GB2159941A

GB2159941A - Coordinate measuring machine

Info

Publication number: GB2159941A
Application number: GB08414716A
Authority: GB
Inventors: David Allan Wright
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1984-06-08
Filing date: 1984-06-08
Publication date: 1985-12-11
Also published as: GB8414716D0; GB2159941B

Abstract

The machine has a measuring table (10) over which is movable a supporting member (13) supported at each side of the measuring table. Movement of the supporting member (13) relative to the measuring table is measured by a scale member (18) attached to the measuring table and a reading head (22) carried by the supporting member. A carriage (17) with a similar movement measuring arrangement is carried by the supporting member (13) for movement relative thereto. Datum means (23, 24) are carried by each side of the supporting member and are operable to locate a fixed datum position on each side of the measuring table 10. Circuitry is provided which uses the outputs of the datum means to correct measurements made over the measuring table by the scale (18) and reading head (22). <IMAGE>

Description

SPECIFICATION Coordinate measuring machine This invention relates to coordinate measuring machines of the type having a measuring probe supported on an arm for movement across a measuring table.

Measuring machines are well-known, and in the smaller machines it is usually adequate to measure the movement of the probe across the measuring table at one side of the machine only. However, on larger machines errors may be introduced due to misalignment of the supporting arm itself, which will affect the accuracy of the measurement of the position of a measuring probe. This applies even if the probe is supported on a gantry extending across the machine. Our British patent No. 2,042,719 describes a measuring machine in which these errors may be avoided by using two or more scales on each axis of the machine, for example by taking measurements from each side of a gantry in order to determine the extent of any twisting of the gantry in use.

Many measuring machines use Moire fringe techniques to measure movements of the probe about various axes, and such techniques require the use of a long measuring scale extending over the full range of movement about an axis. It is not usual to provide absolute scales which will indicate position relative to a fixed datum, and indeed this would introduce its own problems since an object to be measured may be placed anywhere on the table and all measurements would have to be made relative to the machine datum rather than to a datum on the object.

It is therefore simpler to use incremental scales which indicate only successive increments of movement. It is usual therefore to fit a long incremental scale to one part of the machine and to carry a reading head for reading the scale on the relativelymovable part of the machine. The output of the reading head is applied to a counter, and a datum may be established simply by zeroing the contents of the counter at any particular point. This does not, however, make it possible to determine any initial misalignment present in the machine, since the counters associated with the or each scale on an axis would be reset simultaneously, regardless of the presence of any such twist.

It is an object of the invention to provide a coordinate measuring machine having means for measuring and correcting for any permanent or semi-permanent twist in the member supporting the measuring probe.

According to the present invention there is provided a coordinate measuring machine which includes a measuring table, a supporting member extending over the measuring table for movement over the surface thereof, first measuring means operable to measure the movement of the supporting member relative to the measuring table and comprising a first scale member fixed relative to the measuring table and extending over the range of movement of the supporting member and a first reading head carried by the supporting memberfor cooperation with said first measuring scale, a car riage carried by the supporting member for move ment relative thereto in a direction perpendicularto the direction of movement of the supporting member, second measuring means operable to measure the movement of the carriage relative to the supporting member and comprising a second scale member fixed relative to the supporting member and extending over the range of movement of the carriage and a second reading head carried by the carriage for cooperation with the said second measuring scale, datum means movable with the supporting member at each side of the measuring table and operable to locate a fixed datum position on each side of the measuring table, and circuit means responsive to the output of the datum means for calculating and applying corrections to the outputs of the said first reading head.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of part of a coordinate measuring machine incorporating the invention; Figures2 and 3 show similar views of different datum means; Figure 4 illustrates the principle of operation; and Figure 5 is a schematic block diagram of suitable circuit means.

Referring now to Figure 1, this shows the essential features of a gantry-type coordinate measuring machine. A measuring table 10 has a beam 11 secured to one side to form the support for one end member 12 of a supporting member or gantry having a horizontal member 13. The end member 12 is guided on the beam 11 by means of two or more bearing means 14, only one of which is visible. The other end member 15 of the gantry may conveniently run on the surface of the measuring table 10, at one edge thereof, and is similarly supported by suitable bearing means. The beam 11 projects beyond one end of the measuring table 10 in order to accommodate the extra length of the end member 12. The horizontal member 13 of the gantry carries a carriage 16 movable across the gantry in a direction perpendicular to the movement of the gantry over the measuring table 11.The carriage 16 carries a probe support member 17 movable vertically with respect to the carriage. The probe support member is arranged to carry a probe member (not shown) which, by appropriate movements of the gantry, the carriage 16 and the probe support 17, enables the probe to be moved to any point in a volume based on the measuring table 11.

The position of the gantry along the measuring table is measured at at least one side of the gantry. It is convenient to use More' fringe techniques which require the use of a long scale grating extending along one side of the table 11. Such a grating 18 is shown as being carried on a rigid support 19 secured to the side of the table 11. A reading head 22 is carried by end member 15 so as to cooperate with scale grating 18 in known manner. The reading heads will comprise a short scale grating, a light source and a number of light-sensitive detectors, and operate is a well-known manner.

A scale grating will be carried on the horizontal member 13 of the gantry together with a reading head mounted on the carriage 16 for measuring movements of the carriage. Similarly a scale grating will be carried on the probe support 17 together with a reading head mounted on the carriage 16 for measuring movements of the probe supported member.

The measuring machine so far described is simply one example of a gantry-type coordinate measuring machine, and many variations on the arrangement are well-known.

As already stated, it is possible for the gantry assembly to become misaligned, perhaps due to misuse of the machine. Such misalignment will cause errors in the position of the probe along the axis of movement of the gantry, depending upon the position of the carriage 16 on the gantry. Clearly these errors must be measured and corrected, and it is the purpose of the invention to achieve this.

Each end member of the gantry therefore carries a part of a datum position detector, the other part being fixed relative to the table 11. In Figure 1 one datum position detector 23 is visible, whilst the other, 24 is concealed behind scale support 19.

Referring now to Figure 2, this shows one form of datum position detector 23. The stationary part of this is a block 25 secured to the beam 11, which is omitted for clarity. The block 25 supports three photo-sensitive devices 26 and three light sources 27 arranged in three pairs. The outputs of the detectors 26 are connected to circuitry contained on a circuit board 28. The movable part of the datum position detector is an opaque shutter 29 carried for example, on the guide 14 of the gantry. The spacing of the pairs of light sources 27 and photo-sensitive devices 26 and the width of the shutter 29 are such that only when the shutter is in the datum position do equal amounts of light fall on the two outer photosensitive devices and not on the middle one. When the shutter is in any other position light is received by different combinations of photo-sensitive devices.

A similar arrangement is used at the other side of the gantry. Figure 3 illustrates another form of datum position detector in which the reading head 22 carries a shutter 30 movable between three pairs of light sources 31 and photo-sensitive devices.

Other types of datum position detector may be used, such as precision microswitches, magnetic or other pickoffs. Such devices are well known.

The purpose of the datum position detectors isto detect any change in the relative positions of each side of the gantry. The detectors are used in conjunction with the measuring scale, so that, as the gantry is moved, it is possible to detect any travel of the reading head associated with that scale at one side of the gantry passing its datum position and the other side of the gantry doing likewise. Since the effective output of the reading head will normally be in the form of one or more pulse for each increment of movement, a simple circuit will be required to count the pulses which occur between the two datum indications. Any error may then be applied as a correction as will be explained below.It is possible, and may indeed by more convenient, to buiid-in a deliberate known offset between the two datum positions to ensure that a finite time elapses between the two datum indications. It is then necessary only to determine whether the number of pulses counted represents the offset, or differs from it and by how much.

Figure 4 illustrates the manner in which the invention operates. The drawing shows a schematic plan view of part of the machine, with the measuring table 10 and the measuring scale 18 shown. The position of the datum position D1 along the scale 18 is shown, as is the position of the other datum position D2 on the other side of the gantry. The line 40 represents the gantry, with a greatly exaggerated misalignment shown, so that one end of the gantry passes datum position D1 before the other end passes the other datum position D2. It is assumed for simplicity that there is no deliberate offset between the two datum position detectors. The distance between the two datum position detectors perpendicular to the scale 18 (i.e. in the Y direction) is shown as C.Clearly the datum position D1 is passed by one end of the gantry when the scale measurement in XA, and when the gantry has moved to the position shown by the broken line such that the other end of the gantry passes the other datum position, then the scale measurement is nowt8. The inclination of the gantry to its direction of movement (the X-axis) is therefore given by: (X8 - XA)/C.

If the gantry moves to some other position over the measuring table such that the measuring probe is at a position P, then the X-coordinate Xt of that position will not be the measured value X" but will be given by: Xt = X1 - (Y-d) (XB-XA)iC where Xt is the true X coordinate of the probie position and d is the distance between the origin on the Y-axis and the location of the measuring scale 18.

Clearly there will also be an error in the measured Y coordinate of the position P. However, this will depend upon the cosine of the angle of inclination of the gantry to the Y-axis. Since this inclination will in practice be very small, then the error in the Y measurement will be even less, and may for all practical purposes be ignored.

Figure 5 is a schematic block diagram of a circuit suitable for deriving the correction referred to above.

the circuit shown does not include the known circuitry for deriving values of X1, Xa and XB from the reading head outputs. The only variable quantities applied to the circuit of Figure 5 are these X-axis values and the Y-axis value at the measuring point P.

The values C and D are constants. It will be seen, without a detailed explanation being necessary, that the circuit of Figure 5 will operate to determine the required value of Xt. It will be understood that other circuit arrangements may be used to achieve the same objective. The arrangement described is for use when the two datum positions are nominally at the same X-axis coordinate. As already stated the two positions may be deliberately sapced apart along the X-axis in which case the separation must be subtracted from the measured value (Xg - XA) before the remainder of the processing is performed.

The circuitry used to display the coordinates of the measuring point P or to control the movement of the measuring probe is not descrbied as it may take a number of well-known forms.

The above description relates to a simple gantrytype measuring machine having a single measuring scale on each axis. Clearly more complex arrangements may be used, such as that described in our above-numbered British patent. Different arrangements of scales may require the use of a different expression for the determination of the correct values of the coordinates of the point of measurement. The machine may not include a gantry but may have the carriage and probe supported by a cantilever arm supported at one side of the table, with a connecting link passing under the table to the datum means on the opposite side. Other variations are also possible.

Claims

1. A coordinate measuring machine which includes a measuring table, a supporting member extending over the measuring table for movement over the surface thereof, first measuring means operable to measure the movement of the support ing member relative to the measuring table and comprising a first scale member fixed relative to the measuring table and extending over the range of movement of the supporting member and a first reading head carried by the supporting memberfor cooperation with said first measuring scale, a carriage carried by the supporting member for movement relative thereto in a direction perpendicular to the direction of movement of the supporting member, second measuring means operable to measure the movement of the carriage relative to the supporting member and comprising a second scale member fixed relative to the supporting member and extending over the range of movement of the carriage and a second reading head carried by the carriage for cooperation with the said second measuring scale, datum means movable with the supporting member at each side of the measuring table and operable to locate a fixed datum position on each side of the measuring table, and circuit means responsive to the output of the datum means for calculating and applying corrections to the outputs of the said first reading head.

2. A machine as claimed in Claim 1 in which the datum means comprise, on each side of the measuring table, a sensor having a first part attached to the measuring table and a second part carried by the supporting member for movement therewith.

3. A machine as claimed in Claim 2 in which each sensore comprises an electro-optic sensor having one part formed by a radiaton source and a radia- tion-sensitive detector and the other part formed by a shutter member arranged to interrupt the passage of radiation when the supporting member is in the datum position.

4. A machine as claimed in Claim 3 in which each sensor comprises a plurality of radiation-sensitive devices arranged such that the passage of radiation to only a predetermined pattern of said devices is interrupted by the shutter member when the supporting member is in the datum position.

5. A machine as claimed in Claim 4 which includes three radiation-sensitive devices arranged in a line and arranged such that only the centre one of the three devices is shielded from said radiation by the shutter member when the supporting member is in the datum position.

6. A machine as claimed in any one of Claims 3 to 5 in which the radiation-sensitive devices are attached to the measuring table and each shutter member is carried by the supporting member.

7. A machine as claimed in any one of the preceding claims in which the supporting member is a gantry extending across and supported at each side of the measuring table.

8. A coordinate measuring machine substantially as herein described with reference to the accompanying drawings.