WO1997014932A1

WO1997014932A1 - Process and device for the measuring of a three-dimensional shape

Info

Publication number: WO1997014932A1
Application number: PCT/SE1996/001329
Authority: WO
Inventors: Hans ÅHLÉN
Original assignee: Optronic Consult Ab
Priority date: 1995-10-20
Filing date: 1996-10-18
Publication date: 1997-04-24
Also published as: SE9503701L; SE505305C2; SE9503701D0

Abstract

The invention relates to a method and a device for the measuring of the shape of an object (1) to be measured, where the object to be measured has its surface supplied with fluorescent substances in the shape of a pattern (2; 10). The pattern is read by optical reading units (3; 4; 14, 15-19) by reading the same parts of the pattern from mutually different directions. The positions in space of the read-off parts of the pattern are calculated based on the measuring data for the reading units.

Description

Process and device for the measuring of a three-dimensional shape

The present invention relates to a method for measuring of the type which is given in the preamble to claim 1, and a device for performing the method.

TECHNICAL BACKGROUND

With the possibilities to process, transfer and present information concerning measured objects, demands for an effective reading-in of information about the shape of a measured object for the purposes of data processing are increasing. Examples of industrial and medical applications, where such reading-in is advan¬ tageous, are the measuring of models for transferring to CAD-computers (CAD -= Computer Aided Design), checking of the shapes of prototypes and finished products, measuring ofthe shapes of body parts for diagnostic purposes or for later fitting of prostheses, implants etc.

Today's technologies for contactless measuring of a three-dimensional shape are often based on some form of triangulation, where the object to be measured is illuminated with a structured light or by letting a laser point or a laser line scan the object. In these methods the optical characteristics of the measured surface influence the results ofthe measuring. It is, for example, difficult to measure on transparent, highly polished or "milky" surfaces.

Another relatively common way of performing measuring, described e.g. in US-A- 5 305 151, is by means of interferometry, and consists of projecting a grid pattem on the surface of the object to be measured. The grid pattems appear different from different directions and distances. The grid pattem is registered simultaneously by two cameras from two directions. The fringe phases on the pattems registered by the two cameras are computed separately. The coordinates of the object are computed by a combination of the computed fringe phases.

SUBSTITUTE SHEET According to another method for measuring, described in the Swedish Laid-open document 8902748-6, a pattem is projected onto the object to be measured. A pic¬ ture of the pattem on the object to be measured is taken and the projected pattem and the pattem in the picture are compared in order to obtain the three-dimensional geometry of the measured objects, which in this case are teeth in the oral cavity.

The problem involved in projecting pattems is that all the measuring elements comprised in a measuring arrangement must be extremely stably arranged in relation to each another during the measuring, especially the pattem producer, both in rela- tion to the measured object and to the recording means.

The measuring of measuring objects in order to get exact data concerning the shape of a measured object also often takes place by contact measuring in a coordinate measuring machine. SE-9000546 describes an optical method for contactless measuring ofthe shape of an object to be measured with the help of a coordinate machine. In this case an optical system is placed as a measuring probe in the coordi¬ nate measuring machine. The object to be measured is at least partially covered with a fluorescent substance, which is excited by a light beam emitted from the optical system. The optical system has detector means which are sensitive to the fluorescent light. This system gives the advantage that the surface layer of the object to be measured is well defmed for the optical system but it requires an exact measuring with a coordinate measuring machine.

OBJECTS OF THE INVENTION

An object of the invention is to achieve the measuring of the three-dimensional shape of an object to be measured using stereo- and photogrammetric methods.

Another object of the invention is to achieve the measuring of polished and or transparent objects to be measured.

SUBSTITUTE SHEET Yet another object of the invention is to achieve the measuring of an object to be measured with simple subsequent processing of the measuring data.

Yet another object ofthe invention is to achieve the measuring of confined spaces, such as different types of cavities. For this purpose a very small reading unit is necessary.

Yet another object of the invention is to achieve the measuring of an object to be measured by a simple matching of measuring data taken from several observation angles.

Another object of the invention is to achieve the measuring of an object to be measured with a simple and cheap measuring head.

Yet another object of the invention is to achieve a device for measuring, where the measuring device can be placed in a random position in relation to the object to be measured and where measurements taken at different times will be equivalent without the position of the measuring device at the time of measuring influencing the measuring result.

The above mentioned objects are achieved by a method with the characteristics stated in claim 1. Further features and developments ofthe invention as well as a device for performing the method are mentioned in the rest of the claims.

The invention thus relates to a method for measuring the shape of an object to be measured, where the surface of the object to be measured is provided with a fluor¬ escent substance, characterized by the fluorescent substances being applied to the surface of the object to be measured so that the surface receives a pattem; the pattem is read-off by optical reading units with reading-off of the same parts of the patte from mutually different directions; and the positions in space for the read-off patte parts are calculated with the aid of the measuring data from the reading units.

SUBSTITUTE SHEET A pattem formed by fluorescent substances gives a high contrast between pattemed and non-patterned surface parts. The measuring data for the reading units can include their respective reading positions in the reading unit or reading units, the respective positions and the respective reading angles of the reading unit or reading units during reading-in. The pattem is naturally adapted to the reading means so that it is clearly indicatable by it. The pattem can be regular, e.g. comprise a grid net¬ work, or irregular, e.g. comprising randomly placed dots, flakes or the like.

A device according to the invention for measuring of the shape of an object to be measured, where the surface of the object is equipped with a fluorescent substance, includes that the fluorescent substances are provided in the form of a pattem, at least one optical reading unit for the reading off of the same pattem parts of the object to be measured from mutually different directions, and a comparison means is connec¬ ted to the reading unit or reading units for comparing the different readings with each other and individually calculate the position in space of the read-off pattem parts with the aid of differences between the readings for these pattem parts.

The invention is described in more detail below with reference to the appended drawings, where Fig. 1 shows a schematic view of a first embodiment of a device according to the invention, and Fig. 2 shows a schematic view of a second embodiment of a device according to the invention, and Fig. 3 shows schematically an embodiment of an instrument for investigating a plurality of objects to be measured simultaneously.

Fig. 1 shows an object to be measured 1, which can be transparent or have a highly polished or milky surface, from which it is difficult to get an exact measuring result. It should, however, be noted that the invention is not limited to measurements made on objects to be measured with these characteristics but is also suitable for measurements of objects to be measured with any type of surface whatsoever. The

SUBSTITUTE SHEET surface of the object 1 to be measured in the embodiment shown is equipped with a pattem 2, which in the embodiment shown is checkered, but can have any shape whatsoever, e.g. dotted, dashed, wavy, etc.

Measuring against a surface with a fluorescent coating has proven to give good and exact results. The pattem on the surface can consequently be applied with fluor¬ escent paint, which is the preferred embodiment. The pattem can, for example, be applied by spraying or coating the object to be measured with the fluorescent sub¬ stances or by painting the object to be measured with the fluorescent substances with a painting means with a small contact surface, such as a pen or a thin paintbrush. The pattem can also be applied by taping the object to be measured with a fluorescent tape.

The pattem can also be applied by first coating the surface of the object to be measured with a layer to which fluorescent particles can easily adhere, e.g. a sticky surface, and subsequently applying fluorescent particles.

An example of another possibility is to provide the whole surface of the object to be measured with a film of fluorescent paint and applying the pattem over this film with a paint with different qualities, e.g. black and matt.

Another possibility for applying the pattem is to dip the object to be measured into a fluid containing nonfluorescent as well as fluorescent parts, for example to use a fluorescent paint with transparent microspheres which then form the pattem, or a paint with microspheres with fluorescent paint. The pretreatment of the object to be measured can in these cases simply comprise dipping into the paint.

The pattem can also be achieved by spraying fluorescent particles, e.g.microspheres, onto the surface of the object to be measured. The pattem can also occur as flakes in a paint. The flakes can be fluorescent in a nonfluorescent paint or vice versa.

SUBSTITUTE SHEET The pattem can also occur in the material of the object to be measured itself, which in that case is pattemed all the way through. Measuring can, for example, take place on a positive or negative impression of e.g. teeth in order to get CAD-inf ormation (CAD = Computer Assisted Design) for subsequent data processing of the appear- ance of the teeth and subsequently following Computer Assisted Manufacturing (CAM). The impression can for this purpose be cast in a through-patterned material which gives a surface with a suitable fluorescent pattem.

It is also possible to use phototechnical methods. One such method involves apply- ing a film of a light-sensitive or heat-sensitive material, which does not support fluorescent paint, exposing a pattem to light or heat with phototechnics and removing the exposed pattem parts, dipping the object to be measured in the fluorescent paint, which only fastens to the parts of the object to be measured with the uncovered pattem parts, and possibly exposing the remaining film to light or heat and removing it. The above mentioned ways for applying a fluorescent pattem are only given as examples and other ways of application are also conceivable for the man skilled in the art under the guidance of the above.

It is evident from that described above that both regular and irregular pattems can be used according to the invention.

The embodiment shown in Fig. 1 shows two cameras 3 and 4 directed towards the object 1 to be measured. Each camera is equipped with an electronically readable picture sensor in its picture plane. The cameras are preferably of a type with a sensor surface with discrete picture elements, so-called pixels. The picture sensor can be in that case, for example, of the CCD-type (CCD = Charge Coupled Device, i.e. it makes a charge-coupled transmission). It is obvious to the man skilled in the art that another type of picture sensor with discrete picture elements or individually readable sensors can also be used. Altematively, the cameras can be of a type where a scan- ning optic transfers a small partial picture of an object to be measured to a surface- contact-sensing sensor, which gives a signal dependent on the coordinate position of

SUBSTITUTE SHEET the contact point on the sensor, for example a two-dimensional sensor known by the trademark SITEK^®.

Instead of having two cameras, a single camera can be used. This can be moved between two different positions, or the object can also be moved between two exposures with a stationary camera.

Independently ofthe type of camera and carried out measuring operations, the signals from the cameras are transferred to a processing and comparison unit 5. If two cameras are used, this transfer can take place simultaneously. If one camera is used, these transfers take place in sequence one after the other with intermediate storage ofthe first transferred picture. The signals are treated such that only the parts ofthe exposed pictures which correspond to pattem parts on the object to be measured are isolated, and the rest of the picture parts are consequently discrimi- nated away.

The use of fluorescence in connection with the pattem on the object gives a high contrast for the sensed pattem, which gives a secure measuring and a simple isola¬ tion of the corresponding pattem parts. Furthermore, the placing of the pattem directly on the object to be measured gives the advantage that it is stable on the object to be measured so that, without influencing the measuring results, the object to be measured can be freely moved, and also different measuring exposures can be made at different times. Furthermore, measuring of an object can be repeated at different times, between which the shape ofthe object may have changed, for example, it could have been deformed after having been exposed to temperature changes or other types of changes in the surroundings. The measuring device gives sufficient information from the object to be measured in order to give an over- determination ofthe measuring result. This can be used for extra exact positioning ofthe component measuring units in relation to the object to be measured and to each other. This makes the measuring device according to the invention robust and relatively insensitive to how it and its units are placed in relation to the object to be measured during each measuring.

The pattem parts taken with the two cameras 3 and 4 are subsequently compared to each other, pixel for pixel, in the unit 5. For each compared pixel the difference in position on the picture sensor in the two cameras is transferred to a unit 6 for calcu¬ lating the position in space for the point on the object to be measured which corre¬ sponds to the pixel in question. A calculation of the position in space takes place with the aid of knowledge of the respective angle of direction ofthe two cameras in relation to the actual pixel, the distance between the cameras and the respective positions ofthe pixels in the respective picture plane in the cameras, i.e. the position in space calculations take place with so-called triangulation pixel for pixel in the pattem on the object to be measured. It should be noted here that the direction of each camera for the different pixels in the picture sensor can be different but is completely individually known and written in a nonvolatile memory in the picture processing and comparison unit 5.

The current alignment of each camera 3, 4, with the object 1 to be measured, is measured. The alignment of each pixel with the object to be measured is then consequently a combination of the current alignment of the camera and the indi¬ vidual alignment of the pixel in relation to that ofthe camera. According to the invention a photogrammatic method is thus used.

The calculated positions in space are now determined in relation to a fixed point on the measuring arrangement. Subsequently, the calculating unit 6 can perform a transforming of the coordinates in order to place the three-dimensional shape of the object obtained by the measuring into a desired position in a suitable coordinate system. A suitable mathematical representation ofthe shape of the object to be measured is consequently made by the calculating unit 6. This can naturally also comprise other calculations than those given which are known to the man skilled in the art, especially in the art of the technical field for picture processing.

SUBSTITUTE SHEET The result of the calculations made by the position in space calculating unit 6 is transferred to a presentation unit 7, e.g. in the form of a list printed out on a printer, a three-dimensional presentation on a screen, an entry in a computer program of CAD-type with a presentation on a screen, possibly with the possibility to make changes in order to produce a shape which functions better in a particular connection or with adjustment of a mould or the like to the measured shape of the object 1 to be measured.

Fig. 2 shows the principle for a second embodiment. According to this, only one camera is used for taking the, at least two, requisite pictures of an object to be measured. In this embodiment the object 9 to be measured is shown, as an example, provided with a dotted irregular pattem 10, for example achieved through dipping the object to be measured in a paint which contains the pattem points as differing parts in the paint.

The embodiment shown in Fig. 2 is carried out in a way adapted for stereo- measuring especially in confined spaces, for example, measuring of a small hole in an inorganic or organic object in order to investigate a casting, or measuring in a person or animal of the auditory meatus in an ear, in the gullet, in the lungs or the like.

A bunch of fibers 11 are placed with one end 14 on the spot where the picture sensor is to be placed. A sensor plate (not shown) with a sensor element for each light- conducting fiber in the bunch of fibers 11 is placed at its other end. The bunch of fibers 11, which acts like a picture light-conductor, is surrounded by a tube-like, stiff casing 12, which has a part 13 which extends a suitable distance A in front of the surface 14. The outer end of part 13 is equipped with optics 15, which in principle can consist of a number of "camera eyes" placed beside each other along a surface, and controllable so that only one eye at a time is open. The surface can be flat or slightly curved such that each picture ofthe object to be measured through the different "camera eyes" is focussed onto the surface 14.

TITUTE SHEET Instead of forming the end of a bundle of optical fibers, a picture sensor can be placed in the same position as 14. The sensor has then a surface extension transverse to a line of symmetry for the camera and all its pixels can sense a partial picture, for example it can have a separate picture sensor element per partial picture or be a so- called CCD-sensor (CCD = Charged Coupled Device).

The reproducing optics 15 can comprise two or more fixed lenses 16, 17, 18, which from different positions reproduce the object to be measured (not shown) on the end surface 14 of the picture optical conductor. In order to ensure that just the picture from one lense at a time reaches the end surface, each lense is equipped with a shutter in the beam path (only schematically shown as a plate 19 in front of the lenses). The shutter can be mechanical or micromechanical, e.g. a rotating plate with one or more holes where only one hole at a time can be in front of one ofthe lenses 16-18, or it can be a unit with a non-movable shutter function and consist of a plate with controllable LCD-type elements, which can be controlled to optionally open and close the beam path from the object. Instead of lenses, small holes can be used so that the arrangement acts as a pinhole camera. This gives a worse resolution than with lenses but is considerably cheaper. In such a case the elements 15 and 19 can possibly be replaced by a plate with small controllable LCD-type elements, which each can be controlled to form an optical opening.

It is also possible to have a movable lense (not shown in Fig. 2), which is control¬ lable to change position between the exposure of each picture of the clear patte formed by applying the fluorescent pattem parts in or on the object to be measured.

Instead of opening just one camera eye at a time, the plate 19 can have differently coloured apertures. Each sensor element in the remote end (not shown) of the bunch of fibers 11 can comprise several partial elements, each of them individually sensi¬ tive only to one of the colours in the apertures or, altematively, several CCD- sensors, each being sensitive to an individual colour, can be coupled to the end of

SUBSTITUTE SHEET the bunch of fibers. Altematively, a CCD-sensor with possibility of individual sensitivity to different colours can be used. In this case the taking and the storage of the pictures for the comparisons can take place simultaneously instead of sequentially, as can also the comparisons themselves.

As the arrangement according to Fig. 2 functions as a pinhole-camera with optional openings, overlapping ofthe pictures from the object on different parts of the surface 14 occurs, depending on which of the lenses are used.

A shorter projecting part 13 gives a bigger overlapping surface per pair of lenses or holes, which means that fewer lenses or holes can be used and the position in space calculations can be carried out between a small number of overlapping pictures. However, this gives a wider sensing of the object, which is not always desirable during investigation of a small cavity such as a bore hole in a tooth or the like, where the investigating instrument is placed outside the cavity.

A longer projecting part 13 gives a smaller overlapping surface per pair of lenses or holes. The viewing angle per exposed picture will thus be smaller. Thus, the position in space calculations should in most cases be performed by comparison inside the overlapping regions for several pairs of pictures, which requires the storage of more pictures during a picture-taking procedure.

The same type of picture processing, comparison, calculating and presentation units 5-7 as those which are described in connection to Fig. 1, can be used also in the embodiments according to Figs. 2 and 3 and are therefore not shown. However, in most cases, data concerning the pictures taken is transferred in sequence one after the other instead of simultaneously to the picture processing and comparison unit 5, which therefore requires extra memory for intermediate storage of a picture (not shown).

SUBSTITUTE SHEET As mentioned above, the measuring is perfoπned against objects with pattems comprising fluorescent parts. An mstrument comprising a camera of the type shown in Fig. 2 can thereby be equipped with a spray paint nozzle which possibly is extend-able during use (not shown in Fig. 2), either by the side of or concentric with it, or several spray painting nozzles can be fitted around its periphery. Before measuring of an object to be measured a spray painting function is activated so that the object to be measured is provided with its pattem. The measuring is performed subsequently using the camera.

The embodiment shown in Fig. 3 of a measuring instrument is especially usable for flexible positioning by an object to be measured, which can be relatively difficult to approach, for the measuring of it, such as a tooth inside a mouth of a patient and especially for the measuring of a bore hole in a tooth. Here a measuring head 20 with the camera equipment shown in Fig. 2 can be placed at the end of a control means 21. In Fig. 3 the control means 21 is shown as a handle, which is easily maneuver-able by an operator and of which the outer end 20 with the camera equipment is slightly curved in order to give approachability in confined spaces. The control means 21 can instead be controllably bendable and/or also a controllable arm can possibly be connected to the outer end 20 itself in order to control it, and in this case the optical fiber bundle 11 can come directly from the end 20. A controllable light source can also be placed by the measuring head 20 to illuminate the object to be measured during measuring.

The picture collecting end 14 in Fig. 2 is placed inside the outer end 20 in Fig. 3. The fiber bundle 11 is bendable outside the rod 32 and coherently arranged between the end 14 in the measuring head and an electronically readable picture sensor 22. as a second target point detector unit, placed at a distance from the measuring head 20.

The electronically readable picture sensor 22 is coupled to evaluation circuits 23 of the above described type.

SUBSTITUTE SHEET Ln the case of investigating teeth, the teeth are coated before measuring with a coat¬ ing of fluorescent particles, which can be brushed off. This coating can take place by activation of a spray-painting equipment connected to the measuring head 20 and comprising a controllable paint unit 24 coupled by a hose 25 to at least one nozzle (not shown) in the measuring head. It is appropriate that there are anangements (not shown) on the measuring head in order to protect the optics during the spray- painting operation controlled from the unit 24, such as projecting out the nozzle or nozzles and/or temporarily closing of the opening to the optics. It is, however, obvious that the application of the pattem onto the object to be measured can instead be performed separately, and that an instrument without the parts 24, 25 can be used for the actual measuring.

The appropriate pictures are taken and are signal-processed, whereafter the coating is brushed and rinsed away.

Many modifications of the invention are possible within the scope given by the appended patent claims. For example, the measuring of two- as well as three- dimensional surfaces are equally possible. More than two recording units with different positions or altematively mixers with more than two sequentially openable apertures can be used.

It should be noticed that the arrangements shown give more data concerning all the investigated points on the object than what is necessary in order to calculate the shape of the object, which means that the shape can be over-determined. In this way there is sufficient information in order to produce completely equivalent recordings for determining the shape at different opportunities with very different positionings of the measuring instrument in relation to the object to be measured, and also con¬ cerning the constituent components in the instrument, such as cameras 3 and 4 in the embodiment in Fig. 1, or using different types of viewing holes as in the embodi- ments in Figs. 2 and 3. The different types of calculations which are necessary are completely geometrical and are therefore not described. A man skilled in the art

SUBSTITUTE SHEET should be completely intimate with how these are performed and how they can be programmed into a computer.

SUBSTITUTE SHEET

Claims

1. Method for measuring ofthe shape of an object to be measured (1), where the object to be measured is equipped on its surface with fluorescent substances, characterized in that the fluorescent substances are applied to the surface of the object to be measured so that the surface receives a pattem (2; 10); that the pattem is read by optical reading units (3,4; 14, 15-19) with reading of the same parts of the pattem from mutually different directions; and that the positions in space ofthe read-off parts ofthe pattem are calculated based on the measured data from the reading units.

2. Method according to Claim 1, characterized in that the measuring data for the reading units comprise their respective reading positions in the reading unit or reading units, the respective positions of the reading unit (14, 15-19) or reading units (3, 4) and the respective reading directions during reading.

3. Method according to Calim 1 or 2, characterized in that the pattem is applied by spraying or painting the object to be measured with the fluorescent substances.

4. Method according to Claim 3, characterized in that the pattem is applied by painting the object to be measured with the fluorescent paint substances with a painting means with a small contact surface, such as a pen or thin paintbrush.

5. Method according to Claim 1 or 2, characterized in that the pattem is applied by dipping the object to be measured in a fluid which contains non-fluorescent as well as fluorescent parts.

6. Method according to Claim 1 or 2, characterized in that the pattem is applied by taping the object to be measured with a fluorescent tape.

SUBSTITUTE SHEET

7. Method according to Claim 1 or 2, characterized in that the pattem is applied by the surface of the object to be measured first being coated with a layer to which fluorescent particles easily stick, and subsequent application of fluorescent particles.

8. Method according to Claim 1 or 2, characterized in that the pattem is applied by phototechnical means.

9. Device for measuring of the shape of an object to be measured (1), where the object to be measured is provided with fluorescent substances, characterized by a) the fluorescent substances applied in the form of a pattem (2; 10); b) at least one optical reading unit (3, 4; 14, 15-19) for reading of the same parts of the pattem on the object to be measured from mutually different directions; c) calculating means (5, 6; 23) connected to the reading unit or reading units in order to compare the different readings with each other and individually calculate the position in space of the read-off parts of the pattem based on differences be¬ tween the readings for these parts of the pattem.

10. Device according to Claim 9, characterized in that the reading units (3, 4) number at least two and are cameras, each with electronic target point detection of each part of the pattem in the picture plane of the camera.

11. Device according to Claim 9 or 10, characterized in that there is only one reading unit (15-19) but it is equipped with optics for reading of the target point detection, where the optics are equipped with a controllable unit ( 19) in order to provide each reading with mutually different optical paths.

12. Device according to Claim 11, characterized in that a picture formed by the reading unit via an optical fiber bundle (11) is connected to a target point detector unit (22), whereby the target point values are electronically readable; and that the input of the comparison unit (23) is connected to the target point detector unit.

SUBSTITUTE SHEET

13. Device according to Claim 12, characterized in that the reading unit is fitted into a reading head arrangement (20) adapted for flexible positioning by the object to be measured, and that the target point detector unit (22) is placed stationary.

SUBSTITUTE SHEET