WO1992003698A1

WO1992003698A1 - Device for detecting the position of laser beams

Info

Publication number: WO1992003698A1
Application number: PCT/DE1991/000654
Authority: WO
Inventors: Jürgen THIEL
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1990-08-15
Filing date: 1991-08-15
Publication date: 1992-03-05
Also published as: DE4126948C2; DE4126948A1; AU8322791A

Abstract

The description relates to a device for detecting the position of laser beams, e.g. for straightness measurement with a laser, the beam of which impinges on a position-sensitive sensor, the output signal from which is applied to an evaluation unit which determines position deviations in relation to the laser beam. The feature of the invention is that in the path of the laser beam there is a beam divider (RS1) which splits the laser beam into the actual measuring beam (M) and a reference beam (R), there are in the path of the reference beam a mirror system (S1, S2) which multiply deflects the reference beam and, downstream thereof, a second position-sensitive sensor (PSD) on which the reference beam impinges and from the output signal of which the evaluation unit detects spatial directional changes in the laser beam and with that knowledge the position of the beam in the useful branch is corrected.

Description

Device for detecting the beam position of laser beams

Description

Technical field

The invention relates to a device for detecting the beam position of laser beams, for example for straightness measurements with a laser, in which the laser beam impinges on a position-sensitive sensor, the output signal of which is applied to an evaluation unit that determines positional deviations with respect to the laser beam The aim is to detect fluctuations in the beam position of the laser source in order to eliminate errors that arise due to beam position fluctuations.

State of the art

Devices for straightness measurement, which are also referred to as straightness measuring systems, are used, for example, in production measurement technology, where they are used to check the straightness of guides, traversing slides, movements of machine slides, robot movements etc. or for alignment measurements.

A known device for straightness measurement is the laser straightness meter from Vialog, Hanover. In this device for straightness measurement, the laser beam from a HeNe laser is used as a straightness reference, that is to say as an "optical ruler", and the offset to this beam is measured with the aid of a position-sensitive photodiode. The photodiode is shifted in the laser beam direction and the point of impact of the beam on the photodiode is measured in one or two dimensions. Since the directional stability of the laser beam depends on the mechanical stability of the laser tube and the position of the resonator mirrors with respect to one another, the outlay on equipment is great in order to reduce the deviations of the laser beam from the “straight line” below a value of a few micrometers / meters (um / m) hold. For this purpose, it is necessary, among other things, to use special lasers which, for example, have resonator mirror carriers made from a piece of Zerodur. The known device for straightness measurement is therefore comparatively complex and therefore expensive to manufacture.

A device for detecting the beam position is also known, for example, from the dissertation by E. Trapet "A contribution to reducing the measurement uncertainty of laser-based alignment measuring systems". Trapet describes in connection with possibilities of a beam position correction, among other things, a device for straightness measurement, in which the laser beam emanating from a position-stabilized laser head and aligned parallel to the direction of movement is first passed through a fixed beam splitter arranged at the beginning of the measuring section, to which a first reference detector is assigned. The beam entering the measuring section hits another beam splitter on the movable machine part, to which the measuring detector for the straightness measurement is assigned. The beam part running straight through the beam control of the movable machine part strikes a third detector at the end of the measuring section, namely the second reference detector. With such a device for straightness measurement it is possible to make more reliable statements than without beam position correction, but this known device has some serious disadvantages which have a decisive influence on the measurement uncertainty sen. On the one hand, it is disadvantageous that pitching, yawing and rolling movements of the movable machine part are included in the measurement result of the straightness measurement and falsify it accordingly.

Furthermore, a device for beam detection is known from the patent publication DE 34 00 151. In this publication, a device is described in which a partially transparent triple reflector is used on parts of the beam splitter on the movable machine part, with which the beam is positioned on the one hand at a position at the beginning of the measuring section Reference detector is reflected back, on the other hand runs through the partially transparent triple reflector and strikes a second reference detector attached to the end of the measuring section. This arrangement has the advantage over the Trapet solution that pitching, yawing and rolling movements leave the measurement result unaffected by the use of the triple reflector, and in addition straightness deviations are displayed with a double parallel offset.

However, just like the Trapetian arrangement, this device has enormous disadvantages in practical use, which affect beam position fluctuations due to fluctuations in the refractive index of the air (turbulence) and due to refractive index gradients of the air perpendicular to the direction of beam propagation. It has been shown that from the dissertation by H. Schüßler "The Suitability of Laser Beam and Photoelectric Detectors for Measuring the Deviation of Straightness and Flatness in Mechanical Engineering" it follows that a refractive index gradient, which can be caused, for example, by a temperature gradient of 1 degree per Meters, a change in beam radiation of 0.5 µm multiplied by the square of that of the La air gap (in meters). This means, for example, a beam deviation of 12.5 µm for an air distance of 5 m, or a beam deviation of 50 µm for the air line of 10 m from the ideal straight line. Since the second reference detector is positioned at the end of the measuring section in both Trapet and Schüßler, beam position fluctuations due to air turbulence and due to refractive index gradients have a full effect on the measuring results of the reference section. Since Schüßler uses a triple reflector on the moving machine part, the

When the beam is reflected back parallel to itself, the length of the measuring section doubles, but this also doubles the errors due to turbulence, which even quadruples the errors due to refractive index gradients. A further disadvantage of both of the above-mentioned methods in practical terms is that in the event that the space behind the measuring detector is not accessible to a second reference detector because it is covered, for example, by the machine itself, no beam position control can be carried out. Another disadvantage of the large distance from the second reference detector is that deflections of the machine part which carries the second reference detector, for example under load, due to thermal influences or vibrations, are also fully included in the measured values.

Description of the invention

The invention is based on the object of specifying a device for beam position control, for measuring tasks which use a laser beam as a reference line, which detects beam position fluctuations of a laser beam and which are largely independent of disruptive influences caused by air turbulence and refractive index gradients According to the invention, therefore, no attempt is made to stabilize the laser beam with complex measures with regard to its direction; rather, the directional fluctuations of the laser beam that occur in conventional lasers are accepted and measured, and the beam position is corrected arithmetically with the measured values. For this purpose, the laser beam is split into two partial beams by a beam splitter, namely the actual useful laser beam and a reference beam.

In the stationary reference branch, which is preferably encapsulated from external air influences, two position-sensitive sensors, which can be position-sensitive diodes, for example, measure the point at which the beam strikes at two different distances from the laser, so that beam position fluctuations are recognized and their size and direction determined can.

Since beam position fluctuations have a greater effect at a greater distance from the laser, the reference branch should be as long as possible in order to obtain the largest possible correction signal. It is particularly preferred if the length of the reference beam path is in the order of the length of the useful beam path, which can be, for example, several meters. For this reason, the beam path is folded several times and shielded from external influences, so that changes in the point at which the laser beam strikes the position-sensitive sensors, which are not caused by beam position fluctuations, are small compared to the changes caused by beam position fluctuations.

Since beam position fluctuations have the same effect in the reference branch as in the useful branch, can be determined with the and offers a high resolution with the smallest possible space requirement.

An inventive solution to this problem is specified in claim 1. Developments of the invention are the subject of the dependent claims.

Surprisingly, the object of the invention can be achieved in that a beam splitter is arranged in the beam path of the laser beam, which splits the laser beam into the actual useful laser beam and a reference beam, and that another beam in the beam path of the reference beam or the useful beam

A beam splitter is provided with an associated first reference detector, which detects the beam position at the beginning of the reference path, and which has a mirror system in the beam path of the reference beam that folds the reference beam several times and a second position-sensitive sensor is provided after the mirror system, which the reference beam strikes; From the output signals of both position-sensitive sensors, the evaluation unit determines changes in position of the laser beam in space, with the knowledge of which the beam position in the useful branch can be corrected.

According to the invention, it has been recognized that each laser beam is subject to parallel shifts on the one hand and changes in direction on the other hand, the latter being in the range of typically 10 μrad. This means that the position of the laser beam at a distance of one meter in a plane perpendicular to the beam direction can already fluctuate by ± 10 µm relative to the ideal reference. Beam position fluctuations, taking into account the distance between laser and position-sensitive sensor in the useful branch, the position of the laser beam in the room can be corrected by calculation.

The design according to the invention makes it possible to keep the (corrected) beam position fluctuation of the laser in the order of magnitude of 1 μm / m even with commercially available lasers and even with semiconductor lasers.

However, an arrangement according to claim 1 has the disadvantage that beam position correction is only sensibly possible if the stability of the reference path is guaranteed, in particular the stability of the mirror path with respect to tilting of the mirrors with respect to one another. In a further development, therefore, a further optical element is provided in the reference beam path, which The reference beam is split into two beams, with the original reference beam remaining unchanged in its direction, for example, and the second reference beam striking the mirror system at an angle different from the original reference beam, and a third position-sensitive reference detector is provided after the mirror system, the output signal of which is used to tilt the Mirror to each other. It has been shown that tilting two mirrors to each other, between which a light beam is reflected several times, has an effect on the direction of the incident beam which increases approximately quadratically with the number of reflections on one of the mirrors. For example, if one selects the number of reflections of both reference beams in the mirror system such that one reference beam is twice as long as that of the other, beam position fluctuations of A distinction is made between mirror tiltings, since the ratio of the output signals of the reference detectors of the two reference sections is just about 2 due to the factor 2 in the length of the sections in the case of fluctuations in the beam position of the laser, and is approximately 4 for mirror tiltings, due to the approximately square effect with twice the number of reflections . With such an arrangement for checking the stability of the mirror path with an additional optical element, which can be, for example, a beam splitter with a prism or a Wollaston prism or another beam-splitting element, the mirror system is largely independent of mechanical or thermal influences.

A further embodiment, in which the mirror system is attached to a base part, which can also carry the laser, and which consists of a material with a small coefficient of thermal expansion - for example Zerodur, Invar or a fiber composite material - has the advantage that changes in the impact of the Laser beam on the position sensitive sensor that is not from

Beam position fluctuations are minimized. The modulation of the laser light significantly reduces interference from light, particularly when working with light in the visible range, which can also be emitted by semiconductor lasers, for example.

Furthermore, the mirror system can have two mirrors, each of which the reference beam strikes several times. This design not only provides a simple structure, but also a structure that is largely free of interferences, such as those caused by thermal expansion, shocks, etc. As already explained, for the computational correction of the beam position fluctuations for straightness measurements, in addition to the (known) laser / position-sensitive reference sensor distances, it is also necessary to know the laser / position-sensitive sensor distance in the measuring branch, which is generally variable. Therefore, in a further development, a distance measuring system is provided, which measures the distance between the laser and the position-sensitive (measuring) sensor, and whose output signal is applied to the evaluation unit, which determines the beam position fluctuations occurring at the respective location of the position-sensitive sensor. This distance measuring system can in particular be an ultrasonic measuring system in which the ultrasonic transmitter is connected to the base part, for example, and the ultrasonic receiver is connected to the position-sensitive (measuring) sensor.

Brief description of the drawing

The invention is described below by way of example without limitation of the general inventive concept on the basis of exemplary embodiments with reference to the drawing, to which reference is expressly made with regard to the disclosure of all details according to the invention not explained in detail in the text. Show it:

Fig. 1 shows the structure of a device for detecting the

Beam position of laser beams for the application of a straightness measurement, and

Fig. 2 is a block diagram of an evaluation unit. Description of an embodiment

FIG. 1 shows the structure of a device according to the invention for detecting the beam position of laser beams, in which the laser beam is split into a reference beam R and a measuring beam M with the aid of a beam splitter BS1. The reference beam R is with the help of a

Beam splitter BS2 split into the reference beams R1 and R2. The reference beam R1 strikes a position- _sensitive photodiode PSD _Ref1 . The reference beam R2 (dashed) is folded several times by two mirrors S1 and S2 and strikes a position- _sensitive photodiode PSD _Ref2 , which, like PSD _{Ref1, registers} the deviation of the beam beam from the central position, so that beam position fluctuations of the laser beam in the reference beam branch can be detected.

For better understanding, the virtual position of the two reference photodiodes is shown in dashed lines in FIG. The penetration points of the laser beam on these fixed reference detectors define the position of the straight lines spanned by the laser beam in space and can be described by the following formula, where x _p , y _p for the

Parallel offset and α, α stand for an angular change: x _i = x _p + tan (α _x ) * z _i

Y _i = y _p + tan (α _y ) * z _i

The measuring beam M strikes a measuring photodiode PSD _{measuring which} on the one hand registers the straightness deviation to be measured and on the other hand also the beam position fluctuations. With the help of the known distances z ₁ and z _{2 of} the reference detectors, changes in the impact point x _o -Y _o on the measuring photodiode PSD _n result due to beam position fluctuations according to the following formula: x _o = {X ₂ (z _M -z ₁ ) + x ₁ (z ₂ -z _M )} 7 (z ₂ -z ₁ )

y _o = {y ₂ {z _M -z ₁ ) + y ₁ (z ₂ -z _M )} / (z ₂ -z ₁ )

If the measured value x _meas , y _meas deviates from the calculated value

Value x _o, y _o from, so this is a shift in the Meßphotodiode PSD _measurement perpendicular to the laser beam S and provides the result x to be measured deviation y from the nominal straight line is: x = x _measurement - x _o

y = y _meas - y _o

As already mentioned, in addition to the constant distances of the reference detectors from any zero point, the distance of the measuring detector from this zero point must also be known, since a change in direction of the beam has a greater effect at a greater distance than at a smaller one.

The accuracy of the correction is strongly dependent on the beam folding in the reference branch. Therefore, the reference branch R with the mirrors S1 and S2 is housed in a stable, closed housing G, which preferably a mounting plate made of a material with a low coefficient of thermal expansion, such as. B. Zerodur, Invar or fiber composite material.

To detect mirror tilting, the reference beam R2 can be used, for example, with the aid of a beam splitter BS3 (Fig. 1) are divided such that the original reference beam R2 remains unchanged in its direction (dashed beam) and the other beam is reflected perpendicular to the plane of the drawing and deflected with a prism or mirror so that it is at a different angle to R2 strikes the mirror system (solid beam) so that fewer reflections take place. The planes of both beams are (not necessary) preferably parallel. This beam strikes a third position- _sensitive reference _detector PSD _Ref3 , which registers the point of impact of the beam. As already mentioned above, mirror tilts and beam changes can be clearly distinguished from the measured values of the three reference detectors, so that the device is largely independent of mirror tilts.

For the application of the straightness measurement, the ultrasound signal and the signals of the reference diodes and the measuring diode are simultaneously recorded, amplified and, after A / D conversion by an A / D converter, processed digitally by means of a computer, which can be, for example, a commercially available microcomputer. The computer determines the correction value, determines the straightness deviation in the usual way and creates a measurement report etc.

The invention has the advantage, on the one hand, that with the correction of the beam position of the laser described above, it is possible to advance into the accuracy class of 1 μm, while known devices working with lasers have beam direction deviations of 10 μrad, ie 10 μm / m. Beam position fluctuations are thereby corrected, so that the accuracy of the measuring system only depends on the accuracy of the position-sensitive photodiode or the electronic circuit and additionally is independent of the measuring distance. On the other hand, the device according to the invention is significantly cheaper, more compact and more robust than current measuring systems.

Claims

Patent claims

1. Device for detecting laser beams with the following features:

a beam splitter is arranged in the beam path of a laser beam, which splits the laser beam into the actual useful beam and a reference beam,

a further beam splitter and subsequently a position-sensitive sensor are provided in the beam path of the reference beam or of the useful beam, on which the laser beam strikes,

- In the beam path of the reference beam, a mirror system that folds the reference beam several times, and after the mirror system, a second position-sensitive sensor is provided, which the reference beam strikes, and from its output signal together with the output signal of the first position-sensitive sensor, the evaluation unit changes direction or Parallel changes in the laser beam in the room are determined.

2. Device according to claim 1,

characterized in that a further optical element is provided in the beam path of the reference beam, which splits the original reference beam into at least two reference beams which impinge on the mirror system at a different angle and then each on an assigned position-sensitive sensor, and from its output signals changes in the point of impact of the The cause of the laser beam can be separated from one another by changes in the beam position or by tilting the mirrors of the mirror system.

3. Device according to claim 1 or 2,

characterized in that the two reference beams strike the mirror system at the same angle, but that one of the two mirrors is shorter in the plane of a reference beam, so that fewer reflections take place there than in the plane of the other reference beam.

4. Device according to one of claims 1 to 3,

characterized in that the optical element in front of the mirror system is a beam splitter which deflects part of the laser beam perpendicularly zv to the plane of the original reference beam, and in that a 90 ° prism or a mirror is subsequently provided, which preferably converts the beam into one of the original Reference beam deflects parallel plane.

5. Device according to one of claims 1 to 4,

characterized in that the optical element in front of the mirror system consists of a polarization filter and a Wollaston prism, so that the reference beam is split into two beams with an angle dependent on the Wollaston prism.

6. Device according to one of claims 1 to 5,

characterized in that the mirror system is mounted in a closed housing on a base which also carries the laser and which is made of a material with a small coefficient of thermal expansion.

7. The device according to claim 6,

characterized in that material is Zerodur or Invar.

8. Device according to one of claims 1 to 5,

characterized in that the laser is a semiconductor laser.

9. Device according to one of claims 1 to 8,

characterized in that the laser light is modulated.

10. The device according to one of claims 1 to 7,

characterized in that the position sensitive sensors are position sensitive diodes.

11. Device according to one of claims 1 to 10, characterized in that the mirror system has two mirrors on which the reference beam strikes each time several times.

12. The device according to one of claims 1 to 11, characterized in that the longer reference beam is approximately twice as long as the shorter reference beam.

13. Device according to one of claims 1 to 12, characterized in that the length of the reference beam path is of the order of the length of the measuring beam path.

14. The apparatus according to claim 13,

characterized in that the length of the reference beam path is several meters.

15. The device according to one of claims 1 to 14, characterized in that a distance measuring system is provided which measures the distance between the laser and the position-sensitive (measuring) sensor, and its off is applied to the evaluation unit, which determines the beam position fluctuations occurring at the respective location of the position-sensitive sensor.

16. The apparatus of claim 15,

characterized in that the distance measuring system a

Ultrasound measuring system is.

17. The apparatus of claim 16,

characterized in that the ultrasonic transmitter is connected to the base part and the ultrasonic receiver is connected to the position-sensitive (measuring) sensor.

18. Device according to one of claims 1 to 17, characterized in that the correction value of the evaluation unit is used to track the laser beam by means of optical elements, with means of optronics or the laser itself with electronically controlled adjustment units with respect to its position and direction in space .