Aligning optical components of an optical measuring system

Apparatus for aligning an optical measuring system to a desired direction comprises two housings (42,48) attachable to two relatively movable parts of a machine. The housings (42,48) have engagement means (53) such that when they are engaged together the optics inside (44,50) are aligned. The first housing (42) may be provided with attachment means, such as a ball (46), which may be located in a mount (54) on the machine table (56). The position of this ball (46) is determined and the machine spindle housing (48) to be attached to the spindle (60) when the two housings (42,48) are joined and such that the apparatus is aligned with the desired direction. The engagement means (53) between the two housings (42,48) may be compliant along an axis of the apparatus.

The present invention relates to a method of and apparatus for aligning the components of an optical measuring system preparatory to using them in a measuring operation.

One known type of optical measuring system consists of two or more housings, at least one of which is to be fixed to the bed of the machine and another one of which is to be carried by the movable arm or spindle of the machine. One of the housings contains one or more light sources and detectors, and will be referred to hereinafter as the “source housing” while the other housing contains reflectors, and will be referred to hereinafter as the “reflector housing”. Usually the source housing is maintained in a fixed position on the bed of the machine and the reflector housing is mounted on a part of the machine moveable with respect to the machine bed e.g. the machine spindle.

Aligning the optical components is often a time-consuming process which involves firstly the alignment of the source housing so that the beam or beams generated are directed along, or parallel to, one or more of the X,Y and Z axes of the machine. Then the reflectors have to be aligned with the beam or beams so that the reflected beams are directed back onto the detectors. Depending on the type of detectors being used the alignment may have to be accurate to within a few arc seconds.

The present invention provides an optical measuring system, for a machine having two relatively movable parts, the measuring system comprising:two housings attachable to the two relatively movable parts of the machine;wherein each of the housings is provided with a complementary part of an engagement device being such that when the two parts of the engagement device are engaged together, the housings are mutually aligned;and wherein at least one housing is rotatable relative to its machine part to enable the optical measuring system to be aligned with a desired direction.

The optical path between the two housings is thus correspondingly aligned with the desired direction.

Preferably the attachment means comprises an at least part-spherical surface on one of the housing and the machine part and a corresponding mating surface on the other of the housing and the machine part. More particularly, the attachment means may comprise an at least part-spherical surface on each of the housings and a corresponding mating surface on both machine parts. The attachment means may comprise an at least part-spherical surface on one housing with a corresponding mating surface on the corresponding machine part and a mating surface on the other housing with an at least part-spherical surface on the corresponding machine part.

Preferably the engagement device provided between the housings is compliant along an axis of the optical measuring system. The engagement device may comprise one or more protruding elements on one housing and one or more corresponding mating features on the other housing, such that the one or more protruding elements may be inserted into the one or more corresponding mating features.

Preferably each housing contains optical components of the optical measuring system, said optical components being pre-set within the respective housings so that when the two parts of the engagement device are engaged together, the optical components in the two housings are mutually correctly aligned.

A second aspect of the present invention comprises a method of aligning optical components of an optical measuring system in a desired direction on a machine having two relatively movable parts, said system comprising two housings containing optical components of the measuring system, each housing being provided with complementary parts of an engagement device arranged so that when said complementary parts are engaged together the housings are mutually aligned, the method comprising the steps, in any order, of:engaging together the complementary parts of the engagement device of the two housings;mounting the first housing on the first machine part;determining the location of the centre of rotation of the first housing when said housing is mounted on the first machine part;positioning the second machine part at a distance from the centre of rotation of the first housing to enable the second housing to be mounted on the second machine part when the two housings are connected, and wherein the line between the first and second machine parts is aligned with the desired direction; andmounting the second housing on the second machine part.

Referring to the drawings,FIG. 1shows a prior art embodiment of an optical measuring system for mounting on a machine as disclosed in WO02/04890.

The optical measuring system includes a base plate10, a source housing20and a reflector housing22, all of which need to be properly aligned with one or more of the machine axes. The base plate10is connected to the bed of the machine by screws12,14.

The source housing20may contain an autocollimator formed in optical sequence by, a light source24, a beam splitter26, a collimating lens28through which a collimated light beam passes out of the housing, and a detector30which receives a return light beam from the reflector32in the reflector housing22via the beam splitter26.

The source housing20also includes a kinematic seat in the form of three spherical seating elements16arranged in a triangular array and spaced at 120° apart. The seating elements16co-operate with three V-shaped grooves (not shown) on the base plate10to form a conventional kinematic seat for repeatable positioning of the housing on the base plate.

The source housing20has a further kinematic seat18on its front face (i.e. the face which is orthogonal to the beam direction) on which the reflector housing may be seated. The light source24and the reflector32are aligned during the manufacturing stage to ensure that when the reflector housing22is seated in the kinematic seat18on the front face of the source housing20, the light beam and reflector32are properly aligned.

It can be seen therefore that once the source housing20is correctly aligned to direct a light beam along one of the machine axes, e.g. the X-axis, the reflector housing22can be seated on the kinematic seat18on the front face of the source housing20, and will automatically be aligned with the beam from the light source24. Magnets33are used to urge the two housings20,22together at the kinematic seat18.

In order to take care of any mis-match in position between the machine spindle34and the reflector housing22when the two are to be connected together, the reflector housing22is provided with a limited amount of compliance by using an adjustable connector by means of which the reflector housing22can be connected to the spindle34of the machine. The adjustable connector has a ball36which is to be seated in a socket38on the machine spindle34. The ball36is adjustably supported in a retaining device40which, in turn is connected to the reflector housing22, by any suitable means.

An embodiment of the present invention is illustrated inFIG. 2in which the two housings form two parts of a ball-bar. The first part comprises a housing42which contains the light source and interferometer optics of the linear measurement interferometer44. A fibre optic delivery system57A may be used to deliver light to the optics from a remote light source. Fibre optics or wires57B may deliver light from the optics to a remote detector or deliver a signal from a detector in the housing42to an external controller. In an alternative arrangement, the light source may comprise a laser diode inside the housing42and a signal from the detector may be transmitted to an external controller by a wireless communication system, for example a radio or optical link. This arrangement has the advantage that the housing42could be free from wires or optical fibres. The housing42is connected at one end to a ball46. The second part comprises a housing48which contains the retroreflector50of the interferometer and is connected at one end to a ball52. Alternatively the housing48may contain a detector for the interferometer.

The two parts of the ball-bar are joined together at a kinematic joint53formed by seating elements on each part of the ball-bar which are urged into engagement by magnets55. The light source and interferometer optics44in housing42and the retroreflector50in housing48are arranged such that when the two parts of the ball-bar are joined together they are correctly aligned and the direction of the light beam45travelling between the two housings42,48is correspondingly aligned with the longitudinal axis of the ball-bar. It is desirable to align the light beam45travelling between the two housings with a certain direction, for example a machine axis.

Although it is preferable that the two parts of the ball-bar are aligned in order for the light beam45travelling between housings42,48to be both square and centred on the optics in the second housing48, it is only essential that the light beam is square on these optics.

Alignment of the two housings42,48for just squareness is much simpler and may easily be achieved by, for example, machining surfaces of the two housings which will be in contact when they are engaged such that abutment of these surfaces causes alignment of the housings. The housings may then be held together by vacuum or other suitable means.

In order to align the ball-bar and light beam45along a desired direction for taking measurements, for example machines axes X,Y or Z, a cup54is positioned on the machine table56as illustrated inFIG. 3. The cup54may be retained on the machine table56by magnetic means or by any other fixing means. The cup54may have three pads (not shown) such that when a ball of the ball-bar is positioned in the cup it is kinematically seated in order to precisely define its position.

The two parts of the ball-bar are joined together and the ball46of the first part of the ball-bar is seated in the cup54as shown inFIG. 3. A second cup58is mounted in the machine spindle60as shown inFIG. 4. Preferably cup58also contains three pads on which a ball of the ball-bar may be kinematically seated. The spindle60moves the cup58into contact with ball46of the first part of the ball bar. Once the cup58is in contact with the ball46its position can be determined from reading of the machine's scales. This position may now be considered the origin (0,0,0). The position of the ball46may be determined by alternative means, for example by using a probe.

The machine spindle60moves to disengage the cup58from ball46and is moved to a new position a distance L from the origin where L is a distance between the ball centres of the ball-bar. For example spindle60may move the cup58to a new position a distance L from its initial position along the X-axis as shown inFIG. 5. The ball-bar is then rotated about ball46until the ball52of the second part of the ball-bar is seated in cup58. The position of the ball52is now (L,0,0).

In order for the spindle60to move the cup58to the correct position, the balls46and52are preferably the same size. However, as long as the respective sizes of the balls46and52(or the difference between them) is known, the machine can compensate for the difference in size.

The ball-bar and hence light beam45is now aligned along the X-axis. In addition the interferometer optics and the retroreflector are mutually aligned. As shown inFIG. 6the spindle60may now drive cup58along the X-axis to break the connection between the two housings42,48of the two parts of the ball-bar. The spindle60may now move the part of the ball-bar comprising ball52and housing48along the X-axis to enable measurements to be taken.

This invention thus has the advantage that it enables the ball-bar and the light beam45to be aligned in any direction of the machine. Once that direction has been defined the spindle may continue to move in that same direction to take the desired optical measurements.

The cup58may be mounted in the machine spindle60in a compliant manner. This has the advantage that if the spindle60does not move to precisely correct the position to align cup58with ball46, the position of the cup58may be adjusted with respect to the spindle.FIG. 16illustrates the cup58mounted in the spindle60. The cup58is attached via a stem80to a ball82. The spindle60is provided with a cylindrical bore84into which the ball82may be inserted. The position of the ball82and thus the cup58may be adjusted within the cylindrical bore84. When the cup58is in the desired position, a clamp (not shown) is tightened around the cylindrical bore84to maintain the cup's position.

To prevent sagging of the ball-bar when the kinematic joint53is broken the magnets used to hold the balls in the cups are made sufficiently powerful to hold the housings in their positions. The magnetic force may be reinforced with electromagnets. In addition the weight of the ball-bar may be counterbalanced.FIGS. 14 and 15illustrate a counterbalance used on the ball-bar.FIG. 14shows a side view of a part of the ball-bar comprising a housing48and ball52which is mounted in the cup58of the machine spindle60. A counterbalance weight78is attached to the ball52on the opposite side to the housing48.FIG. 15shows a plan view of a part of a ball-bar comprising a housing142including a cup72at one end which is mounted on a ball76of a mount (not shown). In this case a counterbalance weight78is attached to the housing142via arms80such that when the housing142is mounted on the ball76, the counterbalance weight78is on the opposite side of the ball76to the housing142. The shorter the length of the housings, the less forces there are to create sagging.

Alternatively once proper setting between the cups54,58and the balls46,52has been achieved, an adjustment mechanism of the cups may be provided which is tightened to hold the balls in position. An example of such an adjustment mechanism is described in International Application WO02/04890.

To reduce the weight of the ball-bar the light source may be a remote light source connected to the ball-bar by a fibre optic cable.

Once the ball-bar has been aligned along a first axis it is very simple to then align it along a second axis as shown inFIGS. 7–9. InFIG. 7a ball-bar is illustrated with the two parts connected and the ball-bar aligned along the X-axis. The centre of ball46is known and is the ball-bar origin (0,0,0). The centre of ball52is also known and is a distance L from the centre of ball46along the X-axis, i.e. (L,0,0). The spindle may be moved in an arc of radius L about the centre of ball46to position the ball-bar along any other direction. For example as shown inFIG. 8, the spindle60has rotated the ball-bar about an arc of radius L through 90° until it becomes aligned with the Z-axis and the centre of ball52is positioned at (0,0,L). Once the ball-bar is aligned with the Z-axis the spindle60may pull the two parts of the ball-bar apart as shown inFIG. 9to enable measurements to be carried out along this axis.

FIG. 12illustrates a second embodiment of the present invention in which one part of the ball-bar comprises a housing142with a cup72at one end whilst the other part of the ball-bar comprises a housing148with a ball152at one end. In this embodiment the first part of the ball-bar is supported by a mount74having a ball76which fits into cup72. The ball76is held in the cup72by magnetic or other suitable means. A cup158is mounted on the machine spindle160and the spindle160is manoeuvred to position the cup158onto ball76to determine the ball's position. The spindle160then moves the cup158in the direction of desired alignment by a distance L from this position. Where L is the distance between the centre of ball76of mount74and the centre of ball152of the ball-bar. The ball-bar is then aligned with this direction as previously described.

The ball could comprise a part-spherical surface of the housing and the cup could comprise any suitable mating surface. The ball and cup may be replaced by any other suitable rotating means, for example a gimbal or universal joint. Preferably this rotating means allows rotation in three dimensions but rotation within a two-dimensional plane (e.g. XY plane) may be sufficient.

Alternative connections between the two parts of the ball-bar will now be described in more detail with reference toFIGS. 10 and 11. InFIG. 10, one of the housings42is provided with pins62aligned with the optical axis A of the system. There may be two pins62as shown inFIG. 10, or for example, three pins spaced at 120° about a central axis. Housing48is provided with holes64corresponding to the pins62. The two parts of the ball-bar may thus be joined together by inserting the pins62into the holes64. Such an arrangement allows compliance along the longitudinal axis of the ball-bar as the pins slide in and out of the holes, whilst keeping the two parts of the ball-bar accurately aligned in a direction perpendicular to the longitudinal axis of the ball-bar. With this type of connection the accurate alignment perpendicular to the longitudinal axis ensures that the interferometer optics and the retroreflector remain aligned along the longitudinal axis. However movement of the two parts relative to one another along this longitudinal axis does not affect the alignment of the interferometer optics and the retroreflector. Movement in this direction is desirable as it allows some adjustment if the spindle60and cup58have not moved a distance exactly L from the initial position in which case some compliance may be required.

FIG. 11shows an alternative arrangement of the kinematic joint between the two parts of the ball-bar. In this embodiment a protrusion66on one housing42is inserted into a correspondingly shaped aperture68on the other housing48. Tapered surfaces67,69on the protrusion66and aperture68may be provided to ease the location of the protrusion66into the aperture68. The protrusion66and aperture68may be shaped to allow rotation of one part of the ball-bar with respect to another, if the arrangement of the optics inside the housings52,58is such that it is not affected by this rotation. Alternatively features may be provided on the protrusion66and aperture68to prevent rotation of the two parts with respect to one another.

Any joint connecting the two parts of the ball-bar may be used if it fulfils the criteria that the connection must allow compliance parallel to the longitudinal axis of the ball-bar and that this compliance does not affect the squareness of the optics in the two housings. For example, the connection could comprise a kinematic seat as described above with reference toFIG. 2, in which the elements of the kinematic seat are mounted on a compliant material such as a flexure or spring.

The joint between the two parts of the ball-bar may allow further compliance along the longitudinal axis of the ball-bar. Such a joint may comprise a bearing, for example a sleeve bearing.FIG. 13illustrates such a ball-bar with a sleeve bearing70joining the two parts. This apparatus may be used as an optical ball-bar in which the spindle (not shown) drives ball52in a circle of radius R around ball46mounted in a cup (not shown). The use of a ball-bar to calibrate coordinate positioning machines, such as machine tools and coordinate measuring machines is disclosed in more detail in European Patent No. 0508686. Machine errors cause the path of the spindle to deviate from a true circle and thus causes the distance between the two ball centres to vary as ball52is driven round the circle. The joint between the two parts of the ball-bar allows variation in distance between the centres of the two balls46,52and the optics inside the ball-bar are used to measure this variation in distance. This information may be used to calibrate the machine errors in a known manner.