WO2021219239A1

WO2021219239A1 - System and method for calibrating a collaborative robot system

Info

Publication number: WO2021219239A1
Application number: PCT/EP2020/085900
Authority: WO
Inventors: Mikkel Schildknecht HOÉ
Original assignee: Lego AS
Current assignee: Lego AS
Priority date: 2020-04-27
Filing date: 2020-12-14
Publication date: 2021-11-04
Anticipated expiration: 2022-10-27

Abstract

The present invention relates to a calibration device, and a cooperating calibration piece, for the spatial calibration of a robot system. The robot system comprises: a worktable with a worktable surface, the worktable having a worktable coordinate systern and defining a reference frame of the robot system; a robot handler operable for handling parts on the worktable surface, the robot handler having a handling coordinate system for describing movements of the robot handler with respect to the reference frame, and an imaging device operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate systern associated with the field of view. The calibration device comprises engagement elements adapted for engaging the worktable in a pre-determined reference placement with respect to the reference frame, wherein a three-dimensional spatial alignment of the calibration device is defined by a reference plane of the calibration device. The calibration device further comprises on a front side thereof a pre-defined arrange- ment of a plurality of calibration elements, wherein the plurality of calibration elements comprises one or more orientation calibration elements. Each orientation calibration element comprises an orientation contour shaped to define a distinct orientation and adapted for positive mechanical engagement by the cooperating calibration piece according to the distinct orientation. The calibration piece has at a proximal end thereof, a mounting element adapted for mounting the calibration piece to the robot handler of the robot system; and at a distal end thereof, a calibration body shaped as a counterpart for positive mechanical engagement of the orientation contour of the cooperating calibration device according to the distinct orientation defined by said orientation contour.

Description

System and method for calibrating a collaborative robot system

The invention relates to a production line collaborative robot system for handling small parts loosely dispersed on a worktable surface. More particularly, the invention relates to a calibration tool for calibrating a robot system, and to a method of calibrating a robot system.

BACKGROUND OF THE INVENTION

Handling small delicate parts in subsequent processing steps of a production line de mands high precision of the involved production line components. Generally, handling of a small part may include one or more steps of picking up the part, positioning or moving the part, as well as placing the part. Handling of a part in a production line may include receiving the part from an up-stream production line component, and/or passing the part to a downstream production line component. Furthermore, handling of a part in a production line may be adapted to performing one or more processing steps before, during or after handling the part. Processing steps may include e.g. fin ishing steps performed on half-finished parts, steps of applying decorative surface elements to small parts, or assembly steps using small parts. The small parts may have various different shapes, and differently shaped parts may have to be handled in the same production line and in the same production run. Small parts to be handled in a production line may be assembled from small subassembly parts and/or form (part of) a subassembly for a larger assembly themselves. For example, the dimen sions of the small parts may be in the range of a few millimeters or centimeters, and subsequent processing steps may have to be performed in a manner precisely aligned with specified regions of the small part, which may require accumulative alignment tolerances below 1/10^th of a millimeter or significantly less. This imposes severe re quirements on the precision of the handling of parts, in particular of small parts by components, such as robotic components, throughout the production line.

Small parts requiring particularly high attention to precision in production are modular toy elements adapted for being detachably connectable with each other. Such modu lar toy elements have to be produced with extreme precision with respect to specified dimensions in order to ensure interoperability of the produced modular toy elements from any batch produced on any production line used for producing such modular toy construction elements. Modular toy elements may, for example, include well-known toy construction bricks having coupling elements adapted for detachably coupling the modular toy elements to each other, or small parts of a figurine shaped toy assembly with modularly interchangeable elements.

Another challenge arising in the production of small modular toy elements may relate to the reproducibly of performing sequential processing steps on a small part in a highly accurate manner, such as reproducibly applying a decorative element on a small part received from upstream components, precisely on a desired surface portion of the small part. On the other hand, decorations or other finishing steps may vary frequently, and may thus ask for a high flexibility of the production line.

Traditional production lines may employ dedicated machines with highly specialized tooling for each tasks to ensure the desired reproducibility and accuracy in the pro duction line. However, such dedicated production lines are very costly and the high reproducibility and accuracy is achieved at the expense of any flexibility in the pro duction line, as large cost and down time may be involved in modifying the dedicated machines to perform different tasks.

Production lines using robotic components, on the other hand, may provide the de sired flexibility in respect of the varying tasks that may be performed on the production line and how to perform them. Handling a small part in a robotic production line may require, for example, finding the small part at a random location on a worktable, iden tifying the small part, and/or instructing a robot for handling the small part. Such han dling tasks may be performed, e.g. by a robot arm equipped with a suitable end effec tor, which is operable to handle small parts on a worktable surface, and which is guided by a computer vision system. However, in order to ensure the desired repro ducibility and accuracy, frequent spatial calibration of the robotic components with respect to each other is often required.

DE 102014 100538 A1 discloses a method for calibrating a robot and a camera with respect to each other. More specifically, DE 10 2014 100 538 A1 describes a posi tioning calibration method using a calibration body attached to the robot, and a coop erating ring-shaped calibration counter-body to be placed on a positioning surface: in a first step, the calibration counter-body is placed within a field of view of the camera on the positioning surface; in a second step, the calibration body attached to the robot is brought into engagement with the calibration counter-body, and the corresponding robot position data is determined; in a third step, image data of the calibration counter body is acquired, and corresponding camera coordinate data is determined. Using the robot position data and the camera coordinate data, transformation data for the calibration of the robot and the camera with respect to each other is determined. The disclosed method only provides a positioning calibration. Furthermore, the method has the drawback that it requires multiple positioning steps to be performed in order to obtain multiple points for a coordinate calibration of the robot and the camera co ordinate systems with respect to each other. As a consequence, cumulative errors may affect the quality of the calibration.

US 2018/0186004 A1 discloses a robot for performing hand-eye coordinate calibra tion. The robot includes a robot arm including a plurality of joints, a plurality of arm sections, and an end effector, a communication interface, and a control circuit. The control circuit controls the robot arm to place the external object on a worktable after the external object is grasped by the end effector, acquires coordinates of a central point of the external object in a coordinate system of the camera from an image of the external object, and calculates a calibration parameter for defining a relation between a coordinate system of the end effector and the coordinate system of the camera, based on coordinates of the end effector in a base coordinate system of the robot and coordinates of the central point of the external object in the coordinate system of the camera when the external object is placed on the worktable. The disclosed method only provides a positioning calibration. Furthermore, the method has the drawback that it requires multiple positioning steps to be performed in order to obtain multiple points for a coordinate calibration of the robot and the camera coordinate systems with respect to each other. As a consequence, cumulative errors may affect the quality of the calibration.

DE 10 2007 058 293 A1 describes a calibration device and method for calibrating a robot coordinate system with respect to a camera coordinate system. The calibration device has a base carrying a plurality of first markers arranged in a first plane and a plurality of second markers arranged in a second plane parallel to the first plane. The markers are for calibrating the camera coordinate system. The base of the calibration device further carries in a region separate of the first and second markers a plurality of measurement points adapted for calibrating the robot coordinate system with re spect to the calibration device. The disclosed calibration device is apparently com plex, difficult to fabricate, and delicate to handle. Furthermore, the device only pro vides a positioning calibration.

However, inline handling of a small part in production may require delivering the small part to a downstream step of the production line in a precisely aligned arrangement. For example, handling in a collaborative robot system comprising multiple robots may require a precise coordination of the handling between the multiple robots. Also, han dling a small part in a pick and place robot system may require precise coordination of the components. Furthermore, in a collaborative system, multiple different compo nents have to be calibrated with respect to each other in order to ensure a correct and reliable handling in the chain of processing steps. A particular challenge arises, when small parts to be handled may have various different shapes, and may be loosely dispersed and/or arbitrarily arranged on a worktable surface, which is accessed by multiple robotic components for handling the same small part, e.g. in subsequent steps. Therefore, the complexity of the calibration tasks increases, and calibration errors for multiple robotic components may build up when chained together.

Therefore, there is still a need for a simple, efficient and reliable way of calibrating multiple different components of a robotic system with respect to each other.

According to some aspects, an object of the present invention is to overcome at least some the disadvantages of the prior art mentioned above, or to provide an alternative. In one aspect, object of the present invention is to provide a simple and efficient method of spatially calibrating a camera, a robot handler, and a worktable surface of a robot system with respect to each other. According to a particular aspect, the robot system is a collaborative robot system. According to a further particular aspect, the robot system is an inline component of a production line, such as a pick and place infeed. SUMMARY OF THE INVENTION

According to some aspects, the object of the invention is achieved by a calibration device and a method for calibrating a robot system a camera, a robot handler, and a worktable surface of a robot system with respect to each other as recited by the at- tached independent claims, with advantageous embodiments as disclosed in the cor responding dependent claims and in the following.

According to a broad aspect, the invention relates to a calibration device for the spatial calibration of a robot system, the robot system comprising a worktable, a robot han- dler, and an imaging device; wherein the calibration device comprises at least one calibration element, the calibration element comprising an orientation contour shaped to define a distinct orientation and adapted for positive mechanical engagement by a cooperating calibration piece according to the distinct orientation. The calibration de vice has a reference plane for describing the three-dimensional spatial alignment of the calibration device, wherein a direction perpendicular to the reference plane is de noted as the vertical direction, and directions parallel to the reference plane are de noted as lateral directions.

The calibration device may be described as having a front side, a back side opposite to the front side, and a circumferential edge connecting the front side with the back side. Typically, a back side of the calibration device is parallel to the front side. Fur thermore, a calibration device may typically be planar and plate-shaped, wherein the front side and the back side define the plane of the calibration device. Typically, the front side and the back side are parallel to the reference plane of the calibration de- vice.

Preferably according to some embodiments, the calibration device further comprises alignment means and/or engagement elements, said means and/or elements being adapted for engaging the worktable in a pre-determined, preferably fixed, reference placement with respect to a reference frame of the robot system. Preferably, the ref erence plane of the calibration device is parallel to a worktable surface, when the calibration device is in the reference placement. Preferably according to some embodiments, the calibration device comprises one or more further calibration elements, wherein the calibration element and the further cal ibration elements are placed in a pre-defined arrangement.

Advantageously, the calibration element and/or the one or more further calibration elements comprise a positioning envelope shaped to define a unique position and adapted for positive mechanical engagement by a cooperating calibration piece ac cording to the unique position.

The worktable has a worktable coordinate system and defines a reference frame of the robot system. The robot handler is operable for handling parts on the worktable surface, and has a handling coordinate system for describing movements of the robot handler with respect to the reference frame. The imaging device is operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the captured field of view.

The calibration elements are placed in a pre-defined arrangement, in particular in a pre-defined lateral arrangement, on the calibration device. Mounting the calibration device on the worktable in a pre-defined placement provides a pre-defined fixed spa tial relation between worktable coordinate system and the pre-defined arrangement of the calibration elements on the calibration device. Mounting a cooperating calibra tion piece on the robot handler provides a fixed relation between the handling coordi nate system and the calibration piece. Bringing the calibration piece in positive me chanical engagement with the calibration elements then allows for calibrating the han dling coordinate system and the worktable coordinate system with respect to each other. In particular, bringing the calibration piece in positive engagement with the ori entation contour according to the distinct orientation defined thereby, allows for cali brating the orientation of the robot handler with respect to the reference frame. Fur thermore, bringing the calibration piece in positive engagement with the position en velope according to the unique position defined thereby, allows for calibrating the po sition of the robot handler with respect to the reference frame. The orientation contour thus provides an orientation calibration function of the calibration element, and the position envelope provides a position calibration function of the calibration element. The orientation calibration function of the orientation contour and the position calibra tion function of the position envelope may be provided in combination in the same calibration element, thereby providing a placement calibration function. Each calibra tion element may thus comprise an orientation contour shaped to define a distinct orientation of the calibration element on the calibration device and/or a positioning envelope shaped to define a unique position of the calibration element on the calibra tion device, whereby the calibration element is adapted for positive mechanical en gagement by a cooperating calibration piece according to the distinct orientation and/or unique position. A capturing portion of the imaging system, such as a camera or similar imaging sensor, is mounted in a fixed relation with respect to the reference frame of the robot system. The imaging system is configured to capture images from a field of view on the work table surface. By placing the field of view over the calibra tion device, the imaging device can be operated to capture images of the pre-defined arrangement of calibration elements on the calibration device. Analysing the captured images with respect to the position of the calibration elements in respect of information on the pre-defined arrangement allows for calibrating the imaging coordinate system and the worktable coordinate system with respect to each other. Consequently, both the handling coordinate system and the imaging coordinate system are both directly calibrated with respect to the worktable coordinate system.

According to a further aspect, the invention further relates to a cooperating to a cali bration piece for use in combination with the calibration device. The cooperating cali bration piece typically has a head portion at a distal end and a mounting portion at a proximal end. The head portion comprises a calibration body that forms counterpart to the orientation contour and/or positioning envelope of the calibration elements, for positive mechanical engagement of the calibration device according to the distinct orientation and/or unique position.

In a yet further aspect, the invention relates to a calibration kit comprising a calibration device and a cooperating calibration piece according to any of the embodiments dis closed herein. Advantageously, the calibration kit may further comprise a control mod ule comprising programmed instructions to control the robot system for performing steps of spatial calibration of a robot system according to any of the embodiments as disclosed herein. The control module may be implemented at least partly separately and/or at least partly integrated with the existing control system of the robot system.

According to some aspects, the invention relates to a calibration device for the spatial calibration of a robot system, the robot system comprising: a worktable with a work table surface, the worktable having a worktable coordinate system and defining a ref erence frame of the robot system; a robot handler operable for handling parts on the worktable surface, the robot handler having a handling coordinate system for describ ing movements of a handling end of the robot handler with respect to a base frame end of the robot handler fixed to the reference frame; and an imaging device operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the field of view; wherein the calibration device comprises engagement elements adapted for engaging the work table in a pre-determined, preferably fixed, reference placement with respect to the reference frame, wherein a three-dimensional spatial alignment of the calibration de vice is defined by a reference plane of the calibration device; wherein the calibration device further comprises on a front side thereof a pre-defined arrangement of a plu rality of calibration elements; wherein the plurality of calibration elements comprises one or more orientation calibration elements; wherein each orientation calibration el ement comprises an orientation contour shaped to define a distinct orientation in the reference plane of the calibration device and adapted for positive mechanical engage ment by a cooperating calibration piece according to the distinct orientation.

The term “spatial calibration” as used herein refers to establishing a spatial relation of two or more coordinate systems with respect to each other. The spatial relation may be used to determine a transformation between the coordinate systems. The term “position” of an item refers to a specification of the point location of said item by a set of three coordinates, such as expressed in Cartesian coordinates, cylindrical coordi nates, or spherical coordinates. The term “orientation” as used herein refers to the rotational state of said item with respect to an axis of rotation, wherein an orientation in a specified plane or surface corresponds to an orientation of said item with respect to the surface normal to the specified plane or surface taken as the axis of rotation. Unless otherwise specified, the orientation of an item is specified for an axis of rotation passing through the position of the item. The term “placement” of an item as used herein refers to specifying both “orientation” of the item and “position” of the item. When referring to a plane or surface, a vertical direction is a direction parallel to the surface normal at a given position on said plane or surface, and lateral directions are perpendicular to the vertical direction, i.e. directions parallel to said plane or surface. In particular, when referring to the calibration device, the term “vertical” refers to the directions perpendicular to the reference plane of the calibration device, and the term “lateral” refers to directions parallel to the reference plane of the calibration device. The term “lateral position” of an item as used herein refers to the position of said item in the plane or surface concerned, and the term “lateral orientation” as used herein refers to the orientation in the plane or surface concerned. More particularly, when referring to the calibration device, a lateral position and/or a lateral orientation of the item refers to the position and/or orientation with respect to the reference plane of the calibration device.

The robot system may be adapted to the handling of parts, in particular for the han dling of a large number of differently shaped small parts. More particularly, the robot system may be adapted to the handling of small parts, in particular a large number of differently shaped small parts. The robot system may be a collaborative system with a plurality of robotic components working together. According to some embodiments, the robot system may comprise a first robotic component and a second robotic com ponent adapted for cooperating for the handling of the parts. For example, the robot system may comprise a first robot handler and a second robot handler arranged downstream of the first robot handler, wherein the first robot handler is adapted for picking a small part from an arbitrary first placement, i.e. an arbitrary position and orientation, on a worktable surface and placing said small part in a pre-determined second placement, i.e. in a pre-determined position and orientation, on a different portion of the worktable for the second robot handler to pick up from the second place ment in a precise and spatially well-defined manner.

Small parts may have dimensions as typically measured in millimeters (mm) or centi meters (cm). Small parts may thus have a maximum dimension as measured from a first end to a second end opposite of the first end as typically measured in mm or cm, such as up to about 20 cm, such as between 1 mm and 10 cm, such as between 2 mm and 5 cm, such as between 5 mm and 2 cm. Such small parts may also have minimum transverse dimensions of at least ½ mm, such as at least 1mm, at least 2mm, or at least 5mm. The small parts may have various different shapes, which are typically known beforehand. Information on various possible shapes of the small parts may thus be stored in data storage means of the system for use in one or more control modules adapted to control operation of multiple collaborative robot components.

As already mentioned above, the worktable has a worktable coordinate system and defines a reference frame of the robot system. The robot handler is operable for han dling parts on the worktable surface, and has a handling coordinate system for de scribing movements of the robot handler with respect to the reference frame. The imaging device is operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the captured field of view.

When placed in the reference placement, the front side of the calibration device faces towards the imaging device and is arranged within the field of view of the imaging device. According to some embodiments, the calibration device may comprise one or more alignment and/or attachment elements, whereby the calibration device is adapted to engage the worktable in a pre-determined reference placement that is fixed with respect to the reference frame, and thus in a pre-determined fixed spatial relation with respect to the worktable coordinate system.

On the front side, the calibration device comprises a plurality of calibration elements, wherein the calibration elements are placed in a pre-defined arrangement. Advanta geously, the placement of the calibration elements in the arrangement may be gener ally defined with respect to the reference plane of the calibration device. The calibra tion device is adapted to engage the worktable in a pre-determined (fixed) reference placement with respect to the reference frame, such that the calibration elements are arranged within the field of view of the imaging device, when the calibration device is in the reference placement.

The plurality of calibration elements comprises one or more orientation calibration elements, each orientation calibration element has a lateral orientation contour, which is shaped to define a distinct lateral orientation with respect to rotation around a ver tical axis, and which is adapted for positive mechanical engagement by a cooperating calibration piece according to the distinct orientation in a plane parallel to the refer ence plane of the calibration device. By mechanically engaging the one or more ori entation calibration elements with a calibration body of a cooperating calibration piece in a form-locking manner when the calibration device is in its pre-determined refer ence placement, a simple and precise calibration of the orientation of the robot han dler directly with respect to the worktable reference frame is achieved. The mechani cally keyed orientation calibration thus allows for a simple and precise control of an orientation sensitive end effector of the robot handler in the coordinates of the work table, e.g. for correctly picking an item from the worktable surface with a well-defined orientation, or for precisely placing an item in the correct orientation onto the worktable surface.

For a given robot system, the imaging coordinate system is associated with the field of view in an unambiguous manner, thus mapping the captured image data to the field of view on the work table surface. By capturing an image of the pre-defined arrange ment of calibration elements on the front side of the calibration device, when the cal ibration device is in the pre-determined reference placement on the worktable, the imaging coordinate system associated with the field of view can be directly calibrated to the worktable reference frame. To that end, the captured images of the calibration element arrangement may be analyzed to determine the positions of the calibration elements on the calibration device in the imaging coordinate system, and compared to stored or input information on the geometry of the arrangement of calibration ele ments, so as to determine parameters for calibrating the imaging system with respect to the worktable coordinate system, e.g. for translation, rotation, scaling, distortion or other aberration corrections. This allows to precisely capture images of parts in the field of view of the imaging system, and to precisely determine their position and ori entation on the worktable surface in worktable coordinates.

The information on the precise placement of parts on the worktable surface as deter mined by the imaging system is useful, e.g. for controlling the robot handler to cor- rectly approach and pick up said part. By referencing both the imaging system cali bration and the robot handler calibration directly to the worktable, cumulative calibra tion errors are furthermore avoided.

Further according to some embodiments of the calibration device, each calibration element comprises a positioning envelope, wherein the positioning envelope is shaped to define a unique lateral position of the calibration element in the reference plane of the calibration device, and wherein the positioning envelope is adapted for positive mechanical engagement by the cooperating calibration piece according to the unique lateral position.

The plurality of calibration elements may comprise one or more position calibration elements, wherein each position calibration element has a lateral positioning enve lope, which is shaped to define a unique lateral position with respect to translation in lateral directions, and which is adapted for positive mechanical engagement by a co operating calibration piece according to the unique position in the reference plane. In particular, the lateral positioning envelope of the calibration elements is adapted to engage a complementary calibration body of the calibration piece at peripheral points thereof, thereby uniquely defining a lateral position of the calibration piece (and thus of the robot handler to which the calibration piece is mounted) with respect to the position calibration element.

By mechanically engaging the position envelope of one or more calibration elements with a calibration body of a cooperating calibration piece in a form-locking manner when the calibration device is in its pre-determined reference placement, a simple and precise calibration of the position of the robot handler directly with respect to the worktable reference frame is achieved. By performing such a position calibration for a plurality of position calibration elements, a full positional calibration may be ob tained, including calibration parameters for linear translation, scaling, and/or rotation of point positions of handling coordinates with respect to the same point positions in worktable coordinates. For example, using at least three positioning elements at three different unique positions spanning two linear independent vectors, respectively, a full positional transformation of the handling coordinate system into the worktable coordi nate system can be determined with respect to translation, scaling, and rotation of the handling coordinate system into the worktable coordinate system. The mechanically determined position calibration allows for a simple and precise position control of an end effector of the robot handler in the coordinates of the worktable, e.g. for correctly picking an item from the worktable surface with a well-defined orientation, or for pre cisely placing an item in the correct orientation onto the worktable surface.

By combining orientation and position calibration by means of a positive mechanical engagement of the calibration elements with a cooperating calibration device ar ranged on the robot handler, a simple and precise placement calibration of the han dling coordinate system, i.e. including position and orientation of the robot handler with respect to the reference frame defined by the worktable coordinate system is obtained. Furthermore, since the imaging is also calibrated to the same reference frame and using the same calibration device, a simple, efficient, and precise eye-hand calibration is achieved for handling parts on a worktable is achieved, yet avoiding cumulative calibration errors.

Advantageously, the lateral orientation contour is adapted to also act as lateral posi tioning envelope for a unique lateral positioning and distinct orientation of the cooper ating calibration piece with respect to the orientation calibration element on the cali bration device. Furthermore, the distinct placement data thus obtained from one or more such orientation calibration elements with a positioning envelope may be com bined with positioning data obtained from one or more calibration elements with a positioning envelope, but without orientation contour. A lateral positioning envelope, which is adapted to define a unique lateral position, but which does not provide a distinct orientation is e.g. a calibration element with a circular shape as seen in a lateral plane. Since contacting these orientation independent position calibration ele ments may be performed in a simpler and/or faster robot handler control sequence, the critical orientation calibration data can be supplemented easily and quickly by mul tiple position calibration data points, thereby further improving the quality and preci sion of the calibration. Since the further calibration elements are all provided on the same calibration device, which is fixed to the reference frame of the robot system, a precise, cumulative errors are avoided and a further reliable and reproducible calibra tion is obtained. Further according to some embodiments of the calibration device, each of the plurality of the calibration elements comprises one or more guide surfaces adapted to guide the complementary calibration body of the cooperating calibration piece into positive mechanical engagement according to the distinct orientation of the orientation contour and/or according to the unique position of the positioning envelope.

Advantageously, the guide surfaces are inclined with respect to the vertical direction. Thereby, the guide surfaces are adapted to guide the calibration body of the calibra tion piece into the distinct orientation of the orientation contour (and/or unique position of the positioning envelope) as the calibration body, upon approach towards the front side of the calibration device, is moved into positive mechanical engagement with a calibration element thereon. Furthermore, the inclined guide surfaces of a given cali bration element, by guiding the cooperating calibration piece towards the distinct ori entation and/or unique position defined by the calibration element, provide a mechan ical focusing of the robot handler orientation and/or position towards the distinct ori entation and/or unique position of the calibration element. Thereby, a further en hanced precision of the mechanical calibration is achieved.

Advantageously, the one or more guide surfaces comprise guide surfaces that are shaped to guide the calibration body, typically in a sliding movement along the guide surface, towards lateral positive mechanical engagement with the orientation contour and/or positioning envelope, when the calibration body is pushed in a vertical direction towards the front side of the calibration device. The interaction between the calibration body and the guide surface brings about lateral rotational and/or lateral translational forces, which may be sensed and fed to the robot handler control, which in response to the sensed lateral forces orients and/or positions the calibration piece to bring about a balance of lateral forces in the positive engagement position.

For example, an orientation calibration element may have a polygonal orientation con tour as seen in a lateral plane, wherein the shape of the polygon defines a distinct orientation. The guide faces pay then be planar surfaces, each comprising an edge of the polygon and being inclined with respect to the vertical direction, pointing to wards a central axis passing vertically through the geometric centre of the polygonal shape. Furthermore, a calibration element adapted for orientation independent posi tion calibration may have a guide face with a conical shape, such as a circular conical recess in the front side of the calibration device, which opens towards the front side, wherein the conical shape may also be specified by an angle of inclination between a surface normal to the guide surface and the vertical direction defined by the reference plane of the calibration device. According to some embodiments, an angle of inclina tion between a surface normal to the guide surface and the vertical direction may be in the range from 10 degrees to 80 degrees, or from 20 degrees to 70 degrees, or from 30 degrees to 60 degrees, or from 40 degrees to 50 degrees, or about 45 de grees.

Further according to some embodiments of the calibration device, the pre-defined arrangement is a regular arrangement, such as a rectangular matrix arrangement.

By way of example, the regular arrangement may have a rectangular or square sym metry. Arrangements in other regular patterns are also conceivable, such as exhibit ing hexagonal or triangular symmetry, and/or with varying distances between adjacent calibration elements. Thereby a simple calibration routine can be programmed and performed both with respect to the calibration of the imaging coordinate system and of the handling coordinate system with respect to the reference frame defined by the worktable coordinate system.

Advantageously according to some embodiments, the pattern is adapted for recogni tion by the vision system to allow for uniquely determining the position, orientation and scale of the arrangement of calibration elements on the calibration device, for the purpose of calibrating the imaging coordinate system.

Alternatively or in addition thereto, the pattern may be uniquely identifiable by the vision system, e.g. by markings or other codes applied to the front side of the calibra tion device within the field of view. Thereby allowing a control software associated with the imaging system to either directly read relevant calibration parameters, or identify a calibration device, and retrieve relevant calibration parameters for the cali bration device from a local or remote storage medium. The control system may also be configured to prompt a user performing or preparing for a calibration to provide relevant calibration parameters as an input. Furthermore, a unique identification of the calibration device also allows for improved traceability of the calibration in a pro duction line.

Further according to some embodiments of the calibration device, each of the calibra tion elements that are adapted for position calibration comprises a rim with a pre determined silhouette defining a position of the calibration element, wherein the sil houette is recognizable by the imaging system. The rim provides a sharp edge which is easily detectable and recognizable by the imaging system. Preferably the rim de fining the silhouette is arranged in a lateral plane. Thereby an enhanced precision is achieved for the calibration of the imaging coordinate system with respect to the work table coordinate system. Further preferably, all the rims are arranged in the same plane. Thereby precision of the calibration of the imaging coordinate system with re spect to the worktable coordinate system is further enhanced.

Further according to some embodiments of the calibration device, the one or more calibration elements is formed as a through-going hole penetrating the calibration de vice from the front side to a back side of the calibration device.

Advantageously according to some embodiments, the calibration device is planar plate shaped. Further advantageously, the one or more calibration elements are formed as a recess in the front side. Further advantageously, the rim is located at the back side of the calibration device, i.e. the rim defines a silhouette of the opening of the through-going hole at the back side.

Further according to some embodiments of the calibration device, the distinct orien tation is unique within an angular range of acceptance for rotation about a pre-deter- mined axis of rotation, such as within 30 degrees, or within 40 degrees, or within 50 degrees, or within 60 degrees, or within 70 degrees, or within 90 degrees, or within 180 degrees, or within 360 degrees. Most preferably, the pre-determined axis of rota tion is parallel to the vertical direction. Further preferably, the pre-determined axis of rotation intersects a lateral reference plane of the calibration device at the point de fining the lateral position of the calibration element on the calibration device. Typically, the lateral position of a calibration element is defined by the position of the geometric centre of its lateral shape in the reference plane.

By limiting the angular range of acceptance, the orientation is only unique within said angular range. The orientation calibration device may thus be adapted to define mul tiple distinct orientations. Outside the angular range of acceptance, a cooperating cal ibration piece may key into another one of the multiple distinct orientations defined by the orientation contour. This allows for calibrating the orientation of the robot handler with respect to the reference frame of the worktable in multiple distinct orientations.

Preferably according to some embodiments, the orientation contour has an n-fold ro tational symmetry with respect to an orientation axis, such as two-fold, three-fold, four fold, six-fold, eight-fold, twelve-fold, or sixteen-fold. Further preferably, a cooperating calibration body of the calibration piece has a corresponding symmetry. By providing an orientation calibration element and a cooperating calibration piece, which are adapted for positive mechanical engagement in multiple distinct orientations arranged in rotational symmetry, a multiple point orientational calibration may be provided, and an increased precision of the orientational calibration may be achieved. A symmetric orientational calibration may furthermore be useful, or sufficient, e.g. if a given task to be performed by the robot handler is invariant with respect to a specified symmetry. Furthermore, a multiple point orientational calibration may be particularly useful for a fine calibration of the orientation of the handling coordinate system with respect to the worktable coordinate system, if an orientation of the robot handler position is known, or can easily be established, on a rough scale corresponding to or less than the an gular range of acceptance.

Further according to some embodiments of the calibration device, the calibration de vice comprises two or more orientation calibration elements. According to some em bodiments, each one of the two or more orientation calibration elements has the same orientation contour, wherein the orientation contour is arranged to define the same orientation. Alternatively according to some embodiments, the calibration device com prises a plurality of orientation calibration elements, each calibration element com prising an orientation contour with the same shape, wherein the orientation defined thereby is rotated with respect to the orientation of the remaining orientation calibra tion elements. A multipoint orientational calibration can thus be performed at multiple positions, and a further increased precision of the orientation calibration may be achieved.

A further aspect of the invention relates to a calibration piece for the spatial calibration of a robot system with respect to a cooperating calibration device according to any of the embodiments disclosed herein, wherein the calibration device is arranged in a fixed pre-defined reference placement with respect to a reference frame of the robot system. As mentioned above, the robot system comprises a worktable with a workta ble surface, the worktable having a worktable coordinate system and defining a refer ence frame of the robot system; a robotic handler operable for handling parts on the worktable surface, the robotic handler having a handling coordinate system for de scribing movements of the robot handler with respect to the reference frame, and an imaging device operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the field of view. According to some embodiments, the calibration piece comprises at a proximal end thereof, a mounting portion with a mounting element adapted for mount ing the calibration piece to a robot handler of a robot system in a pre-determined reference configuration with respect to the robot handler; and at a distal end thereof, a head portion with a calibration body shaped as a counterpart for positive mechanical engagement of an orientation contour of the one or more orientation calibration ele ments on the cooperating calibration device, according to a distinct orientation defined by said orientation contour.

Further according to some embodiments of the calibration piece, the calibration body is further shaped as a counterpart for positive mechanical engagement of a position ing envelope of one or more, preferably each one, of the plurality of calibration ele ments of the cooperating calibration device, according to a unique lateral position de fined by said positioning envelope.

A yet further aspect of the invention relates to a calibration method as described with reference to exemplary embodiments in the following, using the calibration device and cooperating calibration piece as disclosed herein. By the method at least the same advantages are achieved as described elsewhere herein, with respect to the calibra tion device and calibration piece. Further advantageous embodiments of the method are also evident from the discussion of the embodiments of the calibration device and the cooperating calibration piece.

According to some embodiments, a method of calibrating a robot system using a cal ibration device according to any of the embodiments as disclosed herein and a coop erating calibration piece according to any of the embodiments as disclosed herein is provided, the robot system comprising a worktable with a worktable surface, the work table having a worktable coordinate system and defining a reference frame of the robot system; a robotic handler operable for handling parts on the worktable surface, the robotic handler having a handling coordinate system for describing movements of the robot handler with respect to the reference frame, and an imaging device operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the field of view; wherein the calibration device is placed to engage the worktable in a pre-determined fixed refer ence placement with respect to the reference frame, wherein the calibration elements are located in the field of view of the imaging device, and wherein the calibration piece is mounted to the robot handler in a fixed reference configuration with respect to the handling coordinate system; the method comprising the steps of: contacting at least one of the one or more orientation calibration elements of the calibration device with the calibration piece, thereby bringing the calibration body of the calibration piece in positive mechanical engagement with the calibration contour of the calibration element; and determining an orientation of the robot handler in the handling coordinate system when the calibration body is in positive mechanical engagement with the calibra tion contour; and establishing an orientation of the handling coordinate system with respect to the worktable coordinate system, using the determined orientation.

Further according to some embodiments, the method further comprises the steps of determining a position of the robot handler in the handling coordinate system when the calibration body is in positive mechanical engagement with the calibra tion contour; and establishing a spatial relation of the handling coordinate system with respect to the worktable coordinate system, using the determined position.

Further according to some embodiments, the method further comprises the steps of contacting at least a further one of the plurality of calibration elements of the cali bration device with the calibration piece, thereby bringing the calibration body of the calibration piece in lateral engagement with the further calibration element; determining a further position of the robot handler in the handling coordinate sys tem when the calibration body is in lateral engagement with the further one of the plurality of calibration elements; and establishing a spatial relation of the handling coordinate system with respect to the worktable coordinate system, using the determined further position.

Advantageously, the steps of contacting the calibration elements with the calibration body, and determining an orientation or a position may be repeated for multiple cali bration elements, and a spatial relation of the handling coordinate system with respect to the worktable coordinate system may be established using the determined orienta tions and positions of the respective calibration elements. The established spatial re lation provides a calibration of the handling coordinate system and the worktable co ordinate system with respect to each other. The calibration may be expressed and implemented, for example, as a transformation of handling coordinates system into worktable coordinates, including both the position and orientation of the robot handler.

Further according to some embodiments, the method further comprises the steps of capturing an image of the arrangement of calibration elements on the calibration device; determining the location of the calibration elements in the imaging coordinate sys tem; and establishing a spatial relation for the placement of the imaging coordinate system with respect to the worktable coordinate system, using the determined locations.

The established spatial relation provides a calibration of the imaging coordinate sys tem and the worktable coordinate system with respect to each other. The calibration may be expressed and implemented, for example, as a transformation of imaging coordinates into worktable coordinates, including position and orientation of an item in the field of view. Advantageously, the method uses information on the spatial ar rangement of the calibration elements on the calibration device, as well as the pre determined reference placement of the calibration device in the worktable coordinate system.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described in more detail in connec tion with the appended drawings, which show in

Fig. 1 a perspective view of a robot system under calibration using a calibration kit according to one embodiment;

Fig. 2 a detail of the robot system of Fig. 1 ;

Fig. 3 a front side view of a calibration device according to one embodiment;

Fig. 4 a cross-sectional view as seen along line IV-IV in Fig. 3;

Fig. 5 a perspective view of a calibration piece according to one embodiment, for use in cooperation with the calibration device of Fig. 3; and in

Fig. 6 steps of one embodiment of a method for calibrating a robot system. DETAILED DESCRIPTION

Fig. 1 shows a perspective view of a robot system 1 comprising a frame and panel structure carrying a worktable 10 with a worktable surface, a robot handler 20 opera ble for handling parts on the worktable surface, and an imaging device 30 operable to capture image data from a field of view on the worktable surface. The worktable has a worktable coordinate system and defines a reference frame of the robot system. The robot handler 20 has a handling coordinate system for describing movements of the robot handler with respect to the reference frame. The imaging device 30 has an imaging coordinate system associated with the field of view. Fig. 2 is an enlarged detail of the robot system 1 showing the work table 10 with a calibration device 100 placed thereon in a fixed reference placement with respect to the worktable coordi nate system, and the robot handler 20 with a calibration piece 200 mounted thereto in a fixed reference configuration with respect to the handling coordinate system.

The robot system 1 may be a part of a production line receiving small parts from an upstream production step (not shown), e.g. through a chute 40, and delivering parts at well-defined placements to a downstream production step (not shown). For exam ple, the robot system 1 may be a pick and place infeed receiving small parts from an injection molding machine. The de-molded parts are dumped via a chute 40 onto the worktable surface in random placement. The robot system may find the small parts and determine their position and orientation on the worktable surface by means of the imaging device 30 using computer vision techniques. Using the position and orienta tion of the small parts on the worktable surface as determined by the imaging device 30, the robot handler 20 may be controlled to pick up the small parts at their random placement on the worktable surface, and place the parts in a well-defined, pre-deter- mined placement for subsequent handling and processing. In order to correctly per form these handling steps, the coordinate systems of the worktable 10, of the robot handler 20, and of the imaging device 30 are spatially calibrated with respect to each other using a calibration kit as seen in Figs. 1 and 2. The calibration kit comprises a calibration device 100 and a cooperating calibration piece 200. The calibration device 100 is placed in a fixed pre-determined reference placement on the worktable 10 us ing engagement elements 101, which engage corresponding engagement means on the worktable 10. A three-dimensional spatial placement of the calibration device 100 may be defined by a reference plane 102 of the calibration device 100. The calibration device 100 further comprises on a front side 103 thereof a pre-defined arrangement of a plurality of calibration elements 110, 120. The plurality of calibration elements 110, 120 comprises one or more orientation calibration elements 110. Each orienta tion calibration element 110 comprises an orientation contour 111 , which is shaped to define a distinct orientation I of the orientation calibration element 110 in the reference plane 102 of the calibration device 100, and which is adapted for positive mechanical engagement by a cooperating calibration piece 200 according to the distinct orienta tion I. Turning to Figs. 3 and 4, an embodiment of a calibration device 100 is described, wherein Fig.3 shows a front side elevation, and Fig.4 shows a cross-sectional view taken along line IV -IV in Fig.3. The calibration device has engagement elements 101 for precisely fixing the calibration device 100 to the worktable 10 of the robot system 1 in a pre-determined reference placement, wherein a three-dimensional spatial align ment of the calibration device is defined by a reference plane 102 of the calibration device 100. The calibration device 100 is planar and plate-shaped with a front side 103, a back side 104 opposite to the front side, and a circumferential edge 105 con necting the front side 103 and the back side 104. The calibration device comprises on a pre-defined arrangement of a plurality of calibration elements 110, 120, which in the embodiment shown in Fig.3 is a three-by-three rectangular matrix arrangement. The three-by-three matrix arrangement of calibration elements 110, 120 has a centre row with three identical orientation calibration elements 110, which are all oriented in the same direction. The row of orientation calibration is flanked on either side by a row of three orientation independent position calibration elements 120. However, any suita ble arrangement may be used, as long as the arrangement is pre-determined and known, or made available, to the robot system under calibration, e.g. when performing a calibration procedure on the robot system. For example, a rectangular or square matrix arrangement with a number of n rows by a number of m columns may be con ceived, where n and m are integer numbers larger than 1 , e.g. at least 2, at least 3, at least 4, or at least 5. Furthermore, the distances between adjacent columns and/or rows may be constant, or said distances may be variable, such as increasing. As mentioned above, the plurality of calibration elements 110, 120 comprises one or more orientation calibration elements 110. Each orientation calibration element 110 comprises an orientation contour 111 , which is shaped to define a distinct orientation I of the orientation calibration element 110 in the reference plane 102 of the calibration device, and which is adapted for positive mechanical engagement by the cooperating calibration piece 200 according to the distinct orientation I.

The orientation calibration elements 110 shown in Fig.3 are shaped as equilateral triangles. The orientation contour defines a distinct orientation as indicated by arrow I. Due to the triangular shape, the distinct orientation I is unique within an angle of acceptance of 120 degrees. Due to the three-fold symmetry of the orientation calibra- tion elements 110, two further distinct orientations of the calibration piece 200 in pos itive mechanical engagement with the orientation calibration element 110 are possi ble, as indicated by arrows II and III in Fig.3. As orientation I, the two further distinct orientations, orientation II and orientation III, are also unique within an angular range of 120 degrees.

Each of the calibration elements 110, 120 has a corresponding positioning envelope 112, 122, which is shaped to define a unique position P(i,j) of the calibration element 110, 120 in the reference plane 102 of the calibration device 100, and which is adapted for positive mechanical engagement by the cooperating calibration piece 200 according to the unique position P(i,j). For the orientation calibration elements 110, the orientation contour 111 may also function as the positioning envelope 112, whereas the position calibration elements 120 have an orientation independent posi tioning envelope 122, which is adapted to engage the cooperating calibration piece at peripheral points thereof. The calibration elements 110, 120 are formed as through- going holes extending from the front side 103 to the back side 104. The calibration elements 110, 120 are generally conically shaped, opening in a direction from the back side 104 towards the front side 103. The generally conically shaped side walls of the calibration element thus provides guide faces 113, 123, which are adapted to guide the complementary calibration body 210 of the cooperating calibration piece 200 into positive mechanical engagement according to the distinct orientation I of the orientation contour and/or according to the unique position P(i,j) of the positioning envelope. Thereby a mechanical focusing of the interaction between the calibration elements and the cooperating calibration piece towards the distinct orientation I and/or unique position P(i,j) is achieved. At the back side of the calibration device, each calibration element 110, 120 comprises a rim 114, 124 with a pre-determined circular silhouette centred at the position P(i,j) of the calibration element 110, 120. The silhouette is easily recognizable by the imaging device, in order to reliably deter mine the position P(i,j) even if the condition for image capture in a given robot system are non-ideal, e.g. due to a challenging camera angle.

Fig. 5 shows a calibration piece 200 for the spatial calibration of a robot system 1. The calibration device 200 is for use in combination with a cooperating calibration device 100 as described above with reference to Figs. 1-4. The calibration piece 200 has a distal end and a proximal end, as seen in an axial direction. The calibration piece has a three-fold rotational symmetry with respect to an axis of rotation R parallel to the axial direction, corresponding to the three-fold-symmetry of the orientation cal ibration elements 110 of the calibration device 200. At the distal end the calibration piece 200 comprises a calibration body 210. The calibration body 210 is shaped as a counterpart with a counter contour for positive mechanical engagement of the orien tation contour 111 of the cooperating calibration device 100 according to a distinct orientation I defined by said orientation contour 111. The calibration body 210 is fur ther shaped as a counterpart matching the positioning envelope 112, 122 of calibra tion elements 110, 120 on the cooperating calibration device 100 for positive mechan ical engagement according to the unique position P(i,j) defined by said positioning envelope 112, 122. The calibration body 210 is further shaped with inclined surfaces matching the inclined guide faces of the calibration elements 110, 120 for focussing the positive mechanical engagement towards the distinct orientation I and/or unique position P(i,j) of the calibration element, as applicable. At the proximal end the cali bration piece 200 comprises a mounting element 220. The mounting element 220 is adapted for mounting the calibration piece to a robot handler 20 of the robot system in a pre-defined reference placement with respect to the reference frame, e.g. by means of flanges 221 or similarly suitable alignment means cooperating with corre sponding mounting devices on the robot handler 20. The calibration piece may be configured for mounting in a tool bay, instead of an end effector (not shown) normally placed there. The mounting element 220 may thus be provided with tool alignment means, such as flanges 221, to mimic a general tool mounting element for mounting e.g. an end effector in a well-defined placement corresponding to the pre-defined ref erence placement of the calibration piece 200.

Fig. 6 shows steps of one embodiment of a method for calibrating a robot system using a calibration device and a cooperating calibration piece. The robot system has: a worktable with a worktable surface, the worktable having a worktable coordinate system and defining a reference frame of the robot system; a robot handler operable for handling parts on the worktable surface, the robot handler having a handling co ordinate system for describing movements of the robot handler with respect to the reference frame; and an imaging device operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the field of view. Steps 610, 620, 630, 640, 650, 660, 670, 680, 690 relate to steps of calibrating the handling coordinate system and the work table coordinate system with respect to each other. The steps 622, 624, and 626 re late to steps of calibrating the imaging coordinate system and the worktable coordi nate system with respect to each other. Advantageously, steps 622, 624, and 626 may be performed between step 620 and step 630 connecting point A to point B.

The calibration starts in step 610, where the procedure is initialized. Furthermore in step 610, the calibration device is placed to engage the worktable in a pre-determined fixed reference placement with respect to the reference frame, wherein the calibration elements are located in the field of view of the imaging device. Furthermore in step 610, the calibration piece is mounted to the robot handler in a fixed reference config uration with respect to the handling coordinate system. In step 620, it is decided, if an image calibration is to be performed. If “NO”, the method proceeds directly to step 630. If “YES” the method proceeds via point A to step 622. In step 622 one or more images of the field of view including the calibration device is captured. In step 624, the captured one or more images is analyzed with regard to the position of the cali bration elements using pre-defined calibration data on the arrangement of the calibra tion elements on the calibration device. The calibration data may be obtained by prompting for user input of the relevant calibration data, or may be retrieved from a computer readable storage medium comprising the calibration data. In order to re trieve the correct calibration data, the calibration device may be identified, e.g. uniquely identified or on a more general level by its type. The identification may also occur by user input, or may be directly machine-read by the imaging device, e.g. from a suitable marking applied to the front side of the calibration device within the field of view of the imaging device. Using the calibration data and the positions of the one or more captured images, a spatial relation representing the calibration between the im aging coordinate system and the worktable coordinate system is established in step 626, e.g. in the form of specifying a spatial transformation of the imaging coordinate system to the worktable coordinate system. The method may then proceed via point B to step 630. In step 630, the robot handler may be operated to contact a first cali bration element of the calibration device, so as to bring the calibration piece into pos itive mechanical engagement with the calibration element. In step 640, the position of the robot handler in the handling coordinate system is determined, when the calibra tion body is in positive mechanical engagement with the orientation contour. In step 650 it is decided, if the calibration element is an orientation calibration element with a distinct orientation. If “NO”, the method proceeds directly to step 670. If “YES”, the method continues to step 660. In step 660, the orientation of the robot handler in the handling coordinate system is determined, when the calibration body is in positive mechanical engagement with the orientation contour. The method requires that at least one orientation calibration element be included in the calibration procedure. In step 670, it is decided, if a further calibration element is to be contacted. If “NO”, the method continues with step 680. If “YES”, the method returns to step 630 to repeat steps 630 through 670 until all required calibration elements have been contacted.

Using the unique position and distinct orientation data determined by the method, a spatial relation of the handling coordinate system with respect to the worktable coor- dinate system is established in step 680, e.g. by specifying a spatial transformation of the handling coordinate system to the worktable coordinate system. Thereby, a full calibration of the handling coordinate system, the imaging coordinate system, and the worktable coordinate system with respect to each other is obtained. The calibration procedure may then end in step 690, where the calibration device and the calibration piece may be removed from the robot system.

LIST OF REFERENCE NUMBERS

1 robot system

10 worktable

20 robot handler

30 imaging device

40 feeding chute

100 calibration device

101 engagement element

102 reference plane

103 front side

104 back side

105 circumferential edge

110 orientation calibration element

111 orientation contour

112 positioning envelope

113 guide face

114 rim

120 position calibration element

122 positioning envelope

123 guide face

124 rim

200 calibration piece

210 calibration body

220 mounting element

221 alignment flange

I, II, III distinct orientations

P(i,j) unique position of calibration element (i,j)

610, 620, 630 ... 690 method steps of coordinate system calibration 622, 624, 626 method steps of coordinate system calibration

Claims

1. Calibration device (100) for the spatial calibration of a robot system, the robot system comprising: a worktable (10) with a worktable surface, the worktable having a worktable coordinate system and defining a reference frame of the robot system; a robot handler (20) operable for handling parts on the worktable surface, the robot handler having a handling coordinate system for describing movements of the robot handler with respect to the reference frame; and an imaging device (30) operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the field of view; wherein the calibration device (100) comprises engagement elements (101) adapted for engaging the worktable in a pre-determined reference placement with respect to the reference frame, wherein a three-dimensional spatial align ment of the calibration device is defined by a reference plane (102) of the cali bration device; wherein the calibration device further comprises on a front side (103) thereof a pre-defined arrangement of a plurality of calibration elements (110, 120); wherein the plurality of calibration elements comprises one or more orienta tion calibration elements (110); wherein each orientation calibration element comprises an orientation contour (111) shaped to define a distinct orientation (I) of the orientation calibration element in the reference plane of the calibration device and adapted for positive mechanical engagement by a cooperating calibration piece (200) according to the distinct orientation.

2. Calibration device according to any of the preceding claims, wherein each cali- bration element comprises a positioning envelope (112, 122) shaped to define a unique position (P(i,j)) of the calibration element in the ref erence plane of the calibration device and adapted for positive mechanical engagement by the cooperating calibration piece (200) according to the unique position.

3. Calibration device according to any of the preceding claims, wherein each of the plurality of the calibration elements comprises one or more guide surfaces (113, 123) adapted to guide the complementary calibration body of the cooperating calibration piece into positive mechanical engagement according to the distinct orientation of the orientation contour and/or according to the unique position of the positioning envelope.

4. Calibration device according to any of the preceding claims, wherein the pre-de- fined arrangement is a regular arrangement, such as a rectangular matrix ar rangement.

5. Calibration device according to any of the preceding claims, wherein each cali bration element comprises a rim (114, 124) with a pre-determined silhouette de- fining the position (P(i,j)) of the calibration element on the calibration device, wherein the silhouette is recognizable by the imaging device.

6. Calibration device according to any of the preceding claims, wherein one or more, preferably each calibration element is a through-going hole penetrating the calibration device from the front side to a back side of the calibration device.

7. Calibration device according to any of the preceding claims, wherein the distinct orientation is unique within an angular range of acceptance for rotation about a pre-determined axis of rotation, such as at least within 30 degrees, or at least within 45 degrees, or at least within 60 degrees, or at least within 90 degrees, or at least within 180 degrees, or within 360 degrees.

8. Calibration device according to any of the preceding claims, wherein the calibra tion device comprises two or more orientation calibration elements, each one of the two or more orientation calibration elements comprising the same orientation contour arranged to define the same orientation.

9. Calibration piece (200) for the spatial calibration of a robot system (1) with re spect to a cooperating calibration device (100) according to any one of claims 1- 8, wherein the calibration device is arranged in a pre-defined reference place ment with respect to the reference frame, wherein the calibration piece comprises: at a proximal end thereof, a mounting element (220) adapted for mounting the calibration piece to a robot handler (20) of the robot system; and at a distal end thereof, a calibration body (210) shaped as a counterpart for posi tive mechanical engagement of an orientation contour (111) of the cooperating calibration device according to a distinct orientation (I) defined by said orienta tion contour.

10. Calibration piece according to claim 9, wherein the calibration body is further shaped as a counterpart for positive mechanical engagement of a positioning envelope (112, 122) of the cooperating calibration device according to a unique position (P(i,j)) defined by said positioning envelope.

11. Method of calibrating a robot system (1) using a calibration device (100) accord ing to any one of claims 1-8 and a cooperating calibration piece (200) according to any one of claims 9-10, the robot system comprising: a worktable (10) with a worktable surface, the worktable having a worktable coordinate system and defining a reference frame of the robot system; a robot handler (20) operable for handling parts on the worktable surface, the robot handler having a handling coordinate system for describing movements of the robot handler with respect to the reference frame; and an imaging device (30) operable to capture image data from a field of view on the worktable surface, the imaging device having an imaging coordinate system associated with the field of view; wherein the calibration device is placed to engage the worktable in a pre-de- termined fixed reference placement with respect to the reference frame, wherein the calibration elements are located in the field of view of the imaging device, and wherein the calibration piece is mounted to the robot handler in a fixed refer ence configuration with respect to the handling coordinate system; the method comprising the steps of: contacting at least one of the one or more orientation calibration elements (110) of the calibration device with the calibration piece, thereby bringing the calibra tion body of the calibration piece in positive mechanical engagement with the ori entation contour of the orientation calibration element; and determining an orientation (I) of the robot handler in the handling coordinate sys tem when the calibration body is in positive mechanical engagement with the ori entation contour; and establishing a spatial relation of the handling coordinate system with respect to the worktable coordinate system, using the determined orientation.

12. Method according to claim 11, wherein the method further comprises the steps of determining a position (P(i,j)) of the robot handler in the handling coordinate sys tem when the calibration body is in positive mechanical engagement with the ori entation contour; and establishing a spatial relation of the handling coordinate system with respect to the worktable coordinate system, using the determined position.

13. Method according to any one of claims 11-12, wherein the method further com prises the steps of contacting a further one of the plurality of calibration elements (110, 120) of the calibration device with the calibration piece, thereby bringing the calibration body of the calibration piece in positive mechanical engagement with the further cali bration element; determining a further position (P(i,j) of the robot handler in the handling coordi nate system when the calibration body is in positive mechanical engagement with the further calibration element; and establishing a spatial relation of the handling coordinate system with respect to the worktable coordinate system, using the determined further position.

14. Method according to any one of claims 11-13, wherein the method further com prises the steps of capturing an image of the arrangement of calibration elements (110, 120) on the calibration device; - determining the location of the calibration elements in the imaging coordinate system; and establishing a spatial relation of the imaging coordinate system with respect to the worktable coordinate system, using the determined locations.