GB2188571A - Alignment systems - Google Patents

Abstract

The operating head 22 of a machine tool or other instrument is positioned by an alignment system which detects the actual position of the operating head rather than that of a supporting carriage thereby eliminating any deformation in the arm or support structure which carries the operating head. The system preferably uses optical alignment/detection e.g. by means of a laser beam wherein an emitter carried by a sliding member 12 movable with a head support 16 but isolated therefrom by an appropriate linkage 14, directs a beam 18 to a detector 20 mounted in close proximity to the head 22. A signal indicating undesired displacement of the head may be processed by circuitry 24 and used as a feedback to effect corrective adjustment. <IMAGE>

Description

SPECIFICATION Alignment systems The present invention relates to alignment systems and more particularly to extremely accurate alignment systems for machine tools.

Itis known to control or position the operating parts of a machinetool orother instrument by determining the relative displacement of translating members, either indirectly by measuring the revolutions of the drive screws or, more accurately, by locating measuring scales parallel to the machine guideways. For practical reasons, the measuring scales are normally positioned in close proximitytotheguideways.

Many such instruments or machines are designed to provide controlled movement or translation of displacement along one, two, orthree mutually perpendicular axes, with the objective, for example, of placing a tool or some other device, in a required position relative to a workpiece, and to subsequently execute an operation such as boring or profiling in which the relative movements between tool and workpiece are effected in a controlled manner.

The accuracy and repeatability which can be achieved in any practical application of the type referred to above, although subject to a long list of error-contributing elements, is dependent in the first instance on the precision ofthe machine axis guides or slideways, and the truth or fidelity of the inter-axis orthogonality when more than one axis is operational. Such errors represent an ultimate limit to the intrinsic accuracy of the machine, and will be reproduced with reasonable fidelity by calibration ofthe machine axes by reference to the measuring scales.

In practical applications, the effects of gravitational and other external forces and couples including inertial loads, the compliance of joints and the bending and buckling of members and frames etc., all contribute additional error components, some ofwhich are transient dynamic rather than static.

The absolute magnitude of the spatial errors arising from lack of straightness in the machine guideways, which may be observed at various points on any individual sliding member, will vary, depending on the amount of the 'offset', i.e. the length of a perpendicular from the slide axis to the point of interest,forex- ample, the position of an operating head. In general, the errorwill be a vectorwhich, in the case of a planar mechanism can be resolved into two components, one being parallel to the slide axis and the other perpendicularto it.

If effect of an error in straightness isto cause a driven memberto rotate from its intended position by a skew angle p say, (p being very small), then it is well known that: 1) At points close to the slide axis, the error in the direction oftranslation will be gel =.x(1 -cosp), where 'x' is the resolved component of motion along the slide. The cosine of a small angle is approximately unity, so that, in mose cases, e1 will be negligible.

2) At points on the driven memberwhich are remote from the slide axis, by some distance 'h' say,the errorcomponentinthe direction oftheslide axis will bee2 h.sinp h.p approximately. The magnitude of this error component may become significant.

The preceding brief discussion illustrates several important consequences of lack of straightness in the ways of a machine slide, and the presence of additional errors due to external reactions and moments, the self-deflection of machine elements, and dead-space or clearances should be noted. The importance of achieving the maximum practicable effective length ofthe moving slide member in orderto minimisethe skew angle due to lack of straightness and thus reduce the effect of offset should also be appreciated.

In the analysis of error sources it is convenientto distinguish between: (i) static deformations, (ii) quasi- static deformations such as the effect of 'live' loads which may alter a machine's configuration and hence the deflection characteristics in the course of an operating or machine cycle, but at such a low ratethatthe dynamic effects are negligible, and (iii) deformations dueto axial loads and couples resulting from ac celerations of machine members, including transient and cyclic acceleration components induced byvari able load reactions, etc.

It is an object of the present invention to provide means for measuring the position of the operating head of a machine relative to a datum or reference point in a fixed space with greater precision than hitherto. This is achieved by determining the resolved components of relative displacement between the operating head and the base of each translating memberofthe machine on a continuing dynamic basis, and summing the results with data corresponding to the relative positions of the machine slides as determined by the measuring scales or similar known means.The result represents more correctly the true position of the operating head relative to machine datum, the corrected data including elements of the deflections at the operating head which are due to static or geometric skew, inertial loads, moving or'live' loads, and workpiece reaction or loading, including the reactions from metal-cutting and similar loads. These deflection variables have neglig ible influence on the output data from the measuring scale readouts. The greatly enhanced accuracy of determination of the true position of the operating head will not only permit a corresponding improvement in the steady state accuracy of positioning the machine, but allowforthe correction of many transient or other dynamic effects.

According to the present invention there is provided an alignment system for a machine tool, the machine tool or instrument including a movable arm and a head for holding a tool mounted on the end ofthe arm including measuring means for measuring the position ofthe head, which means is independent of any deflection in the arm.

For the purposes ofthe present invention, the term 'machine tool' includes an instrument for positioning an object such as a piece of scientific apparatus. The term arm is meantforthe purposes of the present invention to include a multiple arm structure such as a portal structure.

Preferably the measuring means includes a light source and a target detector. Preferably the targetdetec- tor is mounted in close proximity to the head and the light source is mounted on a carriage which moves with the arm ofthe machine tool, but is not rigidly attached thereto.

Preferably the light source is mounted on a carriage which is connected to be moved by and with the main arm of the machine tool and which carriage is carried on the same guideway or guideways of the machine tool as the main arm ofthe machine tool.

In a preferred embodiment, wherein the machine tool has a complex arm capable of movement in two or more directions, a plurality oftarget detectors are provided. A single light source may be used to produce an outputfrom each detectorthe light being deflected to form a complex light path by partial light deflectors, a detector being associated with at least one ofthe partial light deflectors and receiving light not deflected by the deflector.

Preferably the partial light deflectors and mirrors which reflect a portion of the light impinging thereon.

The light detectors may be of the quadrant photodetector, lateral effect photodiode or crossed lateral effect photodiode type.

The light source may be ofthe laser type and for greater accuracy a focussing arrangement such as a telescope may be provided.

In an alternative embodiment, the carriage on which the light source is carried is carried on a guideway separatfrom the guideway or guideways on which the main arm of the machinetool is carried.

Embodiments of the present invention will now be described byway of example with reference to the accompanying drawings in which Figure 1 shows schematically in side elevation a machine tool fitted with the alignment system ofthe present invention; Figure2 shows in greater detail one form of mounting from the optical device of Figure 1; Figure 3 shows in greater detail the optical detector of Figure 1; Figure4shows a possible form of coupling for the optical device; Figure 5showthe application of an external preload for bearings; Figure 6shows in schematic representation a single axis column type machine fitted with the alignment apparatus according to the present invention; Figure 7shows in schematic representation a double cantilever machine;; Figure 8shows schematically a comparison between the instrumentation required for various machine configurations; and Figure 9 shows an embodiment giving greater accuracy for the positioning device.

With reference now to Figure 1 ,the measuring system consists of an optical light beam generator 10 mounted on a special sliding member 12,the "position reference stage', which is closely coupled via a universal linkage 14(Figure4) in line with atranslating machine member 16, and thus travels contiguously with it.If care is taken to ensure that the drive to the reference stage 12 from the translating member is purely axial, free of backlash or deadspace, and non-influencing in the lateral plane, the attitude of the reference stage effectively parallels the main machine guideways 17 and the light beam 18 is directed to createa reference perpendicular upon the line of the machine axis, which perpendicular intercepts at a desired target position with a photosensitive detector 20 fixed to the main machine member at a suitable distance from the main guideways, generally as close as practicable to the centreline ofthe operating head 22.Any undesired displacement of the target relative to the position reference datum will be detected and the resulting signal processed in control circuitry 24 to provide output data corresponding to the magnitude and sense of the deviation relative to the point of intersection of the machine guide axis and the light beam axis. The output data may be summed with the data from the measuring machine scale in summation circuit 25 and be used to effect a slight adjustment in the position of the operating head 22 by using the output of circuit 25 as a correction signalforthe main drive motor 26.

The position reference stage 12 will have a low physical profile and relatively small mass. It will not be subjectto any significant external forces or reaction moments with the exception of small preload forces which may be desirable to eliminate the effects of backlash or clearances, orto optimisethe stability of low-friction slides. Due to the proximity of the reference stage 12 to the machine slide axis, offset errors will be negligible, and the attitude ofthe stage will at all times substantially 'mirror' the corresponding attitude of the slideway 17 between the points of contact with the bearing surfaces ofthe position reference stage. The spatial errors oftranslation parallel totheslideaxiswshich may be observed at various points along the optical axis will not, therefore, be influenced at all by the quasi-static or dynamic error sources which affect the load-carrying members of a machine nor, in fact by any of the static deflection referred to, with the exception of those which deform the reference stage slideway itself, and so contribute to skew error.

The arrangement described corresponds, therefore, to a reference cylinder square, where the 'cylinder' is a very narrow and slightly tapering light beam which is translated contiguously with a load-carrying member of a machine. The light beam is aligned to intercept, at a desired target position, with a position-sensitive photodetector array 20 which is fixed to a remote section ofthe load carrying member, so that any relative motion oftranslation of rotation between the reference perpendicular and the loaded member will reuslt in a change in the output from the photodetector 20 thus generating an electrical signal which can be used, with a suitable signal processor 24 for instrumentation or control purposes to indicate the error and/orto make some appropriate adjustment.

If the operating head 22 ofthe machine is significantly offset from the common axis ofthe light beam and main machine member it may be necessary to split the light beam and to embody an additional photodetectortarget near the operating head in orderto obtain the desired performance. The alternative modes of operation are described later.

In a further embodiment, when errors may occur in the transverse plane as well as the plane which includes the guide axis and the optical reference beam axis - the 'principal guideway plane'- a suitable photodetector array or static configuration can be installed which will resolve the intercept point on the target into mutually perpendicular components corresponding to two of the principal axes of the driven member.

The addition of a second similar alignment system perpendicularto thefirst and provided with means to detect relative displacement in the principal guideway plane of a third axis drive is necessary in orderto provide full three-dimensional machine error correction.

In orderto avoid repetition, the combination of a position reference stage 12 optical reference beam 18 and photosensitive target 20, may be referred to as an 'optic perpendicular' in the following text.

Typical guideways 17 for precision powered machines are shown in cross-section in Figure No. 1 a, b. The main guideway 17 is indicated by an unbroken line, and the cross-section of the associated sliding member is shown with a 'dashed' or broken line.

The concave 'V' slide profile shown in the sketches may be replaced with convex forms, or any other type of linear bearing, including crossed rollers of aerostatic bearings, without loss of generality, and thefollowing propositions will applyto any type: When the ratio d:w is very large, as in Figure No.1(b) the transverse section ofthe sliding member (i.e.,the section shown in the drawing), will have a high degree of stability with respect to the transverse axis' of the guideways, so that rolling motion of the sliding member inthetransverse plane can be minimised.With such a configuration,the sliding member is often described as a 'table' as in,forexample, a machinetool.

When the ratio d:w is close to unity as in Figure No. 1(a), the guideways member is usually described as a 'rail'. In this case it is obvious that the stability of the sliding member in the transverse plane will be worse than in case 'b' by the factor d2/d1 approximately.

The reference stage for the dynamic interaxis alignment system should be provided with sufficient length 'L' along the axis of translation in orderto minimise errors of pitch and yaw (Figure No.2), in order to ensure that objective accuracy targets are met. As shown in the preceeding two paragraphs, however, the transverse stability or control of rolling motion, will usually be determined by the geometry of the guideways.

Referring now to Figure No.3, which represents the state of illumination of the target photodetector 20. For the reasons already stated, the stability of the reference stage is likely to be greater in the ZX plane and least in the YZ plane, thus leading to the possibility of errors in the apparent position of the optic perpendicular.

While the component of error Ax in the direction oftranslation represents limiting accuracy due to pitch errors, the component A y in the transverse direction, the result of roll and yawing motion, will have no effect on the accuracy of determination of relative axial motion if the detector is unresponsive to transverse movements of the light beam. A lateral effect photodiode can be operated in such a mode.

With reference to Figure 4, for effective operation, the position reference stage 12 must be connected to the driven machine member 16 by means of a non-influencing linkage such as that which is provided by a double Hooke's joint 30,32,so that relative lateral or pitch and yaw movements of the main member 16 cannot be transmitted to the reference stage 12.

As the system is required to function accurately for alternative directions oftranslation of the driven member, it is important to ensure that the linkage has negligible dead space or backlash, irrespective of whether it is pushing or pulling the load. For similar reasons the attitude of the reference stage oughtto remain substantially unchanged when the direction of motion is reversed.

In the general case, it must be assumed that the point of application ofthe linkage to the reference stage 12 will be determined by practical considerations such that some kind of mechanical couple will be generated.

This will cause the reference stage to pitch and/oryaw depending on the direction and magnitude ofthe couple. There will consequently be a tendency for the stage to rotate in opposite directions wheneverthe drive is reversed. Such rotations will, of course, be strictly limited, first by the physical clearance between slide bearing surfaces and, secondly by the rotational compliance or rigidity of the bearing structure once clearances have been taken up.

The three potential sources of error which have been identified: horizontal and vertical rotations of the reference stage and axial deadspace in the universal joints, together with possible horizontal and vertical translation of the reference stage in directions parallel to the slide axis due to essential bearing clearances, can usually be reduced to negligible proportions by effective design. This will frequently require preloading the couplings and bearings to the extent necessary to ensure that reversal forces and couples are rendered ineffective.

For example, the connecting linkage can be preloaded as indicated schematically in Figure No.4, with a spring preload force 34,36 which is sufficient to prevent any increase in the reference length 'd' overthe linkage. The design, with an ample margin for error, should allowforthe maximum possible accleration of the mass ofthe reference stage, and any static or viscous friction loading of that member.

In the case of machine guideways of the opposing double 'V' design, a well-proven solution for the refer- ence stage is to use crossed roller bearings (not shown). This technique provides a movement with a very low coefficientoffriction, and the bearing clearances can betaken up completely by preloading the bearings.

As a further example, consider a high-precision machine movement using a guide rail and aerostatic bearings. In such a case, the compliance of the bearings might lead to excessive movements of the stage in pitch and yaw. This can be corrected by making provision for appropriate external preload Pfor example by fitting suitable spring-loaded levers 38,40 as indicated schematically in Figure No.5.

The above are but two examples of a number of possible solutions, the exact nature of which will depend on the particular application, the desired accuracy, and other parameters.

The nature of the application requires the capability to detect, with high sensitivity, lateral deviations of a light beam at distances ranging from a few hundred millimetres to several metres. For reasons of standardis ation, it isdesirable that the light emitter 10 and photodetector 20 combination should function with only minor adjustments to the beam focussing and alignment throughout the full range of operation. These re- quirements can be met very satisfactorily by means of a low-power laser 50 (see Figure 1) with special features, which should include a beam diameter 18 of a millimetre or so, relatively low beam divergence, and excellentlong-term pointing stability.Depending on the precise requirements of the application, it may be desirable to incorporate a telescope 52 which will have the effect of increasing beam diameter and reducing the divergence by the same factor.

The cross-section of helium-neon laser beams for alignment applications is basically circular, although a cylindrical lens can be used to change the beam to a fan-shaped segment which will project a narrow line onto a remote surface. One effect of this is, naturally, to reduce the intensity of illumination at the remote surface, but either type of beam may be used for the optic perpendicular system, depending on the applica- tion: For applications requiring moderate resolution and alignment data for a single axis in one plane there is a choice between using a single lateral effect photodiode or a two-segment (bi-cell) detector, in conjunction with a beam of either circular cross-section or with 'line' or rectangularform.

For applications requiring moderate resolution and alignment data along two co-ordinate axes simultaneously, i.e. planar correction, it will be convenient to use a quadrant photodetector or dual, crossed, lateral effect photodiodes, and to retain a circular beam.

When the highest possible sensitivity and accuracy is necessary, the combination of a 'shaped' beam, i.e., a beam projecting a narrow rectangle of illumination with the long axis of the rectangle at right angles to the direction of motion, and a lateral effect photodiode may give the best results, two such beam:detectorcom binations being used for plaparcorrection.

In general,the lateral effect photodiodes will provide better linearity than the bi-cell or quadrant detectors, particularly when the beam cross-section is circular.

The precise manner in which the optic perpendicular may be incorporated in a machine in orderto measure deflection characteristics for instrumentation or control purposes will vary depending on the type and geometry of the machine. Structures based on positioning by reference to cartesian coordinates can be characterised as single-, two-, or three- axis systems, and theirstructural elements will include columns, cantilevers, portal framed and spindles. Some of these members will be more likely than others to influence the positioning accuracy under operational conditions. In most cases, the number of deflection elements which need to be determined in orderto achieve the desired accuracy will be relatively few by comparison with the totality of all deflection components which may be identified for any given structure.

Several embodiments of the invention in forms to suit representative machine structures will now be described commencing with the simplesttype of single-axis machine, by reference to the schematic drawings provided in Figure Nows. 6to 8. The structures shown in these drawings have been considerably simplified and much detail omitted in order to demonstrate the essential principles, and to allow direct comparison between the different cases.

Figures 6a, 6 shows in plan and elevation a typical single-axis machine, consisting of a powered column 60 guided on a flat bed 62 with twin 'V' slides and an operating head 64 which may be considered, for practical purposes, as effectively on the central axis of the column. The location and bearing supports for the position reference stage 66,68,the laser source 50, the target photodetector 20 are shown, the laser source 50 and target 20 being "inside" the arm 60.

In the Figures 6to 8 certain conventions have been assumed in orderto eliminate unnecessarydetail: System of Cartesian Coordinates: In the plane of the drawings, the vertical is taken (unless otherwise indicated) to representthe'Z' axis and the horizontal represents the 'X' axis. The mutual perpendicular to the XZ plane represents theY' axis.

Operating Head: The usual purpose of a coordinate machine isto place an opeating head 64 in a desired position. The operating head might carry out any one of a large number of functions, e.g. metal removal, such as milling cutter or borer, in one system, or it might represent a sensitive metrology probe in another.

For the purpose of this text the operating head is indicated by a circle 64 as its function does not alterthe principles involved.

Optic Perpendicular- Position Reference Stage. The 'n'th position reference stage is indicated bythe symbol 'L#'. Whenever such a symbol is shown it represents a fullytranslating position reference stage coupled to a main machine member by a non-influencing coupling as already described and incorporating a laser-optic system.

The associated photodetectortarget(s): (there may be more than one), are indicated symbolically as 'Tn',' or 'Tn1', 'Tn2 if there are several targets for the position reference stage.

The deflection vectors at the target position are identified by the same subscripts as the target i.e. Ax11, Ay11, Az11 etc. The target photodetectorwill normally be unresponsive to whichever of these elements is parallel to the optic axis.

Guideways and Drive: The presence of a straight machine guideway with an associated drive system and a linear motion transducer is identified by a heavy unbroken line 17 in the schematics.

Before referring to details of the instrumentation it will be helpful to consider the deflection characteristics of two basic machine structure elements.

Figure No. 7a represents a simple column or cantilever mounted on guideways 17 (XX') and incorporating an operating head 64 atthe end of the column 80. If we assume that a reaction force 'P' with vector elements Px, Py, and Pz is exerted at the end ofthe column then we expect to observeatthe operating head: (1) A deflection component bx K1P#in'X' (2) A deflection component A y = K2Py in 'Y' Because of the very much smaller magnitude of the elongation or compression along the column 80 due to Pz, the effect of this element of the reaction force can usually be ignored.

In the model structure represented by Figure 7b, the operating head 64 is offset from the column 80 by an appreciable distance via an additional cantilever82 and the resulting flexure is more complicated. Assuming the same reaction forces as before, we now have three couples or moments acting on each member. If, however, the structure has high torsional rigidity, (an essential condition for most practical designs),the deflectionswill approximate to: (1) A deflection component in 'Y'- Ay = K2Py.

(2) Adeflectioncomponentin'X'-Ax= KXPx.

(3) a deflection component in 'Z' - Az = (d/L)K1 Px+ K3Pz.

where K3 and K4 aretheflexural rigidity constants for the horizontal member in the X and directions respectively. The remaining constants apply to the column AB as in the first model.

The results showthat, if vertical and/or lateral deflections at the operating head are important and there is an offset to the operating head, suitable means to detectthese elements must be incorporated at or nearto the head position, but that the deflection in 'X' may be measured at the column axis. This problem may be solved by incorporating a beam-splitter in the optic path and using two target photodetectors 84,86 as indicated in Figure 7c.

Figures 8a -8b, provide a comparison between the instrumentation requirements for measuring important deflection elements on the two simple structures already examined, and two other more complex but common structures, of which the portal frame is the more interesting. Figures 8a and 8b correspond to the structures of Figures 7a and 7b.

Hereinbefore the general characteristics ofthe deflections applicable to the structures of Figures 8a and 8b were discussed. It is worth noting that, when it is not possible to locate the target exactly on the appropriate centreline ofthe operating head 64, simple geometry oran installed calibration will enabiethe compensation factor to be determined.

The two-axis machine shown in Figure 8c serves to illustrate that no single, or invariable, solution exists for such structures. In this case, it is proposed to use two optic perpendiculars: one for measuring deflections in X andY (50,90): and the other (92,94) responding to deflections in Yand Z. Itisalso possible to adopta solution usingthe single laser source and beam splitter of Figure 8b.

When the distance between the laser source and the target is variable, as in the vertical spindle movement of case 8c,thetarget signals may require correction or compensation to offset the changes in signal intensity and cross-section due to beam divergence. This will normally be less than one milliradian for a typical helium-neon laser source.

Depending on the range of relative displacement and the desired accuracy, such corrections may be achieved in several alternative ways: (1) by incorporating a telescope 52 at the source in order to reduce divergence, (2) by appropriate signal conditioning means, or (3) - in the case of a computerised system - by incorporating the appropriate compensation characteristic in software.

Machines which incorporate portal frame structures and three co-ordinate axes for positioning, as shown in the two views of Figures 8d, e, are very important: they embrace large plano-milling machines,jig-borers, jig-mills, and co-ordinate measuring machines (CMM). Significant deflection variables may include: (a) Rotation in the XZ plane of the entire portal frame 100 about the bearing line of XX' and or column deflections in the same plane.

(b) Lateral sway or rotation of the portal frame 100 in the YZ plane due to changing lateral position of the Y axis carriage assembly.

(c) Simple bending of the horizontal beam 102 of the portal and the associated changes in the centreheight of the spindle and carriage assembly.

(d) Deflections in the ZY plane of the operating head 64 relative to the carriage platform 104 due to external loading (including inertial loads), or curvature of YY' axis due to causes (c) above.

(e) Skew or rotation of the portal frame 100 in the XY plane. This may be especially significant if the portal frame is driven from one side of the bed or table. (In some applications vertical guidance is provided only on the drive side of the table by a rail with two parallel vertical faces and the outside leg may only be provided with horizontal bearings).

Forthe machine shown one possible configuration comprises a first laser 50with targets 106,108 and deflector 110 and a second laser 1 12 with a singletarget 114. Control signals indicating errorarereceived from all three targets and may be combined in control circuitry to produce a correction of the head position.

Alternative loading Up to this point we have considered the possible distortions of machine elements due to changes in static forces, but similarthough variable effects will occurwhen a machine is in motion due to viscous (velocity related) friction or drag and the additional reactions which result from acceleration of the masses as well as time-variant reactions at the operating head. The optic perpendicular allows compensation for these effects, but the accu racy will inevitably be reduced as the highestfrequency components ofthe excitation forces are increased.

Alternative (high accuracy) guidance When the highest achievable accuracy of position control is required as, for example, in precision metrology systems, it may be desirable to incorporate an auxiliary guide rail for the optic perpendicular. Ideally, such an auxiliarywould be located close to the central axis of each main machine guideway. The advantages include: (1) Higher intrinsic accuracy throughout the machine lifetime. A guide rail which carries virtually zero load can be specified for the most desirable properties of stability and accuracy of working thus effectively transferring laboratory standards to the workshop.

(2) When the machine as a whole is mounted on a solid (stiff) bedplate, but has fitted guideways forthe main machine members, the auxiliary rail will isolate the optic perpendicularfrom any distortions ofthe main guideways which resu It from machine loading, etc.

(3) The existence of independent reference guideways as well as the attributes of the optic perpendicular provide practical means for measuring structural deformation during machine operation, with even greater accuracy than can be achieved with the optic perpendicular alone. This provides the designer with considerably more latitude in the choice of design parameters and the essential compromise between performance, cost and efficiency.

A possible embodiment is shown in Figure 9 in which the main machine member 16 operates on guide rails 17,17' and is constructed to be shaped to have a space 1 20 within the member 1 6 for passage of the beam 18, target 20 being mounted at the end of member 16 just above the operating head 64. The additional rail 122 on which the position reference stage 12 is mounted may be made, for example, from machined stone (granite) to extremely high accuracy.

Claims (11)

1. An alignment system for a machine tool as defined, the machinetool including a movable arm and a headfor holding a tool mounted on the end ofthearm including measuring means for measuring the position ofthe head, which means is independent of any deflection in the arm.

2. An alignment system as claimed in Claim 1 in which the measuring means includes a light source and a target detector.

3. An alignment system as claimed in Claim 2 in which thetarget detector is mounted in close proximity to the head and in which the light source is mounted on a carriage which moves with the arm ofthemachine tool, but is not rigidly attached thereto.

4. An alignment system as claimed in Claim 2 or Claim 3 inwhichthe light source is mounted on a carriage which is connected to be moved by and with the main arm of the machine tool and which carriage is carried on the same guideway or guideways of the machine tool as the main arm of the machinetool.

5. An alignment system as claimed in Claim 2wherein the machinetool has a complex arm capable of movement in two or more directions, and in which a plurality oftarget detectors are provided.

6. An alignmentsystem as claimed in Claim Sin which a single light source may be used to produce an output from each detector the light being deflected to form a complex light path by partial light deflectors, a detector being associated with at least one of the partial light deflectors and receiving light not deflected by the deflector.

7. An alignment system as claimed in Claim 6 in which the partial Iightdeflectors are mirrors which reflect a portion ofthe light impinging thereon.

8. An alignment system as claimed in Claim 7 in which the light detectors are of the quadrant photodetec tor, lateral effect photodiode or crossed lateral effect photodiode type.

9. An alignment system as claimed in anyone of Claims 2to 8 in which the light source is ofthe lasertype and for greater accuracy a focussing arrangement such as a telescope is provided.

10. An alignment system as claimed in Claim 2 in which the light source is mounted on a carriage which is carried on a guideway separate from the guideway or guideways on which the main arm of the machinetool is carried.

11. An alignment system for a machine tool substantially as described with reference to the accompanying drawings.