WO2008145184A1

WO2008145184A1 - A method and an apparatus for calibration of a linear track

Info

Publication number: WO2008145184A1
Application number: PCT/EP2007/055248
Authority: WO
Inventors: Peter Fixell; Samuel Brikho
Original assignee: Abb Technology Ab
Priority date: 2007-05-30
Filing date: 2007-05-30
Publication date: 2008-12-04

Abstract

The present invention relates to an apparatus and a method for automatically calibrating a linear track (20) along which a device is moving while it is performing work. The method comprises: moving a mechanical unit (18), provided with a first angle- measuring sensor (1) arranged for measuring an angle relative to the vertical line about a first measuring axis and a second angle-measuring sensor (2) arranged for measuring an angle relative to the vertical line about a second measuring axis essentially perpendicular to the measuring axis of the first angle-measuring sensor, along the track, receiving angular measurements from both angle-measuring sensors for a plurality of locations along the track, and calculating vertical changes in position along the length of the track for both sides of the track based on the received angular measurements from both angle-measuring sensors.

Description

A METHOD AND AN APPARATUS FOR CALIBRATION OF A LINEAR TRACK

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for calibration of a linear track along which a device is moving and carrying out work during operation. The device is, for example, a manipulator of an industrial robot.

PRIOR ART

In a world of industrial robots, the requirement of robot installations to fulfill a good positioning performance is continuously increasing.

There are a number of products available on the market today that are built into the robot controller software in order to modify programmed positions on a robot path. The mechanical tolerances and deflections in the robot arms are identified and cor- rections made to the robot positions, to compensate for any discrepancy. The goal is to make the robot positioning as ideal as possible and to facilitate the use of theoretical positions in robot programs. Those theoretical positions can, for example, be generated by off-line programs, vision guidance, or similar solu- tions. In a robot cell, absolute accuracy is required for more parts than just the robot. Consequently, there is a need to know the correct mechanical behavior of any performed motion in the cell.

In order to extend the work area of a robot it is common to mount the robot on a linear track such that the robot is movable along the track. This is, for example, suitable in the car industry when the robots are used to perform work, such as welding, painting, or gluing, on parts moving on a conveyer. In such a case the linear track is placed along the length of the conveyer. However, linear tracks can be used in many different applications in which the work area of the robot is to be extended.

When a linear track has been installed in a robot cell, it is important that the track is straight in the direction along as well as across the track in order to achieve a good positioning performance. Further, a curved track will increase the stress on bearings and slide bars of the track, thus reducing the life of the track.

Particularly, if the robot is programmed off-line the track has to be completely straight. Otherwise, there will be positioning errors in programmed points on the movement path of the robot due to changes in the height of the track. An often-used solution to this problem is to use high-performance measuring equip- ment, such as laser trackers, and corresponding knowledge of how to perform the needed measurements in order to identify the compensation needed. An estimated five days of work is needed to identify and calculate the necessary corrections using costly equipment, like laser trackers, when installing a linear track. However, there is still a high risk that the performed measurements will be incorrect. Further, laser trackers are very expensive. As an alternative to a completely straight track, it is known to calculate a compensation model based on measurements of the height of the track, thereby enabling exact position- ing of the manipulator.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a cost-effective and fast solution to the above described problems in connection with calibration of a linear track. This object is achieved by a method as defined in claim 1 .

According to the invention, the mechanical unit is provided with a first angle-measuring sensor arranged for measuring an angle relative to the vertical line about a first measuring axis and a second angle-measuring sensor arranged for measuring an angle relative to the vertical line about a second measuring axis essentially perpendicularly to the measuring axis of the first an- gle-measuring sensor. The method comprises moving the mechanical unit along the track, receiving angular measurements from both angle-measuring sensors for a plurality of locations along the track, and calculating changes in vertical position along the length of the track for both sides of the track based on the received angular measurements from both angle-measuring sensors. By both sides are meant both long sides of the track.

The mechanical unit used during the calibration procedure can be any type of mechanical unit suitable for being moved along the track.

For example, the mechanical unit can be a carriage carrying a manipulator. The angle-measuring sensors can be mounted either on the carriage or on the manipulator. It is also possible to only use the carriage during the calibration procedure, and to mount the manipulator on the carriage when the calibration has been finished. It is advantageous to use the device, which is used during operation for carrying out work while moving along the track, as the mechanical unit used during the calibration pro- cedure. However, the mechanical unit does not necessarily have to be the device used for carrying out work while moving along the track. For example, the mechanical unit can be two sliding bearings connected with a rod. What is important is that the sensors have a fixed position relative to the mechanical unit and follow the movements of the mechanical unit during the calibration. Preferably, the mechanical unit is automatically moved along the track during calibration. For example, a computer program controls the movement of the mechanical unit. However, it is possi- ble to manually move the mechanical unit along the track during calibration.

The movement of the robot along the track can either be continuous and the measurements are made during the continuous movement, or stepwise during which the robot is moved an incremental length and the angle measuring is carried out after each incremental movement. The moving of the mechanical unit and the measuring of the angles are preferably repeated along the complete length of the track. Angular measurements from both angular measuring sensors are received and stored for a plurality of locations along the length of the track. Accordingly, changes in inclination along the track are measured. Thereafter, vertical changes in position, i.e. changes in the height of the track, along the length of the track on both long sides of the track are calculated based on the received angular measurements from both angle-measuring sensors.

The invention proposes to use leveling sensors, for example electronic spirit levels, in order to identify the curvature of the track along its length. Such sensors are much cheaper than laser trackers. Although this solution will not cover all degrees of freedom, it will, according to evaluations made, identify at least 80% of the mechanical deviations and drastically reduce the positioning error. According to the invention, the sensors are mounted on a mechanical unit that is used to transport the sensors along the track. This makes it possible to automatically measure the curvature of the track and to calculate the changes in position along the length of the track. The cost and time required for calibration of the track will be reduced to a minimum and the requirement on skilled people will not be as tough as before. A standard service engineer will be able to carry out the calibration of the track.

According to an embodiment of the invention, the method com- prises calculating changes in orientation along and across the track based on the angle measurements, and based thereon calculating changes in position along the length of the track for both sides of the track. Preferably, one of the angle-measuring sensors is arranged such that it measures the inclination of the track in the longitudinal direction of the track and the other angle-measuring sensor is arranged such that it measures the inclination of the track in a direction across the track.

According to an embodiment of the invention, the track com- prises a plurality of adjusting members for mechanically adjusting the height of the track along the length of the track, and the method comprises calculating values as to how much each of the adjusting member has to adjust the height of the track in order to align the track based on the calculated changes in posi- tions along the track, and presenting said values to an operator. For example, the values represent distances that specific positions on the track have to be adjusted upward or downward in order to be aligned with the rest of the track. Thus, a distance including information on the direction, i.e. upward or downward, is presented for each adjusting member. This embodiment makes it easy for an operator to align the track by adjusting the mechanical positions of the track.

According to an embodiment of the invention, the method com- prises calculating compensation parameters for recalculation of positions on a programmed movement path of the mechanical unit based on the calculated changes in positions along the track. It is possible to adjust the mechanical positions of the track to a certain extent. However, in some applications the de- mand on positioning performance is very high. In order to further improve the positioning performance, the moving of the me- chanical unit and the angle measuring along the length of the track are repeated after the mechanical positions of the track have been adjusted, and the calculated changes in positions along the track are used for calculating compensation parame- ters for positions on a programmed path for the mechanical unit. Thus, a mathematical compensation for positioning errors due to alignment errors of the liner track is made based on the angular measurements along the track. This embodiment further improves the positioning performance of the mechanical unit.

According to a further aspect of the invention, the object is achieved by a computer program product directly loadable into the internal memory of a computer or a processor, comprising software code portions for performing the steps of the method according to the appended set of method claims, when the program is run on a computer. The computer program is provided either on a computer-readable medium or through a network, such as the Internet.

According to another aspect of the invention, the object is achieved by a computer-readable medium having a program recorded thereon, when the program is to make a computer perform the steps of the method according to the appended set of method claims, and the program is run on the computer.

According to another aspect of the invention this object is achieved with an apparatus for calibration of a linear track as defined in claim 8.

Such an apparatus comprises: a first angle-measuring sensor arranged on the mechanical unit for measuring an angle relative to the vertical line about a first measuring axis, a second angle- measuring sensor arranged on the mechanical unit for measuring an angle relative to the vertical line about a second measur- ing axis essentially perpendicularly to the measuring axis of the first angle-measuring sensor, and a computer unit to receive an- gular measurements from both angle-measuring sensors for a plurality of locations along the track, and to calculate vertical changes in position along the length of the track for both sides of the track based on the received angular measurements from both angle-measuring sensors. Preferably, the computer unit is adapted to provide orders to the mechanical unit about moving it along the track thereby making the calibration automatic.

According to an embodiment of the invention, the mechanical unit is a manipulator of an industrial robot, the manipulator comprises a base and at least one arm arranged movable relative to the base of the robot, and said angle-measuring sensors are positioned on the base of the robot. Commonly, the base of the manipulator is already provided with a mounting surface for a calibration tool having two perpendicularly arranged angle- measuring sensors, which are used during calibration of the home position of the robot. Therefore, it is convenient to use the same calibration tool and the same mounting position on the manipulator for calibration of the linear track.

According to an embodiment of the invention, said industrial robot comprises a robot controller controlling the positions of the mechanical unit and said computer unit is a part of the robot controller. It is advantageous to utilize the robot controller to perform the necessary calculations for calibration of the track. If the same calibration tool is used for calibration of the linear track as for calibration of the robot home position, the calibration tool is already connected to the robot controller and the controller receives angle-measurements from the calibration tool. A mathematical compensation of positions on the programmed path can also preferably be done internally in the controller.

According to an embodiment of the invention, the apparatus fur- ther comprises a tool adapted to be movably mounted on the rail, which tool is designed to translate horizontal curvature of the rail into a change of the angle in relation to the vertical line, which tool is provided with an angle-measuring sensor arranged for measuring an angle relative to the vertical line about a measuring axis, and said computer unit is adapted to receive angular measurements from both angle-measuring sensors for a plurality of locations along the rail, and to calculate horizontal changes in position along the length of the track based on the received angular measurements from the angle-measuring sensor.

Preferably, the angle-measuring sensor is one of the sensors used for determining changes in the vertical position along the length of the track. However, it is also possible to use another sensor. A second measurement is performed to identify the cur- vature in the horizontal plane using angular measuring sensors and a special tool to be mounted on one of the rails from which the track comprises. Preferably, tool is mounted on the movable mechanical unit. The sequence will be the same, moving the mechanical unit along the track in incremental lengths and re- ceiving angular measurements. The special tool is designed to translate the horizontal curvature to a change of the angle in relation to the vertical line, i.e. the gravity direction, to make it possible to use gravity sensors. It is also possible to manually move the tool along the rail. According to this embodiment, changes in the vertical position along the length of the track is determined in a first step, in which the sensors are mounted on the mechanical unit, and changes in the horizontal position along the length of the track is determined in a second step, in which one of the sensors is mounted on a special tool. Of course, the order of the measurement is of no importance, step two can as well be performed before step one. The two-step methodology will make it possible to cover all degrees of freedom, it will, according to evaluations done, identify at least 95% of the mechanical deviations and drastically reduce the position- ing error. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Figure 1 shows an example of an angle-measuring device suitable for the invention.

Figure 2 shows an industrial robot including a manipulator mounted on a linear track.

Figure 3 shows the manipulator and the linear track in a view from above.

Figure 4 shows an example of a diagram of calculated deviations in vertical positions along the length of a track.

Figure 5 shows a flow diagram of a method according to an em- bodiment of the invention.

Figure 6 shows an example of a tool for calibration of the horizontal changes of position of the track in an upper view.

Figure 7a shows the tool of figure 6 in a view from one side. Figure 7b shown the tool in a view from the opposite side.

Figure 8a shows a view from above of the tool of figure 6 when used in an outwardly bent curve of the rail. Figure 8b shows a side view of the tool when used in an outwardly bent curve of the rail.

Figure 9a shows a view from above of the tool of figure 6 when used in an inwardly bent curve of the rail. Figure 9b shows a side view of the tool when used in an inwardly bent curve of the rail. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows an angle-measuring device for measuring angles in two perpendicular directions. The angle-measuring device comprises two angle-measuring sensors 1 , 2 in the form of inclinometers measuring the angle of an object relative to the vertical line. An inclinometer functions as an electronic level and measures the inclination angle about a measuring axis. However, it is possible to use any type of sensor that measures an angle relative to the vertical line. The sensor can, for example, be optical or mechanical. It is advantageous if the sensor is provided with an electrical communication interface The angle- measuring sensors 1 , 2 are positioned with their measuring axes 3, 4 perpendicular to each other, to be able to measure in two perpendicular directions. Such an angle-measuring device is described in the international patent application with publication number WO 02/084216.

Figure 2 shows an industrial robot including a manipulator 10, a robot controller 12 for controlling the movement of the manipulator, and a teach pendant unit 14 for programming and manually moving the manipulator. The manipulator is programmed to per- form work while moving along the track. The robot program includes a plurality of programmed positions on a path to be followed by a tool or a workpiece held by the robot. The manipulator 10 comprises a base 16, which is mounted on a movable carriage 18, which is positioned on a linear track 20. The robot fur- ther comprises a stand 22, which is rotatable relative to the base about a vertical axis. Further, the robot comprises a plurality of robot arms, which are rotatably mounted about each other. The carriage 18 is movable along the track 20 in the x-direction. The movement of the manipulator, as well as the movement of the carriage 18, is controlled by the robot controller 12. The teach pendant unit 14 is provided with a display device 15. The teach pendant 14 is communicating with the robot controller 12. The manipulator 10 is provided with an angle-measuring device 24 of the type shown in figure 1 . The angle-measuring device 24 is mounted on the base 16 of the manipulator 10. It is also pos- sible to mount the angle-measuring device on the carriage.

Figure 3 schematically shows the angle-measuring device 24, the carriage 18, and the track 20 in a view from above. The track 20 comprises two parallelly arranged rails 30 and 31 . The carriage 18 is arranged movable along the rails 30, 31 in the x- direction. The height of the rails 30, 31 is adjusted by means of adjusting members 34. Each rail is provided with a plurality of adjusting members 34, in the form of feet, arranged at a distance from each other along the rails. The adjusting members 34 include adjusting screws for adjusting the height of the rail upward and downward in dependence on the screwing direction. However, it is possible to use other types of adjusting members. The adjusting screws are manually operated, for example, by a service engineer. As seen from the figure, the angle-measuring device 24 is arranged such that the first angle-measuring sensor 1 measures the inclination of the track 20 in the longitudinal direction of the track, i.e. in the x-direction, and the second angle- measuring sensor 2 measures the inclination of the track 20 in a direction across the track, i.e. in the y-direction.

During calibration of the track, the carriage 18 is moved incrementally along the track, and at each stop the angles identified by the angle sensors 1 , 2 are measured. The smaller the increments taken when doing the measurements, the better the result will be and as this sequence can be made fully automatic, there is no limitation due to the work load. When a measurement is ready, the carriage is commanded to move to the next incremental location for a new measurement to be taken. This sequence is repeated until the complete length of the track is cov- ered. By doing this along the length of the track 20, it is possible to calculate changes in orientation of the carriage along and across the tracks. The calculated changes in orientation are used to calculate changes in positions in a vertical direction for both rails 30,31 along the length of a track. This information can be used in two ways. One way is to give feedback to a service engineer how to adjust the height of the track in order to get a better leveling of the track. This can, for example, be done by presenting values as to how many millimeters that need to be added or subtracted at each and everyone of the adjusting members along the track. The information on how to adjust the track in order to get a better leveling of the track is, for example, presented on the display device 15 of the teach pendant unit 14. The information can also be used to create compensation parameters for the recalculation of the robot positioning in order to compensate for the mechanical inaccuracy due to the mounting of the track.

The movement of the carriage during the calibration procedure, the collection of measurements, and the calibration calculations are performed by software run on a processor. This software is advantageously stored in the robot controller and run on a processor of the robot controller. The angle-measuring device 24 is preferably connected to the robot controller 12. However, it is also possible to use an external computer for performing the calculations and using a display device of the external computer for the presentation of the values. The complete sequence can be automated. With the angle-measuring device connected to the robot controller, the angle information can automatically be collected and synchronized with position information of the track.

The angle-measuring device is preferably mounted to the base of the robot. On most robots, there is already a machine surface available on the base of the robot. This machine surface is used for calibration of the manipulator. The angle-measuring device is connected to the controller and a special program is started and the measuring sequence will begin. The measuring sequence includes measuring the angles provided by the angle-measuring device in each of the incremental steps along the complete length of a track. These values are evaluated and recalculated into a metric deviation.

Figure 4 shows an example of a diagram of calculated deviations (d) in positions in a vertical direction at a plurality of locations along the length (I) of a track. Curve 40 shows calculated deviations in positions in a vertical direction along rail 30 of the track, and curve 42 shows calculated deviations in vertical positions along the rail 31 of the track. As seen from the figure, the deviation is zero for both rails in the beginning of the track and thereafter the deviations are fluctuating. The curves show the topology of the rails in millimeters.

Figure 5 shows a flow diagram of the method and the computer program product according to an embodiment of the present invention. It will be understood that each block of the flow chart can be implemented by computer program instructions.

The carriage is positioned at one end of the track. Angular measurements from both angle-measuring sensors 1 ,2 are received and stored, block 50. A movement order to the carriage 18, carrying the manipulator, is generated, box 52. The carriage is ordered to move an incremental length along the track. When the carriage has been moved the increment, new angle measurements are received and stored. This sequence is repeated until the carriage has been moved to the other end of the track, block 54.

The angle values from the sensor that measures the inclination in the x-direction are used to calculate angle changes along the track, and the angle values from the sensor that measures the inclination in the y-direction are used to calculate angle changes across the track, block 56. Vertical changes in position for both sides of the track, i.e. vertical changes in positions for the left and right rails 30,31 of the track, are calculated based on the calculated angle changes along and across the track, block 58. The deviation can be calculated using sequential lines with direction according to the measured inclination in combination with the length of the incremental steps. At the end of each incremental step a new direction will be obtained by measuring the inclination and moving the incremental length. The deviation of the position of the track is achieved by combining the incremental steps into a diagram (see figure 4).

For each of the adjusting members, the distance to be adjusted including the direction, i.e. upward or downward, is calculated based on the calculated changes in positions, block 60. The calculated distances and the directions are displayed, for example, on the display device 15 of the teach pendant unit 14, block 62. A service engineer adjusts the height of the rails based on the displayed distances. In order to further align the track it is suitable to repeat the steps 50-62 and further adjust the rails. When the positions of the rails have been mechanically adjusted as accurately as possible, it is possible to further improve the accuracy by repeating the steps 50-58 and creating compensation parameters for recalculation of programmed path positions of the manipulator based on the calculated changes in positions along the track. The compensation parameters could be pre- sented as a table using the position along the track as input or they could be presented as a model based solution (mathematical function). The compensation parameters will describe the robot positioning using a real kinematic model.

According to the embodiment of the invention, it is also possible to identify the curvature in the horizontal plane, and based thereon calculate the changes sideways of the track. This is done by using a special tool designed to be movably mounted on one of the rails, and designed to translate horizontal curva- ture of the rail into a change of the angle in relation to the vertical line. The tool is provided with an angle-measuring sensor arranged for measuring an angle relative to the vertical line about a measuring axis. The computer unit is adapted to receive angular measurements from both angle-measuring sensors for a plurality of locations along the rail, and to calculate horizontal changes in the position along the length of the track based on the received angular measurements from the angle-measuring sensor. The special tool comprises two parts generating and transforming the difference in curvature to a change in vertical direction that can be measured by one of the sensors.

Figure 6 shows an example of such a special tool 70 for calibration of the horizontal changes of the position of the track in an upper view. Figure 7a shows the tool in a view from one side. Figure 7b shown the tool in a view from the opposite side. The tool 70 comprises two essentially triangularly shaped parts 71 , 72, each having tree tips 74,75,76 and 78,79,80. The first part 71 comprises a first tip 74 adapted to be movably connected to one of the rails 30 of the track in a joint A, a second tip 75 adapted to be connected to the rail 30 and rotatably joined to- gether with the tip 79 of the second part 72 in a joint B, and a third tip 76 adapted to be connected to the tip 80 of the second part via a vertical shaft 82. The second part 72 comprises a first tip 78 adapted to be movably connected to the rails 30 in a joint C, a second tip 79 adapted to be connected to the rail 30 and rotatably joined together with the tip 75 of the first part 71 in joint B, and a third tip 80 adapted to be connected to the tip 76 of the first part via the vertical shaft 82. The two parts 71 ,72 are joined together in a free rotational joint B.

The third tip 76 of the first part 71 is connected to one end of the shaft 82 by a free rotational joint D. The third tip 80 of the second part 72 is connected to the other end of the shaft 82 by a free rotational joint E. Joints D and E are separated a fixed distance in a vertical direction. The shaft 82 is provided with an angle-measuring device 24 detecting changes in the angle of the shaft relative to the vertical line. The angle-measuring device 24 includes at least one sensor measuring the angle between the vertical line and the measuring direction of the sensor. The shaft 82 will change its angle in relation to the vertical line when the rail changes curvature radius as shown in figure 8a-b and figure 9a-b. Figure 8a shows a view from above of the tool when used in an outwardly bent curve of the rail. Figure 8b shows a side view of the tool when used in an outwardly bent curve of the rail. Figure 9a shows a view from above of the tool when used in an inwardly bent curve of the rail. Figure 9b shows a side view of the tool when used in an inwardly bent curve of the rail.

The tool 70 is, for example, connected to the carriage 18 during the calibration. Thus, when the carriage is moved along the track, the tool follows the movement of the carriage and the tool is moved along the rail. At the beginning of the calibration procedure, the tool is positioned at one end of the rail. An angle measurement from the angle-measuring device 24 is received and stored. The tool is moved an incremental length along the track. When the tool has been moved the increment, a new an- gle measurement is received and stored. This sequence is repeated until the tool has been moved to the other end of the track. By using the known geometry of the tool and the change of angel from the sensor it is possible to calculate a measure of how the curvature of the rail in the horizontal plane. The hori- zontal changes in position along the length of the track are calculated based on the calculated curvature in the horizontal plane and the incremental length, which is known.

The adjusting members 34 of the track are also adapted for me- chanically adjusting the track sideways along the length of the track. The method preferably comprises calculating values on how much each of the adjusting member has to adjust sideways of the track in order to align the track based on the calculated changes in horizontal positions along the track, and presenting said values to an operator. For example, the values represent distances that specific locations on the track have to be ad- justed left or right in order to be aligned with the rest of the track. Thus, a distance including information on the direction, i.e. left or right, is presented for each adjusting member. This embodiment makes it easy for an operator to align the track by adjusting the mechanical positions of the track.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the manipulator does not have to be mounted on the carriage during the calibration, and in that case the measuring device is mounted on the carriage instead of on the manipulator.

Claims

1 . A method for calibrating a linear track (20), wherein the method comprises: moving a mechanical unit (10, 18), provided with a first angle-measuring sensor (1 ) arranged for measuring an angle relative to the vertical line about a first measuring axis and a second angle-measuring sensor (2) arranged for measuring an angle relative to the vertical line about a second measuring axis essentially perpendicular to the measuring axis of the first angle-measuring sensor, along the track, receiving angular measurements from both angle-measuring sensors for a plurality of locations along the track, and calculating vertical changes in position along the length of the track for both sides of the track based on the received angular measurements from both angle-measuring sensors.

2. The method according to claim 1 , wherein the method comprises calculating changes in orientation along and across the track based on the angle measurements, and based thereon calculating vertical changes in position along the length of the track for both sides of the track.

3. The method according to claim 1 or 2, wherein the track (20) comprises a plurality of adjusting members (34) for adjusting the height of the track along the length of the track, and the method comprises calculating values as to how much each of the adjusting members has to adjust the height of the track in order to align the track based on the calculated vertical changes in positions along the track, and presenting said values to an operator.

4. The method according to any of the preceding claims, wherein the method comprises calculating compensation pa- rameters for recalculation of positions on a programmed move- merit path of the mechanical unit based on the calculated changes in positions along the track.

5. The method according to any of the preceding claims, wherei n one of the angle-measuring sensors is arranged such that it measures the inclination of the track in the longitudi nal direction of the track and the other angle-measuring sensor is arranged such that it measures the inclination of the track in a direction across the track.

6. The method according to any of the preceding claims, wherei n the track comprises at least one rail and the method further comprises: mounti ng a tool (70), designed to translate horizontal curva- ture of the rail into a change of the angle in relation to the vertical line, on the rail, which tool is provided with an angle- measuri ng sensor (1 ) arranged for measuring an angle relative to the vertical line about a measuri ng axis, moving the tool along the rail , receiving angular measurements from the angle-measuring sensor for a plurality of locations along the track, and calculating horizontal changes in position along the length of the track based on the received angular measurements from the angle-measuri ng sensor.

7. A computer program product directly loadable into the internal memory of a computer, comprising software for performing the steps of any of claims 1 -5

8. A computer-readable medium, having a program recorded thereon, where the program is to make a computer perform the steps of any of claims 1 -5, when said program is run on the computer.

9. An apparatus for automatic calibration of a linear track (20), characterized in that the apparatus comprises: a mechanical unit (10) arranged movable along the track, a first angle-measuring sensor (1 ) arranged on the mechanical unit for measuring an angle relative to the vertical line about a first measuring axis, a second angle-measuring sensor (2) arranged on the mechanical unit for measuring an angle relative to the vertical line about a second measuring axis essentially perpendicularly to the measuring axis of the first angle-measuring sensor, and a computer unit (12) adapted to receive angular measure- ments from both angle-measuring sensors for a plurality of locations along the track, and to calculate vertical changes in position along the length of the track for both sides of the track based on the received angular measurements from both angle- measuring sensors.

10. The apparatus according to claim 9, wherein the computer unit (12) is adapted to provide orders to the mechanical unit about moving it along the track.

1 1 . The apparatus according to claim 10, wherein the track (20) comprises a plurality of adjusting members (34) for adjusting the height of the track along the length of the track, and the computer unit (12) is adapted to calculate values as to how much each of the adjusting member has to be adjusted in order to align the track based on the calculated changes in positions along the track, and to present said values as to how much each of the adjusting member has to be adjusted to an operator.

12. The apparatus according to any of claims 9-1 1 , wherein the computer unit (12) is adapted to calculate compensation parameters for recalculation of positions on a programmed movement path of the mechanical unit based on the calculated changes in positions along the track.

13. The apparatus according to any of claims 9-12, wherein the computer unit (12) is adapted to calculate changes in orientation along and across the track based on the angle measurements, and based thereon calculate vertical changes in position along the length of the track for both sides of the track.

14. The apparatus according to any of claims 10-13, wherein said mechanical unit (10) is a manipulator of an industrial robot, the manipulator comprising a base and at least one arm arranged movable relative to the base of the robot, and said angle-measuring sensors are positioned on the base of the robot.

15. The apparatus according to claim 14, wherein said industrial robot comprises a robot controller (12) controlling the positions of the mechanical unit and said computer unit is a part of the robot controller.

16. The apparatus according to any of claims 9-15, wherein one of the angle-measuring sensors is arranged such that it measures the inclination of the track in the longitudinal direction of the track and the other angle-measuring sensor is arranged such that it measures the inclination of the track in a direction across the track.

17. The apparatus according to any of claims 9-16, wherein the track comprises at least one rail and apparatus further com- prises: a tool (70) adapted to be movably mounted on the rail, which tool is designed to translate horizontal curvature of the rail into a change of the angle in relation to the vertical line, which tool is provided with an angle-measuring sensor (1 ) ar- ranged for measuring an angle relative to the vertical line about a measuring axis, and said computer unit (12) is adapted to receive angular measurements from both angle-measuring sensors for a plurality of locations along the rail, and to calculate horizontal changes in position along the length of the track based on the received angular measurements from the angle-measuring sensor.