The present invention relates to an apparatus for measuring angular displacement, and may be used, for example, to determine the magnitude of rotation of a rotary table from a reference position.

It is known from U.S. 4,807,997 to mount a rhomboidal- shaped glass element upon an apparatus whose angular displacement is to be measured, and to direct a pair of laser beams through the element, with each beam being incident upon a different face thereof. The beams are subsequently reflected back substantially along their incident path, recombined into an interference beam, and directed onto a photodetecting unit. The phase of the interference beam is determined by the optical path difference between the two component beams and thus varies in dependence upon the angular orientation of the glass element. The phase of the interference beam therefore corresponds to the angular displacement of the apparatus upon which the element is mounted.

The present invention provides an alternative apparatus of this type in which the laser beams are displaced one relative to the other in a direction parallel to the axis of rotation of the apparatus whose angular displacement is being measured.

According to a first aspect of the present invention there is provided an interferometer for measuring angular displacement of an object about an axis comprising: a light source for generating first and second beams of coherent 1ight; first and second retroreflective devices, for reflecting said first and second beams, at least initially, parallel to their incident path; a refractive artefact, mounted for rotation with said object, through which said first and second beams pass on

at least one occasion, the path difference between the first and second beams by virtue of their passage through said artefact varying with varying angular displacement of said artefact about said axis; said first and second beams being combined after reflection by said retroreflecting devices to form an interference beam; and a detector, onto which said interference beam is incident; wherein said first and second beams are axially spaced during their at least one passage through said artefact.

Typically said detector is connected to a counter for generating, on the basis of the output of said detector, an incremental count corresponding to the angular displacement of the object from a notional zero. Preferably, when said laser is a short coherence length laser, e.g. a laser diode, the first and second beams have substantially equal path lengths at said notional zero of angular displacement.

Alternatively when said laser is a long coherence length laser, e.g. a gas discharge laser such as a HeNe laser, or long coherence length laser diode, the path lengths of said beams in air are substantially equal at said notional zero.

Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which:

Fig 1 shows a perspective view of an embodiment of the present invention;

Fig 2 shows a side view of the apparatus of Fig 1;

Figs 3A-C illustrate the operation of the apparatus in Figs 1 and 2;

Figs 4A-C are details of the apparatus of Figs 1 to 3 ; and

Figs 5A and B show alternative configurations of transmissive element for the apparatus of Figs 1-3.

Referring now to Figs l and 2, a table 10 has a base 12, and a rotor 14, rotatable relative to the base 12 about an axis A. In order to measure the angular displacement of rotor 14 relative to base 12, a laser apparatus is provided. The apparatus includes a laser 20, (in the present example, a laser diode, although HeNe lasers for example may be used) which generates a laser beam 22 having orthogonally polarised components 22S,22P. The beam 22 is incident upon a polarising cubic beam splitter 24, which transmits component 22P; component 22S is reflected by beam splitter 24 in a direction substantially parallel to the axis A, and subsequently by a prism 26, thereby to generate a pair of substantially parallel laser beams 22S,22P, separated by a distance x. The beams 22S,22P are incident upon an optically transmissive refractive artefact in the form of a pair of rigidly connected glass blocks 30,32, which are mounted to the rotor 14 of table 10. As will be seen later, the glass blocks 30,32 may be of any suitable shape, but in the present example each have the form of a rectangular parallelepiped, and are oriented one relative to the other in a cruciform configuration when viewed along axis A. The spacing of the beams 22S,22P is such that beam 22S is incident upon the upper glass block 30, and beam 22P is incident upon lower glass block 32. As a result, the two beams 22S,22P will be refracted in opposite directions upon transmission through the blocks 30,32; the beams subsequently continue along a path parallel to their incident path, but in each case with an additional lateral displacement whose magnitude is dependent upon the angle of refraction undergone by the respective beam. Beams 22S and 22P are subsequently incident upon rooftop retroreflectors 40,42 respectively. The retroreflectors 40,42 return the reflected beams 44S,44P along a path substantially parallel to their incident path, whereupon the reflected beams 44S,44P are recombined at polarising beam splitter 24 to generate an interference beam 50 which is incident upon a photodetector (not shown) provided within the laser 20. The photodetector may be connected to a counter (not shown)

whose value has correspondence with the angular displacement of rotor 14 from a datum, or notional zero, which usually corresponds to a zero value on the counter. To reduce phase noise, which might otherwise be a problem when using short coherence length light it is desirable that the path lengths of the beams are equal at the notional zero (i.e. when the counter is zero). In the present example, the notional zero angle occurs when the optical paths of the beams 225,228 in blocks 30,32 are equal. To equalise the path lengths in this position, rooftop retroreflector 42 is offset in the direction of beam travel with respect to rooftop retroreflector 40 by the amount x.

The phase of the interference beam 50 will depend upon the relative optical path lengths travelled by the S and P component beams. This in turn depends upon the degree of refraction undergone by the S and P component beams within the blocks 30,32, which is related to the orientation of the blocks relative to the beams, and therefore the orientation of the rotor 14 relative to the base 12. This can be seen more clearly with reference to Figs 3A-C. Referring now to Fig 3A, a notional "zero" position of rotor 14 relative to the base 12 may usefully be defined when the angle of refraction (as measured from the optical axis O) undergone by the S and P component beams upon their transmission through blocks 30,32, and their path lengths with the blocks 30,32 are identical. At the zero position the path length of the S and P component beams will be substantially the same, and thus the phase of the resultant interference beam 50 may be said to be 0°. If, as in Fig 3B, rotor 14 is rotated through an angle of 30°, it can be seen that the forward face of block 30 subtends a very acute angle with respect to the incoming beams, whereas the forward face of block 32 lies much closer to the perpendicular than previously. As a result, component S will be refracted by a much greater angle upon its passage through block 30 than will component P upon its passage

through block 32. The optical path length of beam component S will therefore be increased, both as a result of the increased distance over which the component beam S must travel through glass, as well as the increased distance which the beam must travel as a result of its larger angle of refraction; the optical path length of component beam P will be decreased due to the shorter distance over which the component beam travels through a glass medium, as well as because of its reduced distance of travel resulting from its smaller angle of refraction. The change in the relative path difference between the component beams S and P will result in a corresponding phase shift of interference beam 50 which may be measured using a photodetector and associated processing electronics such as a counter (not shown) , from which the angular displacement of the rotor 14 relative to the base 12 may be calculated. A corresponding ray diagram is shown in Fig 3C for an angular displacement of -30° (or 330°) in which the optical path length of component P is increased and the path length of component S is decreased.

The configuration of the refractive artefact employed depends upon the range of angular displacements which it is desired to measure; large angular displacements require larger elements. However, we have found that by displacing the beams in a direction parallel to the axis of rotation of the device under test, the radial dimension of the artefact employed may be reduced, since the two differently oriented faces of the artefact upon which the two component beams are incident may be stacked one upon the other. This is particularly important where it is desired to employ an artefact having a relatively low moment of inertia. Since the moment of inertia of the artefact is a function of mr , (where m is the mass of the artefact and r is its radius from its centre of rotation) , increases in the height of the artefact will result only in a corresponding linear increases in the moment of inertia, whereas increases in the size of the radius of the artefact will increase the

moment of inertia as a function of r . The sensitivity of the system is increased by either increasing or decreasing the thickness of the artefact in the direction of propagation of the light beams 22S,22P, or reducing the angle between the front faces of the artefact (this latter measure then reducing, for certain configurations of artefact, the permissible angular range) .

The relationship between the angle subtended by the artefact at the optical axis, and the phase of the interference beam is non-linear. The extent of this non- linearity is, for a given refractive index, dependent upon the angle between the front faces of the artefact. (N.B. In each case above, for the sake of simplicity, the front and rear faces of the blocks forming the artefact are assumed to be parallel although this is not strictly necessary) .

To provide a reference position from which any non-linear compensation may repeatedly be initialised, it is useful to provide the system with an independent "reference mark". Referring now to Figs 4A-C, a collimated light source 60, mounted rigidly with the base 12 of the table generates a beam 62 of light which is incident upon a reflective target 70. The target 70 has two reflective regions 72 and two non-reflective regions 74. When the angular displacement of the table rotor 14, and thus target 72, is such that the reflection of the beam 62 is incident, in a predetermined relationship, upon a photo-voltaic quad cell 80, processing circuitry (not shown) associated with the quad cell 80 emits a pulse which causes the counter connection to photodetector 50 to zero. Referring to Fig 4C, a further pair of reference marks 100 may be provided to define a fixed angular displacement arc a to enable the apparatus to be calibrated to compensate for the thermal coefficients of the blocks 30,32.

Referring now to Figs 5A and 5B, preferred configurations for the optically transmissive refractive artefact are illustrated. Each includes two identical glass blocks, mounted one upon the other; and in other embodiments the blocks have a section substantially similar to that of a kite, and in the other a trapezium.

The invention has been illustrated using an artefact which comprises of a pair of identical prismatic elements, through which both the incident beams 22S,22P and the reflected beams 44S,44P pass; this is not essential. It is possible to configure the apparatus such that the two laser beams pass through the artefact only once, either on the incident (i.e. beams 22S,22P) or reflected (i.e. beams 44S,44P) journeys, and are deflected around the artefact on the other of the journeys. Other artefacts may be provided such as, for example, an element in which the beams 22S,22P are incident upon a single curved face, and as a result have different path lengths within the glass. (Appropriate software and calibration changes would also be required.)

Other light sources and optical elements may be employed to replace elements 24,40 and 42, such as a pair of retroreflectors provided by a single or pair of plane mirrors; the apparatus then requiring appropriate polarising elements.

The present invention need not be embodied by any of the specific features or functions described above, and each feature or function referred to may be substituted by other features performing the same or a similar function, and other functions achieving the same or a similar result.

Further, the different features of the invention described above are not necessarily limited to their association with the embodiments in connection with which they were described. Many aspects of the invention are generally

applicable to other embodiments of the invention described herein.