Optical device for tracking the position of an object

Abstract

A device for determining the position of an object, the object comprising a target retroreflector, including: a laser emitter configured to emit an incident laser beam towards the target retroreflector; a detector configured to detect a laser beam reflected by the target retroreflector; and a centering unit configured to adjust the position of the laser emitter based on the detected laser beam. The device is characterized in that: the device includes a mirror configured to reflect the incident laser beam towards the target retroreflector; and the device includes a processing unit configured to determine the actual position of the target retroreflector, based on the position and orientation of the mirror, and the virtual position of the target retroreflector, after the incident laser beam has been centered with respect to the target retroreflector. Figure 2A

Description

Title of the invention: Optical device for tracking the position of an object. Technical field

[0001] The technical field of the invention is optical metrology, and more specifically laser tracking (position tracking). EARLIER ART

[0002] A laser tracker is a high-precision laser tracking device used to accurately track and measure the position of objects in three dimensions. It is a device used in many industrial fields, for example, but not limited to, aerospace, automotive, and shipbuilding.

[0003] A laser tracker consists of a laser mounted on two perpendicular, motorized axes of rotation and equipped with angular encoders. The object whose position is measured or tracked by the laser, hereinafter referred to as the target, is connected to a retroreflector. The retroreflector is made up of three mirrors, forming a cube wedge, mounted, for example, in a sphere. The cube wedge is positioned at the center of the sphere. This type of retroreflector is designated "SMR: Spherically Mounted Reflector." The laser tracker is notably capable of dynamically tracking the movements of the measurement point, which is the center of the retroreflector.

[0004] Thanks to the two servo motors mounted on the two axes of rotation, the laser tracker can point its beam in all directions. The servo control of the two motors ensures that the beam is constantly directed to the center of the retroreflector. To achieve this, a portion of the reflected beam is directed towards a detector. The signal from the detector serves as a command for the servo control system to keep the beam centered on the retroreflector.

[0005] Figures IA and IB schematically illustrate a target positioning device according to the prior art. The device comprises: - a laser emitter 10, configured to emit an incident laser beam towards an SMR type retroreflector 21, mounted on an object 20 to be controlled; - a detector 11, configured to detect the laser beam reflected by the target; - a camera 12, enabling rapid detection of SMR-type retroreflectors - a support 13, movable in rotation, intended to adjust a position and orientation of the laser emitter and detector; - a centering unit 14, configured to analyze the signal detected by the detector, and to control the position and orientation of the support according to the signal resulting from the detector.

[0006] One of the constraints of using a laser tracker is that the object to be monitored must be directly within the laser's line of sight, without any screen interposed between the tracker and the target. However, the object to be monitored may be located in a cluttered area, making it impossible to position the tracker or difficult for a person to access. The object to be monitored may also be located in an area with an environment unsuitable for a laser tracker: unsuitable temperature and / or humidity, high radiation levels, or the presence of a magnetic or electric field that could interfere with the laser tracker.

[0007] Under the conditions listed in the preceding paragraph, the laser tracker must be located away from the object to be measured, with the potential presence of obstacles, for example screens or protective partitions, between the laser and the object to be monitored. The latter is no longer in the direct line of sight of the laser.

[0008] Patent EPI 171752 addresses this problem and proposes a rigid support incorporating a retroreflector that can be offset from an object to be inspected. The object's position can be determined indirectly by knowing the offset of the retroreflector relative to the object. However, this requires placing a support on the object and possibly moving the support on the object's surface, which can pose operational constraints.

[0009] Patent CN103499293 describes a method for improving accuracy This method uses a laser tracker with mirrors arranged around an object to be monitored. The mirrors are configured to direct the laser beam, emitted by the tracker, to the same point on the object. From the object's perspective, each image forms a virtual image of the tracker. The goal is to leverage the redundancy of measurements by directing the laser beam onto four mirrors, thus obtaining as many measurements as there are mirrors. The four mirrors define four different optical paths between the tracker and the object being monitored.

[0010] The inventors propose a laser tracking device that can be implemented remotely from an object to be controlled, in the absence of a direct line of sight between the laser emitter and the object. Description of the invention

[0011] A first aspect of the invention is a device for determining the position of an object, the object comprising a target retroreflector, the device comprising: - a laser emitter, movable in rotation on a support, and configured to emit an incident laser beam towards the target retroreflector; - a detector, configured to detect a laser beam reflected by the target retro-reflector illuminated by the incident beam; - a centering unit, configured to adjust the position of the laser emitter according to the laser beam detected by the detector;

[0012] the device being characterized in that: - the device includes a mirror, comprising at least one auxiliary retroreflector, the mirror being configured to reflect the incident laser beam towards the target retroreflector; - the centering unit is configured for • control a position of the support according to the laser beam reflected by the target retro-reflector, then by the mirror, so as to direct the incident laser beam towards the target retro-reflector; • determine a virtual position of the target retroreflector, through the mirror; - the device includes a processing unit configured for • receive or determine a position and orientation of the mirror; • determine a real position of the target retroreflector, based on the position and orientation of the mirror, and the virtual position of the target retroreflector, after the incident laser beam has been centered with respect to the target retroreflector.

[0013] According to one possibility: - the mirror is flat; - the mirror includes two or three auxiliary retroreflectors, defining a plane of the mirror; - the processing unit is configured for • center the laser emitter successively with respect to each auxiliary retroreflector; • then determine the position and orientation of the mirror from each beam respectively reflected by each auxiliary retroreflector.

[0014] The device may include a camera, the optical axis of which is parallel to an emission axis of the laser beam, the centering unit being configured to detect a retroreflector on each image generated by the camera.

[0015] According to one possibility: - the mirror is mobile in rotation and / or translation relative to the laser emitter; - the device includes a control unit, configured to control a rotation of the mirror according to a control signal generated by the processing unit.

[0016] The processing unit can be configured to drive the control unit, so that the target reflector is visible, by the camera, through the mirror.

[0017] The processing unit can be configured to drive the control unit, so that the target reflector is positioned in the camera's field of view, through the mirror.

[0018] Advantageously, the incident laser beam extends between the laser emitter and the target retroreflector along a single optical path. This means that there is only one beam propagating between the laser and the object.

[0019] The device may include several mirrors, in series, so that the beam is reflected successively by several mirrors until it reaches the object.

[0020] A second aspect of the invention is a method for determining the position of an object, equipped with a target retroreflector, using a device according to the first aspect of the invention, the mirror being oriented so that the incident beam, emitted by the laser emitter, bypasses an obstacle extending between the object and the laser emitter, the method comprising the following steps: - a) control of a position of the support as a function of the laser beam reflected by the target retro-reflector, then by the mirror, so as to center the incident laser beam with respect to the target retro-reflector; - b) following step a), determination of a position of the target retroreflector as a function of the orientation of the mirror.

[0021] Thus, the incident beam, emitted by the laser emitter, bypasses the obstacle before reaching the target retroreflector.

[0022] The mirror may include at least one auxiliary reflector or at least two auxiliary reflectors or at least three auxiliary retroreflectors, not aligned, defining a plane of the mirror, the method comprising, prior to step a): - centering of the laser emitter successively with respect to the or each auxiliary retro-reflector; - determination of a position and orientation of the mirror from each laser beam respectively reflected by each auxiliary retroreflector.

[0023] The mirror can be movable in rotation and / or translation relative to the laser emitter, in which case the method may include, prior to step a), a rotation and / or a translation of the mirror according to a control signal generated by the processing unit.

[0024] The rotation of the mirror can be carried out so as to form an image of the target reflector, through the mirror, by the camera.

[0025] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in connection with the figures listed below. FIGURES

[0026] Figures IA and IB show a prior art configuration.

[0027] Fig. 2A shows a simplified view of a device according to the invention.

[0028] Fig. 2B represents a mirror forming part of the device according to the invention.

[0029] Fig. 3A schematically represents the reference frame of the tracker and the reference frame of the mirror.

[0030] Figures 3B to 3D illustrate successive rotations to go from the mirror reference frame to the tracker reference frame.

[0031] Fig. 4A shows the main steps for positioning the target reflector using the tracker.

[0032] Fig. 4B shows the detail of step 150 of Fig. 4A. PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0033] Figure 2A represents a device 1 according to the invention. The device comprises the same components as described in relation to the prior art. The device includes a mirror 30, arranged between the laser emitter 10 and the object 20 to be inspected. The object to be inspected is connected to a target retroreflector 21. The mirror 30 is configured to deflect the incident laser beam around an obstacle 2, the latter extending through a direct line of sight from the laser emitter. By direct line of sight is meant a straight line extending between the laser emitter and the retroreflector connected to the object to be inspected. Thus, the mirror is arranged to establish an optical path around the obstacle 2 extending between the object and the laser emitter.

[0034] In this example, the device includes a detector 11, for example based on a PSD (Position Sensitive Device) cell, which is a beam position detector. If the beam does not reach the retroreflector at the center, it also does not reach the PSD cell at the center, creating an error signal. The error signal represents a discrepancy between the direction of emission of the incident laser beam and the direction in which the incident beam is reflected by the target retroreflector disposed on the object.

[0035] The device preferably comprises a camera 12, configured to form an image of the observed scene. The centering unit 14 is then programmed to locate SMR-type retroreflectors from the camera image. This allows for initial positioning of each retroreflector, with more precise positioning being achieved by progressively moving the laser beam to the center of the reflector.

[0036] The centering unit 14 also includes encoders for recording the orientation of the laser beam, as well as the distance traveled by the beam to the retroreflector. The latter can be calculated by a conventional measurement of the time of flight of the laser beam between its emission and its detection by detector 11.

[0037] The device includes a processing unit 15, configured to perform processing steps described below. The processing unit includes, for example, a microprocessor

[0038] Figure 2B shows the mirror 30. The latter is supported by a frame 32, comprising three auxiliary retroreflectors 31a, 31b, and 31c, preferably non-aligned. Each auxiliary retroreflector may be of the same type as the target retroreflector 21 located on the tracked object 20. The mirror may be mounted to rotate and / or translate, the rotation and / or translation being controlled by a control unit 35.

[0039] The presence of three non-aligned retroreflectors is advantageous because it allows the position and orientation of the memory to be determined by measurement, as described below. Alternatively, the mirror may have one or two retroreflectors, which may require additional information, for example, a priori information, to determine the position and orientation of the mirror.

[0040] An important aspect of the invention is that the device is arranged so that the incident laser beam, emitted by the laser emitter 10, is reflected, towards the object 20, by the mirror 30. Symmetrically, the mirror returns, towards the detector 11, the beam reflected by the target retroreflector 21.

[0041] The centering unit 14 is programmed to control the support 13 so that the reflected beam reaches the detector 11 coaxial, or considered to be coaxial, with the incident beam. The smaller the spatial offset between the incident and reflected beams, the more precisely the position of the target retroreflector 21 is controlled.

[0042] We place ourselves in a frame of reference of the device, and defined by an orthonormal basis (ë*. ë^, ëp, represented in figure 3A, centered on an origin point (0,0,0) fixed with respect to the support 13. The origin point and the orthonormal basis (ëj, ë^, ë^) define a frame of reference of the device R. We assign to the mirror 30 an orthonormal basis (ë^, ëX £3), forming the frame of reference of the mirror Rm.

[0043] C2 and C3 denote the coordinates of the respective centers of the auxiliary retroreflectors 21 b 212 and 213. If « is the normal vector to the plane of the mirror,

[0044]

[0045] The Cartesian equation of the mirror plane is:

[0046] ax + by + cz + d = Q (2), a, b, C and being scalars, x, etz being coordinates in the device's reference framework.

[0047] With / a\ (3) n = b \c /

[0048]

[0049]

[0050]

[0051]

[0052]

[0053]

[0054]

[0055]

[0056] can be obtained by applying (6) to the coordinates of each point Ch C2 and C3, the latter being obtained by pointing the laser emitter successively at the center of each auxiliary retroreflector. Each vector ëj, ë^et can be such that: ^ = C^4) = ¾ with CA, = "ACA(5) = n (6) In [Fig. 3A], the coordinates of the real point S and its image S' (or virtual object) by the mirror are shown, with point S corresponding to the center of the target. The real point S corresponds to the center of the target reflector 21. The laser tracker allows us to obtain the coordinates of the virtual point S', in the device's frame of reference. This provides the measurement lxA V .. s ' ' (ëj. Now, we are looking for the coordinates of S in the device's frame of reference; that is [XS\ ys In the mirror frame of reference:

[0057]

[0058]

[0059] IXS\ lxx'] / 1 0 0 \ (7) U — X 0 1 0 \ / Zs' 1 lo 0 -1' The coordinates of each vector e], ë^, and £3 are expressed, in the basis (ëj, ëX ë^) as follows:

[0060] + ve^ + W3 (8)

[0061] ^-u^ + v^ + w'e^

[0062] + v"ë^ + w"e3 (1°)

[0063] A change-of-basis matrix can be formed

[0064] P^ aa V v' w ■ w' (11).11" v" w".

[0065] The transition between the reference frames (ev e2, ëp and (gj, ë^, ë|) is written:

[0066] (^,^,^) = (^,^,^)^(12)

[0067] With p-1^ ^,,^)(13) dei\P) /

[0068]

[0069]

[0070]

[0071]

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[0075]

[0076]

[0077] - det(P) is the determinant of P - Adj(P) is the adjoint matrix of P We can define three rotation matrices respectively according to the Euler angles: a (precession angle), [3 (nutation angle) and y (proper rotation angle) defined respectively in figures 3B, 3C and 3D. The angles a, [3 and y are calculated by the processing unit 15 from the coordinates of points Cb C2 and C3. Figure 3B shows the precession angle α (or first Euler angle), through which a change of coordinate system is performed between the frame Rm (ej, 63) and a first intermediate frame Rm\ Q / , / 1 7 / ) with / / = / . The transition from the frame Rm to the frame Rmi is achieved by a rotation R^#, by the angle α, around the axis: COSfZ sina -sina cosa 0 0 1 (14) 1" Figure 3C shows the nutation angle [3 (or second Euler angle), according to which a change of coordinate system is performed between the first intermediate coordinate system Rmi (gi, gi) and a second intermediate coordinate system Rtn2 (gn, gi) with gn = The transition from the frame Rm] to the frame Rtn2 is achieved by a rotation Rt^, according to the angle [3, around the axis / 7; cos^ 0 sin / Sl (15) , sin / ^ 0 cos^. Figure 3D shows the angle of proper rotation Y (or second Euler angle), through which a change of coordinate system is performed between the second intermediate coordinate system Rm2 (g) and a third intermediate coordinate system Rmi "0 with gYi. The transition from frame Rm2 to frame ^„3 is achieved by a rotation R according to angle Y, around the axis: 0 1(16) cosy -siny siny cosy. We deduce a rotation matrix R, formed from a matrix product of the three matrices defined in (14) to (16). R — Rg^ax Refé x R^,y (17) The basis (gj", g^' of the third intermediate frame is parallel to the basis (^, ^).ct centered on the origin of the mirror frame (^, é\). The term "parallel basis" » corresponds to the fact that the vectors g are respectively parallel to

[0078]

[0079]

[0080]

[0081]

[0082]

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[0084]

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[0087]

[0088] vectors ë^, We go from the third intermediate frame Rmi to the frame of the tracker by a translation T between the origin of the reference frame of the laser tracker and the origin 0' of the reference frame of the mirror, the origin 0' being one of the retro-reflectors. '*()'\ (18) f= yo \ Z (d The coordinates of the center S of the target reflector 21 in the device's frame of reference are obtained from the coordinates in the mirror's frame of reference using the following expression: l%s\ {Xs\ (20) Xd xR+T Figure 4A describes the main steps in implementing a device according to the invention. Step 100: Checking the tracker's aiming During this step, it is ensured that the target retroreflector 21 is within the tracker's field of view, as seen through the mirror 30. This step can be performed using the tracker's camera: it is assumed that the camera's field of view is similar to the tracker's field of view. Depending on the position of the target retroreflector detected by the camera 12, the control unit 35 of the mirror 30 is activated, so as to position the target retroreflector 21 in a central part of the mirror as seen by the tracker. The control unit 35 is then driven by a control signal generated by the processing unit 15. Once the retroreflector is in the tracker's field of view, the tracker is configured to direct the laser beam to the center of the target retroreflector, and to orient itself so that the laser is held, by the centering unit 14, at the center of the retroreflector, by "auto lock" (automatic locking) Step 110: Taking into account the position and orientation of the mirror This involves defining the vectors ë|, ë^, Ë3 of the reference frame linked to the mirror. The mirror's orientation can be fixed and predefined, in which case the vectors εj, ε^, ε| are pre-stored in the processing unit 15. Otherwise, the vectors εj, ε| are determined experimentally by directing the tracker towards each auxiliary retroreflector and successively defining points Cb, C2, and C3. For this purpose, the camera 12 is used to detect each auxiliary retroreflector and direct the laser beam successively towards each one. The precise position of the center of each auxiliary retroreflector is defined by the laser tracker controlled by the centering unit 14. Step 1 2 0: determination of the change-of-basis matrix and the translation vector.

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096]

[0097] Based on the vectors, processing unit 15 calculates the matrix of passage R and the translation vector f as given in (14) to (20). Step 130: Locating the target retroreflector 21 through the mirror 30. During this step, the laser beam is directed towards the mirror. Then the laser emitter is controlled by the centering unit 14, so as to lock onto the position of the center of the target retroreflector 21. Step 140: Determining the virtual coordinates of the target retroreflector, in The device's reference frame: these are the coordinates / x / \ cf. [Fig.3A]. This y ' J s U / L- —ï 5 ■ (6),^3) This step is implemented by the centering unit 14. It can also be implemented by the processing unit 15. Step 1 5 0: Determining the actual coordinates of the retroreflector target, in the device's reference system. This step is implemented by processing unit 15. This step may include the following sub-steps, shown in [Fig.4B]: Substep 151: Determining the coordinates of the virtual point S' in the mirror frame of reference: we apply the relation 'Xs>\ = / iXs'\ ' -T x / T1 / f1 can be determined, from R, according to (13) Substep 152: Determining the coordinates of point S in the mirror frame of reference: relation (7) is applied, so as to obtain lxs from

[0098] Sub-step 153: Determining the coordinates of S in the device's frame of reference, by applying (20) from ' to obtain

[0099] The invention can be implemented when the object to be detected is located in an area where access conditions are difficult, in particular due to clutter, or where environmental conditions (temperature, humidity, irradiation) do not allow the tracker to be placed directly in front of the object to be controlled.

Claims

1. Demands Device for determining the position of an object (20), the object comprising a target retroreflector (21), the device comprising: a laser emitter (10), movable in rotation on a support (13), and configured to emit an incident laser beam towards the target retroreflector; a detector (11), configured to detect a laser beam reflected by the target retro-reflector illuminated by the incident beam; a centering unit (14), configured to adjust a position of the laser emitter according to the laser beam detected by the detector; the device being characterized in that: the device includes a mirror (30), comprising at least one auxiliary retroreflector (31a, 31b, 31c), the mirror being configured to reflect the incident laser beam towards the target retroreflector; the centering unit (14) is configured for • control the position of the support based on the laser beam reflected by the target retroreflector, then by the mirror, in order to direct the beam laser incident on the target retroreflector; determine a virtual position Uxsr\ y ' S s ) of target retroreflector, through the mirror; the device includes a processing unit (15) configured for • receive or determine a position and orientation of the mirror; • determine a real position ( X ) of target retroreflector, depending on the position and orientation of the mirror, and the position virtual of the target retroreflector, after the incident laser beam has been centered with respect to the target retroreflector.

2. Device according to claim 1, wherein: - the mirror is flat; - the mirror has three auxiliary retroreflectors (31a, 31b, 31c), defining a plane of the mirror; - the processing unit (15) is configured to • center the laser emitter successively with respect to each auxiliary retroreflector; • then determine the position and orientation of the mirror from each beam respectively reflected by each auxiliary retroreflector.

3. Device according to any one of the preceding claims, comprising a camera (12), the optical axis of which is parallel to an emission axis of the laser beam, the centering unit (14) being configured to detect a retroreflector on each image generated by the camera.

4. Device according to any one of the preceding claims, wherein: - the mirror is movable in rotation and / or translation relative to the laser emitter; - the device includes a control unit (35), configured to control a rotation of the mirror (30) according to a control signal generated by the processing unit.

5. Device according to claims 3 and 4, wherein the processing unit (15) is configured to drive the control unit (35), so that the target reflector is visible, by the camera, through the mirror.

6. Device according to claim 5, wherein the processing unit is configured to drive the control unit, so that the target reflector is disposed in the field of view of the camera, through the mirror.

7. Device according to any one of the preceding claims, configured so that the incident laser beam extends between the laser emitter and the target retroreflector along a single optical path.

8. A method for determining the position of an object (20), equipped with a target retroreflector (21), using a device according to any one of the preceding claims, the mirror being oriented so that the incident beam, emitted by the laser emitter, bypasses an obstacle (2) extending between the object and the laser emitter, the method comprising the following steps: - a) controlling a position of the support as a function of the laser beam reflected by the target retroreflector, then by the mirror, so as to center the incident laser beam with respect to the target retroreflector; - b) following step a), determining a position of the target retroreflector as a function of the orientation of the mirror.

9. A method according to claim 8, wherein the mirror comprises at least three non-aligned auxiliary retroreflectors (31a, 31b, 31c) defining a plane of the mirror, the method comprising, prior to step a): - centering the laser emitter successively with respect to each auxiliary retroreflector; - determining a position and orientation of the mirror from each laser beam respectively reflected by each auxiliary retroreflector.

10. A method according to claim 9, wherein the mirror is movable in rotation and / or translation relative to the laser emitter, and wherein the method comprises, prior to step a), a rotation and / or translation of the mirror as a function of a control signal generated by the processing unit.

11. A method according to claim 10, wherein the rotation of the mirror is carried out so as to form an image of the target reflector, through the mirror, by the camera.