WO2009024767A1

WO2009024767A1 - Multi-dimensional coordinate measuring apparatus

Info

Publication number: WO2009024767A1
Application number: PCT/GB2008/002787
Authority: WO
Inventors: John Kenneth Dearden; Hyungjoo Kang; Woosup Koh
Original assignee: International Metrology Systems Limited; Ims Company Limited
Priority date: 2007-08-18
Filing date: 2008-08-18
Publication date: 2009-02-26
Also published as: GB2452033A; GB0716185D0

Abstract

Measuring equipment in the form of a portable multi-dimensional coordinate measuring machine (CMM) (10) utilising a simplified construction. The CMM includes a handheld or motorised body (12) in which is arranged a number of XY optical sensors (28, 30) and on which is located a probe (14). The CMM is operated like a computer mouse moveable over a surface (20) and entirely around an object (22) while continuously tracking its position in space. In this way the probe is sequentially brought against at least two points (27, 29) on the object located on the surface to thereby determine the distance between the points. Various embodiments are described to enhance it's dimensional measurement capability including mounting the probe on a vertical measurement system, a horizontal measurement system and/or a rotational measurement system.

Description

Multi-Dimensional Coordinate Measuring Apparatus

The present invention relates to measuring equipment and more particularly, though not exclusively, to a portable multi-dimensional coordinate measuring machine (CMM) utilising a simplified construction.

Industrial needs require to accurately measure dimensions of objects together with their orientation. By defining an XYZ coordinate system, any point on an object can be determined in space relative to the origin of the coordinate system. By determining two points in space an accurate measurement of distance can be made between those two points regardless of the size and shape of the object. In this way the manufacturing industry can ensure that products are made within tolerance levels of the expected dimensions. Currently such tolerances are considered to an accuracy in microns and below.

As products are manufactured in a large variety of sizes e.g watch parts to motor vehicles, CCMs have been developed which are scalable to suit the required environment. The present Applicant, International Metrology Systems provides a CMM which is scalable and operates on accurately measuring points of an object using three or more linear scales and optional rotary transducers arranged on each of the XYZ axis. These structures require an overhead gantry and a sliding bed to access points across the object. They are limited by the requirements of space and the need for the object to locate under the gantry in an orientation where the probes on each linear scale can access. For smaller objects these CMM's include a measuring table upon which the gantry is mounted. The tables are typically constructed with granite to ensure that vibration is prevented from affecting the measurements. A major disadvantage of such instruments is their size, weight and cost. They are dedicated to the environment of use and are typically manufactured in small batches containing precision pieces making them expensive. A CMM is typically installed on a single location on site and all parts requiring to be measured must be transported to it. This creates a major disadvantage in that large and heavy parts are difficult to manoeuvre to the site. As parts must be brought to the machine, a further disadvantage is seen in that component parts are less likely to be measured regularly, with the manufacturer preferring to make the necessary tolerance measurements on the final piece. This wastes parts and time if the component parts are incorrect and the assembled article needs to be taken apart.

In order to overcome some of the above disadvantages a portable measuring system has been developed by Faro Technologies, Inc. The

'FaroArm' is described in at least US 5,402,582. The FaroArm is a portable coordinate measurement machine comprising a multi-jointed (preferably six and/or seven joints) positionable measuring arm for accurately and easily measuring a volume comprising a sphere ranging from six to eight feet in diameter and measuring an accuracy of 2 Sigma +/- 0.003inch.

The FaroArm has at a first end a mount which must be located on a stable surface. This is typically a tripod stand supplied for location in the work place. The FaroArm locates on the base either by a magnetic contact or a vacuum mount which is more suitable for mounting on granite surfaces. At the opposing end is located a probe for contact to a point on the object. The user holds the arm at the probe and manoeuvres it to the desired location on the object. Once touched, the position is stored relative to an origin or datum created by the fixed location of the base. The user can then manipulate the arm to a second desired position on the object. A series of shaft encoders located in each joint compute the position of the probe at any point in space. An adjoining computer operating system stores the information and computes the relative dimensions between the recorded points of measurement.

A major disadvantage of this system is that it cannot access the reverse side of many objects. The arm is fixed, much like a human standing in a single position. The arm can thus only access positions where an operator can reach and thus measurements around an object is difficult if not impossible to be made. Additionally the FaroArm has a complex construction making the equipment relatively expensive.

An object of the present invention is to provide a multi-dimensional coordinate measurement apparatus which can make measurements around an object.

A further object of the present invention is to provide a multi-dimensional coordinate measurement apparatus which can be portable. In this way it can be mounted on a surface such as a granite table.

It is a further object of the present invention to provide a multi-dimensional coordinate measurement apparatus which is of simplified construction for ease of use and cost.

According to a first aspect of the present invention there is provided a multi-dimensional coordinate measurement apparatus comprising a moveable member adapted for travel on a continuous working surface, the member including a plurality of XY tracking means for determining the position of the member across the surface and a measurement probe arranged on the member such that by movement of the member over the working surface the probe is sequentially brought against at least two points on an object located on the surface to thereby determine the distance between the points.

In this way the apparatus is similar to a computer mouse moving over a surface and tracking its position while allowing systematic measurement of distances across objects contacted by the probe.

The XY tracker may be mechanical, electrical or optical. Preferably each XY tracking means is an XY sensor. More preferably there are at least two XY sensors located in spaced relation in the moveable member. More preferably the sensors are twin eye laser sensors arranged to be directed at the working surface. The sensors may be Phillips PLN 2022 twin eye laser sensors. In an embodiment a linear array of XY sensors is located in the moveable member.

Preferably the probe is a force sensitive transducer probe. Alternatively the probe may be a sensing device which does not make physical contact with the object such as a laser interferometer, for example.

The probe may be a touch trigger probe as is known the art. Preferably the apparatus comprises a plurality of interchangeable touch trigger probes. In this way the probes can be switched depending on the object. Additionally the tip of each probe may be spherical, flat bottomed, a ball or a point depending on the application. Alternatively tip may comprise a plurality of spheres in spaced apart relation, detection occurring when any sphere contacts a surface. Optionally the tip may include a scribe or marker. In this way a measurement mark may be left on the surface of the object. Such markings allow later work to be carried out on the object.

Preferably the apparatus includes a datum bracket adapted to locate upon the surface and including means for engaging the member to define a position of the member against which other points can be referenced. This may be considered as the origin and advantageously can be checked at any time. In this way a means for calibrating the measurement apparatus is provided.

In an embodiment the probe is attached to a vertical measurement system. In this way the probe can be moved vertically in relation to the tracking means to provide a 3D coordinate measurement apparatus. The probe may be mounted horizontally or at an angle to the horizontal upon a support of the vertical measurement system. The probe position may be calibrated using the datum bracket and aside from the vertical movement, the probe position is fixed in this embodiment. Preferably the probe is adapted to adjust along a vertical support on the member. There may be means for adjusting the probe such as bearings for easy movement up and down the support. Preferably the apparatus includes data recording means to monitor the height of the probe from a predetermined datum point on the member. In this way the location of the probe can be determined at all times.

In a further embodiment the probe is attached to a horizontal measurement system. In this way the probe can be moved vertically and horizontally in relation to the tracking means to provide a 4D coordinate measurement apparatus. The probe may be mounted horizontally or at an angle to the horizontal initially upon a support of the horizontal measurement system. The probe position may be calibrated using the datum bracket and aside from the movement in an XY axis, the probe position is fixed in this embodiment. Preferably the probe is adapted to adjust along a horizontal support on the member which itself is located upon the vertical support. There may be means for adjusting the probe such as bearings for easy movement up and down, and left and right on the support. Preferably the apparatus includes data recording means to monitor the height and extension of the probe from a predetermined datum point on the member. In this way the location of the probe can be determined at all times.

In a still further embodiment the probe is attached to a rotational measurement system. In this way the probe can be moved rotationally with respect to the vertical and/or horizontal axis in relation to the member to provide enhanced measurement capability. The probe may be mounted on rotational measuring means upon the member, the vertical support or the horizontal support to provide a desired number of axis of measurement. The rotational measuring means may be a motorised rotational head or, alternatively be a probe holder having one or more recesses to carry the probe at a desired orientation. The probe position may be calibrated using the datum bracket. Preferably the apparatus includes data recording means to monitor the height and/or extension and/or rotation of the probe from a predetermined datum point on the member. In this way the location of the probe can be determined at all times.

Preferably the apparatus includes an operating system. More preferably the data is transmitted to the operating system. More preferably the operating system is a computer controller comprising a microprocessor or FPGA. The controller may comprise software loaded onto a standard computer. More preferably, the controller is located in the member and may advantageously provide a display on the member. The controller gathers data and calculates the dimensions required. The data may be transmitted to the controller by wireless means such as RF or infrared signalling.

In this way data can be collected at an origin point in the datum bracket or any other chosen free point in space. By guiding the probe to a first desired position on the object, data is collected at this position and its location in space relative to the origin determined. Moving the probe to a second desired position on the object allows data to be collected to provide a second location in space. Trigonometric analysis then derives the distance between the two positions. Any consecutive measurements made upon the object can then be compared to provide an accurate measurement of the dimension between any two points on the object.

The moveable member may simply be hand held for operation allowing a user to move the apparatus around the object. With wireless communication the member can be moved entirely around the object to access any points thereon. Alternatively the moveable member may be motorised so that its movement is remotely controlled around the object.

To aid movement, the member may include bearings on a lower surface thereof. The bearings may be a system known in the art such as air bearings or roller bearings. It will be apparent that the bearing type will be selected by the weight of the apparatus required and the quality of the working surface. For example, if the working surface has any irregularities, an air bearing would be appropriate as these will be smoothed out. Alternatively the member may include pads such as nylon feet which the member may glide upon over the working surface. The apparatus may optionally include a support including the working surface. Preferably the support is a granite table. More preferably the table is of a weight which may be moved around an environment or located upon a table or other work surface.

In an embodiment of the present invention the working surface is a portion of the object being measured. In this way the apparatus is located directly on the object. The working surface may thus be orientated at any angle and, accordingly, the references to vertical and horizontal become relative.

According to a second aspect of the present invention there is provided a method for determining the distance between two points on an object comprising: (a) locating a multi-dimensional coordinate measurement apparatus according to the first aspect upon a continuous working surface;

(b) moving the member over the working surface to sequentially bring the probe against two points on the object; and

(c) determining the distance between the points.

Embodiments of the present invention will now be described, by way of example only, in which:

Figure 1 is a schematic illustration of a two dimensional coordinate measuring apparatus according to a first embodiment of the present invention;

Figure 2 is an illustration of the measuring apparatus of Figure 1 in use on a first object; Figure 3 is an illustration of the measuring apparatus of Figure 1 in use on a second object;

Figure 4 is a schematic illustration of a three dimensional coordinate measuring apparatus according to a second embodiment of the present invention;

Figure 5 is a schematic illustration of a four dimensional coordinate measuring apparatus according to a third embodiment of the present invention;

Figure 6 is schematic illustration of a five dimensional coordinate measuring apparatus according to a fourth embodiment of the present invention;

Figure 7 is a schematic illustration of a six dimensional coordinate measuring apparatus according to a fifth embodiment of the present invention.

Reference is initially made to Figure 1 of the drawings which illustrates a two dimensional coordinate measurement apparatus, generally indicated by reference numeral 10, according to a first embodiment of the present invention. Apparatus 10 comprises a hand-held unit 12 including a probe 14. The unit 12 is connected wirelessly to a computer 16 upon which suitable software 18 is incorporated to record measurements and determine relative dimensions.

The hand-held unit 12 is shaped for a user to grip the upper surface 24 and move the unit 12 over a surface upon which it is located. The unit 12 has a smooth under surface 26 which includes pads 25 which act as bearings so that the unit 12 can glide over a surface without any interruptions in it's path. Located in the unit 12 and directed towards the under surface 26 are two laser sensors 28,30. Apertures 32,34 in the under surface allow the lasers to be directed against the surface upon which the unit 12 is located. Each laser sensor is arranged at an end

36,38 of the unit to provide the greatest distance possible between the two sensors 28,30.

Each sensor 28,30 is a Philips PLN2020 twin-eye laser sensor. Such a sensor is a high precision, ultra-fast, low-power consuming , small-sized, single-component, laser based tracking device. The sensor is typically used in computer mice, identification devices, printers and mobile phones. The PLN2020 is a fully integrated single-component, robust , self-aligning and small sized (<180 mm³) laser based tracking device. It is an electrical component which includes an 11 pin 3.85 mm high laser taking up a mere 6.8mm x 6.8mm and incorporates lenses for its two lasers in the package assembly. The sensor also has a dedicated laser power control scheme, energy usage is limited to a minimum while power management is maximised, making the device particularly useful for a hand-held cordless unit 12.

The PLN20202 is based on laser tracking technology. It measures changes in position by sensing the scattered laser light that is reflected by the surface, and mathematically by on chip logic and software, determining the direction and magnitude of the movement. It is capable of measuring extremely accurately at a wide range of speeds.

The apparatus 10 further includes a probe 14 mounted upon the upper surface 24 of the unit 12. The probe 14 is an extension arm 15, upon which is located a probe tip 17. Those skilled in the art will appreciate that the probe tip 17 may be of any design such as those provided by Renishaw. Such tips 17 are force sensitive to record a measurement when the tip contacts a surface. Alternatively the tip 17 could be replaced with a non-contact sensor such as an optical system as are known in the art. Yet further the tip 17 may include a scriber or marker to leave an indication on an object when the tip 17 has come into contact with it.

Within the unit 12 is a microprocessor which stores information determined from the sensors 28,30 when a user activates one of the control buttons 33,35. The unit has a wireless transmitter so that the data collected can be relayed to a PC 16 located away from the unit 12. Software 18, loaded on the computer 16 can provide a suitable user interface to offer a user guided instructions for use.

In use, an operator simply places the unit on a suitable work surface 20. Objects for measurement can be brought to the table and located thereon. This is as illustrated in Figure 2 where the object is a machined component of an engine. Like parts to those in Figure 1 have been given the same reference numeral to aid clarity.

The unit 12 is located upon a continuous surface 20. The continuous surface 20 is a working surface upon which the unit can move without losing contact upon. In the embodiment shown the surface 20 is part of a granite table 21 as is known in the art. An object 22 is arranged on the surface 20 in order that measurements can be made thereupon.

The operator then locates the unit 12 upon the surface 20. The unit 12 is wirelessly connected to a PC viewable from the surface 20 so that the operator can view the control commands on the user interface. The unit 12 is initially located in a datum indicator 23 located on a corner of the table 21. The datum indicator 23 provides a single unique position for the apparatus 10 to be located in. This fixed orientation and position of the apparatus 10 is taken as a calibrated origin and the settings on the probe 14 and sensors 28,30 are referenced to it. Once located in the datum, the operator signals to the computer, by pressing a key 33,35 that the unit 12 is in place and the calibration data can be stored. In this way, if the unit 12 is ever lifted off the surface 20, the unit can be placed back in the datum indicator 23 to be recalibrated so that measurements before and after calibration can be used together.

The operator then moves the unit 12, rather like moving a computer mouse across a mouse mat, over the surface to bring the probe 14 to a first desired measurement position 27 on the object 22. It will be appreciated that some adjustment of the probe 14 may be possible, such as by mounting on a flexible arm 15, so that the initial position of the probe is suitable for the object being measured. The datum indicator will, of course, calibrate on a single position of the probe 14 that cannot be adjusted after calibration. However, as the datum indicator 23 is on the table 21 , the probe 14 and datum 23 can be readily adjusted to suit the desired measurement points of the work piece or object 22.

Once the data is recorded for the tip 17 at the first desired measurement point 27, the unit is simply moved around the object 22 and any number of further points, 29 at the same relative height can be measured. On contacting a second point 29, for example, the software 18 will automatically calculate and display the distance between the points 27,29. These calculations are performed by standard trigonometric analysis for calculating the distance between points in space. The sensors 28,30 allow compensation to be made for the rotation of the unit 12 relative to it's original position in the datum indicator 23. Such a measurement is accurate to the sub-micron range and any number of sequential points can be recorded. The software 18 can further provide distance measurements between any two points selected by a user.

It must be remembered that the unit 12 can be slid freely around the object 22 and indeed the operator can walk around the table 21 if this is accessible. In this way no part of the object 22 is inaccessible to the measurement apparatus 10 and the operator.

Reference is now made to Figure 3 of the drawings which illustrates the apparatus 10 used directly upon an object 30 Again like parts are numbered identically to the earlier Figures. In this embodiment, the apparatus is located upon a working surface 32 which is part of the object 22. The datum indicator 23 is now attached to the object 22 at a suitable position on the surface 32.

The unit 12 is first located at the datum 23 to determine a first point or origin for calibration purposes. In this arrangement it is seen that the datum has a straight edged XY piece for the unit 12 to locate against. In order to improve the accuracy of measurements in this arrangement, a linear array of sensors 28,30 say, 4 or 5 are located on the unit 12. In this way, when the unit 12 enters the datum 23 a least squares fit can be made across the sensors in relation to the X and y axis to provide an indication of the 'squareness' between the axis and be used to calibrate the measured data points against. This datum indicator and calculation may be made in any arrangement of the apparatus 10.

Once calibrated at the datum 23, the unit 12 is then moved over a surface of the object 30 from which relative measurements are made at desired location 27,29. As for Figure 2, the distances between successive points can be determined and the curvature and diameter of circular sections 36 can be determined numerically.

At any desired point 27,29 the operator moves the probe against the surface 38 of the object 30. The pressure of the object against the probe tip 17 registers a signal at the microprocessor/controller, consequently the computer records all the measurement data from each of the sensors for every contact point.

The software can display the news that a point has been recorded. The operator can then move the unit to a second desired position. Once there the probe is again contacted to the object and the data recorded. The software than provides a dimensional calculation to determine the distance between any two points in space.

Further measurements can be made so that multiple dimensions can be measured across the object. At any time the user can place the unit back into the origin if they consider that the unit has been moved off the surface.

Reference is now made to Figure 4 of the drawings which illustrates a second embodiment of the present invention. Like parts to those of Figure 1 are given the same reference numeral to aid clarity. As can be seen in the Figure, the probe 14 is now located about a vertical moving column 40. The probe 14 can mechanically or electrically be driven to move up and down the column 40. Such vertical displacement is measured on the microprocessor and provides a third dimension of measurement to the apparatus 100. In this way the apparatus 100 is calibrated at a fixed X,Y,Z position and orientation in the datum indicator 23 (not shown). Once moved over the surface 20 the vertical displacement of the probe tip 17 can be adjusted to provide the additional degree of freedom to measure points on an object at varying heights and depths accordingly.

Reference is now made to Figure 5 of the drawings which illustrates a third embodiment of the present invention. Like parts to those of Figures 1 and 4 are given the same reference numeral for clarity. In this embodiment an additional degree of freedom is provided in the ability for the probe 14 to move horizontally with respect to the unit 12. As can be seen an arm 42 arranged perpendicularly to the column 40 is slidably connected thereto. Movement of the arm 42 on the column is recorded as a vertical displacement, while additionally, the probe 14 is adjustable along the arm 42. The horizontal adjustment is also recorded so that the apparatus 120, can always determine the position of the probe tip 17 in space relative to the location taken in the datum indicator 23 prior to measurements being made. It will be appreciated that this apparatus 120 allows for measurements over the top of objects and into them. This removes the requirement for the object to be located on a rotary table, as the unit 12 can be slid all around the object being measured. It will further be appreciated that counterbalances may be located as required in the unit to stabilise the apparatus 120. If the unit 12 or apparatus 120 becomes heavy thus limiting it's ability to glide over the working surface, an alternative bearing system can be incorporated such a roller or even air bearings, as are known in the art.

Reference is now made to Figure 6 of the drawings which illustrates a fourth embodiment of the present invention. Like parts to those of Figures 1 and 5 are given the same reference numeral for clarity. In this embodiment, the probe 14 is located on a multi-positioner 44. Such multi- positioners 44 are known and consist of a solid block or member in which apertures 46 are precision drilled in relative locations around the block 44. Each location is known and thus the probe can be adjusted, before use, to suit the position of the point being measured. As with the previous embodiment, the probe 14, and now the block 44, is moveable along the horizontally arranged arm 42. This embodiment provides a further degree of freedom as the probe 14 can be positioned in a rotational location upon the arm 42, albeit in only a number of selected positions.

A further feature of the apparatus 130 is illustrated in Figure 6. In this embodiment, the computer has been removed and replaced with a dedicated microprocessor located on the unit 12. The microprocessor takes the form of a field programmable gate array (FPGA) into which the desired algorithms and controls are stored for operating the apparatus 130 and making the desired measurement calculations. The microprocessor has a small LCD display 48 which provides readings of the measurements made and control functions. Such readings may be stored in the unit 12 dependent on the memory space made available. This arrangement has the advantage that a user does not require to keep a pc in view during the measurements. Additionally the apparatus is more transportable as it does not require a PC with dedicated software. This combination of FPGA and display can be incorporated into the apparatus 10, 120, 130, 140 of any embodiment as appropriate.

Reference is now made to Figure 7 of the drawings which illustrates a fifth embodiment of the present invention. Like those previously, like parts have been given the same reference numeral to aid clarity. The apparatus 140 in this embodiment provides six degrees of freedom by operation of the XY sensors 28,30 as the unit 12 moves, vertical adjustment via the column 40, horizontal adjustment via the arm 42 and full 360 degree rotation at the probe 14 by a yaw and pitch positioner 50 located upon the arm 42. In this way the probe can be brought to any desired point on an object for measurement and no location should be inaccessible.

The principal advantage of the present invention is that it provides a multi- dimensional coordinate measurement apparatus which can make measurements between points around an object.

A further advantage of the present invention is that it provides a multidimensional coordinate measurement apparatus which can be portable.

A yet further advantage of the present invention is that it provides a multidimensional coordinate measurement apparatus which is of simplified construction for ease of use and cost.

Various modifications may be made to the invention herein described without departing from the scope thereof. For example while the embodiments described have used a datum indicator to calibrate the apparatus, a location on the object may be selected instead. Additionally, the object is not limited to a machined component but may be any structure locatable in relation to the measuring apparatus.

Claims

1. A multi-dimensional coordinate measurement apparatus comprising a moveable member adapted for travel on a continuous working surface, the member including a plurality of XY tracking means for determining the position of the member across the surface and a measurement probe arranged on the member such that by movement of the member over the working surface the probe is sequentially brought against at least two points on an object located on the surface to thereby determine the distance between the points.

2. Apparatus as claimed in claim 1 wherein the XY tracking means are an XY optical sensor.

3. Apparatus as claimed in claim 2 wherein there are at least two XY sensors located in spaced relation in the moveable member.

4. Apparatus as claimed in claim 2 or claim 3 wherein the sensors are twin eye laser sensors arranged to be directed at the working surface.

5. Apparatus as claimed in any preceding claim wherein the probe is a force sensitive transducer probe.

6. Apparatus as claimed in any one of claims 1 to 4 wherein the probe is a non-contact sensing device.

7. Apparatus as claimed in any one of claims 1 to 5 wherein the probe is a touch trigger probe having a spherical tip.

8. Apparatus as claimed in any one of claims 1 to 4 wherein a tip of the probe includes a scriber to mark the object.

9. Apparatus as claimed in any preceding claim wherein the apparatus includes a datum bracket adapted to locate upon the surface and including means for engaging the member to define a position of the member against which other points can be referenced.

10. Apparatus as claimed in any preceding claim wherein the probe is attached to a vertical measurement system so that the probe can be moved vertically in relation to the tracking means.

11. Apparatus as claimed in claim 10 wherein the apparatus includes data recording means to monitor the height of the probe from a predetermined datum point on the member.

12. Apparatus as claimed in claims 10 or claim 11 wherein the probe is attached to a horizontal measurement system so that the probe can be moved vertically and horizontally in relation to the tracking means.

13. Apparatus as claimed in claim 12 wherein the apparatus includes data recording means to monitor the height and extension of the probe from a predetermined datum point on the member.

14. Apparatus as claimed in any one of claims 10 to 13 wherein the probe is attached to a rotational measurement system so that the probe can be moved rotationally with respect to the vertical and/or horizontal axis in relation to the member.

15. Apparatus as claimed in claim 14 wherein the apparatus includes data recording means to monitor the height and/or extension and/or rotation of the probe from a predetermined datum point on the member.

16. Apparatus as claimed in any preceding claim wherein the apparatus includes an operating system located in the member to transmit data to a computer and/or display it upon the member.

17. Apparatus as claimed in any preceding claim wherein the moveable member may be motorised so that its movement is remotely controlled around the object.

18. Apparatus as claimed in any preceding claim wherein the member includes bearings on a lower surface thereof.

19. Apparatus as claimed in any preceding claim wherein the apparatus includes a support including the working surface.

20. Apparatus as claimed in any one of claims 1 to 18 wherein the working surface is a portion of the object being measured.

21. A method for determining the distance between two points on an object comprising: (a) locating a multi-dimensional coordinate measurement apparatus as claimed in claim 1 upon a continuous working surface;

(c) determining the distance between the points.