Retroreflective target and laser rangefinder device comprising such a target

Abstract

Retroreflective target (100) for a laser rangefinder (1), comprising a spherical reflector (120) and a support (110) on which the reflector is fixed, characterized in that a lens (130) is attached to the support, the lens comprising a spherical face intended to make point contact with a surface (S) of an object (M) to be geometrically characterized, the reflector and the lens extending on either side of the support, and the center of the reflector and the center of curvature of the spherical face of the lens coinciding. Laser rangefinder (1) comprising at least one laser tracker (4) and such a retroreflective target. FIGURE IN ABRIDGED DIAGRAM: Fig. 2

Description

Title of the invention: Retroreflective target and laser ranging device comprising such a target

[0001] The present invention relates to the field of three-dimensional metrology, and more particularly to a retroreflective target for a laser telemetry device. The invention also relates to a laser telemetry device equipped with such a target.

[0002] BACKGROUND OF THE INVENTION

[0003] Three-dimensional metrology is used to geometrically characterize an object by determining the coordinates of points on the surface of the object.

[0004] Three-dimensional metrology machines are known which comprise one or more laser transmitters / receivers (commonly called laser trackers in this application) each emitting a laser beam in the direction of a retroreflective target in point contact with the object.

[0005] When it is based on the use of a single laser tracker, the determination of the coordinates consists of measuring the orientation angles of the laser beam and determining the distance traveled by the laser beam between a reference point of the laser tracker and a center of the retroreflective target by measuring the time taken by said laser beam to make the round trip between the reference point and the center of the target.

[0006] When it is based on the use of several laser trackers, the determination of the coordinates is carried out according to the principle of multilateration: the retroreflective target is targeted simultaneously by the laser trackers which each determine the distance separating it from the retroreflective target, a mathematical processing allowing the recovery of the coordinates of said retroreflective target.

[0007] The retroreflective target generally comprises a spherical reflector, usually made of glass with a refractive index substantially equal to 2, the fragility of which prevents any direct contact with the object, particularly when the surface of the latter is a rough surface of a very hard material, such as silicon carbide. Such surfaces are commonly encountered on telescope mirrors being polished, an application in which the precision of the surface shape is essential and the use of three-dimensional metrology is common.

[0008] To overcome this drawback, it is known to mount the reflector in a support which will make contact with the surface of the object.

[0009] Thus, in SMR (Spherically Mounted Retro-reflector) type targets, the support has the shape of a truncated sphere in which the spherical reflector is placed. The geometric relationship between the center of the reflector and the point of contact with the surface of the object to be inspected then depends on the concentricity between the reflector and the support in contact with the object.

[0010] While it is industrially possible to obtain a concentricity of less than 5 micrometers, the spherical support is generally made of steel or aluminum. However, such a material is not hard enough to withstand, without damage, direct and repeated contact with the rough surfaces of the hardest materials, such as those of the mirrors being polished mentioned above. Indeed, in such a situation, the support might not maintain an acceptable surface condition even for a single measurement cycle. It would be technically feasible to make the support from a harder material, but the cost would be prohibitive.

[0011] Pneumatic supports are also known which are arranged to create an air cushion beneath them. While they have the advantage of avoiding direct contact with the surface of the object, they prove not to be suitable for measuring non-planar and particularly spherical surfaces.

[0012] SUBJECT OF THE INVENTION

[0013] The aim of the invention is to propose a retroreflective target making it possible to at least partially overcome the aforementioned drawbacks. Summary of the invention

[0014] To this end, according to the invention, a retroreflective target is provided comprising a spherical reflector and a support on which the reflector is fixed. A lens is attached to the support and comprises a spherical face intended to make point contact with a surface of an object to be geometrically characterized. The reflector and the lens extend on either side of the support, and the center of the reflector and the center of curvature of the spherical face of the lens are coincident.

[0015] Only the lens is intended to come into contact with the object to be characterized: it can therefore be made of a very hard material. The rest of the support, which is not intended to come into contact with the object to be characterized, can be made of a less hard material than that of the lens. It will be noted that the shape of the lens allows point contact with the object even if the surface of the latter is not flat.

[0016] Furthermore, the concentricity of the lens and the reflector makes it possible to establish a geometric relationship between the center of the reflector and the spherical face of the lens, and therefore to measure the three-dimensional coordinates of the surface without any contact between the reflector and said surface, in other words without causing mechanical wear of the reflector.

[0017] According to a particular characteristic, the retroreflective target comprises means for adjusting the position of the reflector with respect to the lens to allow the concentricity of said reflector and the lens.

[0018] In particular, the support comprises a fixed plate on which the lens is mounted, and a mount on which the reflector is mounted, the adjustment means comprising micrometric screws arranged to adjust the position of the mount with respect to the fixed plate in a median plane of said fixed plate.

[0019] In particular, the reflector is fixed to a reflector holder integral with the mount via a micrometric thread extending around an axis perpendicular to the median plane of said mount.

[0020] In particular, the reflector holder is generally cylindrical in shape and comprises an internal collar having an internal circumference of truncated cone shape on which the reflector rests and is held by gluing.

[0021] According to another particular characteristic, the lens is carried by a ring fixed to the fixed plate.

[0022] According to another particular characteristic, the lens is piano convex.

[0023] According to another particular characteristic, the lens is made of sapphire.

[0024] The invention also relates to a laser telemetry device comprising at least one laser tracker and such a retroreflective target.

[0025] In particular, the device comprises an articulated arm at one end of which the retroreflective target is mounted. Brief description of the drawings

[0026] The invention will be better understood in light of the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended drawings, among which:

[0027] [Fig-1] [Fig.l] is a simplified representation of a laser telemetry device;

[0028] [Fig.2] [Fig.2] is a perspective top view of the retroreflective target equipping the laser telemetry device illustrated in [Fig.l], according to a particular embodiment of the invention;

[0029] [Fig.3] [Fig.3] is a bottom perspective view of the retroreflective target illustrated in [Fig.2];

[0030] [Fig.4] [Fig.4] is an axial sectional view of the retroreflective target illustrated in [Fig.2] ;

[0031] [Fig.5] [Fig.5] is a top view of a portion of the retroreflective target illustrated in [Fig.2];

[0032] [Fig.6] [Fig.6] is a cross-sectional view of the retroreflected target portion deflector illustrated in [Fig.5]. DETAILED DESCRIPTION OF THE INVENTION

[0033] With reference to [Fig.l], the invention is described in application to a laser telemetry device 1 making it possible to characterize a surface geometrically with precision. S optics of a mirror M by multilateration.

[0034] The laser telemetry device 1 comprises a plurality of laser transmitters / receivers or laser trackers 4 (also called tracking lasers), and an articulated arm 2 having a free end on which a retroreflective target 100 is mounted via a connection interface 3. In a manner known per se, each laser tracker 4 emits a laser beam 4.1 towards the retroreflective target 100 which is brought into contact with the surface S of the mirror M by the articulated arm 2. The laser beam 4.1 is reflected by the retroreflective target 100 and follows the same path in the opposite direction to enter the corresponding laser tracker 4. The articulated arm 2 comprises segments connected by joints provided with encoders making it possible to know the angular position of the segments relative to each other and possibly motors making it possible to move the segments relative to each other to form a robot or a cobot.The laser trackers 4 and the articulated arm 2 are known in themselves and will not be described further here.

[0035] With reference to figures 2 and 3, the retroreflective target 100 comprises a support 110 on which are mounted a spherical reflector 120 intended to reflect the laser beams 4.1 of the tracker lasers 4 and a piano convex lens 130 intended to make a point contact with the surface S of the mirror M.

[0036] The support 110 here has the overall shape of a rectangular parallelepiped with a square base and comprises an upper face 111 and a lower face 112 connected to each other by lateral faces 113. The upper face 111 and the lower face 112 are generally square in shape (the corners are cut at 45 degrees) and extend parallel on either side of a median plane XY of said support 110. The lateral faces 113 are rectangular in shape and each connect an edge of the upper face 111 to an edge of the lower face 112. The median plane XY of the support 110 is defined by an axis X and an axis Y each extending perpendicular to two of the lateral faces 113 and which pass through a center On0 of said support 110.

[0037] With reference to figures 4 to 6, the support 110 comprises a central hole 114, of cylindrical shape, opening onto both the upper face 111 and the lower face 112 of the support 110. The hole 114 extends along an axis Z perpendicular to the median plane XY of the support 110, the axis Z passing through the center On0 of the support 110.

[0038] The support 110 also comprises two first grooves 115.1 and two second grooves 115.2 which extend perpendicular to the median plane XY and which open onto both the upper face 111 and the lower face 112: the first grooves 115.1 and the second grooves 115.2 are said to be through grooves.

[0039] The first grooves 115.1 and the second grooves 115.2 are concentric and delimit in the support 110: • a fixed annular plate 110.1, with a generally square external shape, comprising the side faces 113; • an external decoupling part 110.2, annular and of generally square external shape, extending inside the fixed plate 110.1 and attached to said fixed plate 110.1 by two first flexural blades 116.1; • an internal decoupling part 110.3, annular and of generally square external shape; extending inside the external decoupling part 110.2 and attached to said external decoupling part 110.2 by two second flexural blades 116.2.

[0040] The first flexing blades 116.1 are arranged to allow relative mobility, along the Y axis, between the fixed plate 110.1 and the external decoupling part 110.2, and the second flexing blades 116.2 are arranged to allow relative mobility, along the X axis, between the external decoupling part 110.2 and the internal decoupling part 110.3: the first grooves 115.1 and the second grooves 115.2 are called decoupling grooves.

[0041] The precise positioning, along the X axis, of the internal decoupling part 110.3 with respect to the fixed plate 110.1 is achieved via two first micrometric screws 140.1 with ball extending projectingly, along the X axis, on either side of the fixed plate 110.1. Each of the two first micrometric screws 140.1 is screwed into a first threaded ring 141.1 which is tightly mounted in a central orifice provided on one of the lateral faces 113 of the fixed plate 110.1, the balls equipping the ends of the first micrometric screws 140.1 being able to bear against a lateral face of the internal decoupling part 110.3 and thus exert, along the X axis, a thrust force on said internal decoupling part 110.3.

[0042] The precise positioning, along the Y axis, of the internal decoupling part 110.3 with respect to the fixed plate 110.1 is achieved via two second micrometric screws 140.2 with ball extending projectingly, along the Y axis, on either side of the fixed plate 110.1. Each of the two second micrometric screws 140.2 are screwed into a second threaded ring 141.2 which is tightly mounted in a central orifice provided on one of the lateral faces 113 of the fixed plate 110.1, the balls equipping the ends of the second micrometric screws 140.2 being able to bear against a lateral flank of the internal decoupling part 110.3 and thus exert, along the Y axis, a thrust force on said internal decoupling part 110.3.

[0043] The fixed plate 110.1 of the support 110 is fixed to the free end of the articulated arm 2 via the connection interface 3.

[0044] The internal decoupling part 110.3 of the support 110 is said to be mobile and delimits the hole 114 in which a tubular mount 150 of cylindrical shape is mounted, substantially of axis Z. The mount 150 extends projecting from the upper face 111 of the support 110 and is provided externally with a collar 151 for fixing to the part internal decoupling part 110.3, such that said mount 150 is fixed relative to said internal decoupling part 110.3.

[0045] The mount 150 also comprises an end provided internally with a micrometric thread 152 via which a reflector holder 160, generally of cylindrical shape, is screwed along the Z axis into the mount 150. The precise positioning, along the Z axis, of the reflector holder 160 with respect to the mount 150 is achieved via the screwing of said reflector holder 160 into the mount 150.

[0046] As illustrated in [Fig.6], the reflector holder 160 comprises a first external groove 160.1 in which is housed a seal 161 in contact with the mount 150, and a second external groove 160.2 arranged to receive one end of a needle screw 153 screwed into a radial hole provided in the mount 150. The seal 161 and the needle screw 153 make it possible to secure the reflector holder 160 in a sealed manner in the mount 150.

[0047] The reflector holder 160 also comprises an internal collar 162 having an inner circumference of truncated cone shape on which the spherical reflector 120 rests and is held by gluing. The reflector 120 extends in axial projection from the reflector holder 160 so as to offer a large angular acceptance for the aiming of the laser trackers 4, and is here made of glass with a refractive index n substantially equal to two.

[0048] With reference to Figures 3 and 6, the retroreflective target 100 further comprises a ring 170 carrying the lens 130 whose spherical face is intended to be in point contact with the surface S of the mirror M to be inspected. A locknut 171 and a polyurethane film 172 make it possible to tightly secure the lens 130 on the ring 170. The seal 161 and the polyurethane film 172 prevent dirt from entering the micrometric thread 152. The lens 130 is here made of sapphire whose high hardness makes it possible to offer a durability much greater than that of steel or aluminum, and therefore to allow a high number of contacts with the surface S of the mirror M without wear detrimental to the measurement, even if the surface S is rough.

[0049] The ring 170 is fixed on the fixed plate 110.1 of the support 110 and is arranged so that the flat face of the lens 130 extends substantially parallel to the median plane XY, the center of curvature Ono of said lens 130 being substantially coincident with the center Obo of the spherical reflector 120.

[0050] The concentricity of the reflector 120 and the lens 130 is made possible by a fine adjustment of the position of the reflector 120 with respect to the fixed plate 110.1: • along the X axis, via the first micrometric screws 140.1 with here a precision of the order of a micron, • along the Y axis, via the first micrometric screws 140.2 with here a precision of the order of a micron, and • along the Z axis, via screwing the reflector holder 160 into the mount 150 with here a precision of the order of 3 microns.

[0051] This concentricity makes it possible to establish a geometric relationship between the center of the reflector 120 and the point of contact of the lens 130 on the surface of the mirror M, the radius of curvature Rno of the lens 130 being measured with a precision of less than one micrometer.

[0052] It will be noted that the lens 130 can be easily replaced by another lens when its wear due to repeated contact becomes noticeable. The concentricity of the reflector 120 and the new lens 130 is then restored by a new adjustment of the position of said new lens. The effect of the wear of the lens 130 is thus treated by changing only the lens 130, the cost of which is moderate compared to that of the reflector 120.

[0053] The retroreflective target 100 can be moved along the surface S of the mirror M via the articulated arm 2 in order to acquire the coordinates of different points of said surface S via the laser trackers 4. The docking of the retroreflective target 100 on the surface S is done according to the theoretical normal of said surface S so as to minimize the impact of any residual error in positioning the reflector 120 with respect to the support 110.

[0054] The connecting interface 3 connecting the retroreflective target 100 to the end of the articulated arm 2 comprises a contact force sensor between the lens 130 and the surface S of the mirror M. The force sensor acts as a safety device stopping the docking movement of the retroreflective target 100 if the force measured by said sensor is greater than a predetermined value.

[0055] The lifetime of the lens 130 can be increased by carrying out a DLC (Diamond Like Carbon) treatment of its spherical face. The hardness obtained by such a treatment is greater than that of sapphire, which makes the lens 130 less sensitive to wear caused by repeated contact with the surface S.

[0056] Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0057] Although the reflector is spherical here, it can also be partially spherical or mounted on a sphere (SMR type, from the English “Spherically Mounted Retroreflector”).

[0058] Although the lens is here piano convex, it can also be of another shape as long as it includes a spherical face.

[0059] Although the laser telemetry device 1 here comprises a plurality of laser trackers 4 each measuring only the distance traveled by the target (multilateration inspection), it may comprise only a single laser tracker 4 measuring the distance traveled by the laser beam but also the two orientation angles of said laser beam (for example via angular encoders).

[0060] Although the surface S to be inspected is that of a mirror (M), it can be of any other nature.

Claims

Claims

1. Retroreflective target (100) for a laser telemetry device (1), comprising a spherical reflector (120) and a support (110) on which the reflector is fixed, characterized in that a lens (130) is attached to the support, the lens comprising a spherical face intended to make point contact with a surface (S) of an object (M) to be geometrically characterized, the reflector and the lens extending on either side of the support, and the center of the reflector and the center of curvature of the spherical face of the lens being the same.

2. Retroreflective target (100) according to claim 1, comprising means for adjusting (140.1, 140.2, 152) the position of the reflector (120) with respect to the lens (130) to allow concentricity of said reflector and the lens.

3. Retroreflective target (100) according to claim 2, wherein the support (110) comprises a fixed plate (110.1) on which the lens (130) is mounted, and a mount (150) on which the reflector (120) is mounted, the adjustment means comprising micrometric screws arranged to adjust the position of the mount with respect to the fixed plate in a median plane (XY) of said fixed plate.

4. Retroreflective target (100) according to claim 3, in which the reflector (120) is fixed to a reflector holder (160) integral with the mount (150) via a micrometric thread (152) extending around an axis perpendicular to the median plane (XY) of said mount.

5. The retroreflective target (100) of claim 4, wherein the reflector holder (160) is generally cylindrical in shape and includes an internal collar (162) having a frustoconical inner periphery on which the reflector (120) rests and is held by gluing.

6. Retroreflective target (100) according to one of claims 3 to 5, in which the lens (130) is carried by a ring (170) fixed on the fixed plate (110).

7. A retroreflective target (100) according to any preceding claim, wherein the lens (130) is piano convex.

8. A retroreflective target (100) according to any preceding claim, wherein the lens (130) is made of sapphire.

9. Laser telemetry device (1) comprising at least one laser tracker (4) and a retroreflective target (100) according to any one of the

10. preceding claims. Laser telemetry device (1) according to claim 9, comprising an articulated arm (2) at one end of which the retro-reflective target (100) is mounted.

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