APPARATUS AND METHOD FOR DETERMINING THE POSITION AND ORIENTATION OF A FIRST AXIS OF A PART RELATIVE TO A KNOWN FRAME OF REFERENCE

Abstract: Apparatus for determining the position and orientation of an axis (1) of a part (2) relative to a frame of reference (3), including at least two imaging devices (5a, 5b) at least one light source (6), a calibration device (7), releasably holdable by said part (2) about said axis (1), which has the at least one light source (6) on a face (8) thereof, such that when the device (7) is moved through a series of operating positions (9) light is projected or reflected from said source (6), which light is imaged by each of the imaging devices (5a, 5b) at the series of operating positions (9), and the imaging devices (5a, 5b) are operable to output signals (10a, 10b) indicative of the light from said source (6) corresponding to each of the operating positions (9) and a processor (11) for receiving and processing the output signals (10a, 10b) so as to determine the position and orientation of the axis (1) relative to the frame of reference (3).

Description: APPARATUS AND METHOD FOR DETERMINING THE POSITION AND ORIENTATION OF A FIRST AXIS OF A PART RELATIVE TO A KNOWN FRAME OF REFERENCE This invention relates to measurement apparatus for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference, particularly, but not exclusively suitable for use in robotic drilling operations. In robotic drilling operations, the orientation of the drill axis relative to the robot toolframe must be determined in order to drill at a desired angle relative to a part to be worked. Various methods are currently used to measure this relationship, all of which involve the use of an external sensor system, such as a co-ordinate measuring machine (CMM), to probe the drill axis. This is an extremely time consuming task that typically has to be repeated daily off line in order to ensure accurate calibration of the drill axis, interrupting the drilling process and thereby incurring significant time penalties. There is therefore a need for a generally improved apparatus and method for determining the position and orientation of a first axis of a part relative to a known frame of reference that performs the required calibration with minimal disturbance to the working apparatus in a minimum time. According to a first aspect of the present invention there is provided apparatus for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference, including first means for rigidly holding said first part, at least two imaging devices, each attachable to said first means, at least one first light source, which first light source is either an active light source or an illuminable reflecting point, a calibration device, releasably holdable by said first part about said first axis, which device has the at least one first light source on a first face thereof, such that when the device is operatively held by the first part and moved thereby through a series of operating positions light is projected or reflected from said first light source, which light is imaged by each of the imaging devices at the series of operating positions, and the imaging devices are operable to output signals indicative of the light projected or reflected from said first light source corresponding to each of the operating positions in the series, and a processor for receiving and processing the output signals indicative of the light projected or reflected from said first light source at said operating positions so as to determine the position and orientation of a first axis relative to a known frame of reference. Preferably the known frame of reference is a co-ordinate frame common to the at least two imaging devices. Conveniently there is provided indexing means, wherein the first means is an end-effector, which end-effector is in operative association with the indexing means. Advantageously the first part includes clamping means and rotating means. Preferably the calibration device has a shaft of substantially circular cross-section, and wherein the at least one first light source is radially displaced from said shaft. Conveniently the at least one first light source is located on an elongated, substantially rectangular cross-section bar, which bar is rigidly attached to a first end of the shaft and oriented substantially perpendicular thereto. Advantageously the shaft of the calibration device engages with and is releasably held by said clamping means such that when the first means is in operation, the rectangular bar and first end of said shaft protrude from the clamping means and rotate around the first axis. Preferably each of the at least two imaging devices is a metrology sensor operable to create digitisable images. Conveniently there are at least two second light sources, each associated with a respective imaging device. Advantageously the at least one first light source is fabricated from retro- reflective material. Preferably there are provided communication links between the imaging devices and the processor for transmitting the output signals indicative of the reflected light at each of the operating positions, which links include coaxial cables and framegrabber ports. According to a further aspect of the present invention there is provided a method for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference, including the steps of fitting a calibration device to clamping means provided by the first part, which first part is rigidly held by first means and which calibration device has at least one first light source on a first face thereof, which at least one first light source is an active light source or an illuminable reflecting point, rotating the clamping means through at least 360 degrees, imaging on at least two imaging devices light reflected or projected from the at least one first light source, transmitting output signals from the imaging devices, each of which are attached to the first means, to a processor, which output signals are indicative of light projected or reflected from the at least one first light source at a series of operating positions corresponding to rotation of the clamping means, establishing, from the output signals, the first axis position and orientation in a known frame of reference. Preferably establishing the first axis position and orientation includes combining the output signals, which output signals provide a spatial map of the at least one first light source relative to the first axis when the calibration device is rotated by the clamping means, so as to define a first point and a first plane from which the first axis position and orientation is derivable. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is schematic perspective representation of apparatus for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference according to the present invention, Figure 2 is a perspective view of the apparatus of Figure 1 , showing a calibration device having a first light source located thereon, and Figure 3 is a perspective view of the calibration device of Figure 2, showing a series of operating positions of the first light source. In robotic drilling applications, the position and orientation of the drill axis relative to the robot (drill angle) should be known, because the drilling movement effected by the robot will be a function both of the robot movement and the drill angle, and, if the latter is unqualified, it could introduce significant errors to the drilling operation. Apparatus according to the present invention for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference, as shown in the accompanying Figures 1 to 3, is suitable for calibrating any axis of any device where the axis to be calibrated can locate and move a calibration device. In particular, for robots fitted with an end-effector carrying a drill mount, the calibration device can be positioned in the drill mount, and its position recorded by a photogrammetry system. Thus as shown in Figure 1 of the accompanying drawings, apparatus for determining the position and orientation of a first axis 1 of a first part 2 to be moved or operated relative to a known frame of reference 3, includes first means 4 for rigidly holding said first part 2, at least two imaging devices 5a, 5b, each attachable to said first means 4, at least one first light source 6, shown in Figure 2, which first light source 6 is either an active light source or an illuminable reflecting point, and a calibration device 7. The calibration device 7 is releasably holdable by said first part 2 about the first axis 1 , which device 7 has the at least one first light source 6 on a first face 8 thereof, such that when the device 7 is operatively held by the first part 2 and moved thereby through a series of operating positions 9 light is projected or reflected from said first light source 6, which light is imaged by each of the imaging devices 5a, 5b at the series of operating positions 9, and the imaging devices 5a, 5b are operable to output signals 10a, 10b indicative of the light projected or reflected from said first light source 6 corresponding to each of the operating positions 9 in the series. The apparatus further includes a processor 11 , for receiving and processing the output signals 10a, 10b indicative of the light projected or reflected from the first light source 6 at said operating positions 9 so as to determine the position and orientation of a first axis 1 relative to a known frame of reference 3. In the embodiment illustrated in Figure 1 , the first part 2 is a drill mount and the first means 4 is a robot end-effector, and when the robot end- effector is attached to a robot 18, the drill mount is activated to clamp or rotate a part about its axis 1. Figure 1 also shows a co-ordinate frame 3, which is the known frame of reference, and this represents a co-ordinate frame common to both imaging devices 5a, 5b. There are several methods known in the art of establishing a common frame of reference for photogrammetric imaging devices, each of which generically generates a transformation describing the relationship between the two imaging devices, so that measurements from either of the imaging devices 5a, 5b can be mapped to provide a single measurement of position and orientation. The co-ordinate frame is determined off-line, before any measurements are taken, and thus known at the time of measuring the position and orientation of the first axis 1. The calibration device 7 shown in Figure 2 has a shaft 14 of substantially circular cross-section, with the first light source 6 located on an elongated, substantially rectangular cross-section bar 16 and thus radially displaced 15 from the shaft 14. The bar 16 is rigidly attached to a first end 14a of the shaft 14 and is oriented substantially perpendicular thereto, such that when the shaft 14 engages with clamping means 13 provided by the first part 2, and the first part 2 is activated, the rectangular bar 16 and the first end 14a of the shaft 14 protrude from the clamping means 13 and rotate 17 around the first axis 1. Each of the imaging devices 5a, 5b shown in Figure 1 is a metrology sensor, operable to create digitisable images, and there are two second light sources 18a, 18b, each attached to each of the imaging devices 5a, 5b and each projecting light onto the first face 8 of the calibration device 7. Correspondingly the first light source 6 is an illuminable target fabricated from retro-reflective material, reflecting the light projected from the second light sources 18a, 18b. The imaging devices 5a, 5b are operable to create digitisable images, such that the light reflected from the first light source 6 is reproduced as an image of white pixels against a dark background, which white pixels define a two dimensional spatial location of the first light source 6 on each of the imaging devices 5a, 5b. These images are communicated as output signals 10a, 10b by means of communication links 19, which are preferably coaxial cables, to the processor 11 through framegrabber ports 19a, 19b. The apparatus of the invention, described above, is operable to determine the position and orientation of a first axis 1 of a first part 2 to be moved or operated relative to a known frame of reference 3 by implementing a method of the invention, which method includes the steps of fitting a calibration device 7 to clamping means 13 provided by the first part 2, which first part is rigidly held by the first means 4, and which calibration means 7 has at least one first light source 6 on a first face 8 thereof, rotating 17 the clamping means 13 through at least 360 degrees, imaging on at least two imaging devices 5a, 5b light reflected or projected from the first light source 6 at a series of operating positions 9, and transmitting output signals 10a, 10b from the imaging devices to a processor 11. The output signals 10a, 10b are analogue signals that are digitised by a framegrabber in the processor 11 and are stored in memory as bitmaps 20a, 20b for establishing the first axis 1 position and orientation in a known frame of reference. The bitmaps 20a, 20b stored in memory contain a two dimensional array of pixel light intensity values corresponding to a sampling of the output signals 10a, 10b. These are each analysed by the processor 11 to locate the operating positions 9, shown in Figure 3, in the known co-ordinate system 3. The operating positions 9 thus provide a spatial map of the light source 6 relative to the first axis 1 which, for a rotation of the clamping means 13, is a circular loci defining a first plane 21. As the first axis 1 projects through the centre of and is perpendicular to this first plane 21 , the first axis 1 is completely defined.

Claims: CLAIMS 1. Apparatus for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference, including first means for rigidly holding said first part, at least two imaging devices, each attachable to said first means, at least one first light source, which first light source is either an active light source or an illuminable reflecting point, a calibration device, releasably holdable by said first part about said first axis, which device has the at least one first light source on a first face thereof, such that when the device is operatively held by the first part and moved thereby through a series of operating positions light is projected or reflected from said first light source, which light is imaged by each of the imaging devices at the series of operating positions, and the imaging devices are operable to output signals indicative of the light projected or reflected from said first light source corresponding to each of the operating positions in the series, and a processor for receiving and processing the output signals indicative of the light projected or reflected from said first light source at said operating positions so as to determine the position and orientation of a first axis relative to a known frame of reference. 2. Apparatus according to claim 1 , wherein the known frame of reference is a co-ordinate frame common to the at least two imaging devices. 3. Apparatus according to claim 1 or claim 2 including indexing means, wherein the first means is an end-effector, which end-effector is in operative association with the indexing means. 4. Apparatus according to claim 3, wherein the first part includes clamping means and rotating means. 5. Apparatus according to claim 4, wherein the calibration device has a shaft of substantially circular cross-section, and wherein the at least one first light source is radially displaced from said shaft. 6. Apparatus according to claim 5, wherein the at least one first light source is located on an elongated, substantially rectangular cross-section bar, which bar is rigidly attached to a first end of the shaft and oriented substantially perpendicular thereto. 7. Apparatus according to claim 6, wherein the shaft of the calibration device engages with and is releasably held by said clamping means such that when the first means is in operation, the rectangular bar and first end of said shaft protrude from the clamping means and rotate around the first axis. 8. Apparatus according to claim 7, wherein each of the at least two imaging devices is a metrology sensor operable to create digitisable images. 9. Apparatus according to claim 8, including at least two second light sources, each associated with a respective imaging device. 10. Apparatus according to claim 9, wherein the at least one first light source is fabricated from retro-reflective material. 11. Apparatus according to claim 10, including communication links between the imaging devices and the processor for transmitting the output signals indicative of the reflected light at each of the operating positions, which links include coaxial cables and framegrabber ports. 12. Apparatus for determining a first axis of a first part relative to a known frame of reference substantially as hereinbefore described and as illustrated in Figures 1 to 3 of the accompanying drawings. 13. A method for determining the position and orientation of a first axis of a first part to be moved or operated relative to a known frame of reference, including the steps of fitting a calibration device to clamping means provided by the first part, which first part is rigidly held by first means and which calibration device has at least one first light source on a first face thereof, which at least one first light source is an active light source or an illuminable reflecting point, rotating the clamping means through at least 360 degrees, imaging on at least two imaging devices light reflected or projected from the at least one first light source, transmitting output signals from the imaging devices, each of which are attached to the first means, to a processor, which output signals are indicative of light projected or reflected from the at least one first light source at a series of operating positions corresponding to rotation of the clamping means, establishing, from the output signals, the first axis position and orientation in a known frame of reference. 14. A method according to claim 13, in which establishing the first axis position and orientation includes combining the output signals, which output signals provide a spatial map of the at least one first light source relative to the first axis when the calibration device is rotated by the clamping means, so as to define a first point and a first plane from which the first axis position and orientation is derivable. 15. A method for determining a first axis of a first part relative to a known frame of reference substantially as hereinbefore described and as illustrated in Figures 1 to 3 of the accompanying drawings.

Inventors: ALEXANDER RICHARD, GOOCH RICHARD MICHAEL, ALEXANDER, RICHARD, GOOCH, RICHARD, MICHAEL
Applicants: ALEXANDER RICHARD, BAE SYSTEMS PLC, GOOCH RICHARD MICHAEL, ALEXANDER, RICHARD, GOOCH, RICHARD, MICHAEL
Application year: 2000
Main classification: G01B11
IPC codes: B23B49, B23Q17, G01B11