WO1990009562A1

WO1990009562A1 - Optical device and a method for alignment thereof on a machine

Info

Publication number: WO1990009562A1
Application number: PCT/GB1990/000168
Authority: WO
Inventors: Raymond John Chaney
Original assignee: Renishaw Plc
Priority date: 1989-02-15
Filing date: 1990-02-02
Publication date: 1990-08-23
Also published as: GB8903468D0

Abstract

An optical square is described which is provided with secondary reflecting surfaces (21-44) on external or internal faces thereof in fixed relationship to the principal optical surfaces of the square. The secondary reflecting surfaces are arranged with their normals parallel to, or perpendicular to, the operational plane of the square, the operational plane being the plane containing the incident and deflected beams of the square when it is in its operational position. The secondary reflecting surfaces are used to align the square by an optical method which involves directing a light beam onto the secondary reflecting surfaces and orienting the device to correctly align the secondary reflecting surfaces, whereby the operational plane of the square is automatically correctly aligned. Other optical devices for example beam splitter can benefit from the described method of alignment.

Description

OPTICAL DEVICE AND A METHOD FOR ALIGNMENT THEREOF ON A MACHINE

The present invention relates to optical devices ^:~ particularly for use with light beams in making "- measurements on machines, for example co-ordinate measuring machines, machine tools or robotic equipment.

Problems arise with accurate alignment of optical devices with the axes of a machine for use in a measuring operation. In particular, the alignment of an- optical square gives rise to time consuming effort, since in use in a machine, it has to be aligned with at least two axes of the machine simultaneously.

An optical square is a device which deflects an incident light beam through a right angle. There is a plane through the optical device such that an incident beam directed at the device in, or parallel to, the plane will produce a deflected beam in the plane at right angles to the incident beam irrespective of the angle of incidence of the incident beam in the plane. We define this plane as the operational plane of the device since it ±s_t usually desired to orient the device into this plane in us©..

Also for the purpose of this specification the three orthogonal axes of movement of a machine on which the optical square is to be used are defined as x- y- and z-axes, of which the x- and y-axes are in the horizontal plane and the z-axis is vertical. Taking the x-axis as the reference axis, the expressions, roll, pitch and yaw used with reference to the square are then defined as follows: roll is rotation of the square about the x-axis, pitch is rotation of the square about the y-axis, and yaw is rotation of the square about the z-axis.

One form of optical square achieves the deflection of an incident beam at right angles by the use of two principal plane reflecting surfaces rigidly connected together at an included angle of 45°, and with the normals of the principal plane reflecting surfaces lying parallel to the operational plane.

The object of lining up the optical square is to get the operational plane of the square parallel to, or co-planar with, the plane containing the two axes, the squareness of which is to be measured.

The usual method of aligning the optical square is to mount the square as accurately as possible in a housing, and in use, to mount the housing on the bed of the machine, and to align the housing with the machine axes as accurately as possible using dial gauges or other devices.

Such a method however, relies on the accuracy of the mechanical mounting of the optical square in its housing, and the maintenance of the accuracy over the life of the device. Thus great care is required during the manufacture of the component to obtain the required accurac .

The present invention seeks to overcome this problem.

According to one aspect of the invention there is provided an optical device having an operational plane and comprising at least one principal optical surface for performing the desired function of the device, and at least one secondary reflecting surface mounted on at least one face of the device with its normal in a known, fixed angular relationship to the operational plane of the device.

Preferably the normal of any secondary reflecting surface will be parallel with, or perpendicular to, the operational plane of the device.

The alignment of the device may thus be achieved by using the secondary reflecting surface to optically align the device itself to the required machine axes using a light beam, rather than measuring the alignment of the housing in which the device may be mounted.

-**.

The optical device is preferably mounted in a housing and forms together therewith an optical component.

Preferably a plurality of secondary reflecting surfaces are provided on different faces of the device whereby the device may be optically aligned using the present invention in a plurality of orientations'.

The secondary reflecting surfaces may be provided on internal or external faces of the optical device.

According to a particular aspect of the invention the optical device is an optical square which has a, pair of principal plane reflecting surfaces rigidly connected together and the normals of which lie parallel to the operational plane, and wherein secondary reflecting surfaces are connected in fixed relationship with said principal plane reflecting surfaces on i_?.oth intearpal and

,-•^■>j*^***- external faces of the device and each secondary reflecting surface has a normal either parallel to, or perpendicular to, said operational plane.

The invention also includes a method of aligning an optical square in which a laser beam is reflected from a secondary reflecting surface provided on the optical square itself to establish squareness of at least one face of the square to the laser beam.

By this means any errors in the mounting of the square within the housing become irrelevant and can be ignored.

Components other than optical squares can benefit from the provision of the secondary reflecting surfaces of the " invention to assist in the alignment thereof.

For example beam splitters, lenses or other devices which change the direction of an incident beam need to be accurately aligned with an optical beam or a specific direction relative to a machine axis. With these devices the incident beam and the deflected beam will lie in a plane. This plane is referred to as the operational plane of the device, and any secondary plane reflecting surface provided is arranged to have its normal in a known fixed angular relationship, preferably perpendicular to, or parallel to, the operational plane of the device.

Examples of an optical device of the present invention and methods of them aligning will now be more particularly described with reference to the accompanying drawings in which;

Fig. 1 is a sectional plan view of an optical square made in accordance with the present invention through the operational plane thereof,

Fig. 2 is an elevation of the optical square of Fig. 1, in the direction of the arrow A.

Figs. 3 and 4 are plan views of a machine illustrating the steps in one method of aligning an optical square in accordance with the present invention.

Fig. 5 illustrates the positions of the optical square and associated equipment during the set-up operation. Fig. 6 illustrates the optical square in position during a squareness measurement operation on a _^machine.

Fig. 7 illustrates the optical square and ancillary equipment in place on a machine for lining up the optical square by a further alternative method.

Referring now to Figs. 1 and 2 of the drawings one form of optical square 8 is shown, the principal optical surfaces of which are plane reflecting surfaces 10 and 11. The surfaces 10 and 11 are formed on supports which in this

1 example take the form of glass wedges 16,18, and are rigidly interconnected by a connecting device shown as a glass block 12 in the shape of a trapezium. The principal plane reflecting surfaces 10,11 extend away from the glass block 12 to provide a hollow structure which is surrounded by a housing 14. The planes of the surfaces 10 and 11 are convergent and form an included angle of 45°. The normals of the surfaces lie in the operational plane of the square.

The housing 14 has openings 14a, 14b on two adjacent sides to allow for the entry and exit of light beams into the optical apertures 15a,15b of the square. An optical aperture of the square is essentially defined by the size of the space through which a light beam 19 can enter the device and strike both of the principal plane reflecting surfaces. In the present example, the optical aperture 15a extends from the edge of the opening 14a to the corner Cl of wedge 18, and optical aperture 15b extends from the edge of the opening 14b to the corner C2 of wedge 16.

In addition to the principal plane reflecting surfaces 10 and 11, four secondary reflecting surfaces 20,21,22 and 23 are provided on the external faces of the wedges 16,18, with their normals parallel to the operational plane of the square.

Two further secondary reflecting surfaces 24 and 25 are provided on opposite faces of the trapezium with their ^' normals perpendicular to the operational plane of the square. In the example shown the secondary reflecting surface 24 is formed on a face inside the hollow square, i.e. on an internal face of the square while the secondary reflecting surface 25 is formed on an external face of the square.

Alternatively a mirror coating can be provided on either of the faces of the trapezium in place of both of the surfaces 24 and 25. Such a coating would provide secondary reflecting surfaces facing both ways and could be regarded as being on an internal face of the device, or an external face depending on the direction from which an incident beam approached.

The other secondary reflecting surfaces 20 to 23 of the square, or on any other optical device could also be provided as mirror coatings and positioned to be used from either side.

One procedure for aligning the optical square to the x- and y-axes of a coordinate measuring machine or machine tool will now be described in detail with reference to Figures 3 and 4, which illustrate diagrammatically a coordinate measuring machine. The machine has a spindle 30 supported for movement in the direction of the z-axis on a carriage 31. The carriage 31 is supported for movement in the direction of the y-axis on a bridge member 32 which in turn is movable in the direction of the x-axis. The construction of such a machine is well-known and forms no part of the present invention so that it is not necessary to describe it in more detail. The procedure in accordance with the present invention includes the following steps:

1. A target 35 which may be a photo-diode detector, a retro-reflector or a coloured spot, is fitted to the machine spindle 30. A laser 36 is mounted on a translation stage 37 with its beam 38 directed generally along the x-axis towards the target. The laser beam 38 is accurately aligned with the x-axis in known manner by movement of the target alpng the

2. The target is removed and the optical square 8 is mounted on the machine bed at an appropriate height so that the secondary reflecting surface 20 lies in the path of the laser beam and the operational plane of the optical square is generally parallel to the x-y plane of the machine.

The optical square is rotated about the y-axis until the reflected laser beam is parallel to the x-axis in the x-z plane, and thus lies in the x-y plane of the machine as detected by a photo-detector or a screen at the laser position. The plane of the optical aperture 15a is then normal to the x-y plane. >

3. Using the translation stage 37, the laser is translated accurately to position P2 in which the laser beam, while still parallel to the x-axis enters the optical aperture 15a and is deflected at right angles into the direction of the y-axis.

The target 15 is now re-positioned in the path of the deflected laser beam, and by movement of the target along the y-axis while rotating the optical square about the x-axis, the deflected laser beam can be made parallel to the y-axis in the y-z plane of the machine. Since, by virtue of the inherent properties of the square deflector, the angle between the incident beam and the deflected beam will be 90°, the deflected beam will automatically be directed parallel to the y-axis in the x-y plane of the machine, and the planes of both of the optical apertures 15a and 15b will now be normal to the x and y axes respectively.

Thus it can be seen that by using the secondary reflecting surface 20 to align the optical square off one of its own external faces, it does not matter how accurately the optical square is mounted within the housing.

Once the optical square is properly aligned relative to the x- and y-axes it is clamped in position for the measurement of the relative squareness to take place. The measurement operation may be carried out using a straightness interferometer, for example, including a Wollaston prism. The Wollaston prism is mounted on the machine spindle for movement therewith and the laser beam is directed through it. The two divergent beams produced by the prism are passed through the optical square and onto a straightness reflector mounted on a stationary part of the machine. The straightness reflector reflects them back through the square to the interferometer. Such an operation is known in itself and is not described further.

It will be understood that instead of mounting the laser on a translation stage, the same effect can be produced by mounting the optical square on a translation stage.

Although the procedure for aligning the optical square has been described using only one of the secondary reflecting surfaces, the other secondary reflecting surfaces may be used with other reflecting devices if it is not practical to align the laser with a given axis, which may be the case for example, if pillars or other parts of the machine get in the way.

With the number of secondary reflecting surfaces provided on the optical square described herein, the square can be orientated so that its operational plane is parallel to any of the x-z, y-z or x-y planes of the machine with the laser in any convenient position using additional reflectors suitably arranged around the machine.

Figs. 5 and 6 show an alternative method of aligning the optical square so that it is square to the y-z axes with the laser on the x-axis. ^' . ^'

Referring now to Figs. 5 and 6 the method comprises the following steps:

1. The target 35 is used to align the laser beam 38 with the x-axis as described with reference to Figs.. 3 and i ^*• "^** 4.

2. The target is removed and the optical square 8 is mounted on the machine with the secondary reflecting surface 24 in the path of the laser beam and with the operational plane of the optical square generally parallel to the y-z plane of the machine. The optical square is rotated about both the y-e&is and the z-axis until the laser beam is returned along the x-axis and the optical square is clamped in position. The optical square then has its operational plane parallel to the y-z plane and the pitch and yaw errors have automatically been removed. ^"Roll errors about the x-axis are not important because these would only affect the angle of incidence of the measurement beam in the y-z plane, and the squareness of the deflected beam to the incident beam in this plane is unaffected by the angle of incidence. Fig. 6 shows how a measuring operation may be carried out with the optical square correctly aligned and the laser beam directed along the x-axis. A beam deflector 40 directs the laser beam 38 from the x-axis to be parallel to the z-axis where it impinges on a corner cube retro-reflector 42 carried by the machine spindle. The retro-reflector directs the beam back parallel to the z-axis through a Wollaston prism 44 which produces a pair of divergent beams 45,46. The two divergent beams are directed through the optical square and emerge parallel to the y-axis. They are then directed towards a straightness reflector (not shown) on the machine whereby the relative squareness of the machine movement between the y- and z-axes can be measured using known methods.

Fig. 7 illustrates a further embodiment of the invention in which the optical square is to be mounted square to the x-y plane with the laser directed along the x-axis towards the spindle of a machine tool which is horizontal and moves in the direction of the x-axis.

The following steps are needed:-

1. The laser beam 38 is lined up in position A with the x-axis as before using a target (not shown) on the machine spindle 50.

2. The optical square 8 is mounted on the bed of the machine with the secondary reflecting surface 22 on the back of the square in the path of the laser beam.

The optical square is rotated about the y-axis to remove the pitch error, and ensure that the reflected laser beam lies in the x-y plane. 3. The laser 36 (or the optical square 8) is translated in the z direction to position B.

4. The target is removed from the machine spindle and is replaced by a corner cube retro-refϊector 52 which reflects the laser beam back along the x-axis into the optical square, from which it emerges in the direction of the y-axis.

.^■in 5. Using a target (not shown) placed on the machine carriage which moves in the direction of t y-axis, the square is rotated about the x-axis to take out the roll error, and to ensure that the cteflectted beam is parallel to the x-y plane.

Once the square is properly aligned, the squareness of the movements of the machine betwee the x-axis and y-axis direction can be measured by adding a Wollaston prism and a straightness reflector in the beam path as described above.

Variations of the above-described embodiments may be used which also fall within the broad aspects of the present invention.

For example, the laser 36 may be replaced by an auto-collimater which produces a collimated light beam.

The particular form of optical square illustrated had two principal optical surfaces in the form of plane reflecting surfaces inclined at an included angle of 45°. However, other forms of optical square are known and can benefit from the use of the invention. For example, another known form of optical square includes a roof-top reflector. However, in all cases an operational plane can be defined which will determine the orientation of the secondary reflecting surfaces. Further, Figs. 1 and 2 have illustrated a hollow device in which the incident and reflected beams travel through air within the housing 14. As an alternative design the principal optical surfaces may be formed as internal reflecting surfaces within a solid block of transparent material. In this case more of the secondary reflecting surfaces may be formed on internal faces of the device. Also the connecting device 12 can then be dispensed with.

There are other materials which may be used in making the device. For example, in place of glass, the supports may be made from other transparent materials for example glass ceramics, or from opaque materials such as ceramics, or metals and the principal and secondary reflecting surfaces may be formed from polished areas on such opaque materials. The supports may be hollow or solid.

Further, other optical components for example plane mirrors, beam splitters or lenses can benefit from the invention. In every case it will be necessary to define an operational plane relative to the principal optical surfaces of the devices in order to determine the orientation of the secondary reflecting surfaces.

Claims

CLAIMS :

1. An optical device having an operational plane and comprising at least one principal optical surface for performing the desired function of the device, and at least one secondary reflecting surface mounted on at least one face of the device with its normal in a known fixed angular relationship to the operational plane of the device.

2. An optical device as claimed in claim 1 and wherein a secondary reflecting surface is mounted on an external face of the device.

3. An optical device as claimed in claim 1 and wherein a secondary reflecting surface is mounted on an internal face of the device.

4. An optical device as claimed in claim 1 and wherein a secondary reflecting surface has its normal parallel to the operational plane of the device.

5. An optical device as claimed in claim 1 and wherein a secondary reflecting surface has its normal perpendicular to the operational plane of the device

6. An optical component comprising an optical device as claimed in any preceding claim mounted in a housing, said housing having at least one aperture therein for the passage of a light beam therethrough, said secondary reflecting surface being positioned to be visible through said aperture.

7. An optical component as claimed in claim 6 and wherein the optical device is an optical square.

8. An optical component as claimed in claim 7 and wherein said optical square comprises two principal plane reflecting surfaces rigidly connected together, the normals of which lie parallel to the operational plane of the square, and wherein a plurality of secondary reflecting surfaces are connected in fixed relationship with the principal plane reflecting surfaces and each secondary reflecting surface has its normal parallel to, or perpendicular to said operational plane.

9. A method of aligning an optical device with its operational plane in a pre-determined relationship with an axis of a machine, said optical device having at least one secondary reflecting surface on a face thereof and which has its normal in a known fixed angular relationship with the operational plane of the device, the method comprising the steps of: aligning a light beam parallel to a first axis of the machine, placing the optical device in the required position on the machine in the path of the light beam so that the light beam impinges on a secondary reflecting surface of the device, rotating the device about a second axis perpendicular to said first axis so that the secondary reflecting surface attains a pre-determined alignment with said first axis.

10. A method as claimed in claim 9 and wherein the optical device is an optical square, the method further comprises the steps of: producing relative translational movement between the optical square and the light beam normal to the beam whereby the beam enters an optical aperture of the optical square and is deflected at right angles into a direction parallel to said second axis in the plane of the first and second axes, rotating the optical square about the first axis to -15- bring the deflected light beam into parallelism with the second axis in the plane of the second axis and a third axis orthogonal to both of the first second axes.