Fixing device for a multilateration target for three-dimensional metrology machine and robot equipped with such a device.

Abstract

A device (1) for attaching a reflective target (2) to a manipulator arm (101) of a three-dimensional metrology machine, the multilateration target having a support element for bearing against a support (100) along a bearing direction (D), the device comprising: a first plate (5) for attaching the device to the manipulator arm; a second plate (7) for attaching the reflective target (2) to the device; a force sensor (6) arranged to measure a reaction force along the bearing direction; a suspension member (9) from the second plate to the first plate (5), the suspension member (9) being arranged to allow elastic movement of the second plate (7) relative to the first plate (5) along the bearing direction. FIGURE IN ABRIDGED DIAGRAM: Fig. 1

Description

Title of the invention: Device for fixing a multilateration target for three-dimensional metrology machine and robot equipped with such a device.

[0001] The present invention relates to the field of three-dimensional metrology devices.

[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 exist, comprising a frame defining a measuring coordinate system and an armature that is movable within said coordinate system and carries a probe equipped with a feeler for application to different points on the surface of the object to be measured. The coordinates of the different points are determined by means of sensors providing signals representative of the displacements of the movable armature within the measuring coordinate system.

[0005] Three-dimensional metrology machines of this type are suitable for measuring relatively small objects but unsuitable for measuring large objects because it is imperative to limit the maximum slenderness of the moving armature and therefore the risk of deformation of the latter.

[0006] For large parts, three-dimensional metrology machines are known which include one or more laser emitters / receivers (commonly called laser trackers in this application) each emitting a laser beam towards a movable reflective target to be brought into point contact with the object successively at different points of it.

[0007] 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 reflecting target by measuring the time taken by said laser beam to make the round trip between the reference point and the center of the reflecting target.

[0008] When it relies on the use of several tracker lasers, the determination of the coordinates is carried out according to the principle of multilateration: the reflecting target is targeted simultaneously by the tracker lasers which each determine the distance separating it from the reflecting target, a mathematical processing allowing the calculation of the coordinates of said reflecting target.

[0009] The reflecting target generally comprises a cube-shaped reflector mounted on a spherical surface (of the SMR type for Sphere Mounted Retroreflector), having a large angular acceptance (typically 160° from the vertical), and whose fragility precludes any direct contact with the object, particularly when the object's surface is a rough surface of a very hard material, such as silicon carbide. Such surfaces are commonly found on telescope mirrors during polishing, an application in which the accuracy of the surface shape is paramount and the use of three-dimensional metrology is common.

[0010] To overcome this drawback, it is known to mount the reflector in a support comprising a bearing element that makes contact with the surface of the object along a specific contact direction. Thus, in SMR (Spherically Mounted Retroreflector) type targets, the bearing element is shaped like a truncated sphere. The geometric relationship between the center of the reflector and the point of contact of the bearing element with the surface of the object to be measured then depends on the concentricity between the reflector and the bearing element in contact with the object.

[0011] In metrology machines of this type, the reflecting target is relatively heavy, several hundred grams, and is commonly mounted at the end of a robotic arm. It is important that the contact between the support element and the surface of the object to be measured be carried out while minimizing the stresses generated during this contact so as not to damage the surface or the support element. This requires very precise control of the robotic arm, which is made difficult by the necessary slenderness of the arm for relatively large parts on the one hand, and the weight of the target on the other.

[0012] SUBJECT OF THE INVENTION

[0013] The invention aims in particular to provide a device for fixing a target to the end of a manipulation arm, making it possible to remedy at least in part the aforementioned disadvantages. Summary of the invention

[0014] To this end, the invention provides a device for attaching a reflective target to a manipulator arm of a three-dimensional metrology machine, the multilateration target comprising a support element for bearing against a support in a direction of contact, the device comprising: - a first mounting plate for attaching the device to the manipulator arm; - a second mounting plate for attaching the reflector target to the device; - a force sensor arranged to measure a reaction force according to the docking direction; - a suspension element from the second plate to the first plate, the suspension element being arranged to allow elastic movement of the second plate relative to the first plate according to the direction of docking.

[0015] Thus, the combination of the suspension element and the force sensor facilitates the control of the robot while limiting the risk that the support element is applied to the surface of the object to be measured by exerting too much force on said surface.

[0016] According to optional features, used individually or in whole or in combination: - the device includes a damper carried by the first plate and arranged to dampen the elastic movement of the second plate; - the shock absorber is a magnetic shock absorber; - the damper includes at least one magnet attached to the first plate and an electrically conductive core attached to the second plate opposite the magnet to be subjected to a magnetic flux from the magnet in such a way that the magnetic flux induces eddy currents in the core when the core is mobile relative to the magnet; - the damper includes at least one pair of magnets facing each other and defining between them an air gap in which the core extends; - the force sensor includes at least one strain gauge; - the force sensor is placed between the first plate and the second plate; - the force sensor is placed between the first plate and the elastic suspension element; - the elastic suspension element includes at least one blade working elastically in bending along the direction of docking.

[0017] The invention also includes a robot comprising an arm having a free end to which the first plate of the fastening device is attached.

[0018] Other features and advantages of the invention will become apparent from the following description of a particular, non-limiting embodiment of the invention. Brief description of the drawings

[0019] Reference will be made to the attached drawings, among which:

[0020] [Fig. 1] is a partial perspective view of a robot equipped with the fastening device according to the invention;

[0021] [Fig.2] is a schematic representation of the connections between the elements of this device;

[0022] [Fig.3] is a general schematic representation of the device. DETAILED DESCRIPTION OF THE INVENTION

[0023] With reference to figures 1 to 3, the invention is described in application to a laser rangefinding installation allowing to characterize geometrically, by multilateration, a surface 200 of an object as an optical surface of a mirror.

[0024] The laser rangefinding installation here comprises a plurality of laser transmitters / receivers or laser trackers (also called tracking lasers - not shown), and a robot 100 comprising an articulated arm 101, having a first end connected to a base of the robot and a second free end provided with measuring equipment generally designated as 102. Three laser trackers (trilateration) or four laser trackers (quadrilateration) are generally available.

[0025] The manipulator arm 101 comprises segments connected by joints equipped with encoders for determining the angular position of the segments relative to each other and with motors that are controlled to move the segments relative to each other, possibly to form a cobot. The manipulator arm 101 of the robot 100 is further equipped here with an anti-collision sensor 103.

[0026] The measuring equipment 102 includes a target which includes in a known manner a spherical reflector mounted in a support having a spherical support element arranged to bear against the surface 200 of the object in a direction of approach.

[0027] In a manner known per se, each laser tracker emits a laser beam towards the target reflector, the support element of which is brought into contact with the object's surface by the arm 101. The laser beam is reflected by the reflector and follows the same path in reverse to enter the corresponding laser tracker. The laser trackers and the manipulator arm 101 are known per se and will not be described further here.

[0028] According to the invention, the measuring equipment 102 includes a fastening device 1 for attaching the target to the free end of the arm 101. In figures 2 and 3, the reflective target, referenced 2, is symbolized by a ball constituting the reflector and whose lower cap constitutes the support element.

[0029] The fixation device 1 comprises a first assembly linked to the free end of the manipulator arm 101 to form an unsuspended mass 3 connected by an elastic suspension member 9 to a second assembly carrying the target 2 to form a suspended mass 4.

[0030] The unsuspended mass 3 comprises a first plate 5 and a force sensor 6, while the suspended mass 4 comprises a second plate 7, a target holder 8 and the reflector target 2.

[0031] The first plate 5 is a plate, here L-shaped, comprising means for its attachment to the free end of the arm 101, such as tapped holes for receiving screws. The first plate 5 is connected to the manipulator arm 101 of the robot 100.

[0032] The force sensor 6 comprises a deformable bar 6.1 having a first end rigidly fixed (fixed-type connection) to the first plate 5 and a second end rigidly connected (again, fixed-type connection) to the elastic suspension member 9. Strain gauges or strain gauges, not visible in the figures, are fixed to the bar 6.1 so as to be able to measure a stress applied to the bar 6.1 or a deformation of the bar 6.1.

[0033] As regards the suspended mass 4, the second plate 7 includes a plate having means for its attachment to the elastic suspension member 9, such as tapped holes for receiving screws. A target holder 8 extends perpendicularly from the second plate 7 to accommodate the reflector target 2 and to allow the reflector target 2 to be connected to the free end of the arm 101. The target holder 8 includes a frame that is substantially rectangular and defines a central hole in which the reflector target 2 is fixed such that the reflector extends above the target holder 8 and the support element extends below the target holder 8. This arrangement makes it possible to obtain a docking direction (indicated by an arrow D in Figures 1 to 3) that is substantially perpendicular to a principal plane of the target holder 8 and that coincides with a central axis of the hole in the target holder 8.

[0034] The elastic suspension member 9 comprises an upper leaf spring 9.1 and a lower leaf spring 9.2, each having a first end 9.1.1, 9.2.1 rigidly connected to a first spacer 9.3 and a second end 9.1.2, 9.2.2 rigidly connected to a second spacer 9.4. The first spacer 9.3 is fixed to the second end of the bar 6.1 of the force sensor 6 and the second spacer 9.4 is fixed to the second plate 7.

[0035] The upper spring blade 9.1 has a stiffness constant kl and the lower spring blade 9.2 has a stiffness constant k2. The stiffness constants kl and k2 of the blades 9 are identical and chosen based in particular on the weight of the suspended assembly (the target 2, which weighs approximately 700 grams, as well as the second platform 7 and the target holder 8, which weigh approximately 200 grams), such that, in the absence of movement of the manipulator arm 101 and in the absence of any contact of the suspended mass 4 with an external element, the spring blades 9.1 and 9.2 are in a rest state in which they are substantially straight and horizontal or slightly flexed by the weight of the target. The blades 9.1 and 9.2 are thus arranged to exhibit an ability to deform elastically according to a bending movement, offering the second plate 7 a degree of freedom in translation along the direction of docking, relative to the first plate 5.

[0036] This degree of freedom allows the target holder 8 and therefore the reflective target 2 to move in this direction when the support element of the reflective target 2 comes into contact with the surface 200.

[0037] The fastening device 1 also includes a magnetic damper 10, comprising a pair of magnets 11, a core 12 and a support structure 13 supporting the magnets 11.

[0038] The core 12 comprises a plate made of an electrically conductive material, such as copper, which is fixed to the second plate 7 via the second spacer 9.4. The core 12 extends here between the leaf springs 9.1, 9.2 without touching them so as not to hinder their freedom to deform.

[0039] The supporting structure 13 has two main rods 13.1. Each of these main rods 13.1 has a first end connected to a base 13.2 fixed to the first plate 5 and the arm 101, and a second end carrying a slide 13.3 which is adjustable in position along the rod 13.1 and which carries one of the magnets 11. The base 13.2 has two holes, each receiving one of the sliding rods 13.1. Two set screws pass through the base 13.2, each having one end protruding into one of the holes to exert a pressure force on the corresponding rod 13.1, locking the rod 13.1 in position within the hole. It is understood that it is thus possible to adjust the position of the magnets 11 relative to the core 12.

[0040] Within the magnetic damper 10, the magnets 11 then form part of the unsprung mass 3 and the core 12 forms part of the suspended mass 4.

[0041] The two magnets 11 are positioned on either side of the core 12 so as to generate a magnetic flux in the direction of the core 12. When the core 12, which is in the air gap of the magnets 11, is in motion relative to the magnets 11, these magnetic fluxes give rise to eddy currents which circulate in the core 12 and generate a force resisting the movement of the core 12.

[0042] It is understood that the magnets 11 make it possible to dampen the displacement of the core 12, and therefore of the suspended mass 4 relative to the arm 101.

[0043] Consequently, when the manipulator arm 101 brings the support element of the reflector target 2 into contact with the surface 200 in a docking motion, the surface 200 exerts a resisting force on said support element which, if the docking motion continues, causes the leaf springs 9 to bend so that the suspended mass 4 translates along the docking direction relative to the first plate 5 and the free end of the arm 101. The displacement of the suspended mass 4 is damped by the magnetic damper 10. This nevertheless results in the transmission of a force from the spring blades 9 to the bar 6.1 of the force sensor 6 whose gauges will measure the magnitude.

[0044] When the manipulator arm 101 is commanded to bring the reflecting target 2 to another point on the surface 200, the free end of the manipulator arm 101 detaches the support element from the surface 200 and the spring blades 9.1, 9.2 return to their rectilinear rest state, this return movement being damped by the magnetic damper 10 which will limit the risk of oscillations of the suspended part 4 on either side of the rest state.

[0045] In general, the magnetic damper 10 tends to oppose the displacement of the suspended part 4 relative to the unsuspended part 3 during the movements of the arm 101. It is therefore possible to use springs with a lower stiffness constant than that which would be required in the absence of a damper. The combination of this relatively low stiffness constant and the effect of the magnetic damper 10 also allows for greater tolerance in the positioning and displacement of the reflecting target 2 by the arm 101, thus simplifying its control.

[0046] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0047] In particular, the device here comprises three or four laser trackers to perform trilateration or quadrlation but could comprise more.

[0048] The force sensor 6 can be positioned between the first plate 5 and the manipulator arm 101 or the force sensor can comprise only strain / stress gauges directly positioned on the leaf springs 9.1, 9.2.

[0049] The suspension member 9 may comprise only one spring and this spring may have a different structure from that described and comprise, for example, a helical spring having a central axis extending parallel to the direction of docking.

[0050] Although here the magnetic damper 10 comprises a pair of facing magnets 11, it is possible for the magnets 11 to have a different arrangement; for example, they may be arranged in quadrupoles, or the magnetic damper 10 may comprise a single magnet. The magnetic damper may also be arranged to operate by magnetic repulsion / attraction rather than by the use of eddy currents. The damper may also be non-magnetic and, for example, electrostatic or fluidic (hydraulic or gas-powered). It is understood that the damper is preferably non-contact but may alternatively comprise a damper cylinder extending between the first and second plates.

[0051] The shock absorber is also optional.

[0052] Although it is envisaged here that the core 12 is made of copper, other electrically conductive materials such as aluminium can be used.

[0053] Although here the invention operates without a means of guidance, other than the suspension member, to guide the suspended mass relative to the unsuspended mass, it is possible that the device may contain, for example, a slide attached to the unsuspended mass to receive and guide the suspended mass in its movement.

Claims

Demands

1. A device for fixing (1) a reflective target (2) to a manipulator arm (101) of a three-dimensional metrology machine, the multilateration target having a support element for bearing on a support (100) in a direction of approach (D), the device comprising: - a first plate (5) for fixing the device to the manipulator arm; - a second plate (7) for fixing the reflective target (2) to the device; - a force sensor (6) arranged to measure a reaction force in the direction of approach; - a suspension member (9) from the second plate to the first plate (5), the suspension member (9) being arranged to allow elastic movement of the second plate (7) relative to the first plate (5) in the direction of approach.

2. Measuring device according to claim 1, comprising a damper (10) carried by the first plate (5) and arranged to dampen the elastic movement of the second plate (7).

3. Measuring device according to claim 2, wherein the damper (10) is a magnetic damper.

4. A measuring device according to claim 3, wherein the damper (10) comprises at least one magnet (11) attached to the first plate (5) and an electrically conductive core (12) attached to the second plate (7) opposite the magnet to be subjected to a magnetic flux from the magnet (11) such that the magnetic flux induces eddy currents in the core (12) when the core (12) is mobile relative to the magnet (11).

5. Measuring device according to claim 4, in which the damper (10) comprises at least one pair of magnets (11) facing each other and defining between them an air gap in which the core (12) extends.

6. Measuring device according to any one of the preceding claims, wherein the force sensor (6) comprises at least one strain gauge.

7. A measuring device according to any one of the preceding claims, wherein the force sensor (6) is placed between the first plate (5) and the second plate (7).

8. Measuring device according to claim 7, wherein the force sensor (6) is placed between the first plate (5) and the elastic suspension member (9).

9. A measuring device according to any one of the preceding claims, wherein the elastic suspension member (9) comprises at least one blade (9.1, 9.2) working elastically in bending along the docking direction.

10. Robot (100) comprising an arm (101) having a free end to which is attached the first plate (5) of the fastening device according to any one of the preceding claims.

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