Method and apparatus for performing measurements

Abstract: Apparatus is described which can be used to calibrate a machine or as a machine itself. The apparatus comprises two structures each with three supports spaced in a triangular array thereon. The supports may be balls or sockets. The structures are interconnected by six members and each support has the ends of two members connected to it. When used as a calibration device the members are passive extensible measuring bars and the structures are respectively connected to fixed and movable parts of a machine so that movement of the machine parts causes relative movement between the structures and varies the lengths of the measuring bars. From measurements of the lengths of the measuring bars the actual movement of the machine part can be calibrated. When used as a machine, the members are powered struts by means of which one of the structures, which carries a tool or a probe can be manipulated relative to the other, to position the tool or probe relative to a workpiece.

Description: This invention relates to machines in which one member is positioned relative to another. For example, it relates to coordinate measuring machines, machine tools, scanning machines and robots, in which a tool or probe is positioned relative to a workpiece. More particularly, the invention relates to performing measurements on such machines. In some aspects, the present invention is a development of the apparatus and methods described in our International Patent Application No. WO-A-95 14 905, to which reference should be made. Reference is also directed to the apparatus and methods described in International Patent Applications Nos. WO91/03145 (Kearney & Trecker) and WO92/17313 (Geodetic Machines), and in European Patent Application No. 534585 (Ingersoll). All the above applications show machines in which a tool, probe or other operator is mounted on one structure, for movement relative to another structure upon which a workpiece may be mounted. The tool, probe or other operator may be movable and the workpiece fixed, or vice versa. The relative movement is effected by six rams acting between the two structures. These rams can be controlled so as to produce any desired relative movement between the structures, with six degrees of freedom (three translational and three rotational). It is necessary to measure the movements of the rams, and the present invention may be used for such measurement. A problem with the apparatus described in the various above-referenced patent applications is that of calibrating the movement of the machine. To achieve this, it is necessary to measure the relative position and orientation of the moving structure relative to the fixed structure, independently of the measurements of the extension of the rams which produced that position and orientation. The present invention allows such calibration measurements, both on the type of machine described in the above patent applications, and on more conventional machines. One apparatus for calibrating the movements of a machine is described in German Patentschrift No. DE 35 04 464 C1. The apparatus consists of two structures which are connected between relatively movable parts of an industrial robot, and which are interconnected by telescopic measuring bars. One of the structures is connected to the robot in place of a tool so that when the robot is driven throughout its working envelope, the measuring bars can be used to calibrate the relative movements of the two relatively movable parts. Since the apparatus replaces the tool the machine normally carries, the calibration cannot take place while the robot is performing its normal function. According to the present invention there is provided a method of measuring the movements of relatively movable parts of a machine, one of which carries a tool or probe for performing an operation on a workpiece, and to respective ones of which first and second structures are rigidly connected, each of said structures having at least one support thereon, at least one of said structures having three supports thereon spaced in a triangular array, the supports having spherically or part-spherically shaped support surfaces which define calibration points on the supports, the method comprising the steps of: connecting at least three extendable measuring bars between the structures, said measuring bars having shaped surfaces at their ends which engage the support surfaces of the supports in known relationships to said calibration points, characterised by the further steps of: determining the relative positions of the calibration points of the supports, driving the machine to perform an operation on a workpiece while taking readings from the measuring bars, using the readings from the measuring bars to provide information relating to the instantaneous positions of the calibration points of all of said supports relative to each other. An apparatus for carrying out this method is defined in claim 11. According to one embodiment of the invention both of the first and second structures have three spherical or part-spherical supports thereon which are arranged in a triangular pattern, six measuring bars are provided, opposite ends of each bar being connected to one support on each structure. In such an embodiment, each support on both structures has the ends of two measuring bars connected to it. In any one of the above-described embodiments of any of the aspects of the invention, the supports may take the form of spheres or part-spheres, or of spherical or part-spherical sockets. The supported ends of the measuring bars may therefore be provided with correspondingly shaped balls or sockets at either end as appropriate, which may be completely spherical or only part-spherical, in order to fit on the supports. The measuring bars may extend between the supports and be telescopic or extensible in some other way, or may be of fixed length and extend through, or carry extensions which extend through, the supports so as to provide a greater range of relative movement for the structures. According to one feature of the invention, the supported end of a measuring bar lies at an angle to the longitudinal axis of the bar, whereby, when in position on the support, the axis of the bar is aligned with the centre of the support. The measuring bars may be attached to the supports in any convenient manner, but a preferred form of attachment is by means of magnetic attraction. Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Fig 1 is a simplified isometric view of a preferred embodiment of measurement or calibration apparatus, Fig 2 is a side view of part of the apparatus of Fig 1, Fig 3 is a more detailed side view of another part of the apparatus of Fig 1, Fig 4 is a schematic perspective view of a modification of part of Fig 1, Fig 5 shows part of an alternative embodiment of the measurement or calibration apparatus of the invention, Fig 6 is a detail of one of the ball and socket joints showing a modified socket, Fig 7 shows one way of attaching the ends of four struts to a single ball of the apparatus of the invention, Fig 1 shows a device which can be used for calibration measurements. It comprises an upper structure in the form of an open triangular frame 10 and a lower structure in the form of an open triangular frame 12. Each frame is provided with three precision steel balls 14, one at each corner of the triangle. In use, the upper frame 10 is clamped to the moving structure of a machine, while the lower frame is clamped to the fixed structure (or vice versa). The device may be used with conventional machine tools or with the types of machines illustrated in the above-referenced International and European applications. The spheres 14 are linked in use by six members which in this embodiment are passive measuring bars 16. The positions of these are shown merely by broken lines in Fig 1, but more detail is seen in Fig 3. Each measuring bar 16 in this example comprises parts 18,20 which interfit telescopically and extend between the spheres, but may take the form of rigid links which pass through the spheres as described in later embodiments. At each end, there is provided a magnetic socket 22, which holds the bar 16 in a universally pivotable manner to a respective one of the balls 14. The length of the bar 16 at any given time is therefore an accurate measure of the distance between the balls 14. This length is measured by any convenient transducer system, for example a scale 24 within the member 18, over which passes a readhead 26 attached to the member 20. The magnetic sockets 22 are of a type which is known, and commonly used in ball bars which are used for calibrating machines tools and other positioning machines. They contain a trihedral support into which the ball 14 is drawn by a magnet. Given that the ball 14 is accurately spherical, the connection between the ball and the bar 16 is thus defined kinematically with great precision. The magnetic sockets 22 are described in, for example, U.S. Patent No. 4,435,905 (Bryan) and in "A Simple Method For Testing Measuring Machines and Machine Tools" by J.B. Bryan, Precision Engineering, Vol 4, No. 2, April 1982, pages 61-69 and No. 3, July 1982, pages 125-138. The magnetic sockets are not essential and other forms of connection may be used. In use, the triangular frames 10,12 are clamped to the fixed and moving parts of the machine by any convenient means, and the six measuring bars 16 are placed between the respective balls 14. The machine is now exercised so as to move the moving part to a number of different positions and orientations relative to the fixed part. As this happens, a computer as shown at 121 in Fig 11 acquires data from the transducers in each of the measuring bars 16. Using known algorithms, this data is used to determine the precise position and orientation to which the machine has been moved, and thus to calibrate its motion. For example, it is possible to build up an error map showing the true position and orientation of the moving part of the machine for any given demanded position as shown by its own measuring devices. The upper and lower triangular frames 10,12 may each be built as a single unit if desired. However, for convenience, we prefer that it should be of modular construction, made up from a kit of parts which can be dismantled for portability. As seen in Figs 1 and 2, therefore, each frame may be made up of three rods 28, with a screw connection 30 at each end for attachment to the ball 14. If desired, the kit of parts for making the apparatus of the present invention may include a selection of rods 28 of different lengths, for use on machines of different sizes. Of course, the frames 10,12 may also have various clamping features to facilitate clamping to the machine, but these are not shown. The algorithms used to determine the position and orientation of the machine from the lengths of the measuring bars 16 require that the distances between adjacent balls 14 on each of the frames 10,12 should be known. These distances may be determined using one of the measuring bars 16 itself, as shown in broken lines in Fig 2. A total of six such measurements are required before the calibration procedure is carried out, three between adjacent pairs of balls on the frame 10, and three between adjacent pairs of balls on the frame 12. The kit of parts may also include a calibration block so that the measuring bars 16 can be calibrated before.use. Such a calibration block has two precision steel balls, like the balls 14, connected together at a known spacing. This known spacing may be separately calibrated to a traceable standard. The balls of the calibration block may be connected together by a bar of a material having a low or zero coefficient of thermal expansion, such as INVAR™, or ZERODUR™. This ensures that the known spacing between the balls does not change with temperature. Since each ball 14 of the frames 10,12 must receive the magnetic sockets of two of the measuring bars 16, it is necessary to ensure that they do not foul each other during movement. With a measuring bar as shown in Fig 3, it.may be necessary to use balls 14 which have a somewhat larger diameter than the magnetic sockets 22, so that two such sockets can fit on one ball simultaneously. The invention is not restricted to calibrating machines of the type shown in the above-mentioned earlier patent applications. It can equally be used for calibrating a conventional machine tool, coordinate measuring machine, robot, etc. Such conventional machines generally involve a plurality of axes, arranged in series with each other. For example, a five axis machine tool or coordinate measuring machine may involve three slide axes, X,Y,Z, plus two axes of rotation, A and B. The present apparatus may be used to calibrate such a machine by connecting the upper frame 10 to the spindle or quill of the machine, and the lower frame 12 to the table or workpiece holder of the machine. The apparatus of the invention can be installed permanently on a machine, forming the means by which the position and orientation of the moving part of the machine is measured, in order to provide position feedback data for the servo control of the machine's movement. For example, it may replace the extensible instrument arms described in WO91/03145. The six measuring bars 16 of the present apparatus may also be mounted (on balls 14), parallel with the six rams 18 described in our International Patent Application No. WO-A-95 14 905. They then replace the interferometers or other devices described in that application for measuring the machine's movement. Another possibility is for the upper frame 10 to form part of (or to be clamped to) a special metrology frame, which is separate from the moving part of the machine, but connected to it so as to undergo the same motions as the tool or probe. An example of such a separate metrology frame is shown in Fig 6 of our earlier International Patent Application No. WO-A-95 14 905. Instead of the upper frame 10 of Fig 1 of the present application, the three balls 14 are simply connected at three spaced locations on the metrology frame 68 shown in Fig 6 of the earlier application which thus forms the structure. If the measuring bars 16 of the present application are intended to form the permanent measuring devices for the machine, then the balls 14 of the present application will simply be connected in place of the retroreflectors 66 of the interferometers shown in Fig 6 of that earlier application. As an alternative to the triangular frames 10 and 12 shown in Fig 1, there is shown in Fig 5 a frame which consists of three rigid rods 40, each of which has a ball 42 permanently attached at one end and a magnetic socket 44 at the other end. Other arrangements are possible in the construction of the triangular frames 10 and 12. For example the kit of parts described with reference to Fig 1 may consist of six balls, six rigid rods and six measuring bars. The rods and measuring bars terminate in magnetic sockets as described so that they will snap onto the balls. The six rigid rods are arranged to form two rigid triangular frames for the top and bottom of the apparatus and the measuring bars are connected between the triangular frames. Clearly the balls and sockets can be interchanged in other embodiments while still providing the rigid triangular frames interconnected by measuring bars. In Fig 5 the triangular frame is illustrated in place in a calibration jig 46 for determining the spacing of the balls. The jig, which may be made from ZERODUR™ or INVAR™, includes one fixed socket 48 at one end and a movable socket 50 at the other end. The distance between the centres of the sockets gives a measure of the distance between the centres of the balls 42. This distance may be measured by'a telescoping sensing device, for example an LVDT, as shown at 50, or a retroreflector may be mounted on the movable socket to reflect a light beam from a laser interferometer system (not shown) mounted on the fixed socket to measure the distance between the balls interferometrically. In the assembled metrology frame the magnetic sockets of the measuring bars 16 are attached to the balls 42. It is preferable that the axes of the rods and measuring bars should pass through the centres of the balls 42. In order to accommodate this feature where three or more attachments are to be made to one ball, one or more of the sockets on any arm, or on the rods, may be arranged at an angle to the axis of the rod or arm as shown in Fig 6. One of the rods 40 is shown in Fig 6 attached to a ball 42. Each socket has three pads 52 which make contact with the surface of the ball 42. One of the pads lies on the axis of the rod and thus contacts the ball on the line of the axis through the centre of the ball 42. The other two contact points lie preferably just over the orthogonal centre lines of the ball from the first. A magnet 54 is set into the socket to hold the socket in position on the ball. The use of magnetically retained sockets in the two triangular frames 10,12 makes them easy to dismantle for portability of the frame. Sockets as described with reference to Fig 6 can be provided at both ends of the rods. It is not however, essential that the structures are formed as assemblies of rods and balls. A solid or hollow triangular structure could be provided with balls or part-spherical sockets at its apices. Fig 7 shows one arrangement of rods 28, (40) and measuring bars 16 of the above-described embodiments which enables two rods and two measuring bars to terminate on a single one of the balls 14,(42). It can be seen that each of the rods 28,(40) in this example are straight and terminate in magnetic sockets 22 which are concentric with the axes of the rods. The rods are attached to the ball 14, (42) with their axes aligned with the centre of the ball. Each of the measuring bars 16 have angled sockets 60 similar to those illustrated in Fig 6 which enable the measuring bars to be attached to the ball 14, (42) with the axes of the bars passing through the centre of the ball. Magnets 62 are provided in the sockets to hold the bars in place. The sockets are positioned on the ball to enable the maximum freedom for pivoting movement of the measuring bars without interference as the two triangular frames are moved relative to one another on the machine being calibrated. Other types of measuring transducers could be used and disposed either internally or externally of the struts. It is also to be understood that the different types of connections between struts and sockets, or measuring bars and balls are not essential or unique to the embodiments in which they are described, but may in appropriate cases be interchanged between different embodiments. Where the measuring bars alone are used relatively light magnetic forces may be utilised to maintain the bars in position on the balls. Where, however, a magnetic ball and socket arrangement is used to connect a driven strut to a plate or structure, the magnets will need to be sufficiently powerful to prevent detachment of the strut from its support. In such circumstances, or in fact, in any of the ball and socket embodiments, the ball and socket joint may be fed with air to produce an air bearing effect to minimise friction. It is also to be understood that a computer would be used to handle the data coming from the transducers in the passive measuring bars although one has not been specifically identified in these embodiments. To improve the amount of relative movement between the frames of the calibration device, the telescopic measuring bars 16 could be replaced by measuring bars of fixed length which extend through the balls 14 in similar manner to that illustrated for the powered struts in Figs 11 and 12 of the patent application WO 95/20747.

Claims: A method of measuring the movements of relatively movable parts of a machine, one of which carries a tool or probe for performing an operation on a workpiece, and to respective ones of which first and second structures (10,12) are rigidly connected, each of said structures having at least one support (14) thereon, at least one of said structures having three supports thereon spaced in a triangular array, the supports having spherically or part-spherically shaped support surfaces which define calibration points on the supports, the method comprising the steps of: connecting at least three extendable measuring bars (16) between the structures, said measuring bars having shaped surfaces at their ends which engage the support surfaces of the supports in known relationships to said calibration points, characterised by the further steps of: determining the relative positions of the calibration points of the supports, driving the machine to perform an operation on a workpiece while taking readings from the measuring bars, using the readings from the measuring bars to provide information relating to the instantaneous positions of the calibration points of all of said supports relative to each other. A method according to claim 1 characterised in that both of the first and second structures (10,12) have three supports (14) thereon spaced in a triangular array, and each of the supports on the two structures has at least two of the extendable measuring bars (16) connected to it. A method according to claim 1 characterised in that one of the structures (12) has three supports spaced thereon in a triangular array and the other has a single support thereon (70), only three extendable measuring bars (16) being connected between the three supports on the one structure and the single support on the other. A method according to any preceding claim characterised in that the measuring bars are pre-calibrated before attachment to the support on the structures. A method according to any preceding claim characterised in that at least one of said structures (12) comprises three releasably interconnected rods (28) to which three supports (14) having spherical support surfaces are releasably connected. A method according to any preceding claim and wherein the machine includes measuring devices for measuring the relative movements of the machine parts, the method being characterised by the further steps of using the information relating to the instantaneous positions of said calibration points to calibrate the measurements made by the measuring devices of the machine. A method according to any preceding claim and wherein the machine includes a controller for controlling the relative movements of the machine parts, the method being characterised by the further step of passing the information relating to the instantaneous positions of said calibration points to the controller as feedback signals for the control of the relative movements of the machine parts. A method according to any preceding claim, characterised in that at least a portion of each of the support surfaces is part-spherical and for each support said calibration point is the centre of the sphere of which the part-spherical surface forms a part. A method according to any preceding claim, characterised in that the pre-calibrated device for making a determination of the relative positions of said calibration points is one of the extendable measuring bars (16). A method according to any preceding claim and characterised by the further step of pre-calibrating the device and the measuring bars using a calibration block (46) comprising two surfaces (48,50) connected together at a known spacing by a material of substantially zero coefficient of thermal expansion, the surfaces of the calibration block being shaped to co-operate both with the surfaces on the device and the surfaces of the measuring bar. Apparatus for performing measurements on a machine having relatively movable parts one of which is capable of carrying a tool or probe for performing an operation on a workpiece, relative movements of said parts being carried out under the control of a machine controller, the apparatus comprising: first and second structures (10,12) connected to respective ones of the movable parts of the machine and each of which has at least one support thereon, one of the structures having three supports (14) spaced in a triangular array, each of the supports on the first and second structures having a part-spherical support surface thereon, which is centred on a point thereby defining a plurality of calibration points on the structures, the relative positions of said calibration points on said at least one structure being accurately known, at least three extendable measuring bars (16) having part-spherical surfaces at their ends which are shaped to fit the support surfaces on the structures, said measuring bars being connected between the two structures so that the shaped surfaces at their ends are located in predetermined relationships with said calibration points, characterized in that said measuring bars being operable during relative movements of the movable parts of the machine as the machine performs said operation on a workpiece, to provide information as to the relative positions of said calibration points of all of said supports at any instant during said relative movement, said position information being passed to the machine controller as feedback data for the control of the relative movement of the machine parts. Apparatus according to claim 11 characterised in that each of the first and second structures (10,12) has three supports (14) thereon spaced in a triangular array, and six extendable measuring bars (16) are provided, each of the supports on the two structures having two of the extendable measuring bars connected to it. Apparatus according to claim 11 or claim 12 characterised in that at least one of the structures comprises three rods (28) arranged to define a triangle at each apex of which is a said support (14). Apparatus according to claim 11 characterised in that each of the part-spherical surfaces (42) on the supports are convex and each of the part-spherical surfaces on the ends of the measuring bars (16,40) are concave, each measuring bar having an axis which passes through the centres of the universally pivotable connections at each end thereof, and having an angled socket at least at one of its ends on which said concave surface is formed. Apparatus according to claim 14 characterised in that magnetic devices (54) are provided at each of the universally pivotable connections to hold the part-spherical surfaces on the respective measuring bars and supports together.

Inventors: MCMURTRY DAVID ROBERTS, MCMURTRY, DAVID ROBERTS
Applicants: RENISHAW PLC
Application year: 1995
Main classification: G01B21
IPC codes: G01B21, G01B3, B23Q1, B23Q17