WO1993023764A1

WO1993023764A1 - Gauging apparatus

Info

Publication number: WO1993023764A1
Application number: PCT/GB1993/001055
Authority: WO
Inventors: David William Lloyd
Original assignee: Vernon Gauging Systems Limited
Priority date: 1992-05-21
Filing date: 1993-05-21
Publication date: 1993-11-25
Also published as: GB9210862D0

Abstract

Gauging apparatus is described in which it is desired to detect the position of a gauging member as it travels across a work-piece. The invention provides optical means for detecting the position, in particular a rotating light beam on the gauging member which sweeps past a number of detectors. The time intervals taken for the light beam to pass the detectors are utilized to calculate the position of the gauging member, in particular by calculating the angles subtended between lines extending from the gauging member to the detectors.

Description

Gauging apparatus

The present invention relates to gauging apparatus and more particularly to an arrangement for accurately identifying the position of a measuring head forming part of the apparatus.

Gauging apparatus is well known and in one type of apparatus comprises a test-piece engaging measuring head which is arranged to follow the contour of the workpiece. The instantaneous position of the measuring head is monitored whereby the contours across the test-piece can be mapped.

It is known to perform such monitoring with reference to one or more linear scales using an X-Y grating (Moire Fringe) system. However, although such an apparatus is capable of producing very good results it is difficult to set up and to adjust due to the need for the X-Y grating.

The present invention provides a gauging apparatus comprising a base member, a gauging member, means for causing relative movement between the gauging member and the base member whereby to provide an indication of a characteristic of a test-piece, first optical means mounted on the gauging member, second optical means mounted on the base member, and means for causing a succession of light path communications between the first and second optical means whereby to monitor the position of the gauging member.

The apparatus of the present invention provides a simpler and more effective way to monitor the position of the gauging member with respect to the test-piece.

Preferably, the first optical means comprises a rotatable member and the second optical means comprises a plurality of fixed members disposed over at least a portion of the base member. The second optical means preferably comprises optical sensors but may alternatively comprise reflective members in which case the gauging member.may be provided with optical detection means.

The rotatable member may be a prism or mirror. Alternatively, a plurality of fixed light emitting or receiving devices each pointing in a different direction may be sequentially activated whereby to simulate the effect of a rotating beam. Other arrangements are possible.

In another aspect the present invention provides a method for the determination of the position of a gauging member relative to first, second and third locations, said locations being fixed with respect to one another, the method comprising detecting a first angle subtended between first and second straight lines extending between said gauging member and said first and second locations respectively and a second angle subtended between said first straight line and a third straight line extending between said gauging member and said third location, and determining, using said first and second angles, the positions of the gauging member.

Preferably the method is implemented in a gauging apparatus as described above, the three fixed locations being positions around the base member.

In a particularly preferred embodiment the first and second angles are detected by a rotating light beam as described above, the angles between the lines being determined according to the times taken for the light beam to sweep past detectors positioned at the three locations.

The three locations may be in any desired relationship to each other, but preferably they are positioned on three corners of a square or equi-spaced along a straight line.

In order that the present invention be more readily understood, embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which;

Fig. 1 shows a perspective diagrammatic view of gauging apparatus according to the present invention

Fig. 2 is a diagram useful for understanding this invention;

Figs. 3A and 3B show plan views of alternative arrangements suitable for use with the apparatus of Fig. l ;

Fig. 4 shows a timing diagram useful for understanding the operation of the apparatus shown in Fig. 1; and

Fig. 5 shows a further timing diagram.

As shown in Fig. 1, gauging apparatus according to the preferred embodiment comprises a fixed base member 1 on which is mounted a moveable measuring head assembly 2. The measuring head assembly is provided with a workpiece-engaging member 3 and the head assembly is arranged to move in unison with the member 3.

In order to monitor the position of the measuring head assembly 2, a plurality of optical detectors 4 are located at predetermined positions around the perimeter of the base member 1. The angles between imaginary straight lines extending between the gauging member and the respective detectors 4 are measured and used to determine the position of the gauging member 3.

A method of calculating the position of the gauging member from the detected angles will be outlined with reference to Fig. 2. Such calculations can be implemented by computer equipment connected to the detectors 4.

Fig. 2 shows a general case with three detector locations LI, L2, L3. The distance between LI and L2 is A, and the distance between LI and L3 is B. The fixed angle between the two lines between the locations is §. The position of the gauging member is shown as G, and the angles which are measured by the apparatus are α and β. From these it is possible, as explained- in the following, to calculate angle θ and distance r, and hence to determine the position of the gauging member.

As is apparent from Fig. 2 :-

Θ + φ = § eq. 1.

Simple trigonometry with regard to the two triangles in figure 5 gives:-

eq. 2,

eq. 3.

Combining equations 2 and 3 gives:-

This is then a general expression by way of which Θ can be calculated from the constants A, B and $ and the measured angles α and β. Once Θ is known equation 2 may be used to calculate the distance r and thereby to establish the position of the gauging member. In the particular case where the three locations are on three corners of a rectangle, ie Φ = 90° tan e - ^{B cot} β ~ ^A ^{tan Θ ~} A cot α - B

and if the rectangle is a square ^{ie A} = ^{B tan Θ} = gg α - i ^{' ec}- ⁵-

In the particular case where the three locations are along a straight line, ie Φ = 180°

^{tan Θ =} B cot β- A cot α

and if the locations are equi-spaced: ie A = B tan Θ = _{cQt β} I _{∞t g} eq. 6.

The position of the gauging member can therefore be established by the measurement of the two angles α and β shown in Fig. 2.

Figs. 3A and 3B illustrate two preferred arrangements for the detectors in Fig. 1. In Fig. 3A the detectors 4 are positioned at the four corners of a square. Equation 5 above can then be used to calculate the position of the gauging member 3. As is clear from the above discussion of Fig. 2, only three detector positions are necessary to establish the location of the gauging member. However the use of four detectors as shown in Fig. 3A, when used with the rotating beam apparatus to be described below, allows a more continuous up-dating of the gauging iresiber position.

In Fig. 3B the there are three detectors 4 positioned equi-spaced along a straight line. Equation 6 above can then be used to calculate the position of the gauging member 3. In the preferred embodiment illustrated in Fig. 1 the movable measuring head assembly is provided with a rotatable member 6 which is arranged to emit a beam of light 7 which sweeps around at a constant rotational speed. In this case, this is achieved by means of a fixed light source 5 within the measuring head assembly which is an optical communication with a rotating prism or other suitable reflector 6. This is rotated by driving means indicated at 8 to cause a rotating beam of light 7 to strike the detectors 4 around the perimeter of the base member 1 sequentially. The prism 6 is driven by driver 8 at a constant rotational speed which is very accurately maintained by conventional means. As the rotational speed of the prism is constant, the times between the triggering of the photodetectors are proportional to the angles between straight lines extending between the gauging member and the respective detectors.

Any convenient means of indicating the time interval between successive detections can be utilized including a conventional time base or either a micromental encoder or absolute encoder.

In order to understand the operation of the apparatus more clearly, reference is made to Fig. 4 which represents a timing diagram for the arrangement where the detectors are as shown in Fig. 3A with a detector at each corner a square base member. As shown in Fig. 4, with the measuring head assembly 2 at the centre of the base member 1, the time intervals between successive detections of the light beam by the detectors 4 will be equal. However, as the head assembly 2 moves over the base member 1, the intervals between successive detections will vary and these differences can be directly related to the head position.

If the time interval between detections is denoted as ^rt' seconds and the rotational speed of the prism is 'w' revs/sec then the time interval is related to the angle subtended at the gauging member between the detectors can be calculated as follows.

The rotational speed of the light beam is 360.W degrees/sec. Therefore the angle α subtended between the two detectors is ct = 360.w.t degrees. This angle is then used as described above with'respect to Fig.2 to help calculate the position. As is apparent from the discussion of Fig. 2, two sequential angles are required to calculate the current position. Thus each angle calculated as described above can be used in two successive position calculations.

The clock rate will be high and typically 10MHz (or higher) and will be used with a prism rotation of 0.01 seconds, making 'w' above equal to 100 revs/sec. Resolution can be improved by increasing the clock rate or decreasing the prism rotational speed, although this will also result in a decrease in the frequency with which the position of the gauging member can be calculated. A balance between these two factors will be established in different applications of the invention depending on the particular requirements.

It will be understood that the number and location of detectors in the above arrangement can be varied depending on the accuracy and the number of dimensions which it is desired to monitor. A preferred arrangement of detectors for improving the accuracy of the detected position is to have four detectors positioned at the four corners of a square, and four further detectors positioned at the mid-points of the sides of the square. Successive pairs of angles used in the calculation of the position of the gauging member will therefore be alternately from three detectors in a straight line and from three detectors at corners of a square, thus permitting the use of the simplified equations 5 and 6 above.

It will be understood that there are many variations which could be adopted in order to achieve the same desired result and it is not necessary to use a continuous beam of light in all circumstances, pulsing the light may have some advantage in certain circumstances in that the time interval between the emission of the light and reception at the detector could also provide an indication of position.

Fig. 5 shows a timing diagram which indicates that different detector switches could be utilized at each detector location so that the pulse width from the detector could be varied in order to identify unambiguously which detector had been triggered. In this case, one would arbitrarily choose either the leading edge or some other position in the pulse from which to measure the intervals between pulse signals. As shown in Fig. 5, it is preferably decided to utilize the centre of the pulses from the detectors as the datum from which interval measurements are taken.

The advantages of the above described arrangements are that the apparatus is very easy to adjust and is not restricted by size since as the size of the surface area of the measuring head traverse increases, the rotating prism still functions in the same manner. Accuracy may reduce as size increases for a given rotational speed of the prism but to compensate for this, the speed of rotation can by reduced. It follows, therefore, that by varying the speed of rotation of the prism, accuracy can be altered. Preferably, increasing the accuracy is best achieved by reducing the speed of rotation of the prism eg. if the prism rotates at one resolution per second with 100 clock counts, by reducing the speed by a factor of 10, the resolution and accuracy is improved by a factor 10, that is 10000 per resolution. It is thus possible to achieve an infinitely variable resolution from fast prism rotation for coarse movements to slow prism rotations for accurate measurements. In the above description, it is assumed that the head assembly 2 is constrained to move linearly in only X and Y directions. Any rotational movement would complicate the computation of position but would not necessarily invalidate such computation as long as it was anticipated.

Further, although the above embodiments disclose the light paths emanating from the head assembly 2 it would be possible to reverse the arrangement with the rotating beam or beams emanating from one or more sources on the base member and being received by one or more detectors on the head assembly.

Claims

CLAIMS :

1. Gauging apparatus comprising a base member, a gauging member, means for causing relative movement between the gauging member and the base meπlber whereby to provide an indication of a characteristic of a test-piece, first optical means mounted on the gauging member, second optical means mounted on the base member, and means for causing a succession of light path communications between the first and second optical means whereby to monitor the position of the gauging member.

2. Gauging apparatus according to claim 1 in which said second optical means comprises a plurality of fixed members disposed over at least a portion of the base member and wherein means are provided to generate successive respective light path communications between said fixed members and the first optical means.

3. Gauging means according to claim 2 in which said first optical means comprises a rotatable member arranged to generate a rotating light beam which sequentially sweeps past said plurality of fixed members.

4. Gauging apparatus according to claim 3 in which said plurality of fixed members are optical sensing means responsive to said light beam.

5. Gauging apparatus according to claim 3 in which said plurality of fixed members are reflective means and said first optical means comprises optical sensing means.

6. Gauging apparatus according to claim 3, 4 or 5 in which said second optical means comprises three fixed members equi-spaced in a straight line.

7. Gauging apparatus according to claim 3, 4, 5 or 6 in which said second optical means comprises three fixed members positioned at three corners of a square.

8. Gauging apparatus according to claim 7 in which said second optical means further a comprises a further fixed member positioned at the fourth corner of said square.

9. Gauging apparatus according to any of claims 3 to 8 in which said first optical means comprises a continuous light source, a prism in optical communication with said light source and driving means arranged to rotate said prism.

10. Gauging apparatus according to any of claims 3 to 8 in which said first optical means comprises a plurality of fixed light emitting devices, each directed in a different direction, and means arranged to activate said light emitting devices sequentially whereby to generate said rotating light beam.

11. Gauging apparatus according to any of claims 3 to 10 further comprising means arranged to detect the time intervals between said rotating light beam passing respective pairs of said plurality of fixed positions, and means arranged to calculate, from said detected time intervals, the position of the gauging member with respect to the base member.

12. A method for the determination of the position of a gauging member relative to first, second and third locations, said locations being fixed with respect to one another, the method comprising detecting a first angle subtended between first and second straight lines extending between said gauging member and said first and second locations respectively and a second angle subtended between said first straight line and a third straight line extending between said gauging member and said third location, and determining, using said first and second angles, the positions of the gauging member.

13. A method according to claim 12 further comprising detecting said first and second angles by optical means.