GB2169768A - Alignment technique - Google Patents

Abstract

In a method of aligning a measuring instrument (1) with a radiation-reflecting target (2), the measuring instrument emits a direction-modulated target beam (S1). A received beam (S2) reflected by the target (2) is demodulated as a function of the modulation criteria of the transmitted beam and is evaluated with respect to its phase position. The result of this evaluation is used for deriving a control signal for the direction tracking of the measuring instrument. The demodulated signal is evaluated with respect to its phase and as a function of amplitude values for deriving control signals for the directional tracking of the target beam. In apparatus for performing the method, an element (12) modulating the beam direction in controlled manner is provided in the transmitted beam path and a demodulator (23) is arranged in the received beam path, which is synchronized with the modulation frequency of the beam modulating or deflecting element (12). <IMAGE>

Description

SPECIFICATION Alignment technique Background to the invention.

The invention concerns an alignment technique and relates to a method and to an apparatus for the automatic alignment of a measuring instrument with a target.

Field of the invention.

In optical range finders, such as are used for example in geodetic surveying with the odolites, levels, etc., generally the distance from the measuring instrument to a marked target is measured. From the instrument location the target is visually sighted e.g. from a theodolite with an attached or integrated range finder. The sought distance is then determined in electronic equipment by means of a transit time measurement of a light pulse or a phase measurement of a light variation signal. The target is usually constituted by an apparatus with one or more corner cubes, know as a so-called retroreflector.

It would be desirable and advantageous to be able to perform the alignment of the measuring instrument with the retroreflector without visual sighting. This would make it possible to sight the measuring point objectively.

In addition, sighting could be accelerated. In inaccessible terrain or under difficult environmental conditions it would be possible to use the method in conjunction with a remote control or to integrate the method into an automatic measuring process.

The first principles for solving this general objective have already been proposed, e.g. by H. INGENSAND "The Further Development of Automatic Target Setting According to the Intensity Maximum Principle and Development of an Electromechanical Suport for an "observerless" Electronic Tacheometer" in BDVI Forum No. 4/1983, pp.186-192. According to this proposal, the intensity of the light returned by the retroreflector is to be used as a measure for the sought direction. The light emitted by the range finder is used as a sight line or target beam and guided e.g. in a meandershaped or spiral scanning movement over the target reflector until the reflected and re-received light signal reaches a maximum.

It is a disadvantage of this proposal that in the case of an instantaneous measurement there is no indication of the direction in which the maximum is located, so that the search process is too slow for practical use. Morn- over, the maximum constitutes an insensitive measuring criterion, because its first derivative is by definition zero, so that the intensity does not change infinitesimally on moving away from the maximum. Particularly in the case of a flat maximum, the direction determination becomes unreliable. Finally, the light distribution of a light source varies for different distances. Thus, for example, close and far fields can have completely different light distrbutions.The symmetry of the reflected light cone can be destroyed by the angular errors occasionally present in reflector prisms, which can lead to incorrect determinations of the direction to be measured.

It is accordingly an aim of the present invention to provide a usable solution for the alignment of the measuring instrument with the retroreflector without visual sighting, so that the sighting of the measuring point can be performed objectively and more accurately than has hitherto been the case.

Summary of the invention.

In one aspect the present invention provides a method for the automatic alignment of a measuring instrument with a radiation-reflecting target, wherein a direction-modulated target beam is emitted by the measuring instrument, wherein a target-reflected beam is demodulated as a function of the modulation criteria of the transmitted beam and is evaluated with respect to its phase position, and wherein the result of this evaluation is used for deriving a control signal for the directional following of the measuring instrument.

The invention also provides, in a further aspect, apparatus for performing the method of the invention, wherein an element modulating the beam direction in a controlled manner is provided in the transmitted beam path, and wherein a demodulator is arranged in the received beam path and is synchronized with the modulation frequency of the beam-rotating elements.

The invention can enable a much greater setting accuracy when aligning measuring instruments with a target. The process can be controlled and/or automated in a simple manner. This fulfils an essential prerequisite for integrating e.g. theodolites and similar measuring instruments into automatic measuring stations. Brief description of the drawings.

The invention is now described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a representation of a range finder with a target beam moved in planned manner on a retroreflector; Figure 2 is a diagrammatic representation of an alignment device using a rotating prism; Figure 3 is a graph illustrating the time-dependent intensity course of the light reflected by the retroreflector into the measuring apparatus; Figure 4 is a spatial representation of the intensity pattern of reflected tight; and Figures 5A to 5E illustrate further examples of alignment means with a scanning beam moved in a controlled manner.

Detailed description of the drawings.

Figure 1 shows a measuring instrument 1, e.g. a surveying instrument, a retroreflector 2 sighted from the instrument, the rotation axis a about which a light beam S, emitted by instrument 1 is rotated, as well as the target beam S1 at time t, and following a half rotation at time t2. Also shown is light cone A rotating about axis a, and which is in reality three-dimensional.

Figure 2 illustrates further deails of the arrangement for aligning the instrument 1 with the retroreflector 2. Axis b of an optical transmitter 10 is inclined by an angle alpha with respect to the rotation axis a of the transmitting part of the intrument 1. By means of a lens 11, a beam from transmitter 10 is supplied to a rotating prism 12, which is arranged for rotation about axis a at a given speed by a motor 13. After passing a second lens 14, the rotary transmitting beam is emitted as target beam S1 by means of a collective lens 15. The target beam thereby rotates about axis a, whilst including the angle alpha.

Figure 2 also shows the receiver-side optical path for receiving the received beam S2 reflected by the retroreflector, which includes a receiving collective lens 20, a photoelectric receiver 21 located downstream thereof, and a preamplifier 22. The amplified receiving signal is supplied in an appropriate manner from preamplifier 22 to a demodulator 23. The latter is provided with a phase control, which is coupled to the speed control for motor 13.

Due to the optical characteristics of the rotating prism 12, the target beam S, rotates about the axis a at a speed 2f which corresponds to twice the rotational speed f of the rotating prism. Therefore a frequency 2f (double the rotational frequency of the rotating prism) is chosen as the coupling frequency for demodulator 23. The demodulated signals from demodulator 23 are separated according to amplitude and phase and are separately supplied to a computer 24, which derives therefrom coordinate control signals V or H for the vertical and horizontal drive of the measuring equipment adjustment.

If the transmitted target bean S strikes the retroreflector 2 as illustrated in Figure 1, then light energy is reflected and is collected as a received beam S2 in photoelectric receiver 21 via the collective lens arrangement 20 which acts as a telescope. As the target beam rotates, the energy reflected back by the target reflector varies with the rotation period, for example as is shown in Figure 3. However, such intensity fluctuations only occur while the reflector is not precisely located in the axis a of the rotating target beam.

The phase values derived during demodulation in demodulator 23 contain information on the instantaneous rotation angle of rotating prism 12 for which the maximum intensity differences of the received signal S2 are obtained during a rotation. Control signals are derived therefrom in computer 24 for the motor-driven horizontal and vertical drives of the measuring instrument. Controlled by computer 24, these are adjusted to cause movement of the measuring instrument until the amplitude fluctuations of the demodulated received signal disappear.

The corresponding signal pattern is shown in Figure 3. The amplitude signal representing the intensity I of the light reflected back by retroreflector 2 is subject to periodic fluctuations, so long as axis a is not exactly aligned on retroreflector 2. The signal fluctuates with the rotation period T = 1/2f where f is the rotational speed of the rotating prism 12 (or 1/f of motor 13). The first intensity maximum It of the received signal has a phase difference phi compared with the time to indicating the start of the motor rotation period T. The intensity value Ii is compared with a second intensity value 12, which is determined at a time t2, which is displaced by half a rotation period compared with the first time t1.The difference between the two measured intensity values supplies the indicated criterion for the alignment of the transmitted beam with retroreflector 2, which takes place with the aid of control signals V and H.

For the purpose of deriving these control signals, the measured phase values phi are converted in computer 24 into corresponding portions for the V or the H-control signal, as a function of the difference between intensity values Ii and 12. Time t serves as a reference point for a particular angular position of rotating prism 12. Angle phi indicates in which angle coordinate, based on rotation axis a, there is an intensity maximum. The proportional dimensioning of the V and H-control signals take place accordingly. Changes to the measured intensity differences, e.g. the first derivation thereof, are used as a measurement criterion during this following or tracking correction.

The described processes are illustrated in Figure 4. The transmitted beam rotating about rotation axis a is partly reflected by retroreflector 2, the degree of reflection fluctuating as a function of the rotation angle or rotation phase. At time t an intensity amplitude Ii is measured, whereas at time t2 (in the present example half a rotation period later) a much smaller amplitude 12 is measured. Only when the two amplitudes are the same, i.e. when their difference is zero, is the retroreflector 2 positioned precisely on the rotation axis a of the transmitted beam.

In place of the rotating prism shown in Figure 2 it is possible to use various other optical or optoelectrical components for the controlled rotation of the obliquely incident transmitted or target beam. Hereinafter a number of preferred possible alternative embodiments with modified means for rotating the transmitted beam will be described, without there being any claim to completeness. In fact other suitable means can also be used for the same purpose. In such modified embodiments, derivation of the control signals for following the transmitted beam axis can take place on the basis of similar criteria and with similar means to those described hereinbefore in conjunction with the rotating prism arrangement.

According to Figure 5A the optical beam deflection system comprises a rotating optical wedge 30, which is driven by a motor 13 in much the same way as in the previous embodiment. The wedge can be used in place of prism 12 in the arrangement of Figure 2.

Figure 5B shows a construction with a plane-parallel plate 40, which is driven by motor 13 and rotated in such a way that the beams emitted by the optical tansmitter 10 describe a cone about optical axis a after traversing the plate. The cone angle can be determined by inclining the plate relative to optical axis a.

According to another variant, the movement of the transmitted beam can be brought about by a controlled displacement of the optical transmitter. A corresponding embodiment is diagrammatically shown in Figure 5C. A radiation source 50, e.g. a light-emitting diode is placed in a receptacle 51, in which is also fixed an optical light guide 52. On the output side the light guide 52 is inclined by angle alpha with respect to the optical axis a and is mounted in a rotating disk 53 with a guide hole 55 eccentric with respect to the disk rotation axis 54. The latter is preferably identical to the optical axis a of the embodiment of Figure 2. Disk 53 is once again driven by a drive element diagrammatically designated as motor 13.

The controlled movement of the guided output (right-hand) end of light guide 52 can also be produced in any other suitable way. Thus, according to Figure 5D, the end of the light guide 52 remote from the radiation source can be provided with a magnetizable end piece 56, which is arranged in a magnetic field, produced by electromagnets 57, 58 arranged in crosswise manner. By means of a control device 59, the electromagnets can be excited for producing a rotary magnetic field component, so that end piece 56 and with it the end of light guide 52 are rotated about the optical axis of the apparatus.

As a variant of this drive principle, it is possible to use in place of the deflection magnets, piezoelectric pick-ups 60, 61 as shown in Figure 5E which act in a deflecting manner on the free end of light guide 52. A corresponding excitation control of the piezoelectric pick-ups is known per se and need not be represented in detail.

Apart from the aforementiond preferred embodiments, it is also possible to use other controlled, rotating elements, e.g. optoelectrical elements. In accordance with these embodiments for different motion drives, it is also possible to replace the motor drive of the embodiments according to Figures 2, 5A and 5B by other suitable drive means for producing a controlled search movement of the transmitted target beam.

Claims (12)

1. A method for the automatic alignment of a measuring instrument with a radiation-reflecting target, comprising emitting a directionmodulated target beam from the measuring instrument, demodulating a target-reflected beam as a function of the modulation criteria of the transmitted beam and evaluating the demodulated signal with respect to its phase position, and deriving from the result of this evaluation a control signal for the directional following of the measuring instrument.

2. A method according to claim 1, wherein a target beam directed on to the target is deflected with respect to a main beam axis according to a given mathematical relationship, wherein the beam reflected by the target is received and demodulated in accordance with the deflection rules, and wherein the demodulated signal is evaluated with respect to its phase and amplitude for deriving control signals for the directional following of the target beam.

3. A method according to claim 2, wherein the target beam directed on to the target is rotated about its rotation axis with a given frequency and with a deflection angle, and wherein the received beam reflected by the target is demodulated in a demodulator with the rotation frequency of the target beam or a multiple thereof.

4. Apparatus for performing the method according to claim 1, comprising means for modulating the beam direction in a controlled manner provided in the transmitted beam path, and a demodulator arranged in the received beam path and synchronized with the modulation frequency of the beam modulating means.

5. Apparatus according to claim 4, wherein the beam modulating means comprise a motor-driven rotating prism.

6. Apparatus according to claim 4, wherein the beam modulating means comprise a rotary optical wedge.

7. Apparatus according to claim 4, wherein the beam modulating means comprise a periodically moved plane-parallel plate.

8. Apparatus according to claim 4, wherein the beam modulating means comprise means for the controlled displacement of an optical transmitter and consequently for the controlled movement of the target beam from the transmitter.

9. Apparatus according to claim 8 wherein the beam-deflecting or modulating means are in contactless operative connection with the optical transmitter.

10. Apparatus according to any one of claims 4 to 9, wherein the demodulator is arranged to supply signal values characteristic of the phase and amplitude of the received signal to computer means, which are provided with outputs for supplying tracking signals for the direction position of the measuring instrument, the computer means containing means for linking the phase values supplied by the demodulator with amplitude signals.

11. An alignment method substantially as herein described with reference to the accompanying drawings.

12. Alignment apparatus substantially as herein described with reference to, and as shown in, the accompanying drawings.