EP0200723A1

EP0200723A1 - A method of line-of-sight measurement

Info

Publication number: EP0200723A1
Application number: EP19840904206
Authority: EP
Inventors: Svante BJÖRK
Original assignee: Svante Bjork AB
Current assignee: Svante Bjork AB
Priority date: 1983-11-10
Filing date: 1984-11-06
Publication date: 1986-11-12
Also published as: SE439197B; SE8306188D0; SE8306188L; WO1985002253A1

Abstract

Dans une méthode de mesure par ligne de collimation à l'aide d'un rayon laser (B) et un dispositif (13) de détection sensible à la position, on fait effectuer au rayon laser un mouvement combiné orbital et de rotation sur une surface cible (T) du dispositif de détection. La position du centre (M) du chemin orbital (P) sur la surface cible est déterminée et utilisée comme point de référence ou de bornage.In a collimation line measurement method using a laser beam (B) and a position-sensitive detection device (13), the laser beam is made to perform a combined orbital and rotational movement on a surface. target (T) of the detection device. The position of the center (M) of the orbital path (P) on the target surface is determined and used as a reference or boundary point.

Description

A method of line-of-sight measurement Known methods of line-of-sight measurements, such as alignment, straightness, level, position and similar measurements, include the use of a narrow beam of light, such as a laser beam, and photoelectric detector means adapted to intercept the beam and to provide an output indicative of the position where the beam impinges on a target surface of the detector means.

According to one known method the beam is caused to orbit along a circular path on the target surface, and the position of the centre of the circular path is determined on the basis of the length of time the beam illuminates each of a number of segments of the target face during one full cycle of the orbital movement (US-A-3 790 276) . The accuracy of such measurements is dependent on the magnitude of the deviation of the actual position of the axis of the beam from the nominal or supposed position. If the beam is a laser beam, the axis of the beam is typically supposed always to coincide with the axis of a cylindri- cal housing containing the laser tube and to be parallel with a reference plane or line of a support or mount for the housing. In actual practice, however, the actual position deviates from the nominal position.

One cause of such deviation may be instability of the beam with respect to the laser tube. Even if the laser tube is mounted and aligned very accurately, the axis of the beam may thus be offset in an uncontrolled manner from the axis of the laser tube. Another cause may be misalignment of the laser tube with respect to a housing in which the laser tube is fixedly secured. Such misalignment may be caused by deformation resulting from thermal stresses.

An object of the present invention is to reduce substantially or eliminate the influence on the measurement result of any deviation of the actual position of the beam axis from the nominal position.

To this end, according to the invention there is provided a method of line-of-sight measurement in which a narrow beam of light, such as a laser beam, is projected onto a target surface of a photoelectric detector device and caused to orbit along a circular path on the target surface and in which the position of the centre of the circular path is determined, characterised in that the light beam is caused during each completed cycle of its orbital movement to rotate at least a full turn relative to the target surface.

When the beam is a laser beam, the combined rotational and orbital movement may be accomplished simply by rotating the laser tube at a speed substantially higher than the rate at which any uncontrolled changes of the deviation of the actual beam axis position from the nominal beam axis position occur. The centre of the orbital path then will be located on the axis of rotation of the laser tube which axis may be presumed to be stable. Because an offsetting of the beam axis from the axis of rotation is unavoidable in actual practice, the rotation of the laser tube will automatically result in the combined orbital and rotational movement of the beam on the target face.

Alternatively, the laser tube or a housing in which the laser tube is fixedly secured may be attached to a rotatable support, such as a spindle of a machine tool.

The rotational movement may also be accomplished with a stationary source of the beam by positioning a rotating prism (Dove prism) in the beam path. The invention is described in greater detail hereinafter, reference being had to the accompanying diagrammatic drawings in which two embodiments are illustrated by way of example.

Fig. 1 is a diagrammatic representation of a first embodiment comprising a rotating laser tube;

Fig. 2 is a diagrammatic representation of a second embodiment comprising a non-rotating laser tube and a beam-rotating prism.

Referring to the drawings, a laser tube A is fixedly secured within a cylindrical tubular housing 11 which is mounted in bearings 12 for rotation about an axis C. The bearings 12 are mounted on a support S carrying a motor D for rotating the housing 11 at suitable speed. It is here presumed that the axis C coincides with the axis of

- housing 11 and has a stable position so that it can be used as a reference axis.

The axis L of the laser beam B includes a small angle with the axis C. This angle may vary somewhat in operation, e.g. because of thermal drift or deformations resulting from thermal stresses. However, any variations are presumed to be slow in relation to the rotational speed of the housing 11 so that the angle may be considered to be constant over any series of, for example, five or ten successive revolutions. A smaller or larger portion of the angle may be due to misalignment of the laser tube A with respect to the housing 11.

Naturally, the numerical value of the angle included between the axes L and C need not be known. Moreover, the angle need not exist intentionally, the offsetting of the beam axis L from the axis C may be the result of the unavoidable manufacturing and assembling tolerances and changes occurring in operation, such as thermal drift.

During the measurement the beam B impinges on a target surface T of a photoelectric detector 13. In the embodiment illustrated in Fig. 1 the target surface T, which is generally perpendicular to the axis C, is provided by four contiguous planar photodetector elements 13A,13B,13C, 13D having one point 13E in common. Because of the rotation of the housing 11, the illuminated spot S on the target surface T orbits along a circular path P having its centre M positioned on the axis C. In the case illustrated in Fig. 1, the centre M is situated at a distance from the common point 13E of the detector segments. At the same time, the spot S rotates with respect to the target surface T such that it completes one full revolution for each full cycle of the orbital movement.

Electrical output signals produced by the detector 13 are processed in well-known manner in an associated signal processing device 14 producing an output signal indicative of the position of the centre M of the circular path P with respect to the common detector segment point 13E. The determination of the position of the centre M preferably is carried out by successive integration over a series of cycles of the combined orbital and rotational beam movement. The data representing the position of the centre M on the target surface T may be used in different ways, depending on the purpose and nature of the measurement being carried out. For example, in the case of an alignment or position measurement, an adjustable part on which the detector 13 is positioned may be displaced with the guidance of continuously supplied data of the position of the centre M until the centre M coincides with the point 13E.

As is readily understood, the beam axis L need not include an angle with, or intersect, the rotational axis C. Naturally, whether or not the axes intersect, the offsetting of the axes from one another must be limited so that the operative or sensitive portion of the target surface T is capable of intercepting the beam throughout the orbital movement.

The embodiment shown in Fig. 2 differs from that shown in Fig. 1 in that rotation of the laser beam B is accomplished by means of a rotating prism 15 and in that the photoelectric detector 13' is of a different type such that the signal processing device 14' directly produces an output quantity indicative of the position of the centre M' on the target surface T' . An example of detectors of this type is the two-dimensional silicon position sensitive detector produced and marketed under the designations S1200 and S1300 by Hamamatsu Photonics K.K., Hamamatsu City, Japan. Naturally, a detector of this type can also be used with the rotating laser arrangement of Fig. 1. The beam-rotating prism 15 is a so-called Dove prism and causes the beam to orbit about the rotational axis C' of the prism and at the same time to rotate about its axis L at a speed twice that of the rotational speed of the prism.

Claims

1. A method of line-of-sight measurement in which a narrow beam of light, such as a laser beam, is projected onto a target surface of an photoelectric detector device and caused to orbit along a circular path on the target surface and in which the position of the centre of the circular path is determined, characterised in that the light beam (B) is caused during each completed cycle of its orbital movement to rotate at least one full turn relative to the target surface (T) .

2. A method according to claim 1, in which the beam (B) is produced within a laser tube (A) , characterised in that the laser tube is rotated about an axis (C) defined by bearing means (12) supporting the laser tube.

3. A method according to claim 1, in which the rotational and orbital movements of the beam (B) are accomplished by passing the beam through a rotating Dove prism (15).