Tracking instrument with rigid optical measuring unit and coupled reference element

Abstract

A tracking instrument having a rigid optical measurement unit and a coupled reference element. The invention relates to a laser tracker for distance measurement to a target point using a measuring beam. The target axis of the measuring beam can be rotated about a node in a revolution-shaped reference fixedly arranged in a housing of the laser tracker. The laser tracker includes an optical device that provides a measurement path to a target point and an internal reference path. The optical device comprises an optical unit, which is embodied in particular as a mechanical fixture, and the laser tracker comprises a force supply device, which provides a force acting on the optical unit (as a whole), keeping the optical unit in contact with the surface of the reference.

Description

Technical Field

[0001] The present invention relates to the structure of a high-precision laser tracker, which is configured to determine the distance to a target point using a measuring beam. The laser tracker includes a revolution body-shaped datum fixedly arranged in a housing of the laser tracker, and the target axis of the measuring beam can rotate around a fixed node in the datum. Background Art

[0002] High-precision measuring or processing machines, such as coordinate measuring machines, vision measuring machines or articulated robotic arms, need to be calibrated to improve their accuracy performance. Such systems are usually built to provide repetitive behavior rather than absolute accuracy. In other words, typical errors of such systems are systematic and can be electronically compensated after determination. As an example, DE19947374 A1 discloses a calibration method performed by a tracking instrument. From here on, the term laser tracker refers to a universal tracking instrument suitable for performing calibration measurements that are substantially equivalent to the calibration measurements disclosed in DE19947374A1. The universal laser tracker also includes hardware and software components that enable identification of tracking targets and alignment of the laser tracker with the tracking target. The tracking target of the present invention can also represent collaborative targets and non-collaborative targets.

[0003] Because of the sub-micron accuracy required, common laser trackers that rely on an imaginary fixed point as a reference are not suitable for this task. For example, for a typical gimbal mount, a perfect reference requires that the two rotation axes are aligned so that they intersect at a point that also lies on the plane of the beam steering mirror. In addition, the laser beam must be aligned with the intersection point. All of these alignments should be performed with sub-micron accuracy, which is tedious, although not impossible.

[0004] As an alternative, laser trackers based on references have been developed, as disclosed, for example, in EP0919830A2, EP1959226A2 or DE202004007647U1. Such systems are based on measuring distances from a physical reference embodied as a reflecting sphere, in particular based on the interferometric principle. To ensure position and thermal stability, the moving parts of the laser tracker are separated from the reference. The moving parts closely follow the surface of the reference. Due to the presence of the physical reference, small displacements of the parts (i.e. the interferometer) have no effect on the measurement accuracy. Ideally, the accuracy depends only on the manufacturing tolerances of the physical reference, which can be in the range of tens of nanometers.

[0005] One disadvantage of the described structure is the fact that the physical reference acts as an optical reference surface. This requires complex optics and very tight mechanical tolerances to keep the interferometer wavefront (i.e. signal quality) high. Furthermore, the moving elements of the laser tracker need to be mechanically decoupled from the physical reference, which requires high-precision steering mechanisms and controls to follow the reference surface with an accuracy of <5 μm, but in a contactless manner.

[0006] Therefore, systems in which the two functions of a physical reference (ie, a mechanical reference and an optical reference surface) are performed by two separate components are desirable because they can provide similar accuracy with simplified mechanical and optical structures. Summary of the invention

[0007] In view of the above, an object of the present invention is to provide a laser tracker with a simplified structure while improving or maintaining measurement accuracy.

[0008] Another object of the present invention is to decouple the optical reference surface from the mechanical reference.

[0009] Another object of the invention is to prevent or at least reduce the influence of mechanical deformations on the precision.

[0010] The invention relates to a laser tracker for measuring distances to a target point using a measuring beam. The laser tracker comprises a reference piece in the form of a body of revolution which is fixedly arranged in a housing of the laser tracker. The target axis of the measuring beam can rotate about a fixed node in the reference piece. By way of example, from here on, as a specific embodiment, laser will be used interchangeably with measuring beam. Specific features of other types of optical measuring beams may apply accordingly.

[0011] The laser tracker of the present invention belongs to the group of high-precision tracking instruments. In particular, the laser tracker provides distance data with an accuracy better than 1 μm. Contemporary laser trackers achieve this accuracy based on interferometry or time-of-flight measurement. As an example, many features of the laser tracker of the present invention will be disclosed in conjunction with interferometry. However, this should not be interpreted in a restrictive manner. Those skilled in the art understand that aspects of the present invention can be applied in combination with many different types of tracking instruments. Specific features of tracking instruments operating with different distance measurement principles (e.g., pulsed time-of-flight or frequency modulated continuous mode lidar) can be applied accordingly.

[0012] The laser tracker comprises an optical device configured to provide a measurement path of a measurement beam to a target point and an internal reference path. The optical device comprises an optical unit and an optical fiber connected to a source of the measurement beam. The optical unit comprises: (i) a feed-in point for the optical fiber; (ii) a beam outlet for emitting the measurement beam along the target axis; (iii) a contact surface configured to contact a tangent point on the surface of a reference piece. A person skilled in the art understands that the above and any additional numbering is only for readability purposes and does not indicate a specific spatial order or temporal sequence, unless expressly specified to the contrary. Reasonable variations in the geometry or sequence of components for performing methodical steps are within the meaning of the present invention.

[0013] The optical unit is embodied as a mechanical fixture. That is, under the expected forces and torques, the optical unit can be approximated by a rigid body. The optical unit does not include any mechanical intervention options that allow the user of the laser tracker to actively interfere with the mechanical arrangement of the optical unit to change the optical path length between the feed point and the beam outlet. The optical unit also does not include any movable parts intended to change the optical path length between the feed point and the beam outlet during operation of the laser tracker. In other words (except from possible unavoidable but negligible thermal expansion), the optical path between the feed point and the beam outlet is constant. Constant in the sense of the present invention means that the optical path does not change during the measurement operation due to changes in the attitude of the optical unit or the tracker as a whole. The optical unit may include beam deflection and beam shaping elements configured to provide said optical path. The part of the measurement beam inside the optical unit may in particular be implemented as a free beam, while the beam deflection and beam shaping elements may include high reflectivity mirrors, semi-transparent mirrors, collimating lenses and focusing lenses. The optical unit may be implemented without polarization splitters or wave plates. Although one aspect of the invention relates to the realization of a simplified optical unit, the invention is applicable to many reasonable implementations of the optical unit. Some implementations are known from the prior art and the person skilled in the art is able to select a suitable implementation.

[0014] The contact surface is configured to contact the reference, thereby constraining the movement of the optical unit. In other words, the poses of the various components of the optical unit are precisely defined, in particular with respect to the reference itself. Other optical components (e.g., light guides, laser sources, etc.) that are at least partially located outside the housing of the laser tracker are not considered to be part of the optical unit in the sense of the present invention, but they may be part of the overall optical device.

[0015] The laser tracker is configured to use the surface of the reference as a mechanical guide for the movement of the optical unit around the node. The laser tracker includes a force supply device configured to provide a force acting on the optical unit (as a whole). The force acting on the optical unit causes the optical unit to maintain contact with the surface of the reference via a tangent point of the surface of the reference.

[0016] In other words, the reference element defines the optical reference surface. The reference element can define the optical reference directly or indirectly, in particular by realizing a controlled movement of an optical reference element, the attitude of the optical reference element is always defined relative to the reference element. The advantage of indirect definition is that another element, such as an optical plane or an end face of an optical fiber, can be used as an optical reference. This makes the design of the optical unit simpler and the requirements on the individual components are less stringent. In addition, by decoupling the mechanical reference surface and the optical reference surface, the optical properties of the reference element become irrelevant. By way of example, only the indirect definition of the optical reference surface will be discussed in detail.

[0017] In some embodiments, the reference is spherical and the node is the center of mass of the spherical reference. Due to the accuracy requirements, the sphere is to be understood in a strict manner. In particular, the reference has an ISO-21920-2:2021 RMS surface roughness below 20 nm and a shape deviation below 200 nm. In other words, the mechanical reference is foreseen as a perfect sphere and the optical unit is configured to move along the surface of this sphere.

[0018] In some embodiments, the force provided by the force supply device is constant in time and independent of the posture of the optical unit. In other words, the weight of the optical unit and the components that drive it around the reference do not play a role in setting the contact force. Furthermore, in some embodiments, the force is negligible with respect to the compliance of the mechanical reference and its mounting.

[0019] In some embodiments, the contact surface is embodied as a wear-free diamond surface, in particular a wear-free diamond plate.

[0020] In some embodiments, the force supply device comprises a spring, in particular a pre-tensioned helical spring, configured to generate a spring force acting on the optical unit and keeping the optical unit in contact with the surface of the reference piece.

[0021] In some embodiments, the force supply means is provided by a spring pair, which is mounted such that the lines of action of the springs of the spring pair are parallel to the target axis, and the target axis is located between the lines of action. Such an embodiment is advantageous because it is less challenging to balance the tangential components and / or moments of the spring forces using multiple springs. Other geometries including three or more springs are also possible within the meaning of the present invention.

[0022] In some embodiments, the laser tracker is configured to provide a magnetic attraction force between the optical unit and the reference to maintain contact with the surface of the reference. Magnet-based contact is advantageous because no external forces other than friction and gravity are applied to the reference. In other words, the reference and the optical unit can be implemented as a system that is mechanically decoupled from the environment and / or other components of the laser tracker. Magnet-based contact is also advantageous because the magnetic force can be precisely controlled, unlike a spring force, which may drift due to wear of the spring.

[0023] In some embodiments, the force supply means is provided by a magnetic element arranged at the optical unit, in particular at or near the contact surface, more particularly arranged adjacent to a contact point on the contact surface for contacting the tangent point. The reference piece is configured to be magnetically attractable. More specifically, the magnetic element is implemented as a permanent magnet or an electromagnet, and the reference piece includes a ferromagnetic material or is made of a ferromagnetic material. Magnetic elements based on permanent magnets are advantageous because they do not require power lines, i.e. wiring that may apply force to the carriage during movement of the optical unit. On the other hand, electromagnets can allow electronic control of the force between the contact surface and the tangent point.

[0024] In some specific embodiments, the force supply device includes at least one guide member that allows uniaxial translation of the optical unit. In particular, the direction of the uniaxial translation is parallel to the target axis. That is, the displacement of the target axis is prevented by preventing the rotation of the optical unit.

[0025] In some embodiments, the magnetic element is mounted with its polar axis parallel to the target axis.Alternatively or additionally, the force supply means comprises or is provided by a pair of magnetic elements mounted such that the target axis is symmetrically located between the polar axes of the magnetic elements.

[0026] In some embodiments, the laser tracker comprises a suspension device for force-free suspension of the optical unit. In particular, the suspension device is configured to support the weight of the optical unit so that in various relative attitudes of the optical unit with respect to the surface of the reference, gravity-induced interactions between the contact surface and the surface of the reference are minimized, in particular prevented. Despite the direct mechanical contact between the reference and the optical unit, these embodiments allow for improved accuracy, since the forces applied to the reference are reduced exclusively to the friction of the rotary bearings, regardless of the angle or mounting position of the entire system. In other words, these embodiments enable a simpler design with larger tolerances of the interferometer and the mechanism.

[0027] In some embodiments, the suspension device comprises a tilt axis penetrating the center of mass of the optical unit, thereby allowing a forceless tilt of the optical unit about the tilt axis. The tilt axis may be horizontal, i.e. parallel to a defined plane of the laser tracker. In other words, the tilt axis is perpendicular to the direction of gravity in order to justify the attitude of the laser tracker. The tilt axis may be foreseen as a continuous, one-piece physical axis, but may also be an imaginary axis of rotation, for example provided by a suspension device that provides multiple degrees of freedom for the movement of the optical unit.

[0028] In some embodiments, the tilt axis is perpendicular to the target axis and offset from the target axis, and the tilt axis and the tangent point define a plane tangent to the surface of the reference member. In other words, for any position of the optical unit relative to the reference member, the center of mass of the optical unit and the tangent point (contacted by the contact surface of the optical unit during operation) lie on a line tangent to the surface of the reference member. Therefore, the optical unit can only apply a force to the reference member along the normal direction of the reference member.

[0029] In some embodiments, the suspension device comprises a swivel or rotation bearing, in particular embodied as a low-friction ball bearing or roller bearing, to provide or mount the tilt axis.

[0030] In some embodiments, the optical unit comprises at least one counterweight mounted in such a way as to ensure that the tilt axis penetrates the center of mass of the optical unit, in particular wherein the counterweight is mounted in an adjustable manner. Due to the high precision requirements of adjustable in the sense of the invention means "factory adjustable", i.e. the attitude or weight of the counterweight may be adjustable during rare calibration events rather than during daily use of the laser tracker.

[0031] In some embodiments, the laser tracker is configured to perform distance measurements based on the principle of interferometry. In particular, the laser tracker is configured to provide distance measurements to a target point with sub-micron accuracy. Although the laser tracker of the present invention is in principle compatible with non-interferometric methods or partial interferometric methods (e.g., frequency modulated continuous wave lidar), interferometry represents an overall adequate compromise between accuracy and size / weight of the optical device.

[0032] In some embodiments, the optical fiber is embodied as a non-polarizing single mode fiber and the feed point provides an optical reference point for the internal reference path. The distance to the target point can also be measured from the feed point. Such interferometric designs are known from the prior art. In some embodiments, the distal end of the optical fiber is located outside the housing and optically coupled to a fiber splitter. Due to limited space and from a thermal management perspective, it is advantageous to have a laser source outside the housing of the laser tracker. Design elements configured to minimize inaccuracies caused by forces applied by the optical fiber connected to the external point are also known from the prior art.

[0033] In some embodiments, the optical unit includes a first portion having a first thermal expansion coefficient and a second portion having a second thermal expansion coefficient. The first thermal expansion coefficient is less than 10 -5 K -1 , especially less than 10 -6 K -1 The second thermal expansion coefficient is related to the thermal expansion coefficient of the reference piece. The dimensions of the second part are designed so that its thermal expansion compensates the thermal expansion of the reference piece. In other words, the first part is made of invar alloy. Due to machining requirements, the reference piece itself cannot be made of this alloy, so the insert of the optical unit is configured to compensate for the thermal expansion of the reference piece and the change in refractive index due to thermal effects.

[0034] In some embodiments, the laser tracker includes a base plate. The base plate has a fixed spatial relationship with the reference member and is configured to define an orientation of the laser tracker relative to an external coordinate system. The housing is rotatable relative to the base plate.

[0035] In some embodiments, the laser tracker includes a support unit that supports the optical unit. The support unit is rotatably mounted to the base. The housing is mounted to the support unit. The thermal expansion coefficients of the base and the support unit are selected to minimize thermal movement of the reference point relative to the base.

[0036] In some specific embodiments, the housing is configured to enclose the reference piece and the optical unit in a dustproof and moisture-proof manner. The housing includes a fixed exit window transparent to the measuring beam, so that for the distance measurement to the target point, the measuring beam is emitted at the beam outlet of the optical unit and then passes through the exit window. In some specific embodiments, the exit window has a cylindrical shape and has a range of at least 50°, in particular a range of at least 80°, in a direction perpendicular to the base plane. Large exit windows are advantageous because they implement a design in which the exit window is built into the housing with a fixed inclination. This embodiment provides many beneficial characteristics: (i) the housing can be implemented as a hard shell providing better mechanical properties, (ii) due to the lack of moving parts, sealing can be achieved in a more reliable manner, and (iii) the drive device of the exit window and / or the direct mechanical connection between the optical unit and the exit window can be omitted, which eliminates a possible source of measurement inaccuracy. The latter is particularly advantageous because, unlike the prior art, the optical unit and the reference piece are in mechanical contact with each other.

[0037] The exit window may be formed of chemically strengthened silicate-based glass or sapphire-based material.

[0038] Although modern manufacturing techniques cannot produce exit windows with the required uniformity, due to the fixed geometry, this measurement error can be characterized and stored in a lookup table for compensation. In other words, improved accuracy can be achieved by straightforward means.

[0039] In some embodiments, the laser tracker includes a handle in a fixed spatial relationship with the base. The laser tracker is configured to provide a transport mode. In the transport mode, the rotation of the housing is locked and the exit window is located between the reference member and the handle. In some specific embodiments, during the start-up sequence, the exit window automatically locates to the opposite side of the handle. In some embodiments, the handle is removable to allow a full 360° measurement of the orientation axis.

[0040] In some embodiments, the base includes three pads, which protrude from the base on the side opposite to the reference member and provide three-point support for the base. The pad defines the plane of the base. The pad can be implemented as a fixed or leveling foot. The base includes a plurality of boreholes. The boreholes are configured to accommodate fastening elements that provide a mechanical connection to an external object, in particular bolts connected to a corresponding slider. The plurality of boreholes include a first group and a second group. Each borehole of the first group of boreholes is located in one of the three pads. The second group of boreholes includes at least one borehole outside the three pads. It is obvious to a person skilled in the art that the boreholes represent a general feature category configured to accommodate the fixing elements, and different types of closed or open grooves are boreholes in the sense of the present invention.

[0041] In some embodiments, the plane defined by the first borehole from the first group, the second borehole from the second group, and the node is perpendicular to the plane of the base. In other words, the first borehole and the second borehole are located on a line that intersects the projection of the node to the bottom surface. The first borehole and the second borehole can be equidistant from the node.

[0042] In some embodiments, the boreholes of the first group and in particular at least one borehole of the second group are arranged equidistant from the node. In particular, wherein the pads are arranged as an equilateral triangle.

[0043] In some embodiments, the second group includes a plurality of boreholes, in particular two. More specifically, the boreholes of the second group are arranged so that each borehole of the second group is associated with a corresponding borehole of the first group and a plane defined by the two boreholes, and the node is perpendicular to the plane of the base.

[0044] For example, for each pad, the base includes a second set of boreholes in an opposing position. The first set of boreholes and the second set of boreholes may be located on the periphery of the base in a regular hexagonal arrangement. Alternatively, the handle may be arranged in a position opposing one of the pads, and the base includes a second set of two boreholes in a position opposing the other pad.

[0045] In some embodiments, one of the boreholes in the first set and one of the boreholes in the second set provide a fastening axis, and the base has reflection symmetry with respect to the fastening axis.

[0046] The present invention, by itself or in combination with any of the above features, also relates to a laser tracker, wherein a target axis of a measurement beam can rotate around a fixed node in a rotational body-shaped reference member. The reference member is fixedly arranged in a housing of the laser tracker. The laser tracker includes an optical device configured to provide a measurement path of the measurement beam to the target point and an internal reference path. The optical device includes an optical unit and an optical fiber connected to a source of the measurement beam. The optical unit includes (i) a feed point for the optical fiber, and (ii) a beam outlet for emitting the measurement beam along the target axis. The laser tracker is configured to use the surface of the reference member as a mechanical guide for the movement of the optical unit around the node. The optical unit may include a reference point on the surface of the reference member. The laser tracker includes a base. The base has a fixed spatial relationship with the reference member and is configured to define the orientation of the laser tracker relative to an external coordinate system. The housing can rotate relative to the base. The base comprises (i) three pads protruding from the base on a side opposite to the reference member and providing three-point support for the base, and (ii) a plurality of boreholes configured to receive fastening elements providing a mechanical connection to an external object, in particular bolts connected to the corresponding slider. The plurality of boreholes comprises a first group and a second group. Each borehole of the first group is located in one of the three pads. The second group of boreholes comprises at least one borehole outside the three pads.

[0047] The present invention, by itself or in combination with any of the above features, also relates to a laser tracker, wherein a target axis of a measurement beam can rotate around a fixed node in a reference member in the form of a body of revolution. The reference member is fixedly arranged in a housing of the laser tracker. The laser tracker comprises an optical device configured to provide a measurement path of the measurement beam to a target point and an internal reference path. The optical device comprises an optical unit and an optical fiber connected to a source of the measurement beam. The optical unit comprises (i) a feed point for the optical fiber, and (ii) a beam outlet for emitting the measurement beam along the target axis. The laser tracker is configured to use a surface of the reference member as a mechanical guide for the movement of the optical unit around the node. The optical unit may include a reference point located on the surface of the reference member. The laser tracker comprises a base. The base has a fixed spatial relationship with the reference member and is configured to define the orientation of the laser tracker relative to an external coordinate system. The housing (i) is rotatable relative to the base; (ii) is configured to enclose the reference piece and the optical unit in a dust-proof and moisture-proof manner, and (iii) includes a fixed exit window that is transparent to the measuring beam, so that for distance measurement to a target point, the measuring beam exits at a beam outlet of the optical unit and then passes through the exit window.

[0048] In some embodiments, the laser tracker includes a handle in a fixed spatial relationship with the base. The laser tracker is configured to provide a transmission mode. In the transmission mode, the rotation of the housing is locked and the exit window is positioned between the reference member and the handle. In some specific embodiments, during the start-up sequence, the exit window automatically positions to the opposite side of the handle. In some specific embodiments, the handle is removable. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] By way of example only, specific embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which:

[0050] Figure 1a and Figure 1b A schematic diagram of a prior art laser tracker is depicted.

[0051] Figure 2a and Figure 2b A schematic diagram of an embodiment of a laser tracker of the present invention is depicted, the laser tracker comprising a spring pair as force setting element.

[0052] Figure 3 A schematic diagram of an embodiment of a laser tracker of the present invention comprising magnets as force setting elements is depicted.

[0053] Figure 4 A schematic diagram of an embodiment of a laser tracker of the present invention having a suspension arrangement for forceless suspension of an optical unit is depicted.

[0054] Figure 5 A schematic diagram of an embodiment with a large field of view fixed exit window is depicted.

[0055] Figures 6a to 6c The base with pads and drilled holes is shown viewed from the bottom. DETAILED DESCRIPTION

[0056] Figure 1a A laser tracker 1 of the prior art is schematically shown. The laser tracker 1 is configured to determine the distance between a target point 2 and an optical reference point 3. In some embodiments, the target point 2 is a specially designed retroreflector. The laser tracker 1 is configured to track the movement of the target point 2. Various tracking methods are known in the prior art, and the skilled person can provide a suitable tracking method. The present invention is not limited to any known specific tracking method, in particular, the present invention is applicable to improvements, enhancements of the described or suitable alternative methods. In the depicted embodiment, the optical reference point 3 is located on the surface of a mechanical reference member 5. The reference member 5 is rigidly mounted to a base 8 via an axis 85. The base 8 is configured to define the relative orientation of the laser tracker 1 with respect to an external coordinate system, in particular to define a plane 80 of the base 8.

[0057] In order to determine the distance between the target point 2 and the optical reference point 3, the laser tracker 1 includes an optical device 60. The optical device 60 includes an optical fiber 600 and an interferometer. The distal end of the optical fiber 600 is coupled to a laser source, which can be located outside the housing 9 of the laser tracker 1. The components of the optical device 60 located inside the housing 9 of the laser tracker 1 and configured to move substantially together can be regarded as equivalent to an optical unit.

[0058] The interferometer comprises a feed point 69 for the optical fiber 600 and a beam outlet 68 for emitting the measurement beam 4 along the target axis 42. The interferometer transmits the measurement beam 4 and defines a part of the optical path inside the interferometer of the laser tracker 1. Some key features of the depicted interferometer are Figure 1b is shown in more detail in .

[0059] Laser tracker 1 further comprises a bracket configured to align optical device 60 so that target axis 42 corresponding to a portion of the measurement beam path outside housing 9 intersects target point 2 and node 50 of reference piece 5. In other words, bracket is configured to rotate a portion of optical device 60 around node 50 in reference piece 5.

[0060] As an example, the carriage comprises a support unit 71 mounted to a base 8 and a first motor 710, such as a direct-drive DC servo motor, to rotate the support unit 71 around a first rotation axis 79. The first rotation axis 79 coincides with the axis 85 of the reference piece 5. The carriage further comprises a yoke 72 and a second motor 720 to tilt the yoke 72 around a second rotation axis 78 perpendicular to the first rotation axis 79. The intersection 789 of the first rotation axis 79 and the second rotation axis 78 coincides with the node 50 within the required accuracy. Although the interferometric measurement principle allows for small lateral offsets of the interferometer, the components 71, 72, 710, 720 of the carriage require accuracy in the μm range. There are many alternatives to the depicted prior art carriage, and the applicability of the invention is not limited to a specific variant.

[0061] Figure 1bAn embodiment of an optical device 60 is illustrated, in particular an interferometer known in the prior art. The interferometer moves with the carriage. For this reason, the laser source 602 cannot be fed directly to the interferometer, so a polarization-maintaining fiber 600 is used to emit two orthogonal polarizations into the interferometer at a feed point 69. Disturbances in the polarization-maintaining fiber 600, for example due to thermal drift or mechanical movement, will cause mixing of the two orthogonal polarizations within the fiber 600. In order to prevent this mixing from affecting the accuracy of the displacement measurement, a reference counter signal 411 is extracted at the interferometer by a first beam splitter 61. In this way, the effects of polarization mixing are common to the reference counter signal and the measurement counter signal and are therefore eliminated. The signal is fed to a reference count input 612 on a counter card via an optical fiber 601.

[0062] Polarization beam splitter 64 separates two orthogonal polarizations. One passes directly as reference 441. The second 431 is sent to the optical reference point 3 via quarter wave plate 631 and returns 432 to polarization beam splitter 63 via quarter wave plate 631. Each pass through quarter wave plate 631 rotates the polarization state of beams 431, 432 by 45°. The target beam 421 then passes through polarization beam splitter 64 and second quarter wave plate 641 to target point 2. Target 2 reflects the return beam 422 back through quarter wave plate 641, which ensures that when hitting polarization beam splitter 64, measurement beam 442 is reflected out to interfere with reference beam 441. The measurement signal generated by this interference is transmitted via optical fiber 604 to measurement count input 642 on the counter card. The third beam splitter 65 is used to direct a portion 451 of the return beam 422 from target 2 to a position sensitive detector 652, which provides position feedback for tracking servo. For the sake of transparency, collimation, focusing and other beam shaping elements are not shown. Such elements can be provided by a person skilled in the art based on the prior art.

[0063] It can be seen that the prior art system requires a rather complex optical arrangement 60 due to the requirement of moving the interferometer and due to the fact that the mechanical reference directly provides the optical reference point 3. The latter also requires that the mechanical reference should have a surface that can provide the optical reference point 3.

[0064] Figure 2aA schematic diagram of a first embodiment of the laser tracker 1 of the present invention is illustrated. In the implementation of the inventive concept depicted, the mechanical reference element 5 does not directly provide the optical reference point 3. The optical reference point 3, depicted as a feed point 69 of a non-polarized single-mode optical fiber 301, performs a defined (i.e. constrained) movement along a surface. The node 50 of the surface and the center point of the reference element 5 coincide. The optical device comprises a mechanically fixed optical unit 6, i.e. its internal components including the feed point 69 have a defined spatial relationship with each other. The depicted optical unit 6 is mounted on a yoke 72 so that its posture is defined by the posture of the yoke 72. As shown Figure 1a As shown, the yoke 72 is part of a bracket which aligns the optical unit 6 so that the measuring beam 42 strikes the target point 2 and is reflected back from the target point 2 .

[0065] The optical unit 6 comprises a contact surface 675, in particular a wear-free diamond plate, which is configured to contact a tangent point 51 on the surface of the reference piece 5. This contact ensures a defined spatial relationship between the optical reference point 3 and the reference piece 5. A high reflectivity mirror 67 configured for deflecting the measuring beam 4 is mounted with the contact surface 675.

[0066] The laser tracker 1 comprises a force supply device, which is embodied as a spring pair 76, 77, which is configured to provide a contact force 705 between the contact surface 675 and the reference member 5, i.e., a net force generated by spring forces 760, 770. The spring pairs 76, 77 are mounted so that their lines of action 761, 771 are parallel to the target axis 42, and the target axis 42 is located between the respective lines of action 761, 771. Although the spring pairs 76, 77 represent a good compromise between the complexity of the force supply element and the balance between the weight and the tangential component of the contact force 705, the present invention can be implemented with other spring geometries.

[0067] The contact force 705 is arranged such that it does not cause substantial deformation or displacement of the mechanical reference 5. In other words, the spring pair 76, 77 is configured to provide spring forces 760, 770, whose compliance with respect to the reference 5, the shaft 85 or the seat 8 is negligible. In order to achieve the required accuracy in the sub-micron range, the contact force 705 (i.e., the spring forces 760, 770) must not exceed 100 mN, preferably not exceed 10 mN.

[0068] Figure 2b Shows Figure 2a Schematic description of the optical arrangement 60 of the laser tracker depicted in FIG. Since the reference is not an optical component, the requirements on the wavefront quality are significantly reduced. Figure 1bThis makes the interferometer much simpler than the design of the laser tracker. Laser light 40 from a stable laser source 602 is fed to a fiber splitter 66 connected to an unpolarized single mode fiber 301. A feed point 69 located in the housing of the laser tracker and in a fixed spatial relationship with the other components of the interferometer reflects a portion of the laser light back to the fiber 301. The reflected laser light is used as reference light 441, and the feed point 69 is used as an optical reference point 3.

[0069] The light 432 emitted from the reference point 3 is deflected by a corresponding beam deflecting element (e.g., a high reflectivity mirror 67) toward the target 2. The target 2 reflects the transmitted beam 421 back to the mirror 67. The return beam 422 is deflected toward the optical reference point 3 by the mirror 67. The measurement beam 442 enters the optical fiber 301 and interferes with the reference beam 441. The measurement signal generated by the interference is decoupled by the optical fiber splitter 66 and transmitted to the measurement count input 642 on the counter card via the optical fiber 304.

[0070] The design is advantageous not only from the perspective of complexity and cost, but also in terms of measurement accuracy. The interferometer is a moving part that rotates around a reference. Its weight and the deformation caused by this weight are important sources of system errors, so reduced weight is desirable for a general-purpose laser tracker.

[0071] Figure 3 A second embodiment of the laser tracker 1 of the present invention is shown. Figure 2a The key difference between the first embodiment depicted is that the contact force 705 between the reference piece 5 and the contact surface 675 is provided by a magnetic element 75, which is depicted as a permanent magnet and is mounted on the contact surface 675, on the side opposite to the side in contact with the reference piece 5. Advantageously, the magnetic attraction between the reference piece 5 and the magnetic element 75 can be understood as a purely internal force, i.e., anchoring to another component is not required. The reference piece 5 is made of or includes a ferromagnetic material. The skilled person understands that changes in the number, placement or operating principle of the magnetic element 75 are within the meaning of the present invention. For example, a pair of electromagnets or a trio of electromagnets can be used equivalently instead of a single permanent magnet.

[0072] The optical unit 6 is mounted to a bracket represented by a yoke 72 such that a pair of guides 73, 74 and corresponding channels 730, 740 allow uniaxial translation of the optical unit 6. A magnetic element 75 is located between the axes 731, 741 of the channels 730, 740. The axes 731, 741 of the channels 730, 740 are parallel to the target axis 42. This geometry compensates for the tangential component of the contact force 705.

[0073] Figure 4FIG. 1 shows a third embodiment of the laser tracker 1 of the present invention. Figure 3 The key difference between the second embodiment depicted in FIG. 1 and FIG. 2 is that the laser tracker 1 comprises a suspension device 70 for the forceless suspension of the optical unit 6. The suspension device 70 is configured to support the weight of the optical unit 6 so that in each relative attitude of the optical unit 6 with respect to the surface of the reference 5, a gravity-induced interaction between the contact surface 675 and the surface of the reference 5 is prevented.

[0074] In the depicted embodiment, the suspension device 70 comprises a tilt axis 700 penetrating the center of mass of the optical unit 6. By way of example, the tilt axis 700 is horizontal. Horizontal in the sense of the invention may be understood as being parallel to the plane 80 of the base 8. In other words, the depicted tilt axis 700 is perpendicular to the direction of gravity for a correct adjustment of the attitude of the laser tracker 1. The tilt axis 700 may be implemented as a continuous, one-piece physical axis, but may also be implemented as an imaginary axis of rotation. The depicted tilt axis 700 is perpendicular to the target axis 42 and offset from the target axis 42. The tilt axis 700 and the tangent point 51 define a plane 702 tangential to the surface of the reference element 5. The optical unit 6 comprises a counterweight 701 mounted in such a way as to ensure that the tilt axis 700 penetrates the center of mass of the optical unit 6. The suspension device 70 also provides the function of the guides of the second embodiment, so they are not necessary in this embodiment.

[0075] In the depicted embodiment, the force supply means is provided by a pair of magnetic elements 751 , 753 mounted such that the target axis 42 is located between the polar axes 752 , 754 of the magnetic elements 751 , 753 .

[0076] It is clear to those skilled in the art that the first, second and third embodiments described focus on illustrating some specific features of the laser tracker of the present invention. The various features of these embodiments can be combined with each other and / or replaced by suitable alternative features, or some optional features can be completely omitted.

[0077] Figure 5 An embodiment is shown in which the housing 9 comprises a fixed exit window 91, so that for distance measurement to a target point, the measuring beam along the target axis 42 exits at the beam outlet 68 of the optical unit 6 and then passes through the exit window 91. The exit window 91 provides an inclined field of view 910 of at least 50°, in particular an inclined field of view 910 of at least 80°, observed from the node 50. Inclined means perpendicular to the plane 80 of the base 8.

[0078] This design offers many advantages compared to the prior art with a small movable exit window 91. A fixed exit window 91 does not require, for example, a bellows, so the housing 9 can be implemented as a completely closed hard shell, which provides better safety and environmental sealing. In addition, a separate motor synchronized with the optical unit 6 or a direct mechanical coupling between the optical unit 6 and the exit window 91 can also be omitted, which enables a more precise design of the laser tracker 1.

[0079] The depicted embodiment also includes a handle 82 in a fixed spatial relationship with the base 8. The depicted handle 82 includes a mounting element 820 that allows the handle 82 to be quickly installed / removed from the base 8. As depicted, the laser tracker 1 is in an operating mode, that is, the exit window 91 and the handle 82 are located on opposite sides of the reference 5. The laser tracker 1 is configured to provide a transmission mode in which the rotation of the housing 9 is locked and the exit window 91 is located between the reference 5 and the handle 82. The handle 82 covers the exit window 91 and provides shielding from impact damage.

[0080] Figure 6a A schematic diagram of a base 8 according to the present invention is shown. The base 8 has three pads 831-833 protruding from the base 8 and arranged near the periphery of the base 8. In the depicted embodiment, the pads 831-833 are arranged in an equilateral triangle arrangement. The pads 831-833 define the plane of the base 8. The pads 831-833 also have a corresponding first set of boreholes 841-843, which are configured to accommodate fastening elements that provide a mechanical connection to an external object. The fastening elements can be bolts connected to the corresponding sliders. For example, the boreholes 841-843 are embodied as circular holes.

[0081] Due to the geometry of the external object, three-point fixation is not always possible. Therefore, the base 8 has a second set of boreholes 844 spaced apart from the first set of boreholes 841-843. In this particular embodiment, the second set of boreholes 844 is arranged in one of the first set of boreholes, namely borehole 841, and the second set of boreholes 844 provides a fastening axis 84. The base 8 has a reflection symmetry with respect to the fastening axis 84. This allows two-point fastening of the base 8, whose fastening axis 84 is located between the other two pads 832, 833, that is, a three-point support using pads 831-833 is possible. In the depicted embodiment, the fastening axis 84 intersects with the axis 85 of the reference piece 5, or in other words, intersects with the projection of the reference piece 5 to the base 8. The second group may include additional boreholes.

[0082] Figure 6bDepicted is the step of mounting the base 8 (with laser tracker) to an external object 93 having a surface with grooves 92 enabling positioning of a slide 94. A first set of boreholes 841-843 associated with pads 831-833 is implemented as elongated holes. The distance 920 of the grooves 92 makes three-point fastening possible using the first set of boreholes 841-843.

[0083] Figure 6c An alternative mounting situation is depicted. Here, a three-point mounting using the first set of boreholes 841-843 located inside the pads 831-833 is not possible due to the distance 920 of the recess 92 on the external object 93. In contrast, a two-point mounting based on one borehole (here hole 841) of the first set of boreholes 841-843 and one borehole (here hole 844) of the second set of boreholes 844-846 is always possible, because the two points can be aligned in a line. Since the selected first hole 841 and second hole 844 are located between two other pads 832, 833, a three-point support based on three pads 831-833 can also be achieved. For example, the first set of boreholes 841-843 is depicted as a semi-open recess, and the second set of boreholes 844-846 is depicted as a cylindrical hole. The skilled person understands that there are many suitable alternatives, so the placement and shape of the depicted boreholes 841-846 should not be interpreted in a restrictive manner.

[0084] Although the present invention has been described in the foregoing with reference to some specific embodiments, it must be understood that various modifications and combinations of different features of the embodiments are possible. All these modifications are within the scope of the appended claims.

Claims

1. A laser tracker (1) for measuring the distance to a target point (2) using a measuring beam (4), wherein: - the target axis (42) of the measuring beam (4) is rotatable about a fixed node (50) in a solid-of-revolution reference element (5) which is fixedly arranged in the housing (9) of the laser tracker (1), - the laser tracker (1) comprises an optical device (60) configured to provide a measurement path and an internal reference path (441) of the measurement beam (4) to the target point (2), The optical device (60) comprises an optical unit (6) and an optical fiber (301, 600) connected to a source (602) of the measuring beam (4), wherein the optical unit (6) comprises: o a feeding point (69) of the optical fiber (301, 600), o a beam outlet (68) for emitting the measuring beam (4) along the target axis (42), and o a contact surface (675) configured to contact a tangent point (51) on the surface of the reference member (5), - the laser tracker (1) is configured to use the surface of the reference (5) as a mechanical guide for the movement of the optical unit (6) around the node (50), It is characterized in that - the optical unit (6) is embodied as a mechanical fixture, and The laser tracker (1) comprises a force supply device configured to provide a force (705) acting on the optical unit (6), the force (705) keeping the optical unit (6) in contact with the surface of the reference element (5).

2. The laser tracker (1) according to claim 1, wherein: The reference member (5) is spherical, and the node (50) is the centroid of the spherical reference member (5).

3. The laser tracker (1) according to claim 1 or 2, wherein: The contact surface (675) is embodied as a wear-free diamond surface, in particular a wear-free diamond plate.

4. The laser tracker (1) according to any one of the preceding claims, wherein: The force supply device comprises a spring (76, 77), in particular a pre-tensioned helical spring, the spring (76, 77) being configured to generate a spring force (760, 770) acting on the optical unit (6) and keeping the optical unit (6) in contact with the surface of the reference element (5), In particular, - the force supply means is provided by a spring pair, the spring pair being mounted so that the lines of action (761, 771) of the springs (76, 77) of the spring pair are parallel to the target axis (42), and The target axis (42) is located between the lines of action (761, 771), in particular symmetrically between the lines of action (761, 771).

5. The laser tracker (1) according to any one of the preceding claims, the laser tracker (1) being configured to provide a magnetic attraction force between the optical unit (6) and the reference (5) to maintain contact with the surface of the reference (5).

6. The laser tracker (1) according to claim 5, wherein: The force supply device is provided by: - a magnetic element (75, 751, 753), the magnetic element (75, 751, 753) being arranged at the optical unit (6), in particular at or near the contact surface (675), more particularly arranged adjacent to a contact point on the contact surface (675) for contacting the tangential point (51), and - the reference element (5) is configured to be magnetically attractive, In particular, - the force supply means comprise at least one guide (73, 74) allowing a uniaxial translation of the optical unit (6), - the magnetic element (75, 751, 753) is embodied as a permanent magnet or an electromagnet, - the reference element (5) comprises a ferromagnetic material or is made of a ferromagnetic material, More specifically, - the direction of the uniaxial translation is parallel to the target axis (42), and - the magnetic elements (75, 751, 753) are mounted so that their polar axes (752, 754) are parallel to the target axis (42), or the force supply means is provided by a pair of magnetic elements (75, 751, 753), and the pair of magnetic elements (75, 751, 753) are mounted so that the target axis (42) is symmetrically located between the polar axes (752, 754) of the magnetic elements (75, 751, 753).

7. The laser tracker (1) according to claim 5 or 6, comprising a suspension device (70), the suspension device (70) being configured to provide a forceless suspension of the optical unit (6), In particular, The suspension device (70) is configured to support the weight of the optical unit (6) so that in various relative postures of the optical unit (6) relative to the surface of the reference member (5), the interaction caused by gravity between the contact surface (675) and the surface of the reference member (5) is minimized, in particular prevented.

8. The laser tracker (1) according to claim 7, wherein: The suspension device (70) comprises a tilting axis (700) which penetrates the center of mass of the optical unit (6) so as to allow forceless tilting of the optical unit (6) about the tilting axis (700).

9. The laser tracker (1) according to claim 8, wherein: - the tilt axis (700) is perpendicular to the target axis (42) and offset from the target axis (42), and - said tilting axis (700) and said tangent point (51) define a plane (702) tangential to said surface of said reference element (5).

10. The laser tracker (1) according to any of the preceding claims, the laser tracker (1) being configured to perform the distance measurement based on the interferometry principle, in particular being configured to provide the distance measurement to the target point (2) with sub-micrometer accuracy, In particular, - the optical fiber (301, 600) is embodied as a non-polarized single-mode optical fiber (301), - the feed point (69) provides an optical reference point (3) for the internal reference path (441), More particularly, the distal end of the optical fiber (301, 600) is located outside the housing (9) and optically coupled to a fiber optic splitter (66).

11. The laser tracker (1) according to any one of the preceding claims, wherein: The optical unit (6) comprises a first part having a first thermal expansion coefficient and a second part having a second thermal expansion coefficient, wherein: - The first thermal expansion coefficient is less than 10 -5 K -1 , especially less than 10 -6 K -1 , - the second coefficient of thermal expansion is related to the coefficient of thermal expansion of the reference element (5), and The second portion is dimensioned so that its thermal expansion compensates for the thermal expansion of the reference element (5).

12. The laser tracker (1) according to any one of the preceding claims, comprising a base (8), wherein: - the base (8) has a fixed spatial relationship with the reference (5) and is configured to define the orientation of the laser tracker (1) relative to an external coordinate system, and - the housing (9) is rotatable relative to the base (8), In particular, - the laser tracker (1) comprises a support unit (71) supporting the optical unit (6), - the support unit (71) is rotatably mounted to the base (8), - the housing (9) is mounted to the support unit, and - The coefficients of thermal expansion of the base (8) and the support unit (71) are selected so as to minimize thermal movements of the reference point (3) relative to the base (8).

13. The laser tracker (1) according to claim 12, wherein: The housing (9) - is configured to enclose the reference element (5) and the optical unit (6) in a dust-proof and moisture-proof manner, and - comprising a fixed exit window (91) transparent to the measuring beam (4), so that for the distance measurement to the target point (2), the measuring beam (4) exits at the beam outlet (68) of the optical unit (6) and then passes through the exit window (91), In particular, The exit window (91) - has an extent (910) of at least 50°, in particular at least 80°, in a direction perpendicular to the plane (80) of the base (8), -Formed from chemically strengthened silicate-based glass or sapphire-based materials.

14. The laser tracker (1) of claim 13, further comprising a handle (82) having a fixed spatial relationship with the base (8), the laser tracker (1) being configured to provide a transmission mode, wherein: The rotation of the housing (9) is locked, and the exit window (91) is located between the reference piece (5) and the handle (82).

15. The laser tracker (1) according to any one of claims 12 to 14, wherein: The base (8) comprises: - three pads (831-833) protruding from the base (8) on the side opposite to the reference element (5) and providing three-point support for the base (8), and a plurality of boreholes (841-846) configured to receive fastening elements providing a mechanical connection to an external object (93), in particular bolts connected to a corresponding slide (94) on the measuring table, Wherein, the plurality of boreholes (841-846) include: - a first set of boreholes (841-843), wherein each borehole of the first set of boreholes (841-843) is located in one of the three pads (831-833), and - a second set of boreholes (844-846), wherein the second set of boreholes (844-846) comprises at least one borehole outside the three pads (831-833), In particular, a plane defined by a first borehole (841) from the first group, a second borehole (844) from the second group and the node (50) is perpendicular to a plane (80) of the base (8).

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