminated only when it is required to re-calibrate the apparatus. Thus, the back illumination of the reference object is used only intermittently, while, for convenience, the lamp 5, illuminating the sample, is left on permanently. As it is required to view only one of the images at any time, it is necessary to prevent stray light from the light source along the axis C illuminating the reference object 10 via the mirrors 8,9. This is achieved by means of the cross polarised filters 16, 17.

When first used, it is necessary initially to calibrate the apparatus. The reference object is backlit, and no hardness indentation sample is present along axis B. Therefore, the only image visible is .that of the reference object, which is of known size.

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10 -fl"S O 0 IO 04 0 0 o 0 0 a 0 0 0 0 0 00 o 000.

o a 0 4 0 4a t I4 I t a 4 Therefore, the image analysis system calibrates itself by means of the processor, by measuring the number of pixels occupied by the calibration image, and determining a'relationship between the size of the object in millimetres and the measured number of pixels. Then, the light at D is switched off, and a metallic sample bearing a hardness indentation is moved into the focus of the camera along axis B. The camera now records the image of this hardness indentation, and, after making any necessary adjustments to the illumination of the sample, applies the relationship between pixels and millimetres derived from the calibration procedure to the size of the image, in pixels; thus, a measure of the size of the hardness indentation is achieved. Using this system, it is possible for the apparatus to be regularly recalibrated, since the time taken for the calibration is very short, and it may be carried out while one hardness indentation is being removed from the appratus and another is being introduced.

Thus, there is provided an image analysis system, which may be rapidly and accurately calibrated, in order to substantially eliminate the possibility of errors caused by camera drift or operator error. In addition, the invention provides a system which allows accurate analysis of the image to be made, by ensuring correct illumination of the object.

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Claims

1. A static indentation hardness testing machine, for determining the hardness of a sample on the basis of the size of an indentation made in the sample, the machine including a measurement apparatus for comparing the size of the indentation with that of a reference object, the apparatus comprksing:- a means for forming an image of the indentation whose size is to be determined, when the sample is in a viewing %]C4 position; and a reference object, mounted in the apparatus, which may Sbe viewed, using the means for forming an image, instead of, 0 0 Sor in addition to the sample.

2. A machine as claimed in claim 1, wherein the indentation, the size of which is to be measured, and the reference object may be viewed via a beam splitting mirror, and are located such that an image of one is obtained by

4. transmission of light from the beam splitting mirror and an image of the other is obtained by reflection of light from the beam splitting mirror. 3. A machine as claimed in claim 1 or 2 wherein the e4b~jett w:hose size is t----b--termineds located at the to s4 a viewing position, which is located in a focal plane of a a camera lens, and illuminated, while the reference object is also located in a focal plane of the camera lens, and is illuminated, and hence viewed through the camera, only when there is oob in the viewing position, 4. A static indentation hardness testing machine as claimed in one of claims 1 to 3 wherein the reference object comprises an image of a hardness test indentation, etched onto a transparent surface, and is mounted in the machine such that it is viewed from a first side thereof and is illuminated from theAside thereof. A static indentation hardness testing machine substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings. S8911O2,!psspecl13,austin.spe, I 1DATED THIS 2nd November, 1989 SMITH SHELSTON BEADLE Fellows Institute of Patent Attorneys of Australia. Attorneys for the Applicant CHARLES THOMAS AUSTIN 494*4 o 0 0 &94 09 00 09 0 o 4 00 00 o A 9 9040 0 *040 0909 9 @40 9909 99 4 4* 4# 4 9 ii 9 440409 4 9 04 0 4 04 qo 1 1 r) 1) 1-