WO1992016817A1

WO1992016817A1 - A method and an arrangement for determining the positions of a plurality of measuring points on an object, such as an automotive vehicle, said measuring points being moveable in relation to a reference point

Info

Publication number: WO1992016817A1
Application number: PCT/SE1992/000158
Authority: WO
Inventors: Rudolf Wiklund
Original assignee: Geotronics Ab
Priority date: 1991-03-15
Filing date: 1992-03-13
Publication date: 1992-10-01
Also published as: SE468449B; SE9100796L; SE9100796D0

Abstract

The present invention relates to an arrangement for determining the positions of a plurality of measuring points on an object, such as an automotive vehicle, these measuring points being moveable in relation to a reference point. According to the invention, an optical signal emitter (11-1n) is arranged at each measuring point. These signal emitters function to produce, in sequence, an optical pulse of short duration, which is delivered to a computer (3) through the intermediary of an optical receiver means (2), which produces an output signal representative of the position of the signal emitter concerned. The signals are identified in the computer on the basis of the time point at which they were produced. The positions of the measuring points are also calculated by the computer and displayed on a display device (4), for instance a display screen. This measuring arrangement enables several measuring points to be monitored at one and the same time, therewith enabling a total picture of the deformation of an object to be readily obtained. The invention also relates to a method for determining the positions of a plurality of measuring points on an object, these measuring points being moveable in relation to a reference point.

Description

A Method and an Arrangement for Determining the Positions of a Plurality of Measuring Points on an Object. Such as an Automotive Vehicle. Said Measuring Points Being Moveable in Relation to a Reference Point

The present invention relates to a method and to an arrangement for determining the positions of a plurality of measuring points on an object, such as an automotive vehicle, said measuring points being moveable in relation to a reference point.

When straightening cars that have suffered collision damage, the positions of a plurality of measuring points on the car chassis are compared with the original posi- tions of these same points as defined in the car manu¬ facturing specifications, and straightening of the car is continued until all measuring points coincide with the positions specified by the manufacturer, within given tolerance values. Many different measuring systems are known for the purpose of facilitating the work of straightening damaged vehicles. One such known measuring system, retailed by the company Dataliner AB, uses laser beams. The laser beams are brought into alignment with scales connected to the measuring points and the point at which a respective beam impinges on a respective scale shows the deviation between the actual or prevailing position of the measuring point and its correct position. The scales are suspended from the measuring points in a manner such that all the correct measuring-point positions given on the scales will lie in one and the same plane, the so-called measuring plane. Thus, in order to determine a measuring plane and to calibrate the ^' measuring system in relation to said plane, it is Unnecessary to obtain three correct calibration points, i.e. three measuring points on the collision damaged vehicle that are located on completely undamaged, or practically undamaged parts of the vehicle and therewith have correct positions, since the indicated deviation between the position of the measuring points and the correct positions are completely related to the measuring plane. The method used to calibrate the measuring arrangement comprises a number of mutually different working procedures and is relatively time consuming. Furthermore, when using a measuring arrangement of this kind, it is difficult for an operator to monitor more than one measuring point at a time during the course of a vehicle straightening operation, and consequently any negative effect on measuring points other than on the monitored measuring point may not be discovered early enough in the straightening operation to be rectified. Since the positions of the measuring points are related to a fixed measuring plane, it is necessary to establish a new measuring plane and to provide a new calibration if the car, or vehicle, should be moved from the measuring plane during the course of a straightening operation.

The present invention relates to a method and to an arrangement for determining the positions on an object of several measuring points which are moveable in relation to a reference point, said method and said arrangement enabling several measuring points to be monitored simultaneously and also allowing the whole of the object to be moved relative to the reference points without needing to re-calibrate said arrangement.

The inventive method is characterized by the following steps:

an optical signal emitter capable of producing an optical signal of short duration at a given time point is placed at each measuring point;

the signal emitters are initiated in a mutually sequen¬ tial time sequence at specific time points;

each signal produced by an initiated signal emitter is received by an optical receiver means which, upon re¬ ceiving said signal, produces a signal representative of the position of the measuring point;

the signals produced by the receiver means are, in turn, delivered in sequence to a calculating means; and

the measuring point concerned is identified by the calculating means on the basis of the time point at which the associated signal emitter was initiated, and the position of the measuring point in relation to other measuring points and/or in relation to the reference point at a given point in time is calculated, stored and displayed.

Because the positions of all measuring points are dis¬ played simultaneously, it is easy to obtain a total picture of the deformation of the object concerned. This information is also obtained irrespective of whe-ther or not the object is moved in relation to the measuring system, since the positions of the measuring points are given in relation to each other or in relation to a reference point, and it is therefore not necessary for the calibration points always to remain in a specific measuring plane during a straightening operation. Furthermore, when the inventive method is used for measuring a car chassis, no re-calibration is necessary if the whole of the car chassis is moved during the straightening operation, as distinct from earlier known measuring systems, and the measuring arrangement enables calibration to be effected much more quickly than is the case with known measuring arrangements.

According to one preferred embodiment of the inventive method, the time-sequence initiation of the signal emitters is repeated and is continuous. When this method is applied for measuring a car chassis during a straight¬ ening operation, the development of the straightening operation can therewith be monitored continuously for all measuring points, which facilitates discovery and cor¬ rection of an erroneously chosen pulling or pressing direction in the straightening operation.

According to a variant of this embodiment, the position of respective measuring points at a point in time subse¬ quent to signal emitter initiation are calculated by extrapolation of stored values of measuring point posi¬ tions at at least two given time points. This enables the measuring system to be used to predetermine the final result and also the time at which a commenced straightening operation will be completed, and also enables the speed at which each measuring point moves to be determined. This information can be used to monitor and control automatically one or more commenced straigh¬ tening operations that have been started.

According to another variant of the preferred embodiment of the invention, the reference point is fixed in rela- tion to the object concerned, which somewhat facilitates calibration of the inventive measuring arrangement to some extent.

The invention also relates to an arrangement for carrying out the aforesaid method. This arrangement is characterized in that it includes an optical signal emitter at each measuring point, said signal emitter functioning to produce an optical signal of short dura¬ tion at a given time point, and further includes means for initiating the signal emitters in a time sequence at specific time points; an optical receiver which after receiving a signal produced by a signal emitter produces a signal which is representative of the position of the measuring point concerned, means for identifying the measuring point concerned on the basis of the time at which the associated signal emitter was initiated, means for calculating the position of respective measuring points in relation to other measuring points and/or in relation to the reference point at a given time point on the basis of signals received from the receiver, and means for storing the calculated positions and visually displaying said positions.

According to one preferred embodiment of this arrange- ment, each signal emitter includes a light source and the receiver means includes at least two optical receivers which are mutually separated in given positions in relation to the reference point. According to a variant of this arrangement, the light sources produce a light signal whose spectrum deviates from that of normal light, and the optical receivers are comprised of a fish-eye lens and a SITEC-detector arranged in the image plane of the lens.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which

Figure 1 is a diagrammatical, schematic illustration of one embodiment of an inventive arrangement; Figure 2 is a schematic perspective view of the arrange¬ ment shown in Figure 1 used to measure a car chassis;

Figure 3 illustrates schematically the construction of an optical receiver which can be used in an arrangement according to Figures 1 and 2; and

Figure 4 illustrates a variant of the measuring arrange- ment illustrated in Figure 2.

The measuring arrangement illustrated in Figure 1 in¬ cludes a plurality of optical signal emitters 1 -1 , for

1 n instance light-emitting diodes. In order to eliminate disturbances from other light sources in the area in which the measuring arrangement is to be used, the spectrum of the light emitted by the optical signal emitters will preferably differ from the spectrum of normal light and may lie, for instance, in the infrared range.

The inventive measuring arrangement also includes an optical receiver means 2, which receives the signals produced in time sequence by the signal emitters and, on the basis of these signals, produce output signals which are representative of the positions of respective signal emitters 1 -1 in relation to the receiver means. Ac-

1 n cording to one preferred embodiment of the invention, the receiver means 2 includes at least one optical receiver 7, which includes a fish-eye lens 8 and an SITEC-detector

9 placed in the image plane or picture plane of the lens

8. As will be seen from Figure 3, a detector of this kind has a flat, extensive surface and produces an electrical output signal which corresponds to the coordinates of the point at which an incident light beam impinges on the flat surface. Since this impingement point corresponds directly to the direction of the incident light beam, the detector output signal will therefore indicate this direction.

In a preferred variant of this arrangement, the receiver means 2 includes two optical receivers of the aforedes- cribed kind, which are placed apart in fixed positions relative to one another and relative to a fixed reference point. In this way, there are obtained two directional signals which indicate the position of the light source relative to the receiver means, which is sufficient to define unambiguously the position of the light source, i.e. the position of the signal emitter concerned. For reasons of redundancy and measuring accuracy, it may also be appropriate to provide the receiver means with three optical receivers 7.

It is also feasible to use a receiver means which com- prises only one single optical receiver, when each signal emitter includes two light sources which are disposed in fixed positions relative to one another and which are designed to produce a light signal one after the other.

However, this design results in a more complicated construction of each signal emitter and also complicates the calculations required to determine the positions of respective signal emitters.

The output signals from the receiver means 2 are deliv- ered to a computer 3, via an A/D-converter 6. The computer functions to calculate the positions of re¬ spective signal emitters on the basis of the output signal from the receiver means, thereby identifying the signal emitter concerned. Since the signal emitters are initiated in a time sequence, one after the other, as previously mentioned and as described in more detail herebelow, the signal emitters can be identified by the computer on the basis of that point in time at which the output signal of said receiver means is delivered to the computer.

The arrangement illustrated in Figure 1 also includes a display unit 4, such as a terminal display screen for instance, on which the new, calculated position of the signal emitter concerned is shown, preferably together with the earlier calculated positions of the remaining signal emitters.

In order to facilitate identification of the signal emitters and to enable the use of optical receivers which are capable of detecting only one light source at a time, the signal emitters are preferably initiated in a manner such that when one signal emitter is activated or ignited, all remaining signal emitters are inactivated or extinguished. To this end, the arrangement illustrated in Figure 1 includes an initiation circuit or initiation device 5. This initiation device is intended to deliver voltage pulses to the signal emitters 1 -1 , sequentially and at given time intervals, so as to ignite briefly their light-emitting diodes. The device 5 is connected to the computer 3, and is preferably activated thereby.

In its simplest form, the device 5 is constructed so that subsequent to being activated continuously over a given time interval, it will produce voltage pulses which are distributed in sequence to the various signal emitters

1 -1 by a distributor. This process continues until the I n device 5 is switched-off by a stop signal from the computer 3, and each signal emitter is thus activated over given time intervals which are equal to the sum of the time response of n-number of voltage pulses and n- number of intervals between said pulses. With the use of such an initiating device 5, the output signals arriving at the computer 3 can be identified with their associated signal emitters solely on the basis of their order number in said sequence. In order to enable any desired number of signal emitters to be used, the distributor of the initiating device 5 can be constructed in a manner which will enable any desired number of signal emitters to be disconnected from said device without affecting the duration of the time interval between the activation of remaining signal emitters. This embodiment will, of course, require that the number of remaining signal emitters can be programmed in the computer.

Figure 2 illustrates a slightly modified variant of an inventive measuring arrangement. This variant differs from the arrangement illustrated in Figure 1, mainly because each of the signal emitters 1' -1' includes an

1 6 activating device, and because the receiver means 2, the A/D-converter 6 and the computer 3 of the Figure 1 embodiment are incorporated in a single unit 10, which also includes an initiation signal emitter for the cordless activation of the light-emitting diodes of the signal emitters. The initiation signal emitter may have the form of a light-signal transmitter or acoustic-signal transmitter, in which case the signal emitters will include a receiver which activates its respective light- emitting diode upon receipt of a signal which is particular to each signal emitter, e.g. an acoustic signal of given frequency. The signal emitters may also be provided with delay circuits, so that the various signal emitters will be activated sequentially in time sequence through the action of the delay circuits, subsequent to initiation of all signal emitters by means of a common initiation signal.

As will be seen from Figure 2, the unit 10 includes two optical receivers T and 7' . which are of the kind illustrated in Figure 3. These signal emitters are positioned in fixed relationship with one another and with a reference point on the unit 10, which reference point may be chosen as desired. The positions of the optical receivers 7' and 7' and the selected reference

1 2 point are programmed in the computer of the unit 10.

The manner in which the inventive measuring arrangement is used to measure an automotive vehicle will now be described step-by-step with reference to Figure 2.

Figure 2 illustrates a slightly impact-damaged car in broken lines. As a first step in the straightening operation, the signal emitters 1' -1' are placed at

1 6 appropriate measuring points on the car chassis or car body. The positions of these measuring points on an undamaged car of the make concerned are given by the manufacturer of the car and are appropriately programmed in the computer included in the unit 10, subsequent to having positioned the signal emitters. It is mentioned in this respect that the length of respective signal emitters is preferably such that even the light-emitting diode of a signal emitter which is placed on the highest of these measuring points will be located beneath the lowest point of the car, so as to ensure that the light signal produced by the light-emitting diode will not be obstructed by any part of the car. When the signal emitters are given mutually different lengths, it is necessary to program the length of each respective signal emitter in the computer, so that the arithmetical program of the computer is able to take this into account when calculating associated measuring points.

For the sake of clarity, only six signal emitters have been shown in Figure 2, although it will be obvious that the number of signal emitters used may be much greater.

The number of measuring points chosen and their positions on the car will, of course, depend on the extent and the nature of the damage.

The unit 10 and the display screen 4' connected thereto are then positioned such that none of the light-emitting diodes of the signal emitters will be obstructed by any object in the work area in which measurements are taken and so that all light-emitting diodes will lie within the field of view of the wide- angled lens of the unit 10.

In order to determine the position of the unit 10 in relation to the car, three measuring points are chosen as calibration points, for instance the points from which the signal emitters 1 ' , 1' and 1' are suspended.

These three measuring points are preferably placed on parts of the car which are undamaged or only slightly damaged. The initiating signal emitters of the unit 10 are then activated, so as to produce the aforesaid three different signals at given time intervals, these signals activating the signal emitters 1' -1' in sequence. Since the initiating signals are transmitted via the computer, the time at which respective signals are delivered is known and the straightening signals deliv- ered by the optical receivers 7 , 7 can be readily identified, i.e. identified with associated signal emitters. The positions of the three calibration points in relation to the unit 10 and in relation to one another are calculated and stored in the computer. The mutually relative positions of the three calibration points are then compared with corresponding values of corresponding, correct measuring points whose positions have earlier been programmed in the computer. If the deviations between the compared positions lie within the manufacturing tolerances, the positions of the correct measuring points in relation to the unit 10 are deter¬ mined, so as to uniformly distribute said deviations. In other words, if the comparison between the measuring points 1' and 1' illustrate, for example, that the horizontal deviation is l mm, and if one of these points is assumed to have the correct position, the positions of the correct measuring points l' and 1 ' are determined so as to locate these positions at a distance of 0.5 mm from the corresponding relevant measuring point.

Subsequent to thus determining the positions of the three correct measuring points in relation to the calibration points, and therewith relative to the unit 10, the positions of the remaining correct measuring points in relation to one another and in relation to the unit 10 are determined and stored in the computer, for later comparison with the prevailing or actual positions of corresponding measuring points on the car.

On the other hand, if the comparison between the mutu-al- ly relative positions of the calibration points and the positions of the correct measuring points shows that the deviations, or some of said deviations, lie outside the manufacturing tolerances, this will indicate that the car is damaged at one or more of the calibration points. The computer is not then able to determine the positions of the three correct measuring points in relation to the unit 10, at least not in a reliable manner, and the positions of these points must be determined by an operator after assessing the nature and extent of the damage concerned. Consequently, it is important that the three calibration points chosen are situated on undamaged parts of the car to the best possible extent, so as to avoid the necessity of making this assessment and also to facilitate such an assessment.

When calibration and determination of the three correct positions of the measuring points has been carried out by the computer, the positions of the measuring points concerned are determined and compared with the correct positions and the vehicle straightening work can com¬ mence. During the work of straightening the vehicle, the initiating signal emitter of the unit 10 is preferably activated by the computer in a manner to repeatedly and continuously initiate the signal emitters 1' -1', in

1 6 sequence, one after the other. This enables the effect of the straightening operation in progress on all mea¬ suring points to be monitored continuously and to be displayed on the display device 4'.

The computer is preferably programmed so that the display device 4' will display the starting positions of the measuring points prior to commencing the straightening operation in question, the prevailing positions of said measuring points, and their correct positions. This enables the effect of the straightening operation to be easily followed on the display device. The computer may also be programmed so that the operator himself can chose the measuring point or measuring points to be displayed.

In one advantageous variant, the computer can be pro¬ grammed so as to display solely those measuring points whose positions are changed as a result of the straight- ening operation in progress. When the computer is programmed in this way, it is not necessary for the operator to choose the measuring points to be studied, and instead the choice is made automatically while ensur¬ ing, at the same time, that the operator is made aware of all measuring points that are affected by the straightening operation concerned.

When all requisite straightening operations have been completed, the result is verified by comparing all of the relevant positions of the measuring points with their respective programmed correct positions.

It will be understood that the computer may also be programmed to calculate the speed at which a measuring point is moved during a straightening operation, on the basis of the prevailing positions stored at different points in time, and therewith also to calculate the time point at which said measuring point will be located in a given position. The inventive measuring arrangement can thereby be used to control a straightening operation in a manner to stop said operation automatically when said operation has been completed or when movement of the measuring point deviates from a desired movement path; for example, the computer can be programmed to interrupt the straightening operation when the deviation between the prevailing position of the measuring point and the correct position of said point increases during an ongoing straightening operation. This enables several straightening operations to be carried out and monitored simultaneously. In addition to enabling the straighten¬ ing work to be shortened in this way, by allowing a commenced straightening operation to be monitored auto¬ matically, and commencing a new straightening operation, it is possible, in certain instances, to achieve better distribution of the forces applied during the straight¬ ening operations, for instance by carrying out simulta¬ neously two laterally directed pulling operations, such that the resultant forces in the neutral plane of the car, or vehicle, will be zero.

Because large forces are applied during the work of straightening a damaged vehicle, it is possible that the whole of the vehicle will be moved during a straightening operation. In order to prevent such movement from influencing the measuring process, the computer is programmed so that the three correct measuring points which correspond to the selected calibration points are "locked" onto these points, i.e. the computer ensures that these measuring points will follow the movement of the calibration points, and that the correct positions of remaining measuring points are again calculated after such movement of the vehicle.

In the case of the variant illustrated in Figure 4, the optical receiver 2" is connected rotatably to the unit 10", which in addition to including an A/D-converter and computer also includes a computer-controlled mechanism which functions to rotate the optical receiver 2" step- wise through given angles. In the case of this variant, it is not necessary for all signal emitters to lie in the field of vision of the receiver means simultaneously, therewith enhancing flexibility with regard to the positioning of the unit 10" and, for instance, enables the unit to be placed centrally beneath the car. How-ev- er, it is necessary to program the computer with the position to which the receiver means 2" shall be rotated when initiating each signal emitter and receiving its output signal, before the measuring and calibrating process can be commenced. Although the Figure 4 illus- tration does not illustrate the signal emitters included in the measuring arrangement, these emitters may be of the kind illustrated in Figures 1 and 2.

The measuring arrangement may also be used to measure wheel angles with the aid of two or three signal emitters mounted on respective wheels, wherein a reference plane can be determined with the aid of the known measuring points on an undamaged vehicle. Side surfaces, such as side windows, may also be studied with the aid of the measuring arrangement and the signal emitters provided.

Since the optics and electronics used in the measuring arrangement are not completely linear, it is necessary to calibrate the measuring arrangement per se, prior to use. This can be effected by first placing the unit 10 of the Figure 2 embodiment or the unit 10" of the Figure 4 embodiment in its intended position in relation to the straightening bench or the like on which the car is intended to rest during subsequent straightening work.

Three signal emitters are then placed at known points on the straightening bench and the signal emitters then initiated, thereby enabling the position of the unit relative to the straightening bench to be determined. A fourth signal emitter is then positioned in a diamond pattern and its position relative to the three signal emitters arranged on the straightening bench is determ¬ ined at specific places in the pattern. Any deviation between the measured position values and the known values according to the pattern on the basis of the first measuring value are noted and deviations on the basis of these noted values and dependent on where the fourth signal emitter is positioned in the pattern, are calculated by interpolation correction curves for the measuring arrangement so as to take into account any error in said arrangement due to the position of the measuring point. The diamond pattern will preferably extend over the whole of the area within which the signal emitters can be expected to move when straightening a damaged car. A robot can be used for moving the fourth signal emitter in a given diamond pattern. It is also possible to use fixed signal emitters which are placed in a specific pattern, for example the straightening bench used may be equipped with such signal emitters.

The receiver means may also be attached directly to the vehicle chassis in a given position relative thereto, so that its position relative to the correct positions of the measuring points is fully determined. Naturally, in this case, no calibration points are required for deter¬ mining correctly the correct positions of the measuring points in relation to the reference point of the measur¬ ing arrangement. The receiver means should also be attached in this case to an undamaged part of the vehi¬ cle, so as to facilitate the vehicle straightening work on the part of the operator. However, possible damage to the chassis will not have an effect on the final result of the straightening work, but only makes it difficult for the operator to select appropriate straightening operations and the order in which these operations shall be carried out, in order to ensure that the total straightening work shall be kept as small as possible.

As will be understood, the described embodiment of a measuring arrangement may be modified in several ways within the scope of the invention. For example, an optical filter can be placed between the lens of the optical receiver and its detector plate, in order to ensure that only light of a given spectrum will be allowed to reach the detector plate. Furthermore, the measuring arrangement components illustrated schemati¬ cally in Figure 1 need not be assembled in the manner illustrated in Figure 2 and in Figure 4, but may be comprised of separate elements or may be combined in other ways. For instance the display screen may be an integral part of the computer. It will also be under¬ stood that the measuring arrangement can also be used to measure the deformation of different points of other objects than automotive vehicles. The invention is therefore restricted only by the contents of the follow¬ ing Claims.

Claims

1. A method for determining the positions of a plural¬ ity of measuring points on an object, such as an automo¬ tive vehicle, said points being moveable in relation to a reference point, c h a r a c t e r i z e d by the following procedural steps:

an optical signal emitter (1-.-1 ) which is capable of producing an optical signal of short duration at a given time point is placed at each measuring point;

the signal emitters (1 -1 ) are initiated in a mutually sequential time sequence at specific time points;

each signal produced by an initiated signal emitter is received by an optical receiver means (2) which, upon receiving said signal, produces a signal representative of the position of the measuring point;

the signals produced by the receiver (2) are, in turn, delivered in sequence to a calculating means (3) , and

2. A method according to Claim 1, c h a r a c ¬ t e r i z e d by initiating the signal emitters in time sequence repeatedly and continuously. 2υ

3. A method according to Claim l or 2, c h a r a c ¬ t e r i z e d in that the position of the measuring point at a point in time subsequent to initiating the signal emitter is calculated by extrapolation of stored values of the position of the measuring point at at least two given time points.

4. A method according to Claim 1, c h a r a c ¬ t e r i z e d by calibrating the measuring arrangement in accordance with the following steps prior to using said arrangement; holding three measuring points on the object fixed in relation to a reference point and se¬ lecting said measuring points as calibration points; placing a signal emitter at each said calibration point; initiating the signal emitters in the three calibration points in sequence, one after the other and calculating and storing the positions of the calibration points in relation to one another and in relation to the reference point; moving a fourth signal emitter in a given movement pattern; initiating the fourth signal emitter and the three signal emitters in sequence, one after the other at given positions of the fourth signal emitter in said given movement pattern; and calculating the positions of the fourth signal emitters in relation to one another and comparing these positions with said known positions, i.e. the stored positions of the calibration points and the given position of the fourth signal emitter in said pattern; storing any differences between the compared positions as a function of the positions of the fourth signal emitter given by said pattern; calculating correction curves with the aid of interpolation for intermediate positions of the fourth signal emitter in said given pattern; and calibrating the measuring arrangement with the aid of these curves.

5. A method according to any one of Claims 1-3, c h a r a c t e r i z e d by fixing the reference point in a given position relative to the object.

6. An arrangement for determining the positions of a plurality of measuring points on an object, such as a vehicle, said measuring points being moveable in relation to a reference point, c h a r a c t e r i z e d in that the arrangement includes an optical signal emitter (1 - 1 ) at each measuring point, said signal emitter functioning to produce an optical signal of short duration at a given time point; means (3, 5) for initiating the signal emitters in time sequence, one after the other, at given time points; an optical re- ceiver means (2) which subsequent to receiving a signal from a signal emitter produces a signal which represents the position of the measuring point concerned; means (3) for identifying the measuring point on the basis of the time point at which the signal emitter concerned was initiated and for calculating the position of this measuring point in relation to other measuring points and/or in relation to the reference point at a given time point; and means (3) for storing the calculated positions and means (4) for displaying said calculated positions.

7. An arrangement according to Claim 6, c h a r a c ¬ t e r i z e d in that each signal emitter includes two light sources which are arranged in a given relationship with one another and which are intended to produce a light signal one after the other.

8. An arrangement according to Claim 6, c h a r a c ¬ t e r i z e d in that each signal emitter (1 -1 )

1 6 includes a light source; and in that the receiver means (2) includes at least two optical receivers (7) which are mutually spaced apart in given positions in relation to said reference point.

9. An arrangement according to Claim 8, c h a r a c - t e r i z e d in that the receiver means (2) includes three optical receivers (7) .

10. An arrangement according to any one of Claims 6-9, c h a r a c t e r i z e d in that each light source produces a light signal whose spectrum deviates from the spectrum of normal light.

11. An arrangement according to Claim 10, c h a r ¬ a c t e r i z e d in that the light sources emit infra- red light.

12. An arrangement according to any one of Claims 6-11, c h a r a c t e r i z e d in that the optical receivers (7) are comprised of a fish-eye lens and a SITEC-detector placed in the image plane of the lens.

13. An arrangement according to Claim 12, c h a r ¬ a c t e r i z e d in that a filter which allows only light from the light sources to pass through is arranged between the fish-eye lens and the SITEC- detector.