GB2280259A

GB2280259A - Detecting angular displacement.

Info

Publication number: GB2280259A
Application number: GB9315397A
Authority: GB
Inventors: Timothy George Slack; Nicholas Charles Roberts
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1993-07-24
Filing date: 1993-07-24
Publication date: 1995-01-25
Also published as: GB9315397D0

Abstract

To detect simultaneously angular displacement of an object about three axes (X, Y and Z), a reflecting grating (1) is attached to an object whose angular displacement is to be detected and a beam of collimated light shone onto the grating (1) and reflected back through a positive collimating lens (4) onto light sensor means (7) where it forms spaced spots of light. The position and orientation on the sensor means (7) of these spots (8, 9) indicates the angular displacement of the object about the three axes (X, Y and Z). Rotation about the X and Y axes causes translation of the spots (Fig. 3) and rotation about the Z axis causes rotation of the spots (Fig. 4). <IMAGE>

Description

METHOD AND APPARATUS FOR DETECTING ANGULAR DISPLACEMENT This invention relates to a method for optically detecting simultaneously angular displacement of an object about three axes and to apparatus for carrying out this method.

It has been proposed to detect or measure tilt of an object about two axes by optical means in which a mirror is attached to the object and a beam of light shone at this mirror. Movement of the object about two axes is detected by the change in position of the reflected beam of light which moves across a detector. However this proposed system is only suitable for detecting angular tilt of the object about the X and Y axes and in order to measure tilt about the third or Z axes it is necessary to use a separate system based on beam polarisations. This not only introduces undesirable complexity but also has the drawback that the beam polarisation technique is difficult to employ if the object translates along the Z axis as well as tilts.

There is thus a need for a generally improved method and apparatus for optically detecting simultaneously angular displacement of an object about three axes which is simpler, lighter and cheaper than the foregoing proposed method and apparatus and which is basically unaffected by translation of the object along any one of the three axes.

According to a first aspect of the present invention there is provided optical apparatus for detecting simultaneously angular displacement of an object about three axes, including a reflecting grating attachable to the object whose angular displacement is to be detected, a source of a beam of monochromatic light, a positive collimating lens, reflector means for receiving the light beam and reflecting it through the lens, which collimates the beam, towards the grating, and light sensor means located on the side of the lens remote from the grating and operable to receive light reflected back through the lens and reflector means by the grating which forms on the sensor means two or more spots of light whose position indicates angular displacement of the object.

Preferably the light sensor means is a two dimensional array of charge coupled devices and the grating and lens are such as to produce three spots of reflected light from the array of charge coupled devices.

Conveniently the light source is a laser.

According to a further aspect of the present invention there is provided a method for optically detecting simultaneously angular displacement of an object about three axes, including the steps of shining a beam of collimated monochromatic light at near normal incidence onto a reflecting grating attached to an object whose angular displacement is to be detected, and receiving the light reflected by the grating through a positive collimating lens on light sensor means where it forms two or more spaced spots of light whose position and orientation on the sensor indicates angular displacement of the object about the three axes.

Preferably the light beam is a beam of laser light.

Conveniently the sensor is a two dimensional array of charge coupled devices and the grating and lens are such as to produce three spots of reflected light on the array.

Advantageously the spots are produced by three reflected beams, one of which is undiffracted and the other two or which are diffracted.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which; Figure 1 is a diagrammatic view from one side of apparatus according to the present invention for detecting simultaneously angular displacement of an object about three axes, Figure 2 is the pattern of light spots produced on the sensor means of Figure 1 in the absence of any angular displacement, Figure 3 is the pattern of light spots produced on the sensor means of Figure 1 if the object tilts about the X and Y axes, and Figure 3 shows the pattern of spots produced on the sensor means of Figure 1 if the object tilts about the X, Y and Z axes.

Optical apparatus for detecting simultaneously displacement of an object about three axes according to the present invention is shown in general in Figure 1. The apparatus includes a reflecting grating 1 illuminated at near normal incidence which is attachable to an object whose angular displacement is to be detected.

The apparatus also includes a source 2, preferably a laser, of a beam of monochromatic light 3, a positive collimating lens 4, reflector means 5 which reflects the light beam 3 through the lens 4 towards the grating 1 but which allows light 6 reflected from the grating 1 back through the lens 4 to be transmitted to light sensor means 7 located on the side of the lens 4 remote from the grating 1.

A suitable source 1 is a laser diode operating at a wavelength of about 850 nanometres. The reflector means 5 may be a 50% reflecting mirror.

The sensor means 7 is operable to receive light reflected back through the lens 4 and reflector means 5 by the grating 1, which reflected light falls on the sensor means 7 as two or more spots of light whose position indicates angular displacement of the object (not shown).

When the grating 1 produces just two, diffracted, spots of light from the output of the sensor means 7 the location of the centres of the two spots is measured. The angle between the imaginary line joining the spots is then calculated and also the location of the exact centre of this line.

If the object, whose displacement is being measured, rotates about the X or Y axes (see Figure 1) then the angle of the line joining the spots will be unaltered. This shows that there has been no rotation about the Z axis. By measuring the change in location of the mid-point of the line joining the spots and using the known focal length of the lens, the rotations about X and Y are calculated. If there has also been a rotation about the Z axis, the line joining the spots will now be at a different angle. This angle can be directly used to find the Z axis rotation. At the same time, the rotation about the other two axes can be found from the location of the mid-point of the line joining the spots.

The number of spots can be increased by changing the profile of the lines on the grating. This may be done to increase the precision of the system since the imaginary line will then be drawn through more spots. This will help to average out the random errors in the measurement, from the output of the sensor means 7, of the locations of the centres of the spots.

The light sensor 7 preferably is a two dimensional array of charge coupled devices (CCD's) having a plurality of pixels or elements for receiving the reflected light.

The charge coupled devices indicate the intensity of light received by each element or pixel and indicate position by the particular pixels in the array receiving the illumination.

Preferably the grating 1 and lens 4 are such as to produce three spots of reflected light, as shown in Figures 2, 3 and 4 of the accompanying drawings, on the array of charge coupled devices forming the sensor means 7. This increases the accuracy of the detection of angular displacement of the object about the three axes. The light 6 reflected back towards the sensor means 7 comprises several beams resulting from diffraction produced by the grating 1. The exact number of beams 6 and relative intensity depends on the profile of the lines of the grating 1. For the purposes of the Figures 2, 3 and 4 illustrations three beams have been reflected, one of which is undiffracted tracing the path of the original beam and the other two being diffracted beams which make equal but opposite angles to the original beam path. This result is shown in Figure 2 which represents the pattern of spots on the sensor means 7 where no angular movement of the object takes place.

The grating 1 must be made up of straight, parallel lines. The profile of the individual lines will govern the number of spots produced. The separation of the lines is one of the factors which will determine the separation of the individual spots at the output. The other factors are the focal length of the collimating lens 4 and the wavelength of the light. For a lens of focal length f; with a wavelength ; and a grating with a period (separation between rulings) of umm: the separation of the diffracted spots in the output will be A where: A 2fX (1) U If the grating 1 also produces a spot from an undiffracted beam at the sensor means 7, it will be exactly A/2 from each of the diffracted spots.For a sensor means 7, 5mm across; with k= 850nm and a 5cm focal length lens 4; the maximum value of u is about 0.02mm. However, the required size of the sensor means 7 will depend not only on the separation of the spots but also on their maximum expected displacement due to rotations. The greater the value of Athe more sensitive the system will be to rotations about the Z axis (see Figure 1).

If the light source is monochromatic the two diffracted beams will form nearly identically size and shape spots to that formed by the undiffracted beam. Hence in Figure 2 the spot 8 represents the undiffracted beam and the spots 9 represent the two diffracted beams which spots 9 are spaced from the spot 8.

Figure 2 represents the pattern of spots on the sensor means array when the object is aligned with the detection apparatus of the invention. In other words when there is no tilt or angular displacement.

Figure 3 shows the effect of simultaneous tilts about the X and Y axes and the amount of angular tilt or displacement can be calculated from the displacement of the centre spot 8 with respect to its position in Figure 2.

Figure 4 shows the pattern of spots resulting from simultaneous tilts about all three axes X, Y and Z. Thus in Figure 4 the X and Y tilts or angular displacements are determined by displacement of the centre spot 8 as was the case in Figure 3. The tilt or angular displacement about the Z axis has additionally rotated the lines of the grating 1 having the effect of causing the two diffracted spots 9 to rotate about the undiffracted spots 8. Thus the tilt or angular displacement in the Z direction can be deduced from the angle of line during the two diffracted spots 9.

With the apparatus and method of the present invention angular displacement or tilt about all three axes can be detected and measured simultaneously using the same equipment. In addition a translation of the object in the X and Y directions will not affect the results provided the grating 1 is not translated to a position outside the beam of light 6. Apart from the grating 1 no part of the apparatus is in contact with the object.

Claims

1. Optical apparatus for detecting simultaneously angular displacement of an object about three axes, including a reflecting grating attachable to the object whose angular displacement is to be detected, a source of a beam of monochromatic light, a positive collimating lens, reflector means for receiving the light beam and reflecting it through the lens, which collimates the beam, towards the grating, and light sensor means located on the side of the lens remote from the grating and operable to receive light reflected back through the lens and reflector means by the grating which forms on the sensor means two or more spots of light whose position indicates angular displacement of the object.

2. Apparatus according to claim 1, wherein the light sensor means is a two dimensional array of charge coupled devices and wherein the grating and lens are such as to produce three spots of reflected light on the array of charge coupled devices.

3. Apparatus according to claim 1 or claim 2, wherein the light source is a laser.

4. Optical apparatus for detecting simultaneously angular displacement of an object about three axes, substantially as hereinbefore described and as illustrated in Figure 1 as modified or not by any one of Figures 2, 3 and 4 of the accompanying drawings.

5. A method for optically detecting simultaneously angular displacement of an object about three axes, including the steps of shining a beam of collimated monochromatic light at near normal incidence onto a reflecting grating attached to an object whose angular displacement is to be detected, and receiving the light reflected by the grating through a positive collimating lens on light sensor means where it forms two or more spaced spots of light whose position and orientation on the sensor means indicates angular displacement of the object about the three axes.

6. A method according to claim 5, in which the light beam utilised is a beam of laser light.

7. A method according to claim 5 or claim 6, in which the sensor means is a two dimensional array of charge coupled devices and in which the grating and lens are such as to produce three spots of reflected light on the array.

8. A method according to claim 7, in which the spots are produced by three reflected beams, one of which is undiffracted and the other two of which are diffracted.

9. A method for optically detecting simultaneously angular displacement of an object about three axes, substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.