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September-October 2014- Automatic processes for discontinuities indication reading and part-acceptance/rejection

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Written by Laurence
Friday, 19 September 2014 10:21

September 2014

1. INTRODUCTION

In the early ‘70s, many PT specialists came to the conclusion that the only way to improve significantly the reproducibility and the reliability of the fluorescent penetrant inspection would be not to use human eyes, subject to unavoidable variations, but to use instead a system detecting fluorescent indications with a high level of confidence.

As a general matter, at least 60 to 70 % of the parts that an inspector checks would not need to be inspected, with a risk that, as his watchfulness decreases, he could not detect defective parts, a reaction somewhat similar to a driver who falls asleep while driving for too long on a straight main road. Using “ghost parts,” as in the car industry, does not prevent such a failure.

2. HISTORY

2.1. 1972

A patent(1) was filed dealing with means to detect and show indications from fluorescent penetrant testing and magnetic particle testing.

Illumination source

The UV-A sources, which were available at that time, had the following disadvantages:

  • A broad spectral distribution,
  • The spectral distance of wavelengths between the absorption and the peak fluorescent emission of any pigment or dye is rather the same, whatever the emission spectrum. Thus, if a material absorbs strongly in the near ultraviolet, and if it fluoresces in blue, it would absorb blue radiation while emitting a light yellow light,
  • The wide angle covered by these conventional sources.

Knowing this, the choice of a light source came to:

  • Either an helium-cadmium metal-vapour laser emitting a coherent energy at the 441.6 nm wavelength,
  • Or an argon laser emitting a coherent energy at the 488 nm wavelength.

 

Note that the 441.6 nm wavelength is within the electromagnetic spectrum of the actinic blue light, recently used to perform intermediate inspections. A proof that history repeats itself!

As can be seen, using blue light instead of UV-A did apparently come with no objection raising: the situation is different today because of the standards and specifications in force.

This actinic blue light, which has no effect on the optical brightener(2), is absorbed by the yellow dye in the 400-440 nm wavelength range depending on the solvent in which it is dissolved. Under actinic blue light, this yellow dye emits fluorescence centered on the 550 nm(2) wavelength.

This same blue light is suitable for the fluorescent pigments of MT detection media, which have an absorption peak at 470 nm and an emission peak at about 525 nm.

Thus, a helium-cadmium metal-vapour laser is appropriate for both fluorescent penetrant and magnetic particle inspections.

On the other hand, the 488 nm wavelength is not compatible with the aforementioned fluorescent penetrants. The authors mention a dye, which is compatible with this wavelength, uranine, also known as D & C Yellow No. 8, which is the fluorescein disodium salt (CAS No. 518-47-8). Uranine has indeed an absorption peak at 490-500 nm, and it emits a yellow or yellow-green fluorescence at 515 nm.

These kinds of laser had not been taken on, as this would have required changing all the dyes in fluorescent penetrants and all the pigments in all the fluorescent detection media used in magnetic particle testing. This would have led to replacing all the already approved and used PT and MT materials by new materials to be approved: an immense and non-justifiable work!

Scanning system

The laser beam was focused by a lens into a narrow beam which passed through an ultrasonic device which produced the periodic transverse deflection of the beam. The so-deflected beam was then impinged on a longitudinal deflecting means, which comprised eight reflecting surfaces on the octagonal periphery of a drum which could be driven by a motor.

Data recording system

When the scanning spot traversed an indication, energy was transmitted to a photocell to produce an electrical signal. Signals so-produced were amplified by an amplifier and were applied to the control grid of the cathode of a television picture tube. Horizontal deflection of the cathode-ray beam of the picture was controlled from the horizontal sweep circuit while a vertical sweep circuit controlled the vertical deflection. The horizontal and vertical sweep circuits were controlled from a clock circuit which was connected to a control circuit of the motor. The horizontal sweep of the picture tube was, thus, synchronized with the operation of the deflection device while the vertical sweep was synchronized with the rotation of the drum.

The output of the amplifier was also applied to a gate circuit controlled from the clock to a registering circuitry coupled to pattern recognition means. Such circuits worked to detect indications along (a) given direction or directions on the surface of the part being inspected, regardless of their positions on the surface.

 

2.2. 1973

John J. FLAHERTY and Eric J. STRAUTS presented a paper titled “Automatic Fluorescent Indication Detection Using a Flying Spot Laser System” at the ASNT National Spring Conference held in Los Angeles, California (USA). We do not have this text. This system was designed to inspect bearings.

A patent(3) was filed dealing with the inspection by fluorescent technique, limited to the screw threads and upper surfaces, of parts such as beverage bottles after cleaning and just before refilling. One of the objects of the invention was to provide a fluorescent inspection technique compatible with digital data-processing techniques.

Abstract of this patent

“A method for examining the upper part of a glass vessel for defects comprises the steps of applying a fluorescent material to selected surface of the upper part of the vessel while inhibiting the introduction of the fluorescent material into voids or defects existing in coated surfaces, irradiating the coated surface with light having a frequency for exciting the fluorescent material, scanning the surface for discontinuities occurring in fluorescent radiation which corresponds to defects in the vessel, and providing an indication of the existence of such a discontinuity. The selected surfaces, in particular, comprise the helically formed threads and the top surfaces of screw top beverage bottle. An apparatus in accordance with the invention is provided for effecting the examination of a glass vessel.”

The process steps were as follows:

  • Bottle cleaning and rinsing,
  • Coating of the upper portion of the bottles with the fluorescent material containing a small concentration of quinine in sodium alginate or water,
  • Examination,
  • Rejection of defective bottles and filling up of non-defective bottles.

 

The inventor mentioned, as a fluorescent material compatible with human consumption, diluted solutions of quinine (absorption peak: 347 nm, emission peak: 454 nm) in sodium alginate or water that could remain on the bottle after inspection.

When it was practical to rinse the bottle after inspection, the inventor mentioned the use of other fluorescent materials such as:

  • Anthracene in alcohol,
  • Napthalene-red in alcohol,
  • Resorcin-blue in water,
  • And rhodamine in water.

After the application of the fluorescent material, the bottles were carried by the conveyor from a turntable to the inspection booth, where every bottle spun-up while facing a fixed sensor designed to detect discontinuities.

The detection means included:

  • An ultraviolet irradiation source fitted with a visible-light blocking filter, which emitted a beam focused on the upper surface,
  • A scan of the surface by rotating the bottle and sequentially positioning segments of the bottle for their irradiation by the ultraviolet beam,
  • A photo-detector, which sent an electrical signal proportional to the intensity of the incident beam, coupled with the scan and the pre-amplifier,
  • The amplified signal was coupled with a circuit analysing the data from the photo-detector signal, able to determine and display the presence of a discontinuity in the bottle.

The binary pieces of information provided by the photo-detector at each position of the bottle were sent to the computer and stored in its memory. The memory already contained a pattern, configuration or "signature" of a bottle known to be non-defective. This information or signature had been recorded, thanks to the inspection of a bottle known to be non-defective in order to provide signal patterns, then, stored as reference data. The data from a bottle under inspection, temporarily stored in the memory, were, then, transferred to the computing unit for comparison with the "signature" or data from a non-defective bottle.

Typically, bottles having defects half-a-millimetre long or greater along the length of the surface defined after the directions of rotation are considered as potentially dangerous, should be detected, and the bottles should be rejected.

 

2.3. 1975

A patent(4) was filed dealing with an automatic defect-detecting method and apparatus for magnetic particle testing.

Abstract of this patent

“An automatic surface defect-detecting method and apparatus for detecting only effective significant surface defects. The significant defects can be detected from video signals and differential signals corresponding to particular scanning lines respectively by removing the distributed density portions having larger pulse width in the scanning line direction, or removing smaller differential value portions in density, from the examined portions with higher density of fluorescent magnetic powders thereon.”

Radiation source

A UV-A source sent UV radiation onto the surface under inspection in a given direction inside a dark room. No other information is supplied.

Scanning system

A television camera was mounted above the part under inspection, in the dark room; the camera lens was looking downwards. A display screen was built in. An optical filter allowed only radiation with the suitable wavelength to detect the surface discontinuities, the light being selected from among reflected light rays from the examined part. A camera control unit controlled the scanning operation of the television camera, and selected and transmitted only the video signals corresponding to the particular scanned lines.

Signal processing and data recording system

A signal-processing circuit sent a “defect-warning” signal after the video signals from the camera control unit had been processed when a defect-to-be-marked had been detected. A delay circuit sent a signal with some lag after detecting the defect indication. The delay circuit made a mark on the defects’ areas.

 

2.4. 1977

A patent(5) was filed dealing with a method and apparatus for the automatic recognition and evaluation of optical crack indication on the surface of work-parts.

Abstract of this patent

“A method and device for the automatic detection and evaluation of optical crack indication on the surface of workpieces is disclosed. In such method and device, visual displays are converted into electrical bright-dark signals by the use of light sensitive device, for example, an image recording tube. The surface under observation is scanned by the light-sensitive device line by line, the width of each of which (either as individual line or groups of adjacent lines) corresponds to the maximum optical display width from which evaluation is to proceed. The bright-dark signals thus obtained from three lines or lines groups are compared. From the signal of the middle line or line group and the signals of the two other line or line groups there is formed a difference signal which is evaluated to generate an error evaluation signal of a minimum value is exceeded.”

Summary of the invention

“Apparatus for performing the method according to the invention is characterized by a control system for the light-sensitive device which scans the surface under observation in lines, the width of which, either as individual line or as a group of adjacent lines, corresponds to the maximum optical display width from which evaluation is to proceed, and by a logic comparator circuit for the bright-dark signals thus obtained from three lines or line groups adjoining each other or separated from each other by an intermediate line and directly successively or simultaneously scanned, to deliver an error evaluation signal only if the difference between the signal of the middle line or line group and the signals of the two other lines or line group exceeds a minimum value. The control system can effect scanning by the so-called line interlace method in which scanning is performed by the omission of one line at a time.

One advantageous embodiment of apparatus of this kind is characterized in that the light-sensitive device comprises a recording camera for the projection of the observed surface on the screen of a recording tube whose electron beam, which scans the said screen line by line, is controlled by a line frequency oscillator which defines the line traversing rhythm, by a synchronizing pulse oscillator, connected downstream thereof and effecting flyback dark control and by a horizontal and by a vertical oscillator and whose video signals are amplified and are supplied to the logic memory and comparator system.

Advantageously, such apparatus also embodies a playback tube which is driven by the control system for the recording tube and reproduces the image of the observed surface and whose control grid is connected to the output of the logic comparator system.

The surface image is rotated optically, for example through mirror systems, or electronically, for example by modulation of the deflection voltages for the scanning beam or the scanning beams are rotated by means of a sine-cosine oscillator so that it is possible to evaluate indications in any direction and not only those whose direction happens to be orientated in the line direction. Mechanical relative rotation between object and scanning device can also be used to this end.

Three-gun tubes can be used in place of single-gun recording and playback tubes or it is possible to use tubes which simultaneously scan over three lines by means of "chopping", storage of the bright-dark signals being omitted.

The light-sensitive device for scanning the observed surface can also comprise apparatus for light beam or laser beam scanning.”

 

2.5. 1980-1984

2.5.1. First generation of the AEOS® system

In the early ‘80s, a patent(6) was filed and the owner of the patent filed the AEOS® trademark, an acronym for Automatic Electronic and Optical Scanning system.

The previously proposed automatic systems may cause errors when trying to highlight a flaw close to an edge of the part. In this case, the equipment had to differentiate a dark response outside the edges of the part and the edges of the part, and the edge of an insufficiently washed part, which displayed an indication on its surface. The previous automatic inspection systems tended to treat such an insufficiently washed area as flaws; thus, these systems were deceitful and unreliable.

One or us gave a paper(7) and published an article(8) on the AEOS® system.

Keep in mind that the task was not that easy because, more than thirty years ago from now, computers had nothing in common with today equipment: slow operating systems and data processing, small memory and storage capabilities, leading to using large air-conditioned rooms full of electronic boxes, few, unsophisticated software, etc.

Remember: the Macintosh® was launched some years later, on January 24, 1984!

The AEOS® system was an "all-or-nothing" system that rejected all the parts with indications of discontinuities detected by penetrant testing.

An AEOS® machine was built for demonstration, able to automatically inspect up to 250 mm-long turbine engine blades and similar parts, at a rate of three parts per minute.

AEOS® general layout

English translation of the captions (French terms in alphabetical order):

  • Chambre d’essai : Test cell
  • Ecran de contrôle : Control screen
  • Miroirs oscillants : Oscillating mirrors
  • Ordinateur : Computer
  • Photomultiplicateurs : Photomultipliers
  • Pièces à contrôler : Parts to be inspected.
  • Unité d’affichage : Display unit

The basic principle was a scan by the flying-spot coming from a collimated laser beam reflected by the oscillating mirrors.

The visible radiation generated by the laser beam, which excited the penetrant retained in the discontinuities, was focused on photomultipliers.

The signals were then sent to an analog-digital converter and were analyzed by a microprocessor. These signals were used to actuate the electromechanical part-handling equipment, and they were displayed on a control monitor.

AEOS® Optical system

English translation of the captions (French terms in alphabetical order):

  • Amplificateurs: Amplifiers
  • Cellule photoélectrique: Photoelectric cell
  • Convertisseur analogique/digital: Analog/digital converter
  • Diviseur de faisceau: Beam splitter
  • Filtres: Filters
  • Microprocesseur: Microprocessor
  • Miroir fixe: Fixed mirror
  • Modulateur: Modulator
  • Moniteur: Monitor
  • Pénétrant retenu par le défaut: Penetrant retained by the defect
  • Photomultiplicateurs: Photomultipliers
  • Scanner balayage horizontal: Horizontal scanner
  • Scanner balayage vertical: Vertical scanner
  • Source d’alimentation laser: Laser power supply
  • Stockage informations: Data storage
  • Système optique: Optical system

The AEOS® system had three components:

  • The control module,
  • The electromagnetic handling module,
  • The microprocessor.

 

Control module

The UV-A sources then available were based on mercury-vapour bulbs, with two drawbacks:

  • They could provide a UV-A irradiation high enough only when close enough to the surface under inspection; here, this was impossible, due to the presence of the part handling system. Furthermore, it was necessary to focus the UV-A radiation beam,
  • They produced a high level of visible light (illuminance) taken by the sensors along with the visible light coming from the indications.

Using more powerful UV-A sources of the same kind would not have been an answer, as the illuminance level increases along with the UV-A irradiance increase.

The source chosen to excite the dyes of the fluorescent penetrant or of the fluorescent pigments of the MT detection media was a helium-cadmium metal-vapour laser, emitting an actinic coherent radiation at 442 nm (blue visible light).

To prevent any stray visible light coming from the laser, a "black glass" filter was mounted in front of the laser.

The laser beam was reflected by oscillating mirrors. The first mirror oscillated around a horizontal pivot at a frequency between 100 and 400 Hz, so as to generate a moving spot in the vertical mode. The second mirror oscillated at a much lower frequency along a vertical axis so as to generate a moving spot in the horizontal mode.

The beam entered the dark room in which the part under inspection was, and scanned its surface with a circa 1.4 mm (1/16 in) diameter light spot (instantaneous diameter) on a height of circa 250 mm (9 27/32 in), circa 100 mm (3 15/16 in) wide.

The diameter of the light spot and the frequency of oscillations of the mirrors were selected to ensure the illumination of all the points of the test surface (about 250 x 100 mm) (9 27/32 x 3 15/16 in) which could be increased or decreased using diaphragms and deflectors.

The part under inspection was maintained at the center of the dark room, its axis in coincidence with the central vertical line of the scan.

The visible light from the penetrant or the MT detection medium was detected by six photomultipliers suitably placed so as to collect any light from any point within the scanned area. Optical filters mounted in front of each photomultiplier suppressed any unwanted light other than the fluorescent one.

English translation of the captions (French terms in alphabetical order):

  • Chambre noire: Dark room
  • Contrôleur séquentiel: Sequence controller
  • Dispositif de libération de la pièce: Part-releasing device
  • Diviseur de rayon: Beam splitter
  • Ecran de contrôle: Control panel
  • Laser: Laser
  • Manipulateurs: Handling units
  • Microprocesseur: Microprocessor
  • Miroir réglable: Adjustable mirror
  • Miroirs oscillants: Oscillating mirrors
  • Modulateur: Modulator
  • Module d’examen: Inspection module
  • Module de manutention mécanique: Mechanical handling module
  • Module optique: Optical module
  • Obturateur du rayon laser (fermé lorsque la table tourne): Laser-beam gate (closed when the table turns)
  • Photomultiplicateurs : Photomultipliers
  • Porte tournante étanche à la lumière: Revolving light-proof door
  • Poste de chargement des pièces : Parts’ loading stage
  • Robot manipulateur: Manipulator robot
  • Table tournante recevant les pièces acceptées: Turntable for accepted parts
  • Table tournante recevant les pièces rebutées: Turntable for rejected parts
  • Terminal: Terminal
  • Unités de rotation: Rotary units

Electromagnetic handling module

The handling system was designed to present all the faces of the part under inspection. A robot withdrew the part from a specially fitted support at the end of the automatic PT process line and placed it in a work-part holder, vertically rotatable mounted on a turntable.

The turntable, then, made a 180° rotation so that the work-part holder entered the dark room where the scanning took place. Here, the part was rotated 360° in four equal stages, with a pause at each stage to allow for the laser-beam scanning.

Then, a horizontal clamp pneumatically took the part in the middle of its vertical axis. The work-part holder released the part and the clamp was rotated 90° in a vertical plane to present one end of the part to the laser-beam scanning, and was, then, rotated 180° to present the other end to the laser-beam scanning. Then, the clamp put back the part into the work-part holder. The turntable made a 180° rotation and the robot grabbed the part under inspection to put it into the suitable basket, accepted or rejected parts.

The “accepted part” or “rejected part” decision was displayed on the screen and was printed.

Computerized data processing

The microprocessor controlled and operated the whole system.

Any light collected by the photomultiplier was converted into an electric signal, which was first processed by the associated amplifier, then, by an electric signal integrator. The elementary signals were instantaneously combined. The signal was, then, processed in an analog-digital converter and sent to a microprocessor. The microprocessor signal averaged the response of the successive scans, by collecting a complete signal and dividing it into the elements of an M x N matrix, where M and N were figures about 250.

This device actually divided each averaged vertical scan into M sections and each averaged horizontal scan into N sections.

If a surface element produced a high-level signal when compared to a predetermined and programmed standard, the detection threshold value, then, an "indication" was registered.

The coordinates of the matrix element showing abnormal signals were recorded and were used to actuate the electromechanical manipulator.

The microprocessor compared the response of each matrix element with those in close vicinity.

Comparing the intensity of a detected signal in a given location and the intensity of the signals produced by the surrounding area was found out to be an effective method to offset the fluorescent background. When an extremely large indication would extend over several elements of the matrix, the system would treat the signal as if it were a background. Therefore, the system would wrongly accept the part while it should reject it. This event had been foreseen and, in the program, a derogatory absolute maximum value that made the system activate a signal whenever such an intensity level was detected, regardless of the signal intensity of the surrounding area, had been put in place.

This data process is detailed in another patent(9) of the same inventors.

A real-time computer-generated image of the part was displayed on the monitor screen. The detected indications were flashing to draw the attention to their right location in relation to the part geometry, which would make it easier a subsequent inspection using other non-destructive testing (NDT) techniques or methods.

The information was stored in memory; this allowed for removing the part out of the AEOS® and comparing the fluorescent indications as seen by conventional visual inspection with flashing signals on the computer-generated image on the monitor.

The AEOS® was supposed to lower the inspector’s workload by 50 to 75 %.

The AEOS® was subject to a thorough evaluation in the Rolls-Royce, an aircraft engine manufacturer, Bristol plant, in England in 1981, and the second prototype was tested in the Avco Lycoming (known now as Textron Lycoming Turbine Engine, a subsidiary of Textron Inc.), another aircraft engine manufacturer, Charleston plant in South Carolina (USA). The results of these various assessments have proved invaluable, and highlighted some drawbacks (10).

 

2.5.2 J.F. VAERMAN’s works

In 1983, a US patent(11) and a European patent(12) were filed, which refer to some of the aforementioned patents(1) (6).

This device automatically read the discontinuity indications on PT-processed parts. A focused (not necessarily monochromatic) ultraviolet beam, provided by a UV-A source or a laser, illuminated the surface of the part under inspection after successive reflections on a separating mirror (able to reflect UV-A radiation, and not visible light) and on two oscillating mirrors for scanning along two axes of the part surface. The visible-light beam reemitted parallel to the incident beam went through the separating mirror and was received by a photoelectric-type detector after going through a filter system, which let the visible-light go on while blocking the residual ultraviolet light. The output information was sent to a data processing unit, which also got information, thanks to a different electronic circuit, about the position of the oscillating mirror.

2.5.3 Y. F. Cheu’s works

Y. F. Cheu published a paper(13) in 1984 dealing with the automatic crack detection with computer vision and pattern recognition of magnetic particle indications, and presented a conference(14) on the same topic in 1985.

The automatic reading of the indications of discontinuities shown by MT comes with two major well-known hurdles:

  • A low contrast of crack indications,
  • The edge indication is indistinguishable from the crack indication.

Yen Fwu Cheu explains that he overcame these troubles, thanks to a pre-image processing, and an image processing with an intermediate water-rinsing of the connecting rods after applying the water-based fluorescent magnetic ink and magnetization.

The critical rinsing parameters were its duration and the water pressure, which shall be experimentally determined to counter the edge effect while enhancing the indications’ seeability at the same time. Doing so, the author was able to overcome the need of strict positioning of the connecting rods for the automatic reading. Furthermore, the required computing power was found to be lowered at a time when the computing power and computers’ speed had nothing to do with the performance of today computers.

The system comprised two sub-systems, each one supported by its own computer.

  • The vision subsystem, including: four UV-A xenon tubes, four CCD cameras, one scanning-images device with 256 gray levels. The algorithm led to a 16 x 16 pixel image and calculated the average gray level. The discontinuities’ indications reading from the binary images was performed by comparing the indication length and the discontinuity width/length ratio with predetermined acceptance thresholds.
  • The mechanical subsystem came as a circular installation with ten steps (including the marking of the rejected connecting rods, the connecting rods’ demagnetization and corrosion protection after inspection.)

This system was able to inspect 1,200 rods per hour.

The accepted connecting rods were put into crates while those rejected by the system were examined by the inspector.

 

2.6. 1985

Second generation of the AEOS® system

The US subsidiary of the British group which designed the AEOS® system signed a contract with the Cherry Point (North Carolina) U.S. Naval Air Rework Facility (NARF), for the study, the supply and the implementation of an automatic fluorescent penetrant process line, along with an automatic defect detection system for the inspection of the turbine blades from the AV8B VTOL (vertical take-off and landing) aircraft; the AV8B was an improved version of the VTOL HARRIER powered with Pegasus 11-61 (F402-RR-408, according to the nomenclature of the U.S. Air Force) from Rolls-Royce Ltd.

For the very first time, an automatic image-processing, named “AEOS® Mark III Fully Automated Fluorescent Blade Inspection System” was coupled with the most sophisticated automatic PT line that had been built so far.

This system met the following requirements:

  • An automatic PT process line to process blades listed according to their manufacturers’ part numbers,
  • Robotic handling of processed blades, including the withdrawal of blades from the PT process line with their supply to the automatic detection process and blade distribution, depending on the decision made by the data-processing system,
  • Blade illumination using a suitable ultraviolet source, and exposure of the illuminated blade to a vision system coupled with a computer-aided image processing of the detected indications,
  • Blades’ classification in various categories corresponding to the defect locations,
  • Detection of defects in the 0.25 mm (0.0010 in.) size range, which was a 300 % improvement over the resolution of the best systems existing then.

The first systems, which were all based on a rapid flying-spot scanning process, allowed for providing a high and uniform (UV-A) irradiance on the entire surface of the part under inspection. However, this technique led to reflections, which, if too intense, were difficult to process. Another drawback is that it was difficult to get a pixel size well below 1.5 mm (1/16 in) diameter. Finally, the various scanning mechanisms were complex.

On the other hand, then, video cameras, even the cheapest ones, gave a good contrast, even with low (UV-A) irradiance levels. The main trouble was to use a small enough aperture to provide an acceptable depth of field while achieving a sharp contrast in the pixels’ network.

Description of the PT automatic process line

This linear process line included 18 steps with an overhead conveyor for transferring eight-blade hangers. Two post-emulsifiable fluorescent penetrants and the dry developer were applied by the electrostatic spraying technique; the hydrophilic emulsifier aqueous solution was applied by the foam technique. This process line included a rinsing and washing water treatment facility.

After processing, the parts were carried to the robotic system.

Robot and image processing Management

A computer connected the robot control and the image-processing system.

Automated inspection system diagram

The operator controls the operation of the entire system through a computer.

The followings are displayed on the screen:

  • On page 1: the main menu on the colour graphics monitor,
  • On page 2: The multi-coloured diagram of the PT process line with the identification of the different steps by their names with the coloured blocks showing the processing sequence. The entire device, its mode of operation, the time of the full cycle and the position of each step. If everything is ready to operate at a given step, the block representing the position is displayed in green. If there is any fault that shall be corrected, the block would appear in red. The details of the anomaly are displayed on the monochrome status monitor screen as an alarm synopsis. When the fault is fixed, the red block turns to green. When all the indicators are green, it means that the system is back to nominal.
  • On pages 3 and 4: the process program number and the numbering of the blades processed according to this program. The system is designed to inspect up to 99 types of blades,
  • On page 5: the position of the loading stage. When red, a block states there is an anomaly of any kind at the corresponding stage. At the bottom of the screen, when everything returns to normal, the display turns to green. When all the red indications have switched to green, everything is in order at the loading stage to trigger the sending of the first basket of blades,
  • On the following pages: similar information for each programmed stage and, additionally, information regarding each water-treatment system.

Further, three screens show in detail the procedure to follow to run the process line according to its three modes of operation, i.e. manual, single-cycle or automatic.

Thanks to the processing controller and its control panels, simple but very detailed, a single operator is able to launch, programme, monitor the entire system and make it work.

The full system was described in two papers(10) and (15).

 

2.7. 1987

Franz FEIL and Klaus GOEBBELS published a paper(16) dealing with a system which allowed the fully automatic surface defect detection when using PT or MT.

This system included:

  • A robot, which could handle two parts in the “load” or “remote” modes,
  • An automatic PT process line with the following stages: pre-cleaning, drying, colour contrast penetrant application, post-cleaning, non-aqueous wet developer application. Cleaner, penetrant and developer were applied using spray guns. After inspection, a colour sensor checked if the spray applications had been correctly performed.
  • Parts were illuminated using a set of white-light sources.
  • A CCD camera.

The camera signal was digitalized into 512 points per line with an eight-bit amplitude. The decision threshold could be selected to take into account the surface and contrast conditions.

The accept/reject decision was a function of boundary conditions set by software and could be based either on a single defect in excess of a given length or on multiple defect occurrences. Defects down to 2 % of the vertical image extension could be found.

Data acquisition, processing and evaluation, as well as accept/reject decision were done in 40 ms.

The accept/reject signals could be used to mark the parts.

A similar system had been built to PT check cylindrical parts with sintered layers for pores and cracks on the surface, and by ultrasonic testing (UT) for internal pores and bonding defects.

The optoelectronic/computer system was also used to perform successful MT inspection on various car-engine components. Illumination could be carried with white light or ultraviolet (UV-A) irradiation.

 

2.8. LATE ‘80S

Some car manufacturers showed some interest for the automatic reading of indications. Underneath, find some examples.

2.8.1. First French car manufacturer

Gordon D. STEWART, the Engineering Division Manager of the British group which designed the AEOS® system, designed and offered an automatic fluorescent PT process line in which a Level 2 water-washable fluorescent penetrant was used without any developer for the inspection of rocker-arm pads on behalf of one of his suppliers based in Portugal.

This automatic process line was coupled with an automatic reading of defect indications and an automatic sorting device (accepted or rejected parts). The only real problem, which was solved by an appropriate signal processing, was to get rid of the "edge effect" due to the penetrant retention by the pad edges, which measured about 2 cm x 2 cm (circa 25/32 in x 25/32 in).

This project was stopped because the cost of the installation, although very attractive, did not allow for a sufficient return on investment in view of the low cost, even then, of the Portuguese labor work!

2.8.2 Second French car manufacturer

Around the same time, one of us was contacted for a similar need for the MT inspection of steering rods. The aim was to decrease the time for the examination of the parts from nine seconds for an inspector to 7 seconds in automatic reading...but by not inspected some areas where “nothing was ever found”.

Of course, nowadays, with the advances in computer technology, this seven-second time could be certainly reduced.

The question is to know if it is reasonable not to inspect areas where there is never anything to detect. Certainly not in the case of critical parts. For example, in the aerospace industry, Inconel® turbine discs after machining are unitarily PT-inspected.

2.8.3 German Automotive industry

We heard about two automatic reading systems coupled to magnetic benches supplied by a German company. To our knowledge, these systems have not been used since a long time.

 

2.9. IBIS program (1978-1992)

The US Air Force, Army and Navy in collaboration with General Electric, an American aircraft engine manufacturer, had started a program of the utmost importance dealing with the development of the automatic inspection, known as IBIS(9), an acronym for "Integrated Blade Inspection System."

This program, the work of which began in 1978 and was planned to be completed in 1985, included the part-processing operations by fluorescent penetrant testing in automatic process line as well the automatic inspection.

This project has also been described in a document of General Electric.

In this paper, the focus was mainly the importance of the expertise in the field of image-processing and of pattern recognition. A "bleed-back"(17) step was even included.

Three of the leading aircraft engines’ manufacturers and the two world largest penetrant materials manufacturers took part in such projects.

The Ibis system, and the installation Pratt and Whitney commissioned, were both based on a helium-cadmium laser, which emitted a collimated beam with a scanning system using mirrors. Photomultipliers were used as a receptor to catch fluorescence emitted at the wavelength of 520 nm by any indication whatsoever.

This program was the subject of an updated study published in December 2012(18).

We have only the following summary:

“The purpose of this design study was to identify ways to improve the Integrated Blade Inspection System. The Air Force requires inspection of jet engine compressor and turbine blades to locate defects and prevent engine failure. The current inspection process uses fluorescent penetrant as an aid to identify cracked blades. A systems engineering design process was applied to evaluate the current inspection techniques and to develop alternative methods to satisfy the Air Force requirements. Three different inspection systems were developed and compared to the current process: manual, semi-automated, and fully automated inspection. This study made several noteworthy contributions: development of classification software to validate the neural network approach for accurate blade classification, demonstration of potential advantages of charge-coupled device cameras for data gathering, quantification of the cost of incorrectly classifying jet engine blades, examination of the value of a statistical quality control plan for the inspection process, and identification of a method using multiple images to extract additional features from cracks. The study demonstrates that the fully automated system could dramatically outperform the manual inspection process by improving the consistency of the inspection process and raising the quality of the blades returned to service.”

We do not know if this new study led to practical industrial applications.

 

2.10. 1998

A patent(19) was filed dealing with "a non-destructive testing method of the condition of a surface likely to present cracks, by a process based on the viewing of the wave emitted by a dye material applied onto the surface and into the cracks, under the effect of an incident excitation beam of a wavelength appropriate to the dye product, characterized in that it is the rotation of the linear polarized wave emitted by the dye in accordance with the dye-material thickness with respect to a wavelength of the incident linearly polarized light to suppress the viewing of spots due to residual dye material present on the surface."

The device included a laser that generates the incident excitation beam and a camera, which recorded the light emitted by the dye material.

This process was designed for both penetrant testing and magnetic particle testing.

It allowed for the image optimizing and for the cracks’ depth assessing. The invention was based on the fact that, when the dye material was excited by a linear polarized wave, the dye material that was on the surface or in the crack emitted a linear polarized light that made an angle with the incident rectilinear polarized wave, this angle being a function of the dye-material thickness in the considered area or in the cracks.

This angular deviation between the incident polarized rectilinear wave, and the rectilinear polarized wave emitted by the dye was used to delete information from areas where some residual penetrant was left on the surface, to keep only the areas in which some penetrant had entered cracks, taking advantage of the fact that the angular deviation corresponding to a wavelength emitted by the residual dye material on the surface was different from the angle due to the dye material in a crack, because the thickness of the residual dye material on the surface was always less than the thickness of the dye material in a crack.

Furthermore, the angular deviations corresponding to the various cracks were used to determine the crack depth.

Radiation source

Source of non-polarized light to which a polarizer was associated or, preferably, a source of polarized light. For example, a laser which delivered a monochromatic rectilinear polarized wave having parallel edges and that emitted illuminance or irradiance centered on the wavelength appropriate to the dye which was used.

The laser beam could be transmitted through optical fibers to the surface to be inspected.

For the inspection of holes and recess surfaces, the laser beam was transmitted through an optical fiber to an endoscope.

The patent mentioned other devices.

Surface scanning

It could be done by moving:

  • Either the beam, for example, thanks to fixed and oscillating mirrors driven by a programmed computer,
  • Or the surface, by placing the part to be inspected on a turntable, which can undergo a translation or which could move up or down along an axis parallel to the rotation axis of the part.

Viewing means

Camera fitted with a telephoto lens and a wave analyzer to select the rectilinear polarized waves emitted by the dye.

By rotating the analyzer, parasitical spots gradually disappeared, leaving only the images due to cracks for a certain rotation of the analyzer. Continuing to rotate the analyzer, images of cracks were eliminated successively in order of increasing depth. It was then, possible to establish a calibration curve of the rotation of the analyzer related to the depths of cracks in order to assess the depth of the cracks on a surface under inspection.

The elimination of parasitical spots and the assessment of the crack depths could be performed at the same time using two successive analyzers.

 

2.11. 1999

The inventor of the previous French patent(19) filed a US patent(20) very similar to his French patent.

Such a system was launched by the assignee of the US patent(20) who claimed:

  • “A process which is an optical-based and NOT a visual-based solution.”
  • “A system based on the fact that when the fluorescent dyes or pigments are excited by an adapted polarized wave, they have the property to restore a luminous indication whose characteristic is to also have a polarity; this polarity will have been modified according to the thickness of excited product met by the wave. Rotary filter polarizing can eliminate the parasitic traces, such as excess dye penetrant or developer, being always a thickness lower than that of the product contained in a crack, so what remains is only the indications of real defects. As you continue the inspection while turning this filter until the indication of defect disappears, one obtains an indication of depth of crack. The indication of a defect, before total extinction can be measured in pixels on a video monitor thus allowing a measurement in three dimensions (3D) by utilizing an appropriate designed software. In layman terms, the treated piece to be inspected is exposed to a polarized light that is emitted from a UV-A laser. The polarizer is set to the depth of a defect by the user and only defects which are of the set depth or greater will emit visible light. The visible light emitted can be detected manually, a light detector source or even on a CCD color video camera which records the defect images. Because the process has the ability to judge the depth of a defect, as well as, the length and width, this 3 dimensional view of a defect can be utilized in further analysis of the inspected part.”
  • “In penetrant testing: no PT developer is necessary (a major cost savings).”

 

In 2001, the General Manager of this company toured Europe.

Accompanied by a consultant who was an NDT technician retired from a French aircraft manufacturer, he paid visits to the main French PT and MT materials/equipment manufacturers/suppliers, hoping to sign immediately with them some distribution agreements.

He also paid visits to some main jet engine manufacturers and repair centres.

One of us was part of a demonstration performed by an aerospace maintenance workshop based in the Île-de-France (France).

A laboratory evaluation was carried out, also in 2001, by a French manufacturer of PT materials and equipment on car parts such as: steering pivots, camshafts, steering racks, etc.

The undeniable system capabilities and limitations for the automatic detection of the MT defects were identified.

The benefits of the system were:

  • Easy operation and implementation,
  • Ability to locate and quantify the amount of fluorescent material present at a place, exactly like an operator would do. This information can, then, be easily stored in the form of a computer-generated image in gray levels where each level corresponds to a certain amount of materials,
  • Contactless operation and possibility, with only one camera, to inspect a wide area because the angle of incidence of the laser beam may go up to ± 40°,
  • Upgradeable system, thanks to the possible addition of data processing algorithms.

The system problems and limitations were:

  • Insufficient sensitivity of the camera to detect tiny defects, even though when easily visible to the naked eye. A new more sensitive black-and-white camera was thought of in the final system,
  • Difficult focusing. The part/camera distance was critical,
  • In MT, contrary to PT, the fluorescent material does not usually go inside the cracks. The amount of material on the defect is not always greater than the amount of material accumulated on sound areas (retention area, field leak area); therefore, the 3D vision is not achievable,
  • The influence of interference areas could be limited by a specific positioning of the camera vis-à-vis the area to be inspected, but in this case, this led to a substantial decrease in the easiness of implementation.

A magnetic bench with an automatic defect-detection system based on this equipment did not seem possible without the addition of more or less complex image-processing algorithms.

Finally, this system seemed better suited to the decision support and the detection reliability.

The same system was loaned (or rented?) at a French jet engine repair centre and the conclusion of these tests was that the system was well suited for laboratory applications, but not in production.

No company ever bought this system in France.

If we are to believe the statements of the inventor of this system, a major jet engine manufacturer, at the behest of a major airline, altered, or, at least, considered to alter, its standard operating procedures to introduce this system in its inspection means. What did really happen?

In 2002, an American company marketed this system for some time, under its own trademark as a Fluorescent Penetrant Inspection System, for PT only and not for MT.

Anyway, the company that invented this system shut in late 2009 by stating that:

“Unfortunately, the sad state of the economy forced it to halt all operations.”

 

3. THE CURRENT SITUATION

Since January 2008, when we both retired, there is not a fully-automatic inspection system in operation.

If, since the ‘80s, the acquisition and signal-processing systems grew in performance and speed, if many companies offer software, in fact, there is a lack of integrators who can link these systems to the PT process lines or magnetic benches.

One of the main problems is the parts handling to present them to the automatic reading and automatic inspection/decision system. The parts shall be grabbed by the manipulator robot at areas where there is no fluorescent trace, to avoid hiding these areas or withdrawing these traces, or to prevent the manipulator contamination by residual traces of fluorescent penetrant or magnetic particles which, when left over on other parts, could produce misleading indications.

The inspector, with his eyes and his brain, knows skillfully how to take the parts without touching the discontinuities.

Another problem is that no expert automatic detection system has the ability of an inspector to distinguish between an acceptable and an unacceptable indication, or to “feel” a misleading indication. At most, these systems may perform some sorting, and it is always an inspector who will examine the "contentious" parts rejected by the system.

This topic remains exciting and new projects, more or less ambitious, are constantly launched and new patents are filed proving that this topic still arouses much interest.

It looks like the Holy Grail or the search for the philosopher's stone.

 

References

(1)US patent N° 3,774,030 Defect detecting and indicating means for non-destructive testing, filed on June 2, 1972 and published on November 20, 1973. Inventors: Donald T. O’Connor, Bruce C. Graham, David W. Price. Assignee: Magnaflux Corporation, Chicago (Illinois), USA.

(2)Pierre Chemin and Patrick Dubosc, Dyes and fluorescent penetrants, August 2012.

(3)US patent N° 3,829,690 Method and apparatus for the examination of articles for defects, filed on August 29 1973 and published on August 13, 1974. Inventor: Elly P. Snyder.

(4)US patent N° 3,988 530 Automatic defect-detecting method and apparatus filed on September 30, 1975 and published on October 26, 1976. Inventors: Yoshizo Ikegami, Kuniomi Abe, Seijiro Kushibe, Takao Yoshinaga, Tsunemasa OKada. Assignees Konan Camera Reseach Institute; Sumitomo Metal Industries Ltd, Japon.

(5)US patent N° 4,207,593 Method and apparatus for the automatic recognition and evaluation of optical crack indications on the surface of workpieces, filed on July 27, 1977 and published on June 10, 1980. Inventors: Volker Deutsch, Ernst-August Becker, Ulrich Förstermann (Allemagne). Assignee: Karl Deutsch Prüf- und Messgérätebau GmbH & Co. KG, Wuppertal, Allemagne.

(6)European patent N° 0, 050,935 Method and apparatus for examining a workpiece, filed on October 12, 1981 and published on May 29, 1985. Inventors: Martin Edwin Allard and Joseph Augustine Willcox, Assignee: Brent Chemicals International Plc, Ridgeway, Iver SLO 9JJ, Buckinghamshire (Great Britain).

(7)Pierre Chemin, Automatisation de la lecture des indications de défauts en ressuage et magnétoscopie fluorescents sous rayonnement ultra-violet par un dispositif automatique à balayage électronique et optique, (Editor’s note: Automating the defects indications viewing in fluorescent under ultraviolet radiation PT and MT by an automatic electronic and optical scanning system), Journées Nationales sur les Essais Non Destructifs (Editor’s note: National Congress on Non-destructive Testing), organized by the French Confederation for Non-Destructive Testing (COFREND), Rue Olivier de Serres, 75015 Paris (France), January 1982. Congress proceeedings page 147-157.

(8)Jean-Claude Hugues, Pierre Chemin and Karl Marcus Jacobsen, Dispositif de lecture automatique à l’aide d’un scanner des indications fluorescentes des défauts mis en évidence par ressuage ou magnétoscopie (Editor’s note: Automated system assisted by scanner for the detection of fluorescent indications from PT/MT processes), Revue Pratique du Contrôle Industriel, Editions Ampères, Paris, N° 121 bis, June 1983, pages 60-64.

(9)US patent N° 4,428,672 Variable threshold workpiece examination, filed on October 15, 1981 and published on January 31, 1984. Inventors: Martin Edwin Allard and Joseph Augustine Willcox. Assignee: Brent Chemicals International Plc, Ridgeway, Iver SLO 9JJ, Buckinghamshire (Great Britain).

(10)Pierre CHEMIN, La lecture automatique des indications de défauts mis en évidence par ressuage fluorescent (Editor’s note, The automatic indication reading of defect detected by fluorescent penetrant testing). Revue Pratique du Contrôle Industriel. Éditions Ampère, Paris, N°143 bis March 1987.

(11)US patent N° 4,536,654 Device for detecting flaws on a piece, filed on April 21, 1983 and published on August 20, 1985. Inventor: Jean Fernand Vaerman (11 April 1930 – 13.décember 2009). Assignee: Société Nationale d’études et de Construction de Moteurs d’Aviation, Paris, France.

(12)European patent N° 0,093,636, Dispositif de détection de défauts sur une pièce (Editor’s note: Device for detecting flaws on a piece), filed on April 20, 1983 and published on June 25,1986. Inventor: Jean Fernand Vaerman. Assignee: Société Nationale d’études et de Construction de Moteurs d’Aviation, Paris, France.

(13)Yen Fwu Cheu, Automatic Crack Detection with Computer Vision and Pattern Recognition of Magnetic Particle Indications, The monthly journal Materials Evaluation (Volume 42. No 12, pages 1506-1510, November 1984) of the ASNT (American Society for Nondestructive Testing), PO Box 28518, 1711 Arlingate Lane, Columbus, OH 43228-0518, USA.

(14)Yen Fwu Cheu, Automatic Crack Detection with Computer Vision and Pattern Recognition of Magnetic Particle Indications, 11th World Conference on Nondestructive testing, November 1985, Las Vegas (Nevada), USA.

(15)Charles H. Armstrong High Defect-Resolution Capability from a Computer-Controlled Fluorescent Penetrant Processing and Viewing System. The monthly journal Materials Evaluation, Vol. 44, No. 12, November 1986, pp. 1426–1429 of the ASNT (American Society for Nondestructive Testing), PO Box 28518, 1711 Arlingate Lane, Columbus, OH 43228-0518, USA.

(16)Franz Feil and Klaus Goebbels Automation of Surface Defect Detection and Evaluation with Liquid Penetrants: Development and Industrial Application. The monthly journal Materials Evaluation Vol. 45, No. 7, July 1987, pp. 838–840 of the ASNT (American Society for Nondestructive Testing), PO Box 28518, 1711 Arlingate Lane, Columbus, OH 43228-0518, USA.

(17)Pierre Chemin and Patrick Dubosc, The wipe-off technique, DPCNewsletter N° 020, January.

(18)DeLiberato, Tony J. ; Perkins, Steven W. ; Saplin, Steven K. ; Snyder, John G. ; Toussaint, Gregory, Air Force Institute of Technology Wright-Patterson, Air Force Base (AFB), Ohio, USA, School of Engineering. Integrated Blade Inspection System (IBIS) Upgrade Study.

(19)French patent 98 10062 Procédé et dispositif pour le contrôle non destructif de l’état d’une surface au moyen d’un produit colorant (Editor’snote : Method and apparatus for the non-destructive testing of the condition of a surface using a dye material ) filed on August 5, 1998 and published on February 11, 2000. Inventor: Pierre-Marie Paillotet. Patent agent: Regimbeau.

(20)US patent 6,556,298 B1 Method and system for non-destructive dye penetration testing of a surface filed on April 5, 1999 and published on April 29, 2003. Inventor: Pierre-Marie Paillotet. Assignee: Holores Inc., Port St. Luae, Florida (USA).

Last Updated ( Friday, 19 September 2014 14:24 )