GB2341026A

GB2341026A - Direction measuring apparatus adjustable about two non perpendicular axes

Info

Publication number: GB2341026A
Application number: GB9813388A
Authority: GB
Inventors: Barry James Gorham
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-06-23
Filing date: 1998-06-23
Publication date: 2000-03-01
Anticipated expiration: 2018-06-23
Also published as: GB2341026B; GB9813388D0

Abstract

By horizontally scanning a single fan-shaped light beam a number of times across a target and adjusting the fan orientation angle to the horizontal for each scan a measurement of target location may be achieved in the same way as "Laserfix" but using only a single light beam. The direction measuring system, e.g. an automatic laser theodolite, for performing this measurement comprises apparatus rotating about two non perpendicular axes, Z-Z' and Z'-Z'', first and second means operable for rotating the apparatus about the axes, means for projecting a fan-shaped light beam in a plane perpendicular to the second axis and rotatable about the second axis by the second drive means and means for determining the angular position of the apparatus about the first axis. A sensor, for detecting the presence of the beam, is positioned by the object whose direction is to be measured, and means for computing the direction of the sensor relative to a reference point at the origin of the light beam is also provided. Preferably the axes are at 45 degrees to each other and the source of the light beam is fixed in relation to the first axis but the fan is rotated about the second axis. The apparatus can measure the full space vector to a distant sensor using a single light beam where previously two beams have been necessary.

Description

2341026 Apparatus for the measurement of direction This invention relates

to apparatus for the measurement of direction. It is concerned specifically with the design of a form of automatic theodolite that utilises the geometrical properties of shaped beams of electromagnetic radiation in the process of measurement.

In LTK Patent GB 2090096B by this author, a form of automatic scanning laser theodolite is described (a design since named and publicised as LASERFIX) in which a fixed pair of mutually inclined laser fan-shaped beams is caused to rotate about an axis normal to the plane of the single angle scale of the instrument. The process allows measurement of full spatial direction to any distant target within reach of the beams for g 1 1 each revolution of the scanner and in almost real-time.

The present invention represents both an alternative and improved design of automatic theodolite of the broad type described in this earlier patent. Its main advantage over the earlier invention is that, whilst preserving the feature of full spatial measurements using but a single angle scale, a theodolite made according to the present invention can achieve a comparable performance by projecting only a single fan-shaped laser beam.

According to the invention there is provided: a piece of apparatus orientationally adjustable about first and second axes which are nonperpendicular; first and second drive means operable to rotate the apparatus about its first and second axes respectively. means for projecting a fan-shaped light beam in a plane perpendicular to said second axis and rotatable about that axis by said second drive means; sensor means for sensing the presence of said beam. means for determining the angular position of the apparatus in its rotation about the first axis, and means for computing the direction in space of said sensor means relative to a reference point at the source of the radiation.

However, before describing the present invention in more detail and making clear its context with respect to the stated prior art, it would be more apposite to introduce first the main features of the earlier invention.

The principle of measurement of that invention comprises the projection of two fanshaped laser beams which are mutually inclined to the vertical and are rotated as a fixed pair from a single rotary scanner ( Fig. 1). The geometry of the pair of laser beams remains fixed as they rotate about a vertical axis through the centre of the instrument ( 'L') The other necessary ingredient for the scanner to convert it into a theodolite is the incorporation of an electronic angle encoder into the rotor shaft. In one embodiment, this takes the form of a glass moii- 6fringe digitiser disc that generates an electrical signal for every 6 arc minutes of rotation. Sufficient inertia is built into the system to aflow time interpolation between angle markers giving a precision of 1 arc second. From Fig. 1 it is clear that the central and horizontal rays in the two fan beams have a fixed included angle 0' say, and the nominal 45' tilt angle of the two beams has an actual value of 'cc'. Both 0 and'(x are determined by initial instrument calibration and remain sensibly fixed thereafter.

If an electronic photocell is assumed to lie within the field of illumination of the scanning laser beams of Fig. 1, then it would be struck successively by each of the pair of beams thus generating two electrical signals per rotation of the laser scanner. These signals are then used each to latch the instantaneous reading of the instrument encoder scale. Subsequent processing of these readings, together with use of the known system geometrical constants, 0 and cc, leads to a determination of the full spatial vector of the line which joins the instrument optical centre to the centre of the target photocell. This corresponds to the combined horizontal (azimuth) and vertical (elevation) angles of a conventional theodolite, but is here achieved automatically with but a single angle scale.

In more detail, consider the case where the distant photocell is situated on the instrument local horizon, such that the central ray of light in each fan successively strike the photocell. Then if the angle disc in the scanner is latched at the moment the sweeping first beam crosses the cell centre, a disc direction reading R1 would be generated.

2 A corresponding reading R2 for the second beam would also be produced. Since the photocell is in the horizon plane of the scanner it is clear that R2 - R1 = 0 (See Fig. 2(i) where the target point is struck by the horizontal rays of the respective fan laser beams).

It is also clear from Fig. 2(ii) that where the photocell is above the level of the local horizon, R2 - R1 < 0, and where it is below, Fig. 2(51), the angular separation of those parts of the two beams which consecutively strike the photocell are further apart, and so R2 - R1 > 0. Clearly, the difference between the two latched values on the angle scale compared with the constant 0 is a measure of the vertical angle (V. A.) to the target.

For a more detailed analysis using spherical trigonometry, reference is made to Fig. -33. Here, the sloping laser beam shown by the dashed line is the first beam of the pair that is being scanned in a clockwise direction. It is shown at the instant that it meets the 0 target P.

The vertical projection of the point P meets the instrument horizon plane at B. The local elevation angle of P at the instrument is (p and the azimuth angle between point B and the point 1 where the laser fan beam intersects the horizon plane is y. The other information given in the figure is the sideways tilt angle of the laser beam (usually about 45') and designated ct.

Applying Napier's formula to the right-angled spherical triangle IBA, we have TAN (p = TAN (x. SIN xy (1) However, the angle xy cannot be evaluated from a single encoder reading; the encoder reading for the second beam is needed in order to evaluate this quantity. The manner in which these two readings may be combined so as to Cenerate the appropriate elevation angle from the instrument may be seen from Fig. 4. Here, the angle which separates the two encoder readings C4 equivalent to R2 - R1, is smaller than 0 due to the elevated target by the equivalent of 2W. Thus, 0 = ap + 2y or W = (0 - ap) 12 3 It is clear that the local elevation angle from the instrument to the distant target, as given by equation (1) is now: TAN (p = TAN a. SIN [ (0 - ap) 12 1 (2) where only % is an observable; the other parameters are known instrument constants.

The horizontal plane direction or azimuth of the target may be given merely by the mean of the two latched encoder values, namely ctp,/2 + R1 Full spatial direction measures to all suitable point targets within the field of illunUnation of the scanner can thus be obtained for every full circle rotation of its beams. The rate of direction data update is limited largely by the speed of the electronic counters used in the interpolation of the moir disc pulses. A reasonable practical limit is close to 50 HZ for the scanner, although this may have to be significantly reduced due to limitations of photocell detection of scanning beams at longer ranges. At ranges up to 50 metres, a standard deviation for a set of 10 consecutive direction measures of +/- 0.6 arc second has been consistently obtained from a prototype instrument made according to this principle.

In order to use this kind of instrument, which is essentially a theodolite, for the measurement of spatial position, it is necessary to introduce some form of length scale. One way of doing this is to arrange that the targets to be used for the measurements are set into a fixed and known spatial array. The positioning process then comprises a measurement of the direction from the scanner to each individual, and identified, target of the array followed by a spatial resection analysis that generates the individual 3-13 coordinates for each target in the coordinate reference frame of the scanner.

It was this technique which was used in the first application of LASERFIX as a robotic 'Total Station' surveying instrument, first produced as an industrial prototype in 1984; also in the automatic tunnel guidance system LASERGUIDE, first produced in 1987. Subsequent applications of this technology have occurred in both automatic in-situ spatial calibration of production robots, and in the automatic guidance, in respect of both 3-D position and attitude, of site vehicles in road construction.

4 In terms of system design, Laserfix is very simple. Its main components are two independent fan-beam laser projectors, a rotary support table, incremental angle encoder and integral system microprocessor board. Of these, the most expensive items are the high quality laser beam projectors. In keeping with the original design, two projectors must be mounted and adjusted so that their optical axes both intersect the rotation axis of a rotary table at the same point, and additionally, their sideways lean angles (a) must be measured independently for instrument calibration. There would be significant advantages in the manufacture of such an instrument, as well as enhancing its reliability, if the measuring process that uses two beams could be achieved by using a single laser projector.

Unfortunately, if a single laser source were to be employed, and two beams produced from it by appropriate beam-splitting optics, this necessarily reduces the radiant power available to each beam to less than 50% of that generated directly by the source projector, with corresponding loss of working angular range. Moreover, using a high power laser source in a single projector, to allow for losses through beam splitting, still does not provide a solution, since the beam quality generated by such laser diodes is still generally inferior to that of their low powered, and less expensive, counterparts.

More significantly, however, unless more sophisticated and necessarily more expensive optical components are used than for a pair of single projectors, the resulting two derived components of the initially produced laser beam do not appear to emanate from a single point. This introduces an eccentricity error into the geometry of the spatial measurements and generates a further need for correction.

Accordingly, a fresh approach has been sought which exploits the full radiant output from a low-powered laser diode into a single fan-shaped laser beam, so that the beam is presented to a stationary target successively as a right leaning and then lefl leaning fan beam in the manner of the original Laserfix principle.

Early attempts to scan a single fan beam over one or more cycles of rotation and then to rotate it about it's own axis so that it leaned in the opposite sense for further scan cycles, were unsuccessful. It was not possible to maintain the geometry of the beams to within the desired 1 arc second specification when subject to such a reciprocating motion, and in manufacture would have been inordinately expensive.

In the event, the production of a geometrically stable pair of mutually inclined scanning laser beams that derive successively from but a single laser projector was achieved from a design that is both simple and efficient. The main features of this design constitute the present invention. An embodiment of the invention in the form of an automatic laser theodolite is described below and in relation to figures 5 to 9 inclusive.

The basic geometry of the frame of the theodolite is shown in Fig. 5. There are two independently rotatable elements: the encoder, which is integral with the baseplate of the instrument, is rotatable about a vertical axis Z - Z', and the projector disc carrying a single laser projector which generates a fan beam lying parallel to the plane of this disc. The projector disc is independently rotatable about the fixed axis Z'- Z". In practice, and preferably for maximum accuracy, the axis Z'- Z" would be inclined at about 45' to the vertical.

Fig. 6 illustrates the effect of rotation of the body of the instrument about the vertical axis Z - Z',- the single angle encoder in the instrument base is used to monitor in real time this continuous rotationThe projector disc houses the single laser beam fan projector which lies in a plane close to and parallel with the plane of the projector disc itself This is illustrated in Fig. 7, It is also clear from Fig. 7 that rotation of the projector disc about it's inclined axis Z' - Z" has the effect of moving the laser fan in circular fashion in a plane which is inclined to the horizontal by about 45'. If, for a particular setting of the laser fan beam around the face of the projector disc, the main alidade of the instrument were to be rotated about its vertical axis Z - Z', then a band of latitude would be illuminated by the beam.

6 Two pairs of such positions for the projector disc are shown in Fig. 7. they are beams designated 1 and 4 and 2 and 3. Each pair, generated by settings of the laser fan-beam on opposite sides of the projector disc, would form a W' beam occurring at different latitudes about the instrument. The individual beam components would be separated in time according to the rotation of the projector disc between the two beam positions and the speed of rotation of the instrument about its main axis.

The main encoder readings corresponding to incidence of such beams on a distant target can be used in a similar way to that described for Laserfix to deduce both the azimuth and elevation angles for the direction of that target. The main difference occurs here in that the value of the angular separation of the horizontal rays in the planes of each of the two beams, 0, is now always 180', This process of forming the W' shaped beam pairs from non-simultaneous rotations is further illustrated in Figure 8. Here is shown the sequence of passage of the laser fan beams as the theodolite alidade is rotated in azimuth as the laser fans are successively rotated to new positions around the face of the projector disc.

An important feature of a theodolite, made in accordance with the present invention, is that there is neither correlation nor synchronisation required between the angular positions of the instrument alidade and of the projector disc, and hence, no mechanical coupling is required between them. Another significant feature is shared with Laserfix, namely that since the alidade rotations are monitored in angular terms via the encoder, there is no need to maintain constancy of rotation of the scanning mechanism.

Further, rotation of the projector disc about it's own axis does not have to be accurately controlled since the accuracy of the measurements is independent of the particular rotation position of the projector disc. For survey monitoring applications, where fixed targets are observed in sequence over a period of time, the projector disc could be set to rotate slowly and continuously. Rotations about the two axes of the apparatus may be produced by normal electric motor/ flexible drive-belt combinations.

7 In practice, the inclination to the instrument vertical axis Z - Z' of each arm of the 'V' beam will be the same, since they are merely different parts of the same plane. Further, since, in geometrical terms, both beams are different sections of a single plane sheet of light, parallel to the plane of the projector disc, the intersection of the main alidade rotation axis With this plane occurs at a single point - there can be no eccentricity error. These factors considerably simplify instrument calibration.

Since for Laserfix the shape of the individual fan beams represents a compromise, between useful horizontal range and vertical angular coverage, the latter is restricted by the available and eye-safe laser radiant power in the beams. In the case of the present invention, the single beam can be tailored only for required horizontal operating range; the desired vertical angular coverage can be achieved merely by rotation of the projector disc.

A preferred embodiment of the optical projection for the new instrument is shown diagrammatically in Fig. 9. Here, a single laser diode is powered through a slip fing and has a fixed geometrical relationship with the projector disc. The laser beam fan is produced by a cylindrical lens attached to a fight-angle reflecting prism and may be rotated for calibration to ensure that the fan beam lies in a plane parallel to the projector disc. The prism is rotatable independently of the projector disc so that the laser beam illuminates successively different sectors of that disc.

Tests performed on a prototype, constructed on a Laser-fix instrument base, produced the same accuracy as the Laserfix theodolite itself, namely, a standard deviation of less than I arc second for a set of ten consecutive direction measures to a quadrant form photocell at a range of 50 metres. An identical accuracy is also achieved where the electronic photocells are replaced with trihedral ('comer-cube') retro-reflecting prisms and a single photocell is mounted on the projector disc and coupled through a slip-ring to an electronic counter board.

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Claims

1. A piece of apparatus orientationally adjustable about first and second axes which are non-perpendicular; first and second drive means operable to rotate the apparatus about its first and second axes respectively; means for projecting a fan-shaped light beam in a plane perpendicular to said second axis and rotatable about that axis by said second drive means; sensor means for sensing the presence of said beam; means for determining the angular position of the apparatus in its rotation about the first axis, and means for computing the direction in space of said sensor means relative to a reference point at the source of the radiation.

2. A piece of apparatus as claimed in Claim 1 wherein the first and second axes have an angular separation of between 30 degrees and 60 degrees.

3. A piece of apparatus as claimed in Claim 1 or in Claim 2 where the source of the light beam is fixed in relation to the first axis of the apparatus and optical components for forming the fan-shaped beam are rotated about the second axis.

4. A piece of apparatus substantially as herein described and illustrated in the accompanying drawings.

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