WO1990009559A1

WO1990009559A1 - Position determination method and apparatus

Info

Publication number: WO1990009559A1
Application number: PCT/AU1990/000054
Authority: WO
Inventors: Josef Zybert
Original assignee: Josef Zybert
Priority date: 1989-02-15
Filing date: 1990-02-13
Publication date: 1990-08-23

Abstract

This invention provides a method and apparatus for position determination of a worktool in for example positioning equipment such as sensing, moving, marking or machining equipment. The method and apparatus comprises at least one reference pattern (g) in a known relation to a workpiece (m), means for imaging at least a portion of said reference pattern (c and d) with said means in a known relation to said worktool (b) and means for processing said image to determine one or more points on said image whereby said points accurately determine the position of said worktool (b) relative to said workpiece (m). Markings may be included on said reference pattern which specify spatial coordinates of particular points on said pattern. Calibration data may be included which specifies corrections to spatial coordinates of particular points on said reference pattern. Means may be included for calibrating and/or monitoring dimensional changes in at least said reference pattern and/or said workpiece and/or said sensors.

Description

POSITION DETERMINATION METHOD AND APPARATUS

BACKGROUND TO INVENTION

Field of the Invention

This invention relates in general to position determination of a worktool in for example positioning equipment which may take the form of:

(a) sensing equipment such as digitising equipment, coordinate measuring equipment or inspection equipment; .

(b) moving equipment such as pick and place robots;

(c) marking equipment such as mkpen plotters, photoplotters or integrated circuit projection equipment; and

(d) material working equipment such as engraving equipment and drilling equipment; where said worktool comprises respectively:

(a) image producing means such as imaging or sensing components; (b) material handling means such as grippers;

(c) media marking means such as inkpens, photopens, photoheads or X-ray beams; and

(d) material cutting means such as diamond saws.

In said positioning equipment said worktool is usually,located on a workhead which is positioned relative to a workpiece along axes such as the X and/or Y and/or Z axes in a substantially continuous motionbymeans of servomotors under computer control.

Each application for said equipment requires the position of said worktool to be determined whilst in motion with a certain accuracy. For example in the electronics industry manufacturing integrated circuits requires master artwork which is commonly produced on photoplotters with accuracies of at least +-5 micrometers. In the printing and graphic arts industries digitising images or producing master artwork often requires accuracies of +-5 to +-10 micrometers. For other applications accuracies of upto +-100 micrometers are satisfactory'. The accuracy which is obtainable by the said equipment is dependent on many factors and is particularly dependent on the accuracy of positioning a worktool in relation to a workpiece.

Description of the Prior Art

Heretofore a variety of methods have been employed for determining the position of a worktool.

It is known from the prior art that one such method which is commonly used is based on transducers positioned on the motor drives comprising a scale (circular indicia) fixedly attached to the motor shaft and a sensor fixedly attached to the motor body wherein rotation of the motor shaft produces rotation of the circular indicia resulting in on or off signals in said transducers which are counted to extract the incremental rotation of the shaft and the corresponding linear movement of a worktool.

It is also known from the prior art that another such method comprises scales (linear indicia) fixedly attached along the tracks alongwhich the X and/or Y and/or Z members move and sensors adjacent to the scales whereby movement of the said members results in a concurrent movement of said transducers relative to the scales producing on or off signals which are analysed to extract the linear movement.

In this regard there has hitherto been a number of inventions based on the above general methods, including the following United States specifications:

4,499,374, 4,529,964, 4,603,480, 4,606,642,

4,606,643, 4,607,956, 4,631,403, 4,631,404,

4,645,925, 4,649,648, 4,667,096, 4,684,257,

4,700,483, 4,701,615, 4,739,164, 4,750,836.

A common feature of these methods is that the position of the worktool is determined with transducers and scales which are distant from said worktool. Those skilled in the art shall be aware that the mechanical components between these two locations experience dimensional variations due to temperature, flexing under load, mechanical backlash and other distortions which lead to changes in the position of said worktool with respect to said workpiece in an unknown manner and extent in relation to the position indicated by said position transducers.

In many applications where positioning equipment is used the required accuracies are low and as a consequence the abovemen- tioned errors arewithin acceptable limits. In other applications where high accuracies in the order of 5 micrometers or less than 1 micrometer are required these errors are reduced by^"various design approaches. These design approaches are well known to those experienced in the art and it is known that such approaches are often necessarily very expensive.

The prior art also teaches us that another method of position determination is based on laser interferometry wherein a laser beam is directed to mirrors on the moveable members producing interference fringeswhich are countedtopreciselydetermine the spatial coordinates of the worktool. Those experienced in the art are aware that this method although very accurate has certain limitations and requires sophisticated apparatus which may be very expensive.

Therefore, a method is desired which is capable of the determination of position of a worktool with high accuracy.

DEFINITIONS

In this specificationthe following terms are defined as follows:

'worktool' means a firstmember andmore particularlytheworking surface of said first member;

^•workpiece' means a second member and more particularly the working surface of said second member, said surface may also be a non physical plane in space;

¹referencepattern' means anypatternproducingadifferent image from one imaged position on said pattern to the next;

'known relation* means that for two members in any spatial locations the coordinates of each point on the working surface of one member are known and/or are fixed relative to the working surface of the other member; and

'drivemeans' means anymethod formoving saidmountingmeans and/ or said workpiece.

SUMMARY OF THE PRESENT INVENTION

This invention provides a position determination method and apparatus, comprising mounting means holding a worktool in oveable relationship with a workpiece, one or more reference patterns in a known relation to saidworkpiece, means for imaging at least a portion of said reference pattern with said means in aknown relationto saidworktool, means forprocessing said image to determine one or more points on said image whereby said points accurately determine the spatial relation of said reference pattern, the apparatus thereby accurately determining the position of said worktool relative to said workpiece and drive means associated with said mounting means and/or said workpiece for positioning said worktool in relation to said workpiece.

In a preferred feature of this invention there is at least one referencepatternwhichmaybe of similarspatial geometryto said workpiece and in a proximate relation to said workpiece. The said patterns may be said workpiece itself, or produced on said workpiece as physical marks or as projected images, or produced on a seperate material and said material placed in a known relation to said workpiece or as an image in space located in a known relation to said workpiece such as that produced by a hologram or a scanning laser beam. The said referencepatternmay be a photographic reverse image of said reference patternwherein dark areas are replaced with light areas and light areas are replaced with dark areas.

In another preferred feature of this invention said reference patterns may incorporate data marks representing- absolute spatial coordinates of particular points of said reference pattern.

In another preferred feature of this invention calibration data may be used in association with said reference patternwhere said data may be generated by initial calibration of said reference pattern whereby said data represents differences between the actual coordinates ofparticularpoints of said referencepattern and the specified coordinates.

In another preferred feature of this invention there are imaging means which may be arranged in a proximate relatioin to said worktool wherein said imaging means may comprise one or more sensors each said sensor producing an image of at least a portion of said reference patterns where said images may comprise one or more scan lines, said scan lines may comprise one or more scan data points where said data points may have a known spatial geometry and may relate to individual coordinate points on said reference pattern and may perform in part a measuring function with said data points forming a measuring scale.

In another preferred feature of this invention there are processing means for analysing said image which may identify markings of said reference pattern and which may calculate said spatial coordinates in terms of said data points of said image in relation to said markings. Said processingmeans may also pre- calculate images based on possible next positions of said worktool andcompare saidpre-calculated imageswith^ hedetected image at said next position.

In another preferred feature of this invention means may be incorporated for calibrating and/or monitoring dimensional changes in at least the reference pattern and/or workpiece and/ or sensors.

Whilst this specification is largely directed to applications including marking equipment and in particular photoplotters it is to be understood that the invention has application to and covers any other areas where accurate location of a worktool is required.

The novel features of the present invention are set forth with particularity in the appended claims. Other features and advantages of the present invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG-1 is a schematic diagram showing a plan view of a representative marking equipment.

FIG-2 is schematic diagram showing views of the workhead.

Fig-3 is a schematic diagram showing an isometric view illustrating a typical arrangement of the major features of the present invention.

FIG-4 is a schematic diagram showing representative versions of sensors including illumination and magnifying lens.

FIG-5is a diagrammatic representation of several two dimensional reference patterns and includes representative imaged lines and the corresponding position of the worktool.

FIG-6is a diagrammatic representation of four unit cells of the referencepatterns in FIG-5 and FIG-7 including a representative imaged line.

FIG-7is a diagrammatic representation of aportion of a reference pattern, or unit cell, with data therein representing absolute spatial coordinates.

FIG-8 is a graphical representation of the output from a charge coupled device sensor or a scanning laser beam sensor for a typical trace.

FIG-9 is a graphical representation of a portion of the output from a charge coupled device sensor for two velocities, 0 cm/sec and 10 cm/sec.

FIG-10 is a graphical representation of the output from threshhold circuits with the curves displaced vertically for clarity.

FIG-llis a graphical representation of processed data which may be used to determine position.

FIG-12 is a graphical representation of nine curves vertically displaced for clarity to indicate the possible nine curves which could be detected after the next step movement of the worktool from the current central position.

FIG-13 is a diagrammatic representation of a portion of a reference pattern, a representative imaged line and the possible direction of movement of the sensor corresponding to the curves in FIG-12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method and apparatus described herein relates in general to position determination of a worktool and has application in for example positioning equipment such as sensing, moving, marking and machining equipment. Where said worktool is a cutting device in machining equipment for cutting three dimensional profiles fromblocks ofmaterial a three dimensional referencepatternmay be required. Similarly in applications for robots three δ

dimensional referencepatternsmaybenecessary and itmaybemore appropriate for said reference pattern to be an image positioned in space such as may be formed by a hologram or a scanning beam of laser light. For applications where said worktool is a lightbeam such as in photoplotting equipment a two dimensional reference pattern may be more appropriate with said reference pattern being physically produced as marks on a surface or as a projected image displayed on the workpiece or displayed in space as a single or multi dimensional image such as may be produced by a hologram.

The preferred embodiments of this invention are best described by reference to marking equipment and in particular to photoplotters.

Referring now to FIG-1there is a schematic diagram showing a plan view of a representative marking equipment. The apparatus la consists of a flat bed or platten lb onto which a workpiece is placed. An Xmember lcmoves along guides Ifto the left and right inthe Xdirection. On said Xmember is located a Y member Idwhich moves along guides (not shown) on said X member up and down in the Y direction. Movement of said X and Y members is effected by servomotors (not shown) . Attached to said Y member Id is a workhead lh and shown in more detail in FIG-2. Movement of said X and Y members under computer control results in the worktool, 2b of FIG-2, tracing the desired pattern on said workpiece.

In FIG-2 the workhead 2h incorporates imaging means comprising sensors 2cand 2dwhich may be located on said workhead on either side of saidworktool as shown in FIG-2and said sensorsmay detect images from said reference pattern along for example paths 2s. On said workhead may also be located a worktool 2b producing in the case of a photoplotter a light beam 21 and said workhead may be supported above the workpiece on a cushion of air to maintain a small and constant separation from the workpiece.

Refering now to FIG-3 which is a schematic diagram showing an isometric view of the main features of the present invention and is expanded in the vertical axis for clarity. On the reference pattern 3g may be located the workpiece 3m above which may be a workhead 3h on which may be located sensors 3c and 3d detecting images of said reference pattern along paths 3s, there ϊαay also be located illumination means 3i and worktool 3b producing for example a light beam 31 which works the workpiece 3m. -•,

The said reference pattern may be said workpiece itself or on the front or rear surface of said workpiece, or may be seperate from said workpiece in various locations in known relation to said workpiece, for example, on the top surface of the platten lb of FIG-1as blackmarks on awhite background lgof FIG-1,on seperate material where said material may have the same thermal expansion characteristics as said workpiece said material laid onto said platten and said workpiece placed onto said material or alternatively said material with said reference pattern may be located above saidworkheadwith saidworkpieceplacedbelow said workhead onto said platten. The said reference pattern is not limited to physical production on a workpiece and may include for example reference patterns formed as projected images on said workpiece or by using means such as holography three dimensional images may be projected into regions of space in a known relation to said workpiece.

In the preferred emodiment of this invention in photoplotting said reference pattern may be proximate to the recording media and the one of the two surfaces of said media used for recording purposes may substantially cover the surface of said reference pattern and the seperation of said media from said reference patternmay beminimal and of the order of 0.1 millimetre. In this configuration there is a direct correspondence between a particular position on said reference pattern and a particular position on said media and said direct correspondence is preferably maintained throughout the plotting process.

This direct correspondence between said reference pattern and said workpiece can be achieved by other means. For example said reference pattern may have a smaller surface area than said workpiece with a plurality of sensors positioned in relation to saidworkheadwherein said sensors utilise saidreferencepattern in turn to locate said workhead on said workpiece. As another example said reference pattern may be located away from said workpiece to one side, above or below. In each such configuration there shall bemaintained a known relationbetween saidworkpiece and said reference pattern.

To those skilled in the art it shall be apparent that there may be a multiplicity of positions for said reference pattern in relation to said workpiece. It should also be apparent that there may be a multiplicity of methods for the production of said markings of said reference pattern which is not limited to physical markings. It is to be understood that this invention covers any other variations of position and production which satisfy the above general requirements which have not been specifically alluded to in this specification.

The said reference pattern may have a multiplicity of designs, for example, a square grid of 20 micrometer vertical and 30 micrometer horizontal lines seperated by 450 micrometers shown in FIG-5.1, two vertical 20 micrometer lines 20 micrometers apart and seperated by 450 micrometers and horizontal 30 micrometer lines seperated by 450 micrometers shown in FIG-5.2and zigzag lines at 45 degrees to the horizontal with lines sloping to the right being 20 micrometers wide and those sloping to the left being 30 micrometers wide shown in FIG-5.3. Further examples may bethereversephotographic images ofthe abovementionedpatterns wherein for example said pattern shown in FIG-5.1 becomes rectangles of 430 micrometers by 420 micrometers with centre to centre spacings of 450 micrometers.

Designs for said reference patterns are better visualised by utilising a unit cell being a representative cell which when duplicated in all directions forms said reference patterns but it is to be understood that said reference patterns need not necessarily be formed of unit cells. Unit cells for the patterns of FIG-5.1, 5.2 and 5.3are shown in FIG-6.3, 6.2 and 6.3 respectively. The active area of said sensors may be at least as large as the dimension of said unit cell. Markings forming said reference pattern should preferably produce a different image from one imaging position to the next. Where said reference pattern is formed of unit cells it is preferable that markings forming said unit cell produce a different image for each different imagingpositionwithin saidunit cell wherein specific images are indicative of specific positions within said unit cell. It is seen that a multiplicity of designs of unit cells can be conceived which could satisfy the above requirements and it is to be understood that this invention covers any other designs not specifically alluded to in this specification.

A further aspect of said reference pattern is the inclusion of markings giving the absolute coordinate positions of particular points in each unit cell of said reference pattern as indicated by the example shown in FIG-7. These markings may be for example a set of 20 micrometer diameter dots arranged in some code such as the binary code to represent position in the coordinate directions. For example for a plotting area of 1.2 metres by 0.8 metres and a reference pattern of periodicity of 0.5 millimetres the codes would preferably represent numbers upto -2400,^* being 2400 by 0.5 millimetres or 1.2 metres which is achieved using 12 dots to represent the number 4095. For example the dots shown in FIG-7 may represent the spatial coordinates of the bottom left hand corner of the unit cell having said dots therein. The markingsmayalsobedecimal or alphnumericdigits orsomepattern representing the corresponding coordinates. This feature of said pattern may be sensed seperately by, for example, a 512 by 512 pixel charge coupled device imaging 4 of the above unit cells being for example an area of said reference pattern of circa 1 millimetre by 1 millimetre. The said image would thenbe analysed to extract the data, for example a plurality of dots, digits or patterns fromwhichto decodethe coordinatevalues ofsaidpoints in each unit cell.

In certain configurations of this invention ambient light may be used to illuminate said reference pattern. In other configura- tions an illumination sourcemaybeusedwherein said light source emits light in the infrared, visible or ultraviolet regions and said light source may be located in the vicinity of said sensors and said reference pattern as shown in FIG-4where the said light sources are represented by 4i. Thus for photoplotting on ultraviolet film with a reference pattern of black marks on a white background said light source may be infrared light of 875 nanometres suchasthatproducedby light emittingdiodes or laser diodes.

Imaging means are located on said workhead shown in FIG-2 as sensors 2c and 2d. The workhead 2h of FIG-2 is positioned on the Y member Idof FIG-1as indicated by lh of FIG-1. On said workhead 2h is situated the worktool 2b of FIG-2in between said sensors and in this configuration there is a fixed geometrical relationshipbetweenthe said sensors and saidworktool. In other configurations theremaybe a different number of sensors located in different positions in relation to said worktool.

One of the said sensors is shown diagramatically in FIG-4 with three representative variations. In the configuration depicted a two dimensional referencepattern 4g,and lgof FIG-1, is located below the workpiece 4mand substantially covers the whole of the working area lg shown in FIG-1. In one variation as shown in FIG- 4.1the illumination source 4imay be located to one side of the optic axis of the lens 4oand the light beammay be reflected from the partial mirror 4rthrough the lens onto said workpiece along light path 4s. Light reflected from said reference pattern may then pass along the light path 4s through the lens 4o and through the partial mirror 4ronto said sensors 4c/4d. In an alternative configuration said light beam may be directed to said workpiece 4m angularly along the light path 4i either directly from the illumination source as shown in FIG-4.2or indirectly for example through an optic fiber as shown in FIG-4.3. Light reflected from said reference pattern may then pass through lens 4o along path 4s to sensors 4c/4d. In configurations where said reference pattern is located below the upper surface of said workpiece the illumination may traverse said workpiece to illuminate said reference pattern and the light reflected from said reference pattern may then traverse said workpiece to reach said sensor through said lens. In an alternative configuration the optic axis of themagnifying lens maybe arranged atan angle to thevertical.

In a preferred embodiment of this invention said reference pattern may be imaged using sensors such as charge coupled devices. For example a charge coupled device having 1024 pixels whichare output in a 25 microsecondperiodmaybe suitable. Other charge coupled devices with a different number of pixels operating at other data rates may also be suitable. A laser beam scanning for example 0.5 millimetres in 25 microseconds may also be suitable. In order to position said worktool with an accuracy of 1 micrometer the movement of said worktool should preferably not exceed 1 micrometer in 25 microseconds'which is equivalent to 4 centimetres/second. Generally the speed with which said sensors scan said reference pattern influences the maximum velocityof aworktool in relationto a workp^'iece or aparticular resolution and this may be expressed as follows,

velocity = resolution / scan speed

and it is seen that for greater resolution for example 0.1 micrometers the velocity may need to be reduced to 4 mm/sec.

To effect positioning of a worktool with an accuracy of 1 micrometer said reference pattern would preferably need to be imaged with a resolution of at least 1 micrometer. There exists a relationship between the Numerical Aperture of a lens and the resolution of said lens at a particularwavelengthwhich is given by Rayleigh's equation as follows,

R = 0.61 . Wl . SQRT( (1 / NA . NA) - 1 )

where R denotes the resolution, Wl denotes wavelenth, and NA denotes numerical aperture. Thus a lens having a Numerical Aperture of 0.5 may resolve 1 micrometer when illuminated with light of wavelength 875 nm.

An example of data output from a charge coupled device which images a section of the reference pattern of FIG-5.1is shown in FIG-8which specifically relates to a reference pattern printed on a seperate sheet with said pattern produced on the upper surface of said sheet and where a workpiece is placed onto said reference sheet where said workpiece is daylight working film Agfa Printon DL511p with the photographic emulsion on the upper surface. The saidreference pattern is illuminated by an infrared light emitting diode TSHA5203 through said workpiece and the reflected light imaged onto a charge couple device WV-CD50 through a microscope objective of magnification 10. Typically a reference pattern may be imaged with an edgewidth of some 20 micrometers between the low intensity, for example 10%, and the high intensity, for example 90%, levels. The output from said charge coupled device is a voltage variation with time and this may be fed into electronic circuits for further preprocessing. Forexample the output fromthe charge coupled devicesmaybe sent to one or more threshhold circuits which produce outputs as indicatedby 10a, lOband 10cinFIG-10, shownvertically displaced for clarity, representing 25%, 50% and 75% of the maximum signal value. This data may then be further analysed to determine the centre or edge position and the corresponding pixel atwhich this occurs as shown in Fig-ll The data collected by said sensors is preprocessed by electronic circuits then analysed using computer processing means to extract relative displacement, absolute position, velocity and acceleration data.

For the next determination of position it is seen from FIG-12that the resultant line centres or edges calculated from the data represented in FIG-11 may have one of the 9 possibilities representedby curves ato ishown inFIG-12withthe corresponding directions of movement labelled a to i shown in FIG-13. Position determination may also be achieved in the following manner. The sensor output forpossible nextpositionsmaybe calculatedprior to movement to said next position said sensor output may be one of the 9 curves represented in FIG-12. As said sensor moves to saidnextpositionoutput fromsaid sensors for saidnextposition may be compared with each of the nine curves of the type represented in FIG-11 thus identifying said next movement.

Images of said reference patterns detected by certain sensors such as charge coupled devices may be distorted in certain circumstances such as higher velocities of said sensor relative to said reference patterns. For example a charge coupled device with a frame scanning rate of 25 microseconds may produce scan data points as shown in FIG-9 for two velocities, 0 cm/sec indicated by 9a and 10 cm/sec indicated by 9b. The image may be distorted due to the integrated intensity detected by each pixel of said charge coupled device of the relatively moving image during the frame scanning period and may result in an increase in linewidth of 2 micrometers and a shift in the line centre of 1 micrometer in the direction of movement for each 4 centimetres/ second. The calculated coordinates are normalised by adding 1 micrometer for each 1 micrometer/25 microsecond (4 centimetres/ second) velocity of said sensor relative to said reference pattern.

In one embodiment of thepresent inventionposition is determined with accuracies of circa +-5 micrometers. The said reference pattern may be produced from a unit cell with dimensions of the order of but less than the active area of said sensors and the accuracy of markings may be for example of the order of +-100 micrometers. In this embodiment measurement of distance between two coordinatesmayuse datapoints fromsaid sensors, for example the pixels of a charge coupled device, where the number of said data points between markings on said pattern are counted. The markings of said reference pattern may be used as temporary location marks to assist in counting data points between the starting point and end point of movement. It is known to those experienced in the art that the tolerance in the distance between the first and last pixel of a 1024 pixel charge coupled device is of the order of +-0.065 micrometers and that this corresponds to an error on a workpiece in the X and Y directions of +-0.005 micrometers per millimetre with a 13x magnification lens and the cumulative error per metre is +-5 micrometers or +-0.0005 % between two points. In this manner said charge coupled device may be used as a ruler for measuring distance and said reference pattern provides convenient temporary reference marks in the measuring process as the measuring device is shorter than the measured distance. The said distance between said pixels of said charge coupled device is dependent on temperature and it may be preferable that the equipment and workpiece be allowed to thermally stabilise prior to operation.

In a refinement to the above embodiment said workpiece has incorporated calibration marks with an accuracy of circa +-1 or +-5 micrometers at a standardtemperature. An initial calibration run may then be performed by said equipment whereby the number of data points from said sensor, for example the number of pixels of a charge coupled device, between these calibration marks is counted to provide a calibration factor. For example a workpiece having calibration marks seperated by 1 metre may during calibration result in a count of 1,001,000 data points thereby the workpiece has a calibration factor of 1,001,000 data points per 1,000,000 micrometers. Thus to move said worktool 10 millimetres, that is 10,000 micrometers, would require a data point count of 10,010. This refinement may not reduce cumulative errors introducedbythe incrementalmeasurement of distance from which position is determined. Cumulative errors may be reduced by selecting a particular point within the unit cell and calibrating the spatial coordinates of said point in each cell of said reference pattern in terms of the number of data points utilising said sensors or said calibration may be performed independentlyof saidequipment. Inthismannerabsoluteposition may be based on specific positions on said reference pattern ratherthan on incremental distancemeasurements thereby leading to accuracies of at least l micrometre.

The position of a worktool is determined as follows. Let P20 and P30 represent the data points at which the line centres or edges of the vertical 20 micrometer and horizontal 30 micrometer lines 17

of a reference pattern having such a design have been determined. For example with charge coupled devices having a pixel spacing of 13 micrometres and a lens magnification of 13 the coordinates are given as X = 500*C_X - P20 and Y = 500*Cy - P30 where C_x and Cy are the number of X and Y unit cells traversed along thepattern from the origin. A similar calculation may then be performedwith the data from the second charge coupled device and these two* sets of X and Y coordinates have an accurately known geometrical relationship with said worktool which allows the spatial cordinates of said worktool in relation to the workpiece to be derived. These coordinates are refined using the calibration values in the following manner. The measured spatial coordinate at each point on the reference pattern are corrected using calibration values of C_x & Cy+i for P20<P30 and C_x+ & Cy for P2OP30.

Thus a position detection system has been described whichhas the advantage of permitting the coordinate position of a worktool to be accurately determined to at least H—1 micrometer or H—5 micrometers.

Claims

CLAIMS :

1. A position determination apparatus comprising: mounting means holding a worktool in moveable relationship with a workpiece ^~ one or more reference patterns in a known relation to said workpiece; means for imaging at least a portion of said reference pattern with said means in a known relation to said worktool; means for processing said image to determine one or more points on said image whereby said points accurately determine the spatial relationof saidreferencepattern, the apparatus thereby accurately determining the position of said worktool relative to said workpiece; and drive means associated with said mounting means and/or said workpiece for positioning said worktool in relation to said workpiece.

2. An apparatus accordingto claim 1 wherein eachposition on said referencepattern relates to one seperate correspondingposition on said workpiece.

3. An apparatus according to any one of the preceding claims wherein said reference pattern is formed from a representative unit cell whereby eachdifferent imagedpositionwithin said cell produces a different image.

4. An apparatus according to any one of the preceding claims wherein said reference pattern is imaged through a workpiece.

5. An apparatus according to any one of the preceding claims wherein said reference pattern has associated markings specify¬ ingthe coordinates of points on said reference pattern including means for sensing and processing.

6. An apparatus according to any one of the preceding claims including associated calibration data representing differences between the actual coordinates of points on said reference pattern from the specified coordinates.

7. An apparatus according to any one of the preceding claims including means for calibrating and/or monitoring dimensional variations in at least said workpiece and/or said reference pattern and/or said sensors.

8. An apparatus substantially as hereinbefore described with reference to the drawings.

9. A position determination method which comprises the steps of: providing one or more reference patterns in a known relation to a workpiece; providing means for imaging at least a portion of said reference pattern with said means in a known relation to a worktool; and processing said images to determine one or more points on said images where said points accurately determine the spatial relation of said reference pattern where said known relations thereby accurately locates said worktool relative to said workpiece.

10. A method according to claim 9 wherein saiddata points of said image have a known spatial geometry and relate to individual coordinate points on said reference pattern.

11. Amethod according to any one of the preceding claims wherein processor means calculate sensor outputs corresponding to possible nextmovements of said sensor relative to said reference pattern prior to said movement then comparing said calculated outputs with the sensor output after said movement.

12. A method according to any one of the preceding claims wherein said image is analysed to calibrate and/or compensate for distortions of said image.

13. A method substantially as hereinbefore described with reference to the drawings.