WO2003101664A1 - Tool movement and positioning system - Google Patents

Abstract

Apparatus for positioning and holding a tool relative to a workpiece comprising a carriage for conveying the tool over the surface of the workpiece and means for holding the carriage onto the surface of the workpiece. The invention provides a vacuum sealing arrangement, a laser position-finding and guidance system, a multiple vacuum cup robot and a system for determining localised surface discrepancies between the actual position/shape and that which is desired or designed. Such an arrangement provides a robot capable of 'crawling' over a workpiece and carrying out precise manufacturing operations thereon.

Description

TOOL MOVEMENT AND POSITIONING SYSTEM

This invention relates to the positioning and holding of a tool relative to the surface of a workpiece on which it is intended the tool carry out one or more operations, such as a manufacturing operation. The invention is a development of the tool positioning system described in our earlier international patent application No. PCT/G B01/02994 (publication No WO 02/06003), however in several aspects this invention has broader applications than to manufacturing operations, as will be described below. It is common to carry out such manufacturing operations on workpieces as measuring, drilling, milling, cutting, countersinking and inspecting; tools for such operations are well known in the art and form no part of the present invention per se, however the terms "tool" and "tools" as used hereinafter should be understood to embrace all such apparatus for performing any such operation. In addition, the present invention is directed towards any system in which a semi-autonomous or autonomous apparatus moves over a solid surface, such as for cleaning windows on tall buildings, for security and surveillance purposes and the like, and also towards the locating and positioning of any semi-autonomous vehicle at a distance from a base; the terms "tool" and "tools" should therefore be understood to embrace all of these concepts.

The accuracy with which tools are positioned is dependent upon the level of accuracy required in the finished product. In applications in which a high degree of accuracy is required, such as the aircraft industry, a significant proportion of the manufacturing time and cost is accounted for by the need to ensure that tools are accurately positioned relative to the workpiece in order that a number of manufacturing operations can be performed with accuracy and at precisely defined locations on the surface of the workpiece. In conventional manufacturing, tools are located by hand with the assistance of jigs and fixtures, which are costly and complicated items in themselves. Alternatively, tools may be located by robot arms, however such methods are costly and inherently inaccurate, particularly where the workpiece is large and requires successive tool operations to be carried out over a large area.

Further, the ability of a tool positioning system to move a tool carriage over a surface of varied shape, orientation, roughness and/or porosity is wholly dependent on the means holding the carriage to the surface. The holding means must hold the carriage to the surface sufficiently strongly to prevent it from falling off, such as when the carriage is inverted and/or when a manufacturing or other operation is carried out, but whilst allowing the carriage to move over the surface. Our earlier application, PCT/GB01/02994, gave one possible solution to this problem, which was to provide a lower holding force when the carriage needs to move and a higher force to hold the carriage during a manufacturing operation. This system, however, is not as effective in practice as initially hoped.

Accordingly, in a first aspect the present invention provides an apparatus for movement over a solid surface in which a carriage is held in slidable contact with the surface by ambient air pressure, a lower than ambient air pressure region is formed between the carriage and the surface and bounded by a sliding seal mounted to the apparatus and adapted to slide over the surface and to minimise the flow of ambient air into the low pressure region and at least one element is provided in frictional contact with the surface and adapted to move the carriage over the surface, wherein the force holding the carriage to the surface is isolated from the sliding seal.

The divorcing of the force applied to hold the carriage to the surface from the sliding seal which bounds the vacuum plenum where the shroud meets the surface is a significant feature of this aspect of the invention; it enables the carriage to be moved about the surface whilst remaining soundly in contact therewith, and represents a considerable advantage over conventional arrangements using long brush seals and the like, which tend to allow ambient air to leak into the vacuum plenum and are therefore particularly inefficient. In order to provide movement relative to the workpiece, the carriage preferably comprises one or more elements held in frictional contact with the workpiece surface by the holding means and adapted to move the carriage over the workpiece surface.

The elements may comprise one or more wheels, with associated conventional drive and steering mechanisms, or indeed any alternative arrangement capable of engaging with the workpiece surface in order to move the carriage relative thereto, such as caterpiller tracks. The frictional arrangement between the elements may be adjusted in any conventional manner so as to provide the optimum drive for the carriage consist with the requirement not to cause damage to the workpiece surface; the coefficient of friction between the element(s) and the workpiece surface may be adjusted by providing tread patterns or suction cups to the wheels/tracks, for example.

The force holding the carriage to the surface preferably acts through the drive element(s) in frictional contact with the surface. Thus, it is important that there be little suspension, or relative movement between the said element(s) and the body of the shroud defining the vacuum plenum chamber. In contrast, the sliding seal at the periphery of the shroud, or "cup" where it abuts the surface is preferably resiliently mounted thereto, so as to be able to reciprocate towards and away from the surface with an aggregate spring force (i.e. the total spring force along the length of the sliding seal) being substantially less than the total force holding the carriage to the surface. The sliding seal may comprise a bellow arrangement, that is one which is able to reciprocate towards and away from the surface, but which is substantially impervious to airflow parallel to the surface - therefore resistant to ambient air leakage into the vacuum plenum. Alternatively, a sliding "collar" type arrangement might be provided, although this might not be sufficiently flexible along its length to permit the sliding seal to conform to an irregularly-shaped surface. Advantageously a bellows arrangement is inherently flexible along its length, and is thus able easily to conform to such surfaces. Preferably, the sliding seal also comprises a brush seal, which in use is in contact with the surface. The advantages of brush seals in accommodating small surface irregularities, steps and the like are well known in the art, however the removal from the brush seals according to the present invention of the transferred force holding the carriage to the surface enables the carriage to move more easily over the surface whilst optimising the brush seal performance, reliability and longevity as compared to conventional arrangements.

In the arrangement of this invention, a high flow, low pressure vacuum pump arranged to evacuate a plenum chamber adapted to seal against the surface in which the sliding seal is isolated from the force of ambient air pressure forcing the carriage towards the surface, provides a carriage which can reliably move over surfaces having a great variety of shapes, orientation and inclinations, roughness and of differing materials. Such a carriage would have applications outside manufacturing, such as in window cleaning or other maintenance operations on tall buildings, in security, surveillance and reconnaissance and so on.

In a second aspect, the invention provides an apparatus for determining the location of a movable vehicle in terms of its translational and rotation coordinates relative to a base having a laser beam source, the apparatus comprising means for pointing the laser beam at a directional reflector mounted on the vehicle and means for pointing the reflector at the source, sensor means being provided for measuring the elevation and azimuth bearings of the laser beam, the elevation and azimuth bearings of the reflector relative to the vehicle and the attitude of the vehicle about two orthogonal axes, and ranging means for measuring the distance between the base and the vehicle. Such an arrangement provides for determining a vehicle's location and orientation with a high degree of accuracy, and is applicable anywhere such accuracy is required; allied to a suitable movement control means it would provide a very accurate laser guidance system.

As more particularly used in manufacturing, this aspect of the invention provides an apparatus for determining the location of a carriage adapted to move over a surface in terms of its translational and rotational co-ordinates relative to the surface, the apparatus comprising a base fixed at a predetermined position relative to the surface and having a laser beam source, means for pointing the laser beam at a directional reflector mounted on the carriage and means for pointing the reflector at the source, sensor means for measuring the elevation and azimuth bearings of the laser beam, the elevation and azimuth bearings of the reflector relative to the carriage and the attitude of the vehicle about two orthogonal axes, and ranging means for measuring the distance between the base and the source.

In such an arrangement the base is effective to establish the translational co-ordinates of the carriage and the sensors on the carriage establish its rotational co-ordinates. In practice, the carriage would sense the rotation of the reflector about two orthogonal axes and the attitude of the carriage about two orthogonal axes, thus providing a degree of redundancy (over that strictly required to establish the position of the carriage in six degrees of freedom) but ensuring accuracy. Preferably the axes for sensing carriage attitude are substantially in the horizontal plane. The term "laser" is to be understood to mean a very narrow beam of coherent monochromatic light in the ultraviolet, visible or infrared region of the spectrum, and is not to be construed on being restricted to optical wavelengths. Preferably the means for point the laser beam at the reflector and/or the means for pointing the reflector at the source comprise a laser light-sensitive target, the sensitivity of the target varying across its surface according to a predetermined pattern, so as to align the laser beam/reflector according to where on the target the laser light impinges. The target may be either a quadrant detector or a position sensitive device (PSD) as are known in the art, or an equivalent thereof. The system is preferably adapted to operate in a single "bullseye" manner, such that there is a central region within which there is no action, but when the laser beam impinges outside the "bullseye" the pointing means moves the laser/reflector so as to bring the impingement point into the "bullseye". This action is conveniently proportionate to the distance between the impingement point and the "bullseye", so that the greater this distance the faster the pointing means moves the laser/reflector so as to bring it into alignment.

The carriage may comprise a beam splitter interposed between the reflector and the source and adapted to direct a portion of the laser beam arriving from the source towards the means for pointing the reflector at the source.

In this way the reflector and the target need not be coincident; no such arrangement is strictly necessary for the base since here the target may simply be aligned closely adjacent the laser beam source. Preferably the ranging means utilises laser interferometry techniques acting on the laser beam to measure the distance between the base and the carriage.

The apparatus may comprise means for comparing the determined locating and orientation of the carriage relative to a virtual model of the surface. In the broader aspect, the comparing means need not be limited to positioning relative to a surface, but may instead position the vehicle in three-dimensional space according to some virtual model thereof. According to the application, this virtual model may be dynamic, for example in the case where the invention is applied to a guidance system for a missile aimed at a moving target. In another aspect, the invention provides an apparatus for movement over a solid surface in which a carriage is held in slidable contact with the surface by ambient air pressure comprising a plurality of separate, spaced lower than ambient air pressure regions each region being defined by a shroud in sliding contact with the surface, each shroud being mounted so as to pivot independently of each other shroud.

Such an arrangement facilitates the conformability of the carriage to a surface which is very irregular or has a large radius of curvature; it also allows the carriage to cross large discontinuities in the surface without falling off the surface. Where one shroud loses its purchase on the surface, because ambient air leaks in as the shroud passes over the discontinuity, the remaining shrouds serve to retain the carriage in contact with the surface. The optimum number of shrouds is probably three, given that a tripod arrangement with three points of contact, allows an object to be supported firmly on a surface of any shape or curvature.

In a yet further aspect, the invention also provides an apparatus for positioning and holding a tool relative to a workpiece comprising a carriage for conveying the tool over the surface and means for holding the carriage onto the surface of the workpiece, the carriage remaining capable of movement relative thereto, wherein means are provided to measure localised deflection of the surface, caused by the mass of the carriage relative to the tool. This arrangement allows for the precise placement of the tool at a predetermined position on the surface, allowing for localised deflection of the surface caused by, for example, the mass of the carriage, or the failure of the surface to conform exactly its design shape. The measuring means may suitably comprise a laser rangefinder mounted to the carriage. It will be apparent to those skilled in the art that the above aspects may be employed independently, or together in any combination, according to a particular application.

The invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic view, in partial cross-section, of an embodiment of an apparatus for carrying a tool over a solid surface in accordance with the first aspect of the invention;

Figure 2 is a schematic view of an apparatus for determining the location of a tool-carrying carriage, as shown in Figure 1 , in accordance with the second aspect of the invention, and

Figure 3 is a schematic view of a tool carrying apparatus in accordance with the further aspects of the invention.

In the Figures, like numerals denote like elements; however, it should be noted that this is for ease of reference, and it does not denote that the different aspects of the invention must be used in combination. As will be clear from the foregoing general description, the different aspects of the invention illustrated may be used in any combination, they may also be applied independently.

Referring now to Figure 1, a carriage 1 is shown, to which is mounted a tool 3 (a drill having a bit 5 is illustrated). The carriage 1 comprises a cup 7 enclosing a space 9 from which air is evacuated by a high flow, low pressure vacuum pump 11 , creating a lower than ambient air pressure therein. This pressure difference, between ambient and that in space 9, presses the carriage 1 towards the surface 13. Air contract between the cup 7 and the surface 13 is completed by seal 15, which comprises two portions: a brush seal 17 in contact with the surface, and a sliding portion 19, shown here as a bellows-type arrangement, which is able to reciprocate towards and away from the surface but which is impervious to air and thus prevents ambient air from leaking through into space 9. The sliding portion 19 is biased towards the surface 13 so as to maintain contact between the brush seal 17 and the surface. The carriage 1 has wheels 21 which engage the surface and move the carriage relative thereto. The wheels 21 are fixedly mounted to the cup 7 by rigid structs 23, so as to hold the cup 7 at a fixed distance from the surface 13. The bias force pressing the brush seal 17 into contact with the surface is significantly less than the force due to the pressure differential pressing the carriage onto the surface. This enables the carriage to be moved easily over the surface even if the surface is inclined, or even inverted. Because the force holding the carriage onto the surface acts through the wheels 21 this increases the friction therebetween, enabling the wheels to drive the carriage over the surface. Because the bias force acting on the brush seal 17 is significantly less than the force due to the pressure differential acting on the wheels, the carriage has no difficulty in overcoming static friction in order to move over the surface as is experienced with conventional sealing arrangements.

The sliding portion or bellows 19 is sufficiently resilient to flex along its length and thus to allow for variations in the shape or material of the surface around the circumference of the cup 7, and thus allow the carriage to move with ease over steps and other discontinuities in the surface whilst maintaining a low pressure in space 9. As shown in Figure 1 , the right hand wheel 21 is driven and steered by drive unit 25. The wheels 21 have a rolling surface formed of an elastic compound so as to provide good frictional engagement with surface 13 whilst preventing damage thereto. Around part of the drill 3 is a rotary, vacuum seal 27, which is sealingly connected to vacuum cup 7 by a flexible diaphragm 29, so ensuring that the vacuum cup 7 extends in substantially unbroken fashion over its entire surface area, whilst allowing drill 3 to move along three orthogonal axes relative to vacuum cup 7 (and allowing drill bit 5 to rotate about one of these axes). Moving means 31 which is rigidly mounted to the carriage 1 is adapted to move the drill 3 along these three axes, so as to move the drill 3 left and right as shown in the drawing, into and out of the plane of the drawing, and up and down as in the drawing.

Normalisation sensors 33 (only one is shown in Figure 1 ) are mounted to the carriage 1 and are adapted so as to sense the angular orientation of the carriage 1 , and particularly the drill bit 5, relative to the surface on which the carriage 1 rests (not shown, for clarity) and into which a drilling operation is to be carried out. Rotating means 35 is adapted to rotate the drill 3 about two orthogonal axes substantially coplanar with the surface upon which the carriage rests, so as to ensure that the drill bit 5 is normal to the surface into which it is to drill, or so as to enable the drill bit 5 to drill at any desired angle other than normal into the surface. Control unit 37, which suitably comprises a microprocessor unit and a combined power source, powers and controls the operation of the drill 3, drive unit 25, moving means 31 , rotating means 35 and the vacuum pump 11.

In use, carriage 1 is intended to move with a minimum of operator intervention over the surface of a workpiece so as to move the tool 3 into an accurate position and orientation in order to carry out a precise manufacturing operation. In order to hold the carriage 1 onto the surface of the workpiece and to prevent it from falling off as it moves around the workpiece, the interior of the vacuum cup 7 is at least partially evacuated, forming a vacuum plenum, so that ambient air pressure holds the carriage 1 against the workpiece. The seal 15 acts as the seal between the vacuum cup 7 and the workpiece, and is sufficiently flexible and/or flexibly mounted to conform to the surface configuration of the workpiece. When the carriage 1 is in approximately the correct position, the evacuation of the vacuum cup 7 may be increased, compressing the seal 15; as a result, the carriage 1 is very firmly clamped to the workpiece. The low pressure within the vacuum cup 7 is controllable so as to ensure that the carriage 1 is unmoved by the forces, additional to its own weight, arising from the fine positioning of the tool 3 and, more particularly, from the operation of the tool when drilling is taking place. When the carriage 1 is clamped onto the workpiece, the tool 3 and the drill bit 5 are accurately positioned and orientated by way of the moving means 31 and the rotating means 35, as is described in our earlier International patent application, publication No. WO 02/06003.

Referring now to Figure 2, the carriage 1 of Figure 1 is shown additionally provided with a reflector 41 which is mounted on a pivot 43 to the carriage 1. Reflector 41 is a retro-reflector, such as those produced by Virtek

Vision International Inc. Pivot 43 permits the reflector to be rotated about two orthogonal axes; preferably these are perpendicular to the axis of the drill (not shown in Figure 2 for clarity), and the pivot 43 is on the drill axis (so that the axis of member 45 shown in Figure 2 is coincident with the axis of the drill 3 shown in figure 1 - the reflector 41 is therefore moved by moving means 31 and rotating means 35 together with the drill 3 so as to maintain this relationship).

Also provided and mounted so as to pivot with the reflector 41 are a beam splitter 47 and a laser sensitive target 49, which are described further below. A sensing and drive means 50 is provided for sensing the azimuth and elevation bearings of the reflector and for driving the reflector in azimuth and elevation.

Also illustrated in Figure 2 is a base 51 , comprising a laser 53 which is pivotally mounted so as to pivot in azimuth and elevation about pivot 55, a laser sensitive target 57, sensing and drive means 59 for sensing the elevation and azimuth bearings of the laser 53 and for driving the laser so as to pivot in azimuth and elevation, and a control unit 61. The control unit stores a virtual model of the surface 13 (the position of which relative to the base 51 is precisely known) and details of the precise points on the surface at which a drilling operation is required.

The operation of the arrangement shown in Figure 2 is as follows. The laser 53 and reflector 41 are aligned as will be described below. By sensing the azimuth and elevation bearings of the laser 53 and by measuring the range between the laser and the reflector using laser interferometry techniques, the control unit 61 can establish the precise three dimensional position of the carriage 1 (and by comparing this position to a predetermined position on the surface 13 at which a drilling operation is to be carried out, as stored in the control unit memory, the control unit 61 can establish whether or not the carriage needs to move, and in which direction). By sensing the azimuth and elevation bearings of the reflector through means 50 and the angular orientation of the carriage 1 relative to the surface 13 using normalisation (or "tilt") sensors 33 (shown in Figure 1 ), control unit 37 can establish the precise angular orientation of the carriage 1 ; by combining the measurements of the control units 61 and 37 it is possible to establish the precise translational and rotational location of the carriage 1 relative to the base, so that the various drive means 25, 31 , 35 can be actuated so as to carry out a drilling operation in precisely the right position on, direction relative to and depth into the surface 13 (it will be noted that the system described has a degree of redundancy in that it is only necessary to establish the location of the carriage with six degrees of freedom, whereas the system described uses four sensors to establish the three rotational degrees of freedom; this is because the carriage 1 of Figure 1 has two normalisation sensors 29 as well as the two sensor to establish the orientation of the reflector relative to the carriage).

The alignment of the laser 53 and the reflector 41 towards each other is effected using the targets 49, 57 and the beam splitter 47. The beam splitter 47 is interposed between the laser 53 and the reflector 41 so as to divert a portion of the laser beam 63 towards the target 49. The remainder of the laser beam 63 travels, in the direction of the arrows shown, to the reflector 41 there to be reflected back towards the target 57 which is mounted closely adjacent to the laser 53, and so as to pivot therewith. The targets 49, 57 are preferably of variable sensitivity to laser light according to where on their surface the laser light impinges (quadrant detectors or PSDs, as are known in the art, are suitable). By measuring the variation of the signal produced by the target the appropriate control unit 37, 61 can establish what movement of the reflector 41 and of the laser 53 is required in order to keep the laser light impinging in the centre of the relevant target and, because the position of the targets relative to the reflector/laser is known, to keep the laser and reflector pointing at each other. To avoid the system from continually "limiting", the arrangement is preferably to establish a "bullseye", so that when the laser light impinges with the "bullseye" the control units 37, 61 do not actuate the movement of the laser/reflector, but as soon as the laser light impinges outside the "bullseye" the control unit initiates movement to bring the impingement back towards the "bullseye". The speed of movement is preferably proportional to the distance between the point at which the laser light impinges and the "bullseye". Figure 3 illustrates a carriage 101 mounting a drill 103 having three vacuum cups 107 which are independently mounted to the carriage 101 by fixed rigid struts 109, the cups 107 being mounted to pivots 111 so as to conform to the surface 113. Because the weight of the carriage 101 may deform the surface in the region where drilling is to take place, as shown by the phantom line illustration, the undeflected shape of surface 113, a laser range finder 115 is provided to measure the deflection, in order more precisely to position the drill. Positioning of the drill is as described above in connection with Figures 1 and 2, or as in our earlier WO 02/06003, and the vacuum cups 107 may be as described in connection with Figure 1 or as in WO 02/06003. The laser range finder can also be used to account for inconsistencies between the actual surface and the virtual model of the surface.

At any time, data may be input to the control unit 37 or 61 by way of an operator interface (not shown).

Now that a specific embodiment of the invention has been described, numerous modifications and variations will immediately spring to the minds of those skilled in the art. For example, the control unit 37 and pump 11 may either be remote or affixed to the carriage 1 for greater compactness (and the control unit 37 and control unit 61 may comprise a single integrated microprocessing unit); the operator interface 45 might comprise a keyboard and might also be located on the carriage 1 , or it might comprise a remote infra-red or microwave linkage or the like. The vacuum cup 7 depicted is in the form of a rectangular box - this could be of any shape provided it has a circumferential edge at least loosely configured so as to conform to the surface of the workpiece; it could, for example, be toroidal in shape with the manufacturing operation being performed through the central opening thereof, or it could comprise a plurality of small vacuum cups of any shape. The vacuum system described could be complemented by any means capable of providing an attractive force, such as an electromagnetic device, for use on a ferrous or ferro-magnetic structure, or even a system employing a releasable adhesive arrangement.

The illustrated embodiments incorporate a drill, for performing a drilling operation, however the drill could be replaced by any other conventional tool for performing other manufacturing operations, or operations such as welding, testing, painting, testing or other localised treatment. The embodiments may also have applications outside manufacturing, such as in building or structural maintenance and inspection, remote guidance systems, security or surveillance systems and so on.

Claims

1. Apparatus for movement over a solid surface in which a carriage is held in slidable contact with the surface by ambient air pressure, a lower than ambient air pressure region is formed between the carriage and the surface and bounded by a sliding seal mounted to the apparatus and adapted to slide over the surface and to minimise the flow of ambient air into the low pressure region and at least one element is provided in frictional contact with the surface and adapted to move the carriage over the surface, wherein the force holding the carriage to the surface is isolated from the sliding seal.

2. Apparatus as claimed in Claim 1 wherein the force holding the carriage to the surface acts through the element(s) in frictional contact with the surface.

3. Apparatus as claimed in Claim 1 or 2 wherein the sliding seal is resiliently mounted so as to be able to reciprocate towards and away from the surface with an aggregate spring force substantially less than the total force holding the carriage to the surface.

4. Apparatus as claimed in Claim 1 , 2 or 3 wherein the sliding seal comprises a bellow arrangement able to reciprocate towards and away from the surface, but which is substantially impervious to airflow parallel to the surface.

5. Apparatus as claim in any preceding claim wherein the sliding seal comprises a brush seal.

6. Apparatus for determining the location of a carriage adapted to move over a surface in terms of its translational and rotational co-ordinates relative to the surface, the apparatus comprising a base fixed at a predetermined position relative to the surface and having a laser beam source, means for pointing the laser beam at a directional reflector mounted on the carriage and means for pointing the reflector at the source, sensor means for measuring the elevation and azimuth bearings of the laser beam, the elevation and azimuth bearings of the reflector relative to the carriage and the attitude of the vehicle about two orthogonal axes, and ranging means for measuring the distance between the base and the source.

7. Apparatus as claimed in Claim 6 wherein the means for pointing the laser beam at the reflector and/or the means for pointing the reflector at the source comprise a laser light-sensitive target, the sensitivity of the target varying across its surface according to a predetermined pattern, so as to align the laser beam/reflector according to where on the target the laser light impinges.

8. Apparatus as claimed in Claim 6 or 7 wherein the carriage comprises a beam splitter interposed between the reflector and the source and adapted to direct a portion of the laser beam arriving from the source towards the means for pointing the reflector at the source.

9. Apparatus as claimed in Claims 6, 7 or 8 wherein the varying means utilises laser interferometry techniques acting on the laser beam to measure the distance between the base and the carriage.

10. Apparatus as claimed in any of Claims 6 to 9 comprising means for comparing the determined locating and orientation of the carriage relative to a virtual model of the surface.

11. Apparatus as claimed in any of Claims 6 to 10 comprising means for moving the vehicle to a desired location and/or orientation relative to the surface.

12. Apparatus for movement over a solid surface in which a carriage is held in slidable contact with the surface by ambient air pressure comprising a plurality of separate, spaced lower than ambient air pressure regions each region being defined by a shroud in sliding contact with the surface, each shroud being mounted so as to pivot independently of each other shroud.

13. Apparatus as claimed in Claim 12 consisting of three shrouds.

14. Apparatus for positioning and holding a tool relative to a workpiece comprising a carriage for conveying the tool over the surface and means for holding the carriage onto the surface of the workpiece, the carriage remaining capable of movement relative thereto, wherein means are provided to measure localised deflection of the surface, caused by the mass of the carriage relative to the tool.

15. Apparatus according to Claim 14 wherein the measuring means comprises 5 a laser rangefinder mounted to the carriage.

16. Apparatus according to any preceding claim for positioning and holding a tool relative to a workpiece comprising a carriage for conveying the tool over the surface of the workpiece and means for holding the carriage onto the surface of the workpiece, the carriage remaining capable of movement o relative to the workpiece surface.

17. Apparatus according to Claim 16 wherein the carriage comprises one or more elements held in frictional contact with the workpiece surface by the holding means and adapted to move the carriage over the workpiece surface. 5

18. Apparatus according to Claim 16 or 17 wherein the holding means and/or the carriage is/are adapted to conform to the surface configuration of the workpiece immediately adjacent thereto.

19. Apparatus according to any of Claims 16 to 18 comprising means for sensing the position of the carriage and/or the tool relative to the 0 workpiece, and directing means adapted to control the carriage in order to move the carriage over the workpiece surface between pre-determined positions thereon.

20. Apparatus according to any of Claims 16 to 19 comprising means for sensing the position of the tool relative to the workpiece surface and 5 means for moving at least part of the tool relative to the carriage along at least two orthogonal axes.

21. Apparatus according to any of Claims 16 to 20 comprising means for sensing the angular orientation of at least part of the tool relative to the workpiece surface and means for rotating at least part of the tool about at 0 least one axis to adjust said angular orientation.

22. Apparatus according to Claim 20 or Claim 21 , when dependent on Claim 19, wherein the directing means is adapted to control movement and rotation effected by the moving means and the rotating means, respectively, such that the tool reaches a predetermined position and/or orientation relative to the surface of the workpiece, and to actuate the tool.

23. Apparatus according to Claim 22 comprising a programmable controller adapted to control the directing means in order automatically to convey the carriage and to position and orient the tool between predetermined locations and orientations relative to the workpiece surface, and to actuate the tool in order to carry out one or more predetermined tool operations on the workpiece.

24. Apparatus according to Claim 23 wherein the controller is programmable so as automatically to carry out different sequences of predetermined tool operations on a workpiece and/or to carry out different sequences of predetermined tool operations on a variety of different workpieces.